Wet Etching; Wet Processing

Wet Etching (Page 2 of 2)

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 SiO₂etchant 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.

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