Surface Reactions Leading to Contamination of Metal Films Photochemically Deposited from the Hexacarbonyls
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SURFACE REACTIONS LEADING TO CON'IAMINATION OF METAL FILMS PHOTOCHIEMICALLY DEPOSITED FROM Til. I IIXACARBONYLS K. A. SINGMASTER, F. A. IIOULE AND R. J. WILSON IBM Research Division, Almaden Research Center, 650 Ilarry Road, San Jose, CA 95120
ABSTRACT A systematic study of the origin of contaminants in metal films photochemically deposited from the group VI hexacarbonyls is described. Background gas present in the cell during deposition, exposure to air and incomplctc removal or co groups from the surface of the growing film all affect C and 0 incorporation. The data are compared to results of recent experiments examining surface photoproducts of the metal carbonyls.
INTRODUCTION The group VI metal hexacarbonyls present several important advantages as precursors to film deposition by ultraviolet (UV) photolysis: they are reasonably volatile (100 mTorr at room temperature), have a high absorption cross section in the region of 250 nm (10-17 cm 2) and dissociate in the gas phase with near unit quanturm yield to form partly carbonylated species such as M('CO) 4 , M = Cr, Mo and W [1-4]. Their chief disadvantage as precursors for photochemical deposition is that the films are of notably poor quality, being heavily contaminated by carbon and oxygen [3,4]. Although UIV laser induced deposition can lead to good quality films if laser power densities are high enough to cause appreciable heating of the films [5], elevated temperatures are not always desirable. In this report we describe results of a systematic study of sources of contamination in films deposited from Cr and Mo hexacarbonyls by low power photochemical deposition [6]. The goals of this work are twofold: to investigate the possibility of controlling aspects of the deposition process to eliminate contaminants altogether, and to learn as much as possible about surface photochemical and thermal reaction- for comparison to studies of photolysis of condensed metal hexacarbonyls on crystalline surfhaces. EXPERIMENTAL SECTION All films were deposited on Si(Ill), doped n- or p-tvpe (0.1 Q-cm), cleaned in an acid oxidant bath and left covered by its native oxide. The light source was a frequency doubled Ar ion laser, providing up to 4 mW at 257 n11. Afler separation from the 515 nm beam, the UV beam was expanded and focussel to a 5 pin spot at the substrate surface. Films were grown during exposures ranging from 5s to 15 inimusing incident power densities from 40-2300 W/cm 2. Two deposition cells were used7 a stand-alone cell which could be evacuated to the mid 10-4 Torr range, and an ultrahigh vacuum (1.IIV) cell with a base pressure in the low 10"9 range. The latter could be coupled to a vacuum suitcase for transportation of samples to a remote laboratory for analysis without air exposure. For all depositions the cells were backfilled with vapor of crystalline M(CO) 6 which had been subjected to several freeze-pump-thaw cycles. No buffer gases were used. Three sets of measurements were carried out for each itetal: growth in low vacuum and exposure to air prior to
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