Abstract:
A method for forming a base of a bipolar transistor. A narrow base is formed using a flash of boron doping gas in a reaction chamber to create a narrow base with high boron concentration. This method allows for reliable formation of a base with high boron concentration while maintaining manageability in controlling deposition of other materials in a substrate.

Description:
BACKGROUND 
   1. Field 
   Circuit fabrication, including a method for forming a base region of a bipolar transistor. 
   2. Description of the Related Art 
   Bipolar transistors, more specifically heterojunction bipolar transistors (HBTs) are used in devices requiring high frequency operation such as wireless and networking devices. HBTs are used in these devices because of their high cut off frequencies greater than 150 gigahertz (Ghz) even though they consume more power than equivalent metal oxide semiconductor (MOS) based technologies. 
   HBTs typically consist of an emitter region, base region and collector region. The emitter region generally has a larger band gap than the base region to achieve high frequency performance. The speed at which the HBT can switch is referred to as the cutoff frequency, f t . The cutoff frequency of a given HBT is generally related to the width of its base region. The narrower the base region of a HBT, the shorter the base transit time and higher the cutoff frequency, f t . 
   HBTs formed in silicon germanium (SiGe) films typically use boron diffusion or implantation to form a base region. Methods for forming HBTs include chemical vapor deposition (CVD) techniques. However, current methods for forming HBTs are limited in further reliably decreasing the base width because such methods are generally unable to create adequate boron concentration in a base region with reduced base width while controlling the SiGe deposition rate. Boron tends to diffuse rapidly in a SiGe film using current methods causing thick base widths of greater than 25 nanometers (nm). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
       FIG. 1  is a flow chart of a method to fabricate an ultra narrow boron doped SiGe:C base. 
       FIG. 2  depicts a substrate within a reactor chamber and a manifold control flow system of the reactor. 
       FIG. 3  is a Secondary Ion Mass Spectrometry (SIMS) profile of ultra narrow boron doped SiGe:C base of an HBT, including boron and germanium concentration for various flow rates of diborane gas. 
       FIG. 4  is a schematic depicting the placement of in situ doped boron and carbon in the Si/SiGe/Si base stack. 
       FIG. 5  is a schematic of an embodiment of an HBT with a fluorine passivation layer. 
       FIG. 6  is a SIMS profile demonstrating the effect of a fluorine passivation layer on boron diffusion. 
       FIG. 7  depicts a cross-sectional side view of a portion of a substrate on which an HBT is to be formed and shows a layer thereon. 
       FIG. 8  depicts the substrate of  FIG. 7  with a silicon germanium layer formed thereon. 
       FIG. 9  depicts the substrate of  FIG. 8  with carbon substitutionally combined with the silicon germanium layer. 
       FIG. 10  depicts the substrate of  FIG. 8  with a boron doped region of silicon germanium formed thereon. 
       FIG. 11  depicts the substrate of  FIG. 10  with an additional layer of silicon germanium formed over the boron doped region. 
       FIG. 12  depicts a substrate where a fluorine passivation layer has been formed to encompass a boron doped region. 
   

   DETAILED DESCRIPTION 
   Exemplary embodiments are described with reference to specific configurations and techniques. Those of ordinary skill in the art will appreciate the various changes and modifications to be made while remaining within the scope of the appended claims. Additionally, well known elements, devices, components, circuits, process steps and the like are not set forth in detail. 
     FIG. 1  is a flow chart of one embodiment of a method of forming a heterojunction bipolar transistor (HBT).  FIG. 2  illustrates a substrate in a chamber undergoing the method described in FIG.  1 .  FIGS. 7-12  show the formation of an HBT on a substrate according to one method. 
   Referring to  FIG. 1 , a wafer or substrate is placed in a reactor (block  1 ). Referring to  FIG. 2 , Reactor  100  includes reactor chamber  101 . In one embodiment, the chamber is that of an Epsilon® E3000 300 millimeters (mm) Epitaxial Reactor or E2500 or E2000 200 mm Epitaxial Reactor manufactured by ASM, Inc. Within reactor chamber  101  is wafer holder on stage  103  for securing substrate  102  onto which depositions are to be made. Reactor  100  also includes a heat source disposed in reactor chamber  101 , such as in wafer holder  103  (e.g., a thermocouple). The temperature within reactor chamber  101  may be monitored by one or more temperature gauges (not shown). Source gases and carrier precursor gases enter the reaction chamber at port  120  and flow through reaction chamber  101  to chamber exhaust port  121 . Chamber exhaust  104  is coupled to vacuum  105  which maintains a desired pressure within the chamber. The pressure within reaction chamber  101  may be monitored by one or more pressure gauges (not shown). In one embodiment, the temperature gauges and/or the pressure gauges are coupled to a process controller that regulates the temperature and pressure within reaction chamber  101 . 
