MPXM2102AS datasheet(25/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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The failure mechanisms that may affect a sensor or actuator

will be discussed along with the contributors and acceleration

means. Failure mechanisms of interest during media testing

of semiconductor MEMS devices are shown in Table 1. MEMS

applications may involve disposable applications such as a

blood pressure monitor whose lifetime is several days.

General attributes to consider during testing include: lifetime

expectations, cost target, quality level, size, form, and

functionality.

Table 1. Typical Failure Mechanisms for

Sensors and Actuators [6–10]

Failure Mechanism

Uniform Corrosion

Localized Corrosion

Galvanic Corrosion

Silicon Etching

Polymer Swelling or Dissolution

Interfacial Permeability

Adhesive Strength

Fatigue Crack Initiation

Fatigue Crack Propagation

Environment Assisted Cracking

Creep

Methods for performing media compatibility testing to

determine the potential for the various failure mechanisms will

be presented. Attributes of the testing need to be well

understood so that proper assessment of failure and lifetime

approximation can be made. The lifetime modeling is key for

determination of the ability of a sensor device to perform its

intended function. Reliability modeling and determination of

activation energies for the models will provide the customer

with an understanding of the device performance. The

definition of an electrical failure can range from catastrophic,

to exceeding a predetermined limit, to just a small shift. The

traditional pre to post electrical characterization (before and

after the test interval) can be enhanced by

in situ monitoring.

In situ monitoring may expose a problem with a MEMS device

during testing that might have gone undetected once the

media or another environmental factor is removed. This is a

common occurrence for a failure mechanism, such as

swelling, that may result in a shift in the output voltage of the

sensor. Response variables during environmental testing can

include: electrical, visual, analytical, or physical characteristics

such as swelling or weight change.

DEFINITIONS & UNDERLYING CAUSES

The definition of a media compatible pressure sensor is as

follows:

The ability of a pressure sensor to perform its specified

electromechanical function over an intended lifetime in the

chemical, electrical, mechanical, and thermal environments

encountered in a customer’s application.

The key elements of the definition are perform, function,

lifetime, environment, and application. All of these elements

are critical to meet the media compatibility needs. The

underlying causes of poor media compatibility is the hostile

environment and permeability of the environment. The

environment may consist of media or moisture with ionics,

organics, and/or aqueous solutions, extreme temperatures,

voltage, and stress.

Permeability is the product of diffusivity and solubility.

Contributors to permeability include materials (e.g. polymeric

structures), geometry, processing, and whether or not the

penetration is in the bulk or at an interface. The environment

can also accelerate permeation if a concentration gradient,

elevated temperature and/or pressure exist. An example of

material dependence of permeation is shown in Figure 2.

Organic materials such as silicone can permeate 50% of the

relative moisture from the exterior within minutes where

inorganic materials such as glass takes years for the same

process to occur.

Figure 2. Permeation relationship for various materials.

MIN

DAY

MO YR

100

–5

–4

–3

–2

–1

–6

SILICONES

FLUORO–CARBONS

10–6

10–8

10–10

10–12

10–14

10–16

PERMEABILITY (g/cm–s–torr)

TIME FOR PACKAGE INTERIOR TO REACH

50% OF EXTERIOR HUMIDITY *

METALS

EPOXIES

GLASSES

* Richard K. Traeger, “Nonhermiticity of Polymeric Lid Sealants,

IEEE Transactions on Parts, Hybrids, and Packaging, Vol. PHP–13,

No. 2, June 1977.

Gasoline and aqueous alkaline solutions represent two

relatively diverse applications that are intended for use with a

micromachined pressure sensor. The typical automotive

temperature range is from –40

° to 150°C. This not only makes

material selection more difficult but also complicates the

associated hardware to perform the media related testing [11].

A typical aqueous alkaline solution application would be found

in the appliance industry. This industry typically has a

narrower temperature extreme then the automotive market,

but the solutions and the level of ions provide a particular

challenge to MEMS device reliability.

Gasoline

contains

additives

such

as:

antiknock,

anti–preignition

agents,

dyes,

antioxidants,

metal

deactivators, corrosion inhibitors, anti–icers, injector or

carburetor detergents, and intake valve deposit control

additives [12]. To develop a common test scheme for the

liquid, a mixture table was developed for material testing in

gasoline/methanol mixtures. The gasoline/methanol mixtures

developed were intended for accelerated material testing with

a gasoline surrogate of ASTM Fuel Reference “C” (50%

toluene and 50% iso–octane) [13]. Material testing is

performed with samples either immersed in the liquid or

exposed to the vapor over the liquid. The highly aromatic Fuel

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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