JPH04297807A - Phase shift micro fizeau interferometer - Google Patents

Abstract

PURPOSE:To obtain a title instrument of low cost capable of high speed measurement with high precision. CONSTITUTION:This instrument employs a semiconductor laser 12 as the light source and is provided with an injection current adjusting means 5 that adjusts injection current into the semiconductor laser 12 as well as a temperature adjusting means 50 for the semiconductor laser 12. The injection current into the semiconductor laser 12 is changed stepwise to give the interferometer a relative difference in phase, and phase shift interferometry is used to derive test phases; therefore, measurement can be made at high speed and with high precision.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase-shifting microfizeau interferometer, and more particularly to an improved interferometer using a laser light source.

0002

2. Description of the Related Art Interference microscopes using phase shift interferometry are attracting attention when measuring the fine surface shape of an object in a non-contact manner. In this phase shift interferometry, a part of the light emitted from the light source is reflected by a reference mirror, and another part of the light emitted from the light source is reflected by the sample. Then, the reflected light from the reference mirror and the reflected light from the sample are combined to generate interference light, and a phase shift is introduced into the interferometer to scan the interference fringes. This is to obtain shape information.

By the way, this phase shift interferometer generally uses a white light source or a helium-neon laser as a light source, and the phase difference required for phase shift interferometry is obtained by moving a reference mirror using a piezo element. I was getting it.

0004

[Problems to be solved by the invention] However, this piezo element has nonlinearity, instability, hysteresis characteristics, etc., and the device is expensive, and furthermore, there are problems in that a high voltage is required for driving. . That is, it is difficult to strictly control the movement of the reference mirror using a piezo element or the like, making it difficult to improve measurement accuracy, and drive control itself takes time, which limits high-speed measurement. For this reason, although phase shift interferometry is theoretically an extremely excellent method for measuring surface fine shapes, it has not fully demonstrated its features such as high precision and high speed.

The present invention has been made in view of the problems of the prior art described above, and its object is to provide an inexpensive phase shift microfizeau interferometer that is capable of high-accuracy and high-speed measurement.

[0006]

[Means for Solving the Problems] In order to achieve the above object, a phase shift microfizeau interferometer according to the present invention uses a semiconductor laser as the light source, and also has an injection current adjustment that adjusts the injection current to the semiconductor laser. It is characterized by having a means.

[0007] Furthermore, the phase shift microfizeau interferometer according to claim 2 is characterized in that a temperature adjusting means for the semiconductor laser is provided.

[0008]

[Operation] The phase shift microfisseau interferometer according to the present invention uses a semiconductor laser as a light source, changes the injected current in stages to give a relative phase difference to the interferometer, scans the interference fringes, and shifts the phase. Derive the phase to be tested using interferometry. Further, by providing a temperature adjustment means for keeping the temperature of the semiconductor laser constant, more accurate detection can be performed.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the basic configuration of a phase shift microfizeau interferometer according to an embodiment of the present invention.

The interferometer 10 shown in the figure includes a light source 12 made of a red semiconductor laser, a beam splitter 14, a reference mirror 16, and a sample holding means 18. and,
The laser beam emitted from the light source 12 passes through the collimator lens 2
0, the light is made into parallel light, and further enters the beam splitter 14 via lenses 22, 24, and 26. The light reflected downward in the drawing by the beam splitter 14 is again converted into parallel light by the objective lens 28, and further converted into circularly polarized light by the quarter-wave plate 30. Then, the circularly polarized light irradiates the reference mirror 16, and a part of the light is reflected on the surface of the reference mirror 16, and the light transmitted through the reference mirror 16 is transmitted to the sample held by the sample holding means 18. It is reflected by the surface 34 of the sample 32.

