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International Journal of Electronics and Communication Engineering & Technology (IJECET) Volume 7, Issue 1, Jan-Feb 2016, pp. 75-86, Article ID: IJECET_07_01_008 Available online at http://www.iaeme.com/IJECETissues.asp?JType=IJECET&VType=7&IType=1 Journal Impact Factor (2016): 8.2691 (Calculated by GISI) www.jifactor.com ISSN Print: 0976-6464 and ISSN Online: 0976-6472 © IAEME Publication A SURVEY OF RADIATION HARDENING BY DESIGN (RHBD) TECHNIQUES FOR ELECTRONIC SYSTEMS FOR SPACE APPLICATION Rakesh Trivedi Ph.D. Research Scholar, EC Department, Nirma University, Ahmedabad, Gujarat, India Usha S Mehta Professor, EC Department, Nirma University, Ahmedabad, Gujarat, India ABSTRACT Considering the extensive usage of electronic systems in Spacecraft and harsh radiation environment in the Space, radiation effects on electronic systems and development of radiation hardening electronic systems have been a topic of research. Researchers are trying to solve this problem at various abstraction levels, starting from fabrication technology to packaging and at system level. This paper covers an exhaustive survey on Radiation Hardening by Design. This survey has covered the research work from nearly 80’s to current state of art. Key words: Displacement Damage (DD), Radiation, Radiation Hardening by Design (RHBD), Single Event Effect (SEE), Total Ionizing Dose (TID). Cite this Article: Rakesh Trivedi and Usha S Mehta. A Survey of Radiation Hardening by Design (RHBD) Techniques for Electronic Systems for Space Application. International Journal of Electronics and Communication Engineering & Technology, 7(1), 2016, pp. 75-86. http://www.iaeme.com/IJECET/issues.asp?JType=IJECET&VType=7&IType=1 1. INTRODUCTION A radiation source in space includes Solar flares, Coronal mass ejections, Solar wind, Galactic cosmic rays, Van Allen radiation belts, solar particle events etc. This radiation environment consists of particles such as photons, electrons, neutrons, and heavy ions. Widely, two major effects are with respect to radiation effects are 1) http://www.iaeme.com/IJECET.asp 75 [email protected] Rakesh Trivedi and Usha S Mehta Cumulative Effect 2) Single Event Effect (SEE). The classification of radiation effects shown in Fig. 1 Figure 1 Various Radiation effects induced in electronic devices SEE: Single Event Effect TID: Total Ionizing Dose DD: Displacement Damage SET: Single Event Transient MBU & MCU: Multiple-Bit Upset and Multiple-Cell Upset SEFI: Single Event Functional Interrupt SEL: Single Event Latch-up SEB: Single Event Burnout SEGR: Single Event Gate Rupture The charge particles can hit the ICs, results in non-destructive or destructive effects depends on the charge intensity and strike location. Particle hit leads to erroneous performance. When a particle hit introduces IC malfunctioning but does not damage it permanently called soft-error. Whereas any permanent damage to device due to particle hit is regard as a hard-error. Cumulative effect is long-term effect and changes parameters of the device. Further, it divided in to two effects. One is TID effect, which is a result of the hole trapping in gate oxide region. When any energetic particle hit on gate of the MOSFET or at STI (Shallow Trench Isolation) region at this condition holes are trapping inside gate oxide or STI region, which damage the structure of the both, gate oxide as well as isolation oxide. Due to hole trapping in gate oxide, changes the threshold voltages of the device and due to hole trapping in isolation oxide between two devices increasing leakage current. Another effect is Displacement Damage (DD) effect which occurs when any energetic particle displace atoms in silicon or insulator and creates electrically active defects. DD effect also known as TNID (Total Non-Ionizing Dose). SEE effect is further subdividing in to six effects. In Single Event Upset effect, upset the storage node charge in memory due to particle strike and change the logic level. SEU are observing in storage elements like Flip-Flop, latches SRAM cells. If SEU effect occurring multiple storage nodes in memory, it is known as MBU & MCU. Due