# Implementation of a CMOS/MEMS Accelerometer with ASIC Processes

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Process Flow

#### 2.2. Accelerometer Design

#### 2.2.1. Single Axis Accelerometer

_{1}to b

_{5}. The spring constant was found by applying a force balance to each beam segment. According to Hooke’s law, the relation between applied force (F

_{y}), spring constant along y-axis (k

_{y}) and displacement along y-axis (δ

_{y}) is formulated below:

_{y}

_{)}was applied at the end of the spring. The displacement along y-axis for each beam segment was given by:

_{yi}is the corresponding displacement along y-axis, and k

_{yi}is spring constant of the segment along y-axis.

_{y}.

_{2}and b

_{4}were clamped-guided cantilever beams, hence spring constants k

_{y}

_{2}and k

_{y}

_{4}are listed below, where k

_{c}is spring constant along the y-axis, E was Young’s modulus of elasticity, t was the thickness of structure, W is the width of spring, n was the number of cantilever beam segments in series and L was the length of spring [23].

_{1}, b

_{3}and b

_{5}were rectangular beams hence spring constants k

_{y}

_{1}, k

_{y}

_{3}and k

_{y}

_{5}are given by k

_{s}[23]. The beam segments b

_{1}, b

_{3}and b

_{5}were very stiff along the y-axis. There was almost no displacement along the y-axis. The width of the segment was deliberately selected two times larger than the cantilever beam to minimize the displacement of segments b

_{1}, b

_{3}and b

_{5}. The resulting spring constant was about 10

^{5}times larger than k

_{y}

_{2}and k

_{y}

_{4}.

_{1}, b

_{3}and b

_{5}, the serpentine spring only consisted of cantilever beams in series as in Figure 3c.

_{y}) can be obtained by the following equation where m is mass of the proof mass and a

_{y}was the acceleration along y-axis. The 1 g acceleration a

_{y}was around 9.81 m/s

^{2}. The dimension of proof mass was 606 μm × 462 μm × 10.14 μm and m was around 4.32 μg.

^{3}, therefore the width of the spring (W), must be kept small to get higher sensitivity. The spring width (W), was limited to 4 μm by the CMOS/MEMS process. Increasing the spring length (L), or the number of cantilever beam segments in series n in a limited size can produce higher sensitivity. The proposed accelerometer had the displacement of 104.99 nm at 1 g. The FEM simulation results are listed in Table 3.

#### 2.2.2. Tri-Axis Accelerometer

_{I}to F

_{III}are force from these three parts. According to Hooke’s law in angular form, the relation between applied torque (τ), torsion spring constant (k

_{θ}) and rotation angle (θ) is formulated below:

_{z}) was obtained by the following equation where L

_{z}is the distance from the torsion spring to the sensing finger as in Figure 5. For 1 g acceleration τ was around 2.77 × 10

^{−12}N·m, rotation angle was 3.23 × 10

^{−5}rad and δ

_{z}was around 16.24 nm. The displacement of FEM simulation was 20.08 nm. The FEM simulation results are listed in Table 8.

