Surface Micromachined Devices for Microwave and Photonic Applications

Transcription

1 Invited Paper Surface Micromachined Devices for Microwave Photonic Applications M. Frank Chang Ming C. Wu Electrical Engineering Department University of California, Los Angeles Los Angeles, California J. Jason Yao M. Edward Motamedi Rockwell Science Center 1049 Camino Dos Rios Thous oaks, California ABSTRACT As an enabling technology, Micro Electro Mechanical Systems (MEMS) have continuously provided new improved design/implementation paradigms for a variety of scientific engineering applications. In this paper, we review recent advances made in MEMS its derivative MOEM (Micro Opto Electro Mechanical) devices for both microwave photonic applications. In area of microwave, MEMS switch has been regarded as one of most crucial components. The developed switch uses a suspended silicon dioxide micro-beam as cantilever arm, a platinum-to-gold electrical contact, electrostatic actuation as switching mechanism. It functions from DC to microwave frequencies has an excellent electrical isolation (>-50 db at 4GHz >-25 db at 40 GHz) minimum insertion loss (< 0. 1 db at 4 GHz <0.5dB at 40 GHz). Compared to its semiconductor counterparts, MEMS switch is superior in both performance power consumption. In area of photonics, microoptical components, such as diffractive refractive microlenses, micromirrors, beam splitter beam combiners have received considerable attention recently. Optical systems that once were considered to be impractical due to limitations of bulk optics can now easily be designed fabricated with all required optical paths, signal conditioning, electronic controls, integrated on a single chip. On-chip optical processing will enhance performance of devices such as focal plane optical concentrators, beam shapers, Fiber Data Distribution Interface (FDDI) switch, miniature tunable 3-D Fabry-Perot Etalon. In this paper we review advances in MEMS switches microoptical components developed at Rockwell Science Center. We also review potential of on-chip optical processing recent achievement of free-space integrated optics microoptical bench components developed at UCLA. Keywords: MEMS, microwave swtches, microoptics, MOEM, actuators, integration, mirror, microlens, FDDI, 3-D Fary-Perot. 1. INTRODUCTION The continuous growth of MEMS has offered paradigm shifts in miniature device design for a variety of scientific industrial applications. Two major application areas will be addressed in this paper: microwave MEMS switches photonic MEOM devices. Switches are widely used in microwave systems subsystems for signal routing, impedance matching, amplifier gain adjustment. Traditional solid state switches have a significant insertion loss (typically >1 db) in Part of SPIE Conference on Optoelectronic Materials Devices Taipei, Taiwan. July SPIE Vol X/98/$1O.OO

2 'On' state a poor isolation (typically < -30 db) in 'Off' state even at low microwave frequencies (about 1 GHz)1. Here we describe design, fabrication performance of a MEMS switch which can hle broadb microwave signals (up to 40 GHz), while maintaining a minimal insertion loss an excellent electrical isolation. On or h, microoptical components, such as diffractive refractive microlenses, have recently received considerable attention for development of optical systems3. In first generation of miniaturized optical system, microoptical components were hybridized with electronics circuits in some cases with movable components such as scanning mirrors piezo-actuators. As MEM MOEM technologies continue to advance, miniaturized optical systems may also advance to nearly monolithic systems. Integrated devices based on MOEM systems can form free space integrated optics consequently push technology to development of a microoptical bench-on-a-chip. 2. MICROWAVE SWITCH 2.1 Switch Design The MEMS switch is designed for applications with signal frequencies from DC to 40 GHz. Figure 1 shows a schematic illustration of a surface micromachined switch design. Figure 2 contains a topview micrograph of a fabricated MEMS switch. Top E'ectrode Cantflever Sign& Line (Gold) (Not to scale) Contact (Gold) Fig. 1 A schematic illustration of a micromachined RF switch design As shown in Fig. 1, for general design of MEMS switches, silicon dioxide cantilevers are typically between 100 jim 200 jim in length, 10 jim in width, 2 jim thick. The grid capacitor structures in Fig. 2 have an overall areaof 200 jim by 200 jim. The gap between bottom of silicon dioxide cantilever metal lines on substrate is 3 jim. The gold microstrip signal lines are 2 jim thick, between 20 jim 40 jim in width. The gold contact metal is 1 jim thick. The contact area is on average 400 pm2. 215

