Alignment system for lithographic proximity printing

An alignment system is described for lithographic proximity printing apparatus wherein the wafer and lithographic mask are each individually aligned with a third element. An alignment mask carries an alignment pattern corresponding to an alignment mark on the microcircuit wafer and also carries an alignment pattern corresponding to an alignment mark on the proximity printing mask. In one embodiment, the alignment mask is illuminated from the back side by alignment radiation (which need not be visible light) and the alignment patterns carried by the alignment mask are imaged onto the corresponding wafer and proximity printing mask alignment marks. Since the projected alignment patterns are spatially separated at the alignment mask one of the alignment patterns is conveniently shifted in effective optical position to compensate for the difference in axial position of the wafer and printing mask alignment marks. When a projected alignment pattern image correlates with (i.e., overlays) the corresponding alignment mark, reflected or scattered light leaving the alignment mark is either at a maximum or at a minimum (i.e., an extremum) depending upon the system configuration. A light splitter deflects some of the light coming from each of the alignment marks to individual light intensity monitors, such as photomultipliers. Alignment of the wafer and printing mask has been achieved when both intensity monitors reach the correct extremum simultaneously (or reach the midpoint of the correct extremum if the extremum is not sharp).

FIELD OF THE INVENTION 
This invention relates to systems for aligning a microcircuit wafer and an 
overlying closely spaced lithographic pattern mask for lithographic 
proximity printing during manufacture of microcircuit electronic 
components, and more particularly, it relates to a system of this type 
which may be readily automated. 
BACKGROUND OF THE INVENTION 
During conventional fabrication of microcircuit components, patterns are 
successively transferred from masks to resist layers on a wafer. In order 
to build a useful microcircuit component, each successive pattern must be 
accurately aligned with previously transferred underlying patterns. As the 
linewidth of microcircuits gets smaller, alignment accuracy must be 
correspondingly improved. Furthermore, the number of alignment steps 
needed to fabricate a single microcircuit component has been increasing, 
thus making an automatic alignment system highly desirable. 
One method of transferring a pattern from a mask to an optically sensitive 
wafer surface is by optical projection printing. This technique has the 
advantage that the pattern on the mask can be scaled up in size for ease 
in fabricating the mask. A projection system then demagnifies the enlarged 
pattern to the desired size during a subsequent projection printing step. 
An accurate alignment system for projection printing which can be readily 
automated is described in my U.S. Pat. No. 4,232,969. 
A disadvantage with projection printing is that optical focussing elements 
are needed. This limits the wavelength of the light which can be used for 
projection printing since suitable focussing elements do not exist at 
short wavelengths, such as for example in the soft x-ray region of about 6 
.ANG. to 25 .ANG. in wavelength. This region is of particular interest 
because lithographic resist materials are being developed which are 
sensitive to x-rays in this region. Such shorter wavelengths are generally 
of interest for lithography because they promise the possibility of 
improved resolution, smaller linewidths and more dense, compact, faster 
and possibly cheaper microcircuits. 
Since projection printing currently cannot be used for x-ray lithography, 
proximity printing is contemplated instead. This method places the mask 
very close to but not in contact with the wafer because contact would 
cause damage to the wafer surface and the mask. The proximity mask is then 
flooded with actinic radiation. While proximity printing is a generally 
well known lithographic technique, highly accurate alignment techniques 
still do not exist for this method and none of the known alignment 
techniques for proximity printing systems are readily automatable. 
Prior art alignment systems for proximity printing generally involve the 
use of a split field alignment microscope. A person visually observes 
through a microscope an alignment mark on the wafer and an alignment mark 
on the proximity mask and then adjusts the relative position of the 
proximity mask and wafer until the marks are aligned. One problem is that 
the marks are not in the same object plane (typically they are separated 
by 20-60 .mu.m) and microscopes typically can be focussed only on one 
plane at a time. One solution is to use a low numerical aperture optical 
system so that there is a large depth of field, but this results in lower 
resolution and consequently lower alignment accuracy. Another solution is 
to use a bifocal optical system. Such a system is described, for example, 
by A. White in "Simple bifocus element for microscope objectives", 16 
Appl. Optics 549 (1977). Unfortunately, bifocal elements have reduced 
contrast and any microscope which uses visible light has resolution limits 
imposed by the wavelength of the light employed. Automation of such 
systems has not been particularly successful because of the inherent 
complexity involved in aligning (and consequently matching) two images 
electronically. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an alignment system for 
microcircuit lithography proximity printing systems with an alignment 
accuracy suitable for x-ray lithography. 
