Background Statement for SEMI Draft Document 5409 NEW STANDARD: GUIDE FOR METROLOGY FOR MEASURING THICKNESS, TOTAL THICKNESS VARIATION (TTV), BOW, WARP/SORI, AND FLATNESS OF BONDED WAFER STACKS

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Background Statement for SEMI Draft Document
5409

NEW STANDARD: GUIDE FOR METROLOGY FOR MEASURING
THICKNESS, TOTAL THICKNESS VARIATION (TTV), BOW,
WARP/SORI, AND FLATNESS OF BONDED
WAFER STACKS

Notice
: This background statement is not part of the balloted item. It is provided solely to assist the recipient in
reaching an informed decision based on the rationale of the activity that preceded the creation of this Document.


Notice
:
Rec
ipients of this Document are invited to submit, with their comments, notification of any relevant
patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this
context, “patented technology” is defined as

technology for which a patent has issued or has been applied for. In the
latter case, only publicly available information on the contents of the patent application is to be provided.



Background

This Guide
, which is targeted at the 3DS
-
IC community,

is i
ntended to assist
in
the
selection and use of Bonded
Wafer Stack (BWS) metrology tools
and to provide guidance in
performing BWS measurements such as total
thickness variation, bow, warp, and sori.
In addition, t
he Guide provide
s

examples of BWS measureme
nts for
these
metrology tools

and
can assist wafer producers and users of BWS metrology to develop products and conduct
meaningful evaluations.


This document was developed in the
Inspection and Metrology
TF of

the

N.A. 3DS
-
IC Committee.

The SNARF for
thi
s was

approved
April 3, 2012
.
Draft Document
5409

was approved for yellow ballot in Cycle
1

in CY2013
,

by
the 3DS
-
IC
G
lobal Coordinating Subcommittee (G
CS
)
.


Review and Adjudication Information


Task Force Review

Committee Adjudication

Group:

Inspection
& Metrology Task Force

North America 3DS
-
IC Committee

Date:

April 2, 2013

April 2, 2013

Time & Time zone:

8:00 AM to 10:00 AM, Pacific Time

3:00 PM to 5:00 PM, Pacific Time

Location:

SEMI Headquarters

SEMI Headquarters

City, State/Country:

San Jose,
California

San Jose, California

Leader(s):

Victor Vartanian (SEMATECH)

David Read (NIST)

Yi
-
Shao Lai (ASE)

Urmi Ray (Qualcomm)

Sesh Ramaswami (Applied Materials)

Richard Allen (SEMATECH)

Chris Moore (Semilab)

Standards Staff:

Paul Trio (SEMI NA)

408.943.7041 /ptrio@semi.org

Paul Trio (SEMI NA)

408.943.7041 / ptrio@semi.org


This meeting’s details are subject to change, and additional review sessions may be scheduled if necessary. Contact
Standards staff for confirmation.


Telephone and web
information will be distributed to interested parties as the meeting date approaches. If you will
not be able to attend these meetings in person but would like to participate by telephone/web, please contact
Standards staff.




This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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DRAFT

Document Number:
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Date:
11/14/2013


SEMI Draft Document 5409

NEW S
TANDARD: GUIDE FOR METROLOGY FOR MEASURING
THICKNESS, TOTAL THICKNESS VARIATION (TTV), BOW,
WARP/SORI, AND FLATNESS OF BONDED WAFER STACKS

1

Purpose

1.1

Control of parameters, such as bonded wafer stack (BWS) thickness, total thickness variation (TTV), bow,
warp
/sori, and flatness metrology, is essential to successful implementation of a wafer bonding process. These
parameters provide meaningful information about the quality of the wafer thinning process (if used), the uniformity
of the bonding process, and the a
mount of deformation induced
to

the wafer stack by the bonding process. Total
thickness variation is also critical in certain bonded wafer manufacturing process steps, since non
-
planarity can lead
to problems in subsequent processing steps, including litho
graphic overlay and intermittent electrical contact
between metal layers in the bonded wafers. This Guide provides a description of tools that can be used to determine
these key parameters before, during, and after the process steps involved in wafer bond
ing.

2

Scope

2.1

This Guide provides examples of the capabilities and limitations of various measurement technologies
applicable to bonded wafer stacks (BWS) as well as their suitability for different applications.

2.2

The Guide describes metrology techniques tha
t are applicable to both temporary and permanently bonded wafer
stacks.

2.3

This Guide is complementary to existing SEMI Test Methods for measuring these parameters on single wafers,
in some cases extending existing metrology techniques to a bonded wafer stack

and in other cases describing
metrology techniques specific to a bonded wafer stack.

2.4

The Guide focuses on general measurement techniques including IR laser profiling, white light confocal
microscopy, visible and IR interferometry, capacitance, and back
-
p
ressure metrology. Each technology has unique
strengths and weaknesses

some rely on front
-
side illumination, others on back
-
side illumination. Some techniques
can measure the thicknesses of individual layers in the bonded wafer stack, and some are additio
nally capable of
measuring surface nanotopography.

2.5

The metrology examples provided in this Guide originated from industry experts and are believed to be
representative of tool performance as of the year 2012. However, as tool and measurement techniques
continue to
evolve and improve, BWS measurement performance may surpass what is contained in this Guide. The user should
investigate metrology suppliers’ current capabilities.

2.6

The measurements described in this Guide are on bonded wafer stacks with thickn
ess in the range of 50 to
15
50
μ
m.

2.7

The stacks considered include carrier and device wafers and bonding layers, including cases where there are
more than two wafers in a stack. Bonded wafers may be classified as either temporar
il
y bonded (i.e. a device to a

carrier wafer) or permanently bonded. Temporary bonding uses a temporary adhesive; permanent bonding could be
adhesive, oxide, metal
-
metal (e.g. Cu
-
Cu), or hybrid bonding. Two representative two
-
wafer stacks are depicted in
Figures 1 and 2. The first st
ack (Figure 1) is a bonded pair of 775 µm thick wafers following TSV formation and the
bonding operation. The second stack (Figure 2) is a bonded wafer stack with a top wafer thinned to ~50 µm, and
bonded on top of a 775 µm wafer using a temporary adhesiv
e.




This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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Figure 1


Bonded Wafer P
air
Following TSV
Formation

and Wafer Bonding O
peration



Figure 2



Temporarily Bonded Wafer Pair
--
Device Wafer is
Edge
-
Trimmed and Thinned


NOTICE:

SEMI Standards and Safety Guidelines do not purport to address all safety issues associate
d with their
use. It is the responsibility of the users of the Documents to establish appropriate safety and health practices, and
determine the applicability of regulatory or other limitations prior to use.

3

Limitations

3.1

While this Guide provides an
overview on the use of several available techniques, it does not provide the level
of detail typically available in Test Methods. For this purpose, SEMI documents are referenced that the user may
find of interest. In addition, all suppliers may not follo
w SEMI standard measurement methods in their metrology
procedures.

3.2

The information in this guide does not encompass the establishment of specific a methodology for such
measurements but is only a guide to the user describing the various types of metrology
including their capabilities
and limitations.

3.3

This Guide describes metrologies that have been applied to a wafer stack composed of, at most, two wafers.
Thus, the user is cautioned that the methods described may be limited with applied to wafer stacks con
sisting of
more than two wafers.

4

Referenced

Standards and Documents

4.1

SEMI Standards and Safety Guidelines

SEMI M23


Specification for Polished

Monocrystalline Indium Phosphide Wafers


SEMI M43


Guide for Reporting Wafer Nanotopography

SEMI M49



Guide for

Specifying Geometry Measurement Systems for Silicon Wafers for the 130 nm to 22 nm
Technology Generations

SEMI M59


Terminology for Silicon Technology

SEMI M65


Specifications for Sapphire Substrates to Use for Compound Semiconductor Epitaxial Wafers

SE
MI MF533


Test Method for
Thickness and Thickness Variation of Si Wafers

SEMI MF534



Test Method for Bow of Silicon Wafers




This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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SEMI MF657



Standard Test Method for Measuring Warp and Total Thickness Variation on Silicon Slices and
Wafers by a Non
-
contact Sc
anning Method

SEMI MF1390


Standard Test Method for Measuring Warp on Silicon Wafers by Automated Non
-
Contact
Scanning

SEMI MF1451


Test Method for Measuring Sori on Silicon Wafers by Automated Non
-
Contact Scanning

SEMI MF1530



Standard test method for Measuring Flatness, Thickness, and Thickness Variation on Silicon
Wafers by Automated Non
-
Contact Scanning

NOTICE:

Unless otherwise indicated, all documents cited shall be the latest published versions.

