Effect of prior C, Si and Sn implantation on the etch rate of CVD diamond

Diamond & Related Materials 15 (2006) 1266 – 1270 www.elsevier.com/locate/diamond

Effect of prior C, Si and Sn implantation on the etch rate of CVD diamond P.W. Leech a,*, T. Perova b, R.A. Moore b, G.K. Reeves c, A.S. Holland c, M. Ridgway d b

a CSIRO Manufacturing and Infrastructure Technology, Clayton, Victoria, Australia Department of Electronic and Electrical Engineering, University of Dublin, Trinity College, Ireland c RMIT University, School of Computer Systems and Elect. Eng., Melbourne, Australia d Department of Electronic Materials Engineering, ANU, Canberra, Australia

Received 22 March 2005; received in revised form 10 August 2005; accepted 28 September 2005 Available online 2 November 2005

Abstract Diamond films were implanted with C+, Si+ or Sn+ ions at multiple energies in order to generate a uniform layer of implantation-induced disorder. The implant energies of 60, 180, 330 and 525 keV for C+ ions, 200, 500 and 950 keV for Si+ ions and 750 and 2000 keV for Sn+ ions were selected to give an approximately constant vacancy concentration at depths over the range ¨ 0 – 0.5 Am. An analysis of the C+ implanted surfaces by Raman spectroscopy has shown an increase in non-diamond or sp2-bonded carbon at doses in the range 5  1013 to 5  1015 cm 2. In comparison, a completely non-diamond structure was evident after implantation with either Si+ ions at a dose of 5  1015 ions/cm2 or Sn+ ions at  5  1014 cm 2. For a given dose, the etch rate of the diamond film was shown to increase with the mass of the implanted species in the order of C+, Si+ and Sn+. For a given implant species, the etch rate increased with the implant dose and the ion-induced vacancy concentration. The etch rate of the implanted diamond in various gases decreased in the order of O2, CF4/O2 and CHF3/O2 plasmas. D 2005 Elsevier B.V. All rights reserved. Keywords: Ion implantation; Reactive ion etch; Diamond

1. Introduction The extreme electrochemical stability and chemical inertness of diamond of have resulted in its recent application in biosensor devices [1] and microelectrode arrays [2]. But a key requirement in the fabrication of many biosensor devices in diamond has remained the ability to define fine-scale structures. While the etching of diamond by wet methods has proven extremely difficult, the fabrication of patterned surfaces has been achieved by selective growth [3] or by irradiation with plasma or ion beams [4]. One of the most widely applied methods in the patterning of diamond has been reactive ion etching in an O2 plasma. But while oxygen RIE has enabled the definition of fine structures in diamond, the etching has been characterised by surface roughening. The use of alternative fluorine-based gases such as SF6 [5] or CF4

[6] during reactive ion etching has produced significantly smoother surfaces than with O2 plasmas with the limitation of a lower etch rate. In this paper, we have examined for the first time the use of ion implant-induced damage of diamond as a means of increasing the rate of subsequent reactive ion etching. The etch rate of the films has been measured as a function of mass of the implant species (C+, Si+ or Sn+) and ion dose. In addition, the etch rate of the diamond has been measured as a function of the bias voltage in the RIE system in order to evaluate the effect of ion implantation in relation to the etch mechanism. The range of etching gases (CF4/O2, CHF3/O2 or O2) was selected in order to allow a comparison of fluorocarbon gases with oxygen. In the CF4/O2 and CHF3/O2 gas mixtures, the small addition of O2 was intended to reduce the build up of fluorocarbon residues during reactive ion etching. 2. Experimental

* Corresponding author. Private Bag 33, Clayton South MDC, 3169, Victoria, Australia. Tel.: +61 3 9545 2791; fax: +61 3 9545 2844. E-mail address: [email protected] (P.W. Leech). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.09.034

The diamond films used in these experiments were grown by microwave plasma enhanced CVD on 76 mm wafers of

