B08152070蒋敏蕾 外文原件.pdf

MEMS Vacuum Packaging Technology and Applications Jin Yufeng Zhang Jiaxun Peking University Shenzhen Graduate School Shenzhen 518055 China National Key Lab of Micro Nano Fabrication Technology of China Tel 86 10 62752536 Fax 86 10 62751789 jinyf Abstract Many MEMS Micro Electro Mechanic Systems parts have to meet the requirements for vacuum packaging In vacuum packaging leakage and gas permeation which will affect the normal function of the components are major problems Hermetic sealing is one of the most important technologies for reliable vacuum packaging In this paper several hermetic sealing technologies for vacuum packaging will be presented including eutectic bonding adhesive bonding glass frit bonding and silicon glass anodic bonding Furtherfore the author will introduced two approaches to deal with sealing imperfect surface caused by electric feedthroughs which link to the outside of the small cavity of MEMS sensors The getter will be disdussed as it is essential to keep the vacuum environment inside the cavity of device since the inner walls might release gas after hermetic seal 1 Materials Used in MEMS Vacuum Packaging 1 Gas permeation in vacuum packaging has to be considered when choosing materials applied in MEMS packaging For the same quantity permeated gas the pressure deterioration caused by gas permeation in MEMS is much more than that in conventional structure since the volume is smaller in MEMS cavity Furthermore thinner structures are often used in MEMS vacuum packaging This will cause more serious permeation problem for the MEMS devices For instance the permeated gas is hundred times more when the thickness of a wall or diaphragm is reduced from 1mm to 10 m In case of the gas permeation we should choose the packaging materials with low permeation rate Fig 1 compares the permeation rate of moisture or water molecules through several kinds of packaging materials which are used in modern electronic fabrication and packaging Their permeation rate ranges from 10 18 cm3 sec to 10 10 cm3 sec Fig 1 Permeability of water through non hermetic and hermetic materials Featuring lower permeation rate glasses ceramics silicon nitrides metals and some pure crystals are suggested to be the candidates for hermetic packaging Those with higher permeation rate which are regarded as non hermetic materials must be kept away from the catalogue for hermetic packaging In our works glass ceramic and adhesive materials with low permeation rate were chosen as the packaging structures materials 2 Hermetical Sealing for MEMS Structure Hermetic packaging plays an important role in many microsystems Hermetic sealing which protects the microsystems from harmful environmental influences can significantly increase the reliability and lifetime of them Besides anodic bonding a number of other bonding techniques have also been used for hermetic packaging including silicon to gold eutectic bonding glass frit bonding fusion bonding and bonding using evaporated glass The hermetic sealing processes developed in this research work include electrostatic bonding or anodic silicon glass bonding eutectic bonding glass frit bonding 2 1 Solder bonding and eutectic bonding Solder bonding for hermetically sealing wafers is based on solder joining two wafer together Of them eutectic bonding is widely applied in MEMS packaging which takes the advantage of the eutectic alloy to realize a bond between two substrates at a lower temperature Solder of a suitable material set can be formed in the bonding area between substrates of package and device Raise the temperature until the solder flows and creates a bond to seal two substrates The most obvious materials to use are those standard solders used in microelectronic applications but many of such solder materials contain either flux or sufficient impurities These flaws cause significant outgassing during the reflow process This becomes a major problem when trying to use such solders for vacuum packaging Recent development research on new fluxless solder materials may overcome such problem and several groups are pursuing this 2 Comparing with standard solder it is also possible to use alloys of different materials in the form of eutectic solder One of the most common material sets is the eutectic of gold and silicon Silicon gold eutectic is quite attractive because it is formed at a temperature of 363 C with one part silicon and four parts gold This materials is commonly used in MEMS fabrication and when the eutectic is formed the outgassing problem is resolved since the mixture is simply formed by raising the temperature and the starting materials are pure In addition the temperature is low enough for most applications 0 7803 9449 6 05 20 00 2005 IEEE 2005 6th International Conference on Electronic Packaging Technology On one hand for silicon gold eutectic bonding although the eutectic point is 363oC the bonding temperature must be higher A higher temperature can promote the diffusion of gold and silicon into each other and increase the thickness of the diffusion layer where the chemical composition can match with what is needed for eutectic bonding Therefore a higher temperature and a longer bonding time are beneficial to a good bonding On the other hand if the bonding temperature is too high it may cause serious diffusion of gold into silicon which will degrade the function of the silicon devices Fig 2 shows the scanning acoustic microscope SAM micrographs of eutectically bonded sensor wafer and silicon cap wafer at a bonding temperature of 400 450oC During SAM analyzing process the bonded wafers are immersed in deionized water Bubble free interfaces are observed and no water is sucked into the cavities It indicates that