WO2002097807A1

WO2002097807A1 - Nano-fabricated data storage device and production and operation methods therefor

Info

Publication number: WO2002097807A1
Application number: PCT/AU2002/000670
Authority: WO
Inventors: Sverre Myhra
Original assignee: Griffith University
Priority date: 2001-05-28
Filing date: 2002-05-24
Publication date: 2002-12-05
Also published as: AUPR529401A0

Abstract

A nano-fabricated data storage device comprises a data storage medium in the form of a silicon disc substrate (1) having a pre-existing silicon oxide surface layer and a read/write/erase 'head' in the form of a scanning probe microscope tip (2). The amplitude and duration of a voltage signal applied to the tip are controlled by a variable voltage source (3) and a temporal controller (4) respectively, to write data to, and read and erase data from the storage medium. Data is written on the silicon oxide layer by growing spatially resolved silicon oxide structures thereon and/or by forming pits in the layer. Rastering the microscope tip over the layer surface reads the data by detecting the presence or absence of the silicon oxide structures and removal of the layer at data locations erases previously written data.

Description

NANO-FABRICATED DATA STORAGE DEVICE AND PRODUCTION AND OPERATION METHODS THEREFOR

The invention relates to a nano-fabricated data storage device and production and operation methods therefor. In particular, although not exclusively, the invention relates to a data storage device produced by means of Scanned Probe Microscopy (SPM) and nano-lithographic techniques.

BACKGROUND TO THE INVENTION

In the field of computer technology and in particular in the area of data storage, the trend has been towards miniaturisation and improved performance. Digital data storage based on magnetic interactions is a well- established technology that is approaching fundamental limitations in terms of bit size and spacing and in terms of read/write speeds. Current technology is based on exploiting the magnetoresistance effect that yields a bit density of about 1 Gbit/cm² with a read speed of around 200 Mbits/s. Other data storage solutions such as optical discs are cheaper, but store less data than magnetic discs. Transistor-based microchip memory, although able to be read data faster, also stores less data and is more expensive.

Technologies exploiting Scanned Probe Microscopy (SPM) systems have been investigated extensively in the search for smaller scale data storage with higher performance. A pointed probe tip of a scanning probe microscope may be used, for example, to move individual molecules into prescribed, fixed, new positions on a surface and/or to modify molecules without change of position. This leads to the concept of using this technique to write and read data.

For example, IBM has demonstrated "thermal writing" to a substrate at a bit density of 80 Gbit/cm² for an in-principle read speed of 1 Gbit/s.

There exists the possibility of utilising SPM techniques to achieve large storage capacity and high read/write rates.

DISCLOSURE OF THE INVENTION In one form, although it need not be the only or indeed the broadest form, the invention resides in a nano-fabricated, mass data storage device comprising: a storage medium having a silicon substrate and a silicon oxide layer; writing means to write data onto said storage medium in the form of spatially resolved silicon oxide structures; reading means to read data previously written onto said storage medium; and erasing means to erase data previously written onto said storage medium, wherein said writing, reading and erasing means is at least one electrically conducting tip of a scanning probe microscope to which an electrical signal is applied.

Preferably, the amplitude and duration of the signal provided to said tip are selectable and the signal duration required to write said spatially resolved silicon oxide structures may be about 0.1 ms. The duration required to erase said spatially resolved silicon oxide structures may be in the range of about 10-100μs.

The nano-fabricated, mass data storage device may store data in digital format or in analogue format.

Where the data storage device comprises more than one of said electrically conducting tips, said tips are arranged in a parallel array.

In another form, the invention resides in a method of writing data onto a storage medium, said storage medium having a silicon substrate and a silicon oxide layer, said method including the steps of: applying an electrical signal to a tip of a scanning probe microscope; and, growing spatially resolved silicon oxide structures on said storage medium, said structures representing said data.

Preferably, the structures are grown in the form of dots or in the form of lines. In a further form, the invention resides in a method of reading data previously written on a storage medium, said storage medium having a silicon substrate and a silicon oxide layer, said method including the steps of: rastering the tip of a scanning probe microscope over the silicon oxide surface; and, reading said data by detecting a presence or absence of spatially resolved silicon oxide structures on said storage medium.

