Smoothing of ultrasonically drilled holes in borosilicate glass by wet chemical etching

Besides silicon various types of glass play an important role in microsystem technology. One requirement of the glasses is that they have to be microstructurable. In many applications for microsensors and microstructures the glass must be suitable for anodic bonding. Usually borosilicate glass (e.g. Coming Pyrex code 7740 glass) is used, which has a thermal expansion coefficient that is almost equal to the thermal expansion coefficient of silicon and which has the necessary electrical conductivity at the temperature at which the bonding process occurs. For many applications, e.g., microfluidic systems, it is necessary to have fluids flown through ultrasonically drilled holes in the Pyrex glass to the silicon chip. The main objective of the investigation was to obtain smooth walls of ultrasonically drilled holes because every contamination influences the performance of the microsystem. In this paper the ultrasonic drilling method is compared with two other procedures for machining holes (sand blasting and laser machining). Other ways of structuring glass have been presented previously. A process was developed to smooth the walls of the drilled holes with different concentrations of hydrofluoric acid.