Methods and apparatus for detecting and compensating for focus errors in a photolithography tool

An apparatus for detecting and compensating for focus errors in a photolithography tool which holds a reticle and a substrate substantially parallel is described. The apparatus includes at least one proximity gauge for determining a first distance between the reticle and the substrate, thereby determining if a focus error condition exists. The apparatus also includes an actuator for adjusting the position of the reticle with respect to the substrate, thereby compensating for the focus error condition. According to another embodiment, a lens system actuator is employed for adjusting a lens system parameter, thereby compensating for the focus error condition detected by the at least one proximity gauge.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of photolithographic techniques. 
More specifically, the invention relates to the detection of and 
compensation for focal plane location and planarity errors. Still more 
specifically, one embodiment of the present invention provides an improved 
method and apparatus for measuring the distance between the focal planes 
of the reticle and the substrate in a photolithography system which 
employs scanning or stepping of both the reticle and the substrate. 
Another embodiment of the present invention provides an improved method 
and apparatus for adjusting the focus of Wynn-Dyson or other unity 
magnification, telecentric, catadioptic lens system. Another embodiment of 
the present invention provides an improved method and apparatus for 
eliminating or modifying the sag or warp in a reticle or substrate in a 
photolithography system. The present invention will be particularly useful 
in photolithography systems that are used for printing on large 
substrates, printing from large reticles, or where the depth of focus is 
so small that even slight focus errors are important. 
Currently there are a number of problems associated with holding reticles 
and substrates in high resolution photolithography systems so that they 
are flat and parallel to the degree required to maintain good focus. These 
problems are particularly pronounced in photolithography systems which 
rapidly process large area substrates using very large imaging fields as 
in the case of, for example, the manufacture of flat panel displays and 
multi-chip modules. Also problematic in this regard is the processing of 
devices such as microprocessor chips and dynamics memory (DRAM) chips 
which employs relatively large fields and lenses which have very shallow 
depth of focus. 
One focus problem encountered with photolithography systems which move both 
the reticle and the substrate relative to the lens is the creation and 
maintenance of the proper gaps and parallelism between the reticle, 
substrate, and lens. Focus gauging and adjustment techniques exist for the 
more traditional lithography tools that have fixed lenses and reticles, 
but more and different techniques are needed for systems which move both 
the reticle and the substrate and for the new multiple axis scanners. 
Traditionally, the reticle stage (and therefore the reticle) is 
permanently fixed with respect to the lens, and the position of the 
substrate relative to the lens is sensed using, for example, reflective 
laser beams, capacitive techniques, or air gap sensing technology. 
Another problem is encountered when adjusting the focus of a 
photolithography system. The traditional technique is to adjust the 
position of the substrate relative to the fixed lens and reticle. This is 
still a valid technique for some applications, but it would be very 
difficult for a system employing this technique to adequately compensate 
for realtime focus variations in a high-speed scanning application such as 
those used to process large area substrates. 
Another focus problem involves keeping the reticle and substrate planes 
parallel. Keeping the substrate flat is relatively easy. It can be rigidly 
attached over its entire surface to a flat substrate chuck with vacuum. 
However, the reticle must be supported only on its perimeter to allow a 
clear aperture over the majority of its area through which light may be 
projected. This type of support allows any natural warp or stress in the 
reticle to be expressed as a focal plane error. Mounting the reticle on a 
reticle chuck with a predominately horizontal attitude exacerbates the 
problem by inducing sag of the reticle due to gravity. Currently, no 
satisfactory techniques exist which prevent or compensate for this 
problem. 
From the foregoing, it will be understood that improved techniques for 
dealing with focus problems are needed, particularly for multiple axis 
scanning systems, large area substrate processing, and systems employing 
lenses with shallow depth of focus. 
SUMMARY OF THE INVENTION 
The present invention provides new and improved methods and apparatus for 
the detection of and compensation for focal plane location and planarity 
errors during photolithographic processes. 
According to a specific embodiment of the present invention, proximity 
gauges are attached to the lens system located between the reticle and 
substrate to measure the distances from the lens system to the reticle and 
substrate, and to therefrom determine the distance between the reticle and 
substrate planes. These measurements are then used in various embodiments 
to facilitate adjustment of the lens focus, the reticle to substrate 
separation, and the spacing between the lens system and the reticle and/or 
substrate. 
