Near infra red (NIR) spectroscopic photon emission microscopy for semiconductor devices

NEAR INFRA-RED (NIR) SPECTROSCOPIC PHOTON EMISSION MICROSCOPY FOR SEMICONDUCTOR DEVICES LEN WEE BENG NATIONAL UNIVERSITY OF SINGAPORE 2004 NEAR INFRA-RED (NIR) SPECTROSCOPIC PHOTON EMISSION MICROSCOPY FOR SEMICONDUCTOR DEVICES LEN WEE BENG (B.Eng.Hons., NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004 ABSTRACT This report presents the Near Infra-Red Spectroscopic Photon Emission Microscope (NIR SPEM) developed to investigate emission spectrums of mainly silicon devices The system has modes of operations, high speed and high sensitivity mode, and spectral response from 400nm to 1800nm wavelength It was found that emission spectrums of forward biased pn junctions are dependent on junction doping concentration while spectrums of reverse biased junctions are sensitive to the field conditions Mean emission wavelength, λ50%, of nMOSFETs in saturation is found to correlate closely with channel electric field strength The spectral region around silicon bandgap energy (950nm to 1300nm) is found to exhibit abnormalities during electrical malfunctioning of devices ACKNOWLEDGEMENTS This project would not have been successful without the patient guidance and insightful contributions of the following persons: Professor Jacob C H Phang NUS Enterprise, National University of Singapore Professor Daniel S H Chan Department of Electrical and Computer Engineering, National University of Singapore Mr Liu Yong Yu Centre for Integrated Circuit Failure Analysis and Reliability, National University of Singapore In addition, the assistance of Dr Lap Chan in contribution of test samples from Chartered Semiconductor Manufacturing Ltd is valuable for the completion of this project Their assistance in providing the resources and suggestions in the course of the project is greatly appreciated Contents TABLE OF CONTENTS LIST OF FIGURES iii CHAPTER 1: INTRODUCTION 1.1 Overview 1.2 Motivation 1.3 Justification 1.4 Scope of Project 1.5 Scope of Report 1.6 Summary CHAPTER 2: AN INTRODUCTION TO PEM 2.1 Photon Emission Microscopes 2.2 Literature Survey: Spectroscopic Photon Emission Microscopy 14 2.3 Emission Mechanisms 24 2.4 Summary 28 CHAPTER 3: DEVELOPMENT OF A NIR PEM SYSTEM 29 3.1 The NIR Spectroscopic Emission Microscope (NIR SPEM) 29 3.2 Calibration 33 3.3 Characterization 38 3.4 Comparison to Previous Results 44 3.5 Summary 49 i Contents CHAPTER 4: EMISSION FROM PN JUNCTIONS 50 4.1 Forward Biased PN Junctions 50 4.2 Reverse Biased PN Junctions (Silicon) 57 4.3 Reverse Biased Junctions (III-V) 63 4.4 Summary 68 CHAPTER 5: EMISSION FROM MOSFET DEVICES 69 5.1 NIR Emission Spectrum of nMOSFET in Saturation 69 5.2 Case Study of Some Defective Devices 78 5.3 Summary 85 CHAPTER 6: SUGGESTIONS FOR FUTURE WORK 86 6.1 System Capability Improvements 86 6.2 Further Investigations 91 6.3 Summary 94 CHAPTER 7: CONCLUSION 95 REFERENCES 97 ii List of Figures LIST OF FIGURES Figure number and Caption Page CHAPTER Figure 2.1: Block diagram of a typical PEM [8] Figure 2.2: (a) Reflected light, (b) emission and (c) overlay image of a BJT with emitter-base junction under reverse bias with 100X objective Figure 2.3: Absorption coefficient, a (bold line), of a typical substrate for different wavelengths, taking into consideration photon interaction with phonons (dotted line) and free carriers (solid line) [11] Figure 2.4: Spectral sensitivities of non-compound semiconductor detectors [10] Figure 2.5: Spectral sensitivity of an HgCdTe detector array at 77 K [13] Figure 2.6: Emission image of a portion of a microprocessor taken from backside with 50X objective [18] Figure 2.7: Block diagram of PEM with semiellipsoidal mirror collector Figure 2.8: Block diagram of a spectrograph with photodiode array for simultaneous spectrum acquisition Figure 2.9: Emission spectrum library of some common faults in silicon based devices Figure 2.10: Emission spectrum of (a) forward and (b) reverse biased silicon pn junction [23] Figure 2.11: (a) Emission spectrum and (b) correlation of total emission with substrate current of nMOSFET in saturation [24] Figure 2.12: (a) Uncalibrated and (b) calibrated emission spectrums of 1.0mm channel length nMOSFET biased into snapback [5] 10 11 11 12 13 13 15 16 17 Figure 2.13: (a) Gm(max) and Id(lin) changes and (b) difference spectra after Isub(max) and Ig(max) stresses Difference spectra after (c) Isub(max) and (d) Ig(max) stress [7] Figure 2.14: Emission spectrum