But with the continued expansion of MEMS devices

into new areas of application, the limitations of

silicon (Si) usefulness became clear. The need to

combine electronics and MEMS on the same chip

(iMEMS), improve the wear characteristics of moving

parts, and achieve a greater mass of moving parts in

MEMS inertial sensors have led researchers away

from a ‘one-material-fits-all’ approach. Instead the

search is on for materials that more directly serve

specific ends. The need for biocompatibility in the

emerging field of bio-MEMS has added urgency to the

quest for new materials, since Si-based materials

cannot meet every bio-MEMS need.

A more basic problem with Si arises from its surface

properties. At micron dimensions, surface forces are

frequently stronger than the more familiar elastic and inertial

forces that are proportional to volume. Particularly

troublesome is stiction, the tendency for surfaces to stick

together. This is because water – present from humidity or

other sources – in small gaps gives rise to strong capillary

forces. If the materials involved are hydrophilic and the water

evaporates, the materials pull together seeking the few water

molecules remaining. This may stick surfaces together and

render a device inoperative. Though a bare Si surface is

hydrophobic, it quickly oxidizes on exposure to air or water

to become hydrophilic.

Expensive procedures, such as supercritical drying1, are

routinely used to avoid this problem in effecting the final

release of complex Si MEMS structures. A better solution

would be the discovery or creation of a hydrophobic thin film

by Robert Huber and Neal Singer

Out with the oldin with the new

Sandia National Laboratories,Albequerque, NM 87123, USAE-mail: nsinger@sandia.gov

(Image above courtesy of Sandia NationalLaboratories.)

The materials issues facing the

microelectromechanical systems (MEMS) community

can be understood best in terms of the historical

context. The field began almost as an afterthought

among those engaged in integrated circuit

production. These researchers recognized early on

that the same processes used in the production of

circuits could be re-ordered to make very small

mechanical devices. The huge investment made by

the electronics community in silicon technologies –

and the relative ease with which these techniques

could be adapted to device production – made them

an obvious resource for early MEMS designers. It is no

accident that polycrystalline silicon is the most

commonly used structural material.

July/August 200236 ISSN:1369 7021 © Elsevier Science Ltd 2002

REVIEW FEATURE

material, suitable for MEMS fabrication and able to simplify

the release process by allowing water to be removed by a

dehydration bake.

This article describes a few of the promising new materials

being explored at Sandia National Laboratories and other

organizations as solutions to the above problems. Any new

films must have properties that polycrystalline Si does not,

but must be compatible with most of the microfabrication

processes that already exist.

Integrated MEMSAn oft-mentioned advantage of Si surface micromachining

(SMM) is its potential for integrating complete electronic

circuits and complex micromachines on the same chip2. The

advantages are obvious: lower system costs thanks to batch

fabrication of the entire system, better performance because

of lower noise and less capacitive loading, higher reliability

because of fewer interconnects, and simpler packaging.

However in practice, realization of these advantages has

proven to be quite difficult for a number of reasons, including

some very complicated questions involving materials

compatibility.

The material of choice for SMM structures is usually

polycrystalline Si, the same material extensively used in

CMOS (complementary metal-oxide silicon) circuits. In many

ways polycrystalline Si is an ideal material for MEMS. It has

high strength, can be made almost stress-free (an essential

property for free standing structures), and is linearly elastic

at strains up to the fracture point. It can be deposited as

conformal layers that are part of a multilayer structure.

Sandia National Laboratories’ SUMMiT™ process

(see Figs. 1 and 2 for examples) is able to use as many as five

polycrystalline Si layers separated by sacrificial layers of

silicon dioxide (SiO2) and silicon nitride (Si3N4).

Why, then, are there so few integrated MEMS processes

available and why are they so expensive? The answer lies in

the fabrication details.

For micromachines, the polycrystalline Si layers must be

formed stress-free. Stress-free polycrystalline Si requires very

high temperature anneals, in the 1100°C range. But modern

CMOS circuits use submicron dimensions, both horizontally

and vertically. Submicron MOS transistors cannot withstand

the high temperatures over time required to produce stress-

free polycrystalline Si. Different approaches have been taken

by various labs to solve this problem.

