Experimental approaches to well controlled studies of thin-film nucleation and growth

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M E T H O D S 102 (1972) 5 2 1 - 5 3 8 ; ©

NORTH-HOLLAND

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S E S S I O N E: 521-573

E X P E R I M E N T A L A P P R O A C H E S T O WELL C O N T R O L L E D STUDIES OF T H I N - F I L M N U C L E A T I O N AND G R O W T H H. POPPA*, R. D. M O O R H E A D and K. H E I N E M A N N t

Ames Research Center, National Aeronautics and Space Administration, Moffett Field, California, U.S.A. Two experimental approaches are described for reliable quantitative studies of thin-film nucleation and growth processes including epitaxial depositions. System 1 consists of a L E E D - A u g e r instrument modified for the controlled vapor deposition on bulk substrates and is combined with advanced methods of very high resolution electron microscopy for the detailed analysis of film structure and orientation. System 2 is a U H V electron microscope adapted for in-situ deposition studies. The operational and functional features of each system are discussed

leading to the selection of certain phases of a deposition problem that are best investigated in each of the systems. In general, however, the two approaches complement each other almost ideally, and the combined use of both can result in a comprehensive investigation of vapor deposition processes not obtainable with any other known method. Performance characteristics and application of each system to selected problems of vapor deposition studies are illustrated with examples from recent nucleation and growth work.

1. Introduction

new and more reliable information on several epitaxial systems11 - 17). These investigations in some cases took advantage of the availability of such powerful surface analytical tools as LEED (low-energy electron difffraction) and AES (Auger electron spectroscopy) or made appropriate use of RED (reflection-electron diffraction) in an ultrahigh vacuum environment. In this way, it became possible to better characterize and control the structure and composition of the substrate surface or the growing film or both. Although the introduction of these highly improved methods of experimental investigation has not led to the expected better understanding of fundamental epitaxial processes, it has definitely served to uncover the even greater than anticipated complexity of the phenomenon of epitaxy. Accordingly, there is now an even more pressing need for well-controlled studies of thin-film nucleation and growth both from applied technological and basic points of view. More detailed studies of the fundamentals of the interaction of atomic, molecular, and ionic beams with surfaces and of the ensuing growth of a thin-film deposit hold the promise of providing additional information to be used for the eventual formulation of a successful theory of epitaxy. The chances are better than ever that this ultimate goal can eventually be reached with the high degree of experimental sophistication now available in this field. In the meantime, however, it seems only prudent to also use our experimental expertise, at least partially, in a more pragmatic manner. In spite of the complex variety of epitaxial results, it is generally agreed that the process of film formation can conveniently be divided into three major stages: (1) nucleation, (2) growth, and

In spite of more than fifty years of active experimental and theoretical research in the area of thin-film nucleation and growth, a full understanding of the basic concepts and mechanisms needed to explain the multitude of experimental findings is still lacking 1' 2). This is particularly evident when the present state of knowledge in the field of oriented overgrowth (epitaxy) is considered and is in general due to the large variety and complexity of physical and chemical interaction processes involved. Furthermore, the interaction processes in question are usually very complicated because of the extremely small quantities of reactants that take part in the reactions-which, in turn, often are also very surface specific. In the recent past, substantial progress toward a better understanding of some of the important fundamental processes in deposition from the vapor phase onto amorphous substrate surfaces has been made on both the theoretical and experimental sides of the problem. Recent theoretical efforts have been concentrated on heterogeneous nucleation 3- 5) mechanisms and on atommigration and capture-processes on solid surfaces6-S). Experimentally, the introduction of ultrahigh vacuum (UHV) techniques, sometimes combined with in-situ electron microscope observations9'1°), has spurred noteworthy advances. The application of UHV technology to the physical vapor deposition of thin films on cleaved, clean, single-crystal substrates has resulted in * Presented the paper. ~ National Research Council Postdoctoral Resident Research Associate at the Ames Research Center, NASA.

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(3) coalescence and fill-in stage. In the case of oriented overgrowth, experience has shown that the final orientation of the resulting film can be predominantly influenced by any one of the three stages of formation. More and better defined deposition studies in different epitaxial systems should concentrate on a more detailed analysis of the different stages of epitaxial film growth. The information gained should be very helpful for a better understanding of essential film growth mechanisms and eventually pave the way for influencing the structure of the final film by systematically adjusting the deposition conditions during the formation of the film. During several years of active research in the area of nucleation and growth of thin metal films on amorphous and crystalline substrates at the Ames Research Center, we have designed and developed the proper usage of two different experimental systems. Each system is technically rather sophisticated and intended for use in separate phases of our research program. From the start, however, the design philosophy for both systems was dictated by the realization that either experimental approach was necessarily afflicted with some basic shortcomings, and that only the proper combined usage of both systems could lead to reliable experimental results.

Briefly, the two experimental approaches can be characterized as follows. System 1 is a conventional LEED/AES system that was modified so that well controlled vapor depositions on macroscopic bulk substrate materials could be performed. Structure and composition of the substrate surface (cleanliness) can be monitored by LEED/AES techniques, and deposits of different thicknesses, representing different stages of the deposition process, are investigated by transmission high-resolution electron microscopy and diffraction after removal from the deposition chamber. For this analysis, the thin-film deposit is either detached from the bulk substrate or the substrate is sufficiently thinned to become electron transparent. In the latter case, highresolution electron microscopy of the substrate-overgrowth composite is possible. System 2 is an electron microscope modified in such a way as to permit the in-situ, high-magnification observation and recording of the entire process of thin-film formation. The vapor deposition parameters are wellcontrolled and the influence of the imaging electron beam on the deposition process is minimized although not completely eliminated. Particular features and the performance of these two experimental systems will be described in this report.

