JPH04366703A - Optical displacement measurement method and device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for optically measuring displacement, and more particularly to a method and apparatus for optically measuring displacement suitable for positioning a substrate in semiconductor manufacturing equipment such as an EB apparatus.

0002

2. Description of the Related Art When manufacturing semiconductors, a semiconductor substrate to be processed is placed on a stage and processed while moving the stage. At this time, an optical measurement means is used to precisely measure the amount of movement of the substrate, that is, the amount of movement of the stage. A phase shifter is installed that irradiates the reference surface and the object to be measured to cause their reflected waves to interfere with each other, and that allows the amount of phase shift of one optical beam path to be variable by π radians or more. When the phase shift amount is changed using the phase shifter, the amplitude of the interference wave periodically becomes maximum or minimum, so this point is used as a compensation point to detect when the differential value of the interference signal becomes 0. The phase difference between the two optical beam paths is detected from the magnitude of the drive signal applied to the phase shifter. From this and the phase difference obtained when the object to be measured is moved, the amount of movement, that is, the displacement of the object is detected. In addition, in the device disclosed in JP-A-63-11803, the operating principle is the same as that described above, but at the moment when the interference signal takes an intermediate value between the maximum value and the minimum value (when the phase difference becomes nπ+π/2). The only difference is that the current moment) is used as a compensation point and this is detected. Furthermore, the device disclosed in JP-A No. 62-88902 performs highly accurate measurement based on the above principle when the moving speed of the object to be measured is low, and when the moving speed is high, it simply detects changes in the brightness of the interference signal. The movement speed is measured by counting.

0003

[Problem to be solved by the invention] In the conventional example described above,
In both cases, a type of phase shifter that controls the phase difference between two lights of different polarization is used, so the driving voltage for the phase shifter is large, and the phase shifter is designed to have a temperature compensation function. However, there is a problem in that it is difficult to achieve optical integration and lower driving voltage. Furthermore, in the first conventional example, since the interference signal is differentiated and the compensation point is obtained as its zero point, there is a problem in that errors due to noise are likely to occur. Further, in the second conventional example, there is a problem that the circuit for calculating the intermediate value from the maximum value and the minimum value of the interference signal becomes complicated. Furthermore, in the first and second conventional examples, it is difficult to increase the modulation frequency of the phase shifter and increase the response speed due to the frequency limit of the differentiating circuit and the intermediate value detection circuit. The third conventional example has poor measurement accuracy when the moving speed is high, and therefore cannot satisfy the demand for accurately measuring and drawing the position while moving the stage at high speed using, for example, an EB device.

[0004] An object of the present invention is to provide an optical displacement measurement method and apparatus that can be driven with low voltage, has high accuracy, and has a fast response speed.

[0005]

[Means for solving the problem] The above object is to divide coherent light from a light source into two, and to change the phase difference of the coherent light divided into two by a phase shifter driven by a sine wave drive signal. The movable object is modulated so that it changes in a sinusoidal manner of π radians or more, one of the modulated lights is reflected on a fixed reference surface as a reference light, and the other modulated light is used as a measurement light. Changes in the optical path length of the measurement light caused by the displacement of the movable object can be determined by examining the amplitude of the interference signal generated by combining the reflected light from the reference surface and the reflection surface. The phase change of the measurement light due to displacement is detected from the magnitude of the drive signal when the change in phase caused by the change in phase is offset by the change in phase difference caused by the phase shifting means, and the amount of displacement of the movable object is determined from the detected phase change. This is achieved by calculating the amplitude of the drive signal and setting the amplitude of the drive signal so that the value of the zero-order Bessel function for half the total amplitude of the phase difference generated by the phase shifter becomes zero.

[0006]

[Operation] Since the phase shift means individually shifts the phase of two lights having the same polarization plane, it is possible to drive with a low voltage. In addition, by selecting the amount of phase shift by the drive signal as described above,
Regardless of whether the movable object moves or not, the moment when the AC component of the interference signal becomes 0 can be set as the compensation point, making it possible to always perform highly accurate measurements.

