JP3542346B2 - Method and apparatus for measuring thin film thickness - Google Patents

Description

【００３１】
と表される。すなわち、図３の強度変化のピーク間隔で測定された距離Ｌは、測定しようとしている薄膜の機械的な膜厚の１／２ではなく、薄膜の光学膜厚Ｔｏｐｔの１／２である。
【００３２】
被測定物６の薄膜６９の屈折率ｎが既知である場合には、測定された光学膜厚Ｔｏｐｔから、求める薄膜の機械的な膜厚Ｔは、先に述べた光学長と機械長との関係によって
【００３３】
【数２】

【００３４】
で与えられる。以上によって、屈折率ｎが既知である場合には薄膜６９の膜厚Ｔが求められることになる。
【００３５】
被測定物６の薄膜６９の屈折率ｎが未知である場合には、上で測定された光学膜厚
Ｔｏｐｔと独立に被測定物６の屈折率ｎを測定しなければならない。このように、光学膜厚
Ｔｏｐｔと屈折率ｎを互いに独立に求める方法としては、すでに、先にも述べたように、文献１，文献２，特開平９−２１８０１６などがある。
【００３６】
本発明では、よりシンプルで安定な方法として、以下に述べる可動部分の少ない方法を用いている。
先に述べた方法によって、被測定物６の光学膜６９の厚さＴｏｐｔは既に求まっているものとする。このＴｏｐｔの測定中の腕１１のファイバー１７、光投射光学手段であるレンズ１６、そのレンズを光軸方向に移動する移動ヘリコイド１６０、測定物６、測定物６をその被測定薄膜６９の面内で直交する２軸ＸとＹとの回りに２自由度で姿勢微調整するゴニオメータ式調整装置６９０、の相互位置の配置を図４（ａ）に示す。この状態では、ファイバー１７の端面とレンズ１６の間の距離はレンズの焦点距離ｆに保たれていて、レンズ１６からの出射光は並行光となっており、それによって、図３に示したように、干渉強度変化の上に、２つののピークを得ている。この状態において、ファイバー１７の端面と被測定物６の間の距離ｇを、たとえば、レンズ１６の焦点距離ｆの４倍以上５倍までの距離、すなわち、
【００３７】
【数３】

【００３８】
の範囲の所定の距離ｇに固定する。つぎに、このように一旦距離ｇを固定したまま、レンズ１６だけを被測定物の方向に移動させる。その結果、レンズ１６から出射した光は並行光ではなくなり、被測定物６の薄膜６９の表面６Ａ、裏面６Ｂのいずれからの反射光も発散するため、大部分の光はファイバー１７には戻らない。この結果、干渉ピークは消失する。［式３］は、１枚のレンズ１６で、レンズの移動に対して、被測定薄膜６９の表面６Ａ、裏面６Ｂで結像を得るための条件である。
【００３９】
さらに、レンズ１６を被測定物６の薄膜６９の方向に移動させてゆくと、やがて図４（ｂ）に示すように、集光束は被測定薄膜６９の裏面６Ｂで結像する。裏面６Ｂで結像の様子をさらに詳しく示すために、図４（ｄ）に図４（ｂ）の拡大図を示す。集光光線は、被測定薄膜６９の持つ屈折率のため薄膜６９の表面６Ａで屈折する。光線の入射角と屈折角とは屈折の法則を満足するように屈折する。このときのレンズ１６の最初の状態からの移動距離をａとする。ここで、薄膜の裏面６Ｂへの入射光と反射光とは互いに共焦点の関係となっており、反射光の大きな部分が、ファイバー１７に戻り、干渉計中での干渉強度パターン上に１個のピークが現れる。このとき、表面６Ａでは結像しておらず、共焦点状態を満足していないので、表面からの反射光による干渉ピークは現れない。すなわち、裏面と表面とからの２個のピークは同時には現れず、上述のように１個のピークが現れるのみである。
【００４０】
さらに、レンズ１６を被測定物の方向に移動させてゆくと、やがて、図４（ｃ）に示すように、集光束は被測定薄膜６９の表面６Ａに結像する。このとき、裏面６Ｂでの結像の場合と同様、反射光の大きな部分が、ファイバー１７に戻り、干渉計中での干渉強度パターン上に１個のピークが現れる。ここでのレンズ１６の、図４（ｃ）の状態からの移動距離をｂとする。レンズの焦点距離ｆ、上記の距離、ａ、ｂ、ｃ、ｇおよび、薄膜６９の屈折率ｎなどの間には、レンズの結像関係式と屈折の法則から図４を参照して以下の諸式が成立することがわかる。
【００４１】
図４（ｂ）において、ｃはレンズ１６から被測定薄膜６９への収束入射光束の膜６９中への延長線（破線）の交点までの距離である。レンズの結像公式から、
【００４２】
【数４】

【００４３】
が成立する。また、図から明らかなように、
【００４４】
【数５】

【００４５】
の関係がある。被測定薄膜６９の表面６Ａで起こる屈折について、屈折の法則から、次の［式６］が成立する。
【００４６】
【数６】

【００４７】
［式４］、［式５］、［式６］からｃを消去して次の［式７］が得られる。
【００４８】
【数７】

【００４９】
［式７］の最右辺はすべて既知量である。先の［式２］より
【００５０】
【数８】

【００５１】
である。Ｔｏｐｔは既に測定されている既知量である。 ［式６］、［式７］、［式８］から
【００５２】
【数９】

【００５３】
【数１０】

【００５４】
ｄは［式７］により既知量であるから、［式９］［式１０］により薄膜６９の膜厚Ｔと屈折率ｎがそれぞれ独立に得られたことになる。この測定の間、ファイバー出力端と被測定物との間の距離ｇは一定に保たれている。
なお、図４の（ｃ）では、レンズの位置がさらに図中に示したｂだけ前進しており、被測定薄膜の表面６Ａで結像しており、干渉強度パターンにピークが現れるので、表面６Ａの結像を確認することができる。このｂの値と先にも用いたａ，ｆ，ｇの値の間には［式４］と同様なレンズによる結像関係が成立している。このときのファイバー１７の端面とレンズ１６の間の距離、レンズ１６と被測定薄膜の表面６Ａとの間の距離、はいずれもレンズ１６の焦点距離ｆのほぼ２倍となっている。
【００５５】
上の測定の間、ファイバー出力端と被測定物との間の距離ｇは先にも述べたように一定に保たれていた。このような測定が可能なｇの範囲は、ｇが［式３］を満足しておればよく、厳密に定められた値である必要はない。そこで、調節可能で、かつ、固定可能な枠構造（投射光を遮らない）をファイバー出力端に取り付けて、被測定物にこの枠構造先端部を軽くタッチして測定を行うことにより被測定物の設定がシンプルで安定な測定が可能となる。
【００５６】
次に被測定薄膜が、レンズの焦点ｆにくらべて半径の小さい円筒体や球殻体状の曲面状の薄膜である場合について、第２の実施例を図５を用いて説明する。円筒体状薄膜の例としては、細管チューブそのもの、またはその外面上に形成された薄膜、球殻体状薄膜についての例としては、真珠の成長層などがある。これらの場合には、被測定薄膜が平面状の場合の図４（ａ）、（ｂ）、（ｃ）、（ｄ）が図５（ａ）、（ｂ）、（ｃ）、（ｄ）に置き換わる。図４（ａ）では
Ｔｏｐｔを測定する際のファイバー１７とレンズ１６間の距離をレンズの焦点距離ｆに一致させたが、球体薄膜の場合には図５（ａ）に示すように、その距離を焦点距離ｆの２倍付近の値ｈ、すなわち、
【００５７】
【数１１】

