An Investigation on NEG Thick Film for Vacuum packaging of MEMS

Transcription

1 An Investigation on NEG Thick Film for Vacuum packaging of MEMS Y.F. Jin* 1,3, Z.P. Wang 1, L. Zhao 2, P.C. Lim 1, J. Wei 1 1) Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore ) Nanjing Delta King Hi-tech Co Ltd., Nanjing , China 3) Institute of Microelectronics, Peking University, Beijing , China ABSTRACT An approach to maintain vacuum in MEMS devices, by integrating MEMS fabrication process with getter material preparation, is presented in this paper. A coating process for thick film of getter material on silicon and glass wafers, which are common materials in fabrication of MEMS devices and package, has been investigated in detail. The getter material consists of a powder mixture of zirconium, vanadium and iron, which features high sorption capacity to active gas such as H 2, O 2, N 2, CO and H 2 O vapor. Several patterned NEG thick films to simulate different needs in MEMS application have been made. The sorption capacity of the coated getter material was examined. The coating of NEG thick film onto the inner surface of a MEMS pressure sensor and the activation of NEG during anodic bonding process were carried out. Keywords: non-evaporable getter (NEG), micro-electro-mechanical system (MEMS), reliability, thick film, vacuum packaging 1. INTRODUCTION Hermetic packaging and vacuum maintaining technology are essential for various MEMS systems and devices to enhance their reliability. The reliability and performance of resonant MEMS devices, such as micro-resonator, micro-gyroscope, are affected by environmental pressure and composition. Thin diaphragm micro pressure sensors, widely used in automotive and IT industries, could obtain very high sensitivity if the reference pressure within the cavity is kept constant. The product life of a vacuum field emitter device (FED) would be prolonged by maintaining long-term vacuum condition in the sealed structure. The performance of some RF MEMS components is related to the stability of the gas pressure inside the cavity during the component s operation. The humidity and corrosive composition would degrade their reliability due to delamination of electronic thin-films, the accumulation of surface charge in dielectric surfaces, and the stiction of mechanical structures caused by capillary forces [1]. Significant efforts have been made to create advanced packaging of vacuum for MEMS application. For instance, the pressure inside vacuum packaged microstructures without NEG has reached about 10-3 Torr [2-4] based on advanced sealing process, such as sputtering SiO 2 film as a sealing material. By coating active getter, maintaining a vacuum of Torr in a micro cavity was reported. A promising technology on hermetic approaches and vacuum maintenance is to use Non-evaporable getter (NEG). A prototype as low as Torr was attained in a sealed cavity using NEG as the internal getter [6]. With the advantage of high sorption capacity, commercialized NEG were prepared by coating getter materials on strips or sheets as standard products. They can be cut into required shapes and sizes by mechanical scissors or laser beam. The NEG will then be fastened onto the inner surface of microstructure. One of the challenges for this approach is the NEG size, which could not match with the miniaturization of MEMS devices. An additional space, which is called NEG room, has to be made on the backside or adjacent side of the cavity where MEMS chip is located. A tunnel connecting the NEG room to the cavity is also required. Besides, loose particles of getter material generated during the process cutting a NEG sheet and fixing small size NEG onto micro cavity are harmful to MEMS movable sensing and actuating structures. *Tel: ; Fax: ; yfjin@simtech.a-star.edu.sg Reliability, Testing, and Characterization of MEMS/MOEMS II, Rajeshuni Ramesham, Danelle M. Tanner, Editors, Proceedings of SPIE Vol (2003) 2003 SPIE X/03/$

