WO2001041269A1 - Very narrow band excimer or molecular fluorine laser - Google Patents

Abstract

An excimer or molecular fluorine laser system generates a laser output bandwidth of less than 0.6 pm, and preferably 0.4 pm or less. The laser resonator has a line-narrowing unit preferably including a beam expander and a grating, and may include one or more etalons. The grating is a blazed grating having a blaze angle greater than 76°, and is preferably greater than 80°. The grating structure is preferably defined by the surface of the grating substrate. The substrate is preferably aluminum. The outcoupler is preferably a partially transmissive mirror positioned on the opposite side of the laser tube as the line-narrowing unit. Alternatively, outcoupling is performed by a polarization coupled resonator (PCR). A polarization rotator is preferably used in this alternative resonator configuration.

Description

VERY NARROW BAND EXCIMER OR MOLECULAR FLUORINE LASER

BACKGROUND OF THE INVENTION

Excimer, molecular, and molecular flourine lasers having a

narrowed spectral emission band are particularly useful in

microlithography. Narrowed spectral linewidths are desired because

minimum feature size and depth of focus in microlithography are

limited by chromatic aberrations of projection optics. Examples of

such lasers include KrF-, ArF-, XeCI-, XeF- and F₂-lasers, which

exhibit output emission wavelengths in the deep ultraviolet (DUV)

and the vacuum ultraviolet (VU V) regions of the electromagnetic

spectrum. A typical setup of laser systems emitting spectrally

narrowed output beams includes a resonator, a discharge chamber

filled with a gas mixture and connected to a power supply for

generating an output beam, and a wavelength selection module.

Without a wavelength selection unit, the natural output beam

of these lasers can be spectrally very broad (e.g., having a linewidth

around 500 pm) compared with a linewidth desired for applications

in microlithography (around one picometer or less) . The linewidth is

thus narrowed by the wavelength selection module, which also

allows a particular narrow band of wavelengths within the broad

band spectrum of the laser to be selected as the output.

A line-narrowed excimer or molecular fluorine laser used for

microlithography provides an output beam with specified narrow

spectral linewidth. It is desired that parameters of this output beam such as wavelength, linewidth, and energy, energy stability and

energy dose stability be reliable and consistent. Narrowing of the

linewidth is generally achieved through the use of a linewidth

narrowing and/or wavelength selection and wavelength tuning

module (hereinafter "line-narrowing module") consisting most

commonly of prisms, diffraction gratings and, in some cases, optical

etalons. A line-narrowing module typically functions to disperse

incoming light angularly such that light rays of the beam with

different wavelengths are reflected at different angles. Only those

rays fitting into a certain "acceptance angle" of the resonator

undergo further amplification, and eventually contribute to the

output of the laser system.

Conventional wavelength selection units exhibit a fixed

dispersion or beam expansion meaning that the dispersion or

expansion ratio cannot be adjusted during laser operation.

Depending on the type and extent of line narrowing and/or

selection and tuning that is desired, and the particular laser that the

line-narrowing module is to be installed into, there are many

alternative line-narrowing configurations that may be used.

According to the extent of line-narrowing that is desired, excimer

laser systems can be broadly classified into three general groups:

broad-band, semi-narrow band and narrow band. Broad band excimer lasers do not have any line narrowing

modular components. Therefore, the relatively broad (i.e., 300-400

pm) characteristic output emission bandwidth of a KrF or ArF laser,

e.g., is outcoupled from the laser resonator of a broad band excimer

laser system. Fig. 1 A schematically illustrates a typical broad band

laser resonator. The laser resonator includes a highly reflective

mirror ( 1 0), a laser tube ( 1 2) having a discharge chamber including a

pair of main electrodes ( 1 1 ) connected to a discharge circuit and a

preionization unit (not shown) and containing a gain medium, and a

partially transmissive outcoupler ( 1 4) for outcoupling the beam ( 1 6) .

A semi-narrow band laser has a characteristic output that is

line-narrowed using most typically a dispersive prism or gratings.

The dispersive prism or prisms are typically located between the

discharge chamber and a highly reflective resonator reflector. On

the other side of the laser chamber is typically a partially reflective

output coupler. The output emission bandwidth of the semi-

narrowed laser is reduced for a KrF or ArF laser, e.g., from around

300 pm to less than 1 00 pm. The semi-narrow band laser may be

used in combination with catadioptric (reflective) optical imaging

systems for industrial photolithography. The absence of refractive

optics and associated chromatic aberrations in catadioptric imaging

systems permits the linewidths of semi-narrow band lasers to be sufficient, and permit semi-narrow band lasers to be satisfactory

radiation sources for photolithographic applications.

Fig. 1 B schematically illustrates an example of a semi-narrow

band laser. The laser includes a highly reflective mirror ( 1 0), a laser

tube ( 1 2) and an outcoupler ( 1 4) for outcoupling the beam ( 1 6) . A

dispersive prism ( 1 8) is inserted into the resonator between the laser

tube ( 1 2) and the highly reflective mirror ( 1 0) . An aperture ( 1 9) is

also shown inserted between the laser tube ( 1 2) and the outcoupler

( 1 4) which may serve to reduce the acceptance angle of the

resonator and further reduce the output emission bandwidth.

A narrow band laser that typically has a far greater dispersive

power that the dispersive prisms referred to above further includes a

grating. The line-narrowing unit may comprise a Littrow

configuration of beam expanding prisms and a grating. The grating

used is typically an echelle-type blazed reflection grating having a

blaze angle around 76°. The most significant factor in the line

narrowing of this system is the dispersive power of the grating.

Preferably, a plurality of beam expanding prisms are used to magnify

the beam, thus reducing the beam divergence by the same

magnification factor, and contributing to the narrowing of the

bandwidth by spreading the beam over a larger area of the grating.

One or more etalons may also be added for further line narrowing,

for instance, either just before the grating, or between the prisms, or as an outcoupler. There are other related techniques described in

the patents and patent applications referenced above. Such

techniques are used to narrow the linewidth to below 1 pm. As

such, narrow band lasers are used in combination with refractive

optical imaging systems.

A fourth classification, very narrow band, is sometimes

referred to when it is desired to distinguish those lasers in the

narrow band group that have a particularly very narrow output

emission bandwidth (e.g., < 0.6 pm) . For instance, a typical narrow

band KrF excimer laser emitting around 248 nm or an ArF laser

emitting around 1 93 nm has a line-narrowing unit capable of

reducing the bandwidth to between 0.8 pm and 0.6 pm. To improve

the resolution of the projection optics, an even narrower laser

bandwidth is desired. It is particularly desired to have excimer and

molecular fluorine laser systems of high reliability and a very small

bandwidth of less than 0.6 pm and particularly still as low as 0.4 pm

or less.

