JPH0378267A - Excimer laser device - Google Patents

Abstract

PURPOSE:To prevent a laser output from being lowered until impurity concentration is saturated at the initial stage of laser operation by providing a laser gas purifying device, and encapsulating impurity gas produced during laser oscillation in a discharge tube beforehand. CONSTITUTION:A laser oscillator 1 is supplied with given mixing ratio laser gas from a laser gas supply device 2 and a given amount of impurity from an impurity addition device 4. After completion of the supply of the laser gas and the impurity to the laser gas and the impurity to the laser oscillator 1, there are started simultaneously laser oscillation of the oscillator 1, laser gas purification by a laser gas purifying device 4, and laser gas supply by the device 2 for making constant the laser gas mixing ratio. Since impurity concentration in the laser gas remains unchanged so long as operation conditions of the oscillator 1, the device 2, and the device 3 are altered, the laser output is prevented from being lowered owing to the increase of the impurity concentration and hence a stable laser output can be maintained over a long period.

Description

[Detailed description of the invention] Industrial applications The present invention relates to an excimer laser device.

Conventional technology Rare gas halogen excimer lasers are high-output, high-efficiency laser light sources that oscillate in the ultraviolet region, and are used as light sources for various semiconductor and photochemical processes, including VLSI lithography and CVD, as well as for drilling and cutting. It is attracting attention as a tool for ultra-fine processing.

By the way, rare gas halogen excimer lasers use extremely reactive halogen gas as the laser gas, so during continuous operation, the laser output gradually decreases due to a decrease in halogen gas and an accompanying increase in impurities other than the laser gas. It is difficult to avoid going. This short lifetime of the laser gas is one of the reasons that hinders the expansion of the range of applications, and an immediate solution is desired.

Conventionally, in order to extend the life of laser gas, it has been common to use a laser gas cleaning device. There are two types of laser gas purification equipment: the "replaceable type", which discards part of the laser gas and supplies new laser gas at the same time, and the "replaceable type", which removes part of the laser gas, removes halogen gas and impurities from it, and replaces it with new one. There are two types of ``circulatory purification type'' in which halogen gas is added and returned to the laser body.

Problems to be Solved by the Invention Among the impurities generated during continuous operation of a rare gas halogen excimer laser, those that increase in proportion to the input per unit time to the laser discharge have a particularly dominant effect on the decrease in laser output. affect Further, since the source of impurities is the material inside the laser discharge tube, it is thought that impurities continue to be generated semi-permanently. The impurity increase rate can be expressed as equation (1).

(dm/dt)+=a --(1) Here, m is the impurity concentration, and a is the generation rate (constant) of impurities per unit time.

On the other hand, the impurity removal rate by the laser gas purifying device is proportional to the impurity concentration and gas flow rate at that time (the amount of circulation per unit time in the case of a circulation type, the amount of exchange per unit time in the case of an exchange type). Expressed as a formula, it is as shown in formula (2).

(dm/d t)x=mr/V - Old... (2) Here, ■ is the total volume of gas (constant), and r is the gas flow rate per unit time (constant). However, if the gas purification system is a "circulation type", equation (2) only holds true in the ideal case, that is, when all impurities are removed from the laser gas that has passed through the gas purification system. It is.

Rate of change of impurity concentration m at time t d m / dt
can be expressed as (1)+(2), so it becomes a differential equation like equation (3).

dm/dt=a-mr/V...
--(3) When equation (3) is solved, the change in impurity concentration m over time can be expressed by equation (4).

m-(aV/r)11-exp (-r/V)tl...
...(4) That is, the impurity concentration m is a constant value (a
V/r). In the extreme case, if the gas flow rate r is made infinite, m=0 and an increase in impurities can be prevented, but this is unrealistic.

After the impurity concentration is saturated, it is possible to stabilize the laser output by taking measures to keep the mixing ratio of the laser gas constant. However, as the impurities gradually increase, there is no way to prevent the laser output from decreasing until it reaches a certain value, which has become a major problem.