the cavities are well sealed The pull test results show that the bond strength is more than 5 MPa Fig 2 SAM micrographs of sealed wafers 2 2 Adhesive bonding The advantages of adhesive bonding are its low process temperature and the possibility to join different materials 3 This bonding technology makes use of an intermediate adhesive layer to join two substrate materials with different properties The adhesive materials may be epoxies or polymers Sometimes epoxy is acceptable for gas filled MEMS device For instance epoxy is used in micro optical switch for holding optical components together However epoxy in the light path is not desirable as it may age drift or crack at high laser power levels This causes a significant problem for the package since the package has to protect the device and simultaneously provide access to the environment that the device is supposed to contact with As a result a lot of effort has been expended on developing the proper protection encapsulation medium for MEMS Fig 3 is an application example of adhesive bonding for micro optical switch The process of adhesive bonding starts with applying the adhesive layer followed by contacting the wafers and forming bond by a heat curing or ultraviolet UV curing 4 Optical fiber Substrate Ceramic Plate 2 Ceramic Plate 1 Glass cap Adhesive Fig 3 Adhesive packaging for micro optical switch Adhesives are widely used in packaging for MOEMS such as tacking filling and sealing the precision structure joining the ceramic frames glass lid and PCB substrates to form a hermetic package However it is difficult to obtain uniform and hermetic bonding with vacuum grade and humidity insensitive due to the permeation of moisture We could choose an adhesive material with low permeation rate or coat an anti permeation layer such as SiO2 to resolve such a problem 2 3 Glass frit bonding The advantage of glass frit bonding is the capability of producing good hermetic seals Development of a glass frit bonding process is to use an in between glass layer at temperatures below 400 C By combining anodic bonding with glass frit coating on wafers in various materials such as silicon ceramic and metal it is possible to anodically bond wafers except glass wafer with silicon wafer Besides it can be used in hermetic bonding between ceramic layers Fig 4 is one example of its application Silicon lid Glass frit Substrate Fig 4 schematic packaging of glass frit bonding The process can be described as below First apply the frit paste onto the substrate with MEMS chips through screen print process After that the frit must be thoroughly dried Oven drying can be used Then raise the temperature to around 400 C the softening point of the frit and hold for 5 to 10 minutes before cooling down Sealing cycles depends on the geometry and size of sealing interface The important parameters of heating process are starting point for experimentation sealing temperature holding temperature and heating rate of each step which should be followed the specification given by the frit supplier It is necessary to maintain an oxidizing environment at all times in the furnace 2 4 Anodic bonding Anodic bonding can be used to bond two materials such as silicon and glass silicon and silicon ceramic and metal etc In recent years anodic bonding has been widely applied to vacuum packaging of MEMS devices It is a reliable and effective process for hermetically sealing silicon wafers to glass wafers or quartz substrates Anodic bonding is usually carried out under constant temperature and voltage The cathode makes contact with the glass wafer while the anode connects to the silicon wafer By heating at 200 500 C and supplying 200 1500 Volts DC voltage across a silicon glass wafer stack the positive ions in the glass mainly sodium ions which come from the dissociation of NaO2 move to the cathode leaving the non bridging oxygen ions the oxygen ions bonded to only one silicon atom behind Consequently a negatively charged depletion layer is formed adjacent to the anode The electrostatic force between this negative layer and the positive charges induced around the anode makes the two sides intimately contact with each other This force allied to the softening of the glass allows some conforming of glass to the opposing surface and makes possible hermetically bonding between surfaces that are imperfect A low temperature anodic bonding for hermetic sealing was developed The interface integrity was observed under scanning electron microscope SEM Fig 5 shows a typical cross section of bonded Si and glass It proves that silicon and glass were densely bonded together Fig 5 A typical cross section of bonded Si and glass Measurement of the distribution of the elements Si O and Na in the interface between Si and glass also shows that Si content decreases while O content increases from Si wafer side to glass wafer side No obvious change was found for Na element The reason is that low temperature was used in our bonding process Although Na may migrate to cathode the migration level is much lower than that at high temperature 2 5 Hermetic bonding with imperfect surface The electrical feedthroughs which link to the outside of the sealed structure make the surface of the substrate imperfect Therefore the hermetic packaging with electrical feedthroughs is an essential consideration for many microsystems 5 6 The electrical feedthroughs are generally required to connect a micro sensing or actuating element from the inside of the sealed structure to the outside world For example electrical power needs to be supplied to the sealed region and electrical sensing signals need to be extracted from the sealed package Lateral electrical feedthroughs of metal conductors are commonly used for a long