There are restrictions on conventional laser resonators

preventing achievement of very narrow bandwidths of < 0.6 pm,

while maintaining other parameters such as pulse energy, pulse

repetition rate or lifetime of optical components. One of these

restrictions is a limitation on the expansion ratio or magnification of

the beam expander. The expansion ratio is limited by a limitation on size of the prisms that can practically be used in the beam expander

due to the magnitude of wavefront distortions introduced particularly

by larger prisms.

Another restriction is that reduction of the width of slit

apertures in the resonator is limited by energy considerations. That

is, below some minimum slit aperture width, the output energy of

the laser would be insufficient.

Increasing the finesse of an etalon in the resonator is limited

by a reduction in transmissivity of the etalon with increased finesse.

Below some minimum transmissivity of the etalon, the resonator

losses incurred are not tolerable.

In the '520 patent, mentioned above, a laser resonator is

described for generating output pulses having a bandwidth of 0.8

pm. The bandwidth of the pulses described in the '520 patent can

be reduced further to as low as 0.6 pm bandwidth by precise

modification of certain laser specifications. These modifications

include adjusting the composition of the gas mixture, the degree of

output coupling, the material of prisms and the length of the

electrodes.

In the '991 patent, mentioned above, a laser resonator is

described as providing pulses having a bandwidth of 0.5 pm or less

using an etalon output coupler, i.e., in place of the typical partially

reflective mirror output coupler. The addition of an etalon output coupler in the resonator, however, results in a complex resonator

because the etalon outcoupler would require special, complicated

fine tuning, such as by pressure tuning or using piezoelectric

actuators.

As mentioned above, diffraction gratings have been

incorporated into lasers in order to provide spectrally narrowed

output beams. A diffraction grating typically includes a plate or film

with a series of closely spaced lines or grooves (typically many

thousands per inch/hundreds per mm) . Diffraction gratings are

usually planar but gratings with other profiles are often used where

required by the application (e.g., spectroscopes) . See also U.S.

Patent no. 5,095,492. Diffraction gratings may also be formed in a

volume of material.

Diffraction gratings, their design and construction are

described in E.G. Loewen and E. Popow in Diffraction Gratings and

Applications (Marcel Dek er, 1 997) as well as in U.S. Patents No.

5,999,31 8 (Morton et al.) and 5,080,465 (Laude) . Each of these

three references is incorporated herein by reference in its entirety.

Diffraction gratings may be made by actually engraving each line

individually using a very precise ruling or etching mechanism. These

ruled gratings generally are of very high quality and very expensive.

Typically, such gratings are used as masters from which copy or

replica gratings are made. Replica gratings are practically as serviceable while being substantially less inexpensive. The

interference between a pair of laser beams can also be used to

directly generate holographic gratings. This technique allows

gratings with more complex arbitrary shapes and designs to be

manufactured.

Usually, the substrates for the gratings are made of special

glasses or ceramics, such as ULE ™ and Zerodure™ . In one design, a

diffraction grating would have a thin layer of epoxy with a thickness

of about 1 2 to 40 microns over the substrate surface. The epoxy

layer would have a diffraction grating incorporated as part of its

structure as the result of a replica process. The epoxy surface

would then in turn be coated by an aluminum layer on the order of

1 0 -30 microns thick. Aluminum absorbs more than 1 0% of the

radiation in the DUV spectral region within a very thin layer

thickness. An additional layer of a dielectric material might also be

added to the outer surface of the aluminum layer.

The '465 and '31 8 patents also teach the manufacture and

use of diffraction gratings having at least three layers as shown in

Fig. 2: a thin aluminum reflective upper layer (72), an epoxy

intermediate layer (74), and a glass substrate (76) . Optionally, there

may also be a dielectric coating (78) above the thin aluminum layer

(72) . While the thin aluminum layer (72) provides a reflective

surface, which is relatively impervious to the intense light of a laser beam, the laser beam can damage the underlying epoxy layer. Any

discontinuity in the thin aluminum layer allows the laser light to

penetrate to the underlying epoxy layer which is then subject to

photodecomposition reactions and consequential degradation of its

diffractive properties. This damage substantially limits the lifetime of

a diffractive grating and therefore undesirably increases the down

time of the laser.

To increase the lifetime and optical stability of a diffraction

grating, the epoxy substrate needs to be protected from such

photodecomposition. The '31 8 patent discloses application of a

protective aluminum overcoat of approximately 1 00 nm thickness

over the thin aluminum reflective layer. The overcoat is applied

under vacuum conditions by sputtering aluminum or deposition of

aluminum vapor onto the reflective aluminum layer after it has been

separated from the master. It is thought that the discontinuities or

fractures in the reflective aluminum layer are formed during

separation of the aluminum layer from the master. This overcoat of

aluminum protects the epoxy layer from any discontinuities of the

thin aluminum reflective layer and provides a diffraction grating with

an enhanced optical stability and use lifetime.

The intense energies of the laser beam are associated with a

great deal of heat energy resulting from even a relatively small

absorption of the intense laser light by matter. The rate at which heat energy is carried away from the diffraction grating having an

aluminum, epoxy and glass layer is primarily limited by the thermal

conductivities of the epoxy and glass layers which are substantially

less than that of aluminum.

The performance of a diffraction grating is sensitive to

temperature changes. For instance, temperature changes,

particularly nonuniform changes in the temperature, may distort the

wavefront of a back reflected laser beam due to heat related

distortions in the grating structure. Temperature changes distort

surface flatness to adversely affect bandwidth. Temperature

changes also variably alter the distance of the grating lines from

each other to produce a wavelength shift.

If a wavelength shift of less than 0.1 pm is required, the

maximum variation in the temperature of the grating is given by the

formula: δT < (δλ)/α. In this formula, δT represents the temperature

variation, δλ represents the wavelength shift, and α represents the

coefficient of linear thermal expansion of the substrate of the

grating.

The photon energy of a laser can be quite high, especially for

excimer, molecular, or molecular flourine laser operating in the UV

region. For instance, a KrF laser operating around 248 nm generates

photons of about 5eV; an ArF laser operating around 1 93 nm

generates photons of about 6.4 eV; and a F₂ laser operating around 1 57 nm generates photons of about 7.9 eV. Photons with these

energy levels are capable of breaking the molecular bonds of the

epoxy substrate. In addition, the epoxy layer is subject to thermal

decomposition. Thus, there is a need for a more temperature and

laser beam resistant diffraction grating for use in lasers, especially

for highly dispersive diffraction gratings incorporated into a line

narrowing module.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an excimer or

molecular fluorine laser having an output emission bandwidth below

0.6 pm.

