JP4680296B2 - Interferometric measuring method and interferometric measuring apparatus using the same - Google Patents

Description

  The present invention relates to an interference measurement method and an interference measurement apparatus using the same, and more particularly to a method for measuring the surface shape of an optical component protected by a package with an optical window with high accuracy using an interference method and the same. The present invention relates to an interference measurement apparatus.

MEMS (Micro) with micro structure and accompanying displacement such as translation and tilt
Electro Mechanical Systems) include optical filters and optical cross connectors used for wavelength division multiplexing in the example of optical components.

  FIG. 1 is a diagram showing a cross section of a MEMS optical component 10 protected by a package 100 with an optical window 11. In order to evaluate the processing accuracy and dynamic characteristics of the optical component 10, it is known to use an interference measuring device. (For example, Patent Documents 1 and 2).

  The methods using the interference measuring apparatus disclosed in Patent Documents 1 and 2 have an interference optical system that causes object reflected light and reference light to interfere with each other, and observe interference fringes generated by the interference optical system.

  2A and 2B are diagrams illustrating a conventional method for measuring MEMS using such an interference measurement method.

  FIG. 2A is a diagram showing an interference optical system that does not assume measurement through glass. First, the irradiation light Lo is input to the half mirror 4 through the lens 2. The half mirror 4 divides the incident irradiation light Lo into two directions toward the reference mirror 8 and the measurement object 10.

  The light reflected from the reference mirror 8 and the measurement object 10 is overlapped again by the half mirror 4 to generate interference fringes, which are output through the lens 15 as measurement light Lr. Here, lasers and other light sources are used for interference measurement, but lasers use light sources other than lasers because they have a very long coherence distance and cause interference in unnecessary parts. There are many cases.

  Since the coherence distance is relatively short with a light source other than a laser, the optical distance between the reference mirror 8 side and the measurement object 10 side must be approximately equal in order to generate interference fringes. However, when the optical window 11 is in front of the measurement object 10 as shown in FIG. 2A, the optical distance on the measurement object 10 side is increased by (n−1) d. Here, n is the refractive index of the optical window material, and d is the thickness of the optical window.

  Therefore, the optical distance is different between the reference mirror 8 side from the half mirror 4 and the measurement target 10 side, and interference fringes are not generated.

  For this reason, as shown in FIG. 2B, in order to make the optical distance of both the same on the reference mirror 8 side, the compensation glass 7 is inserted, and the optical distance is different between the reference mirror 8 side and the measurement object 10 side. It is conceivable to avoid the problem of end.

  Here, in FIG. 2A, only the difference in optical distance is focused as a problem. However, in the interference measurement through the optical window 11, it is also considered that the interference fringe contrast is lowered due to the reflected light from the optical thin film (antireflection film or the like) formed on the optical window 11 and the interference measurement becomes difficult. Must.

  For the sake of explanation, it is assumed that the optical component to be measured is for communication and the wavelength used is infrared. In such a case, the antireflection film formed on the optical window 11 is designed not to reflect infrared light used for communication. However, other wavelength ranges may be indifferent.

  For example, in the graph showing the relationship between the wavelength region and the reflectance shown in FIG. 2C, the reflectance is kept low in the target wavelength region (in this case, infrared) I, but the other wavelength region (visible light region). In II, the reflectance is not taken into consideration, and there is a case where the measurement target must be measured through the optical window 11 on which the antireflection film 11a having the characteristic of generating the reflected light Lr0 is formed.

  As shown in FIG. 3A, the reflected light Lr1 from the measurement object 10 and the reference light Lr2 from the reference mirror 8 interfere with each other at the half mirror 4, and the interference light Lr is output. Since normal interference measurement is performed in the wavelength region of the visible light region II, as shown in FIG. 3B by enlarging a portion surrounded by a circle in FIG. The portion Lr0 is reflected.

  That is, as shown in FIG. 3B, an antireflection film 11a is formed on the optical window 11 and is designed not to reflect infrared light, as described above, but is reflected in the visible light region II. Is not considered. Therefore, the reflected light Lro in the visible light region II is generated on the surface of the antireflection film 11a. Such reflected light Lro from other than the measurement object 10 causes the contrast of the interference fringes to deteriorate.

FIG. 4 is a diagram for further explaining the problem in the interference measurement through the optical window 11. As shown in FIG. 4a, when interference measurement is performed at wavelengths λ ₁ and λ ₂ in the visible light region II, the reflectance is low at wavelength λ _{2 and} there is little unnecessary reflection. For this reason, interference fringes with high contrast can be acquired (FIGS. 4b and 4c). On the other hand, when the wavelength λ ₁ is used, the reflected light Lro due to the antireflection film 11a of the optical window 11 increases and the interference fringe contrast decreases (FIGS. 4d and 4e).

