40 % tunneling magnetoresistance after anneal at 380 ° C for tunnel junctions with iron – oxide interface layers

Spin tunnel junctions fabricated with one interposed Fe–FeOx layer between the Al 2O3 barrier and the top CoFe pinned electrode show large tunneling magnetoresistance ~TMR! ~40%! for anneals up to 380 °C. The annealing temperature TTMR * , where maximum TMR occurs, increases with the inserted Fe–FeOx layer thickness. For samples with thicker inserted layer, the pinned layer moment ~which usually starts to decay below 300 °C in the normal junctions ! i creases with annealing temperature up to 380 °C and remains at a maximum until 450 °C. The large TMR at high temperature is related with the diffusion of extra Fe ~from the Fe–FeOx layer ! into the electrode interfacial region and the as-deposited paramagnetic FeOx decomposition into metallic Fe, and possibly the formation of some Fe 3O4, which compensate the interface polarization loss associated with Mn interdiffusion. Rutherford backscattering spectrometry analysis confirms partial Fe diffusion into the top CoFe electrode after anneal. Meanwhile, x-ray photoelectron spectra for the Fe 2p core level show that the FeOx contribution in the upper part of the inserted layer decreases upon annealing, while it increases in the inner part near the barrier, suggesting the FeOx decomposition and the oxygen diffusion toward the inner metallic Fe and Al barrier. The study of R3A values and barrier parameters versus annealing temperature for samples with 7 and 25 Å Fe–FeOx also reflects the above structural changes in the inserted layer. © 2001 American Institute of Physics.@DOI: 10.1063/1.1356712 #

Magnetic tunnel junctions ͑MTJs͒ with tunneling magnetoresistance signal ͑TMR͒ in excess of 40% and adjustable resistance-area products RϫA from 10 6 to few hundred ⍀ m 2 can now be fabricated.The present TMR signals and RϫA values are suitable for magnetic random access memory applications.However, good thermal stability for the MTJs is required to cope with standard backend processes ͓sintering: 400-450 °C, plasma enhanced chemical vapor deposition oxide deposition: 350 °C͔ occurring during MTJ integration with a ͑CMOS͒ wafer. 1,2The TMR signal usually decreases above 300 °C due to the polarization loss resulting from the Mn ͑in Mn-X exchange layer͒ diffusion into the CoFe top electrode. 3,4In this work, thermal stability for MTJs is improved up to 380 °C by the insertion of an iron-oxide layer with appropriate thickness between the AlOx barrier and the top CoFe pinned electrode.This brings MTJs one step closer to full CMOS backend compatibility. 5,6unctions with structure of glass/Ta90 Å/ Ni 80 Fe 20 70 Å / Co 80 Fe 20 30 Å/Al9 Åϩoxide / Fe (t Fe ) ϩoxide/ Co 80 Fe 20 40 Å/Mn 76 Fe 24 250 Å/Ti 10 W 90 ͑N͒150 Å were pre-pared by ion beam deposition and oxidation using a Nordiko 3000 IBD tool, and then patterned down to 3ϫ1 m 2 dimensions with a self-aligned process. 7,8Magnetic measurements were done by a vibrating sample magnetometer in unpatterned parts of the sample.X-ray photoelectron spectroscopy ͑XPS͒ experiments were performed to analyze the Fe and FeOx spatial distribution.Since the XPS signal comes from an area within a distance of about 2-3 ͑ is the inelastic mean free path for electrons͒ from the sample surface, low-energy ion beam ͑4 keV, 45°͒ etch was carried out to obtain a depth profile.Steps 1 and 2 correspond to surface spectra, and step 6 is the spectrum taken after sputtering for 6 s so that step n is taken after sputtering for 6 •(nϪ2) s.The etch rate is around 6-10 nm/min.The intensities of the photoelectron lines were recorded using the Al K ␣ emission line.All thermal treatments consist of 45 min anneals at each temperature, done in vacuum (10 Ϫ6 Torr), and followed by cooling in field applied along the common easy axis of the electrodes.
