Scanning Tunnelling Microscopy - Research Paper Example

Scanning Tunnelling Microscopy 1. Abstract Novel technologies to observe the structure of matter at an atomic scale have enabled researchers to gain a better understanding of the processes at work on the atomic scale. This has enabled newer developments such as the miniaturization of computer chips, creation of quantum computing and the development of nano electronics. More and more computing power can now be developed onto the same sized chips as before while innovative aspects of processing instructions and storing data are also in development. The heart of this development lies in the ability to monitor developments at the atomic scale. Since the inception of such technology (namely scanning probe microscopy) only a few decades ago the progress in the fields of electronics and computing has been very rapid. This paper looks at the various techniques in use and being researched in order to provide a sustainable framework for data storage at the atomic scale. The current literature on this matter has been reviewed in sufficient detail to fully appreciate contemporary as well as previous and future developments. Experimental evidence also suggests that the use of atomic scale observations to augment data storage is only bound to increase as time proceeds. There are a few problems with using SPM techniques for data storage as yet such as excessive tip wear but the interest in this field is bound to augment development and research through the development of novel materials and techniques to solve these and other problems. 2. Introduction Scanning probe microscopy (SPM) refers to the branch of microscopy that creates images of surfaces that it scans using physical probes. The probe is mechanically moved on to the specimen’s surface and often raster scans are employed. [Che08] The surface is generally scanned line to line and the corresponding surface features are recorded as functions of the probe’s position. SPM came to prominence following the invention of the scanning tunnelling microscope in the early eighties. [Che08] The resolution of the final surface image varies between techniques depending on the abilities of the piezoelectric actuators employed. These actuators are used to create the fine motions required to image the surfaces in question. [Che08] The data obtained is plotted as a two dimensional grid of captured data points which is then visualised using false colours in the form of a computer image. 2.1. Applications Scanning probe microscopy has grown in use over the years and its applications span multidisciplinary approaches. The various disciplines that are utilising scanning probe microscopy include (but are not limited to) material sciences, biology, physics, engineering, chemistry, computer sciences, medicine, space science etc. [Bhu08] SPM techniques have been utilised to discover the atomic structures and interactions of various materials so as to enhance the use of novel materials in nano applications. [Kas07] Similarly SPM has found extensive use in biological applications such as biomedical research and characterising plant cell walls as well as animal cell walls. [Hau06] SPM is also finding new inroads into the world of computing and this paper will be concerned largely with the application of two SPM techniques to the development and fabrication of nano scale electronics and computers. The fields of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) will be analysed in context of their current positions for advancement in research relating to computing and electronics applications. A typical arrangement for SPM is shown in the figure one below. Figure 1 - Typical SPM Arrangement [Wil06] 2.2. Quantum Computing Quantum computers are computational devices that rely on quantum based mechanical phenomenon for example entanglement and superposition in order to carry out data based operations. Regular computing relies on transistors that have been scaled down and are being scaled down to promote miniaturization. [Che08] Quantum properties are relied on largely in order to represent data as well as to perform operations on data. One of the biggest advantages offered by quantum computers is the fact that they can process many different streams of data and instructions while conventional computers are limited in this respect. [Che08] Given the need for greater computing power along with cryptanalysis and analysis of multi variable systems, it is highly unlikely that conventional computing will be able to keep up with the demand for greater computing power. As yet quantum computing is still in its infancy and most of the investigation carried out relies on operation on qubits (quantum bits). As mentioned before quantum computing relies on the quantum state of particles in order to offer computational power. In order to develop such systems, the interaction of quantum particles needs to be studied at the quantum scale. Moreover the spin states of quantum particles needs to be studied in great detail so as to delineate what each