Differential plane mirror interferometer

Abstract: A differential plane mirror interferometer comprises a source (10) which emits a light beam (12) containing two orthogonally polarized components of different frequencies; a source of a stabilized electrical reference signal (11) of a frequency corresponding to the frequency difference between the two components of the light beam; a tilted shear plate (16) and a half-wave plate (29A) for converting said beam into two parallel beams (30,33) with the same polarization; means including a polarizing beamsplitter (40), for causing each of the beams (30,33) to be reflected twice by one of two plane mirrors (71,70) to produce two parallel output beams (60,63) with the same polarization; means including a half-wave plate (29B) and the tilted shear plate (16) for connecting said two output beams into a single output beam(80)in which the phase difference between the two components is proportional to the optical path length between the two mirrors (70,71); a photoelectric detector (83) to produce the measurement signal (85); and a phase/meter accumulator (90) to indicate the phase difference between the reference and measurement signals (11,85) which is proportional to the changes in the optical path length between said two mirrors.

Description: The present invention relates to apparatus for the measurement of optical path length changes between two plane mirror surfaces. More particularly, the invention relates to optical apparatus which is useful for high accuracy displacement metrology using interferometry. An interferometer is a basic instrument for most high accuracy displacement measurements in dilatometry, material stability studies, the machine tool industry, and in the semiconductor fabrication industry. One type of interferometer representative of the current state-of-the-art is the differential plane mirror interferometer which measures the optical path length changes between two external mirrors and which is described in R.R. Baldwin and G.J. Siddall, "A double pass attachment for the linear and plane interferometer; "Proc. SPIE, Vol, 480, pp. 78-83 (May 1984). A conventional differential plane mirror interferometer consists of a fixed plane mirror and a movable plane mirror, which form the interferometer cavity, and auxiliary optical components (retroreflectors, wave plates, mirrors, beamsplitters). This type of interferometer has an inherent optical resolution of one quarter of the wavelength of the light used and has particularly high stability which is necessary for the ever increasing demand for improved accuracy. Thus, it is particularly insensitive to any tilt of the plane mirrors and motion of the auxiliary optic components. The conventional differential plane mirror interferometer is, however, overly complicated, requiring many auxiliary optical components thereby subjecting the measurement beams to many reflections. These drawbacks ultimately limit the accuracy that can be achieved due to a lower signal-to-noise in the measurement signal as a result of reduced optical beam power and polarization leakage. The present invention retains the basic plane mirror interferometer cavity of the conventional differential plane mirror interferometer; however, the use of a shear plate in the present invention not only reduces the number of optical elements but also reduces the number of reflections by nearly 50%. The improvements of the present invention thus further increase the accuracy that can be attained with this type of interferometer. The instant invention provides a differential plane mirror interferometer system capable of measuring accurately either changes in length or changes in optical path length. This interferometer system, which is of the kind defined in the preamble of claim 1 and known from Journal of Physics E. Scientific Instruments, vol. 16, No 12, December 1983, pages 1208-1213, The Institute of Physics, Dorking, GB; M. OKAJI et al.: "High-resolution multifold path interferometers for dilatometric measurements", is characterized by the features specified in the characterizing part of claim 1. In the drawings, Fig. 1 depicts in schematic form one embodiment of the instant invention where all optical beams are in a single plane. Fig. 2 depicts in schematic form a second embodiment of the instant invention where the optical beams are not in a single plane. Figure 1 depicts in schematic form one embodiment of the instant invention where all optical beams are in a single plane. While the apparatus has application for a wide range of radiation sources, the following description is taken by way of example with respect to an optical measuring system. Light source 10, which most preferably uses a laser, emits input beam 12 which is comprised of two stabilized frequency components which are orthogonally polarized as indicated by the dot and arrow, see, for example, Bagley et al. U.S. Patent No. 3,458, 259 issued July 26, 1969 and commonly owned, co-pending U.S. Patent Applications Serial Nos. 710, 859, entitled "Apparatus to Transform a single Frequency, Linearly Polarized Laser Beam Into a Beam with Two Orthogonally Polarized Frequencies" filed March 12 1985; 710, 947, entitled "Heterodyne Interferometer System", filed March 12, 1985; and 710,927, entitled "Apparatus to Transform a Single Frequency, Linearly Polarized Laser Beam with two