Chapter 4 Clean room, wafer cleaning and gettering

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Nov 27, 2013 (3 years and 8 months ago)

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Chapter 4 Clean room, wafer cleaning and gettering

1.
Introduction.

2.
Clean room.

3.
Wafer cleaning.

4.
Gettering.

5.
Measurement methods.

1

NE 343: Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

Effect of defect and contamination
on semiconductor industry

Importance of unwanted impurities increases with shrinking geometries of devices.

75% of the yield loss is due to defects caused by particles (1/2 of the min feature size).

LLS: localized light
scatters (use laser to
detect and count
particles)


GOI: gate oxide
integrity, by electrical
measurement


Requirement different
for DRAM and logic
chip, due to greater
gate insulator area on
DRAM chip.


10
9
/cm
2



0.0001%
monolayer

2

Silicon Wafer
SiO
2
or other thin films
Photoresist
Au
Cu
Fe
Particles
Interconnect Metal
Na
N, P
Contaminants
may consist of
particles
,
organic
films,
photoresist
, heavy
metals or alkali ions.

Modern
IC factories employ a three tiered approach
to controlling unwanted
impurities
:

1.
clean factories

2.
wafer cleaning

3.
gettering

Type of contaminants

3

Effects on MOSFET: two examples

MOSFET threshold voltage is given by:




If
t
ox
=10nm, then a 0.1V
V
th

shift can be caused by
Na
+

or K
+

of
Q
M
=2.15

10
11

ions

/cm
2

(<0.1% monolayer or 10ppm in the oxide).


0
=8.85

10
-
12
F/m,

ox
=3.9

For MOS DRAM, refresh time of several
msec

requires a generation lifetime of




This requires trap density N
t

10
12
/cm
3
, or 0.02ppb
(10
12
/(5

10
22
)=0.02ppb).

(


is trap capture cross
-
section,
v
th

is minority carrier
thermal velocity;
V
th

10
7
cm/sec,


10
-
15
cm
-
2
)

Deep
-
level traps (
Cu, Fe, Au etc
.) pile
up at the surface where the devices are
located. This causes leak current. Need
refresh/recharge the MOS capacitor.

DRAM: Dynamic Random Access Memory

4

Residual
contaminants affect
kinetics of
processes, here oxidation.

Effects of cleaning on thermal oxidation

5

Particle contaminants

Particle sources: air, people, equipment and
chemicals.

A typical person emits 5
-
10 million particles per
minute.

>0.2

m

>0.5

m

NH
4
OH

130
-
240

15
-
30

H
2
O
2

20
-
100

5
-
20

HF

0
-
1

0

HCl

2
-
7

1
-
2

H
2
SO
4

180
-
1150

10
-
80

Particle density (number/ml)
for ULSI grade chemicals

6

ULSI: ultra
-
large
-
scale integration

Fe, Cu, Ni, Cr,
W, Ti…

Na, K, Li…

Metal contamination

Sources: chemicals, ion implantation, reactive ion etching, resist
removal, oxidation.

Effects: defects at interface degrade device; leads to leak current
of p
-
n junction, reduces minority carrier life time.

Dry etching

Ion
implantation

Photoresist
removal

Wet oxidation

9

10

11

12

13

Log (concentration/cm
2
)

Fe Ni Cu

7

Chapter 4 Clean room, wafer cleaning and gettering

1.
Introduction.

2.
Clean room.

3.
Wafer cleaning.

4.
Gettering.

5.
Measurement methods.

NE 343 Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

8

9

Modern
IC factories employ a three tiered approach
to controlling unwanted
impurities
:

1.
clean factories

2.
wafer cleaning

3.
gettering

Clean factory is the first approach against contamination

Clean factory



Wafer cleaning



Gettering

Factory environment is cleaned
by
:


HEPA filters and recirculation
for the air.


“Bunny suits” for workers.


Filtration of chemicals and
gases.


Manufacturing protocols.

Clean room

HEPA: High Efficiency Particulate Air


HEPA filters composed of thin porous sheets of
ultrafine glass fibers (<1

m diameter
).


It is 99.97% efficient at removing particles from air.


Room air forced through the filter at 50cm/sec.


Large particles trapped, small ones stick to the fibers
due to electrostatic forces.


The exit air is typically better than class 1.

10

Class of a clean room


Air
quality is measured by
the “
class” of the facility
.


Class 1
-
100,000
mean number of particles, greater than 0.5

m, in a cubit foot of air.


A typical office building is about class 100,000.


The particle size that is of most concern is 10nm


10

m. Particles <10nm tend to
coagulate into large ones; those >10

m are heavy and precipitate quickly.


Particles
deposit on surfaces by Brownian motion (most important for those <0.5

m
)
and gravitational sedimentation (for larger ones).

