Andrew B. Graham

A Method for Wafer-Scale Encapsulation of Large Lateral Deflection MEMS Devices

Packaging of microelectromechanical systems (MEMS) is a critical step in the transition from development to commercialized product. This paper presents a thin-film encapsulation process that allows varying trench widths suitable for MEMS devices with lateral deflections as large as 20 ¿m. The process involves the deposition and planarization of a sacrificial-oxide layer of up to 23 ¿m thick, the deposition of a 20 ¿m epitaxial-silicon sealing cap, the release of structures using hydrofluoric acid (HF) vapor, and the sealing of the structure at low pressure. Devices produced using this encapsulation method are capable of surviving standard backend processes such as wafer singulation and wire bonding. Among the numerous types of devices encapsulated, two different types of silicon MEMS resonators were fabricated. These functioning resonators demonstrate the ability of the process to successfully encapsulate devices, taking advantage of both large and small trench widths. Such a generalized fabrication platform greatly expands the possibilities of the wafer-scale encapsulation to numerous MEMS devices and retains the robustness necessary for backend processing.

Characterization of Encapsulated Micromechanical Resonators Sealed and Coated With Polycrystalline SiC

This paper presents the characterization of sealed microelectromechanical devices and their packaging fabricated in a polycrystalline 3C silicon carbide (poly-SiC) thin-film encapsulation process. In this fabrication technique, devices are sealed with a nominally 2-¿m low-pressure chemical vapor deposition poly-SiC, and the device layer is simultaneously coated with a nominally 0.2- ¿m poly-SiC thin film. Device characterization includes measurement of the resonant frequency and the quality factor of double-ended tuning-fork micromechanical resonators, which have a Si-SiC composite beam structure. Experimental results show that the pressure inside the packaging can be controlled from 447 Pa to 15.5 kPa with a 400°C annealing process. The frequency drifts of the encapsulated resonators are less than the frequency noise level (±10.6 ppm) measured over 29 days at 84.6°C ±0.1°C, which suggests that the poly-SiC thin-film packaging technique can offer hermetic packaging for various applications in microelectromechanical systems including inertial sensors. In addition to the packaging performance, the temperature coefficient of Youngs modulus for poly-SiC is derived from the resonant-frequency change of resonators with temperature. The reduction of the quality factor due to the poly-SiC coating, predicted in the theoretical model, is confirmed by measurements.

Fatigue Experiments on Single Crystal Silicon in an Oxygen-Free Environment

The fatigue lifetime of single crystal silicon (SCS) was characterized in an environment free of oxygen, humidity, and organics. Long-term (> 1010 Hz) fatigue experiments performed with smooth-walled SCS devices showed no signs of fatigue damage up to 7.5 GPa. In contrast, experiments using SCS devices with a silicon dioxide (SiO2) coating and rough sidewalls due to scalloping from deep reactive ion etching exhibited fatigue drift at 2.7 GPa and suffered from short-term (<; 1010 Hz) fatigue failure at stress levels >3 GPa. In these SCS-SiO2 experiments, the initiation of fracture occurs in the SiO2 layer. It is concluded that fatigue in this case is likely attributed to a subcritical cracking mechanism; not reaction-layer nor dislocation related. A cross-comparison with other works from literature is developed to show that packaging a pristine device in an inert environment is necessary in order to operate devices at high-stress levels.

Resonant pressure sensor with on-chip temperature and strain sensors for error correction

Temperature and package stress induced errors pose a challenging obstacle for improving accuracy of strain-based resonant pressure sensors. This paper presents a multiple sensor solution where three resonators were built under a shared pressure sensor diaphragm. By manipulating the anchoring scheme and the location of the resonators, temperature and stress signals can be independently captured and used to compensate for the errors in the pressure signal. After compensation, the pressure sensor showed a 20× reduction in temperature dependency and a 2× reduction in stress dependency.

High-cyclic fatigue experiments of single crystal silicon in an oxygen-free environment

We report on the first study of fatigue in single crystal silicon MEMS resonators within an extremely clean and controlled environment using the ‘epi-seal’ encapsulation technology. This packaging technology provides a unique opportunity to investigate controversial issues in silicon fatigue since the devices are not exposed to air, oxygen, organics, or other residues that might complicate the initiation and observation of fatigue. We have conducted fatigue experiments on 10 devices over 1010 actuation cycles with various dynamic loadings ranging from 1.0 to 1.9 GPa at 29°C and from 0.2 to 1.1 GPa at 280°C. No fatigue related phenomena have been observed under these experimental conditions.

A single process for building capacitive pressure sensors and timing references with precise control of released area using lateral etch stop

In this paper, we present a capacitive absolute pressure sensor co-fabricated with a MEMS resonator using an improved epitaxial polysilicon encapsulation process. The process features insensitivity to timed hydrofluoric acid etch variation when releasing structures via sacrificial silicon dioxide. Moreover, the process enables fabrication of structures to drive/sense in both lateral (x, y) and vertical (z) directions, providing a powerful fabrication platform for sensor integration on either bulk silicon or SOI wafer substrates.

The long path from MEMS resonators to timing products

Research on MEMS Resonators began over 50 years ago. In just the last 10 years, there has been a series of important technological developments, and (finally!) success at commercialization. The presentation will highlight some key milestones along this path, describe some of the critical technology steps, and outline some of the important non-technological events within SiTime - all of these factors contributed to the successful outcome.

Encapsulated mechanically coupled fully-differential breathe-mode ring filters with ultra-narrow bandwidth

We present hermetically encapsulated, mechanically coupled, 20 MHz breathe-mode ring filters with ultra narrow bandwidths of 5.9 kHz and 8.9 kHz, which correspond to 0.029% and 0.044% bandwidth with less than 2.7 dB insertion loss. This is the narrowest percent bandwidth achieved by a mechanically coupled MEMS filter. The ability to attain an insertion loss this small for such a small percent bandwidth is made possible by employing encapsulated MEMS resonators with Qs ∼ 280,000. We also explore the effect of the coupling beam width on the filter bandwidth and experimentally demonstrate that the bandwidth is directly proportional to the width of the coupling beam. This work provides guidance for coupling beam design in future MEMS filters.

Epitaxial Silicon Microshell Vacuum-Encapsulated CMOS-Compatible 200 MHz Bulk-Mode Resonator

This paper shows the first successful combination of dielectrically-transduced 200 MHz resonators with the epi-silicon encapsulation process, and demonstrates a set of important capabilities needed for the construction of CMOS-compatible RF MEMS components. The result shows the resonant frequency of 207 MHz and a quality factor of 6,400. The high f.Q (1.2×1012 Hz) makes this encapsulated resonator an excellent candidate for applications in local oscillators and RF spectrum analyzers.

Wafer Scale Encapsulation of Large Lateral Deflection MEMS Structures

Packaging of microelectromechanical systems (MEMS) is a critical step in the transition from product development to production. This paper presents a robust, hermetically-sealed encapsulation method that can accommodate many traditional MEMS devices by allowing large lateral deflection structures within a clean environment. Using the new technology described in this paper, trench widths ranging from 1¿m to 100¿m were successfully encapsulated at the wafer level while maintaining devices as thick as 20¿m. Devices produced with this method have proven durable enough to withstand harsh post-processing such as dicing and wire bonding. Two different types of MEMS resonators are also discussed, demonstrating the use of both large and small trench widths within the encapsulation.