Abstract:
A semiconductor process and apparatus uses a predetermined sequence of patterning and etching steps to etch a gate stack ( 30, 32 ) formed over a substrate ( 36 ), thereby forming an etched gate ( 33 ) having a vertical sidewall profile ( 35 ). By constructing the gate stack ( 30, 32 ) with a graded material composition of silicon-based layers, the composition of which is selected to counteract the etching tendencies of the predetermined sequence of patterning and etching steps, a more idealized vertical gate sidewall profile ( 35 ) may be obtained.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to integrated circuit transistors and in particular the present invention relates to metal oxide semiconductor (MOS) transistor gates.  
       BACKGROUND  
       [0002]     Integrated circuits transistors produced using a standard complementary metal-oxide-semiconductor (CMOS) integrated circuit fabrication process, such as MOS field-effect transistors (MOSFET), have source and drain regions, and a gate electrode. The MOSFETs are typically fabricated such that each have an n-type doped polysilicon gate electrode. The source and drain regions are typically implanted into a substrate of silicon. A channel region is defined between the source and drain regions and beneath the gate electrode. Because of overlap capacitance, gate overlap of the source and drain regions is not desired. That is, a capacitance is created between the gate and source/drain regions where an overlap exits. It is desired, therefore, to minimize this overlap.  
         [0003]     Controlling the amount of overlap between the gate and source/drain is compounded by the need to anneal the implant regions of the source/drain to meet minimum depth requirements. One technique used to control the implant spacing between the source and drain uses spacers attached to side walls of the gate electrode. Additional fabrication steps are required to create these spacers.  
         [0004]     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a transistor which has a reduced overlap capacitance while reducing the required processing steps.  
       SUMMARY OF THE INVENTION  
       [0005]     The above mentioned problems with MOSFET&#39;s and other problems are addressed by the present invention and will be understood by reading and studying the following specification.  
         [0006]     In one embodiment, the present invention provides an integrated circuit transistor comprising a source, a drain, and a gate electrode formed from a single layer and having a top region, a bottom region, and first and second opposite vertical side walls. The first and second vertical side walls have a stepped surface such that a first lateral distance between the first and second vertical side walls in the top region is greater than a second lateral distance between the first and second vertical side walls in the bottom region.  
         [0007]     In another embodiment, an integrated circuit transistor gate electrode comprises a single layer of conductive material. The gate electrode has a top region, a bottom region, and first and second opposite vertical side walls. The first and second vertical side walls have a stepped surface such that a first lateral distance between the first and second vertical side walls in the top region is greater than a second lateral distance between the first and second vertical side walls in the bottom region.  
         [0008]     In yet another embodiment, a method of fabricating an integrated circuit transistor is described. The method comprises fabricating a layer of conductive material, performing a first etch of the conductive material to define first and second opposite vertical side walls of a gate electrode, and performing a second etch of the conductive material to form recess regions in the first and second opposite vertical side walls. The recess regions are located at a bottom of the first and second opposite vertical side walls so that a cross-section of the gate electrode generally approximates a T-shape. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  illustrates a transistor formed with a straight profile gate electrode and oxide spacers;  
         [0010]      FIG. 2  illustrates a “T-shaped” gate electrode formed from multiple layers of material;  
         [0011]      FIG. 3  is a cross section view of an integrated circuit transistor;  
         [0012]     FIGS.  4 ( a )-( f ) illustrate one method of fabricating the transistor of  FIG. 3 ; and  
         [0013]      FIG. 5  is a graph of transistor drain current versus over etch. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The terms wafer and substrate used in the following description include any structure having an exposed surface with which to form the integrated circuit (IC) structure of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.  
         [0015]     A transistor is described herein which has a gate electrode with a “notched” profile. The notch is formed above both source and drain regions to control the location of an initial implant of source and drain extension regions. The notched polysilicon gate electrode enables an offset of the extension from the edge of the gate polysilicon to gate oxide junction. The offset provides enough lateral diffusion distance to perform an anneal operation without resulting in unwanted lateral diffusion under the gate electrode.  
