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Electric field and tip–surface interaction dependence in nanopattern deposition by electropulsed scanning probe microscopy
Materials Science and Engineering C 25 (2005) 756 – 760 www.elsevier.com/locate/msec
Electric field and tip–surface interaction dependence in nanopattern deposition by electropulsed scanning probe microscopy L.V. Melo a,b,*, F. Delgado a, P. Brogueira a,b a
Physics Department, Instituto Superior Te´cnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal b ICEMS, Instituto Superior Te´cnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal Available online 15 August 2005
Abstract The reduced dimensions of novel integrated devices and systems stresses the need for new nanopatterning techniques. We showed that metallic individual pixels, pixilated patterns and smooth patterns can be obtained by applying negative voltage pulses to a tip coated with a CoCr metallic film while scanning in tapping mode AFM. Electropulsed AFM deposition at different negative pulse voltages and frequencies, and different tip – surface interaction conditions is reported in this work. Deposits with pixel diameter between 21 and 34 nm and height between and 2 – 3 nm were obtained by applying 12 V pulses to the tip. When the pulse voltage goes to 16.8 V the pixel diameter increases to 90 nm and height exceeds 4.0 nm. At 10 Hz pulse frequencies the patterns obtained are pixilated, resulting in high surface roughness (Ra = 0.44 nm). At 20 Hz the deposited pixels are fully merged, resulting in flat surfaces with low roughness (Ra = 0.19 nm). For higher pulse frequencies up to 60 Hz the roughness remains between Ra = 0.17 and Ra = 0.22 nm. The deposited thickness increases from values around 1.0 nm at 7 V pulse voltage to values around 4 nm at 16.5 V. By properly tuning the amplitude setpoint for a new tip a similar deposited thickness dependence can be obtained. The deposited thickness increases with the tip – surface interaction. D 2005 Elsevier B.V. All rights reserved. Keywords: AFM; SPM; Nanodeposition
1. Introduction The development of increasingly small electronic and magnetomechanic devices stresses the need for new nanometer-size fabrication techniques. Different SPM-based nanofabrication techniques have been reported [1– 4]. The fabrication of nanosized metal films in a controlled manner using a commercially available AFM by applying negative voltage pulses to the tip was also reported [5,6]. The characteristics of the deposited nanopatterns are expected to depend on deposition parameters. The dependence on the line density and tip scan velocity has already been reported [5]. In this paper, we study the dependence of the thickness,
* Corresponding author. Physics Department, Instituto Superior Te´cnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal. Tel.: +351 218417775; fax: +351 218419013. E-mail address: [email protected] (L.V. Melo). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.045
roughness, pixel size of the deposited patterns on pulse frequency, pulse voltage and tip/surface interaction.
2. Experimental A D3100 Scanning Probe Microscope (SPM) from Digital Instruments was used for the nanofabrication and characterization. The deposition was performed at atmosphere by applying negative voltage pulses ( 7 to 16.8 V) between a commercially available CoCr-coated Si tip (MESP) and a <111> n-doped Si substrate while scanning. The voltage pulses were applied to the tip through a Signal Access Module (DI) from an HP3311A function generator. Square wave signals with frequencies between 5 and 60 Hz were used. The SPM was operated in Tapping Mode TM Atomic Force Microscopy (AFM) during deposition with scanning rates below 2 Hz. The detected free tip amplitude was tuned at 1.0 V. Depositions were performed at different scan line
L.V. Melo et al. / Materials Science and Engineering C 25 (2005) 756 – 760
densities from 0.32 to 1.28 nm 1. Different tip/surface interaction conditions were used, with amplitude setpoints from 0.30 to 0.50 V (these values correspond to 30% and 50% of the free amplitude setpoint, respectively). The Si wafer was connected to ground through a specially designed holder that was electrically insulated from the D3100 chuck. Tapping Modei AFM was used for characterization. The AFM topography measurements were performed immediately after deposition. The characteristic parameters of the deposition were obtained from the topography measurements, namely pixel size, thickness and roughness.