of (a) forward and reverse biased pn junction and (b) nMOSFET in saturation for wavelengths >1.0mm [25] Figure 2.15: (a) Emission spectrums and (b) total emission for different channel lengths of n and p channel MOSFETs in saturation [26] Figure 2.16: (a) Emission image of input protection structure Spectral analysis of different emitting sites showing dominant (b) thermal emission and (c) carrier injection into silicon [26] 18 19 20 21 Figure 2.17: l50% versus electric field of DDD and LATID devices at Vg=3V [6] Figure 2.18: Classification of emission spectrum of a gate oxide rupture using a trained neural network [27] Figure 2.19: Band diagram of a pn junction in (a) thermal equilibrium and (b) forward bias Figure 2.20: E-k diagram of recombination in indirect bandgap materials due to a phonon and (b) phonons and impurities [25] Figure 2.21: Band diagram of emission in reverse bias pn junction Figure 2.22: Schematic of emission mechanism in saturated MOSFETs 22 23 24 25 26 27 CHAPTER Figure 3.1: Block diagram of NIR SPEM Figure 3.2: Block diagram of High Speed mode of operation of spectrograph Figure 3.3: Block diagram of the High Sensitivity mode of the spectrograph Figure 3.4: Wavelength calibration for High Speed mode, showing the known peaks at 1014 and 1357nm and other artefact peaks Figure 3.5: The (a) theoretical spectrum, (b) measured spectrum and (c) correction factor for spectral response calibration for High Speed mode Temperature of tungsten lamp is 3000K Figure 3.6: Wavelength calibration spectrum for high sensitivity mode 29 32 33 34 35 36 List of Figures Figure 3.7: The (a) measured spectrum and (b) correction factor for the spectral response calibration of the high sensitivity mode 37 Figure 3.8: Calibrated spectrums of a forward biased pn junction measured using the high speed and high sensitivity modes Figure 3.9: Spectrum of the NEP for the high speed mode of operation Figure 3.10: NEP of High Speed mode at different exposure time 38 39 40 Figure 3.11: SNR of forward biased pn junction at various IF 41 Figure 3.12: SNR of reverse biased pn junction at various IR Figure 3.13: Noise levels in High Sensitivity mode at various amounts of averaging 41 42 Figure 3.14: SNR of forward biased pn junction at various IF in high sensitivity mode 43 Figure 3.15: SNR of reverse biased pn junction at various IR in high sensitivity mode Figure 3.16: (a) Complete and (b) up to 950nm wavelength emission spectrum of forward biased pn junction in high sensitivity mode Figure 3.17: Forward biased pn junction emission spectrum as published in [22] Figure 3.18: Emission spectrum of saturated long channel nMOSFET at biasing conditions on I-V curve (inset) [22] Figure 3.19: (a) Emission spectrum and (b) I-V characteristic of 1mm gate length nMOSFET in saturation 44 45 46 47 48 CHAPTER Figure 4.1: Forward biased emission spectrum of HSDL-4220 IR LED at various bias current Figure 4.2: Emission spectrum of emitter-base junction of npn BJT under various forward bias Figure 4.3: Emission spectrum of (a) collector-base and (b) collector –substrate junction of npn BJT under forward bias Figure 4.4: Emission spectrum of source/drain-substrate junction of nMOSFET under various forward bias 51 52 54 55 Figure 4.5: Average Peak Wavelength for forward bias junctions at emission sites of different doping concentrations Figure 4.6: Emission spectrums of reverse biased pn junction in (a) visible and (b) NIR region Figure 4.7: Reverse bias emission spectrum of Device in the (a) visible and (b) NIR region Figure 4.8: (a) Normalized spectrums showing the emission difference at 1000nm wavelength and (b) setup for monochromatic emission image acquisition Figure 4.9: (a) Reflected light and 1000nm monochromatic emission images of (a) Device and (b) Device Figure 4.10: (a) I-V characteristic and (b) emission spectrum of InGaP (red) LED under reverse bias 57 58 59 60 62 64 Figure 4.11: (a)I-V characteristic and (b) emission spectrum of HSDL-4220 IR LED under various reverse bias conditions 65 Figure 4.12: Band diagrams of double hetero junction LED under (a) zero bias and (b) reverse bias 66 Figure 4.13: l50% distribution of different types of pn junctions in reverse bias 67 CHAPTER Figure 5.1: (a) I-V characteristics and (b) emission spectrum of MOS1 Figure 5.2: (a) I-V characteristics and (b) emission spectrum of MOS2 Figure 5.3: (a) I-V characteristics and (b) emission spectrum of MOS3 Figure 5.4: (a) I-V characteristics and (b) emission spectrum of MOS4 