JPH01276684A - Detector of wavelength abnormality of narrow band oscillation excimer laser - Google Patents

Abstract

PURPOSE:To detect multiwavelength oscillation securely by a detector with a simple constitution and avoid the degradation of resolution when an excimer laser is employed as a light source of contract projection aligner by a method wherein the output power and central wavelength power of the laser are detected and the abnormality of the spectrum wavelength is detected by observing whether the ratio of those powers is within an allowable range or not. CONSTITUTION:The ratio of the output power and central wavelength power of an excimer laser is calculated from the output of a detector 301 of an oscillation center wavelength and central wavelength power and the output of a power monitor 202 and the ratio is observed to detect whether side peaks are created or not. If the side peaks are detected, a CPU 203 generates a wavelength abnormality signal. The wavelength abnormality signal is transmitted to an aligner (not shown in the figure). With this constitution, the wavelength abnormality of the spectrum of the oscillated laser beam can be detected easily and with a high reliability and the degradation of resolution can be avoided.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a narrow band IJ used as a light source of a reduction projection exposure apparatus.
The present invention relates to a wavelength abnormality detection device for an IT oscillation excimer laser.

[Conventional technology]

Semiconductor equipment! ! ! The use of excimer lasers as light sources for reduction projection exposure apparatuses for industrial use is attracting attention. This is because the wavelength of excimer laser is short (for example, the wavelength of KrF laser is about 248.4 nl1), so the limit of optical exposure is 0.5μ.
m or less, the depth of focus is deeper compared to the q-line and i-line of conventional mercury lamps for the same resolution, and the numerical aperture (NA) of the lens is small, making it possible to reduce the exposure area. This is because it can be expected to have many excellent advantages, such as being able to increase the size and obtaining a large amount of power.

However, there are two major problems that must be solved when using an excimer laser as a light source for a reduction projection exposure apparatus.

One is that the wavelength of the excimer laser is 24
8.4 Because of its short n11, the only materials that transmit this wavelength are quartz, CaF2, and MClF2, and among these materials, only quartz can be used as a lens material in practice in terms of uniformity and processing accuracy. It is impossible. For this reason, it is extremely difficult to design a reduction projection lens with chromatic aberration correction, and it is therefore necessary to narrow the output wavelength band of the excimer laser to such an extent that this chromatic aberration can be ignored.

Another problem is how to prevent the speckle pattern that occurs as the band becomes narrower in excimer lasers, and how to suppress the reduction in power that occurs as the band becomes narrower.

As a technique for narrowing the band of an excimer laser, there is a technique called an injection locking method.

This injection locking method places a wavelength selection element (etalon, diffraction grating, prism, etc.) inside the cavity of the oscillator stage, limits the spatial mode with a pinhole, causes single mode oscillation, and amplifies this laser light. Injection locking by stage. Therefore, its output light has high coherence, and if it is used as it is as a light source for a reduction exposure device, a speckle pattern will occur. It is generally believed that the generation of speckle patterns depends on the number of spatial transverse modes included in laser light. That is, it is known that when the number of spatial transverse modes included in laser light is small, speckle patterns are more likely to occur, and conversely, when the number of spatial modes is large, speckle patterns are less likely to occur.

The injection lock method described above is essentially a technology that narrows the band by significantly reducing the number of spatial transverse modes, and is not suitable for reduction projection exposure equipment because the generation of speckle patterns becomes a serious problem. do not have. .

Another promising technique for narrowing the band of excimer lasers is the use of an etalon, which is a wavelength selection element. As a conventional technique using this etalon, AT&T Bell Laboratories has proposed a technique in which an etalon is disposed between the front mirror of an excimer laser and a laser chamber to narrow the band of the excimer laser. However, this method cannot narrow the spectral linewidth sufficiently;
In addition, there is a problem that power loss due to etalon insertion is large, and there is also a drawback that the number of spatial transverse modes cannot be increased very much.

Therefore, the inventors adopted a configuration in which an etalon with a large effective diameter (about several tens of diameters) is placed between the excimer laser beam mirror and the laser chamber, and with this configuration, 20
A laser beam with an output of about 50 mJ per pulse is obtained by narrowing the spectrum so that the full width at half maximum is about 0.003 nllll or less in the range of x10rRIn2. In other words, by adopting a configuration in which an etalon is placed between the excimerade mirror and the laser chamber, the light source of the reduction projection exposure system can narrow the laser band, secure the number of spatial transverse modes, and reduce power loss due to the insertion of the etalon. be able to solve essential problems as required.