Sandia National Laboratories has developed a process in

which the MEMS components are fabricated first in a

depression etched into the wafer surface3. After the

mechanical elements are formed, the depression is

completely back-filled with SiO2 and the wafer is resurfaced

using chemical mechanical polishing (CMP). At this point, the

July/August 2002 37

Fig. 2 Computer-generated cut-away model of a SUMMiT 5™ device. (Courtesy of JamesAllen, Craig Jorgensen, and Victor Yarberry at Sandia National Laboratories.)

Fig. 1 Computer-generated cut-away view of a SUMMiT 5™ device, in this instance atorsional ratchet actuator. (Courtesy of Stephen Barnes, Craig Jorgensen, and VictorYarbury, Sandia National Laboratories.)

wafer goes into the CMOS process where the electronics are

formed by a near-standard process. Since the mechanical

polycrystalline Si has already been exposed to very high

temperature anneals, the relatively low temperature process

for submicron CMOS has little or no effect on the MEMS

device. After completing the CMOS device, a few additional

low temperature steps are needed to complete the metal

interconnections and release the MEMS structures

(see Figs. 3 and 4). This MEMS-first approach is difficult in

practice because of the very long fabrication throughput

times and restrictions on the MEMS design. Therefore, many

attempts have been made to add MEMS to completed circuits

– a second approach to fabricating integrated MEMS.

But a critical need for the MEMS second approach is much

lower temperatures during MEMS fabrication. This need is

one of the drivers in the search for new MEMS-suitable

materials. Two examples of the MEMS second approach are

the Defense Advanced Research Projects Agency (DARPA)

sponsored Application-Specific Integrated-MEMS Process

Service (ASIMPS) alpha run, a prototyping service supported

through a collaboration of Cronos Integrated Microsystems,

MEMSCAP, and the Carnegie Mellon MEMS Laboratory, and

the service from MOSIS.

The first process uses conventional CMOS processing

followed by a sequence of maskless dry-etching steps to

fabricate composite metal/insulator microstructures.

Currently, the process is designed for a 0.6 µm CMOS process

from Austria Micro Systems International4, but temperature

is still an issue.

The integrated MEMS process from MOSIS has much the

same approach. Processing steps are added to a conventional

CMOS-processed multi-project wafer and the MEMS user

performs a post-processing maskless anisotropic etch. At

present the available CMOS is the 2 µm double

polycrystalline Si SCNA and SCPE Orbit process5.

In with the newAmong the more interesting new MEMS materials that might

solve the temperature problem are polycrystalline germanium

(Ge), polycrystalline silicon germanium (SiGe), polycrystalline

diamond films, and amorphous diamond films. All four of

these can be formed in a stress-free condition at much lower

temperatures than polycrystalline Si.

Researchers at the Berkeley Sensor and Actuator Center at

the University of California, Berkeley, under the leadership of

Roger T. Howe, have demonstrated microstructures built on

standard CMOS wafers using both polycrystalline Ge and

SiGe.

Polycrystalline Ge

This material6 can be deposited by conventional low

pressure chemical vapor deposition (LPCVD). Rapid thermal

annealing (RTA) can then be used to give low resistivity,

tensile polycrystalline Ge films for freestanding

microstructures. The time-temperature cycles are compatible

with standard aluminum (2% Si) metallized CMOS wafers.

The mechanical properties of polycrystalline Ge are

comparable to Si and resonators have shown quality factors,

Q, as high as 30 000 in vacuum. Resonators with integrated

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July/August 200238

Fig. 3 Cross section of a Sandia National Laboratories integrated MEMS structure.

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CMOS amplifiers have been built to demonstrate the

capabilities of the material.

Polycrystalline SiGe

If used in combination with a sacrificial layer of

polycrystalline Ge, this material shows promise as a

micromechanical structural material for use on top of

conventional CMOS wafers with aluminum (Al) metallization

(see, for example, Fig. 5).

