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329978-1-4799-5296-0/14/$31.00 2014 IEEE
PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014
System on Module Total Ionizing Dose Distribution Modeling
A. O. Akhmetov, D. V. Boychenko, D. V. Bobrovskiy, A. I. Chumakov, O. A. Kalashnikov, A. Y. Nikiforov and P. V. Nekrasov
Abstract - The paper presents total ionizing dose (TID) distribution due to trapped electrons and protons at the system on module (SOM) surface. TID calculation was made in 3D_SPACE software (Specialized Electronic Systems). The main goal of this paper is a more precise definition of the radiation hardness requirements for space electronics. A huge TID level dispersion for different ICs in SOM is demonstrated. Basic Al sphere approach for TID spacecraft requirements calculations is shown to provide overestimated conservative results.
When a spacecraft mission is planning, the requirements for the radiation environment on the spacecraft orbit should be specified, considering the date and time of launch and lifetime duration. At present, there are special software [1, 2, 3], that calculates space particles fluxes (heavy ions, electrons and protons) on the spacecraft orbit, based on different models . Then radiation environment inside the spacecraft is set, taking into account (or excluding) internal arrangement of the satellite-borne equipment. Spacecraft shielding should be taken into consideration as well. Modern satellites have rather complex geometry, so the classical Al sphere approach to the radiation environment calculation is too simplified. For an accurate assessment of the radiation environment inside the satellites or separate electronic modules specialized CADs are used, for example, FASTRAD .
Electronic components used in the satellites are certified to meet the radiation hardness requirements . TID requirements are determined during the internal radiation environment specification. Only TID effects are considered in this paper. Weve evaluated the internal radiation environment inside the Al sphere, rectangular parallelepiped, for the various components of an SOM, in order to assess whether the average TID requirements inside the satellites, conform to the calculated actual TIDs for various ICs.
II. 3D_SPACE SOFTWARE
3D_SPACE (SPELS) software was used for the TID calculations. It allows calculating TID inside the Al shield
due to electrons and protons. Input parameters for this software are 3D shielding model and particles spectrum (electrons or protons). 3D models were made in the 3D MAX software and exported to 3DS format. 3D_SPACE allows calculations using different shielding materials. It is based on ray-tracing method for shielding calculation at full spatial angle in selected point (see fig. 1). Thickness of all materials is converted to aluminum equivalent. TID calculation due to protons is carried out by continuous deceleration method and by Monte Carlo method in case of electrons.
Fig. 1. Panoramic depth calculation in 3D_SPACE. Grey and dark grey colors correspond to effective Al shielding more than 2 g/cm2 and red color less than 2 g/cm2. This tool is useful to define weak spots in the shielding.
3D_SPACE features: + Fast algorithm; + Shielding weak-spots visualization; + Geometry export possibility n 3DS format; No possibility of materials exporting; Equivalent shielding thickness calculated to
III. 3D MODELS
Weve used following 3D models for the calculation: Al sphere, Al rectangular parallelepiped and PC/104 SOM (see fig. 2). Sphere and rectangular parallelepiped were chosen because of its simplicity and vast application in classical models of radiation environment. At the same
A.O. Akhmetov, D.V. Boychenko, D.V. Bobrovskiy, A.I. Chumakov, O.A. Kalashnikov, A.Y. Nikiforov and P.V. Nekrasov are with National Research Nuclear University, Moscow, Russia firstname.lastname@example.org
time PC/104 SOM are widely used in spacecrafts in order to decrease the design time. Usually, SOM consists of several ICs (up to hundreds in complex SOMs) that can be irradiated to different TID levels due to its geometrical dimensions and the shielding by PCB, radiators, and other components. 3D models of the processor module CME137 , digital I/O module DM6856HR  and analog/digital I/O SDM7540  were created.
Fig. 2. 3D model inside the Al shape made in 3D MAX.
IV. SPACE ENVIRONMENT
The satellite radiation environment was estimated for the 1000 km circular orbit with 60 inclination angle. At such orbits the main contribution to the TID is made by electrons and protons of the Earth radiation belts . Calculation of the radiation environment was made using SPENVIS  and OMERE  software according to AP8 and AE8 models. The resulting electron and proton spectra obtained by SPENVIS and OMERE corresponded each other.
V. TID DISTRIBUTION
At the first step, the TID distribution inside the Al sphere and rectangular parallelepiped was calculated. TID calculation due to electrons and protons for the Al sphere was held at points located on the sphere radius. The calculation results are shown in Fig. 3. TID calculation due to electrons and protons inside the rectangular parallelepiped was held at points located on the greater geometrical axis of the parallelepiped. The calculation results are shown in Fig. 4. All calculations were performed for the Al sphere and rectangular parallelepiped 1 mm, 4 mm and 10 mm thick.
At the second step, the TID calculations were carried out for the CPU module CME137 inside the Al sphere (SOM located at the center of the sphere). It was done in order to cut off the soft component of the electron spectrum, which could make a major contribution to the
obtained TID level. Sphere in this case simulated the shielding, because electronic components are located inside the shielding in real applications. Cases of external mounting were not considered. 100 uniformly spaced points were selected on the surface of the processor module and at each point the TID was calculated. Calculation results are presented in Fig. 5 and 6.
Fig. 3. Dose due trapped protons vs radius coordinate at the Al sphere for 1, 4 and 10 mm shield thickness.
Fig. 4. Dose due trapped protons vs radius coordinate at the Al rectangular parallelepiped for 1, 4 and 10 mm shield thickness.
Fig. 5. TID distribution due to electrons. Dose calculated for each of one hundred uniformly spaced points as a function of X-Y plane coordinates in SOM DM6856HR-5V. Maximum DMAX/DMIN ratio equals to 6.75.
Fig. 6. TID distribution due to protons. Dose calculated for each of one hundred uniformly spaced points as a function of X-Y plane coordinates in SOM DM6856HR-5V. Maximum DMAX/DMIN ratio equals to 1.63.
Figures 3 and 4 demonstrate that TID in the center of the figures has a maximum value decreasing toward surface because of the effective shielding thickness increase. That ratio is correct for a rectangular parallelepiped too, but is more complex and depends on shielding geometry. As a result, it can be concluded that the assignment of internal radiation environment using Al sphere model gives overestimated conservative TID values. The TID calculations on the surface of the processor module revealed TID levels that could differ several times for different ICs. This is due to the fact that components located at the center of the SOM have most effective shielding than the components at the surface. A considerable contribution of heat sinking and various PCB connectors should be taken into account.
As a result, the TID distribution for the various SOM ICs may have a significant divergence. For test electrons and protons spectrum used in our calculations, almost two times TID divergence was found for different ICs. It should be considered for SOMs in space applications.
For example, calculations were made on SOM SDM7540 . As a result, it was found that the DAC (a part of the SDM7540) hardness level is about 1 krad (Si), while other components have hardness levels up to 10 krad (Si). TID distribution modeling for real operating conditions was done to determine TID level for the DAC and to propose the necessary local radiation shielding, which does not considerably increase the satellite mass.
The simulation results show that TID distribution inside the satellite has a significant divergence (up to 2
times for considered orbit). That appeals to more precise radiation environment requirements inside satellites.
The results calculated in 3D_SPACE software will be compared to other CADs (for example, FASTRAD) based on Monte Carlo method for TID calculation.
The authors wish to thank Andrey Kozlov with the Specialized Electronic Systems (SPELS) for his contribution.
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