Depth Profiling of an Organic FET with XPS and Argon Cluster Ions

Significance of the Topic

Thin, flexible organic field-effect transistors (FETs) are central to next-generation electronics, offering low cost and mechanical flexibility. Copper phthalocyanine (CuPc) based organometallic semiconductors promise enhanced performance compared to purely organic materials. Accurate compositional and chemical state information through the device depth is crucial to optimize charge transport, layer integrity and device stability.

Objectives and Study Overview

This study demonstrates a combined X-ray photoelectron spectroscopy (XPS) depth profiling approach to characterize a CuPc organic FET stack on SiO₂/Si. Key goals include:

• Assessing surface and subsurface chemical states of the CuPc layer without damage.
• Profiling through the insulating SiO₂ to the silicon substrate.
• Generating a continuous quantitative depth profile using a single ion source.

Methodology and Instrumentation

The Thermo Scientific Nexsa XPS system with a MAGCIS dual-mode ion source was employed. The protocol comprised two sputter modes:

• Argon cluster ions (4 keV, average cluster size ~2000 atoms) for gentle removal of the CuPc layer while preserving chemical information.
• Monatomic argon ions for etching through the SiO₂ isolation layer into the silicon substrate.

The organic FET sample was clipped to a standard holder for electrical grounding. XPS spectra were recorded after successive etch cycles to monitor composition and chemical state evolution.

Key Results and Discussion

Surface C 1s spectra of the device closely matched a reference CuPc powder, apart from additional hydrocarbon contamination at ~285 eV. After ~6 nm cluster ion sputtering, the C 1s spectrum aligned with pure CuPc, indicating minimal subsurface damage. Peak fitting revealed:

• Red component: aromatic ring carbon atoms.
• Blue component: carbons bonded to nitrogen in five-membered rings.
• Green loss features: aromatic π–π* transitions preserved by cluster sputtering.

Quantitative analysis yielded elemental atomic percentages (observed vs. expected):

• C: 78.6 at% (78.0 at% expected)
• N: 19.5 at% (19.5 at% expected)
• Cu: 1.9 at% (2.4 at% expected)

The combined profile showed consistent CuPc stoichiometry until the SiO₂ interface, then accurate oxide and silicon composition after switching to monatomic ions, demonstrating a seamless organometallic/inorganic depth profile.

Benefits and Practical Applications

This dual-mode ion approach enables:

• Damage-free chemical profiling of sensitive organic and organometallic layers.
• A single-instrument workflow for both soft and hard materials.
• Quantitative, layer-by-layer compositional mapping essential for device development, quality control and failure analysis.

Future Trends and Opportunities

Advancements may include:

• Optimized cluster sizes and energies for even gentler sputtering of biomolecular and polymeric films.
• Integration with in situ monitoring for real-time process control.
• Extension to multilayer organic/inorganic hybrid devices and emerging 2D materials.

Conclusion

The MAGCIS ion source on the Nexsa XPS platform successfully delivered a continuous, damage-free depth profile of a CuPc organic FET. Cluster sputtering preserved chemical state integrity in the organometallic layer, and monatomic ions efficiently profiled the SiO₂/Si substrate. This methodology enhances the analytical toolkit for organic electronics research and development.

References

No literature references cited in the original application note.