CBE581 chap 15

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14. Packaging

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Introduction Three major steps for electronic and mechanical micromachine fabrication 1. Fabrication with additive and subtractive processes 2. Packaging such as bonding, wafer scribing, lead attachment and encapsulation 3. Testing (leak test, electrical integrity and sensor functionality)

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IC Packing provides 4 functions 1. Signal redistribution 2. Mechanical support 3. Power distribution 4. Thermal management Packaging in IC vs. in Mechanical Micromachines

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Packaging in MEMS Protecting from the environment and interaction with environment. Packaging problem is the least severe for a physical sensor and the most severe for chemical and biological sensor. Hybrid MEMS in which MEMS and electronics are fabricated in separate processes and put together afterward, is recommended. Dicing Final step in fabrication of a 3D microstructure, and the first in packaging. Cutting processes involve surfactants, cleanliness, and blade/depth ratio applied to MEMS wafers If dices are less than 0.5mm on a side, edge definition such as anisotropically etched V-grooves and separation of individual devices are recommended. Most mechanical sensors and actuators are equipped with a bonded cap or cover protecting them during dicing, followed by a final sacrificial release.

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Cavity sealing Sealing of Polysilicon and Silicon Nitride Cavities Cavity Sealing and Bonding Reactive sealing

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Sealant films, such as oxides and nitrides, can be deposited over small etchant holes.

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Packaging shells takes long time due to the long etch process through the etch holes at the perimeter of the shell. Permeable polysilicon windows are used to remove the underlying sacrificial PSG using concentrated HF in 120s.

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Cavity is formed by selective etching of p+ epitaxial Si over more heavily doped Si p++ layers. Epitaxial Cavity Sealing

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Reactive sealing requires 1000oC and thick SiN sealing requires 850oC. Some sealant gas deposits on the encapsulated micro devises. Tethered cap structures are sealed down to the substrate employing a low-temperatures Au-Si eutectic bond at 363oC by HESIL process. HEXSIL Cavity Sealing

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Glass to Silicon (L1 wafer-scale die bonding) Sodium-rich glass and metal make bond. Coring #7070, soda lime #0080, and potash soda lead #0120, and aluminosilicate #1720 are suitable besides pyrex. Si-Pyrex bonding occures between 180 and 500oC. Depending on the thickness of the glass and the temperature, voltages between 200 and 1000V. Bonding Field-Assisted Thermal Bonding (anodic bonding or electrostatic bonding)

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The operating temperatures are near the glass- softening point but well below its melting point, as well as below the sintering temperature of standard AlSi metallization. Advantage of a low-temperature process with a lower residual stress and less stringent requirements for the surface quality of the wafers. Surface roughness Ra < 1 ㎛ Dust free Native or thermal oxide layer on Si must be thinner than 200 nm.

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Anodic bonding mechanism is not clean. At elevated temperatures, the glass becomes a conductive solid electrolyte and the bonding results through the migration of sodium toward the cathode. Sodium migration leaves a space charge (bound negative charges) in the region of the glass/silicon interface. The high electric field between the glass and Si results in an electrostatic force that pulls the glass and Si together. Problems 1. Difficult technology 2. Mismatch in the thermal coefficient between the glass and Si. 3. The viscous behavior of the glass results in degraded long-term stability of the components. Field-assisted Thermal Bonding Modification NES used a Ti mesh bias electrode (400 oC, 600 V less than 5 min bonding). Protecting Si (Glass-Al-SiO2-Si or Glass-poly Si – SiO2 – Si). Two Si wafers between 4 – 7 ㎛ thick borosilicate glass.

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Field-assisted bonding between two Si wafers containing thermally grown oxide film (1 ㎛ thick) is succesful (850 – 950 oC, 30 V for 45 min) Silicon Fusion Bonding Based on a chemical reaction between OH group at the surface of oxide layer of wafer Roughness must be smaller than 4 nm. Oxidize Si/Si, oxidized Si/ oxidized Si, Si/Si, SiN layered Si/Si, SiN layered Si/Si- N layered Si, GaAs/Si, Si/glass. Before fusion bonding, the oxidized Si surface must be hydrated by soaking wafers in H2O2-H2SO4, diluted H2SO4, or boiling HNO3. Oxygen-plasma treatment increases the number of OH groups. Then, the wafers are rinsed in deionized water and dried. Self bonding occurs Polymerization of the silanol group is believed to be the main bonding reaction.

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Thermal bonding with intermediate layers LPCVD PSG (1-2 ㎛ thick) layer with two Si wafers at 1100 oC for 30 min shows good bonding as long as the wafers are clean and reasonably flat.. Low-temperature sealing glasses (glass frits #75xx) with sealing temperatures from 415 to 650 oC. RF sputtering of corning 7593 glass frit to obtain 8000 Å thick glass film. APCVD boron oxide – hygroscopic problem B-doped SiO2 (softening T = 450 oC) – crack problem SOG and sodium silicate layer To bond the unpolished back side of a Si die to another Si part, an aqua-gel based on a hydrophilic pyrogenic silica powder and PVA as a binder may be used. (15 min at RT = 1 Mpa bond strength) Eutectic Bonding Au-Si eutectic bonding at 363 oC. Difficult to obtain complete bonding over large areas.

