JPS61155902A - Interference measuring apparatus - Google Patents

Abstract

PURPOSE:To achieve highly accurate measurement, by controlling a reference mirror by measuring not only the shape of an article to be measured by a coherent beam source and a beam splitter but also the minute change in the reference phase of the reference mirror. CONSTITUTION:The beam of a coherent light source 30 is incident not only to an article 38 to be measured through a first beam splitter 34 but also to a reference mirror 32 through a second beam splitter 36 and the interference fringe obtained by interfering respective reflected beams by the first beam splitter 34 is read by an image pick-up apparatus 40. Further, data are inputted to an interference fringe phase analytical system 42 and receives operational processing by an electronic calculator 44 to measure the shape of the article 38 to be measured. Next. the minute change of the reference mirror 32 having a drive apparatus 46 such as a piezoelectric element attached thereto is detected as phase change by the beam of beam source 30 and beam interference through the second beam splitter 36, reflective mirrors 48, 50 and a third beam splitter 52 in a beam receiving apparatus 54 and a successive fringe phase measuring apparatus 56 and receives operational processing by the calculator 44 before the drive apparatus 46 is controlled through a drive circuit 58.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an interference measurement device using a fringe scanning method.
The present invention relates to an apparatus that can accurately measure the surface shape of an object to be measured, aberrations of an optical element, etc. by performing phase modulation of interference fringes with high precision.

First, the outline of the interference measurement device will be explained with reference to FIG. Laser light emitted from a coherent light source, for example, a laser light source 1o, is converted into a parallel beam (
The beam splitter 14 separates the light into an optical path (b) and an optical path (c). The parallel beam on the optical path (b) is reflected by the reference mirror 16 in the opposite direction to the incident direction,
It returns to the beam splitter 14 again as a reference wave. Further, the parallel beam on the optical path (c) is reflected by the test surface of the object to be measured 18 in a direction opposite to the direction of incidence, and returns to the beam splitter 14 again as a test wave. In the beam splitter 14, the reference wave and the test wave interfere with each other in the optical path (d), and the interference fringes 20 are read by an imaging device 22 such as a television camera. The brightness of the interference fringes 20 is determined by the phase difference between the reference wave and the test wave every 2 times when the wavelength of the laser beam is input. In the early interferometry methods, only the information on the peak position of this interference fringe 20 was focused, so the measurement sensitivity was only human or λ/2, and no higher measurement precision could be expected. . Therefore, an interferometric measurement method called the fringe scanning method or fringe scanning method is a method that focuses on the intermediate concentration distribution other than the peak position of the interference fringes and increases the measurement accuracy to λ1500 or even λ/1000. It has been proposed upstream.

Interferometry using the fringe scanning method is performed by slightly changing the phase of the reference wave mentioned above.
: the number of data sampling), the change in the interference fringes that rises each time is read as a sinusoidal change in the output of each pixel of the imaging device, and the initial phase difference at each measurement point on the object to be measured is calculated from the output. By determining the shapes and piecing them together, the shape of the surface to be measured can be measured with high precision.

The principle of this interference measurement method using the fringe scanning method will be explained with reference to FIG. Beam splitter 4, reference mirror 16, object to be measured 18 and parallel beam (a), optical path (b), (c),
(D) is the same as the case described with the same reference numerals in FIG. , reference mirror 16 by λ/2N (*Ni116 on the caudal side) 8
Rotate slightly. At that time, the pixel outputs of points A' and B' on the interference fringe, which correspond to measurement points A and B of the object to be measured 18, change sinusoidally for one cycle, for example, from bright → dark → bright. . Figure 3 shows the situation. The figure (a) shows that the pixel output intensity (I) at point A' on the interference track, which corresponds to point A of the object to be measured 18, is caused by the minute movement of the reference mirror 16 (n,, n2...
. . . n16). Similarly, the figure (b) shows that the pixel output intensity (I) at point B on the interference fringe corresponding to point B on the object to be measured 18 is caused by the slight movement of the reference mirror 16 (n,, n2...・n
's). When there is an optical path difference with respect to the reference plane at each measurement point, the initial phase of the sinusoidal curve of each pixel determined by the above method differs. In other words, (a) and (b) in Figure 3
, the initial phase difference is δ. Therefore, by joining together the initial phase differences obtained in the same manner at each of the other measurement points, the shape of the object to be measured, etc. can be measured with high precision.

