Editors' Choice—Control of Si₃N₄ Etching Kinetics and Selectivity to SiO₂ by the Additives in Superheated Water

Abstract

Selective etching of Si3N4 to SiO2 is essential in the semiconductor fabrication process. In particular, as the number of alternating Si3N4/SiO2 multi-layered stacks increases, selective removal of Si3N4 without loss of SiO2 becomes difficult. In this study, the dissolution of Si3N4 was demonstrated in superheated water without addition of H3PO4, which has been widely used to etch Si3N4. The dissolution rates of Si3N4 and SiO2 in the superheated water depended strongly on the concentration of OH−, and the activation energy obtained for the dissolution of Si3N4 was 72.65 ± 0.95 kJ/mol. It is believed that the attack of the partially δ+ charged Si atoms in the Si3N4 by nucleophilic OH− was the key step in the dissolution of Si3N4 in the superheated water. Because a tradeoff between the dissolution rate of Si3N4 and the Si3N4-to-SiO2 etching selectivity was observed, H2SiO3 and HF were added to HCl-based superheated water for optimization. The HCl-based superheated water with the addition of 0.005 vol% HF and 0.01 M H2SiO3 allowed successful fabrication of a horizontal SiO2 trench structure on a patterned Si3N4/SiO2 15 pair-layered stack through selective etching of Si3N4 without thinning of the SiO2 layer.

Addition of Carboxylic Acids to Superheated Water for the Environmentally Benign Removal of Silicon Nitrides

“…In contrast, when 0.1 M Ca­(H 2 PO4) 2 , CuSO 4 , CuCl 2 , Cu­(NO 3 ) 2 ·3H 2 O, or ZnBr 2 was added to DI water, the etching rate of Si 3 N 4 was less than 11.4 Å/min with an initial pH of solution less than 4. The results are in agreement with those of our previous study, which found that the etching rate of Si 3 N 4 in superheated water is dependent on the pH of the solution because Si 3 N 4 is etched by the nucleophilic attack of OH – …”

“…According to eq , the Si 3 N 4 etching rate is R Si 3 N 4 ≈ 170­[OH – ] 0.12 in this region. This reaction rate equation shows the same reaction order of [OH – ] and a similar rate constant compared with the rate equation obtained in our previous study, R Si 3 N 4 = 130­[OH – ] 0.12 (indicated by the dotted line in Figure ), where Si 3 N 4 was etched in superheated water with a wide range of OH – concentrations . In contrast, the etching rate of Si 3 N 4 is determined by the concentrations of RCOO – and OH – in superheated water when carboxylic acid is added.…”

“…As this process is repeated, Si 3 N 4 is etched in the form of Si­(OOCR) 4 , and it is thought that Si­(OOCR) 4 later changes into Si­(OH) 4 by reaction with H 2 O, as expected from the results in Figure b. The above mechanism is quite similar to the Si 3 N 4 etching mechanism by OH – in superheated water proposed in a previous study . However, it is very different in the sense that RCOO – , instead of OH – ions, acts as a nucleophile to etch Si 3 N 4 because the OH – concentration is fairly low in carboxylic-acid-containing superheated water.…”

“…For example, breathing H 3 PO 4 induces irritation of the nose, throat, and lungs, and eye contact with H 3 PO 4 causes burns . In addition, a shorter replacement cycle of H 3 PO 4 increases the process cost . HF is also known to etch Si 3 N 4 , − but it is a highly corrosive and dangerous substance that can seriously damage the skin upon contact. , HF also causes various problems in terms of EHS.…”

“…From the reported mechanism of Si 3 N 4 etching in H 3 PO 4 and HF solutions, it is known that nucleophilic substitution of Si 3 N 4 plays an important role in the etching of Si 3 N 4 . , In addition, it is reported that Si 3 N 4 can be removed in hot H 2 O with the addition of 20% citric acid at 80 °C or 20% tartaric acid at 90 °C, but the etching rates were as low as 0.52 and 0.73 Å/min, respectively . However, when H 2 O becomes superheated at a temperature between its boiling point (100 °C) and critical point (374 °C) under pressure, relatively higher etching rates of Si 3 N 4 were obtained. , The self-ionization of H 2 O increases in superheated water, increasing the concentration of H + and OH – in the solution. In particular, it was reported that nucleophilic attack by OH – determines the overall reaction kinetics of Si 3 N 4 etching in superheated water .…”