hai sumit,
1>
normally in our company we will pick a cell [medium dirve strength ] random form all logic ..eX:- and,or,nor,mux,nand etc..,
2>
up to my knowledge spare cells are not connected respect to clock.. So far we havent done separate group spare cells for clock.. if so please explain..
3> I have experience on soc encounter.. In that we will set box with assumption not accurate.. we will place less spare cells if we are have small areas.. based on that we set xstepx and xstepy value..
i herewith attached a sample for u
set spareModuleName "SPARE_CELLS"
set xStep 75
set yStep 75
#Creating Spare module
createSpareModule -moduleName $spareModuleName -cell { ADDFHX8M AND3X1M AO21XLM AO22X1M AO2B2BX1M AOI2BB1X8M BUFX12M CLKINVX1M CLKNAND2X16M CLKXOR2X2M INVXLM NAND4BBXLM NOR2X5M OR3X2M XNOR3X2M MX2X2M } -tie $tie_cells
placeSpareModule -moduleName $spareModuleName -util 0.3 -stepx $xStep -stepy $yStep -offsetx 20 -offsety 20