High Density Data Storage System With Topographic Contact Sensor

The present invention relates to high density data storage apparatus for use in digital processing, and, more particularly, to a data storage system for transferring information between a data processing resource and a topographically altered storage medium.

The need to increase the areal density in data storage devices is critical. This need drives much of the current research in micromagnetics, coding theory and head design, to name just a few areas of inquiry. Although data storage densities of one gigabit per square inch (Gbit/in.sup.2) have been achieved with magnetic recording, and several Gbit/in.sup.2 have been obtained using optical recording, there has existed no path to achieving densities substantially in excess of ten Gbit/in.sup.2.

Accordingly, while much progress has been made in increasing data storage densities using magnetic and optical techniques, there is still a need for a storage device offering significantly improved storage capability with corresponding high data access rates for rapid read/write operations.

It is therefore a primary object of the present invention to provide a data storage system offering the potential of not just incremental increases in areal density, but improvements of several orders of magnitude over that of current magnetic and optical data storage systems. It is a further object of the invention to provide a data storage system with the capability to read and write information at an areal density of at least 20 Gbit/in.sup.2. A still further object of the present invention is to provide high data storage densities in conjunction with rapid data read rates, for example, in the megabit per second range.

To that end, a mechanical data storage system is provided that includes a rotatable storage medium mounted for rotation about a drive axis, and having formed thereon a surface topography representing machine readable information. The storage system further includes a high resolution contact sensor having a microfabricated cantilever arm and a stylus mounted at one end of the cantilever arm. The stylus is positionable to engage the storage medium and follow the surface topography when the storage medium is rotated. A detector system provides an output signal representing cantilever deflections caused by changes in the surface topography of the storage medium.

The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description of exemplary embodiments thereof, when read in conjunction with accompanying drawing, in which:

FIG. 1 is a diagrammatic side view of high density data storage system constructed in accordance with the present invention;

FIG. 2 is a detailed diagrammatic view of an optical light source and detection module constructed in accordance with the present invention;

FIG. 3a is a diagrammatic view of a control sensor stylus engaging a topographic incongruance in an on-track orientation;

FIG. 3b is a diagrammatic view of a control sensor stylus engaging a topographic incongruance on a storage medium in a first off-track position;

FIG. 3c is a diagrammatic view of a control sensor stylus engaging a topographic incongruance of a storage medium an a second off-track position;

FIG. 4 is a perspective view of a contact sensor constructed in accordance with the present invention showing the underside thereof.

FIG. 5 is a graph illustrating the cantilever response rate in a data storage system constructed in accordance with the present invention.

FIG. 6 is a more detailed view of a portion of the graph of FIG. 5.

FIG. 7 is a diagrammatic side view of a system for writing topographic data to a storage medium in accordance with another aspect of the present invention; and

FIG. 8 is a diagrammatic side view of a system for writing topographic data to a storage medium in accordance with a still further aspect of the present invention.

Turning now to FIG. 1, which is diagrammatic in nature and not to scale, a data storage system 2 includes a rotatable storage medium 4 which may be generally disk-shaped and mounted for rotation about a central drive axis in the direction of the arrow labeled A. Although the storage medium may be formed from many conventional materials, a polymer such as one of the methylmethacrylates, PMMA for example, is preferred. There is formed on the upper face of the storage medium 4 a surface topography defining machine readable information. The phrase "machine readable information" is understood herein to mean information that is readable by a digital processing resource such as a programmed computer. More specifically, a plurality of data tracks are provided having a series of surface incongruances arranged in a machine readable pattern. The incongruances may be a series of pits or indentations 6 separated by island portions 7 of the media surface that are not pitted. Alternatively, instead of pits, a series of raised protruberances or bumps could be provided. In either case, the incongruances may be selectively spaced or of variable length in a frequency modulated encoding mode, or may be varied in depth in an amplitude modulated encoding mode. Positioned below the storage medium 4 is a drive motor 5 of conventional design for rotating a spindle 5a that supports the medium 4. The medium is rotated at a selected uniform rotational speed about the drive axis D, shown in phantom lines. Positioned adjacently above the storage medium 4 is a high resolution contact sensor 8 mounted to a support system 9. The contact sensor 8 includes a microfabricated cantilever arm 10 extending at one end from a base 11. A stylus 12 is mounted at the other end of the cantilever arm. The stylus 12 is positionable to engage a selected data track of the storage medium 4 and to follow the data track when the storage medium is rotated. The stylus is downwardly biased so that it tracks the surface topography of the data track over which it is located. When the disk is rotated, the cantilever arm 10 rises and falls as the stylus 12 tracks through the peaks and valleys of the storage medium.

