Spot Size Guide

Getting good results in e-beam lithography means finding the best combination of a lot of different exposure parameters. Two of the most important of these are beam spot size and pattern pixel size.

Spot Size vs. Pixel Size

Pixel size is the pixel diameter (also called “resolution” and “beam step size”) that you choose when exporting your pattern in LayoutBEAMER?. This number can be any multiple of the system’s base resolution unit (0.5 nm), all the way up to the width of your subfield, which is typically about 4 μm.

Spot size (also called “beam diameter”) is the width of the beam and is roughly proportional to the beam current. The table below gives approximate theoretical spot sizes for various beam currents set up on the 300 μm aperture, approximated using column geometry and electron physics. The chart of high-current spot sizes was empirically measured on the tool using the mss command.

For best results, some thought should be put into both of these numbers before you start writing.

Spot Size Considerations

• In order to make a continuous shape, pixels need to overlap. Therefore, choosing a pixel size that’s about 40-60% of the spot size you plan to use is a good idea (this obviously doesn’t apply if you’re trying to make a discrete array of dots). Anything between 10% and 100% will give you an exposure at some dose, but following the 40-60% rule of thumb will give you the widest process window.

• For good linewidth fidelity, your smallest feature should always be a minimum of 4-5 pixels wide if possible. Choose your pixel size and spot size accordingly.

• For obvious reasons, avoid pixel sizes that force you to choose a beam current so low that the pixel clock overflows (frequency >50MHz) in order to get the correct spot. Usefully, the spot sizes from 0.1 to 1 nA are roughly identical if you’re using the 300 μm aperture. There is also a defocus setting in CJob (see below) that can be useful in this situation)

• Finite resist contrast and other external factors will limit your minimum developed feature size to 10-15 nm regardless of your beam settings. Using spot/pixel sizes smaller than this is useful from a feature-fidelity standpoint (for example, a 10 nm line that’s 5 2nm pixels wide will have much less linewidth variation than one that’s a single 10nm pixel wide) but won’t increase your maximum resolution.

• The system-recorded spot size values in the beam profiles are pretty useless, especially for lower beam currents, for various reasons. You can perform a more accurate measurement of the spot size by loading a beam current and running the mss script. The script doesn't really need any input to run, but it does have a few options that you can learn about by running mss --help.

• If you need a wider spot size than can be achieved with the beam current you’re using, an alternative to a higher current is defocusing the beam. You can do that by checking the defocus box next to the beam current selection field in CJob and putting in the desired spot size in nanometers. Note that since the system can only accurately measure spot sizes greater than about 50 nm, this will only work if you need a spot size above that general range.

Spot Size Table, 300um Aperture (theoretical)

Spot Size Chart, 300um Aperture (measured)

* The above chart is only valid at the 300um aperture setting, so if you're using a different aperture make sure you measure the spot yourself with mss!