Project story, Instruments and Skills

Project Overview

During this project, I developed a silicon-based nanomechanical cantilever gas sensor platform using semiconductor microfabrication technology. The goal was to fabricate a highly sensitive miniaturized sensor array capable of detecting different gas molecules by converting molecular interactions into mechanical deformation and electrical signals.

This project covered the complete semiconductor device development cycle — from mask layout design, wafer-level fabrication process development, process optimization, metrology characterization, device packaging, and sensor validation.

The first step of this project was the design of the complete photolithography mask set.

I designed the device layout using semiconductor layout design software and developed multiple Cr masks required for different fabrication layers.

The mask design included:

• Cantilever structure geometry
• Electrode patterns
• Piezoresistor regions
• Metal contact pads
• Front-side device structures
• Backside silicon release windows

A total of six Cr masks were designed and fabricated for the complete MEMS process flow.

After mask design:

• Laser lithography was used for pattern generation
• Cr mask development was performed
• Pattern dimensions and alignment accuracy were verified using optical microscopy

Critical dimensions including cantilever length, width, electrode spacing, and sensing regions were optimized to improve device sensitivity.

The sensor was fabricated using Silicon-On-Insulator (SOI) wafer technology to achieve precise control of the suspended silicon structure.

Starting substrate:

• SOI wafer
• Device Si layer: ~5 µm
• Buried SiO₂ layer: ~0.5 µm
• Handle Si layer: ~560 µm

The complete fabrication process included:

1. Wafer cleaning and surface preparation
2. Front-side lithography patterning
3. Silicon device layer etching using Deep Reactive Ion Etching (DRIE)
4. Piezoresistive sensing structure formation
5. Thin film deposition and patterning
6. Metal electrode fabrication
7. Back-side lithography alignment
8. Deep backside silicon etching
9. Cantilever release process
10. Final device inspection and packaging

A major challenge was achieving a reliable suspended cantilever structure without mechanical failure.

I optimized several semiconductor fabrication parameters:

Lithography Optimization

• Exposure conditions
• Development time
• Pattern resolution
• Mask alignment accuracy

Dry Etching Optimization

• DRIE etching rate
• Etching uniformity
• Sidewall profile
• Selectivity between Si and SiO₂

Structural Optimization

Different cantilever geometries were designed and tested to improve mechanical response.

Parameters optimized:

• Cantilever dimensions
• Thickness
• Mechanical stiffness
• Stress distribution
• Deflection sensitivity

The objective was to maximize gas-induced mechanical response while maintaining device reliability.

Multiple semiconductor metrology techniques were used throughout development.

• Optical Microscopy: Lithography inspection, pattern transfer verification, etching quality analysis, defect inspection
• SEM Analysis: Microstructure verification, etched profile observation, released structure inspection
• Electrical Characterization: Resistance measurement, piezoresistor performance evaluation, sensor response testing

Process Failure Analysis

Root cause analysis was performed to identify fabrication challenges including:

• Over-etching
• Incomplete silicon release
• Pattern deformation
• Mechanical stress-induced failure

Fabrication parameters were modified based on metrology feedback.

After successful fabrication:

• Individual sensor chips were diced
• Devices were packaged
• Wire bonding was performed
• Electrical connection reliability was verified

The completed sensor chips were integrated into testing platforms for gas sensing experiments.

This project demonstrated my ability to develop a complete semiconductor MEMS device from concept to working prototype.

Key expertise developed:

This work represents a full semiconductor product development workflow:
Design → Mask Fabrication → Wafer Processing → Metrology → Optimization → Packaging → Functional Device

This project focused on developing high-performance organic photovoltaic (OPV) devices through systematic process engineering, device architecture optimization, and reliability investigation. The work followed an industry-style semiconductor development approach: design → fabrication → characterization → failure analysis → process improvement.

I developed and optimized multiple organic solar cell device architectures, including inverted and conventional structures with binary and ternary donor–acceptor blends.

Device stacks included:

• Transparent conductive oxide substrates
• Electron transport layers
• Donor-Acceptor bulk heterojunction active layers
• Hole transport layers
• Thermally evaporated metal electrodes

Different donor/acceptor materials, blend ratios, layer thicknesses, processing conditions, and interface layers were optimized to maximize power conversion efficiency (PCE), reproducibility, and device stability.

A Design of Experiments (DOE) strategy was implemented to understand the relationship between fabrication parameters and final device performance.

