Study On The Fabrication Of Sub-micro Beams With Au/Si Galvanic Etch-Stop Technique

Sub-micron and nano beam is the fundamental structure of the device used in MEMS and NEMS.Because nano-beam is sensitive to the intrinsic stress in the structure,it is always made of single-crystalline materials.The fabrication of nano-stuctures needs high precision controllability.The p+ layer used in highly-doped boron etch-stop and pn junction used in electrochemical etch-stop is not thin enough to achieve thickness in sub-micro or nano dimension.Thus,the expensive substrate,for example,SOI wafer,is used to fabricate sub-micro and nano-structures wildly.In this paper,fabrication of the single-crystalline silicon sub-micro beams with the contactless electrochemical etch-stop technique is presented.According to the critical area ratio of Au/Si electrodes in that technique,the etch-stop is realized by controlling the exposed etching area of Si.Before beam is released,Au is contacted with substrate and the area ratio of Au/Si electrodes is less than the critical one.Once the beam is released,the beam and the Au electrode is separated from the substrate.The area ratio of Au/Si electrodes is more than the critical one,thus,the beam will not be etched.This fabrication process has several distinguishing points.Firstly,the structure with the thickness in sub-micro and nano dimension can be realized by avoiding shallow junction diffusion. Secondly,the thickness of the beam is determined by the shallow dry etch,which leads to high precision controllability.Thirdly,the beam is supported by Au electrode and insulated from the substrate.Fourthly,The thickness of the beam does not change with the etching time increases.Finally,this fabrication method with contactless electrochemical etch-stop technique has the advantages of high precision controllability,good uniformity and low cost.This method has not been reported yet,thus,is an original creation.The process to fabricate sub-micro beams on(100)and(111)wafers are designed respectively.The beam with the thickness of 235nm is released on the(111)wafer,which is insulated from the substrate.The thickness of the beam does not become thinner as the etch time increases.The sub-micro beam on the(100)wafer is released successfully too.Meanwhile,the contactless electrochemical etch-stop technique is investigated.The characteristics of the galvanic cell composed of 25%TMAH solution,Si and Au are analyzed.The I-V curves of the n-Si and p-Si electrodes under 60℃,70℃,80℃and the Au electrode are measured respectively.The etch-stop phenomenon as a result of galvanic cell formation is verified by the wafer-level chip and the separated electrodes experiments.