qubit is representing. [Sto08] Nearly all spin based quantum computers have a common difficulty in evaluating the single spin readouts. Previous attempts were made using high resolution optical microscopy but this proved unreliable. [Sto08] Recent advances in scanning tunnelling microscopy (STM) have enabled the accurate determination of the single spin readouts thereby providing greater reliability for quantum computing. The use of STM to determine these spin states is far more reliable than other techniques currently in use including coding and optical microscopy as mentioned before. [Sto08] 2.3. Molecular Electronics The use of electronics has grown beyond imagination over the decades and this fact has been aided by the reduction of the size of electronic devices. The reduction in size means greater portability for the average user and this has introduced electronics in small devices such as watches, music players and the like as well as in larger devices such as tablets, photo voltaic cells and the like. [Inz08] The field of molecular electronics comprises of the study and application of molecular level components that are used to fabricate electronic devices. Molecular electronics consists of the mass applications used in conductive polymers as well as single molecular electronic devices as well as nanotechnology. [Inz08] A typical switching device created through molecular electronics is shown in Figure Two to delineate the scale of fabrication involved. Given the fact that molecular electronics uses techniques comprising of physics, chemistry and material sciences there is dire need to study these devices in greater detail. The reduction in the sizes of transistors was much helped out by optical microscopy but these developments reached an impasse. This impasse was resolved using techniques introduced through SPM techniques. [Fau11] Both research and fabrication of molecular electronic devices is dependent on determining the exact structure of the lattice involved. Moreover the positioning of doping agents within the lattice structure is of the utmost importance such as in TCOs (transparent conducting oxides). [Fau11] SPM techniques have been used in ever increasing numbers to delineate the structures of electronic devices on the molecular scale. For example SPM techniques have been combined with X-ray spectroscopy in order to determine the lattice structures of virgin metallic oxides as well as doped metallic oxides. [Fau11] SPM is also under use to study the biocompatibility of materials used to construct electronic devices. Generally SPM nanolithography is performed on thin films created on substrates in order to determine the material structures and properties. [Kas07] Recent research into molecular electronics to determine development on soft flexible materials as well as hard rigid materials has also been performed using SPM lithography because of the ease of use and the higher reliability of results offered. [Kas07] Figure 2 - Simple Molecular Electronics Switch [CNS04] 2.4. Nano-Scale Device Fabrication Miniaturization of electronic and other devices has meant that novel manufacturing techniques need to be developed in order to ascertain the quality of the devices being produced. Nano scale devices have also assumed greater importance in newer fields especially fields that require precise motion and control. [Wal081] Nano scale devices can be largely classified into two broad categories which are micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS). MEMS based components are distinct from molecular nano-technology and molecular electronics. [Wal081] Such devices rely exclusively on small mechanical devices that are driven using electricity and these devices range in size between 20 micrometers and 1 millimetre. The components that are required to construct MEMS devices range in size from 1 micrometer to 100 micrometers and thus there is great need to observe such devices especially during and after manufacturing. [Wal081] MEMS devices consist of a centralised unit that is used to process data as well as microprocessors and several intermediate devices that interface with input devices such as micro-sensors. When devices these sizes are considered the standard concepts of classical physics are not always usable. [Wal081] The large ratios between the surface areas and the volumes of these devices mean that surface based phenomenon such as electrostatic effects as well as wetting govern the volume based effects such as the thermal mass and the inertia. [Wal081] On the other hand, NEMS is the next logical step in the evolution of MEMS and tends to integrate nano electronics with mechanical actuators at the nano scale. Typical applications for NEMS devices include accelerometers as well as chemical detectors. [Wal081] Figure Three shows a nano scale fabricated device to provide greater understanding of the sizes involved. Most materials used in NEMS are based on carbon and more specifically on carbon nano tubes as well as graphene. This is because carbon offers both the mechanical properties