08. Micro- & Nano-Fabrication

group
Giuseppe Maruccio
giuseppe.maruccio@unisalento.it
Giuseppe Maruccio
Omnics Research Group
Dip. di Matematica e Fisica, Università del Salento,
CNR NANOTEC - Institute of Nanotechnology
Via per Arnesano, 73100, Lecce (Italy)
Micro- and Nano-fabrication

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Calculators and
Miniaturization
trends

VIII. Micro- and Nano-fabrication techniques
└ Miniaturization & Tech.Nodes └ Semiconductor growth └ Lithography: λ, e, i, X, h/i, Altern., Soft
4
1643 Pascalina: precorritrice della moderna calcolatrice,
inventata dal filosofo e matematico francese Blaise Pascal,
permette di addizionare e sottrarre tenendo conto del riporto
1853 Calcolatrice Meccanica Scheutz: calcolatore
automatico meccanico
1943. I think there is no a world market for may-be more than
five computers
1949. Computers in the future may weigh no more than 1.5 tons
1977. There is no reason for any individual to have a computer
(this company does not exist anymore!)
Calculator evolution

Kryder law
RAM
Miniaturization & Moore law
• Nel 1965 predisse la famosa legge omonima: numero di transistor per chip raddoppiato ogni 2 anni.
• Questa legge è stata rispettata negli ultimi 40 anni e si pensa possa valere sino al 2010
1950 2000
5
J.Mat.Chem. 14, 542 (2004)
Moore
law
1970 1980 1990 2000 2010 2020 2030
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0
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8
FEATURE
SIZE
(nm)
YEAR
THE ROAD MAP

6
Evolution of Computer Power/Cost

7

Process of Microfabrication / Lecture Content
Single crystal
growing
Wafer
slicing
Film
deposition
Oxidation
Diffusion Ion
implantation
Etching Lithography
Metallization Bonding Packaging Testing
8

Semiconductor
technologies &
growth

Silicon Electronics & Growth
 Silicon crystal structure is regular, well-understood, and to a large extent controllable.
 In a perfect crystal, each of silicon’s four outer electrons form covalent bonds, resulting in
poor electron mobility (i.e. insulating)
 Doping silicon with impurities alters electron mobility (i.e. semiconducting)
• Extra electron (“N-type”, with phosphorous, for example)
• Missing electron (“P-type”, with boron, for example)
└ Miniaturization & Tech.Nodes └ Semiconductor growth
10
• Slicing step includes
 Slice the ingot into slices using a diamond saw
 Polish the surface, and
 Sort
 Silicon occurs naturally in the forms of silicon dioxide and various silicates and hence,
must be purified by:
• Heating to produce 95% ~ 98% pure polycrystalline silicon
• Using Czochralski (CZ) process to grow single crystal silicon
1 rev/s
10 m/s

From Sand To Chip

Bulk Semiconductor

Microfabrication
Techniques

Clean room environment
14
A clean environment designed to reduce the contamination of processes and materials. This is
accomplished by removing or reducing contamination sources.
 Air is highly (HEPA) filtered (99.99% @ 0.3)
 Layout should minimize particle sources in filtered air stream
 Air flow should remove most particles generated by process
CONTAMINANTS
 Particulate: Dust, skin, hair, makeup…
 Chemical: Oil, grease, metal ions, perfume…
 Biological: Bacteria, fungi,…
 Radiation: Ultraviolet light…
• Personnel Control
• Dress code
• Personal Hygiene
• Gowning
• Environmental Control
• Entrance and exit
• Materials and supplies
• Cleaning and maintenance
• Atmospheric

Particles control
15
• 50 micron particles are visible
• Average human hair is about 100 microns
• Time to fall 1 meter in still air
• 33 seconds for 10 micron particle
• 48 minutes for 1 micron particle
• Humans generate >1x105 particles per minute when motionless (fully
gowned)
• Humans can generate >1x106 particles when walking in the cleanroom

Lithography Process
Prepare
Substrate
Apply
Photoresist
Softbake
Expose
Develop
Post
Exposure
Bake
lithography is generally followed by a process which transfers
the pattern from the resist to a substrate
via etching, growth of a material in the interstices of the
resist , or doping
Photo
Changing light source =
type of lithography
UV Light
X-ray
16

Types of Lithography
A. Photolithography (optical, UV, EUV)
B. E-beam/ion-beam/Neutral atomic beam
lithography
C. X-ray lithography
D. Interference lithography
E. Scanning Probe
F. Shadow mask, Nanotemplates &
Colloidal Lithography
G. Soft Lithography & Nanoimprint
H. Self Assembly
17

Miniaturization & Technology Nodes
18
Research Required Development Underway Qualification/Pre-Production
This legend indicates the time during which research, development, and qualification/pre-production should be taking place for the solution.
2007 2010
2001 2013 2016
First Year of IC Production 2004
Technology
Options
at
Technology
Nodes
(DRAM
Half
Pitch,
nm
248 nm + PSM
193 nm
193nm + PSM
PEL
157 nm
EUV, EPL
ML2
IPL, PEL, PXL
Narrow
Options
EUV
EPL
ML2
IPL, PEL, PXL
EPL, EPL
ML2
Innovative technology
IPL, PEL, XPL
Narrow
Options
Narrow
Options
130
90
65
45
32
22
DRAM Half Pitch
(Dense Lines)
Narrow
Options
EUV
EPL
ML2
IPL, PEL, PXL
EUV = extreme
ultraviolet
EPL = electron
projection
lithography
ML2 = maskless
lithography
IPL = ion projection
lithography
PXL = proximity x-ray
lithography
PEL = proximity
electron
lithography
Lithography
Costs

