mse-630 lithography topics: wafer exposure systems photoresists manufacturing methods &...
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MSE-630
Lithography
Topics:
•Wafer exposure systems
•Photoresists
•Manufacturing Methods & Equipment
•Measurement Methods
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Projected Lithography Requirements
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Projection systems
Contact Printing:•Limited diffraction effects
•Inexpensive
•Contact between mask & resist results in damage – low yield
•Oldest & simplest
Proximity Printing:•Mask/wafer separated 5-25 m
•Separation results in poor resolution
•Limited to m-sized features
•Both systems require 1X masks
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Projection systems
•Produce high resolution w/o defects
•Resolution limited by diffraction effects
•Mask is 4X – 5X image size
•Sub-micron features
•50 wafer/hour throughput
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Diffraction effectsLight passing through
aperture, with an opening ≈, diffracts,
creating a larger image than that on the
mask
Light travels as a plane wave in free space. The spherical wavelets combine to for a uniform front. When it passes through an aperture, the waves from the limited number of wavelets superimpose, spreading out in all directions.
Smaller openings mean greater diffraction
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The light that diffracts at the highest angels carries detailed information about the shape of the aperture,
is not collected passing through the focusing lens
and is lost.
Projected intensity of light through a circular aperture. Rings around center bright spot result from diffraction
d
fD
22.1
D = diameter of central disk
f = focal length
d = objective lens diameter
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A Fourier series is used to describe the path of light:
11
11
)(211
''
,(),(
''
0
1),(
),()(
yxtFffe
writeweshorthandinz
yfand
z
xf
areasopaquein
areasclearinyxt
where
dxdyeyxtyxe
yx
yx
yfxfi yx
The intensity is given by: I(fx,fy) = |(fx,fy)|2 = |
F[t(x1,y1)]|2
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Resolution
sin
sin
61.0
)sin2(
22.122.11
nNA
refractionofindexn
NAk
nfn
f
d
fR
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Depth of Field
222
22
2
2sin
2211
4
,
cos4
NAk
NADOF
NAf
d
smallisif
The Rayleigh criteria for depth of focus states that the path difference for a ray on the center line and one coming from the edge of the aperture should not differ by more than /4.
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Example
Estimate the resolution and depth of focus of an excimer laser stepper using KrF light source ( = 248 nm) and NA=0.6 Assume k1 = 0.75 and k2 = 0.5.
Solution:
R = k1*/NA = 0.75(0.248/0.6) = 0.31 nm
DOF = ± k2*/NA2 = ±0.5(0.248/(0.6)2) = ±0.34 m
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Modulation Transfer Function (MTF)
minmax
minmax
II
IIMTF
The MTF is a measure of the quality of contrast between features. As features move closer together, diffraction affects cause their Airy disks to begin to overlap, changing the degree of intensity between the two features.
Generally, a MTF>0.5 is needed. Smaller values limit the minimum feature size
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Aerial images produced by contact printing (dashed line) proximity, and projection systems. g=0 in contact system and ~25 microns in proximity system.
The quality of the image decreases as the mask is removed from the surface, with gap size g. The image can be calculated when the gap falls between
< g < W2/2where W = mask opening width
The minimum resolvable feature size is: Wmin ≈ √(g)
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Simulated aerial image. Shading corresponds to optical intensity in the aerial image. The black borders correspond to the mask image that is being printed. The exposure system simulated has an NA = 0.43, partially coherent g-line ( = 436nm). Min feature size is 1 m.
Left: same example as above except feature size has been reduced to 0.5 m. Note the much poorer image quality
Below: Same example, but wavelength has been reduced to 0.365 nm and NA has been increased to 0.5. Note improvement in image quality
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Light CoherencyA perfect point source creates parallel beams after passing
through the condensor lens. A light source with finite size
causes light to strike mask at a variety of angles
Coherent Partially Coherent
A definition of spatial coherence of light sources is:
S = light source diameter/condenser lens diameter = s/d
Or S = NAcondenser optics/NA projection optics
As s becomes more incoherent, MTF degrades for
larger features but improves for smaller ones.