as well as the electrical conductivity properties that are desirable for devices at the nano scale. [Wal081] Carbon metallic nano tubes have also been developed as nano electronics interconnects as these materials display high current densities. However as yet there are large problems exhibited by these devices as the exposition of carbon to oxygen in the environment tends to degrade the electrical properties. [Wal081] Moreover there is a large need for research before these devices are fully implemented as their mechanical properties need exploration. Similarly the complicated nature of graphene’s electrical properties is worth investigating too to understand the band gap better. [Wal081] Hence there is as yet a large need to build on the learning from MEMS in order to better construct NEMS which are still largely in a developmental stage only. Figure 3 - Nano Scale Fabricated Device [Adv08] 3. Relevance of SPMs (STM and AFM) 3.1. STM A scanning tunnelling microscope is used to create surface images at the atomic level. The typical resolution levels of STMs in contemporary research range between 0.1 nm and 0.01 nm resolution based depths. [Che08] Given such detailed resolution levels it is possible to image individual atoms and to manipulate them routinely. A major advantage offered by STM instruments is that they can be utilised in air, water, liquids, vacuum and ultra high vacuums. [Che08] Moreover STM instruments can also be used in diverse temperature ranges from near zero Kelvin temperatures to few hundred degrees Celsius. STM techniques utilise the concepts of quantum tunnelling. The conducting tip used for STM is brought near the specimen’s surface where a potential difference is applied between the tip and the surface. Using a potential difference aids electrons to tunnel using the vacuum created between the tip and the surface. [Che08] The current between the tip and the surface is known better as the tunnelling current which depends on the position of the tip, applied voltage as well as the local densities of states (LDOS) of the specimen. The current is monitored as the tip’s position moves across the specimen surface. [Che08] While STM offers ease there are certain disadvantages too as STM relies on highly clean surfaces, on the sharpness of the tips, vibration control as well as highly intricate electronics. 3.2. AFM Atomic force microscopy (AFM) is a form of high resolution SPM that can go for resolution levels of the order of fractions of a nanometre. This technique is far better than optical microscopy because the techniques utilised perform better than the optical diffraction limit. [Hin06] AFM is being used extensively in order to image, measure and manipulate substances at the nano scale. The surface of the specimen is felt using a mechanical probe. The movements of the scanner are controlled very precisely through the use of piezoelectric elements using electronics. [Hin06] Certain versions of AFM instruments allow the scanning of electric potentials through the use of conducting cantilevers. More recent versions of the AFM instruments being developed allow the passage of currents between the tips and the probe. However the use of such machines is highly challenging and only a few research groups have reported their successful use. [Hin06] The figure presented below shows the kind of geometric accuracy that can be achieved using an AFM cantilever. This implies that AFM can be used without doubt to store data in discrete form. Figure 4 - Indentations written and read using a heated AFM cantilever [Kin02] 4. Research Based Proof 4.1. Experimentation Extensive research has been carried out on scanning probe microscopy and its applications in the fields of computing especially data storage and representation as well as data processing. However any such proposals to replace existing data storage mechanisms must offer long terms perspectives so that the developed prototype devices can be scaled down in terms of cost and size to make them more competitive. Using techniques from scanning probe microscopy ensures that existing data storage and handling techniques can be miniaturised and provided with greater reliability but the long term perspectives of cost still need to be dealt with to ensure economic feasibility. 4.1.1. Use of AFM and STM for Data Storage Both AFM and STM are capable enough of providing long term perspectives on data storage as these devices can easily be scaled down to the nano as well as atomic scales as required by the application. The nanometre tips that are utilised for AFM and STM imaging offer the reliability which is centred on the basic function of confining interactions. [Kin02] Polycarbonate based substrates have been used to achieve data densities of up to 100 Gb/inch2 through thermo mechanical heating of the tip used for AFM. More research allowed increases in the data read back speeds up to 10 Mb/s as well as implementing track servoing. [Kin02] The tips used for both AFM and STM pose problems especially when high data rates are concerned. The resonant