- Email: [email protected]
Steps towards the use of silicon drift detectors in heavy ion collisions at LHC
Nuclear Instruments and Methods in Physics Research A 360 (1995) 67-70
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSiCS RESEARCH SectmA
EISEVIER
Steps towards the use of silicon drift detectors in heavy ion collisions at LHC S. Be012 a,* , P. Burger b, E. Cantatore ‘, G. Casse ‘, F. Corsi ‘, M. Cuomo ‘, W. Dqbrowski *, D. De Venuto ‘, P. Giubellino a, G. Gramegna ‘, V. Manzari d, C. Marzocca ‘, F. Navach d, G. Portacci ‘, L. Riccati a, A. Vacchi ’ aDipart. di Fisica Sperimentale dell’(int~~ersitirand INFN, Torino, Italy
’
h Canberra semiconductor, NV 2250 Olen. Belgium ’ Dipart. di Elettrotecnica ed Elettronica de1 Politecnico and INFN. Bari. Italy d Dipart. di Fisica dell ‘Uni[w-sit&and INFN, Bari, Italy e Dipart. di Fisica dell’lJnitwsit& and INFN, Trieste, Italy Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland
Abstract The inner tracking system of the ALICE detector for Pb-Pb collisions at the LHC require a very good granularity in the innermost planes, due to the high particle density, up to 8000 particles per unit of rapidity. The silicon drift detectors are a very good candidate for this application, but up to now no large system using this technology has been industrially produced and operated in experiments. One of the first steps towards large scale production is the study of the doping uniformity in commercially available Si wafers. The understanding of doping fluctuations is of fundamental importance since they introduce deviations of the electron trajectories from the expected ones. In addition, it is also necessary to know the changes possibly introduced by different processing steps in the resistivity profiles. We report here the results of measurements of resistivity profiles for NTD silicon wafers both before and after processing.
1. Introduction Silicon drift detectors will be used for the inner tracking system of the ALICE detector. The aim of ALICE collaboration is to study the physics of strongly interacting matter at extreme energy density where the formation of a quark gluon plasma is expected [I]. The basic functions of the inner tracker are secondary vertex reconstruction of hyperon decays, particle identification and tracking of low momentum particles, and the improvement of the momentum resolution. Because of the high particle density, the three innermost layers need to be truly two dimensional devices, such as pixel or silicon drift detectors. Silicon drift detectors deliver unambiguously the two coordinates of the origin of ionization by measuring the electron cloud drift time and the centroid of the distribution of the charge collected at the anodes. In this way they enhance resolution and multitrack capability at the expense of speed, being ideally suited to this experiment in which very high multiplicities are coupled with relatively low event rates
* Corresponding
author.
016%9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(94)01223-7
[2]. The detector will be fabricated from 4-in., high resistivity, neutron doped silicon wafers of 300 p,m thickness. Neutron doped silicon has been chosen to avoid problems due to inhomogeneous doping of the bulk, as it is explained in the following paragraph.
2. Doping problems In silicon drift chambers the semiconductor n bulk is depleted by suitably biased p+ strips which also provide the drift field parallel to the surface of the detector. Electrons generated by ionization are collected in the middle plane of the detector and then drift towards the n+ anode pads. In the ideal case in which the doping distribution in the bulk is perfectly homogeneous, the electric potential along the drift direction can be written as a solution of Poisson’s equation in two dimensions. The consequent trajectory of the electron cloud is a straight line from the generation point to the collecting anode. Actually, as proved by different studies, the doping distribution is not homogeneous, and this leads to local variation of the depth of the potential energy valley in the depleted wafer and therefore to a perturbing electric field in the transverse
I. TRACKING
68
S. Be&
et al. / Nucl. Instr. and Metil. in PIys. Rrs. A 360 (1995) 67-70
Fig. 1. (a) Schematic
drawing of a test structure. (b) Photograph
of a diode.
S. Be012 et al. / Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 67-70
direction which adds to the ideal drift field. This causes a deflection of electron trajectory along their path, and also variation of the drift time. An example of trajectories deflection has been observed by the laboratory tests of a SDC carried out by the UA6 collaboration. For the UA6 detectors it was used traditionally doped floating zone silicon in which the expected fluctuations, as WackerChemitronic found on n-type phosphorous doped silicon of high resistivity, are expected to be 15% of the mean value [S]. Comparing results obtained by measuring the deviations of the electron trajectories by means of laser injection of electrons, it was confirmed that fluctuations were of that order of magnitude [3,4]. In Ref. [4] it is shown that it is possible to reduce the deviations by increasing the electric field. Unfortunately it is impossible to totally correct the trajectories. For what concerns the problems due to the variations of drift speed, they can be avoided by calibrating the drift time. This is done by electron injection in known locations on the surface of the chamber. In order to obtain a distribution of dopant as homogeneous as possible, a doping tecnique different from the traditional one can be used. Silicon is irradiated by neutrons: the exact resistivity value can be obtained within narrow limits and the material is practically free from the macroscopic and microscopic variation of resistivity observed in conventionally doped silicon [6]. This kind of material is called Neutron Transmutation Doped silicon. The DSI collaboration plans to use NTD silicon for the production of SDCs. Since it was not possible to find exhaustive information about the homogeneity of the dopant distribution in NTD silicon, we decided to carry out a complete set of measurements on NTD silicon wafers.
3. Processing steps Since in the ALICE inner tracking system hundreds of drift chambers are needed, the detectors will be produced industrially. Therefore we have launched an R&D program together with Canberra Semiconductor. This is the first time that a complex device such as a SDC is produced in large scale. So, in addition to the tests of homogeneity of resistivity, it is necessary to carry out tests of all processing steps, in order to see if they can introduce changes in the resistivity profile. At first, polishing of both side of wafers and polysilicon deposition have been studied. The wafer has to be double-side polished because detector structures will be implanted on both sides. A batch of one-side polished wafers has been bought from Wacker-Chemitronic and then some of them have been polished on the second side at Micropolish. After that a process of polysilicon deposition and then remotion has been carried out at Canberra. The process of polysilicon deposition takes place at a very high tem~rature, and this can damage the silicon bulk lattice structure and we would
69
like to be sure that it doesn’t change the expected distribution.
dopant
4. Measurements on test structures At Canberra test structures have been implanted on silicon wafers to measure the resistivity profile and the values of leakage current. Comparing results on differently treated wafers it is possible to evaluate the effects due to industrial processes. The test structures consist of 256 diodes of 3.7 mm diameter arranged in a 16 X 16 matrix implanted on one side of the 4 in. diameter wafer, see Fig. 1. Each diode is a p+ implantation covered by aluminium. On the other side of the wafer a n+ implantation provides bulk contact. Measurements of the C-V curve of each inverse biased diode have been performed in order to obtain the resistivity profile. The voltage point at which the capacitance reaches its minimum value has been evaluated: this value is the so called “depletion voltage”, and it is proportional to the doping concentration at a fixed mobility value. The variation of resistivity values between adjacent diodes on a diode matrix allows us to get information about the doping concentration uniformity on each wafer. We repeated these measurements on different wafers. Leakage current measurements have also been performed. Results are summarized in the next section.
5. Results and conclusions For what concerns leakage current measurements, the values was increased after polishing the wafers on the second side. The mean value of currents at the depletion voltage have been found to be equal to 60 pA before polishing and 120 pA after polishing. The polysilicon deposition process does not seem to change the values of the current. From the resistivity measurements we can conclude that the RMS of the doping concentration distribution is less than 2% of the mean value. Polysilicon deposition caused problems on few diodes as can be seen Table 1 Results of resistivity
measurements
(the RMS value is averaged
over all wafers of that kind) RMS over one wafer
Maximum spread
Wafer
Mean
Wafer to
‘YPe
resistivity (0. cm)
wafer
3227
6.2%
1.1%
6%
3033
10%
1.4%
6%
3232
9%
1.6%
30%
spread
on each wafer
1 side polished 2 sides polished Polys~iicon treated
I. TRACKING
in Table 1. In fact the maximum fluctuations are 30% of the mean value, while before the polysilicon treatment they were of about 6% of the mean value. These results demonstrate that neutron doped silicon provides much better uniformity compared to conventionally doped one. Fluctuations are less than expected, and it should be possible to realize properly working SDCs. For what concerns industrial processing, we point out that all steps must be kept under good control. New test structures are going to be measured in order to increase statistics, and to study other production steps, such as the oxide deposition.
References
[I] ALICE collahoraticm, Lcttcr Of Intent For ALICE, CEKN/LH cc. Y3/16. (3 P. Kehak et al., Nucl. Instr. and Meth. A ?35 (19X.5) 224. (31 A. fastoldi et al., J. Appl. Phys. 71 (1992) 2593. [4] A. Vacchi et al., Nucl. Instr. and Meth. A 306 f 19911 187. [S] W. van Amman and H. Herzcr NucI. Instr. and Meth. 226 (1983) 94. [6] E.W. Haas and MS. Schnoeller, IEEE Trans. Electron DeVlCCS‘3(8) (1976), 803.
