Overview
Silicon nitride films play a key role in MEMS and low-pressure chemical vapor deposition (LPCVD) enables the fabrication of low-stress silicon nitride films.
Applications in MEMS
Silicon nitride is widely used in MEMS as a structural layer, insulating layer, passivation layer, and hard mask. SiN is chemically resistant and hydrophobic, making it suitable as a passivation layer for MEMS pressure sensors and MEMS flow sensors. The bandgap of silicon nitride is about 5 eV, larger than many oxides, and it lacks donor and acceptor levels so it behaves as an insulator. SiN typically exhibits a resistivity on the order of 1e14 Ω·cm and a dielectric strength around 1e7 V/cm, allowing it to function as an insulating layer. It also has low thermal conductivity, about 20 W/(m·K), and a high elastic modulus, around 250 GPa, which makes it suitable for composite support layers with SiO2.
Silicon Nitride Windows for TEM
Silicon nitride windows are a direct application of low-stress SiN films. Although the material volume used is small, the unit price is high. Compared with traditional copper TEM grids, silicon nitride support windows offer higher melting point, stronger chemical inertness, and higher strength, and are used for in situ heating, liquid cell, or carbon-containing sample TEM observation. Typical film thicknesses on silicon nitride windows required by the market range from 20 nm to 200 nm, with stresses close to zero. How can LPCVD be used to produce low-stress or near-zero-stress silicon nitride films?
What Is LPCVD
LPCVD stands for low-pressure chemical vapor deposition. The main difference from atmospheric pressure CVD (APCVD) is the operating pressure: LPCVD typically operates at 10-1000 Pa, while APCVD runs at about 101.3 kPa.
LPCVD Silicon Nitride Deposition
Stoichiometric Si3N4 is typically deposited by LPCVD at temperatures of 700-900 °C and pressures of 200-500 mTorr. Films produced by this stoichiometric process often have high tensile stress above 1 GPa, which can cause cracking and delamination in films with thicknesses on the order of hundreds of nanometers. To obtain LPCVD SiN films thicker than ~100 nm that do not suffer from high stress, low-stress films are required. These are commonly silicon-rich silicon nitride films produced by increasing the silicon content in the deposition process. The silicon excess can be achieved by increasing the SiH2Cl2 (DCS) to NH3 ratio during deposition; in general, a higher DCS/NH3 ratio reduces film stress. For example, with a DCS/NH3 ratio of 6:1, a deposition temperature of 850 °C, and a pressure of 500 mTorr, the deposited film approaches zero stress.
Typical LPCVD Deposition Steps
The deposition process can be summarized as follows:
- Introduce reactant gases with specified compositions and flow rates into the reaction chamber, along with an inert carrier gas. In experiments, SiH2Cl2 (DCS) and NH3 are used as reactants, with N2 as the inert gas.
- Reactant gases flow toward the substrate.
- The substrate adsorbs the reactants.
- Adsorbed atoms migrate and undergo surface chemical reactions to form the film.
- Gaseous reaction byproducts are exhausted from the reaction chamber.
Figure: Schematic of horizontal and vertical LPCVD furnaces.
Process and Equipment Considerations
Two points should be noted during deposition: 1) Temperature differences of up to 20 °C between the furnace tube inlet, outlet, and center can cause large film stress variations across wafers processed in the same furnace. Increasing the center temperature can reduce wafer-to-wafer stress differences to below 3%. 2) Gas concentration decreases farther from the inlet, resulting in reduced deposition thickness at positions far from the inlet. Increasing the reactant gas flow rate can mitigate this.
Compared with horizontal LPCVD systems, vertical LPCVD systems offer several advantages: 1) a double-tube process chamber that improves gas flow uniformity; 2) lower particle generation, easier control, and longer maintenance intervals; 3) lower oxygen concentration in the reaction chamber and thinner native oxide formation. Structurally, the process chamber uses a double-tube design with a straight inner tube to improve gas flow uniformity, and wafer boat rotation is added to improve gas concentration uniformity across wafer surfaces.
