Patent Application
20200243757
The present invention relates to magnetic random-access memory (MRAM) and more particularly to a method for manufacturing a magnetic random-access memory array wherein annealing is performed after memory element pillar definition to increase thermal annealing budget.
Magnetic Random-Access Memory (MRAM) is a non-volatile data memory technology that stores data using magnetoresistive cells such as Magnetoresistive Tunnel Junction (MTJ) cells. At their most basic level, such MTJ elements include first and second magnetic layers that are separated by a thin, non-magnetic layer such as a tunnel barrier layer, which can be constructed of a material such as Mg—O. The first magnetic layer, which can be referred to as a reference layer, has a magnetization that is fixed in a direction that is perpendicular to that plane of the layer. The second magnetic layer, which can be referred to as a magnetic free layer, has a magnetization that is free to move so that it can be oriented in either of two directions that are both generally perpendicular to the plane of the magnetic free layer. Therefore, the magnetization of the free layer can be either parallel with the magnetization of the reference layer or anti-parallel with the direction of the reference layer (i.e. opposite to the direction of the reference layer).
The electrical resistance through the MTJ element in a direction perpendicular to the planes of the layers changes with the relative orientations of the magnetizations of the magnetic reference layer and magnetic free layer. When the magnetization of the magnetic free layer is oriented in the same direction as the magnetization of the magnetic reference layer, the electrical resistance through the MTJ element is at its lowest electrical resistance state. Conversely, when the magnetization of the magnetic free layer is in a direction that is opposite to that of the magnetic reference layer, the electrical resistance across the MTJ element is at its highest electrical resistance state.
The switching of the MTJ element between high and low resistance states results from electron spin transfer. An electron has a spin orientation. Generally, electrons flowing through a conductive material have random spin orientations with no net spin orientation. However, when electrons flow through a magnetized layer, the spin orientations of the electrons become aligned so that there is a net aligned orientation of electrons flowing through the magnetic layer, and the orientation of this alignment is dependent on the orientation of the magnetization of the magnetic layer through which they travel. When the orientations of the magnetizations of the free and reference layer are oriented in the same direction, the majority spin of the electrons in the free layer are in the same direction as the orientation of the majority spin of the electrons in the reference layer. Because these electron spins are in generally the same direction, the electrons can pass relatively easily through the tunnel barrier layer. However, if the orientations of the magnetizations of the free and reference layers are opposite to one another, the spin of majority electrons in the free layer will be generally opposite to the majority spin of electrons in the reference layer. In this case, electrons cannot easily pass through the barrier layer, resulting in a higher electrical resistance through the MTJ stack.
Because the MTJ element can be switched between low and high electrical resistance states, it can be used as a memory element to store a bit of data. For example, the low resistance state can be read as an on or “1”, whereas the high resistance state can be read as a “0”. In addition, because the magnetic orientation of the magnetic free layer remains in its switched orientation without any electrical power to the element, it provides a robust, non-volatile data memory bit.
To write a bit of data to the MTJ cell, the magnetic orientation of the magnetic free layer can be switched from a first direction to a second direction that is 180 degrees from the first direction. This can be accomplished, for example, by applying a current through the MTJ element in a direction that is perpendicular to the planes of the layers of the MTJ element. An electrical current applied in one direction will switch the magnetization of the free layer to a first orientation, whereas switching the direction of the current and such that it is applied in a second direction will switch the magnetization of the free layer to a second, opposite orientation. Once the magnetization of the free layer has been switched by the current, the state of the MTJ element can be read by reading a voltage across the MTJ element, thereby determining whether the MTJ element is in a “1” or “0” bit state. Advantageously, once the switching electrical current has been removed, the magnetic state of the free layer will remain in the switched orientation until such time as another electrical current is applied to again switch the MTJ element. Therefore, the recorded date bit is non-volatile in that it remains intact in the absence of any electrical power.
The present invention provides a method for manufacturing a memory device. The method includes forming an array of magnetic memory element pillars, dielectric isolation material and circuitry associated with the array of magnetic memory element pillars on a wafer. Then, after forming the array of magnetic memory element pillars, dielectric isolation material and circuitry, a thermal annealing process is performed that is configured to both anneal the magnetic memory element pillars and also perform back end of line thermal processing of the circuitry.
The magnetic memory element pillars can be performed by depositing full film memory element material, forming a mask structure over the full film memory element material and performing a material removal process such as ion beam etching or reactive ion etching. After the pillars have been thus formed, a dielectric isolation material which may include silicon oxide and/or silicon nitride can be deposited and planarized by chemical mechanical polishing.
The method for manufacturing the memory device can be performed without any thermal annealing of the memory element material prior to formation of the memory element pillars. This method advantageously prevents any damage to the memory element pillars (such as damage to the MgO barrier layer or surrounding interface) that might otherwise occur as a result of post pillar process Back End Of Line (BEOL) thermal annealing processes.
