Abstract:
According to one exemplary embodiment, a heterojunction bipolar transistor comprises a base having a concentration of a first material at a first depth, where the first material impedes the diffusion of a base dopant. For example, the first material can be carbon and the base dopant can be boron. The first material also causes a change in band gap at the first depth in the base. According to this exemplary embodiment, the base further comprises a concentration of a second material, where the concentration of second material increases at the first depth so as to counteract the change in band gap. For example, the second material may be germanium. The concentration of the second material, for example, may increase at the first depth by amount required to cause a decrease in band gap to be substantially equal to the increase in band gap caused by concentration of the first material.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the present invention is in the field of fabrication of heterojunction bipolar transistors.  
           [0003]    2. Related Art  
           [0004]    In a silicon-germanium (“SiGe”) heterojunction bipolar transistor (“HBT”), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer.  
           [0005]    The SiGe HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Cutoff frequencies in excess of 100 GHz, which are comparable to the more expensive gallium-arsenide based devices, have been achieved for the SiGe HBT.  
           [0006]    The higher gain, speed and frequency response of the SiGe HBT are possible due to certain advantages of silicon-germanium, such as a narrower band gap and reduced resistivity. These advantages make silicon-germanium devices more competitive than silicon-only devices in areas of technology where high speed and high frequency response are required.  
           [0007]    The advantages of high speed and high frequency response discussed above require the realization of a thin highly doped base layer in the NPN SiGe HBT. For example, boron is commonly utilized to provide P-type doping of the base in an NPN silicon-germanium HBT. However, boron has a tendency to diffuse in the base. In other words, the boron profile in the base has a tendency to widen, thus undesirably widening the base. Boron diffusion is further accelerated during subsequent thermal processing steps that occur in the fabrication of the NPN SiGe HBT. The increased boron diffusion can severely degrade the high frequency performance of the NPN SiGe HBT. Thus, suppression of boron diffusion presents a major challenge in the fabrication of a NPN SiGe HBT.  
           [0008]    One method of suppressing boron diffusion in the base of the NPN SiGe HBT is by adding carbon in the base. To effectively arrest the diffusion of boron, a heavy carbon doping level is required. For example, a concentration greater than approximately 0.1 atomic percent of carbon can be added in the base of the NPN SiGe HBT at the point where the concentration of boron peaks. Due to the high carbon concentration, the impact on the lattice is such that the periodicity of the lattice is altered to compensate total strain. Since the in-plane strain is key to band-gap narrowing in SiGe, the addition of carbon doping counters this benefit from which increased NPN performance is derived. Thus, although adding carbon in the base effectively suppresses boron diffusion, the addition of carbon has the undesirable effect of increasing the band gap in the base and consequently diminishing the performance of the NPN SiGe HBT.  
           [0009]    Graph  100  in FIG. 1 shows exemplary boron, carbon, and germanium profiles in a base in an NPN SiGe HBT. Graph  100  includes concentration level axis  102  plotted against depth axis  104 . Concentration level axis  102  shows relative concentration levels of boron, carbon and germanium. Depth axis  104  shows increasing depth into the base, starting at the top surface of the base, i.e. at the transition from emitter to base in the NPN SiGe HBT. The top surface of the base in the NPN SiGe HBT corresponds to “0” on depth axis  104 .  
           [0010]    Graph  100  also includes boron profile  106 , which shows the concentration of boron in the base, plotted against depth, i.e. distance into the base. Boron profile  106  includes peak boron concentration level  108 , which occurs at depth  114 . Graph  100  further includes carbon profile  112 , which shows the concentration of carbon in the base, plotted against depth. The concentration of carbon in carbon profile  112  increases abruptly from 0.0 to a constant level at depth  114 , and remains at a constant level from depth  114  to depth  122 . At depth  122 , the carbon concentration level decreases abruptly to 0.0.  
           [0011]    Graph  100  further includes germanium profile  116 , which shows the concentration of germanium in the base of the present exemplary NPN SiGe HBT, plotted against depth. Germanium profile  116  begins at 0.0 concentration level at depth  110  and ramps up, i.e. increases linearly, to depth  118 . Germanium profile  116  maintains a constant concentration level from depth  118  to depth  120 . At depth  120 , germanium profile  116  ramps down, i.e. decreases linearly, to 0.0 concentration level at depth  122 . Thus, a concentration of carbon is added in the base of the NPN SiGe HBT at depth  114 , which corresponds to peak boron concentration level  108 .  
