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As shown in Figure 2317, the steps in the technique of two-layer resist etch-back planarization are:
          i) A photoresist layer is spin-coated.
          ii) The photoresist layer is countermasked. For instance, [1] this technique used a special mask, with a general oversizing of 0.6 µm and an exclusion of all STI critical dimensions smaller than 1.55 µm, the zones with smaller STI dimensions being masked using a special narrow lines grid. Such a masking method avoids misalignment problems appearing with countermasking and direct etching technique, the resized first photoresist blocks are reflowed in STI topographies, leading to an easy planarization by the second resist layer. Additionally, the use of a conventional i-line resist makes the lithographic step a noncritical step.
          iii) The photoresist layer is developed and thermally cured.
          iv) The second photoresist layer is spin-coated to planarize the residual topography.
          v) The flat surface is transferred into the substrate using conventional plasma etch back. The etch-back planarization was performed with a LAM 4520 Rainbow plasma etcher using (Ar:CF₄ :O₂) gas mixture.
          vi) CMP was performed using a STEAG 550 machine, with Klebosol slurry and Rodel -IC 1400/ Suba 4, under the conditions of 4 PSI pressure and 50 rpm speed.

Figure 2317. Two-layer resist etch-back planarization. [1]

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[1] André Schiltz, Laëtitia Palatini, Maryse Paoli, Maurice Rivoire, and Alain Prola, Plasma etch-back planarization coupled to chemical mechanical polishing for sub 0.18 μm shallow trench isolation technology, Journal of Vacuum Science & Technology A 18, 1313, doi: 10.1116/1.582346, (2000).