According to another specific embodiment of the present invention, the 
focus of a unity magnification, telecentric, catadioptic lens system 
frequently used in photolithography equipment, such as, for example, a 
Wynn-Dyson lens system, can be manipulated quickly and precisely by 
adjusting the axial position of the primary mirror with respect to the 
other lens elements. 
According to still another specific embodiment of the invention, a 
distorted reticle focal plane due to sag or warp is corrected by 
supporting the portion of the reticle around the lens at a fixed distance 
from the lens with an air bearing. According to yet another embodiment, a 
distorted reticle focal plane is corrected by supporting the reticle or a 
portion of the reticle from the side opposite the lens system by applying 
air pressure or vacuum as needed in a either a fixed or dynamic spatial 
pattern. 
Thus, according to the invention, a method and apparatus for detecting and 
compensating for focus errors in a photolithography tool are described. 
The apparatus includes at least one proximity gauge for determining a 
first distance between the reticle and the substrate, or alternatively the 
distances between the reticle and the tool's lens system and the substrate 
and the lens system, respectively. These distances are used to determine 
if a focus error condition exists. The apparatus also includes an actuator 
for adjusting the position of the reticle with respect to the substrate, 
thereby compensating for the focus error condition. According to another 
embodiment for use in such a system, a lens system actuator is employed 
for adjusting a lens system parameter, thereby compensating for the focus 
error condition. 
A lens system is also described which includes a primary mirror, a 
plurality of lens elements including at least one prism, and a primary 
mirror actuator for adjusting the axial position of the primary mirror 
with respect to the plurality of lens elements, thereby moving the image 
plane. 
A reticle is also described which includes a plurality of image fields 
distributed over its surface. At least one stiffener is secured to the 
surface of the reticle in a portion of the non-imaging areas between the 
image fields for reducing warpage of the reticle. 
A reticle chuck assembly is also described which includes a reticle chuck 
frame having an aperture, and a substantially transparent reticle chuck 
secured to the reticle chuck frame which has a surface for holding a 
reticle. 
A further understanding of the nature and advantages of the inventions may 
be realized by reference to the remaining portions of the specification 
and the drawings.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
FIG. 1 is a view of a photolithography system 100 with a focus gauging 
apparatus designed according to a specific embodiment of the present 
invention. The block diagram shows a side view of system 100 which shows 
the relationship between reticle 102, lens 104, substrate 106, and a 
reticle and substrate gauges 108 and 110 which are attached to lens 104. 
According to various embodiments, reticle 102, substrate 106 and lens 104 
are scanned and/or stepped in planes parallel to the surface of the 
substrate and must remain parallel and properly spaced relative to each 
other. Reticle gauge 108 measures the separation between reticle 102 and 
lens 104. Reticle gauge 108 is placed to gauge an area on reticle 102 
adjacent to or surrounding the active area of reticle 102 at any given 
moment. Substrate gauge 110 measures the separation between substrate 106 
and the lens 104. Substrate gauge 110 is placed to gauge an area on 
substrate 110 adjacent to or surrounding the active area of substrate 110 
at any given moment. It will be understood that reticle and substrate 
gauges 108 and 110 may each comprise a plurality of gauges. The 
information from gauges 108 and 110 is used to adjust the focus of the 
system either statically or dynamically. The focus adjustment can be 
accomplished in a variety of different ways including the techniques 
described herein. For example, the gauges could be strategically mounted 
on the lens to read the "terrain" ahead of the relative motion of the lens 
with respect to the reticle and substrate. In addition, gauges mounted on 
both sides of the lens could read immediately in front of the lens whether 
it is scanning or stepping forward or backward. Gauges on the right and 
left sides of the lens could read one row ahead of the lens whether the 
serpentine stepping pattern turns right or left. By extension, an array of 
gauges could be mounted to various stationary or moving structures in the 
system to systematically measure the substrate and/or reticle. 
