Epi-Seal MEMS Process

:: Epi-Seal

Earlier work in our group under the DARPA HERMIT program resulted the development of a encapsulation process for silicon MEMS. The epi-seal encapsulation process [1, 2] is a unique wafer-level encapsulation for silicon structures that is free of contamination and native oxide. Such an encapsulation has been demonstrated to be crucial to achieving high stability [3, 4] with no observable fatigue [5, 6]. This established low cost/high yield process is currently being used by SiTime with great success for the fabrication of silicon MEMS resonators.

:: Fabrication Process

The standard epi-seal process is as illustrated on the right. First, the MEMS structure is defined by etching trenches in the device layer of a silicon-on-oxide (SOI) wafer using deep reactive ion etching (DRIE). A sacrificial oxide layer is deposited to fill the trenches and to provide a oxide spacer layer above the device. Contacts are etched in the oxide and a silicon layer is deposited in an epitaxial reactor. This is crystalline silicon where there is a crystalline seed and epitaxial polysilicon where there is oxide. Vents are then etched into this cap and vapor HF is used to etch the oxide to release the device. After a high temperature hydrogen bake in an epitaxial reactor to remove contaminants and native oxide, a second layer of epitaxial silicon is deposited, sealing the device in a clean cavity. Thereafter, electrical isolation and contacts are defined. The pressure in the cavity is further lowered by diffusing hydrogen in the cavity out, achieving a pressure of a few Pa.
  

Epi-Seal variants

    Standard Epi-Seal (Hot Dog): Only gaps ~1um can be defined. [1]
    Large Gaps (Grand Canyon): Wider gaps possible using wafer bonding. [2]
    Narrow Gaps: Gaps of 200-300nm can be achieved. [3]
    Oxide Coated Silicon Structures (Krispy Kreme): For passive temperature compensation. [4]
    Localized Degenerately Doped Regions: For passive temperature compensation. [5]
    Top Electrodes: Electrodes can be defined in the cap in addition to in-plane electrodes. [6]

:: Platform for Integrated MEMS

There is much interest in integrating microelectromechanical systems (MEMS) today for cross-sensitivity compensation or for applications such as inertial navigation. For example, many silicon MEMS devices are sensitive to temperature and having a thermometer on chip allows for temperature compensation techniques to be applied. Some MEMS devices such as magnetometers may also be sensitive to acceleration and having an accelerometer on chip can then allow for compensation. Integrating these compensated timing references and sensors on the same chip is essential for high stability and low noise sensors. A wide variety of such devices have been fabricated on the epi-seal platform:

• Timing References (Resonators)
• Accelerometers
• Gyroscopes
• Stress/Strain Sensors
• Pressure Sensors
• Thermometers
• Magnetometers
• Switches

:: Timing References

Our group has demonstrated capacitively actuated silicon MEMS resonators with high stability due to the clean encapsulation afforded by epi-seal. Flexural mode resonators as well as bulk mode resonators [1-3] have been demonstrated within the low pressure epi-seal environment of < 1 Pa.

Temperature Stability

Eldwin Ng

Since the stiffness of silicon varies over temperature, any temperature change results in a shift in the resonant frequency (~-30ppm/C). This is not acceptable for many applications and numerous methods have been developed to compensate for this variation. Silicon resonators have been coated with silicon oxide for passive compensation [1], and electronic phase-locked loop techniques have been developed for more accurate compensation [2]. More recently, degenerate doping of silicon has been shown by other groups to reduce the temperature dependence [3-4], and work is on-going to develop temperature-insensitive epi-sealed resonant systems.

Narrow Gaps for Bulk-Mode Resonators

Eldwin Ng

Higher frequency bulk-mode resonators are desired for lower phase noise. As the displacements for such modes are smaller than flexural modes, narrow transduction gaps (<300nm) are required to drive and sense such modes. A fabrication process for narrow trenches in single crystal silicon has been developed using a combination of deep reactive ion etching (DRIE) and selective epitaxial deposition [1]. Trenches of <100nm wide and over 45um deep (aspect ratio of >500:1) in single crystal silicon are demonstrated. Silicon bulk acoustic resonators (SiBARs) developed by the Ayazi group [2] are demonstrated in an epi-seal process with narrow gaps fabricated in single crystal silicon.

Nonlinear Operation

Yushi Yang

Resonators can be driven to higher amplitudes well into the nonlinear regime. Traditionally, this is not done as instabilities can occur in the open-loop case when resonators are operated above the critical vibration amplitude. However, in a closed-loop oscillator, resonators can be driven with much greater amplitudes stably. This improves the power handling performance and reduces the far-from-carrier phase noise.

Modeling of Energy Loss Mechanisms

Shirin Ghaffari

The quality factor of a resonator is a measure of the amount of energy dissipated. Often, high quality factor resonators are desired for low phase noise, motional impedance and power consumption - this means that energy loss has to be minimized. For MEMS resonators, some of the dominant energy loss mechanisms include gas damping, anchor loss, thermoelastic dissipation (TED) [1,2] and the Akhieser effect (AKE) [3]. TED and AKE are seen to place hard limits on the f Q product for silicon resonators as shown in the figure.

:: Gyroscopes

Chae Hyuck Ahn, Sarah Nitzan [Horsley Group, UC Davis]

In addition to accelerometers, gyroscopes for measuring rotational velocity have been demonstrated in the epi-seal process. Based on the Disk Resonating Gyroscope (DRG) design [1], at least two degenerate mode shapes exist and the device is driven on one axis in one of the modes. Coriolis forces from the rotation induce a resonance in the second mode and this can be sensed.
One issue with the DRG is the frequency split between the wineglass modes, due to the material anisotropy of (100) single crystal silicon. A good frequency match is desired for maximum energy transfer between the modes, as this results in large sensitivity. Having the device made of polysilicon reduces the frequency split, but causes roughening of the surface due to silicon migration [3]. The frequency mismatch in (100) silicon can be tuned away by modifying the design in layout - and we show that near-zero frequency splits can be achieved [4].

:: Stress/Strain Sensors

The resonant frequency of a beam changes with the applied axial stress. Resonating beams that are anchored at both ends (for example, the beams in a double-anchored double-ended tuning fork resonator) will measure a frequency change if the anchors are pulled apart or pushed together. This effect can be exploited to measure changes in film stress [1], or package stress [2].

:: Pressure Sensors

Chia-Fang Chiang

Diaphragms can be defined on the cap to measure pressure. A pressure change causes a deflection in the membrane of the diaphragm, which can then be measured capacitively or by using a strain-based resonant approach.
More on Pressure Sensors

:: Thermometers

Eldwin Ng

Since silicon as a material has a temperature dependence, these resonators can be used as thermometers [1-3]. By monitoring the resonant frequency shift with temperature, the temperature can be deduced. Resonators fabricated in the epi-seal process have been shown to be very stable - two resonators side-by-side track each other to within 10ppb over the course of a month, corresponding to a temperature noise of a fraction of a mC with 1s of integration time [3].

Magnetometers

in collaboration with Mo Li [Horsley Group, UC Davis]

Magnetometers have been successfully fabricated in the epi-seal process. Testing is currently underway.

Contact Switches

in collaboration with Scott Lee and Kimberly Harris [Howe Group, Stanford]

Preliminary data show that maintaining an ultra-clean environment is critical for the reliability of contact switches. Results to be added soon.