JP3340792B2 - Microstructure measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a phase shift amount and a phase shifter film thickness of a phase shift mask of a phase shift mask used for manufacturing a semiconductor integrated circuit.
The present invention relates to a technique for measuring fine irregularities, and more particularly to a fine structure measuring apparatus capable of precisely measuring a fine structure using a differential optical heterodyne method.

[0002]

2. Description of the Related Art As the degree of integration of a semiconductor integrated circuit increases, the development of a lithography technique capable of forming a finer pattern has been required.
For example, in the manufacture of 16Mbit DRAM, it is necessary to form a pattern with a minimum line width of 0.5 μm.
A minimum line width of 0.3 μm must be achieved. The use of a phase shift mask has been proposed as one of processing techniques for realizing such a minimum line width of submicron or subhalf micron.

As shown in FIG. 1, the phase shift mask has a metal pattern 2 formed on one surface of a transparent substrate 1, and a transparent shifter 3 is further provided on the metal pattern. When such a shifter 3 is provided, the amplitude distribution of the light transmitted through the phase shift mask is such that the phase is inverted at the shifter portion. Therefore, when the light is projected on the wafer 4, the light intensity distribution has a large contrast. Become. Various types of such phase shift masks have been proposed. For example, when the metal pattern is a periodic pattern, a phase shift mask for a repetitive pattern provided with a shifter every other repetition, a phase shift mask for an isolated pattern provided with a shifter around an isolated pattern, and the like are known. ing.

When a fine pattern is processed using such a phase shift mask, it is important that the amount of phase shift by the shifter is a value that is accurately specified. For this purpose, it is necessary to quantitatively inspect the phase shift amount of the phase shift mask, but conventionally, an ellipsometer that measures the film thickness using multiple reflection between the surface of the shifter and the interface between the transparent substrate and the shifter is used. Had been. However, in the ellipsometer, when the difference between the refractive index of the transparent substrate and the refractive index of the shifter is small, the measurement becomes extremely difficult, and the amount of phase shift is calculated from the thickness of the shifter and its refractive index. When the refractive index is not uniform over the range, the phase shift amount cannot be measured accurately. Also, when the phase shift mask is actually used, the light for exposure passes not only through the shifter but also through the transparent substrate, so that even if only the phase shift amount measured by the phase shifter does not match the actual phase shift amount. There is also.

[0005] To solve the drawbacks of such a phase shift mask inspection apparatus, for example, Japanese Patent Application Laid-Open Nos. 4-151662, 4-181251 and 4-181252 disclose a simple phase shift mask. The interference fringes formed by the periodic pattern on the phase shift mask are moved by changing the angle at which the light beam of one frequency is incident, or by moving the light shielding member arranged in the optical path, and the amount of movement is measured. An apparatus for inspecting a phase shift mask which detects the amount of phase shift by using the method is disclosed.

Further, as a technique for accurately measuring the three-dimensional structure of the surface of a fine structure, a differential optical heterodyne interferometry has been proposed, which is described in, for example, JP-A-59-211810. This measurement method irradiates two light beams having slightly different frequencies onto an object surface, detects a beat component of a light beam reflected from the object surface, and compares the phase with the beat component of the reference light beam. It measures the shape of the object surface. In order to generate two light beams having slightly different frequencies as described above, the acousto-optic device is driven by two AC signals having frequencies independent of each other, and the frequencies diffracted in a direction corresponding to the driving frequency are different. Two light beams are generated, and these light beams are incident on portions of the object surface that are close to each other.

[0007] In the conventional apparatus for forming interference fringes as described above and detecting the amount of phase shift based on the amount of movement, when detecting interference fringes, measurement based on light intensity is performed. Power fluctuations result in measurement errors. In addition, in order to move the interference fringes, it is necessary to change the incident angle of the light beam incident on the phase shift mask or to move the light shielding member. However, an error of the mechanical mechanism is inevitable. The error becomes a measurement error of the movement shift amount. As described above, the conventional phase shift mask inspection apparatus has a disadvantage that the amount of phase shift by the phase shift mask cannot be accurately measured.

Further, if the above-mentioned differential optical heterodyne interferometry is used, there is a possibility that the shape of the surface of the phase shift mask, that is, the thickness of the phase shifter can be measured.
In some cases, the film thickness does not accurately represent the amount of phase shift. That is, the amount of phase shift can be calculated from the thickness of the phase shifter and its refractive index, but the refractive index may not be accurate and the refractive index of the phase shifter may not be uniform,
The amount of phase shift actually introduced often does not match the amount of phase shift calculated. In other words, what is important in the phase shifter is not the film thickness but the phase shift introduced by the thickness, and the phase shift cannot be accurately measured by the above-described conventional differential optical heterodyne interferometry. There is.

