EP2022050A1

EP2022050A1 - Memory data writable and readable by micropoints, box structures and the manufacturing method

Info

Publication number: EP2022050A1
Application number: EP07729586A
Authority: EP
Inventors: Serge Gidon
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2006-05-30
Filing date: 2007-05-29
Publication date: 2009-02-11
Also published as: WO2007138035A1; FR2901909B1; US20090173929A1; JP2009539198A; FR2901909A1

Abstract

This invention concerns data storage memory, writable and readable using at least one writing (60) or reading micropoint which touches on a contact point with a dedicated writing or reading point on the surface of a substrate, either to modify the physical state of the zone, in reading or erasure mode, or to determine the physical state of the zone since the data stored in the zone is defined by the physical state of the zone. The surface of the substrate is subdivided into aggregated individual *batches (75) composed of one layer of a first sensitive material whose state can be modified by the application of a writing micropoint, each *batch (75) being surrounded by a box (80) made of a second material with little or no sensitivity to the application of the writing micropoint, with this second material completely separating the individual *batches from each other. The material of the boxes is the same as that of the *batches although they are differentiated by differentiating impurities. The organization into *batches and into boxes may be achieved using photolithography or through a process of self-organization of materials that can be spontaneously agglomerated into *batches.

Description

MICROPOINT-READABLE INDEXABLE DATA MEMORY, BUILD-IN STRUCTURE, AND METHOD OF

MANUFACTURING

The invention relates to data memories writable or readable via microtips.

In the quest for ever-increasing information storage densities, so-called microtip mass storage memories have been devised in which the data entry and the reading of the stored data are done by applying an apex-sized microtip. extremely small (a few nanometers) against the surface or near the surface of a substrate that carries a sensitive layer that will also be called media.

The application of a writing micropoint on the sensitive layer makes it possible to locally change the physical state of the layer without modifying the state thereof around the zone concerned. The state change may be a change in electrical state such as a resistivity value change, or a larger physical state change (for example, from an amorphous state to a crystalline state) that induces a change in state. elsewhere, most often also a modification of electrical, thermal or even chemical properties.

Conversely, the application of a read micropoint on a sensitive layer that includes zones that have not undergone this change of state, which can be called registered zones and unregistered zones, makes it possible to read the state of the zone.

The principle of the use of a microtip for the storage of data is inspired by the work that has been conducted in the field of atomic force microscopy (AFM for "Atomic Force Microscope"); this work has shown that one can explore a surface using a microtip with an extremely high geometric resolution (nanoscale).

For an atomic force microscope, the microtip is moved over the surface of an object to explore its relief by measuring microtip displacements; for a data memory, the microtip is moved over the surface of the substrate to write data, with a very high density, and reread them. The density is related to the size of the microtip and the position determination accuracy of the microtip on its displacements, as well as to the specific resolution of the media which depends on the grain size of the sensitive layer. To increase the read or write data rate, it has already been proposed to use a multiplicity of microtips in parallel.

The article "The" Millipede ", by P. Vettiger et al., Published in IBM Journal of Research and Development, Vol 44 No. 3 in May 2000, sets out these principles for a data memory in which the data storage is done by so-called "thermomechanical" effect: the microtip locally heats the sensitive layer area (a polymer) on which it comes to rest; this heating begins by softening the layer; the pressure exerted on the point drives it into the layer; a hole is created in the layer. For reading, a thermal effect is also used: the electrical resistance presented by the tip is sensitive to heat, and the temperature that the tip takes depends on whether the tip is in a hole created during registration and increases the heat transfer; it is therefore possible, by placing the tip in an electrical measuring circuit, to detect the presence of holes, in relation to the position of the tip.

More generally, these methods inspired by atomic force microscopy have given rise to various experiments using principles of sensitive layer that may be different from the principle explained in the previous paragraph. In the European patent application EP 0739004 A1 the sensitive layer is an insulating layer in which the microtip applies an electrical breakdown voltage locally creating an electrically conductive area in the middle of the insulating environment. Replay is electrical, by measuring the current flowing through the microtip. It may be noted that this solution does not allow erasure because the breakdown is irreversible, and the memory is not rewritable, which is a drawback.

Phase change materials, typically from the chalcogenide family such as Ge ₂ Sb ₂ Te ₅ , or AgInSbTe, have also been tested: by thermal action of the microtip on a localized area, the material can be passed locally. an amorphous state to a state crystallized. The state is reversible and one can theoretically erase a registered zone by putting it again in the amorphous state, always using a heating but under different conditions (generally with a soak, that is to say say rapid cooling). The article "Electrical probe storage using Joule heating in phase change media", by S. Gidon et al., Published in Applied Physics Letters, vol. 85 No. 26 on December 27, 2004, describes the principles of such a memory, with the peculiarity that heating for crystallization or for return to the amorphous state is made by Joule effect, by applying a current through the media, from the writing microtip. The layer is initially amorphous and poorly conductive; the inscription is made by local crystallization, under the effect of direct heating by Joule effect in the relevant zone of the layer. The crystallized material is more conductive than the amorphous material. The reading is done by applying a voltage (less than that used for rewriting) on a reading microtip and measuring the current flowing, which depends on whether the material has remained amorphous or has been crystallized.

