Abstract:
A storage device comprises a substrate having a recording layer, the recording layer having plural regions associated with respective plural storage cells. A light source generates write light having a first wavelength to write to the storage cells, wherein the storage cells have a size less than the first wavelength.

Description:
BACKGROUND  
       [0001]     Various types of storage media can be used in computers and other types of electronic devices. Examples of storage media include integrated circuit storage devices, such as dynamic random access memories (DRAMs), static random access memories (SRAMs), electrically erasable and programmable read-only memories (EEPROMs), and so forth. Storage media also include magnetic and optical-based storage media, such as floppy disks, hard disks, compact disks (CDs), and digital versatile disks (DVDs).  
         [0002]     Optical DVD technology has enabled the storage of relatively large amounts of data on a relatively small disk. The continued trend towards even higher storage densities on optical storage media such as DVDs has led to development of the Blu-Ray technology, which uses blue-violet laser light instead of red laser light (associated with conventional DVD technology) to write and read bits on the DVD. Blue-violet laser light has a shorter wavelength than red laser light, which enables better focusing and greater precision of the laser light when writing to and reading from storage cells on the optical medium. The use of shorter wavelength blue-violet laser light enables higher density arrangement of data on an optical medium.  
         [0003]     Traditionally, storage cells on optical media are diffraction limited, which means that the storage cell sizes are larger than the wavelength of the laser light used to write to the storage cells. Diffraction limited storage media are therefore unable to achieve even greater storage density.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  illustrates a portion of a storage device according to an embodiment of the invention.  
         [0005]      FIG. 2  illustrates the use of laser to write data to and read data from a storage device, according to an embodiment.  
         [0006]      FIG. 3  is a timing diagram showing laser light pulses for writing storage cells in the storage device, according to an embodiment.  
         [0007]      FIG. 4  is a graph illustrating a temperature profile of a storage cell region in response to a write laser pulse, according to an embodiment.  
         [0008]      FIG. 5  is a block diagram of an example system that incorporates a storage device according to an embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0009]      FIG. 1  shows a storage device according to an embodiment that includes a storage substrate  10  that contains a plurality of storage cells  12 . The storage substrate  10  includes a support structure  14  over which several layers are formed. A first layer  16  formed over the support structure  14  includes a number of electrical electrodes or conductors  18  that extend generally along a first direction (indicated as being the X direction in  FIG. 1 ). According to one embodiment, the conductors  18  are formed of a reflective, electrically conductive material (e.g., aluminum silicon).  
         [0010]     A semiconductor layer  20 , such as a p-type silicon layer, is formed over the first layer  16 . A phase-change layer  22  is formed over the semiconductor layer  20 . In one example, the phase-change layer  22  is formed of an n-type material. In an alternative embodiment, the phase-change layer  22  is formed of a p-type material, while the semiconductor layer  20  is formed of an n-type material. The layers  20  and  22  have different doping types (p-doping type or n-doping type) to form a p-n junction.  
         [0011]     Examples of the phase-change material used to form the phase-change layer  22  include In 2 Se 3 , InSe, Ga 2 Se 3 , GaSbTe, GbSb, and AgGaSbTe. Other phase-change materials can be used in other embodiments.  
         [0012]     Another layer  24  is formed over the phase-change layer  22 , with the layer  24  including electrodes  26  that extend along a second direction, indicated as being the Y direction in  FIG. 1 . The X and Y directions in  FIG. 1  are generally perpendicular to each other. In a different embodiment, electrodes  18  extend in the Y direction, while the electrodes  26  extend in the X direction.  
         [0013]     An anti-reflective coating and a protective layer  28  can be formed over the layer  24 . The anti-reflective coating layer allows laser light, and optionally, electron beams to pass through to the phase-change layer  22  to perform writes and reads of the storage cells  12 .  
         [0014]     The layers of the storage substrate  10  depicted in  FIG. 1  are provided for exemplary purposes. In other implementations, other arrangements and layers can be employed for the storage substrate  10 .  
         [0015]     The phase-change layer  22  is effectively a recording layer that is programmable to store data bits in respective storage cells  12 . Each region of the phase-change layer  26  corresponding to a storage cell  12  has at least two phases, a crystalline phase and an amorphous phase. Alternatively, instead of an amorphous phase, two different crystalline phases can be used for storing data bits. When programmed to a first phase, a storage cell  12  contains a data bit having a first data state or logical value. However, if the phase-change layer portion of the storage cell  12  is programmed to have a second phase, then the storage cell  12  contains a data bit having a second, different data state or logical value.  
         [0016]     A data detector  32  is provided on the storage substrate  10  to perform readback of the data bits contained in the storage cells  12 . The data detector  32  is electrically connected to the electrodes  18  and  26  to detect a voltage across each pair of electrodes  18 ,  26 . If a storage cell  12  contains a first data state, then the data detector  32  detects a first voltage. However, if a storage cell  12  contains a second data state, then the data detector  32  detects a second voltage. Although depicted as being one logical block  32 , the data detector  32  can actually have multiple data detector circuits, one for each respective group (e.g., a column or row) of storage cells.  
         [0017]      FIG. 1  also shows a write/read mechanism  34  that is provided on a second substrate  36 . The second substrate  36  and the storage substrate  10  are movable with respect to each other to position the write/read mechanism  32  over selected storage cell(s)  12  to program (write) or read the storage cells. Note that either the second substrate  36  or the storage substrate  10 , or both, can be movable to achieve relative motion between the write/read mechanism  34  and the storage cells  12 . The write/read mechanism  34 , according to one embodiment, includes laser light sources for propagating laser light onto the storage substrate  10  for purposes of performing writes and reads with respect to the storage cells  12 . In one embodiment, the write/read mechanism  34  includes write laser sources (for performing writes) and read laser sources (for performing reads). Alternatively, the write/read mechanism  34  can include electron beam emitters (instead of read laser sources) that are used for performing reads, and write laser sources for performing writes. More generally, a read laser source or electron beam emitter in the write/read mechanism  34  is referred to as a “read illuminating beam generator” that is able to emit a laser light or an electron beam.  
         [0018]     According to some embodiments of the invention, each write/laser source of the write/read mechanism  34  is able to write data bits onto the storage cells  12  that have sizes that are not diffraction limited. In other words, the write laser light source is able to write storage cells  12  that each has a size (“sub-wavelength size”) smaller than the wavelength of the laser light produced by the write laser source. Storage cells  12  that have sizes smaller than the wavelength of the write laser light are referred to as sub-wavelength storage cells. A storage cell has a size smaller than the wavelength of the write laser light if (1) the diameter of the storage cell, or (2) a width or length of the storage cell, or (3) any other dimension of the storage cell, is smaller than the wavelength of the write laser.  
         [0019]     The ability to achieve a sub-wavelength storage cell is provided by generating a write laser pulse having a power amplitude and duration that does not cause phase change in portions of the phase-change layer  22  outside the phase-change layer region of a targeted storage cell, even though the phase-change layer region of the targeted storage cell is smaller than the wavelength of the write laser light. The characteristics of the write laser pulse that enable writing to and reading from sub-wavelength storage cells are described further below.  
         [0020]      FIG. 2  is a side view of a portion of the storage substrate  10  and the second substrate  36 . Write laser sources  102  are provided on a lower surface  101  of the second substrate  36 . In addition, read illuminating beam sources  100  (which can be electron beam emitters or laser sources) are also formed on the lower side  101  of the second substrate  36 . The write laser sources  102  and read illuminating beam sources  100  are part of the write/read mechanism  34  ( FIG. 1 ). Although multiple write laser sources  102  and read illuminating beam sources  100  are depicted in  FIG. 2 , other embodiments can employ a single write laser source  100  and/or a single read illuminating beam source  102 .  
         [0021]     In one example embodiment, the write laser light produced by each write laser source  100  has a wavelength of about 399 nanometers (nm), while the read laser light produced by each read laser source has a wavelength of about 422 nm. Wavelengths of the write and read laser lights having approximately the exemplary wavelength values above are wavelengths of blue laser lights (which include blue laser light or blue-violet laser light). In other embodiments, other wavelengths can be used for the write and read laser lights.  
         [0022]     In  FIG. 2 , a first write laser source  102  generates a laser light beam  105 A to be directed at a first storage cell  12 A, whereas a second write laser source  102  generates a second laser light beam  105 A to be directed at a second storage cell  12 B.  FIG. 2  also depicts first and second read laser sources  100  generating respective first and second read laser light beams  104 A,  104 B. In the position depicted in  FIG. 2 , for performing a read, the read laser sources  100  are aligned with respect to storage cells  12 A,  12 B to enable the laser light beams  104 A,  104 B from the read laser sources  100  to impact respective storage cells  12 A,  12 B. To perform a write, the write laser sources  102  would be aligned with respect to the storage cells  12 A,  12 B (by relative motion of the storage substrate  10  and second substrate  36 ) to direct laser light beams  105 A,  105 B from the write laser sources  102  to the storage cells  12 A,  12 B.  
         [0023]     In the example of  FIG. 2 , the write laser light beam  105 A directed at the storage cell  12 A causes the region of the phase-change layer  22  that is part of the storage cell  12 A to either remain at, or change to, a first phase (e.g., a crystalline phase). On the other hand, the write laser light beam  105 B directed at storage cell  12 B causes the region of the phase-change layer  22  that is part of the storage cell  12 B to remain at, or change to, a second phase (e.g., an amorphous phase). The region of the phase-change layer  22  that is part of the storage cell  12 A is indicated as crystalline region  114 , whereas the region of the phase-change layer  22  that is part of the storage cell  12 B is indicated as amorphous region  112 . In other examples, the storage cell  12 A can be programmed to the amorphous phase, whereas the storage cell  12 B can be programmed to the crystalline phase.  
         [0024]     In the amorphous region  112  of the storage cell  12 B, the read laser light beam  104 A induces creation of electron-hole pairs. However, since electron-hole pairs in the amorphous region  112  tend to recombine at a relatively rapid rate, little or no current flows from the amorphous region  112  through the semiconductor layer  20  to the electrode  18  in response to the read laser light beam  104 B. However, in the crystalline region  114 , recombination of electron-hole pairs occurs at a slower rate than in the amorphous region  112 ; therefore, in response to the read laser light beam  104 A, a current flow  106  is induced from the crystalline region  114  through the semiconductor layer  20  to the electrode  18 . The p-type phase-change layer  22  and the n-type semiconductor layer  20 , which are adjacent to each other, effectively provide a p-n junction that behaves as a diode.  
         [0025]     In an alternative embodiment, a storage cell is programmable to two different crystalline phases—a first crystalline phase and a second crystalline phase. The two crystalline phases have different recombination rates for electron-hole carrier pairs (free carriers) so that different currents are induced in response to the read laser light beams  104 A,  104 B.  
         [0026]     Current flow through the p-n junction causes a voltage drop across the diode represented by the p-n junction. The voltage drop occurs across electrodes  26  and  18 . The electrode  26  is connected to the + input of an operational amplifier  108 , whereas the electrode  18  is connected to the − input of the operational amplifier  108 . The operational amplifier  108  is part of the data detector  32 . The operational amplifier  108  checks for a voltage drop across electrodes  26  and  18 . If a first voltage drop (corresponding to a first phase of the phase-change layer region of a selected storage cell) occurs between electrodes  26  and  18 , the operational amplifier  108  outputs a first value to a signal Data_Out. However, if a second, different voltage drop (corresponding to a second phase of the phase-change layer region of a selected storage cell) across electrodes  26  and  18  is detected by the operational amplifier  108 , then the operational amplifier  108  outputs a second value to the signal Data_Out. In one embodiment, a resistor  110  is part of a feedback loop associated with the operational amplifier  108 . In other embodiments, other types of circuitry for detecting a voltage drop (or current) across the electrodes  26  and  18  can be employed. Although one operational amplifier  108  is depicted in  FIG. 1 , multiple operational amplifiers  108  can be part of the data detector  32  to detect data states of corresponding multiple storage cells.  
