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The influence of substrate temperature on the properties of Al, Zr and W coatings deposited by closed-field unbalanced magnetron sputtering
Vacuum/volume
Pergamon PII: s0042-207x(97)00127-9
49/number l/pages 43 to 4711998 0 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
The influence of substrate temperature on the properties of Al, Zr and W coatings deposited by closed-field unbalanced magnetron sputtering P J Kelly* and R D Arnell, Research Institute for Design, Manufacture Salford, M5 4WT, UK received 2 1 July
and Marketing,
University of Salford,
1997
Closed-field unbalanced magnetron sputtering (CFLJBMS) is now recognised as a technique capable of producing high quality industrially relevant coatings. As a result of this, BNFL are undertaking a major project with the aim of fully characterizing a CFLJBMS system. As a part of this project, characterization studies have been carried out of the properties of Al, Zr and W coatings deposited by CFUBMS. Deposition parameters, such as target current (over the range 2A to 8A), substrate bias (- 30 V to - 70 V), coating pressure (0.5mtorr to 3mtorr) and substrate-to-target separation (80mm to 750mm), were varied in a systematic manner, using experiments designed using the Taguchi Method. Substrate temperatures were also measured, and were found to range from 200°C to 360°C. The coatings were analysed using X-ray diffraction techniques to determine grain size, micro- and macro-strains and texture. Knoop microhardness measurements were also made. The relationships between the deposition parameters, substrate temperature, and the subsequent coating properties were investigated. 0 7998 Elsevier Science Ltd. All rights reserved
Introduction
Closed-field unbalanced magnetron sputtering (CFUBMS) is now recognised as a technique capable of producing high quality industrially relevant coatingse3 As a result of this, a major project has been undertaken at Salford University, with the aim of fully characterizing the CFUBMS system. In order to achieve this aim, a number of separate, but strongly inter-related projects have been carried out. These include studies of the plasma characteristics of the system4 and the development of mathematical models of the magnetron discharge.5 Extensive studies have also been made of the influence of deposition parameters on the structure of coatings produced by this system.6 Indeed, this has led to the development of a novel structure zone model relating to the CFUBMS system.7 The work reported here shows the relationship between deposition parameters, specifically ion current density and substrate bias voltage, and substrate temperature. It then describes the influence of substrate temperature on the properties of the coatings produced in this study. Experimental
The rig selected for characterization was a Teer Coatings Ltd. UDP 450 CFUBMS system.8 This rig has been fully described elsewhere, as have the standard deposition conditions used during this project6 Aluminium, zirconium and tungsten coatings were deposited under systematically varied conditions. These metals
were chosen because of their interest to the project sponsors, BNFL, and because of the wide variation in their physical properties and crystallographic structures. For each metal, experimental arrays were developed using the Taguchi method.’ The standard array used was the nine run L9 array. This array allows up to four factors to be varied at three levels. However, in these experiments only three variables were used in the arrays, with the fourth column being left blank. The variables selected were target current, substrate bias and coating pressure. Overall, for the coatings described here, target current was varied from 2 A to 8 A, substrate bias from - 30 V DC to - 70VDC, and coating pressure from 0.5 mtorr to 3 mtorr. Deposition times were varied with target current to produce batches of coatings of nominally the same thickness. Substrateto-target separation, &, was also varied from 80 mm to 150 mm. For each coating run, the substrate temperature was monitored using mineral-insulated thermocouples clamped to the substrate holder. However, no attempt was made to actually control the substrate temperature. Table 1 shows a Taguchi L9 array developed for the investigation into zirconium coatings, by way of example. Additional experiments were carried out to determine the ion current drawn at the substrate for all deposition conditions tested. For each set of conditions, this involved increasing the substrate bias voltage until the current drawn at the substrate saturated. In all cases, this occurred at bias voltages greater in magnitude 43
P J Kelly and R DAmekThe
influence of substrate temperature
on properties
Table 1. Taguchi L9 array for investigation of zirconium coatings
Run no.
