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Low capacitance point diodes fabricated with focused ion beam implantation
Solid-State Electronics 47 (2003) 989–993 www.elsevier.com/locate/sse
Low capacitance point diodes fabricated with focused ion beam implantation L. Bischoff *, B. Schmidt Institut f€ ur Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germany Received 8 July 2002
Abstract Low capacitance pþ n point diodes were fabricated by combination of sputtering and implantation of a 35 keV Ga focused ion beam through a thin oxide layer on a silicon substrate. The capacitance of the diodes were determined to be in the range of 1016 F. The current–voltage characteristics show a tendency to a generation/recombination controlled behaviour with increasing dose and with increasing depth of the sputter crater. This is correlated to the big amount of inactive Ga atoms of about 70% in the Si lattice after an annealing of 900 °C; 20 min, N2 . Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Focused ion beam; Point diode; Ga implantation; Sputtering; I–V characteristics.
1. Introduction In the last two decades focused ion beams (FIBs) have become a very useful tool for many tasks in micron and sub-micron technology [1,2]. Probe sizes of <10 nm and current densities of more than 10 A cm2 are now available and allow to use these beams for many applications. Integrated circuit repair and modification [3], failure analysis [4], lithographic mask repair [5] or FIB lithography [6] as well as the writing maskless implantation [7] are the main applications in microelectronic industry. Especially during the R&D phase the FIB is very advantageous because of its high spatial resolution and its flexibility varying dose, energy and pattern design on one chip, or even in one structure detail. Most of the FIB systems employ a Ga liquid metal ion source (LMIS). In spite of the broad spectrum of applications of Ga beams many cases suffer from the impurity incorporation but these ions
*
Corresponding author. Tel.: +49-3512602963; fax: +493512603285. E-mail address: l.bischoff@fz-rossendorf.de (L. Bischoff).
are also suitable for the modification of GaAs structures by local disordering without contamination [8] or also acting as an acceptor in n-type silicon to form pnjunctions [9,10]. This work deals with the fabrication of point-like diodes using a self-aligned writing FIB implantation process.
2. Experimental A schematic flowchart of the preparation process of the samples is given in Fig. 1. On standard silicon wafers, n-type, h1 0 0i-oriented with a resistivity of q ¼ 1–10 X cm a 200 nm thermal oxide was grown. Applying a photolithographic process a 2:5 2:5 mm2 square window per chip was opened. Then in this windows a 100 nm thick thermal oxide was grown. Corresponding to the metallization mask certain positions inside the oxide window were irradiated with Ga ions. For the irradiation the FIB system IMSA-100 was used which is described more in detail elsewhere [11]. Points (stationary beam) and squares of 1 1 lm2 (scanned beam) were sputtered into the thin (100 nm) oxide accompanying by an implantation of Ga with a
0038-1101/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1101(02)00456-2
990
L. Bischoff, B. Schmidt / Solid-State Electronics 47 (2003) 989–993
n-Si, <100>
oxydation, dry 200 nm
photolithography, structuring of an oxide window
oxydation, dry 100 nm
Ga + - FIB-implantation variable in size and dose
Al- contact sputterdeposition
photolithography structuring of the contacts
Fig. 1. Flowchart of the preparation process of the diodes.
35 keV beam at doses of 6 1016 –8 1018 cm2 . The ion beam current was in the range of 160–250 pA and the spot size, FWHM of the Gaussian beam profile, about 200 nm [12]. To prevent a charge build-up during irradiation of the sample a temporary Al covering on the back side of the wafer was done for grounding. After implantation a thermal treatment at 900 °C for 20 min in a dry N2 ambient has been performed to anneal the Si lattice and to activate the Ga atoms. At the last step Al-contacts were deposited, 300 nm on the front and 500 nm on the back side, structured by a standard photolithography and annealed at 400 °C; 30 min. The diodes were analysed measuring the I–V characteristics close to the zero point and also n a wide range of voltage in the reverse bias direction.