## 3. Results

#### 3.1. Surface Topography Measurement

#### 3.2. Mechanical Measurement

## 4. Discussion and Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

- Qu, H. CMOS MEMS fabrication technologies and devices. Micromachines
**2016**, 7, 14. [Google Scholar] [CrossRef] [PubMed] - Tsai, M.; Liu, Y.; Sun, C.; Wang, C.; Cheng, C.; Fang, W. A 400 × 400 µm
^{2}3-axis CMOS-MEMS accelerometer with vertically integrated fully-differential sensing electrodes. In Proceedings of the 2011 16th International Solid-State Sensors, Actuators and Microsystems Conference, Beijing, China, 5–9 June 2011; pp. 811–814. [Google Scholar] [CrossRef] - Yamane, D.; Konishi, T.; Takayasu, M.; Ito, H.; Dosho, S.; Ishihara, N.; Toshiyoshi, H.; Masu, K.; Machida, K. A sub-1g CMOS-MEMS accelerometer. In Proceedings of the IEEE Sensors, Busan, Korea, 1–4 November 2015; pp. 1–4. [Google Scholar] [CrossRef]
- Chuang, W.; Hu, Y.; Chang, P. CMOS-MEMS test-key for extracting wafer-level mechanical properties. Sensors
**2012**, 12, 17094–17111. [Google Scholar] [CrossRef] [PubMed] - Wu, J.; Fedder, G.K.; Carley, L.R. A low-noise low-offset capacitive sensing amplifier for a 50-μg/√Hz monolithic CMOS MEMS accelerometer. IEEE J. Solid-State Circuits
**2004**, 39, 722–730. [Google Scholar] [CrossRef] - Boser, B.E.; Howe, R.T. Surface micromachined accelerometers. IEEE J. Solid-State Circuits
**1996**, 31, 366–375. [Google Scholar] [CrossRef] - Baltes, H.; Brand, O.; Hierlemann, A.; Lange, D.; Hagleitner, C. CMOS MEMS—Present and future. In Proceedings of the Technical Digest, MEMS 2002 IEEE International Conference, Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No. 02CH37266), Las Vegas, NV, USA, 24 January 2002; pp. 459–466. [Google Scholar] [CrossRef]
- Yen, T.; Tsai, M.; Chang, C.; Liu, Y.; Li, S.; Chen, R.; Chiou, J.; Fang, W. Improvement of CMOS-MEMS accelerometer using the symmetric layers stacking design. In Proceedings of the IEEE Sensors, Limerick, Ireland, 28–31 October 2011; pp. 145–148. [Google Scholar] [CrossRef]
- Kuo, F.Y.; Chang, C.S.; Liu, Y.S.; Wen, K.A.; Fan, L.S. Temperature-dependent yield effects on composite beams used in CMOS MEMS. J. Micromech. Microeng.
**2013**, 23. [Google Scholar] [CrossRef] - Tseng, S.; Lu, M.S.; Wu, P.; Teng, Y.; Tsai, H.; Juang, Y. Implementation of a monolithic capacitive accelerometer in a wafer-level 0.18 µm CMOS MEMS process. J. Micromech. Microeng.
**2012**, 22. [Google Scholar] [CrossRef] - Chiang, C. Design of a CMOS MEMS accelerometer used in IoT devices for seismic detection. IEEE J. Emerg. Sel. Top. Circuits Syst.
**2018**, 8, 566–577. [Google Scholar] [CrossRef] - Liu, Y.; Tsai, M.; Fang, W. Pure oxide structure for temperature stabilization and performance enhancement of CMOS-MEMS accelerometer. In Proceedings of the IEEE 25th International Conference on Micro Electro Mechanical Systems (MEMS), Paris, France, 29 January–2 February 2012; pp. 591–594. [Google Scholar] [CrossRef]
- Qu, H.; Fang, D.; Xie, H. A single-crystal silicon 3-axis CMOS-MEMS accelerometer. In Proceedings of the IEEE Sensors, Vienna, Austria, 24–27 October 2004; Volume 2, pp. 661–664. [Google Scholar] [CrossRef]
