- Letter
- Open Access

# Fabrication and characterization of monolithic piezoresistive high-g three-axis accelerometer

- Han-Il Jung
^{1}, - Dae-Sung Kwon
^{1}and - Jongbaeg Kim
^{1}Email author

**Received:**7 December 2016**Accepted:**31 December 2016**Published:**10 January 2017

## Abstract

We report piezoresistive high-g three-axis accelerometer with a single proof mass suspended by thin eight beams. This eight-beam design allows load-sharing at high-g preventing structural breakage, as well as the symmetric arrangement of piezoresistors. The device chip size is 1.4 mm × 1.4 mm × 0.51 mm. Experimental results show that the sensitivity in X-, Y- and Z-axes are 0.2433, 0.1308 and 0.3068 mV/g/V under 5 V applied and the resolutions are 24.2, 29.9 and 25.4 g, respectively.

## Keywords

- Piezoresistive
- Monolithic
- Miniature
- High-g
- Three-axis accelerometer

## Background

High-g accelerometers have been widely used in vehicle crash test, automotive air bag and shock detection applications. It is important to reduce the size of the sensor to extend the field of applications. Silicon microfabrication technique makes it possible to reduce size of the sensor as well as the production cost through batch fabrication, making it suitable for mass production. In previous researches, fabricated high-g accelerometers measure the acceleration in one-axis [1–6] or three-axis with three proof masses [7]. Compared with a three-axis accelerometer with multiple proof masses, a monolithic three-axis accelerometer is free from axis-alignment error and has advantages of smaller device size, better uniformity of the measurement signals in three sensing directions [7]. Design and analysis of monolithic three axis high-g accelerometers have been reported before [8–10], however, to our best knowledge, no actual device and experimental results were reported.

In this work, we developed a monolithic piezoresistive high-g three-axis accelerometer. The detailed description for the design, fabrication and characterization is given in the following.

## Design and fabrication

### Device and circuit design

_{out_x}for Z-axis bridge circuit is given by

_{6}+ R

_{8})/(R

_{10}+ R

_{12}) = (R

_{2}+ R

_{4})/(R

_{14}+ R

_{16}) and η

_{Az}is nonlinear variable of the bridge circuit. The nonlinear term is always zero and r = 1 from Fig. 1c and Table 1, when all the resistors are identical [12]. Accordingly, the simplified equations of the output signal are expressed as follows:

_{x}, the same amount of compressive stress is applied to R

_{9}and R

_{15}, while the same amount of tensile stress is applied to R

_{11}and R

_{13}due to the symmetric arrangement. Resultantly, ΔR

_{9}= ΔR

_{15}= −ΔR

_{11}= −ΔR

_{13}and the change of output voltage V

_{out_y}becomes zero [Eq. (3)]. Similarly, V

_{out_x}and V

_{out_z}are measured by each bridge circuit.

Resistance changes of piezoresistors with axial acceleration

X-axis | Y-axis | Resistors of Z-axis acceleration measurement circuit | ||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

R | R | R | R | R | R | R | R | R | R | R | R | R | R | R | R | |

A | − − − | − − − | − − − | − − − | − | + | + | − | +++ | +++ | +++ | +++ | + | − | − | + |

A | + | − | − | + | +++ | +++ | +++ | +++ | − | + | + | − | − − − | − − − | − − − | − − − |

A | ++ | ++ | − − | − − | ++ | ++ | − − | − − | − − | − − | ++ | ++ | − − | − − | ++ | ++ |

### Fabrication

_{2}) layers were grown on both sides of SOI wafer by thermal oxidation (a). After the SiO

_{2}was patterned and etched, contact pads and p-type piezoresistors were formed by implanting boron ions with the concentration of 5 × 10

^{19}atoms cm

^{−3}(b). Then 300-nm-thick SiO

_{2}layer was deposited on the top surface as an insulating layer using plasma-enhanced chemical-vapor deposition. Etching via holes (c) for interconnection was followed by metallization to form contact pads (d). The top SiO

_{2}, device layer and buried SiO

_{2}layer were sequentially etched to form proof mass and mechanical beams (e–f). Finally, the handle layer was deep-etched from backside to release the beams (g). It is noted that the proof mass is defined on the 500 μm-thick substrate layer, and is attached to the beams defined on the 5 μm-thick device layer.

## Results and discussion

*ε*,

*π*

_{ eff },

*π*

_{ 44 }and

*E*denote the strain, effective piezoresistive coefficient, shear piezoresistive coefficient (85 × 10

^{−11}Pa

^{−1}) and Young’s modulus of silicon (163 GPa), respectively [12]. Thus the average gauge factor of the piezoresistors is calculated as 69.3.

_{x}, S

_{y}, S

_{z}on the X-, Y-, Z-axes were 0.2433, 0.1308, 0.3068 mV/g/V and resolutions were 24.2, 29.8, 25.4 g, respectively. The linearity of each directional measurement is calculated as L

_{Ax}= 0.978, L

_{Ay}= 0.918 and L

_{Az}= 0.997 by least-square fitting. For a specific axial direction, the output signal of the corresponding resistor increased with increasing acceleration, while the others remained constant. Consequently, the fabricated accelerometer is able to detect acceleration in arbitrary direction by vector summation of all three output signals. Although there exist number of non-ideal factors including microfabrication error, misaligned attachment, and electrical noise that may cause cross-axis sensitivity, nonlinearity, and non-zero output voltage, the developed accelerometer can measure both the magnitude and direction of high-g acceleration in arbitrary direction.

## Conclusions

A high-g three-axis accelerometer using single proof mass has been designed, fabricated, and tested. The size of the fabricated accelerometer chip is 1.4 mm × 1.4 mm × 0.51 mm, and the accelerometer is suitable for impact test of portable electronic devices to evaluate mechanical robustness. To decompose acceleration in arbitrary direction into x, y and z components with minimal cross-talk among each axis, we designed sixteen piezoresistors and four external resistors that construct three Wheatstone bridge circuits. The accelerometer could also be applied to detect high-g shock of projectiles or vehicles.

## Declarations

### Authors’ contributions

HJ and DK participated in design, fabrication, experiment and drafted the manuscript. JK conceived the study, reviewed all test methods and results, and finalized the manuscript. All authors read and approved the final manuscript.

### Competing interests

The authors declare that they have no competing interests.

### Funding

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (NRF-2015R1A2A1A01005496) and LG Electronics.

**Open Access**This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

## Authors’ Affiliations

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