Nanostructures: MEMS and MOEMS

David SzopinskiConsumer OptoelectronicsMarch 7, 2001

Topics

? What Is Microengineering?? Photolithography? Silicon Micromachining? Excimer Laser Micromachining? Microsystems, Microsensors, and

Microactuators? Microcomponent Assembly and Packaging

What Is Microengineering?

? Microengineering refers to the technologies and practice of making three dimensional structures and devices with dimensions in the order of micrometers.

? Two constructional technologies ofmicroengineering are microelectronics andmicromachining.

Micromachining and Microelectronics

? Microelectronics is a very well developed technology in producing electronic circuitry on silicon chips.

? Micromachining is the name for the techniques used to produce the structures and moving parts of microengineered devices.

Main Goals Of Microengineering

? Integrate microelectronic circuitry intomicromachined structures

? Produce completely integrated systems (microsystems)

Market Segment

…an estimated $10 Billion market today and a $34 Billion market in 2002. (Micromachine Devices, "European Study Sees MEMS Market at More Than $34 Billion by '02," May, 1997, page 1).

Photolithography

? Photolithography is the basic technique used to define the shape of micromachined structures

? Technique is essentially the same as that used in the microelectronics industry

The Patterning Process

Thin film of silicon dioxide on silcon substrate

Photoresist coated wafer

Exposure to ultraviolet light through mask

Develop resist

Etch pattern into oxide

Strip resist

LIGA (Lithographie, Galvanoformung, Abformung)

? Exposes PMMA (polymethyl methacrylate) plastic with synchrotron radiation through a mask (X-ray lithography)

? Vertical wall structures with spectacular accuracy

? Metal is then plated into the structure

LIGA Process

Silicon Micromachining

? Bulk Silicon Micromachining– pattern thin films that have been deposited on a

silicon wafer – shape the wafer itself, to form a set of basic

microstructures? Surface Silicon Micromachining

– produce complex microstructures on the surface of a silicon wafer using techniques for depositing and patterning thin films

Basic Structures

? Patterning of insulated electrical conductors? Application: electric fields to manipulate

individual cells

Form V shaped grooves,or cut pits with taperedwalls into silicon

Basic Structures

? A combination of dry etching and isotropic wet etching can be used to form very sharp points

column with vertical sidesRIE (Dry Etch Process)

undercut edge mask to form sharp edgeWet Etch Process

Surface Micromachining

? Build up the structure in layers of thin films on the surface of a silicon wafer (or any other suitable substrate)

? Process employs films of two different materials, a structural material (commonlypolysilicon) and a sacrificial material (oxide)

Surface Micromachining

? Films are deposited and dry etched in sequence

? Sacrificial material is wet etched away to release the structure

? The more layers, the more complex the structure, and the more difficult it becomes to fabricate

Simple Surface MicromachinedCantilever Beam

A sacrificial layer ofoxide is depositedon the surface of thewafer

Polysilicon is then deposited and patterned using RIE techniques

Simple Surface MicromachinedCantilever Beam

The wafer is then wet etched to remove the oxide layer under the beam, freeing the structure

Fabrication Of Chamber On Silicon

? Chamber is defined by a volume of sacrificial oxide

? Polysilicon is deposited over wafer surface? Window is dry etched, wet etch removes oxide,

leaves windowed chamber

Excimer Laser Micromachining

? Produce relatively wide beams of ultraviolet laser light

? Material adjacent to the area machined is not melted or distorted by heating effects

Contact Alignment Projection (Stepper)

Excimer Laser Micromachining

? Shape of structure is controlled by using a chrome on quartz mask

? Vertical sides can be created? Adjusting the optics can produce structures

with tapered sidewalls

Microsystems

? A microsystem can be considered to be any device or unit made up of a number ofmicroengineered components

Control System

Model For Microsystem: Control System

? Microsensors detect changes in the parameter to be controlled

? Electronic control logic operate microactuatorsbased on information from the sensors

? Bring the parameter to be controlled within the desired limits

Transducers

?A transducer is a device that converts one physical quantity to another

?Sensors and actuators are special types of transducers

Sensors vs. Actuators

?A sensor is a device that converts one physical or chemical quantity to an electrical one

?An actuator is a device which converts an electrical quantity into a physical or chemical one

