CALIFORNIA STATE UNIVERSITY, NORTHRIDGE
THE LASER FLAT FINDER
ON A PRE-ALIGN STATION
A graduate project submitted in partial satisfaction
of the requirements for the degree of Master of Science in
Engineering
by
Hee-June Choi
January 1982
The Graduate Proiect of Hee-June Choi is Approved:
Dr. N.agi El Naga' I,
9TOfessor
s.
GadomsKi
California State University, Northridge
ii
ACKNOWLEDGEMENTS
I would like to thank Professor J. C. Prabhakar for
his invaluable advise and encouragement during the
preparation of this report, as well as other professors,
Professor Nagi El Naga and Professor S. Gadomski.
I also gratefully
acknowledge the support of T.R.E.
Semiconductor, Corporation and my family who enabled me
to continue my education.
In particular, I am deeply
grateful to Eleanor Silletti for her kind help in typing.
iii
TABLE OF CONTENTS
Figures and Table .
v
Abstract
vii
1.
Introduction
1
2.
Design Criteria of Devices
5
2.1
Optical and Laser Arrangement
5
2.2
Scanner and Scanner Driver . .
g
2.3
Beam Detection Electronic Unit
15
2.4
The Capture Range and Resolution .
for the Pre-Aligner
21
2.5
Micro-computer and Data Process- . .
ing Unit
26
2.
System Analysis
32
3.1
Motor and Tachometer Specification
32
3.2
Breakaway Voltage of the Motor and .
the Position of the Pre-aligner
Servo
34
3.3
The Fixed Speed and Tachometer . . .
Gain Setup
35
3.4
The Overall System Gain
35
3.5
System Analysis
37
Conclusions .
43
References
45
iv
FIGURES AND TABLE
1.1
Pre-align Station Assembly . . . .
2
2.1.1
Optical Arrangement for the Laser
Flat Finder
5
2 .1. 2
Gaussian Irrandiance for 3209 H-PC
Showing Definitions of Beam Radius W
2.1.3
Growth in Radius With Distance . .
Propagated Away from Gaussian Waist
8
2.2.1
Time Diagram for a One Complete Scan
Cycle
10
2.2.2
Connection Diagram for 4423 Function
Generator
13
2.2.3
Signal Driver Circuits for the ALS-300
Scanner
2.3.1
Minimum and Maximum Laser Power for
each sensor
17
2.3.2
Spectral Responsivity for the
SD 172-11-11-021 Sensor
18
2.3.3
Beam Detection Amplifier for 0 1
20
2.3.4
The Detection Circuit for 0
21
2. 4. 1
The Specification of 3" Wafer
22
2.4.2
Minor Flat With Maximum Width and
Major Flat With Minimum Width
23
2.4.3
Variations of Speed of the Laser
Beam During a Cycle Period
25
2.5.1
Programming Flow Chart
30
3.1.1
Dimensions of the Pre-aligner Chuck
32
3.1.2
Transfer Function for the Motor
33
v
2
and 03
. .
.
7
14
. . .
3.4.1
Resolution of the Pre-aligner for
the 5" Wafer
3.5.1
Summing Amplifier
3.5.2
The Overall Transfer Function for
the System
3.5.3
The System Bode Plot
3.5.4
The System Nichol's Chart
3. 5. 5
The Overall System Control Circuit
. . . . . . . .
.
. .
36
37
. . .
. . .
38
41
. . . .
41
42
TABLE
1
The Speed Profile of the First Half
Scan Cycle
vi
26
ABSTRACT
THE LASER FLAT FINDER
ON A PRE-ALIGN STATION
by
Hee-June Choi
Master of Science in Engineering
The precision final alignment of a wafer on the wafer
stepper will lead us to produce the micro-inch geometry
integrated circuits for the semiconductor industry.
The
function of the laser flat finder on a pre-aligner is
to locate the major flat of a wafer within the field of
view on a TV camera for the final alignment.
This work is in regard to the design of the laser
flat finder on a pre-align station.
Since the accurate
alignments on the pre-align station insures a better
resolution of the final alignment on the TV, the required
resolution of the pre-align station and major design
parameters to achieve this goal is discussed.
Also
discussed are the various design criteria of
the devices which are used in the pre-align station such
vii
as the scanner, the micro-computer, the laser, etc.
The
laser beam and the optical arrangements are implemented
to detect the edges of a wafer, and the micro-computer
has been used for control and measurements of the prealigner as well as to replace TTL logic.
The development
of the algorithm of the operational steps of the software
will also be studied.
Finally, the system analysis is
carried on by using control theory.
viii
Chapter 1
INTRODUCTION
Recent developments in the semiconductor industry
demand much more dense circuits on a much smaller size of
an integrated circuit board.
To meet this requirement,
T.R.E. Semiconductor, Corp. developed the "700SLR·Wafer
Stepper".
This wafer stepper is a precise step-and-repeat
system for exposing photoresist coating silicon wafers.
After processing, these silicon wafers become the basic
components of electronic integrated circuits.
The wafer
stepper can be divided into the following major subsystems:
1.
Wafer pre-align station
2.
X-Y stage
3.
Reduction camera and illuminator
4.
Automatic reticle handler
5.
Control console assembly
Since this paper is regarding the design of a laser flat
finder on the pre-align station, the articles of the
discussion will be limited for the pre-aligner station.
