Abstract:
A mask is used for lightly doped drain and halo implants in an integrated circuit device. The mask exposes only portions of the substrate adjacent to field effect transistor gate electrodes. Since the halo implant is made only near the transistor channels, where it performs a useful function, adequate device reliability and performance is obtained. Since the halo implant is masked from those portions of the active regions for which it is not necessary, active region junction capacitances are lowered. Such lowered capacitances result in an improved transistor switching speed. The mask used to define the lightly doped drain and halo implant region can be easily formed from a straight forward combination of already existing gate and active area geometries.

Description:
This application is a continuation of application Ser. No. 08/426,874, filed Apr. 24, 1995 and now abandoned, which is itself a division of Ser. No. 07/769,185 filed Sep. 30, 1991, pending. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the fabrication of integrated circuit devices, and more particularly to a technique for reducing junction capacitances in field effect transistors. 
     2. Description of the Prior Art 
     Integrated circuit device geometries well below one micron feature sizes continue to become increasingly common. In general, the use of smaller devices on integrated circuit chips results in better performance and high packing density, thereby reducing cost while increasing performance. However, with such small feature sizes device performance is significantly adversely impacted by physical effects which can be largely ignored with larger devices. Therefore, transistor reliability becomes an important issue in the sub-micron regime. 
     One problem which is important for small geometry devices is known as the hot electron effect. This effect refers generally to the injection of energetic electrons from the channel region into the gate, causing degradation at transistor parameters such as threshold voltage and transconductants, therefore resulting in worse performance. The effect is more pronounced for n-channel devices. Numerous transistor designs have been proposed and investigated in order to solve the hot electron injection problem. 
     One approach to dealing with the hot electron effect utilizes lightly doped drains (LDD) and a halo implant. Halo implants are moderately doped implants of the same conductivity type as the well or substrate in which the transistor is formed, and which lie in a thin layer generally along the source/drain to substrate/well junctions. A combination of LDD structures and halo implants has proven to achieve good device performance and reliability. However, due to the extra impurities used to form the halo in the junction areas, higher junction capacitances are obtained. This is true for both n-channel and p-channel devices. These increased junction capacitances tend to adversely impact transistor switching speeds. 
     It would be desirable to provide a method for fabricating integrated circuit transistors which provides good protection from hot electron effects. It is further desirable for such a method to minimize source/drain capacitances to improve operating speeds for the device. 
     SUMMARY OF THE INVENTION 
     Therefore, according to the present invention, a mask is used for lightly doped drain and halo implants in an integrated circuit device. The mask exposes only portions of the substrate adjacent to field effect transistor gate electrodes. Since the halo implant is made only near the transistor channels, where it performs a useful function, adequate device reliability and performance is obtained. Since the halo implant is masked from those portions of the active regions for which it is not necessary, active region junction capacitances are lowered. Such lowered capacitances result in an improved transistor switching speed. The mask used to define the lightly doped drain and halo implant region can be easily formed from a straight forward combination of already existing gate and active area geometries. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIGS. 1-3 illustrate a preferred method for forming integrated circuit transistors according to the present invention; and 
     FIGS. 4 and 5 illustrate partial semiconductor integrated circuit layouts illustrating use of the preferred technique. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention. 
     Referring to FIG. 1, an integrated circuit device will be formed on a substrate 10. Field oxide regions 12, 14, 16 are defined and grown as known in the art. A field effect transistor gate electrode, consisting of a gate oxide layer 18 and a polycrystalline silicon layer 20, is formed as known in the art. The polycrystalline silicon layer 20 may be silicided with a refractory metal to increase conductivity. 
     To this point, the processing steps have been conventional. At this stage of the fabrication process, the device is ready for a lightly doped drain (LDD) and halo implant. 
     Referring to FIG. 2, a layer of photoresist 22 is applied to the device and patterned to define an opening 24. The opening 24 exposes the gate electrode and adjacent regions of the substrate 10. Other regions of the substrate are covered, and thus protected from implantation of impurities. 
     Impurities used to define a halo region 26 and LDD region 28 are implanted into the substrate 10 through the opening 24. For an n-channel transistor, regions 28 are doped n - , and halo regions 26 are doped p-type. For a p-channel transistor, the LDD regions 28 are doped p -  and the halo regions 26 are doped n-type. 
     Referring to FIG. 3, the resist layer 22 is removed and sidewall spacers 30 formed as known in the art using an oxide layer deposition and anisotropic etchback. After formation of the sidewall spacers 30, heavily doped source/drain regions 32, 34, and an active region 36 are implanted. Diffusion of the source/drain regions, LDD regions, and halo regions is caused by a thermal annealing step to activate the implants. This results in a structure approximately as shown in FIG. 3, with the halo regions 26 helping to minimize the hot electron effect as is known in the art. 
     Because most of the active regions were masked by the resist layer 22 while the halo regions 26 were implanted, the increased junction capacitance caused by implantation of the halo regions 26 occurs only within the region 24 near the gate electrode. The remainder of active regions 32 and 34, and the entirety of region 36, are not subjected to the halo implant. In these regions, the source/drain junction capacitance has a relatively low value. 
     In the preferred method, the LDD implants are made with the photoresist layer 22 in place. Although the LDD implant also does not extend into active regions not adjacent the gate electrode, this has little effect since the LDD regions 28 are effective only at the ends of the channel region underneath the gate oxide 18. 
