JPH01201975A - Excimer laser oscillator - Google Patents

Abstract

PURPOSE:To increase the life of the device by disposing an ultraviolet ray emission lamp at positions that can irradiate a discharge space. CONSTITUTION:When an ultraviolet ray emission lamp 21 is lighted by a lamp drive power supply 22 prior to the charging of a charging capacitor 13, an ultraviolet ray 15 irradiates a discharge space 16, and a laser gas is ionized. In an oscillating power supply circuit 2, when a main capacitor is charged by a high-voltage power supply 4 via a charging resistance 5, it increases in accordance with its time constant, causing the switch 6 to close and a discharge current to flow through an inductance 8. The charges of the main capacitor 7 moves to the charging capacitor 13, and the potential between the anode 11 and the cathode 12 increases, thus generating a discharge in the discharge space 16. As the laser gas in the discharge space 16 has been ionized, the discharge becomes a glow discharge. This makes it possible to obtain a device that is small, having a long life, and for which repeated operation is possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an excimer laser oscillator with a long life and high repetition rate.

[Conventional technology]

Excimer lasers oscillate efficiently in the ultraviolet region, and due to their short wavelength, high brightness, and short pulse characteristics, they are being widely used in various industrial fields, mainly in semiconductor processing, processing, and medical applications.

Excitation methods for excimer lasers include discharge excitation, electron beam excitation, X-ray excitation, and microwave excitation. Among these, the discharge excitation type is often used in practice because the stirrup configuration is simple and it is easy to downsize. One of the challenges of discharge-excited excimer lasers is how to obtain a uniform glow discharge between electrodes. Therefore, it is necessary to ionize the gap between the electrodes in advance of the main discharge, and measures such as automatic pre-ionization or X-ray pre-ionization are being taken.

The configuration of a conventional automatic pre-ionization discharge excited excimer laser is shown in FIG. The device shown in the figure consists of a laser oscillator 1 and an oscillation power supply circuit 2.

The outer shell of the laser oscillator 1 is a cylindrical sealed container 3, which has a length determined by the oscillator output in a direction perpendicular to the plane of the drawing. Inside the sealed container 3, there is a laser gas as an oscillation medium.
For example, in the case of ArF excimer laser, Ar, F2,
Contains He, etc. Main discharge electrodes consisting of an anode 11 and a cathode 12 are arranged facing each other in the center of the sealed container 3, and a discharge space 16 is formed therebetween. The anode 11 and the cathode 12 are each a cylindrical long integral body. At both ends of the cylinder at positions corresponding to the discharge space 16, laser mirrors (not shown) consisting of a total reflection mirror and an output mirror are arranged.

A pre-ionization electrode 14 is connected to the anode 11 and the cathode 12 via a charging capacitor 13. Each of the preionization electrodes 14 is needle-shaped and has an appropriate gap. A plurality of charging capacitors 13 are connected to the main discharge electrode, and one to one charging capacitor 13 is connected to each charging capacitor 13.
Three sets of pre-ionization electrodes 14 are connected.

A laser oscillator 1 is connected to an oscillation power supply circuit 2 at points A and B. The oscillation power supply circuit 2 operates as follows. Charge is stored in the main capacitor 7 by the high voltage power supply 4 through the charging resistor 5. The potential of the main capacitor 7 increases according to the time constant with the charging resistor 5. When the potential rises to a certain level, the switch 6 made of, for example, a spark camp or thyratron is closed, and the main capacitor 7 is connected to the switch 6.
, a discharge current flows through the inductance 8. As a result, the potential at point A increases in the negative direction with respect to point B.

As the potential at point A increases in the negative direction, the pre-ionization electrode 1
Discharge occurs at cap No. 4. As a result, the charge in the main capacitor 7 is transferred to the charging capacitor 13, and the potential difference between the anode 11 and cathode 12 of the main electrode increases as the charge transfers.

