Sepulveda et al MEE 2004

Microelectronic Engineering 73–74 (2004) 435–440 www.elsevier.com/locate/mee Polycrystalline diamond technology for RFM...

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Microelectronic Engineering 73–74 (2004) 435–440 www.elsevier.com/locate/mee

Polycrystalline diamond technology for RFMEMS resonators N. Sep ulveda-Alancastro *, Dean M. Aslam Micro and Nano Technology Laboratory, Electrical and Computer Engineering Department, Michigan State University, East Lansing, MI 48910, USA Available online 20 March 2004

Abstract A polycrystalline diamond (poly-C) resonator technology, with minimum feature sizes in the range of 1–2 lm and an excellent quality of released structures, is demonstrated for the first time. The poly-C films are grown using a nucleation density of 1  1011 cm 2 . The resonator structures reported in this paper have not been tested yet. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Polycrystalline diamond; Poly-C; Micromechanical resonator; RFMEMS; Q-factor; Resonance frequency

1. Introduction The possible applications of RFMEMS in wireless communications have been the driving force for the development of technology of poly-Si-based vibrating micro-electro-mechanical systems (MEMS) resonators [1–4], which are expected to be employed in the next generation of wireless integrated microsystems (WIMS). Although the fundamental resonance frequencies of up to 274 MHz have been achieved for poly-Si resonators, these fall short of the frequency ranges of 1–6 GHz, used in the conventional wireless systems, and 10–80 GHz expected to be used in the near future. To address the requirements of ever increasing frequency ranges, new resonator mate*

Corresponding author. E-mail address: [email protected] (N. Sep ulveda-Alancastro).

rials with acoustic velocities higher than that of poly-Si have been investigated recently [5,6]. Diamond, with the highest acoustic velocity of 18,076 ms 1 [6], seems to be the most superior among the new materials including SiC. The C–C sp3 bonds in tetrahedral configuration, crucial for the formation of single crystal diamond (SCD), are responsible for a unique combination of diamond properties. However, SCD is too expensive for most applications in wireless systems. Furthermore, SCD technology is not compatible the conventional Si technologies. Fortunately, the polycrystalline diamond (poly-C), with physical properties approaching those of SCD, can be fabricated on Si substrates using chemical vapor deposition (CVD) techniques and its cost is comparable to that of poly-Si for film thicknesses in the range of 1–2 lm. As the poly-C films contain sp and sp2 C–C bonds leading to non-diamond phases at the grain

0167-9317/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2004.03.011

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boundaries, their properties such as acoustic velocity can be affected adversely if the poly-C quality, indicated by the sp3 /sp2 ratio as measured by Raman spectroscopy, is not carefully controlled during the poly-C growth by CVD. An additional difficulty in producing poly-C resonator films is the surface roughness and patterning of poly-C films. Perhaps such problems are responsible for the poor poly-C resonator characteristics reported so far. For example, the typical resonance freTable 1 Parameters for poly-C films Growth parameter

Sample 1

Sample 2

Hydrogen flow rate (sccm) Methane flow rate (sccm) Trimethyl boron flow rate (sccm) Deposition temperature (°C) Microwave plasma power (kW) Deposition chamber pressure (torr) Growth rate (lm/h) Other parameters Resistivity (X cm) Surface roughness (nm) Diamond powder Size (nm) Film thickness (lm) Average surface area of film grains (lm2 )

100 1.5 1

100 1.5 1

600 1.5

725 1.5

35

45

0.1

0.25

5 18 10–25 1.5 0.06

0.3 23 50–100 2 0.2

H2 containing 0.098% tri methyl boron (TMB) was used as the dopant gas.

quency, Q-factor, Young’s modulus and acoustic velocity reported for poly-C resonators are 2.94 MHz, 6225, 305 GPa and 9320 ms 1 , respectively [7]. The reported high-Q value of 36,460 is for a comb-drive resonator with a very low resonance frequency of 27.3 KHz. Thus, the reported data fall short of the expected resonator frequencies in the range of 0.1–50 GHz using resonator dimensions above 1 lm. Reducing the resonator dimensions close to or below 1 lm for poly-Si resonators is expected to lead to size-related limitations, due primarily to enhanced adsorption properties of water-related species to the Si surface. As diamond surfaces are known to be chemically inert to such adsorption, submicron poly-C resonator structures are potentially feasible. However, the technological difficulties encountered in the fabrication of submicron poly-C resonators may need to be overcome before any useful devices can be fabricated. In the present work, two important issues are addressed for the resonator dimensions in the micrometer range; poly-C film technology and resoTable 2 Dry-etching parameters for poly-C Argon Oxygen SF6 Substrate temperature Microwave plasma power Deposition chamber pressure

Fig. 1. SEM, AFM and Raman results for sample 1 (a–c) and sample 2 (d–f).