FIG. 2 is a view of a lens system 200 for use in a photolithography system 
with a focus adjustment apparatus designed according to a specific 
embodiment of the present invention. Lens system 200 is shown as a 
Wynn-Dyson lens system having a positioning actuator 202 attached to 
primary mirror 204. Lens system 200 includes primary mirror 204, multiple 
front lenses 206 and 207, and two prisms 208 which provide image planes 
210 on opposite sides of lens system 200. In a typical Wynn-Dyson 
configuration, all lens elements (including primary mirror 204) are held 
rigidly with respect to each other at precisely fixed locations. All the 
lens elements must remain collinear on the optical axis or undesirable 
aberrations will be induced, particularly for multi-chromatic systems. In 
the embodiment of the invention shown in FIG. 2, positioning actuator 202 
is attached to primary mirror 204 to displace it precisely along optical 
axis 212 and to thereby effect focus shifts in lens system 200. This 
technique capitalizes on the fact that in such a lens system movement of 
primary mirror 204 along optical axis 212 primarily affects focus without 
introducing any other deleterious effects, within a large focus range. 
Referring to FIG. 2, the effect of moving primary mirror 204 toward the 
other elements is to shift image planes 210 away from the surfaces of 
prisms 208. A similar relationship between axial position and image plane 
position exists for some of the other lens elements such as, for example, 
prisms 208. Thus, these elements could be used in a similar manner to 
effect focus shifts. However, primary mirror 204 is generally separated 
from the other elements and more available for coupling to other devices, 
such as positioning actuator 202. Lens system 200 may be employed for a 
technique called "focus drilling" which involves moving the focus during 
an exposure to increase the effective depth of focus of the lens system. 
This technique may be performed in a stepper photolithography system, and 
even for a scanner system by rapidly oscillating the axial position of the 
primary mirror. 
FIG. 3 shows a photolithography system 300 with a focus adjusting apparatus 
designed according to a specific embodiment of the present invention. The 
side view block diagram shows the relationship between reticle 302, lens 
304, substrate 306, and reticle support air bearings 308 which are 
attached to lens 304. Reticle 302, substrate 306 and lens 304 are scanned 
and/or stepped in planes parallel to the surface of the substrate and must 
remain parallel and properly spaced relative to each other. Reticle 
support air bearings 308 exert force on reticle 302 in the direction away 
from substrate 306, pushing reticle 302 away from lens 304 to provide a 
precisely controlled separation between reticle 302 and lens 304. This 
action counteracts the sag of reticle 302 due to gravity as well as any 
natural warp in reticle 302. One or more reticle support air bearings 308 
are strategically placed to apply pressure to an area on reticle 302 
adjacent to or surrounding the active area of the reticle at any given 
moment. The use of air bearings provides precisely controlled application 
of force without a solid object physically contacting reticle 302. In 
addition, the force introduced by air bearings 308, and by extension, the 
position of reticle 302, may be dynamically adjusted by modulation of the 
air pressure to air bearings 308. According to various embodiments, sag or 
warp of reticle 302 away from lens 304 and substrate 306 may be 
counteracted by augmenting air bearings 308 with vacuum ports placed 
adjacent to or surrounding air bearings 308 to pull localized portions of 
the surface of reticle 302 toward air bearings 308. Alternatively, the air 
pressure mechanism depicted in FIG. 4 may be used to push the reticle 
against the reticle support air bearings. Another alternative is to follow 
the lens and the reticle support air bearings with another set of softer 
air bearing on the other side of the reticle to apply force to the 
opposite side of the reticle which opposes the force applied by the 
reticle support air bearings. These softer air bearings could be mounted 
to the illuminator (not shown) which is located on the opposite of the 
reticle from the lens. 
FIG. 4 shows a photolithography system 400 with a focus adjusting apparatus 
designed according to another embodiment of the present invention. This 
side view block diagram shows the relationship between lens 402, substrate 
404, and a reticle 406 on a reticle chuck 408 which forms a sealed chamber 
within its aperture (indicated by the vertical dashed lines) with reticle 
406 acting as one wall. Reticle chuck 408 is constructed as a large stiff 
plate with a reticle chuck aperture (not shown) to allow illumination pass 
through reticle chuck 408 and through reticle 406 to lens 402 and 
substrate 404. Reticle chuck 408 is substantially larger than reticle 406 
so that reticle chuck 408 can support the entire perimeter of reticle 406. 
The reticle chuck aperture is substantially smaller than reticle 406 so 
that reticle chuck 408 can support reticle 406 over a substantial area. 
