The International Space Station (ISS) will provide the ability to synthesize new materials and to manufacture new products in space. For example, semiconductor crystals and protein crystals, which have important applications to computer technology and biotechnology respectively, can be grown to a greater degree of perfection in microgravity than on earth. Repeated experiments since sky lab and subsequent shuttle missions have demonstrated this.
The Space Station will provide a platform that can offer months or even years of microgravity conditions at a time. Among the earliest commercial products to be manufactured on the ISS will be high-purity semiconductor crystals and large protein crystals with enhanced crystallinity.
In order to support a crystal-manufacturing platform in space, it will be necessary to have on the ISS certain instruments that can be used to characterize samples while still in orbit. An ideal instrument for this application would be one that can resolve the structure of crystals down to the atomic scale, is small in size, has low power requirements, and is operationally clean. Such an instrument can be developed in the form of a Scanning Probe Microscope (SPM) for the microgravity environment, or a microgravity SPM system.
Scanning Probe Microscopy is a general term used to describe a type of microscopy in which a local probe is raster scanned over a sample of interest. As the probe is scanned across the sample, a local interaction between the probe and sample is measured. Scanning Probe Microscopes (SPMs) include the Scanning Tunneling Microscope (STM), the Atomic Force Microscope (AFM) and many other types that have been developed from these earlier predecessors.
The ISS is being assembled with various experimental apparatus and instrumentation in mind; however, a scanning Probe Microscope (SPM) has not been intended. It is an ideal instrument for materials processing in space where direct atomic resolution can be obtained of crystalline materials as they form instead of returning them to Earth for analysis.
SPMs are generally very sensitive to mechanical and acoustic noise, especially when imaging at the atomic scale. This usually requires some degree of vibration isolation. The instrument can be isolated from acoustic vibration by placing it under a vacuum. The mechanical vibrations usually require either an elaborate passive vibration isolation system or a sophisticated electronic active vibration isolation system.
Due to the scientific benefits of installing an SPM on the ISS, an experiment has been devised to determine the effectiveness of such an instrument in reduced gravity. In the experiment, an AFM will be mounted to an active vibration isolation system, which will minimize mechanical resonance. A vacuum will be produced around the instrument, which will eliminate acoustic noise. Mounting a hollow acrylic cylinder directly to the vibration isolation unit and evacuating the air inside the chamber will accomplish this.
The AFM requires minor adjustments during operation; therefore, radio controlled servos will be used to adjust certain controls remotely. This will allow for the device to be adjusted without any direct contact from the operations team, eliminating outside vibration. When reduced gravity is achieved, the operations team will attempt to image samples at the micrometer range (1 micron gold grating) and at the atomic scale (2 nm corrugation of Highly Oriented Pyrolytic Graphite (HOPG) or 5nm corrugation of a mica sample). The force curves corresponding to the images will be recorded along with the accelerations. The force curves and images will be acquired according to preset configurations. These microgravity data will then be compared to the results of similar experiments performed on the surface of the Earth.
The preflight included loading the substrate disk with pre-mounted samples onto the sample cylinder in the AFM.
1) The AFM head was placed over the sample and secured with heavy-duty springs. The microscope cantilever was then lowered over the sample by screwing the height adjusting screws in the counterclockwise direction. Initially, the distance between the cantilever and the sample was approximately 0.5mm. Due to the small sizes and distances involved with this process, a monocular, that filters laser light, was used to make final manual adjustments of the microscope.
2) The AFM’s internal laser (which is rigidly enclosed in solid aluminum) was aligned over the cantilever. This is shown in Figure 1. The extremely narrow laser beam was carefully positioned over the tip of the triangular cantilever. Due to the shape of the cantilever, and the shape of the surface of the sample, the Law of Reflection guarantees us the laser cannot be reflected anywhere, except on the photoreceptors, in the AFM head housing.
Figure 1: AFM Head showing Laser Diode/Detector layout
3) We positioned the ellipse shaped reflection of the laser beam onto the photoreceptors, until a voltage of 6.1V or higher, was reached.
4) Once this minimum voltage was reached, we placed the top portion of the acrylic vacuum chamber over the AFM. The AFM was already mounted securely to the bottom plate and secured with 1/4” bolts.
5) A vacuum was pulled with our new Robinair brand Vacuum pump. The vacuum pump was turned on for 1 minute, until a pressure of 2 Torr was reached. The vacuum pump was NOT used again until the next flight.
6) We then turn on both computers, both monitors, and the radio transmitter.
7) We then loaded the Nanoscope II program. All parameters were set to a custom default setting.
