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Serial EEPROM Endurance
AN537
Everything a System Engineer Needs to Know About Serial EEPROM Endurance
The term "endurance" has become a confusing parameter for both users and manufacturers of EEPROM products. This is largely because many semiconductor vendors treat this important application-dependent reliability parameter as a vague specmanship topic. As a result, the system engineer often designs without proper reliability information or under-utilizes the EEPROM as an effective solution. Endurance (the number of times an EEPROM cell can be erased and rewritten without corrupting data) is a measure of the device's reliability, not its parametric performance. As such, endurance is not achieved by somehow making EEPROM devices more durable or robust to extend the life of the intrinsic erase/write cycle, but rather by reducing their defect-density failure rates. This has a direct impact on the design engineer characterizing EEPROM memory needs for an application and evaluating components from various manufacturers. The system design engineer needs to understand not only the relationship between the application, expected use and failure mechanisms, but also how the manufacturer has arrived at published endurance data for its components. This tutorial volume is intended to clarify some of the issues in the industry and provide a tool for the system design engineer, the system reliability engineer, and the component engineer to determine EEPROM reliability and understanding how to apply it to actual application requirements. It will examine four main areas: * * * * CMOS floating gate memory cell operation and characteristics Significant process and design interactions and endurance characterization variables Common misinterpretations of endurance Determining some real world application reliability requirements
EEPROM MEMORY CELL OPERATION AND CHARACTERISTICS
In discussing endurance characteristics of EEPROMs, it's important to review how these components operate, and why and how they fail. Figure 1 illustrates a CMOS floating gate EEPROM cell, including voltage conditions for READ, ERASE, and WRITE operations. To erase or write, the row select transistor must have the relatively high potential of 20V. This voltage is internally generated on chip by a charge pump, with the only external voltage required being VDD. The only difference between an ERASE and a WRITE is the direction of the applied field potential relative to the polysilicon floating gate. When 20V is applied to the polysilicon memory cell gate and 0V is applied to the bit line drain (column), electrons tunnel from the substrate through the 90-angstrom Tunnel Dielectric (TD) oxide to the polysilicon floating gate until the polysilicon floating gate is saturated with charge. The cell is now at an ERASE state of "1". When 0V is applied to the polysilicon memory cell gate and 20V is applied to the bit line drain (column), electrons tunnel from the polysilicon floating gate through the TD oxide to the substrate. The cell then is at a WRITE state of "0". This sequence of the transfer of charge onto the floating gate (ERASE) and the electrical removal of that charge from the floating gate (WRITE) is one ERASE/ WRITE cycle, or "E/W cycle." The field (applied voltage to an oxide thickness) across the tunneling path created by the 20V potential is extremely high in order to transfer the electrons. Over the cell's "application time," as measured by E/W cycles, the EEPROM cell begins to wear out due to the field stress. The EEPROM cell wears out as the number of cycles increase resulting in the voltage margin between the ERASE and WRITE states decreasing until finally there is not enough margin for the EEPROM sense amp to detect a difference in the two states during a READ. Failure is defined as when the sense amp can no longer reliably differentiate logic state changes. Figure 2 (single cell EEPROM endurance characteristics) illustrates that the intrinsic wear out point for a normal cell with specified dimensions and electrical characteristics is very acceptable, in excess of 2 million E/W cycles. Failures at lower cycles are due mostly to very small defects or imperfections in the oxide or silicon-to-oxide interface.
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(c) 1992 Microchip Technology Inc.
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Serial EEPROM Endurance
A key point to remember is that most failures occurring at less than 2 million E/W cycles are due to the number of defects per a given area (defect density dependent.) Thus high EEPROM endurance reliability is achieved by reducing the defect density failure rates, not by increasing the number of intrinsic cycles in the cell's operational design. Error correction circuits are design techniques commonly used by EEPROM manufacturers to increase endurance by reducing the failure rate caused by single bit failures. These circuits are transparent to the user. One typical scheme is using 4 bits of error correction for every 8 "real" bits (one byte). In this scheme, one bit failure in the byte is correctable, while if two bits within the byte fail, the byte is not correctable. Another error correction scheme is to use one "parity" bit for every "cell." Here both EEPROM cells must fail to result in a bit fail.
