Title: Rotational stage with vertical axis adjustment
Abstract: A stage system is disclosed for supporting and positioning a semiconductor wafer for inspection in an optical metrology device. A chuck for supporting a wafer is mounted to the stage system. The stage system can move the chuck along two linear orthogonal axes. A rotational stage is also provided for rotating the chuck. A mechanism is provided for adjusting the vertical position of a chuck to allow for focusing of the probe beam of the metrology device. The vertical adjustment mechanism is designed so that it does impede the rotational positioning of the chuck.
Patent Number: 6,917,420 Issued on 07/12/2005 to Traber
| Inventors:
|
Traber; Thomas (Dublin, CA)
|
| Assignee:
|
Therma-Wave, Inc. (Fremont, CA)
|
| Appl. No.:
|
863305 |
| Filed:
|
June 8, 2004 |
| Current U.S. Class: |
356/237.4; 356/399; 356/244 |
| Intern'l Class: |
G01B 011/00 |
| Field of Search: |
356/2371-2375,399-401,244
355/18,53,54
|
References Cited [Referenced By]
U.S. Patent Documents
| 4328553 | May., 1982 | Fredriksen et al.
| |
| 4457664 | Jul., 1984 | Judell et al.
| |
| 4750141 | Jun., 1988 | Judell et al.
| |
| 4872835 | Oct., 1989 | Tullis et al.
| |
| 5066131 | Nov., 1991 | Iwata et al.
| |
| 5083757 | Jan., 1992 | Barsky.
| |
| 5182615 | Jan., 1993 | Kurosawa et al.
| |
| 5670888 | Sep., 1997 | Cheng.
| |
| 5717482 | Feb., 1998 | Akutsu et al.
| |
| 5906860 | May., 1999 | Motoda et al.
| |
| 5923408 | Jul., 1999 | Takabayashi.
| |
| 6249342 | Jun., 2001 | Cheng.
| |
| 6771372 | Aug., 2004 | Traber.
| |
| 2001/0036742 | Nov., 2001 | Wytman.
| |
| 2002/0021428 | Feb., 2002 | Nakano et al.
| |
| 2002/0159054 | Oct., 2002 | Ebert et al.
| |
| 2003/0003848 | Jan., 2003 | Tobin.
| |
| Foreign Patent Documents |
| 1061558 | Dec., 2000 | EP.
| |
Primary Examiner: Pham; Hoa Q.
Attorney, Agent or Firm: Stallman & Pollock LLP
Parent Case Text
CLAIM OF PRIORITY
The present application is a continuation of U.S. patent application Ser. No.
10/178,623, filed Jun. 24, 2002, ROTATIONAL STAGE WITH VERTICAL AXIS ADJUSTMENT,
now U.S. Pat. No. 6,771,372 B1, which claims the benefit of U.S. Provisional Application
Ser. No. 60/336,530, filed Nov. 1, 2001, ROTATIONAL STAGE WITH VERTICAL AXIS ADJUSTMENT,
each of which is hereby incorporated herein by reference.
Claims
1. A stage system for supporting a sample, comprising:
a horizontal chuck capable of supporting a sample;
a vertical axis actuator coupled to said chuck in order to provide for vertical
movement of the chuck, the vertical axis actuator including a PZT element being
adjustable in length, and a lever element for transferring adjustments in length
of the PZT element to the chuck as vertical movement; and
a rotational driver operationally coupled to the chuck for rotating the chuck
about a central axis.
2. A stage system according to claim 1, further comprising:
a rotational stage positioned to support said chuck, the rotational stage including
at least one flexure member coupling the chuck to the stage such that tilting and
lateral movement of the chuck, with respect to the stage, is significantly minimized
while the chuck is being raised and lowered.
3. A stage system as recited in claim 1, further including first and second linear
stages for moving the horizontal chuck along first and second orthogonal horizontal axes.
4. A stage system according to claim 1, further comprising:
a first annular flexure operationally coupled between the rotatable driver and
the horizontal chuck; and
a second annular flexure operationally coupled between the rotatable driver and
the horizontal chuck, with said second annular flexure being located parallel to
and spaced apart from the first annular flexure.
5. A stage system according to claim 4, wherein each of said first and second
annular flexures is substantially rigid in a direction in the plane of the respective
annular flexure and flexible in a direction along a central axis of the respective
annular flexure.
