JPH0769143B2 - Optical heterodyne edge sensor and scale accuracy measuring device - Google Patents

Description

The present invention relates to an edge sensor for detecting edges of a fine pattern and a scale accuracy measuring device, and more particularly to improvement of an edge detecting mechanism thereof.

[Prior Art] In the field of semiconductor devices, precision measuring instruments, etc., a technique for forming a fine pattern on a substrate is indispensable, and it is required to perform pattern measurement with higher accuracy than its manufacturing precision. Has been done.

For example, high-precision linear encoders are used in precision measuring instruments or NC machine tools, and brush contact types, photoelectric types, electrostatic capacitance types, etc. are well known as linear encoders, but they are all long type. The scale of is needed.

And in the photoelectric encoder, as a long scale,
For example, a glass substrate on which a scale pattern made of chromium is formed is used.

Here, a scale accuracy measuring device is used to measure the accuracy of the scale, and an edge sensor that detects the edge where the chrome scale rises from the glass substrate or a sensor that detects the center position of the scale line is required as a scale reader. Becomes

Conventionally, there are a slit scanning type, a laser scanning type, etc. as the edge sensor, and a photoelectric microscope as the center position detecting sensor of the scale line.

As shown in FIG. 5 (A), the slit scanning type edge sensor is a photometer (not shown) in which the scale pattern 10 is optically enlarged and the image plane thereof is scanned by the light receiving slit 12 in the direction of arrow I. Detect light and dark.

As a result, as shown in FIG. 6B, the amount of light passing through the light receiving slit 12 decreases when scanning the scale pattern 10, and when the amount of light reaches a certain level II, it is determined to be a pattern edge. To do.

Further, there is a photoelectric microscope as shown in FIG.

In the figure, the sensor is an epi-illumination lamp 14, an objective lens optical system 16, splitters 18 and 20, and a vibration slit 2
2, a vibrator 24 that vibrates the vibration slit 22, and a light receiving element 26.

Then, the light emitted from the epi-illumination lamp 14 is reflected by the splitter 18 and radiated via the objective lens optical system 16 onto the scale 28 on which the scale pattern 10 is formed.

The reflected light from the scale 28 is returned to the objective lens optical system 16,
An image is formed on the vibration slit 22 via the splitters 18 and 20,
The light transmitted through the slit is received by the photoelectric element and converted into an electric signal.

The output signal change of the photoelectric element 26 is shown in FIG.

As shown in (A) of the figure, no signal is generated when the image of the scale pattern 10 is out of the amplitude of the slit 22, but when the image comes within the slit amplitude, the slit is generated as shown in (B). A signal with a frequency equal to the vibration frequency is generated.

When the image of the scale pattern approaches the amplitude center of the slit, the amplitude of the signal increases and eventually reaches the maximum at the position of (C), and then one peak begins to dent as in (D).

This means that the vibration slit 22 is scanning up to both sides of the scale pattern 10.

When the scale pattern image further approaches the slit amplitude center, the peak indentation gradually deepens, and when both of them coincide with each other as shown in (E) and the frequency doubles, the scale pattern image becomes slit. It is located in the center.

Therefore, the pattern edge can be measured by detecting the signal shown in (E) in which this frequency is doubled.

However, such photoelectric microscopes or slit scanning type edge sensors are not suitable for measuring a scale pattern having a low contrast because they are directly affected by epi-illumination light because they use an optical image.

In addition, the slit scanning type had to detect a subtle change in transmittance, and the photoelectric microscope type had to vibrate the slit itself. In both cases, the measurement speed was 100 to 200 μm / sec, which was very slow. .

On the other hand, as shown in FIG. 8, the laser scanning type edge sensor includes an objective lens 16 that collects laser light on a scale 26 that is an object to be measured, and a photoelectric sensor that detects scattered light diffusely reflected from the scale 26. It has elements 26a and 26b.

When the edge portion of the scale pattern 10 is irradiated with the laser light, the laser light is diffusely reflected from the edge portion and detected by the photoelectric elements 26a and 26b.

