NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p1
Lecture 12 Process Testing


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
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
Mask Alignment accuracy and lithography resolution
Optical microscopy
Line width measurement
Resistivity measurement
Film thickness measurement
Surface profile measurement
Mask Alignment accuracy and lithography resolution
1. Mask alignment accuracy
Previous
pattern
8 m line
and space
4 m line 2 m line
and space and space
Current
pattern
Alignment accuracy: larger than 4 m
2. Lithography resolution
L
L’
H
(Finger #n)
Mask shape
l (totally N fingers)
Lithography resolution:
lH
Rn
NL
After lithography
NTHU ESS5810
F. G. Tseng

Micro System Fabrication and Lab
Lec12, Fall/2001, p2
Optical microscopy
1. Basic
Three broad categories for optical measurements:
a. Photometric measurements (amplitude change of reflected
or transmitted light)
b. Interference measurements (phase change of reflected or
transmitted light)
c. Polarization measurements (ellipticity change of reflected
light)
Four main optical techniques (reflection, emission, absorption,
transmission):
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p3
2. Optical microscopy
Compound microscope
Overall magnification M=Lateral magnification of the
objective  angular magnification of the ocular
3. Resolution, Magnification, Contrast
Diffraction at a circular aperture of diameter d, the angular
position and the first minimum (by Airy in 1834):
sin(  ) 
1.22
d
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p4
a. Resolution (two objects can be distinguished when the
central maximum of one coincides with the first minimum
of the other, Raleigh)
s
0.61
0.61

n sin(  )
NA
NA=numerical aperture, expressing the resolving power of
the lens and the brightness of the image. However,
increasing NA decreases depth of field and shallow
working distance.
Most NA in air is around 0.4-0.8, to increase NA:
i. Increasing angle .
ii. Immersion objective in oil (increasing n)
iii. Using short wave length light (green light, also most
sensitive to human eye)
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p5
b. Magnification
MaximumNA(microscope )
1.4

 700 
MinimumNA(eye)
0.002
Limit
of
resolution (eye)
0.15mm
M 

 750 
Limit
of
resolution (microscope ) 0.0002mm
M 
Magnification above this is empty magnification!!
Usually M400-500 with air as the immersion medium.
c. Contrast
i. The ability to distinguish between parts of an object.
ii. Contrast of surface-height variation can be improved
by dark-field microscopy—using condenser lens and
light impinging on the sample at an angle to see the
reflect light from the steps.
iii. Smaller steps (down to 30 Armstrong) can be seen by
Differential interference contrast.
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p6
NTHU ESS5810
F. G. Tseng

Micro System Fabrication and Lab
Lec12, Fall/2001, p7
Line width measurement
Line width usually called critical dimensions (CD); measurement
error should be 3-10 times smaller than the process error.
1. Optical method
Usually use optical microscopy, resolution is around 0.5 m.
Optical images are determined by diffraction, aberrations,
focus position, the spectral bandwidth of the illumination,
numerical aperture.
Diffraction effect
2. Electrical method
a. SEM: resolution 0.005 m.

Resistivity measurement
Resistivity measurement by two probes
RT 
V
 2 Rc  2 Rsp  Rs
I
Rc: contact resistance and Rsp spread resist can not be accurately
determined thus Rs can not be accurately extracted from the
measured resistance.
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p8
1. Four point probe
Separate current probe from voltage probe to reduce current
spread and contact effect by reducing current flow into voltage
probe (with large impedance).
Thus, the potential V at distance r from the electrode carrying
a current I in a material of resistivity  is:
V
I
2r
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p9
For probes resting on a semi-infinite medium with current
entering probe 1 and leaving probe 4, the voltage V between
any two point on the same line become:
V
V23 
I 1 1
(  )
2 r1 r4
I 1 1
1
1
(  

)
2 s1 s3 s2  s3 s1  s2
If s=s1=s2=s3,
  2s
V
I
Optimum probe spacing is on the order of 0.5-1.5mm, and
varies with sample diameter and sample thickness.
For arbitrarily shaped sample, the above equation need to be
modified by thickness effect, edge effect, and probe placement
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p10
effects:
i. For thin film, t<s/2

t V
ln( 2) I
ii. For sample diameter d>40s, edge effects become negligible
iii. For 3 to 4 probe spacing or more to wafer boundary, the
probe direction will not effect measurement.

Film thickness measurement
1. Reflectivity measurement (Nanospec)
For insulating layers and epitaxial semiconductor films:
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p11
t1 
i0i
2n1 (i  0 ) cos( ' )
i: complete cycle from 0 to I
For thinner film (<2000 A), it is difficult to find the first
minimum, thus less accurate—using ellipsometry instead.
2. Ellipsometry
i. Measure the thickness of thin dielectric films on highly
absorbing substrates but also be used to determine the
optical constants of films or substrates.
It allows
thickness measurements at least an order of magnitude
smaller than interferometric methods. (10 A possible)
ii. Ellipsometer is based on measuring the state of
polarization of polarized light. (when light is reflected
from a single surface, the amplitude is reduced and phase
is shifted, which is related to index of the material)
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p12
To measure film thickness (cyclical nature of thickness
measurement):
t1 


2 n12  sin 2 ( )
Surface profile measurement (Surface profiler)
NTHU ESS5810
F. G. Tseng
Micro System Fabrication and Lab
Lec12, Fall/2001, p13
Reference:
1. Dieter K. Schroder, “Semiconductor Material and Device Characterization”, John
Wiley & Sons, Inc., chapter 1 and 9, 1990.
2. Dektak3ST surface profile measuring system manual, VeeCO Metrology Group.