Influence of apparatus geometry and deposition conditions on the structure and
`topography of thick sputtered coatings
`John A. Thornton
`
`Citation: Journal of Vacuum Science & Technology 11, 666 (1974); doi: 10.1116/1.1312732
`View online: http://dx.doi.org/10.1116/1.1312732
`View Table of Contents: http://scitation.aip.org/content/avs/journal/jvst/11/4?ver=pdfcov
`Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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`Influence of apparatus geometry and deposition conditions
`on the structure and topography of thick sputtered coatings
`John A. Thornton
`
`Telic Corporation, 1631 Colorado Avenue, Santa Monica, California 90404
`(Received 10 December 1973)
`
`Two cylindrically symmetric and complementary sputtering geometries, the post
`and hollow cathodes, were used to deposit thick (-- 25-,.,.) coatings of various
`metals (Mo, Cr, Ti, Fe, Cu, and At-alloy) onto glass and metallic substrates at
`deposition rates of 1000-2000 A./min under various conditions of substrate
`temperature, argon pressure, and plasma bombardment. Coating surface
`topographies and fracture cross sections were examined by scanning electron
`microscopy. Polished cross sections were examined metallographically.
`Crystallographic · orientations were determined by x-ray diffraction. Microstructures
`were generally consistent with the three-zone model proposed by Movchan and
`Demchishin [Fiz. Metal. Metalloved. 28, 653 (1969)]. Three differences were
`noted: (1) at low argon pressures a broad zone 1-zone 2 transition zone
`consisting of densely packed fibrous grains was identified; (2) zone 2 columnar
`grains tended to be faceted at elevated temperatures, although facets were often
`replaced by smooth flat surfaces at higher temperatures; (3) zone 3 equiaxed
`grains were generally not observed at the deposition conditions investigated.
`Hollow cathode deposition accentuated those features of coating growth that relate
`to intergrain shading.
`
`INTRODUCTION
`
`The structure of thick vacuum-deposited coatings is
`determined by a sequence of morphological changes
`which occur as the coating grows in thickness from
`discrete initial nuclei, and by grain boundary migra(cid:173)
`tion and recrystallization which may occur concur(cid:173)
`rently.1 Thus, in addition to the relevant grain bound(cid:173)
`ary energies, the growth process is dependent on the
`incident coating flux,
`the coating atom adsorption
`probabilities, the density of surface sites, and the
`adatom surface mobility. These parameters depend in
`turn on the coating atom energy and angle of inci(cid:173)
`dence, the exposed crystallographic surfaces, the pres(cid:173)
`ence of foreign atoms, and most important, the sub(cid:173)
`strate temperature. As
`in bulk grain growth and
`recrystallization studies, one expects useful correla(cid:173)
`tions in terms of T /T m, where T is the substrate
`temperature and T m is the coating material melting
`point (K). After examining thick coatings of Ni, Ti,
`W, Ah03, and Zr02, Movchan and Demchishin2
`(M & D) divided the T/T m scale into three zones:
`zone 1 (T /T m <0.25-0.3), consisting of tapered crystal(cid:173)
`lites with domed tops which · increase in width with
`temperature; zone 2 (0.25-0.3<T/Tm<0.45), consist(cid:173)
`ing of columnar grains with smooth matt surface; and
`zone 3 (T /T m> 0.45), consisting of equiaxed grains and
`bright surface. Similar zones have been suggested by
`Sanders. 3 There is considerable evidence of evaporated
`coating structures that support the M & D model.'-7
`
`Little work has been reported on structure of thick
`sputtered coatings. s-u Sputter deposition
`involves
`several factors that make it different from evaporation:
`(1) sputtered atoms have considerable kinetic energy
`(an average of 4-40 eV); (2) energetic sputtered atoms
`may simultaneously approach substrate in several
`directions; (3) ambient gas is always present; and (4)
`substrate may be subjected to plasma bombardment.
`This paper reports on the structure of . relatively
`thick ( "'25-,u) coatings of various metals that were
`de sputter-deposited under various conditions of T /T m 1
`argon pressure, and plasma bombardment, using two .
`cylindrically symmetric and complementary sputtering
`geometries, the post and hollow cathode (see Table I).