     FIG. 4 is a top view of a portion of a hypothetical integrated circuit device illustrating a preferred masking layout for resist layer 22. In FIG. 4, gate electrode 40 overlies rectangular active region 42. A rectangular photoresist mask opening is defined by dotted line 44. The halo and LDD implants are made only into regions 46 within the active region 42. The remaining portions 48 of active region 42 do not receive the halo implant. In addition, active region 50, which does not contain a gate electrode, receives no halo implant at all. 
     The area in which the increased junction capacitance occurs is the region 46 adjacent to the gate electrode 40. Since only a relatively small portion of the total active area has such a halo implant, the total increase in junction capacitance for the device, above that which would exist without any halo implants at all, is relatively modest. 
     FIG. 5 shows another layout, with a gate electrode 52 and mask opening 54 as shown. Active region 56 is one source/drain region for the transistor defined by gate electrode 52, and is used to make a contact with an interconnect contact location 58. Again, it will be seen that the halo implant is confined to the regions near the gate electrode, so that the overall increase in junction capacitance is relatively low. 
     The mask used to pattern the photoresist to define the openings 44 and 54 can be created by generating a mask which is the intersection of the poly gate and the active region geometries. The openings thus formed are then enlarged somewhat in order to accommodate possible misalignment and to ensure that the halo and LDD implants will extend for some distance laterally of the channel region. The n-channel halo implant mask can only be performed in the p-well region, and the p-channel halo implant mask can only be performed in the n-well region. Combining already defined and tested patterns in this way helps to minimize pattern generation complexity. 
     It will be appreciated by those skilled in the art that the described technique can lead to a significant decrease in junction capacitance for integrated circuit devices. The extent of the capacitance decrease is dependent in large part upon the ratio of the area which receives the halo implant to that which does not. The halo implant is eliminated entirely in regions which do not have gate electrodes, such as active interconnections and contact regions used for well strapping. 
     If desired, the mask used to pattern the resist layer 22 for the LDD and halo implant can also be used for the threshold voltage (V T ) implants. Eliminating the (V T ) implants can provide a further reduction in junction capacitance because the implant tends to increase the surface carrier concentration, especially in n-channel devices. Reuse of the same mask for this purpose provides improved transistor performance, and minimal additional cost and complexity to the fabrication process. 
     Various optional steps may be made a part of the process. In one embodiment, the basic method comprises the steps of: a) forming a gate electrode on a semiconductor substrate; b) forming a first masking layer over the device, such masking layer having an opening where the gate electrode crosses an active area of the substrate to define a field effect transistor; and c) performing a halo implant of impurities having a first conductivity type into the substrate through the opening in the masking layer. The masking layer may comprise photoresist. 
     One variation on this further comprises the step of: performing a lightly doped drain implant of impurities having a second conductivity type through the opening in the masking layer. 
     Another variation further comprises the steps of: before said gate electrode forming step, forming a second masking layer over the device, such layer having an opening over the same region as the first masking layer opening; performing a threshold voltage adjust implant through the second masking layer opening; and removing the second masking layer. In this variation, the second masking layer may comprise photoresist. 
     In another variation, the active area crossed by the gate electrode has the first conductivity type, and the method further comprises the steps of: forming a second gate electrode over a second active area of the substrate, the second active area having the first conductivity type; forming a second masking layer over the device, such second masking layer having an opening where the second gate electrode crosses the second active area of the substrate to define a second field effect transistor; and performing a halo implant of impurities having the second conductivity type into the substrate through the opening in the second masking layer. 
     This variation may further comprise the step of: performing a lightly doped drain implant of impurities having the first conductivity type through the opening in the second masking layer. 
     Alternatively, this variation may further comprise the steps of: before said second gate electrode forming step, forming a third masking layer over the device, such layer having an opening over the same region as the second masking layer opening; performing a threshold voltage adjust implant through the third masking layer opening; and removing the third masking layer. 
     Another variation may further comprise the steps of: after said halo implant performing step, forming sidewall spacers alongside the gate electrode; and forming heavily doped source/drain regions in the substrate. 
     In another embodiment, the method comprises the steps of: forming a p-well and an n-well in a substrate, such wells having active areas defined by field oxide regions; forming a first gate electrode over the p-well and a second gate electrode over the n-well; forming a first masking layer over the device having an opening where the first gate electrode crosses the p-well and exposing portions of the substrate in the p-well adjacent to the first gate electrode; implanting p-type impurities through the first masking layer opening to form a halo; removing the first masking layer; forming sidewall spacers alongside the first and second gate electrodes; forming heavily doped n-type source/drain regions in the p-well; and forming heavily doped p-type source/drain regions in the n-well. 
     One variation on this embodiment further comprises the steps of: implanting n-type impurities through the first masking layer opening to form a lightly doped drain region. 
     Another variation further comprises the steps of: forming a second masking layer over the device having an opening where the second gate electrode crosses the n-well and exposing portions of the substrate in the n-well adjacent to the second gate electrode; implanting n-type impurities through the first masking layer opening to form a halo; and removing the second masking layer. 
     This variation may further comprise the step of: implanting p-type impurities through the second masking layer opening to form a lightly doped drain region. 
     This variation may alternatively further comprise the steps of: before said gate electrodes forming step, forming a first VT mask over the p-well, such first VT mask having an opening having a size and location substantially the same as the first masking layer opening; performing a threshold adjust implant into the p-well through the first VT mask opening; and removing the first VT mask. It may further comprise the steps of: before said gate electrodes forming step, forming a second VT mask over the n-well, such second VT mask having an opening having a size and location substantially the same as the second masking layer opening; performing a threshold adjust implant into the n-well through the second VT mask opening; and removing the second VT mask. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.