KEYWORDS: Lithography, Logic, Optical lithography, Etching, Metals, Control systems, Computer simulations, Photomasks, Extreme ultraviolet, Semiconducting wafers, TCAD, Back end of line, Front end of line
As the industry marches on onto the 5nm node and beyond, scaling has slowed down, with all major IDMs & foundries predicting a 3-4 year cadence for scaling. A major reason for this slowdown is not the technical challenge of making features smaller, but effective control of variation that creeps in to the fabrication process. That variability manifests itself as edge placement error (EPE), which has a direct impact on wafer yield. Simply defined as the variance between design intent vs. actual on-wafer results, EPE is one of the foremost challenges being faced by the industry at the advanced node for both logic and memory. This is especially critical at three stages: the front end of line (FEOL) STI patterning; middle of line (MOL) contact patterning; and back end of line (BEOL) trench patterning where the desired tight pitch demands EPE control beyond the capability of 193i multi-patterning or even EUV single pattern. In order to mitigate this EPE challenge, we are proposing self-alignment of blocks & cuts through a multi-color materials integration concept. This approach, termed as “Self-aligned block or Cut (SAB or SACut)”, simply trades off the un-manageable overlay requirement into a more manageable etch selectivity challenge, by having multiple materials filled in every other trench or line.
In this paper we will introduce self-alignment based block and cut strategies using multi-color materials integration and show implementation for BEOL trench block patterning. We will present a breakdown of the key unit process challenges that were needed to be resolved for enabling the self-alignment such as: (a) material selection of multi-color approach; (b) planarization of spin on materials; (c) void-free gap fill for high aspect ratio features; and last but not the least, (c) etch selectivity of etching one material with respect to all other materials exposed. Further, we will present a comparison of our new self-alignment approach with standard approaches where we will articulate the advantages in terms of EPE relaxation and mask number reduction. We will conclude our talk with a brief snapshot of the future direction of our EPE improvement strategies and our view on the future of patterning beyond 5nm node for the industry.
The lithographic processes and resolution enhancement techniques (RET) needed to achieve pattern fidelity are
becoming more complicated as the required critical dimensions (CDs) shrink. For technology nodes with smaller
devices and tolerances, more complex models and proximity corrections are needed and these significantly increase
the computational requirements. New simulation techniques are required to address these computational challenges.
The new simulation technique we focus on in this work is dense optical proximity correction (OPC). Sparse OPC
tools typically require a laborious, manual and time consuming OPC optimization approach. In contrast, dense OPC
uses pixel-based simulation that does not need as much manual setup. Dense OPC was introduced because sparse
simulation methodology causes run times to explode as the pattern density increases, since the number of simulation
sites in a given optical radius increases.
In this work, we completed a comparison of the OPC modeling performance and run time for the dense and the
sparse solutions. The analysis found the computational run time to be highly design dependant. The result should
lead to the improvement of the quality and performance of the OPC solution and shed light on the pros and cons of
using dense versus sparse solution. This will help OPC engineers to decide which solution to apply to their
Current state-of-the-art OPC (optical proximity correction) for 2-dimensional features consists of optimized
fragmentation followed by site simulation and subsequent iterations to adjust fragment locations and
minimize edge placement error (EPE). Internal and external constraints have historically been available in
production quality code to limit the movement of certain fragments, and this provides additional control for
OPC. Values for these constraints are left to engineering judgment, and can be based on lithography
process limitations, mask house process limitations, or mask house inspection limitations. Often times
mask house inspection limitations are used to define these constraints. However, these inspection
restrictions are generally more complex than the 2 degrees of freedom provided in existing standard OPC
software. Ideally, the most accurate and robust OPC software would match the movement constraints to
the defect inspection requirements, as this prevents over-constraining the OPC solution.
This work demonstrates significantly improved 2-D OPC correction results based on matching movement
constraints to inspection limitations. Improvements are demonstrated on a created array of 2D designs as
well as critical level chip designs used in 45nm technology. Enhancements to OPC efficacy are proven for
several types of features. Improvements in overall EPE (edge placement error) are demonstrated for
several different types of structures, including mushroom type landing pads, iso crosses, and H-bar
structures. Reductions in corner rounding are evident for several 2-dimensional structures, and are shown
with dense print image simulations. Dense arrays (SRAM) processed with the new constraints receive
better overall corrections and convergence. Furthermore, OPC and ORC (optical rules checking)
simulations on full chip test sites with the advanced constraints have resulted in tighter EPE distributions,
and overall improved printing to target.
