Silicides as contact material for DRAM applications

TiSi, CoSi, CoSi2 and NiSi are used for a salicide contact metallisation in DRAM devices. The contact resistance is studied: for contacts to tungsten silicide-gates, for self-aligned n-type contacts in the memory cell array and for large borderless n-type and p-type contacts in the chip periphery. It is shown that the contact resistance on p-type contacts is reduced with CoSi by at least 40% compared to TiSi and NiSi, whereas the contact resistance of cell array-, gate- and n-type contacts is comparable for the different silicides. In order to enable a sufficient retention time the leakage current is critical. Due to the low silicon consumption NiSi contacts result in the smallest leakage. Using a Cobalt + Ti-Cap stack the Si leakage currents rise by orders of magnitude. It is shown that with a TiN-cap and a changed chemistry for the selective strip leakage currents as small as for TiSi can be achieved. The role of CoSi-thickness and the formed silicide-phase, i.e., CoSi or CoSi2, on contact resistance and leakage current are discussed.