JPH0695004B2 - Three-dimensional shape measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a three-dimensional shape measuring apparatus that measures the surface shape of an object by utilizing light interference.

[Conventional technology]

Conventionally, the length measuring unit that uses the interference of light used in this type of measuring device is called an incremental type, and integrates the pulse signal obtained according to the amount of displacement (interference fringe displacement) of the measurement target surface. The distance is counted to obtain the distance to the measurement target surface. Therefore, if there is a discontinuous portion such as a step on the surface to be measured, the integrated input will be cut off and measurement will not be possible. Further, if the power is cut off during the length measurement operation, even if the power is turned on again, the measured amount up to that point is reset, and the measured value after that becomes completely meaningless.

In order to solve such a problem, the applicant of the present application has already been able to continue accurate measurement even when there is a discontinuous portion on the measurement target surface as Japanese Patent Application No. 60-277381. A three-dimensional shape measuring device is proposed. This is because, in the length measurement unit using Michelson's interference optical system, at least two or more lights having different wavelengths are switched to sequentially measure the phase delay amount of light according to the distance to the measurement target surface, The distance to the measurement surface is obtained from the relationship between the wavelength and the phase delay amount.

FIG. 2 shows the configuration of this three-dimensional shape measuring apparatus. In the figure, LZ1 is a laser light source that selectively generates a plurality of coherent lights with different wavelengths, AOM1 is an acousto-optic modulator that frequency shifts the light on the reference side in order to heterodyne detect the phase delay amount of the light (hereinafter, OSC is a modulation signal source that drives the AO modulator AOM1 at a constant frequency fb, P2 and P3 are λ / 4 plates, P4 is a polarizing plate, BS2 to BS4 are beam splitters, MR2 is a mirror, D1 is a photodetector, PD1 is a phase detection circuit that detects the amount of phase delay included in the output of the photodetector D1, and CON2 is the relationship between the wavelength of the light used for measurement and the amount of phase delay at that time, up to the measurement target surface OBJ. An arithmetic circuit for obtaining the distance, D3 and D4 are 4-division detectors used for the correction operation described later, SL is a cylindrical cylindrical lens, LE is for converging the measurement light, and the object to be measured OB
It is an objective lens that irradiates J (hereinafter referred to as the measurement target surface OBJ). The laser light source LZ1 is composed of, for example, a light source having a fixed wavelength and a wavelength shifter that shifts the wavelength by an arbitrary amount, and sequentially generates light having an arbitrary wavelength.

DR1 is a lens driving unit that moves the objective lens LE in X, Y, and Z directions according to the outputs of the four-division detectors D3 and D4, and DR2 is a measurement target surface OBJ (measurement object) that is moved in X and Y directions. The measured object drive unit, MN1 and MN2, are the objective lens LE and the measured object (O
BJ) is a movement amount monitor that monitors the movement amount in the X and Y directions. CON1 is the output of movement amount monitors MN1 and MN2, and the measured surface O
This is an arithmetic circuit that calculates the position of the measurement point on the BJ.

Now, in the three-dimensional shape measuring apparatus configured as described above, the light emitted from the laser light source LZ1 is used as the measurement target surface O
The light is made incident on the interference optical system including BJ, and the light returned through the interference optical system is received. From the relationship between the wavelength and the phase delay amount, the distance from the measurement target surface OBJ, that is, the Z-axis direction (light The distance is measured in the axial direction. Therefore, in the interference optical system, it is necessary to correct the tilt of the optical axis with respect to the measurement target surface OBJ and the focus position so that the optical axes of the irradiation light and the reflected light match.

Here, first, the correction mechanism will be described. As shown in FIG. 3 (a), if the measurement target surface OBJ is inclined by θ, the reflected light is inclined by 2θ and the position of the reflected beam incident on the 4-division detector D3 is 2Fsi from the center.
It is displaced by nθ (F is the focal length of the objective lens LE). When this displacement is detected by the 4-division detector D3 and the objective lens LE is moved in the X or Y direction via the lens driving unit DR1, the position of the reflected light is adjusted to the center of the 4-division detector D3. As shown in FIG. 6B, the optical axis of the measurement light can be made perpendicular to the measurement target surface OBJ.

On the other hand, when the focus is not on the measurement target surface OBJ, the reflected light does not return to parallel light. Therefore, a part of the reflected light is passed through the cylindrical lens SL and the 4-division detector D4 is also used.
When it is received by, the reflected light becomes a spot of an inertia circle and hits the 4-division detector D4. At this time, if the measurement target surface OBJ is in front of the focus position, a vertically long spot is obtained, and if it is after the focus position, a horizontally long spot is obtained. Therefore, this spot becomes a circle and the 4-division detector D4
If the objective lens LE is moved in the Z direction via the lens driving unit DR1 so that the respective outputs of the
You can focus on J.

