Abstract:
A base structure for high performance Silicon Germanium (SiGe) based heterojunction bipolar transistors (HBTs) with arsenic atomic layer doping (ALD) is disclosed. The ALD process subjects the base substrate to nitrogen gas or hydrogen gas (in ambient temperature approximately equal to 500 degrees Celsius) and provides an additional SiGe spacer layer. The surface of the final silicon cap layer is preferably etched to remove most of the arsenic. The resulting SiGe HBT with an arsenic ALD layer is less sensitive to process temperature and exposure times, and exhibits lower dopant segregation and sharper base profiles.

Description:
CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY 
   The present application is related to co-pending U.S. patent application Ser. No. 11/367,030 filed Mar. 2, 2006, entitled “HIGH PERFORMANCE SiGe:C HBT WITH PHOSPHOROUS ATOMIC LAYER DOPING.” U.S. patent application Ser. No. 11/367,030 is assigned to the assignee of the present application and is hereby incorporated by reference into the present disclosure as if fully set forth herein. The present application claims priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 11/367,030. 
   TECHNICAL FIELD OF THE INVENTION 
   The present invention is directed, in general, to atomic layer doping (ALD), and more specifically, to a high performance Silicon Germanium (SiGe) based heterojunction bipolar transistors (HBTs) with arsenic ALD. 
   BACKGROUND OF THE INVENTION 
   ALD is a method in which deposition of an atomic layer of material is controlled by a pre-deposited layer of a precursor. ALD processes typically promote the adsorption of gases into a substrate and hence the deposition of an atomic layer of material on the substrate. By alternating the supply of a reaction gas and a purging gas, ALD processes can uniquely control the deposition of material on a substrate at an atomic level. For example, when a reaction gas (or precursor) is exposed to a substrate surface, atoms of the reaction gas are chemically adsorbed into the substrate. The reaction gas is then purged by exposing the substrate to a purging gas. The purging gas ideally only reacts with the substrate where the reaction gas had been previously adsorbed. The resulting chemical reaction eventually forms an atomic layer of material onto the substrate surface. ALD thus provides unique control of the doping dosage and doping location at an atomic level, allowing for selective layer growth and both single and double layer dopant coverage. 
   Conventional ALD applications aid in meeting micro-scaled production requirements. ALD applications are typically ideal for abrupt, localized highly doped structures with relatively sharp profiles and provide little or no interaction with the growing layer. Conventional ALD doping, in a hydrogen gas ambient, generally provides high segregation of the dopant (in most cases phosphorus) and a broad profile. Thus, conventional ALD processes limit (direct current) DC and (radio frequency) RF performance. Important band engineering factors such as the gain, Early voltage, voltage between the base and collector and cutoff frequency are adversely affected. 
   Conventional silicon-based bipolar junction transistors (BJTs) have been a dominant semiconductor device since the advent of the integrated circuit. Many other semiconductor materials outperform silicon-based devices. However, because most semiconductors are incompatible with the silicon-based process technologies, the development of such materials has not been forthcoming. Silicon Germanium (SiGe) and Silicon Germanium: Carbon (SiGe:C) have been recent exceptions. 
   SiGe has an energy gap that varies as a function of the concentration of Germanium. Thus, SiGe allows for band-gap engineering, which in turn provides improvements in high speed and high frequency performance. A principal application of SiGe has been with heterojunction bipolar transistors (HBTs). The base of an HBT is the most heavily doped region of the transistor and is thus a prime area for band-gap engineering. SiGe HBTs generally offer a higher unity gain frequency, lower noise, higher collector currents and better linearity than the conventional silicon BJT. Moreover, SiGe HBTs may be integrated with existing CMOS technologies, keeping production costs for low powered, high performance products relatively low. 
   In conventional doping applications, the dopant, phosphorus (P), exhibits high levels of segregation as Germanium (Ge) concentrations are varied. The overall dopant profile resulting from conventional doping methods is not very sharp, but in fact relatively broad. The steepness or sharpness of the resulting curve due to phosphorus segregation is typically about 20 nanometers per decade (20 nm/dec). High P segregation adversely affects important transistor characteristics such as the gain, Early voltage, voltage between the base and collector, and cutoff frequency. Accordingly, SiGe transistors made in accordance with conventional doping methods exhibit relatively poor RF and DC performance. Moreover, other dopants, such as arsenic, exhibit high migration properties, especially in patterned wafers or oxide windows. Thus, such single crystal base structures result in extremely low doping. 
   There is, therefore, a need in the art for an ALD system in which there is low segregation of a dopant and less sensitivity to temperature and exposure time. There is also a need for an improved system for producing high performance SiGe based HBTs with ALD. 
   SUMMARY OF THE INVENTION 
   The present disclosure generally provides a process and base structure for high performance Silicon Germanium (SiGe) based heterojunction bipolar transistors (HBT) with arsenic atomic layer doping (ALD). 
