All-silicon optical contactless testing of integrated circuits - Lamar ...

All-silicon optical contactless testing of integrated circuits - Lamar ...

INT. J. ELECTRONICS, 2002, VOL. 89, NO. 7, 537±547All-silicon optical contactless testing of integrated circuitsS. SAYIL{, D. KERNS*{ and S. KERNS{A novel approach for replacing mechanical probes used for wafer level testing withan optical contactless method for the application and extraction of test vectorsfrom advanced silicon integrated circuits (ICs) is described. Experimental resultsdemonstrate its feasibility, using a novel silicon light-emitting diode (LED) andsilicon light sensors fabricated on the IC. The proposed method uses visible lightand device structures that are completely compatible with standard silicon ICprocessing. The optical test-head is inexpensive, fabricated with standardmicroscope optics, and allows the simultaneous use of mechanical probes forpower and other signals.1. IntroductionWith increasing chip densities, conventional mechanical probing techniques forinternal fault detection and functional testing face increasing challenges. Mechanicalprobe approaches are limited by their inherent large size and the concomitantparasitic loading e ects. These factors encourage development of non-mechanicaltechniques for debugging and design veri®cation, as well as for functional testing.The SIA National Technology Roadmap for Semiconductors predicts that applicationspeci®c integrated circuits (ASICs) will need over 3000 I/O pads in the ®rst yearsof the new century, with a peripheral pitch distance of 50 mm (NTRS 1997). Suchlarge I/O counts challenge testing reliability in numerous ways; assuring reliableohmic contact to all test pads during repeated die tests becomes a signi®cant concern.`Design for Testability’ approaches can enhance test-vector coverage throughcircuit-level design strategies. Such techniques have extended the usefulness ofmechanical test approaches, but cannot ameliorate the fundamental limitations associatedwith the simultaneous mechanical probing of large numbers of small pads.New approaches will be required.Contactless testing resolves many of the challenges associated with conventionalmechanical wafer testing. A number of contactless techniques has been investigated,but none has yet found acceptance as a routine testing tool. Electron-beam testinghas been used in a variety of ways for many years, and techniques such as photoemissiveprobing electro-optic sampling, charge density probing and photo-excitationtechniques have also been investigated since the 1980s. These contactless techniquescan provide access to determine the logic state of internal nodes of a deviceunder test. These contactless technologies also have many major disadvantages,including such factors as high equipment costs, complex test set-ups, complexmeasurement chamber requirements (often high-vacuum), crosstalk, incompatibilityReceived 5 July 2001. Accepted 19 September 2002.{ Pamukkale University, Turkey.{ Franklin W. Olin College of Engineering, 1735 Great Plain Avenue, Needham,MA 02492, USA, and Vanderbilt University, Nashville, Tennessee, USA.* Corresponding author. E-mail: david.kerns@olin.eduInternational Journal of Electronics ISSN 0020±7217 print/ISSN 1362±3060 online # 2002 Taylor & Francis Ltd 10.1080/0020721021000044313

538 S. Sayil et al.with conventional wafer probing systems, and the risk of damage to the device undertest (Weingarten 1988, Heinrich 1990, Marcus 1990, Solkner and Wolfgang 1994,Bohm 1996, Sayil 2000).The work reported here seeks to develop a new test methodology, completelycompatible with silicon IC technology, that can be implemented simultaneously withconventional mechanical probes using standard equipment. The goal of this combinedapproach is to complement the use of mechanical probes and thereby providethe potential for increased test coverage and reliability. The approach should alsoavoid many of the limitations of other contactless techniques.A fully optical contactless testing technique is presented, utilizing the integrationon the chip or device under test (DUT) of a silicon light emitter or LED (for sendingdata out optically) and a silicon photodiode (for receiving data). In addition theDUT would contain a driver circuit for the LED, and an ampli®er andcomparator circuit to amplify the signal from the photodiode and reconstruct alogic-compatible digital signal. The chip area required for these circuits has beencalculated to approximate that required for a bond-pad and associated drive or inputbu er circuitry. The selected `output’ electrical signals are converted to opticalsignals by on-chip silicon-based LEDs or electroluminescent photon sources(Kerns et al. 1989).Silicon is generally a poor material for light emission, though it is reasonablygood as a photodetector. The use of an optimized silicon light-emitting diode structureas an electroluminescent source allows the entire approach to be fully compatiblewith current silicon technology. The devices designed and tested have measurede ciencies on the order of 2:22 £ 10 ¡5 photons per electron and further improvementis anticipated (Akil et al. 1998, 1999, Dong 1999). This research supportsthe concept that low-level silicon light emission is su cient to support contactlesstesting.The equipment required to implement this approach at the wafer probe consistsof an optical lens system (much like a microscope) and an optical test head.Mechanical probes are used to apply power, ground, and selected test vectors.The optical test head simultaneously monitors the logic state of additional testnodes, both at the die periphery and internal to the chip, as depicted in ®gure 1.Figure 1.Schematic of optical test set-up.