Class

0.1

0.3

0.5

5.0

1

35

3

1

10

350

30

10

100

300

100

1000

1000

7

10000

10000

70

100000

100000

700

Particle diameter (

m
)

by definition

11

Particle contamination and yield


Generally, particles on the order of the technology minimum features size or
larger will cause defect.



75 y
ield loss in modern VLSI fabrication facilities is due to particle
contamination.


Yield models depend on information about the description of particles.


Particles on the order of 0.1
-
0.3

m are the most troublesome: larger particles
precipitate easily; smaller one coagulate into larger particles.

12

Chapter 4 Clean room, wafer cleaning and gettering

1.
Introduction.

2.
Clean room.

3.
Wafer cleaning.

4.
Gettering.

5.
Measurement methods.

NE 343 Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

13


Cleaning involves removing particles, organics and metals from wafer surfaces.


Particles are largely removed by ultrasonic agitation during cleaning.


Organics (photoresist) are removed in O
2

plasma or in H
2
SO
4
/H
2
O
2

(
Piranha
) solutions.


The “RCA clean” is used to remove metals and any remaining organics.

Modern wafer cleaning

Typical person emit 5
-
10
million particle per
minute.

Most modern IC plants
use robots for wafer
handling.

A cassette of wafers

14

RCA
clean is “standard
process”
used
to remove
organics, heavy
metals
and
alkali ions.

Ultrasonic
agitation is used
to
dislodge
particles
.

SC: Standard Cleaning

RCA: Radio Corporation of
America, now makes TV,
stereos…

Standard RCA
cleaning procedure

DI water: de
-
ionized water

120 - 150ÞC
10 min
Strips organics
especially photoresist
DI H
2
O Rinse
Room T
80 - 90ÞC
10 min
Strips organics,
metals and particles
DI H
2
O Rinse
Room T
80 - 90ÞC
10 min
Strips alkali ions
and metals
Room T
1 min
Strips chemical
oxide
DI H
2
O Rinse
Room T
H
2
SO
4
/H
2
O
2

1:1 to 4:1
HF/H
2
O
1:10 to 1:50
NH
4
OH/H
2
O
2
/H
2
O
1:1:5 to 0.05:1:5
SC-1
HCl/H
2
O
2
/H
2
O
1:1:6
SC-2
and
all
contaminants on
top of it, but induces H
passivated surface (bad)

Less NH
4
OH will
reduce
surface roughness

not
removed by
SC
-
1

HF dip added to remove oxide

15

SC
-
1:


NH
4
OH(28%):H
2
O
2
(30
%):H
2
O=1:1:5

-

1:2:7
; 70
-
80

C,
10min, high
pH.


Oxidize organic contamination (form CO
2
, H
2
O…)


Form complex
such as Cu(NH
3
)
4
+2

with metals (IB, IIB, Au, Ag, Cu, Ni, Zn,
Cd
, Co, Cr).


Slowly dissolve native oxide and grow back new oxide, which removes particles on
oxide.


But NH
4
OH

etches Si and make the surface rough, thus less NH
4
OH is used today.

Standard cleaning (SC)

SC
-
2
:

HCl
(73%):H
2
O
2
(30
%):H
2
O=1:1:6
-

1:2:8; 70
-

80

C; 10min, low
pH.


Remove alkali ions and
cations

like Al
+3
, Fe
+3

and Mg
+2

that form NH
4
OH insoluble
hydroxides in basic solutions like SC
-
1.


These metals precipitate onto wafer surface in the SC
-
1 solution, while they form
soluble complexes in SC
-
2 solution.


SC
-
2 also complete the removal of metallic contaminates such as Au that may not
have been completely removed by SC
-
1 step.

16



Si

2
H
2
O

SiO
2

4
H


4
e



M

M
z


ze

(1)

(2)

Principles of metal cleaning

If
we have a water solution with a Si wafer and metal atoms and ions, two reactions take
place
.






The two reactions will proceed in opposite directions, one providing electrons, which will
then be consumed by the other (forming an oxidation/reduction couple). In this couple, the
stronger reaction will dominate.

Generally, (2) is driven to the left and (1) to the right so that SiO
2

is formed and M plates
out on the wafer.

Good cleaning solutions drive (2) to the right since M
+

is soluble and will be desorbed from
the wafer surface.

17

Reaction goes to the left

The strongest oxidants are at the bottom (H
2
O
2

and O
3
). These reactions go to the left,
grabbing electrons and forcing (2) in previous slide to the right.

Fundamentally the RCA clean works by using H
2
O
2

as a strong oxidant.