The general operational steps of the pre-aligner station
are as follows.
The wafer pre-aligner subassembly re-
ceives a wafer from a send cassette through the belt
mechanism as shown in Figure 1.1
The wafer is placed
on the pre-aligner chuck and the wafer is centered by a
6-pin centering mechanism.
The chuck with the vacuumed
wafer is raised into position for the laser flat finder
1
2
to locate the major flat.
A He-Ne laser is used to
generate three laser beams that scan across the edges of
the wafer for a fraction of the whole scan range.
Then,
the three photo sensors detect the laser beams and provide
an amplified signal to the micro-computer.
::l ~ ~r; ~T -:"'"\
(~!,'-=w ...:-uc C."fe) '\
Figure 1.1
Pre-align Station Assembly
The micro-
3
computer measures the time periods, and performs an
analysis of these data to determine where the flat is
located on the wafer and the relative error of the position
alignment of the major flat.
The micro-computer then
drives the torque motor in constant velocity to find a
major flat and servos the motor to optimum alignment after
this major flat is detected.
All the design specifications of the laser flat
finder are worked on the worst cases of each wafer size,
3, 4, and 5 inch.
Therefore, any size wafer is acceptable
for the laser alignment without modifying any portion of
the software or the hardware circuits.
As mentioned
before, the function of the pre-aligner is to locate the
major flat of each wafer.
This pre-alignment of a wafer
flat insures that the alignment mark on the wafer appears
in the field of view of the TV camera.
Therefore, the
viewing range of the camera is the capture range of the
wafer for the final alignment.
If the major flat alignments
of the wafers in the pre-align station were done accurately
enough to locate the alignment mark of the wafer on the
TV screen within the minimum position errors, one is able
to use a high magnification TV camera for the final alignment.
The field of view of the 700SLR is about 2 mils by
3 mils on the TV.
Compared with this, the other leading
model in the photo-lithographic industry uses a TV
camera with the field of view of about 10 mils by 15 mils.
4
The precision final alignment leads one to generate the
micro-inch geometry integrated circuits for the semiconductor industry.
This laser flat finder has turned out to be the most
accurate pre-alignment technique in the industry to date.
During the development of the first flat finder, there were
many difficult problems to overcome.
The major technical
difficulty encountered was to meet the requirement to
operate with many different kinds of wafers such as garnet,
silicon and sapphire.
At the beginning of the design
stage, the Infrared Light Emitting diodes (LED) were used
as a light source of the flat finder mechanism.
The two
L.E.D.'s illuminate even light on the corresponding
position sensors.
These photo couples are separated
farther than the width of the minor flat, but are closer
than the width of the major flat.
This keeps the flat
finder mechanism away from capturing the minor flat of the
wafer.
The photo couple mechanism works satisfactorily for
the silicon wafer, but the transparent characteristic of
the garnet and sapphire wafers transmit extra light through
the wafers.
The amount of extra light totally depends on
the wafer itself.
The integration of the unpredictable
amount of light through the wafers causes a large position
error for the pre-aligner alignments.
This is the reason
for the development of the switching concept of the laser
pre-aligner.
Chapter 2
DESIGN CRITERIA OF DEVICES
This section discusses the various design criteria
of the devices and major factors for selecting each device.
Also the development of operational steps will be studied.
2.1
Optical and Laser Arrangement
A)
The Optical Arrangement
The 3209 H-PC laser is located at the bottom of the
laser flat finder mechanism and the laser beam comes to
the scanning mirror through the right angle prism (Figure
2.1.1).
Right
~50%
Beam
Splitter
1
X Axis
Beam Scan
Direction
Fixed Mirror
Y Axis
Figure 2 .1.1
Optical Arrangement for
the Laser Flat Finder
5
6
With the arrangement of one scanning mirror and two
fixed mirrors, a 150 mils wide scanning beam is reflected
off the scanning mirror.
After a single beam laser goes
through the beam splitters, the three parallel beams are
transmitted to the corresponding photo sensors.
There are
two advantages of the conversion of the three beams from
the single beam originated.
First, it simplifies the
complex optical arrangement which might occur by using
three different laser beams.
Second, it eliminates all
the common errors such as the fluctuations of each laser
beam intensity, the beam position errors, etc.
B)
The Selection of the 3209 H-PC Laser
The 3209 H-PC helium-neon laser has .5 mw output
power and a wavelength of 632.8 nanometers.
The 3209
H-PC (1) is also 500:1 linearly polarized laser.
The
advantage of having the linearly polarized laser is that
the polarized laser beam eliminates the output power
fluctuation which occurs when a randomly polarized beam
reflects off the reflective surfaces.
At the beginning
of the design stage, the randomly polarized laser was in
fact used with a misunderstanding of the polarization
effect.
Since the polarization of the laser beam is varied
from point to point along the beam wave in a random manner,
the output of the laser beam fluctuates too much to have
a steady light source.
Therefore, the 3209 H-PC linearly
polarized laser is selcted as the light source.
The
7
other impottant factor for selecting a laser beam is its
spot size.
When a laser is detecting the edge of a wafer,
the smaller spot size of the laser beam has better accuracy
for detecting the wafer edge position.
However, if the
spot size of the laser beam is too small, the high density
of the laser power may damage the photo sensors.