Further, in the conventional differential optical heterodyne interferometry, two light beams emitted from the acousto-optic element are split by a beam splitting means in order to obtain a reference light beam serving as a phase reference. One is irradiated on the object to be inspected, and the other is incident on the light receiving means as a reference light beam. However, since the light beam is divided and used as described above, the light amount of the light beam applied to the object is reduced, and there is a disadvantage that the S / N is reduced accordingly.

By applying the principle of the above-mentioned differential optical heterodyne interferometry, a phase shift mask capable of accurately measuring not only the amount of phase shift caused by a phase shift mask but also the phase difference introduced by a fine structure. The present inventor has previously proposed an apparatus (Japanese Patent Application No. Hei 5-20291).
issue). The microstructure measuring apparatus includes a light source unit that emits a light beam including two light beams having slightly shifted frequencies and emitted in different directions, and two light beams emitted from the light source unit. Optical means for causing the microstructure to be measured, first light receiving means for generating a first electric signal representing a beat component of two light beams emitted from the light source means, and A second light receiving means for receiving the transmitted light flux and generating a second electric signal representing a beat component of the two light beams; and the first and second light receiving means.
Phase comparing means for comparing the phases of the first and second electric signals generated from the light receiving means and detecting the amount of phase shift by the fine structure.

In such a microstructure measuring and inspecting apparatus, two light waves having slightly different frequencies interfere with each other, instead of measuring the phase shift amount based on the movement amount of the interference fringes of the light, and the interference phase is beaten. Since the amount of phase shift by the phase shift mask is measured based on the principle of the differential optical heterodyne method that converts the signal into the signal phase, it is possible to essentially measure extremely high accuracy and reduce the light intensity fluctuation of the light source. Even if it does, no measurement error occurs and no mechanical moving mechanism is required, so that the amount of phase shift by the phase shift mask can be measured very accurately and with good reproducibility. Further, when the beat component of the light beam transmitted through the microstructure is to be extracted, unlike the conventional differential optical heterodyne interferometry, the beat component transmitted through the microstructure is extracted.
Since the beat component of the light beam of the book is detected, the amount of phase shift given to the light transmitted through the test object can be accurately measured. Further, when the reference light beam is extracted from the zero-order light beam of the acousto-optic element, it is not necessary to split the light beam to obtain the reference light beam, so that the light amount of the light beam applied to the fine structure Is not reduced, a signal having a high S / N can be obtained, and more accurate measurement can be performed.

[Problems to be solved by the invention]

When the phase shift introduced by the phase shifter of the phase shift mask is measured by the improved differential optical heterodyne interferometry as described above, the phase shifter 1 is formed as shown in FIG. It is necessary to irradiate two light beams to a portion where the phase shifter is not formed and a portion where the phase shifter is not formed. On the other hand, various line-and-space masks are used as phase shift masks. When inspecting these, it is necessary to change the distance D between the two light beam spots 2 and 3 in various ways. For this purpose, it is conceivable to change the interval D between the two light beams by adjusting the optical system. However, it is impossible to avoid disturbance of the optical axis due to the movable part, and the measurement accuracy is reduced. is there. Further, there is a disadvantage that the operation of finely adjusting the movable portion is troublesome, and the operability is poor.

An object of the present invention is to eliminate the above-mentioned drawbacks and to measure the accuracy of measurement by electrically changing the distance between two light beams irradiated on the fine structure, not by adjusting the optical system. It is another object of the present invention to provide a fine structure measuring apparatus capable of improving operability.

[0014]

SUMMARY OF THE INVENTION The present invention is directed to a light source that emits a single-frequency light beam and a single-frequency light beam emitted from the light source. An acousto-optic element for generating two different light beams, a drive circuit for supplying first and second drive signals having slightly different frequencies to the acousto-optic element, and two acousto-optic elements emitted from the acousto-optic element An optical unit for causing a light beam to be incident on a microstructure to be measured; a first light receiving unit for generating a first electric signal representing a beat component of two light beams emitted from the acousto-optic element; A second light receiving means for receiving a light beam transmitted through the fine structure or a light beam reflected by the fine structure, and generating a second electric signal representing a beat component of the two light beams; Generated from two light receiving means A fine structure measuring device comprising: a phase comparing means for comparing the phases of the first and second electric signals to detect a phase shift amount due to the fine structure, wherein the frequency supplied to the acousto-optical element is slightly different. A variable oscillation circuit is provided in a drive circuit for generating a signal, the frequency is changed to change the distance between two light beams incident on the fine structure, and the output signal of the variable oscillation circuit is further changed. The first and second electric signals supplied to the phase comparing means and mixed with the first and second electric signals output from the first and second light receiving means are respectively extracted to obtain third and fourth electric signals having a difference frequency. It is characterized in that the phase of the third and fourth electric signals is compared to detect the amount of phase shift due to the fine structure.

