| Through the use of a Quartz Crystal Microbalance (QCM), we measure the differential sputter yield profile of a material over a hemisphere above the target. The QCM allows us to record sputter yield as a function of angle over the target from +90° to -90° with respect to the target normal. These individual data points are then collected and fitted, typically with a modified Zhang Equation, which can then be integrated to find an overall total yield. The QCM apparatus resides in a 0.125 m2 vacuum chamber attached to a Cryo-Torr cryogenic pump. The picture below shows the QCM system setup. | |||
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When a high energy particle such as an ion strikes the surface of another material, the particle starts a "collision cascade". The impacting particle strikes another particle, which in turn strikes another particle, and so forth. These collision cascades may be as short as one or two additional particle impacts, or can be lengthy and have many branches as one particle strikes not one but two or three others. When one branch of this collision cascade reaches the surface of the material, an atom can be ejected from the surface to float freely around in space (or air). This particle ejection is called "sputtering". When sputtering occurs, two pieces of information are important. First, the number of particles coming off the surface will determine how quickly the surface is eroding, and therefore how long the material will last before it becomes useless. Second, the direction from the surface at which the particle leaves is important, because the particle may then impact another surface and either start a new collision cascade and sputtering event, or potentially stick to the other surface. If the particle sticks to, for example, a solar panel, the panel may rapidly lose its functionality as it is screened by these other particles. The picture below illustrates a sputtering event. |
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The QCM operates on the principle of resonant frequencies. A piezoelectrically active crystal is stimulated to vibrate at its resonant frequency, which can then be measured very precisely (9 orders of magnitude). The QCM is then positioned over a sputter target and an ion beam is directed toward the target surface. Sputtered atoms leave the target surface, and those which leave in the direction of the QCM strike and stick to the crystal. As the mass of the crystal changes from this particle accumulation, the resonance frequency changes. The change is recorded and can be converted into a measurement of the mass of particles which have stuck to the crystal, or more specifically, the mass and, knowing the density of the material, the number of particles which left the surface and struck the QCM. The minimum change in frequency that can be detected is 0.001 Hz, which corresponds to roughly 10-12 grams of material. Once a measurement has been made at this initial location, the QCM arm (setup pictured below left) rotates to place the crystal at a new location. In this manner, measurements over an entire semicircle are taken. Once one hemisphere is complete, the target itself can be rotated, and a new semicircle measured. By combining several semicircles, a hemisphere can be constructed, illustrating the yield in any given direction around the target (shown below right). |
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With such a high level of precision, measurements can be made on most materials at ion energies as low as 40 eV, and possibly lower. Boron Nitride, a material of particular interest in electric propulsion, has a very low yield, but has been measured in this facility at 40 eV. Refractory metals, coating materials, complex rubbers and plastics, and ceramics and insulators can all be measured through this method with very high precision. Once data has been taken, they can be functionall fitted using what is called the "Modified Zhang" equation (shown below). Using two free parameters, this function is fitted with a least-squares routine to provide a prediction of yields at any location around the hemisphere. The function can also be integrated to calculate a total particle yield for the test. This information shows not only how many particles come off of a surface, but also the percent which leave the surface in any given direction. Once the directional (differential) yields are known, sputtering can be accounted for in the placement of sensitive instrumentation like solar panels. |
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The QCM chamber employs a cathode-filament ion source with a variety of available grid configurations. A two grid setup is used for high energies (800 - 2500 eV), while a three grid setup is used for intermediate energies (350 - 1200 eV). Where energies overlap, either set can be used. Most uniquely, however, an in-house designed and manufactured four grid setup can be used to achieve collimated beams at energies ranging from 350 eV to as low as 20 eV. The physics of electromagnetic interactions which occur in an ion beam cause the beams produced in two grid ion sources to diverge and become useless at lower energies. To correct for this, three grid sources add a focusing element that helps reduce the divergence. However, even this third grid cannot prevent the degradation of the beam at very low energies, such as those of particular interest to Hall thruster sputtering. The addition of a fourth grid, requiring a number of modifications to the beam parameters, has produced well-collimated beams (divergence angles of 12° or less) at energies as low as 80 eV, with usable beams as low as 20 eV. This has allowed testing of otherwise experimentally unknown values such as threshold sputter energy. |
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