Subtractive patterning operations

In order to form OFETs, it is necessary to pattern at least the source and drain layer of the device. To form circuits, typically four layers need to be patterned. Printing techniques pattern additively, i.e. material is only deposited where it is wanted. When a material is deposited using a large area deposition technique, the unwanted material needs to be defined and removed in a subtractive process. 

Polymer semiconductor materials which are soluble cannot tolerate exposure to solvents, which can cause them to swell and/or dissolve. Many oligomeric organic semiconductors are also not tolerant to solvent exposure, which complicates photolithography. Pentacene, for example, is not soluble to any significant degree in any solvents. When pentacene OFETs are exposed to most solvents, however, their semiconductor character is destroyed. 

The solution to photolithographically patterning the active layer of OFETs is to protect the semiconductor from materials it is not compatible with (i.e. organic solvents and water). One approach is to use water soluble resists which do not induce the phase transformation [57]. Another is to encapsulate the transistors using parylene first, and then proceed with normal photolithography and patterning [58]. A third approach is to use fluorinated or supercritical C02 based resist materials which also do not interact with pentacene or other organic semiconductors [59]. 

Another approach to patterning is to sensitize the target material to directly be photosensitive and wash off the remaining material. A number of custom compounded and commercially available materials have been applied as pho-topatternable gate dielectrics. A similar strategy has also been demonstrated for a soluble pentacene precursor, which converts to an insoluble and well ordered form after treatment [60], and conductor traces in an integrated circuit device [61]. 

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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In addition to the measured values and the analytical values (e.g. content, concentration), latent variables are included in the scheme. Latent variables can be obtained from measured values or from analytical values by means of mathematical operations (e.g. addition, subtraction, eigenanal-ysis). By means of latent variables and their typical pattern (represented in chemometric displays) special information can be obtained, e.g. on quality, genuineness, authenticity, homogeneity, origin of products, and health of patients. 

Due to the above mentioned issues, the values collected from the network interface cannot be directly stored in the FM as a pattern. Instead, a new pattern is generated and stored, as depicted in Fig. 13.2. To each RSS value obtained at a given point it is subtracted a reference value and then the result of this operation is... 

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