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Photoresists
Typical process for g-line and i-line resists
•Resist changes chemical properties when exposed to light
•Resist may be positive or negative. In positive resists, the exposed areas dissolve when developed
•Resist is poured on to wafer, then wafer is spun. Viscosity is controlled by solvents, which evaporate
•Following patterning and developing, resist is baked to increase hardness
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Photoresists
Basic structure of diazoquinone, a photo active compound (PAC) in positive resists
Sensitivity is a measure of the light required to expose a resist. In Deep UV resists (DUV), this is 20-40
mJ/cm-2. Lower sensitivity gives higher contrast and increased processing latitude.
The above diagram illustrates how diazoquinone changes when exposed
to light. The final molecule (bottom left) is carboxylic acid, which is
soluble in common developers (e.g., KOH, NaOH, TMAH (tetramethyl
ammonium hydroxide))
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As wavelengths get shorter, photoresist has to be reformulated to have the desired reactivity and sensitivity
In DUV resists, incoming photons react with a photo-acid generator (PAG) molecule, creating an acid molecule that acts as a catalyst to make resist molecule soluble. This
process can repeat tens or hundreds of times during the post-exposure
bake (PEB).
Sensitivity of these resists is ~ 20-40 mJ/cm-2
This scheme is being exploited in a new generation of resists designed to
work at short wavelengths.
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Properties and Characterization of Resists
Idealized contrast curves for positive and negative resists
Resist is characterized by its
contrast and critical modulation transfer
function (CMTF)
Contrast is its ability to distinguish light from dark areas in the aerial image
Resists with high contrast can sharpen a poor aerial image
o
f
Q
Qlog
1 Qo = dose at which exposure begins to have
an effect
Qf = dose at which exposure is complete
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The MTF was used to measure the “dark” versus “light” intensities in the aerial image. A similar
quantity is used for the resist, called the Critical MTF. It is the minimum optical transfer function
needed to resolve a pattern in the resist.
110
1101
1
0
of
fresist QQ
QQCMTF
Example of how the quality of the aerial image and the resist contrast combine to produce the resist edge profile. The left side shows a sharp aerial image and steep resist edges. The example on the right shows a
poorer aerial image and the resulting gradual edges on the resist profile.
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Resist Exposure Issues
Resist coatings are:
•Non-uniform thickness
•Not exposed simultaneously
•Light absorption falls off exponentially with increasing depth:I = Ioexp(-z)
Highly reflective substrates create standing waves that
systematically over/underexpose resist, resulting in “wavy” lines:
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Mask Engineering: Optical Proximity Correction
Because high frequency detail is lost due to
diffraction defects and apertures and lenses are round, square details on mask are lost on aerial
image (left). OPC adjusts the image on the right to provide greater detail in
the projected pattern
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Mask Engineering: Phase Shifting
In the image at left, diffraction effects result
in a loss of contrast where the waves
overlap.
On the right, a material is added to the mask to cause the light to shift phase 180o, thereby
canceling the effect of overlap.
Phase shifting can be used to improve the quality of the aerial image or to improve the depth of focus of the exposure system at constant resolution by
using a lower NA system
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Wafer Exposure SystemsThe schematic at left illustrates a scanning
system for transferring information to the
wafer. This requires a 1X mask.
Modern systems use a 5X (or larger mask)
and combine scanning with a stepper to
systematically cover small portions of the
wafer.
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Techniques to improve image quality
In the Kohler illumination system (left), light is focused at the
entrance pupil of the projection lens, which captures the diffracted
light from any features on the mask equally well
Off-axis illumination captures some of the higher-order
diffracted light which was lost in the normal illumination process
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Measurement Methods
Mask inspection measurements read
the mask and compare it to another mask or a mask in a
design database
Typical mask flaws include:
•Opaque defects: Cr where it should not be
•Clear defects: Cr isn’t where it should be
•The actual size of the feature on the mask, which is influenced by the size of the e-beam spot used to write the mask (typ. ~0.125 m)
•Proximity effects from electron backscattering in the resist resulting in distortion
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Test Structures
65
43
2ln
I
Vs
This is an example of a typical test
structure built into the edges of a
chip/sheet.
It is used to extract the sheet resistance
The geometry is chosen to define one square of the
material (labeled s)
A current, I5-6 is applied, and voltage, V3-4, is read at terminals 3 & 4. The sheet resistance is then:
Given the sheet resistance, the line width can be calculated
from: 32
51
V
ILW s