Controlling Sodium Contamination in Cleanrooms
Maria A. Lester, Associate Editor -- Semiconductor International, 3/1/2000
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Maria A. Lester,
Associate Editor
Sodium is the sixth most abundant element on the earth, comprising 2.6% of the planet's crust. Though potential device contamination is a concern to the industry, sodium contamination has not been studied in depth. Typically, sodium contamination is not an issue in front-end processes because of its high 882degC boiling point. However, back-end, low-temperature (450degC) processes cannot remove sodium contamination, creating particular problems in non-volatile memory devices. Therefore, a phosphorus-doped barrier layer such as PSG or BPSG is added to protect the active devices. Researchers at LETI (Grenoble Cedex, France) and CRMC (Marseille Cedex, France) studied the deposition and removal of sodium contamination on silicon wafers in a cleanroom environment.
The human body is a primary source of sodium contamination. Therefore, wafers in a cleanroom are susceptible to contamination when placed near human activity. At room temperature, sodium contamination comes from airborne particles rather than in gaseous form. The researchers found that airborne sodium deposition could be reduced by protecting wafers with conventional storage boxes, far from human contact, and storing the wafers for periods no longer than one week.
| 1. Impact of DI water and water spiked with 0.01% HCl on sodium contamination. (Source: LETI) |
Interestingly, during wet processes, sodium deposition was found to occur only during DI water rinses. Sodium deposition was shown to follow a Langmuir adsorption model. Temperature and ionic concentration influenced sodium deposition. The amount of sodium deposited decreased with increased temperature and the presence of competing ions. The researchers found a simple solution: a clean water rinse removes the sodium contamination. Results could be accelerated by adding a strong acid such as HCl (Fig. 1) or by increasing temperature. SC1, SC2 and 0.1% HF cleaning chemistries also decreased sodium contamination (Fig. 2).
Evaluation of Calcium Contaminants
Calcium is a known silicon wafer contaminant that can degrade electrical performance. Now, research from IMEC (Leuven, Belgium) suggests NH+4 is at least as effective as H+ in removing metals from the oxidized silicon surface and preventing metal ion contamination.
| 2. Results of sodium removal from cleaning chemistries. (Source: LETI) |
The researchers studied the removal of calcium using an ammonia-hydrogen peroxide-water solution (APM) — specifically, the effect of NH+4 on Ca2+ adsorption and Ca2+ adsorption from APM and from HNO3 solutions. They used a spin-contamination method to study metal ion adsorption in aqueous solutions. NH+4 was shown to prevent Ca2+ adsorption onto the oxidized silicon surface by an ion exchange.
The study showed that Ca2+ adsorption was low due to its competition for adsorption sites with NH+4, the dominant cation in APM. The adsorption site density of the oxide surface was found to control the number of adsorbed metal ions on the silicon surface by ionic attachment. The research also indicated greater metal ion adsorption from dilute APM than from standard APM. The metal ion adsorption was fast compared to in-solution etching. Therefore, Ca2+ adsorption was not significantly affected. The metal ions on the surface and in solution remained in chemical equilibrium. Finally, the equilibrium constant governing NH+4 adsorption was shown to be comparable to or larger than that of H+.
