Angle grain boundary in Ge

If q is less than 5⁰, the value of D is quite large as compared with the interatomic distance and so each dislocation can be considered as isolated. If q is about 15⁰ than D is only a few interatomic distance and we get a collection of irregular, diffused and deformed vacancies.

At present the etch pit method is the most direct method of determining the dislocation density. The density of dislocations is the number of dislocation lines which intersect a unit area in the crystal. It ranges from 10² – 10³ in the best germanium and silicon crystals to 10¹¹ – 10^‑12 dislocations/cm² in heavily deformed metal crystals. The density of dislocations can be estimated in solids by the following methods:

(i) By plastic deformation of crystals, just like the bending of a pack of playing cards.

(ii) By X-ray transmission method.

(iii) By X-ray reflection.

(iv) By electron microscopy

(v)

Defects in crystals and Elements of Thin Films

By measuring the increase in the electrical resistivity produced by the dislocations in heavily cold – worked metals

(vi) By measurement on magnetic saturation of cold- worked fermagnetic materials.

(vii) .By decoration methods. Decorated helical dislocations can be produced in calcium fluoride by decorating particles of CaO.

(viii) By etch pit methods.

1.13 ETCHING- TYPES OF ETCHING          

In order to form a functional Micro-Electro-Mechanical Systems (MEMS) structure on a substrate, it is necessary to etch the thin films previously deposited and/or the substrate itself. In general, there are two classes of etching processes:

1.Wet etching where the material is dissolved when immersed in a chemical solution

2.Dry etching where the material is sputtered or dissolved using reactive ions or a vapor phase etchant

In the following, we will briefly discuss the most popular technologies for wet and dry etching.

Wet etching: This is the simplest etching technology. All it requires is a container with a liquid solution that will dissolve the material in question. Unfortunately, there are complications since usually a mask is desired to selectively etch the material. One must find a mask that will not dissolve or at least etches much slower than the material to be patterned. Secondly, some single crystal materials, such as silicon, exhibit anisotropic etching in certain chemicals. Anisotropic etching in contrast to isotropic etching means different etch rates in different directions in the material. The classic example of this is the <111> crystal plane sidewalls that appear when etching a hole in a <100> silicon wafer in a chemical such as potassium hydroxide (KOH). The result is a pyramid shaped hole instead of a hole with rounded sidewalls with a isotropic etchant. The principle of anisotropic and isotropic wet etching is illustrated in the figure below.

This is a simple technology, which will give good results if you can find the combination of etchant and mask material to suit your application. Wet etching works very well for etching thin films on substrates, and can also be used to etch the substrate itself. The problem with substrate etching is that isotropic processes will cause undercutting of the mask layer by the same distance as the etch depth. Anisotropic processes allow the etching to stop on certain crystal planes in the substrate, but still results in a loss of space, since these planes cannot be vertical to the surface when etching holes or cavities. If this is a limitation for you, you should consider dry etching of the substrate instead. However, keep in mind that the cost per wafer will be 1-2 orders of magnitude higher to perform the dry etching

Defects in Crystal

If you are making very small features in thin films (comparable to the film thickness), you may also encounter problems with isotropic wet etching, since the undercutting will be at least equal to the film thickness. With dry etching it is possible etch almost straight down without undercutting, which provides much higher resolution.

 

 

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