8) We then used channel 1 of the transmitter to control a servo that adjusts the AFM’s set-point voltage. This was set to –2.00V± 0.1V. After this was set, the transmitter was turned off. We used only the computer to control the AFM from then on.
9) The cantilever was engaged and we set all parameters until we successfully acquire an image of the gold grating.
10) Once a successful image was acquired, we withdrew and disengaged the microscope. This was accomplished by hitting the withdraw key seven times. Then, Everything was powered down.
11) All components were double checked to make sure they were securely mounted to the cart.
12) We were now ready to load our cart onto the KC-135.
The first flight operations consisted of attempts to image pre-mounted samples of a Gold Grating with the Atomic Force Microscope. All initial preparations were conducted on the ground, prior to flight according to the preflight section. Each configuration required approximately twenty seconds to twenty-three seconds of zero gravity to test. We wished to test the thirty-eight different configurations to see which ones were successful, relative to the ones that were tested in a 1-G environment and the impact varying accelerations and resonating vibrations have on the cantilever.
Our Procedures for the first flight was as follows:
1) After having reached our designated altitude in the KC-135, we boot up our two computers, vibration isolation system, accelerometer, and the radio transmitter.
2) The computer program was started and the parameters that were set on the ground, during preflight, were loaded up.
3) The set-point voltage was set again by using channel 1 of the transmitter.
4) The transmitter was turned off once the proper voltages were set.
5) The computer controlled all functions of the microscope thereafter.
6) We then engaged the microscope using the computer.
7) After the microscope was engaged, the force curve was examined.
Our goals for the first flight were as follows:
1) Successfully acquire images of the Gold Grating and the Force Curve on 38 parabolas, each with different, preset configurations.
2) Attempt to discover new configurations for the AFM system in zero-g.
3) Determine the effects of resonating vibration on the AFM.
4) Determine the effects of varying acceleration on the AFM, and what the best configurations are for counteracting these external forces.
Sample change out during flight was eliminated from this procedure since we were unsuccessful in obtaining an image of the mica sample even in the Hangar. Also, after the Test Readiness Review, it was concluded that a sample change out would be too difficult to do in flight.
During the second flight, the operations of the microscope were similar to the first flights operations. The preflight operations were identical.
Our Procedures for the second flight were as follows:
1) In the second flight we were looking at higher resolution images of the Gold Grating
Our goals for the second flight were as follows:
1) Successfully acquire images of the Gold Grating with higher resolution (resolve the spheroids in between the rulings).
2) Formulate a new procedure, basically an “operators manual”, for operation in a zero gravity environment for the entire Microgravity Microscope System, which includes: the AFM, vibration isolation system, our custom vacuum chamber system, radio control system, and all data acquisition tools.
The team acquired two types of data. Acceleration data of the frame during flight and image data from the AFM during zero-g. Our data collection was taken only during the zero g portions of the parabolas. Figure 2 shows the entire experimental assembly mounted in the KC-135 prior to take-off. It was fortunate that we were able to pump down the chamber and perform a test run prior to take off.
During the first flight, it was discovered that the AFM could remain engaged and imaging throughout the entire flight. This allowed us to eliminated the need to disengage and re-engage in between parabolas. This was not planned in the procedure and was discovered only in flight. This facilitated the data acquisition since it eliminated the need for another procedure. Then, when a good image was obtained during zero-g, the data could easily be saved to a buffer. The acceleration data showed that the accelerations of the plane were very nearly what they should have been. The zero-g portions were right around free fall at –10 m/s2 ± 2m/s2.
Figure 6 shows a typical image from flight #1. The grid pattern is clearly resolved, despite the pronounced noise in the vertical scan. The grid pattern is a 1mm x 1mm square pattern. However, the individual gold spheroids were not clearly resolvable in any of the data from this flight#1.
Figure 7 shows a typical image from Flight#2. This image is a slightly higher magnification (smaller scan range) than the image from Flight#1. It is clear once again that the grid pattern is well resolved, and the noise level appears to be less severe. It is not clear why this would be, but a number of reasons are possible, including just lower turbulence on that day. Closer analysis reveals the individual gold spheroids being resolved. These are shown in Figures 8 & 9. The sizes of the spheroids are on average @ (40-80)nm across. This gives us a rough measure of the resolution limits of the microscope during this operation.
Although the noise is less severe in the second flight#2, it is still measurable. The noise appears to be very periodic and could be analyzed by performing a Fast Fourier Transform to isolate the frequency. Figure 10 shows the results of passing only the periodic noise data. This revealed a definite periodicity in the noise with a spectral period of @ 12nm. Based upon the microscope settings that we were using during Flight #2, we estimated the frequency of the noise to be @ 652Hz. This frequency is getting close to the usual resonant frequencies of the microscope (~ kHz). Once the International Space Station is completed, high frequency oscillations should not be as much of a problem due to its greater mass, and therefore, lower resonant frequencies.
