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The influence of surface topography and angle of adatom incidence on growth structure in sputtered chromium
Thin Solid Films, 63 (1979 ) 121 129 © ElsevierSequoiaS.A., Lausanne Printedin the Netherlands
121
THE INFLUENCES OF SURFACE T O P O G R A P H Y AND A N G L E OF ADATOM INCIDENCE ON G R O W T H STRUCTURE IN SPUTTERED CHROMIUM * J. W. PATTEN
Battelle, Pacific Northwest Laboratories, Richland, Wash. 99352 (U.S.A.) (Received April 16, 1979 ; accepted April 25, 1979)
The influences of the substrate surface topography and the angle of adatom flux on the growth structure of thick sputter-deposited chromium were investigated using a single "point" source for adatom incidence angles of 0, 7.5, 15, 30, 45, 60 and 90 °. Two types of columnar growth were observed. (1) A columnar defect structure was associated with geometric shadowing of substrate surface asperities and was always parallel or nearly parallel to the flux of incident atoms. This observation indicates that atomic self-shadowing, as described by the well-known tangent law, does not control defect growth. The results of the present study support a simple geometric shadowing mechanism. The open boundaries of the columnar growth units (defects) in this type of structure are generally associated with reduced corrosion resistance and other (local) degradations of coating properties. (2) A columnar solidification texture was always perpendicular to the substrate surface, with the size of individual columnar regions being related to the topography of the substrate surface.
1. INTRODUCTION Crystalline and amorphous films produced by line-of-sight processes such as evaporation and sputtering have been widely reported to exhibit columnar microstructures, with the details of the microstructure depending on deposition parameters such as the nature of the adatom flux (energy distribution, density, composition, angle of incidence etc.) and the conditions at the substrate surface (nucleation sites, surface finish, impurities, surface temperature etc.) 1-13. It is generally agreed that geometric shadowing is involved in the development of these columnar growth features, with the columnar structure being more pronounced with increasing proportions of low angle incidence adatoms and with increasing surface roughness (more effective shadowing sites)4. The boundaries between these columnar regions (or growth sites) may be grain boundaries or, in more extreme cases, may be gaps between isolated growth structures 8 which therefore result in a reduced deposit density. These open columnar structures are of particular interest because of the implication of the open boundaries for deposit properties, such as mechanical strength and permeability to corrosive agents 9. * Paper presented at the InternationalConferenceon MetallurgicalCoatings, San Diego, California, U.S.A., April 23-27, 1979.
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J.w. PATTEN
Nieuwenhuizen and Haanstra ~ first investigated the relation between the angle of adatom incidence and the orientation of these columnar growth units (or the equivalent open boundaries) and formulated the "tangent rule": 2 tan fi = tan where/~ is the angle between the growth direction and the substrate normal and ~ is the angle between the adatom flux and the substrate normal. In effect this relation means that the columnar structure is always inclined towards the adatom flux, but it is always more nearly normal to the substrate surface than is the adatom flux. Dirks and Leamy ° have surveyed many published results and considerable amorphous deposit data of their own and have concluded that this tangent rule is obeyed for crystalline and amorphous films with 0 ~< ~ ~< 60'. They have also observed that porous boundaries around dense columnar growth cores are more pronounced with decreased adatom surface mobility and more oblique adatom incidence. They have developed a model of simple geometric shadowing of the adatom flux by atoms within the growing film, where self-shadowing by justdeposited atoms produces void formation when the rate of migration of atoms to the shadowed regions is smaller than the rate of void incorporation during growth. The diameters of the thin columnar growth features are of the order of a few atomic diameters. Their more recent results ls'16 agree with this model, but deal with amorphous films having columnar growth units of the order of 50 250 A in diameter. In addition, depositions at large ~ have been found to result in row-like column aggregates in a direction perpendicular to the plane of incidence and the effects of adatoms incident from various angles are thought to be geometrically additive 6, 1 These results seem to apply to the zone 1 microstructure described by Movchan and Demchishin ~ in evaporated materials and verified by Thornton 2 in sputtered materials. However, these results do not completely describe our observations of the substrate topography-related microstructures of thick sputterdeposited materials, particularly with respect to the much coarser columnar defect structures that can be produced with appropriate conditions and observed optically in metallographic cross sections. For this open type of structure, low density (easily etched) columnar grain boundaries (growth unit boundaries) occur with very high frequency and are the dominant metallographic feature. This work was undertaken to determine if this tangent rule related ~ to/3 for thick sputtered deposits and to determine if an atomic self-shadowing mechanism could be applied to describe the growth of this open structure which is often associated with poor coating properties. 2.