【００５８】
にとり、干渉強度パターン上に２つのピークが生じるよう球状被測定物をセットすればよい。これ以後の測定プロセスは、図の４の（ｂ），（ｃ），（ｄ）と同様な図５の（ｂ），（ｃ），（ｄ）のプロセスとなるので、［式４］〜［式７］中のｆをｈに置き換えればよく、最終的には、、［式８］〜［式９］がそのまま適用でき、球面上薄膜についても、膜厚Ｔと屈折率ｎとをそれぞれ独立に測定することができる。
【００５９】
第３の実施例の配置図を図６に示す。本実施例では、腕１１と腕１２の両方に同一の特性を持つ圧電ファイバー長可変装置１９Ａ、１９Ｂを、それぞれに設置している。圧電ファイバー長可変装置１９Ａ、１９Ｂには、それぞれ励起電源２０Ａ、２０Ｂからの励起電圧が印加される。直流を印加するか、低周波交流電圧を印加するか、あるいは、それらを重畳して印加するか、いずれの場合にしても、それらを独立に印加しても、共通の電圧を印加してもよい。特に共通の電圧を印加する場合には、それらの波形が両圧電素子に互いに逆相として働くように印加すれば、同一電圧で両腕の長さの差を図１の単一のファイバー長可変装置の場合の２倍変化させることができ、測定可能膜厚レンジを拡大することができる。また、同一電圧を印加する場合は、電源を１個として、各圧電素子への印加方向を逆にしても、同様の効果が得られることはいうまでもない。
【００６０】
本発明は、実施例とその各種の側面に関して特に説明してきたが、その種々の変更や変形が本発明の精神と目的から逸脱することなくなされ得ることは当該分野に通常に精通したなんびとによっても理解されるところであろう。
【００６１】
【発明の効果】
以上に説明したように本発明によれば、被測定薄膜の直前の光空間伝搬路以外の部分の干渉計の光伝搬路をすべてファイバーとすることが可能となる。また、低コヒーレンス光源として高周波変調半導体レーザーを用いたことで、光伝搬路をほぼファイバー化したにもかかわらず、光源出力の利用効率を高めることができる。しかも被測定物の膜厚と屈折率を単一の装置の単一の可動部分の操作によって測定できる。また、これらの測定方法と測定装置は、平面状薄膜に限らず、球面状、円筒状その他各種の曲面状薄膜の測定にも適用できる。これらの結果、他物体上に形成された薄膜、あるいは、チューブ状、曲面状の薄膜に適用できる。しかもこの膜厚測定装置は組立容易で、使用の都度専門家による調整など不要で、安定性に優れ、膜厚が安定して測定可能である。したがって産業上の効果は極めて大きい。
【図面の簡単な説明】
【図１】第１の実施例の膜厚測定装置の光学的配置図と光源励起電源回路
【図２】第１の実施例に使われる圧電式ファイバー長可変装置の斜視図とその励起電願部の回路図
【図３】第１の実施例における干渉計の腕の長さに対する干渉強度波形の変化
【図４】第１の実施例の測定原理を説明するための、被測定物直前の空間伝搬部の種々の光学的配置状態の図
【図５】第２の実施例の測定原理を説明するための被測定物直前の空間伝搬部の種々の光学配置状態の図
【図６】第３の実施例の膜厚測定装置の光学配置図
【図７】従来の空間伝搬光路型膜厚計測装置の原理を示す光学的配置図
【図８】従来の空間伝搬形の干渉計において、コヒーレンス光源を用いたときの１個の反射鏡の変位に対する両腕からの干渉強度の変化を示す図
【図９】従来の空間伝搬形の干渉計において、低コヒーレンス光源を用いたときの１個の反射鏡の変位に対する両腕からの干渉強度の変化を示す図
【図１０】従来のファイバー化膜厚測定装置の光学的配置図
【図１１】本発明の実施例に使用する磁気ひずみ装置の２つの例を示す斜視図であり、（ａ）は円筒状の構造を持つもの、（ｂ）はトロイダル状の構造を持つものである。
【符号の説明】
１ 光源、半導体レーザー
２ ハーフミラー
３ 入射光
４ 透過光
５ 反射光
６ 被測定物
６Ａ 被測定薄膜の表面
６Ｂ 被測定薄膜の裏面
７ 反射鏡
８、９ 干渉光
１０ 光検出器
１１、１２ 干渉計の腕
１３ 変位装置
１４、１５ 空間伝搬路
１６、２６ レンズ
１７ ファイバー
１８ 長尺ファイバー
１９、１９Ａ、１９Ｂ 圧電ファイバー長可変装置
２０、２０Ａ、２０Ｂ 圧電素子励起電源
２１、２２、２３、２４ 方向性結合器の端子
２５ 高反射コーティング
６９ 膜
１００ バイアスティー
１０１ 直流電源
１０２ 高周波電源
１９１ 圧電素子
１９３、１９４ 電極
２０１ 直流電源
５００ 電気導体巻線
６００ 磁気ひずみ体[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a device for non-destructively and non-contactly measuring and displaying the thickness of a thin film.
[0002]
[Prior art]
The necessity of non-destructively measuring thin films applied and formed on objects by various methods such as coating, painting, printing, coating, laminating, vapor deposition, depositing, sticking, transferring, forming, growing, etc. It often occurs in various fields of industrial, distribution and general consumption economic activities, such as management, quality control of products, and appraisal, identification, and authentication of jewelry made of thin films such as pearls. In addition, non-destructive film thickness of the thin film formed on the outer surface or inner surface of the cylindrical or tubular body, process control by continuous non-contact measurement, and film just under the surface of biological tissue with a layer structure in dentistry, ophthalmology, dermatology, etc. Thickness-related treatments and diagnoses often arise, such as the need for non-destructive measurement of enamel (or enamel) thickness in human and animal teeth.
[0003]
Independent sheet-like thin films such as films, papers, and plastic sheets can be measured with a micrometer or other mechanical film thickness meter. Such a method of mechanical film thickness measurement cannot be applied to a thin film or the like on the surface of a tubular or tubular body. For a thin film formed on an object, an ellipsometer is used if the thin film is relatively transparent. However, the measurement by the ellipsometer is complicated and time-consuming, and is not suitable for general purposes such as process control, quality control, and discrimination.
[0004]
When the thickness of the thin film is 100 μm or more and the required resolution is 10 μm or more, a simpler low coherence interferometer has been used instead of the ellipsometer. First, FIG. 7 is a diagram showing the principle of a conventional low coherence interferometer using light propagation in space. In FIG. 7, when the relative intensity of the incident light 3 that exits the light source 1 and enters the half mirror 2 is 1, the transmitted light 4 and the upward reflected light 5 are each converted to a relative intensity of about 0.5. Occurs. The transmitted light 4 is reflected on the surface of the DUT 6 and again enters the half mirror 2. On the other hand, the upward reflected light 5 is reflected by the reflecting mirror 7 and again enters the half mirror 2. The respective reflected lights re-entering the half mirror 2 interfere on the half mirror 2 to generate two interference lights 8 and 9. One interference light 8 goes to the photodetector 10 and the other interference light 9 goes to the light source 1. The above constitutes a so-called Michelson interferometer.
[0005]
The optical path from the half mirror 2 to the device under test 6 is called one arm 11 of the interferometer, and the optical path from the half mirror 2 to the reflecting mirror 7 is called the other arm 12 of the interferometer. When the coherence of the light source is good, when the length of one arm is fixed and the length of the other arm is slowly changed, interference of two reflected lights re-entering the half mirror 2 causes As shown in FIG. 8, the intensities of the interference light beams 8 and 9 change sinusoidally. And when one interference light becomes strong, the other interference light becomes weak. The output current from the photodetector 10 changes sinusoidally when the length of one arm is changed according to the change in the intensity of the interference light 8. FIG. 7 shows a case where the optical path length of the arm 12 is changed. The displacement device 13 for changing the optical path length of the arm 12 has a reflecting mirror 7 attached to the tip thereof.
[0006]
The principle of the conventional thin film thickness meter using the low coherence interferometer will be described below. As the light source 1 of the interferometer for measuring the film thickness, a light source having a low coherence, that is, a light source having a short coherence length is selected. As a light source having a short coherence length, there is a so-called super luminescent diode (hereinafter, referred to as SLD) or a halogen lamp. If the coherence length is short, the interference light that changes sinusoidally with respect to the change in the length of one arm of the interferometer, and thus the sinusoidal output current from the detector 10, as shown in FIG. Takes the maximum value at the center point M of the envelope on the horizontal axis when the lengths of both arms (FIG. 7) are equal, and the sine wave-like change in a very narrow range (current (light input)) before and after that. (About ten to several tens of cycles). Therefore, the position of the reflecting mirror 7 that gives the maximum value of the envelope of the sinusoidal change in the output current of the photodetector can be used for position measurement by interference. The smaller the half width of the envelope, the smaller the resolvable distance can be.
[0007]
In the measurement of the film thickness of the thin film by the interferometer, two reflection points exist because the refractive index of the front surface and the back surface of the thin film change stepwise. If the resolvable distance of each of these two reflection points is smaller than the film thickness itself, the above-described position measurement can be applied twice, and the difference between the positions is used as the optical film thickness of the thin film. Can be. The optical film thickness is obtained by multiplying the mechanical film thickness to be measured by the refractive index of the thin film. Therefore, when the refractive index of the thin film is known, the actual mechanical film thickness of the film to be measured can be obtained by dividing the optical film thickness by the refractive index. However, if the refractive index of the thin film is not known, it is necessary to measure the refractive index of the thin film independently. As described above, a method for independently determining the optical film thickness and the refractive index is described in, for example, G. Optics Letters, Vol. 20, No. 21, page 2258 (November 1995). J. Tearney et al., "Determination of the refractory Index of high scattering human tissue by optical doherence tomography" (published in Vol. 19, Vol. 19, pp. 1923, October 23, October 23, 1923). T. Fukano et al., "Simultaneous Measurement of Thicknesses and Refractive Indices of Multiple Layers" (hereinafter referred to as Document 2), and the inventor disclosed in Japanese Patent Application Laid-Open No. 2001-141652. Method for Simultaneous Measurement of Refractive Index and Thickness of Measurement Object and Apparatus Therefor ".
[0008]
The interferometer-type film thickness measuring apparatus that relies on light propagation in a space in the above-described prior example as described above requires a high degree of skill and time to arrange, set, and adjust optical components. There is also a problem with the long-term stability after completion, and there has been a problem that, even after the production of the device, it must be constantly adjusted by a skilled person each time the device is used. This is because, in an interferometer that relies on light propagation in space, the arrangement, setting, and adjustment of one optical component such as a mirror or a beam splitter correlates with the arrangement, setting, or adjustment of another optical component. This is because, in order to complete the arrangement and adjustment of the entire apparatus, the adjustment time required for adjustment of each component is approximately N (N-1) times as long as the number of components is substantially N. Further, as for the stability, the stability of each component is required to be √N times the number N of parts.
[0009]
