STRUCTURAL MODIFICATION OF MgO/CoFeB USING A LOW ENERGY ION BEAM FROM AN ASSISTED DEPOSITION SOURCE

Transcription

1 , 6-10 th September STRUCTURAL MODIFICATION OF MgO/CoFeB USING A LOW ENERGY ION BEAM FROM AN ASSISTED DEPOSITION SOURCE Ricardo Ferreira, Susana Cardoso and Paulo P. Freitas

2 Presentation Outline I. Motivation II. Ion Beam Assisted Deposition III. X-ray Diffraction Data of MgO and CoFeB/MgO/CoFeB IV. HR-TEM of CoFeB/MgO multilayers V. Conclusions

3 Motivation Magnetic Tunnel Junctions AlOx MTJs : Magnetic Field Sensors, HDD read heads MgO MTJs : non-volatile memories (MRAM), Spin Transfer Nano-Oscillators (Wi-Fi, novel CMOS magnetic hybrid devices) Slide 1/17

4 Motivation PVD Deposited MgO Magnetic Tunnel Junctions As deposited amorphous CoFeB [100] MgO amorphous CoFeB After annealing bcc [100] CoFeB [100] MgO bcc [100] CoFeB ~ 10% TMR ~300% TMR PVD systems : coherently tunneling is routinely achieved TMR is strongly system dependent best TMR values are still far away from theoretical predictions (>1000%). Small number of parameters available (pressure, RF power, distance to substrate) Slide 2/17

5 Ion Beam Deposition Kauffmann type Ion Beam Guns V + : Ion Kinetic Energy I + : Ion Beam Current V - : Extraction Voltage Number of Ions extracted per unit I - : Ion Beam Divergence of time and the energy per ion are independent parameters! Constant Beam Profile : I + = α (V + - V - ) 3/2 Slide 3/17

6 Ion Beam Deposition MgO Ion Beam Deposition Not Assisted Energy - V+ [V] Ion I+ : +79mA V+ : +600V dep: A/s I+ : +108mA V+ : +800V dep: A/s I+ : +138mA V+ : +1000V dep: A/s 135.4W 84.7W Ion Current - I+ [ma] 47.4W Depo osition Rate [A/s] Equation Weight Residual Sum of Squares y = a + b*x Instrumental Adj. R-Square Value Standard Error DepRate Intercept 0 -- DepRate Slope E-5 I+ : +79mA V+ : +600V dep: A/s I+ : +138mA V+ : +1000V dep: A/s I+ : +108mA V+ : +800V dep: A/s I+ = α V 3/ Beam Power - I + V + [W] As deposited amorphous CoFeB amorphous MgO amorphous CoFeB After annealing amorphous CoFeB [100] MgO amorphous CoFeB Ion Beam deposited MgO is always amorphous in the as-deposited state! Slide 4/17

7 Ion Beam Assisted Deposition Nordiko 3600 Ion Beam Deposition Tool Ar Xe Ion Beam Assisted Deposition of MgO : An assistance Ar ion beam, directed to the substrate is used to provide extra energy to the atoms as they are deposited This extra energy can promote the crystalization of the MgO Slide 5/17

8 Ion Beam Assisted Deposition XRD diffraction of CoFeB/MgO/CoFeB stacks X-ray Diffraction In ntensity [cps] MgO Deposition Conditions: Deposition Gun +138mA; +1000V/-275V Assist Gun +95mA; +350V/-300V MgO 002 As deposited Annealed CoFeB bcc θ Ta CoFeB MgO CoFeB Ta 30Å 150Å 300Å 150Å 30Å Slide 6/17

9 Ion Beam Assisted Deposition Deposition rate versus Assist Gun Beam Power Effective Depositio on Rate [Å/s] Effective Deposition Rate = Deposition Rate Etch Rate Deposition Rate α Dep Gun Beam Power (I + V + ) Etch Rate α Assist Gun Beam Power (I + V + ) +0V/-0V +0mA +0V/-0V +0mA +50V/-650V +90mA +0V/-0V +100V/-350V +0mA +60mA +100V/-450V +30V/-850V +70mA +100mA +50V/-650V +90mA Deposition Gun Parameters: +1000V/-275V; +138mA +800V/-275V; +108mA +600V/-275V; +79mA +200V/-300V +80mA +350V/-100V +100V/-550V +70mA +350V/-175V +100mA +85mA +400V/-100V +100V/-550V +80mA +100mA +300V/-300V +60mA +275V/-175V +300V/-350V +70mA +95mA +350V/-175V +85mA Assist Gun Beam Power - I + x V + [W] Slide 7/17

10 Ion Beam Assisted Deposition Effect of Assist Gun Ion Energy (V + ) Assist V + <100V Surface Effects Assist beam ions enhance the mobility of surface atoms, without removing them. Effective Depositio on Rate [Å/s] Effective Deposition Rate = Deposition Rate Etch Rate Deposition Rate α Dep Gun Beam Power (I + V + ) Etch Rate α Assist Gun Beam Power (I + V + ) +0V/-0V +0mA V + <100V Soft Energy Transfer Regime +0V/-0V +0mA +50V/-650V +90mA +0V/-0V +100V/-350V +0mA +60mA +100V/-450V +30V/-850V +70mA +100mA +50V/-650V +90mA Deposition Gun Parameters: +1000V/-275V; +138mA +800V/-275V; +108mA +600V/-275V; +79mA +200V/-300V +80mA +350V/-100V +100V/-550V +70mA +350V/-175V +100mA +85mA +400V/-100V +100V/-550V +80mA +100mA +300V/-300V +60mA +275V/-175V +300V/-350V +70mA +95mA V + >100V Etching Regime +350V/-175V +85mA Assist Gun Beam Power - I + x V + [W] Assist V + >100V Bulk Effects Etch Rate : ± (Å/s) / W Slide 8/17

