Abstract:
A method of making a bipolar integrated circuit which requires neither an epitaxial layer nor a buried layer. The required doping of a semiconductor substrate, e.g., silicon, is obtained by a series of etching steps alternated with ion implantation steps of a selected impurity type, and heat treatment steps. The emitter and collector zones of a transistor are formed on sloping walls of adjacent troughs formed in a semiconductor substrate. The base zone of a transistor is formed on the confronting sloping wall of one of these troughs. Lead conductors are located in the troughs along sloping wall portions of the troughs.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a bipolar integrated circuit and to a method of making the same. 
     In known bipolar integrated circuits, the production process requires a greater outlay than the process for the production of integrated MOS circuits because an epitaxial layer on the semiconductor substrate and a buried layer beneath the epitaxial layer are required. 
     A further disadvantage of knwon bipolar circuits consists in that the area requirement which is governed in particular by the requisite insulating frames around the individual transistors, by adjustment tolerances and by the requisite safety clearances between the variously doped zones, is relatively large. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide integrated bipolar transistors in which the above mentioned disadvantages are avoided. 
     This aim is realized by a bipolar integrated circuit which is formed by a series of process steps which include masking steps, ion implanation steps and heating steps. 
     An advantage of the invention consists, in particular, in that the process for the production of the bipolar integrated circuits of the invention is simplified in that no epitaxial layer, and consequently, no buried layer are provided. 
     Advantageously, the circuits in accordance with the invention have a very small area requirement on account of the self-adjusting masking steps. 
     A further advantage of the invention consists in that in the production of the circuits of the invention, it is possible to construct emitter zones and contact windows having dimensions of less than 1μ without the need to produce these sub-micron-structures photolithographically. 
     Further details of the invention, and its developments are given in the description and the Figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to 10 show the individual process steps for the production of an integrated bipolar semiconductor circuit in accordance with the invention. 
     FIG. 11 shows a pnp transistor produced in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, the process in accordance with the invention will be explained, making reference to FIGS. 1 to 10. An electrically insulating layer 2 is applied to the semiconductor layer 1, which is, preferably, a homogeneously doped, monocrystalline silicon wafer whose surface 11 is a crystallographic (100) - surface. Preferably, the layer 2 is produced by thermal oxidation and possesses a thickness of, for example, 1.2μm. The homogeneously doped substrate 1 is, for example, a silicon material homogeneously doped with 10 15  boron ions per cm 3  (FIG. 1). 
     In a further process step, using a conventional photo-lithographic technique, as illustrated in FIG. 2, openings 6 are provided in the SiO 2  layer 2 at the location at which the transistor zones are later to be formed, and likewise openings 7 for the insulation frame. The remaining parts of the layer 2 are designated 22. 
     As illustrated in FIG. 3, in a further process step, the silicon which has been exposed in the openings is partially removed in a preferably alkaline etching medium. Etching troughs 61, 611 and 71 possessing a depth of approximately 1μm, for example, are thereby formed beneath the openings 6 and 7 (FIG. 2) within the silicon. 
     In a further process step which in represented in FIG. 4, the two outer openings 7, 71 are masked, whereupon donors, such as, e.g., phosphorus ions are implanted. The masking of the two outer openings 7, 71 is preferably effected with photo resist or aluminum. On account of the masking of the outer openings 7, 71, the implanted phosphorous ions penetrate only in the central openings 6, 61 and 611 into the silicon substratel. In this way, the n-implanted troughs 9 are formed by ion implantation in the p-doped substrate 1. 
     As can be seen from FIG. 5, the photo-resist or aluminum masking layer 8 is removed. In a subsequent annealing process, the arrangement is heated to approximately 1100° C, in an inert gas atmosphere whereby the phosphorus ions diffuse further into the silicon substratel. In this way, a zone 91 having an approximately homogeneous phosphorus ion concentration of, e.g., 10 16  cm -3 , is formed at a distance of approximately 1.5μm from the surface. In the next process step, acceptor ions, for example, boron ions, are introduced into three of the four openings, preferably again by ion implanation. In this way, p+ doped zones 72 are produced beneath the openings 71 which represent the insulation frame, and a p+ doped zone 92 is produced beneath the opening 611. The p+ doped zone 92 serves as base zone of the bipolar transistor. To prevent boron ions from penetrating through the second opening of the openings 61, this must be covered in masking fashion prior to the ion implanation, as already described in respect of FIG. 4. This can be again effected, e.g., by a photo resist layer or a structured aluminum layer produced in the conventional photo-lithographic technique. Like the aforementioned masking step, this photo-lithographic step is self-adjusting, as a misadjustment of the masking structure within the permitted adjustment accuracy (here ± 1μm) has no influence on the position and size of the zone to be adjusted. 
     As represented in FIG. 6, following the boron implanation (for example 10 14  boron ions per cm 2 ), the masking layer is removed above the right opening, and a thermal oxidation follows at a temperature of, e.g., 1100° C. This oxidation ensures that the boron ions reach a desired penetration depth of, e.g., 1.3μm, as a result of which the zones 921 and 721 represented in the Figure are formed. At the same time, a desired penetration depth of the phosphorus ions of, e.g., 4μm is achieved, and the zones 91 expand to such an extent that they join up to form a zone 911. A thermal oxidation layer is formed on the entire surface, whereby parts of the previously existing oxide layer are reinforced, and oxide layers having a desired thickness of, e.g., 0.8μm are formed in the openings. In FIG. 6, these zones of the insulating layer which has been produced by thermal oxidation are referenced 10. 
     In a further process step, the surface of the arrangement is covered with a masking layer, with the exception of the opening arranged above the zone 921 and the half of the adjacent opening lying above the trough which relates to the production of the transistor. This masking step is, likewise, self-adjusting, however, only in the direction marked with the reference 13 in FIG. 7. 
     In FIG. 7, the masking layer is referenced 12. Now, in an ion etching step, the surface of the semiconductor arrangement is exposed to a homogeneous, vertical ion beam 19, which is neutralized by electrons. The oxide is here removed approximately twice as rapidly at the oblique flanks 14 as on the horizontal surfaces. In this way, it is ensured that the silicon material is exposed on the oblique flanks 14 when a residue 101 of the oxide layer 10 is still present on the horizontal surfaces above the opening 611 and half the opening 61. The etching process is terminated when the silicon is exposed or is slightly etched on the flanks 14. The exposed silicon zones 14 represent the contact windows of the integrated circuit. The oxide thickness then amounts to between 0.4 and 1.3μm on the individual zones. Within those zones, which are not covered by the masking layer, and which relate to the transistor production, the thickness of the oxide layer 101 is here approximately 04.μm (FIG. 7). 
     As can be seen from FIG. 8, in a further process step, the masking layer 12 is removed. Then a new masking layer, which preferably again consists of photo resist or aliminum is applied, this masking layer being structured in such a manner that of the exposed. silicon zones, only those which are not to receive the subsequent n+ doping are covered. This n+ doping is effected by ion implanation of donor ions such as, for example, phosphorus, arsenic or antimony in a relatively high concentration for the formation of the emitter zones 16 and the collector terminals 17. The dose of the implanted ion here amounts to approximately 10 16  ions per cm 2 . The preceding photo-lithographic process is again self-adusting. 
     Following the implantation step, the masking layer is again removed, whereupon an annealing process can follow in order to set an emitter penetration depth of, e.g. 0.8μm. 
     In a further process step, as illustrated in FIG. 9, using conventional photo-lithographic processes or lift-off techniques, the conductor paths which preferably consist of aluminum or an aluminum-copper-silicon-alloy are applied. In FIG. 9, the aluminum paths are referenced 18. 
     The minimum space requirement for an integrated bipolar npn transistor produced by the described process, based on minimum structure dimensions of 2μm and an adjustment tolerance of ±1μm is 22μm 1482 m = 308μm (FIG. 10). Details of FIG. 10 which have already been described in association with the other Figures bear the corresponding references. The dash-dotted line indicates the middle of the insulation frame. 
     The process of the invention can be used to produce not only bipolar npn transistors, but also all other important elements of integrated bipolar circuits, such as, e.g., pn diodes, Schottky diodes, resistors or multi-emitters without an extra outly. 
     Conventional lateral pnp transistors can also be integrated without an extra outlay. In addition, the process of the invention is suitable for the production of improved pnp transistors with a higher current amplification (FIG. 11), because the arrangement of emitter zone 161 and collector zone 171 reduces the effects of the parasitic substrate pnp transistor. The construction of such a pnp transistor requires an additional masking step which advantageously is likewise self-adjusting, and an additional boron ion implanation. 
     In an arrangement corresponding to that shown in FIG. 7, but without the zone 91, the emitter zone 161 and the collector zone 171 are produced by boron implantation. By means of a further phosphorus implantation, the base terminal zone 912 is produced. These two last mentioned steps would be interposed between the process stages of FIGS. 7 and 8 in the process sequence illustrated in FIGS. 1 to 10. 
     A n+ collector depth diffusion can optionally be provided in order to reduce the collector bulk resistance. The corresponding masking and implantation is then effected between the process stages represented in FIGS. 4 and 5. 
     The proposed production process is suitable not only for bulk silicon substrates, but also for SOS techniques. 
     It will be apparent to those skilled in the art that many modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.