Patent Publication Number: US-6985377-B2

Title: Phase change media for high density data storage

Description:
PRIORITY CLAIM 
     This application claims priority to the following U.S. Provisional Patent Application: 
     U.S. Provisional Patent Application No. 60/418,619 entitled “Phase Change Media for High Density Data Storage,” filed Oct. 15, 2002. 
     CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     U.S. patent application Ser. No. 10/684,883, entitled “Molecular Memory Integrated Circuit Utilizing Non-Vibrating Cantilevers,” filed Oct. 14, 2003; 
     U.S. patent application Ser. No. 10/684,661, entitled “Atomic Probes and Media for High Density Data Storage,” filed Oct. 14, 2003: 
     U.S. patent application Ser. No. 10/684,760 entitled “Fault Tolerant Micro-Electro Mechanical Actuators,” filed Oct. 14, 2003; 
     U.S. Provisional Patent Application No. 60/418,616 entitled “Molecular Memory Integrated Circuit Utilizing Non-Vibrating Cantilevers,” filed Oct. 15, 2002; 
     U.S. Provisional Patent Application No. 60/418,923 entitled “Atomic Probes and Media for High Density Data Storage,” filed Oct. 15, 2002; 
     U.S. Provisional Patent Application No. 60/418,612 entitled “Fault Tolerant Micro-Electro Mechanical Actuators,” filed Oct. 15, 2002; and 
     U.S. Provisional Patent Application No. 60/418,618 entitled “Molecular Memory Integrated Circuit,” filed Oct. 15, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field the Invention 
     This invention relates to media for high density data storage in molecular memory integrated circuits for use in micro-electric mechanical systems (MEMS). 
     2. Description of the Related Art 
     Phase change media are used in the data storage industry as an alternative to traditional recording devices such as magnetic recorders (tape recorders and hard disk drives) and solid state transistors (EEPROM and FLASH). CD-RW data storage discs and recording drives utilize phase change technology to enable write-erase capability on a compact disc-style media format. Like other phase change media technology, CD-RWs take advantage of changes in optical properties when media material is heated above ambient temperature to induce a phase change from a crystalline state to an amorphous state. 
     Data storage devices using optical phase change media have enabled inexpensive, medium density storage with the flexibility of erase and rewrite capability. Unfortunately, current technology does not enable the very high densities required for use in today&#39;s high capacity portable electronics and tomorrow&#39;s next generation technology such as systems-on-a-chip and MEMs. Consequently, there is a need for solutions which permit higher density data storage, while still providing the flexibility of current phase change media solutions. 
     SUMMARY OF THE INVENTION 
     High density data storage requires a media in which to store data. One such media is a phase change media that alters its resistivity when data is written to the media. The media can include an overcoat. The overcoat can help reduce physical damage inflicted on the media from a device such as a cantilever tip in a molecular memory integrated circuit used to write to or read from the media. Additionally, data written to the media can be in many states. Hence, the media can store digital data and/or analog data. 
     Other objects, aspects and advantages of the invention can be obtained from reviewing the figures, specification and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details of the present invention are explained with the help of the attached drawings in which: 
         FIG. 1  is an embodiment of the invention that shows a cross-section of a media device in an unwritten state. 
         FIG. 2  is another embodiment of the invention that shows a cross-section of a media device including data bit. 
         FIG. 3  is another embodiment of the invention that shows a cross-section of a media device including a data bit where an embodiment of a cantilever tip is connected with the media device. 
         FIG. 4  is another embodiment of the invention that shows a cross-section of a media device including a data bit where another embodiment of another cantilever tip is connected with the media device. 
         FIGS. 5A ,  5 B, and  5 C depict another embodiment of the invention showing another atomic probe with a coating on five sides of a core. 
         FIGS. 6A ,  6 B, and  6 C depict another embodiment of the invention showing another atomic probe with a coating on one side of a core.  FIG. 6D  depict yet another embodiment of the invention. 
         FIGS. 7A ,  7 B, and  7 C depict another embodiment of the invention showing another atomic probe with a coating formed on the end of a core.  FIG. 7D  depicts yet another embodiment of the invention. 
