Abstract:
An improved scalable, resistive element for use in a semiconductor device that can be produced with a small feature size and precise resistance is provided by the present invention. The resistive element includes a base layer positioned on top of a metal line. A seed layer of is deposited on top of the base layer. A thin barrier layer of Al is deposited on top of the seed layer and oxidized. A non-magnetic metal layer is then deposited on top of the barrier layer. The base layer and the non-magnetic metal layer form electrodes on either side of the barrier layer. The barrier layer is thin enough that a tunneling current can travel between the electrodes. The resulting resistive element may be constructed with a high resistance and a very small feature size.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention generally relates to the field of semiconductor manufacturing. More particularly, the present invention disclosure describes an improved process for manufacturing resistive elements that are smaller than conventional resistors and well suited for integration into modern semiconductor devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     Resistors are a basic electrical component of almost every integrated circuit. Modern integrated circuits are typically manufactured from semiconductors using a process whereby layers of materials are deposited upon a semiconductor substrate and then patterned by selectively etching away portions of the deposited layers. Typically, resistors are formed by depositing a length of conductive material having a predetermined resistance per unit of length such that the length of the resistor determines its particular resistance value. Thus, to create a resistor having a relatively high resistance, a relatively long length of material must be deposited. However, as integrated circuits increasingly become more complex with more components, they require more and more space on semiconductor substrates to construct. Thus, there is a need in the prior art to produce smaller and smaller resistors. More particularly, there is a need to produce smaller resistors having larger resistance values. Furthermore, the resistors used in integrated circuits are often required to have a resistance value that is within a very narrow range of resistance values. Thus, there is a need in the prior art for a method of producing a resistive element that is smaller than current resistors, yet has a precise resistance value and can be manufactured to have a high resistance value.  
       SUMMARY OF THE INVENTION  
       [0003]     A preferred embodiment of the present invention is directed toward a method for producing a resistive element. In accordance with the method, a smoothing layer of Ta is deposited over a first electrode. A seed layer of CoFe is then deposited over the smoothing layer and the first electrode. A metal layer is deposited over the seed layer and the metal layer is oxidized in order to form an insulating barrier layer. The barrier layer is constructed so that it is thin enough to allow a tunneling current to flow to a second electrode. The resistance of the resistive element is a function of the thickness of the insulating barrier layer and the area of the resistive element. Accordingly, the resistive element may be patterned and etched such that the resistive element has a predetermined resistance value.  
         [0004]     In accordance with another embodiment of the present invention, a method of producing a resistor for use in a semiconductor device is provided. A base layer of TaN is deposited over a metal contact point. A smoothing layer of Ta is then deposited over the base layer. A seed layer of CoFe is deposited over the base layer. A barrier layer of Al less than approximately 2 nanometers thick is deposited over the seed layer. The barrier layer is then oxidized. A non-magnetic metal layer, preferably of Al, is deposited over the barrier layer. Finally, a protective cap layer is deposited over the non-magnetic metal layer. The resistor is then patterned such that the resistor has a desired resistance value. It is also possible to adjust the Al thickness and oxidation conditions to modify the tunnel barrier and obtain the desired resistance value for a given shape.  
         [0005]     Yet another embodiment of the present invention is directed toward a resistive element for use in a semiconductor device. The resistive element includes a base layer of TaN positioned over a metal contact. A smoothing layer of Ta is positioned over the base layer. A seed layer is positioned over the smoothing layer and base layer. A barrier layer is positioned over the seed layer. The barrier layer is at least partially oxidized. A non-magnetic metal layer is then positioned over the barrier layer. A protective cap layer is positioned over the non-magnetic metal layer.  
         [0006]     Yet another embodiment of the present invention is directed toward a resistor having a top electrode formed from a non-magnetic metal. The bottom electrode includes TaN and the top electrode includes at least one of Al and TaN. A seed layer of CoFe and a thin insulating layer of oxidized Al less than approximately 2 nanometers in thickness are positioned between the bottom electrode and the top electrode. The insulating layer is thin enough to allow a tunneling current to be established between the top electrode and the bottom electrode.  