(111) oriented Si. The polycrystalline films were undoped with a thickness of ¨ 15 Am as supplied by Sumitomo, Japan. A series of host samples of diamond film with dimensions of ¨ 1 1 cm were mounted on the implantation holder using silver paste and then implanted with either C+, Si+ or Sn+ ions. Each of the ion species was implanted separately at doses in the range 5  1013 to 5  1015 cm 2. The holder was cooled directly with liquid nitrogen in order to maintain the samples at  196 -C during implantation. Using this technique of cooling, the level of structural disorder was maximised in the diamond. For C+ ions, a series of sequential energies (60, 180, 330 and 525 keV) were used in order to create an approximately uniform profile of implant-induced C vacancies. The implants of Si+ ions were similarly performed at sequential energies of 200, 500 and 950 keV while Sn+ ions were implanted at 750 and 2000 keV. The implanted surfaces were then analysed with a Renishaw 100 micro-Raman system equipped with an Ar+ ion laser for excitation at 488 nm. This excitation wavelength was used in order to improve the signal from the sp 3 bonding [7,8]. The laser radiation was focused onto the sample using a 50 microscope objective which also acted to collect the scattered light from the surface. The conformal depth of sampling of the spectrometer was ¨ 2 Am. The micro-Raman spectra (400 –1800 cm 1) were acquired from the as-grown and implanted surfaces. The subsequent reactive ion etching of the surfaces was performed in a commercial parallel plate STS320 system with rf powering at 13.56 MHz. In this system, the cathode was continuously cooled to 20 -C during etching. The three different compositions of gases used in the experiments were (i) CF4 (28 sccm)/O2 (2 sccm), (ii) CHF3 (28 sccm)/O2 (2 sccm) and (iii) O2 (30 sccm). The etch rate was measured as a function of rf power (200 – 400 W) and hence bias voltage at a chamber pressure of 50 mTorr. The etch time was constant for each experiment at 10 min. After etching, the resist mask

Intensity (A.U.)

P.W. Leech et al. / Diamond & Related Materials 15 (2006) 1266 – 1270

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1

2 3 4

400

600

800

1000

1200

1400

1600

1800

Raman shift, cm-1 Fig. 2. Raman spectra of unimplanted diamond (curve 1) and after implantation at 5  1013 Si+/cm2 (curve 2), 5  1014 Si+/cm2 (curve 3) and 5  1015 Si+/cm2 (curve 4).

was removed in acetone and the etch depth was measured by stylus profilometry. 3. Results and discussion 3.1. Implantation profiles The TRIM (Transport of Radiation in Matter) computer code was used to simulate the profile of vacancy concentration induced by implants with the 3 different ions. Based on these profiles, the implant parameters (ion energy and dose) were selected such that an approximately constant concentration of vacancies was produced at depths over the range ¨ 0– 0.5 Am. In the case of Sn+ ions, a significantly higher energy (750 – 2000 keV) was required in order to achieve this depth distribution than for C+ ions (60 – 525 keV). Fig. 1 has shown the cumulative distribution curves of C vacancies versus depth for each species. For the C+, Si+ and Sn+ implants, the average value of the cumulative vacancy concentration was ¨ 0.15, 0.4 and 1.75 vacancies/cm3, respectively. 3.2. Raman spectra

Fig. 1. Vacancy concentration versus depth for diamond implanted with C+, Si+ and Sn+ ions. The plots show the sum of the implants at energies of 60, 180, 330 and 525 keV for C+, 200, 500 and 950 keV for Si+ and 750 and 2000 keV for Sn+ ions.

