Phase change memory device and fabrication method thereof

[0001] 1. Field of the Invention

[0002] The invention relates to a memory device and fabrication thereof, and more particularly relates to a phase change memory device and fabrication thereof.

[0003] 2. Description of the Related Art

[0004] Phase change memory devices have many advantages, such as high speed, lower power consumption, large capacity, greater endurance, better process integrity and lower cost. Phase change memory devices can thus be used as independent or embedded memory devices with high data integrity. Due to the described advantages and others, phase change memory device may replace flash memory.

[0005] To serve as a viable alternative, phase change memory technology requires highly reliable, high-speed, low-current, low-voltage operation. Current phase change memory technology employs heating at the interface between a metal electrode contact and the phase change material. This heating method is highly inefficient, however, and requires a lengthy programming duration of up to several hundred nanoseconds, and may also require a substantial amount of current (0.5 mA or greater). The described inefficient heating is also characterized by non-uniform temperature distribution within the phase change region, diminishing reliability due to accumulation of incomplete phase change in the programming volume. It is thus desirable to devise a phase change memory cell structure design that avoids these disadvantages.

[0006] A detailed description is given in the following embodiments with reference to the accompanying drawings. These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by the invention.

[0007] The invention provides a phase change memory device. A first conductive spacer is provided. A second conductive spacer is under the first conductive spacer. A phase change layer comprises a first portion substantially parallel to the first and second conductive spacers, and a second portion on top of the second conductive spacer, wherein the second conductive spacer is electrically connected to the first conductive spacer through the second portion of the phase change layer.

[0008] The invention provides a phase change memory device. A first dielectric layer contains a first opening. A second opening is provided under the first opening. The width of first opening is greater than the width of the second opening. A first conductive spacer is on a sidewall of the first opening. A second conductive spacer is on a sidewall of the second opening. A first dielectric spacer is optionally adjacent to the first conductive spacer. A second dielectric spacer is optionally adjacent to the second conductive spacer. A phase change layer fills both openings and is electrically coupled in parallel with the first conductive layer and covers the top of the second conductive spacer. A dielectric layer is optionally deposited on the phase change layer.

[0009] The invention provides a method for forming a phase change memory device. In an exemplary embodiment of the method, a first dielectric layer is first formed on a bottom electrode. The first dielectric layer is patterned to form a first opening. The dielectric layer is patterned to form a second opening under the first opening, expanding the first opening in the process. A conductive-spacer layer is formed on the first dielectric layer and in the first and second openings. A dielectric-spacer layer is optionally formed on the conductive-spacer layer. The conductive-spacer layer and the optional dielectric-spacer layer are anisotropically etched. A phase change layer is conformally deposited in the first and second opening. An additional dielectric layer is optionally formed on the phase change layer.

[0013] The following description is of the contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. Embodiments of the invention, which provides a phase change memory device, will be described in greater detail by referring to the drawings that accompany the invention. It is noted that in the accompanying drawings, like and/or corresponding elements are referred to by like reference numerals.

[0014] The dominant electrical resistance component in a phase change memory cell is the phase change material itself or an external heater. An external heater component is undesirable due to the poor thermal uniformity thereof (temperature decreases with distance from the heater). Thus, an embodiment of the invention uses the phase change material to be the dominant electrical resistance. In order to reduce both programming current and voltage, the phase change volume must be minimized. Additionally, to minimize non-uniform heating the phase change volume should not be in direct contact with an effective heat sink path. This is achieved by imposing a lateral offset between the top and bottom metal contacts and the phase change material of an embodiment of the invention; thus, thermal resistance is increased in the heat loss path.

[0015] FIG. 1A˜FIG. 1J show intermediate cross sections of fabrications of a phase change memory device of an embodiment of the invention. First, referring to FIG. 1A, a first dielectric layer 104, such as oxide, is deposited on a bottom electrode 102, such as W or TiAlN. The bottom electrode 102 could be one terminal of a transistor or a diode. A photoresist layer is coated on the first dielectric layer 104 and then defined by lithography to constitute a photoresist pattern 108. Thereafter, the first dielectric layer 104 is etched using the photoresist pattern 108 as a mask to form a first opening 106, partially through the first dielectric layer 104, for a time shorter than that required for completely penetrating the first dielectric layer 104. An etch stop layer (not shown) may alternatively be used.

[0016] Referring to FIG. 1B, the photoresist pattern 108 is trimmed, for example, about 10-15 nm is removed laterally from each side of the first opening 106. In an embodiment of the invention, the trimming of the photoresist pattern 108 uses oxygen plasma.

