Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.
The overall goal of this procedure is to produce nanoscale thickness transmission electron microscopy specimens for studying combined electrical and mechanical load effects on material microstructures with negligible temperature rise. This method can help answer key questions in the electrically assisted deformation field regarding the role of current densities in increasing formability of metals by utilizing in-situ TEM observation. The main advantage of this technique is that the nanoscale thickness test section rejects heat to the bulkier supporting frame at a high enough rate to prevent significant temperature increase.
To begin the procedure, spin coda 180 micron thick silicon wafer with sufficient positive photoresist to form a 7.5 micron thick layer. Bake the wafer at 60 degrees Celsius for two minutes and then at 115 degrees Celsius for 90 seconds. Place a chroming glass mask over the coded wafer and expose the wafer to UV light.
Develop the pattern with the appropriate photoresist developer. Bond the patterned wafer to a 500 micron thick silicon support wafer with a low melting point temporary adhesive. Apply the adhesive uniformly to prevent excessive heating and etching damage to the patterned wafer.
Then etch through the patterned wafer with deep reactive ion etching using the Bosch process. Monitor the etching rate with a profilometer. When etching is complete, soak the etched wafer in acetone overnight to dissolve the temporary adhesive and the photoresist.
Then deposit two to three micrometers of silicon dioxide on both sides of the wafer by plasma enhanced chemical vapor deposition at 300 degrees Celsius. Under an optical microscope, use sharp tweezers to carefully detach the silicon frames from the wafer and remove any tabs. To begin preparing the specimen array, attach a five centimeter by five centimeter piece of 99.99%pure copper foil to a glass slide using PET or polyimide tape.
Spin coat both sides of the foil with a one micron layer of photoresist. Bake the photoresist at 115 degrees Celsius for two minutes to form a uniform coating protecting the copper sample from laser damage. Use a laser guided by a high-speed mirror galvanometer to cut a five by four array of specimens into the copper piece.
Briefly immerse the array in a 40 to 60 degree Celsius solution of 40%ferric chloride to remove damaged edges and reduced the widths of the specimen gauges to below 20 microns. Rinse the sample in deionized water. Then dissolve the protective layer of photoresist in successive baths of acetone, methanol, and isopropanol.
Dry the specimen arrays under a stream of nitrogen gas and store the arrays in a dry nitrogen desiccator. Use the laser to cut a box around the specimen array, releasing it from the rest of the copper film. Use scissors to cut a single metallic specimen from the array.
Place a small amount of silver epoxy on the silicon frame and carefully align the specimen with the specimen gauge spanning the narrow gap in the center of the frame. Once the specimen is correctly aligned, use conductive silver epoxy to attach silver wires 50 microns in diameter and 30 millimeters in length to each end of the specimen. Next, use successive passes of focused ion beam milling to cut multiple shoulders at high milling rates and then the gauge section to 100 nanometers by 10 microns by 10 microns at much lower milling rates.
Measure the cross sections with scanning electron microscopy. Then remove the exposed sides of the metallic specimen frame with focused ion beam milling, laser cutting, or mini scissors to finish preparing the MEMTS. To begin the microscopy experiments, under an optical microscope, mount the MEMTS on a single tilt straining TEM holder with both spaced by 0.5 millimeter thick non-conductive washers.
Use silver conductive epoxy to connect the silver wires to the TEM holder pins. Verify that the measured resistance is above 10 megaohms between each end of the MEMTS and the grounded TEM holder. Connect an external DC power supply to the electrical feed throughs in the TEM holder and load the MEMTS into the TEM.
Prepare the TEM to acquire images during the experiments. Apply tensile strain in small steps until the motion of one or more dislocations is observed. Allow the specimen to equilibrate under the strain for one minute before applying an input current density to the specimen.
After every change in mechanical or electrical loading, equilibrate the specimen under the electron beam for one minute and then acquire steady state TEM images of the specimen. A single crystal copper specimen was prepared and characterized using this method. Tensile strain was applied to the specimen until dislocation motions indicated that post yield equilibrium state had been reached.
Plane dislocations were monitored with bright-field images taken in the same camera orientation as was used to view the TEM diffraction pattern. Additional tensile strain was applied resulting in a new dislocation loop. This dislocation loop showed no significant changes after applying a current density of 500 amperes per millimeter squared.
Upon removing the current and further increasing the strain, changes were observed in the shape of the dislocation loop. Similar results have been observed with current densities of up to five kiloamperes per millimeter squared. After watching this video, you should have a good understanding of how to create and test not only copper EAD speciemns, but also specimens from other metals, polymers, and ceramics.
