Ion-enhanced evaporated tungsten for VLSI applications

The continuing advancement of the integrated circuit technology requires innovation in materials and processes. In the area of gate metallurgy and interconnect technology, tungsten is being explored actively for FET applications because of its mid gap work function, low resistivity, and high melting point. Sputtering is perhaps the most commonly used process to deposit low stress tungsten films on oxide surfaces but the technique lacks independent control over individual processing parameters and the resulting film properties. Evaporation produces very pure films but they tend to delaminate from oxide surfaces due to high levels of intrinsic stress (8×1010 dynes cm-2). The high stress can also cause micro-cracking in the gate oxide and significantly degrade thin oxide integrity. Here, we report a novel technique for depositing tungsten on oxide substrates which utilizes concurrent ion-beam bombardment during evaporation. Previous works on tungsten silicide (1), niobium (2), chromium (3) and germanium (4) have demonstrated that film stress can be modified by ion bombardment. These materials, however, fail to retain modified stress upon subsequent heat treatments {especially above recrystallization temperature) because they have low melting point and related thermal mismatch problems. Tungsten, on the contrary, has high melting point (3400°C) and its stress is expected to be thermally stable during high temperature anneals. The use of a separate ion source whose control is independent of the deposition process enables the optimization of the film properties including film stress and oxide damage.