General

JERG-2-143

DESIGN STANDARDSPACE ENVIRONMENT EFFECTS
MITIGATION

Sept. 3 , 2008

Japan Aerospace Exploration Agency

JERG-2-143

This is an English translation of JERG-2-143. Whenever there is anything ambiguous in thisdocument, the original document (the Japanese version) shall be used to clarify the intent of therequirement.

Disclaimer
The information contained herein is for general informational purposes only. JAXA makes nowarranty, express or implied, including as to the accuracy, usefulness or timeliness of any
information herein.

JAXA will not be liable for any losses relating to the use of the information.

Published by
Japan Aerospace Exploration Agency
Safety and Mission Assurance Department
2-1-1 Sengen Tsukuba-shi,Ibaraki 305-8505, Japani

JERG-2-143

Table of Contents
1 General Provisions .................................................................................................................................... 1
1.1 Purpose .............................................................................................................................................. 1
1.2 Scope ................................................................................................................................................. 1
1.3 Related Documents ............................................................................................................................ 1
1.3.1 Applicable Documents .................................................................................................................... 1
1.3.2 Related Documents ......................................................................................................................... 1
1.4 Definitions of Terminology................................................................................................................ 1
1.4.1 Definitions of Terminology ............................................................................................................. 1
1.4.2 Abbreviations.................................................................................................................................. 4
1.4.3 System of Units............................................................................................................................... 5
2 Standards .................................................................................................................................................. 5
2.1 Outline of Radiation Resistance Design ............................................................................................. 5
2.1.1 Design Procedures .......................................................................................................................... 5
2.1.2 Design Evaluation Items ................................................................................................................. 8
2.1.3 Effects from sources other than Cosmic Radiation ........................................................................ 11
2.2 Cosmic Radiation Environment ....................................................................................................... 11
2.2.1 Cosmic Radiation Environment Model ......................................................................................... 11
2.3 Total Dose Design Standard ............................................................................................................. 122.3.1 Total Dose Prediction Method ....................................................................................................... 122.3.2 Total Dose Prediction Model ......................................................................................................... 13
2.3.3 Radiation Deterioration of Electronic Components ....................................................................... 19
2.3.4 Radiation Deterioration of Material ............................................................................................... 22
2.3.5 Design Procedure .......................................................................................................................... 26
2.4 Radiation Resistance Design Standard for Solar Array Panels .......................................................... 29
2.4.1 Radiation Deterioration Prediction Method of Solar Cells ............................................................. 29
2.4.2 Radiation Resistance Design of Solar Cells (Cell Selection).......................................................... 32
2.4.3 Radiation Shielding (Panel Structure)............................................................................................ 34
2.4.4 Other Deterioration Factors ........................................................................................................... 35ii

JERG-2-143

2.5 Single Event Design Standard .......................................................................................................... 36
2.5.1 Outline of Single Event Effect....................................................................................................... 36
2.5.2 Design Procedure .......................................................................................................................... 37
2.5.3 Prediction Method of Single Event Probability of Occurrence ...................................................... 37
2.5.4 Measures for Single Events of Electronic Components ................................................................. 38
2.5.5 Measures for Single Events of Devices ......................................................................................... 39
2.6 Design Standard for Displacement Damage of Semiconductor Devices ........................................... 42
2.6.1 Overview ...................................................................................................................................... 42
2.6.2 Prediction Method of Displacement Damage ................................................................................ 42
2.6.3 Resistance for Displacement Damage of each Device ................................................................... 44
2.7 Affects from other than Cosmic Radiation ....................................................................................... 48
2.7.1 Measures for Atomic Oxygen........................................................................................................ 48
2.7.2 Measures for Ultraviolet Rays ....................................................................................................... 49iii

1 General Provisions
1.1 Purpose
This radiation resistance design standard specifies the basic design requirements for the radiation
resistance design of spacecraft developed by Japan Aerospace Exploration Agency (hereinafterreferred to as JAXA).
1.2 ScopeThis radiation resistance design standard explains the basic conditions of the designs which cause
deterioration or temporary malfunctions of devices and materials due to effects from radiation
received in orbit. This radiation resistance design standard also outlines the effects on thecomponents and materials from the cosmic radiation environment, and highlights procedures forradiation resistance designs and radiation countermeasures.

1.3 Related Documents

1.3.1 Applicable Documents(1) ISO15390 Space environment (natural and artificial) - Galactic cosmic ray model(2) CREME96 Cosmic Ray Effects on Microelectronics 96 https://creme96.nrl.navy.mil/(3) JERG-2-141 Space Environment Standard

1.3.2 Related Documents(1) N/A

1.4 Definitions of Terminology
1.4.1 Definitions of Terminology

The definitions of the terminology used in this design standard are as follows.
(1) Trapped Radiation Particles
A general term for charged particles trapped in the geomagnetic field.
(2) Galactic Cosmic Ray
Are fundamental particles of a high energy which fly from space, and fly from any direction

1

towards the surface of the Earth. Most of these particles are protons, and also include aheavier nucleus than these particles.
(3) Flux
The intensity of radiation at any point is called flux or particle flux. The flux which passes
through a minute flat surface of dS (cm2) in unit time per unit area is called flat surface particle
flux. The particle flux which traverses the surface of a sphere with a cross section of dS perunit time is called spherical surface particle flux. The unit of integration flux is expressed by
m-2s-1. The unit of energy differential flux is expressed by m-2s-1MeV-1. When including thedirection, add sr-1 (steradian).
(4) Fluence
Is a time quadrature of the flux. This is used to express the environment of the operation
period of a spacecraft. The integration fluence unit is expressed by m-2. The energy integral
fluence unit is expressed by m-2MeV-1. When including the direction, add sr-1 (steradian).
(5) Absorbed Dose
The absorbed dose is calculated by dividing the energy imparted into sufficiently small volume
elements of a substance from ionizing radiation, by the mass of the volume elements. This isnormally expressed in Gray or rad.
(6) DoseThis is an idiomatic term which expresses the radiation dose and the absorbed energy, and isused to express various meanings, such as the absorbed dose, exposure dose, etc.
(7) Total DoseThe total absorbed dose received by components or materials to a specific point.
(8) RadA unit of absorbed dose, and 1 rad equals 100erg/g = 10-2J/kg.
(9) Gray: Gy
This is the SI unit of the absorbed dose, and 1 Gy is equivalent to 1J/kg = 100 rad.
(10) Single Event Effect
Refers to effects, such as malfunctions of circuit elements (software errors), or latch up which
are caused by the effect of a single high energy particle.
(11) Bremsstrahlung2Electromagnetic radiation produced when charged particles are slowed down by the behaviorof the nucleus.
(12) Solar FlareThis is an explosion phenomenon which occurs on the surface of the sun, accompanied by therelease of high energy particles.
(13) Spectrum
This generally refers to the items regarding the wave length of light. However, in this standard,
the spectrum refers to the items which express the particle flux density of the radiation for eachof the energies.
(14) L Value
This refers to the distance to a point where the magnetic lines of force intersect with theequatorial plane of the geomagnetic field from the earth's core, with RE (radius of earth) usedas the unit.
(15) Anneal
This refers to the phenomenon where there is a recovery in performance recovers which
previously deteriorated due to radiation.
(16) Linear Energy Transfer
This is the energy which the charged particles lose per unit length or unit surface density. Thisis expressed by MeV/µm or MeV/(g/cm2). This is referred to as LET.
(17) Geomagnetic EffectThis phenomenon refers to when cosmic rays enter into the geomagnetic field, and are drawn
into various orbits according to the energy and the incident direction, and certain particles moveaway again without touching the surface of the earth.
(18) Dose Rate
This refers to the dose per unit of time.
(19) Heavy Ion
These are ion particles with a large atomic number, and generally refer to particles of He or
more.
(20) NIEL (Non-Ionizing Energy Loss)
Damage not caused by ionization of the incidence particles is called non-ionizing energy loss.

3

(21) DDD (Displacement Damage Dose) Method
A method of predicting the deterioration of solar cells using the NIEL value developed by NRLof the U.S. as a parameter.
(22) RDC (Relative Damage Coefficient) Method
A method of predicting deterioration of solar cells using the RDC developed by JPL of the U.S.
as a parameter.

1.4.2 Abbreviations
The abbreviations used in this design standard are as follows.
(1) NASA: National Aeronautics and Space Administration(2) JPL: Jet Propulsion Laboratory(3) NRL: Naval Research Laboratory(4) MOS: Metal Oxide Semiconductor(5) CREME96: Cosmic Ray Effects on Microelectronics 96(6) DDD: Displacement Damage Dose
(7) RDC: Relative Damage Coefficient(8) NIEL: Non-Ionizing Energy Loss
(9) CFRP: Carbon Fiber Reinforced Plastics(10) SEU: Single-Event Upset(11) SET: Single-Event Transient(12) SEL: Single-Event Latch up(13) SEB: Single-Event Burnout(14) SEGR: Single-Event Gate Rupture(15) MCU: Multiple Cell Upset(16) MBU: Multiple Bit Upset(17) EDAC: Error Detection and Correction(18) CTE: Charge Transfer Efficiency(19) CTR: Current Transfer Ratio(20) MSM: Metal-Semiconductor-Metal4

1.4.3 System of Units
The units used in this design standard are SI units in principal, as specified in JIS Z 8203
'International System of Units (SI) and Method of Use', except for the following units.
Used in this Design Standard

SI UnitAbsorbed dose (rad)Gray: Gy (= 100 rad)Energy (electron-volt: eV)

Joule: J (= 1/1.602×10-19eV)

2 Standards
2.1 Outline of Radiation Resistance Design
The damage which radiation causes to components and materials can be classified into cumulative
effects of all incidental radiation is called the total dose effect. Damage which occurs due to an
incident of one charged particle and the former is called the single event effect.

