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A Novel Optical Component for the Development of an Integrated Interferometric System A. Graciasa, N. Tokranova, S. Olson, and J. Castracane College of Nanoscale Science and Engineering, University at Albany – SUNY, Albany, NY 12203 1. ABSTRACT Optical interferometry is a well established technique for high resolution displacement measurements. It is commonly used in the semiconductor industry as a sub-system of manufacturing and metrology tools. As the industry progresses, the tools continue to evolve, requiring the concomitant reduction of size and cost in sensors. Existing interferometric systems are bulky and therefore difficult to incorporate in equipment. Efforts are ongoing to miniaturize these systems but with optical components (beam splitters, detectors and lasers) still in the millimeter range, it is difficult to realize ultra compact systems. Thus, it is imperative to focus on development of micron scale components that would provide the necessary high spatial resolution in a compact format. The focus of this paper is on the development of a micron size optical component that combines multiple optical elements and can be integrated with VCSELs at the wafer level to yield a compact, low cost interferometric system. The design and development of this component containing the beam splitter and reference mirror will be presented including the investigation of suitable polymeric materials with desirable optical properties and appropriate fabrication techniques. Preliminary optical measurements of the integrated system will also be demonstrated. This approach has the potential to impact the next generation of micron scale interferometers as precise position/proximity sensors. Keywords: Interferometer, optical component, PDMS, polyurethane, micromolding

2. INTRODUCTION Optical interferometry is a commericially available technique for precision positioning systems in a variety of applications within the micro-device manufacturing field. These systems provide high resolution alignment for both manufacturing 1 and metrology tools 2. Researchers are investigating a number of interferometeric techniques, including fixed frequency and heterodyne, to address the continued need for more accurate measurements 3. As semiconductor devices evolve, manufacturing and metrology systems increase in both cost and complexity to meet the challenges of the industry. To meet the needs of industry, tool manufactures must reduce cost while building sophisticated equipment, both of which can be achieved by reducing sub-system size and incorporating lower cost parts. A number of researchers have developed polymer optical components as an alternative to the commercially available products. These polymers are particularly advantageous because their mechanical and thermal properties can be controlled and components can be fabricated using simplified techniques producing less expensive but superior components 4. Additionally, these components can be integrated with light sources and detectors into compact subsystems 5. Researchers are investigating a number of techniques to fabricate high quality components, ranging from direct lithography 6 to replication molding 5. This paper discusses the work on the fabrication of a novel optical component for an interferometric system. The optical component combines the beam splitter and reference mirror into a monolithic component that is integrated with a [email protected], phone (518) 956-7313, College of Nanoscale Science and Engineering – SUNY, 255 Fuller Rd, Albany, NY, 12203

Optical Components and Materials VI, edited by Shibin Jiang, Michel J. F. Digonnet, John W. Glesener, J. Christopher Dries, Proc. of SPIE Vol. 7212, 72120P © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.808043 Proc. of SPIE Vol. 7212 72120P-1

the optical source to create a compact interferometer system. Three polymers, SU-8™, poly (dimethylsiloxane) and polyurethane, were considered as the structural material for the optical component, and the fabrication process used was based on the properties of the structural material. The optical components were characterized using scanning electron microscopy and were then integrated with a VCSEL. Preliminary measurements are presented below.

3. BACKGROUND Three materials, SU-8™, poly (dimethylsiloxane) (PDMS) and polyurethane, were investigated as potential structural materials for the optical part discussed. Each of these materials is a suitable structural choice for optical components and has been previously used for different applications. SU-8 has already been applied to the fabrication of waveguides 7, lenses 8 and gratings 9, PDMS has been used to fabricate lenses integrated with microfluidic networks 10, 11, 12 and waveguides 13 and polyurethane has been used in optical waveguides 14 and as the matrix for non-linear optical components 15. SU-8 is an epoxy-based, negative-tone photoresist manufactured by Microchem Corporation (Newton, MA). SU-8 was originally developed by IBM for high aspect ratio patterns for the semiconductor industry 16 and was later adopted by the MEMS community as a structural material for a number of their applications 17, 18, 19. SU-8 is offered encompassing a wide range of viscosities allowing for film thicknesses ranging from nanometers to greater than 250μm. Researchers have been able to successfully expose SU-8 with much shorter wavelengths than the target wavelength of 365nm, and from electron beam 20 to x-ray 21 producing structures of various sizes. In addition to the ease of processing, SU-8 has a number of advantageous characteristics that make it suitable for a number of applications including high thermal stability (>315oC – 5% wt. loss), good resistance to chemicals and plasma etching and the ability to produce structures with vertical sidewalls 22. SU-8 has appropriate optical properties for applications in the micro-photonic devices field; it has low optical absorption in the visible and infrared spectral ranges, evident in the extinction coefficient plot seen in Figure 1 23 and an index of refraction close to that of glass, 1.5 - 2 24 minimizing light intensity loss due to reflections at the interface between the SU8 structure and glass substrate.