Figure 5.5: (a) I-V characteristics and (b) emission spectrums of MOS5 Figure 5.6: (a) I-V characteristics and (b) emission spectrums of MOS6 70 72 73 74 75 76 Figure 5.7: Relation between l50% and the channel electric field of MOSFET devices in the NIR region 78 List of Figures Figure 5.8: IV characteristics of MOS1A at VG = 3V and VG = 0V 79 Figure 5.9: Schematic of (a) working and (b) potential punch through device with potential distribution at VG = VD = VS = 0V 80 Figure 5.10: Schematic of (a) normal and (b) punch through device at applied bias of VS = VG = 0V and VD > 0V 80 Figure 5.11: Emission spectrum of MOS1A at VG = 3V Figure 5.12: IV characteristics of MOS1B 81 82 Figure 5.13: IV curve of (a) IS-VD and (b) ISUB-VG for MOS1B 83 Figure 5.14: Emission spectrum of MOS1B at VG = 3V and various VD 83 Figure 5.15: l50% values of MOS1, MOS1A and MOS1B as a function of VD 85 CHAPTER Figure 6.1:Comparison between emission spectrums with and without clipping Figure 6.2: (a) Steady state signal taking into consideration NIR PMT induced noise and (b) clipping-off of signal by SEMICAPS card after amplification Figure 6.3: Comparing “Integrated signal” from using integrating amplifier to “Sampled signal” using current amplifier Figure 6.4: Block diagram of a Photon counting ssetup Figure 6.4: Spectral response of (a) silicon based CCD and (b) InGaAs array 87 88 89 90 91 Chapter 1: Introduction CHAPTER 1: INTRODUCTION This project aims to investigate near infra-red (NIR) spectroscopy using photon emission microscope (PEM), to determine emission properties of semiconductor devices in operation This chapter presents the motivation and objectives of this project The scope of the project and this report are also presented 1.1 Overview Photon emission microscopy is widely used in the failure analysis and reliability laboratories of semiconductor related facilities primarily as a defect localization tool A major advantage of this technique is that it is a probe-less method, which does not damage healthy devices nor further degrade existing defects Sample preparation only requires decapsulation of packaged devices, allowing high throughput Furthermore, the use of high numerical aperture (NA), low magnification objective lenses allows “pan” view of complex devices to pinpoint the location of likely defects This feature is extremely useful as it dramatically reduces the time of analysts to search through very complicated chips to identify the defect areas for further testing Despite the advantages, photon emission microscopy is not without disadvantages, the major concern being the lack of spatial resolution of this technique Essentially, this is an optical technique and resolution is limited to approximately half the wavelength of the detection wavelength Since a large proportion of the emission for silicon devices has rather long wavelength (> 0.6µm), the spatial resolution would be much poorer than the critical dimensions of today’s devices (~0.18µm) Another disadvantage of Chapter 6: Suggestions for Future Work confusion when acquiring spectrums with weak emission Currently, high gain settings are avoided in spectrum acquisition, which limits the sensitivity of the system This problem can be overcome by replacing the current amplifier to a charge integrating amplifier The integrating amplifier integrates the signal over a fixed duration before output to the data acquisition setup The integration duration should be significantly larger than the mean separation between noise pulses Figure 6.3 shows the “Integrated signal” as compared to the “Sampled signal” with clip-off The data acquisition setup would then average the integrated signal to measure 8.047 units, which is a better reflection of the NIR PMT measurement Figure 6.3: Comparing “Integrated signal” from using integrating amplifier to “Sampled signal” using current amplifier 6.1.2 Photon Counting Another method to overcome the clip-off problem is to use photon counters, which also increases the sensitivity of the system by approximately orders of magnitude [33] Figure 6.4 shows the block diagram of this setup with a high bandwidth, fixed gain amplifier and a photon counter unit, controlled using a personal computer The 89 Chapter 6: Suggestions for Future Work photon counter would count the number of pulses caused by photons striking the NIR PMT The photon counter unit can be programmed to reject very high and very low amplitude pulses caused by shot noise and background noise