At this time, the movement of the objective lens LE and the movement of the object to be measured (OBJ) for scanning the measurement point are calculated by the arithmetic circuit CON1.
And the accurate position of the measurement point is sequentially calculated.

Next, returning to FIG. 2, the operation of the length measuring unit for measuring the distance from the measurement target surface OBJ will be described.

The angular frequency of the light emitted from the laser light source LZ1 is ω, its amplitude Vo is Vo = sinωt (1), and the modulation angular frequency in the AO modulator AOM1 is ωb (= 2
Assuming that the + 1st-order diffracted light of the AO modulator AOM1 is used as πfb), the amplitude V1 of the light modulated by the AO modulator AOM1 becomes V1 = sin (ω + ωb) t (2), and returns through the measurement target surface OBJ. Amplitude of incoming light V
2 becomes V2 = sin (ωt + φ) (3). It should be noted that φ is a phase delay amount generated corresponding to the difference in optical path length between the reference side optical path and the measurement side optical path.

On the photodetector D1, the two lights as shown in the above equations (2) and (3) are superposed, so that the amplitude of the incident light is V1 + V2 = sin (ω + ωb) t + sin (ωt + φ) = 2sin
{Ωt + (ωbt + φ) / 2} ・ cos {(ωbt−φ) / 2}
As in (4), it is the sum of V1 and V2. Here, since the output of the photodetector D1 is proportional to the square of the amplitude of the incident light,
Theoretically (V1 + V2) ² = 4sin ² {ωt + (ωbt + φ) / 2} ・ cos
² {(ωbt−φ) / 2} (5), but the photodetector D1 cannot respond to the frequency of light and shows an average value, so its output Vp is Vp = 1 + cos (ωbt−φ) (6)

Therefore, if the modulation angular frequency ωb in the AO modulator AOM1 is known, the phase delay amount φ can be calculated from the value of the output Vp of the photodetector D1.

By using the Michelson interference optical system, it is possible to measure the phase delay amount φ that changes according to the distance as described above. The value of the phase delay amount φ is (2πN + φ): Since N is equivalent to a natural number, it is not possible to immediately specify the distance to the measurement target surface OBJ from the magnitude of the phase delay amount φ.

Therefore, in the three-dimensional shape measuring apparatus in the figure, the wavelength used for measurement is changed, and the phase delay amount φ corresponding to each wavelength is changed.
Is sequentially measured, and these measurement results are solved as simultaneous equations to obtain the distance to the measurement target surface OBJ.

Now, set the distance from the beam splitter BS2 to the mirror MR2
Assuming that the distance from the beam splitter BS2 to the measurement target surface OBJ is d2, the difference between these optical path lengths is 2d1−2d2 = 2d. Therefore, the wavelength of light used for measurement is λ1, λ
2, λ3, λ4 (λ1 <λ2 <λ3 <λ4), and the phase delay amounts obtained at this time are φ1, φ2, φ3, φ4 (φ1 to φ4 are 0 to 2π), the following measurement results show that Such an expression holds.

2d = n1λ1 + λ1φ1 / 2π (7) 2d = n2λ2 + λ2φ2 / 2π (8) 2d = n3λ3 + λ3φ3 / 2π (9) 2d = n4λ4 + λ4φ4 / 2π (10) n1 to n4 are natural numbers. When d is calculated using the equations 7) and (8), d = A12 (n1-n2) + A12 (Φ1 / λ1-Φ2 / λ2) (11) Φ1 = λ1φ1 / 2π, φ2 = λ2φ2 / 2π. Here, A12 is the least common multiple wavelength at the two wavelengths λ1 and λ2. If the value of A12 is selected to be equal to or larger than the measurement range, A12 in the above equation (11)
The value of the term (n1-n2) can be specified, and the distance d can be uniquely calculated from the phase delay amounts φ1 and φ2 corresponding to these wavelengths λ1 and λ2. That is, if the measurement range is narrower than the least common wavelength A12, then the phase delay amount φ
Is always between 0 and 2π, the distance d and the phase delay amount φ
Since and correspond one-to-one, the distance d can be immediately specified from the phase delay amount φ. For example, if the measurement range is 0 to 1000 mm, the least common wavelength A12 is 1000 mm.
If the sizes of the wavelengths λ1 and λ2 are selected so that, the term of A12 (n1-n2) in the above equation (11) becomes 0, and the distance d is uniquely calculated from the phase delay amounts φ1 and φ2. be able to.

