Sensors and Actuators 78 Ž1999. 41–47 www.elsevier.nlrlocatersna
Diamond microstructures for optical micro electromechanical systems ) H. Bjorkman , P. Rangsten, K. Hjort ¨ Department of Materials Science, Uppsala UniÕersity, Box 534, SE-751 21 Uppsala, Sweden Received 2 November 1998; accepted 16 November 1998
Abstract We have used hot filament chemical vapour deposition ŽHFCVD. to fabricate diamond microstructure components for optical micro electromechanical systems ŽMEMS.. In order to demonstrate the wide application range for diamond technology we have made components for different applications such as diffractive optics, laser-to-fibre alignment, and active cooling of high power devices. The free-standing polycrystalline diamond devices were grown on moulds of silicon and fused silica, creating replicas of the moulds. The structured silicon-on-insulator ŽSOI. wafers as replicating moulds for diamond deposition makes it possible to create shapes that can be very useful for laser-to-fibre alignment. The two-step SOI diamond deposition process enables the creation of long and precise capillaries or capillary arrays with a flat top-side here intended for cooling of high power electronic devices. Furthermore, microstructured fused silica has been introduced as mould material, which gives new possibilities in micro-optical mould design since fused silica technology is much more common in optical industry. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Replication; HFCVD; Optical MEMS
1. Introduction Diamond is one of the most interesting materials under consideration for micro electromechanical systems ŽMEMS., e.g., Refs. w1,2x. It is a low-cost 1 CVD-deposition film material for coverage of silicon moulds. What is particularly interesting for optical MEMS is that diamond is has the widest electromagnetic radiation transparency range and the highest thermal conductivity of all materials known. In addition, it is possible to make it either semiconducting or insulating. In most cases, when diamond microstructures are made, the structures need to be supported by a carrier structure of, e.g., silicon. There are only a few cases where the diamond microstructures are totally free-standing, i.e., a micromotor structure w3x, electron emitting tips w4x, SPM cantilever with tips w5x, nanoindention tips w6x, abrasive tips w7x, gears w8,9x, laser-to-fibre alignment w2,10x, moulds for polymer replication w2x, and free-standing capillaries w2x. ) Corresponding author. Tel.: q46-18-471-3235; Fax: q46-18-4713572; E-mail:
[email protected] 1 See, e.g., information from ASTEX. They claim it is possible to reach full deposition cost of US$84 per carat Žincluding maintenance, capital and labour costs., i.e., US$11.6rmm on a 4-in. wafer or US$5.9rcm2 for a 40 mm thick diamond film Žhttp:rrwww.astex.comr thermal.htm..
To microstructure diamond films, replication w2,6x, dry etching w11x, ablation w9,12x, and selective deposition w5,8x are methods that can be used. Replication is the preferred method to enable geometries that are more complex and facilitate structuring of thick films w2x. To cool a high power device quickly, a diamond chip can be used as carrier w13x. For further improvement of heat transport from high power devices, it would be interesting to introduce active cooling using fluid capillaries in the diamond carrier. For laser-to-fibre alignment devices, it is of high importance that the fibre and the laser are aligned and positioned in close vicinity to each other. Normally, laser-to-fibre alignment devices are made of silicon w14x. By using diamond, a laser-to-fibre alignment structure, with high heat conductivity, and where laser-to-fibre distance is minimised, can be made. In previous studies, diffractive optics devices made of UV-grade fused silica have been used to homogenise eximer laser beams w15x. Since an eximer laser is a high power UV-laser, the optical element has to be made in a material, which can sustain the high optical power. Even fused silica, however, will absorb some light, which can induce dislocations and eventually lead to that the fused silica device is destroyed w16x. Diamond, on the other hand, has potential to withstand higher optical power than
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 2 0 2 - 2
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H. Bjorkman et al.r Sensors and Actuators 78 (1999) 41–47 ¨
Fig. 1. SEM micrograph of the silicon mould for the laser-to-fibre alignment structure before dicing.
fused silica w16x and it is thus of interest to manufacture diffractive optics devices in diamond. We now present a new diamond replication process that allows non-self-supporting silicon structures to act as parts in the mould by using SOI wafers as substrates. The SOI wafer enables us to make precise diamond structures for height alignment of fibre to laser. It also makes it possible to fabricate flat diamond chips with long capillaries or capillary arrays for high power electronic device carriers or other fluidic systems. Furthermore, microstructured fused silica has been introduced as mould material, which gives new possibilities in micro-optical mould design since fused silica technology is much more common in optical industry. We present demonstrator microstructures primarily intended for devices in optical MEMS. Silicon microstructuring and diamond replication has been performed as earlier described in Ref. w2x.
For the laser-to-fibre alignment device, the slow etching Ž111. planes on SOI-silicon wafers were used to create V-ridges. The device layer thickness Ž70 mm. and the width of the mask oxide Žoriented along a &110:-direction. were used to control the height and width of the V-ridges respectively. The SOI technology was used to enable maximum control of the etch depth so that no time-stop was needed. Then the ends of the ridges were cut off with a dicing saw to get vertical ends and thereby decrease the optical path between laser and fibre. This could not have been done by wet etching. See Fig. 1 for a scanning electron microscope ŽSEM. micrograph of the silicon mould before dicing. The active cooling devices were fabricated by a two-step SOI diamond deposition process. The thickness of the device layers Ž70 mm. and the widths of the mask oxide Žoriented along a &100:-direction. were used to control the height and width of the ridges respectively, see Fig. 2 for the resulting silicon mould. The oxide layer was removed and thereafter diamond was deposited. After that, the mould with diamond on one top-side was further etched from the back-side, using the buried oxide layer as etch-stop to prevent the ridges from being etched. Subsequently, diamond was deposited a second time, and a final sacrificial etch of silicon was performed in HFrHNO 3 Ž7:3.. See Fig. 3 for a complete flow-scheme of the process. Prior to diamond deposition on the silicon substrates, the oxide was stripped off in an HF solution Ž1:10.. 2.2. Diamond deposition Polycrystalline diamond films were deposited on the moulds by HFCVD using a mixture of 150 sccm hydrogen
2. Experimental 2.1. Mould fabrication The continuous-relief diffractive optics fused silica mould was fabricated using direct-write analogue electron-beam lithography and reactive ion etching w15x. When manufacturing the moulds for active cooling components, as well as the laser-to-fibre alignment chips, bonded SOI wafers Ž70 mm device layer on 1 mm buried SiO 2 . were used. By standard bulk micromachining processes, i.e., lithography and wet etching, different silicon microstructures were fabricated. The silicon wafers were of Ž100.-orientation we used SiO 2 as masking material. Potassium hydroxide ŽKOH, 40 gr100 ml. at 808C was used as etchant.
Fig. 2. SEM micrograph of the silicon mould for the active cooling device.
H. Bjorkman et al.r Sensors and Actuators 78 (1999) 41–47 ¨
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Fig. 3. The two-step diamond deposition process.
and 1.5 sccm methane gas at substrate temperatures around 9008C and chamber pressure of 50 mbar. The moulds were pre-treated by ultrasonic agitation with diamond seeds in ethanol for 10 min to promote diamond nucleation. The substrate temperature was monitored with a thermocouple positioned on the mould surface. A 1 mm thick tungsten wire was used as filament placed at 5 mm distance from the mould. The wire had been positioned so that it could thermally expand freely during the whole deposition, without changing the distance between wire and mould. The deposition rate was about 1 mmrh. A typical deposition thickness was 20 mm. The experimental set-up is shown in Fig. 4.
was used consisting of 7:3 of HNO 3 :HF, with an etch rate of about 50 mmrmin at 808C etch without stirring. For complex sacrificial etching, e.g., inside diamond capillaries, stirring was used to promote etching.
2.3. Mould remoÕal When using fused silica moulds, the diamond coating most often was automatically released during the cooling process due to large difference in thermal expansion and low adhesion. When necessary the fused silica was removed by HF. For silicon moulds, on the other hand, it is always necessary to sacrificially etch the silicon away. Due to the good chemical resistance of diamond, the most aggressive and fast etching solutions can be used. An isotropic etch
Fig. 4. Principle sketch of HFCVD set-up.
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H. Bjorkman et al.r Sensors and Actuators 78 (1999) 41–47 ¨
Fig. 5. Raman spectra from the diamond–silicon interface and from the diamond top-side.
Fig. 6. LOM micrograph of a diffractive optics device.
H. Bjorkman et al.r Sensors and Actuators 78 (1999) 41–47 ¨
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2.4. DeÕice characterisation Raman spectroscopy was used to characterise the quality of the deposited coatings. The thickness and the filling properties of the coatings were investigated in a SEM and in a light optical microscope ŽLOM.. A CCD camera was used to estimate the etch-rate in the capillaries.
3. Results Raman spectroscopy performed on the deposited coatings displayed a sharp diamond peak at 1332 cmy1 and a
Fig. 8. Ža. SEM micrograph of the active cooling device. Žb. Close-up of the cooling capillaries in Ža..
low and broad amorphous carbon band around 1550 cmy1 indicating good diamond quality, Fig. 5. 3.1. DiffractiÕe optics deÕice The diffractive optics device consisted of an array of 30 = 30 diffractive off-axis microlenses. Typical relief heights and feature sizes of the off-axis lens were 0.5 mm and 10–100 mm, respectively, see Fig. 6. 3.2. Laser-to-fibre alignment deÕice Fig. 7. Ža. SEM micrograph of a laser-to-fibre alignment chip. Žb. Principle of the laser-to-fibre alignment chip. Žc. The laser-to-fibre alignment chip in cross-section.
The laser-to-fibre devices were designed so that the centre of a 125-mm-diameter fibre would be aligned to the laser emission point, see Fig. 7. The edge of the trench was
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sharp as required for minimised distance between laser and fibre. Furthermore, the section of the carrier surface closest to the laser emission point is slightly lower than the rest of the surface. By positioning the laser so that its conducting flip-chip SnrAu bumps are placed in this depression, the good contact between the laser and heat conducting diamond can be achieved, Fig. 7Žc..
diamond’s combined properties such as electrical insulation, heat transport, and optical transparency makes it appealing for the applications suggested here.
3.3. ActiÕe cooling deÕice
We gratefully acknowledge Fredrik Nikolajeff for fabricating the fused silica moulds. The work was carried out within the Centre for Advanced Micro Engineering ŽAME., and SUMMIT, financed by the Swedish Foundation of Strategic Research ŽSSF., and by the Swedish National Board for industrial and Technical Development ŽNUTEK., respectively.
The active cooling device consisted of an in- and an outlet leading to an array of 10 capillaries with 70 = 70 mm inner dimensions covering an area of 1 cm2 . A CCD camera was used to monitor the etch depth and etch rate in the active cooling device capillaries. The insides of the tubes were totally etched producing a continuous network of channels through the cooling device. The etch rate proved to be 3 mmrh. For different views of the active cooling device, see Fig. 8.
4. Discussion With fused silica as mould material, diffractive optics devices in diamond could be produced and used for direct micro-optical applications. With the replication technique, the surface roughness of the final product was comparable to the master material w2x. If the rough side of the device was polished w17,18x, the device could be used as a transmissive, as well as a reflecting, micro-optical element. Obviously, we intend to investigate the optical performance of the produced devices closer in the near future. Using structured SOI wafers as a replicating mould for diamond deposition makes it possible to create shapes that can be very useful for, e.g., laser-to-fibre alignment. Diamond’s good heat conductivity makes it ideal for the task. The two-step SOI diamond deposition process enables the creation of long and precise capillaries or capillary arrays with a flat top-side, here intended as high power electronic device carriers. Future research will focus on polishing the diamond optical devices, e.g., to make transmissive micro-optical elements. We will also mount lasers and fibres on the laser-to-fibre alignment device. Finally, connecting the fluidic devices to pumps is a future goal.
5. Conclusions The devices show that diamond microstructures produced by replication from silicon moulds have a large potential for optical MEMS. The possibility to replicate silicon and fused silica, in combination with the chemical inertness to most etching solutions, makes it possible to produce shapes that are not otherwise possible to fabricate in silicon or other common materials for MEMS. Also,
Acknowledgements
References w1x K.E. Spear, J.P. Dismukes, Synthetic Diamond: Emerging CVD Science and Technology, Wiley, New York, 1994. w2x H. Bjorkman, P. Rangsten, P. Hollman, K. Hjort, Diamond replicas ¨ from microstructured silicon masters, Sensors and Actuators 73 Ž1999. 20–29. w3x M.Y. Mao, T.P. Wang, J.F. Xie, W.Y. Wang, The fine patterning of diamond thin film, Proceedings IEEE Micro Electro Mechanical Systems, Amsterdam, Netherlands, 1995, pp. 392–393. w4x W.P. Kang, J.L. Davidsson, Q. Li, J.F. Xu, D.L. Kinser, D.V. Kerns, A novel low field electron emission polycrystalline diamond microtip arrays for sensor applications, Transducers ’95 and Eurosensors IX, Stockholm, Sweden, June 25–29, 1995. w5x W. Kulisch, A. Malave, G. Lippold, C. Mihalcea, E. Oesterschulze, Fabrication of integrated diamond cantilevers with tips for SPM applications, Diamond and Related Materials 6 Ž1997. 906–911. w6x Ph. Niedermann, W. Hanni, N. Blanc, R. Christoph, J. Burger, ¨ Chemical vapor deposition diamond for tips in nanoprobe experiments, Journal of the Vacuum Science Technology A 14 Ž3. Ž1996. 1233–1236. w7x R. Gahlin, H. Bjorkman, P. Rangsten, S. Jacobson, Designed abra˚ ¨ sive diamond surfaces, to be printed in Wear. w8x M. Aslam, D. Schulz, Technology of diamond microelectromechanical systems, Transducers ’95 and Eurosensors IX, Stockholm, Sweden, June 25–29, 1995. w9x J.D. Hunn, S.P. Withrow, C.W. White, R.E. Clausing, L. Heatherly, Fabrication of single-crystal diamond microcomponents, Applied Physics Letters 65 Ž24. Ž1994. 3072–3074. w10x M.G. Jubber, A.J. McLaughlin, J.H. Marsh, J.S. Aitchison, P. John, C.E. Troupe, J.I.B. Wilson, Micromachined pattern transfer into CVD diamond, Diamond and Related Materials 7 Ž1988. 1148–1154. w11x E. Kohn, P. Gluche, M. Adamschik, Diamond MEMS—A New Emerging Technology, Diamond ’98, Crete, Greece, 14–18 September 1998. w12x V.J. Konov, V.G. Ralchenko, S.M. Pimenov, A.A. Smolin, T.V. Kononenko, Laser microprocessing of diamond and diamond-like films, Proceedings of SPIE, Vol. 2045, pp. 184–192 w13x M.N. Touzelbaev, K.E. Goodson, Applications of micron-scale passive diamond layers for the integrated circuits and microelectromechanical systems industries, Diamond and Related Materials 7 Ž1988. 1–14. w14x P.P. Deimel, Micromachining processes and structures in microoptics and optoelectronics, Journal of Micromechanics and Microengineering 1 Ž1991. 199–222. w15x F. Nikolajeff, S. Hard, ˚ B. Curtis, Diffractive microlenses replicated
H. Bjorkman et al.r Sensors and Actuators 78 (1999) 41–47 ¨ in fused silica for excimer laser-beam homogenizing, Applied Optics 36 Ž1997. 8481–8489. w16x M. Rothchild, Optimal materials for eximer laser applications, Optics and Photonics News, May 1993, pp. 8–15. w17x S. Jin, J.E. Graebner, T.H. Tiefel, Thinning and polishing of diamond films by diffusional reaction with metals, Proceedings of SPIE Diamond Optics V 1759 Ž1992. 57. w18x A.M. Zaitsev, G. Kosaca, B. Richarz, V. Raiko, R. Job, T. Fries, W.R. Fahrner, Thermochemical polishing of CVD diamond films, Diamond and Related Materials 7 Ž1988. 1108–1117. Henrik Bjorkman was born in 1970. He received his MSc in Materials ¨ Physics at Uppsala University, Sweden, in 1994 and has been a PhD ˚ student in the microstructure technology group at the Angstrom ¨ Laboratory, Uppsala University since the autumn of 1995. His PhD work is primarily focused on microstructuring of diamond, mainly using HFCVD and the replicating technique. Moreover, his work also involves diamond wear and characterisation as well as silicon micromachining. Pelle Rangsten was born in 1968 and has been a PhD student since 1994 ˚ in the micromechanics group at the Angstrom ¨ Laboratory, Uppsala University, Sweden. The subject of his work is different microfabrication techniques in silicon, quartz, and diamond. The research related to silicon concerns device and system solutions mainly in the medical field. Considering quartz, more fundamental manufacturing processes are investigated, such as wet chemical etching and direct bonding. His present research is focused on diamond as a material for microstructures.
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Klas Hjort was born in Sater, Sweden, 1964. He received the MSc in ¨ Engineering Physics at Uppsala University in 1988 and the PhD ŽEng.., Materials Science, Uppsala University, in 1993. In 1994, he was a post-doctorate at the Institute of High Frequency Electronics, TH Darm˚ stadt, Germany. He returned to the Angstrom ¨ Laboratory, Uppsala University, in 1995 as Assistant Professor. In 1997 he became Associate Professor at the Department of Materials Science, Uppsala University. Since 1988, basic mechanical characterisation, surface and bulk microstructuring, solid state bonding, and device fabrication of bulk acoustic wave microstructure devices based on GaAsrAlGaAs have been among his interests. His present research areas are InP based micromechanics Žmechanical characterisation, bulk and surface micromachining, wafer bonding, and optoelectronic MEMS., diamond microstructures, ion track micro technology like micromachining by ion track etching ŽMITE., resonant quartz micro structures, and polymeric microstructures.
