JPS6345875A - Multimode narrow band oscillation excimer laser - Google Patents

Abstract

PURPOSE:To construct an excimer laser optimal for use as a source for a reduced projection exposure unit by a method wherein etalon specifications in the excimer laser are so defined as to satisfy time constant conditions for a desired optical area and resolution. CONSTITUTION:An air-gap etalon 3 is installed in the cavity of an excimer laser provided with a resonator built of a total reflection mirror 1 and emission mirror 2. This excimer laser, provided with an air-gap etalon 3 in its cavity, chooses a beam of a desired length, and oscillates, without a reduction in the number of transverse modes, a laser beam optimal for reduced projection exposure with a narrow spectrum line width, which is defined by the transmission characteristics of the etalon 3. The etalon 3 is so specified as to satisfied the formula (1) and oscillates an excimer laser beam carrying a resolution capability necessary for subjecting a resist to exposure as indicated by a reticle pattern. In the formula (1), alpha is the OTF required to subject a resist to exposure, Rlambda(u, v) the homogeneous OTF of an illuminating system and reduced projection lens, and Wlambda the weight at a wavelength lambda.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a multimode narrowband oscillation excimer laser, and particularly to an excimer laser used as an exposure light source in IJ lithography, which requires high resolution.

[Conventional technology]

Recently, an excimer laser that generates light in the ultraviolet region has been used as an exposure light source for IJ lithography.

This excimer laser will be explained using a product from Lambda Fischik as an example. As shown in Fig. 9, this injection lock type laser has a total reflection mirror 1.
1 and an output mirror 12, and an oscillator section 10 consisting of a dispersion prism 13, an aperture 14, 15, and an electrode 16 disposed within this resonator, and optically connected via mirrors 17 and 18. Mirror 21.
22 and an amplifier section 20 composed of electrodes 23.

In this laser, the oscillator unit 10 separates the wavelengths using a dispersion prism 13, and divides the wavelengths into A, IJ, and IJ? The -chambers 14 and 15 function to narrow down the beam, thereby providing a laser beam with a narrow spectral linewidth and coherent beam characteristics. The laser light is injection-locked into the amplifier section 20 constituting the unstable resonator, forced to oscillate in cavity mode, and increases the power.

In addition, the laser announced by AT&T (NIKKEIM
ICRODEVICES May 1986 issue 50-55
page) has a structure as shown in FIG. This laser has a narrow spectrum)/l/ line width by disposing a chamber 34 and two etalons 33 between the chamber 34 and the output mirror 32 in a resonator consisting of a total reflection mirror 31 and an output mirror 32. , and enables laser oscillation with a certain number of lateral bands.

[Problem that the invention seeks to solve]

An excimer laser using an in-disc lock method can obtain laser light with high output and a narrow spectral linewidth, but at the same time it becomes a single mode (coherent light), so when it is used as a reduction projection light source, the specifications However, there was a problem in that high resolution could not be obtained due to the occurrence of errors.

Furthermore, when an AT&T type laser is used for exposure using a normal illumination optical system, speckles still occur due to the insufficient number of transverse modes. Therefore, it is necessary to use a scan mirror type as an illumination optical system, which complicates the structure and control of the exposure apparatus. In addition, since the power of this type of laser is low, the exposure time is 25
It took seconds and lacked practicality.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a multi-mode narrow-band oscillation excimer laser capable of oscillating a multi-mode, narrow-band, high-output laser beam suitable for reduction projection.

[Means and effects for solving the problem] According to the present invention, an excimer laser is provided with an etalon for multi-mode and narrow band oscillation, and the specifications of the etalon are as follows: fAw, 2R, (u − v) dλ≧α However, OTF required to expose the α-resist according to the reticle amount turn Rλ (u, ma): Monochromatic light O of the illumination system and reduction projection lens
TF W,2=weight at wavelength λ, w,2='r(λ) //, T(λ)dλT(λ): Norworth vector of oscillation laser light, T(λ); f(λ ,
β)・,/7. E(k, ・F, , FSRk, λ) fc
λ, β): Power spectrum of virtual spontaneous oscillation due to increase in threshold value due to the presence of an etalon between the chamber mirrors β: Threshold increase rate E (kk-F, to FSRk, λ) Transfer function Ftk of the four worth vector of light due to one etalon :
The total finesse FSRk of each etalon is set to satisfy the fleece vector range kk of each etalon: the finesse coefficient, and an excimer laser having the resolution necessary to expose the resist according to the reticle pattern is oscillated.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an example in which one air-gap etalon 3 is disposed within the cavity of an excimer laser equipped with a resonator consisting of a total reflection mirror 1 and an emission mirror 2. In this excimer laser, the air-gap etalon 3, which is a wavelength-selective It is possible to oscillate a laser beam suitable for reduction projection exposure with a narrow spectral linewidth determined by the transmission characteristics of No. 3.