to SEU effect in processor, FPGAs etc. functionally of the circuits disturbs. This effect called SEFI. Single Event Transient is an energy pulse due to strike on sensitive node of the circuit. Sensitive nodes are off nMOS or pMOS transistors in the circuits. Single Event Transient (SET) effect is observing in Combinational logic circuits. If the duration of SET pulse are more than the clock frequency of the circuit may disturb the function of the circuits or upset the memory if SET pulse is occurs at clock edge. SEL is triggering of the thyristor (PNPN structure) mainly exiting in CMOS circuits due to radiation. SEB effect is occurs in power MOSFET devices when source is forward biased and drain-source current is higher than the breakdown voltage of the parasitic structures. SEGR effect occurs when particle damages the gate oxide insulation of the power MOSFET. A TID, DD, SEL, SEB and SEGR effect creates permanent damage to the devices so these types of http://www.iaeme.com/IJECET.asp 76 [email protected] A Survey of Radiation Hardening by Design (RHBD) Techniques For Electronic Systems for Space Application effects are categorised under hard error. SEU, SEFI, MBU & MCU are affects device or circuits temporary so these effects are categorised under Soft error. The rest of the paper is organised as Section 2 discusses different Radiation Hardening by Design techniques, followed by conclusion. 2. RADIATION HARDENING BY DESIGN This section presents mitigation techniques to be applying at design level to reduce the effect of radiation. Storage elements like Flip-Flops, Latches, and SRAMs are more sensitive elements than combinational circuits. Radiation Hardening by design can done by two ways: 1) By Layout modification 2) By circuit design modification 2.1. Hardness by Layout Design In order to mitigate effects like TID, SEL and SEE, two-layout design techniques are in used 1) Enclosed Layout Transistor 2) Guard Ring. While Enclosed Layout Transistor mitigates TID effect, SEL effects can handled better by using Guard Rings. Layout Design modification can be brought in many different manner. The Fig. 2(a) and Fig.2 (b) shows conventional and enclosed nMOS transistor layout. Enclosed Layout Transistor (ELT) is not the only member of this category. Ringed source and ringed inter-digitated design are two such layout techniques for mitigating TID effects. All these three layout techniques focus on preventing leakage current between source and drain. Fig.2(c) and Fig.2 (d) shows layout for Ringed Source and Ringed inter–digitated Transistors, respectively [1]. Figure 2 (a) Conventional two edge nMOS and (b) Enclosed Layout Transistor nMOS (c) Ringed Source Transistor (d) Ringed inter-digitated Transistor Although, they provide compact design as compared to ELT, sometimes they are not immune to TID effects. The most commonly used layout technique for TID prevention is ELT, which is briefly describing as follow: 2.1.1. Enclosed Layout Transistor In the beginning, this type of layout design proposed to prevent gate leakage current in conventional CMOS process. However, after detailed mathematical analysis performed by A. Giraldo [1] [2] for effective aspect ratio calculations, this layout technique is being increasingly used to prevent radiation effects in nuclear and space environment. [3]. Fig. 3(a) shows enclosed gate layout transistor where, the gate is surrounded from outside by Drain (or Source) and enclosed Source (or Drain). http://www.iaeme.com/IJECET.asp 77 [email protected] Rakesh Trivedi and Usha S Mehta Figure 3 (a) Enclosed Gate Layout of Transistor (b) Guard Ring in nMOS Transistor The effective aspect ratio of ELT transistor can well estimate by below formula [1] [2] [3]: (1) Where, c, d, d' = d – c / √2 and α is as shown in Fig. 3(a). Leff is effective gate length, etching, and diffusion. The parameter α has been take into account for identifying borderline between transistors T1 and T2 in Fig. 3(a). Typical value of α is 0.05 and has been found to be technology independent. The parameter K that multiplies the second term in (1) is geometry dependent. For long channel devices (L greater than 0.5 μm), the K