Positioned on the back side of the cantilever arm 10, adjacently above the stylus 12, is a light reflective surface 14 having a smooth mirror facet for reflecting laser light without substantial distortion. Positioned above the light reflective surface 14 is an optical light source and detection module 16. The module 16 includes a laser light source 18 and an adjacent light detector 20. The light source 18 is positioned to direct an incident laser beam on the reflective surface 14 while the detector 20 is positioned to receive the reflected beam. The light source/detection module 16 is electrically connected to circuitry 22 for processing the output signal of the detector to provide a data signal and a tracking error signal. The data signal is directed to a host controller such as a digital computer and the tracking error signal is directed to a tracking control module 23.

Referring now to FIG. 2, the optical source/detection module 16 and its interaction with the control sensor 8 are illustrated in greater detail. The optical source 18 may include a semiconductor laser device 24 of conventional design in combination with a focusing lens 26. The light detector 20 may include a conventional quadrant cell ("quad-cell") photodiode having four detector segments A, B, C, and D. The quadrant detector 28 is able to sense vertical displacement of the cantilever arm 10 caused by the peaks and valleys of the medium, as well as cantilever twist caused by an off-track condition of the stylus 12. This condition is illustrated in FIGS. 3a, 3b and 3c. In each of these Figures the media is illustrated in section and is moving normal to the plane of the page. The storage medium 4 is shown as having a rounded incongruance 30 over which the stylus 12 of the cantilever arm is drawn as a result of the relative movement between the storage medium and the stylus. The incongruance 30 is formed to have a rounded valley aligned with the center of the data track in which the incongruance is located, and a pair of rounded side surfaces extending away from the valley in the direction of either side of the data track. Although FIGS. 3a-3c illustrate a depression, it will be appreciated that the incongruance could be raised to form a rounded geometry that is essentially a mirror image of that of the concave incongruance 30. FIG. 3a shows a condition where the stylus 12 is properly positioned in the center of the data track and therefore rides over the peak of the incongruance 30. FIG. 3b represents an off-track condition wherein the stylus 12 is positioned on one side of the track center line such that the stylus 12 rides over a lateral portion of the incongruance 30. This imparts a twist to the cantilever arm 10. Similarly, FIG. 3c shows the condition wherein the stylus 12 is positioned on the other side of the track center line which again causes the cantilever arm 10 to twist.

Returning now to FIG. 2, the light source 24 and quadrant detector 28 may be arranged so that vertical deflections of the cantilever arm 10 caused by the topography of the storage medium 4 will result in a lateral displacement of the reflected laser beam across the quadrant detector 28 (up and down in FIG. 2). Likewise, cantilever twisting resulting from an off-track condition of the stylus 12 will result in lateral deflection of the reflected laser beam across the quadrant detector 28 in a direction 90 degrees from the beam deflection resulting from vertical cantilever arm displacement (left and right in FIG. 2). As is conventional, the quadrant segments A, B, C, and D each represent a discreet photodiode detector which generates an electrical output signal having an intensity proportional to the intensity of the incident light. The circuitry 22 includes circuit elements for determining differences in the output levels of the discrete photodiodes. By properly positioning the quadrant detector 28 with respect to the cantilever arm reflector 14, changes in the vertical displacement of the cantilever arm can be directly calculated by the relationship: ##EQU1## Similarly, changes in the twist of the cantilever arm 10 can be directly calculated in accordance with the relationship: ##EQU2##

Thus, the optical source/detection module 16 and circuitry 22 function as a data signal/tracking error signal generating system. The output signal from the detection module 16 is processed by circuitry 22 in accordance with Formulas I and II above to provide a data signal and a tracking error signal, respectively. The data signal component is directed to the host controller. The tracking error signal is directed to the tracking control module 23 for generating a servo signal that controls the position of the contact sensor 8 over a selected data track on the storage medium 4. Cantilever support system 9 includes a drive system 31 which is connected to the tracking control module 23 and is capable of gross and fine movements for track location and track servo operations, respectively. In addition, the drive system 29 includes a conventional piezoelectric control system for controlling cantilever loading.

