Materials
Diamond wafers: A family of materials
Thin-film diamond on silicon or silicon-based wafers is key to enabling the adoption of diamond by the semiconductor industry. Due to advances in reactor design and chemistry, a variety of types of diamond can be deposited onto wafers up to 300 mm in size, with excellent thickness and property uniformity, enabling high- yield production of microdevices. Properties such as film stress, electrical and thermal conductivity, and roughness can be precisely controlled parameters, allowing the device designer enormous flexibility and precision. For instance, it is now possible to make semimetallic diamond films heavily doped with boron with surface roughnesses less than 1 nm and minimal residual stress over an entire 200 mm wafer. This ability to control diamond properties enables the device designer to integrate both electrically conductive and electrically insulating diamond into devices. Thanks to advances in CMP slurries and pad materials, diamond wafers can be processed down to root mean square (rms) roughness values well below 1 nanometer (nm), removing a traditional limitation of the technology; that is, grain coarsening as a function of thickness. An example of a smooth diamond film deposited onto a 100 millimeter (mm) silicon wafer is shown in Figure 1.
Integration into today’s processes
In principle, diamond can be integrated at any arbitray point into an existing process flow at either a micro electrical mechanical systems (MEMS) or CMOS foundry. Deposition temperatures range from 900° C to 400° C with significantly high deposition rates even at low
temperatures. Therefore, a diamond process can be inserted before or after back-end metallization, in addition to front-end integration using, for instance, a diamond DOI (diamond on insulator) “starter wafer” in place of a SOI wafer. Thus, diamond is compatible with the thermal budget of embedded CMOS electronics allowing for structural diamond MEMS devices to be integrated and controlled actively by advanced radio frequency (RF) electronics. Figure 2 shows a custom designed RF MEMS alternating current (AC) switch (designed by MEMTronics) directly integrated with a silicon- on-sapphire high-performance CMOS wafer (provided by Peregrine Semiconductor) designed to drive the switch for Ka-band applications. A key process in current semiconductor manufacturing is the ability to planarize a surface between depositions using CMP. Smooth diamond films can be further processed to have mean roughness values that approach atomic flatness similar to prime grade silicon wafers. Figure 3 shows the difference of an as-deposited diamond thin film with a mean roughness of approximately 7 nm and the same film after CMP, with a mean roughness of less than 1 nm. Another key process in current MEMS manufacturing is the ability to precisely pattern and etch diamond structures. SiOx, aluminum, and other metals (e.g., Cr and Ni) can all be used successfully as hard mask materials to pattern diamond using standard optical lithography processes. Several etch recipes are now publically available. Enabling diamond MEMS with moving components, such as accelerometers, is achieved using silicon dioxide as a sacrificial layer coupled with oxygen-based reactive ion etching (RIE). Etch rates approaching 600 nm/min can be achieved
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www.euroasiasemiconductor.com Issue II 2010
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