For MEMS applications it will be more and more important to fabricate thin membranes with low intrinsic stress while maintaining high mechanical and structural performance in terms of rigidity and resilience. In this paper a new approach to obtain stiff thin membranes is presented as an alternative to conventional membranes obtained by bulk micromaching methods (like chemical or electro-chemical etch stop technique). The reinforcement of the membranes is achieved by vertical structures obtained by filling a mold of deeply etched silicon trenches with different materials (SiO2, Si3N4, polysilicon) and subsequent removal of the bulk silicon. The theoretical estimation of the rigidity of a plate with vertical enforcement strips has been studied by the comparison of the inertial modulus of beams with T-shaped section. The mechanical behavior of membranes stiffened by vertical structures with different dimensions (in terms of depth, thickness and pitch) has been investigated through simulation using ANSYS as finite elements analysis software. The silicon trenches have been fabricated by deep reactive ion etching (DRIE) based on SF6/O2 plasma, using a 500 nm thick TEOS as masking layer. These structures have been subsequently sealed by deposition of a silicon oxide layer and filled by poly-silicon. The silicon oxide has been used both to calibrate the trench dimensions and as etch-stop layer for the TMAH anisotropic etch that is required to release the three dimensional structure. For this reason, some experiments have been carried out to define the best conformality condition in terms of thickness uniformity of the layer between different type of silicon oxide (thermal oxide, TEOS, LTO). In order to minimise the intrinsic stress distribution due to the deposition process and to optimise the polysilicon conformality, the effects of the annealing temperature and the boron doping conditions have been characterised. The mechanical behaviour of these structures has been measured under different load conditions.
Vertically structured thin membranes by a lost mold technique
Faes, Alessandro;Ferrario, Lorenza;Guarnieri, Vittorio;Giacomozzi, Flavio;Lui, Alberto;Margesin, Benno;Zen, Mario
2001-01-01
Abstract
For MEMS applications it will be more and more important to fabricate thin membranes with low intrinsic stress while maintaining high mechanical and structural performance in terms of rigidity and resilience. In this paper a new approach to obtain stiff thin membranes is presented as an alternative to conventional membranes obtained by bulk micromaching methods (like chemical or electro-chemical etch stop technique). The reinforcement of the membranes is achieved by vertical structures obtained by filling a mold of deeply etched silicon trenches with different materials (SiO2, Si3N4, polysilicon) and subsequent removal of the bulk silicon. The theoretical estimation of the rigidity of a plate with vertical enforcement strips has been studied by the comparison of the inertial modulus of beams with T-shaped section. The mechanical behavior of membranes stiffened by vertical structures with different dimensions (in terms of depth, thickness and pitch) has been investigated through simulation using ANSYS as finite elements analysis software. The silicon trenches have been fabricated by deep reactive ion etching (DRIE) based on SF6/O2 plasma, using a 500 nm thick TEOS as masking layer. These structures have been subsequently sealed by deposition of a silicon oxide layer and filled by poly-silicon. The silicon oxide has been used both to calibrate the trench dimensions and as etch-stop layer for the TMAH anisotropic etch that is required to release the three dimensional structure. For this reason, some experiments have been carried out to define the best conformality condition in terms of thickness uniformity of the layer between different type of silicon oxide (thermal oxide, TEOS, LTO). In order to minimise the intrinsic stress distribution due to the deposition process and to optimise the polysilicon conformality, the effects of the annealing temperature and the boron doping conditions have been characterised. The mechanical behaviour of these structures has been measured under different load conditions.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
