Modified Phosphosilicate Glasses for VLSI Applications

In this study, the glass transition temperature T_g is determined from thermal expansion measurements in a series of glass compositions prepared by bulk fusion of a phosphosilicate matrix to which oxides of boron, germanium, or arsenic are added. On a per-molar basis, addition of boron was observed to be the most effective in lowering the glass transition temperature of the phosphosilicate matrix, as compared to arsenic or germanium, which is found to be the least effective. For borophosphosilicate glass with concentrations of B₂O₃ less than 13 m/o, values of T_g are observed to be reduced by increasing the P₂O₅ content. Near 13 m/o B₂O₃ the temperatures become independent of P₂O₅ while at a still higher B2O3 content they are actually raised upon increasing the P₂O₅ concentration. For the germanophosphosilicate glass, no such reversal is noted as values of T_g decrease slightly with P₂O₅ additions at all GeO₂ concentrations. Glass compositions with potential VLSI applications can, thus, have the P₂O₅ content reduced to the minimum value required to provide gettering without significantly affecting the glass transition or associated viscous flow parameters. From a comparison of the measured values of T_g in bulk glasses with reported values of the flow temperature T_f for films with corresponding compositions, a correlation factor relating those two parameters is established. Thus, the relatively simple thermal expansion technique can now be used to survey for glass compositions with promising flow characteristics without the rigorous and time consuming effort of conducting the more difficult bulk viscosity experiments or developing deposition processes for films with possibly unsuitable viscous characteristics.