Interferometer system and method for lens column alignment

A projection exposure apparatus and method aligns a substrate with an optical axis of a lens column using one or more interferometers, mirrors, and a control device. The lens column projects a pattern from a mask onto the substrate. The optical axis of the lens column is perpendicular to the substrate. To ensure precise alignment of the lens column and the substrate, the one or more interferometers use a plurality of beams having respective paths, the lengths of which change in response to movement of the optical axis. In response to the detected changes of the interferometer beams, a control device adjusts the position of the stage so that the substrate is aligned with the optical axis of the lens column.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
The present invention relates generally to a projection exposure system for 
transferring an image of a pattern onto a substrate, such as a 
semiconductor wafer. More particularly, the present invention relates to a 
projection exposure apparatus and method that provides enhanced precision 
in the alignment of a projection optical system and the substrate. 
B. Description of the Prior Art 
Semiconductor fabrication requires precise alignment of an optical system 
with a substrate in order to produce extremely detailed circuitry on the 
substrate. FIG. 1 illustrates a conventional projection exposure apparatus 
10, which includes a light source 12, a stage 14, a lens column 16, a mask 
18, and an interferometer 20. Light source 12 illuminates mask 18, causing 
a pattern from the mask to be projected through lens column 16. Lens 
column 16 projects the pattern onto a photosensitive substrate on stage 
14. The optical axis of lens column 16 is indicated by the broken line 
down the middle of lens column 16. 
Exposure of the pattern onto the substrate requires moving stage 14 to 
provide accurate alignment between the substrate and lens column 16. The 
optical axis of lens column 16 cannot be directly measured without 
disturbing the projection of the image of the pattern to stage 14. 
Therefore, in conventional systems, interferometer 20 detects alignment by 
projecting a first light beam 21 to a mirror on side 24 of lens column 16, 
and a second light beam 22 to a corresponding side 26 of stage 14. Each 
beam is reflected back through a beam splitter to a control device. As the 
beams travel back through the beam splitter they interfere with each 
other. Using the interference information, the control device determines 
the alignment between the optical axis and the substrate on stage 14. In 
response to the alignment determination, the position of stage 14 is 
adjusted in relation to lens column 16 to align the substrate on stage 14 
with lens column 16. 
Charges in beam 21, however, may not provide accurate information for 
alignment because of the sensitivity of lens column 16 to thermal 
expansion and displacement. Fluctuations in the air temperature may result 
in expansion of lens column 16, as generally illustrated in FIG. 2. Note 
that an even expansion (or contraction) of both sides does not displace 
the optical axis. FIG. 3 illustrates shifting of lens column 16. Shifting 
of lens column 16 results in the same reading from beam 21 as is received 
in FIG. 2 even though the optical axis in FIG. 2 has not shifted. 
Therefore, an adjustment of stage 14 based on the interferometer readings 
in FIG. 2 will result in an inaccurate alignment of stage 14 and lens 
column 16. Accordingly, errors arise in the exposure of stage 14 to the 
image of the pattern projected through lens column 16. More particularly, 
because conventional alignment procedures use only one side of lens column 
16, all of the alignment problems are not properly detected. Accordingly, 
the adjustment of stage 14 in relation to lens column 16 is often 
inaccurate and, as such, results in errors in the projection of the image 
of the pattern onto the substrate on stage 14. 
Further still, lens column 16 may be sensitive to vibrations in the 
projection exposure apparatus. Typically, adjusting stage 14 results in 
vibration throughout the projection exposure apparatus. This vibration may 
result in fluctuations of lens column 16, as generally illustrated in FIG. 
3. Because of the fluctuations, the precise alignment of lens column 16 
may no longer be attainable from the conventional alignment procedure. 
Accordingly, adjustment of stage 14 in relation to lens column 16 is often 
inaccurate and results in errors in the projection of the image of the 
pattern onto the substrate on stage 14. 
