AU733537B2 - Digital information storage - Google Patents

Description

WO 98/45847 PCT/AU98/00247 DIGITAL INFORMATION STORAGE Technical Field This invention flows from the application of a newly found property of tetrahedral amorphous carbon. The invention relates to a digital information storage device useful in a range of applications. In further aspects, it also relates to a writable medium.

Background Art Modern computer systems use a combination of three different types of memory in their general run-time operation; as distinct from long-term storage devices such as floppy disk drives or CDs. They are static random access memory SRAM, dynamic random access memory DRAM, and electrically erasable read only memory EEPROM. In addition, a fourth class of memory, known as ferroelectric random access memory FERAM, is expected to be made commercially available within the next five years.

SRAM and DRAM represent the most common type of memory used in computers today. SRAM stores information by the operation of a logical element known as a flip-flop, which is a combination of either four or six transistors in a circuit that can be in either of two states, each state representing either a logical or a SRAM is extremely fast (with access times now better than 20 nS) but as it requires up to six transistors per bit, it does not have a high storage density, and is quite expensive to manufacture.

DRAM, on the other hand, stores information in the form of charge stored on a simple capacitor, accessed through a single transistor, and so is much cheaper and has a much higher storage density. However, the charge stored on a DRAM cell leaks away in around 1 ms, and must be constantly refreshed. As a result DRAM is slower than SRAM (with access times of around 70 ns). Therefore, in most computers a combination of the two type are used: SRAM for speed; and DRAM for high storage density.

The principal drawback of both SRAM and DRAM is that the information is stored on them is volatile in that once power to the device is removed (either through power failure or just switching the computer off) the information is lost. Additionally, both types of memory are susceptible to data loss due to ionising radiation, and so cannot be used in devices such as weapons or satellite systems.

WO 98/45847 PCT/AU98/00247 2 A third type of memory, less common but still widespread, is EEPROM. EEPROMs currently represent the main type of non-volatile memory, in which information can be stored for up to 10 years or more even with the loss of power. Most EEPROM devices work on the principal of storing charge either in a floating poly-silicon gate or floating insulator such as silicon nitride, just above the base of a MOS (metal-oxide-silicon) transistor. Charge is stored on the gate by applying a voltage across the oxide layer which isolates it from the channel of the MOS transistor, and the charge state can be read by measuring the turn-on voltage of the transistor.

EEPROMs are much slower than either SRAM or DRAM with write times from several microseconds to milliseconds (depending on the desired memory retention).

Finally, while no commercial devices exist, there is speculation that ferroelectric RAM (FERAM) will be commercially available within five years.

While SRAMs, DRAMs and EEPROMs record information electrostatically, FERAM stores information in the electric polarisation direction of a thin ferroelectric film. FERAM have a similar device structure to DRAM with a ferroelectric capacitor accessed by a single transistor. Writing to the device is done by applying a voltage to the capacitor (which polarises the ferroelectric), and reading is done by applying a reverse voltage and measuring the current associated with the polarisation state of the device.

FERAMs are expected to have memory retention times of potentially hundreds of years, and have speeds comparable to current DRAMs. While FERAMs should have higher memory densities than SRAMs, due to material and design constraints, it is not expected to have densities as high as achievable with DRAM.

Tetrahedral amorphous carbon is a thin film material fabricated in a magnetically filtered vacuum cathodic arc deposition system. It is deposited at room temperature and so can be coated onto virtually any surface including glass and most plastics, and its fabrication is relatively inexpensive, using pure graphite as a source material. Typical film thicknesses range from 10 to 100 nm. It has a hardness comparable to natural diamond and is a wide bandgap semiconductor with an electric bandgap of approximately 2.5 eV crystalline silicon with a bandgap of 1.12 eV). Ta-C is naturally p-type and can be dopedn-type with the WO 98/45847 PCT/AU98/00247 3 introduction of impurities such as nitrogen and phosphorous during the deposition process.

Summary of the Invention In a first aspect, the invention is a digital information storage device comprising a mass of tetrahedral amorphous carbon, where individual bits of data are stored in the form of reversible changes in both the resistance and small signal capacitance of respective regions of the tetrahedral amorphous carbon.

Electrical means may be used to apply a localised electrical field, in excess of a threshold value, to the tetrahedral amorphous carbon in order to reversibly change both the resistance and small signal capacitance of regions of the tetrahedral amorphous carbon in order to store or delete bits of data.

