Interactions of Polysilicon with Aluminum
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INTERACTIONS OF POLYSILICON WITH ALUMINUM and K.N. TU'*: G. OTTAVIANI*, *Physics Department, via Campi
C. NOBILI* 213/a Modena,
41100 Modena
Italy
"*IBM T.J.
Watson Research Center,
Yorktown Heights,
N.Y.
10598
ABSTRACT In-situ resistivity measurements togheter with Auger electron spectroscopy, MeV 'He" backscattering spectrometry, scanning electron microscope and x-ray diffraction have been used to investigate interactions between Al films and CVD polycristalline silicon layers deposited on thermally grown SiOŽ on silicon. A sharp and well defined increase in resistivity around 450 -'C has been associated to the erosion of the polysilicon and growth of Si crystallites in the metal film. The kinetic of the transformation has been studied by isothermal 1 treatment over the 390 - 450 :C temperature range. An activation energy of 2.2 1 0.2 eV has been measured. Similar results have been obtained by using treatments at constant heating rate. A critical analysis of the available data suggests that the rate limiting step is the nucleation of Si grains in the aluminum film. The driving force for the process can be identified to be the free energy difference between the initial and final states.
INTRODUCTION The metal and silicon interactions' have been widely investigated because of their importance in integrated circuits 4 fabrication. On the basis of reaction products we can classify metals in two groups; metals which form stable compounds with silicon and metals which have a simple eutectic point with Si. Pt, Pd, W, Ti etc belong to the first group and Al, Au, Ag belong to the second group. Among the latter aluminum5o* is the most widely used metal in electronic devices and is in contact with either single crystal silicon or polysilicon. An aluminum film in contact with silicon single crystal 7 10 dissolves the silicon at temperatures below their eutectic point. The reaction ends when the silicon content in the metal film reaches the solid solubility value, e.g. 1.5 at% at 57'? -C. On the other hand if the aluminum film is in contact with amorphous"' or polycristallinel=-'-1 silicon, the reaction keeps going even if the solid solubility have been reached. The growth of crystalline silicon precipitates in the aluminum film takes place, resulting in the erosion of the polysilicon. Finally the original bilayer structure is transformed into micron size and isolated silicon grains which are imbedded in the aluminum film. Interaction of the Al and Si has been detected in the 400 - 500 -C temperature range. The morphology of the final structure is
Mat. Res. Soc. Symp. Proc. Vol. 106. 91988 Materials Research Society
164
thicknesses. relative mainly affected by their has also been grain growth and dissolution Silicon observed to occur with gold and silver films:, Compared to aluminum the reaction temperature is lower with Au and higher with Ag. Germanium 2 ':'• behaves similarly. The objective of this study is to investigate the solid phase reaction between poly-silicon and Al films. By using in-situ resistivity
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