Excimer laser with greater spectral bandwidth and beam stability

An excimer laser having reduced spectral bandwidth variation is provided. The laser has an assembly of components comprising a beam expander having at least two prisms that reduces the spectral bandwidth of the laser beam that is passed through the assembly. At least one of the prisms of the beam expander is made of a thermally stable material, wherein the material has an coefficient of absorption of no more than about 0.1 percent/cm for light having the desired wavelength, a rate of change of refractive index with temperature of no more than about 2.3.times.10.sup.-6 .degree.C..sup.-1, a thermal conductivity of at least about 9.71 W/m-.degree.C. at 20 deg C., and/or a coefficient of thermal expansion of at least about 9.times.10.sup.-6 (deg C.).sup.-1. In a preferred embodiment, the stability of beam profile is improved, the variation in spectral bandwidth of the beam from the laser is reduced, the spectral bandwidth itself is also reduced, and the laser exhibits greater voltage stability.

FIELD OF THE INVENTION 
The invention relates to a laser, particularly an excimer laser for use in 
photolithography, that has spectral narrowing assembly control and which 
laser, consequently, produces a beam of light with an improved beam 
profile and consistently-narrow spectral bandwidth. 
BACKGROUND OF THE INVENTION 
Excimer lasers are used in a number of applications, and one application in 
which excimer lasers have found particular use is photolithography for 
chip manufacture. Light from the laser illuminates a photoresist layer 
spun onto a silicon substrate, and a mask between the laser and the 
substrate allows some of the laser light to illuminate portions of the 
photoresist. The photoresist is subsequently developed, and the substrate 
is etched in areas unprotected by the photoresist. 
For some photolithography applications, a laser beam produced within a 
laser chamber contains light within a range of wavelengths at or near the 
desired wavelength of 248.3 nm for a KrF excimer laser or 193.3 nm for an 
ArF excimer laser. The spectral-narrowing assembly removes much of the 
light at undesired wavelengths and returns the light within a desired 
narrow range back to the laser chamber. It is very desirable to produce a 
laser beam having a narrow spectral bandwidth with little variation in 
spectral bandwidth. 
The laser is part of a larger piece of equipment such as a stepper or a 
scanner that processes the wafers. A typical stepper has a carousel that 
contains a number of wafers to be etched and otherwise processed to form a 
finished product such as a computer chip. The stepper removes one of the 
wafers coated with photoresist from the carousel and positions the wafer 
in the path of the laser light. The stepper also places the appropriate 
mask between the coated wafer and the laser light. The stepper assures 
that all components are properly aligned, and the stepper then instructs 
the laser to fire and expose a portion of the photoresist on the wafer to 
a series of pulses (i.e. a "burst") of laser light that provides 
sufficient energy to alter the exposed portion of photoresist so that its 
chemical composition differs from the chemical composition of the 
unexposed photoresist. The stepper shuts off the laser, repositions the 
wafer, and again the stepper fires the laser and exposes a portion of the 
photoresist to the laser light. The stepper continues this process of 
shutting off the laser, repositioning the wafer, and refiring the laser 
until the laser has exposed the entire layer of photoresist on the wafer. 
The stepper then replaces the wafer into the carousel, advances the 
carousel, and removes another wafer from the carousel. The stepper 
subsequently repeats the process of positioning the second wafer, firing a 
burst, and repositioning the second wafer until the photoresist on the 
surface of the second wafer is completely exposed. The stepper then 
repeats the process until all wafers in a carousel are exposed. The 
carousel is removed from the stepper, and another carousel is inserted to 
begin the process anew. A scanner operates similarly to a stepper, but the 
scanner includes scanning operations during which the beam is scanned 
across portions of the wafer. This is typically accomplished by moving the 
wafer and mask continuously under the beam. 
Because of the stepping mode of operation in a stepper or scanner, the 
laser does not operate in a continuous or steady-state manner. The laser 
could be fired continuously or in a predictable periodic fashion, and the 
power of the beam could be used on demand by opening a shutter on the 
laser in order to have consistent operation of the laser. However, it is 
much more economical to stop firing the laser when the beam is not needed 
instead of firing the laser continuously and discarding the beam most of 
the time. The duty cycle for a laser used in conjunction with a stepper or 
scanner is typically between only about 10% and about 50%. 
