JP2649378B2 - Wavelength anomaly detector for narrow band oscillation excimer laser - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength anomaly detection device for a narrow band oscillation excimer laser used as a light source of a reduction projection exposure apparatus.

[Conventional technology]

Attention has been paid to the use of excimer lasers as light sources for reduction projection exposure apparatuses for manufacturing semiconductor devices. This is because the wavelength of an excimer laser is short (for example, the wavelength of a KrF laser is about 248.4
nm), the limit of light exposure can be extended to 0.5 μm or less, the depth of focus is deeper than conventional mercury lamp g-line and i-line at the same resolution, lens numerical aperture This is because many excellent advantages such as a small (NA), a large exposure area, and a large power can be expected.

However, there is a problem to be solved when using an excimer laser as a light source of a reduction projection exposure apparatus.
There are two major problems.

One is that the wavelength of the excimer laser is
The material that transmits this wavelength is quartz, CaF
Among these materials, only quartz can be used practically as a lens material in terms of uniformity and processing accuracy. For this reason, it is extremely difficult to design a reduction projection lens with chromatic aberration correction.
Therefore, it is necessary to narrow the output wavelength of the excimer laser so that the chromatic aberration can be ignored.

Another problem is how to prevent the speckle pattern generated with the narrow band of the excimer laser and how to reduce the power accompanying the narrow band.

As a technique for narrowing the band of an excimer laser, there is a technique called an injection lock method. In this injection lock system, a wavelength selecting element (etalon, diffraction grating, prism, etc.) is arranged in a cavity of an oscillator stage, a spatial mode is restricted by a pinhole, and a single mode is oscillated. Inject synchronization. For this reason, the output light has high coherence. If this light is used as it is as the light source of the reduction exposure apparatus, a speckle pattern is generated. Generally speckle
It is considered that the generation of the pattern depends on the number of spatial transverse modes included in the laser light. That is, it is known that a speckle pattern is easily generated when the number of spatial transverse modes included in the laser beam is small, and a speckle pattern is hardly generated when the number of spatial modes is large. The above-described injection lock method is essentially a technique for narrowing the bandwidth by significantly reducing the number of spatial transverse modes, and is not suitable for a reduced projection exposure apparatus because the occurrence of a speckle pattern is a major problem. . .

Another promising technique for narrowing the band of an excimer laser is an etalon that is a wavelength selection element.
As a conventional technique using this etalon, AT & T Bell Labs has proposed a technique in which an etalon is arranged between a front mirror of an excimer laser and a laser chamber to narrow the band of the excimer laser. However, with this method, the spectral line width cannot be made sufficiently narrow.
In addition, there is a problem that the power loss due to the etalon insertion is large, and further, there is a disadvantage that the number of spatial transverse modes cannot be increased too much.

Therefore, the inventors have adopted a configuration in which an etalon having a large effective diameter (about several tens of mm) is disposed between the rear mirror of the excimer laser and the laser chamber.
The laser beam with an output of about 50 mJ per pulse is obtained by applying a uniform narrowing of the spectrum width to about 0.003 nm or less at the full width at half maximum in the range of 10 mm ² . In other words, by adopting a configuration in which an etalon is arranged between the rear mirror of the excimer laser and the laser chamber, the narrow projection of the laser, securing of the number of spatial transverse modes, and reduction of power loss due to insertion of the etalon are reduced. An essential problem required as a light source of an exposure apparatus can be solved.

However, the configuration in which the etalon is disposed between the rear mirror of the excimer laser and the laser chamber has excellent advantages in narrowing the band, securing the spatial transverse mode, and reducing the power loss, but the power transmitted through the etalon is extremely low. Physical changes such as temperature fluctuations occur in the etalon,
For this reason, there have been problems that the center wavelength of the oscillation output laser light fluctuates and the power is significantly reduced. This tendency is particularly noticeable when two or more etalons having different free spectral ranges are used for narrowing the band.

[Problems to be solved by the invention]

By the way, if the etalon is arranged during the optical resonance, the etalon transmits light with a very high energy density, so that the surface accuracy and parallelism of the etalon deteriorate over a long period of time. In some cases, finesse was reduced due to damage to the reflection film.