This state is shown in FIG. 9, in which FIG. 9A shows the detection signal of the photoelectric element 26a and FIG. 9B shows the detection signal of the photoelectric element 26b.

Therefore, the edge can be detected by obtaining the peak value of the detection signals of both photoelectric elements.

According to such a laser scanning type edge sensor, there is an advantage that it is not affected by the incident illumination light and is not affected by the contrast of the measured object.

[Problems to be Solved by the Invention] However, the problem with the conventional technology is that the laser scanning edge sensor having various advantages as described above still has a slow measurement speed.

That is, the laser beam is vibrated by the vibrating mirror 30 at a small deflection angle.

This is because the synchronous detection is performed in the signal processing stage, and the vibration speed becomes the rate-determining step, and the measurement speed cannot be increased.

Therefore, with such a laser scanning type edge sensor, there is only a measurement speed of several tens to 100 μm / sec, and for example, a linear encoder of 1000 mm requires a very long time to accurately measure a long scale. The measurement environment changed during long-time measurement, which adversely affected the measurement accuracy.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide an edge sensor and a scale accuracy measuring device with high accuracy and high measurement speed.

[Means for Solving the Problems] To achieve the above object, an optical heterodyne edge sensor according to the present invention includes a laser oscillator, an acousto-optic element, a beam splitter, an objective lens, and two photoelectric conversion elements. , Consisting of a phase meter.

Then, the laser oscillator emits laser light.

The acousto-optic element modulates the laser light into two kinds of laser beams having different frequencies and deflects the laser light in two directions.

The beam splitter splits the two types of modulated laser beams into two directions, a reference laser beam and an irradiation laser beam, respectively.

The objective lens irradiates the object to be measured so that the two types of laser beams included in the irradiation laser beam respectively form two beam spots separated by a predetermined distance on the object to be measured.

The reference side photoelectric conversion element receives the reference laser beam and uses it as a reference signal.

The measurement-side photoelectric conversion element receives the reflected beam of the irradiation laser beam from the object to be measured and uses it as a measurement signal.

The phase meter detects the phase difference between the reference signal and the measurement signal,
Edge signal.

Further, it is preferable that the two beam spots formed on the surface of the object to be measured are adjacent to each other so as not to overlap each other, and the spot area width thereof is equal to or smaller than the scale line width.

The scale accuracy measuring device according to the present invention includes a drive system that relatively drives the scale, a position detection system that detects the position of the scale, and a scale detection system that detects the scale on the scale.

The scale detection system is composed of the optical heterodyne edge sensor.

[Operation] Since the optical heterodyne edge sensor according to the present invention has the above-mentioned means, the reflected beams from the two beam spots formed on the object to be measured by the irradiation laser beam have a flat surface on the object to be measured. If it is reflected from the surface portion, it only causes a constant initial phase difference from the reference beam.

On the other hand, when the two beam spots formed by the irradiation beam are formed to have a step, the reflected beam adds a phase change due to the step to the initial phase and is photoelectrically converted to obtain a measurement signal. Also changes the phase.

However, the reference beam remains with its initial position.

On the other hand, the reference signal and the measurement signal are compared with each other, and the time point at which a phase change occurs in addition to the initial phase difference is a step or edge, and edge detection can be performed by detecting the phase difference between both beams.

At the time of this edge detection, the reference signal and the measurement signal of different frequencies are used to substantially eliminate the limitation on the dynamic range of the phase measurement, and thus the edge can be accurately detected.

Here, unlike the conventional laser scanning type edge sensor, vibration scanning of the beam itself is not required, and edge detection can be performed at extremely high speed.

[Embodiment] A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a state in which an optical heterodyne edge sensor according to an embodiment of the present invention is used as a scale accuracy measuring device for measuring the scale accuracy of a scale, and the reference numeral 100 is given to a portion corresponding to the conventional example. In addition, the description is omitted.

In this embodiment, the scale accuracy measuring device is a scale
A drive system 150 for the 126, a position detection system 152 for the scale 126,
And a graduation detection system 154.

The drive system 150 is composed of a coarse feed mechanism and a minute feed mechanism (not shown). The minute feed mechanism includes a piezoelectric element 156 and a piezoelectric element driver 158 that supplies a drive voltage to the piezoelectric element 156. , And a drive controller 160.

