Optical MEMS

Welcome to Seminar

on

OPTICAL MEMS

Name of Guide Name-Anisha Singhal

Prof. R.S. Meena Roll No.-11/278

MEMS

Acronym for Microelectromechanical Systems

Consist of mechanical microstructures, microsensors,

microactuators and microelectronics, all integrated

onto the same silicon chip.

Have the ability to sense, control and actuate on the

micro scale, and generate effects on the macro scale.

Have the feature size in the range of 1μm-1mm.

Advantages: low weight, low cost, low energy

consumption, quick response time, high resolution and

high sensitivity.

MEMS

Micro

actuators

Micro

sensors

Micro

electronics

Micro

structures

Optical mems

Fusion of three technologies

Why mems used in optics

The wavelength of light is in the µm range as the MEMS

smallest features.

Micro-forces generated by micro-actuators have no

difficulty in acting on mass less photons. A photon has no

mass, so easy to deflect light.

Miniature optical elements capable of moving and

managing light.

Directly manipulate an optical signal eliminating

unnecessary optical-electrical-optical (O-E-O) conversions.

The effect of moving optical elements is stronger than

electrooptic, thermal-optic effects

Very efficient beam steering devices can be made.

Fabrication

Silicon is dominant material for fabrication.

Conventional IC processes (lithography, depositions,

implantation, dry etching, etc.) are often used in

microstructure formation.

Micromachining is the process of shaping silicon or

other materials to realise 3-D mechanical structures

in miniature form and the mechanical devices that

are compatible with the micro electronic devices.

Bulk Micromachining

Surface Micromachining

LIGA

DRIE

Bulk Micromachining For realization of 3D optomechanical structures on Si

substrate for aligning optical fibers or forming optical

MEMS devices.

Single Crystal Silicon is used as the basic material.

The process impacts the substrate.

Use either wet or dry anisotropic etching (or both).

With SOI wafers, a thin top layer of single crystal silicon

(50um—100 um) is separated from a larger silicon

substrate by a thin oxide layer which serves as an etch stop

and a microstructure release mechanism.

Many liquid etchants demonstrate dramatic etch rate

differences in different crystal directions. <111> etch rate is

slowest, <100> and <110> fastest.

Anisotropic Etching of Silicon

• Anisotropic etches have direction dependent etch rates in crystals .

• Typically the etch rates are slower perpendicularly to the crystalline

planes with the highest density.

• Commonly used anisotropic etches in silicon include Potasium

Hydroxide (KOH), Tetramethyl Ammonium Hydroxide (TmAH), and

Ethylene Diamine Pyrochatecol (EDP) .

<111>

<100>

Silicon Substrate

54.7

Surface Micromachining

Deposition and etching of different structural layers on

top of the substrate.

Structural Layer has the desired electrical, mechanical

and thermal properties. Polysilicon is commonly used as

one of the layers.

Sacrificial layer supports the structural layer until it is

etched- during ‘release etch’. Silicon dioxide is used as

a sacrificial layer which is removed or etched out to

create the necessary void in the thickness direction.

Surface micro-machining leaves the wafer untouched, but

adds/removes additional layers above the wafer surface.

Surface Micromachining

Deposit sacrificial layer Pattern contacts

Deposit/pattern structural layer Etch sacrificial layer

Lithographie, Galvanoformung, Abf

ormung

(LIGA) Fabrication technology used to create high- aspect ratio microstructures.

Consists of three main processing steps: lithography, electroplating and molding.

Polymethyl methacrylate (PMMA) is applied as photoresist to the substrate by a glue-down process.

Two main types of LIGA Technology: X-ray LIGA and Extreme Ultraviolet (EUV) LIGA.

X-ray LIGA can fabricate with great precision high aspect ratio microstructures whereas EUV LIGA can fabricate lower quality microstructures.

Deposition of adhesion, seed layer and photoresist is done in lithography process and then it is exposed to the synchrotron radiation

Electroplating is a process to fill in the voids between the polymeric features.

Molding is process of machining the overplated region filling the microstructure.

Deep Reactive Ion

Etching(DRIE) Fabrication technology used to create high- aspect

ratio microstructures.

Highly anisotropic etch process used to create deep

penetration, steep-sided holes and trenches in wafers.

Two main technologies for high-rate DRIE: cryogenic and

Bosch. Both processes can fabricate 90° (truly vertical)

walls.

In cryogenic-DRIE, the wafer is chilled to −110°C.The low

temperature slows down the chemical reaction that produces

isotropic etching. It produces trenches with highly vertical

sidewalls.

In Bosch, a standard isotropic plasma etch is done. The

plasma contains some ions, which attack the wafer from a

nearly vertical direction. Sulfur hexafluoride[SF6] is often

used for silicon. Deposition of a chemically

inert passivation layer is also done alternately.

ACTUATORS

Converts electrical signals to mechanical

displacements of mirrors.

A type of motor that is responsible for

moving or controlling a mechanism or

system.

It is operated by a source of energy, typically

electric current, hydraulic fluid pressure,

or pneumatic pressure, and converts that

energy into motion.

Electrostatic Actuation

Voltage is applied between the movable and the fixed

electrodes.

The moving part rotates about the torsion axis until the

restoring torque and the electrostatic torque are equal.

Magnetic Actuation

The principle of magnetic actuation is based on the Lorentz Force Equation.

Magnetic field can be induced by electrical current which can generate the force exerted on the moving magnetic material.

Electromagnetic coils can be integrated on the movable part, making it quasi-magnetic by current injection.

Can operate in liquid environment.

Thermal Actuation

The mismatch between thermal expansion coefficients of materials yields structural stress after temperature change.