II-A. Optical
Lithography
and EUV

Photolithography
• KrF λ=248nm
• ArF λ=193nm
• F2 λ=157nm
• Smaller features need smaller wavelengths of light
– UV: 365nm - 436nm
– Deep UV (DUV): 157nm - 250nm
– Extreme UV (EUV): 11nm - 14nm
– X-ray: < 10nm
• Photolithography is a process by which an image is optically transferred
from one surface to another, most commonly by the projection of light through
a mask onto a photosensitive material.
• Photoresist is a material that changes molecular structure when exposed to
radiation (e.g. ultraviolet light). It typically consists of a polymer resin, a
radiation sensitizer, and a carrier solvent.
• A photomask is typically manifested as a glass plate with a thin metal layer,
that is selectively patterned to define opaque and transparent regions.
20

Prepare
Substrate
Apply
Photoresist
Softbake
Expose
Develop
Post
Exposure
Bake
2. Apply Positive Photoresist
3. Softbake
1. Prepare Substrate
4. Expose
5. Develop
21

Three types:
– Contact: The mask is directly against the substrate – good minimum feature size, bad for the mask and
substrate to touch
– Proximity: The mask is a few m away from the substrate – degrades minimum feature size but good
for reliability because mask doesn’t touch substrate
– Projection: Lenses are used to focus the mask’s image onto the substrate – good minimum feature
size, good for reliability
A positive photoresist is
weakened by radiation
exposure, so the remaining
pattern after being subject to a
developer solution looks just
like the opaque regions of the
mask
A negative photoresist is
strengthened by radiation
exposure, so the remaining
pattern after being subject to a
developer solution appears as
the inverse of the opaque
regions of the mask.
22
Photolithography

Photolithography – Step by Step
23

Intel –The Making of a Microchip
24

25
Circuit fabrication (PCB)

Lithography Techniques
dot.che.gatech.edu
Light sources: Hg arc lamp (l0=436, 365, 248 nm)
KrF laser (l0=248 nm), ArF laser (l0=193 nm),
F2 laser (l0=157 nm)
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
60
80
100
120
140
160
180
200
TREND OF NANOLITHOGRAPHIES UV 193 nm
UV 157 nm
extreme UV
X-rays
Electron beam
Ion beam
print
Resolution
(nm)
YEAR
Approaching The Limit Of
Top-Down Nanotechnologies
27

ASML - How water can make more powerful chips
28

ASML - Powering the Next Phase of Semiconductor Manufacturing
29

Mask Maker’s
Holiday:
“large” k1
Mask Maker’s
Burden: “small” k1
Why?
Minimum lithographic feature size =
k1: “Process complexity factor” – includes “tricks” like phase-shift masks
λ: Exposure wavelength
NA: Numerical aperture of the lens – maximum of 1 in air, a little higher in immersion lithography (Higher
NA means smaller depth of focus, though)
k1*λ
NA
ftp://download.intel.com/research/silicon/EUV_Press_Foils_080204.pdf
There are only so many “tricks” to increase this
gap, and they are very expensive … we MUST
go to a shorter wavelength!
Adapted from Matt Smith, Penn State University
30
Why EUV? It’s all about the money
By decreasing λ by a factor of 14, we
take pressure off k1 – this makes the
masks less complicated and
expensive because we can skip the
“tricks”
Next Generation Lithography: EUV

Next Generation Lithography: EUV
• Pretty soon UV lithography will hit the limit in terms of
minimum feature size
• EUV is the next step
• Uses very short 13.4 nm light
• Few materials allow EUV light to pass through, so
reflective (instead of transmissive) optics must be used
• Mask pattern must be really absorbent to EUV light, so
heavy metals are used
• Uses reduction optics (4 X)
• Step and scan printing
• Optical tricks seen before all apply: off axis illumination
(OAI), phase shift masks and OPC
• Vacuum operation
• Laser plasma source
• Very expensive system
Mask fabrication is the most difficult task.
31

32

II-B. e-beam
Lithography

Electron Beam Lithography
Iowa State Univ.
dot.che.gatech.edu
Optical Lithography E-beam Lithography
Limiti di Risoluzione
Good for making optical
lithographic masks for use
in UV, EUV, and X-ray
Advantages
• Better resolution
• Direct writing, no mask
needed
• Arbitrary size, shape, order
Disadvantages
• Serial processlow
throughput
• slow, small area
• Exposure source: electron beam
At acceleration voltage Vc=120kV, λ=0.0336Å
Diffraction is not a limitation on resolution
(l < 1 Å for 10-50 keV electrons)
Resolution depends on electron scattering
and beam optics. Beam size can reach ~ 5 nm
Operation: Direct writing with
narrow beam (turning it on and
off to write or not write a pixel)
34

