The single-exposure patterning capability of EUVL
At the 2021 SPIE Advanced Lithography Conference, imec and ASML presented several papers that demonstrate the ultimate single-exposure patterning capability of today’s 0.33NA NXE:3400 extreme ultraviolet lithography (EUVL).
Process optimisations have enabled the patterning of dense 28nm pitch line/spaces with an Inpria metal-oxide resist in one single exposure, relevant for high-volume manufacturing. For the first time, optical and e-beam inspections were correlated with electrical data to gain further insights in improving stochastic defectivity – i.e., both breaks and bridges.
In addition, source optimisations have led to printing the smallest pitch possible with the current NXE:3400 scanner (i.e., 24nm pitch line/spaces and 28nm pitch contact holes), allowing for early material development required for high-NA EUV lithography scanners.
Extreme ultraviolet lithography has reached a critical decision point where one can move to EUV multi-patterning for printing the densest features of the next generation ICs or push further the single print capability on today’s 0.33NA full field scanners.
“While multi-patterning techniques would offer more relaxed pitches, single patterning enables a tremendous cost advantage and simpler process schemes,” said Kurt Ronse, Advanced Patterning Program Director at imec. “Imec and ASML have demonstrated 28nm pitch single-exposure patterning readiness for line/spaces, which corresponds to critical back-end-of-line metal layers of a 5nm technology node. This brings the NXE:3400 scanner very close to its resolution limit for high-volume manufacturing.” The results were obtained using Inpria’s metal-oxide (MOx) resist process.
To increase learnings on stochastic patterning failures, defectivity inspection data obtained with scanning electron microscopy, broadband plasma and e-beam technologies were successfully correlated with data obtained from electrical measurements. The electrical tests were carried out on large-area ruthenium-metallised serpentine structures that allowed to measure electrical opens (and hence bridges in the resist), as well as on metallised fork-fork and tip-to-tip structures that allowed to measure electrical shorts (and hence critical breaks in the resist).
Besides showing a good correlation, the complementary electrical measurements allow to capture important trends across multiple process changes that can help mitigating stochastic printing failures (papers n° 11609-26; 11611-21).
The extendibility of 0.33NA EUV lithography to pitch 28nm resulted from co-optimising the various contributors to the patterning process, including mask templates, illumination settings, metal-oxide resist and etch processes.
For example, the benefits of using bright field mask tonality and controlled lens aberrations were shown to largely improve the printability at small pitch and critical dimensions (papers n° 11609-27; 11609-29).
Besides pushing the boundaries of single-exposure EUVL for high-volume manufacturing, imec and ASML have brought the 0.33NA NXE:3400 to its extreme resolution with the aim to use it as a platform for early material development for the high-NA EUVL tools.
Steven Scheer, VP advanced patterning process and materials at imec, explaind: “Imec and ASML recently also showed the tool’s capability of printing 24nm pitch line/spaces and 28nm pitch contact holes – the latter by optimising pupil and imaging conditions and by using double line/space exposures at a combined dose of 45mJ/cm2.”
“Pattern transfer could be successfully demonstrated on very thin resists that are relevant for high-NA EUV,” added Andrew Grenville, CEO of Inpria. “This will provide the imec patterning ecosystem with the opportunity to develop resist, metrology and etch processes to accelerate the introduction of the next-generation EUVL system, i.e., the high-NA EXE:5000.”
Scheer continued: “These developments will complement the learnings that are currently being obtained from imec’s attosecond analytical and interference lithography lab (the AttoLab), expected to offer a high-NA resist imaging capability to print features down to pitches as small as 8nm.”