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MTJ Sensors for Biochip applications

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By applying an electric current between the two metalic electrodes, electrons are able to cross the thin insolator layer. Tunneling current depends on the relative magnetization of the layers.......

By applying an electric current between the two metalic electrodes, electrons are able to cross the thin insolator layer. Tunneling current depends on the relative magnetization of the layers. Magnetoresistance –
Capacity to alter the electrical resistance of a given material when a magnetic field is applied to it.

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  • 1. Micro and nanofabrication Techniques
    MTJ Sensors for Biochip applications
    Pedro Novo 64407
    António Sousa 64427
    Álvaro Faleiro 66040
    MBioNano
  • 2. Layout
  • 3. 1. Introduction
    Magnetoresistance –
    Capacity to alter the electrical resistance of a given material when a magnetic field is applied to it.
    Occurs in magnetic tunnel junctions (MTJs) and consits of two ferromagnetic layers separated by an insolator.
    By applying an electric current between the two metalic electrodes, electrons are able to cross the thin insolator layer. Tunneling current depends on the relative magnetization of the layers.
  • 4. Sintetic Antiferromagnetic (SAF)
    1. Introduction
    Passivation Layer
    Free Ferromagnetic
    Barrier
    Pinned Ferromagnetic
    Antiferromagnetic
    In the top of the first layer, the orientation of the spin defines the layer orientation
    Using Ru in this layer, will allow a stronger stabilization.
    The last layer has a free orientation.
  • 5. 1. Introduction
    Two electrode magentizations in antiparallel state.
    High resistance state.
    Two electrode magentizations in parallel state.
    Low resistance state.
    Longitudinal magnetic field helps the orientation of the free ferromagnetic layer. Varying an applied transversal magnetic field, its possible to measuere the resistance variation
    TMR (%)
    Resistance (Ohm)
    Applied Magnetic Field (Oe)
    P PFreitaset al, 2008
    It is expected a linear sensor!
  • 6. 1. Introduction
    Sensible to week magnetic fields (nT).
    Motivation:
    Biomedical and Biotechnologic applications
    Alternative to the use of fluorescent markers and optical instruments.
    A probe is funcionalized in a MTJ structure. Biomolecules with magnetic labels hibridez in this surface.
    P PFreitaset al, 2008
  • 7. 2. MicrofabricationProcess
    Two dies were fabricated at the same time during all the work: #407 and #408
    The process started with all layer deposition using Nordiko 3600
    TiWN2 150Å
    Ta 50Å
    Ru 50Å
    CoFeB 20Å
    MgO
    CoFeB 40Å
    Ru 8Å
    CoFe 30Å
    2.1. Step #1 - 1st Exposure, Main pillar definition
    Main pillar definition was performed in the DWL machine using the following mask:
    Main pillar dimensions are 675 um x 105 um
    Both samples were well exposed:
    MnIr 200Å
    Ta 90Å
    Ru 200Å
    Ta 90Å
  • 8. 2. Microfabrication
    This step was carried out using the Nordiko 3600 machine to remove all the material until reach the substrate (glass) keeping the main pillar protected by the photoresist.
    Four repetitions of 200 s etching (60º) followed by 200 s cooling were performed.
    Three additional repetitions were performed since the real etching velocity was inferior to the theoretical one (1A/s)
    Samples were checked after resist strip
    2.2. 1st Ion milling, Total structure etch, followed by resist strip
    Top electrode and junction definition was performed in the DWL machine. For this step, sample alignment was required for correct definition.
    The following mask was used:
    2.3. 2nd Exposure, Top electrode and junction definition
  • 9. TiWN2 150Å
    2.3. 2nd Exposure, Top electrode and junction definition
    Ta 50Å
    Ru 50Å
    CoFeB 20Å
    MgO
    CoFeB 40Å
    CoFeB 40Å
    Ru 8Å
    Ru 8Å
    CoFe 30Å
    CoFe 30Å
    MnIr 200Å
    MnIr 200Å
    Ta 90Å
    Ta 90Å
    Ru 200Å
    Ru 200Å
    Ta 90Å
    Ta 90Å
    Top electrode dimensions are 350 um x 105 um and junction has 70 um x 3 um
    Both samples were well exposed:
  • 10. This step was initiated in the Nordiko 3600 machine with the following parameters:
    2.4. 2nd Ion Milling - Top electrode and junction definition
    * Pan (60º)+(30º)
    Since we obtained an incomplete etch, it was decided to use the Nordiko 3000 machine to end this step. The following parameters were used:
    † Pan (60º)+(30º)
    After optical inspection, neither samples presented any deformations.
  • 11. 2.5. 1st Insulating layer deposition
    This step was performed using the UHV2. It was deposited a layer of of Al2O3 with 600 A thickness.
    2.6. 1st Oxide Lift-off
    It is difficult to perform oxide lift-off, so both samples were left over night in the hot micro-strip.
    Error observed after oxide lift-off
  • 12. Contact leads definition was performed in the DWL machine. For this step, sample alignment was again required for correct definition.
    The following mask was used:
    2.7. 3rd Exposure, Contact leads definition
    Both samples were well exposed:
    2.8. Contact leads deposition
    This step was performed using the Nordiko 7000 with the following parameters:
  • 13. We experienced some technical problems during this step!!
    2.9. Step #8 - Contact leads deposition
    Some structural anomalies after this step were observed. Nevertheless, the process continued.
    2.10. Step #9 - 4th Exposure, Junction top contact plus current line definition
    Junction top contact and current line definition was performed in the DWL machine. For this step, sample alignment was once again required for correct definition.
    Each pad has 200 um x 200 um
  • 14. The following mask was used:
    2.11. 4th Exposure, Junction top contact plus current line definition
    #408, was generally well exposed and aligned at the first time.
    #407, At the first exposure was not well aligned, so the photoresist had to be removed, and a second exposure had to be done.
  • 15. 2.12. 2nd Insulating layer deposition
    This step was performed using the UHV2. It was deposited a layer of of Al2O3 with 1000 A thickness.
    2.13. 2nd Oxide Lift-off
    It is difficult to perform oxide lift-off, so both samples were left over night in the hot micro-strip.
    Final structure overview.
  • 16. Characterization of tunnel magnetic junction
    TMR = 38,5%
    TMR = 5,3%
  • 17. Conclusions
    • Globally the objective of the work was achieved addressing the themes referred in the classes;
    • 18. The majority of the MTJ had undesired TMR (<< 40%) although the structures seamed well constructed;
    • 19. This study provided the opportunity to experience useful techniques in microfabrication.