Packaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Packaging for MEMSKirt R. Williams, Ph.D.Member of the Technical StaffAgilent TechnologiesAgilent Labs, Palo Alto, California, USANovaSensorK. Williams/NovaSensorNovaSensorK. Williams/NovaSensorK. Williams / NovaSensorcourtesy M. Judy/Analog DevicesK. Williams / NovaSensorcourtesy M. Judy/Analog DevicesPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Packaging for MEMS Outline• Packaging goals• Materials properties:Thermal stress, heat transfer,and hermetic sealing• Types of packaging• On-chip• Metal• Glass• Ceramic• Plastic• Comparison• Packaging costs• SummaryPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Packaging for MEMS Outline• Packaging goals• Materials properties:Thermal stress, heat transfer,and hermetic sealing• Types of packaging• On-chip• Metal• Glass• Ceramic• Plastic• Comparison• Packaging costs• SummaryPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Packaging GoalsGoals of Packaging for ICs• Electrical power in• Signal input/output• Remove heat• Protect chip during product manufacturing and in use• Breakage• CorrosionPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Packaging GoalsGoals of Packaging for MEMS• They must interact with the outside world• Each has its own unique additional requirementsExamples:• Contact with outside gas or liquid• Cannot allow packaging stress to affect signal• Hermetic seal• Lifetime (may be long or short--disposable)• And for a product, packaging must be doneat a sellable costApplication and market determine sales price• Microelectromechanical devices/systems usually haveboth electrical and non-electricalinput or output signalsPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000• Accelerometers/gyros• Electrical contact• Inertial force transferred without filtering• Known mounting angle• Maintain atmosphere(e.g., vacuum, overpressure, no humidity)Packaging GoalsPackaging Requirements for Specific MEM Devices• Pressure sensors• Electrical contact• Part of chip contacts pressure to be measured• Another part of chip may contact a different pressure(e.g., atmospheric pressure)Packaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000• Micromachined infrared imager• Electrical contact• IR light enters• Maintain vacuum for thermal isolation• Optical switch• Electrical contact• Light enters and exits• Fibers accurately held in place• May need hermetic seal with high voltagesPackaging GoalsPackaging Requirements forSpecific MEM Devices (cont.)• Projection display• Electrical contact• Light enters and exits without distortion• May need to conduct heat outPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000• Flow sensor• Electrical contact• Flow passes chip in controlled manner• Valves• Electrical contact• Flow enters and exits• Need particle filteringPackaging GoalsPackaging Requirements forSpecific MEM Devices (cont.)• Ink-jet head• Electrical contact• Ink exits• Smooth, low-wear, low-friction surface• Low costPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000• Wet chemical, pH, and gas sensors• Electrical contact• Contact chemical or gasPackaging GoalsPackaging Requirements forSpecific MEM Devices (cont.)• Microfluidics (e.g., electrophoresis)• Electrical contact or electric field• Flow enters• Displaced air is removed• RF devices: filters, mixers, etc.• Signals in and out without attenuation or reflection(good transmission lines)• Good signal isolation/low couplingPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000• Each application has its own unique requirements→ → → → Many different custom-made packages• Some standardization for well-commercialized devices• Pressure sensorsSame engineers and founders at several co.’s• AccelerometersOff-the-shelf packages• Must develop package in conjunction with chip designNot afterwards!• Lose some of MEMS size and cost advantage with packaging• Packaging is usually dominant part of MEMS cost• Including more in a single package can save cost(e.g., sensor and IC; sensor and actuator)Packaging GoalsPoints to ConsiderPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Packaging for MEMS Outline• Packaging goals• Materials properties:Thermal stress, heat transfer,and hermetic sealing• Types of packaging• On-chip• Metal• Glass• Ceramic• Plastic• Comparison• Packaging costs• SummaryPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Thermal StressThermal ExpansionUnconstrained thermal strain εεεε is proportional tothermal coefficient of expansion (TCE or αααα)and temperature riseεεεε = α ∆α ∆α ∆α ∆TTCE often given in ppm/K (= 10-6strain per °C)Silicon has a very low TCE (compared to other materials) ofεεεε = 2.6e-6 K-1at room temperatureTCE of Si rises with temperature,but linear approximation is usually usedPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Thermal StressThermal StressDifferent expansion rates of silicon chipand packaging result in stress on chipthat varies with temperatureStress at top of silicon (where sensitive part usually located)depends on• TCE difference• Thicknesses• Young’s modulus• ...and interface (die-attach)packagesilicondie-attachPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Thermal StressThermal Stress (cont.)Example:63% tin / 37% lead solder melts/freezes at 183°CPackage will shrink more than silicondown to room temperature,resulting in permanent compressive stress in chipCan bend beams, cause offsets, etc.packagesilicondie-attachIf a hard die-attach material is usedand high temperature is needed(e.g., to melt solder),stress will be “frozen in” whendie-attach material hardensPackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Thermal StressTemperature Cycling• With a hard die-attach material (e.g. solder)stress is transferred from package to chip• Large temperature swings can fracture chip or packageor cause plastic deformation of the die-attach material• Temperature cycling can cause fatigue of chip, package,or die-attach material, leading to fracturePackaging for MEMSC. 2000 Kirt Williams BSAC Seminar, Oct. 2000Thermal StressReducing Thermal StressQuestion:How can thermal stress at chip
View Full Document
Unlocking...