The behavior of nickel adsorption during caustic etching of boron-doped silicon wafer is investigated. Caustic solutions considered for investigation are sodium hydroxide (NaOH) and potassium hydroxide (KOH). In addition to the amount of nickel present in the caustic solution, the degree of nickel contamination of the wafer is also affected by the dopant concentration, temperature and nature of the caustic solutions used. Control of nickel contamination by various additives to the caustic solution is found to be effective and comparable to each other. The possible mechanism of nickel adsorption is explained by considering metal stability in specific caustic solution and the electro-chemical potential Wet etching of silicon by caustic solution is extensively used, because of its high anisotropic etch rate, in both silicon wafer manufacturing and device fabrication. Use of KOH in fabrication of microelectricalmechanical system (MEMS) is widespread.1 Strong dependence of silicon etch rate on crystal orientation (<100> vs. <111) and on silicon doping level is observed in caustic etching of silicon. This makes caustic etching a desirable candidate for manufacturing silicon structure requiring anisotropic etching. Numerous studies have been done to explain the etch rate, primarily in KOH solution, in terms of its dependence on temperature, dopant type and concentration, crystal orientation as a function of KOH concentration.
2-4In wafer manufacturing, silicon wafers are subjected to various processes such as lapping, grinding and edge profiling which generate surface defects caused by the stress induced from such processes. Subsequent caustic or acid etching is required to remove these defects. However, nickel contamination by adsorption is observed on the surface and in the bulk during the etching process. Continued reduction of overall metal contamination is proposed in technology roadmaps to improve device yield and performance.5 Deposition of metal ions during acid etching is explained based on the electrochemical potential of the ion compared to the semiconductor surface. In general, metal ions with a higher potential than the semiconductor are irreversibly adsorbed on the surface compared to one with lower electrochemical potential. 6 Higher etch rate, in addition to better flatness, back-side geometry, cost and operation safety is achieved in caustic etching of wafer compared to an acid etch process.
7Commercially available caustic solutions like sodium hydroxide and potassium hydroxide normally used as etchants typically contain a significant quantity of nickel, copper, and other metals. Some of these metals can diffuse easily into the bulk of the silicon wafer during the etching process. The caustic solutions namely NaOH and KOH are used for investigation of nickel adsorption on boron doped (<100>) silicon wafers with varying resistivity. The influence of caustic etchant type, wafer doping level and temperature on metal adsorption is discussed. A possible mechanism for nickel adsorption is discussed along wit...