WO2009155424A1

WO2009155424A1 - Adjustable high stability monolithic double axis interferometer

Info

Publication number: WO2009155424A1
Application number: PCT/US2009/047805
Authority: WO
Inventors: Bing Zheng; Joseph Armand Christman
Original assignee: Bookham Technology Plc
Priority date: 2008-06-20
Filing date: 2009-06-18
Publication date: 2009-12-23

Abstract

An adjustable interferometer includes an optical input head to couple the interferometer to a light source. The interferometer includes a monolithic optics device having at least two 45° surfaces. A first 45° surface splits an input beam of light from the light source into a signal beam and a reference beam. A second 45° surface transmits or reflects the signal beam depending on a polarization of the signal beam. An adjustment assembly in the interferometer includes one or more fasteners and a mirror. The mirror reflects the reference beam back into the monolithic optics device. One or more of the fasteners, when loosened, allow adjustment of a position of the mirror to adjust one or more properties of the reference beam. The fasteners, when tightened, are securely fix the position of the mirror.

Description

TITLE : ADJUSTABLE HIGH STABILITY MONOLITHIC DOUBLE AXIS

INTERFEROMETER

BACKGROUND

1. Field of the Invention

[0001] The present invention relates to interferometers for use in, for example, metrology. Certain embodiments relate to adjustable interferometers.

2. Description of Related Art

[0002] Current interferometers typically use several discrete optical devices to define optical paths in an interferometer and to control the beam paths of the signal and reference beams. Using several discrete optical devices increases the potential for assembly error since each optical device must be accurately positioned. Using fewer optical devices (e.g., only one optical device) decreases the potential for positioning errors during assembly of the interferometer. In addition, using fewer optical devices reduces the number of optical devices that may potentially have manufacturing problems. The complexity of using several optical devices and the potential for problems in using several optical devices reduces the stability and long term reliability of such interferometers. [0003] Some interferometers are adjustable only during assembly of the interferometer.

Thus, once the interferometer is assembled, user adjustment of properties of the interferometer are no longer possible. User-adjustable interferometers may have the problem of drift or movement of adjustable parts used to allow the interferometer to be adjustable. The stability and reliability of such user-adjustable interferometers may thus suffer because of the drift or movement of parts in the interferometer.

[0004] Thus, there is a need for interferometers that use few optical devices to define optical paths in the interferometer and allow user adjustment while maintaining or increasing the stability and reliability of the interferometer. SUMMARY

[0005] In certain embodiments, an adjustable interferometer includes an optical input head. The optical input head may couple the interferometer to a light source. An input beam of light from the light source may be directed to a monolithic optics device in the interferometer. The monolithic optics device may include at least two 45° surfaces. A first 45° surface may split the input beam of light into a signal beam and a reference beam. The reference beam may be reflected by the first 45° surface towards an adjustment assembly. The signal beam may be transmitted through the first 45° surface towards a second 45° surface. The second 45° surface may transmit or reflect the signal beam depending on a polarization of the signal beam. [0006] The adjustment assembly may include one or more fasteners and a mirror. The mirror may reflect the reference beam back into the monolithic optics device. One or more of the fasteners, when loosened, allow adjustment of one or more positions of the mirror. Adjustment of the positions of the mirror may adjust one or more properties of the reference beam. The fasteners, when tightened, may securely fix the position of the mirror. [0007] The signal beam may transmit through the second 45° surface of the monolithic optics device onto a substrate surface. The signal beam may reflect off the substrate surface back along its incident path. The reflected signal beam may be polarized approximately 90° so that the polarized signal beam reflects off the second 45° surface of the monolithic optics device. The signal beam reflects off the second 45° surface towards a cube corner that reflects the signal beam 180° back towards the second 45° surface along a different path.