Optical contactless testing of ICs 539In the testing scheme developed for this feasibility study, an on-chip siliconphotodiode is used to receive optical test signals generated at the optical test head(®gure 1). The modulated light source in the optical test head is a standard GaAsLED (or laser diode), precisely located so that the optics direct this light to thespeci®c location of the receiver diode on the DUT. An on-chip light emitter sendstest signals through the same optical system to a precisely located photodetector andampli®er in the optical test head. In this feasibility demonstration a hybrid approachwas used, with the silicon LED driver and receiver ampli®er located `o -chip’.The proposed technology can substitute for many of the peripheral wafer probes,greatly reducing the mechanical complexity and enhancing the reliability of probecards. Further, the technology provides simultaneous access to internal nodes.2. Experimental approachTo establish and evaluate the capabilities of an all-silicon optical test approach,hybrid breadboards were designed. Figure 2 shows the optical system for the experiments:a Polaroid camera and a B&L microscope optics system. Specially fabricatedsilicon light emitter chips were used as the DUTs. These chips include many siliconlight emitting and detecting structures, including p±n and Schottky photodiodes asshown in ®gure 3.Breadboardof OpticalTest Head inImage PlaneFigure 2.The experimental optical system, with an enlargement of the device under test(DUT).

540 S. Sayil et al.Figure 3.The specially designed experimental DUT.3. Experiments3.1. Transmission of input stimulus data from optical test head to chip (DUT) forencodingThe simultaneous transmission of multiple input optical signals to the DUT forinput data encoding is required for the success of the proposed optical testingapproach. Two spatially separated optical input signals were simultaneously appliedto the DUT to establish this capability. Figure 4 illustrates the experimental set-up,with two GaAs LEDs serving as pulsed light sources. A circuit board located on theimage plane of the Polaroid attachment encloses the LEDs (see ®gure 2). The LEDsare positioned at locations on the image plane corresponding to silicon photodiodeson the DUT. In general a source array will be designed speci®cally for each DUT.The location of sources corresponds to the imaged location of internal nodes to betested, and is, therefore, particular to the optical test head con®guration used fortest. The whole image of the chip under test appears on the image plane when theproper lens combination has been used, and therefore proper location of the LEDs isstraightforward. In this experiment, an 8 £ objective has been utilized.For the experiment, adjacent photodiodes were chosen to represent internalcircuit test nodes. This choice maximizes the probability of crosstalk between thetwo photodiodes, which are separated by 520 mm. The square-wave modulationsignals for these LEDs were set 1808 out of phase.The photodiode outputs are connected to transimpedance ampli®ers housed in ashielded breadboard arrangement. The transimpedance amplifer converts the photodiodecurrent to an output voltage. The minimum transimpedance required to obtaina detectable signal depends on the test head’s optical output power and the attenuationof the particular optical setup. However, it was calculated for nominal values

Optical contactless testing of ICs 541Figure 4.Schematic of transmission of stimulus data from optical test head to chip.that a transimpedance of 20 k« would provide an adequate signal for the comparator.The integration of such a transimpedance ampli®er on a silicon chip has beendemonstrated to be feasible in CMOS (Woodward 1996). The signal is then processedby a comparator circuit with its threshold set near the middle of the voltageswings from the ampli®er, to restore proper logic levels. The schematic drawing ofthe transimpedance amplifer used in the the hybrid breadboard is shown in ®gure 5.The hybrid comparator circuit utilizes a CA3290 in a standard comparator con®guration.Figure 6 shows the output waveforms for the two optical encoding signals.Figure 6(a) shows the left, and 6(b) shows the right side of the light transmissionpath of ®gure 4. These results clearly indicate that it is possible to transmit multipleinput optical signals simultaneously to chip with negligible crosstalk.3.2. Transmission of chip outputs from DUT to optical test head for data extractionThe simultaneous transmission of output signals from the chip to the optical testhead for detection is also required for the success of the proposed method. Thistransmission is a greater challenge than the input signal transmission, because thetransmission source is the relatively weak on-chip silicon emitter. The SiliconPhotonics Groups at Vanderbilt University and Olin College developed lightgeneratingtest structures in silicon, using only standard silicon dopants, boron andphosphorus. These test structures were fabricated by standard silicon processing

