Structuring of Diffusion-Modified Glass

A variation of the composition of the salt melt did not prove successful. The optimal temperature for the diffusion process was 405° C. Temperatures below 405° C resulted in the formation of microcracks in the modified surface layer because of big differences of the GTE between the bulk glass and the silver ion exchanged glass layer. If the temperatures exceeded 405°C visible 

The surface layer modified by ion exchange was indeed photosensitive. Baumgart [25] measured the concentration profiles of various ions and calculated the threshold value of the silver concentration required for photostructuring of the modified surface layer. It was found that Si +, Al and K+ ions do not take a part in the diffusion process, but Na ions diffuse out of the glass whereas Ag and Li ions diffuse into the glass. 

The measured concentration profile and a fitted curve of the silver ions near the surface of the ion exchanged glass are shown in Fig. 9.28. A maximum concentration of about 0.23 mass% silver was found directly at the surface of the sheet. The concentration of silver ions decreases with increasing distance from the surface. The calculated diffusion constants basing on the measured concentration profiles of various ions are given in Table 9.4. 

The main advantage of diffusion modification method to structure glasses is the relatively precise etch stop at the position of the threshold value of the silver concentration. The maximum permitted diffusion time is 20 h which allows for a maximum depth of structures of 400 pm. This hmit of diffusion time is due to the onset of undesired LMS crystallisation at the glass surface during the crystallisation process. The reason is the presence of too much silver in the glass surface which will be reduced also without special UV exposme and forms nuclei dming thermal treatment. If diffusion times in excess of 10 h are required a surface treatment should follow the diffusion process. 

The photostructuring of diffusion-modified FS21 glasses is limited with a depth of 400 pm because of the long diffusion time required, the onset of surface crystallisation and additionally an angle of wall inclination that exceeds 10°. Using this method it is impossible to fabricate holes, trenches and structures of a definedly varying depth in the same substrate. 

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