3 Topviews of RF switch designs 300 pm < Fig. 2: Micrograph image showing topview of a fabricated RF switch. At low RF frequencies, insertion loss is dominated by resistive loss of signal line which includes resistance of signal line contact resistance. At higher frequencies, insertion loss can be attributed to both resistive loss skin depth effect. To minimize resistive loss, a thick layer (2 rim) of gold is used. The width of signal lineis a more limited variable than thickness because wider line, lower insertion loss, but also poorer 'Off' state electrical isolation due to increased line capacitance. Electrical isolation of switch also depends on capacitive coupling between signal lines substrate. Therefore, semi-insulating GaAs substrate is chosen for RF switch over a conductive silicon substrate. GaAs substrates are used instead of or insulating substrates such as glass so that RF switch may retain its monolithic integration capability with MMICs. The capacitive coupling between signal lines may be reduced by increasing gap between signal lines on substrate gold contact metal on bottom of suspended silicon dioxide cantilever (Fig. 1). However, this increase in gap also increases voltage required to actuate switch because gap distance also determines actuation capacitance. The aluminum top metal of capacitive actuator couples to underlying gold ground (GND) metallization across this gap. For a fixed gap distance, voltage required to actuate switch may be reduced by increasing actuation capacitor area. However, this increase in capacitor area increases overall mass of suspended structure thus closure time of switch. If stiffness of suspended structure is increased to compensate increase in structure mass to maintain a constant switch closure time, required voltage to actuate switch will be furr increased. In managing tradeoffs between device parameters for applications in which RF switches will be used, insertion loss electrical isolation are given highest priority, followed by closure time, finally actuation voltage. The grids in actuation capacitor structure introduced in designs shown in Fig. 2 reduce structural mass while maintaining overall actuation capacitance by relying on fringing electric fields of grid structures. A second reason for use of grid structures is to reduce atmospheric squeeze damping between cantilevered paddles substrate. Switches without grid design were observed to have a much larger closure opening time due to squeeze damping effect. 2.2 Switch Fabrication The RF switches are manufactured using a surface micromachining technique with a total of five masking levels2. PECVD silicon dioxide is deposited as structural material, polyimide is used as sacrificial material. Figure 3 is a cross- 216

4 sectional schematic illustration of process sequence. The low process temperature budget of 250 C ensures switch's monolithic integration capability with MMICs. Starting from a 3-inch semi-insulating GaAs wafer, a layer of rmal setting polyimide (DuPont P12556) is spun on cured via a sequence of oven bakes with highest baking temperature of 250 C. A layer of pre-imidized polyimide (OCG Probimide 285) is n spun on baked with highest baking temperature of 170 C. A silicon nitride layer 1500 A thick is n deposited patterned using photolithography reactive ion etched (PIE) in CHF3 02 plasma. The pattern is furr transferred to underlying polyimide layers via 02 RIE (Fig. 3a). This creates a liftoff profile similar to tn-layer resist system except that two layers of polyimide are used. A layer of gold is electron beam evaporated with a thickness equal to that of rmal set DuPont polyimide layer. The gold liftoff is completed using methylene chloride to dissolve pre-imidized OCG polyimide, leaving a planar gold/polyimide surface (Fig. 3b). The cross linked DuPont polyimide has good chemical resistance to methylene chloride. A second layer of rmal setting polyimide (DuPont P12555) is spun on rmally cross linked. A layer of 1 m gold is deposited using electron beam evaporation liftoff to form contact metal (Fig. 3c). A 2 pm thick layer of PECVD silicon dioxide film is n deposited patterned using photolithography ME in CHF3 02 chemistry (Fig. 3d). A thin layer (2500 A) of aluminum film is n deposited using electron beam evaporation liftoff to form top electrode in actuation capacitor structure (Fig. 3e). And lastly, entire RF switch structure is released by dry etching of polyimide film in a Branson 02 barrel etcher. The dry-release is preferred over wet chemical releasing methods to prevent any potential sticking problems during release. Along Bottom Electrode in Fig. la Along Signal Lines in Fig. la a) i'\.111(1 Aiuninun (kd (e _ Prelnklizeci i:)oiy nude Thenuuai Set tiai (]aas Fig. 3 An abbreviated schematic illustration of RF switch process sequence. 217

