Not applicable.
There exist many environments that may be considered hostile to modern electronics. As one example, consider petroleum drilling and production operations, which can require electronics to operate in a borehole miles below the surface. The borehole environment is often hot, with temperatures approaching and exceeding 200 Celsius. At such temperatures, bulk-silicon based memory devices may suffer performance degradation to the point of inoperability.
Memory devices include an array of memory cells and some support circuitry. At high temperatures, bulk-silicon based transistors in the support circuitry suffer from large leakage currents. The leakage currents degrade circuit performance and may even cause permanent damage as the resistive heating from such leakage currents contributes to the problem in a runaway fashion. In the array of memory cells, each memory cell includes one or more elements that store state information in some physical form (e.g., an electrical charge or voltage). In many existing memories, power is consumed to maintain the state information. Such power consumption increases when leakage current increases, thereby causing increased temperatures, increased leakage currents, and increased power consumption in an often-destructive trend. In another, not entirely distinct, group of memories, the state information is maintained as a charge on an electrically insulated conductor. Increased temperatures create thermally excited carriers that may cause the charge to bleed away, thereby destroying the state information.
Given such difficulties with memory device operation at elevated temperatures, it would be desirable to have a memory device that does not suffer from excessive charge bleeding or excessive leakage currents at high temperatures.
Accordingly, there is disclosed herein various nonvolatile integrated device embodiments suitable for use at high temperatures. In some embodiments, a high temperature nonvolatile integrated device comprises a sapphire or spinel substrate having multiple ferroelectric memory cells disposed upon it. In other embodiments, a high temperature nonvolatile integrated device comprises a silicon on insulator substrate or a large bandgap semiconductor substrate having multiple ferroelectric or magnetic memory cells disposed on it. In yet other embodiments, a high temperature nonvolatile integrated device comprises a sapphire, silicon on insulator, or a large bandgap substrate having programmable read only memory (PROM) cells or electrically erasable PROM (EEPROM) cells disposed on it.
A better understanding of the disclosed embodiments can be obtained when the following detailed description is considered in conjunction with the following drawings, in which:
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
Certain terms are used throughout the following description and claims to refer to particular system components and configurations. As one skilled in the art will appreciate, companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.
The following discussion concerns various high temperature non-volatile memory embodiments, including high temperature programmable read-only memories (PROMs), high temperature electrically-erasable PROMs (EEPROMs), high temperature magnetic random access memories (MRAMs), and high temperature ferroelectric random access memories (FRAMs). Also discussed are various embodiments of programmable logic devices (PLDs), including field-programmable gate arrays (FPGAs). A number of semiconductor technologies are first described below as a basis for implementing these high temperature devices.
Regions 210 and 216 are the channel regions, and they are each covered by insulating layers 220 and 222. Insulating layers 220 and 222 may be silicon dioxide, or alternatively may be an insulator with a high dielectric constant. The insulating layers 220 and 222 in turn are topped by conductive gates 226 and 228. The insulating layers 220 and 222 are very thin to allow a significant electric field to penetrate the surface of the channel regions when a small voltage is imposed on the gates. A much thicker insulating layer 224 is provided beneath device interconnects 232 to prevent a significant electric field from penetrating the non-active regions.
When a positive voltage is applied to gate 226, an n-type channel forms in the p-type region 210. The n-type channel provides a low resistance connection between regions 208 and 212. A similar voltage applied to gate 228 eliminates a p-type channel in the n-type region 216, electrically isolating regions 214 and 218. Conversely, when the positive voltage is removed from gate 226, the channel in region 210 disappears, electrically isolating regions 208 and 212. Removing the voltage from gate 228 allows a channel to form in region 216, electrically connecting regions 214 and 218. If electrode 230 is coupled to ground, electrode 234 is coupled to a positive supply voltage, and the gate electrodes are coupled together as an input node, then center electrode 232 is driven to the digital inverse of the voltage on the input node.