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSiCS RESEARCH SectmA
EISEVIER
Steps towards the use of silicon drift detectors in heavy ion collisions at LHC S. Be012 a,* , P. Burger b, E. Cantatore ‘, G. Casse ‘, F. Corsi ‘, M. Cuomo ‘, W. Dqbrowski *, D. De Venuto ‘, P. Giubellino a, G. Gramegna ‘, V. Manzari d, C. Marzocca ‘, F. Navach d, G. Portacci ‘, L. Riccati a, A. Vacchi ’ aDipart. di Fisica Sperimentale dell’(int~~ersitirand INFN, Torino, Italy
’
h Canberra semiconductor, NV 2250 Olen. Belgium ’ Dipart. di Elettrotecnica ed Elettronica de1 Politecnico and INFN. Bari. Italy d Dipart. di Fisica dell ‘Uni[w-sit&and INFN, Bari, Italy e Dipart. di Fisica dell’lJnitwsit& and INFN, Trieste, Italy Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland
Abstract The inner tracking system of the ALICE detector for Pb-Pb collisions at the LHC require a very good granularity in the innermost planes, due to the high particle density, up to 8000 particles per unit of rapidity. The silicon drift detectors are a very good candidate for this application, but up to now no large system using this technology has been industrially produced and operated in experiments. One of the first steps towards large scale production is the study of the doping uniformity in commercially available Si wafers. The understanding of doping fluctuations is of fundamental importance since they introduce deviations of the electron trajectories from the expected ones. In addition, it is also necessary to know the changes possibly introduced by different processing steps in the resistivity profiles. We report here the results of measurements of resistivity profiles for NTD silicon wafers both before and after processing.
1. Introduction Silicon drift detectors will be used for the inner tracking system of the ALICE detector. The aim of ALICE collaboration is to study the physics of strongly interacting matter at extreme energy density where the formation of a quark gluon plasma is expected [I]. The basic functions of the inner tracker are secondary vertex reconstruction of hyperon decays, particle identification and tracking of low momentum particles, and the improvement of the momentum resolution. Because of the high particle density, the three innermost layers need to be truly two dimensional devices, such as pixel or silicon drift detectors. Silicon drift detectors deliver unambiguously the two coordinates of the origin of ionization by measuring the electron cloud drift time and the centroid of the distribution of the charge collected at the anodes. In this way they enhance resolution and multitrack capability at the expense of speed, being ideally suited to this experiment in which very high multiplicities are coupled with relatively low event rates
* Corresponding
author.
016%9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(94)01223-7
[2]. The detector will be fabricated from 4-in., high resistivity, neutron doped silicon wafers of 300 p,m thickness. Neutron doped silicon has been chosen to avoid problems due to inhomogeneous doping of the bulk, as it is explained in the following paragraph.
2. Doping problems In silicon drift chambers the semiconductor n bulk is depleted by suitably biased p+ strips which also provide the drift field parallel to the surface of the detector. Electrons generated by ionization are collected in the middle plane of the detector and then drift towards the n+ anode pads. In the ideal case in which the doping distribution in the bulk is perfectly homogeneous, the electric potential along the drift direction can be written as a solution of Poisson’s equation in two dimensions. The consequent trajectory of the electron cloud is a straight line from the generation point to the collecting anode. Actually, as proved by different studies, the doping distribution is not homogeneous, and this leads to local variation of the depth of the potential energy valley in the depleted wafer and therefore to a perturbing electric field in the transverse
I. TRACKING
68
S. Be&
et al. / Nucl. Instr. and Metil. in PIys. Rrs. A 360 (1995) 67-70
Fig. 1. (a) Schematic
drawing of a test structure. (b) Photograph
of a diode.
S. Be012 et al. / Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 67-70
direction which adds to the ideal drift field. This causes a deflection of electron trajectory along their path, and also variation of the drift time. An example of trajectories deflection has been observed by the laboratory tests of a SDC carried out by the UA6 collaboration. For the UA6 detectors it was used traditionally doped floating zone silicon in which the expected fluctuations, as WackerChemitronic found on n-type phosphorous doped silicon of high resistivity, are expected to be 15% of the mean value [S]. Comparing results obtained by measuring the deviations of the electron trajectories by means of laser injection of electrons, it was confirmed that fluctuations were of that order of magnitude [3,4]. In Ref. [4] it is shown that it is possible to reduce the deviations by increasing the electric field. Unfortunately it is impossible to totally correct the trajectories. For what concerns the problems due to the variations of drift speed, they can be avoided by calibrating the drift time. This is done by electron injection in known locations on the surface of the chamber. In order to obtain a distribution of dopant as homogeneous as possible, a doping tecnique different from the traditional one can be used. Silicon is irradiated by neutrons: the exact resistivity value can be obtained within narrow limits and the material is practically free from the macroscopic and microscopic variation of resistivity observed in conventionally doped silicon [6]. This kind of material is called Neutron Transmutation Doped silicon. The DSI collaboration plans to use NTD silicon for the production of SDCs. Since it was not possible to find exhaustive information about the homogeneity of the dopant distribution in NTD silicon, we decided to carry out a complete set of measurements on NTD silicon wafers.