MATERIALS A N D P R O C E D U R E S
As described previously 18, a triode sputtering system designed in our laboratory was modified to accommodate the target-to-substrate arrangement shown in Fig. 1. The mask allowed any area of the target to be exposed for sputtering so that the relation between the adatom flux and the substrate could readily be changed. The substrate was fabricated in two pieces, with flat faces at 0, 7.5, 20, 30, 45 and 90 ° to the target normal (direction of most concentrated adatom flux). This allowed the effect of a number of angles of flux incidence to be determined from the
GROWTH STRUCTURE IN SPUTTERED Cr
123
results of a single sputtering experiment. Figure 2 is a photograph of a typical substrate just prior to its installation in the sputtering system.
~'~C
Cu
$UBSTRATE
12.7 Cm
k\\\\\\\\\\\"
--~ \ \ \ \ \ \ \ "
-t
I Fig. 1. A schematic representation of the sputtering target-to-substrate arrangement. Fig. 2. A typical copper substrate (two-part) with the inclination of the faces to the adatom flux and the areas sectioned for evaluation indicated. (Top view: magnification, 1.1 x .)
The results described here were obtained with a circular area of diameter 1 in exposed (not masked) in the center of the target--a close approximation to a point source. The dispersion of an incident adatom beam with this geometry was of the order of +- 5 ° from a ray between the substrate and the center of the target. The target was high purity chromium (18 ppm C, 20 ppm 0 2, 30 ppm Fe and all other impurities less than 3 ppm). The substrate was metallographically polished O F H C copper which was water cooled. Calibrating marks were placed on each face at known angles using a scribe of known shape to provide additional information on the defect growth at surface irregularities. Typical sputtering experiments were conducted using a chamber pressure before the introduction of the krypton sputtering gas of 7.7 x 10-6 Pa and a krypton pressure during the experiments of 0.48 Pa. A substrate etch was applied for 10 min prior to sputter deposition using a substrate potential of 200 V and a substrate current of 0.2 A. Typical parameters during deposition were a target current of 0.06 A, a target potential of 2000 V and a substrate temperature of 25 °C. The effective deposition rate normal to the flux direction (i.e. on a 90 ° face) was approximately 0.9 nm s -1. The deposits were metallographically sectioned and were characterized using light microscopy in both polished and etched conditions. The specimens were electrolytically etched at 5 V for 5-10 s in a solution of 5 ml CrOa, 4 ml H E S O 4 and
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200 ml H 2 0 . Prior to sputter deposition the ion-etched substrate surfaces were examined using scanning electron microscopy (SEM) to characterize the substrate surface topography. The outer surfaces of the deposit and the substrate~deposit surfaces (after the removal of the copper substrates in nitric acid) were also characterized using SEM. Sections of the copper substrates were bent in order to fracture the adherent chromium deposits and the fracture surfaces were characterized using SEM. 3. RESULTS AND DISCUSSION Figure 3 is an optical micrograph of a deposit produced with the chromium flux incident at an angle of 60 ° to the copper substrate (top arrow). The two asperities on the substrate surface (bottom arrows) are replicated by the chromium deposit. The deposit (and defect) growth, as represented by the direction of growth of these surface asperities, is very nearly parallel to the incident adatom flux.
60
J~ /65 o
Cu SUBSTRATE
Fig. 3. An optical micrograph of a sputtered chromium deposit. Substrate surface asperities were replicated by the deposit (bottom arrows). (As-polished : magnification, 630 x )
Figure 4 shows three optical micrographs of etched cross sections of the same chromium deposit as that in Fig. 3. The chromium adatom flux was also incident at 6 0 to the substrate in the areas shown (Fig. 3(a), top arrow). Two types of columnar growth structure were revealed by etching. One type produced local defects in the coating and was associated with substrate surface asperities (Fig. 4(a), bottom right arrow). The other type of columnar structure, a solidification structure, always grew perpendicular to the substrate surface regardless of the adatom incidence angle (Fig. 4(a), bottom arrow). 3.1. Columnar dejects For the first type of columnar growth, substrate surface asperities were transmitted through and magnified by the chromium coating as shown in Fig. 4(a) (c). In every case the etchant preferentially attacked the peripheral areas of the defects and left a core area relatively unaffected. This may indicate defect core areas of high density with lower density material surrounding each core. Such a reduction in density would have to be on a very fine scale, however, as no voids were detected within the limits of optical resolution in the polished cross sections (Fig. 3). Each high density core is propagated from the highest point ofa substrate surface asperity
G R O W T H S T R U C T U R E IN S P U T T E R E D
Cr
125
to the highest point of the corresponding growth defect in the chromium deposit and is assumed to represent the direction of growth of the defect. This direction is very close to 60 °, i.e. parallel to the incident adatom flux (Figs. 4(a) and 4(b)). For a large number of such measurements this growth angle was 61 + 2 °, i.e. 60 ° within experimental error. Similar results were obtained for other adatom flux incidence angles of 90, 45, 30 and 20 °. Insufficient deposit thickness for growth angle measurement was obtained at 7.5 and 0 ° adatom incidence.
