Sangjun Park

Surface/bulk micromachined single-crystalline-silicon micro-gyroscope

A single-crystalline-silicon micro-gyroscope is fabricated in a single wafer using the recently developed surface/bulk micromachining (SBM) process. The SBM technology combined with deep silicon reactive ion etching allows fabricating accurately defined single-crystalline-silicon high-aspect-ratio structures with large sacrificial gaps, in a single wafer. The structural thickness of the fabricated micro-gyroscope is 40 /spl mu/m, and the sacrificial gap is 50 /spl mu/m. For electrostatic actuation and capacitive sensing of the developed gyroscope, a new isolation method which uses sandwiched oxide, polysilicon, and metal films, is developed in this paper. This triple-layer isolation method utilizes the excellent step coverage of low-pressure chemical vapor deposition polysilicon films, and thus, this new isolation method is well suited for high-aspect-ratio structures. The thickness of the additional films allows controlling and fine tuning the stiffness properties of underetched beams, as well as the capacitance between electrodes. The noise-equivalent angular-rate resolution of the SBM-fabricated gyroscope is 0.01/spl deg//s, and the bandwidth is 16.2 Hz. The output is linear to within 8% for a /spl plusmn/20/spl deg//s range. Work is currently underway to improve these performance specifications.

Electrostatic actuation of surface/bulk micromachined single-crystal silicon microresonators

In fabricating microelectromechanical systems (MEMS), bulk micromachining using [100] and [110] single crystal silicon and surface micromachining using polycrystalline silicon are used. However, both micromachining methods have drawbacks, and micromachining actuating or sensing MEMS using single crystal silicon has been an active research topic in resent years. This paper presents electrostatic actuation of a resonator fabricated by the SBM (surface/bulk micromachining) process. The SBM process allows fabricating released structures in single crystal silicon. To fabricate electrodes and to electrically isolate them, a junction isolation method using reverse-biased diodes is developed. The breakdown voltage of this isolation method is measured to be larger than 150 volts. A SBM processed microresonator is actuated at 36 kHz. A displacement of several /spl mu/m is achieved in atmosphere with a 20 volts peak-to-peak supply.

A novel 3D process for single-crystal silicon micro-probe structures

A new fabrication method for a three-dimensional (3D), single-crystal silicon micro-probe structure is developed. A probe card structure requires tips that are at least 50 μm tall on cantilevers thick enough to withstand a few mN of force as well as 50 μm of tip bending. The cantilever structure also must be able to move at least 50 μm of vertical motion, requiring a large sacrificial gap. The developed 3D fabrication method is based on the surface/bulk micromachining technology, which can fabricate released, high aspect ratio, single-crystal silicon microstructures with high yield using (111) silicon.

Surface/bulk micromachining (SBM) process and deep trench oxide isolation method for MEMS

This paper presents a new method for micromachining released structures with single crystal silicon, as well as a new method for electrically isolating the released structures with a deep trench oxide. The developed surface/bulk micromachining (SBM) process utilizes (111) silicon wafers. The structural patterns are defined using a reactive ion etcher. Then, the patterns as well as sidewalls are passivated with an oxide film, and bare silicon is exposed at desired areas. The exposed bare silicon is further reactive ion etched, which defines sacrificial gap dimensions. By undercutting the exposed bulk silicon sidewalls in an aqueous alkaline etchant, cleanly released microstructures can be fabricated. For sensing or electrostatic actuation of released microstructures, a deep trench oxide isolation method is developed and applied successfully to actuate a comb-drive actuator. The developed SBM process with deep trench oxide isolation allows fabricating single crystal silicon MEMS with a low parasitic capacitance.

A New Micromachining Technique with (111) Silicon

A new micromachining technology using (111)-oriented silicon is developed. This technology allows advanced surface micromachining with the advantages of using bulk silicon. It utilizes reactive ion etching (RIE) for patterning of microstructures to be released from the substrate, followed by aqueous alkaline etching of bulk silicon under the patterns to release the microstructures. The release etch utilize the high etch selectivity of {111} planes to {100} and {110} planes, and therefore, large plates can be released without additional etch holes, and very smooth and clear undersurfaces and substrate support surfaces can be micromachined.

