Currently, preproduction lithography tools can routinely print dense features at 22 nm and current investigations focus on printing features less than 16 nm. Despite successes in decreasing feature size, issues with pattern fidelity have arisen, which are described as roughness in the patterns, known as line edge roughness (LER). LER can be defined as three standard deviations (3σ) of the line edge from an imaginary straight line. The 3σ LER is a primary limiting factor for manufacturing sub-50 nm features due to the local variations. Effective reduction of LER is, therefore, an important factor for making better performing devices.
The primary aim of this thesis is to develop materials and methodologies to reduce the roughness along lithographically printed lines, as well as to understand the relationship between the structure of the polymers and their capacity to heal roughness. In particular, rationally designed block copolymers will be synthesized and tested for their effectiveness in
healing LER, where electrostatic attraction will be used to direct polymer deposition onto lithographic features and an annealing step will be used to minimise the free energy at the polymer-air interface and result in a decrease of surface roughness. The testing regimes will look at assessing interactions with surfaces and then determining the effectiveness of healing for model rough surfaces and also lithographically printed features.
Three generations of block copolymers for healing LER will be described in this thesis. Chapter 2 describes an amphiphilic block copolymer, Poly(2-(N,Ndimethylamino) ethyl methacrylate)-block-poly(tert-butylmethacrylate) (PDMAEMA-b-PtBuMA), which self assembles into polymersomes in aqueous solution at pH 6.3 and was able to attach to model surfaces and lithographic patterns. Upon annealing the LER was reduced by as much as 58%. However, the polymersomes had diameters of 18 nm in solution and were not expected to be compatible with the small feature sizes.
Chapter 3 describes the effect of molecular weight and relative block sizes of PDMAEMA-b-PtBuMA on the size of the self-assembled particles, with a specific aim of generating smaller particle sizes that could be used to heal small feature sizes. The hydrodynamic diameter and zeta potential of a series of BCPs have been assessed, where it was found the size of the self assembled particles did not follow the expected sizes based on the BCP molecular weight and relative block lengths. The assessment of the size of the particles deposited on a negatively-charged film were conducted using AFM, where the particle size was 50 nm or greater for the entire series. An assessment was made that these particles would not be useful for healing sub-50 nm lithographic features. Therefore, the issue of healing sub-50 nm features is further addressed in the next chapter using double hydrophilic block copolymers.
Chapter 4 describes the second generation of block copolymers for healing LER, which have a double hydrophilic architecture. The primary difference in the design of the double hydrophilic block copolymers (DHBCs) was that the hydrophobic PtBuMA block was replaced with a copolymer of glycerol methacrylate and diethyleneglycol methacrylate. PDMAEMA was kept as the charged block. A series of DHBCs with different molecular weights were prepared and their structure and solution properties were assessed. The deposition of DHBCs on negatively-charged substrates was supported by ellipsometry, X-ray photoelectron spectroscopy and AFM. After confirming the adhesion of the DHBCs on planar flat surfaces, they were assessed for their capacity to heal model rough surfaces. The most effective healing occurs when the weight fraction of the non-charged to charged block is close to 0.5. The same methodology is applied to electron beam patterns and extreme ultraviolet lithography (EUVL) patterns. It was observed that these polymers were able to attach to printed lines and reduce the 3σ LER by as much as 26%.
It was hypothesised that increasing chain flexibility might give a higher degree of healing. Therefore, hyperbranched double hydrophilic block copolymers were investigated in Chapter 5. The synthesis of hyperbranched PDMAEMA using propane-1,3-diyl diacrylate as a branching agent shows good control over branching for the range of experimental conditions that were used. Hyperbranched PDMAEMA was chain extended with solketal methacrylate (SMA). This was followed by RAFT end group modification and hydrolysis to give a series of DHBCs, hyperbranched PDMAEMA-b-Pglycerol MA.
The attachment of hyperbranched polymer onto negatively-charged flat surfaces was supported by ellipsometry and AFM. The polymers were also found to be able to reduce the nanoscale roughness on model rough surfaces. The same methodology was also applied to electron beam lithography and EUVL patterns. It is observed these hyperbranched polymers were able to reduce 3σ LER by up to 28%.
In summary, factors that affect the degree of healing were analysed and it was concluded that key elements in the structure of the block copolymers were important and will form the basis of design rules for future generations of block copolymers.