Outline
•
•
•
•
•
•
Microelectronics Processing
Lithography
Introduction
Clean Rooms
Photoresists
Pattern Transfer
Masks
E-Beam Lithography
Historical Development and Basic Concepts
Photolithography
Electron
Gun
Light
Source
Condenser
Lens
Mask
Focus
Photolithography is the sequence of activities needed for transfer
a pre-designed pattern to the surface of a semiconductor wafer.
Deflection
• Patterning process consists
of mask design, mask
fabrication and wafer
printing.
Reduction
Lens
Mask
CAD System
• Layout
• Simulation
• Design Rule Checking
Uses photosensitive polymer (called “photoresist”), which is a
resistant
i t t coating
ti usedd to
t register
i t an image
i
on the
th desired
d i d surface
f
Features transferred to substrate surface by shining light through
glass plates (called “masks”).
Wafer
Mask Making
Wafer Exposure
• It is convenient to divide the
wafer printing process into three
parts A: Light source, B. Wafer
exposure system, C. Resist.
Aerial
Image
(Surface)
The pattern can be registered on a mask, or supplied directly
from a computer to a scanning radiation source.
P+
P+
N+
N Well
Latent
Image
in Photoresist
Process
N+
P Well
TiN Local
Interconnect Level
(See Chapter 2)
P
• Aerial image is the pattern of
optical radiation striking the top
of the resist.
• Latent image is the 3D replica
produced by chemical processes
in the resist.
Process Sequence
(a) Substrate covered
with silicon dioxide
barrier layer
(b) Positive photoresist
applied to wafer
surface
(c) Mask in close
proximity to surface
(d) Substrate following
resist exposure and
development
(e) Substrate after
etching of oxide layer
(f) Oxide barrier on
surface after resist
removal
(g) View of substrate
with silicon dioxide
Resolution determined by the combination of
pattern on the surface
1. system 2. resist, 3. processing !
1) Clean wafer surface
– bake (get rid of H2O)
– RCA clean
– apply adhesion promoter (HMDS = “hexi-methyl-di-silizane”)
2) Deposit photoresist (usually by spin-coating)
3) Soft bake (or “pre-bake”) - removes solvents from liquid
photoresist
4) Exposure (pattern transfer)
5) Development - remove soluble photoresist
6) Post bake (or “hard bake”) - desensitizes remaining
photoresist to light
7) Resist removal (“stripping”)
Page 1
046880 A. Kolodny
1
Yield
Yield for a 10-mask lithographic process
The Need for Cleanroom
„ Electronics fabrication requires a clean
processing environment for lithography.
„ Goal: minimize dust particles that can
settle on substrates or masks and cause
DEFECTS.
„ Dust on a mask looks like an opaque
feature; will get transferred to underlying
layers; can lead to short circuits or open
circuits.
Graphic Illustration
How Big of a Particle is Tolerable?
„ Particle 1 may result
in formation of a
pinhole in underlying
layer.
– Example: 0.5 μm CMOS technology
• Lateral Features:
– pattern size = 0.5 μm
– pattern tolerance = 0.15 μm
– level-level registration = 0.15 μm
• Vertical Features:
– gate oxide thickness = 10 nm
– field oxide thickness = 20 nm
– film thicknesses = 250-500 nm
– junction depths = 50-150 nm
„ Particle
P ti l 2 may cause
a constriction of
current flow in a
metal runner.
„ Particle 3 can lead to
a short between the
two conducting
regions.
Page 2
046880 A. Kolodny
2
Real Defects
Class
„ Numerical designation taken from maximum
allowable number of particles 0.5 μm and larger
per ft3 (English system).
„ For IC fabrication, a class 100 clean room is
required (about four orders of magnitude lower
than ordinary room air).
Human hair on a
4MB memory-chip
„ For photolithography, class 10 or better is
required.
Masking error due to
a metallicparticle
Particle Size Distribution Curve
Sample Problem
A 300 x 300 mm square substrate is exposed for 1 minute under
laminar flow at 30 m/min. How many dust particles will land on
this substrate in a Class 1000 clean room?