An x-axis single-crystalline silicon microgyroscope fabricated by the extended SBM process

A high-aspect ratio, single-crystal line silicon x-axis microgyroscope is fabricated using the extended sacrificial bulk micromachining (SBM) process. The x-axis microgyroscope in this paper uses vertically offset combs to resonate the proof mass in the vertical plane, and lateral combs to sense the Coriolis force in the horizontal plane. This requires fabricating vertically and horizontally moving structures for actuation and sensing, respectively, which is very difficult to achieve in single-crystalline silicon. However, single-crystalline silicon high-aspect ratio structures are preferred for high performance. The extended SRN/I process is a two-mask process, but all structural parts and combs are defined in one mask level. Thus, there is no misalignment in any structural parts or comb fingers. In this extended SBM process, all vertical dimensions of the structure, including the comb height, vertical comb offset and sacrificial gap, can be defined arbitrarily (up to a few tens of micrometers). For electrical isolation, silicon-on-insulator (SOI) wafers are used, but the inherent footing phenomenon in the SOI deep etching is eliminated and smooth structural shapes are obtained, because the SBM process is used. In the fabricated x-axis microgyroscope, the lower combs used to vibrate the proof mass are vertically offset 12 /spl mu/m from the upper combs. The fabricated x-axis microgyroscope can resolve 0.1 deg/s angular rate, and the measured bandwidth is 100 Hz. The reported work represents the first x-axis single-crystalline silicon microgyroscope fabricated using only one wafer without wafer bonding. We have previously reported several versions of z-axis microgyroscopes and x-, y-, and z-axis accelerometers, using the SBM process. The results or this paper allow integrating x-, y-, and z-axis microgyroscopes as well as x-, y-, and z-axis microaccelerometers in one wafer, using the same mask and the same process.

Mesa-supported, Single-crystal Microstructures Fabricated by the Surface/Bulk Micromachining Process

In fabricating microelectromechanical systems (MEMS), bulk micromachining using (100) and (110) single crystal silicon and surface micromachining using polycrystalline silicon are most commonly used. However, both micromachining methods have drawbacks, and fabricating actuating or sensing microdevices using single crystal silicon has been an active research topic in recent years. The surface/bulk micromachining (SBM) process using (111) silicon, developed by us previously, allows fabricating released structures in single crystal silicon. This paper extends the SBM process to fabricate released structures that are anchored to the substrate via mesa islands. This allows fabricating folded-type comb drive resonators. With the extension of the SBM process developed in this paper, any microstructure fabricated by the one structural polysilicon surface micromachining technique can also be fabricated in single crystal silicon.

The effects of post-deposition processes on polysilicon Young's modulus

Polysilicon films deposited by low pressure chemical deposition (LPCVD) are the most widely used structural material for microelectromechanical systems (MEMS). However, the properties of LPCVD polysilicon are known to vary significantly, depending on deposition conditions as well as post-deposition processes. This paper presents extensive experimental results, investigating the effects of phosphorus doping and texture on Youngs modulus of polysilicon films. Polysilicon films are deposited at 585, 605 and 625 to a thickness of 2 m. Specimens with varying phosphorus doping levels are prepared by diffusion doping at various temperatures and times using both and phosphosilicate glass (PSG) as the source. Youngs modulus is calculated by taking the average of the values calculated from the resonant frequencies of four different-size lateral resonators. Our results show that Youngs modulus decreases with increasing doping concentration, and increases with increasing texture. The polysilicon grain size and grain boundaries could also have an influence on Youngs modulus, which remains to be further investigated.

A novel electrostatic vertical actuator fabricated in one homogeneous silicon wafer using extended SBM technology

In this paper, we present a novel, extended SBM (surface/bulk micromachining) process for vertical actuation and vertical sensing in single-crystal silicon MEMS. Only one homogeneous (111) silicon wafer is used to accomplish this vertical motion and vertical sensing. The developed process includes two photolithography steps and two SBM process steps, but the planar gap between the upper and lower electrodes, which is critical for achieving symmetry, is defined only by the first photolithography step. This ensures that this extended double-SBM process for vertical actuation and sensing is robust to alignment errors. Two vertical resonators are fabricated. The resonators are fabricated with 5 μm thick upper electrodes, 19 μm thick lower electrodes, 1 μm vertical gap, and 6 μm planar gap. This resonator is actuated at 17.2 kHz with an AC voltage.

A novel MEMS silicon probe card

We have developed a novel cantilever-type probe which is capable of less than 70 of pitch and 12g of force. This probe is suitable for wafer-level burn-in testing, function testing and circuit-board O/S testing including memory and RF devices. The probe was fabricated with epitaxial polysilicon on silicon substrate. The through via hole interconnection was formed in silicon wafer by nickel electroless plating and copper electroplating. The electroless plating is easy method and can deposit film uniformly for deep trench and through hole. Especially, it can deposit film on any substrates without seed layer and allow it to be electroplated. The aspect ratio of through via hole was larger than 10:1 and the contact resistance was less than 1 ohm.