Data retention | Fatigue | Data retention after fatigue | Results of product reliability tests |
FUJITSU FRAM Quality & Reliability
Reliability(1/4)
FRAM is a device with ferroelectric film, and therefore, the reliability of the ferroelectric film must be taken into account as a semiconductor device in addition to that of FRAM itself. A fall in the level of polarization must be notable to guarantee the reliability of ferroelectric film because it affects the data retention (the data holding characteristic) and fatigue of a FRAM.
Data retention
FRAM is a nonvolatile memory device that can hold written data even after it is powered off. Its ability to hold data over long period is called data retention. Data retention is strongly dependent on temperature, so that it is generally provided that the guaranteed retention lifetime (e.g., 10 years) is subject to temperature conditions (e.g., 55°C or less).
The test method for data retention, various retention characteristics, and the mode of degradation that determines the lifetime of a FRAM are explained below.
(1) Test method
As explained above, data retention is strongly dependent on the temperature. Based on this characteristic, it is possible to calculate the long term lifetime of FRAM for a short time period by temperature acceleration. [See Figure 5 "Test flow".]
A data pattern (e.g., a checkerboard pattern of 0 and 1) is written into the FRAM, and it is left at a high temperature (e.g., 150°C) for a certain time period. Then the data pattern is read out from the FRAM under actual use (e.g., the lowest power supply voltage: 4.5 V, and the highest operational temperature: 85°C), and compared with the original written pattern. Then the reversed data pattern (where 0 and 1 are reversed) is written into the FRAM and the pattern is checked for correctness. Finally, the original data pattern is written into the FRAM again and it is stored at high temperature. This cycle is repeated until there is an error in either the first reading or the second reading. The retention lifetime of a FRAM stored at high temperature is determined.
Of the two readings in the cycle flow, the first reading is called SS (same state) because the original data pattern is read out as it is from a FRAM stored at high temperature and the second reading is called OS (opposite state) because the reversed data pattern is read out from that FRAM.
Figure 5. Test flow
(2) Temperature acceleration
Table 2 is the storage test result at 85-175°C. Failures occurred at the highest temperature 175°C/2 kh, but there occurred no failure on any other conditions. This means that the FRAM is more likely to fail at a higher temperature. We will continue to conduct this test on the FRAM to obtain the temperature acceleration coefficient to predict the data retention lifetime of a FRAM used in an actual application.
Table 2. Result of high temperature storage test
| Temperature storing | Result of high temperature storage test (No. of failed units / no. of tested) |
||||
|---|---|---|---|---|---|
| Lifetime(h) | 168h | 500h | 1kh | 2kh | 3kh |
| (1) 85°C | 0/80 | 0/80 | 0/80 | 0/80 | 0/80 |
| (2) 100°C | 0/80 | 0/80 | 0/80 | 0/80 | 0/80 |
| (3) 125°C | 0/80 | 0/80 | 0/80 | 0/80 | 0/80 |
| (4) 150°C | 0/80 | 0/80 | 0/80 | 0/80 | 0/80 |
| (5) 175°C | 0/80 | 0/80 | 0/80 | 4/80 | 4/80 |
(3) Dependence on power supply voltage
The writing voltage applied to a ferroelectric capacitor in a memory cell is proportionate to the power supply voltage. On the other hand, the level of polarization of the ferroelectric film is dependent on the applied voltage as shown in Figure 6. The retention tends to be dependent on the level of polarization, so that writing with an insufficient power supply voltage may cause a reduction of retention. The test of data retention as mentioned above is made at the lowest voltage for the guaranteed operation on the catalog (e.g., 4.5 V). Accordingly data retention at a voltage less than that of lowest voltage is not guaranteed.
Figure 6. Graph of QTV characteristic
a) Degradation by depolarization
Depolarization means a decrease in the level of polarization of ferroelectric film, which weakens the hysteresis characteristic as shown in Figure 7. Decrease in the level of polarization concerns errors in reading because the FRAM reads data by detecting the levels of polarization.
Depolarization of the FRAM is greater at a higher ambient temperature. When the FRAM is exposed to a temperature, its depolarization is determined in seconds, and stays almost unchanged after that time. Returning the FRAM to the original temperature and rewriting data in it restores the original level of polarization. Failure in retention caused by depolarization means a defect in the SS reading in the evaluation cycle flow as shown in Figure 5.
Figure 7. Degradation of the hysteresis characteristic by depolarization
b) Degradation by imprint
Imprint means that the FRAM becomes resistive to reversal in polarization according to data that is written in. A FRAM recognizes 0 and 1 by detecting the levels of polarization, and therefore, it is difficult to read/write the reverse data in a FRAM if imprint occurs. Imprint is observed as a rightward or leftward shift of the hysteresis loop as shown in Figure 8.
The OS reading in the evaluation cycle flow in Figure 5 is done to check this mode of degradation.
Figure 8. Degradation of the hysteresis characteristic by imprint
(5) Others
The surface mounting device receives heat stress in mounting. Data retention is considered to be affected by certain temperatures in mounting. The FRAM must, therefore, be exposed under 260°C in mounting. For detailed mounting conditions, please contact our sales division.