Generates motion by thermal expansion amplification. A small amount of thermal expansion of one part of the device translates to a large amount of deflection of the overall device.

The structure deforms due to this built-in stress.

Ability to generate large deflection.

Microfabricated thermal actuators can be integrated into micromotors.

Digital micromirror devices (DMD)

Array of tiny mirrors (up to 2 million).

Each mirror pivots about a fixed axis.

Each mirror acts as a digital light switch

◦ ON: Light is reflected to desired target

◦ OFF: Light is deflected away from target

Pulse Width Modulation (PWM) techniques are

used to perform digital light modulation.

1024 shades of gray and 35 trillion colors possible.

Used in projection systems(Digital Light

Processing (DLP) projectors), TV and theaters.

DMD Specifications

Mirror Size = 16µm x 16µm

Resonant Frequency = 50kHz

Switching Time < 20µSec

Total Rotational Angle = 10°

Total Efficiency of Light Use > 60%

Fill Factor per Mirror = 90%

Mirror mounting mechanism Each mirror is mounted on Hinge Support

Posts. Each mirror rotates about the posts. Torsion hinge restores the mirror to its

default horizontal state when no power isapplied to the circuit.

Mirror Rotation Each mirror rotates ±10° for total rotational

angle of 20°

Landing Electrode provides stop pad for the mirror and allows precise rotational angles.

Bias Bus & Address

Electrodes Bias/Reset Bus

provides stop pad

and connects all

mirrors to allow for a

bias/reset voltage

waveform to be

applied to the mirrors

Address electrodes

are connected to an

underlying SRAM

cell’s complimentary

outputs

SRAM Cell

Complimentary SRAM

cell outputs connected

to the address

electrodes actuate the

mirrors by

electrostatically

attracting/repelling the

free corners of the

voltage-biased

mirrors.

Principle of Operation

Balancing electrical torque with mechanical torque

Telectrical is proportional to (voltage)2

Tmechanical is proportional to (deflection, a)

a

Working of DMD

OPTICAL SWITCHING

Switching is the process by which the destination of an individual

optical information signal is controlled.

A micro-mirror is used to reflect a light beam in MEMS optical switch.

The direction in which the light beam is reflected can be changed by

rotating the mirror to different angles, allowing the input light to be

connected to any output port.

Realized through the fusion of various techniques such as micro-

machining techniques for fabricating the mirror, optical design

techniques for achieving low-loss optical connections, and control

techniques for positioning the mirror accurately.

Can switch large numbers of optical signals simultaneously.

It can be used as a trunk switch for handling large amounts of traffic,

and as a switch in large urban communication networks.

High bit rate transmission must be matched by switching

capacity. Switching can be performed in 10-30 msec.

Optical or Photonic switching can provide such capacity.

Can switch optical signals without converting them into

electrical signals.

CURRENT

64 kbits/sec for

each subscriber

(1 voice channel)

Estimated aggregate

switching capacity is

10 Gbits/sec

PROJECTED

155 Mbits/sec for

each subscriber

(Video + data etc..)

Estimated aggregate

switching capacity is

15.5 Tbits/sec

Example: 100,000 subscriber digital exchange

The need for Optical Switching

Why optical switches

Increasing number of wavelengths and bandwidth in

dense wavelength-division-multiplexed (DWDM)

networks enhanced its need.

Explosive network traffic

Rapidly growing data rate and port count

Bottleneck due to conventional OEO switches

(bandwidth, bit error rate and capacity mismatch).

Cost effective

2D MEMS Switches

Mirrors have only 2 positions (cross or

bar)

Crossbar configuration

N2 mirrors

Mirrors require complex

closed-loop analog control

But loss increases only as a

function of N1/2

Higher port counts possible.

Mirrors can be tilted to any

angles

N or 2N mirrors accomplish

non-block switching.

Good scalability

3D MEMS based Optical Switch Matrix

Fastest Optical Switch

Switch-on and switch-off a semiconductor optical cavity within a

world-record short time of less than 1 picosecond.

Ultrafast optical data communication.

Tiny on-chip light sources and lasers.

Cavities are used for their ability to store light in a volume in space for

a particular duration of time.

A generic cavity consists of two mirrors separated by a length of

transparent material.

In the cavity, light bounces back and forth between the mirrors. Light is

an electromagnetic wave, so only waves whose wavelength (or color)

matches the cavity length can exist in the cavity

Constructive interference where crests and valleys of many waves

coincide, and therefore add up to a high intensity. As a result, the

allowed waves resonate to form a standing wave in the cavity.

When white light is sent to the cavity only blue light constructively

interferes inside and is transmitted. This situation defines a "zero" bit.

When control (indicated by green star) and signal light pulses arrive at the

same time, the cavity is switched by the control light. As a result, red light

is transmitted through the cavity. This defines a "one" bit.

Later the situation returns to the starting situation.

Advantages of optical mems

Offer more than 10 terabits per second of total switching capacity, with each

of the channels supporting 320GB per second which is 128 times faster than

current electronic switches.

Nearly 32 times denser than an electronic switch.

Low optical insertion loss, low crosstalk and low power consumption.

Transparency (wavelength, polarization, bit rate, data format).

Cost effective wafer scale manufacturing.

Use of Si fabrication technology results in stiffer mirrors that are less prone

to drifting out of alignment and which are robust, long lived and scalable to

large no. of devices on wafer.

Challenges

Design and Packaging

Testing

Sensing Latching

Controllability Reliability

Conclusion

This new technology has the potential to

revolutionize the optical components industry.

MEMS optical switches are currently

dominant and promising in the future.

Bit rate independent, MEMS based optical

components will “future proof” next generation

networks.

Optical MEMS accelerate deployment of the

“all optical network”.