High Pressure RHEED

RHEED (reflective high energy electron diffraction) is a powerful tool to monitor the deposition of thin films. An electron beam hits the substrate/film at a very small angle to the surface, so the electrons don’t penetrate deep into the material. Diffraction takes place in the top few atomic layers of the sample, so the resulting diffraction pattern contains information about the film surface. RHEED works well in UHV processes like MBE (molecular beam epitaxy), where the chamber/process gas pressure is very low – typically below 10−6 Torr. The pressure range can be extended somewhat by pumping the RHEED gun (electron source) differentially to prevent cathode filament degradation. But at the process pressures present in a PLD/Laser MBE system (normally up to 1 Torr), the mean free path length of the electrons is reduced (at 10−6 Torr: ≈100 m, at 1 Torr: ≈100 μm). The additional scattering will blur the RHEED pattern and render it unusuable, as electron gun and RHEED screen are typically mounted 30…40 cm away from the substrate. SURFACE offers a solution for this problem: The SURFACE HP-RHEED system. Here the electron beam is enclosed on 90% of its way in a differentially pumped tube. The electron beam is exposed to the deposition chamber pressure only very close to the substrate, so a small spot size and uniform electron energy are maintained at the substrate surface.