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| Line 274: |
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| END | | END |
| </pre>
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|
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| =Beam pitch, beam current and exposure dose relationship=
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| A sample is exposed at a user selected beam current to provide a user defined exposure dose. The system will calculate the shot time (dwell time) of the beam to provide the requested dose. It is clear that high doses will require long dwell times whereas low doses will require short dwell times. There is however another important parameter to the dwell time calculation and that is the beam/shot pitch, i.e. how far beam shots are placed from each other. The beam scanner on the JEOL 9500 system is 100 MHz, thus the temporal resolution is 10 ns and it is not possible to have a beam shot (or dwell time) less than 10 ns. This, in combination with selected beam current and exposure dose will set a lower limit on the beam pitch.
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| The relation might not be obvious at first and is illustrated below. The two identical features are filled with beam shots at two different pitches. the right version has a lower beam pitch (half of the other) and thus there are simply many more beam shots. Consequently, to provide the same area exposure dose the shot time for the right side feature will be much shorter. At half the pitch there will be four times the number of beam shots and thus the shot time will be 1/4 the shot time of the left feature.
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| {| style="border: none; border-spacing: 0; margin: 1em auto; text-align: center;"
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| |-
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| | [[image:BeamPitch.png|500px]]
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| | colspan="1" style="text-align:center;|
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| Example feature filled with beam shots at one times beam diameter (left) and half beam diameter (right). Illustration: Thomas Pedersen.
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| |}
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| The shot time can be calculated as ''t = D·p<sup>2</sup>/I'', where ''D'' is the dose in µC/cm<sup>2</sup>, ''p'' is beam pitch and ''I'' is current. As a practical example let us consider an exposure at 2 nA beam current and a desired dose of 250 µC/cm<sup>2</sup>. At 4 nm beam pitch the shot time will come out to 20 ns, while at 2 nm beam pitch it will come out at 5 ns and thus violate the hardware limitation of 10 ns. In order to expose the pattern with 2 nm shot pitch one would have to choose a lower beam current at the expense of increased writing time.
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| The shot time is in fact such an important number that the system will specifically display the minimum shot time for each sequence at the end of job file compilation as illustrated below. In this case the exposure consists of five sequences with a few different doses and beam currents and the shot pitch (scanstep) has been adjusted to make sure all shot times are below 10 ns. Bear in mind the shot pitch (scanstep) is in units of 0.25 nm, thus sequence 1 is written at 4 nm shot pitch.
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| <pre>
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| Start estimation writing time -----
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| Shot counting start -----
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| thopeQU23008a.v30 at 11-MAR-2023 14:53:40
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| Shot counting end -----
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| Seq. writing time shottime[nsec] resist[uC/cm2] stdcur[nA] scanstep
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| 1 0:52:27 16.000 200.000 2.000 16
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| 2 0:13:46 13.500 200.000 12.000 36
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| 3 0:11:15 12.000 100.000 12.000 48
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| 4 0:12:10 13.500 200.000 12.000 36
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| 5 0:11:14 12.000 100.000 12.000 48
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| Total : 1:40:51
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| Estimation result file : thopeQU23008a.csv
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| End estimation writing making time -----
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| </pre> | | </pre> |
