Gate electrode with depletion suppression and tunable workfunction

Abstract

Semiconductor device ( 100 ) performance is improved via a gate structure ( 120 ) having a tunable effective workfunction and reduced gate depletion effects. According to an example embodiment of the present invention, the design threshold voltage of a semiconductor device ( 100 ) is adjusted in a manner that includes providing a gate having a workfunction that enables operation of the semiconductor device ( 100 ) at a selected voltage. The gate is formed having two different conductive materials ( 130, 135 ) with different electric workfunctions that both significantly contribute to the overall workfunction of the gate. The relative composition, thickness, and arrangement of each of the two conductive material is selected to attain a gate electrode workfunction that is different than the workfunctions of each of the two layers and that sets the threshold voltage of the semiconductor device ( 100 ). In addition, by selecting the order of the layers, carrier depletion in the gate electrode can be avoided. The adjustability of the effective workfunction of the gate electrode can be applied to a variety of semiconductor devices, including any modem MOS transistor, providing independent adjustment of the channel doping, semiconductor alloy composition, and the threshold voltage of the device, thus enabling improved performance. The ability to reduce gate depletion effects also provides enhanced device current drive.

What is claimed is:  
     
         1 . A semiconductor device having a gate electrode workfunction, the semiconductor device comprising a gate electrode structure including first and second conductive materials having respective workfunctions contributing to, and being different than, the gate electrode workfunction.  
     
     
         2 . The semiconductor device of  claim 1 , wherein the semiconductor device is a MOS device having a dielectric between the gate electrode structure and a substrate.  
     
     
         3 . The semiconductor device of  claim 2 , wherein each of the first and second conductive materials, respectively, has a composition and thickness adapted to define a voltage threshold for the MOS device.  
     
     
         4 . A semiconductor device having a gate electrode workfunction and a threshold voltage, the semiconductor device comprising a gate electrode structure including lower and upper conductive materials having respective workfunctions contributing to, and being different than, the gate electrode workfunction, the lower conductive material being between the upper conductive material and a dielectric and having a thickness and composition that significantly contribute to but do not result in the threshold voltage being negligibly contributed to by the upper conductive material.  
     
     
         5 . A semiconductor device comprising: 
 a gate structure having a gate dielectric over a substrate and, over the gate dielectric, an electrode comprising a first conductive material with a first selected thickness and a second conductive material formed over the first gate conductive material and having a second selected thickness, the first and second conductive materials having different workfunctions; and    the first and second conductive materials being configured and arranged with respective compositions and thicknesses that, in response to a voltage applied to the electrode, cause the gate electrode structure to exhibit a workfunction that is different than the workfunction of the first conductive material and different than the workfunction of the second conductive material.    
     
     
         6 . The device of  claim 5 , wherein the electrode exhibits a workfunction that is between the respective workfunctions of the first and second conductive materials and differs from one of said respective workfunctions by between about 5% and 10% of the difference between said respective workfunctions.  
     
     
         7 . The device of  claim 5 , wherein the electrode exhibits a workfunction that is between the respective workfunctions of the first and second conductive materials and differs from one of said respective workfunctions by between 10% and 15% of the difference between said respective workfunctions.  
     
     
         8 . The device of  claim 5 , wherein the electrode exhibits a workfunction that is between the respective workfunctions of the first and second conductive materials and differs from one of said respective workfunctions by at least 15% of the difference between said respective workfunctions.  
     
     
         9 . The device of  claim 5 , wherein the semiconductor device exhibits an effective threshold voltage that is between respective threshold voltages that would be exhibited by the semiconductor device respectively having the first conductive material dominating the workfunction and having the second conductive material dominating the workfunction, and wherein the effective threshold voltage of the semiconductor device differs from one of the respective threshold voltages by between about 5% and 10% of the difference between the respective threshold voltages.  
     
     
         10 . The device of  claim 5 , wherein the semiconductor device exhibits an effective threshold voltage that is between respective threshold voltages that would be exhibited by the semiconductor device respectively having the first conductive material dominating the workfunction and having the second conductive material dominating the workfunction, and wherein the effective threshold voltage of the semiconductor device differs from one of the respective threshold voltages by between 10% and 15% of the difference between the respective threshold voltages.  
     
     
         11 . The device of  claim 5 , wherein the semiconductor device exhibits an effective threshold voltage that is between respective threshold voltages that would be exhibited by the semiconductor device respectively having the first conductive material dominating the workfunction and having the second conductive material dominating the workfunction, and wherein the effective threshold voltage of the semiconductor device differs from one of the respective threshold voltages by at least 15% of the difference between the respective threshold voltages.  
     