- Tseng, S.H.; Yeh, C.Y.; Chang, A.Y.; Wang, Y.J.; Chen, P.C.; Tsai, H.H.; Juang, Y.Z. A monolithic three-axis accelerometer with wafer-level package by CMOS MEMS process. Proceedings
**2017**, 1, 337. [Google Scholar] [CrossRef] - Sun, C.; Tsai, M.; Liu, Y.; Fang, W. Implementation of a monolithic single proof-mass tri-axis accelerometer using CMOS-MEMS technique. IEEE Trans. Electron Devices
**2010**, 57, 1670–1679. [Google Scholar] [CrossRef] - Tsai, M.; Liu, Y.; Sun, C.; Wang, C.; Fang, W. A CMOS-MEMS accelerometer with tri-axis sensing electrodes arrays. Procedia Eng.
**2010**, 5, 1083–1086. [Google Scholar] [CrossRef] [Green Version] - Jiang, K.; Chen, H.; Hsu, W.; Lee, Y.; Miao, Y.; Shieh, Y.; Hung, C. A novel suspension design for MEMS sensing device to eliminate planar spring constants mismatch. In Proceedings of the IEEE Sensors, Taipei, Taiwan, 28–31 October 2012; pp. 1–4. [Google Scholar] [CrossRef]
- Karbari, S.R.; Kumari, U.; Pasha, R.C.; Gowda, V.K. Design and analysis of serpentine based MEMS accelerometer. AIP Conf. Proc.
**2018**, 1966, 020026. [Google Scholar] [CrossRef] - Fang, Y.; Mukherjee, T.; Fedder, G.K. SI-CMOS-MEMS dual mass resonator for extracting mass and spring variations. In Proceedings of the 2013 IEEE 26th International Conference on Micro Electro Mechanical Systems (MEMS), Taipei, Taiwan, 20–24 January 2013; pp. 657–660. [Google Scholar] [CrossRef]
- ADXL327, Datasheet, Analog Devices, Inc. Available online: https://www.analog.com/ADXL327 (accessed on 17 December 2018).
- Liu, Y.; Huang, C.; Kuo, F.; Wen, K.; Fan, L. A monolithic CMOS/MEMS accelerometer with zero-g calibration readout circuit. In Proceedings of the Eurocon 2013, Zagreb, Croatia, 1–4 July 2013; pp. 2106–2110. [Google Scholar] [CrossRef]
- Liu, Y.; Wen, K. Monolithic Low Noise and Low Zero-g Offset CMOS/MEMS Accelerometer Readout Scheme. Micromachines
**2018**, 9, 637. [Google Scholar] [CrossRef] [PubMed] - Kaajakari, V. Beams as micromechanical springs. In Practical MEMS: Analysis and Design of Microsystems, MEMS Sensors, Electronics, Actuators, RF MEMS, Optical MEMS, and Microfluidic Systems; Small Gear Publishing: Las Vegas, NV, USA, 2009; pp. 56–65. ISBN 9780982299104. [Google Scholar]
- Qu, H.; Fang, D.; Xie, H. A monolithic CMOS-MEMS 3-axis accelerometer with a low-noise, low-power dual-chopper amplifier. IEEE Sens. J.
**2008**, 8, 1511–1518. [Google Scholar] [CrossRef]

**Figure 1.**Cross-sectional view of the application-specific integrated circuit (ASIC)-compatible complementary metal-oxide semiconductor (CMOS)/micro-electromechanical system (MEMS) process flow: (

**a**) The standard CMOS process with an additional patterned metal7 (ME7) layer; (

**b**) The thick photoresist passivation layer is deposited for etch protection; (

**c**) The anisotropic silicon oxide dry etching; (

**d**) The isotropic silicon substrate dry etching.

**Figure 2.**The proposed accelerometer: (

**a**) The top view of the proposed accelerometer; (

**b**) Mechanical model of the structure; (

**c**) Circuit model of the structure.

**Figure 3.**The schematic of serpentine spring: (

**a**) Spring design parameters; (

**b**) The free body diagram; (

**c**) The proposed simplified model.

**Figure 7.**The SEM images of the proposed accelerometer: (

**a**) The whole structure; (

**b**) The curl matching frame; (

**c**) The x-axis spring; (

**d**) The torsion spring.

**Figure 9.**The surface profile of the proposed accelerometer: (

**a**) The three-dimensional view; (

**b**) The top view; (

**c**) The cross-section view.

Specifications | Design |
---|---|

Young’s Modulus of Elasticity (E) (GPa) | 70 |

Spring Width (W) (μm) | 4 |

Spring Length (L) (μm) | 370 |

Cantilever Beam Segments (n) (count) | 8 |

Structure Thickness (t) (μm) | 10.14 |

Specifications | Design | FEM | Error (%) |
---|---|---|---|

y-axis Spring Constant (k_{y}) (N/m) | 0.45 | 0.41 | 8.18 |

Specifications | FEM |
---|---|

Displacement at 1 g (nm) | 104.99 |

Initial Capacitance (C_{0}) (fF) | 91.97 |

Capacitance (ΔC) (fF) | 2.35 |

Resonant Frequency (f_{0}) (Hz) | 1562.85 |

Mass (M) (μg) | 4.32 |

Spring Constant (K) (N/m) | 0.415 |

Specifications | x-Axis | y-Axis |
---|---|---|

Young’s Modulus of Elasticity (E) (GPa) | 70 | 70 |

Spring Width (W) (μm) | 5 | 5 |

Spring Length (L) (μm) | 472 | 489 |

Cantilever Beam Segments (n) (count) | 2 | 2 |

Structure Thickness (t) (μm) | 10.14 | 10.14 |

Specifications | Design | FEM | Error (%) |
---|---|---|---|

x-axis Spring Constant (k_{x}) (N/m) | 1.69 | 1.63 | 3.45 |

y-axis Spring Constant (k_{y}) (N/m) | 1.52 | 1.50 | 1.19 |

Specifications | Design |
---|---|

Shear Modulus (G) (GPa) | 79 |

Spring Width (W) (μm) | 4 |

Spring Length (L) (μm) | 300 |

Structure Thickness (t) (μm) | 10.14 |

Part | Area (μm × μm) | Moment Arm Length (μm) |
---|---|---|

Region I | 700 × 40 | 280 |

Region II | 260 × 40 | 130 |

Region III | 700 × 30 | 150 |

Specifications | x-Axis | y-Axis | z-Axis |
---|---|---|---|

Displacement at 1 g (nm) | 85.05 | 62.59 | 20.08 |

Resonant Frequency (f_{0}) (Hz) | 1708.45 | 1991.43 | 2634.14 |

Mass (M) (μg) | 14.15 | 9.58 | 3.88 |

Specifications | Proposed Model | FEM | Experimental Result |
---|---|---|---|

Single Axis: Resonant Frequency (f_{0}) (Hz) | 1575.20 | 1562.85 | 2000.00 |

Tri-axis: In-plane Resonant Frequency (f_{0}) (Hz) | 2036.05 | 1991.43 | 2500.00 |

Tri-axis: Out-plane Resonant Frequency (f_{0}) (Hz) | 3910.12 | 2634.14 | 5354.65 |

Specifications | Spring Width of 3.6 μm | Spring Width of 4 μm | Spring Width of 4.4 μm |
---|---|---|---|

Torsion Spring Constant (k_{θ}) (N·m/rad) | 6.45 × 10^{−8} | 8.57 × 10^{−8} | 1.10 × 10^{−7} |

Resonant Frequency (f_{0}) (Hz) | 3392.79 | 3910.12 | 4436.83 |

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**MDPI and ACS Style**

Liu, Y.-S.; Wen, K.-A.
Implementation of a CMOS/MEMS Accelerometer with ASIC Processes. *Micromachines* **2019**, *10*, 50.
https://doi.org/10.3390/mi10010050

**AMA Style**

Liu Y-S, Wen K-A.
Implementation of a CMOS/MEMS Accelerometer with ASIC Processes. *Micromachines*. 2019; 10(1):50.
https://doi.org/10.3390/mi10010050

**Chicago/Turabian Style**

Liu, Yu-Sian, and Kuei-Ann Wen.
2019. "Implementation of a CMOS/MEMS Accelerometer with ASIC Processes" *Micromachines* 10, no. 1: 50.
https://doi.org/10.3390/mi10010050