ONE-TIME PROGRAMMABLE MEMORY AND METHOD FOR MAKING THE SAME
A one time programmable nonvolatile memory formed from metal-insulator-semiconductor cells. The cells are at the crosspoints of conductive gate lines and intersecting doped semiconductor lines formed in a semiconductor substrate.
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This invention relates to a nonvolatile programmable semiconductor memory, and more particularly, to the making and operating of a one-time programmable (OTP) anti-fuse memory.
BACKGROUNDNonvolatile memory retains stored data when power is removed and is desirable in many different applications. As system-on-chips (SoCs) become more prevalent in consumer electronics and industrial applications, embedded nonvolatile memories have become more common. Embedded memory is incorporated onto the same underlying semiconductor die and non-memory circuitry.
The embedded memory is used for various purposes, among which are chip IDs, analog trimming, yield enhancement, and code storage. It would be advantageous if the embedded memories did not require added masks and process modifications to a standard CMOS flow. “Flash” memory that uses multiple polysilicon layers is not compatible with standard CMOS flow. As a result, gate dielectric based anti-fuse memory increasingly has become the choice of SoC chip designers because it is standard CMOS process based, reliable, and secure.
Gate dielectric anti-fused based memory can be broadly categorized into two groups, depending upon its operating principle. The first type is a cross-point memory consisting of a single capacitor at each gridpoint. The second type has more than two access lines for each cell in the memory array. A typical example is a storage capacitor or transistor coupled in series with a selection device such as a transistor or diode. examples of the first type can be found in U.S. Pat. Nos. 6,898,116, 6,992,925, 7,638,855, and 7,110,278. An example of the second type is U.S. Pat. No. 6,667,902 (and the references cited therein).
Cross-point memory arrays are advantageous due to its compact layout and simple decoding. As a result, embedded OTP memories of this type can be about eight times smaller than those of the second type. However, prior art cross-point OTP memories have drawbacks, such as significant process complexity, array leakage current, and reliability.
Furthermore, for embedded applications, it is very important to comply with logic layout design rules while introducing no extra process steps or only non-critical ones. As shown in prior art
U.S. Pat. Nos. 6,898,116 and 6,992,925, as illustrated in prior art
U.S. Pat. No. 7,110,278 to Keshavarzi discloses a cross-point memory similar to that of Peng except that the source and drain of each MOSFET is disconnected from its neighbors, as shown in prior art
Consequently, there is a need for a cross-point anti-fuse OTP memory that offers not only a compact size but also logic CMOS compatibility, low leakage current, and improved program reliability.
Various embodiments of the present invention are now illustrated in following figures using terms commonly employed by those skilled in the art. It will be understood that they are not intended to limit the invention to these embodiments. The invention can be practiced without one or more of the specific details, or with other methods, components, materials. In other instances, well-known structures, materials, process steps, or operations are not shown or described in detail in order not to obscure aspect of the invention.
In accordance with a disclosed embodiment,
Still referring to
As noted, the embodiments disclosed herein follow standard CMOS process flow except for the addition of a bitline BL implant mask that is used to form the Wines (active stripes) in the substrate.
While the standard n-wells are being implanted, the memory array regions are masked off in addition to the n-MOSFET devices. Thus, the drawn layer CBI serves two purposes: (1) to generate the n-well mask such that the memory array is covered while n-wells are being implanted in other areas of the substrate, and (2) to generate a bitline mask to form the active stripes. In some embodiments, this process may be performed by the combination of the n-well and bitline implants.
Next at box 703, the active stripes are implanted by the n-type dopant. This could be done with phosphorus and/or arsenic with a dose ranging from 1×1014 to 1×1015 and an energy ranging from 20 KeV to 80 KeV. As will be seen below, the active stripes in one embodiment have a super retrograde profile such that there is a deeper n+ band near the bottom and a shallower n− region near the surface. Depending on the particular process technology, well known multiple dose and energy implants can be used just like those used to form the standard n-well. The active stripe implant (also referred to as a cell bit implant (CBI)) may be done either before or after the regular n-well implant, without extra thermal annealing. In this embodiment, the implant is n-type dopant, similar to the n-well implant, but with a lower energy.
It is desirable to have the CBI: (1) have its n-p substrate junction shallower than the isolation STI (see
Indeed, as noted above, the p+ floating regions are not part of the active cell devices and therefore are optional (and can be masked out). However, to avoid additional masking steps, they can be left in (since they are floating and electrically isolated) and are formed from self-aligned source/drain implant when standard CMOS poly gate design rules are used.
One way to eliminate the optional p+ floating regions is illustrated in
Yet another embodiment is illustrated in
It can be appreciated that various combinations of the multiple concepts described herein may be combined into yet other embodiments. For example, the thicker gate oxide technique may be combined with the blocked source drain implant of
Still, in yet another embodiment, the floating doped semiconductor regions can be n+-type. As shown in
For OTP memories of smaller capacity, the memory array itself is a relatively small percentage of the total die area. In these embedded applications, it is advantageous to develop antifuse memories without introducing added mask and process steps in addition to standard CMOS processes. As such, yet another embodiment eliminates the additional CBI mask described above.
