The present application relates to systems, devices and methods for memory access operations involving phase change memory units.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
Phase change memory (“PCM”) is a relatively new nonvolatile memory technology, which is very different from any other kind of nonvolatile memory. First, the fundamental principles of operation, at the smallest scale, are different: no other kind of solid-state memory uses a reversible PHYSICAL change to store data. Second, in order to achieve that permanent physical change, an array of PCM cells has to allow read, set, and reset operations which are all very different from each other. The electrical requirements of the read, set, and reset operations make the peripheral circuit operations of a PCM very different from those of other nonvolatile memories. Obviously some functions, such address decoding and bus interface, can be the same, but the closest-in parts of the periphery, which perform set, reset, and read operations on an array or subarray, must satisfy some unique requirements.
The physical state of a PCM cell's memory material is detected as resistance. For each selected cell, its bitline is set to a known voltage, and the cell's access transistor is turned on (by the appropriate wordline). If the cell is in its low-resistance state, it will sink a significant current from the bit line; otherwise, it will not.
Set and Reset operations are more complicated. Both involve heat. As discussed below, a “set” operation induces the memory material to recrystallize into its low-resistance (polycrystalline) state, while a “reset” operation anneals the memory material into its high-resistance (amorphous) state.
Write operations (Set and Reset) normally have more time budget than read operations. In read mode a commercial PCM memory should be competitive with the access speed (and latency if possible) of a standard DRAM. If this degree of read speed can be achieved, PCM becomes very attractive for many applications.
The phase change material is typically a chalcogenide glass, using amorphous and crystalline (or polycrystalline) phase states to represent bit states.
A complete PCM cell can include, for example: a top electrode (connected to the bit line), a phase change material (e.g. a chalcogenide glass), a conductive pillar which reaches down from the bottom of the phase change material, an access transistor (gated by a word line), and a bottom connection to ground. The phase change material can extend over multiple cells (or over the whole array), but the access transistors are laterally isolated from each other by a dielectric.
FIG. 2A shows an example of a PCM element 2010. A top electrode 2020 overlies a phase change material 2030, e.g. a chalcogenide glass. Note that material 2030 also includes a mushroom-shaped annealed zone (portion) 2070 within it. (The annealed zone 2070 may or may not be present, depending on what data has been stored in this particular location.) The annealed zone 2070, if present, has a much higher resistivity than the other (crystalline or polycrystalline) parts of the material 2030.
A conductive pillar 2050 connects the material 2030 to a bottom electrode 2040. In this example, no selection device is shown, though in practice, an access transistor would normally be connected in series with the phase change material. The pillar 2050 is embedded in an insulator layer 2060.
When voltage is applied between the top 2020 and bottom 2040 electrodes, the voltage drop will appear across the high-resistivity zone 2070 (if present). If sufficient voltage is applied, breakdown will occur across the high-resistivity zone. In this state the material will become very conductive, with large populations of mobile carriers. The material will therefore pass current, and current crowding can occur near the top of the pillar 2050. The voltage which initiates this conduction is referred to as the “snapback” voltage, and FIG. 2C shows why.
FIG. 2C shows an example of instantaneous I-V curves for a device like that of FIG. 2A, in two different states. Three zones of operation are marked.
In the zone 2200 marked “READ,” the device will act either as a resistor or as an open (perhaps with some leakage). A small applied voltage will result in a state-dependent difference in current, which can be detected.
However, the curve with open circles, corresponding to the amorphous state of the device, shows some more complex behaviors. The two curves show behaviors under conditions of higher voltage and higher current.
If the voltage reaches the threshold voltage Vth, current increases dramatically without any increase in voltage. (This occurs when breakdown occurs, so the phase-change material suddenly has a large population of mobile carriers.) Further increases in applied voltage above Vth result in further increases in current; note that this upper branch of the curve with hollow circles shows a lower resistance than the curve with solid squares.
If the applied voltage is stepped up to reach the zone 2150, the behavior of the cell is now independent of its previous state.
When relatively large currents are applied, localized heating will occur at the top of the pillar 2050, due to the relatively high current density. Current densities with typical dimensions can be in the range of tens of millions of Amperes per square cm. This is enough to produce significant localized heating within the phase-change material.
This localized heating is used to change the state of the phase-change material, as shown in FIG. 2B. If maximum current is applied in a very brief pulse 2100 and then abruptly stopped, the material will tend to quench into an amorphous high-resistivity condition; if the phase-change material is cooled more gradually and/or not heated as high as zone 2150, the material can recrystallize into a low-resistivity condition. Conversion to the high-resistance state is normally referred to as “Reset”, and conversion to the low-resistance state is normally referred to as “Set” (operation 2080). Note that, in this example, the Set pulse has a tail where current is reduced fairly gradually, but the Reset pulse does not. The duration of the Set pulse is also much longer than that of the Reset pulse, e.g. tens of microseconds versus hundreds of nanoseconds.
FIG. 2D shows an example of temperature versus resistivity for various PCM materials. It can be seen that each curve has a notable resistivity drop 2210 at some particular temperature. These resistivity drops correspond to phase change to a crystalline (or polysilicon) state. If the material is cooled gradually, it remains in the low resistivity state after cooling.
