The Kinetics of LPCVD Tungsten Deposition in a Single Wafer Reactor

Abstract

The rate equat ion for the low pressure chemical vapor depos i t ion (LPCVD) of tungsten from tungsten hexaf luor ide on (100) si l icon was exper imenta l ly determined in a laboratory scale single wafer vacuum reactor. The reactor was des igned and bui l t as a h igh vacuum stainless teel system with a min imum of heat and mass t ransfer l imitat ions. The tungsten film depos i t ion is in it iated by the si l icon reduct ion of tungsten hexaf luoride. In the absence of hydrogen, the si l icon reduct ion results in a 10-40 nm self- l imiting tungsten deposit, w i th the th ickness dependent upon the nat ive oxide layer pr ior to deposit ion. The hydrogen reduct ion of tungsten hexaf luor ide is one-hal f order in hydrogen, zero order in tungsten hexa-1 f luoride, and has an act ivat ion energy of 73,000 J mol- at temperatures f rom 561 to 683 K, and pressures f rom 0.067-1.3 kPa 4 1 05 8 1 0 (0.5 to 10 torr). The pre-exponent ia l factor was found to be 6.8 • 10 nms Pa " (4.7 • 10 A ra in- torr- ~). A mechan ism is proposed and two possible rate l imit ing steps yield rate express ions cons is tent wi th the observed kinetics. The rate l imit-ing step could be e i ther the addi t ion of adsorbed monatomic hydrogen to adsorbed part ial ly f luor inated tungsten or hy-drogen f luoride desorpt ion. In the absence of hydrogen the depos i t ion was perfect ly select ive over all ranges of tempera-ture and pressure. Dur ing the hydrogen reduct ion se lect iv i ty was lost in less than 600s at temperatures above 653 K (380 ~ Due to the observat ion that loss of selectivity is an act ivated process, it is conc luded that depos i t ion of tungsten on oxide is in it iated by a chemica l react ion rather than by random surface defects. Due to the h igh vacuum procedures, fi lms