In this embodiment, the bit line implant 1501 is the standard n-well implant mask. Instead of covering the whole memory array area, the n-well implant mask covers each active stripe 1503 individually. N-well spacing is designed to prevent BL to BL leakage during programming. The cell size of this embodiment is larger than the others because the regular n-well is deeper than that of STI. Cross-sectional views are given in
Note that the above embodiments are for p-type MIS cells and can be easily switched to n-type MIS cells. Programming and read operations are the same for all p-type implementations. A simple polarity change applies to all n-type MIS cell embodiments.
With
For an un-selected cell at (WLi, BLn), the MIS capacitor is under deep depletion and the cell will not be disturbed. For the un-selected cell at (WLi, BLI), the programmed cell behaves as a reverse biased diode and its leakage current is extremely small. There is no effective voltage developed across MIS cells at (WLj, BLI) and (WLj, BLn). During read operations, bias conditions are similar to those of programming except the change from Vpp to Vread.
Features and aspects of various embodiments may be integrated into other embodiments, and embodiments illustrated in this document may be implemented without all of the features or aspects illustrated or described. One skilled in the art will appreciate that although specific examples and embodiments of the system and methods have been described for purposes of illustration, various modifications can be made without deviating from the spirit and scope of the present invention. Moreover, features of one embodiment may be incorporated into other embodiments, even where those features are not described together in a single embodiment within the present document. Accordingly, the invention is described by the appended claims.
Claims
1. An antifuse-based one-time programmable non-volatile memory cell comprising:
- a buried bitline formed in a substrate, the buried bitline of a first conductivity type;
- a dielectric layer formed over at least a portion of the buried bitline; and
- a conductive gate formed over the dielectric layer, the conductive gate defining a channel region under the conductive gate and dielectric layer;
- wherein the channel region does not have electrical interaction other than to said buried bitline or conductive gate.
2. The memory cell of claim 1 wherein formed on the sidewalls of the conductive gate are sidewall spacers.
3. The memory cell of claim 2 wherein floating regions of a second conductivity type are formed in the substrate spaced away from the channel region by the sidewall spacers.
4. The memory cell of claim 2 wherein regions of higher dopant concentration of the first conductivity type are formed in the bitline spaced away from the channel region by the sidewall spacers.
5. The memory cell of claim 1 wherein the buried bitline has a graded dopant concentration with a lower dopant concentration near the dielectric layer and a higher dopant concentration deeper in the substrate.
6. The memory cell of claim 1 wherein said dielectric layer is thicker proximal to at least a portion of the edge of the channel region than to the center of the channel region.
7. The memory cell of claim 1 wherein the buried bitline is formed from standard n-well implants.
8. A memory array comprised of a plurality of antifuse-based one-time programmable non-volatile memory cells, the memory array comprising:
- a plurality of buried bitlines formed in a substrate, the buried bitline of a first conductivity type;
- a dielectric layer formed over at least a portion of the buried bitlines; and
- a plurality of conductive gate wordlines formed over the dielectric layer, the conductive gate wordlines intersecting with the plurality of buried bitlines, the memory cells located at the intersection of said conductive gate wordlines and buried bitlines, further wherein a channel region is defined under the intersection of said conductive gate wordlines and buried bitlines and dielectric layer;
- wherein the channel region does not have electrical interaction other than to said buried bitline or conductive gate.
9. The memory array of claim 8 wherein formed on the sidewalls of the conductive gate of the memory cells are sidewall spacers.
10. The memory array of claim 9 wherein floating regions of a second conductivity type are formed in the substrate spaced away from the channel region by the sidewall spacers.
11. The memory cell of claim 7 wherein regions of higher dopant concentration of the first conductivity type are formed in the bitline spaced away from the channel region by the sidewall spacers.
12. The memory array of claim 8 wherein the buried bitlines have a graded dopant concentration with a lower dopant concentration near the dielectric layer and a higher dopant concentration deeper in the substrate.
13. The memory array of claim 8 wherein said dielectric layer is thicker proximal to at least a portion of the edge of the channel region than to the center of the channel region.
14. The memory array of claim 9 wherein the sidewall spacers of adjacent conductive gate wordlines completely span the space between the adjacent gate wordlines.
15. The memory array of claim 8 wherein shallow trench isolations are formed between the burled bitlines.
16. The memory array of claim 8 wherein buried bitlines are formed from standard n-well implants.
Type: Application
Filed: Nov 28, 2012
Publication Date: Jun 27, 2013
Applicant: KILOPASS TECHNOLOGY, INC. (Santa Clara, CA)
Inventor: Kilopass Technology, Inc. (Santa Clara, CA)
Application Number: 13/687,925
International Classification: H01L 27/112 (20060101);