5 2.3 Microwave performance of MEMS switches The stiffness of suspended switch structure is designed to be between 0.2 N/rn 2.0 N/rn for various cantilever dimensions. The lowest required actuation voltage is 28 Volts with an actuation current on order of 50 na which corresponds to a power consumption of 1.4 iw. It requires zero power to maintain switch in eir 'On' or "Off' state due to nature of electrostatic actuation. The switch closure time is on order of 10 jis. All characterization was performed in an atmospheric ambient. The silicon dioxide cantilever has been stress tested for a total of sixty five billion cycles (6.5x 1010), no fatigue problem was observed. The maximum current hling capability for prototype switch is 200 ma with cross sectional dimensions of narrowest gold signal line being 1 pm x 20 pm. Figure 4 shows low frequency (1 khz) input output response of RF switch in 'On' 'Off' states. As shown in Fig.5, an electrical isolation of -50 db an insertion loss of 0.1 db at 4 Gllz have been achieved. When tested at 40 GHz, MEMS switch showed an insertion loss of 0.5 db isolation of -25 db. At both frequencies, MEMS switch has outperformed traditional semiconductor switches at least 10 db in isolation 1-2 db in insertion loss. Switch in 'Off' Switch in 'On' input: I KHz Fig. 4 The switch output response in (a) 'Off' (b) 'On' states to a 1 Volt input signal at 1 khz. 218

6 \.. / fl9 -'J.-' c1 - C C iii:. '- -- I IlItI Ill = ;e - e B -u ULi -r-r U Frequency (GHz) 0-10 ' -20 Ar' C -'+) (I) -50 '- -60r Fig. 5 Insertion loss isolation of RF switch from 100 MHz up to 4 GHz. 3. MICROOPTICS Microoptics technology is becoming increasingly essential to development of many optical systems. Optical components such as diffractive refractive microlens, multilevel optical gratings, beam splitters are being incorporated into advanced sensor systems. Microoptics is an enabling technology for applications that cannot be addressed using conventional optics. In following we will discuss some of se microcomponents. 3.1 Diffractive microlens A diffractive microlens is a microoptical component as small as a few tens of a micron in diameter with a thickness on order of an optical wavelength. The microlens speed can be as fast as f/o.3 in air for high index materials such as silicon GaAs about f/i for quartz glass substrates. A diffractive microlens is an approximation of a kinoform, or continuous diffractive lens. The kinoform lens structure can be approximated by multilevel lithography stepwise etching. To fabricate a multistep process, binary optic design is considered Fig.6. Binary optics will reduce number of lithographic masks by a factor of 2m/m, where m is number of required masks. r, r-i r n nnnn I 1 nnn ii nn ri ri Two PHASE-LEVELSTRUCTUPE Jn 1-D J - '' - FOUR PHASE-LEVEL STRUCTURE..".... EIGHT PHASE-LEVELSURUCTUUE Fig. 6 shows a processing cycle for fabrication of an eight-level binary optic microlens. Fig. 6(a), 6(b) 6(c) illustrate process of two-phase, four-phase eight-phase microlenses5. 219

7 The spectral range of short wave infrared (SWIR) focal plane arrays (FPAs) is from rim. These arrays have numerous applications in medicine, security, industrial inspection, JR astronomy, all of which will benefit from diffractive optics. High sensitivity at high operating temperature is often important in se applications. Conventional FPAs have relatively poor sensitivity low signal-to-noise ratio at high temperatures. Integrating binary optic microlenses hybrid EPA technologies leads to a new approach to reduce detector size, while retaining a given image resolution optical collection area. The resultant detector volume reduction has led to a significant decrease in detector dark current, hence, to an increase in device performance. Recent results from Rockwell4 show that integration of microoptics SWJR HgCdTe can produce detectors that perform exceptionally at elevated operating temperatures. The detector configuration of this effort is shown in Fig. 7. Here light incident on backside of substrate is focused by a thin film binary optic into detector forming total internal reflection; mesa sides augment diodes' light-garing abilities beyond physical extent of active layer. Note that with no buffer layer, delineation of active area is incomplete, effective optical area of detector will be wavelength dependent due to shorter wavelengths being more effectively absorbed by mesa valleys which have larger area. Retaining planar geometry on top of mesa preserves passivation advantage of cap layer as a result enhances optical efficiency of system.. Planar As (p-type)implant Wide-b-gab n-type through cap layer cap layer Passivated surface Active absorbing n-type layer isolated from or diodes by mesa (nominal width 12 pm 6 pm, nominal heights - 5 pm) I - 'jransparent n-iype buffer layer ;i Focusing / Incident light ' microlens, Transparent semi-insulating CdZnTe substrate Fig.7 Planar mesa architecture, demonstrating potential for integration of microlens with EPA. 3.2 Refractive microlenses Refractive microlenses may provide an attractive, low-cost alternative to diffractive microlenses. Refractive diffractive microlenses are complementary, in some cases refractive optics is a sole solution to optical system design. For example, in case of a short wavelength optical system (? < 1 jim) requiring high speed microlenses (< f/4), diffractive optics faces process limitation (CD< 1 rim). At Rockwell, we have designed fabricated refractive microlenses by RIE ion milling, in fused quartz, bulk silicon, CdTe, GaAs, hip, GaP; by thin film deposition of Ge films on fused silica A1203. Lens diameters ranged from 30 to 500 rim, lens f numbers were in range of f/o.76 f/6. Reliable reproducible fabrication of microlens arrays depends on accurate control of photolithographic reactive ion etching processes. Photoresist microlenses can be reliably fabricated within a range of aspect ratios. Dimensional control of microlenses in solid material can n be tailored by carefully controlled reactive ion etching. Both mixtures of SF6 or 02 with CF3H were used to accurately control lens sag during etching. Figure 8 is a SEM micrograph of a InP microlens with speed off/i, 5Oim diameter centers spaced at 70 rim. 220

8 Fig.8 SEM micrograph of an f/i, 5O-.tm-diam linear microlens array fabricated in InP. 4. MICROOPTICAL MEMS: MOEM SYSTEMS Micromachining of silicon substrate has been applied to integrated optics realization of a miniature optical bench. Also, surface micromachined hinges spring-latches5 have been used to achieve monolithic fabrication of threedimensional micro-optics.6 This technology opens a new area for integrated optics in free space. Using this new technique, three-dimensional micro-optical components can be fabricated integrally on a single Si chip. The Si substrate serves as a micro-optical bench on which microlenses, mirrors, gratings or optical components are prealigned in mask layout stage using computer-aided design n constructed by microfabrication. Additional fine adjustment can be achieved by on-chip micro-actuators micropositioners, such as rotational translational stages. With hybrid integration of active optical devices, a complete optical system can be constructed, as illustrated in Fig.9. Rotatable grating Rotatable beam splitte 450 Mirror Semiconductor Fresnel lpcr Si Substrate Fig.9 The schematic diagram illustrating micromachined free-space micro-optical system on a single substrate 4.1 Microbase, micromount, microhinge design The micro-optics plates used for mirror, beam splitter, microlens array, grating, collimator are processed by surface micromachining, y are released from substrate by selectively removing sacrificial material (deposited Si02) 221

