sample, implant a million computer chips, whatever is your ®eld of interest, or current task. And ®nally:

(d) examine the sample with the techniques at your disposal!

One can see why it is useful to think of the cleaning the sample (b above) as the ®rst stage of the experimental process, because what you can characterize is determined by what you have bolted onto the system. Even if you have the particular equipment, you might decide not to use it because it takes too long, or doesn't answer the question you are currently asking. And, as implied at the beginning of section 2.3.4, it is helpful not to have too many accessories bolted on to the system at the same time. Not only will the pressure be worse than it might be; none of the accessories will actually be working when you need them!

Of course, as in all design problems, the real situation is a balance between competing tendencies; if you change the vacuum and accessory con®guration too frequently, you pay a large price in inconvenience, down time and loss of output, measured in either scienti®c results or material products. But if you don't build in the possibility for con®gurational changes at the design stage, you risk wasting a very large investment.

2.4.3Sample transfer devices

Increasingly, UHV-based in situ techniques are being applied in engineering and manufacturing situations. Given the availability of quite complex sample transfer devices, whole sequences of surface engineering processes can be performed on samples, as for example in molecular beam epitaxy (MBE) and other (commercial) equipment. Transfer of samples between equipment with a UHV device was ®rst demonstrated by Hobson & Kornelsen (1979), including showing that it was possible to transport the equipment by air across the Atlantic at pressures below 1029 mbar. However, this is still not a routine, nor a necessarily desirable, procedure.

Sample transfer is done on a regular basis when samples are to be examined at major facilities, such as a synchrotron radiation laboratory. It can be much more eYcient to prepare the sample in a dedicated surface science or MBE chamber, and then transport the sample, typically pumped using a moderately sized ion pump, to the measuring station. One such design is indicated in ®gure 2.5, which is specialized for X-ray diVraction measurements at the HASYLab synchrotron in Hamburg (Johnson 1991).

This design consists of a small chamber, built inside the ion pump housing itself, with a ¯ange on the end capped with a thin hemispherical beryllium window, through which the X-rays can pass in and out. The sample sits at the center of a two-axis diVractometer, and can be heated or cooled during the experiment; the whole transfer chamber is mounted on a rigidly engineered rotatable goniometer stage. Transfer to and from the preparation chamber is eVected by closing the gate valve, shown in ®gure 2.5(a), unbolting the assembly from the goniometer, and proceding cautiously. When bolted to the other chamber, the sample can then be withdrawn into the sample preparation position using a transfer shaft ®xed to that chamber.

52 2 Surfaces in vacuum

There are many designs for such transfer shafts which maintain full UHV conditions internally. A common design is to use a shaft with a magnetic slug, coupled to a strong permanent magnet external to the vacuum chamber. A schematic drawing of a set of chambers connected with several such magnetic transfer devices is shown in ®gure 2.5(b). Indicated also are the entry locks, and the place where the X-ray transfer chamber is attached. The fact that it looks like a space station is not entirely coincidental ± it is a space station with deep space on the inside.

2.4.4From laboratory experiments to production processes

I have noted here that UHV-based experimental research and manufacturing technologies are quite capital-intensive, and have argued the case that adequate thought must be put into both the design and operation stages. Once one considers scale-up from the laboratory to production, these points have even greater weight. The dollar ®gures involved in the semiconductor industry are quite astounding, and control of contamination during the surface processing involved is a major concern and expense.

For example, Ouellette (1997) reports that `an 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 contamination'. She goes on to estimate that a single Fab-line can lose $15M/month from contamination-related defects. The de®nition of defects is suitably 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 O'Hanlon (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 won't go away.

The other important industry is based even more directly on chemistry. Estimates for the catalytic industry (Bell 1992, Ribiero & Somorjai 1995) suggest that 17% of all manufactured goods go through at least one step involving catalytic processes. Rabo (1993) reported 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 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 on point defects or steps on the surface, and may be very diYcult 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 (SMPs). There is more surface area associated with a given volume of metal if the particle size is small, and additionally the reactivity of the

542 Surfaces in vacuum

less strongly bound SMP's 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. Some of these topics are discussed in section 4.5.

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 chapter and the next. Although our primary focus here is on allegedly simple systems, and doesn't go very far in a chemical direction, the subjects are in fact seamless, as I believe the examples chosen will show.

