{HEAD ~ The spacer} preferably comprises {TAIL ~ a material} selected from the group comprising refractory metal silicides and nitrides 
preferably comprises
{ HEAD ~ The spacer } preferably comprises { TAIL ~ a material } selected from the group comprising refractory metal silicides and nitrides
HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH