Timing devices -- the heartbeat of digital electronic systems – have generally been based on quartz, but silicon MEMS has opened the door for radical improvements
BY PIYUSH SEVALIA
Executive VP of Marketing
and DR. AARON PARTRIDGE, Founder and Chief Scientist
Timing devices are the heartbeat of digital electronic systems, providing the clock signal to which all other signals are synchronized. Over the past several decades, these timing devices have been based on quartz crystals, available in the form of resonators (a mechanical vibrating element) and oscillators (a resonator combined with an electrical circuit). Quartz crystals for timing circuits are made in specialized factories by companies that are highly focused to this task. The core competency of these quartz companies lies in the precision manufacturing and cutting of quartz crystals to ensure they operate at the correct frequency and are stable over temperature.
Quartz manufacturers have done an excellent job in fulfilling these basic and essential needs. However, quartz companies have not kept pace with the semiconductor industry in terms of performance improvement and features, nor are quartz companies well equipped to provide the variety, fast turnaround, and cost advantages inherent in the semiconductor industry.
The emergence of silicon MEMS (microelectromechanical systems) has opened the door for radical improvements in the once-staid timing industry. MEMS timing solutions are rapidly replacing quartz products due to the many benefits of MEMS-based timing. Here are the top six benefits of using MEMS timing solutions.
1. Higher performance
Quartz crystal oscillators are limited in accuracy to about ±20 ppm for noncompensated devices, and their frequencies are limited to a range of 10 to 60 MHz for small packages. Jitter is in the range of 1 to 2 ps, integrated over 12 kHz to 20 MHz. These limitations are set by the mechanical constraints of quartz devices. While these limits can be exceeded in special cases, the part cost increases significantly. MEMS oscillators do not have these limits, since they use a programmable analog architecture. For instance, the circuit-centric MEMS oscillator architecture readily supports frequencies from 1 to 625 MHz.
There are also subtle problems in quartz not found in silicon MEMS. A phenomenon called “activity dips” is a good example. They cause a resonator’s frequency to sharply jump tens of parts per billion (ppb) as the part is swept over temperature. (In low-cost crystals it can be much worse, with jumps of 1 ppm.) The reason for activity dips is related to how waves propagate laterally through quartz; they are extremely difficult to remove and, for practical purposes, one must assume that all quartz resonators have them. Activity dips generally set the lower limit on crystal stability at about 0.1 ppm, or about 100 ppb. Activity dips are not present in correctly designed silicon MEMS and thus they can provide higher performance.
2. Better features
Quartz companies typically outsource their oscillator circuit development and production to semiconductor companies and focus their resources on manufacturing the quartz crystals. In contrast, silicon MEMS timing companies follow the fabless semiconductor model and have deep expertise in designing the MEMS resonator as well as the analog oscillator circuit. This in-house intelligence results in the availability of unique features that are not available from quartz oscillators. MEMS timing features include:
- Customizable frequency from 1 Hz to 625 MHz with up to six decimal places of accuracy
- Spread-spectrum capability for EMI reduction
- Programmable drive strength control for better signal integrity and EMI reduction
- 1.8-V operation over the entire frequency range and 1.2 to 4.5-V (continuous) operation for battery-powered applications
- Programmable pull range from ±25 to ±1,600 ppm in VCXOs, VCTCXOs, and DCXOs
These features are available in MEMS devices over a variety of operating temperatures. The devices also come in a wide range of industry standard SMD packages that can be used as drop-in replacement for quartz devices. Special packages are also available, such as ultra-small 1.5 x 0.8-mm chip-scale packages or SOT23-5 for higher board-level reliability in harsh environments.
3. Better resiliency and reliability
Silicon MEMS timing solutions exhibit better reliability (operating life) compared to quartz. MEMS-based oscillators have a <2 FIT rate which translates to 500 million hours MTBF (mean time between failure). This is approximately 15 times better than typical quartz devices.
In terms of robustness and immunity to noise, SiTime MEMS oscillators demonstrate the following test results compared to comparable common quartz-based oscillators.
- 54 times better electromagnetic susceptibility (EMS) measured in average spurs (dB)
- 3 times better power supply noise rejection (PSNR) measured in integrated phase jitter per mVp-p
- Up to 30 times better vibration sensitivity measured in ppb/g across various vibration frequencies
- Up to 25 times better immunity to shock measured in peak frequency deviation in PPM
These benefits stem from the size and structure of the resonators. The resonators in quartz oscillators are millimeter-scale cantilevered structures that are sensitive to acceleration. While a crystal may have megahertz electrical resonances, it has kHz structural resonances. These kHz frequencies can be excited by external vibration or shock. And this shows up as vibration sensitivity or failures. MEMS resonators on the other hand, are about 10 times smaller with up to 3,000 times smaller moving mass and have about 10 times higher mechanical modes, and are thus less sensitive to external vibration and shock. It is a matter of simple scaling.
Additionally, packaging and oscillator circuit design, such as those used in SiTime oscillators, make MEMS-based devices more immune to electrical noise. For example, the MEMS resonators are mounted close to the drive circuitry, giving less antenna area for electrical noise pickup compared to quartz packaging. Multilevel on-chip regulators make the oscillator more resilient against power supply noise.
4. Better availability
Because MEMS oscillators have a programmable architecture, most features can be customized using a programmer, such as SiTime’s Time Machine II. This small programmer can create instant samples in any frequency, any stability, and any supply voltage. This gives system designers the capability to program and test a vast array of timing-related features in their own lab and accelerate development time without needing to search, source, and wait for samples.
Silicon MEMS timing devices are manufactured in semiconductor fabs and assembly houses, and MEMS oscillators are held in inventory in the form of programmable die on wafers. When a production-quantity order is placed, devices are packaged, tested, programmed, and shipped with a 3 to 5-week lead time. This is much shorter than the typical 8 to 16-week manufacturing lead time of quartz device makers that follow a material-intense manufacturing flow. The short lead times offered by MEMS timing vendors result in better inventory control, as well as the ability to meet upside demand more quickly and cost-effectively.
5. Better price
Because MEMS devices are manufactured in silicon and packaged in low-cost standard plastic packaging, they offer a lower price trajectory. MEMS timing companies, which use a fabless model, leverage the infrastructure of the semiconductor industry and are therefore better equipped to offer competitive pricing. In addition, the short lead times, increased features, and higher reliability of MEMS timing solutions translate to a lower total cost of ownership for electronics manufacturers.
6. SoC integration
SiTime offers kilohertz and megahertz resonators for customer that want to integrate MEMS resonators into their own products. The kilohertz resonators are suited to timekeeping applications where one would use a 32-kHz quartz tuning fork, and the megahertz resonators are suitable for reference applications such as clocking and RF. It is difficult to embed quartz crystals inside plastic packages as the over-molding causes significant performance and reliability issues.