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Examples applying these capabilities to the study of selected thin-film nucleation, growth, and aftergrowth problems will demonstrate the usefulness of each system for particular problem areas. It will also become clear, however, that the most promising and reliable experimental approach for studying all the complex aspects of a given thin-film deposition problem in a comprehensive way, and one not obtainable with any other known method, is through the proper utilization of the combined capabilities of both systems. 2. Performance characteristics and unique instrumental features of both experimental systems 2.1. LEED-AUGER/HIGH-RESOLUTION ELECTRON-MICROSCOPY APPROACH

This approach is based upon the combined use of several advanced conventional systems that are normally available in most laboratories involved in surface physics and thin-film research: (1) a UHV system with facilities for evaporating, monitoring, and controlling the flux of several depositing materials, a crystal cleavage device, a mass spectrometer, a substrate heater, and LEED-Auger three-grid optics, and (2) a modern highresolution electron microscope with point-to-point resolution of at least 3 A. The modified commercial LEED system we use as a deposition plant is shown in fig. 1. Ref. 18 contains a detailed description of features and step-by-step operational procedures of this system; here, we want to restrict our discussion of the

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apparatus to its more important functional features. The substrate-holder and substrate-heater design incorporating radiation heating and multiple substrate specimen handling is essential for the reliable and economical operation of the UHV deposition system. The multiple specimen holder contains, in the case of mica specimens, two blocks of mica of about 80 x 10 x 0.5 mm. Each block is machined by abrasive jet into eleven separate substrate segments (fig. 2). Each segment is thick enough to permit about three individual cleavages and, thus, a total of 22 air-cleaved and 22 to about 60 vacuum-cleaved mica specimens are available in one pump-down cycle. The radiative, cylindrical specimen heater with a small, downward-facing deposition openingin the middle, is mounted opposite the substrate holder. The use of movable supports for both the substrate holder and the substrate heater then allows the rapid placement of the substrate into the oven and also quick withdrawal from it. In this way, it is possible, for each deposition, to expose a freshly cleaved substrate segment to the vapor flux and also quickly quench a justdeposited film by cleaving its mica substrate off the heated mica block and collecting it in the tray below. Therefore, substrate surface cleanliness can be improved and after-deposition-annealing effects can be controlled to some extent, when desired. In the usual mode of operation, the substrate can be rotated around a horizontal axis to either face the evaporation source or the LEED optics, or can be

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positioned foi cleavage. For these maneuvers, the substrate heater is usually withdrawn. A hot-plate type of substrate heater, which is integral with the substrate holder, may be used for special applications. In this case, it is possible (see fig. 1) to position the substrate surface so that it sees the beam of depositing material from below and the electron beam of the LEED device from the rear at the same time. Therefore, in-situ continuous recording of LEED or Auger data during deposition can be accomplished. A measured and rate-controlled dosage of evaporant is furnished by the combined use of flux monitoring quartz crystal, main shutter, and one of three available evaporation sources. The type of evaporation source used depends mainly on flux requirements. For lowevaporation fluxes we use Knudsen cell sources that also eliminate charged source particles that can influence film growth appreciably 19); for higher deposition rates, a three-hearth electron beam evaporator is used. Furthermore, a retractable SiO source is provided for evaporating a protective coating onto freshly deposited layers should this be desirable. The total pressure during evaporation in this system can be held in the middle of the 10-lo torr range, after a previous thorough outgassing of the substrate, its heater, and the evaporation sources. To achieve this, the pumping speed of a conventional 140 1/s Vac-lon pump was supplemented with a titanium-sublimation pump cooled with LN2. A typical residual gas spectrum determined with a quadrupole mass spectrometer, which is located so as to be in line of sight with the substrate, is shown in fig. 3. The kinds of information obtained on the films deposited in this system are determined to a large extent by the specific combination of substrate and overgrowth

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materials under study. LEED, Auger, and RED measurements of the crystallographic structure, deposited mass, purity, and average "grain size" of the film are made while the film remains on the surface of the bulk substrate. (For the RED measurements, the specimen has to be transferred to another vacuum system.) Usually, however, a much more detailed knowledge of the morphology (including the number density of growing particles) and crystallographic structure of the film is obtained by high-resolution transmission electron microscopy and diffraction techniques requiring electron-transparent thin specimens. It becomes necessary in such cases to either separate the deposited film from the bulk substrate (which in some special cases is possible by chemical dissolution of the substrate material) or leave the film on the substrate and attempt to thin the substrate from the back side sufficiently to obtain an electron-transparent substrate-film composite. One of the obvious advantages of the latter technique is the possibility of directly determining epitaxial film-substrate structural relationships. Layertype substrate materials that cleave easily, like mica, are particularly suitable for preparing composite electron microscope specimens because they can first be cleaved by hand to a thickness of a few microns, and then finally thinned by means of oblique incidence argon ion bombardment (see fig. 4). In principle, the ion-beam thinning process is universally applicable to metals, semiconductors, and insulator substrates; in practice, however, the applicability of this method depends on the availability of a suitable prethinning process because of the inherently small ion-sputtering rates obtainable (microns per hour). 2.2. UHV IN-SITU ELECTRON-MICROSCOPE SYSTEM Characteristic features of our UHV electron-microscope specimen chamber for in-situ studies of vapordeposition processes were first published in 19691°).

