Resolution limits, photolithography

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. 

The role of the novolac resin is not as minor as it may seem as a base-soluble binder. While the aromatic nature of the resin provides high dry etch resistance, the novolac structures and properties such as the ratio of o-cresol to m-cresol, the ratio of ortho to para backbone linkages, the molecular weight, and molecular weight distribution all affect the dissolution behavior, thermal flow resistance, and lithographic performance. The optimization of the novolac and diazonaphthoquinone properties in conjunction with the improvement of the i-line step-and-repeat exposure tools has pushed the resolution limit of photolithography to a sub-0.5-pm regime [4]. 

It has been reported that with thermal nanoimprint it is possible to achieve resolutions of less than 10 nm in pattern transfer. It has been confirmed that no resolution limit exists in nanoimprint, and resolution is controlled by the accuracy of mold fabrication. It is possible to produce fine patterns similar to photolithography without high-cost equipment in nanoimprint. PMMA is superior for thermal plasticity polymers because of its stability in division. 

Unlike photolithography, NIL is free of the resolution limitations determined by optics and resist sensitivity. Essentially,... 

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. 

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. 

Within the last decade, comparatively inexpensive and scaleable tchniques known as soft lithography have been the focus of much development. Patterning of a substrate is afforded by using a master elastomeric stamp that contains a nanostructured pattern, known as a relief on its surface. Contrary to photolithography, the resolution of the final pattern is not limited by light diffraction, but only depends on the... 

Although a mature technique, the application of photolithography is limited by its requirement for expensive facilities, high processing cost, and limited resolution. 

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