It is a further object of the invention to provide such a laser

with a bandwidth as low as 0.4 pm or less.

It is a further object of the invention to provide a line-

narrowing unit for a laser in accord with the first two objects, i.e., to

produce the desired very narrow bandwidth, without complicated

fine tuning of such an element as an etalon outcoupler.

To meet the above objects, an excimer or molecular fluorine

laser is provided for generating a laser output bandwidth of less than

0.6 pm, preferably of less than 0.5 pm, and more preferably 0.4 pm

or less. The laser resonator preferably includes a laser tube

surrounded by a resonator including a line-narrowing unit and an outcoupler. The line-narrowing unit preferably includes a beam

expander and a grating, and may include one or more etalons. The

grating is advantageously configured to provide enhanced dispersion

for reducing the bandwidth in accord with the above objects. The

grating is preferably a blazed grating having a blaze angle greater

than 76°. The blaze angle is preferably particularly greater than 78°,

and more particularly greater than 80°. For example, the blaze angle

may be between 78° and 82°, and more preferably around 81 °. The

grating is preferably an echelle type reflection grating, and as such,

serves also as a highly reflective resonator reflector.

The line narrowing unit preferably has a diffraction grating

with an advantageously high damage threshold with respect to laser

beam radiation and heat associated with laser beam narrowing. This

preferred grating is defined within the surface of the grating

substrate/rigid base body such that the substrate and grating are

substantially formed from a single material which has high thermal

conductivity and is resistant to the destructive action of prolonged

exposure to intense laser beam energy. This grating therefore has an

advantageously high damage threshold owing to the definition of the

grating structure by the substrate surface. In some embodiments,

this grating is preferably has a coating of reflective dielectric

material. The grating disperses and reflects portions of the incident

beam and is resistant to the energy associated therewith. The outcoupler is preferably a partially transmissive mirror,

and preferably is positioned on the opposite side of the laser tube as

the line-narrowing unit. Alternatively, outcoupling may be performed

by reflecting a component, such as a polarization component, of the

beam from a surface of a prism or other optical surface, and the

resonator may be a polarization coupled resonator (PCR) . A

polarization rotator is preferably used in this alternative resonator

configuration. The laser tube includes a discharge chamber filled

with a laser gas mixture and having a plurality of electrodes

connected to a discharge circuit for energizing the gas mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 A schematically illustrates a broad band laser resonator.

Fig. 1 B schematically illustrates a semi-narrow band laser

resonator.

Fig. 2 schematically illustrates a diffraction grating having a

grid structure formed in an epoxy layer attached to a substrate.

Fig. 3A schematically illustrates a laser resonator in accord

with a first embodiment of the present invention.

Fig. 3B schematically illustrates a laser resonator in accord

with a second embodiment of the present invention.

Fig. 4A schematically illustrates a first line-narrowing unit in

accord with the present invention. Fig. 4B schematically illustrates a second line-narrowing unit

in accord with the present invention.

Fig. 5 schematically illustrates a grating having a blaze angle

greater than 76° in accord with the present invention.

Figs. 6A-6D schematically illustrate several diffraction gratings

having a grid structure formed within the surface of the

substrate/rigid base body.

Figs. 7A-7B schematically illustrate how to use ion beams to

form a diffraction grating within the surface of a substrate/rigid base

body.

Fig. 8 is a schematic block diagram of a preferred narrow band

or very narrow band laser according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 3A schematically illustrates a first laser resonator

configuration for an excimer or molecular fluorine laser in accord

with the present invention. The resonator design shown in Fig. 3A

includes a laser chamber or laser tube ( 1 2) containing a laser gas

mixture and having a pair of main electrodes ( 1 1 ) and one or more

preionization electrodes (not shown) connected to a discharge circuit

including a power supply and pulser circuit for energizing the gas

mixture. Preferred circuits (not shown) and circuit components such as main and preionization electrodes are described at U .S. patent

applications no. 08/842,578, 08/822,451 , 09/390, 1 46,

09/247,887, 60/1 28,227 and 60/1 62,645, each of which is

assigned to the same assignee as the present application and which

is hereby incorporated by reference.

The resonator further includes a line-narrowing unit (25), a slit

aperture ( 1 9) and a partially transmissive outcoupling mirror or

resonator reflector ( 1 4) . More than one aperture or no aperture may

be included, and the aperture or apertures may be located in various

positions within the resonator including either side of the laser tube

( 1 2) . Preferred aperture designs and configurations are described at

U.S. patent no. 5, 1 61 ,238 and U.S. patent application no.

09/1 30,277, each of which is assigned to the same assignee as the

present application and hereby incorporated by reference. A

wavelength monitor and stabilization device is included in the laser

system (although not shown) and the preferred system is described

at U.S. patent no. 4,905,243 and U.S. patent application no.

09/41 6,344, each of which is assigned to the same assignee, and

U.S. patents no. 5,420,877, 5,450,207, 5,978,391 and 5,978,394,

all of which are hereby incorporated by reference. The line-

narrowing unit is described in detail below with reference to Figs.

4A-4B and Fig. 5. Fig. 3B schematically illustrates a second resonator

configuration in accord with the present invention. The resonator of

Fig. 3B includes a laser tube ( 1 2) and aperture ( 1 9) as described

above, and a line-narrowing unit (25) (as mentioned, to be described

in detail below). The resonator of Fig. 3B also includes a highly

reflective mirror or resonator reflector (26), a polarization rotator (28)

and a polarizing beam splitter (29) . The polarization rotator (28) in

front of the highly reflective mirror (26) and the polarizing beam

splitter (29) work together to outcouple a polarization component of

the beam. Thus, there is no partially transmissive outcoupler in the

resonator of Fig. 3B, and the line-narrowing unit (25) also includes a

highly reflective component such as a highly reflective grating. A

surface of another optical component such as a prism,, angled

window of the laser tube or tilted etalon may be used to outcouple

the beam instead of the beam splitter (29) .

