RU2376677C2 - Memory cell with conducting layer-dielectric-conducting layer structure - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to micro- and nanoelectronics, and specifically to memory devices made using micro- and nano-electronic methods. The memory cell includes two electrodes with a dielectric layer placed between them. The dielectric layer has defects which provide electrical conduction by tunneling carriers in the defects, and contains semiconductor material impurities near one of the electrodes which provide for acquisition, storage and removal of electric charge, which blocks current from flowing in the defects of the dielectric layer.

EFFECT: design of a two-electrode nonvolatile reprogrammable memory cell with reproducible parametres, using the effect of electrical charge storage.

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Description

The invention relates to the field of micro- and nanoelectronics, and in particular to memory devices implemented using the methods of micro- and nanoelectronics.

The literature describes a memory cell in the form of a metal-oxide-semiconductor structure (MOS structure) (see US patent No. 7209384 from 04.24.2007). Such a MOS structure includes one field effect transistor, consisting of a selected semiconductor region with formed source and drain regions to which metal electrodes are connected. An insulating layer is formed on the surface of this region, in which the control electrode and a special electrically insulated region (“floating gate”) are located, located between the control electrode and the semiconductor region, capable of storing charge for a long time. The distance between the semiconductor and the floating electrode is from 10 to 50 nm. The presence or absence of a charge on a floating electrode encodes one bit of information.

Reading the state of the cell is carried out according to the level of conductivity of the channel "drain-source". When reading, in the absence of charge on the "floating" gate, under the influence of a positive field on the control gate, an n-channel is formed in the substrate between the source and the drain, and a current arises.

The presence of a charge on a "floating" gate changes the current-voltage characteristics of the transistor in such a way that the channel does not appear at the usual reading voltage, and no current arises between the source and drain.

When programming the drain and the control gate, high voltage is applied (moreover, the voltage is supplied to the control gate approximately two times higher). Electrons from the channel are injected onto the floating gate and change the current-voltage characteristics of the transistor.

When erasing, high voltage is applied to the source. Electrons tunnel from the isolated region to the source.

The disadvantage of such a memory cell is the complexity of circuitry solutions for the implementation of memory elements and the technology of their manufacture due to the presence of three electrodes for recording and reading information.

A known memory cell having a layered structure (“sandwich” structure) is a metal-insulator-metal (MIM), in which thin dielectric films of various oxides with a thickness of 10 nm to several microns located between two metal electrodes are used as an insulator (cm Dirnley J., Stoneham A., Morgan D. // UFN, 1974, v.112, issue 1, pp. 83-127). After fabrication of the structure, it is placed in a vacuum and so-called molding is performed, which consists in applying a constant voltage of up to 15 V to the electrodes. After that, the device exhibits N-shaped current-voltage characteristics under the influence of the applied voltage, which allows the use of such devices as memory elements . It was established that the possibility of forming depends on the composition and pressure of the residual atmosphere in vacuum, and the molding itself leads to the formation in the structure of channels similar to the breakdown channels between metal electrodes. An essential factor for obtaining the required current – voltage characteristics is the penetration of residual atmosphere molecules into the formed structure.

The disadvantage of this device is the low reproducibility of characteristics, which is associated with poor controllability of the conditions for the molding operation.

Closest to the claimed (prototype) is a memory cell with a structure of a conductive layer - dielectric - conductive layer (see Mordvintsev V.M., Levin V.L. RF Patent No. 2072591, 1997), which includes: two metal electrodes - an electrode of the first a level located on an insulating substrate, a second-level electrode separating electrodes of two levels, a dielectric layer with a thickness of 3 to 100 nm, an insulating gap in the form of an open end of the dielectric layer in the regions of intersection of the electrodes of the first and the edges of the second-level electrode, located in iruyuschey slit carbonaceous active material, the conductivity of which changes during the passage therethrough of the electron flux, and the medium above the surface of the insulating slits, ensuring delivery starting carbonaceous (organic) material and the ability to remove particles of the carbonaceous material from the surface of the insulating slits. In the case of applying sufficient voltage to the metal electrodes and passing through an insulating gap an electric current from organic matter due to the destruction of its molecules during electron impact, a carbonaceous active material is formed (the process of electroforming), the conductivity of which varies widely depending on its composition and structure, depending on from the values of current and past charge. In this case, the element can be transferred to a highly conductive state. The supply of a voltage pulse of a larger amplitude leads to the thermal removal of the formed carbon-conducting material and the element is transferred to a low-conductive state. Both states persist indefinitely when the power is off, and switching between them is possible repeatedly. Such an element can serve as the basis for constructing a non-volatile electrically reprogrammable memory device. A significant drawback of such an element of the non-volatile memory device is the low reproducibility of characteristics associated with the use of carbonaceous active material that undergoes chemical transformations in the process of recording information in a memory cell.

An object of the invention is the creation of a two-electrode non-volatile reprogrammable memory cell with reproducible parameters, using the effect of the conservation of electric charge.

The problem is solved in that in a known memory cell with a structure of a conductive layer - a dielectric - a conductive layer including two electrodes with a dielectric layer located between them, the dielectric layer has defects that provide electrical conductivity by tunneling charge carriers through defects and contains semiconductor material inclusions located near one of the electrodes and ensuring the acquisition, conservation and removal of an electric charge that blocks the flow of current through defects in the layer e dielectric.

The technical results achieved in this case are as follows.

Firstly, the proposed design eliminates the instability of the properties of the memory cell caused by the use of carbon materials that are subject to degradation during operation due to the absence of materials undergoing chemical transformations during operation.