//•DATOM FLUX CIDENT AT 6 0 *
Cr DEPOSIT Cu SUBSTRATE
(a)
(b)
(c) Fig. 4. Optical micrographsof a sputtered chromium deposit. Adatom flux incident at an angle of 60° to the substrate surface. (Etched: magnification, 525 x .) (a) Large surface asperities. Note the columnar shadowing defect structure (bottom right arrow) and the columnar solidification structure (bottom left arrow). (b) Smaller surface asperities. (c) The sides of growth defects from a small surface asperity are parallel during equilibrium shadowinggrowth. If each defect is assumed to be approximately conical, then sections through the defects should be curved. This was observed for the defects associated with the largest substrate surface asperities (Fig. 4(a)) and for the initial segments of all defects. However, for the smaller asperities (Figs. 4(b) and 4(c)) the sides of the growth defects became nearly straight and parallel to each other after a small thickness of chromium deposit growth (Fig. 4(c)). It was also noted that the sides of the growth defects approached an angle of 60 ° to the substrate surface, i.e. parallel to the adatom flux, as the size of the associated surface asperity decreased and as the chromium deposit thickness increased. This was demonstrated by measuring the angle between the substrate surface and the leading edge (towards adatom flux) of the defects (Fig. 4(c), top arrow). For a large number of measurements the average angle was 55.9 + 4°; i.e. this angle never exceeded 60 °. Also the opposite side of the defects (trailing edge) was never observed to grow at less than 60 ° to the substrate surface, but was observed to grow at a wide range of angles approaching 60 °. These observations do not agree with growth directions predicted by the "tangent rule" described earlier; this can be seen by comparing the predicted and measured values of 0 shown in Table I. Therefore the atomic self-shadowing
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J.w. PATTEN
TABLE 1 P R E D I C F E D A N D M E A S U R E D D E P E C T G R O W T H A N G L E S F O R D [ F f ' E R E N T A D A ~ O M [,L(,'N I N ( I I I E N ( ' E A N G I , L S
90 60 45 30 20
0 30 45 60 70
0 17 27 41 54
90 73 03 49 36
S7 61 45 32 21
is the angle between the adatom flux and the substrate normal. 0 ( = 90 - :0 is the angle between the adatom flux and the substrate surface. fl is the angle between the growth direction and the substrate normal. 0 ( =90 -[~) is the angle between the growth direction and the substrale surlace. Predicted by the tangent rule, 2 tan [~ = tan a. b Measured. mechanism leading to the tangent rule predictions is assumed not to be active for the experiments described in this work. It is suggested instead that atomic mobility on the free surface of the growing c h r o m i u m deposits is of a sufficiently long range to allow local selection of the lowest energy positions (a large number of nearest neighbors~ but is of a much shorter range than the scale of the substrate surface asperities shown here. It is further suggested that defect growth proceeds by a geometric shadowing mechanism with the c h r o m i m n flux behaving as a continuous medium. This result was partly expected, since self-shadowing on an atomic scale would not be expected to dominate over geometric shadowing by large surface asperities for small deposit thicknesses, i.e. early in the deposition. However. if atomic self-shadowing was active at all, then defect growth should have approached the angle predicted by the tangent rule when the film thickness exceeded twice the surface asperity height, and should have reached the tangent rule predicted wtlue in the limit. This was not observed. Instead early defect growth seemed to be very similar to the geometric shadowing behavior predicted for step coverage by Blech et al. ~8. ~9 by Bindell and Tisone 2° and by Tisone and Bindell 2 ~. As growth proceeded, defect cores grew parallel to the a d a t o m flux from the start of deposition and the sides of the defects approached this growth direction as a limit, It is suggested that the geometric shadowing calculations of Tisone and Bindell, if extended to large deposit thicknesses, may be expected to predict the defect growth observed in this work. 3.2. C o l u m n a r solidi[ication s t r u c t u r e
The second type of c o l u m n a r growth structure noted earlier (Fig. 4(aL b o t t o m left arrow) was investigated further by bending a copper substrate in order to fracture the adherent c h r o m i u m deposit. Figure 5 shows a c h r o m i u m fracture surface produced in this way. Some areas fractured along boundaries of the growth defects describes earlier (Fig. 5(a)). Other areas fractured along the c o l u m n a r boundaries oriented perpendicular to the substrate (Fig. 5(b)L F r o m Fig. 5(b), the solidification structure was elongated in the substrate plane along an axis parallel to a projection of the a d a t o m flux and had an approximate grain diameter imajor) of 1.3x 10- ~" cm (1.3 jam). This agreed approximately with the etched optical
GROWTH STRUCTURE IN SPUTTERED
Cr
127
metallography in Fig. 4. Correspondence between the size of these fracture surface features and the grain size was also verified by transmission electron microscopy evaluation of thinned deposits and will be discussed in ref. 22. PHYSICAL SHADOWING GROWTH DEFECT
ADATOM FLUX /
DEPOSIT
SUBSTRATE
(a) COLUMNAR CONDENSATION GRAIN R r}l liMi"l/~I~1~~
Cu SUBSTRATE
......