         [0016]     To more fully understand the notched gate electrode described herein, reference is made to  FIG. 1  illustrating a transistor  100  formed with a straight profile gate electrode  102  and oxide spacers  104 . First a gate oxide layer  106  is formed on a substrate, and then a layer of gate polysilicon is deposited, masked and etched to form an electrode  102 . It is noted that the lateral edges  108  and  110  of the gate polysilicon are straight. A layer of oxide is then fabricated over the gate polysilicon. The oxide is patterned and etched to form spacers  104  attached to the straight edges of the gate polysilicon. Source and drain regions  112  and  114  are then formed into the substrate using the oxide spacers to define a lateral distance between the junctions, or doped regions. An anneal operation is then performed to further vertically diffuse the source/drain regions. The annealing operation also results in lateral diffusion of the dopants under the gate polysilicon. It will be appreciated that the variables experienced in depositing, masking and etching the oxide spacers results in a variable distance between the edges of the spacers and the polysilicon edge. The lateral diffusion of the extension regions, therefore, often results in an uncontrolled overlap with the gate electrode. This overlap results in degraded performance by creating an overlap capacitance. In addition, the extra processing steps of forming the oxide spacers are not desired.  
         [0017]     A transistor having an alternate gate electrode is illustrated in  FIG. 2 . The gate electrode cross section approximates a “T”. That is, the top of the gate electrode is wider than the base. The transistor gate is not formed from a single layer of conductive material, but requires the deposition, patterning and etching of a second polysilicon layer  116 . This transistor provides a larger interconnect conductor, but requires the multi-process steps of forming the oxide spacers and the additional gate polysilicon deposition, pattern and etch to form the top of the electrode.  
         [0018]     To reduce overlap capacitance, while minimizing process steps, a notched gate electrode is described herein which is formed from a single layer of conductive material. Referring to  FIG. 3 , a cross section of a fabricated integrated circuit transistor  200  is illustrated and described. The transistor includes a gate electrode  202  fabricated with notches  204 . The gate electrode is separated from a substrate  250  by a layer of gate oxide  208 . Source and drain regions  212  are formed (such as by ion implanting) into the substrate. The source and drain regions include extension regions  210 . The area between the extension regions, and beneath the gate electrode, is referred to as the transistor body, or channel region. It will be appreciated by those skilled in the art that the notches  204  allow the diffusion of the extension regions to be a controlled distance from a vertical surface of the notch. That is, the depth of the notches defines a lateral diffusion distance which can be used during an annealing step without creating an horizontal overlap between the bottom of the gate electrode and the source/drain regions, as explained below. It will be appreciated by those skilled in the art, that the transistor illustrated in  FIG. 3  is not complete and that electrical contacts to the source, drain and gate are required. To focus on the present invention, these, and other optional features, have not been illustrated.  
         [0019]     The gate electrode  202  is formed from a single layer and has a top region  203 , a bottom region  205 , a first vertical side wall  207  and a second opposite vertical side wall  209 . The first and second vertical side walls have a stepped surface such that a first lateral distance Y between the first and second vertical side walls at the top region is greater than a second lateral distance X between the first and second vertical side walls at the bottom region. In one embodiment, distance Y is approximately 20 nano meters larger than distance X. It will be appreciated that the difference between Y and X can vary over a wider range, including but not limited to 10 to 40 nano meters. The transistor has a general T-shaped gate in a cross section view which intersects the source and drain regions.  
         [0020]     A description of one method of fabricating a transistor having a notched gate electrode is provided as follows, with reference to FIGS.  4 ( a )-( f ).  FIG. 4 ( a ) illustrates a cross-section of a semiconductor substrate  250 , a layer of gate oxide  208  and a layer of material, such as doped polysilicon  252 . It will be appreciated that the substrate in the region of the transistor can be isolated from adjacent circuits and doped accordingly for the type of transistor desired, as known in the art. The polysilicon layer  252  is masked and bulk etched to define the upper edges and the vertical side walls of the gate electrode  254 , as shown in  FIG. 4 ( b ). Once the gate oxide layer is reached, a selective etch is performed to create the notches  204  on the bottom edges of the gate electrode, see  FIG. 4 ( c ). The second etch process is highly selective and does not remove much gate oxide  208 . As such, there is no breakthrough of the gate oxide. The selective etch removes passivation at the polysilicon to gate oxide comer, and allows lateral etching of the polysilicon gate electrode to create the notches  204 . During the selective etch process, the lateral etch rate approaches saturation to enable uniform control of the lateral undercut. Thus, the formation of the notches is close to self limiting. The selective etch is performed with the bulk polysilicon etch process, but can be considered a separate step because the etch control parameters are changed. The selective etch is a low pressure, high power etch which has a duration approximately equal to the duration of the bulk polysilicon etch, in the range of about 20 to 40 seconds.  