Electrical means may be used to apply a localised electrical field, in excess of a threshold value, to the tetrahedral amorphous carbon in order to detect changes is resistivity or dielectric constant, or both, of regions of the tetrahedral amorphous carbon in order to read bits of data stored within it.

The mass of tetrahedral amorphous carbon is conveniently arranged in the form of a layer. the electrical means may be moveable over a surface of the layer in order to write or read data. Alternatively, the electrical means may comprise a fixed arrangement of conductors extending over a surface of the layer.

The memory effect in ta-C was originally observed as a "kink" in the forward direction of the current-voltage characteristic of nitrogen doped ta-C deposited onto thermally evaporated aluminium films on glass. The "kink" was noticed to disappear when the voltage was scanned in the opposite direction, from positive biases to negative biases. This effect is attributable to the storage of electrons with electronic defects, known as charge traps, within the ta-C.

The principal advantage of using ta-C as a basis for digital information storage is its inherent cheapness, since it can be deposited very easily and cheaply over large areas. The material also has the potential for achieving higher storage densities than current non-volatile memories, and possibly higher than even DRAM.

Memory arrays could be created simply by depositing a ta-C film in between perpendicular sets of conductive address lines, with a memory cell, WO 98/45847 PCT/AU98/00247 4 or bit, at the intersections between crossing lines. By putting half the threshold voltage on one line of half on a crossing line each cell could be written or erased individually without significantly affecting neighbouring cells. Such memories have the potential to be fabricated without the need for access transistors for each cell.

Furthermore, increased memory densities may be achieved by stacking successive arrays vertically. This would have the added advantage of allowing address lines within a given plane to be more widely separated and further reduce the possibility of soft errors in adjacent cell during the writing of an individual cell.

Even without stacking, ta-C could be incorporated into existing DRAM device designs, achieving equal memory densities without the chance of soft errors, and with existing writing and sensing circuitry, but without the need for refreshing, and with significantly less susceptibility to errors induced by ionising radiation.

As well as the production of generic memory ICs, ta-C devices could be used as the memory element for the pixels of a flat screen display. Their advantages would be the ability to deposit them directly onto the back of the screens, integrated into the fabrication of the pixel elements themselves.

Apart for its low cost, ta-C has been found to be quite stable at relatively high temperatures, making it ideal in an environment which is potentially hazardous to other semiconducting materials.

As well as the fabrication of discrete memory elements, there exists the potential to use ta-C as a cheap writable medium, that can be deposited onto virtually any surface, and is intrinsically scratch resistant; pure ta-C has a hardness approaching diamond. With the exception of optical CDs, most writable media in current use utilise the magnetisation of ferromagnetic coatings on disks, tapes, and other surfaces. This requires the use of electromagnetic coils for reading and writing, which on the scale of microelectronic components, are extremely large, and so restrict the ultimate information densities that can be achieved. Coatings of ta-C on the other hand, may be written electrostatically with a simple point contact many orders smaller than the write head of magnetic media. In addition, with the use of many point contacts in a row, information could be written to a ta-C film in parallel, not only increasing the density, but the speed as well.

WO 98/45847 PCT/AU98/00247 Such a use could have potential applications from audio and video recording, to bulk computer storage, to the recording of information on plastic swipe cards, such as credit cards or security cards.

In another aspect, a DRAM cell could be constructed having tetrahedral amorphous carbon used in place of the storage capacitor dielectric. This cell has the benefit of isolation with an access transistor.

Brief Description of the Drawings An example of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a pictorial diagram of a digital information storage device embodying the present invention; and Figure 2 is the current voltage characteristic of the device of figure 1.

Best Modes for Carrying Out the Invention Digital information storage device 1 comprises any number of layers of tetrahedral amorphous carbon 2, each one deposited between sets of aluminium address lines 3 and 4. The aluminium address lines 3/4 above each layer are perpendicular to the address lines 4/3 below each layer. The layers are around 80 nanometres thick as indicated, but the figure is not drawn to scale in horizontal direction.

A memory cell, or bit, is created at the intersection of each crossover between lines 3 and 4.

To write charge to the device, half a threshold voltage V/2 is put on one of the address lines 3, and half the threshold voltage -V 1 2 is put on one of the cross lines 4. The voltage is sufficiently high to change both the resistance and the small signal capacitance in the region of ta-C between the two crossings, but is not expected to significantly affect the material elsewhere.

