Patent Application
20240008374
The present invention generally relates to semiconductor device fabrication, and, more particularly, to semiconductor devices that feature reduced reset currents.
Phase change material (PCM) cells can be used to store information in a device. The resistance of a PCM cell may be changed by applying heat that causes a partial or complete change in the phase of the PCM cell. While the resistance can be changed in one direction (e.g., decreased) in a continuous fashion, changing the resistance in the opposite direction is accomplished all at once with a reset operation. This reset operation may use a higher current than incremental resistance changes.
A method of forming a memory cell includes forming a hole in an interlayer dielectric to expose an end of a conductive top electrode. A phase change material is conformally deposited on surfaces of the hole. A remaining portion of the hole is filled with a dielectric material after conformally depositing the phase change material.
A semiconductor device includes a dielectric layer having a depression in a top surface of the dielectric layer. A phase change material layer is on a surface of the depression. A dielectric structure on the phase change material layer fills the depression. A first electrode is in electrical contact with phase change material layer. A second electrode is in electrical contact with the phase change material layer.
A memory cell includes a phase change material liner beneath, and laterally surrounding, a dielectric core. A dielectric layer is around the phase change material liner. A first electrode is in electrical contact with phase change material layer.
These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.
The following description will provide details of preferred embodiments with reference to the following figures wherein:
Phase change material (PCM) cells may be formed using a thin layer of PCM, for example formed conformally on a sidewall of a structure. Multiple electrical contacts may be formed to the thin PCM layer, defining a region of relatively high current flux, which may define a region where amorphization of the PCM may occur. The thin layer of PCM reduces the amount of current needed to heat the PCM to reset the resistance. In addition, this approach to PCM cell fabrication improves size uniformity of the contact areas and provides the ability to tune device resistance using the dimensions of the PCM layer, rather than by doping.
PCMs may include materials where two distinct solid phases of matter are available within the operating conditions of the device. For example, the PCM may be able to stably occupy an amorphous phase and a crystalline phase at a given temperature and pressure. For a PCM, the resistivities of these two phases may differ, for example with the amorphous state having a higher resistivity than the crystalline state. Within a given PCM cell, the two phases may coexist in differing proportions. For example, a PCM cell that is entirely in the amorphous state may have a first resistance, a PCM cell that is entirely in the crystalline state may have a second resistance, and a PCM cell that is partially in the amorphous state and partially in the crystalline state may have a third resistance that is between the first resistance and the second resistance.
During operation, the PCM cell may start in the amorphous state. To reduce the resistance of the PCM cell, a first current pulse may be applied to the PCM cell. The first current pulse may provide enough energy to change the phase of the PCM from the amorphous phase to the crystalline phase. This change may be performed incrementally, with each pulse changing the phase of a portion of the PCM, until the entire PCM has changed to the crystalline phase.
To reverse the change of the resistance, a reset current pulse may be applied. The reset current pulse may generate enough heat to reset the entire PCM to the amorphous phase. The reset current pulse may generally be larger than the magnitude of the first current pulse, and may depend on properties of the PCM, such as the thickness of the layer of PCM. The current flux may be increased due to the relatively small contact area of the side-contact heater. Thus, by using a thin PCM layer as the PCM cell, the magnitude of the reset current pulse can be reduced. The resistance may be tuned by adjusting the dimensions of the contact, and the side-contact structure provides good thermal isolation between devices.
Referring now to
A cross-sectional plane AA is indicated. Subsequent views will show both top-down and cross-sectional views of steps in the fabrication of PCM devices. As used herein, the terms “electrode” and “wire” may be used interchangeably, as both refer to conductive structures.
Referring now to
The bottom electrode 202 may be formed by, e.g., etching a trench in the dielectric layer 204 and then depositing a conductive material to fill the trench. The trench may be formed by, e.g., establishing a pattern mask using any appropriate photolithographic process, followed by a selective anisotropic etch that uses the pattern mask to block the etch in regions outside of the trench. The conductive material may be deposited using any appropriate conformal or directional deposition process, followed by a chemical mechanical planarization (CMP) process that exposes the top surface of the dielectric layer 204.
