Moving interferometer wafer stage

A stage used for positioning and aligning a wafer, as used in photolithography or microlithography in semiconductor manufacturing having a plurality of interferometer laser gauges placed on a movable wafer stage associated with a pair of stationary orthogonal return mirrors. A beam of light parallel to the X axes is directed through a penta prism to an interferometer laser gauges placed on the wafer stage near the wafer plane through a plurality of beamsplitters and fold mirrors. The present invention is less sensitive to rotation or twisting of the wafer stage and eliminates or reduces certain errors introduced by the rotation. Additionally, large stable return mirrors may be used, increasing the travel distance permitted by the wafer stage while reducing weight on the wafer stage. The wafer stage can be more accurately positioned and accommodate larger wafer sizes with improved positioning and alignment accuracies.

FIELD OF THE INVENTION 
This invention relates generally to photolithography as used in 
semiconductor manufacturing, and particularly to a wafer stage with 
accurate positioning and alignment. 
BACKGROUND OF THE INVENTION 
In the manufacture of semiconductor devices and flat panel displays, 
photolithography or microlithography is often used. A substrate or wafer 
stage on which a semiconductor wafer or other substrate is placed is used 
to align and position the wafer during exposure. A wafer stage used in 
aligning and positioning a wafer is disclosed in U.S. Pat. No. 4,952,858 
entitled "Microlithographic Apparatus" issuing Aug. 28, 1990 to Daniel N. 
Galburt, which is herein incorporated by reference. Therein disclosed is 
an electromagnetic alignment apparatus including a monolithic stage, a 
substage, and an isolated reference structure. Another wafer stage is 
disclosed in U.S. Pat. No. 5,285,142 entitled "Wafer Stage With Reference 
Surface" issuing Feb. 8, 1994 to Daniel N. Galburt and Jeffrey O'Connor, 
which is herein incorporated by reference. Therein disclosed is an 
electromagnetic substage and an electromagnetic monolithic stage coupled 
such that one follows the other and having a single reference surface 
extending over the entire range of motion of the monolithic stage. 
Additionally disclosed therein are interferometer return mirrors placed on 
the wafer stage. Interferometer return mirrors are used in an 
interferometer alignment system for accurately positioning and aligning a 
wafer stage. The interferometer return mirrors have always been placed on 
the wafer stage with the interferometers placed off of the wafer stage. 
While this has been acceptable for most photolithographic operations, as 
the wafer size becomes larger and the feature size of the circuit elements 
become smaller, there is a need to improve the structure of wafer stages 
to improve positioning and alignment of the wafer. The return mirrors in 
an interferometer system must be stable, and are therefor usually large 
and heavy. Because of the increasing wafer size, the wafer stage must 
travel longer distances. This results in large, heavy interferometer 
return mirrors being placed on the wafer stage. As a result, it is often 
difficult to quickly and accurately move the wafer stage. Additionally, 
the mirrors being mounted on the wafer stage are sensitive to rotation of 
the wafer stage which results in errors being introduced, often referred 
to as cosine errors. Additionally, when the wafer stage is rotated, an 
optical signal loss occurs which limits stage travel and rotation and 
requires a larger, more powerful laser illumination source to be used with 
the interferometer. Accordingly, there is a need to improve upon the 
conventional wafer stage structure to enhance positioning and alignment 
accuracies as well as reducing the weight and power required in 
conventional wafer stages. 
SUMMARY OF THE INVENTION 
The present invention is directed to a wafer stage having a plurality of 
interferometers placed thereon. The interferometers move with the wafer 
stage. Two stationary orthogonal return interferometer mirrors are placed 
adjacent the wafer stage and are used in conjunction with the 
interferometers placed on the wafer stage in order to obtain accurate 
alignment and positioning information. In one embodiment, the wafer stage 
is mounted vertically and a laser beam is folded using a penta prism and 
directed to the plurality of interferometers on the wafer stage through a 
plurality of beamsplitters and fold mirrors. 
Accordingly, it is an object of the present invention to reduce errors in 
positioning and alignment of a wafer stage. 