Fig. 2 has shown typical Raman spectra for the as-deposited and Si+ implanted diamond in the spectral range 400– 1800 cm 1. For the unimplanted samples, the Raman spectrum was dominated by the sp 3 diamond peak at ¨ 1332 cm 1. Also evident was a broad band at 1400 –1600 cm 1 with centre at ¨ 1500 cm 1 as previously attributed to diamond-like carbon (DLC) [8,9]. In films grown by CVD, the non-diamond signal has typically been localised at grain boundaries [9]. The magnitude of the 1400 –1600 cm 1 signal in the unimplanted samples has indicated that the films in this study contained a significant presence of non-diamond material. However, Raman line-mapping of cross-sections of the film has shown a uniform quality with only a small deviation in sp2 bonded carbon Raman signal detected close (¨ 20– 100 nm) to the interface and to the surface.

P.W. Leech et al. / Diamond & Related Materials 15 (2006) 1266 – 1270

The C+ implanted samples exhibited an sp 3 diamond peak at all implant doses although there was a progressive decrease in the intensity with dose. For the Si+ implants, the spectra in Fig. 2 have also shown a decrease in height of the sp3 peak with dose. At the highest dose examined of 5  1015 Si+/cm2, there was no sign of an sp 3 peak, instead exhibiting a broad Raman band at ¨ 1550 cm 1. For the Sn+ implants, an sp3 peak was only barely evident at  5  1014 Si+/cm2 and was absent at 5  1015 Si+/cm2. A broad peak at 1550 cm 1 has previously been reported following the implantation of diamond with Ar+ (2  1016 cm 2) [10] and B+ (1 1016 cm 2) [11]. The peak was attributed to the formation of non-diamond carbon as either amorphous [10] or graphitic carbon [11]. In previous studies, the onset of a fully amorphous or non-diamond structure has been reported above a critical level of implant dose [12,13]. The critical dose, Dc, decreased with the mass of the ion [12,13]. In the present results, the dose required for the absence of the sp 3 peak was 5  1014 cm  2 for Sn + implantation (equivalent to 6.8  1022 vacancies/cm3 as calculated from TRIM) and 5  1015 cm 2 for Si+ implantation (equivalent to 1.1 1023 vacancies/cm3). The small peak located at 520 cm 1 in many of the spectra has been attributed to the Si substrate. For the experimental setup of the Raman microscope used in this work, the laser waist was ¨ 3Am. Although the beam was focused on the surface of the transparent film, some defocused laser light has evidently caused excitation in the Si substrate. The reduction in this Raman signal at 520 cm 1 at the highest dose of 5  1015 ions/ cm2 in Fig. 2 was consistent with an increase in the adsorption coefficient and hence a shallower beam penetration into the film with increasing damage. 3.3. Measurements of etch rate In Fig. 3, the etch rate of the implanted diamond films at 350 W has been plotted as a function of the ion-induced 70

45 40 35

Etch Rate (nm/min)

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30 25 20 15

5E15 C 5E14 C

10

5E13 C

5

Unimplanted

0 21

22

23

Volts

24

25

1/2

1/2

Fig. 4. Etch rate versus V for diamond implanted with C+ ions at doses of 5  1013 to 5  1015 cm 2 in CF4/O2 plasma.

vacancy concentration in each of the CF4/O2, O2 and CHF3/O2 plasmas. The vacancy concentration was determined for each combination of implant species and ion implant dose using the TRIM code. The calculation was based on the gross, athermal concentration of vacancies without any component of thermal recombination. In each plot in Fig. 3, the etch rate has shown a continuous increase with the gross, athermal vacancy concentration. Each plot was comprised of different specimens with the 3 different doses for the C+, Si+ and Sn+ implants. Hence, the trend in etch rate in Fig. 3 has shown a direct dependence on the induced level of vacancy concentration as influenced by the increase in implant dose or the ion mass (12C, 28Si and 120Sn). At any given level of induced vacancy concentration, the etch rate was also dependent on the gas mixture. Fig. 3 has shown that the highest etch rates were obtained for O2 plasmas and the lowest level for CHF3/O2 gases. The CF4/O2 plasmas produced an etch rate slightly lower than O2 although the slope 60

50

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Etch Rate (nm/min)

Etch Rate (nm/min)

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O2 CF4/O2 CHF3/O2

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5E15 Si 5E14 Si 5E13 Si

10

Unimplanted

0 1.00E+20 1.00E+21 1.00E+22 1.00E+23 1.00E+24 3

Vacancy Concentration (Vacancies/cm )

Fig. 3. Etch rate versus vacancy concentration for diamond implanted with C+, Si+ or Sn+ ions at 5  1013 to 5  1015 cm 2 in CHF3/O2, CF4/O2 and O2 plasma at an rf power of 350 W.

