Evaluation of silicon nitride and silicon carbide as efficient polysilicon grain-growth inhibitors

Abstract

The roughness of the surface of the floating polysilicon layer in many non-volatile memory devices [1–12] has for long been critical to their electrical performance. The degree of smoothness of this polysilicon film determines the quality as well as the structural integrity of the interface that is subsequently formed with the interpoly dielectric. The occurrence of facetal graingrowth in the polysilicon film under high-temperature process conditions is the chief cause of the degradation of the film surface smoothness. Much research has been conducted to improve the surface planarity of the polysilicon surface. Tan et al. [2] discussed the possibility of using chemically-mechanically polished polysilicon film to replace the floating polysilicon layer. The smoothness of the film improved but the undesirable condition of facetal grain-growth, after deposition, in the polysilicon film remains probable during process steps involving oxidation or annealing [13, 14]. The substitution of a floating polysilicon layer by an amorphous silicon layer, which results in a smooth surface, has also been employed by some semiconductor manufacturers. However, amorphous films are not widely used in volume production of memory devices because of the slower deposition process and arduous control. A process is needed that is simple to carry out and yet bears little adverse electrical and structural consequence. In this work, we introduced selected impurities to the surface of a deposited polysilicon film via low-energy ion implantation. During subsequent hightemperature process conditions, e.g.>1000 ◦C, the top surface of the film (≈30 A) will undergo recrystallization [13]. Our purpose was that the surface impurities should act as nucleating centers and facilitate the growth of small grains near the top of the film. In this way, facetal polysilicon grain-growth should be inhibited and a smooth surface obtained. In this experiment, a 110 A thick tunnel oxide layer was thermally grown on a Si (110) p-typed wafer, followed by a Low Pressure Chemical Vapor Deposition (LPCVD) of a 1500 A thick polysilicon layer (floating polysilicon). The polysilicon layer was later implantdoped with phosphorus (dose: 1× 1015 per cm2). The