Use of MEV implantation to form vertically modulated N+ buried layer in an NPN bipolar transistor

#1. Technical Field

#The present invention relates generally to bipolar junction transistors and in particular to bipolar junction transistors formed in the same integrated circuit with complimentary metal-oxide-semiconductor transistors (BiCMOS). Still more particularly, the present invention relates to formation of bipolar transistors with reduced vertical collector resistance.

#2. Description of the Related Art

#The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

#FIG. 1 depicts a vertically integrated bipolar device design commonly employed in the prior art, along with the mask levels utilized to form this device;

#FIG. 2 is the vertically integrated bipolar device of FIG. 1 and its equivalent circuit diagram;

#With reference now to the figures, and in particular with reference to FIG. 3B, a bipolar device in accordance with the preferred embodiment of the present invention is depicted. The figures representing device profiles or cross-sections of portions of an integrated circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention. The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention may be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention.

#In lieu of both a buried subcollector and sinker, a vertically modulated subcollector 304 is employed in FIG. 3B to reduce both the vertical and lateral components of the collector resistance. The differential penetration of a high-energy implant creates both a buried layer and its connection to the collector, simultaneously. Collector resistance is significantly reduced without the need for separate process steps forming buried collector and sinker structures. The resulting technology has a higher performance-to-price ratio.

#Subcollector 304 in FIG. 3B is a vertically modulated buried layer formed by the high energy implant. Subcollector 304 includes a portion beneath the base region 114 which is vertically displaced from the portion of subcollector 304 beneath collector contact region 120. At least the portion of subcollector 304 beneath the base region 114 overlaps or intersects the well forming collector region 112. Subcollector 304 is an integral, heavily doped region formed by a single implant. Subcollector 110 and sinker 302 in FIG. 3A, remain distinct despite overlapping or contacting, and are formed by separate implants. Thus, a danger exists that sinker 302, if not properly implanted, will not contact or overlap subcollector 110. In that event, reduction of collector resistance by sinker 302 is less effective. Use of vertically modulated subcollector 304 as shown in FIG. 3B avoids this potential problem.