JPH0371987A - Laser beam machining method - Google Patents

Abstract

PURPOSE:To perform rational laser beam machining by the optimum heating quantity by calculating the optimum laser beam quantity from reflectance obtained by irradiation of an observation laser beam of the low output. CONSTITUTION:A laser beam machine is constituted of a laser beam irradiation part 1, a reflected beam measurement part 2 and a control part 3 which calculates the optimum laser beam output from the reflected beam quantity and controls the output of the laser beam irradiation part 1. A machining part is irradiated with the pulse observation laser beam of the low output and the reflectance of the machining part is obtained from its reflected beam quantity and based on it, the optimum irradiation laser beam quantity of the machining part is calculated and the control part 3 regulates and controls its value. By this method, variation of heat receiving energy due to variance of reflectance of the machining part is prevented and machining of the optimum heating quan tity can always be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a processing method using laser light, which is used, for example, in laser trimming of resistors in semiconductor devices, laser cutting of wiring conductors, and the like.

[Conventional technology] In conventional laser trimming of monolithic ICs,
As shown in Figure 5, the resistance value is adjusted by irradiating pulsed laser light to a predetermined location of the resistor interposed between the base film and the protective film on the silicon substrate and blowing it out. are doing.

During operation of the device, the laser power remained fixed at a preset constant value.

[Problems to be Solved by the Invention] However, in the above laser trimming device, the laser beam 1 (i.e., the amount of heat absorbed by the resistor) that is absorbed by the white body of the resistor and is involved in fusing it, is absorbed by the resistor and the base film above and below the resistor. Even under a constant laser output, the laser light varies greatly due to changes in the thickness of the protective film and the thickness of the protective film.In other words, the laser light irradiated to the resistor is transmitted to each boundary surface 30 as shown in FIG.
Reflects at -83. Since the base film and the protective film are made of a material that does not absorb laser light, part of the laser light is absorbed by the resistor and the silicon substrate, and the rest is reflected to the outside.

To explain further, the surface reflectance of each boundary surface 5o-83 is determined by the interference of each reflected light component and the irradiated laser beam, and this interference changes depending on the thickness of the six films and the resistor.

In the end, due to variations in the film thickness of the 6 films and the resistor, the reflectance
The absorption coefficient of the resistor and the absorption coefficient of the silicon wafer change. If the amount of laser light absorbed by the resistor is insufficient, it will be impossible to trim the resistor, and if it is excessive, bad effects such as cracks and deformation will occur in the base film and protective film.
These have been a major cause of lowering reliability and yield in laser trimming.

A similar problem occurs, for example, in laser cutting of wiring conductors.

The present invention has been made in view of the above problem, and an object of the present invention is to provide a laser processing method that can keep the amount of laser light absorbed in the processing area of the processing target constant regardless of fluctuations in the reflectance. This is an issue.

[Means for Solving the Problems] The laser processing method of the present invention is a laser processing method in which a processing part of a workpiece is irradiated with a laser beam to change its shape or characteristics, and a small output is observed at the processing part. The process is performed so that the reflectance of the processed area or its vicinity is determined by irradiating a laser beam and the amount of reflected light, and based on the calculated reflectance, the amount of laser light absorbed by the processed area becomes a constant desired value. The method is characterized in that the optimum amount of laser light for one shift to be irradiated to the site is calculated, and the laser beam adjusted to the calculated optimum amount of laser light is irradiated to the processed site.

[Example] FIG. 1 shows an example of a laser trimming apparatus to which the laser processing method of the present invention is applied.

This laser trimming device includes a laser beam irradiation unit 1 that irradiates a pulsed laser beam to cut a predetermined fusing portion of a target to be fused, a reflected light measurement unit 2 that measures the amount of reflected light from the fusing portion, and a The control section 3 calculates the optimum laser output from the amount of reflected light and controls the output of the laser beam irradiation section 1.

The laser beam irradiation section 1 includes a pulsed laser 11, a mounting section 12 on which a wafer to be fused is placed, and a pulsed beam outputted from the pulsed laser 11 by an X perpendicular to the incident optical axis.
It includes a light guide system 13 that scans in the Y direction and guides the light to the wafer. The pulse laser 11 consists of an Nd:YAG laser having an ultrasonic Q switch and excited by a krypton arc lamp, and its laser irradiation output is adjusted by controlling the current supplied to the krypton arc lamp. The light guiding system 13 includes a focusing objective lens system (not shown), a reflecting mirror system 18, and a prism 19 that branches the reflected laser light from the wafer to the reflected light measuring section 2.

