Abstract:
A passive device structure includes an unpatterned metal gate layer formed in a passive device region of a semiconductor device; an insulator layer formed upon the unpatterned metal gate layer; a semiconductor layer formed upon the insulator layer; and one or more metal contact regions formed in the semiconductor layer; wherein the insulator layer prevents the metal gate layer as serving as a leakage current path for current flowing through a passive device defined by the semiconductor layer and the one or more metal contact regions.

Description:
BACKGROUND 
     The present invention relates generally to semiconductor device manufacturing and, more particularly, to improved integration of passive device structures with metal gate layers. 
     In standard complementary metal oxide semiconductor (CMOS) devices, polysilicon is typically used as the standard gate material. The technology of fabricating CMOS devices using polysilicon gates has been in a constant state of development, and is now widely used in the semiconductor industry. One advantage of using polysilicon gates is that they can sustain high temperatures. However, there are also some problems associated with using a polysilicon gate. For example, due to the poly-depletion effect, polysilicon gates commonly used in CMOS devices are becoming a gating factor in chip performance for channel lengths of 0.1 micron and below. Another problem with polysilicon gates is that the dopant material in the polysilicon gate (e.g., boron) can easily diffuse through the thin gate dielectric causing further degradation of the device performance. Thus, one proposed way of improving the performance of sub-micron transistors is to use metal gates in place of conventional polysilicon gates, particularly with the advent of high-k gate dielectric materials. 
     In addition to transistor devices, other types of devices are also formed on integrated circuits. For example, certain passive device structures such as resistors and eFUSEs are formed above the substrate level. In particular, where such devices are integrated with metal gate technology, a silicon layer is formed on top of the metal gate layer (used in forming the metal gates in the transistor region). Thus, while the metal gate layer located in the transistor or active device regions is patterned according to a desired gate structure, the other portions of the metal gate layer residing in the passive device regions remain unpatterned and permanently reside below the passive device structures. 
     SUMMARY 
     In an exemplary embodiment, a passive device structure includes an unpatterned metal gate layer formed in a passive device region of a semiconductor device; an insulator layer formed upon the unpatterned metal gate layer; a semiconductor layer formed upon the insulator layer; and one or more metal contact regions formed in the semiconductor layer; wherein the insulator layer prevents the metal gate layer as serving as a leakage current path for current flowing through a passive device defined by the semiconductor layer and the one or more metal contact regions. 
     In another embodiment, a method of forming a passive device structure, includes forming an unpatterned metal gate layer in a passive device region of a semiconductor device; forming an insulator layer upon the unpatterned metal gate layer; forming a semiconductor layer upon the insulator layer; and forming one or more metal contact regions in the semiconductor layer; wherein the insulator layer prevents the metal gate layer as serving as a leakage current path for current flowing through a passive device defined by the semiconductor layer and the one or more metal contact regions. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  is a cross-sectional view of a cross-sectional view of an existing passive device structure formed over a metal gate layer; 
         FIG. 2(   a ) is a cross-sectional view of another existing passive device structure formed over a metal gate layer; 
         FIG. 2(   b ) is a top view of the passive device structure shown in FIG.  2 ( a ); 
         FIG. 2(   c ) is a cross-sectional view of the passive device structure of  FIG. 2(   a ) in a programmed state; 
         FIG. 2(   d ) is a top view of the passive device structure shown in  FIG. 2(   c ); 
         FIG. 3  is a cross-sectional view of a passive device structure formed over a metal gate layer, in accordance with an embodiment of the invention; and 
         FIGS. 4(   a ) and  4 ( b ) are cross-sectional views of a passive device structure formed over a metal gate layer, in accordance with another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With the presence of a metal gate under a semiconductor (e.g., silicon) layer, the resulting resistor in passive device structures in effect becomes a parallel connection of the silicon resistance (R si ) and the metal gate resistance (R metal ), which lowers the resistance significantly. Moreover, changing the doping concentration in the silicon layer does not modulate the overall resistance to a substantial degree, since metal is much more conductive and dominates the total resistance. For example, the resistance of titanium nitride (TiN) is about 200 Ω/square, whereas passive devices require a target higher resistance (e.g., about 350 Ω/square) for 45 nm technology. 
     Accordingly, disclosed herein is a structure and method for improved integration of passive device structures with metal gate layers. In brief, an insulating layer such as aluminum oxide (Al 2 O 3 ) is formed between the metal gate layer and the silicon layer, thus effectively preventing current from flowing from the passive device layer (e.g., silicon) into the metal gate layer and undesirably decreasing the passive device resistance. In turn the characteristics of the passive structure (e.g., resistor, eFUSE, etc.) may be determined by the silicon (semiconductor) layer, and not the combination of silicon and a metal layer therebeneath. 