Intermittent operation of the laser creates transient phenomena that affect 
the consistency of the laser beam, and the transient conditions themselves 
vary substantially because of the varying "of" times associated with 
repositioning and realigning wafers, changing wafers, and changing 
carousels. 
SUMMARY OF THE INVENTION 
It is thus one object of the present invention to provide a laser having a 
more consistent spectral bandwidth under varying laser firing conditions. 
It is another object of preferred embodiments of the invention to provide 
a laser having a smaller spectral bandwidth than was previously provided. 
It is another object of certain embodiments of the invention to provide a 
laser with improved voltage stability. It is a further object to provide a 
beam expander for a laser with improved control over beam expansion and 
beam angle despite changes in the duty cycle of the laser. 
Among other factors, the invention is based on the inventors technical 
finding that an excimer laser equipped with a spectral narrowing assembly 
as described herein (1) produces a beam with a more consistent bandwidth 
despite temperature variations caused by intermittent operation of the 
laser, (2) provides an expanded laser beam having very consistent 
dimensions, (3) produces a beam with a smaller spectral bandwidth, (4) has 
reduced energy requirements, (5) has improved voltage stability, and (6) 
has a longer life. These and other objects, factors, and advantages are 
apparent from the discussion herein, including the claims and the appended 
drawings. 
The present invention provides for a laser with prisms which: (1) absorb 
little of the laser beam; (2) experience little change in refractive index 
with change in temperature of those prisms; (3) have a high thermal 
conductivity; (4) have a coefficient of thermal expansion that closely 
matches the coefficient of thermal expansion of the mounting system for 
the prisms; and/or (5) have a coefficient of thermal expansion that is 
offset by a change in refractive index with change in temperature. The 
invention also provides a method for reducing the spectral bandwidth 
variations in a beam of light from a laser, which method comprises 
transmitting the beam of light generated by the laser into a 
spectral-narrowing assembly whose prisms experience little change in their 
ability to expand a beam at substantially constant angle and dimensions 
despite changes in temperature of those prisms, as described above. 
The inventor also found that, in a preferred embodiment of the invention, 
the laser produced a beam of light that had a smaller spectral bandwidth 
than was possible with previous lasers that utilized fused-silica prisms 
in the spectral-narrowing assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
It had been determined that intermittent operation of the laser has 
resulted in inconsistent photoresist patterns in a number of instances. 
Intermittent operation has resulted in over- or under-exposure of 
photoresist, which led to semiconductor device reliability problems and 
failure. Continuous firing of the laser has produced a more consistent 
photoresist pattern than intermittent firing produced. 
During experimentation with an approximately two-year old laser in the 
laboratory, the inventor noted that there were significant variations in 
the beam profile and the spectral bandwidth of the laser beam when the 
laser was fired intermittently instead of being fired continuously. These 
variations existed even though the laser had a well-known 
spectral-narrowing assembly having three high-quality fused-silica prisms, 
an aperture in front of the prisms and an aperture intermediate between 
the second and third prisms, an echelle grating, and a mirror to reflect 
light from the prisms to the grating that, in combination, reduced 
spectral bandwidth of the laser beam. Spectral variations and variations 
in the beam profile cause inconsistent photoresist patterns, and the 
pattern is not as sharp and well-defined when the spectral bandwidth is 
wide as when the spectral bandwidth is narrower. The inventor thus decided 
to investigate the source of these spectral variations and provide an 
improved laser that did not have the spectral variation. 
An excimer laser used in photolithography is a complex device having many 
components that cooperate to produce a laser beam of the desired beam 
profile, wavelength, pulse energy, and spectral bandwidth. After a 
substantial amount of experimentation, the inventor's discovered that the 
source of spectral bandwidth and beam profile variation was within the 
spectral-narrowing assembly itself. The inventor determined that the 
prisms of the spectral-narrowing assembly absorbed some of the beam 
energy, and the prisms heated because of the absorbed energy. The 
increased temperature of the prisms caused the laser beam to be refracted 
differently than when the temperature was lower. A critical portion of the 
prisms acted like a thermal lens, diverting the light from its intended 
path so that the light exited the prisms in a slightly but significantly 
different fashion. 
