Formation of silicon and gallium arsenide wafers  (Page 8/11)

Etching silicon

The most commonly used etchants for silicon are mixtures of hydrofluoric acid (HF) and nitric acid (HNO ₃ ) in water or acetic acid (MeCO ₂ H). The etching involves a reduction-oxidation (redox) reaction, followed by dissolution of the reaction products. In the HF-HNO ₃ system the HNO ₃ oxidizes the silicon and the HF removes the reaction products from the surface. The overall reaction is:

The oxidation reaction involves the oxidation of Si ⁰ to Si ⁴⁺ , and it is auto-catalytic in that the reaction product promotes the reaction itself. The initial step involves trace impurities of HNO ₂ in the HNO ₃ solution, [link] , which react to liberate nitrogen dioxide (NO ₂ ), [link] .

The nitrogen dioxide oxidizes the silicon surface in the presence of water, resulting in the formation of Si(OH) ₂ and the reformation of HNO ₂ , [link] . The Si(OH) ₂ decomposes to give SiO ₂ , [link] . Since the reaction between HNO ₂ and HNO ₃ , [link] , is rate limiting, an induction period is observed. However, this is overcome by the addition of NO ₂ ^- ions in the form of [NH ₄ ][NO ₂ ].

The final step of the etch process is the dissolution of the SiO ₂ by HF, [link] . Stirring serves to remove the soluble products from the reaction surface. The role of the HF is to act as a complexing reagent, and thus the reaction shown in [link] is known as a complexing reaction. The formation of water as a reaction product requires that acetic acid be used as a diluent (solvent) to ensure better control.

The etching reaction is highly dependent on the relative ratios of the etchant reagents. Thus, if an HF-rich solution is used, the reaction is limited by the oxidation step, [link] , and the etching is anisotropic, since the oxidation reaction is sensitive to doping, crystal orientation, and defects. In contrast, the use of a HNO ₃ -rich solution produces isotropic etching since the dissolution process is rate limiting ( [link] ). The reaction of HNO ₃ -rich solutions has been found to be diffusion-controlled over the temperature range 20 - 50 °C ( [link] ), and is therefore commonly employed for removing work damage produced during wafer fabrication. The boundary layer thickness ( [link] ) and therefore the dimensional control over the wafer is controlled by the rotation rate of the wafers. A common etch formulation is a 4:1:3 mixture of HNO ₃ (79%), HF (49%), and MeCO ₂ H (99%). There are some etchant formulations that are based on alternative (or additional) oxidizing agents, such as: Br ₂ , I ₂ , and KMnO ₄ .

Alkaline etching (KOH/H ₂ O or NaOH/H ₂ O) is by nature anisotropic and the etch rate depends on the number of dangling bonds which in turn are dependent on the surface orientation. Since etching is reaction rate limited no rotation of the wafers is necessary and excellent uniformity over large wafers is obtained. Alkaline etchants are used with large wafers where dimensional uniformity is not maintained during lapping. A typical formulation uses KOH in a 45% weight solution in H ₂ O at 90 °C.

Etching gallium arsenide

Although a wide range of etches have been investigated for GaAs, few are truly isotropic. This is because the surface activity of the (111) Ga and (111) As faces are very different. The As rich face is considerably more reactive than the Ga rich face, thus under identical conditions it will etch faster. As a result most etches give a polished surface on the As face, but the Ga face tends to appear cloudy or frosted due to the highlighting of surface features and crystallographic defects.