Bonding Surface - Xi Hao
In studies and researches that use microfluidic devices, one of the most challenging part is the fabrication of the microfluidic devices. Since the microstructures inside the microfluidic devices are in very small scales, which a very small failure of the device can cause leakage of the fluids and can potentially lead to failure of the experiment, the microfabrication step is very important. The fabrication steps are usually creating layers of the microfluidic chips, and the assembly of the layers.
Bonding surfaces is a process that bonds two surfaces to assemble them into a complete unit. Bonding Surfaces is a critical fabrication step for microfluidic divice assembly. While creating microfluidic devices, the devices usually contains multiple layers as shown in figure 1, and since the devices contain fluids inside, incomplete or low quality bondings may cause fluid leakage and directly affact the functionality of the devices. According to properties of the surfaces and the requirements for the devices, there are multiple methods to bond surfaces, and a some of the most common methods for bonding surfaces are UV/Ozone treatment, Thermal bonding, Adhesives and Crosslinking.
Methods to bond surfaces
Ultraviolet (UV)–Ozone Treatment
UV-Ozone treatment is also known as UV-Ozone Activating, which is a surface activating process of sample or substrate used in material and device fabrication. In this process, photochemical reaction happens on the surfaces and substrates of the materials. Sometimes, it is not necessary to use UV, which in laboritories, we can create a plasma using RF and achieve the same result as UV-ozone.
UV light has two types of wavelength (185nm and 254 nm). Each wavelength plays a different role in UV-Ozone treatment. 185-nm UV light dissociates molecular oxygen into triplet atomic oxygen, then the triplet atomic oxygen combines with molecular oxygen and generates ozone.
In addition, the 254-nm UV light will dissociate ozone to molecular oxygen and singlet atomic oxygen. Singlet atomic oxygen has strong oxidation power, and it reacts with substrate surfaces. In this reaction, the surfaces were oxidizing on inorganic substrates such as silicon wafers. In the case of organic materials, chain scission of molecules happens and organic residue contaminants are gently removed from the substrates as volatile byproduct molecules such as Carbon dioxide, water and oxygen.
Poly(dimethyl siloxane) (PDMS) and poly(vinylmethyl siloxane) (PVMS) are two types of commonly used polysiloxanes. Both PDMS and PVMS undergo dramatic changes in their properties when exposed to UVO. The surface chemical composition of both PDMS and PVMS at long UVO treatment times changes substantially and features a high density of hydrophilic groups. These oxidized polysiloxanes will then be able to attach to inorganic surfaces such as silican wafers of glass slids to form conformal Si-O-Si bondings.
Thermal bonding is a bonding between materials that their physical properties changes with increasing temperatures. These materials are usually called thermoplastics. Thermoplastics are materials that are reversibly moldable above a certain temperature, and solidify upon cooling. Thermal bonding occurs when thermal plastics are heated to and beyond their Glass transition temperature, and pressures are applied.
The temperature where the materials start becoming moldable is known as the glass transition temperature. Glass transition is the reversible transition in amorphous materials from a hard and relative brittle state into a soften or viscous state when increasing the temperature.
When the thermoplastic materials are heated above its glass transition temperature, the mobility of the polymer chains in the thermoplastic materials start increasing and when the polymer chains are free to move, they can bridge the gap between the two layers and bond them together.
Thermal bonding requires extremely clean and flat surfaces and is typically performed in cleanrooms. In application, thermal bonding can be used to bond glass to glass or silicon to silicon. In thermal bonding, similar materials are bonded at elevated temperatures and pressure.
When the surfaces are photocrosslinking photoresists, they can bond to each other by cross-linking under present of light. Photocrosslinking photoresist is a type of photoresist, which could crosslink chain by chain when exposed to light, to generate an insoluble network. Photocrosslinking photoresist are usually used for negative photoresist. There are cross-linkers in the photocrosslinking photoresists that will be activated when exposed to light, the crosslinker ratios determine the rate of the bonding process. When there are more light sensitive cross-linkers in the photoresists, the the surfaces will bond to each other faster, and in opposite, if there are less light sensitive cross-linkers in the photoresists the bonding process will be slower.
Adhesive is another example of cross-link bonding. An adhesive layer between surfaces can bond the surfaces onto each other. Many common adhesives are epoxy-based, which they can be either cured by exposure to UV light or by heating. The curing process results in bonding between the surfaces.
There are some other methods to create molecular-level cross-linking. For example, organofunctional alkoxysilane molecules can be used to modify the surface of a substrate that contain hydroxyl groups. These hydroxyl groups can attack and displace the alkoxy groups on the silane and form a covalent Si-O-Si bond, and can form bonds across the interface of the substrate, which can be used to bond another substrate like glass or PDMS. These alkoxysilane allows molecular-level cross-linking without a thick layer of glue.
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