[0017] Referring to FIG. 1C, after the trimming process, the first dielectric layer 104 (and optional etch-stop) is again etched, until the bottom electrode 102 is exposed. The etching may be anisotropic etching. A two-level opening structure 112, comprising a first opening 106, and a second opening 110, is thus obtained by self-aligned etching. As shown in the figure, the second opening 110 is under the first opening 106, and the width of the first opening 106 is greater than the width of the second opening 110.

[0018] Referring to FIG. 1D, a conductive-spacer layer 114 is conformally deposited (e.g., by PVD or ALD) on the first dielectric layer 104 and into the opening. Specifically, the conductive-spacer layer 114 covers sidewalls of the first and second openings 106 and 110. In an embodiment of the invention, the conductive-spacer layer 114 is TaN or TiN, and may be about 2-10 nm. When forming conductive spacers, adequately flat contacts at the conductive-spacer layer 114 are required to be provided at the contact surface. Therefore, in one embodiment of the invention, a dielectric-spacer layer 116, such as CVD oxide, may be deposited on the conductive-spacer layer 114.

[0019] Referring to FIG. 1E, the conductive-spacer layer 114 and the dielectric-spacer layer 116 are etched anisotropically, in which the horizontal portions thereof can be substantially removed. The conditions of the etching process are fine tuned to have a substantially close etching rate and selectivity between the conductive-spacer layer 114 and the dielectric-spacer layer 116. Note that the process steps described constitute a specific structure. As shown in FIG. 1E, an adjacent second conductive spacer 122 and second dielectric spacer 124 pair is under an adjacent first conductive spacer 118 and a first dielectric spacer 120 pair, wherein the second conductive spacer 122 and second dielectric spacer 124 pair is not connected to the adjacent first conductive spacer 118 and first dielectric spacer 120 pair.

[0020] Referring to FIG. 1F, a phase change layer 126 is then deposited by, for example, PVD, CVD or ALD, to cover conductive spacers 118 and 122, and dielectric spacers 120 and 124. In an embodiment of the invention, the deposited phase change layer 126 is a layer about 1-3 nm thick. The phase change layer 126 can comprise a chalcogenide compound, for example, a chalcogenide compound comprising Ge—Te—Sb, GeSb, or SbTe. As shown in FIG. 1F, the phase change layer 126 comprises a vertical portion 107 on sidewalls of the first and second dielectric spacers 120 and 124, and a horizontal portion 109 on top of the second dielectric and conductive spacers 122 and 124. As well, the vertical portion 107 and the horizontal portion 109 of the phase change layer 126 are substantially perpendicular. The bottom electrode 102 may be partially etched during the conductive spacer etching process; this however is inconsequential as that portion of the electrode is not in the current path.

[0021] In FIG. 1G, the same tool used for deposition of the phase change layer 126 can optionally deposit a protective layer 128. The protective layer 128 may be an oxide such as zirconium oxide or tantalum oxide and may serve the dual purpose of protecting the phase change layer 126 from oxidation after removal from the deposition tool, and reduction of void formation during program cycling.

[0022] Referring to FIG. 1H, a second dielectric layer 130 is deposited, by CVD, for example, on the phase change layer 126. The first opening 106 is filled with layers to constitute a first via 132, and the second opening 110 is filled with layers to constitute a second via 134. (The labels 106 and 110 are shown in FIG. 1G only.) In FIG. 1I, the second dielectric layer 130 is then planarized, by CMP for example, to present a planar surface and remove unnecessary portions. Note that a portion of phase change layer 126 beyond the first and second openings 106 and 110 may be removed by the planarization with no effect as the removed portion is not critical.

[0023] FIG. 1J shows deposition of the top electrode 136, such as W or TiAlN, and subsequent patterning by lithography and etching, to remove a portion of the phase change layer 126 remaining on the top surface of the first dielectric layer 104. In another embodiment of the invention, the top electrode 136 may electrically connect with the first conductive spacer 118.

[0024] FIG. 1K shows an operating mechanism of the phase change memory device of an embodiment of the invention. Although the phase change layer 126 contacts the electrode, because it is very thin, current 191 will overwhelmingly go through the conductive spacer contacts 118 and 122. Note that the active region 193 will be in the horizontal portion 109 of the phase change layer 126 between the top of the second conductive spacer contact 122 and the foot of the first conductive spacer contact 118. The effective heating cross-section area is thus the product of the thickness of the phase change layer 126 and the circumference of the second via 134. The effective phase change length is the distance between the top of the second conductive spacer 122 and the foot of the first conductive spacer 118.

[0025] According to one of the embodiments described, the structure of the invention allows simultaneous reduction of current and voltage and thermal non-uniformity in the programming region of the phase change memory device. In addition, the fabrication process flow is simple, and, alignment is not a critical issue with the proposed self-aligned process.

[0026] While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.