Orthogonally Polarized Frequencies", filed March 12, 1985. Source 10 provides an electrical reference signal 11 which corresponds to the frequency difference between the two stabilized frequencies, by means of either a frequency stabilized electronic oscillator or a photoelectric mixer with electronic amplifier. Beam 12 is incident on shear plate 16 which is a tilted glass substrate with optically flat surfaces 17 and 18 which are mutually parallel. The function of shear plate 16 is to spatially separate the two frequency components using conventional polarization techniques. Beam 12 passes through surface 17 to become beam 13 which has the same polarization as beam 12. Surface 17 has an antireflection coating 21A over the region where beam 12 passes through it. Polarizing coating 23A on surface 18 splits beam 13 so that one polarized frequency component is transmitted as beam 30 whereas the other orthogonally polarized frequency component is reflected as beam 14. Beam 14 is totally reflected from reflective coating 25A on surface 17 to become beam 15. Beam 15 passes through surface 18 to become beam 31 which has the same polarization as beam 15. Surface 18 has an antireflection coating 27A over the region where beam 15 passes through it. Beam 31 passes through half-wave retardation plate 29A which rotates the linear polarization of beam 31 by 90° so that resultant beam 33 has the same polarization (but still a different frequency) as beam 30. Beams 30 and 33 enter polarizing beamsplitter 40 with polarizing coating 42 and are transmitted as beam 34 and 35 respectively. Beams 34 and 35 pass through quarter-wave retardation plate 44 and are converted into circularly polarized beams 50 and 51, respectively. Beam 51 is reflected from fixed reference mirror 71 to become beam 51A while beam 50 is reflected from movable mirror 70 affixed to the stage whose relative position is being measured to become beam 50A. Beams 50A and 51A pass back through quarter-wave retardation plate 44 and are converted back into linearly polarized beams which are orthogonally polarized to the original incident beams 34 and 35. Beams 50A and 51A are reflected by polarizing coating 42 to become beams 52 and 53. Beams 52 and 53 are reflected by retroreflector 45 to become beams 54 and 55. Beams 54 and 55 are reflected by polarizing coating 42 to become beams 56 and 57. Beams 56 and 57 pass through quarter-wave retardation plate 44 and are converted into circularly polarized beams 58 and 59. Beam 59 is reflected from fixed reference mirror 71 to become beam 59A while beam 58 is reflected from movable mirror 70 to become beam 58A. Beams 58A and 59A pass back through quarter-wave retardation plate 44 and are converted back into linearly polarized beams which are polarized the same as the original incident beams 34 and 35. Beams 58A and 59A are transmitted by polarizing coating 42 and leave polarizing beamsplitter 40 as beams 60 and 63. Beams 60 and 63 are mutually parallel by virtue of the inherent optical properties of retroflector 45, independent of any tilt that may be present between mirrors 70 and 71. Beam 60 passes through half-wave retardation plate 29B which rotates the linear polarization of beam 60 by 90° so that resultant beam 62 has a linear polarization which is orthogonal to beam 63. Beam 62 passes through surface 18 to become beam 64 which has the same polarization as beam 62. Surface 18 has an antireflection coating 27B over the region where beam 62 passes through it. Beam 64 is totally reflected from reflective coating 25B to become beam 65. Surface 17 has reflective coating 25B over the region where beam 64 intersects it. Beams 65 and 63 are recombined to form beam 66 by polarizing coating 23B. Surface 18 has polarizing coating 23B over the region where beams 65 and 63 intersect. Beam 66 passes through surface 17 to become beam 80. Surface 17 has an antireflection coating 21B over the region where beam 66 passes through it. Beam 80, like input beam 12, has two frequency components which are orthogonally polarized. Each frequency component has traversed exactly the same optical path length (through air and glass) except for the optical path, nℓ, between mirrors 70 and 71 where n is the index of refraction of the medium between mirrors 70 and 71 and ℓ is the distance between mirrors 70 and 71. The optical path length corresponding to this distance, ℓ, results in a phase difference between the two frequency components of beam 80. Motion of mirror 70 causes this phase difference to vary. This phase variation is directly proportional to the distance, L, moved by mirror 70 for a constant n and is measured by passing beam 80 through polarizer 81, oriented at 45° to each polarization component, which mixes the two orthogonally polarized frequency components in beam 80 to give beam 82. Similarly, if ℓ is fixed and n varies, then the phase variation is directly proportional to the change in n. The interference between the two frequency components is detected by photodetector 83 as a sinusoidal intensity variation with a frequency equal to the difference frequency between the two components of beam 12. The variation of phase between sinusoidal electrical output 85 and sinusoidal electrical reference signal 11 is measured by the phase meter/accumulator 90, see for example the aforementioned commonly owned co-pending U.S. Patent Application Serial No.710,928, to provide the output 92 which is directly proportional to the change in optical path length between mirrors 70 and 71. This optical configuration is extremely insensitive to measurement error because changes in the other optical components, such as those induced mechanically or thermally, affect both frequency components equally and therefore have no influence on the measured phase variation 92. In addition, environmental effects, such as variations in the refractive index of air, can be minimized by placing mirror 71 close to mirror 70 to reduce the optical path length difference between the two frequency components. It should be noted that half-wave retardation plates 29A and 29B could be a single element with a hole in it to allow beam 63 to pass through it unaffected. Figure 2 depicts in schematic form a second embodiment of the instant invention where the optical beams are not in a single plane. This configuration permits a more compact optical system. the description of this figure is identical to Figure 1 and is numbered correspondingly. The only differences are that now coatings 21A and 21B, 23A and 23B, 25A and 25B, and 27A and 27B in Figure 1 become coatings 21, 23, 25, and 27 respectively; and half-wave retardation plates 29A and 29B in Figure 1 become single half-wave retardation plate 29. Thus, in Figure 2, light source 10, which as previously mentioned, most preferably uses a laser, emits input beam 12 which is comprised of two stabilized frequency components which are orthogonally polarized as indicated by the two arrows. Source 10 also provides an electrical reference signal 11 which again corresponds to the frequency difference between the two stabilized frequencies. Beam 12 is incident on shear plate 16 which is a tilted glass substrate with optically flat surfaces 17 and 18 which are mutually parallel. The function of shear plate 16 is to spatially separate the two frequency components using conventional polarization techniques. Thus, in the embodiment of Figure 2, beam 12 is divided by shear plate 16, with the aid of antireflection coatings 21 and 27, polarizing coating 23 and reflective coating 25, to become vertically polarized beam 30 and horizontally polarized beam 31. Beam 31 passes through the single half-wave retardation plate 29 which rotates the linear polarization of beam 31 by 90° so that resultant beam 33 has the same polarization (but still a different frequency) as beam 30. Beams 30 and 33 enter polarizing beamsplitter 40 with polarizing coating 42 and are transmitted as beams 34 and 35 respectively. Beams 34 and 35 pass through quarter-wave retardation plate 44 and are converted into circularly polarized beams 50 and 51, respectively. Beam 51 is reflected from fixed reference mirror 71 to become beam 51A while beam 50 is reflected from movable mirror 70 affixed to the stage whose relative position is being measured to become 50A. Beams 50A and 51A pass back through quarter-wave retardation plate 44 and are converted back into linearly polarized beams that are orthogonally polarized to the original incident beams 34 and 35. Beams 50A and 51A are reflected by polarizing coating 42 to become beams 52 and 53. Beams 52 and 53 are reflected by retroreflector 45 to become beams 54 and 55. Beams 54 and 55 are reflected by polarizing coating 42 to become beams 56 and 57. Beams 56 and 57 pass through quarter-wave retardation plate 44 and are converted into circularly polarized beams 58 and 59. Beam 59 is reflected from fixed reference mirror 71 to become beam 59A while beam 58 is reflected from movable mirror 70 to become beam 58A. Beams 58A and 59A pass back through quarter-wave retardation plate 44 and are converted back into linearly polarized beams that are polarized the same as the original incident beams 34 and 35. Beams 58A and 59A are transmitted by polarizing coating 42 and leave polarizing beamsplitter 40 as beams 60 and 63. Beams 60 and 63 are mutually parallel by virtue of the inherent optical properties of retroreflector 45, independent of any tilt that may be present between mirrors 70 and 71. Beam 60 passes through the single half-wave retardation plate 29 which rotates the linear polarization of beam 60 by 90° so that resultant beam 62 has a linear polarization which is orthogonal to beam 63. Beams 62 and 63 are combined by shear plate 16, with the aid of antireflection coatings 21 and 27, polarizing coating 23 and reflective coating 25, to become beam 80. Once again beam 80, in the embodiment of Figure 2, like input beam 12, has two frequency components which are orthogonally polarized. Each frequency component, as was true with the Figure 1 embodiment, has traversed exactly the same optical path length (through air and glass) except for the optical path, nℓ, between mirrors 70 and 71 where n is the index of refraction of the medium between mirrors 70 and 71 and ℓ is the distance between mirrors 70 and 71. The optical path length corresponding to this distance, ℓ, results in a phase difference between the two frequency components of beam 80. Motion of mirror 70 causes this phase difference to vary. This phase variation is directly proportional to the distance, L, moved by mirror 70 for a constant n and is measured by passing beam 80 through polarizer 81, oriented at 45° to each polarization component, which mixes the two orthogonally