Note that 5 and 5' are windows, and the chamber sealed by these windows is filled with, for example, a mixed gas of argon and fluorine, a mixed gas of krypton and fluorine, etc., and an excited state called an excimer is generated. Molecules formed by combining atoms in the ground state are used to emit laser light that has a short wavelength (193 nm for ArF and 248 nm for KrF, ultraviolet) and is essentially non-coherent.

FIG. 2 shows an embodiment in which the etalon 3 is disposed outside the excimer laser cavity, that is, toward the outside of the output mirror 2, and the other components are the same as in FIG. 1.

Next, the specifications of the etalon for obtaining the desired laser beam will be considered.

Generally, transmitted light T(λ) when light with a certain power spectrum f(λ) is transmitted through a certain number of etalons is expressed by the following equation.

T(λ)=f(λ)/[1+(4/xF,dn2((
(λ-x)/FSRX)) ... (1) Here, F is the total finesse of the etalon, and this total finesse F is the finesse due to the surface precision in the air gear lug of the etalon FF1 The finesse due to reflection FR9 The finesse due to parallelism F, and the finesse F due to beam expansion of the incident light,
It is expressed by . Further, in the fleece vector range of the FSR etalon, this fleece vector range FSR is expressed by the distance d between the reflective films and the refraction Inn between them.

FSR = x” /2n d = x2/2d (n = 1 in the case of air gap) However, the center wavelength of xFif (λ) (248.35 nm in the case of KrF excimer laser) However, if this etalon is clogged inside the laser cavity, By placing it between the rear mirror and the chamber, the phenomenon of increasing the threshold of laser oscillation and the apparent finesse of the etalon occurs.

The reasons for this are: 1) Since light repeats turns within the cavity, in the case of an internal etalon, the light passes through the etalon many times.

2) In the case of an internal etalon, the light is concentrated at wavelengths with high light intensity within the cavity.

It's for a reason.

The rate of increase in these threshold values and finesse varies depending on the type of laser, the number of transverse modes, the etalon throughput, etc. Therefore, the case of a multimode narrowband oscillation excimer laser needs to be determined experimentally.

Now, if the increase in the threshold of the internal etalon (when an etalon is inserted between the rear mirror and the chamber) is β, then
Virtual power spectrum f1n (
λ)#: is expressed by the following equation.

f(λ,β) (fout(λ)-β)/(1-β)・
...(3) However, 10 ut is the laser spectrum when there is no etalon.

Therefore, the laser spectrum T(λ) when one internal etalon exists between the rear mirror and the chamber can be expressed by the following equation by determining the finesse coefficient k.

T(λ) f(λ, β)//C1+47πskFtm”
(((λ-x)/FSR)7r) ] ・−・(4)n
The power spectrum T(λ) of laser light when there are etalons is expressed by the following equation.

T(λ)=f(λ,β)・II E(kk−F, ni, F
SRk, λ) ...(5)k-Ftk, FSRk, λ) = 1/C1+(4/π")kk-F,,dn"(((
λ-X)/PSR)π)] Generally, the white OTF (optical tran) of the illumination optical system and reduction projection lens
sfer functlon) Rw(u,v) can be found by integrating the monochromatic light OTFRλ(,,v) over the wavelength range from the definition formula of the white OTF.

RW("-') =/λWλ・Rλ(u,v)
dλ ・(6) However, W, = T(λ) //, T(λ) dλ ・
...(7) u, v:! 2 frequency this white OTF ~ (US, resist reticle 14
It is necessary that the OTFα is at least the minimum required value for exposure through the turn.

RW(u,v)≧α・・・・・・(
8) Therefore, in the multimode obtained by equation (5).

And so that the white OTFRy (u, v) obtained by substituting the narrow band oscillation laser light into equation (6) is equal to or larger than α,
A desired resolving power can be obtained by setting various conditions for the etalon to cause the laser beam to oscillate.

Sate, Air Yatsubu Etalon 3 specifications include fleece vector range 0.308 nm (air gap space set to 100 μml1c), finesse 5, reflectance 90% (
As the reflective film, SIO□, At20. , ZrO2゜HfO2, N*F, etc.), surface accuracy of 2730 or more (
λ=633nm), parallelism 1730 or more (λ=633n
An experimental example of the output of the excimer laser shown in FIG. 1 and FIG.