It is a further object of the present invention to reduce the size and 
weight of a wafer stage or to increase the travel distance of a wafer 
stage without increasing the size or weight of the wafer stage. 
It is an advantage of the present invention that a lower power laser 
illumination source may be used. 
It is a further advantage of the present invention that it is more tolerant 
of rotation or twisting of the wafer stage. 
It is a feature of the present invention that interferometers are placed on 
the moving wafer stage. 
It is a feature of the present invention that the stationary interferometer 
return mirrors are placed off of the moving wafer stage. 
It is a further feature of the present invention that an appropriate system 
of mirrors such as a penta prism is used to maintain a beam orthogonal to 
the stationary return mirrors irrespective of some twisting or rotation in 
the substrate plane of the wafer stage. 
These and other objects, advantages, and features will become readily 
apparent in view of the following more detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 schematically illustrates the present invention. A wafer stage 10 
has a wafer 12 placed thereon. The wafer stage 10 has a right-handed, (X, 
Y, Z) Cartesian coordinate system 11 centered over the wafer 12. Also 
placed on the wafer stage 10 are four laser gauge type interferometers 14 
and 16 positioned along the X-axis and the Y-axis respectively, of the 
edges of the wafer stage 10. Light from the interferometers 14 and 16 
travels to two orthogonal reference mirrors 30 and 32. Light from 
interferometers 14 travels in the Y-direction to the stationary return 
mirror 30 located with its face parallel to the X-Z-plane. Light from 
interferometers 16 travels in the X-direction to a second stationary 
return mirror 32 located with its face parallel to the Y-Z-plane. The term 
light as used in this application is meant to refer to electromagnetic 
radiation of any wavelength, and not only to light in the visible 
spectrum. 
In the simplest configuration three single-axis plane mirror type 
interferometers, incorporating corner cubes occupy any three of the four 
interferometer positions. When illuminated each plane mirror 
interferometer has a metrology axis in the direction of the illumination 
that should be set normal to the associated-reference mirror. As shown in 
FIG. 1, it may be desirable to add a fourth interferometer of the same 
type to provide nominally redundant information for error correction and 
other purposes. In this configuration, the four interferometer laser beams 
are aligned in a plane that is nominally parallel to the wafer plane. All 
alignment information furnished by the laser gauges or interferometers 14, 
16 is referenced to a plane parallel to the wafer plane. For the critical 
overlay alignment requirements of microlithography, it is necessary to 
provide small motion control and alignment of the wafer in six 
degrees-of-freedom, three in the wafer plane and three perpendicular to 
the wafer plane. The wafer stage 10 is free to move relatively large 
distances, more than one wafer diameter, in the in-plane or 
X-Y-directions, as long as the interferometer laser beams fall onto the 
reference mirrors 30 and 32, and rotation about the Z-axis is typically 
restricted to less than about several milliradians. For the other three 
degrees-of-freedom, system constraints allow only very small motions. The 
preceding configuration provides information needed to control the three 
in-plane degrees-of-freedom. Alternative means, not necessarily 
optically-based, must be established to provide control information for 
the other three. The faces of the two reference mirrors 30 and 32 define 
five degrees-of-freedom. Five plane mirror type interferometers can be 
located on the wafer stage 10 and directed at the reference mirrors 30 and 
32 to provide useful information for these five degrees-of-freedom. In one 
convenient configuration, the five interferometers may include the two 
interferometers 16 and one of the interferometers 14, for example the 
rightmost one, with the addition of two more interferometers, not shown. 
