Chromium masks

The use of aluminum-based masks in photolithography has been proposed.347 According to the scheme employed, aluminum is deposited onto a polished glass sheet. The regions of the mask that should be light transparent are then converted into porous oxide. As the operation of aluminum anodization exhibits a much better vertical anisotropy than chemical etching, the masks obtained reproduce the parameters of standard masks more precisely than the chromium masks usually used. 

Polishing uses abrasive compounds and mechanical action to remove contaminants. Although used extensively in preparing glass substrates for chromium masks and in obtaining the desired flatness of silicon wafers, it is seldom, if ever, used once the silicon wafer enters a process sequence. 

After photolithographic pattern definition (which is covered in more detail in the next section), gold can be etched in a 30% solution of KI in DI water, then rinsed. The chromium adhesion layer can be etched using any of several etching cocktails [62] [63] or pre-mixed commercial reagents (e.g. Transene Chromium Mask Etchant), then rinsed and stripped. 

Photolithography can be performed using chromium or emulsion masks. Chromium masks can hold tighter resolution and alignment tolerance and remain cleaner than emulsion masks, but quality emulsion masks are available which can be used down to 4 microns or better resolution. 

In an alternative approach developed by NimbleGen (http //www.nimblegen. com/, Madison, WI), arrays are built using photodeposition chemistry based on a maskless array synthesizer (MAS). The MAS system is a high-density DNA fabrication instrument that uses a maskless light projector as a virtual mask instead of the physical chromium masks used by Affymetrix. The virtual mask is an array of hundreds of thousands of individually addressable aluminum mirrors on a computer chip. These mirrors function as virtual masks that reflect the desired pattern of UV light and are controlled by the computer. 

JU-Molar Soln. Eq. o( base present. Days heated to 75 . Per cent, latent chromium. Masked OH-rad1cIes per 100 Cr-atoms. Masked OH>radfcIes per Cr>atoni in latent basic Cr. 

For micro- and nanofluidic devices, sputtering is usually performed after channel fabrication. Areas that do not require sputter coating are masked off using a lift-off layer. This could be a specially fabricated photoresist layer, or it could just be the chromium mask leftover from the channel etching step. The structured substrate is placed into the sputtering chamber where it is sputtered with the desired material. Then it is removed and the lift-off layer dissolves, leaving... 

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