`
`PREPARATION OF COATINGS
`
`Three experimental arrangements were used: (1) hol(cid:173)
`low cathode with fixed-temperature substrate holder;
`(2) hollow cathode with substrate holder capable of
`· maintaining temperature gradient (see Fig. 1); and
`(3) post cathode surrounded by substrate holders
`maintained at various temperatures.
`Axial magnetic fields protected the substrates from
`primary-electron bombardment in both geometries.
`Substrates in hollow cathode were electrically isolated
`and floated at -30 to -50 V relative to the anode. At
`a typical deposition rate of 1500 A/min the substrate
`heat load, due primarily to the kinetic energy and
`heat of condensation of coating atoms and plasma
`
`666
`
`J. Vac. Sci. Technol., Vol. 11, No. 4, July/Aug. 1974 Copyright © 1974 by the American Vacuum Society
`
`666
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`n
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`'C .111 ior'
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`2
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`Ar. pressure
`(µ)
`1 to 30
`1 to 30
`1 to 30
`1 to 30
`1 to 30
`
`100 to 3000
`800 to 20 000
`1000 to 1500
`1000 to 2500
`25 to 1000
`1000
`1000
`1 650
`650
`
`12 1
`
`Deposition
`rate (Å/min)
`
`Substrate
`material b
`Glass and S.S.
`Glass and Ta
`Glass
`Glass
`Glass and Ta
`Glass and Ta
`Ta
`Glass and S.S.
`Glass and S.S.
`
`
`
`
`
`TABLE I. Summary of experimental conditions.
`
`Coating material
`Copper
`Copper
`Al Alloy (type 6061)
`Al Alloy (type 6061)
`Titanium
`Titanium
`Molybdenum
`Chromium
`Iron
`
`Cathode type
`
`Hollow
`Post
`Hollow
`Post
`Hollow
`Post
`Post
`Hollow
` Hollow
`
`Substrate
`Temperature (°C)a
`LN2 to 500
`20 to 800
`LN2 to 500
`LN 2 to 500
`LN2 to 1200
`20 to 1200
`400 to 1100
`LN2
`LN2
`
`aLN2 is the temperature obtained on substrate clipped to holder through which liquid nitrogen was continually flushed. Actual tem-
`perature on substrate surface was not determined.
`b S.S. to stand for stainless steel.
`
`radiation, was about 10_1 W/c m 2. (The ion component
`of ambipolar diffusion flux was about 0.3 mA/c m 2.)
`Post cathodes were typically 3-cm o.d. by 30-cm
`long with substrates located at radii of 10_12 cm.
`Virtually no plasma species reached the substrates,
`which floated at a fraction of a volt relative to the
`anode. The substrate heat load was about 10_1 W/c m 2
`at a deposition rate of 2000 Å/min. Substrate plasma
`bombardment was achieved in post cathode apparatus
`by reducing the magnetic field strength so that the
`plasma extended to the substrates.
`The cathodes (targets) were water cooled. Aluminum
`alloy targets were formed from commercial 6061 tubing,
`copper targets from OFHC tubing, and titanium targets
`from commercially pure tubing. The molybdenum
`target was a pressed and sintered tube. Iron and
`chromium targets were electroplated onto stainless
`steel mandrels.
`Substrates were 0.060-in . glass microscope slides,
`0.03-in. stainless steel sheet, and 0.005-in. tantalum
`foil. Substrate temperatures of greater than 600°C
`were obtained by heating tantalum foil clamped be-
`tween copper electrodes, and measured by a radiation
`thermometer in the hollow cathode and by chromel _
`alumel thermocouples in the post experiments. Tem-
`peratures of 100 _600°C were obtained by clamping
`substrates onto stainless steel holders containing ni-
`chrome heaters and iron_constantan thermocouples.
`Substrate temperatures of approximately 20 and
`_196°C were obtained by flushing water or liquid
`nitrogen through copper or aluminum substrate holders.
`The apparatuses were evacuated to between 5 X 10 _7
`and 1 0 _ 6 Torr with 4-in. oil diffusion pumps prior to
`deposition. Under these conditions, the residual gas
`
`60CM-4
`
`HOLLOWCATHODE
`\
`
`f 1OCM
`
`/
`WATER COOLING
`
`THERMOCWPLES
`'
`
`/ S"BS\TRATE H O L D E R
`HEATER
`
`\
`SUBSTRATES
`
`FIG. 1. Schematic of hollow cathode sputtering apparatus with
`temperature gradient substrate holder.
