The functionality of the "PointDistance" command, the "UpdateRate" command, and the "ConnectPoints" command are intimately related to the operation modes (ContinuousMode and PulsedMode). These parameters are of great importance for obtaining the intended structure. However, their interdependances are sometimes confusing for users. Hence, we will review some special cases to illustrate the underlying principles:
The Fundamentals
| Command |
Description |
| ContinuousMode |
The continuous mode tells an AOM (Acousto-Optical Modulator) to blank out the laser beam only if the "write" command is set. If not, the resist is continuously exposed by the leaser beam. |
| PulsedMode |
The pulsed mode is used for single shots. Thus, each coordinate defined by the GWL structure file is positioned. Then, the laser beam is turned on to expose the photoresist. After a period defined by the ExposureTime the laser is turned off and the stage approaches the next coordinate.
|
| PointDistance |
If two coordinates are given in the GWL structure file, the piezo stage would stupidly run from the first to the second point. Especially, if these two points are diagonal, the piezo might take an arbitrary trajectory and not neccessarily the bee-line.
To circumvent this problem, the PointDistance is used to discretize the line between 2 points by segments whose lengths in nm are defined by the respective PointDistance value.
An example: Let's say we used the command "PointDistance 100" and defined two point coordinates in the GWL structure file: <0 0 0> and <10 0 0>. This would mean that the software feeds the piezo stange with coordinates <0 0 0>, <0.1 0 0>, <0.2 0 0>, ..., <10 0 0>.
|
| UpdateRate |
This parameter defines the rate in 1/s of updating the set values of the piezo stage
|
| ScanSpeed |
ScanSpeed = UpdateRate x PointDistance. Hence, this is the writing speed of the lithography system.
|
| ConnectPointsOn |
This command turns on the discretization of 2 points by line segments. The length of each segment is set by PointDistance.
|
| ConnectPointsOff |
This command turns off the discretization. Setting of the PointDistance does NOT show any effect on the writing process. In this case, the piezo stage connects these points in an arbitrary manner given by the piezo's control circuit.
|
Example 1: How to Write A Straight Line:
Let's have a look at the following code lines:
ContinuousMode
ConnectPointsOff
PointDistance 100
UpdateRate 1000
0 0 0
0 30 0
The ContinuousMode is certainly the right choice since the laser beam is not blanked out between scanning from point <0 0 0> to <0 30 0>. However, ConnectPointsOff would turn off the automatic discretization and would possibly lead to a bent line (although the PointDistance is set to an optimal value). Therefore, it's better to modify the code:
ContinuousMode
ConnectPointsOn
PointDistance 100
UpdateRate 1000
0 0 0
0 30 0
So, does it make sense to use ConnectPointsOff in combination with ContinuousMode? The answer is "YES, sometimes"! For example, if you want to write rounded complex-shaped structures like helices. Here, it is useful to design the structure by using a CAD program to define the curved lines and export the structure as a point cloud. If this point cloud is dense enough, the piezo stage receives many coordinates which are placed close to each other. In short terms: This combination of commands is useful if the required discretization is hard-coded in the GWL structure file.
Example 2: How to Write 2 Separated Dots at A Distance of 30 microns?
Let's start with:
PulsedMode
ConnectPointsOn
ExposureTime 20 % time in ms
PointDistance 1000 % = 1µm
UpdateRate 1000
0 0 0
0 30 0
Here, the pulsed mode is the right choice, but ConnectPointsOn results in a different structure. The system discretizes the given 2 points by 30 line segments. Hence, this results in a dot array with 1µm dot distance. Only if ConnectPointsOff is used, we obtain the intended 2 separated dots. Hence, the correct code would look like
PulsedMode
ConnectPointsOff
ExposureTime 20 % time in ms
UpdateRate 1000
0 0 0
0 30 0
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