Mechanical Properties of Ultrananocrystalline Diamond Thin Films for MEMS Applications

Microcantilever deflection and the membrane deflection experiment (MDE) were used to examine the elastic and fracture properties of ultrananocrystalline diamond (UNCD) thin films in relation to their application to microelectromechanical systems (MEMS). Freestanding microcantilevers and membranes were fabricated using standard MEMS fabrication techniques adapted to our UNCD film technology. Elastic moduli measured by both methods described above are in agreement, with the values being in the range 930 and 970 GPa with both techniques showing good reproducibility. The MDE test showed fracture strength to vary from 3.95 to 5.03 GPa when seeding was performed with ultrasonic agitation of nanosized particles. INTRODUCTION Carbon in its various forms, specifically diamond, may become a key material for the manufacturing of MEMS/NEMS devices in the 21st Century. The new ultrananocrystalline diamond (UNCD) films developed at Argonne National Laboratory [1] may provide the basis for revolutionary microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS). The UNCD films are grown using a microwave plasma chemical vapor deposition technique involving a novel CH4/Ar chemistry. The process yields films with extremely small grain size (2-5 nm), significantly smaller than nanocrystalline diamond films (30-100 nm grain size) produced by the conventional CH4/H chemistry [2,3]. The UNCD films posses many of the outstanding physical properties of diamond, i.e., they exhibit exceptional hardness, extremely low friction coefficient and wear, high thermal and electrical conductivity, the latter when doped with nitrogen [4]. Preliminary results have shown that this unique microstructure results in outstanding mechanical properties (~ 97 GPa hardness and 967 GPa Young’s modulus that are similar to single crystal diamond [5]), unique tribological properties (coefficient of friction of the order of ~0.02-0.03, [6]), and field-induced electron emission (threshold voltage 2-3 V/μm, [7]). Preliminary work by investigators at Argonne has demonstrated the feasibility of fabricating 2-D and 3-D MEMS components that can be the basis for the fabrication of complete MEMS / NEMS devices [8-10]. Components such as cantilevers and multilevel devices such as microturbines have already been produced. These preliminary exercises are promising steps toward full-scale application of UNCD components in functional MEMS devices. However, before full-scale integration can occur, several intrinsic material properties, such as elastic modulus, plasticity and fracture of undoped and doped UNCD must be well characterized to fully exploit the potential of this material. In this paper, we use micro-cantilever deflection and the membrane deflection experiment [11] techniques to gain a better understanding of the elastic modulus and fracture strength for UNCD thin films. We have taken special care to design different specimen characteristics for each technique in an attempt to minimize effects in each that hinder accurate property measurements. EXPERIMENTAL PROCEDURE Two types of specimens were used in this study. Both consist of freestanding, thin-films of UNCD with thickness ranging between 0.55 to 0.65 μm. The films are grown directly onto a Si substrate and specimen structures were microfabricated using standard techniques as described in Moldovan et al. [10]. Two structures were constructed, freestanding cantilevers and fixed-fixed membranes. Figure 1 is an SEM image and 3D schematic view of the cantilever structure. The dimensions of the cantilever are defined on the figure with t as the thickness, b as the width (20 μm for all cantilevers), and l as the cantilever length (200 μm) at the point of contact during deflection. The structure of the cantilevers contained an etching undercut that resulted in the specimens having a “T” shape. This is accounted for in the data reduction procedure and is described later. The second type of specimen consists of specially designed double-dog-bone, freestanding membranes as shown later Figure 4. The specimens are designed to minimize stress concentrations and boundary bending effects. When vertically deflected, direct tension is produced in the gauge regions resulting in uniform specimen stressing. Further details are given in Espinosa et al. [11]. Specimens with gauge lengths of 300 μm and gauge widths of ~ 13.5 μm were tested. Both structures were probed with a nanoindenter to obtain high-resolution loaddeflection signatures. Figure 1. SEM image (a) and 3D schematic (b) of the freestanding UNCD cantilever structures. The figure illustrates an undercut resulting in a “T” shape. Parameters are defined in the text. RESULTS AND DISCUSSION In order to test the load resolution of the nanoindenter a deflection test was performed on a single crystal (110) Si AFM tapping-mode tip, a material for which the elastic properties are well (b) (a) characterized; i.e., E[111] = 185 GPa, E[110] = 170 GPa and E[100] = 130 GPa [12]. Figure 2 shows optical images of the topand bottom-view of the tip architecture illustrating a “T” shape. Dimensions of the tip where b = 46.62 μm, b2 = 197.93 μm, t = 4.1 μm, l = 100 μm, lu = 12 μm, and the taper around the top edge is 5.0 μm in width and 100 nm deep. Simulations were performed with these dimensions and the equivalent length, leq, was found to be 82.58 μm. Figure 2 shows the load-deflection curve for a cantilever test length of 80 μm. The stiffness, k, for different tests was found to vary between 2.58 x10 to 2.61 x10 mN/nm which corresponds to a modulus of 166 to 168 GPa, using the equation k = Ebt/4l(1-v) using v = 0.27, close to that of the [110] direction for Si, 170 GPa.