6. A stage system as recited in claim 4, wherein the first annular flexure has
a different diameter than, and is positioned concentrically with respect to, said
second annular flexure.
7. A stage system as recited in claim 4, wherein, the first annular flexure is
displaced vertically with respect to the second annular flexure.
8. A stage system for supporting a sample, comprising:
a chuck having an upper surface for supporting a wafer, the chuck further having
a central element opposite said upper surface;
an axial actuator operationally coupled to the chuck for raising and lowering
the chuck in a direction orthogonal to a plane of the upper surface, the axial
actuator including a push rod capable of being driven in the orthogonal direction
and an coupling body for coupling the push rod to the central element of the chuck;
a hose element coupled to the coupling body such that a source of vacuum can
be applied through the central element to a vacuum opening in the upper surface
of the chuck; and
a bearing positioned between the coupling body and the central element, the bearing
allowing the central element to be rotated a full 360 degrees with respect to the
coupling body, independent of the raising and lowering of the chuck and without
rotation of the coupling body and hose element.
9. A stage system as recited in claim 8, further including first and second linear
stages for moving the chuck along first and second orthogonal horizontal axes.
10. A stage system according to claim 8, further comprising:
a rotational driver operationally coupled to the chuck for rotating the chuck
about a central axis.
11. A stage system for supporting a sample, comprising:
a chuck having an under surface for supporting a wafer, the chuck further having
a central element opposite said upper surface;
an axial actuator operationally coupled to the chuck for raising and lowering
the chuck in a direction orthogonal to a plane of the upper surface, the axial
actuator including a push rod capable of being driven in the orthogonal direction
and an coupling body for coupling the push rod to the central element of the chuck;
a hose element coupled to the coupling body such that a source of vacuum can
be applied through the central element to a vacuum opening in the upper surface
of the chuck;
a bearing positioned between the coupling body and the central element, the bearing
allowing the central element to be rotated a full 360 degrees with respect to the
coupling body, independent of the raising and lowering of the chuck and without
rotation of the coupling body and hose element; and
a stage positioned to support said chuck, the stage including at least one flexure
member coupling the chuck to the stage, such that tilting and lateral movement
of the chuck, with respect to the stage, is significantly minimized while the chuck
is being raised and lowered.
12. A state system for supporting a sample, comprising:
a chuck having an upper surface for supporting a wafer, the chuck further having
a central element opposite said upper surface;
an axial actuator operationally coupled to the chuck for raising and lowering
the chuck in a direction orthogonal to a plane of the upper surface, the axial
actuator including a push rod capable of being driven in the orthogonal direction
and an coupling body for coupling the push rod to the central element of the chuck;
a hose element coupled to the coupling body such that a source of vacuum can
be applied through the central element to a vacuum opening in the upper surface
of the chuck;
a bearing positioned between the coupling body and the central element, the bearing
allowing the central element to be rotated a full 360 degrees with respect to the
coupling body, independent of the raising and lowering of the chuck and without
rotation of the coupling body and hose element;
a rotational driver operationally coupled to the chuck for rotating the chuck
about a central axis;
a first annular flexure operationally coupled between the rotatable driver and
the chuck; and
a second annular flexure operationally coupled between the rotatable driver and
the chuck, with said second annular flexure being located parallel to and spaced
apart from the first annular flexure.
13. A stage system according to claim 12, wherein each of said first and second
annular flexures is substantially rigid in a direction in the plane of the respective
annular flexure and flexible in a direction along a central axis of the respective
annular flexure.
14. A stage system as recited in claim 12, wherein the first annular flexure
has a different diameter than, and is positioned concentrically with respect to,
said second annular flexure.
15. A stage system as recited in claim 12, wherein, the first annular flexure
is displaced vertically with respect to the second annular flexure.
Description
FIELD OF INVENTION
The present invention relates to metrology tools with rotating stages having
integrated Z-axis adjustment for inspecting a wafer.
BACKGROUND
This invention relates to optical metrology tools of the type described in U.S.
Pat. No. 6,278,519, incorporated herein by reference. Referring to prior art FIG.
1, these types of tools include a light source for generating a probe beam 7,
which is focused onto a semiconductor wafer 4. Changes between the incident
probe beam 7 and the reflected beam are monitored to evaluate characteristics
of the sample 4.