When this potential difference reaches a discharge starting voltage determined from the distance between the main electrodes, the type of laser gas, and the sealing pressure, a main discharge occurs. On the other hand, ultraviolet rays 15 are generated along with the discharge at the camp of the pre-ionization electrode 14. The ultraviolet rays 15 ionize the laser gas in the discharge space 16, and this pre-ionized state is maintained for a certain period of time. Therefore, the main discharge occurring in the discharge space 16 functions as a glow discharge to increase the efficiency of relay laser oscillation.

If this preliminary ionization operation is not sufficient, the main discharge will become a needle-like arc discharge. Arc discharge deteriorates the enclosed laser gas and wears out the anode 11 and cathode 12.

As a result, the lifetime of the laser oscillator is shortened. In other words, for an automatic pre-ionization discharge excited excimer laser,
The pre-ionization is extremely important for increasing efficiency and extending life.

[Problem to be solved by the invention]

As described above, one to three sets of pre-ionization electrodes 14 are connected to each charging capacitor 13. As many sets of pre-ionization electrodes 14 as possible are connected to one charging capacitor 13, and the pre-ionization electrodes 14 are
If it is brought as close as possible to the anode 11 and cathode 12, which are the main electrodes, the ultraviolet rays generated by the discharge between the caps of the pre-ionization electrode 14 will efficiently irradiate the discharge space 16, increasing the efficiency of pre-ionization. However, charging capacitor 13
The high-voltage capacitor used as a capacitor has a certain size limit determined by its withstand voltage, and there is a limit to the number of pre-ionization electrodes 14 that can be connected, so uniformly ionizing a wide area depends on the configuration. There were certain restrictions. Furthermore, in this method, the pre-ionization electrode is needle-shaped and the structure is complicated, so that the inductance resistance R of the circuit is larger than in other methods. Therefore, it is difficult to charge and discharge charges rapidly, which has been a major problem for high-repetition operations. Furthermore, the pre-ionization itself is essentially an arc discharge, and it is inevitable that the laser gas deteriorates, the pre-ionization electrode wears out over time, and dust is generated accordingly. These drawbacks have caused the laser oscillator to have a short lifespan.

= On the other hand, in the X-ray pre-ionization method, X is placed on the side of the laser oscillator.
A window is provided through which radiation can pass, an X-ray tube is placed in close contact with the window, and the X-ray tube is operated prior to discharge to perform preliminary ionization. This method can eliminate the drawbacks of the automatic pre-ionization method described above, and there are examples of it being applied to high-output systems.

However, the interior of a laser oscillator is generally filled with laser gas at diffused pressure, whereas the interior of an X-ray tube is a vacuum. Therefore, the window that connects the laser oscillator and the X-ray tube is
We need to embrace that pressure difference. On the other hand, there is a contradictory proposition that the window portion is required to be as thin as possible in order to transmit X-rays. Therefore, special measures are required in terms of material selection and structure for the window. Furthermore, since the X-ray tube itself has a large volume, the oscillator becomes large as a whole, and a high voltage is required to drive the X-ray tube, resulting in a large drive power source. This has the disadvantage that the entire device is large and expensive.

An object of the present invention is to eliminate the drawbacks of these various discharge-excited excimer laser oscillators, and to provide an excimer laser oscillator that is compact, has a long life, and is capable of high repetition operation.

[Means to solve the problem]

An excimer laser oscillator of the present invention for solving the above problems will be explained with reference to FIG. 1 corresponding to an embodiment. As shown in the figure, the oscillator 10 includes a pair of discharge electrodes 11.12 and a laser mirror (not shown) in the vicinity of the terminal end of the discharge electrodes 11.12 in a sealed container 3 filled with an oscillation medium gas.
have. A discharge space 16 is formed surrounded by the discharge electrodes 11, 12 and their laser mirrors. An ultraviolet light emitting lamp 21 is placed at a position where it can irradiate this discharge space.

[Effect]

The ultraviolet light emitting lamp 21 is lit continuously or in a pulsed manner prior to main discharge. Ultraviolet light emitting lamp 21
A part of the ultraviolet rays 15 generated from the discharge space 16 irradiates the discharge space 16 and uniformly ionizes the laser gas present therein, so that the main discharge occurring in the discharge space 16 becomes a uniform glow discharge.

