Photodefinable Glass

Photodefinable Glass Processing
  
Microstructures made in Apex photodefinable glass
by: Khalid Tantawi

Introduction:
Glass has three main advantages over metals, semiconductors, and PDMS, that made it claim a significant role in the nanotechnology and microfabrication industry:
  1. Its optical transparency makes it the unquestionable material (along with PDMS) for many applications such as in microfluidics. 
  2. Its superior thermal insulative property over metals and semiconductors.
  3. Its chemical inertness to many compounds that attack metals, semiconductors, and PDMS. 
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 [1], 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 [2], 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 [3].  

    

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 [1].  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 [1]). 

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.

As of 2012, there are two commercially available photosensitive glasses, these are Foturan glass by the German Schott Corporation, and APEX  by the US based Lifebiosciences, Inc. (now called 3D Glass Solutions).  Fotoform glass by Corning is no more being produced. 

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.  

References

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

3. Cheng Y, Sugioka K, Masuda M, Shihoyama K, Toyoda K and Midorikawa K 2003 Optical gratings embedded in photosensitive glass by photochemical reaction using a femtosecond laser Opt. Express 11 1809–1816

4. Khalid H. Tantawi, Emanuel Waddell, and J.D. Williams, “Structural and composition analysis of Apex™ and Foturan™ photodefinable glasses”, Journal of Materials ScienceVol. 48, no. 5, pages 5316-5323, (2013).


Journal Publications in Photosensitive Glass Research                     
  1. W. R. Gaillard, K. H. Tantawi,  V. Fedorov, E. Waddel, J. D. Williams, In-plane Spectroscopy with Optical Fibers and Liquid Filled APEX Glass Microcuvettes”, J. Micromech. and MicroEng., 23, 107001, 2013.
  2. Khalid H. Tantawi, Emanuel Waddell, and J.D. Williams, “Structural and composition analysis of Apex™ and Foturan™ photodefinable glasses”, Journal of Materials ScienceVol. 48, no. 5, pages 5316-5323, (2013).
  3. Khalid Hasan Tantawi, William Gaillard, Jake Helton, Emanuel Waddell, Sergey Mirov, Vladimir Fedorov, and John D. Williams, “In-Plane Spectroscopy of Microfluidic Systems Made in Photosensitive Glass”, Journal of Microsystem Technologies, 2013. DOI 10.1007/s00542-012-1626-6
  4. Khalid Hasan Tantawi, Janeczka Oates, Reza Kamali, Nathan Bergquist and John D Williams, Processing of photosensitive APEXTM glass structures with smooth and transparent sidewallsJournal of Micromechanics and Microengineering Vol. 21, 017001 (2011)

Selected Conference Presentations and Proceedings

  1. 1. Khalid H. Tantawi, William Gaillard, and J. D. Williams, “Challenges of Photodefinable Glass Technology in Micromechatronics”, 124th Tennessee Academy of Science Conference, Morrisstown, TN, November 2014

  2. Merits and Limitations of Photodefinable Glass Technology in Micromechatronics Applications, Khalid H. Tantawi, William Gaillard, and John Williams, Jordan Journal of Physics, (in press)

  3. Khalid H.M. Tantawi, Janeszcka Oates, John Williams, “Processing of Smooth Sidewalls on Lithographically Defined Apex Glass”, IMAPS 6th International Conference and Exhibition on Device Packaging,Scottsdale/Fountain Hills, Arizona USA (March 2010)

Multimedia: 
Photosensitive Glass Processing video by Khalid Tantawi and John D. Williams, University of Alabama in Huntsville:


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