This thermal lens effect caused significant variations in laser beam 
expansion and in control of spectral bandwidth. For example, if the laser 
described above was fired in short bursts with a long time period between 
bursts, a significant change in beam expansion occurred, and the spectral 
bandwidth varied much of the time that the laser operated in this manner 
(see FIG. 2A, wherein for the time period between 60 and 1000 seconds, the 
laser was operated at a 10% duty cycle and the spectral bandwidth was 
changing during much of this time period). Also, significant variations in 
spectral bandwidth are observed when the laser is fired in long, rapid 
bursts, the laser is stopped for a sufficiently short period of time that 
a wafer can be repositioned, and the laser is refired. A stepper fires the 
laser rapidly as a portion of a wafer is exposed, stops firing for a short 
time as the wafer is repositioned, refires the laser in another burst or 
series of bursts, again stops the laser for a shorter or longer period of 
time as the wafer is again repositioned, and again stops firing for a long 
period of time as a wafer or carousel is replaced. Because of the manner 
in which a stepper operates, the prisms of the spectral narrowing assembly 
are exposed to rapidly-changing amounts and rates of addition of beam 
energy. Consequently, the refractive properties of the prisms change 
rapidly and often as the stepper processes wafers. In particular, the 
refractive properties of the prisms changed substantially when the duty 
cycle of the laser changed. 
The inventors learned that a laser equipped with a spectral narrowing 
assembly that had optical components which met certain specifications 
expanded the laser beam much more consistently, and consequently the laser 
had greater spectral bandwidth stability than had been achieved 
previously. The inventor's determined that a laser equipped with a 
spectral narrowing assembly having at least two prisms would experience 
reduced variance in beam expansion and spectral bandwidth despite changes 
in how long or how often the laser was fired when at least one of the 
prisms was made of a material that expanded the laser beam at 
substantially the same angle and to substantially the same dimensions 
despite changing conditions. 
The inventors found that, the problems of variance in spectral bandwidth 
and beam profile distortion caused by burst-mode operation of the laser 
and rapid heat-up and cool-down of optics, could be reduced by designing a 
laser having a spectral-narrowing assembly whose elements were more stable 
to thermal changes. In a spectral-narrowing assembly initially having 
three fused silica prisms, substantial improvement in beam quality was 
observed when at least one of the prisms is made of a thermally-stable 
material which provides consistent beam expansion despite changes in the 
rate at which the laser is operating. Best performance was observed when 
all of the prisms were comprised of thermally stable material. 
In one preferred embodiment of the invention, the last of the prisms in the 
expansion chain is made of a material which has a low absorption of light 
at the desired wavelength. (The last prism in this embodiment had the 
longest path for the beam to travel through the prism.) A prism will 
experience less temperature change and greater stability of its optical 
and physical properties discussed below if the material from which the 
prism is made absorbs less of the laser beam. Consequently, the 
coefficient of absorption of a prism is preferably less than 0.5 
percent/cm and more preferably is less than about 0.1 percent/cm. Values 
for the coefficient of absorption at 248 nm (wavelength) of various 
materials are listed in Table 1 below. 
In another preferred embodiment of the invention, at least one of the 
prisms is made from a material that experiences little change in 
refractive index as the temperature of the prism changes. The laser beam 
is expanded at a substantially constant angle if the refractive index 
changes little, and the width of the expanded beam exiting the prism is 
also substantially constant regardless of what temperature the prism has. 
A prism preferably has a rate of change in refractive index with 
temperature (dN/dT) of less than about 15.times.10.sup.-6 (deg C.).sup.-1 
or smaller (absolute value). Values for dN/dT of various materials are 
listed in Table 1 below. 
In another preferred embodiment of the invention, at least one of the 
prisms is made of a material which has a high thermal conductivity. A high 
thermal conductivity provides two benefits. As the prism absorbs energy 
from the beam, the temperature of the prism will rise unless heat is 
conducted away from the prism rapidly. Consequently, a high thermal 
conductivity assures that the prism will remain close to ambient 
temperature. Also, a high thermal conductivity reduces thermal gradients 
within the prism. Thermal gradients can occur because beam energy which is 
absorbed into the prism is not conducted quickly throughout the prism. A 
low thermal conductivity causes the temperature of the portion of the 
prism carrying the beam to increase substantially, and a high thermal 
conductivity permits heat to disperse rapidly through the prism. A high 
thermal conductivity helps to avoid physical distortion of the prism as 
well as unusual refraction caused by e.g. significant variations in the 
refractive index across the prism. Preferably, the thermal conductivity of 
the material from which the prism is made about 9.7 W/m-.degree.C. or 
greater at about 20 deg C. Values of the thermal conductivity for various 
materials are listed in Table 1 below. 