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The design philosophy was based on the idea of providing a low-pressure environment at the site of the microscope specimen only (not in the entire column of" our RCA/EMU-4 microscope) by replacing the original microscope specimen chamber with a small, selfcontained, UHV deposition- and specimen-handling system, and by isolating this chamber from the rest of the 10- 5 torr microscope vacuum system with two lowgas-conductance apertures (see fig. 5). The imaging electron beam enters and leaves the UHV region through these two special apertures. The specimen chamber contains an electrostatically focused, bent-beam electron gun for the sequential downward evaporation of three different materials, a flux-monitoring and controlling quartz crystal, an axially symmetric microscope specimen heater (up to 1000°C), and provisions for the future attachment of specimen surface-analyzing and treating accessories (e.g., ion-bombardment gun or Auger-electron spectroscope or both). In the usual operating mode, the chamber is pumped by two 400 I/s Orb Ion pumps and a liquid-nitrogen-cooled titanium-sublimator pump through a pump-down sequence of the type illustrated in fig. 6 (the final stage, LH 2 pumping, is not normally utilized). Once a low background pressure in the UHV region is reached, it is, of course, possible to introduce different gaseous environments at the site of the microscope specimen to conduct deposition experiments dealing with the influence of gases or vapors upon film nucleation and growth. The mass spectra for various gaseous environments are shown in fig. 7; they were obtained with a

quadrupole mass spectrometer permanently connected to the central part of the U HV chamber for backgroundpressure monitoring. Various gases and vapors can be introduced from a gas-handling manifold section into the main chamber via a bakeable needle valve. As can be seen from fig. 6, it takes ~ 2 0 h of pumping after introducing a new specimen for pressures during evaporation of 1 to 5× 10 - 9 torr (depending on deposit material) to be achieved. It is obvious, therefore, that the addition of a multiple specimen-storage facility in the specimen-treatment portion of the chamber will eventually become an economic necessity if large numbers of specimens are to be investigated during systematic deposition studies. An alternative solution to the problem of working economy in a low-pressure, in-situ microscope system is to prepare a new and clean specimen surface in-situ after each deposition test. This has been done either by (1) re-evaporating the deposit at the end of a particular test, if the specimen-heater capacity is sufficient (e.g., 700°C needed for silver on carbon), and vapor depositing a fresh thin layer of substrate material before the next test (e.g., amorphous carbon o r S i O ) 2 ~), or (2) by repeated thermal cleavage of a single-crystalline substrate in small areas (several/~m in diameter) with the imaging electron beam, as was first reported possible with MgO by Honjo22). The resolving power of an in-situ electron microscope is another area of concern that is receiving continued attention. As was to be expected, the normal highresolution capability of the basic microscope was some-

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what degraded by the addition of the UHV specimen chamber, and attempts are still under way to improve the instrument's resolution from ~ 15 A achievable at present to 10/~ or better. The lower of the two apertures that allow passage of the imaging electron beam through the UHV chamber is causing most of the performance problems due to its critical electron-optical location and its small size (about 250/~m). The aperture is located inside the objective-lens pole piece between specimen and objective lens and is, at present, not adjustable from the outside of the microscope. This has the following consequences: (1) the working distance-or focal length of the objective-is rather large (about 5 mm), (2) the alignment of the microscope is rendered more difficult, and (3) slight traces of aperture contamination have a disastrous effect on microscope performance. There is, however, still considerable room for improvement in each of the three problem areas, and we have good reason to believe that a resolution of I0 A or slightly better can eventually be achieved. An essential part of a useful in-situ electron microscope is an image-intensifier system consisting of the intensifier chain with television output and still camera or video tape recorder for conveniently and efficiently recording the television output. This system fulfills two major objectives by decreasing the current density and, thus, the influence of the imaging electron beam upon the process under study, and by providing a means for recording changes in the electron-microscope final image. The latter changes can be either due to kinetic changes of the nucleation and growth processes that are being studied or caused by thermal drift that is not unusual at the high substrate deposition tempe[attires often employed.

for thin-film nucleation and growth studies are the use of bulk substrate materials, the ability to clean and easily analyze and monitor the condition of the substrate surface, and the very detailed electron-microscope evaluation of the deposited films. For the study of epitaxial film growth on singlecrystal substrate materials, it is particularly helpful to be able to work with macroscopic bulk substrates since we must also have the capability for cleaning (cleavage, ion bombardment, vapor deposition, heating) and analyzing (LEED-Auger) their surfaces prior to film deposition. After deposition, as mentioned earlier, the

3. Principal merits and shortcomings of both experimental systems including some typical applications A prerequisite for any meaningful study of physical vapor deposition is the close experimental control of the important deposition parameters: substrate temperature, impinging vapor flux, and residual gas environment. Careful attention has been given to this point in the two systems discussed here, as demonstrated in the previous section. The remaining functional differences between the two approaches are basic and due to the specific nature of the experimental approach chosen in each case.

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deposit has to be detached or means have to be available for sufficiently thinning the substrate to permit subsequent inspection by transmission electron microscopy. The capability of analyzing the crystallography and chemical composition of the substrate surface before and of analyzing a combination of substrate and film surface during or after deposition is one of the major attractions of the method discussed in this section. LEED and Auger spectroscopy techniques are sufficiently developed that they can be advantageously used 23) for monitoring the substrate surface "cleanliness" before deposition, even on insulating materials (if the secondary electron-emission coefficient is greater than 1), as shown for mica in fig. 8. The actual usefulness of LEED and AES for measurements during or after growth depends on such factors as film structure, thickness, and the tendency for charge-up of the surface under study.