Secondly, the manufacture of the proposed memory cell and memory devices based on it is possible using standard technological methods and designs of modern microelectronics.

No information similar to the proposed memory cell was found in the information sources, which allows us to conclude that it is new.

In addition, the totality of the features of the claimed cell is not obvious to a specialist from the achieved level of technology for solving the task, which confirms the compliance of the method with the criterion of "inventive step".

The rationale for the proposed memory cell, along with information confirming the possibility of its implementation, are given below on the example of a specific cell, implemented by us.

The essence of the invention is illustrated by drawings:

Figure 1. - A schematic representation of a memory cell with a structure of a conductive layer-dielectric-conductive layer, where 1, 3 - a conductive electrode, 2 - a dielectric layer, 4 - inclusion of a semiconductor material.

Figure 2. - Experimental current-voltage characteristics of a memory cell conducting layer-dielectric-conducting layer on the example of a metal-silicon dioxide-semiconductor structure.

Figure 3. - An approximate view of the energy diagram of a memory cell with a metal-silicon dioxide-semiconductor structure.

An example of the implementation of the inventive memory cell conductive layer-dielectric-conductive layer when using the structure of a metal-silicon dioxide-semiconductor with inclusions (clusters) of amorphous hydrogenated silicon with a size of 1-4 nm in silicon dioxide located near the metal electrode. The metal electrode is a sprayed Ni nickel film with an area of ~ 7 mm; a silicon dioxide film of SiO ₂ ~ 70 nm thick is used as the dielectric layer; a semiconductor substrate is a p ⁺ Si silicon wafer with a concentration of the main charge carriers of ~ 10 ^l9 cm ^{- 3} .

The invention is confirmed by the measurement of electrophysical parameters. The electrophysical parameters of the experimental structure are examined using a semiconductor device parameter meter. The current-voltage characteristics of the metal - silicon dioxide - semiconductor structure with inclusions in the insulating layer of the semiconductor material (Figa-2) confirm the possibility of using it as a memory cell.

When a positive voltage is applied to the silicon substrate (Fig. 2a), the current increases to about 250 microamps. At a voltage of ~ 5 volts, the current flowing through the structure drops sharply. The cell goes into a non-conductive state.

When the voltage of the same polarity is reapplied, the current practically does not flow through the cell (Fig.2b). This corresponds to reading a closed state.

When applying a negative voltage to the silicon substrate with a value of the order of ~ 20 V, a sharp increase in current is observed (Fig.2c).

Upon subsequent supply of a positive voltage to the silicon substrate with a voltage of up to 5 V, a current flow of up to 250 microamps is observed (Fig. 2d), which corresponds to reading the open state of the cell.

Thus, the open state is recorded by applying a large negative voltage to the silicon substrate, the closed state is recorded by applying a large positive voltage, and the cell state is read by a small positive voltage.

It is established that the closed and open state of the cells after recording does not change, at least for a month. When current flows at a voltage of 3 volts through an open cell, its value practically does not change over time.

Structures that differ from the described material and the type of conductivity of the substrate (silicon substrate of the electronic type of conductivity n + Si, metal substrate (aluminum, nickel)), a thickness of silicon dioxide of 50-130 nm, the type of material of the metal electrode (aluminum aluminum) were also experimentally studied. The results are similar to those described above.

The proposed memory cell obviously has the following mechanism of operation. Figure 3 shows an exemplary energy diagram of a metal-insulator-semiconductor memory cell with amorphous hydrogenated silicon clusters included in the insulating layer. At low positive voltages, a current flows through the silicon substrate by electron tunneling between silicon dioxide defects. This corresponds to a parabolic view of the current-voltage characteristics of the cell (Figa and 2g). In this case, electrons cannot enter the amorphous silicon cluster, since it has a sufficiently large band gap. At such voltages, it is possible to read the state of the cell (“open” or “closed”).

With a further increase in voltage, the electrons overcome the potential barrier through tunneling, fall into the cluster and are localized there. The result is a blocking of the flow of current through defects of silicon dioxide, the consequence of which is the termination of the flow of current through the cell (Figa). This effect is similar to the phenomenon referred to in the literature as “Coulomb blockade” (See MD Efremov, GN Kamaev et al. Physics and Technology of Semiconductors, 2005, Volume 39, Issue 8, pp. 945-952) . The tunneling of electrons from the cluster to the silicon substrate is negligible, since the distance between the clusters and silicon is much greater than between the metal electrode and the clusters.

When a negative voltage is applied to the silicon substrate, which ensures the tunneling of charge carriers from clusters to a metal electrode, electrons from the clusters tunnel into the metal. In this case, the effect of blocking the current flow chains through silicon dioxide defects disappears. This means that the cell goes into an open state.

Thus, in the proposed memory cell, information is encoded by the presence of a charge in the region of semiconductor material inclusions located near the metal electrode.

From the above description of the mechanism of operation of the memory cell, it follows that a similar mechanism of writing and reading information can be realized when most of the conductive, dielectric materials and semiconductor inclusions material are used in the cell structure.

Claims (1)

A memory cell with a structure of a conductive layer - dielectric - conductive layer, including two electrodes with a dielectric layer located between them, characterized in that the dielectric layer has defects providing electrical conductivity by tunneling charge carriers through defects and contains semiconductor material inclusions located close to one from electrodes and providing the acquisition, conservation and removal of an electric charge that blocks the flow of current through defects in the dielectric layer.

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