Another object is to provide an alignment system for lithographic proximity 
printing systems which is readily automatable. 
These and further objects and advantages are achieved by my invention, 
wherein the wafer and proximity mask are each individually aligned to a 
third element, an alignment mask. 
The alignment mask carries an alignment pattern corresponding to an 
alignment mark on the microcircuit wafer and also carries an alignment 
pattern corresponding to an alignment mark on the proximity printing mask. 
In one embodiment, the alignment mask is illuminated from the back side by 
alignment radiation (which need not be visible light) and the alignment 
patterns carried by the alignment mask are imaged onto the corresponding 
wafer and proximity printing mask alignment marks. Since the projected 
alignment patterns are spatially separated at the alignment mask one of 
the alignment patterns is conveniently shifted in effective optical 
position to compensate for the difference in axial position of the wafer 
and printing mask alignment marks. When a projected alignment pattern 
image correlates with (i.e., overlays) the corresponding alignment mark, 
reflected or scattered light leaving the alignment mark is either at a 
maximim or at a minimum (i.e., an extremum) depending upon the system 
configuration. A light splitter deflects some of the pattern light coming 
from each of the alignment marks to individual light intensity monitors, 
such as photomultipliers. Alignment of the wafer and printing mask has 
been achieved when both intensity monitors reach the correct extremum 
simultaneously (or the midpoint of the correct extremum if the extremum is 
not sharp). 
The optical system is bidirectional so that the intensity monitors and the 
alignment radiation source may be exchanged in position. Preferably the 
light splitter is an apertured mirror which splits the numerical aperture 
of the objective so that dark field illumination may be used for improved 
sensitivity and signal to noise ratio. The alignment mask actually does 
the alignment pattern matching, thereby facilitating automation and 
permitting optimizations in the optical system.

DETAILED DESCRIPTION OF THE INVENTION 
The principles of my invention will now be described in connection with 
FIG. 1, which illustrates a simplified visually monitored and mannually 
controlled embodiment having bright field illumination. Wafer 10 carries 
an alignment mark 12 and is positioned parallel with and is closely spaced 
from a lithographic proximity printing mask 14 carrying an alignment mark 
16. Proximity mask 14 obviously must be transparent to alignment radiation 
in region 19 immediately above wafer alignment mark 12. If the proximity 
mask supporting substrate 17 is transparent to alignment radiation, this 
region 19 overlying wafer mark 12 merely needs to be free from opaque mask 
material 21. Otherwise an aperture must be provided in the proximity mask 
substrate at region 19. As illustrated, plane 18 defined by the wafer 
alignment mark 12 is spaced from plane 20 defined by the proximity mask 
alignment mark 16. A typical spacing is about 20 to 60 .mu.m. The position 
of wafer 10 within plane 18 may be controlled via line 22 by conventional 
position adjustment apparatus 24. Also, the position of proximity mask 14 
within plane 20 may be controlled via line 26 also by apparatus 24. 
Apparatus 24 and consequently the relative position of wafer 10 and 
proximity mask 14 are controlled manually via line 28. 
Alignment mask 30 defines a third plane 32 and carries a first transparent 
mask pattern 34 corresponding to wafer mark 12 and a second transparent 
mask pattern 36 corresponding to the proximity printing mask mark 16. An 
objective 38 images plane 32 and plane 20 onto each other through beam 
splitter 40. A block of transparent material 42 having appropriate index 
of refraction and thickness shifts the image of pattern 34 downward from 
plane 20 to plane 18 so that at alignment the image of pattern 36 will 
fall on mark 16 in focus while the image of pattern 34 will simultaneously 
fall on mark 12 also in focus, even though objective 38 may have a high 
numerical aperture, which is preferred. 
Beam splitter 40 defines a fourth plane 42 such that objective 38 also 
images plane 20 and plane 42 onto each other. Thus, a first bidirectional 
light propogation path 44 is formed from alignment mask pattern 34 through 
beam splitter 40 and objective 38 to wafer mark 12, reflecting then back 
from mark 12 through objective 38 and reflecting from beam splitter 40 to 
position 46 in front of eyepiece 47 and observation eye 48. A second 
bidirectional light propogation path 50 is also formed from alignment mask 
pattern 36 through beam splitter 40 and objective 38 to the printing mask 
mark 16, reflecting then back from mark 16 through objective 38 and 
reflecting from beam splitter 40 to position 52 in front of eyepiece 47 
and observation eye 48. 