The change in resistivity and the dielectric constant is achieved by storing charge within what is believed to be electronic defects with the ta-C; these defects are "charge traps" or "donor traps". Referring to figure 2, the effect can be seen electronically as a kink 5 in the forward (increasing voltage) characteristic 6, which disappears in the reverse characteristic 7.

Before the application of any bias voltage these donor traps are occupied by electrons. The traps are extremely localised and so the electrons WO 98/45847 PCT/AU98/00247 6 held in them are rendered immobile, and thus unable to conduct current.

When a reverse bias voltage is applied to the device, the electrons are excited out of these traps and into the conduction band of the material. Once in the conduction band they are free to move and therefore conduct electricity. Deexcitation of these electrons seems to occur at an extremely slow rate (of the order of months to years), unless a certain forward threshold voltage is applied, by which they de-excite very rapidly. This accounts for the kink observed in the I-V characteristic as the previously excited electrons deexcite from the conduction band, the conductivity of the device will drop, causing a drop in the current, forming the observed kink.

Both the storage and release mechanisms take place only with the application of bias voltages beyond a certain threshold, and biases less than this threshold have little effect on the stored charge. In addition, it has been found that since the excitation of electrons from donor traps reduces the resistivity of the nitrogen doped to-C, a means of measuring the charge state with small bias voltages can be achieved, without altering the amount of charge stored on a device. Due to the nature of the memory effect which utilises traps which are deep in the mobility gap of to-C, the release of that charge, without the application of any positive bias, it quite slow. Therefore, such a device is able to hold a substantial amount of charge for long periods of time, of the order of several months to several years.

The stored charge can be detected during readout, and it can be obliterated by applying the reverse potential in order to clear the memory.

Although the invention has been described with reference to a particular embodiment, it should be appreciated and may be embodied in many other forms. For instance, the tetrahedral amorphous carbon can be deposited on a very large number of substrates to provide memory which can be electrostatically written to and read. It therefore has the potential to replace magnetic swipes and to form storage media such as read only disks.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

WO 98/45847 PCT/AU98/00247 7 TETRAHEDRAL AMORPHOUS CARBON (ta-C) Tetrahedral amorphous carbon or (ta-C) is a diamond-like form of carbon with electrical and physical properties (such as semi-conductivity and extreme hardness) approaching those of crystalline diamond. It is an amorphous network of carbon atoms which are predominantly bonded (around 80%) in a tetrahedral, sp 3 -hybridised (diamond-like) bonding state, with a smaller fraction (around 20%) bonded in a trigonal, sp 2 hybridised bonding state. The predominantly tetrahedral nature of ta-C has been confirmed by both neutron and electron diffraction studies 1 4 Ta-C can be fabricated by a number of thin film deposition techniques involving energetic ion bombardment (with ion energies in the range 20 eV-500 eV), such as filtered cathodic vacuum arc, laser ablation, and ion assisted magnetron sputtering.

Ta-C has a hardness comparable to diamond, and is a wide band-gap semiconductor with a mobility gap of approximately 2.5 eV 3 5 It is naturally p-type (though only weakly so), and can be doped n-type with the introduction of impurities such as nitrogen and phosphorous during the deposition process 6 8 Ta-C can be easily deposited over quite large areas onto virtually any surface including silicon, glass, and plastic. At present there is a limitation on the thickness of as deposited ta-C films that can be achieved (60-100 nm) owing to the presence of relatively high film stresses GPa). These, however, appear to be significantly reduced by n-type doping and with the introduction of boron (which does not seem to act as a dopant in a ta-C) during the deposition process 9 A memory effect based on the ionisation of charges held in deep traps within the mobility gap has been observed and confirmed in medium to heavily doped ta-C films deposited onto aluminium by the author. A similar effect seems also to be present in both undoped and ion damaged ta-C films, although positive confirmation of this has yet to be achieved.

1P. H. Gaskell, A. Saeed, P. Chieux, and D. R. McKenzie, "Neutronscattering studies of the structure of highly tetrahedral amorphous diamondlike carbon", Phys. Rev. Lett 67(10), 1286-1289 (1991).

2 P. H. Gaskell, A. Saeed, P. Chieux, and D. R. McKenzie, "The structure of highly tetrahedral amorphous diamond-like carbon", Phil. Mag. B 66, 155 (1992).

WO 98/45847 PCT/AU98/00247 8 3 D. R. McKenzie, D. A. Muller, E. Kravtchinskaia, D. Segal, and D. J.

