- Email: [email protected]
The effect of Au and O implantation on the etch rate of CVD diamond
Applied Surface Science 221 (2004) 302–307
The effect of Au and O implantation on the etch rate of CVD diamond Patrick W. Leecha,*, Geoffrey K. Reevesb, Anthony Hollandb, Mark C. Ridgwayc a
CSIRO Manufacturing and Infrastructure Technology, Melbourne, Australia School of Computer Systems and Electrical Engineering, RMIT University, Melbourne, Australia c Department of Electronic Materials Engineering, Australian National University, Canberra, Australia b
Received 7 October 2002; received in revised form 26 May 2003; accepted 17 July 2003
Abstract Diamond films were implanted with Au or O ions at multiple energies in order to produce a uniform region of C vacancies. Analysis of the implanted films by Raman spectroscopy has shown that the proportion of non-diamond or amorphous carbon increased with dose (5 1013 to 5 1015 ions/cm2). For implantation with Au ions, a complete amorphization near to the surface was evident at a dose of 5 1015 ions/cm2. We have examined the ion beam etch (IBE) rate of the films as a function of the implant species and dose. The etching experiments were performed using either Ar or Ar/O2 gases at a bias energy of 500– 1000 eV. In Ar gas, the process of sputter etching has produced a similar increase in etch rate with dose for both the Au and O implants. In Ar/O2 gases, the process of ion-enhanced chemical etching produced greater etch rates than obtained in Ar gas with higher rates for the Au than the O implants. # 2003 Elsevier B.V. All rights reserved. Keywords: Diamond films; Ion implantation; Etching; Raman spectroscopy
1. Introduction Diamond has become a preferred material in a range of devices including high frequency SAW filters, inert biomedical chips and ultra-low friction micromechanical systems. In the fabrication of these devices, the substrate of choice has been diamond-on-silicon formed by heteroepitaxial growth. But limitations on the use of CVD diamond films in device processing have included the surface roughness evident with increasing film thickness and the inability to * Corresponding author. Present address: Private Bag 33, Clayton South MDC 3169, Vic., Australia. Tel.: þ613-9545-2791; fax: þ613-9545-2844. E-mail address: [email protected] (P.W. Leech).
chemically etch or polish the surface. Reports of the successful etching of diamond have been largely based on plasma or ion beam techniques [1]. Of these methods, ion beam etching (IBE) has shown an optimum quality of finish of the diamond consistent with lithographic patterning [1,2]. However, the application of IBE has been restricted by a low etch rate. Recent results have shown that an effective means of increasing the rate of ion beam etching was by the prior implantation of the diamond surface with Ge or C ions [3]. The increase in etch rate was dependent on the comparative amount of non-diamond carbon produced during implantation [3]. This result was consistent with an earlier study which has shown that high dose implantation enabled the chemical etching of the previously inert surface of diamond [4].
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00949-8
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307
303
In the present paper, we have examined for the first time, the effect on etch rate of CVD diamond of prior implantation with a heavy ion (Au) and an ion of intermediate mass (O). For each ion species, the implants were performed at multiple energies in order to produce a uniform concentration of carbon vacancies with depth. In the etching experiments, the parameters of ion beam energy and the gas mixtures (Ar or Ar/O2) have been varied in order to evaluate the effect of prior implantation in relation to the etch mechanisms.
the samples, the stainless steel chamber was pumped down to a base pressure of 4 104 Pa. Subsequently, the process gases of either pure Ar (3.25 sccm) or a mixture of Ar (3.25 sccm) and O2 (1.0 sccm) were introduced into the chamber by means of electronic mass flow controllers. A series of experiments was conducted with the energy of the etching ion beam varied from 500 to 1000 eV in both Ar and Ar/O2 gases. The duration of each experiment was maintained constant at 20 min with the measurement of etch rate performed using a Dektak profilometer.
2. Experimental details
3. Results and discussion
The undoped films of diamond used in the experiments were grown by microwave CVD on silicon wafers with a (1 1 1) orientation. The polycrystalline films were 3 mm thick with a grain-size of 1–3 mm and an average surface roughness, Ra 2:5 nm as measured by scanning probe microscopy. The implantation of the diamond films was performed using either Au or O ions at fluences in the range 5 1013 to 5 1015 ions/cm2. These doses have been reported to produce a wide variation in the level of damage in CVD diamond implanted by Ge and C ions [3]. The implant energies were selected on the basis of simulations using the transport of ions in matter (TRIM) code. In the case of the O ions, a sequence of implants at three separate ion energies of 0.10, 0.29 and 0.50 MeV was used to produce a uniform level of damage (0.15–0.30 C vacancies/A/ion) to a depth of 0.5 mm. The Au ions were implanted at dual energies of 1.5 and 3.5 MeV inducing a greater level of damage (1.5–2.5 C vacancies/A/ion) also to a depth of 0.5 mm. During implantation, the samples were held at a temperature of 196 8C in order to minimize dynamic annealing. Raman measurements on the preand post-implanted surfaces were performed using a Renishaw Ramascope 2000 system with a near infrared laser source (780 nm). A 50 objective allowed the focus of the laser onto a position which was approximately 1 mm in diameter. The etching experiments were conducted in a commercial ion beam system incorporating a 5 cm filamentless source. The samples were mounted on the rotating base plate in the system which was continuously cooled by circulating water. After the loading of
Figs. 1 and 2 have compared Raman spectra in the region 1200–1600 cm1 typical of the O and Au implanted diamond, respectively. For the unimplanted samples, the prominent feature of the spectra was, the definitive peak at 1331 cm1 representative of sp3 bonded diamond [5]. Also appearing in the spectra for the unimplanted diamond were two weaker peaks at 1445 and 1560 cm1 which have previously been assigned to the presence of diamond-like carbon [6]. In CVD films, these non-diamond phases have typically been localized in the crystal boundaries [6]. Unimplanted
5E13 O 5E14 O 5E15 O
1200
1300
1400
1500
1600
-1
Raman Shift (cm ) Fig. 1. Raman spectra of CVD diamond before and after implantation with O ions at various doses.
304
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307
Unimplanted 5E13 Au 5E14 Au 5E15 Au
1200
1300
1400
1500
1600
Raman Shift (cm-1) Fig. 2. Raman spectra of CVD diamond before and after implantation with Au ions at various doses.
Following implantation with O ions at doses from 5 1013 to 5 1015 ions/cm2, the spectra in Fig. 1 have shown a progressive reduction in intensity of the 1331 cm1 signal. In addition, at the highest dose of 5 1015 O ions/cm2, a new Raman peak had appeared at 1415 cm1. Sato and Iwaki have previously observed a Raman peak at 1410 cm1 following Ar ion implantation at 200 8C which they attributed to the presence of a disordered graphite structure [8]. Also, Hayward et al. have assigned the appearance of a peak at 1410 cm1 in CVD diamond to regions of disordered graphite within the grains of nano-crystalline diamond [7]. This disordered structure was located near to the interface with the silicon substrate [7]. The Raman peak at 1415 cm1 in the present results has accordingly been assigned to non-diamond or graphitic sp2 bonded phases. The formation of disordered structures by low temperature ion implantation has been described by Prawer et al. in terms of the formation of carbon interstitial and vacancy defects [9] as created within an ion cascade. Fig. 2 has compared the Raman spectra for the unimplanted and Au implanted surfaces. For Au implants at doses of 5 1013 and 5 1014 ions/cm2, there was a reduction in the height of the diamond peak with increasing dose. At the dose of 5 1014
Au ions/cm2, a newly emerged band was also evident at 1415 cm1. At the highest dose of 5 1015 Au ions/ cm2, the spectrum has shown the complete absence of a diamond peak at 1331 cm1 and instead exhibited a broad band at 1400–1550 cm1. These features of the spectrum for high dose Au implants (5 1015 ions/ cm2) have indicated very extensive damage to the structure of the diamond to form a region of amorphous carbon. In previous transmission electron diffraction studies of C ion implantation into diamond, Jiang et al. have identified a critical dose for the onset of amorphization [10]. Above this dose, a large amount of diamond phase was transformed into amorphous carbon. Below Dc, a heavily damaged carbon was formed while retaining the diamond structure [10]. Raman studies of implantation of a variety of ions into diamond have also reported Dc as the critical dose above which a graphitization process was initiated upon post-implant annealing [11]. The magnitude of Dc was reduced with heavier ions such as Sb (1 1014 ions/cm2) compared with lighter ions, for example C (2:5 1015 ions/cm2) and Ar (ð210Þ 1015 ions/cm2) [11]. Similarly, the present results for Au ion implantation have shown an amorphization of the CVD diamond at a dose between 5 1014 and 5 1015 ions/cm2 while for O implants, the onset of amorphization would necessitate a dose greater than 5 1015 ions/cm2. This result was consistent with the greater damage calculated by TRIM for the heavier ion (Au) compared with O implantation. Below Dc, the Raman spectra have suggested the formation of non-diamond or graphitic carbon sp2 bonded phases together with the retention of the diamond peak. The etch rate of the diamond as a function of the square root of (IBE) energy, Ei, has been plotted for the Au and O implanted surfaces in Figs. 3 and 4, respectively. The individual plots have compared the etch rates at each dose with the unimplanted samples. These experiments were performed in Ar gas. Steinbruchel has previously characterized the ion beam etching of various dielectrics, metals and semiconductors as a process of either physical sputtering or ion-enhanced chemical etching [12]. In both types of process, the etch yield Y(E) was given by the expression: 1=2
YðEÞ ¼ AðEi
1=2
Eth Þ
(1)
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307 18
Etch Rate (nm/min)
16 14 12 10 8 6
5E15 O ions/cm2 5E14 O ions/cm2
4
5E13 O ions/cm2
2
Unimplanted
0 0.6
0.7
0.8
0.9
1
1.1
1/2
Ion energy (eV) 1=2
Fig. 3. Ar ion etch rate vs. Ei
for O implanted samples.