In another preferred embodiment of the invention, at least one of the 
prisms is made of a material which has a coefficient of thermal expansion 
that is approximately equal to the coefficient of thermal expansion of the 
mounting material for the prism. The prism is mounted to a rigid structure 
such as metal with glue or metallic clamps. The prism has an overall 
temperature that is approximately equal to its surroundings. As the prism 
heats due to the beam passing through it, the surrounding structure also 
heats because the structure absorbs the energy of the light that is 
discarded by the prism and/or the grating. If the coefficient of thermal 
expansion of the prism is not at least approximately equal to the 
coefficient of expansion of the material to which the prism is mounted, 
the prism and its mount expand at different rates and in different 
amounts, and unusual stresses in the prism and/or mount are created. The 
prism can be distorted and/or moved slightly, and beam dimensions and 
angles are affected. If the coefficient of thermal expansion of the prism 
is at least approximately equal to the coefficient of expansion of the 
material to which the prism is mounted, the prism and its mount expand at 
approximately equal rates and amounts, and unusual stresses in the prism 
and/or mount are avoided. Thus, the beam's dimensions and angles are more 
consistent despite temperature changes when the coefficient of expansion 
of the prism approximates or equals the coefficient of expansion of the 
prism's mount. Metal housings typically have a coefficient of thermal 
expansion between about 9.times.10.sup.-6 (deg C.).sup.-1 for steel and 
about 24.times.10.sup.-1 (deg C.).sup.-1 for aluminum. Thus, for a prism 
mounted within a metal housing, the prism's coefficient of thermal 
expansion is preferably between about 9.times.10.sup.-6 (deg C.).sup.-1 
and 24.times.10.sup.-6 (deg C.).sup.-1. Values of the coefficient of 
thermal expansion for various materials are listed in Table 1 below. 
TABLE 1 
______________________________________ 
Fused Silica 
CaF.sub.2 MgF.sub.2.sup.1 
______________________________________ 
Coefficient of 
0.5-1 &lt;0.1 N/A.sup.2 
absorption (for 
0.1 .div. 0.5 
.lambda. = 248 nm) 
(percent/cm) 
dN/dT 12.8 .times. 10.sup.-6 
-10.6 .times. 10.sup.-6 
2.3 .times. 10.sup.-6 
(deg C).sup.-1 
15 (ab 248) 
-6.3 ab (248) 
and 
1.7 .times. 10.sup.-6 
Thermal 1.38 9.71 0.3 
conductivity 
Wm.sup.-1 .degree. C..sup.-1 
Coefficient of 
0.52 .times. 10.sup.-6 
18.85 .times. 10.sup.-6 
13.7 .times. 10.sup.-6 
thermal and 
expansion 8.48 .times. 10.sup.-6 
(deg C.).sup.-1 
______________________________________ 
.sup.1 MgF.sub.2 is birefringent 
.sup.2 N/A = not available 
In another preferred embodiment of the invention, the coefficient of 
thermal expansion and the rate of change of refractive index with 
temperature have offsetting effects. For example, where the portion of the 
prism contacted by the beam heats locally and causes the prism to distort 
the direction and/or dimensions of beam expansion, an opposite change in 
refractive index can offset this distortion and provide more consistent 
beam dimensions and/or direction. A CaF.sub.2 prism expands as the 
temperature is increased, since its coefficient of thermal expansion is a 
positive value (18.85.times.10.sup.-6 (.degree.C.).sup.-1). However, the 
rate at which the refractive index changes as temperature increases is a 
negative value (-10.6.times.10.sup.-1 (.degree.C.).sup.-1). These effects 
tend to offset each other and provide a laser beam having dimensions at 
the exit of the prism that are more consistent than when both the 
coefficient of thermal expansion and refractive index of the prism do not 
offset each other. 