I 2 3 4 5 6 7 8 9
Coating Target current (A) (run time. (min)) Substrate bias (V) pressure (mtorr) __.0.5 -30 4 (60) 1 -50 4 (60) 3 -70 4 (60) I -30 6 (45) 3 -50 6 (45) -70 0.5 6 (45) 3 -30 8 (30) 0.5 -50 8 (30) I -70 8 (30)
than - 1OOV. This saturation current was converted to an ion current density using the area of the substrate holder. More detailed accounts of this procedure are given in Refs 6 and 7. Coatings were deposited onto various substrate materials selected for their suitability for particular analytical techniques. For example, coatings were deposited onto silicon wafers for structural examination, using the SEM, and for microhardness measurements, using a Knoop microhardness indentor. Silicon was chosen because it fractures cleanly, minimising the distortion of the coating fracture surface. Also, the topography of the coating mirrors that of the substrate. Thus, the highly polished surface of the wafers allows microhardness indentations to be easily identified and measured, which is often not the case for substrates with ground surfaces. For each coating, microhardness measurements were taken at three loads (5 g, 10 g and 15 g). Ten measurements were taken at each load, to give a mean value for that load. The mean values at each load were then extrapolated to zero load to reduce any influence of the substrate material on the hardness measurements. Coatings were also deposited onto copper coupons for XRD analysis. Unlike silicon wafers, this material does not have a strong preferred orientation and should not, therefore, influence the texture of the coatings. Also, the main diffraction peaks of the coating materials do not overlap with those of copper. The X-ray equipment used was the Philips X-PERT system, operating in the 0/20 mode with CuKcc radiation. Each scan provided information on the texture of the coatings. Line profile analyses then gave information on the grain size, lattice strain (also termed ‘microstrain’) and elastic strain (‘macrostrain’) of the coatings. Grain sizes and lattice strains were estimated from measurements of the broadening of the major peaks in the 0/2Q scan, using the Scherrer equation.” Annealed aluminium and zirconium standards were produced and analysed to allow the component of the peak width due to instrumental effects to be determined. It was not possible to produce an annealed tungsten standard. Therefore, the data from the Al and Zr standards were extrapolated to allow the contribution of instrumental effects to the width of the tungsten peaks to be estimated. Elastic strains were found by comparing the position of a suitable high angle peak in the O/20 scan with the position of the corresponding peak for the annealed standard. This value was converted from a ‘throughthickness’ strain to an ‘in-plane’ strain using Poissons Ratio for the coating material. Results Ion current density. Over the conditions tested, substrate ion currents were found to range from 0.24A to 2.5A, which is 44
equivalent to a range of ion current densities from 0.8 mA cm-’ to 8 mAcm-*. Further, ion current density was found to be directly proportional to target current, but to decrease with increasing pressure and d,_,. A similar dependence on target current and rl, , was also observed for deposition rate. However, over the range tested, pressure (and substrate bias) had no significant influence on this parameter. Again, more details of these results are given in Refs 6 and 7. Substrate temperature. The temperature of the substrate holder was monitored throughout each coating run, and the final temperature attained was recorded. Taguchi analyses were then carried out using final substrate temperature as the response variable. Very similar trends were observed for all three metals. An example is given in Figure 1, which shows the results of the Taguchi analysis of the zirconium array data. As can be seen, increasing target current and bias increases the final substrate temperature, whereas increasing pressure and d, tr decreases final substrate temperature. From Taguchi analyses it is also possible to estimate the relative contributions of each array factor to the response variable. For the example given in Figure 1, at ds , = 80 mm, the contributions of target current, bias and pressure to the substrate temperature were 51%, 25% and 24%, respectively, whereas at d,_, = 110 mm the contributions were 35%, 44% and 15%, respectively. This implies that as d3I increases the relative contribution of target current to substrate temperature falls, and the contribution of substrate bias increases. All of these effects can be readily explained in terms of changes to the energy of the ions, and the ion and neutral fluxes incident at the substrate. In sputtering systems, energy is transferred to the substrate, and the growing film, through bombardment by electrons, ions and energetic neutrals; by the heat of condensation of the coating atoms; and by radiant heating.” As mentioned above. both ion current density and deposition rate were found to be directly proportional to target current. Thus, increasing target current increases both the flux of ions incident at the substrate and, since there will be a greater flux of coating atoms, a greater heat of condensation will be liberated. Increasing bias voltage increases the average energy of the bombarding ions. In
Zirconium Array - Taguchi Analysis Substratetemperature 320
,
180
’
r
1
4A 6A 8A targa current
I
-30v bOV -70v substrate bias
1
0.5
1.0 3.0 mating tmssure. mlorr
Figure 1. Zirconium array: Taguchi analysis using final substrate tem-
perature as response variable.
PJ Kelly and R D Ame//: The influence of substrate temperature
on properties
raising pressure or separation reduces both the flux, and the average energy of particles incident at the substrate, through increased gas phase collisions. The power density at the substrate, P,ub, can be estimated by taking the product of ion current density and substrate bias voltage. Since ion current density depends on target current, pressure and d,_,, Psub includes all the parameters identified by the Taguchi analyses as influencing substrate temperature. The energy delivered to the substrate by the condensing coating atoms is not considered. However, despite this, a good correlation was observed between Psub and substrate temperature. By way of example, the relationship between power density at the substrate and substrate temperature for the tungsten array is shown in Figure 2. Regression analysis gives a correlation coefficient, Y,for these data of 0.84, indicating a strong positive correlation in the data. Correlation coefficients of 0.88 and 0.75 were obtained for the equivalent relationship for the Zr and Al arrays, respectively. contrast,
Coating structures. The structures of the coatings produced during this project have been reported elsewhere.6,7 As such, SEM micrographs will not be reproduced here. However, for completeness, brief descriptions of the structures will be given. The aluminium coatings were deposited at homologous temperatures, T/T,,,, (where T is the substrate temperature and T,,, is the melting point of the coating material, both measured in Kelvin) over the range 0.51 to 0.68. SEM examination of the fracture sections of the aluminium coatings indicated that these coatings had fully dense structures and were highly ductile. Good coatingto-substrate adhesion after fracture was also observed. The zirconium coatings were deposited at homologous temperatures over the range 0.22 to 0.28. These coatings were found to have dense columnar coatings, i.e., the coating structures had a clear columnar element, but there were no voids observed between the columns. Very similar structures were observed for the tungsten coatings, which were deposited at homologous temperatures over the range 0.13 to 0.17. As for aluminium, good coating-to-substrate adhesion after fracture was observed for the Zr and W coatings. These results demonstrate the ability of the CFUBMS system to produce dense, ‘high temperature’ structures at relatively low substrate temperatures, in comparison to other sputtering systems,“-” and to suppress the formation of ‘low temperature’ porous structures.
