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Modification of gold coatings by ion bombardment during deposition
Thin Solid Films, 107(1983) 335-344 METALLURGICAL AND PROTECTIVE COATINGS
335
MODIFICATION OF GOLD COATINGS BY ION BOMBARDMENT DURING DEPOSITION* S. S. NANDRA, F. G. WILSON AND C. D. DESFORGES
Engelhard Industries Ltd., Davis Road, Chessington, Surrey (Gt. Britain) (Received March 28, 1983; accepted April 19, 1983)
Experiments were performed in which a copper substrate was bombarded with argon ions (up to 6 keV) before and/or during the thermal evaporation of gold. The adherence, microstructure and through porosity of the coatings were evaluated for various ion energies and current densities over a range of deposition rates and coating thicknesses. Sputter cleaning prior to deposition was found to be primarily responsible for the substantially improved adherence of the coatings. Because the absolute levels of the porosity were found to be very sensitive to the substrate condition and the coating thickness, the porosity was always measured by direct comparison with an in situ control sample. Ion bombardment at low energy levels (0.8-I.5 keV) during deposition had no effect on the final through porosity. The porosity levels were reduced at high energy levels (5-6 keV), provided that the ratio between the flux of the gas ions and the depositing atoms exceeded a critical level.
1. INTRODUCTION
A requirement for any critical thin film application is that the coating should be strongly adherent to the substrate, be fully dense and have a minimum through porosity. Early coating techniques, such as thermal evaporation, exhibit definite limitations with respect to these coating requirements. Recent developments in vacuum coating technology have led to more advanced techniques which can be successfully applied to produce high integrity coatings. These techniques, such as sputtering and ion plating, are versatile and can be used to prepare thin films of materials with specific electrical and mechanical properties. Experience indicates that these high integrity coatings largely result from the kinetic energy of the depositing particles and from the bombardment of the coatings by the energetic ions ~' 2 A common feature of such advanced techniques is the presence of a glow discharge during deposition. Although a glow discharge is easy to produce, it is a complex phenomenon. It is composed of energetic gas and metal ions, neutrals and * Paper presented at the International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 18-22, 1983. 0040-6090/83/$3.00
© Elsevier Sequoia/Printed in The Netherlands
336
s . s . NANDRA, F. G. WILSON, C. D. DEsFORGES
electrons. The energy associated with ions tends to have a beneficial effect on the coating; for example, densification may be increased through the sputtering and redeposition of the coating material. It is also thought that neutral metal atoms gain kinetic energy through momentum transfer from the energetic ions. Because of the complexity of the glow discharge process, the precise reasons for the improvement in the film properties are not entirely clear. For example, it is thought that the adherence is improved because the kinetic energy of the depositing particles tends to induce interdiffusion of the coating and the substrate atoms at the interface 3. The adherence is further enhanced by a sputter cleaning of the substrate and by the reduction in stress 4 in the coating due to the ion bombardment. In contrast, the microstructure is thought to be influenced by the kinetic energy transferred to the substrate by the depositing particles. This produces an annealing effect in the coating analogous to the effect of a heated substrate in a conventional coating process. Although the annealing effect in this case is limited to the top few layers of the substrate, the energy increases the mobility of the depositing particles, thus enabling them to rearrange and form a dense structure. In order to understand the effect of the various species in the glow discharge on the properties of the coating, it is necessary to employ a technique whereby the species can be separated. The use of an independent source of low energy (less than 10 keV) gas ions offers such an opportunity 5. The gas ion source can be used to sputter clean the substrate prior to deposition; it can also be used to bombard the coating in a controlled manner during deposition 6. Such a technique was employed in this work to evaluate the effect of ion bombardment on the properties of gold coatings produced by thermal evaporation onto copper substrates. 2.
EXPERIMENTAL DETAILS
2.1. Sources The deposition system consisted of a gas ion and a resistance-heated source. The gas ion source was a Kaufman type ~ supplied by Ion Tech Inc., Fort Collins, CO, and was capable of producing current densities ranging from 0.05 to 20 mA cm -2 at a maximum ion energy of 1.5 keV. The diameter of the beam was 25 mm. The major attraction of this type of source was that the beam currents and energies could be independently adjusted. The ionization efficiency of the source was high and very high current densities could be obtained (up to 10 mA cm-2 with an argon flow rate of 2 c m 3 s-l). This also allowed the deposition work to be conducted in a high vacuum (less than 10- 2 Pa), using a turbomolecular pump with a pumping speed of about 15001 s- 1. This type of pump also ensured a minimum of hydrocarbon contamination. The resistance-heated evaporation source consisted of a tungsten strip pressed in the middle to form a reservoir for the gold. It was heated by an a.c. power supply, and deposition rates of up to 2 lam rain- 1 could be obtained.
2.2. Preparation of films Figure 1 shows schematically the arrangement of the sources and the substrate within the vacuum chamber. The substrate was held about 250 mm above the thermal evaporation source. The gas ion source was displaced to the side and
ION BOMBARDMENT OF
Au
337
COATINGS
inclined at an angle of 30 ° from the normal to the substrate. A crystal monitor was attached close to the substrate to regulate the deposition rate and the final thickness of the coatings. To determine the effect of the ion beam in any given experiment, onehalf of the substrate (the control sample) was completely shielded from the ion beam while the other half of the substrate was fully exposed to it. This also allowed any effect of the ion beam to be assessed independently of the vacuum conditions, the deposition rate, the substrate surface condition or the substrate temperature. Because of the possible effect of the substrate surface on the structure of the coatings, a consistent precleaning procedure was applied prior to the substrate's being introduced into the vacuum chamber. The substrate was usually polished to a centre-line average (c.l.a.) surface roughness of about 0.24 ~tm with an abrasive paper. It was then degreased ultrasonically in Decon 90 for 5 rain; this was followed by a further ultrasonic clean in distilled water, and the substrate was then thoroughly rinsed. Finally, the substrate was ultrasonically cleaned in isopropanol for a further 5 min and dried with clean air. It was then mounted in the vacuum chamber and, immediately before the chamber door was closed, a jet of clean air was directed at it to ensure that the surface was dust free. A large number of samples were prepared with deposition rates ranging from 0.02 to 2 ~tm m i n - 1 and the final thickness of the coatings ranged from 0.3 to 8.5 Hm. The samples were produced in three different ways, as summarized in Table I. In case Crystal monitor
i Pressure <]O-2Pa
I
"
],L
Substrate - #/
# i
/iiii.,I /1 I
Ion beam
•
i/Ill / /,,#,-/~,/l / ii
"t ,
#L
Partition
I
i~
Shutter Evaporation source
Ion source
Argon gas inlet
Fig. 1. Schematic arrangement o f t h e g a s i o n a n d e v a p o r a t i o n s o u r c e s i n a vacuum chamber.