Reactive Ion Etching (RIE) is a form of plasma etching in which during etching the surface to be etched is placed on a radio-frequency powered electrode. Moreover, during RIE the surface to be etched takes on a potential that accelerates the etching species extracted from plasma toward the surface, in which the chemical etching reaction is taking place in the direction normal to the surface. Other examples of anisotropic etching that can be used at this point of the present invention include ion beam etching, plasma etching or laser ablation.
The conductive material of the bottom electrode 202 may be formed by any appropriate process including, e.g., chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or gas cluster ion beam (GCIB) deposition. CVD is a deposition process in which a deposited species is formed as a result of chemical reaction between gaseous reactants at greater than room temperature (e.g., from about 25° C. about 900° C.). The solid product of the reaction is deposited on the surface on which a film, coating, or layer of the solid product is to be formed. Variations of CVD processes include, but are not limited to, Atmospheric Pressure CVD (APCVD), Low Pressure CVD (LPCVD), Plasma Enhanced CVD (PECVD), and Metal-Organic CVD (MOCVD) and combinations thereof may also be employed. In alternative embodiments that use PVD, a sputtering apparatus may include direct-current diode systems, radio frequency sputtering, magnetron sputtering, or ionized metal plasma sputtering. In alternative embodiments that use ALD, chemical precursors react with the surface of a material one at a time to deposit a thin film on the surface. In alternative embodiments that use GCIB deposition, a high-pressure gas is allowed to expand in a vacuum, subsequently condensing into clusters. The clusters can be ionized and directed onto a surface, providing a highly anisotropic deposition.
CMP is performed using, e.g., a chemical or granular slurry and mechanical force to gradually remove upper layers of the device. The slurry may be formulated to be unable to dissolve, for example, the work function metal layer material, resulting in the CMP process's inability to proceed any farther than that layer.
Referring now to
As shown in the cross-sectional view, additional dielectric material 304 is deposited over the bottom electrode 202. This new dielectric material 304 may be formed from the same electrically insulating material as the first dielectric layer 204 or may be a different dielectric material. Any appropriate deposition process may be used, such as CVD.
Any appropriate number of conductive wires 302 may then be formed over the new dielectric material 304, for example by depositing a layer of conductive material and then patterning the layer of conductive material to form wires. Any appropriate deposition method, such as CVD, may be used to deposit the conductive material. A top layer of dielectric material 306 may then be deposited over the wires 302 using any appropriate process, such as CVD. The conductive wires 302 may have any appropriate thickness, for example between about 5 nm and about 50 nm.
Referring now to
A hole 402 is etched down, through the dielectric layers 304 and 306 and through the wires 302 to expose the bottom electrode 202. Although the hole 402 is shown as having similar dimensions to the bottom electrode 202, it should be understood that the bottom electrode 202 may be wider or narrower than the hole 402.
The wires 302 are separated into contact electrodes 108, with a set of wires on each side of the hole 402. In this example the three wires 302 are separated into six contact electrodes 108. Any appropriate number of such contact electrodes 108 may be formed in this manner. In some embodiments, as described in greater detail below, the hole 402 may be positioned at one end of the wires 302, such that the wires 302 are not split into halves. In such embodiments, an odd number of contact electrodes 108 may be formed.
The etch may be performed using any appropriate selective anisotropic etch that removes material from the dielectric layers but that does not substantially damage the bottom electrode 202. Alternatively, multiple distinct etches may be performed, including a first etch to expose the wires 302, a second etch to remove the exposed material of the wires 302, and a third etch to expose the bottom electrode 202. Using multiple etches can promote selectivity between the final etch and the bottom electrode 202, where a single etch might remove too much of the conductive material of the bottom electrode 202.
Referring now to
A layer of PCM 102 is conformally deposited in the hole 402. It is contemplated that the layer of PCM 102 may be deposited to a thickness between about 2 nm to about 50 nm, but it should be understood that other thicknesses may be used instead. The PCM layer 102 may be formed from any appropriate material, such as GeSbTe in any appropriate proportions, SiGeSbTe in any appropriate proportions, Sb2Te3, GeTe, and Cr2Ge2Te6. In some cases the PCM layer 102 may be doped, such as with nitrogen, while in other embodiments the PCM layer 102 may be undoped. The dielectric structure 104 may be formed from a dielectric material that is distinct from, e.g., the top dielectric layer 306, and may be deposited using any appropriate deposition process to fill the hole 402. The dielectric structure 104 may be formed from, e.g., silicon nitride or a high-k dielectric material. After deposition of the PCM material and the dielectric material, a CMP process may be performed to expose the top dielectric layer 306, with the CMP process stopping on the material of the top dielectric layer 306.
Referring now to
As can be seen from this view, the contact area between the side electrodes 106 and the PCM layer 102 may be small relative to the contact area between the underlying bottom electrode 202 and the PCM layer 102. The lower thermal conductivity of smaller electrodes 106 localizes heat in the PCM layer 20 around the smaller electrodes 106. This decreases the chance that heat will localize between the electrodes, causing amorphized PCM to occur further away from the electrodes, which could result in current finding alternative paths through the PCM layer 102. Localizing amorphization increases the predictability in resistance changes in the PCM layer 102. The contact dimension between small electrodes 106 and the PCM layer 102 may be in a range between about and about 20 nm, while the contact dimension between the bottom electrode 202 and the PCM layer 102 may be between about 20 nm and about 500 nm, in either dimension. It is specifically contemplated that the contact area between the larger electrode and the PCM layer 102 may be greater than about twice that of the contact area between the smaller electrode(s) and the PCM layer 102.
Referring now to
Referring now to
The thermal sink layer 802 normalizes the temperature of the PCM layer 102 near its surface. As a result, the portions of the PCM layer 102 closer to the electrodes 108 will experience higher temperatures during a reset operation, while the portions closer to the thermal sink layer 802 will remain relatively cool. This can limit the region that returns to an amorphous state to the area around the electrodes 108, which can make the resistance of the device easier to control.
Referring now to
Referring now to
Referring now to
In the top-down view, the dielectric material 1106 is not shown, to illustrate the relationship of the upper contact electrode 908 and the lower contact electrode 906. In some embodiments, the upper contact electrode 908 may be narrower than the lower contact electrode 906, to tune the properties of the PCM device by controlling the current flux at the interface between the upper contact electrode 908 and the PCM layer. In some embodiments, the lower contact electrode 906 may be made narrower than the upper contact electrode 908, or the wires may be made with the same cross-sectional dimensions.
Referring now to
PCM material is formed in the hole by any appropriate conformal deposition process, such as CVD. New dielectric material is then used to fill the remaining space in the hole, from a dielectric material that is distinct from the top layer of dielectric material 1106. PCM material and the new dielectric material may then be polished down using a CMP process that stops on the material of the top dielectric layer 1106, leaving PCM layer 902 and dielectric structure 904.
Referring now to
Referring now to
Block 1404 then determines the resistance of the PCM cell using Ohm's law, which may be expressed as R=V/I. Thus, dividing the applied read voltage V by the current I that passes through the PCM cell gives the resistance value R. This resistance may correspond to a predetermined data value, for example with a high resistance value corresponding to a logical ‘one’ and with a low resistance value corresponding to a logical ‘zero.’ Intermediate values are also possible. For example, if the resistance range is divided into regions, then each region may correspond to a different binary sequence. In an example with four such regions, the possible data values may be ‘00, “01,”10,’ and ‘11.’
Referring now to
Block 1504 then applies an adjustment voltage pulse between the first electrode and the second electrode. The adjustment voltage pulse may be larger than the read voltage pulse, and may cause a partial or total phase change of the PCM cell, for example from an amorphous phase to a crystalline phase. The duration of the pulse may determine how much of the PCM cell changes in phase, and thus may correspond to the determined resistance change.
Referring now to
Block 1604 then applies a reset voltage pulse between the first electrode and the second electrode. The reset voltage pulse may be larger than the adjustment voltage pulse, and may return the PCM cell to an amorphous state.
Referring now to
Block 1704 etches a hole 402 through the wires 302 and parts of the substrate to expose, e.g., the bottom electrode 202 or the thermal sink layer 802. The hole 402 may be formed by any appropriate anisotropic etch process, such as an RIE, using a photolithographically patterned mask. The hole 402 cuts into the wire(s) 302, forming contact electrodes 108.
Block 1706 conformally deposits PCM in the hole 402 using any appropriate conformal deposition process, such as CVD. Block 1708 then fills the remainder of the hole 402 with a dielectric material. Block 1709 polishes down the PCM and the dielectric material to form PCM layer 102 and dielectric structure 104. Block 1710 then forms upper wires 106 to meet the contact electrodes 108, for example by etching down through a passivating dielectric layer and depositing conductive material.
In some embodiments, an upper electrode may be formed over the PCM layer 102. The upper electrode may be formed along with the upper contacts, for example etching down through the passivating dielectric layer in a region over the PCM layer 102 and depositing conductive material that makes contact with a top surface of the PCM layer 102.
Referring now to
Referring now to
In such embodiments, read and write currents may be established between one of the top/bottom electrodes and one of the side electrodes to form a memory cell. For example, the left-side electrode and the bottom electrode may form one memory cell, while the right-side electrode and the top electrode may form a second memory cell.
It is to be understood that aspects of the present invention will be described in terms of a given illustrative architecture; however, other architectures, structures, substrate materials and process features and steps can be varied within the scope of aspects of the present invention.
It will also be understood that when an element such as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements can also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements can be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.
The present embodiments can include a design for an integrated circuit chip, which can be created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer can transmit the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed.
Methods as described herein can be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case, the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.
It should also be understood that material compounds will be described in terms of listed elements, e.g., SiGe. These compounds include different proportions of the elements within the compound, e.g., SiGe includes SixGe1-x, where x is less than or equal to 1, etc. In addition, other elements can be included in the compound and still function in accordance with the present principles. The compounds with additional elements will be referred to herein as alloys.
Reference in the specification to “one embodiment” or “an embodiment”, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”, as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment.
It is to be appreciated that the use of any of the following “/”, “and/or”, and “at least one of”, for example, in the cases of “A/B”, “A and/or B” and “at least one of A and B”, is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of both options (A and B). As a further example, in the cases of “A, B, and/or C” and “at least one of A, B, and C”, such phrasing is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of the third listed option (C) only, or the selection of the first and the second listed options (A and B) only, or the selection of the first and third listed options (A and C) only, or the selection of the second and third listed options (B and C) only, or the selection of all three options (A and B and C). This can be extended, as readily apparent by one of ordinary skill in this and related arts, for as many items listed.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as dwell, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, can be used herein for ease of description to describe one elements or feature's relationship to another element(s) or feature(s) as illustrated in the FIGS. It will be understood that the spatially relative: terms are intended to encompass different orientations of the device in use or operation addition the orientation depicted in the FIGS. For example, if the device in the FIGS. is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term below can encompass both an orientation of above and below. The device can be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein can be interpreted accordingly. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers can also be present.
It will be understood that, although the terms first, second, etc. can be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element discussed below could be termed a second element without departing from the scope of the present concept.
The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be accomplished as one step, executed concurrently, substantially concurrently, in a partially or wholly temporally overlapping manner, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.
Having described preferred embodiments of a vertical ultra-thin PCM cell (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.