When the finesse of the etalon is reduced, a large sideband is generated, and multi-wavelength oscillation is caused. When this laser light is used as a light source for reduction projection exposure, there is a problem that the resolving power is greatly reduced.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a wavelength abnormality detection device that can easily and reliably detect the wavelength abnormality of the oscillation laser light spectrum.

[Means for solving the problem]

Therefore, in the present invention, in a narrow-band oscillation excimer laser device, the output power of a laser beam and the center wavelength power are detected, and it is determined whether or not the ratio of these is within an allowable range. Abnormality detection is performed.

[Action]

With the above configuration, the center wavelength and the sideband wave are separated by the diffraction grating spectroscope or the monitor etalon, the power of the center wavelength and the output power of the laser are detected, and whether or not the ratio is within an allowable range is monitored. This makes it possible to easily and reliably detect anomalies in the spectral wavelength.

Here, if the ratio is out of the allowable range, an abnormal signal is generated as a wavelength abnormality.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention. In this embodiment, two etalons 101 and 102 are provided between the laser chamber 107 and the rear mirror 106.

The apparatus of this embodiment controls the laser output power by controlling the component of the laser medium gas in the laser chamber 107 and the excitation intensity control (discharge voltage control) of the laser medium, and the center for controlling the laser output center wavelength. It has a wavelength control system 300 that simultaneously or alternately performs wavelength control and superposition control for superimposing transmission center wavelengths of the etalons 101 and 102.

First, the operation of the power control system 200 and the wavelength control system 300 in the steady state will be described. The properties of the laser medium gas used for the excimer laser as the laser medium gradually deteriorates over time, and the laser power decreases.
Therefore, the excitation control of the laser medium, that is, the discharge voltage is controlled, and the power control system 200 performs component control of the laser medium, that is, output control for keeping the laser output constant by performing gas exchange. That is, the first
As shown in the figure, a part of the oscillated laser light is branched by the beam splitter 104 and incident on the power monitor 202, and the change in laser power is monitored.The CPU 203 controls the excitation intensity of the laser medium via the laser power supply 204. The output is controlled so as to keep the laser output constant by changing the laser medium gas or partially exchanging the laser medium gas via the gas controller 205.

Also, part of the oscillated laser light is beam splitter
At 103, the light is branched as sample light, and is added to an oscillation center wavelength and center wavelength power detector 301. The oscillation center wavelength and center wavelength power detector 301 has an oscillation center wavelength λ of the excimer laser 10 included in the sample light and a power P of the center wavelength.
λ is detected. The oscillation center wavelength and center wavelength power detector 301 is composed of a diffraction grating type spectroscope and an optical position sensor, the details of which are shown in FIG.

In FIG. 2, the oscillation center wavelength and center wavelength power detector 301 includes a diffraction grating type spectroscope 34 including concave mirrors 31 and 32 and a diffraction grating 33, and an optical position sensor 35. The sample light sampled by the beam splitter 103 is condensed by the lens 36 and is input from the entrance slit 37 of the spectroscope 34. The light input from this entrance slit 37 is a concave mirror
The light is reflected by 31, becomes parallel light, and is irradiated on the diffraction grating 33.
Here, the diffraction grating 33 is fixed at a predetermined angle, and reflects at a diffraction angle corresponding to the wavelength of the incident light. The diffracted light is guided to the concave mirror 32, and the reflected light from the concave mirror 32 is guided to the optical position sensor 35 to form an image. That is, a diffraction image of the incident slit 37 corresponding to the wavelength of the incident light is formed on the light receiving surface of the optical position sensor 35, and the center wavelength λ of the sample light can be detected from the position of the diffraction image of the incident slit 37. it can.
The center wavelength power Pλ can be detected from the light intensity of the diffraction image of the entrance slit 37.

Note that a photodiode array, a light spot position detector PSD (Positive Sensitive Detector), or the like can be used as the light position sensor 35. here,
When a photodiode array is used as the optical position sensor 35, the center wavelength is detected by the position of the light receiving channel having the maximum light intensity, and the center wavelength power is the light intensity of the channel corresponding to the center wavelength or the light intensity of the channel near the center wavelength. From the sum of When a PSD is used as the optical position sensor 35, the center wavelength is detected from the output ratio of the PSD, and the center wavelength power is detected from the output.