[0015]

According to the present invention, two light beams incident on a fine structure are effectively utilized by effectively utilizing the characteristic that the direction of a light beam emitted from the acousto-optical element changes depending on the frequency of a drive signal applied to the acousto-optical element. Can be changed very minutely and very accurately. Since the frequency of the drive signal is changed in this manner,
There is no need to provide a movable part in the optical system, so that the measurement accuracy is not reduced and the operability is very good. Further, the output signal of the variable oscillation circuit for changing the drive signal is supplied to the phase comparing means,
And the invention that mixes with the second electric signal,
The same advantages as the superheterodyne method can be obtained, and the phase shift amount due to the fine structure can be measured with extremely high accuracy.

[0016]

FIG. 2 is a diagram showing a configuration of a first embodiment of a fine structure measuring apparatus according to the present invention. In this example, a laser light source 11 that emits laser light of a single frequency is provided. As the laser light source 11, for example, a He-Ne gas laser that emits a laser beam having a wavelength of 6328 ° can be used. The laser light is made to enter the acousto-optical element 12 at a predetermined incident angle. The acousto-optic device 12 includes two oscillators 13a and 13b having slightly different frequencies and a double balanced modulator (abbreviated as DBM for short) and two slightly different frequencies from each other. Give a drive signal. In the present invention, the first oscillator 13
a is configured as a fixed oscillator, and the second oscillator 13b is configured as a variable oscillator. For example, the oscillation frequency of the first oscillator 13a can be set to 70.5 MHz, and the oscillation frequency of the second oscillator 13b can be set to 535/2 to 1605 / 2KHz. In this case, from DBM15, 70.5MHz ± 535/2 to 70.5MHz ± 1605
A / 2KHz drive signal will be output. For the time being,
The description will be made on the assumption that the output frequency of the second oscillator 13b is fixed to 500 KHz. When the acousto-optic device 12 is driven by two drive signals having different frequencies (ν1 and ν2), two laser light beams having different frequencies are emitted from the acousto-optic device in different directions. That is, assuming that the frequency of the incident laser light is ν, a light beam having a frequency of ν + 70 MHz and a light beam having a frequency of ν + 71 MHz are emitted in different directions.

In this way, two light beams emitted from the acousto-optic element 12 are made incident on the beam splitter 15,
The light beam is split into a first light beam reflected by the reflection surface and a second light beam transmitted through the reflection surface. The first light beam is condensed by a first condenser lens 16 and made incident on a first light receiver 17, and a first electric signal generated from the first light receiver is amplified by a first preamplifier 18. I do. The first light incident on the first light receiver 17
Are not affected by the phase shift mask, and therefore, the first electric signal output from the first preamplifier 18 functions as a phase reference signal.

The phase shift mask 20 to inspect the second light beam transmitted through the beam splitter 15 through the objective lens 19
Incident on The light beam transmitted through the phase shift mask 20 is condensed by the second condenser lens 21 and made incident on the second light receiver 22, and the second electric signal generated from the second light receiver is converted into the second electric signal. It is amplified by the preamplifier 23. The first and second electric signals output from the first and second preamplifiers 18 and 23 are supplied to a phase comparator 24 to detect a phase difference between these two signals, and output the phase difference signal. Output to terminal 25. As will be described later, the phase difference signal output from the phase comparator 24 indicates the phase difference of the light transmitted through the portion of the phase shift mask 20 that is the subject, where the phase shifter is provided and the portion that is not provided with the phase shifter. The phase difference generated by the phase shifter of the phase shift mask 20 can be detected based on the phase difference signal.

Next, the operation of the microstructure measuring apparatus of this embodiment will be described. FIG. 3 shows the operation of the acousto-optic device 12 in this example. A laser light beam having a frequency ν is incident on the acousto-optic element 12 at a predetermined incident angle θ. The frequencies of the electrodes of the acousto-optic element 12 are ν1 and ν2, respectively.
When a drive signal (ν1 <ν2) is given from the DBM 14, transmitted light that is not frequency-modulated is emitted in a direction coinciding with the incident direction, and diffracted light with a frequency of ν + ν1 and ν + ν2.
Will be emitted symmetrically with respect to the zero-order diffracted light. As described above, the acousto-optic element 12 simultaneously performs not only the deflection of light but also the modulation of the frequency. Such an operation itself is known, for example, “Optics” published in May 1992, Vol. 5, No. 5, pp. 327-332. The present invention further utilizes the fact that the direction of the diffracted light emitted from the acousto-optic element 12 changes according to the frequency of the drive signal. That is, assuming that the angle with respect to the zero-order diffracted light when the frequency of the drive signal is ν1 and ν2 is θ1, the frequency of the drive signal is ν1 ′,
When the angle changes to ν2 ', the above angle changes to θ2. In the present invention, utilizing the above-described characteristics of the acousto-optic element 12, two light beams whose frequencies are slightly different from each other and whose emission directions change slightly are generated.

FIG. 4 is an enlarged view showing the structure of the phase shift mask 20. A phase shifter 20b is provided on a transparent substrate 20a.
Are selectively formed. Here, when the light beam passes through a portion where the phase shifter 20b is not formed and a portion where the phase shifter is formed, a phase shift occurs. That is, assuming that the thickness of the phase shifter 20b is d, the refractive index is n, the wavelength of the light beam is λ, and the phase difference is Δφ, the relationship expressed by the following equation (1) is obtained. To establish.