Figs. 4A-4B schematically illustrate two preferred line-

narrowing units (25) for use with the first and second resonator

arrangements shown in Figs. 3A-3B. Another line-narrowing unit

that can be used to provide a very narrow bandwidth would include

a prism beam expander, one or more etalons and a highly reflective

mirror, but no grating. However, more preferred embodiments of the

present invention each have a grating, as shown in Figs. 4A-4B. The line narrowing-unit of Fig. 4A includes a prism beam

expander (32), a grating (36) and an optional aperture (38) . The

prism beam expander (32) shown includes two beam expanding

prisms, but the beam expander (32) may comprise a different

number of beam expanding prisms such as one or more than two. A

dispersion prism may also be included. Alternatively, another beam

expander may be used such as a lens configuration including a

diverging and a converging lens. The beam expansion prisms may

each comprise CaF₂, or fused silica, or the prisms may comprise one

each of fused silica and CaF₂, or the prisms may comprise another

material having similar properties such as absorption coefficient,

thermal expansion and resistance to thermal stress at the laser

wavelengths and repetitions being used (e.g. , 248 nm, 1 93 nm and

1 57 nm, and 1 kHz or more) . Preferred beam expanders are set

forth at U.S. patent no. 5,761 ,236, and U.S. patent application no.

09/244,554, each of which is assigned to the same assignee, and

U.S. patent no. 5,898,725, all of which are hereby incorporated by

reference. The grating is described in more detail below with

reference to Fig. 5.

Fig. 4B shows a second preferred line-narrowing unit for use

with either of the first or second preferred resonator arrangements

shown in Figs. 3A-3B. The line-narrowing unit shown in Fig. 4B

includes a prism beam expander (32) (as discussed above with respect to Fig. 4A), an etalon (39), a grating (36) and an optional

aperture (38) . The preferred etalon is described at U .S. patent

applications no. 60/1 62,735, 09/31 7,695 and 09/31 7,527, each of

which is assigned to the same assignee and is hereby incorporated

by reference. More than one such etalon may be included.

A preferred grating (36) is shown in Fig. 5. This preferred

grating may be included in the line narrowing units of each of the

preferred line-narrowing units of Figs. 4A-4B. The grating

dimensions of any of the figures 1 -8 are not drawn to scale. The

separation of grooves is related to the wavelength of the light to be

reflected by the grating and the narrowness of the range of

wavelengths it is required to reflect. Gratings may have grooves on

the order of tens of thousand per cm. An incident beam l₀ reflects

from the surface of the grating, as shown. Preferred distance, or

separation, between grooves, d, is governed by formulae that are

well known in the art (e.g., d • (sin 1 + sin R) = N_DOλ, where I is the

angle of the incident beam to the grating surface, R is the angle of

the reflected beam, λ is the wavelength of the beam, and N_D0 is

according to the diffraction order number. For retroreflected beams,

as at the blaze angle (α_b), the incident and reflective angles are the

same. Thus, the formula reduces at the blaze angle condition to d •

2 (sin cc_b) = N_D0 λ). The grating (36) preferably has a line groove

density of 1 /d. The beam l₀ impinges upon the grating (36) and the rays reflect from the grating (36) according to the standard grating

formula. That is, the beam is dispersed by the grating (36) such that

light rays incident at the grating (36) reflect at unique angles

depending on their particular wavelengths. The wavelengths around

a central wavelength λ₀ are retroreflected back into the laser

resonator, as shown in Fig. 5.

Only those rays having wavelengths within the acceptance

range of angles θ₀ of the laser resonator will be included in the

output emission beam of the laser system. The range of

wavelengths that will be retroreflected back into the laser tube is λ₀ -

Δ λ/2 to λ₀ + Δ λ/2. Thus, the wavelength range has a breadth Δλ

that will determine ultimately the bandwidth or linewidth of the

output emission beam of the laser (see below) . The central

wavelength within the band is λ₀ which is separately controlled

preferably by orienting the grating (36) at a particular selected angle

with respect to the incident beam.

The angular acceptance range θ₀ is fixed by the resonator and

discharge width independently of the dispersion of the grating.

Thus, the wavelength range Δλ which ultimately determines the

output bandwidth of the laser beam may be adjusted by adjusting

the dispersion of the grating (36) based on the formula:

θ₀ = dα/dλ « Δλ ( 1 ), where dα/dλ is the dispersion of the grating (36).

The passive bandwidth or single-pass bandwidth generated by

a grating in Littrow configuration (shown schematically in Fig. 4A) is

particularly described by the following equation:

Δλ' = λ₀ • ΔΘ/[2 • tan( _B)] (2),

where Δλ' is the bandwidth, λ₀ is the central wavelength of the

output emission beam of the laser, α_B is the blaze angle of the

grating (36) used in Littrow configuration, tan (α_B ) corresponds to

the dispersion of the grating (36) in Littrow configuration, and ΔΘ is

the beam divergence.

The final bandwidth Δλ" after n passes or round trips through

the resonator for a gaussian line shape is approximately given by:

Δλ" * Δλ'/(n)^1/2 (3) .

An observation of equations (2) and (3) reveals that the bandwidth

Δλ" can be adjusted (i.e., reduced) in the following ways:

1 . decrease the beam divergence ΔΘ; 2. increase the grating dispersion (tan (α)); and/or

3. increase the number of round trips, n.

Decreasing the divergence according to item 1 is possible by

increasing the magnification of the prism beam expander (32) of

Figs. 4A (or Fig. 4B) . The expansion by magnification factor M

reduces the beam divergence by the same factor M . However, this

is one of the restricted techniques discussed above. That is,

increasing the expansion ratio of the beam expander is limited

because wavefront distortions due to imperfections at the surfaces

of the prisms will substantially inhibit successful bandwidth

narrowing effort beyond a certain magnification M. It is preferred

that the magnification M be maximized in accord with this

restriction, but the desired narrow bandwidth is not fully achieved in

this way according to the present invention.

Increasing the number of round trips in accord with item 3 is

also preferred in accord with the present invention. For example, the

gas mixture composition should be optimized (particularly the

halogen concentration is the gas mixture) as well as the degree of

outcoupling by the outcoupler (components 14 and 29 of Figs. 3A

and 3B, discussed above) (see the '520 patent referred to above).

However, the pulsed discharge mode of the laser has a short lifetime

of gain medium inversion (in the range of < 1 00 nanoseconds) .

Thus, increasing the number of round trips, n, is limited by this short inversion lifetime, and, as with item 1 , the desired bandwidth is not

fully realized in accord with the present invention in this way either,

i.e. , by maximizing the number of round trips.

An optimized resonator of the type as shown schematically in

Fig. 3A, and in accord with items 1 and 3 above produced a

bandwidth around 0.6 pm; not yet fully in accord with the objects of

the invention. The grating used was an echelle type grating having

tan(α) = 5 (known as a R5-grating) . The slit width of the aperture

( 1 9) was optimized in accord with the '277 application mentioned

above. The preferred slit width of the aperture ( 1 9) is 1 -2 mm. The

number and type of prisms in the beam expander (32) was also

optimized, and may generally vary depending on the laser system

and specifications of the industrial application. In addition, the

output pulse energy was around 1 0 mJ, the energy stability was a

deviation around < 3%, the dose stability has a deviation around

< 0.5%, and the repetition rate was around 2 kHz, in accord with

typical specifications delivered requested by stepper/scanner

manufacturers.

