Adaptive servo control system

An adaptive position control system including X, Y and .theta. stages and their associated positioning and motion detection apparatus, position detection apparatus for detecting the position of the stages relative to a fixed reference, X and Y position control subsystems responsive to signals developed by the motion and position detection apparatus and operative to generate coarse mode and precision mode drive signals for application to the X and Y stage positioning apparatus, X and Y adjust subsystems for enabling adjustment of the position detection apparatus, and a .theta. adjust subsystem operative in a coarse mode and precision mode for adjusting of the angular orientation of the .theta. stage. Operator control input to the various subsystems is accomplished by means of a joystick control mechanism and central processing unit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to servo control systems and more 
particularly to a nonlinear, adaptive position control system particularly 
suited for use in multi-axis control systems having dual mode servo 
subsystems for energizing a single drive mechanism per axis to effect 
precision positioning of a work piece relative to a work apparatus and/or 
optical device. 
2. Description of the Prior Art 
Although numerous analog control systems and techniques are known in the 
prior art, there is a class of adaptive position servo systems in which 
the overall positional accuracy and the required wide dynamic range of 
travel do not lend themselves readily to a purely analog solution. For 
example, very few analog reference devices can achieve a linearity in 
excess of one part in 10.sup.6. 
Even though others have used laser distance measuring interferometer 
techniques to accomplish such accuracies, these systems have typically 
required the use of two distinct drive mechanisms. For example, as 
disclosed in the U.S. patents of Hassan et al, U.S. Pat. Nos. 3,904,945 
and 4,016,396, a coarse positioning stage is used in combination with a 
fine positioning stage. More specifically, the coarse positioning stage is 
used to position the apparatus in the vicinity of the desired position and 
is latched in such position while the fine positioning stage, which has a 
much smaller range of travel, is used to make the final precise movement 
to the desired location. Obviously, such systems involve a substantial 
degree of mechanical complexity which could be reduced considerably if a 
single mechanical drive stage per dimensional degree of freedom were 
required to perform both the coarse and precision positioning operations. 
SUMMARY OF THE PRESENT INVENTION 
It is therefore an object of the present invention to provide an adaptive 
work piece positioning control system which operates in both a high slew 
mode and an ultra precise positioning mode to actuate a single drive 
mechanism per dimensional degree of freedom. 
Another object of the present invention is to provide a multiple-axis work 
piece positioning apparatus having adaptive control subsystems which 
precisely and quickly position with high positional accuracy those 
carriage elements which are moved along the several axes. 
Briefly, the preferred embodiment of the present invention includes X, Y 
and .theta. stages and their associated positioning and motion detection 
apparatus, position detection apparatus for detecting the position of the 
stages relative to a fixed reference, X and Y position control subsystems 
responsive to signals developed by the motion and position detection 
apparatus and operative to generate coarse mode and precision mode drive 
signals for application to the X and Y stage positioning apparatus, X and 
Y adjust subsystems for enabling adjustment of the position detection 
apparatus, and a .theta. adjust subsystem operative in a coarse mode and 
precision mode for adjusting of the angular orientation of the .theta. 
stage. Operator control input to the various subsystems is accomplished by 
means of a joystick control mechanism and central processing unit. 
Among the numerous advantages of the present system is that the stages can 
be driven to a selected position very quickly and with a high degree of 
precision. 
Another advantage of the present invention is that operator adjustments of 
stage position can be easily made by the use of a single joystick 
controller. 
An additional advantage of the present invention is that position control 
data from the microprocessor is input in digital form to the several 
subsystems using a single bus line. 
Still another advantage of the present invention is that an extremely 
accurate and stable position detection and control system is provided. 
Other objects and advantages of the present invention will no doubt becom 
apparent to those skilled in the art after having read the following 
detailed description of a preferred embodiment which is illustrated in the 
several figures of the drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A nonlinear adaptive control system in accordance with the present 
invention is illustrated in FIGS. 1, 2 and 3 of the drawing with each 
figure containing a portion of the system. A layout of the entire system 
can be obtained by juxtaposing the three figures as indicated by the 
layout shown as FIG. 4. In FIG. 1 there is shown, in exploded schematic 
form at 10, a positioning mechanism including a .theta. stage 11, an X-Y 
stage 12, a reference plate 13, and a photodetection system 14. Also shown 
are block diagram schematics of an X position control subsystem 16, a Y 
position control subsystem 18, a central processing unit 20 and a joystick 
position controller 22. In FIG. 2 there is shown an X adjust subsystem 24, 
a Y adjust subsystem 26 and a serial-to-parallel shift register 28 for 
loading data into the subsystems 24 and 26. In FIG. 3, the .theta. adjust 
control subsystem is shown at 30. 
Although the system of the present invention can be used to control any of 
a variety of ultra precise positioning devices, for simplicity of 
illustration, the wafer-positioning mechanism of a semiconductor 
processing apparatus with repeatable positional accuracies on the order of 
several microinches is depicted with the several components thereof 
inverted. Such components are typically arranged such that the upper 
surface of .theta. stage 11 (the bottom surface as illustrated) is adapted 
to support a semiconductor wafer W which is to be processed. 
