JP2838171B2 - Phase shift micro Fizeau interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase shift micro-Fizeau interferometer, and more particularly to an improvement in an interferometer using a laser light source.

[0002]

2. Description of the Related Art An interference microscope using a phase shift interferometer has attracted attention when measuring a fine surface shape of a measurement object in a non-contact manner. In this phase shift interferometry, a part of light emitted from a light source is reflected by a reference mirror, and another part of light emitted from the light source is reflected on a sample. Then, the reflected light from the reference mirror and the reflected light from the sample are combined to generate interference light, where a phase shift is introduced into the interferometer, and the interference fringes are scanned.
At this time, information on the surface shape of the sample is obtained from the interference fringes.

In this phase shift interferometer, a white light source or a helium / neon laser is generally used as a light source. The phase difference required for the phase shift interferometry is determined by moving a reference mirror using a piezo element. I was getting in.

[0004]

However, this piezo element has problems such as non-linearity, instability, hysteresis characteristics, etc., and the apparatus is expensive, and a high voltage is required for driving. . That is, it is difficult to precisely control the movement of the reference mirror using a piezo element or the like, and it is difficult to improve the measurement accuracy. In addition, the drive control itself takes time, and there is a limit to high-speed measurement. For this reason, although the phase shift interferometry is theoretically a very excellent method for measuring a fine surface shape, it does not sufficiently exhibit features such as high accuracy and high speed.

An object of the present invention is to provide an inexpensive phase shift micro-Fizeau interferometer capable of high-accuracy and high-speed measurement.

[0006]

In order to achieve the above object, a phase shift micro Fizeau interferometer according to the present invention uses a semiconductor laser as the light source and adjusts an injection current to the semiconductor laser. Means are provided.

A phase shift micro-Fizeau interferometer according to the present invention is characterized in that a semiconductor laser temperature adjusting means is provided and an objective lens having a long working distance is provided . Here, the working distance of the objective lens means the objective lens.
The distance from the front end of the noise to the sample is shown.

[0008]

The phase shift micro-Fizeau interferometer according to the present invention uses a semiconductor laser as a light source, gradually changes the injection current to give a relative phase difference to the interferometer, scans the interference fringes, and shifts the phase shift. The phase to be detected is derived using the interferometry. Further, by providing a temperature adjusting means for keeping the temperature of the semiconductor laser constant, higher detection can be performed . In addition, an objective lens with a long working distance
Increasing the path difference reduces the change in injection current,
Changes in the user output can be minimized.
High-accuracy detection without being affected by changes in injection current
The result is obtained.

[0009]

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

The interferometer 10 shown in FIG. 1 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 light emitted from the light source 12 is collimated by the collimator lens 2.
The light is collimated by 0, and is incident on the beam splitter 14 via lenses 22, 24 and 26. The light reflected by the beam splitter 14 downward in the figure is converted into parallel light again by the objective lens 28, and further converted into circularly polarized light by the 波長 wavelength 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 a test object held by the sample holding unit 18. The light 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, the lens 28 and the beam splitter 14 again
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 observation result by the camera 38 is visually observed by the monitor 40, stored in the frame memory 42, subjected to desired data processing by the microcomputer 44, and output to the XY plotter 46.

A feature of the present invention is that a red semiconductor laser is used as a light source of a phase shift micro-Fizeau interferometer, and the temperature of the semiconductor laser is kept constant.
That is, a relative phase difference is given to the interferometer by changing the injection current in a stepwise manner, the interference fringes are scanned, and the phase to be detected is derived using the phase shift interferometry. For this purpose, in the present embodiment, an 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 The temperature of the laser 12 is kept constant.

[0013] That is, the semiconductor laser as shown in FIG. 2 when the current i is injected above the threshold i _s, starts oscillation of laser light. When the injection current i further increases, the wavelength λ increases in proportion to the increase of the injection current i. When the boundary currents i _a, wavelength increases with a step, also a constant linear region continues. The present invention utilizes this characteristic property of the semiconductor laser.

The derivation of the test phase is performed as follows. Assuming that the reflectance of the surface 34 of the test sample and the reference mirror 16 is low and the influence of the second or higher-order repetitive reflection is negligible, the intensity distribution of the interference fringes on the image plane can be expressed by the following equation (1).

[0015]

I (x, y, λ) = a (x, y) + b (x, y) cos, φ (x, y)｝ where φ (x, y) is 2π · ω (x, y) / λ and ω (x, y)
Is the height distribution of the test surface. Therefore, by obtaining ω (x, y), surface information of the test sample can be obtained. Further, a (x, y) and b (x, y) can be considered as constants at specific points on the surface to be inspected. Therefore, when the phase shift Δφ is introduced into the interferometer by some method, the above equation (1) can be replaced by the following equation (2).

[0016]

## EQU2 ## I (x, y, Δφ) = a (x, y) + b (x, y) cos {φ (x, y + Δφ)} Then, the Δφ is 0, π / 2, π, 3π / 2, and by measuring the respective intensity distributions I ₁ , I ₂ , I ₃ , I ₄ , φ (x, y) can be obtained from the following equation (3).

[0017]

## EQU3 ## φ (x, y) = tan ⁻¹ {(I ₄ −I ₂ ) / (I ₁ −I ₃ )} Information on the surface to be inspected is obtained from φ (x, y). By the way, conventionally, the change of Δφ is 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 of FIG. 1 is schematically shown in FIG. Equation 1 can be rewritten as follows using the wavelength λ as a parameter.