[0007]

[Example] Fig. 1 shows an example in which the present invention is applied to an EB device.
This will be explained using FIGS. 2 and 3. First, an overview of the configuration will be described. In FIG. 1, coherent light from a laser light source 1 is input to a phase shifter 7 installed in a vacuum chamber 19 via a polarizing plate 2 and a polarization maintaining fiber 6. The phase shifter 7 outputs the incident light by dividing it into two beams whose phase difference can be variably controlled. One of these beams is reflected by the reference plane 14 and the other by the mirror 17 fixed to the stage 16. The reflections are combined into an interference signal that is extracted through the optical window 21 of the chamber 19. A mirror 17 and a substrate 18 are installed on the stage 16, and an electronic system (not shown) is installed above the stage 16, which focuses the electron beam onto the substrate 18 and forms a desired pattern on it. be drawn. Since the drawing range for electronic engineering is small,
The entire substrate 18 is moved by the stage 16 to obtain a wide drawing range. The moving distance of this stage is determined by processing the above-mentioned interference signal by the phase difference detection means 22, the up/down pulse generator 34, the counter 42, and the adder 43.

[0008] The details and operation of each part will be explained below. Of the coherent light emitted from the laser light source 1 , only the vibration component in a plane parallel to the plane of the paper is selected by the polarizing plate 2 and focused onto the polarization maintaining fiber 6 by the condensing lens 3 . This fiber 6 preserves the above-mentioned vibration plane of the incident light and guides it to near the chamber 19. The output light is made parallel by the lens 4, enters the lens 5 through the optical window 20 in the chamber 19, and is focused on the waveguide 9a of the phase shifter 7. The phase shifter 7 consists of a lithium niobate (LiNbO3) substrate 8, waveguides 9a to 9c formed by diffusing titanium thereon, a metal cladding 10, and metal electrodes 11a to 11c. The light passes under the metal cladding 10. Originally, the vibration plane of the light incident on the waveguide 9a is almost parallel to the plane of the paper, and almost only the TE mode component is excited in the waveguide 9a, but the polarization maintaining fiber 6 is not ideal, and the polarization plane Since some TM modes are also excited due to alignment errors between the storage fiber 6 and the phase shifter 7, they are absorbed and removed by the metal cladding 10.

FIG. 2 shows a cross section of the phase shifter 7, in which voltage is applied to metal electrodes 11a and 11c (electrode 11b is grounded).
Electric lines of force 45a and 45b generated within the substrate 8 when
It is shown. Here, lithium niobate substrate 8
The crystal axis of wave path 9a,
9c, they are along the Z-axis direction and face in opposite directions. It is known that by doing this, the refractive index of lithium niobate crystal for light with a vibration plane in the Z-axis direction changes best (Light Wave Electronics, Corona Inc.,
1974, p. 274), thereby making it possible to efficiently modulate the phase difference between the measurement light B2 on the waveguide 9c and the reference light B1 on the waveguide 9b. Therefore, when the sinusoidal (frequency ω) output voltage of the oscillator 32 is amplified by the amplifier 33 and applied between the electrodes 11a and 11b in FIG. The phase difference is also sinusoidally modulated at frequency ω. Therefore, if this amplitude is A0, the difference in optical path length can be expressed as A0 sinωt. The reference light B1 exiting the waveguide 9b is made into parallel light by the lens 12, reflected by the reference surface 14, and then reflected by the half mirror 15.
Transparent. On the other hand, the measurement light B2 exiting the waveguide 9c is shaped into parallel light by the lens 13, passes through the half mirror 15, is reflected by the mirror 17 on the stage, reaches the half mirror 15, and is reflected again. In this way, the reference beam B1 and the measurement beam B2 having the same vibration plane cause interference. The optical path length difference between the two lights at this time is the sum of the optical distance s1 from the center of the reference plane 14 to the center of the half mirror 15 and twice the distance s2 from the mirror 17 to the center of the half mirror 15. Since s1+2s2, the difference S in the total optical path length is