3. Processing steps Since in the ALICE inner tracking system hundreds of drift chambers are needed, the detectors will be produced industrially. Therefore we have launched an R&D program together with Canberra Semiconductor. This is the first time that a complex device such as a SDC is produced in large scale. So, in addition to the tests of homogeneity of resistivity, it is necessary to carry out tests of all processing steps, in order to see if they can introduce changes in the resistivity profile. At first, polishing of both side of wafers and polysilicon deposition have been studied. The wafer has to be double-side polished because detector structures will be implanted on both sides. A batch of one-side polished wafers has been bought from Wacker-Chemitronic and then some of them have been polished on the second side at Micropolish. After that a process of polysilicon deposition and then remotion has been carried out at Canberra. The process of polysilicon deposition takes place at a very high tem~rature, and this can damage the silicon bulk lattice structure and we would
69
like to be sure that it doesn’t change the expected distribution.
dopant
4. Measurements on test structures At Canberra test structures have been implanted on silicon wafers to measure the resistivity profile and the values of leakage current. Comparing results on differently treated wafers it is possible to evaluate the effects due to industrial processes. The test structures consist of 256 diodes of 3.7 mm diameter arranged in a 16 X 16 matrix implanted on one side of the 4 in. diameter wafer, see Fig. 1. Each diode is a p+ implantation covered by aluminium. On the other side of the wafer a n+ implantation provides bulk contact. Measurements of the C-V curve of each inverse biased diode have been performed in order to obtain the resistivity profile. The voltage point at which the capacitance reaches its minimum value has been evaluated: this value is the so called “depletion voltage”, and it is proportional to the doping concentration at a fixed mobility value. The variation of resistivity values between adjacent diodes on a diode matrix allows us to get information about the doping concentration uniformity on each wafer. We repeated these measurements on different wafers. Leakage current measurements have also been performed. Results are summarized in the next section.
5. Results and conclusions For what concerns leakage current measurements, the values was increased after polishing the wafers on the second side. The mean value of currents at the depletion voltage have been found to be equal to 60 pA before polishing and 120 pA after polishing. The polysilicon deposition process does not seem to change the values of the current. From the resistivity measurements we can conclude that the RMS of the doping concentration distribution is less than 2% of the mean value. Polysilicon deposition caused problems on few diodes as can be seen Table 1 Results of resistivity
measurements
(the RMS value is averaged
over all wafers of that kind) RMS over one wafer
Maximum spread
Wafer
Mean
Wafer to
‘YPe
resistivity (0. cm)
wafer
3227
6.2%
1.1%
6%
3033
10%
1.4%
6%
3232
9%
1.6%
30%
spread
on each wafer
1 side polished 2 sides polished Polys~iicon treated
I. TRACKING
in Table 1. In fact the maximum fluctuations are 30% of the mean value, while before the polysilicon treatment they were of about 6% of the mean value. These results demonstrate that neutron doped silicon provides much better uniformity compared to conventionally doped one. Fluctuations are less than expected, and it should be possible to realize properly working SDCs. For what concerns industrial processing, we point out that all steps must be kept under good control. New test structures are going to be measured in order to increase statistics, and to study other production steps, such as the oxide deposition.
References
[I] ALICE collahoraticm, Lcttcr Of Intent For ALICE, CEKN/LH cc. Y3/16. (3 P. Kehak et al., Nucl. Instr. and Meth. A ?35 (19X.5) 224. (31 A. fastoldi et al., J. Appl. Phys. 71 (1992) 2593. [4] A. Vacchi et al., Nucl. Instr. and Meth. A 306 f 19911 187. [S] W. van Amman and H. Herzcr NucI. Instr. and Meth. 226 (1983) 94. [6] E.W. Haas and MS. Schnoeller, IEEE Trans. Electron DeVlCCS‘3(8) (1976), 803.