Writing¶
see also CLL Runtime panel
Overview¶
The NanoFrazor patterns the resist (PPA) by bringing the heated tip of the thermal cantilever into contact with this resist material. This causes the resist in contact with the tip to evaporate. When not writing, the tip is separated from the surface by the so-called forward height (the writing height). A capacitive force is used to bring it into contact with the surface. The depth of the patterned feature depends on both the temperature of the tip when it contacts the surface and also the strength of the electrostatic force.
Selection of parameters¶
Writer Voltage¶
The writer voltage sets the temperature of the cantilever. The tSPL resist evaporates at less than 200 °C. However much higher heater temperatures are required to pattern the resist. The reasons for this are two fold. Firstly, the thermal resistance of the tip and the tip-sample interface result in a lower temperature at the sample than at the cantilever’s heater. Secondly, the decomposition should occur on the microsecond time scale of the tip-sample interaction, not the >1 s or timescale typically probed during a sample calorimetry measurement. Consequently, first patterning of the resist is usually observed at temperatures in the range 600 °C - 700 °C. Higher temperatures allow for deeper patterning but this comes at the expense of patterning resolution. A temperature of 900 °C is a good starting point for the temperature.
Forward Height (the writing height)¶
The selection of the writing height is complicated by a “cross-talk” between the forward height and the voltage used to heat the cantilever (the Writer Voltage). When the writer is turned ON, the cantilever bends towards the surface both as a result of the thermal bimorph effect and an electrostatic force arising from the heater voltage. This deflection is larger at higher writer voltages. Typically, the cantilever will deflect by 150 nm when heated to 900 °C.
The forward height must be set to a value which is larger than the sum of this writer voltage deflection plus the adhesion length (see Surface Approach). If this is not the case, the tip will stick to the surface even after the electrostatic force is switched off and produce deep indentations.
If the forward height is set too high, the tip might not reach the target depth even when the NanoFrazor applies maximum possible electrostatic force. Typically, the cantilevers can be actuated by 300 nm although this can vary by as much as 50% within a batch of cantilevers.
For most cantilevers then a selection between of 350 nm should yield good performance.
Force pulse duration¶
When the electrostatic force is switched on, the cantilever is pulled towards the surface. The resonant frequency of the cantilevers is around 150 kHz so roughly 4 µs are required to pull the cantilever onto the surface. Pulse durations shorter than this will not result in a written pattern.
Heat Pulse Duration (Pulsed Mode only)¶
The writer voltage can be applied continuously or via heat pulses, which are applied once per write pixel. This pulsed heating mode can help extend the lifetime of a cantilever by avoiding electromigration in the vicinity of the heater. The heat pulse ends at the same time as the force pulse when the tip separates from the sample. Because the thermal time constant of the cantilever is around 100 µs and as such much longer than the mechanical time constant (3 µs), the duration of the heat pulse must be longer than the duration of the force pulse. This allows the cantilever to warm up before touching the surface. There is a cross-talk between the writer temperature and the heat pulse duration. This arises if the duration of the heat pulse is more than a few µs shorter than the pixel time. In this case the cantilever does not have time to heat to the steady state measured during the thermal calibration. A colder tip may not be able to efficiently remove the resist.
Pre-tension Force (Pulsed Mode only)¶
A substrate voltage can be used to bias the tip position closer to the surface before the application of the force pulse. This biasing force is called the pre-tension force. It is switched off at the end of the force pulse. This is done to produce a higher restoring force to pull the tip off the surface after the pixel has been written. When it is desirable to reduce the force pulse below 5 µs, this pre-tension force should be used [Paul2011].
[Paul2011] | Philip C Paul et al., Rapid turnaround scanning probe nanolithography, Nanotechnology, 22, 275306 (2011). |