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

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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

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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

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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)

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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:

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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