There-
fore the area of the laser is carefully calculated.
The
3209 H-PC laser has a perfect plane wave front and a
Gaussian transverse irradiance profile, as shown in
Figure 2 .1. 2
Irradiance
Io = 100%
13.5%
Wo
Wo Contour Radius
Figure 2. 1. 2
Gaussian irrandiance profile for 3209 H-PC
showing definitions of beam radius Wo
8
The common definition of the diameter of the laser is the
distance in which the transverse field amplitude falls to
a fraction 1/e of its peak value. At this diameter, the
beam intensity falls to l/e 2 (13.5%) of its peak value.
Because it is impossible to have a perfectly collimated
beam, the formula describing the beam spreading is used
W(Z) = Wo
Where
eq (1)
A = 633 x l0- 6mm
Z = 178mm
Wo = .249mm
If we insert all the parameters in eq (1),
W(Z) = .249
+
-6
633 X 178 X 10
( 3.14 X 6.2 X 10 -2
= .28757mm
z
Figure 2.1.3
Growth in radius with distance
propagated away from Gaussian Waist
(2~
9
The area of the laser beam:
nW 2 = 3.14 x (.28757mm) 2 = .26mm 2
The maximum light energy density which the photo sensor
can take is 1.5 mw/mm 2 .
The maximum energy which the photo sensor can have with
the given area of .26mm 2 is:
.26mm
2 x 1 · 5mw
mm 2
= . 39 mw = 390 11w
In section 2.3, the maximum laser power which is delivered
to the photo sensor is l601.1w.
Therefore, the 3209 H-PC
laser is properly applicable in this design.
2.2
Scann and Scanner Driver
A.
ALS-300 Scanner
The ALS-300 scanner is a linear analog device which
follows the signal shape of the input current (3).
There-
fore, when an anticipated scan rate is required to be
changed for better system performance, it can be easily
accomplished.
This is not possible for the resonant type
scanning devices.
This is the main reason why ALS-300
scanner is used in the laser flat finder mechanism in
this project.
The requirement of the scanner in our application is
to generate the 150 mils wide scanning laser beam with an
operable scan frequency rante.
The main considerations for
selecting a proper scan period are the data processing
time for the microcomputer and the error signal response
time for the drive motor.
10
Figure 2.2.1 shows the timing diagram of the major
operational steps for the flat finder within one scan cycle.
Edge detecting time
Data processing time for the
micro-computer
Response time of error signal for
the drive motor
Figure 2.2.la
Time diagram for the operational steps for
the graphical viewpoint
on a wafer edge
t
Start
Counting
r
2
r tDTrl~ve motor
start responding
to error counts
Stop counting, and micro-computer
starts processing the data
Figure 2.2.lb
Time diagram for a one complete scan cycle
11
At the beginning of the time period T , the external
1
counter is started to count just after the laser beam goes
across the wafer edge, and the laser beam keeps travelling
outside of the wafer.
This counter will continue counting
until the laser beam is obstructed by the wafer edge again.
This is the end of the time period T .
1
During the next
time period of T , the microcomputer processes the data,
2
which the external counter generated in the time period
T , and sends the error signal to the drive motor through
1
the Digital/Analog Converter (D/A).
At the end of the time
period T , the laser beam reverses the travel direction and
2
travels back to the wafer edge again.
At time period T ,
3
it is time for the drive motor to respond to the error
signal from the microcomputer.
As many as 200 instruction
steps are required to complete the laser flat finder
function for the microcomputer.
time for the microcomputer is
ing time is .Sms.
Since one complete cycle
2.5~s,
the total data process-
Therefore, the period of the lOms scan
cycle is chosen to minimize the effect of the computer
data processing time.
Also the mechanical positioning of
a wafer on the pre-align chuck should be such that
the
travel time of the laser beam to travel under the wafer
should be long than .Sms.
Then the operational range of
the mechanical wafer positioning on the pre-aligner can
be calculated as follows:
As shown in Figure 2.4.3b, the minimum time duration
12
of T2 is from t
=
2.25 ms to 2.75 ms.
Position of the wafer edges:
For t = 2.25ms,X = 75 sin
For t
=
2. 75ms,X
=
75 sin
(
Z
(z
TI
1r
.00225) = 74
.01
:~~ 275 )
=
X is operation range of the pre-aligner for
74
f cycles.
Therefore, the total acceptable range for a mechanical
wafer positioning is:
150
(75-74)
= 149 (mils).
Which is the total
operating range for half cycle.
It also shows that the tolerances of a wafer position
mechanism can be as much as 149 mils.
B.
The Scan Function Generator
For the ALS-300 scanner unit, all interior coil
connections are well packed.
Therefore, two exterior
leads can be connected directly to a function generator.
The Burr-Brown 4423 (4) is selected as a function generator.
The model 4423 is a precision quadrature oscillator
which has sine and cosine wave outputs available at the
same time.
It is also external resistor programmable
for a selected frequency.
Figure 2.2.2 shows the conn-
ection diagram for 100 Hz frequency.
13
rl5Vr!:l5V rlSV
0
@@
4423
CD !
