JPS63173902A - Method for measuring minute dimension - Google Patents

Abstract

PURPOSE:To detect an edge position with high accuracy even when the difference in level at an edge part is the wavelength of beam or less, by detecting the change in the length of the beam path at the difference-in-level part of an edge by an optical heterodyne interference method. CONSTITUTION:The laser beam from a laser beam source 10 is projected on an acousto-optical element AO 11 which is, in turn, excited at the modulated frequency fd+fm of the frequency (fa) in an AO driver 12 and the frequency fm of an external AC power source 14 and, therefore, the laser beam is divided into beams of two frequencies to be projected on a beam splitter 151. The beam reflected by the beam splitter 151 becomes a reference signal 180 and the transmitted beam from said splitter 151 is reflected from an object to be measured to become an object reflected beam signal 170. When the beam paths of two beams on the measuring surface are different, phase shift is generated by optical heterodyne interference and, therefore, by comparing said phase shift with the phase of the reference beam signal 180, an edge part is detected. The direction of beam is changed by changing the voltage Vd of a DC voltage source 13 and an edge position is detected from the voltage obtained when the edge part is detected.

Description

[Detailed Description of the Invention] [Industrial Field of Application] [Background of the Invention] With the recent progress in the precision machinery industry, workpieces have become increasingly micro-sized and highly precise. Processing is performed, and the finely machined dimensions reach the order of 1 micron≠〒-V# (μmm: ri), and there is an increasing need to measure dimensions with an accuracy of submicron meters or less.

[Conventional technology]

One method in the prior art is to illuminate an object to be measured with minute dimensions, magnify it several thousand times with a microscope, and measure the dimensions by receiving the magnified image with an image sensor. It is often used.

This process binarizes the video signal of the received image emitted by the image sensor at a preset slice level value, and processes the pixels of the image sensor that are included between the falling and rising edges of the binarized signal. The dimensions are determined by counting the pitch.

Another method of the prior art is to irradiate the object to be measured with a laser beam focused on a minute spot diameter, and use an acousto-optic element (A.
) is used to electrically scan the laser beam on the surface between the two edges of the object to be measured, and in particular analyze the intensity changes of the reflected light from near the two edges. There is a technique that detects the position and measures the dimensions from the amount of deflection of the acousto-optic element required to deflect light between two edges. This technology was applied for a patent by the inventor of the present application in Japanese Patent Application Laid-Open No. 58.
This is detailed in Japanese Patent Application Laid-open No. 170762 and Japanese Patent Application Laid-Open No. 59-79772.

[Problem that the invention seeks to solve]

The binarization processing method using an image sensor described in the former part of the conventional technology achieves a good contrast with a good S/N ratio of 4 bright and dark areas and stabilizes slice level values in order to perform stable binarization processing. However, if the contrast ratio between the protrusions or depressions of the part where the dimensions of the object to be measured are to be measured and the base material is poor, the slice level value for binarization will become unstable and the measurement accuracy will deteriorate. descend.

The method using the optical deflection of an acousto-optic element mentioned in the latter part of the conventional technology has a small step (height) between the edge part that constitutes the convex part or the concave part of the part where the dimension of the object to be measured is to be measured and the base material part. If the depth of focus is within the focal depth of the focused laser beam to be irradiated, the change in the intensity of the reflected light from the edge portion and the reflected light from the base material portion will be reduced, making it possible to accurately detect the edge position. This will reduce the accuracy of dimensional measurement.

The present invention solves the problems of the conventional measurement method described above, and
Especially if the contrast ratio of the area where the dimensions are measured is poor,
It is an object of the present invention to provide a method for measuring minute dimensions that can stably measure minute dimensions with high precision when the step difference between an edge part and a base material part is small.

[Means for solving problems]

Laser light emitted from a laser light source is incident on an acousto-optic element, and the optical operation of the acousto-optic element is controlled by an acousto-optic element driver operated by applying direct current voltage and alternating current voltage to the acousto-optic element. generates two beams of light having mutually different frequencies and traveling in mutually different directions, separates the traveling directions of the two beams into two different directions, and focuses the separated two beams of light. The light beam is irradiated onto the surface of the object to be measured, the dimensions of which are to be measured, and the reflected light from the object is received to create an object reflected light signal. When directly receiving light to create a reference optical signal, the frequency of the AC voltage signal input to the acousto-optic element driver is controlled to control the angle formed between the two beams, and the acoustic By controlling the voltage of the DC voltage signal input to the optical element driver, one of the two separated beams is deflected by a predetermined distance on the surface of the object to be measured, and each beam is The phase between the object reflected light signal and the reference light signal is detected for each deflection state, and the phase data of the object to be measured is determined from the phase data that is a function of the DC voltage that causes the light deflection to be performed. The dimensions are calculated from the voltage difference between the DC voltages that are used to deflect light between the phases corresponding to the edges of -.

