RF-MEMS DEVICES: PROBLEMS REGARDING

RELIABILITY AND DEGRADATION MECHANISMS7.1: Introduction

Microelectromechanical Systems (MEMS) consist of mechanical components

ranging in size from a few microns to a few hundred microns. MEMS sensors (like

pressure sensors) sense an aspect of environment and result an output signal. Actuators

(micro-engines and discriminators) act on the specific input and produce a specific

action. Actuators are activated by the user; sensors wait a signal from the environment.

MEMS manufacturing processes and tools are borrowed from the ICs industry.

These are formed by surface micromachining (successive deposition and selective

etching of polysilicon layers on top of the silicon surface), or by bulk micromachining

(etching into the silicon substrate using anisotropic etchants), or by high aspect ratio

micromachining (HARM). Geartrain structures, microengines, micro-steam engines,

micromirrors etc are produced using surface micromachining. Bulk micromachining is

used to fabricate structures like cantilevers, bridges, or channels etc. HARM includes

processes like LIGA can create free standing structures of height up to a few hundred

microns.

Now-a-days, the MEMS technology employed for radio-frequency or microwave

applications is continually developing rapidly. This RF-MEMS technology is creating

and facing various problems related to reliability and degradation mechanics. In this

paper, we will present an overview of the most significant failure /degradation

mechanisms and reliability issues related to RF-MEMS devices. Although, our

knowledge concerning failure mechanisms and reliability problems is still very

incomplete, yet knowledge about this area is increasing day by day. This chapter

discusses reliability issues related to fabrication, metallic contact, electrostatic actuation

and packaging.

RF-MEMS devices are actually the micro-electro-mechanical-systems (MEMS)

used for RF (radio frequency) and microwave applications. In the last ten years, RF-

MEMS technology has become most promising and emerging technology. Recently,

researchers are providing more attention towards this technology because of its skills in

1

implementing reconfigurable passive networks for the coming generation multi-standards

and multi-frequency wireless communication systems.

The main emphasis of the researchers now is on a particular class of RF-MEMS

devices that includes varacters, capacitive switches, ohmic contact based switches and

multiplexers. All these devices operate at RF or microwave frequencies. The major

advantage of RF-MEMS devices is that many limitations exhibited by traditional RF

devices can be easily overcome. RF-MEMS devices make system more reliable, cheaper,

faster and capable of incorporating more complex functions. They offer low insertion

loss, high isolation, good linearity, good economy, good miniaturization, high quality

factor, do not consume power and can be integrated with solid state circuits.

The reliability of RF-MEMS devices is of major concern for long term

applications and is currently the topic of an intense research effort. For example, in DC

contact RF-MEMS switches, the reliability is strongly related to the metal contact used,

whereas in capacitive switches, the reliability is limited by dielectric charging. Now-a-

days, a number of academic and government laboratories and companies have started to

report cycling lifetimes for manufactured prototypes which are in line with future and

present needs of defense and commercial users.

Technologies like Radant MEMS, MIT Lincoln Labs or Raytheon have reported about

100 billion cycles for ohmic contact switches. IMEC has led several studies into the

reliability of RF-MEMS. Consequently, some commercial products have begun

appearing in the market. In future, RF-MEMS technology has to prove in good

performance and reliability. It should provide new functionalities and unexplored circuits

and systems to find its path in to the commercial RF microwave and wireless market

gradually.

In this chapter, we will report some of the major degrading and failure

mechanisms that affect RF-MEMS performance and reliability. In particular, this paper

gives emphasis on reliability issues regarding to fabrication, metallic contact, electrostatic

actuation and packaging for the capacitive and ohmic RF-MEMS devices.

7.2: Most General RF-MEMS Degradation Mechanisms

RF-MEMS technology is still in its infancy and not matures. Along with the

benefits of this technology, there are a lot of shortcomings and limitations. This

2

technology is accompanied with lack of standardization in the manufacturing process and

in the packaging technique. This technology has limited data of reliability compared to

the conventional micro-electronics. Also, there is very less knowledge of failure and

degrading mechanisms of RF-MEMS devices.

