Using Laser Scanning Microscopy to Determine Electromigration in Molybdenum Disilicide

The scope of my research is determining the electromigration phenomena in molybdenum disilicide and having a look at the influencing factors, such as the length of the line under test and the encapsulating material on the parameters effective ion charge and deactivation energy. The current experimental challenge lies in expanding this method to higher temperatures. Compared to other techniques, our protocol uses a laser scanning microscope. Other techniques usually use a scanning electron microscope. For measurements with the scanning electron microscope, you usually have a sample preparation that might influence the measured activation energy and the measured effective ion charge, so in our case, we don't need this elaborate sample preparation. This also makes it faster. We will focus on having an investigation of the effective ion charge in molybdenum disilicide at elevated temperatures, and also investigating the activation energy of molybdenum disilicide at elevated temperatures and undoped molybdenum disilicide doped with different dopant species, and we will also have a look at the changes of artificially generated voids in different materials.

[Instructor] To begin, switch on the laser scanning microscope and open the measurement and analytic software. Using an appropriate sample holder, secure the sample so it remains fixed on the microscope stage during scanning. Prepare an accurate current source and necessary wires for electrical connection and adjust the height of the microscope stage. Now, position the sample in the sample holder under the laser scanning microscope. Align the sample parallel to the table of the microscope and fix it in place to prevent any movement during measurements. Connect the electrical outlet of the current source to the sample or sample holder based on the setup. Confirm that the bond wires are still attached to the sample by optical inspection. Adjust the height difference between the objective lens and the sample to bring the region of interest into focus using the lowest magnification objective lens. Use manual focus or click auto focus in the observe window of the measurement software. Change the objective lens to a higher magnification and refocus on the region of interest. Continue this process until the region of interest is clearly visible at the highest magnification, such as 150X, in the observe window. Set tools, measure, and average count to four, then click options followed by auto save, select a save destination folder, enter a file name prefix and sample, and click okay. Open the measure window, select expert mode, and choose measurement settings followed by surface profile, super fine 2048 by 1536, and high accuracy. To increase the distance between the objective lens and the sample, click the upward arrows until the entire surface appears black in the window, then click set upper POS. Next, decrease the distance using downward arrows until the full surface is visible and continue until the surface turns black again, then click set lower POS. Click auto gain and then start measurement to begin scanning the surface. Increase the distance between the objective and sample by several millimeters up to one centimeter using the upward arrows to defocus the laser before stressing the sample. Apply current stress using the predetermined conditions, such as current density and time, then stop the current flow after the specified time. Three to five minutes after applying current stress, focus the laser scanning microscope on the region of interest when the sample returns to room temperature. Continue focusing until the sample no longer shifts its focus on its own to ensure there are no drifts in surface measurement due to temperature changes. Scan the same region that was scanned before the current stressing using the exact same settings as used earlier. Open the analytics software and click file and open, then locate the correct file. If the file is already open, proceed to correct the tilt of the samples after selecting process image and correct tilt to launch the tilt correction window. In the correction window, set the display image to laser plus optical and choose the correction method plane tilt three points to display three points on the image. Move the guiding line so that most of each line lies in the background, and adjust the three points close to the region of interest. Next, move the three points so that the plane represented by two straight lines in the cross sections aligns with the background. Select do not adjust offset height zero data and auto adjust height range, then click execute, followed by close to apply the corrections. To open the trimming window, click process image and trimming. Choose the trimming width and height according to the region of interest, and adjust the selection rectangle to encompass the entire region of interest. Save the corrected and trimmed image and click file and locate the correct file. To export the region of interest while preserving three-dimensional information, click file, followed by output 3D CAD data to open the output parameter window. Set skip amount to one, actual number display accuracy to 10, XY zoom ratio to X1, and enhance height to 100%, then, choose surface and click set to confirm the settings. Select the point group data to save uniquely labeled data. After the export is completed, a confirmation window will appear. Open the version of the evaluation software and packages. To start the program, click the arrow icon. Navigate to the folder containing the ASC files after clicking open and selecting the appropriate save path. Load the ASC files into the program with the correct sample name from the selection list. Ensure the area option is selected, then click cross, followed by area. Using the mouse, select a rectangle on the substrate surface to define the scale for height. Examine the two height histograms before and after the current stressing, positioned beside the region of interest image, and adjust the selection to ensure both histograms appear normally distributed and similar. Now, click the zero button labeled as background to set this height as the background level. Choose a second rectangle on a flat section on top of the line under test. Again, examine and adjust the histogram so they appear normally distributed and as similar as possible. Click line under test, then click okay to save this height value. Next, click the arrow icon again to rerun the program. Draw a rectangle near the rim of a single hillock, or void in the image labeled IMG compare using the left mouse button. Adjust the rectangle to closely match the rim of the structure using the zoomed image, such as the one labeled relaxed crop. Refine the selected region so that the rectangle precisely encompasses the hillock, or void. Finally, click the save button next to IMG compare to save the integral volume based on the pixel sum. Hillocks formed after current stressing showed heights typically around 190 nanometers, with the smallest clearly detectable hillocks at 34 nanometers, and lateral dimensions of approximately one micrometer. Electromigrated volume increased with the length of the line under test, as shown by the exponential trend line in the plot. Electromigrated volume increased with higher current density, and two different thicknesses of encapsulating high temperature Silicon oxide showed different onset points for electromigration. At a lower current density of 2.56 times 10 to the power of 10 amperes per square meter, usable data demonstrated an increasing trend of electromigrated volume with increasing line length.