Note that this cross-sectional view and the ensuing views are not drawn to scale. The wafer substrate may actually be about 1 mm thick, while the semiconductor layer may (for example) be 10−8 to 10−4 m thick. The thickness of the conducting layers may be around 10-100 nm thick, and the gate insulators may be less than 10 nm thick. Due to the thin semiconductor layer used to construct the integrated circuit, the depletion regions around the doped regions have significantly reduced areas. Because leakage currents result from thermally excited carriers in the depletion regions, the significantly smaller depletion regions allow for greatly improved high-temperature performance due to correspondingly reduced leakage currents. More importantly, SOI devices will operate at significantly higher temperatures than bulk silicon devices can achieve.
Substrate 302 is a large bandgap semiconductor, meaning that it has a bandgap greater than that of silicon (1.12 eV). Silicon carbide (SiC) is suitable, having a room-temperature bandgap of 2.99 eV. Gallium arsenide (GaAs) may also be suitable, with a bandgap of 1.42 eV. The larger bandgaps significantly reduce the number of thermally excited carriers in the depletion region, thereby reducing undesirable leakage currents and providing acceptable performance at higher temperatures than bulk silicon devices.
Regions 410 and 416 are the channel regions, and they are each covered by gate insulators 420 and 422. Gate insulators 420 and 422 may be silicon dioxide, or alternatively may be a crystalline insulator with a high dielectric constant. The gate insulators 420 and 422 in turn are topped by conductive gates 426 and 428. The gate insulators 420 and 422 are very thin to allow a significant electric field to penetrate the surface of the channel regions when a small voltage is imposed on the gates. A much thicker insulating layer 424 is provided beneath electrical interconnects 432 to prevent a significant electric field from penetrating the non-active regions. As described previously, the desired inverting behavior may be accomplished by coupling electrode 430 to ground, electrode 434 to a positive power supply, and coupling the gate electrodes 426 and 428 together to serve as an input node. The center electrode 432 then carries a digital inverse of the voltage on the input node.
As with SOI construction, SOS construction provides for performance at high temperatures. The construction of transistors in a thin silicon layer reduces the depletion region size, thereby reducing leakage currents from thermally excited charge carriers in the depletion region.
The island construction further reduces depletion region size, and eliminates stray leakage paths between devices. Devices constructed in this manner can perform at higher temperatures than bulk silicon devices.
The various device constructions described above may be employed in combination with other techniques for increasing the maximum operating temperature. For example, trenches, guard rings, and other structures may be used to eliminate leakage through semiconductor layers 206 or 406, and through substrate 302. (Guard rings are conductive structures around sensitive areas. The structures are held at or near the same potential as the sensitive areas to reduce the electric field gradient, thereby reducing leakage currents).
A number of semiconductor technologies were described above as a basis for implementing high temperature devices. The following discussion concerns various high temperature non-volatile memory embodiments, including high temperature programmable read-only memories (PROMs), high temperature electrically-erasable PROMs (EEPROMs), high temperature magnetic random access memories (MRAMs), and high temperature ferroelectric random access memories (FRAMs).
The support circuitry 604 receives an address signal that is indicative of the row to be selected for access. The support circuitry further receives a read/write signal that is indicative of the desired type of access to the selected row. For a read access, the support circuitry provides a data signal indicative of the memory cell states sensed in the selected row. For a write access, the support circuitry receives a data signal indicative of the memory cell states to be set in the selected row. A bi-directional data bus may be used to convey the data signals to and from the support circuitry.
In addition to the foregoing functions, support circuitry may implement interface protocols for communicating with other integrated circuit devices. The protocols may include state machine logic for handling sequential operations to perform operations, and may further include level shifter circuitry to convert between internal and external voltage levels. In certain memory architectures, the support circuitry may include charge pumps to generate internal voltages well in excess of the power supply voltage.
Note that memory cells 702 are arranged in a simple rectangular grid of rows and columns. Each memory cell stores one bit of information, though multi-bit cells exist and may also be used. The relationship between the physical array and the logical arrangement of bits into words is determined by the way the address signal is partitioned. The three most significant bits of the address may be used to select one of the eight rows in the illustrated embodiment. The next two bits of the address signal may be used to select one of the four columns associated with each multiplexer. Since only two column groups are provided, each address location contains only two bits (one from each multiplexer). Thus the illustrated embodiment is organized into 32 two-bit words. A practical memory may include over 64 million 32-bit words.
In fuse technology, a thin conductive line is initially provided. The resistance state of selected bits is then changed by driving a large current through the fuse, causing it to heat and change phase to either a liquid or a gas. The mobile phase then separates from the leads, leaving an open circuit. In anti-fuse technology, an amorphous material (e.g., doped silicon) is initially provided. The amorphous material naturally has a high resistance due to the many grain boundaries that inhibit charge conduction. The resistance state of selected memory cells is then changed by driving a large current through the anti-fuse, causing the amorphous material to heat and melt. As the material cools, it crystallizes into a low resistance state with a minimal number of grain boundaries.
Diode 808 allows current to flow to the column lines, but prevents current from leaving the column lines to flow along the row lines. This selectivity is important to prevent currents on the bit lines from bleeding through non-selected row lines and non-selected memory cells to other bit lines. Such bleeding prevents reliable detection of the memory cell states. Accordingly, diode leakage currents will inhibit high temperature performance of the PROM. Operation at high temperatures can be enabled or enhanced though use of SOI processes, SOS processes, and large-bandgap substrates, where leakage currents in the memory cell array and the support circuitry will be significantly reduced.
At elevated temperatures, leakage currents caused by thermally excited electrons make detection of the selected memory cell's charge state more difficult, both because a selected transistor in a “non-conductive” state looks conductive in the presence of sufficient leakage, and because unselected transistors with significant leakage currents fail to isolate the column line sufficiently for accurate measurement of the selected transistor. Operation at high temperatures can be enabled or enhanced though use of SOI processes, SOS processes, and large-bandgap substrates, where leakage currents in the memory cell array and the support circuitry will be significantly reduced.
EEPROMs are non-volatile devices, as the floating gates can retain their charges for well in excess of 10 years, regardless of whether power is supplied or not. However, the floating gates requires a relatively lengthy time to charge or discharge, so EEPROM programming operations are relatively slow. Flash memory is a form of EEPROM that allows multiple memory locations to be erased or written at the same time, thereby significantly reducing the average programming time.
Returning to
With respect to magnetic materials, the terms “hard” and “soft” connote relatively high and low magnetic coercivities, respectively. A soft magnetic material can be oriented by a weaker magnetic field than can a hard magnetic material. Thus, soft magnetic layer 1110 can be re-oriented without altering the orientation of hard magnetic layer 1106, by simply not allowing the magnetic field to exceed the critical level required for re-orienting the hard magnetic layer.
Another factor that determines the orientation of the magnetic layers is the “easy axis.” Each of the layers may have an axis of preferential orientation along which less of a magnetic field is required to orient the layer, and along which the persistent magnetization of the layer will point (e.g., arrows 1107, 1111). Such an axis may be established by the geometry of the layer and/or by a crystalline orientation of the layer and/or by providing an anti-ferromagnetic layer for exchange biasing. Axes perpendicular to the easy axis are “hard” axes, and may require much higher fields to establish a persistent orientation. In some cases, magnetization along these axes may not be stable.
