Abstract:
A method for smoothing variations in threshold voltage in an integrated circuit layout. The method begins by identifying recombination surfaces associated with transistors in the layout. Such recombination surfaces are treated to affect the recombination of interstitial atoms adjacent such surfaces, thus minimizing variations in threshold voltage of transistors within the layout.

Description:
RELATED APPLICATIONS 
   This application is a divisional of co-pending U.S. application Ser. No. 11/757,294 filed 1 Jun. 2007, herein incorporated by reference. 

   BACKGROUND 
   The invention relates to integrated circuit devices, and more particularly to the suppression of layout sensitivity in a transistor array. 
   It has long been known that semiconductor materials such as silicon and germanium exhibit the piezoelectric effect (mechanical stress-induced changes in electrical resistance). See for example C. S. Smith, “Piezoresistance effect in germanium and silicon”, Phys. Rev., vol. 94, pp. 42-49 (1954), incorporated by reference herein. It has also been observed that stress variations in a transistor array can produce variations in carrier mobility, which in turn leads to variations in threshold voltage in the transistors of the array. That problem, and a solution for it, are set out in U.S. patent application Ser. No. 11/291,294, entitled “Analysis of Stress Impact on Transistor Performance”, assigned to the assignee hereof. 
   Further study has shown, however, that beyond stress some variation in threshold voltage remains, suggesting some additional factor at work. Variations encountered have been far from trivial, with swings of over 20 mV being common. The art has not suggested any potential causes for such problems, not has it presented solutions. Thus, it has remained for the present inventors to discover the cause of such variations and to devise solutions, all of which are set out below. 
   SUMMARY 
   An aspect of the claimed invention is a method for smoothing variations in threshold voltage in an integrated circuit. The method begins by identifying recombination surfaces associated with transistors in the MOSFET array. Such recombination surfaces are treated to affect the recombination of interstitial atoms adjacent such surfaces, thus minimizing variations in threshold voltage of transistors within the array 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  illustrates an embodiment of a single transistor constructed according to the claimed invention. 
       FIG. 1   b  illustrates an embodiment of a transistor array constructed according to the claimed invention. 
       FIG. 2  is a plot of threshold voltage and drain current as functions of the distance from the channel to an STI interface (for isolated transistors) or to the next transistor (for nested transistors) 
       FIG. 3  depicts the recombination of interstitial ions during annealing, to repair lattice damage. 
       FIG. 4  depicts the recombination process shown in  FIG. 3 , with the addition of enhancing and suppression regions according to the claimed invention. 
       FIG. 5  shows the results achieved by the claimed invention, reflected in the ion concentration patterns at each transistor, in which the interstitial recombination rate is high at the channel/gate oxide interface and low at the silicon/STI interface. 
       FIG. 6  is a process flowchart of the method according to the claimed invention. 
   

   DETAILED DESCRIPTION 
   The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. 
   The claimed invention can best be understood by first considering an illustrative MOS transistor  10 , shown in  FIG. 1   a , which shows both a plan view (upper portion) and a cross-section taken on line A-A (bottom portion). There, a diffusion region  12  includes a source region  16  and drain region  18  formed in the diffusion region, with a gap between these regions overlain by a gate  14 . The area under the gate is the channel  20 . Spacers  22  lie on either side of the gate (not shown in plan view). It will be understood that materials and fabrication techniques relating to these components, and to the MOS device as a whole, are wholly known in the art and are thus not described in any detail here. It is anticipated that the array would be formed in a Partially Depleted, Silicon-on-Insulator (PDSOI MOSFET) substrate, but the teachings of the present application apply to bulk configurations as well. It will be noted that the drawings depict bulk MOSFET devices. Further, it is well-known in the art that the MOSFET channel is doped to adjust the threshold voltage that determines when the MOSFET turns on and off. Channel dopants employed in typical MOSFET devices include species such as boron. The embodiment depicted in  FIG. 1   a  has been so modified, employing ion implantation techniques in general use. The resulting concentration of B atoms in the crystal lattice of the diffusion region is represented by concentration plot, which depicts an inner high-concentration zone and an outer minimal concentration pattern. As is generally known, concentration of dopant decreases from a high concentration zone  23  near the channel surface, generally outward into the channel, to a selected minimal concentration level  24 . The concentration levels  23  and  24  are lines of equal dopant concentration within the channel, grading from the regular, smooth curve of the maximum concentration area and grading to the irregular form of minimal concentration plot  24 . Although not shown, those in the art will understand that concentration grades from maximum at line  23  to minimal at line  24 . The transistor arrays discussed below employ a number of individual transistors, constructed as set out here. Details related here will be omitted for the sake of focus and clarity in the discussion that follows. 
