wafer inspection system: calibration and measurement
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Wafer Inspection System: Calibration and Measurement
Author: Xiaoliang Wang
Professor: David Pui
TA: Keung Shan Woo
Lab Members: Xiaoliang Wang,
Meghan Kearney,
Bruce Mehdizadeh,
Bob Chenny
Lab Date: April 24, 2002
Lab Location: EE Cleanroom
Report Date: April 25, 2002
1
Wafer Inspection System: Calibration and Measurement
Abstract
Wafer surface scanners are widely used in semiconductor industries to detect
particle contamination on wafers. The performance characteristics of a PMS SAS-3600
wafer surface scanner have been evaluated using ideal polystyrene latex (PSL). It was
also used to measure irregularly shaped silicon particles. Three sizes of PSL spheres
(0.199µm, 0.3µm, and 0.426µm) were used to study the sizing accuracy and counting
efficiency of this wafer scanner. The results show that this scanner sizes 0.199µm and
0.3µm PSL reasonably accurate. But it cannot size 0.426µm PSL very well. The
background noise lever was so high that this scanner could not measure particles near its
nominal lower detection limit: 0.1µm. It over-counted Si particles by 28%.
An electrostatic enhanced wafer deposition chamber was used to prepare the test
wafers. Both the theoretical calculations and experimental results show that electrostatic
force increases the deposition velocity by 2 orders of magnitudes for particles smaller
than 1µm. The sedimentation is the dominant mechanism for particles larger than 1µm.
Introduction:
Particle contamination on the semiconductor is a very important problem in
integrated circuit fabrications. As the feature sizes shrink to smaller than sub-micron
dimensions, particle contamination on the wafer is the leading cause of products yield
loss [1]. Figure 1 shows a relation of failure rate affected by particle with DRAM
memory capacity or minimum feature size. It shows that 95% of failure is caused by
particle contamination at 16 Mbit DRAM [2].
2
Figure 1. DRAM failure rate caused by particles contamination [2]
The wafer surface scanner is the state-of-the-art instrument to monitor defects on
wafers. There are various methods to measure the number and type of defects on wafers,
including chemical (in-situ residual gas analysis), optical (film thickness sensors and
laser-scattering detectors) and beyond (x-ray and electron beam scattering) [3]. By far,
the most common and sensitive instruments for measuring particles are those based on
light scattering, which are called wafer surface scanners. Such scanners are essential for
many applications, such as inspection of incoming bare silicon wafers, measuring particle
contamination added by processing equipment, and evaluation of the efficiency of wafer
cleaning systems [4]. The specifications of several commercial available wafer scanners
are listed in Table 1.
The mechanism of a wafer scanner is similar to that of the airborne or liquidborne
particle laser particle counters. The wafer is swept by a laser beam. The light is scattered
by particles and the surface. The photodetector collects the scattered light and converts it
to a voltage signal, which is a strong function of the particle size and refractive index.
Therefore, light scattering is a high-resolution method for both sizing and counting,
especially for sub-micrometer sized particles. Today’s instrumentation can count single
particles with effective light-scattering diameter as small as 0.05µm [5]. The major
performance parameters of the wafer scanner are: sizing accuracy, counting efficiency,
lower detection limit, and counting repeatability [6].
3
Table 1 Wafer Surface Scanners1
Wafer Surface Scanners
Manufacturers Model LaserIncident
AngleCollection
AngleLower
Detection Measurement Range
Tencor Instruments2400 Charleston RoadMountain View,CA 94043Tel.) 415/9696767Fax.) 415/9696371
Surfscan 4000
Surfscan 5500
Surfscan 6200
Surfscan 7000
2mW He-Ne(0.6328µm)
0º -45º ~ -5º5º ~ 45º
0.2µm (PSL) 0.006-1024µm2
(11 Channels)
Estek Products Division9625 Southern Pine Blvd.Charlotte, NC 28273Tel.) 704/5234808Fax.) 704/5298303
WIS-8500 Ar ion(0.488µm)
-15º -25º ~ 85º 0.2µm (PSL) 0.173~0.46µm(Gain=1.0)
0.269~2.062µm(Gain=0.1)
(10 Channels)WIS-8500 II
PMS1855 South 57th CourtBoulder, CO 80301Tel.) 303/4437100Fax.) 303/4496870
SAS-3600-60º
-10º ~10º
0.1µm (PSL) 0.1~1.2µm(15 Channels)
SAS-5800
5~10mW He-NeP-Polarization
(0.6328µm)S-Polarization
(0.6328/0.543µm)60º
The objective of this experiment is to calibrate the sizing accuracy, counting
efficiency and lower detection limit of a PMS Model SAS-3600 XP Surface Analysis
System and to use it to measure polydisperse silicon particles. The deposition rate of
electrostatic enhanced particle deposition chamber will also be studied.
Experimental Methods:
1. The wafer scanner
The wafer surface scanner used in this project is a model SAS-3600 scanner
produced by PMS (Particle Measuring Systems, Inc.) of Boulder, Colorado. Figure 2
shows the schematic diagram of this scanner [6]. In this system, a S-polarized laser and a
P-polarized laser with a wavelength of 0.6328µm and 0.594µm are simultaneously
incident on the wafer surface with an incident angle of ±60º from the normal to the
surface, respectively. The scanner scans the wafer in a spiral path with the laser beams
sweep the wafer from the center to the edge while the wafer is rotating on a vertical axis.
The scattered light over ±45º solid angle is collected through a microscope dark-field
objective. Then it is separated into S- and P- polarized components again by the
1 From ME 5116 class material provided by Prof. David Pui.
4
polarizing beam splitter, and each component is delivered to the photodiode detector. The
S-polarized component is used for detecting smaller particles (0.1~0.3µm), and the P-
polarized component is used to measure particles larger than 0.3µm [6].
Figure 2 Schematic optics diagram of the PMS SAS-3600 wafer surface scanner [6]
2. Particle Deposition System and Standard Calibration Wafers
In order to calibrate the wafer surface scanner, we need to prepare wafers
deposited with particles of know size and counts. This was done by a particle deposition
system developed at the University of Minnesota, as is shown in Figure 3. The two major
parts of this system are: a monodisperse particle generation system and a particle
deposition chamber. Particles are generated with an atomizer. The differential mobility
analyzer (DMA) classifies the polydisperse input by particle mobilities. The
predominantly monodisperse particles out of the DMA are then introduced into the
deposition chamber, where particles deposit onto the test wafer by electrostatic attraction
and sedimentation. The deposition area can be controlled by controlling the screen
5
voltage applied to the wafer, and the number of particles deposited can be controlled by
controlling the deposition time [7].
Figure 3(a) Schematic diagram of the particle deposition system [7]
Figure 3(b) Schematic diagram of the deposition chamber [7]
Note that two OPCs are placed immediately upstream and downstream of the deposition
chamber to measure particle counts, and a laminar flow element is used to measure the
flow rate to the chamber. The number of particles deposits on the wafer can be calculated
from Equation 1 [7].
downup
downup CQ
QCN −
×= (1)
0.272lpm
0.228lpm 0.228lpm
6
where
N = total number of deposited particles
Cup = upstream CPC count
Cdown = downstream CPC count
Qup = aerosol flow rate to upstream CPC, 0.272 lpm in this experiment
Qdown = aerosol flow rate to downstream CPC, 0.228lpm in this experiment
The wafers used in this experiment were 150mm wafers. Some information about
the prepared test wafers is listed in Table 2.
