Vacuum AFM

Mar 7, 2017 - the Vacuum AFM was a joint project with R. Tanaka

Objective

Existing AFMs operate largely in laboratory temperature and pressure environments. For metallic and high purity samples, this precludes measurements of a clean surface since even brief exposure to air will result in surface organic contamination and oxidation of exposed metal (a tiny, nanometer-thick oxide coating forms on all metals exposed to air and is responsible for preventing metals from disintegrating! Some scientific questions, however, would benefit from direct measurement of a pure metallic surface, which cannot be realistically done in air). The purpose of this instrument is to enable AFM testing of clean surfaces, by combining an Argon ion gun to clean the surface under high vacuum, and a large vacuum chamber housing the AFM. After the sample surface is cleaned of contamination and oxides under vacuum in the ion gun, the sample is transferred under the AFM head under vacuum-tight conditions. The chamber is then back-filled with high purity nitrogen (minimal water and oxygen content, to avoid oxidation) as necessary for the measurements to proceed. To facilitate the sample transfer process, a single glove is attached to the chamber, allowing an operator to move and position multiple samples without exposing the chamber to outside air.

Design

Some aspects of the design may be of interest:

Photos

Overview


From left to right, the front, back, and top views of the device.

From the front you can see the computer interface and valve box. From the back, the vibration isolation table inside the chamber and the argon ion gun power supply are visible. From the top, two glass viewports are visible along with the glove port.

Vacuum


On left, the large vacuum pumps for the main chamber. On right, the smaller vacuum pumps for the ion gun chamber.

Both the main chamber and the ion gun chamber are vacuumed using independent sets of turbopump in line with a roughing pump. The main chamber has a very big oil-filled rotary vane pump which brings the chamber to around 100 mTorr, along with a medium speed (48 kRPM) turbomolecular pump that brings it to 1 mTorr. The ion gun chamber has a small diaphragm pump (to avoid oil contamination of the high vacuum components) which gets to 1 Torr, and a high speed (90 kRPM) turbomolecular pump that brings the vacuum to 1e-6 Torr.

Argon ion cleaner


A closer look at the argon ion cleaner.

In this image the connection of the argon cleaning chamber to the main chamber is visible, along with the small argon cylinder and a microammeter to measure the ion current incident on the sample.

Results