FIG. 4 shows an example of an increase in CF impurity concentration and a decrease in laser output during continuous oscillation of a conventional KrF excimer laser. He=94.75%, F2=0.25
%, Kr=5% is sealed in a laser discharge tube, and the flow rate is 117
As a result of continuously oscillating the laser while cleaning the laser gas with min, it took more than 5 million shots until the concentration of impurity CF4 was saturated at about 50 ppm and the laser output stabilized, and the rate of decrease in laser output during that time was It reached more than 10%.

The present invention has been made to solve these problems, and provides a method for obtaining stable laser output by preventing a decrease in laser output during the initial stage of laser operation until the impurity concentration is saturated. It is something.

Means for Solving the Problem In order to solve this problem, the present invention is equipped with a laser gas cleaning device, and impurity gas generated during laser oscillation is sealed in advance in the discharge tube. .

Effect: According to such a configuration, since the impurity concentration in the laser gas is always maintained at a constant value during continuous laser operation, the laser output does not decrease at the beginning of laser operation and can maintain a stable state.

Embodiment FIG. 1 is a schematic diagram of an excimer laser device according to an embodiment of the present invention, in which a laser oscillator 1. laser gas supply device 2,
It is composed of a laser gas cleaning device 3 and an impurity adding device 4. Here, an exchangeable type gas cleaning device is used. A laser gas supply device 2 supplies laser gas at a predetermined mixing ratio, and an impurity addition device 4 supplies a predetermined amount of impurities to the laser oscillator 1. According to the inventor's experiments,
For example, in the case of a KrF excimer laser, it is known that CF4 is an impurity that has a dominant effect on output reduction. As the impurity to be added, only CF4 is sufficient.

Here, the amount of impurity added was calculated from the impurity generation rate a obtained from the experimental results, the gas flow rate r, and the total gas volume V (a
V/r).

After the supply of laser gas and impurities to the laser oscillator 1 is completed, laser oscillation of the laser oscillator 1, laser gas cleaning by the laser gas cleaning device 3, and laser gas supply for changing the laser gas mixture ratio by the laser gas supply device 2 are simultaneously started. . The impurity concentration in the laser gas does not change unless the operating conditions of the laser oscillator 1, laser gas supply device 2, and replaceable laser gas purification device 3 are changed, so the laser output does not decrease due to an increase in impurity concentration, and the laser output remains stable over a long period of time. Laser output can be maintained.

FIG. 2 is an example of the impurity concentration and laser output when the present invention is used. He=94.75%, F2=0
．． 25%, Kr=5% is sealed in a laser discharge tube at a total pressure of 1 kg/cj, and after adding 50 ppm of impurity CF4, the exchangeable gas purifier is operated at a flow rate of 11/min to clean the laser gas. At the same time, 5% F2 (He
By adding an extra 10 ml/min of dilution), the laser was continuously oscillated while keeping the laser gas mixture ratio constant.

The concentration of impurity CF4 is almost constant at 5Qppm,
No decrease in laser output was observed in the initial stage of laser oscillation, and the laser output remained stable.

FIG. 3 shows an example of the impurity concentration and laser output when a circulating type gas purifying device is used instead of the exchange type shown in FIG. 2, and the oscillation conditions are the same as in FIG. 2. In this case as well, the concentration of the impurity CF4 was approximately constant at 50 ppm, and the laser output was stable without any decrease in the laser output at the initial stage of laser oscillation.

As described in detail, according to the present invention, it is possible to prevent a decrease in laser output due to an increase in the concentration of impurities other than laser gas during the initial stage of operation of an excimer laser, and to maintain stable laser output over a long period of time. The purpose of the present invention is to provide an excimer laser device having excellent features such as:

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the present invention, FIG. 2 and FIG.
The figure shows the impurity concentration and laser output when the present invention is used, and FIG. 4 shows the impurity concentration and laser output of a conventional excimer laser. 1... Laser oscillator, 2... Laser gas supply device, 3... Gas cleaning device, 4...
...Impurity addition device.

Claims (1)

[Claims] An excimer laser device equipped with a laser gas cleaning device, characterized in that impurity gas generated during laser oscillation is sealed in advance in a discharge tube.