The two additional interferometers may be placed, one under the selected 
rightmost interferometer 14, and the other located under either one of the 
interferometers 16. The two additional interferometers, not shown, 
therefore are offset in the negative Z-direction, into the page, with 
respect to the interferometers 14 and 16. Mirrors 30 and 32 would be made 
wider in the same negative Z-direction to accommodate the additional 
interferometer laser beams. Typically, the measurement axis of three 
interferometers would lie in a plane parallel to the wafer. The 
measurement axis of the other two interferometers would lie in a parallel 
plane offset along the Z-axis. The remaining required sixth 
degree-of-freedom, Z-axis location, requires an additional reference 
structure. Two forms of such a reference structure of particular interest 
are both structures parallel to the wafer plane. One structure, not shown, 
located above the wafer 12 surface could hold one or three sensors that 
measure the distance to the wafer thereby providing Z or all three 
out-of-plane information. Alternatively, the structure could be a mirror 
with its face parallel to the X-Y-plane. The reference structure mirror 
provides a flat reference surface parallel to the plane defined by the 
wafer 10. The Z or all out-of-plane metrology information could be 
measured with one or three sensors, including interferometers, on the 
wafer stage. 
All of the reference items must be mechanically stable with respect to each 
other and the photolithography image that will be printed. 
The interferometer metrology axes should be spaced as far apart as possible 
- typically of the order of one wafer diameter. It is possible to replace 
several single-axis interferometers with appropriately selected multi-axis 
interferometers. 
The Hewlett-Packard Company, Test and Measurement Organization, 
manufactures an extensive line of laser gauge components that are useful 
with this invention. These components belong to their commercial product 
line listed as "laser interferometer positioning systems" in their current 
1996 catalogue. Catalogue components relevant to this invention include: 
laser heads; beam directing optics; one-, two-, and three-axis 
interferometers; fiber optic-fed detectors; and the associated metrology 
electronics. 
Each interferometer 14 and 16 has an output 18. The output 18 is coupled to 
a photodetector, not shown, through fiber optic cable 20. The fiber optic 
cable 20 may be mechanically coupled to the interferometer or it may be 
mechanically decoupled and only coupled optically in a manner similar to 
the optical input feed options described below. 
Each of the interferometers 14, 16 should be illuminated so that the 
metrology axis is maintained as perpendicular to the associated-mirror 30, 
32 face as possible. The interferometers 14, 16 can be illuminated as a 
group with the illumination distributed from a less precisely positioned 
single-axis stage to optics on the wafer stage 10, for example as 
illustrated in FIG. 1. Alternatively, the illumination can feed each of 
the interferometers 14, 16 separately with the distribution optics not on 
the wafer stage 10, but on a less precisely positioned stage. This latter 
concept is illustrated in FIG. 2. 
A first penta prism beamsplitter 22 is placed adjacent one of the 
interferometer laser gauges 16. A second penta prism beamsplitter 24 is 
placed adjacent the other interferometer laser gauge 16. A beamsplitter 26 
is placed adjacent penta prism beamsplitter 24 and is used to fold or 
direct light to a second beam folder or fold mirror 28 adjacent one of the 
interferometer laser gauges 14. A laser 34 is positioned off of the wafer 
stage 10 and directs a beam of light 37 parallel to the X axis. A penta 
prism 36 is positioned to receive the light from laser 34 and directs the 
light to penta prism beamsplitter 22 on the wafer stage 10. A beamsplitter 
38 directs a portion of the light from the laser 34 to a wavelength 
monitor 40. 
The penta prism 36 is mounted so as to follow, in the X direction, the 
travel of the wafer stage 10, as indicated by the double headed arrow 35. 
Current technology laser gauge lasers are relatively large and typically 
would be mounted on a stationary platform so that it is convenient to 
illuminate the interferometers as shown in FIG. 1 and FIG. 2 starting with 
penta-prism 36 moving on a single-axis stage that moves in the direction 
of arrow 35 parallel to the laser light and normal to reference mirror 32. 
Although the interferometer metrology axis orthogonality to the reference 
mirror is sensitive to rotations in the wafer plane by the single-axis 
stage, penta-prism 36 can be replaced by a fold mirror if these rotations 
are small enough. 
In principle, the laser 34 could be attached directly to the one-axis stage 
and aligned with its light perpendicular to mirror 30. The wavelength 
monitor 40 is used to monitor and determine the status of the atmosphere 
as it affects the laser 34. 
