Poly-Si films

With the continuing scale-down of ULSI features, the demand on the electrical resistivity of the electrode material is reduced. Many researchers have studied new conductor materials such as W/WN,v/poly-Si film. 

Most poly-Si CMP processes involve patterned oxide filled with polysilicon. The generation mechanism and physical appearance of the scratches for polysilicon CMP are the same as in the oxide CMP described earlier. Most Polysilicon CMP processes are rather selective toward silicon oxide. More specifically, the polysilicon removal rate is usually much higher than that for oxide. Such slurries usually contain low concentration of abrasive, which usually translates to low defect density. At low magnification, scratches on the poly-Si film after CMP usually look like a continuous line rather than chatter marks. At higher magnification, the scratch usually looks like irregular trench with multifaceted walls. 

Pitting or void in the poly-Si is a common mode of defect. The severity of such a defect depends on the type of poly-Si material involved. More specifically, the crystalline structure and the level of doping elements can have a significant impact on the tendency of the poly-Si film to form pits and voids during the CMP process. 

The types of defects existing before polysilicon CMP are similar to the types of defects that can be observed before oxide CMP. The main difference is that the poly-Si film is not as transparent as an oxide film. The thicker the poly silicon film, the less transparent it is. 

T. Matsuyama, M. Tanaka, S. Tsuda, S. Nakano, and Y. Kuwano, Improvement of n-type poly-Si film properties by solid phase crystallization method, Jpn. J. Appl. Phys. 32, 3720, 1993. 

Degree of thermal diffusion is an important factor in treating rapid annealing, since it will govern in-depth crystallinity of poly-Si films formed and thermal damage to glass substrates. One-dimensional thermal diffusion coefficient can... 

Polycrystallization of micrometer-order-thick a-Si films by FLA exhibits curious phenomena different from the case of crystallization of thin (<1 pm) a-Si films, and consequently forms poly-Si films with characteristic microstructures. Figure 11.9 shows surface images of the Si films after FLA under various lamp irradiances [35]. Only the edges are crystallized in the case of the lowest lamp irradiance, and the crystallized area expands towards the center of the... 

Fig. 11.8. SIMS profiles of (a) Cr, (b) P, and (c) B atoms before and after FLA [34]. Schematic diagram of the p-i-n stacked poly-Si films formed by FLA is also shown. The abrupt profiles of the surface B atoms as well as of the bottom Cr and P atoms are maintained after FLA...

Fig. 11.11. Cross-sectional TEM image of the poly-Si film formed by FLA [35]. The region L (containing large stretched grains) and the region F (consisting of 100-nm-sized fine grains) alternatively appear in lateral crystallization direction...

Fig. 11.12. Raman spectra of poly-Si films formed on Cr-coated soda lime and quartz glass substrates [28]. A clear c-Si peak located at 520 cm-1 can be observed, whereas no significant signal relating to a-Si can be observed at approximately 480 cm-1, indicating high crystallinity. A FWHM of the c-Si peaks of 7-9 cm-1 indicates the existence of fine grains less than 10 nm in size, consistent with the TEM image...

Fig. 12.4. Continuous poly-Si film formed by the ALILE process on a 3-in substrate (To obtain this figure the Al on top of the poly-Si film was sele, removed by wet chemical etching), (from [28])...

The crystallized fraction, which was defined as the ratio of the dark area to the total area in the optical micrographs, is shown in Fig. 12.7 as a function of the tx for different annealing temperatures T (from 500 to 530°C). The tx necessary to form a continuous poly-Si layer (i.e., to reach a crystallized fraction of 100%) becomes shorter with increasing annealing temperature. For example, at 530°C a continuous poly-Si film was formed after 19 min. Whereas, it took 90 min to form a continuous poly-Si film at 500°C. The time necessary to finalize the ALILE process can be separated into the above... 

In this section, the most important structural (grain size, grain orientation and intragrain defects) and electrical properties of the poly-Si films prepared by the ALILE process are discussed. 

A typical EBSD map of the surface of a poly-Si film on glass prepared by the ALILE process is shown in Fig. 12.9 (left) [39]. The sample was annealed at 425°C for 16h. Afterwards, the Al(+Si) top layer was removed by CMP. The poly-Si film shown here features an average grain size of 7 pm and a maximum grain size of 18 pm. From such EBSD measurements, not only the grain size but also the crystallographic orientation of the grains can be determined. 

A high preferential (100) orientation of the poly-Si surface is favorable for subsequent epitaxial growth at low temperatures [41-43]. Due to the preferential (100) orientation, the utilization of the poly-Si films formed by the ALILE process as a template (seed layer) for subsequent epitaxial thickening at low temperatures is quite attractive [44]. 

Hall effect measurements were used to investigate the electrical properties of the poly-Si films formed by the ALILE process. Due to the incorporated Al, the poly-Si films are always p-type. At room temperature, a hole concentration of 2.6 x 1018 run 3 and a hole mobility of 56.3 cm2 V 1 s 1 were determined [16]. Temperature dependent Hall measurements revealed both valence band conduction and defect band conduction (two-band conduction). For such highly doped material, the presence of a defect band conduction is expected. The Al concentration in the poly-Si films was measured by secondary ion mass spectroscopy (SIMS). An Al concentration of about 3 x 1019 cm 3 was found, which is about a factor of 10 larger than the... 

The permeable membrane (barrier layer) plays a crucial role for the ALILE process. Usually, the barrier layer is formed by exposure of the Al-coated glass substrate to air. The thickness of this barrier layer (A1 oxide), which is on a nanometer scale, can be influenced by the variation of the exposure time. With increasing exposure time, the thickness of the barrier layer increases. The thicker the barrier layer, the lower is the nucleation density and the longer is the process time necessary to form a continuous poly-Si film [19]. By X-ray photoelectron spectroscopy (XPS) measurements it was shown that the barrier layer stays in place during the whole ALILE process. 

In order to demonstrate the importance of the barrier layer for the layer exchange process, an initial glass/Al/a-Si stack without a barrier layer was prepared [50]. To prevent the formation of a barrier layer the a-Si was deposited directly onto the A1 (without vacuum break). The SEM images in Fig. 12.11 make the influence of the barrier layer clear. Two samples are shown - one with barrier layer (by exposure to air for 2 h) (left hand side) and one without barrier layer (right hand side). Both samples were annealed for 45 min at 480° C. After the annealing step, the A1 was etched off chemically. To show the cross section as well as the surface, the samples were tilted by 30° for the SEM measurements. The sample with barrier layer (left) shows a continuous poly-Si film with Si islands on top (similar to what is shown in Fig. 12.5), whereas the sample without barrier layer (right) does not show a continuous poly-Si film but a porous film structure. The former interface between the A1 layer and the a-Si layer is not visible. In some parts, the... 

Fig. 12.11. SEM images of a sample with barrier layer (left) and a sample without barrier layer (right). The samples were annealed at 480°C for 45min. After the annealing step, the A1 was etched off. The samples were tilted by 30° to show both cross section and surface. The sample with barrier layer (left) features a continuous poly-Si film with Si islands on top. The sample without barrier layer (right) features a porous structure, (from [50])...

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