Previous studies in radiation induced degradation in optocouplers at room temperature indicated that the most affected parameters were CTR and dark current (1,2). This fact is also applicable to high voltage (HV) optocoupler where the working voltage could go up to 10KV.
In space applications, optocouplers may need to work at very low temperatures. Under this low temperature operating conditions, the device degradation subjected to radiation exposure may show different behaviour compared to a standard radiation test under room temperature.
The purpose of this paper is to analyse the effects of proton displacement damages up to 8•1011p/cm2 and total ionisation dose up to 100Krad (Si)) at room and low temperature (i.e. 25°C and -40°C) on custom assemblies of High Voltage (HV) Optocouplers. The HV opto-coupler assembly, radiation setups for low temperature Gamma and Displacement Damage radiation tests are described in detailed.
An optocoupler working at high voltage (up to 12kV) and mounted by IRAP using a commercial optodiode OZ150SG  surrounded by six LEDs SFH4253  will be used in the instruments PAS (Proton Alpha Sensor) and HIS (Heavy Ion Sensor) that are part of the Solar Wind Analyser Suite (SWA). The particle sensors will measure the velocity distribution of the major ionic species at a time resolution equivalent to the ambient proton cyclotron period and determine their moments (density, temperature, velocity, pressure tensor). It is designed to measure the full 3D velocity distribution functions of the major solar wind species, protons and a-particles. For the HIS instrument, it is capable of measuring ions from 0.5 to 60 keV/q, species from 3He to Fe. PAS will be operating more than 50% of the time below -20°C and at high voltage. The radiation resistance of this optocoupler was unknown and it was considered that it could be different at ambient that at the actual low operating temperature. For this reason it was decided to perform the radiation campaigns with some components at room temperature and some components at -40°C.
These custom optocouplers are designed to work up to 12kV, radiation testing board of each optodiode surrounded by the six LEDs is depicted in Figure 1.
The radiation testing board consists of six optocouplers connected in parallel with a protection resistor of 1MΩ. During either proton radiation or gamma radiation exposure, the monitored parameter at low temperature was the Current Transfer Rate when LEDs IF= 1 mA and optodiode VRWM = 5kV.
LOW TEMPERATURE RADIATION SETUP
Both gamma and proton radiation had to be performed with the possibility of keeping an optocoupler at a stable temperature of -40°C at the radiation facility. Several cooling options were considered. Finally, it was decided to use a small chamber with a window being cooled with liquid nitrogen in a close loop configuration to allow precise PID temperature control of the samples. A small (approx. 100 liters) liquid nitrogen vessel was required near the radiation area and it was filled periodically to keep the temperature stable. Figure 2a shows the gamma radiation set up whereas figure 2b shows the proton radiation set-up at proton radiation facility.
Two calibration resistances, blue resistors on the Fig. 1, were measured at each radiation step as reference. These calibration resistances were measured at 1kV and 5kV at each intermediate step for validation of the setup.
Proton radiation campaign was performed at the Cyclotron Resource Centre at the Université Catholique de Louvin (UCL). The Cyclone is a multiparticle, variable energy, cyclotron capable of accelerating protons up to 65 MeV. Table I show proton radiation conditions applied to the HV optocouplers.
Proton radiation test conditions
|Beam Energy (MeV)||60||60||60||60|
|Cumulative Fluence (p/cm2)||1×1011||2×1011||6×1011||8×1011|
|Flux [p/s cm2)||1×108||1×108||1×108||1×108|
|Exposure Time (min)||16.6||16.6||66.6||33.3|
Gamma radiation campaign was performed at the RADLAB (Gamma Radiation Laboratory) at the Centro Nacional de Aceleradores-CNA (University of Seville, CSIC & Junta de Andalucía). The RADLAB is based on a 60Co gamma radiation source, the dose rate uniformity in the radiation field was better than 95% and the expanded uncertainty associated with the measurement is +/- 4.2%. Table II show gamma radiation conditions applied to the HV optocouplers.
Gamma radiation test conditions
|2||Dose Rate||223 rad/h|
|3||Total Dose||109 Krad|
|4||Radiation Steps||20- 50 - 109 Krad|
The intermediate measurements performed after each proton and gamma radiation step are described in table III. The dark current at VRWM=1kV and VRWM=5kV, the forward voltage at IF from 0 to 10 mA and the CTR (Current Transfer Rate) at VRWM=1kV and VRWM=5kV and IF= 0.1 mA and 1 mA, were measured at each intermediate step.
