CBRAM. Gamma Radiation Tolerant CBRAM Technology CASE STUDY. Introduction

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1 CBRAM Gamma Radiation Tolerant CBRAM Technology Introduction Adesto Technologies is the leading developer and provider of CBRAM, an ultra-low power non-volatile memory for standalone and embedded memory applications. CBRAM is also referred to in literature as Programmable Metallization Cells (PMC), or Electrochemical Metallization Cells (ECM). It utilizes electrochemical control of nanoscale quantities of metal in thin dielectric films or solid electrolytes to perform the resistive switching operation for non-volatile data storage. CBRAM is a leading candidate in the emerging memory landscape in the ITRS roadmap for semiconductors, [1] primarily due to its attractive features such as scalability (<10nm), ultra-low energy operations, fast read, write and erase times and low voltage requirements. There is a growing need to incorporate non-volatile memories in medical devices and many of these devices are often sterilized using gamma radiation to ensure protection from biological contamination. Gamma irradiation is an attractive method for the sterilization of medical devices and pharmaceuticals. It results in minimal or no rise in temperature, leaves no residue, and requires no quarantine time post processing. Data stored in conventional non-volatile memories such as EEPROMs and Flash are extremely susceptible to ionizing radiation effects as low as 50 krads. Gamma sterilization is done typically at 2.5 to 5 Mrads where conventional EEPROMs or Flash stand no chance of survival. This is due to the fact that information stored in EEPROMs/Flash is in the form of charges trapped into a floating gate which are easily perturbed by ionizing radiation. CBRAM fundamentally differs in the way it stores information and hence is expected to be resilient to ionizing radiation. [2] Gamma sterilization is accomplished by exposing products to an isotope source of radiation, most commonly in the form of cobalt 60 source pencils. On an industrial scale, this is done by loading boxes of product into containers which are conveyed into a shielded cell where they are then indexed around a radiation source for a predetermined amount of time. The minimum dose delivered to the products is a function of the time and the activity of the source. The dose required to achieve a sterile product depends on the number of organisms present on the products. A dose of 2.5 Mrads is commonly validated as it is a high enough to provide an adequate assurance of sterility for most products. In all industrial irradiator designs, there will be a range of doses delivered within an irradiation container, therefore for a minimum dose of 2.5 Mrads, the actual dose received by products could be as high as 5 Mrads in some cases. CS-CBRAM /2013

2 Experimental Work Two independent studies were conducted to study the tolerance of CBRAM technology to gamma exposure. Study #1 compares CBRAM with Floating Gate memory technology. Both memories were fabricated using 130nm CMOS technology and tested for lower dose levels up to 450 krads (Si). Test units were chosen from two product lines from Adesto Technologies Corporation namely, RM24EP128KS-ATGA (128kB Serial EEPROM based on CBRAM technology) and AT25F512B (512kB Serial Flash based on traditional floating gate technology) [3]. Both products were designed for the consumer market and do not have any special radiation tolerance scheme built into their design. Alternating patterns of logic 1 and 0 (Hex: AA55) were programmed into the memory arrays using memory testers. Data was verified on the arrays before transporting to Arizona State University for gamma irradiation. Two (control) units from each product family were unexposed on the gamma cell to check the baseline data loss during transportation. Study #2 was done on CBRAM technology for much higher doses of radiation typically used Figure 1. GammaFIT Production Irradiator (Nordion) for gamma sterilization. A total of 180 parts from Adesto Gen2 CBRAM family RM24EP128KS-ATGA were chosen for this study and data retention and product functionality was verified after exposures at 0, 1.5, 2.5 and 5 Mrads. Gamma exposure was conducted in collaboration with Nordion. Through their Gamma Center of Excellence, Nordion collaborates with industry and academics to promote innovation in gamma technology by providing access to Nordion s precision irradiation capabilities in return for information gained from the irradiation results. For this study, doses were delivered using a precision underwater irradiator. Twenty devices for each exposure dose were programmed with alternating 0 and 1 (checkerboard pattern) to check if data would survive the radiation exposure. The test flow is illustrated in Figure 2. DC and AC parametric data was collected from five devices for each exposure dose to determine if chip functionality would be affected by gamma radiation. In addition to these, 80 more devices were programmed in a way that the CBRAM cells resistance distributions could be extracted for a large population. This was done in order to study any effect of the radiation to the data storage mechanism of the CBRAM, which stores information as Low or High resistance states. Figure 2. Experiment flow for Study #2 (gamma exposure up to 50 kgy) 2