mechanical frequencies of AFM based cantilevers pose issues in reliability as the tip has to contact the specimen surface which means that forces will be generated at the nano and atomic scales. [Kin02] On the other hand STM tips are limited by the low tunnelling currents permitted as well as the feedback rates that ultimately end up in low data rates for both writing and reading. The single AFM tips in use for data storage functions can operate on the microsecond levels while the traditional magnetic storage systems employ the nanosecond time scales which imply that the AFM based technologies must be improved by at least a thousand times just in order to offer valid competition to current technologies. The current technologies used for data storage will not be able to keep up with the increasing demand for more storage space within the same dimensions of storage devices. [Bhu08] Hence it is essential that the size of data storage devices be taken down to the nano and atomic scales in order to enhance areal densities. Both AFM and STM offer such solutions with areal densities of up to a few tera bytes per square inch being offered. [Bhu08] 4.1.2. The Millipede Concept for Data Storage An alternative offered to such obstacles is the “millipede concept” that delineates the use of multiple tips working simultaneously and alone as required in order to increase the data storage rates. The millipede concept relies on the use of multiple tips that each operate in their own areas. The developmental model by IBM uses an array of 32 x 32 tips which can write 1024 locations simultaneously. [Bhu08] When a writing based operation is initiated the tips all move individually to perform a thermo mechanical writing in their own respective blocks. Figure Five below shows an array of cantilever tips that are utilised for data storage along with their imaging using an electron microscope. Therefore instead of one tip writing in different areas, different tips write data in their own respective areas. [Kin02] It is possible to achieve data densities of up to 500 Gb/inch2 in an area no greater than 9 mm2 using the millipede technique. Figure 5 - Millipede cantilever array data storage chip and scanning electron microscope images of individual cantilevers in the array. [Kin02] 4.1.3. Relevance of Nano Tribology and Nano Mechanics The classical sphere of research relating to SPM based data reading and recording technologies is largely comprised of the tip and the cantilever used to perform the scanning and writing operations. However in recent years there have been greater attempts at finding newer materials and studying their behaviour when exposed to SPM methods. [Che08] In this respect the areas of nano tribology and nano mechanics are of vital importance because any new materials being used in place of conventional magnetic storage mediums must be investigated for their behaviour on the nano and if required the atomic scale. Given the fact that SPM based data recording techniques are under development for high areal densities, it is expected that the probes used will scan up to velocities of 100 mm/s which is higher than current scanning velocities in use. [Bhu08] One approach used AFM based probes that scan over a phase change chalcogenide medium while phase change is achieved through the application of varying current that tends to heat up the interface. Another approach to dealing with this issue is to use ferro electric data storage whereby a conducting AFM probe is scanned over a thin film of lead zirconate titanate (PZT). The ferro electric material can be polarized through the application of short voltage pulses at the interface of the AFM tip and the bottom electrode layer that tend to go beyond the coercive field created in the PZT thin film. [Bhu08] This has the consequence of non volatile changes within the electronic properties under consideration. However whichever approach is considered the issue of tip wear continues unabated. One large problem in commercialising AFM based data storage techniques is the tip wear experienced as a result of the contact between the tip and the specimen material. [Bhu08] Consequently research is being diverted to the nano tribology as well as the nano mechanics of these devices in order to improve their total life. Only a long lasting tip can guarantee that AFM based data storage techniques can become a success. 4.1.4. Characteristics of Films and Tips used for Data Storage The largest advantage of using AFM tips for data storage is the prototyped data densities achieved that is of the order of several terabytes per square inch. Moreover as outlined before the millipede concept can be used in order to enhance the thermo mechanical recording and playback from storage devices. [Bhu08] The films used for such recording are very thin and generally have sizes of up to 40 nm while a harder substrate is used for development. The figure below shows a typical image for a 3D AFM topograph developed on a PET surface with a simple array. [Kas07] The cantilevers used within these arrangements are composed of heaters integrates with the tips. The dimension of the tips is generally of the order of nanometres. These tips are heated up to some 400oC and are then brought into contact with polymer based recording materials. [Bhu08] The cantilever that is originally used for writing is subsequently used for reading operations. As the cantilever displays temperature resistance dependence, it can also be used as a thermal feedback sensor. However as mentioned previously the wear of the tip is an issue. [Bhu08] It cannot be expected that the tip wears down to a large measure but any wear that affects the critical dimensions of the tip is enough to cause unreliability in read and write operations. Figure 6 - 3D AFM topography of an array of four surface pits on the PET sample. [Kas07] 4.1.5. Application of PCM (Phase Change Memory) and OUM (Ovonic Unified Memory) In contrast to the memory systems based on AFM presented above, another system is recording onto systems through phase change memory (PCM). Research into PCM delineates the potential for PCM to emerge as a strong candidate for memory devices. [Bhu08] PCM is closely connected to amorphous semiconductors. The chips that are based on ovonic unified memory (OUM) type storage typically produce different levels of resistance when used on a glass based material. [Bhu] These resistances can be coded and interpreted as the zero and one used in the binary system with the lower resistance denoting zero and the higher resistance denoting one or vice versa. The cells used in OUM applications offer non volatile memory and are composed of a horizontal strip of chalcogenide. [Bhu08] Chalcogenide is an electrically conductive material that is largely glass based. An electrode is connected to the chalcogenide cell and this technology is already being utilised in the rewritable compact disc (CD) field. [Bhu08] The chalcogenide material is heated to some 630oC using an electrode type probe through the use of a high current. This causes the creation of amorphous glass regions that display high resistances. [Bhu1] On the other hand, when a lower current is applied which keeps the chalcogenide cell temperature below 630oC, the resulting structure is crystalline and presents a lower resistance. When reading operation are carried out, a low amount of current is applied through the tip to the specimen’s surface that resultantly measures the resistance of the chalcogenide cell in question. [Bhu08] The probes used for such operation need to be conductive and it is standard practice to use silicon tips that are coated with platinum. Now if faster data read and write rates are desired, the tip can be expected to operate at velocities around 100 mm/s which presents two problems that are high temperatures and high friction. [Inz08] The high temperatures are requisite to the writing operation at start and are generated as a result of the tips movement across the surface. The high velocities create large amounts of friction as it depends on the velocity directly. [Bhu08] The condition of tips after data write and read operation is shown in Figure Seven below which indicates the level of wear experienced. The probe tips used in these applications tend to have short lives and offer less than desired reliable operation. Another contributing factor is the fact that OUM films are protected using DLC (diamond like carbon) films that tend to offer greater hardness than the employed tip. [Bhu08] The constant contact between the surface and the tip tends to wear out the softer medium. Research indicates that the probe tips wear out as a result of three major processes which are adhesive wear, abrasive wear and low cycle fatigue. [SuC03] The figure below shows the conditions of four tips made of platinum in different wear conditions. The fatigue tends to wear down materials as hard as platinum so there is dire need to find more resistant materials in order to enhance the life of the tips. Figure 7 - SEM images of a virgin Pt tip and Pt tips obtained after 1 m sliding at 100 nN for the sliding velocities at 0.1, 1 and 100 mm/s on the PZT film. [Bhu08] 4.1.6. Ferro Electric Memory Applications Another major technique used in SPM based data storage is ferro electric memory that is utilised in the form of ferro electric random access memory (FeRAM) within electrical address based storage systems. The dielectric structure of a typical FeRAM application is composed of ferro electric materials (generally PZT) and when the material is subjected to an external electric field, the positive and negative charges are displaced from their initial positions. [Set06] This behaviour is similar to polarization. The polarization occurring in a ferro electric material is almost instantaneous and represents the displacement that is characteristic to the particular crystal structure of the concerned material. [Bhu08] The reapplication of the electric field can reorient the polarization directions of the material. The polarization directions are designated as zero and one and are then used to store data