Photodefinable Glass Processing
Glass has three main advantages over metals, semiconductors, and PDMS, that made it claim a significant role in the nanotechnology and microfabrication industry:
To a less extent, its electrical insulation also, gives glass the upper hand in many applications.
Photodefinable glass , also called Photosensitive glass, is a special glass that allows one to form microstructures (see figure 1) in the glass without use of any of the conventional drilling and machining processes. The trick by which this happens is that this glass, when exposed to a certain dose of near-to-mid uv radiation, then baked at a certain temperature and duration, it transforms into a new compound, the crystalline-phase lithium metasilicate. This formed material is much more active for reaction with hydrofluoric acid (HF) than the amorphous glass. Thus if one exposes certain parts of a photosensitive glass wafer to uv radiation (assuming that the minimum dose requirements are reached), different microstructures may be created in a top-down approach of microfabrication.
Figure 1. A microcuvette made in photosensitive glass for IR spectroscopic tests on microfluidics.
Fabrication of Microstructures in Photodefinable Glass:
Basically, to fabricate a glass microstructure from photosensitive glass, the glass is processed in four steps , the role that each step plays is explained here below:
Step 1: Exposure to mid UV light
A glass sample is exposed to mid-UV light, the exposure dose has to exceed a certain threshold in Joule/gram. (Note that if the threshold is given in Joules/cm2 then the thickness of the glass has to be specified). The threshold is about 30 J/g, when measured at a wavelength of 280 nm through a Gaussian-distributed intensity UV filter with its center at 280 nm. In other words, if a 0.5 mm thick glass is exposed to a typical light bulb through the UV filter mentioned above, and the measured intensity was 0.10 mW/cm2 at 280 nm, then you need to leave it for about 2 hours under the light. After working with photosensitive glasses for years, it is from the author's experience that the best wavelengths are in the range 300-312 nm. This is likely to be due to the presence of the peak of the absorption band of Ce4+ ions in this range. Exposure with no mask would be useless, unless you want to study the glass properties after exposure, a quartz mask with any pattern on it may be used so that the glass captures a latent image of that pattern. During this step a reaction occurs in which cerium is oxidized to a more stable state , the electrons released are caught by silver ions. This is the reason why the glass must be exposed to mid-UV light, as the absorption bands of the cerium ions Ce3+ and Ce4+ are in the mid- UV range of frequencies. As of January 2011, It is not known yet exactly if a complete oxidation-reduction reaction takes place or not, It was not possible to find any evidence that supports the idea that a complete reaction occurs, the only difference that can be observed is a change in the UV absorption spectra of the glass following this stage. However, the change is likely to be due to the oxidation of cerium. It was not possible to detect the change that occurs with scanning electron microscopy. It is suggested that the electrons released by the cerium ions are trapped by the oxygen atoms to form ion-electron pairs with the silver ions .
Figure 2: The glass is exposed to ultra-violet light to receive a certain dose in a contact aligner (left). The mask used for the exposure (right). Photos taken in the clean room of the University of Alabama in Huntsville.
Step 2: Baking
The glass is then baked in a furnace for temperatures up to about 570 C . After that stage all the exposed parts of the glass will change their color to a dark brownish color, this happens because the silver ions are reduced to silver, and then they agglomorate to form nano clusters. These nanoclustors act as nuclii for the formation of the crystalline lithium metasilicate.
Step 3: Etching in HF Acid
The next step is etching the exposed regions of the glass by HF acid (HF acid is a toxic material, its vapor is also toxic and may lead to serious injuries if it touches the skin or is inhaled. ) At the end of this step, we will have a microstructure that is a 3 dimensional projection of the image of the mask used in the exposure step.
Step 4: Second Baking
This is the annealing stage at temperatures in the range 530-545 C the purpose of it is to reduce the surface roughness significantly, and allow the glass to be used in micro optics. This temperature range is above the glass transition temperature by about 100 C, the glass surface reflows and results in a significant reduction in surface roughness. Measurements performed at the University of Alabama in Huntsville using confocal microscopy and surface profilers show that the average surface roughness is reduced about 20 times (from about 0.5 um to 27 nm) (see ref ).
Figure 3: A Scanning Electron Microscope (SEM) image of a microstructure made in APEX photosensitive glass with a smooth sidewall after the second annealing stage. Image taken at the University of Alabama in Huntsville labs.
Structural and Composition Analysis of Photosensitive Glasses:
The science of photosensitive glasses was studied using Rutherford Backscattering, X-ray diffraction, UV/Vis, IR, and Raman spectroscopies. Rutherford backscattering spectra indicate the presence of cerium in both types of photodefinable glass, other results are also shown in Tantawi et. al. Structural and Composition Analysis of Apex and Foturan Glasses.
1. Khalid H Tantawi, Janeczka Oates, Reza Kamali-Sarvestani, Nathan Bergquist and John D Williams, 2011, Processing of photosensitive APEXTM glass structures with smooth and transparent sidewalls, IOP Journal of Micromechanics and Microengineering 21 017001
2. Dietrich T R, Ehrfeld W, Lacher M, Kr¨amer M and Speit B1996 Fabrication technologies for microsystems utilizing photoetchable glass Microelectron. Eng. 30 497–504
4. Khalid H. Tantawi, Emanuel Waddell, and J.D. Williams, “Structural and composition analysis of Apex™ and Foturan™ photodefinable glasses”, Journal of Materials Science, Vol. 48, no. 5, pages 5316-5323, (2013).
Journal Publications in Photosensitive Glass Research
Photosensitive Glass Processing video by Khalid Tantawi and John D. Williams, University of Alabama in Huntsville:
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