Influence of the deposition conditions on growth and structure of evaporated films

VACUUMVOI. 13, pp. 337-347. PergamonPress Ltd. Printed in Great Britain.

VACUUM Influence of the Deposition Conditions on G r o w t h and S t r u c t u r e of Evaporated Films K. H. B E H R N D T Bell Telephone Laboratories, Inc., Murray Hill, New Jersey, U.S.A. (Received 9 May 1963 ; accepted 27 May 1963)

The properties o f thin films depend to a large extent on the film structure which in turn is influenced by the mechanism o f film growth. The phenomena contributing to adhesion, surface mobility o f the arriving atoms, ordering processes in the films, grain size, and other basic film porperties are reviewed. Their relation to the materials o f film and substrate, the deposition conditions, contamination etc. is discussed. The surface structure o f the substrate and its cleanliness which influence film growth appreciably, are treated in appendices. Thus, a broad survey is presented in an attempt to illustrate the phenomena o f film growth and their relation to controllable deposition conditions.

I. Introduction

2. Adhesion of the film to the substrate

Thin films are now being used commercially for numerous applications. Frequently the properties of the available film materials are not satisfactory for the requirements of a special device and must be changed in a predetermined manner. Such changes can be achieved by several approaches of which alloying is probably the oldest in the case of metallic films. Oxides, nitrides and other compounds have also been utilized in place of the pure materials1.2 and even mixtures of metals and dielectrics (" cermets ,,)2,3 have been employed to enhance resistivity and temperature stability of resistors4,5. One of the possible means for changing the film properties is to alter the film structure. This approach has not been utilized so far because of the difficulties encountered in structure control. In fact, undesired structural variations have hampered the industrial production of films. Many film properties are structure sensitive, e.g. magnetic, superconducting, electrical or optical properties. It is obvious that the structure-property relationship will influence reproducibility since the latter depends on the extent to which the structure of all film elements on a given substrate as well as among substrates can be reproduced. Therefore, the film structure is of extreme importance both for the present production of films and for the inherent possibility of altering the film properties. Most reviews which have appeared so far, have either predominantly discussed the nucleation and growth phenomena without emphasizing their connexion to the deposition conditions6-12, or treated mainly the relation between deposition parameters and film properties (e.g., Ref. 13-15). There seems to be a gap which the present paper attempts to fill by a discussion of film growth and structure as related to the deposition conditions. Since only the most important aspects can be treated, the reader is referred to the original publications referenced for further details.

When an atom approaches a surface (substrate), forces will arise between the former and the atoms of the latter as soon as the distance has become of atomic order. " Van der Waals forces " are caused by the fact that the centers of the positive and negative charges of the atom do not coincide in all time elements, even though this is the case in the time average. The oscillation of the charge results in a dipole moment, and the dipole of one atom induces a dipole in the other. The forces arising depend on the separation of the atoms as well as their polarizability and ionizing energies. When atoms come so close to each other that their electron shells penetrate, then'" interatomic forces " arise. They are I

I

500~--

I

1

-4D.- --..~

I - - - - . 0 - ~.

- 3o0

[

o

400 200

~(~

30C

o 200 ,oo

too

~...O.._..._.~ ,_..d

0

.~_~ _ J ~ r h ~ l 2o0

i

c~ i Au p,... o 40o

600

Time (h)

FIG. 1. Variation of adhesion with time for Fe, Al and Au films deposited on glass at a residual air pressure of 10-5 torr (after Benjamin and Weaver17).

337

338

K . H . BEHRNDT

of the valence or homopolar type when two atoms share one or more electrons, and ionic or heteropolar when one or more electrons from one type of atoms are transferred to the other. The appearance of interatomic forces implies what is normally called a "chemical reaction" between film and substrate. Both covalent and ionic bonds are stronger than the energy arising in " physical " adsorption (see Ref. 16 for further details). From this discussion, it is apparent that the strength of the bond between film and substrate depends on the material of both. The experimental results of Benjamin and Weaver17 shown in Fig. 1, prove that the adhesion differs appreciably for Fe, A1 and Au films on glass substrates. However, it is also apparent that the adhesion changes with time after deposition. The same authors found 18 that these changes are due to the formation of an oxide layer between film and substrate. The adhesion is excellent when this layer is well developed (e.g., in Fe) while poor adhesion (as in Au films) results when no oxide is formed. In this latter case, the adhesion is probably due to Van der Waals forces only. The formation of the oxide layer depends strongly on the affinity of the film material for oxygen. Film structure (pores and/or grain boundaries to permit the permeation of gases) and surface mobility are additional factors.

