Design and Implementation of a Measurement Unit for Laser Testing of Semiconductor Memories Ignacio Garda, Ariel P. Cédola, Member, IEEE, Marcelo A. Cappelletti, Member, IEEE, Federico San Juan, and Eitel L. Peltzer y Blancá
Abstract—A system to monitor pulsed laser beams on semiconductor EPROM, EEPROM and RAM devices is presented. The laser beams can generate single event effects (SEE), which are detected by reading the memory cells during or after laser irradiation. Initial user-defined or preconfigured bit patterns are saved on memory cells before laser pulse strikes. A Visual Basic application running on a computer communicates with the hardware in order to start the writing/reading operations, visualize the cell contents and save the obtained results in files for post-processing. Data related to first tests carried out on an EPROM with a 1 picosecond laser are presented and discussed. Index Terms— Laser testing, pulsed laser beam, radiation effects, semiconductor memories, single event effects (SEE).
I. INTRODUCTION
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ATA storage devices are a fundamental part of any electronic system. Due to the fast paced development of new technologies, the field of application of these devices is continuously growing. In particular, for recent space equipment, it is usual to include several memory integrated circuits, which are inevitably exposed to space radiation damages [1]. These damages are produced by the impact of energetic particles (electrons, protons, neutrons, etc.) on the semiconductor materials, and include permanent or transitory loss of data, interruption of functionality and permanent electronic failures, among others. Reliability of data storage microcircuits under radiation plays a very important role in today’s space missions, so ground tests are mandatory in order to obtain information about device performance and to predict the device and whole system behavior in harsh environments. Since realization of experimental tests in particle accelerator facilities is very expensive, destructive and time-consuming
Manuscript received on April 21, 2013. This work was supported in part by the Universidad Nacional de La Plata, Argentina, under Grant I158. I. Garda, A. P. Cédola, M. A. Cappelletti and E. L. Peltzer y Blancá are with GEMyDE, Grupo de Estudios de Materiales y Dispositivos Electrónicos, Departamento de Electrotecnia, Facultad de Ingeniería, Universidad Nacional de La Plata, 48 y 116 piso 1, CC 91, La Plata (1900) Argentina (e-mail:
[email protected]). F. San Juan is with CIOp, Centro de Investigaciones Opticas, CONICETUNLP, Camino Centenario y 508, La Plata (1900) Argentina (e-mail:
[email protected]).
due to the excessive setup implementation time, alternative laboratory test methods for space radiation effects in electronics have been developed [2]. One of these methods is known as laser testing, and consists in the utilization of pulsed laser beams for the irradiation of electronic devices [3], [4], which have proven to produce, under very specific testing conditions and with acceptable precision, effects similar to the impact of charged particles into devices like heterostructure transistors [5], [6], linear integrated circuits [7]-[10], A-to-D converters [11]-[13], and memories [14]-[16]. The fact that both, charged particles and photons, generate charge when penetrating a semiconductor material makes pulsed laser useful to the study of single event effects (SEE) in electronic devices. However, since the interaction with the semiconductor is different in each case, giving rise to different charge track structures, the laser technique cannot directly replace particle accelerator testing [17]. Laser light is not able to reach all sensitive areas in a die, either by packaging characteristics or by metallization shadowing, so finding a correlation between particle and laser induced SEE is not straightforward. Different techniques have been proposed to deal with these issues [18]-[20]. Anyway, laser testing can be considered a reliable tool complementing particle accelerator testing, for evaluation of the circuit’s sensitivity to SEE and effectiveness of hardware/software fault mitigation techniques, with its main advantage being the possibility to bring information about spatial and temporal dependence of the device sensitivity to radiation [21]. Additionally, its cost and dangerousness for the experimentalists are lower in comparison to the use of particle accelerators. In this paper, the design, fabrication and testing of a system for the detection and analysis of pulsed laser induced single event upsets (SEU) on semiconductor memories is presented. The hardware works connected to a computer running the software for configuration, control, visualization and storage of collected data. The system allows to write to and read the memory cells, and visualize on an intuitive GUI the errors produced as laser pulses are triggered. Three types of memories can be investigated: EPROM, EEPROM and RAM. With minor modifications, the system could be applied to SEU testing under heavy-ion irradiation. In Section II some brief fundamentals of laser testing are reviewed and in Section III the implementation of the measurement system is depicted. In
Section IV, finally, experimental tests carriedd out on EPROM devices are shown and discussed. As far as the authors’ knowledge, this is the first work about SEU U laser testing in microcircuits reported outside the USA, Japann and Europe. II. LASER TESTING FUNDAMENTTALS Although charged particles and laser pulsses may generate the same amount of electron-hole pairs in a seemiconductor, the distribution of these carriers is very differennt for each case. When a charged energetic particle penetrates a semiconductor, a dense and narrow track of electron-hole ppairs is produced within an approximated diameter of 0.5 μm, depending on the mass, charge and energy of the incident parrticle. The energy loss per unit of length or LET (linear energyy transfer) can be considered as a constant along the complete track [22]. For laser pulse irradiation, on the contrary, botth semiconductor absorption and beam spreading lead to a chaarge track with a wider radial extension and a much lower carrrier density than the particle induced track [23], as shown in Fig. 1. This substantial difference between the two phenoomena can affect the amount of charge collected, and conseequently the SEE measurement, given that processes like funnneling and Auger recombination are dependent on the charge density. Nevertheless, both experimental and theoreetical works have shown that laser and particle strikes producee similar transient effects on integrated circuits [17].
