2008 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP 2008) 978-1-4244-2740-6/08/$25.00 © 2008 IEEE

A Reliability Investigation of MEMS Transducers with Comb Structures

Ping AN, Yandong HE, Yufeng JIN, Yilong HAO* National Key Laboratory of Micro/Nano Fabrication Technology / Institute of Microelectronics,

Peking University, 100871, P.R.China anping@ime.pku.edu.cn, jinyf@ime.pku.edu.cn, ylhao@ime.pku.edu.cn

Telephone: 86-010-62752561 Fax: 86-010-62752789

Abstract An overview of reliability investigation of micro electro

mechanical system (MEMS) transducers with comb structures was presented. With the development of the MEMS industrialization, the reliability is underway to meet the need of market. To build a reliability research and develop platform, MEMS center of Peking University made its roadmap of a general developing plan. In this paper, the roadmap was presented and the short-term roadmap about failure analysis (FA) was illustrated. In addition, FA techniques of localization, characterization and sample preparation for MEMS were summarized. Finally, some FA cases of the center of PKU were discussed and a destructive physical analysis (DPA) case about the particle impact noise detection (PIND) was presented.

Introduction Reliability researches about MEMS are particularly

important to break the bottle-neck of MEMS applications. With the advent of MEMS applications in application in automobile industry, consumer electronics, space vehicles etc., the MEMS sensors and actuators will take a more important role for their lower weight, lower capacity and higher reliability than normal transducers. Moreover, electric consumers and car electronics need the products with more reliable, more stable and longer life MEMS devices but low cost. To speed up the application of MEMS devices in these fields, it is necessary to develop the reliability all-round.

However, the general reliability problems have not been discovered sufficiently and systematically yet for the problems do be numerous and complicated. The problems would involve in the material, physics, devices and even system. Furthermore, different samples generated different reliability issues. Radio frequency (RF) switches often relate to creep and lifetime period. But the microengines with gear often involve in the friction and the wear. Moreover, it is very hard to perform systematical experiments for the time consuming and the cost. Only one experiment to determine the life of the devices often takes several months. While the study of MEMS reliability problems was started more than ten years ago in the world, most related researches were pursued separately or specially in different programs. The general studies of MEMS reliability was carried out by Sandia Laboratory of USA in the end of the last century. The groups in Sandia studied on the poly-Si structures which made by surface processes and took a series of experiments on the special micro-engine with gears and combs. [1] The results involved in failure mechanisms, such as the adhesion, the wear and the friction, and FA such like the influence to the lifetime by the humidity etc. French Space Agency

(CNES) performed the general reliability studies five years ago. Their achievements were mainly on the environmental tests and some basic mechanism researches like thermal deformation. [2] In China, many special reliability problems have been studied. However, the general research on MEMS reliability has just started and seldom issued some useable mechanism of failure and some results about lifetime model. Achievements of systematic reliability study had not been issued yet. Therefore, it is very essential to study the MEMS reliability generally and systematically. In this paper, we present a roadmap of how to perform the systematic research in PKU (Peking University).

In addition, Micro comb structure made by bulk-Si undertakes a standard sample in MEMS or MOEMS for it takes great applications in MEMS as a senor or an actuator normally. The sensor with comb structures like resonators，accelerometers and gyros can be easily produced by standard bulk-Si processes and can realize a 2D or 3D direction sensing with a high level sensitivity, a high reliability and an expandable ability. The comb actuator also has the most popular application in RF MEMS as well for its integratability with integrated circuit (IC) in a die. Therefore, the reliability investigation was discussed by using MEMS devices with comb structures as samples.

In the present paper, the roadmap of reliability investigation was introduced first. Then the FA as the basic problems was studied and the failure investigation was presented in details by introducing some novel FA techniques and some results of a MEMS structure with comb structures.

1 Roadmap The roadmap of how to study the complex reliability

problems in MEMS center of PKU follows four parts as showed in Fig. 1: FA, test structure, lifetime and harsh environment. FA involves in investigating failure modes, failure analysis and building failure mechanism. These will be performed in three levels in turns of die level, module level and system level. The test structure part relates the studies of designing test structure like stress gauge to extract the parameters of the material and processes etc. These studies will be carried out to establish a database of the parameters. The lifetime is the studies about mean time to failure (MTTF), Mean time before failure (MTBF), storage lifetime and accelerated stress test and modeling etc. The harsh environment will be explored to find some novel method to anti-harsh stresses and promote the quality of products. As the most important one and the most urgent one, the FA of MEMS was studied first.