In the article "Ultra-high-density phase-change storage and memory", by Hendrick F. Hamann and others, published on www.nature.com on April 9, 2006, the heating is indirect, the laser-heated microtip transfers its heat to the sensitive layer area with which it is in contact; in addition, the reading is done by thermal detection: the tip is heated (less than for writing) and the thermal impedance of the tip is measured. In all these realizations, assuming that erasure is theoretically possible, one realizes that it is probably very difficult to implement it practically. Local control of the binary state of an area punctually affected by a microtip may be questionable because the thermal action on an area has effects on the immediate environment of that area; in particular we understand that the heat generated can not remain completely localized where we want it. For example, ironing (erasing) a crystalline zone in the amorphous state may leave or create an unwanted crystalline ring around the amorphous area. The residual conductivity of this crown device may prevent detection of the amorphous nature of the area that was desired to be erased.

The invention particularly aims to facilitate the recording, reading, erasure, writable data memory and readable by microtips.

For this, the invention proposes a data storage memory, writable and readable by means of at least one writing or reading microtip that comes in proximity (in contact with or in the immediate vicinity) of a zone to write or read on the surface of a substrate, either to change the physical state of this zone, in writing or erasure, or to determine the physical state of the zone, the data stored in the zone being defined by the physical state of the zone, in reading, characterized in that the surface of the substrate is subdivided into a set of individual islands of a layer of a first sensitive material liable to change state under the action of the micropoint writing, each island being surrounded by a box formed by a second material little or not sensitive to the action of the writing microtip, this second material completely separating the individual islands from each other.

Thus, instead of using a uniform and continuous sensitive layer for recording data, a layer previously structured by a network of boxes (ie a mesh network of walls) connected to each other is used, which isolates the individual islets from each other; an individual island delimited by the inner periphery of a box constitutes an individual area corresponding to at least one elementary data stored in memory.

The sensitive layer is preferably constituted by a material capable of changing the crystalline phase by controlled thermal action, in particular a chalcogenide, and in particular a GeSbTe compound of germanium, antimony and tellurium or an AgInSbTe compound of silver, of indium, of antimony and tellurium, able to pass from an amorphous state to a crystalline state in a reversible manner under the effect of controlled heating. For the AgInSbTe material, the crystallization properties depend in particular on the proportion of silver, the silver lowering the crystallization temperature and thus facilitating it. Other materials are conceivable, such as compounds based on germanium and tellurium GeTe or germanium and selenium GeSe. The invention is however applicable to other types of materials, for example polymers capable of changing state and electrical conduction during a temperature cycle including for example a thermal quenching (very fast cooling). The material of the boxes surrounding the sensitive layer is preferably an electrically insulating material; it preferably has a low thermal conductivity. This may be in particular a ZnS zinc sulphide compound and SiO ₂ silica, rather rich in ZnS (70 to 80% by weight for example). Particularly advantageously from the point of view of cost and manufacturing accuracy, it is expected that the second material (that of the boxes), less sensitive to phase changes than the first material (that of the islands), is essentially formed by the same material as the first, but that differentiation impurities are contained in one and / or the other of the two materials. The impurities are chosen in such a way that they facilitate the change of amorphous / crystalline state for the first material (that of the individual islands), and / or that they make it more difficult to change the state for the second material (that of the the caisson network that surrounds the islets). The impurity implanted in the islands will preferably be made of silver which tends to lower the crystallization temperature and thus to facilitate the crystallization of the material. Conversely, the impurity implanted in the boxes will rather increase the crystallization temperature; the impurity may be hafnium or more generally an atom of great atomic number to better tend to hinder any process of crystallization. It may also be advantageous for the impurities implanted in the boxes, that these impurities are impurities tending to reduce the electrical conduction (oxygen, nitrogen, hydrogen, argon, gallium); indeed, the decrease of the conductivity will make it more difficult a phase change (crystallization in particular) in the caissons while this change of phase will remain possible in the islets which will not have received this impurity.

In a first exemplary embodiment, the substrate is covered with a thermal barrier layer of low heat conducting material and a continuous electrode which covers the barrier layer; the continuous electrode is covered with islands of the layer of sensitive material surrounded by boxes formed by the second material; all the islands and caissons are covered by a friction reduction layer of the microtip; this layer acts as a protective layer against microtip and media wear. The substrate may be silicon, glass, or organic material. The barrier layer may be silica, silicon nitride, or preferably ZnS zinc sulphide compound and silica SiO ₂ , the latter compound having a low thermal conduction (about 200 times less than silicon). The thickness of the barrier layer can be from about 10 nanometers to 100 nanometers.