Arrow 1115 shows a field along a hard axis of the soft magnetic layer 1110. Such a field may be established by passing a current along conductor 1102 as shown by arrows 1103. Current flowing in conductor 1102 creates a circular magnetic field around the conductor in accordance with the “right hand rule.” A current flowing in conductor 1102 may make soft magnetic layer 1110 more susceptible to re-orientation by a magnetic field along its easy axis. Such a field may be provided by a current flowing through conductor 1104 as shown by arrows 1105. Current flowing in the direction shown may orient the soft magnetic layer 1110 as shown by arrow 1111. A current flowing in the opposite direction through conductor 1104 while current flows in conductor 1102 may orient the soft magnetic layer in the direction opposite arrow 1111. Thus currents flowing through a row line 1102 and column line 1104 may store information by appropriately orienting a magnetic layer at the intersection of the row and column lines.
Conductors 1102 and 1104 may be in electrical contact through the MTJ. A parallel orientation of layers in the MTJ may be detected as a (relatively) low resistance between conductors 1102 and 1104, while an anti-parallel orientation may be detected as a (relatively) high resistance between these conductors. If position-dependent variation of memory cell characteristics make it difficult to determine when a measured resistance value is high or low, so-called “destructive read” techniques may be employed. In a destructive read, a measurement of the memory cell's existing state is first made. Then a known state is written to the memory cell, and a second measurement is made. If the measurements match, then the pre-existing state matches the known state. Conversely, if the measurements differ significantly, the pre-existing state is the inverse of the known state. In this latter case, the pre-existing state has been destroyed, and a subsequent write operation is required to re-instate the original state.
The easy axes of the magnetic layers may be transverse to the axis of conductors 1206 and 1218. The orientation of soft layer 1212 may be set in the direction shown by arrow 1214 by passing currents through conductors 1206 and 1218 in the directions shown by arrows 1208 and 1220, respectively. (Conductor 1218 may be electrically isolated from the memory cell.) The magnetic fields around conductors 1206 and 1218 may combine to provide a magnetic field strength sufficient to re-orient soft layer 1212, where the fields individually would be insufficient to do so. The orientation of layer 1212 may be set in a direction opposite arrow 1214 by reversing the currents in both conductors.
The structure formed by electrode 1332, ferroelectric layer 1350, and electrode 1352, electrically behaves much like a capacitor, with the following notable difference. When an electric field is applied to the ferroelectric layer, a “charge spike” will occur if the polarity is opposite the polarity of a previously-applied electric field. This difference allows the structure to be used as a bit memory. The polarity of an applied electric field “sets” the ferroelectric material to one of two possible states. The state can be later determined by the presence or absence of a charge spike when a subsequent electric field is applied. The read operation is destructive, so a re-write may be used to reset the state of the ferroelectric material.
Ferroelectric memory 1402 receives an address signal ADDR, a read/write control signal, and a bidirectional data bus. A column decoder 1416 receives a portion of a memory address ADDR and asserts a corresponding row line. Ferroelectric memory 1402 further includes a set of column line pair multiplexers/demultiplexers (MUX/DEMUX) 1418. Each MUX/DEMUX 1418 receives the remaining portion of the memory address ADDR and couples the corresponding column line pair to a Driver/Detector circuit 1420. For write operations, the driver/detector circuits 1420 drive a voltage between the column line pair with a polarity that indicates the received data bit. For read operations, the driver/detector circuits 1420 drive a predetermined voltage between the column line pair and measure the presence or absence of a charge spike. The presence or absence is decoded as a one or zero (or vice versa), and the detected data is provided on the data bus. If a charge spike is detected, the Driver/Detector circuit 1420 drives an opposite polarity across the column line pair to reset the ferroelectric memory element to its original state.
A number of nonvolatile memory cell architectures have now been described. Each of these architectures may be implemented using SOI, SOS, or large-bandgap semiconductor technology. Such construction of the memory arrays and support circuitry may allow the memory to operate at higher temperatures than would be possible with elements implemented in bulk silicon technology. Another technique for enhancing performance in adverse conditions is illustrated in
In GMR MRAM cells, the column lines and row lines run in series with the magnetoresistive elements.
