Dear Collaborators,
Please find below the Proposal on Radiation Hardening on Silicon Detectors(named from now on the ROSE collaboration). The co-spokesmen are Gunnar Lindstroem, Steve Watts and Francois Lemeilleur. It has been submitted to the CERN LHC Committee and will be discussed and we hope agreed by this Committee on 30 May 1996. This document refers to a Table that I will send you later (I am not sure the format can be sent by E-mail) and to an Appendix which is the summary of the CERN October 95 Workshop you already should have received several months ago. As collaborating institutes, Bari, Minsk and Modena have to be included in addition as well as the European Space Agency as observer. We will let you inform of the LHCC decision and after organize the detailed planning of the silicon sample distribution to participant institutions for measurements, irradiations and analyses.
Please any comments from your side should be directed to one of the spokesmen.
Best regards, Francois Lemeilleur.
CERN/LHCC 96-23
P62 / LHC R&D
23 April 1996
PROPOSAL FOR FURTHER WORK ON RADIATION HARDENING OF SILICON DETECTORS
The ROSE Collaboration
(R & d On Silicon for future Experiments)
Co-Spokespersons: Francois Lemeilleur, Gunnar Lindstroem, Steve Watts
Brookhaven National Laboratory, USA.
H. W. Kraner, Z. Li
Brunel University, UK
A. Holmes-Siedle, I. Hopkins, J. Matheson, M. Solanky, S. Watts
Institute of Nuclear Physics and Engineering, Bucharest, Romania
A. Vasilescu
Institute of Physics and Technology of Materials, Bucharest, Romania
T. Botila, D. Petre, I. Pintilie, L. Pintilie
University of California, Dept. of Materials Science, Berkeley , USA
E. Weber
University of Catania, Italy
S. Albergo, R. Potenza
Dortmund University, Germany
C. Becker, A. Rolf, R. Wunstorf
CERN, ECP Division, Switzerland
G.L. Casse, B. Dezillie, M. Glaser, F. Lemeilleur, C. Leroy
CERN, PPE Division, Switzerland
S. Roe, P. Weilhammer
Universita di Firenze, Italy
U. Biggeri, E.Borchi, M.Bruzzi, E.Catacchini, E. Focardi, G. Parrini
Hamburg University, Germany
H. Feick, E. Fretwurst, G. Lindstroem, M. Moll
Imperial College, University of London, UK.
B. MacEvoy, G. Hall
INFN, Pisa, Italy.
R. Dell'Orso, A. Messineo, G. Tonelli, P. Verdini, R. Wheadon
Kings College, University of London, UK
G. Davies
Institute for Nuclear Research, Kiev, Academy of Sciences, Ukraine
P. Litovchenko
Max Planck Institute, Munich, Germany
G. Lutz, R.H. Richter
Universita di Padova, Italy
N. Bacchetta , D. Bisello, A. Giraldo
Czech Technical University of Prague, Czech Republic
S. Pospisil, B. Sopko
PSI, Switzerland
K. Gabathuler, R. Horisberger
Laboratory of Non-Equilibrium Processes in Semiconductors, Ioffe Physico-Technical
Institute, St. Petersburg, Russia
V. Eremin, E. Verbitskaya
University of New Mexico, USA
J.A.J. Matthews, S. Seidel
University of Perugia, Italy
P. Bartalini, G.M. Bilei, P. Ciampolini, D. Passeri, A. Santocchia
Institute of Nuclear Physics Demokritos , Greece
G. Fanourakis, D. Loukas, A. Markou, I. Siotis, S. Tzamarias, A. Vayaki
Observers
European Space Agency, ESTEC, Holland
B. Johlander, Solar system Division
IMEC, Belgium
C. Claeys, E. Simoen
University of Lancaster, UK
A. Chilingarov
LEPES, Grenoble, France
J-C. Bruyere
Max Planck Institute, Munich, Germany
J. Kemmer, N. Meidinger
Contents
1. Introduction
2. Objectives
3. Plan of action
4. Timescale and Milestones
5. Request for resources
6. Industrial Collaboration and Observers
7. Overview of the collaboration
8. Conclusion
1. Introduction
Silicon detectors are key components of many future experiments. In recent years there has been a large amount of work aimed at understanding radiation damage effects in high resistivity silicon detectors. CERN's RD2 and RD20 collaborations have devoted much effort to this generic R&D work. Both collaborations have submitted final reports and most of the groups have joined LHC experiments. However, there has been a substantial amount of progress in the last year, and groups working on radiation effects in RD2 and RD20 are optimistic that more radiation tolerant detectors are possible by the use of "defect engineering". There is a clear desire and need for all the groups working in this area to collaborate on a common programme. This will prevent duplication of effort, focus the activities of the groups, and lead to efficient exchange of ideas, irradiation facilities, test facilities, and samples.