inside of the evaporator, recontamination of the surface from the residual gas atmosphere is possible19. Back streaming of oil vapors from the diffusion pump has been shown to influence film adhesion20. In addition, outgassing of the evaporant material can cause the deposition of a contaminating film. Such a film formed on the areas of a glass slide exposed to the residual gas atmosphere through a mask is shown in Fig. 2. The breath pattern (see Appendix I) on the contaminated areas can clearly be seen. This contamination occurred in spite of a shutter between source and substrate and was due to the gas evolving from the nickel-iron melt 21. Sometimes, baking of the substrate and/or maintaining it at elevated temperatures will improve the film adhesion because the contaminant adsorbed on the substrate may be evaporated or its deposition from the residual gas atmosphere prevented (due to enhanced re-evaporation of adsorbing gas).

3. Surface mobility After an atom has arrived on the surface of a substrate, it has " surface mobility ", i.e. it can travel over a certain distance which depends on the energy available for this process. Energy of condensation and thermal energy corresponding to the substrate temperature are the con-

FIG. 2. Breath pattern on contaminated glass substrate (after Maddocks and Behrndt20. The adhesion is influenced also by the cleanliness of the substrate. Contamination of only a few atomic layers thickness alters the Van der Waals forces appreciably and may also prevent the formation of an oxide layer. Therefore, considerable attention has been given to cleaning of the

substrate (see Appendix I). However, even after cleaning the tributing factors. But for a given pair film-substrate the temperature of the latter is of primary importance since Levinstein22 has shown that the velocity of the arriving atom will, in general, not influence the film structure.* The

*The influence of atomic velocity is still the subject of some controversy22a.

Influence of the Deposition Conditions on Growth and Structure of Evaporated Films distance an a t o m can travel due to the energy of condensation and substrate temperature depends on the strength of the forces between a t o m and surface, i.e. on the material of both. Campbell23 has shown recently that the surface mobility of gold varied by a factor of 10 a m o n g the substrate materials investigated.* C o n t a m i n a t i o n will also influence the conditions, since it alters the bond strength. W h e n an a t o m collides during its motion on the surface with another a t o m of the film material which had arrived previously, then the two atoms m a y combine to form an a t o m pair. Since energy o f association is released in this process 24, the surface mobility of a pair is appreciably lower than that of an individual atom. Therefore, the average surface mobility in the initial stages of growth depends on the number of pairs, triplets etc. formed, i.e. the probability

339

for collisions. The latter is a function of the n u m b e r of high rate is almost continuous, the 175 A film deposited at a rate 600 times lower consists still o f isolated islands and even the 560 A film of Fig. 3b is not completely continuous. Apparently, the larger number o f nucleation centers at the high deposition rate results in the formation of considerably m o r e islands which grow together at a low film thickness. Conversely, the fewer islands formed at the low rate grow up initially in their vertical rather than the planar dimension, and a continuous film is attained at a much higher thickness.**

4. Ordering processes in the film I f the substrate is a single crystal then the arriving atoms will frequently arrange themselves also in an ordered manner,

FIG. 3. Difference in the structure of silver films deposited on formvar in different deposition times (a) 2 sec ; (b) 20 min (after Sennett and Scott25). atoms arriving on the surface per unit time, viz. the rate of film deposition. D u r i n g the initial stages of film growth pairs, triplets, etc. can serve as nucleation centers for crystallites. The importance of the deposition rate is clearly seen in Fig. 3 where electron micrographs of silver films are shown2S. All films of Fig. 3a were deposited simultaneously in 2 sec, while the deposition time for the films of Fig. 3b was 20 rain. While a 180 A film deposited at the

and " e p i t a x i a l g r o w t h " is observed.t However, oriented growth in areas larger than isolated islands requires a sufficient surface mobility. This fact is illustrated by Fig. 4 which shows electron diffraction pictures of 400 A a l u m i n u m films on NaC132. A t 100°C substrate temperature, the film is apparently polycrystalline. Ordering begins at 200°C, and a t a substrate temperature of 300°C, the A1 film is well ordered.