757
voltages and frequencies. Fig. 1a) shows the deposition of isolated pixels performed by applying 12 V and 80 ms wide pulses with a frequency of 0.7 Hz (T = 3.4 s) between the tip and the substrate while scanning at 1 Hz (200-nm/s tip velocity). Several pixels can be observed. Each individual pixel corresponds to one isolated electrical pulse. Pixel diameter between 21 and 34 nm and height between and 2 – 3 nm were measured [6]. Fig. 1b) shows 3D topography AFM images of two square deposits obtained from two consecutive 100 100 nm2 scans with 12 V pulses applied to the tip. The depositions were performed at 200 nm/s tip velocity and 1.28 nm 1 scan line density. The left deposit was performed at a 5 Hz pulse frequency and the right deposit at a 10 Hz pulse frequency. In the left deposit the individual pixels can be clearly observed. This can be understood as follows: at this tip velocity and at 5 Hz pulse frequency an average of 2.5 pulses (pixels) are expected per
3. Results and discussion Fig. 1 shows 3D topography AFM images and height cross-sections of deposits performed at two different pulse
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Fig. 1. 3D topography AFM images and height cross-sections of deposits performed at two different pulse voltages and frequencies: a) deposition of isolated pixels performed by applying 12 V and 80 ms wide pulses with a frequency of 0.7 Hz (T = 3.4 s) between the tip and the substrate while scanning at 1 Hz (200 nm/s tip velocity); b) 100 100 nm2 depositions performed by applying 12 V pulses to the tip at 200 nm/s tip velocity and 1.28 nm 1 scan line density. Left deposit: 5 Hz pulse frequency; Right deposit: 10 Hz pulse frequency; c) and d) two square deposits obtained from 400 400 nm2 scans with 16 V pulses applied to the tip. The depositions were performed at 1600 nm/s tip velocity and 0.32 nm 1 scan line density at 10 Hz pulse frequency (c) and 40 Hz pulse frequency (d); e) and f) show the respective height cross-sections of the patterns along the lines marked on the 3D topography images.
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L.V. Melo et al. / Materials Science and Engineering C 25 (2005) 756 – 760
Fig. 2. Surface roughness vs. pulse frequency plot. All the deposits were fabricated from 400 400 nm2 scans with deposited thickness vs. pulse frequency for the same deposits.
line (some lines will have 2 and others 3). For a pixel size of 20 to 30 nm it is expected that individual pixels can be clearly identified within a 100 nm line. Both deposits are 117 117 nm2 and the average film thickness measured from different sample cross-sections is 3.0 nm for both squares. These results agree with the lateral dimensions expected from the 100 nm scan size and the 28 nm average pixel size determined from Fig. 1a). The roughness of the deposits was found to be Ra = 0.30 nm (left deposit) and Ra = 0.18 nm (right deposit). Fig. 1c) and d) show 3D topography AFM images of two square deposits obtained from 400 400 nm2 scans with 16V pulses applied to the tip. The depositions were performed at 1600 nm/s tip
16 V pulses applied to the tip. Inset:
velocity and 0.32 nm 1 scan line density. The deposit in Fig. 1c) was performed at a 10 Hz pulse frequency and the deposit in Fig. 1d) at a 40 Hz pulse frequency. Fig. 1e) and f) show the respective height cross-sections of the patterns along the lines marked on the 3D topography images. In the deposit performed at 10 Hz individual pixels can be observed. A 90 nm size can be inferred taking into account the superimposition of an average of 2.5 pixels per line along the x-direction (at 1600 nm/s tip velocity and 10 Hz pulse frequency, 2.5 pulses are applied in average during a 400 nm scan) and the high scan line density along the ydirection. This pixel size for the 16 V pulse voltage is in agreement with the lower limit of the lateral pattern
Fig. 3. Deposited thickness vs. pulse voltage (in absolute value) plot. The solid and open circles correspond to two different sets of deposits performed at 35 and 40 Hz pulse frequencies and 0.5 and 0.3 V setpoint amplitudes, respectively. The free amplitude setpoint was 1.0 V for both cases. A different tip was used for each set of deposits.