A source of alignment radiation 54, illuminates the back side of alignment 
mask 30 through a lens 56, thereby causing a pattern of light 
corresponding to wafer mark 12 to be imaged by objective 38 along path 44 
to the position in plane 18 where wafer mark 12 is to be located at 
alignment. Simultaneously another pattern of light corresponding to 
proximity mask mark 16 is imaged by objective 38 along path 50 to the 
position in plane 20 where printing mask mark 16 is to be located at 
alignment. Eye 48 simultaneously observes any reflection of these light 
patterns. When wafer 10 is properly positioned, the projected light 
pattern from the alignment mask pattern 34 correlates (i.e., maximally 
overlaps) with the reflective wafer mark 12 (assuming the wafer mark is 
reflective and the background is not reflective rather than the reverse, 
which is an obvious alternative) and the light intensity of this pattern 
observed by eye 48 will reach a maximum. Also, when proximity mask 14 is 
properly positioned, the projected light pattern from the alignment mask 
pattern 36 correlates (i.e., maximally overlaps) with the reflective 
proximity mask mark 16 (assuming again that the proximity mask mark is 
reflective on a non-reflective background, rather than the alternative) 
and the light intensity of this pattern observed by eye 48 will also reach 
a maximum. 
Thus eye 48 monitors the intensity of both light patterns for a 
simultaneous extremum (maximum or minimum depending upon whether the 
alignment light pattern falls upon a relatively reflective or relatively 
absorptive mark), while adjusting the positions of wafer 10 and proximity 
mask 14. When both patterns are at the appropriate extremum intensity, 
wafer 10 and mask 14 are aligned. It should be apparent that this system 
would function similarly and equally well if the illumination source 54, 
56 and the light intensity monitors (embodied by eyepiece 47 and eye 48) 
were exchanged for each other in position. Furthermore it should be 
apparent that if a light pattern intensity extremum is relatively flat 
(i.e., does not have a sharp maximum or minimum), then alignment occurs at 
the midpoint of the extremum. 
The FIG. 1 embodiment illustrates use of bright field illumination. 
Although bright field illumination can be used in practicing my invention, 
dark field illumination is preferred because it offers better sensitivity 
and a better signal-to-noise ratio and because the nature and 
characteristics of dark field optical signals are more predictable for 
reliable sensing by electronic light intensity detectors. A preferred 
embodiment of my invention is illustrated in FIG. 2, which employs dark 
field illumination. 
An exemplary pattern for the wafer mark 12 and an exemplary pattern for the 
proximity mask mark 16 are shown in FIG. 3. Wafer mark 12 comprises two 
perpendicular and crossing bars 54, 56 oriented in an X direction and Y 
direction respectively for alignment signals in the Y direction and X 
direction respectively. Mask mark 16 comprises four bars, two parallel 
bars 58 being oriented in the X direction for an alignment signal in the Y 
direction and two parallel bars 60 being oriented in the Y direction for 
an alignment signal in the X direction. 
Corresponding alignment mask patterns 34, 36 are shown in FIG. 4 and 
consist of narrow slits in an otherwise opaque mask area. Slits 64 
oriented in the X direction correspond with the edges of bar 54 of the 
wafer alignment mark 12, while slits 56 oriented in the Y direction 
correspond with the edges of bar 56 of mark 12. Slits 68 oriented in the X 
direction correspond with the edges of bars 58 of the mask alignment mark 
12, while slits 70 oriented in the Y direction correspond with the edges 
of bars 60 of the mask alignment mark 12. 
It should be noted that the proximity printing mask alignment mark 
illustrated in FIG. 3 has two bars 60 for the X direction and two bars 58 
for the Y direction, while the wafer alignment mark has only one bar 56 
for the X direction and one bar 54 for the Y direction. The printing mask 
alignment signal thus can be expected to have greater sensitivity than the 
wafer alignment signal. The situation could just as well be reversed and 
serves merely to illustrate a symmetrical mark arrangement as well as a 
mark configuration which is not confined to a single isolated region. The 
alignment marks shown in FIG. 3 are also illustrated in cross-section in 
FIG. 2. However because of the scale of FIG. 2 the fine double slits in 
the alignment patterns shown in FIG. 4 have been represented instead by 
single wider slits carrying the same identifying reference numbers. It 
should be understood that the actual alignment mask pattern described in 
connection with the FIG. 2 embodiment is shown in detail instead in FIG. 