Tools of this type typically include a motion stage for supporting the wafer
4. Various stage motion combinations are available including full X-Y stages;
R/theta stages; and ½X-½Y plus theta stages (where theta means 360 degrees
of rotation). Prior art FIG. 1 exemplarily illustrates an X-stage 22, a
Y-stage 24, and theta stage 26. The motion of the stages is computer
controlled for moving the wafer into position with respect to the focused spot
of the probe beam 7.
These tools also typically include a focusing (preferably autofocusing) system,
which brings the wafer into the focal plane of the focusing optics of the measurement
system 2. A number of these systems operate to translate the focusing optics
in a vertical direction with respect to the sample. Alternatively, the stages themselves
are provided with some form of vertical (Z-axis) movement for focusing purposes.
Since the motion system needs to be designed to fit within the available height
3, conventional Z-axis stages that utilize guide rails are difficult to
integrate. The length of the profile moving along the guide rails directly affects
the Z-axis' stiffness against tilting movement. Where the length of the moving
profile is limited by the available height, the tilting movement of the moving
parts becomes hard to control. In order to reduce the tilting movement, the contact
pressure between the moving profile and the guide rails needs to be increased,
which results in increased friction and consequently increased actuating forces.
High friction and actuating forces again reduce the movement resolution in Z-axis.
Therefore, there exists a need for an apparatus and method for highly
precise vertical micro adjustment of a rotating stage with minimal friction and
a maximum stiffness against tilt movement and lateral movement.
Conventional linear guiding systems define the movement direction by
either a sliding or a rolling contact. This is feasible where an extensive movement
range needs to be covered. In this application, the required Z-axis movement range
is only twenty thousandths of an inch. Providing Z-axis movement over that small
a range with sliding or rolling guides still requires a relatively bulky and heavy
assembly, which increases the moment of inertia of the motion system. As a consequence,
the motion system moves more slowly.
Therefore, there exists a need for a Z-axis guiding system that is low
in mass as well.
BRIEF SUMMARY
A wafer motion system includes one or two conventional linear stages and a rotating
stage, which are mounted on top of each other. The one or two linear axes are horizontal.
The rotating stage is placed at the top and is configured for holding a wafer and
rotating it around a vertical axis of revolution. The one or two linear stages
have a travel range defined in combination with the rotating stage to position
the wafer with respect to the probe beam. The wafer is placed on a chuck and held
down by a vacuum provided between wafer and chuck.
The chuck itself is guided along the axis of revolution within the rotating stage.
Specifically configured and placed flexures or membranes elastically guide the
chuck without any substantial friction. The membranes easily deflect in the vertical
direction while being highly rigid in horizontal direction. Preferably, at least
two ring shaped membranes are vertically positioned relative to each other. The
horizontal stiffness of each membrane in combination with the vertical offset between
them results in a high stiffness against tilt.
The rotationally symmetric design of the membranes allows them to be easily integrated
into the generally rotationally symmetric design of the rotating stage. The relatively
small mounting space required for mounting the membranes results in little additional
volume and mass necessary for integrating the membrane rings in the rotating portion
of the rotating stage.
The membranes provide a substantially friction free guidance of the chuck allowing
for a smooth and precise actuation and adjustment. A horizontally oriented piezo
stack is utilized to provide the vertical actuation of the chuck via a lever system,
which amplifies and transforms the horizontal expansion of the piezo stack into
a vertical movement of the required range. The vertical lever movement is transmitted
to the chuck unit via a central linking assembly, which provides for initial adjustment
and preload of the actuator to the chuck. The linking assembly also receives external
vacuum and/or pressure air and transmits it into the chuck unit.
The vertical movement system is actuated by a voltage applied to the piezo stack,
which expands in accordance to the well-known principles of piezo elements. The
amount of horizontal piezo stack movement is in the micron range. The lever system
amplifies the piezo movement by a factor of approximately 15.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 schematically shows a metrology tool of the prior art.
FIG. 2 schematically illustrates the function of the present invention
FIG. 3 shows a simplified front view of a stage system having a Z-axis motion
system in accordance with the present invention.
FIG. 4 is a perspective view, partially in section, of the subject assembly.
FIG. 5 shows a simplified section view of the rotating stage in a view direction
perpendicular to the view direction of FIG. 3.
FIG. 6 shows a perspective bottom view including the assembled chuck, flexures
and base.
FIG. 7 shows a perspective bottom view including the chuck, the flexures and
the Z-axis motion system.