In yet another preferred embodiment of the invention, at least one of the 
prisms of the assembly is made of a material wherein the coefficient of 
absorption, the rate of change of refractive index with temperature, the 
thermal conductivity, and the coefficient of thermal expansion are 
selected to provide a laser beam bandwidth variance that is no more than 
50% and more preferably no more than 25% of the laser beam bandwidth 
variance supplied by an assembly wherein the assembly is identical to the 
assembly of this invention in all respects except that the prisms of the 
beam expander are made of fused silica. The laser beam bandwidth variance 
is the difference between a laser beam bandwidth measured when the laser 
is fired at a duty cycle of 10% and a laser beam bandwidth measured when 
the laser is fired at a duty cycle of 100%, wherein the laser is fired at 
the duty cycle of 100% immediately after the laser is fired at the duty 
cycle of 10%, and wherein the coherent beam of light has an energy of 12 
mJ and a wavelength of 248.3 nm, as explained in Example 1. The 
coefficient of absorption, the rate of change of refractive index with 
temperature, the high thermal conductivity, and the coefficient of thermal 
expansion may each be selected as described above. Or, the above factors 
can be balanced so that one or two of the factors, for example, offset the 
effects of the other factors and provide consistent beam expansion under 
varying operating conditions. 
FIG. 1 shows a preferred spectral narrowing assembly of this invention that 
produces a coherent beam of light with little spectral bandwidth variation 
and good beam profile. In operation, an excimer laser generates a 
spectrally-broad, tall and narrow beam 100 within its discharge chamber, 
which beam has light of a desired wavelength and light of an undesired 
wavelength. The beam is reflected by an optical resonator that comprises a 
partially-reflective front mirror (not shown) at one end of the discharge 
chamber and an echelle grating 110 at the other end of the discharge 
chamber. The beam expander (contained in box 120 of the Figure) is 
positioned so that a portion of the laser beam generated in the discharge 
chamber passes through the prisms 122, 124, and 126 of the beam expander 
to grating 110. 
The prisms of the beam expander are positioned such that the beam of fight 
from the discharge chamber contacts the beam contact area of the first 
prism, and the beam expands in width as the beam is transmitted through 
prisms 122, 124 and 126. Mirror 140 directs the beam exiting prism 126 
onto the surface of echelle grating 110. The light of the desired narrow 
range of wavelengths is reflected from grating 110 back through the prisms 
of the beam expander and back into the laser chamber; and substantially 
all of the light of the undesired wavelengths is reflected in directions 
such that it does not re-enter the laser chamber. The spectrally-narrowed 
beam re-enters the laser chamber, in further amplified and a portion of 
the amplified beam passes through it to the front mirror as output of the 
laser system. 
The particular application in which the laser is to be placed determines 
the spectral bandwidth of the beam that the laser must produce and thus 
determines the particular configuration of the spectral-narrowing 
assembly. Prisms are very effective for expanding the beam initially and 
removing light of unwanted wavelengths, but prisms are less effective for 
removing light of unwanted wavelengths after much of the light of unwanted 
wavelengths has already been removed. A KrF laser with no spectral 
narrowing produces a beam having a spectral bandwidth of approximately 300 
pm. If a prism is coated with an antireflection coating and is positioned 
to select light of a desired wavelength by expanding the beam and 
reflecting the light from a total-reflection mirror through the prism and 
back into the laser chamber, the spectral bandwidth can be reduced to e.g. 
approximately 100 pm. A second prism in series with the first prism can 
reduce the spectral bandwidth to e.g. approximately 50 pm, and a third 
prism in series with the others can reduce the spectral bandwidth to e.g. 
approximately 10-20 pm, depending on the size of the prisms and the angle 
at which the prisms face the laser beam. A grating positioned after a 
prism reduces the spectral bandwidth to much less than 10-20 pm. Thus, if 
a beam having a spectral bandwidth of approximately 50 pm can be used in a 
given application, the spectral-narrowing assembly needs only two prisms 
under this example. Or, if a beam of less than 10 pm spectral bandwidth is 
required, the spectral-narrowing assembly can use one, two, three, or more 
prisms and a grating. 