           [0012]    Graph  200  in FIG. 2 shows an exemplary band gap curve in the base in the present exemplary NPN SiGe HBT. Graph  200  shows band gap curve  202 , which shows the change in band gap caused by carbon profile  112  and germanium profile  116  in FIG. 1 in the base in the present exemplary NPN SiGe HBT. Graph  200  includes change in band gap axis  208  plotted against depth axis  204 . It is noted that “0” on change in band gap axis  208  refers to the band gap of a reference base comprising only silicon, i.e. a silicon-only base. It is also noted that an upward move on band gap curve  202  indicates a decrease in the band gap of the base of the present exemplary NPN SiGe HBT relative to the band gap of a silicon-only base. Conversely, a downward move on band gap curve  202  indicates an increase in the band gap of the base relative to the band gap of a silicon-only base.  
           [0013]    Depth axis  204  corresponds to depth axis  104  in FIG. 1. In particular, depths  210 ,  214 , and  222 , respectively, correspond to depths  110 ,  114 , and  122  in FIG. 1. At depth  210 , band gap curve  202  begins to decrease at a linear rate. As is known in the art, an increase in the concentration of germanium in a base of an NPN SiGe HBT results in a decrease in band gap. Thus, band gap curve  202  decreases from depth  210  to just prior to depth  214  as the result of a ramp up in concentration of germanium. At depth  214 , the band gap increases abruptly from band gap level  212  to band gap level  216 . This step increase in band gap corresponds to the addition of carbon in the base at depth  114  in FIG. 1. As such, the addition of carbon in the base of an NPN SiGe HBT results in an undesirable increase in the band gap of the base. This increase in band gap creates an electric field in the NPN SiGe HBT that opposes current flow, and thus results in a decrease in the speed that the NPN SiGe HBT can achieve.  
           [0014]    Thus, there is a need in the art to provide a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing a concomitant undesirable increase in band gap in the base.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention is directed to a band gap compensated HBT. The present invention overcomes the need in the art for a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing a concomitant undesirable increase in band gap in the base.  
           [0016]    According to one exemplary embodiment, a heterojunction bipolar transistor comprises a base having a concentration of a first material at a first depth, where the concentration of the first material impedes the diffusion of a base dopant. For example, the first material can be carbon and the base dopant can be boron. The first material also causes a change in band gap at the first depth in the base. For example, the first material may cause an increase in band gap at the first depth in the base.  
           [0017]    According to this exemplary embodiment, the base of the heterojunction bipolar transistor further comprises a concentration of a second material, where the concentration of the second material increases at the first depth so as to counteract the change in the band gap. For example, the second material may be germanium. The concentration of the second material, for example, may increase at the first depth by an amount required to cause a decrease in the band gap to be substantially equal to the increase in band gap caused by the concentration of the first material.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a graph showing boron, carbon, and germanium profiles in the base of a conventional exemplary NPN SiGe HBT.  
         [0019]    [0019]FIG. 2 is a graph showing a change in band gap of the base in a conventional exemplary NPN SiGe HBT of FIG. 1.  
         [0020]    [0020]FIG. 3 illustrates an exemplary structure, including an exemplary NPN SiGe HBT, in accordance with one embodiment of the present invention.  
         [0021]    [0021]FIG. 4 is a graph showing boron, carbon, and germanium profiles in the base of an exemplary NPN SiGe HBT in accordance with one embodiment of the present invention.  
         [0022]    [0022]FIG. 5 is a graph showing a change in band gap of the base of the exemplary NPN SiGe HBT of FIG. 4 in accordance with one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The present invention is directed to a band gap compensated HBT. The following description contains specific information pertaining to the implementation of the present invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order not to obscure the invention. The specific details not described in the present application are within the knowledge of a person of ordinary skill in the art.  
         [0024]    The drawings in the present application and their accompanying detailed description are directed to merely exemplary embodiments of the invention. To maintain brevity, other embodiments of the invention which use the principles of the present invention are not specifically described in the present application and are not specifically illustrated by the present drawings.  
