**Diamond, Diamond-Like Carbon (DLC) and Diamond-Like Nanocomposite (DLN) Thin Films for MEMS Applications**

T. S. Santra1, T. K. Bhattacharyya2, P. Patel3, F. G. Tseng1 and T. K. Barik4 *1Institute of Nanoengineering and Microsystems (NEMS), National Tsing Hua University, Hsinchu, Taiwan 2Department of Electronics and Electrical Communication Engineering, Indian Institute of Technology, Kharagpur, West Bengal, 3Department of Electrical and Computer Engineering, University of Illinois at Urbana Champaign, 4School of Applied Sciences and Humanities, Haldia Institute of Technology, Haldia, Purba Medinipur, West Bengal, 1Republic of China 2,4India 3USA* 

#### **1. Introduction**

458 Microelectromechanical Systems and Devices

Zupan, M., & Hemker, K. J. (2002). Application of Fourier analysis to the laser based

pp. 214-220.

interferomteric strain/displacement gage, *Experimental Mechanics*, Vol. 42, No. 2,

Amorphous carbon films have been utilized in many types of engineering systems and adapted to fulfill a wide variety of applications. The uses of surface coatings are mainly to protect structural materials from high temperature environments or to confine the electric charge largely on interfaces between materials with differing electronic properties mainly for enormous commercial significance. Diamond, Diamond-like Carbon (DLC) and Diamond-like Nanocomposite (DLN) thin films are based on amorphous carbon films. Diamond, DLC and DLN thin films has generated a great interest in the academia due to its fundamental and technological importance. Presently, researchers have given much attention to fabricate the Micro- or Nano- electromechanical systems (MEMS/NEMS) with different types of materials. The characteristics lengths of these technologies are micrometer to nanometer range. MEMS are defined as miniature devices which combining with mechanical, electrical, optical, and biological fields to fabricate integrated circuits (IC) or other similar manufacturing devises. The applications of these MEMS technologies are in different vast areas, like biomedical, environmental, transportation, manufacturing, robotics, space sciences, computing systems etc [1-5]. Researchers have much expectation of these new frontier technologies after silicon-based microelectronic technologies. For excellent MEMS devices, the coating materials should have the properties like high hardness, high modulus of elasticity, high thermal conductivity and tensile strength, high fracture toughness, low surface roughness, very low coefficient of friction, low thermal expansion, high band gap energy, high transmission capability etc. All of these unique and attractive

Diamond, Diamond-Like Carbon (DLC)

[29-31, 6].

Nanocomposite (DLN) thin films.

sp2 content influences the electronic properties of the films.

and Diamond-Like Nanocomposite (DLN) Thin Films for MEMS Applications 461

sp2 (graphite-like) to sp3 (diamond-like) bonds. The amorphous carbon is a mixture of sp2, sp3 and sp1 sites with the presence of nitrogen and hydrogen. The nitrogen free carbon films are shown in Fig. 1 on ternary phase diagram. In this figure, the phase diagram defines the regions of pure carbon (designated a-C), tetrahedral amorphous carbon (ta-C), and hydrogenerated amorphous carbon (a-C:H) with the corresponding extent of hydrogenation [19-21]. To increase the degree of sp3 carbon bonding, better amorphous carbon (a-C) films can be produced by any kind of deposition systems. If sp3 carbon bonding is very high, then this a-C can be denoted as a tetrahedral amorphous carbon (ta-C) [22]. Fig. 1 shows amorphous hydrocarbon (a-C:H) or diamond like films, but it is not higher order due to large hydrogen content. To achieve less hydrogen content with much more sp3 bond, plasma enhanced chemical vapour deposition (PECVD) technique is ideal to generate tetrahedral amorphous carbon films [20]. The sp3 content influence the mechanical properties of the films. The mechanical and wear resistance properties are more prominent with increase of hydrogen content into the films. On the other hand, surface energy and coefficient of friction decreases with greater hydrogen passivation into the films. Again, the

Diamond, Diamond-Like Carbon (DLC) and Diamond-Like Nanocomposite (DLN) thin films can be deposited by different chemical vapor deposition technique like plasma enhanced CVD, plasma assisted CVD, microwave plasma CVD or a hot filament [23-27], ion beam deposition, pulsed laser ablations, filtered cathodic arc deposition, magnetron sputtering etc. The DC plasma jet chemical vapor deposition can be used for Diamond like carbon films deposition also [28]. Table 1 shows the different properties of diamond films

H (atomic %) 30 0 0 <0.1 --- Sp3 fraction <0.5 >0.8 >0.9 ~1.0 0.5-0.8 Density (Kg/m-3) 2350 3260 3500 3515 --- E (GPa) 300 757 300 1050 90-160 Hardness (GPa) <15 >20 >45 45 9-17 Residual stress (GPa) 1-2 8-10 0 0 --- Table 1. Different parameters of Diamond, Diamond-Like Carbon (DLC) and Diamond–Like

In this section, we describe diamond-like carbon deposition by plasma which consists of argon (99.998%), hydrogen (99.9%) and CH4 (99.5%) is used as a carbon source and is mixed into the plasma jet. The plasma jet is sprayed onto a substrate fixed on a water-cooled substrate holder. The hydrocarbon species in the gas phase for the CH4-Ar-H2 gas system, temperatures (500 0C to 6000 0C) and a total pressure of 0.25 atm (25 KPa) has been computed using the thermodynamic computer program. The deposition was performed on Si(111) surface with the growth rate 80 μm/hr The CH4/H2 gas ratio and substrate temperature influence the properties of diamond. Fig. 2 shows the Scanning Electron Microscopy (SEM) morphology of DLC thin films [28]. Diamond-like Carbon (DLC) and Diamond-like Nanocomposite (DLN) are is basically amorphous carbon based films. In amorphous carbon structure, there is a possibility to form both threefold coordinate (sp2 site) as in graphite and fourfold coordinate (sp3 -site) as in diamond [32]. Each of the four valance electron lies in the sp3 -site forms σ-bonds with neighbors [33]. In sp2-site, only

a-C:H (DLC) ta-C (DLC) UNCD Diamond DLN

properties present in Diamond, Diamond-like Carbon (DLC) and Diamond-like Nanocimposite (DLN) based thin films [6-14]. The amorphous carbon films based MEMS are fully dominated the silicon-based MEMS technologies. The silicon-based MEMS with mechanical loading have lack of high fracture toughness facing with high reliability. Under some extreme conditions like very high temperature or very high particle radiation, silicon may fail to sustain these properties. However, silicon have very large coefficient of friction, high surface energy, high wear rate and small band gap energy, which cannot fulfill the all material properties of MEMS [15-18]. To overcome these drawbacks of silicon materials, researchers are continuously trying to look for new materials for MEMS applications. Ceramics (wide band gap), semiconductors (such as SiC), Polymers (PDMS, PMMA), can play important role for MEMS fabrications. Except these materials, diamond, diamond-like carbon (DLC) and diamond-like nanocomposite (DLN) etc are promising materials for MEMS applications. High elasticity and tensile strength of DLC and DLN films can suitable for high frequency MEMS devices. The temperature withstanding capability of both DLC and DLN films is up to 600 0C or slightly more. The biocompatibility of DLC and DLN films is strongly effective for biosensors in diagnostics and therapies, surface coatings for surgical instruments, prosthetic replacements etc. Chemically modified DLC and DLN surfaces can act as sensing trace of gases to detect biomolecules in biological research. We have presented a brief review about the latest properties of different amorphous carbon based diamond, Diamond-like Carbon (DLC) and Diamond-like Nanocomposite (DLN) thin films and their application in MEMS/NEMS devices.
