26-06-2014, 12:51 PM
MAGNETIC RANDOM ACCESS MEMORY
MAGNETIC RANDOM ACCESS MEMORY.doc (Size: 72 KB / Downloads: 13)
ABSTRACT
Magnetic RAM (MRAM) is a new memory technology with access and cost characteristics comparable to those of conventional dynamic RAM (DRAM) and the non-volatility of magnetic media such as disk. That is MRAM retains its memory even after removing power from the device. Such a non-volatile memory has important military applications for missiles and satellites. Clearly such a device could also have important commercial applications if the non-volatility were accomplished without impacting other properties of the memory, notably density, read and write speed, and lifetime. IBM in cooperation with Infineon is promising to launch this new technology that will eliminate the boot-up process of a computer and thus enable it to turn on as instantly as a television or radio using memory cells based on magnetic tunnel junctions. This paper discusses the following aspects in detail: Attractions of this new technology How MRAM works MRAM Architecture Magnetic Tunnel Junctions - future of MRAM Challenges faced Anticipated Applications
INTRODUCTION
You hit the power button on your television and it instantly comes to life. But do the same thing with your computer and you have to wait a few minutes while it goes through its boot up sequence. Why can't we have a computer that turns on as instantly as a television or radio? IBM, in cooperation with Infineon, is promising to launch a new technology in the next few years that will eliminate the boot-up process. Magnetic random access memory (MRAM) has the potential to store more data, access that data faster and use less power than current memory technologies. The key to MRAM is that, as its name suggests, it uses magnetism rather than electrical power to store data. This is a major leap from dynamic RAM (DRAM), the most common type of memory in use today, which requires a continuous supply of electricity and is terribly inefficient. Twenty-five years ago, DRAM overtook ferrite core memory in the race to rule the PC memory market. Now it looks like ferromagnetic technology could be making a comeback, with IBM Corp. and Infineon Technologies charging a joint team of 80 engineers and scientists with the task of making magnetic RAM (MRAM) a commercial reality within
ATTRACTIONS OF THIS NEW TECHNOLOGY
Consider what happens when power goes off while you are typing on your computer? Unless you are connected to an uninterruptible power supply you lose everything you were working on since you last saved the document. That's because your computer's random access memory (RAM), which stores information for fast access, can't function without power. The same goes for your cell phone and PDA. Both require a battery to keep the RAM intact with your phone numbers and personal data. But IBM researchers have developed a new form of RAM magnetic RAM (MRAM) that doesn't forget anything when the power goes out. RAM promises to be Cheap Fast Nonvolatile Low power alternative RAM has these attractions over conventional RAM, which uses electrical cells to store data, as MRAM uses magnetic cells. This method is similar to the way your hard drive stores information. When you remove power from your computer, conventional RAM loses memory, but the data on your hard disk remains intact due to its magnetic orientation, which represents binary information. Because magnetic memory cells maintain their state even when power is removed, MRAM possesses a distinct advantage over electrical. With DRAM (RAM used in PCs and workstations) you store a charge in a capacitor. That charge will leak away over time and it needs to be refreshed frequently that takes power. But with MRAM you have no such problems .You need no power to maintain the state, and toy only need to pass a small current through the memory to read it.
Compared with SRAM (RAM used to build fast memory, cache) MRAMs are as fast as SRAM with read/write speeds better than 2.5 nanoseconds. Moreover MRAMs can be build
smaller than SRAM and hence would be cheaper. Compared to Flash memory (an example for Flash memory is computer's BIOS chip), it’s much faster to write on to MRAM. Thus MRAM threatens to replace not only dynamic RAM, but also Flash memory. (Flash memory is used for easy and fast information storage in such devices as digital cameras and home video game consoles. In fact, Flash memory is considered a solid state storage device. Solid state means that there are no moving parts -- everything is electronic instead of mechanical
MAGNETIC RAM ARCHITECTURE
Like Flash memory, MRAM is a nonvolatile memory a solid-state chip that has no moving parts. Unlike with DRAM chips, you don't have to continuously refresh the data on solid-state chips. Flash memory can't be used for instant-on PCs because it hasn't demonstrated long-term reliability. MRAM will likely compete with Flash memory in the portable device market for the same reason that it will replace DRAM - it reduces power consumption.
In MRAM only a small amount of electricity is needed to store bits of data. This small amount of electricity switches the polarity of each memory cell on the chip. A memory cell is created
READING DATA
To read the bit of information stored in this memory cell, you must determine the orientation of the two magnetic moments. Passing a small electric current directly through the memory cell accomplishes this. When the moments are parallel, the resistance of the memory cell is smaller than when the moments are not parallel. Even though there is an insulating layer between the magnetic layers, the insulating layer is so thin that electrons can "tunnel" through it from one magnetic layer to the other
WRITING DATA
To write to the device, you pass currents through wires close to (but not connected to) the magnetic cells. Because any current through a wire generates a magnetic field, you can use this field to change the direction of the magnetic moment. The arrangement of the wires and cells is called cross-point architecture the magnetic junctions are set up along the intersection points of a grid. Wires called word lines run in parallel below the magnetic cells. Another set of wires called bit lines runs above the magnetic cells and perpendicular to the set of wires below. Like coordinates on a map, choosing one particular word line and one particular bit line uniquely specifies one of the memory cells. To write to a particular cell (bit), a current is passed through the two independent wires (one above and one below) that intersect at that particular cell. Only the cell at the cross point of the two wires sees the magnetic fields from both currents and changes state.
