Method for rapid estimation of layout-dependent threshold voltage variation in a mosfet array

Abstract

An automated method for estimating layout-induced variations in threshold voltage in an integrated circuit layout. The method begins with the steps of selecting a diffusion area within the layout for analysis. Then, the system identifies Si/STI edges on the selected area as well as channel areas and their associated gate/Si edges. Next, the threshold voltage variations in each identified channel area are identified, which requires further steps of calculating threshold voltage variations due to effects in a longitudinal direction; calculating threshold voltage variations due to effects in a transverse direction; and combining the longitudinal and transverse variations to provide an overall variation. Finally, a total variation is determined by combining variations from individual channel variations.

We claim as follows: 
     
         1 . An automated method for estimating layout-induced variations in threshold voltage in an integrated circuit layout, comprising the steps of:
 selecting a diffusion area within the layout for analysis;   identifying STI edges on the selected area;   identifying channel areas in the selected area; and   for each given channel area identified in the step of identifying:
 using a computer, estimating threshold voltage variations due at least to Transient Enhanced Diffusion effects in the given channel area, in dependence upon distances between a point in the given channel area and the STI edges on the selected area; and 
 using a computer, combining the threshold voltage variations estimated in the step of estimating threshold voltage variations due at least to Transient Enhanced Diffusion effects in the given channel area. 
   
     
     
         2 . A method according to  claim 1 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a member of the group consisting of:
 a decay function of the form
   λ i ( r )=1/(( r/α   i )β i+ε   i ); and
 
   a decay function having the form of the error function, erf(r); and   a decay function having the form of the complimentary error function, erfc(r),   where α i , β i , and ε i , are process and material-related factors, and r is a distance between the point in the given channel area and an STI edge.   
     
     
         3 . A method according to  claim 1 , wherein the step of estimating threshold voltage variations comprises the steps of:
 calculating threshold voltage variations due at least to Transient Enhanced Diffusion effects in a longitudinal direction, in dependence upon distances between a point in the given channel area and transversely oriented STI edges on the selected area; and   calculating threshold voltage variations due at least to Transient Enhanced Diffusion effects in a transverse direction, in dependence upon distances between a point in the given channel area and longitudinally oriented STI edges on the selected area.   
     
     
         4 . A method according to  claim 1 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a decay function which decreases with greater distance from the point, the decay function being dependent further upon an amount of crystal lattice damage introduced by source/drain implantation. 
     
     
         5 . A method according to  claim 1 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a decay function which decreases with greater distance from the point, the decay function being dependent further upon a halo implant energy. 
     
     
         6 . A method according to  claim 5 , wherein the decay function is dependent further upon a thermal budget for annealing. 
     
     
         7 . A system for automated estimation of layout-induced variations in threshold voltage in an integrated circuit layout, comprising:
 a digital computer, including a processor and data storage means storing a computer program configured to perform the steps of:   selecting a diffusion area within the layout for analysis;   identifying STI edges on the selected area;   identifying channel areas in the selected area; and   for each given channel area identified in the step of identifying:   estimating threshold voltage variations due at least to Transient Enhanced Diffusion effects in the given channel area, in dependence upon distances between a point in the given channel area and the STI edges on the selected area; and   combining the threshold voltage variations estimated in the step of estimating threshold voltage variations due at least to Transient Enhanced Diffusion effects in the given channel area, in dependence upon distances between a second point in the given channel area and the STI edges on the selected area.   
     
     
         8 . A system according to  claim 7 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a member of the group consisting of:
 a decay function of the form
   λ i ( r )=1/(( r/α   i )β i +ε i ); and
 
   a decay function having the form of the error function, erf(r); and   a decay function having the form of the complimentary error function, erfc(r),   where α i , β i  and ε i , are process and material-related factors, and r is a distance between the point in the given channel area and an STI edge.   
     
     
         9 . A system according to  claim 7 , wherein the step of estimating threshold voltage variations comprises the steps of:
 calculating threshold voltage variations due at least to Transient Enhanced Diffusion effects in a longitudinal direction, in dependence upon distances between a point in the given channel area and transversely oriented STI edges on the selected area; and   calculating threshold voltage variations due at least to Transient Enhanced Diffusion effects in a transverse direction, in dependence upon distances between a point in the given channel area and longitudinally oriented STI edges on the selected area.   
     
     
         10 . A system according to  claim 7 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a decay function which decreases with greater distance from the point, the decay function being dependent further upon an amount of crystal lattice damage introduced by source/drain implantation. 
     
     
         11 . A system according to  claim 7 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a decay function which decreases with greater distance from the point, the decay function being dependent further upon a halo implant energy. 
     
     
         12 . A system according to  claim 11 , wherein the decay function is dependent further upon a thermal budget for annealing. 
     
     
         13 . Data storage means for use with a digital computer having a processor, the data storage means having stored thereon a computer program configured to perform the steps of:
 selecting a diffusion area within the layout for analysis;   identifying STI edges on the selected area;   identifying channel areas in the selected area; and   for each given channel area identified in the step of identifying:   estimating threshold voltage variations due at least to Transient Enhanced Diffusion effects in the given channel area, in dependence upon distances between a point in the given channel area and the STI edges on the selected area; and   combining the threshold voltage variations estimated in the step of estimating threshold voltage variations due at least to Transient Enhanced Diffusion effects in the given channel area, in dependence upon distances between a second point in the given channel area and the STI edges on the selected area.   
     
     
         14 . Data storage means according to  claim 13 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a member of the group consisting of:
 a decay function of the form
   λ i ( r )=1/(( r /α i )β i +ε i ); and
 
   a decay function having the form of the error function, erf(r); and   a decay function having the form of the complimentary error function, erfc(r),   where α i , β i  and ε i , are process and material-related factors, and r is a distance between the point in the given channel area and an STI edge.   
     
     
         15 . Data storage means according to  claim 13 , wherein the step of estimating threshold voltage variations comprises the steps of:
 calculating threshold voltage variations due at least to Transient Enhanced Diffusion effects in a longitudinal direction, in dependence upon distances between a point in the given channel area and transversely oriented STI edges on the selected area; and   calculating threshold voltage variations due at least to Transient Enhanced Diffusion effects in a transverse direction, in dependence upon distances between a point in the given channel area and longitudinally oriented STI edges on the selected area.   
     
     
         16 . Data storage means according to  claim 13 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a decay function which decreases with greater distance from the point, the decay function being dependent further upon an amount of crystal lattice damage introduced by source/drain implantation. 
     
     
         17 . Data storage means according to  claim 13 , wherein the step of estimating threshold voltage variations comprises the step of multiplying a maximum threshold voltage variation by a decay function which decreases with greater distance from the point, the decay function being dependent further upon a halo implant energy.