602 Surfaces in vacuum

LPCVD (Low Pressure CVD) where the pressure is 0.1±1 mbar. MOCVD (Metal±Organic CVD) or OMVPE (Organo±Metallic Vapor Phase Epitaxy) is also a widely used technique, as is PECVD (Plasma Enhanced CVD); there are many variants on this theme (Vossen & Kern 1991).

The question of reaction mechanisms and rate limiting steps in CVD is highly complex. Under LPCVD conditions diVusion processes in the gas are typically not the dominant eVect, so that at the growth temperatures, kinetic processes on the growing surface are rate limiting, as indicated in ®gure 2.9(b) (Vescan 1995). However, we can see from ®gure 2.9(a) that all the reactions, in the gas phase and on the surface of the growing ®lm, are in series and that there is typically very little information on the intermediate states of the reaction. Thus understanding CVD in atomic and molecular terms is very much an ongoing research project, which we will return to later in section 7.3.

Further reading for chapter 2

Dushman, S. & J. LaVerty (1992) Scienti®c Foundations of Vacuum Technique (John Wiley).

Glocker, D.A. & S.I. Shah (Eds) (1995) Handbook of Thin Film Process Technology

(Institute of Physics), especially parts A and B.

Hudson, J.B. (1992) Surface Science: an Introduction (Butterworth-Heinemann) chapters 8 and 9.

Lüth, H. (1993/5) Surfaces and Interfaces of Solid Surfaces (3rd Edn, Springer) chapters 1 and 2.

O'Hanlon, J.F. (1989) A Users Guide to Vacuum Technology (John Wiley). Matthews, J.W. (Ed.) (1975) Epitaxial Growth, part A (Academic).

Moore, J.H., C.C. Davis & M.A. Coplan (1989) Building Scienti®c Apparatus (2nd Edn, Addison-Wesley) chapters 3 and 5.

Roth, A. (1990) Vacuum Technology (3rd Edn, North-Holland).

Smith, D.L. (1995) Thin-Film Deposition: Principles and Practice (McGraw-Hill). Tsao, J.Y. (1993) Materials Fundamentals of Molecular Beam Epitaxy (Academic).

Problems for chapter 2

These problems are to practice and test ideas about vacuum systems, design problems and surface preparation techniques.

Problem 2.1. Design of vacuum systems for speci®c purposes

Use your knowledge of (and appendices on) conductances of standard size tubes, and the characteristics of vacuum pumps, to suggest (and justify semi-quantitatively) design choices in the following situations.

Problems for chapter 2	61

(a)Pumping an approximately spherical chamber of diameter 0.5 m. The chamber is to be let up to air infrequently, and we want to achieve as good a pressure as possible (, 10210 mbar).

(b)Pumping a cylindrical chamber of length about 10 m and diameter 0.1 m. The chamber is to be periodically ¯ooded with rare gases up to about 1023 mbar pressure, and the important point is to be able to achieve pressures below 1028 mbar quickly and economically.

(c)Pumping a state of the art particle accelerator from sections of pipe of length about 50 m and diameter 0.1 m, with a total length in excess of 50 km (kilometers) at a pressure of ,10211 mbar.

Problem 2.2. Design of a Knudsen source for depositing elemental metal ®lms

A Knudsen source is an evaporation furnace which relies on the establishment of the vapor pressure above a solid or liquid source material. A small hole in the furnace above the source material, plus collimating holes in front of the source, allow a beam of the source material to be directed at the sample. Use your knowledge of vapor pressures and kinetic theory to design a source which will deposit one monolayer per minute on a sample held 0.15 m away from the exit of the source, will be uniform on the sample within 1% for the central 0.01 m diameter, and will not deposit any material on the sample outside a radius of 0.02 m. Do this in stages, with discussion, as follows.

(a)Consider the formula R5nv/4 for the number of atoms hitting unit area per second of an enclosure, and how this formula applies to a Knudsen source. Derive the formula by considering the relevant integrations over angles and the Maxwell±Boltzmann velocity distribution.

