Non-Ashing High Dose Implant Strip in IC Manufacturing 1Souvik Banerjee, 2Peggi Cross, 1Ramesh Borade, 2Srini Raghavan, and

3Russell Stevens1BOC Eco-Snow Systems, 2 The University of Arizona at Tucson, 3ATMI

High dose implant strip (HDIS) in front-end-of-line (FEOL) is done after the source and drain (S/D) implants with Arsenic, Boron, or Phosphorous ions at dosage levels of greater than 1E15 atoms/sq. cm. and less than 100 keV implant energy. The resist, used as an implant mask, gets heavily cross-linked and impregnated with ions from the implantation process. The ion implantation also depletes the resist surface of hydrogen atoms thereby transforming it into a dense, carbonized, outer layer known as the crust. Following the S/D implant, the resist is stripped from the wafer surface. Current stripping practice involves 2-step plasma dry strip or ash followed by wet cleaning with SPM and SC1 chemistries. The ash step is done at temperatures of 120 to 350 C using Oxygen plasma. The oxidizing ashing step, results in silicon and oxide losses and increase in equivalent oxide thickness (EOT). The ITRS roadmap limits to less than 1Å of oxide and silicon losses and negligible increase in EOT, conditions not met with current ash process. Therefore a non-ashing method of HDIS is desirable at 65 nm technology node and below. This paper describes a new method of HDIS, using a combination of CO 2

cryogenic and aqueous cleaning. The CO2 cryogenic cleaning works by momentum transfer of the CO2 snow particles

formed during the atomization and expansion of high pressure (850 psi) liquid CO2 through a specially designed nozzle. The expansion of liquid CO2 through the nozzle creates solid and gaseous CO2 in a highly directional and focused stream. Contaminant removal occurs by momentum transfer by the solid CO2 followed by the drag force by CO2 gas. Thus the cleaning mechanism in cryogenic aerosol is suitable for application where impact on underlying film has to be minimized.

Results will be presented on 5000 Å blanket 248 nm DUV resist coated wafers, implanted with 5E14 or 1E16 atoms/sq. cm. of As ions. SEM cross-section of the wafers indicate that dense crust is 30% of the resist thickness for 5E14 and 37% for 1E16 atoms/sq.cm. dosage levels. Results of cleaning experiments with the above wafers will be presented using optical microscope images showing removal in a total time of about 150s for both type of implants. The cleaning efficiency was determined using energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). EDX done on one of the earlier experiments in which 99% of the resist was removed, showed that Carbon decreased to 7% on a clean area of the wafer from 92% on a spot of resist left behind. The Silicon atomic percent increased from 3% when measured on the resist spot to 91% as measured on the clean area. Subsequently, process optimization was done to remove 100% of the resist film by the combination of dry cryogenic aerosol and wet cleaning. The effect of the dry and wet cleaning combination on poly Silicon and oxide films were measured. The data showed no change in film thickness of either films within statistical error limits.

The need for developing a non-ashing HDIS exists at 65 nm technology node and lower in semiconductor manufacturing. Cryogenic aerosol cleaning using CO2 has been demonstrated in the past to be successful in removing sub-micron particles without etching or damage. This same technology has been developed to remove ion implanted photoresist in an unique combination with wet cleaning. The dry-wet combination overcomes many of the problems associated with plasma ashing, which has been the dominant photoresist strip technology in IC industry so far.