         [0007]     The above described preferred embodiments of the present invention represent a number of improvements upon the prior art. First, unlike prior art resistors, resistors can be constructed in accordance with the present invention to have both a small feature size (because of the vertical structure of the resistor) and a large resistance value. Second, the present invention provides resistors that can reliably be constructed to have a particular resistance value with very little variance in their resistance values. Finally, the resistance value of the resistors can be easily modified by patterning them to have a particular resistance value during an etching process already typically present during the manufacturing process. As an option, the resistance value can also be set by a proper choice of Al thickness and appropriate barrier oxidation. Thus, the present invention represents a substantial improvement upon the prior art. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a pictorial representation of a resistive stack created in accordance with a preferred embodiment of the present invention;  
         [0009]     FIGS.  2 ( a ) and  2 ( b ) are graphs of resistance values for resistive stacks created in accordance with a preferred embodiment of the present invention having a CoFe seed layer;  
         [0010]     FIGS.  3 ( a ) and  3 ( b ) are graphs of resistance values for resistive stacks created without a CoFe seed layer; and  
         [0011]      FIG. 4  is a flow chart of a method of constructing a resistive element in accordance with a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]     Referring now to  FIG. 1 , a representation of a resistive stack  2  manufactured in accordance with a preferred embodiment of the present invention is shown. A picture of a resistive stack such as shown in  FIG. 1  may be obtained in practice by viewing the resistive stack with a transition electron microscope (TEM). The resistive stack  2  is constructed upon a metal line or layer  4 . As discussed in more detail below, the metal line  4  provides one electrical contact point to the resistive stack  2 . A Tantalum Nitride (TaN) layer  6  is deposited upon the metal layer  4  to form a base layer  6  of the resistive stack  2 . The TaN layer  6  is depicted in  FIG. 1  as being 150 Angstroms (A) thick. However, the thickness of the base layer  6  is not critical as long as it does not deviate to the point that the functionality of the resistive stack  2  is substantially affected. In order to provide a smoother surface for deposition of a seed layer, a thin smoothing layer of Tantalum (Ta)  8  is deposited on the surface of the base layer  6 . A seed layer  10  of Cobalt Iron (CoFe) is then deposited on top of the smoothing layer  8 . The seed layer  10  of CoFe provides a receptive surface for the deposition of the barrier layer  12 . The barrier layer  12  preferably consists of a thin layer of Aluminum (Al). Most preferably, the barrier layer  12  is less than 2 nanometers thick. However, as discussed in more detail below, the thickness of the barrier layer  12  will depend upon the desired resistance of the resistive stack with the only requirement being that it is thin enough to allow a tunneling current. Once the barrier layer  12  has been deposited, it is oxidized to create an electrical barrier in the resistive stack. Oxidizing the barrier layer  12  dramatically increases its resistance to the point that the barrier layer  12  essentially functions as an insulator. A non-magnetic layer  14  is then deposited upon the oxidized barrier layer  12 . The non-magnetic metal layer  14  is most preferably Al. Finally, a protective layer  16  of TaN is deposited upon the non-magnetic metal layer  14  to protect it from corroding.  
         [0013]     The construction set forth in  FIG. 1  results in two electrodes, base layer  6  and non-magnetic metal layer  14 , that are separated by an insulating barrier, barrier layer  12 . The barrier layer  12  is constructed to be thin enough to allow a tunneling current to flow between the electrodes  6  and  14 . When in operation, and depending upon the direction of current flow, current collects on the base layer  6 , tunnels through the barrier layer  12  and is received by the non-magnetic metal layer  14 . The resistance value of the resistive stack  2  depends upon the surface area of the electrodes  6  and  14 , the thickness of the barrier layer  12  and the extent of the oxidation of the barrier layer  12 . Thus, the resistance of the resistive stack  2  can be made extremely large without a corresponding increase in the size of the resistive stack  2 . Therefore, the embodiment shown in  FIG. 1  overcomes the prior art size problems associated with constructing a resistor having a large resistance value.  
