Patent Grant
7541607
The present invention relates to an electrically rewritable non-volatile memory element and to a method of manufacturing the element. More specifically, the present invention relates to an electrically rewritable non-volatile memory element having a recording layer that includes phase change material, and to a method of manufacturing the element.
Personal computers and servers and the like use a hierarchy of memory devices. There is lower-tier memory, which is inexpensive and provides high storage capacity, while memory higher up the hierarchy provides high-speed operation. The bottom tier generally consists of magnetic storage such as hard disks and magnetic tape. In addition to being non-volatile, magnetic storage is an inexpensive way of storing much larger quantities of information than solid-state devices such as semiconductor memory. However, semiconductor memory is much faster and can access stored data randomly, in contrast to the sequential access operation of magnetic storage devices. For these reasons, magnetic storage is generally used to store programs and archival information and the like, and, when required, this information is transferred to main system memory devices higher up in the hierarchy.
Main memory generally uses dynamic random access memory (DRAM) devices, which operate at much higher speeds than magnetic storage and, on a per-bit basis, are cheaper than faster semiconductor memory devices such as static random access memory (SRAM) devices.
Occupying the very top tier of the memory hierarchy is the internal cache memory of the system microprocessor unit (MPU). The internal cache is extremely high-speed memory connected to the MPU core via internal bus lines. The cache memory has a very small capacity. In some cases, secondary and even tertiary cache memory devices are used between the internal cache and main memory.
DRAM is used for main memory because it offers a good balance between speed and bit cost. Moreover, there are now some semiconductor memory devices that have a large capacity. In recent years, memory chips have been developed with capacities that exceed one gigabyte. DRAM is volatile memory that loses stored data if its power supply is turned off. That makes DRAM unsuitable for the storage of programs and archival information. Also, even when the power supply is turned on, the device has to periodically perform refresh operations in order to retain stored data, so there are limits as to how much device electrical power consumption can be reduced, while yet a further problem is the complexity of the controls run under the controller.
Semiconductor flash memory is high capacity and non-volatile, but requires high current for writing and erasing data, and write and erase times are slow. These drawbacks make flash memory an unsuitable candidate for replacing DRAM in main memory applications. There are other non-volatile memory devices, such as magnetoresistive random access memory (MRAM) and ferroelectric random access memory (FRAM), but they cannot easily achieve the kind of storage capacities that are possible with DRAM.
Another type of semiconductor memory that is being looked to as a possible substitute for DRAM is phase change random access memory (PRAM), which uses phase change material to store data. In a PRAM device, the storage of data is based on the phase state of phase change material contained in the recording layer. Specifically, there is a big difference between the electrical resistivity of the material in the crystalline state and the electrical resistivity in the amorphous state, and that difference can be utilized to store data.
This phase change is effected by the phase change material being heated when a write current is applied. Data is read by applying a read current to the material and measuring the resistance. The read current is set at a level that is low enough not to cause a phase change. Thus, the phase does not change unless it is heated to a high temperature, so data is retained even when the power supply is switched off.
In order to efficiently heat phase change material using a write current, it is desirable to use a structure that does not readily allow diffuse of the heat generated by the write current. In one structure for achieving, the upper surface of a recording layer is covered with a top electrode having low thermal conductivity to cut down on heat dissipation to a bit line that has a large thermal capacity and high thermal conductivity. See U.S. Pat No. 5,536,947, “Writing Current Reduction for High-density Phase-change RAM,” Y. N. Hwang, S. H. Lee, S. J. Ahn, S. Y. Lee, K. C. Ryoo, H. S. Hong, H. C. Koo, F. Yeung, J. H. Oh, H. J. Kim, W. C. Jeong, J. H. Park, H. Horii, Y. H. Ha, J. H. Yi, G. H. Hoh, G. T. Jeong, H. S. Jeong, and Kinam Kim,” IEEE 2003, and “An Edge Contact Type Cell for Phase Change RAM Featuring Very Low Power Consumption, ” Y. H. Ha, J. H. Yi, H. Horii, J. H. Park, S. H. Joo, S. O. Park, U-In Chung, and J. T. Moon, 2003 Symposium on VLSI Technology Digest of Technical Papers.
