Abstract:
A phase change memory device is disclosed, including a substrate, a phase change layer over the substrate, a first electrode electrically connecting a first side of the phase change layer, a second electrode electrically connecting a second side of the phase change layer, wherein the phase change layer composes mainly of gallium (Ga), antimony (Sb) and tellurium (Te) and unavoidable impurities, having the composition range of Ga x Te y Sb z , 5&lt;x&lt;40; 8≦y&lt;48; 42&lt;z&lt;80; and x+y+z=100.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/025,379, filed on Feb. 01, 2008, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to a phase change memory device, and more particularly relates to a phase change memory material of a phase change memory device. 
     2. Description of the Related Art 
     Phase change memory has many advantages, such as fast speed, lower power consumption, high capacity, robust endurance, easy embeddability in logic IC, and lower cost, so that it can serve as a stand-alone or embedded memory device with high integrity. Due to the described advantages, phase change memory has been considered the most promising candidate for the next-generation nonvolatile semiconductor memory which may replace the commercialized volatile memory, such as SRAM or DRAM, and non-volatile memory, such as flash. 
     Chalcogenide is widely used in phase change memory devices. The chemical element of group VI, such as S, Se or Te, is the major material of Chalcogenide and is combined with elements of group IV or V and some dopants for use in phase change memory devices. Ge 2 Sb 2 Te 5  is the most popular material used in phase change memory devices because it can provide a binary state switching with a fast and reversible phase transition between an amorphous phase (with extremely high electrical resistivity) and a crystalline phase (with very low electrical resistivity). Ge 2 Sb 2 Te 5 , however, still has deficiencies which include low crystallization temperature, low electrical resistivity at crystalline state while high melting temperature, containing major amount of Te which is highly volatile and toxic to easily contaminate the processing chamber and its environment, among others. A novel phase change material is required to promote performance of phase change memory devices, as well as to lessen the burden to environment. 
     BRIEF SUMMARY OF INVENTION 
     According to the issues described previously, an embodiment of the invention provides a phase change memory device, comprising a substrate, a phase change layer over the substrate, a first electrode electrically connecting a first side of the phase change layer, and a second electrode electrically connecting a second side of the phase change layer, wherein the phase change layer composes mainly of gallium (Ga), tellurium (Te), antimony (Sb), and unavoidable impurities, having the composition range of Ga x Te y Sb z , 5&lt;x&lt;40; 8≦y&lt;48; 42&lt;z &lt;80; and x +y+z=100. 
     The invention further provides a phase change memory device, comprising a substrate, a phase change layer over the substrate, a first electrode electrically connecting a first side of the phase change layer, and a second electrode electrically connecting a second side of the phase change layer, wherein the phase change layer has two states of a stable phase. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows the designed and studied alloy compositions of the phase change material of an embodiment of the invention. 
         FIG. 2  shows comparison of melting temperature and crystallization temperature of the sample Ga 20 Te 30 Sb 50  with a conventional Ge 2 Sb 2 Te 5 . 
         FIG. 3  shows crystallization temperature and the ratio between crystallization temperature and melting temperature (T x /T m ) of the studied alloys of an embodiment of the invention. 
         FIG. 4  shows electrical resistance after crystallization of the studied alloys. 
         FIG. 5  shows electrical resistance as a function of temperature of the example (Ga 20 Te 30 Sb 50 ) and a conventional Ge 2 Te 2 Sb 5  alloy films. 
         FIG. 6  shows fabrication of a phase change memory device using Ga—Te—Sb alloy as a phase change material of an embodiment of the invention. 
         FIG. 7  shows a thermogram taken using a DTA during heating up of the amorphous Ga 20 Te 30 Sb 50  sample of the embodiment of the invention. 
         FIG. 8  shows electrical resistance as a function of programming current of a memory device made of Ga 20 Te 30 Sb 50  sample of the embodiment of the invention. 
         FIG. 9A  and  FIG. 9B  show failure time as a function of 1/kT to compare data retention of the memory devices made of Ga 20 Te 3 Sb 50  (A) with that made of the conventional Ge 2 Sb 2 Te 5  (B). 
         FIG. 10A  shows resistance as a function of programming current of an example (Ga 20 Te 30 Sb 50 ). 
         FIG. 10B  shows reduced R-ratio as a function of pulse width of an example (Ga 20 Te 30 Sb 50 ). 
         FIG. 11  shows resistance as a function of number of cycles of the example (Ga 20 Te 30 Sb 50 ). 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The following description is of the contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense, not for limiting the invention. 
     Conventional chalcogenide-based phase change material, Ge 2 Sb 2 Te 5 , has many advantages, such as high electrical resistance difference and high crystallization speed, but it has many deficiencies required to be improved. 
     The designed and studied alloy compositions of the phase change material of an embodiment of the invention are shown by the area bounded by points I, II, III, IV, V and VI in  FIG. 1 . There are two series of compositions: A, B, C, D, and E along the Sb 80 Te 20 —GaSb tie-line (tie line 1), and compositions F, G, H, I, and J along the Sb 2 Te 3 —GaSb tie line (tie line 2). All of them can be represented by the formulae:
 
