Introduction to Microeletromechanical Systems (MEMS) · Introduction to Microeletromechanical Systems (MEMS) ... Coupling to outside media ... but research in this area has lagged

11

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)


MEMS Overview

Micromachining: lithography, deposition, etching, …

Processes & Foundries

Devices & Structures

Methodology

History & Market

Introduction &

Background

22


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]


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

33


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


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

44


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


Case Study: Commercial Pressure Sensor

• Motorola Manifold-Absolute Pressure (MAP) Sensor

• Measure mass airflow into engine to optimize air-fuel ratio

55


Design Specifications


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

66


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


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]

77


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


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

88


Micromirror AssemblySandia National Labs,

Albuquerque

Microsnap fastener Prasad, Böhringer and

MacDonald, Cornell University

MicrotweezerKolesar, Jayachandran, Odom and Ruff, TCU

Examples:

Microassembly


• 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

99


• 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


Micro Grippers

Traditional pick-and-place paradigm(one at a time)

Power off, gripper closed

Power on, gripper open

Power off, gripping

1010


HexSil Process

[C. Keller 1998, UC Berkeley]


HexSil MicroTweezers

1111


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


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

1212


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


Wafer-to-Wafer Transfer

Tethered transfer[Singh et al. 1997]

Process flow, includes HexSil fabrication and Indium solder bond.

Target wafer with transferred structures

1313


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


Stochastic Self-Assembly

Principle: use annealing processes that reach desired minimum energy state

1414


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


[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

1515


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.)


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

1616


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]


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

1717


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


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

1818


Fabrication of Parts and Substrate

Cross-section of the part and the substrate

Top view of the substrate

Connected to Potentiostat

Cr/Au

Si


SAM Reductive Desorption on Gold

Broadened Peaks of SAM Desorption

1919


SAM Desorption Accomplished

• Desorption Results (on the left)

Lubricant onHydrophobic Surfaces

No Lubricant onHydrophilic Surfaces


Assembly Results

Assembled parts

1 mm

Free spots

Multi-step assemblywith 2 sets of parts

2020


Summary

• Organization of various parts by electrochemical modulations of surface energies

• Optimization of desorption process• Various fabrication processes tested

Nitride/Oxide/Spin-on Glass


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]

2121


[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


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

Documents

Introduction to Microeletromechanical Systems (MEMS) · Introduction to Microeletromechanical Systems (MEMS) ... Coupling to outside media ... but research in this area has lagged