Nitrogen annealing effect on resistivity of porous silicon

Porous silicon (PS), normally fabricated by electrochemical anodisation of crystalline silicon in an HF/Ethanol solution to introduce nanoporous holes in its microstructures, has drawn significant attention for application in Microelectromechanical systems (MEMS) and sensing, due to the large surface area and control of film properties through porosity. However, as-fabricated PS are not compatible with standard photolithography based MEMS fabrication process, since the alkaline based developer used in photolithography will remove as-fabricated PS in a few seconds. In addition, aged (oxidized) porous silicon film in air will result in unstable change of porosity and optical thickness. Therefore passivation using nitrogen under certain temperature (nitridation) has been developed in our group to prevent the properties of the films from changing over time, and to form a SiOxNy layer on PS surface to make it compatible with photolithography. Based on nitridation of PS film, processes compatible with standard micromachining processes have been developed to release all-mesoporous silicon microstructures with limited residual stress and stress gradient. The released PS has uniform lateral porosity and suggests a path for fabricating complex, scalable structures with porous silicon.
Through control the porosity of PS film during formation process, desired mechanical, thermal, electrical and optical properties can be achieved. Our study and simulation have shown that the noise equivalent temperature difference (NETD) and thermal time constant can be tuned through porosity, which also suggests the much better performance in thermal sensing of porous silicon compare to current dominant commercial materials such as VOx and hydrogenated amorphous silicon (α-Si:H). In addition, to build a thermal detector (bolometer) based on all-silicon structure will avoid material incompatibility issues associated with VOx and α-Si:H. However, to utilise PS as a functional material in thermal sensing, released microstructures based on nitridised PS will be required, therefore the effect of nitridation on PS resistivity needs to be studied, achieving low resistivity after annealing is expected to electrically actuate the released PS microstructure.
In this work, metal/PS/Si/metal structure was employed to study the nitrogen annealing effect on resistivity of PS. Resistivity change abstracted from I-V curves of PS films under annealing temperatures from 300 oC to 600 oC (which is typically used for nitridation of PS in MEMS fabrication) were investigated, as shown in attached figure. Significant change can be observed with the annealing temperature, indicating an 8-order decrease of resistivity after 600 oC annealing. It is also found annealing temperature could affect the contact of metal/PS, resulting in resistance increase with temperature. The role of porosity on film resistivity is still under investigation, but limited results have shown significant resistivity decrease with PS film porosity. In addition, it is found that thermal heating effect is significant between metal/PS contacts under voltage higher than 10V, therefore only low voltage range (-1V to +1V) was employed in the investigation.