Double self-aligned fabrication process for making a bipolar transistor structure having a small polysilicon-to-extrinsic base contact area
This is a division of U.S. patent application Ser. No. 280,142 filed June 30, 1981 which has now been abandoned. 
    
    
     DESCRIPTION 
     1. Technical Field 
     This invention relates to an oxide isolated bipolar transistor structure with a small polysilicon to extrinsic base contact area, fabricated by a double self-aligned process wherein the base is self-aligned to an oxide window and the emitter is self-aligned to the base. 
     2. Background Art 
     U.S. Pat. No. 4,157,269 issued June 5, 1979 to Ning, et al entitled Utilizying Polysilicon Diffusion Sources and Special Masking Techniques describes a process for fabricating a bipolar transistor using a single self-aligned process wherein the emitter is self-aligned to the extrinsic base contacts. The polysilicon contact to the extrinsic base region, being defined by the relative alignment of two mask levels, is much larger than that achieved by the present invention. 
     U.S. Pat. No. 3,398,355 issued Aug. 20, 1978 to Dill, Jr., entitled Transistor Structure with an Emitter Region Epitaxially Grown over the Base Region shows a transistor structure with a relatively large base contact area, and is cited only because the base contact includes an insulating layer thereover. The base contact is aluminum and the insulating layer is aluminum oxide film grown on the aluminum layer to maintain it stable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a cross-section view of a bipolar transistor structure following the principles of the present invention. 
     FIGS. 2-1 through 2-6 are illustrations of the cross-section of the bipolar transistor of FIG. 1 during selected steps in a double-self-aligned fabrication process according to the present invention. 
    
    
     DISCLOSURE OF THE INVENTION 
     The present invention provides an improved bipolar transistor structure and a preferred double-self-aligned process for fabricating such structure. 
     FIG. 1 illustrates a cross-sectional view of an embodiment of a bipolar transistor structure following the principles of the present invention. 
     The structure of FIG. 1, resulting from a double-self-alignment fabrication process to be described, includes a semiconductor substrate 10 with recessed oxide isolation regions 12. A layer 14 of insulation material such as silicon dioxide extends on the surface of substrate 10 over and past the edges of the recessed oxide regions 12 toward the center of the device region and includes a window opening for the emitter region. The insulation layer 14 extends to the edge of the extrinsic base 16. A layer 18 of conductive contact material such as p+ polysilicon is disposed over the insulation layer 14 and extends over the edge thereof and vertically downward to contact the extrinsic base 16. In practice, the extrinsic base 16 is formed by driving in dopants from polysilicon layer 18, therefore polysilicon layer 18 need only be doped p+ in the region 32 over the extrinsic base region rather than throughout the entire layer 18, which needs only to be composed of conductive material. A second insulation layer 20, such as a combination of silicon dioxide 20-1 and aluminum ioxide 20-2 is disposed on and extends over the inner edge of contact layer 18 and vertically downward to the surface of substrate 10 to isolate the emitter region from the base contact 18. Alternatively, layer 20 may be composed entirely of one single suitable insulating material. 
     The remaining elements of the bipolar device include the emitter 22, the intrinsic base 24, an n-epi collector 26, an n+ subcollector 28, and an emitter contact 30 composed, for example, of n+ polysilicon. 
     It should be noted that the base contact area is very small, of sub-micron dimensions, and is disposed between insulating layer 14 and the insulating layer 20, which also serves to isolate the base contact 18 from the emitter contact 30. The reduced area of the extrinsic base 16 reduces the parasitic capacitance, which results in both high speed operation and high packing density for integrated circuit applications. Also, the extrinsic base is self-aligned to the vertical edge of oxide 14 and the emitter is self-aligned to the extrinsic base, as a result of a fabrication process described hereinbelow. 
     A process for fabricating the bipolar device illustrated in FIG. 1 is shown in FIGS. 2-1 through 2-6. The main feature of the process is to first employ an oxide step to define a self-aligned rim of conductive material which serves as the base contacts, then to use this conductive rim to establish an oxide isolated emitter. 
