Self-organized pore formation and open-loop-control in semiconductor etching

Electrochemical etching of semiconductors, apart frommany technical applications, provides an interesting experimental setup for self-organized structure formation capable e.g. of regular, diameter-modulated, and branching pores. The underlying dynamical processes governing current transfer and structure formation are described by the CurrentBurst-Model: all dissolution processes are assumed to occur inhomogeneously in time and space as a Current Burst (CB); the properties and interactions between CB’s are described by a number of materialand chemistrydependent ingredients, like passivation and aging of surfaces in different crystallographic orientations, giving a qualitative understanding of resulting pore morphologies. These morphologies cannot be influenced only by the current, by chemical, material and other etching conditions, but also by an open-loop control, triggering the time scale given by the oxide dissolution time. With this method, under conditions where only branching pores occur, the additional signal hinders side pore formation resulting in regular pores with modulated diameter. Silicon monocrystals are the most perfect material mankind has ever created since they are essential for high integrated computer circuits. Due to this high degree of perfection, i. e. corrosion is not defect-driven, and “ideal corrosion” is possible: Silicon and other semiconductors can be made porous by electrochemical etching giving an outstanding variety of pore sizes, from nanopores to macropores, and geometries, including disordered nanoporous, dendritic-like sidebranching pores, and pores with modulated diameter. Abstracting from the details of the underlying electrochemical processes, the Current Burst Model together with the Aging Concept, and accounting the interactions between Current Bursts, the generation of different pore geometries, oscillations and synchronization phenomena can be explained including the percolation transition to global oscillations. Based on the time scale derived from the Aging Concept, an open-loop control can be applied to suppress sidebranching of pores in a technologically relevant regime.