Nucleation and Growth During Tungsten Atomic Layer Deposition on Oxide Surfaces
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NUCLEATION AND GROWTH DURING TUNGSTEN ATOMIC LAYER DEPOSITION ON OXIDE SURFACES S. M. GEORGE, J.W. ELAM, R.K. GRUBBS AND C.E. NELSON Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309 [email protected] ABSTRACT Nucleation and growth has been studied during tungsten (W) atomic layer deposition (ALD) on oxide surfaces. Auger electron spectroscopy (AES) was utilized to examine the deposition of W during the sequential (A) WF6 and (B) Si2H6 reaction cycles that define W ALD. The AES results displayed an initial nucleation period of ~10 AB cycles to deposit one tungsten monolayer on SiO2. Subsequently, the W and Si AES signals grew and oscillated dramatically versus WF6 and Si2H6 exposures. The increase in the W AES signal in the growth region was consistent with a W ALD growth rate of 3.5 Å per AB cycle. An examination of the oxygen and tungsten AES signals versus AB cycles indicated that W ALD displayed nearly ideal "layer-by-layer", Frank- van der Merwe growth after the nucleation period. On Al2O3, the AES results displayed a much shorter nucleation period for W ALD. Only 3 AB cycles were required to deposit one tungsten monolayer. Subsequently, the tungsten film grew at a rate of 3.6 Å per AB cycle. The initial nucleation period and growth mechanism during ALD are important because they will affect the roughness of the resulting ALD film. INTRODUCTION Atomic layer deposition (ALD) is a useful technique to deposit thin films with high conformality and precise thickness control. ALD is controlled at the atomic level by selflimiting surface reactions [1]. ALD growth rates are typically ≤ 1 monolayer or ~1 Å per AB reaction cycle. Very few studies have examined the nucleation and growth mechanism during ALD. Nucleation is important because film roughness will develop if multiple AB reaction cycles are required to obtain the initial monolayer. The growth mechanism is also critical for film roughness because ALD films may not be deposited in an ideal "layer-by-layer" manner. This paper examines nucleation and growth during W ALD on Al2O3 and SiO2 surfaces. The binary chemical vapor deposition (CVD) reaction: WF6(g) + Si2H6(g) → W(s) + 2SiHF3(g) + 2H2(g) has been used previously to deposit tungsten [2]. Tungsten atomic layer deposition (ALD) can be accomplished by separating the binary reaction into two self-limiting half-reactions [3]. These two reactions can be written as: (A) W-SiHyFz* + WF6 (g) → W-WFx* + SiHaFb(g) (B) WFx* + Si2H6 (g) → W-SiHyFz* + 2H2 (g) + SiHaFb(g)
(1) (2)
where the asterisks designate the surface species. The stoichiometry is left unspecified to allow for the possibility of different surface reactions. Successive application of the WF6 and Si2H6 half-reactions in an ABAB… sequence produced W ALD at substrate temperatures between 425-600 K [3]. Fourier transform infrared O7.7.1
(FTIR) investigations of the surface chemistry indicated that the WF6 and Si2H6 half-reactions were complete and self-limiting at T>400 K [3]. In situ spectroscopic ellipsometry m
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