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Claims  |
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What is claimed is:
1. A linear motor system for providing XY motions comprising:
a planar driver including sets of a number of energizing windings, the
windings of each set having active lengths in the form of linear segments
disposed to be solely along one of two orthogonal directions, the sets
being disposed in a two-dimensional array with at least one set in each of
the two orthogonal directions;
a planar magnetic structure, including a two-dimensional array of spaced
apart magnetized poles thereon in a pattern substantially aligned with the
two orthogonal directions;
means maintaining the driver and magnetic structure in opposed,
substantially parallel relation during relative movement therebetween in
two orthogonal directions; and
means coupled to the energizing windings for energizing the windings of the
two orthogonal directions separately and selectively with driver currents
in accordance with relative position to generate thrusts in one or both of
two orthogonal directions and relative motion between the driver and
magnetic structure by interaction between the energized active lengths and
the opposed magnetic structure.
2. The invention as set forth in claim 1 above, wherein the magnetic
structure is substantially stationary and the driver is movable.
3. The invention as set forth in claim 2 above, wherein the driver
comprises a platform having a central axis and at least four sets of
driver windings disposed in quadrants about the central axis, the active
lengths of windings in diagonally disposed quadrants having like
orthogonality and wherein the magnetic structure comprises a sequence of
poles of like magnetization along each direction of the array, with
alternating magnetization between successive sequences.
4. The invention as set forth in claim 1 above, wherein the driver is
substantially stationary and the magnetic structure is movable.
5. The invention as set forth in claim 4 above, wherein the magnetic
structure comprises a platform having an array of poles of opposite
magnetization thereon, and wherein the planar driver comprises a table
having windings thereon in two orthogonal directions.
6. The invention as set forth in claim 1 above, wherein one of the driver
and magnetic structure is movable and the other fixed, and further
including means coupled to the fixed unit for sensing the position of the
movable unit and controlling the means for energizing the windings.
7. The invention as set forth in claim 1 above, wherein the driver
comprises a movable platform having a symmetrically placed center of
gravity and the magnetic structure comprises a fixed table, and wherein
the windings are disposed in quadrants about the center of gravity of the
platform, with the active lengths of windings in diagonally disposed
quadrants being similarly oriented such that relative rotational movement
is imparted by differential energization of windings of like
orthogonality.
8. The invention as set forth in claim 1 above, wherein the means
maintaining the parallel relation comprises air bearing means coupled to
one of the driver and magnetic structure and in air bearing relation with
the other planar unit, and wherein the active lengths of windings of each
set are parallel along a given direction for that set and the system
further comprises means for commutating the signals to the windings of a
set in accordance with the relative position between the windings and the
magnetic poles.
9. The invention as set forth in claim 1 above, wherein the magnetic
structure is stationary and comprises a magnetic base having a rectangular
array disposed thereon of magnetized areas of polarity alternating between
successive rows and columns and separated by non-magnetic areas, and
further including a thin superimposed flat top surface of non-magnetic
material facing the driver.
10. A system for positioning wafers with high precision, high speed and
short settling times relative to the optical axis in a step and repeat
system comprising:
a flat table surface normal to and encompassing the optical axis;
a platform disposed above the flat table surface and including air bearing
means providing low friction support on the platform, the platform having
its center of gravity in a plane close to the table surface and the air
bearing means including a number of air bearing pads spaced apart about
the center of gravity;
linear motor means coupling the table surface to the platform, the linear
motor means including winding means and permanent magnet means, with one
being mounted on the platform facing the table surface and closely
positioned relative to the plane of the center of gravity and the other
being embedded in the table surface, the winding means comprising an array
of orthogonally disposed linear active lengths of windings disposed such
that forces generated by inductive reaction between active lengths of the
same orthogonal direction and the permanent magnet means are exerted
symmetrically relative to the plane of the center of gravity in the
platform and are symmetrical and substantially perpendicular to an axis
normal thereto;
means for sensing the position of the platform relative to the table; and
means for separately energizing the active lengths in each orthogonal
direction in response to sensed position to provide controlled thrust on
the platform.