As is clear from the above, in interference measurement using the fringe scanning method, highly accurate measurement depends on how accurately the phase of the reference wave can be changed by A/N. Therefore, various methods have been proposed in the past for changing the phase of this reference wave with high precision, for example, methods of performing minute movements of the reference mirror 16 with high precision. One of them is
The amount of movement of the reference mirror 16 is read by a floating capacitive sensor, etc.
This is a method to control its movement. This method directly reads and controls the amount of movement of the reference mirror, so it is excellent in terms of real-time measurement control, but the capacitance sensor itself must be calibrated using other high-precision measuring instruments. In addition to this complexity, there are also errors due to drift, so it is not always satisfactory. As another conventional method, there is a method in which the relationship between the voltage applied to the piezo element and the amount of movement is determined in advance, and the reference mirror is controlled based on the relationship. For example, when a sufficiently accurate plane mirror is placed at the position of the object to be measured 18 in FIG. In this method, the voltage is calculated from the displacement of the interference fringes of the reference mirror, and during actual measurement, the voltage calculated in advance is sequentially applied to control the minute movement of the reference mirror. However, this method does not measure the amount of movement of the reference mirror when actually measuring the object to be measured. Therefore, since real-time measurement control and closed loop control are not possible, the movement accuracy of the reference mirror is poor, and highly accurate measurement of the object to be measured cannot be performed.

The present invention solves the above-mentioned drawbacks of these conventional methods and accurately performs minute displacement of the reference wave to perform phase modulation of interference fringes with high precision, thereby enabling highly accurate measurement of the object to be measured. The present invention provides an interference measurement device that makes it possible.

A first embodiment in which the present invention is applied to an interferometer commonly called a Twyman Green type will be described below with reference to FIG. 30 is a laser light source, and 32 is a reference mirror, which in the present invention is commonly used as a measurement interferometer for measuring the shape of an object to be measured and as an interferometer for measuring minute movements of the reference mirror. By measuring the minute movement of the reference mirror,
The reference mirror is configured to control a drive device that drives the reference mirror.

First, an interferometer for measuring the shape of an object to be measured will be explained. The laser light emitted from the laser light source 30 becomes a parallel beam (A), and the first beam splitter 34
The optical path is separated into (b) and ())).

The parallel beam in one of the separated optical paths (B) is further separated into optical paths (E) and (E) by the second beam splitter 36. The parallel beam of the optical path (e) is shown in reference 11.
It is reflected in the direction opposite to the incident direction by t32 and returns to the $2 beam splitter 36 again as a reference wave. This reference wave is separated into an optical path (g) that travels straight by the second beam splitter 36 and an optical path that is reflected at right angles, travels in the opposite direction on the optical path (b), and returns to the first beam splitter 34. be done. The parallel beam on the optical path (c) that is separated by the one-way beam splitter 34 and goes straight is the object to be measured 3.
8 in the opposite direction to the incident direction, and returns to the first beam splitter 34 again as a test wave. In the first beam splitter 34, the reference wave and the test wave are interfered in the optical path (d), and the interference fringes are read by an imaging device 40 such as a television camera, and further interfere with the interference fringe phase analysis system 42. Data for each pixel of the stripe is captured.

In the interference fringe phase analysis system 42, when the data of the modulated interference fringes generated each time the reference mirror 32 moves minutely is sequentially captured by the method described later, the data is processed by the electronic meter Kfi 44, and each measurement The initial phase difference at each point is determined, and by piecing these together, the shape of the object to be measured 38 can be determined.

Next, an interferometer for measuring the minute movement of the reference mirror 32 will be explained. A driving device 46, such as a piezo element, is attached to the reference mirror 32 to move it minutely. The parallel beam from the laser light source 30 passes through the optical paths (A) and (B), and travels straight through the second beam splitter 36 as described above. It is reflected and separated into an optical path (g) that travels straight. The parallel beam on the optical path (H) is reflected at right angles by the first sufficiently accurate reflecting mirror 48, forming an optical path (H) and passing through the third beam splitter 5.
2. Further, the parallel beam on the optical path (T) is reflected at right angles by the second sufficiently accurate reflecting mirror 50 and enters the third beam splitter 52 as an optical path (S). By the third beam splitter 52, the parallel beam in the optical path (H) which is the test wave of the reflecting mirror 48 and the parallel beam in the optical path (S) which is the reference wave of the reference mirror 32 are interfered in the optical path (N). , the interference fringes are read by the light receiving device 54. The light-receiving device 54 may include two or four light-receiving elements, such as a known mower stripe measuring device.
The two interference fringes used are arranged so as to be shifted in phase by 90 degrees with respect to the moving direction. The light receiving device 54 sequentially outputs changes in the interference fringes that occur when the reference mirror 32 moves minutely to the fringe phase measuring device 56, and the amount of movement of the reference fi 32 is measured based on the signal. This measured value is input to the electronic calculation@44, and the electronic computer calculates whether or not the reference mirror 32 has moved by a preset amount. The voltage applied to the drive device 46 that drives the mirror 32 is controlled.