SOLUTION:
1) Class 1000 => 35,000 particles/m3 (from graph)
2) Air flow volume over wafer/min = 30 m/min (0.3m x
0.3m) = 2.7 m3
3) # of particles = 35,000 x 2.7 = 94,500!!!
If each of these causes a defect, we are in serious trouble!
Sulfuric Acid has the highest number of particles and HF the lowest.
• Adhesion of Particles:
1. Van der Waals Forces.
2. Forces due to the formation of an electrical double layer.
3. Forces due to capillary action around particle.
4. Chemical bond between the particle and the surface.
• Particle removal mechanisms
1. Dissolution.
2 Oxidizing degradation and dissolution.
2.
dissolution
3. Lift-off by slight etching of the wafer surface.
4. Electric repulsion between particles.
• H2O2 can oxidize the silicon surface and OH- group (from NH4OH) provide
negative charge on silicon.
• The deposition of particles is a strong function of pH values of the solution.
With increasing pH value above 10 results in low particle deposition (SC-1
have the highest removal efficiency).
CONTAMINATION NATURE
PARTICULATES CHUNKS OF GRANULAR
MATTER
DUST from abrasion grinding
and handling.
• INORGANIC “GRIT”-abrasive
particles, sand, clay (from airborne or chemicals).
• LINT from clothing, skin, hair organic in nature, bacteria and etc.
FILMS ATOMIC, IONIC OR
POLYMERIC
ORGANIC
Resist residues
left by evaporation of solvents
Oil from water
and handling.
INORGANIC
Metal layers
Ions from resist
and reagents
Residues from
reagents, and
handling
Page 3
046880 A. Kolodny
3
IBM 300 mm FAB
IBM 300 mm FAB wet clean
IBM 300 mm FAB mask clean
Outline
• Introduction
• Clean Rooms
)Photoresist
• Pattern Transfer
• Masks
• E-Beam Lithography
Photoresist
Photoresist (2)
• Photosensitive polymer compound that either gets more
or less soluble when exposed to light.
• Spun onto wafers and prebaked to produce a film of 0.5 to
a few microns thick.
• Photolithography labs have yellow light because
pphotoresist is sensitive to wavelengths
g < 500 nm.
Page 4
046880 A. Kolodny
4
Positive Optical Resist
Requirements for Photoresists
• Exposure to radiation leads to breakdown of PAC
• Dissolution rate in developer (hydroxide) changes
•
•
•
•
Adhesion to substrate
Radiation induced solubility change
Etch resistance
Developabilityin solvent (in aqueous base or
other solvent)
• Pinhole-free thin films
• Transparency
• Easy to Remove
Without sensitizer 150 Å/s
With sensitizer
10-20 Å/sec
After exposure
1000-2000 Å/s
Key idea is the differential solubility of about 100:1
Negative Optical Resist
• Negative optical resist becomes insoluble in regions
exposed to light
– Photochemical reaction generates cross-linking to form 3D
molecular network
– New structure insoluble in developer (usually an organic
solvent)
