Mechanistic Studies of CVD Metallization Processes:  Reactions of Rhodium and Platinum β-Diketonate Complexes on Copper Surfaces

Abstract

The hexafluoroacetylacetonate complexes Rh(hfac)(C 2 H 4 ) 2 and Pt(hfac) 2 are known to serve as chemical vapor deposition precursors to Rh and Pt thin films. In the absence of a reducing carrier gas, the depositions are surface-selective and occur preferentially on copper, but under these conditions, the metallization processes are unexpectedly inefficient relative to the rapid deposition of Pd on Cu seen for the palladium analogue Pd(hfac) 2 . Mechanistic studies of the reactions of Rh(hfac)(C 2 H 4 ) 2 and Pt(hfac) 2 on copper surfaces under ultrahigh vacuum conditions have now been performed in order to elucidate the factors responsible for the differences among these surface-selective metallization processes. The studies demonstrate that adsorption of the rhodium complex Rh(hfac)(C 2 H 4 ) 2 on copper surfaces is accompanied by the loss of the coordinated ethylene groups, even at 100 K. At these low temperatures, the adsorbed Rh(hfac) fragments are oriented in several ways with respect to the surface. Heating the substrate above ∼150 K causes the hfac ligands to realign to a perpendicular orientation relative to the surface. The platinum precursor Pt(hfac) 2 adsorbs molecularly at 100 K with the molecular planes of the hfac ligands oriented parallel to the copper surface. Heating the substrate to temperatures above 150 K again results in a realignment of the hfac ligands to a perpendicular orientation. This reorientation is accompanied by a partial reduction of the Pt centers (as judged from shifts seen in X-ray photoelectron spectroscopy core level data), a result suggesting that the hfac ligands begin to dissociate from the platinum centers near 150 K. At temperatures above 220 K, the transfer of the hfac ligands from both complexes to the copper surface is complete, as signaled by the reduction of the metal centers to the zero-valent state. The copper-bound hfac ligands are further transformed upon heating, either reacting with copper surface atoms to yield Cu(hfac) 2 (which desorbs at temperatures above 250 K) or decomposing (with fragments desorbing above 350 K). The presence of platinum on the copper surface promotes the former reaction as judged by the appearance of a new reaction-limited desorption process for Cu(hfac) 2 . The presence of rhodium on the copper surface does not promote the formation of Cu(hfac) 2 , although autocatalysis is noted in the steady-state reactive scattering data. The inability of the Rh and Pt precursors to engage in a sustained transmetalation reaction with the copper surface is attributed to the slow interdiffusion of copper through the Rh/Cu and Pt/Cu alloys that are produced in the near-surface region.

Rhodium pyrazolate complexes as potential CVD precursors

“…Bond lengths 3 X, but somewhat surprisingly, to the best of our knowledge, this is the first pyrazolate complex of this kind. The compound crystallizes in the space group C2 with four molecules per unit cell.…”

“…CVD processes can also allow for the selective growth of films that have good step coverage compared with traditional techniques such as PVD. [1][2][3] Thin films of rhodium are used as electrical contacts, 4,5 reflective coatings, 6,7 catalysis, 8,9 and wearresistant coatings for extreme conditions. 2,10 Previous studies have focused on volatile compounds which feature traditional organometallic ligands such as CO, C 2 H 4 , C 5 H 5 , and fluorinated hydrocarbons such as trifluoroacetylacetonate and it has been shown that film quality is dependent on the precursor used.…”

“…Impurity levels depend strongly on the precursor and carrier gas used. For example, films grown using argon as a carrier gas using [Rh (μ-Cl)(CO) 2 ] 2 at 180°C contain 49% Rh, 24% C, and 27% O, while films grown under the same conditions from [Rh(η 3 -allyl) 3 ] contain 86% Rh and 14% C. 10 In order to explore the potential to grow films with different compositions and/or new morphologies we have investigated the use of precursors which have non-traditional ligands. New CVD precursors should ideally meet several criteria.…”

“…<200°C), decompose cleanly at a moderately higher temperature, and be readily prepared in high purity and in relatively high yield, especially when precious metal films are desired. 11 In order to impart volatility we have focused our studies on complexes which contain PMe 3 , and on ligands that bear CF 3 groups since they are known to give rise to volatile compounds in many cases. Several groups including our own have had success utilizing appropriately substituted pyrazolate ligands to synthesize volatile complexes for use as CVD precursors.…”

“…Several groups including our own have had success utilizing appropriately substituted pyrazolate ligands to synthesize volatile complexes for use as CVD precursors. [13][14][15] Using this strategy, we have synthesized a series of new rhodium complexes containing bis-trifluoromethyl pyrazolate (3,5-(CF 3 ) 2 -Pz) and PMe 3 . The new compounds reported in this paper, [Rh(η 2 -C 2 H 4 ) 2 (μ-3,5-(CF 3 ) 2 -Pz)] 2 (1), [Rh 2 (μ-Cl)(μ-3,5-(CF 3 ) 2 -Pz)(PMe 3 ) 4 ] (2), [Rh(3, 2 -Pz] (5) were isolated in moderate to good yields, characterized spectroscopically and the solid state structures determined by single crystal X-ray diffraction studies.…”