JPH0834327B2 - Multi-mode narrow band oscillation excimer laser - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a multimode narrowband oscillation excimer laser, and more particularly to an excimer laser used as an exposure light source for photolithography that requires high resolution.

[Conventional technology]

Recently, an excimer laser that emits light in the ultraviolet region has been used as a light source for exposure in photolithography.

The excimer laser will be described by taking a product of Lambda Physic Inc. as an example. As shown in FIG. 9, this injection lock type laser is a total reflection mirror.
A resonator composed of 11 and an output mirror 12, a dispersion prism 13, apertures 14 and 15 and an electrode 16 arranged in the resonator.
And an amplifier section 20 optically connected to each other via mirrors 17 and 18 and composed of mirrors 21 and 22 and an electrode 23.

In this laser, the oscillator section 10 divides the wavelength by the dispersion prism 13 and narrows the beam by the apertures 14 and 15, whereby a laser beam having a narrow spectral line width and coherent beam characteristics can be obtained.
And this laser light is the amplifier part that constitutes the unstable resonator.
Injection-locked to 20, forcibly oscillates in cavity mode to increase power.

In addition, the laser (NIKKEI MICRODEVICE) announced by AT & T
S May 1986, pages 50-55) has the structure shown in FIG. This laser has a total reflection mirror 31 and an emission mirror 32.
By arranging the chamber 34 and the two etalons 33 between the chamber 34 and the emission mirror 32 in the resonator consisting of, it is possible to achieve laser oscillation having a narrow spectral line width and a certain number of transverse modes. There is.

[Problems to be solved by the invention]

The injection-lock type excimer laser can obtain a laser beam with a large output and a narrow spectral line width, but at the same time, it becomes a single mode (coherent light). However, there was a problem that high resolution could not be obtained.

Further, in the AT & T type laser, when exposure is performed using a normal illumination optical system, speckles are also generated because the number of transverse modes is insufficient. Therefore, it is necessary to adopt a scan mirror system as the illumination optical system, which complicates the structure and control of the exposure apparatus. In addition, the laser of this system has a low power and requires an exposure time of 25 seconds, which is not practical.

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

[Means and Actions for Solving Problems]

According to the present invention, in an excimer laser provided with an etalon, the specifications of the etalon are as follows: Here, α: OTF R _λ (u, v) required to expose the resist according to the reticle pattern: monochromatic light of illumination system and reduction projection lens OTF W _λ : weight at wavelength λ, and W _λ = T (λ ) / ∫ _λ T (λ) dλ T (λ): power spectrum of the oscillated laser beam, f (λ, β): Power spectrum of virtual spontaneous oscillation due to the threshold increase due to the presence of the etalon between the chamber and the rear mirror β: Threshold increase ratio E (k _k · F _tk , FSR _k , λ): Transfer function of the power spectrum of the light by one k-th etalon F _tk : Total finesse of each etalon FSR _k : Free spectral range of each etalon k _k : Set to satisfy the finesse coefficient, and set the resist An excimer laser having a resolution necessary for exposing the reticle pattern to light 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 arranged in the cavity of an excimer laser provided with a resonator composed of a total reflection mirror 1 and an emission mirror 2. In this excimer laser, the air gap etalon 3 which is a wavelength selection element is arranged in the cavity and light of a desired wavelength is selected by this etalon, so that the number of transverse modes is not reduced and the etalon 3 is used. It is possible to oscillate a laser beam suitable for reduction projection exposure having a narrow spectral line width determined by the transmission characteristics of the.

Reference numerals 5 and 5 ′ are windows, and a chamber gas sealed by these windows is filled with, for example, a mixed gas of argon and fluorine, a mixed gas of krypton and fluorine, or the like, which has an excited state called an excimer. Lasers with short wavelength (193 nm for ArF and 248 nm for KrF) and essentially non-coherent laser light are generated by using a molecule formed by combining atoms and atoms in the ground state.

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

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

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

T (λ) = f (λ) / [1+ (4 / π ² ) Ft ² sin ² {((λ−χ) / FSR) π}] ...
(1) where F _t is the total finesse of the etalon, and this total finesse F _t is the finesse F _F due to surface accuracy in the air gap of the etalon, the finesse F _R due to reflection, the finesse F _P due to parallelism, and the beam of incident light. Finesse F _d due to spread
By and, _Ft = {(1 / F _F ) ² + (1 / F _R ) ² + (1 / F _F ) ² + (1 / F _d ) ² } ^−1/2 … (2) . FSR is the free spectral range of the etalon, and this free spectral range FSR is represented by the distance d between the reflective films and the refractive index n between them.

FSR ＝ x ² / 2nd ＝ x ² / 2d (n = 1 in case of air gap) where x is the center wavelength of f (λ) (248.35nm in case of KrF excimer laser) However, this etalon is in the laser cavity By putting it between the rear mirror and the chamber,
The phenomenon of increasing the threshold of laser oscillation and the finesse of the apparent etalon occurs.