3. Results and discussion To implant the Si bulk the Ga ions have to penetrate the oxide layer within a sputtering process. The calculated sputtering depth and the corresponding position of the projected range RP as a function of the ion dose is shown in Fig. 2. For this calculation the sputtering yields Y for SiO2 of 2.5, and for Si of 2.6, were taken from previous experiments [13]. The implantation distribution data were obtained using the TRIM code [14]. The amount of Ga in the implanted layer should be due to the high dose close to the saturation concentration of NSi =Y ¼ 2 1022 cm3 [15]. But the solubility limit of Ga in Si is only 1:4 1019 cm3 [16] and the maximum of activated atoms after annealing is in maximum 30% [17] i.e. the acceptor concentration in the p-type region
L. Bischoff, B. Schmidt / Solid-State Electronics 47 (2003) 989–993
can be estimated to be about 4 1019 cm3 . By utilizing a capping layer (Si/SiO2 /Six Ny /SiO2 ) after annealing at 1250 °C for 16 h an activation of about 75% could be achieved [18], but this procedure was not applied in this case. Also about 30% are on interstitial positions, found by RBS investigations. The residual Ga atoms are incorporated in precipitates [12] or Ga clusters [19] within the pþ -layer and they can form a tail by diffusion, and/or move as neutral impurity centres to the interface [17]. The donor concentration in the n-type bulk material is about 1 1015 cm3 , corresponding to q ¼ 5 X cm. The I–V characteristics in the small signal range were measured automatically using a Probe system PM 6 (Karl Suss) and a High Voltage Source-Measure Unit type 237 (Keithley Instruments) from )2 up to þ1 V in steps of 0.1 V shielded from any influences like crosstalk and also from light. The current range was 1014 – 107 A. Typical I–V plots in the low signal case are shown in Fig. 3. The experimental results are compared with the usual description for the diode current [20]: eU I ¼ IS exp 1 ð1Þ mkT where IS is the reverse saturation current, which was determined by fitting the experimental data for the calculations. U is the applied voltage, e the elementary charge, k is the BoltzmannÕs constant and T is the absolute temperature. The ideality factor m has a value of 1 if the current is diffusion dominated and 2 for the generation–recombination current. If both are comparable this value lies between 1 and 2. Two different kinds of the behaviour were found depending on the ion dose. In the range from 2 1017 to 2 1018 cm2 the current in the reverse biased mode was about 1011 –1012 A with a very small scattering from chip to chip. The forward biased diodes showed a slope
Fig. 3. Calculated and measured I–V characteristics for different doses and implantation areas.
for an ideal diode, i.e. m ¼ 1 in Eq. (1). Proper operating diodes were found in the case of the scanned beam as well as in the stationary beam mode implantation beginning at doses higher than 2 1017 cm2 in a good agreement with the sputtering data, shown in Fig. 2. At D 2 1017 cm2 the sputtering crater reaches the Si/ SiO2 interface and the Gaþ ions are able to penetrate into the Si substrate. The behaviour of the ideality factor m as a function of the dose for both implantation modes is shown in Fig. 4. The value of the parameter m was extracted from forward biased characteristics from the averaged slope in the current interval up to I ¼ 107 A using Eq. (1). The saturation current was taken from the experimental data. It is to be seen that a nearly ideal behaviour of the diodes, m is about 1, was found when the implanted profile in the silicon after annealing is very close to the former interface. In this case all Ga remains in a thin sheet in the silicon substrate. In the lower dose range it is revealed a diode based on the tunnelling effect through remaining SiO2 . The implanted Ga forms a
m
Fig. 2. Sputtering depth and position of RP as a function of ion dose.
991
2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8
squares dots
1E17
1E18
1E19 -2
dose (cm ) Fig. 4. Quality factor m (Eq. (1)) as a function of Ga ion dose.
