JPH02247503A - Moire type mask aligning method using photothermoelastic effect - Google Patents

Abstract

PURPOSE:To align an optical mask with high accuracy by detecting elastic wave vibration generated in a wafer according to photothermoelastic effect generated as diffraction gratings provided on the mask and wafer opposite each other are displaced relatively. CONSTITUTION:The 2nd diffraction grating 3 on the optical mask is irradiated with a laser light beam which is intensity-modulated with an acoustic frequency - an ultrasonic wave frequency and the beam is projected to the wafer 4a made of an elastic vibration material so that the projection image on the grating 3 is in the same direction at the same pitch with the 1st diffraction grating cut in the wafer 4a. Then the elastic wave vibration generated in the wafer 4a by the photothermoelastic effect of the irradiation with the intensity-modulated laser light beam through the grating 3 is detected and converted into an electric signal. Then this electric signal is detected synchronously as to the reference phase of the acoustic frequency - ultrasonic wave frequency. Then the relative position of the optical mask to the wafer 4a in the same direction is set according variation in the output intensity of the synchronous detection proportional to variation in the intensity of a moire image formed owing to the position shift between the projection image of the grating 3 and the grating 4.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention applies pattern transfer technology in a lithography process, for example, when manufacturing integrated circuits, the same optical mask and semiconductor wafer or wafer made of elastic vibrating material can be used. Regarding the method of mask positioning by overlapping patterns, in particular, it is possible to achieve much more precise mask positioning than conventional methods by detecting moiré caused by misalignment of overlapping identical patterns using photo-thermoelastic effects. This is how it was done.

(Prior Art) Generally, in order to align an optical mask with respect to a semiconductor wafer during the manufacture of integrated circuits, diffraction gratings with the same pattern are placed in two orthogonal axes at corresponding corners of the wafer and the mask, respectively, so as to face each other. and positioning is performed in two orthogonal axes directions by overlapping the diffraction gratings,
Furthermore, similar pairs of diffraction gratings are provided at diagonal corners of the wafer and mask to correct the mutual inclination of the two orthogonal axes.

However, when the line width of the pattern forming an integrated circuit is relatively large as in the conventional case, the relative positioning of the mask and wafer by pattern transfer as described above depends on the feeding accuracy of the stage on which the wafer is mounted, that is, the moving frame. This has been done within the image resolution accuracy of microscopes conventionally used to align markers such as the above-mentioned diffraction gratings. However, with the recent trend toward higher integration, the line width of patterns forming integrated circuits has become significantly thinner than before, and next-generation integrated circuits are now likely to have line widths on the order of submicrons. Therefore, alignment of the mask and wafer for patterns with such line widths requires precision on the order of several tens of nanometers, so the conventional alignment methods described above are not suitable for use in the production of next-generation integrated circuits. It would be impossible.

(Problems to be Solved by the Invention) The above-mentioned mask positioning in which the diffraction grating image on the optical mask is projected over the diffraction grating on the semiconductor wafer to eliminate positional deviation between the two is highly accurate and sensitive. As a technology to achieve this, a moiré method has been proposed that uses a laser as a light source for mask irradiation.

However, since laser light has extremely good coherence, if you try to overlap a pair of diffraction gratings while keeping them perfectly parallel and detect mutual coincidence, the difference between the reflected light from the diffraction grating surface and the irradiated light Interference is unavoidable, and the interference between the unnecessary reflected light and the signal light interferes with mutual coincidence detection due to the influence of minute changes in the grating gap, making it difficult to measure the lateral displacement of the diffraction grating. As a result, highly accurate positioning cannot be performed, or a complicated position detection method must be used to avoid such interference.

On the other hand, as a measurement technology for laser applications, when semiconductor wafers, dielectric materials, metal materials, and other elastic vibrating materials are irradiated with laser light, the photothermal strain that occurs in these materials and the resulting photothermoelastic effects are used. The generated elastic wave vibrations have been attracting attention as a highly sensitive spectroscopic method or a non-destructive inspection method for defects inside materials, and various studies have been conducted. However, let alone research aimed at applying the elastic wave vibrations generated by such photo-thermoelastic effects to the above-mentioned positioning or degeneration detection, there has been no research that has mentioned the possibility of such applications. Not reported.