338
s.s.
N A N D R A , F. G. W I L S O N , C. D. DEsFORGES
1 the substrate was sputter cleaned for about 30 s using an ion beam energy of 1.5 keV and a current density of 1 mA cm - 2; in a few cases the sputter cleaning time was increased to 8 min. The ion source was then switched off and gold was evaporated to form a deposit. In case 2 the substrate was first sputter cleaned and then, during deposition of the film, the depositing particles were b o m b a r d e d with energetic ions (hereafter referred to as "ion mixed"). A series of coatings was obtained in which the energy of the ions was varied from 0.8 to 6 keV and the current density was varied from 0.02 to 0.18 mA c m - 2. In case 3 the films were deposited without any sputter cleaning of the substrates but, during deposition, they were b o m b a r d e d with ions with an energy of 1.5 keV and a beam current density of 0.1 mA c m - 2. During deposition the substrate temperature never rose above 250 °C, as measured with an attached thermocouple. The final thickness of the films was measured on a 13backscattering instrument. In a typical experimental run, where an ion-mixed film was obtained on a sputter-cleaned substrate, the following procedure was used. First the chamber was pumped down to 1 0 - 4 p a . With the substrate shielded from the sources, the resistance source was switched on and the gold was outgassed above its melting temperature. The temperature of the source was reduced to below the melting point of gold, the shutter was removed and the substrate was sputter cleaned. The ion current density was then reduced to a predetermined level and the temperature of the evaporation source was raised until the desired deposition rate was achieved, as determined with the crystal monitor. The sources were left operating until the desired final coating thickness was achieved.
2.3. Evaluation of the coatings The coating properties of particular interest were the adherence, the microstructure and the porosity. The coating adherence was evaluated by "scratch" and by "pull-off" tests. Scratches were made with a microhardness indenter and TABLE I SUMMARY OF THE GAS ION SOURCE PARAMETERS AND THE DEPOSITION CONDITIONS USED TO PRODUCE THE COATINGS IN CASES 1, 2 AND 3
Case
Parameters for the pretreatment o f the substrate Gas ion energy
Current density
(keV)
(mA c m - 2)
Parameters for the treatment during coating
Description
Time (min)
Deposition conditions Rate
Gas ion energy
Current density
(keV)
(mA e r a - 2) --
1
1.5
1
0.5-8
--
2
1.5
1
0.5
0.8-6
3
--
--
--
Control ~-
--
(lam min - 1)
Final thickness (~tm)
0.1-2
0.3-8.5
0.02-0.2
0.3-6
1.5
S p u t t e r cleaned and evaporated 0.02-0.18 S p u t t e r cleaned a n d ion m i x e d 0.1 Ion mixed
0.1
0.8-2
--
--
As for cases 1, 2or 3
Evaporated
T h e c o a t i n g w a s e v a p o r a t e d o n t o p a r t of the s u b s t r a t e shielded f r o m the ion b e a m .
ION BOMBARDMENT OF
Au
COATINGS
339
observations of any flaking were made with a scanning electron microscope. The pull-off tests were applied using a commercial adherence tester supplied by Quad Group, Santa Barbara, CA. The method involved attaching a stud of known surface area onto the film with epoxy resin and pulling under a known load. The stress at which either the epoxy or the film failed was automatically registered. The instrument was capable of providing stresses of up to 69 MPa. The microstructure of coatings is usually determined by examining fractured cross sections in a scanning electron microscope. However, with gold coatings on copper, it was found difficult to produce fractures through bending the specimens. The coatings tended to deform considerably, which often marred the observation of their microstructures. Attempts to produce fracture by tearing the coating, after chemically dissolving away the substrate, resulted in some success, and a limited number of cross sections were produced for observation. The porosity was determined quantitatively using an electrographic technique 8 which is well established in the telecommunications industry where gold-plated electrical contacts are routinely checked. A filter paper enriched with cadmium sulphide was soaked in distilled water and was placed on the coating. The sample was then pressed between two electrodes, and an electric current was passed through it. The through pores present in the coating allowed the substrate ions to pass through and to stain the filter paper. Each stain mark, usually in the form of a black spot, represented a pore; pore sizes as small as 1 ~tm in diameter could be detected. However, it was not possible to measure the pore sizes directly from the stains because of the spread on the filter paper. 3.
