Depth Feedback¶
see also CLL Forces panel
Overview¶
The amount of resist removed by the NanoFrazor at each point can be controlled via the force applied to the cantilever [Knoll2010]. This enables the formation of 3D structures but requires careful control of the patterning process if the target geometry is to be accurately reproduced. The depth feedback exploits the in-situ inspection capability to adjust the electrostatic force applied during patterning.
Description of the procedure¶
The Layout Editor is used to define a target depth for each write pixel in the write field. Before patterning, an initial estimate is made of the electrostatic force required to achieve the specified write depth. After each write line the Depth Feedback examines the written depth and uses this information to improve the estimate of the electrostatic force which should be applied for the depth levels occurring on the next write line. The Depth Feedback increases the electrostatic force when the measured depth is lower than the target depth, and decreases it when the measured depth is higher than the target depth. When a depth level is not visible, the feedback guesses the correct force with a linear interpolation of the force between the two neighbouring visible levels.
A correct assignment of each measured depth to the corresponding target depth level is critical for the correct behaviour of the Depth Feedback. This is possible only when write and read fields are aligned.
Moreover, because of the finite size of the NanoFrazor tip, the depths measured close to the edges between two levels are influenced by tip convolution. These depths should not be considered in the computation of the depth error used by the feedback. A feedback mask (see Figure 19) is used to exclude these points when collecting information about the written and target depths.
The size of the regions that should be ignored, for a correct behaviour of the feedback, depends on the size of the tip and on the residual misalignment between the read and write fields.

Figure 19 On the left the original write field. On the right the feedback mask, in pink, overlaid to the write field. The pixels under the pink areas are not considered by the depth feedback.
[Knoll2010] | Armin W. Knoll et al., Probe-Based 3-D Nanolithography Using Self-Amplified Depolymerization Polymers, Advanced Materials, 22, 3361-3365 (2010). |