11. The invention as set forth in claim 10 above, wherein the winding means
is mounted on the underside of the platform and comprises at least one
array of four linear multiple coil sets disposed in quadrants about the
center of gravity, the active turns of coil sets in oppositely disposed
quadrants being parallel and orthogonal to the active turns of coil sets
in the remaining quadrants, and wherein the system includes means
responsive to the sensed position for energizing the coil sets to control
position of the platform relative to the table surface.
12. The invention as set forth in claim 11 above, wherein the table surface
comprises an array of permanent magnet elements embedded in the table
surface in a checkerboard pattern of columns and rows, the magnetic
elements being of like polarity along the columns and rows, and the
columns and rows alternating in polarity and conductive non-magnetic areas
filling the spaces between the permanent magnet elements.
13. The invention as set forth in claim 12 above, wherein the multiple coil
sets each have multiple active turn lengths lying in a nominal plane
adjacent the table surface and the permanent magnet elements, wherein the
areas of each coil set span more than one permanent magnet element, and
wherein the system further includes means for commutating energizing
signals applied to the turns in a coil set in accordance with the platform
position for that axis.
14. The invention as set forth in claim 13 above, wherein the means for
sensing the position of the platform includes at least two laser
interferometer means, and means responsive to the inteferometer means for
computing the instantaneous position of the platform.
15. A system for positioning a semiconductor wafer precisely in X and Y
directions within a given horizontal plane comprising:
a planar member having a flat upper surface including a plurality of
embedded square outline permanent magnet elements disposed in rows and
columns along X and Y axes, and a plurality of square outline non-magnetic
separators disposed therebetween in alternating fashion in the rows and
columns, the permanent magnet elements in individual columns and rows
being of like polarity but the polarities of the elements in successive
columns and rows alternating;
a platform including air bearing means maintaining the platform at a
predetermined height above the upper surface of the planar member, the
platform having a predetermined center of gravity;
four coil sets mounted in quadrants symmetrically about the center of
gravity of the platform on the underside of the platform, the coil sets of
one diagonally opposed pair having active linear lengths of turns disposed
substantially along the X axis, and the coil sets of the other diagonally
opposed pair having active lengths of turns disposed substantially along
the Y axis, each coil set having multiple parallel subsets of turns and a
square outline covering a predetermined number of magnetic elements and
non-magnetic elements;
means for sensing the position of the platform;
means providing driving signals for the X and Y directions; and
means responsive to the sensed position of the platform and the driving
signals for commutating the driving signals in appropriate senses to the
subsets of the turns of the coil sets to provide desired thrusts in the X
and Y directions.
16. The invention as set forth in claim 15 above, wherein the coil sets
each have an area equivalent to four of the magnetic elements and
non-magnetic elements, and wherein the means for commutating constantly
energizes a like number of subsets of a coil set such that constant thrust
is exerted for a given driving current amplitude.
17. The invention as set forth in claim 16 above, wherein the coil sets
each have six subsets and four are energized at a time when driving
current is applied for the coil set.
18. The invention as set forth in claim 16 above, whrein the diagonally
opposed subsets are energized in like fashion to generate balanced thrusts
relative to the center of gravity of the platform.
19. The invention as set forth in claim 16 above, wherein at least one pair
of diagonally opposed subsets are energized in differential fashion to
induce angular motion of the platform relative to the X and Y directions.
20. A system for positioning a platform precisely in horizontal plane
comprising:
a planar member having a flat upper surface lying in a horizontal plane,
the planar member including a plurality of permanent magnet elements in a
rectangular array, the elements being spaced apart and being of like
polarity along the lines of the array, with polarity alternating along
successive lines, the planar member including non-magnetic spacers between
the permanent magnet elements;
a platform member including support means movably maintaining the platform
member above the planar member at a selected spacing, the platform member
including a number of coil sets disposed about the vertical axis of the
center of gravity thereof, the coil sets having windings with active turns
disposed only in one of two orthogonal, horizontal directions, the coil
sets and permanent magnet elements having areas and spacings such that the
active turns of the coil sets concurrently span like areas of oppositely
poled permanent magnet elements; and
means coupled to the coil sets for energizing the coil sets to generate
forces on the platform member to provide movement in any direction in the
horizontal plane.
21. The invention as set forth in claim 20 above, wherein the coil sets
comprise four substantially square outline coil sets disposed in quadrants
about the vertical center of gravity of the platform, the active lengths
of windings of one diagonally opposed pair being oriented in one direction
and the active lengths of windings of the other diagonally opposed pair
being oriented in the orthogonal direction, and wherein the permanent
magnet elements are squares having sides one-half the length of the sides
of the coil sets and spaced apart by a like distance, such that each coil
set constantly is coextensive with equal areas of oppositely poled
permanent magnet elements.
22. The invention as set forth in claim 21 above, wherein the coil sets
each comprise a plurality of serially disposed windings at a predetermined
pitch, the windings having active lengths disposed adjacent and parallel
to the plane of the permanent magnet elements.
23. The invention as set forth in claim 22 above, wherein the coil sets
include distributed polyphase windings and wherein the system further
includes means for sensing the position of the platform and means coupled
to the means for energizing and responsive to the sensed position position
to commutate the driving signals between the windings of a coil set at
increments corresponding to the predetermined pitch.
24. The invention as set forth in claim 23 above, wherein the distributed
polyphase windings are Gramme windings having three phases and wherein the
commutation switches the leading phase winding to zero when the leading
winding is coextensive with permanent magnets of opposing polarity.
25. The invention as set forth in claim 23 above, wherein the distributed
polyphase windings of each coil set comprise two groupings of windings,
like windings in each three phase grouping being serially connected and
oppositely wound such that opposite currents pass through them.
26. The invention as set forth in claim 20 above, wherein the planar member
comprises a thick magnetic base supporting the permanent magnetic elements
and providing a magnetic back plane therefor.
27. The invention as set forth in claim 26 above, wherein magnetic base is
a cast iron structure having hollowed out portions, and wherein the system
further comprises a flat non-magnetic surface member disposed over the
permanent magnet elements adjacent the coil sets.
28. A system for positioning a platform precisely on a horizontal plane
comprising:
a planar member having a plurality of windings in first and second sets
with the active lengths of each set being disposed in only one of two
orthogonal directions in a horizontal plane, the windings of the first and
second sets intersecting throughout an area in which the platform is to be
positioned;
platform means disposed and movable above the horizontal plane and
including a number of permanent magnet elements in a preselected pattern
of opposite polarities adjacent the horizontal plane that is substantially
aligned with the two orthogonal directions; and
means for energizing the first and second sets of windings to exert thrusts
on the platform means to the permanent magnet elements by interaction
between the active lengths of the sets of windings and the permanent
magnet elements.
29. The invention as set forth in claim 28 above, wherein the windings of
the first and second sets each comprise a plurality of turns having a
predetermined pitch and wherein the permanent magnet elements have sizes
and spacings proportioned to the predetermined pitch.
30. The invention as set forth in claim 28 above, wherein the permanent
magnet elements comprise a central square of one polarity having sides
that are an integral multiple of the predetermined pitch, and a number of
symmetrically positioned permanent magnet elements of opposite polarity
positioned thereabout.
31. The invention as set forth in claim 30 above, wherein the system
further includes means responsive to the position of the platform means
for controlling the energization of selected sets of windings to generate
desired thrusts on the platform means.
32. The invention as set forth in claim 31 above, wherein the permanent
magnet elements of opposite polarity are squares having one-half the
length per side of the permanent magnet element of one polarity and each
is positioned adjacent a different corner of such permanent magnet
element.
33. The invention as set forth in claim 31 above, wherein the platform
means further includes a flux closure member adjacent the permanent magnet
elements on the opposite side from the planar member. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
In many applications, particularly precision optical systems such as are
used in semiconductor processing, inspection and testing, it is desirable
to be able to move an object very precisely with respect to two orthogonal
axes while maintaining the object in a reference plane. Step and repeat
projection systems provide a good example of the need for a positioner or
aligner that must meet such critical and difficult requirements. Such
systems are used in the semiconductor industry for repeatedly projecting
the image of a photomask onto a wafer at different positions. The array of
patterns on a silicon or other semiconductor wafer must be successively
laid down in columns and rows with submicron accuracy. Semiconductor
wafers of 6" diameters are now being utilized and up to 8" diameters will
be used in the future.