Figure 1: Extinction coefficient of SU8 23 The second material investigated was poly(dimethylsiloxane), a non-photodefinable silicone elastomer manufactured and distributed by Dow Corning (Midland, MI). PDMS is commonly used for the fabrication of micro-fluidic networks for lab-on-a-chip applications 10, 25. The fabrication of these structures is done using a micromolding technique, in which masters are used to define the final device in the PDMS films 26, 27. Although PDMS requires micromolding in order to fabricate microstructures, it is a robust material with good thermal stability between -45oC and 200oC, chemical inertness, low electrical conductivity and elasticity 28. More recently, a positive-tone, photodefinable PDMS, sensitive to short wavelengths (< 365nm), has been developed, widening the possibilities of device fabrication 29. The PDMS also has low optical absorption (Figure 2) in the visible and infrared regions and as mentioned previously, it has been used in the fabrication of micro-optics.

Proc. of SPIE Vol. 7212 72120P-2

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Figure 2: Optical properties of poly (dimethylsiloxane) 25 The final material investigated was polyurethane, a two part, non-photodefinable resin, Crystal Clear 200 available from Smooth-On (Easton, PA). Polyurethane has been used as a structural material or host matrix for applications 30 in a diverse range of fields but is commonly used as a protective lacquer 31. Fabrication techniques range from electrospinning 32 and molding 33 to coating and etching 34 depending on the application. Though polyurethane is not photodefinable, structures can be fabricated using soft lithography, and it still offers a number of suitable optical and mechanical properties. It has an index of refraction close to that of glass (n = 1.491), with a tensile modulus of 70MPa 35 . Once cured, it is rigid and resistant to most solvents and weak bases. Polyurethane has low optical absorption (Figure 3) for wavelengths greater than 300nm.

4

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400 500 Wavelength. nfl

00

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Figure 3: Absorbance of spectra of polyurethane film containing nano-silica particles 36 The interferometer system for this work is based on the Michelson interferometer (Figure 4), which consists of a single light source, a beam-splitter and two mirrors, a reference mirror, the target and detector. The path difference relative to the beam splitter between the reference mirror and the target mirror produces an interference pattern at the detector, which can then be used to determine the position of the target. The position is calculated using a simple equation where the path difference is related to the wavelength, λ, fringe order, m. and the index of refraction, n, of the material the light is traveling through, in standard interferometers would be air (Equation 1).


Target

Source Reference Mirror

Detector

Figure 4: (a) Schematic of a Michelson interferometer (b) example of interference pattern 37

d=

mλ 2n

(1)

The work done focused primarily on the development of an optical part that would integrate the beam splitter and reference mirror into a single component.

4. EXPERIMENTAL DETAIL An investigation of two potential integration schemes was done (Figure 5). This optical component would integrate the beam splitter and the reference mirror (Figure 5a). The first scheme used direct patterning of the optical component on the VCSEL. The photo-definable polymer (SU-8) was coated on the VCSEL, and then selective masking and angled lithography was used to define the beam splitter and reference mirror surfaces. The second scheme (Figure 5b) used micro-molding to create the optical component independent of the VCSEL structure, and then aligned and integrated into the system. Each scheme has its advantages. In the case of the SU-8 based structure, alignment of the optical part over the VCSEL aperture can be done during the lithography process. SU-8 is a low cost material and structures are mechanically and thermally stable. The design and integration of the second optical part includes a 45o angle air gap; this gap prevents the VCSEL surface from overheating and the part can be integrated with any commercially available source without special consideration to index of refraction matching. The angle of the structure and the surface finish are critical to the successful implementation of this novel polymer optical part.

SemiTransparent Reference Mirror VCSEL

Movable Mirror Detector

Target Semi-transparent Mirror

Detector

Reference Mirror

VCSEL

Figure 5: Schematic representation of the two integration schemes: (a) patterning of optical component in SU-8 directly on VCSEL (b) micro-molding of optical component followed by integration with VCSEL The initial experiments investigated SU-8 as the structural material. To fabricate the SU-8 based optical component, a two exposure lithography process would be required including angled lithography to create the 45o angle. Angled SU-8 structures have been previously demonstrated in the literature 38 and a similar technique was used to create the angled structures for this work. The 100 μm SU-8 film was coated directly on the VCSEL array and then, soft baked for 30 minutes at 95 oC. The mask was mounted on the SU-8 film and then, the whole apparatus was tilted and


exposed. A post exposure bake of 35 minutes at 95 oC was done to cross-link the SU-8 film. The film was then immersion developed for at least 1 hour to remove the unexposed SU8. The fabrication of the optical part (Figure 6) from PDMS or polyurethane required a molding process. The process required two master molds and two replica molds to create the final structure. The first mold master was created by patterning the structure in SU-8. The process began by cleaning a silicon substrate in a solution of hydrogen peroxide and sulfuric acid (1:3) and followed by a dehydration bake at 200 oC for 5 minutes. To create the necessary 500 μm film, a two coating process was necessary. After the surface preparation step, a thin layer of SU-8 2010 was used as an adhesion layer. Then, the first SU-8 2100 film was coated on the silicon wafer and the film was then relaxed for 4 hours and baked at 95 oC for 4 hours after which the second layer was coated using the same procedure. After the exposure (2100 mJ/cm2), the film was relaxed again and then baked at 95 oC for 1 hour to ensure that the exposed areas were fully cross-linked. The developed structure was done by immersion develop overnight without agitation. The aluminum mold master for the 45o angle of the beam splitter was a custom part purchased from PolyOptics (Santa Rosa, CA). Prior to creating the replica molds in PDMS, both masters were treated with Sigmacote (Sigma Aldrich, Milwaukee, WI) to ensure that the replicas were easily released from the masters. The PDMS, (Sylgard 184, Dow Corning, Midland, MI) is a two part silicone elastomer which is mixed at a ratio of 10 to 1 of the elastomer to curing agent. The mixed material is then degassed at room temperature in vacuum, the poured over the master molds (Figure X). The film is then cured in an oven at 60 oC for a minimum of 2 hours. The replicas were then released from the masters.

(a) (b)

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Al master

Si

Figure 6: Schematic representation of the fabrication of the replica molds needed to build the optical part (a) fabrication of the master mold either from SU-8 or Aluminum (b) PDMS molding to create the replica mold (c) schematic representation of the replicated part The fabrication of the optical part followed similar processing steps to the replica molding process (Figure 7). The PDMS replicas are used to fabricate the final optical part. To fabricate a PDMS optical part using the PDMS replica molds, the molds were treated with trichlorosilane vapor in order to ensure that the part would be released from the mold. The treatment consisted of a short oxygen plasma treatment to first activate the PDMS surface, a 10 minute vapor based surface treatment at 60 oC followed by a 50 minute thermal treatment at 60 oC. After this treatment, PDMS is prepared as described above and molded into the final optical part. In the case of the polyurethane optical part, there was no need to surface treat the PDMS and the replica molds could be used as is. The polyurethane (Crystal Clear 200, Smooth On, Easton, PA) used for this work was also a two-part kit mixed at ratio of 1 to 1. After mixing the material was degassed, and then injected into the PDMS mold and degassed for a second time. The parts were then aligned and clamped together before the film was thermally treated for at least 2 hours at 60 oC.


Final Optical Part PDMS Polyurethane

Figure 7: Schematic representation of the molding process used to fabrication the optical part. Metal coatings were done using shadow masks and physical deposition of thin AuPd films (Figure 8) for the semitransparent mirror (10nm) and the reference mirror (100nm). The completed structure was then manually manipulated to integrate with commercially available single-mode VCSEL, with an operating wavelength of 850nm (Bookham, San Jose, CA).

Shadow Metal deposition on 45o

Metal deposition on sidewall of structure for reference mirror

Figure 8: Schematic representation of the coating process

5. DISCUSSION The successful fabrication of a SU-8-based optical part (Figure 9a) was hindered by the material properties of SU-8. Using Snell’s law and the index of refraction for both air (n = 1) and SU-8 (n = 1.5) it was determined that angled exposures would not be able to create the 45o angle needed for the beam splitter (Figure 9b).


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Figure 9: (a) SEM micrograph of an angled SU-8 structure and (b) a graph of the resultant angle as a function of the exposure angle based on Snell’s law The molding process was able to successfully fabricate both PDMS and polyurethane based optical parts (Figure 10a, 10b). However, the mechanical stability of the PDMS and the difficulty in getting quality metal coatings for the mirrors prevented it from being used for this application. The polyurethane optical parts were successfully fabricated (Figure 10b) using the soft lithography technique described above. Cured polyurethane was mechanically stable and proved to have suitable surface properties to produce good metal coatings for the beam splitter and reference mirror.

(a)

(b)

loop

WD= 33 Mg 435X

EHT = 5.00 kV

Stg

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I = 90.0 Dtt = SE2

Figure 10: (a) Optical image of a PDMS optical part (b) SEM micrograph of polyurethane optical part After the metal deposition, the part was then integrated with the VCSEL source provided by the optoelectronics group from the College of Nanoscale Science and Engineering (Figure 11a) and the system was tested using the optical test set-up seen in Figure 11b that captured the interference pattern using a CCD camera.


Figure 11: (a) Integration of the optical part with the VCSEL array (b) optical test set-up For these experiments the movable mirror was adjusted manually using a micro-manipulator and the interference was captured using a CCD camera (Figure 12a). The capture software has a cross-hair function that provided a line-scan view of the spot which was viewed at the edge of the image (Figure 12a). Based on the line-scan a fringe peak was selected and monitored for changes in intensity as a function of movement of the target mirror (Figure 12b). The fringe intensity can be calculated from equation 2, where I1 and I2 are the light intensity from the two mirrors, d is the path difference, λ is the wavelength, f is the focal length, and x and y are the coordinates of the focal plane 39. The preliminary experiments showed that the fringe intensity did change with movement of the target mirror. However improvements to the resolution is possible by using VCSEL modulation together with special readout electronics. 900 800

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Figure 12: (a) Interference pattern captured by the CCD camera (b) The change in intensity of an interference fringe as a function of the movement of the target mirror.

⎡ 2 ⋅π Intensity = I 1 + I 2 + 2 ⋅ (I 1 ⋅ I 2 ) ⋅ cos ⎢ ⎢ λ ⎣

2 ⎤⎞ ⎤ ⎛ ⎡ 2 ⎜ 2 ⋅ d ⋅ cos ⎢ x + y ⎥ ⎟ + π ⎥ ⎜ f ⎥ ⎢⎣ ⎥⎦ ⎟⎠ ⎝ ⎦

(2)

6. CONCLUSIONS Successful fabrication of a novel polyurethane optical element for a low-cost, inteferometric system has been demonstrated. A micro-molding process was developed to create the element that integrated the beam splitter and the reference mirror. SEM micrographs confirmed the successful fabrication of the optical part. The element was then integrated with a commercially available VCSEL and preliminary experiments were conducted to evaluate the success of the element construction. The results show that with the displacement of the moveable mirror, changes in light intensity were visible proving the validity of our device design. Taking advantage of the optical and process capabilities of polymers, we demonstrated a promising new material system for the development of novel optical elements for interferometric systems.


7. ACKNOWLEDGEMENTS The authors would like to acknowledge Dr. Serge Oktyabrsky, Dr. Michael Yakimov, Dr. Vadim Tokranov and Dr. Jobert Van Eisden for providing VCSEL structures. Additionally, the authors would like to thank Bruce Altemus for his contribution to various aspects of the research. The work was done with partial financial support from MTI Instruments Inc. (Albany, NY).

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