value is 4. When the channel width of transistors is being shorter (L equal or less than 0.5 μm), the value of K is gradually decreases from 4 to 3.5, since the polysilicon strip starts to hide T2 [3]. The constant polysilicon strip width denoted by A is independent of the gate length L and is equal to minimum gate width. The first part of (1) corresponds to the four linear edge transistors (T1), the second part seven to eight angle corner transistors (T2), the third part to the three liner corner transistors (T3) [4][5]. From experiments it has been found that threshold voltage shift for ELT is less than 5mV for 70kGy (SiO2) of total radiation. In addition, it has been show that threshold voltage variation decreases with increase in gate length L [4] [5]. 2.1.2. Guard Ring Latch up in CMOS due to parasitic thyristor formation, it can leads to device destruction. Due to this conducting parasitic thyristor a low resistance path forms between VDD and VSS, which established a significant high current flow, and leads to device breakdown. The solution to this problem is that a high resistance path should exist between two supply voltages, which will decrease the current value. One way to reduce amount of current is to decrease the gain of parasitic transistors. This can be done by increasing the distance between the adjacent MOSFETS. The limitation of this technique is it decreases circuit density. Fig. 3(b) shows the layout of MOSFET with a guard ring. For efficient prevention of SEL, nMOS should guarded by a p+ ring and pMOS with n+ ring. Guard rings helps to http://www.iaeme.com/IJECET.asp 78 [email protected] A Survey of Radiation Hardening by Design (RHBD) Techniques For Electronic Systems for Space Application prevent inter device leakage. It has reported that devices implementing guard rings show a very high tolerance to SEL (LET > 90 (MeV – cm2) / mg) [5]. 2.2. Hardness by Circuit Design For the study of circuit, behaviour against SEU is recognizing sensitive nodes in the circuit. SEU sensitive nodes are OFF transistors in the circuit. Here, it has tried to mitigate Single Event Effects and especially Single Event Upset using techniques like 1) adding transistor redundancy to existing logic, 2) charge sharing between devices. However, primary focus for preventing SEUs at Circuit Design level has been adding extra redundant logic to existing logic while using same commercial process technologies. Some of those techniques are discussing below: 2.2.1. Rockett Memory Cell Rockett has presented first radiation-hardened latch in 1988 [6]. transistors surround this standard 6-transistor memory element. 6-P channel Figure 4 (a) SEU hardened Memory Cell by Rockett (IBM) [6] (b) SEU-Hardened Memory Cell by Whitaker (NASA) [7] This design leads to a 16 transistor radiation-hardened latch with input buffer, widely known as Rockett Memory Cell and shown in Fig. 4(a). 2.2.2. Whitaker Memory Cell A second solution published first in 1991 [7] and improved in 1992 [8], represents an original memory cell. The storage bits are located in two different places providing a redundancy and maintaining a source of uncorrupted data after an SEU. Here, Whitaker modifies topology of Rockett Memory Cell with use of 12 transistors instead of 16.Fig. 4(b) shows Whitaker Memory Cell. 2.2.3. Improved Whitaker Memory Cell by Liu The static power of the Whitaker Memory cell is high. M. Norley Liu [9] improved over previous cell topology by using of 14 transistors arrangement as shown in Fig. 5(a). http://www.iaeme.com/IJECET.asp 79 [email protected] Rakesh Trivedi and Usha S Mehta Figure 5 (a) Improved Whitaker Memory Cell by Liu [9] (b) SEU hardened AND/NOR gate to design an R-S latch [10] 2.2.4. RS Latch based Memory Cell by J.Canaris J. Canaris proposed a Logic family, which is SEU tolerant. So, using AND-NOR or OR-NAND gates, a Latch can be designed [10]. This is show in Fig. 5(b).Previously designed Memory cells had drawbacks number of transistors is more, static power consumption is high, and recovery time after upset is high. These all drawbacks are eliminate to an extent/removed and proposed as HIT (Heavy Ion Tolerant) Memory Cell. 2.2.5. HIT