         [0027]      FIG. 3  is a timing diagram that illustrates two pulses  200 ,  202  of a write laser light beam for performing writes to a storage cell (or storage cells) of the storage substrate  10  ( FIG. 1 ). The first pulse  200  (having a power amplitude P 1  and pulse width t 1 ) is used to program a storage cell to an amorphous phase. The second pulse  202  having power amplitude P 2  and pulse width t 2  is used to program the storage cell to the crystalline phase.  
         [0028]     The power amplitude and pulse width of each of the pulses  200  and  202  depicted in  FIG. 3  is selected to heat the phase-change layer region in a targeted storage cell such that temperature in the phase-change layer region has a temperature profile similar to profile  300  depicted in  FIG. 4 . The temperature profile depicted in  FIG. 4  generally represents the temperature in the phase-change layer region of a storage cell as a function of distance. The temperature profile  300  has a generally Gaussian shape. In other words, the temperature profile  300  is generally a normal curve, which is a symmetrical bell-shaped curve of normal distribution. More generally, the temperature profile  300  has a generally bell-shaped curve. The peak of the generally bell-shaped curve (representing the maximum temperature induced in the phase-change layer region of a targeted storage cell) is located generally at, or near, the center of the storage cell (represented as point D C  in  FIG. 4 ). The temperature away from this center or near center location D C  in the storage cell drops from the peak according to the generally bell-shaped curve of  FIG. 4 .  
         [0029]     The wavelength of the write laser light is represented by λ As depicted in  FIG. 4 , a portion of the generally bell-shaped temperature profile is above the melting temperature (T melting ), represented by the horizontal dashed line, of the phase-change layer. The portion of the temperature profile above the melting temperature has a width W, which is smaller than the wavelength λ of the write laser light. As a result, in response to the write laser light, only the region of the phase-change layer where the temperature rises above T melting  is programmed. Therefore, the size (diameter, width, or other dimension) of a storage cell can be made as small as the width W depicted in  FIG. 4 . The value of the width W is smaller than the wavelength λ to enable formation of a sub-wavelength storage cell according to some embodiments.  
         [0030]     In one example, a 399-nm write laser light pulse having power amplitude of 3.5 milliwatts (mW) and pulse width of 50 nanoseconds (ns) can be used to form storage cells with a diameter of about 170 nm. In other examples, the power amplitude can be adjusted between 2-10 mW, and the pulse widths can be varied between 10-50 ns, or greater. The values given above are for the purpose of example. In other implementations, other values for the power amplitude and pulse width of the write laser light can be used to effectively write to sub-wavelength storage cells.  
         [0031]     The storage device described above according to some embodiments can be packaged for use in a computing device  204  (e.g., desktop computer, portable or notebook computer, server computer, handheld device, consumer electronic device such as a camera and appliance, and so forth). For example, as shown in  FIG. 5 , the storage device according to some embodiments is referred to as a high-density storage device  200 , which can be attached or connected to an I/O (input/output) port  202  of a computing device  204 . The I/O port  202  can be a USB port, a parallel port, or any other type of I/O port. Inside the computing device  204 , the I/O port  202  is connected to an I/O interface  206 , which in turn is coupled to a bus  208 . The bus  208  is coupled to a processor  210  and memory  212 , as well as to mass storage  214 . Other components may be included in the computing device  204 . The arrangement of the computing device  204  is provided as an example, and is not intended to limit the scope of the invention. In alternative embodiments, instead of being coupled to an I/O port of the computing system, the high-density storage device  200  can be mounted (directly or through a socket) onto the main circuit board of the computing device  204 .  
         [0032]     In the foregoing description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details. While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.