The electrode may be of titanium nitride or carbon; it is preferably made of a material having both an electrical resistivity intermediate between the resistivities of the two crystalline and amorphous states of the medium and a low thermal conductivity. If the electrode is carbon, we can add metal elements such as silver, chromium, nickel, gold, to adjust the electrical conductivity. The thickness of the electrode may be of the order of 2 to 10 nanometers for example.

The wear protection layer of the microtips may be carbon. In a second exemplary embodiment, the substrate is covered with a thermal barrier layer of low heat-conducting material covered with islands of the sensitive layer; the islands are surrounded by caissons consisting of the superposition of the second layer, an electrode, and a third insulating layer electrically and thermally insulating; all the islands and caissons are covered by a friction reduction layer of the microtip; this layer acts as a protective layer against microtip and media wear. In this second embodiment, the electrode (electrically continuous due to the continuity of the boxes which are connected to each other) is somehow pierced with an opening at the location of each island, and the electrical connection between an island and the electrode is made by the edge of the electrode around the periphery of the island.

To define the geometry structured in islands and caissons, it is possible to use etching masks obtained by photolithography, but it is also possible to use so-called "self-organization" methods: in these In the processes, a layer of material is deposited under conditions such that the material automatically agglomerates into small islands separated from each other. This self-organization can produce, on very thin layers, a network of islands with a higher resolution than photolithography allows. The material thus deposited can be used directly as an active material in the final product, or it can constitute a mask for defining a pattern in another layer, this other layer possibly possibly itself constituting a layer of the media, or serving itself. same mask to define a pattern in a third layer. Consequently, the invention proposes a novel method for manufacturing a writable and readable memory via at least one writing or reading microtip that comes into contact with an elementary zone to be written on or read on the surface of a substrate, characterized in that the elementary zones are individual islands of a first material, surrounded by caissons of different material, and in that the islands are defined using a self-organizing step at least one substance which, when deposited on a surface of a substrate, is able to self-organize into a pattern of individual islands separated from each other.

The self-organizing substance may be an impurity intended to be diffused in an underlying layer to define individual islands. It can also be a substance serving as a mask for the treatment of an underlying layer.

The substance which self-organizes may in particular be a polymer, this polymer being deposited at the same time as a second polymer having affinities with the first, the bonding forces between the two polymers generating a self-organization in which the first polymer agglomerates into individual islands surrounded by a matrix of the second polymer.

Other features and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

FIG. 1 represents the principle of a microtip memory;

- Figure 2 shows the writing and erasure in the memory of Figure 1; FIG. 3 represents the principle of a microtip memory, structured according to the invention;

FIGS. 4 to 9 represent the steps of a method of manufacturing a memory according to the invention in a first exemplary embodiment;

- Figures 10 to 15 show the steps of manufacturing a memory in another embodiment.

In Figure 1, we recall the principle of a rewritable microtip memory constituted by a controllable phase change material. By "phase change" is meant above all the transition from an amorphous phase to a crystalline phase. One could at most consider materials that can pass from a first crystalline state to a second crystalline state distinguishable from the first. By "controllable phase change material" is meant a material whose crystallization temperatures are sufficiently low and the conditions for crystallization or return to the amorphous state are sufficiently known for selectively and voluntarily under the effect of an electric control by a microtip, to produce the passage of a state towards the other. In particular, the transition speeds between phases will be less than 10 microseconds.

A substrate 10 is covered by a continuous thermal barrier layer preventing excessive heat dissipation to the substrate during writing or erasure (too much heat dissipation would tend to prevent heat concentration and therefore crystallization).

The barrier layer 20 is covered with a continuous electrode 30 that can be brought to a desired potential (a ground potential for example, to simplify the understanding). The continuous electrode 30 is covered with a continuous layer 40 of a material such as a chalcogenide, especially Ge ₂ Sb ₂ Te ₅ . This material has the property of being able to change reversibly phase, between an amorphous phase and a crystalline phase, by thermal action at relatively easy to reach temperatures and under known conditions with respect to the rates of warming and necessary cooling for these phase changes. The layer 40 is the sensitive layer of the memory, it is she who stores the information; the binary information corresponding to a small area of the layer is the amorphous or crystalline state of this zone. Finally, the sensitive layer 40 is preferably covered with a layer 50 called the tribological layer. This layer serves to facilitate the sliding of the reading or writing microtip on the surface of the substrate for access to the different individual zones of the sensitive layer. This is a layer of protection against wear of the microtip. The writing can be done for example by applying an electrical voltage pulse between the microtip and the electrode (Joule direct heating). Erasing is done by applying a voltage pulse of different characteristics (shorter). The reading is done by applying a lower voltage on the microtip and measuring the current flowing through the tip.

In FIG. 1, there is shown a single read or write tip 60, but a multiplicity of individually controlled network peaks can be used to simultaneously access a large number of individual zones and thus to increase the speed of transmission. writing or reading the media. The writing or reading tips are extremely thin points (of the order of a few nanometers in area at their end) carried by the end of a lever arm cantilever.

