Abstract:
A structure for storing digital data is provided, with a high reflectance layer comprising a noble metal formed over an underlying material layer, and a plurality of low reflectance portions comprising a mixture of a noble metal and an underlying material. The plurality of low reflectance portions have top surfaces comprising a compound of the underlying and the noble metal. A method of changing reflectance on a data storage disk is also disclosed. The method comprises the acts of irradiating a laser light beam onto a noble metal formed over an underlying layer, and raising the temperature of the noble metal above the melting temperature forming a compound of the noble metal and the underlying material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 11/168,317, filed on Jun. 29, 2005, titled Gold-Semiconductor Phase Change Memory For Archival Data Storage, the entire disclosure of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to archival data storage on optical disks, and more particularly to using novel materials that provide greater long-term stability to the stored data. 
     BACKGROUND OF THE INVENTION 
     Personal computer users, businesses, and public offices are faced with a deluge of data in the form of digital information. The question of how to preserve this data for the next decade, and for the ages, has yet to be answered. The most common data backup includes storing the data on a writable compact disk CD, CD-R (compact disk, write once read-only memory), CD-ROM (compact disk, read-only-memory), or a CD-RW, short for CD-ReWritable disk. A CD-ROM is an adaptation of the CD and is designed to store data in the form of text and graphics, as well as sound. A CD-RW is a type of CD disk that enables a user to write onto the disk multiple times. A CD-R comprises an organic layer sandwiched between a transparent base and a reflective layer. When heated by a focused laser, the dye layer melts and forms a series of pits, which are readable by a laser beam as 0&#39;s and 1&#39;s. 
     The technology behind a CD-RW is known as optical phase-change, an optical storage technology in which data is written with a laser that changes dots on the disk between amorphous and crystalline states. Phase change is a type of CD recording technology that enables the disks to be written, erased, and rewritten through the use of a layer of a special material for the recording layer—the phase change layer—that can be changed repeatedly from an amorphous (formless) to a crystalline state. The crystalline areas allow the metalized layer to reflect the laser beam better, while the non-crystalline portion absorbs the laser beam, so the beam is not reflected. An optical head reads data by detecting the difference in reflected light from amorphous and crystalline dots. 
     During writing, a focused laser beam selectively heats areas of the phase-change material above the melting temperature, so all the atoms in this area can rapidly rearrange. The recording phase-change layer is sandwiched between dielectric layers that draw excess heat from the phase-change layer during the writing process. Then, if cooled sufficiently quickly, the random state is “frozen-in,” and the so-called amorphous state is obtained. The amorphous version of the material has different reflection properties where the laser dot was written, resulting in a recognizable CD surface. Writing takes place in a single pass of the focused laser beam, which is referred to as “direct overwriting,” and can be repeated several thousand times per disk. Once the data has been burned, the amorphous areas reflect less light, enabling a “Read Power” laser beam to detect the difference between the lands and the pits on the disk. The recorded tracks on a CD-RW disk are read in the same way as regular CD tracks. That is, by detecting transitions between low and high reflectance, and measuring the length of the periods between the transitions. The only difference is that the reflectance is lower than for regular CDs. 
     A digital versatile disk (DVD) provides an optical disk technology that allows for much greater storage as compared with CDs. With reference to  FIGS. 1 and 2 , a DVD&#39;s sevenfold increase in-data capacity over the CD has been largely achieved by tightening the tolerances throughout the predecessor CD system. The tracks on the DVD are placed closer together, thereby allowing more tracks per disk than found on CDs. As shown in  FIG. 2 , the DVD track pitch  4  is reduced to 0.74 microns, less than half of CD&#39;s 1.6 micron track pitch  2 , as shown in  FIG. 1 . The pits  6 , in which the data is stored, are also a lot smaller, allowing more pits per track. The minimum pit length  10  of a single layer DVD is 0.4 microns, as compared to 0.83 microns pit length  8  for a CD. With the number of pits having a direct bearing on capacity levels, the DVD&#39;s reduced track pitch and pit size alone give DVD ROM disks four times the storage capacity of CDs. The packing of as many pits as possible onto a disk is, however, the simple part. The real technological breakthrough of the DVD was with its laser. Smaller pits mean that the laser has to produce a smaller spot, and the DVD achieves this by reducing the laser&#39;s wavelength from the 780 nanometers infrared light of a standard CD, to 635 nm or 650 nm red light. 
