Silicon Field Emitter Array by Fast Anodization Method
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SILICON FIELD EMITTER ARRAY BY FAST ANODIZATION METHOD Y.M. Fung, W.Y. Cheung, I.H. Wilson, J.B. Xu and S.P. Wong Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, P. R. China ABSTRACT A new fast fabrication method entailing, two step anodization of silicon with different HF solutions was used to form a high aspect ratio silicon Field Emitter Array on n-type silicon (resistivity of 0.01Ωcm). A Silicon oxide mask was used to define the field emitter array. The silicon substrate was pre-anodized with low current density for 1 minute in the dark and then anodized in HF:H2O:Ethanol solution. Finally, the porous silicon was removed by isotropic solution etching. The turn-on voltage of the fabricated field emitters was approximately 27V/µm when the emission current density reaches 1µA/cm2. This compares with the turn-on field of about 35V/µm on silicon tip array fabricated by using an isotropic etching solution of HNO3. We obtained field emitter arrays with good uniformity and reproducibility. INTRODUCTION The fabrication of a conventional silicon field emitter array (FEA) is by isotropic wet etching (nitric acid) or by anisotropic wet etching (potassium hydroxide). The usage of wet solution etching has several problems: (1) the etching rate is too slow and (2) the uniformity of the silicon tip size is not good because the etching rate depends on the solution flow rate onto the wafer. Electrochemical etching by using hydrofluoric acid (HF) is widely used for the formation of porous silicon. This has been studied since the 1950s[1] and much attention has been paid to the formation mechanisms[2]. It is used for the application of fabricating thick buried silicon dioxide layer for silicon on insulator applications[3-4]. Recently, porous silicon has been applied to the fabrication of microelectromechanical systems (MEMS)[5] and silicon field emitter[6-7]. The porous silicon has been formed can be removed easily with nitric acid solution at room temperature. Using this technology for fabrication of field emitter arrays, the problem of variable chemical etching rate of silicon can been overcome. In this work, in order to overcome the problems of conventional silicon FEA fabrication method due to the usage of wet solution etching, we propose a new fabrication method using anodization. This method can produce uniform and symmetric silicon field emitters. Anodization was used to form thick porous silicon layer which was removed by chemical etching. The microstructures of the porous silicon formed, which affects the etching rate of the porous silicon, can be controlled by varying the anodization parameters such as etchant concentration, anodization time, current density, substrate resistivity and substrate type.
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EXPERIMENTAL DETAILS The sample is etched in two different concentrations of etching solution at a constant current density of 90mA/cm2. The schematic of the electrochemical etching cell used for the silicon tip f
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