(Equation 1)

In the present invention, since two light beams having slightly different frequencies are used, these light beams generate a beat. The frequency of this beat is equal to the difference between the frequencies of the two light beams, and has a phase equal to the above-described phase difference Δφ. Therefore, the phase shift amount Δφ introduced by the phase shifter of the phase shift mask can be detected by comparing the phase of the beat component with the reference phase.

FIG. 5 shows that the first and second preamplifiers 18 and 23 transmit the two light beams constituting the second light beam when both light beams pass through the portion of the phase shift mask 20 where the phase shifter 20b is not formed. FIG. 3 shows first and second electric signals S1 and S2 to be output. In this case, there is a phase difference Δ between these first and second electric signals S1 and S2.
t1 occurs. FIG. 6 shows the first and second preamplifiers 18 when one of the two light beams passes through the portion of the phase shift mask 20 where the phase shifter 20b is not formed and the other passes through the portion where the phase shifter is formed. And 23 show the first and second electric signals S1 and S2 output from the first and second electric signals. In this case, a phase difference Δt2 occurs between the first and second electric signals S1 and S2. As described above, the phase of the beat component differs between when two light beams pass through the phase shifter 20b and when they do not. That is, by obtaining these phase differences Δt1 and Δt2, the phase shift amount Δφ introduced by the phase shifter can be obtained from the following equation (2). T is the second
Of the electric signal S2 of FIG.

(Equation 2)

In the present invention, the first and second light receivers 17 and 22 receive two coherent light beams in a superimposed manner, so that the first and second light receivers 17 and 22 output the first and second light beams. The electric signals S1 and S2 indicate the beat components of the two light beams. That is, the beat signal detected by the light receiver is such that the interference fringes due to the two light waves move by the number of fringes equal to the difference between the frequencies of the two light waves per second. Therefore, by detecting the phase difference between the first and second electric signals S1 and S2, the phase difference introduced by the phase shifter 20b of the phase shift mask 20 can be detected. Since this phase difference does not detect the light intensity itself, accurate measurement can be performed without being affected by fluctuations in the light intensity of the light source.

Further, in the present invention, as described above, the direction of the light beam emitted from the acousto-optic device 12 can be changed by changing the oscillation frequency of the second oscillator 13b. The distance between the two light beams can be adjusted to match the space. In this case, the frequency of the light beam also changes, but since this modulation appears in the same way for the two light beams, there is no effect on the beat component.

FIG. 7 shows a second embodiment of the fine structure measuring apparatus according to the present invention, in which the same parts as those in the previous example are denoted by the same reference numerals. He as a single frequency laser 11
Using a -Ne laser, a laser beam emitted from the laser is corrected for a cross-sectional shape of the beam via a beam expander 51, and then is incident on the acousto-optic element 12 at a predetermined angle. As described above, the acousto-optical element 12 is supplied with drive signals of variable frequencies ν1 and ν2 from the DBM 14. As described above, the laser beam incident on the acousto-optic element 12 is emitted as two laser beams having different frequencies and different directions. These two laser beams are made incident on a polarizing beam splitter 55 via relay lenses 53 and 54. The polarization plane of the polarization beam splitter 55 is set so as to transmit these laser beams.

In the above embodiment, a beam splitter is used to extract the reference light beam. In this embodiment, two laser beams pass through the beam splitter 55. In this example, a transmitted light beam emitted from the acousto-optic element 12 is used as the reference light beam. Although the frequency of the transmitted light beam was the same as the frequency of the laser beam emitted from the laser 11, it was confirmed that the luminance was equal to the difference between the frequencies of the two AC drive signals generated from the two-frequency oscillator 52. That is, it has been found that the reference light can be obtained by using the transmitted light beam emitted from the acousto-optic element 12. When the transmitted light beam emitted from the acousto-optic element 12 is used in this manner, a beam splitter for splitting the light beam as in the above-described embodiment is not required, and as a result, a high-brightness light is applied to the phase shift mask. Beam can be incident, S / N becomes good,
There is an advantage that measurement accuracy is improved. Furthermore, in the previous example, the setting and adjustment of the optical system including the photodetector were very delicate because it was necessary to interfere and receive the two light beams in order to take the beat component of the reference beam. In this example, since only a single light beam needs to be received, there is an advantage that the setting and adjustment of the optical system becomes very easy. In order to improve the measurement accuracy, it is desirable to make the optical path length of the measurement light beam coincide with the optical path length of the reference light beam. However, if the optical path length of the two reference light beams is increased, the stability is lacking. However, in this example, since the number of reference light beams is one, there is an effect that even if the optical path length is increased, the reference light beam becomes stable. Further, in order to prevent the light amount of the light beam incident on the test object from decreasing, it is conceivable that a reference signal for giving a reference phase is electrically formed from the drive signal of the acousto-optic element 12. Since the reference signal extracted in this manner does not pass through the acousto-optic element, it does not essentially have the phase accuracy as a reference signal, so that high measurement accuracy cannot be ensured.