FIG. 8 shows two exemplary flexures of the preferred embodiment in concentric
arrangement as assembled for operation.
FIG. 9 shows lower a perspective bottom view including the chuck, the flexures
and the Z-axis motion system.
DETAILED DESCRIPTION
FIG. 1 generally illustrates a wafer support and movement system
2 in
which the subject invention can be incorporated. The system
2 is of the
type that would be incorporated in a metrology tool for optically inspecting semiconductor
wafers. Such metrology tools are described, for example, in U.S. Pat. Nos. 6,278,519
and 5,608,526, the disclosures of which are incorporated herein by reference. Such
tools are capable of performing various measurements including spectroscopic reflectometry
and spectroscopic ellipsometry.
In such metrology tools, a probe beam
7 is focused onto the sample surface.
The reflected probe beam is monitored to evaluate characteristics of the sample.
In these systems it is important to accurately focus the probe beam on the sample.
In some systems, focusing is achieved by moving the optics within system
2
(not shown). In others, focusing is achieved by moving the stage system supporting
the wafer in the Z-axis perpendicular to the sample surface. The subject invention
is intended to permit focusing by Z-axis movement of the stage. The mechanism for
providing such Z-axis motion, while allowing complete 360 degrees of rotation of
the wafer, will be discussed below.
As seen in FIG. 1, the stage system can include linear stages
22,
24
and a rotating stage
26. The first linear stage
22 provides movement
along a linear axis
21 perpendicular to the view plane of FIG.
1.
On top of the first stage
22 is mounted the second stage
24 moving
along a linear axis
23. On top of the second stage is mounted the rotating
stage
26 rotating around an axis of revolution
25. The present invention
introduces a Z-axis motion system that is preferably integrated in the rotating
stage
26, while permitting a full 360-degrees of rotation of the rotating
stage
26. It is noted that the scope of the invention is not limited by
a specific configuration and/or number of stages. The detailed configuration and
operation of the inventive Z-axis motion system is described in the following.
FIGS. 2 to
9 illustrate features of the invention. FIG. 2 is the most
simplified schematic which is useful for illustrating the mechanical Z-axis operation
of the system. FIGS. 3 and 5 are also somewhat simplified to illustrate the overall
structure. FIGS. 4 and 6 to
9 illustrate specific details of the construction
of the preferred embodiment.
As seen in these Figures, an actuator, illustrated generally as
30, is
provided for controlling the vertical movement of the chuck
20 which supports
the wafer
4. The vertical movement is indicated by the arrow A. The actuator
acts via cup
40 directly on push rod
60 which is preferably substantially
coincident with the axis of revolution
25 of the chuck (which is co-axial
with the axis of the rotary stage
26). The vertical movement is transmitted
from the push rod
60 via adjustor
62, coupling body
64, and
a bearing
80 onto the central element
70, which is attached to a
chuck
20. The chuck
20 is attached to and coupled with the base
50
via a flexure
54. The central element
70 is attached to and coupled
with the base
50 via a flexure
52. The central element includes channel
19 which transmits vacuum received through bore
76 in coupling body
64 to grooves
81 in the chuck
20 for holding the wafer in
place during measurement.
The flexures
52,
54 hold the chuck
20 together with the
central element
70 and any other attached elements substantially rigid against
lateral movement and/or tilt movement with respect to base
50. Only movement
in vertical direction along the axis of revolution
25 is provided. The scope
of the invention includes embodiments, where the central element
70 and
the chuck
20 are one piece or separate pieces mechanically connected to
one another.
The base
50 is attached onto a rotating portion
27 of the stage
26. Rotational movement induced by the rotating stage
26 is transmitted
via the base
50 and the flexures
52,
54 onto the central element
70 and the chuck
20. The non-rotating portions
28 and
53
of the stage
26 are affixed on the linear stage
24 as is shown in
FIGS. 1 and 3. Rotational movement is decoupled from the elements
60,
62,
and
64 by the bearing
80.
In the preferred embodiment, the flexures
52,
54 are concentrically
arranged with respect to the axis of revolution
25. Each of the flexures
52,
54 provide rigidity against lateral movement. The concentric
and vertically offset arrangement of the flexures
52,
54 combines
the lateral stiffness of each of the flexure
52,
54 advantageously
to make the chuck
20 and the central element
70 additionally highly
stiff against tilt movement and provides for a highly precise linear movement performed
by the chuck
20 in direction along the axis of revolution
25.