Then, the scale 128, which is the object to be measured, is driven in the direction of arrow I by the piezoelectric element 156 under the control of the drive controller 160.

The position detection system 152 includes a light wave interferometer 162 and an amplifier 164.
And counter 166.

Then, the movement signal of the scale 128 detected by the interferometer 162 is amplified by the amplifier 164 and counted by the counter 166.

A feature of the present invention is that an optical heterodyne edge sensor is used for the scale detection system 154.

The optical heterodyne interferometry method causes interference between the reference light and the measuring light having similar frequencies, and there is an example in which a surface roughness measuring device has been developed by using this principle in the past (Japanese Patent Laid-Open Publication No. Sho.
59-20849).

However, in the surface roughness measuring device, the target unevenness is 2 to 3 μm and the order is different from the film thickness (0.01 to 0.5 μm) of the scale or the like targeted by the present invention, and the optical heterodyne interferometry is used as the edge sensor. I had no idea what to do.

That is, in the region of the thickness of the scale, the complex amplitude distribution of the light changes intricately at the scale edge portion due to the step and the focus position, and further, the thickness of the wavelength scale of the light of the scale scale (for example, chrome). When the light is incident on the metal surface of, unlike the case where the light is incident on a simple metal having a thickness of several μm or more, the depth of the light penetrating into the metal is significantly increased. The phase that occurs at the edge is not directly proportional to the phase of the optical path difference due to the step because it varies depending on the type.

Therefore, with such a film thickness, the phase difference was not proportional to the optical path difference due to the step, and it was considered impossible to use the optical heterodyne interferometry for edge detection.

However, the present inventor has found that even for a thin film as described above, it is possible to obtain a phase difference output although it is not proportional to the step, and it is formed on the substrate by optical heterodyne interferometry. We have developed an optical heterodyne edge sensor that directly detects the steps on the thin film scale.

In the present embodiment, the optical heterodyne essence sensor 154
Includes a laser oscillator 168, a Bragg cell (acousto-optical element) 170, a beam splitter 172, a reference side photoelectric conversion element 174, a measurement side photoelectric conversion element 176, and a phase meter 178.

Then, the laser light emitted from the laser oscillator 168 enters the Bragg cell 170 through a predetermined optical system.

The Bragg cell 170 receives a driving voltage from a Bragg cell driver 182 under the control of the controller 180.

Here, the Bragg cell 170 creates a density wave of the same wavelength as the ultrasonic wave in the internal medium by the ultrasonic wave generated by the piezoelectric transducer, and the density wave causes a periodic change of the refractive index to the incident light. On the other hand, it plays the role of a phase grating and diffracts the light, and since the light is reflected by the kinetic density wavefront, the Doppler effect occurs, and it has two functions of changing the frequency of light by the frequency of ultrasonic waves.

The Bragg cell driver 182 has a high frequency (several tens to several hundreds of MHz) generation circuit for carrier built in, and another frequency (about 50 KHz to 1 MHz) is externally fetched to mix two kinds of frequencies. The thing is applied to the Bragg cell 170 as a drive source.

In this embodiment, for example, fc + f is applied to the Bragg cell 170.
Two frequencies, m (70MHz + 80KHz) and fc-fm (70KHz-80KHz) are applied.

Then, the two laser beams pass through a predetermined optical system and reach the beam splitter 172.

The beam splitter 172 further divides the incident laser beam into two directions, one of which is reflected by the beam splitter 172 and is irradiated as an irradiation laser beam onto the scale 128 via the objective lens 184.

Then, the reflected beam from the scale is again the objective lens.
The light enters the measurement-side photoelectric conversion element 176 via the beam splitter 184 and the beam splitter 172 and is converted into the measurement signal s1.

On the other hand, the other split by the beam splitter 172 passes through the beam splitter 172, enters the reference side photoelectric conversion element 178 as a reference laser beam, and is converted into a reference signal s2.

Both signals s1 and s2 are amplified by the amplifier 186 and input to the phase meter 178.

