Development of low cost set up for anodic bonding and its characterization

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 46, October 2008, pp Development of low cost set up for anodic bonding and its characterization C C Tripathi +, Shruti Jain +, Pawan Joshi +, S C Sood + & Dinesh Kumar* + Ambala College of Engineering. & Applied Research, Devasthli, Ambala + jain.shruti15@gmail.com *Electronic Science Department, Kurukshetra University, Kurukshetra Received 3 September 2007; revised 3 January 2008; accepted 3 April 2008 A low cost experimental set- up for anodic bonding has been developed indigenously in the college laboratory and glass silicon bonding parameters characterized. Anodic bonding between silicon and glass substrates has been characterized in detail. The effects of magnitude of the applied voltage on the time required for complete bonding have also been investigated. The effect of voltage, point contact, bond strength and electrostatic force in anodic wafer bonding process has also been analyzed. The glass to silicon bonding at 1150V, 450ºC has been successfully performed. This enables simple, but highly accurate, alignment of pre-patterned glass and silicon wafers. Fabricated devices have wide benefits like glass transparency at optical wavelengths Keywords: Low cost set-up, Silicon glass bonding 1 Introduction Wafer bonding is one of the key process steps for the construction and packaging of micro electro mechanical systems (MEMS). It is also a cost effective method for zero level MEMS packaging, and it has increasingly become a key technology for material integration in various areas of MEMS, microelectronics and opto-electronics. The manufacturers of MEMS, require wafer level bonding of one silicon wafer to another silicon substrate or a glass wafer. This provides the first level packaging solution that makes these processes economically viable. Different wafer bonding approaches are currently used in MEMS industry: Fusion, adhesive, eutectic, anodic, solder bonding etc 1,2. Fusion bonding process requires a high temperature annealing, which is not always suitable for the devices with aluminium or copper integrated circuits. Also adhesive bonding been non-hermetic, more emphasis is being laid on low temperature bonding, which being not only reduces process cost and time, but also minimize bonding induced stress and warp age after cooling. Anodic bonding is one of the most used wafer level packaging procedures and the most robust process. This process is being widely used for bonding glass substrate to other conductive materials due to its good bond quality. It can serve as a hermetic and mechanical connection between glass and metal substrates or a connection between glass and semiconductor substrate 3,4. Anodic bonding, which is also called field assisted thermal bonding or electrostatic bonding, is commonly used for adhesive free bonding of glass to silicon. This technique is used to join glass with metals and semiconductors at temperature well below the softening point of the glass 5. In view of the large efforts for the development of MEMS devices to take advantage of micro fabrication technique, a new thrust has been generated to initiate work in the field of MEMS devices. This has led to the development of low cost set-up for fabrication of MEMS devices, which are dimensionally not very critical. In this paper, we present the development strategy of a low cost anodic bonding set-up using locally available equipments and facilities. The anodic bonding process for silicon substrates and glass materials has been investigated in detail. Firstly, the time needed for a complete bonding (thereafter as bonding performance) at different applied voltages has been evaluated. Effect of point contact, bond strength and electrostatic force have also been analyzed. The electrostatic force depends not only on the applied voltage, but also on other factors such as the bonding temperature and the sodium content in the glass wafer. The gap between the two wafers has a significant effect on the magnitude of electrostatic

2 TRIPATHI et al.: LOW COST SET UP FOR ANODIC BONDING 739 force, which implies that the bond quality through the anodic bonding technique heavily relies on the surface smoothness. This conclusion has been confirmed experimentally. 2 Experimental Details 2.1 Experimental set-up The bonding system comprises of a dc power supply, electrodes and temperature controlled workstation as shown in Fig. 1. A low cost set- up using laboratory hot plate (550 W) AC mains and klystron power supply (500 Volts, 40 ma) three numbers are connected in series to get the required voltage available. Equipments available in Microwave laboratory of Electronics and Communication Engineering discipline with slight modifications in the hot plate and electrode assembly was used to perform the anodic bonding. The hot plate was modified with PID type temperature controller and chrome alumel thermocouple to monitor the temperature of silicon wafers to be bonded. Different types of electrodes were designed and tested before reaching the final plum type cathode electrodes designed and fabricated in college mechanical workshop. These enable anodic bonding using indigenously developed low cost equipment. Pre-cleaned silicon and glass substrates were sandwiched. Then, the aluminium bars and screws were used to fix the bonding pair including the electrodes. The assembly was heated at different temperatures and a dc voltage was applied to the electrodes, ensuring a positive electrode potential on the silicon side with respect to the glass. 