Double-exposure techniques

Double-exposure techniques (see Fig. 17.14) comprise a sequence of two separate exposures of the same resist layer, with the same mask or two different masks. 

Double exposure is commonly used to pattern features in the same layer that are different or have incompatible densities or pitches. A good example might he the simation where two exposures are made such that one exposure is used to define features oriented in one direction, while the other exposure is used to define the other set of features oriented in another direction that is perpendicular to the first direction. In this way, two-dimensional patterns can be decomposed into two one-dimensional patterns, which are considerably easier to print. In other words, the double-exposure technique enables the patterning of minimum pitch feamres 

Step 3. Second masking exposure with mask position translated relative to the first exposure 

Step 4. Develop the final double-density resist image 

Relative to the other double-patterning approaches, the double exposure is the simplest to implement since it does not require additional follow-up process steps. Its main challenge is meeting alignment tolerance requirements. 

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Figure 9. (top) Forgery of Sasanian silver gilt plate, private collection. Figure 10. (Tjottomj X-ray radiograph of forgery shown in Figure 9. Decorations appear as darker areas indicating hollow area between two surfaces. This plate is manufactured by the double shell technique, a method not used by Sasanian silversmiths. Exposure 200 kV, 5 mA, lead screens.

Other methods of reducing the crosslink density have also led to sensitivity improvements, of which the most promising is the uneven copolymer mixtures with a potential 60-70% reduction in operating dose. However, the solutions must be aged after mixing in order to obtain reproducible performance. This technique also requires work on a developer system. Flood exposure yields a 50% improvement in sensitivity but creates additional processing difficulties due to double exposure effects. 

To overcome the above limitations, we employ double-exposure holographic interferometry [110]. Holographic techniques have the crucial advantage of storing the full field image of the sample before and after irradiation, thereby permitting spatially resolved characterization of the induced effects over the full sample. The technique is applied to the study of polymeric systems, either in the form of plates or mainly in the form of films cast on transparent substrates. 

Figure 17.18 Cross-sectional SEM image of 32-nm half-pitch lines patterned with LFLE double-patterning technique, (a) Line profiles of the two resist layers following the second lithographic exposure, (b) Line profiles after the final pattern transfer to underlying substrate. (Courtesy of S. Holmes.)...

It can be concluded that it is very difficult to predict the result from a polymer macrostructure, but it is relatively easy to measure the secondary species generated on irradiation by using known analytical techniques, such as measuring swelling, tensile tests, analysis using nuclear magnetic resonance (NMR), etc. The yield is then expressed by the G value, which represents the number of cross-links, scissions, double bonds, etc., produced for every 100 eV (1.6 X 10 J) dissipated in the material. For example, G (cross-links), abbreviated G(X), = 3.5 means that 3.5 cross-links are formed in the polymer per 100 eV under certain irradiation conditions. Similarly, the number of scissions formed is denoted by G(S). In order to determine the number of crosslinks or G(X), the number of scissions or G(S), etc., it is necessary to know the dose or dose rate and the time of exposure for these irradiation conditions. From the product yields it is possible to estimate what ratio of monomer units in a polymer is affected by irradiation. ... 

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