DE102005001668A1 - Memory circuit, has reference value unit generating reference value dependent on temperature of material, and comparator finding condition of memory cell depending on comparison result of electrical valve and reference value - Google Patents

Abstract

The circuit has a phase-changeable memory cell (1) with phase changeable material. An evaluating processor unit has a comparator that compares an electrical value that is dependant on a resistance value of the material with a reference value and determines the condition of the cell based on the comparison result. A reference value unit is designed to generate the reference value dependent on temperature of the material.

Description

The The invention relates to a memory circuit having a phase change memory cell.

Phase change memory cells in so-called PCRAM memories (PCRAM: Phase Change RAM) a chalcogenide compound in an amorphous and in a crystalline phase may be present. Typical chalcogenides are e.g. Ge-Sb-Te or Ag-In-Sb-Te compounds. Because the two phases the chalcogenide compound in their electrical conductivity can clearly differentiate the different states of the chalcogenide material by an electrical measurement and thus information in the phase change memory cell stored and retrieved. Describing yourself in the amorphous state located cell in the low-resistance crystalline Phase takes place by a heat pulse, the material over a Heats up crystallization temperature and thereby crystallize. Of the reversed process, i. a deletion is realized by the material above the melting temperature, which is bigger as the crystallization temperature, is heated and then by a quick cooling is brought into the amorphous, high-impedance state.

Of the respective state in which the phase change memory cell is, so by measurement the electrical conductivity of the phase change material. The electrical conductivity However, the phase change material is strongly temperature dependent, so that a reliable one Assessment of conditions the phase change memory cell due to the temperature dependence the conductivities is difficult. In particular, the various states of the Phase change memory cell a significantly different temperature behavior on, whereby the determination of the respective state of the phase change memory cell is difficult. This applies in particular to phase change Seich cells, in which several mixed phase states can also be set. So that can a so-called multi-level operation is possible, in which more as two states can be stored in a phase change memory cell. There there the conductivity ranges are significantly smaller, affects the temperature behavior of the individual phase states considerably more.

The Reading a state from a phase change memory cell is usually done by Determining an electrical conductivity dependent electrical variable, usually a voltage that has a reference voltage that is a limit between two conductivity ranges, the different states define, forms. With a strong temperature dependence the conductivities of the different states The phase change memory cell can be at a fixed reference sizes Incorrect determination of the state to be read come.

It Object of the present invention, a memory circuit with a phase change memory cell to provide, in particular across from strong temperature fluctuations is robust, so that the stored Date reliable read out.

These The object is achieved by the memory circuit according to claim 1.

Further advantageous embodiments of the invention are specified in the dependent claims.

According to the invention is a Memory circuit provided with a phase change memory cell, the having a phase change material. The phase change material can dependent from his degree of crystallinity take different resistance values. The memory circuit has an evaluation unit, which comprises a comparator, which is for detecting a stored in the phase change memory cell state one dependent on the resistance of the phase change material electrical Size with one Reference size compares and the dependent from the comparison result, a state of the phase change memory cell and thus the stored date is determined. The reference size is using a reference size unit generated, wherein the reference size unit is designed to increase the reference size depending on a temperature produce.

The inventive memory circuit has the advantage that as a reference size no fixed as possible constant electrical quantity, e.g. is provided as a fixed reference voltage, but a temperature-dependent, so that the state of a phase change memory cell also with their strong dependence the conductivity reliable from the temperature can be determined.

The reference size unit may be configured such that the reference size is generated depending on the temperature of the phase change material. In particular, the reference variable unit may be designed to generate the reference variable as a function of a predetermined temperature dependence of the electrical conductivity of the phase change material. Especially the different ones crystalline states of the phase change material lead to different temperature behavior of the conductivity, it makes sense to generate the reference size depending on the temperature dependence of the conductivity of the phase change material.

It it can be provided that the reference variable unit has a sensor, which is arranged close to the phase change material. This sensor can for example be provided with a diode with a temperature-dependent forward voltage be. A diode is particularly suitable, a certain temperature dependence the conductivity imitate the phase change material, since the temperature dependence the forward voltage of the diode is known very accurately.

According to one preferred embodiment of Invention, the evaluation unit may comprise a plurality of comparators, those for detecting the predominant in the phase change memory cell Condition one of the resistance value of the phase change material dependent electrical size with one compare to the respective reference size. There are also provided a plurality of reference size units, the generate the respective reference quantities, where at least one of the benchmarks of depending on the temperature is.

The Reference size unit can be designed to produce different temperature dependent reference quantities allowing more than two different states of the phase change memory cell are determinable. In particular, you can the respective reference sizes different temperature dependencies be generated.

According to one another embodiment The invention provides a writing circuit for determining the degree of crystallinity of the Phase change material of the phase change memory cell to be adjusted so that the resistance value corresponds to the predetermined state to be stored.