The actuator
30 includes a PZT stack translator
38 to which members
43 and
44 are attached. Members
43 and
44 have spherical
ends (and are shown as balls in FIG.
2). The lever
34 of the actuator
is a single piece incorporating a stationary base section
34A and a rotating
lever section
34B connected by a thin hinge element
36. One end of
the PZT stack translator is held rigid against the base section of the lever
34B
via member
43, adjusting screw
35 and clamping plate
39. The
opposite end of the PZT stack translator contacts the lever section of the lever
34B via member
44, and retainer plate
45.
The PZT stack
38 is a well known horizontally stacked number of piezo
elements. Increasing the voltage applied to the PZT stack
38 causes it to
expand; decreasing the applied voltage causes the PZT to contract. The expansion
movement of the PZT stack
38 is transmitted via the member
44 onto
the lever
34B, which transforms the horizontal movement into an angular
movement around the fixed hinge
36, while at the same time amplifying it
by a factor of approximately
15. Consequently, the point of contact
41
moves upward. The lever
34B thereby redirects the horizontal expansion movement
of the PZT stack
38 into a vertical movement suitable for Z-axis adjustment
of the chuck
20 and a wafer
4. The upward positioning of the chuck
and the bending motion of the flexures is shown in phantom line in FIG.
2.
The expansion range of the PZT stack
38 is approximately 33 microns and
the angular movement of the lever
34B is 0.30 degrees, such that the point
of contact
41 moves off-axis of the rotary stage and chuck. This misalignment
is allowed and absorbed by the cup
40 and links
60 and
62,
which has a ball and socket joint at each end.
The lever
34B serves to amplify the expansion of the PZT stack
38
by a factor that is essentially defined by the proportion between distance
31
and
32. Distance
31 is between hinges
36 and
44. Distance
32 is between
36 and
41. In the preferred embodiment, the
amplification is approximately 15 times the expansion of the PZT stack
38.
The movement amplification and redirection introduced by the lever
34B provides
for sufficient design space for the PZT stack
38 without compromising the
overall height of the motion system. In the preferred embodiment, the PZT stack
has a diameter of ⅜ of an inch and a length of about three inches.
The PZT stack
38 expands proportionally in response to the applied voltage.
In practice, it is difficult to control height directly based on the applied voltage
due to friction and and/or deformation along the movement path between the PZT
stack
38 and the chuck
20. Therefore, in the preferred embodiment,
the vertical positioning is controlled by a feed back system wherein the vertical
height of the wafer is monitored with a focusing mechanism
110 above the
wafer. Errors in focusing are monitored and used is a feedback loop to the voltage
controlling the PZT stack. In the preferred embodiment, the focus system
110
provides a signal that is processed by a processor
112 into a second signal,
which is transformed preferably by a linear variable differential transformer
114
(LVDT) into a voltage applied to the PZT stack
38. It is clear to one of
ordinary skills in the art, that any device for precise distance or position measurement
may be utilized instead or in addition to the auto focus system
11.
To initially setup the vertical position of the chuck
20, adjustor
62
may be threaded in or out of the push rod
60, which lowers or lifts the
chuck
20. Adjustor
62 is seated and held in coupling body
64,
the detailed function of which will be explained with respect to FIGS. 4 and 5.
As seen in FIG. 4, the lever
34B is preferably configured as a partly
separated
portion of an enclosure
34A-
34B enclosing the PZT stack
38.
The enclosure
34A-
34B has a central cavity configured to encapsulate
the PZT stack
38 and the elements
43,
44. The hinge
36
is preferably configured as a membrane that bridges the fixed portion
34A
of the enclosure with the moveable portion
34B. The membrane hinge
36
is sufficiently flexible to absorb the angular movement of the lever
34B.
A frame
37 is mounted on the enclosure and hold anti-rotation pins
72
(see also FIGS.
4 and
7). Pins
72 keep the elements
60,
62, and
64 aligned and non-rotating.
In the preferred embodiment, the PZT stack
38 is biased at a central voltage
value, which is defined as the mid-range or central Z-axis position of the chuck
at which the flexures
52,
54 may be in a neutral non deflected state.
