Abstract:
A structure and a method for operating the same. The method comprises providing a resistive/reflective region on a substrate, wherein the resistive/reflective region comprises a material having a characteristic of changing the material&#39;s reflectance due to the material absorbing heat; sending an electric current through the resistive/reflective region so as to cause a reflectance change in the resistive/reflective region from a first reflectance value to a second reflectance value different from the first reflectance value; and optically reading the reflectance change in the resistive/reflective region.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to memory devices, and more specifically, to optoelectronic memory devices. 
     2. Related Art 
     A memory cell in a typical semiconductor memory device usually comprises one or more transistors and can store one of two possible values depending on the voltage potential of a certain node of the memory cell. For instance, the memory cell can be considered storing a 1 if the node is at 5V and storing a 0 if the node is at 0V. As a result, the memory cell is an electronic memory cell. In order to write the electronic memory cell, an appropriate voltage potential is applied to the node (or another node) of the electronic memory cell. In order to read the content of the electronic memory cell, the voltage potential of the node (or another node) of the electronic memory cell can be sensed and then amplified by a sensor-amplifier (sense-amp) circuit. However, electrical digital signal transmission is usually slower than optical digital signal propagation. Therefore, it would speed up the write and read cycles of the memory cell if either or both of the write and read cycles can be performed optically. As a result, there is a need for an optoelectronic memory device that (a) can be written optically (i.e., by light) or electrically (by applying voltage) and/or (b) can be read optically (i.e., by light) or electrically (by sensing voltage). 
     SUMMARY OF THE INVENTION 
     The present invention provides a method, comprising providing a resistive/reflective region on a substrate, wherein the resistive/reflective region comprises a material having a characteristic of changing the material&#39;s reflectance due to a phase change in the material; sending an electric current through the resistive/reflective region so as to cause a reflectance change in the resistive/reflective region from a first reflectance value to a second reflectance value different from the first reflectance value; and optically reading the reflectance change in the resistive/reflective region. 
     The present invention also provides a method, comprising providing a resistive/reflective region on a substrate, wherein the resistive/reflective region comprises a material having a characteristic of changing the material&#39;s resistance due to the material absorbing heat; projecting a laser beam on the resistive/reflective region so as to cause a resistance change in the resistive/reflective region from a first resistance value to a second resistance value different from the first resistance value; and electrically reading the resistance change in the resistive/reflective region. 
     The present invention also provides a structure, comprising (a) N regular resistive/reflective regions on a substrate, N being a positive integer, wherein the N regular resistive/reflective regions comprise a material having a characteristic of changing the material&#39;s resistance and reflectance due to the material absorbing heat; (b) N sense-amp circuits electrically coupled one-to-one to the N regular resistive/reflective regions, wherein each sense-amp circuit of the N sense-amp circuits is adapted for recognizing a resistance change in the associated regular resistive/reflective region; and (c) a light source/light detecting device optically coupled to the N regular resistive/reflective regions, wherein the light source/light detecting device is adapted for recognizing a reflectance change in each regular resistive/reflective region of the N regular resistive/reflective regions. 
     The present invention provides an optoelectronic memory device that (a) can be written optically (i.e., by light) or electrically (by applying voltage) and/or (b) can be read optically (i.e., by light) or electrically (by sensing voltage). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an optoelectronic memory device, in accordance with embodiments of the present invention. 
         FIG. 2  illustrates one embodiment of optoelectronic memory cells of the optoelectronic memory device of  FIG. 1 , in accordance with embodiments of the present invention. 
         FIG. 3  illustrates how an applied voltage pulse affects the resistance of the optoelectronic memory cell of  FIG. 2 , in accordance with embodiments of the present invention. 
         FIG. 4  illustrates how an applied voltage affects the resistance of the optoelectronic memory cell of  FIG. 2 , in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an optoelectronic memory device  100 , in accordance with embodiments of the present invention. More specifically, the optoelectronic memory device  100  comprises, illustratively, two optoelectronic memory cells (OEMC)  110   a  and  110   r . The optoelectronic memory device  100  further comprises a switch  120  for applying voltage potentials Vcc and V WR  to the OEMC  110   a  and  110   r  in a manner controlled by the control signals Va and Vr (further details are discussed below). 
     The optoelectronic memory device  100  further comprises a switch  130 , a sensor-amplifier (sense-amp) circuit  140 , and a light source/light detecting device  150 . The switch  130  electrically couples the OEMC  110   a  and  110   r  to the sense-amp circuit  140  in a manner controlled by the control signal Vr, whereas the light source/light detecting device  150  is optically coupled to the OEMC  110   a  and  110   r  (further details are discussed below). 
