TMR Sensor Free Layer Back Edge Definition via Methanol RIE
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Solution Overview
Problem
Conventional methods for manufacturing CPP-TMR sensors with extended pinned layers result in undesirable back edge profiles and inaccurate stripe height due to material removal, affecting the pinning of the reference layer.
Innovation Solution
A methanol-based reactive ion etching process is used to define the free layer back edge, ensuring the MgO spacer layer is not damaged, and optionally extending the reference layer beyond the free layer's side edges, with precise control using patterned photoresist layers and optical end-point detection to maintain uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If conventional methods are used to create an extended pinned layer, then the reference layer extends beyond the free layer, but material removal results in undesirable back edge profiles and inaccurate stripe height
Solution Approach 1:
The patent introduces a sacrificial layer (e.g., Ta or Ru) deposited between the free layer and reference layer. This intermediary layer allows the reference layer to extend beyond the free layer back edge without requiring material removal. The sacrificial layer is later removed through selective etching, leaving a clean back edge profile and accurate stripe height measurement, thus resolving the contradiction between shape and manufacturing precision
Solution Approach 2:
The patent performs preliminary deposition of the sacrificial layer before forming the extended reference layer structure. This preliminary action enables subsequent selective removal to achieve the desired back edge profile without compromising the stripe height accuracy, as the sacrificial layer serves as a temporary placeholder that is removed only after defining the reference layer extension
2Shape
If material removal is used to define the free layer back edge, then the back edge is defined, but the reference layer material is also removed affecting pinning
Solution Approach 1:
The sacrificial layer acts as a protective intermediary between the free layer and reference layer during the etching process. When etchants are applied to define the free layer back edge, the sacrificial layer prevents removal of the reference layer material, thereby maintaining the reference layer pinning reliability while achieving precise back edge definition
Solution Approach 2:
The patent extracts only the necessary portion (sacrificial layer) for removal while preserving the critical reference layer. The selective etching process removes the sacrificial layer and free layer material at the back edge region, but the reference layer remains intact due to its different etch selectivity, thus maintaining pinning functionality
3Shape
If the reference layer is extended beyond the free layer side edges, then extended pinned layer is achieved, but conventional methods require multiple material removal steps
Solution Approach 1:
The sacrificial layer is deposited preliminarily across the entire structure before extending the reference layer. This preliminary deposition simplifies subsequent processing by providing a uniform protective mask that enables single-step selective etching to achieve the extended pinned layer configuration without multiple complex removal steps
Solution Approach 2:
The sacrificial layer serves multiple functions: it protects the reference layer during etching, defines the back edge position, and enables side edge extension. This multi-functionality reduces the overall process complexity by consolidating multiple protective and definitional roles into a single layer
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 achieves a precisely defined free layer back edge and extended reference layer without material removal, improving the accuracy of stripe height and trackwidth dimensions, and maintaining effective pinning of the reference layer.
Implementation Method 1
a methanol (CH3OH)-based reactive ion etching (RIE) removes the unprotected free layer, defining the free layer back edge
Implementation Method 2
Termination of the methanol-based RIE when the MgO layer is detected prevents any removal of the MgO
Data Source
AI summary
A tunneling magnetoresistive sensor has an extended pinned layer wherein both the MgO spacer layer and the underlying ferromagnetic pinned layer extend beyond the back edge of the ferromagnetic free layer in the stripe height direction and optionally also beyond the side edges of the free layer in the trackwidth direction. A patterned photoresist layer with a back edge is formed on the sensor stack and a methanol (CH3OH)-based reactive ion etching (RIE) removes the unprotected free layer, defining the free layer back edge. The methanol-based RIE terminates at the MgO spacer layer without damaging the underlying reference layer. A second patterned photoresist layer may be deposited and a second methanol-based RIE may be performed if it is desired to have the reference layer also extend beyond the side edges of the free layer in the trackwidth direction.