2 Studies on MEMS vacuum packaging using NEG thick film to eliminate gas, which deteriorates the vacuum condition in the cavity of MEMS device, is described in this paper. Experiments on coating of getter films onto silicon wafer and glass wafer were carried out. The sorption capacity was measured on a vacuum system setup. A prototype of applying the getter film onto the cavity of MEMS device was also presented. 2. EXPERIMENTS In order to maintain high vacuum in cavity, it is necessary to eliminate the residual gas, such as degas from materials, micro-leak gas from outside the package, desorption gas from the inner surface of MEMS structures during packaging procedure. An efficient approach is to utilize a getter. Getters can chemically absorb active gases, such as moisture, CO, CO 2, N 2, O 2, and H 2 [7]. In general, they must be activated prior to use, i.e., heated under vacuum at high temperatures for a recommended time. Almost all metals are capable of adsorbing gases on their surface after thorough degassing. Getters are normally made from metals, such as Ta, Zr, V, Al, Ti, Mg, Ba, P or their mixture. Getters may be classified into three groups according to the form in which the getter material is active: flash getter, coating getter and NEG or bulk getter [7]. NEG is widely applied in electronic devices where additional evaporating material and high temperature are not permitted. In this work, a gettering alloy of Zr, V and Fe was selected to develop NEG, since this combination presents the advantages of higher gettering ability and lower activation temperature (around 350~500 ), which matches with the temperature attained during anodic bonding process. It is also very stable and airtight. In addition, the powder used to make thick film can be safely handled in open air, without the danger of spontaneous ignition posed by pure zirconium powder. The activation parameters include temperature, heating time, method of heating, and pressure during activation. In our research, NEG was used as a small vacuum pump placed in a sealed cavity. The proposed method of chipscale vacuum packaging with getter thick film is shown schematically in Fig. 1(b). For comparison, the packaging using traditional NEG is shown in Fig. 1(a). Unlike the method for traditional getters in which materials in sheets, wires or bulks are placed in the MEMS cavity, a thick film of getter is coated directly onto the inner surface of the cavity. The general requirements of the getter used in MEMS packaging were considered. Firstly, an efficient MEMS chip (a) Package using traditional NEG NEG (b) Package using thick film NEG Fig. 1 A comparison of chip-size package using thick film NEG with the package using traditional NEG sorption capacity to residual gas is essential. Secondly, the NEG thick film should have a good adhesion to substrate, such as silicon and Pyrex 7740 wafers, and good mechanical stability so that they can endure vibrations or shocks during fabrication, activation, test and use. Thirdly, they should feature low sorption capacity prior to activation when temperature ranges from ambient to 150 C, and low activating temperature so as to be activated during anodic bonding process, in which a typical anodic bonding temperature is about 400 C. Besides, a fabrication process compatible with IC process is desired. In order to investigate the performance of the getter film, such as stress and adhesion to substrate, samples with sizes ranging from a few square millimeters to the whole surface of a wafer were fabricated. The thickness of the film varied from 50um to 400um. Fabrication process of packaging sensor coating with a NEG thick film is presented in Fig. 2. Preparation for fabrication consists of mask design, making getter paste by mixing K 4 Si, graphite with powder of Zr-V-F e alloy, and fabrication of MEMS chip. Coating of NEG thick film starts with printing the getter paste onto the surface of 500 µm thick, double side polished Pyrex 7740 glass wafer to form a 276 Proc. of SPIE Vol. 4980

3 patterned thick film of getter, followed by pre-baking at temperature of 120, for half an hour. Finally, an anodic bond was conducted to hermetically join the glass wafer to the silicon wafer. The bonding process was carried out at a low pressure of Torr, with an applied voltage of 1000V and at a temperature of 450 for duration of 60 min. High temperature was set to activate the getter, and the bonding process was prolonged to remove adsorption gas on the surface of MEMS chip and the getter film. 3. RESULTS AND DISCUSSION Coating getter on glass wafer MEMS chip fabrication Wafer bonding & getter activating The fundamental experiment starts with NEG coating on the substrates. Thick films of NEG were successfully coated on both silicon and Pyrex glass wafers. A sample was fabricated by coating the whole surface of a 4 silicon wafer with the getter film to investigate the uniformity of the film. Its thickness was measured on a profilometer. Fig. 3 shows the thickness distribution of the getter film. The thickness of the Fig 2 Schematic fabrication process of packaging sensor with a NEG thick film film is 100 µm, and profile of the surface varies within ±15 µm due to the porosity of the film. Another sample was also made through coating of patterned NEG films onto glass wafer, which is often used to fabricate cavity in MEMS sensors or a lid or bottom structure in packaging. A typical glass wafer coated with NEG films is shown in Fig. 4. The areas of the getter are 1 1mm 2, 2 2mm 2 and 5 5mm 2, respectively. 15µm 0-15µm Fig. 3 The profile of the NEG thick film coating on silicon wafer Fig. 4 A photo of patterned NEG film coating on a glass wafer (a) on silicon wafer 1mm (b) on glass wafer 1mm Fig. 5 Micrographs of NEG Thick Film Proc. of SPIE Vol

4 The porosity of NEG is essential, which enlarges the gettering surface for a limited size of surface coated with NEG and thereby, enhances the sorption capacity. Fig. 5(a) and (b) are micrographs showing NEG coating on silicon wafer and glass wafer respectively, which displays the physical porous surface of the thick film coatings. Test of sorption capacity The test of sorption capacity was conducted to examine the performance of the getter film. A schematic diagram of the experimental system is shown in Fig. 6. At the beginning of test experiment, the thermoelectric couple and wafer coated with NEG film were assembled on the heater, followed by placing the assembly into the testing chamber of 650 ml in volume. The chamber was then pumped down a vacuum of Torr. The wafer was heated to the preset temperature of 350 for 10 minutes to activate the getter. After the activation step, the chamber was cooled to ambient temperature. Testing gas, H 2, was then introduced into the chamber through variable leak valve until the chamber pressure reached to 5000 Pa. The sorption capacity can be quantified by examining the pressure variation in the chamber. Experimental pressure variation versus testing time is shown in Fig. 7. Fig 6 A schematic diagram of the experiment of evaluating the sorption capacity for NEG P (Pa) 4000 Without NEG With NEG time(sec) Fig. 7 Pressure variation versus testing time The sorption capacity of the NEG thick film in a confined chamber can be expressed as d ( PV ) Q g + Q c = (1) dt where Q g and Q c are gas quantity adsorbed by getter and inner surface of the chamber, respectively. t is time. P is the pressure inside the chamber, and V is the volume of the test chamber. The minus sign is used because adsorbed quantity of gas is related to the decrease of pressure. As Q g is much larger than Q c, Eq. (1) can be approximately written as 278 Proc. of SPIE Vol. 4980

5 Pt P0 Q g V (2) t t 0 where P t and P 0 are pressure at time being t and t 0, respectively. Substituting V = 650ml, P 0 = 5000 Pa at t 0 = 0, and P t = 1000 Pa at t = 400 sec. the sorption rate of the NEG thick film is about 6500 Pa ml/sec or 6.5 Pa Litre/s. Dividing the total adsorbed gas (3250PaLitre) by the area of getter film, which is about 666 mm 2, the gettering capability of the patterned NEG film is Pascal Litre/m 2. To demonstrate fabrication process of applying NEG thick film in MEMS devices, experiment was carried out to coat the NEG thick film onto the inner surface of a micro pressure sensor. The structure consists of a Pyrex glass lid coated with thick film getter and a silicon substrate with a cavity fabricated by wet etching. Fig 8(a) and (b) are micrographs showing one sample from top-view and bottom-view. The volume of cavity is mm 3 with 37 µm thick diaphragm. The size of getter film is about mm 2, and its thickness is about 100µm. Adhesion of the getter film to the glass lid was observed before bonding process and after activating. There is no any loose particle found during the experiment. Primary test results proved that good mechanical stability was realized. (a)top-view (b) Bottom-view Fig. 8 Micrographs of the sample of MEMS structure with a thick film coating inside the vacuum cavity. The getter film was activated during anodic bonding process, at SUMMARY By coating NEG thick film on both a silicon wafer and a glass wafer, a vacuum maintaining for MEMS devices and packaging has been developed. Along with activating NEG by anodic bonding process, the patterned NEG thick films coated onto inner surface of MEMS cavity has been demonstrated. The finest NEG film realized less than 1 mm 2 in size. The thickness ranges from 50µm to 400um. In addition to save space for MEMS device or chip scale packaging, it will help to maintain vacuum circumstance in hermetic cavity of MEMS devices to obtain their excellent sensitivity. The sorption capacity of the getter film reached to Pascal Litre per square meter. ACKNOWLEDGEMENT This work was supported by Agency for Science, Technology and Research (A*Star), Singapore, project No. 003/101/03. Proc. of SPIE Vol

6 REFERENCES 1. C. Marxer, et al, Vertical Mirrors Fabricated by Deep Reactive Ion Etching for Fiber-Optic Switching Applications, IEEE J. of MEMS 8, pp , Mu Chiao and Liwei Lin, Accelerated Hermeticity Testing of a Glass-silicon Package Formed by RTP Aluminum-to-Silicon Nitride Bonding, The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 14, Brian H. Stark and Khalil Najafi, An Ultra-Thin Hermetic Package Utilizing Electroplated Gold, the same as above 4. Chang Liu and Yu-Chong Tai, Sealing of Micromachined Cavities using Chemical Vapor Deposition Methods: Characterization and Optimization, "IEEE/ASME J. of Microelectromechanical Systems (J. MEMS), Vol. 8, No. 2, June 1999, pp Patty P. L. Chang-Chien, Kensall D. Wise, Wafer-Level Packaging Using Localized Mass Deposition, The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 14, H. Henmi, S. Shoji, Y. Shoji, K. Yosimi, and M. Esashi, Vacuum package for microsensors by glass- silicon anodic bonding, Sensors and Actuators A, 43(1994) pp A. Roth, Vacuum technology, Elsevier North-Holland, Proc. of SPIE Vol. 4980