Atomic Force Microscopy Observation and Characterization of a CD Stamper, Lycopodium Spores, and Step-Height Standard Diffraction Grating

Transcription

1 Atomic Force Microscopy Observation and Characterization of a CD Stamper, Lycopodium Spores, and Step-Height Standard Diffraction Grating Michael McMearty and Frit Miot Special Thanks to Brendan Cross

2 Activity Log April 17, 2013: 4:30 6:00 PM Learned how to use AFM software and practiced setting in tips and scanning samples April 22, 2013: 4:00 6:00 PM Performed practice runs on diffraction gratings Performed two runs on the 3 micron step-height standard diffraction grating Performed two runs on the 20 micron step-height standard diffraction grating April 26, 2013: 12:35 2:10 PM Scanned CD stamper Performed 50 micron by 50 micron scan of CD stamper Repeated scan at 128 scan lines (default 64) Performed 15 micron by 15 micron scan at 128 scan lines May 2, 2013: 2:10 5:45 PM Mounted and scanned Lycopodium spores Performed seven vibrating scans Performed one non-vibrating scan Note: All scans are non-vibrating unless otherwise indicated

3 Abstract In this lab, we attempted to measure microscale features of a step-height standard, of a CD stamper, and of Lycopodium spores. The step-height standard was used to calibrate our measurements. The CD stamper microstructure was evident when we scanned it, as was the structure of the step-height standard. However, we failed to get any meaningful data on the Lycopodium. With our corrected data, we measured the track pitch of a CD to be about 0.51 microns and the pit height to be microns. These values are opposed to our experimental diffraction measurement of pitch of 1.58 microns and the standard pit height of microns. There is a high likelihood that we messed up somewhere along the line in calculating correction constants. Additionally, we did not have the best sampling with which to correct our data. Introduction AFM (Atomic Force Microscopy) is a technique by which a nanometer scale tip on a cantilever traces across the surface of a sample. A laser beam is reflected off the cantilever so that small movements of the lever produce larger angular movements of the reflected laser on a photodetector. In this way, one can use AFM to determine the microscale structure of many objects. In our lab, we used the AFM to investigate the microscale structure of a step-height standard to calibrate our measurements. We also observed the pits on a CD stamper and attempted to observe the surface structure of Lycopodium spores. Description To properly carry out this experiment, it is imperative to have an Atomic Force Microscope (AFM) with an accompanying computer available since they are the sole equipments used in this lab. Due to extremely delicate parts, we had to be careful when mounting the cantilever and tip, and also when mounting a sample into the AFM. Once properly mounted, the device utilizes an extremely fine tip that reacts to atomic scale forces to detect minute disturbances in the topography of the sample. By scanning back and forth for multiple lines, it is able to render something akin to a topographical map of the sample by combining the lines into a single image. We then analyze this image to collect various data about the features that were scanned by the AFM. Image Credit: Wikimedia Foundation

4 We utilized the AFM belonging to the Andrews University Physics Department. This AFM was built by a faculty member at an AFM workshop. It is contained within a latching box to eliminate outside air currents from disrupting scans, and is mounted on a platform suspended from the top of the box by bungee cords to attenuate unwanted vibrations, seismic or otherwise. A camera attached to the computer is utilized for setup and to ensure the sample is in place. We used LabVIEW based software, bundled in with the AFM by AFM Workshop, to collect our data, and used the open-source SPM analysis program Gwyddion to analyze our data. We had step-height standard and CD stamper samples in stock that were already mounted. To mount the Lycopodium, we used double sided tape, with one side adhering to a flat metal cylinder, and the other securing the Lycopodium spores in place. To set up the AFM, several steps are necessary. The first is to carefully put into a tip-holding module a scanning tip using tweezers and a platform that aids placing the tip into the module. The next step is to adjust the direction in which the laser beam is pointing so that it reflects off the back of the tip. This is needed so that small oscillations of the tip will produce much larger oscillations in the path the reflected laser traces out on the photodiode detector. It is then necessary to center the detector so it will pick up the reflected laser beam. Then the tip is manually lowered until it is within half a centimeter of the sample. Following this, one must switch to the software to perform an automated tip approach. This cannot be done manually, because the adjustments are so precise that a manual approach would inevitably break the scanning tip. If the tip solidly contacts the sample, we proceed to scan. Otherwise, if its position drifts significantly, we will retract the tip, and try again for a better contact. Drift usually indicates that a thin layer of air is in between the sample and the tip, which can be a cause of poor data. Once solid contact is made, a scan is begun. Once the scan is completed, the data is saved to be analyzed in Gwyddion.

