LPCVD Nitride vs. PECVD Nitride for MEMS

LPCVD silicon nitride and PECVD silicon nitride are the two primary methods used to deposit silicon nitride films during MEMS fabrication, sensor manufacturing, and semiconductor wafer processing. In MEMS, silicon nitride serves as an etch mask, a passivation layer, an electrical isolation film, and the structural material of membranes and beams. Both deposit silicon nitride from gas-phase precursors, but one process uses a high-temperature furnace while the other uses a low-temperature plasma. That single difference determines film composition, hydrogen content, residual stress, conformality, barrier quality, thermal budget, and where each film belongs within a MEMS fabrication flow.

Choosing the right nitride is often one of the most consequential thin-film decisions an engineer makes, because it influences masking, etch selectivity, membrane behavior, passivation quality, and long-term device reliability.

Same Material, Two Different Processes

Both films are silicon nitride deposited onto the wafer surface, so the distinction is not growth versus deposition but rather furnace chemistry versus plasma chemistry. LPCVD builds a dense, near-stoichiometric, nearly hydrogen-free film at high temperature. PECVD builds a hydrogenated film with tunable properties at low temperature. Nearly every practical difference that follows, from residual stress to which substrates each film can coat, traces back to that temperature and energy source.

How LPCVD Nitride Works

Low Pressure Chemical Vapor Deposition (LPCVD) nitride is a high-temperature deposition process performed in a low-pressure hot-wall furnace. Wafers are loaded vertically into the furnace tube, where dichlorosilane (SiH₂Cl₂) and ammonia (NH₃) react at approximately 750 to 850 °C to deposit silicon nitride. The low pressure and elevated temperature drive a uniform, surface-controlled reaction across the entire load.

Because the reaction is thermally driven rather than line-of-sight, LPCVD nitride is highly conformal. It coats every exposed surface, follows underlying topography faithfully, and deposits on both sides of the wafer and the wafer edge. The resulting film is dense, near-stoichiometric Si₃N₄ that contains very little hydrogen and forms an exceptionally effective barrier against moisture, oxygen, and mobile-ion contamination.

Standard stoichiometric LPCVD nitride carries high tensile stress, which limits how thick the film can be deposited before cracking. By adjusting the gas ratio toward a silicon-rich composition, the tensile stress can be reduced substantially, producing a low-stress nitride suited to thick films and suspended membranes.

Stoichiometric and Low Stress Nitride

Rogue Valley Microdevices offers both film types from a single stable nitride process. Stoichiometric LPCVD silicon nitride is supplied with a tensile film stress at or above 800 MPa and a refractive index of 2.00 ± 0.05 at 632.8 nm. Low-stress LPCVD nitride is supplied with a tensile film stress below 250 MPa and a refractive index of 2.20 ± 0.05 at 632.8 nm. Targeted film stress between these values is available by tuning the gas ratio to match a specific application.

How PECVD Nitride Works

Plasma Enhanced Chemical Vapor Deposition (PECVD) nitride is a low-temperature deposition process performed inside an RF plasma reactor. Silane (SiH₄) together with ammonia (NH₃) or nitrogen (N₂) is introduced into the chamber, where radio-frequency power generates a plasma that dissociates the precursor gases into highly reactive species. These species react at the wafer surface to deposit a silicon nitride film at approximately 200 to 400 °C.

Because the plasma supplies the activation energy, deposition occurs well below the temperature limits of aluminum interconnects, temperature-sensitive materials, and completed device structures. The deposited film is a silicon-rich, hydrogenated nitride (SiNₓ:H) that contains significant bonded hydrogen and is slightly less dense than LPCVD nitride. In exchange, its film stress can be tuned from compressive to tensile through deposition parameters, and its refractive index can be adjusted for optical applications.

Because the wafer rests on a temperature-controlled chuck or electrode that shields the backside from the plasma, PECVD nitride is typically a front-side process. Rogue Valley Microdevices deposits PECVD silicon nitride in thicknesses up to 2 µm at low temperature, with a customized refractive index or film stress, and also offers PECVD silicon oxynitride (SiON) up to 4 µm for optical and dielectric applications.

