A Systematic Review on MEMS Gyroscope.docx

1. A Systematic Review on Micro-Electromechanical System (MEMS)
Gyroscope
Abstract: A micro-electromechanical system(MEMS) gyroscopeis commonly used to monitor the angular rate of a moving
body due to its benefits. The most promising advantages include its small size, low cost, and a high degree of integration.
MEMSgyroscopehas different fabrication processes and micromachining techniques. LIGA (Lithography-Galvanoformung-
Abformung), bulk micromachining, surface micromachining, Silicon-on-glass (SOG) and Deep Reactive Ion Etching (DRIE)
are the known fabrication techniques for MEMS gyroscope. This paper systematically reviewed the fabrication techniques
used to fabricate the MEMSgyroscope. Thecurrent review paper also focuses on the performance of MEMSgyroscopewhich
included several recent developments. For the conclusion of results, the variable typically used is the rate of turn (°/s) for
MEMS angular rate sensors with respect to bandwidth frequency. Finally based on the review some analysis on fabrication
technology, key principles, and performance parameters are discussed.
Keywords: MEMS, Gyroscope, Micromachining, Fabrication
1. Introduction
Gyroscopes are the most often used inertial instruments for measuring the angular velocity of moving objects.
MEMS technology MEMS gyroscopes have achieved a lot of attraction in the industry and academia. MEMS
gyroscopes are vibratory in nature, and their operation is based on Coriolis force. The main component is vibrating
proof mass which is oscillating two orthogonal modes of vibration. MEMS gyroscopes are classified into three
types based on their application: rate grade, inertial grade, and tactical grade gyroscope. Fabrication techniques
used in MEMS gyroscope fabrication are surface micromachining, bulk micromachining, Lithographie-
Galvanoformung-Abformung (LIGA) process [1]. Based on the micromachining process the MEMS gyroscope
is of two types linear and torsional gyroscope. MEMS Gyroscope is less bulky, easy to operate, and low power
consumption, and has various applications in the automotive industry, aerospace industry, and digital device
manufacturing industry. They are classified into two types based on materials one is silicon type and the other is
non-silicon type. The quartz gyroscope is one of the most reliable and high-quality non-silicon gyroscopes. Due
to high fabrication cost, error, and complexity in fabrication [2]. Silicon-based gyroscopes have lower power
consumption and high yield as compared to quartz gyroscopes. This type of gyroscope has several types of
applications in medical, defence, information, and environment, etc. The evolution of the MEMS gyroscope and
its major technologies are summarised in this article.
Figure 1 Schematic of the gyroscope’s mechanical structure.[3]
2. Literature Review
Verma et. al. Silicon wafer is used in the fabrication process and a high aspect ratio is maintained using the DRIE
process and then vapor phase etching is used for etching of the buried oxide. The sputtering process is used for
the deposition of Cr/Au. To expose the pads for wire bonding,the lithography and etching processes are repeated.

2. Baking is done to harden the resist. The two mask SOI process is used for fabrication then the lithography process
is used forpatterning the structure.After lithography DRIE etching process is used foretching.Cleaning ofsilicon
substrate takes place using isotropic etch mode with SF6. As a structural layer, the silicon layer is 15 microns
thick, and the spacing between the comb fingers is 3 microns wide, because this combination produces the
maximum capacitance of roughly 2 pF. FEM analysis is performed, and the resonance frequency for both modes
of driving and sense is calculated; the analytical results are similar to the ideal values [1].
Sung et. al. has proposed the fabrication method for MEMS gyroscope which can provide high displacement
sensitivity. They use silicon wafers for fabrication. Firstly, silicon is etched using the DRIE process.The silicon
oxide layer is developed using a wet oxidation process. Buffered Oxide Etch is utilised to design the oxide layer,
followed by tetramethylammonium hydroxide etching. Then a layer of aluminium was deposited using e-beam
evaporation. Finally, the silicon substrate gets anodically attached with the glass substrate. By this fabrication
method, the problem of footing can be avoided. The displacement sensitivity affects the efficiency of the
gyroscope by using this fabrication method. Footing effects are also less in this method than the conventionalone.
The gyroscope comprises the proof mass, beam springs,and electrodes. The frequency analyzer is used to obtain
the frequency response in a vacuumchamber and the process results in a standard deviation across the wafer[4].