Because constantly smaller images are being generated, ever greater
accuracy in positioning is being demanded. With 6" wafers, for example,
high density circuit patterns are laid down at center-to-center spacings
of the order of 0.10". Thus approximately 2500 images must be exposed or
processed sequentially with a positioning accuracy of 0.1 micron in order
that the complete processing sequence, which typically involves many
projection steps, can be carried out with precise alignments and high
yields. It is also sometimes required to be able to rotate the image
through a small angle (e.g. 3.degree.) in order to align the pattern for
various purposes.
These requirements for accuracy and reliability of operation are such that
older bearing mounted positioners with compounded XY stages cannot be as
precisely level as needed, much less provide rapid but highly precise
movement between successive exposure positions. It has been recognized
that a platform supported on air bearings above a stable, precisely flat
surface might achieve suitable positioning speed and accuracy. However,
this remains no more than a desirable goal at the present time, inasmuch
as an air bearing mounted XY drive system with limited angular motion is
not yet commercially available. Moreover, there are many potential
problems to be considered, such as the settling time required before the
platform is truly stationary once it has been moved to a new position.
With most XY stages, driving thrusts are exerted off axis relative to the
center of gravity of the platform. Thus overturning moments are exerted
that would induce a rocking action at some resonant frequency in air
bearing mounts. Until this rocking action becomes damped to an adequately
low level the next projection step cannot be undertaken.
Linear motors are well known as devices for providing precisely
controllable longitudinal movement. Because the elements of the linear
motor lie along a plane, and the motor can move the elements at high
speed, its promise for use in precision positioning systems is recognized.
Precision sensors employing laser interferometers can be used to provide
signals for servoing position to submicron accuracy. However, no suitable
configuration of two axis or X-Y linear motor drive is available or known.
The added requirements of a semiconductor processing station, including
the ability to maintain the image in an almost perfectly flat plane,
rotate the image through a small angle, and position rapidly with minimal
settling time, also remain to be met.
A two-dimensional linear stepping motor system was initially proposed by
Sawyer in U.S. Pat. No. Re.27,436, and has been used since principally for
controlling the mechanism in a flatbed plotter. In this system two
orthogonally disposed linear stepping motors are defined within a movable
platform above a planar base. The platform can be actuated in X or Y
separately or in a vectorially combined direction, but only in stepping
fashion. Even using known microstep techniques, however, this mechanism
cannot meet the precision and velocity requirements of the semiconductor
industry without much refinement and excessive cost. It also cannot rotate
relative to the XY plane, and thus would require an added mechanism for
this purpose.
A system for positioning an element in two dimensions in semiconductor
manufacturing apparatus is disclosed in recently issued U.S. Pat. No.
4,535,278 to Asakawa. In this proposal an array of permanent magnets is
arranged in a spaced apart rectangular pattern, with magnets of like
polarities disposed along diagonals. Driving coils in the form of flat,
square sided, loops are mounted on the underside of a movable member. Four
such coils are shown, with outer dimensions, widths and spacings being
precisely related to the particular magnetic fields of the array so as to
generate predictable instantaneous force vectors, depending on incremental
position and instantaneous driving current, for each coil. By manipulating
individual driving currents constantly as position changes, net drive
force vectors are said to be generated that induce the needed motion in
the driven device. This system is said to be capable of controlling
angular position as well as X and Y positions. It is, however, extremely
complex because it is based on multiple non-linear interactions which vary
with position. The four sided coils may interact at any instant with from
none to four permanent magnets to give a predictable but highly variable
resultant force vector. Each of the four sides of a coil sees a different
instantaneous force vector that varies in both angle and amplitude and
must be resolved by signal processing into individual driving signals that
somehow yield the desired net X and Y forces. Thus the driving current for
each coil must constantly be computed and changed, requiring input signals
to be converted depending on position in accordance with complex
"distribution factors" in "multiplier type digital-analog converters". For
high precision the system requires virtually infinite resolution, and it
appears likely that some positions may exist at which pure X and Y forces
cannot be resolved. Furthermore, the forces exerted are not symmetrically
applied relative to the center of the device and settling times are highly
uncertain.