Safe operation at the LHC for 10 years is the main goal of current development work. This canot be guaranteed at present for the fluence levels expected over the whole radial coverage of the experiments. Moreover, there are uncertainties which need further study which affect our ability to project our present knowledge of radiation hardness data to the LHC operational scenario. Success in these areas will also be of great benefit to experiments at HERA-B and in space projects.
A Workshop on Radiation Hardening of Silicon Detectors was held at CERN in October 1995. Representatives from about 30 institutions attended. In addition, solid-state physicists are now showing a keen interest in this work, and several attended the workshop. Key staff from the silicon wafer and detector industry in Europe also attended and contributed some vital background information. A summary of the Workshop is appended to this proposal (see Appendix I). It contains a good summary of work performed to the end of 1995.
The most recent results were presented at the Florence Conference in March 1996. Substantial progress has been made by all groups in many areas. Transparencies from this conference are available (CERN Building 13 3-005).
Two important issues highlighted by the October 1995 Workshop were addressed at the Florence conference. These were:
a) Most results to date have used similar silicon which in most cases came from the same source - Wacker. It is vital to obtain data using material that is significantly different. First results using epitaxial material are extremely promising. Diodes made on 900 ohm.cm, 100 micron thick n-type epitaxial silicon have been irradiated with 24 GeV/c protons. As usual, the effective doping concentration falls with irradiation, but by 1014 p cm-2 the diodes have still not inverted and the depletion voltage appears to have plateaued. Further data at higher fluences will be obtained in the near future. In addition, the leakage current damage parameter is about a factor two less than in high resistivity float-zone material and no reverse annealing is observed. Epitaxial material is thought to have substantially increased oxygen and carbon levels compared to float-zone silicon due to auto-doping from the CZ substrate. Measurements are in progress to determine the impurity levels in the epitaxial silicon used to date. More diodes are to be manufactured using this material and we are collaborating with RD19 because such silicon is suitable for pixel detectors.
b) The measured leakage current in neutron irradiated detectors is larger than expected on the basis of a Shockley-Read-Hall (SRH) generation-recombination calculation using deep level defect concentrations measured by DLTS and other techniques. New calculations indicate that inter-centre charge transfer results in enhanced leakage current compared to estimates based on standard SRH processes. Such charge transfer is possible between divacancies in terminal clusters. One consequence of this idea is that the electron occupancy of the divacancies is enhanced and they can contribute significantly to the radiation induced build-up of negative space charge in the bulk silicon. Work is in progress to correlate defect levels in irradiated diodes with leakage current data. This together with ESR techniques and further theoretical calculations will allow us to validate these ideas.
The Florence Conference demonstrated yet again that our understanding of radiation effects in silicon detectors is improving rapidly and that real progress is being made because of regular contact between the groups.
The rest of this proposal describes the scientific objectives, plan of action, schedule, milestones, and request for resources.
2. Objectives
a) To develop radiation hard silicon detectors that can operate beyond the limits of present devices and that ensure guaranteed operation for the whole lifetime of the LHC experimental programme.
b) To make recommendations to experiments on the optimum silicon for detectors and quality control procedures required to ensure optimal radiation tolerance.
3. Plan of Action
The plan of action is given in the Workshop Summary. The schedule is also given in the summary but is repeated here for clarity. We have found a company in the Czech Republic (Polovodice) which can manufacture 3 inch silicon ingots with varying oxygen and /or carbon concentrations. In addition, ingots containing other impurities (Sn, Ge, N) which may act as gettering sites for radiation induced defects will also be grown. The ingots can then be cut and polished into wafers. Such wafers are vital to the project and enable us to check and improve device models, defect kinetics simulations, material characterisation techniques, and processing methods.
The work at this company will be supervised locally by Prof. B.Sopko at the Czech Technical University in Prague. The work has been organised into two phases. In the first phase, the company will produce standard and oxygenated material. If this is successful, then more exotic variants will be tried.