Table 2 Information about the prepared wafers
Sample# 1 2 3 4 Particle material PSL PSL PSL Silicon Particle size (µm) 0.199 0.3 0.426 -- Screen Voltage (V) 4000 4000 4000 -- Deposition Time (s) 70 200 150 20 Upper CPC Count 3406 5863 2732 -- Lower CPC Count 6 8 8 -- Scanner Count (before) 23 45 103 27 Scanner Count (after) 2126 4517 2366 3650
3. Particle deposition rate:
In the manufacturing environment, particles deposit onto the wafer by diffusion,
sedimentation and electrostatic attraction. The rate of particle deposition on wafer surface
at various conditions has been studies previously [8], [1]. Using the analogy between heat
and mass transfer and particle diffusion, Liu and Ahn (1987) proposed that particle
diffusion rate from the ambient air to the surface of wafer which is put horizontally in
vertical airflow could be expressed as Equations 2, 3 based on experiments by Sparrow
and Geiger (1985):
21
31
Re08.1 ScSh = (2)
21
31
0 Re834.0 ScSh = (3)
where
C = slip correction factor
D = kTC/3pµDp, is the diffusion coefficient
7
Din = deposition chamber inlet tube diameter
Dp = particle diameter
Dw = wafer diameter
k = Boltzmann’s constant
K = mass transfer coefficient, which is also the deposition velocity
Q = flow rate to the deposition chamber
Re = U Dw / ? is the Reynolds number
Sc = ?/D is the Schmidt number
Sh = KDw/D is the Sherwood number
Sh = mean Sherwood number for the whole wafer
Sh0 = Sherwood number at the center of the wafer
U = 4Q/pDin2 is the air flow velocity
T = absolute temperature
µ = gas viscosity
? = kinetic viscosity
From these equations, particle deposition rate due to diffusion can be determined by:
ww DDScDDShK /Re08.1/ 21
31
=×=
21
61
213
2
308.1
−−
×= DwU
DpkTC
νπµ
(4)
Similarly,
21
61
213
2
0 308.1
−−
×= DwU
DpkTC
K νπµ
(5)
The particle deposition due to sedimentation is determined by the settling velocity
Vs, which is defined as:
µρ 18/2gCDpV Ps = (6)
where
Pρ = particle density
g = gravitational acceleration
8
Because there is electric field between the wafer and the inlet tube, as is shown in
Figure 3(b), electric force enhances the particle deposition. The velocity due to electric
attraction (VE) is:
DpH
CqVV o
E πµ3= (7)
where
q = particle charge
Vo = applied voltage
H = distance between inlet plate and wafer
For simultaneous diffusion, sedimentation and electric attraction, the deposition V
can be estimated from:
V = K + Vs + VE (8)
On the other hand, the mean deposition velocity V can be defined as the ratio of
particle flux to the wafer J (number of particles deposit on wafer surface per unit area per
unit time) to the particle concentration in the bulk air above the wafer N [1], [8], i.e.,
V = J/N (9)
ww
w
AtN
J = (10)
where
Nw = number of particles deposited on the wafer
tw = deposition time
Aw = wafer area
Both J and N can be determined experimentally. Therefore, we can compare the
experimental results with theoretical predictions. However, both Equation (5) and (9) are
only valid when the wafer is under vertical laminar flow. As we can see from Figure 3(b),
this wafer deposition chamber uses a tube to output aerosols and the flow is not laminar.
Therefore, wafer deposition velocity discussed in this report can only give some
qualitative information. For precise quantitative result, more complicated CFD is needed.
9
4. Experiment procedures:
To calibrate the wafer surface scanner, PSL spheres with diameters of 0.199µm,
0.3µm and 0.426µm were deposited and measured first, then a wafer deposited with
polydisperse silicon was measured. Although this surface scanner has a nominal lower
detection limit of 0.1µm (PSL), the smallest size bin was not used in this experiment
because the background noise (from clean wafer) lever is close to the response of 0.1µm
PSL. The raw data of these measurements are listed in Appendix C.
Results:
1. Sizing accuracy:
Figure 4(a) to Figure 4(d) show the wafer scanner responses to monodisperse
0.199µm, 0.3µm, 0.426µm PSL spheres and polydisperse Si particles with irregular
shapes. Figure 5 compares the nominal PSL sizes and the scanner reported geometric
mean sizes. The statistics of the measurements is listed in Table 3.
From these results, we can see that the sizing accuracies for 0.199µm and 0.3µm
PSL are reasonably good, but it is very poor for 0.426µm PSL. The reason for the poor
sizing of 0.426µm PSL can be explained by the manufacturer provided calibration curve
as is shown in Figure 6. Note that the response of the scanner to PSL in the size range of
0.3~0.5µm is pretty flat, which means that the scanner does not have good sizing
capability in this size range.
Figure 4(a) 0.199µm PSL Figure 4(b) 0.3µm PSL
10
Figure 4(c) 0.426µm PSL Figure 4(d) polydisperse Si
Figure 4 Wafer scanner calibration and measurement results
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7true size (µm)
mea
sure
d s
ize
(µm
)
experimental
theoretical
Figure 5 Comparison of nominal and experimental sizes
Table 3 Statistics of the measurement
PSL size (µm) Geometric mean size Geometric standard deviation 0.199 0.209 1.398 0.30 0.205 1.379 0.426 0.515 1.782
11
Figure 6 Theoretical and manufacture provided responses of the wafer surface scanner [6]
2. Counting efficiency:
The wafer surface scanner counting efficiency is defined as the ratio of measured
particle counts to deposited particle counts. The measured particle counts is the
difference between the scanner counts before and after particle deposition. The ideal
deposited counts can be calculated from Equation 1. Here we assume that particle loss on
the tubing can be neglected. The results are listed in Table 4. Note that the counting
efficiencies for PSL are reasonable. For Si, the counting efficiency is higher than 100%.