If the arrangement, setting, and adjustment of one optical component are not related to the arrangement, setting, and adjustment of the other optical components, the time required to complete the arrangement and adjustment of the entire apparatus is approximately The adjustment time required for the adjustment of the parts is almost the same as the number of parts, and the stability of the entire apparatus is the same as the stability of the individual parts.
As described above, in order to separate and independence of the arrangement, setting, adjustment, and stability of each optical component, the optical propagation between the optical components depends on the spatial propagation and the optical fiber is used. What is necessary is just to make it independent from light propagation.
[0010]
FIG. 10 shows a conventional interferometer film thickness measuring apparatus in which the film thickness measuring apparatus using the interferometer by the light propagating in the space shown in FIG. What is indicated by a thick solid line is a fiber. The components in FIG. 10 that perform the same functions as the components in FIG. 7 are denoted by the same reference numerals as the corresponding components in FIG. The half mirror 2 in FIG. 7 is replaced by a fiber directional coupler 2 in FIG. The function of the fiber directional coupler 2 is the same as that of the half mirror 2 in FIG. 7, and the light entering from the terminal 21 of the directional coupler goes out from the terminals 22 and 24 by an equal amount, but the light from the terminal 23 Do not go out. Light entering from terminal 22 exits an equal amount from ends 21 and 23 but does not exit from terminal 24. Light entering from terminal 24 exits equal amounts at ends 21 and 23 but not from terminal 22. The lenses 16 and 26 serving as projection optical elements are for converting divergent light emitted rightward from the fiber end of each arm into parallel light. The distance between the fiber end and the lenses 16 and 26 is set to match the focal length of each lens so that parallel rays are incident. As shown by 14 and 15 in FIG. 10, each arm has a spatial propagation path.
[0011]
In the conventional fiberized interferometer film thickness measuring device shown in FIG. 10, the arrangement, setting, and adjustment of each optical component are also independent in assembling the system of the interferometer film thickness measuring device using many optical components. You. In addition, the assembly is performed simply by connecting the components with fiber connectors. Therefore, no skilled person is required, and the assembling work is extremely shortened. Further, the stability of the device is remarkably improved for the above-mentioned reason. However, in this conventional apparatus, the spatial propagation path 14 immediately before the object 6 and the spatial propagation path 15 immediately before the reflecting mirror 7 for changing the length of the arm 12 of the interferometer cannot be removed. Reference numeral 13 denotes a reflector moving mechanism provided with the reflector 7 for changing the propagation length of the space propagation path 15 immediately before the reflector 7.
[0012]
[Problems to be solved by the invention]
In the conventional fiberized interferometer film thickness measuring apparatus of FIG. 10, in order to project the measuring light to the object to be measured in a non-contact manner and to make the distance between the arms of the interferometer variable, the space just before the object to be measured 6 is required. A propagation path 14 and a space propagation path 15 immediately before the reflector 7 for changing the length of the arm 12 of the interferometer were required. Therefore, the drawbacks of the prior art due to the existence of the spatial propagation path, namely, the complexity of the assembly and adjustment work of the device, and the inclusion of the spatial propagation path interferometer which is sensitive to disturbance from the surrounding environment. The resulting device instability has not been resolved.
[0013]
In addition, as a light source, a low-coherence SLD, a halogen lamp, or the like has been conventionally used in order to reduce a resolvable distance. However, since these light sources have low spatial coherence, when they are formed into fibers, the coupling efficiency from the light sources to the fibers becomes extremely poor, and the power level of light that can be used for interferometric measurement is extremely lowered. Therefore, the signal-to-noise ratio of the sinusoidal detection current output obtained as a result of the interference is reduced, and the film thickness resolution to be obtained is lowered. In particular, when the thin film to be measured is not a transparent body but a medium having scattering or absorptivity, a low power level of the measuring light is fatal, and in that case, the measurement becomes impossible.
[0014]
As described above, in a fiberized interferometer-type film thickness measuring device, the coupling efficiency from the fiber to the fiber from a low coherence SLD or halogen lamp light source required for high-resolution interferometric film thickness measurement is increased. However, there is a problem that the film thickness resolution ability is extremely lowered due to the formation of the film, and the film thickness resolving ability resulting therefrom is lowered.
[0015]
[Means for Solving the Problems]
In the fiberized interferometer film thickness measuring apparatus of the present invention, first, in order to facilitate the assembling and adjusting work of the interferometer included in the apparatus and to improve the stability of the apparatus, a part of the fiber constituting the interferometer is formed. By making the fiber length itself variable, the spatial propagation path necessary for making the length of the arm of the interferometer variable was eliminated. As a result, the fiber interferometer film thickness measuring apparatus of the present invention thus configured is shown in FIG. In FIG. 1, a spatial propagation path other than an extremely short spatial propagation path 14 immediately before the DUT 6 necessary for projecting the measurement light onto the DUT (for example, a spatial propagation path 15 immediately before the reflecting mirror 7 in FIG. 10). ) Are all fiberized.
[0016]
As a method for changing the physical length of the fiber itself described above, a long fiber wound around a piezoelectric element or a magnetostrictive element is inserted into at least one arm of an interference type, and a variable voltage ( A piezoelectric element) or a variable current (magnetostrictive element) is applied. As a result, all parts other than the space propagation path 14 immediately before the object to be measured are formed into fibers. It facilitates the assembly of the film thickness measurement interferometer and stabilizes the operation.
[0017]
The thin film thickness measuring apparatus of the present invention comprises a semiconductor laser as a light source modulated at a predetermined high frequency on the order of gigahertz, and a measurement optical path arm and a non-measurement optical path arm for guiding low coherence light emitted by the semiconductor laser. An interferometer having an arm of the above, wherein the non-measuring optical path arm is an optical path substantially consisting only of an optical fiber (hereinafter simply referred to as a fiber) and not including a spatial propagation optical path, and the measuring optical path arm is Except for a space-propagating optical path of a predetermined length from the end face of the optical fiber to project the low coherence light emitted by the semiconductor laser to the surface of the thin film to be measured from the object to be measured, the optical fiber comprises only the optical fiber. Features.
The method for measuring the thickness of a thin film according to the present invention includes the steps of: generating a low coherence light by modulating a semiconductor laser as a light source at a predetermined high frequency on the order of gigahertz; Guiding the two arms of the light path arm,
The non-measurement optical path arm is an optical path substantially consisting only of an optical fiber (hereinafter simply referred to as a fiber) and not including a spatially propagating optical path, and the measurement optical path arm transmits the low coherence light emitted by the semiconductor laser to an object to be measured. Except for a predetermined length of the spatially propagating optical path from the end face of the optical fiber to the surface of the thin film to be projected for projecting on the surface of the thin film to be measured, it consists of only the optical fiber, and the thickness of the thin film to be measured is measured by the interferometer. It is characterized by the following.
Further, in the fiber interferometer film thickness measuring apparatus of the present invention, a pulse oscillation semiconductor laser is used, and the excitation current of the semiconductor laser is subjected to high frequency modulation. While utilizing the better spatial coherency, the reduction in temporal coherency (necessary to increase the film thickness resolution) that can be achieved by modulation is also realized. As a result, even in the case of fiber conversion, the light source High efficiency In addition to maintaining good coupling and realizing low temporal coherency, sensitivity and resolution with respect to film thickness are simultaneously increased.
[0018]
In addition, a long fiber wound around a piezoelectric element or a magnetostrictive element to make the length of the arm of the interferometer variable is provided on both arms of the interferometer, and each is in opposite phase. excitation By doing so, the range of measurable film thickness is expanded.
[0019]
In addition, a new method is proposed in which a single device can simultaneously measure the optical film thickness and refractive index required for measuring the film thickness with unknown refractive index by moving only the lens to irradiate the object to be measured. Have been.