2.2 Wafer preparation and pre-cleaning The silicon wafers used in the study were 1, boron doped <100> silicon with a resistivity in the range 2-10 Ω-cm. The glass wafers tried were ordinary microscope glass slides, optical glass and commonly used pyrex borosilicate glass. The optical flatness of the glass surface was of the order of λ, λ/4, 50 λ (λ=0.55 micron) for glass slides, optical glass and one of the pyrex glass, respectively. The thicknesses of both silicon wafer and glass wafer are of the order of 500µm and 1500 micron, respectively. Prior to alignment, the surfaces of the silicon and glass wafers were cleaned in the solution of sulphuric acid and hydrogen peroxide. A solution of hydrogen peroxide and sulphuric acid in the ratio of 1:3 was prepared and samples were dipped in for 10 min, then rinsed in hot distilled water for 5 min and then rinsed in cold distilled water for 5 min. Next dipped in methanol and finally, dried before mounting for bonding. 2.3 Glass to silicon bonding To bond silicon wafer and glass in an anodic bonding process, the cleaned silicon and glass were sandwiched to each other and kept at the hot plate as shown in Fig. 2. The temperature of the hot plate was slowly raised to keep it in bonding temperature range i.e. from 400 to 450 C and then voltage was applied. The bonding voltage was varied and it was dependent upon silicon, glass surface quality, surface preparation and temperature of the hot plate along with sodium content in the glass selected for bonding. However, low temperature bonding between 400 to 450 C is most suitable for all pre-fabricated devices on silicon. The placing of the silicon and glass to be bonded, should be such that the silicon wafer is electrically connected to the anode and the glass to the cathode. Fig. 1 Schematic drawing of the typical apparatus used in the anodic bonding Fig. 2 Experimental set up for anodic bonding

3 740 INDIAN J PURE & APPL PHYS, VOL 46, OCTOBER 2008 Accordingly, the glass wafer was kept on the top and the silicon wafer was kept at the bottom. The temperature was observed by thermocouple attached to the hot plate, which is controlled by temperature controller. The hot plate operating temperature was kept near the glass-softening point, but below its melting point. The bonding process was observed to be complete within min. 2.4 Bonding mechanism To understand how silicon wafer and glass stick together in an anodic bonding process, we must know the element that makes up silicon and the glass that are used in the bonding. Experiments showed that the elements that make up the glass to be bonded (pyrex borosilicate glass or pyrex) have sodium oxide (Na 2 O). It was found that there is a content of 3.5 per cent of sodium oxide in the pyrex. When the silicon wafer and the pyrex are put together and placed at the anodic bonding set-up and heat is added, at a certain temperature the pyrex is hot enough and becomes soft. Since the pyrex is softened, an applied voltage produces an electric field between the silicon wafer and the pyrex. The electric field exists because the applied voltage makes the presence of the mobile metal ions to exploit to the high negative voltage of the pyrex. The high negative voltage pulls most of the positive metal sodium ions (Na+) to the top, attracting them and neutralizing them. As a result, the positive ions move away to the cathode leaving behind permanent negative ions. These permanent negative ions then form a depletion region between the silicon wafer and the pyrex as shown in Fig. 3. This depletion region gives rise to a large electric field, which is between the silicon wafer and the pyrex. As a result of the electric field, the silicon wafer and the pyrex are pulled into contact, the strong electrostatic attraction between the Fig. 3 Schematic showing the joining of silicon wafer and pyrex in anodic bonding silicon wafer and glass wafers, fixing them firmly in place. Increasing the bonding temperatures up to 450 C, can further enhance the mobility of these positive ions. In addition, the electric field makes oxygen from the glass to transport to the glass-silicon interface where it combines with silicon to form SiO 2, creating the permanent bond that bonds the silicon and glass together. Hence, the following assumption is made. The oxygen ions carrying negative charges are assumed to be evenly distributed in the glass wafer and sodium ions are neutralized at the surface once they reach the cathode. This assumption is based on the fact that the oxygen is tightly bonded to the glass networking structure in the glass wafer and there is no free oxygen ion available for movement. 3 Results and Disscusion 3.1 Bonding of different types of glass with silicon Bonding of different types of glass like soda lime glass, optical glass, and pyrex glass with silicon at different temperatures and voltages has been done. Most suitable temperature and voltage values for soda-lime glass are 300 C, 480V but cracks are not controllable, which start appearing during cooling process at temperature of 96 C. Also even if a small portion is left unbonded, the bonds start breaking. Same situation of cracks is there with optical glasses. In case of pyrex glass, there were no cracks at all. A strong and non-reversible bonding has been observed at 1150V, 450 C for optically flat glass to silicon bonding. It is because the thermal expansion coefficient (TEC) of the used pyrex glass wafer is very similar to the TEC of silicon. 3.2 Effect of applied voltage Fig. 4 shows that the required bonding time drops significantly as the applied voltage increases from 700 to 1150 V. This may be as follows: With respect to the bonding mechanism at an elevated temperature, Na+ ions in the glass become so mobile that they are attracted towards the cathode as a result of the applied voltage. This leaves behind relatively immobile oxygen anions at the glass side of the silicon-glass interface, at which a space charge region is formed. This in turn creates an equivalent positive image charge on the silicon side of the silicon-glass interface resulting in a high electric field magnitude of 10 6 V cm -1 across the silicon-glass interface. Under the high electric field, oxygen anions drift away from the Na+ depletion region to the silicon surface. As this

4 TRIPATHI et al.: LOW COST SET UP FOR ANODIC BONDING 741 Fig. 4 Plot of bonding time versus applied voltage for p-type silicon and pyrex glass happens, oxidation of silicon by the oxygen anions is presumed to occur and a thin oxide layer is formed at the interface, which contributes to the migration of the bonding front. However, at a small applied voltage, i.e. a reduced electric field, the drift velocity and the kinetic energy possessed by the oxygen anions cannot sustain a high oxidation rate at the bonding front of the silicon-glass interface, thus, a longer bonding time is required. As the electric field becomes negligible, reaction is extinguished at the interface and no bonding can be achieved. We have observed that in the absence of an electric field, there is no indication of bonding. A large electric field is applied across the joint, which causes an extremely strong bond to form between the two materials.fig. 5 shows a glass plate bonded over a cavity etched into a silicon wafer using anisotropic etching and Fig. 4 shows the plot of bonding time versus applied voltage for p-type silicon and pyrex glass. The bonding current as a function of time is shown in Fig. 6. The results show that the current rises rapidly at the beginning of the bonding. It indicates that the surge of Na+ ions drift to the cathode to generate a current in the circuit. As the migration of Na+ continues, the accumulation of positive charge repels the incoming ions and the current density decreases to an almost steady value. Bonding was done at 1150V for min. If the bonding voltage is decreased the bonding time increases. It has been observed that bonding depends not only upon the voltage, temperature and time but it also depends upon surface uniformity. Bonding voltage and time are considerably reduced in case of optically flat surf 3.3 Effect of point cathode contact The results indicate that point contact electrode configuration provides excellent bonding uniformity Fig. 5 Silicon and glass bonding Fig. 6 Plot of current as a function of time Table 1 Bonding voltage and time Bonding voltage (V) Bonding time (min) as compared to flat contact configuration. Table 1 shows the experimental parameters under which various material combinations were successfully bonded. It was found that bonding time depends upon the applied voltage, temperature and cathode electrode contact configuration (flat/point). Tabulated values also show that bonding time strongly depends on the contact configuration. Bonding, using point cathode electrode contact 6, is much faster as

5 742 INDIAN J PURE & APPL PHYS, VOL 46, OCTOBER 2008 compared to flat cathode electrode contact. It was observed that silicon and pyrex glass bonding takes about one eighth of the time while using point contact as compared to flat contact. Point cathodes, usually the end of a stiff wire, are commonly used in anodic bonding to make electrical contact to the glass. In the experimental studies of anodic bonding, point cathode allows researchers to view the glass-silicon interface through the glass during bonding. A point cathode also causes anodic bonding to spread radially outwards from a spot beneath the point cathode. This radial progression of the bonding front prevents any air between the glass and metal members from being trapped at the interface during the bonding process. 3.4 Bond strength The bond strength obtained in this study is comparable to that (from 5 to 25 MPa) of wafers bonded using higher bonding temperatures by other researchers 7,8. The bond strength increases with an increase in the bonding temperature and voltage. It is believed that glass is annealed during bonding process and the fracture strength of the glass improves. It is observed that the fracture invariably occurs inside the glass substrate. The crack is initiated and propagates in the glass. Occasionally, the crack extends into the silicon wafer through the interface without damaging the bond. This is a demonstration of the quality of the bond. The reactive phenomenon at the interface is largely dependent on the bonding temperature since temperature is a major driving force of ion mobility and atomic diffusion. High bonding temperatures introduce higher energy, drift more oxygen anions to the interface, enhance the diffusion of oxygen into silicon and finally promote greater reaction in the interface. A higher voltage produces a higher electric field, which increases the drift velocity of the sodium and potassium ions. Higher voltage also accelerates the decomposition of the sodium ions from the lattice matrix and contributes to a larger depletion region of the alkaline ions. A larger depletion region enhances the electrostatic force and promoted the bond. This seems to be the likely reason for the higher bond strength. For bonding temperatures higher than 400 C, high bonding strengths (above pyrex toughness) were obtained; the pyrex itself broke before it was debonded. This observation was confirmed by the blade test. The bondings performed at temperatures lower than 400ºC exhibited weak mechanical properties. However, the strength of the samples bonded at 450ºC increased to a value higher than the pyrex toughness. 3.5 Influence of electrostatic force High temperatures generate high ion mobility in the glass substrate. As the temperature is raised, the resistivity of glass decreases exponentially. A rapid build up of the space charge occurs at the interface, giving rise to electrostatic forces, which pull the two wafers into intimate contact. Therefore, high temperatures cause high current and result in good bond quality. At high temperatures, more ions decompose from Na 2 O and K 2 O to migrate to the cathode. The equilibrium state is easily obtained and the transition period is shorter. A higher applied voltage is expected to increase the mobility of the Na+ ions. A higher voltage produces a higher electric field. The higher electric field increases the drift velocity of the sodium ions. Higher voltage also accelerates the detachment of the sodium ions from the lattice matrix and contributes to the concentration of free sodium ions. This is the likely reason for the shorter time required to establish the equilibrium state. Therefore, a higher applied voltage can generate more free sodium ions and subsequently, contribute to a larger electrostatic force. The temperature is a dominant factor, which affects the electrostatic force, which causes the firm contact of two wafer surfaces, facilitating the bonding reaction to occur. The voltage is also significant for the control of the maximum electrostatic force. The bonding temperature, applied voltage and bonding time have an obvious complementary relationship for a certain electrostatic force needed for chemical reaction between silicon and glass wafers. 4 Application to Microsystems Different applications of metal to glass anodic bonding have been reported recently in the field of Microsystems technologies 9,10. Metal to glass anodic bonding has been used to integrate robust fluidic interconnects to a piezoresistive silicon/pyrex liquid flow sensor 11. Moreover, in case of silicon piezoresistive sensors, metal to glass anodic bonding is foreseen to have applications in the packaging field of pressure sensors operating in harsh environments. In the case of titanium, its biocompatible property combined with its assembling to pyrex materials in which micro channels can be structured, make it a good candidate to be used in biocompatible micro

6 TRIPATHI et al.: LOW COST SET UP FOR ANODIC BONDING 743 fluidic systems. Anodic bonding of pyrex can also be successfully performed on thin metallic films. An application of this technology is the encapsulation under vacuum of quartz resonators. Pyrex caps are bonded on thin metallic films forming the electrodes of the resonators. Anodic bonding is gaining significance in Micro-Total-Analysis Systems (µtas) and in miniaturized biological reactors because of its promising use in sealing silicon and glass based micro fluidic devices. The bonding technique can be broadly applied in the fabrication of sensors, actuators, microstructures, 3D integrated circuits and optoelectronics devices. The recent developments in the bonding technique have made the micro mechanical design more flexible. 5 Conclusion We have successfully bonded silicon with glass at 1150V, 450 C in min. It was found that bonding time depends upon the applied voltage, temperature and design of the cathode electrode and the flatness of the surfaces to be bonded. Temperature plays the major role as the mobility of the ions increases and bonding takes less time. Acknowledgement The authors would like to thank Director, Dr. Anil Joshi, ACE & AR, Devasthali, for his constant encouragement during the course of this work. The authors also wish to acknowledge and extend thanks to Sh Nalini Kant mentor of the college without whose support and encouragement the work would not have been possible. We also extend our sincere thanks to our worthy Chairman, Dr Jaidev for his kind support and motivation. The authors are also thankful to Director, CEERI, Pilani, for providing pyrex wafers used in the experiment. References 1 Schmidt M A, Proceedings of the IEEE, 86 (1998) Chen M, Yuan L & Liu S, Sensors and Actuators, (2006). 3 Rogers T & Kowal J, Sensors and Actuators A, (1995) Cheng Y T, Lin L & Najafi K, J Micro- electro Mechanical Systems, 10 (2001) Madou M J, Fundamentals of Micro fabrication, the science of Miniaturization, second edition, (CRC Press, New York), Waris M & Ahmed M, Appl Surface Sci, 252 (2006) Cozma A & Puers B, J Micromech Microeng, 5 (1995) Lee T M H, Hsing I M & Liaw C Y N, J Micro Electro Mech System, 9 (2000) Blom M T, Chmela E, Gardeniers J G E, Berenschot J W, Elwenspoek M, Tijssen R & Van den Berg A, J Micromech Microeng, 11 (2001) Sim D Y, Kurabayashi T & Esashi M, J Micromech Microeng, 6 (1996) Weber P, Briand D & De Rooij N F, Proceedings of Euro sensors XVI, Prague, The Czech Republic, (2002) 282.