[Effect]

When performing minute dimension measurements using the above method, the optical deflection operation and optical frequency shift operation of the acousto-optic element are used to focus two laser beams at a close distance to a minute spot diameter to be measured. Optical heterogain interference is performed while deflecting light on the object surface. When two beams of light are irradiated onto the edge of the object to be measured, the optical path length between the two beams changes due to the step at the edge, causing a phase change due to optical heterodyne interference, which causes an electrical phase change. Detect. The dimensions are determined from the voltage difference when light is deflected between phases corresponding to two edge portions.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a system block diagram illustrating the minute dimension measurement method of the present invention.

A laser light source 10 emits a laser beam 100 by performing, for example, He-N e 'v-laser oscillation. 11 is an acousto-optic element (hereinafter abbreviated as A.0), 12 is A-011
an acousto-optic driver (
(hereinafter abbreviated as A-0 driver), 16 is a DC voltage source that inputs DC voltage to the A-0 driver 12, and generates a DC voltage Vd in the range of 0 to 1 volt, for example. 1
Reference numeral 4 denotes an AC voltage source that inputs an AC voltage to the A.0 driver 12 and generates an AC signal with a frequency fm. Here A
- The 0 driver 12 is composed of elements such as a voltage controlled oscillator (hereinafter abbreviated as vCO), a double balance mixer, a high frequency power amplifier, etc., and is sold by Intraaction, Inc. of the United States under the trade name DE-40M, for example. The A.0 driver 12 forms the A.0 driver 12 when DC voltage Vd is applied from the DC voltage source 13.
A high frequency signal with a frequency fd is emitted by the CO, which is A-M modulated by a double balance mixer with an AC signal with a frequency fm applied from the AC voltage source 14 to obtain a frequency fd±fm.
An alternating current signal with two frequency components is created, amplified by a high frequency power amplifier, and outputted to A.011 as f d :f:f m
A high frequency signal of frequency components is applied. A.011 has the above-mentioned AC signal fd±fm with two frequency components.
It is driven by a zero drive signal 121 to generate two beams of light 101 and 102 having different frequencies and different traveling directions.

The frequency of the laser beam incident on A.011 is f. When closed, the two beams of light 101 and 102 each become fo +f d+
It has a frequency of f m - fo + f df m. At this time, two beams of light 10 emitted from A-011
1,102 is controlled by the DC voltage from the DC voltage source 16, and the angle θ formed between the two beams 101 and 102 is
m can be controlled by the frequency of the AC voltage source 14.

Reference numeral 15 denotes an optical system including A.011, in which the traveling directions of two beams of light 101 and 102 emitted by A.011 are separated into two different directions by a beam splitter 151. One of the two separated beams is beam light 10.
6 and beam light 104, and passes through the objective lens 152.
The light is focused and irradiated onto the object 16 whose dimensions are to be measured. The reflected light from the object to be measured 16 is again diverted by the beam splitter 151 and becomes the object reflected light 107 and 108.
The light is taken out and received by the first photoelectric conversion unit 17, and the current -
Voltage conversion is performed to create an object reflected light signal 170. The other two beams separated by the beam splitter 151 are 1
05 and 106, the light is received by the second photoelectric converter 18, current-voltage conversion is performed, and a reference optical signal 180 is generated.

As mentioned above, optical heterodyne interference occurs when two light waves with different frequencies are made to interfere. For information on optical heterodyne interference, refer to Optics 9.5 (1980) P26
6. The frequency of the two beams of light to be interfered with is f. +fd+fm.

If fo+fd fm, the electric fields of each light E1, E,
is E1=At eXp(i ・2ff(fo +f d+
fm) t+φ,) ...■Ex =A4 eXI)
(i ・27r(fo +f d-fm) t+φ,)
'−...It is represented by ■. Here, A, , A, is the amplitude of the light wave.

