MEMS application fields:

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2011.09.13.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 1 Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework**

Consortium leader

PETER PAZMANY CATHOLIC UNIVERSITY

Consortium members

SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund ***

**Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben

***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg.

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VLSI Design Methodologies

MEMS technologies

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(VLSI tervezési módszerek)

(MEMS tecnológiák)

PÉTER FÖLDESY

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The topics are covered in this chapter:

• MEMS application fields

• Taxonomy of the the MEMS devices and technologies

• Manufacturing processes

• Design methods and tools

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Courtesy of Sandia National Laboratories, SUMMiT(TM) Technologies, www.mems.sandia.gov

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Section I

MEMS application fields, reasons why and where to use MEMS solutions

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What is MEMS for?

• Micro-Electro-Mechanical Systems (MEMS) are integration of mechanical elements, sensors,

actuators, and electronics on a common silicon substrate using microfabrication technology.

MEMS application fields:

• Displays, e.g the DMD chip in a projector.

• Optical switching technology

• Bio-MEMS applications in medical and health related technologies e.g. Lab-On-Chip, …

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MEMS application fields:

• Inkjet printers.

• Accelerometers in modern cars for a large number of purposes including airbag control.

• MEMS gyroscopes used in modern cars and other applications to detect yaw.

• Silicon pressure sensors e.g. car tire pressure sensors, and disposable blood pressure sensors.

• Even greater integration at nanoscale called NEMS (Nano-Electro-Mechanical Systems)

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Why the MEMS are useful and reliable?

• Integrated mass manufactured process, high yield

• There are no assembly steps of the mechanics

• Standard microelectronic friendly manufacturing steps

• Standard photolitography

• Design process is somewhat similar to the classic microelectronic designs, plus, mechanics

• Its raw materials are reliable and available: Si, quartz, glass, polimers

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Why the MEMS are useful?

• In small sizes, the forces (electrostatic, mechanical) can be modeled by classic theories

• The most widely used raw material is the silicon:

• Tensile srength is very good (does not tear)

• Young modulus is near to steel (it bends, does not break)

• Its structure is mechanically stable, no hidden failures

• Up to 500C^o its mechanical properties remain the same

• Piezoresistivity is high

• Other raw material, such as SiGe are used as well.

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Section II

Taxonomy of the the MEMS devices and technologies

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The two mainstreams:

Bulk micromachining

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Surface micromachining

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Bulk micromachining

• Used for pressure, gas, infrared and other sensors

• Cheap and has good yield

• The substrate is formed and etched, mostly with chemical etching (not only)

• After masking, the unnecessary volumes are removed

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Bulk micromachining manufacturing process

• The bulk micromachining is based on patterned etching of the substrate. The used etching process can be various. The figure below shows the basic steps of an anisotropic etch process.

{111}

Silicon frame

Silicon nitride patterned mask

{111}

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More complex example: Manufacturing flow of a pressure sensor

• The need is a piezoresistive film pair hanging on a bridge, that has an airgap below.

• The film should be connected to metal wires to make connection.

• There is also a need for a supporting wafer below

• The supporting wafer is perforated with a hole towards the air gap to avoid the temperature difference caused

mechanical stress on the bridge.

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Insulator deposition Deposition of metal and Diffuse piezoresistors pattern generation on it

Electrochemical backside etch to form cavity

Anodic bonding of the glass handle wafer

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Surface micromachining

• The method to create moving parts

• Not the substrate is manufactured, but structures are build on top of it

• Layer after layer, sandwiches of structural material (e.g. Si) and sacrificial material (SiO₂) are formed

• The moving parts are the remaining structural material

• The sacrificial material is removed to let wholes inside the structures.

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Surface micromachining manufacturing steps

Substrate is deposited by sacrificial material

Structural objects are deposited and the s. material is dissolved

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Microneedles, brain neuroprobes

• Actually a usual bulk micromachining technology is used for neuroprobes, combined with metal electrode formation or CMOS readout and amplifier circuits, but as a final step, the wafer is etched completely around the needles

(Dissolved wafer technology)

Neuroprobes project dissemination

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Section III

MEMS manufacturing process and methods

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How the different layers are built up?

• Liftoff

• The process is useful for patterning materials that cannot be etched without affecting underlying materials on the

substrate.

• In lift-off processes a sacrificial material, e.g. photoresist, is first deposited and patterned on the substrate.

