Abstract:
A method for fabricating improved integrated circuit devices. The method enables selective hardening of gate oxide layers and includes providing a semiconductor substrate having a gate oxide layer formed thereover. A resist is then formed over the gate oxide layer and patterned to expose one or more areas of the gate oxide layer which are to be hardened. The exposed portions of the gate oxide layer are then hardened using a true remote plasma nitridation (RPN) scheme or a high-density plasma (HDP) RPN scheme. Because the RPN scheme used in the method of the present invention runs at low temperature, the patterned resist remains stable through the RPN process, and those areas of gate oxide layer which are exposed by the patterned resist are selectively hardened by the RPN treatment, while those areas covered by the patterned resist remain unaffected.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 10/791,006, filed Mar. 2, 2004, pending, which is a continuation of application Ser. No. 10/198,215, filed Jul. 17, 2002, now U.S. Pat. No. 6,699,743, issued Mar. 2, 2004, which is a continuation of application Ser. No. 09/641,067, filed Aug. 17, 2000, now U.S. Pat. No. 6,458,663, issued Oct. 1, 2002. The disclosure of the previously referenced U.S. patent applications and patents referenced are hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to fabrication of transistors in integrated circuit devices. Specifically, the present invention provides an improved method for fabricating an integrated circuit device including N-channel and P-channel transistors having selectively hardened gate oxides.  
         [0004]     2. State of the Art  
         [0005]     State of the art integrated circuit (IC) devices generally require transistors including hardened gate oxides. As feature dimensions continually decrease, gate oxides are hardened in order to prevent diffusion of dopants from overlying layers into and through the gate oxide, to prevent breakdown at voltages below normal operating parameters, and to confer resistance to hot electron degradation. State of the art transistors including hardened gate oxides, therefore, often possess performance and reliability advantages relative to transistors including non-hardened gate oxides applied in the same context. However, hardened gate oxides are not desirable in all IC applications, and known methods for hardening gate oxides often require significant design and performance tradeoffs.  
         [0006]     While it is generally preferred to harden the gate oxides of P-channel devices due to the nature of P-type dopants, such is not the case for N-channel devices. Hardening of N-channel devices is generally not necessary as N-type dopants do not readily diffuse through non-hardened gate oxides. Moreover, hardening N-channel devices is often undesirable due to compounding performance problems. As is well known, hardening of gate oxides included in N-channel devices leads to significant threshold voltage (V T ) roll-off. While V T  roll-off can be counteracted through enhancement implants, the increased dopant concentration resulting from enhancement implants causes additional performance problems, such as refresh degradation and reduced surface mobility. Therefore, it would generally be advantageous not to harden the gate oxides of N-channel devices included within an IC device.  
         [0007]     Despite the difficulties generally resulting from hardening the gate oxides of N-channel devices, however, the ability to selectively harden the gate oxides of N-channel devices in particular instances would be advantageous.  
         [0008]     Additionally, as is also well known, it is often desirable to include P-channel or N-channel devices having gate oxides of varying thicknesses within a single IC device. For instance, it is beneficial to provide N-channel devices with gate oxides which are thicker than the thin, hardened gate oxides generally included in P-channel devices. Increasing the thickness of non-hardened N-channel gate oxides increases the V T  of the corresponding N-channel devices, thereby decreasing control and performance the same. Moreover, it may also be advantageous to fabricate an IC device including hardened P-channel or N-channel devices incorporating hardened gate oxides of varying thicknesses.  
         [0009]     Therefore, an ideal method for fabricating IC devices would facilitate simple and low-cost fabrication of IC devices including P-channel and N-channel devices which have been selectively hardened and which may include hardened or non-hardened gate oxides of varying thicknesses. However, known methods for hardening gate oxides generally do not provide the flexibility needed to enable selective hardening of gate oxides within an IC device. Furthermore, though it is possible to fabricate hardened gate oxides of varying thicknesses using known techniques, such techniques generally require additional etch steps, which are costly and serve as an additional source of error in a fabrication process.  
         [0010]     For example, known methods for hardening gate oxides included in an IC device, such as a dynamic random access memory (DRAM) device, often require blanket hardening of a gate oxide layer deposited over a semiconductor substrate. During subsequent fabrication steps, both N-channel and P-channel gate oxides must then be formed using the blanket hardened gate oxide layer. Consequently, every one of the N-channel and P-Channel devices included in the subsequently formed IC device includes a hardened gate oxide. Blanket hardening processes simply do not enable selective hardening of particular areas of the gate oxide layer and, thereby, compromise the quality of IC devices fabricated by such methods.  