To prevent the project from being dependent on this one company, material from various sources will be obtained and evaluated. This includes old stock, epitaxial, and silicon from Russia. Silicon containing various impurities is also being developed through Brookhaven National Laboratory.
Wacker have produced oxygenated float zone silicon. Although this is not a commercial product, efforts will be made to evaluate this material. Contacts also exist with Topsil who have provided Si(Ge) wafers in recent months.
4. Timescale and Milestones
Oct. '95 Decide on material options to cover a broad range of oxygen, carbon, boron and phosphorus concentrations.
Jan. '96 Analysis of starting material. onwards Manufacture of test diodes on various material.
Apr. '96 Assessment of devices before and after irradiation. Microscopic and macroscopic evaluation.
Aug/Oct. '96 First results on macroscopic parameters of the test devices.
MILESTONE 1: Workshop on Defect Engineering and Radiation Hardening.
Comparison of data with models.
Dec. '96 Decision on "best choice" of material. Obtain material.
Jan. '97 Analysis of starting material. onwards Manufacture of test diodes on "best choice" material.
Apr. '97 Assessment of devices before and after irradiation. Microscopic and macroscopic evaluation.
Aug/Oct. '97 Results on macroscopic parameters of the test devices.
MILESTONE 2: Workshop on Defect Engineering and Radiation Hardening.
Dec. '97 MILESTONE 3: Report providing recommendations on the silicon to be used for LHC detectors, including quality control procedures to be used during production. Work required for LHC experiments Technical Design
Reports.
Although the project will not be considered by the LHCC until May 1996, work has started using "seedcorn" money in order to keep to the tight timescales required by the LHC experiments. Most material required for the first phase (see section 3) has been delivered to CERN. The project and phase 2 work at Polovodice cannot proceed beyond May without the resources detailed in
Section 5.
5. Request for resources
This is a two year project. The costs for each year are as follows:
Cost (SFr)
"Pure" ingot with standard oxygen and carbon levels 1 000.--
7 ingots introducing various impurity atoms (O, C, Sn, N, Ge) (7 X 3000
SF) 21 000.--
Cut and polishing of wafers (800 wafers) 8 000.--
Processing 15 000.--
Impurity analysis of wafers 15 000.--
Travel and Meetings 30 000.--
Cost of irradiation facilities (see note) 10 000.--
Total for Year 1: 100 000.-- SFr
The second year of the project will have a similar breakdown.
Decisions on the material to order for the second year of the programme will
be made at the end of 1996.
Total for Year 2: 100 000.-- Sfr
Total cost of project: 200 000.-- SFr
Note: Most of the irradiations will use facilities that participating institutions can either resource from their own budgets or which are available at no cost. Facilities at CERN will also be used. However, it is clear that the project will have to pay for radiation facilities in order to keep to schedule. We estimate that access to suitable radiation facilities at short notice is about 1 200 SFr/day. As a result we have requested a contribution to these costs.
6. Industrial Collaboration and Observers
As mentioned above, silicon wafer and detector manufacturers from all over Europe attended the Workshop held at CERN in October 1995. Following this Workshop many manufacturers have indicated a willingness to contribute in some way to the project, and are keen to be kept informed of its progress. We consider it vital to maintain contacts with the semiconductor industry and will invite their representatives to all the Milestone Workshops and keep them up to date with progress.
Some institutions are keen to be kept informed of progress and have experience in radiation effects and material characterisation. However, they are not able to contribute directly to the project at present. These institutions will have "observer" status, which means that they will be invited to Milestone Workshops and will receive reports .
7. Overview of the collaboration
The spreadsheet in the Table provides information about the collaborating groups. The key skills required are material characterisation, defect characterisation, detector characterisaton, modelling, and access to radiation facilities. We believe that a key strength of the collaboration is that it contains physicists from the solid-state commmunity as well as detector experts.
8. Conclusion
In conclusion, although it may not be possible to prevent damage to silicon detectors by irradiation, recent work gives hope that process modifications might lead to harder detectors. Since oxygen and carbon are the dominant capture sites for vacancies and interstitials, these are the key ingredients to alter. Moreover, it will be essential for the LHC experiments to apply strict quality control to the detector starting material. Without such control, it would not be apparent until the detectors had operated for some time that due to variations in impurity levels in the silicon wafers, the radiation hardness had been compromised.