〔Example〕

Examples of the present invention will be described in detail below.

FIG. 1 is a cross-sectional view showing an embodiment of an excimer laser oscillator 10 of the present invention. A sealed container 3, an anode 11, a cathode 12, and a discharge space 1 disposed within the sealed container 3.
The configuration of the portion 6 is the same as that of the oscillator 1 shown in FIG. Of course, a laser mirror consisting of a total reflection mirror and an output mirror (not shown) is also provided, and the sealed container 3 is filled with laser gas.

However, the oscillator 10, unlike the oscillator 1 shown in FIG. Further, the charging capacitor 13 is arranged outside the sealed container 3. The ultraviolet light emitting lamp 21 communicates with the outside of the sealed container 3 through a through terminal (not shown) and is connected to a lamp drive power source 22 (see FIG. 2).

FIG. 2 is a cross-sectional view of the ultraviolet lamp 21 used in the excimer laser oscillator 10.

A thick quartz tube (diameter 15 mm, wall thickness 3 mm) is used as the material for the discharge tube 25 of the lamp, and a coating 28 made of a thin film of magnesium fluoride (OIgF2) is provided on its outer surface. Electrodes 26 and 27 are arranged at both ends of the discharge tube cylinder 25, respectively, and are sealed. Its electrodes 26 and 27 are connected to a lamp drive power source 22. Further, the discharge tube cylinder 25 is filled with low pressure mercury.

Note that the oscillation power supply circuit 2 used is the same as the conventional circuit.

The operation of the excimer laser oscillator 10 of the above embodiment will be explained below.

Although the ultraviolet light emitting lamp 21 may be lit continuously or discharge pulsed every time immediately before discharge, the latter example will be described here. The ultraviolet lamp 21 is turned on by the lamp drive power source 22 at an appropriate timing prior to charging the charging capacitor 13. As a result, the ultraviolet rays 15 irradiate the radiation space 16, and the laser gas therein is ionized.

In the oscillation power supply circuit 2, when a charge is stored in the main capacitor 7 through the charging resistor 5 by the high-voltage power supply 4, the charge increases according to the time constant, the switch 6 closes, and a discharge current flows through the inductance 8. As a result, the charge in the main capacitor 7 is transferred to the charging capacitor 13, the potential difference between the anode 11 and the can 112 increases, and discharge occurs in the discharge space 16. On the other hand, since the laser gas in the discharge space 16 is ionized by the irradiation with the ultraviolet rays 15 as described above, the discharge becomes a glow discharge.

Since the ultraviolet light emitting lamp 21 is long in the shape of a rod, it can be seen as a surface light source, and the discharge space 16 can be uniformly irradiated, whereas the conventional pre-ionization electrode method is a point light source of ultraviolet light. Since the discharge generated there is also a uniform glow discharge, no deterioration of the laser gas occurs. In this example, in particular, since the charging capacitor 13 is disposed outside the container 3, the capacitor and the laser gas do not deteriorate due to a reaction between the capacitor coating material and the laser gas. Since the ultraviolet ray irradiation area from the ultraviolet light emitting lamp 21 is wide, it is possible to set a wide distance between the anode and cathode electrodes 11 and 12, and it is possible to obtain a large area beam similar to X-ray preionization.

Furthermore, since the thin wire or rod-shaped preliminary ionization electrode used in the past is not used, the charging capacitor 13 is connected to the anode 1.
1 and cathode 12 can be used with connection pieces having sufficiently low inductance and resistance values. Therefore, charging capacitor 13, cathode 12, discharge space 1
6.7 The impedance of the discharge circuit configured by the node 11 can be lowered, and the relay laser oscillator can operate at high repetition rates.

The ultraviolet light emitting lamp 21 used in the above embodiment has an MgF2 thin film coating 28 on the outer surface, so that the ultraviolet transmittance is improved. At the same time, resistance to the laser gas sealed in the sealed container 3 being ionized and corroding the outer surface of the ultraviolet light emitting lamp 21 is improved.