In operation, the laser 34 emits a light beam 37, a portion of which is 
split by beamsplitter 38 and directed to the wavelength monitor 40. Most 
of the light beam 37 is directed to the penta prism 36, which redirects 
the light to penta prism beamsplitter 22. The penta prism beamsplitter 22 
permits a portion of the light to continue to penta prism beamsplitter 24. 
A portion of the light is reflected by penta prism beamsplitter 22 and 
directed to the adjacent interferometer 16. One of the emerging beams is 
directed to the adjacent interferometer laser gauge 16 and is 
perpendicular to the beam of light entering the penta prism beamsplitter 
22. The light enters the interferometer laser gauge 16 and is directed to 
and reflected from the return mirror 32. The output 18 is carried to a 
photodetector, not shown, by fiber optic cable 20. From this output, 
information is obtained as to the location of the wafer stage 10, from 
which position and alignment information is calculated using conventional 
known techniques. The light entering penta prism beamsplitter 24 is 
similarly directed to the adjacent interferometer laser gauge 16. A 
portion of the light is split and directed to beamsplitter 26. A portion 
of the light entering beamsplitter 26 is directed to the adjacent 
interferometer laser gauge 14, and a portion of the light is folded to 
beam folder or fold mirror 28. The fold mirror 28 folds the light and 
directs it to the adjacent interferometer laser gauge 14. While four 
interferometer laser gauges 14 and 16 are illustrated, it should be 
appreciated that only three are generally needed. However, four may be 
used for redundancy if desired. 
Typically, the laser 34 may be placed a distance 50 to 150 cm from the 
penta prism 36. In some applications, where the size or heat of the laser 
is undesirable, the laser 34 may be placed even further away. The penta 
prism 36 may be mounted from 20 to 50 cm from the furthest extended travel 
of the wafer stage 10. The two interferometer laser gauges 14 may be 
separated by a distance of approximately 20 cm. Similarly, the two 
interferometer laser gauges 14 may be separated by a distance of 
approximately 20 cm. Each interferometer 14 and 16 is typically placed 2 
to 32 cm from the return mirrors 30 and 32, respectively. The above 
dimensions are only given by way of example, and other dimensions may be 
appropriate depending on the particular application. 
Many advantages are obtained by the structure of the present invention. By 
placing the mirrors off the wafer stage, the mirrors can be made larger 
and more stable, and more accurately manufactured at lower cost. 
Additionally, the wafer stage can be made smaller and of less weight. 
Wafer stage rotation accuracy is also improved by increasing the 
separation of the paired interferometers without increasing the size of 
the wafer stage, which would normally be required if the mirrors were 
mounted on the laser stage as is conventionally done. Additionally, it is 
also possible to mount the interferometers so as to place them in or close 
to the wafer plane, eliminating errors such as Abbe offset error. 
Additionally, with the structure of the present invention, the angle at 
which the input beam enters the interferometer does not influence 
alignment. Therefore, some degree of rotation about the Z axis, of 
approximately two milliradians, is possible without affecting alignment or 
position accuracy, depending upon the type of interferometer laser gauge 
used. Typically, the wafer stage 10 has three laser gauge interferometers 
placed close to the wafer plane for measuring three degrees of freedom, 
X-translation, Y-translation, and Z rotation. The three laser gauge 
interferometers are preferably mounted on the wafer stage 10 forming a 
right triangle, with the two short legs between the 90.degree. angle being 
parallel to the respective longitudinal axis of the return mirrors 30 and 
32. 
