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

Paper PS+MM-MoA7
Via Drilling on Silicon Wafers using the Cryogenic Process

Monday, November 3, 2003, 4:00 pm, Room 315

Session: MEMS Etching
Presenter: T. Tillocher, GREMI, France
Authors: T. Tillocher, GREMI, France
A. Basillais, GREMI, France
X. Mellhaoui, GREMI, France
P. Lefaucheux, GREMI, France
M. Boufnichel, ST Microelectronics
R. Dussart, GREMI, France
P. Ranson, GREMI, France
Correspondent: Click to Email
Plasma etching has an important place in microelectronics and microsystems industries. Some techniques, especially Bosch and cryogenic processes, enable the realization of high aspect ratio structures. The Bosch process is widely used in spite of scalloped profiles whereas the cryogenic process is quicker and presents smooth etched surfaces. Via (12 µm in diameter) drilling on silicon wafers is achieved by cryogenic etching in an industrial ICP reactor (Alcatel 601E). A SF6/O2 plasma is created and expands toward a polarized and cooled silicon wafer. Plasma parameters are optimised so as to obtain vias with an anisotropic profile with a high etch rate. End to end vias were performed with an average etch rate about 7µm/min by etching separately the two sides of the wafer. Yet, all of them did not meet right in the middle of the plate as it should be and their shape is not reproducible in the whole wafer. This seems to be due to a non enough homogeneous cooling system. Moreover, some defects, such as bowing and undercut lead to an increase of the final diameter of the via and hence must be limited. A process including a soft etching step, an over-passivating step, and a standard etching step leads to a significant reduction of these defects. O2/SF6 ratio is adjusted in the two first steps. Additionally, it was shown that bias voltage has a strong influence on the profiles : its increase is not synonym of a deeper etching but a slightly greater volume etched. Crystal orientation dependent etching also appeared at lower temperatures and particularly in the <111> direction what can explain negative slopes in this cases.