How the Two Processes Differ

Mechanical and Barrier Properties

LPCVD nitride is valued for its mechanical robustness, chemical resistance, and barrier performance. Its density and near-stoichiometric composition make it an excellent diffusion barrier and a durable etch mask, and its well-controlled stress makes it the standard choice for suspended membranes and other released structures. PECVD nitride provides reliable passivation, environmental protection, and electrical insulation, with the added flexibility of tunable stress for managing the mechanical behavior of completed devices.

Optical Properties

Both films are useful in optics, but for different reasons. LPCVD nitride offers a stable, repeatable refractive index near 2.0 and very low defect density, which supports waveguides and other photonic structures. PECVD nitride and PECVD silicon oxynitride offer a refractive index that can be tuned through deposition chemistry, which makes them well suited to anti-reflective coatings and made-to-order optical layers deposited at low temperature.

Step Coverage and Topography

LPCVD nitride deposits conformally and follows fine, high-aspect-ratio topography closely, which is why it is preferred for masking and lining demanding features. PECVD nitride provides good coverage over moderate topography and is well suited to blanket passivation and interlayer dielectric layers. For features that demand the highest conformality, engineers may also evaluate ALD processes.

Process Integration

Many production MEMS and sensor flows use both nitrides. LPCVD nitride is deposited early, before metal, to serve as an oxidation mask, diffusion barrier, etch stop, or structural and membrane layer. Once high-temperature processing is complete, PECVD nitride is deposited to passivate and encapsulate the finished device, protect against moisture and handling damage, or provide a low-temperature dielectric over metal. Combining the two lets engineers leverage the strengths of each while respecting the thermal limits of completed structures.

LPCVD Nitride vs. PECVD Nitride at a Glance

Rogue Valley Microdevices Nitride Film Options

Rogue Valley Microdevices produces both LPCVD nitride film types from a single stable nitride process, along with low-temperature PECVD nitride and silicon oxynitride. The charts below summarize the standard specifications for each film.

Stoichiometric LPCVD Nitride

Low Stress LPCVD Nitride

PECVD Silicon Nitride and Silicon Oxynitride

Targeted Stress LPCVD Nitride

Many MEMS structures perform best at a specific film stress rather than at the extremes of the stoichiometric or low-stress recipes. Because film stress in LPCVD nitride is governed by the silicon-to-nitrogen ratio, Rogue Valley Microdevices can tune the gas ratio within its single stable nitride process to deliver a targeted film stress between the low-stress and stoichiometric values. The same adjustment shifts the refractive index, which moves predictably from about 2.00 for a stoichiometric film toward about 2.20 as the film becomes more silicon-rich.

A targeted-stress film is valuable when a suspended membrane, cantilever, or other released structure must hold a particular curvature, flatness, or resonant frequency that a standard recipe does not provide. Rogue Valley Microdevices develops targeted-stress nitride to a customer specification and can include a stress monitor wafer for films thicker than 2,000 Å to verify the delivered stress.

Targeted Stress Specifications

Typical Applications

MEMS Application Examples

Choosing the Right Nitride

Choose LPCVD nitride when conformality, film density, barrier performance, oxidation masking, etch selectivity, or controlled membrane stress are the primary requirements, and when the wafer can tolerate furnace temperatures. Choose PECVD nitride when thermal budget is the limiting factor, when the film must be deposited over metal or completed device structures, or when tunable stress or a tunable refractive index is needed.

1. Can the wafer tolerate furnace temperatures near 750 to 850 °C? If not, choose PECVD nitride.
2. Does the film need to coat both sides, line fine topography, or serve as an oxidation mask or diffusion barrier? Choose LPCVD nitride.
3. Is the film a suspended membrane or other released structure? Choose low-stress LPCVD nitride.
4. Is the objective passivation, encapsulation, or an optical coating over a finished device? Choose PECVD nitride.
5. Remember that many successful flows use both nitrides at different stages.