         FIGS. 8A ,  8 B, and  8 C depict another embodiment of the invention showing another atomic probe with a coating banded on a core.  FIG. 8D  depicts yet another embodiment of the invention. 
         FIGS. 9A ,  9 B, and  9 C depict another embodiment of the invention showing another atomic probe with a coating on one side of a core, where coating has a protective material connected with it. 
         FIG. 10  is a scanning electron microscope view of another embodiment of the invention showing another atomic probe with a coating. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an embodiment of the invention that shows a cross-section of the media device  100  in an unwritten state. The media device  100  includes a substrate  102 , an undercoat  104 , a media  106  and an overcoat  108 . Substrate  102  supports the media device. Undercoat  104  can be formed on top of substrate  102 , however, undercoat  104  is not required. Next, media  106  is formed and then an overcoat  108  is placed after the media  106 . 
     Media device  100  can be made from a variety of materials. For instance, in one embodiment, media device  100  can include an undercoat  104 . Undercoat  104  can be placed over the substrate. The substrate is typically a material with a low conductivity. In one embodiment, undercoat  104  is a highly conductive material. For instance, one embodiment uses a material for undercoat  104  that includes tungsten. Yet another embodiment of undercoat  104  includes platinum. Other embodiments of undercoat  104  can include gold, aluminum, or copper. 
     While undercoat  104  has been described as being a highly conductive material, undercoat  104  can also be an insulator. For instance, undercoat  104  can be made from an oxide or nitride material, thereby insulating the media  106  from the substrate  102 . 
     In another embodiment, media device  100  includes an overcoat  108 . Overcoat  108  is made from a material that is different from media  106 . The overcoat  108  is selected to prevent physical damage to the media or to the probe tip when the probe tip comes into contact with overcoat  108 . The overcoat is selected to reduce wear of the overcoat and probe tip over an extended time period. Overcoat  108  typically has a low conductance characteristic, but a high hardness characteristic. For instance, in one embodiment overcoat  108  is made from titanium nitride, which is a poor conductor, but is hard. In another embodiment, overcoat  108  can be made of a diamond-like carbon. The conductivity of diamond-like carbon can be adjusted in the manufacturing process through a variety of techniques. One such technique includes using a dopant such as nitrogen in the formation of the diamond-like carbon. 
     In yet another embodiment of media device  100 , overcoat  108  can be an insulator. For instance, overcoat  108  can be an insulator such as nitride, for example silicon nitride. If an insulator is used for overcoat  108 , then any current applied to memory device  100  will have to tunnel through the insulator before reaching the media  106 . Thus, in one embodiment, the insulator used for overcoat  108  is kept relatively thin, thereby reducing the amount of tunneling required before a current can interact with media  106 . In another embodiment, the insulator for overcoat  108  is an oxide. A number of different insulators are useful for overcoat  108 , and these insulators have an advantage of being very hard. 
     In other embodiments of media device  100 , media  106  is a phase change material. In still other embodiments of media device  100 , media  106  is a phase change material such as germanium, tellurium and/or antimony and is commonly known as a chalcogenide. As a phase change material is subjected to different temperatures, the phase of the material changes between crystalline and amorphous states. As a result of this phase change, the resistivity of the material changes. This resistivity change is quite large in phase change materials and can be easily detected by a probe tip that has a conductive coating on it by passing current through the tip and the media. Phase change materials are well known in the art and can be found disclosed in numerous references, for example U.S. Pat. Nos. 3,271,591 and 3,530,441 both issued to Ovshinsky and incorporated herein by reference. 
     In yet another embodiment of media device  100 , media  106  can be a magneto optic material. 
     In addition to overcoat  108 , media device  100  can include a lubricant  101  that is placed on top of overcoat  108 . For instance, in one embodiment, lubricant  101  can be molybdenum disulfide. Lubricant  101  can be a liquid. Lubricant  101  can also be any number of thin liquids. Lubricant  101  can be applied to overcoat  108  by many different methods. In one embodiment, lubricant  101  is deposited on top of overcoat  108  using a deposition process. In another embodiment, lubricant  101  is sprayed onto overcoat  108 . 