         [0011]     Additionally, blanket hardening techniques cause difficultly in fabricating IC devices including gate oxides of varying thicknesses. Hardened oxide layers generally will not grow significantly during subsequent thermal oxidation processes. Therefore, to fabricate an IC device having gate oxides of various thicknesses using a blanket hardening process, the gate oxide layer must be formed such that, after hardening, the hardened gate oxide layer is as thick as the thickest desired gate oxide. The hardened gate oxide layer must then be selectively etched back to a desired thickness where P-channel or N-channel devices having thinner gate oxides are to be formed. Such a process is disadvantageous because it adds the cost and complication associated with one or more additional etch steps. Moreover, known etching processes are difficult to control where only minute amounts of material must be removed. Thus, as the thickness of state of the art gate oxides shrinks well below 70 Angstroms, the need to etch back a hardened gate oxide layer becomes increasingly problematic and can only serve as a source of error, decreasing fabrication throughput as well as device reliability.  
         [0012]     As can be appreciated, an improved method for fabricating IC devices including selectively hardening gate oxides is needed. Such an improved method should not only enable fabrication of P-channel and N-channel devices including selectively hardened gate oxides but also enable fabrication of such devices including hardened or non-hardened gate oxides of varying thicknesses without requiring additional etch steps.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     The present invention provides a method for fabricating IC devices which answers the foregoing needs. The method of the present invention includes providing a semiconductor substrate having a gate oxide layer formed thereover. A resist is then formed over the gate oxide layer and patterned to expose one or more areas of the gate oxide layer which are to be hardened. The exposed portions of the gate oxide layer are then hardened using a true remote plasma nitridation (RPN) scheme or a high-density plasma (HDP) RPN scheme. Because the RPN scheme used in the method of the present invention runs at low temperature, the patterned resist remains stable through the RPN process. Therefore, those areas of gate oxide layer which are exposed by the patterned resist are selectively hardened by the RPN treatment, while those areas covered by the patterned resist remain unaffected.  
         [0014]     The method of the present invention may also include additional steps for growing or hardening non-hardened areas of the gate oxide layer remaining after the first RPN treatment. For example, the non-hardened portion of the gate oxide layer, or a portion thereof, may be grown to provide thick, non-hardened gate oxides for N-channel devices. Alternatively, the non-hardened portion of the gate oxide layer, or a portion thereof, may be grown and subsequently hardened to provide P-channel devices or N-channel devices having hardened gate oxides of varying thickness. The portion of the gate oxide layer hardened by the first RPN need not be masked during subsequent thermal oxidation and hardening steps because, once hardened, the gate oxide will not grow significantly when exposed to subsequent thermal oxidation processes. In fact, where implants are required through the gate oxide layer, it is beneficial not to mask the hardened portion of the gate oxide layer during subsequent thermal oxidation, as such a process will heal any implant damage done to the gate oxide layer.  
         [0015]     As can be easily appreciated by one of skill in the art, the present invention enables fabrication of an IC device including N-channel and P-channel devices having hardened or non-hardened gate oxides of varying thicknesses made necessary by known methods. Moreover, the method of the present invention may be easily integrated into known fabrication processes using known technology, and, because the method of the present invention does not require etching of the gate oxide layer to achieve gate oxides of varying thicknesses, the method of the present invention is cost effective and will continue to be useful as device dimensions shrink beyond the dimensions of those devices currently considered to be state of the art.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0016]     The figures presented in conjunction with this description are not actual views of any particular portion of an actual semiconducting device or component, but are merely representations employed to more clearly and fully depict the present invention.  
         [0017]      FIGS. 1 through 8  are cross-sectional views illustrating various steps of three different embodiments of the method of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     According to a first embodiment, the method of fabricating an IC device according to the present invention includes providing a semiconductor substrate  10  upon which N-channel devices and P-channel devices may be formed. Any suitable semiconductor substrate known in the art may be used, though a silicon semiconductor substrate is preferred. Further, if desired, the semiconductor substrate may include wells doped with N-type or P-type impurities, as is known in the art. As is shown in drawing  FIG. 1 , a gate oxide layer  14  is formed over the top surface  12  of the semiconductor substrate  10  using known deposition or thermal oxidation process. Thermal growth of the gate oxide layer  14 , however, is preferred, as it provides an SiO 2  layer that is substantially free of contaminants. The gate oxide layer may be formed to any desired thickness, though, in the context of 0.18 μm technology, it is preferred to provide a gate oxide layer having a thickness in the range of substantially 30 {acute over (Å)} to substantially 50 {acute over (Å)}.  
