Etchant gases

In the etchant-unsaturate model described by Equation 7-10 41), specific chemical species derived from electron collisions with etchant gases are considered. 

TABLE 1 Summary of etch rate results for GaN, AIN and InN from various dry etching techniques for some typical etchant gases. 

In this process step an etchant (gas or liquid) removes the Si02 where it is not protected by the photoresist. Ions are implanted into the unprotected silicium. With the implantation of ions the structure of the surface will be changed. 

The F/C ratio model accounts for the fact that for carbon-containing gases etching and polymerization occur simultaneously. The process that dominates depends upon etch gas stoichiometry, reactive-gas additions, amount of material to be etched, and electrode potential and upon how these factors affect the F/C ratio. For instance, as described in Figure 8, the F/ C ratio of the etchant gas determines whether etching or polymerization is favored. If the primary etchant species for silicon (F atoms) is consumed either by a loading effect or by reaction with hydrogen to form HF, the F/... 

Usually, dry etching of polyimide is performed by RIE with O2, CF, or their mixtures as an etchant gas, utilizing positive photoresist (2), metals such as aluminum (3), spin-on-glass (4), or SiN (5) as an etching mask. However, in the former case, it is difficult to define a fine via-hole as small as 2 pm or less because the resist thickness must be two or more times that of the polyimide as a result of the equal etching rates between photoresist and polyimide. In the latter case, though the fine pattern can be obtained the additional pattern transformation from the photoresist to the masking layer is necessary. 

Phosgene was found to be a suitable reagent for the plasma etching of the semiconductor GaAs, but it reacted more slowly than dichlorine [1910], It has also been used, mixed with CO, as an etchant gas for refractory metals and, particularly, their silicides [701a],... 

As the name implies, the species that do the etching in this technique are gaseous plasmas generated within an evacuated chamber to which is introduced the etchant precursor gas(es) and which is subjected to high-frequency electric power via external electrodes. These plasmas comprise ionic and neutral fragments. For a precursor etchant gas such as carbon tetrafluoride (CF4), species including C, CF" ", CF, C, F, CF, CF2, and CF3 have been detected in the generated plasma. ... 

Dry etching is a commonly used teclmique for creating highly anisotropic, patterned surfaces. The interaction of gas phase etchants with surfaces is of fundamental interest to understanding such phenomena as undercutting and the dependence of etch rate on surface structure. Many surface science studies aim to understand these interactions at an atomic level, and the next section will explore what is known about the etching of silicon surfaces. 

Some nonconductors, such as the polymers polycarbonates and polystyrenes, must be subjected to a surface treatment prior to activation to ensure good adhesion of palladium nuclei. Surface treatment can include the use of chemical etchants for plastics or reactive gas plasma treatments (66). 

Specific Materials. Because the fundamental physics and chemistry of reactive gas discharges used for etching is not yet fully understood, empirical approaches to the design of etch processes abound. As indicated earlier, etchant selection begins with considerations of product volatility. Virtually any gas or vapor that can dissociate to form etchant species has been considered or studied. An abbreviated list of the most common reactant molecules used to etch films of interest in electronic materials processing is given in Table IV. 

An interesting demonstration of profile control via alteration of the specific chemistry is that of silicon etching in C1F3 mixtures (86). Because a pure chemical (isotropic) etchant (F atoms) is combined with an ion-bombardment-controlled (anisotropic) etchant (Cl atoms), a continuous spectrum of profiles with varying anisotropies is generated by changing the gas composition. 

The preparation or etching of compound semiconductors is more complex due to the potential of altering the surface stoichiometry. Shiota et al. (36) used AES to show that the final As/Ga at the surface of GaAs was very dependent on the chemical activity of wet chemical etchants. Bertrand was able to follow the changes in the chemical bonding of Ga and As on p-type GaAs etched in HC1 or Br in methanol and relate this to Schottky barrier heights of similarly prepared surfaces with Pb contacts (37). 

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