technology. III. MEASUREMEN NT SYSTEM The basic architecture of the developed d system can be summarized as follows: a control block, that receives incoming commands from the user and controls all write/read operations on memories; the devicee under test (DUT) block, where any of the three types off supported memories are connected; an excitation block, composed by the laser ng the custom developed equipment and a computer, runnin user-friendly software through whicch the system is managed. The block diagram of the measurem ment unit is shown in Fig. 2 and a more detailed description of the t hardware and software is given below. An external DC pow wer source supplies all the necessary voltage levels to the differrent components.
Fig. 2. Complete diagram of the measurem ment system, including the bus connections between blocks.
Fig. 1. Comparison between particle and laser-inducedd charge tracks on a semiconductor die.
With respect to the laser characteristicss, photon energy, beam intensity, pulse length and spot size muust be determined according to the device and materials to bee investigated. In general, the photon energy must be higher thaan but close to the semiconductor bandgap, in order to minimizze the absorption and maximize the penetration depth. Howeveer, if absorption is too low, the density of generated carriers annd, therefore, the equivalent LET are low, too. So a trade-off iss established. The laser LET can be modified by adjusting the photon energy and also the beam intensity. The pulse length muust be shorter than the response time of the irradiated circuit, in the order of picoseconds. The spot size must be smallerr than the lateral dimensions of the circuit junctions, so the laaser beam can be focalized to accurately measure the circuit uupsets sensitivity. The minimum spot size is imposed by diffracction and depends on the laser wavelength and the focus lenses [3]. Spot mon with current diameters smaller than 2 µm are comm
A. Hardware The core of the control block is a PIC16F877A 40 pin, 5 n functions including the port microcontroller, with its main selection of the memory chip to bee accessed, cell addressing, data reading and writing and a establishing serial communication with the computter that is sending and receiving data. This communicatio on is based on the RS232 protocol and a MAX232 driver/receeiver IC is used to convert the microcontroller digital signals to the adequate levels. The system is designed to work with EPROM, RAM and ontroller fulfills all the EEPROM. The selected microco requirements to operate with thesee devices, i.e. number of ports and I2C communication capab bilities. EPROM and RAM parallel-operated memories considered in this work demand a total of 23 and 19 I/O ports, respecctively. EEPROMs, which the system has been designed to work with, are accessed through the two-wire serial in nterface SCL and SDA microcontroller pins. A schematic of the main part of the system can be appreciated in Fig. 3. he main board with 5V and The DC power source supplies th 12.75V. The first voltage is applied to every integrated circuit, whereas the second one is used as programming voltage for the EPROM. This power source must m be manually enabled
Fig. 3. Schematic of the control and DUT blocks of the developed system.
through a switch by the user when writing to the EPROM. A set of LED diodes indicates the ON/OFF state of the system and the type of memory being used. The write and read operations of the system were successfully tested with the following memory models: EPROM M27C256B (256 Kbit), SRAM HM6116LP-2 (16 Kbit) and EEPROM 24C08 (8 Kbit). B. Laser equipment and measurement setup The equipment chosen to perform the tests was a Ti-Sa laser in complement with an optical parametric oscillator (OPO) with a pulse width of 100 fs, a tunable energy per pulse of 700 pJ to 1 mJ, and a tunable wavelength from 370 to 2500 nm. A wavelength of 800 nm was selected with the aim of preventing effects of higher orders of absorption mechanisms, and to obtain, at the same time, decent penetration distances from 10 to 20 µm. One picosecond pulses were applied in all the measurements. A complete diagram of the measurement setup is presented in Fig. 4. Immediately after the laser output and before the beam enters the OPO, a light divisor is placed in order to deviate half of the energy into a secondary beam, allowing the use of the test facility for other projects simultaneously. The oscillator is configured in order to obtain a beam with a wavelength of 800 nm, a diameter of 8 mm and pulse energy of 700 μJ. The beam is then targeted approximately into the DUT with the aid of two reflective surfaces. Next the beam is focused through a half-wave plate and a polarizer, working together as an attenuator, its output depending on the input angle. At this point, the incident energy is measured and
Fig. 4. Complete laser measurement setup.
