Megasonic Enhanced Photo Resist Removal Using Unique Solvent Free Phase-Fluids on an Advanced Single Wafer Resist Processing System
Authors: Dussault Donald , Weinhart Martin , Zenger Tobias , Eichinger Igor 
Published: February 2020

Removal of photoresist materials (resist stripping) is very straightforward in theory, but can be difficult and complicated in practice. Classical methods have included SPM (sulfuric peroxide module) /piranha, ozone and UV ozone, solvent based fluids and plasmas. All of the legacy methods include hazardous and toxic materials and/or high equipment expense as well as multi-step complexity. The stripping process becomes even more challenging by factors such as thick resist, cross-linked resist or sensitive metals or materials under the resist. This study was carried out In order to determine feasibility of the use of a unique phase-fluid based photoresist-stripper in commercial semiconductor resist removal process steps. Due to their highly dynamic inner structure, phase-fluids penetrate into the polymer network of photoresists and lift the material from the surface. As these water-based stripping fluids are non-aggressive, non-toxic, and require no special handling, the ability to apply them to industry standard resists will result in a reduction of toxic and dangerous chemistries and the environmental impact of their disposal. To investigate the impact of megasonic energy on the process time in both the active and rinse steps and therefore throughput, the study included the introduction of megasonics as a process variable in all test combinations. The study was broken down into two phases. Phase one consisted of a broad spectrum screening process incorporating nine standard photoresists Fig. 1 with various standard thermal treatments as well as a selection of five phase-fluid formulations. The tests were carried out in a single substrate stagnant immersion tank incorporating a megasonic transducer illustrated in Fig. 2. Process time was defined as the time from immersion of the substrate until the photoresist was completely removed. Process variables tested for each resist/phase-fluid combination included three process temperatures, ambient, 40c and 60c as well as the addition of megasonic energy at ambient with a frequency of 925KHz at a dosage of 1.5W/cm2 directly coupled. The results achieved in this phase one screening were used to select the best case phase-fluid, and process conditions for each of the screened photo resists. These conditions became the baseline parameters for phase two. Phase two of the study was carried out in actual single wafer production conditions on an EVG®100 Series Resist Processor equipped with a MegPie® large area megasonic transducer. Process recipes were established using phase one parameters and then refined for optimum total process time, including the actual phase-fluid process time and rinse time. Spin off and dry cycles were kept constant in all process recipes. Complete resist strip process times of <20 seconds were achieved with several photoresist phase fluid combinations while other combinations involving BCB based resist resulted in little to no removal. The results achieved in phase two testing demonstrate the feasibility of a phase-fluid based photo resist strip in a single wafer platform with megasonic enhancement for specific photo resists.

Figure 1