E-Beam sources
• Thermionic emitters:
• Electrons “boiled” off the surface by giving them
thermal energy to overcome the barrier (work
function)
• Current given by Richardson-Dushman equation
• Field Emitters:
• Takes advantage of the quantum mechanical
properties of electrons. –Electrons tunnel out when
the surface barrier becomes very narrow
• Current given by Fowler-Nordheim equation
• Photo Emitters:
• Energy given to electrons by incident photons
• Only photo-electrons generated close to the surface
are able to escape
Electron Column
An electron column to generate
focused e-beam which directly write
to the substrate
35

36
Electron beam lithography

37
Immagini EBL

Two Layer Resist Processing
• The best results of e-beam have been
achieved for making a pattern of a
single layer of evaporated or sputtered
material.
• The two-layer process produces an
undercut structure which functions as a
shadow mask for the deposition
process.
• After deposition from a collimated
source, the resist is removed with a
solvent.
ref: Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
38

II-B2.
Ion beam
Lithography

Focused Ion Beam (FIB)
• Liquid ion source: Ga, Au-Si-Be alloys LMI sources due to the long
lifetime and high stability.
Advantages:
• High exposure sensitivity: 2 or more orders of magnitude higher
than that of electron beam lithography
• Negligible ion scattering in the resist
• Low back scattering from the substrate
• Can be used as physical sputtering etch and chemical assisted etch.
• Can also be used as direct deposition or chemical assisted
deposition, or doping .
Disadvantages:
• Lower throughput, extensive substrate damage.
40

Ion Lithography
 No diffraction and Ions scatter much less than
electrons so a higher resolution is feasible
 In addition, resists are more sensitive to ions than to
electrons. There is also the possibility of a resistless
wafer process.
 Problems:
• Ion Beam source (e.g. Gallium)
• Mask
• Beam forming
• Not as mature as EPL
Ion beam
Step-and-scan
wafer stage
Electrostatic
lens system
(4:1 reduction)
Vacuum chamber
Ion source
Mask
Reference
plate
An important application of ion lithography is the repair of
masks for optical or x-ray lithography, a task for which
commercial systems are available.
41

Comparisons
• IPL Mask
42

43

44

FIB lamella preparation
45

II-C. X-Ray
Lithography
• Became very important in MEMS: LIGA
• Despite huge efforts seems abandoned for NGL for now
Advantages: High resolution, Large area
Disadvantage: Synchrotron production of x-rays is the most favorable. Synchrotron
facility necessary and synchrotrons need to be further developed as a source for x-rays.

X-rays
Wilhelm Conrad
Roentgen (1845-1923)
x-rays are light (l = 10 nm to 0.01 nm)
 Roentgen discovered an unknown ray in 1895 (X-ray) experimenting with
evacuated tubes that he would fill with specific gases and then pass electricity
through.
 Roentgen found that despite shrouding the tube in black paper (as it let off a
glow similar to our incandescent bulbs) it somehow caused a barium
platinocyanide-coated screen to glow.
 He won the first Nobel prize in physics in 1901
X-ray interactions
 x-rays interact with matter through the excitation or ionization of
atomic electrons
 absorption of high energy x-rays causes the creation of
photoelectrons (core shell holes) but does not lead directly to
resist modification [Cerrina, J Phys D, 2000]
 relaxation is what leads to material modifications
• energy release of a higher lying electron via x-ray
fluorescence
• Auger effect – higher lying electron transfers energy to
another atomic electron, which is then ejected[eds. Suzuki, et
al., “Sub-Half-Micron Lithography for ULSIs”, 2000 ]
 photoelectrons and auger electrons are responsible for
modification of resists
47

X-ray Lithography
• After EUV comes X-ray lithography, it
enables super-high resolution pattern transfer
• X-ray lithography employs a shadow printing
method similar to optical proximity printing.
The x-ray wavelength (4 to 50 Å) is much
shorter than that of UV light (2000 to 4000
Å). Hence, diffraction effects are reduced and
higher resolution can be attained. For
instance, for an x-ray wavelength of 5 Å and a
gap of 40 µ, R is equal to 0.2 µ.
• Exposure source: X-rays are produced by
synchrotron radiation from a bending magnet
in a high energy electron storage ring.
• Resist: sensitive to x-ray. Resist, usually
PMMA, is exposed through an x-ray mask in
proximity or in contact with the wafer.
– IBM used resists developed for DUV and obtained
successful results
• Mask: mask production seems to be the
limiting step in the technology (no
demagnification optics possible); expensive to
produce; Ex. SiC membrane covered by high
Z metal; fabricated by e – beam writer
Grenoble Synchrotron
48

X-ray sources
• Types of x-ray sources:
• Electron Impact X-ray source
• Plasma heated X-ray source
• Laser heated
• E-beam heated
• Synchrotron X-ray source
Wilson et al., IBM J Res Develop, 1993
Synchrotron
Synchrotron radiation: a magnetic field to cause an
e-beam to follow a circular orbit at velocities near the
speed of light; the orbiting electron’s emitted radiation
will become sharply peaked in the forward direction;
capable of providing continuous source of soft and hard
x-rays
Wilson et al., IBM J Res Develop, 1993
49

II-D. Holographic
and Interference
Lithography

II-D. Interference Lithography
T.A.Savas et al., J.Appl.Phys.85
(1999) 6160
Patterned Nanostructures
51