The design

procedures and items of the design evaluation for these two effects are as follows.
2.1.1 Design Procedures
The radiation resistance design procedures are indicated in Figure 2.1-1.
The details are described in item 2.2 and later, and the outline is indicated in the following.
(1) Cosmic radiation environment
The types and effects of the cosmic radiation are indicated in Table 2.1-1.
The effects from the trapped radiation particles and solar protons with an abundance of fluence
dominate the total dose effect, and heavy ions and high energy protons with a large LET (heavy
ions created by the nuclear reaction with the material atoms) are applicable to the single event
effect.
(2) Prediction of radiation damage
As indicated in Figure 2.1-1, the prediction of the radiation damage must be performed by
predicting the radiation dose which the components and materials receive and compared to thetest results, in consideration of the value of the environmental conditions of the cosmic radiation
received which declines according to the mass distribution in a spacecraft.
The total dose effect is a phenomenon where incidental radiation occurs to the material, doing
damage proportional to the total amount of energy given, and semi-permanently deteriorating
the characteristics of the material, and eventually destroying its function.
5

The deterioration of solar cells is a type of total dose effect. However, the displacement
damage in the crystals of semi-conductors becomes a significant problem. Therefore, it iscommon to use the incidental number per unit area converted into 1 MeV electron (1 MeV
equivalent fluence) as a radiation dose, not as a total dose. Recently, the method of using thedisplacement damage dose (DDD) was established, which converts to a capacity to generate
the displacement damage using the electrons, protons and NIEL value of each of the energies.
Since the occurrence of a single event effect depends on the LET of the heavy ions, it isnecessary to consider the radiation environment as a function of LET. The fluence of the
protons in which the energy is used as a function, is also necessary depending on the device
and the orbit. Since optical semiconductor devices and bipolar devices deteriorate due todisplacement damage resulting from protons, deterioration prediction using the NIEL value isperformed.

6

Examination of Measures
Selection of Components and Materials

NO

Mass Distribution in Satellite

Circuit Design, etc.
Resistance Data(*2)(*4)

(*2)

(*3)

Prediction of
Deterioration
Rate by
Radiation

Prediction of
Total Dose

Characteristics
Prediction

Total Dose EffectOK
Examination of Measures(*1)
Environmental
Conditions
Setting

Mission Period
Trapped Radiation Particles
Solar Cosmic Ray
Galactic Cosmic Ray

Selection of Components
and Materials
Mass Distribution in Satellite
Resistance Data(*2)

Prediction of Deterioration
Circuit Design, etc.
Rate by Total Dose
(*2)

(*2)

(*3)(*4)
Deterioration
Prediction of
Parameters

Calculation of
Proportionality
Coefficient

Proton Fluence
Calculation

Characteristics
Prediction

OK

Examination of Measures(*2)

Cell Selection(*1) Uncertainty is included in the environmental model(*2) Examination at components level(*3) Examination at assembly level(*4) Is it OK including the design margin andthe environment model margin?

Panel Structure(*2)

Calculation of
1 MeV
EquivalentElectron Ray
Fluence(*2)

Radiation
Resistance
Data

Considerationof Shielding
EffectDisplacement
Damage of
Semiconductor
DevicesNO(*2)
Calculation of
Displacement
Damage Dose

Deterioration by
Other Factors

NO
Panel Design(*3) (*4)(*3)
Considerationof Shielding
EffectRequirement Specifications

OrbitalConditions

EOL Output
Prediction

Deterioration ofSolar Array Panels
OK

Examination of Measures
Component SelectionNO

Mass Distribution in Satellite(*2)
Calculation of
Heavy Ion / Proton
Fluence

Circuit Design

Resistance Data

Single Event Effect(*3) (*4)(*2)
Prediction of
Occurrence Rate ofSingle Events

Criticality
AnalysisOK

Figure 2.1-1 Radiation Resistance Design7(3) Measures against radiation
When the characteristics value is predicted to exceed the tolerance range due to the effects ofradiation, it is necessary to take measures against radiation. The following three methods aremeasures against radiation.
- Applying radiation shielding to reduce the cosmic radiation.
- Changing to components and materials with a higher radiation resistance.
- Margins in the circuits or structure should be in order to absorb deterioration by radiation.
Use of these measures must be carefully considered and selected based on suitability. .

(4) Important points in radiation resistance design
A high level of uncertainty is included in the prediction of radiation damage. Although
regrettable, radiation damage cannot be determined with a high level of precision. Therefore,
it is necessary to ensure an appropriate margin of radiation damage is incorporated into
radiation resistance design. Even though it is difficult to determine this margin quantitatively,
the important items must be sufficiently examined and evaluated in order to perform a complete
and effective radiation resistance design.
As indicated in Figure 2.1-1, there are uncertainties in the cosmic radiation environment model,
which must take into account potential errors in the evaluation of devices in the margin. The
margins must be established as an individual project, and entered into the design standards ofthe project.

2.1.2 Design Evaluation Items(1) Cosmic radiation environment
The environment model used for setting the environmental conditions of cosmic radiation isconsidered to have the greatest number of uncertainties. Therefore, the margin for the valueof the environment model is particularly important for the radiation resistance design, including
the impact to the spacecraft design.
(2) Prediction of total dose
There are many different materials arranged in complicated combinations in a spacecraft, and
it is very difficult to accurately measure the total dose in the spacecraft.

Usually, asimplification of calculation is performed and it is necessary to understand the relatively high8likelihood of error.
(3) Prediction of the deterioration rate of solar cells
When predicting the deterioration rate of solar cells using a 1 MeV electronic conversion
exposure dose as a parameter, which prevents inconsistencies by deriving the factors used forconverting the radiation environment to the exposure dose (relative damage coefficient, and 10
MeV protons / 1 MeV electronic conversion factor) from the experimental results as much as
possible. When predicting the deterioration rate of solar cells using the displacement damage
dose as a parameter, the NIEL value can be acquired from the application data sheet, or using
technical data.
(4) Prediction of the probability of an occurrence of single events
The design must be performed taking into consideration that there are uncertainties in thepresumed accuracy of the induction area of the devices, including the environmental conditionsin the occurrence rate of a single event. If the requirements cannot be satisfied at the device
level, then necessary measures must be taken at the device level to satisfy the requirements.
(5) Deterioration by displacement damage of semiconductor devices (except for solar cells)
Since optical semiconductor devices and bipolar devices deteriorate in the displacement
damage by protons, the deterioration prediction is performed using the NIEL value.
In particular, the radiation effect of the optical semiconductor devices greatly differs from thestructure of the devices, and there are also great individual differences of select components.
The radiation margins must be appropriately considered according to the mission requirements,
while taking into account these differences.
(6) Radiation resistance of components and materials
The radiation resistance of components and materials is normally acquired from the results ofradiation irradiation tests on the ground, and in conditions that are different than the conditionsin space. Therefore, it is necessary to compare both conditions for the items indicated below,
and it is necessary to predict the fluctuation of radiation resistance considering the differencesin the conditions on Earth and in space.
- Types of rays
- Dose rate
- Operating conditions- Surrounding environmental conditions- Target characteristics

9

Table 2.1-1 Types and Effects of Cosmic Radiation
Cosmic Radiation

Total DoseSingle Event

Radiation source

Particles

EffectEffectTrapped radiation

Protonsparticles

ElectronsProtonsElectrons









Protons

Galactic cosmic

Electrons





rays

Heavy ions / HeSolar cosmic rays
Heavy ions / Heparticles

particles(Note 1) : Major effect: Minor effect10

: No effect2.1.3 Effects from sources other than Cosmic Radiation
Although not defined as cosmic radiation, atomic oxygen and ultraviolet rays are factors in thespace environment which enter materials directly exposed to space, such as the thermal control
materials, and may affect the life of the spacecraft. It is necessary to refer to the radiation
resistance design procedures, and evaluate these affects.

2.2 Cosmic Radiation Environment
2.2.1 Cosmic Radiation Environment Model
Refer to Chapter 9 of JERG-2-141 for the cosmic radiation environment model.

(1) Margin of uncertainty of the cosmic radiation environment model
The cosmic radiation environment is under research even today, and the margin of uncertainty
is included in the model.
It is necessary to include the margin using one of the following methods.
(a) Including a margin in the model.
(b) Calculating the parameters applicable for evaluation without including a margin in themodel, and then comprehensively evaluating the results to include the model and othermargins.

11

2.3 Total Dose Design Standard
One type of the radiation damage to components and materials is called the total dose effect, and isa lasting damage caused by the accumulation of total incidental radiation. The total dose effect can
be classified into ionization damage and displacement damage. The energy loss from radiation in
the material is pre-dominantly caused by an ionization phenomenon, and is characterized by theamount of energy (absorbed dose) absorbed by the material due to the passage of radiation. Using
the absorbed dose concept, the radiation damage from different radiation sources can be evaluatedat the same time.

Optical devices ,CCD and solar cells tend be affected more easily by thedisplacement damage, and a deterioration of the output of the solar cells from radiation exposure isespecially dominant due to this mechanism.