The presumable reason is that: due to the irregular shape and high refractive index of Si
particles, some of the noise signals are large enough to be counted by the scanner.
Table 4 Wafer surface scanner counting efficiencies of the measured particles
Sample# 1 2 3 4 Particle material PSL PSL PSL Silicon Particle size (µm) 0.199 0.3 0.426 -- Scanner Count (before) 23 45 103 27 Scanner Count (after) 2126 4517 2366 3650 Measured Count 2103 4472 2263 3623
Theoretical response S-pol. (?=0.633µm) P-pol. (?=0.594µm) PMS calibration ? S-pol. ? P-pol.
PMS SAS-3600 Incident angle: 60º Collection angle: -45º~45º
101
100
10-1
10-2
10-3
10-4
10-5
0.1 1.0 2.0
12
Deposited Count 2826 4862 2179 2826 Counting Efficiency(%) 74.4 92.0 103.9 128.2
3. Particle deposition rate:
As discussed earlier, theoretical particle deposition rate can be calculated using
Equations 4 to 8. The calculation parameters and steps are listed in Appendix B. The
deposition velocity curves for different mechanisms are plotted in Figure 7. Note that for
particles smaller than 1µm, electrostatic attraction is the dominant force for deposition;
for particles bigger than 1µm, sedimentation is most important. Particles with diameters
around 1µm have the minimum deposition rate. Because the electrostatic effect is about
two orders of magnitudes higher than diffusion, particle deposition is not a strong
function of gas velocity for this deposition chamber. As shown in Figure 8, the deposition
velocities are almost the same for two quite different gas velocities. However, Figure 9
shows that deposition rate increases as the voltage increases for particles smaller than
1µm. It demonstrates that electric field can enhance deposition of smaller particles
significantly. Also note that the size with minimum deposition rate increases as voltage
increases.
Deposition rate of different mechanisms
1.E-06
1.E-04
1.E-02
1.E+00
1.E+02
0.01 0.1 1 10
particle diameter (µm)
depo
sitio
n ve
loci
ty (
cm/s
)
diffusion
gravity
electric
overall
Figure 7 Mean deposition velocities by different mechanisms for a 150mm-diameter,
freestanding, horizontal wafer in a VLF clean room (PSL particles, Vo = 2000V,
U=0.12m/s)
13
Mean deposition rate at different gas velocities
0.01
0.1
1
10
100
0.01 0.1 1 10
Particle diameter(µm)
dep
osi
tio
n v
elo
city
(cm
/s)
U=0.12m/s
U=1.00m/s
Figure 8 Mean deposition rates at different gas velocities for a 150mm-diameter,
freestanding, horizontal wafer in a VLF clean room (PSL particles, Vo = 2000V)
Mean deposition rate under different voltages
0.0001
0.001
0.01
0.1
1
10
100
0.01 0.1 1 10particle diameter
dep
osi
tio
n v
elo
city
Vo=0V
Vo=1000V
Vo=2000V
Vo=4000V
Figure 9 Mean deposition velocities under different voltages for a 150mm-diameter,
freestanding, horizontal wafer in a VLF clean room (PSL particles, U=0.12m/s)
Experimental deposition rate can be calculated from Equation 9. The deposited
particles N have been calculated using Equation 1. The free stream concentration can be
calculated using upstream CPC reading upC and flow rate upQ . Therefore, Equation 1 can
be changed to Equation 11 as below:
14
up
up
up
up
AC
NQ
Qt
C
AtN
V =••
= / (11)
where
t = deposit time, as listed in Table 2
A = effect wafer area. In this case, an annular radius of 15mm was not taken
into consideration. A = p (7.5-1.5)2 =113.1cm2
The experimental and theoretical results are plotted together in Figure 10. Note
that the theoretical predictions are systematically higher than experimental data. For
0.199µm, the theoretical data is 4 times higher than the experiment. This discrepancy is
due to the non-laminar flow above the wafer, as was explained earlier.
