Theoretical and experimental investigation of chemical pattern etching
Georgiadou, Maria
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Permalink
https://hdl.handle.net/2142/21828
Description
Title
Theoretical and experimental investigation of chemical pattern etching
Author(s)
Georgiadou, Maria
Issue Date
1990
Doctoral Committee Chair(s)
Alkire, Richard C.
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Language
eng
Abstract
The processes occurring during wet chemical etching of partially masked copper surfaces for pattern definition were investigated. Chemical dissolution of rectangular cavities with active copper base and inert photoresist sidewalls was carried out in the presence of flowing acidic cupric chloride solutions. It was found that for a 3.5M CuCl$\sb2$ + 0.5M HCl + 0.5M KCl etching solution a salt film precipitated on the entire metal surface and dissolution was controlled by mass transfer of the products. The presence of recirculating eddies at the corners added extra resistance to mass transfer there, such that undercutting was suppressed and anisotropic profiles were obtained. For a 0.5M CuCl$\sb2$ + 0.5M HCl + 0.5M KCl etching solution the solubility of the film is higher since the ratio of Cu$\sp{+2}$ to Cl$\sp-$ concentration is lower; the film precipitated preferentially at the corners of a 5:1 cavity, whereas the rest of the surface dissolved film-free. Anisotropic profiles were also obtained. For the same solution and 1:1 cavities, dissolution proceeded under film precipitation on the entire surface.
A mathematical model was developed in order to predict conditions for film precipitation on the entire or part of the metal surface, as well as optimum etching conditions. The model was based on finite difference and accounted for two-dimensional convective mass transport in a cavity geometry, migration, reaction equilibria, multiple species and multiple electrochemical reactions. Very good agreement between the predicted and experimentally measured average etch rates were accomplished. Operation was suggested at high oxidant concentrations and high flow velocities in order to achieve high speed etching.
"In addition, analysis of the chemical environment at the electrode-solution near surface region was performed in-situ by Raman spectroscopy. Indication of existence of at least CuCl$\sp+$, CuCl$\sb2$, and CuCl$\sb3\sp-$ was found, as well as of mixed valence polynuclear ""interaction"" complexes. Due to strong overlapping of the bands of the species, resolution of the Raman spectra to individual complexes was not possible."
The regeneration of the etching solution via oxidation of the products by air was investigated by means of a one-dimensional mathematical model for a flat geometry. It was found that regeneration was favored under flow conditions and at high O$\sb2$ concentrations, for which it took place very close to the metal surface, thus enhancing the etch rate.
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