Brent Schwab

All Wet Stripping of Implanted Photoresist

Introduction Photoresist stripping in IC manufacturing has become more challenging. The number of photoresist levels has increased while the allowable material loss and allowable surface damage has decreased. Heavily implanted photoresist is especially challenging due to the dehydrogenated, amorphous carbon layer that forms on the surface [1]. The carbonized layer can be removed by plasma etching, or can be broken up by physical processes such as ion bombardment or the swelling of the underlying photoresist material. Physical processes, however, tend to leave residues where the carbonized resist contacts the wafer at the edges of features and particularly at the inner boundary of the wafer edge bead removal area. In addition, new plasmadoped (PLAD) implant processes provide very high implant doses and require complicated, multi-step ashing sequences for complete resist stripping. Plasma ashing processes used to strip implanted photoresist tend to oxidize the wafer surface and cause an unacceptable increase in Si material loss in subsequent processing steps. Interest in ash-free, all-wet stripping processes is driven primarily by the desire to reduce surface damage and material loss, but is also by a desire for a simplified stripping process for PLAD implants, and the elimination of a process step for all implants (wet strip/clean vs dry ash followed by a wet clean). A liquid mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2), also known as “piranha” or “SPM”, can be used to remove photoresist that is unimplanted or only lightly implanted, up to about 1x10 ions/cm. When H2SO4 is mixed with H2O2, monopersulfuric acid (H2SO5 or “Caro’s acid”) is formed. Caro’s acid, and to some degree H2SO4 itself, breaks down the undamaged carbon polymer chain, eventually forming H2O and CO2 reaction products [2]. Caro’s acid, however, does not effectively remove heavily carbonized resist. Fortunately, H2O2 and Caro’s acid break down to form radicals of OH and HSO4. These radicals rapidly react with the carbonized layer, but are very short-lived (lifetime ~10s,) and so are present at a very low concentrations. Current piranha processes are heated as high as 150°C in order to accelerate radical formation and achieve sufficient reactivity and stripping rates on partially carbonized resists. If we assume Arrhenius behavior, and a radical formation activation energy of 200 kJ/mol, then the radical formation rate (and therefore concentration) will increase by over 400 times with a temperature increase from 150°C to 200°C. The rate of attack by Caro’s acid will be similarly increased. While beneficial for stripping, this rapid decay makes 200° C immersion processing impractical. In this work, achievement of freshly mixed chemistries with 200°C on-wafer temperature has enabled the wet stripping of implanted photoresist exposed to doses of over 1x10 ions/cm. Solid State Phenomena Online: 2007-11-20 ISSN: 1662-9779, Vol. 134, pp 109-112 doi:10.4028/www.scientific.net/SSP.134.109

Advanced Cryogenic Aerosol Cleaning: Small Particle Removal and Damage-Free Performance

The performance of a new cryogenic aerosol process was evaluated for cleaning nanoparticles and providing damage-free processing. Particle Removal Efficiency (PRE) tests conducted with wet deposited 40 nm, 30 nm and 18 nm silica particles on 300 mm wafers demonstrated cleaning efficiencies above 80%. Damage-free capability of the cryogenic aerosol process was evaluated with poly-silicon lines with an aspect ratio of approximately 9:1. These results highlight the potential of this new cryogenic aerosol to provide semiconductor device yield benefits by reducing small particulate contamination without causing pattern damage.