SURFACE TENSION POWERED SELF-ASSEMBLY OF 3D MOEMS DEVICES USING DRIE OF BONDED SILICON-ON-INSULATOR WAFERS R.R.A Syms, C. Gormley and S. Blackstone Dept. of Electrical and Electronic Engineering, Imperial College, Exhibition Road, London, SW7 2BT BCO Technologies (NI) Ltd., 5 Hannahstown Hill, Belfast, BT17 OLT INTRODUCTION Surface tension powered self-assembly is a technique for mass parallel fabrication of 3D microelectro-mechanical systems (MEMS) from surface micromachined parts, which are rotated outof-plane by the surface tension of pads of a meltable material 1. Recently, we have demonstrated a simple two-mask process based on mechanical parts formed from 4 industry-standard bonded silicon-on-insulator (BSOI) wafers and meltable pads of thick photoresist, Hoechst AZ4562 2,3. Here, we describe enhancements obtained by deep reactive ion etching (DRIE) of the parts using an inductively coupled plasma (ICP), introduce improved hinge designs, and demonstrate a new range of self-assembling mechanisms. The process is first described, and the influence of critical fabrication steps such as lithography and etching on uniformity, yield and accuracy are discussed. Applications in micro-opto-electro-mechanical systems (MOEMS 4 ) are then demonstrated. FABRICATION PROCESS Figure 1 shows the process, which is based BSOI material fabricated at BCO and consisting of 4 (100) Si substrates carrying 5 µm thick bonded Si layers on 2 µm oxide. In previous work, parts have been formed in the bonded layer by conventional RIE using a Cr hard mask. Replacing this process with DRIE allows feature sizes to be reduced from ca 5 µm to 2 µm. This improvement in dimensional control increases the accuracy of the assembled structure. The use of a stop-onoxide etch increases uniformity and reduces the likelihood of failure by resist adhesion to the substrate.
To define the parts, the wafers were patterned by photolithography, using a Quintel Q4000-IR aligner. The surface pattern was then transferred to the bonded layer by deep reactive ion etching in a Surface Technology Systems Single-Chamber Multiplex ICP Etcher, using the BCO/STS Advanced Silicon Etch, a stop-on-oxide DRIE process. The resist mask was then stripped. The ASE process uses alternating cycles of ICP etching and passivation at ca 25 mtorr pressure to etch silicon to depths > 200 µm at high rates and with excellent sidewall verticality. In the etch step, sulphur hexafluoride is used to remove silicon by dissociating SF 6 into fluorine radicals. Although the etch process is isotropic, lateral erosion is prevented by a short polymer deposition step after each etch. This forms a layer of passivation (C x F y ) on the surface of the feature by ionisation and dissociation of octafluorocylcobutane (C 4 F 8 ). To re-initiate etching, fluorine radicals first etch the base of the passivation, and then the silicon itself. Although cyclic processing leads to sidewall scallops, these can be minimised by careful adjustment of the etch parameters. To form the meltable pads, the etched wafers were cleaned in fuming nitric acid to promote adhesion, spin-coated with Hoechst AZ4562 photoresist, pre-baked at 90 C, exposed using the Quintel aligner, and developed in Hoechst AZ400K developer (1 : 4 in DI water) for 6 minutes. A spin speed of 1400 rpm gave hinge driver pad thicknesses of 11.8 µm. To free the mechanical parts, the buried oxide was then removed by etching for 10 hours in 7 : 1 buffered HF, which penetrated through 4 µm square holes in the movable parts. To ensure adhesion of the resist pads during such a long etch, they were premelted at 100 C for 30 mins. Previous demonstrations have used freeze-drying in a water/methanol mixture to dry the structures without surface tension collapse. However, it has been found that most organic liquids (including cyclohexane, methanol, propan-1-ol, propan-2-ol, propylene carbonate etc.) either dissolve the resist pads or weaken their adhesion. The following alternative procedure was therefore employed. Samples were simply placed faced-down in open dishes containing ultrapure distilled water, frozen solid, and then freeze-dried in an Edwards Modulyo freeze-drier. The
dryer was then vented to dry N 2 while warming the substrates to avoid surface tension collapse caused by condensate. Assembly of the structure into its final three-dimensional configuration was carried out by melting in a convection oven for 6 minutes at 145 C. To improve reflectivity and provide electrical connection across the insulating resist pads, devices were then sputter-coated with Au metal. PROCESS AND DESIGN IMPROVEMENTS Previous demonstrations of resist-powered self-assembly have suffered from low yield, caused by a) detachment of the movable parts, b) stick-down, and c) poorly-controlled rotation rates. We have found that the former problem may be substantially eliminated by perforating the silicon lands on either side of the hinge, so that the resist is effectively pinned to the surface of both parts as shown in Figure 2a. Stick-down of structures with component sizes of order 1 mm may be prevented by careful control of the release step, as discussed. Rotation rates may be controlled by improving pad-to-pad uniformity. This is achieved by ensuring that the etched Si surface contains few open areas, so that the resist spin-coating process may accurately planarize the surface. 3D MOEMS components have previously been based on parts rotated 45 out-of-plane. Rotation is powered by melting the resist pads, and the geometry of the assembly is fixed by a mechanical limiter. Figure 2b shows the mechanism used, which involves simultaneous rotation of two parts in opposite directions. Catches on the parts engage to prevent further rotation when each has rotated through 45. The accuracy of the mechanism is high, because it involves a long lever arm, and because the construction is determined by the geometric layout of the parts. However, it is relatively bulky, and is therefore suitable mainly for assembly of major component frames. Simpler mechanisms can be used to set the final angles of smaller components. Figure 2c shows a limiter based on two cranks attached to the moving part close to the hinge. The cranks prevent
