A New Lateral Growth Free Formation Technique for Titanium Silicide Using the Si/W/Ti Trilayer Structure

A Si/W/Ti trilayer structure which consists of a top insulating Si film, a middle lateral growth suppressing W film, and a bottom Ti film is proposed to form the titanium silicide. With the properly chosen thicknesses, the W/Ti films on the oxide layer can be completely converted into the TixW~ alloy after annealing. Since the TixW~ alloy has a higher silicidation temperature than the pure Ti film, the titanium silicide cannot be formed on top of the oxide layer and the problem of lateral growth can be solved. The thickness ranges of the W/Ti films for the TiSi2 formation without the lateral growth has been determined experimentally. Within these ranges, the obtainable sheet resistance of TiSi2 is between 0.8 and 2.5 ~/[~. Moreover, the optimal annealing process is a one-step anneal at 750~ for 30 min, which can be carried out in an N2 flowing open-tube furnace. The mechanism of TiSi2 formation using the trilayer structure has also been explored and verified by experimental data. Recently the salicide (self-aligned silicide) process, which forms a silicide film in both polysilicon-gate and source/ drain regions to reduce their sheet resistance, becomes more and more attractive in VLSI technology. Generally, the refractory metal and its silicide for the salicide process have to satisfy the following requirements: (i) low silicide resistivity; (ii) easy processing; (iii) high etching selectivity between the metal and its silicide and simple etching solution; (iv) compatibility with the existing MOS process. Many candidates such as PtSi (1), CoSi2 (2), and TiSi2 (3) are now under development. Although titanium silicide is a promising candidate for the salicide process, it has to solve two severe problems. First, the annealing ambient for titanium silicide needs a stringent control of residual 02 contaminants to avoid the oxidation of the Ti film, since Ti is a strong reducing agent. This often causes difficulties in process control. The second problem is the so-called bridging or lateral growth phenomenon which may happen between gate and source/ drain regions due to the out-diffusion of Si atoms into the Ti film on top of the oxide layer to form the TiSi2 bridge. To solve the lateral growth problem, a N2 annealing process was proposed in which a TiN film is formed to stop the lateral growth of the silicide (3). In this two-step annealing process, a first anneal is performed to form the silicide at a lower temperature of 600~ After the etching of TiN layer, a second anneal at a higher temperature of 800~ is used to reduce the resistivity. During the first annealing process, the Ti film has to be exposed to the ambient. So this process still has the first problem. In a previous work (4), we proposed a Si/Mo/Ti structure for the formation of TiSi2, which solves the problem of environmental sensitivity. In this approach, the trilayer structure is further improved by replacing the Mo layer with the W layer. It is found that with the suitably chosen thicknesses, the Si/W/Ti trilayer structure can be annealed in a conventional N2 flowing open-tube furnace and the lateral growth can be completely suppressed. Thus, the above-mentioned two problems can be solved. In this paper, the TiSi2 formation technology using the Si/W/Ti trilayer structure will be presented. The experimental results and the reaction mechanism will also be investigated and analyzed.