In line with remarkable developments in an information and telecommunication field, there is an increasing demand for various kinds of memory devices. In particular, nonvolatile memory devices, which can retain data even if power is turned off, are demanded as memory devices available for mobile terminals, MP3 players, or the like. Since nonvolatile memory devices can electrically program and erase data, and retain data even though power is not supplied, they are increasingly applied to a variety of fields at present. However, typical dynamic random access memories (DRAMs) are volatile memory devices, and thus lose stored data when power is not supplied. Therefore, many studies are being conducted on nonvolatile memory devices which can be used in place of DRAMs.
Among various kinds of nonvolatile memory devices, researches are mainly being made on a phase RAM (PRAM) using a phase transition phenomenon, a magnetic RAM (MRAM) using a magnetoresistance, a ferroelectric RAM (FRAM) using a ferroelectric effect, and a resistance RAM (ReRAM) using a resistance switching or conductivity switching phenomenon of a metal oxide thin film. In particular, much attention has been paid on MRAMs recently because MRAMs have an operation speed faster than other nonvolatile memory devices, and excellent durability against the repetitive use.
MRAMs are classified into two memories according to a method for read-out of information, of which one is an MRAM using giant magnetoresistance (GMR) effect and the other is an MRAM using tunneling magnetoresistance (TMR) effect. Since the MRAM using the GMR effect has the magnetoresistance less than 10%, a reading speed of information is slow and a signal-to-noise ratio (SNR) is low. Also, the MRAM using the GMR effect may be affected by a magnetic field applied to adjacent magnetic recording elements, and thus the magnetic recording elements should be spaced apart from each other by a predetermined distance or more, leading to a difficulty in achieving high integration.
The MRAM using the TMR effect has a magnetic tunnel junction (MTJ) structure as a basic structure. The MTJ structure is a stack structure where a read electrode, an anti-ferromagnetic layer, a magnetic pinned layer formed of a ferromagnetic material, an insulating layer, a magnetic free layer formed of a ferromagnetic material, and a drive electrode are formed over a substrate in sequence. Like the MRAM using the GMR effect, the MRAM using the TMR effect stores information using a magnetoresistance difference according to a relative difference in magnetization direction between the magnetic free layer and the magnetic pinned layer. Unlike the MRAM using the GMR effect, however, the MRAM using the TMR effect has a faster reproduction rate and a higher SNR than the MRAM using the GMR effect because it has the magnetoresistance of 20% or more.
In the MRAM using the TMR effect, the resistance of each magnetic recording element greatly varies with a thickness of an insulating layer. Accordingly, information is stored by the use of a resistance difference from an adjacent comparative magnetic recording element at present. If, however, a thickness difference between an insulating layer of a storage magnetic recording element and an insulating layer of a comparative magnetic recording element is 0.2 Å or greater, it is difficult to read out information stored in the magnetic recording element. Therefore, there is a technical problem in that an insulating layer should be conformally formed over a wafer of several inches in radius during a manufacturing process.
As the magnetic recording element shrinks in size, the magnetic free layer and the magnetic pinned layer get close to each other. Therefore, a ferromagnet of the magnetic free layer is affected by a magnetic field of a ferromagnet of the magnetic pinned layer. Such a magnetic field produced by the magnetic pinned layer, that is, a stray field, may have a detrimental effect, for example, a decrease in magnetoresistance, or an increase in coercive force of the magnetic free layer. Especially, the insulating layer in the MTJ structure is thinner than the conductive layer of the MRAM using the GMR effect because the MTJ structure makes use of the TMR effect. Consequently, the magnetic free layer and the magnetic pinned layer get closer and closer to each other, and thus the magnetization of the magnetic free layer is greatly affected by the magnetic pinned layer.