L.V. Melo et al. / Materials Science and Engineering C 25 (2005) 756 – 760
dimensions of 445 nm (notice the incomplete edge definition of the pattern due to discrete pixel number in a line). Due to the rough surface of this deposit the height scatters between 2 and 4.5 nm. The roughness value measured on this deposit is Ra = 0.44 nm. In the deposit performed at a 40 Hz pulse frequency the measured roughness value is Ra = 0.21 nm corresponding to the observed flat surface. The pattern width is 492 nm (as expected for non-pixilated deposit) and the height is 6.0 nm. Fig. 2 shows a roughness vs. pulse frequency plot. All the deposits were fabricated from 400 400 nm2 scans with 16 V pulses applied to the tip. The depositions were performed at 1600 nm/s tip velocity and 0.32 nm 1 scan line density. The measured roughness is Ra = 0.44 nm for the pattern deposited at 10 Hz pulse frequency, and then drops dramatically to Ra = 0.19 nm at 20 Hz, when the deposited pixels are fully merged. Once this happens the roughness does not vary as much, remaining between Ra = 0.17 and Ra = 0.22 nm for higher frequencies. The inset shows the thickness vs. pulse frequency for the same deposits. An average value of 4.1 nm was measured for the 10 Hz pixilated pattern, varying between 2.0 and 4.8 nm within the pattern as indicated by the vertical line on the inset. The values seem to follow approximately the same trend as the roughness values for the other frequencies. A maximum of 6.4 nm is obtained at f = 40 Hz. Fig. 3 shows a thickness vs. pulse voltage (in absolute value) plot. The solid and open circles correspond to two different sets of deposits performed at 35 and 40 Hz pulse frequencies and 0.5 and 0.3 V setpoint amplitudes, respectively. The free amplitude setpoint was 1.0 V for both cases. A different tip was also used for each set of deposits. The deposited thickness grows steadily from values around 1.0 nm at 7 V pulse voltage to values around 4 nm at 16.5 V. Notice that a specific tip/amplitude
759
setpoint combination can be tuned in order to obtain the same deposition behavior. Fig. 4 shows a deposited thickness vs. amplitude setpoint plot. The amplitude setpoint is a measure of the magnitude of the tip/surface interaction. The deposited thickness increases from around 4.5 nm to around 6.0 nm when the amplitude setpoint is reduced from 0.46 to 0.44 V (the tip –surface interaction increases). The inset shows the corresponding roughness vs. amplitude setpoint plot. The experimental data scatter around an horizontal line at Ra = 0.30 nm from 0.20 to 0.30 nm, reaching 0.50 nm in some cases.
4. Conclusions Deposits with pixel diameter between 21 and 34 nm and height between and 2 – 3 nm were obtained by applying 12 V pulses to the tip. When the pulse voltage goes to 16.8 V the pixel diameter increases to 90 nm and height exceeds 4.0 nm. At lower pulse frequencies the patterns obtained are pixilated, resulting in high surface roughness (Ra = 0.44 nm). At 20 Hz the deposited pixels are fully merged, resulting in smooth surfaces with low roughness (Ra = 0.19 nm), remaining in close values for higher pulse frequencies. We also show that the deposited thickness increases with the absolute value of the pulse voltage. Similar deposit characteristics are obtained with different tips by properly tuning the amplitude setpoint. We observed that the deposited thickness increases when the tip –surface interaction increases.
Acknowledgements This work was supported by ‘‘Fundac¸a˜o para a Cieˆncia e Tecnologia’’ through a Pluriannual Contract with ICEMS
Fig. 4. Deposited thickness vs. amplitude setpoint plot. Inset: corresponding roughness vs. amplitude setpoint plot.
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L.V. Melo et al. / Materials Science and Engineering C 25 (2005) 756 – 760
(IST) and by projects POCTI/CTM/38330/01 and POCTI/ FIS/33362/99.
References [1] D.D. Chambliss, R.J. Wilson, S. Chiang, J. Vac. Sci. Technol. B 9 (1991) 933.
[2] K. Bessho, S. Hashimoto, Appl. Phys. Lett. 65 (1994) 2142. [3] K. Bessho, Y. Iwasaki, S. Hashimoto, J. Appl. Phys. 79 (1996) 5057. [4] T. Sato, C. Kaneshiro, H. Okada, H. Hasegawa, Jpn. J. Appl. Phys. Part 1 38 (1999) 2448. [5] P. Brogueira, L.V. Melo, Mater. Sci. Eng., C, Bionim. Mater., Sens. Syst. 23 (2003) 77. [6] L.V. Melo, P. Brogueira, Mater. Sci. Eng. C, Bionim. Mater., Sens. Syst. 23 (2003) 263.