Types Of Sensors

? Thermal Sensors– Thermocouples, Thermoresistors, Thermal flow-

rate sensors

? Radiation Sensors– Photodiodes, Phototransistors, CCD’s,

Integrated Optics

? Magnetic, Chemical, and Mechanical Sensors

Thermocouples

? When two dissimilar metals (e.g. copper and iron) are brought together in a circuit, and the junctions are held at different temperatures, a small voltage is generated and an electrical current flows between them

A Working Thermocouple

? V = ( Pa - Pb )( Ta - Tb )

? The open circuit voltage is related to the temperature difference (Ta - Tb ), and the difference in the Seebeckcoefficients of the two materials (Pa – Pb)

Photodiodes and Phototransistors

? The simplest photodiode is a reverse biased p-n junction

? without light, only a small amount of current flows (the dark current)

? with light, additional carriers are generated, and more current flows

Photodiodes and Phototransistors

? typically work in the visible light - near infrared region of the spectrum

? high impedance devices and operate at relatively low currents (typically 10uA dark current, rising to 100uA when illuminated)

? fairly linear responses to increasing illumination, very fast response times

Microactuators

? Moving micromirrors to scan laser beams, or switch them from one fiber to another

? Drive cutting tools for microsurgicalapplications

? Drive micropumps and valves for microanalysisor microfluid systems

? Microelectrode devices to stimulate nervous tissue in neural prosthesis applications

Electrostatic Actuators

? Comb Drives popular with surface micromachined devices

Comb Drives

? Consist of many interdigitated fingers? Applied voltage causes an attractive force to

develop between the fingers, which move together

? The increase in capacitance is proportional to the number of fingers

? To generate large forces, large numbers of fingers are required

Close-up of Comb Drive Springs

Wobble Motors

A surface micromachined wobble motor design

Magnetic Actuators

? These microstructures are often fabricated by electroplating techniques, using nickel

? Nickel is a (weakly) ferromagnetic material ? Lends itself for use in magnetic microactuators

Linear Motor

The magnet resting in the channel is levitated and driven back and forth by switching current into the various coils on either side of the channel at the appropriate time.

Problems

? The coils are two dimensional (three dimensional coils are very difficult to microfabricate)

? The choice of magnetic materials is limited to those that can be easily micromachined, so the material of the magnet is not always optimum

? Leads to rather high power consumption and heat dissipation for magnetic actuators

Microcomponent Assembly and Packaging

? Microengineered devices have the potential to be as inexpensive as silicon chips are today.

? Two conditions must be met:– 1) the fabrication process has a high yield (most of the

devices on a wafer function properly, and will continue to do soafter packaging)

– 2) batch processing techniques are used for as much of the process as possible (i.e. large numbers of devices per silicon wafer, and a large number of wafers are processed at the same time at each fabrication step)

Exceptions?

? It is difficult to achieve high yields with complicated microsystems.

? If the device does something that is very important, and cannot be done any other way, a low yield and expensive devices can be justified.

Additional Costs

? Assembling complex devices from many microscopic parts generally involves handling the devices individually.

? This adds significantly to the cost of the finished part (tens to hundreds of times the cost of the actual active part of the device -depending on the complexity and requirements of packaging).

Alignment and Assembly

? Various techniques can be used to automatically align different components of a microsystem.

? Due to the small size of the parts involved, surface tension forces (in liquids, such as water) can be used to assemble microengineered devices.

“V” Groove

"V" grooves are relatively easy to fabricate in silicon, and these can be used to align optical fibres to waveguides on the chip

Conventional IC Packaging

? IC packages are usually ceramic (for high reliability applications), or plastic.

Cool Stuff

Mirror mechanism with a spider miteSpider mite has legs anchored on a mirror drive assembly.

More Cool Stuffhttp://www.mdl.sandia.gov/micromachine/images.html

World's smallest microsteam engine

Indexing Motor

Grain of pollen and red blood cellsDrive gear chain and linkages, with a grain of pollen (top right) and coagulated red blood cells (lower right, top left) for scale

References

? http://www.stdmems.com/? http://www.trimmer.net? http://mems.isi.edu/archives/otherWWWsites_tutorial.html? http://www.dbanks.demon.co.uk/ueng/? http://www.people.cornell.edu/pages/akt1/memsmain.html? http://www.cfdrc.com/datab/research/mems/momems.html? http://www.arpa.mil/MTO/MOEMS/index.html? http://www.memsoptical.com/memtut1.htm