Bipolar-CMOS Base Layer Epitaxy and Doping
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Solution Overview
Problem
Integrated circuits containing bipolar transistors and MOS transistors face challenges in achieving desired transfer frequency, collector-base breakdown voltage, and base resistance while also requiring reduced fabrication cost and complexity.
Innovation Solution
The integration of a bipolar transistor with a crystalline collector and a polycrystalline base overlapping field oxide, combined with an MOS transistor having a gate with specific doping density, is achieved through a non-selective epitaxial process and ion implantation, allowing for efficient doping and reduced series resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a bipolar transistor is integrated with MOS transistors using conventional fabrication processes, then the device can be manufactured, but the fabrication cost and complexity increase
Solution Approach 1:
The patent merges bipolar and MOS transistor fabrication into a unified process sequence. The bipolar transistor is formed first with its collector, base, and emitter regions, then the MOS transistor is integrated in subsequent steps using the same substrate. This combining of multiple device types into a single fabrication flow reduces overall process complexity and enables bipolar-CMOS technology integration.
Solution Approach 2:
The fabrication process is designed to be universal, handling both bipolar and MOS device formation through a common sequence of steps. The same ion implantation, thermal processing, and patterning tools are used for both device types, making the fabrication line multi-functional and reducing the need for separate specialized processes.
2Speed
If the base region is made thinner to improve transfer frequency, then the transfer frequency increases, but the base-collector breakdown voltage decreases
Solution Approach 1:
The patent applies local quality by creating a graded base structure where the base width varies spatially. The base is thinner near the emitter to maximize transfer frequency, and thicker near the collector to maintain breakdown voltage. This non-uniform base width profile allows each region to be optimized for its specific function, resolving the contradiction between speed and strength.
Solution Approach 2:
The invention changes the base width parameter as a function of position rather than maintaining a constant width. By varying the base width parameter from emitter to collector, the design achieves high transfer frequency in the emitter-base region while maintaining adequate breakdown voltage in the collector-base region.
3Manufacturing precision
If ion implantation is used to dope the base region, then the doping precision improves, but the process complexity increases
Solution Approach 1:
The patent uses preliminary action by performing ion implantation of the base region before forming the emitter. This early doping establishes the base carrier concentration and width with high precision, and subsequent process steps build upon this pre-established foundation. The preliminary ion implantation avoids the need for more complex in-situ doping methods during emitter formation.
Solution Approach 2:
Ion implantation serves as an intermediary method that decouples the doping process from the structural formation process. By using ion implantation as a separate, controllable step with precise dose and depth control, the patent achieves accurate base doping without requiring complex integrated doping-formation processes.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in a bipolar transistor with improved transfer frequency and higher base-collector breakdown voltage, along with reduced series resistance and fabrication complexity, while maintaining flexibility in performance specifications.
Implementation Method 1
a base layer is then formed on the collector, overlapping the adjacent field oxide, using a non-selective epitaxial process, so that a portion of the base layer on the collector is crystalline and a portion of the base layer on the field oxide adjacent to the collector is polycrystalline
Implementation Method 2
An ion implant operation is performed which provides dopants to the base by ion implanting dopants into the base layer in an area laterally separated from the collector while blocking the dopants from the base layer over the collector, and concurrently ion implanting dopants into a gate layer over an area for the MOS transistor
Data Source
AI summary
A process of forming an integrated circuit containing a bipolar transistor and an MOS transistor, by forming a base layer of the bipolar transistor using a non-selective epitaxial process so that the base layer has a single crystalline region on a collector active area and a polycrystalline region on adjacent field oxide, and concurrently implanting the MOS gate layer and the polycrystalline region of the base layer, so that the base-collector junction extends into the substrate less than one-third of the depth of the field oxide, and vertically cumulative doping density of the polycrystalline region of the base layer is between 80 percent and 125 percent of a vertically cumulative doping density of the MOS gate. An integrated circuit containing a bipolar transistor and an MOS transistor formed by the described process.