Devitrification in Borophosphosilicate Glass Films Used in VLSI

Borophosphosilicate glass (BPSG) films are very widely used as flowable layers in silicon-gate MOS integrated cir-cuits (1-8). Recent industry trends toward lower tempera-ture processing of integrated circuits (4, 9) have been ac-companied by a number of cases of devitrification in BPSG films, with crystalline precipitates forming in the amorphous BPSG matrix. This technical note discusses conditions which result in formation of the precipitates and indicates their identity. BPSG films are generally deposited over patterned poly-silicon (or other refractory gate conductors), flowed to pro-vide a semiplanarized surface, patterned to provide con-tact cuts, and then reflowed to provide rounded contours at the edge of the contact uts. Originally, BPSG composi-tions containing on the order of 4 weight percent (w/o) each of boron and phosphorus were used (1-4, 6, 8, 10), and flow temperature was 900~176 As with PSG films (11, 12), steam ambients were frequently used to produce a greater degree of flow (2, 3, 5, 13-16). In recent years, there has been a tendency to reduce pro-cess temperatures to minimize diffusion effects in the sili-con, particularly in high-density VLSI and ULSI inte-grated circuit wafer fabrication. As a result, device manufacturers have increased the boron and/or phos-phorus content of BPSG films (5, 13, 17-19) to achieve ade-quate flow at lower temperatures such as 850 ~ They have also preferred to control reoxidation and to flow BPSG films in dry ambients to avoid excessive oxidation under the edge of the patterned polysilicon lines [the smiling polysilicon effect or gate bird's beak (9)]. Dry ambients re-sult in less glass flow than wet ambients at the same tem