II-D. Interference Lithography
52
l

Holographic Lithography
53

Alternative
Lithographies

Nanofabrication. Scale bars: 2 m
[Kawata et al., Nature, 2001, 412, 697-698 ]
Two-Photon 3D Lithography
Satoshi Katawa and colleagues at Osaka University used two
laser beams to sculpt the micro-beast from resin, which
solidifies only where the lasers cross. The team refined this
'two-photon micropolymerization' to a resolution of 120
nanometres (120 billionths of a meter).
55

56
Two-photon polymerization (TPP) is a serial
process for the fabrication of three-
dimensional microstructures. Because of its
ability to produce geometries with no
topological constraints with a resolution
smaller than 100 nm
The excited state of a molecule can be
reached by the absorption of one photon of
an appropriate wavelength or by the
simultaneous absorption of two photons,
each having half the energy required for
the transition. In this case, two-photon
absorption is said to be degenerate. When
the photons promoting the transition are of
different energies (wavelengths), the
process is said to be nondegenerate.
Two-photon cross-sections of most molecules
are very small; usually on the order of 1 GM or
less, where 1 GM corresponds to 10-50 cm4 sec
photon-1 (GM stands for GöppertMayer in
honor to the scientist who theoretically
predicted multiphoton absorption). As a result,
high local photon fluxes are needed to promote
two-photon absorption. Commercially available
Ti:sapphire lasers emitting at wavelengths in
the near-infrared region of the spectrum and
producing pulses shorter than 100 fs can meet
this requirement. Each pulse has a peak power
on the order of a kilowatt, while at a typical
repetition rate of 80 MHz the average power is
on the order of milliwatts. When focused into
tight spots, these lasers allow reaching the
photon densities needed for two-photon
absorption to occur.

57
Since the probability of a two-photon absorption event is proportional
to the second power of the light intensity, excitation can be spatially
localized.
- total one-photon (dashed line) and
- two-photon (continuous line) absorption per transverse section of a
focused laser beam.
While the absorption probability is the same along the optical axis for a
one-photon process, a maximum coinciding with the focal point is
present for the two-photon process.

58

II-E.
Scanning Probe
Lithography
• Probe: STM, AFM
• Techniques: Voltage pulse,
CVD, Local
electrodeposition, Dip-pen

Scanning Probe Lithography
60

Manipulation of Atoms
1. Parallel process
2. Perpendicular process
61

STM CVD
62

Nanolithography
• Local anodic oxidation, passivation, localized chemical vapor deposition,
electrodeposition, mechanical contact of the tip with the surface, deformation of the
surface by electrical pulses
63
Nanodeposition
Local Electrodeposition

Dip Pen Lithography
In dip-pen lithography (DPL) a reservoir of “ink” is stored
on the cantilever holding the scanning probe tip, which is
manipulated across the surface, leaving lines and patterns
behind. Lines as thin as 15 nanometers have been drawn.
The attainable resolution depends strongly on the substrate
roughness, the writing speed and the relative humidity.
64

Dip Pen Lithography
65

Thermal Dip Pen Lithography
• A thermal-DPN (tDPN) method was
developed by Georgia Tech’s William
King and NRL’s Lloyd Whitman
• By using easily-melted solid inks and
special AFM probes with built-in
heaters writing can be turned on and off
at will.
A topographic image of a surface scanned with a heated AFM cantilever tip for 256 seconds i n
each of four 500 nan ometer squares. The cantileve r temperature is show n for each of the four
scans. No deposited materialis observed from the tw o low-temperature scans. The scan at 98 ¡C
resulted in light deposition. Robust depositi on occurred during the final scan when the cantileve r
temperature was 122 ¡C. Image courtesy of Naval Research Laboratory
http://gtresearchnews.gatech.edu/newsrelease/tdpn.htm
66

Multi-Pen for Scalability
A dip-pen nanolithography
that has an array of 55,000
pens that can create 55,000
identical molecular patterns
67

Colloidal
Lithography
(Lecce results)

group
Giuseppe Maruccio
Colloidal lithography
Pick up
Metallization
Cu/Au
Tape stripping with
carbon tape
Nanoholes fabrication
Nanodisks fabrication
Metallization before
colloidal assembly
Reactive ion
etching

Soft-Lithography
and Nanoimprinting
 Molding of a polymer using a photoresist
master mold
 Fast, cheap prototyping method
 Down to 10 nm feature resolution.
 Require little capital investment
 Ambient laboratory conditions
 Able to generate features on curved
substrates

What is Soft Lithography
71

Soft Lithography – Summary
1. Rapid prototyping: replication of master
2. Microcontact printing of molecules
3. Microfluidics
Pre-industrialization
72
Rapid Prototyping
• A system of channels is designed in a CAD
program.
• A commercial printer uses the CAD
• file to produce a high-resolution transparency
• This transparency is used as a photomask in
contact photolithography to produce a master.
• A master consists of a positive relief of
• photoresist on a silicon wafer and serves as a
mold for PDMS.
• Liquid PDMS pre-polymer is poured over the
master and cured for
• 1 h at 60 °C. (C) The PDMS replica is peeled
from the master
• (D) the replica is sealed to a flat surface to
enclose the channels.