Figure 2 shows schematically what happens when erasing a memory area, showing the risk of a bad erasure. It is assumed that the erased initial state of the memory is a state in which the entire sensitive layer is in an amorphous state (Fig. 2a). In this amorphous state, the vertical conductivity of the sensitive layer from the microtip to the electrode 30 is low. Information is created by rendering an individual area 70 (Fig. 2b) below the microtip; this is done by applying direct or indirect heating by microtip 60, at a temperature allowing crystallization. Heating at about 200 ^° C. for a hundred nanoseconds allows this to be done; this heating can be generated directly (Joule effect) by the passage of a current between the microtip 60 and the electrode 30. In the crystalline state, the electrical conductivity (vertically) is higher between the microtip and the electrode 30. The erasure can be done by applying a higher heating, of the order of 600 ° C to 700 ° C and soaking, that is to say, very fast cooling (of the order of 10 nanoseconds). This remelting followed by quenching makes it possible to reconstitute an amorphous zone in place of the crystalline zone 70. Unfortunately, this amorphous zone is not perfect and in practice it risks being in the form of an amorphous central portion. surrounded by a peripheral zone 74 which is partly crystalline (FIG. 2c). This results from temperature conditions and cooling rates which are different in the central zone and at the periphery. If this is the case, the memory point is badly erased because the residual conductivity of the peripheral zone can make believe, when reading, the presence of a crystalline zone and not an amorphous zone.

FIG. 3 represents a schematic example of structured memory according to the invention. The memory comprises individual sensitive zones 75 of a controllable phase change material (compound based on tellurium Te or antimony Sb or germanium Ge, such as GeTe or GeSb or SbTe or a chalcogenide such as GeSbTe, for example Ge ₂ Sb ₂ Te ₅ , or AgInSbTe) in the form of individual islands surrounded by boxes 80 of a material different from the material of the zones 75 (preferably a silica and zinc sulphide compound, or possibly a low-conductive carbon such as hydrogenated carbon). The material of the caissons 80 is of a nature such that it is less easy to crystallize than the material of the islands 75. The caissons form a continuous mesh network, or a sort of regular grid, and the sensitive zones 75 are presented as punctual islands in the openings of this network.

In the example of FIG. 3, the caissons, like the sensitive zones, are formed above a continuous electrode 30 (for example made of titanium nitride or of carbon made conductive by metal adjuvants such as Ag, Cr, Ni, Au etc.). The electrode 30 is itself formed above a layer 20 (preferably a silica and zinc sulphide compound) forming a thermal barrier between the electrode 30 and the substrate 10 (silicon or glass substrate or organic matter). A tribological protection layer 50 (preferably sprayed carbon) is preferably formed above the sensitive areas 75 and the caissons 80. the location of the sensitive areas 75, the superposition of layers can be similar to that of Figure 1.

The material of the boxes is chosen of a nature little or not sensitive to the action of a current applied by the microtip: it does not change crystalline state as easily as the material of the islets; it is also insensitive to the heat generated in sensitive islands 75 during their writing or erasure; in other words, this material does not easily change state from the point of view of its electrical conductivity, whether under the direct action of the writing microtip or under the indirect action of the writing of a neighboring island. The material of the caissons is therefore advantageously an electrical insulating material (that is to say more insulating than the material of the sensitive zones, both when the latter is in a crystalline state and when it is in an amorphous state) so that the currents applied between the microtip 60 and the electrode 30 pass into the sensitive zone above which the tip is placed without being deflected in the material of the boxes. The casing material is preferably a poor thermal conductor to help locate heat in the islands.

The writing is done by application, between the microtip (placed on the media above an island 75) and the electrode 30, of sufficient voltage (of the order of 3 to 5 volts), account- Given the high resistivity of the region of the sensitive layer in the amorphous state, to heat the island and bring it to the crystallization temperature for the time necessary for this crystallization. The heating current remains localized in the island and allows the crystallization of the layer preferably over the entire height of the island. The writing is typically by voltage pulses with a duration of the order of 100 to 1000 nanoseconds.

Erasing is done by applying a high voltage to a crystal island through the microtip to melt the island material. The voltage pulse that produces the direct heating current required for this reflow is very short and the falling edge of the pulse is particularly short (less than 10 nanoseconds) to induce a very short cooling time; this produces a kind of soaking, allowing the material to remain in the amorphous state after melting. The small size of the island and the fact that the heating current is well concentrated in the island, the whole of the zone becomes amorphous, without any risk of peripheral crystalline traces around the amorphous zone. The fact that the material of the box is relatively thermally insulating facilitates this quality of erasure, and from this point of view the choice of the ZnS-SiO ₂ compound for the boxes is favorable. Erase pulses can typically last 40 nanoseconds.

It should be noted that the surface layer 50 must have both a sufficiently high conductivity in the vertical direction so that the current applied by the microtip passes well in the vertical direction towards the island 75 and conductivity sufficiently low in the horizontal direction so that the current is not directed to the other islets (among which there may be islands that have passed into the more conductive crystalline state). The tribological layer 50 is very thin, which reduces its horizontal conductivity.

