Mar 7, 2017 - the Vacuum AFM was a joint project with R. Tanaka
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.
Some aspects of the design may be of interest:
- Controls for all valves are situated in one location. Both electrically actuated (for pneumatic lines) and pneumatically actuated (for vacuum lines) valves are used.
- All components are securely attached to the frame, specifically for 'easy' possiblity of relocation of the device. The frame is on caster wheels.
- Independent pumping systems are used for the main vacuum chamber and the Argon ion gun chamber, with the latter capable of achieving a higher ultimate vacuum (to 1e-7 Torr vs 1e-3 Torr in the main chamber) but with a lower pumping capacity.
- For back-filling the chamber with gas, a custom machined orifice plate is used. This ensures that only the vacuum-capable KF and CF joints are exposed to vacuum, while the pipe thread and hose joints are only exposed to pressurized gas. This way air is not inadvertently drawn in.
- Heater tape is used on the Argon ion gun chamber, in order to speed up pumping to high vacuum (we expect that most of the residual gas is from water attached in a thin layer to the metal surfaces, the high surface area of bellows was particularly problematic as evidenced by a poor vacuum when bellows sections are heated). The effect of heating becomes visible at the 1e-5 Torr range. Pumping to ultimate vacuum may take up to 24 hours, with heating on.
- 'Kinematic' mounts, with three adjustable support points, allow fairly precise positioning of the vacuum components of the ion gun, in order to match the angle of the assembly to that of the main chamber, for a good seal between flat surfaces. This way mechanical loads are handled by the support structure rather than the vacuum joints.
- A dual convection and (hot) ionization gauge is used to measure internal pressure all the way from atmosphere to vacuum. This investment proved very worthwhile for this application, where both vacuuming and back-filling are necessary. In addition a thermocouple gauge is used on the main chamber as a backup - it only operates in the 1-1000 mTorr range.
- A vibration isolation table inside the chamber enables higher quality AFM measurements, despite the very shaky lab floor (it is next to a pump room!).
- An interlock system is used to prevent operation of the gate valve between chamber and ion gun in case of electrical power loss (preventing a sudden closure due to the elctrically operated valve) and unless the sample transfer rod is returned to a safe location (this way the gate cannot be shut when the rod is passing through it).
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.
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.
- Measurements form the basis for upcoming paper (TBA)
- Presented at 2016 MRS Conference, Boston MA
- Article in Active Matter book (in publication)
- R. Tanaka M. S. Thesis (in publication)