A comparison between rad-hard float zone silicon diodes as gamma dosimeter in radiation processing

In this work, we report on the results obtained with rad-hard Standard Float Zone (STFZ) and Diffused Oxygenated Float Zone (DOFZ) silicon diodes in radiation processing dosimetry. The dosimetric probes were designed to operate in the direct current mode, as on-line radiation dosimeter. The irradiation of the samples was performed using a Co source with a dose rate of almost 2.4 kGy/h. The current response of each diode was measured as a function of the exposure time in steps from 5 kGy up to 50 kGy to achieve a total absorbed dose of 275 kGy. In this dose range it is observed a significant decrease in the photocurrent generated in both devices due to gamma radiation defects produced in their active volumes. To mitigate this effect, the samples were pre-irradiated with Co gamma rays at 700 kGy. Despite of being less sensitive, these devices presented stable and reproducible current signals with a relative sensitivity decrease of about 19% within the whole range of dose studied. The dose-response curves of the pre-irradiated diodes showed quadratic behavior with correlation coefficient higher than 0.9999 for total absorbed dose up to 275 kGy. The comparison of the diodes’ dosimetric responses evidenced that the best result was achieved with the DOFZ, which exhibited slightly higher sensitivity and stability than the STFZ devices. However, it is important to note that all preirradiated diodes can be used as gamma dosimeters in radiation processing applications.


INTRODUCTION
The use of silicon diodes as on-line gamma dosimeters in radiation processing is a big challenge due to the radiation damage effects suffered by semiconductor devices, such as: sensitivity dependence on dose-rate and the decrease of sensitivity with accumulated dose, the main restriction to their application in high dose dosimetry [1].On the other hand, semiconductor devices have been widely used for photon and electron beams dosimetry, mainly in the field of radiation protection, medical imaging and radiation therapy [1][2][3][4][5], where the doses are considerably lower than those delivered in some typical radiation processing applications, that can easily reach 30 kGy or more [6].
Nevertheless, developments of rad-hard silicon diodes for high energy physics experiments have been changed this reality, allowing the use of these devices in applications under severe radiation environment [7,8].Among several defect engineering technologies used to decrease the radiation damage, one is to diffuse oxygen into the float zone Si bulk, increasing the oxygen concentration when compared with the standard float zone diode.This technology has shown a substantial improvement in radiation hardness, mainly when the diodes are irradiated with gamma rays [9].
In this work, we report the results of the comparison between rad-hard Diffused Oxygenated Float Zone (DOFZ) and Standard Float Zone (STFZ) silicon diodes in radiation processing dosimetry as on-line radiation dosimeter.The main advantage of recording real time currents, and consequently dose rates, is to provide continuous data of the dose delivered to the product for quality assurance in radiation processing.

MATERIALS AND METHODS
The p + /n/n + junction devices used were produced on rad-hard float zone Si wafers of 300 m thickness, active area of 25 mm 2 (surrounded by a multiple guard ring structure), resistivity about 1.4 k•cm and oxygen concentration < 10 15 cm -3 .Additionally, the DOFZ wafer was passivated by oxygen diffusion into the bulk, reaching an oxygen concentration  10 17 cm -3 .Both diodes were manufactured by Okmetic Oyj (Finland) and processed by the Microelectronics Center of Helsinki University of Technology [10] in the framework of the CERN RD50 Collaboration.Two samples of each diode (STFZ and DOFZ) with similar characteristics related to I/V and C/V standard measurements, hereafter referred as STFZ#54, STFZ#75, DOFZ#2 and DOFZ#6, were used in this work.Two of them (STFZ#54; DOFZ#2) were not pre-irradiated, while the others (STFZ#75; DOFZ#6) were primed before measurements with a dose of 700 kGy.This value was previously accessed by monitoring the current sensitivity decay of similar STFZ and DOFZ diodes continuously irradiated with 60 Co gamma rays.The corresponding plots of radiation-induced current as a function of the accumulated dose up to 1 MGy, enabled predictions to be made about the predose needed to stabilize the current response of the diodes.The dose rates used to irradiate each diode are listed in Table 1

RESULTS AND DISCUSSION
The photocurrent response of the STFZ#54 and DOFZ#2 diodes under 60 Co gamma radiation with different exposure times is shown in Figure 3.The sensitivity dependence of the diodes on the dose is clearly evidenced by a fast decay of the photocurrent for accumulated doses of about 30 kGy, followed by a slower decrease at higher doses, as can be seen in the  From Figure 4, it is expected a less pronounced sensitivity decay at accumulated doses higher than approximately 300 kGy.Therefore, in order to improve the diode response stability, by reducing its sensitivity dependence on the dose, the STFZ#75 and DOFZ#6 samples were pre-irradiated with 60 Co gamma rays with a dose of about 700 kGy.Indeed, in Figure 5, the photocurrent response of the STFZ#75 and DOFZ#6 diodes, evaluated for accumulated doses up to 275 kGy, presents very stable and reproducible current signals at all doses studied, as can be seen by the normalized photocurrent as a function of the accumulated dose (Figure 6).This fact can be attributed to the saturation of the traps generated in the sensitive volume of the devices by the radiation and is confirmed by the average photocurrent values and the sensitivity for the pre-irradiated devices showed in Table 2.It is important to note that, even being less sensitive after the high pre-dose, the signal to dark current ratio is about more than 10 3 [11], allowing the safe use of these diodes as dosimeters.Analyzing the results presented in Tab. 2, it is possible to conclude that the STFZ diode is more sensitive than the DOFZ diode.As can be seen in Figure 08, for doses higher than 30 kGy, diodes without pre-dose shown a significant saturation, probably due to point defects created in their volume by the gamma radiation.
Conversely, within the whole dose range studied it was observed a quadratic response of the pre-

Figure 1
Figure 1 shows the dosimetric probe, a PolyMethilMetacrylate (PMMA) box, which protected the diodes from moisture, light and mechanic vibrations.All guard rings of the diodes were left floating, while the back plane (n + ) of each device was properly grounded.The p + front electrode was directly connected, via a 2 m long 50  coaxial cable, to a Fluke 189 multimeter with time resolution of 4 s.The photocurrents as a function of time were acquired with the FlukeView Forms® software.

Figure 1 :
Figure 1: Dosimetric probe based on the rad-hard float zone Si device.
. The diodes were placed in the central position of the 60 Co irradiation chamber and irradiated in consecutive steps of 5 kGy from 50 kGy.It took more than 110 h irradiation time to achieve an accumulated dose of 275 kGy per diode.After each step, the irradiator was switched off, and the leakage current of the dosimetric probe was performed.The temperature during exposure was typically 25 ± 3 ºC.

Figure 4 .
In order to eliminate the influence of dose rate in the generation of the photocurrent, the data in this figure are normalized by the maximum current measured at the beginning of exposition and presented as a function of the accumulated dose.

Figure 3 :
Figure 3: Current response of the (a) STFZ#54 and (b) DOFZ#2 diodes for successive exposure times, irradiated at the dose rate of 2.33 and 2.52 kGy/h, respectively.

Figure 4 :
Figure 4: Normalized photocurrent for the STFZ#54 and DOFZ#2 diodes as a function of the accumulated dose.

Figure 6 :
Figure 6: Normalized photocurrent for the STFZ#54 and DOFZ#2 diodes as a function of the accumulated dose.

Figure 7 :
Figure 7: Average current signals of each successive exposure, normalized to the average current at 5 kGy of respective diode, as a function of the accumulated dose.