Lubricant oil, vacuum grease, high vapor pressure polymer and photoresist samples can introduce hydrocarbon contamination into SEM, FIB and TEM sample chamber. XPS data shows that surface of a clean sample will be contaminated by airborne hydrocarbon contamination after exposing to air for just one hour. For low-landing-energy high-resolution secondary-electron mode imaging on high-resolution FE-SEM, secondary electrons mostly come out of the thin top surface layer. If the sample surfaces are contaminated with a layer of hydrocarbon contaminations, signal electrons are mostly coming out of the contamination layers instead of the material of interest. Therefore, contamination can reduce image contrast and degrade image resolution.
In X-ray analysis, high dose electron beam will irradiate the sample surface for a long time. If the sample chamber is heavily contaminated by hydrocarbon, carbon deposition will build up in the process. Carbon content will be higher than the actual percentage in the materials.
Two components in electrons optics column are exposed to high dose of electron irradiation. Electron apertures are used to limit beam angle and control beam current by cutting off the high angle electrons. Aperture may also be used as beam blanker. SE detector is used to collect emitted secondary electron signals. These two components are not located in ultra-high vacuum section of the electron optics column. They are susceptible to heavily hydrocarbon contamination build-up if the sample chamber is not clean. Hydrocarbon is not a good conductor. It can create unstable charging issue inside the electron optics column. Therefore, focus and beam position may drift during long slow scan because electrostatic charge will accumulate on the hydrocarbon layer. Localized charging inside the electron optics column can also increase electron optics aberrations and reduce electron optics resolution.
Remote plasma source (EM-KLEEN or SEMI-KLEEN) can be attached to the SEM and FIB chamber. Once user closes electron gun gate valve and turns off high voltages. The gas dosing valve inside the plasma cleaner will start introduce process gas (usually air) into the plasma cleaner at very low flow rate (less than 30sccm). Rf energy will ionize the oxygen or hydrogen gas and generate atomic oxygen, atomic hydrogen and OH. Those reactive radicals will then diffuse into the SEM or FIB chamber and react with the hydrocarbon contaminations on the sample and inside the chamber. The byproducts are usually low vapor pressure gas species that can be easily pumped away by vacuum pump. Remote plasma cleaner can clean sample and chamber at the same time.
Principle of downstream plasma cleaning for SEM, FIB, TEM and other high vacuum systems
SEM and TEM sample can be pre-cleaned using Tergeo-EM tabletop plasma cleaners. If air, Ar+O2 or H2 gases are used to generate plasma in Tergeo-EM plasma cleaner, radicals can effectively remove the hydrocarbon contaminations on the sample surface. Tergeo-EM plasma cleaner is the only TEM/SEM plasma cleaner that has integrated both immersion mode plasma cleaning (samples are immersed in plasma) and downstream mode plasma cleaning (samples are placed outside the plasma) in one system. Downstream plasma clean can be used to clean delicate samples that contains carbon, such as graphene, DLC (diamond like carbon), carbon nanotube or for samples with very thin coatings. In addition, unique pulsed mode operation can generate extremely short plasma pulses to further reduced the plasma intensity for delicate samples. Patented plasma sensor technology monitors the plasma strength in real time. It helps user to set up right cleaning recipe for different types of samples.