This makes it possible to integrate the measurement head portion including the spatial propagation path immediately before the object to be measured and to stably measure the film thickness more easily by holding the tip portion in contact with the object to be measured.
[0020]
Combined with the improvement of the above components, easy assembly, improved sensitivity and film thickness resolution, expanded measurable film thickness range, stable operation, and a fiber that enables simultaneous and independent measurement of film thickness and refractive index An interferometer film thickness measuring device can be realized.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described.
[0022]
FIG. 1 shows a first embodiment of the present invention. Corresponding elements having the same functions as those of the conventional example shown in FIG. 6 or FIG. 10 are given the same reference numerals as those in FIG. 6 or FIG. A duplicate description of them will be omitted here.
[0023]
The light source 1 uses a semiconductor laser. As the semiconductor laser 1, a type of semiconductor laser which can be modulated at high speed, has a small junction capacitance and other floating capacitance, has a short device lead wire, and has a small inductive reactance, which is used for an optical LAN or the like, is used. Through such a semiconductor laser, a DC bias from a DC power supply 101 and a sinusoidal high-frequency voltage from a high-frequency power supply 102 of about 0.8 GHz to 3 GHz pass through a bias tee 100 represented by an equivalent circuit shown in the drawing. Applied. In general, such a semiconductor laser capable of high-frequency modulation repeats at the frequency of the applied high-frequency voltage, even if the degree of modulation is about 10 to 20%, due to the nonlinearity of the electron distribution present at the junction. It outputs pulsed light with a pulse width of several tens of picoseconds. Due to this pulsed laser oscillation, the density of the electron distribution at the junction described above rapidly changes in a short time within the pulse width. Therefore, the refractive index of the junction also changes rapidly, and the oscillating light undergoes frequency modulation with a large modulation factor. Generally, it is known that the spectrum of frequency modulation spreads over a wide frequency band. As a result, with DC excitation, a laser that oscillated in a substantially single chip longitudinal mode oscillates in many longitudinal modes, and a broad spectrum can be obtained. At the above modulation frequency of about 0.8 GHz or less, the light intensity changes showing a change almost faithful to the modulation waveform, and does not become a pulsed light output. At a modulation frequency of about 3 GHz or more, the optical output cannot follow the modulation waveform, becomes a constant output, and does not become a pulsed optical output. On the other hand, unlike a halogen lamp or an SLD, laser oscillation has a high spatial coherence which is equivalent to that of a normal semiconductor laser. Therefore, a modulated semiconductor laser can provide a light source that has both low coherence and high spatial coherence, which cannot be obtained with a halogen lamp or SLD, and is most suitable for measuring an interference film thickness.
[0024]
The arm that changes the length of the fiber among the two arms of the interferometer is the non-measuring optical path arm 12 in this embodiment, and the fiber 18, which is the main element constituting the arm 12, is extended by several tens of meters. There are things. This was wound around a cylinder 191 of, for example, a cylindrical piezoelectric element (poled by a known method) such as lead zirconate titanate (so-called PZT) while applying a constant tension and preferably a constant reverse bias voltage. A piezoelectric fiber length variable device 19. An excitation voltage from the piezoelectric element excitation power supply 20 is applied to the cylinder 191 of the piezoelectric element. The excitation voltage may be a constant voltage (DC) or a voltage that changes with a desired waveform at a desired frequency (AC). FIG. 2 shows an example of the configuration of the piezoelectric fiber length varying device 19. In FIG. 2, the long fiber 18 is wound around the cylindrical piezoelectric element 191 many times, and the entire surface of the cylindrical inner wall and the outer wall shown in FIG. A voltage from the DC power supply 201, a voltage from the low-frequency AC power supply 202, or a voltage through the filter circuit (L + C) shown in FIG. Thereby, the diameter of the cylinder 191 is changed constantly or periodically, and tension is applied to the fiber to change the length of the fiber constantly or periodically. In the structure of FIG. 2, a fiber having a normal cladding diameter of 125 mm was used, and when the thickness of the cylindrical piezoelectric element was 5 mm, an elongation of 1 mm per 10 m of the fiber was obtained at an applied voltage of 100 V.
[0025]
As described above with reference to FIG. 9, in the low coherence interferometer according to the present invention, an interference waveform appears only in the vicinity of a state where both arms of the interferometer have the same length. Therefore, in this embodiment in which the non-measuring optical path arm 12 includes a long fiber, the length of the fiber constituting the majority of the measuring optical path arm 11 in order to balance the length with the long fiber of the arm 12 is determined by the non-measuring optical path. The length must be a length obtained by subtracting the space propagation path length 14 (converted to the light propagation path in the fiber) immediately before the measured object 6 from the long fiber length of the arm 12. Therefore, the fiber 17 of the measuring optical path arm 11 in FIG. 1 is made to include the long fiber 199 wound around the air core.
[0026]
Since the length of one arm 12 of the interferometer can be changed by the piezoelectric fiber length changing device 19 described above, the length of the arm 12 of the interferometer is changed in the prior art fiber interferometer shown in FIG. The space propagation path 15 in front of the reflecting mirror 7 provided for this purpose is no longer necessary. Therefore, in the present invention, the high-reflection coating 25 is directly applied to the end face of the end of the fiber constituting the arm 12, and this is used as a reflecting mirror of the arm 12 of the interferometer. Thereby, the spatial propagation path 15 as shown in FIG. 10 can be eliminated, and the easiness of assembling the device and the interference (there is no need to adjust the length of the spatial propagation path 15 and the perpendicularity between the mirror and the light beam). It is possible to contribute to the improvement of the stability of the meter (the calibration of the deviation of the length and the angle of the mirror and the direction of the movement is unnecessary).
[0027]
The above-described piezoelectric fiber length variable device may be replaced with a magnetostrictive fiber length variable device using a magnetostrictive body such as an alphero alloy or a rare earth intermetallic compound (so-called terphenol). An example is shown in FIG. FIG. 11 shows an electric conductor winding 500 shown by a thin line and a fiber 18 shown by a thick line in a diagram for applying a magnetic field to a magnetostrictive body 600 formed in a rod shape (a) or a toroidal shape (b). It is wound. The method of winding the fiber while applying tension at the time of winding the fiber, the method of applying a DC bias having a polarity at which the magnetostrictive body 600 becomes thinner at the time, and the method of applying the DC bias at the time of operation are the same as in the case of a piezoelectric fiber length variable device. Is the same as
[0028]
Next, the measurement operation by the thin film thickness measuring apparatus using interference according to the first embodiment of the present invention shown in FIG. 1 will be described.
The length of one arm 12 of the interferometer is changed by sweeping the voltage from the piezoelectric element excitation power supply 20 applied to the piezoelectric fiber length varying device 19. When the long fiber 18 wound around the piezoelectric element 191 of the piezoelectric fiber length varying device 19 is sufficiently long and the voltage applied from the power source 20 is sufficiently large, the change in the optical length of the arm 12 is made smaller than the measured optical film thickness. Can also be large. Therefore, the change in the length of the arm 12 can be made larger than the optical thickness of the film 69 of the DUT 6. At the end of the light beam of the arm 11, there are two surfaces 6 A and 6 B of the film 69 as the reflection surface of the DUT 6. The waveform changes at two places as shown in FIG. The DUT 6 has a goniometer-type two-degree-of-freedom angle fine adjustment for adjusting the front surface 6A and the back surface 6B of the thin film 69 to be measured almost vertically to the optical axis of light projected from the lens 16 of the arm 11. It is held by the device 690.
[0029]
The horizontal axis in FIG. 3 represents the mechanical length of the arm 12, that is, the actual length in a normal sense, by replacing the DUT 6 with a reflecting mirror in advance and moving it by a predetermined distance. The horizontal axis in FIG. 3 can be represented as the mechanical length of the arm 12 because it can be graduated as a change in height. The distance between two interference peaks on the scale on the horizontal axis is represented by L.
Since the above two interference peaks occur based on the reciprocation of light in the thin film 69, the interval L (measured by the mechanical distance) between these two interference peaks is the optical length of the thin film 69, that is, , The optical thickness of the thin film 69 (the value obtained by multiplying the mechanical thickness of the thin film by the refractive index of the thin film) Topt. Therefore,
[0030]
(Equation 1)

