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3.2.1 Fabrication and Morphology of Polysilicon Layers

  Polycrystalline silicon has a grain oriented structure, with monocrystalline grain interior regions surrounded by grain boundaries. Polysilicon can be fabricated by a wide variety of techniques, and the physical properties and the morphological structures differ significantly.

The most common fabrication methods are:

The first method uses a CW (continuous wave) laser equipment to anneal partial implantation damage within an amorphous silicon layer. Due to the heavy ion bombardment the regularity of the silicon matrix was destroyed. The energy of the light beam melts the silicon crystal and, hence, recrystallization occurs. The grain size of the recrystallized silicon can be controlled by the laser power. CW-laser annealing is applied on a limited area, while the zone melting recrystallization is used to fabricate large-area polysilicon layers. An IR (infrared) or UV (ultra violet) source is scanned across the wafer and, thus, a narrow but large polysilicon recrystallization zone is created.

The molecular beam epitaxy (MBE) is the most expensive method. It operates at lower temperatures than the other methods, so the annoying dopant redistribution is suppressed. An evaporated molecular beam is directed towards the substrate surface. The low vapor pressure of silicon ensures the condensation of the material on the surface.

The most common method for fabrication of polysilicon layers for VLSI circuits is pyrolizing silane tex2html_wrap_inline5221 between tex2html_wrap_inline5223 and tex2html_wrap_inline5225 in a low pressure ambient, which is referred to as low pressure chemical vapor deposition (LPCVD). The chemical reaction is given by (3.2-1).

  equation1046

Polysilicon LPCVD depositions are limited to the above cited temperature range, because at higher temperatures gas phase reactions will result in rough, loosely adhesion, and silane depletion. On the other hand, at temperatures under tex2html_wrap_inline5223 , the deposition rate would be too low for practical applications.

The polysilicon deposition rate is a strong nonlinear function and depends on the partial pressure of silane, temperature and additional process gases as shown in Figure 3.2-1. Data are taken from [Kin83] [Har84] [Sze88]. One major advantage of CVD methods is the simple possibility for the introduction of dopants into the deposited material. This process is known as in-situ technique, where dopant gases, like phosphine tex2html_wrap_inline5229 , arsine tex2html_wrap_inline5231 or diborane tex2html_wrap_inline5233 are added to the silane. The effects onto the deposition rates are different. Diborane increases drastically the deposition rate because of forming tex2html_wrap_inline5235 radicals, which catalyzes the chemical reactions and, hence, increases the deposition rate. Phosphine and arsine are absorbed on the surface and are therefore blocking the aggregation of silane, thus the deposition rate decreases. Data for deposition rates including silane partial pressure dependence and in-situ doped samples are given in Figure 3.2-1.

   figure1055
Figure 3.2-1: Deposition rates for pyrolizing of silane to fabricate polysilicon layers. Pressure dependence (pp tex2html_wrap_inline5237 silane partial pressure, tp tex2html_wrap_inline5237 silane total pressure) is also given [Kin83] [Har84] [Sze88].

The deposition temperature and doping conditions are affecting the microstructure of polysilicon films drastically. Films deposited at tex2html_wrap_inline5241 are amorphous (see Fig. 3.2-2a), with an average grain size below tex2html_wrap_inline5243 . This amorphous layer has similar properties as silicon amorphized by ion implantation ( tex2html_wrap_inline5245 ). Ion implantation is also a possible way to dope the polysilicon layer, but ion implantation destroys the grain structure of polysilicon in any case. Layers deposited at tex2html_wrap_inline5247 are only partially crystalline with a higher fraction of amorphous islands (Fig. 3.2-2b). The grain structure and the grain boundaries are not well defined and the average grain size is about tex2html_wrap_inline5249 . By adding a dopant gas the grain size is significantly increasing ( tex2html_wrap_inline5251 ). Figure 3.2-2c shows the morphology of an in-situ doped sample, where at tex2html_wrap_inline5247 still partially amorphous regions exist. Therefore, a delay time must be considered before the crystalline growth takes place [Kin83]. Polysilicon films deposited at tex2html_wrap_inline5255 show columnar grains with well established grain boundaries as given in Figure 3.2-2d. The average grain size in this case is about tex2html_wrap_inline5257 .

   figure1068
Figure 3.2-2: Microstructures of LPCVD deposited polysilicon films for different deposition temperatures and doping conditions. Note the difference in the amorphous portion of the layer b) undoped and c) in-situ doped. a) is fully amorphized, where d) shows typical grain structure including grain boundaries.

The morphological quantities of polysilicon, like average grain size tex2html_wrap_inline5259 , preferred grain orientation O, surface roughness tex2html_wrap_inline5263 , and the average deposition rate tex2html_wrap_inline5265 are shown in Table 3.2-1 for a deposition temperature range tex2html_wrap_inline5267 from tex2html_wrap_inline5269 up to tex2html_wrap_inline5271 . These subtle data are necessary for an initial setup when modeling the diffusion process. The data were obtained by X-ray diffraction measurements [Har83] [Duf83] [Har84], where the most intense reflections in the tex2html_wrap_inline5273 plane are recorded.

   table1077
Table 3.2-1: Properties of undoped and phosphorus doped polysilicon films for different deposition temperatures and process conditions. Data are taken form [Har83][Duf83][Har84]. Note the unpredictable grain size behavior of the in-situ doped sample.

If polysilicon films of high structural perfection, low strain, and small surface roughness are required, the deposition temperature should not exceed tex2html_wrap_inline5455 . As a drawback it should be noted, that the deposition rate is lower than tex2html_wrap_inline5457 . Higher growth rates can be obtained at higher deposition temperatures, but only at the cost of structural and surface properties.

One of the drawbacks of polysilicon is that it changes its morphological structure at high temperatures. The grain boundaries try to migrate, and grain growth is occurring. Hence, the thorough modeling of grain growth behavior is essential for diffusion modeling within polysilicon layers.


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Next: 3.2.2 Polysilicon Grain Growth Up: 3.2 Diffusion in Polysilicon Previous: 3.2 Diffusion in Polysilicon

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