# Patent

## Title
Method and apparatus for flatband voltage tuning of high-k field effect transistors

## Abstract

In one embodiment, the invention is a method and apparatus for flatband voltage tuning of high-k field effect transistors. One embodiment of a field effect transistor includes a substrate, a high-k dielectric layer deposited on the substrate, a gate electrode deposited on the high-k dielectric layer, and a dipole layer positioned between the substrate and the gate electrode, for shifting the threshold voltage of the field effect transistor.

## Background
BACKGROUND OF THE INVENTION
The present invention relates generally to high-k field effect transistors, and relates more particularly to flatband voltage tuning of high-k field effect transistors.
Flatband voltage is the voltage at which there is no electrical charge in a semiconductor and, therefore, no voltage drop across the semiconductor (i.e., in a band diagram the energy bands of the semiconductor are horizontal (flat)). Flatband voltages of high-k based p-type field effect transistors (pFETs) and n-type field effect transistors (nFETs) are difficult to control.
For instance, in conventional metal-oxide-semiconductor FETs (MOSFETs), the threshold voltage often fails to meet the desired value (e.g., ±0.3 V for both pFETs and nFETs). In some cases, electrode stacks are chosen according to workfunction to provide the desired flatbands, but this often does not work. Moreover, threshold voltage shifts can vary significantly from desired values, and the causes for such shifts, which may be numerous, are not fully understood.
Thus, there is a need in the art for a method and apparatus for flatband voltage tuning of high-k field effect transistors.
SUMMARY OF THE INVENTION
In one embodiment, the invention is a method and apparatus for flatband voltage tuning of high-k field effect transistors. One embodiment of a field effect transistor includes a substrate, a high-k dielectric layer deposited on the substrate, a gate electrode deposited on the high-k dielectric layer, and a dipole layer positioned between the substrate and the gate electrode, for shifting the threshold voltage of the field effect transistor.



BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG. 1 is a schematic diagram illustrating a field effect transistor component of a complementary metal-oxide-semiconductor device, fabricated in accordance with a first embodiment of the present invention; and
FIG. 2 is a schematic diagram illustrating a field effect transistor component of a complementary metal-oxide-semiconductor device, fabricated in accordance with a second embodiment of the present invention.



DETAILED DESCRIPTION
In one embodiment, the present invention is a method and apparatus for flatband voltage tuning of high-k field effect transistors (FETs). Embodiments of the invention introduce a dipole layer in the gate stack of a FET for threshold tuning. The dipole layer achieves voltage tuning in accordance with a simple rule based upon the electronegativity differences of the metal, oxygen (or nitrogen), and high-k dielectric components of the FET.
FIG. 1 is a schematic diagram illustrating a field effect transistor (FET) component 100 of a complementary metal-oxide-semiconductor (CMOS) device, fabricated in accordance with a first embodiment of the present invention. Specifically, the FET 100 is an nFET.
The FET 100 comprises a silicon substrate layer 102, a high-k dielectric layer 104, a dipole layer 106, and a gate electrode 108.
In one embodiment, the silicon substrate layer 102 includes a chemical oxidation (CHEMOX) layer deposited thereon. In one embodiment, the CHEMOX layer has a thickness of approximately 0.5 nm.
The high-k dielectric layer 104 is deposited on the silicon substrate layer 102. In one embodiment, the high-k dielectric layer 104 comprises hafnium oxide (HfO2) or hafnium silicate (HfSiO). In one embodiment, the high-k dielectric layer 104 has a thickness of approximately two to approximately three nm.
The dipole layer 106 is deposited on the high-k dielectric layer 104. In one embodiment, the dipole layer 106 comprises an insulating material, such as a metal oxide or a metal nitride. As explained in greater detail below, the material that forms the dipole layer 106 is chosen to shift the threshold voltage of the FET 100 in a desired direction. In one embodiment, the dipole layer 106 has a thickness of approximately 0.3 to approximately two nm.
The gate electrode 108 is deposited on the dipole layer 106. In one embodiment, the gate electrode 108 comprises titanium nitride (TiN) having a layer of polysilicon deposited thereon. In an alternative embodiment, the gate electrode 108 comprises straight polysilicon.
Although the dipole layer 106 is illustrated as being positioned between the high-k dielectric layer 104 and the gate electrode 108, the dipole layer 106 may be positioned anywhere between the silicon channel and the point where gate bias is applied.
The dipole layer 106 functions as an electronegativity adjustment layer, defined by the shift parameter, γ. The shift parameter, γ, is further defined as:




