Methods for verification of ultra-pure water with air gap membrane distillation : Focusing on applications in the semiconductor industry

• Main Author: Pirouzfar, Pedram
• Format: Others
• Language: English
• Published: Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013) 2020
• Subjects: Nano Technology; Nanoteknik; Other Engineering and Technologies not elsewhere specified; Övrig annan teknik
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-80018

In the semiconductor industry, the purification process of the silicon wafers is of a great importance. If water of sufficient quality is not used, the silicon wafer surface runs a risk of being destroyed by particles and bacteria sticking to its surface. Semiconductors cannot be manufactured on the destroyed surfaces and to achieve the highest efficiency of the circuits, water with high purity is required for the purification process. The silicon wafers produced by the manufacturer have an oxide layer on them as a protective layer. This oxide layer needs to be cleaned off before it can be used for the manufacture of semiconductors. The oxide layer is removed by applying 5% hydrogen fluoride (HF) to the surface which is afterwards cleaned away with water. It is mainly within this part of the purification process that particles and bacteria get stuck on the surface of the silicon wafer. At present, water of poor quality is used which is unable to dilute and purify the mixture that becomes with hydrogen fluoride and the oxide layer.   As development is constantly advancing and the line width of the circuits becomes narrower and smaller, water with almost no particles is needed to clean these small areas. The particle size of the water must not exceed 20 nm in order to effectively clean the silicon wafers and preferably the particle size should not exceed 10 nm.   In the present study, an air gap membrane distillation module was investigated for the purpose of verifying the purity of the water where spherical spheres of 20 nm diameter were added into the purified water and examined in a dynamic light scattering (DLS). Because ultra-pure water (UPW) is a very aggressive water, storage is a problem. Four different container materials ability to store UPW with maintained purity were studied; white borosilicate ice cream, brown borosilicate ice cream, ethylene chlorotrifluoroethylene (ECTFE) and polyvinylidene fluoride (PVDF).   Experiments were also done to further verify the purity of the water by adding ultra-pure water on a silicon wafer and allowing it to dry to study the dry spots. The dry spots were studied in an SEM to see if the water left any particles behind on the surface. The same experiment was also done with tap water and distilled water which was dripped on a silicon wafer and dried. These dry spots were examined in a scanning electron microscope (SEM). To investigate how effectively ultra-pure water cleans a silicon wafer, an amount of 5% hydrogen fluoride on a silicon wafer was added and rinsed with ultra-pure water and tap water respectively. The same experiment was also done with tap water for comparison. These silicon wafers were studied in an SEM to see if any particles were left on its surface from the respective water. An initial methodology was also done when 5% hydrogen fluoride was diluted with ultra-pure water and tap water to compare the amount of respective water it used to dilute this acid.   In the present study, simulations were made on the air gap membrane distillation module in COMSOL where four different geometries were simulated with the aim to see how the temperature profile on the hot and cold side changed as the geometry and area of the membranes changed.   The purity of the water produced with the air gap membrane distillation were verified with DLS and the particle size did not exceed 20 nm. Further experiments showed that with UPW, there were no dry spots on the surface of the silicon wafer and no particles could be seen when the silicon wafer was examined in an SEM. When the tap water was dropped on the silicon wafer and dried, one could clearly see the drying spots. When the silicon wafer was examined in an SEM, there were many particles left on the surface. The distilled water left no drying stains on the surface but on the other hand, it was able to see particles on the surface examined when in an SEM. When 5% hydrogen fluoride had been dropped on the surface and washed away with UPW, no particles could be detected when examined in an SEM. However, particles were found when the same amount of hydrogen fluoride was rinsed off with tap water.   When 5% hydrogen fluoride was diluted to a neutral pH of 6-7, about 200 ml of UPW was used as separated from tap water where it went to the quadruple to dilute the same amount of hydrogen fluoride. This showed the purity of the ultra-pure water compared to tap water.   For the simulations it was possible to see how the temperature profile changed with the area. With a large area, the temperature profile on the hot and cold side became very poor. The temperature on the hot side dropped a lot and on the cold side it increased a lot. The largest area simulated was 255x255 mm. With a smaller area, a more even temperature profile was obtained. The area that gave the best temperature profile was 180x100 mm, which was the smallest area investigated. In contrast, the diffusion area becomes smaller as the area decreases, leading to a reduced production of ultra-pure water.   This study is close to research and is about developing new technology and modifying/improving existing technology.