introduction to microeletromechanical systems (mems) · introduction to microeletromechanical...
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Texas Christian University Department of Engineering Ed Kolesar
Introduction toMicroeletromechanical Systems
(MEMS)Lecture 14 Topics
• Microelectronics packaging issuesProtectionElectrical connections (number of wire bonds)Heat transfer
• MEMS packaging issuesCoupling to outside media (may or may not be necessary)Vacuum packaging (e.g., resonating devices)Custom packaging for specific customer needs (“mass customization” Motorola)Cost (e.g. Motorola integrated pressure sensor: 35% MEMS/CMOS chip, 45% packaging, 20% calibration)
Texas Christian University Department of Engineering Ed Kolesar
MEMS Overview
Micromachining: lithography, deposition, etching, …
Processes & Foundries
Devices & Structures
Methodology
History & Market
Introduction &
Background
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Texas Christian University Department of Engineering Ed Kolesar
Packaging
Process and methods to assemble a device into a housing for • Useful, safe, and reliable interaction with its surroundings• Protection from surroundings
[Sample Packaging Process, Maluf 2000]
Texas Christian University Department of Engineering Ed Kolesar
MEMS Testing• Electrical functionality can usually be tested at wafer
level• Other functionalities may be much more difficult to
testPressureAcceleration...
• May require testing of individual die, or testing after packaging → much more expensive
• MEMS foundries: virtually impossible to test devices submitted by various customersThis has serious consequences for the organization of MEMS industry
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Texas Christian University Department of Engineering Ed Kolesar
MEMS Calibration
• Devices come with variation from fabrication, and their behavior may drift over time
• Calibration: mapping between output signal and desired information
• Two main approaches:Tune device to match output to specKnow output signal mapping
• Techniques:Trimming of resistors, resonator beams, …Program mapping into EPROM
Texas Christian University Department of Engineering Ed Kolesar
MEMS Packaging
MEMS packaging, testing, and calibration are important and expensive, but research in this area has lagged behind
Reasons:• Less attractive as research topic?• Not crucial for proof-of-concept device designs?• Intrinsically industrial and commercial: dependent on
large volumes, customer-specific information, trade secrets
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Texas Christian University Department of Engineering Ed Kolesar
MEMS Packaging
Solutions:Systematic approach and sub-division of problem
[Senturia 2000]:• Design MEMS device and package simultaneously
Often done in different departments or companies
• Partition system wiselyFor example, monolithic vs. assembly
• Define System Interfaces• Design Specifications• Detailed Design
Texas Christian University Department of Engineering Ed Kolesar
Case Study: Commercial Pressure Sensor
• Motorola Manifold-Absolute Pressure (MAP) Sensor
• Measure mass airflow into engine to optimize air-fuel ratio
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Design Specifications
Texas Christian University Department of Engineering Ed Kolesar
System Partitioning
Design decision:• Integrated signal conditioning circuitry and trimmable
calibration resistors on MEMS chip with diaphragm
Reasons:Smaller deviceImproved interconnect reliabilityImproved overall electromagnetic compatibilityLower overall system cost
Higher development and production costHigher device complexity
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Texas Christian University Department of Engineering Ed Kolesar
Interfaces
• 3 leads for operationPowerGroundOutput
• 6-8 leads for calibration
• Calibration after die mount on package → package must support more pins than necessary for regular use
• Embedding of chip in protective silicone gel• Applied after calibration, requiring pre-compensation
Texas Christian University Department of Engineering Ed Kolesar
Next-Level Assembly
• Plastic housing for circuit board with sensor unit
• Protection and ease of handling
• Custom interfaces for specific application
• “… in MEMS devices, especially, the package and the die are inseparable. That is to say, the package affects the electrical output of the device, and, in many cases, the die affects the needed packaging.” [Monk and Shah 1996]
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Texas Christian University Department of Engineering Ed Kolesar
Current Research in Microassembly
•Very large numbers of very small components
• Independent parallel fabrication of components
•Fabrication at high density, assembly at lower density
•Hybrid systemsbuilt from standard components
Texas Christian University Department of Engineering Ed Kolesar
Why Microassembly ?• Hybrid systems fabricated in established processes from
standard components
• Independent parallel design, fabrication, and testing of components
• Fabrication at higher density, assembly at lower density
• Incompatible processes / materials for MEMS
• Unlike CMOS for electronics, there does not exist one standard monolithic fabrication process for complex microsystems (and most likely never will)
ApplicationsDisplays (LED, VCSEL), imaging arrays, wireless amplifier grids,
complex microsystems, ...