In a particularly preferred embodiment, the invention provides a selective 
spectral-narrowing assembly as illustrated in FIG. 1 comprising an 
aperture 130, a beam expander 120 comprising three prisms, a mirror 140 
and an echelle grating 110 arranged to reflect a desired narrow range of 
wavelengths back into the laser chamber. The prisms of the beam expander 
are made of a thermally stable material which transmits the coherent beam 
of light and wherein the prisms experience little change in optical 
properties with change in temperature. The aperture is positioned such 
that the tall and narrow coherent beam of light 100 generated in the laser 
chamber passes through the aperture. 
The spectral narrowing assembly described above has at least one aperture 
that shapes the laser beam. An aperture may be placed between the laser 
chamber and the first prism through which the laser light from the chamber 
is transmitted, as illustrated. Alternatively or additionally, an aperture 
may be placed between prisms or between the last prism and the grating. 
The position of the aperture and the width of the opening in the aperture 
are selected to provide a beam of the desired dimensions. 
There are at least two prisms in the beam expander, and the beam expander 
preferably has at least three prisms to expand the beam and spread it into 
its spectrum. It is desirable to expand the beam so that its expansion 
ratio (i.e. the ratio of the width of the beam exiting the beam expander 
to the width of the beam entering the beam expander from the laser 
chamber) as the beam exits the last prism is appropriately high so that 
the grating can effectively select the desired wavelength of light and 
discard light of undesired wavelengths. An additional benefit of a high 
expansion ratio is that the energy of the laser beam is spread over as 
much of the surface of the grating as feasible, which prolongs grating 
life. In various embodiments of the present invention, expansion ratios of 
between 5 and 30 would be preferred. 
The prisms are made of a thermally stable material, as discussed 
previously. It is not necessary for all prisms in the beam expander to be 
made from a thermally stable material, although this configuration is 
preferred. The prism in which the laser beam is expanded to its greatest 
width is preferably made of thermally stable material such as calcium 
fluoride or magnesium fluoride, followed in preference by the prism in 
which the laser beam is expanded to its second greatest width. Some of the 
combinations of on are specified in Table 2 below. 
TABLE 2 
__________________________________________________________________________ 
Second prism 
Third prism 
Fourth prism 
Prism first 
through which 
through which 
through which 
contacted by 
light from the 
light from the 
light from the 
Number of 
light from the 
discharge 
discharge 
discharge 
prisms in beam 
discharge 
chamber chamber chamber 
expander 
chamber passes passes passes 
__________________________________________________________________________ 
2 FS TSM -- -- 
2 TSM FS -- -- 
2 TSM TSM -- -- 
3 TSM FS FS -- 
3 FS TSM FS -- 
3 FS FS TSM -- 
3 TSM TSM FS -- 
3 TSM FS TSM -- 
3 FS TSM TSM -- 
3 TSM TSM TSM -- 
4 TSM FS FS FS 
4 FS TSM FS FS 
4 FS FS TSM FS 
4 FS FS FS TSM 
4 TSM TSM FS FS 
4 TSM FS TSM FS 
4 TSM FS FS TSM 
4 FS TSM TSM FS 
4 FS TSM FS TSM 
4 FS FS TSM TSM 
4 TSM TSM TSM FS 
4 TSM FS TSM TSM 
4 TSM TSM FS TSM 
4 FS TSM TSM TSM 
4 TSM TSM TSM TSM 
__________________________________________________________________________ 
FS = fused silica; TSM = thermally stable material 
The grating of the spectral narrowing assembly is preferably an echelle 
grating. The echelle grating is also preferably mounted to a curvature 
adjustment device as specified in U.S. Pat. No. 5,095,492, which patent is 
incorporated by reference to provide sufficient and enabling description, 
including description of a best mode of practicing this embodiment of the 
invention. Several other grating configurations well known for spectral 
narrowing could be substituted for the eschelle grating shown in FIG. 1. 
The spectral narrowing assembly optionally has a total-reflection mirror 
140 positioned between the prisms and the echelle grating. The mirror 
reflects the beam exiting the prisms onto the grating and vice versa. The 
mirror may be mounted to a curvature adjustment device as specified in 
U.S. Pat. No. 5,095,492 in addition to or instead of mounting the grating 
to a curvature adjustment device. 
It is not necessary to position all of the components of the spectral 
narrowing assembly within one module or enclosure. For example, the 
aperture can be formed in the assembly which positions the back window 
onto the laser discharge chamber or can be formed in a plate between the 
window and the assembly, and the prisms and grating can be located on 
separate platforms that are attached to beams or rails within the 
enclosure that houses all of the laser components. 
A laser equipped with a spectral narrowing assembly of this invention can 
have a longer lifetime. The spectral narrowing assembly suffers less heat 
damage to its optical components. Heat tends to distort optical components 
and also causes impurities or imperfections within those components to 
damage the components further. The rate of damage increases as more heat 
and imperfections build within the components and as components are heated 
or cooled rapidly. Consequently, the laser equipped with a spectral 
narrowing assembly of this invention can operate for a longer period of 
time before needing repairs or replacement. 
In a preferred embodiment of the invention, a laser with a spectral 
narrowing assembly of this invention provided a narrower spectral 
bandwidth than a laser equipped with a spectral narrowing assembly having 
optical-grade fused silica prisms. The examples below show how the 
bandwidth of a laser beam was reduced from about 1.1 pm where 
optical-grade fused silica prisms were used within the spectral narrowing 
assembly to a low of about 0.75-0.8 pm when the last and/or next to last 
prisms of the spectral narrowing assembly were made of a thermally stable 
material such as CaF.sub.2. Thus, a laser having a spectral narrowing 
assembly of this invention has the additional benefit of reduced bandwidth 
in certain preferred embodiments. 
A laser of this invention has substantially less variation in beam width 
and thus in expansion ratio. The width of the laser beam after expansion 
through the beam expander can vary by up to 50% when the prisms of the 
beam expander are made of materials other than thermally stable materials. 
When at least one of the prisms is made of a thermally stable material, 
the width of the expanded beam varies significantly less, and in many 
instances the variance in beam width is about 10% or less. 
The laser may optionally have a polarizing element that polarizes the laser 
beam for certain applications. The polarizing element may be located 
before any of the components of the spectral narrowing assembly, or the 
polarizing element may be a part of the components of the spectral 
narrowing assembly. The polarizing element may be a separate optical 
component such as a polarizer, or the polarizing element may be a 
polarizing coating on e.g. the prisms of the spectral narrowing assembly. 
Prisms may optionally be coated with an antireflection coating to prevent 
the substantial loss of light that occurs when prisms are mounted at large 
angles to the incident laser beam. 
The spectral narrowing assembly of this invention can be used in many types 
of lasers where it is desired to reduce variations in the spectral 
bandwidth or where it is desirable to reduce the spectral bandwidth 
itself. For example, the spectral narrowing assembly of this invention can 
be used with other gas lasers such as CO.sub.2 lasers or solid-state 
lasers such as NdYAG lasers. The spectral narrowing assembly of this 
invention is particularly suited to lasers that produce ultraviolet or 
near-UV light, such as KrF or ArF excimer lasers. 
The advantages discussed herein are demonstrated in the following examples, 
which are included to illustrate the invention. The scope of the claims is 
not limited to the preferred embodiments or the following examples but is, 
instead, to be interpreted considering the discussion herein, the figures, 
the meaning of terms to one of ordinary skill in the art as used or 
defined herein, and the claims themselves. 
EXAMPLES 1-6 AND COMATIVE EXAMPLES A-B 
In all examples, a KrF excimer laser was used to produce a tall, narrow 
laser beam having a nominal wavelength of 248.3 nm. The laser was 
configured substantially as illustrated in FIG. 1. The laser was operated 
at 100% duty cycle, with a pulse frequency of 600 Hz and a beam power of 
12 mJ. After 60 seconds, the laser was operated at 10% duty cycle, and 
after 1000 seconds, the laser was switched to 100% duty cycle. Duty cycle 
is the percentage of time that the laser fires during a given time period. 
A 100% duty cycle indicates that the laser fired all of the time during 
the time period, and a 10% duty cycle indicates that the laser fired 10% 
of the time period (e.g., firing occurs in a burst for 0.1 sec., the laser 
is idled for 0.9 sec., and firing repeats). 
The material from which each prism was made was varied according to Table 3 
below. 