[Math 1]
It is given by S=s1+2s2+A0 sinωt. Therefore, the intensity I of the interference light is the intensity of B1, B2, I1, I2
, where the wavelength of light is λ

[Math. 2] I=I1+I2+2(I1・I2)1/2 cos(2
πS/λ)=I1+I2+2(I1・I2)1/2・c
os(2π/λ(s1+2s2+A0 sinωt))
It is determined by The cos() term in this equation is

[Equation 3] cos(2π/λ(s1+2s2))・cos(A
sin ωt)−sin(2π/λ(s1+2s2))
・sin(A sin ωt) However

[Equation 4] It can be written as A=2πA0/λ, but cos(Asinωt), cos(Asi
nωt) is calculated using Bessel function Jn, n=0, 1,...

[Math 5] It can be expanded as follows. Substituting this into [Math 3]

[Equation 6] The first term in [Equation 6] is constant when the object to be measured is stationary, but when it is moving, s2 changes with time and becomes an alternating current component with a frequency proportional to the speed of movement. The terms other than the first term are alternating current components that oscillate at integral multiples of the modulation frequency ω. Therefore, in this embodiment, two
By adjusting the amplitude A0 of the phase difference between the two lights B1 and B2, A in [Equation 4] becomes J0(A)=0, that is, A=2.40...
・Set to meet the requirements. Then, J0 of [Math. 6]
Since the term containing (A) is always 0, the alternating current components of ω and higher in the interference signal in [Equation 2] are J2n, J in [Equation 6]
It is expressed only by the term containing 2n-1, and the reason why this is always 0 is that the co of [Equation 2]
It coincides with the moment when s(2πS/λ) becomes 0. At this moment, the total optical path length difference S is S=(λ/2π)(π/2+nπ)(
However, n is an integer). Therefore, at this instant (phase shift compensation point), a pulse is generated by the pulse generator 28 in the phase difference detection means 22. That is, the output of the photodetector 23 that detects the interference signal intensity is passed through a high-pass filter 24 to remove the DC component whose angular frequency component is less than ω.
The output is inverted every time the comparator 25 compares it with the ground potential and it becomes 0 (matches).
R27 converts it into a pulse at the time of generation. This pulse is one period (2π/ω) of phase modulation by the phase shifter 7.
The gate circuit 29 extracts only the first pulse generated after the phase difference of one cycle exceeds -π/2. This is done by appropriately dividing the voltage applied to the phase shifter 7 using a variable resistor 31 and applying it to the gate circuit 29.
This is done by applying a voltage to At the same time, since this time is the phase compensation point, the amount of phase shift at this time, that is, the amount of phase compensation, is detected by the A/D converter 30 from the applied voltage. If we take the phase compensation amount detected in this way on the vertical axis and the position of the object to be measured (half of the total optical path length difference S) on the horizontal axis, we can see that the object moves every time the position changes by π/4 as shown in Figure 3. The phase amount is from π/2 to -π/2 (if the moving direction is opposite to the diagram, it is from -π/2 to π
/2) discontinuously. Here, the number of times of this discontinuous change, including the direction, is measured by the up/down pulse generator 34 and the counter circuit 42. When the reference input values of the digital comparators 35 and 36 are set to the values shown in A and B in FIG. 3, the up/down pulse generator 34 simultaneously changes the outputs of both digital comparators from Low to High only when a discontinuous change occurs. (Hi→Low)
This is detected by the delay line 37, inverters 38, 39, and AND circuits 40, 41 to generate up/down pulses. In this way, when the count number of the counter 42 is m, it means that the discontinuous point in FIG. 3 has been passed m times. The amount of phase shift between one discontinuity point and the next discontinuity point is π, and the optical path length is λ/2, so the change in s2 corresponds to half of this, λ/4, and when this is passed m times, the change is only mλ/4. Corresponds to movement. Therefore, when the initial position of the measurement object 16 is X0, the amount of phase compensation is Φ0, the current position is X, and the amount of phase compensation is Φ, the amount of movement of the stage 16 is X-X0.

