Patent Application
20090175153
The invention relates to a medium for recording data by electrical effect comprising at least a recording layer whereon there is arranged a localized electrical conduction layer formed by a material presenting a low electrical conductivity and wherein inclusions presenting a higher electrical conductivity than that of said material are dispersed.
Data recording, in both the computer and multimedia fields, has to meet an increasing requirement for capacity. Different techniques have been developed ranging from the magnetic hard disk to the recording medium using optics (of DVD type for example) and solid memories. Whatever the recording technique used, the size of the memory dots (bits) is always sought to be reduced. Increasing the recording capacity means increasing the storage density. But the means of accessing the recorded data often fix the maximum possible storage density: for example the size of the laser spot in optics or the line/column addressing circuit for solid memories.
Recently, very large storage capacities of about one Terabit/cm2 have been obtained by implementing microtips of the same type as those used in the field of atomic force microscopy (“The Millipede—More than one thousand tips for future AFM data storage”, P. Vettiger et al., IBM J. RES. Develop., vol. 44, no 3, May 2000, p. 323-340). High density is obtained by localizing the bits by means of microtips with an apex of nanometric dimension. Generally, a plurality of microtips is used, in quasi-contact with the surface of the recording medium, to locally modify the properties of said medium and to therefore encode data before being able to read it back.
Recording media can be sorted into different families depending on the type of properties modified in the medium when recording of the data takes place. Data recording can therefore be achieved by forming holes in a layer of polymer material or by reversible change from an amorphous state to a crystalline state in a phase change material or by magnetic or optic effects.
Data recording is also performed by displacement of electrically charged species (electrons, ions) or of a current from a microtip through the recording medium. Such a technique also enables bits presenting a very good spatial resolution to be obtained.
S. Gidon et al., in the article “Electrical probe storage using Joule heating in phase change media” (Applied Physics Letters, Vol. 85, No 26, 27/12/2004, pages 6392-6394) studied data recording performed by means of microtips in a recording medium by phase change. The recording medium is formed by a stack successively comprising a silicon substrate, a carbon electrode, a Ge2Sb2Te5 recording layer and a carbon protective layer. Recording is achieved by heating an area of the recording layer by Joule effect. Said area is in fact designated by a microtip electrically polarized so that a current flows through the medium, which enables a mark with a dimension of 15 nm to be obtained.
However, the performance of such recording media results from the fact that the recording layer is designated, in near field, by microtips. The protective layer or layers arranged between the microtips and the recording layer, in particular to protect the medium against friction of the microtips, have to be as thin as possible so as not to reduce the spatial resolution. This constraint is not however compatible with long lifetime of recording media, all the more so as the protective layers are often subjected to large stresses such as heating caused by flow of the current or effects linked to the surface electric field.
In Patent application EP-A-0739004, the recording medium does not comprise a protective layer. Data recording is performed by modifying the conduction of the recording medium locally, under an electric field. This modification is achieved by breakdown of a silicon oxide insulating layer formed on a p- or n-doped silicon substrate and used as recording layer. Applying a voltage between a microtip placed on the same side as the insulating layer and said substrate in fact creates a current flux of Fowler-Nordheim type that causes a local decrease of the resistance of the insulating layer. This local decrease of the resistance of the insulating layer is thereby used to record data in the insulating layer. However, the absence of a protective layer makes this type of recording medium extremely fragile.
In the document US2005/0285169, the hysteretic material memory layer of a memory matrix is covered by a conducting layer that is electrically anisotropic. The conducting layer is formed by a method enabling a molecular structure to be obtained containing pass-through conducting dots insulated from one another by a matrix of lower conductivity. The molecular structures used are for example metallic clusters, grains in the form of columns, granular films, fullerenes, nanoparticles, etc. In the document US2005/0285169, the conducting layer with its conducting dots enables the multilayer masked structures designed to delineate the ferroelectric cells from the ferroelectric random-access memories (FeRAM) to be replaced.
In Patent application GB1088117, a moving data recording medium by electric effect comprises an electrically conducting substrate on which there is deposited a molybdenum disulphide film able to be selectively and reversibly changed from a high impedance state to a low impedance state by a controlled electric current applied between an electrode tip and the conducting substrate. The molybdenum disulphide film can be covered by another film, etched in the form of a plurality of conducting portions, the spaces between the contacts being filled with an insulating material. Data recording and erasure are thereby performed in the areas of the molybdenum disulphide film located directly under the conducting portions. Such a film is not however easy to implement and does not enable precise recording of data.
The object of the invention is to remedy the shortcomings of the prior art. More particularly, the object of the invention is to propose a data recording medium by electrical effect presenting high data storage capacities.
According to the invention, this object is achieved by the appended claims.
Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention given for non-restrictive example purposes only and represented in the accompanying drawings, in which:
According to a first embodiment illustrated in
What is meant by electrical-effect data recording medium 1 is a recording medium able to use charge storage effects, charged species (ions, cations) migration effects or sub-field conduction effects. This can cause a localized modification of the local electrical conductivity in the recording layer or memory layer of the medium, for example by breakdown or by current flow. The electrical effect can also be considered as a field effect inducing a concentration of the current lines in a more or less conducting recording layer. In this case, the write process can be based on a thermal effect in a recording layer with phase change or on an electrolytic effect or on any other effect leading to displacements of charged species.
In a recording device by microtips 2, a voltage is applied between at least one microtip 2 and an electrode arranged under the recording layer in such a way as to generally modify the electrical conduction of the recording layer locally.