Electric field and tip–surface interaction dependence in nanopattern deposition by electropulsed scanning probe microscopy L.V. Melo a,b,*, F. Delgado a, P. Brogueira a,b a
Physics Department, Instituto Superior Te´cnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal b ICEMS, Instituto Superior Te´cnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal Available online 15 August 2005
Abstract The reduced dimensions of novel integrated devices and systems stresses the need for new nanopatterning techniques. We showed that metallic individual pixels, pixilated patterns and smooth patterns can be obtained by applying negative voltage pulses to a tip coated with a CoCr metallic film while scanning in tapping mode AFM. Electropulsed AFM deposition at different negative pulse voltages and frequencies, and different tip – surface interaction conditions is reported in this work. Deposits with pixel diameter between 21 and 34 nm and height between and 2 – 3 nm were obtained by applying 12 V pulses to the tip. When the pulse voltage goes to 16.8 V the pixel diameter increases to 90 nm and height exceeds 4.0 nm. At 10 Hz pulse frequencies the patterns obtained are pixilated, resulting in high surface roughness (Ra = 0.44 nm). At 20 Hz the deposited pixels are fully merged, resulting in flat surfaces with low roughness (Ra = 0.19 nm). For higher pulse frequencies up to 60 Hz the roughness remains between Ra = 0.17 and Ra = 0.22 nm. The deposited thickness increases from values around 1.0 nm at 7 V pulse voltage to values around 4 nm at 16.5 V. By properly tuning the amplitude setpoint for a new tip a similar deposited thickness dependence can be obtained. The deposited thickness increases with the tip – surface interaction. D 2005 Elsevier B.V. All rights reserved. Keywords: AFM; SPM; Nanodeposition
1. Introduction The development of increasingly small electronic and magnetomechanic devices stresses the need for new nanometer-size fabrication techniques. Different SPM-based nanofabrication techniques have been reported [1– 4]. The fabrication of nanosized metal films in a controlled manner using a commercially available AFM by applying negative voltage pulses to the tip was also reported [5,6]. The characteristics of the deposited nanopatterns are expected to depend on deposition parameters. The dependence on the line density and tip scan velocity has already been reported [5]. In this paper, we study the dependence of the thickness,
* Corresponding author. Physics Department, Instituto Superior Te´cnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal. Tel.: +351 218417775; fax: +351 218419013. E-mail address: [email protected] (L.V. Melo). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.045
roughness, pixel size of the deposited patterns on pulse frequency, pulse voltage and tip/surface interaction.
2. Experimental A D3100 Scanning Probe Microscope (SPM) from Digital Instruments was used for the nanofabrication and characterization. The deposition was performed at atmosphere by applying negative voltage pulses ( 7 to 16.8 V) between a commercially available CoCr-coated Si tip (MESP) and a <111> n-doped Si substrate while scanning. The voltage pulses were applied to the tip through a Signal Access Module (DI) from an HP3311A function generator. Square wave signals with frequencies between 5 and 60 Hz were used. The SPM was operated in Tapping Mode TM Atomic Force Microscopy (AFM) during deposition with scanning rates below 2 Hz. The detected free tip amplitude was tuned at 1.0 V. Depositions were performed at different scan line
L.V. Melo et al. / Materials Science and Engineering C 25 (2005) 756 – 760
densities from 0.32 to 1.28 nm 1. Different tip/surface interaction conditions were used, with amplitude setpoints from 0.30 to 0.50 V (these values correspond to 30% and 50% of the free amplitude setpoint, respectively). The Si wafer was connected to ground through a specially designed holder that was electrically insulated from the D3100 chuck. Tapping Modei AFM was used for characterization. The AFM topography measurements were performed immediately after deposition. The characteristic parameters of the deposition were obtained from the topography measurements, namely pixel size, thickness and roughness.