As shown in FIG. 7, an optical laser beam 56 which travels straight through the acousto-optic device 12 is made incident on a reflecting mirror 57 via a relay lens 53, and is then condensed by a condensing lens 58 to a photodetector 17 having a first slit. Make it incident. Thus this receiver
A first signal serving as a phase reference is generated from 17 and supplied to one input terminal of a phase comparator 24 via a preamplifier 18. The two laser beams transmitted through the polarizing beam splitter 55 are transmitted through a relay lens 59, a quarter-wave plate 60, and a slit 61.
Then, the light is made incident on the objective lens 19 via the beam splitter 62, and is focused and made incident on two portions of the phase shift mask 20 which are close to each other. The phase shift mask 20 is mounted on the XY stage 63. The two laser beams transmitted through the phase shift mask 20 are converted into parallel light beams by the collimator lens 64, and are incident on the second light receiver 22 having a slit via the beam splitter 65. The output terminal of the light receiver 22 is connected to one input terminal a of the switch 66 via the preamplifier 23. The output terminal of the switch 66 is connected to the other input terminal of the phase comparator 24. Therefore, switch 66 is connected to terminal a.
, The phase of the first electric signal from the first light receiver 17 and the phase of the second electric signal from the second light receiver 22 are compared by the phase comparator 24. As in the previous example, the amount of phase shift introduced by the phase shifter of the phase shift mask 20 can be measured.

In this embodiment, two laser beams reflected on the surface of the phase shift mask 20 are further condensed by the objective lens 19, and are polarized through the beam splitter 62, the slit 61 and the quarter-wave plate 60. The light is incident on the beam splitter 55. Since this laser beam has passed through the quarter-wave plate 60 twice, the polarization direction is rotated by 90 °, and is therefore reflected by the polarization beam splitter 55. The laser beam reflected by the polarizing beam splitter 55 is converted into a parallel light beam by the collimator lens 54, and is incident on a third light receiver 68 having a slit. Therefore, the third light receiver 68 detects a beat component which is an interference component between the two laser beams reflected by the phase shift mask 20. The output signal of the third light receiver 68 is converted to a preamplifier 69.
To the other input terminal b of the switch 66. Therefore, when the switch 66 is switched to the terminal b, the unevenness on the surface of the phase shift mask 20, that is, the thickness of the phase shifter can be measured. The slit 61 constitutes a so-called confocal optical system for mainly removing scattered light from the objective lens 19, and has a small width in a direction perpendicular to the plane of FIG. 7 and a direction parallel to the plane of FIG. It is elongated. The slits provided in the light receivers 17, 22, and 68 increase the measurement sensitivity by limiting the number of interference fringes incident on the light receiving surface, and have a narrow width in a direction parallel to the paper of FIG. .

As described above, in this embodiment, by switching the switch 66, the amount of phase shift introduced by the phase shifter of the phase shift mask 20 and the thickness of the phase shifter can be measured. Conventionally, the film thickness of the phase shifter has been measured in order to specify the characteristics of the phase shift mask, but the actual measurement is performed by a contact method using a probe. However, this method is very inaccurate and the measurement is troublesome. In this embodiment, the thickness of the phase shifter can be measured easily and with very high accuracy in a non-contact manner, and at the same time, the amount of phase shift is also measured. It is. The results of actually measuring the phase shift amount and the film thickness of the phase shifter of the phase shift mask according to the present embodiment are shown in the following table. The phase shift amount is represented by an angle, and the film thickness is represented by a unit of nm. The results in the upper row were obtained by setting the magnification of the objective lens to 10 times, forming a chromium pattern on a quartz substrate, and forming a phase shifter on top of the same material as the refractive index of quartz. Shows a measurement of a phase shift mask having irregularities formed on the surface of a quartz substrate using a 20 × objective lens. Furthermore, the same part is measured five times. However, since the XY stage is moved between successive measurements, exactly the same point is not measured exactly.

[Table 1]

Further, in the embodiment shown in FIG. 7, an illumination lamp 70 is provided, and the light radiated therefrom is condensed by a condenser lens.
71, illuminate the phase shift mask by irradiating from below the phase shift mask 20 through the beam splitter 65, and further take an image of the phase shift mask with the CCD camera 72 via the objective lens 19 and the beam splitter 62. I have. The output image signal of the CCD camera 72 is supplied to the monitor 73 so that the measurement site of the phase shift mask 20 can be observed.