Step and Flash Imprint Lithography (SFIL) is a revolutionary next generation lithography option that has become increasingly attractive in recent years. Elimination of the costly optics of current step and scan imaging tools makes SFIL a serious candidate for large-scale commercial patterning of critical dimensions below ~50 nm. This work focuses on the kinetics of the UV curing of the liquid etch barrier and the resulting densification/contraction of the etch barrier as it solidifies during this step. Previous experimental work in our group has measured the bulk densification of several etch barrier formulations, typically about 9 % (v/v). It remains unknown, however, how much etch barrier contraction occurs during the formation of nano-scale features. Furthermore, it is of interest to examine how changes in monomer pendant group size impact imprinted feature profiles.
This work provides answers to these questions through a combination of modeling and experimental efforts. Densification due to the photopolymerization reaction and the resulting shift from Van der Waals’ to covalent interactions is modeled using Monte-Carlo techniques. The model allows for determination of extent of reaction, degree of polymerization, and local density changes as a function of the etch barrier formulation and the interaction energies between molecules (including the quartz template). Experimental efforts focus on a new technique to examine trench profiles in the quartz template using TEM characterization. Additionally, SEM images of imprinted images from various etch barrier formulations were examined to determine local contraction of the etch barrier. Over a large range of etch barrier formulations, which range from 10 - 20 % volumetric contraction as bulk materials, it was found that dense 100 nm lines printed approximately the same size and shape.
Resist technologies that will enable next-generation lithography (NGL) such as extreme ultraviolet lithography (EUV) will require tighter control of critical dimension (CD) with appropriate reduction of line edge roughness (LER) of resist features to levels that seem unrealizable today. Given the delicate balance existing between LER, resolution and sensitivity that is associated with photoresist patterning, alternative processing methodologies that can address such parameters individually are required. In this work a post-processing method designed to control LER is proposed based on the ability of an additive-containing rinse to condition the surface of photoresist patterns. Organic salts added to the final rinse used to quench the development process are found to be particularly effective towards this end. LER reduction up to 15% was observed for a broad range of 193 nm resist systems, while preserving the integrity of the pattern profiles. The dependence of LER reduction on additive concentration was investigated and the limited improvement observed was explained based on the tendency of the additive to self-aggregate. Finally, the advantage of including an additive in the rinse step instead of using an additive-containing developer is discussed in terms of critical dimension bias and overall image integrity control.
Satellite spot defects are a class of defects widely observed in photoresist processing in 248 nm and 193 nm lithography. These defects become more and more significant as the feature sizes shrink and can potentially become “killer” defects, leading to bridging between lines and/or blocking vias. Traditional potential solutions (i.e., optimization of development rinse step) have yielded improvements in the past but did not eliminate the problem. The use of water-soluble topcoat layers was shown to eliminate these defects but it imposes limitations on throughput and cost and it is incompatible with 157 nm lithography and 193 nm immersion schemes. In this work, we report the use of aqueous surfactant solutions for the suppression of defects in 248 nm and 193 nm lithography, with emphasis on satellite spot defects. Suppression of total defects by up to ~99% and practically complete elimination of satellite spot defects were achieved by use of aqueous surfactant solutions for various resists. A handful of materials that can be incorporated into rinse solution for the successful elimination of blob defects in a variety of resists were identified. It was determined that the two most important factors that enable successful defect elimination are the surfactant concentration and the extent of surfactant adsorption to specific resist systems.
The use of in situ spectroscopic ellipsometry (SE) is demonstrated as a technique for studying photoresist dissolution. Experiments carried out using a J.A.Woollam M-2000 ellipsometer and a custom built cell designed for in situ film measurements show that bulk dissolution rate measurements using the SE technique agree with dissolution rate data obtained using multiwavelength interferometry. SE is also demonstrated as a method for measuring thin film dissolution rates, water sorption, and films that swell. An additional focus of this work was the topic of interfacial “gel” layer formation during photoresist dissolution. Ellipsometry and interferometry were used to test several photoresist resins, with an emphasis on phenolic polymers. Single and multiple layer models were used to analyze the data, and were compared to model calculations predicting formation of a gel layer. For the materials studied, interfacial gel layer formation in low molecular weight phenolic polymers was not detected, within the resolution of the experimental techniques (< 15 nm).