[Formula 7] It is given by X−X0=λm/4+λ(Φ−Φ0)/(4π). According to this embodiment, the phase shifter can be driven at a low voltage, and accurate displacement measurement can be performed even when the stage moves at high speed.

FIG. 4 shows another embodiment of the present invention.
The coherent light from the laser light source 1 passes through the half mirror 51
divided into two parts, one is a polarizing plate 53 and the other is a mirror 52.
The light is incident on the polarizing plate 54 via the polarizing plate, and the light is converted into light having a vibration plane parallel to the plane of the paper by these polarizing plates, and is incident on the phase shifter 55. The phase shifter 55 is composed of electro-optic crystals, for example, lithium niobate crystals 56 and 57, and electrodes 58a to 58d. It is installed so that the positive direction and the left direction are the positive direction of the Z axis. A voltage from the oscillator 32 is applied to the electrodes 58a and 58d (electrodes 58b and 58c).
is grounded), the lithium niobate crystal 56
, 57 in opposite directions are generated, and a phase difference occurs between the two lights as will be described later. These are lenses 59 and 60 and polarization maintaining fibers 61 and 62, respectively.
, lenses 63, 64, and optical windows 65, 66 into a vacuum chamber 69. The light from the optical window 65 is split into two by a half mirror 70 as reference light B1, and part of it is guided to a half mirror 72 and the other part is guided to a polarizing beam splitter 73 via a mirror 71. This polarizing beam splitter 73
transmits light whose vibration plane is parallel to the paper surface, and reflects light whose vibration plane is perpendicular to the paper surface. The light from the other optical window 66 passes through a half mirror 72 and a polarizing beam splitter 73 as measurement light B2 (this has a polarization plane parallel to the plane of the paper), and passes through a quarter-wave plate 74 onto the stage 16. It reaches the mirror 17, where it is reflected and passes through the quarter-wave plate 74 again.
It returns to the splitter 73 via the route. If the direction of the quarter-wave plate 74 is determined so that the plane of polarization of the measurement light is rotated by 90 degrees when the quarter-wave plate goes back and forth in this way and becomes perpendicular to the plane of the paper, the measurement light that returns to the beam splitter 73 can be rotated by 90 degrees. B2 is reflected here and combined with the reference light from the mirror 71 to form an interference signal C1, which is output from the optical window 68. Further, the reference light from the half mirror 70 and the measurement light branched by the half mirror 72 are also combined and output to the optical window 67 as an interference signal C2.
is output from.

The portion consisting of the phase difference detection means 76, the up/down pulse generator 78, the counter 80, and the adder 82 is the same circuit as the one with the same name in FIG. The amount of movement of the stage 16 is calculated from the interference signal C1 with that reflected by the mirror 17 using the same operation as in FIG. On the other hand, in the portion consisting of the phase difference detection means 75, up/down pulse generator 77, counter 79, and adder 81, which have the same circuit configuration as the above, the amount of movement is 0.
The apparent movement amount at this time is calculated from the interference signal C2. This apparent amount of movement is due to the fact that the optical path lengths of the two light beams in the polarization maintaining fibers 61 and 62 change due to the influence of temperature, pressure, etc. This is caused by the addition of a phase shift amount, and this apparent shift amount is also included in the output of the adder 82 by the same amount. Therefore, the true amount of movement can be obtained by subtracting the apparent amount of movement from the calculated amount of movement of the stage 16 using the subtractor 3. The method for correcting this apparent amount of movement is shown in Figure 1.
It goes without saying that this is also applicable to the case of the phase shifter 7.