         [0025]    [0025]FIG. 3 shows a cross-sectional view of structure  300 , which is utilized to describe one embodiment of the present invention. Certain details and features have been left out of FIG. 3 that are apparent to a person of ordinary skill in the art. Although structure  300  illustrates an exemplary NPN SiGe HBT, the present invention manifestly applies to other similar or related structures, such as PNP HBTs. Structure  300  includes collector  302 , base  304 , and emitter  306 . Collector  302  is N-type single crystal silicon, which might be deposited epitaxially using a reduced pressure chemical vapor deposition (“RPCVD”) process in a manner known in the art. Base  304  is a P-type silicon-germanium single crystal that might be deposited epitaxially in an RPCVD process. As seen in FIG. 3, base  304  is situated on top of, and forms a junction with, collector  302 . In the present exemplary embodiment, emitter  306 , which is situated above and forms a junction with base  304 , comprises N-type silicon. Collector  302 , base  304 , and emitter  306  thus form the present exemplary NPN SiGe HBT, which is generally referred to by numeral  308  in FIG. 3.  
         [0026]    As seen in FIG. 3, buried layer  310 , which comprises N+ type material, i.e. heavily doped N-type material, is formed in silicon substrate  312  in a manner known in the art. Collector sinker  314 , which also comprises N+ type material, is formed by diffusion of heavily concentrated dopants from the surface of collector sinker  314  down to buried layer  310 . Buried layer  310 , along with collector sinker  314 , provide a low resistance electrical pathway from collector  302  through buried layer  310  and collector sinker  314  to a collector contact (not shown in FIG. 3).  
         [0027]    As further seen in FIG. 3, deep trench structures  316  and  318  and field oxide structures  320 ,  322 , and  324  provide electrical isolation from other devices on silicon substrate  312 . Deep trench structures  316  and  318  and field oxide structures  320 ,  322 , and  324  might comprise, among other things, silicon oxide material and are formed in a manner known in the art. Dielectric segments  326  and  328 , which can comprise silicon oxide, provide electrical isolation between emitter  306  and base  304 .  
         [0028]    By way of background, characteristics and functionality of the present exemplary NPN SiGe HBT  308  are affected and can be tailored by varying steps of the fabrication process. One useful tool for controlling the resultant performance characteristics of NPN SiGe HBT  308  is the dopant and silicon-germanium profiles. In particular, it is desirable to accurately control the dopant and silicon-germanium profiles of base  304  to achieve a desired NPN SiGe HBT performance. In the present application, a dopant profile in base  304  is also referred to as a concentration of the base dopant, such as boron, in base  304 .  
         [0029]    Graph  400  in FIG. 4 shows exemplary boron, carbon, and germanium profiles in the base of an exemplary NPN SiGe HBT in accordance with one embodiment of the present invention. In particular, boron profile  402 , carbon profile  404 , and germanium profile  406 , respectively, in graph  400  show boron, carbon, and germanium profiles in base  304  in FIG. 3. Graph  400  includes concentration level axis  408  plotted against depth axis  410 . Concentration level axis  408  shows relative concentration levels of boron, carbon, and germanium, respectively, in boron profile  402 , carbon profile  404 , and germanium profile  406 .  
         [0030]    Depth axis  410  shows increasing depth into base  304  in FIG. 3, starting at the top surface of base  304 . Thus, “0” on depth axis  410  indicates the approximate transition from emitter  306  to base  304 . Additionally, depth  422  on depth axis  410  indicates the approximate transition from base  304  to collector  302  of NPN SiGe HBT  308  in FIG. 3. Boron profile  402  shows the concentration of boron in base  304 , plotted against depth, i.e. distance into base  304 . Boron profile  402  includes boron peak  412 , which represents the peak concentration level of boron in base  304 . Boron peak  412  occurs at depth  416  in base  304 . It is noted that boron is used as an exemplary P-type dopant in the present exemplary NPN HBT for the purpose of illustrating the present invention by way of a specific example. However, the principles of the present invention apply equally to an NPN HBT using a different P-type dopant in its base and even to a PNP HBT using an N-type dopant in its base.  
         [0031]    Continuing with graph  400 , carbon profile  404  shows the concentration of carbon in base  304 , plotted against depth, i.e. distance into base  304 . It is noted that carbon is also referred to as a “diffusion suppressant” or as “impeding” diffusion in the present application. As shown in FIG. 4, carbon profile  404  begins at depth  416 , where a concentration of carbon is introduced into base  304 . In other words, carbon doping begins at depth  416  in base  304 . At depth  416 , the concentration of carbon increases abruptly from 0.0 concentration level to carbon concentration level  424 . For example, carbon concentration level  424  can be approximately 0.5 atomic percent of carbon. The concentration of carbon remains at carbon concentration level  424  down to depth  422 . At depth  422 , the carbon concentration level in base  304  abruptly decreases from carbon concentration level  424  to 0.0 carbon concentration.  
