JP3765044B2 - Excimer laser energy control device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an energy control apparatus that is mainly used as a light source of a progressive moving reduction projection exposure apparatus (hereinafter referred to as a stepper) and that controls output energy of an excimer laser apparatus that oscillates laser by discharge excitation.
[0002]
[Prior art]
It is very important for a stepper that exposes a semiconductor wafer or the like to control the exposure amount to be constant. As the light source for exposure, an excimer laser device is widely used in order to meet the recent demand for high integration density of semiconductor circuits. However, since the excimer laser device is a so-called pulse discharge excitation gas laser, there is a problem that the pulse energy of each pulse of the oscillating laser light varies due to various factors, and as a result, the exposure amount is not stable. Therefore, conventionally, in an excimer laser apparatus used for a stepper or the like, in order to reduce this variation and stabilize the exposure amount to a constant value, exposure by so-called multiple pulse exposure is performed in which a plurality of pulse oscillations are continuously performed. Some perform volume control. By this multiple pulse exposure amount control, the exposure amount variation as a whole can be reduced to a predetermined value or less, and a desired exposure amount accuracy can be obtained.
[0003]
In the stepper, since exposure and movement of the stage on which the wafer is placed are repeated alternately, the above excimer laser device is operated in a so-called burst mode. The burst mode refers to repeatedly performing an operation of suspending pulse oscillation for a predetermined time after laser light is continuously pulsed a predetermined number of times. However, as a feature during operation in the burst mode, as shown in FIG. 5, at the initial stage of each continuous pulse oscillation after a pause for a predetermined time (hereinafter referred to as burst oscillation), the oscillation is in a stable state. Although relatively high pulse energy can be obtained, if pulse oscillation is continued, each pulse oscillation gradually becomes unstable due to a laser gas density disturbance or a local temperature rise on the surface of the discharge electrode. The so-called spiking phenomenon in which the output pulse energy decreases as shown in FIG.
[0004]
In order to solve this problem, the applicant of the present invention, for example, in Japanese Patent Application Laid-Open No. 7-106678 uses the property that the magnitude of the output pulse energy is approximately proportional to the magnitude of the charging voltage, and FIG. As shown in the figure, the initial pulse charge voltage at each burst oscillation is reduced, and then the charge voltage of each pulse is gradually increased to prevent an increase in energy at the beginning of burst oscillation due to spiking phenomenon. The technology is disclosed. According to this prior art, each pulse of burst oscillation is considered in consideration of various parameters such as the oscillation pause time ts shown in FIG. 5 and the power lock (trademark) voltage (charging voltage determined according to laser gas deterioration). The charging voltage data for setting the energy to a desired target value is stored in advance for each pulse of burst oscillation, and the energy measurement value for each pulse of each burst oscillation that has already been output until the previous burst oscillation is stored, Based on the comparison result between the measured energy value and the target pulse energy value, the stored charging voltage data corresponding to each pulse is corrected. The control by this correction is hereinafter referred to as spike killer control.
[0005]
According to such a technique, in each burst oscillation, the spike killer control is performed not only in the spike region at the start of oscillation, but also in the plateau region and steady region (see FIG. 5) where the oscillation stabilizes thereafter. . As described above, the spike region is easily affected by the length of the pause time ts, and a larger pulse energy is output than the other regions even at the same charging voltage. However, in the plateau region and the steady region, it is considered that the influence of pulse oscillation (for example, increase in electrode temperature, laser gas disturbance, etc.) immediately before the same burst oscillation is more strongly affected than the influence of the pause time ts. It is done. Therefore, in a region other than the spike region, there is a problem that the effect of suppressing variation in pulse energy by the spike killer control is not sufficient.