     FIG. 1   b  depicts an array  100  of three transistors  110 ,  112  and  114 . As previously described, the array is shown both in plan and cross-section views and each individual transistor is constructed consistent with the description above. As is commonly seen, a transistor array is formed on a chip, on which are formed a number a relatively large diffusion regions  102 . These regions have appropriate dopants added, by conventional processes such as ion implantation, to produce extensive source and drain regions  104  and  106 , respectively. Finally, gate material  108  is overlaid in strips. Transistors are isolated to prevent any cross-coupling, by areas of oxide insulator material, such as the Shallow Trench Isolation (STI) areas  122 . As the name implies, any suitable insulator can be used in an STI, but tetraethyl orthosilicate (TEOS) is preferred. It should be noted that the nature of integrated circuits will result in some individual transistors being isolated by themselves, such as transistor  114 , while others are nested into groups of two or more, such as transistors  110  and  112 . 
   Surprisingly, it has been found that even after eliminating stress-induced threshold voltage variations, a large amount of variation remained within a transistor array. As reflected in  FIG. 1   b , measurements in a typical array revealed V t  variation from 334 mV to 356 mV, a swing of 22 mV. Initial investigation did not immediately uncover the cause of this variation, but it was noted that the variation primarily occurred between individual isolated transistors, such as transistor  114 , and those in nested groups, such as transistors  110  and  112 . 
   It was noted that one difference between a point in the channels of transistors  110  and  112 , compared to a similar point in transistor  114  is the distance from such a point to the two surrounding STI walls. Further investigation led to the data charted in  FIG. 2 , which shows both V t  and I d  as functions of distance (in nm) from the channel to surrounding STI walls (for isolated MOSFETs such as transistor  114 ), and to the next MOSFET (for nested elements such as transistors  110  and  112 ). As shown, at the distances seen in current fabrication technologies, from 100-200 nm, considerable variation exists, but that variation reduces steadily with increasing distance, and becomes negligible at distances of about 500 nm. 
   A clue to what is happening at the lattice level can be gained by returning to  FIG. 1   b . The bottom portion of that drawing includes plots of channel dopant concentration,  110   a ,  112   a , and  114   a . As noted above, dopant such as boron is implanted in channel  128  to adjust threshold voltage. That operation generally is accomplished by ion implantation. Although the implantation for transistors  110 ,  112 , and  114  proceeded identically, one can observe an interesting result in  FIG. 1   b . Namely, the concentration of dopant, as shown by the shape of the profile, skews toward the nearer STI wall. Thus, in profile  110   a , the dopant concentration tilts toward the left, on the drawing page, while that of profile  112   a  tilts in the opposite direction, to right. In contrast, isolated transistor  114  displays a symmetrical concentration pattern  114   a , tilting in neither direction. 
   Based on these results, it was hypothesized that the issue could relate to recombination of damaged areas in the crystal lattice. As shown in  FIG. 3 , and as noted above, dopants (such as boron, phosphorous or arsenic) are introduced into the source and drain regions, usually by ion implantation, to create highly conductive layers in that area. The implantation process produces a damaged area  130  in the target crystal lattice, where the newly implanted ions have displaced the ions (generally Si ions) previously occupying crystal lattice ion sites but the displaced ions are still present within the lattice, as interstitial ions. It is further known that the displaced interstitials tend to migrate through a diffusion process toward a surface of the crystal structure, such as the interface between the crystal structure and the STI  122 , or interface between silicon channel and gate stack  123 , where displaced ions can recombine at the channel surface onto free Si lattice sites that characterize a surface area. This occurs at elevated temperature during the application of the thermal annealing process. Ion paths in  FIG. 3  are shown by arrows  132 . As can be seen, the distance that individual ions must travel to reach a surface and there recombine are different, which makes it more likely that ions located near such a surface will be able to recombine quickly. Before the interstitials displaced by the implantation recombine at the silicon surfaces, they move around and enhance diffusivity of the dopants like boron, phosphorus, or arsenic. This phenomenon is known as Transient Enhanced Diffusion (TED). The amount of TED that the dopants experience in the channel determines the concentration of dopants near the channel surface, and therefore determines the threshold voltage. Therefore, recombination of interstitials at different silicon surfaces affects threshold voltages of the adjacent MOSFETs. 