     One method of making the media device  100  is with a traditional semiconductor manufacturing processes. Yet another method of making media device  100  is to use a shadow mask. Thus, a mask wafer that contains at least one aperture is placed over a final wafer, which will contain a memory device  100 . The mask wafer and final wafer are then subjected to a deposition process. During the deposition process, chemicals pass through the shadow mask and are deposited to form a media device  100 . 
       FIG. 2  is another embodiment of the invention that shows a cross-section of the media device  200  including data bit. Media device  200  includes a substrate  202 , an optional undercoat  204 , a media  206 , and an overcoat  208 . The media  206  further includes a data bit  210 , which represents data stored in the memory device  200 . 
     The media can be of many different types. One embodiment is where media device  200  includes a charge storage type media  206 . Charge storage media stores data as trapped charges in dielectrics. Thus, for charge storage media, media  206  would be a dielectric material that traps charges when media  206  includes a written state. Changing media  206  back to an unwritten state simply requires the removal of the trapped charges. For instance, a positive current can be used to store charges in media  206 . A negative current can then be used to remove the stored charges from media  206 . 
     Another embodiment is where media device  200  is a phase change media. Thus, media  206  can include a material that has a resistance characteristic at an ambient state, but the resistance characteristic changes in response to temperature changes. For instance, as a current is applied such that the current passes through media  206 , the temperature of media  206  is increased. After media  206  is heated to a predetermined temperature, the current is removed from media  206 , causing the temperature of media  206  to decrease back to the ambient state of media  206 . During the cooling of media  206 , the resistivity of media  206  changes from its original state, the state before the current was applied. This resistance change is caused by the thermal writing of a crystalline bit. When the resistive characteristics of media  206  change from its original state, then media  206  is said to be in a written or crystalline state. To erase the written state from memory  206 , a second current is applied to media  206 . The second current causes media  206  to heat to a second and higher temperature. The second current is then removed from media  206 , causing media  206  to cool back to an ambient temperature. As media  206  cools, the resistivity of media  206  returns to the resistivity media  206  had at its original amorphous state, or a close approximation to the original resistivity state of media  206 . 
     Another embodiment of a phase change material for media  206  requires media  206  to be heated to a higher temperature for a written state to exist. For instance, applying a first current to media  206  such that media  206  is heated to a temperature approximately equal to 170° C. to 200° C. As media  206  cools back to an ambient state, then the resistivity of media  206  will decrease. To reset media  206  back to an unwritten state, a second current is applied to media  206  causing media  206  to heat to a temperature somewhere in the range of 600° C. As media  206  cools back to an ambient state, any written state to the area of media  206  subjected to the second current and heated to 600° C. will revert back to the resistivity that media  206  possessed before having been changed to a written state. 
     Different materials can be used for media  206  to adjust the operating range for memory writing and resetting the media  206  back to an unwritten state. Altering the proportions of the elements in a chalcogenide is one way of altering the written and erased temperatures. 
     Yet another embodiment of the media device  200  has the resistivity of media  206  changed in a similar way, except that media  206  is also self-quenching. Thus, media  206  begins at an unwritten, ambient state. A first current is applied to media  206 , thereby heating media  206  to a first predetermined temperature. For instance, a write operation can require that media  206  be heated to a temperature of at least 170° C. Then, the first current is removed from media  206  and media  206  begins to cool. As media  206  cools, the resistivity of media  206  changes such that the new resistivity characteristic can be interpreted as a written state. This state can cause the resistivity of media  206  to increase or decrease, depending on the material used for media  206 . Subsequently, to erase the written memory to media  206 , a second current is applied to media  206 . Media  206  is heated to a second temperature (for instance, media  206  can be heated to a temperature of at least 600° C.). As media  206  cools, the resistivity of media  206  is returned back to a state approximately equal to the original state of media  206 , thereby erasing the written data in media  206 . 