Next, the measurement value d ₁₂ is obtained by the combination of the wavelengths λ1 and λ2 as described above. From the measurement accuracy (measurement error) at this time,
The true value range at the measurement output d ₁₂ is d ₁₂ min <d ₁₂ <d ₁₂ max
Then, the next combination of wavelengths (λ
1, λ3) is selected such that the true value range d ₁₂ min to d ₁₂ max is the measurement range (least common wavelength). Therefore, the measurement range is narrowed down and higher resolution measurement is possible.

In this way, by utilizing the above relationship, selecting a combination of wavelengths (least common wavelength) and narrowing down the measurement range in sequence makes it possible to obtain absolute measurement results over any measurement range. The measurement resolution can be increased to the wavelength unit. In addition, since it is possible to always obtain absolute measurement results, the measurement target surface OBJ
Even if there is a discontinuous portion such as a step above, it is possible to obtain a measurement output corresponding to the change of the surface and continue accurate measurement.

[Problems to be solved by the invention]

However, in the three-dimensional shape measuring apparatus using the length measuring unit as described above, the simultaneous equations are established from the relationship between the plurality of wavelengths and the phase delay amount, and the distance to the measurement target surface is calculated. In order to perform a highly accurate length measurement operation, it is necessary to measure the phase delay amount with respect to the light of each wavelength with high accuracy, and the phase measuring means becomes complicated.

INDUSTRIAL APPLICABILITY The present invention eliminates the above-mentioned drawbacks of the conventional apparatus, can obtain an absolute length measurement output, and has a three-dimensional shape measuring apparatus having a length measuring unit capable of performing high precision and high resolution length measurement. The purpose is to realize the above with a simple configuration.

[Means for solving problems]

The three-dimensional shape measuring apparatus of the present invention uses Michelson's interference optical system as a length measuring unit for measuring the amount of displacement of a surface to be measured and uses two lights having different wavelengths according to the distance to the surface to be measured. In the three-dimensional shape measuring apparatus configured to measure the phase delay amount of each light and determine the distance to the measurement target surface from the relationship between these wavelengths and the phase delay amount, The phase delay amount of one or both of the two lights is changed so that the difference between the phase delay amounts of the two lights is always a constant value. A length measuring unit is provided for calculating the distance to the surface to be measured based on the set phase difference.

[Action]

In this way, in the length measuring unit, if the wavelength difference (frequency difference) of two lights is changed so that the difference between the phase delay amounts on the interference surfaces becomes a constant value, Since the length of the combined wavelength of the two lights is changed in a fixed relationship according to the distance, the subsequent arithmetic processing becomes easy. Further, since it is only necessary to measure the difference in the amount of phase delay between the lights of two wavelengths, the configuration of the phase detecting means can be simplified. In particular, when the difference in the amount of phase delay is set to 0 or 2π, the absolute value of the amount of phase delay with respect to the light of each wavelength does not matter, so the phase detection means can be configured by a simple circuit such as a mixer. it can.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of a three-dimensional shape measuring apparatus of the present invention. In the figure, the same parts as those in FIG. 2 are designated by the same reference numerals. In the three-dimensional shape measuring apparatus of the present invention, the object to be measured (OBJ) and the objective lens L
Since the drive system of E is the same as that of the device shown in FIG. 2, the details thereof are omitted and only the length measuring unit is shown. LZ
2 is a laser light source that emits coherent light, AOM2 gives a frequency shift to the light emitted from the laser light source LZ2,
An AO modulator that splits the light into two lights with different wavelengths (frequency) and controls the wavelength difference arbitrarily. VC0 is a voltage-controlled oscillator that drives this AO modulator AOM2. P1 is the polarization plane of one light 90 °. Λ / 2 plate to rotate, MR1 is a mirror, HMR1 and HMR2 are half mirrors, CC1 is a cube corner on the reference side fixed at a fixed distance, BS1 is a polarized beam that transmits or reflects light depending on the direction of the polarization plane. A splitter, D2 is a photodetector, and DA is a digital-analog converter that generates an offset signal VΔθ for changing a set phase difference Δθ described later. Here, the AO modulator AOM2 and the voltage-controlled oscillator VC0 constitute a phase varying means for two lights, and by changing the drive frequency of the AO modulator AOM2, the wavelength difference (frequency difference) between the two lights is changed. The amount of phase delay in each light is changed. Also, 2
Phase detection circuit according to the difference in the amount of phase delay in wavelength light
The output signal Vθ of PD1 is fed back to the voltage controlled oscillator VC0 and 2
The oscillation frequency of the voltage controlled oscillator VC0 is controlled so that the difference between the phase delay amounts of the two lights becomes an arbitrary constant value Δθ. The arithmetic circuit CON2 determines the wavelength difference (frequency difference Δf) of light according to the drive frequency of the AO modulator AOM2 and the set phase difference Δθ at that time.
The distance to the measurement target surface OBJ is calculated from the relationship with.