RESULTS AND DISCUSSION
3.1. Adherence Three or four samples were cut from each of the coated areas for examination. Scratch tests produced severe flaking of the control samples; there was no flaking of the coatings produced in cases 1 and 2. Furthermore, these coatings failed to be dislodged in the pull test even when a stress of 69 M P a was reached (Table II). This was true even for the thickest coatings (8.5 tam thick) produced at a high deposition rate (0.5 lam m i n - 1), which must be expected to exhibit severe internal stresses. It was clear that the metallurgical bond between the sputter-cleaned substrate and the coating was the dominant factor in the high adherence of the coatings. This is in contrast with the observations of germanium films on glass 4, in which ion mixing was considered to have improved the adhesion through interdiffusion at the substrate-coating interface, together with a reduction in the intrinsic stress. Pull tests applied to case 3 coatings showed some improvement in the coatings over the control samples; the average value of stress at which the coatings failed was 2.4 MPa. This indicated that ion bombardment might have caused some interdiffusion of the atomic species at the coating-substrate interface. However, the tests also indicated that any interdiffusion at the interface was less important than sputter cleaning for good adherence of the coatings. 3.2. Microstructure The observations of cross sections in the scanning electron microscope did not
340
s . s . NANDRA, F. G. WILSON, C. D. DEsFORGES
TABLE II S U M M A R Y OF T H E A D H E R E N C E OF T H E C O A T I N G S P R O D U C E D U N D E R D I F F E R E N T D E P O S I T I O N C O N D I T I O N S O N SPUTTER-CLEANED a SURFACES
Adherence (MPa)
Evaporation conditions Deposition rate
Final thickness
(lam min t)
(lam)
Case 1
Case 2 b
Case 3
Control c (evaporated)
0.1 0.1 0.1 0.2 0.4
0.4 2 6 4.7 2
>69 --> 69 >69
->69 >69 ---
-2.4 ----
-~0 ~0 ~0 ~0
1
4.14
>69
--
--
~0
2 2.26
8.5 4.6
>69 > 69
---
---
~0 --
"Ion beam energy, 1.5 keV; current density, 1 mA cm - 2; sputtering time, 0.5 min. b Ion beam energy, 1.5 keV; current density, 0.08 mA cm -2. c The coating was evaporated onto part of the substrate shielded from the ion beam.
r e v e a l a n y c l e a r d e t a i l s o f t h e m i c r o s t r u c t u r e . I n all t h e s a m p l e s t h e r e w a s n o o b s e r v a b l e d i f f e r e n c e b e t w e e n t h e m i c r o s t r u c t u r e o f c a s e l, 2 a n d 3 c o a t i n g s c o m p a r e d w i t h t h o s e p r o d u c e d as t h e c o n t r o l . T h i s p r o b l e m w a s a t t r i b u t e d t o t h e deformation of the coatings when they were bent or torn. Figure 2 shows a typical c r o s s s e c t i o n o f o n e o f t h e c o a t i n g s , r e v e a l i n g c o n s i d e r a b l e d e f o r m a t i o n a t its e d g e . N o r m a l l y , for s p u t t e r - c l e a n e d substrates, densely p a c k e d c o l u m n a r - t y p e s t r u c t u r e s w o u l d b e e x p e c t e d . T h i s is b e c a u s e t h e r e w o u l d b e a n i n c r e a s e d n u m b e r o f l o c a l i z e d d e f e c t s c a u s e d b y i o n b o m b a r d m e n t w h i c h w o u l d a c t as n u c l e i for t h e d e p o s i t i n g p a r t i c l e s . S u c h s t r u c t u r e s w e r e n o t a p p a r e n t in t h i s i n v e s t i g a t i o n .
Fig. 2. Scanning electron micrograph of the fractured surface of an evaporated (control) coating deposited at a rate of O.1 p.m min - 1.
ION BOMBARDMENT OF
Au COATINGS
341
3.3. Porosity The porosity results were obtained in a form in which the left-hand side represented the porosity in the ion-mixed or sputter-cleaned coatings and the righthand side represented that in the control coatings. This made possible a direct comparison between the experimental and the control coatings. The cases where the porosity decreased owing to the use of the ion beam were denoted as + ; in cases where the porosity increased, - was used; 0 represented no change from that in the control coating. Examples of corresponding porosity results are shown in Fig. 3(a), Fig. 3(b) and Fig. 3(c) respectively. The non-uniformity in the distribution of the pores is most probably related to variations in the structure of the substrate surface. Figure 3 also shows variations in the porosity with the coating thickness. As would be expected, the porosity level reduces as the thickness increases. This is clearly illustrated by the control coatings. Experimental
+
.
Control
.,.: :~'
(a) 0.521am
0.75 tam
Coating thicknesses 1.06 ttm
1.32 tam
Coating thicknesses
{b)
{c) Coating thicknesses
0.3 tam
0.3 t.tm
Fig. 3. The porosity in the coatings of gold on copper: (a) ion-mixed coatings (ion beam energy, 5 keV; current density, 0.1 m A c m - 2; deposition rate, 0.2 tam m i n - 1); (b) ion-mixed coatings (ion beam energy, 1.5 keV; current density, 0.03 mA c m - 2 ; deposition rate, 0. ! I.tm m i n - 1); (c) coatings evaporated onto a sputter-cleaned surface (ion beam energy, 1.5 keV; current density, 0.1 mA c m - 2; cleaning time, 8 rain; deposition rate, 0.1 ttm min-1). ( + , 0 and - indicate a reduction, no change and an increase in the porosity respectively.)
342
s.s.
N A N D R A , F. G. W I L S O N , C. D. DEsFORGES
3.3.1. Effect of sputter cleaning About seven samples were produced in which the surfaces were sputter cleaned for periods ranging from 30 s to 8 min prior to deposition of the coatings (case 1). For a sputter cleaning time of 30 s there was no observable increase or decrease in the porosity. The effect of increasing the sputtering time was to increase the porosity of the experimental sample over that of the control; after 8 min the effect became very marked (Fig. 3(c)). To check the effect of the substrate surface roughness, a coating about 1.1 ~tm thick was deposited onto a polished surface (c.l.a. roughness, 0.06 I~m). The increase in the porosity of the experimental area was similar to that of the conventionally prepared samples (c.l.a. roughness, 0.24 lam).