Process variables investigated:

• Active layer concentration
• Donor/acceptor ratio
• Additive concentration
• Spin coating speed
• Annealing temperature
• Annealing time
• HTL/ETL thickness optimization

Fabricated more than 100 experimental batches. Each batch contained 12–20 substrates (20 mm × 20 mm), with each substrate integrating 8 individual solar cells. Large-scale fabrication statistics enabled identification of process windows, yield improvement strategies, and reproducible high-efficiency conditions.

A process–performance correlation framework was developed to identify the dominant parameters controlling device efficiency.

Statistical analysis was performed between fabrication variables and photovoltaic parameters:

• Power Conversion Efficiency (PCE)
• Fill Factor (FF)
• Short-circuit current density (Jsc)
• Open-circuit voltage (Voc)

Correlation analysis revealed critical process contributors and guided further optimization of device fabrication conditions.

Long-term thermal stability experiments were performed by monitoring device performance degradation over time. When performance degradation was observed, a semiconductor-style root cause analysis workflow was applied:

Problem: Device efficiency and stability decreased after thermal aging.

Investigation: Advanced thin-film metrology was used to analyze degradation mechanisms:

• Active layer morphology evolution
• Interface degradation
• Film uniformity changes
• Material redistribution and phase separation

Root Cause Analysis: The degradation pathway was identified by correlating electrical performance loss with physical and material changes in the device stack.

Solution Strategy: Processing conditions and device structures were optimized to improve morphology stability, device lifetime, and fabrication reproducibility.

This project directly connects with semiconductor and thin-film manufacturing workflows:

• Thin-film deposition process optimization
• DOE-based process development
• Device fabrication and yield improvement
• Failure analysis and troubleshooting
• Materials characterization and reliability engineering
• Data-driven process control

The approach follows industrial engineering methodology used in semiconductor, display, photovoltaic, and advanced device manufacturing.

Based on this work, three peer-reviewed manuscripts are currently in preparation:

1. Process optimization and performance enhancement of organic solar cells
2. Thermal reliability testing and degradation mechanism analysis
3. Morphology-driven structure–property relationship and failure pathway investigation

Project Overview

Developed plasma-polymerized thin films through a complete semiconductor process development workflow including material selection, PECVD deposition, DOE optimization, advanced metrology, root cause analysis, and electrical/optical reliability validation.

The complete development strategy:
Material Design → Plasma Process Development → Thin Film Fabrication → Metrology Characterization → DOE Optimization → Reliable Device Integration

A custom PECVD system was developed for plasma polymer thin-film fabrication.

Major work:

• Designed experimental plasma deposition setup
• Controlled plasma polymerization process
• Optimized deposition parameters
• Fabricated metal–insulator–metal structures for electrical characterization

Different precursor materials were selected and investigated using a material engineering approach.

Performed:

• Molecular structure analysis
• Theoretical calculation
• Material property prediction
• Process parameter optimization using DOE

Optimized parameters: Plasma power, Pressure, Deposition condition, Film thickness, Processing window

Established: Material Structure → Plasma Chemistry → Film Property Relationship

SEM and AFM were used for metrology-driven process optimization.

• SEM: Surface morphology, Film uniformity, Defect inspection
• AFM: Surface roughness, Nanostructure analysis, Film quality evaluation

Root cause analysis identified morphology-related process variation and optimized PECVD conditions for improved repeatability.

EDS and XPS were performed to analyze: Elemental composition, Chemical bonding states, Surface chemistry, Plasma-induced chemical modification

Used these results to optimize chemical structure and improve film reproducibility.

Thermal reliability was investigated using TGA/DTA analysis.

Studied: Decomposition temperature, Thermal degradation pathway, Material stability, Temperature-dependent reliability

Established: Polymer Structure → Thermal Stability → Device Reliability

FTIR spectroscopy was used to investigate: Chemical bonding, Molecular vibration modes, Functional groups, Thickness-dependent structural changes

Used FTIR results to optimize plasma polymer network formation.

Optical metrology was performed to analyze: Optical absorption, Transparency, Electronic structure, Band gap engineering, Extinction coefficient

Correlation developed: Chemical Structure → Optical Properties → Device Performance

Electrical characterization investigated: Leakage current, Current density, Thickness effect, Charge transport behavior

Conduction mechanisms studied: Ohmic conduction, Space charge limited conduction, Schottky emission, Poole–Frenkel emission

Identified relationship: Film Chemistry → Trap States → Electrical Performance

AC electrical characterization evaluated: Dielectric constant, Capacitance response, Frequency-dependent behavior, Dielectric loss, Polarization mechanism

Optimized films showed improved dielectric stability and electrical reliability.

