Optical MEMS devices and their fabrication

1. Welcome to Seminar
on
OPTICAL MEMS
Name of Guide Name-Anisha Singhal
Prof. R.S. Meena Roll No.-11/278

2. 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.

3. MEMS
Micro
actuators
Micro
sensors
Micro
electronics
Micro
structures

4. Optical mems
Fusion of three technologies

5. 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.

6. 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.

7. Bulk Micromachining
Surface
Micromachining
LIGA
DRIE

8. 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.

9. 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

10. 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.

11. Surface Micromachining
Deposit sacrificial layer Pattern contacts
Deposit/pattern structural layer Etch sacrificial layer

12. 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.

14. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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%

23. 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 is
applied to the circuit.

24. 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.

25. 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

26. 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.

27. Principle of Operation
Balancing electrical torque with mechanical torque
Telectrical is proportional to (voltage)2
Tmechanical is proportional to (deflection, a)
a

28. Working of DMD

29. 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.

30.  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

31. 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

32. 2D MEMS Switches
 Mirrors have only 2 positions (cross or
bar)
 Crossbar configuration
 N2 mirrors

33.  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

34. 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.

35. 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.

36. 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.

37. Challenges
Design and
Packaging
Testing
Sensing Latching
Controllability Reliability

38. 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”.