ID MAGNETIC SELECTION
Selected cells receive both Ix and Iy currents, and are switched into the desired memory states. The currents must be selected so that Iy or Ix separately do not disturb the memory state of stored data. Bits on the same x line or y line that are not being written are subjected to "half-select" currents which tend to disturb the data. If very large currents are used to insure the writing of worst case cells, then the half-select currents are also large and tend to disturb the most disturb-sensitive cells. The half-selected memory states are also not nearly as stable as stored and they provide the majority of projected cell failures in time. In addition to half-select currents, these cells must withstand stray fields from neighboring cells and fields from leakage currents and stray environmental fields. Thus, the requirements for uniformity and design margins present challenges in manufacturing the 2D magnetic arrays. Most magneto resistive memory schemes also use a 2D selection scheme for reading data. The original MRAM concept use magnetic 2D selection schemes for reading, which introduce further, disturb conditions.
Magnetic tunnel junction memories (MTJ) use a diode or transistor to select a memory cell for reading, and thus do not have significant disturb conditions for reading, but they still have the constraints of 2D magnetic selection for writing."ID selection" scheme for both reading and writing a magneto resistive memory cell improves reliability. A high current of either polarity (plus current for a "1" and negative current for a "0") is passed through a select transistor and through the memory cell to write. A lower current is used to generate a voltage across the cell which will be higher or lower depending on the data stored lower current can be used for
CURRENT STATUS
Magnetic RAM is not an overnight technological feat. It has taken nearly three decades to develop. To give you an idea of when IBM began working on MRAM, Microsoft didn't even exist when IBM made its first breakthrough in this technology. In 1974, IBM Research developed a miniature computer component called the magnetic tunnel junction. This component was eventually used to store information. The potential market for MRAM is big. It is expected to eventually become the memory standard for future electronics, replacing DRAM. In The potential market for MRAM is big. It is expected to eventually become the memory standard for future electronics, replacing DRAM. In MRAM has the potential to replace today's memory technologies in electronic products of the future," said Bijan Davari, IBM Vice President of Technology and Emerging Products. He added that the announcement of MRAM's impending availability is a major step in moving the technology from the research stage to product development.
CHALLENGES FACED
While progress has been made in determining the structure and materials needed for MRAM development there are still many hurdles to jump before MRAM chips can be made production-worthy. Among the issues to tackle are architectures, materials development, submicron manufacturing, wiring, and the feasibility of integrating MRAM with logic. Present day challenges for MRAM technology include Reducing drive currents Eliminating cell instabilities due to magnetization vortices improving modes of operation at nanometer dimensions fundamental thermal instabilities Finding applications with sufficient volumes and performance advantages to make MRAM manufacturing costs competitive to be practical, dense MRAM cells should operate with less than a few mA currents when the lithography is at the 0.2 - 0.3 micron dimensions. Two reasons are: to stay within the current carrying capability of thin, narrow metal lines, and to be compatible with the center-to-center circuit spacing at the edge of the magnetic array. Reported data shows more than 10 times the desired current densities. Several mitigating ideas have emerged. One is to coat or "keeper" the tops and edges of the strip lines used in the memory array. This is done to reduce word currents by a factor of 3. An additional idea is to reduce the rise time of pulses, which takes advantage of the gyro-magnetic nature of the magnetization. This technique has reduced the required drive currents by a factor of more than 2. Devising methods whereby required current levels scale down with size of the memory cell will continue to be a challenge for MRAM
CONCLUSION
If MRAM chips are to debut, the completely different architecture will make DRAM chips obsolete and issue a new era of memory chips. MRAM solves your problem of losing data typed on your computer unless you are connected to uninterruptible power supply, as MRAM doesn't forget anything when power goes out. The difference is conventional RAM, uses electrical CELLS to store data, MRAM uses magnetic cells. This method is similar to the way your hard drive stores information. When you remove power from your computer, conventional RAM loses memory, but the data on your hard disk remains intact due to its magnetic orientation, which represents binary information. Because magnetic memory cells maintain their state even when power is removed, MRAM possesses a distinct advantage over electrical cells.
There is still a long way to go before MRAM is ready for prime time. Neither IBM nor Motorola, for instance, is expected to go into mass production until they prove that they can make 256 megabit chip set the standard memory module used today. But, as total sales of computer memory in 2000 were estimated by Semico Research Corporation to have been worth $48 billion, manufacturers have a considerable incentive to ensure that MRAM becomes a serious challenger for DRAM's crown.