MEMS applications using diamond thin films
04/01/2006
Diamond thin film offers advantages over silicon and other thin-film materials used in micro- and nanofabrication. The technology for depositing high-quality diamond thin films over large-area substrates has only recently become available and is being used for a number of applications, including MEMS devices.
Michael A. Huff, The MEMS and Nanotechnology Exchange, Reston,Virginia; Dwain A. Aidala, sp3 Diamond Technologies Inc., Santa Clara, California; James E. Butler, Naval Research Laboratory, Washington, DC
Designers of nearly all MEMS devices have used silicon as the material of choice because of its exceptional physical properties, including strength-to-weight ratio greater than that of steel, a high level of hardness, and a nearly perfect linear stress-strain curve, even after high numbers of cycles. But while silicon is an excellent choice for implementing micro-miniaturized devices, there are certain real-world applications-such as in harsh environments and extremely high-performance products-where the material’s properties fall short.
Thin-film diamond offers enhanced mechanical properties, including significantly higher stiffness, strength, hardness, thermal conductivity, and chemical robustness, versus silicon and most other thin-film materials commonly used in microfabrication technologies. Furthermore, the surface chemistry of thin-film diamond lowers the friction and stiction properties of the contracting surfaces in MEMS structures and, thereby, overcomes a significant reliability problem in many surface micromachined MEMS devices. Although the attributes of diamond thin films have long been recognized by MEMS designers, the technology for depositing high-quality diamond films over large-area substrates has only recently become available.
Figure 1 shows a color-enhanced scanning electron microscope (SEM) image of a prototype NEMS resonator fabricated from a 30nm-thick nanocrystalline diamond film (grown at NRL and fabricated at Cornell) and covered by 30nm of sputtered gold. The shadow masking of the gold underneath and the curvature is caused by the stress of gold on diamond.
CVD nanocrystalline growth
After significant development over several decades, a robust chemical vapor deposition (CVD) technology to deposit high quality and highly uniform diamond thin films onto large-diameter substrates has recently become a reality and is enabling diamond thin films to be used in many new micro- and nanofabrication applications, including MEMS technology.
These CVD nanocrystalline diamond thin films have a columnar growth structure with grains ranging in size from 5-15nm up to ~10% of the thickness of the film. High-quality diamond films are deposited directly onto a thin film of thermal oxide grown on a silicon wafer. The diamond deposition process can be performed in a variety of commercial CVD systems employing microwave plasma or hot filament activation of the gases. For example, in a commercial microwave plasma reactor operating at 2.45GHz, nanocrystalline diamond is grown at 750°C with purified (99.999%) gas flows of 900sccm hydrogen and 3sccm methane, a microwave plasma power of 800W, and a chamber pressure of 15torr. The nanocrystalline film thickness is monitored in situ using laser interferometry. Diamond films up to ~4µm in thickness can be grown simultaneously on multiple 100mm and 150mm dia. wafers using a hot filament reactor.
Of primary importance is verification that the material deposited can actually be classified as a true diamond material and not a “diamond-like carbon” material. This is critical because to achieve the properties of a diamond film-such as Young’s modulus, thermal conductivity, optical transparency, etc.-the material must have the actual atomic structure of diamond. For example, some thin films recently reported in the literature have high levels of sp2 carbon and material properties significantly inferior to bulk diamond material. To verify that films are, in fact, >99.9% diamond, a number of techniques have been used, including TEM, Raman scattering, photoluminescence spectra, x-ray diffraction, surface acoustic waves (SAW), etc.
For example, the Raman spectra of the films listed above using a 488nm wavelength excitation displays a well-defined diamond zone center phonon peak at 1332cm-1, as well as a weaker broadband response at 1500cm-1 from defects at grain boundaries and dislocations [1].
More practical for the MEMS engineer interested in using this material to fabricate devices is knowledge of the measured material properties [1]. Diamond thin films have a measured density of 3500 kg/m3, which is equivalent to the density of single-crystal diamond, a Young’s modulus of ~1.12GPa (slightly lower than bulk material), and a thermal conductivity depending on film thickness of 1-14W/cm-K (which is more than half the bulk diamond value). Since these films are polycrystalline in nature, this would explain the lower value of thermal conductivity resulting from phonon scattering caused by the grains in the material. The diamond films can be doped in situ to modify their electrical conductivity by the introduction of boron as a reactant during the deposition.
Nanofabrication
The availability of high-quality diamond material alone is not sufficient to make it useful for real world applications. A set of technologies to pattern and define the material and integrate it with other processing steps is required before its full potential can be realized by device designers. A significant amount of development work has already been done in this area. Because the diamond is deposited directly onto silicon wafers, the material cost is very low and furthermore, the substrates can be pre- or post-processed using conventional micro- or nanofabrication tooling (i.e., fabricators can use the existing MEMS Exchange processing infrastucture to implement devices using diamond thin films).
For pattern transfer, any number of photolithographic techniques can be used, including contact to e-beam lithography, with the selection primarily based on the smallest resolution and ultimate fidelity desired. Thin-film diamond is easily etched using reactive ion etching (RIE) and an oxygen (O2) plasma. Because of the chemical robustness of the diamond material, the etch rates are rather slow compared to other conventional materials used in the industry, and therefore, a hard mask material (Al, Cr, Ni, or oxides) having a higher selectivity than photoresist may be required depending on the diamond thin film layer thickness to be etched. In any case, devices with submicron dimensions are readily made in this thin-film diamond material.
