Demidov, A A

Nikiforov, A Y

Skorobogatov, P K

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AbstractA diffused laser irradiation was used to improve lasersimulation adequacy of dose rate effects in silicon IC’swith high metallization density. The conversion ofcoherent output laser irradiation into the diffused one wasperformed by specialized  homogenizer. Test structureswith various metallization coverage were designed.Numerical simulations together with laser tests wereperformed in order to clarify the advantage of diffusedlaser irradiation before coherent one.1. INTRODUCTIONThe laser simulation of dose rate effects is based onlaser beam capability to ionize IC's semiconductorstructures [1]. Associated shadowing effects have beeninvestigated in details [2,3]. It was shown that the metalcoverage reduces the simulation adequacy if coherentlaser irradiation is used. The influence of shadowing onlatch-up threshold in this case was analyzed in [4].A way to improve dose rate laser simulation adequacywas proposed in [5]. The approach is based on theapplication of noncoherent (diffused) laser radiation thatreduced metallization shadowing effects. The opticalmodel of diffused laser radiation interaction with ICsemiconductor structures and appropriate softwaresimulator were presented. It was shown that in the case ofdiffused laser irradiation the ionizing effect is defined byaverage metallization coverage of structure and theadjacent area.In this work we applied the diffused laser irradiation todose rate ionizing current and latch-up simulation. Thespecialized test structures were manufactured toexperimentally estimate the advantage of diffused laserirradiation before coherent one.2. TEST STRUCTURES DESCRIPTIONTwo structure sets (TSCPHXX and TSCLUXX) aremanufactured in conventional 2-mm bulk CMOS process.A structure's cross-section is presented in Fig. 1. Eachstructure includes well-substrate p-n junction (48x78 mm)with strip contacts and various metallization coverage.Contact region size is 2x2 mm. The structure set TSCLUXXis similar to TSCPHXX but with different disposition ofhighly doped regions in order to form SCR (latch-up). Thestructural parameters are: p-substrate is B doped up to 12W ·cm. The 6 mm n-well is P doped up to 1700 W/. The 0.6mm p+ and 0.35 mm n+ regions are doped up to 110 W/and 40 W/ consequently.The structures TSCPHXX have four leads:  twoanodes (A1 and A2) and two cathodes (K1 and K2) andstructures TSCLUXX have leads: base and emitter ofparasitic p-n-p transistor (B1 and E1) and emitter and baseof parasitic n-p-n transistor (E2 and B2).USE OF DIFFUSED LASER IRRADIATION TO IMPROVE DOSE RATESIMULATION ADEQUACYA.A.Demidov, A.Y.Nikiforov, P.K.Skorobogatov,Specialized Electronic Systems, Moscow, Russia (e-mail: pkskor@aha.ru) Figure 1. TSCPHXX  (TSCLUXX) test structures cross-sectionVarious test structures have the individual metal stripwidths. Metal strip parameters of the structures aresummarized in Table 1.Table 1. Test structures metallization parameters  (Spn is  p-njunction square Sm is p-n junction metallization square).Test structure Metallizationstrip width lm,mmMetallizationcoverage,Sm/Spn,%TSCPH2 (TSCLU2)  2 8.3TSCPH6 (TSCLU6)  6 25TSCPH12 (TSCLU12) 12 50TSCPH18 (TSCLU18) 18 75TSCPH22 22 91.7The minimum and maximum shadowing cases arepresented in Fig.2. The last strip to the right does notshadow the p-n junction and its width is equal to 10 mmfor all structures. Figure 2. TSCPH2 (top) and TSCPH22 (bottom) teststructures view3. NUMERICAL AND EXPERIMENTALCOMPARATIVE RESULTSIn order to perform the test structures transientanalysis the "DIODE-2D" software simulator was usedwhich is the two-dimensional solver of fundamentalsystem of equations taking into account the electrical andoptical processes including multireflection and free carriernonlinear absorption [3]. The numerical simulator opticalmodel was modified in order to take into account thediffuse feature of laser radiation.Pulsed laser simulator "RADON-5E" with 1.06 mmwavelength and 11 ns pulse width was used [6]. Thesimulator was supplied by homogenizer that convertspurely coherent parallel laser irradiation into diffusedmode. The laser pulse maximum intensity was varied from6·102 up to 2.1·106 W/cm2 with laser spot size covering theentire chip. The ionizing current transient response wasregistered with "Tektronix TDS-220" digital oscilloscope.The laser simulation calculated and experimental dataof well-substrate junction ionizing current amplitude vs.metallization coverage of TSCPHXX test structures underVcc=5V are presented in Fig.3.  