JPH022193A - Wavelength-variable laser device - Google Patents

Abstract

PURPOSE:To improve setting accuracy of central wavelength of laser beam by comparing an integral value of intensity at the side of short wavelength of a reference position with that at the side of long wavelength of the reference position of a spectral pattern of laser beam which is space-developed by a wavelength dispersion element. CONSTITUTION:A wavelength detector 11 is provided to control selective wavelength of a wavelength selecting element and to set a central wavelength of laser beam to an object vale. That is, a signal processing circuit 12 measures an integral value I1 of intensity of a fringe 14 which is in the inner side of a channel of a reference of a linear image sensor 15 and an integral value I2 of intensity of the fringe 14 which is in the outer side, and produces the difference DELTA by a subtraction circuit 19. A valve control circuit 20 controls a central wavelength of laser beam by operating valves 8, 9 to set the difference to a fixed value Vr. A peak position of intensity of the fringe 4 can be adjusted and set continuously without receiving any restriction to a channel interval of the linear image sensor 15. As a result, a set wavelength accuracy of central wavelength of laser beam can be remarkably improved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a laser device used as a light source of a projection exposure apparatus.

2. Description of the Related Art In recent years, with the miniaturization of LSI patterns, there has been a tendency for light sources for pattern exposure to have shorter wavelengths. One such light source is a high-pressure mercury lamp, and its q-line (43 snm) or i-line (365 nm) has been used in the LSI manufacturing process. So-called ultra-LSI with even finer patterns
, a light source with a shorter wavelength is required, and laser light sources such as excimer lasers are attracting attention as a means to meet this demand. An excimer laser uses a combination of a rare gas such as krypton or xenon and a halogen gas such as fluorine or chlorine as a laser medium.
It is possible to obtain oscillation lines with sufficient output for exposure of LSI patterns at several wavelengths between m and 193 nm.

The gain bandwidth of these excimer lasers is as wide as approximately 1 nm, and when oscillated in combination with an optical resonator, the oscillation line is 0.
．． It has a line width (full width at half maximum) of about 5 nm.

When a laser beam having such a relatively wide line width is used as an exposure light source, as in the case of a lamp light source, it is necessary to employ an imaging optical system corrected for chromatic aberration in the exposure optical system. However, in the ultraviolet region where the wavelength is 350 nm or less, the range of choices for optical materials for lenses used in the imaging optical system is limited.
It becomes difficult to correct chromatic aberration. Therefore, the wavelength is 350 n
When using a laser light source with a diameter of 0.2 m or less in an exposure apparatus, the line width of the laser oscillation line should be set to 0. By making it monochromatic to about O05 nm and setting its center wavelength to the value required by the exposure device with an accuracy of ±0.001 nm (1 pm) or less, it becomes possible to use an imaging optical system that does not correct chromatic aberration, and the exposure device This makes it possible to simplify the optical system, thereby reducing the size and cost of the entire exposure apparatus.

The present inventors have already proposed a configuration of a laser device that achieves the above object. In other words, it is a wavelength tunable laser device in which means for detecting the center wavelength of laser light is built into the main body, and the oscillation wavelength can be set to an arbitrary value and the wavelength can be maintained. In order to realize such a laser device, a method for precisely detecting and controlling the laser oscillation wavelength is required.

In order to detect the oscillation wavelength of the laser, a Fabry-Bérot etalon or a grating can be used as a wavelength dispersion element, and a photoelectric conversion element that detects the wavelength of the laser light spatially expanded by these wavelength dispersion elements can be, for example, a linear image. Sensors are available.

A linear image sensor can instantly detect the peak position of a nuvector pattern of a laser beam spatially expanded by a wavelength dispersion element, so the wavelength detector and signal processing circuit can be made extremely small.

Therefore, it is one of the most suitable photoelectric conversion elements for the laser device that has a built-in mechanism for controlling the center wavelength, which is proposed by the present inventors.

Problems to be Solved by the Invention However, in such conventional wavelength tunable laser devices, the resolution of the linear image sensor used as the wavelength detector is limited, which limits the accuracy of setting the center wavelength of the laser beam. There was a point.

The present invention has been made to solve these problems, and provides a wavelength tunable laser device with a wavelength setting accuracy of ±1 pm or less.

Means for Solving the Problem In order to solve this problem, the present invention comprises an optical resonator consisting of a laser medium, a total reflection mirror, and an output mirror, and a single or plural wavelength selection elements are installed in the optical resonator. In a laser device in which the central value of the oscillation wavelength is made variable within the gain bandwidth of the laser medium by installing the device in The wavelength detecting means detects the integrated value of the intensity on the shorter wavelength side from the reference position and the intensity on the longer wavelength side from the reference position of the spectral pattern of the laser light spatially expanded by the wavelength dispersion element. It is configured to compare integral values.

