JP3839762B2 - Method and apparatus for controlling laser ablation in solution - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for controlling laser ablation in solution used in the fields of catalysts, paints, chemistry involved in the production of thin films, semiconductor manufacturing, and the like.
[0002]
[Prior art]
It is known that ablation more efficiently than in a vacuum or gas phase can be performed by irradiating a metal placed in a solution with a high-power laser beam (see, for example, Non-Patent Document 1). It is called ablation. Such laser ablation in solution is a useful technique as a method for producing a colloid because nano-sized metal fine particles are generated by agglomeration of metal that jumps out of the target object to be ablated into the solution.
[0003]
FIG. 8 shows a schematic structure of the in-solution laser ablation apparatus. In the figure, 1 is a cell, 2 is a solution, 3 is a target, 4 is a laser, and 5 is a lens. Pulse laser light is generated from the laser 4 and focused on the surface of the target 3 immersed in the solution 2 by the lens 5 to cause ablation of the target material such as metal. The fine particles of the target material that have been peeled off and scattered from the target 3 by ablation are dispersed in a colloidal form in the solution 2.
[0004]
On the other hand, in laser processing in vacuum, an acoustic wave is measured by an acoustic sensor attached to a workpiece, and a processing state is detected (see Patent Document 1). In laser ablation in solution, it is known that sound is generated during laser irradiation (see, for example, Non-Patent Document 2).
[0005]
[Non-Patent Document 1]
Photochemistry, 1999, VOL.30, NO.3, p.260 -261
[Non-Patent Document 2]
Journal of Precision Engineering, 1999, VNVOL 65, NO. 12, p.1709-1712
[Patent Document 1]
Japanese Patent Laid-Open No. 08-090261
[Problems to be solved by the invention]
The condition for efficiently producing fine particles using in-solution laser ablation is to keep the intensity of the sound generated during laser ablation to the maximum. However, a spike-like signal is mixed in this acoustic intensity signal, which makes it difficult to properly adjust the focal position of the laser beam to the target surface.
[0007]
FIG. 4 shows an example of generation of an acoustic intensity signal. The horizontal axis of the figure represents the relative position of the lens with respect to the stage (10 mm / division), and the vertical axis represents the acoustic intensity. Here, the acoustic intensity is represented by the scale of the piezoelectric sensor used for the acoustic sensor. The figure shows the change in sound intensity when the lens is gradually moved away from the stage from the left side where the lens focal position is in the stage. In the conventional measurement methods, the spike signal appears prominently when the focal point of the lens is positioned in front of the target surface. This randomly appearing spike signal is due to the interaction between the fine particles in the solvent generated by laser ablation and the laser light. That is, the generated fine particles absorb high-density laser light in the vicinity of the focal point, and acoustic signals generated when heating and decomposing are mixed as spike signals.
[0008]
An object of the present invention is to realize control that is not affected by spike signals in control of laser ablation in solution using an acoustic intensity signal.
[0009]
[Means for Solving the Problems]
From the knowledge that the spike signal mixed in the acoustic signal generated by laser ablation in solution is generated by the interaction of the laser beam and the fine particles generated by laser ablation in solution, the spike signal is generated by the laser ablation. Considering that it is limited to the time domain after generation, the signal point that is not affected by the spike signal on the time axis is obtained on the basis of the oscillation start point of the laser pulse, and the output of this signal point is maximized. In addition, the present invention proposes an invention of a control method and a control apparatus for laser ablation in a solution configured to control the focal position of a laser. According to the present invention, it is possible to obtain a point where the sound intensity is maximized without being affected by the spike signal, and to perform laser ablation with the maximum efficiency.
[0010]
FIG. 1 shows a basic configuration of a control apparatus for laser ablation in solution according to the present invention. In the figure, 1 is a cell, 2 is a solution, 3 is a target, 4 is a laser, 5 is a lens, 6 is an acoustic sensor such as a piezoelectric element for detecting an acoustic signal in the cell 1, and 7 is controlling the vertical movement of the lens 5. A Z-axis stage controller for performing the operation, an A / D converter for converting an acoustic signal in an analog signal format into a digital signal format, and an acoustic signal extracting unit for extracting a level of the acoustic signal at a timing in a time domain that is not affected by a spike signal. Reference numeral 10 denotes an optimal control unit that optimally controls the Z-axis stage controller 7 so that the level of the acoustic signal extracted at the timing in the time domain not affected by the spike signal is maximized. These elements 1 to 10 constitute one control loop.