Comparison of experimental and theoretical deposition velocities
0.01
0.1
1
10
100
0.01 0.1 1 10
Particle diameter(µm)
depo
sitio
n ve
loci
ty
(cm
/s)
theoretical
experimental
Figure 10 Comparison of experimental and theoretical deposition velocities
Conclusions:
A wafer surface scanner has been calibrated with three sizes of monodisperse PSL
spheres, and it was used to measure particles counts of a wafer with polydisperse silicon
particles. In addition, the particle deposition rate in the electric enhanced deposition
chamber was studied.
The wafer scanner can provide reasonable size information for 0.199µm and
0.3µm PSL. However, both theoretical calculation and experiments show that it has bad
sizing around 0.4µm. Wafer scanner response is a strong function of both wafer and
15
measured particle refractive index. We should be careful when interpreting the PSL
“optical equivalent” sizes.
The counting efficiencies of the wafer were also studied. The noise level was so
high that it is comparable to the responses to 0.1µm PSL. Therefore, we could not use
this scanner to measure particles below 0.2µm. For the three calibrated PSL spheres, the
counting efficiencies were reasonable. However, when measuring particles with irregular
shapes such as Si, the scanner had a counting efficiency much higher than 100%. The
presumable reason is due to noise.
An electrostatically enhanced deposition chamber was used to prepare the test
wafers. Theoretical calculation shows that electrostatic force increases the deposition
velocity about 2 orders of magnitudes for particles smaller than 1µm. Sedimentation is
the dominant mechanism for particles larger than 1µm. The particle around 1µm diameter
has the minimum deposition velocity. Unlike the deposition chamber without
electrostatic force, the flow velocity does not affect the deposition velocity for this
chamber. A comparison between the theoretical and experimental deposition velocity was
made. It shows that the experimental data are systematically lower than the theory.
References:
1. Pui, D.Y.H., Y. Ye, and B.Y.H. Liu, (1990) "Experimental Study of Particle
Deposition on Semiconductor Wafers," Aerosol Sci. Technol. , 12: p. 795-804.
2. Komagata, M.,(1996) A new method of reducing the particle contamination in
semiconductor manufacturing. in 18th IEEE/CPMT International , 1996.
3. Diaz, R.E., On-Wafer Measurement of Particles, in Contamination-Free
Manufacturing for Semiconductors and Other Precision Products, R.P. Donovan,
Editor. 2001, Marcel Dekker, Inc.
4. Liu, B.Y.H. and S.-K. Chae, (1993) "Sizing Accuracy, Counting Efficiency, Lower
Detection LImit and Repeatablility of a Wafer Surface Scanner for Ideal and Real-
World Particles," J. Electrochem. Soc., 140(5): p. 1403-1409.
5. Donovan, R.P., Off-Wafer Measurement of Contaminants, in Contamination-Free
Manufacturing for Semiconductors and Other Precision Products, R.P. Donovan,
Editor. 2001, Marcel Dekker, Inc. p. 27-77.
16
6. Chae, S.K., et al.,(1993) Performance Characteristics of the PMS SAS-3600 Wafer
Surface. in 39th Annual Technical Meeting of the Institute of Environmental Sciences.
Las Vegas, Nevada.
7. Woo, K.S. and B.Y.H. Liu,(1997) A Particle Deposition System for the Preparation
of Standard Calibration Wafers. in 43rd Annual Technical Meeting, IES. Los Angeles,
CA.
8. Liu, B.Y.H. and K.H. Ahn, (1987) "Particle Deposition on Semiconductor Wafers,"
Aerosol Sci. Technol., 6: p. 215-224.
Appendix A: Instruments list
• PMS Model SAS-3600 XP Surface Analysis System
• Electrostatically enhanced wafer deposition chamber
• PSL aerosol generator
• Differential mobility classifier monodisperse aerosol generator (DMA)
Appendix B: Particle deposition rate calculation
1. Parameters:
k = 1.38× 10-23 J/K
T = 293K
µ = 1.83× 10-5 Pa?S
ν = 1.57× 10-5 m2/s
Dw =150mm = 0.15m
Q = 0.228 lpm = 3.80 × 10-6 m3/s
Din = 0.25inch = 0.00635m
U = 4Q/pDin2 =0.12 m/s
H = 0.05m
Q = 1.6× 10-19 C
Pρ = 1005kg/ m3 (PSL)
Vo = 2000V
2. Equations:
17
K 21
61
213
2
308.1
−−
×= DwU
DpkTC
νπµ
µρ 18/2gCDpV Ps =
DpHCqV
V oE πµ3
=
V = K + Vs + VE
Appendix C: Raw data
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