The effect of excessive thinning of the field oxide (dishing), observed in the planarization process by chemical mechanical polishing (CMP) of glass-filled isolation trenches, if reported for isolation spaces below 10 mu m for the first time. For spaces in the range of 1.0 to 10 mu m the amount of dishing was determined as 44 +/- 6 nm and was independent of CMP process parameters. Suggested mechanism for this kind of dishing implies removal of hydroxylated oxide surface in field regions by a noncontact hydrodynamic process when direct wafer-to-pad contact occurs with nitride surface in active regions only. The addition of reactive ion etching step after a shorter CMP step is shown to provide better planarity with no dishing and is suggested as an alternative to the CMP-only process.