SUMMARY OF THE INVENTION
A system in accordance with the invention provides an XY positioning table
a two-dimensional planar array of selectively magnetized areas, with like
magnetization of elements in columns and rows, facing a two-dimensional
planar array of selectively energizable driver coil sets lying in a
horizontal plane. The coil sets have active turn lengths lying along
either the columns or rows of the array. A platform for an object to be
positioned, such as a wafer, contains one part of the drive system while
the table contains the other. Current through the driver coil sets is
varied in accordance with instantaneous platform position and desired
position to induce the desired motion. The interaction between the
magnetic elements and the active turn lengths generates thrust in either
or both of the two orthogonal directions to position the platform in the
horizontal plane. In addition, differential actuation of individual coil
sets in the driver coils may be used to introduce limited angular motion
in the platform. The platform is supported on air bearing mounts above the
table, and the magnetic field interaction takes place just above the
surface of the table, constantly directing thrust through the virtual
center of the moving structure. The coil sets are divided into multiple
subsets which are energized by associated commutating circuits in such
fashion that thrust is constant for a given driving current amplitude.
In one example of a system in accordance with the invention, the
positioning system comprises a flat fixed surface plate having an embedded
array of magnetic elements of individual rows and columns, with elements
of opposite magnetization being offset along different orthogonal lines to
provide a checkerboard array having interspersed non-magnetic areas. The
movable platform is disposed above this fixed surface, and comprises, at
its underside, a number of selectively disposed and symmetrically mounted
coil sets disposed in quadrants about the center of gravity. Each pair of
two diagonally disposed coil sets includes active turn lengths parallel to
one of the two orthogonal directions to generate balanced thrust in the
other orthogonal direction. Positioning is controlled by sensing the
instantaneous position of the platform using laser interferometers,
deriving the drive signals needed for movement to a desired position, and
controlling excitation of the coil sets without processing of the drive
signals. Movement at angles to the X and Y axes is generated by concurrent
control of the relative amplitudes of the forces exerted in the X and Y
directions. Rotation of the movable stage is effected by unbalanced
energization of the primary elements in the corner quadrants. Multi-turn
coil sets are employed for greater thrust, the windings in the coil sets
being energized under control of a commutating system. The position of the
platform is calculated in each of the two orthogonal directions relative
to the magnetic elements and drive signals are commutated between windings
without cogging effects.
In another example of a system in accordance with the invention, the coil
drivers are disposed in the flat surface in mutually orthogonal
directions, and a pattern of magnetized elements is mounted on the
underside of the movable platform. Again, the platform is positioned using
commutated drive signals computed from sensing of instantaneous position.
This arrangement has the advantage that the winding structure is fixed,
but a disadvantage is that the exerted energizing windings extend through
the length of the array, reducing efficiency.
Another feature of the invention is that dimensional stability for the
table is achieved utilizing a magnetic back plate provides a temperature
stable substrate for the checkerboard of magnetic elements, the spaces
between which are filled with insulating material such as glass. The
checkerboard array is covered by a thin, dimensionally stable top surface
such as glass which serves as the reference plane for an air bearing
support system for the superimposed movable platform.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the invention may be had by reference to the
following description, taken in conjunction with the accompanying
drawings, in which:
FIG. 1 is a combined block diagram and perspective view, partially broken
away, of a positioning system in accordance with the invention;
FIG. 2 is a perspective view of the underside of a movable platform used in
the system of FIG. 1;
FIG. 3 is a plan view of energizing winding sets used on the movable
platform of FIG. 2, showing further details thereof;
FIG. 4 is a side sectional view of the energizing winding sets of FIG. 3;
FIG. 5 is a fragmentary plan view of the table surface of FIG. 1, showing
the relationship of the energizing winding sets in different positions to
magnetized areas on the table surface;
FIG. 6 is a side sectional view of a portion of the table surface of FIGS.
1 and 6 showing further details thereof;
FIG. 7 is a perspective view, partially broken away of a different example
of a system in accordance with the invention;
FIG. 8 is a perspective view of the lower portion of a platform used in the
system of FIG. 7, showing the disposition of magnets on the underside
thereof; and
FIG. 9 is a schematic view of a surface table used in the system of FIG. 7
showing the disposition of energizing windings thereon relative to the
permanent magnets on the platform.