Motivation
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Texas Christian University Department of Engineering Ed Kolesar
Micromirror AssemblySandia National Labs,
Albuquerque
Microsnap fastener Prasad, Böhringer and
MacDonald, Cornell University
MicrotweezerKolesar, Jayachandran, Odom and Ruff, TCU
Examples:
Microassembly
Texas Christian University Department of Engineering Ed Kolesar
• Serial MicroassemblyAssembly one-by-one, traditional pick-and place paradigm
• Parallel MicroassemblyMultiple parts (identical or different) assembled simultaneously
deterministic: destination of parts known in advance (→ planning)
stochastic: destination of parts determined by random process (→ annealing)
Taxonomy of Microassembly
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Texas Christian University Department of Engineering Ed Kolesar
• Manual micro assembly: with microscopes and tweezers
• High-precision macroscopic robots: sub-micron resolution e.g. [Quaid, Hollis ‘96], assembly robots by MRSI (US) or Sysmelec (Switzerland)
• Teleoperated and visually based microassembly:For example, [Nelson and Vikramaditya 1997], [Bellouard et al. 1997], [Sulzmann 1997], [Coudourey et al. 1997], [Feddema and Simon 1998]
• Microgrippers: gripper sizes of 100 µm or lesse.g. [Kim, Pisano and Muller 1992], [Pister, Judy, Burgett and Fearing 1992], [Keller and Howe 1995 and 1997], [Dario et al. 1997], [Bellouard et al. 1997]
Serial Microassembly
Texas Christian University Department of Engineering Ed Kolesar
Micro Grippers
Traditional pick-and-place paradigm(one at a time)
Power off, gripper closed
Power on, gripper open
Power off, gripping
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HexSil Process
[C. Keller 1998, UC Berkeley]
Texas Christian University Department of Engineering Ed Kolesar
HexSil MicroTweezers
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Deterministic
• Self-assembling 3D structures: “micro origami” e.g. [Pister et al. 1992], [Syms et al. 1995 and 1998], [Fujita et al.
1996]
• Flip-chip, wafer-to-wafer transfer: combine devices from two (or more) wafers [Cohn and Howe 1997], [Singh et al. 1997]
• Microgripper arrays: parallel pick-and-place[Keller and Howe 1997]
Parallel Microassembly
Texas Christian University Department of Engineering Ed Kolesar
Deployable beam trusses. Nakamura Lab, Aerospace Eng., Nihon University
R. Syms, Dept. of Electrical and
Electronic Engineering, Imperial
College, London
macro
Flip-chip assembly. Cohn, Liang, Howe and Pisano, UC Berkeley
Deterministic Self-Assembly
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Wafer-to-Wafer Transfer
Transfer of vacuum lids [Cohn 1997]
Process flow, includes HexSil lid fabrication and Au/Si eutectic weld
Vacuum lid andcleaved cross section
Texas Christian University Department of Engineering Ed Kolesar
Wafer-to-Wafer Transfer
Tethered transfer[Singh et al. 1997]
Process flow, includes HexSil fabrication and Indium solder bond.
Target wafer with transferred structures
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Stochastic
• Manipulator surfaces (“programmable force fields”):e.g. [Pister, Fearing and Howe 1990], [Fujita et al. 1993], [Böhringer et al. 1994], [Liu and Will 1995], [Suh and Kovacs 1996]
• Fluidic or vibratory agitation & mating parts:“micro APOS”[Cohn, Kim and Pisano 1992], [Yeh and Smith 1994],
[Hosokawa, Shimoyama and Miura 1995]
• Nanomanipulation: inspired by chemical processes[Whitesides et al. 1991], [Requicha et al. 1997]
Parallel Microassembly
Texas Christian University Department of Engineering Ed Kolesar
Stochastic Self-Assembly
Principle: use annealing processes that reach desired minimum energy state
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surface forces:
• electrostatic forces ~ r2
• capillary forces ~ r
• van der Waals forces ~ r
• gravity ~ r3
• ambient pressure ~ r2
[From Fearing 1995]
Reminder: Scaling of Forces
Texas Christian University Department of Engineering Ed Kolesar
[Cohn and Kim and Pisano 1991][Yeh and Smith
1994]
Achieving yields of 99.9999% !
