We develop a cost-effective, high-resolution, and noninvasive imaging technique for thermal mapping of semiconductor edifices in integrated circuits. Initial implementation was done using a power-stabilized optical feedback laser system that detects changes in the optical beam-induced current when the package temperature of the device is increased. The linear change in detected current can be translated to a thermal gradient, which can reveal semiconductor “hotspots”—localized sites with anomalous thermal activity. These locales are possible fault sites or areas susceptible to defects, which are the best jump-off points for failure analysis

Caesar Saloma

Carlo Mar Blanca

Vera Marie Sastine

Vernon Julius Cemine

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High-Resolution Thermography57High-Resolution Differential Thermographyof Semiconductor EdificesVera Marie Sastine*, Vernon Julius Cemine, Carlo Mar Blanca, and Caesar SalomaNational Institute of Physics, University of the Philippines,Diliman, Quezon City 1101E-mail: vsastine@nip.upd.edu.phABSTRACTScience Diliman (July–December 2004) 16:2, 57–60*Corresponding authorWe develop a cost-effective, high-resolution, and noninvasive imaging technique for thermal mapping ofsemiconductor edifices in integrated circuits. Initial implementation was done using a power-stabilizedoptical feedback laser system that detects changes in the optical beam-induced current when the packagetemperature of the device is increased. The linear change in detected current can be translated to athermal gradient, which can reveal semiconductor “hotspots”—localized sites with anomalous thermalactivity. These locales are possible fault sites or areas susceptible to defects, which are the best jump-offpoints for failure analysis.INTRODUCTIONTo perform successful failure analysis on asemiconductor device, precise knowledge of the exactfail site location is important. One of the most rapidways to locate a fault zone is to uncover “hotspots”—edifices with temperatures above the accepted operatingrange of the device. Various hotspot isolationtechniques have been in existence since the earliest daysof failure analysis and have been satisfactory to varyingdegrees for the location of suspected damage sites ondifferent integrated circuits.Thermal imaging as a failure analysis tool has beenimplemented using infrared cameras (Rogalski, 2003)or array detectors (Christofferson et al., 2001).However, the high thermal discrimination and simpleoperation of these detectors are offset by the highpurchase cost. Other detection techniques exist suchas liquid-crystal microscopy (Lee & Pabbisetty, 1993)and scanning microscopy (Shi et al., 2003), but theyare too invasive and less sensitive.Aside from being expensive, all these methods solelydetect remnants of thermal conduction bleeding throughthe surface of the integrated circuit. Because of theirpoor resolution in the axial direction, they cannot mapout thermal propagation of an embedded defect. Thechallenge, therefore, is to develop a cost-effective,noninvasive thermographic technique that would enableus to localize and map out areas of high thermalgradient.The method we propose will utilize a custom-built opticalfeedback microscope operating in the near-IR range(Cemine et al., 2004). The thermal response of theindividual IC sections is then monitored and analyzedby measuring the optical beam-induced current (OBIC)as the package temperature is increased. Becauselonger wavelengths λ suffer less scattering (~1/λ4), themethod can be used to noninvasively tunnel throughthick layers of semiconductor die in contrast to thesurface-limited probes of the scanning electronmicroscope (SEM) (Shi et al., 2004). High-resolutionSastine et al.58sample dissection can then be done as a verificationprocedure. Since there is no downtime for samplepreparation, the analysis is simple, fast, and does notrequire a vacuum environment so essential to SEMimaging. This ease of maintenance is further supportedby the system’s simplification of the fault identificationprocess: both defect localization and identification canbe done in the same setup, cutting cost and experimentalhours.METHODOLOGYA schematic diagram of