3.3.2. Effect of ion mixing The results of the porosity tests on the coatings produced in case 2 are summarized in Tables III and IV. Each result is representative of up to six samples produced under identical conditions. The variables to which particular attention was paid are the beam energy, the beam current density, the deposition rate and the film thickness. TABLE III EFFECT OF THE GAS ION BEAM ENERGY AND THE CURRENT DENSITY ON THE POROSITY IN THE COATINGS IN CASE 2 a Gas ion energy (keV)
Effect o f the following current densities on the porosity 0.02-0.04 mAcm- 2
0.8 1 1.5
0.05-0.06 mAcm - 2
0b
5 6
0.8-1,0 m A cm - 2
0.12-0.13 mAcm - z
0d
0d 0d 0d
0b
0d
+d +d
+¢
a 0 d e n o t e s no c h a n g e b e t w e e n the e v a p o r a t e d a n d the i o n - m i x e d c o a t i n g s ; + d e n o t e s s o m e r e d u c t i o n in the p o r o s i t y in the i o n - m i x e d c o a t i n g c o m p a r e d with that in the e v a p o r a t e d coating. b D e p o s i t i o n rate, 0.02 lam m i n - ~. c D e p o s i t i o n rate, 0.2 ~tm m i n - ~. a D e p o s i t i o n rate, 0.1 I~m rain - 1. T A B L E IV EFFECT OF THE DEPOSITION RATE ON THE POROSITY IN THE COATINGS IN CASE 2 FOR VARIOUSGAS ION BEAM ENERGIES AND CURRENT DENSITIES Deposition rate (lam m i n - 1 )
0.02 0.05 0.1 0.2
Effect o f the following gas ion beam current densities on the porosity 0.02-0.04 mA cm- 2
0.05-0.06 mA cm- 2
0~
0~
0•
+b 0b
I o n b e a m energies, 1 a n d 1.5 keV. b I o n b e a m energies, 5 a n d 6 keV.
0.09 mA cm- 2
0~ 0~
O. 1 macro-2
O. 12 m A cm
+b + b
(p
2
O. 18 mAcro
+~
2
ION BOMBARDMENT
OF Au
343
COATINGS
For gas ion energies up to and including 1.5 keV, there was no noticeable effect on the porosity. With energies of 5-6 keV there was usually a reduction in the porosity, although (Table IV) there was no change when the deposition rate was 0.2 I~m min - ~ and the gas ion current density was between 0.05 and 0.06 mA c m - 2. (The higher gas ion energies were achieved by negatively biasing the substrate.) It is generally thought that the microstructure of coatings is modified through the transfer of momentum from the ions to the depositing atoms. Thus at any fixed beam energy the number of ions bombarding a coating relative to the number of depositing atoms should be important. Too low a number of ions may not produce the desired overall effect on the depositing atoms. To illustrate this, the data from Tables III and IV have been converted (Table V) to indicate the effect of the ratio of ions to depositing particles on the observed porosity. The results in Table V confirm that for beam energies of 1.5 keV or less there is no noticeable reduction in the porosity in the ion-mixed coatings (even when the ion-to-atom ratio is as high as 0.23). When the beam energy is increased to 5 keV or above, a significant reduction in the porosity in the ion-mixed coating is obtained when the ion-to-atom ratio is 0.038 and above. However, with the ratio of 0.023, TABLE
V
EFFECT OF THE ION BEAM ENERGY AND THE CURRENT DENSITY ON THE POROSITY IN THE COATINGS IN CASE 2 FOR KNOWN DEPOSITION RATES
Gas ion energy (keV)
Effect on the porosity of the following ratios of incident ions to the number of gold atoms depositing onto the substrate 0.023
0.038
0.046
0.8
0.076
0.098
0.15
0.23
0
0
0
1 1.5 5 6
0
0 + +
+ +
0
0
0
0
+
corresponding to a deposition rate of 0.02 i.tm min- ~ and a current density of 0.05-0.06 mA cm -2, there was no observed change in the porosity. Therefore, it seems that there is a threshold level for the beam energy and the ion-to-atom ratio below which the porosity cannot be reduced by ion mixing. Although the porosity in the coatings could be reduced consistently by ion bombardment at an energy level of 5 keV or higher, it was not possible to reproduce the exact levels of the porosity reduction in repeat samples produced under identical conditions. Furthermore, the porosity could not be completely eliminated even when the coatings were deposited to a thickness of 6 txm. The production of coatings by ion mixing is a relatively new technique, and there is much that needs to be understood, particularly regarding the mechanisms of momentum transfer from the energetic gas ions to the depositing particles. 4. CONCLUSIONS (1) Argon ion bombardment (up to 6 keV) before and/or during the thermal
344
s . s . NANDRA, F. G. WILSON, C. D. DEsFORGES
evaporation of gold onto copper has been found to influence the adherence and the through porosity of the coatings. (2) Sputter cleaning prior to deposition was the most effective means of producing highly adherent coatings; ion bombardment during deposition improved the adherence only slightly. (3) When the duration of the sputter cleaning was increased significantly, there was a marked increase in the through porosity in the final coatings. (4) Ion bombardment during deposition reduced the final through porosity when the energy levels were high (5-6 keV) and when the ratio between the flux of the gas ions and the depositing atoms exceeded a critical level. REFERENCES 1 D.M. Mattox, in R. F. Bunshah (ed.), Deposition Technologies for Films and Coatings, Noyes, Park Ridge, NJ, 1982, pp. 254-258. 2 D.G. Teer and B. L. Delcea, Thin Solid Films, 54 (1978) 295. 3 G. Carter and D. G. Armour, Thin Solid Films, 80 (1981) 13. 4 E.H. Hirsch and I. K. Varga, Thin Solid Films, 52 (1978) 445. 5 C. Weissmantel, G~ Reisse, H.-J. Erler, F. Henny, K. Bewilogua, U. Ebersbach and C. Schiirer, Thin Solid Films, 63 (1979) 315. 6 D.W. Hoffman and M. R. Gaerttner, J. Vac. Sei. Technol., 17 (1980) 425. 7 H, R. Kaufman, J. Vac. Sei. Technol., 15 (1978) 272. 8 M. Clarke, in R. Sard (ed.), Properties of Electrodeposits, Electrochemical Society, Princeton, N J, 1975, pp. 122 141.
