Electrochemical Etching of Semiconductors

Introdution

Many systems from all fields of science possess a common and most fascinating property: They may form patterns in space and/or time from originally uniform states when held far from thermodynamic equilibrium. The basic concepts explaining such selforganization phenomena are often independent of the nature of the specific system, i.e. they are universal.
We are interested in self-organizing processes occurring during electrochemical reactions at the solid/liquid interface (especially at the electrolyte/semiconductor interface). Among the latter, our current interest is the influence of long-range or nonlocal spatial coupling on spatio-temporal dynamics. In electrochemical systems the range of the spatial coupling can be tuned continuously from local or diffusive coupling to global coupling by easily accessible control parameters; thus they are an ideal model system for the study of the effect of coupling range on pattern formation.

Electrochemical Reactions at the Semiconductor-Electrolyte Inferface

Research of the anodic dissolution of semiconductors in different electrolytes yields an increasing number of phenomena which may be most generally characterized as current oscillations in time and space. Oscillations in time comprise not only stable current (or voltage) oscillations at constant voltage (or current, respectively) but also damped current oscillations and oscillations hidden in noise. Current oscillations in space are better known as pits or pores, and it is the large variety of pores obtainable in Si by probing the available parameter space together with the potential for novel products that finds increasing interest in the scientific community. Pore geometry's and morphologies comprise spongelike structures in the nanometer range (generally addressed as micropores), mesopores in the 10 nm – 100 nm range always following <100> directions, “well-formed” macropores in the micrometer range - straight, smooth, and with aspect ratios of 500 and larger - heavily branched macropores with rather peculiar morphologies, and two-dimensional structures termed "trenches" and "wings". In addition, the dependence of the pore morphology on crystal orientation shows peculiar effects, e.g. pore growth directions in <113>. Many attempts have been made to understand the dissolution process and the mechanisms responsible for its apparent instability in space and time. While much pro-gress has been made in recent years, much remains mysterious and there is a general lack of predictive power. Moreover, a whole new world of electrochemically formed pores emerges in other semiconductors with many similarities to Si, but also many differences and most likely a wealth of phenomena not yet discovered. It appears to be an over ambitious and hopeless goal to understand these and other phenomena from just a few basic principles – and that may well be the realistic of view of the subject.
 
 

THE ESSENTIALS OF THE CURRENT BURST MODEL

The current burst model is based on the following major assumptions which will be made more specific later on: While these assumptions project a radical departure form conventional views of the (homogeneous) electrode processes, it is obvious that the three basic chemical processes at the electrode – direct dissolution, oxidation and oxide dissolution – can not all occur at the same time and space - the electrode state thus must be inhomogeneous at some scale by necessity. The CBM essentially exploits this point by specifying the sequence of reactions as a current burst and the resulting electrode state by the interaction of current bursts.

Voltage Osciallations during Pore Growth

AGING CONCEPT

While at high current densities the semiconductor surface is completely covered with oxide at low current densities, most of the semiconductor surface will be in direct contact to the elec-trolyte. It is well known that after chemical dissolution the free surface is passivated, i.e. the density of surface states reduces as a function of time which increases the stability of the sur-face against further electrochemical attack. For the example of silicon the speed and the per-fection of passivation of the (111) crystallographic surface is larger than for the (100) surface. This selective aging of surfaces leads to a self amplifying dissolution of (100) surfaces (which will become pore tips) and a preferential passivation of (111) surfaces (which will become pore walls). Under optimized chemical conditions with an extremely large passivation differ-ence between (111) and (100) surfaces a self organized growth of octahedral cavities occurs. The octahedra consists of (111) pore walls. As soon as the complete surface of the octahedra reaches a critical value, it is easier to start a new cavity at a (100) tip of the old cavity, since the current density in the new, small cavity is larger and no surface passivation will occur until the surface again becomes to large. This growing mechanism leads to an oscillation of both the current through each pore and the diameter of each pore as a function of time – see Figure 1. As in the case of a compound semiconductor the surface aging of III-V compounds is more complicated since there exist two (111) surfaces, e.g. GaAs only the {111} A planes (Ga-rich planes) appear as stopping planes. So in most III-V compounds not ocahedrons are etched but thetrahedra with four (111)A planes as stopping planes. An other example of the aging effect in case of compound semiconductors is the formation of a nucleation layer.


Figure 1 - Selfinduced Oscialltions during Pore Growth in InP


 
  A rather spectacular consequence of the surface aging concept is demonstrated in Figure 1 for InP. A lateral interaction of pores can occur by a next neighbor interaction due to an overlap of the space charge region. Etching InP at very high current densities is accompanied by self induced voltage oscillations (for details see publications). Each peak in the voltage corresponds to an increase of the pore diameters for all pores as seen in Figure 1.

Globale Selforganization

There is, however, a most spectacular additional manifestation of self-organization: the formation of a two-dimensional single pore crystal in InP under certain conditions; Fig. 2 shows some examples. In contrast to e.g. self-organized pore crystal in Al2O3, the InP pore crystals are single crystals (demonstrated by the Fourier transform in Figure 3) which, to the best of our knowledge, makes then unique.


Figure 2 - Selorganized Pores in InP


Figure 3 - FFT

Conclusions

While there are many known modes of pore etching in Si (and possibly more to be discovered), a general understanding of the formation processes has not yet been achieved. Technical uses of macropores are nevertheless possible, and a rapidly growing number of applications is under investigation. While large area etching is not easy, it is possible and has been demonstrated.
 

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Last modified: 04/25/04
URL: http://www.marc-christophersen.de