Failure Mechanisms in MEMS - CiteSeerX

Failure Mechanisms in MEMS Jeremy A.Walraven Sandia National Laboratories. Albuquerque, NM USA

Abstract MEMS components by their very nature have different and unique failure mechanisms than their macroscopic counterparts. This paper discusses failure mechanisms observed in various MEMS components and technologies. MEMS devices fabricated using bulk and surface micromachining process technologies are emphasized.

1.

Introduction

Many types of MEMS components are currently employed in a variety of applications. These devices are different from their bulk or IC counterparts and have new failure mechanisms that do not occur in traditional ICs or bulk components. Most MEMS devices are so small that their surface area to volume ratio is relatively high (1,000:1 through 10,000:1 surface area: volume ratio). To properly categorize these devices and those currently under development, a general taxonomy of MEMS devices has been developed. This taxonomy is based on the complexity of the design and, to an extent, operation. The devices are categorized as: •

Class 1 devices have no moving parts. These include consists of accelerometers, pressure sensors, ink jet print heads, strain gauges, etc.

•

Class 2 devices incorporate moving parts without rubbing or impact on their surfaces. These include gyros, comb drives, resonators, and filters.

•

Class 3 structures contain moving parts with impacting surfaces. These include relays and valve pumps.

•

Class 4 devices contain moving parts with impacting and rubbing surfaces. These include shutters, scanners, micro-locking mechanisms, and optical switches [1].

Many of the MEMS components available commercially are fabricated using LIGA, surface micromachine, bulk micromachine, or integrated MEMS processing technologies. Regardless of the technology used, MEMS design and functionality fall into one of the 4 classes. Paper 33.1 828

Each classification of MEMS has failure mechanisms associated with it. Some are specific to that category of devices, while others overlap with other categories of devices. Class 1 devices have failed via particle contamination and stiction (release, and shock-induced). Class 2 devices suffer similar failure mechanisms as in Class 1. With moving components, mechanical properties such as fatigue may induce failure depending upon the material system used. Class 3 devices have all the same potential failure mechanisms as Class 2. Impacting surfaces can generate structural damage leading to cracking, fracture, etc. Class 4 devices may suffer potential failure mechanisms shown in Class 3. The addition of rubbing surfaces may induce heat resulting in friction failures, wear, etc. Other failure mechanisms found in MEMS include component degradation, charging, ESD, shock, vibration, stress corrosion cracking, corrosion, and creep. Certain failure mechanisms tend to be application specific and are not typically observed in one class of devices vs. another.

2.

MEMS Failure Mechanisms

2.1 Class 1 MEMS components with no moving parts are unknown to fail due to operation. However, as has been demonstrated in ICs, particulate contamination can and typically will induce failure in a MEMS device. Particles can be difficult to detect because they may not electrically interfere with the operation of a device. Particulate contamination may serve to mechanically obstruct the device while its electrical integrity is maintained. This failure mechanism may be subtle and difficult to detect because particles may not electrically bridge a structure resulting in a short. An example of particle contamination mechanically obstructing a device is shown in Fig. 1. Here, the inertial sensor cannot move, and therefore, cannot create an output signal to send to the surrounding electronics.

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Other failure mechanisms found in Class 1 devices include stiction (adhesion of structural components to one another or to the substrate) and process specific issues.

To study the effects of fatigue, electrostatically driven devices fabricated with a notch at the hinge are used to observe fatigue failure and crack growth and propagation. As shown in Fig. 3, a polysilicon comb structure with a fabricated notch at the base acts as a stress concentration site allowing fatigue and crack growth to occur more readily. Fig. 3b shows a crack initiated at the notch. A crack reduces the lifetime of the device and makes it more susceptible to failure by fracture.

a

Fig. 1. Particle obstruction in an older inertial sensor. Photo courtesy of Analog Devices [2]. 2.2 Class 2 These devices have intentionally designed moveable parts that interact with the rest of the device to perform a given function. The DMD™ fabricated by TI is an example of a Class 2 device whose micromirrors function by moving +/- 10o to redirect light. This device uses aluminum as the structural material. Reports have described hinge and yoke regions of the DMD™ as being susceptible to fatigue [3]. The mechanical fatigue failure of this region would result in a micromirror not returning to its original position [3]. The DMD™ and associated hinge and yoke regions are shown in Fig. 2. Changes in design with slight modifications in the fabrication process have significantly reduced mechanical fatigue as a failure mechanism in the DMD™.