       FIG. 2  illustrates one embodiment of the OEMC  100   a  of  FIG. 1 , in accordance with embodiments of the present invention. More specifically, the OEMC  100   a  comprises a resistive/reflective region  210  (comprising tantalum nitride TaN in one embodiment) embedded in a dielectric layer  240 . The dielectric layer  240  (comprising silicon dioxide SiO 2  in one embodiment) is formed on a semiconductor (e.g., silicon) layer  250 . 
     In one embodiment, the TaN resistive/reflective region  210  is electrically coupled to two electrically conducting lines  220   a  and  220   b  through two vias  230   a  and  230   b , respectively. Illustratively, the electrically conducting lines  220   a  and  220   b  comprise aluminum (Al) or Copper (Cu) or any other metals, whereas the vias  230   a  and  230   b  comprise tungsten (W) or Cu or any other metals. In one embodiment, the Al line  230   a  of the OEMC  100   a  is coupled to the switch  120  ( FIG. 1 ) whereas the Al line  230   b  of the OEMC  100   a  is coupled to ground ( FIG. 1 ). 
     In one embodiment, the fabrication of the OEMC  100   a  can start out with the silicon layer  250 . Next, in one embodiment, a SiO 2  layer  240   a  (the lower portion of the silicon dioxide layer  240 ) is formed on top of the silicon layer  250  by, illustratively, CVD (chemical vapor deposition) of SiO 2 . 
     Next, in one embodiment, the TaN resistive/reflective region  210  is formed on top of the SiO 2  layer  240   a . Illustratively, the TaN resistive/reflective region  210  is formed on top of the SiO 2  layer  240   a  by (i) blanket depositing a TaN layer (not shown) on top of the SiO 2  layer  240   a  and then (ii) directionally and selectively etching the deposited TaN layer such that what remains of the deposited TaN layer is the TaN resistive/reflective region  210 . 
     Next, in one embodiment, a SiO 2  layer  240   b  (the upper portion of the silicon dioxide layer  240 ) is formed on top of the SiO 2  layer  240   a  and the TaN resistive/reflective region  210  by illustratively CVD of SiO 2 . As a result, the TaN resistive/reflective region  210  is embedded in the SiO 2  layer  240 . 
     Next, in one embodiment, the two W vias  230   a  and  230   b  are formed (i) in the SiO 2  layer  240   b  and (ii) in electrical contact with the TaN resistive/reflective region  210 . Illustratively, the two W vias  230   a  and  230   b  are formed by (a) creating two holes  230   a  and  230   b  by any conventional lithographic process such that the TaN resistive/reflective region  210  is exposed to the surrounding ambient via the holes  230   a  and  230   b , then (b) blanket depositing a tungsten layer (not shown) so as to fill the two holes  230   a  and  230   b  with tungsten, and then (c) planarizing the deposited tungsten layer until a top surface  242  of the SiO 2  layer  240   b  is exposed to the surrounding ambient. 
     Next, in one embodiment, the two Al lines  220   a  and  220   b  are formed (i) on top of SiO 2  layer  240   b  and (ii) in electrical contact with the two W vias  230   a  and  230   b , respectively. Illustratively, the two Al lines  220   a  and  220   b  are formed by (a) blanket depositing an Al layer (not shown) on top of the SiO 2  layer  240   b  and the two W vias  230   a  and  230   b , and then (b) directionally and selectively etching the deposited Al layer such that what remain of the deposited Al layer are the two Al lines  220   a  and  220   b.    
     In one embodiment, the structure of the OEMC  110   r  is similar to the structure of the OEMC  110   a  described above. Moreover, the OEMC  110   r  is coupled to the switch  120  and ground ( FIG. 1 ) in a manner similar to the manner in which the OEMC  110   a  is coupled to the switch  120  and ground ( FIG. 1 ). 
       FIG. 3  shows a plot  300  illustrating how an applied voltage pulse affects the resistance of the TaN resistive/reflective region  210  of the optoelectronic memory cell  110   a  of  FIG. 2 , in accordance with embodiments of the present invention. More specifically, the inventors of the present invention have found that a voltage pulse applied across the TaN resistive/reflective region  210  ( FIG. 2 ) changes both the resistance and the reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ). The reflectance is defined as the ratio of the photon flux reflected by a surface to the photon flux incident on the surface. In one embodiment, the applied voltage pulse can have a triangular shape. More specifically, the applied voltage pulse comprises an increase from 0V to a peak voltage and then a voltage drop back to 0V. 