Recently, the method of using the displacement

damage dose (DDD) for converting to the ability to cause displacement damage using electrons,
protons and the NIEL value of each energy, has been established for the predicted deterioration ofsolar cells. Refer to the items of the radiation deterioration prediction method of solar cells fordetails.

2.3.1 Total Dose Prediction Method
The types of radiation which must be considered to predict the total dose are trapped electrons,
trapped protons and solar protons, as indicated in Table 2.1-1. When these types of radiation
penetrate materials, the energy of the radiation will decrease due to the interaction and absorptionof the radiation with the materials. In the case of electrons, the bremsstrahlung will be generated
due to their interaction with the materials. Since the range of the bremsstrahlung is longer thanelectrons, it can become dominant when a thicker shield is used.
As indicated in Figure 2.3-1, the total dose is calculated for parallel incidence radiation.
Accordingly, this is equivalent to the spherical shell model as indicated in item 2.3.2, when a shieldof equal thickness is used for the isotropic radiation incidence from all directions.
The details of the total dose calculation method are indicated in JERG-2-143-HB001.

The

radiation absorbed dose and the generated amount of the bremsstrahlung differs according to thetypes of materials used for the shielding and the target components. However, the total dose iscalculated by using aluminum as the shielding material, and is sufficient by converting the density
into an equivalent aluminum thickness for shielding materials of other aluminum components. The
results of the total dose calculation are generally provided by the relationship between the shieldthickness (depth) and the absorbed dose, i.e., the Dose-Depth Curve.

12

An example of a

Dose-Depth Curve is indicated in Figure 2.3-2.

2.3.2 Total Dose Prediction Model
The following two methods can be considered as the prediction models, to calculate the actual
radiation dose which the components receive.

Radiation φ (E)Depth Z
Targetφ (E): Incidence Radiation Fluence (particles/cm2)
Z: depth (g/cm2)
Figure 2.3-1 Total Dose Calculation Model

13

Figure 2.3-2 Dose-Depth Curve (Example)

14

- Simplified method
The purpose of this prediction model is to acquire the relationship between the shield thickness
and the total dose (Dose-Depth Curve) assuming that the material and configuration of complex
shielding is a simple shape, such as a spherical shell or a plate. Moreover, its purpose is also toacquire the total dose directly, calculated by using the shielding thickness and surface density.

- Three-dimensional model
The purpose of this prediction model is to acquire the distribution of the shielding thickness ineach direction when viewed from the target (calculation point of the total dose), and to acquire thetotal dose using this calculated value.

(1) Simplified method
When calculating the Dose-Depth Curve, three types of shielding material configurations, suchas spherical shell model, semi-infinite plate model and finite plate model will be used, inconnection with the use of the shielding effect calculation code for space (SHIELDOSE-II).
(a) Spherical shell modelThis model assumes that the model is covered with a shielding material of equal thickness forthe isotropic radiation incidence. This model is the most basic model for performing theshielding calculation, and is acquired by the calculation method indicated in item 2.3.1.
However, since the actual shielding configuration is close to the plate in many cases, theradiation dose calculated using this model may result in a larger value.
This model is used for the shielding calculations for shielding configurations close to sphericalshell or semi-spherical shell, and the basic data for the semi-infinite plate model and thethree-dimensional model indicated in the following diagram.
(b) Semi-infinite plate modelThis model assumes that the model is covered with a shielding material of an infinite widthwith a uniform thickness, having an infinite plane surface shielding material, and a shielding
material of infinite thickness in the opposite direction.
(c) Finite plate modelThis model assumes that the model is covered with a shielding material of a finite thickness,having a semi-infinite plane surface shielding material, and a shielding material of a finite
thickness in the opposite direction (radiation incidence from one surface of the plate).

15

(1) Spherical Shell Model(2) Semi-infinite Plate Model(3) Finite Plate Model
Figure 2.3-3 Model Configurations(Note) These figures are typically presented in order to easily understand the models, with the backdiffusion scattering taken into consideration in the actual calculation.

16

In the actual mounted devices, there are many cases where one direction of the shielding has asufficiently large thickness compared to the other direction, when viewed from the component’s
position. There are also many cases where the configuration of the shielding material is a plate,or close to a plate. Therefore, when calculating the radiation absorbed dose in a simplified
method, the semi-infinite plate model is effective.

(2) Three-dimensional modelThis model is used for calculating the total dose of the components and materials moreaccurately. The spherical shell model and the semi-infinite plate model are simple. However,
the total dose of the components and materials used in spacecraft changes according to theconditions of the surroundings, such as the mounting position, etc., and the actual configurationof the shield is complex and may not necessarily work in conjunction with them. Therefore, inorder to calculate the total dose more accurately, it is necessary to acquire the mass
distribution for each direction with the target as the center. This model converts the mass
distribution into the shielding thickness of standard materials, calculates the absorbed dose ineach direction using the Dose-Depth Curve of the spherical shell model, and integrates thevalues for all solid angles. The formulas are as follows.

17

DT =

1
4π

π

2π

∫ ∫ D ( t (θ, ϕ ) )cosθ dϕ dθ2−π(2.3-2)0

2DT: Absorbed dose of the target

D(x ): Absorbed dose of the spherical shell model when assuming the shield thickness is xt(θ ,ϕ ): Shield thickness of the (θ ,ϕ ) directionϕ: Longitude directionθ: Latitude direction

A conceptual diagram of the mass distribution in space craft is indicated in Figure 2.3-4.

Satellite

TargetMounted Device
Figure 2.3-4 Mass Distribution in Space Craft

18

2.3.3 Radiation Deterioration of Electronic Components(1) Semiconductor electronic components(a) Outline of radiation deterioration
When there is constant radiation, the effect of a positive charge remains in the insulating
layers, such as the SiO2, and the occurrence of the interface state density on the insulating
layer and interface of the silicon crystals are the largest issues with silicon semiconductors.
These issues are expressed by a decrease in the current amplification factor, and an increasein the leakage current for bipolar transistors. In ICs, the circuit characteristics indicate aremarkable fluctuation due to the variations of these parameters.

The main variable

parameters and tendencies of various semiconductor devices for the total dose are indicated inTable 2.3-1. The parameters with the largest effect are indicated in the following.

i) Bipolar transistors
Deterioration in the current amplification factor hFE is the most remarkable in the low current
range.
Recently, Enhanced Low Dose Rate Sensitivity (ELDRS) of linear bipolar components has
been discovered in ground radiation tests, and with dose rates of 50 rads/sec or less, thedeterioration of the gain was larger in some of the linear bipolar components in which radiation
tests were performed at low dose rates.
ii)

TTL
The output voltage (VOL) and the propagation delay time (tPHL) increases remarkably due tothe decrease in the current amplification factor hFE.

iii) CMOS
The input level VIH and VIL fluctuates, the output driving capability IOH, IOL decreases, andthe propagation delay time tpd increases remarkably. The power current IDD increases when
static due to internal leakage. The refinement means that thinning of the gate oxide film andthen total dose tolerance is improved in general. However, the leakage phenomenon isincreased specifically by irradiation has also been confirmed in the oxide film of several nm
order, it should be noted.

19

iv)

Memory devices (SRAM, DRAM, etc.)
The same tendency as CMOS basically occurs. Particularly, there are many cases where agreat fluctuation can be observed in the power current IDD and the input leakage current when
static.

v)

FPGA (anti-fuse type)The input level VIH and VIL fluctuates, the output driving voltage VOH and VOL decreases,
and the propagation delay time tpd increases remarkably. The power current IDD increases
when static due to internal leakage.

(b) Notes for use
Generally, the radiation tolerated dose of the electronic components greatly depends on
manufacturing process factors, device design factors, operating conditions (existence of a bias
application, etc.), type or energy of the radiation and the dose rate. There are also cases where
the radiation tolerated dose greatly changes according to the manufacturer, production lot etc.
and therefore, it is necessary to understand the radiation tolerated dose data of each electronic
component sufficiently when designing the circuitry. An important point for the design of thecircuitry which can be predicted at the electronic component level is that it is necessary toconsider the acceptable level of characteristic deterioration and how to use it, and examine
whether to introduce a derating design.

Examples of cautionary measures for the main

variable parameters for the total dose are as follows.

i) Bipolar transistors
Since the current amplification factor hFE changes greatly when the collector current issmaller, it must be designed so that the collector current and the minimum necessary hFE
after exposure are compatible in the actual circuit.
ii) TTL
In consideration of the deterioration of the low level output voltage (VOL), it is necessary totake a derating of fan-out.
iii) CMOS, memory devices, FPGA
It is necessary to be cautious in terms of preventing a conflict between signals, even though
there is a variation of the propagation delay time tpd from the early stages to the final stages

20

of exposure.

Table 2.3-1 Deterioration Tendency of Semiconductor Devices by Radiation

Semiconductor
DeviceBipolar transistors

Main Variable Parameters
Items

Symbols

Current
amplification factor

hFE

Decrease

Leakage current

ICEO
ICBO

Increase
Increase

Saturation voltage

VCE (SAT.)