where Ei is the ion energy and A and Eth are the constants representing the slope and threshold energy of etching, respectively. The constant, A, has been shown to depend on the specific combination of the projectile ions and the material of the target [12]. For physical sputtering, the mechanism of etching has been well characterized as the collision-induced removal of target atoms by chemically inert ions. 18
305
During sputtering, A / U 1 where U was the surface binding energy required for the removal of products (the minimum energy of a surface atom required for ejection from the target) [13]. In chemical sputtering, the ionized gases (in this case O2) have been the main reactants with resulting volatile etch products. The values of A have been shown as significantly larger in ion-enhanced chemical etching than in purely physical sputtering [12–14]. This characteristic has been attributed to the virtually zero binding energy of volatile etch products [12]. A least squares fit of equation (i) to the data in Figs. 3 and 4 has also shown a linear dependence of 1=2 Y(E) on Ei . As an indication of the quality of fit of the data to the equation, the magnitude of r2 (the square of correlation coefficient) was calculated from each plot. For the implanted samples, the value of r2 was in the range 0.91–0.99, indicating a good quality of fit. A summary of the values of the slope constant, A, and the etch rate at Ei ¼ 1000 eV has been given in Table 1. The constant, A, was essentially independent of the treatment by implantation, the ion species and dose. This result has indicated that the same mechanism of etching has applied in implanted and unimplanted samples. However, the etch rate in Ar gas for both Au and O implants increased with the magnitude of dose. In comparison, etching of the diamond in Ar/ O2 gases produced a similar trend in etch rate versus Ei although at a higher rate than in Ar. The etch rate versus implant dose in Ar or Ar/O2 gases has been plotted in Fig. 5. During etching in each
16
Etch Rate (nm/min)
14
Table 1 Values of the slope constant, A, and etch rate of CVD diamond in Ar gas
12 10
A (nm/(min V1/2))
Etch rate (nm/min) at Ei ¼ 1000 eV
11.0
10.7
Au implant (ions/cm ) 51013 51014 51015
10.8 11.3 10.8
11.9 13.3 16.3
O implant (ions/cm2) 51013 51014 51015
10.7 12.0 10.8
12.8 13.7 16.3
Implanted sample/dose
8 6
5E15 Au/cm2
Unimplanted
5E14 Au/cm2
4
2
5E13 Au/cm2
2
Unimplanted
0 0.6
0.7
0.8
0.9
1
1.1
1 /2
Ion Energy (eV) 1=2
Fig. 4. Ar ion etch rate vs. Ei
for Au implanted samples.
306
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307
25
Etch Rate (nm/min)
20
15
10 Au implant - Ar/O2 IBE O implant - Ar/O2 IBE
5
Au implant - Ar IBE O2 implant - Ar IBE
0 1E+13
1E+14
1E+15
1E+16 2
Implant Dose (ions/cm ) Fig. 5. Etch rate of diamond at Ei ¼ 1000 eV as a function of prior dose of O or Au implants using either Ar (3.25 sccm) or Ar (3.25 sccm)/O2 (1 sccm) gases.
