by Ed Korczynski, Senior Technical Editor, Solid State Technology
April 23, 2008 – Making photomasks has historically been an incredibly difficult yet profitless part of the IC fab industry, and maskmaking equipment has to be innovative to handle the creation of phase-shift masks with sub-resolution assist features (SRAF). Cleaning has always been essential for mask manufacturing, of course, but there is even more concern with mask cleanliness today. Fabs (particularly memory) may soon start cleaning masks in-house during production use to prevent haze buildup. Also, both EUV and NIL could be in volume production within five years, and since neither approach allows for the use of a pellicle the mask would almost certainly need to be cleaned in production.
Addressing this potential need for mask cleaning is Applied Materials with its new Tetra Reticle Clean tool, based on a cluster of dry and wet processing chambers around an atmospheric robot handler mounted on a linear track. A top-down schematic provided by the company (see figure) shows a typical configuration with three wet and one dry processing chambers. The wet chambers have in-situ drying capability such that masks are handled “dry-in/dry-out” by the robot.
The wet chamber cleans both sides of the mask simultaneously. The mask is held at corners to float it over a chuck, which supplies cleaning chemistry as well as megasonic energy. The megasonic energy is coupled through the backside aqueous chemistry to the mask, so that uniform cavitation is supplied to the topside through the mask itself. Topside cleaning also occurs due to micro-droplets with kinetic energy from a point-source rastering across the spinning topside. SC1 plus surfactants, or ozone/DIW chemistries can be used.
Applied Materials has shown 99% particle removal efficiency (PRE) on blank masks, and zero damage seen on patterned masks with sub-50nm SRAFs which would be used for 32nm node processing. Since sulfur is known to contribute to haze growth, all chemistries including those used for resist-strip used are sulfur-free.
The dry (or plasma) etch chamber uses an inductively-coupled plasma (ICP) source to provide remote RF energy, and a gas distribution plate through which ions diffuse to reach the heated processing pedestal.
The company claims nine current customer engagements — which corresponds to every “Tier-1” mask shop in the world. Currently running in production at 45nm, the tool is reportedly cutting 30%-50% of the haze problem at a commercial fab. –E.K.