further motion after they have reached the substrate. The accuracy of this mechanism is low, because the lever arm is now very short, and because the construction is determined by layer thicknesses in addition to geometric layout. However, it is sufficient for subcomponent assembly. 3D MOEMS COMPONENTS The improvements described above have increased the yield of self-assembled 3D structures from ca 5% to ca 70%, and allowed the construction of more sophisticated MOEMS devices. In particular, improved adhesion of mechanical parts has allowed the span of 45 micromirrors to be increased to 1 mm (Figure 3). Similarly, improvements in the definition of the parts has allowed the angular error of 45 rotated structures to be reduced to ca ± 4 and the finger gap in comb drive electrodes to be reduced 2 µm. Torsion mirror scannners have previously been demonstrated by polysilicon surface micromachining, using electrostatic 5,6 and electrothermal 7 drives. In each case, an indirect drive was used, in which the mirror was driven by an actuator on the substrate via a hinged link. Direct actuation, which allows high-q operation, has been demonstrated by surface tension selfassembly, using a skewed electrostatic drive 3. However, the skewed electrode layout resulted in a high drive voltage, because of the weak electrostatic field. Furthermore, because the fixed electrodes lay between the moving electrodes and the substrate, there was a trade-off between the drive voltage and maximum scan angle; reducing the drive voltage by extending the moving electrodes restricts the angle through which the mirror can rotate before it strikes the substrate. These limitations have been overcome by rotating the fixed drive electrodes out-of-plane by an additional self-assembly operation, using the below-substrate limiter of Figure 2c to set their final angle. Repositioning the fixed electrodes also allows the construction of electrostaticallydriven mirror scanners with different rotation axes, as shown in Figure 4. These may be driven directly through a wide angle without the electrodes clashing, and also have high Q-factors.
Time-sequential surface tension self-assembly allows the demonstration of 90 rotated structures. The limiter mechanism of Figure 2b is first used to construct a stop in mid air above the substrate, which then prevents further rotation of an additional component when it has rotated through 90. These structures may act as fixed mirrors; however, they may also carry other optical components. For example, Figure 5 shows arrays of 80 µm diameter refractive microlenses, which have been formed by reflow molding of photoresist 8 without introducing any additional process complexity. CONCLUSIONS We have shown that the use of deep reactive ion etching and improved design allows a substantial increase in the yield and complexity of surface tension self-assembled 3D MOEMS components. REFERENCES 1. Syms R.R.A., Yeatman E.M. "Self-assembly of fully three-dimensional microstructures using rotation by surface tension forces" Elect. Lett. 29, 662-664 (1993) 2. Syms R.R.A., Blackstone S. 3-D self-assembly of optomechanical structures using bonded SOI 1999 Annual Meeting of the Electrochemical Society, Honolulu, Hawaii, Oct. 17-22, paper 1026 (1999) 3. Syms R.R.A. Surface tension powered self-assembly of 3-D micro-optomechanical structures IEEE/ASME J. Microelectromech. Syst. 8, 448-455 (1999) 4. Wu M.C., Lin L.-Y., Lee S.-S., Pister K.S.J. "Micromachined free-space integrated micro-optics" Sensors and Actuators A50, 127-134 (1995) 5. Tien N.C., Solgaard O., Kiang M.H., Daneman M., Lau K.Y., Muller R.S. "Surface micromachined mirrors for laser-beam positioning" Sensors and Actuators A52, 76-80 (1996) 6. Kiang M.-H., Solgaard O., Muller R.S., Lau K.Y. Micromachined polysilicon microscanners for barcode readers IEEE Photon. Tech. Lett. 8, 95-97 (1996) 7. Butler J.T., Bright V.M., Reid J.R. Scanning and rotating micromirrors using thermal actuators Proc. SPIE 3131, 134-144 (1997) 8. King C.R., Lin L.Y., Wu M.C. Out-of-plane refractive microlens fabricated by surface micromachining IEEE Photon. Tech. Lett. 8, 1349-1351 (1996)
BSOI wafer Etch oxide Pattern; DRIE bonded layer Freeze dry Spin-coat resist Pattern resist Silicon Thermal oxide Photoresist Premelt resist Figure 1. Melt resist Fabrication process for surface tension powered 3D microstructure self-assembly. Fixed land Hinge driver Moving part Fixed land Latch #1 Fixed land Crank Moving part Hinge driver Latch #2 Keying Buried oxide Resist Bonded layer Latch #1 Latch #2 Substrate Figure 2. a) Keyed hinges; b) above and c) below substrate latches for 45 rotated structures.
Figure 3. 1 mm x 1 mm self-assembled 45 mirror, showing detail of mechanical limiter. Figure 4. Self-assembled torsion mirror scanner with self-assembled drive electrodes. Figure 5. Self assembled 90 rotated structure carrying reflow molded collimating lenses.