/I FLUX qT AT 4 5 °
DEPOSIT
(b) Fig. 5. SEM fractographs of sputtered chromium deposited with an adatom incidence angle of 45 ° onto a metallographically polished copper substrate maintained at 25 °C. (a) Fracture along the low density boundary of a physical shadowing growth defect growing parallel to the adatom flux. (Magnification, 1800 x .) (b) Fracture along columnar condensation grain boundaries growing perpendicular to the substrate surface. (Magnification, 900 x .)
The grain size of the columnar solidification structure also corresponded to topography-raised surface features on the ion-etched substrate. Figure 6(a) is a SEM micrograph of an ion-etched copper substrate and indicates grain boundaries in the copper (bottom arrow), protrusions approximately 1-3 gm in diameter corresponding to the solidification grain size in the chromium deposits (center arrow) and a particle of surface contaminant with a depression etched around it (top arrow). These contaminants were found by X-ray fluorescence analysis to be high in magnesium, silicon and calcium, and provided nucleation sites for shadowingproduced columnar defects in the chromium deposits. Figure 6(b) is a SEM micrograph of the substrate side of a chromium deposit with the substrate chemically removed. This photograph indicates that the chromium deposits clo-ely replicate the etched copper substrate surfaces. Figure 6(c) is a SEM micrograph of the deposit-free surface. Here the tops of the shadowing-produced columnar defects (top arrow) are surrounded by the dimpled deposit surface, with the dimple (depression) size (bottom arrow) corresponding to the grain size and the size of the substrate surface protrusions. This agrees with the observation in Fig. 4(b) that the
128
J.W. PATTEN
columnar solidification grains have parallel sides, with the free gurface end of each column being the same size as the substrate end.
lal
(b)
(c) Fig. 6. SEM micrographs of substrate and deposit surface topography. (Magnification. 480 x .) (a) Ionetched substrate surface: bottom arrow, copper grain boundary: center arrow, surface protrusions: top arrow, surface contaminant rich in magnesium, silicon and calcium. (b) Substrate side of the chromium deposit. (c) Deposit-free surface: top arrow, shadowing-produced columnar defect: bottom arrow, dimpled deposit surface.
4. CONCLUSIONS Two different types of microstructural columnar growth structure were observed in sputter-deposited chromium coatings. One type produced local defects in the coatings and was associated with geometric shadowing of substrate surface asperities. These defects consisted of high density cores surrounded by lower density (more easily etched) areas and always grew parallel to the incident adatom flux. These defects are the type responsible for the open columnar defect structures that can dominate and degrade the properties of thick line-of-sight-deposited coatings. The other type of columnar structure, a columnar solidification structure, always grew perpendicular to the substrate surface regardless of the incidence angle. No evidence of the tangent rule relating the column orientation to the adatom flux
G R O W T H STRUCTURE IN SPUTTERED
Cr
129
incidence direction was observed for either type of structure, so that atomic selfshadowing was not important in determining the growth direction. ACKNOWLEDGMENTS
This research was supported by the Department of Energy, Basic Energy Sciences. Special thanks are also due to R. W. Moss and M. A. Bayne for experimental assistance, E. D. McClanahan for helpful discussions, R. F. Stratton for the sputter depositions, H. L. Butts and J. H. King for apparatus design, R. H. Beauchamp for optical metallography and H. E. Kjarmo for SEM microscopy. REFERENCES 1 2 3 4
5 6 7 8 9
10 11 12 13 14 15
16
17 18 19 20 21 22
