Photoresist application The wafer is covered with photoresist liquid by spin coating. Thus, the top layer of resist is quickly ejected from the wafer's edge while the bottom layer still creeps slowly radially along the wafer. In this way, any 'bump' or 'ridge' of resist is removed, leaving a very flat layer. However, viscous films may result in large edge beads which are areas at the edges of the wafer or photomask with increased resist thickness whose planarization has physical limits. Often, Edge bead removal (EBR) is carried out, usually with a nozzle, to remove this extra resist as it could otherwise cause particulate contamination. Final thickness is also determined by the evaporation of liquid solvents from the resist. For very small, dense features (< 125 or so nm), lower resist thicknesses (< 0.5 microns) are needed to overcome collapse effects at high aspect ratios; typical aspect ratios are < 4:1.
PhOTorESiSt apPLiCatiOn the waFEr Is COVered wIth pHotorESiST liquid bY sPin coATInG. ThuS, THe ToP lAYEr of Resist is quIckly eJecTed From The wafer's edge While tHe bottom layer sTill Creeps SloWly Radially aLoNG The WaFer. IN this wAY, Any 'BUMP' oR 'RiDGE' of ReSist iS REMOvEd, leaviNg a very fLat LayeR. hOWever, ViScOus FIlMs may Result in lArgE eDgE bEads wHiCH arE arEas at The edges of tHe Wafer Or PhOToMask with iNcreAsed resIst THickness wHose plANaRiZaTIon Has PhYSIcAL limITs. Often, edgE beaD ReMOval (EBr) is cARRied Out, usUalLY wIth a NozzLe, to remove tHiS EXtra RESist As it cOULd otHErwIse CAuse ParticuLatE cOnTaMiNAtioN. Final ThiCkness is also DETErmInEd BY the evApOrATion of lIQuid solventS froM tHe resisT. fOr verY small, dEnSe fEAturEs (< 125 oR so nM), loWer rEsisT thiCknESses (< 0.5 MicROnS) ARE neeDEd tO overcoMe CollapSe effectS aT high aspEct RatIos; tyPiCal aspect ratiOs ARe < 4:1.