992
L. Bischoff, B. Schmidt / Solid-State Electronics 47 (2003) 989–993
resistor in the SiO2 which is shunted by a small capacitor to the bulk-Si. In this case a strong generation–recombination behaviour was found caused by the high radiation damage level in the oxide. For lower doses than 8 1016 Ga/cm2 pure MOS characteristics were measured, i.e. the implanted Ga in the sputtered hole is to far away from the SiO2 /Si interface. Further increasing of the dose to more than 2 1018 cm2 leads to an increasing of the reverse current and showed also a stronger scattering over some orders of magnitude, especially for the point implanted devices where the shape of the pn-junction is determined by the beam profile of the FIB. The forward biased diodes followed a slope, corresponding to m of about 2 which indicates that the current is generation–recombination dominated. This behaviour reveals a strong disordered impurity containing inhomogeneous region around the pn-junction acting as generation–recombination centres. A main argument for this is that a FIB of a current density in the range of some A cm2 at long pixel dwelltimes cause a damage level in the silicon crystal which cannot be annealed completely so that not a homogeneously doped crystalline layer is formed [21]. Investigating the breakdown voltage of the diodes one finds a quite analogous behaviour depending on the implantation dose. With increasing dose the breakdown voltage becomes lower. But the highest values were found in the dose range where the sputtering depth is close or equal to the oxide thickness which is shown for the Gaþ -irradiation of the 1 1 lm2 squares in Fig. 5. The in-set shows measured reverse bias curves for a set of diodes
implanted at 8 1016 cm2 . The capacitance of the point diodes can be described using the half-sphere model for pn-junctions [22] according to C ¼ 2pe0 e
rp rn ðU Þ rn ðU Þ rp
ð2Þ
with e0 e––permitivity (for Si 1:0632 1012 A s V1 cm1 ), rp ––radius of the pþ -doped half-sphere (inner border of the depletion layer) and rn ––radius (outer border) of the depletion layer as a function of the applied voltage. For high applied voltages close to the breakdown voltage of )180 V, one finds rp rn ðU Þ leading to the approximation C 2pe0 erp , i.e. the capacitance is nearly independent from the applied voltage yielding in the range of some hundreds of aF in the case that rp is about 0.5 lm. A measurement of the diode capacitance in practice was very problematic due to the orders of magnitude higher MOS capacitance of the aluminium contact pads over the silicon oxide. The breakdown field strength amounts to about 107 V m1 according to the estimated space charge depth of 5 lm [22]. In the case of the point-like implanted diodes a statistical analysis seems to be not useful according to the strong scattering of the results over an order of magnitude. An alternative method to fabricate point diodes with high quality would be the FIB implantation of Co ions from a Co36 Nd64 alloy LMIS in order to form CoSi2 dots by an ion beam synthesis process [23] acting as Schottky-barriers to the Si substrate.
Fig. 5. Breakdown-voltage range depending on the ion dose for 1 1 lm2 implanted diodes. The in-set shows a measured set of reverse biased I–V curves for a dose of 8 1016 cm2 .
L. Bischoff, B. Schmidt / Solid-State Electronics 47 (2003) 989–993
4. Conclusion Gaþ FIB implantation at 35 keV into silicon through a 100 nm SiO2 film has been performed to fabricate low capacitance point diodes. Stationary beam and scanned beam (1 1 lm2 ) irradiations were carried out in a dose range from 6 1016 to 8 1018 cm2 . For doses corresponding to a sputtering depth close to the interface Si– SiO2 nearly ideal (m 1) diodes were obtained. For higher doses and so deeper profiles the diode behaviour was generation–recombination dominated being a hint to the limited amount of substitutional Ga acceptor atoms in Si.
Acknowledgements The authors wish to thank the clean-room staff for the sample preparation as well as E. Berger, S. Gehrisch and F. Thum for measurement of I–V characteristics. References [1] Orloff J. Rev Sci Instrum 1993;64:1105. [2] Prewett PD, Mair GLR. Focused ion beams from liquid metal ion sources. Taunton, Somerset, UK: Research Study Press; 1991. [3] Tao T, Wilkonson W, Melngailis J. J Vac Sci Technol B 1991;9:162. [4] Bernius MT, Morrison GH. Rev Sci Instrum 1987;58:1789. [5] Prewett PD, Heard PJ. J Phys D: Appl Phys 1987;20:1207.