B.A. Movchan and A. V. Demchishin, Fiz. Met. Metalloved., 28 (4) (1969) 653-660. J . A . Thornton, J. Vac. Sci. Technol., 11 (4) (1974) 666 670. J . A . Thornton, J. Vac. Sci. Technol., 12 (4) (1975) 830-835. J.A. Thornton, The influence of bias sputter parameters on thick copper coatings deposited using a hollow cathode, Int. Conf. on Metallurgical Coatings, San Francisco, Calif., 1976, in Thin Solid Films, 40 (1977) 335. R . F . Bunshah and R. S. Juntz, Metall. Trans., 4 (1973) 21-26. A . G . Dirks and H. J. Leamy, Thin Solid Films, 47 (1977) 219 233. J . W . Patten and E. D. McClanahan, J. Appl. Phys., 43 (11) (1972) 4811 4813. J . W . Patten and E. D. McClanahan, J. Less-Common Met., 30 (1973) 351 359. J.W. Patten, D. D. Hays, R. W. Moss and J. W. Fairbanks, Recent developments in the application of high-rate sputtering technology to the formation of hot corrosion-resistant metallic coatings on marine gas turbine first-state vanes and blades, presented at the 1977 Tokyo Joint Gas Turbine Congress, Tokyo, May 22 27, 1977. R.J. Hecht and J. R. Mullaly, J. Vac. Sci. Technol., 12 (4) (1975) 836-841. D . H . Boone, T. E. Strangman and L. W. Wilson, J. Vac. Sci. Technol., 1I (4) (1974) 641 646. R . F . Bunshah, J. Vac. Sci. Technol., 1l (4) (1974) 633-638. W . F . Weston, T. C. Baker, C. J. Smith, A. L. Chavez, V. K. Grotzky and J. F. Capes, J. Vac. Sci. Technol., 15 (1) (1978) 54 58. J . M . Nieuwenhuizen and H. B. Haanstra, Philips Tech. Rev., 27 (1966) 87-91. A. G. Dirks and H. J. Leamy, Magnetic and structural properties of some a m o r p h o u s rare earth transition metal thin films, presented at the lntermag Conf., Florence, May 1978, to be published in J. Appl. Phys. A . G . Dirks and H. J. Leamy, Microstructure and magnetism in a m o r p h o u s rare earth-transition metal thin films, presented at the 23rd Conf. on Magnetism and Magnetic Materials, Minneapolis, Minn., November 1977, to be published in J. Appl. Phys. H . J . Leamy and A. G. Dirks, Microstructure and magnetism in a m o r p h o u s rare earth-transition metal thin films I : microstructure, J. Appl. Phys., in the press. I.A. Blech, Thin Solid Films, 6 (1970) 113. I.A. Blech, D. B. Fraser and S. E. Haszko, J. Vac. Sci. Technol., 15 (1) (1978) 13-19. J.B. B i n d e l l a n d T . C. Tisone, Thin Solid Films, 23(1974) 31 47. T . C . Tisone and J. B. Bindell, J. Vac. Sci. Technol., 11 (2) (1974) 519-527. J . W . Patten, to be published.
121
THE INFLUENCES OF SURFACE T O P O G R A P H Y AND A N G L E OF ADATOM INCIDENCE ON G R O W T H STRUCTURE IN SPUTTERED CHROMIUM * J. W. PATTEN
Battelle, Pacific Northwest Laboratories, Richland, Wash. 99352 (U.S.A.) (Received April 16, 1979 ; accepted April 25, 1979)
The influences of the substrate surface topography and the angle of adatom flux on the growth structure of thick sputter-deposited chromium were investigated using a single "point" source for adatom incidence angles of 0, 7.5, 15, 30, 45, 60 and 90 °. Two types of columnar growth were observed. (1) A columnar defect structure was associated with geometric shadowing of substrate surface asperities and was always parallel or nearly parallel to the flux of incident atoms. This observation indicates that atomic self-shadowing, as described by the well-known tangent law, does not control defect growth. The results of the present study support a simple geometric shadowing mechanism. The open boundaries of the columnar growth units (defects) in this type of structure are generally associated with reduced corrosion resistance and other (local) degradations of coating properties. (2) A columnar solidification texture was always perpendicular to the substrate surface, with the size of individual columnar regions being related to the topography of the substrate surface.
1. INTRODUCTION Crystalline and amorphous films produced by line-of-sight processes such as evaporation and sputtering have been widely reported to exhibit columnar microstructures, with the details of the microstructure depending on deposition parameters such as the nature of the adatom flux (energy distribution, density, composition, angle of incidence etc.) and the conditions at the substrate surface (nucleation sites, surface finish, impurities, surface temperature etc.) 1-13. It is generally agreed that geometric shadowing is involved in the development of these columnar growth features, with the columnar structure being more pronounced with increasing proportions of low angle incidence adatoms and with increasing surface roughness (more effective shadowing sites)4. The boundaries between these columnar regions (or growth sites) may be grain boundaries or, in more extreme cases, may be gaps between isolated growth structures 8 which therefore result in a reduced deposit density. These open columnar structures are of particular interest because of the implication of the open boundaries for deposit properties, such as mechanical strength and permeability to corrosive agents 9. * Paper presented at the InternationalConferenceon MetallurgicalCoatings, San Diego, California, U.S.A., April 23-27, 1979.
