How to make glass microfluidics device? chemical wet etching and room temperature bonding

[First part of this post is here where I wrote step by step procedure for mask design and photolithography.]

After completing the photolithographic procedure, we use two different solutions for selectively removing chromium and glass from the channel network on the bottom substrate. 

Chemical wet etching solutions. On left is BOE with temperature control
and on right is Chrom etchant. Middle one is DI water.

At first, chromium layer from channel network is removed by immersing the substrate plate into a Teflon coated jar containing chromium etchant solution for about 15 min. One should note that leaving substrate plate in this solution for long time (hours and hours-this happens when you forget to remove it) could eat away even the photoresist layer from the entire plate. We buy the chrome etchant from Transene Company, INC, MA, USA and according to manufacturer specification the solution contains ceric sulfate (5-10%), nitric acid (5-10%), sulfuric Acid (1-5%), water (~ 75%). The entire substrate is then washed with DI water and dried by blowing N₂ gas. At this point one can clearly see through the substrate in channel patterning region.

Cartoon depicting the removal of photoresist and chromium layer from channel pattern

After this step, we cut the substrate into individual chips (1" x 2") using a glass-cutter (altogether 8 chips from one 8” x 8” substrate). Then, put extra layer of photoresist, dry in oven (80 ^o C), take out from the oven, and let it cool.

The substrate plate-individual chip is then immersed into a 1:10 buffered oxide etchant (BOE) solution bought from Transene Company, INC, MA, USA. According to the company specifications, this solution contains a mixture of hydrofluoric (HF) (1-25%), ammonium fluoride (NH₄F) (2-40%), distilled water. Hydrofluoric acid is a hazardous chemical, therefore must be handled with extra precaution. The BOE solution is continuously stirred at the rate of 120 rpm using a magnetic stirrer and the temperature is set at 55 ^oC. The substrate plate is removed from the BOE solution at a certain interval of time ~10-15 min (depending on the etching rate of BOE and channel depth desire), washed with DI water, and is dried with N₂. With our settings, etching rate is ~0.5 micron per minute. 

However, this rate can be manipulated based on your need. 

Cartoon showing the substrate plate after removing glass material.
Remember the glass etching is in all direction (not like in this cartoon)

The depth of the channel is measured using a XP series stylus profiler (Ambios Technology, CA, USA). This step is repeated until one gets desired channel depth.  

Profiler

Once desired channel depth is obtained, ~1 mm diameter access holes were punched at the channel terminals using a microabrasive power blasting system (Vaniman) by blowing sand particles from back side of the substrate. The miroabrasive system utilizes mechanical erosion on the substrate by bombarding with high-kinetic energy sand particles and results in conically shaped holes. This step is followed by removing the photoresist and chromium layer on remaining parts of the bottom substrate by using acetone and chromium etchant, respectively. 

Sandblaster 

Bonding two glass plates: The etched channels in the bottom substrate were sealed using cover plate. The two plates (bottom substrate and cover plate) were cleaned and brought together in a beaker containing DI water for bonding. They were removed from the beaker together with a tweezer, water was removed by gently pressing the plates with hand in paper towel, and then put under couple of heavy books for ~2 hrs. Then the bonded chip was kept in an oven at 80 ^oC for ~1hrs to strengthen the bonding between two glass plates. Bonding is a crucial step to create closed fluidic microchip networks. 

The bonding strength between two surfaces is proportional to the density of individual chemical bonds established between the two plates. Our bonding method is simple and does not require clean room facilities, programmed high-temperature furnaces, pressurized water sources, and adhesives. The bonding process can be completed in ~3 hrs. One of the most important factors affecting successful bonding of planner glass chips is the cleanliness of the bonding surfaces of glass substrates. Unsuccessful bonding events are often associated with solid particles or organic material remaining on the glass surfaces, for example, dust, residual photoresist, chromium or glass particles etc., prior to bonding. Some other critical factors for glass bonding are flatness of the glass plates and chip area.

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. 

click HERE for second part of this post

Instruments/equipments in a microfluidic laboratory

I am presenting the instruments/facilities that we use in our microfluidic/nanofluidics instrumentation laboratory. We primarily fabricate glass based microfluidic devices ourselves and use them for different biological applications.

1. We have a basic photolithography setup that includes a dark room, UV light source and developing solution.

2. Then we perform wet chemical etching to make channel networks on the substrate glass plate using BOE (buffered oxide etchant) which is an appropriate mixture of HF and NH4F.

Chemical wet etching setup

The BOE solution is heated to certain temp to fasten the etching rate and stirred continuously to have uniform etching. The solution on the right is Chromium etchant which removes chromium layer from the substrate plate.

3. Sand blaster is used to make holes to have access to the channels. It uses 25-50 micron  sand particles which are bombarded on the surface of glass substrate. It makes holes of ~1mm diameter.

Sand blaster to make holes

First Microfluidic Chip Ready to Use

As I have started working on microfluidic devices for last couple of weeks, my first chip is ready for use. This is a glass microchip intended to enhance the sensitivity of ELISA. Design of the this chip is not complex. Fabrication of such chips requires more art than science, I believe. My chip is not perfect but good enough to carry out some of the tests that I am planning in following days. I am happy for this. 

The most difficult and uncertain part of chip fabrication was bonding two glass plates-the substrate in which channels were developed and a cover plate to cover the substrate plate. I will write in details about each steps involved in fabrication with my experience in upcoming posts. Keep visiting this blog. In brief photo-lithography and wet etching process were employed and room temperature bonding was used. Below is the picture of my chip.