         [0032]    Carbon is introduced into a base of the present exemplary NPN SiGe HBT to suppress boron diffusion, which can desirably decrease the effective base width. For example, the thermal annealing process utilized in the fabrication of the NPN SiGe HBT can cause boron to diffuse into adjoining silicon regions of the NPN SiGe HBT, which can severely degrade the performance of the NPN SiGe HBT. Although carbon effectively suppresses boron diffusion, the addition of carbon into the base results in an undesirable increase in band gap in the base. For example, an introduction of 1.0 atomic percent of carbon in the base increases the band gap by approximately +10.0 milli-electron volts (meV).  
         [0033]    Continuing with graph  400  in FIG. 4, germanium profile  406  shows the concentration of germanium in base  304 , plotted against depth, i.e. distance into base  304 . Germanium profile  406  begins at depth  414  in base  304 , and it (i.e. germanium profile  406 ) ends at depth  422 , which corresponds to the approximate start of collector  302  in FIG. 3. The germanium concentration level in silicon-germanium base  304  starts at 0.0 germanium concentration at depth  414  and ramps up, i.e. increases linearly, to germanium concentration level  426  just prior to reaching depth  416 .  
         [0034]    By way of background, increasing the concentration of germanium in a base of an NPN SiGe HBT results in a reduction in band gap in the base. For example, 8.0 atomic percent of germanium is equivalent to a reduction in band gap of approximately 10.0 meV. The reduction in band gap allows an electric field to build up in the base, which produces the desirable result of increasing performance of the NPN SiGe HBT. In addition, increasing the concentration of germanium in a base of an NPN SiGe HBT correspondingly increases the in-plane stress and changes the electronic band structure favorably to enhance carrier mobility, thereby increasing performance of the NPN SiGe HBT.  
         [0035]    According to an embodiment of the present invention, at depth  416 , which coincides with the addition of carbon in base  304 , the concentration of germanium is stepped up, i.e. abruptly increased, to germanium concentration level  428 . As a result of the step increase in the concentration of germanium at depth  416 , the band gap is correspondingly decreased in base  304 . However, as discussed above, the introduction of carbon at depth  416  results in an increase of band gap corresponding to the amount of carbon added at depth  416 . Thus, the increase in concentration of germanium at depth  416 , in accordance to one embodiment, offsets the increase in band gap resulting from the addition of carbon at depth  416 .  
         [0036]    For example, an addition of 0.5 atomic percent of carbon at depth  416  results in an increase in band gap of approximately 5.0 meV, since, as discussed above, 1.0 atomic percent of carbon results in an increase in band gap of approximately 10.0 meV. However, as discussed above, 10.0 atomic percent of germanium is equivalent to a reduction in band gap of approximately 10.0 meV. Thus, the amount of increase in concentration of germanium necessary to offset 0.5 atomic percent of carbon would be approximately 5.0 atomic percent. Accordingly, to offset the addition of 0.5 atomic percent of carbon at depth  416 , the concentration of germanium at depth  416  is stepped up, i.e. abruptly increased, by 5.0 atomic percent of germanium.  
         [0037]    As shown in Graph  400 , the concentration of germanium continues to ramp up, i.e. increase linearly, from depth  416  to depth  418 . The ramp up of germanium concentration builds a desirable electric field in base  304 . For example, the distance between depth  414  and depth  418  can be approximately 200.0 Angstroms. When, for example, germanium ramps up to a concentration level of 25.0 atomic percent at depth  418 , which corresponds to a band gap change of approximately 25.0 meV over 200.0 angstrom (10.0 atomic percent of germanium is equivalent to a reduction in band gap of approximately 10.0 meV). The corresponding electric field gradient would be approximately 25.0 meV/200.0 angstroms or approximately 1.25*10 4  volts per centimeter.  
         [0038]    At depth  418 , the concentration of germanium reaches germanium concentration level  430 . The concentration level of germanium remains at germanium concentration level  430  from depth  418  to depth  420 . For example, germanium concentration level  430  can be 25.0 atomic percent of germanium. Between depth  420  and depth  422 , the germanium concentration level ramps down, i.e. decreases linearly, from germanium concentration level  430  at depth  420  to a germanium concentration level of 0.0 at depth  422 .  