993
[6] Kubena RL, Joyce RJ, Ward JW, Garwin HL, Stratton FP, Brault RG. Appl Phys Lett 1987;50:1589. [7] Teichert J, Bischoff L, Hausmann S. Microelectron Eng 1998;41/42:233. [8] Hirayama Y, Suzuki Y, Taruha S, Okamoto H. Jpn J Appl Phys 1985;24:L516. [9] Stephen J, Grimshaw GA. Rad Eff 1971;7:73. [10] Tamura M, Shukuri S, Moniwa M, Default M. Appl Phys A 1986;39:183. [11] Bischoff L, Hesse E, Hofmann G, Naehring FK, Probst W, Schmidt B, et al. Microelectron Eng 1993;21:197. [12] Schmidt B, Bischoff L, Teichert J. Sens Actuators A 1997;61:369. [13] Bischoff L, Teichert J. Focused ion beam sputtering of silicon and related materials. Report FZR-217, 1998. [14] Ziegler JF, Biersack J, Littmark U. The stopping and range of ions in matter. New York: Pergamon Press; 1985. [15] Teichert J, Bischoff L, Hesse E, Schneider P, Panknin D, Gessner T, et al. J Micromech Microeng 1996;8:272. [16] Trumbore FA. Bell Syst Tech J 1960;39:205. [17] Baron R, Shirim GA, March OJ, Mayer JW. J Appl Phys 1969;40:3702. [18] Watanabe M, Ishiwata O, Nagano M, Kirihata H. J Electrochem Soc 1992;139:1748. [19] Capano MA, Kavanagh KL, Bensoussan S, Hobbs LW. The Rigaku Journal 1988;5:3. [20] Shockley W. Electrons and holes in semiconductors. Princeton, NY: Van Nostrand; 1950. [21] Hausmann S, Bischoff L, Teichert J, Voelskow M, M€ oller W. J Appl Phys 2000;87:57. [22] Paul R. Halbleiterdioden. Berlin: Verlag Technik; 1976. 184 pp. [23] Teichert J, Bischoff L, Hausmann S. Microelectron Eng 1997;35:455.
Low capacitance point diodes fabricated with focused ion beam implantation L. Bischoff *, B. Schmidt Institut f€ ur Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germany Received 8 July 2002
Abstract Low capacitance pþ n point diodes were fabricated by combination of sputtering and implantation of a 35 keV Ga focused ion beam through a thin oxide layer on a silicon substrate. The capacitance of the diodes were determined to be in the range of 1016 F. The current–voltage characteristics show a tendency to a generation/recombination controlled behaviour with increasing dose and with increasing depth of the sputter crater. This is correlated to the big amount of inactive Ga atoms of about 70% in the Si lattice after an annealing of 900 °C; 20 min, N2 . Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Focused ion beam; Point diode; Ga implantation; Sputtering; I–V characteristics.
1. Introduction In the last two decades focused ion beams (FIBs) have become a very useful tool for many tasks in micron and sub-micron technology [1,2]. Probe sizes of <10 nm and current densities of more than 10 A cm2 are now available and allow to use these beams for many applications. Integrated circuit repair and modification [3], failure analysis [4], lithographic mask repair [5] or FIB lithography [6] as well as the writing maskless implantation [7] are the main applications in microelectronic industry. Especially during the R&D phase the FIB is very advantageous because of its high spatial resolution and its flexibility varying dose, energy and pattern design on one chip, or even in one structure detail. Most of the FIB systems employ a Ga liquid metal ion source (LMIS). In spite of the broad spectrum of applications of Ga beams many cases suffer from the impurity incorporation but these ions
*
Corresponding author. Tel.: +49-3512602963; fax: +493512603285. E-mail address: l.bischoff@fz-rossendorf.de (L. Bischoff).
are also suitable for the modification of GaAs structures by local disordering without contamination [8] or also acting as an acceptor in n-type silicon to form pnjunctions [9,10]. This work deals with the fabrication of point-like diodes using a self-aligned writing FIB implantation process.
