JP2652170B2 - Excimer laser wavelength controller - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a wavelength control device for an excimer laser,
In particular, the present invention relates to a wavelength control device of 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 due to the short wavelength of the excimer laser (the wavelength of the KrF laser is about 248.35 nm)
Therefore, the limit of light exposure can be extended to 0.5 μm or less, the depth of focus is deeper than the conventional mercury lamp g-line and i-line at the same resolution, and the numerical aperture (NA) of the lens This is because many excellent advantages can be expected, such as a small size, a large exposure area, and a large power.

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 as short as 248.35 nm, so the only material that transmits this wavelength is quartz, CaF2, MgF2, etc., and only quartz can be used as the lens material for uniformity and processing accuracy. . For this reason,
It becomes impossible to design a reduction projection lens with chromatic aberration corrected. Therefore, it is necessary to narrow the band of the excimer laser to such an extent that this chromatic aberration can be ignored.

Another problem is how to prevent a speckle pattern generated with the narrow band of the excimer laser and how to reduce the power with 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 between a laser chamber in an oscillator stage and a rear mirror, and a spatial mode is limited by a pinhole to oscillate a single mode. The laser light is injection-locked by the amplification stage. For this reason, the output light has high coherence. When this light is used as a light source of a reduction exposure apparatus, a speckle pattern is generated.

It is generally considered that the generation of a speckle pattern depends on the number of spatial transverse modes included in a laser beam. That is, it is known that when the number of spatial transverse modes included in the laser beam is small, a speckle pattern is easily generated, and when the number of spatial transverse modes is large, the speckle pattern is unlikely to be generated. . The above-described injection lock method is essentially a technique for narrowing the bandwidth by significantly reducing the number of spatial transverse modes. Since the occurrence of a speckle pattern is a major problem, it cannot be used in a reduced projection exposure apparatus. .

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, this method has a problem that the spectral line width cannot be reduced too much and the power loss due to the etalon insertion is large.
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 two etalons E1 and E2 having a large effective diameter (about several tens of mm) are arranged between the rear mirror 1 of the excimer laser and the laser chamber 2 as shown in FIG. According to the configuration, a narrow band of a spectrum width of about 0.003 nm or less at full width at half maximum in a range of 20 × 10 nm ² is obtained, and a narrow band oscillation excimer laser beam with an output of about 50 mJ per pulse is obtained. In other words, by employing a configuration in which an etalon is arranged between the rear mirror of the excimer laser and the laser chamber, the reduction in the bandwidth of the laser, the securing of the number of spatial transverse modes, and a reduction in power loss due to insertion of the etalon reduce the size of the reduced projection exposure apparatus. It solves an essential problem required as a light source.

However, the configuration in which the etalon is arranged between the rear mirror of the excimer laser and the laser chamber has excellent advantages in narrowing the band, securing the number of spatial transverse modes, and reducing power loss, but the power transmitted through the etalon is low. Since the etalon becomes extremely large, a physical change such as a temperature change occurs in the etalon, which causes a problem that the center wavelength of the oscillation output laser light fluctuates, multi-wavelength oscillation occurs, and the power is significantly reduced. This tendency is particularly remarkable when two or more etalons having different free spectral ranges are used for narrowing the band.

FIG. 5 schematically shows a decrease in the power of the output laser light whose band has been narrowed as described above, in which the relationship between the transmission wavelength spectrum of the etalon E2 and the actual laser oscillation spectrum is shown by etalon E1. It is shown under the relationship between the superposition of E2 and E2. In FIG. 5, the dotted line shows the transmission spectrum of etalon E2, and the solid line shows the actual laser oscillation spectrum. ΛE1 indicates the transmission center wavelength of the etalon E1, and λE2 indicates the transmission center wavelength of the etalon E2. As is clear from FIG. 5, the state shown in FIG. 5 (b) in which the transmission center wavelength λE1 of the etalon E1 and the transmission center wavelength λE2 of the etalon E2 are completely overlapped provides the maximum output power max Pλ. It can be seen that the maximum output laser light power cannot be obtained in the state of FIG. 5 (a) or FIG. 5 (c) where the alignment is insufficient.

