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Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures
166
Surface and Coatings Technology, 59 (1993) 166—170
Reactive magnetron sputtering of zirconium carbide films using Ar—CH4 gas mixtures J. Bruckner Technische Universität Chemnitz, Fachbereich Physik, PSF 964, 9010 Chemnitz (Germany)
T.
Mäntylä
Institute of Materials Science, Tampere University of Technology, P.O. Box 589, SF-33101 Tampere (Finland)
Abstract Zirconium carbide coatings have been produced by reactive r.f. sputtering on cold-rolled steel samples. The films were deposited in an argon—methane atmosphere. The influence of total pressure, methane partial pressure, substrate temperature and bias voltage on the structure and properties was investigated. The coatings were mainly characterized by X-ray diffraction (XRD), scanning electron microscopy and hardness measurements. Energy dispersive X-ray spectroscopy analysis was used to detect impurities such as argon. It could be shown that an increasing methane partial pressure results in an increasing lattice parameter. The films have a B 1 NaC1 structure with a (ill) preferred orientation. With increasing bias voltage the structure becomes more dense and the deposition rate decreases. A very thin intermediate iron carbide layer was observed by XRD analysis ofZrC films about 100 nm thick.
1. Introduction The production of hard and tough materials becomes more and more an industrial standard. Diamond-like carbon and the transition metal carbides and nitrides form a broad group of coating materials. However, there is still a lack of knowledge of how coating conditions influence the properties of a film. Adhesion problems and problems with stresses in the films are still present. TiC and TiN are the most studied and used transition metal carbides and nitrides. Transition metal carbides form the refractory carbides which have similar properties such as very high melting point (2000—4000 °C), great hardness and a good corrosion resistance [1]. In this report the relatively unstudied coating material zirconium carbide is investigated,
13.56 MHz was used. An L-type matching network was installed between the power supply and the cathode to minimize the reflected power. A magnetron (US Gun II, 2 in) was installed behind the target. Pressures of about io~ Pa could be achieved by a full automatic pumping system consisting of a rotary pump, a diffusion pump and a liquid nitrogen cold trap. The target—substrate distance was 40 mm. The arrangement of the samples relative to the target is shown in Fig. 1. A quartz lamp was placed in the substrate holder to heat the substrates. The substrate temperature was measured by a K-type thermocouple
dummy—sub st rate
substrate
2. Experimental details Polycrystalline zirconium carbide coatings were deposited in a commercial Nanotech 4 r.f. diode sputtering unit on cold-rolled steel samples. The depositions were carried out with a polycrystalline zirconium target (diameter, 2 in) with a purity of 99.9%. Argon (purity, 99.999%) was used as inert gas and methane (purity, 99.95%) as reactive gas. The gases were mixed outside the chamber and introduced through a tube (diameter, 2—3 mm) ending about 20 mm from the target. An r.f. power supply working at a frequency of
0257—8972/93/$6.00
2
holder
3
target
crater
target Fig. 1. Arrangement of the samples relative to the target.
©
1993
—
Elsevier Sequoia. All rights reserved
/
J. Bruckner, T. Mdntyld
Reactive magnetron sputtering of ZrC using Ar—CH
4
using a dummy substrate. A d.c. power supply was connected to the, substrate holder to bias the substrates. Before deposition the substrates were pretreated by polishing with SiC paper and alumina powder and ultrasonic cleaning in acetone. Finally they were ion in the chamber at argon pressures between 3 Scanning electron microscopy (SEM) was used to determine the structure, surface conditions and deposition rate of the films. Cross-sections (fracture surface) of the films were produced for this purpose. Energy dispersive X-ray spectroscopy (EDS) analysis was carried out to detect impurities such as argon in the films. X-ray diffraction (XRD) using Cu K~xradiation was used to measure lattice parameters and to determine the preferred orientation of the films. A Shimadzu microhardness tester was used to measure the Vickers hardness. The stoichiometry of the films was determined by
167
-,
__________________________
.
-
____________ ____
_____
__________________________________________ (a) .
(b)
______
.
~.TTT~ -
I4-I $~
(a)
_ __
r~ ~
__ ~
_
J
~
F
~
..—~—.
