Dimensional Constraints on High Aspect Ratio Silicon Microstructures Fabricated by HF Photoelectrochemical Etching

Abstract

The fabrication of macropores in crystalline silicon by photoelectrochemical etching in a hydrofluoric acid electrolyte is investigated. It is shown that the dimensional constraints on the pore diameters, which, in previous literature, are considered to depend on substrate doping, can be significantly relaxed. We show that it is possible to fabricate arrays of square section macropores with sides ranging from 2 to 15 m using the same n-doped ͑2.4-4 ⍀ cm͒ silicon substrate. Moreover, we demonstrate that macropore arrays with pitch variation up to 100% ͑8 and 16 m͒ can be simultaneously grown on the same silicon sample. The same process is used to fabricate arrays of silicon walls with different spacing and pitch as well. A simple model, based on the coalescence in a single pore of multiple stable pores, is proposed to explain the experimental data.Electrochemical etching of silicon in hydrofluoric acid ͑HF͒ electrolyte is a well known technique for the formation of porous silicon. 1,2 The dissolution of silicon in the electrolyte is activated by holes. In p-silicon electrodes holes are the majority carriers, so that it is possible to produce porous silicon simply by etching the sample in an HF-based solution, even in the dark. For n-type silicon, the majority carriers are electrons, so that it is necessary to generate holes in order to produce porous silicon. Depending on the silicon doping and on the type of the anodized silicon substrate, different pore morphologies can be obtained. In fact, microporous layers characterized by random pores with nanometric dimensions can be easily grown on p-type substrates, 3 while macroporous layers with micrometric straight pores can be obtained from n-type substrates. 4 In the last case, by illuminating the rear surface of the wafer with sufficiently energetic photons, holes can be generated in the substrate by a process of photon absorption. Under anodic biasing conditions, the photogenerated holes move to the front surface and silicon dissolution takes place. Initially, the electric field concentrates at sharp defects on the flat wafer surface. Surface defects therefore act as seeding points for macropore formation. As the etch progresses, the electric field still concentrates at the pore tips, where most of the injected holes react with the electrolyte. Fewer holes are then available for the dissolution of the sidewalls, that are therefore protected against dissolution. 5 By prepatterning the wafer surface with defect sites it is possible to determine where macropores will form. KOH etching after a standard photolithographic step can be used to create pyramidal notches in the required positions which can act as an array of defects. Both random 5,6 and prepatterned 7 macropore arrays with high aspect ratio ͑up to 250͒ were fabricated through the wafer thickness 8 and on the whole wafer. 9-11 The macropore morphology significantly depends on the anodization conditions, such as current density, etching time, HF concentration, temperature, and bias, as well as on substrate pro...

Fabrication of regular silicon microstructures by photo‐electrochemical etching of silicon

“…[9] regular pores with sides ranging from 2 µm up to 15 µm and pitch variation up to 100% were fabricated on the same 2.4÷4 Ω·cm silicon substrate. Moreover, we showed that macropore formation is just a feature of photo-electrochemical etching and that by changing the initial pattern it is possible to fabricate a variety of silicon microstructures with more complicated shape [8]. As a matter of fact, fabrication of regular arrays of walls, tubes, spirals, pillars was demonstrated.…”

“…Applications of such structures to IR filters [5], photonic crystals [6] and micromechanical systems [7] have been reported as well. However, it was only very recently that photo-electrochemical etching has been verified as a tool for bulk silicon micromachining [8]. As a matter of fact, in ref.…”

Fabrication of one-dimensional photonic crystals by photoelectrochemical etching of silicon

“…Studies of optical characteristics of such photonic crystals showed that they can be successfully used in the mid IR region of the spectrum [2,3]. Later, simi lar structures were obtained using photoelectrochemi cal etching (PECE) of silicon with (100) orientation [4][5][6][7]; previously, this method had been mainly used to obtain a 2D periodic lattice in macroporous silicon [8][9][10]. The advantages of the PECE method as com pared to anisotropic etching consist in the use of a sub strate with the standard (100) orientation, in the absence of a necessity to precisely orient the pattern of the photomask with respect to crystallographic axes, and the possibility of fabricating simultaneously the trenches and macropores in a unified process [11,12].…”

Fabrication and doping of silicon micropillar arrays for solar light harvesting

“…[39] The shapes of these structures can either be defined by the shape of a physical object that is brought in close proximity of the substrate, or by using a masking layer, which can be deposited by various methods. [39,40] Using this technique, many complex shapes have been created, when the proper masking steps were used. [41,42] For example, in an comprehensive study on electrochemical etching, Bassu et al were able to fabricate a MEMS device, with high-aspect ratio (100) comb fingers suspended by high-aspect ratio folded springs.…”