FIG. 8 shows another embodiment of the signal processing portion of the fine structure measuring apparatus according to the present invention, which is applicable to the measuring apparatuses shown in FIGS. 2 and 7. The first oscillator 13a for generating a fixed frequency signal is constituted by a local oscillator having a crystal oscillator which oscillates stably. The oscillation frequency of the first oscillator 13a is, for example, 7
1.5MHz. In this example, a varicap is used for the second oscillator 13b that generates a signal of a variable frequency, and the variable oscillator is configured to control the bias voltage by a variable resistor to change the oscillation frequency. In this example, the oscillation frequency of the variable LC oscillator 101 is 990 to 2060 KH
It can be changed in the range of z. The output signal of the oscillator 101 is supplied to one input terminal of the first DBM 102. A 455 KHz signal is supplied from the crystal oscillator 103 to the other input terminal of the DBM 102. Therefore, DB
M 102 outputs a signal of the sum and difference of the frequencies of these two input signals, that is, a signal of 990 ± 455 KHz to 2060 ± 455 KHz. This output signal is filtered 104
Through the lower frequency band signal, i.e. 535 ~ 1650KH
Extract only the signal of z. In addition, this signal is
After amplification at 5, the signal is supplied to a 1/2 frequency divider 106 to be 535/2 to 162
Generates 0 / 2KHz signal. Since the waveform of this signal is close to a square wave and is not suitable for driving the acousto-optic element 12, the signal passes through the filter 107 to be a signal close to a sine wave.
This is supplied to the second DBM 14 via the amplifier 108. Therefore, from DBM 14, 71.5MHz ± 535 / 2KHz ~ 71.5MHZ
A drive signal of z ± 1620 / 2KHz will be output.

The output signal of the first light receiver 17 for receiving the reference light beam is supplied to the phase comparison circuit 24 via the preamplifier 18. In this example, the phase comparison circuit 24 includes the first
DBM111 is provided, and one input terminal
Output signal, and the other input terminal
The output signal of the LC oscillator 101 is supplied. Since the output signal from the preamplifier 18 is a beat component of two light beams,
The frequency is between 535 KHz to 1605 KHz, and the frequency of the output signal from the variable LC oscillator 101 is 990 to 2060 KHz, so from the first DBM 111 990 ± 535 KHz to 2060 ± 16
05KHz signal is output, but the difference frequency is always 455KHz
Becomes That is, no matter how the oscillation frequency of the variable LC oscillator 101 is changed, the output signal of the DBM 111 includes a 455 KHz signal. The output signal of the DBM 111 is passed through a filter 112 to extract the 455 KHz signal. After the output signal of the filter 112 is amplified by the amplifier 113,
The signal is supplied to one input terminal of the second DBM 114, and the other input terminal is supplied with a 456 KHz signal generated from the fixed oscillator 115. The output signal of the second DBM 114 is filtered 11
6 to extract the difference frequency component, and supply this as a first electric signal to one input terminal of the phase comparator 117.

The signal output from the second light receiving element 22 that receives the light beam having the fine structure and having undergone the phase shift is processed in the same manner as described above.
, A filter 119, an amplifier 120, a fourth DBM 121 and a filter 122, and the fourth DBM 121 has a fixed oscillator 11.
5 output signal. Thus, the filter 122
Is supplied to the other input terminal of the phase comparator 117 as a second electric signal. In this embodiment, the phase comparator 117 is constituted by a digital phase comparator. However, since the frequencies of the first and second electric signals are 1 KHz as described above, the number of samplings can be drastically increased. Therefore, the phase difference can be measured with high accuracy. The phase difference measured in this way is output from the display 123.

FIG. 9 shows a signal waveform in this embodiment. For example, 455 KHz and 99 shown in FIGS. 9A and 9B.
When the 0 KHz signal is supplied to the DBM 102, the DBM outputs a signal in which the 535 KHz signal and the 1525 KHz signal are superimposed as shown in FIG. 9C. This is passed through a filter 104 to extract a 535 KHz signal. This signal is close to a sine wave as shown in FIG. 9D, but the frequency of this signal is
When the signal is converted into a rectangular wave as shown in FIG. 9E in order to reduce the frequency to 1/2 and supplied to the frequency divider 106, the output signal becomes a rectangular wave as shown in FIG. 9F. As described above, since such a square wave is not suitable for driving the acousto-optic element 12,
The signal passes through a filter 107 and is converted into a sine wave as shown in FIG. 9G.

In the above embodiment, the phase comparison circuit
24, a DBM is provided to mix the output signal of the variable LC oscillator 101 for generating the drive signal of the acousto-optic element 12 with the signals output from the first and second light receivers 17 and 22, Next, the operation will be described. In the present invention, the frequency of the drive signal of the acousto-optic element 12 is changed to adjust the interval between two light beams incident on the fine structure. Therefore, the frequency of the output signal of the first and second photodetectors 17 and 22 is 535 to 550 in the above numerical example.
It will change in the range of 1605KHz. Therefore, a circuit for processing the output signals of these light receivers needs to have a wide band of at least 1070 KHz as shown in FIG. 10A. On the other hand, a large noise N is superimposed on the output signals of the light receivers 17 and 22 over the entire band. Therefore, the first and second electric signals supplied to the phase comparator 117 contain a very large amount of noise, and the phase difference between these electric signals cannot be accurately detected. On the other hand, in the above-described embodiment, the DBMs 111 and 118 are provided, so that the
Since a signal of 455 KHz is taken out, a circuit having a very narrow band of 3 to 5 KHz is sufficient as shown in FIG. 10B. Therefore, even if noise N of the same level as in the case of FIG. 10A is present, the S / N is increased, and the phase difference can be accurately detected in the phase comparator 117, thereby improving the measurement accuracy. be able to.