[0006]
In order to solve such a problem, the same applicant has proposed the following laser apparatus according to Japanese Patent Laid-Open No. 9-248682. In other words, in the initial spike region of each burst oscillation, the charging voltage at the time of each pulse oscillation is changed to the oscillation pause time, the order of pulses within the same burst oscillation, and the measured value (monitor value) of the output pulse energy. In association with the spike region, the charging voltage at the time of each pulse oscillation is stored in correspondence with the measured value of the pulse energy output. When each burst oscillation is performed, in the spike region, among the stored past pulse oscillation data, the oscillation pause time and the order of pulses in the burst oscillation are equal, and the current burst oscillation At least one set of the measured value of the output pulse energy close to the energy target value and the charging voltage of the pulse at that time is read, and the charging voltage at the time of the current pulse oscillation is calculated based on the read value, and the calculated charging Based on the voltage, pulse oscillation (corresponding to the spike killer control) is performed. (Hereinafter, such control is referred to as learning control.) Also, in the region after the spike region, the pulse energy measurement value of the pulse output immediately before the current burst oscillation that has already been stored and the charging voltage at that time are stored. Based on the read value, a charging voltage at the time of the current pulse oscillation is calculated, and pulse oscillation (hereinafter referred to as every pulse control) is performed based on the calculated charging voltage.
[0007]
[Problems to be solved by the invention]
However, even in the conventional energy control based on the learning control and the pulse control as described above, for example, as shown in FIG. There is a phenomenon that the value becomes lower than the target value. However, if the oscillation continues, it converges to the energy target value. When attention is paid to the pulse energy in a continuous region in each burst oscillation, this variation is not so large. For this reason, until now, there was no problem even if there was such a variation in pulse energy. However, in stepper devices, the demand for variation accuracy corresponding to the recent higher integration specifications of semiconductors has become very strict, and the influence of this variation cannot be ignored.
[0008]
It is considered that the cause of the energy reduction phenomenon as described above is the spiking phenomenon when the laser is oscillated, the subsequent decrease in output energy, and the random swell of the output energy. That is, for example, as shown in FIG. 8, when the burst voltage oscillation is intermittently performed with the charge voltage command value kept constant, the output energy decreases sharply from the beginning of the oscillation start after the pause. In order to keep the energy constant, it is necessary to increase the charging voltage command so as to cancel the change in the output energy. (See FIG. 6) When the range in which the learning control is applied is shortened as much as possible and the pulse control is applied in a range where the pulse energy varies, the pulse control is essentially feedback control as described above. Therefore, the deviation between the target value of energy and the measured value is unavoidable. Further, when the output energy for each burst is observed when the charge voltage command value is kept constant and the burst oscillation is intermittently performed, as shown in FIG. Is superimposed. That is, even if the pulse order in each burst oscillation is the same, the fluctuation of the pulse energy is greatly different for each burst oscillation, that is, the Nth burst and the (N + 1) th burst. For this reason, it is difficult to apply learning control over a range in which the pulse energy varies. As described above, the fluctuations and deviations, which have not been regarded as problems in the past, can no longer be ignored as the demand from the application field of excimer laser devices (for example, stepper devices) becomes higher. Yes.
[0009]
The present invention has been made paying attention to the above-mentioned problems, and provides an energy control device for an excimer laser device that can reduce the deviation amount of each pulse energy without being affected by random undulation of output energy. It is an object.
[0010]
[Means, actions and effects for solving the problems]
In order to achieve the above object, according to the first aspect of the present invention, when pulse oscillation is performed in a burst operation mode in which a continuous pulse oscillation for a predetermined time and a pause for a predetermined time are alternately repeated, Learning control is performed for a predetermined number of pulse oscillations, and after this learning control, each pulse control is performed. During the learning control, at the time of the i-th pulse oscillation from the start of oscillation, Depending on the voltage command value Vi of the i-th pulse and the deviation value between the energy target value Ed and the measured energy value E of the pulse oscillated corresponding to the i-th voltage command value Vi, the current i-th voltage The command value Vi is calculated and output, and at the time of each pulse control, the voltage command of the (I-1) th pulse oscillation within the same continuous pulse oscillation at the time of the Ith pulse oscillation from the start of oscillation. The value VI-1 and The I-th voltage command value VI is calculated and output according to the deviation value between the energy target value Ed and the energy measurement value E of the pulse oscillated in response to the (I-1) -th voltage command value VI-1. In the energy control device of an excimer laser device, which includes an output control unit 10 that oscillates pulses and controls the discharge voltage based on the voltage command values Vi and VI to control the pulse energy of the laser, the output control unit 10 During each pulse control, a correction value ΔE calculated based on a deviation value between the energy target value Ed and the energy measured value E is added to the energy target value Ed to correct, and the corrected new energy target value Based on Ed0, the voltage command value VI is calculated and output.