   In an embodiment of the disclosure, an atomic layer doping method is provided. The method comprises flowing a first gas adjacent to a substrate; purging the substrate with arsenic; reacting a surface of the substrate to form an atomic layer of a compound; and flowing a second gas adjacent to the substrate. The compound may be an arsenic ALD layer. The concentration of arsenic may be about 3.5*10 13  atoms per square centimeter. The first gas may be one of: hydrogen and nitrogen, while the second gas may be hydrogen. The atomic layer doping method is preferably performed in an ambient of about 500 degrees Celsius. The method may further comprise growing a first SiGe layer prior to flowing the first gas adjacent to the substrate. The method may further include growing a SiGe spacer layer and a silicon cap layer. The method may still further include etching the silicon cap layer to reduce the presence of arsenic. 
   In another embodiment of the disclosure, a transistor base structure is provided. The transistor base structure comprises a SiGe layer; an ALD layer adjacent to the SiGe layer, wherein the ALD layer is doped with arsenic; and a SiGe spacer adjacent to the ALD layer, wherein the SiGe spacer layer is grown in a gas ambient. The gas ambient may be one of: hydrogen gas and nitrogen gas. The transistor base structure may include an ALD layer doped with arsenic. The concentration of arsenic may be 3.5*10 13  atoms per square centimeter. The transistor base structure may also include a silicon buffer layer adjacent to the SiGe layer and a silicon cap layer adjacent to the SiGe spacer. The surface of the silicon cap layer may be etched to remove at least some of the arsenic. The transistor may be a SiGe HBT. A secondary ion mass spectrometry (SIMS) profile of arsenic concentration as a function of depth for the transistor base structure is preferably about 6 nanometers per decade. 
   In still another embodiment of the disclosure, a method for building a SiGe HBT base structure is provided. The method comprises growing a silicon cap layer adjacent to a silicon buffer; and purging the Ge into the silicon cap layer to form a SiGe layer. The method also includes exposing the SiGe layer to a gas ambient at about 500 degrees Celsius; purging the SiGe:C layer with arsenic; and growing an ALD arsenic layer adjacent to the SiGe layer at about 500 degrees Celsius. The method further includes growing a SiGe spacer layer in an N 2  ambient, wherein the SiGe spacer layer is adjacent to the ALD dopant layer; flowing a gas adjacent to the SiGe spacer layer; and growing a silicon cap layer adjacent to the SiGe spacer. The gas ambient may be one of: hydrogen and nitrogen. The concentration of arsenic may be 3.5*10 13  atoms per square centimeter. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. It is also noted that the term “layer” may mean a single layer, a portion of a layer, a layer within a layer, a sub-layer and/or multiple layers. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
       FIG. 1  depicts a simplified cross section of a base structure  100  of a Silicon Germanium (SiGe) based heterojunction bipolar transistor (HBT) in accordance with an embodiment of the present disclosure; 
       FIG. 2  depicts an exemplary process diagram for atomic layer doping (ALD) in accordance with an embodiment of the present disclosure; 
       FIG. 3  depicts a simplified cross section of a base structure of an embodiment of the present disclosure; and 
       FIG. 4  depicts an example of a secondary ion mass spectrometry (SIMS) profile of the dopant concentration (atoms/cm 3 ) as a function of depth (μm) when exemplary doping methods in accordance with an embodiment of the present disclosure are used. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 4 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged device. 
     FIG. 1  depicts a simplified cross section of a high performance Silicon Germanium (SiGe) based heterojunction bipolar transistor (HBT) base structure  100 . Two layers of SiGe  101  and  103  sandwich a dopant layer  102 . Unlike conventional methods which typically use phosphorus (P) as the dopant, the base region of an exemplary SiGe HBT in accordance with an exemplary embodiment of the present disclosure uses arsenic (As) as the dopant to form dopant layer  102 . 
     FIG. 2  depicts an exemplary process diagram  200  for atomic layer doping (ALD) in accordance with an embodiment of the present disclosure. Process  200  begins at step  201  with a silicon surface layer  301   a  (see FIG.  3 ) in an ambient temperature of approximately 400 degrees Celsius (400° C.). Silicon surface layer  301   a  is baked at about 900° C.) in step  202  to remove any residual contaminant from the surface. Then, in step  203 , silicon surface layer  301   a  is cooled to appropriately 600-650° C. At an ambient temperature of approximately 600° C., a silicon buffer layer  301   b  is grown on top of the silicon surface layer  301   a  in step  204 . The thickness of silicon buffer layer  301   b  is grown to about 2-10 nm. Preferably, silicon buffer layer  310   b  is grown to about 5 nm. 
   The concentration of Ge is preferably controlled to remain substantially the same during steps  205  through  210 . Process  200  continues in steps  205  and  206 , where the ambient temperature is kept at approximately 600° C. and two epitaxial layers of germanium (Ge) are purged into the silicon cap layer grown in step  204 . Steps  205  and  206  control Ge grading from essentially zero to about 20%. Preferably, Ge grading is sustained at about 15% Ge. After purging the silicon cap layer grown in step  204  with Ge, a SiGe layer  302  (see  FIG. 3 ) is formed in step  207 . The thickness of SiGe layer  302  is generally kept between 30-100 nm. Preferably, SiGe layer  302  is about 50 nm thick. SiGe layer  302  is then exposed to a hydrogen (H 2 ) ambient and cooled to approximately 500° C. in step  208 . Alternatively, in step  208 , SiGe layer  302  may be exposed to a nitrogen (N 2 ) ambient. 