In a yet further form, the invention resides in a method of writing data onto a storage medium, said storage medium having a silicon substrate and a silicon oxide layer, said method including the steps of: applying an electrical signal to a tip of a scanning probe microscope; forming a pit in said silicon oxide layer at a location of said data to be recorded, said pit representing said data.

Suitably, the pit may be formed in the silicon oxide layer to a depth substantially equal to a thickness of the silicon oxide layer.

The duration of the signal required to form said pit may be in the range of about 10-1 OOμs.

In a yet further form, the invention resides in a method of reading data previously written on a storage medium, wherein the storage medium has a silicon substrate and a silicon oxide layer and the data is in the form of pits, said method including the steps of: rastering the tip of a scanning probe microscope over the silicon oxide surface; and, reading said data by detecting the presence or absence of said pits on said storage medium.

In a yet further form, the invention resides in a method of erasing data previously written on a storage medium, said storage medium having a silicon substrate and a silicon oxide layer, said method including the steps of: applying an electrical signal to a tip of a scanning probe microscope; and, removing a portion of said silicon oxide layer at a plurality of locations of said data to be erased, the removal of said portion of said silicon oxide layer erasing said previously written data. BRIEF DESCRIPTION OF THE DRAWINGS To assist in understanding of the invention and to enable the invention to be put into practical effect preferred embodiments will now be described by way of example only and with reference to the accompanying drawings, wherein:

FIG. 1 shows an image of a 3x3 array of silicon oxide "bits" produced by tip-induced, spatially resolved growth in accordance with one of the methods of the present invention; FIG. 2 shows an image of a sequence of silicon oxide "bits" produced by tip-induced, spatially resolved growth in accordance with one of the methods of the present invention;

FIG. 3 shows a contour line of two of the silicon oxide "bits" of FIG 2; FIG. 4 shows an image of a tip-induced pit formed in a silicon oxide layer in accordance with one of the methods of the present invention;

FIG. 5A shows an image of a feature produced by tip-induced silicon oxide removal in accordance with one of the methods of the present invention;

FIG. 5B shows a contour line of the feature in FIG 5A; and, FIG. 6 shows a schematic diagram of the data storage device in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Scanned Probe Microscopy (SPM) is used to thermally grow silicon oxide layers on industry standard silicon wafer substrates using an electrically conducting SPM tip. The wafer samples selected were approximately 2x12mm² and comprised an oxide layer of approximate thickness of 2.4nm, 4.2nm or 6.4nm. The samples were cleaned by spinning under iso-propyl-alcohol and rinsed with distilled water. The samples were then dried in a stream of nitrogen gas and placed in a vacuum envelope of 10^"2 - 10^"3 Pa of a standard JEOL instrument.

The surface of each sample was imaged to check that it was free from artefacts by imaging a 1 - 4μm² area in the contact mode with a lever- imposed force loading of 10-100nN and zero bias. In accordance with the invention, the electrically conducting SPM tip may then be manipulated to grow spatially resolved silicon oxide structures, of the type shown in FIGS. 1 and 2, on the silicon wafer substrate comprising the pre-existing silicon oxide layer. The silicon oxide structures are formed beneath the surface of the silicon oxide layer, essentially at the silicon/silicon oxide interface, using a range of bias voltage values, scan speeds and force loadings.

A larger field of view, centred on the location of the surface growth, may then be re-scanned with zero bias in order to reveal the topographical effects induced by the tip-to-substrate bias. FIG. 1 shows a 3x3 array of silicon oxide "bits", each with an apparent diameter of about 30-40nm and a height of about 8-1 Onm. The dots were written to a 2.4nm thick silicon oxide coating by applying a positive sample bias voltage of 8V and by keeping the W₂C coated tip stationary for approximately 1 s. The duration of the signal applied to the SPM tip was approximately 0.1 ms. The tip radius of curvature at the apex was approximately 20-30nm with a height of approximately 15-20μm. The beam characteristics were: length 300μm, width 35μm and thickness 1 μm. The irregularities in the shapes of the dots shown in FIG. 1 were caused by thermal drift. FIG. 2 shows a sequence of lines denoting the numerals 1-7 in binary code from left to right of the image, with the least significant bit being at the top of the image. The sequence was written as lines approximately 20- 30nm in width and about 2.5 - 4nm in height above the 2.4nm thick silicon oxide layer. A W₂C coated tip and a sample bias of 8V were again used. The contour line in FIG. 3 illustrates the heights, widths and separation of two of the lines. The radius of curvature of the tip apex is approximately 20nm or greater and thus the line width of the lines shown in FIG.2 are less than approximately 15nm.

