[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.
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.
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.