the microscope setup is shownin Fig. 1. A semiconductor laser (SL) (SharpLT024MD, λ = 793 nm at laser case temperature =25°C) was used as a light source. It has a built-inphotodetector (PD) that is used to detect the confocalreflectance signal. The SL is controlled by a laser diodecontroller that is connected to the computer. Thediverging elliptical illumination beam was collimatedby a collimating lens (C) (Melles Griot 06GLC003,numerical aperture = 0.276, focal length = 14.5 mm).Mounted anamorphic prisms (AP) (Melles Griot06GPA004, magnification = 3x) were used tocircularize the beam. An iris diaphragm (D) wasinserted to lessen the effect of astigmatism.An objective lens (O) (Olympus UPlan Fl, nominal NA= 0.5, working distance = 1.7 mm) was used to focusthe beam on the sample. The current produced uponsample irradiation is the one-photon optical beam-induced current (1P-OBIC) signal. The light reflectedfrom the sample forms the confocal reflectance signal.The sample is a photodiode array (MN8090 Matsushita5017) biased at 3.56 V and is mounted on a triaxisservomotor stage (S) (Thorlabs PT3-Z6) that iscontrolled by a motion-controller card inside thecomputer. The sample is heated using a thermoelectriccooler (TEC) controlled by a current controller (CC),while the temperature is monitored using a multimeter.Confocal reflectance and OBIC signals are detectedusing a 12-bit data-acquisition board (NationalInstruments 6024E, conversion rate = 200 kHz) insidethe computer. Stage-scanning and image-acquisitionprotocols are coded using LabVIEW 7.0 (NationalInstruments).Confocal and OBIC images were acquired for sixsample temperatures: 25.1 (room temperature), 29,38.2, 48.9, 61.6, and 72.1°C. The effect of the sampletemperature on the OBIC signal generation isdetermined. OBIC thermal-gradient mapping of thesample image is undertaken to reveal die defects andsites that are susceptible to failure.RESULTS AND DISCUSSIONFigure 2 shows the 190 mm x 190 mm 1P-OBIC andconfocal reflectance images taken at the focus (Z = 0)for three sample temperatures: (a) 25.1, (b) 48.9, and(c) 72.1°C. For each measurement the object’s axiallocation was monitored and adjusted to compensatefor any package expansion that may have occurred.Identical confocal images mean that we are alwaysinvestigating the same focal plane.Notice the significant decrease in intensity on the OBICimage as temperature increases. This signifies that asthe sample temperature increases, the current generatedby the beam (the 1P-OBIC signal) at each point on theFig. 1. Optical setup. Confocal and OBIC tandem microscopeusing an optical feedback laser system (SL). Collimating optics(C, AP) expand the beam which is focused by an objective (O)on the sample. The IC specimen is mounted on a thermoelectriccooler (TEC) to increase the package temperature by adjustingthe current controller (CC).IC samplePDSLC APDCCOGNDSzaxisTECHigh-Resolution Thermography59observation area decreases. We operated the diodelaser in power-stabilized mode ensuring a constantillumination power. The decrease in OBIC signal is thencaused by the increase in resistance of the sample asthe temperature of the sample increases. Quantificationof this observed phenomenon will be presented later.We determine the semiconductor and metal parts ofour observation area by multiplying the OBIC andconfocal images, and OBIC complement and confocalimages, respectively. This method is already anestablished procedure for semiconductor and metaldiscrimination (Daria et al., 2002; Miranda & Saloma,2003). As inputs, we use the OBIC and confocal imagesat room temperature (T =25.1°C). Figure 3 shows theconfocal and OBIC images at room temperature andtheir corresponding semiconductor and metal sites. Theportions that are low in intensity on both thesemiconductor and metal images are either the dielectricor the substrate.Five different portions (each of area 20 mm x 20 mm)on the OBIC images are selected to quantitativelydetermine the OBIC signal decrease as the temperatureincreases [Figs. 4(a) and 4(b)]. Figure 4(c) shows theplots of the OBIC average intensities versustemperature for the five portions. Their respectiveslopes, which represent the OBIC signal rate of changewith temperature, are also shown.Fig. 2. The 190 mm x 190 mm confocal and 1P-OBIC images at(a) 25.1, (b) 48.9, and (c) 72.1ºC. OBIC intensity decreases assample temperature increases.OBIC Confocal(a)(b)(c)Confocal OBICSemicon Metal, etc.Fig. 3. Confocal and 1P-OBIC images for room temperatureand their corresponding semiconductor and metal sites.Temperature (oC)Ave. OBIC intensity(a.u.)