Stressed Phase Change Memory Cells Inhibit Hexagonal Phase Formation
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Solution Overview
Problem
Phase change memory devices using Ge2Sb2Te5 materials face reliability issues due to the undesirable hexagonal phase formation, which leads to grain-size variations and voids at the interface, resulting in low yield and reliability concerns.
Innovation Solution
Applying stress to the substrate, either compressive or tensile, to inhibit the transformation from the rock salt to the hexagonal phase in phase change materials, by inducing stress into the GST (GexSbxTex) materials during the manufacturing process of memory cells, such as mushroom-type, bridge-type, active-in-via, and pore-type structures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the temperature of as-deposited amorphous Ge2Sb2Te5 material is increased to form crystalline phases, then the material exhibits desired phase change properties, but the hexagonal phase forms which causes grain-size variations and interface voids leading to low yield and reliability issues
Solution Approach 1:
The patent applies preliminary anti-action by depositing a stress-induced layer (such as silicon nitride or silicon oxide) on the backside of the substrate before forming the phase change memory structure. This layer pre-applies mechanical stress to the substrate that counteracts the stress-induced hexagonal phase formation during subsequent high-temperature processing, thereby preventing the harmful phase transformation before it occurs
Solution Approach 2:
The patent changes physical parameters by controlling the stress state of the substrate through the stress-induced layer. By adjusting the thickness and material properties of the stress-induced layer, the patent modifies the mechanical stress parameter on the phase change material, shifting the phase stability conditions to suppress hexagonal phase formation while maintaining rock salt phase stability during high-temperature processing
2Manufacturing precision
If high temperature processing is applied to form crystalline phases in phase change materials, then the material achieves desired electrical properties, but film shrinkage and phase transformation occur causing manufacturing defects
Solution Approach 1:
The stress-induced layer is deposited in advance to create a pre-compressive stress state on the substrate. This preliminary action counteracts the tensile stress that would normally develop during high-temperature processing, preventing film shrinkage and interface void formation before they occur
Solution Approach 2:
The patent exploits differential thermal expansion between the substrate, stress-induced layer, and phase change material. The stress-induced layer is designed to have different thermal expansion characteristics that, when combined with mechanical stress, compensate for the film shrinkage that occurs during high-temperature crystallization processing
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
The induced stress effectively prevents the formation of the hexagonal phase at high temperatures, improving the yield and reliability of phase change memory devices by maintaining the rock salt phase, thus enhancing the manufacturing process and device performance.
Implementation Method 1
The stress on the substrate can be released, inducing a selected one of tensile or compressive stress into the phase change materials
Implementation Method 2
Phase change materials that exhibit a large resistivity contrast between crystalline (low resistivity) and amorphous (high resistivity) states
Data Source
AI summary
A memory device includes a substrate and a memory array on the substrate. The memory array includes memory cells including stressed phase change materials in a layer of encapsulation materials. The memory cells may include memory cell structures such as mushroom-type memory cell structures, bridge-type memory cell structures, active-in-via type memory cell structures, and pore-type memory cell structures. The stressed phase change materials may comprise GST (GexSbxTex) materials in general and Ge2Sb2Te5 in particular. To manufacture the memory device, a substrate is first fabricated. Memory cells including phase change materials in a layer of encapsulation materials are formed on a front side of the substrate. A tensile or compressive stress is induced into the phase change materials on the front side of the substrate.