Mechanical etching or ion-assisted etching (bombarding ions remove non-volatile etch

products) is driven by the acceleration of the charged species in the plasma towards the

oppositely charged sample due to the applied RF power as illustrated in Figure 7. Thus

chemical etching is more probable to occur than direct ion bombardment on the feature

sidewalls. For example, an increased flow rate of Cl2 increased the concentration of Cl

that are able to form volatile species with the GaAs substrate resulting in sidewalls that

are more rounded [13] (Figure 8a). It is also expected that as the concentration of Cl is

increased, a layer of increasing thickness of GaCl3 forms, limiting the etch rate [3, 14].

The GaCl3 forms at the surface of the GaAs. Its low volatility prevents it from desorbing

off of the GaAs substrate unless it is energetically removed by ion bombardment. The

etch depth and sharpness of the feature dimensions decreased with increasing Cl2 flow

rate (Figure 8b). Similar results are expected for increasing Cl2 flow rates from our

experimentation. Future TEM analysis would show the crystallographic selectivity of

GaAs planes with the As rich planes having a higher etch rate than the Ga rich planes due

to the formation of more volatile As chloride species. Werbaneth et al, reported that an

increased Cl2 flow decreased the etch profile, more tapered sidewalls, and increased the

Au etch rate. At a high enough Cl2 flow, it is expected that some reduction in the gold

layer will occur.


 flfl (- Mask 7

 -Substrate

 Figure 6. a) Anisotropy due to b) Isotropy due to chemical
 physical sputtering etch mechanism
 etch mechanism

Figure 6. Sputtering versus chemical etching.