     The written states within media  206 , however, can be also be changed back to the ambient state of media  206 , or an unwritten state, by applying heating to a large region of the memory device  200 . For instance, memory device  200  can apply a current to a buried heater under the memory device  200 . This heating can be applied to all of the memory locations in the memory device  200  such that the resistivity characteristics of media  206  is returned to the ambient state throughout the entire memory device  200 . 
     As described above in some of the various embodiments of media device  200 , the ambient, or unwritten, state of memory device  200  has the media  206  having a high resistivity, and the written state of media  206  having a low resistivity. In other embodiments these states can be reversed such that the ambient, or unwritten state, for memory device  200  has media  206  having a low resistivity and a written state has media  206  having a high resistivity. 
     Another embodiment of memory device  200  can have media  206  capable of having a plurality of resistivity states. For example, at the ambient state, the media  206  of memory device  200  can have a first resistivity. Media  206  can then be heated to different temperatures and then cooled, thereby changing the resistivity of media  206 . One embodiment senses whether the resistivity for media  206  is at or near the ambient state for media  206  or at some state that is sufficiently different to be measured as a state different than ambient, or unwritten. Another embodiment is able to sense a plurality of resistivity states that media  206  can possess. 
     For instance, media  206  begins with some first resistivity characteristic at an ambient, or unwritten state. A first current is then applied to media  206 , thereby heating media  206  to a first temperature. The first current is then removed from media  206 , which thereby begins to cool. As media  206  cools, media  206  gains a second resistivity characteristic. In one embodiment, the second resistivity characteristic of media  206  can be measured more precisely than simply whether the second resistivity characteristic is different from the first resistivity characteristic. The second resistivity characteristic can vary depending on the temperature that the media  206  is heated to by the first current. Thus, the different resistivity characteristics that can be represented by the second resistivity characteristic can be representative of a range of data values. This range of data values can be classified in discrete ranges to represent analog values. Alternatively, the precise value of the resistivity characteristic for media  206  can be measured for more precise analog data storage. Measurements of the resistivity are preferentially obtained by taking measurements which are relative to a first state of the media, but can also be obtained by taking absolute value measurements. Another method of measurement extracts the data as the derivative of the measured data. 
     Media  206  can posses a large dynamic range for resistivity states, thereby allowing analog data storage. The dynamic range for the resistivity characteristic of media  206  can be approximately 1000-10,000 (or 10^3 to 10^4) orders of magnitude. In one embodiment, however, heating from the probe on the phase change material can cause only a very small area of media  206  to undergo a change in its resistivity. In this form a smaller dynamic range maybe observed, as only a small region of the media is altered. 
       FIG. 3  is another embodiment of the invention that shows a cross-section of the media device  300  including a data bit where an embodiment of an atomic probe  311  is in contact with the media device  300 . Atomic probe  311  includes a core  310  and a coating  312 . Media device  300  includes a substrate  302  connected with undercoat  304 . Undercoat  304  of memory device  300  is connected with media  306 . Media  306  includes a data bit  314 . Media  306  and data bit  314  are connected with overcoat  308 . Atomic probe  311  is in contact with overcoat  308 . Memory device  300  can be any one of the embodiments described above in  FIGS. 1 and 2 . 
     One embodiment of the atomic probe  311  includes core  310  and coating  312  with a generally conical shape. For instance, atomic probe  311  has a generally conical shape, however, atomic probe  311  can also be described as having a generally trapezoidal shape. Atomic probe  311  can have a radius of curvature  313  from a few nanometers to as much as fifty nanometers or more. The radius of curvature  313  is measured from a line  309  generally extending down the center of the atomic probe  311 . Atomic probe  311  is generally symmetrical along the line  309 , but as will be seen below, atomic probe  311  is not limited to being symmetrical along the line  309 . 
     In one embodiment, either to write a data bit  314  or read a data bit  314  to the memory device  300 , the atomic probe  311  should make contact with the memory device  300 . This contact can occur where atomic probe  311  contacts the overcoat  308  of the memory device  300 . The point of contact on the atomic probe  311  is generally conductive. Thus, in one embodiment, the atomic probe  311  includes a core  310 , which is an insulator, that is partially covered by a conductive coating  312 . The coating  312  of atomic probe  311  makes contact with overcoat  308  of memory device  300  during access of memory device  300 . 