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Bonding with organic photopatternable layers Native oxide prevent the bonding to take place Great mounting stress, causing long-term drift due to the relaxation of the built-in stress. Lithographic patterning of thick resist layers AZ-4000 ans SU-8 photoresist, Liga resist PMMA. Low bonding temp, high bond strength, no metal ions, reduced stress. L1 and L2 packaging are possible Impossible hermetic seals, high vapor pressures, poor mechanical properties. Photopatterned bonding

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Bonding of plastic to plastic Using adhesives, tapes, plastic welding (hot plate and ultrasonic welding), and selected solvents by partially dissolving the bonding surfaces. Development of low-cost, high-speed, and reliable bonding techniques for microfluidic devices is challenging.

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Alignment during Bonding Guiding holes -> 50 ㎛ accuracy Bonding machine equipped with an in situ optical alignment set up (- 2.5 ㎛ accuracy) Imaging and Bond Strength and Package Hermeticity Tests Imaging a bonded pair of Si wafers: IR transmission, ultrasound and X-ray topography Mechanical test Alignment during Bonding Guiding holes -> 50 ㎛ accuracy Bonding machine equipped with an in situ optical alignment set up (- 2.5 ㎛ accuracy)

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Hermeticity test was carried out by He leak detection. (5 X 10-11 – 5 X 10-10 Torr l/s leak rate) FTIR measurement of N2O inside sealed Si cavity. To control cavity pressure for critical damping of packaged micromechanical devices, non-evaporable getters (Ni/Cr ribbon covered with a mixture of porous Ti and Zr-V-Fe alloy that absorbs gases after ativation at 400 oC)

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Higher Levels of Packaging – L2 to L5 Sensor Die attach and Wire Bond in a TO-8 Header After dicing a sensor die, die was attached to a TO-8 header.

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Die Protection Vapor-deposited organics for mildly aggressive environments (2 – 3 ㎛ poly(p-xylene)). Silicone oil over the die. Coating of the die surface with soft substances. SiC coating for harsh environments. Plastic or ceramic cap for particle and handling protectioin. Welded-on Ni cap with pressure pore.

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Stress Isolation and Thermal Management Sensor elements should not be subject to undesirable mechanical stresses originating from their packaging structure. Multichip Packaging Micromachined chips can be packed laterally as in multichip modules (MCM).

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Connections between Layers (Vias) Wet etching (aspect ratio <1) Dry etching (aspect ratio ≈ 30) Through-wafer electrical interconnect fabrication compatible with standard CMOS processing (High-density SF6 plasma ; Bosch Process) Laser Drilling (aspect ratio ≈ 50) Ultrasonic Drilling Temperature Zone Melting (TZM) Via formation and metal deposition are one and the same process Interconnects between plastic layers Radiant heat (1000 – 1200 oC) Al SiAl eutectics

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Partitioning Partitioning is one of the major challenges in MEMS How far can we push integration of electronics with the MEMS sensing function? What can we include with the MEMS disposable ? What can we put into the fixed reader instrument ? On board or off-board fluidics ? Battery or main power ? Monolithic vs. Hybrid MEMS Hybrid integration means combining thin film Ics with thick film technology Hybrid sensor keeps the electronics separate from the sensor Tow pieces of Si on the same substrate connected by a short wire bridge. Si sensor mounted in a header plugged into an electronics board. In monolithic MEMS, electronics and MEMS elements are cofabricated within one single sequential silicon process, in which yield is low.

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Partitioning in a Microfluidic Instrument Nozzle, pump, channel, reservoir, column, mixer, oscillator, diode, amplifier, valves

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Fluid Propulsion Methods and MEMS Integration Mechanical pumps : Piezoelectric, electro-osmotic, electrowetting, electrohydrodynamic pumping. Acoustic streaming : Constant fluid motor induced by an oscillating sound field at a solid/fluid boundary. Mixing is possible Electrophoresis / Electro-osmosis Centrifugal pumping Vacuum Pressure Reservoir Heating and Cooling and MEMS Integration Electrical current passing through resistors integrated on thin membrane. External heating and cooling system Heating fluid within the micro instrument using radiation (ir, rf or microwave)

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Sample Introduction Flow injection analysis (rotary of sliding valves) Creating wells into which the sample is dropped. Then the sample is then wicked into an internal chamber by capillary action. Micro and Nano Assembly By humans with tweezers and microscopes or pick-and-place robots.

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Scaling of the Assembly Process Surface forces dominate over volume forces. To avoid some of these problems, the smallest components are often manipulated in a liquid medium. Micro Assembly Examples Serial micro assembly Optical tweezer : light has momentum and can be used to catch and manipulate objects in a size range from nanometers to micrometers. 0.7 – 1.06 ㎛ wave length 25 – 500 mW in a focal spot between 0.5 and 1.0 mm in diameter Laser scalpel : cutting biological objects inside cells. STM to manipulate individual atoms Parallel micro assembly HELIX flip-chip process Microgripper arrays

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Stochastic approaches Assemble magnetically coated semiconductor parts employing an array of magnetic sites. Solvent-surface force micro assembly Self-assembly using electrostatic levitation At nanogen, electric fields are used to transport and the control the placement of proteins, RNA and DNA.

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DNA-Meditated Assembly