Thus, when the contrast of the interference fringes decreases, noise increases in the measurement result of the height of the MEMS that is the measurement target 10, and measurement itself may be impossible depending on the degree of the contrast decrease.
Anti-reflection film JP-A-2002-5619 JP 2004-177225 A

  Accordingly, an object of the present invention is to provide an interference measurement method and an interference apparatus using the same, which solve the problem caused by the decrease in contrast of interference fringes caused by unnecessary reflected waves by such an antireflection film.

A first aspect of the present invention that achieves such an object is an interference measurement apparatus that measures the surface shape of a measurement target protected by an optical window in which an optical thin film is formed, and that has a broad wavelength distribution characteristic. An illuminating device to output, a wavelength filter selecting means for inputting light output from the illuminating device, switching a plurality of wavelength filters having different center wavelengths, and selectively outputting at least one wavelength light; and the wavelength A half mirror that splits the wavelength light selected by the filter selection means into reference light and irradiation light directed to the measurement target, and further generates and outputs reflected light from the measurement target by the irradiation light and interference light by the reference light Have
The wavelength filter selection unit is configured to select a combination of wavelength filters that make the contrast of interference fringes in the interference light from the half mirror be equal to or greater than a predetermined threshold value.

  In the first aspect, the wavelength filter selection means selectively outputs a plurality of wavelength lights, and a combination of wavelength filters that makes an average contrast of interference fringes of interference light in the plurality of wavelength lights equal to or greater than a predetermined threshold value. May be selected.

  In the first aspect, the wavelength filter selection unit selects another wavelength filter when a spatial average value of interference fringe contrast acquired by the currently selected wavelength filter is equal to or less than a predetermined threshold value. Alternatively, a combination of wavelength filters with which the interference fringe contrast is equal to or higher than a predetermined threshold may be selected.

  In the first aspect, the optical filter further includes a spectroscope, the reflection characteristic from the optical window is evaluated in a state where the reference light is shielded, and the wavelength filter selection unit is configured to reflect the reflection characteristic evaluated by the spectroscope. Based on the above, the optimum wavelength filter may be selected.

  Furthermore, in the above, the reflection characteristic evaluation in the spectroscope is input to the control PC, and the PC is configured to cause the wavelength filter selection means to select the optimum wavelength selection filter based on the reflection characteristic evaluation data. May be.

  In the first aspect, each of the plurality of wavelength filters is a wavelength cut filter having a different center wavelength, and the light from which the wavelength component has been removed by the wavelength cut filter selected by the wavelength filter selection unit is the wavelength filter. You may comprise so that it may input into a Harmirror.

  Furthermore, in the first aspect, the illuminating device includes a plurality of light emitting diodes that output light having different central wavelengths, and the wavelength filter selecting means is configured to emit at least one light of the plurality of light emitting diodes. The diode may be selectively driven to selectively output the corresponding one wavelength light from the selectively driven emitting diode.

  A second aspect of the present invention that achieves the above object is an interference measurement method for measuring the surface shape of a measurement object protected by an optical window in which an optical thin film is formed, and is a method for measuring light having a broadband wavelength distribution characteristic. Input to the wavelength filter selection means, the wavelength filter selection means to selectively output at least one wavelength light, the selected wavelength light is branched into reference light and irradiation light directed to the measurement object, and the irradiation The reflected light from the measurement target by the light and the interference light by the reference light are generated, and the wavelength filter selecting means is a wavelength filter for which the contrast of the interference fringes in the interference light from the half mirror is greater than or equal to a predetermined threshold. A combination is selected.

FIG. 1 shows a cross section of a MEMS optical component protected by a package with an optical window. FIG. 2A is a diagram illustrating an interference optical system that does not assume measurement through glass. FIG. 2B is a diagram illustrating a configuration that avoids the problem that the optical distance is different between the reference mirror side and the measurement target side. FIG. 2C is a diagram illustrating a visible light region and a wavelength region of an antireflection film. FIG. 3A is a diagram (part 1) for explaining problems in interference measurement through an optical window. FIG. 3B is an enlarged view of a portion surrounded by a circle in FIG. 3A. FIG. 4 is a diagram (part 2) for explaining problems in interference measurement through an optical window. FIG. 5A is a diagram showing a basic conceptual configuration of the present invention. FIG. 5B is a diagram for explaining the selection of a specific wavelength in the relationship graph between the wavelength and the reflectance distribution. FIG. 5C is a diagram for explaining selection of a plurality of wavelength regions in a relationship graph of wavelength and reflectance distribution. FIG. 5D is a diagram for explaining a cut in a wavelength region with many unnecessary reflections in the relationship graph between the wavelength and the reflectance distribution. FIG. 6 is a diagram showing an interference measuring apparatus according to an embodiment of the present invention in accordance with the principle of the present invention described with reference to FIGS. FIG. 7 is an example flow of a selection method of the narrowband wavelength selection filter. FIG. 8 is a diagram illustrating a configuration in a case where interference fringes of two wavelengths are acquired independently. FIG. 9 is a diagram showing multiple filter changers each having a filter in which a plurality of wavelength cut filters can be arranged on the optical path. FIG. 10 is a diagram illustrating an example in which an LED (light emitting element) is used as an illumination light source.

