Ambient controlled two-step thermal treatment for spin-on coating layer planarization

A method for forming a semiconductor structure, comprising:

• etching a dielectric layer on a substrate to form a plurality of first trenches having a first pattern density in a first region of the substrate and a plurality of second trenches having a second pattern density in a second region of the substrate, the first pattern density being greater than the second pattern density;
• applying a spin-on coating (SOC) layer comprising a cross-linkable material on the substrate to fill the plurality of first trenches and the plurality of second trenches, a portion of the SOC layer overlying the plurality of first trenches being thicker than another portion of the SOC layer overlying the plurality of second trenches;
• baking the SOC layer at a first temperature ranging from 80° C. to 110° C. under a first atmosphere with an oxygen level greater than 20% to remove a solvent from the SOC layer;
• heating the SOC layer to a second temperature ranging from 120° C. to 200° C. under a second atmosphere with an oxygen level less than 5% to reduce a thickness variation of the SOC layer in the first region and the second region, wherein the second temperature is higher than the first temperature, and the thickness variation of the SOC layer is reduced by at least about 50%;
• and
• cross-linking the cross-linkable material at a third temperature ranging from 150° C. to 300° C. under a third atmosphere with an oxygen level greater than 20%, wherein the third temperature is higher than the second temperature.

A method for forming a semiconductor structure, comprising:

• depositing a dielectric layer over a substrate, the dielectric layer having a planar top surface;
• forming trenches in the dielectric layer, the trenches comprises a first set of the trenches in a first region of the substrate and a second set of the trenches in a second region of the substrate, the first set of the trenches having a first pattern density, and the second set of the trenches having a second pattern density less than the first pattern density;
• depositing a bottom layer comprising a cross-linkable precursor polymer over the substrate, the bottom layer filing in spaces between the trenches and having a non-planar top surface, wherein a top surface of a first portion of the bottom layer over the first set of the trenches is located above a top surface of a second portion of the bottom layer over the second set of the trenches;
• heating the bottom layer at a first temperature and under a first atmosphere with a first oxygen level of less than 5% to reduce a height difference between the top surface of the first portion of the bottom layer and the top surface of the second portion of the bottom layer;
• heating the bottom layer at a second temperature and under a second atmosphere with a second oxygen level of greater than 20% to crosslink the cross-linkable precursor polymer, thereby forming a crosslinked bottom layer;
• depositing a middle layer over the crosslinked bottom layer;
• and
• forming a photoresist layer over the middle layer.