Use of a commercial RF Plasma Cleaner in eliminating adventitious carbon contamination in an XPS system

 

Atomically clean surfaces are necessary for many applications.  XPS is often used to confirm surface cleanliness.  We have observed that XPS can in fact be a source of contamination but that a plasma decontamination system can ameliorate the problem.  During the thirty minute setting time before introduction into the main chamber of the XPS system, the antechamber contaminated the surface of samples with approximately 0.6 nm of organic materials.  We noted this buildup on the native oxide surface of a single crystal silicon wafer samples (Figure 1).  Previous to their exposure to the XPS system, these samples were cleaned using atomic oxygen in air, by placing the samples within 1 cm of an excimer lamp in air for at least 5 min.  The oxidation process frees the SiO2 of adventitious carbon (AC).  Use of spectroscopic ellipsometry (SE) provides a method of characterizing surface cleanliness.  With the use of the lamp, the apparent thickness of the oxide layer (really a layer of oxide and AC) would decrease to 1.8 nm, the thickness of the native oxide.    When we introduced the clean samples to the XPS antechamber, we saw an increase in the apparent oxide thickness and XPS scans showed a large carbon presence.  However, after we installed Evactron-C plasma decontamination system, adventitious carbon was virtually eliminated as seen in XPS (Figure 2) and confirmed by SE. 

 

 

Figure 1: Before the use of the plasma cleaner XPS data continually demonstrate the presence of carbon on samples that should have been clean

Figure 2: After the use of the plasma cleaner XPS data demonstrate the virtual elimination of the adventitious carbon.