The solutions DIH20: NH40H: H202 and DIH20: HCL: H202 have been used extensively in the semiconductor industry for the removal of transition and alkali metals from wafer surfaces. The former, when used in a ratio of approximately 5:1: 1 effects the wet oxidation removal of organic surface films and trace metals (Cu, Au, Ag, Ni, etc.) while the latter, in an approximate 6:1:1 ratio dissolves alkali ions and hydroxides of Al+3, Fe+3, and Mg+2 desorbing by complexing residual metals. Of concern is that these mixtures are known to plate out charged ionic species, particularly Fe, which remains on the Si surface after exposure to these solution. This effect adds an enhanced surface potential barrier which can be negative or positive depending on the concentrations of species deposited.
While the effects of Fe as deposited by the NH40H solution is well-known, we are concerned in this study with the deposition of Ca, K, and the lighter ions, Na and Al. Conventional analytical methods such as ICP-MS or AAS or TXRF have been used to detect these ions but are problematic since each has a limited range of detectable species or is destructive to the surface when used with VPD to increase resolution. In this study, we have verified the deposition of these species by a ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) analysis since it has the ability to detect the complete range of the desired ions. In addition, we have applied high-injection surface photovoltage (SPV) to obtain an average surface charge on each sample. To illustrate the use of these methods, we have applied them to the analysis of various closed acid processing systems. Our interest here is the amount of these selected ions being deposited from these solutions and the influence of using a standard canister or bulk acid supply system. Comparisons were also made to standard and megasonic RCA cleans and hot phosphoric nitride strips. We will discuss ToF-SIMS and how it was applied to analyze the addition of these charged trace ions. This work suggests that the problem of trace ion detection on Si must include the entire range of transition and alkali metals, and that ToF-SIMS is a promising analytical tool for this purpose.