2. Experimental A schematic flowchart of the preparation process of the samples is given in Fig. 1. On standard silicon wafers, n-type, h1 0 0i-oriented with a resistivity of q ¼ 1–10 X cm a 200 nm thermal oxide was grown. Applying a photolithographic process a 2:5 2:5 mm2 square window per chip was opened. Then in this windows a 100 nm thick thermal oxide was grown. Corresponding to the metallization mask certain positions inside the oxide window were irradiated with Ga ions. For the irradiation the FIB system IMSA-100 was used which is described more in detail elsewhere [11]. Points (stationary beam) and squares of 1 1 lm2 (scanned beam) were sputtered into the thin (100 nm) oxide accompanying by an implantation of Ga with a
0038-1101/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1101(02)00456-2
990
L. Bischoff, B. Schmidt / Solid-State Electronics 47 (2003) 989–993
n-Si, <100>
oxydation, dry 200 nm
photolithography, structuring of an oxide window
oxydation, dry 100 nm
Ga + - FIB-implantation variable in size and dose
Al- contact sputterdeposition
photolithography structuring of the contacts
Fig. 1. Flowchart of the preparation process of the diodes.
35 keV beam at doses of 6 1016 –8 1018 cm2 . The ion beam current was in the range of 160–250 pA and the spot size, FWHM of the Gaussian beam profile, about 200 nm [12]. To prevent a charge build-up during irradiation of the sample a temporary Al covering on the back side of the wafer was done for grounding. After implantation a thermal treatment at 900 °C for 20 min in a dry N2 ambient has been performed to anneal the Si lattice and to activate the Ga atoms. At the last step Al-contacts were deposited, 300 nm on the front and 500 nm on the back side, structured by a standard photolithography and annealed at 400 °C; 30 min. The diodes were analysed measuring the I–V characteristics close to the zero point and also n a wide range of voltage in the reverse bias direction.
3. Results and discussion To implant the Si bulk the Ga ions have to penetrate the oxide layer within a sputtering process. The calculated sputtering depth and the corresponding position of the projected range RP as a function of the ion dose is shown in Fig. 2. For this calculation the sputtering yields Y for SiO2 of 2.5, and for Si of 2.6, were taken from previous experiments [13]. The implantation distribution data were obtained using the TRIM code [14]. The amount of Ga in the implanted layer should be due to the high dose close to the saturation concentration of NSi =Y ¼ 2 1022 cm3 [15]. But the solubility limit of Ga in Si is only 1:4 1019 cm3 [16] and the maximum of activated atoms after annealing is in maximum 30% [17] i.e. the acceptor concentration in the p-type region
L. Bischoff, B. Schmidt / Solid-State Electronics 47 (2003) 989–993
can be estimated to be about 4 1019 cm3 . By utilizing a capping layer (Si/SiO2 /Six Ny /SiO2 ) after annealing at 1250 °C for 16 h an activation of about 75% could be achieved [18], but this procedure was not applied in this case. Also about 30% are on interstitial positions, found by RBS investigations. The residual Ga atoms are incorporated in precipitates [12] or Ga clusters [19] within the pþ -layer and they can form a tail by diffusion, and/or move as neutral impurity centres to the interface [17]. The donor concentration in the n-type bulk material is about 1 1015 cm3 , corresponding to q ¼ 5 X cm. The I–V characteristics in the small signal range were measured automatically using a Probe system PM 6 (Karl Suss) and a High Voltage Source-Measure Unit type 237 (Keithley Instruments) from )2 up to þ1 V in steps of 0.1 V shielded from any influences like crosstalk and also from light. The current range was 1014 – 107 A. Typical I–V plots in the low signal case are shown in Fig. 3. The experimental results are compared with the usual description for the diode current [20]: eU I ¼ IS exp 1 ð1Þ mkT where IS is the reverse saturation current, which was determined by fitting the experimental data for the calculations. U is the applied voltage, e the elementary charge, k is the BoltzmannÕs constant and T is the absolute temperature. The ideality factor m has a value of 1 if the current is diffusion dominated and 2 for the generation–recombination current. If both are comparable this value lies between 1 and 2. Two different kinds of the behaviour were found depending on the ion dose. In the range from 2 1017 to 2 1018 cm2 the current in the reverse biased mode was about 1011 –1012 A with a very small scattering from chip to chip. The forward biased diodes showed a slope
Fig. 3. Calculated and measured I–V characteristics for different doses and implantation areas.
