Shadow mask technique

A shadow-mask technique has been applied for the local metal deposition to exclude metal residues on other designs processed on the same wafer (Fig. 4.2b). Such metal residues may be caused by imperfections in the patterned resist due to topographical features on the processed CMOS wafers or dust particles. The metal film is only deposited in those areas on the wafer, where it is needed for electrode coverage on the microhotplates. This also renders the lift-off process easier since no closed metal film is formed on the wafer, so that the acetone has a large surface to attack the photoresist. Another advantage of the local metal lift-off process is its full compatibility with the fabrication sequence of chemical sensors based on other transducer principles [20]. 

In response to requirements and manufacturing approaches, different patterning techniques have been developed. Photolithographic techniques similar to those used in Si technology [42, 43] are used. Shadow mask techniques are well... 

The shadow-mask CRT is the workhorse of the video display industry. Used in the majority of color video displays since the introduction of color television in the early 1950s, the shadow-mask technique has been refined to yield greater performance and lower manufacturing cost. 

Each of the techniques described above has unique strengths and weaknesses, and the optimum device structure for commercial full-color displays will also be heavily influenced by the ease with which it can be mass-produced. Currently full-color OLED displays have been manufactured commercially by using two of the above described techniques only, i.e., (a) side-by-side pixels deposited by high-precision shadow masking and (b) using white OLEDs and color absorption filters. 

The first step in sample preparation is the deposition of a thin metal film on an insulating substrate (e.g. a glass microscope slide). This base electrode is deposited by conventional vacuum deposition techniques with the electrode geometry defined by a shadow mask. Next, this electrode is oxidized either by exposing the film to room air or oxygen, or by establishing an oxygen plasma within the vacuum chamber. In the case of Al-electrodes, a remarkably uniform oxide layer is formed, typically 1-2 nm thick. The oxide film may then be dosed with the compound of interest this is achieved in one of three ways. 

A number of other polymer deposition techniques may be used in sensor fabrication. Some of the most exciting are techniques adopted from the semiconductor industry. For example, uncross-linked hydrogel precursors can be cast onto the surface of a substrate and UV cross-linked through a shadow mask. This technique allows patterning of the hydrogel to make complex sensor anays on a single sensor substrate, for example, an integrated O2, CO2, and pH sensor array [23]. 

In shadow masking, a stencil intercepts unwanted material and allows the desired material to reach the substrate. The shadow mask can be a freestanding or tensioned sheet like an etched metal foil, or a patterned layer on the substrate which can be easily removed. The technique is relatively general - evaporation processes, some solution processes, and some sputtering processes can be patterned in this way. The features which can be formed... 

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