Selective low temperature microcap packaging technique through flip chip and wafer level alignment

Transcription

1 INSTITUTE OFPHYSICS PUBLISHING JOURNAL OFMICROMECHANICS ANDMICROENGINEERING J. Micromech. Microeng. 14 (2004) PII: S (04) Selective low temperature microcap packaging technique through flip chip and wafer level alignment CTPan Department of Mechanical and Electro-Mechanical Engineering, and Center for Nanoscience & Nanotechnology, National Sun Yat-Sen University, Kaoshiung 804, Taiwan Received 8 September 2003, in final form 10 November 2003 Published 19 January 2004 Online at stacks.iop.org/jmm/14/522 (DOI: / /14/4/012) Abstract In this study, a new technique of selective microcap bonding for packaging 3-D MEMS (Micro Electro Mechanical Systems) devices is presented. Microcap bonding on a selected area of the host wafer was successfully demonstrated through flip chip and wafer level alignment. A passivation treatment was developed to separate the microcap from the carrier wafer. A thick metal nickel (Ni) microcap was fabricated by an electroplating process. Its stiffness is superior to that of thin film poly-silicon made by the surface micromachining technique. For the selective microcap packaging process, photo definable materials served as the intermediate adhesive layer between the host wafer and the metal microcap on the carrier wafer. Several types of photo definable material used as the adhesive layer were tested and characterized for bonding strength. The experimental result shows that excellent bonding strength at low bonding temperature can be achieved. 1. Introduction With recent trends in microelectronics moving more and more towards incorporation with MEMS devices, reducing the overall cost becomes vital. One major cost driver in today s MEMS is the packaging process. Packages for microdevices have all the same functions as for ICs but are considerably more complex. Since MEMS include moving structures, requirements for packaging of such components must take that into account. Some functions also include interaction with the surrounding environment, such as pressure sensors. Packaging should provide mechanical support to the sensitive chip. MEMS packaging includes microfabrication and wafer bonding processes. There are several wafer bonding techniques offering permanent wafer protection, such as anodic and fusion bonding. Most of these methods use a glass or silicon wafer as a passive lid over the MEMS structures. Cavities between the top lid and the MEMS wafer allow the device structures to move freely [1]. But conventional anodic and fusion [2 7] wafer bonding techniques can not be employed in all MEMS and IC packaging processes due to high temperature or high electric fields during the bonding process, which might damage IC or MEMS devices. These problems have prompted researchers to study low-temperature bonding techniques [8 10], and use eutectic bonding [11, 12]. A low temperature selective encapsulation bonding technique was presented, including localized silicon gold eutectic bonding, localized silicon glass fusion bonding, localized solder bonding and localized CVD bonding processes [13]. There are also methods such as the donor/target procedure [14], using two wafers, one for the micromechanical component and the other for the MEMS component. They are bonded together using a flip-chip technique. On the other hand, hermetic packaging of microdevices is required in many applications. Resonant, and tunneling devices, infrared and pressure sensors, voltage controlled oscillators and vacuum displays all utilize vacuum sealing to improve performance [15, 16]. Vacuum sealing can be accomplished at the package level using brazing, soldering or welding [17, 18]. Wafer-to-wafer bonding has also been employed to hermetically seal microdevices [18, 19]. However, these approaches require bonding temperatures over 220 C to finish the process. Although the temperature is much lower than that of anodic and fusion bonding process, it will still damage some of the IC and MEMS structures during the /04/ $ IOP Publishing Ltd Printed in the UK 522