[0011]
In FIG. 1, a laser 4 is continuously pulse-driven and emits pulsed laser light at regular intervals. The pulse laser beam is focused by the lens 5 and irradiates the surface of the target 3 in the solution 2 at almost the focal position. The acoustic sensor 6 attached to the side wall of the cell 1 detects an acoustic wave generated in the solution 2 by ablation and outputs an acoustic signal. The acoustic signal is converted into a digital signal by the A / D converter 8 and input to the acoustic signal extraction unit 9. The acoustic signal extraction unit 9 is set in advance with a predetermined time value in a time region not affected by the spike signal, with the pulse laser transmission start time as a base point on the time axis, and the level of the input acoustic signal is The time value is extracted and sent to the optimum control unit 10. The optimum control unit 10 monitors the change in the level of the acoustic signal extracted for each radiation of the pulsed laser light, calculates the error signal in the direction in which the level of the acoustic signal always takes the maximum value, and the Z-axis stage controller 7. To enter. The Z-axis stage controller 7 drives the Z-axis stage according to the given error signal and controls the focal position of the lens 5. Thus, the entire control system can be operated so as to always obtain the maximum ablation efficiency without being affected by the spike signal.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
A specific embodiment will be described with reference to FIGS.
FIG. 2 is a configuration diagram of the in-solution laser ablation control device according to the embodiment. In the figure, 11 is a quartz cell, 12 is a solution, 13 is a target, 14 is a laser, 15 is a lens, 16 is a piezoelectric piezoelectric microphone, 17 is a preamplifier for amplifying an acoustic signal, 18 is an acoustic signal waveform monitor and digital signal conversion. The oscilloscope 19 is a personal computer, which analyzes the waveform of the acoustic signal, detects the spike signal generation area, determines the extraction timing position of the acoustic signal, and calculates optimal control to maximize the level of the extracted acoustic signal. Data processing such as. Reference numeral 20 denotes a Z-axis stage controller, and reference numeral 21 denotes an electric Z-axis stage that moves the lens 15 up and down.
[0013]
The Nd: YAG laser triple wave (355 nm) was used as the ablation light, the intensity was adjusted to 4 mJ / pulse, and the surface of the target 13 was condensed and irradiated by the lens 15. In order to precisely control the surface of the target 13 and the position of the focal point of the laser beam to be irradiated, the position of the lens 15 was controlled by an electric Z-axis stage 21. The acoustic wave was detected by a piezoelectric piezoelectric microphone 16 attached to the side surface of the quartz cell 11. In order to prevent a decrease in reproducibility due to displacement of the position of the piezo piezoelectric microphone 16, the piezo piezoelectric microphone 16 was fixed to the quartz cell 11 with an adhesive. Grease for impedance matching is applied to the joint surface between the cell and the piezo. An acoustic signal converted into a current by the piezoelectric piezoelectric microphone 16 was amplified by a preamplifier (input impedance 50Ω) 17 and then input to an oscilloscope 18 and measured as an electric waveform. Stage position control and signal capture were performed from the personal computer 19 via GPIB.
[0014]
First, an embodiment in which optimization of the focal position is attempted by a conventional measurement method will be described. FIG. 3 shows an example in which the main part of the acoustic signal waveform is observed with the acquisition time scale set to 500 microseconds. Here, the amplitude indicated by the arrow was selected as a measurement value for signal intensity monitoring. Next, FIG. 4 shows the change in the intensity of the acoustic signal obtained by moving the lens position while monitoring this measured value. The lens position indicates that the right side is farther from the target. A colloid-derived signal appears in a spike shape in the latter half of the signal.
[0015]
Next, the solution according to the present invention will be described. FIG. 5 shows the measurement with a high resolution by reducing the time scale of uptake compared to FIG. FIG. 5 (a) shows a signal when the focal point of the laser beam is in front of the target, and is a waveform containing many colloid-derived spike signals. FIG. 5B shows a signal when the focal point of the laser beam is in the back (or the surface) of the target. The two signals are compared, and the point not affected by the spike signal is determined from the signal waveform of FIG. This procedure is specifically shown in FIG. 7 as follows.
[0016]
(1) Move the lens while monitoring the total sound signal intensity. At this time, the lens moves away from the side closer to the target.
(2) Measurement of acoustic signal and spike signal generation position by ablation. If no spike signal is generated, the lens position where the signal intensity is maximum is determined as the focal point.
(3) Measure the waveform at the ablation signal start position (eg, point indicated by a in FIG. 4).
The lens position (for example, in FIG. 4) farther from the spike signal generation start position than the distance between the position of (4), (3) and the spike signal generation start position (for example, the point indicated by b in FIG. 4). Measure the waveform at point c). The former is waveform A and the latter is waveform B.
