Study On The Adhesion Strength Of Metal Films On PMMA Substrates By UV Surface Treatment

Polymethylmethacrylate (PMMA) is widely used to fabricate microfluidic chips due to its good transparency, good mechanical property, good biocompatibility and ease of fabrication. The deposition of metal thin films onto the PMMA substrate is a main method to fabricate metal micro/nano structures on microfluidic chips. However, the adhesion strength between the metal film and PMMA substrate is very poor, which limits the application of metal micro/nano structures in microfluidic chips. To this end, an ultraviolet (UV) surface treatment method to improve the adhesion strength between the metal film and PMMA substrate was studied.A method of UV surface treatment was presented, and gold microelectrodes were fabricated by using photolithography and etching. The adhesion strength between the gold film and the PMMA substrate was tested by immersion test, blowing test, ultrasonication test and tape test, and compared with other method for improving adhesion strength. The results indicated that the adhesion strength between the gold Au film and the PMMA substrate was higher, and the consistency of the metal film was better.The mechanism of UV surface treatment to improve the adhesion between metal film and PMMA substrate was studied. We used X-ray photoelectron spectrometer, fluid analyzer, atomic force microscope and three-dimensional surface contour graph equipment to study the change of chemical bond, functional group content as well as the change of surface hydrophilicity and roughness. We concluded that UV treatment made the PMMA surface hydroxyl groups, at the same time other polarity the content of oxygen containing functional groups has improved. Those make PMMA surface oxidation. Because of those changes, the adhesion strength between metal atoms and PMMA substrate was improved. In addition, the surface roughness after UV treatment was increased, which also improved the adhesion between the metal film and the PMMA substrate.As the application of the method above, a PMMA nanofluidic electrochemical chip with integrated silver microelectrodes was fabricated. The chip consisted of a PMMA substrate with sub-micrometer channels and a cover plate with micro channels. The PMMA substrate integrated the sub-micrometer channels and silver microelectrodes. Sub-micrometer channels as a sieve was used for trapping silica nanoparticles modified with streptavidin and assembled the nanoparticle crystal (NPC). Silver electrodes were used to measure the conductance of nanoparticles crystals. The cover plate integrated four microchannels for liquid transmission and four liquid storage pools. The oxygen plasma etching rate of PMMA was studied. The average etching rate was 15 nm/min with the optimized parameter. The roughness of channels was less than 2 nm. A method of UV assisted bonding was studied to avoid the collapse of the sub-micrometer channels during the bonding process. This method significantly reduced the bonding temperature, and the width and depth direction deformation of the sub-micrometer channels were 2% and 5%, respectively. Using the chip, the detection limit of biotin was 1 aM, and the range of detection was from 1 aM to 0.1 nM.