   Introduction of gases into the chamber are managed by manifold control valves  107 ,  109 ,  111 ,  113 , and  115 . Source and carrier gases are generated at source points  106 ,  108 ,  110 ,  112 , and  114 . In one embodiment, the control valves are coupled to a system controller. In another embodiment, the gases introduced through the control valves are carrier precursor gases. A carrier precursor gas is a gas or energized gas of one or more of ions or radicals of a constituent that upon introduction into a substrate produces carriers such as electrons or holes. Such gases are distinguished from inert carrier gases such as nitrogen (N 2 ) or Hydrogen (H 2 ) that may be used in delivering a carrier precursor gas to reaction chamber  101 . 
   In one embodiment, substrate  102  may be a semiconductor substrate such as a silicon wafer (e.g., a 300 millimeter (mm) diameter silicon wafer). Alternatively, substrate  102  may be a silicon-on-insulator (SOI) substrate such as a single crystal silicon film on an insulator. 
   In one embodiment, a system controller controls the environmental conditions and process elements in the reactor chamber  101  including manifold control valves  107 ,  109 ,  111 ,  113  and  115  and other process related devices. The system controller, for example, controls reactor chamber  101  temperature, flow rates of source and carrier gases into the reactor chamber  101  and the timing of source gas release into the reactor chamber. In one embodiment, the system controller receives input from a user to set any of the environmental conditions, process steps, or to create a set sequence of changes for the process elements or environmental conditions. In one embodiment, the system controller is coupled to a memory storage device  140  comprising a machine-readable medium having a machine-readable program embodied therein for directing operation of the system. In one embodiment, user input is given to the system controller using a system controller interface  135 . 
     FIGS. 7-12  show the formation of an HBT on substrate  102  according to one representation technique. 
     FIG. 7  shows a 10 nm layer of single crystal semiconductor  703  formed on the substrate  102  by introducing a 20 standard cubic centimeter per minute (sccm) flow of silicon-based gas through the associated manifold  107  into the reaction chamber  101  from source  106  ( FIG. 1 , block  2 ). In one embodiment, the silicon based gas is silane (SiH 4 ). Other silicon based gases such as disilane or dichlorosilane can also be used for this purpose. This thin single crystal silicon layer acts as a seed layer and helps nucleation and growth of the SiGe layer. In one embodiment the SiH 4  source is 1% SiH 4  in a SiH 4  and H 2  mixture. 
   In one embodiment, after the single crystal silicon layer is formed, a germanium based gas is introduced into the reaction chamber  101  ( FIG. 1 , block  3 ). In one embodiment, the germanium-based gas is germane (GeH 4 ). Other germanium based gases include dichlorogermane. 
   One initial flow rate of the germanium based gas GeH 4  is 45 sccm. The GeH 4  is introduced into reactor chamber  101  through manifold  109  from source  108  (See FIG.  2 ). In one embodiment, the percentage of GeH 4  in the source gas is one percent with the remainder a carrier gas such as hydrogen H 2 . 
   A collector region  705  is formed over the single crystal silicon layer. In one embodiment, the germanium based gas flow is ramped up to 140 sccm within 13 seconds of introduction into reactor chamber  101  ( FIG. 1 , block  5 ). In this embodiment, the silicon based gas flow is held constant at 20 sccm during the ramp up period. This mixture of gases in the reaction chamber  101  results in the concentration of germanium in the deposition on the substrate  102  to rapidly grade from zero to 17 percent. This forms silicon germanium (SiGe) layer  705  over the pure silicon layer as illustrated in FIG.  8 . In one embodiment, the GeH 4  and SiH 4  flow through the reaction chamber  101  for 29 seconds at 140 (sccm) and 20 sccm, respectively ( FIG. 1 , block  6 ). The layer of SiGe that is formed is 20 nm wide with an overall germanium concentration of 17 percent measured by Secondary Ion Mass Spectrometry (SIMS). 
   In one embodiment, carbon based gas is introduced into reaction chamber  101  after the germanium based gas is introduced ( FIG. 1 , block  4 ). In this embodiment, a carbon based gas such as methyl silane (CH 3 SiH 3 ) is introduced into reactor chamber  101  through the associated manifold flow control valve  111  from source  110 .  FIG. 9  shows carbon substitutionally combines with the forming SiGe layer to create SiGe:C layer  707 . The percentage of methyl silane in the source is approximately 2 percent with the remainder a carrier gas such as H 2 . In another embodiment, carbon-based gas is introduced into the reactor chamber  101  by manifold flow control valve  111  approximately when the germanium-based gas is introduced and continues, in this embodiment, at a constant flow rate of 20 sccm until completion of depositions related to forming the base region are complete. Other possible carbon carrier gases include Methyl Silane (CH 3 SiH 3 ). 
   Carbon substituted in the manner described above into the SiGe lattice serves as a diffusion suppressant for boron. In one embodiment, the SiGe:C layer limits initial diffusion of boron to a narrow base region. Widening of the doped base region during subsequent processing, especially during annealing of the substrate during refinement subsequent to the formation of the bipolar transistor are likewise limited by the SiGe:C layer. For example, subsequent annealing processes may exposes the bipolar transistor to temperatures up to 1080 degrees Celsius (° C.). However, carbon subsitutionally combined in the SiGe lattice as in this embodiment can limit diffusion of, for example, boron and maintain the width of a boron doped base region of 10 nm width to a width of 14 nm after annealing. 
   In one embodiment, after a 20 nm SiGe or SiGe:C layer  707  is formed over the single crystal silicon layer  703 , the germanium based gas flow is slowly ramped down from its peak flow of 140 sccm ( FIG. 1 , block  7 ). In this embodiment, during the period when the germanium-based gas is ramped down, manifold flow control valve  115  for diborane (B 2 H 6 ) (See  FIG. 2 ) is saturated with diborane gas ( FIG. 1 , block  8 ). The concentration of diborane in source  114  is 1 percent with the remainder a carrier gas such as H 2 . In this embodiment, manifold flow control valve  115  associated with the source  114  for diborane directs the diborane gas flow directly to reactor exhaust  104 . A steady state of diborane gas flow is established in associated manifold flow control valve  115 . In another embodiment, the diborane flow is directed to the exhaust  104  by manifold flow control valve  115  before the germanium flow ramp down begins. 
   In one embodiment, when the germanium-based gas flow grades down to 75 sccm, the diborane will be in a steady state and introduced into reaction chamber  101  by manifold flow control valve  115 . In this embodiment, a diborane gas flow of 20 sccm is used. The diborane gas flow introduction is “flashed” into the reaction chamber  101 . A representative flash of diborane gas is on the order of a few seconds, e.g, three seconds (block  9 ). In this embodiment, during the diborane flash period, the germanium gas flow is reduced from 75 sccm to 70 sccm. A 10 nm doped base region of the transistor is thereby formed ( FIG. 1 , block  10 ). A 20 sccm flow of diborane gas forms a 8E19 carriers per cubic centimeter peak concentration doped base region. In other embodiments, the diborane gas flow rate is varied up to 100 sccm. At 100 sccm, a peak concentration of 2E20 carriers per cubic centimeter is achieved.  FIG. 10  shows the boron doped region  709  formed over emitter region  705  and substrate  703 . 
   The variables of the diborane gas flow flash including length of the flash period, rate of gas flow and concentration of source gas can be varied to achieve predictable results. The diborane gas flash can be for a lengthened period of time resulting in a wider doped base region including flash periods up to 20 seconds in length. Diborane flashes with duration of less than three seconds tend not to produce base regions with sufficient boron concentration levels according to current processing limitations. 
   In one embodiment, a SiGe layer continues to form after the diborane flash is completed and the boron doped region has been formed. The SiH 4  and GeH 4  continues to flow into the reactor chamber  101 .  FIG. 11  shows a substrate with a SiGe layer  711  formed over the boron-doped region  709 . The flow of SiH 4  and GeH 4  is halted to complete the layer  711  (block  11 ). Alternatively, a CH 3 SiH 3  flow is stopped with the SiH 4  and GeH 4  where CH 3 SiH 3  is used to form SiGe:C. In one embodiment, the flow of SiH 4  continues after the GeH 4  and CH 3 SiH 3  flows have halted. A 20 nm silicon cap is formed over the SiGe:C structure ( FIG. 1 , block  12 ). The SiH 4  flow is subsequently stopped once the cap is completed ( FIG. 1 , block  13 ). 
