IBM's Millipede. Conor Walsh Friction and Wear of Materials RPI Hartford 12/13/12

Transcription

1 IBM's Millipede Conor Walsh Friction and Wear of Materials RPI Hartford 12/13/12

2 The Millipede data storage device was developed by IBM and first demonstrated as a prototype at the 2005 CeBIT computer expo in Germany. The project was a natural evolution from the creation of the scanning tunneling microscope (STM), developed at IBM Zurich in 1981 by Gerd Binning and Heinrich Rohrer, who earned the 1986 Nobel Prize in Physics for this invention (Chang). The STM was able to create nano-scale images by bringing a very sharp conducting probe tip near a surface, applying a voltage between the two, and measuring the current that results (the phenomenon of electronic transfer in this case is called tunneling). This measured current is a function of the density of the surface and the gap between the surface and the tip, which can be processed into an image of the surface as the tip is passed back and forth along it. Binning and Rohrer soon realized that the highly sensitive and precise silicon tips that this form of microscopy required could also be used to contact a surface, and make marks in some cases. In fact, the STM's direct descendent the atomic force microscope (AFM) can be used to modify materials and even machine thin films. The AFM differs from the STM mainly in that the tip is attached to a cantilever and contacts the surface, and a beam of electrons are reflected off of the cantilever into a collector in order to take height measurements as the tip is dragged across a surface (Figure 1). The AFM is usually used to created highly precise topographical maps of surfaces, but in some cases is used to create micrometer-sized structure (or smaller) on a substrate by increasing the tip-surface interaction strength and applied load on the probe. The probe causes mechanical deformation to create these structures, which can be as small as 50 nm (Bai).

3 Figure 1: Comparison of STM and AFM (from Scanning and Atomic ) However, Binning and Peter Vettiger, another IBM employee, began pursuing a different route in the mid-1990s, in the field of data storage (Chang). Using the innate precision and conducting nature of the silicon tip, they contrived to make extremely small dents or pits (on the order of nm) in a polymer surface by pulsing a current through the cantilever until the tip is heated above the glass temperature of the polymer, which is usually polymethyl methacrylate, or PMMA (Durig). This requires temperatures near 750 degrees Fahrenheit, much greater than the temperature with which the polymer transitions from a glass state to a liquid, which is necessary because of the poor heat transfer between tip and surface due to low contact area. The polymer melts and allows the applied load of the tip to dip below the surface before the temperature is reduced, which effectively writes a bit of data (Figure 2). The polymer layer is kept thin, on the order of 40 nm, and is backed by a silicon substrate in order to prevent the probe tip from penetrating too deep. This also minimizes the size of each of the bits, which can potentially be as small as 10 nm (Vettiger). The entire surface of the polymer can be considered a grid of bits,

4 where a pit is equivalent to a 1 and a continuous flat surface is a 0. In order to read the surface's data, the tip is translated across the surface while heated to a lower temperature, roughly 570 degrees Fahrenheit. The polymer will not melt at this temperature, but whenever the tip falls into a pit, some increased heat transfer occurs as the tip and the surface experience a greater contact area. The temperature change in the cantilever that results can be detected and a 1 is read by the device. Further, if the tip is reheated to the write temperature while inserted into a pit and then the applied load is removed, the polymer will flow back into the indentation and create a flat surface before returning to a glass state, thus re-writing the bit as a 0. This technique is equivalent to erasing the bit. Figure 2: Artist's Rendering of the Bit-Writing Process (from Millipede ) The real advantages of this device become apparent when the cantilevers and tips are put into use in massively parallel fashion (Figure 3). The IBM prototype featured 1024 cantilevers (hence the name Millipede ), and has the theoretical potential for ultra-high storage densities of multiple Terabits per square inch (Bhushan). The parallel operation of the tips, which are

5 simultaneously translated by micro-mechanical actuators, allows for high data read/write speed, which is further improved by using high-frequency cantilevers. Using thin, narrow and short cantilevers, which have the benefit of low mass, can allow for a high resonant frequency without making the cantilevers too stiff (Mamin). In this way the Millipede technology can compete with current data storage devices, which fall mainly into two categories: magnetic and flash. Magnetic storage, such as that in CDs, has a relatively high data storage density but slow read/write speeds. However, Millipede actually has roughly 4 times the current storage density of magnetic storage devices ( About ). Flash memory (used in such devices as the ubiquitous flash drive), is a form of electronic data storage and has lower storage densities but extremely high read/write times. Millipede can be competitive with these high read times due to the highly parallel operation with high-frequency cantilevers (potentially in the 10 Gigabits/second range) (Durig). Thus, it appears that a Millipede device can combine the advantages of the various other storage devices currently available. Figure 3: Massively Parallel Cantilever Construction (from Mamin)

