Cryo-HIM-SIMS on the npSCOPE: Correlative Topographic, Transmitted and SIMS Imaging at Cryogenic Temperatures

Multimodal helium ion microscopy (HIM) and secondary ion mass spectrometry (SIMS) provide in situ spatial imaging and chemical analysis at nanoscale resolution. Operation at cryogenic temperatures is needed to overcome artifacts related to sample preparation and beam damage during analysis of beam-sensitive samples (including frozen-hydrated specimens). Here, we present a gas field ion source HIM, the "npSCOPE", that was retrofitted with a liquid nitrogen-cooled copper-shielded instrument chamber and a 5-axis cryo-sample stage. A dedicated liquid nitrogen-cooled ultrahigh vacuum sample transfer system paired with a humidity-controlled cryo-glovebox reducing contamination complements the setup for analysis of frozen-hydrated samples. Thanks to its three detection modalities - secondary electron (SE) imaging, chemical analysis based on an integrated double-focusing magnetic sector SIMS system and scanning transmission ion microscopy (STIM) - this prototype allows for correlative topographic, volumetric and chemical imaging under room temperature (RT) and cryogenic conditions. No significant ice buildup or edge effects were observed when monitoring samples over 36 h at < -139 °C. Cryo-SE imaging confirmed reduced beam-induced modification of beam-sensitive silica-coated gold nanoparticles, and cryo-STIM imaging on a polycrystalline gold foil showed a 34% average improved ion transmission signal intensity as compared to RT. The full cryo-transfer and analysis workflow was successfully tested on cryo-lamellae of plunge-frozen cells prepared by cryo-focused Gallium ion beam milling (cryo-FIB), providing proof-of-principle confirmation for correlative cryo-analysis at the nanoscale. In conclusion, correlative cryo-ion-beam imaging and analysis is a promising solution for nanoscale, multimodal investigations of both close-to-native biological and other beam-sensitive materials.