The semiconductor industry continues to produce ever smaller devices that are ever more complex in shape and contain ever more types of materials. The ultimate sizes and functionality of these new devices will be affected by fundamental and engineering limits such as heat dissipation, carrier mobility and fault tolerance thresholds. At present, it is unclear which are the best measurement methods needed to evaluate the nanometre-scale features of such devices and how the fundamental limits will affect the required metrology. Here, we review state-of-the-art dimensional metrology methods for integrated circuits, considering the advantages, limitations and potential improvements of the various approaches. We describe how integrated circuit device design and industry requirements will affect lithography options and consequently metrology requirements. We also discuss potentially powerful emerging technologies and highlight measurement problems that at present have no obvious solution.
In the past few years, novel methods of patterning have made considerable progress. In 2011, extreme ultraviolet (EUV) lithography was the front runner to succeed optical lithography. However, although EUV tools for pilot production capability have been installed, its high volume manufacturing (HVM) readiness continues to be gated by productivity and availability improvements taking longer than expected. In the same time frame, alternative and/or complementary technologies to EUV have made progress. Directed self-assembly (DSA) has demonstrated improved defectivity and progress in integration with design and pattern process flows. Nanoimprint improved performance considerably and is pilot production capable for memory products. Maskless lithography has made progress in tool development and could have an α tool ready in the late 2015 or early 2016. But they all have to compete with multiple patterning. Quadruple patterning is already demonstrated and can pattern lines and spaces down to close to 10-nm half pitch. The other techniques have to do something better than quadruple patterning does to be chosen for implementation. DSA and NIL promise a lower cost. EUV promises a simpler and shorter process and the creation of 2-D patterns more easily with much reduced complexity compared to multiple patterning. Maskless lithography promises to make chip personalization easy and to be particularly cost effective for low-volume chip designs. Decision dates for all of the technologies are this year or next year.
We have studied the photolysis of tin clusters of the type [(RSn) 12 O 14 (OH) 6 ] X 2 using extreme ultraviolet (EUV, 13.5 nm) light, and developed these clusters into novel high-resolution photoresists. A thin film of [(BuSn) 12 O 14 (OH) 6 ][p-toluenesulfonate] 2 (1) was prepared by spin coating a solution of (1) in 2-butanone onto a silicon wafer. Exposure to EUV light caused the compound (1) to be converted into a substance that was markedly less soluble in aqueous isopropanol. To optimize the EUV lithographic performance of resists using tin-oxo clusters, and to gain insight into the mechanism of their photochemical reactions, we prepared several compounds based on [(RSn) 12 O 14 (OH) 6 ] X 2. The sensitivity of tin-oxide films to EUV light were studied as a function of variations in the structure of the counter-anions (X, primarily carboxylates) and organic ligands bound to tin (R). Correlations were sought between the EUV sensitivity of these complexes vs. the strength of the carbon-carboxylate bonds in the counteranions and vs. the strength of the carbon-tin bonds. No correlation was observed between the strength of the carboncarboxylate bonds in the counter-anions (X) and the EUV photosensitivity. However, the EUV sensitivity of the tinoxide films appears to be well-correlated with the strength of the carbon-tin bonds. We hypothesize this correlation indicates a mechanism of carbon-tin bond homolysis during exposure. Using these tin clusters, 18-nm lines were printed showcasing the high resolution capabilities of these materials as photoresists for EUV lithography.
Pure thin films of organotin compounds have been lithographically evaluated using extreme ultraviolet lithography (EUVL, 13.5 nm). Twenty compounds of the type R 2 SnðO 2 CR 0 Þ 2 were spin-coated from solutions in toluene, exposed to EUV light, and developed in organic solvents. Exposures produced negative-tone contrast curves and dense-line patterns using interference lithography. Contrast-curve studies indicated that the photosensitivity is linearly related to the molecular weight of the carboxylate group bound to tin. Additionally, photosensitivity was found to be linearly related to free radical stability of the hydrocarbon group bound directly to tin (R ¼ phenyl, butyl, and benzyl). Dense-line patterning capabilities varied, but two resists in particular show exceptionally good line edge roughness (LER). A resist composed of an amorphous film of ðC 6 H 5 CH 2 Þ 2 SnðO 2 CCðCH 3 Þ 3 Þ 2 (1) achieved 1.4 nm LER at 22-nm half-pitch patterning and a resist composed of ðC 6 H 5 CH 2 Þ 2 SnðO 2 CC 6 H 5 Þ 2 (2) achieved 1.1 nm LER at 35-nm half-pitch at high exposure doses (600 mJ∕cm 2 ). Two photoresists that use olefin-based carboxylates, ðC 6 H 5 CH 2 Þ 2 SnðO 2 CCH═CH 2 Þ 2 (3) and ðC 6 H 5 CH 2 Þ 2 SnðO 2 CCðCH 3 Þ═CH 2 Þ 2 (4), demonstrated better photospeeds (5 mJ∕cm 2 and 27 mJ∕cm 2 ) but worse LER.
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