- 10o

b

+ 10o Fig. 3. a) Electrostatically driven polysilicon device with a notch fabricated at the anchor (black arrow) is used to test for fatigue. b) crack initiation at the notch. Photo courtesy of Stuart Brown at Exponent, and Dennis Freeman at MIT

Hinge Yoke

Spring Tip CMOS

Fig. 2. Critical mechanical components of the DMD™.

By initiating a crack, the powered component becomes susceptible to fracture or particulate contamination. A fracture failure is considered a catastrophic failure. In Paper 33.1 829

this failure mechanism, the fractured component acts as a loose piece of material with the potential to obstruct other moveable components within the device. Although mechanical fatigue is not commonly found in MEMS devices, fatigue may lead to fracture but is typically diagnosed before catastrophic failure occurs. Process and handling improvements have helped reduce and eliminate fracture in MEMS. Devices may also be redesigned to minimize stress concentration sites.

3.

Global Failure Mechanisms

3.2 ESD Some failure mechanisms described earlier in this paper will affect a MEMS device regardless of its class. Failures due to stiction and particle contamination have been shown to cause failure in all 4 classes of devices.

2.2 Class 3 MEMS devices with impacting surfaces have the potential to create debris, fracture components, induce cracks, etc. Impact failures are very dependent upon the force exerted on the opposite MEMS structure. MEMS structures may be resilient to impact if the impact force is low, reducing the risk of cracking the contacting structure. One family of devices that sustain impact damage are RF MEMS switches. Here, two surfaces are designed to contact and conduct a signal from one line to another. When a device is cold switched (no power applied during mechanical switching), the surfaces collide. Depending on the contact force, asperities may break off during impact resulting in debris or contamination altering the contact properties. 2.2 Class 4 Like Class 3, these devices have moving, impacting structures with the addition of rubbing surfaces. Rubbing creates friction and often will result in the creation of wear material or debris. The formation of this material may result in several different failure mechanisms. These are: failure by particle contamination binding the device, particles causing third body wear changing the motion tolerance, particulate contamination preventing or obstructing motion, and adhesion of rubbing or contacting surfaces [4]. The mechanism for wear may depend on the temperatures reached during rubbing. Material may be rubbed off the contacting surfaces; surface material may be oxidizing – then rubbed off, etc. Many parameters must be examined to determine the root cause of wear, making analysis straight forward but time consuming. An example of a microgear that failed due to wear of rubbing surfaces is shown in Fig. 4. Note the debris in Fig. 4a (black arrow) and the adhered spot of Fig. 4b. Redesigning the structures to reduce wear will increase reliability. Experimental evidence has shown that wear resistant films such as SAM (Self Assembled Monolayer) coatings are removed during the wear process. Other wear resistant films such as tungsten have shown promise. Paper 33.1 830

a

b Fig. 4. a) wear debris on the surface of a microengine operated to 600,000 cycles. b) FIB cross section through the worn region showing adhered surfaces caused by rubbing. The microengine failed by seizing of the gear. Many MEMS components are sensitive to electrostatic discharge (ESD) and electrical overstress (EOS) [5]. Typical EOS/ESD events have been shown to damage input protection circuitry for ICs. Unfortunately, standalone MEMS structures do not have input protection mechanisms to avoid electrical overstress damage. As shown in Figs. 5a and b, electrostatic actuators from a microengine and a torsional ratcheting actuator were damaged by a human body model ESD event. The ESD transient resulted in a welding of the polysilicon finger to the a) ground plane, and b) other MEMS structure. EOS/ESD transients may not cause the structural damage shown in Fig. 5, but the device may exhibit movement when it is in a powered down state providing a false positive signal such as inadvertent motion, deflection, etc.

a

b

removed, surface tension forces can pull the MEMS component into contact with the substrate resulting in a stuck, or stiction-like failure. Other causes of stiction include shock induced stiction (mechanical overstress) and voltage overstress resulting in a large contact area with surface tension forces keeping the two in contact. These stiction events can occur horizontally or vertically depending on component density. In these stiction induced cases, the restoring force on the device is not enough to bring the component back to its original position. Restoring forces can be enabled through the fabrication of a spring-like structure with a constant restoring force or biasing a device in the opposite direction. An example of a stiction failure of freefloating beams in contact with the substrate is shown Fig. 6. Simple, diagnostic devices such as free standing and fixed-fixed cantilever beams are processed and released with product die as monitors for particulates and release stiction issues. Traditional fabrication and assembly defect reduction programs can be implemented to reduce particulates. Anti-stiction coatings can be used to reduce and/or eliminate the effects of stiction.

Fig. 5. ESD failures induced in electrostatic actuators. a) ESD transient caused a short bridging a polysilicon comb finger to ground of a microengine, b) short from the fixed comb fingers to the moveable comb fingers of a TRA. An ESD pulse may also induce partial motion, setting a device off its as-fabricated position. This may be important if device operation relies upon previous or known positions. Proper ESD handling techniques and precautions reduce the probability of occurrence of this failure mechanism. Device design has a significant effect on ESD robustness of a MEMS device. In most instances, a beam will deflect in one direction, resulting in a short. Designs that implement stiffer, more rigid components will result in an increase in the ESD voltage required to induce failure in the device. 3.2 Stiction Stiction can cause catastrophic failure in many MEMS devices. This failure mechanism can be induced in a variety of ways. During release, when the sacrificial material is removed, a meniscus can be formed between the structure and the substrate. When the liquid is

Beams contacting the ground plane Fig. 6. Stiction induced failure on an inertial sensor. Note, stiction was induced by breathing on the device. Image courtesy of Analog Devices.

4.

Microfluidic Failure Mechanisms

Microfluidic MEMS (including Bio MEMS) are an interesting class of devices where failure mechanisms not only result from the device operation itself, but also from the environment/material the device is used in. Packaging materials are also important when examining failures in microfluidic devices. Many packaging materials outgas. This out-gassed material may bind to other materials present in the environment, leading to clogging or a build-up of material in strategic active regions. Paper 33.1 831

a

5 µm

of spec. An example of a clogged electrostatic drop ejector is shown in Fig 7 [6]. Microfluidic MEMS are exposed to many different materials and environments. It is important to be sure the material used in the device does not result in obstruction or physical attack of the structural component when the device is biased. In Bio MEMS, obstruction may become more prevalent if the fluid or biological material changes viscosity with the addition of an electric field. Proteins, cells, etc. may act differently in the presence of an applied field. This may result in obstruction, or a reduced flow rate by added resistance to the pumps in the device.

b

10 µm

Many devices in Bio and microfluidic MEMS are under research and development. Although there have been successful commercial products (Ink jet print heads [7], µTAS medical systems on a chip), many technical hurdles are being overcome in the design, fabrication, and test processes.

5.

MEMS devices offer uniqueness in their application, fabrication, and functionality. Their uniqueness creates various failure mechanisms not typically found in their bulk or IC counterparts. In ICs, electrical precautions are taken to mitigate failure. In MEMS, both electrical and mechanical precautions must be enacted to reduce the risk of failure and increased reliability. Unlike ICs, many MEMS components are designed to interact with their environment, making the fabrication, testing, and packaging processes critical for the success of the device.

c

7.

10 µm Fig. 7. a) SEM image of a contaminated orifice, b) FIB cross section of a piston stuck to the foreign material, and c) top view SEM image showing contamination at the center and edge of the piston (arrows). Resulting contamination or material build up may likely render the device inoperable by obstructing motion, binding two or more structures together (similar to stiction but a physical material binds the material together), or restricting fluid flow bringing the device out Paper 33.1 832

Conclusions

Acknowledgements

The author would like to thank the microelectronics development laboratory at Sandia National Laboratories for their processing efforts, Bonnie McKenzie and Alex Pimentel for their SEM and FIB work respectively, ASI for ESD testing, and Brad Waterson for material and fruitful MEMS discussions. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration for the United States Department of Energy under Contract DE-AC04-94AL95000.

8. [1] [2] [3] [4]

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