2.4 Surface Preparation and Cleaning Procedures : In-Situ Experiments

2.4 Surface Preparation and Cleaning Procedures: In-situ Experiments a) Cleaning and Sample Preparation

There are two aspects of cleaning: (i) cleaning of sample chambers, pieces of equipment; and (ii) sample cleaning. The first is a rather obvious combination of dirt removal, degreasing, ultrasonic rinsing, use of solvents, etc. This requires care, and is time-consuming; it is a clear candidate for 'more haste less speed', since it is essential to be systematic; thinking that this is a 'low-level' activity which you should be able to race through does not help. Cultivate high level thought in parallel, but concentrate on the details. A typical, but not unique set of prescriptions is handed out as a guide. Typically, each laboratory will have its do's and don'ts, and the recipes will use commercial and brand-name solvents available locally.

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 lengthier, 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;

W(110): This bcc, close-packed, substrate has been used many times because it was possible to clean it reproducibly. Fe(110), which is arguably more interesting, was 'known to be difficult', so that workers have shied away. 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.

Si(111): This semiconductor substrate can be prepared in various ways, and it is known that the equilibrium reconstruction is the 7x7 structure (see section 1.4). But temperatures above 900oC are needed to clean the surface by (resistive or focussed high power lamp) heating, and this is above the 7x7 to '1x1' transition at 837oC. Thus the procedure is typically to heat to say 1000oC at < 10-9 mbar until clean, then cool slowly through the phase transition to allow large domains of 7x7 to grow, followed by a more rapid cool to room temperature. By contrast, the Ge(111) surface, which has the c2x8 to '1x1' transition at 300oC, and has a much more 'mobile' surface, is quite a lot easier to clean. It is less reactive to oxygen, and can be cleaned by heating at 500- 600oC after an initial light ion bombardment. b) Examples of In-Situ Experiments

Most surface experiments are performed in-situ, i.e without breaking the vacuum. Given the availability of quite complex sample transfer devices, whole sequences of surface engineering proceeses can be performed on samples, as for example in Molecular Beam Epitaxy (MBE) and other (commercial) equipment. The progress of such experiments proceeds along the following lines:

(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 characterizing 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 characterize 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.