AVS 50th International Symposium
    Plasma Science and Technology Monday Sessions
       Session PS+MM-MoA

Paper PS+MM-MoA2
Improvement of Anisotropy and Aspect Ratio of a Pattern Etched in Bosch Process by using a Faraday Cage

Monday, November 3, 2003, 2:20 pm, Room 315

Session: MEMS Etching
Presenter: J.-H. Min, Seoul National University, Korea
Authors: J.-H. Min, Seoul National University, Korea
G.-R. Lee, Seoul National University, Korea
J.-K. Lee, Seoul National University, Korea
S.H. Moon, Seoul National University, Korea
C.-K. Kim, Ajou University, Korea
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Bosch process, which consists of sequentially alternating etch and deposition steps using SF@sub 6@ and C@sub 4@F@sub 8@ plasmas, has been widely used for deep silicon etching in the fabrication of MEMS due to its advantages for obtaining patterns of high aspect ratio and anisotropy. Because the opening sizes of many MEMS structures are considerably large (about 1~100 µm), the electric field at the convex corner of a micro feature is locally distorted such that ions travel inside the etched pattern with a broad angular distribution. As a result, the flux of ions incident on the bottom surface is decreased with an increase in the etch depth, which eventually limits the maximum aspect ratio obtained in Bosch process. This limiting factor cannot be overcome by optimizing process variables. In this study, a Faraday cage, defined as a box made of conductor walls, was used to overcome this limitation. In the Faraday cage system, ions enter perpendicular to the sheath formed along the top grid plane of the cage and travel inside the cage maintaining the initial incident direction because electric potential in the cage is unaffected by outside voltages and therefore is the same throughout. Accordingly, the trajectory of ions, which has a narrow angular distribution determined by the grid pitch of and the sheath thickness on the top plane, is not changed at the convex corner of the micro feature or inside the pattern located in the cage. It was confirmed by an ion angular-energy distribution analyzer that the angular distribution of ions entering the pin hole of 10-µm-diameter, which is the same as the size of pattern opening, is narrower in the cage system than in the case of no cage. As a result, the aspect ratio and the anisotropy of the etched pattern were improved by using a Faraday cage in Bosch process.