Saturday, February 15, 2014

How to make glass microfluidic device? mask design and photolithography

During my PhD at the University of Wyoming, I may have made ~400-500 microfluidic devices all from glass substrate. My PhD laboratory uses borosilicate based substrate/cover plate to make the chips for variety of applications such as immunoassay development, separation experiments, fuel cell units. Glass is one of the oldest materials in microfluidic field and has certain advantages (see below) over other materials that are being used in this field of research.
  1. Inertness to many chemicals
  2. Optical transparency
  3. Low fluorescence
  4. High resistance to mechanical stress
  5. Well established surface modification procedures
Below are the steps taken for the fabrication of glass microfluidic device: starting from designing the channel network using CADopia software to bonding the glass plates. I will take you through each of these steps.

1. Mask design: We create a channel design using CAPopia drawing software. I had never used any drawing softwares before and found this one easy to use. Once the design is complete, we send it out to a printing company (Fineline Imaging, Colorado) to get the mask. 

Mask with channel design

2. Photolithographic procedure: Once the photo-mask arrives from the printing company, we perform basic photo-lithography procedure to transfer the channel pattern design onto the glass substrate.  We buy borosilicate glass substrate (Telic, CA, USA) which is 4” x 4” in dimension and 1.65 mm thick. One side of this glass substrate is coated with a layer of chromium and photoresist on top of each other. Both chromium and photoresist layers are about 100-200 nm in thickness. The photoresist coated on the substrate is polymer based positive photoresist which is sensitive to UV radiation. Therefore the box containing the substrate plates must be opened only in dark room.

In photolithography room turn on the UV light source before at least 15 min you plan to shine it to the substrate plate. We use a custom made box where one can place photo-mask on top of substrate plate. This assembly is then gently pressed by putting a glass plate on top of it. The whole thing is inside a box in which a shutter can be opened when ready to expose to UV radiation.

Typically we expose the substrate to the UV radiation for ~ 30-45sec. During this step the polymer of photoresist breaks down and is removed by soaking the substrate in a photo-developing solution (MF-319, Rohm and Haas) for ~ 5 min. The substrate is then washed with deionized (DI) water (DirectQ Water Purification System, Millipore) and dried by blowing N2 gas. After this washing step, one can clearly see the channel pattern on the glass substrate. 

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  1. Hi
    Its very interesting article. I have few doubts like as you mentioned in one of the step that you gently press the mask with glass slide coated with chromium and photoresist, does that affects the feature size of the channels? What is the range of feature size that one can obtain by using this protocol?


    1. I used 100 um size channel patterns towards smaller channel side.

    2. Charmi,
      I have used this technique (contact photolithography) to make features as small as 10 microns. In theory, it can go smaller, but especially with these plastic masks the failure rate is quite high. If you want to make features between 10-1 microns, projection photolithography is often used. There is no real upper limit on the size of features that can be prepared, as long as your substrate is rigid enough to not sag and collapse the feature.

  2. Interesting article! It’s really very helpful as it contains lots of information about fabrication of glass based micro fluidic devices. As we all already know that Micro fluidic devices are the system in which the small volume of fluid can be handled and glass supposed to be for glass substrate. The way of your article writing is wonderful and showing that you’re master in content writing. I also read your article of chemical wet etching and room temperature bonding of glass micro fluidic devices. I would be grateful if you continue posting and keep looking for your next updates.Just train yourself at VLSI field. Contact us at VLSI Engitech Pvt Ltd now.