Figure 2 Overview of the fabrication process of the gyroscope [4]
Chun et al. developed a doubly decoupled MEMS gyroscope with a wide frequency range of operation. In both
sensing and driving modes, the resonance bandwidth is raised without impacting the Q factor. It can be operated
at any frequency within 240 Hz. MetalMUMPs, a MEMSCAP-developed fabrication technique, is used. The
substrate is an n-type silicon wafer. The thicknesses of the various thermal oxide layers, such as nitride,
polysilicon, and electroplated nickel layer, vary. KOH is used for etching, and polysilicon acts as an electrical
conductorbetween the probing pad and the moveable structure.To do FEM simulation, the damping ratio 5x10 -3
and 10 micro-N force are selected. The resonance frequency of the driving and sensing modes is investigated,and
the first two peaks of the three peaks are closely spaced, resulting in a quasi-'resonance band' with a broad
bandwidth [5].
Cao et. al. has designed a double U-beam vibration ring mems gyroscope.The device is fabricated using a 300-
micron thick silicon wafer. The etching process is done using the DRIE etching technique. The Silicon on glass
process is used to fabricate the device in which sputtering ofmetal takes place. the anodic bonding is used between
the wafer and the glass layer. Glass chips are etched to protect the microstructure. To determine the vibration
characteristics like natural frequency, mode shape and stability of the gyroscope modal analysis is performed.
After the analysis the rationality of the design structure is verified. [6]

3. Figure 3 Main process flow of fabrication of DUVRG[6]
Ma et. al. has discussed the key processes of Mems fabrication technology for gyroscope using silicon on glass.
It involves many processes like patterning, DRIE, anodic bonding,and polishing. Chemical Mechanical Polishing
(CMP) is an important process for maintaining the thickness and quality of the surface. Polishing quality is
affected by some parameters like the flow of the polishing slurry, load, and rotational speed. TMAH etchant is
used to wet etch the silicon substrate for the anchor region. Electrodes are made of Cr and Au that have been
sputtered and patterned.silicon and glass are bonded using anodic bonding. At last polishing and DRIE process
takes place [7].
Haitao et. al. discussed high-resolution gyroscope operating at atmospheric pressure. they have proposed the
design and fabrication of the gyroscope. Due to symmetry in structure, the matching of resonant frequencies is
achieved and results in higher sensitivity. There are two groups of cantilevers and each of the groups has 12
cantilevers which are attached to the proof mass. Both drive and sense modes do not result in each other motion.
Due to this double decoupled of both the modes is achieved. The fabrication process involved silicon on glass
(SOG) and the DRIE technique. Silicon wafers are etched using the DRIE process to specify anchor. lift-off
process is used to make electrical connections by layering the Ti/Pt/Au on the glass. the silicon wafer is bonded
with glass using an anodic bond.KOH is used to thin the silicon substrate.Afterthe DRIE process again the wafer
is diced and bonded. The proposed design is utilised to minimise the gyroscope's sensitivity [8].
Figure 4 The resonant frequency of the gyroscope
Where k is the spring constant for all cantilevers, m is the mass of the movable component, E is the Young's
Modulus of silicon, b is the breadth, l is the length, s is the area in plane, and ρ is the density of silicon. Aspect
ratio (b/l) is the most critical design parameter for operating modes.
Kou et al. introduce the VRG structure, which contains an axisymmetric ring resonator as well as capacitance
transducers. Etching, sputtering, and anodic bonding are all part of the VRG fabrication process. The process
begins with a 4-micron thick AZ4620 PR layer spun on a silicon wafer. DRIE is used to build anchors at a depth
of about 40 microns, then a 2-micron PR layer is spun and patterned on the wafer, then Ti and Au are sputtered
on the glass to form electrode leads, then silicon and glass wafers are anodized, and a 7-micron thick PR layer is
coated on the backside, followed by a 160-micron deep etching. The simulation results demonstrate that the

4. working mode of the gyroscope has a natural resonant frequency of 8.805 kilohertz and 8.807 kilohertz, with a
frequency splitting of 2 hertz.[9]
Alper et al. postulates an SYMDEC gyroscope administered by utilizing liquefied wafer MEMS procedure on an
insulating substrate.For temperature and sensitivity,dependent drifts frequencies are allowed to be matched for
a similar installation. While prevention of instability due to mechanical operations is accomplished by using
Decoupled drives and sense modes.The capacitance of 130 and 20 fF is generated with the support of a 12-15 µm
thick silicon layer having an aspect ratio of 10. Based on the dissolved wafer procedure an uncomplicated three
cover process is used for fabrication. Firstly, for the formation of gyroscope patterns, the front side is diffused
with a deep-boron of 12-15 µm to a high doping density. Then it is followed by another boron-doped based on
20-25 µm DRIE etch. Finally, the undoped silicon wafer can be dissolved in an EDP solution by filliping the
wafer and bonding from the anchor regions. An additional IC based on CMOS is fabricated and connected to the
gyroscope. The measured resonance frequencies of the gyroscope are 40.65 and 41.25 kHz for the drive and
sensing modes, respectively. Initials test helps in the conclusion of net 0.030deg/s resolution rate at atmospheric
pressure while 0.017deg/s in vacuum at 50 Hz respectively.[10]
Cao et al. used a typical SOG and DRIE technique to build the DUVRG of 300 micron silicon wafers containing
eight DUVRG symmetrical supporting springs and twenty-four silicon capacitor electrodes. The procedure starts
with sputtering on a glass layer to create a terminal, then etching the bottomsurface of a silicon wafer to create
an anchorand anodizing it with a glass layer, then DRIE to etch the wafer, and finally etching glass caps to create
a three-layer structure. The actual and predicted resonant frequency values diverged by 5.33 and 5.36 percent,
respectively. The bias instability and ARW of the resulting DUVRG were both under 8.86 (◦)/h and 0.776 (◦)/h,
respectively. [11]
Alper et al. presents an improvisational decoupling configuration between drive and sense nodes of an angular
rate sensor systemby utilizing a 100 μm thick MEMS gyroscope. The installation helps in the enhancement of
robustness against drive oscillations, therefore, decreasing the mechanical instability between components.