Figures 11-14 show image data of the same sample and under similar conditions, but obtained in the lab. The greatest difference between the plane data and the lab data was the noise. The lab data being much less noisy. This is not surprising and is consistent with our expectations. The image data obtained from the flights did exceed our expectations, since any image at all in such an environment would have been significant. To the best of our knowledge, this is the first test done ever of such an instrument in an unfriendly environment. (i.e. not a lab environment).
In closing, we can see that the resolution limit of the lab image data is only slightly better than that of Flight#2. (32.60nm for the lab and 52.2nm for the Flight #2 data) both imaging the gold spheroids.)
Experimental Setup on the KC-135
Figure 2 : Final assembly installed in the KC-135 prior to take off.
Image Data: Flight #1. All images are unfiltered without any image processing.
Figure 6: Image data from Flight #1. The image shows the high frequency noise oscillation in the vertical direction but clearly shows the grid pattern being resolved.
Image Data: Flight #2. All images are unfiltered without any image processing.
Figure 7: Image of Gold calibration grating obtained from Flight #2. Notice that the gold spheroids can also be resolved. (unfiltered data)
Figure 8: Image of Gold spheroids obtained from Flight #2, showing resolution of the gold spheroids. (unfiltered data)
Figure 9: Cross sectional analysis of Gold spheroids on Gold Calibration Grating. A small feature measures at 52.16nm. (unfiltered data)
Image data is unfiltered and a periodic noise feature along the vertical axis can also be seen. However, in spite of the high frequency noise, the spheroids could still be resolved. This demonstrates the feasibility of such an instrument for space station work.
Figure 10: Noise analysis from Figure 9 data. (Flight#2) (unfiltered data)
Analysis of the noise data shows a high frequency oscillation in one direction (vertical scale). It has a spatial period of 12 nm. This is believed to be due to some resonance of the cantilever due to the high frequency vibrations of the microscope itself. These high frequency vibrations should be much less severe on a more massive platform like that of a building or the ISS.
The modulation of this noise feature at 12 nm can be related to an oscillation frequency by the following:
f = frequency
scan rate = 24.06Hz
Dx = 325 nm (scan range)
L = 12 nm (Modulation wavelength)
This is close to the resonant frequencies of scanning probe microscope (~kHz), which may explain pronounced appearance of this noise in the image.
Figure 11: Image of Gold calibration grating obtained in the lab. Notice the noise free image quality.
Figure 12: Cross sectional analysis of Gold Calibration Grating
obtained in the Lab. (unfiltered data)
Figure 13: Image of Gold spheroids obtained in the lab, showing resolution
of the gold spheroids. Note the noise free image quality.
Figure 14: Cross sectional analysis of Gold spheroids on Gold Calibration
Grating. A small feature is measures at 32.60nm.
This KC-135 experiment has successfully demonstrated the operation of a scanning probe microscope in a reduced gravity, vibrationally intensive environment.
The gold calibration grating was imaged during both flights. Higher magnification showed resolution of the smaller gold spheroids. Typical resolution of spheroid sizes is in the range (50-60) nm. These could be measured in spite of the noisy environment.
Due to intensive vibration noise, the interaction of the cantilever with the sample was too large to get a proper force curve, but this did not prevent the microscope from successfully imaging the grid pattern or the spheroids.
Since the ISS will be much more massive than the KC-135, and the noise environment is expected to be less severe, the results of this project show that the chances of successful operation of such an instrument onboard the International Space Station to support scientific research is very good indeed.
Future work may include further flights of related instruments on the KC-135, such as the Scanning Tunneling microscope, to test the atomic resolution limits in such an environment.
Although the active vibration isolation table had minimal effect in isolating the instrument from the large amplitude vibrations encountered on the plane, the vacuum was very effective in isolating the instrument from acoustic noise. It should be possible with a better-designed vibration isolation system, to completely isolate the instrument, or to develop a free-float experiment that would completely alleviate the mechanical vibration isolation problem. Nevertheless, the results from this first test of such an instrument outside the traditional laboratory environment are very encouraging.
As far as we know, this was the first test of its kind in considering the application of Scanning Probe Microscopes to Space Station science.
The following presentation was presented by Steve Parker to Humble High School.
The following presentation was presented by Steve Parker and Ryan Williams at the Texas SPS meeting,
Texas Christian University, Oct. 4-6, 2001.
The following presentation was presented at our seminar at Stephen F. Austin State University. These seminars are open to the public.
The following article appeared in the Dailey Sentinel, which covered the event.