One can see the betteragreement of the diffused model calculation curve withtest data. The usage of homogenizer improves the laserintensity uniformity within chip area and decrease theinfluence of metallization shadowing on test structureionizing current. For TSCPHXX test structure set whenmetallization coverage increase from 8.3 to 91.7% (11times) the ionizing current amplitude  decrease only from10 to 5 mA (2 times) at laser intensity 9.7·103 W/cm2.Figure 3. Theoretical (curve) and experimental (dots)test structures photocurrent amplitudes vs.metallization coverage at laser intensity 9.7·103 W/cm2:1- coherent model; 2 - diffused modelThe comparison between theoretical and experimentaldata for TSCLU2, LU6, LU12 and LU18 test structuresunder Vcc=5V is presented in Fig. 4. One can see thatdiffused laser irradiation model provides the betteragreement with experimental data. Figure 4. Theoretical (curves) and experimental (o) teststructures latch-up levels vs metallization coverage: 1 -coherent model; 2 - diffused modelThe experimentally defined TSCLU18 and TSCLU2 teststructures latch-up levels equal to 4.3.103 and 2.7.103W/cm2 accordingly. The ratio of latch-up thresholds doesnot exceed 1.7 when metallization coverage increase from8.3 to 75 % (9 times). It can be explained by nonparallelfeature of diffused irradiation and by contribution ofreflected irradiation from chip bottom [5].However the numerical estimation for diffused modelgives 3.38 value of the ratio. The difference betweennumerical and experimental values may be explained bycontribution of light refraction and scattering inpassivation layers, SiO2 and Si neglected undercalculation. These factors tend to improve laser ionizationuniformity and decrease the difference latch-upthresholds.To investigate the influence of metallization shape onlatch-up threshold level the structures TSCLU23, LU63and LU18b were designed additionally. They have theindividual metal strip topologies shown in Fig. 5. It wasfound that maximum difference in latch-up thresholds inexperiments did not exceed 3% between TSCLU6 andLU23 structures, 5% between TSCLU18 and LU18bstructures and 10% between TSCLU18 and LU63structures. This difference is within the accuracy of laserintensity dosimetry and so the structure pares listedabove may be considered as equivalent under diffusedlaser irradiation.a)b)c)Figure 5. TSCLU23 (a), TSCLU63 (b) and TSCLU18b (c)test structures viewThe reasons of diffused laser irradiation advantagemay be explained on the base of optical model. Ahomogenizer converts the coherent output laser radiationinto the diffused radiation of finite-size source as shownin Fig.6. It causes the mixing of various laser modes andpermits the nonparallel radiation to partially penetrateunder metallization. The additional effect is due to thereflection of the nonparallel laser beam from thesubstrate’s bottom (see Fig. 6). As a result the laserradiation can penetrate under metallization from backsideand reduce the ionization nonuniformity.Figure 6. The shadow creation and backside reflectionin the case of diffused laser radiation formed by ahomogenizer (1)4. CONCLUSIONSA method for improving of dose rate simulationadequacy based on the application of noncoherent(diffused) laser radiation was applied and adopted to p-njunction ionizing current and CMOS latch-up thresholdestimation. The diffused irradiation was supplied byspecialized laser simulator “RADON-5E” withhomogenizer. Numerical simulation results were verifiedagainst experimental data under specialized test structureswith individual metallization coverage and topology.It was found that usage of homogenizer improves thelaser intensity uniformity within chip area and decreasethe influence of metallization shadowing on CMOS latch-up threshold. For TSCPHXX test structure set whenmetallization coverage increase from 8.3 to 91.7% (11times) the ionizing current amplitude  decrease only from10 to 5 mA (2 times). As for TSCLUXX structures whenmetallization coverage increase from 8.3 to 75 % (9 times)the latch-up threshold level increase only from 2.7.103 to4.3.103 W/cm2  (1.7 times).The measured difference between latch-up thresholdsof structures with equal metallization coverage anddifferent topology did not exceed 3 - 10%.Obtained results demonstrate the advantage ofdiffused laser irradiation for dose rate effects simulation inhighly-metallized IC’s.5. REFERENCES [1]. D.H.Habing, "Use of Laser to Simulate Radiation -induced Transients in Semiconductors and Circuits",IEEE Trans. Nucl. Sci., Vol. NS-12, No.6, p.91-100,December 1965.[2]. M.Jonsson, S.Mattsson, "Transient RadiationResponse of VLSI Circuits: Shadowing Effects andPulse Widths Dependence in Laser" IEEE Trans. Nucl.Sci., Vol. NS-38, No.6, p.1429-1433, December 1991.[3]. A.Y.Nikiforov, P.K.Skorobogatov, "Dose Rate LaserSimulation Tests Adequacy: Shadowing and HighIntensity Effects Analysis", IEEE Trans. Nuc. Sci., Vol.NS-43, No.6, p.3115-3121, December 1996.[4]. P.K.Skorobogatov, A.Y. Nikiforov, B.A.Ahabaev“Laser Simulation Adequacy of Dose Rate InducedLatch-up”, in Proceedings of the 4th Europ. Conf."Radiations and Their Effects on Devices and Systems,Cannes, France, Sept. 15-19, 1997, pp.I21-I23.[5]. P.K.Skorobogatov, A.Y.Nikiforov, A.A.Demidov “AWay to Improve Dose Rate Laser SimulationAdequacy”, IEEE Trans. Nuc. Sci., Vol. NS-45, No.6,December 1998.[6]. A.Y.Nikiforov, O.B.Mavritsky, A.N.Egorov,V.S.Figurov, V.A.Telets P.K.Skorobogatov,S.A.Polevich “RADON-5E” Portable Pulsed LaserSimulator: Description, Qualification Technique andResults, Dosimetry Procedure”, in 1996 IEEE RadiationEffects Data Workshop Record, Indian Wells, July 15-19,1996, pp. 49-54.

Use of diffused laser irradiation to improve dose rate simulation adequacy

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