Figure 1 shows the pinned layer moments versus annealing temperature for samples with t Fe ϭ0, 7, 10, 20, and 25 Å.The Fe and Al layers are oxidized by 10 s exposure to a remote oxygen plasma except for the Al barrier in the refer-a͒ Author to whom correspondence should be addressed; electronic mail: zhongzhi.zhang@inesc.ptence sample with t Fe ϭ0, which is oxidized for 20 s.There is a net increase in the moment of the pinned layer for increasing Fe thickness.For junctions with 7 and 10 Å Fe, there is a strong moment decrease starting around 300 °C, similar to the reference sample without Fe, corresponding to Mn diffusion into the CoFe top electrode.Figure 2͑a͒ shows the TMR versus annealing temperature for the samples discussed above.There are striking differences in the as-deposited TMR and annealing temperature T TMR * where maximum TMR occurs, as shown in the inset for various Fe-FeOx thicknesses.In the as-deposited state, full oxidation occurs for t Fe ϭ10 Å, and zero TMR results, consistent with formation of a nonmagnetic or paramagnetic Fe oxide.The 8% signal for 7 Å Fe is probably due to a discontinuous Fe layer.For thicker Fe layers, the asdeposited TMR signal is 10%, implying that incomplete oxidation occurs ͑FeOxϩFe formed͒.T TMR * increases with the inserted Fe layer thickness, and reaches a maximum of 380 °C at t Fe ϭ20 Å. Upon annealing, the TMR signal starts to increase from 0% at 220 °C to 36% at 270 °C in the sample with 10 Å Fe, indicating that the FeOx decomposition starts at 220 °C.For the thicker Fe layers (t Fe у20 Å), TMR first decreases to zero as the annealing temperature increases to 360 °C, and then shows a sharp peak at 380 °C.The TMR decrease is related with the reduced interfacial polarization, occurring when the oxygen from the FeOx layer moves toward the inner metallic Fe, resulting in the paramagnetic FeOx formation near the barrier.The sharp peak at 380 °C is probably associated with the final decomposition of this interfacial FeOx. Figure 2͑b͒ illustrates the stability of such junctions with t Fe ϭ25 Å upon reanneal.The sample was first annealed up to 380 °C and showed maximum TMR ͑solid square͒.It was then reannealed up to 365 °C ͑open square͒ maintaining the large TMR.
In order to clarify what is happening after anneal to the as-deposited Fe-FeOx layer, XPS measurements were performed on a special sample with a structure of Si /Al 2 O 3 1000 Å/Co 80 Fe 20 30 Å/Al9 Åϩ10 s oxide/Fe20 Å ϩ10 s oxide/Co 80 Fe 20 40 Å/Ta30 Å. Normalized XPS spectra for the Fe 2p core level ͑2 p 3/2 and 2 p 1/2 ͒ are displayed in FIG. 1. Dependence of the normalized pinned layer moments on the annealing temperature for various inserted Fe layer thicknesses.The inset shows hysteresis loops for a multilayer with structure of (Fe20 Å/10 s oxide) 8 before and after anneal.Figure 4 shows RϫA values and barrier parameters for two samples with t Fe ϭ7 and 25 Å.The current-voltage curves were fitted using the Simmons model.The asdeposited RϫA value for 7 Å Fe is about 9.6 M⍀ m 2 due to full oxidation of the Fe layer, 4 orders larger than that in the sample with 25 Å Fe which is only about 600 ⍀ m 2 , but after anneal, both samples reach a similar value.In the as-deposited state, the sample with 7 Å Fe has an effective thickness of about 20 Å and barrier height of 0.5 eV.After anneal at 250 °C where the maximum TMR occurs, the barrier height increases up to 2.5 eV and barrier effective thickness decreases to 10 Å.For 25 Å Fe the as-deposited barrier parameters and the RϫA value are characteristic of pure 9 Å barrier junctions.Upon annealing, the barrier height de-creases to 0.5 eV, but barrier thickness doubles to 20 Å at 360 °C, the barrier parameters at this temperature are similar to the sample of 7 Å Fe in the as-deposited state, indicating the possible same Fe and FeOx distribution existing in these two states.This can be caused by the oxygen migration from the FeOx toward the inner pure Fe near the barrier.Resistance drops at 380 °C where maximum TMR occurs, and at this point barrier height increases from 0.5 to 2.5 eV, and barrier thickness decreases to 10 Å due to the eventual FeOx decomposition at the Al 2 O 3 /Fe interface, in agreement with the XPS results. 9his work was supported by PRAXIS XXI Project No. P/CTM/10220/98 and Sapiens Project No. 34116/99.Z.Z. and S.C. are grateful for the support of post-doctoral Grant No. PRAXIS/BPD/22112/99 and PhD Grant No. PRAXIS/ BD/11533/97, respectively.X.B. is indebted to Dr. J. Alay ͑University of Barcelona͒ for the XPS measurements.Financial support from both the Spanish CICYT through Project No. MAT97-0404 and the Catalonian CIRIT through Project No. 1998SGR0048 are greatly appreciated.