within specified data storage cells. A number of different approaches exist for the case of ferro electric materials and the ensuing data storage. The case of ferro electric transistors is a relevant example as well as ferro electric tunnel junctions and ferro electric data storage based on resistance. [Zhu05] Ferro electric data storage can also be carried out using mechanically addressed systems that use probes like other AFM storage systems to record data. The tips in these cases are placed in contact with storage materials (typically strontium titanate, STO that has been coated with PZT) to form a double layer. [Bhu08] In this arrangement the SRO (strontium oxide) tends to act as the bottom electrode while the PZT film acts as the ferro electric material. The ferro electric material can be easily polarized using voltage pulses with magnitudes of nearly 10 volts over durations of 100 micro seconds that go from the tip to the SRO electrode which aids in exceeding the coercive field created by the PZT layer. [Bhu08] This has the effect of creating localised and non volatile changes in the basic electronic properties of the film beneath. Within the cases of epitaxial c-axis based PZT films the resulting polarization vector could either be parallel to or anti-parallel to the c axis. Research into such ferro electric systems has shown that reduction in the film thickness tends to increase the activation energy. The use of thin PZT films is expected to create miniature domains and greater storage densities. [Bhu08] Moreover if the temperature increase during the write or record operation is of the order of 80oC then the total wear experienced at the tip decreases significantly. Another advantage of these systems is the fact that the tip need not be contacting the film while a read operation is being carried out. [Bhu08] The reading operations carried out in ferro electric thin films are distributed into two broad categories. One approach relies on the piezoelectric properties of ferro electric materials as well as the presence of any statically charged regions. [Bhu08] The ferro electric material is scanned using non contact SPM which aids in the detection of the normal component of the polarization vector. In contrast to the method outlined above, another method is based on contact SPM through the application of an alternating current (AC) voltage to the ferro electric material’s polarized region. [Bhu08] The resulting piezo response is force based and this can be detected using the probe’s tip. The domain structure of the ferro electric film can be measured using piezo response that comes in the form of vibration and can be measured. [Bhu08] It is essential that the tip contact the film in order for this vibration to be detected. When the piezo electric response mode is applied, the voltage is applied through the probe’s tip that tends to act as the moving top end electrode. 4.1.7. STM Based Data Storage STM has been used extensively to manipulate matter at the atomic scale but it has not been found suitable for data storage generally due to the need for low temperatures and vacuums during operation. [Coo00] The production of such low temperatures and vacuums in commercial applications would make them uneconomical for the average consumer. STM has been used to navigate individual atoms and to align C60 molecules on copper lattices which reveals that STM can be used reliably but only under certain conditions. Albrecht et al. have studied the effects of using STM to ablate graphite in order to create 10 nm holes. The study conclusively revealed that STM was unstable in normal operating conditions as the ablation would obscure the imaging process and would also damage tips regularly. [Alb89] This limits the practical aspects of using STM for data storage applications of any kind at all. On the other hand, the use of STM to locally oxidise titanium surfaces has been well researched in order to deal with data storage. A negative bias voltage is applied to the titanium substrate using a scanning probe in the presence of water absorbed on the surface. The appearance of local oxidation has been studied in detail through the manipulation of tip diameter, substrate roughness, field strength, scanning rate, tip sample distance as well as the environment. The research conclusively proved that the diameter of the proximity probe tip as well as the surface roughness of the substrates were the ultimate limiting factors in achieving the lowest feature size on substrate. [Coo00] Based on these issues, the use of STM for data storage has been shelved as STM requires special conditions in order to be used for data storage and still offers far lower areal densities than AFM. 5. Limitations in Experimentation and Prototyping As mentioned previously, one of the largest problems experienced in the case of probe tips is that of consistent wear. As yet the materials used to create the probe tips are softer than the surfaces that they scan. [Bhu08] Two lines of action can be adopted for further research to solve this problem. One