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J.w. PATTEN
Nieuwenhuizen and Haanstra ~ first investigated the relation between the angle of adatom incidence and the orientation of these columnar growth units (or the equivalent open boundaries) and formulated the "tangent rule": 2 tan fi = tan where/~ is the angle between the growth direction and the substrate normal and ~ is the angle between the adatom flux and the substrate normal. In effect this relation means that the columnar structure is always inclined towards the adatom flux, but it is always more nearly normal to the substrate surface than is the adatom flux. Dirks and Leamy ° have surveyed many published results and considerable amorphous deposit data of their own and have concluded that this tangent rule is obeyed for crystalline and amorphous films with 0 ~< ~ ~< 60'. They have also observed that porous boundaries around dense columnar growth cores are more pronounced with decreased adatom surface mobility and more oblique adatom incidence. They have developed a model of simple geometric shadowing of the adatom flux by atoms within the growing film, where self-shadowing by justdeposited atoms produces void formation when the rate of migration of atoms to the shadowed regions is smaller than the rate of void incorporation during growth. The diameters of the thin columnar growth features are of the order of a few atomic diameters. Their more recent results ls'16 agree with this model, but deal with amorphous films having columnar growth units of the order of 50 250 A in diameter. In addition, depositions at large ~ have been found to result in row-like column aggregates in a direction perpendicular to the plane of incidence and the effects of adatoms incident from various angles are thought to be geometrically additive 6, 1 These results seem to apply to the zone 1 microstructure described by Movchan and Demchishin ~ in evaporated materials and verified by Thornton 2 in sputtered materials. However, these results do not completely describe our observations of the substrate topography-related microstructures of thick sputterdeposited materials, particularly with respect to the much coarser columnar defect structures that can be produced with appropriate conditions and observed optically in metallographic cross sections. For this open type of structure, low density (easily etched) columnar grain boundaries (growth unit boundaries) occur with very high frequency and are the dominant metallographic feature. This work was undertaken to determine if this tangent rule related ~ to/3 for thick sputtered deposits and to determine if an atomic self-shadowing mechanism could be applied to describe the growth of this open structure which is often associated with poor coating properties. 2.
MATERIALS A N D P R O C E D U R E S
As described previously 18, a triode sputtering system designed in our laboratory was modified to accommodate the target-to-substrate arrangement shown in Fig. 1. The mask allowed any area of the target to be exposed for sputtering so that the relation between the adatom flux and the substrate could readily be changed. The substrate was fabricated in two pieces, with flat faces at 0, 7.5, 20, 30, 45 and 90 ° to the target normal (direction of most concentrated adatom flux). This allowed the effect of a number of angles of flux incidence to be determined from the
GROWTH STRUCTURE IN SPUTTERED Cr
123
results of a single sputtering experiment. Figure 2 is a photograph of a typical substrate just prior to its installation in the sputtering system.
~'~C
Cu
$UBSTRATE
12.7 Cm
k\\\\\\\\\\\"
--~ \ \ \ \ \ \ \ "
-t
I Fig. 1. A schematic representation of the sputtering target-to-substrate arrangement. Fig. 2. A typical copper substrate (two-part) with the inclination of the faces to the adatom flux and the areas sectioned for evaluation indicated. (Top view: magnification, 1.1 x .)
The results described here were obtained with a circular area of diameter 1 in exposed (not masked) in the center of the target--a close approximation to a point source. The dispersion of an incident adatom beam with this geometry was of the order of +- 5 ° from a ray between the substrate and the center of the target. The target was high purity chromium (18 ppm C, 20 ppm 0 2, 30 ppm Fe and all other impurities less than 3 ppm). The substrate was metallographically polished O F H C copper which was water cooled. Calibrating marks were placed on each face at known angles using a scribe of known shape to provide additional information on the defect growth at surface irregularities. Typical sputtering experiments were conducted using a chamber pressure before the introduction of the krypton sputtering gas of 7.7 x 10-6 Pa and a krypton pressure during the experiments of 0.48 Pa. A substrate etch was applied for 10 min prior to sputter deposition using a substrate potential of 200 V and a substrate current of 0.2 A. Typical parameters during deposition were a target current of 0.06 A, a target potential of 2000 V and a substrate temperature of 25 °C. The effective deposition rate normal to the flux direction (i.e. on a 90 ° face) was approximately 0.9 nm s -1. The deposits were metallographically sectioned and were characterized using light microscopy in both polished and etched conditions. The specimens were electrolytically etched at 5 V for 5-10 s in a solution of 5 ml CrOa, 4 ml H E S O 4 and
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J.W. PATTEN
200 ml H 2 0 . Prior to sputter deposition the ion-etched substrate surfaces were examined using scanning electron microscopy (SEM) to characterize the substrate surface topography. The outer surfaces of the deposit and the substrate~deposit surfaces (after the removal of the copper substrates in nitric acid) were also characterized using SEM. Sections of the copper substrates were bent in order to fracture the adherent chromium deposits and the fracture surfaces were characterized using SEM. 3. RESULTS AND DISCUSSION Figure 3 is an optical micrograph of a deposit produced with the chromium flux incident at an angle of 60 ° to the copper substrate (top arrow). The two asperities on the substrate surface (bottom arrows) are replicated by the chromium deposit. The deposit (and defect) growth, as represented by the direction of growth of these surface asperities, is very nearly parallel to the incident adatom flux.