@J
0 El
= 10 sin 21tft
.Ol]..!f
E2 = 10 cos 2Tift
Figure 2.2.2
Connection diagram for 4423 function generator
In Figure 2.2.2, the frequency f can be expressed by:
f
=
100 Hz
42.05R
=
(C+ .001)
where
c
f is in Hz
is in ]lf
R
is in KSG
(3. 785
=
---2.2.1
2R)
If we choose .Ol]lf as the value of the capacitor,
the value of the resistor will be lOOSG according to the
eq 2.2.1.
The 150 mils wide scanning range is equivalent
to about 3.5 degrees deflection of the scanning mirror.
Since the sensitivity of the ALS300 scanner is 50 MA/deg,
14
the supply current for the scanner should be more than
175 .MA.
The maximum output signal of the 4423 function
generator goes up to lOV, and the D.C. resistance of the
scanner is 8Q.
To generate the required signal level for
the scanner driver, the operational amplifier station is
implemented between signal source and signal driver.
Also, this type of pre-amplifier does not have sufficient
power output.
The model 3571 amplifier (5) is used for the power
amplifier.
Figure 2.2.3 shows the signal driver circuits
for the scanner.
100 pf
15k
4423
3571
ALS-300
Figure 2.2.3
Signal Driver Circuits for the ALS 300 scanner
15
2.3
Beam Detection Electronic Unit
To have a proper conversion from the light energy to
electrical energy on the laser flat finder, all variable
factors of the light energy on the laser flat finder
mechanism should be considered.
The maximum and minimum
output power of the 3209 laser beam was varied from
.3mw to .7mw for each unit.
Since the laser beam reflects
off twice from the scanning mirror and once from each of
the two fixed mirrors, the output power drops to about
58% of the total input power.
The calculation of the
light output is made as follows:
The reflectivity of the scanning mirror (Bulova)
~
85%
The reflectivity of the fixed mirrors (Melles Griot)
Two reflections
from the scanner
~
(.85 X .85)
Two reflections
from the fixed
mtrror
X
(.9
X
.9)
~
90%
Total
Ref~:c:vity
=
So, the fluction of the input powers to the beam splitter
are:
Minimum light input;
.3mw x .5852
=
.176mw
Maximum light input;
.7mw x .5852
=
.4lmw
These are the minimum and maximum values of the laser
output powers which will be delivered to each photo sensor,
according to the output variations of each laser beam alone.
16
Also, we have to consider the loss of the light energy
when the laser beam goes through the prism, the beam
splitters and the protection glass of the photo sensor.
From the Fresnel equations (6), 4% of the light incident
normally on an air-glass interface will be reflected back.
Since the reflections occur at both sides of the air-glass
boundaries, 8% of total light intensity will be dropped
when the laser beam passes through each glass component.
The light transmittances of each sensor are as follows:
For
01 Sensor:
. 92
X
.92
X
1
Transmittance
of the prism
.92
=
.779
Protelion glass of
the photo sensor
First team
splitter
For
02 and 03 Sensor:
.92
X
Transmttance
of the prism
.92
1
First beam
splitter
X
.92
t
Second beam
splitter
X
.92 = .716
i
Protection glass of
the photo sensor
Figure 2.3.1 shows the minimum and maximum values of the
laser beam which are delivered to each photo sensor.
17
Intensity of
the laser
beam
MIN.
.176mw
MAX.
.4lmw
Figure 2.3.1
Minimum and maximum laser power for each sensor
The spectral responsivity of the SD - 172 - 11 - 11 021 sensor is .27A/W for the wavelength of 632.8nm laser
beam (see Figure 2.3.2).
The worst cases of the photo
sensor, the spectral responsivity of the sensor deviates
~
10% from the linear responsivity of the sensor.
18
.6
•5 '
.4
•3
•2
~
.1
oo ____,___~~~----~--~4-~---+----~--+---~--2 0
300
400
500
600
700
800
900
1000 1100
632.8nm
Figure 2.3.2
Spectral Responsivity for the
SD172-ll-ll-021 Sensor (7)
Therefore, the minimum and maximum spectal responsivity of the sensor will be:
Minimum
Maximum
. 243A/W
.297A/W
The corresponding minimum and maximum output current for
photo sensor ~1 ,~2 and ~3 will be:
Minimum
. 69mw x
f-.1aximum
.16mw x
For ~i:
243 :tvlA
1000 mw
297lVIA
lOOOmw
=
16.767 llA
=
47.52
11A
19
For
~
2 and
~
Minimum
.032mw x
243MA
lOOOmw
=
7.7)..1A
Maximum
.073mw x
297MA
lOOOmw
=
21.68)..1A
3:
With this current input, the photo-conductive mode
is used for the first stage amplifier of the detection
circuit, because the application of the reverse bias
voltage on the sensor will minimize the capacitance of
the photo sensor (see Figure 2.3.3).
To complete the photo detection circuits as shown
in Figure 2.3.3, the feedback resistance for ~1 is
calculated.
Since a typical maximum output voltage of
an operation amplifier is 13V (8), the required maximum
and minimum feedback resistances of the detection amplifier
are calculated as follows:
For minimum input current:
13V
16.76)..1A
=
.775M~
For maximum input current:
13V
47.52)..1A
=
.274M~
Therefore, these different output power levels of each
laser beam will cause the mismatches of the output voltage
levels of the operational amplifiers.
1M~
resister and
1M~
So, the
lOOK~
and
potentiometer as a gain adjustment
are used to cover the whole range of the light intensity
variations.