However, because in the structure described in above literary documents the planar recording layer is sandwiched between a top electrode and a bottom electrode, the thickness of the recording layer has to be greatly increased to provide a sufficient decrease in the dissipation of heat to the bit line. Increasing the thickness of the recording layer means it takes longer to form the layer, in addition to which it takes a stronger electric field to induce the phase change, especially the change from a high resistance state to a low resistance state. Since the voltage for inducing the phase change therefore has to be increased, so the structure is not one that is suitable for achieving low-voltage devices.
Thus, it is difficult to adequately raise heating efficiency with devices having a conventional structure. This makes it difficult to reduce write current, and also makes it difficult to speed up write operations.
It is therefore an object of the present invention to provide an improved electrically rewritable non-volatile memory element having a recording layer that contains phase change material, and a method for manufacturing the memory element.
Another object of the invention is to provide an electrically rewritable non-volatile memory element with high heating efficiency having a recording layer that contains phase change material, and a method for manufacturing the memory element.
The above and other objects of the present invention can be accomplished by a non-volatile memory element comprising a bottom electrode, a bit line provided on the bottom electrode, and a recording layer containing phase change material connected to the bottom electrode and the bit line, wherein the bit line is in contact with a growth initiation surface of the recording layer.
Thus, the memory element of this invention has a structure in which the bit line is in contact with the growth initiation surface of the recording layer. This structure can be obtained by forming the bit line before the recording layer, resulting in a three-dimensional structure unlike that of the prior art. This decreases the area of contact between the recording layer and the bit line, decreasing heat dissipation to the bit line without increasing the thickness of the recording layer. With this three-dimensional structure, moreover, there is no top electrode between the bit line and the recording layer, keeping down the complexity of the fabrication process.
The memory element may also include an interlayer insulation layer between the bottom electrode and the bit line, and preferably at least part of the recording layer is formed in a through-hole provided in the interlayer insulation layer. This makes it easier to implement the recording layer as a three-dimensional structure.
The growth initiation surface of the recording layer may be in contact with the growth termination surface of the bit line, or may be in contact with the etched surface of the bit line formed by patterning.
The memory element may also include an etching stopper provided on the bit line, and preferably the through-hole passes through the interlayer insulation layer, the bit line and the etching stopper. Covering the bit line with the etching stopper prevents the bit line being damaged when processing the recording layer.
The above and other objects of the present invention can also be accomplished by a non-volatile memory element comprising a bottom electrode, a bit line provided on the bottom electrode, and a recording layer containing phase change material connected to the bottom electrode and the bit line, wherein the bit line is in contact with at least one of a side surface and a bottom surface of the recording layer.
This structure, too, can be obtained by forming the bit line before the recording layer, resulting in a three-dimensional structure unlike that of the prior art, that decreases heat dissipation to the bit line. In this case too, there is no need for a top electrode between the bit line and the recording layer.
A method for manufacturing a non-volatile memory element according to the present invention comprising a first step for forming a bottom electrode, a second step for forming an interlayer insulation layer covering the bottom electrode, a third step for forming a bit line on the interlayer insulation layer, a fourth step for forming a through-hole in the interlayer insulation layer, and a fifth step for forming a recording layer containing phase change material in the through-hole in contact with the bottom electrode and the bit line.
In accordance with this aspect of the present invention, the bit line is formed before the recording layer, and the recording layer is formed in the through-hole, resulting in a three-dimensional structure having high heating efficiency.
As described above, an electrically rewritable non-volatile memory element having improved heating efficiency, and a method of manufacturing the element can be provided. Therefore, not only a write current can be decreased but also a writing speed can be enhanced.
The above and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein:
Preferred embodiments of the present invention will now be explained in detail with reference to the drawings.
With reference to
A material such as silicon oxide or silicon nitride may be used for the interlayer insulation layers 11 and 13. While it is not an essential requirement, it is preferable to form the interlayer insulation layers 11 and 13 using materials having different etching rates. For example, if the interlayer insulation layer 11 is formed of silicon nitride, it is preferable to use silicon oxide for the interlayer insulation layer 13. The bottom electrode 12 is formed in the shape of a cylinder around the inside wall of a through-hole 11a formed in the interlayer insulation layer 11. Thus, as shown in
As shown in
The bottom electrode 12 is used as a heater plug, constituting part of the heater during data writes. For this, the bottom electrode 12 is preferably formed of a material having a relatively high electrical resistance, such as metal silicides, metal nitrides and nitrides of metal silicides. These include, but are not limited to, TiAlN, TiSiN, TiCN and other such materials.