(c)Semicon OBIC(a) (b)Fig. 4. [(a) and (b)] Semiconductor and OBIC images. (c)Average OBIC intensity of the five monitored sites decreasesversus the temperature of the sample. The numbers are theslopes of the curves.  Image size is 190 x 190 mm.Sastine et al.60For the five portions, OBIC linearly decreases withtemperature. However, the rate of decrease of theOBIC signal at these five regions are different, asexhibited by their differing slope values. This impliesthat some portions of the integrated circuit (IC) heatup faster than other areas. Those regions that heat upfaster reflect a faster OBIC signal decrease or asteeper slope, while those that heat up slower exhibit agradual decrease in OBIC signal. This inhomogeneityin heating rate can reveal the locations of those sitesthat are either defective or highly susceptible to failureswhen the IC is operated at extreme conditions.We can map those defective and failure-susceptiblesites by getting the thermal gradient map using theOBIC images for the six temperature levels. Figure 5shows the thermal-gradient map of the observationarea. Notice the thermal-gradient inhomogeneity onthe observation area as exhibited by the different colorson the thermal-gradient map. Areas that heat up fastare reflected as red on the map (pointed by arrows),while areas that heat up slow are blue. From thisobservation, imaging and localization of those sitesthat are either defective or susceptible to defect, isrealized.CONCLUSIONWe have demonstrated the performance of our newlydeveloped noninvasive imaging technique for thermalmapping of semiconductor edifices in integratedcircuits. The technique detects thermal-gradientinhomogeneity on the IC observation area. Those sitesthat heat up fast are well discriminated from those sitesthat heat up at a slower rate.With this capability, our technique cannot only act as afailure analysis tool but can be used for precursor orpredictive analysis as well. Possible defect sites canbe mapped out even before failure happens. In sucheventuality, the onset and evolution of a failure can berecorded and investigated. Semiconductor architecturecan also possibly benefit from the technique by derivingthermal accumulation points in a particularsemiconductor design. The technique can provide suchbasic tests to expose weaknesses in prototypeintegrated circuits under thermal stress.REFERENCESCemine, V.J., B. Buenaobra, C.M. Blanca, & C. Saloma, (inpress). High-contrast microscopy of semiconductor andmetal sites in integrated circuits by optical feedbackdetection. Opt. Lett. 29:  2479-2481.Christofferson, J., et al., 2001. High-resolution noncontactthermal characterization of semiconductor devices. Proc.SPIE. 4275: 119–125.Daria, V.R., J. Miranda, & C. Saloma, 2002. High-contrastimages of semiconductor sites via one-photon optical beam-induced current imaging and confocal reflectancemicroscopy. Appl. Opt. 41: 4157–4161.Lee, T.W. & S.V. Pabbisetty (eds.), 1993. Microelectronicsfailure analysis. ASM International, Ohio: Chap. 5.Miranda, J.J. & C. Saloma, 2003. Four-dimensionalmicroscopy of defects in integrated circuits. Appl. Opt. 42:6520–6524.Rogalski, A., 2003. Infrared detectors: Status and trends.Prog. Quantum Electron. 27: 59–210.Shi, L., et al., 2003. Nanoscale thermal and thermoelectricmapping of semiconductor devices and interconnects. AIPConf. Proc. 683: 462–468.Figure 5.  Semiconductor image and thermal gradient map of thesample. Red portions (pointed by arrows) indicate highest rate ofchange in temperature while blue portions indicate lowest rate ofchange in temperature.

High-Resolution Differential Thermography of Semiconductor Edifices

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High-Resolution Differential Thermography of Semiconductor Edifices

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