Function: With this configuration, it is possible to improve the setting accuracy of the center wavelength of the laser beam.

Embodiment FIG. 1 is a block diagram of an excimer laser which is an embodiment of the present invention. In FIG. 1, the laser device according to the embodiment of the present invention uses a discharge tube 1 which uses a mixture of rare gas and halogen gas as a laser medium, an optical resonator consisting of a total reflection mirror 2 and an output mirror 3 to emit laser light in the ultraviolet region. oscillate. Air space etalons 4, 4' are placed within the optical resonator. The air space etalons 4 and 41 are a type of Fabry-Bérot etalon in which two parallel plane quartz plates having appropriate reflectivity at the laser wavelength are faced with a small gap between them, and the distance between the gaps and the distance between the gaps are It is a type of wavelength selective element that transmits only a specific wavelength determined by the refractive index of the gas. Air space etalons 4, 4' are airtight containers ε
installed inside.

A high pressure air source 6 and a low pressure air source 7 are connected to the airtight container 5 via pulps 8 and 9, respectively, and an air space etalon 4. 4' (7) Can 7' (D) The atmospheric pressure can be changed. Also, the sample beam taken out by the semi-transmissive mirror 1o is guided to the wavelength detector 11. The wavelength detector 11 changes the wavelength of the sample beam. Accordingly, an electric signal is output to the signal processing circuit 12. The signal processing circuit 12 outputs a signal to open or close the valve 8 or valve 9 according to the signal sent from the wavelength detector 11.

Next, the operation of the wavelength tunable laser device of the present invention having the above configuration will be explained. Generally, the spectral linewidth of an excimer laser is about 0.5 nm, as shown in FIG. The line width is 0.005n as shown in Figure 4 0)).
When monochromating to about m, it is necessary to use a wavelength selection element whose transmission bandwidth is about the same. Such a wavelength selection element can be realized, for example, by combining two air space etalons as shown in the embodiment.

The center wavelength of the monochromatic laser beam can be freely changed within the gain bandwidth of the laser medium by controlling the transmission center wavelength of the wavelength selection element. The laser device according to the present invention is provided with a wavelength detector 11 to control the wavelength selected by the wavelength selection element and to match the center wavelength of the laser beam to a target value. In order to control the selected wavelength of the wavelength selection element, for example, the air space etalons 4 and 4' shown in the embodiment of FIG. depending on,
By opening and closing the pulp 8 between the high-pressure air source 6 or the pulp 9 between the low-pressure air source 7 and changing the air pressure in the airtight chamber S, the refractive index between the gaps between the air space etalons 4 and 4' can be adjusted. Just adjust it.

In order to realize the wavelength detector 11, conventionally, as shown in FIG. The position was read using a linear image sensor 16 or the like. The diameter of the fringe 14 changes depending on the wavelength of the laser beam incident on the air space etalon 13. Since the intensity in the radial direction is still distributed in a mountain shape as shown in FIG. 5, among the light receiving channels of the linear image sensor 16, channel n where the signal intensity is maximum corresponds to the center wavelength of the laser beam. I will do it. Therefore, the center wavelength of the laser beam may be set so that the channel number n matches the reference channel number. However, in such a wavelength detection method, it is determined that the fringe 14a and the fringe 14b in FIG. 6 have the same wavelength. Therefore, when such a wavelength detector method is employed, it is clear that there is a limit to the wavelength setting accuracy.

FIG. 2 explains the principle of the wavelength detection method according to the present invention. The configuration of the wavelength detector according to the present invention is the same as the conventional example shown in FIG. 6, and the linear image sensor 15 is
A fringe 14 is placed within the projected plane. on the other hand,
The signal processing circuit 12 calculates an integral value 11 of the intensity of the fringe located inside the channel of the linear image sensor serving as a reference.
and the integral value of the strength of the outer fringe. is measured, and the subtraction circuit 19 creates the difference Δ. The pulp control circuit 2o controls the pulps 8 and 9 so that the difference Δ becomes a constant value Va.
to control the center wavelength of the laser beam. (2) can be given as a continuous amount, so the peak position of the intensity of the fringe 4 can be continuously adjusted and set without being limited by the channel spacing of the linear image sensor. As a result, the accuracy of the setting wavelength of the center wavelength of the laser beam is dramatically improved.

