GaAs Surface Preparation for Thin Film Deposition Using Sodium Hypochlorite
Motorola Comp;ound Semiconductor One: CS-1
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Keywords : Bleach, surface, hypochlorite, process, improvement
Preparation of Gallium Arsenide (GaAs) surfaces, prior to a thin-film deposition, serves the prevailing functions of lowering defectivity, contamination removal and promoting adhesion. When native oxides, metallics or particulates are a concern, the common GaAs clean includes a dilute acid (HCI, HF) or base NH4OH). When adhesion is the primary concern, the GaAs surface may need to be roughened in order to increase the exposed surface area. This etching is generally achieved using an oxidizing chemistry (H2 O2) in conjunction with an acid or base (HCI, NH4OH). Unfortunately, a number of oxidizing chemistries are often too strong or unstable to be useful in particular processes, such as metal adhesion. Sodium Hypochlorite Bleach (NaOCI) is an oxidizing chemistry that attacks GaAs and is also fairly stable as a diluted solution. The condition of the resulting surface is an indication of process control and effectiveness and is measured using reflectivity.
The purpose of this paper is to discuss the use of active scientific investigation to systematically define the implement changes to a critical NaOCI surface preparation process. Opportunities for improvement of the process were discovered through process stability and manufacturability issues. A hypothesis was formulated and focused on the theoretical breakdown of the bleach during the process. The NaOCI solution degraded at a very fast rate and allowed for only 3 hours of processing per bath, with low statistical process capability (SPC). The investigation began with a literature search into the breakdown mechanisms associated with NaOCI. Following the theoretical stage, wet chemical experiments were conducted in a laboratory. The results of the lab work proved that the breakdown mechanism discussed in research literature was occurring in the existing process. Additionally, this data was verified through experiments in the factory. Process improvement work was completed using experiments in the factory. This work included recipe alteration, bath preparation changes, chemical storage improvements, and in-toll chemical containment improvements. The results of this scientific investigation are improvements involving: a 40% decrease in qualification frequency, 83% decrease in cycle time, 300% SOC improvement, 50% cost decrfease, 100% elimination of reworks, and 1200% improvement in bath-life.
This paper concludes with a discussion on future process improvements with NaOCI as well as applications of the lessons learned to other processes that use unstable chemistries. The work discussed in this paper in an example of the success application fo the scientific method and active research used to determine a root cause and solution for a critical process issue.