Table 2. Data table for analysis of Al, Zr and W coatings.
Final substrate temperature (“C) Homologous temperature, T/Tm Knoop microhardness (MPa) Grain size (nm) Lattice strain (%) Macrostrain
(%)
min. max. min. max. min. max. min. max. min. max. min.
Aluminium
Zirconium
Tungsten
200 357 0.51 0.68 360 860 138 301 0.08 0.13 0.1 0.3
194 331 0.22 0.28 3080 6120 36 132 0.16 0.42
199 336 0.13 0.17 _
I5
0.8 I.2
_
Knoop microhardness. The extrapolated microhardness values for the Al coatings ranged from 360 MPa to 860 MPa. Details of the properties of the coatings examined in this project are listed in Table 2. Figure 3 shows the relationship between coating microhardness and final substrate temperature for the aluminium coatings. As can be seen, there is a clear trend of decreasing coating hardness with increasing substrate temperature. Regression analysis gives a correlation coefficient for these data of -0.9, indicating a strong negative correlation in the data. Similar results were obtained for the zirconium coatings. In this case, the extrapolated microhardness values ranged for 3080 MPa to 6120 MPa. The relationship between coating microhardness and final substrate temperature for the zirconium coatings is shown in Figure 4. Again, a significant negative correlation was observed in the data (r = -0.79). It proved difficult to measure accurately the microhardness of the tungsten coatings. High levels of intrinsic stress and the inherent brittleness of the coatings meant that the regions surrounding the hardness indentations tended to break-up and deadhere from the substrate. The hardness of these coatings can only be given as approximately of the order I 1 to 15GPa. No relationship with substrate temperature was observed for the tungsten coatings. Coating grain size and lattice strain. The aluminium coatings all displayed a strong (111) preferred orientation. Indeed, in several cases only the (111) and its associated (222) reflection were observed. Consequently, estimates of grain size and lattice strain
900
$l3o=o 1 J =_ 700 I
i_
200
250 Final substrate
300 temp.,deg
350
1
400
C
Figure 2. The relationship between substrate power density and final substrate temperature for tungsten coatings.
!400
250 300 Final subsbate temp.. deg C
Figure 3. The relationship between Knoop microhardness strate temperature for aluminium coatings.
and final sub-
45
P J Kelly and R D Amek
The influence of substrate temperature
on properties 300
0.5 )
1
250
E 200 E 2 ‘D 150 .c !! 0
2,500
1
180
100
I 220
200
240 260 280 Final substrate temp.. deg C
300
320
340 200
Figure 4. The relationship between Knoop microhardness and final substrate temperature for zirconium coatings
250
300
Finalsubstrate Temp.,deg C
Figure 6. The relationships between grain size, lattice strain and final substrate temperature for zirconium coatings. were made from measurements of the broadening of the (I 11) peak for each coating. Grain sizes ranged from 138 nm to 30 I nm, with lattice strains varying from 0.08% to 0.13%. As shown in Figure 5, a strong positive correlation (r = 0.85) exists between grain size and substrate temperature for these coatings, whereas a negative correlation (r = -0.81) exists between lattice strain and temperature. There are three main diffraction peaks in the zirconium pattern, namely the (loo), (002) and (101) peaks. However, the zirconium coatings all displayed a strong (002) texture. Estimates of grain size and lattice strain were, therefore, made from measurements of the broadening of the (002) peak for each coating. Grain sizes ranged from 36nm to 132 nm and lattice strains ranged from 0.16% to 0.42%. Again, grain size was found to increase with increasing substrate temperature (Y = 0.84), and lattice strain to decrease with increasing substrate temperature (r = -0.85). These relationships are shown in Figure 6. The tungsten coatings displayed four main diffraction peaks, of which the (110) and (211) had the highest intensities, although no strong texture was displayed. Very broad diffraction peaks were a feature of the tungsten XRD traces, indicating very small grain sizes (5 nm to 13 nm) and high levels of lattice strain (0.8% to 1.2%). These values were averaged from the values obtained from the (110) and (211) peaks for each coating. No relationships were observed between substrate temperature and grain size, or lattice strain. This is not surprising, bearing in mind the low homologous temperatures at which the tungsten coatings were deposited.
Elastic ‘macrostrain’. Elastic strains were estimated for the Al coatings by comparing the position of the (222) peak for each coating (20 approximately 82”) with the position of the corresponding peak in the trace obtained from the Al annealed standard. These ‘through-thickness’ strains were converted to ‘in-plane’ strains using Poissons Ratio for aluminium. All the ‘inplane’ strains were found to be tensile, with the magnitude of the strain increasing with increasing substrate temperature, as shown in Figure 7 (Y = 0.7). The respective coefficients of linear expansion of the coating and substrate (copper) materials are 24 x lo”/ ‘C and 17 x 106/“C. Thus, on cooling the coating will attempt to contract more than the substrate, placing the coating under tension, Also shown in Figure 7 are the theoretical strains which would arise, purely from the difference in thermal expansion coefficients. Mechanical strains are not considered in this analysis, because the substrate is massive compared to the coating and will not, therefore be distorted by stresses in the coatings. Both curves are very similar, strongly supporting the hypothesis that the origin of the elastic strains in the coatings is due to the mis-match in the expansion coefficients of the coating and substrate. Similar trends were not observed for the zirconium or tungsten coatings. The reasons for this are not clear at this stage and further analysis of these coatings is taking place.