Comments on Comparative Example A 
The laser utilizing optical-grade fused silica prisms exclusively had a 
large variation in spectral bandwidth with change in duty cycle, as 
illustrated in FIG. 2A. The difference between the spectral bandwidth when 
the laser was operated at 10% duty cycle (about 1.1 pm) and the spectral 
bandwidth when the laser was operated at 100% duty cycle (about 0.75 pm) 
was approximately 0.35 pm. Such optical-grade fused silica typically has a 
coefficient of thermal expansion of about 0.5.times.10.sup.-6 
(.degree.C.).sup.-1, a rate of change in refractive index with temperature 
of about 12.8.times.10.sup.-6 (.degree.C.).sup.-1, a thermal conductivity 
of about 1.38 Wm.sup.-1 .degree.C..sup.-1, and a coefficient of absorption 
of about 0.5-1%/cm for light having a wavelength of 248.3 nm. FIG. 2B 
shows that the voltage required to produce a laser beam fluctuated between 
about 740 and 785 volts at a 10% duty cycle and between about 750 and 795 
volts for a duty cycle of 100%. 
Comments on Example 1 
This laser, utilizing one fused silica prism and two CaF.sub.2 prisms, had 
little variation in spectral bandwidth with change in duty cycle, as 
illustrated in FIG. 3A. The difference between the spectral bandwidth when 
the laser was operated at 10% duty cycle (about 0.78 pm) and the spectral 
bandwidth when the laser was operated at 100% duty cycle (about 0.76 pm) 
was only about 0.02 pm. Further, the spectral bandwidth for the laser of 
this invention at 10% duty cycle was much lower than the spectral 
bandwidth for Comparative Example A at 10% duty cycle (0.78 pm v. 1.1 pm). 
Also, the laser of this invention has less variation in voltage when 
producing the laser beam. The voltage only fluctuated between about 610 
and 640 volts for a laser of this invention for a 10% duty cycle and 
between about 670 and 690 volts for a 100% duty cycle. Thus, this example 
illustrates many benefits of this invention: spectral bandwidth stability 
is improved, spectral bandwidth is reduced, and power requirements are 
more stable. 
Comments on Examples 2 and 3 
Examples 2 and 3 also show that a substantial reduction in spectral 
bandwidth variance occurs when fewer than all of the prisms of the 
spectral narrowing assembly are made from a thermally stable material. It 
is especially surprising that the spectral bandwidth variance is reduced 
substantially when only the second and/or third prisms (i.e. the prism 
with the second shortest light path to the grating and the prism with the 
shortest light path to the grating, respectively, of a three-prism 
assembly) are made of a thermally stable material. 
TABLE 3 
__________________________________________________________________________ 
Second 
Prism prism Third prism 
Spectral bandwidth 
first through 
through 
variance (pm) (difference 
contacted 
which light 
which light 
between spectral 
by light 
from the 
from the 
bandwidth at 10% duty 
from the 
discharge 
discharge 
cycle and 100% duty 
discharge 
chamber 
chamber 
cycle, as discussed in the 
Example # 
chamber 
passes passes examples) 
__________________________________________________________________________ 
Comparative 
FSUC FSUC FSUC 0.35 
Example A 
Comparative 
FS FS FS 0.25 
Example B 
1 FS CAF2 CAF2 0.02 
2 FS CAF2 FS 0.10 
3 CAF2 CAF2 FS 0.15 
__________________________________________________________________________ 
FS = opticalgrade fused silica coated with an antireflective coating 
FSUC = opticalgrade fused silica, uncoated 
CAF2 = CaF.sub.2, uncoated 
Examples 4-6 
The examples above are repeated using the configurations detailed in Table 
4 below. 
TABLE 4 
______________________________________ 
Prism first 
contacted by 
Second prism Third prism through 
light from the 
through which light 
which light from the 
discharge from the discharge 
discharge chamber 
Example # 
chamber chamber passes 
passes 
______________________________________ 
Comparative 
FSUC FSUC FSUC 
Example A 
Comparative 
FS FS FS 
Example B 
4 FS FS MGF2 
5 FS MGF2 MGF2 
6 MGF2 MGF2 MGF2 
______________________________________ 
FS = opticalgrade fused silica coated with a common antireflective coatin 
FSUC = opticalgrade fused silica, uncoated 
MGF2 = magnesium fluoride