The case demonstrated here (fig. 8), where the LEED pattern obtained is the same for both air-cleaved and vacuum-cleaved mica surfaces, is a good example that AES is often the more useful and easily applied technique for defining the nature of the substrate surface, particularly if that substrate is an insulator. If the substrate is metallic or semiconducting, however, the additional information from the LEED-pattern geometry on the crystallographic order of the substrate surface is indeed most valuable and, of course, not obtainable with AES. Actually it is possible, then, through the controlled adsorption of active gases, to prepare a large variety of different, well-defined substrate surfaces on which to study the epitaxial behavior of the deposited layers 24) even though the true nature of the substrate surfacereconstructed or not-may still be undecided. As the state of the art of interpreting all the information con-

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tained in a LEED pattern-including reflex intensities and energy dependence-is advanced 25'26) and made more readily applicable, the usefulness of LEED for characterizing substrate and deposit surfaces will undoubtedly increase even more and provide challenging new insights into the nature of epitaxial processes. In the past, the bulk of our knowledge concerning early stages of vapor deposition was accumulated mainly by employing in-situ, high-energy, RED techniques/V'2S), the principal advantage being ease of application to many different substrate-overgrowth material combinations and rather good sensitivity. It is becoming more and more apparent, however, that in addition to area-average structural information we need to find out more about even earlier stages of deposition (thinner films), the number density and size distribution of the growing deposit crystallites as a function of film thickness, and the quantitative distribution ofepitaxial orientations among nuclei. Only then can we hope to contribute substantially to the understanding of the basic mechanisms ofepitaxial nucleation and growth. High-resolution electron microscopy and advanced concepts of structural analysis of the deposit

Fig. 10. Simultaneous resolution of substrate (d(330) = 1.49 A) and overgrowth (d(220) = 1.42 A) lattice planes for gold on mica.

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are, therefore, clearly needed. This is one of the most obvious advantages derived from using our L E E D Auger electron-microscope approach. Fig. 9 shows an example of the kind of conventional, high-resolution, bright-field electron micrographs of three different stages of deposition that can be obtained by transmission microscopy of a thinned substrateovergrowth composite of gold on mica. The deposits were prepared by exposing different substrate areas for differing lengths of time while maintaining a constant vapor flux. Nuclei of 10A or smaller can be detected in this way. Deposit-particle size distributions determined at each stage of deposition provide another way for evaluating details of the nucleation and growth kinetics. Orientational relationships between substrate and overgrowth can also be determined by transmission electron diffraction and microscopy. Fig. 9d shows the usual transmission electron diffraction pattern for a later stage of growth. However, in very early growth stages the diffraction information from the deposit is completely buried in the huge substrate background scatter, a direct result of the area-averaging character of the diffraction method. Furthermore, it is always difficult and unsatisfactory to try to extract a quantitative measure of "the degree of epitaxy" existing at a certain stage of deposition from diffraction patterns of the kind shown in fig. 9d, although this has often been attempted15). High-resolution, electron-microscope imaging methods can, by virtue of the necessarily small substrate area imaged, be used much more accurately than diffraction methods for measuring deposit orientationsZ3). Examples for the two methods used are shown in figs. 10 and 1 1. If lattice dimensions of the deposit crystallites can be resolved directly (fig. 10 for gold on mica), then the orientation of such a crystallite can be determined (to be exact, two nonparallel sets of lattice planes have to be resolved), and it is sometimes even possible to assess the degree of lattice distortion in small nuclei (pseudomorphism, size-dependent lattice constant). Simultaneous resolution of the substrate and overgrowth lattices makes it possible to measure substrate-overgrowth misorientations very accurately and can provide a reliable standard for precise latticeconstant measurements. However, it is usually difficult to achieve the very high instrumenta! res~,m tions needed on a routine basis. An alternative method of'orientation determination in thin island films (fig. I1) does not suffer from such a restriction. This technique is called the BRI method (Bragg-reflex-image) and relies upon measurement of systematic shifts of Bragg reflexes (bright field, fig. l lc) when changing the focus setting SESSION E

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of the microscopeZ3). Operation of the microscope in the central dark field mode enhances the shadow contrast (figs. 1la,b). Fig. 1ld summarizes schematically the orientation determination made in this example. The gold crystallites are found to be in a (112)-orientation by measuring shadow image shifts (a) upon defocusing (Az), and using the relation of d = 2Az/a to determine the spacing, d, of the gold lattice planes perpendicular to the substrate surface (2 = wavelength of imaging electrons). Azimuthal misorientations of the gold crystals can also be easily and accurately deduced. Complete and quantitative statistical evaluations of the structure of thin epitaxial island films as a function of thickness, and of the major experimental deposition parameters, have become feasible now for the first time by properly and systematically applying the BRI method to various cases of epitaxial deposition. In spite of all the advantageous featules and possibilities connected with the use of our LEED-Auger/EM system for thin-film deposition studies, there are a

number of significant shortcomings. Studying the kinetics of nucleation and growth in a quantitative manner [for gold/mica~8)] is a very time-consuming and tedious undertaking because of the large number of samples to be prepared when attempting to obtain meaningful statistical data, and because of the extremely close control we have to exercise over all important deposition parameters. Each stage of deposition studied is generated by a full deposition cycle and is to be compared with other deposition stages produced by varying the deposition time only, under otherwise exactly identical conditions of substrate temperature, impinging flux, surface state, and residual gas environment. The second most significant disadvantage of the LEED-Auger/EM system is caused by the necessity of preparing the specimen so that it can be analyzed by high-resolution transmission electron microscopy at some point in time after the end of each deposition. In other words, what are the chances that the film structure that we finally observe in the electron microscope is

