Thermally assisted MRAM with a multilayer encapsulant for low thermal conductivity

#The present invention relates generally to magnetic memory devices, and more specifically, to utilizing a multilayer encapsulant.

#Magnetoresistive random access memory (MRAM) is a non-volatile computer memory (NVRAM) technology. Unlike conventional RAM chip technologies, MRAM data is not stored as electric charge or current flows, but by magnetic storage elements. The elements are formed from two ferromagnetic plates, each of which can hold a magnetic field, separated by a thin insulating layer. One of the two plates is a reference magnet set to a particular polarity; the other plate's field can be changed to match that of an external field to store memory and is termed the “free magnet” or “free-layer”. This configuration is known as a magnetic tunnel junction and is the simplest structure for a MRAM bit. A memory device is built from a grid of such “cells”. In some configurations of MRAM, which includes the type discussed below, both the reference and free layers of the magnetic tunnel junctions can be switched using an external magnetic field; in this case, heat is used to depin the reference layer, and the data is stored in the reference layer.

#One method of reading is accomplished by measuring the electrical resistance of the cell. A particular cell is (typically) selected by powering an associated transistor which switches current from a supply line through the cell to ground. Due to the magnetic tunnel effect, the electrical resistance of the cell changes due to the orientation of the magnetization in the two plates. By measuring the resistance of any particular cell, the direction of magnetization of the writable plate (i.e., the free layer) can be determined.

#The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

#FIG. 1 illustrates a structure for a thermally-assisted magnetoresistive random access memory (TAS-MRAM) device according to an embodiment.

#FIG. 2 illustrates an enlarged cross-sectional view of a multilayer dielectric encapsulant utilized in FIG. 1 according to an embodiment.

#Thermally-assisted magnetoresistive random access memory (TAS-MRAM) entails heating of the magnetic tunnel junction (MTJ) stack to a write temperature (Twrite) higher than the operating temperature (Top) in order to write to the device. This is typically done by Joule heating from a bias current that is applied during the write process. The amount of power required to heat the device to Twrite depends on the thermal conductivity between the device and the surrounding structures and substrate, and the surrounding structures and substrates are at the temperature Top<Twrite.

#For typical TAS-MRAM devices and structures, the power required (to generate the bias current for Joule heating) is undesirably large because the thermal conductivity is large. The thermal conductivity of the device allows heat to leak from the MTJ stack to the surrounding structures and substrates.

#Embodiments of the invention reduce the power required to heat the device to Twrite by reducing the thermal conductivity between the device and dielectric that surrounds the device, which have an operating temperature Top. Embodiments utilize interfacial thermal resistance (also referred to as boundary resistance or Kapitza resistance in the literature) in the encapsulation of the TAS-MRAM device to allow small thermal conductivity between the device and the surrounding dielectric and other structures. In insulators, where heat conduction is primarily via phonons, interfacial resistance from multilayers of dissimilar dielectric layers can substantially decrease the thermal conductivity compared to a single dielectric layer of the same thickness according to embodiments.