JPH04269602A - Interference measuring apparatus and adjusting method - Google Patents

Abstract

PURPOSE:To measure the shape of the surface of a substance under test highly accurately when the substance under test such as an optical element or a semiconductor wafer is measured. CONSTITUTION:In an interference measuring apparatus, a laser light source 1, a beam expander 2 and a beam splitter 3 are sequentially arranged. One luminous flux which is obtained by splitting the light with a beam splitter into two beams is reflected with a first reference plane mirror 4 and thereafter returned to the beam splitter 3. The other luminous flux is transmitted through the beam expander 5 and then reflected from a second reference plane mirror 6. The luminous flux is transmitted through the beam expander 5 again and returned to the beam splitter 3. The luminous fluxes which are reflected from both the first reference plane mirror 4 and the second reference plane mirror 6 are made to interfere, and the interference fringes are formed and can be observed with an image sensing device 7. The internal aberration of an interferometer appears in the interference fringes when the plane accuracy of the reference plane mirror is sufficiently low.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference measurement apparatus and adjustment method for measuring the surface shape and wavefront aberration of an object to be examined, such as an optical element or a semiconductor wafer, using optical interference.

0002

2. Description of the Related Art When measuring a surface shape with high precision using an interferometer, it is most common to use a Fizeau type interferometer, which is less susceptible to aberrations inside the interferometer. In particular, when measuring spherical surfaces such as lenses, there are many optical elements inside the interferometer, such as collimator lenses and reference lenses.
Usually, a Fizeau type interferometer is used, which is less susceptible to aberrations inside the interferometer. However, when measurements are performed with high accuracy of several tenths of a wavelength or higher, internal aberrations of the interferometer itself cannot be ignored. For this reason, several methods have been proposed in which the internal aberration data of the interferometer itself is measured and subtracted from the measured value of the object.

For example, in the invention disclosed in JP-A-62-129707, two prototypes are moved in the direction of rotation around the optical axis and in the direction of the optical axis to capture interference fringe data in three different postures. A method has been proposed in which the internal aberration of an interferometer is calculated using a calculation device and subtracted from subsequent measurement data to measure the shape of the object with high precision.

0004

However, in the above-mentioned conventional technology, a reference prototype is measured multiple times, and the measurement data is processed to calculate the internal aberration of the interference measurement device and stored in the storage device. However, in the case of a Fizeau type interferometer, the method of accurately determining the shape of the object by subtracting it from the measurement data of the object requires that the internal aberration of the interferometric measurement device be suppressed to below the wavelength of the test light. It is known that the internal aberrations of the interferometer cannot be accurately corrected because the measured values are affected by the internal aberrations.

[0005] In order to solve this drawback, the present invention aims to provide an interference measurement device and adjustment method that can improve the accuracy of optical adjustment and correct aberrations more accurately in order to suppress the aberrations inside the interferometer. purpose.

[0006]

[Means and effects for solving the problems] In the present invention,
It is possible to easily configure both a Twyman-Green type interferometer and a Fizeau type interferometer by simply adding or removing a reference plane mirror to the configuration of an interferometric measurement device,
In the configuration of a Twyman-Green interferometer, optical adjustments are made to minimize internal aberrations of the interferometer, and by removing the reference plane mirror that makes up the Twyman-Green interferometer from the interferometer configuration, a Fizeau-type interferometer is created. As an interferometer,
This is used to measure the subject.

FIGS. 1 and 2 are conceptual diagrams of the present invention. A laser light source 1, a beam expander 2, and a beam splitter 3 are arranged in this order as shown in FIG.

[0008] Also, one of the light beams split into two by the polarizing beam splitter 3 is reflected by the first reference plane mirror 4 and then returns to the beam splitter 3. The other beam passes through the beam expander 5 and then is reflected by the second reference plane mirror 6.
The light passes through the beam expander 5 again and returns to the beam splitter 3.

[0009]The light beams reflected by both the first reference plane mirror 4 and the second reference plane mirror 6 interfere to form interference fringes, which are observed by the imaging device 7. If the surface precision of the reference plane mirror is sufficiently small, these interference fringes represent internal aberrations of the interferometer.

In the case of the configuration shown in FIG. 1, the internal aberration of the interferometer is mainly caused by the beam expander 5, so by adjusting the beam expander 5 while observing the interference fringes observed by the imaging device 7, It becomes possible to perform optical adjustment of the interferometer with high precision. In this adjusted state, the reference plane mirror 4 is removed from the Twyman-Green interferometer configuration, the reference plane 8 and the object 9 are added, and the interferometer is transformed into a Fizeau type configuration as shown in FIG. The surface accuracy and wavefront aberration of the specimen 9 are measured. At this time, if the internal aberration of the interferometer is suppressed to about one wavelength, the surface shape of the object 9 is measured by changing the posture of the object 9 described above in three ways.
An effective method is to obtain the internal aberration of the interferometer and subtract it from the measurement data of the object 9.

