Scream'03: a Single Mask Process for High-q Single Crystal Silicon Mems

We present a single-mask single-crystal silicon (SCS) process for the fabrication of suspended MicroElectroMechanical devices (MEMS). This is a bulk micro-machining process that uses Deep Reactive Ion Etch (DeepRIE) of a silicon-on-insulator (SOI) substrate with highly doped device layer to fabricate movable single-crystal silicon MEMS structures, which can be electrically actuated without metal deposition. The buried oxide layer is not removed afterwards and no wet process release is involved in the whole process sequence, which makes this process different from others works based on SOI wafer. Several MEMS oscillators have been made using this process. Dynamic behavior is characterized using a laser vibrometer. Quality factor is improved by more than 1 order compared to the same oscillator made using SCREAM process. INTRODUCTION In some MicroElectroMechanical System (MEMS) application including accelerometers, high-aspect-ratio structures are desired to avoid unwanted out of plane movement. Such structures can be fabricated using bulk micromachining methods by etching into a silicon wafer or siliconon-insulator (SOI) wafer using either wet chemical etching or dry reactive ion etching (RIE). SCREAM (Single-Crystal Reactive Etching And Metallization) [1] is one of the bulk micromachining processes, based on reactive ion etching (RIE) to fabricate suspending structures. Using silicon deep etching technique, SCREAM can make high-aspect-ratio, suspending silicon MEMS structures. Since RIE is independent of crystallographic orientation, more flexibility can be achieved compared to other bulk micromachining methods using wet etching. Here are the key steps of SCREAM process flow. After pattern is transferred to the silicon oxide layer on top of a silicon wafer, the substrate is deep etched anisotropically (BOSCH process) to certain depth to form the MEMS structure. A conformal layer of PECVD or thermal oxide is deposited to protect the structure from release etch. This oxide on the floor of gap is etched away using RIE, leaving the oxide on sidewall as a protection layer. After 1 another deep anisotropic etch (BOSCH), the structure is released using isotropic RIE etch. To be able to electrically actuate the device, a metal layer, such as gold or aluminum, is sputtered on the surface. The undercut in the anchors formed during the release step electrically isolates different parts of the structure. Figure 1 shows a typical cross sectional profile of a microbeam fabricated using SCREAM process. It is a sandwiched structure with silicon core inside, silicon oxide and metal layer outside. In vibration, the lattice mismatch of different materials in the interface consumes energy from oscillation [2]. There can be defects in the interface because of the bonding nature (PECVD and sputtering) and from the undercut after release, which can cause extra energy dissipation. Therefore it is difficult to achieve high quality factor using SCREAM process. Quality factor of many of our devices made using SCREAM process falls less than 1000 in about 10 mTorr. This sandwiched structure may also limit the further minimization of the MEMS devices. Certain thickness of silicon oxide is needed to protect the sidewall to make highaspect-ratio structures. To guarantee good metal coverage, there is also certain gap limits between structures. This may sacrifice the capability to sense and actuate the motion. Stress problem can appear in MEMS structure fabricated using SCREAM process, especially when the size of the structures in sub micron level. The stress accumulated in the different process, such as conformal coverage of silicon oxide protection layer, can cause the MEMS structure to buckle. As SOI wafer gets popular, more and more efforts in bulk micro-machining has been put in making MEMS device out of SOI wafer [3-5]. The process is very straightforward. After patterning, a deep silicon etch process, such as BOSCH, is performed to form the micro structure, following by wet chemical release of the buried silicon oxide layer to form the suspended MEMS structure. The depth of the device is defined by the thickness of the top device layer. A pure single crystal silicon device can be made using SOI wafer. The thickness of Copyright © 2004 by ASME the device is more uniform and the quality factor can be much higher than that of SCREAM process. However, stiction can occur after the final wet chemical etching release process. A critical drying step might be necessary to avoid stiction problem, especially for sub micron devices. Furthermore, the gap between the released structure and the substrate is decided by the thickness of silicon oxide isolation layer, which is usually less than 2 μm, and this clearance might not be large enough in some applications, such as in an in-plane moving oscillator, where the oscillator can easily touch the substrate because of the coupling of out-of-plane mode shape. In this paper, we present a new bulk micro-machining process, SCREAM’03, to fabricate a pure single-crystal-silicon MEMS oscillator. This novel process combines SCREAM process with SOI wafer to make pure single crystal silicon MEMS devices. The structure is released using isotropic RIE etch and no wet release is necessary. No silicon oxide sidewall protection is needed in the release step and stress problem caused by thermal expansion mismatch among different materials is avoided. Since the structure is made of singlecrystal silicon, higher Q is expected compared to SCREAM fabricated devices. The paper is organized as follows. After introduction, we describe this new process flow in detail, following by design and fabrication rules. Then MEMS devices fabricated using SCRAM’03 and SCREAM are described and dynamic characterizations are shown and compared. Before conclusion, we discuss some key issues related to this novel process. PROCESS OUTLINE SCREAM’03 process flow is shown in Fig.2. We start with a SOI wafer with highly doped device layer (ρ~0.01ohm-cm) Figure 1 cross sectional profile of a MEMS beam fabricated using SCREAM process. 2 (Fig.2(1)). After standard optical photolithography (Fig.2(2)), the patterned wafer is deep etched using BOSCH process to the desired depth (Fig.2(3)), followed by deposition of a thick layer of polymer (fluorocarbon) [6] (Fig.2(4)), which is used in BOSCH process for sidewall passivation. This polymer layer protects the structure from later isotropic release etch. An extension etch (another BOSCH process) is performed to etch the polymer on the floor and silicon left in the device layer (Fig.2(5, 6)). The device is released by isotropic etch of the unprotected silicon (Fig.2(7)) using SF6 ICP, the same step as in SCREAM. The polymer on the sidewall and photo resist left are finally removed using oxygen plasma etch (Fig.2(8)). Here, we use polymer to protect the sidewall of structure from release. By changing the time of polymer deposition cycle, polymer thickness can be adjusted as required. To provide enough protection of the structure, the thickness need to be characterized. Since polymer (fluorocarbon) is a soft material, stress problems caused by thermal expansion in SCREAM process can be avoided. We use SOI wafer with highly doped device layer instead of normal silicon wafer. Therefore, the device is electrically conductive and can be actuated without extra oxide isolation and metal deposition. The buried silicon oxide layer is not removed afterwards and provides good isolation among Figure 2 Process flow of SCREAM’03. (1) Deposit photo resist, (2) Pattern, (3) BOSCH deep etch, (4) Deposit polymer, (5) Floor clean polymer, (6) Extension etch, (7) Isotropic release, (8) O2 plasma cleaning. Step (3)-(7) can be performed in Deep Etcher in one run. Copyright © 2004 by ASME different parts. The polymer and photo resist left can be easily cleaned using O2 plasma. So the fabricated device is composed of “pure” single-crystal silicon structure. In SCREAM’03 process, the suspended structure is released using dry etch instead of wet HF release. Stiction problem, which happened often in SOI bulk micro-machining processes and so need special treatment, is avoided. It is possible to fabricate devices with sub-micron features. There is much larger clearance between the moving structure and the substrate than traditional SOI process, which makes large or flexible in-plane moving devices less possible to fail. And outof -plane moving device can have more space to move without affected by the substrate. Except for patterning step and oxygen plasma cleaning step, we program all other steps together in one run in the DeepEtcher. Unlike SCREAM process, which requires equipments with special capabilities, such as conformal coverage of silicon oxide and metal, the whole process of SCREAM’03 requires only three equipments in our fabrication lab, Stepper, Deep Ether, and Rie etcher for oxygen plasma cleaning. DESIGN RULES To successfully fabricate single-crystal silicon MEMS device with SCREAM’03 process, certain design rules should be followed. 1 Beams in moving components should generally be of the same or close width. Wider beams may need more time to release than narrow ones, which will result in more undercut. Therefore, non-uniform beam width may ends out with nonuniform device depth. 2 Because of loading effect, beams with wider gap will etch faster than those with dense structures. This will result in non-uniform device depth as well. Design dummy blocks in open spaces can solve this problem. 3 The fixed parts, such as anchors, should be designed much wider or attached to larger structures, such as fixed fingers, so that those parts will not be etched away after RIE release.