JPH0218979A - Excimer laser device - Google Patents

Abstract

PURPOSE:To enable high repetition operation by generating periodical trigger signals which generate high repetition pulse signal in a first period and stop the signals in a second period. CONSTITUTION:A trigger circuit 5 generate high repeat trigger pulses in a period t1 so as to generate laser beams 8 by discharge 3, and in the next period t2 it stops the oscillation so as to provide the period to blow off the effect of discharge 3 by gas flow 9. As a whole, with period t3, i.e., t1+t2 as one period, it repeats the operation. And the first period t1 is set shorter than three times the time which is required for laser output to settle to certain low output, and the second period t2 is set to the time which is required for the laser output in the next first period to be restored to the initial output. Hereby, high repeat becomes possible without being accompanied by enlargement of the device or lowering of pulse output.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an excimer laser device suitable for high repetition rates.

[Prior art] Fig. 7 is shown in the literature r Appl, Phys, Lett.
, 35 (1979), pp. 912-914.
cs ora closed cycle now r
1 is a schematic diagram illustrating a conventional high repetition rate excimer laser device shown in J.

In the figure, (1) and (2) are electrodes, (3) is a discharge generated between two electrodes (1) and (2), (4) is a high voltage circuit for generating discharge (3), (5) is a trigger circuit for obtaining a trigger signal for pulse operation of the high voltage circuit (4), (6) is a total reflection mirror, (7) is a partial reflection mirror, (8) is a laser beam, ( 9) is the gas flow.

The operation of such a configuration will be described next.

Among excimer lasers, for example, a KrF laser that emits light at a wavelength of 248 nm uses electrodes (1) and (2) in a gas mixture of K, F2, and a buffer gas such as He or Ne.
), and the laser medium is KrF, a molecule that exists for an extremely short period of time.

Now, in Fig. 7, the electrodes (1) and (2) are generally several tens of centimeters in length, and the interval between the two electrodes is 2cl.
The accuracy is maintained at around 1. Now, when a high voltage is applied to this electrode pair from the high voltage circuit (4), a uniform discharge (3
) occurs. Incidentally, in order to efficiently oscillate an excimer laser, a high power density of about IMW/c113 is required. However, it is extremely difficult to maintain this power density for a long time while maintaining uniform discharge. Therefore, most excimer lasers are several 10 n5c
This is a pulse oscillation laser with a circuit that generates a high voltage between electrodes (1) and (2) only between electrodes (1) and (2). The light travels back and forth to generate a laser beam (8). Moreover, if an average output is desired, pulse oscillation may be repeated several tens to 100 times per second. In this case, a switching element capable of high repetition rate, such as a thyratron or thyrisk, is provided in the high voltage circuit (4) and switched according to a signal from the trigger circuit (5).

However, even if high repetition rates of switching are possible, if the number of repetitions is increased too much, the output per pulse will decrease. This is as shown in the literature cited above, and in order to achieve high repeatability, it is necessary to flow the mixed gas using a blower or the like. This is the gas flow (9).

On the other hand, Reference r J, Appl, Phys, 44 (197
3) page 5061 to page 5063 “High”
-pulse-rate glow-discharg
e 5tabllzatlon by gas No.
As shown in w''J, C indicates how many times the gas in the discharge space is replaced for each pulse defined from the flow velocity V, the width d of the area where the discharge is occurring, and the repetition frequency f.
The R value is CR-V/(d-f)...
It is expressed as (1), and it has been shown that a high repetition rate is possible if it is a certain value, for example, 4 or more.

The critical CR value at which stable discharge can be maintained varies depending on the device, and can be reduced to 4 or less by increasing the power density by 10 degrees or changing the gas composition.

Now, in the literature, it is said that the main cause of the deterioration of discharge is density changes due to shock waves, but it is also said that ions, metals, etc. generated by discharge are also a cause. After all, the output of the next pulse is lower than the previous one, and the output of the subsequent pulse is affected by the previous two discharges, although it may be influenced by the previous discharge or the subsequent discharge. Therefore, it becomes even lower than the second pulse output.

When the flow rate is slow, the effects of past discharges accumulate one after another, and eventually uniform discharge cannot be maintained and arc discharge occurs. In order to prevent arc discharge, the gas in the discharge space must be replaced at least four times, as described above. In this state, the accumulation of past discharge and its cancellation by the gas flow are balanced, and a stable discharge is obtained around a certain output value, although it is lower than the initial pulse output.

