ADVANCEMENT IN RESIST MATERIALS FOR SUB-7 NM PATTERNING
AND BEYOND
Li Li1*, Xuan Liu1, and Shyam Pal1
1Advanced Technology Development, GlobalFoundries, Malta, New York, 12020, USA *Corresponding Author’s Email: li.li1@globalfoundries
ABSTRACT
The rapid development in dense integrated circuits requires significant advancement in small scaling patterning technology. EUV technology is considered as a powerful solution for the sub-7 nm node pattering and beyond. The high performance resist development is required for the practical applications of the EUV patterning for high volume manufacturing. In the current work, the requirements for the development of next generation resist materials is reviewed and summarized to propose the design criterion for high performance photoresist materials.
INTRODUCTION
Advanced lithography techniques are becoming more and more critical for the fabrication of modern microelectronics and energy devices,and high performance resist materials are very significant for transitioning those devices from the research lab to the industrial manufacturing.[1-2]With consideration of the high volume manufacturing (HVM) of wafer-based microelectronics devices, patterning technology with extremely small lithography dimensions and improved cost is required for fast integrated devices manufacturing.[3]Extreme ultraviolet (EUV) lithography is considered as one of the most promising technologies to achieve fabrication of devices smaller than 10 nm nodes.[4] Compared with traditional lithography technologies, EUV lithography has stringent material requirements including high etch resistance, high sensitivity and resolution, low line edge roughness (LER) or line width roughness (LWR), appropriate UV absorption, and small molecular size, among others.
It is reported that the EUV technology has begun to be introduced in trial for the processing in some critical layers in 7 nm technology and replaced the traditional ArF immersion technology combined with multi-patterning. [5] For sub-7 nm technology which is close to the atomic level patterning, the use of EUV can greatly decrease the processing complexity if its throughput and processing availability can meet the manufacturing requirements. Performance requirements for EUV resists will require the devel
opment of entirely new resist platforms, which including the development of resist materials, auxiliary materials and the optimization of materials related processing. Herein, we will provide a brief review about the development for the resist materials required for sub-7 patterning and beyond.
RESIST REQUIREMENTS
Absorbance
A challenge in designing new photoresists for EUV wavelengths is the selection of molecular structures that have minimal absorbance and superior characteristics in imaging and etch performance. As we know, at EUV wavelengths, the absorption of all materials is very strong and only dependent on their atomic composition. Figure 1 shows the photo absorption cross-section of different elements under EUV. Note that elements commonly used in photoresists at other wavelengths, such as fluorine, are highly absorbing at ~13 nm, rendering them problematic for EUV applications. Other elements including carbon, silicon, zirconium and hafnium have very high transmission, allowing EUV photons to pass through the entire resist film. Ober and Giannelis in Cornell University are now focused on the development of Hf/Zr based hybrid inorganic/organic resist materials and have made an excellent progress for EUV patterning.[6]
Figure 1. EUV absorption of different elements.  Solubility
xposedFor the high performance resist development, solvent selection is also very crucial considering its processing stability and cost used in the industrial fab. A systematic approach is required for the designing and optimizing the resists. Negative tone resists where the solubility characteristics of the ex
posed and unexposed areas are fairly similar, leading to the challenging selection of an appropriate developer. The Hildebrand solubility
parameter is an excellent tool for predicting the solubility of the resist materials in various solvents and can be used to evaluate if the resist material is eligible for use. It is defined as
δ = (E/V m)1/2                                                      (1) where E and V m are the cohesive energy and molar volume, respectively. The Hildebrand solubility parameter can be divided into three components for contribution and they represent the dispersive, polar and hydrogen bonding interactions. These components are the Hansen solubility parameters and are related to the Hildebrand parameter by the equation:
δt2= δd2+ δp2+ δh2                                                                        (2) Using the Hansen three solubility parameters the solubility behavior of any material can be systematically determined. In this approach the solubility behavior of any material is plotted in a three dimensional graph with each Hansen parameter represented in each of the three axes. The interaction radius R of the spherical volume of solubility, can be used to judge if the solvent is a good developer for the specific resist material or not (shown in Figure 2). The use of Hansen solubility parameters can be extended to investigate how the c
hemistry of the resists affects solubility and different processing/development conditions (e.g. post apply bake, PAB, and post exposure bake, PEB) as well as various additives on solubility.
Figure    2. Hansen solubility parameter plot for    a material.[7]
Dispersion
The inorganic/organic based nanoparticle EUV resist is reported to show the particle size change after the UV exposure,[2, 8-10]and particle size typically affects solubility and dispersion of resist materials. For example, small nanoparticles will be easily dissolved and removed while larger agglomerated ones will not be dissolved. Therefore in parallel to the solubility studies, the influence of processing and development conditions including additives on the particle size should also be studied. Charge stabilization and the corresponding Debye electrostatic double layer involved have been exploited in dispersing and stabilizing nanoparticles. Simply, the electrostatic double layer acts as a shield which prevents the nanoparticles from getting close enough for the van der Waals attractive interaction to set in. When the latter commences the particles are forced to aggregate. The key of the electrostatic stabilization is to develop a repulsive interaction before the attractive interaction begins. This requirement typically translates in having an electrostatic double layer with the required thickness so that at any given point of the interaction vs. distance plot the total interaction is repulsive. For a fixed charge on the particles the electrostatic double layer is inversely proportional to the concentration of ions and to the square of the ions present. In other words as the concentration of the ions increases or
higher charge ions are present, the double layer decreases and aggregation can take place. The interaction energy as a function of distance for well dispersed particles and aggregated particles are shown in
Figure 3.
Figure 3. Interaction energy as a function of distance for well dispersed particles (top) and aggregated particles (bottom). The key is to have a repulsive interaction because of charge commencing earlier than the attractive
interaction due to van der Waals forces. From www.malvern
Defect
Defect level is also an important parameter for the resist evaluation and resist outgassing issue also should be carefully analyzed. The resist outgassing can lead to the deformation of patterning and more importantly contaminate the EUV optics. Witness sample testing has been proposed by ASML to qualify the photoresist materials for not excessively contaminating the scanner optics and other parts in the vacuum environment of the tool.[4, 11]The candidate resist is firstly irradiated on a witness substrate and a nearby resist-coated wafer with EUV radiation simultaneously before HVM use. Currently, more work needs to be focused on the detection and control of the defect level and outgassing issue of the resist materials for their use in sub-7 nm patterning.  OUTLOOK
The advancement in novel resist materials for sub-7 nm patterning materials requires the candidate materials should meet the requirements in absorbance, solubility, dispersion and outgassing from the aspect of materials design. Furthermore, the concerns in etch resistance, UV out of band, resist homogeneity, shelf-life and pattern collapse, etc., also need to consider for their use for HVM. Additional efforts should be focused on the resist materials design and optimization to balance and improve the triangle relationship between resolution, LER/LWR, and sensitivity (RLS).  ACKNOWLEDGEMENTS
Thanks for the support from GlobalFoundires for publication.
REFERENCES
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