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What is Silicon Nitride? How Does It Apply onto Silicon?

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  • icon2 January 7, 2024
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Silicon nitride is a highly advanced ceramic made of silicon and nitrogen atoms bonded together, with the chemical formula Si3N4. This stuff displays some really impressive strengths and capabilities that make it useful for applications where you need to withstand intense heat, pressure, and wear over long periods.

Some key properties of silicon nitride include:

  • Extremely high hardness and wear resistance
  • High strength at both low and high temperatures up to 1000°C
  • Good thermal shock resistance
  • Good chemical resistance and oxidation resistance
  • Low thermal expansion
  • Electrically insulating

These unique properties make silicon nitride suitable for applications where high temperature, wear, and corrosion resistances are critical.

*If you are looking to buy high quality silicon wafers with nitride films, you can check out WaferPro's silicon nitride wafer product page.

How is Silicon Nitride Produced?

There are two main commercial processes used to produce silicon nitride:

Reaction Bonding

In reaction bonding, silicon powder is molded into the desired shape and reacted under nitrogen gas atmosphere at high temperatures around 1400°C. This nitrides the surface of the silicon particles creating a strong silicon nitride bonding layer.

The resulting microstructure contains some unreacted silicon, hence the process is sometimes referred to as reaction-bonded silicon nitride (RBSN).

Table 1 shows typical properties of reaction bonded silicon nitride:

PropertyValue
Density (g/cm3)2.4
Hardness (GPa)12 - 15
Strength (MPa)550
Max Use Temperature (°C)1150

Hot Pressing

In hot pressing, pure silicon nitride powder is compacted under heat and pressure resulting in a fully dense nitride ceramic with no residual silicon. This is often referred to as hot pressed silicon nitride (HPSN).

Table 2 shows typical properties of hot pressed silicon nitride:

PropertyValue
Density (g/cm3)3.2
Hardness (GPa)16 - 18
Strength (MPa)1000
Max Use Temperature (°C)1300

Hot pressed silicon nitride has higher purity and better properties but is more expensive to manufacture than reaction bonded silicon nitride.

What is Silicon Nitride Used For?

Silicon nitride has become an important advanced ceramic material used in many demanding applications thanks to its excellent properties. Some common uses include:

Automotive Industry

  • Glow plugs, rocker arm pads, turbocharger rotors and impellers that must withstand extreme temperatures and stresses.
  • Lightweight engine components such as air intake valves, oil retainers, timing belt pulleys for better fuel efficiency.

Cutting Tools

The high hardness and temperature resistance make silicon nitride well suited to metal cutting applications such as:

Bold

  • End mills and drills for machining superalloys, titanium and other tough materials. Lasts up to 20 times longer than carbide tools.
  • Wire drawing dies for manufacturing steel cables and wires.

Bearings

Silicon nitride balls and rollers have good wear resistance and fatigue strength for use in electric motors, turbines, machine tools and other equipment as hybrid bearings alongside steel components.

Electronic Applications

As a high strength electrical insulator able to withstand high temps, silicon nitride has uses such as heating elements in plasma etching equipment for electronics fabrication.

Medical Industry

The biocompatibility and chemical resistance allows use for percutaneous devices as well as the ceramic hip joint balls and sockets.

How is Silicon Nitride Applied Onto Silicon?

a Silicon Nitride Wafer
A Silicon Nitride Wafer

In semiconductor device fabrication, thin layers or films of silicon nitride can be directly applied onto silicon wafers in processes including:

Low Pressure Chemical Vapor Deposition (LPCVD)

In LPCVD, gaseous silicon containing precursors like silane (SiH4) and ammonia (NH3) are fed into a chamber under vacuum conditions. The molecules thermal decompose and react on the heated wafer surface to deposit a uniform and conformal silicon nitride film with precisely controlled thickness and composition.

Temperatures around 700 - 900°C are typically used to achieve deposition rates practical for manufacturing. By tuning the process conditions like temperature, pressure and gas flow rates, the silicon nitride film properties such as stress and hydrogen content can be optimized for the application.

Plasma Enhanced CVD (PECVD)

PECVD similarly deposits silicon nitride films from precursor gases but utilizes a radio frequency plasma to provide activation energy for the deposition rather than solely thermal energy.

This allows depositions at much lower temperatures of 250 - 400°C ideal for temperature sensitive materials. However, the films tend to have higher hydrogen content and lower density and conformality compared to LPCVD.

Oxidation of Silicon in Ammonia

Heating silicon wafers directly in an atmosphere of pure ammonia (NH3) at 900 - 1200°C results in oxidation where nitrogen from the ammonia reacts with the silicon surface forming a silicon nitride layer.

By controlling the temperature and exposure times, this direct nitridation process can produce films ranging from a few angstroms to a micron thick.

Applications Exploiting the Silicon/Silicon Nitride Interface

The combination of silicon nitride films deposited onto silicon substrates is utilized in many advanced devices across the electronics industry.

