Specific Process Knowledge/Lithography/EBeamLithography/JEOLPatternPreparation: Difference between revisions

Content and illustration by Thomas Pedersen @ DTU Nanolab unless otherwise noted.

Pattern placement is a bit different compared to UV mask design convention and different to how our Heidelberg MLA systems work. Pattern placement is very dependent on the boundary box of the design. All patterns are placed using the '''ARRAY''' command and they are placed with respect to the center of the design boundary box. This is illustrated with the two designs, L1 and L2, below. L1 is a design nicely centered around (0,0) with a symmetric bounding box. If placed by the '''ARRAY''' command with no offset this will come out entirely as expected. L2 is however offset from (0,0) and if exported with its local bounding box and placed by the '''ARRAY''' command with no offset, it will be exposed at the center of the substrate. Thus, it is vital to keep the bounding box in mind as the '''ARRAY command places patterns with respect to the center of the bounding box.''' To get the desired result in this example, one can define an appropriate offset in the '''ARRAY''' command for exposure of L2.

== Alignment preparation ==

=== Boundary box alignment ===

In continuation of the example above, if two layers/exposures are to be aligned to each other in subsequent process steps it is vital that the two layers boundary boxes are identical or at least symmetric around (0,0). With reference to the example below, the boundary boxes can be made identical by either

Corner marks can be 1x1 nm boxes placed outside the substrate to avoid having them exposed onto the substrate.

[[File:BoundingBoxAlign.png|1000px|center|frameless]]

If alignment is needed it is vital to consider this already at the process design and mask design level to ensure that a visible mark is produced. Alignment is done using the electron beam in either SEM mode or beam scan mode. In SEM mode the stage is moved to the expected alignment mark position and the user will manually observe the SEM image and adjust stage position to center the mark at as high magnification as required by alignment tolerance of the design. In beam scan mode the user sets up a routine that will scan the beam across the expected mark position and the Backscatter Electron Detector (BED) will detect the backscattered signal. This is done in both x- and y-directions and system will calculate the mark center based on this.

[[File:JEOL9500Alignment.png|700x700px|center|frameless|Alignment modes of JEOL 9500.]]

Patterns that contain a mix of fine and large dimension structures can advantageously be broken into a low current and a high current exposure. In this way the fine features can be written at low current (and low speed) and the large structures can be written at high current (and high speed). Some patterns, for instance a fine electrode pattern with large bonding pads, can easily and manually be broken into a fine and coarse part and placed in two different layers. For many patterns this can however be very cumbersome but the process can also be automated with Beamer using "bulk and sleeve" setup.  The concept is illustrated below. In this setup, Beamer will extract the periphery (sleeve) of a pattern into one layer and the bulk of structures into another layer. The bulk is oversized slightly (adding a bias) to ensure overlap between the two patterns at exposure. In this way structures can be defined with best possible edge definition while keeping exposure time down. [https://labmanager.dtu.dk/view_binary.php?fileId=5412 A Beamer flow with a default bulk and sleeve setup can be found here.] A 200 nm sleeve with a 50 nm overlap is a good starting point.

The pre-computed PSF can be loaded from either a separate file, from the local archive or from the global archive. In any case the file will probably originate from a Tracer simulation, for more on this visit our [[Specific_Process_Knowledge/Lithography/EBeamLithography/TRACER|Tracer page.]] It can be advantageus to place your PSF in the global (network) archive such that it is available on the Beamer PC in the cleanroom as well as the Beamer PC outside the cleanroom.