335
MODIFICATION OF GOLD COATINGS BY ION BOMBARDMENT DURING DEPOSITION* S. S. NANDRA, F. G. WILSON AND C. D. DESFORGES
Engelhard Industries Ltd., Davis Road, Chessington, Surrey (Gt. Britain) (Received March 28, 1983; accepted April 19, 1983)
Experiments were performed in which a copper substrate was bombarded with argon ions (up to 6 keV) before and/or during the thermal evaporation of gold. The adherence, microstructure and through porosity of the coatings were evaluated for various ion energies and current densities over a range of deposition rates and coating thicknesses. Sputter cleaning prior to deposition was found to be primarily responsible for the substantially improved adherence of the coatings. Because the absolute levels of the porosity were found to be very sensitive to the substrate condition and the coating thickness, the porosity was always measured by direct comparison with an in situ control sample. Ion bombardment at low energy levels (0.8-I.5 keV) during deposition had no effect on the final through porosity. The porosity levels were reduced at high energy levels (5-6 keV), provided that the ratio between the flux of the gas ions and the depositing atoms exceeded a critical level.
1. INTRODUCTION
A requirement for any critical thin film application is that the coating should be strongly adherent to the substrate, be fully dense and have a minimum through porosity. Early coating techniques, such as thermal evaporation, exhibit definite limitations with respect to these coating requirements. Recent developments in vacuum coating technology have led to more advanced techniques which can be successfully applied to produce high integrity coatings. These techniques, such as sputtering and ion plating, are versatile and can be used to prepare thin films of materials with specific electrical and mechanical properties. Experience indicates that these high integrity coatings largely result from the kinetic energy of the depositing particles and from the bombardment of the coatings by the energetic ions ~' 2 A common feature of such advanced techniques is the presence of a glow discharge during deposition. Although a glow discharge is easy to produce, it is a complex phenomenon. It is composed of energetic gas and metal ions, neutrals and * Paper presented at the International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 18-22, 1983. 0040-6090/83/$3.00
© Elsevier Sequoia/Printed in The Netherlands
336
s . s . NANDRA, F. G. WILSON, C. D. DEsFORGES
electrons. The energy associated with ions tends to have a beneficial effect on the coating; for example, densification may be increased through the sputtering and redeposition of the coating material. It is also thought that neutral metal atoms gain kinetic energy through momentum transfer from the energetic ions. Because of the complexity of the glow discharge process, the precise reasons for the improvement in the film properties are not entirely clear. For example, it is thought that the adherence is improved because the kinetic energy of the depositing particles tends to induce interdiffusion of the coating and the substrate atoms at the interface 3. The adherence is further enhanced by a sputter cleaning of the substrate and by the reduction in stress 4 in the coating due to the ion bombardment. In contrast, the microstructure is thought to be influenced by the kinetic energy transferred to the substrate by the depositing particles. This produces an annealing effect in the coating analogous to the effect of a heated substrate in a conventional coating process. Although the annealing effect in this case is limited to the top few layers of the substrate, the energy increases the mobility of the depositing particles, thus enabling them to rearrange and form a dense structure. In order to understand the effect of the various species in the glow discharge on the properties of the coating, it is necessary to employ a technique whereby the species can be separated. The use of an independent source of low energy (less than 10 keV) gas ions offers such an opportunity 5. The gas ion source can be used to sputter clean the substrate prior to deposition; it can also be used to bombard the coating in a controlled manner during deposition 6. Such a technique was employed in this work to evaluate the effect of ion bombardment on the properties of gold coatings produced by thermal evaporation onto copper substrates. 2.
EXPERIMENTAL DETAILS
2.1. Sources The deposition system consisted of a gas ion and a resistance-heated source. The gas ion source was a Kaufman type ~ supplied by Ion Tech Inc., Fort Collins, CO, and was capable of producing current densities ranging from 0.05 to 20 mA cm -2 at a maximum ion energy of 1.5 keV. The diameter of the beam was 25 mm. The major attraction of this type of source was that the beam currents and energies could be independently adjusted. The ionization efficiency of the source was high and very high current densities could be obtained (up to 10 mA cm-2 with an argon flow rate of 2 c m 3 s-l). This also allowed the deposition work to be conducted in a high vacuum (less than 10- 2 Pa), using a turbomolecular pump with a pumping speed of about 15001 s- 1. This type of pump also ensured a minimum of hydrocarbon contamination. The resistance-heated evaporation source consisted of a tungsten strip pressed in the middle to form a reservoir for the gold. It was heated by an a.c. power supply, and deposition rates of up to 2 lam rain- 1 could be obtained.
2.2. Preparation of films Figure 1 shows schematically the arrangement of the sources and the substrate within the vacuum chamber. The substrate was held about 250 mm above the thermal evaporation source. The gas ion source was displaced to the side and
ION BOMBARDMENT OF
Au
337
COATINGS
inclined at an angle of 30 ° from the normal to the substrate. A crystal monitor was attached close to the substrate to regulate the deposition rate and the final thickness of the coatings. To determine the effect of the ion beam in any given experiment, onehalf of the substrate (the control sample) was completely shielded from the ion beam while the other half of the substrate was fully exposed to it. This also allowed any effect of the ion beam to be assessed independently of the vacuum conditions, the deposition rate, the substrate surface condition or the substrate temperature. Because of the possible effect of the substrate surface on the structure of the coatings, a consistent precleaning procedure was applied prior to the substrate's being introduced into the vacuum chamber. The substrate was usually polished to a centre-line average (c.l.a.) surface roughness of about 0.24 ~tm with an abrasive paper. It was then degreased ultrasonically in Decon 90 for 5 rain; this was followed by a further ultrasonic clean in distilled water, and the substrate was then thoroughly rinsed. Finally, the substrate was ultrasonically cleaned in isopropanol for a further 5 min and dried with clean air. It was then mounted in the vacuum chamber and, immediately before the chamber door was closed, a jet of clean air was directed at it to ensure that the surface was dust free. A large number of samples were prepared with deposition rates ranging from 0.02 to 2 ~tm m i n - 1 and the final thickness of the coatings ranged from 0.3 to 8.5 Hm. The samples were produced in three different ways, as summarized in Table I. In case Crystal monitor
i Pressure <]O-2Pa
I
"
],L
Substrate - #/
# i
/iiii.,I /1 I
Ion beam
•
i/Ill / /,,#,-/~,/l / ii
"t ,
#L
Partition
I
i~
Shutter Evaporation source
Ion source
Argon gas inlet
Fig. 1. Schematic arrangement o f t h e g a s i o n a n d e v a p o r a t i o n s o u r c e s i n a vacuum chamber.