         [0019]     After formation of the gate oxide layer  14 , a first resist layer  16  is deposited over the upper surface  18  of the gate oxide layer  14 , as is shown in drawing  FIG. 2 . Any suitable resist may be used, and the resist may be deposited using well-known techniques, such as, for example, spin coating. Using known exposure and developing techniques, the first resist layer  16  is then patterned to expose an area  20  (seen in drawing  FIG. 3 ) of the gate oxide layer  14  which is to be hardened. The exposed area  20  is then subjected to a first RPN treatment (indicated by arrows  22 ), which selectively hardens only the exposed area  20  of the gate oxide layer  14 .  
         [0020]     RPN treatments are well known in the art, and in the context of this invention, either a true RPN treatment or an HDP RPN treatment may be used. Generally, the process parameters of the RPN treatment used to harden the exposed area  20  of the gate oxide layer may be varied to produce desired results in various fabrication contexts. However, the temperature of the RPN treatment must be low enough that the patterned first resist layer  16  remains stable through the entire process. For example, it is presently preferred to use an HDP RPN conducted for approximately 1 second to approximately 30 seconds at about 30° C. to about 90° C. using about 800 watts to 3000 watts of power. Such a method effectively hardens the exposed area  20  of the gate oxide layer  14 , yet runs at a temperature well below that which might render the resist unstable.  
         [0021]     As can be seen in drawing  FIG. 4 , following the first RPN treatment  22 , the remaining portions of the first resist layer  16  are removed, resulting in a first intermediate structure  24 . The first intermediate structure  24  includes the semiconductor substrate  10  with a partially hardened gate oxide layer  26 , which may be used as desired in the fabrication of N-channel or P-channel devices. Generally, the non-hardened portion  28  of the partially hardened gate oxide layer  26  will be used to fabricate at least one N-channel device, while the hardened portion  30  of the partially hardened gate oxide layer  26  will be used to fabricate at least one P-channel device. As will be appreciated by one of skill in the art, various methods for fabricating both N-channel and P-channel devices are well known, and after selectively hardening the gate oxide layer, the method of the present invention may include any suitable fabrication process necessary to complete fabrication of a desired IC device.  
         [0022]     In a second embodiment of the method of the present invention, the partially hardened gate oxide layer  26  of the first intermediate structure  24  is further processed to provide an area of increased oxide thickness. As depicted in drawing  FIG. 5 , the second embodiment of the method of the present invention includes subjecting the first intermediate structure  24  to a thermal oxidation process (indicated by arrows  23 ). Thermal oxidation of the first intermediate structure causes the growth of the non-hardened portion  28  of the partially hardened gate oxide layer  26 . Though the non-hardened portion may be grown to any desired thickness, where the original thickness of the gate oxide layer is in the range of substantially 30 {acute over (Å)} to substantially 50 {acute over (Å)}, it is presently preferred to grow the non-hardened gate oxide to a thickness in the range of substantially 50 {acute over (Å)} to substantially 70 {acute over (Å)}. Moreover, though the hardened portion  30  of the partially hardened gate oxide layer  26  will not grow substantially, the thermal oxidation process will heal any implant damage caused to all portions of the gate oxide layer due to any necessary implant steps. Thermal oxidation of the first intermediate structure  24 , therefore, results in a second intermediate structure  31  (illustrated in drawing  FIG. 6 ) including a second partially hardened gate oxide layer  32  characterized by a hardened portion  30  and a thick, non-hardened portion  34 .  
         [0023]     As was true with the partially hardened gate oxide layer  26  of the first intermediate structure  24  formed by the first embodiment of the method of the present invention, the second partially hardened gate oxide layer  32  may be used as desired to form gate oxides for N-channel or P-channel devices. For example, the thick, non-hardened portion  34  of the second partially hardened gate oxide layer  32  may be used to form a gate oxide for one or more N-channel devices, while the hardened portion  30  of the second partially hardened gate oxide layer  32  may be used to form a gate oxide for one or more P-channel devices. Again, various methods for fabricating an IC device including N-channel and P-channel devices using an intermediate structure, such as the second intermediate structure  31  illustrated in drawing  FIG. 6 , are well known, and the method of the present invention may include any such suitable method.  
         [0024]     The second embodiment of the method of the present invention is desirable because it allows fabrication of an IC device including both an array of P-channels having thin, hardened gate oxides and an array of N-channel devices having thick, non-hardened gate oxides. Thickening the non-hardened gate oxides of N-channel devices results in N-channel devices characterized by a higher V T , and increasing device V T  provides increased device control and enables better differentiation between N-channel and P-channel devices.  
         [0025]     In a third embodiment of the method of the present invention, the second intermediate structure  31  is further processed to produce a third partially hardened gate oxide layer  33  having a first hardened portion  36 , a second hardened portion  38  (shown in  FIG. 8 ), and a non-hardened portion  40 . As can be easily appreciated from drawing  FIG. 8 , both the second hardened portion  38  and the non-hardened portion  40  are thicker than the first hardened portion  36  of the third partially hardened gate oxide layer  33 .  