Reticle 406 is attached to reticle chuck 408 by vacuum. A multitude of 
vacuum ports (not shown) are manufactured into the surface of reticle 
chuck 408 in the area to which reticle 406 is attached. A second large 
transparent plate 410 similar to a reticle but completely transparent is 
permanently attached to the surface of reticle chuck 408 opposite reticle 
406. This reticle chuck cavity cover 410 must be completely transparent to 
allow illumination to pass through it. Reticle chuck cavity cover 410 with 
reticle 406 transforms the reticle chuck aperture into a pressurizable 
volume. A pressure control port 412 is provided to introduce pressure. The 
pressure in the volume can be set to counteract sagging of reticle 406 due 
to gravity. The pressure in the volume can also be dynamically modulated 
to distort the surface of reticle 406 slightly at any point to counteract 
localized warpage. The dynamic distortion technique would require advanced 
knowledge of the topology of the reticle or active feedback of position or 
flatness at the area of interest. This could be accomplished by many 
techniques including use of the apparatus depicted in FIG. 1. 
Another way to compensate by design for sag in a reticle suspended in a 
vacuum chuck frame would be to lap a spherical or otherwise curved surface 
into the surface of the vacuum chuck such that when the reticle is 
attached to the reticle chuck the induced bow in the reticle due to the 
curved surface of the reticle chuck perfectly compensates for the sag due 
to gravity. 
FIGS. 5A and 5B are two views of a reticle 500 designed according to a 
specific embodiment of the invention. Reticle 500 includes a reticle 
stiffener 502. As shown in FIG. 5B, reticle stiffener 502 is only secured 
to areas of reticle 500 which are not image areas 504, i.e., non-imaging 
spaces between product images. In many cases, multiple image fields are 
provided on a single reticle with space between the various field. Thus, 
the geometry of reticle stiffener 502 is determined by the locations of 
these spaces. A particular arrangement of image field might result in a 
number of separate stiffeners on the same reticle. Reticle stiffener 502 
is rigidly adhered to reticle 500 and is as large as possible to maximize 
its stiffness. Reticle stiffener 502 is also manufactured from a highly 
rigid material. According to specific embodiments, reticle stiffener 502 
has the same temperature coefficient of thermal expansion as reticle 500 
to prevent heat induced warpage. This may be accomplished by fabricating 
the reticle stiffener from material used to make reticles, such as 
borosilicate glass or fused silica. Alternatively, a carefully chosen 
steel may be employed. A less invasive alternative is to build reticle 
stiffeners directly into the reticle chuck. According to specific ones of 
such embodiments, the reticle stiffeners include vacuum attachment means. 
FIG. 6A is a side view of a photolithography system 600 which shows the 
relationship between lens 602, substrate 604, reticle 606, and a reticle 
chuck assembly 608 which includes a reticle chuck frame 610 and 
transparent reticle chuck 612. Reticle 606 is adhered to transparent 
reticle chuck 612 by vacuum or electrostatics. Transparent reticle chuck 
612 is adhered to a reticle chuck frame 610 by vacuum or other means. This 
technique allows for reticle 606 to be relatively thin because of the 
structural support supplied by the relatively thick and stiff transparent 
reticle chuck 612. It will be understood that if reticle chuck 612 is made 
thick enough, reticle chuck frame 610 would not be required. Such an 
embodiment 600' is shown in FIG. 6B. Moreover, transparent reticle chucks 
612 and 612' may be further augmented using the techniques described with 
reference to FIGS. 1-5. 
According to one embodiment, if electrostatics are used to adhere the 
reticle to the transparent reticle chuck, the transparent reticle chuck 
includes a pattern of transparent and oppositely charged areas on the 
surface that faces the reticle. The transparent charged areas may be 
created using a thin patterned layer of indium tin oxide, or, 
alternatively, thin metal traces may be employed. The conductive areas 
and/or traces are routed to electrodes for external electrical connection. 
If, according to another embodiment, vacuum is used to adhere the reticle 
to the transparent reticle chuck, the transparent reticle chuck includes 
means to induce a vacuum between the two surfaces. For example, as shown 
in FIG. 7, transparent reticle chuck 700 may include an O-ring seal 702 
around the perimeter of image fields 704 to enclose a substantially 
air-tight volume. The vacuum supply may be introduced by bypassing the 
O-ring, or, if a stiffener 706 is included in transparent reticle chuck 
700, vacuum supply may be introduced through apertures and plenums in 
stiffener 706 or in reticle chuck 700. 