6 sccm 1.5 sccm 1 sccm 30°C 400 W 5 mtorr

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high sp3 /sp2 ratios, surface roughness in the range of 18–23 nm and very-clean released resonator structures. To our knowledge, such high-quality polycrystalline diamond resonator structures, with smallest feature sizes of 1 lm, are being reported for the first time.

2. Poly-C film technology

Fig. 2. Resonator fabrication process flow.

nator fabrication process development. These are crucial for achieving resonance frequencies expected from poly-C films. The results show very

The growth of thin (0.2–1 lm) and smooth poly-C films on Si substrates requires densities of diamond nucleation (seeding) in the range of 1011 cm 2 [8,9]. The size and density of nucleation particles impact the eventual film thickness [8], film continuity and surface roughness of poly-C films. A seeding solution was prepared by mixing 0.25 g of commercially diamond powder (see Table 1) in 250 mL of deionized water (DI). Typically, the Si

Fig. 3. Resonator Structures using sample-1 film technology.

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wafers were etched in NH4 F/HF/H2 O (BHF) solution (4:1:6) for approximately 10 s before they were treated for 30 min in the seeding solution in an ultrasonic bath (Branson 1200). Two groups of samples, sample 1 and sample 2, were seeded using a nucleation density of 1  1011 cm 2 , and poly-C films were grown at 600 and 725 °C in microwave plasma CVD (MPCVD). The poly-C deposition parameters of MPCVD are

shown in Table 1. The film thickness is typically 2 lm. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy were used to monitor the poly-C films as shown in Fig. 1. The film in sample 1 shows a very smooth surface (18 nm), but the surface of film in sample 2 shows a higher roughness but better quality than the sample 1. Higher growth temperatures typically lead to better film quality (high

Fig. 4. Resonator Structures using sample-2 film technology.

N. Sepulveda-Alancastro, D.M. Aslam / Microelectronic Engineering 73–74 (2004) 435–440

sp3 /sp2 ratio) but rough surface. Dry-etching was used for patterning of poly-C films. The etching parameters are shown in Table 2.

3. Poly-C resonator technology A test chip containing cantilever beams, bridges and comb-drives was designed using a four-mask process. The feature sizes of the devices were in the range of 1–100 lm. The process shown in Fig. 2 was used to fabricate resonator structures with minimum feature sizes as small as 1 lm. Starting with an n-type (1 0 0) Si wafer coated with a 2-lm  tri-layer SiO2 layer, a Cr/Au/Cr (300/800/300 A) metal stack was evaporated on SiO2 . After patterning the metal, a sacrificial layer of low temperature SiO2 with a thickness of 0.1 lm was deposited at 300 °C. After etching the anchor holes in the oxide layer, a layer of poly-C was deposited and dryetched using the parameters shown in Table 2. The resonator structures were released by etching the sacrificial layer as shown in Fig. 2(f). The final metal contact layer was Al with a thickness of 300 nm (not shown in Fig. 2). Two different process runs were carried out for the study of resonator structures; one using the sample 1 films and the other using the sample 2 films.

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First, using the film technology developed for sample 1, cantilever beams, bridges and combdrives were fabricated without using the final metal mask. As shown in Fig. 3, very clean resonator structures are obtained. Second, using sample 2 film, resonators were fabricated as shown in Fig. 4. Close-up views of comb-drives and bridges indicate a very successful release of the resonator structures with gaps of (0.1 lm for sample 2 and 2 lm for sample 1). When compared with results of Fig. 3, a relatively rough side walls are seen, which is due to the different etch rates of poly-C grains and grain boundaries (GB). The grains consist of sp3 C–C bonds in tetrahedral configuration, while the GB contains mostly sp1 and sp2 C–C bonds, which are typically found in non-diamond phases of carbon. A comparison of film technologies developed in this work (sample 1 and sample 2) and that developed by other researchers [6] is shown in Fig. 5. The scale bars in Fig. 5(a)–(c) have been adjusted to the same length for comparison purposes. It may be pointed out that the resonator structures shown in Fig. 5(b) are the smoothest poly-C resonators ever fabricated. However, a comparison of the resonator frequencies and Q-factors will show which of the structures shown in Fig. 5 are really better. The measurements of the resonators in the current work are in progress and will be the subject of a subsequent publication.

Fig. 5. Comparison of resonator structures: (a) from [6], (b) sample-1 film technology and (c) sample-2 film technology.

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4. Conclusions

References

A diamond (poly-C) resonator technology for minimum feature sizes in the range of 1–2 lm, with an excellent quality of released structures, is demonstrated for the first time. The poly-C films are grown using a nucleation density of 1  1011 cm 2 .

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Acknowledgements This work was supported in part by the Engineering Research Centers Program of the National Science Foundation under Award Number EEC9986866. One of the authors (N.S.A.) is thankful to the Bill and Melinda Gates Foundation for support through their Gates Millennium Scholarship program. The authors are indebted to Angela Moon for the design of masks used in this study and to Wan-Thai Hsu for a number of discussions in RFMEMS resonators design and testing.