FIGURE 1 - CMOS FLOATING GATE EEPROM CELL
Memory Cell Gate Poly-Silicon, Level 2 Row Select Transistor Inter-Level Dielectric Poly-Silicon, Level 1 Poly-Silicon, Level 2
90 ANG Td Oxide Source Drain
(Not to scale) Common Source Bit Line
Memory Cell Gate Read Bit Line Row Select Gate Memory Cell Gate Common Source 1.6 volts 5.0 volts 5.0 volts 0.0 volts Write 18.0 volts 20.0 volts 0.0 volts Float
Row Select Erase 0.0 volts 20.0 volts 20.0 volts 0.0 volts Standby 0.0 volts 0.0 volts 0.0 volts 0.0 volts
DS00537A-page 2
(c) 1992 Microchip Technology Inc.
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Serial EEPROM Endurance
FIGURE 2 - SINGLE EEPROM CELL ENDURANCE CHARACTERISTICS
Erase Failure will occur when either the ERASE or WRITE margin crosses the sense amp level Sense Amp Level
Cell Margin (Volts)
Write 1 1,000,000
Erase/Write Cycles
PROCESS AND DESIGN VARIABLES AFFECTING ENDURANCE
There are many subtle process and design variables that have a strong impact on endurance. These interacting variables will play very different roles depending on the different process technologies of various semiconductor manufacturers.
The primary interaction is the amount of TIME at the HIGH VOLTAGES that is ultimately applied to the cell. A finite amount of time at finite voltages are required to achieve "optimal" ERASE and WRITE thresholds. If the time is too short and the voltage is too low, the EEPROM will not program to the proper threshold. Also, if the programming ramp time is too fast and the voltage is too high, the EEPROM's endurance will be reduced. Unfortunately, there is most often a trade-off between fast reliable programming performance or high endurance reliability. Some of the significant process and design variables are shown below and their impact on programming performance and endurance performance.
PARAMETER Internal High Voltages Internal High Voltage Ramp Rate Programming Time TD Oxide thickness Temperature
PROGRAMMING HIGHER = Faster Programming FASTER = Faster Programming LONGER = Improved Voltage Margin on the Cell THICKER = Slower Programming LOWER = Faster Programming
ENDURANCE LOWER SLOWER = Increased Endurance = Increased Endurance
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SHORTER = Less Oxide Stress for Increased Reliability THINNER LOWER = Reduced Endurance = Increased Endurance
(c) 1992 Microchip Technology Inc.
DS00537A-page 3
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Serial EEPROM Endurance
COMMON MISINTERPRETATIONS
In examining industry EEPROM literature on the topic of endurance, it's easy to misunderstand or misinterpret endurance concepts due to incomplete databook statements. The following are clarifications to some of the common misinterpretations:
Cycles/Day
In many cases, a serial EEPROM is used for widely varying functions in an application. These functions have different E/W usage requirements (cycles/day), resulting in different endurance requirements and, usually, different reliability results for each function. For example, assume a given end-product application will have a 10-year life. For each function within that application, an assumption must be made for the expected E/W cycles per day for a given segment of bytes. If a function has a segment of bytes cycled 1 time per day, then this segment of bytes will have 3,650 cycles in its lifetime (365 days per year for 10 years at 1 cycle per day and 7 days per week operation). Any given segment of bytes would have to cycle 274 times per day everyday to reach 1,000,000 E/W cycles in its 10-year application lifetime. Such a frequency is, of course, very rare in actual applications. For reference purposes, Figure 3 indicates typical cycles per day for some common applications. Although many manufacturers routinely discuss very high numbers of E/ W cycles, the amount of applications actually utilizing 1 million cycles is very small. A further and very important incorrect assumption often made is that ALL bits in an application need the same number of cycles and endurance ratings. In most applications, however, functions that require a high
Endurance and Read Cycles
READ operations are unlimited since they impose virtually no stress on the cell. Endurance data apply only to E/W cycles.
Erase/Write Ratings
E/W ratings are based on each byte in the application, not on the number of opcodes or control byte commands utilized. For example, if a part is rated to 100K E/W cycles, then each individual byte can be erased and written 100K times. The part is NOT limited to only a total of 100K E/W opcodes or control bytes. This is probably the most common misinterpretation made by system designers. Endurance is thus an interactive applicationspecific reliability parameter. It is not a typical data sheet specification, such as a parametric AC/DC specification with benchmark standards for measurement.
FIGURE 3 - TYPICAL SERIAL EEPROM E/W CYCLES/DAY BY APPLICATION
Maintenance Log Last Number Redial Electronic Lock Access Power Down Storage Digital Potentiometer Look Up Table Tuner Controls System Configuration Anti-Lock Brakes Speed Dial Airbag 0.001 0.01 0.1 1 Cycles/Day 10 100 1000
DS00537A-page 4
(c) 1992 Microchip Technology Inc.
