Jongpal Kim

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.

Why is (111) Silicon a Better Mechanical Material for MEMS

In this paper, we explain the mechanical properties of single-crystalline silicon with respect to deflectional and torsional motions. Young’s modulus, Poisson’s ratio, and shear modulus are isotropic on silicon (111), whereas the variations on silicon (100) and (110) are quite significant. We newly derive formulae for bulk shear modulus of silicon (100), (110) and (111) and show that bulk shear modulus differs from conventional shear modulus because of the anisotropic characteristics of single-crystal silicon. Furthermore, we show that the bulk shear modulus (which governs torsional motion) varies minimally on silicon (111), with respect to crystallographic directions, as compared to silicon (100) and (110).

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.

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.

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 planar, x-axis, single-crystalline silicon gyroscope fabricated using the extended SBM process

A planar, x-axis, single-crystalline silicon gyroscope is fabricated using one (111) SOI wafer using the extended SBM (sacrificial bulk micromachining) process. The gyroscope 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. The extended SBM process is a simple two-mask process, and because all structural parts and combs are defined in one mask level, there is no misalignment in any structural parts or comb fingers. Furthermore, all vertical dimensions of the structure, including the comb height, comb offset and sacrificial gap, can be defined arbitrarily. In addition, the inherent footing phenomenon in the SOI deep etching is completely eliminated and smooth structural shapes are obtained. The fabricated x-axis gyroscope can resolve 0.1 deg/sec angular rate, and the measured bandwidth is 100 Hz. The reported work represents the first x-axis single-crystalline silicon gyroscope fabricated using only one wafer without wafer bonding. In this paper, SOI wafer was used for electrical isolation, but the same device can be fabricated using other available electrical isolation techniques using only one ordinary (111) wafer, albeit fabrication processes are more complicated.

Why Is (111) Silicon a Better Mechanical Material for MEMS: Torsion Case

This paper shows that using the Finite Element Method (FEM), the torsional stiffness of silicon varies by the least amount on silicon (111) with respect to crystallographic directions, when compared to silicon (100) and (110). The used simulator is ANSYS 5.7 with the element type of Solid 64. As a simulation model, we use a simple torsion system, in which a rotational inertia is attached to the center of clamped-clamped beam with a rectangular cross-section. From the results of the modal analysis, the torsional stiffness is derived using the formula between the natural frequency and the torsional stiffness. Simulation results show that the maximum variations of the torsional stiffness on silicon (111), (100) and (110) are 2.3%, 26.5%, and 31.2%, respectively. This implies that on and silicon wafers, substantially different physical dimensions are necessary for devices with the same torsional characteristics, but with different orientations. Therefore, silicon wafers represent a more suitable substrate to design and fabricate torsional micro and nano systems.Copyright

A Novel Micromachining Technique to Fabricate Released GaAs Microstructures with a Rectangular Cross Section

The wet etching properties of GaAs in NH4OH–H2O2–H2O mixed solutions were investigated, and a fabrication method for producing released GaAs microstructures with a rectangular cross section using a (001) GaAs substrate was developed. To obtain the wet etching properties with respect to the crystallographic orientations, the etch rates and cross sectional etch profiles of (001) GaAs with 16 different compositions were measured. Based on the experimental data, a novel GaAs micromachining method in bulk (001) GaAs is proposed. In this proposed method, anisotropic wet etch, sidewall passivation, and wet undercut etch steps are performed, in order to release the beams with a rectangular cross section. This proposed micromachining method is used to fabricate a released microbridge with a rectangular cross section. The developed GaAs micromachining method will be very useful for low loss, highly-tunable capacitors for RF components.

Low-Power Photoplethysmogram Acquisition Integrated Circuit with Robust Light Interference Compensation

To overcome light interference, including a large DC offset and ambient light variation, a robust photoplethysmogram (PPG) readout chip is fabricated using a 0.13-μm complementary metal–oxide–semiconductor (CMOS) process. Against the large DC offset, a saturation detection and current feedback circuit is proposed to compensate for an offset current of up to 30 μA. For robustness against optical path variation, an automatic emitted light compensation method is adopted. To prevent ambient light interference, an alternating sampling and charge redistribution technique is also proposed. In the proposed technique, no additional power is consumed, and only three differential switches and one capacitor are required. The PPG readout channel consumes 26.4 μW and has an input referred current noise of 260 pArms.