Richard A. QuinnellEE Times -- (05/20/2008 4:07 AM EDT)
Many applications need to archive data or retain system information after power-down. These tasks fall to nonvolatile memory that must be in-circuit writable at least once, and often many times. Floating-gate data storage, the traditional technology used to create such NV memory, faces increasing challenges as process lithography shrinks, prompting evolving new technologies to take over.
The uses of in-circuit writable NV memory are legion. In consumer electronics, NV memory stores user data and preferences, as well as photographs and media files. In embedded systems, it may store configuration parameters or calibration data as well as system state information for power-up resets. In virtually any application that needs to download application software or update system software without swapping out components, NV memory can serve as alterable program storage.
The technology at the heart of most NV memory is the floating gate. The amount of charge on the gate of a field-effect transistor determines its ability to conduct current. A floating-gate transistor has two gates: one connected in circuit (as in any other FET) but the other electrically isolated (floating), allowing any charge that the gate holds to remain indefinitely. No current can flow to a floating gate under normal circuit operation, but a high voltage can move electrons to or from the gate by causing charge tunneling through the insulating layers around the gate. A floating-gate transistor can thus serve as a memory element, with the floating gate's charge representing the stored data value. Since the technology's introduction, many memory device types have arisen that use the floating gate, including E2
PROM and flash memory.
Floating-gate technology is running into problems, however, as semiconductor processes continue their trend toward smaller lithography dimensions. "[Manufacturers] are getting to the end of what they can do with floating-gate technology, and they are looking for alternatives," said John Nation, director of corporate marketing at memory maker Spansion. One of the challenges stems from the floating gate's need for isolation from surrounding circuits. As circuit dimensions shrink, the insulating layers surrounding the gate must also shrink. This leads to increased capacitive coupling between memory bits and greater likelihood of small "pinhole" manufacturing defects in the insulating layers, creating a discharge path.
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