Minimizing Mobile Ion Damage during the Ash Process

It is generally accepted that mobile ion contamination and the subsequent device damage is a function of the amount of positively charged ions in the ash gas. As device dimensions shrink and gate dielectric thicknesses decrease, the potential for device damage at the photoresist strip level increases. Ozone ashing traditionally has been known to have extremely low ion concentrations in the gas, while barrel, radio frequency, and downstream microwave ashers have posed more damage issues. At the same time, implant and etch conditions frequently make resist harder (or with a carbonized top layer) and resist removal becomes more difficult without device damage. Fusion has developed a downstream microwave asher that produces extremely low ion concentrations while still maintaining excellent ash rates for different applications. Data from the Fusion asher is compared with commercially available ozone and plasma systems under a variety of temperature and RF power conditions. MOS device wafers were used as the primary method of testing mobile ion damage. The mobile ion results of wafers ashed with the Fusion downstream plasma asher are superior to those of wafers ashed with other plasma ashing systems and are comparable to those obtained when resist is removed by ozone asher or conventional wet stripping methods.