MEMS applications
One notable example of diamond’s benefits in MEMS technology is found in radio frequency (RF) MEMS resonators (Fig. 2) [2, 3]. The extremely high modulus of the diamond material combined with its relatively modest density allows the operational frequencies of MEMS RF resonators to be pushed into the GHz frequency bands, which opens up large potential markets in the communications industries.
 Figure 2. A 3MHz CVD diamond resonator by Wang, Butler, Hsu, and Nguyen. |
Micromachined disk resonators operating at a frequency of >1.5GHz, a quality factor of Q>11,500, and a frequency-Q product of 1.74 x 1013 have been demonstrated. This frequency-Q product compares to the best available quartz crystals and indicates that the performance of diamond MEMS resonators is suitable for RF pre-select and image-reject filters that are used in wireless receiver front ends. These MEMS RF resonators are very small and can be directly integrated onto a microelectronics wafer; they have performance levels high enough to make them suitable for low-loss, front-end filters with selectivities suitable for channel selection. Some have even envisioned that these resonators can enable ultralow power receivers that do not require the conventional frequency-down conversion, but instead directly sub-sample using analog-to-digital (A/D) converters. Arrays of coupled resonators (Fig. 3) are also being explored.
Examples of the benefits of diamond materials’ properties are the optical transparency, chemical robustness, and the extremely large thermal conductivity of diamond thin films, which is about 10× higher than competing materials, thereby opening up opportunities for much improved thermal management (e.g., removing heat from hot spots) in many MEMS, photonic, and microelectronic devices. Since the diamond material is in thin-film form and can be integrated directly onto the substrate with relative ease, various possibilities for heat spreaders are enabled that can potentially improve the performance of a large variety of active devices.
Another application of diamond is the coating of surfaces in MEMS and nanodevices, especially where the potential for significant wear is present, such as in devices with contacting surfaces. It has been shown that silicon is prone to wear at contacting surfaces and therefore other materials, with higher levels of hardness, such as silicon nitride, are used to mitigate wear effects. Diamond, however, has a much higher level of hardness compared to any other available material choice. Diamond is also not as prone to stiction effects (i.e., where two contacting surfaces are stuck together), as are other material choices, such as silicon nitride. There are many other applications not presented here, including medical devices, sensors for harsh environments, etc., for which diamond offers many benefits.
Conclusion
With investment from the Defense Advanced Research Projects Agency, sp3 and the MEMS Exchange have been working with leading researchers at the Naval Research Laboratory (NRL) in the thin-film diamond growth field to transfer recently developed diamond deposition technology and put into place the infrastructure-facilities, expert staff, and equipment-to make this material and associated processes widely and cost effectively available to the community for MEMS development. Multiple wafer hot-filament CVD reactors with the required deposition uniformities for high-quality thin films over 100mm and 150mm substrates are a proven technology. These thin films can be deposited with a thickness appropriate for real device applications, i.e., several microns in total thickness. Furthermore, these diamond films have relatively smooth surfaces and have been successfully integrated into the process sequences of several types of MEMS devices with respectable yield numbers [4]. Through the MEMS Exchange, designers can get access to the diamond deposition technology as well as other micro- and nanofabrication technologies to fully implement the most advanced devices.
References
- J. Philip, P. Hess, T. Feygelson, J.E. Butler, S. Chattopadhyay, K.H. Chen, L.C. Chen, “Elastic, Mechanical, and Thermal Properties of Nanocrystalline Diamond Films,” Journal of Applied Physics, Vol. 93, No. 4, Feb 15, 2003, p. 2164.
- L. Sekaric, J. M.Parpia, H.G. Craighead, T. Feygelson, B.H. Houston, J.E. Butler, “Nanomechanical Resonant Structures in Nanocrystalline Diamond,” Applied Physics Letters, Vol. 81, No. 23, Dec 2, 2002, p. 4455.
- J. Wang, J. E. Butler, T. Feygelson, T.-C. C. Nguyen, “1.51-GHz Nanocrystalline Diamond Micromechanical Disk Resonator with Material-mismatched Isolating Support,” Proceedings, 17th Int. IEEE Micro Electro Mechanical Systems Conf., Maastricht, The Netherlands, Jan. 25-29, 2004, p. 641.
- Interested parties should contact the MEMS Exchange at http://www.mems-exchange.org.
Michael A. Huff received his PhD in electrical engineering and computer science (EECS) from the Massachusetts Institute of Technology (MIT) and is the founder and director of The MEMS and Nanotechnology Exchange, 1895 Preston White Drive, Reston, VA 20191; ph 703/262-5368; e-mail [email protected]; www.mems-exchange.org.
Dwain A. Aidala received his BSEE and MBA from Northeastern U. and is president and COO at sp3 Diamond Technologies Inc., e-mail [email protected].
James E. Butler received his SB at MIT in 1966 and his PhD at the U. of Chicago in 1972, and is research chemist at the Naval Research Laboratory, e-mail [email protected].