0.4 Sbain
‘r
6OC
0.14
t,
0.12
500
0.1
+
@ 400 .$
IE 0.08 00
s 300
8 0.06 9
S - 0.3 .r g 8 ‘u a 0.2 E -lD 5 x i 0.1 ?
(actual) _.*_. StJaWl (IhcaretHat)
200 0.04
150
100 tI 150
200
250
300
Finelsubstratetemp.,dagC
350
8
p.02
Figure 5. The relationships between grain size, lattice strain and final substrate temperature for aluminium coatings.
200
250 300 350 Final substrate temp., deg C
400
‘Thearet& strains from difference in expemim coeffs of substrate(01) and mating
Figure 7. The relationship between coating ‘in-plane’ macrostrain and final substrate temperature for aluminium coatings.
P J Kelly and R D Ame//: The influence of substrate temperature
on properties 6,500
Discussion The analysis of the Al and Zr coatings has produced a consistent set of results. By considering the power density at the substrate. the relationship between ion current density, bias voltage and the resulting substrate temperature was investigated. As would be expected, a strong positive correlation was observed in the data. Further strong relationships were subsequently observed between specific coating properties and substrate temperature. For example, clear trends of increasing grain size with increasing substrate temperature were identified. As grain growth occurs, the lattice becomes more ordered and fewer grain boundaries are present. This accounts for the corresponding decreases observed in lattice strain and microhardness. Such trends were not found for the tungsten coatings, probably as a result of the relatively low homologous temperatures of these coatings. The relationship between coating grain size and microhardness can now be considered. The well-known Hall-Petch equationI states that hardness, H, is inversely proportional to the square root of grain size:
H = H,+kd-“’
2z50t.w Figure 9. The relationship zirconium
where Ho is the intrinsic hardness for a single crystal, d is the grain size and k is a material constant. The relationships between microhardness and d-O.’ for the Al and Zr coatings are shown in Figures 8 and 9, respectively. In both cases, the data are in good agreement with eqn (1) (r = 0.91 and 0.76, respectively). The actual values obtained by regression analysis for Ho were 70 MPa for aluminium and 660 MPa for Zr. However, due to the scatter in the data, these values can only be taken as a general indication of the actual magnitude of Ho for these materials.
m
7. 600
I i
8.
0 700
9.
f b ._ E
10.
8600 5
Figure 8. The relationship aluminium
coatings.
between
grain
0.16
size and microhardness
0.16
for
between
grain
size and microhardness
would like to thank BNFL for funding this project.
References
5. 6.
f
grain size)
Acknowledgements
1. Dense well-adhered coatings of Al, Zr and W have been deposited by CFUBMS under systematically varied conditions. 2. Fully dense and dense columnar structures were obtained at homologous temperatures well below those required by other sputtering systems.
u‘
0.14
l/(root
coatings.
The authors
%
0.12
A strong correlation was observed between the power density at the substrate and the final substrate temperature. Taguchi analyses of the deposition process indicated that all the deposition parameters investigated influenced the final substrate temperature. Strong correlations (Irl>O.7) were, in turn, observed between substrate temperature and grain size, lattice strain and microhardness for the Al, and Zr coatings. And additionally between elastic macrostrain and temperature for the Al coatings, due to the mis-match in thermal expansion coefficients of the coating and substrate materials. Little variation was observed in the properties of the tungsten coatings.
(1)
Conclusions
,
1
11.
0.06
12. 13.
for
14.
Efeoglu, I., Arnell, R. D., Tinston, S. F. and Teer. D. G., Surf: Coat. Technol, 1993,57, 61-69. Monaghan, D. P., Teer, D. G., Laing, K. C.. Efeoglu. I. and Arnell, R.D., Surf: Coat. Technol, 1993,59,21-25. Monaghan, D. P., Teer, D. G., Logan, P., Efeoglu, 1. and Arnell, R.D., Surf. Coat. Technol, 1993,60, 525-530. Bradley, J. W., Welzel, T., Hahn, J., Pintaske, R. and Arnell, R. D., Observations of plasma density peaks in magnetron discharges. Poster presentation at The Inst. of Physics Annual Congress. Leeds, UK, 2427 March, 1997. Bradley, J. W., Plasma Sources. Sci. Technol., 1996,5, 622-631. Kelly, P. J. and Arnell, R. D., Surf: Coot. Tvchnol, 1996.86-87,425p 431. Kelly. P. J. and Arnell, R. D., Proc. 5th Int’l Conf. on Plasma Surface Engineering, PSE’96, Garmisch-Partenkirchen, Germany, Sept. 1996. Accepted for publication in Su$ Coat. Technol. Teer, D. G., UK Patent, GB 2 258 343 B, Magnetron Sputter Ion Plating. Roy, R., A Primer on the Tquchi Method. Van Nostrand Reinhold, New York, 1990. Cullity, B. D., Elements of‘ X-Raj, D$%trcrion. Second Edition. Addison-Wesley, London, 1978. Andrischky, M., Guimaraes, F. and Teixeira, V., Vacuum. 1993, 44(E), 809-813. Thornton, .I. A., J. Vat. Sci. Technol., 1974, 11(4), 66&670. Messier, R., Giri, A. P. and Roy, R. A.. J. Var. Sci. Techno/., 1984, A2(2), 500-503. Sundgren, J. E. and Henztell. H. T. G., J. Vuc. Sci. Techno/., 1986, A4(5). 2259-2279.
