Reading

Overview

The imaging capability of the NanoFrazor provides a high-resolution topography image of the surface [Duerig2005]. This allows for the depth feedback control and the markerless approach to pattern overlay.

Description of the procedure

The NanoFrazor images the surface topography in contact mode: the tip maintains continuous contact as it is raster scanned over the surface. The deflection of the cantilever is measured by monitoring the heat exchange between the cantilever’s read heater and the substrate. To achieve this, a constant voltage is applied to the heater via a series resistance and the NanoFrazor measures the current flowing through the heater. When the cantilever bends up due to a feature on the surface, the heat exchange with the surface decreases and the reader warms up. When the cantilever bends down due to a recess in the surface topography, the reader cools down. The NanoFrazor detects these changes in temperature via the change in current flow through the heater. The reader signal is converted into depth using the calibration given by the Piezo Approach.

Selection of parameters

Read height and the Read Force

If the tip moves over a sufficiently deep hole it may lose contact with the surface since the Z-Piezo position is not changed within a readline. This loss of contact leads to an inaccurate measurement of the topography. To avoid this loss of contact between tip and sample, the reading height set by the Z-Piezo should be adjusted to match the expected maximum hole depth. This maximum will in general not exceed the resist (PPA) thickness. Hence, when patterning a 10 nm thick resist film, the reading (backward) height can be set to -10 nm. Alternatively an electrostatic force can be used during reading to maintain the contact between tip and sample.

CLL_pars7

Figure 10 Read patterned area.

If the read force is too high or the reading height too low (i.e. a large negative value), the PPA surface can be damaged during reading. This is of particular concern when producing high resolution patterns or using a blunt tip.

Read Upsampling

In order to accurately reconstruct the real surface topography, a read pixel size smaller than half of the size of the smallest expected feature should be used. The Read Upsampling factors for X and Y in the Floorplan Panel controls how many read pixels are recorded for each write pixel.

Unnecessary use of high Read Upsampling should be avoided particularly in the Y direction. High Read Upsampling requires the reading of more lines reducing the throughput. Also, repeated reading of the surface can lead to damage if the tip has become blunt (exhibits a high adhesion as outlined in the section on estimation of the tip diameter). This is visible as wave like features in the topography with a period of a few 100 nm and an amplitude of a few nanometers.

Correction Schemes

The NanoFrazor software contains a number of schemes for ensuring it provides accurate topography data.

Thermal conductance image

Since the NanoFrazor interprets changes in the reader temperature as topography under the tip, changes of reader temperature not caused by topography under the tip produces read artefacts. A typical example is the artefact induced by a topography sitting under the reader resistor. Artefacts can also occur as a result of the substrate. Typically, the cooling of the resistor is dominated by the thermal resistance of the air gap but in the case of, for example, substrates containing ultrathin membranes, the substrate can have sufficient thermal resistance to yield artefacts. As the reader flies above the topography, or the different material, the thermal conductance with the substrate changes and so does the reader temperature. This change in temperature is interpreted as topography by the NanoFrazor. Because of this, a thermal conductance image is superimposed to the real topography image. The resolution of this thermal conductance image is limited by the size of the reader, that is 4.5 µm x 2.0 µm.

An example of non-compensated thermal conductance artefact is shown in Figure 11. The real topography image is in the top half of the image, the artefact is visible as a blurred recession in the lower half of the image. The overlaid cantilever shape (red line) shows that, when the tip is on the area where the artefact is visible, the reader is over some existing topography.

To reconstruct the real topography image, the NanoFrazor subtracts the thermal conductance image from the raw read data. The user can enable/disable this correction scheme from the Write Forces Tab.

reading_ghost_image

Figure 11 Non-compensated thermal conductance artefact caused by pre-existing topography. The shape of the cantilever is overlaid to the read image. The positions of the tip and the reader sensor are highlighted.

Flattening

The NanoFrazor uses the right margin of the images for image flattening. It assumes that no topography is present in the right read margin (average depth 0 nm). Any topography in the right margin will cause a flattening artefact as shown in Figure 12. When the Reader Drift correction is enabled (see Write Forces Tab), the flattening is done on both right and left read margins.

read_artefact

Figure 12 a) Topography in the right read margin causes a read artefact. b) Image flattened manually.

Read-back

When writing and reading, the NanoFrazor writes in the forward direction and reads in the backward direction. In order to avoid reading parts of the surface that are later modified by subsequent write lines, the reading is typically performed a few lines behind the writing. The exact number of lines is calculated by the NanoFrazor on the basis of the tip diameter, its opening angle (that are set in the Cantilever Panel) and the target depth specified from the Layout Editor Panel. As shown in Figure 13, if the number of lines between read and write is too small, the image produced by the NanoFrazor does not correspond to the surface topography once the patterning is complete.

readback

Figure 13 a) Reading with 0 readback during write-read. The NanoFrazor reads areas that are successively modified. The final read image does not correspond to the real final topography. b) Readback of a few lines guarantees that the NanoFrazor reads only areas that are not successively modified.