New Era in Thermal Scanning Probe Lithography

Tevis Jacobs and his collaborators from IBM and SwissLitho were achieved sub-10 nanometer feature size in Silicon using thermal scanning probe lithography.

 Fabrication of today’s nanoscale devices requires mask-less techniques  for prototyping of ultimately scaled devices and to fabricate the masters and mask for nanoimprint and for high-volume lithography. Today, there are several mask-less techniques which demonstrate sub-10 nm resolution capabilities in patterning dense lines with tight pitch. Scanning probe lithography (SPL) is also one of these techniques with minimal substrate interference and proximity effects. One advantage of SPL is its ability to operate in ambient conditions and to allow in situ nondestructive inspection. In addition to SPL’s advantageous over other techniques, thermal SPL (t-SPL) has demonstrated an overlay accuracy of better than 5 nm and the capability to fabricate 3D depth profiles with nanometer scale accuracy. The resolution in t-SPL is highest for shallow patterns which pose a challenge for transferring the patterns into functional structures. In one of their recent works they had developed and optimize a versatile three-layer stack that is able to transfer sub-20 nm lateral and 5 nm vertical amplitude t-SPL patterns into a substrate. At such dimensions, little is known about the mechanisms controlling pattern formation. In this work, they  the t-SPL parameters that influence high-resolution patterning on the transfer stack and demonstrate that sub-15 nm half-pitch resolution patterning and transfer by t-SPL are feasible. They have looked at parameters such as the height “h” of the rim and the line depth “d” which are measured from the surface level

First they investigated  the plasma etch behavior of the polyphthalaldehyde (PPA) and poly(methyl methacrylate) PMMA polymer layers on the pattern transfer stack. They showed that the etch process is controlled with a precision better than 0.5 nm. Furthermore, a reliable transfer process is achieved using 6 s etch time if the written patterns fulfill the two criteria h > 0.5 nm and d > 3 nm.

Then they investigate the pattern formation as a function of the applied force and temperature. They varied temperature between 525 to 800 °C with increments of 25 °C. They used forces from 0 to 30Nn. For cold patterning, a threshold force is required to initiate the patterning. They found that for cold patterning h become positive and d decreases. They showed that h is negative and d is highest for hot patterning. Necessity of a threshold force to initiate the patterning at cold temperature was observed. They found that high forces are required to reach significant patterning depths. The optimal force and temperature values for the best patterning were found to be 25 ± 6 nN and 550−700 °C, respectively. They have showed that their results are independent of the tip which is used for patterning.

They also studied the geometry of the patterned lines as a function of the half-pitch of the dense line patterns for optimized patterning conditions. For this, they patterned several arrays of dense lines with the thicknesses of 14, 17, 21, and 30 nm,  with the same tip at 600 °C and 24 nN. They showed that for 14 nm half-pitch, a pattern transfer is still possible, but the 10 nm half-pitch pattern is too shallow for a successful transfer.

Their experiments reveal that, for highest resolution patterning of substrates by t-SPL, the details of the etch transfer process including the thickness of the polymeric layers on the SiO2 hard mask and the line shape of the patterned lines have to be considered jointly to achieve a successful sub-15 nm half-pitch pattern transfer. Moreover, their analysis of the line shape provides insight into the physical processes that govern the line formation at these highest resolutions and thereby limit the resolution of t-SPL patterning on the patterning stack.

In short, they found that the resolution in t-SPL is limited by the extent of the plastic zone in thermo-mechanical indentation on the pattern transfer stack because, at temperatures approaching the resist’s decomposition temperature, the line shape widens, reducing the achievable resolution. They showed that the properties of their pattern transfer stack and etching process determine the required minimum film thickness of the topmost imaging layer in a successful transfer. They achieved reliable transfer of patterned dense lines down to 14 nm half-pitch and in the best case 11 nm half-pitch. Furthermore, there is evidence that an enhanced resolution below 10 nm half-pitch might be possible on a mechanically different transfer stack. It can be summarized that their patterning process and the transfer characteristics  favor wider trenches and narrower walls.

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