Memory Cell HIT Memory Cell designed to improve static power consumption and high recovery time after upset. It designed using 12 transistors [11] (Fig.6 (a)). HIT Memory cell had no any direct path between VDD to VSS which was a problem found in previously defined memory cell. Therefore, lower static power consumption is guarantee in this memory cell compared with previously defined memory cell. Figure 6 (a) HIT Memory Cell [11] (b) DICE Storage Cell [12] http://www.iaeme.com/IJECET.asp 80 [email protected] A Survey of Radiation Hardening by Design (RHBD) Techniques For Electronic Systems for Space Application 2.2.6. DICE (Dual Inter clocked Storage Cell) DICE [12] is composed of two cross-coupled inverter lathes. Feedback to each cell comes from a previous cell. However, nMOS and pMOS are not connecting to the same input. Radiation induced upset at node A can recovered from state at node Cin Fig. 6(b). The limitation of this design is that simultaneous upsets at any two nodes (e.g. nodes A and C in Fig. 6(b)) cannot recover. A variant to the standard DICE implementation has proposed in [13] as Dual DICE, which incorporates interleaving between two DICE cells in order to make them more resistant to SEU, caused by charge sharing. 2.2.7. Temporal DICE Flip Flop Design To address problem of two simultaneous strikes induced upsets in DICE, J. E. Knudsen and L. T. Clark, shown that all four inputs should be made independent to prevent SET while DICE is transparent as well as a delay mechanism along with a majority voter should be setup to decrease SEU more effectively [14]. Figure 7(a) Master Temporal Latch and Slave DICE D Flip Flop (TDFF) [14] (b) Quatro Storage Cell [17] The temporal latch shown in Fig. 7(a) employs a triple redundant feedback mechanism, which separates the signal in time. This makes it immune to both SET and SEU effects [14] [15] [16]. In Fig. 7(a) is showing a temporal latch, which employs delay blocks marked with δ. This shows that the cell is SET hardened up to the duration of one δ delay. 2.2.8. Quatro Latch The Quatro latch is an eight transistors storage cell with the similar basic design compared to a DICE latch but different interconnection topology [17]. The four storage nodes in Quatro cell are namely A, B, C, and D (Fig. 7(b)). Table I shows that over DICE-FF, Quatro-FF is an improvement in area, power, and maximum clock to Q delay while keeping operational characteristics unchanged. http://www.iaeme.com/IJECET.asp 81 [email protected] Rakesh Trivedi and Usha S Mehta TABLE I The DICE Storage Cell [17] Design Area Power Max C-Q delay PDP D-FF 1 1 1 1 DICE-FF 2.95 1.8 1.4 2.5 Quatro-FF 2.5 1.4 1.6 2.2 2.2.9. Gate Sizing The radiation environment in space is uniform in space and time, which indicates that radiation on a chip, is proportional to its active area. When any particle strikes on sensitive node, it generates a current pulse at that node. For a logic upset, generated current pulse needs to be of sufficient value. The current is given by equation (2) [18]. −t −t α β Q τ τ I i n (t)= (e − e ) (τ α − τ β ) (2) Where Q is the charge generated (positive or negative) deposited as result of the particle strike, τα is the collection time constant of the junction, and τβ is the ion-track establishment time constant. τα and τβ are constants that depends on process-related factors. Consider 2 input NAND gate driving a lumped capacitance Cp at its output N in Fig. The total capacitance at N is [18] Ctot al= Cunit (WL )+ C p (3) (a) (b) Figure 8 (a) 2-input NAND gate (b) SEU effects (Q, (W/L), (τα, τβ)). [18] http://www.iaeme.com/IJECET.asp 82 [email protected] A Survey of Radiation Hardening by Design (RHBD) Techniques For Electronic Systems for Space Application So, if (W/L) of the node is increase then Capacitor is charged with more charges. As the size of any gate increase means driving strength of any gate is increase than current produce by the particle strike on sensitive node is decrease. So if particle strike on output node of 2-input NAND gate in Fig. 8(a) than voltage generate by that particle decreases with increasing W/L ratio of the gate shown in Fig. 8 (b). The idea of the authors [18] is that increased output capacitance of any gate can make it is less sensitive to particle hit than a normal gate. This technique was applicable to only older technology. As in the advance technologies, initial value of output capacitance is fF. If this technique applied to advance technologies then increased capacitance values required to satisfy the transient effect become high [19] [20] 2.2.10. Radiation Hardened Memory Cells Previously discussed cells have partially SEU immunity and dependent on polarity on single node or required more power. As CMOS technology scaling down, angled particle strike becoming more severe [21] [22].In [23], an 11 T SRAM cell has proposed [also shown in Fig. 9(a)]. Compared with DICE, as claimed by the authors, this circuit (called 11 T) offers (little) smaller area and considerable lower Power Delay Product (PDP). To have a better robustness against Single Event Multiple Effects (SEME), the authors of [23], proposed another SRAM cell in [24] [also shown in Fig. 9(b)]. They showed that, this latter circuit (called 13 T) can tolerate the effect of Single Event Multiple Effect(SEME) better than their proposed circuit in [23].As the authors reported in [24], their circuit has lower delay, higher power consumption and larger area as compared with DICE. In [25], new SEU hardening Incorporating an extreme low power bit-cell design (SHIELD) proposed. The authors of [26] compare proposed design with proposed design in [12] in terms of technology scaling and effect on it shown in Fig. 9 (c). Figure 9 (a) Proposed in [23] (b) Proposed in [24] (c) The SHIELD SRAM [25] [26] In [27][28] novel Radiation Hardened Memory cell which has 12- Transistor and it is capable of fully SEU immunity and also can tolerant multi bit upset as shown in Fig. 10(a). This technique is show better result against different radiation hardening techniques [27]. http://www.iaeme.com/IJECET.asp 83 [email protected] Rakesh Trivedi and Usha S Mehta Figure 10(a) Proposed RHM (Radiation Hardened Memory) cell [28] (b) Proposed 13-T RHM in [29] In authors of [29] applied similar approach as applied by [24] and authors [24] shows low design overhead, higher robustness to radiation effects in present of SEMEs. Fig. 10(b) shows proposed 13-T RHM in [29]. 2.2.11. Device based SEU Clamping Circuit Equation (2) shows current produced by strike of particles on sensitive node with the help of equation method proposed by [30]. The idea of this method is used same gate with higher threshold than normal gate in circuit. Consider the circuit in Fig. 11; assume that out node is at logic 0. When energetic particle strike on out node, it causes a rising pulse on out node but out node, voltage does not increase because protecting gate is at -0.4V.When the voltage at protecting node outp starts to rise, the nMOS device turns on, thus clamping the protected node. If the particle strike on outp than voltage at protected node is unchanged because protected node initially at much lower voltage (-0.4V) and as the voltage at protecting node increases, the clamping nMOS device turns off and if voltage of the protecting node rises above 0.4V than the clamping pMOS device turns on. Figure 11 Device based SEU Clamping Circuit [30] In a similar manner, the clamping pMOS device helps to protect a gate from falling pulse due to a radiation event. http://www.iaeme.com/IJECET.asp 84 [email protected] A Survey of Radiation Hardening by Design (RHBD) Techniques For Electronic Systems for Space Application 3. CONCLUSIONS Considering the present need of low power, low cost radiation hardened systems; RHBD will be helpful to resolve the issue. In this regard, this paper has tried to cover the entire spectrum of various approaches for Radiation Hardening Design level. As radiation hardening by process is a costly approach, one would prefer to focus on Radiation Hardening by Design. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] A.Giraldo, A. Paccagnella, A. Minzoni, Aspect ratio calculation in n-channel MOSFETs with a gate-enclosed layout, Solid-State Electronics, vol. 44, 1st June 2000, pp. 981-989. 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