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Serial EEPROM Endurance
number of E/W cycles per day require only a small number of bits. Last number redial in a telephone, for example, consumes many E/W cycles per day, but utilizes only a few bytes for this function. By contrast, speed dial storage in that same telephone consumes only a fraction of E/W cycles per day, but requires a relatively large segment of bits to accommodate the many speed dial options. In such an application, the same serial EEPROM normally performs both functions at different address locations. First, memory cell failure rates are defect density driven up to the intrinsic wear out point. Existing defects in a cell, while not causing failure initially, are stressed during every transfer of electrons through the TD oxide until they eventually cause cell failure. Worst case testing would be to erase and write each bit, which is what a write all "0"s pattern with an auto-erase of "1" routine will perform. Indeed, this write all "0"s test pattern will produce very different results than a checkerboard test pattern of alternating "1"s and "0"s within a byte, since cells are changed more often writing all "0"s than in an alternating "1" and "0" write pattern. The resultant failure rate differences are indicated on the pattern effect graph in Figure 4. In actual use, however, a system will experience a random pattern much more like the alternating "1"s and "0"s pattern than the more stressful all "0"s pattern. The key point for system designers is to determine how accurate a test routine has been used to determine a particular manufacturer's endurance data, and make the appropriate judgement on that part's expected endurance in the application.
ENDURANCE DATA FROM THE CUSTOMER'S PERSPECTIVE
Unfortunately, an industry standard for an endurance test method has yet to be adopted. Since endurance data is not baselined, the process of evaluating endurance becomes that much more complicated for the system designer and reliability engineer. It is not uncommon for customers to request endurance data from many semiconductor vendors. All vendors would be expected to comment that they experience a low failure rate through 100K E/W cycles. While this can be a true statement, it can also be a very incomplete statement. It is extremely doubtful that all vendors test their components to the same conditions. Yet the variables within endurance testing are extremely significant. Small differences in text protocol can have enormous differences. Pattern, cycling mode, temperature and array size, for example, are the most significant testing variables.
FIGURE 4 - PATTERN EFFECT ON ENDURANCE TESTING
Byte Write 0 Pattern Cumulative Failure Rate
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Alternating "1" and "0" Pattern
Erase/Write Cycles
(c) 1992 Microchip Technology Inc.
DS00537A-page 5
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Serial EEPROM Endurance
Second, the cycling mode graph in Figure 5 indicates that significantly different results can be achieved in endurance testing using a block cycle mode than using a byte cycle mode. The block mode is commonly used by manufacturers to "speed up" the endurance characterization process. However, endurance results usually will appear much better for the block mode than the byte mode, due to the high voltage variables discussed earlier. The reason is that the voltage ramp rate is significantly SLOWER, the high voltages are slightly lower, thus less stressful for block cycling since the capacitive load of the entire array is on the high voltage charge pump. The capacitive load is much lower with a single row or byte, which thus has a significantly faster ramp rate. Also, there is not a polysilicon to polysilicon stress for adjacent cells since all cells are at the same potential. Most often, these factors combine to yield lower failure rates for block cycling than for byte cycling. Again, the test conditions must match the system conditions. Finally, increasing temperature also increases stress on the cell. Microchip's endurance characterization data indicates that increasing temperature adds an activation energy (Ea) of 0.12eV on the cell. From a 25C to 85C ambient the acceleration factor is approximately 2.1. Therefore, the higher the temperature, the higher the stress. These results will vary significantly with each EEPROM manufacturer.
RECOMMENDED EEPROM ENDURANCE TESTING
Microchip believes that EEPROM components should be endurance tested to reflect system conditions. Therefore, units are cycled to an alternating ONE and ZERO pattern (checkerboard), then to an alternating ZERO and ONE pattern (inverse checkerboard). Again, since endurance characterization data indicates that a random single bit fail is the primary first order failure mode, endurance is defect density (def/cm2 or segment of bit size) dependent. Therefore an expected failure rate range by density can be established.
DETERMINING THE RELIABILITY CALCULATIONS
There are three primary components for the system design engineers to use in determining the endurance reliability required for a defect density limited application. These three components are: * Erase/write cycles/day estimated for the function. * The number of bits in the function (or segment size). * Case operating temperature of the Serial EEPROM. Let's look at three typical examples utilizing the above information to predict a cumulative failure rate at different points in a system lifetime. Please note that Industry endurance perceptions have improved from a very high (> 2%) failure rate expectation to a very low actual PPM level failure rate in the past few years.