3 Observations After exposure, it was observed (Figure 3) that the transparent plastic sleeve in which the chips were transported showed some discoloration which was more visible with higher doses. This type of discoloration is a function of the interaction between the radiation and the polymer chains common in plastics which are less radiation resistant. Results Study 1: Table 1 illustrates the results of Study #1. The effect of gamma irradiation on data retention was obtained by verifying the data pattern (AA55) post irradiation. The amount of data loss was correlated to the number of IOs in the memory array with corrupted data. The effect of gamma radiation on the functionality of the device was obtained by studying the standby current drawn by the chip. (1.5 Mrad) (2.5 Mrad) (5 Mrad) Figure 3: Discoloration of plastic after gamma exposure Table. 1 Summary of Measured Standby Current and Data Loss with Different Dose of Gamma Exposure for CBRAM and Floating Gate Products Device Technology DUT ID Gamma Dose (krad Si) Data Loss (# Bit Errors) Result AT25F512B Floating Gate FAIL AT25F512B Floating Gate FAIL AT25F512B Floating Gate FAIL AT25F512B Floating Gate FAIL AT25F512B Floating Gate PASS AT25F512B Floating Gate PASS RM24EP128KS CBRAM PASS RM24EP128KS CBRAM PASS RM24EP128KS CBRAM PASS RM24EP128KS CBRAM PASS RM24EP128KS CBRAM PASS RM24EP128KS CBRAM PASS 3

4 Figure 4(a) is a plot of the chip standby current measured for units exposed to different radiation doses. It can be deduced that the floating gate product AT25F512B shows degradation to the chip standby leakage current while the CBRAM product does not show degradation dependence to the radiation dose. It is to be noted that the standby current could be affected by many factors including the architecture of the memory array and could be product specific. Figure 4(b) shows the number of bit errors measured after reading back the AA55 checkerboard pattern. It can be observed that the CBRAM product shows no errors even after 447krad exposure. However, the data loss or loss of functionality of the non volatile memory is evident for the floating gate product. Figure 4: Comparison between CBRAM and Floating Gate based NVM technologies tolerance to gamma radiation (a) Chip Standby Current and (b) Data Retention shows CBRAM is more gamma tolerant. Study 2: The experiments bundled in Study #2 (Data Retention Study) are below. Data Retention and Functionality Check All the parts received back from the gamma exposure were tested for data retention by reading back the content in the memory and comparing them with the checkerboard pattern. It was observed that none of the parts showed any failures including the 20 parts exposed at 50kGy (5 Mrad). It was determined that the parts did not lose any data to gamma radiation exposure. Through a separate test-mode in the product, the resistance states of individual bits were extracted and compared to the values before and after the Gamma exposure of 50kGy. Figure 5 shows that there is no visible change in the resistance distribution of bits in programmed state (Low Resistance State or LRS) or on the erased state (High Resistance State or HRS). This shows that the mechanism by which the CBRAM stores information is immune to gamma radiation up to 50kGy. Some parts exposed at 50kGy were further tested for functionality by programming different data patterns and erasing them multiple times, simulating a typical customer usage. All tests reported no errors leading us to conclude no visible damage to CBRAM parts from gamma exposure up to 50 kgy. 4

5 Figure 5. Program State and Erase State Resistance distribution of bits exposed to 50kGy Gamma radiation shows no difference before and after the exposure, suggesting robust immunity of the stored information in CBRAM products to Gamma radiation. Conclusions In summary, tests carried out jointly by Adesto and Nordion show the gamma radiation tolerance up to 5 Mrad (50kGy) from CBRAM technology. This feature combined with its ultra low power and high reliability makes it an attractive option for non volatile storage for applications involving low power and high gamma exposure tolerance. Acknowledgements Adesto wishes to thank Professor Hugh Barnaby and Dr. Yago Gonzalez Velo from Arizona State University for their assistance in Study #1 and Ms. Emily Craven from the Nordion Gamma Center of Excellence for their support with the gamma exposure in Study #2. References [1] International Technology Roadmap for Semiconductors Emerging Research Devices section 2011 edition available online at [2] Marinella, Matthew. "The future of memory." Aerospace Conference, 2013 IEEE. IEEE, [3] Details on CBRAM and Flash products from Adesto Technologies are available online at [4] Boulghassoul, Y., et al. "TID Damage and Annealing Response of 90 nm Commercial Density SRAMs." 5

6 Corporate Office California USA Adesto Technologies 1250 Borregas Avenue Sunnyvale, CA Phone: (+1) Adesto Technologies. All rights reserved. / Rev.: Adesto, the Adesto logo, CBRAM, and DataFlash are registered trademarks or trademarks of Adesto Technologies. All other marks are the property of their respective owners. Disclaimer: Adesto Technologies Corporation makes no warranty for the use of its products, other than those expressly contained in the Company's standard warranty which is detailed in Adesto's Terms and Conditions located on the Company's web site. The Company assumes no responsibility for any errors which may appear in this document, reserves the right to change devices or specifications detailed herein at any time without notice, and does not make any commitment to update the information contained herein. No licenses to patents or other intellectual property of Adesto are granted by the Company in connection with the sale of Adesto products, expressly or by implication. Adesto's products are not authorized for use as critical components in life support devices or systems. For Release Only Under Non-Disclosure Agreement (NDA)