In order to sample high density data at high data rates, a contact sensor, such as the sensor 8 in FIG. 1, is preferably formed using microfabrication techniques such as those used to form the atomic force microscope (AFM). Thin film cantilevers can be formed from a variety of materials, including thermally grown oxide films and vapor deposited metals or dialectrics. FIG. 4 illustrates a conventional cantilever design which may be used advantageously to form the contact sensor 8 shown in FIG. 1. In FIG. 4, a generally triangular, dual-legged free cantilever 32 arm extends from a cantilever base 34 mounted on a substrate 36. In accordance with conventional constructions, the cantilever 32 arm and support base 34 may be produced from SiO.sub.2 or Si.sub.3 N.sub.4, thermally grown on a substrate 36 by low-pressure chemical vapor deposition (LPCVD). The substrate 36 may be formed from silicon wafer material. Following appropriate etching, it provides a support for the cantilever base 34 while leaving the cantilever arm 32 free for relative movement. An additional lower layer 38 formed from thermally-grown SiO.sub.2 or Si.sub.3 N.sub.4 may be provided on the bottom of the substrate 36 for additional support and to facilitate handling of the cantilever and mounting to the tracking arm 9. To complete the control sensor, a stylus 39 is formed at the end of the cantilever arm 32. Many techniques are known for stylus fabrication. In a preferred method, the stylus 39 is integrally formed with the cantilever arm 32 by creating a depression in the substrate 36 prior to depositing the film used to fabricate the cantilever arm 32 and cantilever base 34. Alternatively, the stylus 39 could be deposited on the cantilever arm 32 after the arm is formed. In either case, the stylus may be conical, pyramidal or similarly shaped. Preferably, the construction of the contact sensor assembly will be incorporated in a batch fabrication process which enables rapid, low cost production of the assembly.

The size and configuration of the contact sensor assembly may be determined from the required operating characteristics of the storage system. Lateral resolution, vertical resolution and data rate will all guide the design of the assembly. Generally, lateral resolution is determined by the size of the stylus tip. Using conventional fabrication techniques, lateral resolutions from 50 angstroms to atomic resolution have been demonstrated. Vertical resolution and data rate are determined principally by the characteristics of the cantilever arm itself. Vertical resolutions of an angstrom or less have been obtained. This will be more than adequate for data storage purposes where topographic variations of the media will be on the order of 100 angstroms to provide a good signal to noise ratio.

The nominal data rate limit of the contact sensor may be determined by assuming the cantilever arm operates as an undamped cantilever spring which is caused to vibrate under the action of a force applied laterally at its tip. The second order linear differential equation governing the undamped simple harmonic motion of the cantilever is given as: ##EQU3## where x is the lateral deflection of the cantilever tip, m is the cantilever mass and k is the cantilever spring constant. From this equation, the resonant angular frequency of the cantilever arm is given by the relationship:

In the free vibrational mode, the mechanical response time of the cantilever arm to return from the fully deflected state to the undeflected state may be determined by the relationship:

This corresponds to a nominal data rate limit of f.sub.0 =1/T which, in the free vibration mode at resonant frequencies, is limited by the cantilever's inertial mass. Thus, it may appear that data rates in excess of the resonant frequency may not be obtained since the response of the cantilever arm must be sufficient to maintain the stylus in contact with the media. However, it is also true that the required condition wherein the tip of the cantilever follows the media surface may be obtained by maintaining the force exerted on the cantilever at a level which increases the resonant tip acceleration at all times. Thus, given a media whose topography includes a series of surface incongruances of height h, increased data rates may be obtained if the cantilever is loaded a distance z.sub.L >>h, so that the applied force is roughly kz.sub.L. Under these conditions, the acceleration a of the tip as it encounters a surface incongruance is given by the relationship:

The time t for the cantilever to move a distance h as the incongruance passes the stylus is given by the relationship:

Applying these relationships, the time required for the cantilever to deflect across the distance h of the surface incongruance may be determined in relation to the resonant frequency using the relationship: ##EQU4##

Now, considering a series of sequential peaks and valleys varying sinusoidally on the media surface at a frequency f.sub.signal, the sinusoidal variation having a peak to peak amplitude of h, it will be observed that the relationship between the signal frequency f.sub.signal and the resonant frequency f.sub.0 is given by the relationship:

Thus, although the signal frequency f.sub.signal is proportional to the natural resonance frequency f.sub.o of the cantilever, it can actually be substantially greater than the natural frequency if z.sub.L >>h. It will be further noted that for a given amount of load on the cantilever tip and a given feature depth, the only relevant parameter is the mass of the cantilever-stylus assembly, which should be as small as possible. Using conventional microfabrication techniques, it is possible to make cantilevers with a mass as low as 10.sup.-9 g.

