Silicon Nitride in University Research

Introduction

Silicon nitride wafers are essential in academic research for various applications spanning materials science, engineering, and biomedicine. Their unique mechanical, thermal, and optical properties make them a versatile material for experimentation and innovation.

Universities use silicon nitride wafers extensively in research for various applications across engineering, materials science, and biomedical fields. Here are some of the key ways silicon nitride wafers are used in university research:

1. Micro- and Nano-Fabrication

• MEMS (Micro-Electro-Mechanical Systems): Silicon nitride is commonly used as a structural material or as a masking layer in MEMS devices because of its excellent mechanical and chemical properties.
• Sensors and Actuators: Silicon nitride is used in the fabrication of pressure sensors, accelerometers, and other micro-scale devices.

2. Optics and Photonics

• Waveguides: Silicon nitride has a high refractive index, making it ideal for creating waveguides in photonic circuits and optical communication systems.
• Microring Resonators: Researchers fabricate silicon nitride microrings for optical filtering and signal processing.

3. Material Science and Surface Chemistry

• Passivation Layers: Silicon nitride acts as a robust passivation layer to protect underlying silicon or other materials from oxidation and contamination.
• Etching Studies: Universities often study the etching behavior of silicon nitride in wet and dry etching processes to develop improved fabrication methods.

4. Biotechnology and Biomedical Applications

• Biosensors: Silicon nitride serves as a biocompatible surface for creating sensors that detect proteins, DNA, or other biological markers.
• Implants and Coatings: Thin films of silicon nitride are used in biomedical applications due to their chemical stability and resistance to biofouling.

5. Thin Film Research

• Deposition Studies: Silicon nitride is used in experiments involving chemical vapor deposition (CVD) and plasma-enhanced chemical vapor deposition (PECVD) to explore film uniformity, stress, and adhesion.
• Dielectric Properties: Its dielectric properties are studied for applications in capacitors and other electronic components.

6. Semiconductor Research

• Gate Dielectrics: In advanced transistor research, silicon nitride is used as a dielectric layer or in conjunction with other materials.
• Isolation Layers: It is used as an insulating layer in integrated circuits or for device isolation.

7. Quantum and Nanotechnology

• Quantum Optics: Silicon nitride wafers are used to fabricate micro-cavities and resonators for trapping light and studying quantum effects.
• Nanopore Technology: Researchers use silicon nitride membranes to create nanopores for DNA sequencing and molecular analysis.

8. Mechanical and Thermal Studies

• Stress and Strain Analysis: Universities study the mechanical properties of silicon nitride thin films, such as their tensile strength and stress levels.
• Thermal Conductivity: Research often involves exploring its thermal properties for heat dissipation in electronic devices.

9. Environmental Sensors

• Silicon nitride-based membranes and devices are used to detect gases, pollutants, and other environmental parameters due to their chemical inertness and stability.

10. Educational Purposes

• Hands-On Training: Students in engineering and physics programs use silicon nitride wafers for learning fabrication techniques like lithography, etching, and deposition.
• Experimental Setup: Labs use silicon nitride wafers for testing new designs and processes in a controlled environment.

If you're interested in any specific use case or application, I can provide more detailed insights or examples!

The specifications of substrates and silicon nitride (Si₃N₄) thicknesses vary based on the application, but here are the most common choices for research and industrial use:

Common Substrate Specifications

1. Material:

   • Silicon (Si): Most common substrate material for silicon nitride deposition.
   • Quartz (SiO₂): Used for optical applications due to its transparency.
   • Sapphire (Al₂O₃): Selected for high-temperature or high-stress environments.

2. Wafer Size:

   • 100 mm (4-inch): Widely used in research.
   • 150 mm (6-inch): Common in pilot-scale research and small production runs.
   • 200 mm (8-inch): Rare in academic labs, mostly industrial.

3. Crystal Orientation (for Silicon):

   • <100>: Standard orientation for general research and fabrication.
   • <111>: Used for anisotropic etching or mechanical strength studies.

4. Polishing:

   • Single-Side Polished (SSP): Typical for most applications.
   • Double-Side Polished (DSP): Preferred for optical alignment or applications where both sides are processed.

5. Doping and Resistivity:

   • Undoped (Intrinsic): Often used for electrical or optical research.
   • P-Type or N-Type: Specific dopants like boron or phosphorus are selected based on electrical needs.
   • Resistivity: High resistivity (> 10 Ω·cm) is common for MEMS and sensors to minimize parasitic effects.

6. Thickness:

   • Standard Thicknesses: 500 μm to 725 μm for mechanical robustness.
   • Special Applications: Thin wafers (~200 μm) are used in specific MEMS or optical setups.

Silicon Nitride Thicknesses

The thickness of silicon nitride depends on its intended use:

1. Thin Films (<100 nm):

   • Applications: Passivation layers, electrical insulation in transistors, and some optical coatings.
   • Common Thickness: 20–50 nm.

2. Medium Films (100–500 nm):

   • Applications: Dielectric layers, stress-engineered MEMS structures, and waveguides.
   • Common Thickness: 200–300 nm.

3. Thick Films (>500 nm):

   • Applications: Hard masks for etching, mechanical support structures, and photonic devices.
   • Common Thickness: 1–2 μm.

4. Ultra-Thick Films (>2 μm):

   • Applications: High-stress mechanical devices, deep reactive ion etching (DRIE) masks, and thermal insulation layers.
   • Common Thickness: 2–5 μm or more.

Deposition Methods and Properties

1. Low-Pressure Chemical Vapor Deposition (LPCVD):

   • Produces stoichiometric Si₃N₄ (higher density and stress).
   • Film Thickness: Typically 50 nm to 2 μm.

2. Plasma-Enhanced Chemical Vapor Deposition (PECVD):

   • Results in silicon-rich or hydrogenated silicon nitride.
   • Film Thickness: Typically 50 nm to 5 μm.

3. Stress Tailoring:

   • Low-Stress Silicon Nitride: Common for MEMS to reduce warping (200–500 MPa tensile stress).
   • High-Stress Silicon Nitride: For applications requiring rigidity and strength (up to 1 GPa tensile stress).

Popular Combinations

• Substrate: Silicon <100>, 4-inch, SSP, 500 μm thick, undoped, >10 Ω·cm resistivity.
• Nitride Thickness: 200 nm low-stress LPCVD Si₃N₄ for MEMS or 1 μm PECVD Si₃N₄ for optical devices.

If you need advice tailored to a specific research area, let me know!

Common Substrate Specifications

• Material: Silicon (most common), Quartz, Sapphire.
• Wafer Size: 100 mm (4-inch), 150 mm (6-inch), 200 mm (8-inch).
• Crystal Orientation: <100> for general use, <111> for specific etching or strength requirements.
• Polishing: Single-Side Polished (SSP) or Double-Side Polished (DSP).
• Thickness: Standard 500–725 μm; thin wafers (~200 μm) for specialized applications.
• Resistivity: High resistivity (>10 Ω·cm) for MEMS and sensors.

Common Nitride Thicknesses

Deposition Methods

• Low-Pressure Chemical Vapor Deposition (LPCVD): Produces high-density, stoichiometric Si₃N₄ films.
• Plasma-Enhanced Chemical Vapor Deposition (PECVD): Yields silicon-rich or hydrogenated films, useful for thick layers.
• Stress Tailoring: Options for low-stress (<200 MPa) or high-stress (>1 GPa) films based on application.