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Wafer Bonding — a Powerful Tool for MEMS Indian Journal of Pure & Applied Physics Vol. 45, April 2007, pp. 311-316 Wafer bonding — A powerful tool for MEMS K N Bhat +, A Das Gupta, P R S Rao, N Das Gupta, E Bhattacharya, K Sivakumar, V Vinoth Kumar, L Helen Anitha, J D Joseph, S P Madhavi & K Natarajan* Electrical Engineering Department, Indian Institute of Technology Madras, Chennai 600 036 +Present address: ECE Department, Indian Institute of Science Bangalore 560 012 *Bharat Electronics Ltd, Jalahalli, Bangalore 560 013 Received 7 June 2006; accepted 16 October 2006 Wafer bonding techniques play a key role in the present day silicon bulk micromachining for MEMS based sensors and actuators. Various silicon wafer bonding techniques and their role on MEMS devices such as pressure sensors, accelerometers and micropump have been discussed. The results on the piezoresistive pressure sensors monolithically integrated with a MOSFET differential amplifier circuit have been presented to demonstrate the important role played by the Silicon Fusion Bonding technique for integration of sensors with electronics on a single chip. Keywords : Silicon fusion bonding, Silicon on insulator, Piezoresitive pressure sensor, MOSFET amplifier integration with sensor IPC Code : B81B7/02 1 Introduction 2 Wafer Bonding Techniques for MEMS Wafer- level bonding of a silicon wafer to another Silicon wafer 1 bonding for MEMS is achieved by silicon substrate or to a glass wafer plays a key role in several different approaches such as (1) anodic all the leading-edge Micro-Electro-Mechanical bonding, (2) direct bonding and (3) intermediate layer Systems (MEMS). When used along with the wet or bonding which includes eutectic and glass-frit bonds. dry etching techniques, the wafer bonding technique Even though, the process conditions used for all the can be used to realize (1) membranes of thickness three bonding techniques vary, the general process of varying from couple of microns to several microns, the wafer bonding follows a three step sequence suitable for pressure sensors over a wide range of consisting of surface preparation, contacting and pressures, (2) complicated three dimensional annealing. structures for accelerometers for sensing acceleration Anodic bonding involves bonding a silicon wafer and (3) multilayered device structures such as and a glass wafer with a high content of sodium. Fig.1 micropump suitable for biomedical and microfluidic shows the schematic of the anodic bonding applications, and (4) high aspect ratio structures arrangement. The anodic bonding is carried out at which can compete with the LIGA process. The 450 °C by applying a high voltage in the range manufacturers of MEMS require wafer-level bonding 500 -1000 V as shown in Fig. 1 to attract NA + ions to of one silicon wafer to another silicon substrate or a the negative electrode where they are neutralized. glass wafer. This provides a first level packaging This leads to the formation of a space charge at the solution that makes these processes economically viable. In this paper we first discuss the various silicon wafer bonding techniques and illustrate their role on MEMS devices such as pressure sensors, accelerometers and micropump. The results obtained in our laboratory on the piezoresistive pressure sensors monolithically integrated with a MOSFET differential amplifier circuit are presented to demonstrate the important role played by the Silicon Fusion Bonding technique for the integration of sensors with electronics on a single chip. Fig. 1 — Silicon –glass anodic bonding arrangement 312 INDIAN J PURE & APPL PHYS, VOL 45, APRIL 2007 glass silicon interface, thus creating a strong multiple bonding waves lead to warpage and gases electrostatic attraction between glass and silicon can be trapped in pockets formed by multiple waves, wafer, enabling the transport of oxygen from the glass and result in areas of poor bonding. After this pre- to the glass-silicon interface and converts silicon to bonding step, subsequent annealing is carried out at SiO 2 creating a permanent bond. Processes for anodic temperatures in excess of 1000 °C. During this bonding of silicon to bulk glass and silicon to annealing step, the hydroxyl groups from water 2 silicon using thin glass layer have been reported. molecules create Si-O-Si bond as hydrogen diffuses Typically Pyrex 7740 or Schott 8330 glass are away. Oxygen also diffuses into the crystal lattice to used. The Thermal Expansion Coefficient (TEC) of create a bond interface that is not distinguishable from these glasses match closely matches with the the rest of the silicon structure. Although the high TEC of silicon, resulting in low stress in the bonded annealing temperature involved in this process is a devices. drawback for some applications, the silicon fusion Silicon Direct Bonding (SDB) which is usually bonding technique permits the formation of cavities, referred to as Silicon Fusion Bonding (SFB) is used as well as all- silicon, stress free bonded structures. It for bonding two or more silicon wafers and is based has been reported 4 that surface activation methods on the initial bonding by hydroxyl radicals present on such as argon beam etching to create a clean surface the silicon wafer surfaces prepared by standard RCA 3 prior to bonding result in excellent bond strengths of clean prior to bonding . Mechanical spacers are 10-12 MPa even when the Si-Si bonding is carried out placed at the edges of the wafers as in Fig.2 (a) to at room temperature. maintain physical separation, so that pressing the Intermediate-layer bonding techniques involve middle of the wafers creates an initial point contact deposition of either metallic or glass intermediate that originates the bond . Removing the mechanical films prior to bonding and they are referred to as spacers as in Fig.2 (b) allows a single bonding wave glass-frit bonding and the eutectic bonding. The to propagate from the center of the wafers. The eutectic bonding makes use of the existence of a mechanical spacers are important in establishing a eutectic melting temperature which is considerably single bond front that propagates outward because lower than the melting point of individual constituent elements. For gold and silicon system, the eutectic melting point is 363 °C and corresponds to a eutectic composition of 3.16% silicon and 96.84% gold by weight (19 % silicon and 81% gold by atomic per cent). The eutectic bond is performed by evaporating and plating gold on to one of the silicon wafers and then exposing the gold to UV light just before bonding to remove organic contaminants that preclude gold surface contact with the second silicon wafer into which it is bonded. To accomplish good bond, the second silicon wafer surface preparation must remove any oxide film that can hamper diffusion of gold into silicon. The eutectic bonding method uses pressure applied with the wafers held at a temperature slightly higher than the eutectic temperature. A detailed optimization study 5 has revealed that maximum bond strength of 18 MPa can be achieved with the bonding temperature of 400 °C and the gold layer thickness of 1.0 µm. In the glass-frit bonding process a thin glass layer such as lead borate is deposited on the silicon substrate. The wafers are then brought into contact under pressure at the Fig. 2 — Silicon fusion bonding (a) Wafers placed in position with spacers (b) Wafers are bonded by removing the mechanical melting temperature of the glass, which is generally spacers and pressurizing < 600 °C. BHAT et al .: WAFER BONDING 313 3 MEMS Devices using Silicon Fusion Bonding actuation as follows. When an attractive potential is and Etching applied to the counter electrode, the membrane Among the various wafer bonding methods, Silicon deflects upwards, decreasing chamber pressure, thus Fusion Bonding (SFB) approach results in stress free opening the inlet check valve and drawing fluid into bonds with bond strength as high as that of silicon the chamber through the inlet port. When the voltage itself. This approach has attracted wide interest for is removed, the membrane relaxes, increasing the MEMS as well as for microelectronics and has chamber pressure, and forcing the fluid out of the opened up new avenues to realize complicated chamber through the outlet port. structures with multiple wafer bonding along with In the above structure, the reliability of operation is Deep Reactive Ion Etching (DRIE) and wet chemical limited due to the clogging of the mechanical check etching of silicon . We illustrate the impact of this valve and due to the fatigue and failure of the moving powerful technology for microstructures with part in this valve. In order to overcome this problem, examples drawn from literature as well as from the valve less micropumps have been designed and structures realized in our laboratory. reported in the literature 8. However the actuation Figure 3 shows a generalized process flow reported voltages required for the operation of the micropumps in the literature 6 for building very tall suspended reported in the literature were rather high in the range structures made entirely from single crystal silicon of 50 to 60 V. Actuation voltages need to be low for using SFB and DRIE. Figure 3(a) shows a spacer applications in drug delivery and drug dosage control cavity etched into a bottom wafer. A second silicon for biomedical applications. Figure 5 shows a wafer is bonded on to the cavity wafer by SFB and it schematic structure of the micropump we have is then thinned down to the desired thickness designed for operation below 10 V. In this structure, Fig. 3(b). This is followed by patterning as shown in the inlet and outlet to the pump chamber are realized Fig. 3(c) for DRIE. The micromechanical structure by nozzle and diffuser type dynamic valves prepared shown in Fig. 3(d) is released by etching through the by etching the silicon substrate which is bonded to the top silicon wafer by DRIE into the buried cavity.
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