Successfully demonstrated a complete semiconductor thin-film engineering cycle:

Material Selection → Computational Prediction → PECVD Fabrication → DOE Process Optimization → SEM/AFM/XPS/EDS/FTIR/UV–Vis/TGA Electrical Metrology → Root Cause Analysis → Reliable Thin Film Process Development

Why I selected wearable sensors: There is a critical need for continuous health monitoring in everyday life. However, conventional rigid sensors are uncomfortable and lack the flexibility required for human motion. My research hypothesis was that flexible, nanomaterial-coated everyday fabrics could serve as high-performance, comfortable wearable sensors.

Selection of MWCNT + cotton fiber: Carbon Nanotubes (CNTs) were selected due to their high conductivity, high aspect ratio, and mechanical robustness.

Problem: CNTs suffer from severe aggregation and poor dispersion.

Solution: I applied citric acid and oxygen plasma functionalization to introduce COOH/OH functional groups. This created a highly stable CNT ink, optimizing the cotton coating mechanism for uniform, durable sensor layers.

Material characterization was performed to understand the CNT-fiber interaction and pressure sensing mechanism.

SEM Analysis: Used for cotton fiber morphology, CNT coating uniformity, surface coverage analysis, and fiber network structure after compression.

TEM Analysis: Used for CNT morphology confirmation, nanotube network formation, and conductive pathway analysis.

SEM/TEM results confirmed successful CNT attachment on cotton fibers, forming a conductive nanoscale network.

Initial Challenge: Sensor performance depended strongly on mechanical compression behavior and CNT network formation.

Root cause investigation: Using SEM/TEM and electrical-force characterization, I identified that sensor response originated from:

• Increased CNT-to-CNT contact points during compression
• Reduced tunneling distance between CNT networks
• Increased conductive pathways
• Larger fiber contact area under applied pressure

Pressure applied ➔ Fiber compression ➔ CNT network connection increases ➔ Resistance decreases ➔ Electrical signal generated

This mechanism study enabled optimization of sensitivity and repeatability.

Electrical and mechanical validation included: Pressure sensitivity measurement, Loading/unloading response, Response and recovery time, Hysteresis analysis, Repeatability testing, and Long-term durability (>11,000 cycles).

Optimization improved:

• Sensitivity
• Response stability
• Mechanical reliability
• Wearable compatibility

After sensor optimization, multiple sensors were integrated into a shoe platform for real-time biomechanical monitoring.

Sensor locations were selected to capture: Heel pressure, Toe pressure, Forefoot loading, Pressure shifting, Center of gravity movement.

Human Motion Validation: The smart shoe platform was tested during Standing, Walking, Running, Jumping, and Different activity patterns.

The system successfully monitored: Step cycle, Ground contact force variation, Loading/unloading behavior, Gait characteristics, Body movement patterns.

The platform was further expanded toward: Self-powered wearable sensors, Energy harvesting from body motion, Breathing monitoring, Muscle activity sensing, Human activity classification.

Machine learning analysis: Feature extraction, MRMR feature selection, Activity classification.

Project Outcome:

• Developed complete wearable pressure sensor platform
• Established CNT composite material optimization process
• Performed DOE-based sensor improvement
• Used SEM/TEM-driven root cause analysis
• Integrated sensor into functional smart shoe system
• Demonstrated real-world human activity monitoring

Watch the carbon nanotube functionalization process and the smart shoe in action.

Detailed findings from this project have been published in the following peer-reviewed journals:

• Journal of Sensors (2019)
• Surface and Coatings Technology (2020)
• Analysis & Sensing (2024)

Ask yourself one question: When was the last time you measured your blood pressure?

For most people, the answer is only during a doctor visit or occasional home measurement. However, between those measurements, we do not continuously know our cardiovascular condition.

Conventional cuff-based BP monitors are accurate, but they are bulky, uncomfortable, and not accessible anytime and anywhere. This limitation creates the need for a wearable solution that can continuously support cardiovascular health monitoring.

Commercial smart rings such as Oura Ring and other wearable rings have created a new generation of health monitoring by tracking heart rate, sleep, activity, and physiological trends.

