Abstract:
One embodiment is a method for fabricating the base of a bipolar transistor where the method comprises placing a first wafer in an undoped epi chamber. Next a first undoped base layer is grown over the first wafer. After growing the first undoped base layer, the first wafer is transferred from the undoped epi chamber into a separate doped epi chamber. A first doped base layer is then grown over the first undoped based layer in the doped epi chamber. While the first wafer is being processed in the doped epi chamber, a second wafer can be processed in the undoped epi chamber. Another embodiment is a structure produced by the disclosed method and yet another embodiment comprises a transfer chamber, a transfer arm, a bake chamber, and a separate undoped epi chamber and a doped epi chamber for practicing the disclosed method.

Description:
This is a divisional of application Ser. No. 10/163,661 filed Jun. 4, 2002 now U.S. Pat. No. 6,589,850. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is generally in the field of semiconductor fabrication. More particularly, the present invention is in the field of fabrication of bipolar transistors. 
   2. Related Art 
   Bipolar transistors are commonly used in electronic devices and, in particular, in radio frequency applications. A particular type of bipolar transistor, which is used as an example in the present application, is the silicon-germanium (“SiGe”) heterojunction bipolar transistor (“HBT”) in which a thin layer of silicon-germanium is grown over the bipolar transistor&#39;s collector region to operate as the base of the bipolar transistor. The silicon-germanium HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon-only bipolar transistor, for instance. Cutoff frequencies in excess of 100 GHz, which are comparable to the more expensive gallium-arsenide based devices, have been achieved for the silicon-germanium HBT. 
   The higher gain, speed and frequency response of the silicon-germanium 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 superior speed and frequency response are required. 
   Reference is now made to  FIG. 1 , which illustrates bipolar structure  100 , which in the present example is a silicon-germanium HBT structure. As shown, structure  100  includes, among other components, collector  130 , silicon-germanium base  120 , and emitter  140 . In structure  100 , collector  130  is N type single-crystal silicon and base  120  is P type single-crystal silicon-germanium. As will be discussed in greater detail below, base  120  can be fabricated epitaxially, for example, in a reduced pressure chemical vapor deposition process (“RPCVD”), to grow a silicon-germanium film over top surface  132  of collector  130 . A suitable dopant, such as boron, is typically introduced during the silicon-germanium film growth to attain the desired electrical properties of the base. 
   Continuing with  FIG. 1 , structure  100  further includes emitter  140 , which is situated above and forms a junction with base  120 , and which is comprised of N type polycrystalline silicon. The interface between emitter  140 , base  120 , and collector  130  is the active region of the NPN silicon-germanium HBT, i.e., structure  100 . In structure  100 , dielectric segments  142  provide electrical isolation to emitter  140  from base  120 . 
   As further illustrated in  FIG. 1 , buried layer  134 , which is composed of N+ type material, is formed in semiconductor substrate  110 . Collector sinker  136 , also composed of N+ type material, is formed by diffusion of heavily concentrated dopants from the surface of collector sinker  136  down to buried layer  134 . Buried layer  134  and collector sinker  136  provide a low resistance electrical pathway from collector  130  through buried layer  134  and collector sinker  136  to a collector contact (not shown). Deep trench structures  133  and field oxide regions  138  provide electrical isolation from other devices on semiconductor substrate  110 . 
   Referring now to  FIG. 2 , graph  200  illustrates the doping profile of the base in an exemplary silicon-germanium HBT, such as the NPN silicon-germanium HBT structure of structure  100  in  FIG. 1 . In graph  200 , y-axis  202  plots the concentration level of materials (e.g., germanium and other dopants) that are deposited along with silicon over the collector as part of growing the base, and x-axis  204  plots the thickness of the base as the deposition proceeds. Thus, the origin (i.e., the intersection of y-axis  202  and x-axis  204 ) of graph  200  corresponds to the top surface of the collector over which the silicon-germanium base is grown, at the point where fabrication of the base is to begin with the deposition of silicon only. 