of these gas environments, the etch rate steadily increased with dose of the implants, the greatest rise being evident between 5 1014 and 5 1015 ions/ cm2. The etch rate in Ar gas for the O and the Au implanted samples was similar at a given dose. However, in Ar/O2 gases, the Au implants produced a slightly higher etch rate than the O implanted surfaces. At the highest dose (5 1015 ions/cm2), the etch rate of implanted samples in Ar/O2 gas increased more rapidly than in pure Ar. The Ar ion beam etching of diamond has been previously identified as a process of physical sputtering [14]. In the present results, the slope constant, A, as summarized in Table 1 was independent of the implant conditions, indicating that both implanted and unimplanted samples were etched by the same sputter process. In addition, the dependence of etch rate on dose was the same for the Au and O implants (Fig. 5), both, in this study and for the Ge and C implanted surfaces reported previously [3]. The Au ions induced a greater level of damage (1.5–2.5 C vacancies/A/ion) and a lower threshold dose to amorphization than O ions (0.15–0.30 C vacancies/A/ion). Evidently, for IBE in Ar gas, the effect of mass of the implanting ion was not a significant factor in determining etch rate despite the Au implants producing an amorphization
of the structure. However, the trend of increase in etch rate with dose (Fig. 5) has suggested that the nondiamond forms of carbon resulting from ion implantation were more easily removed by sputter etching than the sp3 diamond. The results have indicated that the rate of sputter etching of the samples was dependent on the proportion of non-diamond carbon as a function of implant dose rather than the implant species. The etching of diamond in Ar/O2 gases has been identified as a process of ion-enhanced chemical etching [13]. The higher etch rate in Ar/O2 than in Ar gas has been attributed to the formation of the volatile etch products of CO and CO2. During IBE in Ar/O2 gases, the higher etch rate of Au implanted surfaces than the O implants has indicated that the increasing degree of damage obtained using the heavier ions acted to further enhance the reactivity of the surface compared with sp3 diamond. In this respect, the highest etch rates correlated with the amorphization of the diamond for the Au implants at 5 1015 ions/cm2.
4. Conclusions The implantation of Au or O ions into CVD diamond has resulted in an increasing proportion of nondiamond or amorphous carbon with dose (5 1013 to 5 1015 ions/cm2). The implantation with Au ions at a dose of 5 1015 ions/cm2 resulted in an amorphous structure. An increasing proportion of non-diamond forms of carbon with dose has resulted in an enhanced IBE in either Ar or Ar/O2 gases. The highest etch rates were obtained for Au implanted surfaces at a dose of 5 1015 ions/cm2 using Ar/O2 gases.
References [1] A. P. Malshe, B.S. Park, W.D. Brown, H.A. Naseem, Diamond Rel. Mater. 8 (1999) 1198. [2] D.F. Grogan, T. Zhao, B.G. Bovard, H.A. Macleod, Appl. Opt. 31 (10) (1992) 1483. [3] P.W. Leech, G.K. Reeves, A.S. Holland, M.C. Ridgway, F. Shanks, Diamond Relat. Mater. 11 (2002) 837–840. [4] J.F. Prins, Radiat. Effects Lett. 76 (1983) 79. [5] B.A. Fox, in: A. Elshabini-Riad, F.D. Barlow III (Eds.), Thin Film Technology Handbook, vol. 7, McGraw-Hill, New York, 1997. [6] Z. Sun, J.R. Shi, B.K. Tay, S.P. Lau, Diamond Relat. Mater. 9 (2000) 1979.
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307 [7] I.P. Hayward, K.J. Baldwin, D.M. Hunter, D.N. Batchelder, G.D. Pitt, Diamond Relat. Mater. 4 (1995) 617. [8] S. Sato, M. Iwaki, Nuclear Inst. Methods Phys. Res. B32 (1988) 145. [9] S. Prawer, A. Hoffman, R. Kalish, Appl. Phys. Lett. 57 (21) (1990) 2187. [10] N. Jiang, M. Deguchi, C.L. Wang, J.H. Won, H.M. Jeon, Y. Mori, A. Hatta, M. Kitabatake, T. Ito, T. Hirao, T. Sasaki, A. Hiraki, Appl. Surf. Sci. 113–114 (1997) 254.
307
[11] C. Uzan-Saguy, C. Cytermann, R. Brener, V. Richter, M. Shaanan, R. Kalish, Appl. Phys. Lett. 67 (9) (1995) 1194. [12] C. Steinbruchel, Appl. Phys. Lett. 55 (19) (1989) 1960. [13] C. Steinbruchel, in: Proceedings of the MRS Symposium on Laser and Particle Beam Chemical Processes on Surfaces, vol. 129, Pittsburgh, PA, 1989, p. 477. [14] P.W. Leech, G.K. Reeves, A.S. Holland, F. Shanks, Diamond Relat. Mater. 11 (2002) 833.
