Article

Structural Assessment of (Sub-)Monolayer Coatings in Device Processing at High Spatial Resolving Power by TOF-SIMS Tandem MS Imaging

Surface Analysis Spotlight: TOF-SIMS

by Jacob Schmidt Staff Scientist

Surface characterization plays a pivotal role in the manufacturing process of next-generation electronic and heterogeneous devices. Characterizing the interfaces between different surfaces is especially challenging, primarily due to a lack of background information and the absence of reference spectra. This poses a significant challenge in supporting the analytical observations of emerging properties at these interfaces. Two case studies are presented here where there was little prior knowledge of the observed properties, but detailed answers were desired. In the first case, a carbon residue was observed by Auger electron spectroscopy (AES) imaging of etched e- beam-patterned structures, but the source of the carbon was indeterminate. In the second case, a bi-functionalized monolayer film was ostensibly loaded with catalytically active metals to form the metal-organic ligands, but previous methods to characterize the film fell short in verifying the structure of the metal-organic ligands.

In this review, a general time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging method for unknown identification and structure elucidation is demonstrated. This method makes use of a TOF-TOF spectrometer designed for simultaneous TOF-SIMS (MS1) imaging and tandem MS (MS2) imaging^1-4. Ions for TOF-SIMS tandem MS imaging were produced by a Bi³⁺ primary ion nanoprobe whereby MS1 and MS2 spectral data were generated from the same pixel in one duty cycle at a resolving power of 300 nm. Ion fluences were within the static limit (i.e. < 10¹³ Bi³⁺/cm2) for nondestructive analysis, even where multiple analyses were conducted at the same position of the sample. Tandem MS peak attributions were made to a calculated mass accuracy (Δm/z) of ≈ 1 ppm which provides confidence in the accuracy of the molecular identifications.

In the first example, TOF-SIMS tandem MS imaging was applied to characterize the carbon-containing residues of the lithography-patterned and etched structures. Several sets of peaks were observed at m/z > 150 in the MS1 spectra. It was established and verified by tandem MS imaging that the carbon observed by AES imaging on the patterned sidewalls arose from two fatty acid (FA) contaminants; specifically, the [M-H]^- ions of FA(12:0) and FA(16:0) were identified. Further, a ≈ 2:1 ratio of FA(12:0) to FA(16:0) was calculated. The high m/z peaks, detected only on the Cu-plated surfaces, were attributed to metal-organic complexes of Cu with the fatty acids. In the positive ion polarity, FACu²⁺ and FA₂Cu³⁺ ions were identified; in the negative ion polarity, FA^- and FA₂Cu^- ions were identified. In each case, a natural Cu isotope composition was confirmed.

Figure 1 – Tandem MS spectra of the FA(12:0) molecular and copper complexed ions. On the top, CID fragmentation of the fatty acid was used for identification by observing the repeated loss of CH₂. On the bottom, CID fragmentation of the high mass peak at m/z 461 reveals the presence of a fatty acid dimer and copper atom. 

In the second example, TOF-SIMS tandem MS imaging was also employed to confirm the presence and elucidate the structure of metal-organic ligands of Au, Pd or Pt atoms attached to bipyridine or triphenylphosphine within patterned monolayer films. Metal atom loading was achieved after monolayer immobilization⁵. The pattern areas were readily observed based on the atomic metal ion signatures. There were a number of MS1 precursor ions potentially related to the metal-organic ligands based on the m/z values within the anticipated range for the target structures. The 2D distribution and structure of complexed bipyridine and triphenylphosphine ligands, together with observing the natural isotope distribution of the metals, was achieved by tandem MS imaging⁶.

Figure 2 – Tandem MS spectrum of a selected region of interest of the bipyridine patterned monolayer film. Isotopic analysis was used to confirm the presence of Pt and Cl within the metal-ligand complex.

Gain deeper insights into TOF-SIMS tandem MS imaging by joining PHI at the Asia Pacific Microscopy Congress, held February 2-7 in Brisbane Australia, where Dr. Jacob Schmidt will present the information at the poster session.

 

References:

[1] G.L. Fisher, et al, Anal. Chem. 88 (2016) 6433-6440.

[2] G.L. Fisher, et al, Microscop. Microanal. 23 (2017) 843-848.

[3] C.E. Chini, et al, Biointerphases 13 (2018) 03B409.

[4] T. Fu, et al, Nature Sci. Rep. (2018) accepted 06 December 2018.

[5] R. Müller, et al, Chemistry (2018) DOI: 10.1002/chem.201803966.

[6] R. Müller and A. Welle at Karlsruhe Institute of Technology (KIT) and C. Barner-Kowollik at Queensland University of Technology (QUB) are acknowledged for providing the samples for analysis.