Thus, in
Localized electrical conduction layer 4, arranged on recording layer 5, is a thin layer comprising localized areas having a higher electrical conduction than that of the rest of said thin layer 4. More particularly, thin layer 4 is formed by a low electrical conductivity material 8 in which inclusions 9 of higher electrical conductivity than that of material 8 are dispersed. Inclusions 9 thereby define the higher electrical conductivity areas. In
The presence of a localized electrical conduction layer 4 on recording layer 5, and more particularly between recording layer 5 and microtips 2, protects recording layer mechanically, in particular against friction caused by microtips 2 but also against any mechanical compression stresses resulting from contact of the microtips on the recording device.
Furthermore, arranging a localized electrical conduction layer 4 on recording layer 5 means that the electric field effect is focused, enabling not only large storage capacities to be obtained while preserving a good spatial resolution but also the energy required for recording said data to be reduced by localizing the electrical effect very finely. The presence of localized areas with a high electrical conductivity, such as inclusions 9, in fact takes advantage of an electrical effect, in medium 1, amplified by the form factor of the volume of said area and more particularly of the volume of inclusion 9. An inclusion 9 of oblong shape thereby leads, when a microtip 2 is located near said inclusion 9, to prolonging the tip effect in localized electrical conduction layer 4, before reaching recording layer 5 in which marks will be formed. Prolonging the tip effect in localized electrical conduction layer 4 can occur by electrical influence or by conduction by proximity of dots, conduction by proximity of dots being equivalent to a conduction of jump conduction type in a medium that is not very conducting. Thus, in
More particularly, numerical modelling of the electric field near an inclusion 9 such as the one represented in
The thicknesses of layers 4 and 5 range from a few nanometers to a few tens of nanometers. For example, the thickness of localized electrical conduction layer 4 may reach a value of 50 nm.
Furthermore, the fact that material 8 presents a non-linear electrical conductivity presents the advantage of obtaining a caisson effect when an electrical field is applied between a microtip 2 and an electrode 6. A material with non-linear electrical conduction does in fact enable the electrical field to be directed onto the inclusion 9 that is closest to said microtip 2. This means that a single mark is created in recording layer 5, said mark being located substantially under the corresponding inclusion 9. On the contrary, with a material with linear electrical conduction, applying an electrical field near several inclusions leads to several marks being formed in the recording layer. This feature of material 8, combined with the oblong shape of the inclusions, is particularly advantageous in so far as the recording medium benefits from a double focusing effect of the field lines, on account of the oblong shape of the inclusions and of the non-linear conduction property of material 8.
Advantageously and as represented in
For example, a recording medium 1 such as the one represented in
a) deposition of the thin layer forming electrode 6, for example by physical vapor deposition (PVD) of amorphous carbon that may be charged by adding metallic inclusions (for example made of gold or silver) designed to increase the electrical conductivity of the thin layer, either in the target or by co-sputtering,
b) deposition of recording layer 5, for example by PVD or by chemical vapor deposition (CVD) or by vacuum evaporation. Recording layer 5 is for example formed by a quasi-insulating material that is able to break down by electric field effect, such a material being for example silica or alumina with a thickness of the thin layer of less than 2 nm. Recording layer 5 can also be formed by a low electrical conductivity material the value whereof is modified by localized current flow, such as carbon charged with impurities that are able to coalesce when an electrical effect takes place and therefore to locally modify the electrical conductivity of the layer,
c) growth of nanotubes forming inclusions 9, for example carbon nanotubes, using for example the method described by M. Chhowalla et al. in the article “Growth process conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition” (Journal of Applied Physics, Vol. 90, No 10, 15/11/2001, pages 5308-5316),
d) deposition of a material 8 with non-linear electrical conductivity so as to form localized electrical conduction layer 4 covering the carbon nanotubes.
e) planarization of localized electrical conduction layer 4, for example by chemical mechanical polishing (CMP), so as to control the thickness of layer 4 and more particularly the height of inclusions 9,
f) deposition of protective layer 3, for example achieved by PVD deposition of carbon that may be charged by metallic inclusions enabling a good electrical conductivity to be obtained.
Such a method for producing recording medium 1 enables the thickness of recording layer 5 to be controlled precisely. This is particularly important in that the latter has to be very small to enable a large electrical field to develop. The thickness of recording layer 5 is more particularly controlled as this layer is produced in the first steps of the method and deposition thereof can not tolerate any shape dispersions of the medium.
According to a second embodiment represented in
In the alternative embodiment represented in
For example, the inclusions of spherical shape can be drops obtained by low-temperature dewetting of a metallic layer, for example made of nickel or iron, with a thickness of less than 10 nm previously deposited on recording layer 5. Inclusions of spherical shape 9 can also be obtained by etching a metallic layer through a mask. This mask can be a photolithographic mask or it can be obtained by a self-organization method such as the one described by K. W. Guarini et al. in the article “Process integration of self-assembled polymer templates into silicon nanofabrication” (J. Vac. Sci. Technol. B 20(6), November/December 2002, pages 2788-2792). Etching presents the advantage of being performed at ambient temperature, which allows phase change material to be used for recording layer 5.
Likewise the material forming recording layer 5 can be identical to material 8 of localized electrical conduction layer 4, as represented in
The invention is not limited to the embodiments described above. For example, the electrical-effect data recording medium can be a medium of memory solid type, electric memory with phase change type (Phase Change RAM or PCram), of CBram type (Conductive Bridge ram) or of solid WORM type (non-rewritable medium or media).
| Number | Date | Country | Kind |
|---|---|---|---|
| 0604458 | May 2006 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2007/000760 | 5/3/2007 | WO | 00 | 11/4/2008 |
