Composition and Properties of PECVD Silicon Nitride Films Deposited from SiH 4 , N 2 , He Gases

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COMPOSITION AND PROPERTIES OF PECVD SILICON NITRIDE FILMS DEPOSITED FROM SiH 4 , N2 , He GASES J.H. Souk, G.N. Parsons and J. Batey IBM Research Division T.J. Watson Research Center Yorktown Heights, NY ABSTRACT Amorphous silicon nitride films deposited from a gas mixture of SiH 4 and N2 with a large flow of He have shown many interesting characteristics. The films show a wide variety of electrical, optical, and mechanical properties with varying amounts of SiH 4 and N2 . The effect of N2 flow rate on film composition in N2-SiH 4 processes is quite different from that of NH3 flow in NH3 -SiH 4 processes. The films were characterized by measurements of (1) Si-H and N-H bond density and bonded hydrogen content, both from infrared absorption, (2) Si/N ratio, (3) refractive index, (4) film stress, and (5) wet chemical etch rate and (6) electrical properties including current-voltage (I-V) and capacitance-voltage (C-V). We find that adding helium to the PECVD process enhances the incorporation of nitrogen in the film and an optimized flow of SiH 4 improves the electrical properties. Films with optimum electrical properties with minimum charge trapping are obtained with N/Si ratio close to 1.33. These films have a small amount of Si-H and N-H bonds, and a low etch rate (< 100 A/min) in aqueous HF solution. The properties of these low temperature (250 0C) PECVD nitrides have many similarities with LPCVD nitrides. Compared with films deposited from SiH 4 , NH3 mixture, these films exhibit very low wet etch rates and much lower H contents, but greater hysteresis in C-V characteristics.

INTRODUCTION Silicon nitride is used for a variety of applications in silicon devices. The structure and properties of silicon nitride (SiN×:H) vary widely with the details of the deposition process and conditions. LPCVD nitride film is commonly deposited at temperatures between 700 to 900'C, and have very low hydrogen content (•5 at. %), high density and well defined electrical properties. However, many applications require low temperatures (below 400'C), and in these cases PECVD is commonly used. In PECVD, the silicon source gas is usually silane and various gases, including N2 , NH3 , NF3 , and N2/NH3 mixtures are used as nitrogen sources. For electrical applications requiring high resistivity and low charge trapping, nitrogen-rich SiN×:H films are typically deposited with NH3/SiH 4 ratio near 12/1 [1,2]. However, the high hydrogen content in these films can degrade the chemical and barrier properties and may cause device degradation when used as passivation layer [3]. High density films with good chemical resistance can be deposited using mixtures of SiH 4 and N2 , although the electrical properties tend to be poor [4,5]. The objective of this study is to improve the electrical properties of PECVD nitrides processed from N2-SiH 4 at lower temperatures (