47
Pergamon PII: s0042-207x(97)00127-9
49/number l/pages 43 to 4711998 0 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
The influence of substrate temperature on the properties of Al, Zr and W coatings deposited by closed-field unbalanced magnetron sputtering P J Kelly* and R D Arnell, Research Institute for Design, Manufacture Salford, M5 4WT, UK received 2 1 July
and Marketing,
University of Salford,
1997
Closed-field unbalanced magnetron sputtering (CFLJBMS) is now recognised as a technique capable of producing high quality industrially relevant coatings. As a result of this, BNFL are undertaking a major project with the aim of fully characterizing a CFLJBMS system. As a part of this project, characterization studies have been carried out of the properties of Al, Zr and W coatings deposited by CFUBMS. Deposition parameters, such as target current (over the range 2A to 8A), substrate bias (- 30 V to - 70 V), coating pressure (0.5mtorr to 3mtorr) and substrate-to-target separation (80mm to 750mm), were varied in a systematic manner, using experiments designed using the Taguchi Method. Substrate temperatures were also measured, and were found to range from 200°C to 360°C. The coatings were analysed using X-ray diffraction techniques to determine grain size, micro- and macro-strains and texture. Knoop microhardness measurements were also made. The relationships between the deposition parameters, substrate temperature, and the subsequent coating properties were investigated. 0 7998 Elsevier Science Ltd. All rights reserved
Introduction
Closed-field unbalanced magnetron sputtering (CFUBMS) is now recognised as a technique capable of producing high quality industrially relevant coatingse3 As a result of this, a major project has been undertaken at Salford University, with the aim of fully characterizing the CFUBMS system. In order to achieve this aim, a number of separate, but strongly inter-related projects have been carried out. These include studies of the plasma characteristics of the system4 and the development of mathematical models of the magnetron discharge.5 Extensive studies have also been made of the influence of deposition parameters on the structure of coatings produced by this system.6 Indeed, this has led to the development of a novel structure zone model relating to the CFUBMS system.7 The work reported here shows the relationship between deposition parameters, specifically ion current density and substrate bias voltage, and substrate temperature. It then describes the influence of substrate temperature on the properties of the coatings produced in this study. Experimental
The rig selected for characterization was a Teer Coatings Ltd. UDP 450 CFUBMS system.8 This rig has been fully described elsewhere, as have the standard deposition conditions used during this project6 Aluminium, zirconium and tungsten coatings were deposited under systematically varied conditions. These metals
were chosen because of their interest to the project sponsors, BNFL, and because of the wide variation in their physical properties and crystallographic structures. For each metal, experimental arrays were developed using the Taguchi method.’ The standard array used was the nine run L9 array. This array allows up to four factors to be varied at three levels. However, in these experiments only three variables were used in the arrays, with the fourth column being left blank. The variables selected were target current, substrate bias and coating pressure. Overall, for the coatings described here, target current was varied from 2 A to 8 A, substrate bias from - 30 V DC to - 70VDC, and coating pressure from 0.5 mtorr to 3 mtorr. Deposition times were varied with target current to produce batches of coatings of nominally the same thickness. Substrateto-target separation, &, was also varied from 80 mm to 150 mm. For each coating run, the substrate temperature was monitored using mineral-insulated thermocouples clamped to the substrate holder. However, no attempt was made to actually control the substrate temperature. Table 1 shows a Taguchi L9 array developed for the investigation into zirconium coatings, by way of example. Additional experiments were carried out to determine the ion current drawn at the substrate for all deposition conditions tested. For each set of conditions, this involved increasing the substrate bias voltage until the current drawn at the substrate saturated. In all cases, this occurred at bias voltages greater in magnitude 43
P J Kelly and R DAmekThe
influence of substrate temperature
on properties
Table 1. Taguchi L9 array for investigation of zirconium coatings
Run no.