Principles of metal cleaning

18

H
2
O


H
+
+OH
-



[H
+
]=[OH
-
] = 6x10
-
13
cm
-
3

Diffusivity of:

H
+

≈ 9.3x10
-
5
cm
2
s
-
1



µ
H+
=
qD
/
kT
=3.59cm
2
V
-
1
s
-
1


of :

OH
-

≈ 5.3x10
-
5
cm
2
s
-
1



µ
OH
-
=
qD
/
kT
=2.04cm
2
V
-
1
s
-
1

cm
M
OH
H
q
OH
H








5
.
18
)
]
[
]
([
1



Ultrasonic cleaning and DI water

RCA cleaning with ultrasonic agitation is more effective in removing particles.

Ultrasonic cleaning:


Highly effective for removing surface contaminants


Mechanical agitation of cleaning fluid by high
-
frequency vibrations (between 20 and 45
kHz) to cause
cavitation

-

formation of low pressure vapor bubbles that scrub the surface.


Higher frequencies (>45kHz) form smaller bubbles, thus less effective.


However,
megasonic

(1MHz) cleaning is also found effective in particle removal.

DI (de
-
ionized) water is used for wafer cleaning.

One monitors DI water by measuring its resistivity, which should be >18M

cm.

Einstein relation: µ=
qD
/
kT
, http://en.wikipedia.org/wiki/Einstein_relation_%28kinetic_theory%29

19

Other cleaning methods

Dry (vapor phase) cleaning:

Energy may come from plasma, ion
beam, short
-
wavelength (UV)
radiation or heating.


HF/H
2
O

vapor cleaning


UV
-
ozone cleaning (UVOC)


H
2
/
Ar

plasma cleaning


Thermal cleaning

Ohmi

cleaning:
room temperature, fewer chemicals

20

Chapter 4 Clean room, wafer cleaning and gettering

1.
Introduction.

2.
Clean room.

3.
Wafer cleaning.

4.
Gettering.

5.
Measurement methods.

NE 343 Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

21

Gettering


For the alkali ions, gettering generally uses dielectric layers on the topside; PSG for
trapping, or Si
3
N
4

layer for blocking them from getting into the device region.

PSG:
phosphosilicate

glass, is a P
2
O
5
/SiO
2

glass that is normally deposited by CVD,
usually contains 5% by weight phosphorus.

PSG traps alkali ions (Na
+
, K
+
) and form stable compounds.

At higher than room temperature, alkali ions can diffuse into PSG from device region
and trapped there.

Problems with PSG: it affects electric fields since dipoles exist in PSG, and it absorb
water, leading to Al corrosion.

22


For metal ions, gettering generally uses traps on the wafer backside or in the wafer bulk.
Here gettering works because the metals (Au…) do not “fit” in the silicon lattice easily
because of their very different atomic size, thus they prefer to stay at defect sites.


Therefore, the idea of gettering is to create such defect sites outside of active device
region.


Backside =
external

gettering: roughing/damaging the backside of the wafer, or depositing
a poly
-
silicon layer, to provide a low energy “sink” for impurities.


Bulk =
intrinsic

(or internal) gettering: using internal defects to trap impurities, thus moving
them away from the active region of the wafer.

H

1.008
1
3
4
11
12
19
20
Li

6.941
Be

9.012
Na

22.99
Mg

24.31
K

39.10
Ca

40.08
Rb

85.47
Cs

132.9
Fr

223
Sr

87.62
Ba

137.3
Ra

226
37
38
55
56
87
88
Sc

44.96
Ti

47.88
V

50.94
Cr

51.99
Mn

54.94
Fe

55.85
Co

58.93
Ni

58.69
Cu

63.55
Zn

65.39
21
22
23
24
25
26
27
28
29
30
Y

88.91
Zr

91.22
Nb

92.91
Mo

95.94
Tc

98
Ru

101.1
Rh

102.9
Pd

106.4
Ag

107.9
Cd

112.4
39
40
41
42
43
44
45
46
47
48
La

138.9
Hf

178.5
Ta

180.8
W

183.9
Re

186.2
Os

190.2
Ir

192.2
Pt

195.1
Au

197.0
Hg

200.6
57
72
73
74
75
76
77
78
79
80
Ac

227.0
Unq

261
Unp

262
Unh

263
Uns

262
89
104
105
106
107
B

10.81
Al

26.98
Ga

69.72
In

114.8
Tl

204.4
C

12.01
Si

28.09
Ge

72.59
Sn

118.7
Pb

207.2
N

14.01
P

30.97
As

74.92
Sb

121.8
Bi

209.0
O

16.00
S

32.06
Se

78.96
Te

127.6
Po

209
F

19.00
Cl

35.45
Br

79.90
I

126.9
At

210
He

4.003
Ne

20.18
Ar

39.95
Kr

83.80
Xe

131.3
Rn

222
5
6
7
8
9
10
2
13
14
15
16
17
18
31
32
33
34
35
36
49
50
51
52
53
54
81
82
83
84
85
86
Period
1
2
3
4
5
6
7
I
A
II
A
III
B
IV
B
V
A
I
B
II
B
III
A
IV
A
V
B
VI
B
VII
B
VIII
VI
A
VII
A
Noble
Gases
Shallow Donors
Shallow Acceptors
Elemental
Semiconductors
Deep Level Impurites in Silicon
Alkali Ions

Deep level impurities in silicon:

large diffusivity, easily trapped by
mechanical defects or chemical traps.

Gettering

Figure 4
-
6 Periodic table indicating the elements that are of most concern in gettering.

23

Fast diffusion of various impurities

Heavy
metal gettering relies
on metal’s very high diffusivity (when in
interstitial sites) in silicon, and its preference to segregate
to “trap” sites.

Diffusivity (cm
2
/sec)

Those metal diffuses fast
because they do so as
interstitials
.

Whereas dopants are
substitutional
and diffuse by
interacting with point defects.

I: interstitial

S: substitutional

They can diffuse from front
-
side to backside of the wafer
(>0.5mm distance)

24

PSG layer

Devices in near
surface region

Denuded zone
or epitaxy layer




Intrinsic
gettering region






Backside
gettering region

500+

m

10
-
20

m

Gettering mechanism

25

Gettering
consists
of:

1.
Making
metal atoms mobile.

2.
Migration
of these atoms to
trapping sites.

3.
Trapping
of atoms.

Step
1 generally happens by
kicking out
the
substitutional (s)
atom into an
interstitial (
i
)
site. One possible reaction is
: (I = interstitial Si)

Step
2 usually happens easily once
the metal
is interstitial since most metals
diffuse
rapidly
in this form.

Step
3 happens because heavy metals
segregate
preferentially to damaged
regions
(dislocation or stacking fault) or to N
+

regions, or pair with effective getters like P (
AuP
).


Step 1 can be facilitated by introducing large amount of Si interstitials, by such as high
density phosphorus diffusion, ion implantation damage or SiO
2

precipitation.



Au
S

I

Au
i
Gettering mechanism

26

Intrinsic

gettering

1
2
0
sec
53
.
2
exp
13
.
0









cm
kT
D
Oxygen diffusivity:




D
0
>>
D
dopants

but D
0
<<
D
metals

1100



900



700



500

Out
-
diffusion of O

Nucleation of SiO
2

Precipitation

(growth of SiO
2
)

denuded zone =
oxygen free;
thickness several
tens of µm

50
-
100nm
in size

Slow ramp

1
-
3 nm min size of nuclei,
concentrations ≈ 10
11
cm
-
3


Temperature
o
C

Time

In intrinsic gettering, the metal atoms segregate to dislocations (formed because of volume
mismatch of SiO
2

and host Si lattice) around SiO
2

precipitates.

15
to 20
ppm

oxygen
wafers are
required:

<10
ppm

-

precipitate density is too sparse to be an effective
getterer
.

>20
ppm

-

wafers tend to warp during the high temperature
process.

Note
: devices that use the entire wafer as the active region (solar cells,
thyristors
, power
diodes, etc...) can not use this technique, but can use extrinsic
gettering.

Today
, most wafer manufactures perform this
intrinsic gettering task that is better controlled.

Precipitates (size) grow @ high T

Density
of nucleation sites grow @ low
T

Therefore, low T to increase density, and
high T to grow its size.

27

Intrinsic
Gettering: SiO
2

precipitates


no SiO
2


SiO
2

precipitates
(50
-
100nm)

SiO
2

precipitates (white dots) in
bulk Si

No SiO
2

on top surface

(denuded zone)

Figure 4
-
13

28

Chapter 4 Clean room, wafer cleaning and gettering

1.
Introduction.

2.
Clean room.

3.
Wafer cleaning.

4.
Gettering.

5.
Measurement methods.

NE 343 Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

29

Un
-
patterned
wafers (blank)



Count particles in microscope



Laser scanning systems
that give maps
of particles down to ≈ 0.2 µm

Patterned
wafers


Optical
system compares a die with a “known good reference” die
(adjacent die, chip design
-

its appearance)



Image processing identifies
defects



Test structure (not in high volume manufacturing
)


Particle contamination detection

Test structures design
to detect defects

30

Monitoring the wafer cleaning efficiency

Concentrations of impurities determined by surface
analysis.

Excite


Identify (unique atomic signature)


Count concentrations

Primary
beam electron


good
lateral
resolution (e can be focused, but not x
-
ray)

Detected
beam electron


good
depth resolution and surface sensitivity



X
-
ray


poor
depth resolution and poor surface sensitivity



ions
(SIMS)


excellent



ions
(
RBS) good
depth resolution, reasonable sensitivity (0.1 atomic%)

works with SEM

He
+

1
-
3
MeV

O
+

or Cs
+

sputtering
and mass
analyses

emitted

31