The effect of Au and O implantation on the etch rate of CVD diamond Patrick W. Leecha,*, Geoffrey K. Reevesb, Anthony Hollandb, Mark C. Ridgwayc a
CSIRO Manufacturing and Infrastructure Technology, Melbourne, Australia School of Computer Systems and Electrical Engineering, RMIT University, Melbourne, Australia c Department of Electronic Materials Engineering, Australian National University, Canberra, Australia b
Received 7 October 2002; received in revised form 26 May 2003; accepted 17 July 2003
Abstract Diamond films were implanted with Au or O ions at multiple energies in order to produce a uniform region of C vacancies. Analysis of the implanted films by Raman spectroscopy has shown that the proportion of non-diamond or amorphous carbon increased with dose (5 1013 to 5 1015 ions/cm2). For implantation with Au ions, a complete amorphization near to the surface was evident at a dose of 5 1015 ions/cm2. We have examined the ion beam etch (IBE) rate of the films as a function of the implant species and dose. The etching experiments were performed using either Ar or Ar/O2 gases at a bias energy of 500– 1000 eV. In Ar gas, the process of sputter etching has produced a similar increase in etch rate with dose for both the Au and O implants. In Ar/O2 gases, the process of ion-enhanced chemical etching produced greater etch rates than obtained in Ar gas with higher rates for the Au than the O implants. # 2003 Elsevier B.V. All rights reserved. Keywords: Diamond films; Ion implantation; Etching; Raman spectroscopy
1. Introduction Diamond has become a preferred material in a range of devices including high frequency SAW filters, inert biomedical chips and ultra-low friction micromechanical systems. In the fabrication of these devices, the substrate of choice has been diamond-on-silicon formed by heteroepitaxial growth. But limitations on the use of CVD diamond films in device processing have included the surface roughness evident with increasing film thickness and the inability to * Corresponding author. Present address: Private Bag 33, Clayton South MDC 3169, Vic., Australia. Tel.: þ613-9545-2791; fax: þ613-9545-2844. E-mail address: [email protected] (P.W. Leech).
chemically etch or polish the surface. Reports of the successful etching of diamond have been largely based on plasma or ion beam techniques [1]. Of these methods, ion beam etching (IBE) has shown an optimum quality of finish of the diamond consistent with lithographic patterning [1,2]. However, the application of IBE has been restricted by a low etch rate. Recent results have shown that an effective means of increasing the rate of ion beam etching was by the prior implantation of the diamond surface with Ge or C ions [3]. The increase in etch rate was dependent on the comparative amount of non-diamond carbon produced during implantation [3]. This result was consistent with an earlier study which has shown that high dose implantation enabled the chemical etching of the previously inert surface of diamond [4].
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00949-8
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307
303
In the present paper, we have examined for the first time, the effect on etch rate of CVD diamond of prior implantation with a heavy ion (Au) and an ion of intermediate mass (O). For each ion species, the implants were performed at multiple energies in order to produce a uniform concentration of carbon vacancies with depth. In the etching experiments, the parameters of ion beam energy and the gas mixtures (Ar or Ar/O2) have been varied in order to evaluate the effect of prior implantation in relation to the etch mechanisms.
the samples, the stainless steel chamber was pumped down to a base pressure of 4 104 Pa. Subsequently, the process gases of either pure Ar (3.25 sccm) or a mixture of Ar (3.25 sccm) and O2 (1.0 sccm) were introduced into the chamber by means of electronic mass flow controllers. A series of experiments was conducted with the energy of the etching ion beam varied from 500 to 1000 eV in both Ar and Ar/O2 gases. The duration of each experiment was maintained constant at 20 min with the measurement of etch rate performed using a Dektak profilometer.