20
+lSV
1'
R2
lOOKQ \w
Rl
lMQ \w
-v
i!
+V
-v
Figure 2.3.3
Beam detection amplifier for 0 1
The calculation of the feedback resistors for 0 2 and
03 is made the same way as for 01. By using the current
which we calculated for 02 and 03' the minimum and
maximum resistances are:
For 0z and 0 3 .
Max resistance:
R =
13V
7.77fJ.A
=
1.67MQ
Min resistance:
R =
13V
21.68fJ.A
=
.6MQ
Since the required feedback resistance with the minimum
light intensity is 1.67MQ for 02 and 0 3 sensors, the 7SOKQ
fixed resistor and lMQ of the potentiometer are used in
the feedback circuit for the
02 and 03 (see Figure 2.3.4).
21
+15V
l
A.
750KS1
lMS"G
I\Mr----1AAt-""""\
To the
gate
input
to the 8253
counter
ov
Figure 2.3.4
The detection circuit for
~
2
and
~
3
When the edge detection signal is being processed
through the first stage of the amplifier, the output of the
first stage amplifier should be set below the saturation
range of the operational amplifier for the linear mode of
the edge detection operation.
Since the output of each
detection circuit is the analog signal, the model 2311
comparator is used to convert the analog signal to the
digital signal for the gate input for the 8253 counter.
2.4
The Capture Range and Resolution for the Pre-aligner.
A.
Determination of the Pre-aligner Capture Range
The tightest specification between the major flat and
the minor flat occurs on the 3 inch wafer.
Therefore,
the calculation of the capture range is performed with
22
the 3 inch wafer specification.
From the specification
sheet (9), the width of the major and the minor flats in
a 3 inch wafer are shown as Figure 2.4.1.
3.0" !' .015"
Major Flat:
location (1-l-0) + 1°
·Minor Flat:
width
location
45° ± 5°from
the major flat
Width 3/8"
~ 1 "
~
Figure 2.4.1
The Specification of 3" Wafer
The major function of the pre-align operation is to
find the major flat without capturing a minor flat.
Therefore, the worst case occurs when the pre-aligner has
a wafer with minimum width of the major flat and maximum
width of the minor flat.
The working capture range should
be within the difference of the radial distances between
the major flat and the minor flat (see Figure 2.4.2).
The calculation of the capture range is as follows:
23
Minor flat with maximum width
Major flat
with minimum
width
Figure 2.4.2
Minor flat with maximum width and
major flat with minimum width
The radial distance
for major flat
The radial distance
for minor flat
=
=
JcL5)
f .....2
~(1.5)
1.47902 - 1.452369
2
~
-
(.375)
-
(.25)
2
2
=
1.45236
=
1.47902
26 mils
Therefore, the distance of 13 rriils is taken as the capture
range of the pre-aligner, which is half the distance
between the major flat and the minor flat.
B.
The Resolution of the Pre-aligner
The field of view of the TV camera is 2 mils by 3 mils,
which is the capture range of the final alignment.
The
alignment of the pre-aligner should be much smaller than
2 mils.
As will be seen later, the null band width of the
D/A output is ~ 1 count.
But, before the microcomputer
outputs the error counst to DAC 82, the error counts are
24
divided by 2 to have better control over error position
range.
Since the nature of the round trip mode of the
laser beam for the error distance doubles the accuracy of
alignment, the total null band width of the pre-align servo
remains 2 laser counts.
The period of one count is
total time of the death band width is
2.5~s;
Therefore, the
S~s.
position error of the wafer alignment, given
S~s
death band,
will mainly depend on the speed of the scanning laser beam.
Because the scanner is driven by a sine wave signal, the
speed of the scanning beam is varied depending on a beam
position within the scan range.
The three different wafer
locations on the pre-align chuck are shown in Figure 2.4.3a,
and the variations of the speed of the laser beam are shown
in Figure 2.4.3b during one cycle time of the scan driver.
3
Figure 2.4.3a
Three different wafer location
on a pre-align chuck
25
0
1
2 ,_ __
2.25 ___
2.75--~5 ~---------;
3
6
7
75
mils
•
9
10
Figure 2.4.3b
Variations of speed of the laser beam during a cycle
period. Dotted lines show the required minimum time
period for the micro-computer (see Section 2-2) .
. In Case 2 of Figure 2.4.3a, the major flat of a wafer
is located in the middle of the scanning range, and the
resolution of the alignment is the worst case because the
speed of the laser beam has reached the maximum speed.
General equation for the laser beam location during
the scan cycle is:
t
X = 75 sin (2TI .Ol)
If we differentiate eq. (1) :
_ dX _
2Tit
v - -dt - 75 X 2TI
.01 cos .01
At peak velocity:
TI
150 X .Ol
COS 2TI
X ~
---(1)
(mil/sec)
= 47100 mil/sec
26
Since each half period of the scan cycle has the same
speed profile except for the direction of the laser movement, Table 1 shows the speed profile of the first half
scan cycle.
TABLE 1
Speed Profile of the First Half Scan Cycle
Time Period, T,
for each point
2. 5
2.6
2.75
3
3.4
3.8
4. 2
4.6
5
Velocity
0
2.957"/Sec.
7.368"/Sec.
14.554"/Sec.
25.237"/Sec.
34.334"/Sec.