To calculate the data rate, a value for the load is assumed. Using an average pressure of 10 GPa and a contact radius of 50 angstroms, a load of 10.sup.-6 N may be assumed. This load is applied to an assumed rectangular Si cantilever of dimensions 50 microns times 10 microns times 1 micron, which is well within the range of cantilever dimensions producible using known microfabrication techniques. If a feature depth of 10 nm is selected, the signal frequency f.sub.signal is determined to be 4.3 MHz. Assuming standard coding techniques are employed, for example, a (2, 7) code, the calculated data rate is approximately 13 Mbit/sec. Because the spring constant k is about 2.5 N/m, a loading distance z.sub.L of 0.4 microns needs to be maintained. Loading distances in this range can be readily maintained with piezoelectric positioners. With stylus forces in the range of 10.sup.-6 N, the sharpness of the stylus tip is somewhat limited. However, tip sizes providing resolutions in the 100 angstroms range can easily withstand such pressures. It is expected that run-out caused by media warpage or other construction imperfections will change the loading force on the cantilever arm. However, an air-bearing spindle can be used to reduce the repeatable run-out, and the run-out can easily be accounted for by incorporating a piezoelectric tube or bimorph in the support system 9.

Alternatively, the problem of run-out may be advantageously avoided by placing the microfabricated cantilever on a weak suspension and/or an air-bearing slider. Thus, the support system 9 may include a conventional slider 40 (FIG. 1) mounted on a conventional suspension 41 of the well known type used in Winchester disk drives. The slider 40 flies at a fixed height above the surface of the storage medium 4, so that it tracks the low frequency variations in disk height. The high frequency variations in topography due to the encoded data will give rise to deflection of the cantilever 10 as before. The load on the cantilever will be determined by the angle and height at which the cantilever is placed on the slider 40, as well as the fly height. By choosing the appropriate geometry, the load will be maintained at a predetermined value, independent of disk run-out. Similarly, the microfabricated cantilever could be placed directly on the suspension 41, whose spring constant and resonant frequency would then be selected to be much lower than the cantilever's. As the disk moves up and down due to run-out, the suspension 41 will deflect much more than the cantilever, so that the load on the cantilever will remain nearly constant. Again, the suspension 41 will not be able to respond to the high frequency variations due to the data, which will be detected by the microfabricated cantilever. In this case, the loading force on the cantilever will be given by the product of the spring constant of the suspension and the loading distance.

Other data rate issues are the frequency response of the deflection detector and the signal-to-noise ratio at a given bandwidth. FIG. 1 illustrates an optical detector incorporating the optical source/detection module 16. Using conventionally available optical detection apparatus, sensitivities in the range of 10.sup.-4 angstroms/Hz.sup.1/2 may be obtained. Since this noise is the shot noise limit, even higher sensitivity can be obtained with higher optical power. Thus, if a 20.sub.db signal-to-noise ratio is required on a 100 angstrom feature, then the signal-to-noise ratio may be calculated in accordance with the relationship:

Thus, the signal-to-noise ratio is easily achieved for a detector with this noise level. A more serious limitation would be the frequency response of the particular detector used. In the case of optical beam deflection, photo detectors can be operated at many MHz.

An opto-mechanical storage system constructed in accordance with the present invention has been tested to evaluate the characteristics of a practical system. The system was constructed in accordance with FIG. 1 using a dual-legged triangular cantilever arm made with commercially available silicon nitride, together with a silicon nitride stylus. The cantilever had a resonant frequency of about 90 KHz. The sample was a test sample for a scanning electronic microscope SEM which had a pattern of two micron deep grooves etched into it. FIGS. 5 and 6 illustrate the response of the cantilever as the medium was spun at 2400 rpm (about 0.4 m/sec. at the radius used). As illustrated in FIG. 6, the rise and fall times were about 1 microsecond. Based on the equations set forth above, shallower features would be expected to reduce the response time, as would smaller, stiffer cantilevers.

