AN1012 datasheet(11/33 Pages) STMICROELECTRONICS

11 / 33 page

AN1012

4T cell devices

Doc ID 6395 Rev 4

11/33

4T cell devices

In moving to the newer process technologies (e.g., M48Z58 (8K x 8) device),

STMicroelectronics has chosen to reduce the active current as well as decrease the die

size. The STMicroelectronics HCMOS4PZ process is a 0.6

μm, double-level metal process.

In the standard SRAM memory cell, 6 transistors are formed into a pair of cross-coupled

inverters. In the 4T memory cell, the top two p-channel devices are replaced by poly-silicon

load resistors (poly-R). This combination allows for significant die size reduction because

the poly-R structures can be stacked on top of the active n-channel devices.

There is always at least one direct path constantly leaking current to ground in each cell

because of the poly-R structures in each SRAM cell. However, the value of the resistor is

extremely high (about 3T

Ω at 25 °C), so at a cell voltage of 3 V, this leads to a leakage

current of 1 pA. Multiplying by the number of cells within the array, the array standby current

can be calculated (i.e. 65.5 nA for a 65536-cell array).

The poly-R structure values are dependent on temperature, so the entire array current is

very strongly temperature-dependent. Appendix B: ZEROPOWER products on page 26

shows the expected battery lifetime of an M48Z58 device versus working temperature with a

VCC duty cycle of 0%.

The original specification was an expected lifetime of greater than 10 years at 25 °C but, in

fact, this target is typically achieved even at 70 °C. By reducing the temperature, the

expected lifetime rises to greater than 20 years (i.e., when the device is operated at 50 °C).

This change is defined entirely by the temperature sensitivity of the poly-R structures within

each SRAM cell.

The M48Z35 also employs the STMicroelectronics HCMOS4PZ process, 4T SRAM cell

technology. Appendix B shows the expected battery lifetime of an M48Z35 device versus

working temperature with a VCC duty cycle of 0%. From this we can see that expected

lifetime is typically greater than 20 years when operated at 30 °C with no external VCC

applied, and falls to approximately 2.6 years for continuous battery backup at 70 °C. This is

to be expected, due to the increased current consumption inherent in the 4T SRAM cell

architecture. It should be noted that this data is based on usage of the SNAPHAT® product

which includes a 48 mAh battery.

AN1012 Datasheet(PDF) 11 Page - STMicroelectronics