41.27"/Sec.
45.62"/Sec.
47.1"/Sec.
Error Distance
for 1 Count
()()
2.957 X 2. 5 = 7.4]1 11
18.42]1 11
36.38]1 11
63.1]1 11
85.8]1 11
103.18]1 11
114.05]1 11
117.75]1 11
Since the resolution of the pre-aligner is worst at
peak velocity, the worst position error for the pre-aligner
is:
2 x 117.75 = 235.5]1 inches.
2.5
Micro-Computer and Data Processing Unit
The micro-computer measures the time periods of input
pulses for three laser beams, and performs an analysis of
this data to determine where the major flat is located on
a wafer and the relative error of the alignment flat.
micro-computer
motor.
The
then sends the error signal to the drive
The micro-computer and the data processing unit
27
consists of three main components.
a)
Model 8253, 3 independent 16-bit counters (10).
b)
Model 8748, single component micro-computer (11).
c)
DAC82, 8-bit digital to analog converter (12).
The 8253 is a programmable counter designed for use
as an Intel micro-computer peripheral.
It consists of
three independent 16-bit pre-settable, down counters.
The
selection of mode configuration for the counter is software programmable by writing a control word into the
control word register.
Mode 4 is selected in the gate to
enable straight down counting application.
The Intel 8748 is a totally self sufficient microcomputer containing the following functions in a single
40-pin package.
1)
8-bit CPU
2)
lK x 8 ROM program memory
3)
64 x 8 RAM data memory
4)
27 I/0 lines
5)
8-bit timer/event counter
6)
2.5~sec
7)
More than 90 available instructions; all
instructions are of 1 or 2 cycles
or
5~sec
cycle time
The 8748 is user-programmable and erasable EPROM
program memory for prototype system.
The functional
operation of the wafer alignment begins with the Servo ON
signal which is corning from the complete function of the
Raise Chuck at the TO input of the micro-computer.
Then
28
the micro-computer immediately outputs the Fixed Speed
signal to the drive motor which rotates a wafer with a
constant speed until the major flat of the wafer is within
the capture range.
During the time period of T2 (as shown
in Figure 2.2.1) the 8748 micro-computer initializes each
counter by setting the control word and presetting the
bias number.
While the laser beams are scanning under
the wafer, the counts will be inhibited because the gate
inputs are low during that time.
As soon as each laser
beam finds a wafer edge at the beginning of Tl, the gate
input goes to high logic state.
Each counter will start
to count and keep on counting until the laser beam is
obstructed by a wafer edge again.
When each gate input
goes to low state, the corresponding counter will stop
counting and latch the last counter number.
After all the
logic states of the gate inputs of the counters become
the low state, the input of Tl of the micro-computer
transits from the high state to the low state.
Then the
micro-computer reads the data from the counters and
processes the data data until the major flat is within
the capture range.
The DAC82 then servos
the drive motor
until the alignment of the major flat is within the
tolerance of the pre-aligner alignment range.
The 8748
micro-computer has two versions of the cycle times,
2.5~s
or
5~s.
The selection of 6MHz on board oscillator provides
a
2.5~s
cycle time for the micro-computer.
There are
29
two reasons for selecting the faster cycle time.
First,
this faster cycle time of the micro-computer allows
shorter software data processing time.
As shown in
Section 2.2, the shorter data processing time will increase
the acceptable range for mechanical wafer positioning.
The
second reason for faster cycle time is that the null band
width of the pre-aligner is 2 counts.
Since the clock
input of the 8253 counters are driven by ALE signal from
the micro-computer, the resolution of the alignment for
the flat finder will be doubled by having twice as fast
instruction cycle.
The ALE signal from the 8748 computer
occurs only once during each micro-computer cycle.
The operational programming flow chart is shown in
Figure 2.5.1.
30
Wait for
Wafer Edge
*Scanner ON
Reset
(Status Bit and
D/A output)
*Chuck Up
Read Counter
01, 0z, 03
No
...
~~----@
Turn a Wafer
Fixed Speed
cW 3
Set Counti::Jg
Mode for
01, 0z, 03
~o~iut~~-:~--1
----.-----·~
Set The Bias
for 01,02,03
Figure 2.5.1
Programming Flow Chart
31
[
EXIT
Yes
Compute
03 - 01
Is it "-~~--->- -No
-----.,
Negative
Sign?
Yes
Set Sign Bit
Set Sign Bit
c.c.w
c.w.
J
f4-----_ _ _
Compute
(03 - 01)
_Qut:Qut
~o
j
DAC
•·--
Figure 2.5.1
(Continued)
Chapter 3
System Analysis
This chapter discusses the overall system performance
through servo control theory.
3.1
Motor and Tachometer Specification
The only external load the drive motor has is the
pre-aligner chuck itself.
Therefore, the total moment of
inertia of the pre-aligner consists of the moment of
inertia of the motor, tachometer and the pre-aligner chuck.
Figure 3.1.1 shows the dimensions of the pre-aligner chuck
and the calculation of the moment of inertia.
a
J,
b =.219"
t
=
1.8" _ _ _....:......,....1
ll.
