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Multi-bit memory cell using long-range non-anchored actuation for high temperature applications
Keywords: non-volatile memory, non-anchored actuation, long-range motion
Anchored micro-electro-mechanical (MEM) memories suffer from elastic fatigue and thermal drift. High temperatures can cause unwanted deflection of cantilevers or membranes. Since anchored element is under stress from restoring force when it is in contact mode any instability in the contact or environment can cause failure. Consequently for very high temperatures (>300C) non-anchored designs are required. To increase the storage density, in addition to scaling down the dimensions, multi-bit cells are introduced. Since multi-bit memory cells require less writing and sensing electrodes they will also allow to decrease control CMOS circuitry, therefore lower power consumption is achieved. Since more than one bit is written in one writing process, writing time is also reduced. Multi-bit indicates multi-stability, which means several minimums in the energy of the system that achieving each of them will make the system stable when there is no command or actuation. For a non-anchored element it means that there should be several positions that are stable as well as the element resides in them. For having a n-bit cell, 2^n stable position states are needed. For achieving more bits in a cell control of motion of the non-anchored element becomes the main issue in the design. It also requires long-range motion of the non-anchored element to be in different positions. The only actuation method that can satisfy all these requirements is magnetic. A novel MEM based non-volatile memory (NVM) is proposed. The storage principle is based on Lorentz’s transduction, utilizing long-range motion of a non-anchored element which has current carrying sliding contact with a conductive path. Position of the moving element indicates the stored data in the multi-bit cell. Data is written in the cell with displacing the moving element by Lorentz’s force, is read by utilizing differential port resistances, and is held by adhesion forces. Data writing at up to 300C, and data retention and reading for higher temperatures are reliable. Each memory cell comprises a magnetic field generator current loop, magnetic flux guiding core, two circular conductive paths with ports for writing and reading, and a free moving element, that has sliding contacts with the paths. Applied current signal to one of the ports will pass through the moving element current line. Consequently, Lorentz’s force is induced on moving element. Moving element current signal shape, defines the position in which moving element resides or in other words, the bit word that is stored in the cell. The reading process of this device is based on differential port resistances. Permanent retention of the data is obtained by adhesion forces in the contact of moving element and the conductive paths. Friction and adhesion behavior in the sliding contact which is carrying current is studied and characterized. Also, high temperature effect on the structure and performance of the device is characterized. For 30um diameter of each cell, storage density and average writing time, will be 1.4Kbits/mm^2 and 90us/bit respectively. Energy consumption in each data writing process is dominated by the magnetic field generator current loop, which is typically 0.45uJ/bit.
Mehrdad Elyasi, PhD student
Institute of Microelectronics (IME), Agency for Science, Technology and Research (A*STAR)
Singapore, Singapore

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