4. 
Referring now to FIG. 2, beam splitter 40 is replaced by a different light 
splitting device mirror 72 having a hole or aperture 74 in the center 
thereof. The aperture 74 acts as an aperture stop and limits the size of 
the cone of light which can pass from plane 32 to planes 18, 20 and vice 
versa. Within this central cone, light is not deflected at all by this 
light splitting device. Outside of the aperture 74, this device reflects 
all of the light coming from planes 18, 20 onto plane 42 and vice versa. 
As will be described in further detail in connection with FIG. 8, if the 
light source is placed behind plane 42 instead, the apertured mirror 72 
instead will illuminate planes 18, 20 only from directions outside of the 
central cone defined by the aperture 74 and transmit light travelling from 
planes 18, 20 to plane 32 only within the central cone. Thus, the 
numerical aperture of objective 38 is split into two parts, a central cone 
and the remaining portion of the numerical aperture, either part being 
within the dark field when observing through the other part. As would be 
apparent to one of ordinary skill in optical systems, if 100 percent dark 
field illumination is desired, aperture 74 should be centered about the 
optical axis and should project upon the object and image planes 
substantially as a circle. Thus aperture 74 ideally is oval in shape. 
Wobble plates 76, 78 are positioned across the optical path between 
apertured mirror 72 and the alignment mask 30 and function to shift or 
scan the image projected by objective 38 back and forth in either the X 
direction or the Y direction. The wobble plates are suitably thick sheets 
of transparent material of appropriate index of refraction such that when 
either is oriented at an angle to the optical axis of objective 38, the 
image projected by objective 38 is laterally shifted in a direction 
perpendicular to the axis of tilt rotation. Wobble plate 76 is angularly 
tilted back and forth (angularly reciprocated) about the Y axis by motor 
80 and causes reciprocating X axis shifts of the objective image. Wobble 
plate 78 is angularly tilted back and forth about the X axis by motor 82 
and causes reciprocating Y axis shifts of the objective image. 
I have described the apertured mirror and equivalent light splitting 
structures as well as wobble plates in further detail in my U.S. Pat. No. 
4,232,969, which is hereby totally incorporated by reference. The function 
and operation of these optical elements are similar in both systems. 
In the FIG. 2 embodiment, light intensity monitoring is done electronically 
by photomultipliers 84, 86 via optical fibers 88, 90, 92. Light from the 
alignment mask pattern corresponding to the wafer mark is received by 
fiber 90 and monitored by photomultiplier 84. Light from the alignment 
mask pattern corresponding to the printing mask mark is received by fibers 
88, 92 and monitored by photomultiplier 86. Since the mask mark light 
pattern has more than one spatially separated part, different fibers are 
used to receive each of the various parts of the light pattern. Since the 
mask mark actually has four parts corresponding respectively to the four 
bars of the mask mark illustrated in FIG. 3, four fibers are used, all 
leading to PMT 86. Only two of the four fibers are illustrated because the 
other two are receiving light from the two mask pattern parts which are 
not illustrated and are farther back and farther forward in the Y 
direction. 
Control system 94 receives the PMT signals and automatically controls 
scanning of the alignment image via motors 80, 82 as well as the position 
of wafer 10 and proximity mask 14 via mask position adjustment motors 96 
and table position adjustment motors 98. The wafer 10 is actually carried 
by table 100, which is moved instead of the wafer itself. 
Alignment proceeds as follows. Control system 94 oscillates one of the 
wobble plates, say wobble plate 76 via motor 80, while monitoring the 
light intensities with PMT 84 and PMT 86. FIG. 7 illustrates typical light 
intensity signals. Signal 102, for example, may represent the light 
intensity sensed by PMT 84 while signal 104 may represent the light 
intensity sensed by PMT 86. As illustrated, light scattered from the edge 
of a mark and collected by a PMT reaches a maximum somewhere during the 
wobble plate scan and then reaches a maximum again as the wobble plate 
returns. When the wafer and proximity mask are not aligned with each 
other, the maximums (extremums) of the wafer alignment signal 102 do not 
positionally correspond to the maximums of the printing mask alignment 
signal 104. The difference in position of the maximums, .DELTA., is a 
measure of the amount of misalignment between the wafer and printing mask 
in the one direction (the X direction when wobble plate 76 is causing the 
image scan). Control system 94 senses this difference and changes the 
relative position of the wafer and printing mask in the X direction until 
the signal maximums coincide or correlate. The printing mask and wafer are 
now aligned in the X direction and the same procedure may be followed with 
the other wobble plate instead to achieve alignment also in the Y 
direction. 
It should be apparent that the relative position of the wafer and printing 
mask may be changed either by moving the printing mask, or the wafer, or 
both. If the printing mask is held at one position and the wafer is 
aligned to the printing mask, the control system is similar to the control 
system described in detail in my U S. Pat. No. 4,232,969, except that here 
the X and Y wobble plates act upon the same image rather than separate 
images and the alignment light patterns for both the X and Y directions 
are sensed by the same rather than separate monitors. Suitable 
modification of the control circuit details described in that patent would 
be apparent to one of ordinary skill and thus will not be described in 
further detail. 
When the wafer and printing mask have been aligned, the next step involves 
flood exposure of the printing mask with actinic radiation. Obviously, 
objective 38 may not physically block the actinic radiation. One 
possibility is that the alignment optical system could be moved away. This 
is practical because the alignment optical system need not be precisely 
positioned in the X-Y plane. Thus, it is possible to move the alignment 
system to one side along supporting tracks or rails. FIG. 6 illustrates 
how a step-and-repeat alignment system might be set up so as to avoid any 
need to move the alignment optical system. Wafer 10 is shown with 
alignment marks located in the center of each square step-and-repeat field 
each field comprising four chip areas. Assume that printing of field 106 
is desired, comprising chip areas 108, 110, 112, 114, if alignment mark 
116 is used, the optical system would block subsequent irradiation of 
field 106. However, other alignment marks are available to use instead. 
For example an alignment optical system could be positioned on each of the 
four sides of the field to be aligned, in this case looking at alignment 
marks 118, 120, 122, and 124. At least two of these alignment systems 
would then be positioned over an alignment mark even when aligning a field 
located along an edge. At least two alignment marks are needed to make an 
accurate rotational alignment. By having four separate alignment optical 
systems redundant data is also available for implementation of more 
sophisticated data processing functions. 
In general, direct separate sensing of rotational errors in alignment is 
not needed. Overall system alignment begins by rotationally aligning the 
alignment mask with respect to the X-Y directions of the table using marks 
at spaced locations. 
Then, a printing mask is rotationally aligned to the alignment mask and 
finally the wafer is aligned to the alignment mask. Once rotational 
alignment is thus achieved, the X-Y table system can be relied upon to 
maintain rotational alignment, or the rotational alignments can be 
periodically corrected. 
FIG. 5 illustrates a bright field alignment mask which can be used with the 
marks illustrated in FIG. 3. It should be noted that the widths of the 
slits are smaller than the widths of the alignment mark bars. Therefore, 
signal extremums will not be sharp. There will be a sudden change as the 
slit image encounters and begins to overlap a bar. Then the signal will 
not change much until the slit image begins to encounter the opposite edge 
and correlation of the slit image and bar begins to decline. Narrow slits 
are used so that edges can be sharply detected. In this situation however, 
alignment occurs when the slit image is centered on the bar, a position 
which must be electronically extrapolated from the location of the edges. 
FIG. 8 illustrates the dark field alignment system shown in FIG. 2 with the 
alignment radiation source and optical fibers exchanged in position. As 
shown, the mirror 72 now reflects the outer illumination light down onto 
the alignment marks. The illuminated marks are then imaged by objective 38 
onto plane 32. When the wafer is properly aligned, the light scattered 
from the edges of the wafer mark will pass through the mask pattern 34 and 
be received by fiber 90. Light scattered from the edges of the printing 
mask mark will pass through the mask pattern 36 when the printing mask is 
in alignment. Alignment mask 30 may or may not be carried on a transparent 
support. A transparent mask support 126 is illustrated in FIG. 8 and FIG. 
2. 
Many changes and modifications can be made to my invention which will be 
apparent to anyone of ordinary skill and which can be made without 
departing from the spirit and scope of my invention. For example, block 42 
may be replaced with an appropriate lens element. Also, the positions of 
the alignment mask patterns could be axially separated to compensate for 
the difference in position between planes 18, 20, thereby eliminating the 
need for a separate physical compensation element. It should also be 
apparent that separate alignment optical systems could be used for the X 
and Y directions, much as described in my U.S. Pat. No. 4,232,969.