Increase

Input current

IIH
IIL

Increase
Decrease

Output voltage

VOH
VOL

Decrease
Increase

Power current

ICC

Decrease

Propagation delay
time

tPHL
tPLH

Increase
Increase

Input level

VIH
VIL

VariesI/O leakage

II
IOZ

Increase

Power current

IDD

Increase

Drive capacity

IOH
IOL

Decrease

Propagation delay
time

tpd

Increase

Input level

VIH
VIL

VariesPower current

IDD

Increase

Drive voltage

VOH
VOL

Decrease

Propagation delay
time

tpd

Increase

TTL

CMOS, Memory
devices

FPGA(anti-fuse type)

Change Tendency

21

2.3.4 Radiation Deterioration of Material

The radiation changes the mechanical and physical characteristics of almost all materials, andgenerally leads to the deterioration them. Metals and ceramics among the components of a spacestructure excel in radiation resistance. Polymeric materials, such as plastics, elastomers etc.,
however, are inferior, and polymeric materials receive remarkable affects from radiation dependingon the dose in the space environment.
The affects on the materials from radiation may change as a result of the type of radiation, energy
and dose rate. Therefore, it is necessary to pay attention to the evaluation data.

(1) Metals
As one guide of the fluence which must take into consideration the radiation deterioration of
metals, the values of 1017 - 1019 protons/cm2 or more are mentioned for protons. However,since the values are several orders of magnitude smaller than that, even though the fluence ofa spacecraft with missions that can last decades, the radiation deterioration of metals may bedisregarded in the actual design.

(2) Polymeric materials
Polymeric materials are materials which are most affected by radiation, and the radiation
resistance changes greatly according to its type and composition. Even though the same
dose is exposed, the radiation resistance of the material changes according to temperature, theexistence of air etc. (generally the radiation resistance deteriorates more in the air). Therefore,
it is necessary to pay attention to the evaluation of the selection data of the material. The
parameters which are affected by radiation exposure of materials are the density, tensile
strength, elongation, Young's modulus, adhesive strength, impact strength, and thermal
conductivity etc.

(a) Thermosetting resins
There are materials of resin elements which can be used with doses up to 107 rads.
However, if 107 rads is exceeded, it is necessary to consider adopting fiber reinforced plastics
which are strengthened with carbon fibers, glass fibers or using epoxy, as a material.
The tensile strength, density and elasticity modulus of fiber reinforced plastics which generally22use thermosetting resin as a material increases due to radiation exposure, and tends todecrease in impact strength and elongation. However, the mechanical properties up to about
108 rads do not change greatly. When about 108 rads are exceeded, there are cases where
the characteristics tend to change according to the type and composition of the matrix. As aresult of NASA performing electron beam irradiation tests on CFRP (carbon fiber/epoxy),its
employment as a material of space structures has demonstrated that CFRP has sufficientresistance for use up to 109 rads. However, since the radiation resistance changes according
to the composition of the material even with the same epoxy CFRP, it is necessary to
sufficiently confirm the characteristics through experiments for high level doses.
(b) Thermoplastic resins
Since the tensile strength and hardness of thermoplastic resins (Teflon, etc.) which include
fluorine generally decreases with high level doses, it cannot be used with doses which exceed
5x104 rads. Other thermoplastic resins are thought to be safe for use up to about 106 rads.
However, radiation resistance for 106 rads or more changes greatly according to the type ofmaterial.
(c) Adhesives
Deterioration of the tensile strength was not observed up to doses of 5x107 rads forthermosetting resin type adhesives used for structures, and can be used safely. The strength
deteriorates slightly at 3x108 rads. However, the level of deterioration changes greatly
according to the type and composition of the material with doses which exceed 3x108 rads. As
a result of performing electron beam irradiation (105 electrons/cm2) on silicon type adhesives
for solar cell coverings, which is a typical non-structure adhesive, it has been reported that
there was almost no deterioration in the permeability.
(d) Elastomers
Elastomers are materials most affected by radiation among the polymeric materials, and therange for safe use from radiation is from 105 to 5x106 rads, and its stability changes according
to its type.
Generally, the elongation decreases by radiation exposure, and the tensile strength decreases
substantially after the initial increase or decrease, while the compression residual strain
increases 50 to 100% in most elastomers. There is a tendency of the hardness to increase
due to radiation exposure in most elastomers. However, butyl rubber and Thiokol rubber
become softer. Natural rubber, urethane rubber and neoprene rubber among the elastomers

23excel in radiation resistance; however, caution is required regarding neoprene rubber, as theradiation resistance changes greatly according to its composition.

(3) Ceramic, graphite and glass
The physical characteristics of these materials, such as the density, mechanical properties,
thermal characteristics, optical quality and electrical properties are all affected by radiation.
The characteristics which are affected by radiation and the level of affect changes according to
the type and composition of the materials and the type of radiation. Generally, their radiation
resistance is higher as compared to organic materials. The items which should be taken into
consideration in terms of design are indicated in the following.
(a) Ceramic
Generally, the physical characteristics of ceramic do not change significantly with ionizing
radiation of 109 rads, or neutron ray exposure at 1019n/cm2 or less. When ceramic is exposed
to a large dose of radiation, the density, elastic modulus, heat conductivity, optical quality andelectrical properties will change. However, composites containing beryllium or boron will beaffected by the thermal neutron fluence at the 1017n/cm2 level.
(b) Graphite
Graphite is slightly more sensitive to radiation as compared to ceramic. The mechanicalproperties (size, intensity, elasticity), thermal characteristics, and electrical properties changewith exposure to radiation, particularly fast neutron rays (E > 1keV). The fluence level in whichremarkable changes occur is at about 1019n/cm2.

24

(c) Glass
The optical quality of glass changes when exposed to radiation equivalent to neutron rays of1015 to 1016n/cm2 or more. Particularly, caution is required as the transparency deteriorates
greatly with the color.
The radiation resistance of other physical characteristics excluding the optical quality iscomparable to ceramic.

(4) Thermal control coating materials
The items which are affected the most among the damage of thermal control coating materials,
is the deterioration of the optical properties, such as the solar absorption rate and thermal
emissivity. The deterioration of the optical properties is also caused by ultraviolet rays andhigh vacuum environments, not only from radiation, particularly organic based thermal control
coating materials, but also by ultraviolet rays.

Generally, the radiation tolerated dose of

metalbased thermal control coating materials is higher, compared to inorganic-based materials
and organic-based materials, which have increasingly lower tolerance to radiation.

Themechanism of deterioration of the optical properties is complicated by the complex behavior of
the ultraviolet rays, radiation, and high vacuum environments. It is difficult to predict thedeterioration quantitatively using laboratory data unless the testing equipment which is used
can apply several of these space environments at the same time.
Therefore, it is preferable to use flight data of spacecraft in the thermal design. However, whenlaboratory data is used, adequate caution is required.

25

2.3.5 Design Procedure
The flow of the design for the total dose is indicated in Figure 2.3-5.
Environmental
conditions
Cosmicradiation
environment
model (*)

Energy spectrumof electrons &protons(*) Includes the reliability levelfor solar flares

Orbitalconditions,launch year,
mission period

Systemrequirements

Total dose prediction
Dose-Depth
Curve
Safety factors

Examinationof measures

thickness data

Satellite
衛星機器配置equipment layout
質量分布mass distribution

Total dose ofcomponents andmaterials

Mounteddevice shieldthickness data

Mounting designfor mounted
devices

System shield

Deterioration
rate prediction

Measures?

Within acceptablevalue by change of
operating conditions

Resistance data

Radiation resistancecharacteristics ofcomponents &materials

Change ofcomponents
and materials

Increase inshieldthickness

Mounted搭載機器device
radiation
放射線環境
environmentNo

Is the radiation
deterioration
within theacceptable value?

Radiation tolerated
dose data ofcomponents &materials

Unknown

Yes

End

Figure 2.3-5 Design Flow for Total Dose26

Documentation
Components &materials data

Component &materials radiationtests

(1) Cosmic radiation environment
The energy spectrum of the electrons and the protons are calculated using orbital conditions,launch year and the mission period determined in the system requirements, based on item 2.2.
The calculated energy spectrum is described in the system specifications as one of theenvironmental conditions in orbit.
(2) Total dose calculation
Based on the energy spectrum, the Dose-Depth Curve is calculated based on item 2.3.1.
Based on the shield data of this Dose-Depth Curve and the system, and the shield data of themounted device, the absorbed dose of the components and materials is calculated based on
item 2.3.2. Normally, it is efficient to perform this calculation by separating it into a systemlevel analysis and a mounted device analysis.
The analysis of results at the system level are provided to the designer of the mounted device
as shielding thickness data equivalent to the absorbed dose on surfaces other than themounted device, or the shielding effect on the system. Based the mounting design, the total
dose on the components and materials of the mounted device is calculated according to item
2.3.2.

(3) Comparison of tolerated dose of electronic components and materials
Based on the calculated total dose and the radiation resistance characteristics data of thecomponents and materials indicated in item 2.3.3 and item 2.3.4, the radiation deterioration of
the characteristics of the components and materials to be used is estimated.

The

deterioration of characteristics is considered within an acceptable value when viewed from theapplication of the components and materials, and the application is determined to beappropriate. When the deterioration of characteristics deterioration exceeds the acceptablevalue, it is necessary to take measures according to item (4). When the radiation resistancecharacteristics of the components and materials are unknown, it is necessary to acquire thedeterioration data by performing radiation tests. Since the acceptable value of the radiation
deterioration of the components and materials changes with individual application conditions,even though it can be used for one application for the same total dose, there are cases when
it cannot be used for other applications.
When the radiation tolerable dose of the components and materials depends on the
production lot, and is determined to be critical judging from the predicted total dose and the27operating conditions, it is necessary to confirm radiation resistance characteristics byperforming radiation tests on each production lot.

(4) Measures
When the deterioration of the components and materials in the calculated total dose exceeds
the acceptable value, it is necessary to take the following measures.

