Patent Document

This is a divisional of application Ser. No. 10/066,872 filed Feb. 4, 2002, now U.S. Pat. No. 6,639,256. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Related Art 
     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. 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. 
     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. 
     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. 
     One method of suppressing boron diffusion in the base of the NPN SiGe HBT is by adding carbon in the base. For example, a concentration of greater than 1*10 19  of carbon atoms per cubic centimeter can be added in the base of the NPN SiGe HBT at the point where the concentration of boron peaks. Although adding carbon in the base effectively suppresses boron diffusion, the addition of carbon has the undesirable effect of causing a band gap discontinuity at the collector-base junction. As a result of the band gap discontinuity at the collector-base junction, the electrical performance of the NPN SiGe HBT is accordingly diminished. For example, the above band gap discontinuity can increase the base transit time of electrons moving from the emitter to the base, thereby limiting the cut-off frequency of the NPN SiGe HBT. 
     Graph  100  in FIG. 1 shows conventional 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 . The bottom surface of the base, i.e. the collector-base junction, corresponds to depth  122  on depth axis  104 . 
     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. 
     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 increases to depth  118 . Germanium profile  116  maintains a constant concentration level from depth  118  to depth  120 . At depth  120 , germanium profile  116  decreases 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 . 
     Graph  200  in FIG. 2 shows a conventional exemplary band gap curve in the base and at the collector-base junction in the conventional 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 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 relative to the band gap of a silicon-only base. 
     Depth axis  204  corresponds to depth axis  104  in FIG.  1 . In particular, depths  210 ,  214 ,  218 ,  220 , and  222 , respectively, correspond to depths  110 ,  114 ,  118 ,  120 , and  122  in FIG.  1 . At depth  210 , band gap curve  202  begins to decrease. 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 , band gap curve  202  indicates an abrupt increase in band gap. This step increase in band gap corresponds to the addition of carbon in the base at depth  114  in FIG.  1 . 
     Band gap curve  202  decreases from depth  214  to depth  218  as the result of a ramp up in concentration of germanium. Between depth  218  and depth  220 , band gap curve  202  remains constant as a result of a constant concentration of germanium. Between depth  220  and depth  223 , band gap curve  202  increases as a result of a ramp down in concentration of germanium. At depth  223 , band gap curve  202  continues to increase to band gap level  224  as a result of a constant concentration of carbon and the ramp down in concentration of germanium. 
     At approximately depth  222 , i.e. at the approximate collector-base junction of exemplary NPN SiGe HBT, band gap curve  202  abruptly decreases to the reference band gap of a silicon-only base. Distance  226  refers to the distance between depth  223 , i.e. the depth at which band gap curve  202  crosses depth axis  204 , and approximately depth  222 , i.e. the approximate depth where band gap curve  202  abruptly decreases to the reference band gap of a silicon-only base. For example, distance  226  can be approximately 50.0 to 100.0. The band gap discontinuity, i.e. the abrupt decrease in band gap, at approximately depth  222  is caused by the abrupt decrease in the concentration level of carbon at depth  222 . As such, the rapid decrease in carbon at the collector-base junction of conventional exemplary NPN SiGe HBT results in an undesirable band gap discontinuity at the collector-base junction. 
     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 an undesirable band gap discontinuity at the collector-base junction. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to method and structure for eliminating collector-base band gap discontinuity in an 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 an undesirable band gap discontinuity at the collector-base junction. 
     According to one exemplary embodiment, a heterojunction bipolar transistor comprises a base having a concentration of germanium, where the concentration of germanium decreases between a first depth and a second depth in the base. For example, the concentration of germanium may decrease at a linear rate between the first depth and the second depth. 
     According to this exemplary embodiment, the base of the heterojunction bipolar transistor further comprises a concentration of a diffusion suppressant of a base dopant, where the concentration of the diffusion suppressant decreases between a third depth and a fourth depth so as to counteract a change in band gap in the base between the first depth and the second depth. For example, the diffusion suppressant can be carbon and the base dopant can be boron. The concentration of the diffusion suppressant, for example, may decrease at a linear rate between the third depth and the fourth depth, and the third depth may be situated in a germanium plateau region in the base. For example, the concentration of the diffusion suppressant may decrease between the third depth and the fourth depth so as to counteract the change in band gap in the base at approximately the second depth. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a graph showing boron, carbon, and germanium profiles in a base in a conventional exemplary NPN SiGe HBT. 
     FIG. 2 illustrates a graph showing a change in band gap of the base and collector-base junction in a conventional exemplary NPN SiGe HBT of FIG.  1 . 
     FIG. 3 illustrates an exemplary structure, including an exemplary NPN SiGe HBT, in accordance with one embodiment of the present invention. 
     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. 
     FIG. 5 is a graph showing a change in band gap of the base and collector-base junction of the exemplary NPN SiGe HBT of FIG. 4 in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to method and structure for eliminating collector-base band gap discontinuity in an 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. 
     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. 
     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 which might be deposited epitaxially in a “nonselective” 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 polycrystalline 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 . 
     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 ). 
     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  comprise 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 . 
     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 profiles. In particular, it is desirable to accurately control the dopant 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 base dopant, such as boron, in base  304 . 
     Graph  400  in FIG. 4 shows exemplary boron, germanium, and carbon profiles in the base of an exemplary NPN SiGe HBT in accordance with one embodiment of the present invention. In particular, boron profile  402 , germanium profile  404 , and carbon profile  406 , respectively, in graph  400  show boron, germanium, and carbon 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, germanium, and carbon respectively, in boron profile  402 , germanium profile  404 , and carbon profile  406 . 
     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 collector-base junction, i.e. the 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. 
     Continuing with graph  400  in FIG. 4, germanium profile  404  shows the concentration of germanium in base  304 , plotted against depth, i.e. distance into base  304 . Germanium profile  404  begins at depth  414  in base  304 , and it (i.e. germanium profile  404 ) ends at depth  422 , which corresponds to the collector-base junction, i.e. the transition from base  304  to collector  302  in FIG.  3 . In one embodiment, depth  422  substantially corresponds to the collector-base junction of exemplary NPN SiGe HBT  308 . The germanium concentration level in base  304  starts at 0.0 germanium concentration at depth  414  and increases to germanium concentration level  430  at depth  418 . 
     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, 10.0 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. 
     Continuing with graph  400 , 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 20.0 atomic percent at depth  418 , which corresponds to a band gap change of approximately 20.0 meV over 200.0 Angstroms (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 20.0 meV/200.0 Angstroms or approximately 1.00*10 4  volts per centimeter. 
     At depth  418 , the concentration of germanium reaches 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 20.0 atomic percent of germanium. In the present application, it is noted that the constant germanium concentration level between depth  418  and depth  420  is also referred to as the “germanium plateau region.” Between depth  420  and depth  422 , the germanium concentration level decreases from germanium concentration level  430  at depth  420  to a germanium concentration level of 0.0 at depth  422 . 
     Continuing with graph  400 , carbon profile  406  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  406  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 . 
     Carbon is introduced into a base of the present exemplary NPN SiGe HBT to suppress boron diffusion, which can undesirably increase the effective width of the base. 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). 
     Continuing with graph  400 , 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 increase in concentration of carbon at depth  416  also results in a corresponding increase in band gap in base  304 . For example, the addition of 0.5 atomic percent of carbon at depth  416  can result in an approximate 5.0 meV increase in band gap in base  304 . The concentration of carbon remains at carbon concentration level  424  down to depth  419 . According to an embodiment of the present invention, at depth  419 , the concentration of carbon is decreased from carbon concentration level  424  to 0.0 carbon concentration at depth  421 . 
     As shown in graph  400 , depth  419 , i.e. the depth at which the concentration of carbon starts to ramp down, is situated in the germanium plateau region, which extends from depth  418  to depth  420  as discussed above. It is appreciated that the concentration of carbon can start to ramp down at any depth in the germanium plateau region or at any depth in the germanium ramp down region. In one embodiment, the concentration of carbon can start to ramp down at depth  418 , i.e. the start of the germanium plateau region. 
