FIELD OF THE INVENTION

The present invention relates to magnetic random-access memory (MRAM) and more particularly to a memory element structure having a Ru hard mask layer for increased data density and reduced parasitic electrical resistance.

BACKGROUND

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 is 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 a “0”, whereas the high resistance state can be read as a “1”. 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 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 current 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 data bit is non-volatile in that it remains intact in the absence of any electrical power.

SUMMARY

The present invention provides a magnetic memory structure that includes a tunnel junction structure and a Ru hard mask layer formed over the tunnel junction structure.

The use of a Ru hard mask in a memory element structure provides multiple advantageous benefits. For example, Ru does not form an insulating oxide and therefore imparts little to no parasitic electrical resistance to the memory element structure.

In addition, Ru has a high resistance to removal by ion etching and chemical mechanical polishing (CMP). This allows the Ru hard mask to be very thin, while still being able to withstand an ion etching process used to remove tunnel junction memory element material to form a memory element pillar. The reduced thickness results in less shadowing between memory elements, especially during high angle ion etching used to remove redeposited material from the memory element pillars. This reduced shadowing effect allows the memory elements to be spaced closer together than would otherwise be possible using a different, thicker hard mask layer.

A memory element pillar having a Ru hard mask can be formed by a process that includes depositing a plurality of memory element layers over a substrate. A Ru hard mask is deposited over the plurality of memory element pillars, and a mask structure is formed over the Ru hard mask layer. An ion etching is then performed to transfer the image of the mask structure onto the Ru hard mask layer and the plurality of memory element pillars, thereby forming a memory element pillar.

The mask structure formed over the Ru hard mask layer can be formed by depositing a material that can be removed by reactive ion etching (RIEable material), and then forming a photoresist mask over the RIEable material. A reactive ion etching can then be performed to transfer the image of the photoresist mask onto the underlying RIEable material.

The RIEable material can be one or more of tantalum, tantalum nitride or silicon oxide, and the reactive ion etching can be performed using a chemistry that is chosen to readily remove the RIEable material. For example, if the RIEable material is tantalum or tantalum nitride, the reactive ion etching can be performed in an atmosphere that includes fluorine or chlorine. If the RIEable material is silicon oxide, the reactive ion etching can be performed using an atmosphere that includes fluorine.

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.