     The first-generation CD players used a 780 nm AlGaAs laser diode developed in the early 1980s. With this technology, a CD-ROM stored about 650 Mbytes of information. The shortest wavelength commercially-viable device that was made in this system was about 750 nm. Further shortening of the wavelength called for a different material, and in the late 1980s red-emitting laser diodes were developed in the AlGalnP system, grown lattice-matched on a GaAs substrate. This material has provided the laser for new DVDs, which store about 4.7 Gbytes of information. Different materials are used to make a laser emit blue light, e.g., at wavelengths in the range of 430 nm to 480 nm. One technique reported has been laser action at 77K from a GaN-based device by researchers at Nichia Chemical Industries in Japan. Nichia announced pulsed room temperature operation at the end of 1995, and continuous operation in early 1997. By August 1997 the room temperature operating life had reached 300 hours. Based on accelerated life-testing at elevated temperatures, Nichia reported in 1999 a room temperature operating life of about 10000 hours at room temperature. A wide variety of solid state laser diodes are now available for use in CD-ROM or CD-ROM like technology. 
     While current optical disk technologies such as DVD, DVD±R, DVD±RW, and DVD-RAM use a red laser to read and write data, a new format uses a blue-violet laser, sometimes referred to as Blu-ray. The benefit of using a blue-violet laser (405 nm) is that it has a shorter wavelength than a red laser (650 nm), which makes it possible to focus the laser spot with even greater precision. This allows data to be packed more tightly and stored in less space, so it is possible to fit more data on the disk even though it is the same size as a CD or DVD. This together with the change of numerical aperture to 0.85 is what enables Blu-ray Disks to hold 25 GB. Blu-ray technology should become available in the 2005 to 2006 time frame. Some new techniques proposed for archival storage have included “a polymer/semiconductor write-once read-many-times memory” and some “novel concepts for mass storage of archival data” using energetic beams of heavy ions to produce radiation damage in thin layers of insulators. 
     Current CD-ROM memories based on changes in organic dyes or phase changes in layers may degrade over time and become unreadable. Although at normal temperature and humidity the life span of CD could be in excess of 100 years, the life span of data on a CD recorded with a CD burner could be as little as five years if it is exposed to extremes in humidity or temperature. And, if an unprotected CD is scratched it can become unusable. What is needed is a data storage medium that can provide greater long-term stability for the stored data. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention relates to a writable optical data storage medium with good long term stability. The data storage medium, which can be a disk, for example, comprises a substrate having a dielectric layer formed thereon, an underlying material layer formed over the dielectric layer, and a noble metal layer formed over the underlying material layer. A protective layer may be formed over the noble metal layer. 
     Data can be written onto the medium by a laser which causes formation of a mixed material portion in the noble metal layer and the underlying material layer. The mixed material portions of the medium have a lower reflectivity than other portions of the medium having the undisturbed noble metal layer, enabling the medium to be read. 
     The invention also relates to a system for writing data and reading data from an optical data storage medium. The system comprises a device capable of irradiating a laser beam onto a medium, which has a substrate with a first dielectric layer, an underlying material layer formed over the first dielectric layer, and a noble metal layer formed over the underlying material layer. The invention provides a writing laser beam capable of forming mixed material portions, e.g., inter-metallic compounds, on the medium, which contain both noble metal and underlying material. The system also provides a reading laser beam that can read data from the medium. 