Although these inaccuracies of conventional alignment procedures may be 
tolerable for the level of precision required under certain semiconductor 
manufacturing design constraints, the precision required in modern 
semiconductor manufacturing processes requires a reduction in or 
elimination of these inaccuracies. To attain such precision, errors caused 
by thermal deformation, displacement, and vibration in lens column 16 must 
be substantially eliminated. 
Therefore, there is a need for a projection exposure apparatus and method 
that can substantially reduce inaccuracies in the alignment of the lens 
and the stage. 
SUMMARY OF THE INVENTION 
The advantages and purpose of the invention will be set forth in part in 
the description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The 
advantages and purpose of the invention will be realized and attained by 
means of the elements and combinations particularly pointed out in the 
appended claims. 
To attain the advantages and in accordance with the purpose of the 
invention, as embodied and broadly described herein, the projection 
exposure apparatus of the present invention includes a projection optical 
system, one or more interferometers, and a control device. To align the 
lens column and the substrate, the one or more interferometers pass a 
plurality of beams having respective paths that travel to respective first 
and second sides of the lens column, thus detecting true shifts in the 
optical axis. In response to the beams, the control device adjusts the 
position of the stage so that the substrate on the stage is properly 
aligned with the optical axis of the lens column. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be obvious from 
the description, or may be learned by practice of the invention. The 
objects and advantages of the invention will be realized and attained by 
means of the elements and combinations particularly pointed out in the 
appended claims. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory only and are 
not restrictive of the invention, as claimed.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Reference will now be made in detail to an embodiment of the invention, 
examples of which are illustrated in the accompanying drawings. 
In accordance with the invention, there is provided a projection exposure 
apparatus and method for transferring an image of a pattern formed on a 
mask through a lens column along an optical axis perpendicular to a 
substrate, onto the substrate. Precise alignment of the substrate with the 
optical axis is achieved by the present invention, which detects proper 
alignment using one or more interferometers that pass at least one beam of 
light to an opposite side of the lens column. 
The projection exposure apparatus of the present invention includes a 
projection optical system, one or more interferometers, and a control 
device. The one or more interferometers are oriented such that a plurality 
of beams are created. Each beam has a respective path, the length of which 
changes in response to a change in a dimension of the lens column along 
the path of the beam. The beams are reflected back to a control device 
which receives the beams, determines the changes in a dimensions of the 
lens column, and adjusts the stage on which the substrate is positioned so 
that a particular area of the substrate is aligned with the optical axis 
of the lens column. 
FIG. 4 is a block diagram showing one embodiment of projection exposure 
apparatus 30 consistent with the principles of the invention. Projection 
exposure apparatus 30 includes lens column 32, mask 36, stage 38 and 
interferometers 34 and 35. Lens column 32 projects an image of a pattern 
from mask 36 onto a substrate on stage 38. The broken line down the center 
of lens column 32 is optical axis 40. The present invention ensures 
precise alignment of a substrate on stage 38 with lens column 32 by 
properly detecting inaccuracies introduced by thermal expansion, 
displacement, and vibration of lens column 32 and adjusting stage 38 
accordingly. 
Interferometers 34 and 35 are positioned on mount 42. Stage 38 is 
positioned at the other end of base 44. In one embodiment, each 
interferometer 34 and 35 includes a separate light source, preferably a 
laser beam, and projects beams to beam splitters 46 and 47, respectively. 
Interferometer 34 projects laser beam 41 to beam splitter 46. Beam 
splitter 46 splits the beam and projects a first beam to first reflective 
surface 50 positioned proximate to side 37 of stage 38, and a second beam 
to mirror 52. Mirror 52 then deflects the second beam to reflective 
surface 54 positioned proximate to side 33 of lens column 32. 