   Referring back to  FIG. 1   b , it will be appreciated that the expected recombination pattern for interstitial ions in the channel of transistor  114  would be symmetrical, as the distances to an STI wall are the same on either side of that transistor. For transistors  110  and  112 , however, application of this discovery would lead one to expect concentration patterns skewed toward the STI wall, and in fact that is exactly the result found. 
     FIG. 4  illustrates a solution to the variation problem presented by the transistor structure of  FIG. 3 . At the interface between the crystal structure and the STI, there is added a layer of material  140  that suppresses recombination of displaced silicon ions. Several materials are known to possess properties that would serve in this role. Notably, an oxide layer containing species such as N or F would tend to suppress interstitial recombination. The exact amounts of these elements required in a specific application to even out the TED effects between sides of a transistor adjacent to an STI wall and the side distant from such a structure. In one embodiment, oxynitrides are employed, produced by adding N to SiO 2 . Additionally, the TEOS in an STI could be replaced by nitride, or a nitride liner could be applied before the STI is deposited, producing a layer  140 . In either instance, nitride would suppress interstitial recombination. 
   In addition to, or instead of suppression recombination at the SIT interface, recombination could be enhanced at the gate interface  142 . A sufficient enhancement would have the identical effect as suppression at the SIT. One embodiment of the claimed invention employs materials including high-K dielectric material such as hafnium oxide (HfO 2 ). 
   Finally, it is possible to select dopants that are insensitive to interstitial-driven TED effects, such as arsenic and antimony. Such species diffuse in Si mainly by interacting with lattice vacancies rather than with interstitials. Thus, they are less sensitive to TED, which in turn results in lowered sensitivity to layout variations in threshold voltage. As in known by those in the art, implantation creates excess interstitials, not excess vacancies, and thus the number of vacancies is determined by the annealing temperature. 
     FIG. 5  illustrates the results of balancing the recombination of interstitial ions. The interstitial recombination rate at the channel/gate oxide interface is high, whereas it is low at the silicon/STI interface. As can be seen, the ion concentration profiles  110   a ,  112   a  and  114   a  are all symmetrical and very similar to each other. Confirming the hypothesis underlying the claimed invention, it can also be seen that the measured V t  across the three transistors now varies by only a single mV, not 22 mV. 
   A process  170  for implementing the claimed invention is shown in  FIG. 6 . As seen there, the process includes two basic steps: First, in step  172 , the MOSFET array is analyzed to select those individual transistors that require further processing. Then, in step  174 , action is taken to balance the recombination rate. Each of those steps need consideration in detail. 
   The analysis and selection step requires determination of which transistors are likely to exhibit imbalances. It is the discovery underlying the claimed invention that one can accurately select such transistors as those in nested configurations—that is, those transistors having another transistor adjacent on one side and an STI adjacent to the other side. That configuration, it has been found, requires action. Fortunately, that configuration is straightforward to identify in a transistor array, making it a simple matter to make such a selection from a system layout, using any of a number of automated design programs. In one embodiment, it is preferred to apply both suppression and enhancement measures globally to the entire MOSFET array. Other embodiments employ the measures singly—that is, employing either recombination enhancement at the gate surface or recombination suppression at the Si/STI interface, but not both. Yet other embodiments use analysis tools to identify particular target devices or sets of devices where enhancement or suppression, or both, would be most helpful. 
   Step  174  requires the implementation of one of the processes identified above to accomplish the rebalancing of recombination rates. For example, in one embodiment the TEOS material of the STI is replaced by nitride, or in another embodiment a nitride layer is deposited in the STI trench before the primary oxide is deposited. In other embodiments, the balancing step is accomplished by enhancing recombination at the gate interface. One method for accomplishing that would be to increase the permittivity of the oxide layer (increase k). Such an increase can be achieved by substituting oxynitride for the SiO 2  in the gate oxide, producing a medium-k material that offered enhanced recombination. Another embodiment first deposits or grows SiO 2 , followed by a layer of high-k material, such as HfO 2 . In either event, it will be helpful to avoid employing a nitrogen-based material, which would tend to suppress recombination. 
   Yet another embodiment proceeds by combining both enhancement of recombination at the gate interface and suppression at the STI interface. 
   While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.