542 S. Sayil et al.Figure 5.Schematic drawing of the hybrid transimpedance photodiode ampli®er.(a)(b)Figure 6.Comparison of each output signal with its original input signal. Upper waveformsshow the original signals, and lower ones show the outputs.techniques and resulted in silicon p±n junctions that emit visible light when operatedin avalanche breakdown (Akil et al. 1999).Figure 7 shows the `postage-stamp’ test structure and also a close view of lightemission on the right corner of this diode. Typical emission spectra can be seen in®gure 8. The spectral peak is at ¹ 2 eV. Despite its relatively low electro-opticalconversion e ciency (quantum e ciency), su cient light is generated for applicationsinvolving contactless testing.The curve in ®gure 8 was measured at a current of 20 mA and a breakdownvoltage of 8 V. Measurable light emission has been observed at lower breakdownvoltages, and at currents as low as 1 mA. There is a linear relationship betweenbreakdown current and the peak in spectral intensity over the range of 5 mA to20 mA.

Optical contactless testing of ICs 543Figure 7.Silicon light-generating test structure.Figure 8.Emission spectra from silicon photodiodes in avalanche.The experimental set-up for output extraction is shown in ®gure 9. A pulsegenerator drives an ampli®er that drives the silicon light-emitting structure.Emitted light is focused through the microscope and detected by a detector placedat the proper location in the image plane of the microscope. Either a photomultipliertube or an avalanche photodiode with ampli®er circuitry can be used as the detector;this experiment utilized the Hamamatsu avalanche photodiode module C5460-0,which has a gain of ¡1:5 £ 10 8 V/A. The resulting waveform at the detector outputcan then be compared with the original waveform driving the silicon light-emitter onthe DUT.Two output optical signals from the DUT were sent simultaneously to the opticaltest head to characterize the extraction of multiple output signals. Two adjacentsilicon light emitting structures on the DUT (®gure 3) were modulated at 10 kHzfor transmission. This frequency was the upper frequency limit of the availabledetection instrumentation; however, the time response of hot-carrier luminescencein silicon LEDs has been shown to be in the range of several picoseconds (Kash andTsang 1996, Tsang and Kash 1997). This would suggest testing frequencies above a

544 S. Sayil et al.Figure 9.Simultaneous transmission of output signals for extraction.gigahertz are possible. The silicon light emitters were modulated at 1808 phasedi erence. Figure 10 compares the resulting output waveforms to the original waveforms.In this ®gure, the output signal image on the oscilloscope screen has beeninverted to account for the negative ampli®er gain. The excellent correspondencebetween these signals demonstrates that multiple output signals can be simultaneouslyextracted with negligible interference.3.3. The simultaneous transmission of data in both directionsThis experiment was performed in order to examine the possibility of sending andreceiving data from the chip simultaneously. Figure 11 shows such an arrangementin which data is transmitted to a DUT; these data are received optically, converted tologic signals, processed by the chip, and the output data transmitted back to the testhead optically.In this experiment, both an avalanche photodiode and a GaAs LED are placed atspeci®c separate locations on the image plane of the microscope, corresponding tothe light-emitter and photodetector on the DUT, respectively.The test vectors are encoded and extracted using the processes described above. Itwas found that when light pulses were simultaneously sent from the optical test headand from the DUT, re¯ections in the microscope from the strong GaAs LED signalcreated crosstalk and blocked out the weak Si LED signal.

Optical contactless testing of ICs 545a- b-Figure 10.Detected signals versus input signals applied to Si LED. Upper waveforms showthe detector outputs and lower waveforms show the original signals.Figure 11.Experimental set-up for simultaneous encoding and extraction of test data.This problem can be easily avoided by special design of the test procedure, and isnot considered to be a major limitation of the contactless testing method. The timingof the test process can be arranged such that the chip output is observed at a di erenttime from the test input data. Also, higher quality non-re¯ective optics may help toalleviate this problem.