Soft Lithography – Summary
73
Advantages:
• Convenient and low cost
• Rapid prototyping
• Deformation of PDMS provides route
to complex patterns
• No optical diffraction limit
• Non-planar or curved surfaces
• Generation of 3D -structures
• Control over surface chemistry
• A broad range of materials
• Applicable to manufacturing
• Patterning over large areas
Disadvantages:
• Distortion of patterns
• Poor registration/alignment
• Compatibility with IC processes
• Defects and their densities
• μCP can only be applied to a number
of surfaces
• MIMIC is a relatively slow process
Soft lithography: advantages
and disadvantages

Replica molding
74

A) Replica molding (REM)  Resolution and roughness of the master is
reproduced
 Advantage: easily bonded to most of
surfaces (Si, SiO2, other plastic) facilitates
fabrication of multilayered structures
Master (Au) Replica (PU)
Resolution 30 nm
Channels of a miniaturized CE device created by
molding PDMS against a lithographic master
1. Master mold is replicated in PDMS by casting and curing PDMS
pre-polymer.
2. PDMS mold is oxidized in oxygen plasma for 1 minute and
exposed to fluorinated silane for 2 hours to make a surface with
low adhesion to PDMS.
3. PDMS is cast against this negative replica, cured and peeled off.
4. This will make the positive replica of the original master.
Replica Molding (REM)
75
Cure on hotplate for few hours
Peel off PDMS

Micromolding

Mechanical compression
for feature size reduction
Creating curved surface
Molding on curved surface
Y. Xia et al., Science 1996
Using mold elasticity
77

The patterning process has high fidelity, with little feature size loss.
78

79

Micro-contact
printing
80

Micro - contact printing (μCP)
81

II) large-area
printing on a
planar surface with
a rolling stamp
I) Printing with a planar stamp. (A) Fabrication of
the stamp; (B) Transfer of the “ink” to substrate
• An “ink” is spread on a patterned PDMS stamp
• The stamp is then brought into contact with the substrate
• The “ink” is then transferred to the substrate where it
can act as a resist against etching
• “Ink” can be a SAM or a biological sample
B) Microcontact printing (µCP) Resolution 300 nm
82

100mm stamp
20m
 It uses a PDMS stamp to form patterns
of self-assembled monolayers (SAMs)
on the surfaces of substrates.
 PDMS stamp is coated with an ink of
the molecules and pressed onto the
solid surface
 Inking creates a Self Assembled
Monolayer on the solid surface
 the use of elastomers allows the
micropatterned surface to come into
conformal contact with the surfaces
over large areas
83

Xia & Whitesides, Angew. Chem. Int. Ed. 1998, 37, 550-575.
A. Kumar & G. Whitesides, Applied Physics Lett. 1993
a. Printing on a planar surface
with a planar stamp.
b. Printing on a planar surface
with a rolling stamp
c. Printing on a non-planar
surface with a planar stamp
Micro - contact printing (μCP), with roller
84

Applying alkanethiols on stamp to form SAM
• These monolayers allow control over wettability, adhesion, chemical reactivity, electrical
conduction, and mass transport to underlying metal
in ethanol
Xia, Y.; Whitesides, G. M. Angew.
Chem., Int. Ed. 1998, 37, 550.
Michel, B.; Bernard, A., et al. IBM
J. Res. & Dev. 2001, 45, 697.
Substrate Molecules
Popular “ink” molecules
85

Patterning of organic single crystals
Nature 444, 913-917 (14 December 2006)
http://www.nature.com/nature/journal/v444/n7121/pdf/nature05427.pdf
a. Procedure used to grow organic
single crystals on substrates that have
been patterned by microcontact
printing. To grow the patterned single
crystals, the patterned substrate is
placed in a glass tube with the organic
source material, vacuum-sealed (0.38
mmHg), and placed in a temperature
gradient furnace tube.
b–d, Patterned single-crystal arrays of
different organic semiconductor
materials. The dotted square in each
image indicates the size and location
of one of the OTS-stamped domains,
while the molecular structure of the
organic material used is shown next to
the image of its single-crystal array.
(optical micrograph)
OTS: octadecyltrichlorosilane
The above three materials are all
hydrophobic and they
binds/sticks to OTS
86

Conformal micro-contact printing on rough surfaces
Dependence of maximal roughness amplitude for
spontaneous formation of conformal contact on the
roughness wavelength (l) for a stamp with Young's
modulus of 2.5 MPa and work of adhesion of 0.1J/m²
(Sylgard 184, solid line) and for a stamp with modulus of 9
MPa and work of adhesion of 0.03J/m² (dotted line).
Larger l allows for higher roughness.
Conformal contact between a soft stamp and a
hard substrate.
87

Micro-contact printing on curved substrates
Whitesides, “Fabrication of submicrometer features on curved
substrates by microcontact printing”, Science, 269, 664 (1995);
Rogers and Whitesides, “Microcontact Printing and
Electroplating on Curved Substrates: Production of Free-
Standing Three-Dimensional Metallic Microstructures”, Adv.
Mater. 9, 475 (1997).
PDMS is soft, it can
roll onto curves
88

Pattern transfer into Si(100) by anisotropic etching
KOH anisotropic etching of Si
is the most popular pattern
transfer technique when the
etching mask is too thin for
liftoff or direct etch.
But it is limited to Si along
certain crystalline directions.
Au
Si
89

Micro-contact printing of proteins
BSA: bovine serum albumin, bovine albumin
(anti-Goat IgG – Alexa 488 and 594)
Fluorescent image
90

Micro-contact printing of DNA
S.A. Lange, V. Benes, D.P. Kern, J.K.H. Horber, A. Bernard, Anal. Chem. 2004, 76, 1641.
A. Scheme of DNA printing. The surface of PDMS was modified such that it exposed positive charges on its surface.
The stamp was incubated with target DNA molecules in a solution of low pH. The stamp was then rinsed, blown dry,
and printed to deliver the DNA to the target surface.
B. Fluorescence images of patterned FITC-labeled DNA on a glass surface after printing.
C. AFM images revealing the printed DNA molecules deposited as patterns on mica substrates. AFM images (tapping
mode in air) of stamped 1-μm lines of oligonucleotides (left, 20-bp oligos; right, 500-bp PCR fragments).
91

Micro-molding in
capillary (MIMIC)
92

• Continuous channels formed upon
contact of PDMS stamp with substrate
• A polymer precursor fills channels with
capillary action
• Polymer is cured and stamp is removed
Resolution 1 μm
C) Micromolding in capillaries (MIMIC)
Micro-molding in capillary (MIMIC)
Liquid
pre-
polymer
Nano-molding in
capillaries is possible
Line-width: 100nm
93

Uses capillary forces to fill the gaps between substrate
and PDMS master
1. Push the PDMS stamp against the substrate.
2. Elastic PDMS seals off walls and creates capillary
channels.
3. prepolymer is placed at the ends of these channels
and fills them automatically due to capillary force.
4. Prepolymer fills the channels using capillary forces.
PDMS can absorb the solvent, which creates a partial
vacuum inside the PDMS cavity and helps to draw in
liquid polymer.
5. Cure the polymer
6. Peel off the PDMS mold
Micro-molding in capillary (MIMIC)
Kim & Whitesides et al, Nature, 1995, 376, 581
Xia, Y.; Whitesides, G. M. Ann. Rev. Mater. Sci 1998, 28, 153.
a: PU (polyurethane) on Si b: polyaniline
c: ZrO2 d: polystryene colloids
e+f: free standing PU
94

Micro - transfer
molding (μTM)
95

• PDMS stamp is filled with a
prepolymer and placed on substrate
• Polymer cured and stamp removed
•Able to generate multilayer structures
Resolution 250 nm
D) Microtransfer molding (µTM)
Micro - transfer molding (μTM)
TM fabrication of a). one-layer microstructures;
b). three-layer polymer microstructures.
PDMS
Pre-polymer
Substrate
96

Zhao & Whitesides et al, Advanced Materials, 1996, 8, 837.
Microstructures fabricated using TM.
a) An SEM image of a fractured sample showing a
pattern of isolated stars of UV-cured polyurethane
(NOA 73) on Ag.
b) An array of parallel lines of spin-on glass on Si with
an aspect ratio (height/width) of 8.
c) A two-layer structure: isolated micro-cylinders (1.5m
in diameter) on 5m-wide lines, supported on a glass
cover slide.
d) A two-layer structure: a continuous web over a layer of
5m-wide lines, supported on a glass cover slide.
e) A three-layer structure on a glass cover slide. The
layers of 4 m-wide lines are oriented at 60o from
each other.
Structures in c-e were made of heat-cured epoxy
(F109CLR).
Micro - transfer molding (μTM)
97

Solvent assisted
microcontact
molding
98

replica bagnata con
solvente e posta a contatto
con substrato
Solvent-assisted micromolding (SAMIM)
D) solvent-assisted micromolding (SAMIM)
Comparison of micro-molding technologies
99

Solvent assisted microcontact molding (SAMIM)
(solvent assisted imprinting)
Kim & Whitesides et al, Advanced Materials, 1997, 9,651.
Substrates
Silicon
Glass
Flexible transparency
Polymer
SU-8 (1μm)
Shipley 1805 Photoresist (500nm)
3% PMMA (70nm)
Solvent
SU-8 (ethanol)
Shipley 1805 photoresist (ethanol)
3% PMMA (acetone)
Uses a solvent to wet the PDMS stamp and soften the structure polymer.
Dissipate and evaporate the solvent through PDMS.
(PDMS stamp can absorb the solvent because of the solvent permeability of PDMS.)
The molded polymer structure becomes solidified in a
few minutes after evaporation of solvent, while the
stamp is still in conformable contact with the substrate.
100

SAMIM of SU-8 in ethanol
SU-8
2m diameter
101

SAMIM of PMMA in acetone
Au structure fabrication
102

Hole array in PMMA Au dot array after liftoff
SAMIM of PMMA in acetone
Dot diameter 160nm.
103

Imprinting
104

Low cost mass production technique
 Stamps can be Si or metal
 Heating of the plastic at its softening temperature at lower pressures or at room-
temperature at elevated pressures
 Materials: PS, PMMA, PVC, …
Hot embossing/imprinting
Si stamp Imprinted channel in
PMMA
Metal mechanical
micromachined tool
Microplate
with 96 CE
devices
Resolution 25 nm
105

