SEMINAR ON MICRO-ELECTRO-MECHANICAL SYSTEMS (MEMS)

A.RADHAKRISHNAN MECHANICAL ENGINEERING, CPT COLLEGE

THIRUVANANTHAPURAMPART-1

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MEMS

What is MEMS Technology ? What are MEMS ? Design Fabrication Applications

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What is MEMS Technology?

Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through micro-fabrication technology.

While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes).

The micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.

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What are MEMS?

Micro-electro-mechanical systems.

Very small mechanical assemblies.

Hinged plate

Hinge close-up

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Micro-Electro-Mechanical Systems (MEMS)

MEMS promises to revolutionize nearly every product category by bringing together silicon-based microelectronics with micromachining technology, making possible the realization of complete systems-on-a-chip.

MEMS is an enabling technology allowing the development of smart products, augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators and expanding the space of possible designs and applications.

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Micro-Electro-Mechanical Systems (MEMS)

How small exactly?

The photo above shows a silicon motor together with a strand of human hair.

MEMS systems are measured in micrometers (microns), that is, millionths of a meter. The diameter of human hair is about 100 microns.

silicon motor together with a strand of human hair

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Micro-Electro-Mechanical Systems (MEMS)

Microelectronic integrated circuits can be thought of as the "brains" of a system and MEMS augments this decision-making capability with "eyes" and "arms", to allow microsystems to sense and control the environment.

Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena.

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Micro-Electro-Mechanical Systems (MEMS)

The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose. Because MEMS devices are manufactured using batch fabrication techniques similar to those used for integrated.

MEMS micromotor

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Micro-Electro-Mechanical Systems (MEMS)

Microelectromechanical systems (MEMS) consist of extremely tiny mechanical elements, often integrated together with electronic circuitry.

MEMS devices are made in a similar fashion to computer microchips. The advantage of this manufacturing process is not simply that small structures can be achieved but also that thousands or even millions of system elements can be fabricated simultaneously. This allows systems to be both highly complex and extremely low-cost.

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What is Micro-engineering ?

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

The two constructional technologies of microengineering are microelectronics and micromachining.

Micro-electronics, producing electronic circuitry on silicon chips, is a very well developed technology.

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

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What is Micro-engineering ?

One of the main goals of Micro-engineering is to be able to integrate microelectronic circuitry into micromachined structures, to produce completely integrated systems (microsystems). Such systems could have the same advantages of low cost, reliability and small size as silicon chips produced in the microelectronics industry. When considering such small devices, a number of physical effects have different significance on the micrometer scale compared to macroscopic scales. Interest in microengineering has spawned or renewed interest in a number of areas dealing with the study of these effects on microscopic scales. This includes such topics as micromechanics, which deals with the moving parts of microengineered devices, and microfluidics, etc.

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What is Micro-engineering ?

The some of micromachining techniques that are in use / under development. Silicon micromachining is given most prominence, since this is one of the better developed micromachining techniques. Silicon is the primary substrate material used in the production microelectronic circuitry (ie, bettersilicon chips), and so is the most suitable candidate for the eventual production of microsystems.

The Excimer laser is an ultraviolet laser which can be used to micro machine a number of materials without heating them, unlike many other lasers which remove material by burning or vaporising it. The Excimer laser lends itself particularly to the machining of organic materials (polymers, etc).

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MUMPS Design Rules

Sep

ara

tion

Cut outCut in

Enclose

14 mandatory design rules, 4 classes

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Behavior

Displacement scales as linearly Capacitance scales as size2

Mass scales as size3

(Forget most of what you know about electromechanical systems).

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Design Tools

L-Edit (Tanner) extends SPICE for mechanical analysis.

MEMCad (Microcosm).

VHDL in the works, but not ready yet.

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Silicon Wafer

A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuit and other micro devices.

The wafer serves as the substrate for microelectronic devices built in and over the wafer and undergoes many micro fabrication process steps such as doping or ion implantation, etching, deposition of various materials,and photolithographic patterning.

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Fabrication

Bulk micro-machining etch into the substrate.

Surface micro-machining build up layers above the substrate and etch.

LIGA deep structures.

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Bulk micro-machining

Pressure sensing diaphragm example.

Etch with KOH (potassium hydroxide).

Etch is anisotropic. stopped in (1,1,1) direction

SiC also stops etch and forms diaphragm.

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Bulk micro-machined example

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Surface micromachining

Specifically - MUMPS (Multi-User MEMS Processes). MUMPS has been called “MOSIS

(Metal Oxide Semiconductor Implementation Service)

for MEMS”. Three poly layer process derived from

Berkeley Sensors and Actuators Center

Seven total material layers. N mask steps.

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Processing Layers (micro motor)

7 material layers/ Isolation (Nitride) conductor (poly) 1st sacrificial (oxide) 1st structural (poly) 2nd sacrificial (oxide) 2nd structural (poly) metal

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Patterning Poly 0

Initial layers: substrate/nitride/poly0/

resist. Expose resist.

Dissolve exposed resist.

Etch poly0.

Wash remaining resist.

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First Sacrificial (oxide) Layer

This layer will eventually be dissolved. Its purpose is to support the structural layer (poly1) above it.

Note: all layers are conformal.

Two timed etches - short for dimples, longer for anchor.

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First structural (poly1) layer

Deposit 2m of polysilicon.

Coat with resist, expose.

Etch with RIE (reactive ion etch).

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2nd sacrificial (oxide) layer

Deposit 0.75m oxide.

Two etch steps - first contacts poly1, second contacts substrate.

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2nd structural (poly2) layer + metal Deposit second poly

layer. Apply resist and etch

(RIE). Apply resist and etch. Deposit metal, gold in

MUMPS.

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Released mechanism

Rinse in solvent to remove resist and overlying metal.

Soak 2 minutes in concentrated HF to dissolve sacrificial oxide layers.

Micro motor

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LIGA

LIGA is a technique that can be used to produce moulds for the fabrication of micromachined components. Microengineered components can be made from a variety of materials using this technique, however it does suffer the disadvantage that currently the technique requires X-rays from a synchrotron source.

LIthograpie, Galvanoformung, Abformung

A quick introduction to mask design is provided following discussion of techniques and structures, rather than directly following the photolithography section. This is so that we are able to become acquainted with the concept of creating structures by sequential photolithography and machining steps first, which hopefully makes it easier to understand what mask design software is trying to achieve.

synchrotron source

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LIGA

200m deep structures.

Coat with thick resist. Pattern with Xrays. Electroplate exposed

area with Ni. Machine to +-5m. Use titanium and Cu

as sacrificial layers.

LIthograpie, Galvanoformung, Abformung

Deep Reactive Ion Etching

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Photolithography Process - Steps

8) Develop inspect

5) Post-exposure bake

6) Develop 7) Hard bake

UV Light

Mask

4) Alignment and Exposure

Resist

2) Spin coat 3) Soft bake1) Vapor prime

HMDS

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Vapour Prime

Promotes Good Photoresist-to-Wafer Adhesion.

Dehydration Bake .

Primes Wafer with Hexamethyldisilazane(HMDS).

Vacuum chuck

Spindle connected to spin motor

To vacuum pump

Photoresist dispenser

Photolithography

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MEMS DEVICES

• Infrared Imager (S)

• Inertial Measurement Unit (S)

- Accelerometer

- Gyros

• Air Velocity (S)

• Pressure (S)

• Chemical Species (S)

• Micromotor (A)

• Micromirror (A)

Microbolometer,

Accelerometer, Analog Device

Vibration ring for gyros

Micromirror/optical switch