[0011]

[Embodiments] Hereinafter, embodiments to which the present invention is applied will be explained in detail.

0012

Embodiment 1 FIG. 3 shows the configuration of an interference measurement apparatus according to Embodiment 1 of the present invention. A laser light source 21, a beam expander 23 that expands the luminous flux from this laser light source 21 to a predetermined beam diameter, and this beam expander 23.
Beam splitter 25 that transmits and reflects the luminous flux from
Furthermore, a 1/2 wavelength plate 24 held between the beam expander 23 and the polarizing beam splitter 25 so as to be freely rotatable, and a rotation mechanism 35 having a function of rotating this 1/2 wavelength plate 24. and the light beam reflected by the polarizing beam splitter 25 has a 1/4
A wavelength plate 26 and a reference plane mirror 27 are held by a parallel movement mechanism 34.

Furthermore, a polarizing plate 31 is provided on the opposite side of the parallel moving mechanism 34 with the polarizing beam splitter 25 in between.
It is held via an arm 36. On the other hand, the light beam transmitted through the polarizing beam splitter 25 is provided with a 1/4 wavelength plate 28, a beam expander 29 for converting the light beam into parallel light of a required diameter, and a reference plane mirror 30. Further, in order to observe the interference fringes of the two reflected lights combined by the beam splitter 25, a polarizing plate 31, a screen 32, and an imaging device 33 are arranged to constitute an interference measuring device.

FIG. 4 shows a rotation mechanism 35 for the 1/2 wavelength plate 24.
FIG. The half-wave plate 24 is attached to a pulley 50, and the pulley 50 is rotatably held by a holding portion 52 via a bearing 51. The pulley 50 is connected to a drive pulley 54 via a belt 53 and is configured to be rotated by a motor 55.

FIG. 5 shows a quarter-wave plate 26 and a reference plane mirror 27.
FIG. 3 is a perspective view showing details of the parallel movement mechanism 34 of FIG. 1/4
The wave plate 26 and the reference plane mirror 27 are held by a guide rail 60 above a base 61 which is held so as to be able to move in parallel, via support plates 66 and 67.
1. An angle fine adjustment mechanism 62 with respect to the support plate 67 on the upper side
The angle can be finely adjusted through. Further, the polarizing plate 31 is fixed to an arm 36 attached to support plates 66 and 67 erected on the base 61. Furthermore, the base 61
A rack 63 is fixed to the side of the motor 64.
A pinion 65 fixed to a drive shaft is engaged with a rack 63 and a motor 64 is driven to move the base 61 along the guide rail 60.

[0016] In the interferometer having the above configuration, a Twyman Green interferometer is configured, and the interference fringes of the reference plane mirrors 27 and 30 are observed. At this time, the half-wave plate 24 is rotated by the rotation mechanism 35 to rotate the polarization plane of the laser beam so that the contrast of the interference fringes is the longest, and the balance of the amount of light incident on each reference plane is adjusted. . If the flatness of the reference plane mirrors 27 and 30 is sufficiently good, the interference fringes observed in this state will be
This represents the aberration inside the interference diameter that does not include the reference lens, especially the aberration of the beam expander 29. If the distance between the surfaces of the lenses constituting the beam expander 29 is adjusted to reduce this internal aberration, the aberration within the interference diameter can be suppressed to about one wavelength.

Therefore, with the internal aberrations sufficiently reduced in this way, the quarter-wave plate 26 is moved by the motor 64.
The reference plane mirror 27 and the polarizing plate 31 are removed from the optical path, the interference measuring device is configured as a Fizeau type interferometer, and the 1/2 wavelength plate 24 is rotated to adjust the amount of light incident on the imaging device 33 to be appropriate. After that, a reference lens 37 and a subject 38 are placed in place of the reference plane mirror 30, and the subject 38 is
Measures surface accuracy and wavefront aberration.

In this embodiment, the quarter-wave plate 26, the reference plane mirror 27, and the polarizing plate 31 can be integrated and removed from the optical path by the motor 64, so the interferometer can be switched without touching the inside of the interferometer. There are advantages to being able to do it.

[0019]

Embodiment 2 FIGS. 6 and 7 show Embodiment 2 of the present invention.

The main configuration of the interference measurement device is shown in FIG. 3 of Embodiment 1.
It is similar to In Example 1, switching between the Twyman-Green type interferometer and the Fizeau type interferometer is performed by removing or inserting the quarter-wave plate 26, reference plane mirror 27, and polarizing plate 31 from the optical path. In Example 2, switching is performed by inserting or removing a shutter 39 between the quarter-wave plate 26 and the beam splitter 25 using a shutter switching mechanism 40.