The electrode 30 preferably has a conductivity which is neither too low nor too high, for example a conductivity intermediate between that of the material of the islands 75 in the crystalline state and that of this material in the amorphous state. The order of magnitude is 1 ohm-cm and the carbon, optionally doped with adjuvants increasing or reducing its conductivity, is adapted to the realization of the electrode. The reading is done by applying a lower voltage (1 to 2 volts) between the microtip and the electrode 30. The current that is measured is measured and the amorphous or crystalline nature of the island under a microtip is deduced therefrom. . The current remains well confined in an island below the microtip, especially when the island is again amorphous, due to the lack of electrical conductivity of the boxes surrounding the island.

With a memory thus structured, one can choose indifferently that the material in the non-registered state is amorphous or crystalline, whereas in the continuous layer memories it is essential that the non-registered state is the state in which the material to phase change is the most insulating possible (in practice the amorphous state).

Figures 4 to 9 show by way of example the manufacturing steps of such a memory in a first embodiment. In this embodiment, the buried electrode is continuous as in FIG. 3 and the phase-change material is deposited on the electrode. We start (FIG. 4) a substrate 10 (silicon or glass or plastic) on which a layer 20 forming a thermal barrier (preferably 10 to 100 nanometers of silica or silicon nitride) is deposited uniformly by vapor phase deposition. or a ZnS-SiO ₂ compound known for its low thermal conduction). A layer 30 constituting the common continuous electrode is then deposited, for example a layer less than 5 nanometers of titanium or carbon nitride whose resistivity can be adjusted by incorporation of metallic elements (Ag, Cr, Ni, Au for example ). The proportion of sp3 and sp2 orbital bond hybridization carbon atoms can also be adjusted from the pressure and deposition temperature conditions to better control the resistivity.

A layer 40 of controllable phase change material is deposited on the electrode, preferably a chalcogenide such as Ge ₂ Sb ₂ Te ₅ (the exact proportions of the constituents may vary, for example it may be Ge22.2Sb22.2Te ₅₅ , ₆ ) or AgInSbTe. The thickness of this layer can be a hundred nanometers. The material is generally deposited in amorphous form.

By photolithography (the simplest solution) or by other methods (such as processes of self-organization of particles of which we will speak later, which allow a higher resolution, or processes of stamping with the help of an engraved mold with the desired pattern), a patterning mask of the layer 40 is formed to delimit individual islands 75 of phase change material layer which constitute the elementary points of the memory. The mask may be a resin mask or a mineral mask obtained by image transfer of a resin mask. Once the mask 77 has been formed, the layer 40 is attacked, for example by reactive ionic etching, where it is not masked, to form the islands 75. FIG. 5 represents the islands covered by the masking layer 77 which has served to protect them during this structuring phase. The masking layer may be removed at this stage or may be retained depending on its nature. In this example, it is considered to remain.

Then deposited (Figure 6), on the substrate thus covered with islands 75, a layer of a material which is electrically insulating and preferably poor conductor of heat. This material fills the gaps between the individual islands 75 forming a network of boxes 80 in which each box surrounds a respective island. The material may be a compound of silicon oxide and zinc sulfide. Its thickness is greater than the height of the islets 75.

The excess layer height 80 (and the mask 77 if it has not been removed before) is then removed (FIG. 7) by any known method (plasma, chemical mechanical polishing CMP).

Then a layer 90 of encapsulating material is deposited (FIG. 8) which protects the phase-change layer. This material may be titanium nitride or carbon made conductive by the presence of metal impurities. The thickness of the layer 90 may be about 5 to 20 nanometers.

Finally, the substrate is planarized for example by mechanical and chemical polishing and is deposited (Figure 9) a thin layer called "tribological layer" 50 which facilitates the sliding of the microtip and protects it from excessive wear. This layer may be of the same nature as the layer 90, in particular of carbon; it is very thin (less than 10 nanometers), it must be sufficiently vertically conductive to allow the passage of a current through the phase change layer, but it must be conductor horizontally so that there is no diverting current to another island 75 when the microtip is applied over an island. The material of the encapsulation layer 90 and the material of the tribology layer can also be deposited in a single step.

Another embodiment of the invention will now be described, in which the boxes surrounding the islands of phase-change material consist of the superposition of a first insulating layer (electrically and thermally), an electrode and a second insulating layer. . The phase change layer is not deposited above the electrode but passes through holes in the electrode. These holes physically correspond to the position of the islets.

We start (FIG. 10) with a substrate 10 (silicon or glass or plastic) on which a layer 20 forming a thermal barrier (preferably 10 to 100 nanometers of silica or silicon nitride) is uniformly deposited by vapor phase deposition. or a ZnS-SiO ₂ compound known for its low thermal conduction). We file a first layer 82 of a thermally and electrically insulating material which will constitute in part the material of the boxes surrounding the islands of phase change material. A layer 30 constituting the common continuous electrode is then deposited, for example a layer less than 10 nanometers of titanium or carbon nitride whose resistivity can be adjusted by incorporation of metallic elements (Ag, Cr, Ni, Au for example ). The proportion of sp3 and sp2 orbital bond hybridization carbon atoms can also be adjusted from the pressure and deposition temperature conditions to adjust the resistivity. And there is deposited a second layer 84, similar to the layer 82. The boxes will be constituted by the superposition of the layers 82, 30 and 84.

The steps of defining the box patterns are then carried out. The simplest is to use a photolithography operation by depositing and engraving a mask 77 whose pattern is that of the boxes to be made (Figure 1 1). Note that the mask is complementary to that used in the previous example (Figure 5).

The etching mask may be made from an insolated photosensitive resin or a layer of deformed material by any molding or stamping process. It can also be realized from self-organization processes which will be discussed later.

The mask thus obtained makes it possible to transfer the masking pattern into the underlying layers 84, 30, 82. This is done by reactive ion etching or ion beam etching. Holes are thus formed in this stack of layers and only the box pattern remains; the etching is stopped on the bottom of the layer 20; the mask 77 is then removed by chemical or mechanical action (FIG. 12).

A layer 40 of controllable phase change material is then deposited which at least partially fills the openings formed in the boxes. This material forms islands separated from each other and these islets constitute the individual memory areas (Figure 13). The phase change material is preferably sprayed over the entire surface and it is preferable to heat the substrate so that the material migrates to the bottom of the cavities. It is also possible to facilitate the migration of the phase change material at the bottom of the cavities by increasing the wettability of the surface and this is possible by first spraying a very fine layer of carbon or material having wettability properties on the surface having the cavities (eg chromium, nickel).

If there is an excess of phase change material, it is removed by etching so that this material does not completely fill the holes formed in the boxes.

Then, in a manner similar to that explained with reference to FIG. 8, an encapsulation material 90, for example titanium nitride or carbon (FIG. 14), is deposited.

The surface of the substrate is planarized by a mechanical and chemical polishing process and the process is terminated by depositing a thin layer of tribology 50 (FIG. 15), in the same manner as explained with reference to FIG. layer may be carbon atomized with metal adjuvants to adjust its resistivity.

The encapsulation material and the tribology layer may possibly be subject to a single deposit.

In the two preceding embodiments, the material of the individual islands (first material) is very different from the material of the caissons (second material) since it is respectively a chalcogenide and a ZnS-SiO ₂ compound. In a third embodiment, it is possible to use for the islands and caissons materials which are very close to one another but differentiated from the point of view of the crystallization properties. A structured memory is then used in which there is a layer of material which, for the most part, is a material capable of changing phase in a controlled manner during a thermal process, and the composition of the material is different in the islands. and the caissons that surround them so that the phase change is easier in the islands than in the caissons. In other words, the areas of memory consist generally of the same material as the boxes that surround them, but the compositions are slightly different in the islands and caissons. To obtain this structure, the composition of a uniform layer deposited on the substrate is locally modified; this modification is made either in the islets or on the contrary in the caissons which surround them. The composition modification can be made by implantation, diffusion, doping with species that chemically or structurally combine with the controllable phase change material. The method is based either on a mask for delimiting the implantation, doping or diffusion zones, or on a self-organization of the material to be diffused before proceeding to a step of actual migration of the dopant in the controllable phase change layer. In one example, a basic stack is carried out as previously described (FIG. 4) with a uniform deposition of a layer of phase-change material (chalcogenide, GeSbTe or InSbTe); then an open mask is made according to the pattern of the boxes to be made, by photolithography or stamping or self-organization; then gaseous species such as oxygen or nitrogen are diffused through the openings of the mask, which will reduce the conductivity of the phase-change material and thus make it very difficult to change the phase by application of write current in the scattered areas, that is to say in the boxes. This diffusion process is controlled by the conditions of pressure, temperature, and the duration of the bringing into contact with the species to be diffused. Finally, after removing the mask, deposit the tribology layer already mentioned. The presence of the tribology layer contributes to stopping the diffusion of oxygen or nitrogen in the material. In addition to oxygen and nitrogen, one could use hydrogen, or a heavy dopant such as hafnium or gallium or argon. These heavy atoms tend to increase the crystallization temperature of the phase change material, thus making the phase change more difficult.

The structure then comprises individual islands of controllable phase change material, surrounded by wells whose conductivity has become much lower than that of the islet material (in their amorphous or crystalline state) so that the writing microtip can not not to pass in the caissons a current which would be likely to change the structure or the electrical conductivity. In another example, the base material used is a compound of indium In, antimony Sb and tellurium Te (possibly containing gallium which tends to increase the crystallization temperature), and is added locally in the islets but not in the caissons impurities of a chemical species (silver in particular) which tends to facilitate the control of the crystallization (for example because the incorporation of this species lowers the crystallization temperature). For example, a mask is formed on a layer of InSbTe or InSbTeGa material, the masking pattern being open at the locations corresponding to the individual islands to be formed; silver impurities are deposited on this mask and diffuse into the InSbTe layer where the mask is open. Again, the mask can be formed from photolithography steps or by using a self-organizing method.

