Low-stress and high-reflectance Mo/Si multilayers for EUVL by magnetron sputtering deposition with bias assistance

To explore the potential of achieving low-stress and high-reflectance Mo/Si multilayers deposited by conventional magnetron sputtering with bias assistance, we investigated the effects of varying Ar gas pressure, substrate bias voltage and bias-assisted Si ratio on the stress and EUV reflectance of Mo/Si multilayers. To reduce the damage of ion bombardments on Si-on-Mo interface, only final part of Si layer was deposited with bias assistance. Bias voltage has strong influence on the stress. The compressive stress of Mo/Si multilayers can be reduced remarkably by increasing bias voltage due to the increase of Mo-on-Si interdiffusion and postponement of Mo crystallization transition. Properly choosing gas pressure and bias-assisted Si ratio is critical to obtain high EUV reflectance. Appropriately decreasing gas pressure can reduce the interface roughness without increasing interdiffusion. Too much bias assistance can seriously reduce the optical contrast between Mo and Si layers and lead to a remarkable decrease of EUV reflectance. Thus, by appropriately choosing gas pressure, bias voltage and bias-assisted Si ratio, the stress values of Mo/Si multilayers can be reduced to the order of -100 MPa with an EUV reflectance loss of about 1%.