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Electron Beam Lithography

Electron Beam Lithography

A good introduction: The "SPIE Handbook of Microlithography, Micromachining, and Microfabrication", Volume 1: Microlithography, Chapter 2: Electron Beam Lithography. This chapter is available online at http://www.cnf.cornell.edu/cnf_spietoc.html.
Equipment Properties
The "Vistec EBPG 5000plus" has a gaussian beam shape and uses the vector scan patterning strategy. The gaussian beam shape allows nanometer scale resolution. The maximum scan frequency is 50MHz. Beam current may be adjusted to between 100pA and 150nA. Beams with lower currents have smaller diameters.
The resolution may be selected in a wide range, usually between 1nm and 50nm. At the same dose a smaller resolution leads to longer writing times, if due to the limited scan frequency the beam current has to be chosen smaller.

Example: Writing time for a 200µm square at a dose of 250µC/cm2:

Beam current [nA]150100.1
Writning Time [s]0,7101000

The electron energy can be set to 20keV, 50keV and 100keV. The standard setting is 50keV.
The substrate movement is done with step and repeat, i.e. during table movement there is no exposure. The field, which can be written without moving the table only by beam deflection is called main field. The size of the main field depends on resolution.

Examples:

Resolution [nm]1251012.5
Max main filed size [µm]180260320650800

The offset created in a continuous structure at a main field limit is called stitching. These structures are written with great deflection. The error occurring at the calibration of the beam deflection to the distance on the sample causes the stitching. Critical structures should therefore be placed in the middle of main fields. (see: Design Rules.) The specification is 80nm at 50kV.
Overlay accuracy is the accuracy, with which two independent EBL steps can be written with respect to each other. For the adjustment special markers are used (see Design Rules). The specification is 80nm at 50kV, depending on design rules for the markers and processing can be done up to an overlay accuracy of 5nm.

Design Rules

General

Structures to be written must be presented as closed polylines (=areas). Lines without width and points without dimensioning cannot be written. Single-pixel lines have to be designed as lines with the resolution as width, dots as squares with the resolution as length of the edge. The totality of all structures to be written is called pattern.

Format

The designs can be present in various CAD formats: GDSII, CIF, DXF, TEXTLIB. Due to the hierarchical structure the other formats are to be preferred over DXF. Just for large arrays of nanostructures DXF files are much larger.
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Example: File size for a 100µm dot field composed of 10nm dots at 100nm spacing:

Format:GDSIICIFDXF
File Size:2KB30KB93M

TEXTLIB is a text based format. Instructions and a viewer for this format are at your disposal if needed.

Grid

The structures should be placed on a grid which corresponds to the resolution required or a multiple of it. Each node point will be shifted to a grid point later at fractioning unless it is not yet placed on a grid point. Therefore rounding errors may be created at structure sizes and positions. In particular at lines of a defined width the corner points (not the end points) have to be placed on the grid.

Usage of Layers

  1. It is frequently advisable to write fine and coarse structures with different beams. For that these structures have to be present in different layers. A small overlap (e.g. 200nm) prevents gaps at drift and rounding errors.
  2. Holes can be generated by placing the spared area in a separate layer. With the XOR function at the pattern processing the holes can be defined easily. It also applies to circular rings (e.g. zone plates). Ask after details if needed
  3. Critical structures should be placed in the center of main fields. This can be enforced by laying boxes (rectangles) around the critical structures. These boxes may at most be as big as the main fields. Structures outside the boxes will conventionally be tiled into regular main fields.
  4. Within a pattern different structures may get different dose factors. The actually written dose is the product of dose factor and basic dose. The structures of one dose have to be summarized on one layer. Dose factors must be greater than 0.5.
    Layout Rules
  5. A layout can be flat or be composed as an array of cells. In an array of cells every cell layout has to be designed only once. The repetitions are done in the write job. An array of cells is useful in particular, if in every cell there must be a new adjustment or if a dose progression is written.
  6. In an array of cells frequently a coarse layer is to be written in addition individually (labelling layer). The orientation of the array with respect to the coarse layer becomes easier, if both array cell and coarse layer are surrounded by a box. The center of the box for the coarse layer and the center of the array of boxes should be identical and, as a rule, correspond to the middle of the sample.

LayoutregelnLayoutregeln b)

Layout rules

In an array of cells different designs may be written in the array. For distinction, either the different designs must be in different layers, or the different designs are in different cells with different boxes. The different designs of an array may however only be written with the same resolution and the same beam current. The dose may be different.

Usage of Markers

  1. Automatically discernable/detectable markers are squares having a positive (metal markers) or negative (etched markers) contrast with respect to the background. Typical sizes of markers are 5-20µm. The surrounding area must be homogeneous, the distance to neighboring markers or other structures should be about 100µm.
  2. A set of markers consists of four markers arranged in a rectangle on the corners of the layout. Structures to be written shall be in the inside of the rectangle, critical structures in the center, if possible. Structures outside the rectangle of markers will be written with a greater positioning error.
  3. During each adjustment a set of markers is exposed and later processed with the structures. Therefore there must be a set of markers for each EBL step. The marker group for different EBL steps in each corner should be written centered in a main field. For this purpose the method with the box in a separate layer (see above) is preferable.
  4. An array of cells must be arranged regularly and orthogonally. Single cells, columns or rows may be dropped.
  5. Normally the accuracy of adjustment is in the order of 20nm. The limit is the writing of markers and the offset of the marker positions with respect to each other.

Advanced Marker Positioning

If required, the markers may be positioned in a better way. This procedure is based on writing each marker in a main field of its own with a very small beam deflection. For the design of the layout there is proper software with the maximum possible control of the write sequence of the markers. In order to minimize thermal drift, which is the limiting factor, there is a delay of 10 hours after the loading of the holder before writing the markers. Due to the expense of resources this method is only applied if required. Hereby the overlay accuracy is in the order of 5nm.

Additional Information

Contact

Dr. Stefan Trellenkamp

Tel.: +49-2461-61-3360
Fax: +49-2461-61-2440
e-mail: st.trellenkamp@fz-juelich.de


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