Xuesong Liu

Fabrication of keyhole-free ultra-deep high-aspect-ratio isolation trench and its applications

An ultra-deep (40–120 µm) keyhole-free electrical isolation trench with an aspect ratio of more than 20:1 has been fabricated. The process combines DRIE (deep reactive ion etch), LPCVD insulating materials refilling and TMAH or KOH backside etching technologies. Employing multi-step DRIE with optimized etching conditions and a sacrificial polysilicon layer, the keyholes in trenches are prevented; as a result the mechanical strength and reliability of isolation trenches are improved. Electrical tests show that such an isolation trench can electrically isolate the MEMS structures effectively from each other and from on-chip detection circuits. The average resistance in the range of 0–100 V is more than 1012 Ω, and the breakdown voltage is above 205 V. This technology has been successfully employed in the fabrication of the monolithic integrated bulk micromachining MEMS gyroscope.

Integrated bulk-micromachined gyroscope using deep trench isolation technology

A novel single-chip bulk-micromachined integrated gyroscope was designed and fabricated in a compatible CMOS process. Using high aspect ratio (30:1) trench etching technology, deep trench electrical isolation materials refilling, and backside etching technologies, bulk MEMS structures and circuits (a part of preamplifier and temperature sensor) were integrated into a single-crystal silicon wafer. A Z-axis integrated gyroscope has been fabricated using this technology, which is compatible with the standard CMOS process. The integrated gyroscope device is 3.5 mm by 3.5 mm in size and has a 100 /spl mu/m thick single-crystal silicon proof mass. At atmosphere pressure, the packaged gyroscope exhibited a low noise floor (0.008/spl deg//sec//spl radic/Hz), large scale factor (3.7 mV//spl deg//sec), small nonlinearity (0.08%), and good excursion stability (40/spl deg//hr//spl radic/Hz).

Design and Fabrication of a lateral axis Gyroscope with Asymmetric Comb-Fingers as Sensing Capacitors

A novel comb capacitor, the fixed and movable fingers of which have different positions but the same height, is proposed. The comb capacitor can be used to measure out-of-plane movement through differential method and has low air damping. Using this kind of comb capacitor, a bulk micromachined y-axis gyroscope is designed, whose structure is doubly decoupled. The gyroscope is fabricated by silicon/glass wafer bonding and deep reactive ion etching process, in which composite etching mask technique is used twice to realize the novel comb structure. The process is compatible with previous reported fabrication process of lateral accelerometers and z-axis gyroscopes.

An improved method employed in anodic bonded glass-silicon gyroscopes to avoid footing effect in DRIE

The SGADER (silicon glass anodic-bonding and deep etching release) technology is developed by Peking University. Many MEMS devices have used this technology, as accelerometers, changing capacitor and micro-gyroscope etc. In the fabrication of these devices by SGADER technology, the bottom damage of the silicon combers and cantilever beams caused by the footing effect is always a serious problem. We have developed feasible methods by patterning about 0.2 /spl mu/m metal film to evacuate charges during over etching time, and got a manifest benefit in the SGADER process. Some comparison has been made to illustrate the effectivity of this addition disposure. A serious consideration must be taken during the design of the whole layout to minimize the parasitic problem brought by metal film left on the glass. It is very important to pay attention to the electrical relationship between the metal film and every silicon structure.

A highly double-decoupled self-oscillation gyroscope operating at atmospheric pressure

In this paper, a highly double-decoupled selfoscillation gyroscope is presented. Deliberate frame demonstrates small cross talk between drive mode and sense mode of less than 0.5% simulated by FEM tool and 1.35% verified by experimental measurements. Tested results exhibit that the bandwidth of the structure has a good immunity to fabrication imperfections. With the deployment of area-varied capacitors and high aspect-ratio structures, large quality factors and low drive voltage are achieved operating at atmospheric pressure, which are 217 and 97 for drive mode and sense mode, respectively, with a 1Vpeak-peak AC drive voltage and 10 V DC bias. Measured scale factor is 10.7 mv/deg/s with a R2-nonlinearity of 0.12% in the range of plusmn300deg/s. The noise equivalent rate is 0.0013deg/s/Hz1/2 (=4.68deg/h/Hz1/2) over 100 Hz bandwidth. The bias stability can achieve 1deg/s for a 6-hour measurement and 0.3deg/s for 120 seconds without any temperature control method employed (1-sigma).

Vertical profiles and CD loss control in deep RIE technology [MEMS fabrication applications]

Deep reactive ion etching (DRIE) technology is widely used to fabricate high aspect ratio structures in MEMS devices. The vertical profiles of the trenches etched by normal advanced silicon etching are not always satisfactory enough for some applications such as a vibratory lateral gyroscope. Bad vertical profiles will decrease the capacitance signal of a lateral comb, change the designed stress of a cantilever beam, reduce the weight of the mass and finally affect the sensitivity and stability of MEMS devices, especially small dimension MEMS structures. We introduce a new technology to improve the vertical profiles, which is realized by decreasing the etching time each cycle. This anisotropy parameter improves from 25 to 175. Moreover, we introduce a method to reduce CD loss. It is realized by adding descending passivation gasses C/sub 4/F/sub 8/ in the first several minutes and decreasing the etching time in the switch periods. The CD loss is decreased from 145 nm to 55 nm.

Post-CMOS process for high-aspect-ratio monolithically integrated single crystal silicon microstructures

A novel modular fabrication process for bulk integrated single-crystal-silicon microstructures designed and manufactured in a post-CMOS process is presented in this paper, which can increase the accuracy and reliability of MEMS sensors as well as lower the fabricating cost. The process involves the conventional CMOS circuit formation, the electrical isolation trench etching and refilling, backside silicon etching, interconnection formation, and structure releasing. The performance of integrated Schottky diodes was tested to have reverse leakage of 10/sup -7/A, and breakdown voltage of 57V. A new method for fabricating void-free isolation trenches is also developed. The resistance of void-free isolation trench is more than 10/sup 12//spl Omega/. The influence of LPCVD high temperature on the doping distribution is simulated.

A bulk micromachined z-axis single crystal silicon gyroscope for commercial applications

In this paper, a bulk micromachined z-axis single crystal silicon gyroscope is presented. The symmetrical and double decoupled structures can greatly suppress the mechanical coupling and easily realize the match of resonant frequencies between the driving mode and sensing mode. Finite element method (FEM) simulation shows that the mechanical intercoupling is less than 0.6% in orthogonal direction and 0.1% in parallel direction. Silicon on glass (SOG) process is utilized to fabricate the gyroscope, therefore large proof mass and capacitance can be achieved with an aspect ratio about 20. The resonant frequencies of driving mode and sensing mode are 4,027 Hz and 4,111 Hz, respectively, which are in good agreement with calculated values. The deviation of bandwidth from design caused by fabrication imperfection is less than 1%. Large quality factors of the working modes are obtained which are 216 and 85 at atmosphere environment, respectively, because large gap between proof mass and substrate is fabricated, and slide film air damping is designed to dominate the operation. Test results show that the scale factor of the gyroscope is 0.9 mv/deg/s in the range of plusmn250deg/s. The signal-to-noise ratio (SNR) is more than 100 at a 10 Hz angular vibration input with a 0.4deg amplitude. The noise equivalent angular rate is 0.008deg/s/Hz1/2.

Simulation and Test of a Doubly Decoupled Lateral Axis Gyroscope

A novel doubly decoupled bulk micromachined lateral gyroscope, proposed in our former work, can efficiently suppress the undesired mechanical coupling, while remaining symmetric in structure. Both theoretical simulations and experiments are carried out to analyze the decoupling property of this gyroscope. The simulated results show that the driving and sensing mode frequencies are 3.821 kHz and 3.956 kHz, respectively. Furthermore, the displacements of the driving mode and sensing mode are also simulated by ANSYS™. In driving mode, simulation result shows that the mechanical coupling from the driving mode to sensing mode is 8%. Similarly, in sensing mode, the mechanical coupling from the sensing mode to driving mode is 0.9%. Micro Laser Doppler vibration meter is employed to detect mechanical coupling in the sensing mode. The test result is about 2%, close to the simulated value.Copyright

Investigation of fabricating ultra deep and high aspect ratio electrical isolation trench without void

This paper reports an improved method of fabricating an ultra deep (40-120 /spl mu/m) and high aspect ratio (more than 25: 1) electrical isolation trench without void to increase the mechanical strength and reliability of the isolation trench in monolithic integration of bulk micromachining MEMS sensors. Before etching and refilling the trench, 0.1 /spl mu/m oxide and 3-4 /spl mu/m polysilicon are LPCVD deposited on the wafer surface as sacrificial films. After etching the trench by DRIE, such sacrificial films are removed to enlarge the trench opening. Then this trench is refilled well without voids using LPCVD oxide and polysilicon. Such an electrical isolation trench has been used in the monolithic integration of a bulk-micromachining MEMS gyroscope, which has shown high performance. Such an isolation trench has sufficient mechanical strength to sustain the bulk MEMS structure. Electrical test shows that such an isolation trench can electrically isolate the MEMS structures effectively from one another and from the on-chip detection electronics. The average resistance in the range of 0-100V is more than 10/sup 11/ /spl Omega/ and no breakdown under 100V.