SOI SRAM Body Contacts via Source Drain Diffusion
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Solution Overview
Problem
In silicon on insulator (SOI) technology, the floating body effect causes device non-uniformity and instability in SRAM cells due to localized body effects and parasitic bipolar interactions, leading to issues like history-dependent threshold voltage shifts and reduced SRAM stability.
Innovation Solution
The implementation of body contacts for SOI FETs by etching an opening through the source/drain diffusion to expose the floating body and forming a silicide contact, allowing the body to be biased to a selected voltage, thereby reducing floating body effects and improving device stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If body contacts are implemented for SOI FETs to reduce floating body effects, then device stability and uniformity improve, but manufacturing complexity and area increase
Solution Approach 1:
The body contact structure is merged with the source/drain diffusion regions by forming P-type body contacts through the N-type source/drain diffusion. This integration eliminates separate body contact structures and reduces overall device complexity while maintaining the ability to reduce floating body effects and improve SRAM stability.
Solution Approach 2:
The source/drain diffusion regions serve multiple functions: they provide electrical connection for source/drain terminals and simultaneously serve as the pathway for body contacts. This multi-functionality reduces the number of separate structures needed and simplifies the overall device architecture while achieving improved device stability.
2Reliability
If body contacts are implemented for SOI FETs, then floating body effects are reduced, but manufacturing process complexity increases
Solution Approach 1:
The P-type body contact regions are formed during the source/drain diffusion process itself, before final contact formation. By preparing the body contact pathway in advance through the diffusion process, subsequent steps are simplified and overall manufacturing complexity is reduced while still achieving the goal of reducing floating body effects.
Solution Approach 2:
The source/drain diffusion process automatically creates the body contact pathway as a byproduct of the diffusion itself. The P-type doping introduced during source/drain formation naturally extends into the body region, providing the body contact function without requiring separate dedicated processing steps.
3Reliability
If openings are etched through source/drain diffusion to expose the body, then body contact is achieved, but diffusion integrity near the channel may be compromised
Solution Approach 1:
The etching process is applied locally and selectively only to specific regions where body contacts are needed, rather than uniformly across the entire source/drain diffusion. This localized approach ensures that the diffusion integrity near the channel remains intact while still achieving body contact exposure in the designated contact regions.
Solution Approach 2:
The P-type doping region serves as an intermediary structure that extends from the source/drain contact region into the body. This intermediary P-type region provides a pathway for body contact formation through the N-type diffusion without requiring direct etching through the entire diffusion structure, thereby preserving diffusion integrity.
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 significantly reduces variation across memory cells, enhances SRAM stability, and minimizes the area penalty typically associated with body contact implementation, improving overall device performance.
Implementation Method 1
etching an opening through the source/drain diffusion to expose the floating body
Implementation Method 2
forming a silicide contact
Data Source
AI summary
Contact with a floating body of an FET in SOI may be formed in a portion of one of the two diffusions of the FET, wherein the portion of the diffusion (such as N−, for an NFET) which is “sacrificed” for making the contact is a portion of the diffusion which is not immediately adjacent (or under) the gate. This works well with linked body FETs, wherein the diffusion does not extend all the way to BOX, hence the linked body (such as P−) extends under the diffusion where the contact is being made. An example showing making contact for ground to two NFETs (PG and PD) of a 6T SRAM cell is shown.