JP3019411B2 - Wavelength stabilizer for narrow band laser - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength stabilizing device for a narrow band laser, and more particularly to a wavelength stabilizing device for an excimer laser emitting ultraviolet light.

[Prior Art] FIG. 8 shows a magazine (OPTICS AND LASER THCHNO
LOGY AUGUST 1986 p187) is a configuration diagram showing a conventional wavelength stabilizing device for a narrow band laser. In the figure, 1 is a laser beam, 2 is an Ar laser for wavelength calibration, 3
Is an etalon, 4, 5, and 6 are lenses for creating interference fringes, 7 is an interference fringe, 8a and 8b are photodetectors, 9a and 9b are differential amplifiers, 10a and 10b are etalon controllers, 11a and 11b are piezos The element 12 is an etalon for a narrow band.

Next, the operation of the wavelength stabilizing device for a narrow band laser will be described.

The beam 1 of the narrow band laser and the beam of the Ar laser 2 enter the etalon 3 respectively. For the emitted beam, only a specific angle component is selected by the etalon. When this beam passes through the lens 5, the diameter of the beam is 2fθ at the focal position of the lens.
(F is the focal length and θ is the emission angle of the beam). The lens 4 is provided because the laser has only a specific angle component, and is used to generate various angle components from a parallel beam by converging once. It can be replaced with a scattering plate. The lens 6 is a magnifying lens used to increase the equivalent focal length when considered in combination with the lens 5 to increase the diameter of interference fringes.

First, consider the upper half system in the figure. The interference fringe generated by the Ar laser 2 is detected by the optical sensor 8a. In this detector, the light receiving surface is divided into two, and each light receiving surface is electrically coupled to the differential amplifier 9a. If the position of the interference fringes is shifted to one of the light receiving surfaces, the differential amplifier The output appears at Based on this output, the etalon controller 10a adjusts the voltage applied to the piezo element 11a, and changes the etalon gap distance until the bias of the interference fringe is eliminated. As a result, the inter-gap distance of the etalon is kept constant. On the other hand, in the system in the lower part of the figure, the angle of the etalon 12 provided in the resonator is changed in order to select the wavelength of the narrow band laser based on the interference fringe caused by the laser beam. This operation is automatically continued until the laser beam illuminates the two light receiving surfaces of the photodetector 8b without bias.

If you want to set the wavelength of the narrow band laser to the desired wavelength, observe the wavelength with another spectrometer, and move the position of the photodetector when the desired wavelength is reached so that the interference fringes illuminate the light receiving surface without bias Do it.

[Problems to be Solved by the Invention] The conventional wavelength stabilizing device for a narrow band laser is configured as described above, and uses a bright and sharp wavelength of an Ar laser. In addition, it is necessary to control the gap interval of the monitor etalon with high accuracy and to keep the position of the light receiving element stable. Furthermore, when an excimer laser is used as a narrow-band laser, the interference fringes can be generated unless the difference in refractive index due to the wavelength or the change with environmental changes such as temperature and pressure is accurately grasped because the wavelength is significantly different from that of the Ar laser. Even if it is brought to a specific position, there is a problem that the wavelength cannot be changed to a desired wavelength, and a problem that the wavelength shifts due to an environmental change occurs.

As a result, when it is desired to shift the desired wavelength, another high-precision spectroscope is required, and the etalon for monitoring has to be put in a thermostat to reduce environmental changes.

Further, in a wavelength control device of a laser device disclosed in Japanese Patent Application Laid-Open No. Sho 64-22086, the interference fringe by the calibration light source and the interference fringe by the laser are controlled to be equal to each other or controlled at a constant interval. However, also in this case, if the etalon for monitoring changes, the wavelength goes wrong even if the position of the interference fringe is the same, so that it is necessary to keep the environment of the etalon stable.

The present invention has been made in order to solve the above problems, and a wavelength stabilizing device for a narrow band laser capable of stably maintaining a laser wavelength at a desired wavelength even with environmental changes. The purpose is to obtain.

[Means for Solving the Problems] A wavelength stabilizing device for a narrow band laser according to the present invention comprises:
Two diameters of the interference fringes generated by the calibration light source is measured, by calculating e _H below from below equation (2), using the below equation (3) from the wavelength that is required with this e _H Calculated value e
_L (calculation) is calculated, two diameters of the interference fringes generated by the laser are measured, and a measured value e _L (measurement) to be described later is calculated from the measured two diameters according to equation (2). Then, there is provided means for controlling the oscillation wavelength of the laser so that the difference between the measured value e _L (measurement) and the calculated value e _L (calculated) becomes equal to or smaller than a predetermined value.

