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ETE444/544

Introduction to

Micro Fabrication

Springer Handbook of
Nanotechnology

Chapter 5

http://www.springer.com/materials/nanotechnology/book/978
-
3
-
540
-
29857
-
1

Introduction


Recent innovations in the area of
micro
fabrication
have created a unique opportunity for
manufacturing structures in the nanometer

millimeter range.


We can fabricate novel electronic, optical,
magnetic, mechanical, and chemical/biological
devices with applications ranging from sensors
to computation and control.

http://www.flickr.com/photos/ibm_research_zurich/5668772519/sizes/l/in/photostream
/

Nanofabrication


Basic
Microfabrication

Techniques


Lithography


Thin Film Deposition and Doping


Etching and Substrate Removal


Substrate Bonding

Moore’s Law and Transistor
Count 1971
-
2008

Moore's law

describes a long
-
term trend in the
history of
computing hardware

whereby the
number of
transistors

that can be
placed inexpensively on an
integrated circuit

doubles
approximately every two years.
The period often quoted as "18
months" is due to David House,
an Intel executive, who predicted
that period for a doubling in chip
performance (being a
combination of the effect of more
transistors and them being
faster).
[1]

he law is named after
Intel

co
-
founder
Gordon E.
Moore
, who described the trend
in his 1965 paper.
[6][7][8]

The
paper noted that the number of
components in integrated circuits
had doubled every year from the
invention of the integrated circuit
in 1958 until 1965 and predicted
that the trend would continue
"for at least ten years"


http://
en.wikipedia.org
/wiki/
Moore’s_law

Fabrication


Top
-
down:

Chisel away material to make
nanoscale objects


Bottom
-
up:

Assemble nanoscale objects
out of even smaller units (e.g., atoms and
molecules)


Ultimate Goal:

Dial in the properties that
you want by designing and building at the
scale of nature (i.e., the nanoscale)

Top Down vs. Bottom Up

http://nanopedia.case.edu/NWPage.php?page=nanofabrication

Top
-
Down:
Photolithography

Mark C. Hersam: Northwestern University

Top
-
Down: Nanoimprint
Lithography

Mark C. Hersam: Northwestern University

Top
-
Down: Nanosphere
Lithography

Mark C. Hersam: Northwestern University

Bottom
-
Up: Carbon
Nanotube Synthesis

Bottom
-
Up:

Molecular Self Assembly


Spontaneous organization of molecules into stable,
structurally


well
-
defined aggregates (nanometer length scale).


Molecules can be transported to surfaces through liquids to
form self
-
assembled monolayers (SAMs).


Merging of two approaches: Top
-
down
and bottom
-
up machining
methodologies



Most human manufacturing methods of small
devices involve top
-
down approaches. Starting
from larger blocks of material we make smaller
and smaller things. Nature works the other way,
i.e., from the bottom
-
up.


All
living things are made atom by atom ,
molecule by molecule; from the small to the
large. As manufacturing of very small things
with top
-
down techniques (NEMS or
nano

mechanical devices) become too expensive or
hit other barriers we are looking at nature for
guidance (
biomimetics
).


Further
miniaturization might be inspired by
biology but will most likely be different again
from nature
--

the drivers for human and
natural manufacturing techniques are very
different.

Seeman

Eigler

Daunert
-

Madou

Montemagno


Merging of two approaches: Top
-
down and
bottom
-
up machining methodologies
--
NEMS


MEMS’ little brother is NEMS, the top
-
down
approach to nano devices. This biomimetic approach
to nano devices I like to call nanochemistry. To
succeed in the latter we will need :


self
-
assembly and directed assembly (e.,g, using
electrical fields
-
see next viewgraph)


massive parallelism


understanding of molecular mechanisms
--
chemomechanics


engineers/scientists who understand ‘wet’ and ‘dry’
disciplines


Example nano chemistry approaches:


Natural polymers: e.g., NAs and proteins not only as
sensors but also as actuators and building blocks
(Genetic engineer NA’s and proteins
-
rely on
extremophiles for guidance)


Mechanosynthesis


NEMS/biology hybrids
--
to learn only



Lithography


Lithography is the technique used to transfer a computer
generated pattern onto a substrate (silicon, glass,
GaAs
, etc.).
This pattern is subsequently used to etch an underlying thin
film (oxide, nitride, etc.) for various purposes (doping,
etching, etc.). Although photolithography,
i
. e., the lithography
using a UV light source, is by far the most widely used
lithography technique in microelectronic fabrication, electron
-
beam (e
-
beam) and X
-
ray lithography are two other
alternatives that have attracted considerable attention in
theMEMSand

nanofabrication areas.

