Chemical States of Copper Contaminants on SiO2 Surfaces and Their Removal by ppm-order HCN Aqueous Solutions

Abstract

Cu contaminants of $1 Â 10 13 atoms/cm 2 on SiO 2 surfaces can be removed to less than $3 Â 10 9 atoms/cm 2 by immersion in 3 ppm HCN aqueous solutions within 2 min at room temperature. The Cu removal process by HCN aqueous solutions consists of the initial fast and subsequent slow steps. X-ray absorption fine structure (XAFS) measurements with the total reflection X-ray fluorescence (TXRF) geometry (TXRF-XAFS) have been used for determination of chemical states of Cu contaminants before and after cleaning with HCN aqueous solutions. Cu contaminants are composed of Cu þ and Cu 2þ species, latter being present above the former. In the former species, a Cu atom is linearly bound to two oxygen atoms (Cu 2 O-like species), while in the latter species, a Cu atom is bound to four equatorial oxygen atoms and two axial oxygen atoms [Cu(OH) 2 -like species]. CN À ions react with outer Cu(OH) 2 Àlike species with a high rate, leading to initial fast removal of Cu contaminants. When the Cu concentration decreases to $1 Â 10 10 atoms/cm 2 , Cu þ species becomes a main species and its removal rate becomes much lower.Metallic contaminants even with a minute concentration (i.e., <1 Â 10 11 atoms/cm 2 ) cause degradation of semiconductor device characteristics, e.g., an increase in the interface state density, 1 shifts of the threshold voltage, 2 and decreases in the minority carrier diffusion length 3 and the minority carrier lifetime. 4,5 Cu has been used as alternative wiring materials to aluminum alloys due to the lower resistivity. Cu induces energy levels near the Si midgap, which greatly decreases the carrier lifetime. Therefore, it should be removed completely. For next generation devices, the International Technology Roadmap for Semiconductors (ITRS) (Ref. 6) suggests that surface concentrations of metal contaminants should be less than 5 Â 10 9 atoms/cm 2 . During cleaning, surface etching should be avoided since it may result in roughened surfaces and formation of defect states. To clean surfaces of Si devices, wet cleaning techniques based on the RCA cleaning method, i.e., cleaning with hydrogen peroxide plus ammonia aqueous solutions (APM) and hydrochloric acid plus hydrogen peroxide mixture (HPM), 7 have been usually employed. To avoid re-adsorption of metal species, incorporation of chelating agents such as ethylene-diamine-tetraacetic acid, 8,9 1,2-cyclohexane-diamine-tetraacetic acid, 8,10 and tetramethylammonium hydroxide 11 in the cleaning solutions has been proposed. The HPM cleaning solutions can remove metal contaminants. However, the HPM cleaning method should be performed at elevated temperatures at around 80 C using relatively high concentration (i.e., a few %) solutions, since the cleaning solutions do not possess high ability of metal removal.We have developed semiconductor cleaning solutions containing cyanide ions (CN À ). 12-19 CN À ions form metal-cyanide complex ions, leading to removal of metal species from surfaces without etching. Moreover, because of the high reactivity of CN À ions with metal...

Improvement of Minority Carrier Lifetime by HCN Treatments

“…In our previous paper, we have found that ammonium hydroxide aqueous solutions cannot remove Cu contaminants below 10 11 atoms/cm 2 , but addition of low concentration (e.g., 2.7 ppm) HCN in the solutions can remove Cu to the concentration below ∼3 × 10 9 atoms/cm 2 . 20 Fig. 3 shows changes in the minority carrier lifetime of the assliced Si wafers.…”

Metal Removal and Defect Passivation Performed on Si Wafers for Solar Cell Use by HCN Treatments

“…We have already reported that HCN solutions possess high activity for metal removal, which activity enables cleaning with ultra-low concentration solutions at room temperature. [6][7][8][9][10] Since metal species removed from surfaces are present in the form of metal-cyanide complex ions (e.g., Cu(CN) 4 3− ) which are extremely stable in aqueous solutions, 11 re-adsorption of the removed metals does not proceed, which makes it possible to use the cleaning solutions repeatedly. Avoidance of re-adsorption also enables to remove metal contaminants to extremely low levels.…”

“…6-10 CN − ions possess high metal removal activity resulting from formation of metal-cyanide complex ions, 11 and therefore, cleaning can be performed using ultra-low concentration HCN solutions at room temperature. 10 Due to high stability of metal-cyanide complex ions in the cleaning solutions, re-adsorption is completely prevented, and thus, metal contaminants can be removed completely even when HCN solutions are used repeatedly for cleaning. Moreover, CN − ions are selectively adsorbed on defect states such as Si dangling bonds, resulting in defect passivation, and thus improving conversion efficiency of Si solar cells.…”