é MICROELECTRONIC ernGincErT EMT 452 MEMS SEMESTER I! 2010/2011 MEMS PackagingOutlines Introduction Key Design and Packaging Considerations Three Levels of MEMS Packaging Interfaces in Microsystems Packaging Enabling Packaging Technologies Die Preparation Surface Bonding / Die-Attach Processes - Wire Bonding - Sealing Types of Packaging Solutions - Ceramic Packaging - Metal Packaging - Molded Plastic Packaging Quality Control, Reliability, and Failure Analysis - Quality Control and Reliability Standards - Statistical Methods in Reliability - Accelerated Life Modeling - Major Failure ModesIntroduction e For example, the packaging of a pressure sensor must ensure that the sensing device is in intimate contact with the pressurized medium yet protected from exposure to any harmful substances in this medium.Introduction e For example, the packaging of a pressure sensor must ensure that the sensing device is in intimate contact with the pressurized medium yet protected from exposure to any harmful substances in this medium.Introduction e For example, the packaging of a pressure sensor must ensure that the sensing device is in intimate contact with the pressurized medium yet protected from exposure to any harmful substances in this medium.Key Design and Packaging Considerations Designing packages for micromachined sensors and actuators involves a number of important factors. Some are shared with the packaging of electronic integrated circuits, but many are specific to the application. These factors also bear significance on the design of the micromachined components themselves. As a result, the design of the package and of the micromachined structures must commence and evolve together.Figure 8.1 A simplified process flow for MEMS packaging. Tebespectandtet wafer 2.Sawanddcewoler 3. Separate dice 4 Petrocss (optional) 1, Upon completion of wafer-level fabrication, inspection and first > » tests take place. The wafer is then mounted on a special sticky tape and sawed. The individual dice are separated. Some post processing, such as removal of a sacrificial layer, may occur at this point. One die or many dice are attached to a ceramic, a metal header, or a premolded plastic lead frame. 3. Electrical interconnects are made by wire bonding, flip chip, or another method. 4. Aceramic, metal, glass, or plastic cap seals the assembly. Alternatively, the die or dice are attached to a metal lead frame. 5. After the electrical interconnects are made, plastic is molded over the assembly. A final test and 5. Die attach and calibration conclude the process. interconnects This simple process does not allow for fluidic or optical connections. N 7, Calibration and 6, Package seal final testCritical factors and considerations in MEMS packaging e The required costs in manufacturing, assemblies and packaging of the components. e The expected environmental effects, such as temperature, humidity, chemical toxicity, etc. that the product is designed for. e Adequate over capacity in the packaging design for mishandling and accidents. e Proper choice of materials for the reliability of the package. e Achieving minimum electrical feed-through and bonds in order to minimize the probability of wire breakage and malfunctioning.Three levels of MEMS packaging Level 1: The “die level”, Level 2: The “device level”, Level 3: The “system level”. System Packaging: fete eee lee eee tee ce eee, ! Device packaging peceeneenennenee | I Die Packaging: I \ Toput ! ' Y Sensing ' Action yt {Element \fSignat tapping} Sigal i! [& Transduction Conditioning & 1_| Actuating 1) Processing (Motion 1Die-level packaging Dies in most microsystems are the most delicate components, which require adequate protection. The objectives of this level packaging are: e To protect the die or other core elements from plastic deformation and cracking, e To protect the active circuitry for signal transduction of the system, e To provide necessary mechanical isolation of these elements, and e To ensure the system functioning at both normal operating and over- load conditions. This level involves wire bonds for electronic signal transmission and transduction such as the embedded piezoresistors in a pressure sensor die and the circuits that connect to them.Die-level packaging often involves wire bonding: Pressure sensor with metal casing: Pressure sensor with plastic encapsulation: Inferconnect Metal cover 4 “A Fond i ce ae) Plastic encapsulant \ La inletDevice-level packaging « electric bridges « signal conditioning circuits Input 7 [Sensing action/—I| element Signal conditioning Signal mapping 8 transduction ‘Actuating element |_/Power ite J supply * Proper regulation of input power Major interface problems: «The interfaces of delicate dies and core