• The material of interest is then deposited on top and the

sacrificial material is subsequently removed, leaving behind only the material deposited directly on the substrate.

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• Evaporation

• In low pressure vacuum, the evaporated material is placed near the target where the material condensates. The

evaporation can be achieved by various methods

• E-beam heating, Resistive, RF-induktiv

• Oxidation

• The substrate is heated up and the chamber in which it is placed is filled with oxygen rich atmosphere. It can be high pressure or wet chemical as well.

• SiO2

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• LPCVD

• Low-pressure chemical vapor deposition: the heated substrate reacts with the source gas.

• Si nitride, polysilicon

• PECVD

• Plasma enhanced CVD, requires lower temperature and plasma speeds up the reaction.

• Spin casting

• Centrifugal force and heating is used to produce castings from a dissolved liquid on a substrate.

• Photoresist in photolithography

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• Sputtering

• In a high vacuum or in inert reactor chamber plasma evaporates (mechanically) the source material that condensates on the substrate.

• Aluminium, Nickel, Gold, Titan, Tungsten (W)

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Etching processes, the main categories:

• Isotropic

• The etching process is the same in all directions.

• Anisotropic

• Crystal plane dependent etching

• Wet (chemical)

• Dry (plasma)

Wet etch Plasma (dry) etch

IsotropicAnisotropic

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• The wet etching is the simplest etching technology. All it requires is a container with a liquid solution that will dissolve the material in question.

• The dry etching technology can split in three separate classes called

• reactive ion etching (RIE)

• sputter etching,

• vapor phase etching.

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• Reactive Ion Etching (RIE),

• The substrate is placed inside a reactor in which

several gases are introduced. Ions in plasma react at the surface forming another gaseous material

(chemical part). Physical part is similar to sputtering.

• A special subclass of RIE is deep RIE (DRIE). Etch depths of hundreds of microns can be achieved with almost vertical sidewalls (remember for the 3D

integration VST). The technology is so-called

"Bosch process“.

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• Example for DRIE process

http://www.mechatronictips.com/2009/07/

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• Sputtering

• Sputter etching is essentially RIE without reactive ions. The substrate is now subjected to the ion

bombardment.

• Vapor phase etching

• Simpler dry etching method. The wafer to be etched is placed inside a chamber, in which one or more

gases are introduced. The material to be etched is dissolved at the surface.

• SiO₂ etching using hydrogen fluoride (HF) and silicon etching using xenon diflouride (XeF₂)

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• Special methods

• LIGA is an x-ray lithography method to form deep and narrow holes, valleys.

• Integrated technologies

• Combination of microelectronics and micromachining.

Classical CMOS technology on top of the substrate, while typically bulk micromachining is applied.

Accelerometers, digital microphones, gas sensors, etc.

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Section IV

Design flow and combined CMOS+MEMS technologies

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MEMS / MEMS design example

• The design of structures at different layer is very similar to the design of standard layout objects.

• So, usual standard layout drawing and checking CAD tools can be used as input.

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MEMS / MEMS plus IC design flows

• Process selection – as many process as many

products, the designer needs to understand the steps and can usually make the right cocktail of steps.

• Multidisciplinary design and 3D modeling

• Exists design environments (Coventor, Intellisense, Tanner’s L-Edit, SoftMEMS, )

• A few standardized cell library exists

• Models for mechanical, electrical simulations

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• Multidisciplinary design and 3D modeling

• Unlike ICs, which consist only of electronic

components, MEMS devices combine technologies from multiple physical domains and may contain

electrical, mechanical, optical or fluidic behavior that should be simulated in parallel:

• Fininte element simulators (FEM)

• Boundary element (BEM, electrostatic)

• Volume of fluid

• Electrokinetics for microfluidics

• Analog behavioral description languages

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Conclusions

• The MEMs technologies are enabling technologies for miniature electro-mechanics

• Many possible technological steps are available and there is no standardization as in IC design.

• The MEMs design requires other engineering

competences, not only electrical, like mechanics, hydrodynamics, physics

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Recommended literature

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http://www.mems-exchange.org/

Rich collection of technology descriptions about advance MEMS and nanoscale projects through prototyping and production.

The MEMS Handbook, Second Edition - 3 Volume Set Mohamed Gad-el-Hak (Editor)

Publisher: CRC Press;

2 edition (December 22, 2005)

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Comprehension questions:

I. What is MEMS stand for?

II. What are the major MEMS technologies, what are their application fields?

III. List layer building and removing technologies.