[0011]
According to the first aspect of the invention, for example, the correction value ΔE is calculated according to the magnitude of the deviation value between the energy target value Ed input from an external control device such as a stepper and the energy measurement value E, and the energy target Correction is made by adding to the value Ed. The corrected new energy target value Ed0 is used as an energy target value for each pulse control to calculate a voltage command value, and the calculated voltage command value is output to the laser power supply unit to control the pulse energy. As a result, when the pulse energy fluctuates due to the undulation during pulse oscillation and the deviation becomes large, the energy target value is corrected according to this deviation, so the voltage command value is also corrected. Therefore, the deviation amount becomes small, and the pulse energy can be stably controlled to a constant value.
[0012]
According to a second aspect of the present invention, in the energy control device of the excimer laser device according to the first aspect, the correction value ΔE is obtained by integrating a deviation value between the target energy value Ed and the measured energy value E by an integrator 20a. And is calculated according to the integrated value.
[0013]
According to the second aspect of the present invention, the deviation value between the input energy target value Ed and the measured energy value E is integrated by an integrator, and the correction value ΔE is calculated according to the magnitude of the integrated value. Is added to the energy target value Ed, and the corrected new energy target value Ed0 is used as the energy target value at the time of each pulse control to calculate the voltage command value. As a result, when the pulse energy varies due to the undulation and the integrated value of the deviation value becomes large, the energy energy target value is corrected according to the integrated value, so that the voltage command value is corrected. Therefore, the deviation amount is reduced, and the pulse energy can be stably controlled to a constant value as a whole.
[0014]
According to a third aspect of the present invention, in the energy control device of the excimer laser device according to the first aspect, the correction value ΔE is a deviation value between the target energy value Ed and the measured energy value E by a first-order lag element 20b. Are integrated while reducing the influence of the deviation value that is older in time, and are calculated according to this integrated value.
[0015]
According to the third aspect of the present invention, the deviation value between the input energy target value Ed and the measured energy value E is integrated by a first-order lag element. In this first-order lag element, the deviation value that is older in time is integrated such that the degree of influence (addition degree) on the integrated value decreases with time. Therefore, the correction value ΔE calculated according to the calculated integrated value is a value that takes into account a new deviation value in terms of time. As a result, the voltage command value is calculated by using the new energy target value Ed0 corrected by the correction value ΔE as the energy target value at the time of each pulse control, compared with the case where the correction is performed by the pure integrator as described above. Thus, the pulse energy can be controlled to a constant value more stably.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments will be described with reference to the drawings.
FIG. 1 is a block diagram showing a configuration example of a stepper in which an energy control device for an excimer laser device according to the present invention is used. In the figure, a laser gas is sealed in a laser chamber 2 of an excimer laser apparatus 1. A predetermined discharge voltage is applied from a laser power source 8 to electrodes (not shown) disposed inside the laser chamber 2, and discharge is performed between the electrodes. Laser oscillation is performed by the laser gas excited by this discharge, and the oscillated laser light is resonated by an optical resonator having a narrow-band element 6 such as a grating or a prism and a front mirror 7, and laser light is emitted from the front mirror 7. 4 is emitted. The laser beam 4 passes through the beam splitter 3 and is guided to the stepper 30, and a part of the laser beam 4 is sampled by the beam splitter 3 and incident on the energy sensor 5 of the output monitor unit. 4 energy per pulse, that is, pulse energy is measured. This energy measurement value E is fed back to the output control unit 10.
[0017]
Further, the stepper 30 as a reduction projection exposure apparatus includes an exposure control apparatus 31. The exposure control apparatus 31 irradiates the wafer to be exposed with the laser light 4 taken in, or sequentially applies the stage on which the wafer is mounted. It is controlled to move by a predetermined distance. Then, in order to obtain a desired exposure amount, the exposure control device 31 outputs an energy target value Ed per one pulse of the oscillation pulse to the output control unit 10, and the total oscillation pulse within each burst oscillation. Number im is output.
[0018]
Based on the deviation value between the target energy value Ed and the measured energy value E, the output control unit 10 uses a predetermined control algorithm to be described later so that the output pulse energy becomes equal to the input target energy value Ed. The voltage command value V is calculated for each pulse, and the calculated voltage command value V is output to the laser power supply unit 8. Thereby, the laser beam 4 is oscillated by being discharged at a predetermined discharge voltage. At this time, pulse oscillation is performed until the number of output pulses reaches the total number of oscillation pulses im. In addition, the output control part 10 can be comprised mainly by computer apparatuses, such as a microcomputer, for example.
[0019]
FIG. 2 is a control block diagram conceptually showing the basic control function configuration inside the output control unit 10 according to the present invention.
In the figure, a target value correction calculation unit 20 calculates a correction value ΔE of the energy target value Ed based on a deviation value H between the energy target value Ed and the energy measurement value E. The correction value ΔE is added to the energy target value Ed, and this added value is input to the pulse control unit 12 as the energy target value Ed0.
[0020]
The learning control unit 11 performs spike killer control by learning control in the initial spike region (see FIG. 5) at the time of each burst oscillation. That is, a voltage command value table (referred to as a voltage table) corresponding to a predetermined energy target value Ed is stored in advance. In the first burst oscillation, the i-th voltage command value Vi in the stored initial voltage table is read and output to the laser power supply unit 8 at the i-th pulse oscillation from the first pulse. Thereafter, based on a deviation value H between the energy target value Ed and the energy measurement value E measured at the time of laser oscillation, the read voltage command value Vi is updated (this is referred to as learning), and the updated voltage The command value Vi is stored as the i-th voltage command value Vi of the voltage table. Thereafter, the updated voltage table is used in the next burst oscillation, and the i-th voltage command value Vi corresponding to the i-th pulse oscillation is read and output to the laser power supply unit 8. When the predetermined i0th oscillation is completed, the final voltage command value Vi0 is transmitted to the pulse control unit 12.
[0021]
Each pulse control unit 12 performs control to reduce the variation of each pulse energy at the time of each pulse oscillation after the spike region. That is, every pulse control unit 12 at the time of the i-th pulse oscillation from the beginning of the burst oscillation, the voltage command value Vi-1 immediately before the (i-1) -th pulse oscillation in the same burst oscillation. The deviation value H between the input energy target value Ed0 and the measured energy value E with respect to the input energy target value Ed0 is output to the laser power supply unit 8 as the current voltage command value Vi. Thereafter, the pulse control unit 12 updates the i-th voltage command value Vi based on the deviation value H between the energy target value Ed0 of the pulse oscillated at this time and the energy measurement value E, and the updated voltage. The command value Vi is stored. Since the stored i-th voltage command value Vi is output at the (i + 1) -th pulse oscillation of the same burst oscillation, basically, feedback control is performed. Note that when the learning control is switched to the pulse-by-pulse control, the voltage command value Vi0 input from the learning control unit 11 is set as an initial value at the time of pulse-by-pulse control.
[0022]
The selector 13 determines whether the current control processing is for the spike region or a region after this, selects and outputs the voltage command value Vi output from the learning control unit 11 when controlling in the spike region, At the time of control after this region, the voltage command value Vi output from the pulse control unit 12 is selected and output.
[0023]
Next, an example of the target value correction calculation unit 20 will be described.
FIG. 3 shows an example in which the target value correction calculation unit 20 is provided with an integrator 20a. That is, the integrator 20a integrates the deviation value H between the input energy target value Ed and the energy measurement value E for each burst oscillation, and the following equation 1 To obtain a correction value ΔE.
[Expression 1]
Correction value ΔE = K × M
Here, K is a predetermined constant.
[0024]
According to this configuration, the energy target value Ed is corrected by the correction value ΔE corresponding to the integrated value of the deviation value H between the energy target value Ed and the energy measurement value E, and the energy target value Ed0 thus corrected is Input to the pulse controller 12. Therefore, when the pulse energy fluctuates due to the influence of the swell or the like, the delay is reduced because the feedback control delay at the time of each pulse control is corrected. Further, when the pulse energy becomes stable, the average value of the deviation becomes close to 0, and the integrated value M of the deviation becomes small, so that the correction by the correction value ΔE does not work.
[0025]
FIG. 4 shows an example in which the first-order lag element 20 b is used in the target value correction calculation unit 20. The first-order lag element 20b obtains a correction value ΔE by the following formula 2 based on the deviation value H between the input energy target value Ed and the measured energy value E.
