Control of Metal Outplating from HF Solutions
Staff -- Semiconductor International, 6/1/1998
Processes in which hydrofluoric acid (HF) is used in the final stage are of great interest, as they typically leave extremely low levels of metallic contamination on a silicon surface. Nevertheless, trace amounts of noble metal ions, especially copper, but also silver, gold, etc., can be deposited on the silicon surface by an electrochemical process that involves mobile charge carriers. Thus, the semiconductor properties of the silicon substrate play a very important role in the metal ion deposition process. This deposition has been studied in detail by the Inter-University Microelectronics Centre (IMEC, Leuven, Belgium).
At IMEC, wafers were immersed in a 0.5% solution of HF in water at 190°C, this solution being contaminated with various concentrations of copper ions over the range 10-1000 w-ppb. The immersion times were in the range 5-600 sec. Illumination was found to enhance the copper ion deposition rate by at least an order of magnitude (Fig. 1).
The addition of chloride ions as hydrochloric acid to the copper-contaminated solution drastically increased the copper outplating behavior. However, the chloride ions greatly reduced the large effect of the illumination. A strong dependence on the chloride concentration was found (Fig. 2).
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| Fig. 1. The copper surface concentration on p-type and n-type silicon wafers after a 10 min immersion is shown as a function of copper ion weight concentration. |
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| Fig. 2. The copper surface concentration is plotted against the chloride concentration in darkness and under illumination. The solution contained 0.5% HF and 100 w-ppb of copper ions. |
The dominating process in the initial stage of copper deposition is the formation of metal nuclei on the wafer surface. After this nucleation stage, the metallic copper nuclei grow in size. Illumination during the immersion enhances the copper deposition because of the generation of minority carriers that are consumed in the deposition process. Small concentrations of chloride ions increase the copper deposition owing to the catalyzing effect of the chloride ions on the Cu2+/Cu + reaction by a chloride-bridged electron transfer pathway and the formation of copper (I) chloride. The addition of large amounts of chloride suppress the copper deposition owing to the formation of soluble higher copper (I) chloride complexes.
IMEC demonstrated that an ion exchange purifier at the point-of-use can be employed for silicon processing in HF. Metallic impurities in diluted HF are removed to sub-ppb levels in less than five bath turnovers without additional particle or organic contamination. This provides an additional insurance against metal plating from HF baths.
IMEC said these results could provide a cheap and controllable way of avoiding contamination by copper in cleaning technology. They show that the cleaning baths could be used over longer periods to reduce costs. Alternatively, the technique could be applied to the control of the deposition of layers of copper.
