Control of dopant diffusion from buried layers in bipolar integrated circuits

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 ( 28 c) 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 ( 28 c) 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, 44 c) 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.