FIG. 9 is a block diagram showing another embodiment of the signal processing circuit of the fine structure measuring apparatus according to the present invention. In the embodiment shown in FIG. 8, the output signal of DBM 102 is 1 /
The output signal of the variable oscillator 101 is supplied to the DBMs 111 and 118 as it is, while being divided by 2 and supplied to the DBM 108, but in this example, the output signal of the DBM 102 is not divided but divided.
The output signal of the variable oscillator 101 is supplied to the DBM 108 and doubled by the multiplier 130 to be supplied to the DBMs 111 and 118. Also, in this example, the amplifiers 113 and 120
Is supplied to the phase comparator 117 as it is. Other configurations are the same as those of the embodiment shown in FIG. Also in this example, by changing the frequency of the drive signal of the acousto-optical element 12, the interval between the two light beams incident on the fine structure can be adjusted, and the output signal of the variable LC oscillator 101 is compared with the phase comparison circuit. 24 DBMs 11
1 and 118 are mixed with the signals output from the first and second photodetectors 17 and 22 to extract a signal of a constant frequency, so that the phase difference can be accurately detected. .

The present invention is not limited to the above-described embodiment, but can be variously modified and modified. For example, in the above-described embodiment, the phase shifter of the phase shift mask is measured. However, other fine structures can be measured. Further, the embodiment shown in FIG. 7 can be modified so that the second light receiver is omitted and only the shape of the surface of the test object is measured. Further, in driving the acousto-optic element, two oscillators having different oscillation frequencies may be used, and one or both of the oscillation frequencies may be made variable to adjust the interval between the two light beams.

[0038]

In the above-described measuring apparatus for a fine structure according to the present invention, since the principle of differential optical heterodyne interferometry is applied, the measuring accuracy can be extremely increased, and the driving of the acousto-optical element can be performed. By changing the frequency of the signal to change the interval between the two light beams irradiated to the fine structure, there is no movable part in the optical system, and the beam interval can be changed accurately and easily. Further, since the phase difference between the output signals from the first and second photodetectors is detected, even if the intensity of the light beam emitted from the light source fluctuates, no measurement error occurs,
For example, the phase difference introduced by a phase shift mask can be measured very accurately. Further, by supplying the drive signal of the acousto-optic element to the phase comparison circuit and mixing it with the output signal of the light receiver, the same effect as that of a so-called superheterodyne is obtained, and the phase difference is accurately detected without being affected by noise. be able to. In addition, according to the measuring apparatus for a fine structure according to the present invention, which receives not only a light beam transmitted through a test object but also a light beam reflected thereby, not only the phase shift amount but also the surface shape Can be measured accurately, and more useful information can be obtained. Further, according to the fine structure measuring apparatus according to the present invention in which the reference light beam is obtained from the transmitted light beam of the acousto-optic element, a high-brightness light beam can be made incident on the test object and the measurement stability can be improved. Can be further improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a phase shift mask irradiated on a phase shift mask.
It is a top view which shows the positional relationship of the spot of a book light beam, and a phase shifter.

FIG. 2 is an overall view showing an entire configuration of an embodiment of a microstructure measuring apparatus according to the present invention.

FIG. 3 is a diagram showing a situation in which the interval between two light beams is changed in the present invention.

FIG. 4 is a diagram showing a phase difference between a light beam transmitted through a phase shifter and a light beam not transmitted.

FIG. 5 is a diagram illustrating a phase relationship between first and second electric signals detected when the electric signal does not pass through the phase shifter.

FIG. 6 is a diagram showing first and second electric signals detected when one light beam passes through a phase shifter.

FIG. 7 is a diagram showing a configuration of a second embodiment of the fine structure measuring apparatus according to the present invention.

FIG. 8 is a circuit diagram showing a configuration of a first embodiment of a signal processing circuit according to the present invention.

9A to 9G are signal waveform diagrams showing the same operation.

FIGS. 10A and 10B are waveform diagrams showing the same operation.

FIG. 11 is a block diagram showing the configuration of a second embodiment of the signal processing section according to the present invention.