     The inventors of the present invention have found that, for a particular size of the TaN resistive/reflective region  210  ( FIG. 2 ), if the peak voltage of the applied voltage pulse is between V 1  and V 2  (for example, V 1  and V 2  can be 3V and 4V, respectively, represented by segment B-C of the plot  300  is applicable), then both the resistance and the reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) change as a result of the applied voltage pulse. More specifically, in this segment B-C of the plot  300 , the higher the peak voltage of the applied voltage pulse, the higher the resulting resistance and the reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ). For example, assume the resistance of the TaN resistive/reflective region  210  ( FIG. 2 ) is originally R 0 , and that a voltage pulse having a peak voltage of V 2  (i.e., 4V in the example above) is applied across the TaN resistive/reflective region  210  ( FIG. 2 ). After the pulse is removed, the resistance of the TaN resistive/reflective region  210  ( FIG. 2 ) is R 1 . Also after the pulse is removed, the TaN resistive/reflective region  210  ( FIG. 2 ) has a higher reflectance. 
     It should be noted that the change in resistance and reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) in the example above with respect to segment B-C of the plot  300  is irreversible. For illustration, assume that after the pulse described above is removed, another voltage pulse having a peak voltage of V 1  (i.e., 3V in the example above) is applied across the TaN resistive/reflective region  210  ( FIG. 2 ). The resistance and reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) would not change back to original values, but remain essentially unchanged (i.e., R 1  for resistance). 
       FIG. 4A  shows a plot  400  illustrating how an applied voltage affects the resistance of the optoelectronic memory cell of  FIG. 2 , in accordance with embodiments of the present invention. More specifically, the inventors of the present invention have found that both the resistance and the reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) change reversibly in response to the applied voltage between 0V and V 1  (in one case, V 1 =3V). For instance, when the applied voltage changes from 0V to V 1 , the resistance of the TaN resistive/reflective region  210  ( FIG. 2 ) increases from R 0  to R 3 . Also, although not shown, the reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) increases (i.e., less transparent). However, when the applied voltage changes from V 1  back to 0V, the resistance and reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) change back to the original values (i.e., reversible). 
     With reference to  FIGS. 1-4A , in one embodiment, the operation of the optoelectronic memory device  100  is as follows, assuming that the OEMC  110   a  and  110   r  operate in the segment B-C of the plot  300  ( FIG. 3 ). 
     In one embodiment, the OEMC  110   a  can be electrically written. Assuming that a 1 is to be written into the OEMC  110   a , then Va and Vr can be adjusted such that the switch  120  electrically couples OEMC  110   a  to signal V WR  such that a voltage pulse of signal V WR  having a peak voltage of V 2  (i.e., 4V in the example above) is applied across the OEMC  110   a . As a result of the pulse, the resistance of the TaN resistive/reflective region  210  ( FIG. 2 ) change from R 0  (initial resistance) to and stays at R 1 . Also as a result of the pulse, the reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) increases (and remains high even after the pulse is removed). 
     In one embodiment, the content of the OEMC  110   a  can be electrically read. More specifically, Va and Vr can be adjusted such that the switch  120  electrically couples the OEMCs  110   a  and  110   r  to Vcc and such that the switch  130  electrically couples the OEMCs  110   a  and  110   r  to the sense-amp circuit  140 . Because the resistance of the OEMC  110   a  is high (R 1 ) while the resistance of the OEMC  110   r  stays at R 0 , the sense-amp circuit  140  can recognize such a difference (by comparing the voltage drops across the OEMC  110   a  and  110   r ) and accordingly generates a 1 at its output Vout, indicating that the OEMC  110   a  stores a 1. It should be noted here that the OEMC  110   r  is used as a reference memory cell for reading the content of the OEMC  110   a.    
     In an alternative embodiment, the content of the OEMC  110   a  can be optically read. More specifically, the light source/light detecting device  150  can generate identical incident beams  162   a  and  162   r  (e.g., lasers) to the OEMCs  110   a  and  110   r , respectively, and receives the reflected beams  164   a  and  164   r  from the OEMCs  110   a  and  110   r , respectively. In one embodiment, the incident laser beams  162   a  and  162   r  have 1.3 μm wavelength with a laser pulse duration of 15 ns and with a laser energy in a range of 0.035 μj to 0.095 μj. Because the OEMCs  110   a  is more reflective than the OEMC  110   r  (as a result of the applied voltage pulse during the write cycle described above), the light source/light detecting device  150  can recognize the difference in the intensities of the reflected beams  164   a  and  164   r  from the OEMCs  110   a  and  110   r , respectively, and accordingly generates a 1 indicating that OEMC  110   a  stores a 1. It should be noted here that the OEMC  110   r  is used as a reference memory cell for reading the content of the OEMC  110   a.    