The reflected light measuring section 2 is composed of a photoelectric conversion device that converts reflected laser light into a voltage signal.

The control unit 3 consists of a microcomputer device, and although not shown, the microcomputer device controls the output voltage from the reflected light measurement unit 2 by A.
It has a built-in A/D converter that performs /D conversion in its input interface, a D/A converter that converts the calculated output current value into D/A, and a D/A converter that amplifies the analog output current output from the D/A converter. The output interface has a built-in driver that supplies the krypton arc lamp.

The operation of this laser trimming device will be explained below.

First, as shown in FIG. 1, a wafer is set on the mounting section 12, the trimming position of the wafer is aligned with the optical axis, and then the depth of focus is adjusted, and then the light emission control routine shown in the flowchart of FIG. 2 is performed. Execute.

In addition, as shown in FIG. 5, the wafer has a silicon substrate and
Base films, CrS i
It consists of a resistor and a protective film. Note that the incident optical axis is set perpendicular to each boundary surface 5o-33.

In this light emission control operation routine, first
1, the observation light adjusted to a constant light intensity of about 5% of the fusing output is irradiated to the trimming position of the wafer (510), and the reflected light amount is determined by checking the output signal voltage of the reflected light measuring section 2 at a predetermined ratio. The reflectance R of the fused portion of the wafer is calculated from (312).

The reflectance R is the amount of reflected light equal to the amount of observation light irradiated by υ1.

Next, the laser irradiation output (hereinafter referred to as fusing output) P when fusing the resistor is calculated based on the reflectance scale (S14).
. This calculation is performed based on the following formula.

P=E/ (Ar)-E/((1-RXCI)xC2> Here, E is the optimal amount of heat absorbed by the resistor/pulse, and is determined in advance according to the resistor's film thickness, specific heat, melting point, etc. Ar is the absorption rate of the resistor, and the absorption rate A is
It is determined by the formula r-(1-RXC1)XC2.

Note that multiplying the reflectance R by a constant ratio C1 is a correction of the reflectance R to a higher value in consideration of the presence of reflected light in a direction different from the incident optical axis.
The reason why 1) is multiplied by a constant ratio C2 is to set the absorption rate A' of the resistor itself to be low, since absorption occurs in both the resistor and the silicon substrate.

Note that although the absorption rates of the resistor and the silicon substrate actually vary depending on variations in the respective film thicknesses, it is assumed here that they are in a proportional relationship for the sake of simplicity. Furthermore, as the film thickness of the resistor increases, the thermal energy required to blow it out also increases, but this will also be ignored for the sake of simplicity. In addition, the absorption rate when a resistor is melted is different from that when it is a solid, but if these Hookters are to be taken into account, the above equation can be corrected by finding a correction coefficient through experiments.

FIG. 3 shows the relationship between the measured reflectance R and the laser irradiation output P calculated from it.

Next, calculate the current value to be supplied to the krypton arc lamp of the laser beam irradiation unit 11 with reference to a pre-stored table so that the pulse laser 11 irradiates the laser beam with the laser irradiation output P calculated above. 516),
The pulse laser 11 is driven with the calculated current value to output laser pulses at regular intervals, and the irradiation position is moved by the light guide system 13 to completely blow out the resistor (318).

According to the laser trimming apparatus of the present embodiment described above, in laser trimming, it is possible to always apply an optimum amount of thermal energy to each resistor regardless of variations in the anti-fM ratio for each wafer, and the resistor It is possible to prevent defective fusing due to an insufficient amount of heat received by the resistor, and deterioration or deformation of the protective film and base film due to an excessive amount of heat received by the resistor.

(Example 2) Another example of a laser trimming apparatus using the laser processing method of the present invention is shown in FIG.

This laser trimming device consists of a laser beam irradiation section 1
, a reflected light measurement section 2, a control section 3, and an irradiation light measurement section 4.The irradiation light measurement section 4 is added to the laser trimming device of the first embodiment, and the prism 14 is added to the laser light irradiation section 1. is added.

The irradiated light measurement section 4 has the same structure as the reflected light measurement section 2, and photoelectrically converts the irradiated laser light branched from the prism 14 and inputs it to the control section 3.