also:chemically inspired microassembly - Whitesides et al. - Hosokawa, Shimoyama and Miura
Stochastic Microassembly
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Fluidic Self-Assembly
Self-Orienting Fluidic Transport (SOFT) Assembly: MOS transistors are positioned on etched glass or plastic panel. Electric interconnects are evaporated and etched after SOFT assembly. [Yeh, Hadley and Smith 1994-1997] (Beckmen Display, Inc.)
Texas Christian University Department of Engineering Ed Kolesar
Driving force for assembly:Strong attraction between hydrophobic surfaces in water
Hydrophobic surfaces: Alkanethiol SAMs on AuOrganic lubricant
[X. Xiong et al. 2000, University of Washington]
Surface Tension Driven Self-Assembly Strategy
Hydrophobic Lubricant
Water
Substrate
Part
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Attraction Between Two Hydrophobic Surfaces in Water
Lubricant on Hydrophobic Area
Hydrophilic Area
1 mm
Demonstrated for massively parallel assembly of micro parts onto a substrate [Srinivasan et al. Transducers 1999]
Texas Christian University Department of Engineering Ed Kolesar
Controlled Self-Assembly
Organization of different parts onto desired locations
No Assembly
SAM adsorption on all the gold areas
Desorption of SAM from undesired areas
Hydrophobic
Substrate
Hydrophilic
Substrate
Hydrophobic
Substrate
Assembly
Assembly parts on desired areas
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Modulation of Surface Energies
Hydrophilic Hydrophobic
Adsorption of alkanethiol SAMs
Hydrophobic Hydrophilic
Reductive desorption of SAMs CH3(CH2)nS -Au+e-→ CH3(CH2)nS -+Au
SubstrateAu
e- e-e-
SubstrateAu
Adsorption is done by soaking surfaces in ethanolic alkanethiol solution for 2 hours or more.
AuSubstrate
AuSubstrate
Texas Christian University Department of Engineering Ed Kolesar
Optimization of desorption (C3H8S, C8H18S,C12H26S, C18H38S)
SAMs Reductive DesorptionCharacterization on Gold (111)
Experiment setup:Au (111) surfaces with SAMs Reference Electrode: SCEElectrolyte: 0.5M KOH
[Walczak and Porter 1991][Weisshaar and Porter 1992]
Peaks of SAMs Desorption
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Fabrication of Parts and Substrate
Cross-section of the part and the substrate
Top view of the substrate
Connected to Potentiostat
Cr/Au
Si
Texas Christian University Department of Engineering Ed Kolesar
SAM Reductive Desorption on Gold
Broadened Peaks of SAM Desorption
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SAM Desorption Accomplished
• Desorption Results (on the left)
Lubricant onHydrophobic Surfaces
No Lubricant onHydrophilic Surfaces
Texas Christian University Department of Engineering Ed Kolesar
Assembly Results
Assembled parts
1 mm
Free spots
Multi-step assemblywith 2 sets of parts
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Texas Christian University Department of Engineering Ed Kolesar
Summary
• Organization of various parts by electrochemical modulations of surface energies
• Optimization of desorption process• Various fabrication processes tested
Nitride/Oxide/Spin-on Glass
Texas Christian University Department of Engineering Ed Kolesar
Micro Fasteners and Locks
Instead of screws and bolts, use simple compliant locking mechanisms and “motion diodes.”
Advantages:• Self-alignment• Linear assembly motion
[Shear-lock structure after assembly, Cohn et al. 1997]
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[Compliant mechanism Donald and Pai 1993][Micro snap fastener Böhringer et al.
1995]
Design of Compliant FastenersCompliant mechanisms (“snap fasteners”):exact simulation of compliant motion (algebraic curves in C-space) [Donald, Pai ‘93]
Automatic designIdea: search of design parameter space
Texas Christian University Department of Engineering Ed Kolesar
Micro fixture array for massively parallel positioning
Fabrication in MOSIS CMOS allows local, distributed sensing and actuation
Thermal actuator
[Tahhan, Böhringer and Goldberg, UC Berkeley]
Micro Fixtures