Silicon Nitride Passivation Films

Passivation refers to the use of a protective insulating layer that blocks charge carriers and prevents surface reactions while allowing desired electrical contact through openings. Silicon nitride is the most prevalent passivation material used in silicon semiconductor devices including:

Integrated Circuits

A plasma enhanced silicon nitride passivation layer is nearly universal in modern integrated circuits with submicron feature sizes. Conformal nitride layer prevents copper wiring corrosion as chips become more dense. It also serves as a barrier to mobile ion contamination and related reliability issues.

LEDs and Laser Diodes

Silicon nitride passivates LED and laser diodes made of materials like gallium nitride to enhance light output while protecting the optoelectronic regions from environment exposure.

Openings in the silicon nitride are strategically patterned where necessary for electrical contacts and light output.

Solar Cells

Silicon nitride deposited by PECVD provides excellent surface and sidewall passivation for silicon solar cells. This is improves cell efficiency by reducing surface recombination. The high quality dielectric also offers reflection and anti-reflection properties that maximize light capture and conversion efficiency in the silicon.

MNOS Memory Devices

Metal nitride oxide silicon (MNOS) devices take advantage of the interface between a very thin silicon nitride layer and the underlying silicon substrate to create memory cells for storage applications.

The thin layers of silicon nitride (5-50 nm) and oxide insulate a metal or polysilicon control gate from the silicon substrate channel. By applying a charge to the gate, electrons can be injected into and stored in traps near the silicon/nitride interface to represent a 1 or 0 bit value.

The stored charge remains even when power is removed enabling non-volatile memory behavior. MNOS memory offers high storage density and was used in early microprocessor firmware storage applications but has been replaced by more advanced technology.

Silicon Nitride Etch Masks

In microfabrication, materials are selectively removed from areas where they are not desired by etching processes. This requires masking off regions where etching is not wanted.

Silicon nitride's extreme chemical resistance allows it to function as an effective etch mask. Openings are patterned into the nitride layer by etching with fluorine based plasmas. The silicon nitride masks underlying materials during wet or dry etching exposures.

Once etching of the underlying material is complete, the nitride can be removed leaving only the desired etched patterns.

This enables selective patterning silicon on nanometer scales for applications such as trenches for advanced 3D transistor channels and fins forFinFETs.

Here a thin (50-100 nm) layer of LPCVD silicon nitride was deposited on silicon and patterned to remain only where needed to mask areas during silicon etching to form mechanical supports and channel structures.

Silicon Nitride Spacers

Silicon nitride films can be anisotropically etched to leave thin spacers along vertical sidewalls.

These spacers insulate adjacent structures separated on nanoscales while taking up minimal space. Key applications include:

  • Protecting sidewalls during subsequent processing steps.
  • Forming offsets enabling aggressive scaling of device dimensions.
  • Separating multiple gates in advanced transistors.

And many others. The high conformality and uniformity of LPCVD silicon nitride combined with directional RIE etching makes it ideal for forming nanoscale spacers where dimensions and reliability matter.

Conclusion

As an advanced ceramic material, silicon nitride enables extreme demands from turbine engines and ball bearings to machining titanium alloys thanks unique and extreme properties.

It’s unique compatibility with silicon processing makes silicon nitride a multifunctional films critical to modern semiconductors where it protects, insulates, masks, is patterned, selectively removed, and exploited for properties at interfaces and boundaries unattainable in bulk materials.

Applications spanning electronics to renewable energy to transportation rely on the material synergies of silicon nitride and silicon rendered through precision chemical vapor processes uniformly depositing films precisely where needed on nanoscales.

Ongoing research seeks to expand applications by further refining deposition techniques to coat silicon nitride onto exotic materials and exploring properties arising in new material systems.

FAQs about Silicon Nitride and Silicon

Is silicon nitride naturally occurring?

No, silicon nitride does not occur naturally. It is purely a synthetic compound made by chemical and manufacturing processes developed to exploit its useful properties.

What gives silicon nitride its strength?

Silicon nitride derives its high strength from the covalent bonding between the silicon and nitrogen atoms along with the crystalline structure. Strong atomic bonds resist dislocation movement which is the mechanism allowing most other materials to deform and fail.

Is silicon nitride brittle like other ceramics?

Unlike traditional ceramics, silicon nitride displays unusually high toughness thanks to some inherent ability to interrupt crack growth. However, it remains more brittle than metals and composites.

Can silicon nitride be machined like metals?

Yes, silicon nitride can be machined by conventional grinding, lapping and diamond cutting tools to achieve finishes, dimensional tolerance and turn complex shapes like metals. Avoid abrasives containing silicates that chemically attack silicon nitride.

How does thermal expansion compare between silicon and silicon nitride?

Silicon nitride has a very low thermal expansion coefficient that is around one-third that of silicon over a wide temperature range up to 1000°C. This helps minimize thermal stress and delamination effects at interfaces.

How pure can silicon nitride films be made?

LPCVD silicon nitride films with impurity levels less than 10 ppm have been synthesized for specialty applications requiring the highest electrical insulation properties. Most semiconductor and MEMS applications use films with > 99% purity.

Is silicon nitride hazardous to humans or environment?

No, silicon nitride is non-toxic, chemically inert in bulk form, and remains safely stable over exceptionally long periods making it suitable for biomedical and implant applications as well as environmentally friendly applications. Machining operations should control and contain fine particulates like other dusts.

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