9 using hydrofluoric acid after fabrication. After release etching, poly-plates with microoptics patterns are free to rotate around hinge pins (Fig. 10). When plate is lifted up, top portion of spring-latch slides into a slot on plate, snaps into a narrower part of slot, thus preventing furr motion of plates. The torsion-spring connecting springlatch to substrate creates spring force, which tends to force spring-latch back to substrate, refore locking plate in its place. The length of spring latch defines angle between plate substrate. After threedimensional micro-optical element is assembled, a layer of gold is coated on lifted poly surfaces. In binary-amplitude Fresnel zone plates or micro-mirrors, a thick layer of gold is needed to completely block light passing through dark. zones or to make a perfectly reflecting mirror. On or h, thinner gold is desired for partially transmitting mirrors or beam splitters. ;< # ç% ; * 3 #, :i:, - t:. :Jfft'j I* : V * St Fig.10 SEM photograph of a Fresnel microlens with precision lens mount 4.2 Fresnel microlenses Figure 1 1 shows a SEM photograph of a three-dimensional Fresnel micro lens after assembly. The diameter of this lens is 280 pim, with a designed optical axis of 254m for passive integration of an edge-emitting semiconductor laser. The optical performance of three-dimensional Fresnel microlens has been tested by collimating a divergent beam emitted from a single mode fiber at 2c 1.3 pm. The intensity FWHM divergence angle is reduced from 5.O to 0.33 by lens. The collimated beam profile fits Gaussian shape (95%fit) very well. 4.3 Translation stage One unique feature of implementing a micro-optical bench using surface micromachining is that micropositioners microactuators can be monolithically integrated in same fabrication processes for different types of optical translators. This allows alignment of optical systems to be fine adjusted, in addition to coarse alignment done at design stage using CAD layout tools. UCLA researchers have successfully integrated three-dimensional micro-optical elements with rotational stages using this process. Figure 10 shows a SEM photograph of a rotatable mirror. The rotatable plate is fabricated on first polysilicon layer, axis hub are defined on second polysilicon layer. The indicator on lower part of picture, originally pointing at O tick, has been rotated counterclockwise by 200after mirror was assembled, as shown on picture. A diffraction grating integrated with rotational stage has been successfully demonstrated using same technology, as shown in Fig

10 t$, : LI, r V ' r r 4 4 Fig.1 1 The diffraction grating integrated with a rotational stage 4.4 Potential applications With newly developed micro-bench, micro-optical components MEMS mechanical parts, we are now ready to demonsirate ir use in micro-optical systems as described as follows: D Tunable FabryPerot Tunable Fabry-Perot (FP) etalons are very useful for wavelength-division-multiplexed (WDM) optical communications, optical sensing, spectral analysis applications. There has been a great deal of interest in applying micromachining technology to realize compact tunable HP etalons, since most Fl? etalons are tuned mechanically. We demonstrated a novel three-dimensional tunable filter implemented on a surface-rnicromachined free-space microoptical bench (FS-MOB).7'8 It can be readily integrated with or microoptical elements (e.g., collimating focusing lenses, or cascaded tunable filters for WDM demultiplexers) or fiber alignment V-grooves. Both parallel-plate VP etalons solid FP etalons have been realized using FS-MOB technology. A schematic drawing a SEM micrograph of a solid FP etalon are shown in Fig. 12(a) 12(b), respectively. The transmission wavelength versus rotation angle of etalon is shown in Fig. 1 3 (a). A very broad tuning range of 58.5nm is obtained when etalon is rotated by 700. The experimental data agrees very well with a oretical analysis using transmission matrix approach. Figure 13 (b) shows transmission spectrum of etalon at 50 rotation. The finesse of etalon is measured to be 1 1, currently limited by scattering loss due to granular surface of polysilicon plate. 223

11 I' a a*_ k,m _#_ Fig. 12 The schematic SEM of tunable solid Fabry-Perot etalon. Fig. 13 The wavelength tuning range transmission spectrum of tunable filter 4.6 FDJM optical bypass switch Si bulk-micromachining wafer bonding techniques have been used to implement free-space opto-mechanical switches.9 However, monolithic integration is difficult substantial assembly is still required. Here, we propose to use surface- micromachining technology to monolithically integrate free-space microoptical switch with microactuators fiber optic alignment guides. They can be made compact light weight, are potentially integrable with optical sources/detectors controlling electronics. The switch consists of a three-dimensional movable mirror four optical fiber guiding rails, as shown in Fig. 14. Four multimode fibers are arranged in a "cross" configuration, with a movable micromirror placed in center. The switch operates in two states: CROSS BAR. When mirror/sliding-plate is moved away from fibers ( center), fibers along same diagonal directions are allowed to communicate with each or. This is defined as CROSS state. In BAR state, mirror/sliding-plate is slid into center light signal is reflected to orthogonal fiber Rotation Angte (degree) Wavelength (nm) 1350

12 Fig. 14 The SEM of three-dimensional mirror sitting on a sliding plate The optical performance of optical switch is characterized by attaching multimode fibers to guiding rails of optical fibers. The insertion loss of switch for both operating states has been measured with an LED source operating at 1.3 im wavelength. The total insertion loss of switch has been measured to be 2.8 db for CROSS state 3. 1 db for BAR state. From se two measurements, reflectivity of mirror is estimated to be 93%. The crosstalk between two states is measured to be db. The excess loss measured here includes Fresnel loss of fiber ( 0.34 db), possible misalignment of fibers. Recently, insertion losses have been reduced to 1.4 db 1.9 db, respectively, with an improved design. 5. CONCLUSIONS MEMS MOEM technologies including both microwave photonic devices have advanced significantly in past several years. These technologies are anticipated to rapidly gain in maturity become available for many microwave photonic system applications. We have presented here a few recent developments in this emerging field have described examples of devices that are being developed. Furr significant advances can be expected for on-chip microwave photonic processing as researchers recognize full range of functionalities that can be realized using se exciting new technologies. 7. REFERENCES 1. Gopinath, A Rankin, J. B. "GaAs FET RF Switches," IEEE Tans. On Electron Dev., vol. ED-32, no. 7, July 1985, pp Or Kobayashi, K. W; Oki, A. K; Umemoto, D. K; Claxton, S. K. Z; Streit, D. C. "Monolithic GaAs HBT p-i-n Diode Variable Gain Amplifiers, Attenuators, Switches," IEEE Trans. On Microwave Theory Tech., vol. 41, no. 12, December 1993, pp J.J. Yao M.F. Chang, "A surface micromachined miniature switch for telecommunication applications with signal frequencies from DC up to 4 GHz" Transducers '95, pp , June, M.E. Motamedi, B. Anderson, R. De La Rosa, W.J. Gunning, R.L. Hall, M. Khoshnevisan, "Binary Optics Thin Film Microlens Array", Proceedings, SPIE, Vol. 1544, pp , July M.E. Motamedi, W.E. Tennant, R. Melendes, N.S. Gluck, S. Park, J.M. Arias, J. Bajaj, J.G. Pasko, W.V. McLevige, M. Zian, R.B. Hall, K.G. Steckbauer, P. D. Richardson, "FPAs thin film binary optic microlens integration," SPIE Proceeding of Miniature Micro-optics Micromechanics, Vol. 2687, pp , (invited paper) 225

13 5. K.S.J. Pister, M.W. Judy, S.R. Burgett, R.S. Fearing "Microfabricated hinges", Sensors Actuators A, 33, pp , L.Y. Lin, S.S. Lee, K.S.J. Pister, M.C. Wu, "Three-dimensional Micro-Fresnel Optical Elements Fabricated," Micromachining Technique by Electronics Letters, Vol. 30 No L.Y. Lin, J.L. Shen, S.S. Lee, M.C. Wu, A.M. Sergent, "Tunable three-dimensional solid Fabry-Perot etalons fabricated by surface-micromachining," IEEE Photonics Tech. Lett., Vol. 8, no. 1, pp , J.L. Shen, L.Y. Lin, S.S. Lee, M.C. Wu, A.M. Sergent, "Surface-micromachined tunable three-dimensional solid Fabry-Perot etalons with dielectric coatings," Elec. Lett., Vol. 31, no. 25, pp , M.F. Dautartas, A.M. Benzoni, Y.C. Chen G.E. Blonder, "A silicon-based moving-mirror optical switch", J. of Lighiwave Technology, Vol. 10, No. 8, August,