     FIG. 3  is a Secondary Ion Mass Spectrometry (SIMS) profile of one embodiment. The profile charts the concentration of boron and germanium over the depth of the transistor being formed. Such as the transistor illustrated in  FIGS. 7-12 . The profile charts germanium concentration and diborane concentration for flow rates of diborane at 11, 17, 23, 29 and 35 sccm. The graph illustrates that increased boron concentration levels can be achieved while maintaining a very narrow base width. The graph illustrates the ramping up of germanium concentration. Then during the diborane flash (e.g., 400 Å to 600 Å) increased boron concentration levels are achieved at increased diborane flow rates while maintaining the 10 nm width of the base region. 
     FIG. 4  is a graph of one embodiment where carbon is substitutionally combined with the silicon germanium to minimize boron out diffusion. The graph illustrates carbon germanium and boron depositions over time. The hard black line slopping up depicts the rising germanium levels combining with the carbon and silicon (not shown) to create the emitter region  707 . Diborane is flashed into the chamber to form the 10 nm the boron doped base  709 . Germanium continues to deposit to form collector region  711 . 
   In another embodiment, a fluorine passivation layer is used to minimize out-diffusion of boron in the base layer during processing subsequent to the initial deposition of the boron to form the base.  FIG. 5  is a SIMS profile of this embodiment with emitter region  501 , base region  502 , and collector region  503 . The fluorine passivation layer  504  encompasses the base region  502 . 
   Referring to  FIG. 2 , in one embodiment, the fluorine passivation layer is formed by introducing a fluorine gas such as fluorine (F 2 ) into the chamber  101 . In another embodiment, the fluorine passivation layer is formed by cracking SiF 6  using plasma techniques, non-plasma techniques or external fluorine passivation. 
   The fluorine gas may be introduced before the diborane flash and ending the F 2  flow into the chamber after the diborane flow is stopped. In another embodiment, the fluorine flow is started after the diborane flow is started but before it is complete. In another embodiment, the fluorine flow is started before the diborane flow but stopped before the diborane flow into the chamber is stopped. This results in a fluorine passivation layer that does not entirely encompass the boron doped region thereby only limiting out diffusion of boron on one side of the base region. It is believed that the fluorine minimizes boron out diffusion by combining with the silicon germanium substrate at substitutional sites in the silicon germanium lattice.  FIG. 12  illustrates a substrate where a fluorine passivation layer  713  has been formed to encompass the boron doped region  709 . In this embodiment, the fluorine passivation layer suppress out diffusion of boron into SiGe layers  705  and  711 . 
     FIG. 6  is a graph illustrating the effect of fluorine passivation layer on boron out diffusion after annealing at 1080° C. The graph illustrates a range of thickness for the boron doped region over concentrations from 1E+16 to 1E+21 before annealing (line  605 ). The graph also illustrates the range of thickness after annealing at 1080° C. without fluorine passivation layer (line  601 ) and with a passivation layer (line  603 ). The graph thereby demonstrates the effect of fluorine to limit out diffusion of boron during annealing. For example, a boron doped region with a width as deposited of 1300 Å may expand to a width of approximately 1750 Å if no fluorine passivation layer is present. However, if a fluorine passivation layer is present, the expansion of the boron doped region is limited to 1550 Å. 
   In one embodiment, using a CVD chamber, all depositions are carried out with a reactor chamber temperature of 600° C. The rate of deposition is generally affected by temperature. The temperature also affects the substitution of carbon into the SiGe lattice. If carbon is introduced at temperatures above approximately 600° C., the carbon tends not be introduced substitutionally to the SiGe lattice. At 600° C. carbon concentration will be approximately 0.3 percent in the SiGe:C layer. In other embodiments, deposition temperature and carbon concentration in the SiGe:C layer can vary. 
   In one embodiment, using the CVD chamber, all depositions are carried out at a chamber pressure of 80 torr. In other embodiments chamber pressure can vary from a few mtorr to atmospheric pressure. 
   In another embodiment, a carrier gas is introduced into the reaction chamber throughout the depositions. In one embodiment this carrier gas is hydrogen (H 2 ). H 2  is introduced into the reaction chamber as a carrier gas at 20 standard liters per minute (slpm) through associated manifold flow control  113  from hydrogen source  112 . In other embodiments, H 2  flow can vary from 5 slpm to 50 slpm. 
   Having disclosed exemplary embodiments, modifications and variations may be made to the disclosed embodiments while remaining within the spirit and scope of the invention as defined by the appended claims.