Increasing dispersion in accord with item 2 advantageously

allows the desired narrow bandwidth to be achieved in accord with

the present invention. This increased dispersion is achieved in

accord with the present invention by using a grating (36) having a

blaze angle greater than 76°. By using a grating (36) having a blaze angle greater than 76° with the line-narrowing unit of either Fig. 4A

or 4B, an object of the invention set forth above is met, i.e.,

providing an excimer or molecular fluorine laser with a bandwidth

less than 0.6 pm. By using a grating (36) having a blaze angle

greater than 80°, the second object of the invention is met, i.e., an

excimer laser is achieved having an output emission bandwidth of

0.4 pm or less. The third object is also met because no fine-tuning

of an optical element such as an etalon outcoupler is necessarily

performed to achieve the desired very narrow bandwidth.

The particularly preferred grating (36) for use with a line-

narrowing unit (25) in accord with the present invention is a

specially designed R6.5-grating (i.e. , tan(α) ~ 6.5) . The blaze angle

of this grating is about 81 °. With this resonator configuration

including a preferred grating (36) having a blaze angie around 81 °, a

bandwidth less than 0.4 pm was achieved with an excimer laser.

The measured bandwidth was around 0.3 pm. The spectral purity of

the laser beam was less than 2.0 pm.

It is recognized that there is an upper limit on how much the

blaze angle can be increased to achieve advantageously narrower

bandwidths in accord with the present invention. For example,

clearly the grating (36) cannot have a blaze angle of 90°, and thus

the blaze angle α_B of a grating (36) in accord with the present

invention will be less than 90°. There is a real limit that is still less than 90° and may be 86°-87°. Although it may be difficult to

manufacture a grating (36) having a blaze angle as high as 86°-87°,

one skilled in the art would understand that it is possible to make

them. Thus, these higher gratings also may be advantageously

used with an excimer laser in accord with the present invention.

The present invention can advantageously achieve an excimer

or molecular fluorine laser having a very narrow output emission

bandwidth Δλ" by increasing the dispersion dα/dλ of the grating (36) .

This increasing of dα/dλ of the grating (36) is achieved by increasing

the blaze angle α_B of the grating (36) from the typical blaze angle

around 75°-76° to more than 76°. Preferably, the blaze angle of the

grating (36) is more than 78°, and more preferably the blaze angle of

the grating (36) is more than 80°. It is specifically preferred to have

a blaze angle around 81 °, in accord with the present invention.

The present invention provides a very narrow band excimer

laser having a resonator as efficient and simple as possible resulting

in a laser system of high reliability. The laser system of the present

invention meets the above objects and the demands of

stepper/scanner manufacturers who desire an excimer laser having

line-narrowing capability such that a laser output beam may be

provided having a bandwidth of 0.4 pm or less.

A preferred grating has a substrate having a surface upon

which the grating structure is preferably machined or etched directly. The grating surface is preferably coated by a highly UV reflecting

layer system of dielectric material or an aluminum layer with an UV

reflection enhancing dielectric coating or coating system or an

aluminum layer with a dielectric protecting layer. This structure is

much more stable against heating and aging effects associated with

increased dispersion of laser radiation. This resistance is largely due

to the absence of the organic epoxy layer which can be adversely

affected by heat and UV radiation. If the body of the grating is

made of metal, a second advantage is the high thermal conductivity

of the grating body compared to glass or ceramic which minimizes

the generation of thermal gradients. Preferably, the temperature of

the grating has to be kept constant within the constraints of the

thermal expansion expression: δT < (δλ)/α.

A dielectric reflecting layer provides a preferred grating that

has a higher UV reflectivity and a greater lifetime as compared to a

pure aluminum surface layer without the dielectric reflecting layer.

A substrate may be virtually of any thickness as long as it is

sufficiently thick to provide material to furnish the grating structure

and to resist deformation or fractures due to the stress associated

with an intended use. Preferred gratings for use in a line narrowing

unit for an excimer laser would have substrate dimensions on the

order of about 30 mm x 1 60 mm x 30 mm to about 35 mm x 300

mm x 35 mm. Preferably, the length of the grooves would vary according to or with the grating substrate dimensions (e.g., groove

lengths from about 30 to 35 mm) . More particularly, a preferred

grating substrate would have the dimensions of 30 mm x 1 60 mm x

30 mm with a groove length of 30 mm; another preferred grating

substrate would have dimensions of about about 35 mm x 300 mm

x 35 mm and a groove length of 35 mm.

The substrate of a preferred diffraction grating is metal, more

preferably, aluminum. Other reflective metals and materials (e.g.,

chromium, magnesium fluoride, silicon and germanium) are also

suitable.

A preferred grating has a coating combining an aluminum layer

with a MgF₂-layer.

Figures 6A-6D show several preferred diffraction gratings.

These gratings have a substrate body (80) with a grating structure

defined therein. The gratings of Fig. 6 differ from prior art gratings

where the grating structure is formed on the surface of a thin epoxy

layer placed on the surface of a substrate body made of glass or

ceramic material (Fig. 2).

In a preferred embodiment, the substrate body (80) of Fig. 6 is

made of metal (e.g., aluminum) . In this preferred embodiment, rapid

(within < 1 second) temperature variations could also be a problem if

they are greater than the temperature variation indicated by the

above thermal expansion expression. In preferred embodiments, the grating surface (92) is coated by a highly UV reflecting dielectric

material (88) (Fig. 6A) or a thin reflective aluminum layer (90) (Fig.

6B) or an aluminum layer coated (90) with a dielectric protecting

layer (88) (Fig. 6C) or an aluminum layer (90) with an UV reflection

enhancing dielectric coating (89) (Fig. 6D).

This structure is much more stable against heating and aging

effects because of the absence of the organic epoxy layer which

could be easily affected by heat and UV radiation. A second related

advantage of the preferred embodiment made of aluminum is the

high thermal conductivity of a grating body made of a metal as

compared to one made of glass or ceramic.

Preferred groove distances for diffraction gratings are

ascertained according to the above discussed formula. These

preferred distances can be readily determined for a particular laser by

substituting the wavelength of the laser beam and the incident and

reflective angles of the grating. Preferred groove distances

correspond to blaze angles in excess of 76° and wavelengths

between about 1 50 nm and 350 nm. More particularly, preferred

groove distances correspond to blaze angles between 76° and 82°

and wavelengths of about 248 nm, 1 93 nm, 351 nm, 222 nm, 266

nm, 355nm, 308 nm, and 1 57 nm.

Fig. 7A shows a way for making a preferred diffraction grating

(50). An ion beam (41 ) is used to irradiate the surface of the substrate itself (45) after passing an attenuator (43) providing an

attenuation corresponding to a desired grating pattern. If the ion

beam cross section is smaller than the substrate surface, the beam

may be scanned across the surface.

A special procedure of this ion beam etching is depicted in

Fig. 7B. In Fig. 7B, an intermediate diffraction grating replica (47) is

first made according to methods available to one of ordinary skill in

the art. For instance, a master grating is first formed by etching

with a diamond stylus. However, the master grating may be formed

by other processes and may even be a replica of another master.

The diffraction grating surfaces of the master may then be treated

with a release agent, such as silicone, so as to facilitate separation

of the replica from the master. The release layer is preferably very

thin, only a few nanometers in thickness. Then, a replica is built up

on the master using known techniques. See U.S. Patent no.

5,999,31 8. In a preferred embodiment, the intermediate replica

diffraction grating structure (44) is made of epoxy and the

intermediate replica substrate (45) is made of aluminum.

As shown in Fig. 7B, the intermediate replica (47) is then

subject to etching by an ion beam (41 ) which removes the epoxy

diffraction grating (44) and forms a diffraction grating (50) at the

same time. As indicated in Figure 7B, the ion beams (41 ) remove

both the epoxy (44) and some of the substrate body (45) in order to form the grating (50) . As the epoxy (44) covers the substrate body

in varying thicknesses, the substrate body (45) is variably etched

according to the overlying thickness of the epoxy grating structure

(44) . As a result, a diffraction grating structure which corresponds

to the structure of the intermediate epoxy grating structure is

reproduced in the surface of the substrate body (50).

A preferred embodiment of the invention therefore is a narrow

line width excimer laser system for use in optical lithography which

incorporates a reflective diffraction grating for use in line narrowing.

The diffraction grating has a particularly high damage threshold as

the diffraction grating is etched directly in the surface material of the

substrate which is preferably aluminum.

A preferred embodiment of the invention is an excimer or

molecular fluorine laser for generating a laser output bandwidth of

less than 0.6 pm, and preferably 0.4 pm or less. The laser resonator

preferably includes a laser tube surrounded by a resonator including

a line-narrowing unit and an outcoupler. The line-narrowing unit

includes a beam expander and a diffraction grating as taught herein,

and may include one or more etalons. The diffraction grating is

advantageously configured to provide enhanced dispersion for

reducing the bandwidth in accord with the above objects and

designed to better withstand the intense UV light and heat associated with a laser application and the increased dispersion of

laser radiation.

A preferred narrow band laser resonator according to the

invention incorporates a diffraction grating having a diffraction grid

directly formed in the surface of the substrate. The substrate is

preferably aluminum. This grating forms part of a line-narrowing unit

providing a Littrow configuration of beam expanding prisms and the

diffraction grating. The grating is substituted for the highly reflective

mirror of the semi-narrow band laser, described above. The grating

is preferably an echelle-type blazed reflection grating having a blaze

angle above 76° and, more preferably, between 78° and 82°.

Perhaps the most significant factor in the line narrowing is the

dispersive power of the grating. Preferably, a plurality of beam

expanding prisms are also used to magnify the beam, thus reducing

the beam divergence by the same magnification factor, and

contributing to the narrowing of the bandwidth. One or more

etalons may also be added for further line narrowing, either just

before the grating, or between the prisms, or as an outcoupler.

The present invention will be particularly described for use

with a KrF-excimer laser emitting around 248 nm, although the

present invention may be advantageously used for spectral

narrowing of other lasers, especially pulsed gas discharge lasers

such as excimer and molecular and molecular flourine lasers emitting in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) . These

lasers have particularly become very important for industrial

applications such as photolithography. Such lasers generally include

a discharge chamber containing two or more gases such as a

halogen and one or two rare gases. Examples of such lasers include

KrF (248 nm), ArF ( 1 93 nm), XeF (350 nm), KrCI (222 nm), XeCI

(308 nm), and F2 ( 1 57 nm) lasers. The inventive methods are

preferably applied to a wide variety of such gas discharge laser

systems.

Fig. 8 schematically illustrates a first laser resonator

configuration for an excimer or molecular fluorine laser in accord

with the present invention. In this system, there is a gas discharge

chamber ( 1 2) containing the laser gas mixture, a fan (not shown)

and heat exchanger (not shown) . Pressure and temperature gauges

for monitoring the gas pressure and temperature within the tube may

also be provided. The chamber ( 1 2) contains a pair of main

electrodes (1 1 ), the anode and cathode, which define between them

a main discharge gas volume ( 1 3) . It also may contain a

preionization unit (not shown) . The electrical pulse power and

discharge module (6) is connected to the main discharge electrodes

( 1 1 ) .

The tube includes resonator units in optic modules at each

end: a rear optics module (2) and a front optics module (3) . The rear optics module (2) contains a high reflective means (21 ) . Preferred

rear high reflective means can be a mirror or reflective grating for

line narrowing and additional optical elements for beam steering or

forming like mirrors or prisms. A wavelength calibration module (23)

is preferably included with the rear optics module (2) . Wavelength

calibration units or devices and techniques are disclosed in U.S.

Patent No. 4,905,243 and U.S. patent applications no.09/1 36,275,

09/1 67,657 and 09/1 79,262, each of which is assigned to the same

assignee as the present application and is hereby incorporated by

reference. The diffraction gratings described in detail above are

readily substituted by one of ordinary skill in the art for the gratings

disclosed in these references. These diffraction gratings are

according to structures preferably etched on the surface of a

substrate. This substrate is preferably metal, and more preferably,

aluminum. The preferred blaze angles are as described above.

The front optic module (3) contains an outcoupling means (31 )

and optionally additional elements for beam steering and shaping the

output beam ( 1 6) . The front optics module (3) preferably contains

an output coupling resonator reflector (31 ) and optional elements,

such as mirrors, beam splitters, prisms or dispersive elements (e.g.,

gratings, etalons) for beam steering splitting or forming. Such

optical elements and techniques are described in U .S. Patents No.

4,399,540, 4,905,243, 5,226,050, 5,559,81 6, 5,659,41 9, 5,663,973, 5,761 ,236, and 5,946,337, and U.S. patent

applications no. 09/31 7,695, 09/1 30,277, 09/244,554,

09/31 7,527, 09/073,070, 60/1 24,241 , 60/1 40, 532, 60/1 40,531 ,

and 60/1 71 ,71 7 each of which is assigned to the same assignee as

the present application, and U.S. Patents No. 5,095,492,

5,684,822, 5,835,520, 5,852,627, 5,856,991 , 5,898,725,

5,901 , 1 63, 5,91 7,849, 5,970,082, 5,404,366, 4,975,91 9,

5, 1 42,543, 5,596,596, 5,802,094, 4,856,01 8, and 4,829,536,

which are each hereby incorporated by reference into the present

application, as describing line narrowing, selection and/or tuning

elements, devices and/or techniques. The high damage threshold

diffraction gratings described in detail above are readily substituted

by one of ordinary skill in the art for the gratings disclosed in these

references. These diffraction gratings are according to structures

preferably etched on the surface of a substrate. This substrate is

preferably metal, and more preferably, aluminum. The preferred

blaze angles are as described above.

In a preferred embodiment, dispersive gratings are employed

for spectral narrowing. See, e.g., U.S. Patent No. 5,095,492 to

Sandstrom; and U.S. Patent No. 4,696,01 2 to Harshaw. Prisms

may also be used as wavelength selection devices. See U.S. Patent

No. 5,761 ,236. Fabry-Perot etalons may also be employed as

wavelength selection devices. See M. Okada and S. Leiri, Electronic Tuning of Dye Lasers by an Electro-Optic Birefringent Fabry-Perot

Etalon, Optics Communications, vol. 1 4, No. 1 (May 1 975) .

Birefringent plates are also used for wavelength selection. See A.

Bloom, Modes of a Laser Resonator Containing Tilted Birefringent

Plates, Journal of the Optical Society of America, Vol. 64, No. 4

(April 1 974); See also U.S. Patent No. 3,868,592 to Yarborough et

al. Unstable resonator configurations may be employed within

pulsed excimer lasers. See, e.g., U.S. Patent No. 5,684,822 to

Partlo. U.S. Patent No. 4,873,692 to Johnson et al. discloses a

solid state laser including a rotatable grating and a fixed beam

expander for narrowing the linewidth and tuning the wavelength of

the laser. Further background information on methods of spectral

linewidth narrowing of lasers can be found in textbooks on the

tunable lasers. See, e.g., A.E. Siegman, Lasers ( 1 986) . Each of the

above references of this paragraph is herein incorporated by

reference.

An electrical pulse power and discharge unit (6) energizes the

laser gas mixture. The pulse power and discharge unit provides

energy to the laser gas mixture via a pair of main electrodes ( 1 1 )

within the discharge chamber. An electrical pulse power and

discharge unit (6) energizes the laser gas mixture. The pulse power

and discharge unit provides energy to the laser gas mixture via a pair

of main electrodes ( 1 1 ) within the discharge chamber. Preferably, a preionization element of the pulse power and discharge unit (not

shown) is also energized for preionizing the gas just prior to the main

discharge. The discharge circuit includes a power supply and pulser

circuit for energizing the gas mixture. Preferred circuits (not shown)

and circuit components such as main electrodes ( 1 1 ) and

preionization electrodes (not shown) are described at U.S. patent

applications no. 08/842,578, 08/822,451 , 09/390, 1 46,

09/247,887, 60/1 28,227 and 60/1 62,645, each of which is

assigned to the same assignee as the present application and which

is hereby incorporated by reference.

The energy of the output beam ( 1 6) has a known

dependence on driving voltage of the pulse power module (6) . The

driving energy is preferably adjusted during laser operation to control

and stabilize the energy of the output beam. The processor (9)

controls the driving voltage based upon the beam energy information

received from the energy monitor (4) . Suitable energy monitors

include photodetectors, photodiodes, and pyroelectric detectors.

Means for regulating laser operation and conditions to control the

output beam are described in U.S. patent application no.

60/1 30,392 and its related non-provisional U.S. patent application

no. 09/550,558 which are assigned to the same assignee and

hereby incorporated by reference in their entirety. The gas mixture of an excimer or molecular fluorine laser is

characterized as being strongly electronegative and maintained at an

elevated pressure (e.g., a few bars) . The gas mixture for an excimer

laser includes an active rare gas such as krypton, argon or xenon, a

halogen containing species such as fluorine or hydrogen chloride,

and a buffer gas such as neon or helium. A molecular fluorine laser

includes molecular fluorine and a buffer gas such as neon and/or

helium.

The gas mixture is naturally heated as it is excited by the

electrical discharge in the discharge area. The heat exchanger (not

shown) cools the heated gas after it exits the discharge area. The

portion of the gas mixture that participates in a laser pulse is

replaced by fresh gas before the next laser pulse occurs. A gas

supply unit (7) also typically supplies fresh gas to the system from

outside gas containers ( 1 7) to replenish each of the components of

the gas mixture. In particular, halogen is typically supplied because

the halogen concentration in the gas mixture tends to deplete during

operation, while it is desired to maintain a constant or near constant

halogen concentration in the gas mixture. Means for releasing some

of the gas mixture is also typically provided so that the gas pressure

can be controlled. Preferred gas replenishment procedures are set

forth in U.S. provisional patent application no. 60/1 24,785 and U .S.

provisional patent application no. 60/1 30,392 and its related non- provisional U .S. patent application 09/550,558 which are assigned

to the same assignee and hereby incorporated by reference in their

entireties.

Preferred gas mixtures and methods of stabilizing gas mixtures

of these excimer lasers and other lasers such as the XeF, XeCI, KrCl

excimer lasers, as well as the molecular fluorine laser, and laser tube

configurations with respect to the gas flow vessel are described at:

U.S. patents no. 4,393,505, 4,977, 573 and 5,396,51 4, and U.S.

patent applications no. 09/31 7,526, 09/41 8,052, 09/379,034,

60/1 60, 1 26, 60/1 28,227 and 60/1 24,785, each of which is

assigned to the same assignee as the present application, and also

U.S. patents no. 5,440,578 and 5,450,436, all of the above U.S.

patents and patent applications being hereby incorporated by

reference into the present application. Gas purification systems,

such as cryogenic gas filters (see U.S. Patent No. 4,534,034,

5, 1 36,605, 5,430,752, 5, 1 1 1 ,473 and 5,001 ,721 assigned to the

same assignee, and hereby incorporated by reference) or

electrostatic particle filters (see U.S. Patent No. 4,534,034,

assigned to the same assignee and 5,586, 1 34, each of which is

incorporated by reference) may also be used to extend excimer laser

gas lifetimes.