     Referring initially to  FIG. 1 , there is shown a cross-sectional view of an existing passive device structure  100  formed over a metal gate layer  102 . Here, the passive device structure  100  is a resistor formed in a semiconductor (e.g., polysilicon) layer  104  that may be doped in accordance with the desired characteristics (e.g., resistance value) of the device. Metal contact regions  106  (e.g., silicide) define opposing terminals of the resistor. 
     Ideally, the resistance of the doped semiconductor layer  104  dominates the total resistance of the passive device  100 . However, as further shown in  FIG. 1 , in addition to the current paths from one metal contact region  106  to another through the semiconductor layer  104 , there is also another current path substantially vertically down through the semiconductor layer  104 , through the metal gate layer  102 , back up through the semiconductor layer  104  and to the other metal contact region  106 . As such, the total effective resistance of the passive device  100  becomes a parallel connection of the semiconductor layer resistance (R si ) and the metal gate resistance (R metal ). 
     Referring now to  FIGS. 2(   a ) and  2 ( b ),  FIG. 2(   a ) is a cross-sectional view of another existing passive device structure  200  formed over a metal gate layer  202 , while  FIG. 2(   b ) is a top view thereof In this example, the passive device structure  200  is an eFUSE formed in a semiconductor (e.g., polysilicon) layer  204  and having a top metal strip  206  (e.g., silicide) characterized by wide terminal portions  208  and a thin neck portion  210  connecting the terminal portions  208 . In one example, an unprogrammed or low-resistance state of the structure  200  leaves the top metal strip portion of the device intact, and thus the presence of the metal gate layer  202  does not have as much of an impact on the desired device resistance as was the case for  FIG. 1 . 
     On the other hand, in a programmed or “blown” state of the eFUSE device  200  as shown in  FIGS. 2(   c ) and  2 ( d ), a void  212  or hole is intentionally created in the neck portion  210  of the strip  206  (such as by laser ablation or passing a high current therethrough) in order to set the eFUSE to a high-resistance state. By severing the metal strip connection between the end terminals, the device  200  is ideally in a high-resistance state, where a small amount leakage current passes through the undoped semiconductor layer  204 . However, as further shown in  FIG. 2(   c ), the metal gate layer  202  once again provides a relatively low resistance parallel current path that can adversely affect desired device resistance characteristics. 
     Referring now to  FIG. 3 , there is shown a cross-sectional view of a passive device structure  300  formed over a metal gate layer  302 , in accordance with an embodiment of the invention. In the embodiment shown, an electrically insulating layer  303  is formed over the metal gate layer  302  prior to deposition of the semiconductor layer  304 . After formation of the insulator layer  303 , the passive device (e.g., a resistor) is formed as known in the art such as by appropriate doping of the semiconductor layer  304  and silicide contact  306  formation to define opposing terminals of the passive device structure  300 . As the passive device structure  300  is formed in a passive region of a semiconductor device, the metal gate layer  302  remains unpatterned therein. 
     The insulator layer  303  may be an oxide layer or any suitable layer that acts as a barrier for electrical current from flowing through semiconductor layer  304  into the metal gate layer  302  and lowering the desired resistance of the device  300 . Exemplary suitable materials for the insulator layer  303  include, but are not limited to, aluminum oxide, zirconium oxide, hafnium oxide, hafnium silicate, silicon oxide, silicon nitride, lanthanum oxide, and combinations thereof Further, the insulator layer  303  may be formed by any suitable techniques such as physical vapor deposition (PLD), atomic layer deposition (ALD) and the like. 
     An exemplary thickness for the insulator layer  303  may be on the order of about 10 to about 200 nanometers (nm), and more specifically about 10 to about 100 nm. In one specific example, a 12-angstrom (Å) layer of aluminum oxide (Al 2 O 3 ) is shown to effectively block an amorphous silicon/metal gate layer conductive path and provide a resistance of about 6×10 4  Ω/square in the absence of doping in the silicon layer. Accordingly, specific resistance targets for passive devices may now be easily achieved through appropriate semiconductor layer doping, without adverse effects due to the now more common metal gate technology in CMOS devices. 
     The effectiveness of the insulator  303  is also schematically shown in  FIG. 3 , where it is seen that no substantial leakage current path from the semiconductor layer  304  is allowed to pass through the metal gate layer  302 . Finally,  FIGS. 4(   a ) and  4 ( b ) are cross-sectional views of a passive device structure  400  formed over a metal gate layer  402 , in accordance with an embodiment of the invention. In this embodiment, the insulator layer  403  is formed over the metal gate layer  402  prior to deposition of the semiconductor layer  404  and formation of the passive device structure  406 , which in this example is an eFUSE. In  FIG. 4(   a ), the device  400  is in an unprogrammed state, whereas in  FIG. 4(   b ), the device  400  is in a programmed or “blown” state. In either instance, it will be seen that no substantial leakage current path from the semiconductor layer  404  is allowed to pass through the metal gate layer  402 . 
     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.