Ga x Te y Sb z  
 
     5&lt;×&lt;40; 8≦y &lt;48; 42&lt;z &lt;80, and x+y+z=100; whereas three Reference compositions located at x=20, y=30, z=50; x=18, y=12, z=70; and x=25, y=8, z=67 were designed to represent Ga 20 Te 30 Sb 50 , Ga 17.6 Te 11.8 Sb 70.6 and Ga 25 Te 8 Sb 67 , respectively. 
     Any methods known in the prior art can be used in the preparation of the designed alloys, and a target for forming a layer of the designed alloys of the present invention. Any deposition methods known in the prior art can be used to form the phase change layer of the phase change memory device of the present invention, which include (but are not limited to): evaporation methods in vacuum such as thermal evaporation and E-beam evaporation; sputtering methods such as DC, RF, magnetron, symmetric, and non-symmetric sputtering, etc.; and vacuum ion plating. In addition, any chemical vapor deposition methods known in the prior art can also be used to deposit the phase change memory alloys. In the embodiments shown below, magnetron sputtering was adopted for the deposition of films. Two targets were used simultaneously, GaSb and Sb 80 Te 20  for the compositions along tie line  1  (compositions A to E), while GaSb and Sb 2 Te 3  were used for the compositions along tie line  2  (compositions F to J). Film composition was tuned and adjusted by the respective sputtering power of the targets. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                   
                 Specific 
               
               
                 Power 
                 (Composition %) 
                 temperatures (° C.) 
               
             
          
           
               
                 Ratio, W 
                   
                 Ga 
                 Sb 
                 Te 
                 Sb/Te 
                 T x   
                 T m−1   
                 T m−2   
               
               
                   
               
             
          
           
               
                  0/50 
                 S8T2 
                 0 
                 82.1 
                 17.9 
                 4.59 
                 123 
                 541 
                 — 
               
               
                 25/50 
                 A 
                 9.9 
                 75.7 
                 14.4 
                 5.26 
                 195 
                 513.9 
                 559.5 
               
               
                 50/50 
                 B 
                 17.1 
                 71.2 
                 11.7 
                 6.08 
                 232 
                 573.6 
                 — 
               
               
                 50/25 
                 C 
                 26.4 
                 65.2 
                 8.4 
                 7.76 
                 277 
                 567.5 
                 — 
               
               
                 75/25 
                 D 
                 31.6 
                 62.1 
                 6.3 
                 9.86 
                 269 
                 567.3 
                 666.5 
               
               
                 75/15 
                 E 
                 38.2 
                 57.7 
                 4.1 
                 14.4 
                 275 
                 564.9 
                 686.8 
               
               
                 50/0  
                 GS 
                 51.4 
                 48.6 
                 0 
                 — 
                 275 
                 564.5 
                 687.1 
               
               
                   
               
             
          
         
       
     
     Table 1 is the quantitative analysis results of the films under study. The designation S8T2 represents Sb 80 Te 20 , and GS for GaSb. They were prepared as References, and the compositions of A to E are denoted in this table. 
     In the embodiment, the Ge of Ge 2 Sb 2 Te 5  is replaced by Ga, wherein Ga has an atomic number is only 1 less than that of Ge. Hence, Ga has a similar atom radius with Ge; and lattice arrangement can be stable upon this replacement. Further, as shown in  FIG. 2 , since the melting temperature of Ga is only 29.8° C., melting temperature of the alloy (Ga 20 Te 30 Sb 50 ) can be effectively reduced to decrease operation energy of the device and to eliminate thermal cross-talk issues for smaller feature-sized devices. 
       FIG. 3  shows crystallization temperature of the studied alloys. When the Ga content is increased, the crystallization temperature (T x ) and the ratio between crystallization temperature and melting temperature (T x /T m ) of the alloys are increased, which means that the Ga—Te—Sb alloy provided good thermal stability. 
       FIG. 4  shows electrical resistance after crystallization of the as-deposited amorphous alloys. When the Ga content is increased in a specific range, crystallized resistance (R c ) and the ratio between electrical resistance of amorphous and crystalline state (R a /R c ) of the alloys are increased. The higher electrical resistance at crystalline state provided by the Ga—Te—Sb alloy can reduce RESET current of the phase change memory device, and size of the device can be reduced and numbers of unit cell per area can be increased. 
       FIG. 5  shows electrical resistance as a function of temperature of the example (Ga 20 Te 30 Sb 50 ) and conventional Ge 2 Te 2 Sb 5 . This figure shows that conventional Ge 2 Sb 2 Te 5  has a first phase change at about 170° C. and a second phase change at about 300° C., and it is clear that the resistance is sensitive to variation of temperature between the first and second phase change points. This phenomenon may lead to resistance variation due to residue heat during operation of the device, thus the stability of the device is affected. In contrast, the sample (Ga 20 Te 30 Sb 50 ) of the invention presents stable crystallized electrical resistance, which is not greatly changed when temperature is increased. 
     Fabrication of a phase change memory device using a demonstrating Ga—Te—Sb alloy as a phase change material and a Ge 2 Sb 2 Te 5  material for REFERENCE, with cell-size of 200 nm×200 nm is illustrated in accordance with  FIG. 6 . A substrate  502 , such as silicon, is provided, and a buffer layer (not shown), such as silicon oxide can be formed on the substrate  502 . A bottom electrode  504  is deposited over the substrate  502 , in which the bottom electrode  504 , in this case comprises of a TiN layer with thickness of about 50 nm and a Ti layer with thickness of about 150 nm. The bottom electrode  504  is patterned by photolithography technology to define a contact area. An insulating layer  506 , such as an oxide, is formed on the bottom electrode  504  and then patterned to form an opening  512 . A phase change layer  508 , which includes Ga—Te—Sb alloy, or reference Ge 2 Sb 2 Te 5 , and is about 100 nm thick, is deposited on the insulating layer  506  and filled into the opening  512 . Next, a top electrode  510 , such as TaN, is formed on the phase change layer  508 , followed by placing the device into a furnace for annealing the phase change layer  508  to change it into crystalline state. 
       FIG. 7  shows the thermogram obtained using differential thermo-analysis (DTA) during heating up of a Ga 20 Te 30 Sb 50  film sample of the embodiment of the invention. It is noted that the phase change material has an incongruent melting. Due to this characteristic, thise phase change Ga 20 Te 30 Sb 5 O material has two endothermic peaks in a DTA or differential scanning calorimetry (DSC) analysis, as shown in the DTA curve of  FIG. 7 . Accordingly, as shown in  FIG. 8 , which shows resistance as a function of programming current, the phase change material has two states of stable phases (state  1  and state  2 ). This is due to the fact that the material has two endothermic peaks. When the material is applied with current to a specific temperature, the first incongruent composition is melted to form a transient liquid which is subsequently quenched by the surrounding into an amorphous phase, a fraction of the cell volume. This firstly formed amorphous phase mixes with the remaining crystalline phase in the cell forming a metastable intermediate state which has an electrical resistance higher than that of the crystalline state, but less than that of the amorphous state. Due to the intermediate electrical resistance state, this phase change material can have an extra memory bit per cell. That is to say, the invented phase change material has the capability to memorize three bits per cell. For example, the memory device using the phase change material can have three bits ( 0 ,  1 ,  2 ) and the memory capacity can be increased from the conventional 2 n  to 3 n . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 T x  (° C.) 
                 T m  (° C.) 
                 T x /T m   
                 R c  (Ω-cm) 
                 R a /R c   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Ge 2 Sb 2 Te 5   
                 157 
                 613 
                 0.485 
                 3.0e−3 
                 2.5e5 
               
               
                 Ga 20 Te 30 Sb 50   
                 237 
                 563 
                 0.61 
                 6.5e−3 
                 4.4e4 
               
               
                 Ga 18 Te 12 Sb 70   
                 232 
                 573.6 
                 0.596 
                 1.45e−3  
                 6.8e3 
               
               
                 Ga 25 Te 8 Sb 67   
                 277 
                 567.5 
                 0.65 
                 1.9e−3 
                 1.1e4 
               
               
                   
               
             
          
         
       
     
     Table 2 shows comparisons among a conventional Ge 2 Sb 2 Te 5  and the three examples Ga 20 Sb 30 Te 50 , Ga 18 Te 12 Sb 70  and Ga 25 Te 8 Sb 67  of the invention. According table, the sample Ga 20 Te 30 Sb 50  presents higher electrical resistance at crystalline state (R c ) than that of a conventional Ge 2 Sb 2 Te 5  to reduce RESET current of the phase change memory device. The high crystallization temperature and T x /T m  renders less problems encountered by the conventional Ge 2 Sb 2 Te 5 , so that size of the device can be reduced and numbers of memory cells per unit area can be increased. In addition, Table 2 also shows that the three examples have much higher T x /T m  to have much better thermal stability. The other two exemplified compositions, Ga 18 Te 12 Sb 70  and Ga 25 Te 8 Sb 67 , have R c  values close to that of the Ge 2 Sb 2 Te 5  while have lower melting temperatures to reduce the energy required for the transient melting (RESET) the memory cells. Hence are applicable for use in high-density phase change memory. 
       FIG. 9A  and  FIG. 9B  show failure time as function of 1/kT to compare data retention of the sample Ga 20 Te 30 Sb 50  with a conventional Ge 2 Sb 2 Te 5 . As shown in  FIG. 9A  and  FIG. 9B , since Ga 20 Te 30 Sb 50  has higher activation energy, which is proportional to the barrier energy between the amorphous state and the crystal state, devices with the material of the example can keep data extrapolating to more than one million years under the temperature of 120° C. However, devices with a conventional Ge 2 Sb 2 Te 5  can keep data for only 4.2 hours under the same condition. Hence, the phase change material of the embodiment of the invention has very good data retention characteristics. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Pulse width 
                   
                 ΔR = 
                 Normalized 
                 Percentage 
               
               
                 (ns) 
                 Rset (Ω) 
                 Rreset − Rset (Ω) 
                 vs. DR500 ns 
                 (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 20 
                 2046 
                 14423 
                 0.920299 
                 92.0 
               
               
                 40 
                 1318 
                 15150 
                 0.966728 
                 96.7 
               
               
                 60 
                 1109 
                 15360 
                 0.980081 
                 98.0 
               
               
                 80 
                 1020 
                 15448 
                 0.985739 
                 98.6 
               
               
                 100 
                 964 
                 15504 
                 0.989308 
                 98.9 
               
               
                 300 
                 856 
                 15612 
                 0.996204 
                 99.6 
               
               
                 500 
                 797 
                 15671 
                 1 
                 100 
               
               
                   
               
             
          
         
       
     
     Table 3 is an analysis of programming speed of the example (Ga 20 Te 30 Sb 50 ) applied with pulse width from 20 ns to 500 ns. The resistance as a function of programming current of the example (Ga 20 Te 30 Sb 50 ) is shown in  FIG. 10A . In table 3, Rset is measured under various pulse-width conditions and average reset resistance of the example reset by pulse widths from 20 ns˜500 ns is calculated as 16468Ω. ΔR is calculated with average reset resistance Rreset (16468Ω) subtracted from Rset. The ΔR (15671) of 500 ns pulse-width condition is set as a base to compare ΔR of various pulse-width conditions to the ΔR of 500 ns pulse-width condition. The result is shown in the final column of table 3 and charted as  FIG. 10B . According to table 3 and  FIG. 10B , we can see that the example (Ga 20 Te 30 Sb 50 ) applied with 20 ns pulse width can achieve about 92% resistance difference between reset and set condition comparing that applied with 500 ns pulse width. Therefore, the example of the invention having very fast programming speed is concluded. 
       FIG. 11  shows resistance as a function of number of cycles of the example (Ga 20 Te 30 Sb 50 ). As shown  FIG. 11 , the example can achieve a cycle number greater than 2×10 5  and this result shows that the example has good endurance performance. 
     The merits of phase-change memory device using the Ga—Te—Sb materials disclosed in this invention are manifest. First of all, they have a reasonably higher crystallization temperature (T x ) while lower melting temperature than state-of-the-art Ge 2 Sb 2 Te 5  alloys. This leads right away to the benefits of less cross-talk problems, while lower energy to RESET, the device of this invention. Second, the phase-change materials disclosed in this invention have both high Tx and activation energy, resulting in memory devices which has much higher thermal stability and can be operative at a temperature 161° C. for 10 years. Third, memory devices with three bits per cell are possible in the some compositions of this invention, leading to much higher memory capacity at the same feature size. Fourth, the phase-change materials disclosed in this invention contain much less Te, hence a cleaner process and less impact to the environmental burden that that of Ge 2 Sb 2 Te 5  alloys can be expected. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.