757
voltages and frequencies. Fig. 1a) shows the deposition of isolated pixels performed by applying 12 V and 80 ms wide pulses with a frequency of 0.7 Hz (T = 3.4 s) between the tip and the substrate while scanning at 1 Hz (200-nm/s tip velocity). Several pixels can be observed. Each individual pixel corresponds to one isolated electrical pulse. Pixel diameter between 21 and 34 nm and height between and 2 – 3 nm were measured [6]. Fig. 1b) shows 3D topography AFM images of two square deposits obtained from two consecutive 100 100 nm2 scans with 12 V pulses applied to the tip. The depositions were performed at 200 nm/s tip velocity and 1.28 nm 1 scan line density. The left deposit was performed at a 5 Hz pulse frequency and the right deposit at a 10 Hz pulse frequency. In the left deposit the individual pixels can be clearly observed. This can be understood as follows: at this tip velocity and at 5 Hz pulse frequency an average of 2.5 pulses (pixels) are expected per
3. Results and discussion Fig. 1 shows 3D topography AFM images and height cross-sections of deposits performed at two different pulse
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Fig. 1. 3D topography AFM images and height cross-sections of deposits performed at two different pulse voltages and frequencies: a) deposition of isolated pixels performed by applying 12 V and 80 ms wide pulses with a frequency of 0.7 Hz (T = 3.4 s) between the tip and the substrate while scanning at 1 Hz (200 nm/s tip velocity); b) 100 100 nm2 depositions performed by applying 12 V pulses to the tip at 200 nm/s tip velocity and 1.28 nm 1 scan line density. Left deposit: 5 Hz pulse frequency; Right deposit: 10 Hz pulse frequency; c) and d) two square deposits obtained from 400 400 nm2 scans with 16 V pulses applied to the tip. The depositions were performed at 1600 nm/s tip velocity and 0.32 nm 1 scan line density at 10 Hz pulse frequency (c) and 40 Hz pulse frequency (d); e) and f) show the respective height cross-sections of the patterns along the lines marked on the 3D topography images.
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L.V. Melo et al. / Materials Science and Engineering C 25 (2005) 756 – 760
Fig. 2. Surface roughness vs. pulse frequency plot. All the deposits were fabricated from 400 400 nm2 scans with deposited thickness vs. pulse frequency for the same deposits.
line (some lines will have 2 and others 3). For a pixel size of 20 to 30 nm it is expected that individual pixels can be clearly identified within a 100 nm line. Both deposits are 117 117 nm2 and the average film thickness measured from different sample cross-sections is 3.0 nm for both squares. These results agree with the lateral dimensions expected from the 100 nm scan size and the 28 nm average pixel size determined from Fig. 1a). The roughness of the deposits was found to be Ra = 0.30 nm (left deposit) and Ra = 0.18 nm (right deposit). Fig. 1c) and d) show 3D topography AFM images of two square deposits obtained from 400 400 nm2 scans with 16V pulses applied to the tip. The depositions were performed at 1600 nm/s tip
16 V pulses applied to the tip. Inset:
velocity and 0.32 nm 1 scan line density. The deposit in Fig. 1c) was performed at a 10 Hz pulse frequency and the deposit in Fig. 1d) at a 40 Hz pulse frequency. Fig. 1e) and f) show the respective height cross-sections of the patterns along the lines marked on the 3D topography images. In the deposit performed at 10 Hz individual pixels can be observed. A 90 nm size can be inferred taking into account the superimposition of an average of 2.5 pixels per line along the x-direction (at 1600 nm/s tip velocity and 10 Hz pulse frequency, 2.5 pulses are applied in average during a 400 nm scan) and the high scan line density along the ydirection. This pixel size for the 16 V pulse voltage is in agreement with the lower limit of the lateral pattern
Fig. 3. Deposited thickness vs. pulse voltage (in absolute value) plot. The solid and open circles correspond to two different sets of deposits performed at 35 and 40 Hz pulse frequencies and 0.5 and 0.3 V setpoint amplitudes, respectively. The free amplitude setpoint was 1.0 V for both cases. A different tip was used for each set of deposits.