0 21

22

23

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24

25

1/2

Fig. 5. Etch rate versus V for diamond implanted with Si+ ions at doses of 5  1013 to 5  1015 cm 2 in CF4/O2 plasma.

P.W. Leech et al. / Diamond & Related Materials 15 (2006) 1266 – 1270 60

Etch Rate (nm/min)

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20

5E15 Sn 5E14 Sn 10

5E13 Sn Unimplanted

0 21

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24

25

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1/2

Fig. 6. Etch rate versus V for diamond implanted with Sn+ ions at doses of 5  1013 to 5  1015 cm 2 in CF4/O2 plasma.

of etch rate versus vacancy concentration was similar to the CHF3/O2 gases. The similarity in etch rate in the CF4/O2 and O2 plasmas occurred despite a difference in the energetic reactive ions in each case (C and CFn+ in CF4 plasmas [14] and dissociated oxygen radicals in the O2 plasma [15]). The etch rate of the unimplanted and implanted diamond films in CF4/O2 plasmas increased steadily with rf power in the range 200 to 400 W. The results have been plotted in Figs. 4 –6 as a function of the square root of the bias voltage, V b, induced at the cathode by the application of rf power. The reactive ion etching of a range of dielectrics and metals has been previously shown to occur by basic processes of either physical sputtering or ion-enhanced chemical etching. In both types of process, the number of atoms removed per incoming ion, Y(E) was described by the expression [16]:   1=2 1=2 Y ð E Þ ¼ A Ei  Eth ð1Þ where E i = ion energy and was the slope and threshold energy constants, respectively. These constants (A and E th) were dependent on the projectile – target combination. The magnitude of E i has been shown as directly proportional to V b [17]. Eq. (1) was based on the generalised relation for physical sputtering:
ð2Þ Y ð E Þ ¼ Cpt Sn E=Ept in which C pt was a constant depending on the projectile, p, and target, t. This constant C pt provided a description of the energy transport within the collision cascade and was inversely proportional to the surface binding energy. The function S n(E/ E pt ) defined the nuclear stopping power. An accurate expression for the dependence of S n(E/E pt ) a E 1/2 and the inclusion of a threshold energy, E th, have been combined in Eq. (1) [16]. Eq. (1) was established at relatively low ion energies [16]. However, the etch rate of SiO2 in CF4 and CHF3 plasmas has been shown as directly dependent on ion density and energy over a wide range of rf power and bias voltages, V b [17].

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A least squares fit of Eq. (1) to the data in Figs. 4 –6 has also shown a linear dependence of the etch rate on V b1/2 for both the unimplanted and implanted samples. As an indication of the quality of fit of the data to this equation, the value of r 2 (the square of the correlation coefficient) was determined in each plot. The magnitude of r 2 for these series of data was predominantly in the range 0.92– 0.99 indicating a good fit. The exception was the diamond implanted with 5  1015 Sn+/ cm2, for which r 2 = 0.88. In addition, the values of A have been calculated from the trendlines in Figs. 4 –6 and summarised in Table 1. In the results in Table 1, the relatively high values of the slope constant (A = 4.0 – 6.6) and of etch rate (28.6 – 52.5 nm/min) have indicated a process of ion-enhanced chemical etching rather than physical sputter etching. The sputter etching of diamond has been previously characterised by relatively low values of these parameters (A < 1.0 nm/min/V 1/2 and an etch rate of < 10 nm/ min) [6]. The reactive ion etching of diamond in CF4/O2 plasmas has previously been identified as a process of ion-enhanced chemical etching [6]. In this process, the main chemical reactants of C and CFn+ have been reported to adhere to the diamond, leading to the formation of highly volatile etch products [14]. The values of the parameters A and etch rate in Table 1 have been shown to increase with the implant dose for each ion species. These parameters also increased with the mass of the implanting ion (12C, 28Si and 120Sn) and the implantationinduced disorder. At the highest dose of 5  1015 cm 2, the etch rate at 400 W power was shown to increase in the order of C+ (38.5 nm/min), Si+ (47.5 nm/min) and Sn+ (52.5 nm/min). In comparison, the etch rate of unimplanted diamond was ¨ 27.1 nm/min. This compared with an etch rate of 23.3 nm/min previously measured for unimplanted diamond of standard crystalline quality [18]. An increase in the base etch rate of ¨ 4 nm/min has resulted from the lower quality of the diamond in the present experiments. The trends in etch rate have been interpreted in terms of the variation in implant-induced vacancy concentration. For low temperature implants such as used in this study, the collision cascades of ions have been shown to predominantly comprise point defects [19]. Prins has reported that the damage induced in diamond by low temperature implantation with a wide variety of ions including massive ions such as As+ and In+ consisted primarily of vacancies and interstitial atoms [19]. An increase in Table 1 Parameter values from a least squares fit of Eq. (1) for etching of diamond in a CF4/O2 plasma Diamond sample

A (nm/min/V 1/2)

Etch rate at 400 W (nm/min)

r2

Unimplanted 5E13 C/cm2 5E14 C/cm2 5E15 C/cm2 5E13 Si/cm2 5E14 Si/cm2 5E15 Si/cm2 5E13 Sn/cm2 5E14 Sn/cm2 5E15 Sn/cm2

4.40 4.00 4.66 5.31 4.84 6.03 6.22 4.99 5.60 6.61

27.1 29.6 38.4 47.5 28.6 33.3 38.5 31.3 43.1 52.5

0.95 0.99 0.99 0.92 0.99 0.99 0.92 0.88 0.92 0.94

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implant dose has been shown to increase the concentration of defects within the individual cascades [12,13]. In the present results, a higher etch rate has correlated with an increase in vacancy concentration in the diamond either by means of a larger ion dose or a heavier ion mass. An increase in vacancy concentration in the diamond resulted in an increased reaction rate at the substrate surface and the enhanced formation of volatile products. The dependence of etch rate on vacancy concentration in these results was not specifically influenced by whether the structure contained sp 3 diamond or was a fully nondiamond phase. The results in Table 1 have also shown that the slope constant, A, increased with both implant dose and ion mass. Evidently, the formation of higher defect concentrations has resulted in an increased dependence of the reaction rate on V b1/2. For a given value of V b1/2, and hence level of energy distribution of the ions in the plasma, a higher value of A has indicated a greater ease of initiation of the reaction. It has been assumed in these experiments that the etch rate was approximately constant within the disordered region. An approximately constant level of vacancy concentration versus depth was calculated in the TRIM profiles (Fig. 1). Also, previous experiments in the O2+ implantation of LiNbO3 have demonstrated that the depth profile of the ion induced damage obtained by TRIM closely correlated with the reactive ion etch rate [20]. 4. Conclusions The implantation of CVD diamond with C+, Si+ and Sn+ ions at doses of 5  1013 to 5  1015 cm 2 has resulted in an increase in etch rate with ion-induced vacancy concentration. During etching in a CF4/O2 plasma, a process of ion-enhanced chemical etching has been identified for both the unimplanted and implanted diamond. For each of the implant species, the etch rate of the diamond film was shown to increase with the implant dose. At a given dose, the etch rate increased with the mass of the implanted species in the order of C+, Si+ and Sn+. The etch rate of the implanted CVD diamond was significantly higher in the CF4/O2, CF4/CHF3, and O2 plasmas than in the CHF3/O2 plasma.

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