Repeating the above oxide growth processes with thicker thermally grown oxide layers, at correspondingly higher bias voltages, tended to result in less uniform oxide growth thicknesses. Higher bias voltages also make it more difficult to maintain the integrity of the tip conditions, possibly due to excessive ohmic heating at the point contact.

The growth of the silicon oxide structures may be attributed to a diffusion-limited mechanism, whereby oxidants are transported as anions to the silicon/silicon oxide interface by the electrostatic field established between the SPM tip and the silicon substrate. The oxide grows at the aforementioned interface.

Factors affecting the mechanism that controls the growth rate of the structures include the electrostatic field strength, the thickness of the pre- existing silicon oxide film, the availability of mobile oxidants, the existence of charge compensation and the existence of a hypothetical insulating layer on the tip. Furthermore, there is likely to be a native oxide coating of approximately 0.5-1 nm covering the tip. The spatial resolution depends on the shape of the tip, the size of the effective tip-to-surface contact area and the extent of the thermal drift. The size of the structure then depends on the dwell time of the tip at the particular location (i.e. the scan speed).

This technique may be used to write data onto and read data from silicon substrates, the "bits" being in the form of the grown, spatially resolved structures of silicon oxide. Direct writing of nano-lithographic patterns on to the silicon substrate is achievable with a high resolution, i.e. with a line width down to about 15-20nm.

The data read rate is essentially given by the raster rate of the scanning probe microscope and the mechanical eigenmode of the probe. However, the write speed is likely to be limited by the transit time for ionic diffusion. Existing SPM technology delivers the simultaneous writing of up to 10 lines at a linear speed of about 50-1 OOμm/s, which is equivalent to 10⁴ - 10⁵ pattern elements or line segments per second. It is envisaged that SPM technology, with smaller and stiffer probes and faster electronics, will add an additional factor of 100 to the speed with a further factor of 100 achievable with the implementation of two-dimensional parallel probe arrays.

In accordance with the invention, the electrically conducting SPM tip may be manipulated to write pits in the pre-existing silicon oxide layer on the silicon substrate. The formation of a pit (writing a data bit) may be dependent on a dielectric breakdown mechanism. The electrostatic field strength used is typically 10-30% higher than the field strength required for optimum oxide growth conditions. The fields used are also comparable with those that result in dielectric breakdown for a Metal Oxide Semiconductor (MOS) gate dielectric. Accordingly, there will be a non-linear breakdown event that progresses until the field is removed, the result of the breakdown being the removal of the silicon oxide in the oxide layer, i.e. the formation of a pit.

The rate of formation of the bit (the pit in the silicon oxide) may be limited by the capacitive rise time of the electrostatic field at the junction between the SPM tip and the oxide layer. The area and depth of the pit is then controlled by the amplitude of the signal above breakdown and the duration of the signal.

This technique may also be used to write data onto the silicon substrate, the extremely fast write speed being defined by the duration of the signal, which is of the order of 0.01 ms.

An example of such a pit is shown in FIG. 4. The pit was formed in a pre-existing oxide layer 2.4nm thick by applying a single signal of amplitude 11 V and duration 0.1 ms, although a pit is achievable with a signal duration in the range 10-1 OOμs. The inner diameter of the pit is less than about 10nm, allowing for tip shape convolution. The pit has a depth corresponding to the thickness of the pre-existing silicon oxide layer and a slightly raised rim around the perimeter of the pit. The mechanism is potentially much faster than that accounting for deposition of oxide dots, although pit formation has not been exhibited for signal durations shorter than 10μs. In accordance with the invention, the electrically conducting SPM tip may be manipulated to remove an area of the pre-existing oxide layer of the silicon substrate. An image of a feature produced by the removal of the preexisting oxide layer is shown in FIG. 5A and a profile of the feature is shown in FIG. 5B. Tip-induced oxide removal over an area 300x300nm² was achieved using a Nanosensor Pointprobe™ tip with a radius of curvature at the apex of about 10-20nm and a height of about 15μm with a substrate-to- tip bias of 3V and a scan speed of 2.8μm/s. The beam length was 450μm with a width of 50μm and a thickness of 2μm. The sample initially comprised approximately a 2.4nm thickness of pre-existing oxide and the depth of the feature was approximately 2nm, as illustrated in FIG. 5B.