     The inventors of the present invention have found that an incident beam (e.g., a laser) can have the same effect as a voltage pulse with respect to changing the resistance and reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ). This is because both the laser beam and the voltage pulse have the same effect of generating heat in the TaN resistive/reflective region  210  ( FIG. 2 ), resulting in a phase change in the material of resistive/reflective region  210  ( FIG. 2 ) leading to the change in the resistance and reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) as described above. In the case of the voltage pulse, the voltage pulse generates an electric current that passes through and hence generates heat in the TaN resistive/reflective region  210  ( FIG. 2 ). In case of the laser, the energy of the laser transforms into heat in the TaN resistive/reflective region  210  ( FIG. 2 ). 
     As a result, in an alternative embodiment, instead of being electrically written as described above, the OEMC  110   a  can be optically written. More specifically, assuming that a 1 is to be written into the OEMC  110   a , then the light source/light detecting device  150  can generate the incident beam  162   a  (e.g., a laser) at sufficient intensity to the OEMC  110   a  such that it is as if a voltage pulse with a peak voltage of V 2  (i.e., 4V in the example above) were applied across the OEMC  110   a . In one embodiment, the incident laser beams  162   a  and  162   r  have 1.3 μm wavelength with a laser pulse duration of 15 ns and with a laser energy in a range of 0.6 μj to 1.5 μj. As a result, both the resistance and reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) increase. This increase in the resistance and reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) can be subsequently detected electrically and optically as described above. 
     In an alternative embodiment, instead of operating in the segment B-C of the plot  300  as described above, the OEMC  110   a  operates in the segment X-Y of the plot  400  ( FIG. 4 ). Operating in the segment X-Y of the plot  400  ( FIG. 4 ), the OEMC  110   a  can simultaneously be written electrically and read optically, and as a result, can be used to convert an electrical signal into an optical signal. 
     More specifically, in one embodiment, when the applied voltage is 0V (a 0 for the electrical signal), the TaN resistive/reflective region  210  ( FIG. 2 ) of the OEMC  110   a  has a first reflectance. The light source/light detecting device  150  can detect the same reflectance for both OEMCs  110   a  and  110   r  and accordingly generates a 0 for the optical signal. When the applied voltage is V 1  (a 1 for the electrical signal), the TaN resistive/reflective region  210  ( FIG. 2 ) of the OEMC  110   a  has a second reflectance higher than the first reflectance. The light source/light detecting device  150  can detect the reflectance difference between the reflectances of the OEMCs  110   a  and  110   r  and accordingly generates a 1 for the optical signal. In other words, the OEMC  110   a  can be used to convert an electrical signal into an optical signal. In one embodiment, laser wavelengths of 532 nm, 1064 nm, or 1340 nm can be used for the incident laser beams  162   a  and  162   r  used to read the content of the OEMC  110   a  while the OEMC  110   a  operates in the segment X-Y of the plot  400  ( FIG. 4 ). 
     Similarly, operating in the segment X-Y of the plot  400  ( FIG. 4 ), the OEMC  110   a  can simultaneously be written optically and read electrically, and as a result, can be used to convert an optical signal into an electrical signal. In one embodiment, regarding the incident laser beams  162   a  and  162   r , the energy of the lasers used for optically writing the OEMC  110   a  when the OEMC  110   a  operates in the segment X-Y of the plot  400  ( FIG. 4 ) can be higher than the energy of the lasers used for optically reading the OEMC  110   a  when the OEMC  110   a  operates in the segment B-C of the plot  300  ( FIG. 3 ) but lower than the energy of the lasers used for optically writing the OEMC  110   a  when the OEMC  110   a  operates in the segment B-C of the plot  300  ( FIG. 3 ). 
     More specifically, in one embodiment, when the intensity of incident laser beam  162   a  is zero (a 0 for the optical signal), the TaN resistive/reflective region  210  ( FIG. 2 ) of the OEMC  110   a  has a first resistance. The sense-amp circuit  140  can detect the same resistance for both OEMCs  110   a  and  110   r  and accordingly generates a 0 for the electrical signal. When the intensity of incident laser beam  162   a  is at a higher level (a 1 for the optical signal), the TaN resistive/reflective region  210  ( FIG. 2 ) of the OEMC  110   a  has a second resistance higher than the first resistance. The sense-amp circuit  140  can detect the resistance difference between the resistances of the OEMCs  110   a  and  110   r  and accordingly generates a 1 for the electrical signal. In other words, the OEMC  110   a  can be used to convert an optical signal into an electrical signal. 