   Continuing with  FIG. 2 , at point A on x-axis  204 , germanium is introduced into the deposition process and grows along with the silicon over the collector. Profile  206  illustrates the doping profile of the germanium. At point B, a suitable dopant is introduced into the mix with the silicon and germanium, and the concentration level of the dopant is shown by profile  208 . The dopant can be boron, for example. By the time the thickness of the base reaches point C, the germanium has been ramped down, and the deposition then continues with silicon and the dopant only. Finally, at point D, the growth of the base in the present example is complete and, as such, point D corresponds to the interface between the base and emitter. 
   Conventional methods for growing the base in a bipolar transistor generally involve a series of steps. In one approach used for forming the silicon-germanium base in a SiGe HBT, for example, a wafer having a transistor region over which the silicon-germanium base is to be grown is initially baked in a reactor chamber at approximately 900° C. for approximately five minutes. Subsequently, the chamber is cooled down to between 600° C. and 750° C. so that the desired base materials, for example silicon-germanium and boron, can be deposited. According to this method, the formation of the base typically requires between approximately five and ten minutes to complete, after which time the wafer is removed from the reactor chamber. 
   Once the base has been formed and the completed wafer has been removed, the chamber has to undergo extensive conditioning in preparation for the next wafer. The conditioning is necessary due to, for instance, the accumulation of materials on the chamber wall from previous deposition procedures that can adversely impact the processing of subsequent wafers. More specifically, a primary concern is the presence of residual dopant materials, such as boron, on the chamber walls which can contaminate subsequent wafers and compromise the electrical properties of the base layer formed on these subsequent wafers. 
   To eliminate the threat of contaminating subsequent wafers, a chamber etching step is needed after the processing of each wafer to remove the undesired materials from the chamber. Typically, the chamber temperature has to be raised to approximately 1100° C., and an etchant, for example HCl gas, is supplied to etch the dopant or undesired materials, along with the silicon and germanium, from the chamber walls. Once the chamber has been cleaned, the etchant is evacuated out of the chamber. The chamber temperature has to then be lowered to approximately 900° C. before the next wafer can be processed. 
   The need to clean the chamber after each wafer introduces a significant time and cost budget on manufacturers. The time required to clean the chamber translates to lower throughput and productivity and to higher manufacturing cost. Some manufacturers have tried to increase throughput by, for example, baking the wafers in a separate bake chamber and coupling the bake chamber to multiple epi chambers, wherein deposition of the base can occur. In this manner, wafers can be processed more quickly since one epi chamber can be depositing while the other epi chamber is being cleaned. The separate bake chamber supplies a steady number of wafers to the epi chambers. However, with this approach, an epi chamber still needs to be cleaned after every wafer to remove the undesired materials accumulated on the chamber walls. Manufacturers have also tried to increase production by simply investing in more equipment, such as more epi and bake chambers. Approaches known currently, however, remain inefficient. Whether due to lower throughput or due to higher costs as a consequence of the need for more equipment, conventional processing methods impose significant burdens on manufacturers. 
   There is thus a need in the art for an approach for fabricating bipolar transistors, such as SiGe HBT transistors, that is more efficient than conventional approaches, and which will increase throughput without imposing additional significant costs. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to method and system for fabricating a bipolar transistor and related structure. The present invention overcomes the need in the art for an approach for fabricating bipolar transistors, such as a silicon-germanium (“SiGe”) heterojunction bipolar transistor (“HBT”), that is more efficient than conventional approaches, and which will increase throughput without imposing additional significant costs. 
   In one exemplary embodiment, the invention is a method for fabricating the base of a bipolar transistor where the method comprises placing a first wafer in an undoped epi chamber. In one implementation, the first wafer is baked in a separate bake chamber prior to placing it (i.e. the first wafer) in the undoped epi chamber. Next a first undoped base layer is grown over the first wafer. As an example, the first undoped base layer can comprise a SiGe layer when the exemplary bipolar transistor is a SiGe HBT. After growing the first undoped base layer, the first wafer is transferred from the undoped epi chamber into a separate doped epi chamber. A first doped base layer is then grown over the first undoped based layer in the doped epi chamber. In one implementation, the dopant might be boron. Moreover, while the first wafer is being processed in the doped epi chamber, a second wafer can be processed in the undoped epi chamber. 
