Final Report Summary - DIAMOND (The Development of an Innovative, Accurate, Monolithic, CVD Diamond Array based Radiation Dosimeter System)
The main aim of diamond project is to develop an innovative, accurate, monolithic, CVD diamond array based radiation dosimeter system. This is to improve the levels of accuracy, and performance required by intensity-modulated radiation therapy (IMRT) technology and help SME's to open a new market and meet market needs. Such a system is include three main challenges, the development of the CVD diamond array detectors, the development of advanced readout electronics and integrations of the system main components.
The project has been completed successfully according to the proposed time scale and good results have been achieved. An array of one dimension (1D), size of 50 detectors with a resolution of 500 micron have been developed and integrated into the read out electronics. A major part of the dosimeter specifications have been met: short priming, short rise time, short decay time. Due to time limitations it has not been possible to fully characterise these dosimeters for dose and dose-rate dependence and phonon energy dependence, so it is not possible to say that the full specification has been achieved. Many aspects required to achieve this optimisation were learned during the last quarter of the project and these should prove to be very useful for the production of a fully working dosimeter.
The major tasks involved during the full span of the project can be summarised as follows: validation of the development platform, the development of a suitable quality of CVD diamond, the optimisation of electrical contacts, general device structure, the study of post growth treatment such as polishing and neutron irradiation, fabrication of a one dimensional array including deposition of contacts and bonding. Throughout these tasks the material quality and device performance have been continuously characterised in order to provide feedback for optimisation.
The electronics tasks were also one of the major tasks undertaken and good progress have been made. The consortium manage to develop a new analogue front end X-ray microchip to handle the signal conditioning system function for the CVD diamond array read out system (DAS). The essential units for the system was also developed, integrated and some initial test has been carried.
The developed one dimension CVD diamond array was integrated into the developed read out electronics but not fully tested with an X-ray source. This is because of unforeseen circumstance we faced during the development process of the CVD diamond material and ASIC.
A total of 17 wafers and 166 samples were produced over the 24 month period. At the end of the first reporting period only 24 samples had been fabricated into dosimeters and characterised. Over the remaining 12 months of the project the number of dosimeter structures fabricated and tested increased to well over 100. This reflects the increased fabrication and characterisation effort devoted to the project after the basic device fabrication technology was developed.
Initial neutron irradiation studies on samples from ERVII-164 showed that very significant improvements in the priming time, rise and decay times and the size of the persistent photoconductivity are achieved. The best of the 164 samples came close to the desired specification and it was decided to test further wafer structures and processes in order to meet the full specification. In these samples the original wafer was over 0.6 mm thick and the samples had over 0.25 mm removed from the nucleation side. For economical and practical reasons we devoted considerable effort to see if similar performance could be achieved with thinner wafers (less expensive to produce and process). It was towards the end of the 24 month period that we had to conclude that the quality of the top section of a thick wafer is still the best possible quality and probably necessary for a working dosimeter, in spite of cost considerations.
It was thought that an alternative to neutron irradiation could be nitrogen doping which would result in a less costly process and probably samples with less structural damage. To test this hypothesis five wafers were synthesised with various concentrations of added nitrogen. A total of 80 samples were produced from these wafers so that tests could be performed independently at all centres: IAF, PTW, Florence and Sheffield. Results have been disappointing. Many samples exhibited a high level of dark current and without irradiation not much improvement compared with pure samples. There was one exception: samples from wafer 191 which exhibited a better performance, not yet meeting the spec. This suggests that the idea may have some merit and if time had allowed, further studies could have been conducted.
The project has been completed successfully according to the proposed time scale and good results have been achieved. An array of one dimension (1D), size of 50 detectors with a resolution of 500 micron have been developed and integrated into the read out electronics. A major part of the dosimeter specifications have been met: short priming, short rise time, short decay time. Due to time limitations it has not been possible to fully characterise these dosimeters for dose and dose-rate dependence and phonon energy dependence, so it is not possible to say that the full specification has been achieved. Many aspects required to achieve this optimisation were learned during the last quarter of the project and these should prove to be very useful for the production of a fully working dosimeter.
The major tasks involved during the full span of the project can be summarised as follows: validation of the development platform, the development of a suitable quality of CVD diamond, the optimisation of electrical contacts, general device structure, the study of post growth treatment such as polishing and neutron irradiation, fabrication of a one dimensional array including deposition of contacts and bonding. Throughout these tasks the material quality and device performance have been continuously characterised in order to provide feedback for optimisation.
The electronics tasks were also one of the major tasks undertaken and good progress have been made. The consortium manage to develop a new analogue front end X-ray microchip to handle the signal conditioning system function for the CVD diamond array read out system (DAS). The essential units for the system was also developed, integrated and some initial test has been carried.
The developed one dimension CVD diamond array was integrated into the developed read out electronics but not fully tested with an X-ray source. This is because of unforeseen circumstance we faced during the development process of the CVD diamond material and ASIC.
A total of 17 wafers and 166 samples were produced over the 24 month period. At the end of the first reporting period only 24 samples had been fabricated into dosimeters and characterised. Over the remaining 12 months of the project the number of dosimeter structures fabricated and tested increased to well over 100. This reflects the increased fabrication and characterisation effort devoted to the project after the basic device fabrication technology was developed.
Initial neutron irradiation studies on samples from ERVII-164 showed that very significant improvements in the priming time, rise and decay times and the size of the persistent photoconductivity are achieved. The best of the 164 samples came close to the desired specification and it was decided to test further wafer structures and processes in order to meet the full specification. In these samples the original wafer was over 0.6 mm thick and the samples had over 0.25 mm removed from the nucleation side. For economical and practical reasons we devoted considerable effort to see if similar performance could be achieved with thinner wafers (less expensive to produce and process). It was towards the end of the 24 month period that we had to conclude that the quality of the top section of a thick wafer is still the best possible quality and probably necessary for a working dosimeter, in spite of cost considerations.
It was thought that an alternative to neutron irradiation could be nitrogen doping which would result in a less costly process and probably samples with less structural damage. To test this hypothesis five wafers were synthesised with various concentrations of added nitrogen. A total of 80 samples were produced from these wafers so that tests could be performed independently at all centres: IAF, PTW, Florence and Sheffield. Results have been disappointing. Many samples exhibited a high level of dark current and without irradiation not much improvement compared with pure samples. There was one exception: samples from wafer 191 which exhibited a better performance, not yet meeting the spec. This suggests that the idea may have some merit and if time had allowed, further studies could have been conducted.