[Expression 2]
ΔEn + 1 = A × Hn + (1-A) ΔEn
Here, ΔEn is a correction value ΔE at the time of the nth pulse oscillation, Hn is a deviation value H at the time of the nth pulse oscillation, and A is a predetermined constant in the range from 0 to 1. According to this equation 2, the old deviation value Hn in the past is forgotten according to the elapsed time, that is, the old deviation value Hn is integrated while decreasing the influence of the old deviation value Hn on the integrated value according to the elapsed time, This integrated value is set as a correction value ΔE.
[0026]
According to this configuration, the correction value ΔE is obtained according to the integrated value of the deviation value H between the energy target value Ed and the energy measurement value E, whereby the energy target value Ed is corrected and a new energy target value Ed0 is obtained. Input to the pulse controller 12. At this time, since the influence of the deviation value H that is older in time is reduced by the first-order lag element, the entire control system is more stable than the pure integrator as described above, and the deviation of the pulse energy is reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration example of a stepper in which an energy control device of an excimer laser device according to the present invention is used.
FIG. 2 is a control block diagram schematically showing a basic control function configuration inside the output control unit.
FIG. 3 shows an example in which an integrator is provided in the target value correction calculation unit.
FIG. 4 shows an example in which a first-order lag element is used in a target value correction calculation unit.
FIG. 5 is an explanatory diagram of a change in pulse energy during burst oscillation according to the prior art.
FIG. 6 shows an example of charging voltage output during spike killer control.
FIG. 7 shows an example of pulse energy fluctuation during spike killer control according to the prior art.
FIG. 8 is an explanatory diagram of pulse energy swell in charge voltage constant control during spike killer control according to the prior art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Excimer laser apparatus, 4 ... Laser beam, 5 ... Energy sensor, 8 ... Laser power supply part, 10 ... Output control part, 11 ... Learning control part, 12 ... Every pulse control part, 20 ... Target value correction | amendment calculating part, 20a ... integrator, 20b ... first order lag element, 30 ... stepper, Vi ... voltage command value, ΔE ... correction value, Ed ... energy target value, E ... energy measurement value.

Claims (3)

When pulse oscillation is performed in the burst operation mode in which continuous pulse oscillation for a predetermined time and pause for a predetermined time are alternately repeated, learning control is performed for a predetermined number of pulse oscillations at the initial stage of each continuous pulse oscillation. Performs pulse-by-pulse control, and during the learning control, during the i-th pulse oscillation from the start of oscillation, the voltage command value (Vi) of the i-th pulse at the previous continuous pulse oscillation and the energy target value (Ed ) And the measured energy value (E) of the pulse oscillated in response to the i-th voltage command value (Vi), the current i-th voltage command value (Vi) is calculated and output. In the pulse control, when the I-th pulse is oscillated from the start of oscillation, the voltage command value (VI-1) of the (I-1) -th pulse oscillation within the same continuous pulse oscillation, and The target energy value (Ed) and the I-1th voltage command value (VI-1) An output control unit (10) for calculating and outputting the I-th voltage command value (VI) according to a deviation value from the energy measurement value (E) of the pulse oscillated and oscillating the pulse, In the excimer laser device energy control device that controls the pulse voltage of the laser by controlling the discharge voltage based on the command values (Vi) and (VI),
The output control unit (10), during each pulse control, calculates a correction value (ΔE) calculated based on a deviation value between the energy target value (Ed) and the energy measurement value (E). The energy control device of the excimer laser device, wherein the voltage command value (VI) is calculated and output based on the corrected new energy target value (Ed0). In the energy control device of the excimer laser device according to claim 1,
The correction value (ΔE) is obtained by integrating the deviation value between the energy target value (Ed) and the energy measurement value (E) by an integrator (20a), and calculating according to the integration value. Excimer laser device energy control device. In the energy control device of the excimer laser device according to claim 1,
The correction value (ΔE) is integrated by the first-order lag element (20b) so that the deviation value between the target energy value (Ed) and the measured energy value (E) is less affected by the old deviation value. An energy control device for an excimer laser device that is calculated according to the integrated value.

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