Therefore, the center wavelength power of the output laser light having a narrow band is detected, and the angle control (superposition control) of the two etalons is performed so that the transmission center wavelengths overlap each other so that the power is maximized. The maximum power can always be obtained.

[Problems to be solved by the invention]

However, as shown in FIG. 4, when the etalon E1 and the etalon E2 are arranged between the rear mirror 1 and the laser chamber 2, the excimer laser has a high gain, and thus has a problem that parasitic oscillation easily occurs. Etalon
When the angles θ1 and θ2 of E1 and E2 meet certain conditions, the reflected light at the etalons E1 and E2 or the reflected light between the two etalons is reflected by the front mirror 3 as shown by broken lines A and B in FIG. It passes through the oscillation region. Therefore, a plurality of parasitic oscillation lights P1 and P2 are generated for the main oscillation light indicated by the solid line ELZ. When this parasitic oscillation light occurs, the output laser light contains many wavelength components as shown by the spectrum characteristics of the solid line A in FIG. 6, and the output laser light when no parasitic oscillation shown by the broken line B occurs. , The wavelength purity is extremely reduced. In addition, as shown in FIG. 7, the beam cross section of the output laser light includes the main oscillation light, which includes many parasitic oscillation lights P1 and P2 whose optical axes are shifted with respect to the ELZ, so that the line width is widened. In addition, the power becomes unstable, and the performance such as the resolution of the reduction projection exposure apparatus is deteriorated.

An object of the present invention is to provide an excimer laser wavelength control device capable of obtaining a narrow band excimer laser beam having high wavelength purity and stable power.

[Means and actions for solving the problems]

In order to achieve the above object, the present invention provides an excimer laser wavelength control device that performs narrow-band oscillation by arranging at least two etalons between a rear mirror and a laser chamber; The etalons are arranged so that reflected light repeatedly reflected between the etalons does not enter an oscillation beam passage area of a front mirror of the excimer laser.

Here, the etalon comprises two etalons of a first etalon and a second etalon, and a first plane including a normal to a light incident surface of the first etalon and a laser optical axis of the excimer laser. And a normal to a light incident surface of the second etalon and a second plane including a laser optical axis of the excimer laser are parallel to each other, and the first etalon and the second etalon with respect to the laser optical axis.
The first etalon and the second etalon can be arranged so that the angles of the etalons are inclined in directions different from each other.

Also, the present invention provides a wavelength control of an excimer laser in which at least two etalons are arranged between a rear mirror and a laser chamber, and a wavelength of an output laser beam is controlled by controlling an angle of the etalon with respect to a laser optical axis. In the apparatus, wavelength detection means for detecting the wavelength of the output laser light, wavelength control means for controlling the angle of the etalon with respect to the laser optical axis in accordance with the detection result of the wavelength detection means, reflection by the etalon The wavelength control means regulates each control angle of the etalon with respect to the laser optical axis so that the reflected light that has been reflected or the light that has been repeatedly reflected between the etalons does not enter the oscillation beam passage area of the front mirror of the excimer laser. And a control angle regulating means.

By doing so, parasitic occurrence does not occur, the wavelength of the output laser light becomes uniform, and the power of the output laser light becomes stable.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention.
A rear mirror 1 and a front mirror 3 are arranged on the laser optical axis at both ends of the laser chamber 2. Further, the laser beam between the rear mirror 3 and the laser chamber 1
Two etalons E1, E2 are arranged next to each other.

On the other hand, a beam splitter 7 is provided on the output side on the optical axis of the excimer laser light that is output after being narrowed by the two etalons E1 and E2.
The narrow-band excimer laser light separated by the above is input to a wavelength detector 10 via a lens 8 and an optical fiber 9, and the center wavelength and the power of the center wavelength are detected.

A detection signal of the center wavelength and the center wavelength power of the output laser light detected by the wavelength detector 10 is input to a controller 11 composed of a microcomputer or the like. The controller 11 determines the center wavelength λ of the output laser light and its power P.
Is input, the deviation Δλ between the detected center wavelength λ and the set wavelength λ0 is calculated, and the angles θ1 and θ of the etalons E1 and E2 via the driver 12 so that the deviation Δλ becomes zero.
2 is controlled. Thereafter, the angles θ1 and θ2 of the etalons E1 and E2 are controlled so that the calculated power P of the center wavelength becomes maximum.

Here, the wavelength detector 10 is configured by a spectroscope, a monitor etalon, and the like.

The angles of the etalons E1 and E2 are controlled by stepping motors 13 and 14, respectively.

By the way, when controlling the angles θ1 and θ2 of the etalons E1 and E2, the controller 11 controls the angles so that the reflected light from each etalon or the reflected light between the etalons does not pass through the oscillation region of the front mirror 3. I do. That is, the angle is controlled so that parasitic oscillation does not occur.

2 (a) and 2 (b) are etalons in which no parasitic oscillation occurs
It is explanatory drawing which shows the conditions of the angle of E1, E2, 20 shows the virtual light source of a parasitic oscillation, l is the distance from the etalon E1 to the front mirror 3, d is the distance between the etalons E1 and E2, a is the front mirror 3 is the outer diameter of the oscillation region, x is the distance from the etalon E1 to the virtual light source 20 of the parasitic oscillation, b is the outer diameter of the virtual light source 20, θ1 is the inclination angle of the etalon E1, and θ2 is the etalon E2
Indicates the oscillation angle of the parasitic oscillation.

In the parasitic oscillation, the reflected light of the etalons E1 and E2 itself or the reflected light between the etalons is generated by the oscillation region a of the front mirror 3.
In order to prevent this parasitic oscillation from occurring, the following condition should be satisfied: ltan θ1> a + b / 2 (ld) tan θ2> a + b / 2 (x + e) tanγ> a + b / 2.

 Here, when θ2> 0, γ = 2 (θ2-θ1) x = dsin2θ2 / sin2 (θ2-θ1).

For example, when d = 75 mm, e = 1130 mm, a = 10 mm, and b = 10 mm, parasitic oscillation occurs when θ1 and θ2 become the angles shown by the hatched portions in FIG. 2 (b). It is sufficient to control θ1 and θ2 in the area other than the part.

Therefore, inside the controller 11, means for regulating the angle of the hatched portion in FIG. 2 (b) as a prohibited area is incorporated in the control procedure.

FIGS. 3A and 3B are flowcharts showing the etalon angle control process executed by the controller 11, and the output laser light detected by the wavelength detector 10 in step 30 of the main program of FIG. 2A. The detection signal of the center wavelength and its power is read, and in the next step 31, etalons E1 and E
Then, the shift amounts Δλ1 and Δλ2 of the transmission center wavelength 2 are calculated.
Next, in step 32, an etalon angle control subroutine is started. This subroutine is shown in FIG. 3 (b). First, in step 320, the shift amount Δλ
1, Δλ2 is multiplied by angle conversion coefficients k1 and k2, and Δλ1, Δλ2
Are converted into angle shift amounts Δθ1 and Δθ2. Next, in step 321, the current tilt angles θ10,
After adding Δθ1 and Δθ2 to θ20 to calculate angles θ1 and θ2 as control targets, in the next step 322, it is checked whether θ1 and θ2 correspond to the angle of the prohibited area, and the angle of the prohibited area is checked. Otherwise, in the next step 324, Δθ1, Δ2 are set so that the angles of the etalons E1, E2 match
Tilt by θ2.

However, if the angle corresponds to the angle of the forbidden area, the free spectral ranges Δλ1FSR and Δλ2FSR of the etalons E1 and E2 and the angle conversion coefficients k1 and k2 with respect to the angle shift amounts Δθ1 and Δθ2 calculated in step 320 in step 323. , And the result of the addition is calculated as a new shift amount Δθ1,
The value is set to Δθ2, and the process returns to step 321.

As a result, a new target angle is calculated at an angle shifted by an angle corresponding to the free spectral range of the etalons E1 and E2, and the angle of the etalons E1 and E2 is set so as to coincide with the target angle unless the target angle is a prohibited area. Is controlled.

The angles θ1 and θ2 of the etalons E1 and E2 are shown in FIG.
Defines the angle between the transmission center axis of the etalons E1 and E2 and the output laser optical axis. The axis perpendicular to the transmission center axis of the etalons E1 and E2 (the axis in the depth direction in the figure) and the output A configuration in which the angle is defined by the angle of the laser beam with respect to the beam cross section and this angle is controlled, or a configuration in which the angle is defined in combination with the angle defined in FIG.

Alternatively, a limit switch or the like may be provided in the etalon holder, the angle at which parasitic oscillation occurs is detected by the limit switch, and the angle control of each etalon may be performed so that parasitic oscillation does not occur.

〔The invention's effect〕

As described above, in the present invention, the means for regulating the angle of the wavelength selection element is provided so that the reflected light from the wavelength selection element itself or between the wavelength selection elements does not enter the oscillation region of the front mirror. Parasitic oscillation due to light can be prevented. This has the effect of increasing the wavelength purity of the output laser light and stabilizing its power. Therefore, if the present invention is applied to the light source of the reduction projection exposure apparatus, stable exposure with high resolution can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is an explanatory diagram for explaining conditions that do not cause parasitic oscillation,
FIG. 3 is a flowchart showing an etalon angle control process;
FIG. 4 is a block diagram showing the configuration of a conventional excimer laser device, FIG. 5 is an explanatory diagram for explaining a phenomenon in which the power of the narrowed excimer laser light is reduced, and FIG. FIG. 7 is a characteristic diagram showing a spectrum characteristic of the output laser light when the laser beam is illuminated, and FIG. E1, E2 etalon, 1 rear mirror, 2 laser chamber, 3 front mirror, 4, 5 emission window, 7
…… Beam splitter, 10 …… Wavelength detector, 11 …… Controller, 12 …… Driver, 13,14 …… Stepping motor.

Claims (3)

(57) [Claims] 1. A wavelength control device for an excimer laser which performs narrow band oscillation by arranging at least two etalons between a rear mirror and a laser chamber, wherein the light reflected by the etalon or the light reflected repeatedly between the etalons is provided. An excimer laser wavelength control apparatus, wherein the etalons are arranged so that reflected light does not enter an oscillation beam passage area of a front mirror of the excimer laser. 2. The etalon comprises two etalons, a first etalon and a second etalon, a first etalon including a normal to a light incident surface of the first etalon and a laser optical axis of the excimer laser. Plane, a normal to the light incident surface of the second etalon and a second plane including the laser optical axis of the excimer laser are parallel to each other, and the first etalon and the 2. The wavelength control device for an excimer laser according to claim 1, wherein the first etalon and the second etalon are arranged so that the angle of the second etalon is inclined in directions different from each other. 3. An excimer laser wavelength controller for arranging at least two etalons between a rear mirror and a laser chamber, and controlling an angle of the etalon with respect to a laser optical axis to control a wavelength of output laser light. In the above, wavelength detection means for detecting the wavelength of the output laser light, wavelength control means for controlling the angle of the etalon with respect to the laser optical axis corresponding to the detection result of the wavelength detection means, reflected by the etalon The control angle of each of the etalons with respect to the laser optical axis by the wavelength control means is controlled so that the reflected light or the reflected light repeatedly reflected between the etalons does not enter the oscillation beam passage area of the front mirror of the excimer laser. Excimer array characterized by comprising a control angle restricting means. The wavelength control device.