     In another embodiment of the invention, during a read operation, atomic probe  311  does not have to make direct contact with overcoat  308  of memory device  300 . Instead, atomic probe  311  is brought in close proximity with memory device  300  such that coating  312  can sense whether a piece of data  314  exists. For instance, if memory device  300  were of a charged storage type memory, then atomic probe  311  would sense the electric and/or magnetic field strength of data  314  through coating  312 . 
     In one embodiment, the core  310  of atomic probe  311  can include oxides, amorphous silicon, or other insulators. The coating  312  of atomic probe  311  is a conductor and can include any number of different components. For instance, coating  312  can include titanium nitride, platinum, gold, aluminum, tungsten, tungsten carbide, tungsten oxide, diamond-like carbon, platinum iridium, copper, doped silicon or a mixture of such conductors. The choice of material for coating  312  of atomic probe  311  is influenced by the chosen application for the atomic probe  311 . For instance, some applications require the coating  312  to be an exceptional conductor like platinum, while others do not require such a high conductivity. Titanium nitride can be used for coating  312  and would be beneficial if the application valued a hard coating  312  over a highly conductive coating  312 . 
     One embodiment of the invention controls the shape of the coating  312  such that it is rectangular in shape. Thus, during a write function to the memory device  300 , the data  314  formed will have a generally rectangular shape as opposed to a spherical shape. 
       FIG. 4  is another embodiment of the invention that shows a cross-section of the media including a data bit where another embodiment of another cantilever tip, or atomic probe  411 , is connected with the media. Again, a media device  400  is shown that can be any of the memory devices previously discussed. Media device  400  includes a substrate  402 , an undercoat  404 , a media  406 , and an overcoat  408 . Media device  400 , as shown, has been written to and a bit of data  414  formed. Atomic probe includes a core  410  connected with a coating  412 . 
     Shown in  FIG. 4 , is an embodiment of the invention where the coating  412  of atomic probe  411  is only on one side of atomic probe  411 . Thus, when atomic probe  411  makes contact with media device  400 , a smaller portion of atomic probe  411  can potentially affect media device  400 . For instance, during a write operation by the atomic probe  411  where media  400  is a phase change media, coating  412  provides a path for conducting a current through media device  400 . The contact area of coating  412  is smaller than the contact area of coating  312  from FIG.  3 . Thus, the amount of media  406  that can be influenced by a current flowing through coating  412  is less than the amount of media  306  that can be influenced by a current flowing through coating  312  from FIG.  3 . 
       FIGS. 5A ,  5 B, and  5 C depict another embodiment of the invention showing another atomic probe  520  with a coating  524  on five sides of a core  522 .  FIG. 5  shows three views of atomic probe  520 .  FIG. 5A  is a side view of atomic probe  520  where coating  524  is connected with core  522 . As can be seen in  FIG. 5A , coating  524  covers an entire side of atomic probe  520 .  FIG. 5B  is a top view of atomic probe  520 .  FIG. 5B  shows that the coating  524  protects the core  522  along five surfaces. The center square portion of  FIG. 5B  is the area that makes contact with a media device. One such media device can be similar to media device  300  of FIG.  3 .  FIG. 5C  shows a three dimensional view of atomic probe  520 . As can be seen from  FIG. 5C , coating  524  extends away from core  522 . 
       FIGS. 6A ,  6 B, and  6 C depict another embodiment of the invention showing another atomic probe  620  with a coating  624  on one side of a core  622 .  FIG. 6  shows three views of atomic probe  620 .  FIG. 6A  is a side view of atomic probe  620  where coating  624  is connected with core  622 . Coating  624  is only on one side of core  622  in FIG.  6 . Also, coating  624  extends past the tip  623  of core  622  or can be even with the end of core  622 . Thus, as the atomic probe  620  is brought into contact with a media device, coating  624  will make contact with the media device.  FIG. 6B  shows a top view of atomic probe  620  and  FIG. 6C  shows a three dimensional view of atomic probe  620  with the coating  624  only on one side of core  622 . 
