1. ECE614: Device Modelling and
Circuit Simulation
Unit 2 Alignment and Exposure System
By Dr. Ghanshyam Singh
Sharda University

2. Outline
• Subsystems:
– Alignment
• The capability
• Aligning steps
– Exposure
• Sources
• Exposure System
– Contact Printing
– Proximity Printing
– Projection Printing

3. The Subsystems
• An optical A&E system is composed of two major
subsystems. One is the subsystem required to correctly
position and pattern on the wafer surface. The second
part is the exposure system.
• Capability
– Resolution: Ability of the machine to produce a particular size
image. (Entire wafer)
– Registration: The capability of placing the images in the correct
position. (Entire wafer)
– The cost of ownership: Initial purchase cost, wafer throughput,
maintenance cost..
– Free from Vibration: Clean room design

4. Alignment System
• It consists of mechanical parts and vacuum system that can hold down the
mask and wafer firmly such that no movement of mask or wafer is
allowed during the exposure sequence.
Mask
Vacuum system
Vacuum Vacuum chuck

5. Aligning steps
• A 3D transition platform that allows a lateral adjustment of wafer
position by x-y-z-θ stage.
• Wafer:
• First mask is aligned with respect to 90º to the major wafer flat.
• Subsequent masks are aligned to a previously patterned mask with the use of
alignment marks.
• The alignment marks are special pattern located in a easily found position on
the edge of each chip pattern or in the scribe line.
• Automatic alignment is also possible
• Cleaved wafer:

6. Mask to Wafer Alignment
• Modern steppers use automatic pattern recognition and alignment
systems. It takes 1-5s to complete the jobs (align and expose).
• Skilled human operators usually take 30-45 s will well-designed
mark.
• Normally requires at least two alignment marks set on opposite sides
on the wafer.
Alignment mark on wafer,
created from the previous step
Alignment mark on mask. Aligned

7. Exposure System:
• Exposure Sources (UV):
– Xenon arc lamps: Near-continuous spectrum in the visible 200-750 nm
with Xe lines above 800 nm
– Mercury arc lamps: High energy output in the UV with intense lines
between 240-600 nm
– Hg-Xe lamps: combines the spectra form Hg and Xe; the Xe gas improves
start-up and extends operating life

8. Xe/Hg spectra
I
H
G
E
Intensity(arbitraryunit)

9. Minimum Feature Size with UV Source
• Minimum Feature Size (micron) Wavelength (nm)
• 1.00 G-line 436 nm
• 0.70-0.35 I-line 365 nm
• 0.25 KrF 248 nm
• 0.18 ArF 193 nm
• The advantage of confining the exposure source to a short wavelength
(high energy) and narrow line-width is to improve resolution capability of
the light source.
• High energy light allowing shorter exposure times, in turn limiting poor
resolution coming from scattering of the light in the resist.

10. Diffraction Effect
• The smaller or narrower the exposing wavelength, the higher the
resolution capability. This is due to the diffraction effect.
• This phenomenon limits resolution of the optical lithography system
and the effect increases with longer wavelength.
Ultra short wavelength: X-ray

11. Diffraction Gratings
when discussing the resolution of a system it is customary
to discuss a series of lines and spaces called a diffraction
grating rather than a single aperture. If the Fraunhofer
criterion is met, one can roughly approximate the areal
image by the superposition of the individual intensities.

12. Modulation Transfer Function
• The modulation transfer function (MTF) of an
image can be defined as :
• MTF = (Imax-Imin) / (Imax+Imin)
• MTF is a strong function of the period of
diffraction grating. As the period of the grating
decreases, MTF decreases.
• The higher the MTF, the better the optical
contrast.

13. CMTF
• Contrast can be thought of as a measure of the
ability of a resist to distinguish between light and
dark portion of the mask.
• Another resist figure of merit that can be
determined from the contrast is the critical
modulation transfer function(CMTF).
• It is approximately the minimum optical
modulation transfer function necessary to obtain
a pattern.

14. CMTF
• It is defined by :
• CMTFresist= (D100-D0) / (D100 +D0) ----(1)
• It can be found using the contrast as
• CMTFresist= (101/γ
-1) / (101/γ
+1) -----(2)
• A typical value of (2) is 0.4.