In addition, in the above, as a method of slightly changing the phase of the reference wave, a case was explained in which the driving device 46 such as a piezo element is directly attached to the reference @ 32 and the reference mirror 32 is slightly moved. It is not limited to this.

That is, for example, the reference mirror 32 is fixed, a wedge-shaped lath plate with two planes slightly inclined is inserted between the optical path (E), and this wedge-shaped lath plate is slightly moved in a direction perpendicular to the optical path (E). In a method of changing the phase of the reference wave by changing the optical path length, it is of course applicable to the case where a piezo probe is attached to a wedge-shaped lath plate and moved minutely.

Next, a second embodiment in which the present invention is applied to an interferometer commonly called a Fizeau type will be described with reference to FIG. In the case of the second embodiment, the same laser light source and reference mirror are used to measure the measurement interferometer to measure the shape of the object to be measured, and to measure the minute movement of the reference mirror to minutely change the phase of the reference wave. It is also used as an interferometer. α is the same. In addition, in the figure! Components with the same reference numerals as those in Figure @4 have exactly the same functions. The laser light emitted from the laser light source 30 becomes a parallel beam, passes through the first beam splitter 60 and the second beam splitter 64, and enters the half mirror 66 as a reference mirror. The half mirror 66 has a reference surface slightly inclined with respect to the optical path, and by slightly moving this reference surface perpendicular to the optical path, the optical path length can be changed. A part of the parallel beam incident on this reference surface is reflected in a direction opposite to the direction of incidence and becomes a reference wave. Also Half Mira 6
A part of the parallel beam that has passed through the half mirror 66 is reflected by the object to be measured 38, becomes a test wave, and returns to the half mirror 66, where it interferes with the reference wave to generate interference fringes. This interference fringe is read by the imaging device 40 via the @2 beam splitter 64, and is read by the interference fringe phase analysis system 42.
The shape and the like of the object to be measured 38 are measured by electronic calculation [44] in exactly the same way as in the first embodiment.

Next, an interferometer for measuring the minute movement of the reference mirror will be described. A driving device 46, such as a piezo element, for example, is attached to the bar 7mi 266, which has the reference surface inclined with respect to the optical path, to move it minutely in a direction perpendicular to the optical path. Laser light source 3 described above
The parallel beam from 0 is split by the first beam splitter 60 into an optical path that is reflected at right angles and an optical path that travels straight. The parallel beam reflected at right angles is reflected in a direction opposite to the incident direction by a sufficiently accurate reflecting mirror 70, and returns to the first beam splitter 60 as a reflected test wave @70. Further, the parallel beam traveling straight through the fPJ1 beam splitter 60 is reflected by the half mirror 66 in a direction opposite to the direction of incidence, and returns to the first beam splitter 60 as a reference wave. The test wave from the reflecting mirror 70 and the reference wave from the half mirror 66 interfere with each other via the 1i beam splitter 60, and the interference fringes are read by the light receiving device 54. Based on the change in the interference fringes read by the light receiving device 54, the movement amount of the half mirror 66 is measured by the fringe phase measuring device 56, and the drive device 46 is controlled by the electronic meter via the drive device drive circuit 58 by the rock 44. α is the same as in the first embodiment.

In the above embodiment, a half mirror having a reference surface perpendicular to the optical path is used as the driving device for minutely changing the phase of the reference wave, and this half mirror is moved along the optical path using a piezo element or the like. Of course, it is also possible. In FIG. 5, the polarizing plate 62 and the λ/4 plate 68 are provided so that the detection wave of the object to be measured 38 enters the imaging device 40 but does not enter the light receiving device 54. That is, the parallel beam from the linearly polarized laser 30 becomes a linearly polarized beam whose polarization plane is limited to one by passing through the polarizing plate 62.

When this linearly polarized beam passes through the λ/4 plate 6 sheet metal 8, it becomes a circularly polarized beam and is further reflected by the object to be measured 38. When it passes through the λ/4 plate 6 sheet metal 8 again, the plane of polarization changes with respect to the original linearly polarized beam. This results in a linearly polarized beam shifted by 90°. Therefore, this linearly polarized beam whose polarization plane is shifted by 90 degrees does not pass through the polarizing plate 62 and does not enter the light receiving device 54.