NanoImprint

Wet and Reactive
Ion etching & ICP

└ Miniaturization & Tech.Nodes └ Semiconductor growth └ Lithography └ Etching └ Implantation └ Deposition
Etching – Mask for a Mask
• The mask must withstand the
chemical environment.
• A typical mask/substrate
combination is oxide on silicon.
• Resilient masks are typically
grown or deposited in whole
films, and must therefore be
patterned through a
photosensitive mask
Reusable mask
Photoresist coating
Functional mask
108

Dry Etching Mechanisms
109
Dry etching technology uses plasma to generate chemically reactive
species, starting from relatively inert molecular gases. Such species
reacting with solid materials form volatile compounds, which are
subsequently removed from pumping systems which are essential
components of the machines used. In a dry etching machine, essentially
two types of chemical reactions occur:
- CHEMICAL-PHYSICAL REACTIONS IN THE GASEOUS
PHASE: the collisions between the electrons and the molecules during
plasma generation (excitement, ionization, dissociation, recombination)
bring to the formation of reactive ions and molecules
- SURFACE CHEMICAL REACTIONS: reactive ions and
molecules act in the erosion of the material film on which the
geometries of electronic devices are to be created.

Dry Etching Mechanisms
• PHYSICAL: High-speed positive ions impact on the
surface and transfer mechanical energy by removing
substrate material.
• Removal based on impact & momentum transfer
• Poor material selectivity
• Good directional control
• High excitation energy
• Lower pressure, <100 mTorr
• CHEMICAL: the highly reactive (radical) neutral
species produced in the plasma interact with the surface
of the material and combine through reactions to the
substrate forming volatile products.
• Highest removal rate
• Good material selectivity
• Generally isotropic
• Higher pressure, >100 mTorr
• PHYSICAL/CHEMICAL
• Good directional control
• Intermediate pressure, ~100 mTorr
110

Etching – a Comparison
ISOTROPIC
• Wide variety of materials
• No crystal alignment
required
• May be very fast
• Sometimes less demand for
mask resilience
ANISOTROPIC
• Predictable profile
• Better depth control
• No mask undercutting
• Possibility of close feature
arrangement
Multiple layers are common
111

Isotropic Wet Etching
Isotropic Wet Etching
• Etch occurs in all crystallographic directions at the same rate.
• Most common formulation is mixture of hydrofluoric, nitric and acetic acids
(“HNA”: HF + HNO3 + CH3COOH).
• Etch rate may be very fast, many microns per minute.
• Masks are undercut.
• High aspect ratio difficult because of diffusion limits.
• Stirring enhances isotropy.
• Isotropic wet etching is applicable to many materials besides silicon
112

Anisotropic Wet Etching
• Etch occurs at different rates depending on exposed crystal
• Usually in alkaline solutions (KOH, TMAH).
• Heating typically required for rate control (e.g. > 80 oC).
• Etch rate typically ~1 µm/min, limited by reactions rather than diffusion.
• Maintains mask boundaries without undercut.
• Angles determined by crystal structure (e.g. 54.7º).
• Possible to get perfect orthogonal shapes outlines using 1-0-0 wafers.
113

Etching – a Comparison
114

DRIE System
Plasmalab System 100 Modular ICP-RIE Etching System
Oxford Instruments
• Reactive ion etching (RIE) is a dry
etching method which combines plasma
etching and ion beam etching principles.
– Choice for most advanced product
line
– It’s not well suitable for deep
etching (>10μm)
• Inductively coupled plasma (ICP)
reactors have been introduced for silicon
RIE process leading to the deep reactive
ion etching (DRIE) technique.
– Higher plasma density
– Higher etching rate, either for
anisotropic and isotropic etching
– Higher aspect ratio (AR)
– Reduction of parasitic effects
Use ICP etcher to create deep etching in silicon
substrate
115

Deposition
techniques
• PVD
• Used to deposit metals
• High purity
• Line of sight
• CVD
• Reactive gases interact
with substrate
• Used to deposit Si and
dielectrics
• Good film quality
• Good step coverage

PVD
• Physical methods produce the atoms that deposit on the substrate
• Process usually done in an evacuated chamber
Advantages:
 Versatile – deposits almost any material
 Very few chemical reactions
 Little wafer damage
Limitations:
 Line-of-sight
 Shadowing
 Thickness uniformity
 Difficult to evaporate materials with low
vapor pressures

Evaporation
• Material to be deposited is heated until it becomes vapor phase
• The heated material flies into the substrate
• The hotter the substrate, the better the film quality
• Can deposit very fast relative to other methods, but not always good
quality film (up to 200 nm/s film growth)
• Rely on thermal energy supplied to the crucible or boat to
evaporate atoms
• Evaporated atoms travel through the evacuated space between
the source and the sample and stick to the sample
• Few, if any, chemical reactions occur due to low pressure
• Can force a reaction by flowing a gas near the crucible
• Surface reactions usually occur very rapidly and there is very
little rearrangement of the surface atoms after sticking
• Thickness uniformity and shadowing by surface topography, and
step coverage are issues
118

e-beam Evaporation
119

Sputtering
 The target (material to be deposited) and substrate are placed facing each other
 A plasma is ignited between them under vacuum
 A voltage bias between them causes ions from the plasma to ram into the target
 The ions eject pieces of the target that “sputter” onto the substrate
Magnetron Sputtering
 A big magnet is used to force the electrons into spiral paths so that they spend more time ionizing
neutral gas particles
 This increases the number of ions
 More ions increases the chances of knocking out some of the material to be sputtered
 Increases efficiency
120