elements with other parts of the packaged products at radically different sizes «The interfaces of these delicate elements with environmental factors, such as temperature, pressure and toxicity of the working and the contacting media.System-level packaging e Involves the packaging of primary signal circuitry with the package of the die or core element unit. Major tasks involve proper mechanical and thermal isolation as well as electromagnetic shielding of the circuitry. e Metal housings usually give excellent protection for mechanical and electromagnetic influences. e MEMS devices or microsystems at the end of this packaging level are ready to be “plug-in” to the existing engineering systems. Packaged inertia Sensor for airbag Deployment systemInterfaces in Microsystems Packaging e Various parts, in particular, the delicate dies of microsystems are expected to be in contact with various working media, e.g. chemicals, optical, corrosive gases, etc. e Interface between these parts with working media becomes a major design issue in packaging.Biomedical interfaces The packaged systems need to be biologically compatible with human systems and they are expected to function for a specified lifetime. Every micro biosystem must be built to satisfy the following requirements that are related to interface: . It is inert to chemical attack during the useful lifetime of the unit. . It follows mixing with biological materials in a well-controlled manner if it is used as biosensors. . It causes no damage or harm to the surrounding biological cells in the cases of instrumented catheters such as pace makers. . It causes no unwanted chemical reactions such as corrosion between the packaged device and the contacting human body fluids, tissue and cells.Optical interfaces ¢ There are two principal types of optical MEMS: 1.The direction of the lights in devices, e.g. micro switches involving mirrors and reflectors. 2.Optical sensors.* Optical MEMS require: Proper passages for light beams to be received and reflected. Proper surface coating for receiving and reflect lights. The quality of the coating must be enduring during the lifetime of the device. The surfaces must be free of contamination of foreign substance. The enclosure must be free of moisture. The presence of moisture may cause stiction of the enclosed components.Electromechanical interface ¢ Electrical insulation, grounding and shielding are typical problems to be dealt with in MEMS and microsystems packaging.Interfaces in microfluidics - Require precise fluid delivery, hermal and environmental isolation and mixing. - Material compatibility of microchannel and contained solvent - Major interface problems: Usealing of the fluid Uinterface between the contacting channel walls and fluid containment wallEnabling Packaging Technologies Die preparation « Dies, or substrates in MEMS, are normally cut (sliced) from single wafers using thin diamond saw blades. Spacing between dies: = 50 jim with saw blade thickness of 20 pm. Cutting wheel: 75 - 100 mm diameter Cutting speed: 30,000 - 40,000 rpm.Die-Attach / Surface Bonding Processes « After wafer dicing, each individual die is mounted inside a package and attached (bonded) onto a platform made of metal or ceramic, though plastic is also possible under limited circumstances. There are four (4) techniques available for surface bonding in MEMS and microsystems: (1) Adhesives (2) Eutectic soldering (3) Anodic bonding (4) Silicon fusion bonding (SFB)Bonding by adhesives e Epoxy resin and silicone rubbers are two commonly used adhesives. e Good bonding by epoxy resin rely on surface treatments and curing process control. Avoid glass transition temperature at 150-175°C. e Soft silicone rubbers are used for bonding parts require “flexibility.” It is vulnerable to chemicals and air. Mechanical Pressure Bonding part pl adhesive a Occ |Eutectic bonding Eutectic bonding involves the diffusion of atoms of eutectic alloys into the atomic structures of the materials to be bonded together. Must first select a candidate material that will form a eutectic alloy with the materials to be bonded. A common material to form eutectic alloy with silicon is thin films made of gold or alloys that involve gold. Gold-tin (80% Au+20% Sn) films around 25 ym thick is commonly used. Bonding takes place at about 300°C. Offers much solid bonding than adhesives. Ce Au/Sn FilmAnodic bonding The working principle of Glass-to-silicon wafer bonding: [Weight for contacting pressure (Cathode) Siemens Heated mechanical support (Anode) Applied DC voltage: 200-1000 volts sr Aiea) Wot Plate (Anode) ‘Weight for contact Pressure (Cathode) Na Depletion | —>| b- =20nm Laver = 1. um Bonding interfaceAnodic bonding The working principle of