I 2 3 4 5 6 7 8 9
Coating Target current (A) (run time. (min)) Substrate bias (V) pressure (mtorr) __.0.5 -30 4 (60) 1 -50 4 (60) 3 -70 4 (60) I -30 6 (45) 3 -50 6 (45) -70 0.5 6 (45) 3 -30 8 (30) 0.5 -50 8 (30) I -70 8 (30)
than - 1OOV. This saturation current was converted to an ion current density using the area of the substrate holder. More detailed accounts of this procedure are given in Refs 6 and 7. Coatings were deposited onto various substrate materials selected for their suitability for particular analytical techniques. For example, coatings were deposited onto silicon wafers for structural examination, using the SEM, and for microhardness measurements, using a Knoop microhardness indentor. Silicon was chosen because it fractures cleanly, minimising the distortion of the coating fracture surface. Also, the topography of the coating mirrors that of the substrate. Thus, the highly polished surface of the wafers allows microhardness indentations to be easily identified and measured, which is often not the case for substrates with ground surfaces. For each coating, microhardness measurements were taken at three loads (5 g, 10 g and 15 g). Ten measurements were taken at each load, to give a mean value for that load. The mean values at each load were then extrapolated to zero load to reduce any influence of the substrate material on the hardness measurements. Coatings were also deposited onto copper coupons for XRD analysis. Unlike silicon wafers, this material does not have a strong preferred orientation and should not, therefore, influence the texture of the coatings. Also, the main diffraction peaks of the coating materials do not overlap with those of copper. The X-ray equipment used was the Philips X-PERT system, operating in the 0/20 mode with CuKcc radiation. Each scan provided information on the texture of the coatings. Line profile analyses then gave information on the grain size, lattice strain (also termed ‘microstrain’) and elastic strain (‘macrostrain’) of the coatings. Grain sizes and lattice strains were estimated from measurements of the broadening of the major peaks in the 0/2Q scan, using the Scherrer equation.” Annealed aluminium and zirconium standards were produced and analysed to allow the component of the peak width due to instrumental effects to be determined. It was not possible to produce an annealed tungsten standard. Therefore, the data from the Al and Zr standards were extrapolated to allow the contribution of instrumental effects to the width of the tungsten peaks to be estimated. Elastic strains were found by comparing the position of a suitable high angle peak in the O/20 scan with the position of the corresponding peak for the annealed standard. This value was converted from a ‘throughthickness’ strain to an ‘in-plane’ strain using Poissons Ratio for the coating material. Results Ion current density. Over the conditions tested, substrate ion currents were found to range from 0.24A to 2.5A, which is 44
equivalent to a range of ion current densities from 0.8 mA cm-’ to 8 mAcm-*. Further, ion current density was found to be directly proportional to target current, but to decrease with increasing pressure and d,_,. A similar dependence on target current and rl, , was also observed for deposition rate. However, over the range tested, pressure (and substrate bias) had no significant influence on this parameter. Again, more details of these results are given in Refs 6 and 7. Substrate temperature. The temperature of the substrate holder was monitored throughout each coating run, and the final temperature attained was recorded. Taguchi analyses were then carried out using final substrate temperature as the response variable. Very similar trends were observed for all three metals. An example is given in Figure 1, which shows the results of the Taguchi analysis of the zirconium array data. As can be seen, increasing target current and bias increases the final substrate temperature, whereas increasing pressure and d, tr decreases final substrate temperature. From Taguchi analyses it is also possible to estimate the relative contributions of each array factor to the response variable. For the example given in Figure 1, at ds , = 80 mm, the contributions of target current, bias and pressure to the substrate temperature were 51%, 25% and 24%, respectively, whereas at d,_, = 110 mm the contributions were 35%, 44% and 15%, respectively. This implies that as d3I increases the relative contribution of target current to substrate temperature falls, and the contribution of substrate bias increases. All of these effects can be readily explained in terms of changes to the energy of the ions, and the ion and neutral fluxes incident at the substrate. In sputtering systems, energy is transferred to the substrate, and the growing film, through bombardment by electrons, ions and energetic neutrals; by the heat of condensation of the coating atoms; and by radiant heating.” As mentioned above. both ion current density and deposition rate were found to be directly proportional to target current. Thus, increasing target current increases both the flux of ions incident at the substrate and, since there will be a greater flux of coating atoms, a greater heat of condensation will be liberated. Increasing bias voltage increases the average energy of the bombarding ions. In
Zirconium Array - Taguchi Analysis Substratetemperature 320
,
180
’
r
1
4A 6A 8A targa current
I
-30v bOV -70v substrate bias
1
0.5
1.0 3.0 mating tmssure. mlorr
Figure 1. Zirconium array: Taguchi analysis using final substrate tem-
perature as response variable.
PJ Kelly and R D Ame//: The influence of substrate temperature
on properties
raising pressure or separation reduces both the flux, and the average energy of particles incident at the substrate, through increased gas phase collisions. The power density at the substrate, P,ub, can be estimated by taking the product of ion current density and substrate bias voltage. Since ion current density depends on target current, pressure and d,_,, Psub includes all the parameters identified by the Taguchi analyses as influencing substrate temperature. The energy delivered to the substrate by the condensing coating atoms is not considered. However, despite this, a good correlation was observed between Psub and substrate temperature. By way of example, the relationship between power density at the substrate and substrate temperature for the tungsten array is shown in Figure 2. Regression analysis gives a correlation coefficient, Y,for these data of 0.84, indicating a strong positive correlation in the data. Correlation coefficients of 0.88 and 0.75 were obtained for the equivalent relationship for the Zr and Al arrays, respectively. contrast,
Coating structures. The structures of the coatings produced during this project have been reported elsewhere.6,7 As such, SEM micrographs will not be reproduced here. However, for completeness, brief descriptions of the structures will be given. The aluminium coatings were deposited at homologous temperatures, T/T,,,, (where T is the substrate temperature and T,,, is the melting point of the coating material, both measured in Kelvin) over the range 0.51 to 0.68. SEM examination of the fracture sections of the aluminium coatings indicated that these coatings had fully dense structures and were highly ductile. Good coatingto-substrate adhesion after fracture was also observed. The zirconium coatings were deposited at homologous temperatures over the range 0.22 to 0.28. These coatings were found to have dense columnar coatings, i.e., the coating structures had a clear columnar element, but there were no voids observed between the columns. Very similar structures were observed for the tungsten coatings, which were deposited at homologous temperatures over the range 0.13 to 0.17. As for aluminium, good coating-to-substrate adhesion after fracture was observed for the Zr and W coatings. These results demonstrate the ability of the CFUBMS system to produce dense, ‘high temperature’ structures at relatively low substrate temperatures, in comparison to other sputtering systems,“-” and to suppress the formation of ‘low temperature’ porous structures.
Table 2. Data table for analysis of Al, Zr and W coatings.