DETAILED DESCRIPTION OF THE INVENTION
A positioner in accordance with the invention, as utilized in conjunction
with a step and repeat system for die-by-die alignment on a semiconductor
wafer is shown in general form in FIG. 1. A generally triangular platform
10 floats on air bearing mounts 12 positioned at each outwardly extending
leg of the platform. A semiconductor wafer 14 resting on a top surface on
the platform is to be positioned precisely, at different X and Y
positions, relative to the optical axis of an overlying photohead 16,
shown only in generalized form inasmuch as a wide variety of these systems
is known. In addition, the wafer 14 is to be rotated through a small
angle, such as a maximum of 3.degree., so as to precisely align the dies
(not shown) on the wafer 14. In a typical situation, the wafer is covered
with a photosensitive layer (not shown) and the photohead 16 exposes a
precision pattern defined by a photomask 17 (shown only generally). In a
step and repeat process, similar exposures are made at a rectangular array
of die positions on the wafer 14. These patterns may be the first on the
wafer 14, or may be deposited over prior layers. After complete exposure
in the step and repeat sequence, a washing step removes material, as
determined by the exposure pattern, following which the next deposition
step in the process can be utilized. The wafer 14 is of standardized form,
having a major flat and a minor flat along different peripheral sections
from which the XY position and angular attitude can be precisely
calculated.
The triangular platform 10 floats on the air bearings 12 on a table surface
20 which has a precisely flat and horizontal upper surface and includes
embedded magnetic elements 22 separated by non-magnetic elements 23 in a
precise pattern as defined hereafter. The platform 10 includes, as
described in greater detail hereafter, coil sets which form linear motor
structures with the permanent magnet elements 22. Air pressure for the air
bearings 12 is provided through an air line 24 and driving signals for the
linear motors are provided through electrical lines 26 arranged in a
flexible umbilical cord 28, the majority of the weight of which may be
suspended from an overhead device (not shown).
With this system, the platform 10 is free to be positioned anywhere on the
table surface 20, and in order to control such position with high accuracy
remote sensors in the form of three beam steered laser interferometers 32,
33, 34 receiving coherent light from a common laser 36 are utilized. Each
laser interferometer 32, 33 or 34 directs a collimated narrow beam onto a
different V-shaped reflector 40, 41 or 42 respectively. The V-shaped
reflectors each have a small spherical curvature at the apex of the V,
such that light impinging on the reflector is reflected directly back to
the source even though the platform 10 may be misaligned within the
3.degree. limit relative to the X and Y axes defined on the table surface
20. The beam from the laser 36 is divided by beam splitters (not shown in
detail) into separate beams for each interferometer 32, 33, 34, and guided
by appropriate angled mirrors (also not shown in detail). At the third
laser interferometer 34, which is shown in simplified schematic form as
typical of all three devices, the beam passes a beam splitter 45 toward a
variable angle galvanometer mirror 46, from which it is directed to the
associated reflector 42 on the platform 10. The galvanometer 47 rotates
the coupled pivoted mirror 46 about a central vertical axis so as to steer
the beam onto the center of the V on the reflector 42. The direction is
controlled by reflecting the returned beam off the pivoted mirror 46 and
passing it onto a radiation sensitive detector 49. The detector 49 feeds
servo circuits 50 controlling the galvanometer 47 in accordance with
signal amplitude so as to track the platform reflector 42 with the beam.
The other beam from the mirror 46 is directed through the beam splitter 48
to the interferometer circuits 51 which are of conventional design, and
which also require a reference from the first beam splitter 45. The waves
and phases of the two beams are compared to provide data for use in
determination of XY and angle (.theta.) position of the platform 10. The
angle of the galvanometer 47 can be used for computation or control, for
which purposes an angle encoder 52, preferably of the optical type, is
coupled to the galvanometer 47 to provide a digital signal. To compute
instantaneous position, however, only the three distance signals from the
interferometer circuits 51 need be applied to a position processor 54,
such as a microprocessor. The processor 54 is programmed to compute, by
software, firmware or hardwired circuits, the instantaneous position of
the wafer 14 on the X and Y axes relative to the optical axis of the
photohead 16. The processor 54 also computes angular deviation of the
wafer 14 about the central vertical axis defined by the optical axis.
Presently used laser interferometers establish positional accuracy within
small fractions of a micron. While the laser sampling time is relatively
slow, typically being measured in tens of cycles per second, the
mechanical system is extremely stable and settling time is not a
significant factor.
