Microstructural and optical properties of as-deposited LPCVD silicon films
Introduction
Polycrystalline silicon (polysilicon) has important technological applications in RAM, EEPROM and microelectronics of large-area liquid crystal displays (LCD), where it plays an active role in the device operation and reliability [1], [2]. Surface roughness, grain size and trap density of the active polysilicon layers are very important variables for this last type of application as they determine the speed of the active matrix (AM) LCD.
Since 1980, several works [3], [4], [5] have been devoted to the study of the structure of as-deposited LPCVD silicon films in connection with the deposition parameters (pressure and temperature). This paper presents the influence of these parameters on surface roughness, grain structure and optical constants of silicon thin films.
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Experimental details
The LPCVD silicon films (250–400 nm in thickness) were prepared in ASM, standard, horizontal, hot wall reactor heated by electrical heaters. Silicon wafers (3-inch) covered by a thermally grown SiO2 layer with a thickness of 130 nm were used as substrates vertically located in the quartz tube. Pure silane with a flow rate kept constant at 40 sccm was been used as a gas source during all depositions. The temperature of the hot zone was varied in the range from 500 to 615°C while the deposition
Results and discussions
The CVD parameters determining the deposition kinetics are able to influence the structure and/or morphology of the as-deposited LPCVD silicon films [3], [4], [5], [15], [16]. In Fig. 1 the XRD spectra of the as-deposited LPCVD silicon films are presented, deposited at 500 and 550°C, while the deposition pressure was varied from 20 to 100 Pa. According to this figure, the presence of 〈211〉 preferred crystallographic orientation at temperatures as low as 500°C is evident.
Our result for 500°C is
Conclusions
We demonstrated for the first time (to our best knowledge) that it is possible to obtain a polycrystalline LPCVD silicon film prepared from silane at temperatures as low as 500°C. This result was sustained by three complementary techniques (XRD, SE and AFM).
Structural, morphological (grain size, crystalline phase and film roughness) and optical properties of LPCVD films are directly controlled by the deposition temperature and/or pressure.
A good agreement between the values of polysilicon films
Acknowledgements
The authors would like to thank Mrs Nicoleta Nastase and Mr S. Nastase for the AFM measurements.
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