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GCIB Depth Profiling - Do you know the size of the cluster you're using?

Surface Analysis Spotlight: XPS

by Kateryna Artyushkova

Director of Analytical Lab & Marketing

Cluster ion beams have become popular in recent years for soft materials because of their capability for low damage to organics, but sputter yields of metal and inorganic materials are very low. Gas cluster ion beams (GCIBs) with small cluster sizes have advantages of both monatomic ion beams and conventional large-cluster GCIBs (>Ar2000+ ). A smaller-cluster GCIB results in higher energy per atom with a sputtering behavior similar to that of the monatomic ion beam and leads to enhancement of the sputter yield in metal and inorganic layers, making them much more efficient in depth profiling of samples with hybrid architecture that contain metals/inorganics/organics.

All modern XPS instruments now have GCIB ion guns with a range of cluster sizes available. The manufacturers usually provide the parameters of the GCIB as predefined settings, and the actual cluster size distribution cannot be verified. One of the exciting options present on PHI Versaprobe 4 and PHI Quantes instruments is the GCIB cluster measurement tool, which allows tuning and measuring GCIB cluster size for depth-profiling applications.

The cluster size is measured using a time-of-flight mass separation method. After Arn + ions are extracted, the cluster ion beam is deflected to eliminate neutral species and monomer through bending and a magnetic Wien filter, respectively. The current profile is recorded from the flight-starting time until the largest mass ions arrive at the sample. The cluster size distribution is obtained by converting the flight time to mass as shown in Figure below.

In a recent paper published in ACS Applied Nano Materials, we have tuned GCIB for a specific cluster size and energy per atom and performed depth profiles of different inorganic, polymeric and hybrid materials.  

Sputter rates were measured for 5 different settings of GCIB (from 500 to 2000 cluster size) for SiO2, NiOx, and crystal TiO2 thin films, which have been widely adopted in semiconductors. The sputter rate on inorganics can be increased by using a smaller GCIB cluster size. The chemical damage to sputter-sensitive inorganic materials is alleviated with a larger GCIB cluster size but at the cost of a reduced sputter rate.

PET, PMMA and PI, widely used for IC packaging, were selected to investigate damage induced by smaller cluster size GCIB sputtering. From the change in the spectral shape in PET and PMMA, the data suggest that differential sputtering between O and C was enhanced with small cluster size. PI, the most prone to ion-induced damage, was changed significantly using GCIB cluster sizes below 2000.

It isn't easy to balance the practical measurement cycle time and the preservation of chemical information in depth profiling of inorganic/organic hybrid devices. The perovskite solar cell was analyzed using 2 GCIB cluster sizes as shown in Figure. The XPS depth profile obtained smaller cluster size - Ar600+ - showed a well-preserved layer structure of the perovskite solar cell.

To read more on how cluster size measurement can help investigate optimized GCIB settings for balancing the sputter damage and sputter rate for novel nanomaterials and hybrid samples, please read the full paper.

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