  Embodiments of the present invention will be described below with reference to the drawings. The embodiments are for understanding the present invention, and the technical scope of the present invention is not limited thereto.

  FIG. 5A is a diagram showing a basic conceptual configuration of the present invention.

  In the present invention, a wavelength filter selection means (in which a plurality of wavelength filters 25 having different center wavelengths can be switched between the illumination device 1 of the illumination light Lo and the half mirror 4 in the interference measurement device shown in FIG. Filter changer) 23.

  Among the plurality of wavelength filters 25, a wavelength filter with less unnecessary reflection and high interference fringe contrast is automatically selected to perform interference measurement. This solves the problems described above.

  For example, among the irradiation light Lo having a broad wavelength distribution emitted from the illumination device 1, a specific wavelength range A (preferably, selected from the visible light range II in the relationship graph between the wavelength and the reflectance distribution shown in FIG. 5B. The wavelength filter 25 for extracting the wavelength region in which the reflectance is lowest is selected by the wavelength filter selection means 23 to perform interference measurement.

  As a result, as shown in FIGS. 4B and 4C, it is possible to obtain an interference fringe image with good contrast. Further, when interference is performed by selectively using a plurality of wavelength regions A and B instead of interference using light of a single wavelength, it is used in the wavelength and reflectance distribution graph as shown in FIG. 5C. A pair of wavelength filters whose average contrast is improved by a plurality of wavelength regions A and B is automatically selected.

  The example of FIG. 5C is the case of two wavelengths A and B. Further, for example, when using illumination light having a wide wavelength range such as a white light source, such as vertical scanning interferometry, a filter that cuts off wavelength regions X, Y, and Z having a lot of unnecessary reflection as shown in FIG. 5D. By using it, the interference fringe contrast can be improved.

According to the principle of the present invention as described above, interference fringes with high contrast can be obtained even through an optical window on which an optical thin film 11a (see FIG. 3B) that mainly reflects visible light is formed. Become.
[Example]
FIG. 6 is a diagram showing an interference measuring apparatus according to an embodiment of the present invention in accordance with the principle of the present invention described with reference to FIGS. 5A to 5D.

  Light emitted from the illumination device 1 having a broad wavelength distribution such as a xenon light source is guided by the light guide 2 to one filter 25 selected by the wavelength filter selection means (filter changer) 23, and the wavelength used for measurement is selected. To do.

  The selected wavelength light is guided toward the measurement target 10 by the half mirror 4, and interference fringes between the measurement target 10 and the reference mirror 8 are generated by the interference objective lens 9. The interference objective lens 9 includes an imaging lens 5 for condensing measurement light on the measurement object 10 and the reference mirror 8, a half mirror 6 for dividing the measurement light in the direction of the measurement object 10 and the reference mirror 8, and a reference mirror. 8. An optical distance compensator 7 for compensating for an increase in optical distance due to the optical window 11, and an optical path shield 36 for temporarily shielding the measurement light by the reference side optical path.

  Light reflected from the measurement object 10, the reference mirror 8, and the optical window 11 is again overlapped by the half mirror 6 to cause interference. The interference fringes imaged by the imaging lens 15 are imaged by a CCD camera 16 and recorded on a PC (personal computer) 17. The PC 17 analyzes the interference fringe from the interference fringe image captured by the CCD camera 16 and calculates the surface shape of the measurement target 10.

  When performing the phase shift method, which is a known technique for calculating the height from the interference fringes with high accuracy, a plurality of interference fringe images are acquired while moving the measurement object 10 in the vertical direction by the piezo stage 18. The surface shape of the measurement object 10 is obtained from a plurality of interference fringe images.

  The measurement visual field and the posture of the measurement target can be adjusted using the coarse movement stage 20.

  A plurality of narrowband wavelength selection filters 25 having different center wavelengths are set in the filter changer 23 in advance.