2 MEMS microdevice IC Selective low temperature microcap packaging technique through flip chip and wafer level alignment bonding pad passivation treatment metal microcap host wafer heater carrier wafer heater Figure 2. EV 501 bonder with PC controller. (c) (d) Figure 1. Process flow chart of microcap package with flip chip and wafer level alignment. Host wafer and carrier wafer, flip chip and wafer level alignment in vacuum environment, (c) bonding process and (d ) microcap separated from carrier wafer and transferred to selected area. bonding process. Another encapsulation method for MEMS was developed. A glass wafer was etched to accommodate the microcap. The transparent characteristics of the glass wafer facilitate the alignment process during the bonding process. But the glass etching process is time consuming and imprecise [20]. Furthermore, for MEMS devices to be successful as commercial products, the cost of packaging must be kept down. It would be very advantageous if first-level or device-level packaging were performed in such a way that the remaining packaging procedures for MEMS chips would be the same as those for IC chips, using common procedures with existing IC packaging equipment [21]. In order to accomplish these goals, in this study an innovative wafer bonding process is proposed. It includes using flip chip and wafer level alignment to batch fabricate to lower the manufacturing cost, fabricating a microcap to protect MEMS devices, developing a passivation technique to transfer the microcap to the selected area of the host substrate, offering a lower bonding temperature and an electric field-free bonding process to protect the IC and MEMS devices from damage, and exploring various photo definable materials to find the highest bonding strength. The process flow chart of wafer level bonding using flip chip packaging is shown in figure 1. Photo definable material with patternable characteristics served as the bonding adhesive layer between the microcap and host wafer. A bonding experiment was carried out with several types of photo definable material used for the bonding process. After the bonding process, bonding strength was tested. The result shows that excellent adhesive strength between microcap and host wafer can be obtained. 2. Bonding process In this paper, we explore a new method using flip chip and wafer level methods to form a microcap on a selected area. The packaging process can be divided into the following steps: first, the metal Ni microcap is fabricated on the carrier substrate with help of a passivation treatment as illustrated in figure 1. Then wafer level and flip chip methods are used to transfer the metal microcap to a selected area on the host substrate (see figures 1 and (c)). Finally, the two wafers are separated as shown in figure 1(d). The metal microcap will be bonded to the selected area of the host substrate. The bonder used in the study is a commercially available EV501 (Electronic Visions) with PC controller, as shown in figure 2. Its alignment accuracy is 1 µm. As shown in figure 1, the bonding pad has to be precisely defined on the microcap. Later, through bonder alignment, the wafers were bonded together. Misalignment of the bonding pad will reduce the bonding strength. Therefore, the accuracy of alignment is a key factor in the bonding process. For the packaging process, photo definable materials served as the bonding adhesive layer. Several types of photo definable material were used as the adhesive layer and tested for bonding strength. In the study, AZ-4620 (a positive photoresist from Shipley), JSR-137N (a negative photoresist from Japan Synthesis Rubber Co.), SU-8 (a negative photoresist from Microchem Co.), and SP-341 (a positive photoresist from Toray Co.) were selected for the bonding experiments Fabrication of the metal microcap Figure 3 shows the fabrication process for the metal microcap. A four-inch (100) single-side polished silicon wafer 525 µm in thickness is used as the carrier wafer. Wet thermal oxidation is grown to a thickness of 1.5 µm at 1050 C to define an etching mask on the carrier silicon wafer (see figure 3). Subsequently, the silicon is anisotropically etched using 30 wt% KOH at 70 C to form a cavity. This etchant does not attack the silicon oxidation. The cavity is formed by the (111) silicon surfaces which are etched at a much lower rate than the (100) surfaces in the anisotropic etchant. The exact cavity shape is defined by the edges of the pattern due 523

