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The chemicals utilized for etching are aggressive acids, such as hydrofluoric, nitric and phosphoric. Other chemicals are used to clean the surface of the wafer, preparing it for the next stage in the fabrication process. All of these chemicals are potential sources of particulate contamination. With respect to etchants there are two major sources for particulate contamination: the etching process itself and those sources associated with the delivery of the acid to its point-of-use.
By its nature etching is a self-contaminating process. During etching acids react with thin film material exposed on the wafer and remove this film from the surface to create a pattern in the film as defined by the way that the photoresists were laid
down. This process can generate particles, inorganic colloids and reaction residues.
The second source of particulate contamination is the acids themselves, as they are received. These acids are the most likely of all process fluids to contain damaging particles. Contamination levels as high as 100 million particles per litre, derived from storage containers, sinks and process equipment, are possible.
The thin films used in electronic device fabrication can have a thickness in the range of 0.02um (200Å), which means that sub-micrometre particles can have a disastrous effect on device yields. In fact, particle generated pinholes in thin gate oxide
films are one of the major causes of low yields in most integrated circuit production.
Metallic impurities (colloids and particle) are of special concern because of their ability to be readily incorporated in exposed areas by substitution in the crystal lattice, or by being chemisorbed to active surfaces. Wafer surfaces are especially vulnerable to chemisorption during the etching and cleaning processes. Acids utilized for these processes continually generate chemically active and reactive surfaces.