- Change to components and materials with less radiation deterioration- Decrease the total dose by increasing the shield- Relax the acceptable value by changing the operating conditions

When it is necessary to increase the shielding, it must be considered that the increase in themass be kept to a minimum. Increasing the thickness of the shielding at the system level willlead to a great increase in the mass.
When the radiation deterioration of a specific component only is critical, spot shielding is
generally performed at the component level. However, by increasing the shielding thickness ofthe entire mounted device, an increase in the mass can be reduced. On the other hand, it isnecessary to consider that there are cases where there is little affect according to the orbital
conditions and the mounting position.

28

2.4 Radiation Resistance Design Standard for Solar Array Panels
Because of their function, solar cells are equipped on solar array panels or on the surface of
spacecraft structures.

Therefore, the solar cells are exposed to a more severe radiation

environment compared with other semiconductor devices. The output of the solar cells deteriorates
from radiation damage, which in turn deteriorates the power generation of the panels. Therefore, inorder for the generation of power from the solar array panels to meet the power required for the
spacecraft until the end of its mission, the amount of deterioration from the radiation must be
predicted and reflected in the panel design (estimate the portion of deterioration, and consider anappropriate margin to determine the initial generation of power).
Normally, the solar array panels consist of a structure in which an aluminum honeycomb, which iscalled the substrate, is sandwiched between CFRP and hardened by a resin. Polymide (Kapton) film
is attached to the surface for insulation (the surface where the solar cells are attached). Solar cells,
cover glass, diodes, electrical wiring are held together with adhesives and silver evaporation coating.
Teflon, which is a thermal control material, may be attached to the back side as required. Since thischapter indicates the design standard for radiation resistance regarding power generation, only thesolar cells and cover glass are described here.
In the radiation deterioration of the output power of the solar array panels, there is radiation
deterioration (color) from sunlight transmissivity of the cover glass or the adhesives, in addition to thesolar cells. The amount of deterioration to the power generation due to the above must be predicted,
and reflected in the panel design.
2.4.1 Radiation Deterioration Prediction Method of Solar Cells
As a method to predict the radiation deterioration quantity of solar cells, there are currently two
types of methods, the relative damage coefficient (RDC) method by JPL, and the displacement
damage dose (DDD) method by NRL. Generally, until now, the former method is most often used.
However, a considerable number of ground radiation irradiation tests must be performed in order toacquire the RDC which is a parameter required for prediction. The latter method is a relatively new
method, and there are still few application results. However, this method has an advantage where
only 3 ground radiation irradiation tests are required in principal (any 2 types of electron ray energies,
and 1 type of proton ray energy), if the parameter called the NIEL value is acquired by calculations
and used in this type of prediction. Therefore, it is suitable to use the RDC method when sufficientground radiation test data is available, such as JAXA space authorized solar cells, or the RDC has
already been acquired, and use the DDD method when sufficient ground radiation test data has not29yet been acquired for new solar cells, or when the three types of irradiation tests could not be
implemented.
When either of the methods is used, it is necessary to estimate the exposure dose of the radiation
for the spacecraft mission period which is applicable for design. Use the radiation environment
codes, such as the JAXA model, NASA AP8 or AE8 and acquire the energy spectrum of the total
exposure dose (fluence) for electron rays and proton rays, from the orbit and mission period. The
number of other existing particles (ions), i.e. the collision frequency, is extremely low in space whencompared to electrons and protons, and even though radiation incidences occur, it will be mitigatedby the cover glass and will not reach the solar cells. Therefore, it is not necessary to consider thisas a deterioration factor.
Next, it is necessary to acquire the deterioration curve (fluence dependency data of the remaining
factors), by 1 MeV electron rays and 10 MeV proton rays of the solar cells to be adopted, or when
there is no data, it is necessary to acquire the data by implementing irradiation tests.

The

remaining factors refer to the ratio of the performance data before radiation exposure and after thedeterioration of the initial performance value.
Use either the RDC method or the DDD method mentioned above for the radiation deterioration
prediction of the solar cells to be adopted. Two types of deterioration prediction methods aredescribed in the following. Refer to this description when selecting the deterioration prediction
method. Refer to the description in JERG-2-143-HB001 for the detailed deterioration prediction
calculation method by the RDC method and the DDD method.
(1) Relative damage coefficient (RDC) method
In the case of the RDC method, first prepare the RDC (using no shielding material, with one
directional radiation incidence) for the electrons and protons of the solar cells to be adopted,
and the output deterioration curve by 1 MeV electron rays from the tests or data sheet. On theother hand, it is necessary to calculate the energy distribution of the fluence of the electronsand protons during the mission period from the radiation environment code.
Once the RDC is acquired from ground radiation tests with no shielding material and a one
directional radiation incidence, it is necessary to convert the value taking into account theconditions with the isotropic incidence and shielding material (considering the cover glass,adhesives and thickness). Using the converted RDC value and the particle energy distributionof the fluence of the electrons and protons, then calculate and derive the equivalent fluence of 1
MeV electron rays for electrons, and the equivalent fluence of 10 MeV proton rays for the30

protons. Then convert the equivalent 10 MeV proton ray fluence to a 1 MeV electron ray
fluence using a 10 MeV protons/1 MeV electron conversion factor, and add to the previous
equivalent 1 MeV electron ray fluence of the electron rays. In this way, the entire radiation
exposure dose is converted as an equivalent 1 MeV electron ray fluence.

Then, it isnecessary to acquire the deterioration quantity (remaining factor) of the solar cell output fromthe equivalent 1 MeV electron ray fluence acquired from the deterioration curve by 1 MeV
electron rays which were acquired, or acquired by these tests.

This is the deterioration

prediction amount (rate) of the solar cells for the mission concerned.

The prediction

procedure is indicated in Figure 2.4-1.
Implement radiation irradiation tests for thetarget solar cells (without cover glass) andacquire the deterioration curve (horizontal
line: fluence)Calculate the relative damage
coefficient (RDC), and 1 MeV
electron/10
MeV
protonconversion factor

Deterioration curve by 1 MeV electron rays

Determine the energy spectrum of thenumber of incidence ray particles (electronsand protons) from the environment
prediction code

Convert the relative damage
coefficient with the conditions ofomnidirectional incidence andcover glass (desired thickness)

Calculate the equivalent 1 MeV electron
ray fluence of the mission period

Estimate the deterioration quantityof the solar cells in the last stage ofthe mission (EOL)

Figure 2.4-1 Prediction Procedure of Solar Cell Deterioration Quantity by Relative Damage
Coefficient (RDC) Method(2) Displacement damage dose (DDD) method
In the case of the DDD method, first prepare the NIEL value of each element which consists ofsemiconductor materials of the solar cells to be adopted based on documentation or
calculations. Then it is necessary to calculate the energy distribution of the fluence of theelectrons and protons during the mission period from the radiation environment code. Since
these distributions are the conditions without shielding material, this is converted inconsideration of the effect of the shielding material (considering the cover glass, adhesives and31thickness). Use the energy distribution of the fluence of the electrons and protons after thisconversion and the NIEL value mentioned above, and then calculate and derive the DDD value
during the mission period. On the other hand, it may be necessary to convert the horizontal
axis of the deterioration curve data acquired by ground radiation irradiation tests into the DDD
value using the NIEL value of the electron or proton fluence, to acquire the new deterioration
curve (horizontal axis is the DDD value).

In this case, it is necessary to acquire thedeterioration quantity (remaining factors) of the solar cell output from the DDD value during themission period, which was acquired from this deterioration curve.

This is the predicted

deterioration amount (rate) of the solar cells for the concerned mission.

The prediction

procedure is indicated in Figure 2.4-2.
When the deterioration rate of the solar cells acquired by the above two methods is larger thanthe deterioration rate of the solar cells which is required for the panels/power supply design, it isnecessary to change the solar cells or enhance the shielding material (such as increasing thecover glass thickness).
Acquire/derive the non-ionizing energy loss
(NIEL) value of the component elements ofthe target solar cells

Determine the energy spectrum of the numberof incidence ray particles (electrons andprotons) from the environment prediction code

Convert the energy spectrum of the number ofincidence radiation particles with theconditions of omnidirectional incidence andcover glass (desired thickness)

Implement radiation irradiation tests for thesolar cells (without cover glass) and acquire
the deterioration curve (horizontal line:
displacement damage dose Dd)

Calculate the displacement damage dose
during the mission period

Estimate the deterioration quantityof the solar cells in the last stage ofthe mission (EOL)

Figure 2.4-2 Prediction Procedure of Solar Cell Deterioration Quantity by Displacement
Damage Dose (DDD) Method
2.4.2 Radiation Resistance Design of Solar Cells (Cell Selection)
Basically, there are two types of solar cells currently authorized by JAXA as components for use
in space: highly efficient silicon cells and 3 junction solar cells. When radiation resistance is the32only consideration for the materials and structure, the 3 junction solar cells are superior (the detailsof their radiation resistance capabilities are outlined in JERG-2-143-HB001). However, the siliconcells provide superior weight, price, and ease of handling. Determination of the type of solar cellto be used must consider the above.
The 3 junction solar cells provide superior radiation resistance. However, since the cells are
made of a laminated structure, the amount of radiation damage changes according to the amountof penetration from the surface, which is determined by the energy - particularly in proton rays(refer to JERG-2-143-HB001 for details). For this reason, the radiation deterioration of the 3
junction solar cells indicates a complex behavior. Therefore, it is necessary to consider theradiation environment and the shielding materials before adopting the 3 junction solar cells.
The measures for radiation deterioration in the structural design of the above two types of solar
cells are described in the following. Refer to this description when selecting the type of solar
cells.
(1) Silicon solar cells
The minority carrier diffusion length of silicon (Si), which is the most important property valueof solar cells, is larger than other solar cell materials, by approximately several 100 µm or more.
On the other hand, the property value which deteriorates greatly due to the effects of significant
radiation damage is also the minority carrier diffusion length. Therefore, in order to control
radiation deterioration, the thickness of the solar cells should be designed for between 50-100µm to prevent the emergence of an effect of the deterioration of the minority carrier diffusion
length, to improve the radiation resistance. By this design, the initial output is sacrificed tosome extent.
Each of the remaining factors of the open circuit voltage (Voc), short circuit current (Isc) andthe maximum power (Pmax) is about 85%, 80% and 67%, when a 1 MeV electron ray of aspace Si solar cell #109 (NRS/BSF type 100 µm), which is authorized by JAXA, is exposed at afluence of 1x1015 cm-2.