FIG. 2 is a perspective view generally illustrating an embodiment 
implementing the schematic drawing of FIG. 1. A wafer stage 10' has a 
wafer chuck 12' placed thereon. A wafer, not shown, is placed on the wafer 
chuck 12'. An interferometer 14' is positioned at one corner of the wafer 
stage 10'. A pair of interferometer laser gauges 16' are positioned 
parallel to the Y axis. Accordingly, in this embodiment only three 
interferometer laser gauges 14' and 16' are needed in order to obtain 
sufficient positioning and alignment information. The wafer stage 10' 
typically has three degrees of freedom in the Y-translation, 
X-translation, and Z-rotation. Between the pair of interferometers 16' are 
placed the first beamsplitter 22', which may be a penta prism, and a 
second beamsplitter 24', which also may be a penta prism. A pair of beam 
folders or fold mirrors 44 and 52 are used to direct the light beam to one 
of the interferometers 16'. A beam folder or fold mirror 46 is associated 
with the other interferometer 16' and directs the light beam thereto. Beam 
folder or fold mirror 48 and fold mirror 50 are associated with 
interferometer 14' and directs the light beam thereto. Associated with 
each interferometer 14' and 16' is an arm 54 having a reference mirror 56 
thereon. The wafer stage 10' rides on air bearings 42, preferably three 
are used in a triangular arrangement, with only two being illustrated. The 
air bearings 42 ride on a plane surface, not shown. When the wafer stage 
10' is positioned vertically, a counter force cylinder 58 is used to 
compensate for the weight of the wafer stage 10'. The counter force 
cylinder 58 is attached to a support 60 which is coupled to a motor or 
linear drive 62. The motor or linear drive 62 permits the wafer stage 10' 
to be moved in the X direction, indicated by arrow 64. A stationary return 
mirror 32' is placed parallel to the Y axis formed by the pair of 
interferometer 16'. The return mirror 32' has a length sufficient to 
accommodate the entire travel distance of the wafer stage 10' in the Y 
direction. A stationary return mirror 30' is positioned parallel to the X 
axis, and has a length sufficient to accommodate the entire travel 
distance of the wafer stage 10' in the X direction. Accordingly, the 
stationary return mirrors 30' and 32' can be made relatively large and 
stable because they are not placed on the wafer stage 10'. Also associated 
with the wafer stage 10' is a calibration detector 66. Calibration 
detector 66 is used in some alignment and positioning operations. 
In operation, a laser source 34' provides a beam of light 37' which is 
directed parallel to the X axis. The beam enters a penta prism 36' which 
folds or redirects the beam 90.degree. to a beamsplitter 22', which may be 
a penta prism. The beamsplitter 22' directs a portion of the beam to a 
fold mirror 46 and another portion of the beam to another beamsplitter 
24', which may be a penta prism. The fold mirror 46 directs the beam to a 
first interferometer 16'. Beamsplitter 24' directs a portion of the beam 
to another fold mirror or beam folder 48 and a portion of the beam to a 
fold mirror or beam folder 44. The fold mirror or beam folder 48 directs 
the beam to a fold mirror or beam folder 50, which directs the beam to 
interferometer 14'. The beam received by the beam folder or fold mirror 44 
directs the beam to beam folder or fold mirror 52. Beam folder or fold 
mirror 52 then directs the beam of light to the second interferometer 
laser gauge 16'. The three interferometer laser gauges 16' and 14' are 
used in association with the stationary mirrors 30' and 32' to accurately 
obtain position and alignment information on the precise location of the 
wafer stage 10' in X-translation, Y-translation and Z-rotation. The three 
interferometers 14' and 16' preferably form a right triangle in a plane 
parallel to the planar surface of the wafer chuck 12'. Preferably, the 
interferometer laser gauges 14' and 16' are placed near the wafer plane, 
thereby eliminating errors such as abbe offset errors. Additionally, the 
structure of the present invention with the use of penta prisms helps to 
maintain the alignment of the interferometer laser gauges irrespective of 
small rotations or twisting about the Z axis, and therefore, is less 
sensitive to rotation than conventional alignment systems using 
interferometers. 
The present invention improves the range of travel of a wafer stage having 
less mass or weight, while at the same time improving alignment and 
positioning accuracies and being less sensitive to certain errors 
introduced by rotation, or having the interferometers positioned away from 
the wafer plane. Accordingly, the present invention improves and advances 
the art. 
Although the preferred embodiment has been illustrated, it will be obvious 
to those skilled in the art that various modifications may be made without 
departing from the spirit and scope of this invention.