The operation of the present invention explained above will be explained by taking as an example the case of FIG. 4 in which the reference mirror is slightly moved in the same direction as the optical path as a method of slightly changing the phase of the reference wave. Now, when the reference mirror 32 is gradually moved by the driving device 46, the amount of movement is measured by the fringe phase measuring device 56 via an interferometer for measuring minute amounts of movement of the reference mirror. When the amount of movement of the reference mirror 32 does not reach a predetermined amount, for example, in FIG. Device 46 is controlled and reference mirror 32 continues to move. The reference mirror 32 moves by a predetermined amount,
When it is determined that the n1 position has been reached, the movement of the reference mirror 32 is stopped by a command. At this time, data on interference fringes created by interference between the reference wave from the reference mirror 32 and the test wave from the object to be measured is collected via the imaging device 40 by a measurement interferometer for measuring the shape of the object to be measured. is captured by the interference fringe phase analysis system 42. Next, the drive device 46 is driven again by a command, the reference mirror 32 starts moving, and the reference I!
32 stops when it reaches the n2 position, and as in the case above, phase modulated interference fringe data is captured by the interference fringe phase analysis system 42. In this way, in the example of Fig. 3, when the reference mirror is evenly moved minutely 16 times (n+, 1□...n16), all the data of the interference fringes that have been changed g4 are captured. The initial phase difference of each measurement point is calculated by the electronic computer 44, and the shape etc. of the object to be measured can be measured with high precision from this.

As described in detail above, the present invention includes an interferometer for measuring the shape of the object to be measured and an interferometer for determining the minute movement amount of the drive device for minutely changing the phase of the reference wave. It is characterized by having a structure in which one laser light source and one reference wave are shared by both interferometers, so that minute changes in the reference wave can be caused by driving for making minute changes in the reference wave. It is possible to perform direct measurement with a laser interferometer that incorporates the device, and the phase change of the reference wave can be performed in real time using closed loop control based on the measurement results. Therefore, highly accurate positioning of the reference mirror, etc. is possible. , there is no need for calibration like when using a capacitance sensor, and the two interferometers mentioned above are composed of independent optical systems except for the laser light source and reference mirror, so there is no possibility of any negative effects on each other. It can produce excellent effects such as not giving anything.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the principle of interference measurement! @Figure 2 is a diagram explaining the principle of interference measurement using the fringe scanning method. FIG. 3 is an output waveform diagram thereof, FIG. 4 is a detailed explanatory diagram of the present invention, and FIG. 5 is an explanatory diagram of another embodiment of the present invention. 30: Laser light source 32: Reference [38: Object to be measured 40
: Imaging device 44: Electronic computer 46: Drive device 52.
60: Beam splitter 54: Light receiving device 56: Fringe phase measuring device 66: Herr 7 mirror 70: Reflector

Claims (3)

[Claims] (1) A reference mirror installed on the optical path of the parallel beam from the coherent light source, an object to be measured installed on the optical path of the parallel beam from the coherent light source, and the beam reflected by the reference mirror. The device includes a device that slightly changes the phase of a reference wave, and a device that causes interference between the reference wave reflected by the reference mirror and the test wave reflected by the object to be measured, and the device slightly changes the phase of the reference wave. Capturing phase modulation information of interference fringes that sometimes occurs between the test wave and the test wave through an imaging device,
A parallel beam from the coherent light source and a reference wave reflected by the reference mirror are input to an interference measuring device using a fringe scanning method, which is configured to measure the surface shape, etc. of the object to be measured by analyzing it. an interferometer for measuring the amount of phase change of a reference wave, the interferometer comprising a beam splitter that measures the amount of phase change of a reference wave, and a reading device that reads interference fringes generated by interfering the parallel beam and the reference wave through the beam splitter; An interferometric measurement device characterized by controlling a device that slightly changes the phase of the reference wave based on the amount of phase change of the reference wave measured by the method. (2) A device for causing interference between a reference wave reflected by the reference mirror and a test wave reflected by the object to be measured,
A parallel beam from a coherent light source is separated by a beam splitter, one of the separated parallel beams is reflected by a reference mirror and returned to the beam splitter as a reference wave, and the other separated parallel beam is reflected by a reference mirror. The interference measurement according to claim 1, characterized in that it is a device that reflects the reflected wave from a measurement object and returns it to the beam splitter as a test wave, and causes the reference wave and the test wave to interfere in the beam splitter. Device. (3) A device for causing interference between a reference wave reflected by the reference mirror and a test wave reflected by the object to be measured,
A device that causes interference between a reference wave reflected by a half mirror provided on the optical path of a parallel beam from a coherent light source and a test wave that passes through the half mirror, is reflected by an object to be measured, and passes through the half mirror again. An interference measuring device according to claim 1, characterized in that: