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).
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.
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.
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.
Examples of the phase-change material used to form the phase-change layer 22 include In2Se3, InSe, Ga2Se3, GaSbTe, GbSb, and AgGaSbTe. Other phase-change materials can be used in other embodiments.
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
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.
The layers of the storage substrate 10 depicted in
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.
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.
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.
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.
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.
In
In the example of
In the amorphous region 112 of the storage cell 12B, the read laser light beam 104A 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 104B. 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 104A, 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.
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 104A, 104B.
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
The power amplitude and pulse width of each of the pulses 200 and 202 depicted in
The wavelength of the write laser light is represented by λ As depicted in
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.
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
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.