The reasons for this are: 1) Light repeats its turn inside the capital, so in the case of the internal etalon, the light passes through the etalon many times.

2) In the case of the internal etalon, the power concentrates on the wavelength with high light intensity in the cavity.

This is because.

The threshold and the increase rate of finesse vary depending on the type of laser, the number of transverse modes, the etalon throughput, and the like. Therefore, it is necessary to experimentally determine the case of a multimode narrowband oscillation excimer laser.

Now, assuming that the increase in the threshold of the internal etalon (when the etalon is inserted between the rear mirror and the chamber) is β, a virtual power spectrum f _in due to the increase in the threshold
(Λ) is represented by the following equation.

f (λ, β) ≈ (f _out (λ) −β) / (1−β) (3) where f _out is the spectrum of the laser in the absence of an etalon.

Therefore, the laser spectrum T (λ) in the case where one internal etalon has an etalon between the rear mirror and the chamber is expressed by the following equation by defining the finesse coefficient k.

T (λ) ≈f (λ, β) / [1+ (4 / π ² ) K ² Ft ² sin ² {((λ−χ) / FSR) π}] (4) There are n etalons The power spectrum T (λ) of the laser light in the case of performing is expressed by the following equation.

However, E (Kk · Ftk, FSRk, λ) = 1 / [1+ (4 / π ² ) (Kk · Ftk) ² sin ² {((λ −χ) FSR) π}] In general, illumination optics and reduced projection White OTF of lens (opt
ical transfer function) Rw (u, v) is obtained by integrating the monochromatic light OTFR _λ (u, v) in the wavelength range from the definition formula of white OTF.

However, W _λ = T (λ) / ∫ _λ T (λ) dλ (7) u, v: Spatial frequency This white OTFR _w (u, v) is the minimum required to expose the resist according to the reticle pattern. It is necessary to be above OTFα.

R _w (u, v) ≧ α (8) Therefore, in the multimode obtained by the equation (5),
Set the etalon conditions and oscillate the laser light so that the white OTFR _w (u, v) obtained by substituting the shoe narrow-band laser light into the equation (6) becomes α or more. By doing so, a desired resolution can be obtained.

As a specification of the air gap etalon 3, a free spectrum range of 0.308nm (air gap space of 100μ
m), finesse 5, 90% reflectance (as a reflective film
SiO ₂ , Al ₂ O ₃ , ZrO ₂ , HfO ₂ , NaF, etc.), surface accuracy λ / 30
Or more (λ = 633 nm), parallelism λ / 30 or more (λ = 633 nm),
An experimental example of the output of the excimer laser shown in FIGS. 1 and 2 when an effective area of 10 × 20 nm is used is shown.

FIG. 3 shows a gain curve a in the case of the so-called internal etalon of FIG. 1 in which the etalon is arranged in the cavity of the excimer laser, and a gain curve b in the case of the so-called external etalon of FIG. 2 in which the etalon is arranged outside the cavity. Is a figure,
Curve c is a gain curve in the case of spontaneous oscillation. According to the experiment, the full width at half maximum due to the configuration of the internal etalon and the configuration of the external etalon are 0.00925 nm and 0.0616 nm, respectively. The finesse of etalon is 33.3 and 5 respectively,
The internal etalon has 6.7 times the finesse compared to the external etalon. The finesse coefficient k according to the experiment was 5 to 7.

Further, in FIG. 3, in the case of the internal etalon, the side peaks that cause the reduction of the resolution are reduced to about 1/10 of those in the case of the external etalon. In this way, the phenomenon that the light intensity ratio of the side peak decreases is
It was observed even when more than one was placed. The reason for this is considered to be that the loss of gain increases due to the light passing through the etalon, and the threshold value of laser oscillation increases.

Estimating the increase rate β of this threshold value from the decrease rate of the side peak was about 0.10 to 0.06. Moreover, when the power spectrum f _out (λ) of the spontaneous oscillation of the KrF excimer laser was measured by the spectroscope, it was as shown in FIG.

From the power spectrum f _out (λ) of this natural oscillation, the power spectrum f _in (λ, β) of the virtual KrF excimer laser when the internal etalon is used is expressed by the following equation from equation (3).

f (λ, β) ≈ ( _out (λ) -β) / (1-β) However, β = 0.06 to 0.10 For example, when only one etalon is provided between the rear mirror and the chamber as shown in FIG. Spectral waveform T
(Λ) is expressed by the following equation.