These and other features and advantages of the invention will be apparent upon reading of the following detailed description of the embodiments taken in conjunction with the figures in which like reference numeral indicate like elements throughout.
For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale.
The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein.
Referring now to
The magnetic reference layer 102 can be part of an anti-parallel magnetic pinning structure 112 that can include a magnetic keeper layer 114, and a non-magnetic, antiparallel coupling layer 116 located between the keeper layer 114 and reference layer 102. The antiparallel coupling layer 116 can be a material such as Ru and can be constructed to have a thickness such that it will ferromagnetically antiparallel couple the layers 114, 102. The antiparallel coupling between the layers 114, 102 pins the magnetization 108 of the reference layer 102 in a direction opposite to the direction of magnetization 118 of the keeper layer 114.
A seed layer 120 may be provided near the bottom of the memory element 100 to initiate a desired crystalline structure in the above deposited layers. A capping layer 122 may be provided near the top of the memory element 100 to protect the underlying layers during manufacture, such as during high temperature annealing and from exposure to ambient atmosphere. Also, electrodes 124, 126 may be provided at the top and bottom of the memory element 100. The electrodes 124, 126 may be constructed of a non-magnetic, electrically conductive material such as Ta, W, Cu and Al can provide electrical connection with circuitry 128 that can include a current source and can further include circuitry for reading an electrical resistance across the memory element 100.
The magnetic free layer 104 has a perpendicular magnetic anisotropy that causes the magnetization 110 of the free layer 104 to remain stable in one of two directions perpendicular to the plane of the free layer 104. In a write mode, the orientation of the magnetization 110 of the free layer 104 can be switched between these two directions by applying an electrical current through the memory element 100 from the circuitry 128. A current in one direction will cause the memory element to flip to a first orientation, and a current in an opposite direction will cause the magnetization to flip to a second, opposite direction. For example, if the magnetization 110 is initially oriented in a downward direction in
On the other hand, if the magnetization 110 of the free layer 104 is initially in an upward direction in
In order to assist the switching of the magnetization 110 of the free layer 104, the memory element 100 may include a spin polarization layer 130 formed above the free layer 104. The spin polarization layer can be separated from the free layer 104 by a coupling layer 132. The spin polarization layer 130 has a magnetic anisotropy that causes it to have a magnetization 134 with a primary component oriented in the in-plane direction (e.g. perpendicular to the magnetizations 110, 108 of the free and reference layers 104, 102. The magnetization 134, of the spin polarization layer 130 may either be fixed or can move in a precessional manner as shown in
The memory element 100, which is shown in side, cross-sectional view in
The nanofabrication of an array of magnetic memory elements such as the memory element 100 described above involves use of Back End of Line (BEOL) fabrication technology. This fabrication technology includes high temperature annealing processes, at temperatures of around 400 degrees C. for durations of 600 to 100 minutes. This BEOL high temperature process is referred to as post process formation annealing, because it is performed after the formation of the magnetic memory element pillars. This presents a challenge in that the post process annealing can be at a higher temperature than the thermal budget of the magnetic memory element pillars. For example, the high temperature BEOL process can cause changes to the crystalline structure of the barrier layer (106 in
An electrically conductive lead 306 can be formed over the CMOS circuitry 304. Also, a dielectric material 308 can be provided at some regions adjacent to the CMOS circuitry 304 and lead layer 306. The top of the lead layer 306 and dielectric layer 308 can be planarized by a process such as chemical mechanical polishing (CMP).
With reference now to
With reference now to
After the mask 402 has been formed, a material removal process such as reactive ion etching, ion milling, etc. is performed to remove portions of the series of memory element layers 310 that are not protected by the mask structure 402 to leave a pattern of memory element pillar structures 310, one of which is shown in cross section in
It should be pointed out that at this point, no thermal annealing processes have been performed at the full film level after pMTJ layers deposition i.e prior device formation process. Thermal processing, such as to ensure a desired crystal structure in the barrier layer 316 is not performed prior to forming the memory element pillars as described above.