~
~
5
~
~
(b)
(c) Fig. 3. (a) Scanning electron micrograph of a film surface (methane
Fig. 2. (a) Scanning electron micrograph of a cross-section (methane content, 1.9%). (b) Scanning electron micrograph of a cross-section (methane content, 2.5%).
content, 1.8%). (b) Scanning electron micrograph of a film surface (methane content, 2.4%). (c) Scanning electron micrograph of a film surface (methane content, 2.9%).
~
168
J. BrOckner, T. Mantyla
/
Reactive magnetron sputtering of ZrC using Ar—CH
4
Rutherford backscattering spectroscopy measurements using H + ions with an energy of 1750 keV.
3. Results and discussion The zirconium carbide films were deposited for different argon pressures, methane partial pressures, substrate temperatures and bias voltages. The deposition parameters used are shown in Table 1. Studies with SEM showed that with increasing methane partial pressure the structure of the films changes from fibrous (Fig. 2(a)) to open (Fig. 2(b)). Simultaneously the film surface changes from smooth (Fig. 3(a)) to flaky (Fig. 3(b)) and open (Fig. 3(c)). The deposition rate decreases with increasing methane content in the sputtering gas. Sundgren et al. [2] showed that the hardness of TiC films increases with increasing carbon concentration. High substrate temperatures and high bias voltages were used to improve the surface conditions of films with high carbon concentrations. Very dense and hard coatings could be produced (Fig. 4). Adhesion problems and compressive stresses in the films were observed, Increasing the bias voltage also influences the film structure. A cross-section of a very dense coating is shown in Fig. 5. With increasing bias voltage the structure becomes more dense and the deposition rate ,
TABLE 1. Deposition parameters Total pressure (Pa) Methane partial pressure (Pa) Methane content (%) Bias voltage (V) Substrate temperature (°C) Deposition time (mm)
0.41.1 0.0 1—0.03 1.25—3.5 from —50 to —215 380—600 3, 180, 300, 420
______
~ Fig. 5. Scanning electron micrograph of a cross-section (bias voltage, —215 V).
decreases. EDS analysis showed that with increasing bias voltage the argon content in the films increases. The results obtained are summarized in Table 2. The lattice parameters were calculated from the distance between the ZrC(l 11) planes measured in XRD. It could be shown that an increasing methane content in the working gas results in an increasing lattice parameter (Fig. 6). A possible explanation could be the incorporation of carbon at interstitial sites in the ZrC lattice. The position of the samples (Fig. 1) has a big influence on the lattice parameter. Possible explanations could be the different sputter conditions or a temperature difference between the samples. The values obtained are given in Table 2. TABLE 2. Results
Deposition rate (j.tm h Lattice parameter (A) Preferred orientation Hardness (HV) C/Zr
)
0.3—2.7 4.58—4.77 (Iii), (220) 1000—5000 0.22—1.5
Lattice parameter a/A
41
~
~
-
+
~ 462 4.6 4.58 4.56 4.54
+ * -
0.007
Fig. 4. Scanning electron micrograph of a cross-section (methane content, 2.5%; substrate temperature, 510 °C).
0.009
0•0l1~
~~0~13
00150.017
0.019
0.021
Fig. 6. Lattice parameter vs. methane partial pressure for the three sample positions. •, Sample 1; +, sample 2; *, sample 3.
J. Bruckner, T. Mdntyld
/
Reactive magnetron sputtering of ZrC using Ar—CH
4
The preferred orientation of the films was determined by comparing the measured XRD intensity ratios with the intensity ratios on the ASTM card. Nearly all coatings have a (111) preferred orientation. The preferred orientation is influenced by the position of the samples (Fig. 7(a)), by the substrate temperature (Fig. 7(b)) and by the methane partial pressure (Fig. 7(c)). With increasing methane partial pressure the preferred orientation changes from (111) to (220) or as shown in Fig. 7(c) to no preferred orientation. Using small amounts of methane in the sputtering gas (1.25 %—2%) a zirconium peak was observed (Fig. 8). The intensity of this Zr(lOO) peak relative to the intensity
I ZrC(222)
_A_...~.________________________
2 3
______________________________________________ (b) ZrC(220)
Z~(t t t)
)
Fc(tm)
~
J~ 2tI~t.
u
ZrC(3t t) ~
t
.
