wires, which are very sensitive to the exact bakeout temperature, say between 150 and 220°C. Despite this caution, the availability of such plastics, coated wires, and even electric motors which work under UHV, has made surface science techniques much more widespread and routine.

2.3.5Pressure measurement and gas composition

As with pumps, the practitioner needs to know what the diVerent types of gauge can do, and what principles they are based on. There are three general purpose gauges for accurate pressure measurement: the ion gauge, the Pirani gauge and the capacitance gauge. The ion gauge works by ionization of the gas molecules, and the ®ne wire collector reduces the low pressure limit due to X-ray emission of electrons, which mimics an ion current. It should only be used below 1021 mbar, works well below 1023 mbar, and has a lower limit typically below 10211 mbar, depending on the design. The cold cathode (Penning) gauge also works by ionizing the gas molecules, and works over the range 5´1028 to 1022 mbar; but it also functions as a sputter ion pump to some extent, and so the pressure tends to be underestimated.

The Pirani gauge utilizes the thermal conductivity of the gas molecules, and works over a range from about 1023 to 102 mbar; it typically is used for semi-quantitative monitoring of the fore-vacuum. A capacitance gauge is extremely precise above 1024 mbar, but requires diVerent heads for diVerent pressure ranges. This is sometimes referred to as a baratron, but (spelt with a capital B) this is the trade name of a company making such equipment; these gauges are used very widely in all aspects of pressure measurement, process and ¯ow control, for example in chemical vapor deposition (CVD) reactors. An outline description of such process equipment is given by Lüth (1993/5, section 2.5); more details are given in various sections of Glocker & Shah (1995).

A list of such gauges is given in table 2.2. There are some relatively new ones, including the spinning rotor gauge, based on gas viscosity, which has been developed and marketed over the last ten years. To ®nd out more about such a development requires a two-pronged approach. One needs manufacturer's catalogues to ®nd out what is actually available commercially. The second line of enquiry is to search the vacuum

Note: True pressure5indicated pressure divided by sensitivity quoted.

journals: further development of the spinning rotor guage is described by Bentz et al. (1997) and Isogai (1997). The basic high and ultra-high vacuum gauge is still the ionization gauge, developed originally by Bayard & Alpert (1950) as described, for example, by Redhead et al. (1968). Commercial gauges are typically calibrated for N2. Other gases have diVerent sensitivities, as set out in table 2.3.

The determination of gas composition is also very important, and is typically done with a compact mass spectrometer known as a residual gas analyzer, or RGA. This produces a characteristic mass spectrum, as in the example shown in ®gure 2.4(a), taken from an American Vacuum Society educational monograph (Drinkwine & Lichtman 1979). A more recent example at better pressure after bakeout is shown in ®gure 2.4(b). It is helpful to record such spectra, and to store examples of when your system is working well, as the spectrum when you have a real leak is typically quite diVerent from if you have performed an inadequate bakeout, or have let unwanted or corrosive gases into your system. As we have implied in section 2.1, the vacuum composition for a well outgassed system is typically dominated by H2, CO and H2O, very diVerent from the atmosphere (see Table 1.3 in Roth 1990). With a real leak, the O2 peak at mass 32 is much higher than in these examples, where it is very, or unmeasurably, small. The second spectrum also shows that some peaks around mass 62 are due to reactions with the hot ®lament in the ion source of the mass spectrometer, in this case Re31 ions.

Most of the less expensive RGAs are based on a quadrupole mass spectrometer, or QMS, whose principle is explained by Lüth (1993/5, Panel 4) and by Moore et al. (1989, section 5.5). Higher mass resolution is obtained in more specialized magnetic sector or time of ¯ight instruments (Duckworth et al. 1986), which are typically attached to specialist facilities for cluster research, secondary ion mass spectrometry, atom probe microanalyis, or isotope dating (e.g. in archaeology). In these latter cases, the mass spectrometer represents a major fraction of the overall cost of the equipment.