60
J~ /65 o
Cu SUBSTRATE
Fig. 3. An optical micrograph of a sputtered chromium deposit. Substrate surface asperities were replicated by the deposit (bottom arrows). (As-polished : magnification, 630 x )
Figure 4 shows three optical micrographs of etched cross sections of the same chromium deposit as that in Fig. 3. The chromium adatom flux was also incident at 6 0 to the substrate in the areas shown (Fig. 3(a), top arrow). Two types of columnar growth structure were revealed by etching. One type produced local defects in the coating and was associated with substrate surface asperities (Fig. 4(a), bottom right arrow). The other type of columnar structure, a solidification structure, always grew perpendicular to the substrate surface regardless of the adatom incidence angle (Fig. 4(a), bottom arrow). 3.1. Columnar dejects For the first type of columnar growth, substrate surface asperities were transmitted through and magnified by the chromium coating as shown in Fig. 4(a) (c). In every case the etchant preferentially attacked the peripheral areas of the defects and left a core area relatively unaffected. This may indicate defect core areas of high density with lower density material surrounding each core. Such a reduction in density would have to be on a very fine scale, however, as no voids were detected within the limits of optical resolution in the polished cross sections (Fig. 3). Each high density core is propagated from the highest point ofa substrate surface asperity
G R O W T H S T R U C T U R E IN S P U T T E R E D
Cr
125
to the highest point of the corresponding growth defect in the chromium deposit and is assumed to represent the direction of growth of the defect. This direction is very close to 60 °, i.e. parallel to the incident adatom flux (Figs. 4(a) and 4(b)). For a large number of such measurements this growth angle was 61 + 2 °, i.e. 60 ° within experimental error. Similar results were obtained for other adatom flux incidence angles of 90, 45, 30 and 20 °. Insufficient deposit thickness for growth angle measurement was obtained at 7.5 and 0 ° adatom incidence.
//•DATOM FLUX CIDENT AT 6 0 *
Cr DEPOSIT Cu SUBSTRATE
(a)
(b)
(c) Fig. 4. Optical micrographsof a sputtered chromium deposit. Adatom flux incident at an angle of 60° to the substrate surface. (Etched: magnification, 525 x .) (a) Large surface asperities. Note the columnar shadowing defect structure (bottom right arrow) and the columnar solidification structure (bottom left arrow). (b) Smaller surface asperities. (c) The sides of growth defects from a small surface asperity are parallel during equilibrium shadowinggrowth. If each defect is assumed to be approximately conical, then sections through the defects should be curved. This was observed for the defects associated with the largest substrate surface asperities (Fig. 4(a)) and for the initial segments of all defects. However, for the smaller asperities (Figs. 4(b) and 4(c)) the sides of the growth defects became nearly straight and parallel to each other after a small thickness of chromium deposit growth (Fig. 4(c)). It was also noted that the sides of the growth defects approached an angle of 60 ° to the substrate surface, i.e. parallel to the adatom flux, as the size of the associated surface asperity decreased and as the chromium deposit thickness increased. This was demonstrated by measuring the angle between the substrate surface and the leading edge (towards adatom flux) of the defects (Fig. 4(c), top arrow). For a large number of measurements the average angle was 55.9 + 4°; i.e. this angle never exceeded 60 °. Also the opposite side of the defects (trailing edge) was never observed to grow at less than 60 ° to the substrate surface, but was observed to grow at a wide range of angles approaching 60 °. These observations do not agree with growth directions predicted by the "tangent rule" described earlier; this can be seen by comparing the predicted and measured values of 0 shown in Table I. Therefore the atomic self-shadowing
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TABLE 1 P R E D I C F E D A N D M E A S U R E D D E P E C T G R O W T H A N G L E S F O R D [ F f ' E R E N T A D A ~ O M [,L(,'N I N ( I I I E N ( ' E A N G I , L S
90 60 45 30 20
0 30 45 60 70
0 17 27 41 54
90 73 03 49 36
S7 61 45 32 21
is the angle between the adatom flux and the substrate normal. 0 ( = 90 - :0 is the angle between the adatom flux and the substrate surface. fl is the angle between the growth direction and the substrate normal. 0 ( =90 -[~) is the angle between the growth direction and the substrale surlace. Predicted by the tangent rule, 2 tan [~ = tan a. b Measured. mechanism leading to the tangent rule predictions is assumed not to be active for the experiments described in this work. It is suggested instead that atomic mobility on the free surface of the growing c h r o m i u m deposits is of a sufficiently long range to allow local selection of the lowest energy positions (a large number of nearest neighbors~ but is of a much shorter range than the scale of the substrate surface asperities shown here. It is further suggested that defect growth proceeds by a geometric shadowing mechanism with the c h r o m i m n flux behaving as a continuous medium. This result was partly expected, since self-shadowing on an atomic scale would not be expected to dominate over geometric shadowing by large surface asperities for small deposit thicknesses, i.e. early in the deposition. However. if atomic self-shadowing was active at all, then defect growth should have approached the angle predicted by the tangent rule when the film thickness exceeded twice the surface asperity height, and should have reached the tangent rule predicted wtlue in the limit. This was not observed. Instead early defect growth seemed to be very similar to the geometric shadowing behavior predicted for step coverage by Blech et al. ~8. ~9 by Bindell and Tisone 2° and by Tisone and Bindell 2 ~. As growth proceeded, defect cores grew parallel to the a d a t o m flux from the start of deposition and the sides of the defects approached this growth direction as a limit, It is suggested that the geometric shadowing calculations of Tisone and Bindell, if extended to large deposit thicknesses, may be expected to predict the defect growth observed in this work. 3.2. C o l u m n a r solidi[ication s t r u c t u r e
