Reflow mechanisms of contact vias in VLSI processing.

During glass reflow of contact vias in VLSI processing, occurrences of a side-wall overhang (i.e., bulging) and, in some extreme cases, closure of vias were observed when a low-temperature flowing borophosphosilicate glass was used with closely-spaced vias. A detailed consideration of the forces and mechanisms involved in reflow indicates that neither the chemical nature of the glass nor the specific processing parameters of reflow are factors in controlling this phenomenon. Consideration of an idealized geometry shows that a side-wall overhang within vias will occur after reflow if the glass thickness is more than a critical fraction of the spacing between vias; this critical fraction is calculated to occur near a value of 0. 393. At all glass thicknesses a certain amount of overhang will develop at an intermediate stage of reflow, but then this overhang disappears for a sub-critical glass thickness or grows for a super-critical glass thickness. In extreme cases, a large overhang can lead to spreading at the glass - substrate interface which can contribute to total closure of vias. Examination of via profiles with varying aspect ratios of BPSG films subjected to relevant reflow cycles is used to provide a detailed experimental confirmation of this model.