In the laser system of Fig. 8, a processor preferably (9)

receives signals from both the energy monitor (4) and the power supply unit. The laser system of Fig. 8 accommodates still additional

signals indicative of the laser operational status from other devices

(not shown) monitoring discharge chamber gas status (e.g.,

discharge chamber gas temperature and pressure gauges, discharge

chamber gas composition monitors) and devices measuring other

laser operational status parameters such as a driving voltage meter.

These additional signals would also be received by the processor (9) .

In the systems according to Fig. 8, the processor (9)

preferably applies algorithms to generate its control signals based

upon input signals from the energy monitor (4) and any other system

status monitors. These algorithms may utilize reference values for

the monitor signals and information based upon the history of past

gas actions signals to generate control signals. These control signals

are received by the gas supply unit (7) which regulates the flow of

replenishment gases into the discharge chamber ( 1 2) and any release

of the discharge chamber gas mixture according to the control signal

from the processor (9) .

All of the references cited in the Background section of this

application are herein incorporated by reference. The specific

embodiments described in the specification are not intended to limit

the scope of the invention, but are only meant to provide illustrative

examples within the spirit and scope of the invention. While

particular embodiments of the subject invention have been described, it would be obvious to those skilled in the art that various

changes and modifications to the subject invention can be made

without departing from the spirit and scope of the invention. All

such modifications are within the scope of this invention.

Claims

What is claimed is:

1 . An excimer or molecular fluorine laser, comprising:

a laser tube including a discharge chamber filled with a laser gas

mixture;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the gas mixture for generating a laser

beam; and

a line-narrowing unit including a blazed grating having a blaze

angle greater than 76°.

2. The laser of Claim 1 , wherein the blaze angle is greater than 78°.

3. The laser of Claim 1 , wherein the blaze angle is greater than 80°.

4. The laser of Claim 1 , wherein the blaze angle is between 78° and

82°.

5. The laser of Claim 1 , wherein the beam has a bandwidth less

than 0.6 pm.

6. The laser of Claim 2, wherein the beam has a bandwidth less

than 0.5 pm.

7. The laser of Claim 3, wherein the beam has a bandwidth less

than 0.4 pm.

8. The laser of Claim 1 , wherein the line-narrowing unit further

includes a beam expander.

9. The laser of Claim 8, wherein the beam expander includes one or

more prisms.

1 0. The laser of Claim 8, wherein the resonator further includes a

partially transmissive output coupling mirror.

1 1 . The laser of Claim 8, wherein the resonator further includes

means for polarization outcoupling the beam.

1 2. The laser of Claim 1 1 , wherein the resonator further includes

a highly reflective mirror.

1 3. The laser of Claim 1 , wherein the line-narrowing unit further

comprises a beam expander including one or more prisms.

4. The laser of Claim 2, wherein the line-narrowing unit further

comprises a beam expander including one or more prisms.

5. The laser of Claim 3, wherein the line-narrowing unit further

comprises a beam expander including one or more prisms.

6. The laser of Claim 1 , wherein the resonator further includes a

partially transmissive output coupling mirror.

1 7. The laser of Claim 1 , wherein the resonator further includes

means for polarization outcoupling the beam.

1 8. The laser of Claim 1 , wherein said laser includes a substrate

comprising a plurality of grooves formed in a surface thereof, said

plurality of grooves substantially defining the structure of said

grating.

1 9. The laser of Claim 1 8, wherein said grating has a dielectric

reflective coating on its surface.

20. The laser of Claim 1 8, wherein said grating has a thin

aluminum coating on its surface.

21 . The laser of Claim 20, wherein said coating is about 1 00 nm

thick.

22. The laser of Claim 20, wherein said thin aluminum coating is

coated with a dielectric reflecting layer.

23. An excimer or molecular fluorine laser, comprising:

a laser tube including a discharge chamber filled with a laser gas

mixture;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the gas mixture for generating a laser

beam; and

a line-narrowing unit for narrowing the bandwidth of said laser,

said line-narrowing unit including a grating and narrowing said

bandwidth to less than 0.6 pm.

24. The laser of Claim 23, wherein said bandwidth is less than 0.5

pm.

25. The laser of Claim 23, wherein said bandwidth is less than 0.4

pm.

26. The laser of Claim 24, wherein the line-narrowing unit further

comprises a beam expander including one or more prisms.

27. The laser of Claim 25, wherein the line-narrowing unit further

comprises a beam expander including one or more prisms.

28. The laser of Claims 23, wherein the resonator further includes

a partially transmissive output coupling mirror.

29. The laser of Claim 23, wherein the resonator further includes

means for polarization outcoupling the beam.

30. The laser of Claim 23, wherein the resonator further includes

a highly reflective mirror.

31 . The laser of claim 23, wherein said laser includes a substrate

comprising a plurality of grooves formed in a surface thereof, said

plurality of grooves substantially defining the structure of said

grating.

32. The laser of Claim 23 wherein said grating has a blaze angle

between 78° and 82°.

33. The laser of Claim 31 wherein said grating has a blaze angle

between 78° and 82°.

34. The laser of Claim 31 , wherein said grating has a blaze angle

greater than 80°.

35. The laser of Claim 31 , wherein said grating has a coating

comprising a reflective dielectric material.

36. A diffraction grating for line-narrowing in an excimer or

molecular fluorine laser, said grating having a blaze angle greater

than 78° and being substantially defined by grooves formed in

the surface of a substrate.

37. The grating of Claim 36 having a blaze angle greater than 80°.

38. The grating of Claim 36 having a blaze angle between 78° and

82°.

39. The grating of Claim 36 further comprising a coating of

reflective dielectric material.

40. The grating of Claim 36, wherein said grating has at least

1 0,000 grooves per centimeter.

41 . A method of forming a diffraction grating in the surface of a

substrate, said method comprising the steps:

generating an ion beam;

patterning said ion beam;

impinging said patterned beam onto said surface to thereby form

said grating therein.

42. A method of Claim 41 wherein said patterning comprises

passing said beam through an attenuator having a structure

according to the structure of said grating.

43. A method of Claim 41 wherein said attenuator is substantially

made of epoxy.

44. A method of forming a diffraction grating in the surface of a

substrate, said method comprising the steps:

providing an ion beam;

attenuating said ion beam according to the structure of said

diffraction grating;

irradiating said surface with said attenuated beam; whereby said attenuated ion beam forms said grating in said

surface.

45. A laser resonator for narrowing beam bandwidth, said

resonator comprising a diffraction grating having a blaze angle

greater than 78°.

46. The laser resonator according to claim 45 wherein said blaze

angle is between 78° and 82°.

47. The laser resonator according to claim 46, said resonator

further including a substrate having a plurality of grooves in a

surface, said surface substantially defining said grating.

48. The laser resonator according to claim 47 wherein said

substrate is substantially made of aluminum.