         [0014]     The benefits of using a seed layer  10  of CoFe in conjunction with the resistive structure of  FIG. 1  can readily be seen by referring to FIGS.  2 ( a ) and  2 ( b ) and FIGS.  3 ( a ) and  3 ( b ). As depicted in FIGS.  2 ( a ) and  2 ( b ), resistive stacks produced in accordance with preferred embodiments of the present invention have relatively high resistance values in relation to their small size and can be produced within relatively tight tolerances. More particularly,  FIG. 2 ( a ) shows the resistance value  20  in ohms for five resistive junctions  22  created from a resistive stack consisting of 150 Å TaN/50 Å Ta/20 Å CoFe/15 Å Al, oxidized/100 Å Al/100 Å TaN/100 Å Ru. All of the resistive junctions  20  have resistance values of approximately 5×10 5  Ohms. Thus, resistors can be manufactured from the type of resistive stack shown in  FIG. 2 ( a ) that have a predictable resistance value within a narrow range of values. In  FIG. 2 ( b ), the 100 Å layer of Al in  FIG. 2 ( a ) has been replaced with a 100 Å layer of Ta. Although the resistance values  24  of the resistive junctions  26  shown in  FIG. 2 ( b ) are different than those of  FIG. 2 ( a ), they also display a low variation in resistance values. Thus, resistors can be designed to have a particular resistance value with a low deviation from their desired resistance value in accordance with preferred embodiments of the present invention.  
         [0015]     The use of the seed layer in the resistive stacks shown in FIGS.  2 ( a ) and  2 ( b ) produces a more uniform barrier layer and, thus, limits the variance in the resistance values of resistors produced in accordance with preferred embodiments of the present invention. When the deposition of the seed layer is omitted from the manufacturing process, the resistance values of the resistors created varies to a much greater degree. For example, referring now to FIGS.  3 ( a ) and  3 ( b ), graphs of resistance values for resistive stacks created without the use of a seed layer are shown. In  FIG. 3 ( a ), the resistance values  36  for a set of resistive junctions  38  similar to those shown in  FIG. 2 ( b ), except without the seed layer of CoFe, are shown. In  FIG. 3 ( a ), it can be seen that the resistance values  36  vary to a much greater degree than those of the resistive junctions  26  shown in  FIG. 2 ( b ). Thus, the seed layer of CoFe significantly diminishes the variance in the resistance values  36 . Similarly, the resistive junctions  42  shown in  FIG. 3 ( b ) correspond to those of  FIG. 2 ( a ) without the seed layer of CoFe. The resistance values  40  of the resistive junctions  42  again display a greater variance in values. Thus, the resistive stacks constructed without a seed layer have a resistance value that is less predictable. Therefore, the use of a seed layer when constructing a resistive stack is a substantial improvement upon the prior art.  
         [0016]     The resistive stacks produced in accordance with the present invention are easily scaled to produce resistors having different resistance values. The resistance of the stacks for a given unit area is substantially constant. Thus, the resistance of the resistive stack may be predictably modified by simply modifying the area of the resistive stack. This is preferably accomplished by masking the resistive stack and etching away the unmasked areas to produce a patterned resistive stack having a predetermined area. Since the resistance of a particular resistive stack is primarily dependent upon its area, almost any desired resistance can be achieved by simply altering the dimensions of the resistive stack. Thus, the preferred embodiments of the present invention are well suited for use in semiconductor chips that require resistors having varying resistances.  
         [0017]     Referring now to  FIG. 4 , a flow chart of a preferred method of constructing a resistive stack is set forth. The method commences with the deposition of a base layer of TaN on top of a metal line as shown in block  100 . A smoothing layer of Ta is then deposited on top of the base layer in block  102 . The base layer and smoothing layer create a lower electrode for the resistive stack. In block  104 , a seed layer of CoFe is deposited on the smoothing layer of Ta. A thin layer of Al or a similar metal is then deposited on the seed layer in block  106 . In block  108 , this thin layer of Al is oxidized to produce a barrier layer. The final resistance of the resistive stack will depend upon the thickness of the barrier layer, the oxidation time of the barrier layer and the overall dimensions of the resistive stack. The smoothing layer of Ta provides a good surface for deposition of the seed layer and substantially limits the variance in resistance values that may result from irregularities in the surface of the base layer and, thus, the thickness and uniformity of the barrier layer. A non-magnetic metal layer is deposited over the barrier layer in block  110 . This non-magnetic metal layer creates an upper electrode for the resistive stack. Although any suitable non-magnetic metal could be used, Ta and Al have proven to be particularly well suited. Finally, a protective layer is deposited over the non-magnetic metal layer as set forth in block  112 .  
         [0018]     The present disclosure includes that contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention.