The subject matter of the following applications, including this application, is related: Swift and Chindalore, Programmable Structure Including Discontinuous Storage Elements and Spacer Control Gates in a Trench, Ser. No. 11/188,585, filed Jul. 25, 2005 and Swift and Chindalore, Method of Fabricating Programmable Structure Including Discontinuous Storage Elements and Spacer Control Gates in a Trench, Ser. No. 11/188,584, filed Jul. 25, 2005.
The invention is in the field of semiconductor devices and, more particularly, nonvolatile storage devices.
Nonvolatile storage is an important element in the design of substantially all electronic devices. In the field of wireless and portable electronic devices, nonvolatile storage must be compact and consume little power. Various nonvolatile storage cells have been proposed and implemented. Included among these conventional cells are planar storage cells and storage cells employing floating gates as a charge storage element. A planar storage cell is characterized by a planar transistor channel region typically located in proximity to an upper surface of the wafer substrate. While planar technology is mature and well understood, planar devices consume an undesirably large amount of wafer area.
With respect to the charge storage element, conventional floating gates have been made of a contiguous strip of a conductive material such as polysilicon. Conductive floating gates present a problem in devices with very thin dielectrics. Thin dielectrics are particularly susceptible to pin hole defects. With a conductive floating gate, all of the stored charge on the floating gate can leak off through a single pin hole defect in the dielectric. Moreover, conventional floating gates are not suitable for localized programming in which injected electrons are confined to a specific location of the charge storage element. Localized programming offers the prospect of multiple bit storage cell, where each bit is associated with a specific region of the charge storage element. Accordingly, it would be desirable to implement a multiple bit storage device suitable for use in an advanced processes employing very thin dielectrics where the design of the implemented device consumes less area than planar devices and devices employing conventional charge storage elements.
The present invention is illustrated by way of example and not limited by the accompanying figures, in which like references indicate similar elements, and in which:
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.
In one aspect, a semiconductor-based storage cell and a corresponding fabrication process employ a trench etched into a semiconductor layer, a select gate formed in the trench, a charge storage stack formed in the trench overlying the select gate, and a control gate overlying the charge storage stack. The depth of the trench exceeds the height of the select gate so that a gap exists between the top of the trench and the top of the select gate. The charge storage stack preferably includes a set of discontinuous storage elements (DSEs). In this embodiment, the DSEs may be silicon nanocrystals or nanoclusters, which are small, discreet silicon structures embedded in a dielectric layer and capable of holding a positive or negative charge. Because DSEs are not physically or electrically connected to each other, DSEs are less susceptible to charge loss through pin holes in the dielectric layer than conventional storage elements such as conventional polysilicon floating gate structures.
Referring to the drawings,
In one embodiment, dielectric liner 104 is silicon oxide, which may be thermally formed (grown) or deposited using CVD (chemical vapor deposition). Hard mask 106 is preferably a dielectric that can be selectively etched with respect to semiconductor layer 102. Hard mask 106 is preferably CVD silicon nitride, which is desirable for its ability to inhibit oxidation of an underlying semiconductor thereby providing a mask for a thermal oxidation process.
Referring now to
In
In
In
Referring to
Turning now to
In
Turning now to
Following formation of bottom dielectric 135, a layer of DSEs are formed overlying bottom dielectric 135. In the depicted embodiment, DSEs 140 (sometimes referred to as nanocrystals) are a set of discreet accumulations of a material capable of storing a charge. Suitable materials include silicon, polysilicon, other semiconductors, metals such as titanium, tungsten, tantalum, aluminum, copper, platinum, and the like, and dielectrics such as silicon nitride or silicon oxynitride. In the preferred implementation, DSEs 140 are silicon DSEs (silicon nanocrystals). In this implementation, DSEs 140 may be formed in any one of a variety of ways, preferably without requiring any photolithography steps. One DSE formation technique includes depositing an amorphous silicon layer and heating it to form the nanocrystals. Another technique is to deposit the nanocrystals using chemical vapor deposition (CVD). DSEs 140 may have various shapes, including hemispherical and spherical, depending upon the deposition technique employed. In one implementation, DSEs 140 are approximately 5 nm in diameter and are spaced at a predominantly uniform spacing of approximately 5 nm. Regardless of the formation technique used, each DSE 140 is a particle of silicon that is electrically and physically isolated from its neighbors.
Referring now to
Turning now to
A top view of storage device 100 depicted in
Storage device 100 includes a pair of injection regions 170-1 and 170-2 programmable using source side injection (SSI) programming. Programming table 190 of
Programming a first bit that is associated with SSI injection 170-1 includes biasing source/drain region 112-1 to a first programming voltage (VP1), biasing control gate 160 to a second programming voltage (VP2), biasing first and select gates 130-1 and 130-2 to a third programming voltage (VP3), biasing source/drain region 112-2 and semiconductor layer 102 to a fourth programming voltage (VP4). For one NMOS embodiment of storage cell 100, VP1 (source/drain programming voltage), VP2, control gate programming voltage, and VP3 (select gate programming) are all in the range of approximately 5 V to 9 V while VP4 is 0 V (ground).
Exemplary programming values are depicted in
Erasing the programmed injecting region includes biasing the control gate to a first erase voltage (VE1) and biasing the semiconductor layer to a second erase voltage (VE2). The select gates 130 may be biased to VE1 or some other voltage during erase to insure complete erase. In addition, erase can be accomplished in either polarity. Thus, for example, VE1 can be +/−6V, while VE2 is −/+6V. The erase conditions apply to each of the programming tables.
A second embodiment of a storage cell 200 is depicted in the cross section of
Programming table 191 of
Programming table 192 of
A third embodiment of storage cell 200 is depicted in the cross section of
This embodiment of storage cell 200 includes four storage devices 100-1 through 100-4. Storage device 100-1 includes control gate 160, select gate 130-1, source/drain region 112-1, and diffusion region 164-1. Storage device 100-2 includes control gate 160, select gate 130-1, source/drain region 112-1, and diffusion region 164-2. Storage device 100-3 includes control gate 160, select gate 130-2, source/drain region 112-2, and diffusion region 164-1. Storage device 100-4 includes control gate 160, select gate 130-2, source/drain region 112-2, and diffusion region 164-2.
In the depicted embodiment desirable for its symmetrical design, diffusion regions 164-1 and 164-2 are arranged in a straight line fashion with both contacts being equidistant from source/drain regions 112-1 and 112-2. In another embodiment of storage cell 200, diffusion regions 164-1 and 164-2 are arranged in a diagonal configuration with diffusion region 164-1 being closer to source/drain region 112-1 and diffusion 164-2 being closer to source/drain region 112-2. This embodiment simplifies the design of back end metalization (not depicted) that will connect to the contact structures.
Each storage device 100-1 through 1004 has a corresponding SSI injection region 170-1 through 1704. By including contacts on opposing sides of control gate 160, this third embodiment is able to program two SSI injection regions within a single charge storage stack 155.
Programming table 193 of
In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, although the depicted embodiment is an NMOS transistor embodiment, PMOS embodiments are equally encompassed. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention.
Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
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