[Problem to be Solved by the Invention] Since the conventional excimer laser device is configured as shown below, it is necessary to increase the flow rate in order to realize high repetition operation. If it is 100 to 200 pps, a conventional blower will suffice, or if you want to achieve a higher repetition rate, you can avoid increasing the size of the blower.Also, if you lower the power density, you will only be able to obtain a laser with a low output per pulse. When used as a buffer gas, there were problems such as increased running costs.

The purpose of this invention was to solve the above-mentioned problems.
l! The first objective is to obtain an excimer laser device that achieves high repetition rate.

[Means for Solving the Problems] The excimer laser device of the present invention includes a pair of electrodes installed on a laser medium [+], and a resonant means for generating laser light by pumping light due to a discharge generated between the electrodes. and means for generating a periodic l-rigger signal that generates a high repetition pulse signal during a first period and no signal during a second period;
Means is provided for applying a high voltage pulse between the electrodes based on a trigger signal.

In the two-legged means, the first period is set to be shorter than three times the time required for the laser output to drop to a certain lower power, and the second period is set to the next laser output of the first period. by setting it to the time required for it to recover to its initial output.
High output operation is achieved using an excimer laser.

[Example] Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1(A) is a schematic diagram of an excimer laser device according to an embodiment of the present invention. In the figure, the trigger circuit (5
) is a high voltage circuit ( 4) generates a signal for pulse operation, and this signal is used to perform a high repetition operation for a period t as shown in Figure 1 (B).
Period t3 that continues for 1 and then pauses for the next period t2
This is a repetitive signal in which one cycle is . This rest period t2 corresponds to a recovery period of the gas flow (9).

The operation of this configuration will now be described.

Excimer molecules are generated when a discharge (3) is generated between the electrodes (1) and (2) by the high voltage circuit (4) in an appropriate mixed gas as in the conventional case. The electrodes (1) and (2) for generating the discharge (3) are several tens of centimeters long in the length direction, and the spacing between them is precisely maintained at about 2 cm. When a pulsed high voltage is applied between the electrodes (1) and (2) by applying a pulsed high-repetition trigger signal from the trigger circuit (5) to the high voltage circuit (4) having a switching element, the electrode (1) ), (2), a uniform discharge (3) occurs, and excimer molecules are generated and emit light for a short time while this discharge (3) is occurring. This light is amplified by pumping while reciprocating between the total reflection mirror (6) and the partial reflection mirror (7), and becomes a laser beam (8), or disappears when the discharge (3) ends. The excimer Φ laser is a laser that operates in pulses, and the output of each pulsed laser beam is quite large. If a higher average output is required, the discharge may be caused many times within a unit time.

In order to perform repeated operations, gas flow (9
) is necessary to blow away the effects of the previous discharge.

Figure 2 is a measurement diagram showing the output change per pulse when a discharge (3) is generated in a mixed gas of K, F2, and He. The average output per pulse is calculated by dividing the output obtained here by the repetition frequency.The time constant of this power meter is about 1 degree, so it is possible to calculate the average output per pulse. , it does not capture fast changes.As I wrote before, when the operation of the -za starts, the influence of the discharge is integrated into the mixed gas, resulting in a decrease in output.Then, the gas flow (9 ) The output becomes constant when it is balanced with the cancellation of the influence of the discharge (3).Of course, even this output fluctuates by several percent due to the instability of the discharge.

This variation was expressed as error par. In FIG. 2, a duty of 100% represents the characteristics of a conventional excimer laser. According to this, the average value Pave of the output starts to decrease already at a repetition frequency of 100 pps, and the output becomes extremely unstable at 250 pps and 300 pps. This is due to the instability of the discharge (3), and bright arc-shaped discharges are often observed during the uniform discharge (3). Also, 350
At pps, arcing occurred continuously and no output could be obtained. In other words, at this repetition frequency, the discharge (
This is thought to be because the cumulative effect of 3) has become too large.

Therefore, in the configuration of this embodiment, the trigger circuit (
5). In other words, the trigger circuit (5)
As shown in Figure (B), in period 11, the high repetition trigger
A laser beam (8) is generated by generating a pulse and discharging (3).
) is generated, and during the next period t2, the oscillation is stopped and a period is provided for blowing off the influence of the discharge (3) by the gas flow (9), and the operation is repeated as one cycle. Here, the period t
The ratio of the period t3 to the period t3 will be referred to as duty, and the operation as described in -1- will be referred to as duty operation.