*On single crystalline substrates, the bond strength differs for the various crystallographic planes. There are even preferred directions of travel for adsorbed atoms, e.g. along the potential troughs between adjacent rows of atoms in the substrate. See for instance Ref. 16, p. 14 ff., where the pertinent literature is also quoted. **The influence of deposition rate was also investigated by Levinstein22 (structure of Sb films) ; David26, Perrot and Tortosa 27 and Philip2S (optical properties of Ag and Au) ; Hass29 (optical properties of SiO) ; Scow and Thun30 (structure and electrical properties of CO ; Engelman and Hardwick31 (structure and magnetic properties of Ni-Fe) ; and Campbell23 (growth of LiF and Au). tFor review concerning growth of films and epitaxy see Ref. 6-12 and 14 also Ref. 33. The formation of films is discussed in Ref. 16, Chapter I, 2.

340

K.H.

In "single crystal" films, two of the crystallographic axes have fixed directions while the crystallites are randomly oriented in "polycrystalline" films. The latter type usually develops on amorphous substrates, at least in most metal films. However, between randomness and complete orientation, a state of order is possible in which one or two crystallographic axes are favored over the others. This "fiber axis texture " h a s frequently been observed for numerous materials on amorphous and polycrystalline substrates (e.g., Ref. 16, p. 106 ff and Ref. 33 ; for a review see Ref. 34). Oriented growth is also possible on polycrystalline substrates to the extent that films can grow on the crystallites comprising the substrate. Figure 5a shows an electron micrograph of a 50 A nickel film electroplated on a Cu substrate and Fig. 5b a picture of a 200 A film on a Zn surface3L It is obvious that the structure of these films differs appreciably and, as documented by further pictures in Ref. 35, conforms to the surface structure of the substrate. However, only electroplating generally yields this replication. In evaporated films, the surface mobility must be sufficiently high before the film atoms will orient themselves on the crystallites of the substrate. Figure 6 shows electron micrographs of nickel films36 evaporated on to the same Cu substrate as employed for Fig. 5. On the 20°C substrate of Fig. 6a, the nickel film consists of 'very small crystallites which do not replicate the surface structure. In Fig. 6b, the substrate temperature during deposition was 300°C. Now, the crystallites of the film are much larger and the orienting influence of the substrate can be seen clearly. It should be pointed out that it was possible to establish by employing both dark and light field electron microscopy, that some of the largest grains were single crystals, e.g. the two grains appearing dark in the upper center of Fig. 6b. This fact implies that there is, beyond replication of the surface structure of the substrate, true oriented growth of the film on grains of the substrate.

5. Grain size In the initial stages of condensation, the atoms on the substrate can form a continuous film or islands, as was illustrated in Fig. 3. The mechanism of island formation was considered as a direct transition from the vapor phase into the crystalline state by Frenkelz4, while Semenov37 assumed a vapor-liquid transition. Which of these mechanisms prevails depends on the material of the film (and substrate) Wh~e Z n generally consists of~hexagonal crystallites in the initial stages of growthS8, Sn is o f t e n deposited in the form of round particles39. It was recently shown by Palatnik and Komnik40 that Bi can condense as crystallites as well as in the form of liquid (supercooled drops on the same substrate material). For a given deposition rate, the mode of growth depends only on the substrate temperature. Little is known about the initial stages of condensation