15. Advantages and Disadvantages of Contact Printing
• Simple and inexpensive
• Little diffraction effect
• Scattering minimised
• Resolution with most common
sources is ~0.5 micron
• Widely used.
• Performance may be affected by
contamination. (Contact)
• Resist/mask may be damaged in
hard contact mode.
• Required Cleaning every 15-25
exposures
• Causing an unwanted defect
when dirt adhering to the clear
portions of the mask
• Alignment of larger-diameter
wafers presents a light
uniformity problem.
gkW λ∝min
g = gap, g ~ 0 contact
mode, Wmin ~ 0
Resist/g~1 micron

16. Advantages and Disadvantages of Proximity Printing
• Proximity printing was
developed to avoid defect
printing
• Separations gap of 10-50
micron are typical. Since there is
no longer any contact between
wafer and mask, defect
generation is sharply reduced
• No damage to wafer and mask
• Do not find much use in VLSI
photomasking processing
• Reduction in the resolution
• Is a trade-off between resolution
capability and defect.
• Some diffraction effects and
also light scattering take place.
Mask is separated from the wafer by a thin gas cushion (2.5-25 micron).

17. Advantages and Disadvantages of Projection Printing
• Projection printing offers higher resolution than proximity printing
together with large separation between mask and wafer and there
is no damage occurs to the mask during process.
• Mask is of higher quality and easy to fabrication. (10:1)
• Each chip is aligned individually. Higher precision
• -------------------------------------------------------------------------
• Optical defects (lens aberrations) increase very rapidly with
increasing NA. (Large NA=more light flux, shorter exposure time)
• A high precision stepper needed. Very complex alignment control
• Slow
Were developed to obtain a high resolution without the defects

18. Factors affecting focus and resolution
a. Rayleigh’s criterion:
The resolution limit due to lens is referred to as Rayleigh's criteria and is
given by
where k is a constant that depends on the ability of the resist to
distinguish between small changes in intensity (typically k is of order 0.6
to 0.75).
Wmin is minim
feature capability
NA (numerical aperture) = n.sin(α).
i.e. an aligner with an NA of 0.4, together with a 436 nm source, can be
used to image lines as small as 0.8 um.






≈
NA
kW
λ
min

19. Factors affecting focus and resolution
b. Depth of focus:
The depth of focus can be described as the distance along
the optical train that the wafer can be moved and still
keep the image in focus. For a projection system this is
given by
One route to get finer lines/resolution is to develop higher
NA lenses.
Increasing the numerical aperture increases the resolution
linearly, but decreases the depth of focus quadratically.
2
NA
λ
σ =

20. Factors affecting focus and resolution
c. Spatial Coherence:
• The image resolution is also a function of
the spatial coherence of the source, which
is the distance along the optic axis the
wafer can be moved and still be kept in
focus. The spatial coherence can be
approximated as
• The ordinate of the plot is the normalised
spatial frequency, given by
vap = 1/2W
• The spatial frequency has been normalized
to Rayleigh criterion, given by
vo = NA/0.61l
terPupilDiame
eDiameterSourceimag
S =

21. Projection Printing: Scanning Type
Scanning Type: Perkin Elmer Company
avoided the problems of a full mask
projection exposure in favor of a scanning
technique.
It used a mirror system with a slit blocking
part of the light coming from the light
source. The slit allows a more uniform
portion of the light to shine on the mirror
system, which is in turn projected onto the
wafer. Since the size of the slit is smaller
than the wafer, the light beam is scanned
across the wafer.
They are called 1:1 aligners, since the image
dimensions on the mask are the same size as
the intended image dimensions on the wafer
surface.

22. Projection Printing: Scanning Type
Advantages:
• exposure optics are reflective
• no large expensive quartz lenses are
required
Disadvantages:
· NA is about 0.16b and thus the resolution
below 1 um is difficult to achieve with ease.

23. Projection Printing: Stepper
Stepper Type: A stepper is basically a
projection printing method that uses the same
technique used to make masks. A reticle,
carrying the pattern of one or several chips, is
aligned and exposed, then is stepped to the next
site and the process is repeated. Some steppers
are 1:1, that is, the image on the reticle has the
same dimensions as those required on the wafer.
However, most use reticles with 5 to 10 times
the final dimensions. These are called reduction
steppers.

24. Projection Printing: Stepper
Advantages:
• A reticle is of higher quality than a full-size mask, so fewer defects occur.
• There is better overlay and alignment because each chip is individually
aligned.
• The procedure of stepping allows precise matching of larger-diameter wafers.
• Resolution improvements because a smaller area is being exposed each time
and a lessened vulnerability to dust and dirt.
Disadvantage:
• relatively lower throughput, say 50 - 80 wafer per hour compared to projection
printing system of 150- 200 wafer per hour.
• Automatic alignment is used and accomplished by passing light beams
through alignment marks on the reticle and reflecting them off corresponding
alignment marks on the wafer surface.