13 December 2011
Diamond based MEMS devices to solve future communications problems?
The perceived wisdom is that diamonds are a girl's best friend. But the highly coveted allotrope of carbon is beginning to find friends amongst those developing technologies for the electronics industry.
One area where diamond is finding application is in high frequency switching designs, where its stability and stiffness bring benefits. But there is also growing interest in using diamond to create microsystems or, more accurately, nanosystems
A researcher with extensive experience is the field is Dr Oliver Williams from Cardiff University's department of astronomy and physics. He said diamond is useful for MEMS devices, but it does depend on the application. "Most MEMS devices are made from silicon and that will continue to be the case. But when it comes to high frequencies, silicon runs into problems." And diamond is seen as a possible solution.
The variety of diamond being applied in these instances is nanocrystalline diamond, in which the grain size is generally less than 100nm.
Many MEMS devices are required to oscillate or feature cantilevers for various purposes. One of the critical performance factors of such devices is the Q factor. A dimensionless parameter, the Q factor provides an indication of how quickly stored energy is lost from an oscillator; the higher the Q, the longer oscillations will continue. A tuning fork, for example, will have a Q of around 1000, but nanocrystalline diamond devices exhibit Q factors at least 10 times greater. In fact, Q factors can exceed 100,000.
A further definitive property is the product of the Q factor and the resonant frequency (see fig 1). A material that can demonstrate a Q factor-resonant frequency product of 1013 will be attractive to those looking to build high frequency switching devices, for example.
Underpinning this performance is the fact that diamond has the highest Young's Modulus of all known materials. Young's Modulus, which relates stress and strain, provides an indication of material stiffness. Where a single crystal of silicon has a Young's Modulus of around 150GPa, diamond's modulus is around 1100GPa. Other benefits include a low coefficient of friction, temperature stability and the largest thermal conductivity of any material – important when downscaling devices.
Yet interest in diamond for such applications is not a recent phenomenon; work has been underway for many years. But there has been a resurgence in interest, as Dr Williams explains. "Nanocrystalline diamond can be processed in pretty much the same way as polycrystalline silicon, so existing technology can be exploited. It is also compatible with existing cmos technologies, which competing MEMS materials are not. Meanwhile, nanocrystalline diamond offers better performance and price advantages. People think diamond is expensive; nanocrystalline diamond is not, it's a lot cheaper than competing III-V materials or silicon carbide."
Dr Williams is working on developing a process by which ultrathin films of diamond are created on a silicon substrate. "Nanocrystalline diamond can be grown in very thin films using chemical vapour deposition (CVD)," he pointed out. "And its properties are similar to those of bulk diamond. In fact, it's much the same process as that used in the silicon industry, although the gases are different."
According to Dr Williams, 'diamond likes to grow on diamond'. "We 'sprinkle' nanodiamond particles on standard silicon wafers and then use CVD. The diamond film grows on these nanodiamond particles but, as the film grows, it gets rougher. This is a problem as roughness 'kills' the Q factor."
The solution is chemical mechanical polishing (CMP). "The silicon wafer usually has a bow of several microns, far exceeding the film thickness of around 150nm. This means we cannot use standard planarisation techniques to polish away the roughness," he continued. "CMP allows high quality wafers to be produced that have a surface roughness of less than 1nm; the smoother the wafer, the better."
Roughness plays a critical role with MEMS cantilevers. "If you are producing a 100nm cantilever," he pointed out, "surface roughness must be less than a few nm and that's tricky to achieve." Roughness is also important when surface acoustic wave (saw) filters are constructed. "If you want to build a 16GHz saw filter," Dr Williams noted, "you need to start from very smooth diamond."
One area where diamond shows its potential for use in MEMS is the lack of need for critical point drying. When a liquid flows over a solid structure, surface tension will tend to pull against that surface. If the structure is delicate, such as in a MEMS device, there is a tendency for it to break apart. "With silicon microfabrication," Dr Willliams noted, "you have to use critical point drying during the etching process. Diamond's inherent strength is so great that it doesn't matter and the etching process is simpler."
In general, silicon MEMS devices have been produced by bulk or surface micromachining. Nanocrystalline diamond wafers remain compatible with existing processes; in particular, they are suitable for reactive ion etching.
While diamond MEMS hold great potential for rf switches and similar products, there is also the potential to use the technology in biocompatible electronics. "Diamond is chemically inert," Dr Williams said, "while silicon is toxic. That makes diamond a real winner."
One potential application is the 'lab on a chip' device, allowing the detection of particular antigens (see fig 2). The concept involves the use of a vibrating diamond cantilever. Dr Williams explained: "Diamond offers the most stable surface available for antibody molecule attachment. In operation, an antibody will 'grab' an antigen molecule, which causes mass loading and hence a downshift in the resonant frequency of vibration. Thus, one can detect viruses, bacteria and pathogens by attaching the relevant antigen and monitoring the resonant frequency."
Dr Williams believes bioMEMS devices haven't had the success envisaged because silicon doesn't offer the required stability. "Diamond allows a very robust solution and if the solution isn't robust, it isn't any use," he concluded.
Diamond enables high speed switch
Looking to meet high performance requirements in communications applications and demands for portability and compact size, the US based Argonne National Laboratory (ANL), along with industrial partners, has developed an integrated rf MEMS switch/cmos device.
Designed for phase shifters, phase array antennas and advanced mobile communications, the monolithically integrated rf switch was developed using MEMS design and ultra nanocrystalline diamond (UNCD) technology to facilitate the integration of all necessary passive rf front end functions on one chip.
Integrated for the first time on a cmos chip, the UNCD dielectric's high conductivity enables fast charging and discharging, improving reliability of the MEMS switches. These advances are said to yield a competitive device switching in less than 10µs, but with an operational lifetime of up to 50 times that of competitive devices.
Collaborating with ANL in the project were MEMtronics, Innovative Micro Technologies, Advanced Diamond Technologies and Peregrine Semiconductor.