Center wavelength λ and center wavelength power Pλ of the sample light detected by the oscillation center wavelength and center wavelength power detector 301
Is a central processing unit (CPU) 302 constituting a wavelength controller.
Is input to

The CPU 302 controls the wavelength selection characteristics (the transmission center wavelength and the selection center wavelength) of the etalons 101 and 102 via the drivers 303 and 304, and the center wavelength of the sample light, that is, the output light of the excimer laser matches the desired wavelength set in advance ( (Center wavelength control) and the center wavelength power is maximized (superposition control). Here, the control of the wavelength selection characteristics of the etalons 101 and 102 by the drivers 303 and 304 is performed by controlling the temperature of the etalon 4, controlling the angle, controlling the pressure in the air gap, controlling the gap interval, and the like.

The center wavelength control specifically shifts the transmission wavelength of the etalon 101 or 102 by controlling the angle or the like of the etalon of at least the smaller free spectral range, such as the etalon 101, thereby shifting the transmission center wavelength of the etalon. And the center wavelength power detector 301 controls the wavelength to be a desired wavelength. The superposition control is based on the etalon with the smaller free spectral range described above.
Etalons other than 101, that is, the transmission center wavelength of the etalon 102 having the larger free spectral range is shifted by a predetermined unit wavelength, the transmission center wavelengths of the etalons 101 and 102 overlap, and the oscillation center wavelength and the center wavelength power detector 301
Is controlled so that the center wavelength power detected by the above becomes maximum.

By the way, when PSD is used as the optical position sensor,
The size of the light receiving surface of the optical position sensor 35 of the oscillation center wavelength and center wavelength power detector 301 is set so that only the center wavelength is detected as shown in FIG. Is) set. This is to prevent sideband waves from being detected. That is, the etalon
When the superposition of 101 and 102 is defective, a sideband is generated. If the optical position sensor 35 detects this sideband, superposition control cannot be performed and accurate center wavelength control cannot be performed. Therefore, the size of the light receiving surface of the optical position sensor 35 is limited so as not to detect the sideband even if the sideband occurs. Generally, when the etalons 101 and 102 are not properly superimposed, a sideband is generated.
The etalons 101 and 102 are designed such that almost no sideband occurs when the superposition of 1 and 102 is completed.

However, if the surface accuracy and parallelism of the etalons 101 and 102 are deteriorated, or the reflection film is damaged, and the finesse of the etalons 101 and 102 is reduced, sideband waves may be generated even when the superposition is completed. Next, this phenomenon will be described.

First, etalon 10 with a small free spectral range
It is assumed that 1 and etalon 102 having a large free spectral range are normal. In this case, when the etalons 101 and 102 are completely overlapped, almost no sideband is generated as shown in FIG. However, when the finesse of the etalon 102 having a large free spectral range is reduced due to deterioration of the surface accuracy or the like, the transmission wavelength of the etalon 102 becomes wider, and as a result, a plurality of sidebands are generated as shown in FIG. Wavelength oscillation occurs. The same applies when the finesse of the etalon 101 is reduced.

When the laser has multi-wavelength oscillation in this way, the resolving power is greatly reduced even if this laser beam is used as a light source for reduction projection exposure.

Therefore, the CPU 203 calculates the ratio between the laser output power obtained from the output of the power monitor 202 and the center wavelength power obtained from the output of the optical position sensor 35, and determines whether or not this value is within a predetermined allowable range. Is monitored constantly, and a wavelength abnormal signal is output when the wavelength is out of the allowable range. By indirectly detecting the sideband in this manner, the sideband of the multi-wavelength oscillation can be easily detected, and the decrease in the resolving power can be detected.

As described above, in this embodiment, the ratio between the laser output power and the center wavelength power is calculated from the output of the oscillation center wavelength and center wavelength power detector 301 and the output of the power monitor 202, and this ratio is monitored. Thereby, it is detected whether or not a sideband wave is generated.

When the sideband is detected, the CPU 203 generates an abnormal wavelength signal. This wavelength abnormality signal is sent to an exposure device (not shown).

Next, the operation of the wavelength abnormality detection will be described with reference to a flowchart.