Turning now to FIGS. 7 and 8, an alternative embodiment of the present invention is illustrated which includes a data writing capability. Thus, a data storage system 42 includes a rotatable storage medium 44 mounted for rotation about an axis (not shown) in the direction of the arrow labeled B. The storage medium 44 has an initially flat surface topography upon which incongruances 46 may be imprinted in a pattern to define machine readable information. As in the case of the previously described storage medium 4, the storage medium 44 is preferably formed from a polymer substrate such as one of the methyl methacrylates, PMMA, for example. As in the system of FIG. 1, there is positioned above the storage medium 44 a contact sensor 48 including a cantilever arm 50 and a stylus 52. The contact sensor 48 is positioned above the storage medium 44 such that an initial load deflection is imparted to the cantilever arm 50 and a load force is applied to the stylus 52. It will be understood that the storage system 42 includes data read hardware such as the detector system of FIG. 1. This data read hardware, however, is not shown in FIG. 7 in order to clearly illustrate the data writing elements thereof. These elements include a laser power source 54 providing a pulsed output signal. The pulsed output signal is provided to a diode laser 56 to produce a pulsed infrared laser output which is directed through a lens 58. The lens 58 focuses the laser beam on the upper side of the cantilever arm 50, behind the stylus 52. Preferably, the stylus 52 is coated with a light absorbing material such that its tip heats up as a result of the incident laser beam energy. With the stylus 52 in contact with the medium 44, the heat will transfer to the medium and raise its temperature. If the medium is of a low melting point material, or has a low-temperature softening point, such as PMMA, the loading force present on the stylus 52 combined with the heating from the laser will cause the tip of the stylus 52 to sink into the medium 44, thus forming a pit. By rotating the storage medium under the stylus 52, and controllably driving the laser 56, a series of incongruances 46 may be created in the medium 44 in a pattern corresponding to the frequency and intensity of the laser pulses. In a test using a non-rotating PMMA substrate, a 30 milliwatt, 30 microsecond pulse and a loading force of about 10.sup.-7 N, a pit was formed in the medium 44 having a diameter of about 1,400-2,000 angstroms. In addition to the pit, a ridge of displaced substrate material was formed around the pit such that approximately half of the diameter represented the pit and half represented the ridge. Reducing the laser power leads to shallower and correspondingly narrow pits. For example, pits as small as 700 angstroms across and 300 angstroms deep can be made. In addition, marks may be made with laser pulses as short as 1 microsecond with a 5 microsecond duty cycle.

It will be understood that alternative methods for heating the stylus can be used. FIG. 8 illustrates a storage system 60 wherein stylus tip heating may be accomplished more efficiently from below the storage medium 62. Again, the storage medium 62 is rotatable about an axis for relative movement in the direction of the arrow labeled C. The storage medium 62 includes a series of surface incongruances 64 which may be formed in a pattern defining machine readable information. A contact sensor 66 positioned above the storage medium 62 includes a cantilever arm 68 and a stylus 70. The contact sensor 66 is positioned above the storage medium 62 at a distance sufficient to impart a pre-load deflection on the cantilever arm 68 and a pre-load force on the stylus 70. Positioned below the storage medium 62 is a laser power supply 72 providing a pulsed output signal to a laser source 74. The laser source 74 produces a pulsed infrared laser output beam which is focused by a focusing lens 75 on the stylus 70. Again, controlled pulsing of the laser 74 results in a surface topographic pattern in the storage medium defined by the frequency and intensity of the laser pulses.

An advantage of the data storage system of FIG. 8 is that data write and data read functions may be combined. Thus, a beam splitter 76 and quarter-waveplate 78 may be placed in the path of the infrared laser beam emitted by the laser source 74. The beam splitter 76 passes the incident beam to the stylus 70. By proper selection of materials, the reflective properties of the stylus 70 and the medium 62 may be utilized to generate a reflective diffraction pattern having at least one interference peak aligned with the beam splitter 76. This high intensity reflected light is deflected by the beam splitter to a light sensitive diode detector 80 by virtue of the change in polarization provided by the quarter-waveplate 78. When the stylus 70 is displaced into an incongruance 64, the intensity of the reflected beam is modulated due to optical interference between light reflected off the cantilever and light reflected off the surface of the disk. This intensity modulation is detected at light sensitive diode 80. In this manner, an intensity modulated signal corresponding to changes in stylus position can be produced. In principle, the reflected light off the surface of the disk will also be affected by the presence of the data features. As these are much smaller in depth and spatial extent than the wavelength of the laser light, this effect will be small compared to the dominant interference between the disk and the cantilever. In the data read mode, the laser source 74 may be operated at a relatively low power level so that substantial stylus heating will not occur. When the data write mode is selected, the power level of the laser source may be increased to provide sufficient stylus heating.

Accordingly, although several preferred embodiments of an high density data storage system with a topographic contact sensor have been disclosed, it will be understood and appreciated that modifications and adaptations will occur to persons skilled in the art. For example, as an alternative to infrared laser heating in the data write mode, it may also be possible to integrate a miniature resistive-heater into the stylus for variably heating and deflecting the stylus toward the storage medium to write data thereto. In addition, media materials other than PMMA may well prove satisfactory in the storage system herein. Structures with integrated optical detection systems could also be used for detecting cantilever deflections. With an integrated sensor, it should be possible to use multiple tips for highly parallel operation. Also, the deflection of the cantilever could be sensed capacitively or inductively. For example, the cantilever could form one plate of a capacitive sensor, or could operate in conjunction with an inductive coil. Moreover, the contact sensor could include a piezoresistive element to detect topographic changes in the storage medium. Therefore, the protection afforded the invention should not be limited except in accordance with the spirit of the appended claims and their equivalents.