φ8 and φ are phases. When light waves in electric field E1 and light waves in electric field E2 are caused to interfere and photoelectric conversion is performed by a photodetector, the light intensity is I = A1" + A2" +2A, A, C03 (2π
Δft+Δφ)...■ However, Δf=2fm, Δφ=φ1
−φ2.

In this way, optical heterodyne interference creates an electrical beat signal with a frequency difference between the light waves, and the phase difference Δ
By measuring φ, information in the region of light included in the light waves of the electric field C1 and the light waves of the electric field E can be detected.

Here, the reference optical signal 180 and the object reflected optical signal 170 are beat signals having the same frequency of 2 fm, but have different phases. Since the two beams 105 and 106 of the reference optical signal 180 are kept in the same phase state temporally, the phase term (Δφ in the equation ■) when they are interfered with in the second photoelectric converter 18 is always a constant value φr.
It is.

On the other hand, the object reflected light signal 170 is the two-beam light 107
．． Since 108 changes the optical path difference due to the unevenness of the object to be measured 16, the phase term changes. The phase of the object reflected light signal 170 is generally expressed as φS.

A phase comparator 19 compares the phases between the reference optical signal 180 and the object reflected optical signal 170.

■As is clear from the formula, the electrical signal representing the interfered light intensity has a DC component of A, ``+A2%, and an AC component of 2A, A.
, C03 (2TΔft+Δφ), and it is desirable to cut the DC component and perform phase detection using the AC component. Further, the phase does not have any meaning when φr and φS alone, but the absolute value of the phase difference 1φr−φsl has meaning.

20 is a light amount detection unit that detects the DC component A-+A2" or the AC component amplitude 2AIA of the object reflected light signal 170. At this time, two beams of light 103 and 104 are optically deflected between the edges of the object to be measured 16. For each state in which the light is deflected, the amount of light of the object reflected light signal 170 is (Px, Pt...
..., Pn), the light amount data (Pi), the phase difference is (φS
, -φr, φSt-φr...φSn-φr)
The data (φi) is obtained. However, n is the number of times the light is deflected.

21 is a dimension calculation unit that determines the edge and μ position of the object to be measured 16 from the phase data (φi) and the light amount data (Pi),
DC voltage source 13 required to deflect light between the edges
The distance between the edges, that is, the dimension of the object to be measured 16 is calculated from the voltage difference. Furthermore, the DC voltage source 16 and the AC voltage source 14 are also controlled.

Here, the main information for determining the edge position is the phase data (φi), and the light amount data (Pi) is used for checking, so depending on the object to be measured 16, the light amount data (Pi) is not necessarily required.

FIG. 2 shows a state diagram when two beams of light are optically deflected on an object to be measured. In FIG. 2, the upper plane part of the object to be measured 16 is 161, the lower plane part is 162, the left edge is 16,
6. The right edge is represented by 164, the step difference between the upper flat part 161 and the lower flat part 1620 is represented by Δh, and the distance 10 between the left and right edges is measured.

The distance between the two beams 103 and 104 is the AC voltage source 1
Frequency fm from 4 and objective lens 152 of optical system 15
It can be determined by the focal length of the beam, etc., and is preferably set to a value approximately equal to the diameter of each beam spot to be irradiated. Optical deflection of two beam light ((a)→(b)→e1→)
Have them do it in the order of →(e). At this time, the voltage emitted from the DC voltage source 16 is changed stepwise. It is preferable that the light deflection step be performed in steps smaller than the beam spot diameter of each of the two beams of light.

In the states (a), (c), and (e), there is no optical path difference between the two beams, but in the cases (b) and (b), the edges 166 and 1
64, an optical path difference exists between the two beams of light.

If the wavelength of the two beams is λ, the optical path difference (step) is Δh, and the measured phase difference is φ, it is expressed as Δh=λ°φ 4 T "'■. In the case of a He-N e laser, λ= 0.
Since it is 63 μm, the phase difference φ corresponds to an optical path difference of 8.8 angstroms per 1°. The phase difference is electrically detected, but if the phase difference is detected in the ±E region↓ then 1Δh1≦1”=i0.16μm
However, the field of optical path difference results in a phase angle exceeding ±T.

Even in the case where the phase angle is not the only information, the phase angle can be corrected by simultaneously measuring the change in the reflection angle of the reflected light and the amount of reflected light.