2.5Thin ®lm deposition procedures: sources of information

2.5.1Historical descriptions and recent compilations

Thin ®lms have been prepared ever since vacuum systems ®rst became available, but deposition as a means of producing ®lms for device purposes is a development of the past 40 years. Thin metallic ®lm coatings on glass or plastic were among the ®rst to be exploited for optical purposes, ranging from mirrors to sunglasses, and this still continues as a major, typically high vacuum, high throughput business. Most such ®lms are examples of polycrystalline island growth; models of island growth are described here in some detail in chapter 5. An early survey of laboratory-based production methods of single crystal epitaxial metallic ®lms on a range of single crystal substrates is given in the articles in Epitaxial Growth, part A (Matthews 1975). As thin ®lm deposition processes have developed very rapidly over the past 25 years, particularly in the context of semiconductor devices, processes have become highly specialized, and have been described in textbook form (Smith 1995), and in updateable compilations such as the Handbook of Thin Film Process Technology (Glocker & Shah 1995), where the individual sections have themselves been edited and have multiple authors. The following sections describe some of these developments in outline.

2.5.2Thermal evaporation and the uniformity of deposits

This technique is the simplest conceptually, corresponding to raising the temperature of the source material, either in an open boat, suspended on a wire, or by any other convenient means so that the material evaporates or sublimes onto the substrate. The boat/wire is typically chosen as a high temperature material such as W or Mo, and must not react adversely with the evaporant. Unless particular precautions are taken, the evaporant will be deposited all over the inside of the vacuum system, and will therefore be both ineYcient in the use of the source material, very messy for the vacuum system, and will not produce a uniform deposit.

Source

(c) Dome planetary geometry

Figure 2.6. The use of substrate rotation to produce more uniform ®lms: (a) substrates mounted on a dome, which is rotated about the axis; (b) substrates on a drum with the sources placed along the center line, with the drum rotating about it; (c) use of planetary motion in a dome geometry for ultimate uniformity (after Graper 1995, redrawn with permission).

The production of uniform deposits with high throughput is a requirement for industrial processes, and this usually means that the substrate has to be rotated. This movement is required because the emission from the source is more or less peaked in a particular (forward) direction, so that the deposited ®lms are thinner at the edges. Three examples of using substrate rotation to alleviate this eVect are indicated in ®gure 2.6 (Graper 1995). Planar solutions are often used because of their simplicity, but the drum solution is preferred for highest throughput. Planetary solutions are required for best thickness uniformity, especially in the dome geometry as illustrated in ®gure 2.6(c). These are used for this reason even at the expense of throughput and reliability.

The vapor pressure of the source material is exponentially dependent on the temperature, as described in section 1.3.1, and shown for some elements in ®gures 1.9 and 1.10. The deposition rate is determined by the source area and temperature, and by the distance between the source and substrate. One should note that diVerent materials have very diVerent relations between the vapor pressure and the melting point, so that a satisfactory deposition rate may only be obtained if the material is liquid, which may drip oV a wire or inclined boat.

56 2 Surfaces in vacuum

Figure 2.7. A small eVusion source using a PBN oven (after Davies & Williams 1985, reproduced with permission).

To avoid such problems, one can use an oven, which can easily be mounted in an orientation such that the liquid does not spill out. With an oven or crucible, it is relatively simple to construct it so that the evaporant does not evaporate in all directions, but comes out in a more or less well directed beam, which can be further collimated so that the source material is directed preferentially onto the substrate. The sources can be characterized as eVusion sources, with a relatively large area opening, or as Knudsen sources, where a small hole is used; in the latter case the model is that the material coming through the hole samples the vapor pressure of the source material inside, and that standard kinetic theory formulae are applicable. Small sources have been developed for use with graphite (Kubiyak et al. 1982) or pyrolytic boron nitride (PBN) ovens, as illustrated in ®gure 2.7 (Davies & Williams 1985). The design of such an evaporation source can be explored via problem 2.2. In practice considerable thought and eVort is required to achieve a uniform temperature enclosure, via careful design of the crucible, of heater windings, radiation shields and water cooling, and by the use of anticipatory electronic control of heater currents based on thermocouple measurements. Such sources and controllers are now commercially available, and a pilot plant system (e.g. to deposit multilayers) may have many of them in action at any one time.

In order to deposit high temperature materials, or materials which interact with the crucible, electron beam evaporation is required. The design typically includes a heavy duty ®lament to emit many milliamps of current, and several kilovolts of high voltage in order to deliver the necessary power. The electron beam is directed onto the sample surface by a shaped magnetic ®eld, typically using an inbuilt permanent magnet (Graper 1995). The heating so produced is very localized, and care is required to be sure that it is localized where it should be; this is also the case when using pulsed ultra-violet eximer