Sputtering
121

Pulsed Laser Deposition
• Similar to Evaporation method, except uses a laser to heat the material to be deposited
• Different because the intense energy creates a plasma
• Plasmas not only contain inert material, but also ions and radicals which could chemically
react with the surface
• Depends on chemistry of reactants
122

Pulsed Laser Deposition
123

CVD
Gases react with substrate
Various types of CVD:
 Atmospheric pressure – APCVD
 Low pressure – LPCVD
 Plasma enhanced – PECVD
 High density plasma - HDPCVD

Chemical Vapor Deposition
• In chemical deposition, the material being deposited on the
substrate reacts / form bonds with the surface
• The substrate as well as reactant temperature play a role in
the rate of reaction
• Precursor gas is pumped into the reaction chamber
• It’s heated until reactive species form
• Ex) SiH4  SiH2 + H2
• The reactive species chemically interact with the surface to
stick to (or react with) it
• Surface properties and temperature can determine how well
something sticks
125

Steps in CVD
1. Transport reactants via forced convection to reaction region
2. Transport reactants via diffusion to wafer surface
3. Adsorb reactants on surface
4. Surface processes: chemical decomposition, surface migration, site
incorporation, etc.
5. Desorption from surface
6. Transport byproducts through boundary layer
7. Transport byproducts away from deposition region

Plasma Enhanced CVD
• Uses an RF or microwave E-field to strip electrons off the precursor gasses
• Since e- are so much lighter than the rest of the molecule (ion), they accelerate in the E-field
faster than the molecules
• By the time the E-field changes direction (at
RF or microwave frequencies) the electron
has gained a lot of momentum and the
remaining molecule (ion) has barely started
to move
• Thus, the e- have a high temperature and the
molecules (ions) have a low temperature
• This means that the substrate can have a
lower temperature, too
• Enables new substrates like glass and
plastic
• This is how TFT-LCD displays can be made
Electrode
Substrate
RF
Source
Plasma
Gas
Optical CVD
• Not always applicable
• Uses different wavelengths of light to break
precursor gas bond to form reactive species
• Ex) Cl2 + h (photon)  2Cl (radicals)
• Also enables low temperature deposition
127

Atomic Layer Deposition (ALD)
128

Atomic Layer Deposition (ALD)
129

Self-Assembly

Alternative strategies
Top down
• Optical lithography  Semiconductors
• Electron beam lithography  Semiconductors
• X-ray lithography  Semiconductors
• Soft Lithographies  Organics, semiconductors
Bottom-up
• Self organized epitaxy  Semiconductor nanostructures
• Self-assembling  Supramolecular structures (organics)
• Biomolecular self organization  Biophysics, physiology
131

Self-Assembly
8 Ǻ
Courtesy of Prof. A.Rowan, IMM, Nijmegen (The Netherland)
18 Ǻ
32 Ǻ
Inner Diameter 8 Ǻ
45
Ǻ
85 Ǻ

• Definition: spontaneous organization of molecules (objects) into stable, well-
defined structures by non-covalent forces.
• Driving force: thermodynamic equilibrium.
• Final structure: determined by the subunits.
• Biological 3D self assembly: folding of proteins, formation of DNA helix…
Self assembled monolayer (SAM)
Chemi-sorption and self-
organization of long-chain organic
molecules on flat substrates.
Alkanethiolates CH3(CH2)nS-
Au(111)
-SH also binds to Ag, but Ag surface
not as stable as Au.
Laibinis,Whitesides, et al. JACS 1991, 113, 152
Self - assembling, classical –SH and Au bonding
133

DNA origami

Chemical Solution
Deposition
• Material is deposited on the substrate in the liquid state
• Spin Coating: Some liquid is placed on the substrate and it’s spun really fast until only
a thin coating is left
• Dip Coating: Dunk the substrate in solution
• Spray Coating: Like spray painting the substrate
• Screen Printing: Put a stencil on the substrate and use a squeegee to pull solution across
• Ink-jet Printing: Same as in an ink-jet printer for a PC

Langmuir-Blodgett (LB) Films
• A form of dip coating
• You have a solution with a layer of special molecules on the surface
• One side of the molecule is water-soluble, and the other is not (like soap)
• Thus all the molecules are aligned on top of the solution
• When you dunk the substrate in, you
get a monolayer (one layer) of aligned
molecules on the substrate
• If you keep dunking it you’ll get a new
layer each time
• The water soluble side of one layer
aligns with the water soluble side of
the next (alternating alignment)
136

137

138

Bonding &
Packaging

Bonding and Packaging
• Wires (=25 m) are bonded to package leads: The bond wires are attached using
thermocompression, ultrasonic, or thermosonic techniques
• Packaging is done by
surface mount technology
└ Miniaturization & Tech.Nodes └ Semiconductor growth └ Lithography └ Etching └ Implantation └ Packaging
140

Copper wire Ball bonding
141

Gold Ball Bonding
142

Wire Bonding
143

Pick and Place
144

08. Micro- & Nano-Fabrication

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