2. Experimental details
3. Results and discussion
The undoped films of diamond used in the experiments were grown by microwave CVD on silicon wafers with a (1 1 1) orientation. The polycrystalline films were 3 mm thick with a grain-size of 1–3 mm and an average surface roughness, Ra 2:5 nm as measured by scanning probe microscopy. The implantation of the diamond films was performed using either Au or O ions at fluences in the range 5 1013 to 5 1015 ions/cm2. These doses have been reported to produce a wide variation in the level of damage in CVD diamond implanted by Ge and C ions [3]. The implant energies were selected on the basis of simulations using the transport of ions in matter (TRIM) code. In the case of the O ions, a sequence of implants at three separate ion energies of 0.10, 0.29 and 0.50 MeV was used to produce a uniform level of damage (0.15–0.30 C vacancies/A/ion) to a depth of 0.5 mm. The Au ions were implanted at dual energies of 1.5 and 3.5 MeV inducing a greater level of damage (1.5–2.5 C vacancies/A/ion) also to a depth of 0.5 mm. During implantation, the samples were held at a temperature of 196 8C in order to minimize dynamic annealing. Raman measurements on the preand post-implanted surfaces were performed using a Renishaw Ramascope 2000 system with a near infrared laser source (780 nm). A 50 objective allowed the focus of the laser onto a position which was approximately 1 mm in diameter. The etching experiments were conducted in a commercial ion beam system incorporating a 5 cm filamentless source. The samples were mounted on the rotating base plate in the system which was continuously cooled by circulating water. After the loading of
Figs. 1 and 2 have compared Raman spectra in the region 1200–1600 cm1 typical of the O and Au implanted diamond, respectively. For the unimplanted samples, the prominent feature of the spectra was, the definitive peak at 1331 cm1 representative of sp3 bonded diamond [5]. Also appearing in the spectra for the unimplanted diamond were two weaker peaks at 1445 and 1560 cm1 which have previously been assigned to the presence of diamond-like carbon [6]. In CVD films, these non-diamond phases have typically been localized in the crystal boundaries [6]. Unimplanted
5E13 O 5E14 O 5E15 O
1200
1300
1400
1500
1600
-1
Raman Shift (cm ) Fig. 1. Raman spectra of CVD diamond before and after implantation with O ions at various doses.
304
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307
Unimplanted 5E13 Au 5E14 Au 5E15 Au
1200
1300
1400
1500
1600
Raman Shift (cm-1) Fig. 2. Raman spectra of CVD diamond before and after implantation with Au ions at various doses.
Following implantation with O ions at doses from 5 1013 to 5 1015 ions/cm2, the spectra in Fig. 1 have shown a progressive reduction in intensity of the 1331 cm1 signal. In addition, at the highest dose of 5 1015 O ions/cm2, a new Raman peak had appeared at 1415 cm1. Sato and Iwaki have previously observed a Raman peak at 1410 cm1 following Ar ion implantation at 200 8C which they attributed to the presence of a disordered graphite structure [8]. Also, Hayward et al. have assigned the appearance of a peak at 1410 cm1 in CVD diamond to regions of disordered graphite within the grains of nano-crystalline diamond [7]. This disordered structure was located near to the interface with the silicon substrate [7]. The Raman peak at 1415 cm1 in the present results has accordingly been assigned to non-diamond or graphitic sp2 bonded phases. The formation of disordered structures by low temperature ion implantation has been described by Prawer et al. in terms of the formation of carbon interstitial and vacancy defects [9] as created within an ion cascade. Fig. 2 has compared the Raman spectra for the unimplanted and Au implanted surfaces. For Au implants at doses of 5 1013 and 5 1014 ions/cm2, there was a reduction in the height of the diamond peak with increasing dose. At the dose of 5 1014
Au ions/cm2, a newly emerged band was also evident at 1415 cm1. At the highest dose of 5 1015 Au ions/ cm2, the spectrum has shown the complete absence of a diamond peak at 1331 cm1 and instead exhibited a broad band at 1400–1550 cm1. These features of the spectrum for high dose Au implants (5 1015 ions/ cm2) have indicated very extensive damage to the structure of the diamond to form a region of amorphous carbon. In previous transmission electron diffraction studies of C ion implantation into diamond, Jiang et al. have identified a critical dose for the onset of amorphization [10]. Above this dose, a large amount of diamond phase was transformed into amorphous carbon. Below Dc, a heavily damaged carbon was formed while retaining the diamond structure [10]. Raman studies of implantation of a variety of ions into diamond have also reported Dc as the critical dose above which a graphitization process was initiated upon post-implant annealing [11]. The magnitude of Dc was reduced with heavier ions such as Sb (1 1014 ions/cm2) compared with lighter ions, for example C (2:5 1015 ions/cm2) and Ar (ð210Þ 1015 ions/cm2) [11]. Similarly, the present results for Au ion implantation have shown an amorphization of the CVD diamond at a dose between 5 1014 and 5 1015 ions/cm2 while for O implants, the onset of amorphization would necessitate a dose greater than 5 1015 ions/cm2. This result was consistent with the greater damage calculated by TRIM for the heavier ion (Au) compared with O implantation. Below Dc, the Raman spectra have suggested the formation of non-diamond or graphitic carbon sp2 bonded phases together with the retention of the diamond peak. The etch rate of the diamond as a function of the square root of (IBE) energy, Ei, has been plotted for the Au and O implanted surfaces in Figs. 3 and 4, respectively. The individual plots have compared the etch rates at each dose with the unimplanted samples. These experiments were performed in Ar gas. Steinbruchel has previously characterized the ion beam etching of various dielectrics, metals and semiconductors as a process of either physical sputtering or ion-enhanced chemical etching [12]. In both types of process, the etch yield Y(E) was given by the expression: 1=2
YðEÞ ¼ AðEi
1=2
Eth Þ
(1)
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307 18
Etch Rate (nm/min)
16 14 12 10 8 6
5E15 O ions/cm2 5E14 O ions/cm2
4
5E13 O ions/cm2
2
Unimplanted
0 0.6
0.7
0.8
0.9
1
1.1
1/2
Ion energy (eV) 1=2
Fig. 3. Ar ion etch rate vs. Ei
for O implanted samples.
