Vacuum-deposited gold films

Surface Science 264 (1992) 312-326 North-Holland

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Vacuum-deposited gold films I. Factors affecting the film morphology

Yuval Golan, Lev Margulis and Israel Rubinstein * Department of Mutetials und Interfaces, The Weizmann Institute of Science, Rehor>ot 76100, Israel

Received 8 July 1991; accepted for publication 18 October 1991

Thin (300-1000 A) gold films were deposited on glass, mica and silicon substrates (preheated or held at room temperature) by sputtering or evaporation. The films were characterized by transmission electron microscopy (TEM), electron diffraction (ED) and voltammetric measurements. Gold sputtering produces pebble-type structures with very small grains and no crystallographic texture. Evaporation of gold onto glass or mica produces large, flat crystallites, with a pronounced (Ill} texture, white on smooth silicon (100) it results in non-textured films. Annealing of the films at 250°C always has the effect of grain enlargement, and, in the case of gold on glass or mica, enhancement of the (111) texture.

1. Introduction

The interest in the nucleation, growth and orientation of thin metal films began shortly after the first experiments on the interaction between electron beams and crystals were carried out by Kikuchi [ll and the interpretation of electron diffraction (ED) patterns first presented by Lin- nik [2]. Briick [3] studied the influence of the substrate on the orientation of deposited metal films. Using polished quartz plates or smooth molybdenite (both polycrystalline) as substrates, he obtained oriented polycrystalline Au films even at high substrate temperatures (500 to 700°C). When condensed on quartz at 480°C the gold showed an orientation with the {ill) pIane parai- lel to the surface, and without any preferred azimuthal direction. On freshly cleaved, heated single-crystal rock-salt substrates, areas of single- crystal gold were readily obtained. He also reported a marked dependence upon the temperature of the substrates. Above a certain critica temperature, (the “epitaxial temperature”), well- defined orientation was obtained, while below this temperature the growth included at least

* To whom correspondence should be addressed

some randomly oriented deposit, Studies by Riidiger [4] were devoted to metallic films obtained by condensation on various heated surfaces in&ding mica and quartz. His results con- firmed the dependence of the orientation on the substrate temperature, as reported by Briick. Gold evaporated onto heated mica at a temperature of 300 to 400°C revealed crystallites of octahedral faces, azimuthally oriented within the film. He also reported that the texture was poorer in thin films with respect to that observed in thick films.

Various studies on epitaxial growth of fee metals on mica substrates were reviewed by Pashley Es]. These films are characterized in general by the occurrence of mixtures of orientations which are temperature dependent. The texture with the (Ill} plane parallel to the surface is reported as dominant; metal crystallites in the form of hexagonal plates were visible under the optical microscope. For gold, the reported epitaxial temperature was 450°C resulting in the {Ill} plane being parallel to the mica surface, with very large misfits up to -68%.

Detailed investigations on the epitaxy of gold deposited at high temperatures (HTI on mica were carried out by Poppa et al. [6] using ED. They found two basically different “epitaxial” orientations, both aligned parallel to the pseudo-

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Y. Golan et al. / Vacuum-visited gold fdms. I 313

hexagonal mica surface structure: a fill) orientation and the not previously reported (211) orientation of the deposited gold crystals, either of which were predominant in different, small regions of the substrate surface. They proposed that heterogeneities in the actual structure of the cleaved mica surface may be responsible for the simultaneous appearance of the two different “epitaxial” gold orientations.

X-ray diffraction studies by Reichelt and Lutz (71 ~nfirmed the existence of two possible orientations. In the case of room temperature (RT) deposition, both orientations of the gold (liO> parallel to the mica (Ol?O) and (OilO) were observed. Upon increasing the substrate temperature above 4OO*C, only the orientation with the gold (ITO) parallel to the mica (OliO) was found. They also reported a strong dependency of the evaporation rate on the transition temperature required to obtain a given crystal quality; the higher the evaporation rate (evaporation rates from 0.07 to 25 As-’ were used), the higher the substrate temperature necessary to obtain well- oriented surfaces.

Renewed interest in the deposition of smooth, well-oriented metal films has arisen lately, to a large extent related to the development of the scanning tunneling microscope ISTM). The fasci- nating possibilities of imaging with atomic resolution led to a need for homogeneous, atomically smooth and electrically conducting surfaces, besides graphite, silicon and some other semicon- ductors, presently used as specimens for STM studies. A number of papers describing imaging of gold films by STM have been published in recent years [8-121. The STM has significant advantages in examining topography of surfaces, including the normal axis (the STM z-direction) with atomic resolution.