H. Cockayne, "Synthesis, structure and applications of amorphous diamond", Thin Solid Films 206, 198-203 (1991).

4 D. R. McKenzie, D. A. Muller, B. A. Pailthorpe, Z. H. Wang, E.

Kravtchinskaia, D. Segal, P. B. Lukins, P. D. Swift, P. J. Martin, G. A. J.

Amaratunga, P. H. Gaskell, and A. Saeed, "Properties of tetrahedral amorphous carbon prepared by vacuum arc deposition", Diamond Relat.

Mater. 51-59 (1991).

D. R. McKenzie, D. A. Muller, and B. A. Pailthorpe, "Compressivestress-induced formation of thin-film tetrahedral amorphous carbon", Phys, Rev. Lett. 67, 773 (1991).

6C. A. Davis, D. R. McKenzie, Y. Yin, E. Kravtchinskaia, G. A. J.

Amaratunga, and V. S. Veerasamy, "Substitutional nitrogen doping of tetrahedral amorphous carbon", Phil. Mag. B 69(6), 1133-1140 (1994).

7 C. A. Davis, Y. Yin, D. R. McKenzie, L. E. Hall, E. Kravtchinskaia, V.

Keast, G. A. J. Amaratunga, and V. S. Veerasamy, "The structure of boron-, phosphourus- and nitrogen-doped tetrahedral amorphous carbon deposited by cathodic arc", J. Non-Cryst. Solids 170, 46-50 (1994).

8 V. S. Veerasamy, G. A. J. Amaratunga, C. A. Davis, A. E. Timbs, W. I.

Milne, and D. R. McKenzie, "n-type doping of highly tetrahedral diamondlike amorphous carbon", I. Phys.: Condens. Matter. 5, 169-174 (1993).

9M. Chhowalla, Y. Yin, G. A. J. Amaratunga, D. R. McKenzie, and T.

Frauenheim, "Highly tetrahedral amorphous carbon films with low stress", Appl. Phys. Lett. 69(16), 2344-2346 (1996).

Claims (14)

1. A digital information storage device, comprising a mass of tetrahedral amorphous carbon, where individual bits of data are stored in the form of reversible changes in both the resistance and small signal capacitance of respective regions of the tetrahedral amorphous carbon.

2. A digital information storage device according to claim 1, where electrical means are used to apply a localised electrical field, in excess of a threshold value, to the tetrahedral amorphous carbon in order to reversibly change both the resistance and small signal capacitance of regions of the tetrahedral amorphous carbon in order to store or delete bits of data.

3. A digital information storage device according to claim 1 or 2, where electrical means are used to apply a localised electrical field, in excess of a threshold value, to the tetrahedral amorphous carbon in order to detect changes is resistivity or dielectric constant, or both, of regions of the tetrahedral amorphous carbon in order to read bits of data stored within it.

4. A digital information storage device according to claim 1, 2 or 3, where the mass of tetrahedral amorphous carbon is arranged in the form of a layer.

A digital information storage device according to claim 4, where electrical means are moveable over a surface of the layer in order to write or read data.

6. A digital information storage device according to claim 4, where electrical means comprise a fixed arrangement of conductors extending over a surface of the layer.

7. A digital information storage device according to claim 6, where the electrical means are conductors which apply a bias voltage to the tetrahedral amorphous carbon.

8. A digital information storage device according to claim 7, where the layer of tetrahedral amorphous carbon is a film deposited in between sets of conductive address lines.

9. A digital information storage device according to claim 8, where successive layers of tetrahedral amorphous carbon are stacked one above another with conductive address lines between each layer. A digital information storage device according claim 8 or 9, where the conductive address lines above a layer of tetrahedral amorphous carbon are perpendicular to the conductive address lines below that layer.

SUBSTITUTE SHEET (RULE 26) WO 98/45847 PCT/AU98/00247

11. A flat screen display comprising digital information storage devices according to any preceding claim, deposited directly onto the back of the screen, and where each digital information storage device is integrated into a respective pixel element.

12. A writable medium comprising, a coating of tetrahedral amorphous carbon to a localised region of which data is written electrostatically by contacting the region with a simple point which is electrically chargeable.

13. A writable medium according to claim 12, where there are many point contacts arranged in a row to write simultaneously to the coating in parallel.

14. A DRAM cell, having tetrahedral amorphous carbon used in place of the storage capacitor dielectric. SUBSTITUTE SHEET (Rule 26)