   Process  200  continues by maintaining the deposition temperature at about 500° C. in steps  209  and  210 . This is a reduction in temperature over conventional ALD doping processes. At 500° C., doping segregation effects are minimized while maintaining a high epitaxial growth rate and complying with any other manufacturing requirements. In step  209 , the epitaxial growth process is temporarily interrupted and the surface of the SiGe layer  302  is exposed to dopant, preferably As, for about one minute. The result is arsenic ALD layer  303  (see  FIG. 3 ). The concentration of dopant, As, is maintained between about 1×10 13  Atoms/cm 2  and 1×10 14  Atoms/cm 2 . Preferably, the concentration of dopant, As, is about 3.5×10 13  Atoms/cm 2 . 
   After exposure to arsenic in step  209 , SiGe spacer  304  (see  FIG. 3 ) is allowed to grow in an N 2  ambient for a predetermined amount of time in step  210 . Step  210  may occur in an H 2  ambient, but an N 2  ambient is preferred to reduce segregation. SiGe spacer  304  is grown to a thickness between about 2-20 nm. Preferably, SiGe spacer  304  is grown to about 10 nm. In step  210 , the top surface of the SiGe spacer  404 , is preferably exposed to an N 2  ambient to aid eventually reducing vapor pressure (VP) auto-doping due to any hydrogen carry-over or memory effect later in process  300 . Alternatively, in step  210 , the top surface of the SiGe spacer  404  may be exposed to an H 2  ambient. In step  211 , the SiGe spacer  304  is exposed to a hydrogen ambient (H 2 ). At this stage, process  200  preferably exposes the SiGe spacer  304  to an H 2  ambient rather than an N 2  ambient. At higher temperatures, an N 2  ambient would adversely react with silicon, while an H 2  ambient facilitates building a silicon cap faster than the same in an N 2  ambient. 
   Process  200  continues in step  212  by increasing the ambient temperature to about 650° C. and growing a final silicon cap layer  305  (see  FIG. 3 ). Silicon cap layer  305  is grown to a thickness between about 20 nm and 60 nm. Preferably, silicon cap layer  305  is about 40 nm thick. Finally, step  212  ends with optional hydrochloric acid etching. In other words, silicon cap layer  305  is preferably exposed to an in-situ etch deposition that reduces arsenic surface contamination or surface poisoning. The etching in step  212  may alternatively occur after the epitaxial process or ex-situ. After cooling the temperature to about 600° C. in step  213 , the resulting base structure  300  (see  FIG. 3 ) may be removed. Thus, a SiGe HBT base structure  300  is formed with an arsenic ALD layer  303 . Notably, in SiGe, arsenic exhibits a much lower diffusion coefficient than the same in phosphorus. In addition, arsenic does not require the addition of carbon to the SiGe film to control any transient enhanced diffusion. Thus, base structure  300  does not require any Silicon Germanium Carbon (SiGe:C) layers. 
   In summary, process  200  results in the exemplary base structure  300  illustrated in  FIG. 3 . Silicon surface layer  301   a  is topped with silicon buffer layer  301   b . SiGe layer  302  is grown on top of silicon buffer layer  310   b . An arsenic ALD layer  303  is grown on top of the SiGe layer  302 . The SiGe layer  302  is topped with a SiGe spacer  304 . The resulting base structure  300  is finished off with a silicon cap layer  305 . 
     FIG. 4  depicts an example of a secondary ion mass spectrometry (SIMS) profile  400  illustrating dopant concentration (Atom/cm 3 ) as a function of depth (μm) when exemplary doping methods in accordance an embodiment of the present disclosure are used. The concentration of dopant, As, is shown by plot  401  in  FIG. 4 . On the other hand, the concentration of Ge is shown by plot  402 . The steepness of the profile is optimized to about 6 nm/dec and full width at half maximum in less than 10 nm at 500° C. Preferably, the steepness of the profile should be minimized. 
   Accordingly, a robust process with sharp base profiles conducive for use in, for example, complimentary high speed BiCMOS where ALD techniques are is disclosed. Such techniques yield less sensitivity to process temperatures and make it possible to reduce exposure times while minimizing outdiffusion. 
   It is important to note that while the present invention has been described in the context of a fully functional process, those skilled in the art will appreciate that at least portions of the process are capable of adapting to a variations within the process without deviating from the preferred embodiments described above. Although the present invention has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, enhancements, nuances, gradations, lesser forms, alterations, revisions, improvements and knock-offs of the invention disclosed herein may be made without departing from the spirit and scope of the invention in its broadest form.