MICRO AND NANOFABRICATION TECHNIQUES

MTJ Sensors for Biochip applications

Pedro Novo 64407António Sousa 64427Álvaro Faleiro 66040MBioNano

Layout

1. Introduction

2. Microfabrication process

3. Characterization of tunnel magnetic junction

4. Conclusion

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.

1. Introduction

Antiferromagnetic

Pinned Ferromagnetic

Free Ferromagnetic

Barrier

Sintetic Antiferromagnetic (SAF)

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.

Passivation Layer

Longitudinal magnetic field helps the orientation of the free ferromagnetic layer. Varying an applied transversal magnetic field, its possible to measuere the resistance variation

TM

R (

%)

Applied Magnetic Field (Oe)

Resi

stan

ce (

Oh

m)

1. Introduction

Two electrode magentizations in parallel state.

Two electrode magentizations in antiparallel state.

Low resistance state.

High resistance state.

It is expected a linear sensor!

P P Freitas et al, 2008

1. Introduction Sensible to week magnetic fields

(nT).

Motivation:Biomedical and Biotechnologic

applicationsAlternative to the use of fluorescent markers and optical instruments.

P P Freitas et al, 2008

A probe is funcionalized in a MTJ structure. Biomolecules with magnetic labels hibridez in this surface.

2. MicrofabricationProcess

Ta 90Å

Ru 200Å

MgOCoFeB 40Å

Ru 8ÅCoFe 30Å

MnIr 200Å

CoFeB 20Å

TiWN2 150Å

Ta 90Å

Ta 50Å

Ru 50Å

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

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:

2. Microfabrication

2.2. 1st Ion milling, Total structure etch, followed by resist strip

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

2.3. 2nd Exposure, Top electrode and junction definition

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:

Ta 90Å

Ru 200Å

MgOCoFeB 40Å

Ru 8ÅCoFe 30Å

MnIr 200Å

CoFeB 20Å

TiWN2 150Å

Ta 90Å

Ta 50Å

Ru 50Å

Ta 90Å

Ru 200Å

CoFeB 40ÅRu 8Å

CoFe 30Å

MnIr 200Å

Ta 90Å

Top electrode dimensions are 350 um x 105 um and junction has 70 um x 3 umBoth samples were well exposed:

2.3. 2nd Exposure, Top electrode and junction definition

2.4. 2nd Ion Milling - Top electrode and junction definition

This step was initiated in the Nordiko 3600 machine with the following parameters:

Samples Etching Turn Time Observations

#407 and #408 1st 275 +100* Incomplete etch

2nd 150+100* Incomplete etch

3rd 0+130* Incomplete etch

* Pan (60º)+(30º)

† 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:

Samples Etching Turn Time Observations

#407 and #408 4th 50+30† Minor etch

5th 30+0† Nearly etched

6th 60+0† Full etch

After optical inspection, neither samples presented any deformations.

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

2.7. 3rd Exposure, Contact leads definition

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:

Both samples were well exposed:

2.8. Contact leads deposition

Seq.48 (svpad) Function Mode Power Pressure Ar N2

1’ soft sputter etch Mod.2 – f.9 60 W/40 W 3 mTorr 50 sccm -

1200 A Al, 32’’ Mod.4-f.1 2 KW 3 mTorr 50 sccm -

150 A TiW, 27’’ Mod.3-f.19 0,5 KW 3 mTorr 50 sccm 10 sccm

This step was performed using the Nordiko 7000 with the following parameters:

We experienced some technical problems during this step!! Some structural anomalies after this step were observed. Nevertheless, the process continued.

2.9. Step #8 - Contact leads deposition

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

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.

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.

Characterization of tunnel magnetic junction

-200.0 -150.0 -100.0 -50.0 0.0 50.0 100.0 150.0450.0500.0550.0600.0650.0700.0750.0

Tunnel magnetic resistance

Magnetic field (Oe)

Resi

stan

ce (

Ω)

-200.0 -150.0 -100.0 -50.0 0.0 50.0 100.0 150.0152.0153.5155.0156.5158.0159.5161.0

Tunnel magnetic resistance

Magnetic field (Oe)Resi

stan

ce (

Ω)

TMR = 38,5% TMR = 5,3%

Conclusions

• Globally the objective of the work was achieved addressing the themes referred in the classes;

• The majority of the MTJ had undesired TMR (<< 40%) although the structures seamed well constructed;

• This study provided the opportunity to experience useful techniques in microfabrication.