In the manufacture of integrated circuits, dopant distributions are produced in semiconductor substrates of, for example, single-crystal silicon. These dopant concentrations are usually topically limited. The dopant distributions thereby have a given profile with a corresponding gradient perpendicular to the substrate surface.
Dopants are introduced into substrates by ion implantation or diffusion. In ion implantation, the distribution in the substrate is limited by a mask that is produced by photolithography. In the production of dopant distributions using diffusion, doped layers, for example, are arranged at the surface of the substrate, the dopant diffusing out of these doped layers. The form of the distribution in the plane of the substrate surface is thereby defined, for example, by a corresponding shaping of the doped layer. To that end, the doped layer is structured according to corresponding photolithographic definition.
The dopant profile perpendicular to the surface of the substrate is critically defined in the implantation by the dose and by the implantation energy. Given diffusion from a doped layer, the dopant profile perpendicular to the substrate surface is defined by the quantity of dopant contained in the dopant layer, by the segregation at the boundary surface to the substrate and by the length of diffusion.
In the production of shallow dopant profiles, i.e. of profiles having a slight penetration depth of the dopants, limits are set given employment of ion implantation in that the range of the ions is extremely great in certain crystal directions due to the channeling effect. A further practical problem is that only a few implantation systems are offered with which an implantation is possible with less than 10 keV implantation energy. Such low energies, however, are required in order to produce shallow dopant profiles with steep gradients. Extremely slight penetration depths for emitter/base and base/collector transitions are desirable in the manufacture of bipolar transistors having extremely short switching times. In a self-aligned double polysilicon process, both base terminals, as well as, emitter terminals are formed by a correspondingly doped polysilicon. The base terminal is insulated from the emitter terminal by an etch residue, what is referred to as a SiO.sub.2 spacer, that remains in place after an unmasked, anisotropic etching of an oxide layer (see H. Kabza et al., IEEE-EDL (1989) Vol. 10, pages 344-346). The active base is produced inside the area defined by the etching residues. The contact between the base terminal and the active base is guaranteed by a doped region in the substrate under the base terminal, this region angularly surrounding the active base. This region is referred to as an inactive base, external base terminal or extrinsic base. The inactive base must overlap the active base given an adequate dopant concentration in order to supply a low-impedance base terminal resistance. The inactive base is thereby produced by drive-out from the base terminal.
When the active base is manufactured after the formation of the etching residues, then a high-impedance base terminal resistance arises because the active base and the inactive base do not overlap or only overlap with a dopant concentration that is too low (see K. Ehinger et al., Proc. of ESSDERC, J. Phys., C4, pages 109-112 (1988)).
Extremely steep base and emitter profiles having low penetration depth are achieved on the basis of what is referred to as double diffusion. What is understood by double diffusion is the successive diffusion of various dopants from the same polycrystalline silicon layer. In this case, the polycrystalline silicon layer for the emitter terminal is first doped with boron, the drive-out thereof leading to the formation of the active base, and is then doped with arsenic whose drive-out leads to the formation of the emitter. The overlap of active base and inactive bas is inadequate in this manufacturing method (see K. Ehinger et al., Proc. of. ESSDERC, J. Phys., C4, pages 109-112 (1988)).
This problem can be resolved in various ways: the base can be produced by implantation before the formation of the etching residues. As a result thereof, the base is laterally expanded up to the edge of the base terminal. Although a good overlap between active and inactive base is thus established, implantation damage in the active region of the transistor nonetheless occurs, which potentially leads to a reduction in the yield. Moreover, the penetration depth of the dopants for the base cannot be made arbitrarily small because of the employment of the implantation.
Another possibility is implanting a part of the dopant concentration for the active base with low energy before the formation of the etching residues. Subsequently, the ultimate value of the base doping is set by drive-out from the polycrystalline silicon layer provided for the emitter terminal (see T. Yamaguchi et al., lEEE-ED (1988) Vol. 35, pages 1247-1256). However, as a result thereof the dopant concentration of the active base is increased under the emitter window and the base profile is thereby broadened.
A further possible solution is doping the etching residue that insulates the base terminal from the emitter terminal with boron (see M. Nakamae, Proc. of ESSDERC (1987), pages 361-363). Due to drive-out from the etching residue, the dopant concentration is locally elevated at the terminal location of the inactive base to the active base. The dopant concentration thereby achieved is prescribed by the quantity of dopant deposited in the etching residue, by the segregation at the boundary surface to the substrate and by the length of diffusion. In order to be able to set the base terminal resistance in a controlled fashion, the etching residue must be formed of a SiO.sub.2 compound whose drive-out is suitably variable with a prescribed temperature budget.
It has been shown that boron-doped SiO.sub.2 compounds dopable in an arbitrary concentration, as employed in M. Nakamae, Proc. of. ESSDERC (1987), pages 361-363, are in a metastable condition. As a result thereof, crystallizations of the glass occur at certain locations. The characteristic of the drive-out is then changed at these locations, so that large topically dependent inhomogeneities of the dopant distribution in the substrate occur. This has a negative effect on the uniform setting of a low base terminal resistance.