     More particularly, in FIG. 2-1, an insulation layer 14 (i.e., silicon-dioxide) is disposed on a silicon substrate 10 having recessed oxide isolation regions 12, an epi collector 26 and an n+ subcollector 28, disposed therein by conventional techniques. A layer 18 of conductive material (i.e., p+ polysilicon, or refractory metal) is deposited over layer 14 and a second insulation layer 20 is deposited over layer 18. Insulation layer 20 must have the properties of being a good reactive ion etching (RIE) mask for polysilicon and be a good mask for silicon oxidation. Aluminum oxide (Al 2  O 3 ) or silicon dioxide (SiO 2 ) is a suitable material for layer 20, however, other materials or combinations of materials are possible. 
     A hole or window is then etched in layer 20 (i.e., using H 3  PO 4  for Al 2  O 3 ) and layer 18, and then part of layer 14 is etched using reactive ion etching. The remainder of layer 14 is then etched by chemical etching so that part of layer 14 is undercut to provide the structure illustrated in FIG. 2-1. The chemical etching which provides the undercut may be optional, and the structure may be fabricated without such undercut. 
     Next, a p+ polysilicon layer 32 is deposited over the structure of FIG. 2-1 as shown in FIG. 2-2. Due to the conformal nature of the deposition, the polysilicon layer 32 will also deposit in the undercut region. This forms a protective bridge connecting the first conductive layer 18 with substrate 10. 
     The polysilicon layer 32 is etched using reactive ion etching in order to remove the polysilicon from the exposed horizontal surfaces, and partially from the sidewall of the aluminum oxide layer 20-2, leaving a vertical connective bridge from layer 18 to the substrate 10 as illustrated in FIG. 2-3. 
     In the next step the exposed silicon substrate 10 and polysilicon 32 is oxidized as well as any residual polysilicon on the sidewalls of the aluminum dioxide layer 20-2 to provide layer 34 as shown in FIG. 2-4. During this step, p+ impurities such as boron is driven into substrate 10 from the remaining p+ polysilicon sidewall remainder of layer 32, thereby forming the extrinsic base regions 16, as also shown in FIG. 2-4. If necessary, an additional annealing treatment can be applied to drive the boron in deeper. 
     Additional silicon dioxide 36 is now chemically vapor deposited over the structure to thicken the oxide layer 34, as illustrated in FIG. 2-5. 
     The horizontal portions of silicon dioxide layers 34 and 36 are removed by directional reactive ion etching, leaving silicon dioxide sidewalls 20-1 on the sidewalls of polysilicon layer 32. This leaves the emitter region of substrate 10 exposed. 
     It is important to note that the polysilicon base contacts 32 and extrinsic base 16 are self-aligned to the window formed by the silicon dioxide sidewalls 20-1. The window serves as the emitter opening and therefore the emitter will be self-aligned to the extrinsic base 16 as shown in FIG. 2-6. 
     The remainder of the device fabrication process cosists of conventional steps. An intrinsic base 24 is formed by boron diffusion or implantation through the window formed by the oxide sidewalls 20-1, and an n+ emitter 22 is implanted (i.e., arsenic). An n+ polysilicon emitter 30 contact is then formed by depositing a layer of arsenic doped polysilicon and masking and etching to produce the structure illustrated in FIG. 1. Alternatively, the emitter contact 30 can be conventional metallurgy. One skilled in the art will appreciate that the base may be implanted before the emitter or that the emitter may be implanted before the base. 
     The above described fabrication process for FIGS. 2-1 through 2-6, and the resultant structure of FIG. 1 provides an npn device. However, using complimentary material of opposite conductivity a pnp device having a p+ doped emitter, an n+ doped extrinsic base, and a p type collector may be provided by following the same fabrication steps. 
     What has been described is a unique bipolar transistor device having a structure wherein a layer of insulating material extends over the device substrate up to the extrinsic base around the emitter region. A very small area conductive base contact is provided to the extrinsic base, and a protective sidewall of insulating material is located on the sidewall of the base contact to isolate it from the emitter contact. This unique structure is made possible by a fabrication process incorporating a double self-alignment technique. 
     While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.