In a single-bit PCM, as described above, only two phases are distinguished: either the cell does or does not have a significant high-resistivity “mushroom cap” 2070. However, it is also possible to distinguish between different states of the mushroom cap 2070, and thereby store more than one bit per cell.
FIG. 2E shows an equivalent circuit for an “upside down” PCM cell 2010. In this example the pass transistor 2240 is gated by Wordline 2230, and is connected between the phase-change material 2250 and the bitline 2220. (Instead, it is somewhat preferable to connect this transistor between ground and the phase-change material.
FIG. 2F shows another example of a PCM cell 2010. A bitline 2220 is connected to the top electrode 2020 of the phase-change material 2250, and transistor 2240 which is connected to the bottom electrode 2030 of the PCM element. (The wordline 2230 which gates the vertical transistor 2240 is not shown in this drawing.) Lines 2232, which are shown as separate (and would be in a diode array), may instead be a continuous sheet, and provide the ground connection.
FIG. 2G shows an example of resistance (R) over time (t) for a single PCM cell following a single PCM write event at time t=0. The resistance curve 2400 for a cell which has been reset (i.e. which is in its high-resistance state) may rise at first, but then drifts significantly lower. The resistance curve 2410 for a cell in the Set state is much flatter. The sense margin 2420, i.e., the difference between set and reset resistances, also decreases over time. Larger sense margins generally result in more reliable reads, and a sense margin which is too small may not permit reliable reading at all. 2G represents the approximate behavior of one known PCM material; other PCM material compositions may behave differently. For example, other PCM material compositions may display variation of the set resistance over time.
The downwards drift of reset resistance may be due to, for example, shrinking size of the amorphous zone of the phase-change material, due to crystal growth; and, in some cells, spontaneous nucleation steepening the drift curve (possibly only slightly) due to introducing further conductive elements into the mushroom-shaped programmable region.
FIG. 2H shows an example of a processing system 2300. Typically, a processing system 2300 will incorporate at least some of interconnected power supplies 2310, processor units 2320 performing processing functions, memory units 2330 supplying stored data and instructions, and I/O units 2340 controlling communications internally and with external devices 2350.
FIG. 2I shows an example of a PCM single-ended sensing memory. Two different PCM cells 2400 on different ends of a sense amplifier can be selected separately. Selected elements 2410 are separately sensed by a single-ended sense amplifier 2420.
FIG. 2J shows an example of a known PCM single-ended sense amplifier 2500. Generally, in a single-ended sense amplifier, a cell read output conducted by a selected bitline BLB is compared against a reference current to provide a digital output OUT. When the PRECHARGE signal turns on transistor 2530, voltage V04 (e.g., 400 mV) precharges the bitline BLB. After precharge ends, the READ signal turns on transistor 2550. Transistor 2550 is connected, through source follower 2560 and load 2580, to provide a voltage which comparator 2600 compares to Voltage_REF, to thereby generate the digital output OUT.
A variety of nonvolatile memory technologies have been proposed over recent decades, and many of them have required some engineering to provide reference values for sensing. However, the requirements and constraints of phase-change memory are fundamentally different from those of any other kind of nonvolatile memory. Many memory technologies (such as EEPROM, EPROM, MNOS, and flash) test the threshold voltage of the transistor in a selected cell, so referencing must allow for the transistor's behavior. By contrast, phase-change memory simply senses the resistance of the selected cell. This avoids the complexities of providing a reference which will distinguish two (or more) possibilities for an active device's state, but does require detecting a resistance value, and tracking external variations (e.g. temperature and supply voltage) which may affect the instantaneous value of that resistance.
The possibility of storing more than one bit of data in a single phase-change material has also been suggested. Phase-change memories implementing such architectures are referred to here as “multibit” PCMs. If the “Set” and/or “Reset” operations can be controlled to produce multiple electrically distinguishable states, then more than one bit of information can be stored in each phase-change material location. It is known that the current over time profile of the Set operation can be controlled to produce electrically distinguishable results, though this can be due to more than one effect. In the simplest implementation, shorter anneals—too short to produce full annealing of the amorphous layer—can be used to produce one or more intermediate states. In some materials, different crystalline phases can also be produced by appropriate selection of the current over time profile. However, what is important for the present application is merely that electrically distinguishable states can be produced.
For example, if the complete layer of phase-change material can have four possible I/V characteristics, two bits of information can be stored in each cell—IF the read cycle can accurately distinguish among the four different states.
(The I/V characteristics of the cells which are not in the fully Set state are typically nonlinear, so it is more accurate to distinguish the states in terms of current flow at a given voltage; resistance is often used as a shorthand term, but implies a linearity which may not be present.)
In order to make use of the possible multibit cell structures, it is necessary to reliably distinguish among the possible states. To make this distinction reliably, there must be some margin of safety, despite the change in characteristics which may occur due to history, manufacturing tolerances, and environmental factors. Thus, the read architecture of multibit PCMs is a far more difficult challenge than it is for PCMs with single-bit cells.