The above-described memory architectures are nonvolatile and provide relatively fast read accesses. Nonvolatile memory cells may be employed in other devices to provide a nonvolatile configuration of such devices. For example, reconfigurable logic such as PLDs and FPGAs may employ one or more of the foregoing architectures and may be implemented with a semiconductor process that allows operation at elevated temperatures. Alternatively, a general purpose (high temperature) processor may be coupled to a nonvolatile memory to operate in accordance with program instructions and data stored in the nonvolatile memory. Otherwise volatile configurable devices may similarly be coupled to a nonvolatile memory to retrieve configuration information stored therein whenever power is restored. Examples of such devices include programmable gain amplifiers, configurable analog-to-digital or digital-to-analog converters, latching relays, “sticky” switches that remember their positions after power loss, and circuits needing factory calibration or periodic calibration in the field, such as temperature compensated thermometers, voltage references, digital trimpots, and configurable ASICs.
The memories described herein operate at temperatures greater than bulk silicon integrated memories. Accordingly, these memories may be particularly suitable for operation in high temperature environments such as deep boreholes, jet engines, internal combustion engines, automotive environments, and power generation environments.
Many integrated circuits are subject to performance degradation or failure at moderately elevated temperatures, while other integrated circuits may continue to perform adequately at such temperatures. In various circuits that may be desirable for long-term installation at moderately elevated temperatures, continuous operation is not necessary. Rather, certain portions of a circuit may need to be accessed only briefly and at infrequent intervals, e.g., nonvolatile program memory may only need to be accessed at power-on and reset events. Voltage references may only be needed at infrequent calibration events. In such circuits, refrigeration efforts may be localized to just that portion of the circuit that requires cooling. Further, the refrigeration may be performed only when the operation of the temperature-sensitive circuits is needed. In such circuits, refrigeration operations may be performed directly on the die or package containing the temperature-sensitive circuitry, greatly reducing the thermal mass that needs to be cooled. Further, since the refrigeration operations may be brief and infrequent, the refrigeration system may be small, and the heat sink may be reduced in size or eliminated. In this manner, the size and power requirements for electronics cooling may be drastically reduced. Thus pinpoint refrigeration may be an efficient and desirable way to enable or improve device performance in high temperature environments.
In the MCM of
Depending on the various parameters for cooling the electronics and the performance of the cooler, a dedicated heat sink may be unnecessary. In the MCM of
Die 1908 may include a Flash memory and a voltage reference. Flash memory can generally retain information at temperatures above the point where the read and write circuitry fails. Upon needing to access the Flash memory to retrieve or store data, a controller may energize the Peltier cooler and pause for a predetermined time interval to allow the memory to cool to an operating temperature range. Once the interval ends, the controller may perform the needed memory accesses and de-energize the cooler. A volatile memory may be used to buffer data traveling to and from the Flash memory, thereby reducing the frequency of accesses to the nonvolatile memory.
In a similar fashion, a controller may energize the Peltier cooler and pause for a predetermined cooling interval before performing a calibration operation with a voltage reference. The accuracy of the voltage reference may be increased by limiting the temperature range in which it is employed.
Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the disclosed invention embodiments may be applied in elevated temperature environments unrelated to wells. For example, the disclosed embodiments may be employed for engine monitoring, heat-driven power generation, materials processing, and oven controls. It is intended that the following claims be interpreted to embrace all such variations and modifications.
The present application claims priority to the following patent applications: U.S. Provisional Patent Application 60/520,950, filed Nov. 18, 2003, U.S. Provisional Patent Application 60/520,992, filed Nov. 18, 2003, and U.S. Provisional Patent Application 60/523,150, filed Nov. 18, 2003. The foregoing applications are hereby incorporated by reference.
Number | Date | Country | |
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60523150 | Nov 2003 | US | |
60520992 | Nov 2003 | US | |
60520950 | Nov 2003 | US |