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Fig. 1 Subjects in MEMS reliability.

2 FA FA is a procedure involving failure sites localization, root

cause investigation and failure model establishment. As the first step of FA, the failure sites localization was pursued by using a series of techniques in three levels of production: die level, module level and system level (Fig. 2). While many FA techniques are based on IC techniques, some of the MEMS FA techniques are quiet different with the ones of IC for FA of MEMS involves in not only electrical analysis, but also mechanical, material and circumstance analysis. In this section, the techniques for MEMS failure analysis were discussed in three parts: 1) Failure localization techniques; 2) Characterization techniques; 3) Subsidiary techniques. In addition, some FA cases by using parts of these techniques were presented in turns of Fig. 2 as: 1) Yield rate investigation; 2) Failures localization in die level; 3) Failure analysis in die level; 4) Failure investigation in module level; 5) Exploratory experiment of PIND test.

Failure in Die Level

Failure in Module Level

Failure in System Level

Design

Process

Assembly

Package

Assembly

Package

Fig. 2 Failure localization in production

2.1 FA Techniques In MEMS failure studies, the main FA techniques can be

parted into failure modes localization, failure characterization and other subsidiary techniques for preparing samples.

In first step of FA, the failures were localized in macroscopic analysis and microscopic analysis first. The techniques for localization were used in MEMS FA and were listed in Table 1. In which, optic microscopy and SEM are both popular in MEMS FA to detect the surface failures, and the confocal IR image, X-Ray inspection, ICT and SAM are used in localizing the defects under the surface layer.

In the following steps, the spotted failures were determined to be which kind of failure modes and found the root causes by characterization techniques (Table 2). Where, AFM, Raman Spectroscopy, X-Ray Diffraction, Nano Indenter, Optical Scanning Profilometer, and Full Field Optical Profilometer can be used in detecting the deformation of the structure to calculate the residual stress, and the material characterization techniques like TEM, EDXS,

EELS, AES and SIMS can be used to determine the contaminations or other special failures related to the material.

During the analysis, some subsidiary techniques were used to prepare specimens or test plane for special uses (Table 3).

Table 1 Techniques for failure modes localization

Technique Applications in MEMS FA Key Features

Optic Microscopy

Surface inspection: wear,

broken, contamination etc.

XY Resolution: 0.25μm Magnification: 1000Х

XY Resolution(3keV): 20～30Å

Scanning Electron

Microscopy (SEM)

Surface inspection: wear,

broken, contamination etc. Magnification:

300000Х Confocal IR

Laser Microscopy

Delamination, failures under the

surface

Limited depth of field, but perfect in focus

X-Ray Inspection

Metal contaminations,

package

XY Resolution: 1μm

Industrial Computerized Tomography

(ICT)

Metal contaminations,

package

XY Resolution: 10μm

Scanning Acoustic

Microscope (SAM)

Delamination

XY Scanning range: 1.3mm2 ～76mm2, XYZ Resolution: 5μm

Table 2 Techniques for characterization

Technique Applications in FA Key Index

Resolution: 10 pm Atomic Force Microscopy

(AFM)

Surface topography and roughness, friction

Surface: conductive or insulate

Raman Spectroscopy Stress

Depth: 1μm XY Resolution: 1μm

Spectrum resolution: 0.2 cm-1

X-Ray Diffraction Stress Spot size: Ф50μm

Nano Indenter Stress, fracture

Optical Scanning

Profilometer

3D topography / roughness /deformation / stress

Z Resolution:1μm

FA Test Structure Lifetime Harsh Environment

1.Chip Level FA

2.Module Level FA

3.System Level FA

1. Structure Design

2. Database of material,

processes, etc.

1. MTTF,MTBF 2. Storage Lifetime

3. ALT Stress Tests

1. High Stress Test

MEMS Reliability

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Full Field Optical

Profilometer/ （Vibrometer

）

3D topography /stress

Dynamic deformation

XY Resolution: < 1μm ,

Z Resolution: <10nm,

（Stroboscope）

Transmission Electron

Microscopy (TEM)

Wear/ contaminations /defects in atomic scale

Resolution: 2 Å

Energy Dispersion X-

ray Spectroscopy

(EDXS)

Wear / contaminations Range: 1um

Electron Energy Loss Spectrum

(EELS)

Wear / contaminations Range: 2 nm

Auger Electron Spectroscopy

(AES)

Wear / contaminations Resolution: <12nm

Second Ion Mass

Spectrometry (SIMS)

Contaminations /delamination / doping

Spatial resolution : 0.5um，depth

resolution: 100A

Table 3 Techniques for sample preparation

Technique Applications in FA Key Index

Focus Ion Beam

High precision dicing

Precision：Å No induced contaminations Range: < 50×30μm2

Parallel Polishing Layer removing Precision:1μm～

1nm

Lift-off Layer or parts removing

Laser Ablation Velocity: several picoseconds

2.2 Failure investigation In this sub-section, some cases were presented in turns to

investigate all the failure modes of a kind of MEMS transducer with comb structures in the die level and the module level.

2.2.1 Yield rate investigation In the large scale production, failures of the comb structure were investigated to improve the reliability. The static capacitance test and the surface check by microscope and SEM were carried out to screen the unqualified structures. In the screening, almost all of the failure modes were found as: over thin finger, hogging finger, structure disjunction, etch pits and some intact structures. The former three failures turned out to be over etched and the broken fingers normally were induced by

random shock operating or man made errors. However, about 19% structures in die level after dry etching release showed an over lager static capacitance (Fig. 3). For the structures looked like qualified ones with a perfect surface, the failures needed to be investigated and analyzed in details.

Fig.3 The yield map-graph of comb devices

2.2.2 Failures localization By surface inspection, failures like contaminations, etch

pits, and the failures by over etching were recognized. Fig. 4 showed some contaminations by SEM/EDX in 10 kV, where (a) was a SEM photo of a contamination and (a'), an EDX curve with a Si peak, shows that the contamination was a broken comb. So did the (b)~(e): (b') with C peak shows us that the contamination is a kind of organics. The organic turned out to be photosensitive resist (PR); (c') with Si and O peaks represented that it could be some dust of SiO2 etc; (d') with peaks of C, Na, Cl etc. shows that the contamination must be a furfur from operator; And (e') with peaks of Fe, Ni and Cr represents that it is a metal particle which could come from a stainless tweezers. The failure modes with over etched structures were showed in Fig. 5, where (a), (b), (c) were caused by over etch and broken fingers (d) were caused randomly by manmade. The etch pits (e) were induced by the wet etching. The last one (f) showed connection combs under the structure layer. Its root cause turned out to be the un-uniformity of the dry etching.

Fig. 4 SEM photos of contaminations and material analysis

by EDX. (a) and (a’) SEM photo and EDX analysis of a broken comb respectively. (b) and (b’) Some PR points. (c)

and (c’) A particle of dust. (d) and (d’) A furfur from operator. (e) and (e’) A metal particle from tweeter.

(a’) (b’) (c’) (d’) (e’)

(a) (b) (c) (d) (e)

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Fig. 5 Micrographs of failure modes

(a) Structure disjunction. (b) Hogging Finger. (c) Thin Finger. (d) Break Finger. (e)Etch Pits.

(f)Connection combs with perfect surface After investigation of a 3000 samples, a classifier was

built for automatic classifying the potential failure modes. The classifier was built by an improved Back-Propagation (BP) neural-net and can be used with combining auto pattern recognition to failure modes. To build the classifier, a series of investigations of failure modes of devices were carried out. As an application, the classifier was built by investigating a kind of comb capacitance micro-engine. The result of training indicates that the classifier is effective with a very low error which is less than one chip in 300 chips in one wafer. The result of testification shows that the classifier fits to the wafers from different batches (Fig. 6). [1,3-6]

Fig. 6 Training errors of the classifier

2.2.3 FA in die level In failure investigation, the 19% failed structures with the

perfect surface needed the further analysis to find the root cause of the failure. To determine that there were some metal particles ling under the combs, X-Ray inspection with a

lower than 1um precision was carried out as in Fig. 7. However, the figure showed that there were not any metal particles which made the capacitance abnormal. To find what had happened under the combs, the technique of sample preparation, parallel polishing, needed to be performed. [7-9] The combs were filled in epoxy and were polished physically after the epoxy solidified. The precision of the polishing was less than 1μm. After removing the structure layer, the pattern under the combs was obtained as in Fig. 8. The black field turned out to be the connection of the combs which was testified by EDX. By comparing the different fields in line (a) and line (b) by EDX, the black field was determined to be Si which was same as the combs. By further analyzing the processes, we found that the connection of the combs was caused by un-uniformity of the dry etching to the combs with high aspect ratio.

Fig.7 X-Ray transmission inspection showed that there were

no metal particles located between structure and substrate

Fig. 8 The optical and Close-up SEM micrograph of the

structure showed some abnormal phenomena occurred in the bottom

In die level, some issues about stress were focused for it caused the deformation of the combs and further induced the static capacitance drifting with the temperature outside. One of the techniques to study the residual stress in micro field is Raman spectroscope. The stress was calculated by the shifts of the spectrum. From Fig.9, we found in the area of the bonding, the shift of spectrum was obvious as the line in 10um which was the bonding area.

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Fig.9 The spectrum shift due to the residual stress in the

interface of bonding

2.2.4 FA in module level In module level, MEMS reliability was related to the

assembling and packaging. As to the assembling, the adhesive strength, the adhesive uniformity will determine the quality of the assembling. In addition, the un-uniform adhesion would cause a different stress to the MEMS die which could be very important to the structure’s reliability. Therefore, a series of experiments were carried out to study the failures of the assembling. One of the studies used the SAM to detect the adhesive uniformity as Fig. 10. After investigated the acoustic reflect wave of the materials of the die substrate and the package shell, the interface of the die and the shell was detected. In Fig.10, the interface was spotted with some gaps especially in the corners of the die. The further examinations showed it was caused by surface tension during the epoxy solidifying. As to the package, a FA case was about gas leakage. After the plat bonding, some modules showed unqualified in atmosphere test for their over large vapor concentration. The interface of the bonding board was checked by X-Ray inspection and the reason turned out to be some holes lied on the sealing board (Fig. 11). The holes kept some gas which released after the sealing.

Fig. 10 The acoustic microscopic figure of failed assembly

Fig.11 Air leakage points of the sealing by X-Ray

inspection

2.2.5 Exploratory experiment of the PIND test With the development of MEMS into the market and into

the space applications, some standard should be built. However, only a very few standard for MEMS have been built until now. In fact, most products of MEMS were test and screened by using the methodologies and standards of IC or other traditional discrete components. But many of that can not be used in MEMS for many MEMS modules have movable parts. One of these tests is PIND which is used to detect the movable particles concealed in the module normally. As some parts of the MEMS transducers with comb structures are movable, whether the PIND test is still usable or not should be re-testified. In one of the experiments, the traditional PIND (4511L) was used to detect a module with some broken combs. The result of the test showed not any particles in the module. It meant that the PIND can not detect the broken comb for the comb weight only about 3×10-10g, which was much lower than the normal particles in other modules which were more than 2×10-6 g. Therefore, the PIND should be developed to detect much lighter and smaller particles to meet the MEMS requirements.

Another experiment was performed to find if the PIND could destroy the combs or not. The results showed that the combs can not destroy under a very low frequency of (25~250Hz).

Therefore, traditional PIND can be used in MEMS DPA. However, it needs to be developed to detect the super lighter particles and the frequency of the equipment needs to be tunable. Moreover, the further test method to build a new standard needs to be testified.

Conclusions MEMS reliability is very important and essential to help

the products into the space applications and consumer electrics market. To build a reliability research platform and realize the design reliability, the roadmap was built to subsumption the complex and difficulty problems. The short-term roadmap was focused on FA and the study logic was presented by steps of different production status. In MEMS FA, many conventional FA techniques were used in localization and characterization. Some subsidiary techniques were used in the sample preparation too. In addition, some

2008 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP 2008)

FA cases of a MEMS transducer with comb structures were presented in turns of FA: yield investigation, failures localization and root causes analysis in die level and some failure modes in module level etc. The last case was an exploratory experiment of PIND used in MEMS DPA. MEMS center of Peking University developed a know-how in MEMS FA and related techniques to promote the reliability of the products with comb structures.

Acknowledgments The authors would like to thank Mr. Wei Cai and Mrs.

Aibin Xu of CEPREI Certification Body for their technique support.

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