Influence of a RF Plasma on the Nucleation of Aluminum Using N-Trimethylamine-Alane (TMAA) as Precursor
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INFLUENCE OF A RF PLASMA ON THE NUCLEATION OF ALUMINUM USING N-TRIMETHYLAMINE-ALANE (TMAA) AS PRECURSOR
A. WEBER, U. BRINGMANN, K. SCHIFFMANN AND C.-P. KLAGES Fraunhofer-Institut for Schicht- und Oberfliichentechnik (Fh-IST) Vogt-Kolln-Str. 30, W-2000 Hamburg 54, F.R.G.
ABSTRACT Aluminum films deposited on silicon with MOCVD methods generally exhibit a high surface roughness due to the hindered nucleation. To decrease the nucleation barrier a pretreatment of the wafer with TiC14 vapor as a chemical activator is commonly suggested. This work examines the influence of a rf hydrogen plasma on the nucleation of aluminum on Si and Si0 2 using TMAA as precursor. Atomic force microscopy (AFM) was used to determine the surface roughnesses and crystallite sizes of the aluminum deposits. Without plasma activation the nucleation barrier on Si (111) is substantially higher than on oxidized Si and therefore the pretreatment of the wafer plays an important role. The hydrogen plasma causes a substantial increase of the nucleus density leading to deposits with lower surface roughnesses compared to conventional MOCVD aluminum. The influence of the substrate on the nucleation is almost "switched off' by the plasma activation and the grain sizes of Al on Si (111) and Si0 2 differ only slightly. Relatively smooth films with a resistivity of 2.7 p.ocm are obtainable by employing a plasma activated nucleation step followed by a thermal MOCVD process. The increased nucleation rate is discussed in terms of kinetic effects of plasma activation. INTRODUCTION The deposition of Al and its alloys is of interest as IC metallization material for the development of VLSI and ULSI devices [1,2]. In the past several years MOCVD techniques mostly using triisobutyl aluminum (TIBA) were intensively investigated and today a good knowledge concerning growth kinetic and decomposition mechanism exists in the literature [3-5]. An alternative to TIBA and other alkyl aluminum compounds is trimetyhlamine-alane (TMAA). During the last three years TMAA gained interest as MOCVD source material for the deposition of pure Al films [6-8]. The lack of Al-C bonds in the TMAA molecule prevents an intramolecular carbon incorporation during the CVD process. Aluminum deposited by MOCVD methods on silicon generally exhibits a high surface roughness due to the hindered nucleation and appears as matte white film. An in situ pretreatment of the wafer with TiC14 vapor is the mostly used method to decrease the nucleation barrier [9,101. The drawback of this chemical activation is the long term corrosion of Al caused by residual chlorine. Other in situ methods are photochemical prenucleation processes achieved by employing an UV laser [11-13] using TIBA as precursor. These techniques offer the advantage of patterned Al growth. Investigations on the Al nucleation employing TMAA as precursor are rare. Gross et al. [7] described the nucleation activation of Cu and TiN interlayers but presented no quantitative surface roughness data. Kobayashi et al. [14] reported on the depositio
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