Deep reactive ion etching process and microelectromechanical devices formed thereby

#The present invention generally relates to micromachined devices, and particularly microelectromechanical system (MEMS) devices formed by etching processes. More particularly, this invention relates to a micromachining process and design elements for a MEMS device using a deep reactive ion etching (DRIE) process to improve yields and device reliability.

#A wide variety of MEMS devices are known, including accelerometers, rate sensors, actuators, motors, microfluidic mixing devices, springs for optical-moving mirrors, etc. As an example, rotational accelerometers that employ MEMS devices are widely used in computer disk drive read/write heads to compensate for the effects of vibration and shock. Other applications for rotational accelerometers that use MEMS devices include VCR cameras and aerospace and automotive safety control systems and navigational systems. Rotational rate sensors and accelerometers have been developed whose MEMS devices are fabricated in a semiconductor chip. Notable MEMS devices that employ a proof mass for sensing rotational rate or acceleration include a plated metal sensing ring disclosed in U.S. Pat. No. 5,450,751 to Putty et al., and an electrically-conductive, micromachined silicon sensing ring disclosed in U.S. Pat. No. 5,547,093 to Sparks, both of which are assigned to the assignee of this invention. Sparks' sensing ring is formed by etching a chip formed of a single-crystal silicon wafer or a polysilicon film on a silicon or glass handle wafer. A sensor disclosed in U.S. Pat. No. 5,872,313 to Zarabadi et al., also assigned to the assignee of the present invention, employs a sensing ring and electrodes with interdigitized members. The positions of the interdigitized members relative to each other enable at least partial cancellation of the effect of differential thermal expansion of the ring and electrodes, reducing the sensitivity to temperature variations in the operating environment of the sensor. Each of the above sensors operates on the basis of capacitively sensing movement of their rings. The sensing rings are supported by a central hub or pedestal. Surrounding the rings are drive electrodes that drive the rings into resonance, while sensing electrodes that also surround the rings serve to capacitively sense the proximity of the ring (or nodes on the ring) which varies due to Coriolis forces that occur when the resonating ring is subjected to rotary motion.

#Another notable MEMS device that employs a silicon proof mass for sensing rotational acceleration is disclosed in U.S. patent application Ser. No. 10/410,712 now U.S. Pat. No. 6,257,062 to Rich, incorporated herein by reference. Rich discloses a disk-shaped proof mass supported above a cavity formed in a substrate. Instead of being centrally supported by a pedestal, Rich's proof mass is suspended from its perimeter with tethers anchored to the substrate rim surrounding the proof mass. The tethers allow the proof mass to rotate about an axis perpendicular to the plane containing the proof mass and tethers. Fingers extend radially outward from the proof mass and are interdigitized with fingers extending radially inward from the substrate rim. The cantilevered fingers of the proof mass and rim are capacitively coupled to produce an output signal that varies as a function of the distances between adjacent paired fingers, which in turn varies with the angular position of the proof mass as it rotates about its axis of rotation in response to a rotational acceleration.

#FIG. 1 is a plan view of a MEMS device in accordance with a preferred embodiment of this invention.

#FIGS. 2 and 3 are plan and cross-sectional views, respectively, of a portion of the MEMS device of FIG. 1.

#FIG. 4 is a detailed plan view of several interdigitized proof mass and rim fingers of the MEMS device of FIG. 1.

#FIGS. 1 through 5 represent a MEMS device 10 fabricated with a DRIE process in accordance with the present invention, with masking steps of the DRIE process being represented in FIGS. 6 through 8. The device 10 is represented as a rotational accelerometer of the type disclosed by Rich, which is incorporated herein by reference. However, those skilled in the art will appreciate that the device 10 could be employed and modified for a variety of applications, including the rate sensors and accelerometers taught by Putty et al., Sparks and Zarabadi et al.

#As illustrated, the device 10 includes a proof mass 14 formed in a sensing die 12. The die 12 is shown as including a semiconductor layer 12b on a substrate 12a (FIG. 3). A preferred material for the semiconductor layer 12b is epitaxial silicon and a preferred material for the substrate 12a is single-crystal silicon, though it is foreseeable that other materials could be used. For example, the substrate 12b could be formed of quartz, glass or any other advantageous substrate to which the semiconductor layer 12b could be bonded. In a preferred embodiment, a wafer containing the die 12 is processed by a known bond-etchback process, by which the substrate 12a is etched to form a cavity 20 and then oxidized to form a bond oxide layer (not shown) on its surface including the cavity 20, and the semiconductor layer 12b is epitaxially grown on a second wafer (not shown) and then bonded to the bond oxide layer of the substrate 12a. The second wafer is then selectively removed to leave only the epitaxial layer 12b on the substrate 12a and overlying the cavity 20, as shown in FIG. 3. While a bond-etchback process is preferred, it is foreseeable that other techniques could be used to produce the die 12 and enclosed cavity 20 of FIG. 3.

#As also seen in FIGS. 1 through 3, the proof mass 14 is defined in the semiconductor layer 12b so as to be suspended above the floor 18 of the cavity 20 between a central hub 16 and a rim 22 formed by the semiconductor layer 12b. The proof mass 14 is attached to the bulk of the die 12 (through the semiconductor layer 12b) with four equiangularly-spaced tethers 30, and is completely separated from the hub 16 by a trench 17, whose width may be on the order of about seven micrometers. The tethers 30 provide that the primary and desired translational mode of the proof mass 14 is rotation within the plane of the proof mass 14 about the hub 16.