Microcontact printing (µCP) is a simple and yet excellent method to functionalize surfaces by using a structured and "inked" stamp to transfer patterns of the desired substances directly onto the substrate surface. One of the key elements in this process is quality of the stamp, which is the negative copy of a master.
In most cases the stamp is made by simply pouring liquid PDMS over a structured Si master. This normally results in a considerable lateral thickness variation of the stamp which is unfavourable for the printing quality since it induces a variation of the printing pressure. Spin-casting can reduce the thickness variation problem but suffers from edge bead and thin stamps which are difficult to handle. We produced 4" PDMS stamps directly on a transparent rigid back-plate, which supports the 900 µm thick PDMS layer and therefore ensures save handling and compatibility to the printing device.
Both master and stamp were evaluated using SEM and AFM. The suitability of these stamps for large area µCP was tested by microcontact printing alkanethiols, which form self assembled monolayers (SAMs) on a gold surface. These structured SAMs were used as etch mask to structure the gold surface with a thiourea solution. The microcontact printing process itself was performed on a EVG®620 mask aligner under clean-room conditions. This guaranteed best possible control over important process parameter like, contact pressure, contact time, and uniformity of pressure. The etch results were investigated using AFM.
The resolution of the PDMS stamp is primarily limited by the resolution of the master. To overcome the resolution limitation of the conventional photolithographic fabrication process for the Si master we additionally used Si masters coated with a UV-curable polymer, which was structured using UV nanoimprint lithography (UV-NIL). The smallest feature size so far characterized is in the range of 700 - 900 nm.
There are numerous potential applications for µCP in life science, the field of organic sensors and semiconductors.
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