Memory cells are typically embedded in current electronic devices, e.g., computers, cell phones, cameras, and games. They may be classified as either volatile or non-volatile memory cells. A volatile memory cell stores data only as long as power is provided to the memory cell. A non-volatile memory cell, on the other hand, continues to retain data long after power has been removed from the memory cell. One type of non-volatile memory cell is a one-time programmable memory cell.
Conventional one-time programmable memory cells, such as fuses and antifuses, typically require a large current to either blow an element and make an open circuit (in the case of a fuse) or to lower the resistance of an element and make it conductive (in the case of an antifuse). This requirement of a large current, which may be as high as about 250 mA, in turn leads to high (and expensive) silicon die area requirement to house large transistors for driving the current and wide propagation tracks for conducting the current. Furthermore, conventional one-time programmable memory cells are typically fabricated on the silicon surface and consume silicon die area for this additional reason.
Other conventional non-volatile memory cells, such as EPROM, EEPROM and FLASH memories, are transistor-based cells that utilize a floating gate to store data in the cell. The presence of a charge on the floating gate indicates a first logic state, while the absence of a charge on the floating gate indicates a second logic state. These conventional memory cells typically require large silicon die area due to, e.g., the size of transistors needed to apply a high voltage to the memory cell. Furthermore, conventional memory cells are also typically fabricated on the silicon surface and consume silicon die area for this additional reason.
Accordingly, there is a need for memory cells which consume less silicon die area while providing the performance characteristics of conventional memory cells.
The systems and methods described herein address deficiencies in the prior art by enabling fabrication of non-volatile memory cells using micro-electromechanical (MEMS) technology. In some embodiments, MEMS-based memories utilize the position of a mechanical member to store data in the memory cell. When the mechanical member is in a first position that is spaced away from an electrode, the memory cell exhibits a first electrical property that indicates a first logic state. When the mechanical member is in a second position that contacts the electrode, the memory cell exhibits a second electrical property that indicates a second logic state. MEMS-based memories may be advantageously formed on the top surface of the interconnect layers of an integrated circuit, thereby consuming less silicon die area.
In one aspect, the systems and methods described herein provide for a one-time programmable memory having multiple memory cells. Each memory cell includes an electrode disposed within the memory cell and a conductor material having two ends disposed proximate to the electrode. The programmable memory provides means for applying a voltage between the electrode and the conductor material, e.g., a voltage source. The conductor material is physically attached to the memory cell at its two ends. The applied voltage generates an electrostatic force sufficient to alter the conductor material from a first state to a second state to program the memory cell. In some embodiments, the electrostatic force is sufficient to permanently alter the conductor material from the first state to the second state. In some embodiments, permanently altering the conductor material comprises bending the conductor material beyond a yield strength or an ultimate tensile strength of the conductor material. This may lead to permanent deformation or fracture of the conductor material. In some embodiments, the conductor material includes a metal bridge or a metal plate. In some embodiments, the conductor material of the memory cell includes one or more anchors. The conductor material is physically connected to the memory cell via the anchors. In some embodiments, the electrostatic force generates stress at the anchors in order to permanently alter the conductor material.
In some embodiments, each memory cell includes a second electrode. The programmable memory provides means for applying a second voltage between the second electrode and the conductor material, e.g., a voltage source. The second electrode may be disposed on a side of the conductor material opposite to the initial electrode. In some embodiments, the electrodes are disposed at the top and the bottom of the conductor material, respectively. The applied second voltage may generate an electrostatic force sufficient to force the conductor material into its original form, thereby reprogramming the memory cell. In some embodiments, the electrostatic force is sufficient to temporarily alter the conductor material of each memory cell from the first state to the second state. The conductor material may return to the first state after a period of time.
In some embodiments, each memory cell includes a current source for driving through the conductor material an applied current that heats the conductor material. The applied voltage and the applied current may be proportionally configured to alter the conductor material from the first state to the second state. In some embodiments, altering the conductor material changes a resistance of the conductor material, thereby altering a current flow through the conductor material when a sensing voltage is applied. In some embodiments, each memory cell is read by applying the sensing voltage between the electrode and the conductor material, and measuring the current flow through the conductor material. The current flow may be about 3 μA or lower. The applied voltage may be about 15V or lower. In some embodiments, the memory is fabricated within a MEMS device, a NEMS device, or any other suitable device.