Further, a calibration light source having a wavelength in a range that satisfies the effective number of the refractive index corresponding to the required wavelength accuracy of the laser is provided, and when the oscillation wavelength of the laser is calibrated by the spectroscope,
The diameter of the interference fringes generated by the calibration light source is D _H ′, the diameter of the interference fringes generated by the laser is D _L ′, and the diameter of the interference fringes generated by the calibration light source during subsequent laser operation is D _H , when the diameter D _L of the interference fringes generated by, those having a means for controlling the oscillation wavelength of the laser so that the ^{_{D H 2 -D H '2 =}} D L 2 -D L' 2.

[Operation] In the wavelength stabilizing device for a narrow band laser according to the present invention, the diameter of the interference fringes according to the calibration light source and the diameter of the interference fringes generated by the laser are measured two by two, a total of four diameters. Since the oscillation wavelength is controlled, even if the arrangement or environment of the optical system of the wavelength stabilizing mechanism is slightly changed, it does not affect the determination of the laser wavelength.

In addition, the diameter of the interference fringe caused by the calibration light source and the diameter of the interference fringe caused by the laser generated when the laser oscillation wavelength is calibrated in advance are measured. Controls the oscillation wavelength of the laser, the diameter of the interference fringe measured during laser operation is small, and the laser wavelength can be accurately maintained at a desired wavelength by a simple measuring means.

Example An example of the present invention will be described below. In FIG. 1, reference numerals 1 to 6 function similarly to the conventional example. The light source 2 for calibration uses a low-pressure mercury lamp. 13 is an image sensor for reading interference fringes, 14 is an optical fiber, 15 is an interference fringe analyzer, 16 is a controller, and 17 is an etalon controller.

Next, the operation will be described. When the beam 1 of the excimer laser guided from the optical fiber 14 and the light of the low-pressure mercury lamp 2 pass through the etalon 3, only the light having a specific angle component is generated. This light is collected by the lenses 5 and 6 and forms interference fringes on the image sensor. FIG. 2 shows the light intensity distribution of the interference fringes appearing as the output of the image sensor 13, wherein (A) shows the interference fringes when the mercury lamp is turned off and only the laser beam is used, and (B) shows the mercury fringes. This is an interference pattern when only the lamp is turned on. DL and DH indicate diameters of interference fringes by a laser and a mercury lamp, respectively. According to a textbook on optics (for example, the principle of (Born & Wolf) optics), when the diameter of an interference fringe is D, the following equation is established.

D ² = 4λf ² (p−1 + e) / n · d (1) where λ is the wavelength, nd is the optical distance between the etalon gaps, and p is the p-th interference fringe from the center, e is a number called a fraction. If the image sensor 13 is arranged so that the diameters of a plurality of interference fringes can be measured, the following calculation is performed in the interference fringe analysis unit 15 based on the measurement results to determine e.

e = ((D (P = 2) / D (P = 1)) 2 -1)
^-1 (2) On the other hand, for a desired wavelength λ of the laser, e _L (e for the laser) can be calculated from the following equation.

m is the order of the interference fringes with respect to the wavelength, n is the refractive index, the subscript H indicates mercury, and L indicates the laser. Incidentally, e _H measures the interference fringes for a mercury lamp, a value obtained from equation (2).

In other words, if the image sensor 13 is used as the interference fringe measuring means and a plurality of interference fringes can be measured, even if the desired wavelength slightly changes (to the extent that the order m does not change), the calculation of the equation (3) is performed. calculated values e _L more demanded, compares the measured values e _L obtained from the from the measurement of the diameter (2) in an interference fringe analysis unit 15, the wavelength of the narrow band laser as above e _L is equal By changing the wavelength, it is possible to obtain a desired wavelength without a separate spectroscope.

FIG. 3 is a flowchart for explaining the operation of the wavelength stabilizing device for a narrow band laser according to one embodiment of the present invention;
In step S1 Measure the diameter of the interference fringes of the calibration light source 2, the e _H of the calibration light source in Step S2 (2) calculating from the equation. In step S3 using this e _H (3) to calculate the e _L from the equation. Next, in step S4, a laser interference fringe is measured,
In step S5, e _L is calculated using equation (2). Steps
In S6, the difference between e _L (referred to as e _L (calculation)) obtained in step S3 and e _L (referred to as e _L (measurement)) obtained in step S5 is taken, and this difference is sufficiently small. Find out if. If not, change the wavelength of the laser in step S7,
Returning to step S4, the diameter of the interference fringes is measured. If it is smaller, it is checked in step S8 whether there is a stop command. If not, in step S9, it is checked whether the operation time is smaller than t ₀ (a value obtained from a time constant of an atmosphere change during laser operation). That is, the diameter of the interference fringes of the calibration light source is measured again after a lapse of time that causes a change in atmosphere, and the diameter of the interference fringes of the laser is measured again within the above time.

