The second type of cleaning is very specific to the material concerned, and to the experiment to be performed. Indeed it may be helpful to think of it as the first stage of the experiment, rather than cleaning as such. For example, in semiconductor processing under UHV conditions, where there are many such preparation stages, 'clean' means 'good enough so that the next stage is not messed up'. Thus, acting quickly, transferring under inert gas, or any trick that will work (i.e. increase throughput/ reliability), all count under this heading; there is no absolute standard.
For research purposes the criteria are remarkably similar. Thus a cleaning process which is good enough for one experiment or technique, may not be sufficient for a more refined technique. An example is that the surface has to be reasonably clean at the sub-ML level to give a sharp LEED pattern; however it does not have to be particularly flat. Once people began to examine surfaces by a UHV microscopy technique, it became clear that many of the cleaning treatments employed (e.g high T oxidation followed by a 'flash' anneal) did not produce flat surfaces at all. Back to the drawing board. Some systems are 'known to be difficult'. This means that a large part of thesis time can be taken up with such work, and that the results may well depend on satisfactory resolution of such problems. Again, don't fret: this is science as she is lived; but that doesn't stop it being frustrating.
The various possibilities for sample cleaning include the following: heating, either resistive, using electron bombardment or laser annealing; ion bombardment; cleaving; oxidation; in-situ deposition and growth. These may be applied singly, or more often in combination or in various cycles. Typically, the first time a sample is cleaned, the procedure is more lengthy, or more cycles are required. Thereafter, relatively simple procedures are needed to restore a once-cleaned surface.
Two examples will be sufficient to give the flavor of such UHV preparation treatments, which typically follow specific external treatments including cutting, X-ray orientation, diamond, alumina and/or chemical polishing and degreasing;
Both substrates can be cleaned on a holder equipped for electron bombardment of the rear side of the sample. Tungsten is typically cleaned by heating in oxygen at around 10-6 mbar at 1400-1500oC for around an hour (to convert C and impurities into oxides), alternated with flash heating to 2000oC to desorb and/or decompose the oxides. Only electron bombardment heating can readily deliver sufficient power density to reach such temperatures.
However, Fe cannot be heated anywhere near such temperatures, since there is a crystal phase transition (bcc to fcc) at T = 911oC, and one might also be nervous about going above the (ferro- to para-) magnetic phase transition at 770oC. The solution is typically to use ion bombardment at room temperature, followed by annealing at moderate T, say 5-600oC. This removes C and O, but promotes surface segregation of sulphur, which is a major impurity in Fe; so a lengthy iterative process is required to reduce S to an acceptable level. This cleaning process is typically monitored by Auger Electron Spectroscopy (AES), which we will discuss in section 3.
(i) Degassing components during and after bakeout. This may apply to masks for deposition, evaporation sources, gauge and TSP pump filaments. The main point is that such equipment will degas during use, worsening the pressure, often directly in the neighborhood of the sample; prior degassing will lessen, but rarely eliminate these effects. A typical procedure is to leave evaporation sources (say) powered up during the later stages of bakeout, but at a low enough level so as not to cause significant evaporation.
(ii) Cleaning the sample and characterising it for cleanliness, typically with AES, for crystallography, e.g. by LEED or Reflection High Energy Electron Diffraction (RHEED), and maybe on a microscopic scale using, say Scanning Electron (SEM) or Scanning Tunnelling (STM) Microscopy.
(iii) Perform the treatment or experiment: deposit/anneal, react with gases, bend the sample, whatever is your field of interest.
(iv) Examine the sample with the techniques at your disposal. One can see why it is useful to think of the cleaning the sample (ii above) as the first stage of the experiment, because what you can characterise is determined by what you have bolted onto the system: even if you have the kit, you might decide not to use it because it takes too long. And, as I have indicated, it is helpful not to have too many accessories bolted on to the system, or none of them will actually be working when you need them.
For example, I read a recent report that Ďan estimated 80% of equipment failures in silicon wafer process lines arise from contamination related defects. Since most wafer fabrication lines average an 80% yield, as much as 16% of the total loss of yield may be due to contaminationí. The report went on to estimate that a single Fab-line can lose $15M /month from contamination-related defects. The definition of defects is suitable wide: anything from peeling paint, worn bearings, bits of PTFE seals, particles, right down to the individual atomic defects incorporated into the materials themselves.
As pointed out by OíHanlon in another paper in 1994, the major drive for UHV in the semiconductor industry comes from the need to control the purity of reacting gases at the parts per billion level. This is understandable, given the predominance of chemical vapor deposition systems using good high vacuum, rather than UHV technology. This is a problem that simply wonít go away.
The other important industry is based even more directly on chemistry. Estimates for the catalytic industry suggest that 17% of all manufactured goods go through at least one step involving catalytic processes. A 1992 report predicted that the yearly catalyst market was projected to be $1.8 billion in 1993, with auto emissions catalysts the fastest growing component. With sensors also a growing market, and environmental concerns growing all the time, these industrial applications are becoming more rapidly more important. Most of this activity involves heterogeneous catalysis, in which gases react over a surface.
There are three major types of catalyst which are the subject of intense study: these are (single crystal) metal and oxide catalysts, and supported metal catalysts, where small metal particles are suspended, typically on oxide surfaces. In all these cases, the properties of the catalyst may be dependent of point defects or steps on the surface, and may be very difficult to analyze. In the case of supported metal catalysts, the properties are very dependent on the dispersion of the metal, i.e. on the size and distribution of the small metal particles (SMPís).
There is more surface area associated with a given volume of metal if the particle size is small, and additionally the reactivity of the less strongly bound SMPís may be enhanced. Examples of SMP catalysts are Pt, Pd and/or Rh dispersed on polycrystalline alumina, zirconia and/or ceria; a selection of these form the principal components of the catalytic converters in car exhaust pipes, converting partially burnt hydrocarbons, CO and NOx (nitrous oxides) into CO2, N2 and H2O.
While I am not claiming that all this economic activity is directly concerned with UHV and surface technology, it is certainly true that this is the reason why semiconductor device engineers and catalytic chemists, and behind them society at large, are interested in the instrumentation described in this section and the next. Although our primary focus here is on allegedly simple systems, and doesnít go very far in a chemical direction, the subjects are in fact seamless, as I believe the examples chosen will show.
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