SiTime: Using SOI Technology to Develop High-Performance MEMS Timing Solutions

A radical SOI-based approach puts  SiTime at the top of the fast-growing silicon-based timing market.

SiTime, an analog semiconductor company, is revolutionizing the timing components industry with silicon MEMS timing solutions that replace legacy quartz products. SiTime offers oscillators (XO, SPXO), voltage-controlled oscillators (VCXO), temperature compensated oscillators (TCXO), and multi-PLL, multi-output clock generators. All these timing products provide reference clock signals in a variety of electronics products, and in a sense, these timing signals are the heartbeat of electronics.

SiTime oscillators
Figure 1. SiTime oscillators combine a MEMS resonator die with an analog oscillator IC in a stacked die arrangement.

SiTime’s high performance MEMS resonators (mechanical vibrating elements) are manufactured using SOI wafers. The MEMS resonators, which are available in both kHz and MHz frequencies, are then co-packaged with programmable analog die in a plastic QFN (quad flat no-lead) package to create MEMS oscillators with industry-standard footprints (Figure 1). These MEMS oscillators offer benefits in features, performance, size, cost and reliability over competing quartz products.

As electronics become more sophisticated, the performance requirements of reference timing components increase dramatically. With the move towards mobility, low power consumption becomes critical. To meet these stringent requirements, a MEMS resonator must have excellent frequency stability that remains constant over time, temperature, voltage and process variations. Additionally, the resonator, along with the analog circuits that drive it, must consume very little power while providing excellent phase noise and jitter characteristics.

Sealed Inside

A resonator that meets these requirements must be in vacuum. A vacuum can be created during packaging, which is the method used by the quartz companies. However, this entails the use of an expensive ceramic package with additional (non-batched) steps that are required to seal the package using welding. Ceramic packages have been known to leak over time. Breaking the vacuum seal results in frequency shifts that can cause catastrophic failures in electronics.

SiTime MEMS-based oscillators
Figure 2. SiTime MEMS-based oscillators and clock generators in industry standard QFN packages.

SiTime took a radically different approach to this problem by sealing the MEMS resonators inside the silicon wafer. The processing for this encapsulation step is done in a high-temperature, ultra-clean epitaxial silicon reactor. There are many benefits of this approach, called the MEMS FirstTM process. The MEMS resonator cavity is extremely clean, resulting in virtually zero frequency drift. The process is cost effective as all resonators on the wafer are sealed at the same time. The MEMS resonator is extremely robust and is compatible with low-cost industry standard plastic packages (Figure 2).

SOI Wafers

SOI wafers are crucial to the successful fabrication of SiTime’s MEMS resonators. The oxide in the SOI wafer is a sacrificial layer that allows for the release of the MEMS structure during fabrication. SiTime’s SOI wafers are made with bond-and-grind back technology, resulting in a single-crystal device layer for fabrication of micromechanical resonators. The MEMS device layer has greater thickness compared to typical SOI for integrated circuits. This leads to enhanced mechanical robustness and improved electrical performance of the micromachine. The resonators and electrodes are created in the SOI layer using a proprietary deep reactive ion etching (DRIE) process. (Figure 3) The majority of fabrication steps in the process use equipment that is commonly available in CMOS fabs.

Side view of SiTime MEMS resonator structure
Figure 3. Side view of SiTime MEMS resonator structure sealed with thick poly silicon cap during MEMS First(TM) process using 10µm SOI wafers.

The superior mechanical properties of single-crystal silicon give SiTime’s oscillators significantly better reliability and smaller size compared to quartz crystal-based products. Single-crystal silicon has extremely reproducible characteristics, i.e. every resonator made with single-crystal silicon is virtually identical in behavior. The highly repeatable mechanical behavior of MEMS resonators ensures that the final test and calibration costs remain low.

Excellent Specs

Marrying this resonator technology to programmable analog CMOS circuits allows SiTime to offer excellent specifications such as very high stability (0.1 PPM), low jitter (500 femtoseconds), and unique features such as spread spectrum, SoftEdgeTM programmable drive strength, and single-chip, multi-protocol signaling. SiTime’s products can be used in a variety of applications. Consumer applications such as digital cameras, storage media, smart cards, gaming solutions, hard disk drives and peripherals drive high volume, while demanding telecom and networking applications drive performance requirements.

Unlike quartz technology, SiTime’s silicon MEMS devices benefit from Moore’s law. SiTime is able to leverage the continuous infrastructure improvements of the electronics industry to enhance scalability, quality, reproducibility and delivery time. Large electronics companies are recognizing these benefits and are rapidly adopting MEMS timing products.

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