  
    
      
        γ
        =
        
          
          
            
              
                χ
                g
              
              -
              
                χ
                d
              
            
            
              
                χ
                
                  h
                  -
                  k
                
              
              -
              
                χ
                d
              
            
          
          
        
      
    
    
      
        (
        
          EQN
          .
          
            
          
          ⁢
          1
        
        )
      
    
  




where χg is the electronegativity of the gate electrode 108, χd is the electronegativity of the dipole layer 106, and χh-f is the electronegativity of the high-k dielectric layer 104.

For the dipole layer 106 of the nFET 100, the metal oxide or metal nitride comprising the dipole layer 106 satisfies the condition γ>1. As illustrated by the arrow Vfb in FIG. 1, this results in the threshold voltage being shifted to a negative voltage.
FIG. 2 is a schematic diagram illustrating a field effect transistor (FET) component 200 of a complementary metal-oxide-semiconductor (CMOS) device, fabricated in accordance with a second embodiment of the present invention. Specifically, the FET 200 is a pFET.
The FET 200 comprises a silicon substrate layer 202, a high-k dielectric layer 204, a dipole layer 206, and a gate electrode 208.
In one embodiment, the silicon substrate layer 202 includes a chemical oxidation (CHEMOX) layer deposited thereon. In one embodiment, the CHEMOX layer has a thickness of approximately 0.5 nm.
The high-k dielectric layer 204 is deposited on the silicon substrate layer 202. In one embodiment, the high-k dielectric layer 204 comprises hafnium oxide (HfO2) or hafnium silicate (HfSiO). In one embodiment, the high-k dielectric layer 204 has a thickness of approximately two to approximately three nm.
The dipole layer 206 is deposited on the high-k dielectric layer 204. In one embodiment, the dipole layer 206 comprises an insulating material, such as a metal oxide or a metal nitride. As above, the material that forms the dipole layer 206 is chosen to shift the threshold voltage of the FET 200 in a desired direction. In one embodiment, the dipole layer 206 has a thickness of approximately 0.3 to approximately two nm.
The gate electrode 208 is deposited on the dipole layer 206. In one embodiment, the gate electrode 208 comprises titanium nitride (TiN) having a layer of polysilicon deposited thereon. In an alternative embodiment, the gate electrode 208 comprises straight polysilicon.
Although the dipole layer 206 is illustrated as being positioned between the high-k dielectric layer 204 and the gate electrode 208, the dipole layer 206 may be positioned anywhere between the silicon channel and the point where gate bias is applied.
For the dipole layer 206 of the pFET 200, the metal oxide or metal nitride comprising the dipole layer 206 satisfies the condition γ<1. In contrast to the dipole layer 106 illustrated in FIG. 1 (for which γ>1), this results in the threshold voltage being shifted to a positive voltage, as illustrated by the arrow Vfb in FIG. 2.
Thus, the general rule with respect to the dipole layers 106 and 206 is that for metal oxides and metal nitrides that satisfy the condition γ>1, the threshold voltage will shift to negative voltages. These materials will therefore be used in nFET devices. For metal oxides and metal nitrides that satisfy the condition γ<1, the threshold voltage will shift to positive voltages. These materials will therefore be used in pFET devices.
Table 1 illustrates a list of metal oxides (and their associated electronegativities) that will satisfy this rule. In one embodiment, a FET according to the present invention uses any one or more of the metal oxides listed in Table 1 in the dipole layer.












TABLE 1





Electronegativity







(Pauling)
0.9-1.1
1.1-1.2
1.3-1.4
1.5
1.6-1.8





















SrO2
Y2O3
HfO2
Al2O3
V2O3



La2O3
CeO2
Sc2O3
AlN
NbO2



LaN
MgO
ZrO2
Ta2O5
ZnO



CaO
Pr2O3

TiO2
Ga2O3



BaO
SmO2

TiN
GeO2




Eu2O3

MnO2
SiO2




Gd2O3


RuO2




Er2O3




Yb2O3










The materials in the first two columns of Table 1 either have experimentally demonstrated or are expected to demonstrate negative flatband shifts, and therefore may be used in nFET devices in accordance with the present invention. Some of the materials in the fourth and fifth columns (i.e., Al2O3, AlN, Ta2O5, MnO2, V2O3, NbO2, GeO2, and SiO2) either have experimentally demonstrated or are expected to demonstrate positive flatband shifts, and therefore may be used in pFET devices in accordance with the present invention. Metal nitrides are expected to behave similarly if insulating.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. Various embodiments presented herein, or portions thereof, may be combined to create further embodiments. Furthermore, terms such as top, side, bottom, front, back, and the like are relative or positional terms and are used with respect to the exemplary embodiments illustrated in the figures, and as such these terms may be interchangeable.

## Claims

1. A field effect transistor, comprising:
a substrate;
a high-k dielectric layer deposited on the substrate;
a gate electrode deposited on the high-k dielectric layer; and
a dipole layer positioned directly between the high-k dielectric layer and the gate electrode, for shifting a threshold voltage of the field effect transistor.

2. The field effect transistor of claim 1, wherein a material from which the dipole layer is formed comprises titanium oxide.

3. The field effect transistor of claim 1, wherein a material from which the dipole layer is formed comprises titanium nitride.

4. The field effect transistor of claim 1, wherein the dipole layer comprises a metal oxide or a metal nitride.

5. The field effect transistor of claim 1, wherein a material from which the dipole layer is formed is selected based on a desired direction of threshold voltage shift.

6. The field effect transistor of claim 1, wherein a material from which the dipole layer is formed comprises: hafnium oxide, scandium oxide, zirconium oxide, zinc oxide, gallium oxide, or ruthenium oxide.

7. The field effect transistor of claim 1, wherein the dipole layer has a thickness of approximately 0.3 to approximately two nm.

8. The field effect transistor of claim 1, wherein the dipole layer has an associated shift parameter defined by electronegativities of the high-k dielectric layer, the gate electrode, and the dipole layer, wherein the shift parameter defines a direction in which the threshold voltage will shift.

9. The field effect transistor of claim 8, wherein the shift parameter is approximately equal to a first quantity divided by a second quantity, the first quantity comprising an electronegativity of the gate electrode minus an electronegativity of the dipole layer, and the second quantity comprising an electronegativity of the high-k dielectric layer minus the electronegativity of the dipole layer.

10. The field effect transistor of claim 8, wherein the shift parameter is greater than one.

11. The field effect transistor of claim 10, wherein a material from which the dipole layer is formed comprises lanthanum oxide.

12. The field effect transistor of claim 10, wherein a material from which the dipole layer is formed comprises lanthanum nitride.

13. The field effect transistor of claim 10, wherein the field effect transistor is an n-type field effect transistor.

14. The field effect transistor of claim 10, wherein a material from which the dipole layer is formed comprises: strontium oxide, calcium oxide, barium oxide, yttrium oxide, cerium oxide, magnesium oxide, praseodymium oxide, samarium oxide, europium oxide, gadolinium oxide, erbium oxide, or ytterbium oxide.

15. The field effect transistor of claim 8, wherein the shift parameter is less than one.

16. The field effect transistor of claim 15, wherein a material from which the dipole layer is formed comprises aluminum oxide.

17. The field effect transistor of claim 15, wherein a material from which the dipole layer is formed comprises aluminum nitride.

18. The field effect transistor of claim 15, wherein the field effect transistor is a p-type field effect transistor.

19. The field effect transistor of claim 15, wherein a material from which the dipole layer is formed comprises: tantalum oxide, manganese oxide, vanadium oxide, niobium oxide, germanium oxide, or silicon oxide.