However, existing smart rings still cannot directly measure absolute systolic and diastolic blood pressure because true BP measurement requires applying controlled pressure to interact with and partially occlude the artery.

Without controlled mechanical squeezing of the artery, direct cuff-like BP measurement is extremely challenging.

We developed a smart ring with a custom-designed mechanical actuator capable of applying controlled radial pressure on the finger to occlude the digital artery.

This approach miniaturizes the fundamental principle of conventional cuff-based BP measurement into a wearable ring platform.

Key developments:

• Custom mechanical actuator design
• Controlled finger artery compression mechanism
• Pressure and optical sensing integration
• Signal processing for systolic and diastolic BP estimation

This actuator concept can potentially be integrated into future commercial smart ring platforms to enable direct BP monitoring.

Several mechanical actuation approaches were designed and evaluated to achieve a compact wearable system.

More than five actuation concepts were investigated with a focus on:

• Miniaturization
• Wearability and comfort
• Repeatable pressure control
• Reliable artery interaction
• Compatibility with commercial smart ring architecture

The mechanical components of the smart ring were designed using CAD software and fabricated via high-resolution 3D printing. The systematic development and optimization process included:

• CAD design of individual ring components separately
• Design optimization for assembly and functionality
• Optimization of 3D printing layer thickness for high precision
• Optimization of UV curing processes to ensure structural integrity
• Achieving mechanically stable and reusable parts
• Final assembly of the fully optimized ring for continuous wear and use

The prototype was evaluated through human studies.

Data were collected from more than 25 subjects, and ring-estimated systolic pressure (SP) and diastolic pressure (DP) were compared with conventional cuff BP measurements.

The results demonstrated the feasibility of wearable ring-based BP monitoring with user maneuvers, reducing dependency on occasional cuff measurements.

This work provides a pathway from traditional intermittent BP measurement toward convenient wearable cardiovascular monitoring.

The project combines:

• Wearable medical device design
• Mechanical actuator engineering
• Sensor integration
• Human physiological measurement
• Data analysis and validation

The technology has potential applications for next-generation smart rings and continuous cardiovascular health platforms.

In this project, I investigated the structural, electronic, and optical properties of copper-based thin films (Cu3N and Cu-TiC) through a combination of computational modeling and experimental validation.

Using the Density Functional Theory (DFT) framework with the Local Density Approximation (LDA) and the Hubbard (U) term, I modeled the crystal structures, band structures, and electron density distributions of these materials. The simulations revealed strong hybridization of atomic orbitals in the valence band regions, which significantly influenced the optical and electronic characteristics.

To validate these theoretical models, thin films were deposited on Si substrates using the DC magnetron sputtering technique. Experimental optical characterizations were then performed to extract the optical bandgap and refractive index, which were subsequently compared against the theoretical approximations.

A comprehensive first-principles study was conducted on Cu3N thin films using Density Functional Theory (DFT) with the Local Density Approximation and Hubbard (U) term (LDA + U). Band structure analysis revealed strong hybridization of Cu 3d and N 2p orbitals due to their antibonding states in the near-valence band region. Experimentally, DC magnetron-sputtered Cu3N films exhibited a polycrystalline structure with a nonstoichiometric phase. The experimentally determined optical band gap (1.44 eV) and refractive index (2.14) showed excellent agreement with our theoretical approximations.

We investigated the structural geometry and electronic properties of Cu-TiC thin films using the DFT framework with LDA. The simulated Cu-TiC band structure demonstrated strong hybridization across Cu-Ti, Cu-Cu, and Cu-C valence band regions. To validate these findings, thin films were deposited on Si via co-sputtering of Cu, Ti, and C targets using DC magnetron sputtering. While the experimentally derived lattice constant aligned closely with the theoretical model, the measured optical bandgap (2.09 eV) highlighted variations from the simulated value (1.10 eV) obtained via linear optical response theory.

We performed First-principles Density Functional Theory (DFT) calculations using the Cambridge Serial Total Energy Package (CASTEP) to investigate the structural, electronic, and optical properties of CuO and Zn-doped CuO thin films. The computed electronic band structures, alongside Total and Partial Density of States (TDOS/PDOS), revealed a significant transition in the band gap upon Zn doping. Optical simulations demonstrated that both materials are transparent with a small energy gap and exhibit maximum reflectivity in the infrared region. The calculated lattice parameters and band gap values were in good agreement with our experimental data.