By Yin Xu
Yin Xu, Vlad Tarasov, Wei Chen, Koukou Suu, ULVAC Technologies, Inc., Senior Supervisor, Process Engineering, 401 Griffin Brook Drive, Methuen, MA 01844, USA, Ph: (001) 978-686-7550, Fx: (001) 978-689-6301, Corresponding Author: firstname.lastname@example.org
The single foremost cause of yield failure in the manufacturing of MEMS devices is “stiction”: the unintentional adhesion of structural components to one another or to the substrate. Release processes and their specific recipes must be developed and optimized based on the combination of structural and sacrificial materials involved. In this study, stiction free removal of organic sacrificial layer in the MEMS release process is discussed. The exposed and embedded organic sacrificial layer is removed by an oxygen based microwave remote plasma process with a small amount of fluorine in a proprietary way. This process results in fully released MEMS structures without stiction failures.
The fabrication of Micro-Electro-Mechanical Systems (MEMS) derived from the integrated circuits (IC) industry, but has developed in its own ways and directions never anticipated by its IC counterpart. Now a highly specialized discipline in its own right, MEMS manufacturing utilizes not only all of the modern IC process techniques, but also novel fabrication methods and uses non-microelectronic materials to create complete microsystems with sophisticated structures and empty space inside the structure. MEMS devices have a wide variety of applications performing basic signal transduction operations as sensors and actuators. The unique nature of MEMS devices introduces new challenges and failure mechanisms in the manufacturing process, which is different from the IC device fabrication.
The single foremost cause of yield failure in the manufacturing of MEMS devices is “stiction”: the unintentional adhesion of structural components to one another or to the substrate. Because of the complicated topography of the MEMS devices, their surface area to volume ratio is very high, typically 100:1 through 10,000:1. At the same time, they are manufactured just a few microns above their supporting substrate. The combination of these characteristics makes MEMS devices very susceptible to surface forces, which can deflect the suspended members towards each other or the substrate. If the deflecting force is sufficiently strong, the MEMS structures can contact with and permanently adhere to the underlying substrate, causing stiction failure.
Release processes and their specific recipes must be developed and optimized based on the combination of structural and sacrificial materials involved. In this work, an all dry plasma ashing process is utilized to remove the photoresist organic sacrificial layer in the polysilicon MEMS release process. The exposed and embedded photoresist sacrificial layer is removed by a low temperature oxygen based microwave remote plasma process with a small amount of fluorine in a proprietary way. This process results in fully released MEMS structures without stiction failures. Existing fabs are able to use this MEMS release process with higher throughput and higher yield than their traditional processes.
The release step in the MEMS fabrication process selectively etches the sacrificial layer and releases the microstructures, creating the freestanding micromechanical structures such as cantilever beams. Several important criteria need to be considered here: (i) Release of MEMS structure without stiction failure, (ii) High ashrate removal of the sacrificial layer (in this case, photoresist), (iii) Complete residue removal, (iv) High selectivity of sacrificial layer to MEMS structure (in this case, MEMS structure is made of polysilicon).
All work was performed on the ULVAC EnviroTM dry resist and polymer removal system. This system incorporates both a microwave downstream plasma source and a non-damage RIE plasma source for resist stripping and residue cleaning. The effect of temperature, fluorine content, plasma source is investigated. The final stiction free release process is a low temperature, oxygen based microwave process with a small amount of fluorine. The nominal ashrate of the process is about 2 um/min, and the selectivity of sacrificial layer to MEMS structure is about 900:1.
Figure 1. Selectivity of photoresist to polysilicon with varying fluorine gas percentage.
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