As shown in
Specifically, as shown in
The recording layer 15 is formed of phase change material. While the phase change material is not particularly limited and may be any material that takes two or more states and in which each state has a different electrical resistance, it is preferable to choose a chalcogenide material. Chalcogenide material means an alloy containing one or more elements such as germanium, antimony, tellurium, indium, and selenium. Examples include binary system alloys such as GaSb, InSb, InSe, Sb2Te3, and GeTe; ternary system alloys such as Ge2Sb2Te5, InSbTe, GaSeTe, SnSb2Te4, and InSbGe; and quaternary system alloys such as AgInSbTe, (GeSn)SbTe, GeSb (SeTe), and Te81Gel15Sb2S2.
The phase change material containing the chalcogenide material can take an amorphous state or a crystalline state. In the amorphous state the resistance is relatively high, and in the crystalline state the resistance is relatively low.
The phase change material is changed to the amorphous state by heating it to or above the melting temperature Tm and then cooling it, as shown by the curve a in
When the recording layer 15 is subjected to a write current, the region of contact between the recording layer 15 and the bottom electrode 12 becomes a heating region P. That is, the phase state of the chalcogenide material in the vicinity of the heating region P can be switched by applying a write current to the recording layer 15. Doing this changes the electrical resistance between the bit line 14 and the bottom electrode 12.
While there is no particular limitation on the thickness of the recording layer 15, in this embodiment, as one example, it is set at a thickness that does not completely fill the through-hole 13a. However, it may be set at a thickness that does completely fill the through-hole 13a. In this embodiment, in addition to being inside the through-hole 13a, a portion of the recording layer 15 is provided outside the through-hole 13a.
The bit line 14 is formed on the interlayer insulation layer 13, in contact with the growth initiation surface 15a of the recording layer 15. As shown in
For the bit line 14, a metallic material is chosen that has low electrical resistance. Examples include aluminum (Al), titanium (Ti), and tungsten (W), or alloys, nitrides or silicides thereof. Specific examples that can be cited include W, WN and TiN. Generally, a metallic material that has a low resistance has a high thermal conductivity, and also, since the bit line 14 has a large thermal capacity, contact with the recording layer 15 in the vicinity of the heating region P causes heat to readily diffuse to the bit line 14, degrading the heating efficiency. However, because in the case of the memory element 10 the recording layer 15 has a three-dimensional structure, it is possible to increase the distance between the heating region P and the bit line 14, without increasing the thickness of the recording layer 15.
An insulation layer 16 is provided on the bit line 14. All of the growth termination surface 15b of the recording layer 15 is covered by the insulation layer 16 (or by the protective insulation layer 17 described below).
The non-volatile memory element 10 thus constituted can be formed on a semiconductor substrate in the form of a matrix to constitute an electrically rewritable non-volatile memory device.
The non-volatile memory device shown in
The structure of the memory element 10 is as has been described with reference to
The structure of each of the memory cells MC used by the memory element 10 is as shown in
As shown in
In the non-volatile semiconductor memory device thus constituted, data reads and writes can be carried out by using the row decoder 101 to activate one of the word lines W1 to Wn, and then passing a current through at least one of the bit lines B1 to Bm. That is, the transistors 103 are switched on in the memory cells of the activated word line, connecting the corresponding bit lines to ground via the memory element 10. In this state, phase changes can be effected in the recording layer 15 by passing write current through the bit lines selected by the column decoder 102.
Specifically, when a prescribed amount of current is used to raise the temperature of the phase change material of the recording layer 15 to at least the melting temperature Tm, shown in
Data can be read by using the row decoder 101 to activate one of the word lines W1 to Wn and then passing a read current through at least one of the bit lines B1 to Bm. A memory cell MC in which the recording layer 15 is in the amorphous phase has a high resistance and a memory cell MC in which the recording layer 15 is in the crystalline phase has a low resistance, so the phase state of the recording layer 15 can be known by using a sense amplifier (not shown) to detect this.