338
s.s.
N A N D R A , F. G. W I L S O N , C. D. DEsFORGES
1 the substrate was sputter cleaned for about 30 s using an ion beam energy of 1.5 keV and a current density of 1 mA cm - 2; in a few cases the sputter cleaning time was increased to 8 min. The ion source was then switched off and gold was evaporated to form a deposit. In case 2 the substrate was first sputter cleaned and then, during deposition of the film, the depositing particles were b o m b a r d e d with energetic ions (hereafter referred to as "ion mixed"). A series of coatings was obtained in which the energy of the ions was varied from 0.8 to 6 keV and the current density was varied from 0.02 to 0.18 mA c m - 2. In case 3 the films were deposited without any sputter cleaning of the substrates but, during deposition, they were b o m b a r d e d with ions with an energy of 1.5 keV and a beam current density of 0.1 mA c m - 2. During deposition the substrate temperature never rose above 250 °C, as measured with an attached thermocouple. The final thickness of the films was measured on a 13backscattering instrument. In a typical experimental run, where an ion-mixed film was obtained on a sputter-cleaned substrate, the following procedure was used. First the chamber was pumped down to 1 0 - 4 p a . With the substrate shielded from the sources, the resistance source was switched on and the gold was outgassed above its melting temperature. The temperature of the source was reduced to below the melting point of gold, the shutter was removed and the substrate was sputter cleaned. The ion current density was then reduced to a predetermined level and the temperature of the evaporation source was raised until the desired deposition rate was achieved, as determined with the crystal monitor. The sources were left operating until the desired final coating thickness was achieved.
2.3. Evaluation of the coatings The coating properties of particular interest were the adherence, the microstructure and the porosity. The coating adherence was evaluated by "scratch" and by "pull-off" tests. Scratches were made with a microhardness indenter and TABLE I SUMMARY OF THE GAS ION SOURCE PARAMETERS AND THE DEPOSITION CONDITIONS USED TO PRODUCE THE COATINGS IN CASES 1, 2 AND 3
Case
Parameters for the pretreatment o f the substrate Gas ion energy
Current density
(keV)
(mA c m - 2)
Parameters for the treatment during coating
Description
Time (min)
Deposition conditions Rate
Gas ion energy
Current density
(keV)
(mA e r a - 2) --
1
1.5
1
0.5-8
--
2
1.5
1
0.5
0.8-6
3
--
--
--
Control ~-
--
(lam min - 1)
Final thickness (~tm)
0.1-2
0.3-8.5
0.02-0.2
0.3-6
1.5
S p u t t e r cleaned and evaporated 0.02-0.18 S p u t t e r cleaned a n d ion m i x e d 0.1 Ion mixed
0.1
0.8-2
--
--
As for cases 1, 2or 3
Evaporated
T h e c o a t i n g w a s e v a p o r a t e d o n t o p a r t of the s u b s t r a t e shielded f r o m the ion b e a m .
ION BOMBARDMENT OF
Au
COATINGS
339
observations of any flaking were made with a scanning electron microscope. The pull-off tests were applied using a commercial adherence tester supplied by Quad Group, Santa Barbara, CA. The method involved attaching a stud of known surface area onto the film with epoxy resin and pulling under a known load. The stress at which either the epoxy or the film failed was automatically registered. The instrument was capable of providing stresses of up to 69 MPa. The microstructure of coatings is usually determined by examining fractured cross sections in a scanning electron microscope. However, with gold coatings on copper, it was found difficult to produce fractures through bending the specimens. The coatings tended to deform considerably, which often marred the observation of their microstructures. Attempts to produce fracture by tearing the coating, after chemically dissolving away the substrate, resulted in some success, and a limited number of cross sections were produced for observation. The porosity was determined quantitatively using an electrographic technique 8 which is well established in the telecommunications industry where gold-plated electrical contacts are routinely checked. A filter paper enriched with cadmium sulphide was soaked in distilled water and was placed on the coating. The sample was then pressed between two electrodes, and an electric current was passed through it. The through pores present in the coating allowed the substrate ions to pass through and to stain the filter paper. Each stain mark, usually in the form of a black spot, represented a pore; pore sizes as small as 1 ~tm in diameter could be detected. However, it was not possible to measure the pore sizes directly from the stains because of the spread on the filter paper. 3.