5

Terminology

NOTE 1:

Refer to
the
SEMI
Standards
Compilation of Terms (COT
s
) for a list of the current
Abbreviations, Acronyms,
Definitions, and Symbols.

5.1

Abbreviations and Acronyms

5.1.1

AM


acoustic microscopy

5.1.2

BWS


bonded wafer stack

5.1.3

CD



critical dimension

5.1.4

CWS


chromatic white light sensor

5.1.5

CSI


coherence scanning interferometry

5.1.6

FAMM



focus/acquire/measure/move

5.1.7

FPD



focal plane deviation

5.1.8

HVM



high volume manufacturing

5.1.9

IR


infra
-
red

5.1.10

ISO



international organization for standardization

5.1.11

RPD


reference plane deviation

5.1.12

SLED


superluminescent light emitting diode

5.1.13

TIR



total indicated runout

5.1.14

TS


through
-
silicon via

5.1.15

TTV


total thickness variation

5.1.16

WLI



white light interferometry

5.2

Bonded Wafer Metrology Overview

5.2.1

explanation of wafer metrology terms
1



the parameters primarily m
easured, wafer bow, warp, and sori need
some explanation. Figure 3 shows schematic diagrams representing these terms.

5.2.1.1

bow



the deviation of the center point of the median surface of a free, unclamped wafer from a median
surface reference plane established

by three points equally spaced on a circle with diameter a specified amount less
than the nominal diameter of the wafer; SEMI MF23 provides a standard test method for measuring the bow of a
single wafer. Note that bow is a signed value. The method can be

adapted to a bonded wafer stack.

5.2.1.2

warp



the difference between the most positive and most negative distances of the median surface of a
free, unclamped wafer from a reference plane; SEMI M59 provides a standard test method for measuring the warp
of a sing
le wafer. Warp can be zero, even for a wafer with curvature, if the curves are mirror images of each other.
SEMI MF657 measures median surface warp using a three
-
point back
-
surface reference plane, resulting in thickness
variation being included in the war
p value. The use of a median surface reference plane in SEMI MF1390



1

SEMI International S
tandards: Compilation of Terms (COT)




This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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Date:
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eliminates this effect. SEMI MF1390 provides a standard test method for measuring the warp of a single wafer.
The method can be adapted to a bonded wafer stack.

5.2.1.3

sori



the algebraic di
fference between the most positive and most negative deviations of the front surface
of a wafer that is not chucked from a reference plane that is a least squares fit to the front surface within the fixed
quality area.

5.2.1.4

flatness



the deviation of the fron
t surface, expressed in TIR or maximum FPD relative to a specified
reference plane when the back surface of the wafer is ideally flat, as when pulled down by a vacuum onto an ideally
flat chuck.

5.2.1.5

non
-
contact



metrology that allows a wafer to be measured wi
thout physical contact to the wafer surface,
preventing contamination or damage to the wafer substrate.

5.2.1.6

c
hromatic white light sensor



a chromatic white light sensor (CWS) is based on the principal of confocal
optics and relies on chromatic scanning. A le
ns is used that refracts white light differentially based on its
wavelength in order to carry out distance measurements. Resolution depends on the intensity of reflected light.

5.2.1.7

i
nterferometry



a technique that relies on the principal of superposition of m
ultiple beams of light to
determine the effect that a material has on the state (phase and amplitude) of the original light beam.

It is this
introduced phase difference that creates the interference pattern between the initially identical waves. If a singl
e
beam has been split along two paths, then the phase difference is diagnostic of
anything

that changes the phase along
the paths. This could be a physical change in the path length itself or a change in the refractive index along the path
.

5.2.1.8

gravity
compensation for horizontally
-
supported wafers



unlike thickness and TTV measurements which
are
independent

of how the wafer is held, bow and warp measurements are complicated not only by wafer stress but
by how the wafer is supported and by gravity. Gra
vity causes substantial deformation of large diameter or thin
wafers, whether they are supported at the edge or in the middle. Unless compensated, gravity will induce a large
error in warp measurements.
SEMI MF1390

describes three compensation approaches
. One approach is to correct
for the gravitational effect of warp measurements by inverting the wafer and measuring both the top and bottom
surfaces of the wafer. Any differences between two values at the same site are due to the effect of gravity and ca
n
be used to correct for single
-
side measurements. Another approach is to use an analytical expression for
gravitational deformation and subtract it from a single
-
side warp measurement
2
. Measurements obtained on
representative wafers can also be performed

and the gravity value determined.

5.2.1.8.1

In addition,
the wafer can either rest on a chuck or be supported by three points (MF657).

5.2.1.9

gravity compensation for vertically supported
-
wafers



on tools that support the wafer vertically, effects
due to gravity are negl
igible and therefore do not require compensation. Bow is only measured on one side so there
is no need to invert the wafer. The manufacturing tolerance for sori is very low, such that a 300 mm diameter
silicon wafer must achieve sori in the nanometer ran
ge in order to achieve the maximum yields in semiconductor
device processing.


Figure 3

Wafer
W
arp,
B
ow, and
S
ori
D
epictions
2




2

W. R. Runyan, T. J. Schaffner, “Semiconductor Measurements and Instrumentation,” McGraw
-
Hill, 1998, pp. 218
-
221.




This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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5.3

Measurement Considerations

5.3.1

thickness and total thickness variation

(SEMI MF
533
)



Wafers that are excessively thin may break during
processing or have inadequate thermal mass or electrical resistance during certain processing steps. Wafers that are
too thick may jam during wafer handling. Excessive thickness variation may encounter problems during mechanical
handling. Excessive devi
ation from flatness may cause focus problems in photolithography steps.

5.3.2

warp

(SEMI MF1390)



Warp can adversely affect the yield of semiconductor devices and can affect wafer
handling and processing. Warp may be caused by unequal stresses on the two ex
posed surfaces of a wafer. Warp
cannot be determined from measurements on a single exposed surface. The median surface may contain regions
with upward or downward curvature or both. In some cases, the median surface may be flat. In all cases, warp is a

zero or positive quantity. Warp is a measure of the distortion of a median surface of a wafer. SEMI MF657
measures median surface warp using a three
-
point back
-
surface reference plane. The back
-
surface reference plane
results in thickness variation bei
ng included in the reported warp value. The use of a median surface reference plane
in SEMI MF1390 eliminates this effect.

5.3.3

bow

(SEMI MF534)



If the median surface of a free, unclamped wafer has a curvature that is everywhere
the same, bow is a measure of

its concave (dished) or convex (mounded) deformation, independent of any thickness
variation that may be present. Measurement of a semiconductor wafer, the deviation of the center point of the
median surface of a free, unclamped wafer from a median
-
surf
ace reference plane established by three
hemispherical points equally spaced on a circle with diameter a specified amount less than the nominal diameter of
the wafer.

5.3.3.1

sori
(
SEMI

MF1451)



Sori solves the ambiguity of warp by measuring the difference betwee
n the
maximum and minimum distances from the front surface of the wafer to a reference plane outside the surface of the
wafer.
SEMI
documents
MF
59, M65, and MF1451 provide

a standard test method for measuring the
sori

of a single
wafer
. A wafer is suppor
ted on a small
-
area chuck with the front surface facing up. Both external surfaces of a
wafer are simultaneously measured by an opposed set of probes to obtain a set of values at the same x and y
coordinates of the distances between each surface and the n
earest probe. The paired distances are used to construct
the median surface. Gravity correction can be performed. One half of the thickness at each point is added to the
corrected median surface to construct the corrected front surface. A least
-
squares

reference plane is constructed
from the corrected front surface. The reference plane deviation (RPD) is calculated at each pair of measurement
points. Sori is then reported as the algebraic difference between the most positive RPD and the most negative
RPD.

