PECVD vs LPCVD silicon nitride: which film for which job

Silicon nitride turns up in almost every process flow: as a hard mask for KOH etching, a membrane for MEMS and X-ray windows, a waveguide core in photonics, a gate or capacitor dielectric, and a passivation layer on finished devices. Two deposition methods dominate in research and pilot fabs.

Low-pressure chemical vapour deposition (LPCVD) in a hot-wall furnace gives dense, stoichiometric films at high temperature. Plasma-enhanced CVD (PECVD) in a parallel-plate reactor gives films at a few hundred degrees that can go on almost any substrate. They are not interchangeable, and choosing the wrong one is a common source of failed membranes and leaky masks. This article compares the two and what each means when you are buying a used deposition tool or furnace.

How each process works

LPCVD nitride is deposited in a horizontal or vertical furnace tube at reduced pressure, well below atmospheric. The classic chemistry is dichlorosilane (SiH2Cl2) with ammonia (NH3) at temperatures typically in the region of 700–800 °C. Wafers stand in a boat, often in large batches, and the film grows on both sides of every wafer. Because the reaction is driven by temperature rather than plasma, coverage over topography is very uniform.

PECVD nitride is deposited in a vacuum chamber where an RF plasma breaks down silane (SiH4) with ammonia and/or nitrogen. The substrate sits on a heated electrode, typically at 250–400 °C. Deposition is single-sided and one wafer or a small batch at a time. The plasma supplies the energy that the furnace supplies thermally, which is why the temperature can be so much lower.

The two also make different demands on a building. A furnace bank needs a large, steady power supply for the heaters, a scavenger or exhaust around the tube ends, and a gas cabinet for dichlorosilane. A PECVD tool is smaller, but silane is pyrophoric and needs its own ventilated cabinet, leak detection and interlocks. Both need exhaust treatment suited to their by-products.

Film properties side by side

The figures here are typical ranges. Actual values depend on the recipe and the tool.

Stress and why it matters

Stress is the property that most often decides between the two.

Stoichiometric LPCVD nitride has high tensile stress, which limits how thick a film can be before it cracks or bows the wafer. For free-standing membranes, labs use silicon-rich LPCVD nitride, made by increasing the dichlorosilane-to-ammonia ratio. The excess silicon brings the stress down to a level where large, thin membranes survive release. This low-stress nitride is a standard MEMS film.

PECVD nitride stress depends on the plasma conditions. On dual-frequency systems, high-frequency RF tends to produce tensile films and low-frequency RF produces compressive films, because of the different ion bombardment. Mixing the two lets you tune stress close to zero. This makes PECVD useful for thick passivation and for stacks on substrates that cannot go into a furnace. If you plan to buy a used PECVD tool for stress-controlled films, check whether it has a low-frequency generator and that the switching or mixing works.

Hydrogen, density and etch resistance

PECVD nitride contains bonded hydrogen from the silane and ammonia, in Si-H and N-H bonds. Hydrogen lowers the film density, increases the wet etch rate in buffered HF and can outgas or shift during later high-temperature steps. For passivation on finished devices, this is usually acceptable. For a mask that must survive long etches, it can be a problem.

LPCVD nitride has much less hydrogen and is dense, with a low etch rate in HF. That is why it is the standard mask for anisotropic KOH or TMAH etching of silicon, where a pinhole or a slow erosion of the mask ruins the wafer. PECVD nitride can serve as a KOH mask for short etches if deposited carefully, but expect higher pinhole density and a shorter safe etch time.

The same hydrogen matters in integrated photonics. N-H bonds absorb in the near infrared close to the 1,550 nm telecom band, so waveguides made from hydrogen-rich PECVD nitride tend to show higher propagation loss there. Low-loss nitride photonics usually relies on LPCVD films, sometimes with a high-temperature anneal, while PECVD remains attractive where the thermal budget rules out a furnace.

Refractive index is a useful quick check for both. Stoichiometric nitride sits close to 2.0 at visible wavelengths, while silicon-rich films have a higher index. A reflectometer or ellipsometer measurement on a monitor wafer catches drifting recipes early.

When to choose which

Choose LPCVD when:

• The substrate can take the temperature and has no metal on it yet
• You need a dense mask for KOH or long HF exposure
• You need low-stress membranes with good uniformity
• Both sides of the wafer can or should be coated
• You run enough wafers to fill a batch

Choose PECVD when:

• The wafer already carries metals, polymers or temperature-sensitive layers
• You need passivation late in the process
• You need to tune stress for thick films or stacks
• You need single-sided deposition
• You run single wafers or pieces in small numbers

Many research cleanrooms keep both: an LPCVD furnace for masks and membranes early in the flow, and a PECVD tool for back-end dielectrics.

What to check on a used tool

For a used LPCVD furnace, such as a Tystar Tytan or a Tempress system:

• Heater element condition and temperature profile along the flat zone
• Quartz tube, boats and end caps; nitride builds up and eventually flakes
• Vacuum system, including the pump's tolerance of ammonium chloride by-products
• Door seals, exhaust heating and any cold traps for by-products
• Gas cabinet and lines for dichlorosilane and ammonia

For a used PECVD system, such as an STS 310PC:

• RF generators and match, including the low-frequency supply if fitted
• Heater and electrode uniformity
• Showerhead condition and chamber clean recipe (usually a fluorine-based plasma clean)
• Pump and exhaust, which see silane and its by-products
• Silane gas handling and safety interlocks

In both cases, ask for a test deposition with thickness, uniformity and refractive index measured.

What we can do

We inspect, refurbish and service PECVD systems and LPCVD furnaces, including tube and boat replacement, heater profiling and RF checks. Our preventive maintenance plans cover the chamber cleans and by-product management that keep nitride processes stable. Tell us the films you need, and we will suggest a tool and recipe starting point.