As a result, the light reflected on the surface of the reference mirror 16 and the light reflected on the surface 34 of the test sample 32 are superimposed again by the reference mirror 16 to form interference light. Then, again the lens 28 and the beam splitter 14
and is guided upward in the figure. This light is imaged by the imaging lens 36 on the light receiving surface of the CCD camera 38. The results of observation by the camera 38 are visually observed by a monitor 40, stored in a frame memory 42, subjected to desired data processing by a microcomputer 44, and then output to an XY plotter 46.

The characteristic feature of the present invention is that a red semiconductor laser is used as the light source of the phase shift microfisseau interferometer, and the temperature of the semiconductor laser is kept constant.
The injected current is changed stepwise to give a relative phase difference to the interferometer, the interference fringes are scanned, and the phase to be detected is derived using phase shift interferometry. For this purpose, in this embodiment, the injection current control command from the microcomputer 44 is given to the injection current control means 50 via the interface 48 to control the injection current to the semiconductor laser 12, and the temperature control means 52 Keep the temperature of the laser 12 constant.

That is, as shown in FIG. 2, the semiconductor laser starts oscillating laser light when a current i equal to or greater than the threshold value is injected. Then, when the injection current i is further increased, the wavelength λ becomes larger in proportion to the increase in the injection current i. Then, when it comes to the boundary current ia, the wavelength increases with steps, and a constant linear region continues. The present invention utilizes this characteristic property of semiconductor lasers.

The phase to be tested is derived as follows. Assuming that the reflectance of the test sample surface 34 and the reference mirror 16 is low and that the influence of secondary and higher-order repeated reflections can be ignored, the intensity distribution of interference fringes on the image plane can be expressed by the following equation 1.

[0015]

[Equation 1] I (x, y, λ) = a (x, y) + b (x, y
)cos{φ(x,y)}where φ(x,y) is 2π
- ω(x, y)/λ, where ω(x, y) is the height distribution of the test surface. Therefore, by determining this ω(x, y), surface information of the test sample can be obtained. Furthermore, a(x, y) and b(x, y) can each be considered to be constants at specific points on the surface to be inspected. Therefore,
If a phase shift Δφ is introduced into the interferometer by some method,
The above number 1 can be replaced with the following number 2.

[0016]

[Formula 2] I (x, y, Δφ) = a (x, y) + b (x, y) co
s{φ(x, y+Δφ)} and the said Δφ is 0, π/
2, π, 3π/2, each intensity distribution I1
, I2, I3, and I4, φ(x, y) can be obtained from the following equation 3.

[0017]

[Formula 3] φ(x, y) = tan-1 {(I4-I2)/(I1-
I3)} Information on the surface to be inspected is obtained from this φ(x, y). Incidentally, conventionally, this change in Δφ was obtained by moving the reference mirror 16 using a piezo element or the like, but in the present invention, it is obtained by shifting the oscillation wavelength of the laser. That is, the interference microscope shown in FIG. 1 is schematically shown as shown in FIG. 3. The above equation 1 can be rewritten as follows using the wavelength λ as a parameter.

[0018]

[Formula 4] I (x, y, λ) = a (x, y) + b (x, y) cos
{(2π·2ω(x,y)+L)/λ0−Δφ} Then, by changing the above Δφ, I1 to I4 are obtained. Note that L is the difference in optical path length between the reference mirror 16 and the surface of the test sample 32. Here, when the injection current of the semiconductor laser 12 is changed, not only the oscillation wavelength but also the laser output changes. Therefore, it is necessary to monitor the laser intensity and normalize the intensity of the interference fringes.
Alternatively, the optical path difference of the interferometer may be increased to reduce the change in injection current required to obtain the necessary phase difference, thereby minimizing the change in laser output. As a result, even when the laser wavelength is shifted as in the present invention, the bias a and amplitude b of the intensity distribution of the interference fringes shown in Equation 4 can be regarded as constant.