where Ei is the ion energy and A and Eth are the constants representing the slope and threshold energy of etching, respectively. The constant, A, has been shown to depend on the specific combination of the projectile ions and the material of the target [12]. For physical sputtering, the mechanism of etching has been well characterized as the collision-induced removal of target atoms by chemically inert ions. 18
305
During sputtering, A / U 1 where U was the surface binding energy required for the removal of products (the minimum energy of a surface atom required for ejection from the target) [13]. In chemical sputtering, the ionized gases (in this case O2) have been the main reactants with resulting volatile etch products. The values of A have been shown as significantly larger in ion-enhanced chemical etching than in purely physical sputtering [12–14]. This characteristic has been attributed to the virtually zero binding energy of volatile etch products [12]. A least squares fit of equation (i) to the data in Figs. 3 and 4 has also shown a linear dependence of 1=2 Y(E) on Ei . As an indication of the quality of fit of the data to the equation, the magnitude of r2 (the square of correlation coefficient) was calculated from each plot. For the implanted samples, the value of r2 was in the range 0.91–0.99, indicating a good quality of fit. A summary of the values of the slope constant, A, and the etch rate at Ei ¼ 1000 eV has been given in Table 1. The constant, A, was essentially independent of the treatment by implantation, the ion species and dose. This result has indicated that the same mechanism of etching has applied in implanted and unimplanted samples. However, the etch rate in Ar gas for both Au and O implants increased with the magnitude of dose. In comparison, etching of the diamond in Ar/ O2 gases produced a similar trend in etch rate versus Ei although at a higher rate than in Ar. The etch rate versus implant dose in Ar or Ar/O2 gases has been plotted in Fig. 5. During etching in each
16
Etch Rate (nm/min)
14
Table 1 Values of the slope constant, A, and etch rate of CVD diamond in Ar gas
12 10
A (nm/(min V1/2))
Etch rate (nm/min) at Ei ¼ 1000 eV
11.0
10.7
Au implant (ions/cm ) 51013 51014 51015
10.8 11.3 10.8
11.9 13.3 16.3
O implant (ions/cm2) 51013 51014 51015
10.7 12.0 10.8
12.8 13.7 16.3
Implanted sample/dose
8 6
5E15 Au/cm2
Unimplanted
5E14 Au/cm2
4
2
5E13 Au/cm2
2
Unimplanted
0 0.6
0.7
0.8
0.9
1
1.1
1 /2
Ion Energy (eV) 1=2
Fig. 4. Ar ion etch rate vs. Ei
for Au implanted samples.
306
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307
25
Etch Rate (nm/min)
20
15
10 Au implant - Ar/O2 IBE O implant - Ar/O2 IBE
5
Au implant - Ar IBE O2 implant - Ar IBE
0 1E+13
1E+14
1E+15
1E+16 2
Implant Dose (ions/cm ) Fig. 5. Etch rate of diamond at Ei ¼ 1000 eV as a function of prior dose of O or Au implants using either Ar (3.25 sccm) or Ar (3.25 sccm)/O2 (1 sccm) gases.
of these gas environments, the etch rate steadily increased with dose of the implants, the greatest rise being evident between 5 1014 and 5 1015 ions/ cm2. The etch rate in Ar gas for the O and the Au implanted samples was similar at a given dose. However, in Ar/O2 gases, the Au implants produced a slightly higher etch rate than the O implanted surfaces. At the highest dose (5 1015 ions/cm2), the etch rate of implanted samples in Ar/O2 gas increased more rapidly than in pure Ar. The Ar ion beam etching of diamond has been previously identified as a process of physical sputtering [14]. In the present results, the slope constant, A, as summarized in Table 1 was independent of the implant conditions, indicating that both implanted and unimplanted samples were etched by the same sputter process. In addition, the dependence of etch rate on dose was the same for the Au and O implants (Fig. 5), both, in this study and for the Ge and C implanted surfaces reported previously [3]. The Au ions induced a greater level of damage (1.5–2.5 C vacancies/A/ion) and a lower threshold dose to amorphization than O ions (0.15–0.30 C vacancies/A/ion). Evidently, for IBE in Ar gas, the effect of mass of the implanting ion was not a significant factor in determining etch rate despite the Au implants producing an amorphization
of the structure. However, the trend of increase in etch rate with dose (Fig. 5) has suggested that the nondiamond forms of carbon resulting from ion implantation were more easily removed by sputter etching than the sp3 diamond. The results have indicated that the rate of sputter etching of the samples was dependent on the proportion of non-diamond carbon as a function of implant dose rather than the implant species. The etching of diamond in Ar/O2 gases has been identified as a process of ion-enhanced chemical etching [13]. The higher etch rate in Ar/O2 than in Ar gas has been attributed to the formation of the volatile etch products of CO and CO2. During IBE in Ar/O2 gases, the higher etch rate of Au implanted surfaces than the O implants has indicated that the increasing degree of damage obtained using the heavier ions acted to further enhance the reactivity of the surface compared with sp3 diamond. In this respect, the highest etch rates correlated with the amorphization of the diamond for the Au implants at 5 1015 ions/cm2.
4. Conclusions The implantation of Au or O ions into CVD diamond has resulted in an increasing proportion of nondiamond or amorphous carbon with dose (5 1013 to 5 1015 ions/cm2). The implantation with Au ions at a dose of 5 1015 ions/cm2 resulted in an amorphous structure. An increasing proportion of non-diamond forms of carbon with dose has resulted in an enhanced IBE in either Ar or Ar/O2 gases. The highest etch rates were obtained for Au implanted surfaces at a dose of 5 1015 ions/cm2 using Ar/O2 gases.
References [1] A. P. Malshe, B.S. Park, W.D. Brown, H.A. Naseem, Diamond Rel. Mater. 8 (1999) 1198. [2] D.F. Grogan, T. Zhao, B.G. Bovard, H.A. Macleod, Appl. Opt. 31 (10) (1992) 1483. [3] P.W. Leech, G.K. Reeves, A.S. Holland, M.C. Ridgway, F. Shanks, Diamond Relat. Mater. 11 (2002) 837–840. [4] J.F. Prins, Radiat. Effects Lett. 76 (1983) 79. [5] B.A. Fox, in: A. Elshabini-Riad, F.D. Barlow III (Eds.), Thin Film Technology Handbook, vol. 7, McGraw-Hill, New York, 1997. [6] Z. Sun, J.R. Shi, B.K. Tay, S.P. Lau, Diamond Relat. Mater. 9 (2000) 1979.
P.W. Leech et al. / Applied Surface Science 221 (2004) 302–307 [7] I.P. Hayward, K.J. Baldwin, D.M. Hunter, D.N. Batchelder, G.D. Pitt, Diamond Relat. Mater. 4 (1995) 617. [8] S. Sato, M. Iwaki, Nuclear Inst. Methods Phys. Res. B32 (1988) 145. [9] S. Prawer, A. Hoffman, R. Kalish, Appl. Phys. Lett. 57 (21) (1990) 2187. [10] N. Jiang, M. Deguchi, C.L. Wang, J.H. Won, H.M. Jeon, Y. Mori, A. Hatta, M. Kitabatake, T. Ito, T. Hirao, T. Sasaki, A. Hiraki, Appl. Surf. Sci. 113–114 (1997) 254.
307
[11] C. Uzan-Saguy, C. Cytermann, R. Brener, V. Richter, M. Shaanan, R. Kalish, Appl. Phys. Lett. 67 (9) (1995) 1194. [12] C. Steinbruchel, Appl. Phys. Lett. 55 (19) (1989) 1960. [13] C. Steinbruchel, in: Proceedings of the MRS Symposium on Laser and Particle Beam Chemical Processes on Surfaces, vol. 129, Pittsburgh, PA, 1989, p. 477. [14] P.W. Leech, G.K. Reeves, A.S. Holland, F. Shanks, Diamond Relat. Mater. 11 (2002) 833.