Hence, the depth of the feature is comparable with the thickness of the original oxide layer. Removal of the oxide layer in this way was achieved for bias voltages in the range 2 - 4V.

The removal of the oxide layer may be attributed to a mechanism in which anions are transported laterally by the tip. A thin, aqueous surface chemical environment may result from atmospheric degradation of the silicon oxide, in which the combination of kinetics and thermodynamics promote tip-induced lateral transport.

In accordance with another aspect of the invention, the three aforementioned techniques may be implemented in the operation of a nano- fabricated data storage device. An example of such a data storage device is shown schematically in FIG. 6. The device comprises a silicon disc substrate 1 comprising a pre-existing silicon oxide surface layer or film, which acts as the data storage medium. An electrically conducting SPM tip 2, which is of the order of nanometres across at the tip, acts as the read/write/erase "head". A parallel array of such tips may be provided, which would increase read and write speeds as described below. An electrostatic field is established between the tip and the silicon substrate using a variable voltage source 3, via which the signal amplitude may be controlled. A temporal controller 4 allows the signal duration to be selected.

The aforementioned techniques of growing silicon oxide structures or creating pits provide high speed data write/read functions and the aforementioned technique of oxide removal provides a very selective data erase function of previously written information. The third aforementioned technique provides the additional function of erasing entire regions of data. Thus, the present invention supports the complete data storage sequence of write-read-erase-rewrite-reread events.

Bit spacing of about 30-50nm at densities of up to 10¹⁵/m² are achievable using the first two techniques. Using a single probe, a writing speed of around 100 kHz and a reading speed of around 10-100 kHz has been achieved. It is envisaged that using parallel probe arrays in combination with SPM technology will achieve writing speeds in the region of 10 GHz and improve the reading speed by a factor of about 10⁴.

As previously illustrated, the silicon oxide structures may be grown for example, as dots or, for example, as lines. Although the lines require a longer time to produce than a dot, the lines may be grown more densely on the substrate than the dots, thus increasing the data storage capacity for a given substrate area.

Furthermore, it is likely that the lines will be read more reliably than the dots since the lines comprise an extended raised region in comparison with the dots, which constitute a more limited raised region. The extended raised region of the lines provides a larger area to be read than the area provided by the dots. Alien particles such as dust, or any small, spuriously- formed silicon oxide structures present a relatively smaller area in comparison to the lines than in comparison to the dots and are therefore less likely to be confused with the bits of data represented as lines. Conversely, reading of data represented by dots is more likely to be affected by dust or the like and thus cause errors in reading because of the smaller area presented by the dots. As previously stated, the height of the grown structures is dependent on controllable factors and thus the height of the structures may be selectable. It is therefore envisaged that an analogue data storage medium comprising spatially resolved silicon oxide structures of varying heights can be achieved using the aforementioned nano-fabhcation techniques, in addition to the digital data storage medium.

Throughout the specification the aim has been to describe the invention without limiting the invention to any one embodiment or specific collection of features. Persons skilled in the relevant art may realize variations from the specific embodiments that will nonetheless fall within the scope of the invention.

Claims

CLAIMS:

1. A nano-fabricated, mass data storage device comprising: a storage medium having a silicon substrate and a silicon oxide layer; writing means to write data onto said storage medium in the form of spatially resolved silicon oxide structures; reading means to read data previously written onto said storage medium; and erasing means to erase data previously written onto said storage medium, wherein said writing, reading and erasing means is at least one electrically conducting tip of a scanning probe microscope to which an electrical signal is applied.

2. The mass data storage device of claim 1 wherein amplitude and duration of the signal provided to said tip is selectable.

3. The mass data storage device of claim 2 wherein the signal duration to write said spatially resolved silicon oxide structures is about 0.1 ms.

4. The mass data storage device of claim 2, wherein the signal duration to erase said spatially resolved silicon oxide structures is in the range of about 10-100μs.