[0008] The signal beam reflects off the second 45° surface towards the substrate surface along a second incident path. The signal beam reflects off the substrate surface back along the second incident path. The reflected signal beam is polarized approximately 90° so that the polarized signal beam transmits through the second 45° surface. The signal beam and the reference beam are combined in the monolithic optics device to produce an interference beam. The interference beam may be used to assess one or more properties of the substrate surface such as displacement on the substrate surface.

BRIEF DESCRIPTION OF THE DRAWINGS [0009] Features and advantages of the methods and apparatus of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings in which: [0010] FIG. 1 depicts a perspective representation of an embodiment of an interferometer.

[0011] FIG. 2 depicts a top-view representation of the interferometer.

[0012] FIG. 3 depicts a top view representation of beam paths through the interferometer.

[0013] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF EMBODIMENTS [0014] FIG. 1 depicts a perspective representation of an embodiment of interferometer

100. FIG. 2 depicts a top-view representation of interferometer 100. FIG. 3 depicts a top view representation of beam paths (as shown by the arrows) through interferometer 100. Interferometer 100 may be used for metrology displacement measurements or other metrology applications. For example, interferometer 100 may be used in semiconductor equipment metrology displacement measurements such as those used in stepper machines. Interferometer 100 allows for real-time, high precision displacement measurements in systems that require nanometer accuracy.

[0015] In certain embodiments, interferometer 100 includes one or more components coupled to (e.g., fastened to) base 101. In some embodiments, interferometer includes optical input head 102. Optical input head 102 may be used to couple interferometer 100 to a light source such as, but not limited to, a laser light source. In some embodiments, optical input head 102 is a fiber knob or other fiber optic coupling device.

[0016] An input beam of light enters interferometer 100 through optical input head 102. The input beam may be directed towards folding mirror 104. Mirror 104 may be a fixed mirror that reflects the input beam 90° towards lens 106. Lens 106 may focus input beam into a focused or collimated input beam. The input beam may then pass through beam splitter 108. In certain embodiments, beam splitter 108 is a clean-up polarizing beam splitter that cleans up the input beam. The input beam may be cleaned to a p-polarized beam by beam splitter 108. For example, the p-polarized component of the input beam may be transmitted through beam splitter 108 and the s-polarized component maybe reflected 90° by the beam splitter.

[0017] The input beam enters monolithic optics device 110 after passing through beam splitter 108. In certain embodiments, monolithic optics device 110 includes at least two 45° surfaces 112A, 112B, as shown in FIGS. 1-3. Monolithic optics device 110 may, in some embodiments, include additional surfaces as desired or needed. In certain embodiments, surface 112A is a p polarizing beam splitting surface. Surface 112A may split the input beam into a signal beam and a reference beam. The signal and reference beams may each be approximately 50% of the input beam. Monolithic optics device 110 is a single, integrated optics device that performs selected optical functions, which might otherwise be performed by multiple optical devices, hi one embodiment, monolithic optics device 110 is formed by multiple pieces of glass that are bonded together to form a single optics device. Other embodiments are also possible in which other optical materials are bonded together to form monolithic optics device 110.

[0018] hi certain embodiments, the reference beam is the beam reflected by surface 112A and the signal beam is the beam transmitted by surface 112 A. The reference beam may be directed to folding mirror 114 by surface 112A. Mirror 114 may be a fixed mirror that reflects the reference beam 90° towards adjustable assembly 118.

[0019] In certain embodiments, adjustable assembly 118 includes adjustment mirror 116. Adjustable assembly 118 may also include mount 120, fasteners 122, and/or fasteners 124. Fasteners 122 may fasten mirror 116 to mount 120. Fasteners 124 may fasten mirror 116 and/or mount 120 to base 101. Fasteners 122 and fasteners 124 may be, for example, set screws or other secure fastening devices that are operated (i.e., loosened or tightened) with a small tool such as an Allen wrench, a hex key, or a screw driver, hi certain embodiments, adjustment assembly 118 includes three fasteners 122 and three fasteners 124. Other numbers of fasteners may be used as desired or needed. Fasteners 122 and fasteners 124, when tightened, securely fix a position of mirror 116 and mount 120 on base 101.