time The standard fabrication process in semiconductor industry such as electronic plating vapor deposition and sputtering makes the lateral electrical feedthrough technique very convenient However thick metal coating on the interface of silicon wafer or glass wafer will results in failure in anodic bonding between silicon and glass due to the air leakage or debonding from the bonding interface There are two approaches to realize hermetic sealing via anodic bonding of silicon to glass with thick metal feedthrough on the surfaces 7 They are the patterned embeded electrode and the vertical via electrode method for interconnection of MEMS devices Fig 6 A process flow of the embeded electrode method The fabrication process of embeded electrode method is shown in Fig 6 Both glass and silicon wafers are available in the process First a patterned shallow trench was etched on the surface of the wafer Second for silicon wafer an additional SiO2 film was deposited in order to form an insulation layer After that metal was deposited on the patterned wafer to form embedded thick electrodes inside the trench Chemical mechanical polishing CMP process was then carried out to form a smooth and leveled bonding surface Finally the silicon and glass wafers were bonded together by anodic bonding Test result shows that after CMP the roughness of the glass wafer planarization is as low as 13 nm which is good enough for silicon to glass anodic bonding with hermetic sealing The embeded electrodes both on glass wafer and silicon wafer have been successfully fabricated The width of the electrodes can be fabricated from 20 m to 80 m and the thickness from 0 5 m to 1 9 m Using MEMS pressure sensor to investigate the hermeticity we found that the leveled and polished glass wafer with embeded electrode was anodically bonded with silicon wafer The other approach is called the vertical via electrode method Micro structure was fabricated through standard MEMS process and anodic bonding After etching of via holes vertical electrodes were formed through the vias by deposition and patterning of metal film Then a metal via process was applied to form 3 D electric interconnection At the same time it also serves as the hermetic sealing process by filling metal material into the vias Finally post processes such as filling polymer insulation PI depositing UBM and wafer bumping were performed and three dimension 3 D interconnection by vias was formed Fig 7 shows a micrograph of an electric via on silicon wafer Vertical interconnection Bottom electrode PI Via Top electrode Fig 7 A micrograph of an electric via on Si wafer 3 Vacuum Maintenance for MEMS Packaging After hermic sealing the inner walls of the small scale cavity might release gas which affect the vacuum maintenance With the advantage of high sorbing capability commercial non evaporable getter NEG has been used in vacuum maintenance of electronic packaging It is prepared by coating getter materials on strips or sheets and cut them into the desirable shape and size by mechanical cutting or by laser beam Then the NEG is fastened on the inner surface of device s structure However it is difficult to apply the commercial NEG to maintain higher vacuum environment in micro scale cavity in order to match with the miniaturization of MEMS devices The method of deposing thin film or thick film of getter materials onto inner surface of micro structures becomes a solution to maintain vacuum in micro cavity 8 9 A schematic of the key process steps is presented in Fig 8 The pre process consists of mask design making of getter paste by mixing K4Si graphite with powder of Zr V Fe alloy and the fabrication of MEMS chip First print getter paste on the surface of double side polished Pyrex 7740 glass wafer to form a pattern Then coat NEG thick film on the surface After pre baking at 120 C for half an hour the glass wafer and silicon wafer with MEMS structure are cleaned to eliminate particles and other contamination on the surfaces Anodic bonding was then applied to hermetically join the glass wafer to the silicon wafer The bonding process was carried out with EV 501 Bonder at a pressure of 1 10 3 Torr and DC voltage of 1000 Volts for 60 minutes The bonding temperature is 450 C We tested the sorption capability to examine the performance of the getter film Experimental pressure variation against time is shown in Fig 9 Good sorption capability of 4 88 106 Pascal Litre m2 has been measured with the gettering of 6 5 Pa Litre s Fig 8 Packaging flow for MEMS with thick film NEG Fig 9 Sorption capability test pressure variation vs time The flashing getter material has also been used in the MEMS packaging research because of its attractive features such as steady performance consistent yield of gettering materials and minimal outgassing during evaporation By evaporation it can be easily deposited onto the inner walls of the micro cavity in the form of thin film The getter used in our research is commercially available under the trade name of BI5U1HFG21 whose active ingredients are Ba Al and Ni alloys Besides its high efficient adsorption performance the experimental results also show that it has the good adhesion of the thin film The thickness of getter film coated on the wafer can be controlled in the range of several to hundreds microns by adjusting the heater temperature and process time It is also feasible to form a patterned getter film on the lid surface using a physical mask between getter source and the target 4 Conclusions Silicon gold eutectic bonding forms a soft eutectic to allow bonding over non planar surfaces it can be done at above the eutectic temperature 363 C and itself does not have the out gassing problem that other approaches produce Adhesive bo