Etch rate other materials

Other important factors to consider in etching are etch rate and selectivity. Etch rate describes how fast the etching is achieved. Selectivity is the ratio between the etch rate on the target material and the rate on another material that is present. Ideally, only the target material should be etched while other materials are left intact that is, the ideal selectivity should be infinity. Unfortunately, this is not possible in reality. 

Etch Profiles. The final profile of a wet etch can be strongly influenced by the crystalline orientation of the semiconductor sample. Many wet etches have different etch rates for various exposed crystal planes. In contrast, several etches are available for specific materials which show Httle dependence on the crystal plane, resulting in a nearly perfect isotropic profile. The different profiles that can be achieved in GaAs etching, as well as InP-based materials, have been discussed (130—132). Similar behavior can be expected for other crystalline semiconductors. It can be important to control the etch profile if a subsequent metallisation step has to pass over the etched step. For reflable metal step coverage it is desirable to have a sloped etched step or at worst a vertical profile. If the profile is re-entrant (concave) then it is possible to have a break in the metal film, causing an open defect. 

In addition to the visualization of topographic transformations, sequences of in-situ images yield a measure of the local kinetics of the reaction. The etch rate of Si has been evaluated in [20] by using the expression R = (AS/S) /i/Af, with AS/S the surface area of terraces removed per cm of electrode in one sequence, h the step height (3.14 A) and At the time elapsed. The quantity (AS/S)h in fact represents the volume of material which has been removed per cm of electrode, because the dissolution occurs layer by layer. The experimental determination of AS is sketched in Fig. 22 f, in which the hatched area represents AS. In other sequences AS includes the surface of eventual pits. The bias dependence of the etch rate and the current voltage curve are shown in Fig.26 for n-Si(lll) in a 2M NaOH solution [20]. 

HNO3 concentration. Also, when there is little etching of the materials, no significant difference exists between p and n types. On the other hand, an appreciable potential difference exists between n andp types in solutions in which the etch rate is high. 

The etch rate of silicon relative to other materials is also important in device fabrications. The data on relative etch rate, which is specific to a specific set of conditions, are widely spread in the literature. For example, the relative etch rate of silicon has been reported for Sii. GC , in NH4OH, boron monophosphide epitaxial layer in HF-HNO3, SiOj in HF solutions, SiOj in KOH, SiOj and nitride in TMAH, ° ° Si02 in different oxides and metallic films, various glasses... 

The etch rate of silicon relative to other materials is also important in device fabrications. The data on the relative etch rate, which is specific to a specific set of conditions, is widespread in the literature. Some examples are Si jGe, in... 

The third method for endpoint detection uses a mass spectrometer that monitors a particular species whose concentration changes dramatically at the endpoint. In all three methods, the etch rate for a particular material can be determined. However, the interferometer technique has two advantages over the other methods. Changes in the etch rate with time can be detected and quantified, and significant etching of photoresist can be detected from the output of the strip chart recorder. In the next section, we take up the applications of plasma etching. 

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