`
`flux was a factor of about 10 _ 5_ 10 _ 6 smaller than the
`typical sputtered flux. Argon (99.998%) was used as
`working gas. Deposition conditions are summarized
`in Table I. Hollow cathode discharge voltages were
`in range 500_800 V, post cathodes in range 700_1000 V.
`
`Coating surface topographies and tensile-fracture
`cross sections were examined by scanning electron
`microscopy. Cross sections of coatings that could not
`be fractured were polished and examined metallo-
`graphically. Sample SEM micrographs are shown in
`Figs . 2 and 3. The dependence of gross structural
`
`DEGCOPPERTlTy ~0.3 H ALUMINUM
`
`ALU~MINUM
`BIAS 80”
`A, ,o h,ICRONS 0 IMAIC~V~
`AT 30 MlCRONS
`Rr 10 MICRONS
`FIG . 2. Fracture cross sections of coatings deposited in hollow
`cathode at various substrate temperatures and argon pressures.
`
`TlTh, = 0.6 1
`
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`J. A. Thornton: Structure and topography of sputtered coatings
`667
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`J. Vac. Sci. Technol., Vol. 11, No. 4, July/Aug. 1974
`EXPERIMENTAL RESULTS
`

`
`J. A. Thornton: Structure and topography of sputtered coatings 668
`
`with a zone consisting of columnar grains which ex-
`tended through the coating thickness, increased in
`width with T/Tm
`(see Figs. 2C and D), and generally
`exhibited faceted surfaces, particularly in the hollow
`cathode case. This structure is very similar to the
`zone 2 structure described by M & D. The gross
`features of the columnar structure were independent
`of argon pressure (compare Figs. 2D and H). Coatings
`deposited at the low T/Tm
`end of the transition zone
`tended to be brittle. The ductility increased with
`and coatings with the zone 2-type columnar
`T/Tm,
`structure tended toward bulk strength and ductility.
`Cu and Al-alloy coatings with fine grained transition
`zone structures yielded tensile strengths of 20 000_
`75 000 psi and 60 000_75 000 psi, respectively. (Other
`investigators have reported high strengths for fine-
`grained W, Ni, and Ti coatings and a trend toward
`bulk properties for high T/Tm
`columnar structures. 2,5,6
`The columnar structure tended to persist in most
`of the sputtered coatings to the highest T/Tm
`values
`examined, although equiaxed grains did form in copper
`coatings deposited at high deposition rates (10 000_
`20 000
`However, at high T/Tm (~0.75) a
`distinct surface structure consisting of smooth flat
`grains with grooved boundaries was consistently ob-
`served, even on the columnar M o , Ti, Cu, and Al-alloy
`coatings (Fig. 3D). Figure 3E shows a Mo coating in
`the transition range where a few faceted grains remain.
`Similar structures are seen at higher argon pressures,
`as shown by Cu coating in Fig. 3F. (This particular
`coating, deposited at 10 000 Å/min, exhibited an equi-
`axed structure.)
`It has been shown that relatively high-energy
`(_ 500 eV) ion bombardment can suppress the forma-
`tion of a distinct columnar structure in Cr,8,12 Ta, 13
`and Be.14 Figure 21 shows coating deposited under
`conditions of Fig. 2F but with ion bombardment flux
`equal to about 80% of the coating atom flux. Distinct
`Al columnar structure is significantly densified even
`at low (80 V) ion energies.
`The crystallographic orientations of about twenty-
`five representative coatings were examined by x-ray
`diffraction. All exhibited texture. At low argon pres-
`sures and T/Tm
`fcc Cu and Al-alloy coatings deposited
`
`FIG. 4. Schematic representation of dependence of coating struc-
`ture on substrate temperature and argon pressure.
`
`FIG. 3. Surface view and fracture cross sections of coatings de-
`posited with post cathode at various substrate temperatures and
`argon pressures.
`
`features on T/Tm
`and argon pressure is summarized
`in Fig. 4. Figure 4 was constructed primarily from Cu
`and Al-alloy data obtained at deposition rates of from
`1000 to 2000Å/ min, but is consistent with considerable
`additional data obtained with Mo, Cr, Fe, and Ti.
`The basic features were essentially independent of the
`substrate material and deposition rate (for the range
`1000_2000 Å/min). (Variations in the extent of the
`structural regions were observed for significantly higher
`and lower deposition rates. These influences will be
`reported in a future communication.)
`Structural zones can be identified, consistent with
`M & D. At very low T/Tm
`coatings exhibited a dark
`gray surface and consisted of tapered crystallites sepa-
`rated by voids, similar to M & D’s zone 1 (see Figs.
`2A, E, and 3A). The tendency of this structure to
`persist to higher values of T/Tm
`increased with argon
`pressure and was greatest in hollow cathodes (compare
`Figs. 2F and 3B). This structure was not observed
`with post cathodes at low argon pressures. In hollow
`cathode at high argon pressure, where voids were more
`exaggerated, a crystallite width-to-length increase with
`increasing T/Tm
`identical to the zone 1 character
`described by M & D, was apparent (compare Figs. 2E
`and F).
`At higher T/Tm
`the intergrain voids began to fill in
`(compare Figs. 2F and G); and the structure passed
`into a transition zone of tightly packed fibrous grains,
`which generally did not extend through the coating
`thickness (Figs. 2B and 3C), increased in both width
`and length with T/Tm
`and exhibited a relatively
`surface. This transition zone
`smooth “fine-domed”
`covered a relatively large range of T/Tm
`and at low
`argon pressures was the dominant low temperature
`structure, particularly in the post cathode geometry.
`This region terminated at T/Tm
`in the range 0.3_0.5
`
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`J. Vac. Sci. Technol., Vol. 11, No. 4, July/Aug. 1974
`

`
`669 J. A. Thornton: Structure and topography of sputtered coatings 669
`
`on glass and stainless steel substrates in both the post
`and hollow cathodes had a <111> preferred orientation,
`while bcc Cr and Mo deposited under similar condi-
`tions had a <110> orientation. These observations imply
`a preference for orientations that place the most den-
`sely populated atomic planes parallel to the substrate.
`Such orientations are commonly seen in evaporated
`thin films 15_19 and have been reported for thick-sput-
`at low argon
`tered coatings. 8,9,20 An increase in T/Tm
`pressures caused Cu and Al-alloy coatings deposited
`with both the post and hollow cathodes to assume
`a <110> orientation.Minor <311> orientations were con-
`sistently observed with the post cathode. Elevated
`argon pressures in the hollow cathodes caused Cu to
`assume a <110> and Al-alloy to assume a <100> orien-
`tation. (Similar observations have been reported by
`others. 15,21) However, Cu and Al-alloy coatings de-
`posited at high argon pressures with post cathode had
`.
`<111> orientation at all T/Tm
`Polymorphic Ti coatings deposited on liquid-nitro-
`gen-cooled glass substrates were in bcc ß-phase with
`<110> orientation. Those deposited on 20°C glass sub-
`strates were in hcp -phase with high degree of basal
`plane orientation. Ti deposited on Ta substrates in the
`temperature range 900-1200°C consisted of a p-phase
`with substrate orientation and random a-phase, which
`was probably a transformed ß-structure. 5 A thick
`interface diffusion zone developed at high substrate
`temperatures, probably as consequence of the fact
`that ß-Ti and Ta form a continuous series of solid
`solutions.
`
`T/Tm .
`
`<0.l there is little adatom surface mobility,
`At
`and initial nuclei tend to grow in the direction of
`available coating flux.3 The growth morphology is af-
`fected significantly by intergrain shading. Tapered
`crystallites develop. Intergrain boundaries are voids
`rather than true grain boundaries, so that coatings
`have poor lateral strength and are underdense, although
`individual crystallites have near bulk density. At high
`argon pressures, adsorbed argon limits the adatom
`mobility and permits this structure to persist to
`higher
`(Th ere is evidence that residual gas
`adsorption can cause mobility variations over exposed
`crystallite surfaces. 2 2 - 2 6 ) Hollow cathode deposition is
`characterized by a large side flux as shown schemat-
`ically in Fig. 1, and these devices are more vulnerable
`than post cathodes to argon pressure effects because
`of intercrystallite shading and the possibility that ad-
`sorbed argon can accumulate in crevices that are
`protected from coating atom impact. The trend toward
`formation of voided structures at high argon pressures
`was found to be considerably less pronounced at low
`deposition rates (~-100 Å/min), probably because high
`deposition rates, like argon pressures, tend to limit
`the adatom surface mobility. Voided structures were
`suppressed at high pressures in hollow cathodes when
`
`m
`
`m ~
`
`m
`
`T/Tm
`
`m
`
`shields were used to limit both the side flux and the
`deposition rate.
`(0.1-0.3) self-diffusion becomes ap-
`At higher T/T
`preciable, and coatings consist of a dense array of
`fibrous grains separated by more nearly conventional
`grain boundaries, probably due to the occurrence of
`a sintering type coalescence during growth. Such
`coatings yield high lateral strengths.
`0.3-0.5 surface mobility is even
`A b o v e T/T
`greater and grain boundary migration and recrystal-
`lization are possible. 3 , 27 Columnar grains extending
`through the entire coating thickness and separated
`by true grain boundaries develop, possibly by surface
`recrystallization during growth. Argon pressure has
`'s
`reduced influence at these
`because of decreased
`surface adsorption.28 Surfaces tend to be faceted in
`range 0.5–0.75. Greater tendency for developing
`T/T
`faceted surfaces in hollow cathode at moderate T/T
`is believed to be due to relatively low energy (30-50 eV)
`ion bombardment etching8 and oblique flux of coating
`atoms. At very high T/T
`(~0.75) equilibrium
`surface structure apparently consists of relatively
`flat grain tops with grooved grain boundaries in
`both apparatuses.
`at elevated
`Coating structure dependence on T/T
`argon pressure is similar to that described by M & D
`with two exceptions : (1) zone 2 columnar grains
`tended to be faceted, and (2) zone 3 equiaxed grains
`were generally not observed. At low argon pressure
`sputtered coatings yielded a broad zone 1-zone 2
`transition region of densely packed fibrous grains not
`specifically described by M & D, although they do
`note that zone 1 grain outlines often were difficult
`to identify. Failure to observe equiaxed grains in the
`pure metals in this work was probably due to fact
`that deposition rates were lower (l0–50 times) and
`that coatings were not as thick (2.5 µ) as those studied
`by M & D (250-2000 µ). Copper coating deposited at
`consisted
`rates of 10 000-20 000 Å/min and high T/Tm
`of large equiaxed grains which appear to have formed
`during deposition by a concurrent recrystallization
`process. Lower Cu deposition rates (1000-2000 Å/min)
`yielded columnar structures. A similar deposition rate
`dependence on the occurrence of room temperature
`after-deposition recrystallization of Cu has been re-
`9 High-temperature (recrystal-
`ported by Patten
`lized) equiaxed grains of M & D should not be confused
`with the fine-grained growth type formed by bias
`sputtering,13 electroplating,29 and CVD.30
`was not
`Structural similarity scaling with T/T
`observed for all materials. Although each basic struc-
`tural region was observed in most cases, transition
`points varied, possibly because of alloy influences and
`the fact that surface mobilities, heats of condensation,
`and vapor pressures do not vary linearly with melting
`point. Departures were greatest for low melting point
`materials, probably because the adatom kinetic energy
`on
`and heat of condensation made low values of T/T
`the condensate surface difficult to achieve for such
`
`m
`
`m
`
`m
`
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`LMBTH-000200
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`DISCUSSION
`T/T,
`J. Vac. Sci. Technol., Vol. 11, No. 4, July/Aug. 1974
`
`m
`et al.
`

`
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`
`J. A. Thornton: Structure and topography of sputtered coatings 670
`
`materials. (The M & D work was limited to the
`melting point range 1453-3410°C.)
`
`The author would like to acknowledge the able
`assistance of V. L. Hedgcoth throughout the course of
`this investigation; also the efforts of W. E. Gardner
`of Sloan Research Industries, Inc. who did the SEM
`work. This investigation was sponsored by the Armco
`Steel Corporation, Middletown, Ohio.
`
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`
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`Bunshah (Interscience, New York, 1968), Vol. 1, Pt. 3, p. 1377.
`
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`ACKNOWLEDGMENT
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`J. Vac. Sci. Technol., Vol. 11, No. 4, July/Aug. 1974