The present invention relates to the manufacture of semiconductor integrated circuits. More particularly, the invention is illustrated with regard to memory cell structures for a flash memory cell or flash E.sup.2 PROM or EPROM cell, but it will be recognized that the invention has a wider range of applicability. Merely by way of example, the present invention can be applied to a variety of embedded memory cell structures such as microprocessors ("MICROs"), microcontrollers, application specific integrated circuits ("ASICs"), and the like.
A variety of memory devices have been proposed or used in industry. An example of such a memory device is an erasable programmable read-only memory ("EPROM") device. The EPROM device is both readable and erasable, i.e., programmable. In particular, an EPROM is implemented using a floating gate field effect transistor, which has binary states. That is, a binary state is represented by the presence of absence of charge on the floating gate. The charge is generally sufficient to prevent conduction even when a normal high signal is applied to the gate of the EPROM transistor.
Numerous varieties of EPROMs are available. In the traditional and most basic form, EPROMs are programmed electrically and erased by exposure to ultraviolet light. These EPROMs are commonly referred to as ultraviolet erasable programmable read-only memories ("UVEPROM"s). UVEPROMs can be programmed by running a high current between a drain and a source of the UVEPROM transistor while applying a positive potential to the gate. The positive potential on the gate attracts energetic (i.e., hot) electrons from the drain-to-source current, where the electrons jump or inject into the floating gate and become trapped on the floating gate.
Another form of EPROM is the electrically erasable programmable read-only memory ("EEPROM" or "E.sup.2 PROM"). EEPROMs are often programmed and erased electrically by way of a phenomenon known as a Fowler-Nordheim tunneling process. Still another form of EPROM is a "Flash EPROM," which is programmed using hot electrons and erased using the Fowler-Nordheim tunneling phenomenon. Flash EPROMs can be erased in a "flash" or bulk mode in which all cells in an array or a portion of an array can be erased simultaneously using Fowler-Nordheim tunneling, and are commonly called "Flash cells" or "Flash devices."
A limitation with flash memory devices is the oxide layer or "tunnel oxide" layer must be high quality to accept repeated programing of the floating gate. High quality oxide is often formed by way of thermal oxidation. However, high quality oxide simply does not have sufficient qualities for higher density or sub-micron sized device structures. Accordingly, nitridized layers have been proposed to replace high quality oxide layers. Unfortunately, the thermal processing techniques used for nitridized layers often cause irregularities, including irregular gate oxide thickness, by way of a phenomenon known as the "Kooi" effect. Such irregularities are generally undesirable in the fabrication of integrated circuits.
From the above, it is seen that an improved flash memory cell structure that is easy to fabricate, cost effective, and reliable is often desired.