     It should be noted that because the changes of the resistance and the reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ) when the OEMC  110   a  operates in the segment X-Y of the plot  400  is smaller than when the OEMC  110   a  operates in the segment B-C of the plot  300 , the sense-amp circuit  140  and the light source/light detecting device  150  need to be more sensitive so as to detect small changes of the resistance and the reflectance of the TaN resistive/reflective region  210  ( FIG. 2 ). 
     In summary, operating in the segment B-C of the plot  300 , the OEMC  110   a  can function as a one-time write optoelectronic memory cell which, after be written, can be read many times either electrically or optically. In contrast, operating in the segment X-Y of the plot  400 , the OEMC  110   a  can function as an electrical-optical converter for converting back and forth between electrical digital signals and optical digital signals. 
     In one embodiment, with reference to  FIG. 1 , the optoelectronic memory device  100  can comprise N OEMCs (not shown) essentially identical to the OEMC  110   a  each of which can store one bit of information (N is a positive integer). For each of these N OEMCs, there needs to be (i) a write switch (not shown but similar to the switch  120 , (ii) a read switch (not shown but similar to the switch  130 ), and (iii) a sense-amp circuit (not shown but similar to the sense-amp circuit  140 ). Moreover, each of these N OEMCs is optically coupled to the light source/light detecting device  150  in a manner similar to that of the OEMC  110   a . The operation of each of these N OEMCs is similar to that of the OEMC  110   a . In one embodiment, all the N OEMCs of the optoelectronic memory device  100  share the same reference OEMC  110   r . Alternatively, each of the N OEMCs of the optoelectronic memory device  100  can have its own reference OEMC. 
     It should be noted that the description of the embodiments above is sufficient such that a person with ordinary skill in the art could practice the invention without undue experimentation. 
     With reference back to  FIG. 2 , in the embodiments described above, the resistive/reflective region  210  comprises tantalum nitride TaN. In general, the resistive/reflective region  210  can be a TaN composite stack including multiple layers (not shown). In one embodiment, the resistive/reflective region  210  can be a SiN/TaN/SiO2/SiN composite stack, a SiN/TaN/SiN composite stack, SiN/SiO2/TaN/SiN composite stack, or any other composite stack that includes a TaN core layer. Also, in the embodiments described above, the dielectric layer  240  comprises silicon dioxide SiO 2 . Alternatively, the dielectric layer  240  can comprise a low-K material such as SiCOH, SiLK, and polymers, etc. 
     In the embodiments described above, the resistance of the resistive/reflective region  210  ( FIG. 2 ) increases when it absorbs a small heat amount that comes from either a voltage source or a low-energy laser. This is the case shown in  FIG. 4A  when the material of the resistive/reflective region  210  ( FIG. 2 ) has a positive temperature coefficient of resistance (TCR). Alternatively, the resistance of the resistive/reflective region  210  ( FIG. 2 ) can decrease when it absorbs a small heat amount that comes from either a voltage source or a low-energy laser. This is the case shown in  FIG. 4B  when the material of the resistive/reflective region  210  ( FIG. 2 ) has a negative TCR. 
     It should be noted that TaN can have either positive or negative TCR depending on the TaN film fabrication process. However, whether the material of the resistive/reflective region  210  ( FIG. 2 ) has a positive or negative TCR, the light source/light detecting device  150  can recognize a difference (if any) in the reflected beams from the OEMCs  110   a  and  110   r  and operate accordingly. 
     It should also be noted that the voltage values in  FIGS. 3 and 4  are for illustration only. Therefore, the scope of the claims are not in any way restricted to these values. 
     Similarly, in the embodiments described above, the reflectance of the resistive/reflective region  210  ( FIG. 2 ) increases when the resistive/reflective region  210  ( FIG. 2 ) absorbs a small heat amount that comes from either a voltage source or a low-energy laser. Alternatively, the reflectance of the resistive/reflective region  210  ( FIG. 2 ) can decrease when the resistive/reflective region  210  ( FIG. 2 ) absorbs a small heat amount that comes from either a voltage source or a low-energy laser. 
     It should also be noted that a resistance increase does not necessarily occurs hand-in-hand with a reflectance increase in either reversible or irreversible case. Similarly, a resistance decrease does not necessarily occur hand-in-hand with a reflectance decrease in either reversible or irreversible case. For instance, the inventors of the present invention have found that for a particular resistive/reflective region  210  ( FIG. 2 ), at some heat absorption level, the resistance of the sample  210  increases above the original resistance value while the reflectivity goes below the original reflectance value. But at a certain higher heat absorption level, the resistance of the sample  210  goes below the original resistance value while the reflectivity goes above the original reflectance value. 
     While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.