It will be appreciated by those skilled in this particular art that the 
.theta. stage and the X-Y stages are of standard air-bearing supported 
configuration with the circular stage 11 and its several components being 
carried by the X stage 32 which, along with the Y stage 34, is 
air-supported upon an ultra flat and stable base platform which for 
simplicity is not shown. Fixed to the surface (lower as illustrated) of X 
stage 32 is a .theta. drive servo motor 36 which drives an eccentric cam 
38 and a servo potentiometer 40. Cam 38 engages a side of the arm 39 and 
serves to rotate the stage 11 about its axis in response to a control 
signal applied to motor 36. A spring (not shown) biases the bar 39 against 
cam 38. Positional feedback information is obtained from a linear voltage 
differential transformer (LVDT) 42 which has its stator mounted to X stage 
32 and its core connected to a slip clutching mechanism 41 attached to 
.theta. stage 11. Also attached to stage 32 are limit stops 43 and 45, the 
end of which are spaced apart approximately 0.020 inches in the preferred 
embodiment so as to accommodate the .+-.0.010 inch travel limits of LVDT 
42. As will be explained in more detail below, this allows the same drive 
mechanism to be used for both coarse and fine positioning of the stage. 
Y stage 34 is driven by a servo drive motor which is affixed to the stage 
support platform (not shown) and provides stage drive by means of a metal 
drive capstan 46 which provides metal-to-metal engagement with a drive arm 
33 of Y stage 34. Motor 44 also drives a tachometer 48. An X stage servo 
motor 50 is carried by a projection 35 of X stage 32 such that metal 
capstan 52 is likewise in metal-to-metal driving engagement with a drive 
arm 37 of Y stage 34. Servo motor 50 also drives a tachometer 54. 
The reference plate 13, as perhaps better shown in the partial illustration 
of FIG. 5, is typically a glass plate upon which by photolithographic 
techniques a reference pattern is deposited having an array of rectangular 
openings 15 that are precision configured and precision located relative 
to each other so as to provide reference apertures that can be optically 
followed by the photodetection system 14. In addition, a continuous border 
aperture 17 is also included to allow for automatic initial stage 
positioning and limit stop sensing. Alternatively, the inverse pattern 
could, of course, be used. In the preferred embodiment the apertures 15 in 
the reference plate are squares that are approximately 10 microns on a 
side and arrayed on 20 micron centers. A typical reference plate contains 
approximately 50 million reference apertures. Where the desired 
positioning resolution is greater along one axis than along the other, 
rectangular reference apertures may be used instead of square ones. 
The photodetection system is attached or referenced to the base platform 
and includes self-illuminating, magnifying and/or focusing optics 60, a 
detection reticle 62, four photodiodes 64, and servo motors 66 and 68 for 
driving eccentric cams 70 and 72, respectively, that impart fine trim 
adjustment to the system relative to the reference plate 13. Circular 
eccentric cams mounted within a precision ball bearing are used in the 
preferred embodiment because of their ease of manufacture, but precision 
lead screws, etc., can also be used. It is very important that their 
surfaces be very smooth and free of "negative bumps". The optics 60 in the 
preferred embodiment provides approximately 10.times. magnification of the 
reference apertures but could just as well provide any other desired 
magnification. For example, an increase in magnification will permit 
larger absolute positioning error in the X-Y adjust stage for the same 
overall system accuracy. Thus, the positional accuracy of the X-Y adjust 
stage does not have to be as precise as the main X-Y stage if the 
magnification is greater than unity. 
Reticle 62 includes four sets of elongated openings, one pair 61 of which 
is comprised of elongated rectangles having their longitudinal dimensions 
oriented in the X direction, and a second pair 63 of elongated rectangles 
which are similarly oriented in the Y direction. The photodiodes 64 are 
positioned such that each receives light focused through one of the four 
sets of apertures in the reticle 62. As indicated in FIG. 5 by dashed 
lines 61a and 61b, and 63a and 63b, one of the aperture sets in each pair 
is aligned relative to the other set in its pair such that its apertures 
are approximately 90.degree. out of phase relative to the apertures of the 
other set when compared to the images of the reference plate openings cast 
thereupon by optics 60. This allows the system to not only determine 
alignment with a particular X-Y location but to also determine the 
direction in which the stage is moving relative to the X and Y axes. For 
example, as illustrated in FIG. 6, if at the time of signal coincidence 
the amplitude of the signal detected through apertures 61a is decreasing, 
and that detected through apertures 61b is increasing, as indicated at 65, 
then the stage is known to be moving in the positive Y direction. If, on 
the other hand, the opposite is true, then the stage must be moving in the 
negative Y direction, etc. 
For automatic alignment, it is desirable that the sensing means be able to 
detect an edge along one axis without interference from an edge along 
another axis. One way in which this can be insured is to arrange the 
diodes and mask as illustrated in FIG. 7 rather than as shown in FIG. 1. 
In this embodiment the photodiodes 64' and corresponding apertures 61' and 
63' of reticle 62' are positioned as illustrated so that, for example, 
when one of the sets of apertures 61' is focusing light from a reference 
plate border onto its photodiode, neither one of the sets 63' are focusing 
border light onto their photodiodes. 
In the preferred embodiment, the optics 60 is secured to the system base 
platform, and the photobodies 64 and reticle 62 are movable relative 
thereto as a unit. Positioning feedback signals indicating displacement of 
system 14 in the X and Y directions are provided by LVDT devices 74 and 
76, respectively. Obviously, the clearance between the outside diameter of 
the LVDT core and the inside diameter of its coil must be slightly in 
excess of the peak-to-peak travel of the X or Y adjust subsystems 
approximately 0.030 inch in this particular application. 
