Reflow Mechanisms of Contact Vias in VLSI Processing

Abstract

During glass reflow of contact vias in VLSI processing, occurrences of a sidewall overhang (i.e., bulging and, as a result, formation of a reentrant angle at the base of a via) and, in 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 altering this phenomenon. Consideration of an idealized geometry shows that a sidewall 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 0.4. At all glass thicknesses, a certain amount of overhang will develop at an intermediate stage of reflow, but then this overhang disappears for a subcritical glass thickness or grows for a supercritical glass thickness. In extreme cases, a large overhang can lead to spreading at the glass‐substrate interface, which contributes to closure of vias.