Figure 3 shows the gain curve 1 in the case of the so-called internal etalon shown in Figure 1, when the etalon is placed inside the cavity of the excimer laser, and the gain curve 1 in the case of the so-called external etalon, shown in Figure 2, in which the etalon is placed outside the cavity. Is it a figure or a song? Qe is a gain curve in the case of natural oscillation. According to experiments, the full width at half maximum of the internal etalon configuration and external etalon configuration is 0.00925 nm and 0.0616 nm, respectively.
It's lc&.

Also, the finesse of the etalon is 33.3, and the finesse of the internal etalon is 7 compared to the external etalon.
It's doubled. According to experiments, the finesse coefficient has a value of 5 to 7.
Next.

Also, in Figure 3, in the case of the internal etalon, the side peaks that cause a decrease in resolution are reduced to about 1/10 compared to the case of the external etalon. This is thought to be due to the fact that this increases the loss relative to the gain and increases the threshold for laser oscillation.

When the increase rate β of this threshold value was estimated from the decrease rate of the side peak, it was about 0.10 to 0.06. Also 1
zJ of spontaneous oscillation of KrF excimer laser by spectrometer
? Worth vector f. When ut(λ) was measured, the result was as shown in FIG.

The power spectrum f of this spontaneous oscillation. The power of a hypothetical KrF excimer laser when using an internal etalon from ut(λ) (kutor/ln(λ, β') a (3
) is expressed as the following equation.

f (λ, β) (10ut (λ) - β) / (1 - β) where β = 0.06 to 0.10 For example, as shown in Figure 1, there is only one etalon between the rear mirror and the chamber. Spectral waveform T(λ) when equipped
is expressed by the following equation.

T(λ)*/ln(λ,β)−E(k−F, ,FSR
, λ)...(8) However, β=0.06 to 0.
10. =5~7 Also, as shown in Figure 2, external etalon 1
In the case of β=0. If there are two external etalons as shown in Figure 5, and two etalons between the laser chamber and the mirror as shown in Figure 6, in equation (9), β = 0.06 to 0. .10 *
kl m k1,5-7 As shown in Figure 7, if there is one etalon between the laser chamber and the mirror and one etalon outside, β = 0.06-0.10 in equation (9), k ,=
5-7. The f-Worth vector waveform T(λ) of the laser beam when n@ etalons are arranged as shown in FIG. 8 is expressed by equation (5).

However, β=0.06 to 0.10: 5 to 7 when the etalon is between the laser chamber and the rear mirror and 1 when the etalon is outside the cavity.

The spectral waveform calculated using the above formula and the spectral waveform obtained experimentally matched well.

Next, Table 1 consists of synthetic quartz lens materials designed monochromatically for excimer lasers with an exposure wavelength of 248.35 nm.
For the reduction projection lens shown in (d), calculations were made using the spectral waveform of the experimental results and equations (6) and (7), and the result was that the resolution was 0.51rnL at the predetermined exposure area.
Examples of specifications of etalons with white OTFRW of 0.4 or more are shown in Tables 2 (a) to (d).