     
         12 . The device of  claim 5 , wherein the semiconductor device exhibits an overall threshold voltage that is defined by a difference between the workfunction of the gate electrode and a workfunction of the substrate, and wherein the overall threshold voltage of the device is adapted to cause a conductive channel to form in the substrate in response to a selected voltage being applied to the electrode.  
     
     
         13 . The device of  claim 5 , wherein the first and second conductive materials are adapted such that the first conductive material is supersaturated with carriers in a manner that inhibits gate depletion effects.  
     
     
         14 . The device of  claim 7 , wherein the gate electrode exhibits a carrier type that is electron dominant, wherein the first conductive material exhibits a workfunction that is higher than the workfunction of the second conductive material and wherein the first conductive material is supersaturated with carriers from the second conductive material.  
     
     
         15 . The device of  claim 7 , wherein the gate electrode exhibits a carrier type that is hole dominant, wherein the first conductive material exhibits a workfunction that is smaller than the workfunction of the second conductive material and wherein the first conductive material is supersaturated with carriers from the second conductive material.  
     
     
         16 . The device of  claim 5 , wherein the first and second conductive materials are adapted to achieve a selected threshold voltage of the semiconductor device while doping in the semiconductor device below the gate dielectric is different than the doping would be for achieving the threshold, absent the multilayer gate structure.  
     
     
         17 . The device of  claim 5 , wherein the first and second conductive materials are adapted to achieve a selected threshold voltage of the semiconductor device while operating the device at temperatures below room temperature.  
     
     
         18 . The device of  claim 5 , wherein the first and second conductive materials are adapted to sufficiently control a threshold voltage of the device without adjusting the dopant concentration below the gate dielectric for the purpose of controlling the threshold voltage of the device.  
     
     
         19 . The device of  claim 5 , wherein carrier mobility in the substrate is greater than the carrier mobility would be, were the substrate subjected to the introduction of dopants to control the threshold voltage of the device.  
     
     
         20 . A method for manufacturing a semiconductor device having a gate electrode structure and a gate electrode workfunction, the method comprising constructing the gate electrode structure with first and second conductive materials having respective workfunctions contributing to, and being different than, the gate electrode workfunction.  
     
     
         21 . The method of  claim 20 , wherein the semiconductor device is a MOS device having a dielectric over a substrate and wherein constructing the gate electrode includes constructing the gate electrode over the dielectric.  
     
     
         22 . The method of  claim 21 , wherein constructing the gate electrode structure includes constructing each of the first and second conductive materials, respectively, with a composition and thickness adapted to define a voltage threshold for the MOS device.  
     
     
         23 . A method for manufacturing a semiconductor device having a gate structure including a gate dielectric over a substrate and an electrode over the gate dielectric, the method comprising: 
 providing a gate electrode workfunction that is defined as a function of the conductive portion of the gate having a first conductive material and a second conductive material, the provided gate electrode workfunction being adapted to cause threshold gate operation of the semiconductor device when a selected gate voltage is applied to the conductive portion;    forming the first conductive material with a first selected thickness over the gate dielectric; and    over the first conductive material, forming the second conductive material with a second selected thickness, wherein the first and second conductive materials are configured and arranged with respective compositions and thicknesses that cause the gate electrode structure to exhibit a workfunction that is different than the workfunction of the first conductive material and different than the workfunction of the second conductive material.    
     
     
         24 . The method of  claim 23 , wherein providing a gate workfunction includes selecting the composition and thickness of each of the first and second conductive materials to cause the gate electrode structure to exhibit, via the combination of the first and second conductive materials, the provided gate workfunction.  
     
     
         25 . The method of  claim 23 , wherein the semiconductor device includes a transistor having a channel region in the substrate below the gate dielectric, wherein providing a gate electrode workfunction comprises: 
 determining a first workfunction of the first conductive material;    determining a second workfunction of the second conductive material;    selecting an operating voltage threshold for the semiconductor device; and    selecting a thickness and arrangement of the first and second conductive materials such that, when the first and second materials are used in combination, the gate electrode exhibits a workfunction between that of the first and second workfunctions that causes the channel to transition between a conducting and non-conducting state when the selected threshold voltage is applied to the conductive portion of the gate electrode.    
     
     
         26 . The method of  claim 23 , wherein providing a gate workfunction includes selecting the thickness of the first conductive layer to selectively tune the gate threshold voltage.  
     