Next, a specific embodiment of the wavelength detection method according to the present invention will be described with reference to FIG. In the embodiment shown in FIG. 3, a scanning type is used as the linear image sensor 15, and the received light signal intensity of each channel is sequentially read out by a three-phase clock pulse from a clock generator 23, and is amplified by an amplifier 21. Ru.

On the other hand, the counter circuit 24 counts clock pulses,
The signals from the x−mth channel to the Xth channel are sent to the low-pass filter 25a, and the signals from the
The switching circuit 22 is operated so as to distribute the +1 channel signal to the low-pass filter 26b. Here, the first channel is a reference channel near the fringe 14, and the measurement width m is set to be approximately the band width of the fringe 14. Subtraction circuit 19. is integrated by the low-pass filter 26.
Smoothed intensity signal 11. I. Create a difference signal Δ from
The signal is sent to the differential amplifier 26. The differential amplifier 26 compares the difference signal Δ with the set value vr, and if there is a difference, opens or closes the pulp 8 or 9 via the pulp drive circuit 27. As a result, the center wavelength of the laser beam is maintained so that the fringe 14 is always located at the same position.

When slightly changing the center wavelength of the laser beam, set value V
If you change , you can sweep continuously.

According to experiments, a setting accuracy of 0.5 pm or less was obtained.

Even in the conventional wavelength detection method, a linear image sensor with an increased focal length of the lens 16 for fringe projection is used.
It is possible to improve the wavelength setting accuracy by narrowing the light receiving area per channel of 6. However, the signal strength was weakened, and it was not possible to obtain wavelength setting accuracy exceeding that achieved by the wavelength detection method according to the present invention. Incidentally, rough setting of the wavelength can be realized by changing the number X of the reference channel.

With the above configuration, the wavelength tunable laser according to the embodiment of the present invention can realize very precise wavelength setting.

Note that although the example uses an air space etalon as the wavelength dispersion element, a similar effect can be obtained by using a grating or the like. Furthermore, although the explanation has been given using an excimer laser as an example of a laser device, it goes without saying that the present invention can be applied to any wavelength tunable laser device.

As described in detail, the present invention improves wavelength setting accuracy by comparing the intensity on the long wavelength side and the intensity on the short wavelength side of the spectral pattern of laser light spatially expanded by a wavelength dispersion element. This makes it possible to provide an improved wavelength tunable laser device.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a wavelength tunable laser device that is an embodiment of the present invention, FIG. 2 is a diagram showing the principle of a wavelength detection method according to the present invention, and FIG. 3 is an embodiment of a wavelength detector according to the present invention. 4 is a diagram illustrating monochromatic laser light, and FIG. 5 is a diagram illustrating a conventional example of a wavelength detector. 1...discharge tube, 2...total reflection mirror, 3.
...Output mirror, 4.4' ...Air space etalon, 6...Airtight container, 6...
High pressure air source, 7...Low pressure air source, 8゜9...
...Pulp, 10...Semi-transparent mirror, 11...
... Wavelength detector, 12 ... Signal processing circuit, 1
3... Air Space Etalon, 14, 14a,
14b...Fringe, 16...Linear image sensor, 16...Lens, 17.
...Inner diffuser plate, 18...Reflector, 19...
...Subtraction device, 20...Valve control circuit, 21
・・・・・・Amplifier, 22・kawa・switching circuit,
23.-m. clock generator, 24.. river counter circuit, 26a. 26b...Low pass filter, 26...Variational differential amplifier, 27...Pulp drive circuit. Agent's name Patent attorney Toshio Nakao Haga 1 Meikazu/-m-Discharge tube 5--Kizai-maki n--One-liquid fleas 512- Signal M, from Riro 13- Air Space Etalon /4--Fringe 15 ゛-゛Nia image assembly Fig. Fig. Fig. IQ-Fishing circuit 20--- High frequency division I8WJ Fig. 23- Clock land habitat 11-9-1 What is the hole?

Claims (1)

[Claims] It is equipped with an optical resonator consisting of a laser medium, a total reflection mirror, and an output mirror, and a single or plural wavelength selection elements are installed in the optical resonator to select the center of the oscillation wavelength within the gain bandwidth of the laser medium. In a laser device having a variable value, a means for controlling the selected wavelength of the wavelength selection element and a means for detecting the oscillation wavelength of the laser beam are built into the laser device, and the wavelength detecting means A wavelength tunable laser device is characterized in that the integrated value of the intensity on the shorter wavelength side from a reference position and the integrated value of the intensity on the longer wavelength side from the reference position of the spectrum pattern of laser light spatially expanded by a dispersion element are compared.