11 Ion Beam Assisted Deposition Effect of Assist Gun Beam Current (I + ) Soft Energy Transfer Regime I + >=80mA is required I + [ma ma] Assist Gun I V -0V -0V -850V -650V -450V -350V -300V -175V -350V -300V I + <= 60 ma Amorphous MgO region -175V -175V -100V -100V Deposition Gun Parameters: +1000V/-275V; +138mA +800V/-275V; +108mA +600V/-275V; +79mA Etch Regime I + >=70mA Crystalline MgO Assist Gun V + [V] Slide 9/17

12 Ion Beam Assisted Deposition Effect of Assist Gun Beam Current (I + ) X-ray Diffraction Inte ensity [cps] MgO Deposition Conditions Dep Gun (+138mA; +1000V/-275V) Assist Gun (+100mA; +100V/-550V) Dep Gun (+108mA; +800V/-275V) Assist Gun (+100mA; +100V/-550V) θ MgO ~600Å Glass MgO texture is stronger for conditions with larger Ion/Atom ratio (Assist Gun I + / Dep Gun I + ). The assist gun current required to promote MgO crystallization increases with decreasing assist gun voltage. Soft Energy Transfer Regime is hard to get : very large currents are required for very low voltages in order to crystallize MgO Slide 10/17

13 X-ray Diffraction Etching Regime versus Soft Regime : impact on MgO structure X-ray Diffraction Inte ensity [cps] MgO Deposition Conditions Dep Gun ( +74mA; +600V/-275V) Assist Gun (+100mA; +30V/-850V) Dep Gun (+108mA; +800V/-275V) Assist Gun ( +70mA; +275V/-175V) Soft Regime 19.2º 19.6º Etching Regime θ [º] Tail indicating lower quality texture MgO ~600Å Glass XRD Diffraction shows that: 1) Textured MgO can be obtained in both regimes 2) There are structural differences between the MgO deposited in the two regimes considered Slide 11/17

14 X-ray Diffraction MgO lattice constant tuning Constant [A] MgO Lattice Å Assist Gun Power (V+I+) / Dep Gun Power (V+I+) MgO ~600Å Glass MgO lattice constant can be changed by setting the ration between the assist gun power and the deposition gun power. This feature can be used to reduce the mismatch between the MgO and CoFeB lattices. Slide 12/17

15 X-ray Diffraction Etching Regime versus Soft Regime : structural differences X-ray Diffraction Inten nsity [cps] MgO Deposition Conditions Dep Gun ( +74mA; +600V/-275V) Assist Gun (+100mA; +30V/-850V) Dep Gun (+108mA; +800V/-275V) Assist Gun ( +70mA; +275V/-175V) Etching regime Soft regime MgO 002 Annealed 1h@Tset=360ºC CoFeB bcc θ [º] Ta CoFeB MgO CoFeB Ta 30Å 300Å 20Å 300Å 30Å MgO deposited in the soft energy transfer regime is more efficient in promoting the texture of CoFeB upon annealing. Slide 13/17

16 HRTEM Stacks prepared thin MgO and thin CoFeB layers Ta 10 bi-layers t-mgo t-cofeb t-mgo t-cofeb t-mgo t-cofeb Ta... 30Å 20Å 30Å 20Å 30Å 20Å 30Å 30Å MgO deposition conditions : 1) Soft Energy Transfer Regime Deposition Gun : +600V; +74mA; 4.0 sccm Xe Assist Gun : +30V/-850V; +100mA; 10sccmAr 2) Concurrent Etching Regime Deposition Gun :+800V; +108mA; 4.0 sccm Xe Assist Gun :+275V/-175V; +70mA; 10sccmAr CoFeB and Ta deposition conditions : Deposition Gun : +1200V/-275V; +170mA; 4.0 sccm Xe Slide 14/17

17 HRTEM Etching Regime versus Soft Regime : CoFeB crystallization Soft Energy Transfer Regime Deposition Gun : +600V; +74mA; 4.0 sccm Xe Assist Gun : +30V/-850V; +100mA; 10sccmAr Concurrent Etching Regime Deposition Gun :+800V; +108mA; 4.0 sccm Xe Assist Gun :+275V/-175V; +70mA; 10sccmAr Mostly crystalline CoFeB Mostly amorphous CoFeB Slide 15/17

18 HRTEM Etching Regime versus Soft Regime : CoFeB crystallization Soft Energy Transfer Regime Deposition Gun : +600V; +74mA; 4.0 sccm Xe Assist Gun : +30V/-850V; +100mA; 10sccmAr Concurrent Etching Regime Deposition Gun :+800V; +108mA; 4.0 sccm Xe Assist Gun :+275V/-175V; +70mA; 10sccmAr Selected Area Diffraction Pattern (SAD) shows strong out of plane [001] texture in all CoFeB and MgO layers Indexed reflections in the FFT suggest [001] texture of the MgO and mixed orientations for the CoFeB CoFeB does not exhibit crystalline properties everywhere Slide 16/17

19 Conclusions Ion Beam Assisted Deposition is an effective technique to obtain strong textured CoFeB/MgO There are two assisted deposition regimes : soft energy transfer and concurrent etching regime XRD data and TEM show that the crystallization of CoFeB depends strongly on the assisted deposition conditions of MgO Data suggests that the best conditions to obtain high textured CoFeB/MgO are obtained with high current and low voltage assistance beams The impact of the assisted deposition conditions on the transport properties of magnetic tunnel junctions is currently being determined Slide 17/17

20 Thank you for your attention! Questions?