2.4Surface preparation and cleaning procedures : in situ experiments

2.4.1Cleaning and sample preparation

There are two aspects of cleaning: (a) cleaning of sample chambers, pieces of equipment; and (b) sample cleaning. The ®rst 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 discussion of possible sets of prescriptions is given in Appendix H.

The second type of cleaning is very speci®c to the material concerned, and to the experiment to be performed. Indeed it may be most helpful to think of it as the ®rst stage of the experiment itself, rather than as a separate cleaning operation. For example, in semiconductor processing under UHV conditions, where there are many such cleaning and 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 suYcient for a more re®ned technique. An example is that the surface has to be reasonably clean at the subML level to give a sharp LEED pattern; however it does not have to be particularly ¯at. Once people began to examine surfaces by a UHV microscopy technique, it became clear that many of the cleaning treatments employed (e.g. high temperature oxidation followed by a `¯ash' anneal) did not produce ¯at surfaces at all, so it was necessary to reconsider options carefully. Some systems are `known to be diYcult'. This means that a large part of one's (e.g. thesis) time can be taken up with such work, and that the results may well depend on satisfactory resolution of such problems.

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 ®rst 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, provided it has been kept under vacuum.

Two examples will be suYcient to give the ¯avor of such UHV preparation treatments, which typically follow speci®c external treatments including cutting, X-ray orientation, diamond, alumina and/or chemical polishing and degreasing.

(i) W and Fe(110)

The b.c.c., close-packed, W(110) substrate has been used many times because it was possible to clean it reproducibly. Fe(110), which is arguably more interesting, is more diYcult because of its reactivity and internal impurities. 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 1026 mbar at 1400±1500°C for around an hour (to convert C and impurities into oxides), alternated with ¯ash heating to 2000°C to desorb and/or decompose the oxides. Only electron bombardment heating can readily deliver suYcient power density to reach such temperatures.

However, Fe cannot be heated to anywhere near such temperatures, since there is a crystal phase transition (b.c.c. to f.c.c.) at T5911°C, and one might also be nervous

502 Surfaces in vacuum

about going above the (ferroto para-) magnetic phase transition at 770°C. The solu-

tion is typically to use ion bombardment at room temperature, followed by annealing at moderate temperature T,5±600°C. This removes C and O, but promotes surface

segregation of sulfur, 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), as discussed in chapter 3.

(ii) Si and Ge(111)

These semiconductor substrates can be prepared in various ways, and it is known that the equilibrium reconstruction of Si(111) at moderate temperatures is the 737 structure (see section 1.4). But temperatures above 900°C are needed to clean the surface by (resistive or focused high power lamp) heating, and this is above the 737 to `131' transition at 837°C. Thus the procedure is typically to heat to say 1000°C at ,1029 mbar until clean, then cool slowly through the phase transition to allow large domains of 737 to grow, followed by a more rapid cool to room temperature. By contrast, the Ge(111) surface, which has the c238 to `131' transition at 300°C, 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±600°C after an initial light ion bombardment, or by cycles of ion bombardment and annealing at around 400°C.

The above Fe(110) example is described at greater length by Noro (1994) and Noro et al. (1995), and there are many other examples locked up in doctoral theses around the world, and in recipes (patented or not), ®ercely guarded by ®rms whose livelihood depends on similar tricks. Discussion often does not appear in article or book form; for this reason, conference proceedings on the topic can oVer useful insights (e.g. Nemanich et al. 1992, Higashi et al. 1993, 1997).

2.4.2Procedures for in situ experiments

Most surface experiments are performed in situ, i.e. without breaking the vacuum. The progress of such experiments and manufacturing processes proceeds along the following lines.

(a)Degassing components during and after bakeout. This may apply to masks for deposition, evaporation sources, gauge and TSP pump ®laments. 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 eVects. 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 signi®cant evaporation.

(b)Cleaning the sample and characterizing it for surface cleanliness, typically with AES, for surface crystallography, e.g. by LEED or Re¯ection High Energy Electron DiVraction (RHEED), and maybe on a microscopic scale using, say Scanning Electron (SEM) or Scanning Tunneling (STM) Microscopy.

(c)Performing the treatment or experiment: deposit/anneal, react with gases, bend the