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truly representative of the actual film structure at the time the deposition was terminated? Is the film structure influenced, for example, by unavoidable post-deposition annealing (the shortest possible annealing time in our system is m 30 s), by deposition of protective layers, by cooling to room temperature, by exposing to atmosphere, by preparation into electron transparent form, and, finally, by the electron microscope observation itself? All of the procedural steps questioned have previously been found to represent a possible source oferror29- 34) and have to be checked carefully for each substrate-overgrowth combination, particularly for corrosion-susceptible materials. It is possible to investigate and sometimes eliminate some of the above mentioned specimen preparation artifacts. Protective layers of SiO

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or carbon have been successfully employed in this respect, but truly reliable and comprehensive studies in this problem area can only be conducted by properly using the in-situ capabilities of the second major deposition system discussed in this paper, the UHV in-situ electron microscope. 3.2. UHV IN-SITU ELECTRONMICROSCOPE The UHV in-situ electron microscope complements the LEED-Auger system almost ideally for film nucleation and growth studies and vice versa. In the two areas where the use of the LEED-Auger system was found to be deficient, the in-situ electron microscope with UHV deposition capabilities was particularly efficient and successful. That is, true kinetic-growth measurements

Fig. 12. High-magnification image-itensifier recordings o f nucleation and growth o f silver on amorphous carbon substrate. The same substrate surface area is imaged at different times during deposition (a, b, c), while the total particle number density is plotted in (d) as a function o f time. SESSION E

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and the study of all aftergrowth effects can be made directly and easily. Figs. 12 and 13 illustrate the particular suitability of in-situ microscopy for investigating details of kinetic deposition processes recorded in real time. The measurement of number density, n (t), as a function of deposition time or thickness (fig. 12), and the observation of size and shape changes of individual crystallites through all stages of deposition (fig. 13a, a typical micrograph of the specimen area evaluated in this test) are particularly prominent examples. Measuring the size distribution of the deposited particles and monitoring changes of the crystallographic structure on an in-situ basis are also feasible but experimentally more difficult. They would involve the simultaneous use of a costly electronic particle-size analyser (e.g., Quantimet by IMANCO) in combination with in-situ electron microscopy or the simultaneous display and recording of electron microscope image and the corresponding selected area electron diffraction pattern35). Almost as important as the capability of recording kinetic changes during film growth is that of pursuing changes in film structure after termination of the deposition, such as the effects of annealing (fig. 14), cooling to room temperature, exposure to the atmosphere, and re-evacuation, or application of protective layers by an additional "structureless" vapor deposit. Studies of this kind are not only indispensable for

assessing the magnitude of possible preparation artifacts introduced during the preparation of high-resolution electron-microscope specimens (see previous section), but they are also valuable for analyzing post-deposition changes of mechanical, electrical, and magnetic properties of technologically important thin films and for studying such basic surface physical processes as surface self-diffusion (film-particle coalescence) or the diffusion of film material over the bare substrate surface (Oswals ripening). The influence of the imaging electron beam upon the various processes of film formation in an in-situ microscope is often a point of major concern, and should always be considered a major problem area with this technique until proven otherwise. A usually sufficient and easy check on beam effects is automatically provided by careful and deliberate use of the microscope illumination system. Only a small substrate area of about 1 pm in diameter is illuminated by the imaging electron beam at all times before and during film growth. After termination of growth, we compare deposit structures inside and outside the beam area. Most beam effects will be disclosed in this way. [Electron-beam influences upon film nucleation and growth can sometimes be very pronounced and are actually used to good advantage in technological applications in the area of micromachining of thin films36).] In general, beam effects can be expected if the imaging electrons interact with the substrate, with adsorbates on the substrate and deposit surface, or with the depositing species in such a way as to change the nature of the interface of the substrate or of the depositing material. Electron-beam influences on film nucleation and growth are, therefore, sometimes due to the structure and composition of the substrate 800

GROWTH OF INDIVIDUAL Ag CRYSTALLITES ON si O AREA

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300

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Fig. 13. T h e growth o f silver on a m o r p h o u s SiO. An electron micrograph of the small substrate area evaluated during in-situ observation (a) a n d particle-diameter m e a s u r e m e n t s (b) illustrate two different m o d e s of growth.

WELL CONTROLLED

S T U D I E S OF T H I N - F I L M

material only. The destructive effects of low and very high dosages of electron irradiation upon otherwise quite inert, thin mica substrates are shown in fig. 15. Low dosages lead to partial destruction of substrate lattice order, as demonstrated in fig. 15 by the imaging of mica lattice planes in selected small substrate areas only. High dosages of electron irradiation result in total destruction of all lattice order in the entire beam area, as seen in the small magnification insert of fig. 15. More frequently, however, beam effects me caused by the interaction of the imaging beam with surface contaminants not removed during cleaning or due to specific residual gas environments in the microscope. In some cases, similar effects on film growth as caused by the beam are simply a consequence of initiall~y dirty substrate surfaces and, therefore, are difficult to distinguish from beam effects. The appearance of nuclei during in-situ deposition on amorphous substrates of SiO and carbon that exhibit an abnormal, i.e., twodimensional growth behavior (figs. 13 and 16), can serve as a good example. Large numbers of "thin growth areas" are found in the beam area under environmental conditions of nucleation and growth that would favor beam-adsorbate interactions. Fig. 16b illustrates this case for the growth of silver on carbon in a water-vapor

•. ,

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NUCLEATION

AND GROWTH

533

environment. The particle number densities are also much higher inside the beam area (upper curve in fig. 16d and left picture in fig. 16b); whereas nucleation on clean substrates in U H V shows only normal threedimensional growth of deposit particles (see fig. 12a-c) and much smaller particle number densities for the same substrate temperature and rate of deposition, as shown in the lower curve of fig. 16d. It is obvious that we must be extremely cautious when in-situ n(t)-measurements are made undel deposition conditions favoring electronbeam effects. It should be noticed, however, that some two-dimensional particle growth was also found outside the beam area (fig. 16b right side) that has to be caused by the adsorption of H 2 0 o n the carbon substrate surface without any beam action (the partial pressure of H 2 0 was ~ 10 -6 torr). In the case of nucleation in a H 2 0 vapor environment, the contamination was controlled, but often the

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--:

xe O

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A I

Fig. 14. Post-deposition changes (e = re-evaporation, c = coalescence, r = ripening) of a silver film deposited at 400 °C (dashed line) and annealed at 600 °C for 300 s (solid lines).

Fig. 15. Radiation damage in thin mica substrates by a 100 keV electron beam. The effect of a small dosage (50 A s/cm 2) is partial destruction of the mica lattice (large micrograph); a 10 times higher dosage (small insert) leads to total destruction of lattice order. SESSION E

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H. POPPA et al.

substrate surfaces were simply just not sufficiently clean for well-defined film growth experiments despite previous cleaning treatments of various kinds. It can be seen from the strong beam influence (fig. 16a) on the growth of silver on a carbon substrate that this type of surface cleaning procedure is not sufficient. (The carbon substrate film was prepared by the usual means outside the in-situ microscope and cleaned by heat treating it in a low 10 -9 torr background pressure environment for an extended period of time at 600°C before the beginning of the silver deposition.) Even the deposition of a fresh thin layer of carbon onto a dirty carbon substrate film prior to the silver deposition has sometimes

been found to be inadequate, probably because of the easy diffusion of contaminants through the thin carbon overlayer at the rather high deposition temperatures used (300-500 °C). Only the sequential deposition of a thicker layer of SiO and carbon reproducibly generated fresh carbon surfaces clean enough to exclude any trace of two-dimensional growth of silver. From the above examples it is obvious that one of the major and, in more general terms, probably the major remaining problem area with in-situ nucleation and growth studies is the reproducible preparation of clean electron-transparent and structurally well-defined substrate films that are insensitive to high-energy (100 kV)

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ON AMORPHOUS CARBON

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(d) NUCLEATION KINETICS IN DIFFERENT ENVIRONMENTS Fig. 16. Effects of substrate cleanliness, residual gas e n v i r o n m e n t a n d imaging electron b e a m u p o n nucleation a n d growth o f silver on carbon.

WELL C O N T R O L L E D STUDIES OF T H I N - F I L M N U C L E A T I O N AND G R O W T H

535

(el): LEAD SELENIDE ON ;;EEAD SELENIDE

Fig. 17. Two promising single-crystal substrate materials for controlled in-situ studies of epitaxial nucleation and growth. (a) PbSe cleaned by homo-epitaxial vapor deposition, (b) MgO cleaned by in-situ electron beam cleavage.

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POPPA

electron irradiation. From the point of view of radiation damage, elemental and compound film materials with predominantly covalent binding are certainly preferable to compound materials with strong ionic character that are more susceptible to heavy-electron bombardment damage. Furthermore, a technique must be available for in-situ cleaning of the surface of the substrate material chosen. This can be accomplished, in principle, by properly applying any of the following methods: evaporation of a fresh layer of substrate material 21), chemical cleaning of the substrate by gaseous reactive etching (particularly of metals), low-energy noble-gas ion bombardment combined with annealing of the substrate (metals and semiconductors37), and finally cleavage of the substrate (electronbeam cleavage of MgO, ref. 22). The overall quality and reliability of in-situ studies of film growth will critically depend on the proper selection of, in the above sense, suitable substrate materials, whether they are in amorphous, polycrystalline, or single-crystal form. This point, it has been proven by experience, cannot be emphasized enough and is judged to be the main reason for the rather limited extent of reliable in-situ film growth data found in the literature at present. This is particularly true for single-crystal substrate materials and resulting epitaxial growth studies; not a single, quantitative, epitaxial, in-situ nucleation investigation has been reported in the literature to date that was conducted under stringent control of the important deposition parameters discussed, while quantitative in-situ nucleation measurements on amorphous substrates have been available for some time 21) because of the relative ease of preparing clean amorphous surfaces by vapor deposition. Fig. 17 presents two examples of single-crystal substrate materials that show promise of fulfilling most of the stated requirements of substrate materials for reliable epitaxial in-situ studies. Chemically prethinned (100)-oriented MgO has been found a2) to cleave under the influence of strong, local, electron bombardment and, furthermore, the epitaxial-growth behavior on MgO does not seem to be affected by electron irradiation16,38). These material properties and the fact that both (100)- and (111)-oriented thin substrate areas are usually generated by this cleavage technique make it feasible to study simultaneously the epitaxial growth on two different substrate orientations (see fig. 17b for Au/MgO). Thus, MgO seems to be an ideal singlecrystal substrate material. The technique has the added advantage of permitting the preparation of a number of fresh substrate areas within a single substrate specimen for several deposition runs, but it proves difficult

et al.