Exposure Systems


Depending on the separation between the mask
and the wafer
, three different exposure systems
are available:

1.
Contact :
gives better
resolution than proximity.

2.
Proximity

3.
Projection


uses
a
dual lens optical
system to project the mask
image onto
the wafer.


Widely used
system in
microfabrication

and can yield
superior resolutions
compared to contact and
proximity methods

Thin Film Deposition and Doping

Exposed area

Unexposed area / blocked area

Step Coverage and
Conformality

Oxidation


Oxidation of silicon is a process used to obtain
a thin film of SiO2 of excellent quality (very
low density of defects) and thickness
homogeneity.

The oxidation of silicon also occurs at room temperature, however, a layer of about
20Å (native oxide) is enough to passivate the surface and prevent further oxidation.

Oxidation Furnace

Doping


The introduction of certain impurities in a
semiconductor can change its electrical,
chemical, and even mechanical properties.


Typical impurities, or
dopants
,
used in silicon
include boron (to form p
-
type regions) and
phosphorous or arsenic (to form n
-
type regions).


Doping is the main process used in the
microelectronic industry to fabricate major
components such as diodes and transistors.

N
-
Type / P
-
Type Junction

Chemical Vapor Deposition and
Epitaxy


Reaction of chemicals in a gas phase to
form the deposited thin film


LPCVD (Low Pressure CVD)


PECVD (Plasma Enhanced CVD)


MO
-
CVD tutorial on epitaxial deposition at
http://www.memc.com/index.php?view=Epitaxial
-
Deposition
-

Metal Organic Vapor Phase Epitaxy

Metalorganic

vapour

phase epitaxy

(MOVPE), also known as
organometallic

vapour

phase epitaxy (OMVPE)

or
metalorganic

chemical
vapour

deposition (MOCVD)
, is a
chemical
vapour

deposition

method of
epitaxial growth

of
materials, especially
compound
semiconductors
, from the surface reaction
of
organic compounds

or
metalorganics

and
metal hydrides containing the required
chemical elements
. For example,
indium
phosphide

could be grown in a reactor on a
substrate by introducing
Trimethylindium

((CH
3
)
3
In) and
phosphine

(PH
3
). Formation
of the epitaxial layer occurs by final
pyrolysis

of the constituent chemicals at the
substrate surface. In contrast to
molecular
beam epitaxy

(MBE) the growth of
crystals

is by chemical reaction and not physical
deposition. This takes place not in a
vacuum
, but from the
gas

phase at
moderate
pressures

(2 to 100
k
Pa
). As
such, this technique is preferred for the
formation of devices incorporating
thermodynamically
metastable

alloys, and it
has become a major process in the
manufacture of
optoelectronics
.

http://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy

LPCVD


LPCVD process is typically carried out in electrically heated
tubes, similar to the oxidation tubes, equipped with
pumping capabilities to achieve the needed low pressures
(0
.1 to 1.0
torr
). A large number
of wafers can be processed
simultaneously, and the material is deposited in both sides
of the wafers.


The process temperatures depend on the material to be
deposited, but generally are in the range of 550 to 900

C.


As in the oxidation, high temperatures and contamination
issues can restrict the type of processes used prior to the
LPCVD. Typical materials deposited by LPCVD include silicon
oxide (e.g., SiCl2H2 + 2N2O

SiO2+2N2+2HCl at 900

C)

PECVD


The PECVD process is performed in plasma systems such as
the one represented in Fig. The use of RF energy to create
highly reactive species in the plasma allows for the use of
lower temperatures at the substrates (150 to 350

C).
Parallel
-
plate plasma reactors normally used in
microfabrication

can only process a limited number of
wafers per batch.


The wafers are positioned horizontally on top of the lower
electrode, so only one side gets deposited. Typical
materials deposited with PECVD include silicon oxide,
nitride, and amorphous silicon. Conformality is good for
low
-
aspect
-
ratio structures, but becomes very poor for
deep trenches (20% of the surface thickness inside
through
-
wafer holes with aspect ratio of ten).

Epitaxial Growth


In this process, a single crystalline material is
grown as an extension of the crystal structure
of the substrate.


It is possible to grow dissimilar materials if the
crystal structures are somehow similar
(lattice
-
matched).


Silicon
-
on
-
sapphire (SOS) substrates and some
heterostructures

are fabricated in this way.