Chidsey and coworkers [8] studied 1200 A gold films evaporated on mica, using STM, X-ray diffraction and transmission electron microscopy (TEM). By X-ray diffraction, the gold films deposited near RT show weak “epitaxy” (i.e., poor in-plane orientation) with the underlying mica. They are, however, highly textured, with the gold 1111) planes parallel to the substrate surface with an angular spread of 1-2”. The STM images are

characterized by a “rolling-hill” topography. Gold deposited on mica at 225°C is characterized by larger crystallites with flatter tops. Most of the surface consists of high, flat plateaus, with grooves and depressions at the grain boundaries. Accord- ing to the X-ray diffraction measurements, the orientation of the crystallites in the HT samples is epitaxial, with both the (111) texture spread and the in-plane mosaic spread smaller than those obtained by deposition near RT. Upon increasing substrate temperature even flatter films were obtained, but the quality of epitaxy was poorer, in contrast with the results of Reichelt and Lutz [7]. The single TEM micrograph presenfied [8] shows a holey film at a thickness of 500 A, with twins and dislocations observed within the crystallites. The effect of annealing of RT samples (RT-A) is reported to improve the smoothness of the films, approaching the morphology of the samples deposited at higher temperatures, but with a much better epitaxial quality in the RT-A samples than in the HT samples.

STM images of gold evaporated on mica were also obtained by Vancea and coworkers [9] as part of a comprehensive study of gold deposited at HT on a variety of substrates. They classify gold under the “mixed surfaces” category - surfaces with varying topographi~l character. Atom- ically smooth plateaus of 600 to 1000 A lateral extension are observed, along with typical polycrystalline surfaces with, hillocks in the height range of a few tens of A. Such a mixed surface with HT evaporated gold on mica has been previously reported by Salmeron and coworkers [lo]. They reported an atomically smooth surface over regions thousands of A wide, combined with ar- tas consisting of occasional bumps, a few tens of A high. As previously suggested by Poppa et al. [6], the authors [9] suspect that the imperfect cleavage of mica in air gives rise to regions with different defect characteristics and densities; therefore, different incipient growth mechanisms may coexist on the same substrate surface.

The nucleation and growth of gold evaporated at RT on glass have been extensively studied by Andersson and coworkers using TEM [13,14], although the crystallographic structure and orientation have not been investigated. These authors

314 Y. Golan et al. / Vacuum-deposited gold films. I

reported a large spread in the island size, accom- panied by a liquid-like coalescence at thicknesses of SO-120 A. Gardiner and Stiddard [15] studied the growth and orientation of thin films of silver on glass by high-energy reflection ED. On substrates at RT a (111) texture developed as a function of increase in the film thickness. Anneal- ing at 300°C led to an enhanced texture. On substrates evaporated at 300°C similar texture was obtained, and in addition, arrays of multiply twinned crystallites were formed. Vancea and coworkers [93 used STM to image surfaces of gold deposited at HT on fire-polished Corning-glass substrates. They have found a rough, uncorre- lcted surface, with a typical z-corrugation of 110 A, and the grain sizes varying roughly between 200 and 400 A.

Gold evaporated at HT on silicon (1001 single crystals is reported [9] to be rougher than on glass. A “hill-like” landscape was observed, with an average height variation of about 160 A. The lateral extension of the hillocks is increased from 400 A for gold on glass to 800 A for gold on silicon (1001, with a more homogeneous grain-size distribution. Mathieu et al. [161 used TEM and ED to study deposits of gold on silicon (111) (experimental deposition conditions are not reported). The surface was found to contain very large crystallites of about 5000 A in size, with the [OOl] direction of the crystallites parallel to the [Oil] direction of the silicon substrate. A mechanism for the nucleation of gold on silicon has been proposed [9,161, based upon the formation of a presumably amorphous gold silicide (Au-Si alloy) layer on the silicon surface. With increasing film thickness, new layers of pure gold crystallites are deposited.