     As discussed above, the addition of carbon in base  304  results in an increase in band gap. Likewise, a decrease in carbon concentration in base  304  results in a corresponding decrease in band gap. Also, as discussed above, an increase in concentration of germanium in base  304  results in a decrease in band gap. Likewise, a decrease in concentration of germanium in base  304  results in a corresponding increase in band gap. Thus, by appropriately ramping down the carbon concentration between depth  419  and depth  421 , the present invention partially offsets the increase in band gap resulting from the decrease in germanium concentration between depth  420  and depth  422 . As a result, the band gap in base  304  increases relative to a reference band gap of a “silicon-only” base, i.e. the band gap of a base comprising only silicon, at approximately depth  422 , while substantially eliminating any band gap discontinuity, i.e. a decrease in band gap, at approximately depth  422 . In one embodiment, the band gap discontinuity, i.e. a decrease in band gap, is prevented at a depth approximately equal to depth  422 . 
     Thus, the present invention provides the advantage of preventing a band gap discontinuity at approximately depth  422  by ramping down the carbon concentration between depth  419  and depth  421  to counteract the effect of a ramp down of germanium concentration between depth  420  and depth  422 . Furthermore, 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 . 
     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 tin band gap caused by carbon profile  406  and germanium profile  404  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. 
     Depth axis  510  corresponds to depth axis  410  in FIG.  4 . In particular, depths  514 ,  516 ,  518 ,  520 ,  521 , and  522 , respectively, on depth axis  510  correspond to depths  414 ,  416 ,  418 ,  420 ,  421 , and  422  on depth axis  410  in FIG.  4 . As shown in graph  500 , band gap curve  502  indicates a decrease in band gap of base  304  from depth  514  to depth  516 . The decrease in band gap from depth  514  to depth  516  is caused by the increase in germanium concentration from depth  414  to depth  416  in FIG.  4 . At depth  516 , band gap curve  502  indicates an abrupt increase in band gap caused by the abrupt increase in carbon concentration at depth  416 . 
     As shown in graph  500 , band gap curve  502  indicates a decrease in band gap from depth  516  to depth  518  as a result of an increase in germanium concentration from depth  416  to depth  418  in FIG.  4 . Band gap curve  502  shows constant level between depth  518  and depth  519 , which is caused by the constant concentration level of germanium between depth  418  and depth  419  in FIG.  4 . Band gap curve  502  indicates a decrease in band gap between depth  519  and depth  520 . The decrease in band gap between depth  519  and depth  520  is caused by the decrease in carbon concentration between depth  419  and depth  420 . 
     Band gap curve  502  indicates an increase in band gap between depth  520  and depth  521 . The increase in band gap between depth  520  and depth  521  is caused by the interaction of the invention&#39;s ramp down in carbon concentration from depth  419  to depth  421  and the ramp down in germanium concentration from depth  420  to depth  421 . Band gap curve  502  indicates an increase in band gap between depth  521  and depth  522 . The increase in band gap between depth  521  and depth  522  is caused by the decrease in germanium concentration between depth  421  and depth  422 . Thus, the present invention&#39;s ramp down in carbon concentration from depth  419  to depth  421  results in a band gap curve with substantially no discontinuity at approximately depth  522 , i.e. the approximate collector-base junction of exemplary NPN SiGe HBT  308 . 
     In contrast to the present invention, conventional band gap curve  202  in FIG. 2 shows an increase in band gap from depth  220  to depth  222 , followed by an abrupt decrease, i.e. a discontinuity, in band gap at approximately depth  222 . Thus, conventional band gap curve  202  in FIG. 2 shows an undesirable discontinuity in band gap at approximately depth  222 , resulting from the abrupt decrease in carbon in the base of the exemplary NPN SiGe HBT. 
     Thus, by providing a decrease in carbon from depth  419  to depth  421  to compensate for a decrease in germanium from depth  420  to depth  422 , the present invention prevents a decrease in band gap at approximately depth  522 . Accordingly, the present invention provides the advantage of a decreasing electric field, i.e. a decreasing electric field without a discontinuity, at approximately depth  522 , i.e. the approximate collector-base junction of NPN SiGe HBT  308 . The elimination of the discontinuity in the electric field at approximately depth  522  provided by the present invention results in an increase in performance of NPN SiGe HBT  308 . 
     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. It is noted that although reference is made to germanium as a band gap altering material throughout the present application, the principles of the present invention apply to any other band gap altering material which causes a change in the band gap where such change is used to counteract an opposing change caused by another material, such as a diffusion suppressant like carbon. 
     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. 
     Thus, method and structure for eliminating collector-base band gap discontinuity in an HBT have been described.

Technology Category: h