This Letter presents simulation, fabrication and testing of a high-performance quad-beam silicon piezoresistive accelerometer with very low cross-axis sensitivity. Cross-axis sensitivity in piezoresistive accelerometers is an important issue particularly for high-performance applications. In the present Letter, low cross-axis sensitivity is achieved by improving the device stability by placing four flexures in line with the proof mass edges. The accelerometer device is realised in a single-step double-sided bulk micromachining technique using a 5 dual-doped tetra methyl ammonium hydroxide solution as an anisotropic etchant. Test results of four fabricated devices show an average prime-axis sensitivity of 559.5μV/g/5V, a maximum cross-axis sensitivity of 0.62 full scale (FS acceleration=13g) of the prime-axis sensitivity and nonlinearity at a level of 0.5 of FS which are comparatively better than already reported devices and commercially available piezoresistive sensors. © 2012 The Institution of Engineering and Technology.

Ravi Sankar A

Electronics

School of Electronics Engineering

Chennai Campus

Grace Jency J

Ashwini J

Das S.

Vellore Institute of Technology (VIT) is a private university located in&nbsp;Tamil Nadu, India. Founded in 1984, as Vellore Engineering College, the institution offers 20 undergraduate, 34 postgraduate, four integrated and four research programs. It has campuses in Vellore, Amravati, Bhopal and Chennai.

VIT is one of the top ranked private universities in India according to NIRF, THE and QS Rankings.&nbsp;Govt. of India has recognized&nbsp;VIT, Vellore as an&nbsp;Institution of Eminence. This has allowed VIT to take independent quality initiatives and move up in world ranking.

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VIT University

Micro & Nano Letters

The article aims to analyze the use of different compensation structures in MEMS micromachining technology to determine the minimum release time as fast as possible where undercutting is desirable. A high undercutting rate is advantageous for the formation of suspended structures. Thus, the implication of the present research includes analysis of corner undercutting behavior, their etching time, and etching characteristics using KOH and TMAH etchants. In this paper, the effective time and etchant concentration are studied using 33% KOH at 80, °C and 25% TMAH at 85, °C. It is found that wide bar with slit structure is the best compensation structure with minimum space competence. © Springer Nature Singapore Pte Ltd. 2018.

Lecture Notes in Electrical Engineering Microelectronics, Electromagnetics and Telecommunications

Etch Time Optimization in Bulk Silicon MEMS Devices Using a Novel Compensation Structure

Piezoresistive acceleration sensors have found a wide range of applications for the last four decades. However, in recent times, high performance piezoresistive acceleration sensors with lower footprint are in demand, especially in the fields of consumer electronics and biomedical engineering. The design of such sensors remains a challenging task due to the competing requirements of high electrical sensitivity (∆R/R) and resonant frequency (f0) along with low device footprint. In this paper, we elaborate the design and optimization of a highly miniaturized doubly clamped acceleration sensor with an integrated silicon nanowire as a piezoresistor. The diminutive size of the silicon nanowire helps in reducing the device foot print. The design is performed using two methods: (1) electrical approach and (2) mechanical approach. In the electrical approach, the conventional bulk silicon diffused piezoresistors are replaced with a silicon nanowire without changing the dimensions of the original structure. It is shown that compared to the conventional design, a silicon nanowire based sensor exhibits 2.51 times better performance in terms of electrical sensitivity. One of the major constraints with miniaturized high performance acceleration sensors is reduced resonant frequency. Therefore, in the mechanical approach we have devised a geometrical optimization technique to miniaturize the sensor geometry by keeping its resonant frequency constant. Numerical simulations are performed to validate the proposed miniaturized sensor with an integrated silicon nanowire as the piezoresistor. The sensor performance is evaluated with a new performance factor given as (ΔR/R)f02/Diesize, which takes into account the electrical sensitivity, the resonant frequency and the die size of the sensor. Results show that the proposed sensor design with an integrated silicon nanowire has 48.2 times better performance factor than a bulk piezoresistor based conventional acceleration sensor. © 2016, Springer-Verlag Berlin Heidelberg.

Microsystem Technologies

Design and optimization of a doubly clamped piezoresistive acceleration sensor with an integrated silicon nanowire piezoresistor

This paper presents fabrication and testing of a high performance quad beam silicon piezoresistive Z-axis accelerometer with a very-low cross-axis sensitivity. Cross-axis sensitivity in piezoresistive accelerometers, mainly caused by the asymmetric structural design is an important issue primarily for high performance applications. In the present study, symmetry of the structure is achieved by shifting the center of mass of the proof mass toward the beam plane by selectively electroplating a 20 μm thick gold layer atop the proof mass and simultaneously reducing the silicon thickness from backside of the proof mass surface. Silicon shadow mask technique was used to deposit a Cr/Au seed layer on the selected regions of the proof mass followed by an electroplating process to achieve the 20 μm thick gold layer. Theoretical analysis shows that for an in-plane acceleration, the rotation of the thickness reduced proof mass with the electroplated gold layer is reduced by 69% compared to the accelerometer device with normal all-silicon proof mass keeping the resonant frequency of both the structures nearly equal. The accelerometer device was realized by a bulk micromachining technique using a 5% dual doped tetra methyl ammonium hydroxide (TMAH) solution. A new figure of merit called the performance factor (PF) defined as the ratio of the product of the prime-axis sensitivity and the square of the resonant frequency to the maximum cross-axis sensitivity is used as a quantitative index to evaluate the fabricated sensors with already reported devices. Test results of a fabricated device with 30 μm flexure thickness show a very-low cross-axis sensitivity of 0.316 μV/Vg for an in-plane acceleration and a PF of around 2900 MHz2. © 2012 Elsevier B.V.

Sensors and Actuators A: Physical

A very-low cross-axis sensitivity piezoresistive accelerometer with an electroplated gold layer atop a thickness reduced proof mass

This paper presents design, fabrication and testing of a quad beam silicon piezoresistive Z-axis accelerometer with very low cross-axis sensitivity. The accelerometer device proposed in the present work consists of a thick proof mass supported by four thin beams (also called as flexures) that are connected to an outer supporting rim. Cross-axis sensitivity in piezoresistive accelerometers is an important issue particularly for high performance applications. In the present study, low cross-axis sensitivity is achieved by improving the device stability by placing the four flexures in line with the proof mass edges. Various modules of a finite element method based software called CoventorWare™ was used for design optimization. Based on the simulation results, a flexure thickness of 30 μm and a diffused resistor doping concentration of 5 × 10 18 atoms/cm 3 were fixed to achieve a high prime-axis sensitivity of 122 μV/Vg, low cross-axis sensitivity of 27 ppm and a relatively higher bandwidth of 2.89 kHz. The designed accelerometer was realized by a complementary metal oxide semiconductor compatible bulk micromachining process using a dual doped tetra methyl ammonium hydroxide etching solution. The fabricated accelerometer devices were tested up to 13 g static acceleration using a rate table. Test results of fabricated devices with 30 μm flexure thickness show an average prime axis sensitivity of 111 μV/Vg with very low cross-axis sensitivities of 0.652 and 0.688 μV/Vg along X-axis and Y-axis, respectively. © 2011 Springer-Verlag.

Design, fabrication and testing of a high performance silicon piezoresistive Z-axis accelerometer with proof mass-edge-aligned-flexures

Coupled effects of an electroplated gold layer and damper holes drilled by Electro Chemical Discharge Machining (ECDM) process on the performance improvement of a quad beam capacitive accelerometer is presented in this paper. A simple quad beam-proof mass configuration with its beams located symmetrically at the centre of all the edges of the proof mass and connected to an outer supporting frame is considered in the present study. For a fixed damping ratio, prime-axis sensitivity of the sensor is increased by the damper holes whereas an electroplated gold layer improves the prime-axis sensitivity, cross-axis sensitivity, and Brownian Noise Equivalent Acceleration (BNEA). Moreover, the increased weight of the proof mass due to an electroplated gold layer further reduces the damping of the device which in turn helps to increase the prime-axis sensitivity more. A new figure of merit called Performance Factor (PF), defined as the ratio of the product of the prime-axis sensitivity and resonant frequency to the cross-axis sensitivity at a fixed damping ratio of 0.7 is used as a quantitative index to evaluate the performance improvement caused by the coupled effects of gold electroplating and ECDM processes. Simulation results show that for a device with damper holes of 8 μm diameter and electroplated gold layer of dimensions 3,000 μm × 3,000 μm × 20 μm, the prime-axis sensitivity is increased by more than 500 times, rotational cross-axis sensitivity and BNEA are reduced by around 10 and 30%, respectively and the PF is improved by around 482 times at a fixed damping ratio of 0.7. © 2011 Springer-Verlag.

Journal	Micro & Nano Letters
Publisher	Institution of Engineering and Technology (IET)
ISSN	1750-0443
Open Access	No