Phase states of the recording layer 15 can be associated with stored logical values. If an amorphous phase state is defined as “0” and a crystalline phase state as “1”, for example, one binary bit of information can be stored in a single memory cell. Also, when moving from the amorphous phase to the crystalline phase, the crystallization ratio can be controlled in multiple levels or linearly by adjusting the time the recording layer 15 is held at a temperature that is not lower than the crystallization temperature Tx and lower than the melting temperature Tm. With multilevel control of the ratio between the amorphous phase and the crystalline phase, two or more bits of data can be stored in a single memory cell, while with linear control, information can be stored as analog data.
The method of manufacturing the non-volatile memory element 10 will now be described.
First, with reference to
The bit line 14 is formed by forming the selected metallic material on the interlayer insulation layer 13, and then patterning it. As a result, growth initiation surface 14a of the bit line 14 is in contact with the whole of the interlayer insulation layer 13. As shown in
Next, with reference to
Next, as shown in
Preferably, as shown in
Thus, in accordance with this embodiment, the recording layer 15 formed on the inside wall of the through-hole 13a has a three-dimensional structure. Moreover, because the bit line 14 is formed before the recording layer 15, there is no forming of the bit line 14 in the through-hole 13a. As a result, the bit line 14 can be sufficiently distanced from the heating region P, and the contact area between the recording layer 15 and the bit line 14 can be reduced which, by decreasing heat dissipation to the bit line 14, results in higher heating efficiency. Furthermore, while a conventional configuration normally has a top electrode provided between the bit line 14 and the recording layer 15, the memory element of this embodiment does away with the top electrode, keeping down the complexity of the fabrication process.
In the case of the embodiment described in the foregoing, part of the bit line 14 is etched during the forming of the through-hole 13a. However, the through-hole 13a may instead be formed in a region where there is no bit line 14.
With this configuration, the distance between the bit line 14 and the heating region P can be increased. However, if the through-hole 13a is too far from the bit line 14, heating efficiency is decreased by the increased volume of the recording layer 15 and power loss from the lower voltage. As such, these factors have to be taken into account when deciding where to locate the through-hole 13a.
Also, in the foregoing embodiment the plane shape of the through-hole 13a and of the recording layer 15 is substantially circular, the through-hole 13a may have a shape that is elongated in the X direction along which the bit line 14 extends, as shown in
If, as shown in
Specifically, when the plane shape of the recording layer 15 is substantially circular, as in
However, if the recording layer 15 is elongated too much in the X direction, the heat diffusion property is increased due to the increase in the contact area S2, and the volume of the recording layer 15 is increased, decreasing the heating efficiency. Therefore, this has to be taken into account when deciding the shape of the recording layer 15.
A non-volatile memory element 20 according to a second embodiment of the present invention will now be described.
The difference between the memory element 20 and the memory element 10 of the first embodiment is that the memory element 20 has an etching stopper 21 on the bit line 14, and the through-hole 13a passes through the etching stopper 21, the bit line 14 and the interlayer insulation layer 13. Other parts and elements are the same and are denoted by the same symbols, so further explanation thereof is omitted.
The etching stopper 21 covers the whole of the upper surface (growth termination surface 14b) of the bit line 14, and functions as an etching stop when the recording layer 15 is being patterned. The bit line 14 is therefore not exposed to the etching atmosphere, which, by preventing the bit line 14 from being decreased in thickness, ensures the bit line has the prescribed resistance.
In this embodiment, contact between the bit line 14 and the recording layer 15 only takes place in the through-hole 13a. More specifically, the recording layer 15 only contacts the etched surface 14c formed by patterning. The contact area between the bit line 14 and recording layer 15 is therefore reduced, further decreasing heat dissipation to the bit line 14.
Instead of patterning the recording layer 15, the whole surface may be etched back to leave just the portion of the recording layer 15 that is on the inside wall of the through-hole 13a.
As shown in
The provision of the etching stopper 21 on the bit line 14 prevents the bit line 14 being damaged when the recording layer 15 is being etched.
The present invention is in no way limited to the aforementioned embodiments, but rather various modifications are possible within the scope of the invention as recited in the claims, and naturally these modifications are included within the scope of the invention.
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