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A side-wall spacer process for releasing and sealing of post-CMOS MEMS pressure sensor membranes

: Walk, Christian; Görtz, Michael; Mokwa, Wilfried; Vogt, Holger

Verband Deutscher Elektrotechniker e.V. -VDE-, Berlin; VDE/VDI-Gesellschaft Mikroelektronik, Mikro- und Feinwerktechnik -GMM-:
MikroSystemTechnik Kongress 2017. Proceedings : MEMS, Mikroelektronik, Systeme, 23.-25. Oktober 2017 in München
Berlin: VDE Verlag, 2017
ISBN: 978-3-8007-4491-6
MikroSystemTechnik Kongress <2017, München>
Fraunhofer IMS ()
post-CMOS; MEMS; pressure sensor

In this work a sub-step of the fabrication of an absolute capacitive pressure sensor MEMS on top of a CMOS substrate is discussed. For low pressure applications membrane diameters of more than 100 μm are required. To apply an isotropic HF-vapour phase etching process for successful release of MEMS structures, an access to the sacrificial layer is required. Since vapour phase etching of a sacrificial layer by side openings is shown to be inappropriate due to the addressed diameters, vertically arranged openings in the structural layer of the MEMS-structure have been utilised. Different designs of possible etch accesses are simulated regarding their impact on stress. It is shown that a diameter minimisation of vertical etch-access tunnels is beneficial with respect to stress (under an applied pressure of 1500 hPa) and regarding the conditions for sealing the cavity. Therefore, a side-wall spacer process, which minimises the diameters of vertical etch-access tunnels was developed and discussed. By the use of two applied hardmasks the diameter of lithographically structured tunnels is reduced by about 260 nm (40%).