Preferably the electrical size is one of the resistance of the phase change material dependent voltage and the reference size one Reference voltage.

preferred embodiments The invention will be described below with reference to the accompanying drawings explained in more detail. It demonstrate:

1 the temperature dependence of the conductivities of an exemplary chalcogenide material at different phase states;

2 a block diagram of a memory circuit according to the invention with a multilevel phase change memory cell and a corresponding evaluation unit;

3a and 3b exemplary circuits for generating a reference voltage for evaluating the state of the phase change memory cell.

Phase change memory cells usually a phase change material capable of assuming various crystalline phase states. Usually a chalcogenide compound is used as phase change material, such as Ge-Sb-Te or Ag-In-Sb-Te compounds. Phase change materials can phase states assume between amorphous and crystalline and indicate accordingly their phase state different conductivities. This will be included a phase change memory cell used to provide information store in the form of the phase state. That one stored in a phase change memory Information leaves over the determination of the electrical resistance of the phase change material or over determining one of the electrical resistance of the phase change material dependent determine electrical size. In particular, the phase change material of the phase change memory cell Part of a voltage divider, for example, using a measuring resistor is constructed to one of the electrical resistance of the phase change material dependent To obtain read voltage, the corresponding phase state the phase change material is assigned.

The different crystalline phase states of the phase change material are subject to significantly different temperature dependencies, so that it is at temperature changes to a significant fluctuation of the electrical conductivities at the different phase states can come.

This may result in a threshold value comparison necessary for determining the phase state of the phase change material in a certain temperature range resulting in a false result, since the electrical conductivity has changed such that the electrical comparison variable exceeds the threshold determined by a reference variable. Particularly when more than two states are stored in such a phase change memory cell, the conductivity regions which define the different phase states come together, so that the high temperature dependency common in phase change materials results in a significant shift in the conductivity ranges of the respective regions leads to phase conditions.

In 1 a diagram is shown by way of example which represents the dependence of the resistance value of a phase change material in relation to the temperature (1 / kT). It can be seen that the larger the amorphous portion of the phase change material, the higher the temperature dependence of the phase change material is.

Several phase states of the phase change material are characterized in that the Phase change material in both an amorphous and crystalline State as well as in intermediate phase states, with the interphase states passing through different amorphous or crystalline fractions are determined. The Setting of the respective phase state can be done by different Current pulses occur with which the phase change material is warmed up. The transition in the high-resistance amorphous state can be achieved e.g. through a short heating of the active material above the melting point and a fast Cooling down, where the amorphous state of crystallinity is "frozen." When programming in the amorphous phase state, the crystallization kinetics is crucial. This is due to the thermal conductivity of the structure and the Current profile determined when switching off the heating current. A very fast cooling leads to a purely amorphous phase with a relatively high electrical resistance. You cool slower, resulting in more crystalline components in the phase change material, whereby the electrical resistance is set correspondingly lower becomes. The low-resistance crystalline state can be reached by longer Heating to a temperature below the melting point, at the crystallization process can take place, so that after a certain period of time the crystalline phase state is reached.

In 2 is a preferred embodiment of a memory circuit according to the invention with a phase change memory cell 1 shown. The phase change memory cell 1 in the illustrated embodiment, only a phase change device 2 from a phase change material, which is arranged for example on (not shown) word and bit lines of a memory cell array also not shown. For reading the phase change memory cell is the phase change device 2 with a measuring resistor 3 connected in series between a _read potential V _read and a ground potential GND, so that a read voltage across the series circuit of the phase change device 2 and the measuring resistor 3 is applied. The voltage divider thus formed supplies an evaluation potential V _A , that of an evaluation unit 4 is made available.

The evaluation unit 4 indicates according to the number of different phase states of the phase change component to be detected 2 a number of comparators 5 (Four in the present example), at whose first inputs in each case the evaluation potential V _{A is} applied. A second input of each comparator 5 is with its own reference voltage source 6 connected to a corresponding reference _potential V _Ref0 to V _Ref3 to the respective comparator 5 provides. The comparators 5 compare the evaluation _potential V _A with the respective reference _potentials V _Ref0 to V _Ref3 . Depending on the comparison result when comparing the evaluation _potential V _A with the respective reference _potential V _Ref0 to V _Ref3 , the output variables V _Out0 to V _Out3 at the respective outputs of the comparator 5 which can be interpreted in a digital manner. In the illustrated embodiment with four comparators, five different phase states of the phase change device 2 be detected. By the number of output bits of the evaluation unit 4 to reduce the output data can be combined by an appropriate link, so that the five different phase states can be coded by three bits of data.

Since the individual phase states of the phase change device 2 have different temperature dependencies in terms of their conductivity, are the reference voltage sources 6 provided so that they individually a temperature-dependent reference _voltage V _Ref0 to V _Ref3 to the comparator 5 invest. Possible curves of the reference voltages over the temperature are shown in the diagrams of the 1 shown with dashed lines.

Since the temperature dependence of the crystalline state of the phase change material is less than the temperature dependence of the conductivity of the phase change material in the amorphous phase state, the reference _voltages V _Ref0 to V _{Ref3 have} different temperature dependencies. Since the evaluation potential V _A is lower in the case of an amorphous phase state than in the crystalline phase state, the lower reference _potentials V _Ref0 to V _Ref3 must therefore have a higher temperature dependence than the higher reference potentials.