Table 7.1: General RF-MEMS degradation mechanisms.

Degradation Mechanisms Accelerated Factors / Causes

Fracture Overload Fracture

Fatigue Fracture

Creep/Plastic Deformation Applied Stress

Thermal Stress

Intrinsic Stress

Stiction Capillary Forces

Van der Waals molecular Forces

Electrostatic Forces

Solid Bridging

Wear Adhesive

Abrasive

Corrosive

Degradation of Dielectrics Charging

Leakage

Breakdown

Electro migration Current density

Temperature

Delamination Thermal shock

Mechanical shock

Surface Contamination Absorption

Oxidation

Pitting in Surface Number of Cycles

Electrostatic Discharge

The reliability of RF-MEMS devices is determined by understanding the root

cause of all concerned degradation modes using a rigorous physics-based approach. One

must expose degradation modes by applying drive factors (like overvoltage) or

environmental factors (like humidity and temperature) which could accelerate

3

degradation. After knowing the degradation modes, we can perform the experiments to

find the acceleration factors and can estimate the lifetime of a well designed and

packaged device. A significant strategy is existing design, also mentioned as design-for-

reliability.

The process is very iterative. Once a degradation mechanism is disclosed, the

design, fabrication, packaging etc can then be improved to minimize or eliminate the

degradation mode. For some high volume markets and safety-critical applications,

reliability has been extensively studied and has resulted parts with degradation rates at

the ppm level of lifetimes of over ten years. For small volume markets where reliability is

very critical (telecom), extremely reliable RF-MEMS devices have also been

demonstrated. Depending upon the fabrication materials and environmental stress

conditions, these devices are subjected to diverse failure modes. A list of general failure

mechanisms of RF-MEMS devices is given in table 7.1. Many degradation modes listed

here can be eliminated through suitable design and packaging.

7.3: Reliability classes of RF-MEMS

The classification of RF-MEMS devices is recently becomes a hot issue in such a

way that it has tendency to include any device which is made with at least one step of

micro-machining technology. So, it has become necessary to make a division of various

RF-MEMS devices in such a way that it becomes significant for studies regarding

reliability. This is important to make some common criteria for the accelerated tests and

ageing models. Three different classes of reliability of RF-MEMS devices are briefed in

the table 7.2. This classification of RF-MEMS devices has been done in accordance with

the level of mechanical complexity and boundary conditions.

The class-I has all the passive components that have been designed for diminished

losses through micro-machining fabrication. Mechanical movements of any part of the

structure of this class of RF-MEMS devices are not required during the functioning and

working. However, some deformations might take place during various processes

involved in fabrication. Reliability and stability of this class of RF-MEMS devices in the

long term do not alter significantly from those of conventional RF passive components.

Stability problems of the structures of these devices might take place to thin

dielectric membranes. Often, these dielectric membranes are used for high quality factor

4

passive components fabricated by using micro-machining. When heat diffusion of the

bulk material is not good, then the membranes also have tendency to expose thermal

problems. In addition to these problems, the devices which are under repeated

temperature cycles during assembly and packaging process can be prone to structural

deformations that cause failure of the device.

Table 7.2: Classification of RF-MEMS Devices.

Class I II III

Micro-machined

Structures

Yes Yes Yes

Movable Parts No Yes Yes

Impact No No Yes

Examples of RF-

MEMS Devices

High-Q Suspended

Inductors: spiral, self

assembled coils; low-

loss RF-Membranes;

RF-CMOS substrate

removal post-processing

Very High-Q

micro-electro-

mechanical

resonators;

continuously

tuning capacitors.

Ohmic contacts RF-

MEMS relays;

switched capacitors;

capacitive coupling

RF-MEMS switches

and multiplexers.

The second class of RF-MEMS devices demands mechanical movement of some

part during the working of the devices. This class of RF-MEMS devices consists of

devices having micro-machined structure and moveable parts. Notable examples of this

class are very high quality factor micro-electro- mechanical resonators and continuously

tuning capacitors. Due to repeated mechanical movements and vibrations, novel stress

mechanisms are introduced on the constituted parts of these devices.

