Lecture 23 MEMS Packaging
MEMS Packaging A simplified process flow for MEMS Packaging
MEMS Packaging Key Design and Packaging consideration Wafer or wafer-stack thickness Wafer Dicing concerns Thermal Management Stress isolation Protective coatings and media isolation Hermetic packaging Calibration and compensation Die-attach process Wiring and Interconnects Electrical interconnects Microfluidic interconnects Types of Packaging Ceramic packaging Metal packaging Molded plastic packaging A MEMS accelerometer chipset is used for automotive airbag applications. Motorola Inc. researchers are applying wafer-level techniques and using a glass paste to achieve a hermetic seal.
Key Design and Packaging Consideration Wafer or wafer-stack thickness 4-in. wafer 525 µm thick 6-in. wafer 650 µm thick Current packaging facilities require the total thickness of the MEMS devices be less than 1 mm Wafer dicing concerns Batch fabrication Diamond or carbide saw blades, 75- to 250 µm wide Each MEMS design merits its own distinctive approach on how to Minimize the adverse effects of dicing Thermal management Cooling of heat-dissipating devices. Thermal management at two levels:» the die level and the package level
Key Design and Packaging Consideration Thermal management (con.) Thermal actuators can dissipates significant power. With efficiencies typically lower than 0.1%, most of the power is dissipated as heat. With high thermal conductivity, ceramics and metals make excellent candidate materials for the packaging housing Stress isolation Stress isolation becomes critical for MEMS devices using piezoelectricity and piezoresistivity. Long-term drift resulting from slow creep in the adhesive or epoxy that attaches the silicon die to the package housing Protective coating and media isolation Environmental effects Coating materials:» Parylene, usually deposited by CVD near room temp.» Silicon carbide, deposited by PE-CVD
Key Design and Packaging Consideration Protective coating and media isolation (con.) Gas sensors: direct exposure to surrounding media Hermetic packaging Theoretically defined as package that prevents the diffusion of helium For small volume packages, leakage rate of helium must be lower than 5 x 10-8 cm -3 /s Prevention of moisture, water diffusion Made of ceramic, metal, and glass can significantly increase the long term reliability of devices
Die-Attach Processes After dicing the substrate, each individual die, is mounted inside a package and attached onto a platform made of metal or ceramic Metal alloys or organic or inorganic adhesives are used as the intermediate layers solders epoxy silicones inorganic adhesives are usually glass metrices embedded with silver and resin
Die-Attach Processes Silicon and glass cannot be directly soldered to, and thus must be coated with a thin metal film to wet the surface Pt, Pd, or Au Organic adhesives are inexpensive, easy to automate and can cure at lower temperature epoxies are thermosetting plastic with cure temperature between 50 and 175 o C filled with silver or gold, they can become thermally and electrically conductive Room-temperature vulcanizing (RTV) silicones can also be used for automated manufacturing
Wiring and Interconnects Electrical interconnects: wire bonding and flip chip Wire bonding thermosonic gold bonding» The gold wire forms a ball bond to the aluminum bond pad» a stitch bond to the package lead gold wire less than 50 µm (2 mils) Aluminum bond wire up to 560 µm
Wiring and Interconnects Flip chip bonding Bonding the die, top-face-down, on the package substrate. Electrical contact are are made by means of plated solder bumps between bond pads on the die and metal pads on the substrate High density of input/output (I/O) connections than wire bonding The effective inductance is miniscule because of short height of the solder bumps Can closely package a number of distinct dice on a single package substrate
Wiring and Interconnects Processing Sputtering of a Ti layer over bond pads, and sputtering of Cu Patterning and etching of the Ti and Cu Electroplating of thick Cu layer Solder bump(a Sn-Pb alloy) is electroplated over the copper In a separate preparation process, solder paste is screen-printed on the package substrate in patterns corresponding to the landing sites of the solder bumps Automated pick-and place machines position the die, and align the bond pads to the solder-paste patterns on the package substrate. Heating in an oven or under IR radiation melts the solder into a columnar, smooth and shiny bump Flip chip may not be compatible with packaging of MEMS that includes microstructures exposed to the open environment. May lead to thin diaphragm damages.
Wiring and Interconnects Y.C. Lee, University of Colorado, Boulder
Wiring and Interconnects Microfluidic Interconnects Micropumps, microvalves Via holes Fluidic sealing Most microfluidic interconnects schemes remain at the level of manually inserting a capillary into Silicon cavity or via-hole, and sealing the assembly with silicone or epoxy
Types of Packaging Ceramic packaging Metal packaging Molded plastic packaging
Types of Packaging Ceramic packaging Process flow for the fabrication of a co-fired, laminated ceramic package with electrical pins and access ports
Types of Packaging The process fills via holes with metal (W). Via left unfilled can be used as fluid or pressure access port Pins or leads are usually brazed to the package, typically made of ASTM F-15 alloy (Fe 52 Ni 29 Co 18 ), known as Kovar Ceramic package consists of a base or a header onto which dice are attached by adhesives or solders Substrate materials: Al 2 O 3, AlN, Borosilicate, BeO thermal conductivity thermal expansion electrical resistivity dielectric permittivity
Types of Packaging Ceramic package example DMD TM type-a ceramic package for SVGA displays Ceramic package for optical MEMS can be complex and costly 114-pins Al 2 O 3 ceramic header Wire bonding establish electrical connectivity between the die and the metal traces on the ceramic header A Cu-Al brazed Kovar seal ring Transparent window consists of a polished Corning 7056 glass fused to a stamped gold-nickel-plated Kovar frame Resistance seam welding of the the seal ring on the ceramic base to the Kovar glass frame Zeolite getters to ensure long-term desiccation
Types of Packaging Metal package Metal package is robust and easy to assemble Standard TO-type (transistor outline) Can only accommodate fewer than 10 pins. TO-type package remain in use in a few applications Metal packages are attractive to MEMS, especially for microfluidic devices A metal hermetic package is often made of ASTM F-15 (Kovar ), stainless steel is also common Holes are punched, either through the bottom for plug-in packages, or sides for flat packages Metal leads are placed through the holes and beads of borosilicate glass (Corning 7052). The metal packaging of media isolated Pressure sensor
Types of Packaging Molded plastic package Not hermetic Two general approaches» postmolding» premolding In postmolding, the plastic housing is molded after the die is attached top a lead frame (a supporting metal sheet). The process subjects the die and the wire bonds to the harsh molding environment. In premolding, the die is attached to a lead frame over which plastic was previously molded. It is attractive in situations where the risk of damaging the die is high, or openings through the plastic are necessary. More expensive.
Types of Packaging Molded plastic package