Stress free polycrystalline SiGe can be deposited by CVD at

temperatures low enough to be compatible with completed

CMOS wafers7. These films can be formed into complex

mechanical structures by surface micromachining with post-

CMOS compatible steps. The built-in strain in the

polycrystalline SiGe layers can be controlled from -145 MPa

to +60 MPa by deposition conditions. Surface micromachined

structures have been successfully formed directly on standard

CMOS electronics8. Folded flexure lateral resonators with a Q

value as high as 15 000 in vacuum have been reported.

Amorphous diamond films

Two other new materials being considered for MEMS are

amorphous diamond (aD), grown by pulsed-laser deposition9,

and CVD polycrystalline diamond films10.

Diamond and hard amorphous carbon have recently

emerged as a promising class of materials to improve the

mechanical performance and reliability of MEMS (see, for

example, Fig. 6). Diamond has the highest hardness (~100

GPa) and elastic modulus (~1100 GPa) of all materials.

Amorphous forms of carbon – specifically the hard carbons,

aD, tetrahedral amorphous carbon (ta-C), and diamond-like

carbon (DLC) – can also approach crystalline diamond in

hardness (up to ~90 GPa) and modulus (800+ GPa). It is not

the hardness that is the main appeal of these materials for

the MEMS designers, however, it is their extreme wear

resistance (up to 10 000 times greater than Si),

hydrophobicity, and chemical inertness.

The inherent stiction resistance of these carbon-based

materials (hydrophobic surfaces do not stick together) and

their chemical inertness permit their use in aggressive

chemical environments. Crystalline diamond also possesses

the highest thermal conductivity and the widest range of

July/August 2002 39

Fig. 4 Integrated MEMS chip with complex CMOS circuits and MEMS components on the same chip. The three-axis accelerometer prototype is a joint effort by the University of California,Berkeley and Sandia National Laboratories.

optical transparency (from far IR to UV) of any material,

making it useful for thermal heat sink structures or micro-

optics.

MEMS structures fabricated from mostly CVD

polycrystalline and nanocrystalline diamond, both in the area

of surface micromachining and in mold-based processes, are

making progress. Already-demonstrated diamond

microstructures include: substrates with integrated channels

for active-cooling, micromachined fresnel optics, optical fiber

alignment structures, free-standing capillary tubes,

acceleration sensors, thermally-actuated liquid ejectors,

electrical microswitches, micro-tweezers, diamond cantilevers

with tips for scanning probe microscopy (SPM) applications, a

diamond motor structure, diamond gears, and

nanoindentation tips.

One disadvantage of CVD polycrystalline diamond is

surface roughness – typically quite high, ~1 µm RMS. This

limits the minimum device dimensions to typical layer

thicknesses of several microns, with tens of microns in the

lateral dimension. This inhibits the use of multi-level

processes to create complex MEMS structures.

A new type of hard carbon material recently introduced is

termed stress-free aD. The material is an amorphous mixture

of nanophases of tetrahedrally-coordinated carbon – about

70% of the total – with three-fold coordinated carbon

comprising the remaining 30%. The three-fold coordinated

carbon is not distributed randomly. Instead, it clusters in

conjugated chain-like or sheet-like structures. In the

deposited state, the material exhibits extremely high levels of

compressive film stress (8 GPa), but, surprisingly, with

suitable control of film deposition, 100% stress relief (down

to 0 ±10 MPa) can be achieved. The resulting stress-free films

are hard, optically transparent, moderately conductive, and

best of all, nearly atomically smooth (0.1 nm RMS roughness

on Si, 0.9 nm RMS roughness on SiO2). These properties are

favorable to the use of this material for surface-

micromachined MEMS applications, where small dimensions

and complicated structures are desired.

The low temperatures needed by the aD films open the

possibility of their application to a second approach for

integrated MEMS. It should be possible by post-processing to

add MEMS to previously processed chips. Al metallization

may have to be added after formation of the aD structures,

but there is no reason why this could not be done.