Glass-to-silicon wafer bonding: [Weight for contacting pressure (Cathode) Siemens Heated mechanical support (Anode) Applied DC voltage: 200-1000 volts sr Aiea) Wot Plate (Anode) ‘Weight for contact Pressure (Cathode) Na Depletion | —>| b- =20nm Laver = 1. um Bonding interfaceSilicon Fusion bonding e It is the induced chemical forces that bond the pieces together. e Wafer surfaces need to be extremely flat (at 4 nm) to be bonded. e Bonding strength between silicon wafers can be as high as 20 MPa. e The SFB process begins with thorough cleaning of the bonding surfaces. These surfaces must be polished, then make them hydrophilic by exposing them in boiling nitric acid. e These two surfaces are naturally bonded even at room temperature. e Strong bonding occurs at high temperature in the neighborhood of 1100°C to 1400°C.Silicon Fusion bonding e It is the induced chemical forces that bond the pieces together. e Wafer surfaces need to be extremely flat (at 4 nm) to be bonded. e Bonding strength between silicon wafers can be as high as 20 MPa. e The SFB process begins with thorough cleaning of the bonding surfaces. These surfaces must be polished, then make them hydrophilic by exposing them in boiling nitric acid. e These two surfaces are naturally bonded even at room temperature. e Strong bonding occurs at high temperature in the neighborhood of 1100°C to 1400°C.Wire Bonding Wire bonding techniques provides the electrical connection to or from the core elements. ET —— 7 tom VA i aca a on Saal Maw if ooen| Be “= astic encapsulant x Pressed The three (3) wire bonding techniques used in IC industry are adopted for MEMS and microsystems. * Thermocompression wire bonding * Wedge-wedge ultrasonic bonding ¢ Thermosonic bonding. * Common wire materials are Au, Ag, Al, Cu and Pt with diameters at 20-80 ym. ‘* Wire bonding is fully automatic.Thermocompression wire bonding Wire bonding is accomplished with mechanical compression at elevated temperatures at about 400°C. ¢ The bonding process is illustrated as: - Heat the wire to form a bead Feed the bead to the pad by pulling down the capillary tool: Metal “ (HEA eer? © Compress the bead to at pad mechanically: y Metal Pad © Retract the capillary tool pura | after the bead is bonded to the pad: W SubstrateWedge-wedge ultrasonic bonding This bonding process takes place at room temperature. e The energy supply to the bonding is from ultrasonic vibration of the tool at 20 - 60 kHz. e The process is illustrated as: Tool Direction Wedge Bonding h\ Wire bard vee Tool Teal Nith mechanical sompression: bing ea Metal Pads Nate Pads Thermosonic bonding e This process uses ultrasonic energy with thermocompression. e As such, wire bonding can take place at 100-150°C. Joints can be in either ball-wedge or wedge-wedge form.Sealing Sealing is a key requirement in MEMS and microsystems packaging. Hermetic sealing is essential in devices or systems such as: microfluidic, optoMEMS, bioMEMS, pressure sensors, etc. There are generally 3 sealing techniques available for MEMS and microsystems: (1) Mechanical sealing technique: « Epoxy for microfluidics. It is flexible but ages with time. e Eutectic soldering for hermetic seals. (2) 2) Sealing by microfabrication processes - Sealing by micro shells: Doped silicon PSG sacrificial Doped silicon layer micro shell Die (a) With sacrificial layer (b) After the removal of sacrificial layer(3) Sealing by chemical reactions: Sealing * Sealing is accomplished by “growing” the sealant using chemical reactions. « Example is the production of SiO, as the sealant for sealing a delicate die with a silicon shell. « The growth of SiO, from the silicon encapsulant to the constraint base provides reliable and hermetic seal for the die Silicon shell SiO, Si Constraint base SiO, film Si Constraint base (@) Unsealed encapsulant (b) Sealed encapsulation by oxide grown from silicon shellTypes of Packaging A package is a protective housing with an enclosure to hold one or multiple dice forming a complete microelectromechanical device or system. The package provides where necessary electrical, optical, and fluid connectivity between the dice and the external world There are three general categories of widely adopted packaging approaches in MEMS; ceramic, metal, and plastic.Ceramic Packaging « Alumina (Al,0;) is the most common of all ceramics, having been used over the centuries in porcelain and fine dinnerware. Aluminum nitride (ALN) and beryllia (BeO) have superior material properties (e.g., better thermal conductivity), but the latter is very toxic. Aluminum nitride substrates tend to be costly in particular because of required complex processing due to the difficulty of sintering the material.Ceramic Packaging « Alumina (Al,0;) is the most common