(5) Select several phase points with zero amplitude in waveform B.
Among the phase points selected in (6) and (5), the phase point having the maximum amplitude in waveform A is selected.
(7) Move the lens while monitoring the amplitude of the selected phase point. The lens position with the maximum signal strength is determined as the focus.
[0017]
First, a signal is measured by selecting a lens position where the focal point is sufficiently in front of the target and the ratio of the acoustic signal component generated by the ablation of the target in the acoustic signal is sufficiently small (for example, FIG. 5A). (The lens position is centered on the position where the generation of the colloid-derived spike signal starts, and can be identified as a position far away from the position that is almost in contrast to the acoustic signal generation start position when the focal point is behind the target.) . Next, select some points where the amplitude of this signal waveform is zero. Next, an acoustic signal at a lens position where no spike signal is generated is measured, and one point having a signal with sufficient amplitude at the same phase (position on the horizontal axis) as the point selected above is selected. One of the measurement points selected in this way is a point indicated by a dotted line in FIG. FIG. 6 shows the signal intensity obtained by moving the lens position using this point as a measurement point. By controlling the Z-axis stage 21 in FIG. 2 using this signal intensity, efficient laser ablation can be performed. In FIG. 3, it is difficult to specify the maximum signal intensity point. However, if the signal intensity of FIG. 6 according to the present invention is used, the maximum signal intensity point can be easily specified, and efficient laser ablation can be performed.
[0018]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the control which irradiates the focus of a laser beam to a target surface can be performed accurately and stably, and efficiency improvement of a laser ablation can be aimed at. Further, since the accuracy of the focal position increases, the irradiation spot of the laser beam can be minimized, so that the fine workability is improved.
[Brief description of the drawings]
FIG. 1 is a basic configuration diagram of an in-solution laser ablation control device according to the present invention.
FIG. 2 is a configuration diagram of an in-solution laser ablation control device according to an embodiment of the present invention.
FIG. 3 is a diagram showing a waveform of an example of observation of the entire main part of an acoustic signal.
FIG. 4 is a diagram showing a waveform of an acoustic signal intensity change obtained by moving the position of a lens.
FIG. 5 is a diagram showing a waveform of an acoustic signal observation example measured with high time resolution.
FIG. 6 is a diagram showing a waveform of an acoustic signal intensity change measured at a time position without a spike signal.
FIG. 7 is a flowchart of an embodiment procedure for performing an optimum lens position control using an acoustic signal while avoiding a spike signal according to the present invention.
FIG. 8 is a view showing a schematic structure of a conventional laser ablation apparatus in solution.
[Explanation of symbols]
1: Cell 2: Solution 3: Target 4: Laser 5: Lens 6: Acoustic sensor 7: Z-axis stage controller 8: A / D
9: Acoustic signal extraction unit 10: Optimal control unit

Claims (7)

In the laser ablation control method that controls the optimal irradiation position of laser light by detecting the acoustic signal generated when high-power laser light is irradiated to the target metal placed in the solution, the spike signal is detected in the detected acoustic signal. A method for controlling laser ablation in solution, wherein the irradiation position of the laser beam is controlled so that the amplitude of the signal portion not affected is maximized. 2. The method for controlling laser ablation in solution according to claim 1, wherein the spike signal is generated when the colloid generated by laser ablation in solution reabsorbs the laser beam. 3. The method of controlling laser ablation in solution according to claim 2, wherein the position of the signal portion that is not affected by the spike signal in the detected acoustic signal is determined based on the oscillation start time of the laser pulse. . In a laser ablation control device that detects the acoustic signal generated when a target metal placed in a solution is irradiated with a high-power laser beam and controls the optimal irradiation position of the laser beam, the spike signal is detected in the detected acoustic signal. An apparatus for controlling laser ablation in solution, wherein the irradiation position of the laser beam is optimally controlled so that the amplitude of an unaffected signal portion is maximized. 5. The apparatus for controlling laser ablation in solution according to claim 4, wherein the spike signal is generated when the colloid generated by laser ablation in solution reabsorbs the laser beam. An acoustic signal extraction unit that extracts the level of the acoustic signal in the signal portion that is not affected by the spike signal in the detected acoustic signal, and the laser light irradiation position is optimized so that the level of the extracted acoustic signal is maximized The apparatus for controlling laser ablation in solution according to claim 5, further comprising: an optimal control unit for controlling. The solution according to claim 6, further comprising a pre-processing unit that determines a timing position of a signal portion that is not affected by the spike signal in the detected acoustic signal with reference to an oscillation start time of the laser pulse. Control device for medium laser ablation.

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