Plasma-Enhanced CVD of Silicon Nitride




Plasma enhanced techniques are often employed to deposit silicon nitride, since unlike silicon dioxide there are no thermal approaches which operate below 600 °C. Deposition is usually performed in showerhead configuration reactors, operating at from a few hundred mTorr to a few Torr. Temperatures of 250 to 400 °C are typically employed. Ammonia is the most common oxidant, although pure nitrogen is also used.

Chemistry



At low power density and low ammonia concentrations, the gas phase is dominated by SiH2, SiH3, and polymerized silanes. In this case, nitrogen must be added at the surface. Films produced under these conditions have copious Si-H and are relatively poor insulators. Increasing the power density and the ammonia:silane ratio produces triaminosilane, Si(NH2)3 in the gas phase, with lesser amounts of hydrogenated species. Films deposited from triamonisilane are almost free of Si-H bonds (though N-H is still present) and have excellent electrical properties. The conditions of the reactor can be easily diagnosed by monitoring the Si2H6 signal in the exhaust by mass spectroscopy. As ammonia is increased the amount of Si2H6 will decrease rapidly; optimal operating conditions are obtained when this signal decreases to nearly zero, in combination with a maximum in deposition rate. The same technique can be used with N2O or N2 as oxidants. The total hydrogen content in the film is controlled primarily by the temperature.

reference: “ Controlling the plasma chemistry of silicon nitride and oxide deposition from silane" D. Smith, J. Vac. Sci. Technol. A11 1843 (1993)

Stress



Nitrides are often under large tensile stress, presumably resulting from the elimination reactions (Si-NH + Si-NH2 => Si-N-Si + NH3, etc.) which form the film. However, in plasma deposition, ion bombardment can be used to densify films and make them more compressive. The stress is influenced by the gas mixture, deposition rate, temperature, and ion bombardment. Ion bombardment is most easily adjusted by using dual frequency reactors, and this approach is often employed for facile adjustment of stress of deposited nitrides. Here the low-frequency component of the plasma excitation is employed to change the plasma sheath voltage somewhat independently of the electron density, so that the ion energy distribution can be changed with modest effects on the plasma chemistry.

reference: "Advantages of Dual Frequency PECVD for Deposition of ILD and Passivation Films" E. van de Ven, I. Connick and A. Harrus, VMIC 1990

Applications



Plasma nitride films are almost universally used for final passivation layers in IC fabrication, often in conjunction with a deposited oxide or PSG layer. Plasma nitride films are effective sodium barriers, with deposition temperatures compatible with aluminum or copper metallization. In this application, stress control is important to avoid film cracking and degradation of metal reliability. Control of total hydrogen and bonding is also important to avoid hot-electron reliability problems. The highest temperature compatible with the metallization must be used to ensure good barrier properties. PECVD silicon nitride is also often employed for passivation of GaAs MESFETs, to avoid oxidation of the exposed GaAs surface (since GaAs oxides are stable in the presence of the substrate).

Plasma nitride films are also used as stop layers for dual Damascene processing. In this technique, trenches and via holes are etched in two separate steps into an interlayer dielectric, and then filled with metal to form wires and interlevel connections. The nitride layers are used to ensure that the trench depth is constant across a wafer, independent of variations in etch rate. However, the use of silicon nitride, which has a high dielectric constant and by the nature of the process is located at the corners of features (where electric field lines concentrate), leads to increased interline capacitance. This problem may be addressed by a more complex etch process which displaces the nitride layer away from the via-line intersection, or by replacing the nitride with a plasma-deposited silicon carbide layer.

Another important area of application of plasma deposited silicon nitride is for the gate dielectric in active-matrix liquid crystal displays (AMLCD's). These displays employ MOS transistors fabricated on glass substrates to locally control access to liquid crystal cells, and are universally used to achieve high-resolution color displays in laptop computers and other portable, low power applications. In this application, very low RF powers are employed, and a large hydrogen concentration (bonded to nitrogen) is desirable to ensure good stability of the interface with the amorphous silicon which forms the transistor active layer. Plasma nitride is often sandwiched with an anodic oxide or deposited silicon dioxide to obtain improved electrical insulation.


Still more old references:
"Sodium diffusion in plasma-deposited amorphous oxygen-doped silicon nitride (a-SiON:H) films" J. Osenbach and S. Voris, J. Appl. Phys. 63 4494 (1988)
"Dependence of Electromigration Lifetime for Via Chains on Slope Angles of Via Holes" H. Nishimura, Y. Okuda, and K. Yano, J. Electrochem. Soc. 142 3565 (1995)
"BLOK: a low K Dielectric Barrier/Etch Stop Film for Copper Damascene Applications", P. Xu et. al., IITC 1999 p. 109
"Gate dielectric and contact effects in hydrogenated amorphous silicon - silicon nitride thin film transistors" N. Lustig and J. Kanicki, J Appl Phys 65 #10 3951 (1989)
"PECVD Nitride as a Gate Dielectric for Amorphous Silicon Thin Film Transistor" Y. Kuo, J. Electrochem. Soc. 142 186 (1995)


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