for an ideal diode, i.e. m ¼ 1 in Eq. (1). Proper operating diodes were found in the case of the scanned beam as well as in the stationary beam mode implantation beginning at doses higher than 2 1017 cm2 in a good agreement with the sputtering data, shown in Fig. 2. At D 2 1017 cm2 the sputtering crater reaches the Si/ SiO2 interface and the Gaþ ions are able to penetrate into the Si substrate. The behaviour of the ideality factor m as a function of the dose for both implantation modes is shown in Fig. 4. The value of the parameter m was extracted from forward biased characteristics from the averaged slope in the current interval up to I ¼ 107 A using Eq. (1). The saturation current was taken from the experimental data. It is to be seen that a nearly ideal behaviour of the diodes, m is about 1, was found when the implanted profile in the silicon after annealing is very close to the former interface. In this case all Ga remains in a thin sheet in the silicon substrate. In the lower dose range it is revealed a diode based on the tunnelling effect through remaining SiO2 . The implanted Ga forms a
m
Fig. 2. Sputtering depth and position of RP as a function of ion dose.
991
2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8
squares dots
1E17
1E18
1E19 -2
dose (cm ) Fig. 4. Quality factor m (Eq. (1)) as a function of Ga ion dose.
992
L. Bischoff, B. Schmidt / Solid-State Electronics 47 (2003) 989–993
resistor in the SiO2 which is shunted by a small capacitor to the bulk-Si. In this case a strong generation–recombination behaviour was found caused by the high radiation damage level in the oxide. For lower doses than 8 1016 Ga/cm2 pure MOS characteristics were measured, i.e. the implanted Ga in the sputtered hole is to far away from the SiO2 /Si interface. Further increasing of the dose to more than 2 1018 cm2 leads to an increasing of the reverse current and showed also a stronger scattering over some orders of magnitude, especially for the point implanted devices where the shape of the pn-junction is determined by the beam profile of the FIB. The forward biased diodes followed a slope, corresponding to m of about 2 which indicates that the current is generation–recombination dominated. This behaviour reveals a strong disordered impurity containing inhomogeneous region around the pn-junction acting as generation–recombination centres. A main argument for this is that a FIB of a current density in the range of some A cm2 at long pixel dwelltimes cause a damage level in the silicon crystal which cannot be annealed completely so that not a homogeneously doped crystalline layer is formed [21]. Investigating the breakdown voltage of the diodes one finds a quite analogous behaviour depending on the implantation dose. With increasing dose the breakdown voltage becomes lower. But the highest values were found in the dose range where the sputtering depth is close or equal to the oxide thickness which is shown for the Gaþ -irradiation of the 1 1 lm2 squares in Fig. 5. The in-set shows measured reverse bias curves for a set of diodes
implanted at 8 1016 cm2 . The capacitance of the point diodes can be described using the half-sphere model for pn-junctions [22] according to C ¼ 2pe0 e
rp rn ðU Þ rn ðU Þ rp
ð2Þ
with e0 e––permitivity (for Si 1:0632 1012 A s V1 cm1 ), rp ––radius of the pþ -doped half-sphere (inner border of the depletion layer) and rn ––radius (outer border) of the depletion layer as a function of the applied voltage. For high applied voltages close to the breakdown voltage of )180 V, one finds rp rn ðU Þ leading to the approximation C 2pe0 erp , i.e. the capacitance is nearly independent from the applied voltage yielding in the range of some hundreds of aF in the case that rp is about 0.5 lm. A measurement of the diode capacitance in practice was very problematic due to the orders of magnitude higher MOS capacitance of the aluminium contact pads over the silicon oxide. The breakdown field strength amounts to about 107 V m1 according to the estimated space charge depth of 5 lm [22]. In the case of the point-like implanted diodes a statistical analysis seems to be not useful according to the strong scattering of the results over an order of magnitude. An alternative method to fabricate point diodes with high quality would be the FIB implantation of Co ions from a Co36 Nd64 alloy LMIS in order to form CoSi2 dots by an ion beam synthesis process [23] acting as Schottky-barriers to the Si substrate.
Fig. 5. Breakdown-voltage range depending on the ion dose for 1 1 lm2 implanted diodes. The in-set shows a measured set of reverse biased I–V curves for a dose of 8 1016 cm2 .
L. Bischoff, B. Schmidt / Solid-State Electronics 47 (2003) 989–993
4. Conclusion Gaþ FIB implantation at 35 keV into silicon through a 100 nm SiO2 film has been performed to fabricate low capacitance point diodes. Stationary beam and scanned beam (1 1 lm2 ) irradiations were carried out in a dose range from 6 1016 to 8 1018 cm2 . For doses corresponding to a sputtering depth close to the interface Si– SiO2 nearly ideal (m 1) diodes were obtained. For higher doses and so deeper profiles the diode behaviour was generation–recombination dominated being a hint to the limited amount of substitutional Ga acceptor atoms in Si.
Acknowledgements The authors wish to thank the clean-room staff for the sample preparation as well as E. Berger, S. Gehrisch and F. Thum for measurement of I–V characteristics. References [1] Orloff J. Rev Sci Instrum 1993;64:1105. [2] Prewett PD, Mair GLR. Focused ion beams from liquid metal ion sources. Taunton, Somerset, UK: Research Study Press; 1991. [3] Tao T, Wilkonson W, Melngailis J. J Vac Sci Technol B 1991;9:162. [4] Bernius MT, Morrison GH. Rev Sci Instrum 1987;58:1789. [5] Prewett PD, Heard PJ. J Phys D: Appl Phys 1987;20:1207.
993
[6] Kubena RL, Joyce RJ, Ward JW, Garwin HL, Stratton FP, Brault RG. Appl Phys Lett 1987;50:1589. [7] Teichert J, Bischoff L, Hausmann S. Microelectron Eng 1998;41/42:233. [8] Hirayama Y, Suzuki Y, Taruha S, Okamoto H. Jpn J Appl Phys 1985;24:L516. [9] Stephen J, Grimshaw GA. Rad Eff 1971;7:73. [10] Tamura M, Shukuri S, Moniwa M, Default M. Appl Phys A 1986;39:183. [11] Bischoff L, Hesse E, Hofmann G, Naehring FK, Probst W, Schmidt B, et al. Microelectron Eng 1993;21:197. [12] Schmidt B, Bischoff L, Teichert J. Sens Actuators A 1997;61:369. [13] Bischoff L, Teichert J. Focused ion beam sputtering of silicon and related materials. Report FZR-217, 1998. [14] Ziegler JF, Biersack J, Littmark U. The stopping and range of ions in matter. New York: Pergamon Press; 1985. [15] Teichert J, Bischoff L, Hesse E, Schneider P, Panknin D, Gessner T, et al. J Micromech Microeng 1996;8:272. [16] Trumbore FA. Bell Syst Tech J 1960;39:205. [17] Baron R, Shirim GA, March OJ, Mayer JW. J Appl Phys 1969;40:3702. [18] Watanabe M, Ishiwata O, Nagano M, Kirihata H. J Electrochem Soc 1992;139:1748. [19] Capano MA, Kavanagh KL, Bensoussan S, Hobbs LW. The Rigaku Journal 1988;5:3. [20] Shockley W. Electrons and holes in semiconductors. Princeton, NY: Van Nostrand; 1950. [21] Hausmann S, Bischoff L, Teichert J, Voelskow M, M€ oller W. J Appl Phys 2000;87:57. [22] Paul R. Halbleiterdioden. Berlin: Verlag Technik; 1976. 184 pp. [23] Teichert J, Bischoff L, Hausmann S. Microelectron Eng 1997;35:455.