However, the configuration in which an etalon is placed between the excimer laser mirror and the laser chamber has excellent advantages in terms of narrowing the band, securing the number of spatial transverse modes, and reducing power loss, but the power transmitted through the etalon is As it becomes very large, physical changes such as temperature fluctuations occur in the etalon,
For this reason, there have been problems in that the center wavelength of the oscillated output laser light fluctuates and the power is significantly reduced. This tendency became particularly noticeable when two or more etalons with different fleece vector ranges were used to narrow the band.

[Faculty FA to be solved by the invention] By the way, if an etalon is arranged during this optical resonance, light with extremely high energy density will pass through this etalon, so the etalon will be damaged over a long period of time. surface accuracy,
Finesse was sometimes reduced due to deterioration of parallelism or damage to the reflective film.

When the finesse of the etalon decreases, large sideband waves are generated, resulting in multi-wavelength oscillation, and when this laser light is used as a light source for reduction projection exposure, there arises the problem that the resolution is greatly reduced.

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

(Means for Solving the Problems) Therefore, in the present invention, in a narrow band oscillation excimer laser, the output power of the laser and the center wavelength power are detected, and it is determined whether the ratio of these is within an allowable range. By doing so, an abnormality in the spectral wavelength is detected.

[Operation] With the above configuration, the center wavelength and sideband waves are separated by the diffraction grating spectrometer or 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 the allowable range is determined. This makes it possible to easily and reliably detect abnormalities in spectral wavelengths.

If the ratio is outside the allowable range, an abnormal signal is generated as a wavelength abnormality.

[Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention. In this embodiment, two etalons 101 and 102 are arranged between the laser chamber 107 and the mirror 106.

The apparatus of this embodiment includes a power control system 200 that controls the laser output power by a component control wJ83 of the laser vX'fl gas in the laser chamber 107 and an excitation intensity control I (discharge voltage control O) of the laser medium, and a laser output center Center wavelength control and etalon 101 to control wavelength
A wavelength control system 300 that simultaneously or alternately performs superposition control that performs alignment of the transmission center wavelength with the wavelength control system 300
It has

First, the operations of the power control system 200 and the wavelength control system 300 in a steady state will be described. The properties of the laser medium gas used in the ximer laser gradually deteriorate over time, and the laser power decreases.

Therefore, in addition to controlling the excitation intensity of the laser medium, that is, the discharge voltage, the excitation intensity control system 200 performs output control to keep the laser output constant by controlling the components of the laser medium, that is, by performing gas exchange. That is, as shown in FIG. 1, a part of the oscillated laser light is split by the beam splitter 104 and enters the power monitor 202, and changes in laser power are monitored. Output control is performed to keep the laser output constant by changing the excitation intensity or partially exchanging the laser medium gas via the gas controller 205.

Also, a part of the oscillated laser light is transmitted to the beam splitter 1.
The sample light is branched at 03 and added to the oscillation center wavelength and center wavelength power detector 301. The oscillation center wavelength and center wavelength power detector 301 detects the oscillation center wavelength λ and the power Pλ of the center wavelength of the excimer laser 10 contained in the sample light. This oscillation center wavelength and center wavelength power detector 301 is composed of a diffraction grating type spectrometer 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 spectrometer 34 consisting of a concave mirror 31, 32J3 and a diffraction grating 33, and an optical position sensor 35. The sample light sampled by the beam splitter 103 is collected by the lens 36 and inputted from the entrance slit 37 of the spectroscope 371 . This entrance slit 3
The light input from 7 is reflected by the concave mirror 31, becomes parallel light, and is irradiated onto the diffraction grating 33. The diffraction grating 33 is fixed at a predetermined angle here, and reflects the incident light at a diffraction angle corresponding to the wavelength. This diffracted light is guided to the four surfaces m32, and the reflected light from the concave mirror 32 is guided to the optical position lens 35 where it is imaged. That is, a diffraction image of the entrance 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 determined from the position of the diffraction image of the entrance slit 37. can be detected. Further, the center wavelength power Pλ can be detected from the light intensity of the diffraction image of the incident slit 37.

Note that as the optical position sensor 35, a first diode array, a light spot position detector PSD (Positive Sensitive Detector), or the like can be used. 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 with the maximum light intensity, and the center wavelength power is the light intensity of the channel corresponding to the center wavelength or the channel near the center wavelength. It is detected from the sum of the light intensities. Also, the optical position sensor 35
When a PSD is used as the PSD, 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 the central processing unit I! that constitutes the wavelength controller. I! Equipment (C
PU)3o2.

The CPU 302 controls the wavelength selection characteristics (transmission center wavelength and selection center wavelength) of the etalons 101 and 102 via the drivers 303 and 304, so that the center wavelength of the sample light, that is, the output light of the excimer laser, is a preset desired wavelength. (center wavelength control) so that the center wavelength power is maximized (superposition control). Here, the wavelength selection characteristics of the etalons 101 and 102 are controlled by the drivers 303 and 304 by controlling the humidity of the etalon 4, controlling the angle,
Control the pressure in the air gap, control the gap spacing! I
etc.

Specifically, the center wavelength control is performed by controlling the angle of at least the etalon 101 having a smaller free spectral range among the etalons 101 and 102, and shifting the transmission wavelength of the etalon, thereby changing the output center wavelength, that is, the oscillation wavelength. Center wavelength and center wavelength power detector 30
1 to control the desired wavelength. In addition, the superposition control shifts the transmission center wavelength of etalons other than the above-mentioned etalon 101 with a smaller free spectral range, that is, the etalon 102 with a larger free spectral range, by a predetermined unit wavelength.
Control is performed so that the transmission center wavelengths of 02 overlap and the center wavelength power detected by the oscillation center wavelength and the center wavelength power detector 301 becomes maximum.

By the way, the oscillation center wavelength and center wavelength power detector 30
As shown in FIG. 3, the size of the light receiving surface of the first optical position sensor 35 is set so as to detect only the center wavelength (so that even if a side peak occurs, it will not be detected). This is to avoid detecting side peaks. That is, when the etalons 101 and 102 are improperly overlapped, a side peak occurs, and if the optical position sensor 35 detects this side peak, it becomes impossible to control the overlay and also makes it impossible to accurately control the center wavelength. Become. Therefore, the size of the light receiving surface of the optical position sensor 35 is limited so as not to detect the side peak even if the side peak occurs. In general, side peaks occur when the etalons 101 and 102 are improperly overlapped, but the etalons 101 and 102 are stacked so that almost no side peaks occur when the etalons 101 and 102 are perfectly overlapped.

However, the surface roughness and parallelism of etalons 101 and 102 deteriorated, and the anti-11 film was damaged, resulting in etalons 101 and 102.
If the finesse of 102 is reduced, side peaks may occur even if the superposition is perfect. Next, this phenomenon will be explained.

First, the small etalon 10 with fleece vector range.
1 and 7 Lee Specj・Larrange large etalon 10
2 is normal. In this case, when the etalons 101 and 102 are completely overlapped, almost no side peaks occur as shown in FIG. 4 [a]. However, when the finesse of the etalon 102, which has a large free spectral range, decreases due to the deterioration of the surface viscosity, the etalon 102
As a result, the transmission wavelength becomes wider, and therefore multiple side peaks occur as shown in FIG. 4(b), resulting in multi-wavelength oscillation. The same applies when the finesse of the etalon 101 decreases.

When a laser emits multi-wavelength oscillation in this way, the resolving power is significantly reduced even if this laser light 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 this value is within a predetermined tolerance range. The wavelength abnormality signal is output when the wavelength is outside the permissible range. By indirectly detecting the sideband waves in this way, it is possible to easily detect the side peaks of this multi-wavelength oscillation, and it becomes possible to detect a decrease in this resolution.

In this way, in this embodiment, the oscillation center wavelength and the output of the center wavelength power detector 301 and the power monitor 202
The ratio between the laser output power and the center wavelength power is determined from the output of the laser beam, and by monitoring this ratio, it is possible to detect whether or not a side peak is occurring.

If a side peak is detected here, CPU2
03 generates a wavelength abnormality signal. This wavelength abnormality signal is sent to an exposure device (not shown).

Next, the operation of this wavelength abnormality detection will be explained using a flowchart.

FIG. 5 shows the main flow of the apparatus of this embodiment. In this operation, it is first determined whether the elapsed time T has exceeded a predetermined time α (step 401), and if it has not elapsed, the process moves to step 402 and normal control is performed, that is, Executes 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 moves to a sideband detection subroutine 403.

This sideband detection subroutine 403 is shown in FIG.

In the sideband detection subroutine 403, first, the laser output power Pt and the center wavelength power Pλ are read into the CPU 203 from the output of the center wavelength power detector 301 and the output of the power monitor 202, respectively (step 50
1).

Next, the CPU 203 calculates the ratio P=Pλ/Pt between the Rade output power pt and the center wavelength power Pλ (step 502).

Then, it is determined whether this ratio P is larger than a predetermined value (step 503), and if it is larger, that is, if the center wavelength power Pλ is sufficiently larger than the laser output power Pt, it is determined that there is no abnormality and this subroutine end. In this way, the sideband detection subroutine 40
3, the time T is reset to 0 and the process returns to step 401.

On the other hand, in the judgment step 503, if this ratio P is smaller than the predetermined value k, that is, if the center wavelength power Pλ is not sufficiently large with respect to the laser output power Pt, the shutter is closed (step 504).
, outputs a wavelength abnormality signal (step 505).

Thereafter, the process returns to judgment step 401 (FIG. 5) in the same manner.

In this way, by periodically transitioning to the sideband detection subroutine 403 to monitor sidebands, it is possible to reliably detect multi-wavelength oscillation due to a decrease in finesse of the etalon. can be prevented in advance.

In the above embodiment, the case where two etalons are used is described, but the same configuration can be made using three or more etalons.

Further, in the above embodiment, a case was explained in which a diffraction grating type spectrometer was used to detect the center wavelength power, but as shown in FIG. The interference light may be separated into interference light due to the center wavelength and interference light due to sideband waves, and guided to the optical sensor 43 for detection. Here, for the monitor etalon, the difference between the sideband wave and the center wavelength Δλs id
e and monitor etalon fleece veritral range FSR
Ilonitor and Δλ5ide′f:F S Rl1onitor(n=
1.2. ...), it is possible to separate the interference light due to the center wavelength and the interference light due to the sideband waves.

〔Effect of the invention〕

As explained above, according to the present invention, the output power of the laser and the center wavelength power are detected, and it is determined whether the ratio of these is within the allowable range.
- Multi-wavelength oscillation can be reliably detected with an extremely simple configuration, and this prevents unexpected decreases in resolution when used as a light source for reduction projection exposure. I can do it.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a diagram without details of the oscillation center wavelength and center wavelength power detector, and Fig. 3 is a block diagram showing the oscillation center wavelength and center wavelength power detector shown in Fig. 2. FIG. 4 is a diagram showing the detection state of the wavelength power detector, FIG. 4 is a diagram showing the state of occurrence of side beak in this embodiment, FIGS. 5 and 6 are flowcharts explaining the operation of this embodiment, and FIG. 7 is a diagram showing the detection state of the wavelength power detector. FIG. 3 is a diagram showing another example of the center wavelength power detector. 101.102...Etalon, 103...Beam splitter, 200...Power control system, 203...C
PU, 204...Laser power supply, 205...Gas controller, 3oo...Wavelength control system, 302...CP
U, 303.304...driver, 31.32...
Concave mirror, 33... Diffraction grating, 34... Diffraction grating type spectrometer, 35... Optical position sensor number, 36... Lens, 3
7... Input slit, 41... Optical fiber, 42...
...Monitor etalon, 43...light sensor. 200 power 卍I wholesale^ -L m Fig. 1 Fig. 3 Fig. 1 chamber wavelength (a) 200 power moxibustion length (/V) Fig. 4 (b) Fig. 5 Nol 1F-Manzai Connecting fiber; L-run Figure 6 41 Optical fiber 8 43 Figure 7

Claims (1)

[Claims] A narrowband oscillation excimer laser configured to include at least two etalons in a resonator of a laser oscillator and to control the wavelength selection characteristics of the etalons, comprising: output power detection means for detecting; center wavelength power detection means for detecting center wavelength power; and a ratio between the output power detected by the output power detection means and the center wavelength power detected by the center wavelength power detection means. 1. A wavelength abnormality detection device for a narrowband oscillation excimer laser, comprising abnormality detection means for detecting a spectrum abnormality of a laser beam by monitoring whether the spectrum is within a preset allowable range.