6

Bonded Wafer Metrology Techniques

6.1

Optical Techniques

6.1.1

Chromatic White Light

6.1.1.1

Chromatic white light sensor metrology utilizes the principle of wavelength
-
dependent focal length to
determine distance (Figure
4
).
The

spectrum of light reflected
from

a su
rface generates a
n intensity profile as a
function of focal length

that is used to determine distance to the sample surface.

The peak in intensity occurs at the
optimal focal point for each wavelength. This technique is useful for determination of 2D p
rofile, 3D topography,
planarity, roughness, and wafer contour (bow and warp). Figure 5 shows a typical set of measurement locations
used in an analysis. The resulting 2D m
easurement

profile
s

for t
hickness, TTV, and
w
arp for
an
825 µm
thick
bonded wafer
are shown in Figure 6, and a wafer thickness contour map is shown in Figure 7.





This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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Figure 4

Chromatic

W
hite
L
ight
S
ensor
U
sed for
D
istance
M
easurements for
B
ow,
W
arp, and
T
opographical
M
easurements

NOTE:

Focal lengths are wavelength dependent, providing distance
measurements.




Figure 5

Data Collection Locations Showing Profile (a
-
d), and Points (A
-
I)





This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
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Figure 6

Profile Measurements for Thickness, TTV, and Warp for 825 µm Thin Bonded Pair Wafer




Figure 7

Wafer Map Results for 825 µm Thin Bonded Pair Wafer


6.1.2

Infra
-
red Interferometry

6.1.2.1

Interferometry is a technique in which light waves are superimposed to obtain information on surface
profiles or displacements between two surfaces. Infra
-
red interferometry is useful for thickness measurements of
substrates that are transparent in near
IR light. Thickness measurements can be performed on single or multi
-
layer
films, including adhesion layers in BWS. At each film interface, some incident light is reflected back to a detector.

6.1.2.2

When two light waves combine, the resulting pattern is dete
rmined by the phase difference between the two
waves

waves that are in phase undergo constructive interference while waves that are out
-
of
-
phase undergo
destructive interference. Typically, a beam of coherent light is split into two beams by a beam splitt
er. Each beam
travels a different path and the two are recombined before arriving at the detector. The path difference, the
difference in the distance traveled by each beam, creates a phase difference between them.

6.1.2.3

For a thin transparent medium, the refl
ection from the top surface interferes with the reflection from the
bottom surface. For some wavelengths, the interference is constructive, and for others it is destructive. In general,
the reflected intensity follows the Fabry
-
Perot equation (eq. 1):




This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
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|




















|









(1)

6.1.2.4

Where r
1

is the reflection coefficient at the first surface of the cavity, r
2

is the reflection coefficient at the
second surface, and δ = 2 π n l / λ, n is the index of refraction, l is the cavity thickness, and λ is the wavelength.
Because of the complex exponential, the reflected intensity is periodic in δ. Since δ is proportion
al to optical
frequency (c / λ), the reflected intensity is periodic in optical frequency with period c / 2nl, where n is the index of
refraction, l is the layer thickness, and c is the speed of light in a vacuum. Then the basic procedure to measure the
th
ickness is to: 1) measure the reflectance for a spectrum of wavelengths; 2) analyze the measured spectrum for a
periodic intensity as a function of optical frequency; 3) calculate thickness from the period and index of refraction.

6.1.2.5

The light source has a wa
velength in the neighborhood of 1.3 µm, and is placed facing the back of the wafer.
The sensor measures both surfaces of the wafer interferometrically and returns a thickness measurement (Figure
8
).
The fast (millisecond) and non
-
destructive measurements
based on backside IR illumination allow

immediate HVM
process feedback.
The
sensor
is used for measuring thickness of thick wafers, thinned wafers, and etch depth of
TSVs and other features.



Figure 8

Infra
-
R
ed
I
nterferometric
S
ensor
U
sed for
D
istance
M
easurem
ents for
B
ow,
W
arp, and
T
opographical
M
easurements

NOTE:

Focal lengths are wavelength
-
dependent, providing distance measurements.


6.1.2.6

Some tools utilize a

dual
-
sided interferometer
to measure

wafer geometry parameters such as thic
kness,
shape and flatness.
The wafer is held vertically between 2 sets of optics so that each surface is measured
simultaneously. The tool is mainly
used

by

bare silicon wafer suppliers to improve their process for wafer geometry
and to qualify wafers before shipment to customers.

6.1.2.7

W
afer geometry data

acquired in 3D

is capable of
providing
nanotopography and
wafer
edge
roll
-
off.
Wafer manufacturing issues can affect geometry at the edge of the wafer, causing die defocus
and CMP removal
non
-
uniformity
.

6.1.2.8

The thickness and warp of the tw
o individual wafers in a bonded pair can be measured using interferometry
utilizing the interference between the light reflected from the wafer and the light reflected from a moving mirror.
The general scheme of such an interferometer is shown in Figure 9.





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Figure 9

Schematic
V
iew of an
I
nterferometer
S
ystem

NOTE:

Beam 2, which is reflected from the bottom surface

of the wafer
, is delayed with respect to beam 1 refle
cted from the top
surface. The delay between beam 1 and
beam 2 is
proportional to
the thickness
of

the wafer. In a real experiment, the incident
beam is perpendicular to the surface of the wafer.


6.1.2.9

A SLED (superluminescent light
-
emitting diode) with a broad line width was used and retro
-
reflecting
mirrors were mounted in a nano
-
motor stage. The dual
-
pro
be scheme for total thickness and warp measurement is
shown in Figure 10.

Fiber
10
X Microscope
Objective
Reference Mirror
Bottom Probe
Si Substrate
Fiber
Reference Mirror

Figure 10

Dual Probe Scheme for the Measurement of the Thickness and Warp


6.1.2.10

In the dual
-
probe setup, each probe has a reference and a measurement arm. Light from
each arm interferes
with light from the scanning mirror to generate the first two interference signals in the upper probe. Similarly, light
from the reference arm and the back surface of wafer produces the first two interference signals in the lower probe.

The distance between the two probes can be determined using the known thickness of a block gauge. This
information in conjunction with the two signals from the upper and lower probes makes it possible to measure the
total thickness of the wafer. If the wa
fer surfaces are rough and/or the substrate is opaque to the incident light, the



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dual probe is a good way to measure its thickness. A typical signal collected by an interferometer system on a bare
Si wafer is shown in Figure 11. For warp measurement, the d
istance between the reference signal and the signal
from the front surface of a wafer into the top probe is measured. In a warp measurement, a 300 mm blank wafer is
used as the un
-
warped reference.



Figure 11

T
ypical
S
ignal from an
I
nterferometer
S
ystem


6.1.2.11

In
Figu
re

12, TTV maps are shown with measurement locations (left) as well as
2D
TTV (center)
and 3D

TTV (right)

maps

for

an

825 µm
two wafer stack
.

Typical wafer thickness results are shown in Figures 13
-
15 for a
1550

m thick two wafer stack.



Figure 12

25
-
Point 2D and 3D Image
Files

of TTV for 825 µm
Two Wafer Stack





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Figure 13

1550 µm

Two Wafer Stack

Results Showing
Bottom

Wafer
Thickness

Map



Figure 14

Two Wafer Stack


Results Showing Top
Wafer
Warp

Map



Figure 15

1550 µm

Two Wafer Stack


Results Showing Top (Thinned) Wafer Thickness Map





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6.1.3

Coherence Scanning Interferometry (CSI) or White Light Interferometry (WLI)

6.1.3.1

White light interferometry
3

is a non
-
contact optical profiling system for measuring step heights and surface
roughness
in preci
sion engineering applications. The technology utilizes a white light beam which passes through a
filter and then a microscope objective lens to the surface of the wafer. The light reflecting back from the surface is
combined with the reference beam and cap
tured for software analysis. After obtaining data for each point, the
system can generate a 3D image (topography) of the surface. The vertical (i.e. height) resolution of this technique is
extremely good, better than 0.01 nm (0.1 Angstrom), which makes it
a potentially practical tool for assessing wafer
surfaces for roughness and nanotopography. However, the lateral resolution can be limited to the spot size, in the
range of 0.35
μ
m. With its broad capability, it is possible to measure local step height, c
ritical dimensions (CD),
overlay, multilayer film thickness and optical properties, combined topography and film thickness, and wafer bow.
The technology has particular strength in new advanced packaging applications for process control of through
-
silicon
vias, microbumps, redistribution layers, and copper pillars.

6.1.3.2

Figure 16 illustrates the principle of CSI. A broadband light source produces a narrow region of optical
interference co
-
located with the focus of the interferometer microscope objective. As th
e objective scans in Z,
orthogonal to the surface, interference is detected by the imaging camera as the measured sample comes into focus.
At focus, each pixel detects the interference peak. Analysis software produces a map of surface height variation
fr
om the pixel
-
by
-
pixel interference peak detection. A key CSI advantage is that Z resolution is independent of
field
-
of
-
view and lateral resolution. This is not the case for confocal microscopes, where Z resolution decreases with
increasing field
-
of
-
view d
ue to decreased numerical aperture. The minimum surface feature height measurable by
CSI depends on the manufacturer’s measurement algorithm, and ranges from ~10 nm to <0.1 nm. CSI imaging
resolution depends on the numerical aperture (NA) of the objectiv
e used and the number of pixels in the camera
array. The area, or field
-
of
-
view measured depends on the objective magnification, which may range from 2.5X
-
100X. Nominal resolution and field
-
of
-
view combinations are 0.7 µm with 500 µm field
-
of
-
view to 5 µ
m with 2.5
mm field
-
of
-
view. Nominal resolution for CSI is two times worse than optical imaging microscopes as expected for
a given NA. CSI systems report height, whereas image microscopes report contrast. It is possible to resolve
features in an imagin
g microscope until contrast equals the background noise. CSI must separate height features,
which is only possible at twice imaging microscope resolution.



Figure 16

Coherence Scanning Interferometer for the
M
easurement of
S
urface
T
opography

NOTE:

Z height is measured

at each pixel as the interferometric objective is scanned in Z and the camera detects the peak of the
coherence envelope as shown.




3

Blunt,
R.T., “White light interferometry


a production worthy technique for measuring surface roughness on semiconductor wafers,” CS
MANTECH Conference, April 24
-
27, 2006, Vancouver, British Columbia, Canada.




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6.1.3.3

CSI can measure polished and rough surfaces, as well as flatness to nm


to mm high stepped and sloped

surfaces. When combined with image analysis software, CSI can measure and position three
-
dimensional features
to sub
-
micron repeatability in X and Y, combined with sub
-
nanometer repeatability in Z. Typical applications are
surface roughness (commercial
tools provide industry standard results, but often lack ISO specified software
bandpass filters), nanotopography, local step height, critical dimensions (CD) and overlay. The technology has
particular strength in new advanced packaging applications for pro
cess control of through
-
silicon vias (TSVs),
microbumps, redistribution layers, copper studs and pillars.

6.1.3.4

A major strength of CSI is ease
-
of
-
use and speed of measurement. Advanced tools incorporate autofocus
and automation capabilities to speed the measur
ement process. Typically no special preparation is required of the
measurement sample, if the sample is homogeneous and without film layers. Microbump/pillar measurements have
been demonstrated in production with 0.5 second Focus/Acquire/Measure/Move (FA
MM) times, while maintaining
nanometer Z performance. Laboratory tools can experience one to five
-
minute FAMM’s primarily due to instrument
set up time.

6.1.3.5

The presence of films and films stacks are both an opportunity and technology limitation of CSI. Pre
sent
CSI algorithms are limited to reliably measuring in the presence of films to thicknesses greater than 1.5
μ
m optical
thickness (thickness times index of refraction). Some manufacturers claim this capability down to 1
-
μ
m optical
thickness. Reported measurement results are film thickness (if the index of refraction is known), surface profile of
the top and also the bottom of the film. Advanced work is occurring to measure features in thin film structures,
improved CD an
d other edge of resolution features. As these technologies emerge new applications will be possible.
When films are less than the measurable optical thickness unpredictable errors occur in the measured data, and the
data is unreliable in Z height.

6.1.3.6

Meas
uring non
-
homogeneous materials limits CSI accuracy. CSI measures Z height by measuring
interferometric phase. Phase is dependent on the material measured. Insulators experience a constant 180° phase
shift, whereas conductors and semi
-
conductors induce
varying phase shifts. These phase shifts appears as surface
topography errors up to 10’s of nanometers. It is possible to correct for these by identifying material types by
regions and region
-
by
-
region apply a correction factor. Some manufacturers have
seen success correcting
algorithmically but this has seen little commercial utilization. For process control, phase change on reflection is a
constant offset and therefore can be ignored when process variation is being tracked when absolute values are not

critical.

6.1.3.7

CSI requires minimal to no surface preparation. Rapid measurements, nanometer level Z resolution and
optical level image resolution make it a preferred tool for many applications. Measurement errors due to films and
phase change of reflectio
n of non
-
homogeneous materials are major limitations for applications based on
semiconductor processes.

6.1.3.8

Five sets of 49 wafer bow measurements (top and bottom) for a 1550
μ
m bonded wafer pair are averaged in
Figure 17, and the 3 sigma standard deviation is

shown in Figure 18.





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Figure 17

Results for 49 Point Dynamic Repeat Bow Measurement on
a

1550

m Bo湤敤e坡f敲 a楲i(
3


5 剥灥Rts)

Average Bow of Top and Bottom Measurement (3 Point Suspension)



Figure 18

Results for 49 Point Dynamic Repeat Bow Measurement on
a

1550


Bo湤敤e坡f敲 a楲i(
3


5 剥灥Rts)

Average Standard Deviation of Top and Bottom Measurement (3 Point Suspension)


6.1.4

Laser Profiling

6.1.4.1

The diagram
in

Figure
19

demonstrates the pri
ncipal of laser profiling with a single optical head where the
laser is focused fi
rst on the top and then on the bottom of the wafer. The laser peak positions that are used to
determine the wafer thickness are also shown.

6.1.4.2

Two focused laser beams can also be used
, one focused on the top and the other on the bottom of the
wafer
stack (Fi
gure 20)
. The measurement is made by moving the focus position from the
surface of the top wafer

to the

top surface of the

bottom

wafer
.
The laser can be focused in the objective to a spot as small as 1

m.
The dual
optical measurement system provides acc
urate wafer thickness measurements independent of material properties,
especi
ally useful for patterned or
bumped wafers, GaAs and other wafer types,
and
after back grinding and dicing
.
Laser profiling provides thickness resolution of approximately 0.1

m
and has also been applied to TTV, bow, warp,
and surface roughness.





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Figure 19

Principle of Single Beam Wafer Thickness Measurement System Showing Wafer Top and Bottom Signal
Peak Positions




Figure 20

Dual
-
Beam Thickness Description


6.1.4.3

The plot in Figure
21

is a plot of the laser focused on the wafer surface

as well as
a glass surface. The
difference in the “in
-
focus” locations equals the thickness of the glass.

OPTICAL THICKNESS
MODULE
BEAM
EXPANDER
BEAM
SPLITTER
OBJECTIVE
MEASUREMENT
SAMPLE
SIGNAL DETECTION
MODULE
OPTICAL FIBER
DETECTOR
PINHOLE
FOCUSING
LENS
BOTTOM
OF WAFER
TOP OF
WAFER



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Figure 21

Example Plot Thickness Measurement



6.1.4.4

Figures 22
-
26 illustrate measurements for a set of five
static repeatability runs obtained on an 825

m thick
bonded wafer
pair

stack using laser profiling. A 2D thickness map is shown for each set of measurements in Figure
22, as well as 3D thickness maps for the five runs (average and 3


sigma standard devia
tion) in Figures Figure 23
and 24. Two
-
dimensional bow and warp maps are shown in Figures 25 and 26.