Plastic deformations, mechanical relaxations, fatigue, creep etc can disturb the

stability of electro-mechanical behavior of these devices. All these failure and

degradation mechanisms cause the mechanical failure of second class of RF-MEMS

devices. In addition to this, oxidation and absorption like surface effects can cause

stresses in moving and oscillating part. As a result complex stability problems are

introduced that help in the failure of device.

The third class of RF-MEMS devices comprises of all the devices demanding two

distinguished mechanical moveable parts to attain and keep contact during a definite time

of cycle of the operation. Novel problems related to reliability are caused due to the

5

presence of mechanical contact between the moving parts of device. These reliability

problems may be of mechanical type and electrical type.

The major effect that diminishes the working of devices is the stiction of

mechanical parts that keep the mechanical contact. Due to stiction of mechanical parts

restoration of resting position becomes almost impossible even after the removal of

actuation force. The stiction can happen due to many factors like redistribution and

accumulation of electric charge in dielectric slabs, capillary effects due to humid

environment, micro –welding of metals due DC or RF power etc.

Examples of this class of devices are ohmic-contact RF- MEMS relays, switched

capacitor, capacitive coupling RF-MEMS switches and multiplexers .Electrical ohmic

contacts between two metallic surfaces may be affected from stability problems that arise

due to number of cycles, variation in resistance of ohmic contacts, transfer and erosion of

material, surface contaminations and other surface effects like absorption and oxidation.

7.4: Specific Reliability Problems of RF-MEMS Devices

To compute the reliability of RF-MEMS, one must be aware of the root cause of

all related failure modes using a careful physics-based approach. This begins with a test

plan to disclose failure modes by applying drive conditions (e.g., overvoltage) or

environmental conditions (e.g., humidity, thermal and mechanical shock) that could

accelerate failures. Once one recognizes the failure modes, one can perform experiments

to determine the acceleration factors, and run tests that can estimate the lifetime well

before.

One may need to work with test structures designed to exhibit only one main

failure mode if the RF-MEMS device is too obscure to real particular degradation

mechanism. Once a failure mode is uncovered, the design, fabrication process, packaging

or materials can then be improved to eliminate or minimize that failure mechanism.

Failure modes depend on the materials used for the device, the fabrication approach, the

packaging, and of course the design. Failure modes of RF-MEMS devices can be

categorized into:

1) Mechanical failure modes, and

2) Electrical failure modes.

Common mechanical failure modes are:

6

(a) Stiction

(b) Creep and Plastic deformation

(c) Wear and friction

(d) Shock and vibration induced fracture

(e) Fatigue and curvature change

(f) Delamination

Common electrical failure modes are:

(a) Dielectric charging and breakdown

(b) Short and open circuits

(c) Arcing across small gaps

(d) Electrostatic Discharge

(e) Electrical Overstress

(f) Corrosion

There exist a large numbers of possible reliability problems and failure

mechanism happening in RF-MEMS devices. These failure mechanism and reliability

problems paint a very complex scenario for the life time testing of these devices. This

demands novel methodologies, accelerated tests and new degradation models of RF-

MEMS devices. The diverse reliability problems of RF-MEMS devices can be related to

their fabrication process, related to electrostatic actuation, metallic contact, radiation,

electrical characterization and packaging.

7.4.1: Reliability Problems Related to fabrication: --

Poor functioning of RF-MEMS can appear directly after the manufacture or after

a relatively short lifetime. This poor functioning of devices is typically to fabrication

related issues. Two types of problems relating to fabrication can take place – one

mechanical and the other is electrostatic type. The presence of residual tension within the

structural material of the device, or bad tension slopes, if not taken into consideration

within the models during the fabrication design, may cause permanent deformations of

the released structures. All this will lead to failure of the device or may result in bad

performances at least. These problems can be identified by non-invasive inspection

techniques like optical interferometery quickly and easily.

7

The reliability problems associated with electrostatic fabrication affects badly the

dielectric layers which are used for isolation in capacitive switches. The electrostatic

reliability problems are also due to accumulation of charge during different steps

involved in fabrication process. As a result, there occurs a shift in actuation from the

original design value. This shift leads to failure of device because it is not according to

electro-mechanical specifications of the RF-MEMS device.