Now, consider a case where calibration is performed by an Ar laser as a calibration light source as in the conventional example. As shown in FIG. 4, the refractive index differs depending on the wavelength. In particular, in an ultraviolet laser such as an excimer laser, the change in n is large, and there is a difference of 2 × 10 ^{−5 from} the wavelength n of the Ar laser. Therefore, unless the wavelength change of the refractive index is obtained very accurately, the wavelength accuracy can be guaranteed only about 5 digits.

On the other hand, when a mercury lamp (wavelength 253 nm) is used with respect to a KrF excimer laser (248 nm), the difference in refractive index is smaller by another digit, and the wavelength accuracy can be reduced to 6 digits or less. Similarly, for an ArF excimer laser (193 nm), a mercury lamp of 185 nm
May be used. In order to guarantee the accuracy of six digits as described above, it is preferable to use a light source having a wavelength (for example, about 300 nm in this case) having the same refractive index in the fifth digit from FIG. Although a mercury lamp has been described here, other light sources such as an iron hollow cathode lamp (248 nm) can be used as long as the wavelength is close to that of a laser. If a light source having a similar wavelength is prepared for other narrow band lasers, the wavelength can be calibrated by the same operation.

As described above, when the wavelength of the laser and the wavelength of the light source for calibration are close to each other, improvement in accuracy can be expected. Further, in this case, the following parameter Q can be introduced instead of the equations (2) and (3).

Q = (D _L1 ² −D _H1 ² ) / (D _H2 ² −D _H1 ² ) (4) where D _L1 is the diameter of the first interference fringe to the laser, D
_H1 and D _H2 are the diameters of the first and second fringes, respectively, for the mercury lamp. On the other hand, Q = m _H + m _L λ / λ _H (5) Here, with respect to Q expected from the equation (5), Q obtained from the equation (4) using observation data of the diameter of the interference fringe is If the wavelength of the narrow band excimer laser is changed so as to be equal, the wavelength can be set to a desired value. In this case, only one interference fringe of the laser needs to be measured, and the measurement time can be reduced.

Further, in the case where the desired wavelength is determined and the spectroscope can be used only at the beginning, if the diameter of each interference fringe when the wavelength is calibrated by the spectrometer is D ′, the following equation is established. By doing so, the wavelength can be maintained even if the gap length or the refractive index of the monitor etalon slightly changes.

F is a parameter in this case. Especially when the laser and the wavelength is short, so put the _{m L λ L / m H λ} H ≒ 1, wherein the even ^{_{easier, G = D H 2 -D '}} H 2 = D L 2 -D' L ² (7) G is a parameter in this case. In this case, the measurement time can be shortened because only one interference fringe of the mercury lamp is required, and the interference fringes are enlarged by the lenses 5 and 6 so that each interference fringe enters the image sensor one by one to improve the measurement accuracy. It can also be done.

In the above example, two peaks were detected across the center of the interference fringes to calculate the diameter. However, if the center position of the interference fringes was known in advance, one peak was detected. May be obtained, and a similar parameter may be calculated from the radius.

FIG. 5 shows points to note when a mercury lamp is used as a wavelength calibration light source. The spectrum of a low-pressure mercury lamp has a spread of around 5pm due to the presence of some isotopes of mercury. However, when the lamp is turned on for a long time, the saturation vapor pressure of mercury increases, as shown in FIG. 6, so that the spread of the wavelength and the depression due to self-absorption are observed. As a result, the accuracy of reading interference fringes of the mercury lamp decreases. To prevent this, a variable resistor is provided between the power supply and the power supply to limit the power supplied to the lamp. The input power was set to 1 W / cm ³ as a guide. Alternatively, the vapor pressure of mercury can be suppressed by attaching a fin 18 to the mercury lamp and air-cooling it, or by cooling a part of the discharge tube with water.

FIG. 7 shows another example of the wavelength monitor, in which a container 19 for sealing an etalon for monitoring, a thermostatic container 20 and a narrow band filter 21 are added.

If you always compare the laser and the calibration lamp, you do not need to pay much attention to the stability of the monitor etalon, but to perform wavelength control at high speed, you need to reduce the number of times the calibration lamp is turned on. Therefore, while keeping the density of the gas in the etalon environment constant by the sealed container and the thermostatic container, the temperature of the etalon rises by cutting off light and heat rays of other wavelengths emitted from the mercury lamp 2 by the narrow band filter 21. When this was prevented, the wavelength could be kept constant, and the number of times the calibration lamp was turned on could be reduced.