Molecular Beam Epitaxy

The molecular beam epitaxy technique (MBE) was developed initially for the crystalline growth of the
semiconductors. It is an ultra
-
high vacuum (P < 10
-
6 mbar) technology based on the sequential evaporation of
the elementary components placed in Knudsen effusion cells. One of the advantages of this method rests on the
control of the growth in real time thanks to the in situ use of the high energy electron diffraction in grazing
incidence (RHEED).


http://iramis.cea.fr/en/Phocea/Vie_des_labos/Ast/ast_sstechnique.php?id_ast=494

Complex Oxide Molecular Beam Epitaxy
--
5

This technology allows pure complex oxides films to be grown
epitaxially
; of special interest are films that are ferroelectric, ferromagnetic, or
superconducting. Alternating layers can be deposited to allow the observation of novel properties at the boundary or interfac
e.

http://www.flickr.com/photos/argonne/3366236801/in/photostream/

Physical Vapor Deposition


In physical deposition systems the
material to be deposited is transported
from a source to the wafers, both being in
the same chamber. Two physical
principles are used to do so:


Evaporation


Sputtering

Electron Beam Deposition

Sputtering


In sputtering, a target of the material to be deposited is
bombarded with high
-
energy inert ions (usually argon).



The outcome of the bombardment is that individual
atoms or clusters are removed from the surface and
ejected toward the wafer.


The physical nature of this process allows its use with
virtually any existing material: metals, dielectrics,
alloys, and all kinds of compounds (for example,
piezoelectric PZT).


The inert ions bombarding the target are produced in
DC or RF plasma.

Sputter Deposition

http://en.wikibooks.org/wiki/Microtechnology/Additive_Processes

Etching / Substrate Removal


Very often the substrate (silicon, glass,
GaAs, etc.) also needs to be removed in
order to create various mechanical micro
-
/nanostructures (beams, plates, etc.). Two
important figures of merit for any etching
process are selectivity and directionality.


Wet Etching


Dry Etching

Wet Etching


Due to the lateral undercut, the minimum
feature achievable with wet etchants is
limited to
> 3μm.


Silicon dioxide is commonly etched in a dilute
(6:1, 10:1, or 20:1 by volume) or buffered HF
(BHF, HF+NH4F) solutions (etch rate of


1
,000Å/min in
BHF).


Photoresist

and silicon nitride are the two
most common masking materials for the wet
oxide etch.

Isotropic etching


Isotropic etching of silicon using
HF/HNO
3
/CH
3
COOH.


Silicon anisotropic wet etch constitutes an
important technique in bulk micromachining.
The three most important silicon etchants in
this category are potassium
hydroxide (KOH),
ethylene diamine pyrochatechol
(EDP), and
tetramethyl

ammonium hydroxide (TMAH).

Dry Etching


Most dry etching techniques are plasma
-
based.
They have several advantages compared with wet
etching.


These include smaller undercut (allowing smaller
lines to be patterned) and higher
anisotropicity

(allowing high
-
aspect
-
ratio vertical structures).


The three basic dry etching techniques:


high
-
pressure plasma etching


Reactive ion etching (RIE)


ion milling

Other Materials and Processes


GaAs,
InP
, and
Ga
, In, Al, mixtures


CIGS semiconductor technology


Silicon photovoltaic modules


Silicon cantilever technology


Carbon nanotube, graphene electronics


Biological systems / junctions

MOCVD / Epitaxial Deposition

http://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy

Contamination Control

Particle Control

http://www.roomdesigne.net/2011/09/12/semiconductor
-
clean
-
room/

V
-
TEK’S CLASS 10,000 CLEAN ROOM

Summary


50 years of Semiconductor processing
technology. Moore’s Law still holds true!


Lithography is the workhorse technology


Thin film deposition (epitaxial layers)


Oxidation (oxide layer growth)


Doping (n and p type junctions)


Etching (dry and wet etch)


Extreme particle / contamination control


References


http://www.slideshare.net/mashiur/ete444lec
5microfabricationpdf
-
1800043


ETE444/544 Introduction to

Micro Fabrication


Springer Handbook of Nanotechnology


Chapter 5


III lithography

Vocabulary Review


Moore’s Law


Top down


Bottom up


Self assembly


Lithography


Masking


Etching


Oxidation


Doping


CVD (Chemical Vapor
Deposition) MO
-
CVD, PE
-
CVD


Epitaxial Growth


MBE (Molecular
Beam Epitaxy)