[Explanation of symbols]

 11 Single frequency laser 12 Acousto-optic device 13a Fixed oscillator 13b Variable oscillator 14 Double balanced modulator (DBM) 15 Beam splitter 16, 21 First and second condenser lenses 17, 22 First and second receiver 18, 23 First and second preamplifiers 19 Objective lens 20 Phase shift mask 24 Phase comparison circuit 51 Beam expander 52 Two-frequency oscillator 53, 54, 59 Relay lens 55 Polarizing beam splitter 56 Transmitted light beam 57 Reflecting mirror 58, 71 Condenser lens 60 1/4 wavelength plate 61 Slit 62, 65 Beam splitter 63 XY stage 64, 67 Collimator lens 66 Switch 68 Third light receiver 69 Preamplifier 70 Illumination lamp 72 CCD camera 73 Monitor 101 Variable LC oscillator 102 DBM 103 Fixed Oscillator 104, 107 Filter 106 Divider 111, 118 DBM 112, 116, 119, 122 Filter 114.121 DBM 115 Fixed oscillator 117 Digital phase comparator 130 Multiplier

Claims (7)

(57) [Claims] 1. A light source that emits a single-frequency light beam, and receives a single-frequency light beam emitted from the light source and generates two light beams having slightly different frequencies and different emission directions. Acousto-optic element, and first and second acousto-optic elements having slightly different frequencies
A driving circuit that supplies a driving signal of the following; an optical unit that causes two light beams emitted from the acousto-optic element to enter a fine structure to be measured; and two light beams emitted from the acousto-optic element A first light receiving means for generating a first electric signal representing a beat component of: a light flux transmitted through the fine structure or a light flux reflected by the fine structure; and a beat component of the two light beams. A second light receiving means for generating a second electric signal, and a phase shift amount by the fine structure by comparing the phases of the first and second electric signals generated from the first and second light receiving means. A fine structure measuring device comprising: a variable oscillation circuit provided in a drive circuit for generating a signal having a slightly different frequency to be supplied to the acousto-optical element; By configured to vary the distance between the two light beams incident on the microstructure, and further supplies an output signal of the variable oscillation circuit to said phase comparing means,
The third and fourth electric signals having different frequencies are extracted by mixing with the first and second electric signals output from the first and second light receiving means, respectively, and these third and fourth electric signals are extracted. An apparatus for measuring a fine structure, wherein a phase shift of the fine structure is detected by comparing signal phases. 2. The apparatus according to claim 2, wherein said first light receiving means is configured to receive a zero-order light beam emitted from said acousto-optic element and generate said first electric signal. The microstructure measuring apparatus according to the above. 3. The optical means is provided with a beam splitting means for splitting two light beams radiated from the acousto-optic element, and one of the light beams split by the beam splitting means is incident on the fine structure. The first light receiving means is configured to receive the other light beam split by the beam splitting means and generate the first electric signal. Structure measuring device. 4. A variable oscillation circuit provided in the drive circuit,
A variable oscillator, a first local oscillator, a first double balanced modulator for mixing the output signals of the variable oscillator and the first local oscillator, and an output signal of the first double balanced modulator. Frequency dividing means, a second local oscillator, and a second double balanced modulator for mixing an output signal of the frequency dividing means and an output signal of the second local oscillator. The two balanced signals output from the double balanced modulator are supplied to the acousto-optic element as the first and second drive signals, and the phase comparing means includes the variable signal. Third and fourth double balanced modulators for mixing the signal output from the oscillator with the output signals of the first and second light receiving means and outputting the third and fourth electric signals, respectively; From the output signals of the third and fourth double balanced modulators, A first and a second filter for extracting a signal representing a phase shift component by a fine structure, and a phase comparator for comparing the phases of output signals of the first and the second filters are provided. Item 3. An apparatus for measuring a microstructure according to Item 2. 5. A variable oscillation circuit provided in the drive circuit,
A variable oscillator, a first local oscillator, a first double balanced modulator for mixing the output signals of the variable oscillator and the first local oscillator, a second local oscillator, and the first double balanced A second double balanced modulator for mixing the output signal of the modulator and the output signal of the second local oscillator, wherein the frequency output from the second double balanced modulator is slightly different; The first and second drive signals are supplied to the acousto-optic element as the first and second drive signals, and the phase comparing means includes a multiplying circuit for multiplying a frequency of a signal output from the variable oscillator. Third and fourth double balanced modulators for mixing the output signal of the multiplier circuit with the output signals of the first and second light receiving means and outputting the third and fourth electric signals; Output signals of third and fourth double balanced modulators A first and a second filter for extracting a signal representing a phase shift component due to a fine structure from a signal, and a phase comparator for comparing phases of output signals of the first and the second filters. The apparatus for measuring a microstructure according to claim 2. 6. An apparatus for measuring a fine structure according to claim 5, wherein said phase comparator comprises a digital phase comparator. 7. The phase comparison means includes a third local oscillator, an output signal of the third local oscillator, and third and fourth electric signals output from the first and second filters. And a fifth and a sixth double balanced modulator that mixes the output signals of the first and second double balanced modulators, and the output signals of the fifth and sixth double balanced modulators are supplied to the phase comparator. An apparatus for measuring a fine structure according to claim 6.