Trench planarization techniques

A method of planarizing over-filled trenches in a semiconductor device, consisting essentially of:

• forming a trench in a substrate by applying a patterned mask layer of nitride on a surface of the substrate and etching the trench through openings in the mask layer, said trench having a relatively large width dimension;
• over-filling the trench and mask layer with low-temperature-oxide (LTO) trench-fill material, said trench-fill material exhibiting a trough above the trench, the trough being narrower than the trench and generally centered above the trench;
• applying a cap layer of nitride over the LTO trench-fill material at least in the trough;
• said cap layer and said mask layer being substantially harder than the trench-fill material, vis-a-vis chemical-mechanical polishing;
• a portion of said cap layer material in the trough being substantially coplanar with the mask layer and generally centered above the trench;
• chemical-mechanical polishing the trench-fill material and the cap layer material until the nitride mask layer and the coplanar portion of the cap layer are exposed;
• and
• subsequently stripping away the mask layer and the portion of the cap layer;
• whereby the portion of the cap layer overlying the trench fill material in the trench and substantially at the level of the mask layer causes the relatively wide trench to behave effectively as two smaller (narrower) trenches during the polishing process.