5. The mass data storage device of claim 1 , wherein the silicon oxide layer has a thickness of approximately 2.4nm.

6. The mass data storage device of claim 1 , wherein the spatially resolved silicon oxide structures are in the form of dots having an apparent diameter in the range of about 30-40nm.

7. The mass data storage device of claim 6, wherein the spatially resolved silicon oxide structures have a height in the range of about 8-1 Onm.

8. The mass data storage device of claim 1 , wherein the spatially resolved silicon oxide structures are in the form of lines having a width in the range of about 10-30nm.

9. The mass data storage device of claim 8, wherein the spatially resolved silicon oxide structures have a height in the range of about 2-5nm.

10. The mass data storage device of claim 1 , wherein the tip of the scanning probe microscope has a radius of curvature at its apex in the range of about 20-30nm.

11. The mass data storage device of claim 1 , wherein the tip of the scanning probe microscope has a height in the range of about 15- 20μm.

12. The mass data storage device of claim 1 wherein the data is stored in said storage medium in digital format.

13. The mass data storage device of claim 12 wherein the spatially resolved silicon oxide structures are of approximately the same height.

14. The mass data storage device of claim 1 wherein the data is stored in said storage medium in analogue format.

15. The mass data storage device of claim 14 wherein the spatially resolved silicon oxide structures are of varying heights.

16. The mass data storage device of claim 1 wherein a plurality of said electrically conducting tips are arranged in a parallel array.

17. A method of writing data onto a storage medium, said storage medium having a silicon substrate and a silicon oxide layer, said method including the steps of: applying an electrical signal to a tip of a scanning probe microscope; and growing spatially resolved silicon oxide structures on said storage medium, said structures representing said data.

18. The method of writing data of claim 17, wherein the spatially resolved silicon oxide structures are grown in the form of dots.

19. The method of writing data of claim 17, wherein the spatially resolved silicon oxide structures are grown in the form of lines.

20. The method of writing data of claim 18, further comprising the step of keeping the tip of the scanning probe microscope stationary for approximately 1s. 0

21. A method of reading data previously written on a storage medium, said storage medium having a silicon substrate and a silicon oxide layer, said method including the steps of: rastering the tip of a scanning probe microscope over the silicon s oxide surface; and reading said data by detecting a presence or absence of spatially resolved silicon oxide structures on said storage medium.

22. The method of reading data of claim 21 , wherein said reading step o includes detecting the presence or absence of one or more dots on said silicon oxide layer.

23. The method of reading data of claim 21 , wherein said reading step includes detecting the presence or absence of one or more lines on 5 said silicon oxide layer.

24. The method of reading data of claim 21 , wherein said reading step includes detecting the presence or absence of one or more pits in said silicon oxide layer.

25. A method of writing data onto a storage medium, said storage medium having a silicon substrate and a silicon oxide layer, said method including the steps of: applying an electrical signal to a tip of a scanning probe microscope; forming a pit in said silicon oxide layer at a location of said data to be recorded, said pit representing said data.

26. The method of writing data of claim 25, wherein the pit is formed in the silicon oxide layer to a depth substantially equal to a thickness of the silicon oxide layer.

27. The method of writing data of claim 25, wherein controlling an amplitude and duration of the electrical signal controls an area and depth of the pit.

28. The method of writing data of claim 25, wherein the duration of the signal to form said pit is in the range of about 10-100μs.

29. The method of writing data of claim 25, wherein the pit is formed in the silicon oxide layer to a diameter of less than about 10nm.

30. A method of erasing data previously written on a storage medium, said storage medium having a silicon substrate and a silicon oxide layer, said method including the steps of: applying an electrical signal to a tip of a scanning probe microscope; and, removing a portion of said silicon oxide layer at a plurality of locations of said data to be erased, the removal of said portion of said silicon oxide layer erasing said previously written data.

31. The method of erasing data of claim 30, further including the step of moving the tip of the scanning probe microscope over the silicon oxide layer at a speed of about 2.8μm/s.

32. The method of erasing data of claim 30, wherein the portion of said silicon oxide layer is removed to a depth substantially equal to the thickness of the silicon oxide layer.

33. The method of erasing data of claim 30, wherein the portion of said silicon oxide layer is removed to a depth of about 2nm.

Dated this Twenty-Fourth day of May 2002 GRIFFITH UNIVERSITY By their Patent Attorneys FISHER ADAMS KELLY