   In a related embodiment, the invention is a structure produced by the above method and, in yet another embodiment, the invention comprises a transfer chamber, a transfer arm, a bake chamber, and a separate undoped epi chamber and a doped epi chamber for practicing the invention&#39;s method. In a manner described in more detail below, the present invention results in a fabrication approach that is more efficient than conventional fabrication techniques, and which will increase throughput without imposing additional significant costs. Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a cross sectional view of some of the features of a bipolar transistor fabricated utilizing conventional methods. 
       FIG. 2  shows the doping profile of one type of bipolar transistor. 
       FIG. 3  illustrates a system for growing a base in a bipolar transistor in accordance with one embodiment of the present invention. 
       FIG. 4  shows a flowchart illustrating some exemplary steps taken to implement an embodiment of the invention. 
       FIG. 5  illustrates a cross sectional view of some of the features of a bipolar transistor fabricated in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is directed to method and system for fabricating a bipolar transistor and related structure. 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 mere example embodiments of the invention. To maintain brevity, other embodiments utilizing the principles of the present invention are not specifically described in the present application and are not specifically illustrated by the present drawings. 
   Illustrated in  FIG. 3  is exemplary wafer processing system  300 , which is used to describe the fabrication of an exemplary base in an exemplary bipolar transistor, in accordance with one embodiment of the present invention. In the present application, an exemplary silicon-germanium (“SiGe”) base in a SiGe heterojunction bipolar transistor (“HBT”) is used for the purpose of illustrating the invention&#39;s concepts by referring to specifics. However, it is apparent to an artisan of ordinary skill that the concepts and techniques of the present invention applies also to bipolar transistors other than SiGe HBTs and that the invention is not limited to the specific examples provided in the present application. Moreover, certain details and features, which are apparent to a person of ordinary skill in the art, have been left out of  FIG. 3  in order not to obscure the concepts of the present invention. 
   As shown in  FIG. 3 , system  300  includes transfer chamber  310 , in which is situated transfer aim  312 . Loadlock  314  is attached to transfer chamber  310  and contains wafers awaiting further processing. For example, loadlock  314  may have end-patterned wafers comprising transistor regions over which the base is to be fabricated. It is to be understood that end-patterned wafers comprise, among other features, transistor regions having components of a bipolar transistor prior to the formation of a base layer over the transistor regions. For example, the bipolar transistor can include a collector, certain oxide and isolation regions, and other components known generally to those in the art. Wafers in loadlock  314  can be picked up and handled by transfer arm  312 . 
   Continuing with  FIG. 3 , system  300  further includes bake chamber  316 , “undoped” epi chamber  318 , and “doped” epi chamber  320  connected to transfer chamber  310 . Bake chamber  316  can be any suitable bake chamber known in the art configured for baking wafers. Similarly, undoped epi chamber  318  and doped epi chamber  320  can be suitable reactors known in the art, in which materials for the base can be deposited. For example, undoped epi chamber  318  and doped epi chamber  320  can be configured for chemical vapor deposition (“CVD”) and/or related deposition methods. The temperature of undoped epi chamber  318  and doped epi chamber  320  can be maintained at a desired temperature to facilitate deposition of the base materials. 
   In the present embodiment, transfer arm  312  can transfer an end-patterned wafer from loadlock  314  and place the wafer in bake chamber  316  to be baked. The temperature of bake chamber  316  can be maintained separately from undoped epi chamber  318  and doped epi chamber  320 . As such, for the purpose of preparing the wafer for base fabrication, the temperature of bake chamber  316  can be maintained at between approximately 800° C. and 1000° C., for example. After a wafer has been in bake chamber  316  for a sufficient length of time, transfer arm  312  can transfer the wafer to undoped epi chamber  318 . 
   In undoped epi chamber  318 , fabrication of the base starts when a first semiconductor, such as silicon, is introduced into epi chamber  318  and begins growing over the end-patterned wafer, including over the transistor regions of the wafer. At a desired point (see, e.g., point A in graph  200 ), a second semiconductor, such as germanium, may be introduced and grown with the first semiconductor over the wafer. As the deposition process progresses, the relative concentrations of the first and second semiconductor materials can be controlled in a manner known in the art. Other materials, such as carbon, may also be introduced into epi chamber  318  and grown with the semiconductor materials. 