Interferometer 35 projects laser beam 43 to beam splitter 47. Beam splitter 
47 splits the beam and projects a first beam to second reflective surface 
54 positioned proximate to side 33 of lens column 32, and a second beam to 
mirror 56. Mirror 56 deflects the second beam to third reflective surface 
58 positioned proximate to side 39 of lens column 32. Lens column 32 
includes tunnel 60 which allows the second beam to travel through lens 
column 32 to third reflective surface 58. Tunnel 60 is oriented 
substantially parallel to the image plane. In one embodiment, lens column 
32 includes a plurality of lenses, and tunnel 60 is disposed in a space 
between adjacent lenses. 
The beams created by beam splitter 47 are reflected back by second 
reflective surface 54 and third reflective surface 58 and detected by 
detector 24. Detector 24 determines changes in the beam lengths by the 
interference of the beams. Similarly, detector 25 determines changes in 
the beam lengths of the beams created by beam splitter 46. Based on the 
changes in beam length detected by detectors 24 and 25, a control device 
(not shown) moves stage 38 to align the substrate on stage 38 with lens 
column 32. Movement of stage 38 is well-understood in the art and will not 
be detailed here. 
The beams from interferometers 34 and 35 allow measurement of displacement 
of each side of the lens column 32 in a direction perpendicular to optical 
axis 40, which allows determination of alignment of lens column 32 in 
relation to stage 38 in the direction perpendicular to optical axis 40. 
From the output of interferometer 35, movement of optical axis 40 may be 
obtained with a high degree of precision because changes in the dimensions 
of lens column 32 in the direction perpendicular to optical axis 40 can be 
detected and compensated for if necessary. 
A similar configuration of interferometers 34 and 35, first reflective 
surface 50, and second reflective surface 54 and third reflective surface 
58, is replicated for determining similar fluctuations of lens column 32 
in a direction perpendicular to the fluctuations detected in FIG. 4. That 
is, whereas the configuration of elements in FIG. 4 detects fluctuations 
of lens column 32 in the left and right direction, additional elements are 
used to detect fluctuations of lens column 32 in a direction perpendicular 
to left and right. 
FIGS. 5 and 6 illustrate an embodiment wherein placement of reflective 
surfaces requires a beam to travel around, rather than through, lens 
column 32. The output of interferometer 34 provides the location of the 
corresponding sides of the stage and the projection optical system with 
respect to one another with a high degree of precision. 
Using the information from interferometers 34 and 35, stage 38 is adjusted 
so that optical axis 40 and the substrate on stage 38 are properly 
aligned. Once aligned, projection exposure apparatus 30 may accurately 
transfer the pattern on mask 36 onto the substrate of stage 38. 
Accordingly, the alignment of optical axis 40 to a particular area of a 
substrate on stage 38 enhances the precision of the exposure of the 
substrate. 
FIG. 5 illustrates one variation of the projection exposure apparatus 
consistent with the invention. The projection exposure apparatus of FIG. 5 
is similar to the exposure apparatus of FIG. 4. In contrast to the 
projection exposure apparatus 30 of FIG. 4, lens column 68 does not 
include a tunnel, and thus the beams are routed differently than in FIG. 
4. Interferometer 35 is configured so that one beam projects to side 39 of 
lens column 68. To accomplish this, interferometer 35 projects laser beam 
43 to beam splitter 47. Beam splitter 47 splits the beam and projects a 
first beam to second reflective surface 54 positioned proximate to side 33 
of lens column 68 and a second beam to mirror 56. Mirror 56 deflects the 
second beam to mirror 62 positioned proximate to side 33 of lens column 
68. Mirror 62 deflects the beam to mirror 64 positioned proximate to a 
corner of side 33 of lens column 68. Finally, mirror 64 projects the 
second beam to third reflective surface 66 positioned proximate to side 39 
of lens column 68. Thus, interferometer 35 provides a measurement of the 
dimension of lens column 68 in the direction perpendicular to optical axis 
40. The output of interferometer 35 provides the position of optical axis 
40 of lens column 68 with a high degree of precision. Also, the output of 
interferometer 34 provides the position of stage 38 in relation to lens 
column 68. Using these measurements, the position of stage 38 may be 
appropriately adjusted, thus resulting in the precise alignment of optical 
axis 40 with a particular area of a substrate on stage 38. 
