Wet Etching                                 

In wafer fabrication, etching refers to a process by which material is removed from the wafer, i.e., either from the silicon substrate itself or from any film or layer of material on the wafer.  There are two major types of etching: dry etching and wet etching.

Wet Etching is an etching process that utilizes liquid chemicals or etchants to remove materials from the wafer, usually in specific patterns defined by photoresist masks on the wafer.  Materials not covered by these masks are 'etched away' by the chemicals while those covered by the masks are left almost intact.  These masks were deposited on the wafer in an earlier wafer fab step known as 'lithography.'

A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent, without changing the chemical nature of the dissolved material. In general, however, a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species.

A basic wet etching process may be broken down into three (3) basic steps:  1) diffusion of the etchant to the surface for removal; 2) reaction between the etchant and the material being removed; and 3) diffusion of the reaction byproducts from the reacted surface. 

Reduction-oxidation (redox) reactions are commonly encountered in wafer fab wet etching processes, i.e., an oxide of the material to be etched is first formed, which is then dissolved, leading to the formation of new oxide, which is again dissolved, and so on until the material is consumed.

Wet etching is generally isotropic, i.e., it proceeds in all directions at the same rate.  An etching process that is not isotropic is referred to as 'anisotropic.'  An etching process that proceeds in only one direction (e.g., vertical only) is said to be 'completely anisotropic'. 

Figure 1. Example of a Wet Etching Station; source: www.futurefab.com

In semiconductor fabrication, a high degree of anisotropy is desired in etching because it results in a more 'faithful' copy of the mask pattern, since only the material not directly under the mask are attacked by the etchant.  Isotropic etchants, on the other hand, can etch away even the portion of material that's directly under the mask (usually in the shape of a quarter-circle), since its horizontal etching rate is the same as its vertical rate. 

When an isotropic etchant eats away a portion of the material under the mask, the etched film is said to have 'undercut' the mask. The amount of 'undercutting' is a measure of an etching parameter known as the 'bias.'  Bias is simply defined as the difference between the lateral dimensions of the etched image and the masked image. Thus, the mask used in etching must compensate for whatever bias an etchant is known to produce, in order to create the desired feature on the wafer.

Because of the isotropic nature of wet etching, it results in high bias values that are not practical for use in pattern images that have features measuring less than 3 microns. Thus, wafer patterns with features that are smaller than 3 microns must not be wet-etched, and should instead be subjected to other etching techniques that offer a higher degree of anisotropy.

Another important consideration in any etching process is the 'selectivity' of the etchant. An etchant not only attacks the material being removed, but the mask and the substrate (the surface under the material being etched) as well. The 'selectivity' of an etchant refers to its ability to remove only the material intended for etching, while leaving the mask and substrate materials intact. 

Selectivity, S,  is measured as the ratio between the different etch rates of the etchant for different materials.  Thus, a good etchant needs to have a high selectivity value with respect to both the mask (Sfm) and the substrate (Sfs), i.e., its etching rate for the film being etched must be much higher than its etching rates for both the mask and the substrate.

Despite the resolution limitations of wet etching, it has found widespread use because of its following advantages: 1) low cost; 2) high reliability; 3) high throughput; and 4) excellent selectivity in most cases with respect to both mask and substrate materials.  Automated wet etching systems add even more advantages: 5) greater ease of use; 6) higher reproducibility; and 7) better efficiency in the use of etchants.

Of course, like any process, wet etching has its own disadvantages. These include the following: 1) limited resolution; 2) higher safety risks due to the direct chemical exposure of the personnel; 3) high cost of etchants in some cases; 4) problems related to the resist's loss of adhesion to the substrate; 5) problems related to the formation of bubbles which inhibit the etching process where they are present; and 6) problems related to incomplete or non-uniform etching.  

Silicon (single-crystal or poly-crystalline) may be wet-etched using a mixture of nitric acid (HNO₃) and hydrofluoric acid (HF).  The nitric acid consumes the silicon surface to form a layer of silicon dioxide, which in turn is dissolved away by the HF.  The over-all reaction is as follows:  Si + HNO₃ + 6 HF --> H₂SiF₆ + HNO₂ + H₂ + H₂O. 

Silicon dioxide may, as mentioned above, be wet-etched using a variety of HF solutions.  The over-all reaction for this is:   SiO₂ + 6 HF --> H₂ + SiF₆ + 2 H₂O.  Water-diluted HF with some buffering agents such as ammonium fluoride (NH₄F) is a commonly used SiO₂ etchant formulation  

Wet etching of aluminum and aluminum alloy layers may be achieved using slightly heated (35-45 deg C) solutions of phosphoric acid, acetic acid, nitric acid, and water.  Again, the nitric acid consumes some of the aluminum material to form an aluminum oxide layer. This oxide layer is then dissolved by the phosphoric acid and water, as more Al₂O₃ is formed simultaneously to keep the cycle going.

Other materials on the wafer may be wet-etched by using the appropriate etching solutions.