     
         27 . The method of  claim 23 , wherein forming the first conductive material includes forming the first material having a thickness of between about 1 and 500 Angstroms.  
     
     
         28 . The method of  claim 27 , wherein forming the second conductive material includes forming a material having a thickness larger than that of the first conductive material.  
     
     
         29 . The method of  claim 23 , wherein forming a first conductive material includes forming a conductive material having a higher workfunction than the workfunction of the second conductive material in response to the gate electrode exhibiting an electron-dominant carrier type.  
     
     
         30 . The method of  claim 23 , wherein forming the first conductive material includes forming a conductive material having a lower workfunction than the workfunction of the second conductive material in response to the gate electrode exhibiting a hole-dominant carrier type.  
     
     
         31 . The method of  claim 23 , wherein providing a gate workfunction includes determining the workfunction as a function of material in a channel in the substrate over which the electrode is to be formed.  
     
     
         32 . The method of  claim 31 , further comprising repeating the steps of providing a gate workfunction, forming the first conductive material and forming the second conductive material, thereby forming a plurality of semiconductor devices, at least two of the devices exhibiting different gate workfunctions, the gate workfunction for each device being selected to cause each of the plurality of devices to undergo threshold gate operation at the selected gate voltage.  
     
     
         33 . The method of  claim 32 , wherein at least one of the first conductive material and the second conductive material in a first one of the plurality of devices is different from at least one of the first and second conductive materials, respectively, in a second one of the plurality of devices.  
     
     
         34 . The method of  claim 32 , wherein determining the workfunction as a function of material in a channel of the device includes determining a workfunction that causes the channel to conduct electricity when a selected voltage is applied to the conductive portion.  
     
     
         35 . The method of  claim 23 , wherein forming the first layer includes supersaturating the first layer with electrons.  
     
     
         36 . The method of  claim 23 , wherein forming the first and second conductive materials includes forming a conductive structure that exhibits a selected gate operating threshold while maintaining a higher carrier mobility in the device than the device would exhibit, absent the combination of the first and second layers.  
     
     
         37 . The method of  claim 23 , wherein forming the first and second conductive materials includes forming first and second conductive materials having different carrier concentrations.  
     
     
         38 . The method of  claim 23 , wherein forming the first conductive material includes forming a thin layer using atomic layer deposition (ALD) to form the first conductive material having a thickness of about one atomic layer.  
     
     
         39 . The method of  claim 23 , wherein forming the first conductive material includes forming a thin layer using molecular beam epitaxy (MBE) to form the first conductive material having a thickness of about one atom.  
     
     
         40 . A system for manufacturing a semiconductor device having a gate electrode structure with a conductive portion over a gate dielectric that is over a substrate, the system comprising: 
 means for providing a gate electrode workfunction that is defined as a function of the conductive portion of the gate having a first conductive material and a second conductive material, the provided gate electrode workfunction being adapted to cause threshold gate operation of the semiconductor device when a selected gate voltage is applied to the conductive portion;    means for forming a first conductive material with a first selected thickness over the gate dielectric; and    means for forming, over the first conductive material, a second conductive material with a second selected thickness, the first and second conductive materials forming a portion of the conductive portion, the composition and thickness of each of the first and second conductive materials being selected and formed so the gate electrode structure exhibits a workfunction that is different than the workfunction of the first conductive material and different than the workfunction of the second conductive material.    
     
     
         41 . A system for manufacturing a semiconductor device having a gate electrode structure with a conductive portion over a gate dielectric that is over a substrate, the system comprising: 
 a control mechanism adapted to provide a gate electrode workfunction that is defined as a function of the conductive portion of the gate having a first conductive material and a second conductive material, the provided gate electrode workfunction being adapted to cause threshold gate operation of the semiconductor device when a selected gate voltage is applied to the conductive portion;    a formation arrangement adapted to form a first conductive material with a first selected thickness over the gate dielectric; and    a formation arrangement adapted to form, over the first conductive material, a second conductive material with a second selected thickness, the first and second conductive materials forming a portion of the conductive portion, the composition and thickness of each of the first and second conductive materials being selected and formed so the gate electrode structure exhibits a workfunction that is different than the workfunction of the first conductive material and different than the workfunction of the second conductive material.    
     
     
         42 . A semiconductor device comprising a gate electrode having conductive upper and lower layers, wherein the upper layer influences a workfunction exhibited by the gate electrode.