STM has some unique features, notably the capability to image surfaces with atomic resolution. TEM, on the other hand, has some comple- mentary capabilities: (i) a magnification associated with a resolution better than 10 A; (ii) ability to identify the crystallographic structure of specific regions by using selected area diffraction and dark-field techniques; (iii> interpretation of TEM images is well understood and rather straightforward. The main limitations of TEM are (i) inability to examine the topography in the

normal axis; (ii> imaging in the transmission mode of gold at, e.g., 120 kV requires film thicknesses below - 500 A. The latter limitation seems less serious considering the STM study [8], which finds minor morphologicalodifferences between films of 500, 1200 and 4800 A.

This paper presents a comparative TEM study of thin gold films deposited on several substrates, using two deposition techniques under various conditions. The present study has been stimu- lated by the increase in the number of STM publications on gold deposition on the one hand, and the clear need for a comprehensive TEM study on the other hand. Also, to our knowledge, sputter-deposited gold films have not been sys- tematically studied or imaged before.

2. Experimental

2.1. Chemicals

HF (Fluka, AR), H,O, (Frutarom, chemically pure), NaOH (Merck, AR), and H,SO, (Palacid, AR) were used without further purification. H,O was triply distilled.

2.2. Substrates

Microscope glass cover slides (Deckglass, Su- perior) were cut into 22 x 11 mm2 pieces, chemically cleaned with acetone, and dried under a stream of argon.

Optically-polished p-type single-crystal silicon (1001 wafers (Aurel GMBH, Landsberg, Ger- many) were cut into - 25 x 10 mm2 slides, soni- cated in absolute ethanol, and washed with triply distilled water. They were then left in 0.5 M NaOH + 10% H,O, for 10 min, washed as above, and transferred to 33% H,SO, + 10% H,O, for 10 min, washed, left in water at 90°C for 30 min, and dried under an argon stream.

Mica plates (SPI) were cut into - 20 x 10 mm2 pieces, and mechanically cleaved. The freshly cleaved faces were used without further treatment.

Y. Golan et al. / Vacuum-deposited gold films. I 315

2.3. Gold deposition

Sputtered gold films (350 and 1000 A thick) were prepared at room temperature (RT) in an Edwards S150B sputter coater using an argon plasma at _ 0.t mbar and 10 mA. Deposition rate was m 1.4 A/s.

Evaporated gold samples (350 and 1000 A thick) were prepared by mounting the substrates in a cryo-HV evaporator (CT&KEY) equipped with a stainless steel shutter for degassing of the metal source. The distance between the source and the substrates was 23 cm. Homogeneous deposition was obtained by moderate rotation of the substrate holder. Gold (99.99%) was evaporated from a tungsten boat at lO-‘j Torr, at a deposition rate of 1.0 A/s. The film thickness was measured with a Maxtek TM-100 thickness monitor. Heating the samples for HT deposition was done radiatively with two halogen lamps fac- ing the samples. The temperature was monitored by a thermocouple positioned 2-3 mm from the substrate surface. Deposition temperature for RT slides was 50-75°C #l, and for HT slides was 250-275°C #l. Ti was evaporated at RTOas de- scribed above, at a deposition rate of 0.1 A/s, to a final thickness of 50 A.

Post-deposition thermal treatment (annealing) of both sputtered and evaporated Au-covered slides was carried out in air, at 250°C for 180 min (overshooting often occurred up to 265°C). Heat- ing rate was lO”/min; after annealing the slides were left to cool to ambient temperature.

2.4. Transmission electron microscopy (TEM)

Samples for TEM Danalysis were prepared by carefully floating 350 A gold films in aqueous HF. Samples peeled off readily from glass substrates (5% HF); gold on mica or silicon required a more concentrated solution (10% HF). The floated gold films were lifted onto copper G-400 (Polaron) TEM grids, and dried at 45°C for 30 min. The samples were examined at room temperature using a Philips analytical EM-400T electron micro-

” The gold morphology was not affected by variations of the temperature within this temperature range.

scope operating at 120 kV. Bright field (BF) images were taken at a magnification of 100000 X

(magnification error was f5%, as obtained by magnification calibration). ED patterns were obtained at a camera length of 800 mm.

2.5. Cyclic voltammetry

Slides coated with 1000 8, gold were plasma- cleaned (in an Edwards S150B sputter, with Ar plasma at 0.4 mbar and 2.5 mA) and mounted as working electrodes in a conventional three-electrode cell. The counter and reference electrodes were, respectively, a Pt coil and a mercurous sulfate reference electrode (+400 mV versus KCl-saturated calomel electrode, SCE). The elec- trolyte was O.lON H,SO,. The electrodes were connected to a potentiostat and an electrochemical programmer (both from the Department of Chemistry, Technion, Haifa) and a Houston In- struments 100 recorder.

3. Results

3.1. Sputtered gold

Figs. la and lb show TEM BF micrographs of 350 A thick gold films sputtered at room temperature (SP) #* on glass (G) and mica (M) substrates. The pictures are quite similar; the gold surface resembles a pebble-type construction with a typical grain size (TGS) #3 of about 100 A in diameter (see table l), with white areas which are holes in the film. The image of a gold film sputtered under the same conditions onto an optically polished silicon (100) wafer (Si) (fig. lc) discloses quite a different picture. The TGS is about 300 A, with larger crystallites and flatter-looking film. The films on silicon (100) are continuous at this thickness (350 A>, with no apparent holes. The ED patterns obtained from all three images are identical (fig. 2a). The concentric rings in the

X2 See list of abbreviations. #3 The TGS was estimated by visually examining numerous

TEM micrographs and choosing typical grains for a certain set of deposition conditions.


Fig. 1. TEM BF images of (a) SP-G gold; (b) SP-M gold; (c) SP-Si gold.

Fig. 2. Electron diffraction patterns for (a) SP-Si gold (similar

patterns are obtained for SP-G and SP-M gold); (b) SP-Si-A

gold.

diffraction patterns, showing very intense (11 l} reflections, are characteristic of a non-textured fee polycrystalline film, with no exposure of a preferred plane onto the surface.

Fig. 3 represents the annealed analogs of the samples in fig. 1. In all three samples, annealing leads to enlargement of the grain siz:. An increase to a TGS of about 180 and 250 A, respectively, is observed for gold on glass and mica. In both cases larger grains appear, although the pebble-type construction is still dominant. The gold crystals on silicon tppear flat, and the TGS increases to about 450 A. The grain enlargement

Y Golan et al. / Vacuum-deposited gold films. I 317

Table 1 Typical grain size (TGS) of gold samples (estimated from the TEM micrographs) and the charge under the gold oxide removal peak (from cyclic voltammograms of the kind shown in fig. 12)

Substrate Gold {1111 TGS Charge

deposition texture (iQ W/cm*)

glass SP-A - 180 1090 glass RT + 850 710 glass RT-A + 1500 580 glass HT + 1250 990 glass HT-A + 1500 675

mica RT + 600 900 mica RT-A + 850 635 mica HT + 1250 975 mica HT-A + 1500 675

Si(100) RT - 250 730 SiWIO) RT-A _ 1200 485 Si(100) HT - 550 802 Si(lO0) HT-A - 1000 421

is also reflected in a somewhat more granular appearance of the ED rings (fig. 2b).

3.2. Gold evaporated at room temperature

Fig. 4 represents gold films evaporated at RT on glass, mica, and silicon (100). On glass, a flat-looking film is obtained% with plate-shaped gold crystallites of about 850 A TGS. The gold on mica is similar in shape, except for the TGS which is somewhat smaller, i.e., N 600 A. The sample on silicot is distinctly different, with a TGS of N 250 A and a surface which is very similar to the gold sputtered on silicon. The ED pattern obtained from the glass sample (fig. 6a) indicates a highly textured polycrystalline surface, with a reflection of a very high intensity from the (220) planes. The ED pattern of gold on mica (not shown) is very similar. The ED pattern of gold on silicon (100) is typical of a non-textured polycrystalline specimen, similar to that shown in fig. 2a.

The annealed samples of gold on glass, mica, and silicon (fig. 5) all show an increase in grain size and in flatness. The effect of annealing on glass and silicon substrates is quite remarkable - an increase in the TGS from = 850 to N 1500 A

Fig. 3. TEM BF images of (a) SP-G-A gold; (b) SP-M-A gold; (c) SP-.%-A gold.


Fig. 4. TEM BF images of (a) RT-G gold; (b) RT-M gold; (c) RT-Si goid.

Fig. 5. TEM BF images of (a) RT-G-A gold; (b) RT-M-A

gold; fc) RT-52-A gotd.

K Golan et al. / Vacuum-deposited gold films. I 319

Fig. 6. Electron diffraction patterns for fa) RT-G gold; (b) RT-G-A gold.

for glass, and from “* 250 to N 1200 A for silicon. TGS of annealed mica s?ples increates less dramatically - from N 600 A to N 850 A. The ED pattern of the RT-Si gold remains essentially unchanged upon annealing. With gold on glass (fig. 6b) and mica (not shown), annealing results in an enhancement of the { 111) texture.

Since the Bragg angles for ED are very small (typically 19, the highly intense reflection of the 1220) planes in the case of gold on glass and mica indicates that these planes lie parallel to the electron beam, i.e., normal to the surface, This observation on the one hand, and the weak reflection of the (111) planes (which are perpendic-

ular to the (220) planes) on the other hand, indicate a preferred texture wherein the { 1111 planes lie parallel to the surface, namely a (111) textured gold.

The large grains of the RT annealed glass specimen provide the possibility to obtain selected-area diffraction patterns from a single crystal, by exposing part of one large crystal in the selected-area aperture. Fig. 7b presents a selected-area single-crystal diffraction pattern obtained from a sample of gold evaporated on glass at RT, and post-deposition annealed CRT-G-A)

200 nm I I

Fig. 7. (a) TJZM BF image of RT-G-A gold. Note the selective aperture image, positioned on a large gold single crystal. (b) Selected-area electron diffraction pattern obtained from (a).

320 Y Golan et al. / Vacuum-deposited gold films. 1

Fig. 8. (a) TEM BF image of RT-G-Ti gold. (b) Electron

diffraction pattern obtained from (a).

(fig. 7a). The spot pattern with sixfold symmetry and prominent {220] reflections indicates the exposure of the (111) plane at the surface of the crystallite #4. All large crystals analyzed by selected-area diffraction gave a similar pattern for all three substrates.

It is common practice to deposit a thin underlayer of, e.g., titanium (Ti) or chromium in order

#4 The weak (111) spots (the points of smallest radius in fig. 7b), unexpected for this orientation, appear to result from

a superposition of zero-order and first-order Laue zones

[17], occurring with very thin foils.

to improve the adhesion of the gold to glass or oth$r substrates. The effect of evaporation of a 50 A underlayer of Ti on a glass substrate (G/Fil prior to the gold deposition is shown in $g. 8a. The TGS significantly decreases to - 250 A, with a marked decrease in the relative intensity of the (220) reflections (fig. 8b), i.e., little or no crystallographic texture. Annealing has a pronounced grain enlarging effect (fig. 9a), but with no notice- able change in the ED pattern (fig. 9b).

3.3. Gold evaporated at high temperature

In contrast to the continuous RT evaporated gold films, the high iemperature (HT) evaporated gold samples (350 A> on glass or mica (figs. 10a and lob) contain elong$ed holes in the film. TGS for both is N 1250 A, although much larger and much smaller grains can be observed. In general, the crystallites appear large, flat, and with a high density of imperfections. The annealed analogs of the HT evaporated gold films on glass or mica (not shown) are similar, with somewhat larger TGS, i.e., N 1500 A for both. The ED patterns of HT gold evaporated on glass (fig. 10~) and mica (not shown) indicate pronounced 1111) texture. The BF images of HT gold deposited on silicon (1001 (figs. lla and lib) demonstrate a strong effect of annealin&, i.e., the TGS increases from N 550 to N 1000 A for the annealed sample. The ED pattern (fig. 11~1 indicates a non-textured polycrystalline film, and remains unchanged after annealing.

3.4. Measurement of the overall relative surface roughness

Cyclic voltammetry (CV> 1181 of gold electrodes, i.e., gold oxide formation and removal in acid solutions (fig. 121, can be used to determine the relative roughness of gold surfaces [19-211, and indicate crystallographic texture [22].

CV’s were obtained for the various gold samples, after cleaning with Ar plasma. Accurate integration of the area under the oxide removal peak, i.e., the amount of charge required for removal of a gold oxide monolayer from the surface, may serve as a direct measurement of the


200 nm I I

200 nm I I

Fig. 9. (a) TEM BF image of RT-G-Ti-A gold. (b) Electron diffraction pattern obtained from (a).

relative surface area 119,233. The charge required for the reduction of a geometric area of 1.0 cm2 gold oxide is summarized in table 1 (the smaller this charge, the smoother the gold surface). Ac- cording to this data, the following trends may be summarized:

(1) Evaporated gold is ~nsiderabIy smoother than sputtered gold. According to the voltammet-

Fig. 10. (a) TEM BF image of HT-G gold. (b) Same, for HT-M gold. Cc) ELectron diffraction pattern obtained from (a).


200 nm I i

b

1 I I I I I I 0.20 0.60 1.00 1.40

E (V vs. SCE 1

Fig. 12. Qclic voltammograms in O.lON H,SO, for (a) SP-G-A

gold; (b) RT-G-A gold (scan rate: 0.10 V/s; electrode area:

1.0 cm*).

ric measurements (only G-A gold samples were compared), the roughness of sputtered gold is nearly twofold that of the evaporated analog (SP- G-A/RT-G-A = 1.88).

(2) RT deposits are smoother than HT deposits. In all samples, the roughness of HT samples is larger than that of RT samples, except for annealed silicon, where the opposite is obtained CRT-Si-A/HT-Si-A = 1.15). This effect is more pronounced with glass substrates (HT-G/RT-G = 1.39; HT-G-A/RT-G-A = 1.16) than with unannealed silicon (HT-Si/RT-Si = 1.10) and mica (HT-M/RT-M = 1.08; HT-M-A/RT-M-A = 1.06).

(3) Annealed gold films are smoother than unannealed ones, for all samples. This effect is most pronounced with silicon (100) substrates (HT-Si/HT-Si-A = 1.90, RT-Si/RT-Si-A = 1.5 l), in agreement with the enlargement in the TGS observed by TEM. For mica CRT-M/RT-M-A = 1.42; HT-M/HT-M-A = 1.44) and glass (HT- G/HT-G-A = 1.47; RT-G/RT-G-A = 1.22) a smaller decrease in roughness is obtained.

The shape of the CV can provide information about the crystallographic texture of the electrode surface. Fig. 12 shows CV’s of sputtered (SP-G-A) and evaporated CRT-G-A) gold samples on glass. Note the spike-shaped peak around 1.3

Fig. 11. (a) TEM BF image of HT-Si gold. (b) Same, for HT-S-A gold. (c) Electron diffraction pattern obtained from

(a).


V in the CV of the evaporated sample, which indicates a {ill} textured gold [22], and the ab- sence of this spike form in the CV of sputtered gold. This observation fully agrees with the ED results detailed above.

4. Discussion

Sputtered gold films have been used as substrates in a number of publications on organic films on electrodes [19,23-251. To the best of our knowledge, no studies on the imaging of such films have been published prior to this work, and therefore the results given in figs. 1, 2 and 3 cannot be compared with any published data. The sputtering technique makes use of the bom- bardment of a target surface by energetic parti- cles. Target atoms are thus ejected, and can be condensed onto a substrate to form a film [26,27]. A major difference between the modes of growth of sputtered and evaporated deposits is that the sputtered atoms would have a much higher kinetic energy. As a result of the energetic impact with the surface, the deposited atoms are probably embedded within the substrate surface atoms, thereby limiting their surface mobility. This, in addition to surface damage which may be caused by the energetic impact, may explain the formation of a high density of nucleation centers, resulting in the poorly oriented pebble-like structure that is typical of the sputtered films.

As shown in sequential growth of gold films [28], the nuclei grow and become larger until a totally continuous film is formed. Figs. la and lb and 3a and 3b may be an intermediate stage in such a sequence, so that the voids observed in these films probably disappear in thicker films. As expected from studies on evaporated films 18,151, post-deposition thermal treatment has a significant effect on the grain size and orientation. In sputtered films, the effect of annealing ranges from a moderate effect of TGS enlargement (28%) on silicon (100) substrates, to a large effect on glass (180%) and mica (250%) substrates.

Vacuum-evaporation is the most commonly used technique for deposition of thin gold films.

When a material is evaporated onto a substrate, the deposit undergoes a number of distinct stages. First, nucleation occurs on the substrate surface. Further deposition makes these nuclei grow via impingement or surface diffusion of single atoms, i.e., ordinary coalescence. Depending upon the substrate conditions and nuclei density, the dis- continuous film may then enter a stage where liquid-like coalescence of individual islands is the dominant growth mechanism. Finally, the islands make metallic contact with one another to form an electrically continuous film [28].

Evaporated gold films appear to be substan- tially different from sputtered films in terms of grain size and shape, as well as in crystalline structure preference. The relatively low kinetic energy of the deposited atoms allows a more controlled deposition compared with the sputtering technique, enabling the attainment of energetically preferred structures with less nucleation centers. The smaller number of nucleation sites results in larger, plate-shaped grains which form a continuous film at a thickness of 350 A on all three substrates studied.

The results obtained in this work for gold films evaporated at RT on mica substrates are in good agreement with the corresponding reported features [8,9]. The TGS of N 600 A is in marked agreemznt with the reported lateral extension of N 500 A [S]. The {ill] texture observed is also in agreement with other publications [5,8]. Note that these authors [5,8] refer to the results as “epitaxial growth” of gold on mica, despite the very high misfits.

The “epitaxial temperature” is believed to be the critical substrate temperature required for epitaxial deposition. There is often disagreement regarding the values of the epitaxial temperature. In epitaxial growth of gold on mica, temperatures ranging from 297 191 to 450°C 151 have been reported. Annealing at 250-265°C is reported [S] to produce much smoother films, approaching the morphology of the samples deposited at higher temperatures, with a much better epitaxial quality in the RT-A samples than in the HT samples. This may probably be attributed to the proximity to the “epitaxial temperature”, allowing surface mobility and sintering. The high-temperature


treatment thus provides some of the activation energy which is required to form the energetically favored organization, i.e., larger grains.

It is notable (and not previously reported) that evaporated gold grows with the same morphology on the non-crystalline glass substrate and on mica. This renders the widely claimed role of the structure of monocrystalline mica as a substrate for epitaxial deposition of gold quite less convincing. The well-recognized tendency of certain metals, such as silver [15], to nucleate and grow onto smooth non-crystalline substrates with low index planes parallel to the surface on the one hand, and the high misfits reported for “epitaxial” growth of gold on mica on the other hand [.5], suggest that the growth of textured gold on both glass and mica may not require any epitaxy to bring {ill] planes parallel to the smooth substrate surface. The similarity in the film morphology on both substrates is more likely due to the chemical similarity between the single-crystal sili- cate structure of the mica surface and that of the amorphous glass. The poor wetting of both surfaces by the deposited gold allows the mobilities of adatoms and clusters on the two substrates to be high and similar in magnitude, resulting in essentially the same low-energy structure.

The poor texture of the gold on silicon (100) further emphasizes the substrate effect on the crystallographic properties of the deposited Au. It is well known [5] that several fee metals (notably Al, Au, Ag) naturally tend to crystallize with a (111) texture, unless the substrate dictates other- wise. Evidently, the cubic symmetry of silicon (1001 interferes with this natural tendency and inhibits the development of a (111) texture.

High-temperature evaporation of gold is probably the most popular and widely used deposition mode. Increasing the substrate temperature may be advantageous in several ways: (i> to provide some of the activation energy required to allow the deposited atoms to assume an energetically preferred situation, associated with a more or- dered phase; (ii) to increase surface and volume diffusion, important for accommodating the misfits which occur as neighboring nuclei grow to- gether; (iii) to aid cleaning of the substrate surface. The removal of contaminations is expected

to lower the density of nucleation centers on the surface, which may explain the holey films obtained on glass and mica at high temperature, while an instantly formed alloy layer on silicon may dictate the formation of a continuous film.

HT gold on mica was reported to yield larger crystallites with flatter tops; at a deposition temperature of 225°C [8], the STM images exhibited flat grains of 1200-1400 A in diameter. These findings are in agreement with the results obtained in this work for HT gold on mica, with flat, smooth crystallites of - 1250 A TGS (which would probably be even larger with the filling of the holes at higher thicknesses), although very small grains of 200-300 A lateral extension can also be found. A strong { 1 ll} texture is obtained, in agreement with the literature reports [3,5,8]. HT evaporated gold on mica is classified in the mixed surfaces category [9], which, according to several authors [6,9], is due to a problematic cleavage of mica in air. This process fails to produce smooth and homogeneous substrates with a large lateral extension due to strong electro- static forces which accompany the cleavage process. The resulting surface consists of large, flat, atomically smooth plateaus in the size range of several thousands of A on the smooth areas of the mica surface. On the cleavage-affected regions, polycrystalline gold was observed with flat hillocks of 600-1000 A lateral extension and 20 to 50 A in height [6,9].

The results obtained in the present work for HT evaporated gold on glass substrates disagree with some of the conclusions drawn from the STM studies. Gold on glass was reported to present a rough polycpstalline surface with hillock? of about 50-200 A in height and 400-1000 A lateral extension [9]. In the present case, at deposition temperatures approaching the epitaxial temperature, the gold domains obtained on glass appear to be as smooth as those on mica, if not smoother. This finding strongly suggests a similarity in the mobility of the deposited gold atoms on the two substrate surfaces, unlike the different coalescence mechanism for gold on glass suggested by Vancea et al. 191. The relatively minor differences in the annealed analogs on glass and mica (20% enlargement of the TGS in both) can

Y Golan et al. / Vacuum-deposited gold films. I 325

be explained by the already high deposition temperature. The (111) texture is enhanced in the annealed samples on both mica (in agreement with ref. [S]) and glass.

The results with HT gold on silicon (1001, i.e., a polycystalline non-textured film with a TGS of N 550 A, are in agreement with the rough polycrystalline surface with 400 to 1000 A lateral extension reported by Vancea and coworkers [9]. The marked twofold increase in TGS after annealing is likely to reflect a rearrangement to an energetically favored morphology, inhibited dur- ing the deposition by the silicon (100) cubic symmetry.

The electrochemical roughness measurements can reinforce our understanding of the TEM images obtained for the various gold samples. Several trends that are observed by TEM are well reflected in the voltammetric measurements: (1) The sputtered gold surfaces are found by voltammetry to be far rougher than the evaporated analogs. (2) The grain enlarging effect of annealing finds systematic expression in the decrease in the electrochemical roughness. The smaller effect of annealing in RT-M compared with RT-G samples is reflected by a larger decrease in the roughness of the glass samples. (3) The tremendous increase in the TGS of annealed gold on silicon (100) is expressed by a drastic decrease in the roughness of RT and HT samples after annealing. Namely, these trends support the intuitive concept stating that in general, smoother surfaces are expected for samples with a larger TGS.

On the other hand, the increase in the relative roughness of HT versus RT deposits, while HT deposits are characterized by a larger TGS than in RT deposits, suggests that additional factors influence the smoothness of the surface besides the grain size, e.g., the surface mobility of the deposited atoms, determined, among other factors, by the deposition temperature. These effects are not entirely clear at this point.

Annealing is a time-dependent process that causes the already deposited atoms to rearrange by sintering to form larger and energetically preferred crystals. This mechanism may rationalize the advantages in depositing gold at RT, perhaps allowing the first few layers to align along the

smooth substrate surface profile, and then to enlarge the crystal size by annealing. According to the voltammetric roughness measurements, RT-A gold turns out to be the smoothest among all the glass and mica samples.

5. Conclusions

The preferred deposition method for obtaining large and oriented gold grains is clearly the evaporation method, and not sputtering, which failed to produce large and oriented crystallites on any of the substrates used. Gold evaporation on mica produces films with large grains, while on glass even larger grains with a narrower size distribution are obtained (along with a better voltammetric smoothness of the surface), with both substrates promoting a fill] preferred texture. It is apparent that annealing has a significant role in grain size enlargement with all three substrates examined. While on silicon (100) no change in the crystallographic texture is observed, annealing of Au on glass or mica promotes an enhancement of the 1111) texture. ~ne~ing of RT glass or mica samples produces gold of a similar or better quality compared with gold deposited at HT. Silicon (100) is suitable for growth of large gold grains, with poor crystallographic texture. The influence of the specific silicon crystallographic plane ex- posed to the surface on the morpholo~ of the deposited gold is currently being studied in more detail [29].

Acknowledgement

We wish to thank the reviewer of this paper for the enlightening remarks.

List of abbreviations

TGS: typical grain size; SP: sputtered at room temperature; RT: evaporated at room temperature; HT evaporated at high temperature; M: mica substrate;


G: Si: Ti: A: TEM: ED: BF: SA: CV: SCE:

glass substrate; silicon (100) substrate; titanium underlayer; annealed at 250°C for 3 h; transmission electron microscope; electron diffraction; bright-field TEM imaging; selected-area; cyclic voltammetry; KCl-saturated calomel electrode.

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Vacuum-deposited gold films

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