7.4.2: Reliability Problems of Contact Material: --

The main considerations in designing the ohmic contact are the contact area and

adherence force. The reliability and stability of direct metal-to-metal ohmic contact

during the working of the RF-MEMS devices can be diminished by many degradation

mechanisms like electro-migration, micro-welding of metals due to DC or RF power,

softening of metal, transfer of material, erosion of material, surface contaminations and

other surface effects occurring due to oxidation and absorption. Both the number of

cycles and the total time spent by the switch in the actuated state are critical factors for

these degrading mechanisms. Lifetime of the ohmic contact in these devices is also

defined by the hot and cold switching requirements.

Most of the contact metal studies do not tackle the issues occurred in the low

contact force regime (sub 100μN), in which most RF-MEMS operate. At such low forces,

the contact resistance is extremely sensitive to even a trace amount of contamination on

the contact surfaces. Significant work was done to develop wafer cleaning processes and

storage techniques for maintaining the cleanliness. To keep contact cleanliness over the

device lifetime, many hermetic packaging technologies were developed and their

efficiency in protecting the contacts from contamination was examined.

The contact reliability is also subjective to the contact metal selection. When pure

Au, a relatively soft metal, was used as the contact material, significant stiction problems

occurred when clean switches were cycled in an N2 environment. In addition, various

mechanical damages occurred after extensive cycling tests. Harder metals that are more

resistant to deformation and stiction are more sensitive to chemical reactions like

oxidation. They also lead to higher contact resistance because of their lower electrical

conductivity and smaller contact areas at a given contact force.

8

The requirements for the contact metal are very rigorous. The contact metals must

have the following properties:-

(a) Excellent electrical conductivity for low loss, high melting point to handle

power,

(b) Appropriate hardness to avoid stiction and

(c) Chemical inertness to avoid oxidation and other chemical reactions.

Finally, hermetic packaging technology using low vapor pressure metal or solvent

free glass-frit must be used to achieve low leakage and to avoid contaminating devices

during bonding.

7.4.3: Reliability Problems Related to Dielectric Charging: --

Dielectric materials have tendency to be affected by accumulation of steady and

slowly moving charges. The charges may be due to different mechanisms like dielectric

polarization, contamination of surface due to oxidation and absorption, defects in the

crystalline structures of the dielectrics. This charge distribution with the isolation layer

will cause a shift in effective electrostatic force at a given applied voltage. There occur

two different effects, according to literature, which depend upon the accumulation of net

charge within the dielectric.

One possibility is when net charge accumulates within the dielectric either during

fabrication process or during the lifetime cycling due to applied voltages. In this case, a

shift in the d-V and C-V curves is observed. Both the pull-in and pull-out voltages will

get changed. As a result, the device will fail to actuate at a required voltage.

The second possibility arises due to the non-uniform distribution of charge across

the geometry of device, while keeping the charge zero in the dielectric layer. Rottenberg

explained how the variations in charge can be responsible for failure of device. The

failure took place due to disappearance of the pull-out bias. Non-uniform distribution of

the charge may occur due to fabrication processing issues and from non-uniform field

distribution during the device actuation and due to the residual air-gaps present in the

position of down-state.

7.4.4: Reliability Problems Related to Electrical Characterization:-

An additional vital aspect concerning the reliability of MEMS devices in general

is related to the Electro Static Discharge (ESD) or Electrical over Stress (EOS).

9

EOS/ESD phenomena are commonly known as a major reliability issue for all kinds of

devices and, unplanned protection structures have been developed in order to stop any

failure or degradation of the electrical characteristics of the stressed devices. EOS/ESD

stress can lead to structural damage also to MEMS switches, impairing either the

mechanical functionality of the devices or electrical characteristics such as scattering

parameters.

Unlike microelectronic circuits, a stand-alone RF-MEMS structure has not

protection mechanism to prevent damage due to electrical overstresses. The electrical

cycling characterization of devices submitted to ESD stresses have taken to strange

results. It is found in fact that scattering parameters not only do not degrade, but usually

they show an improvement in the switch insertion loss and present a lower degradation

rate.

Electrical ohmic contacts occurring between two metallic surfaces can also suffer

from stability problems due to cycling, resulting in changes in ohmic contact resistance.

The causes can be miscellaneous, such as surface contaminations, material transfer and

erosion, and surface changes due to absorption or oxidation. This yet non-exhaustive

compilation of possible reliability and failure mechanisms taking place in RF-MEMS

devices give a picture of a very complex scenario for lifetime testing.

For this reason this new class of MEMS devices, requires the definition of novel

methodologies, device degradation models, and accelerated tests criteria, all covering

different and cross-coupled physical domains. The failure modes are alike to those in the

microelectronics world. One must use watchfulness however because MEMS devices

often use combinations of materials not used in microelectronics, as well as high voltages

(greater than 100V) for numerous actuators.

Accelerating degrading factors related to short and open circuits are electric field,

temperature, and humidity. Accelerating degrading factors related to arcing across small

gaps are electric field, gas pressure and composition. Such factors for dielectric charging

are electric field, temperature, radiation, and humidity. Corrosion is accelerated by the

factors such as humidity, applied voltage and polarity (if anodic corrosion), and

temperature.

7.4.5: Packaging Related Reliability Problems: --

10

Reliability issues of various RF-MEMS packaged devices are application

dependant; hence, there is not a common set of reliability problems. From the knowledge

of failure mechanisms of a system, we come to understand the reliability of that system.

The main degradation mechanism of the RF-MEMS device is stiction. In stiction,

microscopic adhesion takes place when two surfaces come in contact with each other.

Before the integration of contact metal RF-MEMS devices into communication systems

becomes a reality, stiction problem needs to be resolved.

Many researchers have proposed to reduce stiction by either selecting contact

materials with less adhesion [Schimkat, Contact materials for micro relays], applying

chemical surface treatment or by eliminating contamination with plasma cleaning or by a

mechanical approach to provide enough restoring force to overcome the adhesion force

generated at the interface.

RF-MEMS devices can fail due to the delamination of bonded thin film materials.

Bond failure of dissimilar materials and similar metals in such a wafer-to-wafer bonding

can cause delamination also. Dampening is also critical for RF-MEMS due to the

mechanical nature of the parts and resonating frequency. It is mainly due to the

atmospheric gases. Hence, RF-MEMS devices should be properly sealed. Since, RF-

MEMS devices have mechanical moving parts; they are more susceptible to

environmental degradation.

Thermal and heat transfer issues become more complicated by packaging various

functional components into a tight space. Heat dissipation from the packaged system to

environment becomes significant. The miniaturization also increases such problems. In a

thin package, heat spreads in surrounding electronic components and Microsystems.

7.4.6: Reliability Problems Related to Radiation:-

The most of the mechanical properties of silicon and metals such as Young’s

modulus, yield strength etc does not vary with low or with very high radiation doses.

Silicon as a structural material is fundamentally hard to any type of the radiation.

Therefore, most of the RF-MEMS devices are not affected by radiation significantly by

default. Only much greater anxiety is for the drive or control electronics that may require

to be shielded or built with a radiation-hard design and parts.

11

However, radiation damage typically causes latch-up or single event upsets for

electronics, or a continuous deterioration of optical coatings and lenses. Microelectronic

circuits typically consist of millions of transistors, separated by small fractions of a

micron, with very thin (in nanometers) dielectrics or gate oxides, and with each device’s

conductivity controlled by locally applying a gate voltage to doped semiconductors.

RF-MEMS devices can operate on many actuation schemes such as electrostatic,

thermal, and piezo-electric. All these actuation schemes require a good electrical contact

between a bond pad and the actuator (electrode) of the device, but the exact level of

doping is not important given that the material is adequately conductive. Obviously, RF-

MEMS characteristically have no p-n junctions or semiconducting regions where doping

and carrier concentration has a significant role (except for some strain gauges). These do

not have active areas like a transistor, only zones where the electric potential requires

being précised.

As the dielectric films are thick (of order a micron), so, MEMS devices are mostly

not sensitive to radiation. For MEMS devices the main failure mode at high radiation

doses is the accumulation of charge in dielectric layers that first causes a change in the

calibration of the device (essentially by applying a quasi-constant electrostatic force), and

finally can result in complete failure by a short circuit or continuous actuation even at

zero volt.

It is clear that the design and materials play an important role in total acceptable

dose. For example, micro-engines from Sandia National Labs in Albuquerque, NM, USA

were reported to only change their behavior at doses of order 10 MRads. Those devices

did not contain dielectric whose charging could influence device operation. However

tests on accelerometers showed a change in calibration at doses of 100 krad, yet these

devices remained functional. The failures were due to trapped charge in dielectric films.

Radiation doses may affect unpackaged devices because then sensor element is

directly exposed. Similar doses on packaged devices would cause much less damage. RF

switches showed no change in operation at doses of up to 150 k-Rad for one design, but

for a different design the device’s calibration started to vary slowly at doses of 10 k-Rad.

The difference in dose required for degradation between the two devices is due to the

different location of the dielectric layers.

12

In particular, devices that have mechanical motion governed by electric fields

(across insulators) are sensitive to radiation. Hence, there is a chance that performance of

these devices decreases in the space. In a typical radiation test, a set of surface micro-

machined components exposed to gamma radiation doses around 25 k-Rad had harsh

performance degradation. However, this gives no information about the overall radiation

tolerance of MEMS technology; it only says that MEMS devices can be affected by

radiation badly.

7.5 Summary: --

The research of the degradation mechanisms and failure modes of RF-MEMS devices

and the physics behind them is very challenging. This paper presented a brief report of

the interesting complex degradation and failure mechanisms concerning to the reliability

of novel RF and microwave devices which are fabricated using micro-machining

technology. The RF-MEMS technology is facing various problems related to reliability,

stability and lifetime estimation. Although our knowledge about reliability issues is still

incomplete, yet this paper is presented to create interest about this field in future.

Also, another most important effect (that is not described above) impairing device

functionality is the stiction of the mechanical parts that reached contact that is the failure

to restore the device to its resting position after the actuation stimulus has been removed.

Several factors have been known to cause stiction:

(a) Capillary effects due to changed environment conditions,

(b) Electrostatic charge accumulation or redistribution within dielectric layers, and

(c) Micro-welding of metals due to DC or RF power.

The RF-MEMS technology has to prove as performing and reliable as its

traditional counterparts. It should have fully new functionalities and new circuits and

systems solutions, to steadily find its way into the commercial RF microwave market.

The presence of mechanical contact introduces a new class of reliability issues related to

both mechanical and electrical phenomena. Cycled mechanical deformations and

vibrations cause new stress mechanisms on the structural parts of these devices.

Mechanical relaxation of residual material stress, plastic deformations under large

signal range, creep formations and fatigue can all impair the stability of electro-

mechanical device behavior and in the end cause device mechanical failure. Finally, other

13

surface effects like oxidation or absorption can lead to variations of effective mass or

stress of a moving or vibrating structure, resulting stability issues and device failures.

Contact material reliability, fabrication related issues, radiation related issues,

electrostatic characterization related, reliability problems regarding dielectric charging

and packaging related reliability problems are reported here. Two main factors for the life

time durability of RF-MEMS devices (ohmic RF-MEMS switch devices and capacitive

switches) were identified as ohmic contact reliability and charge distribution within the

dielectric. Packaging plays a key role in ensuring the long-term reliability of RF-MEMS

devices.

Reliability is the hindering issue to prevent commercialization and utilization of

RF-MEMS device in significant applications. The reliability and failure mechanisms

happening in RF-MEMS devices are depicting a very complex scenario for the lifetime

testing of these devices, asking for the new methodologies, device degradation models

and accelerated tests criteria, all covering diverse and cross-coupled physical domains.

Reliability is the challenging topic in RF-MEMS technology expansion and

commercialization. The reliability issue of RF-MEMS devices is a simple combination of

electrical reliability, material reliability and mechanical reliability. Fabricating multiple

devices on the same chip will have to deal with more failure modes.

14