Final substrate temperature (“C) Homologous temperature, T/Tm Knoop microhardness (MPa) Grain size (nm) Lattice strain (%) Macrostrain
(%)
min. max. min. max. min. max. min. max. min. max. min.
Aluminium
Zirconium
Tungsten
200 357 0.51 0.68 360 860 138 301 0.08 0.13 0.1 0.3
194 331 0.22 0.28 3080 6120 36 132 0.16 0.42
199 336 0.13 0.17 _
I5
0.8 I.2
_
Knoop microhardness. The extrapolated microhardness values for the Al coatings ranged from 360 MPa to 860 MPa. Details of the properties of the coatings examined in this project are listed in Table 2. Figure 3 shows the relationship between coating microhardness and final substrate temperature for the aluminium coatings. As can be seen, there is a clear trend of decreasing coating hardness with increasing substrate temperature. Regression analysis gives a correlation coefficient for these data of -0.9, indicating a strong negative correlation in the data. Similar results were obtained for the zirconium coatings. In this case, the extrapolated microhardness values ranged for 3080 MPa to 6120 MPa. The relationship between coating microhardness and final substrate temperature for the zirconium coatings is shown in Figure 4. Again, a significant negative correlation was observed in the data (r = -0.79). It proved difficult to measure accurately the microhardness of the tungsten coatings. High levels of intrinsic stress and the inherent brittleness of the coatings meant that the regions surrounding the hardness indentations tended to break-up and deadhere from the substrate. The hardness of these coatings can only be given as approximately of the order I 1 to 15GPa. No relationship with substrate temperature was observed for the tungsten coatings. Coating grain size and lattice strain. The aluminium coatings all displayed a strong (111) preferred orientation. Indeed, in several cases only the (111) and its associated (222) reflection were observed. Consequently, estimates of grain size and lattice strain
900
$l3o=o 1 J =_ 700 I
i_
200
250 Final substrate
300 temp.,deg
350
1
400
C
Figure 2. The relationship between substrate power density and final substrate temperature for tungsten coatings.
!400
250 300 Final subsbate temp.. deg C
Figure 3. The relationship between Knoop microhardness strate temperature for aluminium coatings.
and final sub-
45
P J Kelly and R D Amek
The influence of substrate temperature
on properties 300
0.5 )
1
250
E 200 E 2 ‘D 150 .c !! 0
2,500
1
180
100
I 220
200
240 260 280 Final substrate temp.. deg C
300
320
340 200
Figure 4. The relationship between Knoop microhardness and final substrate temperature for zirconium coatings
250
300
Finalsubstrate Temp.,deg C
Figure 6. The relationships between grain size, lattice strain and final substrate temperature for zirconium coatings. were made from measurements of the broadening of the (I 11) peak for each coating. Grain sizes ranged from 138 nm to 30 I nm, with lattice strains varying from 0.08% to 0.13%. As shown in Figure 5, a strong positive correlation (r = 0.85) exists between grain size and substrate temperature for these coatings, whereas a negative correlation (r = -0.81) exists between lattice strain and temperature. There are three main diffraction peaks in the zirconium pattern, namely the (loo), (002) and (101) peaks. However, the zirconium coatings all displayed a strong (002) texture. Estimates of grain size and lattice strain were, therefore, made from measurements of the broadening of the (002) peak for each coating. Grain sizes ranged from 36nm to 132 nm and lattice strains ranged from 0.16% to 0.42%. Again, grain size was found to increase with increasing substrate temperature (Y = 0.84), and lattice strain to decrease with increasing substrate temperature (r = -0.85). These relationships are shown in Figure 6. The tungsten coatings displayed four main diffraction peaks, of which the (110) and (211) had the highest intensities, although no strong texture was displayed. Very broad diffraction peaks were a feature of the tungsten XRD traces, indicating very small grain sizes (5 nm to 13 nm) and high levels of lattice strain (0.8% to 1.2%). These values were averaged from the values obtained from the (110) and (211) peaks for each coating. No relationships were observed between substrate temperature and grain size, or lattice strain. This is not surprising, bearing in mind the low homologous temperatures at which the tungsten coatings were deposited.
Elastic ‘macrostrain’. Elastic strains were estimated for the Al coatings by comparing the position of the (222) peak for each coating (20 approximately 82”) with the position of the corresponding peak in the trace obtained from the Al annealed standard. These ‘through-thickness’ strains were converted to ‘in-plane’ strains using Poissons Ratio for aluminium. All the ‘inplane’ strains were found to be tensile, with the magnitude of the strain increasing with increasing substrate temperature, as shown in Figure 7 (Y = 0.7). The respective coefficients of linear expansion of the coating and substrate (copper) materials are 24 x lo”/ ‘C and 17 x 106/“C. Thus, on cooling the coating will attempt to contract more than the substrate, placing the coating under tension, Also shown in Figure 7 are the theoretical strains which would arise, purely from the difference in thermal expansion coefficients. Mechanical strains are not considered in this analysis, because the substrate is massive compared to the coating and will not, therefore be distorted by stresses in the coatings. Both curves are very similar, strongly supporting the hypothesis that the origin of the elastic strains in the coatings is due to the mis-match in the expansion coefficients of the coating and substrate. Similar trends were not observed for the zirconium or tungsten coatings. The reasons for this are not clear at this stage and further analysis of these coatings is taking place.
0.4 Sbain
‘r
6OC
0.14
t,
0.12
500
0.1
+
@ 400 .$
IE 0.08 00
s 300
8 0.06 9
S - 0.3 .r g 8 ‘u a 0.2 E -lD 5 x i 0.1 ?
(actual) _.*_. StJaWl (IhcaretHat)
200 0.04
150
100 tI 150
200
250
300
Finelsubstratetemp.,dagC
350
8
p.02
Figure 5. The relationships between grain size, lattice strain and final substrate temperature for aluminium coatings.
200
250 300 350 Final substrate temp., deg C
400
‘Thearet& strains from difference in expemim coeffs of substrate(01) and mating
Figure 7. The relationship between coating ‘in-plane’ macrostrain and final substrate temperature for aluminium coatings.
P J Kelly and R D Ame//: The influence of substrate temperature
on properties 6,500
Discussion The analysis of the Al and Zr coatings has produced a consistent set of results. By considering the power density at the substrate. the relationship between ion current density, bias voltage and the resulting substrate temperature was investigated. As would be expected, a strong positive correlation was observed in the data. Further strong relationships were subsequently observed between specific coating properties and substrate temperature. For example, clear trends of increasing grain size with increasing substrate temperature were identified. As grain growth occurs, the lattice becomes more ordered and fewer grain boundaries are present. This accounts for the corresponding decreases observed in lattice strain and microhardness. Such trends were not found for the tungsten coatings, probably as a result of the relatively low homologous temperatures of these coatings. The relationship between coating grain size and microhardness can now be considered. The well-known Hall-Petch equationI states that hardness, H, is inversely proportional to the square root of grain size:
H = H,+kd-“’
2z50t.w Figure 9. The relationship zirconium
where Ho is the intrinsic hardness for a single crystal, d is the grain size and k is a material constant. The relationships between microhardness and d-O.’ for the Al and Zr coatings are shown in Figures 8 and 9, respectively. In both cases, the data are in good agreement with eqn (1) (r = 0.91 and 0.76, respectively). The actual values obtained by regression analysis for Ho were 70 MPa for aluminium and 660 MPa for Zr. However, due to the scatter in the data, these values can only be taken as a general indication of the actual magnitude of Ho for these materials.
m
7. 600
I i
8.
0 700
9.
f b ._ E
10.
8600 5
Figure 8. The relationship aluminium
coatings.
between
grain
0.16
size and microhardness
0.16
for
between
grain
size and microhardness
would like to thank BNFL for funding this project.
References
5. 6.
f
grain size)
Acknowledgements
1. Dense well-adhered coatings of Al, Zr and W have been deposited by CFUBMS under systematically varied conditions. 2. Fully dense and dense columnar structures were obtained at homologous temperatures well below those required by other sputtering systems.
u‘
0.14
l/(root
coatings.
The authors
%
0.12
A strong correlation was observed between the power density at the substrate and the final substrate temperature. Taguchi analyses of the deposition process indicated that all the deposition parameters investigated influenced the final substrate temperature. Strong correlations (Irl>O.7) were, in turn, observed between substrate temperature and grain size, lattice strain and microhardness for the Al, and Zr coatings. And additionally between elastic macrostrain and temperature for the Al coatings, due to the mis-match in thermal expansion coefficients of the coating and substrate materials. Little variation was observed in the properties of the tungsten coatings.
(1)
Conclusions
,
1
11.
0.06
12. 13.
for
14.
Efeoglu, I., Arnell, R. D., Tinston, S. F. and Teer. D. G., Surf: Coat. Technol, 1993,57, 61-69. Monaghan, D. P., Teer, D. G., Laing, K. C.. Efeoglu. I. and Arnell, R.D., Surf: Coat. Technol, 1993,59,21-25. Monaghan, D. P., Teer, D. G., Logan, P., Efeoglu, 1. and Arnell, R.D., Surf. Coat. Technol, 1993,60, 525-530. Bradley, J. W., Welzel, T., Hahn, J., Pintaske, R. and Arnell, R. D., Observations of plasma density peaks in magnetron discharges. Poster presentation at The Inst. of Physics Annual Congress. Leeds, UK, 2427 March, 1997. Bradley, J. W., Plasma Sources. Sci. Technol., 1996,5, 622-631. Kelly, P. J. and Arnell, R. D., Surf: Coot. Tvchnol, 1996.86-87,425p 431. Kelly. P. J. and Arnell, R. D., Proc. 5th Int’l Conf. on Plasma Surface Engineering, PSE’96, Garmisch-Partenkirchen, Germany, Sept. 1996. Accepted for publication in Su$ Coat. Technol. Teer, D. G., UK Patent, GB 2 258 343 B, Magnetron Sputter Ion Plating. Roy, R., A Primer on the Tquchi Method. Van Nostrand Reinhold, New York, 1990. Cullity, B. D., Elements of‘ X-Raj, D$%trcrion. Second Edition. Addison-Wesley, London, 1978. Andrischky, M., Guimaraes, F. and Teixeira, V., Vacuum. 1993, 44(E), 809-813. Thornton, .I. A., J. Vat. Sci. Technol., 1974, 11(4), 66&670. Messier, R., Giri, A. P. and Roy, R. A.. J. Var. Sci. Techno/., 1984, A2(2), 500-503. Sundgren, J. E. and Henztell. H. T. G., J. Vuc. Sci. Techno/., 1986, A4(5). 2259-2279.
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