L.V. Melo et al. / Materials Science and Engineering C 25 (2005) 756 – 760
dimensions of 445 nm (notice the incomplete edge definition of the pattern due to discrete pixel number in a line). Due to the rough surface of this deposit the height scatters between 2 and 4.5 nm. The roughness value measured on this deposit is Ra = 0.44 nm. In the deposit performed at a 40 Hz pulse frequency the measured roughness value is Ra = 0.21 nm corresponding to the observed flat surface. The pattern width is 492 nm (as expected for non-pixilated deposit) and the height is 6.0 nm. Fig. 2 shows a roughness vs. pulse frequency plot. All the deposits were fabricated from 400 400 nm2 scans with 16 V pulses applied to the tip. The depositions were performed at 1600 nm/s tip velocity and 0.32 nm 1 scan line density. The measured roughness is Ra = 0.44 nm for the pattern deposited at 10 Hz pulse frequency, and then drops dramatically to Ra = 0.19 nm at 20 Hz, when the deposited pixels are fully merged. Once this happens the roughness does not vary as much, remaining between Ra = 0.17 and Ra = 0.22 nm for higher frequencies. The inset shows the thickness vs. pulse frequency for the same deposits. An average value of 4.1 nm was measured for the 10 Hz pixilated pattern, varying between 2.0 and 4.8 nm within the pattern as indicated by the vertical line on the inset. The values seem to follow approximately the same trend as the roughness values for the other frequencies. A maximum of 6.4 nm is obtained at f = 40 Hz. Fig. 3 shows a thickness vs. pulse voltage (in absolute value) plot. The solid and open circles correspond to two different sets of deposits performed at 35 and 40 Hz pulse frequencies and 0.5 and 0.3 V setpoint amplitudes, respectively. The free amplitude setpoint was 1.0 V for both cases. A different tip was also used for each set of deposits. The deposited thickness grows steadily from values around 1.0 nm at 7 V pulse voltage to values around 4 nm at 16.5 V. Notice that a specific tip/amplitude
759
setpoint combination can be tuned in order to obtain the same deposition behavior. Fig. 4 shows a deposited thickness vs. amplitude setpoint plot. The amplitude setpoint is a measure of the magnitude of the tip/surface interaction. The deposited thickness increases from around 4.5 nm to around 6.0 nm when the amplitude setpoint is reduced from 0.46 to 0.44 V (the tip –surface interaction increases). The inset shows the corresponding roughness vs. amplitude setpoint plot. The experimental data scatter around an horizontal line at Ra = 0.30 nm from 0.20 to 0.30 nm, reaching 0.50 nm in some cases.
4. Conclusions Deposits with pixel diameter between 21 and 34 nm and height between and 2 – 3 nm were obtained by applying 12 V pulses to the tip. When the pulse voltage goes to 16.8 V the pixel diameter increases to 90 nm and height exceeds 4.0 nm. At lower pulse frequencies the patterns obtained are pixilated, resulting in high surface roughness (Ra = 0.44 nm). At 20 Hz the deposited pixels are fully merged, resulting in smooth surfaces with low roughness (Ra = 0.19 nm), remaining in close values for higher pulse frequencies. We also show that the deposited thickness increases with the absolute value of the pulse voltage. Similar deposit characteristics are obtained with different tips by properly tuning the amplitude setpoint. We observed that the deposited thickness increases when the tip –surface interaction increases.
Acknowledgements This work was supported by ‘‘Fundac¸a˜o para a Cieˆncia e Tecnologia’’ through a Pluriannual Contract with ICEMS
Fig. 4. Deposited thickness vs. amplitude setpoint plot. Inset: corresponding roughness vs. amplitude setpoint plot.
760
L.V. Melo et al. / Materials Science and Engineering C 25 (2005) 756 – 760
(IST) and by projects POCTI/CTM/38330/01 and POCTI/ FIS/33362/99.
References [1] D.D. Chambliss, R.J. Wilson, S. Chiang, J. Vac. Sci. Technol. B 9 (1991) 933.
[2] K. Bessho, S. Hashimoto, Appl. Phys. Lett. 65 (1994) 2142. [3] K. Bessho, Y. Iwasaki, S. Hashimoto, J. Appl. Phys. 79 (1996) 5057. [4] T. Sato, C. Kaneshiro, H. Okada, H. Hasegawa, Jpn. J. Appl. Phys. Part 1 38 (1999) 2448. [5] P. Brogueira, L.V. Melo, Mater. Sci. Eng., C, Bionim. Mater., Sens. Syst. 23 (2003) 77. [6] L.V. Melo, P. Brogueira, Mater. Sci. Eng. C, Bionim. Mater., Sens. Syst. 23 (2003) 263.