(b)Consider the geometry of the design, and the constraints on the uniformity and area of the deposit. Show that this will limit the size of the hole in the furnace, and suggest a suitable size for holes in both the furnace and the collimator.

(c)Choose an elemental metal of interest to you, and ®nd out the formula for the vapor pressure as a function of furnace temperature. Using the relationship between density n and pressure p for this material, coupled with your design from part (b) work out the temperature at which the source will have to operate to satisfy the deposition rate requirement, explaining your assumptions.

(d)If you actually want to design a real source for this material, consider carefully the materials of which the source can be made, whether you should be using a Knudsen or some other type of source, and how to power the furnace to achieve suYcient temperature uniformity, etc.

Note: short descriptions of most possible deposition techniques are given by Smith (1995)

and by Glocker & Shah (1995); some speci®c designs are in Yates (1997).

62 2 Surfaces in vacuum

Problem 2.3. Some questions on surface preparation and related techniques

Questions about surface preparation are always very speci®c to the materials concerned, but here are a few which may be relevant and which spring from the text of this section.

(a)Why should one either cool the sample slowly through a surface phase transition (e.g. as in the case of Si(111)), or not anneal the sample above a bulk phase transition (e.g. in preparing b.c.c. Fe surfaces)?

(b)What is the main reason why Si(111) produces a 231 reconstruction after cleavage, when the equilibrium surface structure is the 737?

(c)Device engineers always grow a `buVer layer' on Si(100) before attempting to grow a device, e.g. by molecular beam epitaxy. Why is this precaution taken, and how does it improve the quality of the devices grown on such surfaces?

(d)Mass spectrometry shows a range of mass numbers (M/e ratios) for the contents of the vacuum system, but they don't seem to be simply related to the molecules,

e.g. O2, N2, CO, H2O, CO2 which are present. What range of processes are responsible for this discrepancy?

(e)GaAs often evaporates to leave small liquid Ga droplets on the surface. Why does this happen, and how can it be prevented?

3Electron-based techniques for examining surface and thin ®lm processes

This book presumes that the reader is interested in experimental techniques for examining surface and thin ®lm processes; however, there are many books devoted to surface physics and chemistry techniques, some of which are given as further reading at the end of the chapter. There are even several books which are just about one technique, such as Pendry (1974) or Clarke (1985), both on low energy electron diVraction (LEED) in relation to surface crystallography. By the mid-1980s it was already stretching the limits of the review article format to compare the capabilities of the available surface and thin ®lm techniques (Werner & Garten 1984).

Since then, the various sub-®elds have proliferated, so we cannot be comprehensive, or give all the latest references here. In section 3.1 we discuss ways of classifying the large number of techniques which exist, and thereafter the chapter is restricted to techniques based on the use of electron beams. Section 3.2 discusses the most widely used (elastic scattering) diVraction techniques used for studying surface structure. Section 3.3 discusses forms of electron spectroscopy based on inelastic scattering, which are used to obtain composition and chemical state information. Individuals can look in more detail into a particular technique. Students have been asked to present a talk to the class, followed by questions and discussion; some of the topics considered in this way are listed, along with selected problems, at the end of the chapter. As examples, some case studies are given in section 3.3 on Auger electron spectroscopy, in section 3.4 on quanti®cation of AES, and in section 3.5 on the development of secondary and Auger electron microscopy. Stress is placed on the relationship between microscopy and analysis: microstructure and microanalysis. The frontier is at nanostructures and nanometer resolution analysis.

3.1Classi®cation of surface and microscopy techniques

3.1.1Surface techniques as scattering experiments

Most physics techniques can be classi®ed as scattering experiments: a particle is incident on the sample, and another particle is detected after the interaction with the sample. Surface physics is no exception: we can think of an incident probe and a response as set out in table 3.1.

63