  FIG. 7 is an example flow of the selection method of the narrowband wavelength selection filter 25. The wavelength selection filter 25 to be used first is set on the optical path 3 (step S1). An interference fringe image is acquired (step S2), and an average E of interference fringe contrast in a predetermined region or the entire field of view is calculated (step S3).

  When the spatial average E of the interference fringe contrast is less than or equal to the preset threshold T (step S4, NO), change to another wavelength selection filter 25 set in the filter changer 23 (steps S6, NO, S7), As in the beginning, the interference fringe contrast average E is compared with the threshold value T.

  If the interference fringe contrast average E is larger than the threshold T (step S4, YES), the surface shape of the measurement object 10 is calculated from the interference image acquired assuming that the contrast is sufficient for interference fringe analysis (step S5). If, for example, 0.1 is set as the threshold T, it is considered that a measurable interference fringe can be obtained.

  If no contrast equal to or higher than the threshold value T is obtained for all the wavelength selection filters 25 (step S6, YES), the wavelength selection filter 25 set in the filter changer 23 cannot measure, and work from the monitor of the PC 17 Send a message to the person.

  In addition to the wavelength selection method shown in FIG. 7, only the reflected light from the measurement object 10 and the optical window 11 is temporarily collected by the optical path shielding plate 36 and the spectroscope 14 is condensed by the half mirror 12 and the imaging lens 13. Then, the best combination of the wavelength selection filters 25 may be determined from the result of evaluating the spectral characteristics of the optical window 11.

  As another wavelength selection method, when the reflection characteristic from the optical window 11 is known in advance as data, the characteristic data is input to the PC 17 and the optimum wavelength selection filter is selected based on the data. Good.

  When selecting a plurality of wavelengths as shown in FIG. 5C, the optical system having the configuration shown in FIGS. 8 and 9 is used. That is, when the interference fringes of two wavelengths are acquired independently, the configuration of FIG. 8 can be used. After the interference fringes are acquired by the wavelength selection filter 25 of the center wavelength A by the optical system including the half mirror 35, another configuration is obtained. The interference fringes are obtained again after changing to the center wavelength B.

  When a light source in which two different wavelengths are mixed is necessary, in FIG. 8, in addition to the light source 1, another light source 1-2, a light guide 2-2, a filter changer 23-2, and a wavelength selection filter 23-2 are prepared. , Half-mirrors 34 and 35 are used to produce measurement light mixed with a plurality of wavelengths.

  When two or more wavelengths are used, the number of changes of the light source or wavelength selection filter may be increased as in the case of the two wavelengths.

  As shown in FIG. 5D, when it is desired to cut a plurality of wavelength bands X, Y, and Z among wideband wavelengths, it is necessary to simultaneously cut a plurality of wavelengths for one light source. In this case, as shown in FIG. 9, a multiple filter changer 26 having filters 27 each having a plurality of wavelength cut filters on the optical path 3 may be used. FIG. 9 shows an example in which three wavelength regions can be cut simultaneously.

  The PC 17 controls the coarse copper stage 20 and the piezo stage 18 by a coarse movement stage controller 21 for changing the position and orientation of a measurement target and a piezo stage controller 19 used when performing a high-accuracy phase shift method. Further, measurement is performed while controlling the filter controller 24 for selecting a necessary wavelength and the illumination controller 22 for appropriately maintaining the amount of light that changes when the wavelength selection filter 25 is switched.

  Further, the wavelength selection filter 25 may use not only the visible light band but also, for example, light in the infrared or ultraviolet band as measurement light.

  Here, in the above description, in order to perform wavelength selection, an example in which the light source of the illumination device 1 having a broadband wavelength distribution and the wavelength selection filter 25 are used as the illumination light source has been described. However, as shown in FIG. A plurality of LEDs 28 may be prepared as light emitting elements having different wavelengths, and when the wavelength is to be switched, the LED 28 to be used may be switched by causing the LED controller 29 to emit light. The light emission of the LED of the selected wavelength is adjusted to the half mirror 4 by the stage controller 31.

  As explained above, when performing interference measurement through an optical window on which an optical thin film is formed, interference fringes with high contrast can be obtained by using a wavelength with little reflected light from the optical thin film as measurement light. As a result, high-accuracy surface shape measurement with reduced measurement noise is possible, which greatly contributes to the industry.