   At the point in the base fabrication process where introduction of the base dopant material is to occur (for example, at point B in graph  200 ), the deposition process in undoped epi chamber  318  ends. In other words, no dopant is introduced into undoped epi chamber  318 . As a result, the film deposited over the wafer in undoped epi chamber  318  comprises undoped semiconductor material, and the material formed on the walls of undoped epi chamber  318  likewise consists of only semiconductor(s). It is noted that the undoped semiconductor film deposited over the transistor regions of the wafer as part of the base fabrication process is also referred to as an “undoped base layer” in the present application. 
   Once the undoped base layer has been formed in undoped epi chamber  318 , transfer arm  312  can transfer the wafer to doped epi chamber  320 , wherein more materials, including the base dopant, are deposited to complete fabrication of the base. Thus, in doped epi chamber  320 , deposition may begin with the introduction of the desired concentrations of semiconductor materials and the dopant into doped epi chamber  320 . In the example of a silicon-germanium HBT, the semiconductor materials can comprise silicon and germanium, while the dopant can be boron, for example. In certain embodiments, other materials, such as carbon, may also be introduced into doped epi chamber  320  and grown with the semiconductor materials and the dopant. The relative concentrations of the materials introduced into doped epi chamber  320  can be controlled to achieve the desired electrical properties for the base. From the process performed in doped epi chamber  320 , a second, or “doped,” base layer comprising the semiconductor materials and the dopant material is grown over the undoped base layer formed in undoped epi chamber  318 . It is appreciated that the doped base layer may also comprise other materials, such as carbon, in some embodiments. 
   As part of the deposition process performed in doped epi chamber  320 , the materials introduced into doped epi chamber  320  accumulate on the walls of doped epi chamber  320 , in addition to growing on the base layer. Consequently, the walls of doped epi chamber  320  may be blanketed with residual dopant material, following formation of the doped base layer. However, because doped epi chamber  320  is used only for growing the doped base layer, and not the undoped base layer, the presence of dopants on the walls of epi chamber  320  has little effect on the processing of a subsequent wafer in doped epi chamber  320 . Stated differently, even if some of the residual dopant materials on the walls of epi chamber  320  should ultimately deposit onto a subsequent wafer as part of the doped base layer, the impact on the resulting base would be minimal, because the dopant is a desired constituent of the doped base layer. 
   Thus, by fabricating the base of a bipolar transistor in individual steps whereby undoped and doped layers of the base are grown separately in separate chambers, the present invention achieves greater throughput than conventional bipolar transistor fabrication techniques. The increase in throughput is possible because the threat of residual dopant materials on the chamber walls depositing uncontrollably and contaminating the base is averted, since the dopant material is introduced into only the epi chamber where the doped base layer is grown. At the same time, there is no residual dopant material in the undoped epi chamber to contaminate the undoped base layer. An advantage of the present invention, therefore, is that the chambers do not have to be cleaned or conditioned as frequently as conventional fabrication methods. As a result, the present invention achieves greater throughput and substantially reduces manufacturing costs. 
   Reference is now made to  FIG. 4 , illustrating exemplary process  400  for fabricating wafers and for growing the base of a bipolar transistor, such as a silicon-germanium HBT, in accordance with one embodiment of the present invention. Certain details and steps have been left out of process  400  which are apparent to a person of ordinary skill in the art. For example, a step may consist of one or more substeps or may involve specialized equipment, as known in the art. Process  400  begins at step  410  and continues to step  412  where transfer arm  312  places a first wafer comprising transistor regions over which a base is to be fabricated in bake chamber  316  to be baked. For example, a wafer placed in bake chamber  316  can be baked for between approximately one and five minutes at between approximately 800° C. and approximately 1000° C. 
   Next, at step  414  of process  400 , transfer arm  312  removes the first wafer from bake chamber  316  and places it (i.e. the first wafer) in undoped epi chamber  318 , and a second wafer is placed in bake chamber  316  by transfer arm  312 . At step  416 , an undoped base layer is deposited over the first wafer in undoped epi chamber  318 . The undoped base layer comprises semiconductor materials, such as silicon and germanium, for example. In some embodiments, other materials, such as carbon, may also be deposited as part of the undoped base layer. Also at step  416 , the second wafer is baked in bake chamber  316 . Thereafter, at step  418 , transfer arm  312  removes the first wafer from undoped epi chamber  318  and places it (i.e. the first wafer) in doped epi chamber  320 , while the second wafer is transferred into undoped epi chamber  318  by transfer arm  312 , and while a third wafer is placed in bake chamber  316  by transfer arm  312 . 