(2) 3 junction solar cells
The 3 junction solar cells have a laminated structure laminated comprising types of solar cells(sub cells): InGaP on the euphotic surface, GaAs in the middle and Ge on the back side. Eachof the sub cells are called the top cell, middle cell and bottom cell respectively. These 3 cells
are electrically connected in a series, and tunnel junctions are inserted between the cells for33

current flow.

InGaP and GaAs are III-V group compound semiconductor materials, andgenerally excel in radiation resistance more than the IV group semiconductor materials, suchas Si or germanium. Particularly, InGaP provides excellent radiation resistance, and its output
deterioration is the lowest among the 3 sub cells. For this reason, a thinner InGaP top cell isdesigned into the 3 junction solar cells as a current limiting cell to reduce light absorption, i.e.
the current output. Since the 3 sub cells are connected in a series, the sub cell with the lowest
output current determines the output current of the 3 junction cell. By this design, the current
output of the 3 junction solar cells provides extremely high radiation resistance. On the other
hand, the voltage output becomes the total of the 3 cells due to the series connection. There
have been no specific measures taken to improve the radiation resistance.
Each of the remaining factors of the open circuit voltage (Voc), short circuit current (Isc) andthe maximum power (Pmax) is about 90%, 97% and 87% respectively, when a 1 MeV electron
ray of space 3 junction solar cells #502 (Epi wafer A), which is authorized by JAXA, is exposed
at a fluence of 1x1015 cm-2.

2.4.3 Radiation Shielding (Panel Structure)
When actually mounting the solar cells to the substrate of the solar cell panels of a satellite, glass(cover glass, usually with a thickness of about 50-100 µm), is attached to the euphotic surface as ashielding material, to control deterioration caused by radiation damage. The radiation, especiallythe proton energy which comes from space, can be reduced with this glass and adhesives (usuallywith a thickness of about 20-30 µm), and low energy protons, will stop in the glass or adhesives,
and will not reach the core of the solar cells.
The panel substrate on the back side of the solar cells also has a shielding effect. As mentioned
previously, the typical structure of a substrate consists of a sandwiched aluminum honeycomb with
CFRP, with a polyimide film attached on the cell attachment surface. There are cases where athermal control material is attached to the opposite side. These materials have a shielding effectagainst radiation.
As mentioned in the items of the deterioration prediction method, the effect of these shields must
be considered in the radiation deterioration prediction of solar cells.

The shielding effect is

proportional to the surface density (g/cm2) of the shielding material.
Therefore, the shielding thickness on the surface and the back side must be acquired based on
the surface density before performing the deterioration prediction calculation. It is necessary to34use the value considering the portion for the shielding effect in the RDC of the RDC method, andthe radiation fluence in orbit for the DDD method. Then, it is necessary to consider the shielding
effect for each of the radiation incidences from the surface and back side, acquire the equivalent 1
MeV electron fluence from the RDC or the DDD value from the fluence in orbit, total the values ofthe surface and back side, and determine the deterioration prediction calculations.
There are cases where the acceptable deterioration rate of the solar cells is determined by theconstraints, such as the area, size and weight of the solar array panels. When this is the case, itis necessary to calculate the amount of deterioration using the thickness of the shielding material,
such as the cover glass as a parameter, to derive the required shielding thickness and then adopt
a shielding material of that thickness.
2.4.4 Other Deterioration Factors
There is the possibility of deterioration from the sunlight transmissivity of the cover glass (there
are many cases where an antireflection film coating of MgF is used on the surface of thephosphate glass) attached to the euphotic side of the solar cells, and the adhesive (siliconadhesives, typically Dow Corning DC93-500) used to attach the cover glass deteriorate (color)from radiation and UV rays. Therefore, it is also necessary to consider the deterioration in the
power generation of the solar cells due to this kind of deterioration. However, the resistance tonormal radiation and UV ray environments (mission of about 5 to 10 years in a low orbit orgeostationary orbit), have been sufficiently secured for the cover glass and adhesives which are
currently used. In this case, it would not be necessary to take deterioration into consideration.
For missions with particularly severe radiation and UV ray environments other than the above, it isnecessary to perform radiation/UV ray irradiation tests under suitable conditions, evaluate thedeterioration of transmissivity, and reflect any necessary changes in the panel design.

35

2.5 Single Event Design Standard
2.5.1 Outline of Single Event Effect
When charged particles, such as high energy protons, He ions, or heavy ions are exposed toelectronic components, the particles will ionize along the trajectory of the particles and an electron
hole pair will be generated. When a portion of a generated charge flows into the circuit of adevice, malfunctions, over current will occur due to a noise current. Such an effect can begenerated with just one incidence particle, and is called a single event effect (SEE). Single event
effects can be classified into a number of types due to the differences in the types of devices andthe generating mechanism.

(1) SEUSEU is a phenomenon which occurs in memory devices, microprocessors, etc., in which acharge occurs from the incidence of high energy charged particle flowing into the memory
circuit, causes malfunctions and reverses information in the memory. There are cases where
adjoining multiple cells are upset by an incidence of one ion in recently-made components withmicrostructures, which is called MCU. MCU is used differently from MBU, in which multiple
bits are upset in one phase.
(2) SETSET is a non-destructive behavior which is generated in linear ICs, and logic circuits , such as
optical semiconductor devices, OP amplifiers and comparators. The noise pulse generated
from the charged particles which entered at the input stage is retained in the latch circuitry, ortransmitted to the output stage, where malfunctions occur in the circuitry.
(3) SELSEL is mainly generated in devices of a CMOS structure.

The thyristor structure is

specifically included in CMOS structures. The thyristor changes to the ON state according to
the noise current generated by incidences of high energy charged particles, and a large current
continues to flow. Becausee a large current continues to flow locally unless the power supply
is turned OFF, it is necessary to turn OFF this particular thyristor before there are malfunctions
caused by electrodes fusing within the device and a voltage drop within the same power supply
system.
(4) SEBSEB is mainly generated in power MOSFET. The particular transistor is triggered by an36

incidence of high energy charged particles to the particular transistor which is included in the
structure of a power MOSFET, and a large current flows into the device and destroys thedevice.
(5) SEGRSEGR is a phenomenon which is generated in power MOSFET, which destroys the gate
oxide.

Other new single event effects have been discovered accompanying the miniaturization ofdevices. However, the mechanism has not yet been completely clarified, and there are namesof events which have not yet been standardized.

2.5.2 Design Procedure
The design flow for SEE is indicated in Figure 2.5-1. First, orbital conditions and the mission
period are determined according to the mission requirements, and the fluence of the heavy ionsand protons on the conditions is calculated from the space environment model. , When theradiation resistance data on the components selected based on the system requirements is
confirmed, (an irradiation test is performed), The SEE rate of incidence in orbit is calculated fromthe data, with the spectrum of the heavy ions and protons, and a criticality analysis is performed on
the device. If the results are not acceptable for the system, it is necessary to re-select thecomponents, or take measures to protect the device.

2.5.3 Prediction Method of Single Event Probability of Occurrence
Charged particles originating in galactic cosmic rays and solar cosmic rays (protons and heavyions) and protons trapped in the geomagnetic field contribute to single events in cosmic radiation
environments. CREME96, which was developed by NRL, can be used for the prediction of theprobability of a single occurrence. When the information of the target semiconductor devices,such as the resistance data of a single event, the space environment, orbit, and shielding thickness
are input into the program, the probability of occurrence in orbit will be output. If data on theheavy ions does not exist, it is necessary to perform irradiation tests on the heavy ions to acquire
the occurrence cross section - LET curve, and calculate the probability of occurrence. However,the devices in which the LETth exceeds the maximum LET (119 (MeV/(mg/cm2))) of uranium whichis the heaviest atom in space, it can be handled as a device in which a single event is not37

generated. The LETth is calculated from the test results by the Weibull fitting, or simply use theLET of the reverse cross section will be 1/100 of the saturated reverse cross section. In the casewhere LETth < 10 (MeV/(mg/cm2)), the calculation for the probability of a single event occurrence
by protons is also performed. When there is no data on the protons, it is necessary to acquire thedata of the occurrence cross section for the energy of the protons using irradiation tests, andcalculating the probability of occurrence.
In the case of a single event in a destructive mode, since a large number of test pieces are
required to determine the occurrence cross section - LET curve, there are cases where the tests
are performed only to confirm that no event is occurring in the provided LET. In such a case, themaximum occurrence cross section is calculated assuming that a destructive event occurred onceat maximum LET, where a destructive event did not occur, and the probability of occurrence iscalculated.