FIG. 3 shows a phase waveform diagram when the optical deflection shown in FIG. 2 is performed.

As described above, the phase difference changes greatly at the edge portion, and in the case (b) and (b) of FIG. 2, the sign of the change in phase angle is reversed. Therefore, in the edge portion 163 corresponding to (b) in Fig. 2, the phase difference is convex upward as shown in the waveform 31, (in Fig. 2)
At the edge portion 164 corresponding to the waveform 62, the phase difference is convex downward and exhibits the maximum value.

In other parts, the phase difference is zero. As described above, this phase difference is given by (φS - φr), but for example, the phase φr of the reference optical signal may be adjusted so that the phase difference in the plane portion becomes zero. Waveform 31 and waveform 3 where this phase difference is maximum
The dimensions are calculated from the voltage difference ΔVe between the optical deflection voltages Vl and Vr at the two points of 2.

For this purpose, the above-mentioned ΔVe may be measured in advance using the method of the present invention for an object to be measured whose dimensions are known, and the correlation coefficient between ΔVe and the dimensions may be calculated.

In the example described in Figures 2 and 3, the peak of the phase difference was directly obtained because the stage at the edge, the λ difference Δh, was smaller than 1 and the angle of the phase difference did not exceed lff+. Met. In this case, the upper plane part 161 and the lower plane part 1
If 62 is made of a material having a similar reflectance, there will be almost no change in the light intensity of the reflected light at each point even if the light is deflected.

(This is because Δh is smaller than 0.16 μm, which is within the depth of focus of the irradiated laser beam.) The method of the present invention is particularly effective for dimension measurement in such cases. FIG. 4 shows an example of phase difference detection when the phase difference due to the step difference Δh at the edge exceeds 11, corresponding to equation 0.

FIG. 4(A) shows the relationship between the phase angle and Δh. When measuring the phase angle at ±r, for example, 0, A, Pl
In the case of a phase angle (θ, <7'r) of 01 in the state of , the phase angle θ2 (
The case of −π〈θ2×O) similarly corresponds to a state in which the optical path length of the beam 106 is long, but (the embodiments in FIGS. 2 and 3 are the above-mentioned cases), for example, the phase angle of θ2 In the corresponding state, Δh exceeds In and the phase angle θ of 0, C, B, P,
Even in the state of 1-2T, the phase angle actually measured is θ
, the phase is not continuous and a jump occurs in the sign and magnitude of the phase angle.

FIG. 4 (b) is a diagram showing the change in phase angle when Δh becomes thick (corresponding to FIG. 3).
Since it is measured by φS - φr), if we set φr = 0, then 2
The phase difference between the beams of light is directly observed. When the phase difference due to Δh exceeds T, the waveform 41 is in a state where the phase difference is π, so when it exceeds T, a sudden phase jump occurs and the waveform 42 becomes the state in which the sign is inverted. The phase state of the waveform 43 is at the edge position 163, but from the waveform 42 to the waveform 46, the absolute value of the phase changes in the direction of decreasing. Therefore, as a phase waveform, a waveform that is symmetrical with respect to the waveform 460 phase state can be obtained. Further, at the edge position 164, a waveform whose phase sign is inverted is obtained. In the case of this embodiment, the edge position can be determined by detecting the minimum value of the absolute value of the phase within a region where a sudden jump in phase occurs.

Next, when Δh becomes larger and becomes equal to or greater than the phase angle of In7N (n>2), the phase change becomes a more complicated waveform. In such a case, the optical path difference between the two beams of light becomes even larger, so that a change in the amount of reflected light appears.

Figure 4 (/'1 shows the change in the amount of reflected light. Upper flat part 1
If the focus of the two beams of light irradiated onto the lower plane portion 162 is adjusted, the amount of reflected light will decrease because the focal point will be shifted on the lower plane portion 162. Naturally, this area d where this change in the amount of reflected light occurs
There will be an edge position at , ,d. Since the change in phase angle at this time shows a complicated pattern, it is difficult to determine the edge from the maximum or minimum value of the phase change within a predetermined area. A threshold level is set for the magnitude of the change in phase angle near the regions dl and d, and the phase change is binarized, for example, as shown in the rectangular wave 44 and the rectangular wave 45. The dimensions may be measured by determining the voltage difference ΔVe between the center portions of the phase. This case is also the same as the previous case (it is necessary to calculate the conversion coefficient for the relationship between the dimensions and the optical deflection voltage difference ΔVe. In the case of this example, there is no need to process fine fluctuations in the phase angle, and changes in the amount of light It is only necessary to detect the beginning and end of the change in the phase angle that becomes the varnish register for each edge in conjunction with the ! (Pi) is also effective when there is contrast.

FIG. 5 shows an example of the configuration of an optical path diagram of an optical system when two beams of light are generated and focused and irradiated onto the surface of the object 16 to be measured.

Figure 5 (a) is a diagram showing the beam shape of light, Figure 5 (b)
is a diagram showing an optical path. In FIG. 5, 51 and 54 are cylindrical lenses with a focal length of l, 52 and 53 are convex lenses with a focal length of l, and 55 is a focal length of ! , 151 is a polarizing beam splitter, 156 is a one-wavelength plate, and 152 is an objective lens having a focal length of 10.

A-011 performs various modulations of light waves through the interaction of light and ultrasonic waves, and since it is necessary to increase the interaction time, it is desirable that the light incident on A-011 has a wide beam diameter, so the cylindrical lens 51 The combination of the convex lens 52 and the convex lens 52 converts the circular beam shape emitted from the laser light source 10 into a substantially one-dimensionally expanded fan-like shape.

A convex lens 53 and a trigonal lens 54 are used to convert the beam shape of the emitted light from A-011 into a circular beam shape again. At this time, the surface of the cylindrical lens 54 having a refractive effect on the incident light beam is set in a direction different from that of the cylindrical lens 51. Next, circular parallel light having an enlarged diameter is created using the convex lens 55 and condensed by the objective lens 152 .

In the above optical system configuration, the diameter of the beam spot irradiated onto the surface of the object to be measured 16 can be easily changed by particularly selecting the focal lengths of the convex lens 55 and the objective lens 152. Further, the amount of light deflection is 12 .io .theta./is (deflection angle in the range of 0 volts to 1 volt), where θ is the light deflection angle.

Also, the distance between the two beams is lt"lo"θm/is.

Now, although the state of separation of the two beams of light is not shown in the diagram showing the shape of the light beam in FIG. 5(a), they are actually separated by a minute distance.

Furthermore, in the diagram showing the optical path in FIG. 5(b), the 0th order diffracted light must be cut off because it is not used for measurement.

〔Effect of the invention〕

As is clear from the above explanation (by irradiating the measured object with two beams of light with different frequencies and detecting the change in optical path length at the step due to the edge using optical heterodyne interferometry, the step at the edge can be It is possible to detect the edge position with high accuracy even when the wavelength is below the wavelength of , and by performing stable optical deflection, highly accurate dimensional measurement is possible.

[Brief explanation of the drawing]

Figure 1 is a system block diagram explaining the minute dimension measurement method of the present invention, Figure 2 is an explanatory diagram explaining light deflection at the object to be measured, and Figure 3 is an optical path diagram showing an example. be. 10... Laser light source, 11... Acousto-optic element, 12... Acousto-optic element driver, 13... DC voltage source, 14...
...AC voltage source, 15...Optical system, 16
...Object to be measured, 19...Phase comparison Fig. 2 Fig. 3 vl vr Fig. 4

Claims (1)

[Claims] Laser light emitted from a laser light source is incident on an acousto-optic element, and the optical operation of the acousto-optic element is controlled by an acousto-optic element driver that is operated by inputting DC voltage and AC voltage. Two beams of light having different frequencies and traveling in different directions are generated from the element, the traveling directions of the two beams are separated into two different directions, and one of the separated two beams is focused. The beam is irradiated onto the surface of the object whose dimensions are to be measured, and the reflected light from the object is received to create an object reflected light signal, and the other two separated beams are directly When receiving light to create a reference light signal, the frequency of the AC voltage signal input to the acousto-optic element driver is controlled to control the angle formed between the two beams, and the acoustic By changing the voltage of the DC voltage signal input to the optical element driver, one of the two separated beams is deflected by a predetermined distance on the surface of the object to be measured, and each beam is The phase between the object reflected light signal and the reference light signal is detected for each deflection state, and the measured object is detected from the phase data that is a function of the DC voltage that controls the light deflection. A method for measuring microscopic dimensions, characterized in that dimensions are calculated from the voltage difference between the DC voltages that cause light to be deflected between phases corresponding to two edges of an object.