On the other hand, when the injection current i is changed and the oscillation wavelength is shifted from λ1 to λ2, the phases can be expressed as follows. λ1: φ1=2π・(L/2×2)/λ1
=2πL/λ1 λ2: φ2=2π・(L/
2×2)/λ2=2πL/λ2 Therefore, the phase difference Δφ=φ
2-φ1=2πLΔλ/λ12. For this reason, Δφ
It can be expressed as =2πLΔλ/λ2. That is, Δφ=2πLΔλ/λ2=0 Δφ=2πLΔλ/λ2=π Δφ=2πLΔλ/λ2=3π/2 Δφ=2πLΔλ/λ2=2π By varying the injection current i, I1,
All you have to do is obtain I2, I3, and I4.

By the way, since the amount of change in wavelength Δλ is proportional to the change in injection current Δi, Δλ=α・Δi, that is, 2πLΔλ/λ2=2πLαΔi/λ2=2π
Taking the case of Δi=λ2/Lα as an example. For a typical red laser, 670 at 20℃
Since α=0.017 nm/mA for the standard wavelength of nm, if L=24 mm, the current change required for a change of 2π is Δi (mA) = (670×10-3)2/(24 ×0.
017)=1.100mA.

Therefore, the injection currents of π/2, π, and 3π/2 are respectively 0.275 mA, 0.550 mA, and 0.
It is sufficient to change it by 825 mA. Since the linear region of a semiconductor laser is about 10 mA, it is easy to change the current to this extent.

FIG. 4 shows the case where the injection current i is increased stepwise with time according to this embodiment. As shown in FIG. 5, it is understood that the interference fringe intensity distribution I changes as the injection current i changes, and the phase to be detected can be derived from each light reception result.

As explained above, according to the micro-Fizeau interferometer according to this embodiment, the system is stabilized because there is no mechanically movable part such as a piezo element. Also,
The device can be made smaller and cheaper, and has excellent operability. Furthermore, since this embodiment uses an objective lens with a long working distance of 30 mm, changes in the injection current i can be reduced, and changes in the output of the semiconductor laser can be reduced.

Note that the oscillation wavelength λ of the semiconductor laser also depends on the temperature. Therefore, when the injection current i is used as a parameter, it is preferable to maintain the temperature constant by the temperature control means 50. Furthermore, it is also possible to change the oscillation wavelength λ of the semiconductor laser depending on the temperature of the semiconductor laser. Incidentally, in the case of the red semiconductor laser, the rate of change in the oscillation wavelength due to temperature when the injection current was set to 49 mA was 0.061 nm/°C.

[0025]

[Effects of the Invention] As explained above, according to the phase shift micro-Fizeau interferometer according to the present invention, a semiconductor laser is used as a light source, and the injected current is changed stepwise to give a relative phase difference to the interferometer. Since we decided to derive the phase to be tested using phase shift interferometry, we could perform measurements at high speed.
Moreover, it can be performed with high precision.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the configuration of a phase shift microfizeau interferometer according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of the relationship between injection current and wavelength of a red semiconductor laser.

FIG. 3 is a schematic diagram of the interferometer shown in FIG. 1.

[Figure 4],

FIG. 5 is an explanatory diagram of changes in injection current and interference fringe intensity distribution in the interferometer shown in FIG. 1;

[Explanation of symbols]

12 Semiconductor laser 16 Reference mirror 18 Sample holding means 32 Test sample

Claims (2)

[Claims] 1. A reference mirror that reflects a part of the light emitted from the light source, a sample holding means that holds a sample that reflects another part of the light emitted from the light source, and a sample holding means that holds a sample that reflects the other part of the light emitted from the light source; combining light and reflected light from the sample;
A phase shift micro-Fizeau interferometer comprising a combining means for generating interference light, characterized in that a semiconductor laser is used as the light source, and injection current adjustment means is provided for adjusting the current injected into the semiconductor laser. A phase-shift microfizeau interferometer. 2. The phase shift microfizeau interferometer according to claim 1, further comprising means for adjusting the temperature of the semiconductor laser.