BEHRNDT

for various materials and substrates since experimental evidence is very difficult to obtain.* Several theories have been advanced11,12,33,43-49 but the situation is complicated by the fact that minute amounts of contamination can alter the conditions of growth completely. If the contaminant forms a continuous layer, then it may become the "substrate material" in terms of the energies arising between film and substrate. Conversely, if the contamination is present in the form of isolated islands, then the film may preferentially nucleate either at the islands or at the exposed substrate surface, depending on the energies between film-substrate and film-contaminant. It has been shown by LangmuirSO that the presence of nucleation centers will influence film growth to a very large extent. TraubSXand Zehender52 have employed an extremely thin film of Ag (down to 1/1000 of a monoatomic layer) to provide nucleation centers artificially. Figure 7 shows the grain size of Cd films for different amounts of Ag nucleation, and the reduction in grain size is very apparent. The results indicated that the effectiveness of the nucleation depends primarily on the materials of center and film, while other deposition parameters may be of secondary influence. It was pointed out in the previous section that the grain size of the substrate will influence that of the film, and the surface roughness of the substrate is important also (see Appendix II for further details on surfaces). Therefore, it may be advantageous to employ amorphous substrate materials. Fire polished soft glass has been reported to be particularly smooth with a short range surface roughness below 10 A53. It has also been attempted to cover surface irregularities by the predeposition of a layer of SiO of several 1000 A 54 up to 10,000 A thickness55, and electron micrographs show that the SiO indeed results in a smoother surface. Rubbing the surface with chalk will also reduce the roughness. Although this treatment was. mentioned by Strong (Ref. 56, p. 153), it was only recently proven to be effective with respect to the surface roughness57. Both methods, SiO-predeposition and chalk rubbing, improve magnetic properties and reproducibility of ferromagnetic films54,55,57,58 because a smoother, more uniform substrate surface is attained. In addition to influencing adhesion, surface mobility and ordering processes, the substrate temperature will also affect the grain size. Figure 8 shows aluminum films of about 500 A thickness which were deposited at varying substrate temperatures59. It is apparent that the grain size increases appreciably with substrate temperature.** The properties of thin ~ 8 a_r_efrequently found t Q ~ d ~ r appreciably from those of the bulk material, Numerous authors have pointed out that film structure and grain size can be responsible for these discrepancies. For instance, Drumheller61 has shown that the resistance of evaporated Bi films increases appreciably with decreasing grain size, i.e., increasing number of grain boundaries. However, several

*Depositions inside of the electron microscope are likely to provide more information. This technique has considerably progressed since the experiments of McLauchlan, Sennett and Scott39 such that motion pictures of the film growth were recently obtained by Pashley41 and Poppa 4z. **The influence of the substrate temperature is also discussed in Ref. 16, Chapter I, 2 (numerous references) ; for magnetic films, a summary is given in Ref. 13. Recently, the influence of the substrate temperature was discussed by Bourn0 (structure of cryolite films), Engelman and Hardwick31 and Scow and Thun30.

Influence of the Deposition

Conditions

on Growth and Structure

of Evaporated

FIG. 5. Nickel films 50 and 200 A thick, electroplated on to (a) copper (15,000 X)

Films

; (b) zinc

(15,000 X) (after Reimeds).

FIG. 6. Nickel films, 200 A thick, evaporated on to Cu at different substrate temperatures (a) 20°C (60,ooOX) ; (b) 300°C (3,500X) (after Reimerss).

341

342

K.H.

BEHRNDT

FIG. 7. Grain size of Cd films (a) with 10-2 monoatomic layer of Ag ; (b) with 10-3 monoatomic layer of Ag ; (c) without artificial nucleation (after Zehender52).

FIG. 8. Aluminum films, approximately 500 A thick, deposited at different substrate temperatures (a) 20°C ; (b) 150°C ; (c) 450°C (after Hass and SeottS9). investigators have succeeded in obtaining bulk properties even in very thin films. F o r instance, Reynolds and Stilwel162 obtained A g and Cu films with bulk resistivities (if the reduction of the m e a n free path of the conduction electrons is taken into account) down to 100/~ thickness. Neugebaner63 deposited N i films with bulk values of saturation magnetization down to about 20/~. In both cases,

special deposition conditions (deposition rates of 500 to 1000 A/see at pressures of 10-5 torr and pressures of 10-9 torr at low rates, respectively) were maintained. It was pointed out recently by Neugebauer64 that the properties of " u l t r a t h i n " films (below 20 or 30/~ thickness) depend critically on size and separation of the islands comprising the films.