The instantaneous position computed by the position processor 54 is
compared to command positions provided in digital form by a command
processor 56, which indicates the positions desired in X, Y and .theta..
Servo circuits 58 compare the actual to desired position signals and
generate one pair each of X and Y drive signals for energizing the coil
sets on the platform 10. These pairs of X and Y signals, designated
X.sub.1, X.sub.2, Y.sub.1, and Y.sub.2 are applied through commutator
circuits 60 to driver circuits 62, to energize appropriate windings in
each of the coil sets, dependent upon instantaneous platform position.
Different individual drivers in the driver circuits 62 energize specific
windings in each of the coil sets dependent upon their physical relation
to adjacent permanent magnet elements 22 on the table surface 20. The
commutation times are determined by the position processor 54 which
provides an X or Y signal, designated X.sub.c or Y.sub.c to denote each
physical position at which signal commutation should take place, this
corresponding to the pitch of the windings.
FIG. 2 depicts the underside of the platform 40 and the manner in which
four coil sets 64, 65, 66 and 67 are disposed in a specific array on the
underside thereof. The four coil sets 64-67 are disposed symmetrically
about the vertical axis of the center of gravity of the platform 10. Two
coil sets 64, 66, in diagonally opposed quadrants relative to the center,
lie with active lengths of turns parallel to a given axis (here X) on the
underlying table surface 20. It can be seen from the view in FIG. 2 of the
underside of the platform 10 that the turns or active lengths of the coils
64, 66 are those linear lengths that are closest to the table surface 20.
These lengths move in a plane just above the table surface 20 and exert
thrust on the platform 10 in the direction perpendicular to their length
(e.g. along the Y axis). The other diagonally opposed coil sets 65 and 67
are oriented with the active lengths of the windings parallel to the
orthogonal or Y axis and therefore control X axis motion. All of the coil
sets lie in the same plane just above the table surface 20, being held in
position by the cruciform air bearings 12 which are capable of "flying"
the platform 10 above the precisely flat table surface 20 at spacings of
the order of 5-10 microns if desired.
Further details of the coil sets are seen in the plane and cross sectional
views of FIGS. 3 and 4. Each coil set is divided into six separate
windings 70-75 wrapped around a common bobbin 77. Only one of these coil
sets need be described inasmuch as the others are arranged in like
fashion. The separate windings 70-75 are disposed with a uniform pitch
(.theta.) in a three phrase, Gramme-wound configuration. That is, as shown
in FIG. 4, the polyphase winding is distributed by returning the winding
from the active length back around the bobbin 77 at the opposite side, and
disposing corresponding windings of each set of three in opposed phase
relation. This arrangement enables two out of the three phases to be
energized at all times, and thrust to be constant, whichever windings are
energized, a feature which cannot be realized with other types of
polyphase distributed windings.
The area of each coil set 64-67 is of a selected nominal value, and is
furthermore related to the area and geometry of the permanent magnet
elements 22 on the table surface 20. In this instance, the permanent
magnet elements are square in outline and 1" on a side, while the
individual coil sets are square in outline and 2" on a side. The permanent
magnet elements 22 are disposed in a checkerboard arrangement of columns
and rows, with each permanent magnet element 22 alternating with a like
non-magnetic area 23, and with the permanent magnet elements 22
alternating in polarity along the columns and rows alternating as well.
This arrangement assures that each coil set covers an equal area of
permanent magnets 22 of different polarity at any position. For most
economic commutation, the coil sets are separated in the X and Y
directions by the same spacing (i.e. 1") as the length of the side of an
element 22.
The construction of the table surface 20 includes the permanent magnet
elements 22, non-magnetic spacers 23 and other features that enable the
table surface 20 to complete the complex linear motor combinations and to
function as a precise horizontal reference for movement of the platform
10. As best seen in the side view of FIG. 6, the table comprises a heavy
base 80 having concavities 82 on the underside thereof. The upper surface
84 of the base is machined and lapped flat and horizontal. The base 80 may
be molded of cast iron but is non-warping and temperature stable, and much
easier to fabricate than the surface granite blocks widely used in optical
systems. The permanent magnet elements 22 and non-magnetic spacers 23 are
secured on the upper surface 84 of the base 80, as by epoxy or other high
strength adhesive. If it is desired to use a granite base, however, a
magnetic back plate (not shown) can be inserted between the magnetic
elements and the base, to complete the flux paths.