     The invention also relates to a method of changing reflectance of selected areas on a data storage medium, comprising the acts of irradiating a laser light beam onto a noble metal formed over an underlying material layer to raise the temperature of the noble metal above its melting temperature causing the creation of a compound containing the noble metal and the underlying material layer. The invention also provides a recorded optical medium that has a support structure, a first material layer formed over the support structure, and a second light reflective material layer formed over the first material layer. The second light reflective material has a first light reflectance property. The first and second material layers have a property such that a light beam applied to a region of the second material layer heats the first and second material layers and causes a mixture of materials from the first and second material layers, and also causes a second light reflectance property for the region which is different from the first light reflectance property. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention, in which: 
         FIG. 1  shows data storage characteristics of a typical CD; 
         FIG. 2  shows data storage characteristics of a typical DVD; 
         FIG. 3  is a perspective view of a data storage medium in accordance with an embodiment of the invention; 
         FIG. 3A  is a perspective view of a data storage medium in accordance with another embodiment of the invention; 
         FIG. 3B  is a perspective view of a data storage medium in accordance with another embodiment of the invention; 
         FIG. 4  is a partial side-sectional view of the data storage medium of  FIG. 3  during a write operation; 
         FIG. 5  is a partial side-sectional view of the data storage medium of  FIG. 3  during a write operation; 
         FIG. 6  shows an equivalent electrical circuit for the data storage medium of  FIG. 3 ; 
         FIG. 7  is a graph of temperature versus time during a write operation; 
         FIG. 8  shows an equivalent electrical circuit for the data storage medium of  FIG. 3 ; 
         FIG. 9  shows a partial representation of a layer of the data storage medium of  FIG. 3 ; 
         FIG. 10  shows a partial equivalent electrical circuit of the circuit of  FIG. 3 ; 
         FIG. 11  is a graph of temperature versus time during cooling; 
         FIG. 12  shows a partial side-sectional view of the data storage medium of  FIG. 3  after a write operation; 
         FIG. 13  shows a partial side-sectional view of the data storage medium of  FIG. 3  after a write operation; 
         FIG. 14  is a graph showing optical properties of gold; 
         FIG. 15  is table showing combinations of materials used in the invention; 
         FIG. 16  shows a system for using the data storage medium of  FIG. 4 ; and 
         FIG. 17  shows a block diagram of a device for writing and reading from the data storage medium device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying Figures, which form a part hereof and show by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical, and other changes may be made without departing from the spirit and scope of the present invention. 
     The invention relates to an archival storage medium, and is based on a reflectance change between a reflective metal portion of the medium and a mixed noble metal material and underlying material portion of the medium. In particular, the invention relates to an archival storage medium based on a noble metal layer forming a compound with an underlying material layer. Noble metals are metals, or metal alloys, that are highly resistant to oxidation and corrosion. According to the invention, the noble metal layer is selected from the following group of noble metals: Gold (Au), Iridium (Ir), Osmium (Os), Palladium (Pd), Platinum (Pt), Rhenium (Re), Rhodium (Rh) and Ruthenium (Ru). It is preferable that the noble metal layer comprises one noble metal. However, the noble metal layer can also comprise an alloy of more than one metal, or an alloy comprising at least one member selected from the group consisting of Au, Ir, Os, Pd, Pt, Re, Rh and Ru. The underlying material layer can be a metal selected from the following group of underlying materials: Chromium (Cr), Hafnium (Hf), Niobium (Nb), Tantalum (Ta), Titanium (Ti), Zirconium (Zr) and Vanadium (V). It is preferable that the underlying material layer comprises one metal. However, the underlying material layer can also comprise an alloy of more that one metal, or an alloy comprising at least one member selected from the group consisting of Cr, Hf, Nb, Ta, Ti, Zr and V. The underlying material can also be a semiconductor material layer selected from the following group: Silicon (Si) and Germanium (Ge). 
     With reference to  FIG. 3 , an archival storage medium in the form of a memory disk  50  according to an exemplary embodiment of the invention is shown having a carrier wafer or substrate  60 , over which is deposited a dielectric layer  58 , for example an oxide layer. Although the invention is illustrated in the form of a disk in the exemplary embodiment, it may take other forms as well, for example, an optical card or other arrangement. The carrier wafer or substrate  60  can comprise a polycarbonate material, and the dielectric layer  58  can comprise a silicon dioxide layer, for example, or another dielectric layer. An underlying material layer  56  is formed over the dielectric layer  58 . The underlying material layer  56  can comprise any of the underlying materials listed above. In accordance with the invention, a thin noble metal layer  54  is deposited, by evaporation, for example, onto the underlying material layer  56 . Conventional techniques may be used to form the layers, and are well known to one skilled in the art. The noble metal layer  54  is thereafter preferably covered with a protective transparent layer  52 . The protective layer  52  can be a dielectric layer, for example, an oxide layer, which can comprise a silicon dioxide layer. The protective layer  52  can be another passivating material which has an adequate degree of transmission for the wavelength of a laser beam chosen to write data. In a preferred embodiment, the protective layer  52  is a transparent oxide layer so that the noble metal layer  54  retains its reflective properties. A protected noble metal layer  54  combines the natural optical and spectral performance of noble metal together with the durability of hard dielectrics. In addition, coated noble metals can be cleaned regularly using standard organic solvents, such as alcohol or acetone. 
     Referring again to  FIG. 3 , the noble metal layer  54  can be deposited to a thickness of approximately a few 100 Å. The noble metal layer  54  can be deposited by evaporation, for example. In a preferred embodiment, the noble metal layer  54  has a thickness of approximately 50 Å to approximately 300 Å. The thickness of the underlying material layer  56  is preferably several times greater than the thickness of the noble metal layer  54 . In a preferred embodiment, the thickness of the underlying material layer  56  is approximately 200 Å to approximately 2000 Å. The thickness of the protective dielectric layer  52  is approximately 300 Å to approximately 1000 Å. 
     The archival storage medium according to the invention can have other constructions. For example, although  FIG. 3  shows a storage medium with a recordable layer—noble metal layer  54 —next to the protective layer  52 , in conventional art the recordable layer is typically formed next to a substrate. The invention is equally applicable to such a conventional configuration. With reference to  FIG. 3A  of the invention, a storage medium  250  has a substrate  260  on top of a noble metal layer  254 . The substrate  260  can comprise a polycarbonate material. A thin adhesive layer (not shown) may be deposited between the substrate  260  and the noble metal layer  254  to promote adhesion of the noble metal layer  254  to the substrate  260 , or a surface of the noble metal layer  254  may be activated with a plasma to promote adhesion. An underlying material layer  256  is disposed underneath the noble metal layer  254 . The storage medium  250  also comprises a dielectric layer  258  and a label layer  270 . Alternatively, the storage medium can be a double-sided storage medium, as shown in  FIG. 3B , wherein the storage medium  252  is recordable on both sides because it has two noble metal layers  254  and two underlying material layers  256 , one set on each side of the medium  252 . 
     As discussed above, data is written to the data storage medium, e.g., a storage disk, using a laser beam. With reference to  FIG. 4  and the storage medium of  FIG. 3 , a laser beam  70  irradiated onto disk  50  passes through the protective layer  52 , and heats the noble metal layer  54  and the underlying material layer  56 . In the embodiments shown in  FIGS. 3A and 3B , the irradiated laser beam  70  passes through the substrate  260  and heats the noble metal layer  254  and the underlying material layer  256 . Hereinafter, for the sake of brevity, the invention is described with reference to the embodiment of  FIG. 3  although the description is similarly applicable to the embodiments of  FIGS. 3A and 3B . The light of the laser beam  70  has a wavelength, 405 nm to 480 nm, for example, such that it is not significantly absorbed by the protective layer  52  (or substrate  260 ) so the laser beam  70  maintains its energy as it passes through the protective layer  52 , and reaches the noble metal layer  54 . The noble metal layer  54  absorbs the energy from the laser beam  70 , and the heat from the laser beam  70  causes the noble metal layer  54  to melt and diffuse into the underlying material layer  56 . 
     As a result, with reference to  FIG. 5 , a molten region  72  is formed in the medium  50 . The molten region  72  comprises a mixture of materials from the melted noble metal layer  54  and underlying material layer  56 . In particular, the top surface  74  of the molten region  72  comprises a mixture of materials from the noble metal layer  54  and underlying material layer  56 . 
     After the exposure of the laser beam  70  on the molten region  72  is stopped, the molten region  72  cools and becomes a solid mixture region comprising inter-metallic compounds and/or materials from the noble metal layer  54  and underlying material layer  56 . The solid mixture regions are not as reflective as areas of the noble metal layer  54  that were not melted. The difference in reflectance can be detected by a laser during a read operation that has a lower energy than the laser that writes data onto the data storage medium. It has been determined by the inventors that for maximum change in reflectance, which is desirable for data storage, the molten region  72  should contain approximately equal atomic volumes of the underlying material  56  and the noble metal  54 . In accordance with the invention, the molten region  72  may contain more or less of the underlying material  56 , by atomic volume with respect to the atomic volume of the noble metal layer  54 , with good results. This is achieved by forming the underlying material layer  56  thicker than the noble metal layer  54 , as discussed above, and by supplying energy sufficient to melt not only the noble metal layer  54 , but also the underlying material layer  56 , as discussed below. 
     An exemplary laser for the above-described write operation is a blue laser, having a wavelength of 405 nm to 480 nm. Such a laser works best for highest density data storage. The blue laser produces more transmittance and absorption of the light (or not as much reflectance) than other lasers when irradiated onto the noble metal layer  54 . Advantageously, however, the blue laser produces most absorption of the light by the underlying material layer  56 , which is desirable because it allows for low reflectance from surface regions comprising, at least in part, the underlying material layer  56  and the noble metal layer  54 . 
     Thermal design considerations of the disk  50  are now discussed. During writing of data to the disk  50 , the energy of the laser beam  70  is absorbed by a small area of the noble metal layer  54  and underlying material layer  56 , thereby producing the molten region  72 , as discussed above. The range of the laser beam&#39;s high temperature penetration into the disk  50 , and thereby the size of the molten region  72 , can be determined by the heat capacity of the layers of the disk  50 . The temperature reached by the top layer of the disk  50 , when exposed to a laser beam, is dependent upon three fundamental factors: the amount of energy per unit area that is introduced; the rate at which the energy is introduced; and the heat capacity and heat conductivity of the underlying material. The greater the thermal conductivity (or lower thermal resistance) of the underlying layer, the lower the temperature will be at the top layer. Conversely, the lower the thermal conductivity (or higher the thermal resistance) of the underlying material, the higher the temperature will be on the top surface. 
     In accordance with principles of the invention, a laser beam  70  having a short duration, on the order of 0.5 to 5 nanoseconds, for example, is used to melt the noble metal layer  54  and underlying material layer  56 . For a short duration of a laser beam  70 , the heating rate is determined primarily by the energy supplied by the laser beam  70 , as discussed below. The heat capacity of the disk  50  can be modeled using an equivalent electrical circuit model. Approximation of thermal models with electrical circuits is well known, and is not explained in detail herein. 
     An exemplary equivalent electrical circuit  80  of the disk  50  is shown in  FIG. 6 . On the left side of dashed line  84  is the electrical circuit representation  82  of the underlying material layer  56 . To the right of dashed line  84 , the dielectric layer  58  used in the preferred embodiment is represented by a series of electrical components, and a unit length of the dielectric layer—1 micron for example—is represented by electrical circuit  86 . In the equivalent electrical circuit  80 , temperature is analogous to voltage, and the rate of change in temperature, or heat flux, is analogous to current. The heat dissipation is dependent upon the thermal conductivity of each layer of the disk  50 . 
     If the heat dissipation for each layer on a disk  50  is known, the amount of heat required to raise the temperature of the noble metal layer  54  and the underlying material layer  56  to the point where the noble metal layer  54  and the underlying material layer  56  will melt can be determined. If a short duration laser beam  70  is applied to the disk  50 , the temperature change will be determined mostly by the heat capacity of the molten region  72 , in which the energy of the laser beam  70  is absorbed. An equivalent circuit representation can be made for the total amount of heat transferred to the molten region  72 . The total amount of heat transferred into the molten region  72  can be represented as follows:
 
C=C p ρs 2 t units: J/° K  (1)
 
     In equation (1), C is the total amount of heat transferred, C p  is the heat capacity of the underlying material layer  56 , ρ is the density of the underlying material layer  56 , s 2  is the surface area of the spot irradiated by the laser beam  70 , and t represents the thickness of the semiconductor layer  56 . The units are Joules (J) per degree Kelvin (K). This equation can be used to estimate the energy of the laser beam  70  required to melt the noble metal layer  54  and the underlying semiconductor layer  56 , and create the molten region  72  of the noble metal and underlying material. 
     In an exemplary embodiment, a 1 nano-second long 0.65 milli-Watt laser pulse, having a wavelength of approximately 405 nm to 480 nm, may be used to irradiate the noble metal layer  54 . Such a laser pulse delivers an energy, ΔE, of 650 femtojoules to the surface of the noble metal layer  54  to create the molten region  72 . For example, if the size of the molten region  72  is about 0.1 μm 3 , and assuming that the underlying material layer  56  is a silicon layer having a heat capacity of 1.63 J/° K cm 3 , the temperature change in the noble metal layer  54  and underlying material layer  56  can be represented by the following equation:
 
 ΔT=ΔE/ ( C   p   ρs   2   t )=400° C.  (2)
 
     In  FIG. 7 , the temperature ramp up of the noble metal layer  54  and underlying material layer  56 , which turn into the molten region  72 , is shown. Using the parameters discussed above with respect to equation (2), the molten region  72  reaches over 400° C. after about 1 nanosecond. The time duration of the laser pulse can be adjusted to increase the beam&#39;s penetration of the noble metal layer  54  into the underlying material layer  56 . The time duration of the laser pulse may be increased, for example, if the amount of energy delivered by the laser pulse is insufficient to melt the noble metal layer  54 . If the amount of energy is more than sufficient to the melt the noble metal layer  54 , then the duration of the laser pulse may be reduced to increase the speed of the writing process, i.e., the laser pulse may be applied to a different location on the disk  50  in a quicker manner. Lower laser powers may require longer temperature ramp-up times. 
     Cooling of the molten region  72  is now discussed with reference to  FIG. 3 . Cooling of the molten region  72  is accomplished primarily via heat dissipation into layers of the disk  50  that are underneath the noble metal layer  54 . Both the heat capacity and the thermal conductivity of the materials underneath the molten region  72  are determining factors of the rate of cooling. In a preferred embodiment, the underlying material layer  56  is formed over dielectric layer  58  comprising a silicon dioxide layer, which has moderate heat capacity and high thermal resistance. The heat capacity of the oxide layer  58  is about the same as that of the underlying material layer  56 , if the underlying material layer is a silicon or germanium layer. The thermal conductivity of the oxide layer  58  is about one hundred times lower than the underlying material layer  56 , if it is a silicon or germanium underlying layer  56 . 
     The rate of cooling, or quenching, of the molten region  72  can be determined by an equivalent electrical circuit representation of heat conduction, as shown in  FIGS. 8-10 .  FIG. 8  shows an equivalent circuit  120  having a series of electrical components  122  representing the thermal properties of the dielectric layer  58 . In the equivalent electrical circuit  120 , temperature is analogous to voltage and heat flux analogous to current. In  FIG. 9 , a wedge  90  is shown, which is a slice of the dielectric layer  58  having a depth d and a cross-sectional area A. The wedge  90  terminates at a heat sink  91 . The time invariant steady state solution for the wedge  90  of the dielectric layer  58 , terminated by a heat sink  91  with an infinite heat capacity, is a linear variation in temperature, where the thermal resistance of the sample is:
 
 R=d/ ( KA ) units: ° K/W  (3)
 
     In the above equation, d is the depth of the wedge  90  (as seen in  FIG. 9 ), K is the heat conductivity of the dielectric layer  58 , and A is the cross-sectional area of the wedge  90 . For a silicon dioxide dielectric layer  58 , the heat conductivity K=0.014 J/(sec·cm·° K). The rate of cooling is then determined by the R·C time constant of the equivalent electrical circuit. The R·C time constant is, generally, the time required for half of the heat to dissipate, or, in terms of the present invention, the time required for the ΔT to be reduced by 50%. 
       FIG. 10  shows a portion of the equivalent electrical circuit  120  of  FIG. 8 . In the portion of the equivalent electrical circuit shown in  FIG. 10 , the numeral  92  represents a 0.1 μm segment of the dielectric layer  58 . Each 0.1 μm segment  92  of the dielectric layer  58  is represented by its thermal resistance R and heat capacity C o .  FIG. 10  also shows the heat capacity C s  of the underlying material layer  56 . In a simple model, the resistance R of the first segment of the oxide layer is 7×10 7  ° K/W, and its thermal capacity is 1.6×10 −15  J/° K. According to these parameters, the R·C time constant is (7×10 7  ° K/W) (1.6×10 −15  J/° K), or approximately 100 nanoseconds. Thus, the exemplary embodiment, ΔT will be reduced by 50%—from 400° C. to 200° C., in 100 nanoseconds. 
       FIG. 11  is a graph showing the rate of cooling after the laser pulse is no longer irradiated onto the molten region  72 . As discussed above, the time constant R·C of cooling in the equivalent electrical circuit simulation is approximately 100 nanoseconds. It has been found that such rapid cooling may result in the molten region  72  forming a meta-stable mixture, which is discussed below. 
     With reference to  FIGS. 12 and 13 , a read operation for the disk  50  is now described. As discussed above, heating above the melting temperature causes the formation of a molten mixture comprising the noble metal layer  54  and the underlying material layer  56 . Upon cooling or quenching, the noble metal layer  54  and the underlying material layer  56  separate into a two-component solid material, where first component is usually an inter-metallic compound. The second component can be either the underlying material or the noble metal, depending upon the amount of the underlying material that has been melted. If more underlying material melts than can combine with the melted noble metal to form an inter-metallic compound, then the second material comprises the underlying material. If more noble metal melts than can combine with the melted underlying material to form an inter-metallic compound, then the second material comprises the noble metal. Alternatively, if ideal proportions of the noble metal and the underlying material are melted, only an inter-metallic compound remains. If the underlying material is Silicon or Germanium, then the cooled portion comprises an inter-metallic compound and a second component as discussed above. 
     After being melted and cooled, the noble metal layer  54  no longer comprises the entirety of the upper surface of the molten region  72 . As a result, there is a large change in the reflectance between the areas irradiated by the laser beam  70 , and other areas where the gold film  54  was not heated by the laser beam. In  FIGS. 12 and 13 , numeral  100  represents molten region  72  after it has been cooled or quenched. The cooled region  100  comprises the inter-metallic compound  103 , and may comprise a second material  105 , as discussed above. If the underlying material is a semiconductor, the cooled region  100  comprises a mixture of an inter-metallic compound and a second component as discussed above.  FIGS. 12 and 13  also show areas  106  where the noble metal layer  54 , unexposed to the laser beam during the write operation discussed above, has not been melted and still has a high reflectance. 
     Data is read by observing the reflection at the surface of a disk of a low power laser beam, on the order of 100 micro-Watts or less, or approximately one third of the write power, for example. Where the noble metal layer  54  has diffused into the underlying material layer  56 , the reflectance of the laser beam is reduced due to significant absorption or scattering of the laser beam on the top surface  108  of the cooled region  100  ( FIGS. 12 and 13 ). This is because the top surface  108  is now comprised, in substantial part, of the inter-metallic compound. During a read operation, the high reflectance of areas  106  of noble metal layer  54  is easily distinguished from the low reflectance of the top surface  108  of the cooled region  100 . 
     A low power red laser having a wavelength of 650 nm—a wavelength where the reflectivity of noble metals is very high—can be used. The blue (or blue-violet) laser used for the write operation can also be used for the read operation, but at a lower power, on the order of 100 micro-Watts or less, or one third of the write power, for example. While the reflectivity of gold is not high at the wavelength of a low power blue laser, the read operation needs only to distinguish between the presence and absence of gold and can be accomplished using blue laser light. 