respectively Control Personal Computer High bandwidth, fixed gain amplifier NIR PMT Photon counter Data Figure 6.4: Block diagram of a Photon counting ssetup The use of photon counters requires discrete pulses as input to the photon counter, therefore only very weak emissions may be measured using this method Although the use of this setup increases the sensitivity, it cannot be used for strong emissions and the current setup using either integrating or current amplifiers and a data acquisition card is still required 6.1.3 NIR capable CCD camera The current system uses an enhanced silicon based CCD, with spectral response shown in figure 6.5(a), to acquire emission images The CCD has a cut off wavelength of approximately 1.1µm while the spectral response of the InGaAs linear array extends all the way to 1.8µm, as shown in figure 6.5(b) Therefore, the effective overlapping spectral region is only from 850nm to 1050nm and correlating of emission image to spectrum is extremely difficult 90 Chapter 6: Suggestions for Future Work (a) (b) Figure 6.4: Spectral response of (a) silicon based CCD and (b) InGaAs array The installation of a NIR capable imaging array, with spectral response range as least equal to the spectrograph would improve the applicability of the system tremendously Typically, NIR cameras of this spectral range are made of Mercury-CadmiumTelluride (HgCdTe), lead (Pb) based compounds and ternary or quaternary Indium (In), Gallium (Ga), Arsenic (As) and Phosphorous (P) compounds 6.2 Further Investigation This section discusses some results observed in this project, which has not been thoroughly investigated due to constraint in resources One aspect of this is to extend 91 Chapter 6: Suggestions for Future Work the study the effects of oxide trapped charges and interface states in the NIR spectral region Another is to study the effects on the NIR emission spectrum after accelerated stress tests 6.2.1 Effects of Interface States and Oxide Trapped Charges in nMOSFETs In an earlier publication [7], the effects of oxide trapped charges and interface states on the emission spectrum on nMOSFETs were investigated This study was limited to the visible range (up to 850nm wavelength) due to limitations in the detector system’s spectral response With the NIR SPEM, it is now possible to observe the respective effects in the NIR region It was observed in chapters and that most degradation in device performances are translated to abnormalities in the emission spectrum at approximately 1000nm wavelength region This wavelength is very close to the silicon bandgap emission at 1107 to 1150nm, based on doping concentration, and is very likely caused by energy levels introduced by defects in the silicon [34] These energy levels effectively reduce the bandgap energy, resulting in higher emission at wavelengths close to the silicon bandgap Therefore, it will be valuable to investigate the effects on the emission spectrum in the wavelength range from 900 to 1300nm, to reinforce the understanding of the Isub(max) and Ig(max) stress tests However, the current system operation is still not sensitive enough to observe these effects accurately This situation may be improved with the use of photon counting described in section 6.1.2, to improve the system sensitivity 92 Chapter 6: Suggestions for Future Work 6.2.2 Effects of Current Leakage on the Emission Spectrum It was concluded in section 5.1 that a relation exists between the mean emission wavelength, λ50%, in the NIR region and the electric field strength in an nMOSFET It is also observed from sections 4.2 and 5.2 that the current leakage phenomenon in junctions and MOSFETs causes relative increase in the emission at wavelengths from 950 to 1150nm It is generally understood that defects within the silicon structures causes trap formation within the bandgap close to the conduction band, explaining the increase in emission intensity at shorter wavelengths for defective devices However, a quantitative relationship between the emission spectrum and the defects is not firmly established Therefore, with data obtained from planned experiments on the NIR SPEM, a complete range of relevant spectroscopic information could be made available for this study The availability of a non-destructive defect assessment tool, which is capable of observing leakages in semiconductor devices would be extremely useful With increasing chip complexities and packaging technologies, the scope of failure analysts is currently limited