5 Data & Calculations Calibration and Correction To correct our data, we calibrated our measurements using a step height standard using a square grid with 3 µm pitch and 0.1 µm step height. X-Y Calibration Width Ratio: : 3 = : 1

6 Z Calibration Height Ratio: : 1 Data and Corrections Data

7 Data leveled by mean plane subtraction Error Stamper Measurements Profile used:

8 Width: Mean track pitch: 2.39 µm Corrected track pitch: 2.39 µm / = 0.51 µm Using a CD and a laser, we measured the track pitch of a CD to be 1.58 µm, according to the instructions in the Phys. for Sci. II diffraction lab. 1 Height: Measured pit height: µm Corrected pit height: / = µm Industry Standard: µm 2

9 Lycopodium Spores Data We performed seven runs and none produced any recognizable pattern or detail, as one can observe below:

10 7 After further research, we discovered Lycopodium spores have a diameter of about 30 µm, whereas the sampling area of our AFM was 15 x 15 (µm)^2. Error Analysis To analyze our data, we used the open-source program Gwyddion, an analysis tool for scanning probe microscopy (SPM) techniques such as AFM. Unfortunately, the graphical nature of our analysis does not lend itself to traditional techniques of calculating propagated error. Our track pitch as measured by the AFM (0.51 µm) was less than one third the track pitch determined by our diffraction measurement (1.58 µm). This is an unreasonable measurement. Further research is needed to investigate the potential causes of this discrepancy. The first cause that comes to mind is the calibration. One difficulty with our calibration was that it was not obvious where to mark the pitch, as the edge of the upper pit from our calibration data was on the edge of the image. A scan of the same 3 micron grid with obvious places to mark and measure the width of the pits would be the first step towards correct calibration. Additionally, our pit height was measured to be nm, whereas the standard is 100 nm. This second measurement is much more reasonable, however, a rigorous calibration of the AFM itself is necessary to determine the correct height. Additionally, in AFM, the tip may not reach the bottom of narrow pits, and may overshoot the top of ridges, lending an inherent uncertainty to our data. 3 We would have to determine the dimensions of the tip itself in relation to the track pitch to determine whether this would actually be a problem. Another potential source of unwanted error could be warping of the sample. In the picture below, note that the stamper bulges near the middle. This is not too big of a problem, but makes analysis slightly more inconvenient, and can be potentially corrected.

11 For the Lycopodium spores, there may have been excess spores left on the tape that remained after the sample was tapped on the table. For the non-vibrating scans, one of the loose spores may have been caught on the tip and dragged along, resulting in useless data. Using a can of compressed air, such as the ones used to clean keyboards, to blow away excess grains may have solved this problem. Another problem is that the sample window of the AFM is about one quarter the size of a Lycopodium grain. So unless one is lucky enough to have the tip land directly on a spore, no detail will be observed.

12 Conclusion The primary goal of this lab was to gain experience working with an AFM by measuring a variety of sources. We measured a step-height standard to get initial experience, and later returned to that data to correct our CD stamper data. We measured a CD stamper under the AFM as well as by diffraction. Our diffraction value of 1.58 microns is close to the accepted value of 1.6 microns. However our AFM measurement was 0.51 microns. This was most likely due to faulty correction/calibration. Additionally, our calibration measurement was less than ideal, making it difficult to get an accurate correction. Our Lycopodium scans produced no meaningful results. For further research, we could rigorously calibrate the AFM using the step-height standard. Additionally, compressed air could be used to eliminate excess Lycopodium grains. Improved mounting techniques would probably be necessary. References