In the measuring unit configured as above, the AO modulator AOM2
The light divided into two wavelengths λ1 and λ2 by the λ / 2 plate
P1 and half mirror HMR1 are superimposed in a state where the polarization planes are orthogonal, and half mirror HMR2 and cube corner CC
It is incident on the interference optical system consisting of 1. Further, the light passing through the interference optical system is separated by the polarization beam splitter BS1 according to the direction of its polarization plane, and enters the photodetectors D1 and D2, respectively. That is, the light of wavelength λ1 enters the photodetector D1 after passing through the interference optical system, and the light of wavelength λ2 enters the photodetector D2 after passing through the interference optical system. Therefore, in the length measuring unit configured as described above, two light beams having different wavelengths do not interfere with each other, and
Since it is incident on D1 and D2, it can be considered that independent measurement systems are configured for light of different wavelengths (λ1, λ2), and phase delay for light of two wavelengths (λ1, λ2) The quantity can be measured simultaneously under the same conditions. Therefore, it is possible to realize a length measuring unit that is less likely to be affected by air fluctuations in the measurement optical path.

Now, with respect to the two lights of wavelengths λ1 and λ2 that are incident on the interference optical system, their amplitudes are respectively V ₁ = A ₁ sin (ω ₁ t + φ ₁ ) V ₂ = A ₂ sin (ω ₂ t + φ ₂ ) φ ₁ , φ When ₂ is set to the initial phase, in the light of V _1, the amplitude V ₁₁ on the interference plane passing through the interference optical system Becomes If this is received by the photo detector D1 and only the AC component is taken, Becomes

Similarly, when the output obtained from the photodetector D2 is obtained for the light of V ₂ , Becomes

Therefore, V ₁₂ , V detected by the phase detection circuit PD1
The phase difference θ of ₂₂ is And from this, C: speed of light Δf: frequency difference between two lights Λ: synthetic wavelength Is required.

Here, the output Vθ of the phase detection circuit PD1 and the output VΔθ of the digital-analog converter DA together with the voltage controlled oscillator VC
The driving frequency of the AO modulator AOM2 is changed so that the phase difference θ becomes equal to the set phase Δθ by being fed back to 0. Therefore, the measurement target surface OBJ is changed from the oscillation frequency of the voltage controlled oscillator VC0.
You can find the distance to. That is, if the oscillation frequency of the voltage controlled oscillator VC0 is determined, the difference (frequency difference Δf) between the two wavelengths λ1 and λ2 generated by the AO modulator AOM2 is determined. Therefore, in the arithmetic circuit CON2, the frequency difference Δf and the set phase difference are set. By substituting Δθ into the equation (15), the distance to the measurement target surface OBJ can be obtained.

When Δθ = 2π, (d1−d2) = C / 2Δf, and if Δf is obtained from the drive frequency of the AO modulator AOM2, the distance to the measurement target surface OBJ can be calculated immediately. At this time, the phase detection circuit PD1 only needs to detect the difference in the amount of phase delay between the light of the two wavelengths, and the absolute value of each amount of phase delay does not matter, so the phase detection circuit PD1 can be changed by a simple circuit such as a mixer. Can be configured. In such a case, the output Vθ of the phase detection circuit PD1
Since it is not necessary to superimpose the offset signal VΔθ on, the digital / analog converter DA can be omitted.

As described above, in the length measuring unit used in the present invention, the difference between the phase delay amounts at the two wavelengths is fed back to the voltage controlled oscillator VC0 so that the phase difference becomes a constant value. By controlling the (frequency difference), the length of the combined wavelength of the two lights can be changed according to the distance to the measurement target surface OBJ. From this frequency difference, the distance to the measurement target surface OBJ can be easily I can know.

Although the above description has been made with reference to the embodiment shown in FIG. 1, the present invention is not limited to the embodiment shown in FIG. 1, and includes other embodiments as shown below. Is.