In another aspect, the systems and methods described herein provide for trimmer device to trim or reconfigure a device or circuit, e.g., to adjust the electrical characteristics of the device or circuit. The trimmer device includes an electrode disposed within the trimmer device, a conductor material having two ends disposed proximate to the electrode, and means for applying a voltage between the electrode and the conductor material, e.g., a voltage source or any other suitable means. The conductor material is physically attached to the trimmer device at its two ends. The applied voltage generates an electrostatic force sufficient to permanently deform or fracture the conductor material such that its resistance is changed, thereby trimming or reconfiguring the circuit. In some embodiments, fracturing the conductor material changes a resistance of the conductor material, thereby altering a current flow through the conductor material when a sensing voltage is applied. In some embodiments, the conductor material includes a metal bridge or a metal plate.
In yet another aspect, the systems and methods described herein provide for a method for manufacturing a chip comprising a programmable memory arranged in an integrated circuit. The method includes producing layers that form electrical and/or electronic elements on a semiconductor material substrate followed by an Inter Level Dielectric (ILD) layer. The method further includes producing interconnection layers including an etch resistant bottom layer of conductor material and a top layer of conductor material, separated by at least one layer of dielectric material. The at least one etch resistant bottom layer of conductor material is layed over and in contact with the ILD layer. The method further includes forming a portion of the programmable memory within the interconnection layers by applying gaseous HF to the at least one layer of dielectric material. The programmable memory includes memory cells, and each memory cell includes an electrode disposed within the memory cell, a metal bridge or a metal plate disposed proximate to the electrode, and means for applying a voltage between the electrode and the one of the metal bridge and the metal plate, e.g., a voltage source. In some embodiments, the portion of the programmable memory is formed above the etch resistant bottom layer of conductor material in contact with the ILD layer.
Other advantages and characteristics of the systems and methods described herein may be appreciated from the following description, which provides a non-limiting description of illustrative embodiments, with reference to the accompanying drawings, in which:
To provide an overall understanding of the systems and methods described herein, certain illustrative embodiments will now be described. However, it will be understood by one of ordinary skill in the art that the systems and methods described herein may be adapted and modified as is appropriate for the application being addressed and that the systems and methods described herein may be employed in other suitable applications, and that such other additions and modifications will not depart from the scope thereof.
To program the memory cell, the voltage source 106 applies a voltage between the electrode 102 and conductor material 104 to generate an electrostatic force on the conductor material 104. In some embodiments, the applied voltage is about 15V in magnitude. The electrostatic force is applied such that the conductor material is pulled towards to the electrode 102 until it is permanently altered, e.g., either fractured or permanently deformed, as shown in
Permanently altering the conductor material via fracture or deformation, i.e., programming the memory cell, may change the resistance of the conductor material. As a result, when a sensing voltage is applied to the memory cell before and after programming, the resulting current flow may be different. This resulting current flow may be used as an indication of the logic state or value (zero or one) stored in the memory cell. In some embodiments, the applied current is about 3 μA in magnitude. Alternatively, when a sensing current is applied to the conductor material before and after programming, the resulting voltage in the memory cell may be different. This resulting voltage may be used as an indication of the logic state or value (zero or one) stored in the memory cell.
In some embodiments, the memory cell includes means for driving a current through the conductor material, e.g., a current source, to aid the electrostatic force in permanently altering the conductor material. The current heats the conductor material making it more susceptible to deformation. This may lower the voltage applied to generate the electrostatic force, thereby reducing the required size of the voltage source. In some embodiments, the current is adjusted to heat the conductor material, reducing the strength of the conductor material and, thereby, making the conductor material less resistant to the electrostatic force used to alter the physical state of the conductor material. The proportion of voltage applied and current applied may vary or be adjustable. For example, if a higher voltage is applied (resulting in a higher electrostatic force), then a lower current can be applied (resulting in less heating) or vice versa.
The yield strength is typically determined by the “offset yield method,” by which a line is drawn parallel to the linear elastic portion of the curve and intersecting the abscissa at some arbitrary value (generally from 0.1% to 0.2%). The intersection of this line and the stress-strain curve is considered to be the yield strength point 204. The elastic region till yield strength point 204 is the portion of the curve where the material will return to its original shape if the load is removed. The plastic region beyond yield strength point 204 is the portion where some permanent deformation will occur, even if the electrostatic force is removed. The conductor material fractures when the curve reaches fracture point 208.
In order to program or switch the current logic state of the memory cell, the voltage source 308 applies a voltage between the electrode 302 and conductor material 304 to generate an electrostatic force on the conductor material 304. In some embodiments, the applied voltage is about 15V in magnitude. The electrostatic force is applied such that the conductor material is pulled towards to the electrode 302 until it is permanently altered, e.g., permanently deformed, as shown in
Compared to the memory cell of
In some embodiments, the memory cell includes means for driving a current through the conductor material, e.g., a current source, to aid the electrostatic force in permanently altering the conductor material. The current heats the conductor material making it more susceptible to deformation. This may lower the voltage applied to generate the electrostatic force, thereby reducing the required size of the voltage source. In some embodiments, the current is adjusted to heat the conductor material, reducing the strength of the conductor material and, thereby, making the conductor material less resistant to the electrostatic force used to alter the physical state of the conductor material. The proportion of voltage applied and current applied may vary or be adjustable. For example, if a higher voltage is applied (resulting in a higher electrostatic force), then a lower current can be applied (resulting in less heating) or vice versa.
In order to program the memory cell, the voltage source 506 applies a driving voltage (Vd) between the electrode 502 and conductor material 504 to generate an electrostatic force on the conductor material 504. In some embodiments, the applied voltage is about 15V in magnitude. The electrostatic force is applied such that the conductor material is pulled towards to the electrode 502 until it is permanently altered, e.g., either fractured or permanently deformed. The alteration changes the resistance of the conductor material, thereby programming the memory cell.
To determine the logic state or value stored in the memory cell, voltage source 508 applies a sensing voltage (Vs). In some embodiments, the applied voltage is about 1V in magnitude. Output voltage (Vo) across resistor 510 is measured to determine the logic state or value of the memory cell. In some embodiments, the output voltage (Vo) across resistor 510 is about 0V (indicating a fractured conductor material) or about 1V (indicating the conductor material is unaltered). In this embodiment, the measured voltage across resistor 510 is based on a ratio between the resistance of the conductor material 504 (e.g., about 10Ω or any other suitable resistance) and the resistor 510 (e.g., about 100 kΩ or any other suitable resistance).
The proposed memory cell configuration requires significantly less silicon die area for implementation compared to a conventional programmable memory. For example, a conventional antifuse implemented in a 0.4 μm process may require a driving current of about 250 mA and a corresponding transistor size of about 50 μm×50 μm and a propagation track having a width of about 250 μm. However, the proposed memory cell configuration implemented in a 0.4 μm may operate using a driving current of only about 3 μA, with a corresponding transistor size of only about 5 μm×5 μm, and with a propagation track having a width of only about 0.4 μm. Therefore, the proposed memory cell and driving circuitry substantially reduce the silicon die area compared to conventional programmable memories.
In some embodiments, the memory cell described with respect
Described below are process flow steps for fabricating a programmable memory cell via a CMOS MEMS-based process. For example, the memory cell may be fabricated using a CMOS MEMS-based process described in commonly-owned U.S. Patent Application Publication No. 2010/0295138, entitled “Methods and Systems for Fabrication of MEMS CMOS Devices.” However, fabrication processes for the memory need not be limited to CMOS MEMS-based processes, and may include MEMS-based processes, NEMS-based processes, and other suitable processes.
In some embodiments, a programmable memory is arranged in an integrated circuit. The process flow steps of
Applicants consider all operable combinations of the embodiments disclosed herein to be patentable subject matter. Those skilled in the art will know or be able to ascertain using no more than routine experimentation, many equivalents to the embodiments and practices described herein. For example, though the fabrication of a memory cell is described with respect to
This application claims priority to U.S. Provisional Patent Application No. 61/435,057, filed Jan. 21, 2011, entitled “Methods and Systems for Fabrication of MEMS CMOS Devices,” hereby incorporated by reference in its entirety.
Number | Date | Country | |
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61435057 | Jan 2011 | US |