Flow chart of a typical resist process
Types
1. Positive: gets more soluble after exposure
Substrate
cleaning
HMDS
Spin
coat
Develop
Pre-bake
900C
2. Negative: gets less soluble after exposure.
Post
exposure
treatment
Expose
1400C
Plasma
de-scum
Post
bake
Etch
Strip
*Steps
in dashed (pink)
lines are not always used
Page 5
046880 A. Kolodny
5
Image Reversal
Basic Properties of Resists
Basic Properties of Resists
Contrast:
Di= threshold exposure energy dose for resist removal or gel dose
Df= exposure energy dose for complete resist removal or complete insolubilization
Typical valuesγp= 2-3 (Df= 100 mJ/cm2) & γn= 1.5, γ(DUV)= 5-10
(Df= 20-40 mJ/cm2)
Resists with higher contrast result in better resolution because of
morevertical resist profile
Non-ideal Exposure
Critical MTF (CMTF)
CMTF is the minimal MTF value of the optical
system that results in a fully resolved pattern
in the photoresist
Page 6
046880 A. Kolodny
6
Other issues in Photoresist Exposure
Other issues in Photoresist Exposure
Other issues in Photoresist
Exposure
Post Exposure Bake
Constructive
interference
Destructive
interference
Development
Page 7
046880 A. Kolodny
7
Pattern Transfer
Photo Printing Process:
• Light sources
• Exposure
p
techniques
q
• Mask engineering
Energy Sources: Waves or Particles
Metrics of Lithography Systems
• Resolution (smallest dimension)
Wavelength Energy
– Determined by optical system, resist, etch process
Light
• Registration (alignment 3s=1/3 resolution)
– Determined by optical system
• Dimensional
e so
Control
Co o (dev
(device,
ce, die,
d e, wafer,
w e , lot
o uniformity)
u o
y)
– Determined by optical system, mask, resist, etch process
• Throughput (how many wafers/hour)
– Determined by optical system, resist
Particles
UV
400 nm
3.1 eV
Deep UV
250 nm
4.96 eV
EUV
13 50 nm
13-50
25 eV
V
X-Ray
0.5 nm
2480 eV
Electrons
0.62 Å
20 keV
Ions
0.12 Å
100 keV
• Depth of focus (how flat of surface)
– Determined by optical system
• Field of view (how large an area to print)
• Energy sources are
required to modify the
photoresist.
• The energy source is aerial
imaged on the photoresist.
• The imaging can be done
by scanning the energy
beam or by masking the
energy beam.
• Bright sources are usually
required for high
throughput.
• Decreasing feature sizes require
the use of shorter wavelength.
– Determined by optical system
Mercury Arc Lamp
Excimer Lasers
• Traditionally Hg vapor lamps have been used which generate many spectral lines
from a high intensity plasma inside a glass lamp.
• Electrons are excited to higher energy levels by collisions in the plasma.
Photons are emitted when the energy is released.
Brightest sources in deep UV
i-line (365 nm)
for 0.5, 0.35 um
g-line (435 nm)
Excimer laser etched hair
Mercury xenon arc lamps
h-line (405 nm)
Kr + NF3 ⎯energy
⎯⎯→ KrF → photon emission
• KrF • ArF • FF -
Damaged Hg vapor lamp is very dangerous
due to excessive UV irradiation.
(~100 times greater than daily allowable limit)
(used for 0.25 µm, 0.18µm, 0.13 µm)
(used for 0.13µm, 0.09µm, . . . )
(used for ??)
Excimer Wavelength
F2
157 nm
ArF
KrF
193 nm
248 nm
XeBr
282 nm
XeCl
308 nm
XeF
351 nm
CaF2
193 nm
KrCl
222 nm
Cl2
259 nm
Page 8
046880 A. Kolodny
8
Contact Printing*
Proximity Printing
• Contact between the resist and mask provides a
resolution of ~1 μm.
• Drawback: dust particles on the wafer can be
imbedded into mask where mask makes contact with
the wafer.
• Imbedded particles cause permanent damage to mask
and result in defects with each succeeding exposure.
• Small gap (10 – 50 μm) between the wafer and the
mask.
• Minimizes mask damage, but …
• Gap results in optical diffraction at feature edges
that degrades resolution to 2–5 μm.
• Minimum linewidth (or critical dimension):
* We use this in lab.
CD ≅ λg
when λ = wavelength and g = gap
Contact Printer
Projection Printing
• Wafer many centimeters from mask
• To increase resolution, only small portion of the mask
is exposed at a time.
• Small image area is scanned or stepped over the
wafer
f to cover the
h entire
i wafer
f surface.
f
• After exposure of one site, wafer is moved to next
site and the process is repeated.
• Called step-and-repeat projection, with a
demagnification ratio M:1
Scanner: Projection Printer
~$25 M
Proximity
Printer
Page 9
046880 A. Kolodny
9