FIG. 3 is a cross-sectional view showing another embodiment of the excimer laser oscillator of the present invention. Ultraviolet reflecting mirrors 18 are arranged outside the ultraviolet light emitting lamps 21, respectively. The reflecting surface of the ultraviolet reflecting mirror 18 can be a flat or concave surface, but in the illustrated example, a concave curved surface is shown in order to increase the reflection efficiency.

The shape of the concave curved surface is such that the cross section is circular, parabolic, or ellipsoidal so that the reflected light is efficiently and uniformly focused in the discharge space, and an ultraviolet light emitting lamp 21 is placed at the center or near the focal point of the concave curved surface.
Place. As the material of the ultraviolet reflecting mirror, for example, a polished nickel plate is used. In this way, the laser gas is ionized and active, but sufficient durability can be obtained. A polished metal plate with gold plating is more preferable in terms of durability.

In the above embodiment, a low-pressure mercury lamp with good luminous efficiency was used as the ultraviolet lamp 21, but other than this, for example, a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a rare gas halide lamp, or a xenon lamp can be used.

The thin film coating 28 on the outer surface of the ultraviolet light emitting lamp 21 is made of an alkaline earth metal halide film, and in addition to MgF2, it may contain CaF2, other fluorides, or other halogens depending on the type of laser gas. It could even be a monster. Furthermore, the coating 28 is preferably a non-reflective coating that has the maximum ultraviolet light transmittance, and a multilayer coating is particularly preferable.

〔Effect of the invention〕

As described above, the excimer laser oscillator of the present invention has a number of advantages although it is comparable in size to the conventional automatic preionization excitation type excimer laser. First, uniform pre-ionization can be achieved due to the excellent performance of ultraviolet light emitting lamps. The conventional pre-ionization electrode method does not cause deterioration of laser scum due to arc discharge that inevitably occurs. Furthermore, since there is no pre-ionization electrode, there is no deterioration of the laser gas due to its discharge. All of these contribute to extending the life of the oscillator. Problems associated with conventional pre-ionization electrode systems, such as increased circuit inductance and resistance, wear and tear on the pre-ionization electrode, and generation of dust particles, can be solved.

Therefore, the excimer laser oscillator of the present invention can realize excellent characteristics such as small size, long life, large beam area, and high repetition rate operation.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing an embodiment of an excimer laser oscillator and an oscillation power supply circuit to which the present invention is applied, Fig. 2 is a cross-sectional view of an ultraviolet light emitting lamp, and Fig. 3 is a cross-sectional view showing another embodiment of the excimer laser oscillator. The cross-sectional view of FIG. 4 is a diagram showing a conventional excimer laser oscillator and an oscillation power supply circuit. 1.10 Laser oscillator 2 Oscillation power supply circuit 3 Sealed container 4 High pressure power supply 5 Charging resistor 6 Switch 7
Main capacitor 8 Inductance 11 Anode 12 Cathode 13
Charging capacitor 14 Pre-ionization electrode 15 Ultraviolet light 16 Discharge space 18
Ultraviolet reflector 21 Ultraviolet light emitting lamp 22 Lamp drive power source 25 Tube tube 26-27
Electrode 28 Coating patent applicant Japan Radio Co., Ltd.

Claims (4)

[Claims] 1. A pair of discharge electrodes and a laser mirror are provided in the vicinity of the terminal ends of the discharge electrodes in a sealed container, a discharge space is formed surrounded by the discharge electrodes and the laser mirror, and an oscillation medium is provided in the sealed container. 1. An excimer laser oscillator filled with gas, characterized in that an ultraviolet light emitting lamp is disposed at a position capable of irradiating the discharge space. 2. 2. An excimer laser oscillator according to claim 1, wherein the surface of the ultraviolet light emitting lamp is coated with a thin film of alkaline earth metal halide. 3. 3. An excimer laser oscillator, characterized in that the ultraviolet light emitting lamp according to claim 1 or 2 has an ultraviolet reflector disposed at a rear portion facing the discharge space. 4. 4. The excimer laser oscillator according to claim 3, wherein the ultraviolet light reflecting mirror is a concave mirror, and the ultraviolet light emitting lamp is located near the focal point of the concave mirror.