After defining the magnetic element pillar structures as described above, a dielectric, non-magnetic isolation layer is deposited and planarized by a process such as chemical mechanical polishing, leaving a structure as shown in
With reference now to
After the memory element pillars 310, surrounding isolation layer 602, and lead 802 have been formed as described above, a novel thermal annealing process is performed that serves to both anneal memory element pillars (such as to form a desired crystalline structure in the barrier layer 106 (
The post pillar formation annealing process can consist of annealing the wafer as a last step of the BEOL CMOS process. After forming the magnetic pillar elements 310 and forming the surrounding dielectric isolation material 602 on a wafer, the wafer is inserted into a vacuum annealing oven. Then, the temperature is raised from about 25 degrees C. to about 400 degrees C. (e.g. 350-450 degrees C.) within a period of about 40 minutes (e.g. 30-50 minutes). The wafer is kept at this constant temperature for about another 60 minutes (e.g. 40-100 minutes) in a vacuum of at least 1×10−4 Torr Then, the wafer is cooled to about 120 degrees C. (e.g. 100-140 degrees C.) within a period of about 40 minutes (e.g. 30-50 minutes) at a vacuum of at least 1×10−4 Torr. A final cooling step can be performed in an N2 atmosphere to cool the wafer from about 120 degrees C. to about 40 degrees C. (e.g. 30-50 degrees C.) in a period of about 20 minutes (e.g. 10-30 minutes). The presence of the N2 atmosphere can be beneficial in reducing the cool-down time. Another method of post pillar annealing process can include so called Rapid Thermal Annealing (RTA). In this method the temperature rise to about 400 degrees C. and cooling to room temperature of the wafer is achieved in less than 60 seconds in the annealing oven at a vacuum at least 1×10−4 Torr. RTA method reduces significantly temperature ramp up and cool down time without reducing the constant temperature time (soak time). RTA annealing process may be advantageous in certain applications of post pillar formation annealing process required by the BEOL CMOS technology.
The various temperatures and time of annealing can be varied depending on the design of the memory element pillars 310 and associated circuitry. For instance, Boron rich CoFeB alloys (which may be employed in one or more of the layers 114, 102, 104 (
Use of the above described post pillar formation annealing process advantageously allows for high TMR and also thermal stability (PMA) improvement without the need to anneal the memory element structure on the thin film level (i.e. before pillar formation). This process results in no shunting of the memory devices 310, thereby producing high yields. The process achieves desired MgO barrier layer 106 crystallization and proper interface formation of the MTJ junction (i.e. interface between barrier layer 106 and reference and free layers 102, 104).
The above described process can be summarized with reference to
The design of a pMTJ device stack is specific to the particular MRAM application. For example, in a particular design of the pMTJ device, the MgO capping layer 1014 formed on top of the free layer 1004 provides improved perpendicular magnetic anisotropy (PMA) in the free layer 1004.
The resistivity of the device 1002, which influences switching voltage, is sensitive to the Post Pillar Annealing process. Since the overall resistivity of the device 1002 is predominantly determined by the thickness of the MgO layers in the pMTJ stack (e.g. barrier layer 1008 and capping layer 1014) the area resistance (RA) product of the MgO main barrier of a tunnel junction 1008 needs to be in particular ratio with respect to the MgO capping layer 1014. The ratio of the RA product of MgO main barrier 1008 to the RA product of MgO capping layer 1014 can be chosen specifically for the MRAM product application. For instance, RA ratios (Ra-barrier/Ra-cap) of ˜1-3 will lead to higher free layer thermal stability which improves data retention and breakdown voltage. High thermal stability and endurance is required in non-volatile Memory class (NVM) MRAM application and for use in high temperature applications such as automotive applications which can require operation at up to 150 degrees C. At the same time, the switching current and switching voltage may be increased due to thicker MgO barrier 1008 and capping layer 1014. As an example, the MgO main barrier layer 1008 can be chosen to have an RA=8 Ωμm2 with MgO cap RA=4 Ωμm2.
On the other hand, higher RA ratios of 4-16 will lead to a decrease in the resistivity of the device 1002 during post pillar annealing process. Therefore, higher RA ratio (Ra-Barrier/Ra-Cap) devices can be used for MRAM application where low switching current and switching voltage is necessary, such as in SRAM applications. At the same time, thermal stability can be lower compared to devices with RA ratio 1-3. As an example, for SRAM application MgO main barrier can be chosen to be RA=4 Ωμm2 with MgO cap RA=1.0 Ωμm2.
Therefore, the performance parameters of the magnetic element 1002 can be controlled by controlling the ratio of barrier layer area resistance to cap layer area resistance (RA barrier/RA cap). A higher (RA barrier/RA cap) ratio allows for higher thermal stability of the pMTJ device 1002 when used with the above described post pillar annealing process. On the other hand, a lower (RA barrier/RA cap) ratio allows for lower switching current and switching voltage of the pMTJ device 1002 when used with the above described post pillar annealing process.
In both cases, post pillar annealing process is required for CMOS BEOL process but the choice of the MgO main barrier 1008 RA and MgO capping layer 1014 RA for the design of devices would ultimately determine the function of the MRAM device and its particular product application. Therefore, the Post Pillar Annealing process can be universally implemented in CMOS BEOL process for different MRAM product applications but it also requires adjustments to the RA of MgO main barrier and MgO capping layer of the pMTJ device.
While various embodiments have been described above, it should be understood that they have been presented by way of example only and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the inventions should not be limited by any of the above-described exemplary embodiments but should be defined only in accordance with the following claims and their equivalents.