For 25 Å Fe, the pinned layer moment increases almost linearly with annealing temperature up to 380 °C, and remains at a maximum value up to 450 °C.RBS experiments on a special sample of Si/Al 2 O 3 500 Å/Co 80 Fe 20 30Å/Al9 Åϩ10 s oxide/Fe20 Å ϩ10 s oxide/Co 80 Fe 20 40 Å/Al 2 O 3 15 Å shows that 16.5 ϫ10 ϩ15 atom/cm 2 of Fe are found in the as-deposited Fe-FeOx.After anneal at 385 °C, 7ϫ10 ϩ15 atom/cm 2 of Fe have moved into the top CoFe electrode ͑about 8 Å increase in the top CoFe thickness, and an enrichment in Fe of the interfacial CoFe region͒.The remaining 9.5 ϫ10 ϩ15 atom/cm 2 of Fe either form a pure 11 Å thick Fe layer at the AlOx/CoFe interface, with the excess oxygen becoming incorporated into the AlOx barrier, or remaining as FeOx (Fe 3 O 4 ).To check the magnetic nature of the FeOx grown in this experiment, a multilayer of glass /͓Fe20 Å/10 s oxide͔ 8 was made and was subject to the same annealing procedure described before.The inset shows the as-deposited M -H loop ͑characteristic from a paramagnetic FeOx͒ and loops annealed at 280 and 385 °C ͑remanence and large coercivity suggest the formation of a ferromagnetic or ferrimagnetic oxide layer, possibly Fe 3 O 4 ͒.This proves the nonmagnetic characteristic of the as-deposited FeOx layers.From the above analyses, the moment increase upon anneal comes from the as-deposited nonmagnetic or paramagnetic FeOx decomposition into pure Fe and/or Fe 3 O 4 , and partial Fe diffusion into the CoFe electrode.It is found that the moment decrease normally due to Mn interdiffusion has been pushed to high temperature for the samples with a thicker Fe-FeOx layer.

FIG. 2 .
FIG. 2. ͑a͒ TMR vs annealing temperature for junctions with different interfacial Fe-FeOx layers.The inset shows the dependence of the asdeposited TMR value and T TMR * ͑where maximum TMR value occurs͒ on the inserted layer thickness.͑b͒ TMR and RϫA values for junctions with 25 Å Fe, first annealed to get the maximum signal ͑solid square͒ at 380 °C, then subject to further annealing starting at 180 °C ͑open square͒.

Fig. 3͑a͒ ,
Fig. 3͑a͒, showing the metallic Fe and FeOx contributions, after different anneals.The two spectra shown here correspond to two different regions of the Fe-FeOx layer: the upper part near the top CoFe interface ͑step 8, 36 s sputter-ing͒, and the inner part near the barrier ͑step 16, 84 s sput-tering͒, respectively.Figure 3͑b͒ shows the relative intensity of the FeOx contribution in the as-deposited state and annealed at 400 °C, as a function of sputtering time.Near the CoFe interface ͑step 8͒, the FeOx contribution decreases upon anneal, revealing the decomposition of the FeOx layer and oxygen moving into the inner Fe layer.Near the AlOx interface ͑step 16͒, the relative intensity of the FeOx peak increases with annealing due to the oxygen coming from the decomposition of the initial ͑outer͒ FeOx layer, while the intensity of the metal peak decreases.Figure4shows RϫA values and barrier parameters for two samples with t Fe ϭ7 and 25 Å.The current-voltage curves were fitted using the Simmons model.The asdeposited RϫA value for 7 Å Fe is about 9.6 M⍀ m 2 due to full oxidation of the Fe layer, 4 orders larger than that in the sample with 25 Å Fe which is only about 600 ⍀ m 2 , but after anneal, both samples reach a similar value.In the as-deposited state, the sample with 7 Å Fe has an effective thickness of about 20 Å and barrier height of 0.5 eV.After anneal at 250 °C where the maximum TMR occurs, the barrier height increases up to 2.5 eV and barrier effective thickness decreases to 10 Å.For 25 Å Fe the as-deposited barrier parameters and the RϫA value are characteristic of pure 9 Å barrier junctions.Upon annealing, the barrier height de-