Planarization of high topography surfaces with deep holes and cavities using two-step polymer coating

• A unique two-step coating of PPC and PI has been developed to planarize surfaces with high topography.
• Substrates with 70 μm deep holes and 100 μm large cavities have been well planarized for photolithography.
• Silicon dioxide films are allowed to be deposited on the planarized substrates using PECVD.

Pattern transfer on structured surfaces is still a technical challenge in MEMS fabrication due to the accumulation of photoresist at the structure base and the discontinuity at the structure opening. This paper reports a planarization method which employs a two-step polymer spin-coating to cap the structures to facilitate pattern transfer on structured surfaces. The first spin-coating uses heat-depolymerizable poly (propylene carbonate) (PPC) to either partially fill or completely intersect the structures, and the following spin-coating uses polyimide (PI) to form a complete capping film on the structured surfaces. The thermal decomposable feature of PPC allows it to decompose as a sacrificial layer during PI curing, leaving only PI films on the structured surfaces after the two-step coating. High topography surfaces with deep holes with diameter of 25 μm, long trenches with length of 500 μm, and large cavities with width of 100 μm, have been well planarized using the proposed method. Subsequent photolithography verifies that photoresist can be well applied onto the planarized surfaces, and the pattern transfer results are satisfactory. Silicon dioxide deposited using plasma enhanced chemical vapor deposition (PECVD) and aluminum deposited using sputtering are also demonstrated on planarized surfaces. The preliminary results demonstrate the feasibility of planarization of high topography surfaces using the two-step coating method.