Vertical Trench Gate MOSFET Threshold Voltage Control
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Solution Overview
Problem
The existing methods for manufacturing vertical trench gate power ICs face challenges in maintaining the desired relationship between the gate threshold voltages of output stage and control MOSFETs, leading to increased costs and complexity, while also experiencing issues with the thickness of the LOCOS oxide film decreasing, which affects device isolation and breakdown voltage.
Innovation Solution
A semiconductor device manufacturing method that involves forming a trench, a first gate oxide film, and a gate electrode, followed by a selective oxide film and a second gate oxide film of different thicknesses, allowing independent setting of threshold voltages and preventing the decrease in LOCOS oxide film thickness, thereby maintaining the desired voltage relationship without additional ion implantation steps or increased complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional manufacturing methods are used to maintain gate threshold voltage relationship, then device isolation and breakdown voltage are compromised due to LOCOS oxide film thickness decrease, but manufacturing complexity and cost increase
Solution Approach 1:
The patent segments the oxide film formation process into two distinct stages: first forming a thick LOCOS oxide film for device isolation, then selectively removing oxide from specific regions and forming a thinner gate oxide film where needed. This segmentation allows independent optimization of isolation thickness and gate threshold voltage without compromising either function.
Solution Approach 2:
The patent applies local quality by creating different oxide film thicknesses in different regions of the semiconductor device. The LOCOS region maintains a thick oxide film for isolation, while the gate region receives a thinner oxide film for proper threshold voltage control. This spatial differentiation resolves the contradiction between maintaining uniform thick isolation and achieving variable gate characteristics.
2Manufacturing precision
If additional ion implantation steps are added to control gate threshold voltages, then voltage relationship is achieved, but manufacturing steps and cost increase
Solution Approach 1:
The patent extracts the gate threshold voltage control function from the ion implantation process and transfers it to the oxide film thickness control process. By removing the need for additional ion implantation steps and instead using selective oxide removal and formation, the method achieves precise threshold voltage control while maintaining high manufacturing throughput.
Solution Approach 2:
The patent changes the controlling parameter for gate threshold voltage from dopant concentration (ion implantation) to oxide film thickness. This parameter change allows control through standard oxide formation and removal processes that are already part of the manufacturing flow, avoiding the need for additional ion implantation steps and maintaining productivity.
3Ease of manufacture
If LOCOS oxide film thickness is reduced to allow subsequent processing, then device isolation performance deteriorates, but processing continuity is improved
Solution Approach 1:
The patent performs preliminary action by forming the thick LOCOS oxide film for device isolation first, before any gate oxide formation or removal steps. This preliminary isolation structure is then protected while selective oxide removal and gate oxide formation proceed in subsequent steps, ensuring isolation performance is maintained throughout processing.
Solution Approach 2:
The patent resolves the contradiction by operating in different dimensional spaces: the thick LOCOS oxide film provides isolation in the vertical dimension, while selective removal and thin gate oxide formation occur in the lateral dimension. This dimensional separation allows both thick isolation and thin gate oxide to coexist without compromising either function.
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 method effectively maintains the required gate threshold voltage relationship between output stage and control MOSFETs, suppresses the increase in manufacturing steps, and prevents the degradation of LOCOS oxide film thickness, ensuring reliable device operation and reduced production costs.
Implementation Method 1
a first gate oxide film formation step of forming the first gate oxide film along the internal wall of the trench
Implementation Method 2
a selective oxide film formation step of selectively forming the selective oxide film on the second main surface of the first conductivity type semiconductor substrate
Data Source
AI summary
A method of forming a device in each of vertical trench gate MOSFET region and control lateral planar gate MOSFET region of a semiconductor substrate is disclosed. A trench is formed in the substrate in the vertical trench gate MOSFET region, a first gate oxide film is formed along the internal wall of the trench, and the trench is filled with a polysilicon film. A LOCOS oxide film is formed in a region isolating the devices. A second gate oxide film is formed on the substrate in the lateral planar gate MOSFET region. Advantages are that number of steps is suppressed, the gate threshold voltage of an output stage MOSFET is higher than the gate threshold voltage of a control MOSFET, the thickness of the LOCOS oxide film does not decrease, and no foreign object residue remains inside the trench.