Electronic Material For Microsensors

and Microactuators of MEMs

(Microelectric-mechanical system)

Rhidiyan WarokoRhidiyan Waroko

0806331935

Departemen Teknik Metalurgi dan Material

Fakultas Teknik Universitas Indonesia

Outline

• Introduction

• Application• Application

• Design Process and Material Selection

� Material for Microsensor

� Material for Microactuator

• Material Selection : Case Study• Material Selection : Case Study

• Reference

Introduction

• Electronic materials are material which used forelectronic application, such as Microelectro-electronic application, such as Microelectro-mechanical systems (MEMS), due to of theirelectrical properties.

• MEMS are small integrated devices or systemsthat combine electrical and mechanicalcomponents.components.

• Microsensors and microactuators are vital organsof MEMS, forming the interfaces betweencontroller and environment.

Application

MEMS device applications include :

� Inkjet-printer cartridges� Inkjet-printer cartridges

� Miniature robots

� Microengines

� Microtransmissions

� Optical scanners� Optical scanners

� Chemical, pressure and flow sensors

� Ext.

Design Process and Material Selection

Materials selection for engineering design needs aclear understanding of the functionalclear understanding of the functionalrequirements for each individual component andvarious important criteria/factors need to beconsidered.

For MEMS designers, one of the key jobs forachieving the high level of reliability, low unit costachieving the high level of reliability, low unit costand optimal function performance ofmicroelectro-mechanical devices is to carefullychoose materials from a limited set.

Step involved in material design

Design Process and Material Selection

Performance index is such a criterion in ‘Ashby

method’, providing a comparison betweenmethod’, providing a comparison between

material candidates for a given design.

Three things specify the design of structural

elements: The functional requirements, theelements: The functional requirements, the

geometry, and the properties of the material.

Material Selection for Microsensors

The suitability of a microsensor for a particular

application is essentially determined by itsapplication is essentially determined by its

characteristic performance.

Different applications require different sensor

performance.

� Microsensors for pressure sensors� Microsensors for pressure sensors

� Microsensors for resonant application

� Microsensors for microcantilever sensors

Microsensors for pressure sensors

Diaphragm of radius a and thickness t, which is

used to measure pressure in an indirect route:used to measure pressure in an indirect route:

the diaphragm deflects by pressure, and then

the deflection can be converted to an

electrical signal via sensing the variance of the

capacitance.

Microsensors for pressure sensors

The deflection d of the center of the

diaphragm caused by DP is:diaphragm caused by DP is:

To maximize the deflection d for measuring

higher pressure, so, the maximum stress inhigher pressure, so, the maximum stress in

the diaphragm is:

Microsensors for pressure sensors

Substitute t from first equation to second

equation:equation:

The best material for the diaphragm is that with

the largest value of Mthe largest value of M

Microsensor for Resonant Application

Mechanical resonant elements can oscillate at high

frequencies and be used as radio frequency devices.frequencies and be used as radio frequency devices.

The natural vibration frequency f of a high frequency

vibrating element depends strongly upon its material

properties as:

Therefore, high gives high natural vibrationTherefore, high gives high natural vibration

frequency, the best material for this application is the

one with the largest value of M.

Microsensors for Microcantilever Sensors

Microcantilever with specific coating, which isused to detect mercury vapor, moisture, orused to detect mercury vapor, moisture, orvolatile mercaptans by showing the resonancefrequency variation. The resonance frequency,f , of an oscillating cantilever can be expressedas:

For the case of a rectangular cantilever,m*=0.24mb where mb is the mass of the beam.

Microsensors for Microcantilever Sensors

Assuming that the contribution from variation inthe spring constant is small, a mass dependenceof the fundamental frequency can be written as:of the fundamental frequency can be written as:

The mass sensitivity of the structure is given by:

For high mass sensitivity, the best material for acantilever is that with the largest value of M

Material Selection for Microactuators

Microactuators provide drive and motion for avariety of requirements. In some applications,variety of requirements. In some applications,the actuating elements are expected to storeenergy, such as microsprings and flywheels ofmicromotors.

Quite a number of microactuators are based onshape-changing mechanism, such as thermalshape-changing mechanism, such as thermalexpansion, piezoelectric, shape memory alloyand magnetostrictive.

Rotating Disks for Micromotors and

Micropumps

Micromotors and micropumps manage liquid or

gas at microlevel.gas at microlevel.

The energy stored in a flywheel of radius R,

thickness t and density r is:

Rotating Disks for Micromotors and

Micropumps

The best material for high performance

flywheels is that with the largest value of theflywheels is that with the largest value of the

performance index,

Rotating disks for (a) micromotor ; (b) micropump

Compact single stroke actuators

(levers)

Consider the following generic problem: amicroactuator is required to be capable ofmicroactuator is required to be capable ofproviding a prescribed force F, and a prescribeddisplacement d in a single stroke. The volume, V,of the actuator is to be minimized. The actuatorhas length L, cross sectional area A andmechanical advantage r.

There are constraints both on L and A. TheThere are constraints both on L and A. Theconstraint on length a rises because the actuatormust achieve displacement d, but has a limitedstrain, .

Compact single stroke actuators

(levers)

So,

To minimize the volume, theproduct must beproduct must bemaximized, so is theperformance index for thisproblem.

Case StudyMaterial selection for microelectronic heat sinks

Heat sinks are the most common and cost-effective

hardware employed for the thermal management ofhardware employed for the thermal management of

microelectronic circuits and microelectromechanical

systems (MEMS) devices.

Heat sinks function by efficiently transferring thermal

energy (‘‘heat”) from an object at a relatively high

temperature to a second object at a lowert emperature

with a much greater heat capacity. This rapid transfer ofwith a much greater heat capacity. This rapid transfer of

thermal energy quickly brings the first object into

thermal equilibrium with the second, lowering the

temperature of the first object, fulfilling the heat sink’s

rolecas a cooling device.

Material Properties

Material Selection

To prevent electrical coupling and stray capacitance

between a microchip and heat sink, the heat sinkbetween a microchip and heat sink, the heat sink

must be a good insulator, meaning a high resistivity.

But at the same time it must also have the highest

possible thermal conductivity to drain away heat as

fast as possible from the chip. The translation step is

summarized as follows:summarized as follows:

Material Selection

From the graph shown we find that Aluminum Nitride (AlN) or Alumina

(Al2O3) satisfies the constraints and our objective to maximize thermal

conductivity and resistivity is achieved with these materials.conductivity and resistivity is achieved with these materials.

Material Selection

Thermal stress is the stress that appears in a

body when it is heated or cooled butbody when it is heated or cooled but

prevented from expanding or contracting. It

depends on the expansion coefficient of the

material and on its modulus. A development

of the theory of expansion leads to the

relation:relation:

Material Selection

We can make the following observations based onthese equations:these equations:

i. As temperature of the material used in heat sinkincreases (T), the value of thermal expansionalso decreases.

ii. For high values of Young’s Modulus, the value ofthermal expansion must be significant(i.e.neither too high nor too low).(i.e.neither too high nor too low).

iii. Despite satisfying the above mechanicalproperties, the material should have highelectrical resistivity.

Material Selection

From the graph we can conclude that Al, AlN, Al2O3 and to some extent even

Cu and Zn alloys can be best possible materials for microelectronic heat

sinks.sinks.

Material Selection

Thermal contact resistance. Consider a case wherein 2solid bars are brought into contact as. Suppose bar A issolid bars are brought into contact as. Suppose bar A isthe microchip and bar B is the heat sink. Let the lengthof bar A be DxA and that of bar B be DxB performing anenergy balance on the two materials, we obtain:

Where the quantity (hCA)-1 is called the thermal contactresistance and hC is called the contact coefficient.

Material Selection

Therefore, we must note thatthere are two principal

Therefore, we must note thatthere are two principalcontributions to the heattransfer at the joint:

i. The solid – solidconduction at the spotsof contact.of contact.

ii. The conduction throughentrapped gases in thevoid spaces created bythe contact.

Material Selection

Designating the contact area by Ac and the void area by

Av, the equation for heat transfer can be written as:Av, the equation for heat transfer can be written as:

From standard values, aluminum is the best possible

material to satisfy this requirement. We assume that

solid A is silicon based material/substrate

Result of Material Selection

We find that consistent results are obtained in all the

three cases for different mechanical and electricalthree cases for different mechanical and electrical

properties of the heat sink.

Hence aluminum based are very promising materials

for microelectronic heat sink.

Reference

1. S.M. Spearing.” Materials Issues inMicroelectro-mechanical System (MEMS)”.Microelectro-mechanical System (MEMS)”.Actamater. 48 (2000) 179-196

2. G. Prashant Reddy, Navneet Gupta.”Materialselection for microelectronic heat sinks: Anapplication of the Ashby approach”. Materialsand Design 31 (2010) 113–117

3. Jin Qian, Ya-Pu Zhao.”Materials selection in3. Jin Qian, Ya-Pu Zhao.”Materials selection inmechanical design for microsensors andmicroactuators”. Materials and Design 23 (2002)619–625