Photolithography limitations

Magnetic nanostructures possess promising features due to their characteristic sizes that are comparable to the spin-flip diffusion length and magnetic domain-wall width. In practice, their fabrication and characterization become increasingly difficult with feature sizes decreasing beyond the standard photolithography limit [1,2]. 

The minimum size of the monochrome pixels (we consider color in Section 13.7.3) that can be fabricated using OLEDs is dictated primarily by the ability to pattern the electrode which is deposited on top. OLEDs are not sufficiently robust to withstand the normal processes of photolithography. Among the schemes which have been suggested for high resolution patterning is one in which the substrate is pne-pattemed to provide its own shadow mask [1911. By this means, pixel sizes down to 300 p have been demonstrated, and a lower limit of about 100 p is estimated. 

Photolithography Highest Highest as expensive equipments and particular expertise are required Limited to 25-mers or less Photolithography method is protected by patent and currently only Affymetrix has the rights to use this method... 

The sputtering technique has been used in the preparation of SOFC electrodes, and is generally combined with photolithography in the production of thin-lilm patterned electrodes that are mainly used in fundamental reactivity and mechanistic studies [120], Although it is a versatile technique that allows for excellent control of composition and morphology, and relatively low temperatures that help to prevent unwanted reactivity observed at higher temperatures, its major limitations lie in the equipment costs and in the slow deposition rates ( 5 pm/h) [120],... 

After consideration of all factors which limit resolution such as exposure hardware, resist systems, registration, alignment, and linewidth control, there is a general consensus that the useful resolution limit of photolithography may lie somewhere between 0.4 and 0.8 /im and depends on such factors as the implementation of short wavelength UV and the ability to accurately place images from a projection tool onto a silicon wafer. 

Particle size control can also be afforded by constraining the colloid to vesicles , membranes or zeolites An intriguing recent report achieves size limitation of semiconductor electrodes by preparing them by photolithography techniques... 

Limitations. The number of transistors present on a chip lias doubled approximately every 18 months since the integrated circuit was first developed. The main reason for this continuing decrease in the minimum feature sizes of transistors (and consequent increase in density of transistors on the chip) has been the development of photolithography. The most important limitation for further size reduction remains the development of new photo- and utlier lilliugraphic techniques. 

An elastomeric stamp or mold with relief structures on its surface is the key to soft lithographic techniques (see Sect. 3.2). As a result, the utility of these techniques is often limited by the availability of appropriate masters. In general, the masters are fabricated using photolithography. Chrome masks are available commercially from custom fabricators,but the time required for vendors to produce a chrome mask from a design presented in a computer aided design (CAD) file can be weeks, and they are expensive ( 300 per square inch for features larger than 20 pm, and 500 - 1000 per square inch for features between 1 and... 

Microfabrication is growing in importance in a wide range of areas outside of microelectronics, including MEMS, microreactors, micro analytical systems and optical devices. Photolithography will continue as the dominant technology in the area of microelectronics for the foreseeable future. Photolithography has, however, a number of limitations for certain types of applications, as discussed in Sect. 3.1. 

The resolution of the photolithographic process determines the maximum achievable density of the array (i. e. the amount of sequence information encoded on the chip). Table 2 shows the relationship between the resolution, in terms of smallest feature size, and the maximum density at which an array can be printed . Application of the photolithographic process using photolabile protecting groups currently provides a spatial resolution that allows arrays to be fabricated with densities on the order of 106 sequences/cm2, which corresponds to an individual feature size of 10 X 10 m. This feature size is near the limit of resolution that can be achieved by this method using standard photolithography equipment. 

FIGURE 2.14 Electron micrograph of a turn in the channel fabricated in a PDMS chip, created by casting the polymer against a positive relief, which is made of photoresist patterned on a glass substrate. The roughness in the side wall arises from the limited resolution of the transparency used as a photomask in photolithography [1033]. Reprinted with permission from the American Chemical Society. 

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