of all ceramics, having been used over the centuries in porcelain and fine dinnerware. Aluminum nitride (ALN) and beryllia (BeO) have superior material properties (e.g., better thermal conductivity), but the latter is very toxic. Aluminum nitride substrates tend to be costly in particular because of required complex processing due to the difficulty of sintering the material.Ceramic Packaging « Alumina (Al,0;) is the most common of all ceramics, having been used over the centuries in porcelain and fine dinnerware. Aluminum nitride (ALN) and beryllia (BeO) have superior material properties (e.g., better thermal conductivity), but the latter is very toxic. Aluminum nitride substrates tend to be costly in particular because of required complex processing due to the difficulty of sintering the material.Aceramic package is made of laminates, each formed and patterned separately, then brought together and cofired (sintered) at an elevated temperature—typically between 1,500°C and 1,600°C. LEF LFF, _ eal, hey, eR, yy 1. Cast ceramic 2, Punch holes 3. Fill 4, Metalize 5. Cut, stack, and laminate “ eee J 9.Gold or nickel plate 8. Braze pins 7. Nickel plate 6. Sinter Figure 8.8 Process flow for the fabrication of a cofired laminated ceramic package with electrical pins and access ports. (Courtesy of: the Coors Electronic Package Company of Golden, Colorado.)Metal Packaging The silicon pressure sensor measures pressure transmitted via the steel diaphragm and through the oil. The robust steel package offers hermetic protection of the sensing die and the wire bonds against adverse environmental conditions. Each stainless steel package is individually machined to produce a cavity. The die is attached to a standard header with glass-fired pins and wire bonded. This header is resistance welded to the stainless- steel package. Arc welding of a stainless steel diaphragm seals the top side of the assembly. Oil filling of the cavity occurs through a small port at the bottom that is later plugged and sealed by welding a ballMetal Packaging The silicon pressure sensor measures pressure transmitted via the steel diaphragm and through the oil. The robust steel package offers hermetic protection of the sensing die and the wire bonds against adverse environmental conditions. Each stainless steel package is individually machined to produce a cavity. The die is attached to a standard header with glass-fired pins and wire bonded. This header is resistance welded to the stainless- steel package. Arc welding of a stainless steel diaphragm seals the top side of the assembly. Oil filling of the cavity occurs through a small port at the bottom that is later plugged and sealed by welding a ballMetal Packaging The silicon pressure sensor measures pressure transmitted via the steel diaphragm and through the oil. The robust steel package offers hermetic protection of the sensing die and the wire bonds against adverse environmental conditions. Each stainless steel package is individually machined to produce a cavity. The die is attached to a standard header with glass-fired pins and wire bonded. This header is resistance welded to the stainless- steel package. Arc welding of a stainless steel diaphragm seals the top side of the assembly. Oil filling of the cavity occurs through a small port at the bottom that is later plugged and sealed by welding a ball@ Steel diaphragm Silicone oil Stee! housing Sealed fill port Glass fired pins (o) Figure 8.13 (a) Photograph, and (b) cross-sectional schematic of a pressure sensor mounted inside an oibfilled, stainless-steel package. Pressure is transmitted via the stainless-steel diaphragm and through the oil to the silicon sensor. (Courtesy of: GE NovaSensor of Fremont, California.)Molded Plastic Packaging In postmolded plastic packaging, the lead frame is spot- plated with gold or silver on the paddle and the lead tips to improve wire bonding. The die is then attached with adhesive or eutectic solder. Wires are bonded between the die and the lead tips. Plastic molding encapsulates the die and lead frame assembly but leaves the outer edges of the leads exposed. These leads are later plated with tin or tin-lead to improve wetting during soldering to printed circuit boards. Finally, the outer frame is broken off and the leads are formed into a final S-shape (see Figure 8.14).Molded Plastic Packaging In postmolded plastic packaging, the lead frame is spot- plated with gold or silver on the paddle and the lead tips to improve wire bonding. The die is then attached with adhesive or eutectic solder. Wires are bonded between the die and the lead tips. Plastic molding encapsulates the die and lead frame assembly but leaves the outer edges of the leads exposed. These leads are later