estimated in 7 μJ, after which the beam is put through a set of two optical filters with attenuation factors of 100x and 1000x, producing a 70 pJ laser beam output. This last value was estimated, given that the instrument’s resolution was not enough for measuring it. Respecting the laser spot size, a plane convex lens was placed in order to concentrate the beam in an estimated spot diameter of 6 μm reaching the DUT. This was calculated from the expression d 2λf⁄πr [3], where λ is the wavelength of the incident beam, r its radius and f the focal distance of the selected lens. With the objective of aiming the laser spot into the sensitive zones of the DUT, a secondary low-energy visible wavelength (633 nm) He-Ne laser was placed and aligned with the Ti-Sa laser. A photograph of the whole measurement setup is shown in Fig. 5.
Fig. 5. A picture of the measurement setup taken during experiments reported in this work.
C. Software User-friendly management software was developed in order to facilitate the configuration of the system and the collection of data. It consists of a Visual Basic executable that allows the user, in an intuitive way, to set the serial port parameters, connect/disconnect with the main board, choose the type of memory to investigate and the operation to perform (reading or writing). Fig. 6 shows an image of the main screen. In the case of data writing, the user is allowed to select the data pattern to save into the data blocks: all bits to 1, all bits to 0 or alternating 1’s and 0’s. It is also possible to write arbitrary data, by filling the field next to the “Send Data” button and clicking on it. The operations of reading and writing are initiated after clicking on the respective button, located at the bottom. When reading, the retrieved cell contents are both shown on the software screen and saved into a text file for their subsequent analysis. There is no synchronization with laser pulse firing given that SEU, and not transient responses, are the only effects of interest.
Fig. 6. Snapshot of the main screen of the software developed for the management of the measurement system. Cell memory contents are shown in hexadecimal format after a reading operation.
IV. EPROM LASER TESTING RESULTS The main reason to include EPROM as one of the testable memory types lies in the fact that laser spot can reach the silicon die through the fused quartz window, permitting the chip to be exposed to ultraviolet light for memory erasure, and therefore, avoiding the need to decapsulate the device. The only drawback of this procedure is the difficulty to determine the spot size and the effective energy deposited by laser photons within the semiconductor. However, and despite being an obsolete device, it was considered highly convenient for the development of the first laser testing experiences. The DUT is a 256 Kbit EPROM model M27C256B, with 15 address bits and 8 data bits. The experiments were performed at CIOp (Centro de Investigaciones Opticas, Universidad Nacional de La Plata) facilities. The first test was made by continuously firing 70 pJ laser pulses with a repetition rate of 10 Hz over the EPROM, with all the cells filled with 1’s. The reading operation of the device exhibited the appearance of the column errors observed in Fig. 7. As can be seen, 1 to 0 transitions in half of several
Fig. 7. A portion of the bit pattern observed after irradiation with 70 pJ pulses, 10 Hz frequency.
cells were induced by the laser strikes. Posterior exposures to UV light and readings proved that the memory was affected by permanent damages, because none of the altered cells was able to return to the original FF state. Additional tests demonstrated that continuous irradiation leads to an accumulation of energy high enough to permanently disrupt the memory’s functionality, so that, in order to observe SEU
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Fig. 8. Error mapping showing two different types of anomalies. Colored points indicate the location of detected errors, in all cases 1 to 0 transitions.
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Fig. 10. MBU’s distributed all along the cells array after application of 8 μJ laser pulses.