     In an alternative embodiment, coating  624  can be flush with the tip  623  of core  622  similar to the atomic probe  411  shown in FIG.  4 . 
     In another embodiment of the invention, the coating  634  of  FIG. 6D  does not rest to the side of core  642 , as shown in  FIG. 6A , but rather, coating  634  is embedded into core  642 . An example of this would be applying a preferential doping to one side of a single crystal silicon core, causing the embedded area to be conductive, while the core would remain substantially an insulator. 
       FIGS. 7A ,  7 B and  7 C depict another embodiment of the invention showing another atomic probe  720  with a coating  724  formed on the end of a core  722 . In the embodiment of the invention shown in  FIG. 7 , coating  724  is has a generally button like shape. Unlike the embodiments of the invention shown in  FIGS. 5C and 6C , the coating  724  in  FIG. 7C  has a generally cylindrical shape.  FIG. 7B  shows a top view of atomic probe  720  and  FIG. 7C  shows a three dimensional view of atomic probe  720 . 
     The material used for coating  724  can be any of the materials previously disclosed. In an alternative embodiment, however, coating  724  can include carbon nano-tubes. Carbon nano-tubes are useful because they are very small with known dimensions. Thus, the cross-section of a carbon nano-tube is very small. Furthermore, carbon nano-tubes are very strong, thus they can provide the hardness needed for extended life of an atomic probe. Moreover, the structural make-up of a carbon nano-tube coating is capable of handling temperatures well in excess of 600° C. 
     In another embodiment of the invention, the coating  734  of  FIG. 7D  does not rest top of core  742 , as shown in  FIG. 7 , but rather, coating  734  is embedded into core  742 . 
     In these embodiments the core is conductive, or the core is coated with a conductive coating, such that a conductive path exists between the carbon nanotube and the probe. There also exists a conductive path to the cantilever conductor on which the atomic probe sits. 
       FIGS. 8A ,  8 B, and  8 C depict another embodiment of the invention showing another atomic probe  820  with a coating  822  banded on a core  824 .  FIG. 8A  shows coating  824  draped over the core  822  of atomic probe  820 .  FIG. 8B  shows how coating  824  is a thin piece of material relative to the dimensions of the core  822 .  FIG. 8C  is a three dimensional view of atomic probe  820  showing how the coating  824  is draped over core  822 . In another embodiment,  FIG. 8D , coating  834  is also recessed into core  842 . The reduced contact area of coating  824 , or coating  834 , shrinks the affected media, of a media device, when signals are passed through the coating  824 , or the coating  834 , contact is made with a media device by the coating  824 , or the coating  834 . Thus, during a write operation, less media is influenced by the atomic probe  820 . Consequentially, more data can be stored in a media device than if a coating  824 , or coating  834 , with a larger contact surface were used. 
       FIGS. 9A ,  9 B and  9 C depict another embodiment of the invention showing another atomic probe  920  with a coating  924  on one side of a core  922 , where coating  924  has a protective material  926  connected with it.  FIG. 9A  shows a side view of atomic probe  920 . Core  922  is connected with coating  924 . Coating  924  is also connected with a protective material  926 .  FIG. 9B  shows a top view of atomic probe  920  and  FIG. 9C  shows a three dimensional view of atomic probe  920 . 
     As atomic probe  920  makes contact with a media device, the atomic probe  920  can be dragged or pushed along the media device. Such actions can cause mechanical stresses on the atomic probe  920 , and coating  924  in particular. The addition of protective material  926  gives additional support to coating  924 , reducing the chance of coating  924  suffering damage due to contact with the media device. 
       FIG. 10  is a scanning electron microscope view of another embodiment of the invention showing another atomic probe. As can be seen from  FIG. 10 , an atomic probe  1020  is shown with a tip  1030 . The diameter of the base of the atomic probe  1020  is less than two micrometers as can be seen by the scale  1032 . 
     The foregoing description of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.