The side dimensions of the table 20 are ultimately determined by the
maximum size of the semiconductor wafer 14 that is to be used. As seen in
FIG. 1, in which the wafer 14 is of 6" diameter, the permanent magnet
elements 22 are 1" on a side and in a 16.times.21 array, and there is
approximately a 6" margin on three sides and a 3" margin on the fourth
side. This slightly non-symmetrical arrangement enables the platform 10 to
be shifted in all directions so that the coil sets 64-67 remain in
operative relation to the permanent magnets 22, while the legs of the
platform can ride out to the side margins of the table 20 at the limit
positions. The narrower margin along one side can be used because the
geometry of the triangular platform 10 and the permissible but small angle
of rotation do not require a greater margin on this side. With this
configuration, much larger semiconductor wafers can be used in the step
and repeat system than the 6" wafers now predominantly used and the 8"
wafers now in development, provided that the wafer is approximately
centered on the platform. Thus a centered 10" wafer on the platform need
only be moved 5" in any radial direction from the optical axis of the
photohead 16 or a total of 10", whereas a minimum of 16" is currently
available. The present arrangement, however, does enable the wafer 14 to
be loaded virtually anywhere on the top of the platform 10, and still have
all areas accessible to the optical axis of the photohead 16.
The non-magnetic spacers 23 separating the permanent magnet elements 22 may
be of any suitable material, such as fiberglass reinforced resin, ceramic
or the like. With the elements 22 and 23 in position, this surface is
subjected to a final precision lapping which assures that the surface
plane of the elements is flat and horizontal. A thin (e.g. 1/16") float
glass or other nonmagnetic top surface layer 90 is superimposed on the
elements 22, 23 to serve as the reference base for the air bearing mounts
12 for the platform 10. The float glass 90 cover does not interfere with
magnetic intercoupling between the coil sets 64-67 on the platform 10 and
the permanent magnet elements 22 on the table 20.
Vectorial movement of the platform 10 and the wafer 14 in the horizontal
plane above the table 20 is controlled by energization of the coil sets
64-67 on the underside of the platform 10. This may be better understood
by reference to FIG. 5, which shows the active lengths of windings in the
first and third coil sets 64, 66 as disposed along the X axis, with the
windings in the second and fourth sets 65, 67 being disposed along the Y
axis (the directions being designated arbitrarily but in any event being
orthogonal). Thus the first coil set pair 64, 66 has active turns which
generate a force in the Y direction when current is carried in these
windings and the inducted magnetic flux coact with the magnetic field
emanating from a permanent magnet 22. Concurrently, the linear lengths of
the other pair of coils sets 65, 67 generate forces in the X direction
when they carry current. The direction, whether forward or reverse, along
the given axis, and the level of thrust, are determined by the number of
turns, the magnetic field strength, and the amplitude of the current, as
well as the length of winding coextensive with the magnetic element.
Referring to FIG. 5, it is seen that superimposing the coil sets 64-67 over
the permanent magnet elements 22 demonstrates that, as previously stated,
a given coil set covers an equal area of north polarized and south
polarized elements 22. To move in a single direction, such as the X
direction, the individual windings 70-75 (FIG. 4) in each coil set 64 and
67 (FIGS. 2, 3 and 5) with active turns parallel to the Y axis are
energized in accordance with the instantaneous position of those coil sets
relative to the permanent magnet elements 22. Consequently as seen in the
superimposed view of FIG. 5, turns 70 and 71 which are coextensive with
north polarized regions are energized with currents in one direction while
turns 73 and 74 which are coextensive with south polarized elements, are
energized with currents in the opposite direction. All currents are of
amplitude determined by the driving signal. The thrust generated in all
windings which coact with adjacent permanent magnet 22 of either polarity
thus contribute to movement in the selected direction. In order to obtain
movement precisely along the X axis, both coil sets 65, 67 are equally
energized. Thus, the vectorial components of force exerted by each coil
set 65, 67 are equally spaced from and on opposite sides of the center of
gravity and no turning moment about the center of gravity is introduced.
Likewise, Y direction forces are generated by the other two diagonally
disposed coil sets 64, 66, w | | |
