Nickel Contamination from Caustic Etching of Silicon Wafers
Authors: Sinha Drew 
Published: February 2020

Silicon etch rate with caustic is widely investigated due to its anisotropic etch behavior which is important for semiconductor device fabrication. In wafer manufacturing, caustic or acid etching is used to remove surface defects produced from prior processing steps. Prevention of metal deposition is an important consideration for use of caustic in silicon wafer manufacturing process (1). Deposition behavior of nickel on doped silicon wafer from caustic solution is investigated here. A number of process parameters like type of caustic, doping level, and process temperature on deposition behavior of nickel from KOH and NaOH solution are considered. The mechanism of nickel deposition is discussed along with prevention of deposition from caustic solution.
Boron doped P-type silicon wafer of low and high resistivity and <100> orientations is used for the investigation. The wafers were etched approx. 48%-50% by wt. caustic solution at temperatures and duration indicated for investigation. The level of nickel concentration in caustic solution is less than 100 ppb. Surface nickel is investigated following treatment in caustic solutions followed by rinsing with DI water. For bulk measurements of nickel, the wafers were subjected to standard polishing and cleaning procedures following the indicated caustic etch treatment indicated below.
Both surface and bulk contamination of silicon wafer by caustic solutions is investigated. Surface adsorption of nickel from these two caustic solutions is shown in Figure 1. Contamination is found to be higher with NaOH compared with KOH. Bulk contamination of nickel from each of these caustic solutions as a function of resistivity is shown in Figure 2. The amount of deposited nickel tends to increase with increasing doping level for both the NaOH and KOH used here. The increased level of nickel adsorption from NaOH is likely due to lower stability of the nickel ion in NaOH compared to KOH based caustic solution. This is explained by increased availability of water molecule in KOH based caustic compared to NaOH based solution (2). The impact of doping level on deposition is explained by higher reduction potential of boron compared to silicon which enhances the oxidation by OH ion as substrate resistivity is reduced. Furthermore, deposition of nickel ion from alkaline solution also favors the deposition on substrate due to higher reduction potential for the metal compared to silicon in solution (3).
The amount of deposited nickel from caustic solution is also found to increase with solution temperature due to increase in metal diffusivity with increasing solution.
Finally, the prevention of nickel deposition from the caustic solution is found to be effective with the addition of chelating agent like TTHA and DTPA in caustic solution due to higher stability index of nickel ion with these agents in high pH caustic solution. Addition of peroxide in caustic is also found to be effective in preventing nickel deposition by enhancing the stability of nickel ions in solution.
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