Typical ion polishing systems involve cross-section milling, where a sample is ion polished with the ion beam normal to a pre-cut and/or polished surface (e.g., Figure 1 A). Cross-section polishing is often used to prepare microelectronics, where microanalysis of the internal structure is of interest. However, cross-section polishing is spatially limited to only a few millimeters that can be ion polished. As such, it is not well-suited for investigation of geologic materials, where sufficient surface area must be prepared to provide the necessary petrologic context for geochemical investigations of heterogeneous materials. Moreover, sample mounts that are typical for geologic materials are often circular to accommodate sample mount holders in analytical instruments (e.g., EPMA, SEM, NanoSIMS, etc.) and larger than can be accommodated in cross-section polishing systems (10 – 25 mm diameter sample mounts).

Recent development of broad area flat milling capabilities has addressed the issues with cross-sectional polishing (Figure 1 B). Flat milling incorporates three additional variables:

• variation in the ion beam angle relative to the sample surface
• sample rotation
• sample offset relative to the axis of rotation (i.e., eccentricity)

With flat milling, the Ar ion beam is positioned at an angle (Θ), rather than normal to the sample surface; this defocuses the ion beam on the sample surface, permitting a larger area to be polished. The rotational component ensures that the sample is continuously rotated to achieve a more uniformly polished sample surface. Finally, the sample can be offset from the axis of rotation; increasing the area impacted by the ion beam, and therefore, the area of the sample that can be polished. By incorporating these three changes to the ion milling process, large areas (e.g., ~50 mm in diameter) can be uniformly polished. Broad area flat milling is therefore suitable for sample preparation of planetary materials.

The addition of cryogenic stage cooling with temperature control has also resulted in improvements of the ion milling systems that minimize sample damage. The cryogenic model of the Hitachi ArBlade 5000 (i.e., ArBlade5000C) provides active cooling of both the cross-section and flat area milling stages during ion polishing. Liquid nitrogen is supplied to the sample holders to remove heat from both the sample and the milling mask. Following ion milling, the specimen stage is incrementally warmed to prevent ice formation, water condensation, and sample fracturing. The addition of cryogenic cooling permits samples that are temperature sensitive to be prepared damage-free, a common problem for previous attempts to ion polish planetary materials. Finally, cryogenic cooling will be greatly beneficial to future sample return missions that require sample preparation at low temperatures to preserve important characteristics of delicate samples (e.g., volatiles and organics) and precious samples (e.g., Apollo 17 frozen basalts)

Figure 1: A) Cross-section milling setup. The sample is mounted in between the sample stage and an ion beam mask. Samples are typically small (e.g., 1 mm x 1 mm); however, the sample holder can be rotated to mill a larger area of 10 mm x 2 mm. B) Broad area flat milling setup. The sample stage can accommodate a sample up to 2” in diameter. The sample is mounted to the stage and the ion beam is positioned at an angle (e.g., theta) relative to the sample surface. Different ion beam angles can be used to achieve the desired effect (e.g., milling rate, exaggeration of surface topography, etc). An eccentricity offset (d) can be applied to increase the effective surface area of ion polishing. Changes in the eccentricity can also cause changes in the removal rate of material, where a large eccentricity results in a slower polishing rate and a larger polished area. No eccentricity offset (d=0) results in a focused ion beam on the sample surface and results in a narrow, polished area (e.g., 4 mm) and ~25 microns deep within 30 minutes, which can be used to selectively ion polish specific regions of interest. Typical conditions for geologic samples are θ = ~85 and d = 5 mm; these conditions result in a 25 mm area polished to a depth of 3 microns within several hours. The ion polishing protocol can be automated to intermittently polish the sample for 99 hours to increase the overall polished area and depth of polishing.

The ArBlade 5000C is a broad beam Ar ion polishing system with cryogenic cooling and air protection capabilities and has greatly improved capabilities over earlier cross-section ion polishing systems, which include:

• A hybrid milling system that incorporates flat milling capabilities to permit preparation of typical SIMS and EPMA mounts
• Cryogenic stage cooling temperature control that minimizes sample damage
• Air protection capabilities for sample preparation without exposure to atmosphere
• Increased ion gun milling rate
• In-situ monitoring of sample polishing

The NanoMill system uses an ultra-low energy, concentrated ion beam to produce high quality specimens free from amorphous and implanted layers. Ideal for polishing FIB-milled specimens for transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD).

NanoMill Specifications:

• Ultra-low-energy, argon gas ion source (50-2000 eV)
• Concentrated ion beam with scanning capabilities
• Secondary electron detector allows for real-time viewing of the specimen
• Removes damaged and implanted layers without redeposition
• Room temperature to cryogenically cooled stage (temperatures down to -170 °C)
• Precise stage angle adjustment (range -10° to +30°)
• Ideal for preparing specimens from heterogeneous materials
• Inert gas/vacuum transfer option

Model 1040 NanoMill®

• Cressington 208HR (Au, Pt, and Ir coatings)
• Cressington 208C Carbon Coater
• Plasma Cleaner
• Buehler VibroMet2
• Buehler Minimet 1000 Grinder-Polishers
• Buehler IsoMet 1000 Precision Saws
• Buehler EcoMet 30 Semi-Automatic Grinder and Polisher
• Buehler PetroThin Sectioning System
• Zeiss Stemi 508 Stereomicroscope
• Zeiss Axioscope 5 Petrographic Microscope
• 2 Ton ICL Press with In-situ, Heated Platens with Temperature Control
• Vacuum Ovens and Dessicators

Electron Microprobe - Cameca SX-100 | Electron Microprobe - JEOL 8530F | NanoSIMS 50L | Scanning Electron Microscopes | Transmission Electron Microscopes