In the above description, the case where the laser light source LZ2 of one wavelength and the AO modulator AOM2 generate light of two different wavelengths λ1 and λ2 has been exemplified, but the means for generating two light of different wavelengths is not limited to this. It is not limited. Also, λ / plate P1
The case where the measurement of the phase delay amount for two wavelengths of light is performed at the same time by using the and polarization beam splitter BS1 has been described as an example, but the phase delay amount for each light is alternately measured without overlapping the two lights. Even if it does so, the same operation can be performed.

In the above description, the voltage controlled oscillator VC0 is used as the phase changing means for changing the phase delay amount at the combined wavelength of two wavelengths.
The AO modulator AOM2 driven by the output of AO2 has been exemplified, but in order to change the phase delay amount of the composite wavelength, for example, AO
The phase of the drive signal in the modulator AOM2 may be changed, or an optical crystal such as a crystal may be inserted in the optical path on the reference side to change the optical path length on the reference side equivalently. Good.

In the above description, the case where the phase delay amount φ of the light is heterodyne-detected has been illustrated, but the position of the interference fringe that changes depending on the phase delay amount φ may be detected by using the photodiode array. The measurement can be performed. Generally, in a photodiode array, by scanning the photodiode elements that make up the photodiode array at a constant speed, the photodiode array itself can have a spatial filter characteristic, and the phase can be adjusted without using an AO modulator. The position of the interference fringe corresponding to the delay amount φ can be easily detected.

Furthermore, in the above description, the case where only the length measurement in the Z-axis direction is performed with high accuracy has been illustrated, but the length measurement principle of the present invention is used, and the drive amount (X, Y) in the DUT drive unit DR2 is used. By also measuring, the shape of the measured object can be measured more accurately. Further, in such a case, since the length measuring unit used in the three-dimensional shape measuring apparatus of the present invention can always perform absolute measurement, the Z-axis and the X-axis are measured.
Even if the measurement is performed by switching the optical paths in the axis and Y-axis directions, there is no problem in the continuity of the measurement results, and one length measuring unit can commonly perform length measuring operations in three directions.

〔The invention's effect〕

As described above, in the three-dimensional shape measuring apparatus of the present invention, the Michelson interference optical system is used in the length measuring unit that measures the amount of displacement of the measurement target surface, and the two measurement targets are used. The amount of phase delay of light according to the distance to the surface is measured, and the distance to the surface to be measured is obtained from the relationship between these wavelengths and the amount of phase delay 3
In the three-dimensional shape measuring apparatus, one or both of the two lights are position-delayed so that the difference between the phase delays of the two lights on the interference surface in the interference optical system is always a constant value. Since the distance measuring unit is configured to calculate the distance to the measurement target surface by changing the frequency difference between the two wavelengths of light and the set phase difference at this time, The length of the combined wavelength of the two lights can be changed in a fixed relationship according to the distance, and an absolute length measurement output can be obtained only by measuring the difference in the amount of phase delay between the lights of the two wavelengths. At the same time, it is possible to realize a three-dimensional shape measuring device having a length measuring unit capable of performing length measurement with high accuracy and high resolution with a simple configuration.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a three-dimensional shape measuring apparatus of the present invention, and FIGS. 2 and 3 are block diagrams showing an example of a conventional three-dimensional shape measuring apparatus. LZ1, LZ2 ... laser light source, P1 ... λ / 2 plate, P2, P3 ... λ / 4 plate, P4
... Polarizer, BS1 to BS4 ... Beam splitter, MR1, MR2 ... Mirror, HMR1, HMR2 ... Half mirror, CC1 ... Cube corner, D1, D2 ... Photo detector, D3, D4 ... 4-division detector, SL ... Cylindrical lens, LE ... Objective lens, OBJ
… Object to be measured (measurement surface), DR1… Lens drive, DR2…
DUT drive unit, MN1, MN2 ... Movement amount monitor, CON1, CON2 ...
Arithmetic circuit, AOM1, AOM2 ... AO modulator, OSC ... Modulation signal source, VC
0 ... Voltage controlled oscillator, PD1 ... Phase detection circuit, DA ... Digital / analog converter.

Claims (1)

[Claims] 1. A light according to a distance to a surface to be measured by using Michelson's interference optical system for a length measuring unit for measuring a displacement amount of the surface to be measured and using at least two lights having different wavelengths. In the three-dimensional shape measuring apparatus configured to measure the phase delay amount of each of the two and determine the distance to the measurement target surface from the relationship between the wavelength and the phase delay amount, It is characterized by comprising a length measuring unit for changing the phase delay amount of one or both of the two lights so that the difference between the phase delay amounts of the two lights is always a constant value. Dimensional shape measuring device.