[0018]

## EQU4 ## I (x, y, λ) = a (x, y) + b (x, y) cos ｛(2π · 2ω (x, y) + L) / λ ₀ −Δφ｝ Then, I _{1 to} I ₄ are obtained. Here, L is the optical path length difference 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, so that the laser intensity is monitored to normalize the interference fringe intensity, or the optical path difference of the interferometer is increased. The change in the injection current necessary to obtain the required phase difference is minimized, and the change in the laser output is minimized. As a result, even when the laser wavelength is shifted as in the present invention, the bias a and the amplitude b of the intensity distribution of the interference fringe of the above equation 4 can be regarded as constant.

On the other hand, when the injection current i is changed to change the oscillation wavelength from λ ₁ to λ ₂ , the phases can be expressed as follows. λ ₁ : φ ₁ = 2π · (L / 2 × 2) / λ ₁ = 2πL / λ ₁ λ ₂ : φ ₂ = 2π · (L / 2 × 2) / λ ₂ = 2πL / λ ₂ Therefore, the phase difference the _{Δφ = φ 2 -φ 1 = 2πLΔλ} / λ 1 2. Therefore, it can be expressed as Δφ = 2πLΔλ / λ ² . That is, Δφ = 2πLΔλ / λ ² = 0 Δφ = 2πLΔλ / λ ² = π Δφ = 2πLΔλ / λ ² = 3π / 2 Δφ = 2πLΔλ / λ ² = 2π ₁ ,
What is necessary is just to obtain I ₂ , I ₃ and I ₄ .

Incidentally, the change amount Δλ of the wavelength is proportional to the change Δi of the injection current. Therefore, in the case of Δλ = α · Δi, that is, 2πLΔλ / λ ² = 2πLαΔi / λ ² = 2π, for example, Δi = λ ² / Lα. In the case of a general red laser, 670 at 20 ° C.
Since α = about 0.017 nm / mA with respect to the reference wavelength of nm, if L = 24 mm, the current change required for the change of 2π is Δi (mA) = (670 × 10 ⁻³ ) ² / (24 × 0.017) = 1.100 mA.

Therefore, π / 2, π, and 3π / 2 indicate injection currents of 0.275 mA, 0.550 mA, and 0.82 mA, respectively.
That is, it is only necessary to change 5 mA at a time. Since the linear region of the semiconductor laser is about 10 mA, it is easy to make a current change of this order.

FIG. 4 shows a case where the injection current i is increased stepwise with time according to this embodiment. Then, as shown in FIG. 5, it is understood that the interference fringe intensity distribution I changes with the change of the injection current i, and the test phase can be derived from the respective light receiving results.

As described above, according to the micro-Fizeau interferometer according to the present embodiment, since there is no mechanically movable part such as a piezo element, the system is stabilized. Also,
The apparatus can be reduced in size and cost, and has excellent operability. Further, in this embodiment, since the objective lens having a long working distance of 30 mm is used, the change in the injection current i can be small, and the change in the output of the semiconductor laser can be reduced.

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 that the temperature is kept constant by the temperature control means 50. Also, the oscillation wavelength λ of the semiconductor laser
Can be changed by the temperature of the semiconductor laser. Incidentally, in the case of the red semiconductor laser, the rate of change of the oscillation wavelength with temperature when the injection current was set to 49 mA was 0.061 nm / ° C.

[0025]

As described above, according to the phase shift micro-Fizeau interferometer according to the present invention, a semiconductor laser is used as a light source, and a relative phase difference is given to the interferometer by changing its injection current stepwise. Since the phase to be detected is derived using the phase shift interferometry, the measurement is performed at high speed,
Moreover, it can be performed with high accuracy . In addition, long working distance
The objective lens is provided, and the optical path difference is increased.
Minimize changes in flow and minimize changes in laser power
To be affected by changes in the injection current.
A highly accurate detection result can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a configuration of a phase shift micro-Fizeau interferometer according to one embodiment of the present invention.

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

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

FIG.

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

[Explanation of symbols]

 Reference Signs List 12 semiconductor laser 16 reference mirror 18 sample holding means 32 test sample

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yukihiro Ishii 808-48, Oishi, Musashimurayama-shi, Tokyo (56) References JP-A-60-211301 (JP, A) JP-A-62-129707 (JP, A JP-A-3-238309 (JP, A) JP-A-61-155902 (JP, A) JP-A-64-35304 (JP, A) Japanese Utility Model Showa 56-65411 (JP, U) (58) Field (Int.Cl. ⁶ , DB name) G01B 9/00-9/10 G01B 11/00-11/30

Claims (1)

(57) [Claims] 1. A reference mirror that reflects a part of light emitted from a light source, a sample holding unit that holds a sample that reflects another part of the light emitted from the light source, and a reflection from the reference mirror Combining means for combining light and reflected light from the sample to generate interference light; using a semiconductor laser as the light source and adjusting injection current to the semiconductor laser; and adjusting the temperature of the semiconductor laser. In a phase shift micro-Fizeau interferometer comprising: a temperature adjusting means for keeping the temperature constant; and an objective lens having a long working distance, the injection current of the semiconductor laser is obtained by dividing a wavelength change amount / an injection current.
The emission of the semiconductor laser is changed stepwise in the linear region.
The phase shift within 2π.
A phase shift micro-Fizeau interferometer characterized in that