By increasing or decreasing the voltage to the PZT stack
38, the chuck
20
can be raised or lowered. This is compensated by the flexures
52,
54
by bending with their inner portion upwards or downwards. In the preferred embodiment,
the chuck can be moved ± ten thousandths of an inch. The flexures
52,
54 are configured to absorb this vertical movement well within their elastic
deformation range. The chuck
20 is pre-loaded via springs (not shown) vertically
downward relative to the base
50. This ensures that, as the PZT stack expands
and contracts as a result of the varying the applied voltage, intimate mechanical
contact is maintained through parts
34B,
41,
60,
62,
64,
80,
78, (
52) and
20. The movement can be
used to focus the probe beam of light onto the wafer
4.
One important aspect of this invention is that the structure allows the chuck
20 to be rotated by 360 degrees. Bearing
80 serves thereby to transfer
the upward thrust forces while at the same time minimizing friction torque and
eliminating surface-to-surface rubbing during rotation. The pins
72 prevent
push rod
60 and coupling
64 from being rotated by the torque resulting
from the remaining friction in the bearing
80. In this manner, hose
74
(see FIG. 5) does not become wrapped around the mechanism when the stage
26
is rotating. Adjustment element
62 is threaded and can be rotated with a
tool inserted into the hex recess
63 at the top of element
62 to
vary its vertical position. The hex recess
63 is accessible through the
top of chuck
20 and the central bore
19. Adjustor
62 is used
in the initial set up to adjust the height of the system and compensate for tolerances.
Coupling body
64 is connected to a source of vacuum delivered by hose
74
(see FIGS.
5 and
7). Through opening
76 the fluid flow communicates
to the bore
19.
In FIGS. 6 and 7, the flexures
52,
54 are visible in their assembled
position. The flexures
52,
54 are preferably configured as thin membranes
made, for example, from full hard stainless steel with a thickness 0.004 inch.
Such membranes may feature circular perforations
79 (see FIGS. 6,
7
and
8) to reduce resistive axial forces resulting from their elastic deformation.
In the preferred embodiment, the flexures
52,
54 have an annular
width of approximately 1.25 inch. The width is defined as the difference between
the outside diameter and inside diameter of the flexures
52,
54.
FIG. 8 shows in that context two exemplary flexures
52,
54. The hole
pattern along their inside and outside edges correspond to the screw holes of the
clamping rings
73,
75 for the flexure
54 and clamping rings
71,
78 for the flexure
52 (FIG.
5). The clamping rings
71,
73,
75,
78 assist in mounting flexures
52,
54 since they are too thin to be screwed down directly without risk of damage.
Clamping ring
78 also holds the bearing
80 and the seal
77
at the rotating portion of the assembly.
In the preferred embodiment, the stage assembly is provided with three leveling
feet (one of which is shown in FIG. 4 at
90). The leveling feet allow the
chuck to be set to the required height relative to the optics. In addition, the
leveling feet allow the chuck to be leveled, or made parallel to the optics and
at the same time make the chuck perpendicular to the axis of rotation of the rotational
stage. A screw (not shown) passes through the center of each of the leveling feet
to attach the Z-stage to the rotary stage.
In the preferred embodiment, the stage assembly is further provided with three
damper assemblies (one of which is shown at
92). The damper assemblies provide
critical damping to the mechanical "mass-spring" system, such that optimum closed-loop
servo control is achieved.
The Z-axis motion system may operate as follows. To receive the wafer
4,
the chuck
20 may be brought into a predetermined receiving position by having
the processor apply an initial voltage to the PZT stack
38. After the wafer
4 has been placed on the chuck
20, a vacuum is communicated to the
gap between wafer
4 and chuck
20 along the hose
74, the opening
76, central bore
19 and the grooves
81. Once the wafer
4
is secured to the chuck, the processor
112 controls one or more of the stages
22,
24,
26 to bring a predetermined measurement area of the
wafer
4 within the range of the probe beam. The auto focus system
110
recognizes and communicates the offset between focal plane and the measurement
area to the processor
112, which in turn adjusts the voltage correspondingly
until the offset is substantially eliminated. In the preferred embodiment, the
movement resolution and consequently the adjustment precision in vertical direction
is within a range of 0.02-0.003 microns, depending upon particular elements incorporated
into the feedback loop.
While the subject invention has been described with reference to a preferred
embodiment, various changes and modifications could be made therein, by one skilled
in the art, without varying from the scope and spirit of the subject invention
as defined by the appended claims
*