6 However, there are concerns with the lifetime of the Millipede device, as the polymer undergoes cyclic indentation and reformation and as thousands of microscopic cantilevers travel repeatedly back and forth across the surface. This is the major drawback of the Millipede concept, as electronic forms of data storage will not suffer from the majority of these concerns. The Millipede surface and the tip will experience wear over time, mainly through mechanisms such as adhesive wear, abrasive wear, and low cycle fatigue (Bhushan). These mechanisms will act in concert with chemical reactions due to the high velocities and temperatures at the tipsurface contact. Perhaps most significantly, any knowledge of macroscopic contact and interaction between tip and surface may no longer be applicable in the nanoscale regions. As the tip is harder than the polymer surface, it is expected that polymer wear will be more critical, although wear is expected to occur on both components. Reducing the applied load to levels at or below 50 nn can bring down the wear expected on the tip to manageable levels (Mamin). Other possible solutions sacrifice performance: increasing the contact area between the tip and surface by using a larger tip would improve the contact stresses but hurt the storage density. Similarly, lowering the contact force by using less stiff cantilevers would have the unwanted effect of lowering data rate at the same time, as discussed above. In one experimental case, a Hertzian analysis was used with the hard tip approximated as a sphere on the soft, flat polymer surface. The maximum storage density calculated that would still avoid wear was on the order of 65 Gigabits per square inch, a major reduction in the overall potential of the device (Mamin). When producing a massively parallel array of cantilevers, it is essential that the tips and cantilevers are all uniform. These measurements will have an effect on the load input to the surface by each cantilever, and thus the wear that results. Probes are often formed by a lithographic process where either plasma or wet chemical etching removes material from beneath

7 the cantilever, leaving the tip intact. This allows for a highly repeatable and reliable micromachining process that will ensure similar dimensions, and thus similar contact forces (Vettiger). However, the tip generally should have a high hardness, as will be discussed later. One method is to have a wear-resistant tip deposited, particularly if the cantilever contains noble metals (for high conductivity and other properties), as these metals have a lower hardness than even that of silicon. A wear-resistant tip can be formed from an alloy such as the alloy of platinum, iridium and tungsten developed by Allied Signal, Inc. for use in spark plugs (Bhushan). An interesting phenomenon has been observed in some experimental cases where the rate of wear of the tip is extremely non-linear. It appears that some initial fracture may occur to the tip until it develops a more stable geometry, at which point the tip wear drops significantly (Mamin). This may be due to the decrease in stresses as the real contact area between the tip and the surface increases, as well as an increase in hardness of the tip surface as plastic deformation occurs. In fact, the effect can be simulated by modifying the material properties of either the tip or the surface. The surface can be cured for a shorter period of time, resulting in a polymer of increased softness, or the tip can be fashioned from a harder material. However, it must be recognized that any decrease in the wear of the tip through methods such as these will result in a corresponding increase in the wear of the polymer, which may or may not be acceptable. The solution may be to increase the hardness of both materials. An optimal situation was postulated where the tip is made of a material with extremely high hardness, such as diamond or silicon carbide, while the polymer is made just hard enough to keep wear rates to an acceptable lifetime level (Mamin). Of course, the other possible answer to this issue would be to include extra tips or to provide a means to replace tips or even the substrate itself. It has also been reported that wear increases as the logarithm of relative velocity between tip and surface, stemming from

8 thermally activated stick-slip events. Additionally, as velocity increases, the adhesive and impact wear mechanisms begin to have a more significant impact (Bhushan). Further, increased wear is also correlated with higher temperature, an a difference can be seen even between operating temperatures 70 degrees and 180 degrees Fahrenheit. Unfortunately, this problem is hard to avoid as high temperatures are a necessity in the bit-writing and erasing techniques. Finally, humidity plays a role in the wear produced by tip-surface interactions, particularly higher humidity leading to increased wear. Overall, empirical sliding friction coefficients have been found to be in the range of 0.05 to over 0.20, for unlubricated and lubricated contact with an applied load between 50 and 100 nn. (Bhushan) Figure 4: The Effect of Wear on Probe Tips (from Bhushan) Another potential problem with the lifetime of the Millipede device is contamination. Whether contributed from the external environment or the result of wear within the device, contaminate particles could lead to greater wear of the tip or surface, particularly through

