New FIB/SEM micro-heating stage permits nanoscale imaging and analysis of dynamic processes at temperatures up to 1200°C

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Libor Novák, Min Wu, Petr Wandrol, Tomáš Vystavěl
Thermo Fisher Scientific, Vlastimila Pecha 12, Brno, 627 00, Czech Republic
 
Biography
Libor Novák works as a scientist in the Advanced Technology team at Thermo Fisher Scientific in Brno, Czech Republic. He received his PhD in theoretical physics and astrophysics from the Masaryk University in Brno in 2011. His main research topics are focused on signal formation and processing in charged particle devices. Since 2012 he leads the development of technologies and solutions for in-situ electron and ion microscopy.  
 
Abstract
Heating stages in electron microscopes allow investigators to observe dynamic processes, such as phase transformations, recrystallization and oxidation/reduction reactions at the micro to nanometer scales. Conventional heating stages have a large thermal mass which limits maximum temperature and ramping speed, introduces instability in the stage, causes the sample to drift in the field of view and can influence or damage nearby accessories. A new micro heating stage addresses these limitations for samples up to 100 micrometers in size. The heated volume is so small it can achieve temperatures as high as 1200°C at rates up to 104 °C/s with stability and without sample drift. It does not transfer significant heat to surrounding apparatus and is specifically designed to permit the use of imaging and analytical detectors, including X-ray spectroscopy for elemental analysis and electron backscatter diffraction for crystallographic analysis. Fine particles and powders may be analyzed directly. In a DualBeam (focused ion beam/scanning electron microscope – FIB/SEM) system, an automated workflow allows site-specific microsamples to be cut from a bulk specimen with the focused ion beam and transferred to the heating stage with a micromanipulator for immediate imaging and analysis.
 
Corresponding author
Dr. Libor Novák
Materials & Structural Analysis
Thermo Fisher Scientific
Vlastimila Pecha 12, Brno 
627 00, Czech Republic
Telephone: +420513246243
 
Stage description
 
A scanning electron microscope (SEM) is used to look at very small objects. Most SEM images and data are acquired from fields of view smaller than a hundred micrometers. Why must a heating stage accommodate a larger sample? It may be convenient to be able to look at samples large enough to hold easily in your fingers or a pair of tweezers, but the disadvantages of a heating stage large enough to handle such samples arguably outweigh the convenience advantage. Larger samples/stages consume (and must dissipate) much more power. They are limited in their ability to attain high temperatures rapidly, with typical ramping speeds of a few degrees per second. Thermal expansion of stage parts and the sample itself can introduce significant sample drift. The heat and radiation they generate can damage detectors and interfere with signal detection.
 
The new micro-heating module (µHeater, Thermo Fisher Scientific) is an innovative microelectricalmechanical system (MEMS)-based module designed to heat samples smaller than 100 µm quickly and precisely to temperatures as high as 1200°C1. It is compatible with both high-vacuum and low-vacuum operating modes. Its low thermal mass allows for temperature ramps as high as 104 °C/s, reaching 1200°C and settling to within 1°C in just 100 ms. High ramp speeds enable a whole new class of experiments. The uniformly heated area of the MEMS heating chip is 100 μm in diameter, readily accepting samples up to the recommended maximum dimensions of 50μm x 50μm x 50μm.
 
Transparent silicon nitride grids provide excellent support and limit reaction with the sample while also supporting SEM and scanning transmission electron microscope (STEM) imaging. The device is specifically designed to preserve high-resolution imaging performance at high temperatures (Figure 1) and to provide full access to a wide range of imaging and analytical signals. Its low thermal radiation enables the use of any available detector, including secondary electron (SE), backscattered electron (BSE), transmitted electrons (STEM), energy dispersive X-ray Spectroscopy (EDS) and electron backscatter diffraction (EBSD) analysis at high temperatures. Until now EDS and EBSD have been difficult or impossible with larger heating stages due to excessive thermal radiation. 
 
Figure 1. SBA-15 nanoparticles on µHeater. The nanoparticles were heated from room temperature to 1200°C at a heating rate of 1000°C/s. SE images were taken at room temperature and at 1200°C. The images demonstrate the high-resolution imaging capability of the system, even at elevated temperatures.
 
The heater module is mounted on a multi-purpose sample holder (MPH) that enables 360° endless rotation and maintains full utilization of all common capabilities of a FIB/SEM stage including tilting to more than 70°2. The heater module also allows electrical measurements. Contact pads located on the MEMS chip permit the application of four independent electrical biases and allow resistivity measurement of the heated sample with a four-point probe method.
 
The module may be controlled manually or programmed for automatic operation and is fully integrated with the microscope user interface. It provides direct read-out of the temperature value during use as well as recording of experimental data (heater power, temperature and sample resistance). Software control permits user definition of custom ramp/soak profiles to easily manage an experiment or process simulation. The module provides unprecedented mechanical, thermal, and electrical stability during in situ experiments.
 
