Unique Applications of Megasonic Energy to MEMS Single Substrate Wet Processes

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The use of non-standard materials (e.g. specific substrates or polymer materials) for MEMS applications impose a requirement for the development of new techniques for even well-established process steps. Acoustic energy in the MHz frequency range has been used in the semiconductor industry for various processes such as post CMP cleaning, pre-bond and pre-deposition cleaning as well as photoresist development and stripping.The work presented in this paper is introducing two different types of applications of acoustic energy in Micro- Electro- Mechanical Systems (MEMS) manufacturing processes: photoresist processing and single wafer cleaning for high cleanliness applications (wafer bonding).A single wafer spin processer with triangular-shape transducer, the MegPie®, was used for both resist processing and substrate cleaning. The transducer covers a sector of the wafer and is placed in proximity to substrate surface (fig. 1).The small gap between transducer and substrate is filled with the required process fluid. The shape and positioning of the transducer assure optimal acoustic energy transfer to the process fluid resulting in high uniformity.Acoustic energy was used in the past in photolithography for enhancing the uniformity of, and accelerate resist development process, initially in LIGA applications.The new MEMS applications, particularly the ones based on microfluidic structures built in polymer layers coated on a substrate raised the demand of new photolithography processes able to resolve high aspect ratio structures. Such an optimized process addresses new coating/soft baking procedures, new UV exposure techniques as well as new development methods or new resist stripping procedures (fig. 2).Substrate cleaning during a process flow plays a crucial role for the final success of the process. The use of acoustic energy in the megasonic range in either batch or single wafer configurations is known to significantly enhance the cleaning results.Wafer bonding is one of the MEMs processes that is very sensitive to particle contamination and demands the use of high efficiency cleaning processes with good uniformity and repeatability.In a different process flow, particle neutrality and high particle removal efficiency were demonstrated for single wafer cleaning of Si wafers prior to fusion bonding. Experimental results showing the benefits of using acoustic energy for enhanced photoresist development and single wafer cleaning will be presented.Figure 1. Horizontal spinner chuck system with fluid delivery and MegPie® transducer.