Chemical Vapor Deposition of Copper for Advanced On-Chip Interconnects
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S BULLETIN/AUGUST 1994
However, as with any new process trying to break into the mainstream, significant improvement in reliability and performance over that achievable with aluminum alloys must be demonstrated first. Toward this purpose, processes need to be developed that deposit conformal copper films of high purity with acceptable throughput, and integration schemes need to be developed which produce interconnects and multilevel metal structures with reliability significantly better than that of aluminum. This article focuses on efforts at Motorola to deposit copper films by chemical vapor deposition (CVD). The first part briefly describes the available copper precursors. This is followed by a discussion of processes for the deposition of either blanket or selective copper films. Introduction to Copper Deposition Copper films have been deposited successfully by a number of methods.6"8 Among them, CVD is one of the more promising because of its ability to deposit highly conformal films. The need for conformal films
becomes very important in the deep submicron regime, where the aspect ratio of interconnects, and especially vias,9 is expected to significantly exceed 2.0. In the CVD method, an organometallic copper precursor is delivered to a heated wafer; when the precursor contacts with the hot wafer, it decomposes into copper and volatile organic byproducts that are pumped away. A number of copper precursors are available, and many of them have been used successfully in the deposition of pure copper films. In general, they can be divided into two groups; in the first group the copper ion is maintained in the +2 charge state. The most studied member of this group is the precursor Cu+2(hfac)2 or copper bis(hexafluoroacetylacetonate). In the second group, the copper ion is maintained in the +1 charge state by the replacement of one hfac radical by a neutral ligand. Table I summarizes the main properties of representative members of each group from the point of view of the process development engineer. The Cu group offers precursors with high thermal stability and high vapor pressure, which permit their delivery to the deposition chamber at high concentrations. On the other hand, they tend to be very inefficient under most deposition conditions and most publications report deposition rates of ~ 100-200 A/min under a wide range of conditions." In addition, high substrate temperatures are required for decomposition (~400°C) while pure copper films are obtained only in the presence of reducing agents such as hydrogen.10 The precursors in the Cu+1 group are of fairly low vapor pressure and low decomposition temperature. As a result, delivery of these precursors at high concentrations is a problem. On the other hand, the conversion efficiency of these precursors to pure copper is quite high, and deposition rates as high as 10/300 A/min have been demonstrated.11 In addition, the deposition of pure copper does not require the presence of reducing agents; the decomposition of the precursor takes place via the disproportiona
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