FIGURE 5 - CYCLING MODE EFFECT ON ENDURANCE TESTING
Byte Mode
Cumulative Failure Rate
Block Mode
Erase/Write Cycles
DS00537A-page 6
(c) 1992 Microchip Technology Inc.
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Serial EEPROM Endurance
EXAMPLE #1 - AUTOMOBILE ELECTRONIC COMPASS
A 16-byte (128-bits) segment from a 2K array stores data every time the power is turned off in an automobile electronic compass. The design engineers expect the system to power down an average of 5 times/day over a 10-year life in an 85C case temperature environment. The projected Microchip cumulative failure rate under these conditions at the 10 year point is less than 8 PPM or 0.0008% failures in 10 years. The cumulative PPM failure rate graph is referenced on Figure 6.
EXAMPLE #3 - LASER PRINTER
A serial EEPROM could have many functions in a laser printer. A function that would likely require the most cycles/day is the maintenance log storage of the pages printed (estimated to be 100 cycles/day). Three bytes are utilized to store this number. The case temperature environment is estimated to be between 55C and 85C The high number of cycles at an extreme temperature of 85C indicate a failure rate of less than 1200 PPM through the first 5 years and 2600 PPM through the first 10 years. This failure rate can be dramatically reduced if the operating temperature is reduced to 55C. The same 5 and 10 year PPM levels are reduced to 450 PPM and 1600 PPM at 55C. These failure rates are illustrated on Figure 8.
EXAMPLE #2 FULL-FEATURED TELEPHONE
An 8-byte (64-bit) segment is used to store the last number redial on a stationary full feature phone operating in a room temperature environment. A 12K bit (12,288-bits) segment is used for storage of speed dial numbers on the same phone. This application has two major functions and therefore it will have two separate failure rate calculations. The last number redial function is expected to be utilized an average of 20 times/day over a 10 year life. Each speed dial is expected to be updated an average of 0.1 times/day over a 10 year life. Figure 7, the cumulative failure rate graph for these two conditions indicates an extremely low failure rate in the projected 10 year lifetime. The failure rate begins beyond 20 years as shown on the attached cumulative PPM failure rate graph.
SUMMARY
Microchip Technology Inc. has recognized that increasing reliability in serial EEPROMs through increased endurance is not a function of extending the life of the intrinsic erase/write cycle, but depends on reducing defect-density failure rates. Design engineers characterizing EEPROM memory needs for an application and evaluating EEPROM components from various manufacturers need to understand not only the relationship between the application and expected use and failure mechanisms, but also how the manufacturer has arrived at published endurance data for its components.
FIGURE 6 - EXAMPLE #1: THE AUTOMOBILE COMPASS
24LC02, 85C, 5V, 16bytes, 5cycles/day, RANDOM, BYTE 100 80 60 PPM = 82 40 20 0 0.000 0.005 0.010 , 18,250 Cycles, 0.015 10.0 Yrs 0.020
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(c) 1992 Microchip Technology Inc.
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Serial EEPROM Endurance
Microchip has developed a program to calculate the cumulative PPM failure rates for specific system conditions, as shown on the previous pages. This program is being transferred onto menu driven DOS floppy disks. These disks will be available in December 1992 to the system designers, reliability engineers, and component engineers to project endurance failure rates for specific system conditions. These disks will be periodically updated to reflect Microchip's most recent endurance data.
FIGURE 7 - EXAMPLE #2: THE FULL FEATURED PHONE
400 350 300 250 PPM 200 150 100 50
2.77E-05 0.0000584 8.91E-05 1.20E-04 1.50E-04 1.81E-04 0.0002117 2.42E-04 2.73E-04 3.02E-04 3.33E-04 3.64E-04 0.006132 0.012264 0.018396 0.024528 0.03066 0.036792 0.042924 0.049056 0.055188 0.06132 0.067452
55C 85C
0
24LC16, 22C, 5V, K Cycles, 10 Yrs.
FIGURE 8 - EXAMPLE #3: THE LASER PRINTER
Laser Printer Cumulative 365k Cycles 10 Yrs. 3000 2500 2000 1500 1000 500 0
0.12994 0.14454 0.15914 0.17374 0.18834 0.20294
0.05694 7.15E-02
0.29054 0.30514 0.31974 0.33434 0.34894
0.08614 0.10074 0.11534
0.21754 0.23214 0.24674 0.26134 0.27594
8/27/93 0.01314 0.02774 0.04234
Author:
Steve Drehobl Memory Products Division
DS00537A-page 8
(c) 1992 Microchip Technology Inc.
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