546 S. Sayil et al.4. Experimental results and discussionExperimental results demonstrate that multiple optical test vectors can be inputon the periphery or within the core of a DUT with negligible crosstalk using thistechnique. Thus, optical signals can be simultaneously supplied to a silicon chip fordata encoding through multiple test vectors. Multiple simultaneous output signalshave also been successfully extracted with negligible crosstalk, indicating the possibilityof high observability and coverage with the technique. Optical input and outputfor test vectors o ers signi®cant reductions in the mechanical complexity andreliability of chip-level testing relative to mechanical contact probe techniques.The limitations of the technique have not yet been fully explored. Measurementspeed will be limited by the switching speed of Si LEDs and associated opticalreceivers. Based on previous hot carrier luminescence work (Kash and Tsang1996, Tsang and Kash 1997) which reports that switching speeds exceeding10 GHz can be measured using hot carrier luminescence, the switching time forsilicon LEDs is expected to be on the order of picoseconds.The frequency limitation on the performance of the breadboards used in thisexperiment is due to limitations of the electronic instrumentation system, and is notrepresentative of more sophisticated implementations of the approach. Further studyis required to determine the ultimate measurement bandwidth of the proposed testingapproach.5. ConclusionThis work demonstrates the feasibility of an all-silicon contactless testingapproach. Experimental data illustrate that multiple, simultaneous optical signalscan be simultaneously input to or output from a silicon chip.A contactless optical testing approach can address challenges associated withincreases in the number of physical test probes required to test increasingly complexICs. The proposed method appears economically practical, given the availability andlow cost of required system components. The method should have implementationcosts signi®cantly lower than other contactless methods, and is compatible withcurrent production test equipment and processes.ReferencesAkil, N., Kerns, S. E., and Kerns, D. V., 1998, Photon generation by silicon diodes inavalanche breakdown. Applied Physics Letters, 73, 871±872.Akil, N., Kerns, S. E., Kerns, D. V., Jr., Hoffmann, A., and Charles, J.-P., 1999, Amultimechanism model for photon generation by silicon junctions in avalanche breakdown.IEEE Transactions on Electron Devices, ED-46, 1022±1028.Bohm, C., 1996, Electric force microscope. Microelectronic Engineering, 31, 171±179.Dong, J., 1999, internal report (Vanderbilt University, silicon photonics group).Heinrich, H. K., 1990, Picosecond noninvasive optical detection of internal electrical signalsin ¯ip-chip mounted silicon integrated circuits. IBM Journal of Research andDevelopment, 34, 162±172.Kash, J. A., and Tsang, J. C., 1996, Full chip, optical imaging of logic state evolution inCMOS circuits. IEEE International Electron Devices Meeting (IEDM 96), pp. 5.9.1±5.9.3 (December 8±11, 1996, San Francisco, CA, USA).Kerns, D. V., Arora, K., Kurinec, S., and Powell, W., 1989, The silicon diode underavalanche breakdown as a light emitting source for VLSI optical interconnect.Proceedings of the 21st Southeastern Symposium on System Theory (SSST-89),Tallahassee, FL, USA.

Optical contactless testing of ICs 547Marcus, R. B., 1990, Measurement of high-speed signals in solid state devices.Semiconductors and Semimetals, 28 (Boston, MA: Academic Press).NTRS, 1997, The National Technology Roadmap for Semiconductors;Technology Needs (USA:Semiconductor Industry Association).Sayil, S., 2000, All-silicon optical contactless testing of integrated circuits. PhD Dissertation,Vanderbilt University, Nashville, TN, USA.Solkner, G., and Wolfgang, C., 1994, Advanced diagnosis techniques for sub mm IC’s,Microelectronic Engineering, 22, 11±16.Tsang, J. C., and Kash, J. A., 1997, Picosecond hot electron light emission from CMOScircuits. Applied Physics Letters, 70, 889±891.Weingarten, K. J., 1988, Picosecond optical sampling of GaAs IC’s. IEEE Journal ofQuantum Electronics, 24, 198±220.Woodward, T., 1996, Optical receivers for optoelectronic VLSI. IEEE Journal of SelectedTopics in Quantum Electronics, 2, 106±116.

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