polarized frequency components in beam 80 to give beam 82. Similarly, if ℓ is fixed and n varies, then the phase variation is directly proportional to the change in n. As was also true on the Figure 1 embodiment, the interference between the two frequency components is detected by photodetector 83 as a sinusoidal intensity variation with a frequency equal to the difference frequency between the two components of beam 12. The variation of phase between sinusoidal electrical output 85 and sinusoidal electrical reference signal 11 is measured by the phase meter/accumulator 90, to provide the output 92 which as previously mentioned with respect to the Figure 1 embodiment, is directly proportional to the change in optical path length, nℓ, between mirrors 70 and 71. Thus, both the Figure 1 and 2 embodiments employ optical configurations which are extremely insensitive to measurement error because changes in the other optical components, such as those induced mechanically or thermally, affect both frequency components equally and therefore have no influence on the measured phase variation 92. In addition, as was previously mentioned with reference to the Figure 1 embodiment, environmental effects, such as variations in the refractive index of air, can be minimized by placing mirror 71 close to mirror 70 to reduce the optical path length difference between the two frequency components. The principal advantages of the instant invention are: (1) smaller number of optical components, (2) simpler beam paths, (3) fewer reflections, (4) greater light throughput efficiency, (5) lower waveform distortion, (6) reduced optical leakage, (7) reduced non-linearity errors, and (8) lower cost.

Claims: A differential plane mirror interferometric system comprising : a pair of plane mirrors (70, 71) separable by a variable optical path length (ℓ); source means (10) for emitting an input beam (12) comprising two stabilized orthogonally polarized optical components of respective frequencies having a frequency difference (fo), said source means further comprising means for providing an electrical reference signal (11) corresponding to said frequency difference between said two stabilized optical frequencies; means (16) optically coupled to said input beam (12) for separating said input beam components into two spatially separated parallel orthogonally polarized input beams (30, 31) of differing frequency; retardation means (29; 29A) optically disposed in the path of one of said two separated parallel orthogonally polarized input beams (30, 31) for converting said two separated parallel orthogonally polarized beams into two separated parallel beams (30, 33) having the same polarization; multifold beam-folding means (40, 45) optically coupled to said two separated parallel same polarized beams (30, 33) for causing one of said two separated parallel same polarized beams to be reflected a predetermined number of times by one of said pair of plane mirrors (70, 71) and the other of said two separated parallel same polarized beams to be reflected the same predetermined number of times by the other of said pair of plane mirrors to produce two separated parallel output beams (60, 63) having the same polarization; retardation means (29; 29B) optically disposed in the path of one of said two separated same polarized parallel output beams (60, 63) for converting said two separated same polarized parallel output beams into two separated orthogonally polarized parallel output beams (62, 63); means (16) optically coupled to said two separated parallel orthogonally polarized output beams (62, 63) for combining said two separated parallel orthogonally polarized output beams into a single output beam (80) having two orthogonally polarized components of differing frequency, with a phase difference therebetween being directly proportional to said variable optical path length (ℓ) between said pair of plane mirrors (70, 71); means including a polarizer (81) and a photodetector (83), optically coupled to said single output beam (80) for mixing said orthogonally polarized components thereof and producing an electrical measurement signal (85) therefrom, and means (90) operatively connected to said electrical measurement signal (85) and said electrical reference signal (11) for indicating a difference in phase between said electrical reference signal und said electrical measurement signal, said indicated phase difference (92) being proportional to said variable optical path length (ℓ) between said pair of plane mirrors (70, 71), characterized in that said multifold beam-folding means (40, 45) is a two-fold beam-folding means, said predetermined number of times being two, and said means for spatially separating said input beam components (12) into two spatially separated parallel orthogonally polarized input beams (30, 31) and said means for combining said two separated parallel orthogonally polarized output beams (62, 63) into a single output beam (80) are formed by a tilted shear plate means (16) comprising a glass substrate having mutually parallel optically flat surfaces (17, 18) on which two sets of local coatings are disposed, namely a first set of regions of reflection, antireflection and polarization coatings (21A, 27A, 23A, 25A), and a second set of regions of reflection, antireflection and polarization coatings (21B, 27B, 23B, 25B) the first set of coatings being arranged such as to perform said spatial separation, wherein the first surface (17) of the tilted shear plate means (16) has an antireflection coating (21A) over the region through which the input beam (12) passes to be split by a polarizing coating (23A) on the second surface (18) which transmits one polarized frequency component (beam (30)) whereas it reflects the other orthogonally polarized frequency component (beam (14)), which is in turn totally reflected form a reflective coating (25A) on the first surface (17) (beam (15)), and subsequently passes through the second surface (18) via an antireflection coating (27A) as the other (beam (31)) of said spatially separated parallel orthogonally polarized input beams, the second set of optical regions having the same arrangement as the first one and comprising an antireflection region (21B) and a reflection region (25B) both on the first surface (17) of the tilted shear plate means (16), and a polarizing region (23B) and an antireflection region (27B), both on the second surface (18) of said tilted shear plate means (16), so as to perform said combination of said two separated orthogonally polarized output beams incident on the second surface of the tilted shear plate means whereby an optical configuration extremely insensitive to measurement error and misalignment is provided for said interferometric system. An interferometric system in accordance with claim 1, wherein said tilted shear plate means comprises a common tilted parallel plate (16) comprising both said first and second set of regions of coatings. An interferometric system in accordance with any of claim 1 or 2, wherein said means for converting said two separated parallel orthogonally polarized input beams (30, 31) into said two separated parallel same polarized beams (30, 33) comprises a half-wave retardation plate means (29A). An interferometric system in accordance with any of claims 1 to 3, wherein said means for converting said two separated parallel same polarized output beams (60, 63) into said two separated parallel orthogonally polarized output beams (62, 63) comprises a half-wave retardation plate means (29B). An interferometric system in accordance with any of claim 1 or 2, wherein said means for converting said two separated parallel orthogonally polarized input beams (30, 31) into said two separated parallel same polarized beams (30, 33) and said means for converting said two separated parallel same polarized output beams (60, 63) into said two separated parallel orthogonally polarized output beams (62, 63) comprise a single half-wave retardation plate means (29). An interferometric system in accordance with any of claim 1 or 5, wherein said means for causing each of said separated parallel same polarized beams (30, 33) to be reflected twice by one of said pairs of plane mirrors (70, 71) comprises a polarizing beam splitter means (40) and a retroreflector means (45). An interferometric system in accordance with claim 6, wherein said means for causing each of said separated parallel same polarized beams to be reflected twice by one of said pairs of plane mirrors (70, 71) further comprises a quarter wave retardation plate means (44). An interferometric system in accordance with any of claims 1 to 7, wherein said means for producing said electrical measurement signal (85) comprises a polarizer means (81) for mixing said single output beam orthogonal components. An interferometric system in accordance with claim 8, wherein said means for producing said electrical measurement signal (85) comprises a photoelectric detector (83). An interferometric system in accordance with any of claims 1 to 9, wherein said phase difference indicating means composes a phase meter/accumulator means (90). An interferometric system in accordance with any of claims 1 to 10, wherein said means (10) for providing an electrical reference signal (11) comprises a frequency stabilized electronic oscillator. An interferometric system in accordance with any of claims 1 to 11, wherein said means (10) for providing an electrical reference signal (11) comprises a photoelectric mixer and an electronic amplifier means. An interferometric system in accordance with any of claims 1 to 12, wherein one of said pair of plane mirrors (70, 71) is fixed and comprises a reference mirror (71) and the other of said pair of plane mirrors is movable for providing said variable distance (ℓ) between said pair of separable plane mirrors. An interferometric system in accordance with any of claims 1 to 12, wherein the distance (ℓ) between said pair of plane mirrors (70, 71) is fixed for providing the variations in the index of refraction of the medium between said pair of plane mirrors. An interferometric system in accordance with any of claims 1 to 14, wherein all of said beams are in a single plane. An interferometric system in accordance with any of claim 1 to 14, wherein said optical beams are in a plurality of planes. An interferometric system in accordance with any of claims 1 to 16, wherein all of said beams are optical beams. An interferometric system in accordance with any of claims 1 to 17, wherein said source means (10) comprises a laser.

Inventors: SOMMARGREN GARY E, SOMMARGREN, GARY E.
Applicants: ZYGO CORP, ZYGO CORPORATION
Application year: 1986
Main classification: G01J9
IPC codes: G02B27, G01D5, G01B9, G01J9