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Fig. 9. Error mapping showing the column errors generated after 20 μJ (blue points) and 10 μJ (green points) laser strikes.
rather than irreversible damages, all later experiences consisted in the application of single laser pulses. The next test was performed with 20 μJ single pulses over a memory again fully filled with 1’s. The value of energy was arbitrarily set once corroborated that single pulses with energies in the range of 70 pJ to 7 μJ were not able to cause any event. Two different results were obtained from the same measurement. Assuming a 256 row by 1024 column bit distribution for the EPROM DUT, the error mapping after the irradiation is represented in Fig. 8. Red points denote transient 1 to 0 transitions that disappeared by themselves after memory reading. More precisely, a second reading operation performed a few seconds following the first one showed only the 1 to 0 transitions identified with blue points, demonstrating the transient nature of the effect. The blue points indicate a full column error, a severe error similar to that found in SEE studies on DRAM devices, after exposure to laser, heavy ions and protons [15], [24], [25]. According to these works, the observed column error could be classified as a temporary block error, correctable by writing new data or, in the EPROM case, by erasing under UV light. After this reading a new test was conducted, for which the laser pulse energy was reduced to 10 μJ and the location of the incident spot was changed. The resultant error mapping can be seen in Fig. 9, in which the stable errors generated after the previous irradiation are still present. Green points represent the bit flips from 1 to 0 induced by the 10 μJ pulses. No
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Fig. 11. Number of bit flips as a function of laser energy.
transient errors were detected in this measurement, but as above, a full column error was induced by incident laser photons. The origin of this severe error could be attributed to an upset in an internal address register, produced by the strike of the laser pulse [25]. Another experiment with a different initial bit pattern was carried out on a new EPROM sample. Even columns were entirely prefilled with 1’s, whereas odd columns were all prefilled with 0’s. The energy of laser pulses was further lowered to 8 μJ, and the result of memory reading after laser beam irradiation is shown in Fig. 10. Multiple bit upsets (MBU) can be observed, being in all cases, unlike previous tests, 0 to 1 transitions. It is worth noting that only seven columns were affected by the laser, namely: columns 73, 201 and 329 (separated by 128 columns from each other), 705 and 833 (separated by 128 columns, too) and columns 193 and 449 (separated by 256 columns). All bits correctly returned to 1 after one hour of UV illumination. Despite the fact that experiments were made with different initial cell patterns and laser spot locations, a summary of the number of bit errors as a function of the laser beam energy was found useful to establish a qualitative relation between laser intensity and damage generation. A curve bringing together all the experimental results, including both transient and stable errors, is shown in Fig. 11.
V. CONCLUSIONS An instrument for detecting pulsed laser beam induced SEU in semiconductor memories has been developed and successfully tested with EPROM devices. The system is configured and controlled from a custom-made software running in a computer attached to the main board. Three types of memories can be used with this system: EPROM, EEPROM and RAM ICs. The software allows writing arbitrary or preconfigured bit patterns on the memory cells, and reading the content of the whole circuit during or after laser irradiation. All data are saved to txt files. Testing of a 256 Kbit EPROM with a transparent lid irradiated with 800 nm, 1 ps laser pulses yielded interesting results, in accordance with published works related to similar studies on different kinds of memories. In particular, severe errors like full column bit flips were observed. Measurements corroborated that the number of laser induced bit errors is proportional to the laser beam energy. By separating the DUT stage from the main board, the system would be able to be used in particle accelerator facilities, for SEU heavy-ion testing.
[13]
[14]
[15] [16] [17] [18]
[19] [20]
ACKNOWLEDGMENT The authors would like to thank Dr. Jorge Tocho, from CIOp (Centro de Investigaciones Opticas, CONICET-UNLP), for his valuable collaboration and support with laser equipment. REFERENCES [1]