2.5.4 Measures for Single Events of Electronic Components
As basic measures at the component level, it is necessary to give priority for the use of devices
with a larger resistance to a single event, devices in which the level of resistance has been clarified,or actually measure the resistance of the devices in which the resistance is unknown and can be
mentioned using basic measurements. The target devices for each single event are indicated inTable 2.5-1. Be sure to perform an examination, an SET can be generated in all analog ICs.

(1) IC
Numerous tests on the single event of ICs have been conducted worldwide, and data has
been accumulated. It is necessary to collect information while referring to the data base forselection, and compare it to the requirements of the mission determine its appropriateness.
The error detection correction (EDAC) circuit is effective for the SEU. The resistance of the
SEL changes greatly according to the structure of the device. However, since a parasitic
thyristor does not exist in a device with an SOI structure, the SEL will not occur. The collectedcharge can also be suppressed, and is also effective for improvement in the SEU resistance.
The effect of SET, which is generated in linear ICs such as operational amplifiers andcomparators, changes greatly with the operating conditions of the device. For example, it issaid that it is easier for the comparator to be affected by SET when the difference between the
input voltage and reference voltage is smaller, and when the degree of amplification is larger for38

the operational amplifier. Therefore, when referring to the existing SET data, it is necessary toconfirm whether the data was taken under the worst possible conditions.

When actually

evaluating the resistance, it is necessary to perform the evaluation in the worst possible
conditions, or at least in the actual operating conditions if it is difficult to determine the worst
possible conditions. For the selection of the components, it is necessary to examine theselection of the components which can be used in the operating conditions where the effectfrom SET is as low as possible.
(2) Electronic components other than ICs
Since the power MOSFET is mainly used as a switching element of the power system, it isnecessary to take precautionary measures as there is a high possibility of an immediate loss offunctions occurring when SEB or SEGR occur. It is preferable to select a power MOSFET
which has single event resistance as much as possible.

SEB occurs in n channel type

MOSFET with a vertical mold structure. Since SEB does not occur in p channel type MOSFET,
one of the methods is to adopt the p channel type depending on application. SEB will also notoccur if used at a voltage sufficiently lower than the rated voltage. There is a possibility of
SEGR occurring in both the p channel type and the n channel type. When reverse bias is
applied to gate voltage, SEGR tends to occur. Therefore, it is necessary to be careful not toapply a large reverse bias during operation.

In order to adopt a power MOSFET, it isnecessary to verify the safe operating area (SOA) for a single event, and set an appropriate
derating according to the system requirements.
Over currents can be prevented by the load even when SEB occurs in bipolar transistors if
there is a resistance load. There are many cases where damage of the devices can be
avoided.

However, if used for a switching application, it is necessary to take the samemeasures as the power MOSFET.

2.5.5 Measures for Single Events of Devices
Measures for devices are not required if electronic components free of single events can be
adopted. However, if there is no electronic component with SEE resistance which meets therequirements, it is necessary to take measures at the device level. In this case, it is necessary to
select components in consideration of the mission and probability of occurrence . from the view
point to the system. In addition, since the single event effect occurs due to high energy particles,
it is necessary to keep in mind that a reduction in the probability of occurrence cannot be expected39

by increasing the shield thickness.

(a) SEU measures- Correct the errors caused by SEU by an error correcting code or the EDAC circuit. If there isa possibility of MBU occurring due to the structure of the devices, consider distributing the databits over multiple devices to prevent MBU, or using a code which will possibly correct theerrors of multiple bits.
- Prevent errors by having 3 or more of the same circuits as a redundant system, to handle themajority of the output.
- Provide a watchdog timer. This is a measure to prevent the CPU running from out of control,
and the system is reset when a specific address is not accessed for each fixed time.

(b) SET measures- Provide the same multiple circuits, and send the signals through a delay circuit with different
time delays, to handle the majority of the output.
- Provide a filter circuit to remove false pulses.
- Use circuit parameters which will reduce the affect of SET.
- Take the same measures as SEU.

(c) SEL measures- Provide a current limiting circuit to prevent the ICs from being destroyed due to an over current.
The threshold value at constant and when abnormal, must be set in consideration of anincrease in the power current due to the total dose of the concerned IC.
- Use two or more of the same circuits as a redundant system.

(d) SEB measures- The probability of a SEB occurrence increases as the operating voltage approaches the rated
voltage. Therefore, adopt a circuit configuration which is set with an appropriate derating value
according to the mission requirements in consideration of the SEB resistance of each device.

(e) SEGR measures- In consideration of the SEGR resistance of each device, adopt a circuit configuration set with40

an appropriate derating value according to the mission requirements within the specified rangeof the safe operating area.
Cosmic radiation
environment model

Heavy ion and proton
LET / energy spectrum

Missionrequirements
SystemOrbitalconditions

Components
selectionNo

Yes

Is it acceptable?

Radiation testby heavy ionsNo

Is there any
SEE resistance
data?
Yes

SEE measuresfor devices
Radiation tests
by protons

No

Is LETth > 10?

No
Yes

End

Is it acceptable?

SEE occurrence
rate predictionin orbit

Figure 2.5-1 Design Flow for Single Events

Table 2.5-1 Target Device for each Single EventType

Target Device

SEU

Memory devices, microprocessors

SET

Analog IC (comparators, operational amplifiers, regulators, driverICs, ADC, DAC, etc.), logic circuits, high-speed photo couplers,
high-speed MPUs, high speed memory devices, etc.

SEL

CMOS devices

SEB

N channel power MOSFET, NPN bipolar transistors

SEGR

Power MOSFET

41

Yes

2.6 Design Standard for Displacement Damage of Semiconductor Devices
2.6.1 Overview
When a semiconductor is exposed to particles, such as electrons, protons and ions, most of the
kinetic energy is lost due to ionization of the materials. The remaining small amount of energy
drives out the nucleus in the semiconductor from the original position in the lattice, and causes
permanent damage.

This is called displacement damage (DDD), which is the cause ofdeterioration of the characteristics in optical semiconductor devices . In semiconductor devices
covered with a thick shielding, the deterioration by displacement damage of electrons is less thanthe deterioration by the displacement damage of the total dose or protons. Therefore, it is not an
issue for semiconductor devices, except for solar cells, which normally have a very thin shielding.
Since an abundance of high energy protons exist in a space environment, the displacement
damage by protons is an issue for semiconductor devices. The susceptibility for displacement
damage changes greatly according to the application or method of use of the device. (Refer toitem 2.4 for the deterioration by the displacement damage of solar cells, which is excluded fromthis section.)
When electrons, protons, and ions enter semiconductors and the nucleus is driven out, the drivenout atom becomes an interstitial atom, and a hole will be generated in the original position. The
re-coupling of this interstitial atom and hole is unstable, and most return to the original condition
through recombination without leaving any damage. However, some holes move inside the lattice
avoiding recombination, and become a stable defect by bonding with the impurities in thesemiconductor. The level of this defect is formed in the band gap of the semiconductor, andgenerates electron hole pairs, recombination, capture of the carrier, compensation and tunneling atthat level. For that reason, effects, such as an increase in the dark current, a decrease of thecarrier, an increase in the leakage current at the time of the reverse bias, will occur. Since theminority carrier lifetime and the lifetime of the generated carrier among the characteristics ofsemiconductor devices are the most easily affected, the semiconductor devices utilizing thesecharacteristics have the greatest susceptibility to displacement damage.

2.6.2 Prediction Method of Displacement Damage
The energy used to cause displacement damage to materials is expressed as NIEL. When
semiconductor devices receive displacement damage, the parameters of the devices will
deteriorate, and it is known that the degree of deterioration and NIEL are proportionally related.

42

Therefore, it is necessary to acquire the proportional coefficient K from past data or by irradiation
tests of the protons first. Next, it is necessary to calculate the NIEL value in an actual proton
environment and by multiplying the proportional coefficient K by the NIEL value, where the degreeof deterioration in orbit of the parameters of the device concerned can be calculated.

For

example, in the case of a CCD, the charge transport efficiency (CTE) drops by the displacement
damage. However, the change ∆CTE (E) of the CTE for the proton energy is expressed by thefollowing formula using the proportional coefficient K.

∆CTE(E)=K ⋅ NIEL(E) ⋅ Φ(E)(2.6-1)

Φ(E) is the fluence of the protons. Therefore, ∆CTE in orbit is calculated by the following formula.

∆CTE = K ∫ NIEL(E)
∞

0

dΦ(E)
dE
dE

(2.6-2)The prediction flow for displacement damage based on the above is indicated in Figure 2.6-1.

43

Orbital conditions
Mission period
Shielding thickness,
etc.

Fluence of protons

Missionrequirements

Requirement
specification

Environment
conditions

OK

Components
selection

Yes
Requirements
satisfied?

No
Deterioration
parameters

Characteristics
prediction

Radiation tests
by protons

Measures for devices(examination ofshielding thickness, etc.)

No

Is there any DD
resistance data?
Yes

No
Yes

Requirements
satisfied?

* Only for devices which require
implementation of an impact
evaluation by the total dose.