Although FIG. 5 shows the beam traversing over the top of lens column 68, 
interferometer 35 and mirrors 62, 64 and 66 could be arranged so that the 
beam traverses around the bottom of projection optical system 68. 
Furthermore, similar to FIG. 4, there is another set of interferometers 
and mirrors which measure fluctuation in a direction perpendicular to the 
fluctuations measured by the elements shown in FIG. 5. 
FIG. 6 illustrates another embodiment of the projection exposure apparatus 
of the present invention wherein only one interferometer 70 is used for 
measuring the dimension of lens column 32 in a direction perpendicular to 
optical axis 40, as well as the alignment of lens column 32 in relation to 
stage 38 in the direction perpendicular to optical axis 40. Interferometer 
70 is positioned on mount 72. Mount 72 and stage 38 reside on base 44. 
Interferometer 70 projects laser beam 71 to first beam splitter 76. Beam 
splitter 76 splits the beam and projects a first beam to second beam 
splitter 78 and a second beam to third beam splitter 80. Second beam 
splitter 78 splits the first beam and projects a beam to first reflective 
surface 50 positioned proximate to side 37 of stage 38 and a beam to 
mirror 84 positioned on mount 72. Mirror 84 deflects the beam to second 
reflective surface 54 positioned proximate to side 33 of lens column 32. 
Third beam splitter 80 splits the second beam and projects one beam to 
third reflective surface 86 positioned proximate to side 35 of lens column 
32 and another beam to mirror 90 positioned on mount 72. The beam projects 
to third reflective surface 86 by traversing through lens 88 of lens 
column 32. Mirror 90 deflects the beam to second reflective surface 54 
positioned proximate to side 33 of lens column 32. This embodiment may be 
used in situations where there is insufficient space between adjacent 
lenses of lens column 32 to position a tunnel. 
Interferometer 70 provides the beams for measurement of the dimensions of 
lens column 32 in a direction perpendicular to optical axis 40, and 
alignment of lens column 32 in relation to stage 38 in the direction 
perpendicular to optical axis 40. From the output of interferometer 70, 
the location of optical axis 40 of lens column 32 and the location of 
stage 38 in relation to lens column 32 may be determined with a high 
degree of precision. By using measurements from the reflected beams, stage 
38 may be appropriately adjusted, thus resulting in the precise alignment 
of optical axis 40 and the substrate on stage 38. 
A second set of elements (not shown) is also included to perform similar 
measurements of fluctuations perpendicular to the fluctuations measured by 
the elements of FIG. 6. 
FIG. 7(a) is a block diagram illustrating detection of deformation 
utilizing beams passing through the projection optical system, similar to 
the technique illustrated in FIG. 4. More particularly, in the embodiment 
shown in FIG. 7(a), two beams are used. A first beam 92 is reflected off 
of a second reflective surface 54 on the left side 33 of the lens column 
32. A second beam 93 is reflected through lens column 32 to a third 
reflective surface 58 on the right side 35 of lens column 32, and back 
through the opening 60 on the left side 33 of lens column 32. Using 
measurements from beams 92 and 93, it can be determined whether the 
deformation of the lens column 32 has had any impact on the optical axis, 
and whether adjustments need to be made. Note that conventional systems 
take interferometer readings from a single reflective surface on the side 
of the projection optical axis which no shift actually took place. 
Interferometer system 34 is utilized for aligning stage 38 position. Since 
interferometer system 34 measures and detects the difference between the 
length of reference axis 91 and axis 90, stage position can be measured if 
reference axis is stable. When optical axis 40 is aligned with a target 94 
on stage 38, the difference between the length of axis 91 and of axis 90 
that can be measured by interferometer system is defined as: Length of 
axis 90-Length of axis 91=A. 