     Combinations of one material for the noble metal layer and one material for the underlying layer are chosen from the groups of materials discussed above. It is desirable that, after the noble metal  54  diffuses into the underlying layer  56 , a thermodynamically stable compound is formed. Combinations of the noble metal  54  and underlying layer  56  are chosen to form stable inter-metallic compounds. An inter-metallic compound is an intermediate phase in an alloy system having a narrow range of homogeneity and relatively simple stoichiometric proportions. 
     Although all of the combinations of the above-listed noble metals and underlying layer materials are satisfactory for the purposes of the invention, certain combinations have been found to be preferable. For the noble metal, Au (gold) and Os (osmium) are preferable. For example, gold has desirable properties for long term archival stability, for it combines good tarnish resistance with consistently high reflectance throughout the near, middle, and far infrared light wavelengths. In  FIG. 14 , a graph of pure gold&#39;s reflectance versus wavelength in nanometers is shown. Pure gold provides over 96% average reflectance from 650 nm to 1700 nm, and over 98% average reflectance 2000 nm to 1600 nm. In addition to its good optical properties, gold is also effective in controlling thermal radiation. 
     For the underlying material, characteristics such as easy deposition and stability are important, in combination with forming a stable compound with the noble metal layer. Preferably, materials for the underlying layer are Ti (titanium) and Zr (zirconium), and also Si (silicon) when the noble metal is Os (osmium). Preferred combinations for a noble metal layer and underlying material layer include: Au (gold) and Ti (titanium); Os (osmium) and Ti (titanium); and Pt (platinum) and Si (silicon). With reference to  FIG. 15 , desirable combinations of the noble metal layer and underlying material layer are shown. Combinations that have good results, in accordance with the invention, are marked by an “X” in  FIG. 15 . 
     The noble metal layer forms a stable inter-metallic compound with the underlying material. The resulting compound will also have a high degree of resistance to oxidation. The compound will have a significantly different reflectivity than the noble metal alone when exposed to a laser light beam—while reflectivity of the compounds will be significantly less that that of the noble metal. For maximum change in reflectance, the molten volume should contain a sufficient amount of the underlying material, so that the entire volume of the noble metal mixes, when heated, with the underlying material. In most cases, substantially equal atomic volumes of the underlying material layer  56  and the noble metal layer  54  will be sufficient for a complete reaction and mixing of the two materials. 
       FIG. 16  shows a system  150  for reading and writing data to disk  50  of the invention. The system  150  may comprise a central unit  152 , a visual display  154  and a user interface  156 . The disk  50  may be inserted into a slot  158  of the central unit  152 . Inside the central unit  152  is a device for reading and writing data to the disk  50  in accordance with the invention. An example of such a device, generally designated by numeral  170 , is shown in  FIG. 17 . The device  170  has a device controller  172 , a processor  174 , an optical controller  178  and an optical pickup  180 . The processor  174  transmits and receives data which has been read from the disk  50  through the optical pickup  180  and data which will be written to the disk  50  through the optical pickup  180 . The optical controller  178  controls the optical pickup  180 . The device controller  172  controls the overall operation of the device  170 . The device  170  also has a motor  182  for rotating a spindle  184  and the disk  50 . The device  170  is capable of irradiating the disk  50  with a laser beam to melt the noble metal  54  ( FIG. 3 ) and underlying material  56  ( FIG. 3 ) in order to write data to the disk  50 , as discussed above. After data is written to disk  50 , the device  170  can also read data from the disk  50  by irradiating a low power laser beam onto the disk  50 . 
     The above discussed embodiments provide desirable results for long-term stability of archival data storage, and improve the maximum density of the recorded data. Archival storage requires long term stability of the materials, and gold is one of the least reactive materials known to mankind. The lifetime of a gold film and a semiconductor layer between two dielectric oxide layers should be essentially infinite. To the outside world the archival memory will look like a noble metal layer full of sub-micron size holes that are not very reflective. The hole/non-hole areas in a track represent data. 
     While the invention has been described in detail in connection with exemplary embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, the oxide layers may be replaced with glass layers, and the invention can be used with lasers of different wavelengths that expose smaller areas of the noble metal. Also, the noble metal layer can comprise an alloy consisting of more than one noble metal, and the underlying material layer can comprise an alloy consisting of more than one metal. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.