Data retention | Fatigue | Data retention after fatigue | Results of product reliability tests |
FUJITSU FRAM Quality & Reliability
Reliability(2/4)
== Fatigue ==
Fatigue means a decrease in the level of polarization caused by repeated reversals of polarization of the ferroelectric film (See Figure 9.). Fatigue caused by repeated reversals of polarization occurs not only in writing cycles but also in reading cycles, because there is a reversal in the polarization in a cycle process of operation from reading from a FRAM memory cell to rewriting in it because the reading is destructive to data. Accordingly the number of reversals in the polarization of a FRAM is specified as a number of times of possible access to the memory cells, whether they are in read or write cycles.
The test method for evaluation of fatigue, various fatigue characteristics, and the modes of degradation are explained below.
Figure 9. Mechanism of fatigue
(1) Test Method
The test method for fatigue is applying fatigue stress to a FRAM by repeating write or read cycles and then checking that it operates correctly. Assuming that the time of an operation cycle is 250 ns for a 64 k bit FRAM in an organization of 8 k words x 8 bits, the test time it takes to apply the fatigue stress to the FRAM is calculated as follows. Eight bits are simultaneously applied with stress at one time of access, so that it takes 8 k (8,192) cycles to apply fatigue to all the memory cells. For example, the times it takes to implement some test times are calculated as follows.
10EXP6 ; 250ns x 8,192 x 10EXP6 = 0.57 hour
10EXP8 ; 250ns x 8,192 x 10EXP8 = 57 hour (=2.37 days)
10EXP10 ; 250ns x 8,192 x 10EXP10 = 5700 hour (=237 days)
10EXP12 ; 250ns x 8,192 x 10EXP12 = 65 years
As shown above, performing the test of fatigue at some large numbers of cycles is unrealistic. The following methods are taken to substitute for such evaluation.
a) Acceleration of fatigue by applying an excessive voltage
b) Limiting the number of cells to be stressed (sampling test)
c) Applying stress to many devices simultaneously (simultaneous measurement test)
For example, a.) voltage acceleration accelerates by approximately 10 times the fatigue in a FRAM by applying an increment of 1 V to FRAM. The method b.) is generally combined with the method c.) not to under-evaluate fluctuation in the characteristic among cells.
(2) Various characteristics
It has been confirmed that no failures occur in a 64 k bit FRAM after writing in 10EXP10 times under the following conditions. [See Table 3 Fatigue test data.]
- Power supply voltage: 6 V
- Temperature in applying stress: 85°C
- Temperature in an operation test after stress applied: 85°C
- Stress area: 4 k bit/device
- Stress pattern: writing 1 and 0 alternately
- Number of devices evaluated: 457
Table 3. Fatigue test data
| Fatigue conditions | Fatigue results (No. of failures/no. of tested) |
||||
| cycle | 10EXP6 cycles | 10EXP7 cycles | 10EXP8 cycles | 10EXP9 cycles | 10EXP10 cycles |
| 85°C 6.0V | 0/457 | 0/457 | 0/457 | 0/457 | 0/457 |
(3) Mode of degradation
The detailed mechanism of degradation by fatigue has not fully been elucidated. It is, however, known that fatigue causes some phenomena that are similar to those caused by depolarization as the mode of retention degradation. It is different from the depolarization in that the FRAM is not restored to the original level of polarization by rewriting.