[0031]
It is expressed as That is, the distance L measured at the peak interval of the intensity change in FIG. 3 is not 1/2 of the mechanical thickness of the thin film to be measured, but 1/2 of the optical thickness Topt of the thin film.
[0032]
When the refractive index n of the thin film 69 of the device under test 6 is known, the mechanical thickness T of the thin film to be determined from the measured optical thickness Topt is the difference between the optical length and the mechanical length described above. By relationship
[0033]
(Equation 2)

[0034]
Given by As described above, when the refractive index n is known, the thickness T of the thin film 69 is obtained.
[0035]
If the refractive index n of the thin film 69 of the device under test 6 is unknown, the optical thickness measured above
The refractive index n of the device under test 6 must be measured independently of Topt. Thus, the optical film thickness
As methods for independently determining Topt and the refractive index n, as described above, there are Literature 1, Literature 2, and JP-A-9-218016.
[0036]
In the present invention, as a simpler and more stable method, the following method with a small number of movable parts is used.
It is assumed that the thickness Topt of the optical film 69 of the device under test 6 has already been determined by the method described above. The fiber 17 of the arm 11 during the measurement of the Topt, the lens 16 which is a light projection optical unit, the moving helicoid 160 which moves the lens in the optical axis direction, the measured object 6, and the measured object 6 are positioned in the plane of the thin film 69 to be measured. FIG. 4A shows an arrangement of mutual positions of a goniometer-type adjusting device 690 for finely adjusting the attitude with two degrees of freedom around two axes X and Y orthogonal to each other. In this state, the distance between the end face of the fiber 17 and the lens 16 is kept at the focal length f of the lens, and the light emitted from the lens 16 is parallel light, and as a result, as shown in FIG. In addition, two peaks are obtained on the change in interference intensity. In this state, the distance g between the end face of the fiber 17 and the DUT 6 is set to, for example, a distance from 4 times to 5 times the focal length f of the lens 16, that is,
[0037]
[Equation 3]