at present to control the inclination of the cleaved specimen areas with respect to the incident vapor flux. This particular deficiency is not found with the other promising single-crystal substrate material, PbSe. Thin, perfectly epitaxial films of lead selenide were grown 39) on KC1, floated off in water, and picked up on standard electron-microscope specimen grids. A heating treatment at 275°C in the in-situ UHV chamber of the microscope was then found sufficient to remove most of the film-surface contaminants stemming from previous preparation procedures and to permit the homoepitaxial, in-situ thickening of the original PbSe substrate with a freshly evaporated layer of PbSe at a substrate temperature of 150°C (fig. 17a, right side). No evidence of three-dimensional overgrowth of PbSe on PbSe could be found in the clean portions of the substrate film, and the corresponding electron-diffraction pattern showed perfect epitaxy. However, in a substrate area exposed, prior to the PbSe deposition, to an exceptionally large dosage of electron irradiation (200 A s/cm2), we found polycrystallinity and threedimensional overgrowth of PbSe (fig. 17a, left side) that is possibly due to a surface-diffusion-aided contamination of that substrate area by the electron beam or is a consequence of another more direct beam effect. 4. The combined use of both experimental systems and concluding remarks The major operational and functional features of two different experimental systems for the systematic and reliable study of thin-film nucleation and growth from the vapor phase have been described and are summarized in table 1. Both systems are designed so that the most important experimental deposition parameters such as substrate temperature, impinging vapor flux, and residual gas environment can be well controlledthese are basic prerequisites for meaningful film growth studies. In some important functional features, and in their application to various problem areas, the two systems exhibit significant differences. The L E E D Auger high-resolution, electron-microscope system permits the use of easily manageable, bulk-substrate materials that can be analyzed for crystallographic structure and chemical surface composition with LEED-Auger techniques, and details of the deposit structure down to atomistic dimensions can be studied by advanced methods of very high resolution electron microscopy. Major shortcomings of this experimental approach are, on the other hand, considered particularly strong points of the second system, the UHV, in-situ electron-microscope system. The particular advantages of the second system include continuous, real-time

537

WELL C O N T R O L L E D STUDIES OF T H I N - F I L M N U C L E A T I O N AND G R O W T H TABLE 1

Summary of important functional factors of the two experimental systems used for controlled thin-film vapor deposition studies. Important functional and performance characteristics I. Background pressure

LEED-Auger high-resolution electron microscope (EM) system 5 x 10-10 torr

1-5 x 10-a torr

2. Gaseous deposition environment 3. Impinging vapor flux

UHV in-situ electron microscope (EM) system

Quadrupole monitored and controllable Measured and controlled with oscillating quartz crystal

4. Vapor sources (sequential deposition from three sources)

Low rates: Knudsen cells Higher rates: electron beam evaporator (direction: upward)

Electron beam heated crucibles (direction: downward)

5. Form of substrate material

Bulk crystals, thin films

Thin, electron transparent films only

6. Crystallinity of substrate

Amorphous, poly- or single crystalline

7. Substrate surface cleaning

Mechanical cleavage, ion bombardment and anneal (metals, semiconductors), high temperature cleaving and gaseous etch, vapor deposition

Electron beam cleavage (MgO), vapor deposition (amorphous, poly- or single crystalL low temperature gaseous etch (ion bombardment feasible)

8. Analysis of substrate surface structure and composition ("cleanliness ")

LEED/Auger electron spectroscopy (RED feasible)

None (LEED/Auger feasible)

9. Susceptibility to electron bombardment damage of substrate materials

Not important (destruction of substrate crystallinity in composite EM specimens)

Essential: determines usefulness of kinetic nucleation and growth measurements

10. Multiple substrate capability

Multiple mechanical cleavage or multiple vapor deposition

Multiple beam cleavage, fresh substrate vapor deposit combined with repeated re-evaporation of overgrowth material

11. Preparation of electron microscope specimens for evaluation of deposit results

Overgrowth must be separable from substrate (extraction), sufficient thinning of substrate to prepare EM composite (no preparation for RED)

None

12. Measurement of kinetic processes during nucleation and growth

Representative discrete stages of deposition evaluated. Problem: reproducibility, time consuming, area averaging

Continuous recording in realtime, pursue changes of individual deposit crystallites; time saving

13. Artifacts introduced when preparing EM specimens

Must consider artifacts due to annealing, fixing layer, exposure to atmosphere, thinning, or extraction

None; can be used to simulate EM specimen preparation in-situ

14. Analysis of deposition results by EM and ED (electron diffraction)

High spatial resolution (small nuclei, lattice imaging), measure particle number density and size distribution, determine orientation of single deposit particle; no time limit for evaluation

Lower spatial resolution (10-15 A), measure particle number density and size distribution, determine orientation of single particle by shape or area average orientation by ED (simultaneous EM and ED difficult)

15. Imaging electron beam effects

Negligible (easily introduced, if desired)

Often very serious problem; monitoring of influence of electron irradiation on film growth necessary

16. After growth changes of deposit structure (see also 13)

Difficult to determine; in-situ RED desirable

ln-situ measurements most valuable

SESSION E

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H. P O P P A et al.

measurements of kinetic deposition processes, including the ability to follow kinetic growth changes on individual deposit crystallites and the elimination of undesirable aftergrowth effects and electron-microscope preparation artifacts. Other respective disadvantages of the two experimental approaches have also been discussed, in particular the necessity for finding suitable singlecrystal substrate materials for in-situ epitaxial studies that can be prepared in electron-transparent form, that are electron-radiation resistant, and that can be thinned or cleaned in-situ. It is obvious that the two experimental systems and approaches complement each other very well, in some respects ideally. Consequently, the most comprehensive and reliable study of the vapor deposition of one material on the surface of another can be expected to result from the properly integrated use of both systems. It is also clear that the more aspects of a theoretically and experimentally complex problem, such as the epitaxial growth of thin films, we are able to study, the better are our chances of understanding the complicated basic processes involved. To date, we have not accomplished such a complete epitaxial study due mainly to our lack of experience in the selection of substrate materials suitable for both methods of investigation. However, in-situ electron microscopy has often been successfully used to help solve specific problems in deposition studies carried out with the LEED-Auger electron-microscope system, and vice versa, and our working experience with both systems is now approaching the point where a complete epitaxial deposition analysis in the above sense can be performed. The PbSe films used in these experiments were supplied by A. K. Green and J. Dancy of the Naval Weapons Center, China Lake, California. Their cooperation is gratefully acknowledged.

References 1) K. L. Chopra, Thin film phenomena (McGraw-Hill, New York, 1969). 2) j. p. Hirth and G. M. Pound, Condensation and evaporation, Progress in materials science (Macmillan, New York, 1963). z) D. R. Frankl and J. A. Venables, Advan. Phys. 19 (1970) 409. 4) D. Walton, J. Chem. Phys. 37 (1962) 2182. 5) G. Zinsmeister, Vacuum 16 (1966) 529; Thin Solid Films 2 (1968) 497, 4 (1969) 363, 7 (1971) 51.

6) V. Halpern, J. Appl. Phys. 40 (1969) 4627. 7) B. Lewis, Surface Sci. 21 (1970) 273, 289. 8) K. J. Routledge and M. J. Stowell, Thin Solid Films 6 (1970) 401. 9) H. Poppa, J. Vac. Sci. Technol. 2 (1965) 42. 10) V. Valdre, D. W. Pashley, E. A. Robinson, M. J. Stowell, K. J. Routledge and R. Vincent, Proc. 6th Intern. Congr. Electron microscopy, Kyoto 1966 (Maruzen-Tokyo, 1966). 11) j. W. Mathews and E. Grfinbaum, Phil. Mag. 11 (1965) 1233. 12) E. Bauer, A. K. Green, K. M. Kunz and H. Poppa, Basic problems in thin film physics (Vandenhoeck and Ruprecht, GOttingen, 1966). 13) A. K. Green, E. Bauer and J. Dancy, Molecular processes on solid surfaces (McGraw-Hill, New York, 1968). 14) T. N. Rhodin, P. W. Palmberg and C. J. Todd, Molecular processes on solid surfaces (McGraw-Hill, New York, 1968). 15) R. W. Adam, Z. Naturforsch. 23a (1968) 1526. 16) A. K. Green, E. Bauer and J. Dancy, J. Appl. Phys. 41 (1970) 4736; J. Vac. Sci. Technol. 7 (1970) 159. 17) C. A. O. Henning and J. S. Vermaak, Phil Mag. 22 (1970) 269. 18) A. Grant Elliot, P h . D . Thesis (Mat. Sci. Dept., Stanford University, 1971) to be submitted. 19) D. J. Stirland, Appl. Phys. Letters 8 (1966) 326. 2o) R. D. Moorhead and H. Poppa, Proc. 27th Ann. EMSA Meeting, St. Paul, Minn. (Claitors Publ. Co., Baton Rouge, 1969). 21) H. Poppa, J. Appl. Phys. 38 (1967) 3883. 22) G. Honjo, S. Shinozaki and H. Sato, Appl. Phys. Letters 9 (1966) 23. 2a) H. Poppa, K. Heinemann and A. Grant Elliot, J. Vac. Sci. Technol. 8 (1971) 471 ; H. Poppa and A. Grant Elliot, Surface Sci. 24 (1971) 149. 24) E. Bauer and H. Poppa, to be published. 25) C. W. Tucker and C. B. Duke, Surface Sci. 23 (1970) 411. 26) W. D. Robertson, J. Vac. Sci. Technol. 8 (1971) 403. 27) A. Green, E. Bauer, R. L. Peck and J. Dancy, Kristall Tech. 5 (1970) 345. 2s) D. W. Pashley, Recent Progr. Surface Sci. 3 (1970) 23; Adv. Phys. 5 (1956) 174. 29) L. Bachmann, D. L. Sawyer and B. M. Siegel, J. Appl. Phys. 36 (1965) 304. 3o) L. Bachmann and H. Hilbrand, Basic problems in thin film physics (Vandenheock and Ruprecht, G6ttingen, 1966). 31) j. van de Waterbeemd, Philips Res. Rep. 21 (1966) 27. 32) M. J. StoweIl, Thin Solid Films 0 (1969) 1. 33) R. F. Egerton and C. Juharez, Thin Solid Films 4 (1969) 239. 34) L. Reimer, Elektronenmikroskopische Untersuchungs- und Priiparationsmethoden (Springer-Verlag, Berlin/Heidelberg, 1967). 35) R. Speidel and P. Holl, Optik 24 (1966) 298. 36) A. F. Kaspaul and E. E. Kaspaul, Trans. 10th Nat. Vacuum Symp. (Macmillan, New York, 1963). 37) E. Bauer, in Techniques for the direct observation of structure and imperfections (Interscience, New York, 1969) ch. 16. 38) p. W. Palmberg, T. N. Rhodin and C. J. Todd, Appl. Phys. Letters 11 (1967) 33. 39) A. K. Green, J. Dancy and E. Bauer, J. Vac. Sci. Technol. 8 (1971) 165.