T (λ) ≈f _in (λ, β) · E (k · F _t , FSR, λ) ...
(8) However, in the case of β = 0.06 to 0.10, k = 5 to 7 and one external etalon as shown in FIG.
In equation (8), β = 0, k = 1 In the case of two external etalons as shown in FIG. 5, T (λ) ≈f (λ, β) · E (k ₁ · F _t1 , FSR ₁ , λ) · E (k ₂ · F _t2 , FSR ₂ , λ) (9) β = 0, k ₁ , k ₂ = 1 When there are two etalons between the laser chamber and the rear mirror as shown in FIG. In equation (9), β = 0.06 to 0.10, k ₁ , k ₂ = 5 to 7 When there is one etalon between the laser chamber and the rear mirror and one etalon outside as shown in FIG. In equation (9), β = 0.06 to 0.10, k ₁ = 5 to 7, k ₂ = 1 The power spectrum waveform T (λ) of the laser light when n etalons are arranged as shown in FIG. It is represented by a formula.

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

The spectral waveform calculated using the above formula and the experimentally obtained spectral waveform were in good agreement.

Next, Table 1 (a) to (d) consisting of synthetic quartz lens material monochromatically designed for an excimer laser with an exposure wavelength of 248.35 nm.
As a result of calculation using the spectral waveform of the experimental result and the equations (6) and (7) for the reduction projection lens shown in Fig. 5, the white OTFR _w is 0.4 at a resolution of 0.5 µm in a given exposure area.
Table 2 (a) ~ Example of etalon specifications above
It shows in (d).

Table 2 (a) Specification range of the etalon used in the experiment Reflectivity 50 to 93% Reflection finesse F _R = 4.4 to 43.3 Surface accuracy λ / 20 to λ / 100 (633 nm) Surface accuracy finesse F _F = 3.9 to 19.5 Parallel Degree λ / 20 to λ / 100 (633nm) Parallelism Finesse F _F = 3.9 to 19.5 Angle between laser optical axis and etalon When etalon is between chamber and exit mirror 0 to
When the 2.5 ° etalon is in front of the exit mirror 0 ~
When the 2.5 ° etalon is in the rear mirror and chamber 0.1 ~
Finesse due to 5 ° beam divergence F _d = 2 to 500 Effective diameter 20 mmφ or more Total finesse is in the range of F _t = 1.5 to 13.1. The air gap is in the range of 50 μm to 5000 μm and the free spectral range is in the range of FSR = 0.0062 to 0.62 nm.

If a laser beam of an excimer laser provided with an etalon having specifications as shown in Table 2 (a) to (d) is used as a light source, exposure with a resolution of about 0.5 μm is possible.

It was also found that the laser output was dramatically increased by disposing the etalon between the laser chamber and the rear mirror.

Furthermore, when the laser oscillation coherence test of the present invention was conducted, almost no coherence was found. However, if you make a pinhole in the cavity and adjust the pinhole diameter to perform a coherence test, speckle occurs at a pinhole diameter of about 2 mm. Therefore, the effective diameter of the slit and etalon in the cavity must be about 2 mm or more.

〔The invention's effect〕

As described above, according to the present invention, the specifications of the etalon provided in the excimer laser can be calculated by the following equation. By satisfying the above condition, a desired exposure area and resolution can be obtained.

Further, since the laser oscillation with a high output and a large number of transverse modes is possible despite the narrow oscillation line width, the laser becomes very compact and the exposure time is about several hundred ms. Moreover, the scan mirror system is not required as the illumination optical system of the exposure apparatus, and the exposure of the normal integrator system is possible.

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

[Brief description of drawings]

1, 2 and 5 to 8 are schematic configuration diagrams showing an excimer laser provided with an air gap etalon, and FIG. 3 shows a gain curve when the etalon is arranged inside or outside the cavity. The graph shown in FIG. 4 is a graph showing the power spectrum of the spontaneous oscillation of the KrF excimer laser,
9 and 10 are conceptual diagrams showing a configuration example of a conventional excimer laser. 1 ... total reflection mirror, 2 ... exit mirror, 3 ... air gap etalon, 4 ... chamber, 5,5 '... window.

Claims (1)

[Claims] 1. In an excimer laser provided with an etalon, the specifications of the etalon are as follows: ∫ _λ W _λ R _λ (u, v) d _λ ≧ α where α: is required to expose the resist according to the reticle pattern. OTF R _λ (u, v): monochromatic light O of illumination and system and reduction projection lens
TF W _λ : Weight at wavelength λ, W _λ = T (λ) / ∫ _λ T (λ) dλ T (λ): Power spectrum of lasing laser light, Power spectrum of virtual spontaneous oscillation due to increase of threshold value due to existence of etalon between f (λ, β) chamber and rear mirror β: increase rate of threshold value E (k _k , F _tk , FSR _k , λ): Transfer function of the power spectrum of light by one k-th etalon F _tk : Total finesse of each etalon FSR _k : Free spectral range of each etalon k _k : Finesse coefficient Multi-mode narrow-band excimer laser.