[0038]
Is fixed at a predetermined distance g in the range of. Next, while the distance g is fixed once, only the lens 16 is moved in the direction of the object to be measured. As a result, the light emitted from the lens 16 is no longer parallel light, and the reflected light from both the front surface 6A and the back surface 6B of the thin film 69 of the device under test 6 diverges, so that most of the light does not return to the fiber 17. . As a result, the interference peak disappears. [Equation 3] is a condition for obtaining an image on the front surface 6A and the back surface 6B of the thin film 69 to be measured with one lens 16 with respect to the movement of the lens.
[0039]
Further, when the lens 16 is moved in the direction of the thin film 69 of the device 6 to be measured, as shown in FIG. 4B, the condensed light beam forms an image on the back surface 6B of the thin film 69 to be measured. FIG. 4D is an enlarged view of FIG. 4B to show the state of image formation on the back surface 6B in more detail. The condensed light beam is refracted on the surface 6A of the thin film 69 due to the refractive index of the thin film 69 to be measured. The incident angle and the refraction angle of the light beam are refracted so as to satisfy the law of refraction. The movement distance of the lens 16 from the initial state at this time is defined as a. Here, the incident light and the reflected light on the back surface 6B of the thin film have a confocal relationship with each other, and a large portion of the reflected light returns to the fiber 17 and is reflected on the interference intensity pattern in the interferometer by one. Appears. At this time, since no image is formed on the surface 6A and the confocal state is not satisfied, no interference peak due to light reflected from the surface appears. That is, two peaks from the back surface and the front surface do not appear at the same time, but only one peak appears as described above.
[0040]
Further, when the lens 16 is moved in the direction of the object to be measured, the condensed light beam forms an image on the surface 6A of the thin film 69 to be measured as shown in FIG. At this time, as in the case of image formation on the back surface 6B, a large portion of the reflected light returns to the fiber 17, and one peak appears on the interference intensity pattern in the interferometer. Here, the moving distance of the lens 16 from the state of FIG. Between the focal length f of the lens, the above-mentioned distances, a, b, c, g, the refractive index n of the thin film 69, and the like, the following expressions are used with reference to FIG. It can be seen that the equations hold.
[0041]
In FIG. 4B, c is the distance from the lens 16 to the intersection of the extended line (broken line) into the film 69 of the convergent incident light beam to the thin film 69 to be measured. From the lens imaging formula,
[0042]
(Equation 4)