FIG. 6 shows a state in which the shutter 39 is inserted into the optical path, forming a Fizeau type interferometer. FIG. 7 is a perspective view showing details of the shutter switching mechanism. In FIG. 7, the shutter 39 is held along a guide rail 70 via a support plate 75 that stands above a base 71 that is held so as to be able to move in parallel, and a rack 72 is provided on the side of the base 71. A pinion 74, which is fixed to the drive shaft of the motor 73, is meshed with the rack 72. Note that the shutter 39 needs to have a surface that does not reflect much light.

Therefore, the measurement method and necessary adjustment method in the interference measurement apparatus having the above configuration are as described in Example 1.
Since it is possible to implement the method in accordance with the case of , the explanation thereof will be omitted.

In particular, in this embodiment, it is possible to switch between the Twyman-Green interferometer and the Fizeau interferometer without touching the inside of the interferometer, and the switching does not involve moving optical elements. , there is an advantage that alignment adjustment of the reference plane plate 27 does not have to be performed every time.

[0024]

Embodiment 3 FIG. 8 shows Embodiment 3 of the present invention.

The main configuration of the interference measurement device is shown in FIG. 3 of Embodiment 1.
With a similar configuration, a return light is provided between the laser light source 21 and the beam expander 23 to prevent the reflected light from subsequent optical elements from returning to the laser light source 21 and making the laser oscillation unstable. The configuration of a removal device 22 has been added. In addition, in the first embodiment, switching between the Twyman Green type interferometer and the Fizeau type interferometer is performed using the quarter wavelength plate 26 and the reference plane mirror 27.
The third embodiment is characterized in that the switching is performed by rotating the quarter-wave plate 26 using the wavelength plate rotation mechanism 41, whereas the polarizing plate 31 is removed from or inserted into the optical path. shall be. However, this wavelength plate rotation mechanism 41
This can be carried out by applying the same structure as the wavelength plate rotation mechanism 35 of FIG. 4 shown in the first embodiment.

[0026] In the interference measuring device having such a configuration, the quarter-wave plate 26 is rotated by the wavelength plate rotation mechanism 41.
When rotated, the polarization state of the light beam reflected by the reference plane mirror 27 and returned to the beam splitter 25 changes depending on the angle of the 1/4 wavelength plate 26, so that it may be directed toward the imaging device 33 or toward the laser light source 21. It may return. In the former case, the interferometer works as a Twyman-Green interferometer, and in the latter case as a Fizeau interferometer. At this time, since the returning light beam from the laser light source 21 is removed by the return light removal device 22, it will not have any adverse effect on the laser light source 21.

Therefore, especially in the case of this embodiment, since a parallel movement mechanism is not required, a Twyman-Green type interferometer and a Fizeau type interferometer can be continuously operated with a simpler configuration than in Example 2. can be switched.

[0028]

According to the present invention, when measuring an object such as an optical element or a semiconductor wafer, the surface shape of the object can be measured with higher precision than when measuring with a conventional Fizeau interferometer.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of the present invention.

FIG. 2 is a conceptual diagram of the present invention.

FIG. 3 is an explanatory diagram showing Embodiment 1 of the present invention.

FIG. 4 is a perspective view of the rotation mechanism in FIG. 3.

FIG. 5 is a perspective view of the parallel movement mechanism in FIG. 3;

FIG. 6 is an explanatory diagram of Embodiment 2 of the present invention.

FIG. 7 is a perspective view of the shutter switching mechanism in FIG. 6;

FIG. 8 is an explanatory diagram of Embodiment 3 of the present invention.

[Explanation of symbols]

1 Laser light source 2,5 Beam expander 3 Beam splitter 4, 6 Reference plane mirror 7 Imaging device 8 Reference plane 9 Test surface

Claims (2)

[Claims] Claim 1: The light beam emitted from the laser light source is divided into two by a beam splitter, one of which is incident on the reference surface and the other is incident on the test surface, and the reflected light from the reference surface and the test surface is divided into two. In an interference measurement device that measures the shape of a surface to be measured or the wavefront aberration of a lens based on interference fringes obtained by interference, there is a beam expander to obtain a laser beam of a desired diameter, and one of the two divided beams. A reference plane mirror that can be removed from the configuration of the interferometric measurement device is placed in the mirror, and an imaging device is used to observe the interference fringes of the light beams that are reflected from the reference surface and the test surface and overlap again at the beam splitter. Equipped with an interference measurement device. Claim 2: A reference plane mirror is added to the configuration of a Fizeau type interferometer to form a Twyman Green type interferometer configuration.
2. The method of adjusting an interferometer according to claim 1, wherein optical elements inside the interferometer are adjusted so as to reduce internal aberrations of the interferometer.