(Means for solving the problem) However, in the so-called photoacoustic method that utilizes elastic wave vibrations generated in elastic vibrating materials due to photo-thermoelastic effects,
Generally, elastic wave vibrations at acoustic or ultrasonic frequencies are generated with an intensity proportional to the amount of light absorbed. Therefore, if such a photoacoustic method is applied to positioning or displacement detection using the moiré method described above, the detection output signal can be obtained exclusively by light absorption, and the influence of interference between undesired reflected light and signal light can be completely eliminated. It is hoped that this can be eliminated. In order to realize these expectations, the present invention applies a photoacoustic method to so-called laser diffraction moiré displacement measurement, thereby making it possible to achieve positioning and displacement detection with much higher precision than in the past. .

An object of the present invention is to solve the above-mentioned conventional problems, and to reduce the irradiation intensity that changes depending on the mutual positional shift of the diffraction gratings of laser light that is projected by overlapping the diffraction grating on the optical mask with the diffraction grating on the semiconductor wafer. , a moire-type mask positioning method using photo-thermo-elastic effects that enables highly accurate detection by detecting elastic wave vibrations generated by photo-thermo-elastic effects with an intensity proportional only to the amount of absorption of the irradiated light. Our goal is to provide the following.

Another object of the present invention is to detect a change in moiré intensity caused by a positional shift of a projected image of a diffraction grating on an optical mask, which is projected onto a diffraction grating on a semiconductor wafer, with much higher precision than in the past. An object of the present invention is to provide a moiré mask positioning method using photo-thermoelastic effects.

Still another object of the present invention is to provide an optical system capable of accurately and highly accurately detecting changes in the intensity of elastic wave vibrations that occur in proportion to the moiré intensity that changes depending on the mutual positional deviation of the diffraction gratings. An object of the present invention is to provide a moiré mask positioning method using thermoelastic effects.

Still another object of the present invention is to provide a moire-type mask positioning method using photo-thermoelastic effects, which allows the diffraction grating on the mask to be accurately overlapped with the diffraction grating on the wafer to project a Buddha statue or the like. I have something to do.

In other words, the Moire mask positioning method using the photo-thermoelastic effect of the present invention irradiates the second diffraction grating on the optical mask with a laser beam whose intensity is modulated at an acoustic frequency or an ultrasonic frequency to generate elastic vibrations. a step of projecting the second diffraction grating onto a wafer of the material so that the projected image of the second diffraction grating is in the same direction and with the same pitch as the first diffraction grating carved on the wafer; and the intensity modulation. detecting elastic vibrations generated in the wafer due to the photo-thermoelastic effect of the irradiation of the laser beam through the second diffraction grating, and converting the elastic vibrations into electrical signals; A process of synchronous detection with respect to a reference phase of a sound wave frequency, and a change in the output intensity of the synchronous detection that is proportional to a change in intensity of a moiré image caused by a positional shift between the projected image of the second diffraction grating and the first diffraction grating. setting a relative position of the optical mask in the same direction with respect to the wafer based on;
It is characterized by the fact that it has disappeared.

(Function) The present invention is the first to employ elastic wave vibration due to photo-thermoelastic effects as a detection signal for detecting the relative position of a pair of projected and superimposed diffraction gratings. Ba,
Conventionally, this kind of relative positional deviation detection has been hindered by using elastic wave vibrations with an intensity proportional only to the amount of transmitted and absorbed light that accurately corresponds to the relative positional deviation to detect changes in moiré intensity caused by relative positional deviations of the diffraction gratings. This enables highly accurate detection without being affected by the amount of reflected light.

In addition, the intensity of moire caused by the relative positional shift of the diffraction grating changes with the pitch of the grating as a period with respect to lateral displacement, and the change in moire intensity with respect to minute displacement, that is, the maximum differential coefficient is caused by the diffraction The relative position between the grids is 9
According to the present invention, each of the pair of diffraction gratings is divided into two halves, and the relative positions of the diffraction gratings between one half and the other half have opposite phases. By configuring each diffraction grating in this way, the relative position with a 90° phase difference where the change in moiré intensity is maximum can be set as the positioning point, and relative position detection with the highest sensitivity can be performed.