[Effects of the Invention] As described above, according to the present invention, the oscillation wavelength of a laser is measured using interference fringes generated by an etalon, and the oscillation wavelength of the laser is controlled based on the measurement result. , comprising a calibration light source for calibrating the oscillation wavelength, the two diameters of the interference fringes generated by the calibration light source is measured, by calculating e _H from equation (2), request the e _H The calculated value e _L (calculation) is calculated from the obtained wavelength by using the equation (3), two diameters of the interference fringes generated by the laser are measured, and (2) is calculated from the two measured diameters. The measured value e _L (measurement) is calculated according to the formula, and the oscillation wavelength of the laser is controlled so that the difference between the measured value e _L (measurement) and the calculated value e _L (calculated) is equal to or less than a predetermined value. Means, the wavelength of the calibration light source is slightly different from the wavelength of the laser. Also arrange some change in the optical system of the wavelength stabilization mechanism and an atmosphere of the etalon (temperature, pressure) even such changes, the effect of so maintain the laser wavelength.

A calibration light source having a wavelength within a range that satisfies an effective number of a refractive index corresponding to a required wavelength accuracy of the laser; and the calibration light source when the oscillation wavelength of the laser is calibrated by a spectroscope. The diameter of the interference fringes generated by the above is D _H ′, the diameter of the interference fringes generated by the laser is D _L ′, and the diameter of the interference fringes generated by the calibration light source during the subsequent laser operation is D _H , by the laser when the diameter of the resulting interference fringes and D _L, since with a means for controlling the oscillation wavelength of the laser so that the ^{_{D H 2 -D H '2 =}} D L 2 -D L' 2, during the laser operation The diameter of the interference fringes requiring measurement is small, and the laser wavelength can be accurately maintained at a desired wavelength by a simple measuring means.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a wavelength stabilizing device for a narrow band laser according to an embodiment of the present invention. FIGS. 2 (A) and 2 (B) are explanatory diagrams showing interference fringes according to an embodiment of the present invention. FIG. 3 is a flowchart showing the operation of the wavelength stabilizing device for a narrow band laser according to one embodiment of the present invention, FIG. 4 is an explanatory diagram showing the relationship between wavelength and refractive index, and FIG. 5 is an embodiment of the present invention. FIG. 6 is an explanatory diagram showing a wavelength spread of a mercury lamp according to one embodiment of the present invention, and FIG. 7 is a diagram for a narrow band laser according to another embodiment of the present invention. FIG. 8 is a configuration diagram showing a wavelength stabilizing device, and FIG. 8 is a configuration diagram showing a conventional narrow band wavelength stabilizing device. In the figure, 1 is a laser beam, 2 is a calibration light source, 3 is an etalon, 13 is an image sensor, 15 is an interference fringe analyzer, 16
Is a control unit, and 17 is an etalon controller. In the drawings, the same reference numerals indicate the same or corresponding parts.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-1-101683 (JP, A) APPLIED OPTICS Vol. 26 No. 17 (1987) p. 3659−3662 (58) Field surveyed (Int.Cl. ⁷ , DB name) H01S 3/00 H01S 3/13-3/139

Claims (2)

(57) [Claims] An apparatus for measuring an oscillation wavelength of a laser using interference fringes generated by an etalon and controlling the oscillation wavelength of the laser based on the measurement result. A light source, measuring two diameters of interference fringes generated by the calibration light source, calculating a calculated value e _L (calculation) from the measured two diameters and a required wavelength; The two diameters of the interference fringes generated by the measurement are measured, and a measured value e _L (measurement) is calculated from the two measured diameters, and the measured value e _L (measurement) and the calculated value e _{L are} calculated.
A wavelength stabilizing device for a narrow band laser, comprising: means for controlling the oscillation wavelength of the laser so that the difference of (calculation) becomes equal to or less than a predetermined value. 2. A method for measuring an oscillation wavelength of a laser using interference fringes generated by an etalon and controlling the oscillation wavelength of the laser based on the measurement result. A calibration light source having a wavelength in a range that satisfies the effective number of refractive indices to be provided, and when the oscillation wavelength of the laser is calibrated by a spectroscope, the diameter of the interference fringe generated by the calibration light source is D _H ', When the diameter of the interference fringes generated by the laser is D _L ′, the diameter of the interference fringes generated by the calibration light source during the subsequent laser operation is D _H , and the diameter of the interference fringes generated by the laser is D _{L ^{, D H 2 -D H '2}} = D L 2 -D L' 2 and so as wavelength stabilizing system for the narrow-band laser, characterized in that it comprises means for controlling the oscillation wavelength of the laser.