plated with tin or tin-lead to improve wetting during soldering to printed circuit boards. Finally, the outer frame is broken off and the leads are formed into a final S-shape (see Figure 8.14).Die with first evel silicon packaging Plastic molding compound Figure 8.14 Schematic showing a sectional view of a post-molded plastic package. The die is first mounted on a center platform (the paddle) and wires bonded to adjacent electrical leads. The paddle and the leads form a metal lead frame, over which the plastic is molded. A MEMS die should include a frst level of packaging (e.g., a bonded silicon cap) as protection against the harsh effects of the molding process. This particular illustration is of a plastic quad-flat pack (QFP) with electrical leads along its entire outer periphery.Molded Plastic Packaging In postmolded plastic packaging, the lead frame is spot- plated with gold or silver on the paddle and the lead tips to improve wire bonding. The die is then attached with adhesive or eutectic solder. Wires are bonded between the die and the lead tips. Plastic molding encapsulates the die and lead frame assembly but leaves the outer edges of the leads exposed. These leads are later plated with tin or tin-lead to improve wetting during soldering to printed circuit boards. Finally, the outer frame is broken off and the leads are formed into a final S-shape (see Figure 8.14).¢ Finally, a premolded plastic cap is attached using an adhesive or ultrasonic welding. If necessary, the cap itself may also contain a fluid access port. (see Figure 8.15). Encapsulation gel Premolded plastic cap meted Bond wire ‘Adhesivelepoxy Metal lead frame Premolded plastic body Pressure sensing die ‘Adhesive die attach Figure 8.15 Illustration of a premolded plastic package [24]. Adapting it to pressure sensors involves incorporating fluid ports in the premolded plastic housing and the cap.Design case: Packaging of Micro Pressure Sensor Dies Primary packaging considerations The die in a pressure sensor is to support the thin diaphragm that senses the medium pressure by the induced stresses. For accurate sensing the medium pressure, the stresses that the diaphragm has sensed should be those stresses induced by the medium pressure ONLY. e Unfortunately, there could be stresses induced in the diaphragm by sources other than the medium pressure - the “parasite stresses”. « A major source of parasite stress is from the thermal stresses induced by significantly different CTE of various components attached to the diaphragm: Silicon die: 9: 2.33 ppm" Die attach (G0Sn40Pbeader) 3526 ppc How to ISOLATE the die/diaphragm from these sources of parasite stresses become a primary consideration in the packaging design.Die down ¢ It is a process to bond the die to the constraint base with “die attach”. e Three commonly used bonding techniques: Anodic bonding e Eutectic soldering Adhesive Normal die down Die down with “spacer” for die isolation: The extension of the height by the spacer increases the flexibility and thereby reduces the parasite thermal stress. Disadvantage: takes up extra space.Die protection e The delicate die in a pressure sensor needs to be protected from possible damage by the contact pressurized medium. e There are three (3) ways to do this: (1) By vapor-deposited organic on the die surface: The deposited organic coating will insulate the die surface from the contact medium. Unfortunately the deposited organic also serve as a “reinforcement” and make the diaphragm undesirably stiff. Thin organic protective layer- Silicon die Glass constraint base(2) By coating with silicone gel: @ Silicone gel containing one or two parts of siloxanes has very low Young’s modulus. So, it is very soft. e Being soft, it would not add unwanted stiffness to the diaphragm. e A few mm thick coating gives sufficient protection to the die. e The only problem is aging and become contaminated with impurities from the contact medium.(3) Indirect pressure transmission: @ This method is used in situation in which the pressurized medium is so environmentally hostile that direct contact of the die and medium is not possible. e A special arrangement is made for a special case that involved: e P= 70 kPa - 350 MPa @ Impact force = 10-20,000 g e T = 5,000°F in milliseconds ¢ Media contain high-velocity dusts e Die and wirebonds are submerged in silicone oil. e Pressure from the media was transmitted to the diaphragm through silicon oil. e The stainless steel diaphragm has compliance is 100 times less than that of silicon diaphragm. e Minimum volume of silicone oil in order to mitigate thermal expansion. Ceramic volume compensator Diaphragm in contact withQuiz * Explain how a micropump works? * List THREE (3) applications of micropump.