FIGS. 5 and 6 are explanatory diagrams of the operation of the phase shifter 55. FIG. 5 shows a conventional structure, which consists of lithium niobate crystals 84, 85 and electrodes 86 to 89 of the same length and axially shifted by 90 degrees from each other. The reason why the two crystals are provided with their axial directions shifted is to compensate for temperature changes. Lithium niobate crystal 84, 8
5, a voltage V of opposite polarity is applied in the Z-axis direction, and reference light and measurement light whose polarization planes are perpendicular to each other are incident in the Y-axis direction. At this time, the phase difference △Φ newly generated between the reference light and the measurement light emitted from the phase shifter is

[Equation 8] △Φ=(ne3r33-n03r13)sV/
It is given by d. Here, no and ne are the extraordinary ray refractive index and the ordinary ray refractive index, r33 and r13 are Bockels constants, s is the length of the crystal, and d is the thickness of the crystal. no=2
．． 29, ne=2.2, r33=30.8, r13=8
．． 6 (by A. Yariv, “Optical Elec
tronics” p281, Halt-Sound
rs) into [Equation 8] to find the sensitivity when s/d=1, 224×10−12 (rad/V) is obtained. On the other hand, FIG. 6 shows the phase shifter 55 of this embodiment. When reference light and measurement light having the same polarization are incident, the phase difference ΔΦ caused by passing through the phase shifter 55 is

[Equation 9] It is given by △Φ=ne3r33sV/d. Lithium niobate crystals 56, 57
If the conditions are the same as above and s/d=1 is also the same, the sensitivity is found to be 327×10-12 (rad/V). Therefore, in this example, the sensitivity is improved by about 46%. Furthermore, there is a limit to the distance that a light beam can pass through a crystal while keeping its beam diameter below the thickness of the crystal. Since the length of the phase shifter is 2 s in the conventional configuration shown in FIG. 5 and s in the configuration shown in FIG. 6 of this embodiment, the sensitivity improvement in this embodiment reaches 92% when the above beam diameter is taken into account.

[0013]

According to the present invention, the electric circuit can be simplified and the driving voltage of the phase shifter can be reduced, so that the modulation frequency can be increased and the response speed can be increased.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an apparatus showing an embodiment of the present invention.

FIG. 2 is a cross-sectional view of the phase shifter of the embodiment of FIG. 1;

FIG. 3 is an explanatory diagram of the operating principle of the embodiment of FIG. 1;

FIG. 4 is a configuration diagram of an apparatus showing another embodiment of the present invention.

FIG. 5 is a diagram showing the configuration of a conventional phase shifter.

FIG. 6 is a diagram showing the configuration of a phase shifter according to the present invention.

[Explanation of symbols]

1 Laser light source 7 Phase shifter 8 Litium niobate substrate 9a Titanium diffusion waveguide 9b Titanium diffusion waveguide 9c Titanium diffusion waveguide 11a Metal electrode 11b Metal electrode 11c Metal electrode 16 Stage 17 Mirror 22 Phase difference detection means 55 Phase shifter 75 Phase difference detection means 76 Phase difference detection means

Claims (10)