Turning now to the adaptive control system of the present invention, it 
will be noted that the CPU system 20 includes a keyboard 19 for inputting 
new "desired position data" into a microprocessor 21, and input and output 
interface units 23 and 25, respectively. Inputs to CPU 20 for causing 
additional position adjustments are made through a joystick device or 
other suitable position controller 22. It will further be noted that the X 
position control subsystem 16 and the Y position control subsystem 18 are 
essentially identical (except for inertia compensation), and each includes 
a rate feedback amplifier 80, a variable attenuator 82, a dc feedback gain 
and equalizer subcircuit 84, a photodiode detector, pulse generator and 
differential amplifier circuit 86, a servo amplifier 88, an up/down 
counter 90, a comparator/difference generator 92 and a shift register 94. 
For convenience of description, the components of subsystem 16 are 
indicated by unprimed call-out numbers while the corresponding like 
components of the Y position control subsystem are indicated by primed 
call-out numbers. 
The system is initially aligned by positioning the X and Y stages with a 
particular region, such as a corner or two sides of the border surrounding 
the reference rectangular patterns of the reference plate 13 disposed in 
the viewing area of optics 60. The up/down counters 90 are at the same 
time reset to zero or any other convenient reference count by a signal 
from CPU 20 so as to indicate a start position in the X and Y directions. 
For those applications in which automatic alignment is to be included, X 
and Y edge detectors 91 and 93 are provided for sensing the border 
aperture 17 and causing an appropriate signal to be input to CPU 20. Such 
detection is accomplished when the aperture 17 causes the output of one of 
the photodiodes 64 to exceed a threshold level such as is shown at 69 in 
FIG. 6. 
In order to move the stages to a new position, a set of X-Y "desired 
position data" is input to the shift registers 94 via keyboard 19, the 
controller 22 or (under automatic control) by microprocessor 21 for 
comparison with the counts in counters 90. As a result, the difference 
generator 92 will generate signals on one of the lines 102-108 
representing the difference therebetween depending upon whether the 
comparison indicates A=B, A&gt;B, A&lt;B, or .vertline.A-B.vertline.&lt;.delta., 
where A is the actual position and B is the desired new position and 
.delta. is the distance within which the system can stop without 
overrunning the desired stopping point by more than one-half the 
center-to-center spacing of adjacent openings in the reference plate 13. 
The signals developed on line 102-108 are voltages which are used to vary 
the gain control applied to the servo amplifiers 88. The gain control 
networks of the servo amplifiers 88 include resistor 110, 112, 114 and 
116, the variable attenuator 82, the inverter 118 and the switch 120. In 
one embodiment, attenuator 82 is driven to one of two predetermined values 
determined by the signal developed on line 108 and the output is varied 
relative thereto by the output of rate feedback amplifier 80 as input at 
111. Since line 108 is, as indicated, caused to have one signal state when 
the difference between A and B is less than .delta. and is caused to have 
another signal state when the difference between A and B is greater than 
.delta., it will be appreciated that it is this output which determines 
whether the control system is operating in its coarse alignment mode or in 
its fine alignment mode. 
In the case where A.sub.x is less than B.sub.x, as for instance when the 
stage is to move, or is moving, in the positive X direction and wherein a 
positive voltage signal is developed by generator 92 on the appropriate 
output line (i.e., line 106 when the A.sub.x &lt;B.sub.x condition is 
satisfied), the signal is inverted by inverter 118 and passed through 
resistor 112 to the servo amplifier 88. This causes the servo output to 
have a polarity which energizes servo motor 50 in the direction to drive 
the X stage in the positive direction. As the speed of the servo motor 
increases, an output signal from tachometer 54 develops (increases 
positively) and this output is amplified by rate feedback amplifier 80 and 
input to the variable attenuator 82. The attenuated signal is then passed 
through resistor 114 to the current node 113. The rate feedback current 
input through resistor 114 is subtracted from the A.sub.x &lt;B.sub.x current 
signal passing through resistor 112 such that the resultant current input 
to servo amplifier 88 is reduced and in turn reduces the drive signal 
applied to servo motor 50. Ultimately, an equilibrium condition is reached 
with the servo motor 50 driving the tachometer 54 at a speed such that its 
output is at a level whereby the difference in the currents flowing 
through resistors 112 and 114 is exactly that required to cause the servo 
amplifier 88 to drive the servo motor 50 speed at its current value. 
When the condition A.sub.x &lt;B.sub.x occurs, generator 92 will produce a 
positive digital output signal on line 104 (instead of line 106). This 
signal is not inverted, and after passing through resistor 110 causes 
servo amplifier 88 to drive servo motor 50 in the opposite direction from 
the previous case. This, of course, results in a similar reversal in 
polarity of the output signal from tachometer 54. And similarly, the 
currents flowing into summing point 113 through resistors 110 and 114 
oppose one another to produce an equilibrium speed for a given applied 
voltage at 104. 
In either case, where the difference between A and B becomes less than 
.delta., an output will also be developed on line 108 causing attenuator 
82 to change to its more sensitive range. Where A equals B, line 102 will 
be raised and cause switch 120 to close and couple the output of the dc 
feedback gain and equalization circuit 84 to the summing point 113 through 
resistor 116. As a result, the servo loop will be caused to lock into the 
desired position without becoming unstable. 
In general, it can be said that there is an inverse relationship in a 
linear feedback servo system between positional accuracy and overall 
system speed of response which results from satisfying the Nyquist 
Stability Criterion. To overcome some of the drawbacks of the inverse 
relationship between accuracy and speed of response, the above-described 
electronics switching and/or processing schemes are included in the servo 
system to produce a highly nonlinear system having overall performance 
characteristics which yield a fast response time with high positional 
accuracy. 
Typically, and as in the illustrated embodiment, the reference devices and 
their associated detection circuitry produce a pair of signals in phase 
quadrature to one another which are cyclic or repetitive. As was 
previously indicated, in the illustrated embodiment shown in FIG. 5, this 
is accomplished by aligning the focused image of the grid pattern with one 
of the reticle sets so that light from the reference plane grid pattern 
reaches the reticle sets detector while the apertures of the other set of 
the pair permits only one-half of the subtended grid pattern to fall onto 
its corresponding detector. Thus, as the reference plane, and thus the X-Y 
stages, move relative to the photodetection system, the output of the 
photodetectors will be approximately triangular but 90.degree. out of 
phase with each other, as indicated in FIG. 6. 
Each intersection of the photodetector-signals input to differential 
amplifier 86 represents a potential analog stopping position. To stop at 
the intersections indicated in the lower half plane 67, the polarity of 
both signals must be inverted by additional circuitry not shown in the 
drawing. However, without digital processing the analog servo amplifier 88 
cannot distinguish the difference between nuls in adjacent cycles; 
therefore, the number of cycles traversed is recorded in the digital 
up/down counter 90 which remains in the active state even during the 
analog stopping or locking mode. The appropriate phase quadrature 
condition is required for the accurate bidirectional counting of the 
cycles. It is not a required condition for the analog locking mode. In 
fact, for analog locking, a pair of signals from the reference element 
with 180.degree.-phase relationship may be preferred, but is not required. 
As previously indicated, the present adaptive servo system involves the use 
of two feedback paths: (1) a position feedback path through amplifier 86; 
and (2) a rate or higher derivative feedback path through rate feedback 
amplifier 80. The output of both paths are to be adjusted in a manner 
based upon the relative position of the stage from its desired position. 
This method of adjustment results in a very nonlinear system. In general, 
the output of the position feedback path, containing the frequency 
compensation and equalization networks required (when used in conjunction 
with the rate feedback path which will render the system stable), is 
disabled from the analog servo until the desired cycle has been counted. 
Although there are numerous ways to disable this position feedback path, 
normally it is best to achieve the disabling without adversely affecting 
the stored charge on the frequency compensation networks (which could 
result in transient delays). In the illustrated embodiment, the switch 
120, which responds to the output developed on line 102 of the comparator 
92, is shown positioned in the circuit connecting resistor 116 and the dc 
gain and equilization circuitry 84. It will be appreciated, however, that 
this is a schematic illustration and the actual location of the switching 
means 120 will most likely be within the circuitry 84 so as to eliminate 
any adverse effect of charge stored in the circuit during open switch 
conditions. 
The rate feedback path receives its input from a precision, wide dynamic 
range tachometer such as that indicated at 54 and which is connected to 
the motor or drive assembly 50. During the high speed coarse mode of the 
stage, for example, when either the A&lt;B or A&gt;B output is present at 
circuit mode 119, the amount of rate feedback applied to the summing point 
113 is 0, or is relatively small compared to the amount of rate feedback 
signal applied near the desired stopping position. 
In one of the simplest embodiments, a small amount of rate feedback is 
applied to maintain a stable and known speed .omega..sub.1 for a fixed 
coarse input signal during the fast travel mode of the stage. Then the 
distance d.sub.1 required to decelerate from .omega..sub.1 to 
.omega..sub.2 can be calculated from the dynamics of the system. In this 
embodiment .omega..sub.2 is the speed from which the servoed stage can 
stop within less than approximately one-half cycle overshoot of the 
reference feedback element, i.e., 10 microns in the preferred embodiment. 
If the allowed overshoot exceeds one-half cycle, then extra precautions in 
the design must be taken to prevent an unstable limit cycle which can 
result from the non-linear switching of the system. The rate feedback path 
output to the control loop must be increased from a value which yields a 
system speed of .omega..sub.1 to a value which reduces stage speed to 
.omega..sub.2 when the difference between the actual position of the stage 
and the desired position is slightly in excess of .delta.. 
In a more general embodiment, one may increase the rate feedback or the 
higher order derivatives per some functional relationship to 
.vertline.A-B.vertline. in order to achieve a desired deceleration 
response. Also, in the case where small stage steps are anticipated, the 
stage may never reach .omega..sub.1. Thus, to begin decelerating at 
.vertline.A-B.vertline.=.delta. may result in unnecessary time delay. In 
lieu of using the more complex functional embodiment one could use a 
multi-step process of increasing the rate feedback to achieve an overall 
faster response time and a smoother deceleration. However, the simple 
embodiment involving a single step method of increasing the rate feedback 
until .omega..sub.2 has been obtained should not be overlooked because in 
many actual applications the inertia of the system will adequately smooth 
out the step applied rate feedback signal. It may be desired to increase 
the rate feedback even further when the dc position feedback path is 
switched into the servo drive loop as this may make it easier to stabilize 
the resulting position servo system. 
By adaptively adjusting the rate feedback path in accordance with the 
present invention and as noted above, it is not necessary to change the 
input coarse mode signal until the desired cycle of the reference element 
has been obtained. The coarse mode input signal (A&gt;B or A&lt;B) can be 
applied and removed in digital manner. Thus, it is not necessary to apply 
a D-to-A converter to the .vertline.A-B.vertline. output signal and to 
apply the resulting analog signal to the servo amplifier input as shown in 
U.S. Pat. No. 4,015,396. This feature simplified the electronic circuitry. 
Although the basic X-Y control system has been described above with respect 
to a particular application, it will be appreciated that certain 
alterations and modifications may be made. For example, the stage driving 
mechanisms could be comprised of any type device that gives a mechanical 
displacement for some known excitation input signal condition. Such 
devices are typically electromagnetic, hydraulic, pneumatic or 
piezoelectric motors, etc. 
Moreover, the illustrated rate feedback path could include any rate or 
higher order derivative determining elements whose output signal magnitude 
is varied adaptively in some prescribed relationship to the final stopping 
position. In general, this feedback path will have increased output 
intensity (gain) as the stage nears its final stopping position. The gain 
of the rate feedback path is to be varied according to the desired 
deceleration and the amount of overshoot to be allowed. The gain may be 
varied in a stepped mode by a switching circuit or by smoothly adjusting a 
gain determining element (such as by modulating the ON resistance of a 
field effect transistor by varying its gate signal). Likewise, the rate 
feedback signal may be processed by a computer and its output increased 
automatically through the use of a D-to-A converter and fed into the 
analog rate feedback path. It is predominately the proper wide range 
adjustment of this path that allows the high speed coarse travel mode and 
also allows the final stopping position to be stable for extremely small 
displacement errors. 
The system requires a stable reference element to which the actual system 
position can be compared. Whereas, as illustrated in the preferred 
embodiment, one may use a very precise pattern contained on a very stable 
substrate material mounted either to the stage or to a fixed reference 
point of the system, alternatively, and as will be discussed below 
relative to the embodiment of FIG. 8, one may use a distance measuring 
laser interferometer or the like. The system also requires a suitable 
sensing means for detecting and comparing the pertinent information on the 
reference element. If the reference element is attached to the movable 
stage, then the sensing element should be attached to some fixed reference 
position within the system. Alternatively, these two elements could be 
interchanged. 
During the high speed coarse mode the position feedback path is essentially 
disconnected from the analog portion of the servo system to prevent 
interference. However, this dc position information which is in the form 
of digital counts or pulses must be accurately stored and monitored 
because it is needed to determine when and how far the stage is from its 
desired position. Thus, the position information is used to initiate and 
control the tailoring of the gain in the rate feedback path. Finally, when 
the stage position is very near its desired position, the dc position 
feedback path is energized in the analog system and the coarse control 
signals are removed. However, the coarse control signals are automatically 
energized if the stage position error exceeds a predetermined value. It is 
important to note here that the system must still satisfy the normal 
stability criterion and provide subsequent compensation networks for both 
the rate feedback path and the dc position path. One aspect of this 
invention is that the frequency compensation networks in the DC position 
path can remain fixed for large variations in gain of the dc position path 
provided the gain of the rate feedback path is adjusted and 
correspondingly tracks. 
From the above, it will be appreciated that the system will always stop the 
X and Y stages in precisely the position identified by the input 
coordinates. However, it will be also appreciated that an occasion may 
arise wherein it is desirable that the stages be smoothly or incrementally 
adjusted to a position in between one of the stop positions dictated by 
the reference plate or be rotated relative to the reference plate. In 
order to permit adjustments in the X and Y directions, the X and Y trim or 
adjust subsystems 24 and 26 are provided so as to allow one viewing the 
wafer W or other work piece on the X-Y stage through a microscope (not 
shown) to make the desired incremental adjustment by simply moving the 
joystick J of position controller 22 in the appropriate direction. 
In the preferred embodiment, the position controller 22 is a switching 
device having a joystick J which when pushed a short distance to one side 
or another causes a particular fine adjust signal to be applied to the 
corresponding line leading to CPU 20. When joystick J is pushed sideways 
to its limit, it causes a coarse adjustment signal to be applied to the 
corresponding input line. In addition, when joystick J is twisted in the 
clockwise direction, it causes a -.theta. signal to be input to CPU 20 and 
when twisted in the counterclockwise direction causes a .div..theta. 
signal to be input to CPU 20. When the joystick is twisted in either 
direction to its limit, it causes a coarse alignment signal to be input to 
CPU 20. 
Although the joystick control unit may take many forms and may even include 
separate joystick units for providing X-Y and .theta. inputs, the 
illustrated device is comprised of a grounded ring contractor (not shown) 
which is attached to the joystick J. Positionally arrayed around the body 
of controller 22 at orthogonal positions are four inner contacts (not 
shown) each of which are connected to one of the CPU input leads 1-4. The 
inner contacts are positioned so as to be engaged by the contractor when 
joystick J is tilted slightly in the direction of the contact(s). In 
addition, an outer ring contact is provided outside the inner contacts and 
is positioned so as to be engaged by the contactor when J is tilted to a 
larger degree in the direction of any of the orthogonal contacts. The 
outer ring contact is coupled to a CPU input line 5. Since each of the 
lines 105 is normally coupled to a voltage source V+ through a resistor R, 
the effect of causing the ring contactor to contact one or more of the 
line contacts is to pull that particular line or lines to ground and thus 
provide a corresponding input to the CPU 20. Similarly operative structure 
responsive to twisting of joystick J provides the .theta. drive signals. 
It is important to note, however, that when the joystick control is 
embodied in a single unit, provision must be made to insure that in making 
.theta. control adjustments, the operator does not inadvertently input X-Y 
control input and vice versa. This could be accomplished by providing a 
suitable lockout feature or by merely providing adequate dead bands 
between the OFF and ON stick positions. 
In response to the inputs received from controller 22, the microprocessor 
21 develops digital position signals which are serially input to shift 
register 28. In this case where coarse control is desired, the digital 
signal stored in shift register 94 is incremented or decremented. When 
fine control is desired, the digital signal stored in shift register 28 is 
incremented or decremented. The incrementing or decrementing occurs at a 
predetermined rate per the programmed sampling rate of the microprocessor 
as long as the joystick is actuated. 
In the fine adjust control mode, the X coordinate is input into register 28 
and the latch 120 is enabled by the CPU 20 so that is receives the X drive 
signal and outputs it into the D-to-A converter 122. This signal is then 
amplified by amplifier 128 and fed through resistor 130 and summing point 
132 into servo amplifier 134 which drives the corresponding servo motor. 
Positional feedback to the X and Y adjust systems is taken from the LVDT 
units 76 and 74, and is input to the corresponding amplifier and 
demodulator circuitry 136, the outputs of which are fed into the 
differentiator and rate feedback amplifiers 138 for input to the summing 
points 132 through resistors 140, and to the dc position feedback 
amplifiers and equalization circuitry 142 for input to the summing points 
132 through the resistors 144. Adaptive control of the rate feedback path 
is not required because for this subsystem a dynamic range of 1000:1 is 
adequate. On the other hand, the "main" positioning subsystems 16 and 18 
have a dynamic range of approximately 10,000,000:1. 
Referring now to FIG. 3 of the drawing, a similar application of the 
above-described control circuitry is depicted as used to provide smooth or 
incremental phase adjustment of the .theta. stage. As in the X-adjust and 
Y-adjust embodiment, both coarse and fine adjustment inputs are provided 
to CPU 20 via controller 22. Although many types of joystick 
configurations could be used to obtain the desired control inputs, in the 
preferred embodiment, a simple two-position right and two-position left 
rotary switch is coupled to joystick J so that when the joystick is turned 
a few degrees in either direction, either CPU input line 6 or 7 is 
grounded. If joystick J is turned further in either direction, the coarse 
alignment line 5 will also be grounded. 
In this embodiment, a first shift register 150 and D-to-A converter 152 are 
provided for receiving the coarse control signal and a second shift 
register 151 and D-to-A converter 153 are provided for receiving the fine 
alignment signal. A bank of switches 154 shown in the coarse alignment 
configuration are actuated by an output developed by the inverting 
amplifier 155 in response to the grounding of coarse alignment line 5. As 
illustrated, the coarse aligment signal is entered into register 150, 
converted to analog form by converter 152 and input through switch 156 and 
resistor 157 to dc position amplifier 158. The output of amplifier 158 is 
then fed into servo amplifier 159 which develops a drive signal on line 
160 for input to motor 36. 
In response to the coarse drive signal, motor 36 turns cam 38 against arm 
39 to rotate the stage 11 about its axis. If the rotation exceeds the 
spacing between the ends of stops 43 and 45, the slip clutch arm 41 will 
engage the stop and slip relative to stage 11. Note that during the coarse 
alignment cycle the switches 161 and 163 are open and thus any output of 
LVDT 42 is ignored. However, during the coarse alignment operation servo 
potentiometer 40 develops an output which is fed back along line 162 and 
through switch 165 and integrating capacitor 166 into the rate feedback 
amplifier and equalization circuitry 168 which in turn provide rate 
feedback control to servo amplifier 159. The servo potentiometer output is 
also coupled into amplifier 158 via switch 167 and resistor 169 to provide 
a position feedback signal input to servo amplifier 159. 
When the coarse alignment is completed, if fine alignment is desired, the 
joystick is turned back to one of its fine alignment positions maintaining 
a ground on one of the lines 6 or 7 and a fine alignment input developed 
by CPU 20 is loaded into register 151. At the same time, as the grounding 
contact to line 5 is broken, the input to inverter 155 will again go high 
and cause switches 154 to swing into their opposite positions. As a 
result, the analog output developed by converter 153 is coupled through 
switch 170 and resistor 171 into position amplifier 158. 
In this mode, as motor 36 causes stage 11 to slowly revolve, the output of 
servo potentiometer 40 is not fed back to servo amplifier 159 but instead, 
the output of LVDT device 42 is amplified and demodulated by unit 172 and 
the resultant output thereof is fed into amplifier 158 through switch 161 
and resistor 173. The output of unit 172 is also fed into the rate 
feedback amplifier and equalization circuitry through switch 163 and 
integrating capacitor 174. The feedback signals developed by units 158 and 
168 are then fed into servo amplifier 159 to provide the required control. 
When the operator, who might for example be viewing the wafer W through a 
microscope, has determined that the wafer is in the correct position, he 
merely releases the spring-loaded joystick and allows it to return to its 
0 position and the positioning operation is complete. 
One of the features of the present invention is that joystick information 
is converted into digital signals which are processed by the CPU and sent 
over a single data line 100 to any one of the several control subsystems 
and then converted to analog signals for driving the appropriate servo 
motors. More specifically, X position data is loaded from line 100 into 
register 94 by raising enable line 181, Y position data is loaded from 
line 100 into register 94' by raising enable line 182, X adjust data is 
loaded from line 100 into register 28 and then latch 120 by first raising 
line 183 and then line 184, Y adjust data is loaded from line 100 register 
28 and then latch 124 by first raising line 183 and then line 185, coarse 
.theta. adjust data is loaded from line 100 into register 150 by raising 
line 187, and fine .theta. adjust data is loaded from line 100 into 
register 151 by raising enable line 186. As a result, the number of lines 
running between CPU 20 and the control subsystems are very small. 
Note that a single joystick is used to simultaneously control the main X-Y 
servo motors 44 and 50, the trim or adjust servo motors 66 and 68, and the 
.theta. adjust servo motor 36. This concept can, of course, be extended to 
any number of servo subsystems where it is desired that a single joystick 
or other multi-axis control mechanism and/or a single digital output data 
line from the CPU be used to control the various subsystems. Note also 
that even though individual enable lines are shown for each subsystem, a 
group of common lines could be used to enable the subsystems in 
binary-coded fashion, i.e., two lines including the single common line 100 
could be used to enable two subsystems, three lines could be used to 
enable up to four subsystems, four lines up to eight subsystems, etc. 
However, in this particular case, the use of a few extra control lines is 
considered simpler than the introductions of a binary decoder complete 
with transient eliminator. 
This CPU single line control concept is very good for servo control systems 
because the CPU serial output data bus is so much faster than the response 
times of the servo control subsystems. This control concept would be 
suitable for any type of control system wherein the device to be 
controlled is slow compared to the serial output data bus rate. The use of 
the joystick in the manner described results in the provision of a 
quantized output signal for application to the servo subsystems. As such, 
the servo motors, or stage motion, also becomes quantized. This means that 
the stage will step in finite steps. Although it it possible to make the 
steps as small as desired, there may be applications where this 
quantization is not desired. If this be the case, then the joystick can be 
connected to servo potentiometers or be entirely replaced by servo 
potentiometers. The output of such control potentiometers can then be 
connected directly to the servo motor amplifiers, in which case no CPU 
processing of these inputs is required. 
Referring now to FIG. 8 of the drawing, there is shown a modification of 
the present invention in which a laser interferometer assembly 200 is used 
in place of the grid reference plate 13 and its associated optical 
detection equipment. The assembly 200 includes a laser 202, a beam 
splitter 204, a pair of beam benders 206 and 208, an X-direction measuring 
interferometer 210, and a Y-direction measuring interferometer 212. The 
reference plate 213 has two orthogonal side edges 214 and 215 precision 
ground so as to be precisely 90.degree. relative to each other. These two 
edges are used as reflective mirrors for the interferometers 210 and 212. 
The light outputs of the interferometers 210 and 212 are then respectively 
input to laser receivers (such as the HP 10780A) 201 and 203 which in the 
drive up/down pulse generators 286 in the X-position and Y-position 
control circuits as in the previously described circuit. However, since 
the output of a laser interferometer operating in a near infrared 
frequency range is typically quantized about 100 to 400 times finer than 
the digital output obtainable from the above-described grid reference 
plate, the pulse generators 286, the up/down counters 290, 
comparator/difference generators 292, and shift registers 294 must have 
their resolution capabilities increased by the same amount, i.e., by 
approximately eight additional binary bits. Furthermore, by inputting the 
data contained in the added approximately eight least significant bits 
into a digital-to-analog converter 295, an output signal can be obtained 
which is suitable for input to the dc feedback gain and equalization 
circuitry (see 84 in FIG. 1), and the remainder of the circuitry of the 
previously described X-position and Y-position subsystems can be retained. 
Note, however, that the extra resolution from the laser interferometer, it 
is no longer necessary that the X-adjust and Y-adjust subsystems shown in 
FIG. 2 be employed and, therefore, the previously described drive elements 
66 and 68 and their associated elements 60 through 76 can also be 
eliminated. Thus, a very fine resolution high speed, single drive per axis 
system is obtained. 
The extra eight least significant bits added to the elements 290-294 
permits the stage drive units 44 and 50 (FIG. 1) to locate the stage in 
previously unaccessible positions. Obviously, the digital adjust 
information which the CPU 20 previously applied to the adjust subsystems 
24 and 26 is now added to the digital input applied to the shift registers 
294. 
Although the laser system may appear to be simpler than the previously 
described embodiment, it must be remembered that a laser interferometer 
must be enclosed within a tightly controlled environmental chamber to 
achieve its inherent accuracy. Thus, the ambient temperature, atmospheric 
pressure and humidity must be known or controlled. 
On the other hand, with proper selection of the materials used for the 
previously described reference plate 13, the environmental conditions may 
not be as severe. For example, there is a readily available glass material 
suitable for use in the previously described reference plate which has a 
temperature coefficient less than 0.2 ppm/.degree.C. 
With the grid reference plate, the orthogonal relationship between the X 
and Y stages is maintained as a result of the appropriate placement of the 
grid squares on the reference plate. Thus, it is not necessary to align 
the X and Y assemblies at precisely the correct angle, i.e., 90.degree. 
for an X-Y stage. However, with a laser interferometer reference element, 
one must use two orthogonally aligned reference mirrors. The alignment of 
these two mirrors is very critical, and each time the mirrors are 
diassembled, the critical alignment process must be repeated. But by using 
two precision ground edges of a stable reference plate, (ground to 
precisely the desired angle, i.e., typically 90.degree.) as the reference 
reflecting mirrors, one may avoid any further precision alignment in order 
to obtain the precision orthogonal relationship between the stage drive 
assemblies. With presently available rotary index tables this angle may be 
ground to an accuracy of approximately 0.1 arc second. 