Electrical parameters measured at each proton and gamma intermediate step.
|VR||Forward Voltage||IF = 0.1mA; 0.2mA; 0.3mA; 0.5mA; 0.7mA; 1mA; 2mA; 3mA; 5mA; 7mA; 10mA||V|
|IR||Dark Current||VRWM=5kV VRWM=1kV||μA|
|CTR||Current Transfer Rate||IF= 0.1 mA, 1 mA VRWM= 5kV VRWM= 1kV||%|
Gamma Radiation Test Results
Results of the gamma radiation test have shown that there was no optocouplers degradation. For example the dark current at low temperature was obviously lower than at room temperature, but it showed no degradation during the low temperature radiation test. It clearly recovered the same value when measured during annealing stage at room temperature.
The following picture shows the CTR evolution after each gamma radiation step. It did show no degradation. All the other results obtained were similar in the sense that only variations with respect to different temperature was observed, but no gamma radiation related degradation has been noticed.
Figure 5 shows the CTR monitoring during gamma radiation at -40°C considering also the time for intermediate measurements. The vertical lines indicate the moment of ending each radiation step. CTR values clearly indicate the no degradation of all optocouplers during gamma radiation exposure.
Proton Radiation Test Results
However, the proton radiation tests have given rather more interesting results. In this case, there is real optocouplers degradation. The device degradation has been found to be different at low temperature (-40°C) and at room temperature. As an initial summary, the table below shows the main results obtained:
Comparison of main electrical parameters results at two different temperatures.
|TEMP 25°C||TEMP -40°C||TEMP25°C|
|DARK CURRENT (pA)||At 1KV||2224.5||19.3||101.1||17343.3|
|CTR at 0.1mA (%)||At 1KV||0.056||0.149||0.053||0.048|
|CTR at 1mA (%)||At 1KV||0.564||0.857||0.377||0.249|
Table V presents the CTR percentage reduction which was calculated from the above data at the indicated conditions. Note that for a LED forward current of 1mA results at 25°C and -40°C are equivalent, but for lower LED forward current of 0.1mA results at 25°C and -40°C are completely different as CTR reduction at -40°C is almost 5 times higher than the CTR reduction at 25°C.
Comparison of CTR reduction at two different temperatures
|PARAMETER||% CTR Reduction at 25°C||% CTR Reduction at -40°C|
|CTR at 0.1mA (%)||At 1KV||14.29||64.43|
|CTR at1mA (%)||At 1KV||55.85||56.01|
Figure 6 shows the CTR monitoring during proton radiation at -40°C considering also the time for intermediate measurements.
The arrows at each step indicate the moment when the proton radiation started and when it was removed. It was detected some annealing effect after each radiation step. CTR degradation during the test had a similar rate in all steps. However, this rate was different for some minutes during the 3rd step. It is believed that the cause of this change was that the flux was unintentionally increased up to 2x108 p/s/cm2 during those minutes. Although, the final step fluence was 6x1011 p/s/cm2 as specified in the test plan procedure.
The graph below shows the dark current evolution measured at each intermediate step. Note that the first and the last measurements were taken at 25°C while the other measurements were taken at -40°C. These results are not uncommon as the dark current strongly depends on the temperature.
Although the CTR was monitored during the entire proton radiation, it was also independently measured at each intermediate step. The CTR reduction at low temperature was confirmed also at room temperature when the samples were characterized at the end of the radiation and the measured values were compared with initial data. The following picture shows this CTR variation at IF=1mA and V=5kV.
However, when LEDs forward current was lower as shown in figure 9, the CTR degradation was much higher at low temperature than at room temperature. This fact shows the necessity of testing the radiation at the operational temperature because it could lead to different behavior.
The studied optocouplers did not show any degradation under gamma radiation testing, however, some important remarks should be made regarding proton radiation test. Those remarks are summarised as follows;
• A reduction of CTR and an increase of dark current were observed.
• The reduction of CTR is higher at low forward currents (0.1mA) and at -40°C, and it is lower at low forward currents (0.1mA) at 25°C.
• The radiation induced reduction of the CTR at -40°C for low forward current (0.1mA) (almost 65%) was roughly 5 times higher than the reduction at 25°C (i.e. between 11.8% to 14.2%).
• The CTR degradation is not dependent of the high output voltage level obtaining similar behaviour at output voltages of 1kV and 5kV.
• The dark current at 1 & 5 kV increases but all measured values are lower than the specification limit (<1 μA).
• All samples work correctly after 8×10n p/s/cm2.
Therefore, it has been shown that these optocouplers can be used for the intended application, but it is important to properly select the operating conditions because the radiation induced degradation is highly dependent of not only the LED forward conditions but also the operating temperature of the final application.
This paper has shown how complex this custom device behaves from a radiation perspective. It clearly show how important is to have knowledge of the device construction and what it is more important to evaluate the device radiation resistance at the required operating conditions of the final application as non-expected results may be found.
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