Control of dopant diffusion from buried layers in bipolar integrated circuits

Abstract

An integrated circuit and method of fabricating the integrated circuit is disclosed. The integrated circuit includes vertical bipolar transistors (30, 50, 60), each having a buried collector region (26′). A carbon-bearing diffusion barrier (28c) is disposed over the buried collector region (26′), to inhibit the diffusion of dopant from the buried collector region (26′) into the overlying epitaxial layer (28). The diffusion barrier (28c) may be formed by incorporating a carbon source into the epitaxial formation of the overlying layer (28), or by ion implantation. In the case of ion implantation of carbon or SiGeC, masks (52, 62) may be used to define the locations of the buried collector regions (26′) that are to receive the carbon; for example, portions underlying eventual collector contacts (33, 44c) may be masked from the carbon implant so that dopant from the buried collector region (26′) can diffuse upward to meet the contact (33). MOS transistors (70, 80) including the diffusion barrier (28) are also disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] Not applicable. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0002] Not applicable. BACKGROUND OF THE INVENTION [0003] This invention is in the field of semiconductor integrated circuits, and is more specifically directed to the formation of buried doped layers in bipolar transistors in such circuits. [0004] Modern bipolar integrated circuits now typically use vertical bipolar transistors as their active elements. These transistors are vertical in the sense that the active base and emitter regions overlie the collector region, with collector-emitter current traveling through the base in substantially a vertical orientation relative to the plane of the surface of the integrated circuit at which the transistor resides. To provide a robust breakdown voltage, the portion of the collector region adjacent the base is relatively lightly doped. This region is often referred to as the “subcollector”. These lightly-doped subcol...

AI Technical Summary

Benefits of technology

[0014] It is therefore an object of the present invention to provide an integrated circuit and method of fabrication that reduces diffusion of dopant from buried doped layers, such as buried collector layers in bipolar transistors.

[0016] It is a further object of the present invention to provide such an integrated circuit and method that retards the diffusion of one dopant species while enhancing the diffusion of a different dopant species, for example to provide a symmetric emitter profile for complementary devices.

Problems solved by technology

These lightly-doped subcollectors are relatively resistive, however.

Therefore, many modern bipolar structures reduce the effective collector resistance by providing a heavily-doped buried collector layer underlying the subcollector.

This construction results in a significant dopant concentration gradient at the interface between the buried collector regions and the much more lightly-doped overlying subcollector.

A particularly troublesome high temperature process is the epitaxial formation of the subcollector itself, which exposes the wafer to high temperatures for a relatively long period of time.

This updiffusion of dopant from the buried collector can cause significant limitations in the performance and precision of modern bipolar circuits.

Efforts to reduce this updiffusion are known to have detrimental device effects.

Increasing the thickness of epitaxial layer 8 to compensate for the updiffusion effect not only exacerbates the diffusion itself (by increasing the time or temperature of the process), but also is incompatible with the fabrication of high performance and high-speed devices.

Not only does the undesired diffusion from each buried layer reduce the performance of individual transistors 10m 10n, but the difference in diffusion rate also causes mismatch between the complementary transistors in a given circuit.

This mismatch renders the delicate balance of the tradeoff between NPN and PNP performance, and the necessary optimization techniques, even more difficult.

Besides compromising the ultimate performance of the circuit, this device type asymmetry also can reduce the power efficiency of the complementary design.

In addition to the loss of control over the buried layer-subcollector interface, undesired diffusion from heavily-doped buried layers can also contaminate structures away from the buried layers, due to auto-doping during epitaxial growth.

Such contamination can result in device leakage, shifts in threshold voltages, and poor breakdown characteristics in bipolar and MOS devices, as well as in diodes and passive devices.

These constraints have resulted in very complex processing that is not only costly, but also typically results in the inability to maximize the performance of the NPN and PNP devices in a symmetric manner (i.e., without sacrificing the performance of one for the performance of the other).

Besides impacting device performance, as noted above, diffusion from the buried collector layers also impacts the breakdown voltage of the individual devices.

In addition, the asymmetric diffusion of the dopant species also creates mismatching of the device characteristics of NPN and PNP devices in a complementary circuit; such mismatches are especially undesirable, considering that the matching of device characteristics is a primary reason for realizing a circuit in complementary bipolar technology in the first place.

In addition, the tight constraint on thermal budget because of the rapid diffusion of boron from the buried collectors of the PNP devices also increases the likelihood of mismatch among the PNP devices themselves, as these devices can become quite sensitive to the processing conditions that fall within the thermal budget constraints.

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