Using Photosensitive Glass for Photolithography

Photolithography is a process used in microfabrication to place a design or pattern on areas of a thin film or substrate. Photolithography allows patterns to be ‘printed’ onto a substrate in microscale dimensions, enabling it to be an invaluable tool in the electronics and semiconductor industries.

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Photolithography, as the name indicates, uses light for transferring a symmetrical pattern from a photomask to a light-sensitive chemical (photo resist) on the substrate. A series of chemical treatments are then employed to either engrave the exposure pattern, or enable the deposition of a new material in the preferred pattern on the material under the photo resist.

Sometimes, this process has to be repeated multiple times in order to produce the desired results. For instance, a CMOS wafer might have to endure photolithographic cycling/etching up to 50 times.

The process of photolithography enables exact control over the size and shape of the final product, and it is an extremely inexpensive process for generating patterns over an entire surface.

Photolithography on Glass

Photolithography is an effective technique for the mass-production of microstructures on glass and silica substrates. Photo-sensitive glass, a technical glass which crystallizes following exposure to UV light and subsequent heat treatment, enables a permanent pattern to be developed on a substrate before etch processing.

The crystallized areas are then capable of etching away with a high aspect ratio, resulting in very fine structures, which have applications in semiconductors and microfluidics. Continued processing of photosensitive glass (a second exposure and tempering process) allows the glass to be converted into a glass ceramic if needed. Anodic bonding can also be performed.

This is a wafer bonding process, which allows the glass to be sealed to either metal or silicon without introducing an intermediate layer, permitting the glass to seal silicon wafers in microfluidics and electronics.

What is Photosensitive Glass?

Photosensitive glass (PSG) is a transparent glass in the lithium-silicate family of glasses, in which it is possible to capture an image of a mask by the formation of microscopic metallic particles in the glass, following exposure to electromagnetic radiation such as ultraviolet light. This change is due to a redox process, where cerium ions are oxidized to a more stable state discharging electrons, and silver ions are reduced to metallic silver utilizing the electrons discharged from cerium.

If the glass is exposed to UV light of wavelength 280–320 nm, a latent image is produced. The glass continues to be transparent at this stage, but the latent image will be visible when the glass is heated to temperatures in the range 550-560 °C. Upon the heat treatment, a crystalline phase lithium metasilicate forms on the metallic silver particles. The lithium metasilicate phase is etched in order to produce a 3D microstructure.

PSG is a greatly promising material for the production of components for several complex microsystems. High aspect ratio microstructures can be developed by using just slightly modified semiconductor equipment and relatively low manufacturing costs are possible with small-scale production. PSG has high potential in an extensive range of applications as it can be applied in both high temperature and corrosive environments. In addition to this, it is transparent, demonstrates good electrical and thermal insulation, and has a high Young's modulus.

Photo-Structural Glass-Ceramics

In 2003, photo-structural glass ceramics were developed for true MEMS and 3D microfabrication processes, which surpassed the 2D limitations of standard photolithography.

The glasses unique properties make it perfect for the integration of the optical, mechatronic, electrical, and packaging functionality needed in the production of MEMS structures. If heat and UV light are applied, PSG is transformed into a metasilicate material that is 30 times more etchable than the native glass material.

This accelerated etch capability results in considerably sharper and steeper wall angles than other standard wet etch techniques. By varying the furnace temperature profile, PSG can also be ‘ceramized’ to develop particular phase changes for customizable mechanical, light absorption, or electronic features.

Developing Photolithography

Photolithography is an exceptional process for the microfabrication of electronic and optoelectronic devices at a resolution of 50 nm or less, such as micro-organic memory devices, microsensors, micro-circuits, lab-on-a-chip/micro-total analysis systems (µ-TAS), and even piezo electric devices and micro-solar panels for energy harvesting.

The potential to take a particular piece of specially developed photosensitive glass and subject it to a lithographic process to develop thousands of accurate micro-scale devices is apparent. The process can be performed rapidly and (relatively) inexpensively, making it an attractive method for the semiconductor industry.

Photolithography is continuously developing. A new proton lithography mechanism permits the micropatterning of photosensitive etchable glass based on the crystallization of the glass following irradiation with MeV protons and heat treatment. The MeV protons result in a considerably reduced minimum feature size, when compared to UV irradiation, and the threshold dose for etching is extremely low (4000 protons µm⁻²), offering the potential of producing complex microstructures by direct writing using extremely short exposures.

The structure depth in the glass is determined just by the range of the protons in the glass, which allows structures with varied depths to be fabricated. This technique could be valuable in the manufacture of high aspect ratio microstructures such as micro-optical devices and fluid networks.

Technical Glass from Mo-Sci

Mo-Sci can offer a variety of photosensitive glasses that are perfect for photolithography.

Mo-Sci has been playing a vital role in the photosensitive glass market since 1985 and has a privileged record of innovation. The company’s development laboratories are second to none in the production of custom glass formulations for specialist applications.

Mo-Sci has products in all major manufacturing areas and is mainly active in the analytical microfluidics and electronics sectors.

References

1. Dong Qin, Photolithography with transparent reflective photomasks, J. Vac. Sci. Technol. B 16„1…, Jan/Feb 1998, p98-103.
2. Ciprian Iliescu, et al., A practical guide for the fabrication of microfluidic devices using glass and silicon, Biomicrofluidics. 2012 Mar; 6(1): 016505–016505-16.
3. T.R.Dietrich, W.Ehrfeld, M.Lacher, M.Krämer, B.Speit, Microelectronic Engineering Fabrication technologies for microsystems utilizing photoetchable glass, Microelectronic Engineering, Volume 30, Issues 1–4, January 1996, Pages 497-504.
4. C.H. Lin, Fabrication of microlens arrays in photosensitive glass by femtosecond laser direct writing, Appl Phys A (2009) 97: 751–757, DOI 10.1007/s00339-009-5350-8
5. I Gomez-Morilla, Micropatterning of Foturan photosensitive glass following exposure to MeV proton beams, Journal of Micromechanics and Microengineering, Volume 15, Number 4.

This information has been sourced, reviewed and adapted from materials provided by Mo-Sci Corp.

For more information on this source, please visit Mo-Sci Corp.