Fabrication of the gyroscope is done by using the Scale on Glass methodology through etching using a 100 μm
silicon substrate. The etching was done using deep reactive ion etching with a higher aspect ratio of 30. The latest
four mask layer process is used for etching. Firstly, the appearance of the anchors on the glass substrate followed
by an Aluminum safeguard of 100 μm on the bottom. Finally, the anodic bonding of silicon was followed by the
final etching mask via DRIE. Now, the removal of the metal layer to etch the photoresist mask. Though SOG
process is sufficient in removing the notch effects still it experienced problems in the prevention of detrimental
effects. For the prevention of heating and notching deformations, a patterned layer is introduced. The capacitance
gaps in the gyroscope core help in the large diffusional 18.2 fF capacitance. The current installation concluded in
the net bias stability of 14.3deg/hr with a range of 50deg/s and 0.6% scale factor.[12]
Alper et al. present a new SOIMEMS gyroscope with decoupled oscillation modes which is high-performance.
The gyrostat is created utilizing ordinary SOIMUMPS interaction of MEMSCAP. The interaction incorporates
anterior face what's more rear designing of a SOI slice utilizing DRIE. The basic layout and transversion of
cantilever structure organized with the SOIMUMPS procedure has structural slab whose width may be 10 or 25
µm. DRIE totally removes the section of the handle slice under the prorogate design from the back of the handle
wafer. Under the prorogate design, precise scratching of the basis reduces air damping and produces high
involuntary standard variables. One more benefit of eliminating the basis layer under the prorogate designs is
simply prominent dc possibilities may securely commanded the portable constructions, as the impacts of
electrostatic levitation are totally disposed of. At atmospheric pressure, the portrayal of the hybrid -connected
gyrostat reveals a minor observed commotion comparable rate of 90◦/h/Hz1/2, obviating the necessity for a
vacuum container in many applications. The gyroscope's R2-non-linearity estimated one step ahead compared to
0.02 percent.The gyrostat has a 70◦/s minor quadrature signal and a 1.5◦/s transient inclination consistent quality
at climatic strain, the gyroscope's angular rate sensitivity is 100 µV/(◦/s), while in vacuum, it refines to 2.4
mV/(◦/s). The gyroscope's noise-equivalent rate in 20 mTorr vacuum is 35◦/h/Hz1/2, that may enhanced more by
lowering the electromechanical noise.[13]

5. 3. Analysis and Discussion:
After analysing the above papers, the most common fabrication technique used for the fabrication of MEMS
gyroscope is Silicon-on-glass (SOG) and Deep Reactive Ion Etching (DRIE) process. This guarantees that the
aspect ratio and Q-factor are both high. The silicon wafer is utilised as a beginning layer, after which the gap
between the moving parts and the substrate is etched. Deposition of Au/Cr/Al takes place by various deposition
techniques the most common is the sputtering technique. The bonding of silicon substrate and glass takes place
using an anodic bonding process. In some papers Chemical Mechanical Polishing (CMP) polishing takes place.
Finally, the DRIE method is used to etch the silicon.
The MEMS Gyroscope consists of beam springs, Proof mass, and electrodes – sense and driving. The Coriolis
acceleration is produced by angular velocity when the mass oscillates in the y-direction because ofan electrostatic
force. It allows the oscillation of mass in the x-direction. Sense and drive mode are the two options available. The
sensing area determines the gyroscope's sensitivity. It can be improved by increasing the area of the electrodes
and minimising the space between them.
In the aforementioned articles, various FEM software is used to model and analyse the MEMS gyroscope.
Resonance frequency for both the modes sense and driving mode is calculated and compared with the theoretical
results. The proposed fabrication methods in the above papers could achieve high sensitivity.
Conclusion: This research examines the fabrication technique and performance parameters of MEMS Gyroscopes
in light of their applications.MEMS gyroscopes are widely employed in a variety of applications and have become
a necessary component. Modern fabrication technologies such as SOG and DRIE are employed to increase the
device's measurement performance. To avoid notching slowness and footing issues, the method has been
streamlined.

6. Reference
[1] P. Verma et al., “Design, simulation, and fabrication of silicon-on-insulator MEMS vibratory decoupled
gyroscope,” Comput. Opt., vol. 40, no. 5, pp. 668–673, 2016, doi: 10.18287/2412-6179-2016-40-5-664-
668-673.
[2] G. Zhanshe,C. Fucheng,L. Boyu, C. Le, L. Chao, and S. Ke, “Research development of silicon MEMS
gyroscopes:a review,” Microsyst. Technol., vol. 21, no. 10, pp. 2053–2066, 2015, doi: 10.1007/s00542-
015-2645-x.
[3] J. Watson, “MEMS Gyroscope Provides Precision Inertial Sensing in Harsh, High Temperature
Environments,” Analog Devices, pp. 1–4, 2016, [Online]. Available:
https://www.analog.com/en/technical-articles/mems-gyroscope-provides-precision-inertial-
sensing.html#.
[4] J. Sung et al., “A gyroscope fabrication method for high sensitivity and robustness to fabrication
tolerances,” J. Micromechanics Microengineering, vol. 24, no. 7, 2014, doi: 10.1088/0960-
1317/24/7/075013.
[5] C. W. Tsai, K. H. Chen, C. K. Shen, and J. C. Tsai, “A MEMS doubly decoupled gyroscope with wide
driving frequency range,” IEEE Trans. Ind. Electron., vol. 59, no. 12, pp. 4921–4929, 2012, doi:
10.1109/TIE.2011.2177612.
[6] H. Cao, Q. Cai, Y. Zhang, C. Shen, Y. Shi, and J. Liu, “Design, Fabrication, and Experiment of a
Decoupled Multi-Frame Vibration MEMS Gyroscope,” IEEE Sens. J., vol. 21, no. 18, pp. 19815–19824,
2021, doi: 10.1109/JSEN.2021.3095762.
[7] Z. Ma, Y. Wang,Q. Shen, H. Zhang, and X. Guo, “Key processes ofsilicon-on-glass MEMS fabrication
technology for gyroscope application,” Sensors (Switzerland), vol. 18, no. 4, 2018, doi:
10.3390/s18041240.
[8] H. Ding et al., “A high-resolution silicon-on-glass z axis gyroscope operating at atmospheric pressure,”
IEEE Sens. J., vol. 10, no. 6, pp. 1066–1074, 2010, doi: 10.1109/JSEN.2010.2043669.
[9] Z. Kou et al., “Design and fabrication of a novel MEMS vibrating ring gyroscope,” Proc. 2017 IEEE 3rd
Inf. Technol. Mechatronics Eng. Conf. ITOEC 2017, vol. 2017-Janua, pp. 131–134, 2017, doi:
10.1109/ITOEC.2017.8122396.
[10] S. E. Alper and T. Akin, “A Single-Crystal Silicon Symmetrical and Decoupled MEMS Gyroscope on an
Insulating Substrate,” vol. 14, no. 4, pp. 707–717, 2005.
[11] H. Cao, Y. Liu, Z. Kou, Y. Zhang, and X. Shao,“Design , Fabrication and Experiment of Double U-Beam
MEMS Vibration Ring Gyroscope,” 2019, doi: 10.3390/mi10030186.
[12] F. Decoupled, S. M. Gyroscope, S. E. Alper, Y. Temiz, and T. Akin, “A Compact Angular Rate Sensor
System Using a,” vol. 17, no. 6, pp. 1418–1429, 2008.
[13] S. E. Alper, K. Azgin, and T. Akin, “A high-performance silicon-on-insulator MEMS gyroscope operating
at atmospheric pressure,” Sensors Actuators, A Phys., vol. 135, no. 1, pp. 34–42, 2007, doi:
10.1016/j.sna.2006.06.043.