   A doped base layer is then deposited over the undoped base layer of the first wafer in doped epi chamber  320  at step  420 . In the present embodiment, the doped base layer comprises semiconductor materials, such as silicon and germanium, and a desired dopant, such as boron. In some embodiments, the doped base layer may also comprise other materials, such as carbon. Also at step  420 , an undoped layer is grown over the second wafer in undoped epi chamber  318 , and the third wafer is baked in bake chamber  316 . Process  400  then proceeds to step  422  where the first wafer, now having a base comprising an undoped layer and a doped layer, is removed from doped epi chamber  320  by transfer arm  312 . Further processing of the first wafer subsequent to step  422  can be performed in a manner known in the art and is not discussed in detail here. At step  422 , the second wafer is removed from undoped epi chamber  318  and transferred to doped epi chamber  320  by transfer arm  312 , while the third wafer is removed from bake chamber  316  and placed in undoped epi chamber  318  by transfer arm  312 . 
   Process  400  continues at step  424  where a doped base layer is deposited in doped epi chamber  320  over the undoped base layer of the second wafer. Also, at step  424 , an undoped base layer is formed over the third wafer in undoped epi chamber  318 . Next, at step  426  of process  400 , the second wafer having a base comprising an undoped base layer and a doped base layer is removed from doped epi chamber  320  by transfer arm  312 , while the third wafer is transferred from undoped epi chamber  318  to doped epi chamber  320  by transfer arm  312 . Then, at step  428 , a doped base layer is deposited over the undoped base layer of the third wafer in doped epi chamber  320 , following which the third wafer is removed from doped epi chamber  320 . Process  400  then ends at step  430 . 
   Referring now to  FIG. 5 , exemplary structure  500  is used to describe fabrication of a base in a bipolar transistor, such as a silicon-germanium HBT, in accordance with one embodiment. Certain details and features have been left out of  FIG. 5  which are apparent to a person of ordinary skill in the art. As shown, structure  500  comprises collector  530  having top surface  532  formed in substrate  510 . Structure  500  further includes field oxides  538 . It is appreciated that collector  530  having top surface  532 , substrate  510 , and field oxides  538  in structure  500  are respectively equivalent to collector  130  having top surface  132 , substrate  110 , and field oxides  138  in structure  100  illustrated in  FIG. 1 . 
   Continuing with  FIG. 5 , it is shown in structure  500  that base  520  is made up of two separate layers, i.e., undoped base layer  522  and doped base layer  524 . Undoped base layer  522  in the present embodiment comprises semiconductor materials only, while doped base layer  524  comprises semiconductor materials doped with a desired dopant. The semiconductor materials in undoped and doped base layers  522  and  524  can comprise silicon and germanium, while the dopant in doped base layer  524  can be boron, for example. In some embodiments, base  520  may comprise additional materials, such as carbon. 
   It is appreciated that base  520  can be fabricated according to the steps of process  400  in  FIG. 4 , which can be performed by system  300  in  FIG. 3 . In other words, undoped base layer  522  may be formed first in undoped epi chamber  318  wherein only semiconductor materials are deposited, and doped base layer  524  may be separately formed in doped epi chamber  320 , wherein a dopant such as boron is deposited along with the semiconductor materials. It is appreciated that further processing, including the fabrication of an emitter over base  520 , results in a bipolar transistor. 
   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. Also, it is appreciated that certain details have been left out in order to not obscure the invention but that these details are known to those of skill in the art. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skills in the art would appreciate that changes can be made in form and detail without departing from the spirit and the scope of the invention. For example, although the application has referred occasionally to the silicon-germanium HBT structure, it will be apparent to a person of ordinary skill in the art how the invention can be applied in similar situations where greater throughput and lower costs are desired in fabricating bipolar transistors. 
   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. 
   Thus, method and system for fabricating a bipolar transistor and related structure have been described.