approach would have to rely on making the tips harder through denser coatings or harder core materials. [Bhu08] The second approach would depend on making the surface of the memory device softer. [Bhu08] In the case of permanent storage mediums that are not operated on or exposed to environmental factors (such as hard disks) this may be feasible but in the case of exposed mediums such as CDs, DVDs, other optical discs this may lead to faster wear of the memory storage surface. It has also been noticed that the amount of wear increases as the logarithm of the velocity increases and this has been explained using thermally activated atomic scale stick and slip which indicates that the tip is held down and released by particles as it moves across a surface. [Kin02] Moreover an increase in the operating temperature increased the overall wear rate even when the temperature was kept around 80oC only which is significantly lower than the operating temperature. [Bhu08] Current research does not provide satisfactory answers to the mechanisms behind nano wear of the probe’s tip. This is all the more true for tips where a precious metal is coated on top of a softer core material. [Bhu08] It is agreed upon that tip wear is a function of multiple factors such as tip speed, relative hardness of film and tip materials, operating temperature and temperature increase during operation especially writing / recording operations. Other than these factors new research has introduced newer factors too which need further investigation such as relative humidity that has been found to affect tip wear but was previously not considered. [Bhu08] The figure attached below shows the wear conditions in applications with and without lubrication and the resulting tip wear. As could be expected the lubricated tip tends to wear less although as mentioned before more factors need to be investigated in order to manufacture more resistant tips. Figure 8 – (a) SEM images of an unused Pt tip and Pt tips obtained after 1m sliding at 50 nN and after additional 1m sliding at 100 nN and 0.1 mm/s on unlubricated and Z-TETRAOL lubricated surface (b) after 1 m sliding at 100 nN and 100 mm/s. [Bhu08] As yet research has placed far greater focus on the optimisation of the electrical properties of tips without paying much attention to the nano mechanical properties of the coatings and substrates being used. [Inz08] There is still dire need to investigate the hardness, toughness, wear resistance and elastic behaviour of tip materials in order to produce tips that can provide greater reliability and wear resistance so that SPM techniques can be used commercially on a larger scale. [Bhu08] Another major problem with the use of PZT thin films is the change in thickness resulting from the continued application of voltage. The application of voltage affects ferro electric materials in the shape of ferro electric fatigue. [Bhu08] This can also be interpreted as the reduction in switchable polarization as polarization is repeatedly applied to the surface being used. If polarization fatigue can be overcome, there is every chance that this process may be exploited commercially. 6. Conclusion The field of SPM has come a long way since its inception in the early eighties and since the development of instruments in the mid eighties to perform reliable SPM scans. The current research indicates that most of the work carried out in SPM in regards to AFM and STM is based on the creation of high areal density storage mechanisms. These mechanisms include permanent storage solutions such as hard disks as well as non permanent storage mediums such as CDs, DVDs etc. At this point in time the mechanism utilised to store data through SPM is fully understood but its commercial exploitation is still far off into the future because of certain limitations. The limitations are posed by both film materials as well as probe tip materials. The greater part of research on SPM for these applications has been utilised for optimising the electrical properties of these materials while the nano mechanical properties still need to be optimised. Similarly there is as yet need to investigate nano tribology more in order to delineate the mechanisms that carry out film and tip wear. If AFM based data storage techniques can be perfected then there is little doubt that these technologies will dominate the data storage industry for decades to come. 7. Future Outlook Given the current trends in data storage technology, it can be safely declared that magnetic storage methods will be shelved in the future as more reliable AFM tips are manufactured. However STM will not play a major part in data storage because of the specialised demands and lower areal densities offered in comparison to AFM. 8. Bibliography Che08: , (1), Bhu08: , (2), Kas07: , (3), Hau06: , (4), Wil06: , (5), Sto08: , (6), Inz08: , (7), Fau11: , (8), CNS04: , (9), Wal081: , (10), Adv08: , (11), Hin06: , (12), Kin02: , (13), Bhu: , (14), Bhu1: , (15), SuC03: , (16), Set06: , (17), Zhu05: , (18), Coo00: , (19), Alb89: , (20), Read More