Furthermore, if the plate also uses an optically and/or electromagnetically 
detectable border such as that shown at 217 and which corresponds to the 
boundary of the travel limits of the X-Y stage, then the stage can be 
automatically aligned using this border. Although the inverse situation 
could also be employed, typically the laser would be mounted in stationary 
relationship to the reference frame, and the plate would be attached to an 
moved with the X-Y stage. 
Use of the border on the plate for automatic alignment and automatic limit 
stop also requires the use of appropriate border detection apparatus such 
as the mask 262, a linear displacement photodiode 264 (and/or other plate 
detection means) and X-edge and Y-edge detectors 291 and 293. Obviously, a 
simplified border could be produced by using the edges of the plate 213 as 
the border. However, to achieve the required alignment accuracy, it is 
believed to be more appropriate to photochemically form a precision 
border, such as that shown at 217, onto the plate. 
FIG. 9 is an enlarged view of section 9--9 in FIG. 1. This cross sectional 
view shows the inter-relationship between X-Y-.theta. stages 32, 34, and 
11, the reference substrate 13, and the self-illuminating microscope 60. 
Most self-illuminating microscopes produce a background noise light at the 
viewing position because of the internal reflections off the microscope 
lenses that are common to both the viewing optics and the light source 
illuminating optics. Also, the reference substrate on which the reference 
pattern is deposited typically has a non zero reflection coefficient. 
These undesired reflections produce a background light level to the photo 
detectors even when the reflective grid pattern is not within optical 
view. Subsequently, the maximum signal-to-noise (S/N) ratio that can be 
obtained from the detection system is reduced. 
In those applications where it is possible to use a transparent substrate 
material, it is possible to eliminate the need for a self-illuminating 
microscope. FIG. 10 shows a transparent reference plate substrate 313 
illuminated from the back side. In this embodiment the grid pattern would 
typically be the inverse or "negative" of the pattern shown in FIG. 1, 
i.e., the grid squares are transmissive rather than reflective in this 
case. Microscope 360 is required only when it is desired to provide 
magnifying and/or focusing optics to focus the grid pattern onto the 
reticle 62 of FIG. 1 as before. 
For space considerations it may be desireable to locate light source 302 
external to the stage and to employ a light pipe or fiber optic bundle 304 
to carry the light to the desired location behind the grid reference 
plates as shown in FIG. 10. In this case a beam reflector 306 containing a 
mirrored surface would typically be connected or affixed to the end of the 
light pipe or fiber optic bundle to provide a compact means of deflecting 
the light path by 90.degree. and directing the light beam to the back of 
transparent or translucent reference plate 313. Obviously, the 
illuminating light source pattern in the region of interest 307 (area 
focused by microscope onto detector 64 of FIG. 1) should be as uniform as 
possible for optimum signal results. 
As the need for greater and greater positional accuracy and alignment 
developes for manufacturing the processing semiconductors containing 
smaller and smaller geometries, it becomes apparent that in order to 
obtain the alignment accuracies required it will be necessary to get the 
positional feedback information as close as possible to the surface of the 
silicon wafer being processed. Ideally, one would like to include 
positional indicia for automatic alignment and positional control directly 
on the wafer as is the current practice for manual alignment. However, 
although numerous alignment marks have been incorporated into the mask 
deposition patterns, these techniques to date do not lend themselves 
readily to completely automatic alignment processing equipment. Among 
other problems, to illuminate the wafer adequately to enable detection of 
such markings could cause exposure of the photo resist coatings. Also, the 
various mask overlays and photo resist coatings tend to alter the markings 
and the appearance of their surroundings making each exposure step in the 
manufacturing process correspond to a different pattern recognition 
problem. In addition, the markings can interfer with the overall yield 
obtainable from a given wafer. 
One way to overcome many of the difficulties is to apply the reference 
positional pattern directly to the back side of the wafer, i.e., use the 
wafer itself as the reference substrate material referred to herein-above. 
Numerous types of patterns could then be used including absolute, binary 
coded, positional type patterns. FIG. 11 shows a configuration with the 
reference position information 4B being deposited directly on the back of 
the wafer W. In this embodiment the reference substrate is positioned 
directly on the final .theta. stage 411. The .theta. stage contains a high 
quality optical transparent plate 412 for permitting observation of the 
reference pattern by the viewing optical self-illuminating microscope 460. 
In this case a pattern border is used to initially align the .theta. stage 
as well as to provide initial alignment to the X stage 434, and Y stage 
432. 
Since the placement of the wafer on the optical plate 412 cannot guarantee 
exact centering, the initial alignment of the X, Y and .theta. stage is a 
complex process. By using a reticle detector such as that shown in FIG. 7, 
two corners of the border may be detected through the use of an 
appropriate stage control program stored in CPU 20. Then, with the 
combined use of a control program and a computational program, the 
required alignment of the .theta. stage can be made. At this point the 
automatic alignment of the X and Y stages can proceed as before. Use of 
the embodiment of FIG. 11 makes it possible to provide a completely 
automatic alignment control system for wafer processing. 
Although the present invention has been described in terms of presently 
preferred embodiments for precisely controlling the positioning operation 
of a semiconductor processing mechanism, it will be appreciated that the 
disclosed system is equally applicable to other types of precision control 
apparatus which utilize controls in other than the X, Y and .theta. axes. 
Accordingly, it is intended that the appended claims be interpreted as 
covering all alterations, modifications, extensions and applications as 
fall within the true spirit and scope of the invention.