Avva Sai Pranav

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Project · MEMS

MEMS Inertial Measurement Unit

M2D2 Lab, IISc · with ISRO · Guided by Prof. G K Ananthasuresh · Jun 2022 – Jul 2024

12.1µm/rad/s sensitivity
6.77kHz eigenfreq
181.6kQ factor
<5%Sim-to-hw error
SEM micrographs of fabricated ADRG showing accordion-like ring structure at multiple magnifications
SEM micrographs of fabricated ADRG showing accordion-like ring structure at multiple magnifications (28X, 200X, 450X)

Over a two-year collaboration between IISc and ISRO, this project aimed to develop a complete MEMS Inertial Measurement Unit for space navigation applications. The IMU comprised two core sensors: a dual-axis capacitive accelerometer and an Accordion-like Disk Resonator Gyroscope (ADRG). Starting from an existing ring-and-spoke gyroscope concept, extensive parametric optimization was performed on both devices using COMSOL multiphysics simulations. Both devices were successfully fabricated at CeNSE, with the gyroscope validated via laser Doppler vibrometry achieving <5% eigenfrequency error.

12.1 µm/rad/s Mechanical sensitivity — 3–4× prior designs
6.77 kHz Drive-mode eigenfrequency — matched within <5% in test
181.6k Q factor Highest of the five topologies tested
46.4% ΔC₀/C₀ Largest relative sense-capacitance change

Problem Statement

  • Need: ISRO required high-precision inertial sensors for space navigation applications
  • Prior art: Existing DRG topologies — standard DRG, Honeycomb DRG (HDRG), Concentric DRG (CDRG), and Gear-tooth DRG (GDRG) — each advanced sensitivity incrementally but always traded electrode placement against mechanical performance
  • Open question: Could a novel ring geometry significantly improve mechanical sensitivity and quality factor without compromising noise performance or manufacturability?

Methodology

The project followed a comprehensive design-simulate-fabricate-test workflow spanning both accelerometer and gyroscope development. Click a stage to jump there.

Design & Simulation

Accelerometer Design: The MEMS accelerometer employs a proof mass suspended by folded-beam flexures with interdigitated comb-drive electrodes for differential capacitive sensing.

5–8 µmBeam width (swept)
10–60 µmThickness (swept)
~3 kHzTarget eigenfrequency
Optimization Accelerometer optimization parameters

A comprehensive parametric study varied beam width (5, 6, 7, 8 µm) and out-of-plane thickness (10, 20, 30, 60 µm) to optimize:

  • Eigenfrequency (~3 kHz target)
  • Displacement sensitivity
  • Capacitance sensitivity (ΔC/g)
  • Cross-axis sensitivity
  • Quality factor
  • Brownian noise equivalent acceleration

Gyroscope Topology Comparison: Five DRG topologies were evaluated using COMSOL Multiphysics. The novel Accordion-like DRG (ADRG) introduces accordion-shaped spoke connections that increase mechanical compliance while maintaining structural integrity.

9Rings
8 mmOuter diameter
10 µmRing width
60 µmThickness
0.5Anchor : outer radius

Held constant across all five topologies for a fair comparison.

COMSOL eigenfrequency mode shapes of ADRG analyzed by FEM
Mode shapes of ADRG analyzed by FEM showing eigenfrequencies from 2344 Hz to 8068 Hz
Note Why accordion spokes?
Accordion-shaped spokes increase radial compliance while maintaining tangential stiffness. This geometry allows for larger modal displacements under Coriolis forces, directly improving mechanical sensitivity without compromising structural integrity or introducing additional thermoelastic damping paths. Eigenfrequency studies identified drive and sense mode frequencies, with mode matching critical for gyroscope sensitivity.

Mechanical Sensitivity (×10⁻⁶ µm/rad/s):

DRG
3.83
HDRG
4.27
CDRG
3.61
GDRG
2.76
ADRG
12.1
Data Complete topology comparison table

Disk Resonator Gyroscope Topology Comparison (9 rings, 8mm OD, 10µm ring width, 60µm thickness)

DRG Type Freq (Hz) Q (k) ΔC₀/C₀ (%)
DRG 12,720 103.62 14.37
HDRG 11,306 105.42 16.39
CDRG 12,611 99.99 26.36
GDRG 14,761 91.58 29.40
ADRG 6,771 181.55 46.44

Fabrication

Mask Layout Generation: Once mechanical and electrode parameters were finalized through simulation, designs were converted into semiconductor fabrication mask layouts.

  • CleWin — initial layout design and layer definition
  • KLayout — advanced editing and DRC (Design Rule Check) verification

SOI Microfabrication: Devices were fabricated at the Centre for Nano Science and Engineering (CeNSE) at IISc using SOI-based surface micromachining.

60 µmDevice thickness
10 µmRing width
Batch of fabricated MEMS gyroscope dies from CeNSE
Batch of fabricated MEMS gyroscope dies from CeNSE fabrication run

Wire Bonding: Wire bonding connected the MEMS die pads to the PCB carrier for electrical interfacing.

16Addressed outer electrodes
4Common connections
Wire-bonded GDRG die showing M2D2 lab markings
Wire-bonded DRG die mounted showing M2D2 lab markings and concentric ring structure
Process SOI microfabrication steps
  • Start with SOI wafer (60 µm device layer, 2 µm BOX)
  • Photolithography for device patterning
  • Deep reactive ion etching (DRIE) through device layer
  • HF vapor release of buried oxide
  • Critical point drying to prevent stiction
  • Metal deposition for electrodes and bond pads
Info Pinout details
Custom pinout schematics were designed specifying electrode-to-pin mappings: the gyroscope required 16 individually addressed outer electrodes plus 4 common connections. This allows independent drive/sense control of each electrode pair for differential operation.

Test Setup

The fabricated ADRG was characterized using a Polytec MSA-500 Micro System Analyzer (Laser Doppler Vibrometer) at atmospheric pressure and room temperature.

1 atmPressure
25°CTemperature
0°/180°Drive phase
Method How the measurement worked
The device was electrostatically driven with AC+DC excitation applied to opposing drive electrodes (0° and 180° phase). Frequency sweeps measured the mechanical frequency response, with the resonance peak identifying the eigenfrequency. The LDV measured out-of-plane velocity at multiple points across the resonator to verify mode shapes matched FEM predictions.
Gyroscope wire bonding pinout with 16 outer electrodes
Gyroscope pinout: 16 outer electrodes + 4 common
Dual-axis accelerometer pinout with X/Y sense and feedback electrodes
Accelerometer pinout: X/Y sense and feedback electrodes
Equipment Full test equipment list
  • Polytec MSA-500 Micro System Analyzer
  • Keysight 33500B Waveform Generator
  • Stanford Research SR830 Lock-in Amplifier
  • Keithley 2400 SourceMeter (DC bias)
  • Custom vacuum chamber (for Q-factor measurements)

Results

LDV frequency response showing measured resonance at 6.8 kHz
LDV frequency response validating 6.8 kHz resonance prediction
6.8 kHzMeasured resonance
6.771 kHzFEM prediction
~0.4%Error

Target was <5% — simulation methodology held up in silicon.

Key Finding ADRG performance advantages
  • ADRG achieved highest quality factor (181.55k) among five DRG topologies tested
  • ADRG demonstrated best relative sense capacitance (46.44% ΔC₀/C₀)
  • Mechanical sensitivity of 12.1×10⁻⁶ µm/rad/s — 3-4× higher than prior designs
  • LDV validation showed <5% eigenfrequency error between simulation and measurement
Publication Conference paper details
Sai Pranav Avva, Suman Acharya, Kandula Eswara Sai Kumar, Sudhanshu Shekar, Karthik Raveendranath, and G K Ananthasuresh. "A Novel Accordion-like Disk Resonator Gyroscope." International Conference on Micro, Nano, and Smart Systems (ICMNS), July 11, 2024.

The paper presents a comprehensive comparison of five DRG topologies (DRG, HDRG, CDRG, GDRG, ADRG) across mechanical sensitivity, Q factor, capacitance change, and Brownian noise floor metrics.

Additional Information

Conference presentation slides.

Skills Technical skills demonstrated
  • Simulation: COMSOL Multiphysics, FEM eigenfrequency/thermoelastic analysis
  • CAD/Layout: CleWin, KLayout, mask design, DRC
  • Fabrication: SOI micromachining, DRIE, HF vapor release
  • Characterization: LDV, impedance analysis, wire bonding
  • Analysis: MATLAB, Python data processing
Team Contributions & credits

I led parametric design optimization for both sensors, performed COMSOL multiphysics simulations achieving <5% eigenfrequency error, generated CleWin/KLayout fabrication masks, designed wire bonding pinouts, and first-authored the ICMNS 2024 conference paper.

Guided by Prof. G K Ananthasuresh and Karthik Raveendranath (ISRO), with simulation support from Sai Kumar and Suman Acharya, and fabrication by Sudhanshu Shekhar.

Funded by ISRO; fabricated and characterized at CeNSE.