LDMOS Drain Extension Alignment via Field Oxide Hard Mask
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Solution Overview
Problem
Aligning the drain implant directly with the field oxide element in extended drain MOS transistors is challenging due to a lack of topological features in the drain implant, which complicates the minimization of gate-drain capacitance and stress buildup around the field oxide element.
Innovation Solution
A method involving the etching of a field oxide hard mask layer to define a drain field oxide trench area, followed by a drain implant and thermal drive, allowing for the formation of a deeper diffused drain region under the field oxide element, which is either self-aligned or directly aligned to the field oxide hard mask, facilitating the formation of a deeper drain junction compared to the drift region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the drain implant pattern is directly aligned to the field oxide pattern, then gate-drain capacitance is minimized, but alignment is problematic due to lack of topological features in the drain implant
Solution Approach 1:
A field oxide hard mask layer is introduced as an intermediary element to enable alignment between the drain implant and field oxide. The hard mask layer provides etchable topological features that are not present in the drain implant itself, serving as a mediator for the alignment process while allowing direct alignment to achieve capacitance minimization.
Solution Approach 2:
The field oxide hard mask layer is formed in advance before the drain implant process. This preliminary action creates the necessary alignment features beforehand, enabling subsequent direct alignment of the drain implant to the field oxide pattern without compromising manufacturing precision.
2Stress or pressure
If a thermal drive is performed for the drain implant prior to forming the field oxide element, then stress buildup around the field oxide element is reduced, but the field oxide must be formed after the thermal drive
Solution Approach 1:
The thermal drive process is performed as a preliminary action before forming the field oxide element. This sequence allows stress reduction to occur in advance, and the field oxide is then formed on top of the pre-stress-relieved structure, simplifying the overall process by establishing a fixed sequence.
3Reliability
If the drain junction is made deeper under the field oxide element, then gate-drain capacitance is reduced, but the drain implant must extend deeper into the substrate
Solution Approach 1:
The drain junction depth is increased locally only in the region under the field oxide element, rather than uniformly across the entire drain region. This localized deepening achieves capacitance reduction where most needed while maintaining shallower depths in other areas, optimizing the trade-off between capacitance and implant depth.
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 enables the formation of an extended drain MOS transistor with a deeper drain junction under the field oxide element, reducing gate-drain capacitance and stress buildup, and allowing for improved operational potential of the MOS transistor.
Implementation Method 1
Drain dopants are implanted through the drain field oxide trench area
Implementation Method 2
a thermal drain drive is performed
Implementation Method 3
to reduce stress buildup around the field oxide element
Data Source
AI summary
An integrated circuit and method having an extended drain MOS transistor, wherein a diffused drain is deeper under a field oxide element in the drain than in a drift region under the gate. A field oxide hard mask layer is etched to define a drain field oxide trench area. Drain dopants are implanted through the drain field oxide trench area and a thermal drain drive is performed. Subsequently, the drain field oxide element is formed.