Now, in Figure 2, the duty is 20% and 40%.
The average output power per pulse is decreased when the time interval t3 is 1 second. When the duty is made smaller and the pause period t2 is made longer, the characteristics are improved. Further, even at these duties, even at 4001) ps, although the output decreases, arc discharge does not occur and the output fluctuates, making it possible to continue the duty operation for a long time.

Next, Fig. 3 is a measurement diagram of how the peak of the laser pulse output changes during duty operation using a PIN photodiode. In particular, under the same conditions as in Fig. 2, 400 pps, duty 4096 This shows the case where the operation is performed. Initially, the maximum output value Pmax (this is
(approximately equal to the output at low repetition) decreases monotonically, and after 0.25 seconds the minimum output fi? iPmin(
This becomes an output (approximately equal to the output when the duty is 100%). Then, the operation stops after 0.4 seconds, and resumes operation again after another second. Duty 20%
It shows the same output change during operation, and repeats the operation of stopping once after 0.2 seconds. The dotted line indicates the average output value Pave, which corresponds to the output value shown in FIG. As can be seen from the above, 0.25 seconds is the time during which the accumulation of the discharge (3) and the blow-off by the gas flow (9) are balanced. If this time is the output reduction time τ, then τ1 and the minimum output value Pm1n are determined by the gas mixture ratio, power density, gas
Determined by flow (9) etc. The two values are constant if operated under the same conditions. Furthermore, since the improvement in the output characteristics in Fig. 2 is recognized by adding the output change up to the time τ■, if the operating time is too long, the ratio after the minimum output value Pmjn is As the number increases, the improvement in characteristics becomes smaller in appearance. Furthermore, if the time is increased, the discharge may become unstable as in the case of 100% duty. For example, at 400 pss, if the duty was set to operate for more than 0.7 seconds, a phenomenon was observed where the device became unstable. Therefore, it is preferable to set the period t to at least three times the time τ1. On the other hand, on the other hand,
The pause time is preferably 0.25 seconds or more. If it is less than that, the first output of the next cycle will be lower than the output of the previous cycle. If the pause time required to avoid leaving any influence on the next cycle is the output recovery time τ2, the period t2 needs to be longer than the time τ. In other words, tl × 3τI
...(2) t2 τ2
...(3).

FIG. 4 is a schematic diagram of an excimer laser device according to another embodiment of the present invention, illustrating a configuration in which the characteristics are more significantly modified using the same duty operation. In the figure, (10) is a total reflection mirror (6) and a partial reflection mirror (7).
) is an etalon (10) provided on the output side from the discharge (3) of the laser beam (8).

The operation will be explained below in the configuration of L.

Normally, when an excimer laser beam is separated, it is 0.4
It has a wide line width of nm. Therefore, a total reflection mirror (6
) and a partial reflecting mirror (7), and if an etalon (10), which is a spectroscopic element, is placed in the optical resonant type, the wavelength is 0.4 nm.
It is possible to oscillate within a narrow wavelength range. That is, although there are grating prisms and the like as spectroscopic elements, etalons (10) are often used because of their light transmittance and stability against vibrations. The figure shows an air-gap type etalon in which two mirrors face each other with a certain distance between them. The selected wavelength range can be changed by appropriately adjusting this interval and mirror reflectance. Since one etalon (10) can only narrow the wavelength range, it is effective to use a plurality of etalons or to use a grating prism together.

By the way, these spectroscopic elements require extremely high precision processing. For example, in the etalon (10), if the mirror surface changes by several nanometers, the spectral characteristics change, the oscillation wavelength shifts, the beam shrinks, and the output decreases. especially,
When a plurality of etalons (10) are used, the output decreases significantly. The main cause of this change in the mirror surface is thermal distortion caused by the laser beam. The coatings and substrate materials applied to the surface of the etalon (10) each have an absorption coefficient of 96 commas. Therefore, the laser is absorbed at these locations and generates heat, causing the mirror surface to become distorted.

Now, Figure 5 shows 3 p using two etalons (10).
This is a laser output characteristic with a narrow band down to m.

A duty of 100% indicates the output change when operated in the conventional method. K to improve repeatability
Even though four types of mixed gases, r, F2, and He%Ne, were used, the output per pulse gradually decreased from 100 pps. The output here is a constant minimum output value P
It shows the value when m1n is reached. When four types of mixed gases are used, if the etalon (10) is not present, there is almost no decrease in output up to 200 pps, and the decrease in output here is mostly due to thermal strain of the etalon (10).

In order to suppress such thermal distortion, it is sufficient to reduce the thermal power, and for this purpose, it is possible to suppress the relay laser output by reducing the absorption rate of the etalon (10), but it is also effective to perform duty operation. It is true.

Thermal strain decreases relatively slowly, determined by the size of the etalon (10) and the thermal contact between the etalon (10) and its surroundings. For example, an example is shown in the measurement diagram of Figure 6, but 2
When operating continuously at 50 pps with a duty of 100%, the initial output of 3001J becomes the time constant τ3 (1~
2 minutes), it decreased to 15 mJ. Minimum output value Pm1n
The output at this point is 3.75W. Therefore,
Considering the case where the period t3 is 1 second and the duty is 20%, the decrease due to heat is a slow decrease of 1 minute or more, and during 1 second, F3 is 1 second, so there is almost no change in the output.

In this case, it may be considered that an average amount of heat enters the etalon (10) including the rest period t2. That is, 1.
This means that the equivalent of 5W of heat is added.

It can be seen from FIG. 6 that this corresponds to when the device is operated at 50 pps and a duty of 10096, and that no output drop occurs. In fact, when operating continuously at 20% duty, there was almost no single drop in output.

The point A where the single-shot output becomes 90% of the original output is the limit output P
Let the frequency at this time be f,! Then, the product of single-shot output P, frequency f, and duty (-11/13) is Pf
By determining each value so as not to exceed lf, the etalon (10) can be operated with reduced heat input. For example, the single output is 30+n
J and the duty is 20%, 0°2 seconds is 60
It may be possible to repeat the operation of operating at 0 pps and pausing for 0.8 seconds. In other words, 3 3...(4) P-f
-t/l<P-f...(5)1
If the conditions 3 and 1 are met, the output decrease can be prevented and the etalon (
10) It is possible to reduce the heat input to.

Such duty operation is not for special purposes; for example, in semiconductor exposure equipment, a part of the wafer 1 is exposed for 0°2 seconds, and the wafer 1- is moved in the next tenth of a second to expose another area. It is often used when the action of "doing" is repeated. In such cases, the present invention is extremely effective; by using a small excimer laser that can only produce a small output during continuous operation, a high pulse output can be obtained for the period required for semiconductor exposure, achieving high repetition rate operation. can.

Furthermore, in each of the above embodiments, the case where the period t3 is one cycle has been illustrated, but since the duty operation is intended to blow away the influence of discharge during the rest period or to only have a cooling effect on the spectroscopic element, the period t3 is one period. t3 and duty,
Alternatively, it is of course possible to achieve the initial objective even if the repetition frequency during operation changes somewhat.

[Effects of the Invention] As described above, according to the present invention, by operating the excimer laser on duty, it is possible to achieve high repetition operation, which was previously impossible. In addition, it is possible to obtain an excimer laser device that can perform high-repetition operations while reducing the influence of thermal distortion of a spectroscopic element used for narrowing the band.

[Brief explanation of the drawing]

FIG. 1(A) is a schematic diagram of an excimer laser device according to an embodiment of the present invention, FIG. 1(B) is an output waveform diagram of a trigger circuit configured as shown in FIG. 1, and FIG. 2 is a diagram of an excimer laser device according to an embodiment of the present invention. FIG. 3 is a fllll constant diagram showing the temporal change in laser output; FIG. 4 is a schematic diagram of an excimer laser device according to another embodiment of the present invention; Figure 5 shows the U+ constant diagram of the output during high repetition operation in Figure 4 (1'4 configuration), Figure 6 is a measurement diagram showing the temporal change in laser output, and Figure 7 shows the conventional high repetition rate operation. It is a schematic diagram of an excimer laser device. In the figure, (1) and (2) are electrodes, (3) is a discharge, and (
4) is a high voltage circuit, (5) is a trigger circuit, (6) is a total reflection mirror, (7) is a partial reflection mirror, (8) is a laser beam,
(9) is the gas flow and (10) is the etalon. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] A pair of electrodes installed in a laser medium, a resonant means that generates a laser beam by pumping light due to a discharge generated between the electrodes, and a resonant means that generates a high repetition pulse signal during a first period and generates a high repetition pulse signal during a second period. An excimer laser device comprising: means for generating a periodic trigger signal with no signal; and means for applying a high voltage pulse between electrodes based on the trigger signal.