Effect of Process and Layout on Strain Enhancement from Dual Stress Liners

Tensile and compressive stressed nitride liners have been used to increase the carrier mobility in n-channel and p-channel silicon transistors respectively. Simulations indicate how much of the stress in the film is transferred to the channel region and the magnitude of the stress in different directions. A simple bulk piezoresistive model was used to estimate the effect on carrier mobility. It is shown in the case of the n-channel transistors that the enhancement is due to the vertical stress component whereas in the case of p-channel devices the enhancement is due to the in-plane stresses. The effect of different process conditions such as film stress, thickness and method of deposition, on mobility enhancement, was also characterized. It is shown that the enhancement saturates with increasing nitride thickness but scales proportionally with the film stress. Detailed studies of the effect of the circuit layout on the final channel stress allow the critical layout parameters to be identified. The variation of device performance with the layout parameters is quantified and can be used to define design rules as well as equations to modify the device characteristics based on layout.