A package deal: enabling silicon resonators
By Joe Brown, SiTime
Silicon resonators are by no means novel technology. They have been demonstrated in laboratories for more than 30 years. In comparison to quartz crystals, performance for silicon vibrating beams was elusive due to several factors that have significant influence, such as drift, temperature sensitivity, instability and hysteresis. Today silicon-based resonator technology has dramatically advanced with the use of standard semiconductor processing and low cost packaging, meeting the size, cost and reliability expectations of customers seeking to integrate these devices.
MEMS processing has enabled micro-scale mechanical elements, that when packaged with CMOS circuitry, create a silicon-based oscillator capable of unmatched performance. To differentiate and advance from typical MEMS-based products was the motivation behind technology invented by Bosch, now licensed by SiTime, an integrated circuit company developing silicon timing, clock and RF chips. The chips incorporate MEMS timing reference devices inside standard silicon electronic chips, eliminating the need for quartz crystals.
It has been said that 70 percent of the cost of MEMS devices is due to packaging and testing where special processing is often required. For decades, MEMS-based “killer applications” had been expected but in reality had been limited to sensors and actuators. These days MEMS have emerged with high quality, high reliability and competitive prices to advance optical display technology, microphones and time reference.
SiTime is seeking to unseat quartz with its MEMS chip. Photo courtesy of SiTime
The time reference application described in this article uses mainstream semiconductor processing and has been accepted as compatible with CMOS at many factories producing nano-scale features on 200-millimeter wafers. The inventions of the MEMS First and EpiSeal processes have provided process conditions that create a pure - ultra-clean - encapsulated environment that enables silicon to resonate without deviation. As a result the silicon area is optimized for active device performance, not requiring a protective cap as in other MEMS-based solutions that adds cost, process complexity and is not readily available at advanced CMOS factories.
Today quartz crystals are common and often the only alternative for precise time-based solutions. Quartz is a wonderful material but has limits in size and cannot be integrated with silicon. When polished to specific levels quartz will vibrate at one frequency, so multiple crystals are required for multiple frequencies. On the other hand, silicon can be shaped and fabricated using photolithographic and etching processes, which are the basis for defining the frequency of a device. As a result, many different frequencies can be fabricated next to each other on a single chip.
The process begins with a silicon on insulator (SOI) wafer where the device layer - above the oxide - of single crystal silicon will be formed into the resonant structure(s). Another Bosch creation, deep reactive ion etching, or DRIE (also known as ICP, or inductively coupled plasma) then forms the beam structure. Following this etch process, the entire surface is covered with a thin layer of silicon oxide (SiO2). A thin layer of silicon (Epi/Poly) is grown and deposited on top of the oxide, and this layer becomes the foundation of the wafer level encapsulation that follows.
Small holes are then drilled into the silicon cap just above the mechanical element. The moveable structure at this time is fully surrounded in silicon oxide. It is ready to be set free to move and is released with the introduction of hydrofluoric acid. The acid etches away the sounding oxide from the beam. With the beam free to move (vibrate), the protective cover is processed further, built up using common process conditions for this process step (high temperature) that create the ultra-pure environment required for high performance. Silicon is again grown and deposited, sealing the small holes. This high temperature process has a self-cleaning effect on the area surrounding the structure.
Performance is solid since contaminants such as moisture and organics have been removed from the device environment. Single-digit part-per-million stability is realized, maintained under aggressive temperature cycling (-45 degrees Celsius to +85 degrees Celsius) and has been demonstrated over long-term drift analysis over one year. Reliability is also improved: Silicon sensor technology with a similar makeup has demonstrated complete shock and vibration immunity at the range of use.
The existing market consists of 10 billion sockets ready to adapt to a change from quartz to silicon. With advantages in size, cost, reliability, function and ease of use, silicon is at the forefront of another technological revolution.
Joe Brown is head of strategic alliances at SiTime Corp. (www.sitime.com) in Sunnyvale, Calif. and a board member of MANCEF.