2 9~SC. L~....r
~ (c)
ZrC(Itt)
ZrC(2~)
Fe(t~)
ZrC(220)
ZrC(3tt) ZrC(222) 2Lh~t.
Fig. 8. XRD spectrum showing the Zr(lOO) peak intensity relative to the ZrC(lli) peak intensity (methane content, 1.2%; substrate temperature, 380 °C).
poration carbon in the zirconium lattice. During ofXRD investigations of thin coatings (about 100 nm) FenCm peaks were measured (Fig. 9). At the beginning of the deposition process an iron carbide layer is formed simultaneously. The Vickers microhardness of the films was measured using loads between 15 and 50 gI depending on the film and then the average value was calculated. Values between thickness. At least five measurements were carried out 1000 and 5000 HV were determined. The hardness of the films depends on the methane partial pressure, the substrate temperature and the bias voltage. With increasing methane partial pressure the hardness increases (Fig. 10). The hardest film was produced using a methane content of 2.5% and a substrate temperature of 510°C. Measurements of the stoichiometry of the films were carried out. A separate publication is in preparation [4]. It could be shown that with increasing methane content,
ZrC(208) I
ZrC(Ltt)
ZrC(31 t) ZrC(222)
_____ 2t~t. ~
Zr(l5))
content in the gas and with increasing substrate temperature. The C/Zr ratio of these films was less than 0.56. According to the phase diagram of the Zr—ZrC system [3] a zirconium phase exists in this range. The peak position of the Zr(lOO) peak does not conform with the ASTM value. A possible explanation could be the incor-
I
~
ZrC(Itl)
of the ZrC(l 11) peak decreases with increasing methane
(a)
ZrC(ttt)
.dJ\~J Fe,,C
1711. Linm
169
O9.~U)
Fig. 7. (a) XRD spectra showing the influence of the sample position on the ZrC(l 11) peak relative intensity: curve 1, sample 1; curve 2, sample 2; curve 3, sample 3. (b) XRD spectra showing the influence of the substrate temperature on the ZrC(220) peak relative intensity: curve 1, T1=600°C;curve 2, T1=440°C.(c) XRD spectra showing the influence of the methane partial pressure on the ZrC(l 11) peak relative intensity: curve 1, 0.02 Pa; curve 2, 0.027 Pa.
(
8~o~
I
Fe(lO0) Z~(22O)
~I ~
F~C
2th.t~ a
/
i
Fe(2~)
F~C~
~ 213. Lin..r
Fig. 9. XRD spectrum showing the iron carbide peaks of thin ZrC coatings.
170
J. Bruckner, T. Mantyla
/
Reactive magnetron sputtering of ZrC using Ar—CH
4
are similar to the well-known properties of sputtering TiC coatings. It could be shown that an increasing carbon concentration improves the hardness but
Hardness/HV 4000 3500 * -i-
3000
decreases the film quality. Adhesion problems and problems due to the different thermal expansion coefficients of the film and substrate material were observed.
2500 2000 1500 + 1000
Acknowledgment
500
c 0.007
I
0.009
0.011
0.013
0.015
..L
0.017
I
0.019
0.021
Partial pressure/Pa
The authors wish to express their gratitude to Mr. J. Koskinen for the stoichiometry measurements.
Fig. 10. Measured hardness vs. methane partial pressure for the three sample positions. •, Sample I; +, sample 2; *, sample 3.
increasing bias voltage and increasing substrate temperature the carbon concentration in the films increases. The films become harder with increasing carbon content, 4. Summarizing remarks Hard zirconium carbide coatings with hardness values up to 5000 HV have been produced. The results
References I J. E. Sundgren and H. T. G. Hentzell, A review of the present state of the art in hard coatings grown from the vapor phase, J. Vac. Sci, Technol. A, 4(5) (1986) 2259. 2 J. E. Sundgren, B.-O. Johansson and S.-E. Karlson, Reactive sputtering of Ti—N and Ti—C, Parts I—Ill, Thin Solid Films, 105 (1983) 353, 367, 385. 3 E. K. Storms, The Refractory Carbides, Part II, Academic Press, New York, 1967. 4 J. Bruckner and J. Koskinen, in preparation.
