Channel hot electron programmed memory device having improved reliability and operability

#The present invention relates to semiconductor devices and, more particularly, to a memory devices such as EPROMs and Flash EPROMs and, even more particularly, to a channel hot electron (CHE) programmed memory device having improved reliability and operability.

#CHE programming of electrically erasable programmable read-only memories (EPROMs) and flash (EPROMs) can generate between 50 μA-100 μA of cell current. For state of the art devices, which are often built onto P-type epitaxial layers over a low resistivity substrate, this substrate current sinks into the low resistivity layer and does not generate significant voltage drop. However, for EPROMs and flash EPROMs that use a non-epitaxial layer substrate, or ones that are built on an isolated P-well surrounded by a deep N-well, such as in a triple well process, the substrate current that is generated during programming can build up large voltage drops across the resistive FAMOS P-well region.

#If voltage drop exceeds the diode turn-on voltage of approximately 0.7 volts, then the source junction will forward bias. This problem of the resistive P-well situation may be seen in a lowering of the floating-gate avalanche-injection metal oxide semiconductor (FAMOS) BVCEO voltage value. Lowering the BVCEO values, unfortunately, reduces the maximum drain potential of the device. For a constant current programming load line, this reduces the gate current and degrades programmability of the EPROM or flash EPROM.

#For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description which is to be taken in conjunction with the accompanying drawings in which like reference numerals indicate like features and wherein:

#FIG. 1 shows a plot of BVCEO as a function of drain current that occurs in a flash EPROM that uses a non-epitaxial layer over a deep N-well, such as in a triple well process;

#FIG. 2 provides a table summarizing the disadvantageous efforts of the rising body potential on bit line stress;

#Preferred embodiments of the present invention are illustrated in the Figures like numerals being used to refer to like and corresponding parts of the various drawings.

#FIG. 3 illustrates a BVCEO line 14 showing the results of applying the concepts for which RB=15 Ohms and the deep N-well current is constant at −5 μA, to yield a deep N-well potential of approximately −0.4 V. By pumping a slightly negative voltage into the deep N-well no shift in the BVCEO characteristic curve occurs, as was the case in the BVCEO plots of FIG. 1. Accordingly, no reduction in Vt after bitline stress occurs. The result is that no degradation in programmability or gate current reduction occurs with the present invention.

#FIG. 4 shows one embodiment of the present invention that includes memory cell 30 formed on P-substrate 32 which itself is grounded at ground connection 34. Forming the components of memory cell 30 may use any of a variety of known fabrication processes. Deep N-well 36 surrounds isolated P-well 38. Instead of grounding deep N-well 36, a slightly negative voltage VDN of approximately −0.4 V is applied to deep N-well 36 at connection 40. Isolated P-well 38 is grounded at ground potential connection 42. Within isolated P-well 38, N+ region 44 is for forming the memory cell source region and N+ region 46 is for forming the associated drain region. Gate dielectric region 48 is positioned over isolated P-well 38 between source N+ region 44 and drain N+ region 46.