[0020] The position of mirror 116 may be adjusted to adjust properties of the reference beam. Properties of the reference beam that may be adjusted include, but are not limited to, the yaw and the pitch of the reference beam. The yaw and the pitch of the reference beam may be adjusted in combination or independently as desired using fasteners 122 and fasteners 124 because fasteners 122 are independently operated from fasteners 124, and vice versa.

[0021] hi certain embodiments, fasteners 122 are loosened to allow adjustment of the pitch of the reference beam. Loosening fasteners 122 may allow y-movement (up and down movement) of mirror 116 relative to base 101 (i.e., in and out of the paper, as shown in FIG. 2). After adjustment of the pitch of the reference beam, fasteners 122 may be tightened to secure the y-position of mirror 116.

[0022] hi certain embodiments, fasteners 124 are loosened to allow adjustment of the yaw of the reference beam. Loosening fasteners 124 may allow x-movement (left and right movement) of mirror 116 (i.e., movement of the mirror along the reflective plane of the mirror).

After adjustment of the yaw of the reference beam, fasteners 124 may be tightened to secure the x-position of mirror 116.

[0023] hi certain embodiments, fasteners 122 and fasteners 124 are independently operated to independently adjust the x- and y-positions of mirror 116. For example, fasteners 122 may be loose and fasteners 124 tight so that only y-movement of mirror 116 is allowed, or vice versa. Thus, the x- and y-positions of mirror 116 may be independently adjustable (adjustment of one of the positions does not affect the other position). These independent position adjustments allow the yaw and the pitch of the reference beam to be independently adjusted without affecting each other, hi some embodiments, fasteners 122 and fasteners 124 are loosened at the same time so that both the x- and y-positions of mirror 116 can be adjusted simultaneously or in combination with each other.

[0024] Fasteners 122 and/or fasteners 124 may be repeatedly loosened and tightened to adjust and secure, respectively, the position of mirror 116. Because the position of mirror 116 is repeatedly adjustable, interferometer 100 is adaptable to a variety of situations and/or uses as desired or needed by a user. Additionally, because fasteners 122 and fasteners 124, when tightened, securely fasten (lock-in) mirror 116 to base 101, the mirror is inhibited from movement and the stability and reliability of interferometer 100 is high. Thus, adjustment assembly 118 (e.g., mirror 116, mount 120, fasteners 122, and fasteners 124) provides a repeatedly adjustable, highly stable, and highly reliable interferometer 100.

[0025] As shown in FIG. 3, the reference beam is reflected 90° by mirror 116 back into monolithic optics device 110. Surface 112A transmits approximately 50% of the reference beam towards output 126 and reflects the remaining portion (approximately 50%) of the reference beam towards output 128 through beam shifter 136. [0026] hi certain embodiments, the signal beam portion of the input beam transmitted by surface 112A propagates towards surface 112B. hi certain embodiments, surface 112B is a polarizing beam splitting surface. The polarization of the signal beam incident on surface 112B allows for approximately 100% transmission of the signal beam through surface 112B. The signal beam then passes through quarter- wave plate 130 and onto surface 132. Surface 132 includes surfaces to be measured using interferometer 100 such as, but not limited to, semiconductor substrate surfaces.

[0027] Surface 132 reflects the signal beam back towards surface 112B of monolithic optics device 110 along the same path. As the signal beam passes through quarter-wave plate 130, the signal beam undergoes 90° polarization. Because of this polarization, the signal beam is approximately 100% reflected by surface 112B and is reflected 90° towards corner cube 134.

[0028] Corner cube 134 turns the signal beam around 180° and redirects the signal beam along a new path back into monolithic optics device 110. Because of the current polarization of the signal beam, the signal beam is approximately 100% reflected by surface 112B in this direction and is reflected 90° towards quarter-wave plate 130 and surface 132 along a new path. Surface 132 once again reflects the signal beam back towards surface 112B along the same incident path and the signal beam undergoes 90° polarization as the signal beam passes through quarter-wave plate 130. Because of this second polarization, the signal beam is approximately 100% transmitted by surface 112B and is transmitted through towards surface 112A.

[0029] Surface 112A reflects approximately 50% of the signal beam towards output 126 and transmits the remaining portion (approximately 50%) of the signal beam towards output 128 through beam shifter 136. At this point on surface 112A, the signal beam is combined with the reference beam to produce interference beams that propagate to outputs 126 and 128. Approximately 50% of each of the signal and reference beams are in each of the interference beams. The interference beams may be substantially identical to each other so that the beam at output 126 is substantially identical to the beam at output 128. In some embodiments, output 126 is a main output and output 128 is a monitor output. [0030] Properties of one or more of the interference beams may be assessed to assess properties of surface 132. For example, fringe spacing and/or angles of an interference beam may be assessed to assess displacement on surface 132. The properties of the interference beams may be assessed in real-time to allow for real-time assessment of surface 132.

[0031] Because the signal beam encounters surface 132 twice and passes through monolithic optics device 110 twice, the sensitivity of the assessment of the surface is increased at least two times. Further increases in sensitivity may be possible by allowing the signal beam to encounter surface 132 more than two times (e.g., three times, four times, etc.). It is contemplated that other embodiments may include additional optics within interferometer 100 and/or rearrange the depicted optics such that the signal beam passes through monolithic optics device 110 a different number of times (e.g., three or four times) and encounters surface 132 at least once for each pass through the monolithic optics device.

[0032] The use of monolithic optics device 110 in interferometer 100 increases the stability and long term reliability of the interferometer. Monolithic optics device 110 may also allow the use of one optical device instead of several discrete optical devices in interferometer 100. The use of monolithic optics device 110 thus may reduce the possibility of manufacturing errors by reducing the number of optical devices used in interferometer 100. Additionally, monolithic optics device 110 may decrease the complexity of interferometer 100 while providing stability, reliability, and measurement sensitivity.

[0033] It is to be understood the invention is not limited to particular systems described which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification, the singular forms "a", "an" and "the" include plural referents unless the content clearly indicates otherwise. Thus, for example, reference to "a mirror" includes a combination of two or more mirrors.

[0034] Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.

Claims

WHAT IS CLAIMED IS:

1. An adjustable interferometer, comprising: an optical input head configured to couple the interferometer to a light source; a monolithic optics device comprising at least two surfaces each angled at approximately

45° with respect to a light beam path, wherein a first 45° surface is configured to split an input beam of light from the light source into a signal beam and a reference beam, and wherein a second 45° surface is configured to transmit or reflect the signal beam depending on a polarization of the signal beam; and an adjustment assembly comprising one or more fasteners and a mirror, wherein the mirror is configured to reflect the reference beam back into the monolithic optics device, wherein one or more of the fasteners, when loosened, allow adjustment of a position of the mirror to adjust one or more properties of the reference beam, and wherein the fasteners, when tightened, are configured to securely fix the position of the mirror.

2. The interferometer of claim 1, wherein the first 45° surface splits the input beam approximately 50% into the signal beam and approximately 50% into the reference beam.

3. The interferometer of claim 1, wherein the first 45° surface comprises a p polarizing beam splitting surface.

4. The interferometer of claim 1 , wherein the second 45° surface comprises a polarizing beam splitting surface.

5. The interferometer of claim 1 , wherein the signal beam is configured to pass through the monolithic optics device at least twice along separate parallel optical axes of the monolithic optics device.

6. The interferometer of claim 1, wherein the signal beam is configured to encounter a surface of a substrate at least twice, and wherein one or more properties of the surface are configured to be assessed using the interferometer.

7. The interferometer of claim 1, wherein the signal beam and the reference beam are recombined into at least one interference beam by the monolithic optics device.

8. The interferometer of claim 7, wherein the signal beam and the reference beam are recombined into at least one interference beam at a location on the first 45° surface different from a location where the first 45° surface splits the input beam into the signal beam and the reference beam.

9. The interferometer of claim 7, wherein the interference beam is configured to be used to assess one or more properties of a surface of a substrate encountered by the signal beam.

10. The interferometer of claim 1, wherein the position of the mirror is adjusted to adjust the yaw and/or the pitch of the reference beam.

11. The interferometer of claim 10, wherein the yaw and the pitch of the reference beam are adjusted independently.

12. The interferometer of claim 1, further comprising at least one fixed mirror configured to reflect the input beam from the optical input head towards the monolithic optics device.

13. The interferometer of claim 1, further comprising at least one lens configured to focus the input beam.

14. The interferometer of claim 1, further comprising at least one quarter- wave plate located between the monolithic optics device and a surface of a substrate encountered by the signal beam, wherein the quarter-wave plate is configured to adjust the polarization of the signal beam by 90° so that the signal beam is either reflected or transmitted by the second 45° surface of the monolithic optics device.

15. The interferometer of claim 1 , further comprising at least one corner cube reflector configured to reflect the signal beam incident from the monolithic optics device 180° back into the monolithic optics device along a different path from the incident path.

16. An adjustable interferometer system, comprising: a light source configured to provide an input beam of light; an interferometer, comprising: an optical input head configured to couple the interferometer to the light source; a monolithic optics device comprising at least two surfaces each angled at approximately 45° with respect to a light beam path, wherein a first 45° surface is configured to split the input beam into a signal beam and a reference beam, and wherein a second 45° surface is configured to transmit or reflect the signal beam depending on a polarization of the signal beam; and an adjustment assembly comprising one or more fasteners and a mirror, wherein the mirror is configured to reflect the reference beam back into the monolithic optics device, wherein one or more of the fasteners, when loosened, allow adjustment of a position of the mirror to adjust one or more properties of the reference beam, and wherein the fasteners, when tightened, are configured to securely fix the position of the mirror; wherein the signal beam and the reference beam are configured to be recombined into an interference beam in the monolithic optics device; and a substrate surface configured to be encountered at least twice by the signal beam, wherein one or more properties of the substrate surface are configured to be assessed using one or more properties of the interference beam.

17. A method for measuring one or more properties of a substrate surface using an adjustable interferometer, comprising: providing an input beam of light to a monolithic optics device; splitting the input beam of light into a signal beam and a reference beam using a first 45° surface in the monolithic optics device; reflecting the reference beam back into the monolithic optics device using an adjustable mirror; transmitting the signal beam through a second 45° surface in the monolithic optics device onto the substrate surface along a first incident path; reflecting the signal beam off the substrate surface back along the first incident path; polarizing the reflected signal beam on the first incident path approximately 90° so that the polarized signal beam reflects off the second 45° surface; reflecting the polarized reflected signal beam off the second 45° surface towards a cube corner that reflects the signal beam 180° back towards the second 45° surface along a different path; reflecting the signal beam off the second 45° surface towards the substrate surface along a second incident path; reflecting the signal beam off the substrate surface back along the second incident path; polarizing the reflected signal beam on the second incident path approximately 90° so that the polarized signal beam transmits through the second 45° surface; and combining the signal beam and the reference beam in the monolithic optics device to produce an interference beam.

18. The method of claim 17, further comprising adjusting a position of the adjustable mirror to adjust one or more properties of the reference beam.

19. The method of claim 18, further comprising securely fixing the position of the adjustable mirror after adjusting the position of the adjustable mirror.

20. The method of claim 17, further comprising assessing one or more properties of the substrate surface using one or more properties of the interference beam.