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Photoemission Electron Microscopy Ultrafast Dynamics

Prepare for beamtime

General information

To obtain an access card for the ALS, new users need to register by filling out a Participating Guest Information Form. Please fill out the form before traveling to the ALS to expedite the check in at the ALS. Foreign Nationals need to bring a passport and their immigration documents. Please follow this link for information about entering Lawrence Berkeley Lab and read these directions to get to the lab. If this is your first beamtime at the ALS you will also be asked by the users office to fill out a user agreement.

Safety

Your safety is very important to us. The ALS is a research facility, which contains equipment that, if used incorrectly, can lead to serious injuries to users and staff, and to the damage of equipment. Your help is needed to avoid any unsafe practices.

All users at the ALS are required take a safety training before they begin work at the ALS. Please mention in your 'Request for beamtime form' any equipment and materials that are potentially hazardous. This document will be reviewed by ALS safety staff and you will be contacted, if further steps are necessary to ensure a safe operation. User advisories explain the ALS procedures for the most common hazards, in particular lasers, chemicals, radiactive materials, and biological materials. Please also contact the beamline scientist if your experiments should involve any hazards.

Beamline-specific information for BL 7.3.1 and 11.0.l

Samples

Samples for Photoemission Electron Microscopy experiments (PEEM) should have a smooth, conductive surface. A smooth surface can either be obtained by polishing a substrate or by depositing thin films on smooth substrates. Patterned materials can be imaged but the spatial resolution of the microcope may be degraded, if the aspect ratio of structure height and structure size approaches unity.

Ideally, samples should consist only of conductive materials. Conductive films on insulating substrates can be imaged by contacting the surface using a metallic cover or carbon tape contact to the surface. Ideal substrates are metallic single crystals, polished metals, semiconductors wafers (Si), even with thin oxide layers, and also insulating crystals, but only if a continuous conductive film covers the substrate. Thin insulating films on conductive substrates can sometimes be measured. The maximum thickness depends on the conductivity of the film and ranges from 1 nm for highly insulating materials to tens of nm for polymers. If the surface is smooth, we can attempt to evaporate a very thin metal film in situ on top of the insulator to improve the conductivity. Isolated, metallic patterns on highly insulating substrates will likely charge and cannot be measured. Bad insulators with high defect density can sometimes be imaged without capping.

Cap layers protecting a a sample from oxidization need to be thin enough so that x-ray generated electrons can penetrate the cap and leave the sample. Depending on the capping material the film should ideally be between 1 nm (e.g. Pt) and 2 nm (e.g. Al) thick. Thicker cap layers will significantly reduce the signal from the covered sample and increase the background intensity from the cap. A very strong contrast at modest spatial resolution can be detected through cap layers up to 5 nm in thickness, or even thicker. A cap can be partially removed through in situ Ar ion milling but this seldom leads to a full recovery of the signal. Ideal cap materials that prevent surface oxidization of metals are 2 nm of Al and 1-2 nm of Pd, Ru, Pt.

Samples should ideally be 5-15 mm in diameter (circular or square-shaped). Depending on the type of experiment and optional sample heating, this range may be further restricted. No imaging is possible of the outer 2 mm of the sample because of distortion of the electric acceleration field by the sample edge or the edges of a cover. For example, on a sample measuring 10 mm in diameter, only the center 6 mm can be imaged well. Samples smaller than 10 mm in size will be mounted underneath a cover with an opening smaller than the diameter of the sample, to shield the edges of the sample and to avoid field emission and discharges originating from those. Smaller samples down to about 3 mm can be measured through a cover with a very small opening but the scan area is then very limited (~200 um). Ideally, the metal cover should be very thin, have a tapered edge, and should directly sit on the sample surface. Covers with holes of different size are available at the beamline. The xy-stage of the PEEM-2 manipulator can be scanned over about 8x8mm.

Please discuss the sample structure with the beamline scientist before manufacturing samples. On request, it may be possible to test a sample in PEEM before your beamtime.