Then, the phase difference between the two signals is detected, and the edge pulse circuit 18
The edge signal s3 is output by 8.

The edge signal s3 is output to the printer 190 together with the output of the counter 166, and the edge position is printed out.

The operation of the optical system of this embodiment is shown in more detail in FIG.

As is clear from the figure, the laser oscillator 168 is a He-Ne
It is composed of a laser light source 168a and a beam expander 168b, and is arranged with a slight angle shift with respect to the reference axis 1.

The laser light emitted from the laser oscillator 168 enters the Bragg cell 170 via the cylindrical lens 200 and the spherical lens 202.

In the Bragg cell 170, two laser beams L1 and L2 whose frequencies have been changed as described above are formed, respectively diffracted, and emitted from the slit 206a of the stopper 206 via the spherical lens 204 substantially along the reference axis 1.

The laser light component L3 not diffracted by the Bragg cell is emitted from the Bragg cell 170 while being deviated from the reference axis 1 and absorbed by the stopper 206.

Laser beams L1 and L2 emitted from the slit 206a
Enters the beam splitter 172 through the cylindrical lens 208 with a slight difference in diffraction angle.

In the beam splitter 172, some of the laser beams L1 and L2 pass through as they are, and enter the reference photoelectric conversion element 174 to be converted into a reference signal.

On the other hand, the laser beams L1 and L2 reflected by the beam splitter 172 toward the scale 128 form a spot on the scale 128 via the objective lens 184.

As shown in FIG. 3, spots 212a and 212b are formed corresponding to the minute diffraction angle differences of the laser beams L1 and L2, and the diameters of the spots are approximately 1 μm, and both spots are adjacent to each other. .

Therefore, if both spots are positioned with a step, an optical path difference (phase difference) is generated between the reflected beams, a difference in quantum mechanical interaction between light and glass, and a difference between light and metal. Optical factors will be added to change the phase.

Then, the reflected beams from both spot positions again enter the measurement side photoelectric conversion element 176 via the objective lens 184 and the beam splitter 172, and are converted into a measurement signal.

The scale measuring system of the measuring apparatus according to the present embodiment is configured as described above, and its operation will be described below with reference to FIG.

First, as shown in FIG.
When spot scanning is performed in the direction, a signal as shown in FIG. 7B is obtained from each photoelectric conversion element.

Here, the solid line is the reference signal s from the reference side photoelectric conversion element 174.
2, the dotted line indicates the measurement signal s1 from the measurement-side photoelectric conversion element 176.

As is clear from the figure, when both spots are on the scale or on the substrate, there is only an initial phase difference between them, and the beat frequency 2fm of fc + fm and fc−fm is drawn, but at the stepped portion, the phase is further increased. Cause a change.

That is, the measurement signal s1 has a phase lead with respect to the reference signal s2 at the time of shifting from the scale to the substrate, and has a phase delay at the time of shifting from the substrate to the scale.

If the phase advances and the phase lags, + and-in the Z-axis direction of the optical system.
It may be reversed depending on how to take and focus position.

Therefore, when the phase difference between the two signals s1 and s2 is calculated by the phase meter 178, peaks are generated on the positive and negative sides corresponding to the steps, as shown in FIG. 4 (C).

Then, when the threshold levels + SH and -SH set respectively for positive and negative are reached, the edge pulse circuit 188 generates an edge pulse as shown in FIGS.

The measurement result obtained in this way is output to the printer 190 together with the edge position signal.

According to the optical heterodyne edge sensor configured as described above, it is possible to measure the scale accuracy of a line width of about 2 μm.
A measurement speed of about 1 mm / sec to 5 mm / sec can be obtained, which is 10 to 100 times faster than the conventional edge sensor.

Moreover, at a measurement speed of 1 mm / sec, repeatability σ =
0.01 μm can be obtained.

By shortening the measurement time, changes in environmental factors during measurement can be reduced, and measurement can be performed under constant conditions, and errors due to thermal expansion that deteriorate measurement accuracy can be reduced.

[Effects of the Invention] Since the present invention is configured as described above, it has the following effects.

In the optical heterodyne edge sensor according to claim (1), since the edge of the object to be measured is directly detected by the optical heterodyne interferometry, it is necessary to vibrate the beam by using a vibrating mirror or the like as in the conventional case. Is eliminated, and edge detection can be performed at high speed.

At the time of this edge detection, the reference signal and the measurement signal of different frequencies are used, thereby substantially eliminating the limitation on the dynamic range of the phase measurement, so that the edge can be accurately detected.

In the optical heterodyne edge sensor according to claim (2), since the beam spots are adjacent to each other so as not to overlap each other and the spot area width is set to be equal to or smaller than the scale line width, the scale edge can be accurately measured. .

In the scale accuracy measuring device according to claim (3), since the optical heterodyne edge sensor is used as the scale detection system, the edge can be accurately detected, and the accuracy of the long scale is short. The accuracy can be measured with, and the measurement accuracy is improved because it is less affected by environmental changes.

[Brief description of drawings]

FIG. 1 is an explanatory view of a scale accuracy measuring device using an optical heterodyne edge sensor according to an embodiment of the present invention, FIG. 2 is an explanatory view of an optical system of the edge sensor shown in FIG. 1, and FIG. FIG. 4 is an explanatory view of a beam spot of the edge sensor shown in FIG. 1, FIG. 4 is an explanatory view of a signal processing state of the edge sensor shown in FIG. 1, and FIG. 5 is a conventional slit scanning type edge. FIGS. 6 and 7 are explanatory views of a conventional photoelectric microscope, and FIGS. 8 and 9 are explanatory views of a conventional laser scanning edge sensor. 10,110 …… Scale pattern 28,128 …… Scale 150 …… Drive system 152 …… Position detection system 154 …… Scale detection system 168 …… Laser oscillator 170 …… Bragg cell (acousto-optic device) 172 …… Beam splitter 174 …… Reference side Photoelectric conversion element 176 …… Measurement side photoelectric conversion element 178 …… Phase meter

Claims (3)

[Claims] 1. An edge sensor for detecting a difference in level of a thin film scale formed on a substrate, wherein a laser oscillator for emitting a laser beam and the laser beam are modulated into two types of laser beams having different frequencies and are bidirectional. An acousto-optic device that deflects, a beam splitter that separates the two types of modulated laser beams into two directions, that is, a reference laser beam and an irradiation laser beam, and the two types of laser beams included in the irradiation laser beam are measured. An objective lens for irradiating the object to be measured so as to form two beam spots separated by a predetermined distance on the object, a reference side photoelectric conversion element which receives the reference laser beam and serves as a reference signal, and the irradiation laser. A photoelectric conversion element on the measurement side that receives a reflected beam of the beam from the object to be measured and outputs a measurement signal, and the reference signal. An optical heterodyne edge sensor comprising: a phase meter that detects the phase difference between the signal and the measurement signal and uses it as an edge signal. 2. The sensor according to claim 1, wherein
An optical heterodyne edge sensor characterized in that the two beam spots formed on the surface of the object to be measured are adjacent to each other so as not to overlap each other, and the spot area width is equal to or less than the scale line width. 3. A scale measuring device including: a drive system that relatively drives a scale; a position detection system that detects the position of the scale; and a scale accuracy detection system that includes a scale detection system on the scale, The graduation detection system includes a laser oscillator that emits laser light, an acousto-optic device that modulates the laser light into two types of laser beams having different frequencies and deflects the laser beams in two directions, and the two types of modulated laser beams. A beam splitter that separates into two directions as a reference laser beam and an irradiation laser beam, and the two kinds of laser beams included in the irradiation laser beam respectively form two beam spots separated by a predetermined distance on the object to be measured. An objective lens for irradiating the object to be measured, and a reference side for receiving the reference laser beam and using it as a reference signal An electric exchange element, a measurement-side photoelectric conversion element that receives a reflected beam of the irradiation laser beam from the measured object side, and uses it as a measurement signal, detects a phase difference between the reference signal and the measurement signal, and uses it as an edge signal. A scale accuracy measuring device comprising a phase meter and an optical heterodyne edge sensor including the phase meter.