Influence of the Deposition Conditions on G r o w t h and Structure of Evaporated Films 6. F i l m c o n t a m i n a t i o n The properties of m a n y materials, e.g. semi-conductors, resistors or ferromagnetics, depend strongly on the presence of impurities, and introduction of c o n t a m i n a n t into the film must be prevented13,65. Since impurities in the evaporant can appear in the film, the purest materials available must be employed for film fabrication. A second source of contamination is the evaporation of the material supporting the evaporant, i.e. the crucible, b o a t or other device. Heavens 66 reported that A g and G e films evaporated f r o m W or Ta boats contained varying amounts of the boat material. F o r other film materials requiring higher evaporation temperatures or forming alloys with W and Ta, the danger o f evaporation of container material is certainly even higher13,65. Finally, " g a s e o u s i m p u r i t i e s " are important. They consist of the gas molecules trapped in the film during its formation. Gas occlusion depends on several f a c t o r s : adhesion to the substrate of the gas species present in the residual gas atmosphere and their desorption probability, which increases with the substrate temperature ; the residual gas pressure p and the rate of film deposition r 67,68. Gawehn69

343

has calculated the upper limit for trapped gas. H e found that for deposition rates below 300 A/see the p h e n o m e n a occurring at the substrate are predominant while at higher rates collisions between metal atoms and residual gas molecules during the flight of the atoms f r o m source to substrate are m o r e important. Several experimental investigations concerning the relation between residual gas pressure and film properties were performed 67,70-72. The electron micrographs of Fig. 9 show that the structure of tin films deposited in ultra-high v a c u u m differs appreciably f r o m those formed in the presence of oxygen at a p/r ratio of 0.09 oxygen molecules per tin atom71. It is not surprising that the film properties are influenced n accordance with these structural differences.

7. O t h e r influences Some film materials are easily oxidized, and oxide layers at the interface of f i l l and substrate can improve the adhesion. However, if oxide is dispersed through the film or formed on its surface, then the film properties can be influenced appreciably (for a detailed discussion see Ref. 13). The composition of the residual gas atmosphere, in particular the presence

TABLE I

Relations between Film Properties and Deposition Parameters Film Property

Grain size

Influenced by Type of material of the pair film-substrate. Contamination of substrate (nucleation centers). Surface mobility of film atoms (substrate temp., rate of deposition). Surface structure of substrate (surface roughness, grains). Annealing of film.

t

* S C T S

Crystallographic order in film

~

S C T

Adhesion of film

f Material of pair film-substrate. Additional processes (e.g. oxide formation at film-substrate interface). Contamination of substrate. Surface mobility of film atoms.

S S C T

Type of substrate (single- or polycrystalline or amorphous). Contamination on substrate (destruction of ordering influence). Substrate temperature (sufficient surface mobility required).

f Purity of evaporant. "~Evaporation of container material.

Contamination of film

Trapped gas

composition of residual gas atmosphere material of substrate and film substrate temperature residual gas pressure p rate of film deposition r probability for in-flight collision

~

S T

Oxygen affinity of film material.

Oxidation

I Composition of residual gas atmosphere. -~ Adsorbed water layers on substrate. l Substrate temperature. p[r ratio.

Stress

f"Material of film and substrate. t Substrate temperature. Grain size, impurities, crystallographic defects in film. Annealing. Angle of incidence of vapor stream (also affects other film properties).

S T S T

*The last column contains symbols (S = substrate, C = contamination, T = substrate temperature) for the most frequent parameters to permit a quick survey.

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K . H . BEHRNDT

of water vapor, or water layers adsorbed on the substrate surface, will influence the oxide formation. Annealing of the film frequently enhances the amount of oxide, even when it is performed in vacuo or a "protective atmosphere ". In the latter case, small oxygen impurities in the atmosphere are sufficient to cause oxidation because of the elevated substrate temperatures and prolonged time of exposure. The annealing temperature also enhances the surface mobility, and increased grain size is generally found afterwards. Some film properties are stress sensitive13. A planar stress system frequently arises from differences in the coefficients of thermal expansion of film and substrate upon cooling from the deposition temperature. Besides, internal imperfections and impurities can create a complex stress system74. Annealing often reduces the stresses74-76, but a detailed discussion of this subject goes beyond the scope of the present paper.* Some film properties are sensitive to the angle at which the vapor is incident upon the substrate. Effects have been observed for optical properties (e.g., Ref. 84, p. 330 IT) and especially for magnetic properties (e.g. Ref. 77, 79, 85-89).

by the deposition parameters are summarized in Table I. It is seen that each film property depends on several of the deposition conditions. Conversely, the same deposition parameter frequently influences several of the film properties. For instance, the material of the substrate and its surface structure is of importance for all film properties listed. Therefore, it is very essential to choose the most suitable substrate for a given film material and application. Also, contamination of the substrate surface will alter several film parameters, hence the attention given to cleaning procedures (see Appendix I). Finally, the temperature of the substrate affects every aspect of film growth, and its measurement and control is of primary importance. However, this is not meant to imply that those parameters which affect only one or two of the properties were unimportant. In fact, if reproducible films are to be fabricated then all parameters should be maintained constant during and among depositions, and the detrimental influences causing contamination, oxidation and stress must be eliminated. This brief summary of the relations between film structure and deposition parameters illustrates the complexity of the whole situation. On the same substrate material and under

FIG. 9. Structure of Sn films (a) deposited in ultra-high vacuum; (b) deposited at a ratio of oxygen moleculesto Sn atoms (p/r) = 9 per cent (after Caswel171). I n the latter case, the results were explained by stresses 77,79 which were recently shown to be also introduced into SiO films90. However, the majority of the workers concluded that the magnetic properties are predominantly influenced by a structural anisotropy in the film resulting from the angle of incidence91. Since a detailed discussion would go beyond the scope of this paper, it should only be noted that angle-of-incidence effects can be important, and their elimination must be considered when the fixtures for the evaporator are designed92.

8. Summary and conclusions The influences exerted on growth and properties of films

identical deposition conditions, different film materials will vary in their behaviour due to changes of surface mobility and of the energies between film and substrate. For instance, superconducting materials usually exhibit larger grain sizes on room temperature substrates than are found in ferromagnetic films deposited at 300°C substrate temperature. Because of this substantial effect of the f i l l materials it is impossible to make precise predictions concerning grain size and structure of a given material, even if all deposition conditions are known. It was the purpose of the present paper to describe general relations and the reasons for their occurrence. It is hoped that they may be valuable as guide in the vacuum deposition of various film materials.**

*For stresses in metal films see Ref. 73-80, in dielectrics Ref. 23, 81-83, summaries in Ref. 13, 79; and stress measurements, e.g. Ref. 23, 79, 80. **For a review of deposition methods, see Ref. 92 ; control of the deposition parameters is discussed Ref. 65, 92.

Influence of the D e p o s i t i o n Conditions on G r o w t h a n d Structure o f E v a p o r a t e d Films

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APPENDIX

I

Detection and achievement of clean surfaces Observations concerning the cleanliness o f glass surfaces were already m a d e by L o r d Rayleigh 93. H e noticed the appearance of " black breath p a t t e r n s " u p o n achieving a high degree o f cleanliness, and the " breath t e s t " has been employed since that time as a simple but sensitive means for assessing cleanliness. The breath test is performed by breathing on the surface such that moisture condenses on it. O n clean surfaces, a continuous film is formed which does not scatter the light and therefore appears black. In contrast, the moisture film on contaminated surfaces consists of small droplets and appears grey (see Fig. 2, also Ref. 94 or Ref. 56, p. 165). This m e t h o d permits detection of about one monolayer of contamination. The " w a t e r break test ,,95 yields the same sensitivity while the " atomizer test "95,96 indicates a contamination of about 1/10 monolayer. L o r d Rayleigh's experiments, which resulted in black breath patterns on glass surfaces cleaned with h o t acids, initiated numerous papers on chemical cleaning (e.g. Ref. 97-100). These processes frequently included a step o f " a c i d c l e a n i n g " in which the glass surfaces were etched on a microscopic scale (e.g. Ref. 54, 58). A l t h o u g h etching will certainly introduce etch pits and grooves in the surface, an over-all improvement is still possible in cases where large disturbances (up to 1 0 0 0 A high and several microns in diameter were detected in a particular brand o f microscope slides) located in the surface layer are removed by the etching processl0t. However, drastic treatments should be avoided generally, and it was shown recently that even cleaning of soda-lime glasses in a hot detergent solution can result in n o n u n i f o r m etching exposing ridges or lines 102. Therefore, m a n y recent cleaning techniques are based on v a p o r de-

greasing (e.g. Ref. 84, p. 71, Ref. 103, 104) and ultra-sonic cleaningt03,105,106 often combined with a continuous flow o f deionized water107,109. It should be noted that the cleanliness achieved by any process or c o m b i n a t i o n of methods depends not only on the effectiveness o f the individual steps but also on the m a n n e r in which t h e s u b s t r a t e is d r i e d after the final washing. Chemical agents like acetone can recontaminate the surface. Even wiping with a cloth m a y leave a residue (e.g. Fig. 5 of Ref. 104). Therefore, some people use c o m pressed filtered air to blow off the moisture from the substrate surface immediately before inserting it into the v a c u u m system, while others dry them in a well cleaned oven. Substrates are frequently subjected to additional cleaning inside the v a c u u m system. Electron or ion b o m b a r d m e n t are a m o n g the methods m o s t widely used (e.g. Ref. 59, 110-113 or Ref. 84, Ch. 3), and even surfaces clean on an atomic scale could be obtained (Ref. 114-116). Other techniques for achieving atomical cleanliness, but less c o m m o n l y applicable, are heating of the substrate to 1500°C or higher117 or breaking of a crystalline substrate inside of the v a c u u m chamber. It is likely that melting of the surface o f the (glass) substrate in vacuo I04 will yield comparable results. A quantitative evaluation of the various cleaning methods is difficult because slight differences in procedure or apparatus can influence the results. This was shown by Holland 111 w h o found that the parameters in glow discharge cleaning and the composition o f the residual gas atmosphere greatly affect the processes at the substrate. Nielsen104 c o m p a r e d several techniques and found that v a p o r degreasing and (his) ion b o m b a r d m e n t yielded relatively clean surfaces while careful " f l a m i n g - 1 1 0 or vacuum melting were even m o r e effective.

Influence of the Deposition Conditions on Growth and Structure of Evaporated Films

APPENDIX - Surface structure

Since structure and roughness of the substrate surface influence the film properties and their reproducibility54.55,57,58,101,119,120 knowledge of the material employed is important. Several methods have been utilized for investigating surface roughness, such as electron microscopy 121,122, multiple beam interferometry123-125 and a stylus method126,127. Although the sensitivity of the latter is lower than electron microscopy or interferometry, recorder traces of the investigated surface can be obtained in which 2500 A of roughness correspond to 1 in. on the chart paper and 0.01 in. of surface travel to 1 in. on the chart, or 105/~ linear dimension of a disturbance to 1 m m on the chart120. The same instrument has even permitted measurements of step heights to ~ 2 5 /~23,127. Therefore, the sensitivity is sufficient for evaluations of surface roughness, and the recorder traces can be employed to obtain average values. Since such measurements can be performed rapidly, the technique is particularly valuable for continued monitoring of the surface roughness of substrates in order to assure their similarity. Tests of this type are essential since it was found that microscope slides obtained from several suppliers displayed different types of surface inhomogeneities. Besides, the cleaning process can also

347

II

of the substrate

influence the surface roughness (see Appendix I). However, continued measurements of the substrate have scarcely been made so far because they were too time consuming. The stylus method may be suitable to fill this gap. As mentioned in Section 6, soft glass was found to display a very small surface roughness, particularly after fire polishing. Electron microscopy indicated qualitatively the following sequence of increasing surface roughness 122 : glass, glazed alumina, " L u c a l o x " (a translucent alumina), mica, lapped alumina, alumina with SiO overcoat and untreated alumina. More quantitative results were obtained by the stylus method. The smoothest material listed in Ref. 120 was quartz optical flat which was superior to soft glass drawn sheet. The aluminas investigated improved appreciably after polishing, and polished photosensitive ceramic was equal to the quartz optical fiat. Polishing can also improve the smoothness of glass appreciably125A2s. The surface roughness decreases with increasing hardness of the material, until 36 A is attained as the root mean square value for fused quartz12S. This figure is lower by almost a factor of 3 than that reported for the quartz optical flatl20. It is not known whether the discrepancy is due to differences in polishing or measuring technique.