Specific Process Knowledge/Lithography/EBeamLithographyManual

Purpose, location and technical specifications

The JEOL JBX-9500FS electron beam lithography system is a spot electron beam lithography system designed for use in writing patterns with dimensions from nanometers to sub-micrometers.

Substrates that needs e-beam exposure should be mounted in a cassette and transferred into the writer via the robot loader (autoloader).

To request for an e-beam training session, contact e-beam@danchip.dtu.dk; a DTU Danchip personnel will hereafter provide a time slot. For safety reasons, even fully trained users are only authorized to mount substrates into the e-beam cassettes but not authorized to load the cassettes into the autoloader.

To use the e-beam writer, book the machine via LabManager, and ask help from DTU Danchip staff to load your cassette into the robot loader (autoloader).

After your exposure, fully trained users can unload their cassettes from the autoloader, unmount their substrates and re-load an empty cassette into the autoloader.

If you are prohibited to unmount your substrates before another user requires the cassette, you must accept that either the next user or DTU Danchip personel unmount your substrates.

Location

The e-beam writer is located in a class 10 (ISO 4) cleanroom with tight temperature and moisture control. The room must only be entered when the machines or equipment inside the room is intended to be used. Always wear face-mask and an extra pair of gloves when handling cassettes.

The computer controlling the e-beam (EWS/9500) is located in the controller room which is a class 100 cleanroom area. The computers supporting the conversion of the e-beam files are also located in the controller room. Manuals

There are 3 manuals for the e-beam writer; apart from the main manual (this manual) there is a sdf and jdf-file manual, and a BEAMER manual. They can both be accessed from LabManager under Technical documents.

The original JEOL manual for the e-beam writer FS9500 is located on the O-drive: O:\CleanroomDrive\_Equipment\E-beam Technical Specification The system can be characterized as follows:

1. The spot beam for electron beam writing is generated by a ZrO/W emitter and a four-stage electron beam focusing lens system, see illustration below.
2. Electron-beam scanning speeds, f, up to 100 MHz are available (which is maximum scan speed).
3. The acceleration voltage is 100 kV.
4. The e-beam writer can pattern structures with a minimum resolution of 12 nm.
5. The maximum field-size without stitching is 1000µm x 1000µm.
6. The machine has 4 cassettes that can contain
   1. 6 wafers of 2” in size
   2. 2 wafers of 4” in size
   3. 1 wafer of 6” in size
   4. Special cassette for chips (slot sizes 4 mm, 8 mm, 12 mm, and 20 mm)

Pattern writing using the e-beam is implemented on a wafer or chip which has been coated with an electron beam sensitive resist. Both positive and negative types of resists for pattern writing can be used. In either case, the resist sensitivity Q (C/cm2) is a function of the beam current, I (A), the pattern writing area, A (cm2), and the pattern writing time t (s), as given below:

Q = It/A

The e-beam scanning speed f (Hz) is a function of the e-beam scanning step, p (shot step), as shown below:

f = I /(Qp2)

The e-beam writer has scan speeds up to 100 MHz available. The dose, Q, shot step, p, and current, I, is chosen to meet the requirement of the pattern to be written, the writing time available, and also to meet the requirement f<100MHz.

Rough estimation of exposure time

Based on the equations above, a rough estimate of the exposure time is easily calcualted. In the second sheet of the e-beam logbook, a simple program for calculating the scan speed frequency and an estimation of the exposure time can be found. Note, that the actual writing time will exceed the exposure-time, as the exposure-time calculation doesn’t include pre-calibrations and stage movement during exposure.

The area of your pattern can easily be found in L-edit by selecting all of your structure and use the area calculator in Tools/Add-Ins. If your pattern contains instances, you have to flatten your pattern (Cell/Flatten) before you use the area calculator; notice, however, that the flattening cannot be undone.

The machine has a number of objective apertures (no. 15 on above illustration) in order to obtain different beam diameters in different current ranges. The available apertures are:

The beam diameter changes as a function of aperture size and beam current. Simulated beam diameter in the current range 0.01-1000 nA for the available aperture sizes is plotted in the graph below.

Mounting of chips or wafers into cassette

NOTE that wafers in some cassette are mounted with major flat upwards and in some cassette mounted with flat down-ward. Make sure your pattern is designed in the same way.

Wafer cassettes must be handled with great care and with an extra pair of clean gloves (to be found inside the e-beam room). Prevent any form of impact of the cassette and never touch the delicate parts of the cassette, i.e. the reference plane, reference marks or grounding pins. Take care that the cassette is not scratched against the metal table, always keep cleanroom side-sealed lintfree tissues between cassette and table.

General rules for handling cassettes and mounting wafers and chips into cassettes:

• Always wear face-mask and a new pair of gloves
• Never touch the reference planes, i.e. the six polished areas on the front side of the cassette
• Never lift the cassette in the hook
• Always check the wafer for loose parts or flakes before loading. In case of loose parts or flakes, stop mounting, discard or rework the sample and rethink your process.
• Always check the cassette for loose parts, dust and particles.
• For chips: Make sure the chip size is at least 2 mm larger than the slot opening. Be careful when mounting the plate; it will get stuck easily if mounted askew. Always check that the 12 large screws are tight.
• For wafers: When mounting a wafer, hold the spring (wafer securing pin) back when putting the wafer and the plate into the cassette, especially on the 2" cassette (the spring can lift the wafer). Tighten the leaf spring first, then release the spring and tighten the rotation lock lever.
• Inspect the front side of the cassette carefully after mounting the chip/wafer; a small gap between substrate and cassette will lead to a failure in HEIMAP and no exposure is thus possible.

Seen from the front-side of the cassette with the hook to the right, the workpiece windows (wafer positions) are named A, B, C etc in reading-direction from top left to bottom right.