Advantages of Creating Buffered HF at the Point-of-Use
A well-established solution, buffered HF, is made cleaner.
Laura Peters, Senior Editor -- Semiconductor International, 6/1/1998
A new way of forming buffered hydrofluoric acid (BHF) at the point-of-use (POU) offers substantial advantages compared with conventional pre-mixed BHF solutions. The new method involves co-injection of HF and ammonium hydroxide (NH4OH) immediately prior to use. Results show better etch uniformity, lower metallic impurities, lower particle densities and no need for surfactants. These are the findings of Dr. Steven Verhaverbeke, director of research for CFM Technologies Inc. (West Chester, Pa.). Verhaverbeke presented his work at the 14th International Symposium on Contamination Control, held April 26-May 1 in Phoenix, Ariz., part of the 44th Annual Meeting of the Institute of Environmental Sciences (IES, Mt. Prospect, Ill.).
Cost benefits to the POU approach result from the higher concentration of the NH4OH (14.5 mol/liter) relative to ammonium fluoride NH 4F (10.9 mol/liter), making the chemical delivery cycle more productive. In addition, pre-mixed BHF is an expensive chemical that requires dedicated chemical delivery lines. POU generation allows the flexibility to mix the solution at a variety of concentrations. In addition, BHF will crystallize at temperatures less than 16°C, posing problems during transportation of pre-mixed BHF solutions. The POU strategy avoids possible crystallization and freezing. Surfactants are typically used with BHF to improve wettability in high-aspect ratio contacts. With co-injected BHF, the contacts are first wetted with DI water, eliminating the need for surfactants.
Buffered HF is a mixture of NH4F and HF, but it can also be made by mixing NH4OH and HF since NH4OH and HF react to give NH4F. The CFM Full-Flow system was first used to test traditional NH4F:HF solutions, with concentrations of 6:1 and 7:1 (%vol.). Next, etch rates and etch uniformity of BPSG, thermal oxide, thermal nitride and TEOS oxide films were compared.
CFM analyzed etch rates as a function of mixing H2O, HF and NH 4OH in different ratios at 40°C. From this study, it was found that buffered HF provides higher etch rates than dilute HF with the same fluoride concentration. This is due to the fact that the etching species in BHF, HF2-, etches SiO2 faster than the active species in dilute HF, (HF)2. BHF is especially useful for very thick oxide etches. The co-injected NH4OH becomes increasingly important for dilutions starting at 100:1 (H2 O:HF). BHF etch rate is 20% higher than dilute HF at 100:1; at 70:1, BHF rate is 50% higher; and at 63:1 it is 100% faster than in dilute HF solutions with the same concentration of HF. At the highest HF concentrations, the co-injected NH4OH will accelerate the HF etching up to a factor of 4 compared to dilute HF. For solutions more dilute than 100:1, no appreciable difference in etch rates is obtained.
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| Fig. 1 . All etch rates extrapolate to zero, showing that etching is more controlled with point-of-use created BHF. |
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| Fig. 2. The difference in activation energy between BPSG etch rate and oxide etch rate allows high-selectivity etching. Every 15°C increase in process temperature doubles the etch rate and improves selectivity (BPSG:oxide) by 10%. |
Finally, Verhaverbeke determined the temperature dependence of BPSG and thermal oxide etching in the co-injected BHF solution (Fig. 2). Thermal oxide's activation energy was found to be 9.37 kcal/mol and BPSG's activation energy was determined to be 10.54 kcal/mol in BHF. Processing at different temperatures can thus be used to improve the selectivity of BPSG etching to thermal oxide etching. Selectivity improves by ~10% for every 15°C increase in process temperature.