According to another specific embodiment shown in FIG. 8, in a projection 
photolithography tool 800 the reticle 802 and substrate 804 are maintained 
substantially parallel by a pair of air bearings 806 and 808 coupled to 
lens system 810. One air bearing 806 is directed toward reticle 802 and 
another 808 is directed toward substrate 804. In embodiments having double 
telecentric lens systems the two air bearing do not need to be directly 
referenced to the lens system because the distance between the image plane 
(i.e., reticle) and object plane (i.e., substrate) is the critical 
distance, not the distance between the lens system and either of the image 
or object planes. Therefore, where the lens system is double telecentric, 
the air bearings merely need to be directed normal to their corresponding 
plane and in reasonably close proximity to the lens system. In embodiments 
having lens systems which are not double telecentric, the distances 
between the lens system and the image and object planes are critical and 
therefore the air bearings must be directly referenced to the lens system. 
Depending upon the relative masses and sizes of the reticle, lens system 
and substrate, one of these elements may be held fixed and the positions 
of the others determined by the air bearings. According to various 
embodiments of the invention, the use of air bearings in this manner can 
replace or augment a separate focus detection system while simultaneously 
improving both focus and leveling accuracy and speed. It is important that 
the air bearings are stiff and that their air gaps are well regulated. 
Careful regulation of the air gaps allows precise control of the focus 
separation distance. According to more specific embodiments, a means to 
support and capture the air bearings is provided for situations in which 
the reticle and/or substrate is not in place. 
According to yet another specific embodiment shown in FIG. 9, in a 
proximity photolithography tool 900 the gap 901 between reticle 902 and 
substrate 904 is established and controlled by the use of air 
bearing/reticle chuck 906. According to the embodiment shown, where the 
image area of reticle 902 is smaller than substrate 904, an air bearing is 
incorporated into the reticle chuck and oriented such that air 
bearing/reticle chuck 906 flies over the surface of substrate 904. To 
ensure correct flying stability, air bearing 906 is preloaded toward 
substrate 904. This may be accomplished, for example, by pushing the 
reticle chuck toward the substrate with spring mechanisms (not shown), or 
by attracting the reticle and reticle chuck toward the substrate with 
vacuum mechanisms concentrically disposed about the air bearing (also not 
shown). 
Additionally, because the air pressure to air bearing 906 is precisely 
regulated, it may be used to effect small changes in gap 901 between 
reticle 902 and substrate 904. According to one embodiment, the lateral 
positions of air bearing 906 along the surface of substrate 904 is 
controlled by alignment actuators 912 attached to substrate chuck 914. 
Electromagnets 916 (shown in view A--A) affixed to small vertical motion 
mechanisms 918 are provided to capture and separate air bearing 906 and 
reticle 902 from substrate 904. 
During the period when the air bearing is allowed to fly over the 
substrate, gas is deposited in the gap between the reticle and the 
substrate. If the gap is not properly ventilated, catastrophic damage to 
the reticle may occur. Therefore, the pressure in the gap is controlled 
not only to prevent such reticle damage, but also to compensate for 
gravity induced sags and other distortions of the reticle. 
The above descriptions are illustrative and not restrictive. Many 
variations of the invention will become apparent to those skilled in the 
art upon review of this disclosure. Merely by way of example, the reticle 
and substrate gauging technique could be used with any style of lens. Many 
different types of gauges could be used, including optical or capacitive. 
The focus adjustment technique using the primary mirror could be done with 
most types of catadioptic lens types. A variety of materials of 
construction could be used for the various components if chosen carefully. 
The invention can be mounted in various orientations, and with a variety 
of physical mounting techniques. The apparatus may incorporate various 
sensors to facilitate operation or safety. Different operating sequences 
may also be used to tailor the operation of the invention to a particular 
application. This invention may also be incorporated into many different 
types of equipment. The scope of the invention should therefore be 
determined not just with reference to the above description, but instead 
should be determined with reference to the appended claims along with 
their full scope of equivalents.