Spin-on-Carbon

Spin-on carbon (SoC) is becoming increasingly attractive as the bottom layer of multi-level stacks used to enable high aspect ratio transfer from thin resist layers during plasma etching. Fullerene based SoC materials outperform existing state of the art materials across several critical performance metrics, whilst maintaining the advantages of spin-on materials over CVD deposited carbon.

The high etch resistance of the fullerene based material allows high aspect ratio plasma etching from a very thin patterned film. Our fullerenes have extremely low levels of aliphatic carbon-hydrogen, and are low in hydrogen content even in comparison to the aromatic polymers typically used in this application. This has been shown to be critical to solve severe pattern deformation ("wiggling") of features below 20nm defined in spin-on-carbon hardmasks (or organic planarising layers), during the plasma etch step to transfer of the features to the underlying layer. In addition fullerene SoC films have high temperature stability, exceeding 450 °C and are reasonable transparent to wavelengths above 300 nm improving alignment capabilities.

Irresistible Materials' fullerene derivative based spin-on-carbon (SoC) has been developed for use as the bottom layer of a multi-level patterning stack to improve pattern transfer capability from any photoresist.

Figure 1 illustrates the successful transfer of a semi-dense 12 nm pattern (patterned by EUV exposure of a thin HSQ top resist layer) into silicon, with a 10:1 aspect ratio using room temperature ICP etching. More than half of the original thickness of the SoC remains after etching indicating that significantly higher aspect ratios can be achieved with appropriate etching equipment.

Figure 2 shows an semi-dense 11 nm feature transferred to silicon with an aspect ratio of 16:1. Electron beam lithography was used to pattern HSQ resist on the SoC and oxygen plasma was used to transfer the pattern into the carbon. The silicon etch step was undertaken using cryo-etching. As before a significant thickness of SoC remains after the etch.

Fig. 1 Semi-dense 12nm pattern transfer into silicon with EUV patterned fullerene hardmask
Fig. 2 Semi-dense 11nm pattern transfer into silicon with EBL patterned fullerene hardmask

Directed self assembly is an increasingly promising approach to pattern at sub-10 nm resolution. In Figure 3 a PDMS-PS block copolymer film spin coated on a fullerene spin-on-carbon layer is shown. After annealing features with a half pitch of 10 nm are clearly seen (no guide features have been applied).

After assembly of the block copolymer pattern an oxygen plasma was used both to remove the PS and to transfer the pattern into the spin on carbon, as shown in Figure 4.

Fig. 3 Dense 10nm features in a PDMS-PS block copolymer on the spin-on-carbon
Fig. 4 A Thin spin-on-carbon underlayer patterned from the block coplymer using an oxygen plasma.