Surface and Coatings Technology, 59 (1993) 166—170
Reactive magnetron sputtering of zirconium carbide films using Ar—CH4 gas mixtures J. Bruckner Technische Universität Chemnitz, Fachbereich Physik, PSF 964, 9010 Chemnitz (Germany)
T.
Mäntylä
Institute of Materials Science, Tampere University of Technology, P.O. Box 589, SF-33101 Tampere (Finland)
Abstract Zirconium carbide coatings have been produced by reactive r.f. sputtering on cold-rolled steel samples. The films were deposited in an argon—methane atmosphere. The influence of total pressure, methane partial pressure, substrate temperature and bias voltage on the structure and properties was investigated. The coatings were mainly characterized by X-ray diffraction (XRD), scanning electron microscopy and hardness measurements. Energy dispersive X-ray spectroscopy analysis was used to detect impurities such as argon. It could be shown that an increasing methane partial pressure results in an increasing lattice parameter. The films have a B 1 NaC1 structure with a (ill) preferred orientation. With increasing bias voltage the structure becomes more dense and the deposition rate decreases. A very thin intermediate iron carbide layer was observed by XRD analysis ofZrC films about 100 nm thick.
1. Introduction The production of hard and tough materials becomes more and more an industrial standard. Diamond-like carbon and the transition metal carbides and nitrides form a broad group of coating materials. However, there is still a lack of knowledge of how coating conditions influence the properties of a film. Adhesion problems and problems with stresses in the films are still present. TiC and TiN are the most studied and used transition metal carbides and nitrides. Transition metal carbides form the refractory carbides which have similar properties such as very high melting point (2000—4000 °C), great hardness and a good corrosion resistance [1]. In this report the relatively unstudied coating material zirconium carbide is investigated,
13.56 MHz was used. An L-type matching network was installed between the power supply and the cathode to minimize the reflected power. A magnetron (US Gun II, 2 in) was installed behind the target. Pressures of about io~ Pa could be achieved by a full automatic pumping system consisting of a rotary pump, a diffusion pump and a liquid nitrogen cold trap. The target—substrate distance was 40 mm. The arrangement of the samples relative to the target is shown in Fig. 1. A quartz lamp was placed in the substrate holder to heat the substrates. The substrate temperature was measured by a K-type thermocouple
dummy—sub st rate
substrate
2. Experimental details Polycrystalline zirconium carbide coatings were deposited in a commercial Nanotech 4 r.f. diode sputtering unit on cold-rolled steel samples. The depositions were carried out with a polycrystalline zirconium target (diameter, 2 in) with a purity of 99.9%. Argon (purity, 99.999%) was used as inert gas and methane (purity, 99.95%) as reactive gas. The gases were mixed outside the chamber and introduced through a tube (diameter, 2—3 mm) ending about 20 mm from the target. An r.f. power supply working at a frequency of
0257—8972/93/$6.00
2
holder
3
target
crater
target Fig. 1. Arrangement of the samples relative to the target.
©
1993
—
Elsevier Sequoia. All rights reserved
/
J. Bruckner, T. Mdntyld
Reactive magnetron sputtering of ZrC using Ar—CH
4
using a dummy substrate. A d.c. power supply was connected to the, substrate holder to bias the substrates. Before deposition the substrates were pretreated by polishing with SiC paper and alumina powder and ultrasonic cleaning in acetone. Finally they were ion in the chamber at argon pressures between 3 Scanning electron microscopy (SEM) was used to determine the structure, surface conditions and deposition rate of the films. Cross-sections (fracture surface) of the films were produced for this purpose. Energy dispersive X-ray spectroscopy (EDS) analysis was carried out to detect impurities such as argon in the films. X-ray diffraction (XRD) using Cu K~xradiation was used to measure lattice parameters and to determine the preferred orientation of the films. A Shimadzu microhardness tester was used to measure the Vickers hardness. The stoichiometry of the films was determined by
167
-,
__________________________
.
-
____________ ____
_____
__________________________________________ (a) .
(b)
______
.
~.TTT~ -
I4-I $~
(a)
_ __
r~ ~
__ ~
_
J
~
F
~
..—~—.
~
~
5
~
~
(b)
(c) Fig. 3. (a) Scanning electron micrograph of a film surface (methane
Fig. 2. (a) Scanning electron micrograph of a cross-section (methane content, 1.9%). (b) Scanning electron micrograph of a cross-section (methane content, 2.5%).
content, 1.8%). (b) Scanning electron micrograph of a film surface (methane content, 2.4%). (c) Scanning electron micrograph of a film surface (methane content, 2.9%).
~
168
J. BrOckner, T. Mantyla
/
Reactive magnetron sputtering of ZrC using Ar—CH
4
Rutherford backscattering spectroscopy measurements using H + ions with an energy of 1750 keV.
3. Results and discussion The zirconium carbide films were deposited for different argon pressures, methane partial pressures, substrate temperatures and bias voltages. The deposition parameters used are shown in Table 1. Studies with SEM showed that with increasing methane partial pressure the structure of the films changes from fibrous (Fig. 2(a)) to open (Fig. 2(b)). Simultaneously the film surface changes from smooth (Fig. 3(a)) to flaky (Fig. 3(b)) and open (Fig. 3(c)). The deposition rate decreases with increasing methane content in the sputtering gas. Sundgren et al. [2] showed that the hardness of TiC films increases with increasing carbon concentration. High substrate temperatures and high bias voltages were used to improve the surface conditions of films with high carbon concentrations. Very dense and hard coatings could be produced (Fig. 4). Adhesion problems and compressive stresses in the films were observed, Increasing the bias voltage also influences the film structure. A cross-section of a very dense coating is shown in Fig. 5. With increasing bias voltage the structure becomes more dense and the deposition rate ,
TABLE 1. Deposition parameters Total pressure (Pa) Methane partial pressure (Pa) Methane content (%) Bias voltage (V) Substrate temperature (°C) Deposition time (mm)
0.41.1 0.0 1—0.03 1.25—3.5 from —50 to —215 380—600 3, 180, 300, 420
______
~ Fig. 5. Scanning electron micrograph of a cross-section (bias voltage, —215 V).
decreases. EDS analysis showed that with increasing bias voltage the argon content in the films increases. The results obtained are summarized in Table 2. The lattice parameters were calculated from the distance between the ZrC(l 11) planes measured in XRD. It could be shown that an increasing methane content in the working gas results in an increasing lattice parameter (Fig. 6). A possible explanation could be the incorporation of carbon at interstitial sites in the ZrC lattice. The position of the samples (Fig. 1) has a big influence on the lattice parameter. Possible explanations could be the different sputter conditions or a temperature difference between the samples. The values obtained are given in Table 2. TABLE 2. Results
Deposition rate (j.tm h Lattice parameter (A) Preferred orientation Hardness (HV) C/Zr
)
0.3—2.7 4.58—4.77 (Iii), (220) 1000—5000 0.22—1.5
Lattice parameter a/A
41
~
~
-
+
~ 462 4.6 4.58 4.56 4.54
+ * -
0.007
Fig. 4. Scanning electron micrograph of a cross-section (methane content, 2.5%; substrate temperature, 510 °C).
0.009
0•0l1~
~~0~13
00150.017
0.019
0.021
Fig. 6. Lattice parameter vs. methane partial pressure for the three sample positions. •, Sample 1; +, sample 2; *, sample 3.
J. Bruckner, T. Mdntyld
/
Reactive magnetron sputtering of ZrC using Ar—CH
4
The preferred orientation of the films was determined by comparing the measured XRD intensity ratios with the intensity ratios on the ASTM card. Nearly all coatings have a (111) preferred orientation. The preferred orientation is influenced by the position of the samples (Fig. 7(a)), by the substrate temperature (Fig. 7(b)) and by the methane partial pressure (Fig. 7(c)). With increasing methane partial pressure the preferred orientation changes from (111) to (220) or as shown in Fig. 7(c) to no preferred orientation. Using small amounts of methane in the sputtering gas (1.25 %—2%) a zirconium peak was observed (Fig. 8). The intensity of this Zr(lOO) peak relative to the intensity
I ZrC(222)
_A_...~.________________________
2 3
______________________________________________ (b) ZrC(220)
Z~(t t t)
)
Fc(tm)
~
J~ 2tI~t.
u
ZrC(3t t) ~
t
.
2 9~SC. L~....r
~ (c)
ZrC(Itt)
ZrC(2~)
Fe(t~)
ZrC(220)
ZrC(3tt) ZrC(222) 2Lh~t.
Fig. 8. XRD spectrum showing the Zr(lOO) peak intensity relative to the ZrC(lli) peak intensity (methane content, 1.2%; substrate temperature, 380 °C).
poration carbon in the zirconium lattice. During ofXRD investigations of thin coatings (about 100 nm) FenCm peaks were measured (Fig. 9). At the beginning of the deposition process an iron carbide layer is formed simultaneously. The Vickers microhardness of the films was measured using loads between 15 and 50 gI depending on the film and then the average value was calculated. Values between thickness. At least five measurements were carried out 1000 and 5000 HV were determined. The hardness of the films depends on the methane partial pressure, the substrate temperature and the bias voltage. With increasing methane partial pressure the hardness increases (Fig. 10). The hardest film was produced using a methane content of 2.5% and a substrate temperature of 510°C. Measurements of the stoichiometry of the films were carried out. A separate publication is in preparation [4]. It could be shown that with increasing methane content,
ZrC(208) I
ZrC(Ltt)
ZrC(31 t) ZrC(222)
_____ 2t~t. ~
Zr(l5))
content in the gas and with increasing substrate temperature. The C/Zr ratio of these films was less than 0.56. According to the phase diagram of the Zr—ZrC system [3] a zirconium phase exists in this range. The peak position of the Zr(lOO) peak does not conform with the ASTM value. A possible explanation could be the incor-
I
~
ZrC(Itl)
of the ZrC(l 11) peak decreases with increasing methane
(a)
ZrC(ttt)
.dJ\~J Fe,,C
1711. Linm
169
O9.~U)
Fig. 7. (a) XRD spectra showing the influence of the sample position on the ZrC(l 11) peak relative intensity: curve 1, sample 1; curve 2, sample 2; curve 3, sample 3. (b) XRD spectra showing the influence of the substrate temperature on the ZrC(220) peak relative intensity: curve 1, T1=600°C;curve 2, T1=440°C.(c) XRD spectra showing the influence of the methane partial pressure on the ZrC(l 11) peak relative intensity: curve 1, 0.02 Pa; curve 2, 0.027 Pa.
(
8~o~
I
Fe(lO0) Z~(22O)
~I ~
F~C
2th.t~ a
/
i
Fe(2~)
F~C~
~ 213. Lin..r
Fig. 9. XRD spectrum showing the iron carbide peaks of thin ZrC coatings.
170
J. Bruckner, T. Mantyla
/
Reactive magnetron sputtering of ZrC using Ar—CH
4
are similar to the well-known properties of sputtering TiC coatings. It could be shown that an increasing carbon concentration improves the hardness but
Hardness/HV 4000 3500 * -i-
3000
decreases the film quality. Adhesion problems and problems due to the different thermal expansion coefficients of the film and substrate material were observed.
2500 2000 1500 + 1000
Acknowledgment
500
c 0.007
I
0.009
0.011
0.013
0.015
..L
0.017
I
0.019
0.021
Partial pressure/Pa
The authors wish to express their gratitude to Mr. J. Koskinen for the stoichiometry measurements.
Fig. 10. Measured hardness vs. methane partial pressure for the three sample positions. •, Sample I; +, sample 2; *, sample 3.
increasing bias voltage and increasing substrate temperature the carbon concentration in the films increases. The films become harder with increasing carbon content, 4. Summarizing remarks Hard zirconium carbide coatings with hardness values up to 5000 HV have been produced. The results
References I J. E. Sundgren and H. T. G. Hentzell, A review of the present state of the art in hard coatings grown from the vapor phase, J. Vac. Sci, Technol. A, 4(5) (1986) 2259. 2 J. E. Sundgren, B.-O. Johansson and S.-E. Karlson, Reactive sputtering of Ti—N and Ti—C, Parts I—Ill, Thin Solid Films, 105 (1983) 353, 367, 385. 3 E. K. Storms, The Refractory Carbides, Part II, Academic Press, New York, 1967. 4 J. Bruckner and J. Koskinen, in preparation.