By the reference voltage sources 6 a temperature-dependent reference voltage to the evaluation unit 4 supply, the temperature range in which the phase change memory cell is too reliable, can be increased and the overall security of reading the stored data can be increased. In particular, the temperature of a phase change device 2 significantly from the number of previous writers ge depends because there the phase change material is heated to high temperatures, it makes sense, the temperature of the phase change device 2 to measure directly at his position. This is close to the phase change device 2 a thermal sensor 7 arranged, which is a measured variable to the reference voltage sources 6 supplies, so that the corresponding reference _voltages V _Ref0 to V _Ref3 depending on the on the phase change device 2 measured temperature can be generated.

Furthermore, a writing circuit 8th be provided, which are connected to the phase change device to adjust the state of the phase change material in the manner described above according to a predetermined date to be stored.

For example, as in the 3a and 3b represented the heat sensor 7 be formed in the form of a diode, wherein the temperature coefficient of the forward voltage of the diode is used. In 3a For example, a reference voltage source for a reference voltage having a negative temperature coefficient is shown. For this purpose, the diode is operated in the flow direction with a fixed _bias current I _bias so that a reference potential V _Ref falls across the diode. This reference potential has, for example, a temperature dependence in the range of the threshold voltage potential of -2.3 mV / Ke.

In 3b a circuit is shown how to achieve a reference potential with a positive temperature dependence. In addition the diode becomes like with 3a operated with a Biasstromm I _bias and the resulting voltage applied to an inverting element, which may be formed, for example in the form of an amplifier with a gain of -1. In the example shown, the amplifier is fed back with a differential amplifier which is correspondingly implemented in order to implement an amplifier with a gain of -1. Also other negative gain factors are via a positive feedback resistor 11 and a negative feedback resistor 12 adjustable to realize appropriate temperature dependencies.

Depending on the desired accuracy of the adjustment of the reference voltage to the prevailing temperature can for each phase change device 2 a thermal sensor 7 or for a group of multiple PMC devices 2 one heat sensor each 7 be provided.

Instead of in the embodiment of the 2 the illustrated non-transitory memory cell, the phase change memory cell may also have a selection transistor which connects the corresponding phase change device with the measuring resistor depending on a selection signal. Also, at the location of the potential comparison between the evaluation potential V _A and the reference potentials V _Ref0 to V _Ref3 , a comparison of a current dependent on the conductivity of the phase change component and a reference current can also be carried out.

1

Phase change memory cell

2

Phase change component

3

measuring resistor

4

evaluation

5

comparator

6

Reference voltage source

7

heat sensor

8th

Writing suppression Tung

Claims (10)

Memory circuit with a phase change memory cell ( 1 ), which has a phase change material, wherein the phase change material is suitable for assuming different resistance values depending on its degree of crystallinity, and with an evaluation unit ( 4 ) comprising a comparator adapted to detect a phase change memory cell (in 1 ) compares a value dependent on the resistance value of the phase change material electrical quantity with a reference variable, and which determines, depending on the comparison result, a state of the phase change memory cell, with a reference variable unit ( 6 ) for generating the reference variable, characterized in that the reference variable unit ( 6 ) is designed to produce the reference quantity depending on a temperature. Memory circuit according to claim 1, characterized in that the reference variable unit ( 6 ) is designed to produce the reference size depending on the temperature of the phase change material. Memory circuit according to claim 1 or 2, characterized in that the reference variable unit ( 6 ) is designed to generate the reference quantity depending on a temperature dependence of the electrical conductivity of the phase change material. Memory circuit according to one of claims 1 to 3, characterized in that the reference variable unit ( 6 ) a sensor ( 7 ), wherein the sensor is disposed near the phase change material. Memory circuit according to Claim 4, characterized that the sensor is a diode with a temperature-dependent forward voltage having. Memory circuit according to one of claims 1 to 5, characterized in that the evaluation unit ( 4 ) several comparators ( 5 ) for detecting in the phase change memory cell 1 ) compare a dependent of the resistance value of the phase change material electrical quantity with a respective reference size, and that a plurality of reference size units ( 6 ) are provided for generating the respective reference variables, wherein at least one of the reference variables is dependent on the temperature. Memory circuit according to claim 6, characterized in that the reference variable units ( 6 ) are designed to generate different temperature-dependent reference quantities, such that more than two different states of the phase-change memory cell ( 1 ) are determinable. Memory circuit according to claim 7, characterized in that the reference variable units ( 6 ) are provided to generate the respective reference quantities with different temperature dependencies. Memory circuit according to claim 8, characterized in that a write circuit ( 8th ) is provided to increase the degree of crystallinity of the phase change material of the phase change memory cell ( 1 ) so that the resistance value corresponds to the predetermined state to be stored. Memory circuit according to one of claims 1 to 9, characterized in that the electrical size a from the resistance value of the phase change material from pending voltage is, and the reference size is one Reference voltage is.