The second type of c o l u m n a r growth structure noted earlier (Fig. 4(aL b o t t o m left arrow) was investigated further by bending a copper substrate in order to fracture the adherent c h r o m i u m deposit. Figure 5 shows a c h r o m i u m fracture surface produced in this way. Some areas fractured along boundaries of the growth defects describes earlier (Fig. 5(a)). Other areas fractured along the c o l u m n a r boundaries oriented perpendicular to the substrate (Fig. 5(b)L F r o m Fig. 5(b), the solidification structure was elongated in the substrate plane along an axis parallel to a projection of the a d a t o m flux and had an approximate grain diameter imajor) of 1.3x 10- ~" cm (1.3 jam). This agreed approximately with the etched optical
GROWTH STRUCTURE IN SPUTTERED
Cr
127
metallography in Fig. 4. Correspondence between the size of these fracture surface features and the grain size was also verified by transmission electron microscopy evaluation of thinned deposits and will be discussed in ref. 22. PHYSICAL SHADOWING GROWTH DEFECT
ADATOM FLUX /
DEPOSIT
SUBSTRATE
(a) COLUMNAR CONDENSATION GRAIN R r}l liMi"l/~I~1~~
Cu SUBSTRATE
......
/I FLUX qT AT 4 5 °
DEPOSIT
(b) Fig. 5. SEM fractographs of sputtered chromium deposited with an adatom incidence angle of 45 ° onto a metallographically polished copper substrate maintained at 25 °C. (a) Fracture along the low density boundary of a physical shadowing growth defect growing parallel to the adatom flux. (Magnification, 1800 x .) (b) Fracture along columnar condensation grain boundaries growing perpendicular to the substrate surface. (Magnification, 900 x .)
The grain size of the columnar solidification structure also corresponded to topography-raised surface features on the ion-etched substrate. Figure 6(a) is a SEM micrograph of an ion-etched copper substrate and indicates grain boundaries in the copper (bottom arrow), protrusions approximately 1-3 gm in diameter corresponding to the solidification grain size in the chromium deposits (center arrow) and a particle of surface contaminant with a depression etched around it (top arrow). These contaminants were found by X-ray fluorescence analysis to be high in magnesium, silicon and calcium, and provided nucleation sites for shadowingproduced columnar defects in the chromium deposits. Figure 6(b) is a SEM micrograph of the substrate side of a chromium deposit with the substrate chemically removed. This photograph indicates that the chromium deposits clo-ely replicate the etched copper substrate surfaces. Figure 6(c) is a SEM micrograph of the deposit-free surface. Here the tops of the shadowing-produced columnar defects (top arrow) are surrounded by the dimpled deposit surface, with the dimple (depression) size (bottom arrow) corresponding to the grain size and the size of the substrate surface protrusions. This agrees with the observation in Fig. 4(b) that the
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columnar solidification grains have parallel sides, with the free gurface end of each column being the same size as the substrate end.
lal
(b)
(c) Fig. 6. SEM micrographs of substrate and deposit surface topography. (Magnification. 480 x .) (a) Ionetched substrate surface: bottom arrow, copper grain boundary: center arrow, surface protrusions: top arrow, surface contaminant rich in magnesium, silicon and calcium. (b) Substrate side of the chromium deposit. (c) Deposit-free surface: top arrow, shadowing-produced columnar defect: bottom arrow, dimpled deposit surface.
4. CONCLUSIONS Two different types of microstructural columnar growth structure were observed in sputter-deposited chromium coatings. One type produced local defects in the coatings and was associated with geometric shadowing of substrate surface asperities. These defects consisted of high density cores surrounded by lower density (more easily etched) areas and always grew parallel to the incident adatom flux. These defects are the type responsible for the open columnar defect structures that can dominate and degrade the properties of thick line-of-sight-deposited coatings. The other type of columnar structure, a columnar solidification structure, always grew perpendicular to the substrate surface regardless of the incidence angle. No evidence of the tangent rule relating the column orientation to the adatom flux
G R O W T H STRUCTURE IN SPUTTERED
Cr
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incidence direction was observed for either type of structure, so that atomic selfshadowing was not important in determining the growth direction. ACKNOWLEDGMENTS
This research was supported by the Department of Energy, Basic Energy Sciences. Special thanks are also due to R. W. Moss and M. A. Bayne for experimental assistance, E. D. McClanahan for helpful discussions, R. F. Stratton for the sputter depositions, H. L. Butts and J. H. King for apparatus design, R. H. Beauchamp for optical metallography and H. E. Kjarmo for SEM microscopy. REFERENCES 1 2 3 4
5 6 7 8 9
10 11 12 13 14 15
16
17 18 19 20 21 22
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