Particles and powders
 
Particles and powders may be deposited directly on the heater module for imaging and analysis. The typical procedure is to dilute the powder in a liquid (ethanol for example) followed by placing a droplet onto the active area of the heater. Once the droplet dries out, the chamber can then be pumped down to start the experiment. Multiple imaging and analysis methods described above can then be used to characterize particles and powders at high temperatures. An example of EDS mapping in Figure 2 shows the development of an iron rich phase (magenta) while heating a mixture of magnetite and hematite nanoparticles. A lack of oxygen (yellow) in the iron phase is clearly noticeable. 
 
Figure 2. A mixture of magnetite and hematite particles at room temperature (top row) and at 1200°C (bottom row). Columns left to right – SE, SE with oxygen EDS overlay, SE with iron EDS overlay. Scale bar size is 1 µm.
 
Bulk specimens - in situ sample preparation
 
Using the heater module in a DualBeam system equipped with a gas injection system (GIS) and micromanipulator, the platform provides a unique in situ sample preparation capability, extracting a micro-sample from a larger specimen and transferring it to the heating module within the sample chamber (Figure 3). The operator selects the area of interest and uses the FIB to cut a chunk of material from the bulk specimen, leaving it connected at only one edge. The micromanipulator needle is moved into position and attached to the chunk using a beam-induced deposition in the presence of a GIS needle. A final FIB cut frees the chunk from the bulk. Once freed, the operator can then use the FIB to shape the sample as desired and clean the surface to remove any milling residue or surface damage. The chunk is transferred to the heater module, fixed in position with beam induced deposition, and cut free from the micromanipulator. The same technique can be used to create and position a thin lamella in the heater for in-situ STEM imaging in the DualBeam using a detector located below the stage.
 
Figure 3. The Ti6Al4V chunk sample (a) fabricated in bulk and in-situ lifted out with micromanipulator, (b) surface polished and (c) attached to a MEMS heating chip. The chunk surface is 50 µm x 40 µm.
 
Such an approach makes it possible to obtain unique results that were not accessible before as shown by the in situ EBSD characterizations on deformed dual phase (α+β) Ti-6Al-4V alloy. The sample was analysed during heat treatment using the micro heating stage in a Thermo Scientific DualBeam Helios G4 equipped with a standard EBSD detector. The metallic chunk of 50μm x 40μm x 20μm size was first heated at a rate of 1000°C/s to 850°C and subsequently went through a cycling heating and cooling process between 850°C and 1200°C using identical heating rate. In situ EBSD maps were acquired at room temperature and during each isothermal annealing step. The average hit rate (indexing percentage) for the maps are above 95% and did not decrease during the heating. EBSD patterns obtained at room temperature and 1200°C prove that high quality patterns can be collected even at elevated temperatures on the micro heating stage attributed to the small thermal mass of this miniature heating device (see Figure 4). Real time microstructure evolution such as dynamic recrystallisation, grain boundary migration (Figure 5) and phase transformation (Ti α‹-›β) (Figure 6) was observed during the continuous heat treatment sequence. All analytical work was conducted at 20 kV and 11 nA. 
 
Figure 4. Electron backscatter diffraction patterns (EBSP) of (a) Ti-α phase (hcp) at room temperature and (b) at 1200°C.
 
 Key benefits
Rapid and precise heating in a high-vacuum environment. Fast heating of materials to 1200°C in 100 milliseconds.
EDS and EBSD analysis at >1000°C temperature.
Low thermal radiation.
Stable solution for in situ micrometer to nanometer scale imaging – developed specifically to maintain highest imaging performance at elevated temperatures.
Uniform temperature distribution - the MEMS-based design delivers consistent, reproducible, and uniform temperature distribution over the heated area.
Full integration with microscope control software (Thermo Scientific DualBeams and SEMs).
 
Figure 5. Ti-6Al-4V alloy sample was heated gradually from room temperature to 1200°C then cooled down to 850°C with various isothermal steps at a heating rate of 1000°C/s. In situ EBSD inverse pole figure z (IPFZ) maps were collected at room temperature and at each isothermal step during the heating and cooling cycle. Sequential original IPFZ maps show a distinct recrystallization and grain growth (grain boundary migration) process at elevated temperatures. Scale bar in each map is 20µm. Each map was acquired in approximately 7 minutes.
 
Figure 6. Ti-6Al-4V alloy sample was heated gradually from room temperature to 1200°C with various isothermal steps at a heating rate of 1000°C/s. The overlaid phase maps with band contrast maps acquired at room temperature, 900°C, 1000°C and 1200°C show a transformation cycle from Ti α(hcp) to Ti β(bcc) to Ti α(hcp) in the in situ heating sequence. The scale bar in each map is 20µm. Each map was collected in approximately 7 minutes.
 
Conclusion
 
A new micro-heater stage module permits high-resolution imaging and analysis at temperatures as high as 1200°C. Its ability to operate in high- or low-vacuum conditions allows experimental observations of dynamic processes, including recrystallisation, phase transformation, oxidation and reduction, over a range of pressures and gas environments. Its low thermal mass permits use of EDS and EBSD detectors to obtain chemical and crystallographic information. High ramping speeds enable an entirely new class of experiments. In situ preparation provides for extraction of micro-samples from bulk specimens and ensures pristine sample condition.
 
References
 
1. L Mele et al, Microsc. Res. Tech. 79 (2016), 239-250.
2. L Novák et al, Microsc. Microanal. 22 (Suppl. S3) (2016), 184-185.
 
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