by the means available, as mentioned in section 1.1 NIR emission microscopy (imaging and spectroscopy) would be extremely useful both as a localization tool and defect assessment tool, providing a non-destructive first step approach to further analysis Furthermore, with quantitative methods now possible due 93 Chapter 6: Suggestions for Future Work to the ability to observe the full spectrum, it is also possible to extend the use of emission microscopy to a reliability assessment tool 6.2.3 Spectral Analysis Techniques Although the λ50% statistical parameter is shown to provide information on the conditions of the tested devices, it is incapable of providing information regarding more subtle differences in the spectrums, i.e minor peak and changes in spectrum shape It is seen in this project and previous publications that degradation or defects in the devices are generally represented by slight changes in spectral distribution or/and emergence of minor peak(s) Development of a method that is capable of decomposing the individual peaks, using either de-convolution or neural networks, is required before extracting the λ50% parameter from each individual peaks may be more meaningful in the quantitative studies of emission spectrums 6.3 Summary Possible improvements to the system performance are presented in this chapter, including the use of integrating amplifiers, photon counting and NIR cameras to supplement the current system Further investigations to study the effects of oxide trap and interface state effects and current leakages are also presented The relevance of suggested proposals is also discussed 94 Chapter 7: Conclusion CHAPTER 7: CONCLUSION A spectroscopic photon emission microscope (SPEM), sensitive in the NIR region of up to 1800nm wavelength, is assembled in this project The system integrates NIR detectors, an InGaAs linear array and NIR PMT, allowing it to function in high speed or high sensitivity mode Experiments were performed to compare the emission spectrums of pn junctions of various materials and study the fundamental (forward and reverse bias) emission spectrums of these junctions While the forward biased spectrums are found to be predominantly dependent on doping concentrations, reverse biased emission spectrums are found to be very sensitive to field conditions Furthermore, most observations of abnormal behaviors in the spectrums occur in the NIR region and are almost unobserved in the visible region Investigations on nMOSFETs show statistical parameter, λ50%, of the emission spectrum to be closely correlated to the channel electric field Also, most of the emission contents for both healthy and defective devices are contained within the NIR region Some studies of defective devices reveal an increase in the emission intensity at 950 to 1300nm wavelength region, resulting in prominent peaks unobserved in healthy devices During the course of the project, certain areas of the instrument requiring improvements were identified Suggestions to better the performance for extended 95 Chapter 7: Conclusion applications relevant to semiconductor failure analysis were presented and discussed in chapter for implementation in future efforts Lastly, the successful implementation of this instrument allows for observation of emission spectrums previously unseen The availability of such an instrument would no doubt be useful to fellow researchers who have intentions for further development of techniques in NIR spectroscopy 96 References REFERENCES [1] Nishi Y., McPherson J.W, “Impact of New Materials, Changes in Physics and Continued ULSI Scaling on Failure Mechanisms and Analysis”, Proceedings 7th International Symposium on the Physical & Failure Analysis of Integrated Circuits, pp 1-8, July 1999, Singapore [2] Ng T.H, Chim W.K, Chan D.S.H, Phang J.C.H, Liu Y.Y, Lou C.L, Leang S.E, Tao J.M, “An Integrated 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International Symposium on the Physical & Failure Analysis of Integrated Circuits, pp 60-65, July 1995, Singapore [33] O’Connor D.V, Phillips D, “Time Correlated Single Photon Counting”, Academic Press, New York, USA, 1984 [34] Ng K, “Complete Guide to Semiconductor Devices”, McGraw Hill Inc., New York, USA, 1995 102 References 103 [...]... spectrums for comparison, or new information in the NIR wavelength region may show significant enough disparities to discern the spectrums easily It is likely that the combination of the 2 approaches mentioned is required to make spectroscopic photon emission microscopy a much more reliable failure analysis tool than it currently is [21,22] 2.2 Literature Survey: Spectroscopic Photon Emission Microscopy Spectroscopic. .. development of a spectroscopic emission microscope with NIR capabilities The experience and capabilities of CICFAR in the field of emission microscopy development as a motivation and the justification for this project is presented The scope of the project and this report is also presented 6 Chapter 2: An Introduction to PEM CHAPTER 2: AN INTRODUCTION TO PHOTON EMISSION MICROSCOPY (PEM) Photon emission microscopy. .. wavelength [8,9] In conclusion, NIR spectroscopic emission microscopy provides information that is useful in assessment of silicon devices This project aims to extend the measurement capability of spectroscopic emission microscopy at CICFAR into the NIR spectral region 1.4 Scope of Project The objective of this project is to develop an NIR capable spectrograph for the emission microscope at CICFAR Diffraction... differences in the emission spectrums due to interface states and oxide trapped charges [6, 7] These works illustrate the wealth of reference and experience in the field of spectroscopic emission microscopy present in CICFAR, which is valuable for this project 3 Chapter 1: Introduction 1.3 Justification Current capabilities in CICFAR for emission microscopy comprises of panchromatic imaging and spectroscopic. .. field emissions in small dimension devices The observation of spectrums in the NIR spectral region is important for silicon devices as the bandgap energy of silicon, at 1.12eV (equivalent emission wavelength of 1107nm), is not detectable by available systems However, it is believed that majority of the emission and information content is centered about this wavelength [8,9] In conclusion, NIR spectroscopic. .. Figure 2.15(b) shows the total emission intensity for n and p channel MOSFETs in saturation for different channel lengths between 1.1µm to 1.4µm wavelength range The exponential increase in emission intensity at shorter channel length indicates that IR emissions are more prominent in newer technology devices [26] (a) (b) Figure 2.15: (a) Emission spectrums and (b) total emission for different channel lengths... studies have found the emission spectrums to be closely correlated to the device operation modes and defect mechanisms can be identified simply by reference to a spectrum library Quantitative studies in this area have found that emission characteristics may be correlated to the device parameters like the electric fields [2,3] The usage of near infra red detection in photon emission microscopy has gained... observation of emissions from silicon pn junctions in forward and reverse bias and n-channel MOSFETs in saturation These fundamental 14 Chapter 2: An Introduction to PEM spectrums were investigated to understand the mechanism for light emission of devices under the respective operation modes Figure 2.10(a) shows the spectrum for a forward biased pn junction at different biasing currents The emission mechanism... and spectroscopic mode of a PEM and their application in failure analysis A literature survey examining some previous work on the development spectroscopy in PEM and quantitative methods is also presented Lastly, the theory of emissions in devices is briefly discussed 2.1 Photon Emission Microscopes The photon emission microscope can be operated in 2 modes; panchromatic or imaging mode and the spectroscopic. .. literature review of previous works on emission microscopy in the NIR spectral region 5 Chapter 1: Introduction Chapter 3 presents the equipment setup and describes the components used in the instrument Chapter 4 presents the results from investigating pn junctions of silicon and compound semiconductor devices under forward and reverse bias The results for backside detection of emission from silicon pn junctions .. .NEAR INFRA-RED (NIR) SPECTROSCOPIC PHOTON EMISSION MICROSCOPY FOR SEMICONDUCTOR DEVICES LEN WEE BENG (B.Eng.Hons., NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF... to make spectroscopic photon emission microscopy a much more reliable failure analysis tool than it currently is [21,22] 2.2 Literature Survey: Spectroscopic Photon Emission Microscopy Spectroscopic. .. Introduction to PEM CHAPTER 2: AN INTRODUCTION TO PHOTON EMISSION MICROSCOPY (PEM) Photon emission microscopy (PEM) is widely used in semiconductor failure analysis for fault localization and defect type