9 abrasive wear. Further, extra particles could fill in the 1 bits or modify the heat transfer in such a way as to provide false readings for the system. This could severly impair the reliability of the device. It is an unfortunate side effect of mechanical data storage devices that also allows the possibility of damage to the tip or surface due to vibrations, shock, or even temperature extremes. These same concerns plague any mechanical data storage system, including magnetic storage, although electronic storage mechanisms have a significant advantage in the particular case. Regardless, the Millipede device must be well sealed and protected to minimized the impacts of contamination. In order to ensure that the polymer used has appropriate properties for the Millipede device, some analysis must be performed on the mechanics of the bit-writing process. One group of researchers chose to evaluate the tip as acting in a viscous medium (the polymer transitioning from glass to liquid phase). As the applied load on the tip must be greater than the viscous drag forces of the liquid in order to penetrate the surface, if the applied load F is known a maximum useful viscosity η can be determined using Stokes' equation: where v is the mean velocity and R is the radius of curvature of a spherical tip (Vettiger). However, this analysis produces unreasonably low threshold viscosities (the viscosity of PMMA is roughly 7 times greater than the calculated maximum). The explanation for this lies in the time-dependent polymer mechanics. During the short time period over which the bit-writing process takes place (as low as 1 ms), entanglement of the polymer molecules largely does not have time to come into effect, which allows the molecules to deform as individually rather than as a connected chain. The result is a perceived decrease in the shear strength, allowing indentation to occur. Thus, it is the high speed nature of the bit-writing process that allows for

10 use of PMMA and similar polymers at low applied loads to the cantilever. A byproduct of this is an increase in internal elastic stress in the polymer when it resolidifies with an indentation (Vettiger). In fact, these elastic stresses provide one of the mechanisms for the return of the 1 bit depressions to the 0 bit flat surface when sufficient heat is reapplied. The other major mechanism for erasing bits is the force from surface tension, which is dependent on the polymerair free surface energy. The liquid-gas interface contracts in order to maximize the volume to surface area ratio, which can readily be seen in the formation of a soap bubble and normally contributes to formation of a sphere. However, in this case the polymer surface has a greater area when an indentation is present, and the contraction of the surface is exactly what happens when the flat profile is restored. It should also be noted that as material properties play such a large role in the formation of the 1 bits, making indentations with identical cantilever/tip inputs will result in significantly different marks in different materials, even if those materials are very similar (Figure 5). This is not to say that different materials would not work satisfactorily; rather, it appears that the Millipede design would work with most polymer surfaces (Vettiger). The main variable to consider would be the temperature with which the polymer will exceed its glass transition point (as discussed above, the cantilever and tip would have to be heated significantly higher than this, due to high inefficiencies in the heat transfer between tip and surface).

11 Figure 5: Written 1 Bits for Different Materials, Given the Same Inputs Ultimately, the Millipede device represents an enticing new technology that is still only in its infant stages. It presents some unique challenges through tribological concerns, particularly due to the atomic-scale nature of the contact interactions between probe tip and data surface. Despite these concerns, the promise of both a rapid read/write rate and an incredibly high bit density gives this concept a bright future.

12 References: About IBM Millipede Memory. July 29, < about-ibm-millipede-memory/> Atomic Force Microscope. Wikipedia. < Atomic_force_microscope> Bai, Chunli. Scanning Tunneling Microscopy and its Application. September 15, Bhushan, Bharat, Kwang Joo Kwak and Manuel Palacio. Nanotribology and Nanomechanics of AFM Probe-based Data Recording Technology. Journal of Physics: Condensed Matter 20 (2008). Chang, Kenneth. A New System for Storing Data Think Punch Cards But Tiny. New York Times. June 11, < Durig, U. et al. 'Millipede' - An AFM Data Storage System at the Frontier of Nanotribology. Tribology Letters 9 (2000). Mamin, Jonathon H. et al. High-Density Data Storage Based on the Atomic Force Microscope. IEEE. Rev. February 16, Millipede. IBM's Zurich Research Lab. < st/storage/millipede.html> Scanning Tunneling Microscope. Wikipedia. < Scanning_tunneling_microscope> Vettiger, P. et al. The 'Millipede' - Nanotechnology Entering Data Storage. IEEE Transactions on Nanotechnology, Vol. 1, No. 1, March 2002.

13 Cover Image: IBM's Millipede (from Durig)