[0043]
Holds. Also, as is clear from the figure,
[0044]
(Equation 5)

[0045]
There is a relationship. With respect to refraction occurring on the surface 6A of the thin film 69 to be measured, the following [Equation 6] is established from the law of refraction.
[0046]
(Equation 6)

[0047]
By eliminating c from [Equation 4], [Equation 5], and [Equation 6], the following [Equation 7] is obtained.
[0048]
(Equation 7)

[0049]
The rightmost sides of [Equation 7] are all known quantities. From [Equation 2] above
[0050]
(Equation 8)

[0051]
It is. Topt is a known quantity that has already been measured. From [Equation 6], [Equation 7], and [Equation 8]
[0052]
(Equation 9)

[0053]
(Equation 10)

[0054]
Since d is a known amount according to [Equation 7], the film thickness T and the refractive index n of the thin film 69 are independently obtained from [Equation 9] and [Equation 10]. During this measurement, the distance g between the fiber output end and the device under test is kept constant.
In FIG. 4C, the position of the lens is further advanced by b shown in the figure, an image is formed on the surface 6A of the thin film to be measured, and a peak appears in the interference intensity pattern. 6A can be confirmed. An imaging relationship by a lens similar to [Equation 4] is established between the value of b and the values of a, f, and g used earlier. At this time, the distance between the end face of the fiber 17 and the lens 16 and the distance between the lens 16 and the surface 6A of the thin film to be measured are almost twice the focal length f of the lens 16.
[0055]
During the above measurement, the distance g between the fiber output end and the device under test was kept constant as described above. The range of g in which such measurement can be performed is only required to satisfy g [Equation 3], and does not need to be a strictly determined value. Therefore, an adjustable and fixable frame structure (which does not block the projected light) is attached to the fiber output end, Suffered By lightly touching the front end of the frame structure to the object to be measured, the setting of the object to be measured is simple and stable measurement is possible.
[0056]
Next, a second embodiment will be described with reference to FIG. 5 in a case where the thin film to be measured is a cylindrical or spherical shell-shaped curved thin film having a smaller radius than the focal point f of the lens. Examples of the cylindrical thin film include a thin tube formed on the tube itself, or a thin film formed on the outer surface thereof, and examples of the spherical shell thin film include a pearl growth layer. In these cases, FIGS. 4 (a), (b), (c) and (d) when the thin film to be measured is planar are shown in FIGS. 5 (a), (b), (c) and (d). Is replaced by In FIG. 4A,
The distance between the fiber 17 and the lens 16 when measuring Topt was set to be equal to the focal length f of the lens. In the case of a spherical thin film, as shown in FIG. A value h near double, ie,
[0057]
(Equation 11)

[0058]
In addition, the spherical object may be set so that two peaks are generated on the interference intensity pattern. The subsequent measurement process is the same as the processes of (b), (c), and (d) in FIG. 5 similar to (b), (c), and (d) of FIG. It is sufficient to replace f in [Equation 7] with h. Finally, [Equations 8] to [Equation 9] can be applied as they are, and the film thickness T and the refractive index n are each set to the spherical thin film. It can be measured independently.
[0059]
FIG. 6 shows a layout of the third embodiment. In this embodiment, the piezoelectric fiber length variable devices 19A and 19B having the same characteristics are provided on both the arms 11 and 12 respectively. Excitation voltages from excitation power supplies 20A and 20B are applied to the piezoelectric fiber length variable devices 19A and 19B, respectively. Regardless of whether DC is applied, low-frequency AC voltage is applied, or they are applied in a superimposed manner, whether they are applied independently or a common voltage is applied Good. In particular, when a common voltage is applied, if the waveforms are applied to both piezoelectric elements so as to act in opposite phases, the difference between the lengths of the two arms at the same voltage can be changed by a single fiber length variable in FIG. It can be changed twice as much as the case of the apparatus, and the range of the film thickness that can be measured can be expanded. Further, when the same voltage is applied, it is needless to say that the same effect can be obtained even if one power source is used and the application direction to each piezoelectric element is reversed.
[0060]
Although the present invention has been particularly described with respect to embodiments and various aspects thereof, it will be appreciated by those of ordinary skill in the art that various changes and modifications can be made without departing from the spirit and purpose of the invention. It will be understood.
[0061]
【The invention's effect】
As described above, according to the present invention, it is possible to make all the optical propagation paths of the interferometer other than the optical space propagation path immediately before the thin film to be measured be fibers. In addition, by using a high-frequency modulated semiconductor laser as the low coherence light source, it is possible to increase the utilization efficiency of the light source output despite the fact that the optical propagation path is substantially fiberized. In addition, the thickness and the refractive index of the object to be measured can be measured by operating a single movable portion of a single device. In addition, these measuring methods and measuring devices can be applied not only to the measurement of a flat thin film but also to the measurement of a spherical thin film, a cylindrical thin film and various other curved thin films. As a result, the present invention can be applied to a thin film formed on another object or a thin film having a tube shape or a curved surface. Moreover, this film thickness measuring device is easy to assemble, does not require adjustment by an expert every time it is used, has excellent stability, and can measure the film thickness stably. Therefore, the industrial effect is extremely large.
[Brief description of the drawings]
FIG. 1 is an optical arrangement diagram of a film thickness measuring apparatus according to a first embodiment and a light source excitation power supply circuit.
FIG. 2 is a perspective view of a piezoelectric fiber length varying device used in the first embodiment and a circuit diagram of an excitation electrode portion thereof.
FIG. 3 shows a change of an interference intensity waveform with respect to an arm length of the interferometer in the first embodiment.
FIG. 4 is a diagram illustrating various optical arrangement states of a space propagation unit immediately before an object to be measured, for explaining a measurement principle of the first embodiment.
FIG. 5 is a view showing various optical arrangement states of a space propagation unit immediately before an object to be measured, for explaining a measurement principle of a second embodiment.
FIG. 6 is an optical arrangement diagram of a film thickness measuring apparatus according to a third embodiment.
FIG. 7 is an optical layout diagram showing the principle of a conventional space propagation optical path type film thickness measuring apparatus.
FIG. 8 is a diagram showing a change in interference intensity from both arms with respect to a displacement of one reflector when a coherence light source is used in a conventional space propagation interferometer.
FIG. 9 is a diagram showing a change in interference intensity from both arms with respect to displacement of one reflector when a low coherence light source is used in a conventional space-propagation interferometer.
FIG. 10 is an optical layout diagram of a conventional fiberized film thickness measuring apparatus.
11A and 11B are perspective views showing two examples of a magnetostrictive device used in an embodiment of the present invention, in which FIG. 11A has a cylindrical structure, and FIG. 11B has a toroidal structure. is there.
[Explanation of symbols]
1 light source, semiconductor laser
2 Half mirror
3 Incident light
4 transmitted light
5 Reflected light
6 DUT
6A Surface of thin film to be measured
6B Back side of thin film to be measured
7 Reflector
8, 9 interference light
10 Photodetector
11,12 Interferometer arm
13 Displacement device
14, 15 Spatial propagation path
16, 26 lenses
17 Fiber
18 long fiber
19, 19A, 19B Piezoelectric fiber length variable device
20, 20A, 20B piezoelectric element excitation power supply
21, 22, 23, 24 Terminals of directional coupler
25 High reflection coating
69 membrane
100 bias tee
101 DC power supply
102 High frequency power supply
191 Piezoelectric element
193, 194 electrode
201 DC power supply
500 Electric conductor winding
600 magnetostrictive body

Claims (22)