         [0026]     According to the third embodiment, a second resist layer  42  is formed over the top surface  43  of the third partially hardened gate oxide layer  33  of the second intermediate structure  31  (illustrated in drawing  FIG. 7 ). The second resist layer  42  is then patterned by techniques known in the art to expose at least an area  44  of the thick, non-hardened portion  40  of the third partially hardened gate oxide layer  33 . The exposed area  44  is then subjected to a second RPN treatment  47 , such as the preferred RPN treatment discussed herein. The second RPN treatment selectively hardens the exposed area  44  of the non-hardened portion  40  of the third partially hardened gate oxide layer  33 . However, those areas of the thick, non-hardened portion  40  of the third partially hardened gate oxide layer  33  underlying the second resist layer  42  are not affected by the RPN treatment because the second resist layer  42  remains stable through the RPN process. Therefore, after the remaining portions of the second resist layer  42  are removed, the third embodiment of the method of the present invention provides a third intermediate structure  48  that may be further processed by known techniques to provide a desired IC device. For example, the first hardened portion  36  of the third partially hardened gate oxide layer  33  may be used to form one or more P-channel devices having thin, hardened gate oxides, the second hardened portion  38  of the third partially hardened gate oxide layer  33  may be used to form one or more P-channel devices having thick (preferably in the range of substantially 50 {acute over (Å)} to substantially 70 {acute over (Å)}), hardened gate oxides, and the thick, non-hardened portion  40  of the third partially hardened gate oxide layer  33  may be used to form N-channel devices including thick (preferably in the range of substantially 50 {acute over (Å)} to substantially 70 {acute over (Å)}), non-hardened gate oxides.  
         [0027]     Of course, it should be understood that the three embodiments of the method of the present invention discussed herein are provided for illustrative purposes only. The method of the present invention is easily varied to provide IC devices having any desired combinations of hardened or non-hardened gate oxides of varying thicknesses. For example, the first resist layer used in the first and second embodiments of the method of the present invention may be patterned such that, following the RPN process, multiple hardened or non-hardened portions are formed within the gate oxide layer. Additionally, where multiple non-hardened portions are formed within the gate oxide layer, the intermediate structure may be processed according to the second embodiment of the method of the present invention to produce a partially hardened gate oxide layer including one or more hardened portions as well as multiple thick, non-hardened portions, which may be used in fabricating thick gate oxides for N-channel devices. Finally, use of additional masking, growth, and RPN steps can produce virtually any number of different hardened areas of varying thicknesses within a single gate oxide layer. Such a gate oxide layer can be used to form an IC device having any desired combination of selectively hardened N-channel or P-channel devices having gate oxides of different thicknesses.  
         [0028]     Moreover, though the described embodiments have discussed the use of the non-hardened portions of a gate oxide layer for fabrication of N-channel devices, it should be understood that, as mentioned, it is often desirable to create an IC device including one or more N-channel devices having a hardened gate oxide, particularly where long, peripheral N-channel devices are included in an IC device. Hardening of long N-channel devices increases the V T  of such devices and decreases problems with charge leakage, and, as can be easily appreciated by the ordinarily skilled artisan, any of the hardened portions of the partially hardened gate oxide layers produced in any embodiment of the present invention may be used to fabricate one or more hardened N-channel devices. Further, the patterned resist used in any embodiment of the method of the present invention may be patterned such that one or more specific areas of the gate oxide layer to be used for fabrication of hardened N-channel devices are hardened by the RPN treatment.  
         [0029]     In many of its embodiments, the method of the present invention is advantageous from more than one perspective. The method of the present invention enables fabrication of IC devices including any desired combination of selectively hardened N-channel and P-channel devices. Selectively hardening the various devices included in the IC device enhances performance characteristics of the IC device and avoids the compounding performance problems generally presented by hardened N-channel devices. Moreover, the method of the present invention allows fabrication of hardened or non-hardened N-channel and P-channel devices having gate oxides of varying thicknesses without the need for the additional etch steps required by known fabrication processes. By eliminating the need for one or more etch steps, the method of the present invention provides a more cost effective alternative to known methods and eliminates at least one unnecessary source of error in the fabrication process.  
         [0030]     It should be understood that the method of the present invention is broadly applicable and is easily adapted for use in any desired process for fabricating IC devices. Therefore, even though the method of the present invention is described herein with reference to specific examples and figures, such examples and figures are provided for illustrative purposes only. The scope of the present invention is defined by the appended claims and is not limited by the preceding description and drawings.