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Figure 22

Bonded Pair Wafer
Thickness

Map (
F
ive
R
epeats)



Figure 23

Bonded Wafer Pair
25 Site Average Thickness Map (
F
ive
R
epeats)





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Figure 24

Bonded Wafer Pair
25 Site Thickness Map (3


却搮 䑥D.,
F
楶攠
R
数敡ts)



Figure 25

Bonded Pair Wafer Bow Map (
F
ive
R
epeats)






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Figure 26

825 µm Bonded Pair Wafer
Warp Maps (
F
ive
R
epeats)


6.2

Electrical Technique for Measuring Proximity

6.2.1

Capacitance Displacement Metrology

6.2.1.1

Capacitive sensors can
be divided into two categories based upon their performance and intended use. High
resolution sensors are typically used in displacement and position monitoring applications where high accuracy,
stability and low temperature drift are required. Quite frequ
ently, these sensors are used in process monitoring and
closed
-
loop feedback control systems. Proximity type capacitive sensors are typically used to detect the presence of
a part or used in counting applications. The following describes characteristics of

high resolution systems, their
operating principle and application.

6.2.1.2

The capacitance sensor shown in Figure 27 is used throughout a variety of industries to provide highly
stable, accurate measurements of displacement, vibration, position, thickness and ru
nout. Capacitance probes are
typically modeled as a parallel plate capacitor. If two conductive surfaces are separated by a distance and a voltage
is applied to one of the surfaces, an electric field is created. This occurs due to the different charges sto
red on each
of the surfaces.




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Figure 27

Typical Capacitance Sensor


6.2.1.3

Capacitance refers to the ability of the surfaces to hold a charge. In a typical sensor system the probe is one
of the plates and the target being measured is the other plate. If a constant
current is applied, the capacitance change
can be monitored as a linear voltage charge related to the distance between the plates (Figure 28).



Figure 28

Typical Capacitance Sensor Measurement Parameters


6.2.1.4

This distance, or gap, is a function of the area of the ca
pacitance sensor according to the following equation
(Eq. 2):



































(2)

Where capacitance is the area of sensor, A =

r
2

times the dielectric constant of air,



divided by the gap, d.


6.2.1.5

From t
his relationship, capacitance is directly proportional to the area of the sensor and the dielectric
property of the
material

between the sensor and target (typically air). The greater the area of the capacitance sensor
the larger the measurement range, or
gap. If it is assumed that the area and the dielectric constant between the plates
remain constant for a specific probe, any change in capacitance is inversely proportional to the change in distance
between the probe and target being measured and this chan
ge is converted to a voltage for monitoring. The amount
of voltage output change for a given distance change is commonly referred to as the sensitivity of the system. For
example, if a distance change of 1 mm corresponds to a voltage output change of 10 V
the sensitivity would be 1
mm/10 V, or 0.1 mm/V.




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6.2.2

Capacitance Sensors Electric Field

6.2.2.1

A typical high performance capacitance sensor consists of three basic elements: the sensor tip, the guard and
the ground shell. Figure 29 illustrates a typical capacitance
probe. When a voltage is applied to the sensor tip, an
electric field is established between it and any other local conductive material. To maintain accuracy and linearity it
is essential that the electric field in the measurement area be linear, directed
toward the target, and not distorted. To
protect this field, each capacitive sensor has a guard electrode. This creates an additional field around the probe
sensing tip that is driven at the same phase and voltage potential as the sensor. By being equal, t
he auxiliary field
protects the area from becoming warped and cancels any stray capacitance between the two elements.

6.2.2.2

Because the ground shell is at a different potential than the guard, a partial distortion of the guard field to the
ground shell occurs. A
lthough undesirable, the distortion of the guard is acceptable as long as the sensor tip field
remains linear. For best performance, the width of the guard should be at least 2X the system measurement range.


Figure 29

Capacitance Sensor Field


6.2.2.3

Capacitance probes
should be designed with sufficient guard electrodes to protect the sensing area under
normal operating conditions. This probe’s range should not be greater than the width of the guard electrode or poor
linearity will result.

6.2.2.4

In addition to improving linear
ity and accuracy, the guard is also used to reduce noise and external
interference. Each capacitance probe is driven by a low noise coaxial cable. The shield of the cable is used to deliver
the voltage to the guard, at the same voltage and phase. This elim
inates any stray capacitance that might be created
between the center conductor and the shield of the cable, or any other part that may be close to the cable. By design,
this protection significantly reduces external influences from RFI and EMI. It is impo
rtant to note that ordinary
coaxial cable usually does not provide adequate protection or shielding for the system and special cable is generally
required.

6.2.3

Characteristics of Capacitive Sensors

6.2.3.1

Non
-
Contact

6.2.3.1.1

Capacitive
displacement

sensors are non
-
contact by

design. That is, they are able to precisely measure the
position or displacement of an object without touching it. Because of this the object being measured will not be
distorted or damaged and target motions will not be dampened. Additionally, they can m
easure high frequency
motions because no part of the sensor needs to stay in contact with the object, making them ideal for vibration
measurements or high speed production line applications.

6.2.3.2

Range/Standoff Distance

6.2.3.2.1

As mentioned above, the range of a capaci
tance sensor is dictated by the diameter, or area, of the sensor.
The larger the area, the larger the measurement range. Measurement range is typically specified starting when the
probe is touching the target. At this point the output from the amplifier is

0 V. When the gap is increased to equal
the full scale
measurement

range of the capacitive system the amplifier output is 10 V (V
dc
). In theory, the probe can
operate anywhere between these two extremes, however, it is not recommended to operate below 10%

of the gap.
Thus, the ideal operating or standoff distance is between 5 V and 7 V
dc
, permitting the target to move closer to or



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further away from the probe without going out of range. Figure 30 is a simplified diagram showing range, output
voltage and rec
ommended standoff for a typical capacitance sensor.



Figure 30

Capacitance Probe Operating Range


6.2.3.3

Resolution

6.2.3.3.1

The resolution of a displacement sensor is defined as the smallest amount of distance change that can be
reliably measured by a specific system.
Capacitance sensors offer extremely high resolution and stability, often
exceeding that of complex laser interferometer systems. Because of their ability to detect such small motions, they
have been successfully used in many demanding measurement applicati
ons including computer disk drive runout,
microscope focusing and nano
-
positioning within highly complex photolithography tools.

6.2.3.3.2

The primary factor in determining resolution is the system’s electrical noise. If the distance between the
sensor and target is

constant, the voltage output will still fluctuate slightly due to the “white” noise of the system. It
is assumed that, without external signal processing, one cannot detect a shift in the voltage output of less than the
random noise of the instrument. Bec
ause of this, most resolution values are presented based on the peak
-
to
-
peak
value of noise and can be represented by the following formula: Resolution = Sensitivity × Noise.

6.2.3.3.3

Sensitivity is simply the measurement range divided by the voltage output swing o
f the capacitance
amplifier, that for a fixed sensitivity the resolution is solely dependent upon the noise of the system, so that the
lower the noise, the better the resolution.

6.2.3.3.4

It is important to note that some manufacturers specify resolution based on p
eak or rms noise, resulting in
claims that are 2X and 6X respectively better than peak
-
to
-
peak. Although an acceptable method, it is somewhat
misleading as most users do not have the ability to measure voltage changes less than the peak
-
to
-
peak noise value
.

6.2.3.4

Bandwidth

6.2.3.4.1

The bandwidth, or cutoff frequency, of a system is typically defined as the point where the output is
dampened by 3 db. This is approximately equal to an output voltage drop of 30% of the actual value. In other words,
if a target is vibrating w
ith an amplitude of 1 mm at 5 kHz and the bandwidth of the capacitance sensor is 5 kHz the
actual sensor output would be 1 mm × 70% = 0.7 mm.

6.2.3.5

Push or Range Extension

6.2.3.5.1

Typical capacitive amplifier systems operate over a specific capacitance range, limiting t
heir ability to
measure large motions or operate at comfortable standoff distances. To overcome this problem some suppliers use a
proprietary circuit that, with minor component modifications, can be adjusted to change the range and meet a wider
variety of
customer requirements. For example, a small diameter probe with a ½ mm measurement range can be
“pushed” to have a measurement range of 1 mm or even 2 mm. This allows capacitive probes to be used in
applications where space is limited or the target being m
easured is small. It is important to note, however, that a
pushed probe should have a guard width sufficient enough to maintain the performance required, as mentioned



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above. Additionally, pushing a sensor also amplifies system noise, reducing probe resolut
ion. Noise increase is
proportional to push; 2 × Push = 2 × Noise

6.2.3.6

Spatial Resolution

6.2.3.6.1

The field established between a capacitance probe tip and measured object is typically larger than the
diameter of the probe tip (Figure 31). This is because there is an e
poxy gap between the tip and guard elements. The
field diameter is equal to D plus 1X the epoxy gap. When obtaining measurements, capacitance probes provide a
distance equal to the average surface location within the spot area. They are not capable of accu
rately detecting the
position of features smaller than the size of the spot. However, they can repeatedly measure rough surfaces. Because
of this, the probe tip should always be 25% smaller than the smallest feature targeted for measurement. Smaller
sensor
s can distinguish smaller features on an object.



Figure 31

Effective Spot Size of a Capacitive Sensor


6.2.3.7

Linearity

6.2.3.7.1

Capacitance sensors may have an output of 0

10 V
dc

over the full scale measurement range (FSR). In an
ideal world this output would be perfectly lin
ear and not deviate from a straight line at any point. However, in reality
there are slight deviations from linearity. Typically, linearity is specified as a percentage of the Full Scale
Measurement range. During calibration the output from the amplifier
is compared to the output of a highly precise
standard and differences are noted. Some capacitance systems exceed ±0.05% FSR with some achieving ±0.01% or
better.

6.2.3.7.2

Accuracy is a function of linearity, resolution, temperature stability and drift, with linearity being the
majority contributor. Calibration reports provide data that can be used to correct for the non
-
linearity of a system
with inexpensive computers and c
orrection software.
Digital correction t
ypically yields a linearity of
±0.01% or
better
.

6.2.3.8

Stability

6.2.3.8.1

Stability is a function of a variety of different internal and external factors. For short term or relative
measurement applications stability is typically
not an issue. However, if high accuracy is required over a long period
of time, care must be taken when designing fixtures, selecting components and specifying materials of construction.

6.2.3.8.2

Temperature is typically the biggest factor that affects stability.
Temperature swings not only cause
electronic drift but can also cause fixture and probe expansion and contraction. For critical applications high
-
quality
capacitors, resistors and inductors specifically designed for stability to minimize the electronic dri
ft should be used.
To minimize mechanical drift, probes can be manufactured from special low thermal coefficient materials such as
Invar. Thermal correction coefficients can also be provided and used for real
-
time compensation.

6.2.3.8.3

Active capacitance probe sy
stems should never be used in high stability applications because any localized
temperature change surrounding the sensor will result in drift.




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6.2.3.9

Calibration

6.2.3.9.1

For low
-
end proximity sensors calibration is typically not important because linearity of the sensor

is not
critical. Most high performance systems are, by design, inherently linear to approximately ±0.1% of the full scale
measurement range. Some capacitance manufacturers offer sensors with this performance, however, they are
typically not suitable for h
igh precision applications. During calibration, the manufacturer adjusts circuit gain, offset,
and typically performs a proprietary linearity adjustment. Improved performance can be obtained by adding
adjustable break point linearization circuitry within
the amplifier circuit or by providing digital correction.

6.2.3.9.2

During calibration the output of the amplifier verses position of a target is recorded. A best fit straight line
is generated based on this data. Each recorded point is then compared to the generate
d straight line and the percent
deviation is calculated and plotted. Based upon the results adjustments can be made to improve the deviation to
within acceptable limits.

6.2.3.10

Applying Capacitive Probes

6.2.3.10.1

Target Material and Grounding

6.2.3.10.1.1

A capacitance measurement sy
stem mimics a parallel plate capacitor with the sensor as one plate and
the target being measured the other. To create the electric field between the two plates the target must be made of a
conductive material. The
composition

or thickness of the target is

not important, allowing them to be used in many
applications not suitable for eddy current type sensors. In fact, the surface can even be a few hundred ohm
-
cm.

6.2.3.10.1.2

To complete the capacitance circuit the target should be grounded back to the amplifier. For op
timal
performance a conductive path is required, however, capacitive coupled targets can work well if the capacitance is
0.01 µf or higher. An example of a capacitively
-
coupled target is a shaft rotating on air bearings. In theory, air
bearings are non
-
con
tact but the gap between elements is small, and their area is relatively large, creating a high
capacitance path. Thousands of successful applications world
-
wide have been installed with this type of ground.

6.2.3.10.1.3

If the target is poorly grounded, the system is
susceptible to external noise and interference. Care
should be taken when designing the ground return path.

6.2.3.11

Target Size

6.2.3.11.1

The lines of flux in the electric field established between the probe and target always leave the
capacitance sensor normal (90°) to its

surface and always enter the target normal to its surface. If the target being
measured is large enough, and the sensor is within range, the field within the sensing area will be consistent and
linear. If the target is not large enough to support the fiel
d it will tend to wrap around the edge and enter normal to
the target side (Figure 32). This field distortion will create measurement errors by degrading the sensor linearity and
changing its measurement range. Because of this the original factory calibrat
ion can no longer be used and an in
-
place calibration is required, however, accuracy may still be compromised.



Figure 32

Field Distortion from an Insufficient Target Size





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6.2.3.11.2

Targets that are too small to support the electric field can also provide false displaceme
nt signals from
lateral motions. Capacitive sensors are typically used to measure the gap or movement in the direction of the sensor
axis. If the target is large enough any lateral motion will not distort the electric field, however, if the field is wrappe
d
around the target any lateral motion will change the shape of the field causing a change in output even if the gap
remains constant. As a general rule, the target should be 30
-
50% larger than the capacitance sensor.

6.2.3.12

Target Shape

6.2.3.12.1

Capacitance probes will m
easure the average distance to the target under the area of the sensor. If a tilted
or curved target is being measured the electric field will be distorted and the accuracy compromised. When sensors
are calibrated to a flat target the output voltage is the
oretically zero when the probe is in contact with the target. This
is not true when measuring curved or tilted surfaces because the surface prevents the probe from full target contact.
The result will be a shift in the zero point from its original calibrat
ion which will be reflected as an offset in the
measurement, not a sensitivity change. To overcome both issues an in
-
place calibration is possible to correct for the
sensitivity change, however, the sensor measurement range may be reduced. As a general rul
e, a curved target
should be 10 times larger in diameter than the sensing element of the capacitance sensor.

6.2.3.13

Spatial Resolution

6.2.3.13.1

Capacitance sensors have a relatively large sensing area in relationship to their measurement range. As
mentioned above, these
types of sensors take an average measurement to the surface in question. If this surface has
features that are smaller than the sensing element the feature may not be detected or the sensor output may not
respond accordingly. Figure 33 shows how the probe
size can affect the sensor output when measuring a stepped
object, demonstrating sharper voltage changes from a smaller diameter sensor (Sensor C).




Figure 33

Spatial Resolution

6.2.3.13.2

Similarly, the output will depend on the surface roughness. If the roughness changes

over an area the
output from the capacitance sensor will change when the target translates beneath the probe because the average
distance to the surface has changed. The amount of sensor output shift will depend on the magnitude of the surface
roughness.

6.2.3.14

Environmental Conditions

6.2.3.14.1

Capacitance changes as the distance to the target changes and also depends upon the dielectric property
of the material in the gap. Because of this, it is important that there is a homogeneous, non
-
conductive material
between the p
robe and target. In most applications, this material is air, however, many times oil or some other
dielectric fluid is successfully used. If it is not homogeneous, or if the dielectric properties in the gap change, then
accuracy will be affected. This is t
ypically not a problem for changing air properties because the effects are small.



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For instance, the dielectric constant of air changes by ~1.4 ppm/1% of relative humidity. This represents a potential
offset in the senor output of less than 1 mV with a rela
tive humidity change of 50%. Care must be taken to avoid
dielectric changes from other materials and ensure that dirt and debris do not accumulate in the capacitive probe gap.

6.2.3.14.2

The most common environmental problem that can affect the accuracy of a capaciti
ve sensor is
temperature. Not only do the electronics exhibit temperature drift but also expansion and contraction of the probe
and fixturing physically changes the probe gap. Custom probes manufactured of highly stable materials, such as
Invar, are availa
ble for extreme stability applications. It is also possible to adjust the temperature coefficient of
components for custom high
-
stability applications.

6.2.3.15

Advantages and Disadvantages

6.2.3.16

Advantages

6.2.3.16.1

As with any sensing technology, capacitive systems have both ad
vantages and disadvantages. Perhaps
their greatest attribute is their ability to resolve measurements below one micro
-
inch (< 25 nm), at a fraction of the
cost of other high performance technologies. Most are "passive" by design allowing them to be used in

extreme
environments while still maintaining stability. Sensors can be easily customized, allowing them to be adapted into a
variety of applications or settings. They are immune to target composition and work equally well on all conductive
targets, unlike

eddy current probes. They are largely immune to ultrasonic noise,
electromagnetic fields
, lighting
conditions, humidity and temperature.

6.2.3.17

Disadvantages

6.2.3.17.1

Capacitance technology dictates that the probe be mounted close to the target. This increases the
probab
ility of crashing the sensor or damaging the material being measured. Some suppliers have provisions to
extend the measurement range and standoff of the sensor, however, this distance is rarely greater than 15 mm.

6.2.3.17.2

Capacitance sensors should also be kept cl
ean. Dirt or other foreign debris can cause an offset in the
measurement so frequent cleaning may be required depending on the application.

6.2.3.18

Applications

6.2.3.18.1

Thickness Measurements

6.2.3.18.1.1

Thickness quality control monitoring is better applied on line during the manufa
cturing process instead
of periodic sampling after a product has been manufactured. This way process adjustments can be made “on the fly,”
reducing or eliminating the continued production of product that does not meet specification. In some applications,
c
ontact methods can be utilized. H
owever, they are slow, can damage the product and are subject to wear. Non
-
contact sensors are commonly used in these applications.

6.2.3.18.2

A typical application consists of two capacitance probes, one on either side of the materia
l being
measured
.
The difference between the
output

of each sensor is directly related to the thickness of the material being
measured. By taking a differential measurement, any positional movement of the material within the probe gap is
cancelled.

6.2.3.18.3

Figure

3
4

shows a typical thickness setup with A and B representing the sensor outputs. The gap between
the probes, G, is equal to A + B + T. If an initial sample of known thickness, T, is placed within the gap, G can be
determined and used in future calculation
s. Since thickness = G


(A + B), and G is constant, the thickness can be
calculated by simply subtracting the two outputs.




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Figure 34

Typical Thickness Measurement Using
Dual
Capacitance Probes


6.2.3.18.4

Single
-
sided thickness measurements can also be successfully made
using capacitive sensors if the back
side of the material being measured can be referenced to some fixed plane. Figure 3
5

shows a typical single
-
sided
measurement. From this figure it is apparent that the product thickness is directly proportional to the g
ap between
the probe and the surface of the material. Figures 3
6

and 3
7

show a typical bonded pair wa
fer bow map and raw
thickness data from edge (point 0) to center of the wafer.



Figure 35

Single

Point Thickness Measurements





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Figure 36

825 µm Bonded Pair Wafer
Bow

Map



Figure 37

825 µm Bonded Pair
Raw
Thickness

Data


6.3

Pneumatic Technique for Measuring Proximity

6.3.1

The pneumatic technique for measuring proximity relies on the dependence of backpressure in an orifice
conducting flowing gas toward a flat surface on the proximity

of the surface to the orifice. Dual sensors allow
differential measurements.

6.3.2

Differential Backpressure Metrology

6.3.2.1

With a dual backpressure sensor, one can measure thickness (similar to a white light confocal sensor), as
well as bow and warp. The advantage

of differential backpressure sensing technology is that it works nearly
independent of any surface condition (smooth or rough) or material property (conductive or non
-
conductive)
requiring only a low supply pressure (10 psi or less) and only CDA or N
2
. I
n this technique, back
-
pressure is
converted to voltage; a calibration curve is generated and the operating point is determined on the curve. A



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reference backpressure measurement is made on a gauge block prior to every wafer measurement to compensate for
ambient temperature variation effects.

6.3.2.2

An air gauge is a broader terminology used for this technology, but in a broader sense, it includes any kind
of sensing set up using a pressure sensor to detect a backpressure. Usually it uses higher pressure, has poo
r
resolution, and is not stable. Differential backpressure technology is part of the air gauging technology but requires
the use of specific sensor layout (Wheatstone bridge equivalent) to meet the characteristics needed for its use in the
wafer
industry.

6.3.2.3

Differential backpressure sensing technology can be used to achieve non
-
contact, high
-
resolution wafer
thickness measurements. The

differential backpressure sensors (PEL sensors) use clean, dry, compressed air or an
inert gas, such as nitrogen.

6.3.2.4

The PEL se
nsors utilize a pneumatic “Wheatstone bridge” arrangement (Figure 38) with equal flow
restriction and a low
-
pressure differential sensor. The sensing orifice uses a calibrated high
-
precision sapphire
nozzle. Any small variation of the distance between the

sensing nozzle and the target is detected by the PEL sensor
and converted into a corresponding output voltage. The PEL sensor includes built
-
in temperature compensation and
voltage regulation to provide extremely high stability over time and resolution d
own to 0.02

m.


Figure 38

Backpressure Sensor Configuration

6.3.2.5

The
nominal

distance between the sensing nozzle and the target is set at approximately 100

m. This is the
optimal distance to operate within the linear portion of the backpressure response curve with the highest sensitivity.
Under normal operating conditions, the distance between the sensing nozzle and the target may vary by ±20

m
around the pr
eset nominal distance. With a response time of 5 ms the sensor is ideally suited for relatively fast
measurement operations of fixed targets.

6.3.2.6

Source gas is supplied to the sensor at 10 to 12 psi (P
S
). Any variation in the backpressure (P
B
) resulting
from

the changes in the distance (X) between the sensing nozzle (R
x
) and the gauged object (Figure 3
8
) will
generate a change in the output voltage at a rate of 50 mV/

m. Adjusting the setting of the valve (R
V
) provides a
wide range of settings and sensitivit
ies that can vary the pressure reference (P
R
).

6.3.2.7

Measurement System

6.3.2.7.1

Figure 39
shows

a block diagram of the sensor subassembly and the associated interconnecting electronic
and pneumatic controls. The servomotor in the measurement head is a voice coil that m
oves the probe relative to the
sample such that the backpressure reading falls on the calibration curve in the linear working region.





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Figure 39

Measurement System Block Diagram


6.3.2.7.2

The Encoder Interface conditions the signal from the Encoder, which determines the p
osition of the probe.
The Backpressure Sensor converts the backpressure to a voltage output to be supplied to the computer. The
computer processes all data from the measurement head as well as from the robotic handling system and the Y
-
Theta or X
-
Y stage
to control the three component subassemblies.

6.3.2.8

Thickness Calculation

6.3.2.8.1

The system employs a measurement sensor on the underside of the workstation as well as the topside.
From the drawing in Figure 40 the thickness, t, of a sample is calculated in Equation 3

as:




t = t
0

+ (D
-
D
0
)


(a
-
a
0
)


(b
-
b
0
)







(3)


Where:


t
0

is the thickness of the gauge block,

D and D
0

are the relative positions of the probe,

a
0

is the distance from the top probe tip to the gauge block,

a is the distance from the top
probe tip to the sample
,

b
0

is the distance from the bottom probe tip to the backside of the gauge block, and

b is the distance from the bottom probe tip to the back of the sample
.




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Figure 40


Probe
Measurement

Technique


6.3.2.8.1.1

An example of a thickness map using differential back
-
pressure monitoring is shown in Figure 41. The
mean thickness is 872

m.


Figure 41


Thickness Map Obtained by Differential Back Pressure Monitoring


6.3.2.8.1.2

An example of a warp map using differential back
-
pressure
monitoring is shown in Figure 42. The warp
values are for a
n

875

m two wafer stack.





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Figure 42


Warp
Map Obtained by Differential Back Pressure Monitoring


6.4

Acoustic Microscopy for Measuring Bonded Wafer Thickness

6.4.1

A
-
Mode Scanning

6.4.1.1

Acoustic data
can be
collected
at the smallest X
-
Y
-
Z region defined by the limitations of the given acoustic
microscope. An A
-
mode display contains amplitude and phase/polarity information
(Figure 43 on left)
as a function
of time of flight at a

single point in the X
-
Y plane, as shown i
n Figure 43 (Figure 43 on right).



Figure 43

Example of an A
-
Mode Scan and
S
pectrum



6.4.2

B
-
Mode Scanning

6.4.2.1

Acoustic data
can also be
collected along an X
-
Z or Y
-
Z plane versus depth using a reflective aco
ustic
microscope, as shown in Figure 44 on the left. A B
-
mode scan, shown in Figure 44,
contains amplitude and
phase/polarity information as a function of time of flight at each point along the scan line. A B
-
mode scan furnishes
a two
-
dimensional (cross
-
se
ctional) description along a scan line (X or Y)
, as shown in Figure 44 on the right
.




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ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
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Figure 44

Example of a B
-
Mode Scan and
its
R
esulting
I
mage


6.4.3

C
-
Mode Scanning

6.4.3.1

Acoustic data collected in an X
-
Y plane at depth (Z) using a reflective acoustic microscope is shown
in
Figure 45 on the left. A C
-
mode scan contains amplitude and phase/polarity information at each point in the scan
plane. A C
-
mode scan furnishes a two
-
dimensional (area) image of echoes arising from reflections at a particular
depth (Z), as shown in Figu
re 45 on the right.



Figure 45

Example of a C
-
Mode Scan and
its Resulting Image


6.4.4

While acoustic microscopes are typically used to image and assess the bond quality between bonded wafers,
the intrinsic data used to image the bond quality also contains time/distanc
e information (Figure 46 on left). In order
to image at a particular interface of interest, the time/distance to that interface is also known at each X
-
Y location
and is displayed in an A
-
Scan waveform format. The A
-
Scan at each X
-
Y location can be proce
ssed to determine
the thickness of each layer at that point and the surface contours of the front, internal interface or back of the wafer
stack (Figure 46 on right).






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ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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Figure 46

Typical

A
-
Scan
W
aveform
D
isplay (
L
eft) and C
-
Scan
S
howing the
B
ond
Q
uality at
S
evera
l
L
ayers of a
W
afer
B
ond
S
tack

NOTE:

White and light gray areas indicate delamination present at several layers


6.4.5

Thickness Measurements with AM

6.4.5.1

As
m
entioned above, the A
-
Scan waveform consists of time and material interface data at an X
-
Y location
on a bonded wafer. Knowing the speed of sound within a material converts the time data to a distance between one
material interface and another, as shown i
n Figure 47 on the left. The time/distance data can be processed to color
code the defects at different layers, as shown in Figure 47 on the right.



Figure 47

An
E
xample of a
C
olor
C
oded
P
rofile
I
mage
T
hat
M
aps the
D
istance from the
S
urface of a
P
art to
A
nomalies
at a
S
pecific
D
epth
W
ithin a
S
tacked
W
afer

NOTE:

All anomalies of the same color are at the same depth and the color map indicates that the blue anomalies are closer to
the top surface, deeper green and orange being the deepest in the stack.


6.4.5.2

In additio
n, thickness measurement of a layer within a wafer stack can be obtained, as shown in Figure 48.
In this image, the C
-
scan image is shown on the left, while the A
-
Scan waveforms showing the full bonded wafer
thickness, the delamination region, and defect
ive regions within the stack are shown on the right. The data shown in
Table 1 shows the time and distance measurements, based on the speed of sound of the material, made on a sample
at five points of interest.





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ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
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development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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Figure 48

C
-
Scan
I
mage of a
S
tacked
W
afer
S
howing

3 A
-
Scan
W
aveforms

NOTE:

Waveform #1 shows the complete thickness of the stacked wafer. Waveforms #2 and #3 show the distance from the top
surface to the delamination and anomalous regions within the wafer stack
.


Table 1

Example
T
able of
V
arious
T
hicknesses
M
easured a
t 5
P
ositions on a
S
ample.



6.4.6

Bonded
Wafer

Thickness, TTV, Warp, Bow, Sori, and Flatness Metrology with Acoustic Microscopy (AM)

6.4.6.1

Acoustic microscopy (AM) collects data similar to a depth finder in that it can measure the transit time for
sound from a reference point, in this case the transducer, to the first surface it sees. The contour of that surface over
the X
-
Y area scanned can

then easily be displayed in a simple color coded map or process further to provide specific
warp, bow, sori or
global

flatness data. In Figures 49 and 50
,

examples are provided of both types of data displays
for 3D measurement of wafer stack surface contours.

Waveform
Thickness (ns)
Thickness (

m)
1
87
108
2
81
101
3
60
75
4
70
87
5
69
86



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ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
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development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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Figure 49

Example of an AM
S
urface
F
latness
C
olor
-
C
oded
D
isplay with
D
ata
O
verlaid

NOTE:

In this case the wafer stack is bowed 140
μ
m from center to edge.



Figure 50

Exa
mple of a 3D AM
S
urface
F
latness
M
ap with the
D
ata
P
rocessed and
D
isplayed.






This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an offic
ial or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or
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development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.


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6.4.6.1.1

A summary of the techniques described in this guide is shown in Table 2 showing whether they are a high
-
volume manufacturing tool, and whether they measure the thickness of the w
afer stack or the individual wafer
components. The content is based on supplier information and is subject to change.


Table 2

Summary
of

Techniques in Guide



NOTICE:

Semiconductor Equipment and Materials International (SEMI) makes no warranties or representati
ons as
to the suitability of the Standards and Safety Guidelines set forth herein for any particular application. The
determination of the suitability of the Standard or Safety Guideline is solely the responsibility of the user. Users are
cautioned to refe
r to manufacturer’s instructions, product labels, product data sheets, and other relevant literature,
respecting any materials or equipment mentioned herein. Standards and Safety Guidelines are subject to change
without notice.

By publication of this Stand
ard or Safety Guideline, SEMI takes no position respecting the validity of any patent
rights or copyrights asserted in connection with any items mentioned in this Standard or Safety Guideline. Users of
this Standard or Safety Guideline are expressly advise
d that determination of any such patent rights or copyrights,
and the risk of infringement of such rights are entirely their own responsibility.


Metrology Technique
HVM
Non-
HVM
Measurements
Made
Capable of
Measuring
Each Wafer in
Bonded Wafer
Pair
Capable of
Measuring Bond
Layer
White Light Confocal
Microscope
Y
Y
Thickness, TTV,
Bow, Warp
N
N
IR Laser Profiling
Y
Y
Thickness, TTV,
Bow, Warp
Y
Y
White Light
Interferometry
Y
Y
Thickness, TTV,
Bow, Warp
N
N
Infrared
Interferometry/Confocal
Microscope
Y
Y
Thickness, TTV,
Bow, Warp
Y
Y
Capacitance Metrology
Y
Y
Thickness, TTV,
Bow, Warp
N
N
Differential Backpressure
Metrology
Y
Y
Thickness, TTV,
Bow, Warp
N
N
Acoustic Microscopy
Y
Y
Thickness, TTV,
Bow, Warp
Y
Y