In this approach, one can even deposit money in conditions where it self-organizes into individual islands. Indeed, the money lends itself to self-organization by dewetting, that is to say without the need for prior photolithography steps to define the islets. For example, with a phase-change material which is a germanium-antimony-tellurium alloy, rich in antimony and tellurium, and even possibly doped with gallium to increase the crystallization temperature and thus make the phase change more difficult, it is possible to perform the following steps of realization: one establishes the base stack as described previously (figure 4), with a substrate, a thermal barrier layer, an electrode, and a layer of material with phase change; it is possible to add a carbon layer of very small thickness (less than 2 nanometers) to facilitate the dewetting of the silver layer. An extremely thin layer of continuous silver is deposited, of thickness of the order of a few nanometers; by self-organization assisted thermally (at a temperature of about 400 ⁰ C) the silver agglomerates into individual islands separated from each other in a fairly regular pattern. By increasing the temperature the silver migrates into the phase-change material at the point where it has agglomerated, and it modifies by lowering the crystallization temperature at these places which become the individual islands of the memory.

Another approach, symmetrical to the approach of creating islets by silver diffusion, is to do the opposite: the mask is opened at the place of the caissons and not the islets, one starts from a layer of material easily crystallizable (for example a compound AgInSbTe) and deposited on the mask, for the purpose of diffusion of species where the mask is open, an impurity such as hafnium (more generally atoms of great atomic number) which tends to impede any process of crystallization. The material of the starting layer remains a controllable phase change material outside the boxes defined by the mask, and it becomes a material without possibility of phase control in the masked islands. Thus, more generally, in these modes where the islands and caissons are made essentially from a common base material, one starts from a homogeneous layer of an active material and makes

individual islands more active in terms of ease of phase change than the caissons that surround them; - or conversely caissons less active in terms of ease of phase change than the individual islands surrounded by these boxes.

The invention has been described primarily with respect to phase change materials capable of reversing from an amorphous state to a crystalline state. It is applicable more generally to other materials which, without having properly speaking an amorphous phase and a crystalline phase, can have two states whose electrical conductivities, or even other properties, can be detected by a micropoint in phase. reading, the material can pass from one state to another under the effect of an action of the microtip in write phase. In the foregoing, reference has been made several times to steps of defining island patterns or caissons that would be obtained by self-organization rather than by conventional photolithography steps. An important aspect of the present invention is the fact that it is proposed to define the elementary zones of a writable and readable memory using a microtip using a step of self-organization of a thin layer in islands. in order to obtain higher resolution patterns than can be obtained by photolithography. In practice, the self-organization step will be a step of forming a mask from which it will be possible to perform a selective operation in the non-masked areas of at least one layer of material located under the mask. The self-organized mask may be a positive mask protecting areas corresponding to individual islands defining the elementary points of the memory, or on the contrary a negative temporary mask defining these islands but serving to define a complementary positive mask protecting the surrounding boxes. individual islands. In the latter case, the mask defined by self-organization will be eliminated before proceeding to steps of processing a layer under the complementary positive mask.

In one example, a self-organized mask is constituted in the following manner: a layer of a few tens of nanometers consisting of a mixture of two different polymers, which are polystyrene and polymethylmethacrylate, respectively, is deposited on the surface to be masked. a solvent such as toluene which allows sufficient mobility of the polymers. The two polymers organize themselves spontaneously by separating in a regular way: the polymethacrylate is formed in hexagonal cylindrical blocks embedded in a regular matrix of polystyrene. The diameter of the blocks and the periodicity of the network depend in particular on the molecular weights of the compounds. A long-term heat treatment (several tens of hours at a temperature of the order of 150 ° C.) stabilizes this organization. To obtain this self-organization, it is desirable to first treat the surface (silicon or silicon oxide or nitride, for example) on which the polymers are deposited, for example by rubbing it with a random mixture of the polymers; the surface thus treated may be the surface of the encapsulation layer of the memory layer (carbon, silicon, oxide, etc.). This avoids that one of the two polymers wets the surface more than the other and then prohibits the formation of cylindrical blocks.

After polymerization, one of the polymers can be selectively removed by a chemical which dissolves it without attacking the other polymer. For example, ultraviolet exposure degrades polymethacrylate by paralleling the polymerization of polystyrene, and it is only necessary to remove the polymethacrylate residues with acetic acid aided by ultrasonic agitation.

This gives a mask whose pattern is a network of polystyrene boxes having regular holes. This mask can be used for example to define by etching or diffusion patterns in the underlying layer, these patterns corresponding to the holes and to individual islands. For example, if the mask is deposited on a silicon oxide layer, holes can be made in the oxide layer corresponding to the holes of the mask by etching with CHF3 in reactive ion etching in the presence of argon; CH F3 does not attack the polystyrene but attacks the oxide. The silicon oxide layer thus etched with the self-organized pattern can itself serve as a mask.