Deterioration predictionof parameters in orbit

Calculation of
proportionality
coefficient K

TID resistance*

NIEL value

Figure 2.6-1 Deterioration Prediction Method by Displacement Damage to Devices in Orbit
2.6.3 Resistance for Displacement Damage of each Device
The semiconductor devices which will be affected by displacement damage are summarized inTable 2.6-1. It is necessary to perform evaluations on the resistance for displacement damage inadvance, and perform a deterioration prediction in orbit for devices which may be greatly affectedby displacement damage. It is also necessary to perform an evaluation on the affects of a single
event or total dose as required. There are cases where an evaluation on the effects of both thetotal dose and displacement damage are required depending on the device.

Generally, thedevices in which the minority carrier lifetime and the generation carrier lifetime, or the devices inwhich extremely high performance is required in specific parameters, have low resistance todisplacement damage.

44

Table 2.6-1 Semiconductor Devices Affected by Displacement Damage
DeviceEffect of Displacement

Resistance to Displacement Damage

DamageCCD

Deterioration of charge

Easily affected by displacement damagetransport efficiency (CTE),

Linear CCD are highly resistantincrease in the dark current
Photo detectors

Deterioration in speed of

Resistance changes with applications

response

Photo-transistors tend to be affected easily

Increase in dark current

by displacement damage
MSM photo diodes are highly resistantLight emitting

Deterioration of optical

Affect by displacement damage is relatively

devices (LED,outputlowlaser diodes)

Increase in current

LED have less resistance than laser diodes

threshold value (laser

Amphoterically doped LED are less resistantdiodes)Photo couplers

Deterioration of current

Response to radiation is complex

transfer ratio (CTR)

Resistance changes greatly with purpose &applications
Photo couplers which use photo diodes havethe highest resistancePhoto couplers which use photo transistors
have less resistance* Photo couplers with a high speed operationof 1 MHz or more require an examination on
SET

Si bipolar

Deterioration of gain

Less resistance with horizontal type pnp

transistors,

transistors

linear ICs

High resistance heterojunctions(1) CCD
When a CCD receives displacement damage, deterioration of CTE increases in the averagedark current with an extremely large dark current in each pixel, resulting in an increase of noisein the output amplifier.
The CTE is the most important parameter for a CCD. However, when the signal charge
captured by the defective level formed by the radiation of the protons, and by the release of thesignal charge after the clock cycle ends at the time of capture, the signal will appear in theimage of the cycle when the signal is released. This does not produce an image of the original
cycle and the quality of the image deteriorates. Therefore, the affect from the displacement45

damage to the CTE will be determined by the read method or clock frequency of the device.
Since the clock frequency of linear CCD is as fast as 1 MHz or more, and the signal istransmitted without being captured by the defective level, it is not easily affected bydisplacement damage. The measures for CTE deterioration are currently under research.
However, the deterioration of CTE can be slightly reduced by cooling the device to reduce thecapture of the carrier. When additional measures are required, a thicker shielding should be
used. However, the effect from the displacement damage by secondary particles which
occurred due to collisions between the protons and shielding material becomes larger.
There is an average dark current and an extremely large dark current (spike) which appears ineach pixel, and the signal to noise ratio (S/N) can be improved by using large area pixels.
The following items can be considered as measures for displacement damage of CCD. It isnecessary to consider the effect on the devices which are easily affected by the total dose as
well.
- Shielding of the devices- Cooling of CCD- Selection of devices which are highly resistant to displacement damage- Selection of operating conditions which can not be easily affected by displacement damage,and signal processing- Use of CCD with the large area pixel(2) Photo detectors
There are PN junction photodiodes, PIN diodes, and photo-transistors. in photo detectors, andan increase in the dark current by the carrier generated in the depletion layer, as well as
deterioration of the response due to the deterioration of the minority carrier lifetime which are
mentioned as effects due to displacement damage. Since the gain of photo transistors isgreatly affected by the lifetime of the minority carrier, it is easily affected by displacement
damage. However, the MSM photo diode is the majority carrier component, and has a highresistance to displacement damage. The resistance to displacement damage changes greatly
by the application of each device.
Photo diodes specific to various applications must be provided, and the effect from radiation
changes greatly according to the design.

In addition to the permanent damage by

displacement damage, there may be an effect from a temporary increase in the output current

46

due to ionization depending on the application.

(3) Light emitting devices
The resistance of laser diodes to displacement damage is relatively high. However, there is apossibility of the output deteriorating when the current threshold value changes greatly.
Even though LEDs are inferior to laser diodes, there are many LED which have a highresistance to displacement damage.

However, the resistance changes according to theapplications, such as a large output, and use at high speeds. As an exception, it is known thatamphoterically doped LED tends to be affected by displacement damage.

(4) Photo couplers
Photo couplers are hybrid modules which consist of LED (light sources), photo detectors andoptical couplings, and are extensively used for the purpose of separating circuits electrically.
An important parameter of photo couplers is the current transfer ratio (CTR), and is expressedin a ratio for the LED forward direction (drive) current of the collector current of photo detectors.
There are various designs of photo couplers, and the applications can also be largely divided
into digital photo couplers and linear photo couplers. The response of photo couplers toradiation greatly depends on the design and the application. Photo couplers are also affectedby the total dose or single events. In addition to displacement damage, it is necessary toconsider the effects of SET in photo couplers which are used at high speeds of 1 MHz or more.
Amphoterically doped AlGaAs LED tend to be affected by displacement damage the mostamong the LED. Current photo couplers have less resistance than the earlier photo couplers.
The response of photo couplers to radiation becomes complex for the following reasons.

(a) Since a photo coupler is a hybrid module, the individual difference in the radiation resistance is
too large.
(The origin of the configured components cannot be clarified.)(b) The deterioration which can be observed is a combination of the total dose and displacement
damage, and the impact is dependent upon the design or application of the photo coupler.
(c) When a photo coupler is a part of a much larger hybrid module, such as a DC-DC converter,
there is a possibility where the photo coupler may receive an effect from other components inthe module as well.

47

CTR deterioration of photo couplers is mainly determined by the displacement damage
resistance of the LED. When photo transistors are used for the photo detector, an effect fromdeterioration of the optical response may also be received.

Photo couplers using photo

diodes are the most resistant to displacement damage overall.
When performing an evaluation of the CTR, it is necessary to consider it according to theapplication. For digital photo couplers, it is necessary to evaluate the CTR across the entire
wave range, and an evaluation on the CTR in the specific wave range to be used is required forlinear photo couplers.

(5) Bipolar transistors
Bipolar transistors themselves have a relatively high resistance to displacement damage.
However, analog bipolar ICs tend to receive displacement damage.

Even though theresistance of linear ICs changes greatly according to the process and device design, it is known
that linear ICs which include horizontal type pnp transistors tend to receive the mostdisplacement damage, and there is a possibility of linear ICs which use horizontal type pnp
transistors at low currents on the input portion having less resistance. In devices where highperformance is demanded, such as extremely small input offset, bias current and low noise also
tend to be affected by displacement damage. On the other hand, bipolar transistors withheterojunctions, such as SiGe, have a high resistance to displacement damage.

2.7 Affects from other than Cosmic Radiation
2.7.1 Measures for Atomic Oxygen(1) Materials which have a resistance to atomic oxygen must be used, especially on the outersurfaces of spacecraft which travel at, or pass through, altitudes of 1,000 km or less.
(2) Even though a location is not directly exposed, if there is a possibility of atomic oxygen
penetrating an opening, avoid using materials with notable reactions to atomic oxygen.
(3) When it is unavoidable to use such materials, use the appropriate coverings or coatings.
(4) Calculate and specify the fluence of the atomic oxygen for each mission, to predict whether thedeterioration by atomic oxygen is acceptable for the mission period, based on the existing data.
When no data exists, perform verifications as much as possible through testing..

48

The design flow regarding the measures for atomic oxygen is indicated in Figure 2.7-1. Refer
to JERG-2-143-HB001 for details.

Examination of measures
Selection of materials(*1)
Resistancedata?

Prediction ofdeterioration rates
by other factors

Irradiation
tests, etc.

YES

NO

(*2)
Prediction ofdeterioration byatomic oxygen

Prediction ofdeterioration rate

Requirement specifications

Prediction of
atomic oxygen
fluenceNO

YESOK(*1) Includes the environment model margin(*2) Is it OK to include the design margin and the environment model margin?

Figure 2.7-1 Design Flow Regarding Measures for Atomic Oxygen

2.7.2 Measures for Ultraviolet Rays
(1) Do not use materials which are notably affected by ultraviolet rays (wave length of about 400
nm or less), in locations directly exposed to space.
(2) Calculate and specify the fluence of the ultraviolet rays for each mission and location used, andpredict whether the deterioration by ultraviolet rays is acceptable for the mission period basedon the existing data. When no data exists, perform verifications through testing as much as
possible. Confirm the wave length range of the irradiation light source in the resistance toultraviolet ray data of noncommercial use materials.
(3) Consider that the materials will be affected by reflected lighting or concentrated lighting, andthat the ultraviolet ray fluence will increase depending on the location where the materials areused.

The design flow regarding the measures for ultraviolet rays is indicated in Figure 2.7-2. Refer to49

JERG-2-143-HB001 for details.

Examination of measures
Selection of materials(*1)
NO
Resistancedata?

Irradiation
tests, etc.

Prediction ofdeterioration rates
by other factors

NO

YES

(*2)
Prediction ofdeterioration by
ultraviolet rays

Prediction ofdeterioration rate

YES
OK(*1) Includes the environment model margin(*2) Is it OK to include the design margin and the environment model margin?

Figure 2.7-2 Design Flow Regarding Measures for Ultraviolet Rays

50

Requirement specifications

Prediction ofultraviolet ray
fluence