3 CTPan MEMS device IC devices carrier wafer (c) (c) (d) (e) ( f) (g) Figure 3. Illustration of fabrication process of metal microcap and bonding pad. Define etching mask (SiO 2 ), etch cavity, (c) sputter Ni seed layer and do passivation treatment, (d ) define template for microcap, (e) electroplate metal microcap, ( f ) define bonding pad and (g) remove photoresist template. to orientation-dependent etching, and the (111) surface forms an angle of with respect to the (100) surface. After the layers of the remaining oxide are removed, the cavity is finished as shown in figure 3. Then, a Ni layer 1500 Å in thickness is sputtered on the cavity surface, functioning as a seed layer for the electroplating process, followed by a passivation treatment on the Ni layer surface (figure 3(c)). Figure 4. Illustration of bonding process of the metal microcap on selected area. MEMS/IC devices on host wafer, host and carrier wafer brought together through flip chip and wafer level alignment and (c) remove carrier wafer. Later the passivation treatment on the Ni surface will help the metal microcap to separate from the carrier substrate easily. Then a thick photoresist is coated to form the electroplating template for the metal microcap structures (figure 3(d )). A Ni metal microcap structure 15 µm in thickness is formed by an electroplating process (see figure 3(e)). Then, photo definable material, as an adhesive layer, is coated onto the surface of the microcap. To evaporate the solvent contained in the photo definable materials, the material is baked at 85 C for a period of time, such as SU-8 for 7 minutes, JSR for 5 minutes, SP-341 for 3 minutes, and AZ-4620 for 2 minutes. When the solvent in the photo definable material is evaporated, the viscosity of the photo definable material will increase significantly, especially for SU-8, which will enhance the bonding strength. The sample is then exposed and developed to define the bonding pad on the metal microcap as shown in figure 3( f ), which serves as an adhesive layer for the subsequent bonding process. Finally, the electroplating template is stripped off (figure 3(g)) Flip chip and wafer level bonding The recent trends in microelectronics are moving more and more towards incorporating with MEMS devices, as shown in figure 4. One of the most important issues is the MEMS packaging process. The flip chip method is utilized to transfer a microcap onto the selected area of the host wafer through wafer level alignment. The two wafers are placed into the bonder (figure 4). During the bonding process, the bonding temperature is adjusted based on the adhesive layer material. The bonder allows wafers to contact with preset applied bonding force and to be annealed between plate heaters. The final procedure is to separate the carrier wafer from the host silicon wafer. When the carrier silicon wafer is 524

4 Selective low temperature microcap packaging technique through flip chip and wafer level alignment Temperature (C) Hour Figure 5. Chart of temperature versus time for passivation treatment. pulled upwards, the microcap will be separated from the carrier silicon wafer, and left on the host substrate (figure 4(c)). It is worth noting that the metal microcap can be easily separated from the carrier substrate because the Ni seed layer on the carrier wafer surface has been given a passivation treatment in advance. The process of the passivation treatment is to put the Ni layer into an oven being flushed with air for a period of time. The chart of annealing time versus temperature for the treatment is shown in figure 5. Different combinations of annealing temperature and time have been tried in the study. The preliminary result reveals that the recipe shown in figure 5 exhibits the best passivation effect for separating the microcap from the carrier wafer. In the first stage from the beginning to 2 h, the temperature is increased at a rapid rate to 100 C to evaporate the moisture attached to the Ni layer efficiently. In the second stage from 2 to 4 h, the passivation NiO layer is formed in the Ni seed layer surface. In the final stage, the Ni layer is annealed in the oven to reduce the residual stress between the Ni and NiO layers. When the passivation layer does not form uniformly, the microcap can not be separated successfully from the carrier wafer. Due to the treatment, the carrier wafer can be re-used to fabricate more metal microcaps. The bonding method also can be applied on 8-inch wafer. Therefore, the throughput can increase. In this approach, the bonding process allows a microcap to be transferred to the selected area of the host substrate. The schematic illustration of an IC integrated with a MEMS device is schematically shown in figure 6. Moving MEMS devices can be protected by the metal microcap. This method is particularly suitable for the integration of micro-structures with microelectronics involved in MEMS packaging. Once the microcap is transferred to the host wafer, the microdevices are then protected. Therefore, a host wafer protected by a microcap can be treated the same as an IC wafer during the subsequent dicing process. With the package method, devices can be batch-fabricated and batch-packaged. Individual packaged chips can then be obtained by cutting along the dicing line. Figure 7 illustrates schematically now the individual MEMS devices with microcaps can be diced in a standard IC dicing process. In the study, the microcap with passivation technique has been successfully demonstrated. Microcaps from 50 µm 50 µm to 1.5 mm 1.5 mm in area can be made to protect microdevices, as shown in figure Bonding inspection Several methods of nondestructive and destructive test exist for mechanical characterization of the bonding process. The most common techniques are the bond imaging method, cross-sectional analysis, and bond-strength measurement. The bond imaging method is nondestructive and can be used as an in-process monitor, while the cross-sectional analysis and bond strength measurements are destructive for characterization. There exist three dominant methods of bond imaging to inspect bonded pairs including infrared (IR) transmission, ultrasonic, and x-ray topography. In the study, IR transmission was set up to inspect the bonding structure. The IR method has the advantage of being simple, fast, and inexpensive. It can be used directly in the clean room to image the bonded wafers. The other two imaging methods offer higher resolution at the expense of speed, cost, and incompatibility with clean room processing [22, 23]. A simplified IR imaging system was set up in the study, schematically shown in figure 9. It consists of an IR source and an IR sensitive camera with excellent sensitivity in the near-ir range. The bonded wafer pair is host wafer metal microcap without metal microcap IC movable MEMS device Figure 6. Schematic drawing of the flip-chip method transferring the microcap onto the selected area through wafer level alignment. 525

5 CTPan standard IC dicing line camera microcap interconnection bonded pair contact pad IR source Figure 7. Schematic illustration of dicing process after microcap package. 1.5mm Figure 8. SEM photographs of electroplated Ni microcap. located between the IR source and camera. Any defect after the bonding will show up. Examples of the images obtained by the method for two bonded 4-inch silicon wafer pairs are shown in figure 10. It shows the method can successfully examine the bonding result. The bonding pad can be clearly identified as shown in figure 10. On the other hand, as shown in figure 10, if Newton s Ring appears, it means a void or defect exists in the bonding area. This imaging method cannot generally image voids with dimensions of less than one quarter of the wavelength of the IR source. Figure 9. Schematic set up of the IR imaging system Low temperature and electric field free bonding process Photo definable material was applied as the intermediate adhesive layer in this bonding process. It has several features; low bonding temperature, electric field free, strong bonding strength and excellent surface planarization properties. In the experiments, single polished silicon wafers were bonded with other Ni coated wafers using different photo definable materials as an adhesive layer. The influences of the bonding material, the bonding force, and the bonding temperature were investigated. After the bonding process, the bonding strength was examined. The wafer bonding strength, with various photo definable materials as the adhesive layer material, is discussed later. The bonding pad can be patterned on the silicon wafer through photolithography. Therefore, its resolution can reach as small as 10 µm. The bonding result shows that the adhesive layer at low bonding temperature can attach firmly between the bonding interfaces. Also the photo definable material is a liquid-like material, which can planarize the interfaces between the host wafer and the microcap to minimize the residual stress during the wafer bonding process. Bonding temperatures ranging from 50 to 150 Cwere tested. When the adhesive layer reaches the pre-set bonding temperature, the two wafers are then contacted and bonded together. After the process, excellent bonding strength between the two wafers can be formed. For the tensile strength test, bonded pairs are cut into chips using a dicing saw. The bonded chip is attached using clamping apparatus in the MTS (Material Test Station) for the bonding strength test. The clamping apparatus is pulled apart using a pre-set force and speed. Software is available to control this machine and record tensile strength and strain data. Various bonding temperatures for four kinds of photo definable materials were tested. 3. Results and discussions The test results of the bonding strength are shown in figures Each data point represents the average of 526

6 Selective low temperature microcap packaging technique through flip chip and wafer level alignment Figure 10. IR transmission images of two bonded wafer pairs. Successful wafer bonded pair and failed wafer bonded pair with void. Bonding strength (Kg/cm2) SU8 SP-341 AZ-4620 JSR Temperature (C) Bonding strength (kg/cm2) SU-8 JSR AZ-4620 SP Strain (E-3) Figure 11. Experimental result of bonding strength versus bonding temperature for various intermediate adhesive layers. Bonding strength (Kg/cm2) SU8 SP-341 AZ-4620 JSR Bonding force (x10n) Figure 12. Experimental result of bonding strength versus applied bonding force for various intermediate adhesive layers. three measurements. Figure 11 shows the tensile strength test curve as a function of bonding temperature at a constant bonding force (50 N). The result shows that when the bonding temperature for SU-8, JSR, AZ-4620, and SP-341 was between 80 C and 120 C, the bonding strengths for the above adhesive materials reached their maximum. When the bonding temperature was higher than 140 C, AZ-4620 was scorched. When the bonding temperature was higher than 200 C, SU-8, JSR, and SP-341 exhibited the same bonding strength as that at 100 C. It also reveals that SU-8 serving as the adhesive layer, at a bonding temperature of 90 C, reaches a maximum bonding strength about 213 Kg cm 2 (20.6 MPa). SU-8 has many attractive properties as an intermediate adhesive layer, described as follows, SU-8 has epoxy features, with very high bonding strength. SU-8 is a negative photoresist and Figure 13. Experimental result of bonding strength versus bonding strain for various intermediate adhesive layers. is crosslinked after exposure to UV light. It exhibits excellent chemical resistance after UV exposure. In addition, it requires a very low bonding temperature of 90 C to form excellent bonding strength. On the other hand, for AZ-4620, when the bonding temperature was at 90 C, the bonding strength was measured about 86 Kg cm 2. When a thinner bonding layer is required, AZ-4620 photoresist is a good choice for the bonding process. Regarding SP-341, when the bonding temperature was at 90 C, the bonding strength of SP-341 was measured to be about 100 Kg cm 2. When the bonding temperature for JSR was at 90 C, the bonding strength was measured to be about 88 Kg cm 2. Figure 12 shows the tensile strength test curve under different applied bonding forces. Each data point represents the average of three measurements. The bonding temperature was set at 90 C to test SU-8, JSR, AZ-4620, and SP-341. The results show that the bonding strength increases with increase in applied bonding force. It is worth noticing that with a lower applied bonding force, the adhesion between the host wafer, microcap and adhesive layer can not be formed tightly because the applied bonding force is not strong enough to eliminate gaps and voids trapped within the photo definable material. Gaps are caused by total thickness variation of the unsmooth host wafer and microcap surface. But, when a larger bonding force is applied, the adjacent microstructure and IC may be destroyed. The selection of bonding force is an important parameter in the bonding process. It plays a very important role in the wafer bonding process. 527

7 CTPan microcap SU-8 as adhesive layer ductile fracture host wafer 1.5mm Figure 15. SEM of SU-8 fracture surface at the bonding interface. microcap Figure 14. Experimental bonding result with microcap. Thickness of adhesive layer less than 20 µm and microcap on selective area. How to choose the photo definable material as an intermediate layer is also important issue. In this paper, the tensile strength versus strain relationship of photo definable material was also examined. The laser displacement sensor is arranged in the normal direction of pull on the MTS for the strength test. The measurement range and resolution of the laser displacement sensor is ±0.2 mm and 0.04 µm, respectively. The laser displacement sensor measures the microdisplacement before the adhesive layer fractures. The strain of the intermediate layer can be read from the microdisplacement. The stress strain relationship of the intermediate adhesive layer can be plotted by the MTS. Figure 13 shows the stress strain relationships of various adhesive layers. It reveals that when four types of photo definable materials reached their maximum bonding strengths, ranging from Kg cm 2, the maximum strain of SP-341 is 155E-3, SU-8 is 60E-3, AZ-4620 is 75E-3, and JSR is 65E-3. It is worth noticing that SU-8 as the intermediate adhesive layer exhibits ductile behavior in the interface after the bonding process, while SP-341 shows elastic characteristics after the bonding process. The choice of intermediate layer material depends on the requirements for bonding strength. A thickness of the adhesive layer of less than 20 µm can be achieved (see figure 14). The thickness of the intermediate adhesive layer can be controlled precisely by the spin-coating process. Due to the photolithography process, the lateral width of the bonding pad can be controlled to within 10 µm. Therefore, the bonding method has great potential for substrates with a higher density of IC and MEMS devices. Figure 15 shows the surface morphology of SU-8 as an adhesive layer after the tensile strength test. On the left side of the figure, a ductile fracture can be observed. The result means that SU-8 material can offer excellent attachment to the host wafer and exhibit excellent bonding strength. In the study, photo definable material is used as an adhesive layer in this bonding process. Though it has several features; low bonding temperature, electric field free, strong bonding strength and excellent surface planarization properties, it is not recommended for high vacuum hermetic sealing due to its polymeric properties such as moisture absorption, the outgassing problem, and decomposition at high temperature. The outgassing particles from the adhesive layer might destroy the function of MEMS devices. The glass transition temperature (T g ) of the adhesive layer is also a critical issue for the bonding technique. The T g of SU-8 and AZ-4620 is around 180 and 115 C, respectively. It means that when the working temperature is higher than that, the adhesive layer will become softened and liquified. Therefore, it will reduce the bonding strength. A Ni metal microcap with 15 µm in thickness can be formed by the electroplating process. The metal Ni microcap is superior to those made from thin film poly-si by the surface micromachining technique due to its higher stiffness. In the study, the uniformity of the electroplating microcap can be controlled as precisely as 15 µm ± 7%. The total thickness variation of the microcap could cause stress concentration and reduce the bonding strength. However, the effects of stress concentration can be eased and reduced by the bonding method. Since the photo definable material used as the adhesive layer is a liquid-like material before being cured in the bonder, it can planarize the total thickness variation between the electroplated microcap and host wafer to minimize the stress concentration and residual stress after the wafer bonding process. A further study will investigate in more detail the effect of total thickness variation between host wafer and microcap on bonding strength in the future. 4. Conclusion In this study, a silicon wafer bonding technique for 3D microstructures, using photo definable material as an adhesive layer, was presented. Flip chip and wafer level bonding methods were successfully demonstrated to transfer a microcap onto a selected area of the host substrate. The 528

8 Selective low temperature microcap packaging technique through flip chip and wafer level alignment passivation treatment can help the metal microcap to separate from the carrier wafer. The microcap, with the passivation treatment, can be used as a new packaging technique to protect MEMS devices. A Ni metal microcap structure of 15 µm ± 7% in thickness can be formed by the electroplating process. Therefore, the Ni metal microcap is superior to those using poly-si made by surface micromachining techniques due to its higher stiffness. Photo definable material with patternable characteristics can serve as the adhesive layer between the host wafer and microcap. Several types of photo definable materials were tested for bonding strength. The resolution of the bonding pad and thickness can be down to 10 µm and 20 µm, respectively, which is very suitable for highdensity IC and MEMS packaging processes. The preliminary result indicates that SU-8 is the best adhesive material with a bonding strength up to 213 Kg cm 2 (20.6 MPa) at 90 C bonding temperature. When high resolution bonding pad, strong bonding strength, electric field free, and low bonding temperature are required, the best solution is to choose photo definable material as the bonding adhesive layer. It can offer a desirable bonding result. Acknowledgments The author would like to thank Dr Tung-Chuan Wu and Dr Min-Chan Chou at MIRL of ITRI in Taiwan for their guidance, and National Science Council (NSC) for their financial supports to the project (granted number: NSC E CC3 and NSC E ). References [1] Baltes H, Brand O and Waelti M 2000 Packaging of CMOS MEMS Microelectron. Reliab [2] Shimbo I M, Furukawa K, Fukuda K and Tanzawa K 1986 Silicon-tosilicon direct bonding method J. Appl. Phys [3] Albough K B and Cade P E 1988 Mechanism of anodic bonding of silicon to pyrex glass Tech. Dig., IEEE Solid-State Sensor and Actuator Workshop (Hilton Head Island, SC, 1988) pp [4] Wallis G and Pomerantz D I 1969 Field-assisted glass-metal sealing J. Appl. Phys [5] Shimbo M, Furukawa K, Fukuda K and Tanzawa K 1986 Silicon-to-silicon direct bonding method J. Appl. Phys [6] Krause P, Sporys M, Obermeier E, Lange K and Grigull S 1995 Silicon to silicon anodic bonding using evaporated glass Proc. Int. Conf. on Solid-State Sensors and Actuators (Transducers 95) (Stockholm, Sweden, June 1995) vol 1 pp [7] Ito N, Yamada K, Okada H, Nishimura M and Kuriyama T 1995 A rapid and selective anodic bonding method Proc. Int. Conf. on Solid State Sensors and Actuators (Transducers 95) (Stockholm, Sweden, June 1995) vol 1 pp [8] Field L A and Muller R S 1990 Fusing silicon wafers with low melting temperature glass Sensors Actuators A A21 A [9] Tong Q-Y, Cha G, Gafiteanu R and Gösele V 1994 Low temperature wafer direct bonding IEEE J. Microelectromech. Syst [10] Quenzer H J and Benecke W 1992 Low-temperature silicon wafer bonding Sensors Actuators A [11] Lee A P, Ciarlo D R, Krulevitch P A, Lehew S, Trevino J and Northrup M A 1995 A practical microgripper by fine alignment, eutectic bonding and SMA actuation Proc. Int. Conf. on Solid-State Sensors and Actuators (Transducers 95) (Stockholm, Sweden, June 1995) vol 2 pp [12] Cohn M B, Liang Y, Howe R T and Pisano A P 1996 Wafer-to-wafer transfer of microstructures for vacuum packaging Tech. Dig., IEEE Solid-State Sensor and Actuator Workshop (Hilton Head Island, SC, 1996) pp 32 5 [13] Lin L 2000 MEMS post-packaging by localized heating and bonding IEEE Trans. Adv. Packag [14] Milanovi V et al 2000 Microrelays for batch transfer integration in RF systems The 13th Int. Conf. on Micro Electro Mechanical Systems (MEMS 2000) (23 27 Jan. 2000) pp [15] Krauter G, Schumacher A and Gosele U 1998 Low temperature silicon direct bonding for application in micromechanics: bonding energies combinations of oxides Sensors Actuators A [16] Putty M and Najafi K 1994 A micromachined vibrating ring gyroscope IEEE Solid-State Sensor and Actuator Workshop pp [17] Sparks D, Jordan L and Frazee J 1996 Flexible vacuum packaging method for resonating micromachines Sensors Actuators A [18] Sparks D 1998 Component integration and packaging of automotive microsystems Proc. Microsystem Symp. (Delft, The Netherlands, Sept. 1998) pp [19] Sparks D, Jiang G, Chilcott D and Kearney M 1998 Silicon micromachined motion sensor and method of making US Patent Specification (US ) [20] Chen J-Y, Huang L-S, Chu C-H and Chang Peizen 2002 A new transferred ultra-thin silicon micropackaging J. Micromech. Microeng [21] Shivkumar B and Kim C J 1997 Microrivets for MEMS packaging: concept, fabrication, and strength testing J. Micromech. Microeng [22] Maszara W, Jiang B-L, Yamada A, Rozgonyi G A, Baumgart H and de Kock A J R 1991 Role of surface morphology in wafer bonding J. Appl. Phys [23] Schmidt M A 1998 Wafer-to-wafer bonding for microstructure formation Proc. IEEE