Further, according to the present invention, the intensity of the irradiated light projected by overlapping the diffraction gratings is modulated at an acoustic frequency or an ultrasonic frequency, and elastic wave vibrations at the same frequency as the modulation frequency are generated by photo-thermoelastic effects. , it becomes possible to easily and reliably detect elastic wave vibrations exhibiting an intensity proportional to the moiré intensity corresponding to the relative positional deviation between the diffraction gratings, and also convert the detected elastic wave vibrations into electrical signals of the same frequency. At the same time, the electrical signal is synchronously detected with respect to a reference phase of the same frequency, thereby making it possible to detect a change in the intensity of elastic wave vibration corresponding to the moiré intensity with high sensitivity and a high S/N ratio.

Note that the following method is suitable for detecting elastic wave vibration and converting it into an electric signal.

a) Since the elastic vibration of the wafer generates pressure waves in the surrounding gas, the pressure waves are detected by a microphone and converted into electrical signals. In addition, the microphone effectively detects pressure waves generated by thermal expansion of the surrounding gas when the heat generated when the projected image of the diffraction grating on the mask is absorbed by the diffraction grating cut into the wafer and diffuses into the surrounding gas. It can be converted into an electrical signal.

b) Since the elastic wave vibrations generated in the wafer propagate through the wafer itself or its support, by bringing a piezoelectric element into contact with the wafer itself or its support, the elastic wave vibrations are converted into electrical signals due to the piezoelectric effect.

C) If a probe laser beam is applied to an elastically vibrating material in which elastic vibrations are occurring and the deflection of the reflected light beam in response to the elastic vibrations is detected, the elastic vibrations of the material can be detected completely without contact. If highly sensitive laser beam vibration measurement is performed, elastic vibrations can be detected effectively and accurately. In other words, if the beam spot diameter of the reflected beam of the probe laser beam projected onto a wafer undergoing elastic vibration is smaller than the wavelength of the elastic vibration, periodic changes in inclination due to the elastic vibration of the wafer surface occur. Accordingly, the direction of reflection is periodically deflected. Therefore, if the reflected light beam spot is detected at a position appropriately away from the reflection point, elastic wave vibrations can be detected with high sensitivity despite the relatively simple configuration of the measurement system.

d) If a fixed reflection point for obtaining a reference light beam can be found near the wafer mounted on a movable stand, the probe laser light beam is wavefront-split and split into two, and the wafer and fixed reflection point are separated. If the interference between each reflected light beam from the wafer and fixed reflection point is detected, the optical path length difference between each reflected beam from wavefront division to wavefront synthesis for interference detection can be detected. When can be set to 1/4 wavelength, extremely sensitive detection of elastic wave vibration amplitude can be achieved.

In the moire type mask alignment method of the present invention, depending on the state of the elastic vibrating material wafer to be aligned,
The elastic wave vibration can be detected by selecting the most appropriate method from among the detection methods described above.

Further, as a method for accurately overlapping the diffraction grating on the mask with the diffraction grating on the wafer and projecting the Buddha statue, the following method is suitable.

a) When the diffraction grating on the mask is irradiated with parallel lines, that is, parallel light beams from the light source and the Buddha statue or image is projected onto the wafer, the pattern of the projected image will not be degraded due to the diffraction effect of the grating on the mask. If so-called proximity transfer is performed with the mask sufficiently close to the wafer, mask positioning can be accurately achieved by the method of the present invention without being affected by reflected light in the gap between the mask and the wafer.

b) When a reduction optical system is interposed between the mask and the wafer, and when the reduction optical system reduces the diffraction grating on the mask and projects it onto the wafer, the reduction optical system must be set appropriately to remove the mask. The projected image of the upper diffraction grating can be accurately superimposed on the diffraction grating on the wafer, and as a result, elastic wave vibrations due to photo-thermoelastic effects can be accurately generated in the portion of the wafer where the diffraction grating is carved. can.

(Example) The present invention will be described in detail below with reference to the drawings.

First, an example of the basic configuration of a mask positioning device using the photo-thermoelastic effect according to the method of the present invention is shown in FIG. 1(a), and FIG. The basic configuration shown in Figure (C) uses the detection method C) described above to detect elastic wave vibrations, and detects the deflection of the reflected light beam when the probe laser light is projected onto the wafer. This is an example of how to do this.

In the illustrated basic configuration example, a parallel light beam with a beam diameter of 1 mm, for example, emitted from a laser light source l is periodically driven by a chopper 2 that is rotationally driven by an oscillation output of an acoustic frequency or an ultrasonic frequency f from an oscillator 8. In a state where the intensity is modulated intermittently, the light is projected onto a wafer 4a made of an elastic vibrating material, such as a semiconductor wafer, on which the first diffraction grating 4 is carved on the surface, through a second diffraction grating 3 forming an optical mask. Note that the first and second diffraction gratings 4 and 3 are formed at the same pitch of, for example, 25 μm, and the second diffraction grating 3 is formed on the first diffraction grating 4 by proximity transfer of a parallel light beam with a beam diameter of 1 m, for example. Therefore, when the first and second diffraction gratings completely overlap with each other with a phase difference of 0°, the amount of irradiation light reaching the wafer 4a is maximum, and the gratings with a phase difference of 180° are projected onto the wafer 4a. When the beams are completely alternated, the amount of irradiation light becomes almost zero. As a result, the frequency f
The amplitude of the elastic wave vibration of the frequency f that is generated on the wafer 4a by the photothermoelastic effect with the amplitude proportional to the intensity of the irradiated light when the wafer 4a is irradiated with the parallel light beam whose intensity is modulated by the first and second diffraction gratings. 4 and 3, or more precisely, as the horizontal relative position between the projected images of the first diffraction grating 4 and the second diffraction grating 3 changes from the above-mentioned phase difference of 0° to 180°, the amplitude changes from maximum amplitude to zero amplitude. , changes with the period of the pitch of the diffraction grating 3°4, forming a so-called moiré pattern. By detecting the state of the amplitude change of the elastic wave vibration generated on the wafer 4a and forming moiré, it is possible to perform so-called mask positioning by adjusting the degree of overlapping of the first and second diffraction gratings 4.3. .

In the illustrated basic configuration example, in order to detect such elastic wave vibrations, a laser beam from the auxiliary laser light source 5, which serves as the probe laser beam described above, is projected onto the back surface of the wafer 4a, and the reflected light beam is The beam is guided to a beam spot position detector 6 made of, for example, a photodiode for photoelectric conversion, and the converted output electrical signal of frequency f is guided to a lock-in amplifier 7. In the lock-in amplifier 7, the converted output electric signal is synchronously detected by using the oscillation output of the frequency r from the oscillator 8 as a reference signal, and is deflected by the change in the slope of the reflecting surface due to the elastic wave vibration of the wafer 4a. A change in the intensity of the converted output electrical signal corresponding to a change in the amount of light received by the reflected light beam detector 6 is detected.

On the other hand, the illustrated experimental apparatus detects elastic wave vibrations that change in intensity corresponding to moiré caused by the relative positional deviation of the diffraction gratings on the mask and the wafer. - This is for confirming the thermoelastic effect, and the general configuration is the same as the basic configuration described above, but changes have been made to make it easier to obtain confirmation data.

That is, the wavelength 830nI11 is intensity-modulated by the oscillation output of the frequency 1330KHz from the oscillator 8, and the output 20
25 is formed on a transparent mask substrate 3a by a parallel light beam from a laser light source 1 consisting of a mW semiconductor laser.
The transmission type diffraction grating 3 with a μm pitch is irradiated, and the mask substrate 3 is
The light beams are projected onto a reflection type diffraction grating 4 with a pitch of 25 μm formed on a transparent wafer 4a arranged close to and parallel to a. The mask substrate 3a is mounted on a so-called X-Y stage that is movable in two orthogonal axes directions, and controlled by a microcomputer 10 via a position controller 11 so as to move in 1 μm steps in the lateral direction of the diffraction grating 3.4. Control its position. In addition, as described above, in order to detect the elastic wave vibration generated on the wafer 4a due to the photo-thermoelastic effect of the moire irradiation light by the diffraction gratings 3 and 4, the He-N
A laser light beam from an auxiliary laser light source 5 consisting of an e-laser is projected onto a mirror 4b attached to a wafer 4a, and the beam spot position is detected by the deflection state of the reflected light beam caused by the vibration of the mirror 4b in response to the elastic vibration of the wafer 4a. vessel 6
The frequency 13 obtained as the detection output is detected by
An electrical signal of 30KIIz is guided to a lock-in amplifier 7 via an amplifier 9. The lock-in amplifier 7 performs synchronous detection with reference to the reference phase signal of frequency 1330KIIz from the oscillator 8, and generates a detection output signal exhibiting an amplitude change corresponding to the intensity change of the elastic wave vibration generated in the wafer 4a. It is taken out and supplied to the microcomputer 10. The microcomputer IO controls the operating state of the lock-in amplifier 7 according to the detection output signal, and also controls the diffraction grating 3 on the mask 3a and the wafer 4a, which are represented by amplitude changes of the detection output signal as will be described later. A position correction signal corresponding to the relative positional relationship with the diffraction grating 4 is calculated, and the position of the mask 3a is corrected via the position controller 11 so that the diffraction gratings 3 and 4 have a predetermined relative positional relationship. control and achieve the desired high-precision mask alignment.

Typical measurement results of changes in the detection output signal intensity of the beam spot position detector 6 with respect to changes in the lateral relative displacement of the reflective first diffraction grating 4 and the transmission type second diffraction grating 3, both of which have a pitch of 25 μm. An example is shown in FIG. 2(a). In the measurement results shown, the detected output signal intensity is plotted in arbitrary units on the vertical axis, and the relative displacement between each diffraction grating 3゜4 is plotted on the horizontal axis in μm units, and the measured values taken at approximately 2 μm steps are plotted sequentially. The detected output signal intensity, which changes periodically with a pitch of 25 μm of the diffraction grating 3.4, is caused by the elastic wave vibration generated in the wafer 3a causing Moiré fringes due to the relative positional deviation of the diffraction gratings 3 and 4. This shows that this is caused by the photo-thermoelastic effect of the amount of light absorbed by the wafer 4a.

In addition, in order to confirm that the operating principle of the moire type mask alignment method of the present invention is the generation of elastic vibrations due to the photo-thermoelastic effect of the amount of absorbed light that exhibits moire fringes due to the relative positional deviation between the respective diffraction gratings 3 and 4. FIG. 2(b) shows the measurement results obtained using the experimental apparatus shown in FIG. 2(b).

The measurement results shown are the same as those shown in Figure 2(a), with the spacing Z between each diffraction grating 3°4 as a parameter.
The figures are plotted sequentially for each stage of change. Therefore, the parallel light beam that irradiates the transmission type diffraction grating 3 is reflected at the line portion of the grating 3, and the parallel light transmitted through the space portion forms an image behind the grating 3 due to the diffraction effect of the grating. A so-called self-image phenomenon occurs, and the contrast of the grating image changes periodically according to the distance Z from the grating 3. That is, with respect to the pitch p of the grating 3 and the wavelength λ of the irradiation light, the interval Z is Z
= np2/λ, and when the coefficient n is a natural number, the contrast of the grating image becomes maximum with the period p2/λ, but the phase of the maximum contrast is the period p”/λ.
It is reversed every λ, and at an intermediate interval, the contrast becomes minimum and the grating image disappears, presenting uniform illuminance. Therefore, Fig. 2 (Fig. If the measured values shown in a) show the above-mentioned self-image characteristics depending on the change in the spacing Z between the gratings 3 and 4, then the operating principle of the Moiré mask alignment according to the present invention can be explained by the fact that the light transmitted through the gratings This means that it has been confirmed that elastic vibrations occur due to thermoelastic effects. The measurement results shown in Figure 2(b) are the same as those in Figure 2(a) when the coefficient n mentioned above was changed in three steps to 3.3.5°4. - It clearly shows the above-mentioned characteristics of self-image.

Note that in the configuration shown in the first figure, the above-mentioned detection method C
), the occurrence of elastic wave vibrations in the wafer was detected, but similar measurement results could be obtained using other detection methods, such as detection method a), in which pressure waves in the surrounding gas generated in response to elastic vibrations were detected using a microphone. Of course, this was obtained.

Next, when performing mask alignment based on the change in elastic vibration detection signal intensity corresponding to moiré fringes due to the relative displacement between the diffraction gratings 3 and 4, the interval Z between the diffraction gratings 3 and 4 must be adjusted appropriately. Of course, the settings should be made to obtain the maximum contrast characteristic, but in the characteristic curve shown in Figure 2 (a), the required mask position is usually obtained at the maximum point or This is near the minimum point, and the change in signal intensity with respect to change in relative displacement, that is, the differential coefficient, is small, and the accuracy or sensitivity of mask alignment is reduced. In contrast, diffraction gratings are provided on the mask substrate and the wafer substrate, respectively, so that mask alignment can be performed at the relative displacement point where the differential coefficient is maximum in the characteristic curve shown in FIG. 2(a). An example of the pattern is schematically shown in FIG.

In the illustrated diffraction grating pattern, the grating pattern 12
a and 12b are provided on one substrate, for example, separately on the left and right, and grating patterns 13a and 13b are similarly provided on the other substrate on the left and right, and the grating patterns 12a and 13a have a phase difference of 0°. The grating patterns 12b and 13b are made to face each other with a phase difference of 180°.
Alternately face each other.

Diffraction gratings 12a, 12b and 13a having such configurations,
13b is the mask diffraction grating 3 and the wafer diffraction grating 4 in the basic configuration shown in FIG. Figure 2(a), (
If the same measurement results as described above for b) are obtained simultaneously by time division, the measurement results will be as shown in FIG. 4, for example. That is, in the measurement results shown, a characteristic curve 14 indicated by a solid line corresponds to moiré fringes occurring between grating patterns 12a and 13a, and a characteristic curve 15 indicated by a broken line corresponds to the moiré fringes generated between grating patterns 12b and 13b. In both cases, the elastic vibration detection output signal strength changes in response to a change in relative displacement with a period equal to the pitch of the diffraction grating, but in both cases, the strength of the elastic vibration detection output signal changes with respect to a change in relative displacement with a period equal to the pitch of the diffraction grating. It exhibits a phase difference. If the diffraction grating positions on the mask substrate and the wafer substrate are determined so that the intersection of these two characteristic curves becomes the desired mask alignment point, the point of the maximum differential coefficient on both characteristic curves can be aligned. Therefore, it is possible to achieve mask positioning with much higher sensitivity and precision than in the past.

In addition, as described above, the detection of the elastic wave vibration generated in the wafer is performed not only by using the probe laser beam according to the detection method C) in the configuration shown in the first figure, but also by using the various detection methods a. ) to d), the most suitable one for the condition of the subject can be selected to obtain similar effects, and the configuration of the elastic wave detection means according to each method is shown in the figures below. It is preferable that each is shown schematically.

FIG. 5 shows a schematic configuration of an elastic wave vibration detection means using a microphone according to the detection method a) described above. That is, a wafer 4a with a reflective first diffraction grating 4 carved on its surface.
Microphone 16 is placed close to or in close contact with the back surface of
is arranged, and the microphone 16 detects the sound waves produced by the surrounding gas in response to the elastic vibrations generated in the wafer 4a due to photo-thermoelastic effects and converts them into electrical signals.

Further, FIG. 6 shows a schematic configuration of an elastic wave vibration detection means using a piezoelectric element according to the above-mentioned detection method b). That is, a wafer 4 with a reflective first diffraction grating 4 carved on its surface.
A piezoelectric element 17 is placed in close contact with the back surface of the wafer 4a, and the elastic vibration itself generated in the wafer 4a due to photo-thermoelastic effects is directly converted into an electric signal.

Note that since the elastic wave vibration propagates not only to the wafer 4a itself but also to its support, the piezoelectric element 17
Similar effects can be obtained not only by itself but also by bringing it into close contact with its support.

Furthermore, FIG. 7 schematically shows a schematic configuration of an elastic wave vibration detection means to which the effects of a laser interferometer are applied in accordance with the above-mentioned detection method d). In other words, a laser interferometer applied to detecting elastic wave vibrations uses a diffraction grating arranged in such a way that the laser beam guided from the laser light source 5 via the half mirror 22 is separated and then the respective reflected beams are combined. 18 is used in the form of a two-beam laser interferometer,
A wafer 4a on which a first diffraction grating 4 is provided on the surface of the two light beams whose wave fronts have been separated by a diffraction grating 18 is transmitted through a lens 9.
The reflected light beams are focused on a reflection point on the back surface of the camera and a fixed reference point 20 as a standard, and the reflected light beams traveling backwards along the same optical path are superimposed on each other by a diffraction grating 18, and the light intensity is determined through a half mirror 22. The optical path length difference between the two separated and combined beams is set to 1 with respect to the wavelength of the laser beam.
By detecting the intensity of the interference component of the two light beams, which becomes maximum when the wavelength is set to /4, elastic wave vibration can be detected with high sensitivity.

(Effects of the Invention) As is clear from the above description, according to the present invention, when aligning an optical mask with respect to a semiconductor wafer, there is By efficiently and accurately detecting the elastic wave vibrations generated on the wafer based on the photo-thermoelastic effects of moiré fringes, we have achieved positioning of optical masks with much higher precision than before, making it possible to achieve next-generation super The fabrication of high-density integrated circuits can be adequately addressed.

[Brief explanation of drawings]

FIGS. 1(a) and (b) are block diagrams showing the basic configuration and experimental configuration of a mask alignment apparatus according to the method of the present invention, respectively. FIGS. 2(a) and (b) are photo-thermoelastic effects. Figure 3 is a diagram showing an example of a diffraction grating pattern between a mask and a wafer. Figure 4 is an example of a moire signal waveform using a Nil diffraction grating. FIG. 5 is a block diagram showing an example of an elastic wave vibration detection means, FIG. 6 is a block diagram showing another example of an elastic wave vibration detection means, and FIG. 7 is an elastic wave vibration detection means. FIG. 7 is a block diagram showing still another example of means. 1... Laser light source 2... Chopper 3...
Second diffraction grating 3a... Optical mask substrate 4...
First diffraction grating 4a... Wafer substrate 5... Auxiliary laser light source 6... Beam spot position detector 7... Mouth/ink/amplifier. 8... Oscillator 9... Amplifier 10...
Microcomputer II...Position controllers 12a, 12b, 13a, 13b...Diffraction grating patterns 14, 15...Moiré signal 16...Microphone 17...Piezoelectric element 1B...Beam separation and synthesis diffraction grating

Claims (1)

[Claims] 1. A second diffraction grating on an optical mask is irradiated with a laser beam whose intensity is modulated at an acoustic frequency or an ultrasonic frequency, and the second diffraction grating is projected onto a wafer of elastic vibration material; forming a projected image of the diffraction grating in the same direction and with the same pitch as the first diffraction grating carved on the wafer; a step of detecting elastic vibrations generated in the wafer due to the photo-thermoelastic effect of irradiation through the wafer and converting it into an electric signal; a step of synchronously detecting the electric signal with respect to a reference phase of the acoustic frequency or ultrasonic frequency; The same optical mask is applied to the wafer based on a change in the output intensity of the synchronous detection that is proportional to a change in the intensity of a moiré image caused by a positional shift between the projected image of the second diffraction grating and the first diffraction grating. A moiré mask positioning method using a photo-thermoelastic effect, characterized by a process of setting a relative position in a direction, and a step of setting a relative position in a direction. 2. The first diffraction grating and the second diffraction grating each include a first portion and a second portion, and the projected image of the first and second portions on the second diffraction grating; and the first and second portions in the first diffraction grating so that their respective relative positions are in opposite phases to each other, and the intensity-modulated laser beam in the second diffraction grating is Output intensities of the synchronous detection corresponding to electrical signals obtained by independently detecting and converting elastic vibrations generated in the wafer due to photo-thermoelastic effects of irradiation through the first and second parts, respectively. The moire type mask positioning method using photo-thermoelastic effects according to claim 1, characterized in that the relative positions of the optical mask with respect to the wafer in the same direction are set so that the wafers are equal to each other. . 3. The optical system according to claim 1 or 2, wherein the elastic vibration generated in the wafer is detected by a microphone or a piezoelectric element and converted into an electrical signal.
Moiré mask alignment method using thermoelastic effect. 4. The elastic vibrations generated in the wafer are detected by detecting the acoustic frequency to ultrasonic frequency components in the scattered reflected light of another laser beam projected in the vicinity of the projected image of the second diffraction grating on the wafer. A moire type mask positioning method using photo-thermoelastic effects according to claim 1 or 2, characterized in that: 5. The elastic vibration generated in the wafer is detected by detecting interference components between respective reflected lights near the projected image of the second diffraction grating on the wafer and from other fixed points. Claim 1:
A moiré mask positioning method using photo-thermoelastic effects according to item 1 or 2. 6. The second diffraction grating on the optical mask is projected onto the first diffraction grating on the wafer by proximity projection of parallel light. A moiré mask positioning method using photo-thermoelastic effects as described in item 2. 7. The second diffraction grating on the optical mask is projected to overlap the first diffraction grating on the wafer by reduction projection.
Moiré mask positioning method using photo-thermoelastic effects as described in Section 1.