FIG. 5 shows a main flow of the apparatus of this embodiment. In this operation, first, it is determined whether or not the elapsed time T has passed a predetermined time α set in advance (step 401), and if not, the process proceeds to step 402, and normal control, that is, , Power control, superposition control, and center wavelength control. This normal control is repeated until the elapsed time T reaches a predetermined time α. When the elapsed time T reaches a predetermined time α, this is determined in step 401, and the process proceeds to the sideband detection subroutine 403. This sideband detection subroutine 403 is shown in FIG.

In the sideband detection subroutine 403, first, the output of the center wavelength power detector 301 and the output of the power monitor 202 are used to determine the laser output power Pt and the center wavelength power P, respectively.
is read into the CPU 203 (step 501).

Next, the CPU 203 calculates a ratio P = Pλ / Pt between the laser output power Pt and the center wavelength power Pλ (step 50).
2).

Then, it is determined whether or not the ratio P is larger than a predetermined value k (step 503). If it is larger, the subroutine is terminated without any abnormality. When the sideband detection subroutine 403 ends, the time T is reset to 0, and the process returns to step 401 again.

On the other hand, if the ratio P is smaller than the predetermined value k in the judgment step 503, the shutter is closed (step 504) and a wavelength abnormal signal is output (step 5).
05).

Thereafter, the determination step 401 (fifth step)
Return to figure).

As described above, the process periodically shifts to the sideband detection subroutine 403 to monitor the sideband, so that the multi-wavelength oscillation due to the decrease in the finesse of the etalon can be reliably detected, thereby causing an inconvenience based on the multi-wavelength oscillation. Can be prevented in advance. After performing the normal control, the process may always proceed to the sideband detection subroutine to constantly monitor the sideband.

In the above embodiment, the case where two etalons are used has been described. However, the same configuration can be obtained by using three or more etalons.

Further, in the above embodiment, the case where the diffraction grating type spectroscope is used for the detection of the center wavelength power has been described. However, as shown in FIG. Alternatively, the light may be divided into interference light based on the center wavelength and interference light based on the sideband, and guided to the optical sensor 43 for detection. Here, for the monitor etalon, the difference Δλside between the sideband and the center wavelength and the free peripheral range FSRmonitor of the monitor etalon are set so that Δλside ≠ FSRmonitor (n = 1, 2,...) It is possible to separate the interference light due to the wavelength and the interference light due to the sideband.

〔The invention's effect〕

As described above, according to the present invention, the output power of the laser and the center wavelength power are detected, and it is determined whether or not the ratio is within an allowable range. Since it is performed, multi-wavelength oscillation can be reliably detected with an extremely simple configuration, and thereby an unexpected decrease in resolution when used as a light source for reduction projection exposure can be prevented.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a diagram showing details of an oscillation center wavelength and a center wavelength power detector, and FIG. 3 is an oscillation center wavelength and a center wavelength shown in FIG. FIG. 4 is a view showing a detection state of the power detector, FIG. 4 is a view showing a state of occurrence of a side peak in this embodiment, and FIG.
FIG. 6 and FIG. 6 are flowcharts for explaining the operation of this embodiment, and FIG. 7 is a diagram showing another example of the center wavelength power detector. 101,102 ... Etalon, 103 ... Beam splitter, 200
... Power control system, 203 CPU, 204 Laser power supply, 2
05 ... gas controller, 300 ... wavelength control system, 302 ...
CPU, 303, 304 …… Driver, 31, 32 …… Concave mirror, 33 ……
Diffraction grating, 34: Diffraction grating type spectroscope, 35: Optical position sensor, 36: Lens, 37: Incident slit, 41: Optical fiber, 42: Monitor etalon, 43: Optical sensor.

Claims (1)

(57) [Claims] 1. A narrow-band oscillation excimer laser device, comprising: an output power detector for detecting an output power of an output laser beam; a center wavelength power detector for detecting a center wavelength power; Abnormality detecting means for detecting an abnormality in the spectrum of the laser light by monitoring whether or not the ratio between the output power and the central wavelength power detected by the central wavelength power detecting means is within a predetermined allowable range; A wavelength anomaly detection device for a narrow-band oscillation excimer laser, comprising: