- Email: [email protected]
Characterization and TCAD modelling of termination structures for silicon radiation detectors
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 518 (2004) 362–365
Characterization and TCAD modelling of termination structures for silicon radiation detectors$ S. Dittongoa,c,*, M. Boscardinb, L. Bosisioa,c, M. Ciacchia, G.-F. Dalla Bettab,d, P. Gregorib, C. Piemonteb, I. Rachevskaiac, S. Ronchinb, N. Zorzib Dipartimento di Fisica, Universita" di Trieste, Via A. Valerio 2, I-34127 Trieste, Italy ITC-IRST, Divisione Microsistemi, Via Sommarive 18, I-38050 Povo, Trento, Italy c INFN-Sezione di Trieste, Via A. Valerio 2, I-34127 Trieste, Italy d Dipartimento di Informatica e Telecomunicazioni, Universita" di Trento, Via Sommarive 14, I-38050 Povo, Trento, Italy a
b
Abstract We have recently proposed a novel junction termination structure for silicon radiation detectors, featuring all-p-type multiguard and scribe-line implants, with metal field-plates completely covering the gap between the implanted rings. The structure is intended for detector long-term stability enhancement even in adverse ambient conditions and for fabrication-process simplification. A thorough static characterization, including stability measurements in varying humidity conditions, has been carried out on a variety of samples fabricated at ITC-irst. Comparisons with diodes featuring an n-type implant along the border—or no edge structure at all—have been performed. The new structures show stable behaviour at relatively high bias ðB200 VÞ; also in the presence of wide humidity changes (1–90%). A good qualitative agreement has been obtained between experimental results and simulation predictions, allowing to gain deep insight into the physical behaviour of the device. r 2003 Elsevier B.V. All rights reserved. PACS: 85.30.De; 29.40.Wk; 73.40.Qv Keywords: Semiconductor-device characterization; Design and modeling; Solid-state detectors; Metal-insulator-semiconductor structures
1. Device description The devices under test [1,2] consist of a square diode surrounded by a biased guard ring (LG), 100 mm wide, and by a set of floating pþ guard $ This work has been partially supported by the National Institute for Nuclear Physics of Italy (INFN) and by the ‘‘Provincia Autonoma di Trento’’ under the Contract ‘‘Fondo per i progetti di ricerca 2002-progetto PDX’’. *Corresponding author. E-mail address: [email protected] (S. Dittongo).
rings, the last of which extends to the cut edge. The guards are characterized by metal field-plates that completely cover the region between adjacent rings; in this way the n-type substrate just beneath the Si2SiO2 interface is electrostatically screened from the external environment. Two different designs have been developed, differing in the configuration of field-plates: in the first one (P1), each field-plate is oriented inward and overlaps the implant of the preceding ring; in the second one (P2), the field-plates between the rings extend
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.023
ARTICLE IN PRESS S. Dittongo et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 362–365 Table 1 Experimental values of the main technological parameters extracted from test structures made on different types of wafers
Orientation Nsub ð1010 cm3 Þ tox ðnmÞ Nf ð109 cm2 Þ tg ðmsÞ s0 ðcm=sÞ
Wafer A
Wafer B
Wafer C
Wafer D
ð1 1 1Þ 3575 97474 340750 1074 8:672:2
ð1 0 0Þ 2571 86573 243714 4:373:2 5:371:8
ð1 1 1Þ 2872 85572 28479 113750 6:070:4
ð1 1 1Þ 1771 665760 40977 80740 33:770:8
Substrate-donors concentration ðNsub Þ; oxide thickness ðtox Þ; oxide-charge density ðNf Þ; bulk-generation lifetime ðtg Þ; surfacegeneration velocity ðs0 Þ:
alternately outward and inward, so that only every second implanted ring has to be contacted by a metal layer, allowing a larger number of rings (20 w.r.t. 12) to be fitted within the same area. For both designs, four layouts (P1:1; y ; P1:4 and P2:1; y ; P2:4) have been implemented, differing in the spacings between the rings. More details on the geometrical characteristics of samples can be found in [1,2]. For comparison with more traditional structures, simple diodes with a biased guard ring (LG), 100 mm wide, but no floating rings have also been fabricated. Some of these have no edge structure, the others feature an n-type implant along the border, 300 mm wide. The latters have been implemented in five layouts ðN1; y ; N5Þ; differing in the distance ð1002300 mmÞ between LG and n-type edge implant. All devices have been fabricated at ITC-irst on high resistivity n-type silicon wafers, ð1 1 1Þ or ð1 0 0Þ oriented and 300 mm thick. The main technological parameters, extracted from dedicated test structures, are summarized in Table 1 for four different wafer types. 2. Electrical characterization The voltage-handling capability of the structures has been tested by measuring the large-guard current ILG as a function of the reverse bias voltage Vrev : As already observed in [1,2], the lateral punch-through from the LG to the edge is the voltage limiting mechanism in all multiguard structures, whereas in the samples with the n-type
363
Table 2 Experimental values of Vlim ; expressed in V, for all the multiguard structures measured on four types of wafers
P1.1 P1.2 P1.3 P1.4 P2.1 P2.2 P2.3 P2.4
Wafer A
Wafer B
Wafer C
Wafer D
8575 18677 104711 153714 n.a. n.a. n.a. n.a.
4972 11079 5776 9378 n.a. n.a. n.a. n.a.
n.a. 11876 n.a. 9675 13676 16175 13276 15574
n.a. 6975 n.a. 5473 9676 11379 9675 10773
Table 3 Experimental values of VBD ; expressed in V, for all the structures without multiguard measured on two types of wafers
N1 N2 N3 N4 N5
Wafer A
Wafer B
14878 15475 151713 15475 15376
29076 29275 29175 29177 28979
implant along the border breakdown at the pþ 2n junction edge is observed. Following the same approach used in [1], we represent the sensitivity of the LG current to the reverse voltage in the multiguard devices by means of the adimensional function K ¼ ðdILG =dVrev Þ ðVrev =ILG Þ: By adopting the same limit value as in [1], we calculated the maximum voltage, Vlim ; for which KoKlim ¼ 2 (see Table 2). The values of Vlim differ according to the different geometries of the guard rings. As far as the samples without multiguard structures are concerned, Table 3 reports the measured breakdown voltage VBD : No significant difference has been observed in the VBD among different layouts for devices fabricated on a given wafer. As expected, VBD is higher in ð1 0 0Þ w.r.t. ð1 1 1Þ wafers because of the lower oxide charge density.
3. Stability measurements All tested all-p-type multiguard structures exhibited excellent time stability of the leakage
ARTICLE IN PRESS S. Dittongo et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 362–365
current even in the most adverse conditions: no passivation, high humidity (90% RH), and bias voltage close to the operational limit. This result was expected, because the metal field-plates cover the whole region in between consecutive rings. The picture is more complex for devices with a single guard ring. All tested samples, either with or without n-type implant along the cut edge, have shown a large increase of the guard ring current when biased at high voltage ð1202180 VÞ in highhumidity conditions. The time required for the onset of this instability varied (from a few hours to a few days) depending on the bias voltage, the humidity level, the oxide charge (ð1 1 1Þ vs. ð1 0 0Þ wafers) and the presence/absence of a passivating oxide. In all cases, care has been taken to set the bias voltage below the value causing breakdown by avalanche multiplication at the edge of the guard ring. On devices without n-type edge implant, the current has generally been observed to stabilize at a value of a few mA (for a perimeter of 20 mm), while on the structures with edge implant the current always reached compliance (set at 10 mA) before showing signs of stabilization. This different behaviour has been attributed to the onset of avalanche multiplication at the internal edge of the nþ -implant as a result of the field-plate effect of ions drifting on the oxide surface in high humidity, eventually making the surface equipotential with the pþ guard ring [3,4]. By contrast, on structures without edge implant the final current is due mainly to the carriers generated at the damaged cut region, which is collected because of the lateral extension of the space-charge region caused by the formation of the ion field-plate. Measurements made on uncut devices (on wafer) support this view: in the presence of edge implant, the behaviour is very similar to what observed for cut dice, while for structures without n-implant the current stabilizes at a much lower value (tens of nA), likely due to bulk and interface generation in the laterally extended depletion region. Fig. 1 illustrates these points by showing the I2t curves for four devices: with and without n-implant, cut and on wafer. Simulations performed not accounting for the effect of surface ions show that at high voltage (but still below VBD ) the space-charge region
1e-05
1e-06
log(ILG) [A]
364
1e-07
1e-08
+
1e-09
n , cut, RH=47%-70% n , on wafer, RH=15%-78% + no n , cut, RH=30%-75% + no n , on wafer, RH=76%-82% +
1e-10 0
5
10 15 time [hours]
20
25
Fig. 1. Experimental LG current as a function of time for diodes having a single guard ring and kept in a high humidity environment.
Fig. 2. Simulated hole current density in a structure with nimplant on the edge, biased below breakdown voltage. The cut line is on the right edge. A channel of hole current flowing in the bulk from the edge to the LG can be clearly seen.
approaches the lateral cut surface deep in the bulk, causing a high hole current to flow toward the guard ring (Fig. 2); the presence of the nimplant has no effect in this case, because when no ionic charges are considered, the interface is in any case accumulated up to a short distance from the pþ guard ring due to the presence of the positive
ARTICLE IN PRESS S. Dittongo et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 362–365
oxide charge. The drifting ions on the surface change the situation, by compensating the oxide charge progressively extending the depletion region outward; the ion drift stops at the edge of the n-implant, if present, while otherwise continues until it reaches the cut edge of the device. In the first case we get increasingly high-field values at the edge of the n-implant, leading to avalanche breakdown; in the second case the depletion region eventually reaches the damaged cut surface of the device. In order to reproduce the effect of the ions on the outer surface, we added in the simulation a field-plate electrode on top of the oxide, extending from the guard ring to the n-implant, and biased at the same potential of the guard ring [4]. Simulation data clearly show that in this case avalanche breakdown at the edge of the n-implant occurs, at a lower voltage than required to cause breakdown at the edge of the LG in the absence of field-plate.
365
full depletion voltage, so that, at normal operating bias, avalanche breakdown does not occur. In these conditions, the n-implant plays an useful role, because it stabilizes the device by preventing the depletion region from reaching the cut edge. However, for thicker or higher doping substrates, operated in high humidity conditions, the breakdown voltage at the edge of the n-implant may be comparable to the total depletion voltage. In this case the all-p-type multiguard structures offer a reliable solution, with the additional advantage of simplifying the fabrication process. With TCAD simulations we were able to reproduce qualitatively the experimental results. Work is under way to improve the quantitative agreement between simulations and measurements, in order to optimize the design for different process parameters and operating conditions of the devices.
4. Conclusions References Measurements show that all-p-type multiguard structures are very stable at relatively high voltages, even in adverse environmental conditions. For devices without floating rings, long-term stability measurements show that the presence of the n-type implant eventually leads to a decrease of the breakdown voltage, as a result of a shift of the high field region from the edge of the LG to the edge of the n-implant. However, in all tested devices the breakdown voltage is well above the
[1] G.-F. Dalla Betta, M. Boscardin, L. Bosisio, S. Dittongo, P. Gregori, I. Rachevskaia, et al., IEEE Trans. Nucl. Sci. NS49 (4) (2002) 1712. [2] M. Boscardin, L. Bosisio, A. Candelori, G.-F. Dalla Betta, S. Dittongo, P. Gregori, et al., IEEE Trans. Nucl. Sci. NS50 (4) (2003) 1001. [3] A. Longoni, M. Sampietro, L. Struder, . Nucl. Instr. and Meth. A 288 (1990) 35. [4] C. Piemonte, A. Rashevsky, Method of punch-through voltage stabilisation in silicon detectors, INFN/TC-02/08, 2002.
Nuclear Instruments and Methods in Physics Research A 518 (2004) 362–365
Characterization and TCAD modelling of termination structures for silicon radiation detectors$ S. Dittongoa,c,*, M. Boscardinb, L. Bosisioa,c, M. Ciacchia, G.-F. Dalla Bettab,d, P. Gregorib, C. Piemonteb, I. Rachevskaiac, S. Ronchinb, N. Zorzib Dipartimento di Fisica, Universita" di Trieste, Via A. Valerio 2, I-34127 Trieste, Italy ITC-IRST, Divisione Microsistemi, Via Sommarive 18, I-38050 Povo, Trento, Italy c INFN-Sezione di Trieste, Via A. Valerio 2, I-34127 Trieste, Italy d Dipartimento di Informatica e Telecomunicazioni, Universita" di Trento, Via Sommarive 14, I-38050 Povo, Trento, Italy a
b
Abstract We have recently proposed a novel junction termination structure for silicon radiation detectors, featuring all-p-type multiguard and scribe-line implants, with metal field-plates completely covering the gap between the implanted rings. The structure is intended for detector long-term stability enhancement even in adverse ambient conditions and for fabrication-process simplification. A thorough static characterization, including stability measurements in varying humidity conditions, has been carried out on a variety of samples fabricated at ITC-irst. Comparisons with diodes featuring an n-type implant along the border—or no edge structure at all—have been performed. The new structures show stable behaviour at relatively high bias ðB200 VÞ; also in the presence of wide humidity changes (1–90%). A good qualitative agreement has been obtained between experimental results and simulation predictions, allowing to gain deep insight into the physical behaviour of the device. r 2003 Elsevier B.V. All rights reserved. PACS: 85.30.De; 29.40.Wk; 73.40.Qv Keywords: Semiconductor-device characterization; Design and modeling; Solid-state detectors; Metal-insulator-semiconductor structures
1. Device description The devices under test [1,2] consist of a square diode surrounded by a biased guard ring (LG), 100 mm wide, and by a set of floating pþ guard $ This work has been partially supported by the National Institute for Nuclear Physics of Italy (INFN) and by the ‘‘Provincia Autonoma di Trento’’ under the Contract ‘‘Fondo per i progetti di ricerca 2002-progetto PDX’’. *Corresponding author. E-mail address: [email protected] (S. Dittongo).
rings, the last of which extends to the cut edge. The guards are characterized by metal field-plates that completely cover the region between adjacent rings; in this way the n-type substrate just beneath the Si2SiO2 interface is electrostatically screened from the external environment. Two different designs have been developed, differing in the configuration of field-plates: in the first one (P1), each field-plate is oriented inward and overlaps the implant of the preceding ring; in the second one (P2), the field-plates between the rings extend
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.023
ARTICLE IN PRESS S. Dittongo et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 362–365 Table 1 Experimental values of the main technological parameters extracted from test structures made on different types of wafers
Orientation Nsub ð1010 cm3 Þ tox ðnmÞ Nf ð109 cm2 Þ tg ðmsÞ s0 ðcm=sÞ
Wafer A
Wafer B
Wafer C
Wafer D
ð1 1 1Þ 3575 97474 340750 1074 8:672:2
ð1 0 0Þ 2571 86573 243714 4:373:2 5:371:8
ð1 1 1Þ 2872 85572 28479 113750 6:070:4
ð1 1 1Þ 1771 665760 40977 80740 33:770:8
Substrate-donors concentration ðNsub Þ; oxide thickness ðtox Þ; oxide-charge density ðNf Þ; bulk-generation lifetime ðtg Þ; surfacegeneration velocity ðs0 Þ:
alternately outward and inward, so that only every second implanted ring has to be contacted by a metal layer, allowing a larger number of rings (20 w.r.t. 12) to be fitted within the same area. For both designs, four layouts (P1:1; y ; P1:4 and P2:1; y ; P2:4) have been implemented, differing in the spacings between the rings. More details on the geometrical characteristics of samples can be found in [1,2]. For comparison with more traditional structures, simple diodes with a biased guard ring (LG), 100 mm wide, but no floating rings have also been fabricated. Some of these have no edge structure, the others feature an n-type implant along the border, 300 mm wide. The latters have been implemented in five layouts ðN1; y ; N5Þ; differing in the distance ð1002300 mmÞ between LG and n-type edge implant. All devices have been fabricated at ITC-irst on high resistivity n-type silicon wafers, ð1 1 1Þ or ð1 0 0Þ oriented and 300 mm thick. The main technological parameters, extracted from dedicated test structures, are summarized in Table 1 for four different wafer types. 2. Electrical characterization The voltage-handling capability of the structures has been tested by measuring the large-guard current ILG as a function of the reverse bias voltage Vrev : As already observed in [1,2], the lateral punch-through from the LG to the edge is the voltage limiting mechanism in all multiguard structures, whereas in the samples with the n-type
363
Table 2 Experimental values of Vlim ; expressed in V, for all the multiguard structures measured on four types of wafers
P1.1 P1.2 P1.3 P1.4 P2.1 P2.2 P2.3 P2.4
Wafer A
Wafer B
Wafer C
Wafer D
8575 18677 104711 153714 n.a. n.a. n.a. n.a.
4972 11079 5776 9378 n.a. n.a. n.a. n.a.
n.a. 11876 n.a. 9675 13676 16175 13276 15574
n.a. 6975 n.a. 5473 9676 11379 9675 10773
Table 3 Experimental values of VBD ; expressed in V, for all the structures without multiguard measured on two types of wafers
N1 N2 N3 N4 N5
Wafer A
Wafer B
14878 15475 151713 15475 15376
29076 29275 29175 29177 28979
implant along the border breakdown at the pþ 2n junction edge is observed. Following the same approach used in [1], we represent the sensitivity of the LG current to the reverse voltage in the multiguard devices by means of the adimensional function K ¼ ðdILG =dVrev Þ ðVrev =ILG Þ: By adopting the same limit value as in [1], we calculated the maximum voltage, Vlim ; for which KoKlim ¼ 2 (see Table 2). The values of Vlim differ according to the different geometries of the guard rings. As far as the samples without multiguard structures are concerned, Table 3 reports the measured breakdown voltage VBD : No significant difference has been observed in the VBD among different layouts for devices fabricated on a given wafer. As expected, VBD is higher in ð1 0 0Þ w.r.t. ð1 1 1Þ wafers because of the lower oxide charge density.
3. Stability measurements All tested all-p-type multiguard structures exhibited excellent time stability of the leakage
ARTICLE IN PRESS S. Dittongo et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 362–365
current even in the most adverse conditions: no passivation, high humidity (90% RH), and bias voltage close to the operational limit. This result was expected, because the metal field-plates cover the whole region in between consecutive rings. The picture is more complex for devices with a single guard ring. All tested samples, either with or without n-type implant along the cut edge, have shown a large increase of the guard ring current when biased at high voltage ð1202180 VÞ in highhumidity conditions. The time required for the onset of this instability varied (from a few hours to a few days) depending on the bias voltage, the humidity level, the oxide charge (ð1 1 1Þ vs. ð1 0 0Þ wafers) and the presence/absence of a passivating oxide. In all cases, care has been taken to set the bias voltage below the value causing breakdown by avalanche multiplication at the edge of the guard ring. On devices without n-type edge implant, the current has generally been observed to stabilize at a value of a few mA (for a perimeter of 20 mm), while on the structures with edge implant the current always reached compliance (set at 10 mA) before showing signs of stabilization. This different behaviour has been attributed to the onset of avalanche multiplication at the internal edge of the nþ -implant as a result of the field-plate effect of ions drifting on the oxide surface in high humidity, eventually making the surface equipotential with the pþ guard ring [3,4]. By contrast, on structures without edge implant the final current is due mainly to the carriers generated at the damaged cut region, which is collected because of the lateral extension of the space-charge region caused by the formation of the ion field-plate. Measurements made on uncut devices (on wafer) support this view: in the presence of edge implant, the behaviour is very similar to what observed for cut dice, while for structures without n-implant the current stabilizes at a much lower value (tens of nA), likely due to bulk and interface generation in the laterally extended depletion region. Fig. 1 illustrates these points by showing the I2t curves for four devices: with and without n-implant, cut and on wafer. Simulations performed not accounting for the effect of surface ions show that at high voltage (but still below VBD ) the space-charge region
1e-05
1e-06
log(ILG) [A]
364
1e-07
1e-08
+
1e-09
n , cut, RH=47%-70% n , on wafer, RH=15%-78% + no n , cut, RH=30%-75% + no n , on wafer, RH=76%-82% +
1e-10 0
5
10 15 time [hours]
20
25
Fig. 1. Experimental LG current as a function of time for diodes having a single guard ring and kept in a high humidity environment.
Fig. 2. Simulated hole current density in a structure with nimplant on the edge, biased below breakdown voltage. The cut line is on the right edge. A channel of hole current flowing in the bulk from the edge to the LG can be clearly seen.
approaches the lateral cut surface deep in the bulk, causing a high hole current to flow toward the guard ring (Fig. 2); the presence of the nimplant has no effect in this case, because when no ionic charges are considered, the interface is in any case accumulated up to a short distance from the pþ guard ring due to the presence of the positive
ARTICLE IN PRESS S. Dittongo et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 362–365
oxide charge. The drifting ions on the surface change the situation, by compensating the oxide charge progressively extending the depletion region outward; the ion drift stops at the edge of the n-implant, if present, while otherwise continues until it reaches the cut edge of the device. In the first case we get increasingly high-field values at the edge of the n-implant, leading to avalanche breakdown; in the second case the depletion region eventually reaches the damaged cut surface of the device. In order to reproduce the effect of the ions on the outer surface, we added in the simulation a field-plate electrode on top of the oxide, extending from the guard ring to the n-implant, and biased at the same potential of the guard ring [4]. Simulation data clearly show that in this case avalanche breakdown at the edge of the n-implant occurs, at a lower voltage than required to cause breakdown at the edge of the LG in the absence of field-plate.
365
full depletion voltage, so that, at normal operating bias, avalanche breakdown does not occur. In these conditions, the n-implant plays an useful role, because it stabilizes the device by preventing the depletion region from reaching the cut edge. However, for thicker or higher doping substrates, operated in high humidity conditions, the breakdown voltage at the edge of the n-implant may be comparable to the total depletion voltage. In this case the all-p-type multiguard structures offer a reliable solution, with the additional advantage of simplifying the fabrication process. With TCAD simulations we were able to reproduce qualitatively the experimental results. Work is under way to improve the quantitative agreement between simulations and measurements, in order to optimize the design for different process parameters and operating conditions of the devices.
4. Conclusions References Measurements show that all-p-type multiguard structures are very stable at relatively high voltages, even in adverse environmental conditions. For devices without floating rings, long-term stability measurements show that the presence of the n-type implant eventually leads to a decrease of the breakdown voltage, as a result of a shift of the high field region from the edge of the LG to the edge of the n-implant. However, in all tested devices the breakdown voltage is well above the
[1] G.-F. Dalla Betta, M. Boscardin, L. Bosisio, S. Dittongo, P. Gregori, I. Rachevskaia, et al., IEEE Trans. Nucl. Sci. NS49 (4) (2002) 1712. [2] M. Boscardin, L. Bosisio, A. Candelori, G.-F. Dalla Betta, S. Dittongo, P. Gregori, et al., IEEE Trans. Nucl. Sci. NS50 (4) (2003) 1001. [3] A. Longoni, M. Sampietro, L. Struder, . Nucl. Instr. and Meth. A 288 (1990) 35. [4] C. Piemonte, A. Rashevsky, Method of punch-through voltage stabilisation in silicon detectors, INFN/TC-02/08, 2002.