FIG. 7(b) shows the expansion of 1 lens column 32 by x. The left wall 95 of 
lens column 32 shifts x because of lens column expansion. The reference 
axis length becomes shorter by x. Thus, Length of axis 90-Length of axis 
91=A+x. As the stage controller corrects for x by moving stage 30, stage 
38 moves x to left. Since optical axis 40, however, does not move in 
response to lens expansion, target 94 shifts a distance x from optical 
axis 40. Using interferometer system 35, x can be measured. If lens column 
32 expands x, the length of 92 becomes shorter by x, and the length of 93 
becomes longer by x. Thus, the difference between the length of 92 and the 
length of 93 changes 2x. The interferometer system 35 can measure this 
change of 2x, and interferometer system 34 can respond properly. 
FIG. 8 illustrates lens displacement. The left wall 98 of lens column 32 
shifts x because of lens column displacement. Thus, the reference axis 
length becomes shorter by distance x: Length of axis 90-Length of axis 
91=A+x. As the stage controller corrects for x by moving stage 38, stage 
30 moves by distance x to left. In the case of displacement, since optical 
axis 40 also moves by lens displacement, target 94 keeps the same position 
as optical axis 40. If lens column 32 shifts by x, the length of 92 
becomes shorter by x, and the length of axis 93 also becomes shorter by x. 
The difference between the length of 92 and the length of 93 does not 
change. The detection by interferometer system 35 does not affect the 
result of interferometer system 34. Thus, lens expansion that could not be 
distinguished with lens displacement can be measured by utilizing the 
interferometer system 35 and can be corrected. 
FIG. 9 illustrates replacing the interferometer system 34 and 35 of FIG. 8 
with the interferometer system 96, 98, and 100. In FIG. 9, each 
interferometer system is called absolute type. The system 98 has beam 43, 
beam splitter 47, detector 24 and reference mirror 101 instead of second 
reflective surface 54. This system measures the difference between the 
length of 93 and the length of 101. Thus, system 98 measures the position 
of third reflective surface 58. In the same way, system 96 measures the 
position of first reflective surface 50 (stage) and system 100 measures 
the position of second reflective surface 54. The relationship between 
stage position and lens column 32 can be calculated electrically as: 
Result of system 96-Result of system 100. Lens expansion can be calculated 
electrically as: Result of system 98-Result of system 100. Thus, this 
system can provide a result similar to that of the system shown in FIG. 
7(a). 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the exposure of the present invention 
without departing from the scope or spirit of the invention. For example, 
although the systems and methods described include one or two 
interferometers for detecting movement of the optical axis in a particular 
direction, the projection exposure apparatus may include any number of 
interferometers, and the positions of the interferometers are not limited 
as shown. In addition, the system may include alternative configurations 
of mirrors. Furthermore, the one or more interferometers may measure the 
dimension of the projection optical system in a dimension perpendicular to 
the optical axis at multiple locations of the projection optical system. 
The invention is not limited to use as an apparatus or method for the 
exposure of a mask pattern onto a substrate. The apparatus and method for 
detecting alignment error described consistent with the invention may be 
employed in any system requiring a high degree of accuracy and in which 
external factors may influence alignment. 
Furthermore, the exposure system in the present embodiment can also be 
applied to a scanning-type exposure system (See, e.g., U.S. Pat. No. 
5,473,410), where a mask and a wafer are moving synchronously to expose a 
mask pattern. 
In addition, the exposure system in the present embodiment can be applied 
to a step-and-repeat type exposure system, where a mask pattern is exposed 
while the mask and the wafer are stationary, and the wafer is stepped and 
moved in succession. 
In addition, the invention may also be applied to a proximity exposure 
system, where a mask pattern is exposed by closely placing the mask and 
the wafer, without using projection optics. 
The use of the exposure system does not need to be limited to semiconductor 
manufacturing. For example, it can be used in an LCD exposure system, 
where an LCD pattern is exposed onto a rectangular glass plate, or an 
exposure system for manufacturing a thin film magnetic head. 