Claims (13)

An interference measurement device for measuring the surface shape of a measurement object protected by an optical window in which an optical thin film is formed,
An illuminator that outputs light having a broadband wavelength distribution characteristic;
Wavelength filter selection means for inputting light output from the illumination device, switching a plurality of wavelength filters having different center wavelengths, and selectively outputting at least one wavelength light;
The wavelength light selected by the wavelength filter selection unit is branched into reference light and irradiation light directed to the measurement target, and further generates and outputs reflected light from the measurement target by the irradiation light and interference light by the reference light. Have a half mirror,
The wavelength filter selection means selects a combination of wavelength filters that make the contrast of interference fringes in the interference light from the half mirror equal to or greater than a predetermined threshold value;
An interference measuring apparatus configured as described above. In claim 1,
The wavelength filter selection means selectively outputs a plurality of wavelength lights,
An interference measuring apparatus, wherein a combination of wavelength filters that makes an average contrast of interference fringes of interference light in the plurality of wavelength lights equal to or greater than a predetermined threshold value or maximum is selected. In claim 1,
The wavelength filter selection means selects another wavelength filter when the spatial average value of the interference fringe contrast acquired by the currently selected wavelength filter is equal to or less than a predetermined threshold, and the interference fringe contrast is a predetermined threshold. An interference measuring apparatus characterized by selecting a combination of wavelength filters as described above. In claim 1,
Further, a spectroscope is provided, the reflection characteristic from the optical window is evaluated in a state where the reference light is shielded, and the wavelength filter selection means is configured to optimize the wavelength filter based on the reflection characteristic evaluated by the spectroscope. An interference measurement apparatus that performs selection. In claim 4,
Input the evaluation of reflection characteristics in the spectrometer into the control PC,
An interference measuring apparatus characterized in that the wavelength filter selection means selects the optimum wavelength selection filter based on the reflection characteristic evaluation data by the PC. In claim 1,
The plurality of wavelength filters are wavelength cut filters each having a different center wavelength,
An interference measurement apparatus, wherein light from which a wavelength component has been removed by a wavelength cut filter selected by the wavelength filter selection means is input to the her mirror. In claim 1,
The lighting device includes a plurality of light emitting diodes that output light having different center wavelengths,
The wavelength filter selecting means selectively drives at least one light emitting diode of the plurality of light emitting diodes, and selectively outputs a corresponding one wavelength light from the selectively driven emitting diode. Measuring device. An interference measurement method for measuring a surface shape of a measurement object protected by an optical window in which an optical thin film is formed,
Input light having broadband wavelength distribution characteristics to the wavelength filter selection means,
The wavelength filter selecting means selectively outputs at least one wavelength light,
Branching the selected wavelength light into reference light and irradiation light directed to the measurement object;
Furthermore, the reflected light from the measurement object by the irradiation light and the interference light by the reference light are generated,
The wavelength filter selection means selects a combination of wavelength filters that make the contrast of interference fringes in the interference light from the half mirror equal to or greater than a predetermined threshold value;
An interference measurement method characterized by the above. In claim 8,
The wavelength filter selection means selectively outputs a plurality of wavelength lights,
Selecting a combination of wavelength filters in which the average contrast of the interference fringes of the interference light in the plurality of wavelength lights is greater than or equal to a predetermined threshold, or
An interference measurement method characterized by the above. In claim 8,
When the spatial average value of the interference fringe contrast acquired by the wavelength filter currently selected by the wavelength filter selection means is equal to or smaller than a predetermined threshold, another wavelength filter is selected and the interference fringe contrast is a predetermined threshold. An interference measurement method comprising selecting a combination of wavelength filters as described above. A method of measuring a surface shape of an optical component protected by an optical window on which an optical thin film is formed, and a method of manufacturing the optical component, the step of measuring the surface shape comprising:
Input light having broadband wavelength distribution characteristics to the wavelength filter selection means,
The wavelength filter selecting means selectively outputs at least one wavelength light,
Branching the selected wavelength light into reference light and irradiation light directed to the optical component;
Furthermore, the reflected light from the optical component by the irradiation light and the interference light by the reference light are generated,
The wavelength filter selection means selects a combination of wavelength filters that make the contrast of interference fringes in the interference light from the half mirror equal to or greater than a predetermined threshold value;
An optical component manufacturing method characterized by the above. In claim 11,
The wavelength filter selection means selectively outputs a plurality of wavelength lights,
Selecting a combination of wavelength filters in which the average contrast of the interference fringes of the interference light in the plurality of wavelength lights is greater than or equal to a predetermined threshold,
An optical component manufacturing method characterized by the above. In claim 11,
When the spatial average value of the interference fringe contrast acquired by the wavelength filter currently selected by the wavelength filter selection means is equal to or smaller than a predetermined threshold, another wavelength filter is selected and the interference fringe contrast is a predetermined threshold. A method of manufacturing an optical component, wherein a combination of wavelength filters as described above is selected.

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