RESULTS AND DISCUSSION
3.1. Adherence Three or four samples were cut from each of the coated areas for examination. Scratch tests produced severe flaking of the control samples; there was no flaking of the coatings produced in cases 1 and 2. Furthermore, these coatings failed to be dislodged in the pull test even when a stress of 69 M P a was reached (Table II). This was true even for the thickest coatings (8.5 tam thick) produced at a high deposition rate (0.5 lam m i n - 1), which must be expected to exhibit severe internal stresses. It was clear that the metallurgical bond between the sputter-cleaned substrate and the coating was the dominant factor in the high adherence of the coatings. This is in contrast with the observations of germanium films on glass 4, in which ion mixing was considered to have improved the adhesion through interdiffusion at the substrate-coating interface, together with a reduction in the intrinsic stress. Pull tests applied to case 3 coatings showed some improvement in the coatings over the control samples; the average value of stress at which the coatings failed was 2.4 MPa. This indicated that ion bombardment might have caused some interdiffusion of the atomic species at the coating-substrate interface. However, the tests also indicated that any interdiffusion at the interface was less important than sputter cleaning for good adherence of the coatings. 3.2. Microstructure The observations of cross sections in the scanning electron microscope did not
340
s . s . NANDRA, F. G. WILSON, C. D. DEsFORGES
TABLE II S U M M A R Y OF T H E A D H E R E N C E OF T H E C O A T I N G S P R O D U C E D U N D E R D I F F E R E N T D E P O S I T I O N C O N D I T I O N S O N SPUTTER-CLEANED a SURFACES
Adherence (MPa)
Evaporation conditions Deposition rate
Final thickness
(lam min t)
(lam)
Case 1
Case 2 b
Case 3
Control c (evaporated)
0.1 0.1 0.1 0.2 0.4
0.4 2 6 4.7 2
>69 --> 69 >69
->69 >69 ---
-2.4 ----
-~0 ~0 ~0 ~0
1
4.14
>69
--
--
~0
2 2.26
8.5 4.6
>69 > 69
---
---
~0 --
"Ion beam energy, 1.5 keV; current density, 1 mA cm - 2; sputtering time, 0.5 min. b Ion beam energy, 1.5 keV; current density, 0.08 mA cm -2. c The coating was evaporated onto part of the substrate shielded from the ion beam.
r e v e a l a n y c l e a r d e t a i l s o f t h e m i c r o s t r u c t u r e . I n all t h e s a m p l e s t h e r e w a s n o o b s e r v a b l e d i f f e r e n c e b e t w e e n t h e m i c r o s t r u c t u r e o f c a s e l, 2 a n d 3 c o a t i n g s c o m p a r e d w i t h t h o s e p r o d u c e d as t h e c o n t r o l . T h i s p r o b l e m w a s a t t r i b u t e d t o t h e deformation of the coatings when they were bent or torn. Figure 2 shows a typical c r o s s s e c t i o n o f o n e o f t h e c o a t i n g s , r e v e a l i n g c o n s i d e r a b l e d e f o r m a t i o n a t its e d g e . N o r m a l l y , for s p u t t e r - c l e a n e d substrates, densely p a c k e d c o l u m n a r - t y p e s t r u c t u r e s w o u l d b e e x p e c t e d . T h i s is b e c a u s e t h e r e w o u l d b e a n i n c r e a s e d n u m b e r o f l o c a l i z e d d e f e c t s c a u s e d b y i o n b o m b a r d m e n t w h i c h w o u l d a c t as n u c l e i for t h e d e p o s i t i n g p a r t i c l e s . S u c h s t r u c t u r e s w e r e n o t a p p a r e n t in t h i s i n v e s t i g a t i o n .
Fig. 2. Scanning electron micrograph of the fractured surface of an evaporated (control) coating deposited at a rate of O.1 p.m min - 1.
ION BOMBARDMENT OF
Au COATINGS
341
3.3. Porosity The porosity results were obtained in a form in which the left-hand side represented the porosity in the ion-mixed or sputter-cleaned coatings and the righthand side represented that in the control coatings. This made possible a direct comparison between the experimental and the control coatings. The cases where the porosity decreased owing to the use of the ion beam were denoted as + ; in cases where the porosity increased, - was used; 0 represented no change from that in the control coating. Examples of corresponding porosity results are shown in Fig. 3(a), Fig. 3(b) and Fig. 3(c) respectively. The non-uniformity in the distribution of the pores is most probably related to variations in the structure of the substrate surface. Figure 3 also shows variations in the porosity with the coating thickness. As would be expected, the porosity level reduces as the thickness increases. This is clearly illustrated by the control coatings. Experimental
+
.
Control
.,.: :~'
(a) 0.521am
0.75 tam
Coating thicknesses 1.06 ttm
1.32 tam
Coating thicknesses
{b)
{c) Coating thicknesses
0.3 tam
0.3 t.tm
Fig. 3. The porosity in the coatings of gold on copper: (a) ion-mixed coatings (ion beam energy, 5 keV; current density, 0.1 m A c m - 2; deposition rate, 0.2 tam m i n - 1); (b) ion-mixed coatings (ion beam energy, 1.5 keV; current density, 0.03 mA c m - 2 ; deposition rate, 0. ! I.tm m i n - 1); (c) coatings evaporated onto a sputter-cleaned surface (ion beam energy, 1.5 keV; current density, 0.1 mA c m - 2; cleaning time, 8 rain; deposition rate, 0.1 ttm min-1). ( + , 0 and - indicate a reduction, no change and an increase in the porosity respectively.)
342
s.s.
N A N D R A , F. G. W I L S O N , C. D. DEsFORGES
3.3.1. Effect of sputter cleaning About seven samples were produced in which the surfaces were sputter cleaned for periods ranging from 30 s to 8 min prior to deposition of the coatings (case 1). For a sputter cleaning time of 30 s there was no observable increase or decrease in the porosity. The effect of increasing the sputtering time was to increase the porosity of the experimental sample over that of the control; after 8 min the effect became very marked (Fig. 3(c)). To check the effect of the substrate surface roughness, a coating about 1.1 ~tm thick was deposited onto a polished surface (c.l.a. roughness, 0.06 I~m). The increase in the porosity of the experimental area was similar to that of the conventionally prepared samples (c.l.a. roughness, 0.24 lam).