With respect to the light source for the exposure system, the g-line (436 
nm), i-line (365 nm), KrF excimer laser (248 nm), ArF excimer laser (193 
nm), and the F2 laser (157 nm) may be used, as well as charged particle 
beams such as the x-ray and electron beams. If an electron beam is used, 
thermionic emission type lanthanum hexaboride (LaB.sub.6) or tantalum (Ta) 
can be used as an electron gun. Furthermore, when an electron beam is 
used, the structure could use a mask, or it could be structured such that 
a pattern can be formed directly onto a wafer without using a mask. 
The magnification of the projection optical system does not have to be 
limited to a reduction system. For example, it could be 1x or a 
magnification system as well. 
With respect to the projection optical system, when far ultra-violet rays 
such as the excimer laser is used, glass materials that transmit far 
ultra-violet rays such as quartz and fluorite should be used. When the F2 
laser or the x-ray is used, the optical system should be either 
catadioptric or refractive (the reticle should be a reflective type). 
Finally, when an electron beam is used, electron optics should consist of 
electron lenses and deflectors. Needless to say, the optical path for 
electron beams should be in vacuum. 
When linear motors (See, e.g., U.S. Pat. No. 5,623,853 or U.S. Pat. No. 
5,528,118) are used in a wafer stage or a reticle stage, they may be any 
of a variety of available linear motors, such as the air levitation type 
using air bearings or the magnetic levitation type using the Lorentz force 
or reactance force. In addition, the stage could move along a guide, or it 
could be a guideless type where no guide is installed. 
The stage drive system may be implemental using a planar motor which drives 
the stage by electromagnetic force, in which a magnet unit having magnets 
arranged two-dimensionally and an armature coil unit having coils arranged 
two-dimensionally are facing with each other, can be used. In this case, 
either one of the magnet unit or the armature coil should be connected to 
the stage, and the other should be mounted on the moving plane side of the 
stage. 
Reaction force generated by the wafer stage motion can be mechanically 
released to the floor (ground) by using a frame member, as described in JP 
Hei 8-166475 published patent (U.S. Pat. No. 5,528,118). 
Reaction force generated by the reticle stage motion can be mechanically 
released to the floor (ground) by using a frame member, as described in JP 
Hei 8-330224 published patent (U.S. Ser. No. 08/416,558). 
As described above, an exposure system consistent with the principles of 
the present invention can be built by assembling various subsystems, 
including the elements listed in the claims of the present application, in 
such a manner that the prescribed mechanical accuracy, electrical accuracy 
and optical accuracy are maintained. To maintain accuracy of various 
subsystems, every optical system is adjusted to achieve its optical 
accuracy, every mechanical system is adjusted to achieve its mechanical 
accuracy, and every electrical system is adjusted to achieve its 
electrical accuracy before and after its assembly. The process of 
assembling each subsystem into an exposure system includes mechanical 
interfaces, electrical circuits wiring connections and air pressure 
plumbing connections. Each subsystem is assembled prior to assembling the 
exposure system from various subsystems. Once the exposure system is 
assembled with various subsystems, overall adjustment is performed so as 
to ensure that every accuracy is maintained in the total system. 
Incidentally, it is desirable to manufacture an exposure system in a clean 
room where the temperature and the cleanliness are controlled. 
Conventional semiconductor devices are fabricated by going through the 
following steps: the device's function and performance are designed; a 
reticle is designed according to the previous designing step; a wafer is 
made from a silicon material; a pattern from the reticle is exposed on a 
wafer by the exposure system in the aforementioned apparatus and methods 
consistent with the principles of the invention; the device is assembled 
(including the dic6ing process, bonding process and packaging process); 
and the inspection step, etc. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification and examples be 
considered as exemplary only, with a true scope and spirit of the 
invention being indicated by the following claims.