A semiconductor laser as a light source modulated at a predetermined high frequency on the order of gigahertz;
An interferometer having two arms, a measurement light path arm and a non-measurement light path arm, for guiding low coherence light emitted by the semiconductor laser;
Has,
The non-measurement optical path arm is an optical path substantially consisting only of an optical fiber (hereinafter simply referred to as a fiber) and not including a space propagation optical path,
The measurement optical path arm excludes a predetermined length of spatially propagating optical path from the end face of the optical fiber for projecting the low coherence light emitted by the semiconductor laser to the surface of the thin film to be measured from the object to be measured. Consisting only of optical fiber,
An apparatus for measuring the thickness of a thin film. At least the measurement optical path arm of the interferometer includes a piezoelectric fiber length variable device in which a fiber is wound around a piezoelectric element,
The end face of the optical fiber of the non-measuring optical path arm is a reflecting mirror coated with high reflection,
The apparatus for measuring the thickness of a thin film according to claim 1, wherein: Both arms of the interferometer include a piezoelectric fiber length variable device in which a fiber is wound around a piezoelectric element,
The end face of the optical fiber of the non-measuring optical path arm is a reflecting mirror coated with high reflection,
The apparatus for measuring the thickness of a thin film according to claim 1, wherein: At least the pre-measurement optical path arm of the interferometer includes a magnetostrictive fiber length variable device in which a fiber is wound around a magnetostrictive body,
The end face of the optical fiber of the non-measurement channel arm is a reflecting mirror coated with high reflection,
The apparatus for measuring the thickness of a thin film according to claim 1, wherein: Both arms of the interferometer include a magnetostrictive fiber length variable device in which a fiber is wound around a magnetostrictive body,
The end face of the optical fiber of the non-measuring optical path arm is a reflecting mirror coated with high reflection,
The apparatus for measuring the thickness of a thin film according to claim 1, wherein: The magnetostrictive body is substantially cylindrical, and a fiber is wound around the cylindrical surface,
The apparatus for measuring the thickness of a thin film according to claim 4 or 5, wherein: The magnetostrictive body is substantially toroidal, and a fiber is wound around the toroidal surface;
The apparatus for measuring the thickness of a thin film according to claim 4 or 5, wherein: Fixing means for fixing the relative spatial positional relationship between the end face of the fiber of the measurement optical path arm and the surface of the thin film to be measured,
Moving means for moving the projection optical means along the spatially propagating optical path placed on a spatially propagating optical path of a predetermined length from the end face of the fiber to the surface of the thin film to be measured;
The thin film thickness measuring device according to claim 1, further comprising: The thin film to be measured is any plane or curved surface,
The apparatus further comprises a holder for finely adjusting and holding the posture of the DUT so that the normal of the plane or the curved surface substantially matches the optical axis of the space propagation optical path,
9. The apparatus for measuring the thickness of a thin film according to claim 1, wherein: 9. The thin film thickness measuring apparatus according to claim 8, wherein the curved surface of the thin film to be measured has a cylindrical shape, a spherical shell, or an irregular shape. The apparatus according to any one of claims 1 to 10, wherein the predetermined high frequency is in a range of 0.8 to 3 GHz. Modulating a semiconductor laser as a light source with a predetermined high frequency on the order of gigahertz to generate low coherence light; and guiding the low coherence light to two arms of a measurement optical path arm and a non-measurement optical path arm of the interferometer. ,
Has,
The non-measurement optical path arm is an optical path substantially consisting only of an optical fiber (hereinafter simply referred to as a fiber) and not including a space propagation optical path,
The measurement optical path arm excludes a predetermined length of spatially propagating optical path from the end face of the optical fiber for projecting the low coherence light emitted by the semiconductor laser to the surface of the thin film to be measured from the object to be measured. Consists of only optical fiber,
Measuring the thickness of the thin film to be measured by the interferometer,
A method for measuring the thickness of a thin film, comprising: At least the measurement optical path arm of the interferometer includes a piezoelectric-type fiber length variable device in which a fiber is wound around a piezoelectric element, and the fiber length is significantly changed by the piezoelectric action.
The end face of the optical fiber of the non-measuring optical path arm is always a stable non-measuring optical path arm length by being a reflecting mirror coated with high reflection,
The method for measuring the thickness of a thin film according to claim 12, wherein: Both arms of the interferometer include a piezoelectric fiber length variable device in which a fiber is wound around a piezoelectric element,
The end face of the optical fiber of the non-measuring optical path arm is a reflecting mirror coated with high reflection,
By significantly changing the fiber length of both arms by piezoelectric action, the thickness of the thin film to be measured is measured by the interferometer,
The method for measuring the thickness of a thin film according to claim 12, wherein: At least the measurement optical path arm of the interferometer includes a magnetostrictive fiber length variable device in which a fiber is wound around a magnetostrictive body, and the fiber length is significantly changed by the magnetostrictive action,
The end face of the optical fiber of the non-measuring optical path arm is always a stable non-measuring optical path arm length by being a reflecting mirror coated with high reflection,
The method for measuring the thickness of a thin film according to claim 12, wherein: Both arms of the interferometer include a magnetostrictive fiber length variable device in which a fiber is wound around a magnetostrictive body, and the fiber length is significantly changed by the magnetostrictive action,
The end face of the optical fiber of the non-measuring optical path arm is always a stable non-measuring optical path arm length by being a reflecting mirror coated with high reflection,
The method for measuring the thickness of a thin film according to claim 12, wherein: The magnetostrictive body is substantially cylindrical, and a fiber is wound around the cylindrical surface,
17. The method for measuring the thickness of a thin film according to claim 15, wherein: The magnetostrictive body is substantially toroidal, and a fiber is wound around the toroidal surface;
17. The method for measuring the thickness of a thin film according to claim 15, wherein: Fixing the relative spatial positional relationship between the end face of the fiber of the measurement optical path arm and the surface of the thin film to be measured by a fixing means, and on a spatially propagating optical path of a predetermined length from the end face of the fiber to the surface of the thin film to be measured. Moving the placed projection optical means along the spatially propagating optical path,
The method for measuring the thickness of a thin film according to any one of claims 12 to 18, further comprising: The thin film to be measured is any plane or curved surface,
Further comprising a step of finely adjusting and holding the posture of the measured object with a posture adjustment holder so that the normal of the plane or the curved surface substantially coincides with the optical axis of the space propagation optical path,
20. The method for measuring the thickness of a thin film according to claim 12, wherein the thickness of the thin film is measured. 21. The method for measuring the thickness of a thin film according to claim 20, wherein the curved surface of the thin film to be measured has a cylindrical shape, a spherical shell shape, or an irregular shape. 22. The method according to claim 12, wherein the predetermined high frequency is in a range of 0.8 to 3 GHz.

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