In this example, the amount of irradiated light obtained by converting the output signal voltage output from the irradiated light measuring section 4 at a predetermined ratio and the amount of reflected light measured by the reflected light measuring section 2 as in the first embodiment are used. The reflectance R is calculated.

Other control operations are the same as in the first embodiment.

In this way, variations in the amount of heat received by the resistor due to variations in the output of the pulsed laser 11 can be suppressed.

That is, even if the current supplied to the krypton arc lamp is constant, the output of the pulsed laser 11 fluctuates due to fluctuations in power supply voltage, temperature changes, changes in characteristics over time, dust, and the like.

In the laser trimming device of the first embodiment, a decrease in the output of the pulsed laser 11 is mistakenly recognized as a decrease in reflectance, that is, an increase in the absorption rate of the resistor, and in this case, the control unit 3 reduces the laser output. As a result, the laser irradiation output is significantly influenced by these synergistic effects and is reduced, and as a result, the amount of heat received by the resistor is significantly reduced.

According to this embodiment, the reflectance R is calculated from the amount of observation light actually emitted from the pulsed laser 11 and the amount of reflected light, so the measurement of the reflectance R becomes more accurate.

In addition, since you can see the relationship between the current supplied to the krypton arc lamp and the amount of light irradiated by the observation light, for example, if the amount of light irradiated by the observation light decreases, the current will be supplied to the krypton arc lamp at the time of fusing according to the rate of decrease. Increasing the amount of current will compensate for the decrease in fusing output, and conversely, if the amount of light irradiated by the observation light increases, the amount of current supplied to the krypton arc lamp at the time of fusing will be reduced according to the rate of increase. It is also possible to compensate for increases in power.

Furthermore, in each of the embodiments described above, since the observation light and the processing light are outputted from the same laser, the optical axes of both can be made to coincide and the configuration can be simplified.

In each of the above embodiments, there are various control methods for controlling the fusing output in addition to controlling the amount of current supplied to the krypton arc lamp. Examples include using various attenuators and controlling the applied voltage using a semiconductor laser.

The above-described laser trimming of a resistor is an example of the application of the present invention. In addition, in semiconductor devices, the present invention can be applied to fusing wiring conductors used for repairing defects, controlling the amount of heating heat in laser annealing, etc. In that case,
It is also possible to use a continuous wave laser instead of the pulsed laser 11.

Furthermore, the present invention can heat various objects other than semiconductor devices,
It can be used for constant heating regardless of variations in reflectance. For example, even if the surface of the heated object has stains, patterns, color changes, or irregularities, even if a constant laser beam is irradiated, the amount of heating may vary for each heated object or for each irradiated area. By applying the laser processing method of the present invention, each object to be heated or each irradiation site can be heated with a constant thermal energy.

[Effects of the Invention] As explained above, the laser processing method of the present invention irradiates a processed part with observation laser light of a low output that does not substantially cause a change in the shape or characteristics of the part, and reduces the amount of reflected light. Since the reflectance is calculated from the calculated reflectance and the optimum laser light amount is calculated from the calculated reflectance, it prevents fluctuations in the received heat energy of the processing area due to variations in the reflectance of the processing area, and always performs galoeing with the optimum heating amount. be able to.

[Brief explanation of drawings]

Figure 1 shows the laser processing using the laser processing method of the present invention.
FIG. 2 is a block diagram showing the trimming device; FIG. 2 is a flowchart showing its control operation; FIG. 3 is a diagram showing the relationship between the reflectance R of the wafer used as a sample and the fusing power P calculated from it; The figure is a block diagram showing a second embodiment of a laser trimming apparatus using the laser processing method of the present invention. FIG. 5 is a partial cross-sectional view showing an example of a wafer used for laser trimming. 1...Laser light irradiation section 2...Reflected light measurement section 3...Control section

Claims (1)

[Claims] (1) In a laser processing method in which a laser beam is irradiated onto a processing part of a workpiece to change its shape or characteristics, the processing part is irradiated with a low-output observation laser beam, and the amount of reflected light is determined based on the amount of reflected light. Determine the reflectance in the vicinity thereof, and calculate the optimum amount of laser light to be irradiated to the processed area so that the amount of laser light absorbed by the processed area becomes a constant desired value based on the calculated reflectance. A laser fusing method characterized in that the processing area is irradiated with a laser beam adjusted to an optimum laser beam intensity value.