[Claims] [Claim 1] Split coherent light from a light source into two,
A phase shifter driven by a sinusoidal drive signal modulates the phase difference of the divided coherent light into a sinusoidal waveform with a variation width of π radian or more, and one of the modulated lights is The reference light is reflected by a fixed reference surface, the other modulated light is reflected by a reflective surface fixed to a movable object as a measurement light, and the reflected light from the reference surface and the reflective surface are combined. By examining the amplitude of the generated first interference signal, it is possible to determine the above when the change in phase due to the change in optical path length of the measurement light caused by the displacement of the movable object is canceled out by the change in phase difference caused by the phase shift means. A method for optically measuring displacement, comprising detecting a phase change of measurement light due to displacement from the magnitude of a drive signal, and calculating the amount of displacement of the movable object from the detected phase change. 2. The phase shifting means is configured by disposing a Y-shaped branch path for dividing the coherent light into two and two waveguides generated from the Y-shaped branch path on a crystal substrate, wherein the two waveguides 2. A refractive index change having an opposite sign is generated by applying electric fields in mutually opposite directions to the waveguide, and the phase difference of light on the waveguide is changed by the change in the refractive index. Optical measurement method of displacement. 3. The phase shifting means is composed of two bulk electro-optic crystals of the same length, and by applying electric fields in opposite directions to the two electro-optic crystals, refractive indexes of opposite signs are changed. 2. The method for optically measuring displacement according to claim 1, wherein the change in refractive index causes a change in the phase difference of light passing through the electro-optic crystal. 4. The amplitude of the drive signal is set so that the value of the zero-order Bessel function for half the total amplitude of the phase difference generated by the phase shifter is zero, and the amplitude of the first The phase difference detection means determines that the phase difference generated by the phase shift means cancels the phase difference caused by the displacement of the movable object when the alternating current component of the first interference signal becomes zero. 4. The method for optically measuring displacement according to claim 1, characterized in that: 5. Branching the reference light and measurement light output from the phase shifting means, combining the branched lights to generate a second interference signal, and using the second interference signal as an input. The amount of displacement determined from the phase difference detected by the second phase difference detection means having the same configuration as the operating first phase difference detection means was determined from the phase difference detected by the first phase difference detection means. 5. The optical displacement measuring method according to claim 1, wherein a value subtracted from the displacement amount is output as the displacement amount of the movable object. [Claim 6] Divide coherent light from a light source into two,
a phase shifting means driven by a sinusoidal drive signal to shift the phase of each of the coherent lights so that the phase difference of the two divided coherent lights changes in a sinusoidal manner with a variation width of π radians or more; A fixed reference surface for reflecting the reference light using one output light as a reference light, and a movable object for reflecting the measurement light using another output light from the phase shifting means as the measurement light. By examining the amplitude of the first interference signal generated by combining the reflected light from the reflective surface, the reference surface, and the reflective surface, it is possible to determine whether the change in optical path length of the measurement light caused by the displacement of the movable object is and a first phase difference detection means for detecting a phase change of the measurement light due to displacement from the magnitude of the drive signal when a change in phase is offset by a change in phase difference caused by the phase shift means. An optical displacement measuring device characterized by: 7. The phase shifting means is configured by disposing a Y-shaped branch path for dividing the coherent light into two and two waveguides generated from the Y-shaped branch path on a crystal substrate, wherein the two waveguides 7. A refractive index change having an opposite sign is generated by applying electric fields in mutually opposite directions to the waveguide, and the phase difference of light on the waveguide is changed by the change in the refractive index. Optical measuring device for the displacement of. 8. The phase shifting means is composed of two bulk electro-optic crystals of the same length, and by applying electric fields in opposite directions to the two electro-optic crystals, refractive indexes of opposite signs are changed. 7. The optical displacement measuring device according to claim 6, wherein the change in refractive index causes a change in the phase difference of light passing through the electro-optic crystal. 9. The amplitude of the drive signal is set so that the value of the zero-order Bessel function for half the total amplitude of the phase difference generated by the phase shifter is set to zero, and the amplitude of the first The phase difference detection means determines that the phase difference generated by the phase shift means cancels the phase difference caused by the displacement of the movable object when the alternating current component of the first interference signal becomes zero. 9. An optical displacement measuring device according to claim 6, characterized in that: 10. Light combining means for branching the reference light and measurement light output from the phase shifting means and combining the branched lights to generate a second interference signal; A second phase difference detection means having the same configuration as the first phase difference detection means which operates with input is added, and the displacement amount obtained from the phase difference detected by the means is added to the first phase difference detection means. 10. The displacement detector according to claim 6, 7, 8, or 9, wherein a value subtracted from the displacement amount determined from the phase difference detected by the means is output as the displacement amount of the movable object. Optical measuring device.