Two-Step Annealing for TMR Amorphous Free Layer
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Solution Overview
Problem
Current annealing processes for magnetic tunnel junction (MTJ) sensors with MgO tunnel barriers and amorphous free layers fail to achieve a high magnetoresistive (MR) ratio while maintaining low areal resistance (RA) and magnetic softness, which is crucial for high-performance read head applications.
Innovation Solution
A two-step annealing process is employed, where the first anneal is performed at 200° C. to 270° C. with a magnetic field for 0.5 to 15 hours to develop a pinned magnetic state, followed by a second anneal at 260° C. to 400° C. for 0.1 to 3 hours, ensuring crystallization of the amorphous free layer without increasing coercivity, thereby achieving robust pinning and a high MR ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single-step high-temperature annealing process is used to crystallize the amorphous free layer, then the MR ratio is improved, but the coercivity increases and magnetic softness is lost
Solution Approach 1:
The annealing process is divided into two distinct steps: first a low-temperature anneal (200-270°C) to establish pinned magnetic state with minimal coercivity increase, then a high-temperature anneal (260-400°C) to crystallize the amorphous free layer and maximize MR ratio. This segmentation allows each step to optimize for its specific purpose without compromising the other.
Solution Approach 2:
The pinned magnetic state is established in advance during the first low-temperature annealing step before the free layer is crystallized. This preliminary action ensures that when the high-temperature crystallization occurs, the pinned layer structure is already optimized, preventing excessive coercivity development while enabling high MR ratio achievement.
2Measurement precision
If annealing temperature is increased to crystallize the amorphous free layer, then the MR ratio improves, but the areal resistance RA increases
Solution Approach 1:
The annealing process is segmented into two temperature stages: the first stage (200-270°C) establishes magnetic pinning with minimal impact on RA, while the second stage (260-400°C) crystallizes the free layer to maximize MR ratio. This segmentation allows optimization of both MR ratio and RA by controlling the sequence and duration of each annealing step.
3Reliability
If annealing time is extended to develop pinned magnetic state, then pinning strength improves, but the manufacturing productivity decreases
Solution Approach 1:
The annealing process is segmented into two distinct time-based stages: a first anneal (0.5-15 hours) dedicated to developing pinned magnetic state, followed by a second, shorter anneal (0.1-3 hours) for free layer crystallization. This segmentation allows optimization of pinning strength in the first stage while minimizing total process time through the efficient second stage.
Solution Approach 2:
The annealing parameters (temperature and time) are changed between two distinct steps: the first step uses lower temperature (200-270°C) with longer duration to develop pinning, while the second step uses higher temperature (260-400°C) with shorter duration for crystallization. This parameter optimization balances pinning strength development with manufacturing productivity.
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 results in improved MR ratio and reduced RA, maintaining low coercivity and magnetostriction, enhancing the performance of MTJ sensors for high-density recording applications.
Implementation Method 1
a first anneal step at a first temperature of 200° C. to 270° C. and by applying a magnetic field of 1000 to 20000 Oe
Implementation Method 2
a second anneal step at a second temperature of 260° C. to 400° C. to convert the amorphous material in the free layer to a crystalline state
Implementation Method 3
The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons
Implementation Method 4
The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment
Data Source
AI summary
An annealing process for a TMR or GMR sensor having an amorphous free layer is disclosed and employs at least two annealing steps. A first anneal at a temperature T1 of 200° C. to 270° C. and for a t1 of 0.5 to 15 hours is employed to develop the pinning in the AFM and pinned layers. A second anneal at a temperature T2 of 260° C. to 400° C. where T2>T1 and t1>t2 is used to crystallize the amorphous free layer and complete the pinning. An applied magnetic field of about 8000 Oe is used during both anneal steps. The mechanism for forming a sensor with high MR and robust pinning may involve structural change in the tunnel barrier or at an interface between two of the layers in the spin valve stack. A MgO tunnel barrier and a CoFe/CoB free layer are preferred.