Polycrystalline diamond films

Polycrystalline Si vibrating micromechanical (‘µmechanical’)

resonators with frequencies in the hundreds of MHz that

retain Q values of around 10 000 are now being

demonstrated in research efforts9. The idea is to further

extend frequency ranges into the high-UHF and S-Band

ranges needed for front-end RF applications in today’s

wireless transceivers.

Several approaches to increasing frequency ranges have

been successfully demonstrated, including:

(1) Scaling of devices to smaller and smaller dimensions

through brute force;

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July/August 200240

Fig. 5 Schematic cross-section of an iMEMS showing the combination of CMOS built with the usual n-type polycrystalline Si and MEMS made with polycrystalline SiGe. (Courtesy of RogerHowe and Andrea Franke (now at Motorola) of the Berkeley Sensors and Actuator Center.)

REVIEW FEATURE

(2) Strategic geometries that take advantage of higher

frequency mode shapes and eliminate Q-degradation

caused by anchor losses; and

(3) alternative materials such as SiC that boost the acoustic

velocity of the resonator structural material, thereby

making it easier to achieve higher frequency without the

need for overly tiny dimensions.

Because of the possibility of ‘scaling-induced’ performance

degradations of the above techniques, (2) and (3) show the

most promise for extending the micromechanical resonator

frequency range while retaining the Q values and degree of

stability needed for application to the low-low bandpass

filters and ultra-stable reference oscillators needed in

communication transceivers.

One variant utilizes CVD polycrystalline diamond with an

acoustic velocity potentially more than two times higher

than that of polycrystalline Si. This material can be used as a

structural material for clamped-clamped beam and folded-

beam micromechanical resonators. Polycrystalline diamond

has great potential to achieve the desired UHF frequencies of

0.3-3 GHz needed in wireless communication transceivers

more easily because the resonance frequency of a mechanical

device is generally directly proportional to the acoustic

velocity of its structural material. Because of its inherent

stability and chemical inertness, diamond can potentially

offer better aging characteristics than polycrystalline Si – an

important consideration for frequency and reference

applications. Finally, polycrystalline diamond can be

machined nearly as easily as polycrystalline Si, making it

amenable to conventional surface micromachining. With

deposition temperatures below 600°C, CVD polycrystalline

diamond can be more easily integrated with highly

conductive metal electrodes for lower losses and higher

power handling at GHz frequencies.

Since CVD polycrystalline diamond CC-beam

micromechanical resonators have demonstrated measured

resonance frequencies from 2.7-9.9 MHz and Q values up to

6225, they easily equal the current output of polycrystalline

Si resonators in this frequency range. In addition,

approximately 39 kHz folded-beam, comb-driven resonators

have been demonstrated with Q values in the range of

20 000. The frequencies seen in these polycrystalline

diamond devices are 15% higher than that of equivalently

sized polycrystalline Si versions. While this is not the value

potentially achievable, even moderate success has

encouraged further work in this area.

A radical approachThe four new materials discussed above, while not standard

for MEMS, still have most attributes in common with

July/August 2002 41

Fig. 6 Surface micromachined structure of amorphous diamond. (Courtesy of John P. Sullivan and Thomas A. Friedmann, Sandia National Laboratories.)

polycrystalline Si. Some form of CVD deposits the films. The

fabrication processes involved are near-standard surface

micromachining. The final structures look pretty much like

the standard product. But still more radical approaches are

being taken. High aspect-ratio structures are being made by

using a deep reactive ion etch (DRIE) to create a mold from a

Si wafer. The mold is backfilled with thin sacrificial layers and

a thicker structural layer of polycrystalline Si, or

polycrystalline SiGe, or in some very recent experiments at

Sandia National Laboratories, by tungsten (W). Researchers at

the Berkeley Sensors and Actuator Center at the University of

California, Berkeley11 have produced microactuators using Si

molds that are 100 µm deep, but only 6 µm wide.

Efforts to use W as a microelectronics structural material

are underway at Sandia National Laboratories12. This effort

fabricates freestanding W devices that are supported above a

Si substrate by columns of Si3N4. The process starts with the

fabrication of a Si mold, as described above. Some trenches

are filled with Si3N4 to provide the support columns, while

other shallower trenches are filled with W, which forms the

MEMS device. The parts are released by etching away the top

layer of the Si wafer, leaving the W free, but the deeper

nitride-filled trenches are only partly exposed to provide the

support columns (see Fig. 7).

This process has a number of very attractive attributes

when compared to polycrystalline Si SMM MEMS. Among

them are high ‘Z’ direction stiffness, much greater mass than

Si SMM parts, higher capacitance between elements, high

lateral forces, and much higher electrical conductivity

compared to thin polycrystalline Si. There also exists the

potential to integrate W parts with CMOS processes because

all processing steps are at low temperatures. The density of

W is eight times that of Si, and the layers can be many

times thicker than polycrystalline Si layers. Thus, the mass

per unit area of the ‘proof mass’ in a SMM inertial

sensor could, possibly, be increased by a factor of 50.

Since the resolution of some MEMS designs is now limited

by thermal noise, such a large mass increase would allow

a major increase in the sensitivity of these inertial

sensors.

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July/August 200242

Fig. 7 Tungsten MEMS structures made by the Si micromolding process. The support posts are Si3N4. (Courtesy of James G. Fleming, Sandia National Laboratories.)

Micromoldingadvantages

• Deep trench technology allows depths of tens

of microns

• Molding maintains planarity

• Molding greatly reduces or compensates for

stress

• High Z stiffness

• High force attenuation

• High capacitance

• Tungsten for heat spreading?

REVIEW FEATURE

The futureThe new MEMS materials discussed here are just a few of the

many being developed in labs around the world. It is too early

to tell which, if any, will find widespread use. In many cases a

new material is chosen to solve a specific problem with a

specific MEMS device or application. Unless the specific

application generates a large enough volume to support the

generally high cost of both development and specialized

production equipment needed for the unique material, the

material will not become part of the established technology.

Both microelectronic and MEMS manufacturing are very

expensive undertakings. It takes a very high volume to support a

production line economically. Evidence of this is found in the

number of integrated circuit (IC) companies that have gone ‘fab-

less’, using contract-manufacturing services known as foundries

instead. This strategy works with IC chips because a wide variety

of applications can all use the same fabrication line. This is not

necessarily so for MEMS, especially if the MEMS product uses a

new material to solve some unique problem. If the device has

too limited an application, the unique material will remain a

laboratory curiosity, and Si, with all its handicaps, will remain

the material of choice. MT

July/August 2002 43

REFERENCES

1. Mulhern, G.T., et al., Proc. 7th Int. Conf. Solid-State Sensors, Actuators, andTransducers, Yokohama, Japan, (1993) pp. 296-299

2. Sniegowski, J.J., and de Boer, M. P., Annu. Rev. Mater. Sci., (2000) 30, p. 299-333

3. Smith, J.H., et al., Proc. IEDM (1995) p. 609

4. See http://www.ece.cmu.edu/~mems/asimps_short_course/

5. See http://www.mosis.org/Technical/Designsupport/nist-mems-1.html

6. Franke, A.E., et al., 12th International Workshop on Micro Electro MechanicalSystems (1999) Orlando, USA

7. Sedky, S., et al., J. Microelectromechanical Systems (1998) 7(4), p. 365-372

8. Franke, A.E., et al., Solid State Sensors and Actuators Workshop (2000) SouthCarolina, USA

9. Sullivan, J.P., et al., Mat. Res. Symp. Proc. (2001) 657

10. Wang, L., et al., 15th IEEE International Conference on Micro Electro MechanicalSystems (2002) pp. 657-660

11. Heck, J.M., et al., Transducers '99, Sendai, Japan (1999) pp. 328-331

12. Fleming, J., Mat. Res. Symp. Proc., (2002) 729, to be published