         [0021]     In one embodiment, the selective etch is performed using a commercially available Hitachi M511 plasma etcher. The process is performed using the settings shown in Table 1.  
                                                             TABLE 1                               Breakthrough       Over   Over       Parameter   Units   Etch   Bulk Etch   Etch 1   Etch 2                                TCR temp   deg C.   5   5   5   5       EL height   mm   80   80   80   80       Pressure   Pa   0.4   0.4   0.4   1.2       RF Power   W   60   25   20   25       uW Power   W   400   400   400   400       Gas A, Cl   ccm   25   25   25   0       Gas B, O 2     ccm   3   3   3   5       Gas C, HBr   ccm   75   75   75   100       Coil 1   A   14   14   14   14       Coil 2   A   17   17   17   14       Coil 3   A   3   3   3   3       Time   sec   5   EP   24   12       He Backside   kPa   1   1   1   1       Cont Plasma   y/n   n   y   y   n                  
 
 The process uses a first etch, or Break through etch, to remove surface oxide. The bulk etch removes polysilicon to the gate oxide layer. The end point (EP) of this etch is based on measuring gas chemistry in the etch chamber to physically determine when all the polysilicon has been removed. The over etch 1 step straightens the polysilicon profile to forms the final profile with the above defined notches. An optional over etch 2 process can be used to remove any residual of polysilicon remaining after the over etch 1 step. 
 
         [0022]     Referring to  FIG. 4 ( d ), after the notched profile of the polysilicon gate is formed, a shallow implant operation is performed to form the extension regions of the source and drain. The implant is spaced laterally from the bottom of the gate electrode. That is, the top of the gate electrode patterns the shallow implant regions by defining lateral boundaries so that the implant regions do not extend under the gate. The implant regions, therefore, do not substantially extend under the gate, and beyond vertical planes  257  and  258  defined by the side wall surfaces of the top region  203  of the gate electrode.  
         [0023]     The extension regions of the source and drain must vertically extend into the substrate a minimum depth to reduce current spreading resistance. Thus, the shallow implant is thermally processed, or annealed, to further diffuse the implant vertically,  FIG. 4 ( e ). The anneal operation also laterally diffuses the implant regions. By controlling the anneal operation, the lateral diffusion distance can be tailored to match the notch depth. As such, overlap between the gate electrode and the source/drain regions is reduced. After the thermal processing, the source and drain regions  210  extend under the gate electrode beyond the vertical planes  257  and  258 . The source and drain regions  210 , however, do not appreciably extend under the gate electrode beyond vertical planes defined by the interior surface of the notches  204 . Finally, a deep implant is performed to form the full source and drain regions. It will be appreciated that the deep implant is performed to provide low resistance contacts. Doping of the elements and regions of the transistor is considered well known in the art, and is not discussed further herein.  
         [0024]     Several benefits are provided by fabricating transistor gate electrodes with a notched profile. The first benefit is provided in patterning the polysilicon layer. As transistor dimensions reduce, patterning the process layers becomes more difficult. The present transistor gate allows the polysilicon to be patterned using the larger area of the top of the gate electrode, while providing a smaller gate oxide interface area. Second, the overlap capacitance of the transistor is reduce, as explained above.  FIG. 5  is a graph of transistor circuit performance versus over etch depth. The graph illustrates the percent improvement (increase) in oscillation frequency of a ring oscillator using notched transistors. The notch depths were created using increased etch times. The first sample (a) did not contain notches, while the remaining samples had increasing notch depths. Sample (e) had a depth of approximately 15-20 nm, and sample (I) had a depth of approximately 20-25 nm. A ring oscillator was used to illustrate the performance increase attributed to the reduced capacitance of the transistors, with other variable remaining constant. It can be seen that as the depth of the notches increase, the oscillator performance also increases. This performance increase has limitations, and will plateau or decrease as the notch depth continues to increase. Thus, the performance of the last sample (j) begins to decrease due to an increased resistance between the extension regions which do not fully reach the gate polysilicon (negative over lap).  
       CONCLUSION  
       [0025]     A method of reducing overlap capacitance in an integrated circuit transistor has been described herein. The method comprises forming a transistor gate electrode, having a T-shaped cross section, from a single layer of material using an etching process. In one embodiment, a two process etch is performed to form side walls having a notched profile. The notches allow source and drain regions to be implanted and thermally processed without creating excessive overlap capacitance. The reduction of overlap capacitance increases the operating performance of the transistor.  
         [0026]     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.