3.3.2. Effect of ion mixing The results of the porosity tests on the coatings produced in case 2 are summarized in Tables III and IV. Each result is representative of up to six samples produced under identical conditions. The variables to which particular attention was paid are the beam energy, the beam current density, the deposition rate and the film thickness. TABLE III EFFECT OF THE GAS ION BEAM ENERGY AND THE CURRENT DENSITY ON THE POROSITY IN THE COATINGS IN CASE 2 a Gas ion energy (keV)
Effect o f the following current densities on the porosity 0.02-0.04 mAcm- 2
0.8 1 1.5
0.05-0.06 mAcm - 2
0b
5 6
0.8-1,0 m A cm - 2
0.12-0.13 mAcm - z
0d
0d 0d 0d
0b
0d
+d +d
+¢
a 0 d e n o t e s no c h a n g e b e t w e e n the e v a p o r a t e d a n d the i o n - m i x e d c o a t i n g s ; + d e n o t e s s o m e r e d u c t i o n in the p o r o s i t y in the i o n - m i x e d c o a t i n g c o m p a r e d with that in the e v a p o r a t e d coating. b D e p o s i t i o n rate, 0.02 lam m i n - ~. c D e p o s i t i o n rate, 0.2 ~tm m i n - ~. a D e p o s i t i o n rate, 0.1 I~m rain - 1. T A B L E IV EFFECT OF THE DEPOSITION RATE ON THE POROSITY IN THE COATINGS IN CASE 2 FOR VARIOUSGAS ION BEAM ENERGIES AND CURRENT DENSITIES Deposition rate (lam m i n - 1 )
0.02 0.05 0.1 0.2
Effect o f the following gas ion beam current densities on the porosity 0.02-0.04 mA cm- 2
0.05-0.06 mA cm- 2
0~
0~
0•
+b 0b
I o n b e a m energies, 1 a n d 1.5 keV. b I o n b e a m energies, 5 a n d 6 keV.
0.09 mA cm- 2
0~ 0~
O. 1 macro-2
O. 12 m A cm
+b + b
(p
2
O. 18 mAcro
+~
2
ION BOMBARDMENT
OF Au
343
COATINGS
For gas ion energies up to and including 1.5 keV, there was no noticeable effect on the porosity. With energies of 5-6 keV there was usually a reduction in the porosity, although (Table IV) there was no change when the deposition rate was 0.2 I~m min - ~ and the gas ion current density was between 0.05 and 0.06 mA c m - 2. (The higher gas ion energies were achieved by negatively biasing the substrate.) It is generally thought that the microstructure of coatings is modified through the transfer of momentum from the ions to the depositing atoms. Thus at any fixed beam energy the number of ions bombarding a coating relative to the number of depositing atoms should be important. Too low a number of ions may not produce the desired overall effect on the depositing atoms. To illustrate this, the data from Tables III and IV have been converted (Table V) to indicate the effect of the ratio of ions to depositing particles on the observed porosity. The results in Table V confirm that for beam energies of 1.5 keV or less there is no noticeable reduction in the porosity in the ion-mixed coatings (even when the ion-to-atom ratio is as high as 0.23). When the beam energy is increased to 5 keV or above, a significant reduction in the porosity in the ion-mixed coating is obtained when the ion-to-atom ratio is 0.038 and above. However, with the ratio of 0.023, TABLE
V
EFFECT OF THE ION BEAM ENERGY AND THE CURRENT DENSITY ON THE POROSITY IN THE COATINGS IN CASE 2 FOR KNOWN DEPOSITION RATES
Gas ion energy (keV)
Effect on the porosity of the following ratios of incident ions to the number of gold atoms depositing onto the substrate 0.023
0.038
0.046
0.8
0.076
0.098
0.15
0.23
0
0
0
1 1.5 5 6
0
0 + +
+ +
0
0
0
0
+
corresponding to a deposition rate of 0.02 i.tm min- ~ and a current density of 0.05-0.06 mA cm -2, there was no observed change in the porosity. Therefore, it seems that there is a threshold level for the beam energy and the ion-to-atom ratio below which the porosity cannot be reduced by ion mixing. Although the porosity in the coatings could be reduced consistently by ion bombardment at an energy level of 5 keV or higher, it was not possible to reproduce the exact levels of the porosity reduction in repeat samples produced under identical conditions. Furthermore, the porosity could not be completely eliminated even when the coatings were deposited to a thickness of 6 txm. The production of coatings by ion mixing is a relatively new technique, and there is much that needs to be understood, particularly regarding the mechanisms of momentum transfer from the energetic gas ions to the depositing particles. 4. CONCLUSIONS (1) Argon ion bombardment (up to 6 keV) before and/or during the thermal
344
s . s . NANDRA, F. G. WILSON, C. D. DEsFORGES
evaporation of gold onto copper has been found to influence the adherence and the through porosity of the coatings. (2) Sputter cleaning prior to deposition was the most effective means of producing highly adherent coatings; ion bombardment during deposition improved the adherence only slightly. (3) When the duration of the sputter cleaning was increased significantly, there was a marked increase in the through porosity in the final coatings. (4) Ion bombardment during deposition reduced the final through porosity when the energy levels were high (5-6 keV) and when the ratio between the flux of the gas ions and the depositing atoms exceeded a critical level. REFERENCES 1 D.M. Mattox, in R. F. Bunshah (ed.), Deposition Technologies for Films and Coatings, Noyes, Park Ridge, NJ, 1982, pp. 254-258. 2 D.G. Teer and B. L. Delcea, Thin Solid Films, 54 (1978) 295. 3 G. Carter and D. G. Armour, Thin Solid Films, 80 (1981) 13. 4 E.H. Hirsch and I. K. Varga, Thin Solid Films, 52 (1978) 445. 5 C. Weissmantel, G~ Reisse, H.-J. Erler, F. Henny, K. Bewilogua, U. Ebersbach and C. Schiirer, Thin Solid Films, 63 (1979) 315. 6 D.W. Hoffman and M. R. Gaerttner, J. Vac. Sei. Technol., 17 (1980) 425. 7 H, R. Kaufman, J. Vac. Sei. Technol., 15 (1978) 272. 8 M. Clarke, in R. Sard (ed.), Properties of Electrodeposits, Electrochemical Society, Princeton, N J, 1975, pp. 122 141.