Lens Lagoon R(I) D
(I) (1) 345.6500
7.7000 (2) 121.0600
10.8500 (3) -123,07
007,7000 (4) 242°0700
105.5800 (5) -306,4
10014,0000 (6) -170,91
006,7800 (7) 662.8500
19.4000 (8) -214,8
400415,2100(9) 224.0
800 8.8000αd 4
05.3500 86.9800 (+1)
212.7300
10.2000CIll 43
3.6000 1.7500 (Is)
153.4500 9.500ON
'4-) 89.6600 6
．． 4200 (I'5) 232.5900
8.0000 (1/7) 409
．． 1000 6.9800" -28
2,72008,5000 items) -274,7
2007,8700(J'?) -99
．． 6300 28.2300 (1)
-124.7700 80.8
300 (-Ino 178.580
0 13.2800 (μ)
-325,610032,2900c,i)
113.9100 9.2700 (4
) 711.0600 59.9600
u'5) 1g9.1900 29
．． 0500 (24ri 539.8200
4.8500?7) -87.3
700 4.8300(9) -713,
5200,0000αG 95.561
14 6.000000aη IN
FINITY 17.5 million ladders
-232,82731114,000000α9
401.67018 30.000000
@ INFINITY 18.
000000@193.30636
24.455593(1) 178.64
39 17.0000 (2) -400,
8164,2000 (3) 160.0g3
3 24.0000 (4) 76.7
476 18.5000 (5) -105
,071621,0000(6) 194.
6593 76.5000 (7) IN
FINITY 23.5000 (8)
-171,4472,2000 (9)
399.5347 32.0000α0 −
301.4124. 2000 αυ
195.3320 21.000002)
-1183,7886,2000 (13146,883
516,0000α4 830.5295
14.0000 (151INFINITY
5.0000 (lfD 74.
3100 14.5000αη −185
,03365,0000 [Fj 143.2
734 130.1956αI 301
9゜3042 14.0000 (201-376
,4888,2000 (Foundation) 947.02
81 31.0000 stations -310,65
45143,0000g3 658.8269
23.5000 (goods) -405,32
01,2000 (to) 124.4284
28.5000) -1515,6514
,2000@131.8725 2
0.5090 @ 296.6548
4.5000 (I) -1309,55862
4,6766 (7) 463.9672
20.50020 force 2292.9016
24.000002 181.0752
．． 000004 548.9339
8 26.168679 Table 2 (Otori) Specification range of etalon used in the experiment Reflectance 50~93cm Reflection finesse FR=4.4~43.3 Surface accuracy λ/2
0~λ/100 (633nm) Surface accuracy finesse F,
=3.9~19.5 Parallelism λ/20~λ/100 (
633nm) Parallelism finesse F, = 3.9 to 19.5
Angle between the laser optical axis and the etalon When the etalon is between the chamber and the exit mirror 0 to 2
．． 5゜When the etalon is in the position in front of the exit mirror
0~2.5° When the etalon is in the rear mirror and chamber 0.1~5° Finesse due to beam spread Fd
=2~500 Effective diameter 20■φ or more Total finesse #1Ft = 1.5~13. The range is 1.

The air gap is in the range of 50 μm to 5000 μm and the fleece vector range is FSR = 0.0062 to 0.6
2 nm range.

Exposure with a resolution of about 0.5 pm is possible if the laser light of an excimer laser equipped with an etalon having specifications as shown in Table 2 (a) to (d) is used as a light source.

It has also been found that the laser output can be dramatically increased by arranging an etalon between the laser chamber and the mirror.

Furthermore, when we tested the coherence of actual laser oscillations, we found almost no coherence.

However, when a pinhole is made in the cavity and an interference test is performed by adjusting the pinhole diameter, speckles occur when the pinhole diameter is about 2 m.

For this reason, the effective diameter of the slit and etalon in the cavity must be about 2 diameters or more.

〔Effect of the invention〕

As explained above, according to the present invention, the desired exposure area and resolution can be obtained by determining the specifications of the etalon provided in the excimer laser so as to satisfy the following equation (%).

Furthermore, despite the narrow oscillation linewidth, it is possible to oscillate a laser with high output and a large number of transverse modes.
It will be about 3. Further, a scan mirror type is not required as the illumination optical system of the exposure apparatus, and exposure using a normal integrator type is possible.

Therefore, it is possible to construct an excimer laser that is optimal for use as a light source for a reduction projection exposure apparatus.

[Brief explanation of drawings]

1, 2, and 5 to 8 are schematic configuration diagrams showing an excimer laser equipped with an air gap etalon, and FIG. 3 shows a gain curve when the etalon is placed inside or outside the cavity. FIG. 4 is a graph showing the Al Worth vector of spontaneous oscillation of a KrF excimer laser, and FIGS. 9 and 10 are conceptual diagrams showing configuration examples of conventional excimer lasers. 1... Total reflection mirror, 2... Outgoing mirror, 3...
Air gap etalon, 4...chamber, 5,5'.
...wind. Figure 1 Figure 2 Figure 5 Figure 6 Figure 7 Figure 8

Claims (1)

[Claims] In an excimer laser equipped with an etalon for multi-mode and narrowband oscillation, the specification of the etalon is as follows: ∫_λW_λR_λ(u,v)dλ≧α where α: resist pattern OTF required for normal exposure R_λ(u,v): Monochromatic light OTF of illumination system and reduction projection lens W_λ: Weight at wavelength λ, W_λ=T(λ)/∫_λT(λ)dλ T(λ ): Power spectrum of the oscillated laser beam, ▲ includes mathematical formulas, chemical formulas, tables, etc. ▼ f (λ, β): Virtual spontaneous oscillation due to increased threshold due to the presence of an etalon between the chamber and rear mirror. Power spectrum β: Threshold increase rate E (k_k・F_t_k, FSR_k, λ): Transfer function of optical power spectrum due to one k-th etalon F_t_k: Total finesse FSR_k of each etalon
: Free spectral range of each etalon k_k : A multi-mode narrow band oscillation excimer laser characterized by being set to satisfy the finesse coefficient.