Application Of Supercritical CO₂in Photoresist Removal And Low-k Dielectric Repair

It is called supercritical carbon dioxide(scCO2) when the temperature and pressure of CO2are both higher than the critical value (Tc=31.1℃. Pc=7.38MPa). CO2is non-toxic, non-flammable, no pollution, and cheap, so it is a kind of green chemical solvent as its "environmentally friendly". scCO2has been tried to use for photoresist removal in semiconductor manufacturing because it is easy to penetrate into microholes and nano-scale trenches owing to its zero surface tension and high diffusivity. However, scCO2microemulsions containing toxic fluorous surfactants are usually employed for the photoresist removal due to the high solubility of fluorous surfactants in scCO2. Few investigations with hydrocarbon-based surfactants and co-solvent in scCO2have been reported. In this paper, the scCO2system was used to remove the residue of the photoresist on the surface and in holes of silicon wafer. The influencing factors have been investigated systematically including type of surfactants, type of co-solvents, temperature, pressure, time and so on. In addition, the repair function of scCO2was explored on the low-k dielectric material.The major work and results are as follows:1. The effects of two kinds of surfactants on the removal rate in scCO2system were compared.In this study, two hydrocarbon-based nonionic surfactants2,6,8-trimethyl-4-nonyl polyethylene ether (TMN-3) and2-ethyl hexanol polyoxyethylene-polyoxypropylene ether (EH-3) were used to form scCO2microemulsions. The results of surface characterization and composition analysis for the processed samples by metallographic microscope, SEM, FTIR and XPS revealed that comparing with TMN-3, EH-3was more effective to strip the photoresist under the same condition. Furthermore, the optimum concentration of3wt.%and optimum Wwater/WEH-3ratio of1:1were got through experiments.2. The effects of two kinds of co-solvents on the removal rate in scCO2system were compared. With the assistant of two kinds of co-solvents isopropanol and dimethyl sulfoxide (DMSO), supercritical cleaning effects have been improved. At0-8mL different co-solvent amounts, characterization of surface of the processed samples was observed by metallographic microscope. It was found that4mL DMSO was enough to improve the photoresist removal efficiency from15%to85%, more dose did not cause visible higher efficiency. Comparing with isopropanol, DMSO was more effective to strip the photoresist under the same condition.3. The effects of temperature, pressure, time and magnetic stirring on the removal rate in scCO2system were studied.The photoresist was removed in the scCO2microemulsions under the pressure of10-25MPa at40-80℃. The results revealed that the removal rate increased with the cleaning temperature when the temperature was lower than70℃. The removal rate of90%was achieved at70℃and then it gradually decreased as the temperature higher than70℃due to the lower photoresist solubility in scCO2arising from the lower scCO2density at higher temperature. The photoresist removal efficiency improved from40%to85%with the cleaning pressure changing from10MPa to25MPa.The effect of the stirring was studied under different magnetic agitation conditions:without magnetic agitation, with low-speed magnetic agitation and with high-speed magnetic agitation. The results showed that significant promotion of magnetic agitation to the photoresist removal efficiency was observed.4. The effects on cleaning results in the scCO2system and traditional solution were compared.Comparing with traditional wet cleaning method, scCO2cleaning did not lead to any collapse or deformation of patterns and the cleaning efficiency was significantly enhanced.5. The repair function of scCO2system on the low-k dielectric material was explored.Low-k material MSQ films were treated in scCO2system and the traditional SPM solution, respectively. Comparing with traditional wet cleaning method, supercritical system did not cause any damage to low k materials, while the k value of the sample treated in SPM solution increased from2.38to3.61owing to the formation of Si-OH group by water absorption. However, the k value recovered to3.1after the sample was repaired in scCO2system due to the passivation of Si-OH group by scCO2.