...._____
r z-rUpp_e_,.r________
c = .8"
Iz Bottom
---:.1
1~
d
= .37"
f
Figure 3.1.1
Dimensions of the pre-aligner chuck
Volume of upper part of the chuck:
3
2
rr x C-1-)
x b = .556 in
Volume of bottom part of the chuck:
TI
X
(-¥-) 2
X
.8
=
.086 in 3
32
33
Since the density of the aluminum is 1.394 Oz/in 3 ,
the moment of inertia of the pre-aligner chuck is:
Iz
= ~MR 2 = ~ . density . volume . R2
Iz upper = ~ x 1.394 Oz/in 3 x .556 in 3 x (.9) 2 in 2
1
1 sec 2
ft
2
x 3 2 x y 2 ft
. in
.0008175 Oz - in
sec
2
IZ bottom = ~ X 1.394 X .086 X (.185) X ~ 2 X 2
i
= .0000053 Oz
Iz = Iz upper
in - sec 2
+
Iz bottom = .000823 Oz-in-sec
2
From the specification (13):
Moment of inertia of the motor, Jm; .0006 Oz - in - sec 2
Moment of inertia of the tach, Jth; 1.3 x lo-4 Oz-in-sec 2
Therefore, the total moment of inertia is:
Jtot
= Jmotor
+ Jtach + Jchuck
= .001553 Oz - in - sec 2
The block diagram for a motor and the transfer
function for the motor is as follows:
e
Vin
,Figure 3 .1. 2
Transfer function for the motor
8
Vin =
1
Kb
JR S
Lm
S ( 1 + f t + Kb Kt) ( 1 + -R- S) , Kb Kt > > BR
34
From the specification sheet:
Kb
=
.042 V/rad/sec,
Kt
=
5.920z, J
Lm
=
2mH , R
=
=
.001553 Oz - in
sec
2
9.9~
Then, the transfer function for the motor:
8
Vin =
3.2
23.8
S (1 + .062 S)
(1 + .000202 S)
Breakaway Voltages of the Motor and the Position
of the Pre-align Servo.
To determine the required system gain, the calculation
of the breakaway volts is carried out:
From the specification:
Static friction (Tf)
Voltage at Tp ; Vp
Peak torque
= 15
=
; . 5 Oz - in
25V
Oz - in
So, the breakaway voltage
25V x .5 Oz - in
15 Oz - 1n
for the 1500C - 050 motor is:
= .83V
Therefore, the minimum input voltage for the pre-aligner
motor should be more than .83V, but the maximum breakaway
voltage for the motor reaches as high as 1.6V by individual
test of the 1500C - 050 motor itself.
The DAC 82 is an
8-bit D/A converter and is connected for complementary
binary format.
8-bit D/A;
128 counts for lOV
1 count
=
78.12MV
To achieve the breakaway voltage with 1 count:
35
78.12MV
3.3
X
KA = 1.6V,
KA = 20.5
The Fixed Speed and the Tachometer Gain Setup
Because the operation of the pre-aligner is parallel
to the other subsystem operations, the throughput of the
pre-aligner operation is not a critical item.
Therefore,
3 seconds per revolution is taken as the period of the
pre-aligner chuck during the fixed speed mode.
Therefore,
the fixed speed of the pre-align chuck is about 2rad/sec.
The calculation of the tachometer voltage gain for the
fixed speed mode is as follows:
The voltage level for the fixed speed
= .0645V/rad/sec x 2 rad/ sec x KA
I ...
Tac h Sens1t1v1ty
1
Tach Voltage Gain
Since 12.5V is chosen as the output voltage for the fixed
speed,
KA =
12.5V
.0645 V/rad/sec x 2 rad/sec
= 193.8
This gain is implemented at the tachometer feedback circuit.
3.4
The Overall System Gain
Since the pre-aligner station is capable of handling
any size wafer (3", 4", or 5"), different system gain
occurs for each wafer size.
The highest gain is achieved
for 5" wafer because the separation of the two outside
position sensors are the widest for this wafer, as it
36
is the most unstable case as far as the servo analysis
is concerned.
As shown in Section 2.4, the resolution of
the pre-alignment at t = 2.75ms is
18.42~".
Therefore,
the resolution of the pre-aligner for the 5" wafer is,
(see Figure 3.4.1):
2
X 18.42~"
=
36.84~"
f
Counts for the null band width
Figure 3.4.1
Resolution of the Pre-aligner for 5" Wafer
tan '9 =
the resolution of the pre-aligner
the separation of the two outside sensors
=
9
=
36.8~"
1.4
X 106~ 11
tan-l 2.63 x 10- 5 ~ 2.627 x 10- 5 rad
Th:e. two laser counts for the null band width of the
D/A will generate 156.24mv.
The overall system gain will
be:
The D/A output from the null band width
The rotational position error
=
156.24 X lo- 3v
.02627 x l0-3 rad
~
5945.88 V/rad
37
3.5
System Analysis
The transfer function for the summing junction (see
Figure 3.5.1) is:
Figure 3.5.1
Summing Amplifier
=
3
-3
51 X 10
(1 + .54 X 10 S)
(2754 x lo-6s2 + 14.82 x lo-3s + 1)
Figure 3.5.2 shows the block diagram of the overall
transfer function of the system.
38
8;
5945
----@--
21.25 (1+.54xlo-3s)
(.27Sxlo-os2+14.82xlo-3s+l)
I
-
I
.r~(1+.062S)23.8(1+.0002S) I eo -
6.4SS
1
-
Figure 3.5.2
The Overall Transfer Function for the System
To simplify the overall transfer function of the
system, the inside feedback loop of the transfer function
will be solved as follows:
Inner Loop
= T = 1 + GxGxH
505.75 (1+.54xl0- 3 S)
T = S(.27Sxlo-6s 2 +14.82xlo-3s+l) (l+.062S+l2.4xlo-6s 2 )
l+ sos.7s x 6.45 (1+ .54 x lo-3s) s
S(.275xlo-6s2+14.82xl0-3s+l) (l+.062S+l2.4xl0- 6 s 2 )
s
=
.155 (1+ 1852)
s
S(l + 50815)
s
)
(1 + 3776.8
2
(l + 1.244S +
s
)
2237.97
2237.97
505.75(1+.54xl0- 3 S)
Let Gx = S(.27Sxlo-6s2+14.82xlo-3s+l)(l+.062S+l2.4xl0-5s2)
39
Thus, the open loop transfer for the overall system will
be:
~
--
921.5
s
(1 + 1852)
s
s
(l+l.244S s2
S(l+ 50815)
(l+ 3776.8)
+ 2238)
2238
From the Bode plot (Figure 3.5.3), the OdB crossover
frequency of the system is 900 rad/sec with a slope of
20 dB per decade, which is about 1/6 the electrical break
frequency.
This will keep the amount of phase lead to a
minimum which will minimize the noise signal.
The phase
margin of the system is about 57° and the gain margin is
about 13 dB.
Therefore, the pre-alignment system is in
fairly stable servo range.
From the Nichol's chart (see
Figure 3.5.4), we can get the peak frequency Wm, at 900
rad/sec.
We can also get the peak overshoot, Mm = 1.06
(20 Log Mm = . 5 db, Mm = 1.06).
The dumping ratio of the
pre-aligner system for the peak over shoot is
using the equation Wm = Wn
~
1
~
= . 7.
By
the dominant
frequency of the system Wn, is:
J
Wm = 9 0 0 = Wn
1 - 2 ( . 7)
Wn = 6365 rad/sec.
2
From the servo control theory, the system settling time is
the time required for the oscillations decrease to a
40
specified absolute percentage of the final value and thereafter remain less than this value.
A common error percent-
age which is used to determine settling time is 2%.
There-
fore the calculation of the settling time is as follows:
.
.
S ett 1 1ng t1me
=
s4 Wn
: : : . 9ms ( ± 2%)
Figure 3.5.5 shows the overall system control
circuit.
41
Figure 3.5.3
The System Bode Plot
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The System Nichol's Chart
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Chapter 4
Conclusions
As a result of this design of the laser flat finder
mechanism, the wafer pre-alignment by a scanning laser
beam has proved to have accomplished several improvements
compared to the previous photo coupled mechanism of the
pre-aligner station.
The switching concept of the laser
beam not only enables alignment of transparent wafers,
which was not possible with the photo coupled mechanism,
but also provides lesser dependence on the sensitivity of
the photo sensor, thus improving averall alignment accuracy.
The implementation of the micro-computer as a function
controller simplifies the system hardware logic.
It also
improves the reliability of the pre-align station.
The only disadvantage of the laser pre-aligner is
the added complexity of the optical arrangement, necessitating extra adjustment steps of the pre-aligner station
for the optical components.
The most important improvement made by the laser
pre-aligner is that any kind of transparent wafer can
be used on the pre-aligner station.
43
44
Also, further improvement of the alignment accuracy
can be achieved by reducing junction capacitance of the
photo sensor.
This will improve the laser response time.
Thus, future application of a pin diode as a photo sensor
will improve this alignment accuracy.
A limitation of the pre-aligner in multi-system
operation is the matching of pre-aligner orientations
from one system to the other.
This rotational mis-match
from one system to the other can be corrected by future
addition of wafer rotational capability in the main system
controller.
REFERENCES
(1)
Hughes Laser Products, "Operating and Maintenance
Manual for Helium Neon Laser Systems", pp. 2 "-' 18,
1977.
(2)
Melles Griot, "Helium Neon Laser Oxide", pp. 15 "-' 17.
(3)
Bulova Electronic Division, "Linear Scanners",
Bulletin 106, Rev. 1.
(4)
Burr-Brown, "General Catalog", pp. 4-79"-'4-89, 1979.
(5)
Burr-Brown, "General Catalog", pp. 1-103"-'l-108,
1979.
(6)
Hecht Zajac, "Optics", pp. 75"-'80, 1976
(7)
Silicon Detector, "General Purpose Detectors", 1977.
(8)
National Semiconductor, "Linear Data Book",
pp . 3- 191 ""' 3 -19 3 ' 19 7 6 .
(9)
Book of Semi Standards, "Materials", pp. Ml.l.STD.2
"-' Ml.l.STD.7, 1979.
(10)
Intel, "MCS - 48™ Family of Single Chip Microcomputers", pp. 7-93 "-' 7-103, 1978.
(11)
Intel, MCS - 48™ Family of Single Chip Microcomputer", pp. 2-1 "-' 6-8, 1978.
(12)
Burr-Brown, "General Catalog", pp. 5-68 "-' 5-74, 1979.
(13)
Magnetic Technology, "DC Servo Motors and DC
Tachometers, pp. 53, 1975.
45