If necessary, the mask thus produced can be used to define a complementary mask by a lift-off operation, that is to say an operation in which a material is deposited both on the mask and in the holes of the mask and then eliminates both the mask and the product that covers it leaving the product where it has been deposited in the holes of the mask.

The article by Guarini and others "Nanoscale patterning using self-assembled polymers for semiconductor applications", published in the Journal of Vacuum Technology Science B19 (6) Noc / Dec 2001, as well as the article by Guarini and others "Process integration of self-assembled polymer templates into silicon nanofabrication "in the same journal B20 (6) Nov / Dec 2002, outline the principles of this self-organization. Note that to facilitate a regular self-organization, it is preferable to divide the overall area of the area on which the copolymers are deposited in small surface elements separated from each other (for example we can define lines and columns without of polymers, to delimit a network of rectangles of limited size (for example a few hundred nanometers in side) within which the organization will be more uniform than if the whole surface self-organized a block.

Claims

1. A data storage memory, writable and readable via at least one read or write microbip that comes close to a point area to be written on or read from the surface of a substrate, or to change the physical state of this zone, in writing or erasure, or to determine the physical state of the zone, in reading, the data stored in the zone being defined by the physical state of the zone, characterized in that the surface of the substrate is subdivided into a set of individual islands (75) of a layer of a first sensitive material capable of changing state under the action of the writing microtip, each island (75) being surrounded by a box (80) formed by a second material with little or no sensitivity to the action of the writing microtip, this second material completely separating the individual islands from each other.

2. Memory according to claim 1, characterized in that the first sensitive material is constituted by a controllable phase change material, in particular a compound based on tellurium Te or antimony Sb or germanium Ge, such as GeTe or SbTe or a chalcogenide, and in particular a compound of GeSbTe or AgInSbTe, capable of passing from an amorphous state to a crystalline state in a reversible manner.

3. Memory according to one of claims 1 and 2, characterized in that the material of the boxes which surround the islands of sensitive layer is an electrically insulating material.

4. Memory according to claim 3, characterized in that the material of the boxes has a low thermal conductivity.

5. Memory according to one of claims 1 to 4, characterized in that the substrate (10) is covered with a layer (20) forming a thermal barrier, a material with low thermal conductivity, and a continuous electrode ( 30) which covers the barrier layer, the continuous electrode being covered with islands (75) of the layer of sensitive material surrounded by boxes (80) formed by the second material.

6. Memory according to one of claims 1 to 4, characterized in that the substrate is covered with a layer (20) forming a thermal barrier, a low heat conducting material, covered with islands (75) of the material of the sensitive layer, and the islands are surrounded by caissons consisting of the superposition of the second layer (82), an electrode (30), and a third layer (84) insulating electrically and thermally insulating, the electrical connection between an island and the electrode being made by the edge of the electrode around the periphery of the island.

7. Memory according to one of claims 5 and 6, characterized in that all the islands and caissons is covered by a layer (50) for reducing the friction of the microtip, preferably carbon.

8. Memory according to one of claims 5 to 7, characterized in that the substrate is silicon, glass, or organic material.

9. Memory according to one of claims 5 to 8, characterized in that the barrier layer is silica, silicon nitride, or preferably ZnS zinc sulfide compound and silica SiO ₂

10. Memory according to one of claims 1 to 9, characterized in that the material of the boxes is formed essentially by the same material as the material of the islets, differentiation impurities being contained in one or other of the two materials, these impurities being chosen such that they facilitate the change of state for the first material and / or they make more difficult the change of state for the second material.

1. A memory according to claim 10, characterized in that the differentiation impurities are contained in the material of the islets and comprise silver.

12. Memory according to one of claims 10 and 1 1, characterized in that the impurities are contained in the material of the caissons and comprise hafnium, oxygen, nitrogen, hydrogen, hydrogen, gallium, or argon.

13. A method of manufacturing a writable and readable memory via at least one write or read microtip that comes close to an elementary zone to be written on or to be read on the surface of a substrate, characterized in that the elementary zones are individual islands of a first material, surrounded by insulating caissons composed mainly of the same material as the islands, the material of the caissons being doped differently from the material of the islets, and in that the islets are defined by using a step of self-organization of at least one substance which, when deposited on a surface of a substrate, is able to self-organize into a pattern of individual islands separated from each other .

14. The method of claim 13, characterized in that the self-organizing substance is an impurity intended to be diffused in an underlying layer to define the individual islands.

15. The method of claim 13, characterized in that the self-organizing substance is a substance serving as a mask for the treatment of an underlying layer.

16. The method according to claim 15, characterized in that the substance which self-organizes is a polymer, and in that this polymer is deposited together with a second polymer having affinities with the first, the forces of bonding between the two polymers generating a self-organization in which the first polymer agglomerates into individual islands surrounded by a matrix of the second polymer.