Mechanisms anisotropic etching

The (100) surface tends to roughen quicker than the (111) surface and the roughness tends to be permanent on the (100) surface whereas it is transient on the (111) surface. " Such crystal orientation-dependent roughness can also be explained by the anisotropic etching mechanism illustrated in Fig. 7.41. The preferential etching at the (111) steps of the (111) terraces results in the removal of the terraces and reduction of the (111) steps and a reduction of microroughness. 

Figure 16. Surface damage and surface inhibitor mechanisms for ion-assisted anisotropic etching. (Reproduced with permission from Ref. 103J...

Mechanism of Anisotropic Etching. The rich details as to anisotropic etch rates, nature of reactions, and surface topography indicate that a complex mechanism is involved in silicon etching in alkaline solutions. A coherent mechanistic model has to address three basic aspects (1) the physical cause of the difference in the removal rates of atoms from the surface of different crystal orientations, (2) the kinetic processes that actualize such physical cause, and (3) the surface condition that determines the global removal rate of the surface atoms. 

The complexity of the system implies that many phenomena are not directly explainable by the basic theories of semiconductor electrochemistry. The basic theories are developed for idealized situations, but the electrode behavior of a specific system is almost always deviated from the idealized situations in many different ways. Also, the complex details of each phenomenon are associated with all the processes at the silicon/electrolyte interface from a macro scale to the atomic scale such that the rich details are lost when simplifications are made in developing theories. Additionally, most theories are developed based on the data that are from a limited domain in the multidimensional space of numerous variables. As a result, in general such theories are valid only within this domain of the variable space but are inconsistent with the data outside this domain. In fact, the specific theories developed by different research groups on the various phenomena of silicon electrodes are often inconsistent with each other. In this respect, this book had the opportunity to have the space and scope to assemble the data and to review the discrete theories in a global perspective. In a number of cases, this exercise resulted in more complete physical schemes for the mechanisms of the electrode phenomena, such as current oscillation, growth of anodic oxide, anisotropic etching, and formation of porous silicon. 

U. Heim, Some aspects of the mechanism of the wet anisotropic etching of crystals and their consequences for a process simulation. Sensors Actuators N6A, 191, 1998. 

Many models have been proposed for the mechanism of silicon anisotropic etching in alkaline solutions. They can be grouped into two categories those that attribute the relative slower etch rate of (111) planes to the presence of a passive oxide film on the surface, and those that consider the etch rate difference among different orientations to be governed by reaction kinetics. 

For production of micro-mechanic devices etching paths parallel to the wafer substrate surface are often required. Bromine trifluoride (BrF3) is used for this special kind of anisotropic etching. Because of the extreme reactivity of this compound, electrical excitation and plasma formation is not necessary. 

Silicon nitride as a passivation layer on top of an electronic circuit or a metal structure is an excellent diffusion barrier against water and protects the electronic device from organic and metallic (e.g., Na, K) contaminants. Silicon nitride is also used as a masking layer for wet anisotropic etching of silicon (in KOH), as part of a dielectric membrane, and for mechanical protection in micromechanics during face-down handling of the front of electronics while processing the back of the wafer, a silicon nitride passivation layer prevents defects and scratches on the sensitive front side. 

Etching of silicon by fluorine atoms is the most practically important and the best characterized surface etch process (Flamm, 1989, 1990 Winters Cobum, 1992 Lieberman Lichtenberg, 1994). We consider first the pure chemical mechanism of the process, leading to isotropic silicon etching then we discuss ion energy-enhanced anisotropic etching. 

The reaction mechanism of the etch process is not fully clear. Several researchers proposed physical models for silicon anisotropic etching from the viewpoints of energy band gap [4, 5] and Gibbs free energy [6]. Over these models, one equation describing the simplified reaction mechanism is addressed as follow ... 

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