F. B. McLean, “Interactions of hazadours environments with electronics devices,” IEEE Nuclear and Space Radiation Effects Conference (NSREC) Short Course, Section I, 1987. [2] S. Buchner, D. McMorrow, J. Melinger and A. B. Campbell, “Laboratory tests for single-event effects,” IEEE Trans. Nuclear Science, vol. 43, no. 2, pp. 678-686, 1996. [3] S. Buchner et al., “Laser simulation of single event upsets,” IEEE Trans. Nuclear Science, vol. 34, no. 6, pp. 1228-1233, 1987. [4] S. Buchner et al., “Pulsed laser induced SEU in integrated circuits: a practical method for hardness assurance testing,” IEEE Trans. Nuclear Science, vol. 37, no. 6, pp. 1825-1831, 1990. [5] D. McMorrow et al., “Single event dynamics of high performance HBTs and MESFETs,” IEEE Trans. Nuclear Science, vol. 40, no. 6, pp. 18581866, 1993. [6] D. McMorrow et al., “Charge collection mechanisms of heterostructure FETs,” IEEE Trans. Nuclear Science, vol. 41, no. 6, pp. 2055-2062, 1994. [7] R. L. Pease et al., “Comparison of SETs in bipolar linear circuits generated with an ion microbeam, laser light, and circuit simulation,” IEEE Trans. Nuclear Science, vol. 49, no. 6, pp. 31633170, 2002. [8] S. Buchner, J. Howard Jr, C. Poivey, D. McMorrow and R. Pease, “Pulsed laser testing methodology for single event transients in linear devices,” IEEE Trans. Nuclear Science, vol. 51, no. 6, pp. 3716-3722, 2004. [9] C. Weulersse et al., “Probing SET sensitive volumes in linear devices using focused laser beam at different wavelengths,” IEEE Trans. Nuclear Science, vol. 55, no. 4, pp. 2007-2012, 2008. [10] P. Jaulent, V. Pouget, D. Lewis and P. Fouillat, “Study of single event transients in high speed operational amplifiers,” IEEE Trans. Nuclear Science, vol. 55, no. 4, pp. 1974-1981, 2008. [11] S. Buchner, T. Meehan, A. Campbell, K. Clark and D. McMorrow, “Characterization of single event upsets in a flash analog-to-digital converter (AD9058),” IEEE Trans. Nuclear Science, vol. 47, no. 6, pp. 2358-2364, 2000. [12] V. Pouget, D. Lewis and P. Fouillat, “Time-resolved scanning of integrated circuits with a pulsed laser: application to transient fault
[21] [22]
[23] [24] [25]
injection in an ADC,” IEEE Trans. Instrum. Meas., vol. 53, no. 4, pp. 1227-1231, 2004. A. Zanchi et al., “A comprehensive methodology to rate SETs of complex analog and mixed-signal circuits demonstrated on 16-bit A-toD converters,” IEEE Trans. Nuclear Science, vol. 59, no. 6, pp. 27392747, 2012. S. Buchner, J. B. Langworthy, W. Stapor, A. B. Campbell and S. Rivet, “Implications of the spatial dependence of the single event upset threshold in SRAMs measured with a pulsed laser,” IEEE Trans. Nuclear Science, vol. 41, no. 6, pp. 2195-2202, 1994. S. Duzellier et al., “Application of laser testing in study of SEE mechanisms in 16-Mbit DRAMs,” IEEE Trans. Nuclear Science, vol. 47, no. 6, pp. 2392-2399, 2000. T. Oldham et al., “Correlation of laser test results with heavy ion results for NAND flash memory,” IEEE Trans. Nuclear Science, vol. 59, no. 6, pp. 2831-2836, 2012. J. S. Melinger et al., “Critical evaluation of the pulsed laser method for single event effects testing and fundamental studies,” IEEE Trans. Nuclear Science, vol. 41, no. 6, pp. 2574-2584, 1994. D. McMorrow, W. Lotshaw, J. Melinger, S. Buchner and R. Pease, “Subbandgap laser induced single event effects: carrier generation via two-photon absorption,” IEEE Trans. Nuclear Science, vol. 49, no. 6, pp. 3002-3008, 2002. D. Lewis et al., “Backside laser testing of ICs for SET sensitivity evaluation,” IEEE Trans. Nuclear Science, vol. 48, no. 6, pp. 2193-2201, 2001. N. Kanyogoro et al., “A new approach for single event effects testing with heavy ion and pulsed laser irradiation: CMOS/SOI SRAM substrate removal,” IEEE Trans. Nuclear Science, vol. 57, no. 6, pp. 3414-3418, 2010. F. Miller et al., “Laser mapping of SRAM sensitive cells: a way to obtain input parameters for DASIE calculation code,” IEEE Trans. Nuclear Science, vol. 53, no. 4, pp. 1863-1870, 2006. W. J. Stapor, P. T. McDonald, A. R. Knudson, A. B. Campbell and B. G. Glagola, “Charge collection in silicon for ions of different energy but same linear energy transfer (LET),” IEEE Trans. Nuclear Science, vol. 35, no. 6, pp. 1585-1590, 1988. P. Fouillat et al., “Fundamentals of the pulsed laser technique for single event upset testing,” in Radiation Effects on Embedded Systems, R. Velazco et al. Ed., Springer, 2007, pp. 121-141. P. Calvel et al., “Space radiation evaluation of 16 Mbit DRAMs for mass memory applications,” IEEE Trans. Nuclear Science, vol. 41, no. 6, pp. 2267-2271, 1994. K. A. LaBel et al., “Anatomy of an in-flight anomaly: investigation of proton induced SEE test results for stacked IBM DRAMs,” IEEE Trans. Nuclear Science, vol. 45, no. 6, pp. 2898-2903, 1998.