         [0039]    Thus, by abruptly increasing the germanium concentration level at depth  416 , the present invention compensates for the addition of carbon in base  304 . As such, the present invention preserves the advantage of a narrow boron profile in base  304  by utilizing carbon to prevent the diffusion of boron from increasing the effective size, i.e. widening, base  304 . Furthermore, the present invention provides the advantage of a continuous decrease in band gap from depth  414  to depth  418  by abruptly increasing the germanium concentration at depth  416  to compensate for the increase in band gap resulting from the introduction of carbon at depth  416 .  
         [0040]    Graph  500  in FIG. 5 shows an exemplary band gap curve in the base in the exemplary NPN SiGe HBT in accordance with one embodiment of the present invention. Graph  500  shows band gap curve  502 , which shows the change in band gap caused by carbon profile  404  and germanium profile  406  in FIG. 4 in base  304  in FIG. 3. Graph  500  includes change in band gap axis  504  plotted against depth axis  510 . It is noted that “0” on change in band gap axis  504  refers to the band gap of a reference base comprising only silicon, i.e. a “silicon-only” base. It is also noted that an upward move on band gap curve  502  indicates a decrease in the band gap of base  304  relative to the band gap of a silicon-only base. Conversely, a downward move on band gap curve  502  indicates an increase in the band gap of base  304  relative to the band gap of a silicon-only base.  
         [0041]    Depth axis  510  corresponds to depth axis  410  in FIG. 4. In particular, depths  514 ,  516 ,  518 ,  520 , and  522 , respectively, on depth axis  510  correspond to depths  414 ,  416 ,  418 ,  420 , and  422  on depth axis  410  in FIG. 4. As shown in graph  500 , band gap curve  502  indicates a linear decrease in band gap of base  304  from depth  514  to depth  518 . The linear decrease in band gap from depth  514  to depth  518  is caused by the increase in germanium concentration from depth  414  to depth  418  in FIG. 4. The increase germanium concentration from depth  414  to depth  418  includes a step increase of germanium at depth  416  provided by the present invention to offset the increase in band gap caused by the addition of carbon at depth  416 .  
         [0042]    In contrast to the present invention, conventional germanium profile  116  in FIG. 1 shows no step increase in germanium at step  114  to offset the introduction of carbon at step  114  in a conventional base in the present exemplary NPN SiGe HBT. Thus, conventional band gap curve  202  in FIG. 2 shows an undesirable abrupt increase in band gap at depth  214 , i.e. an increase from band gap level  212  to band gap level  216 , resulting from the addition of carbon in the base of the exemplary NPN SiGe HBT.  
         [0043]    Thus, by providing a step increase of germanium at depth  516  to compensate for the addition of carbon, the present invention provides a linear decrease in band gap from depth  514  to depth  518 . Accordingly, the present invention provides the advantage of a constant electric field, i.e. an electric field without a discontinuity, in base  304  from depth  514  to depth  518 . The above constant electric field in base  304  from depth  514  to depth  518  provided by the present invention results in a corresponding increase in performance of NPN SiGe HBT  308 .  
         [0044]    Although carbon and germanium are used as exemplary materials to illustrate an embodiment of the present invention, the present invention is generally directed to increasing a concentration of a second material to counteract a change in band gap caused by a first material. Thus, in the present embodiment, where the first material is carbon and the second material is germanium, the concentration of germanium is increased to counteract an increase in band gap caused by a concentration of carbon. In one embodiment, a first material may be different than carbon and/or a second material may be different than germanium. In that embodiment, the first material may affect the band gap differently than carbon. For example, the second material might be increased to counteract a decrease in band gap caused by a concentration of the first material.  
         [0045]    From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skills in the art would recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. For example, the principles of the present invention are also applicable to NPN SiGe HBTs using a P-type dopant other than boron, or using a diffusion suppressant other than carbon. Moreover, the present invention is applicable to NPN HBTs using semiconductors other than silicon or germanium. Further, the present invention&#39;s principles can also be applied to PNP SiGe HBTs or to non-SiGe PNP HBTs. As such, the described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein, but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention. For example, the specified layouts, dimensions, and doping levels are provided solely for the purpose of illustrating the present invention by way of a specific example and such dimensions, layouts, and doping levels can be manifestly varied without departing from the scope of the present invention.  
         [0046]    Thus, a band gap compensated HBT has been described.