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2/ After UV (or electron beam) lithography and development

5/ The manganite patterns after etching (IBE or RIE)

3/ After evaporation of the etching metallic mask

3/ After evaporation of the etching metallic mask

Al or Ti

6/ The patterned manganite film after removal of the metallic mask

Figure 6. Description of an etching process based on nanolithography (steps 1 and 2) followed by a lift-off process (steps 3 and 4).

broken chains and reveals the resist patterns (see Fig. 6, step 2). Below 10 nm, the Van der Waals intermolecular forces between the irradiated resist molecules prevent this dissolution and development should be assisted by an ultrasonic (US) agitation. Wide lines of 5 nm can be obtained in 140 nm thick PMMA resist using high resolution SEBL and US development [78]. More recently, nanoimprinting appears as a low cost alternative replication method compared to SEBL and XRL, which need expensive steppers or beam pattern generators. During nanoimprinting, the thin thermoplastic resist film is pressed by a mould which has been previously worked out using SEBL and dry etching. This technique allows elaboration of lines and dots as small as 20 nm in width [79,80].

After nanolithography, the pattern transfer can be achieved using direct etching, with the resist as mask, or using a metallic lift-off process followed by etching. The liftoff process, which is explained in Figure 6 (steps 3 and 4), is the preferred method for manganite etching since these CMR oxides are very hard materials compared to metals. In contrast to the extensive studies of reactive ion etching (RIE) for the ferromagnetic metallic films, the plasma etching process for manganite materials has not been studied in depth and the literature is very poor. However, these highly spin polarized CMR oxides are very promising materials for realizing spintronics nanodevices.

The majority of magnetoresistive nanodevices are thus ordinarily etched using conventional Ar ion beam etching (IBE) after an Al lift-off process. Al is the more frequently used metallic mask since it is easily oxidized and because it appears to be the hardest metal for this application. For IBE of manganites, this bilayer Al/Al2O3 presents the best selectivity (selectivity is the ratio of the manganite etch rate over the Al mask etch rate), which never exceeds the value of 2. To avoid removal of the Al mask in solutions that always attack the manganite film (step 6 in Fig. 6), the etching time is often increased in order to fully etch the Al mask. Typical

6/ The patterned manganite film after removal of the metallic mask micrometer dots etched in a LSMO film using such a lift-off process are shown in Figure 7. Note that both the edge and surface of the LSMO dots are very smooth. This is due to the very smooth morphology of both LSMO and Al films, as well as the high quality of the XRL patterning (see Fig. 7a). With such an optimized IBE process, nanopatterning at a sub-100 nm scale of LSMO films can easily be achieved. The nanodevice presented in Figure 8 exhibits 80-nm-wide contact arms, and 120-m-wide trenches have been successfully etched on the top of the microbridge [81].

To avoid lateral damages from the high energy Ar bombardment that can modify the magnetoresistive nature of the manganite, RIE is the alternative method. It allows an increase of selectivity for deep etching and also smoother edges. Wang et al. [82] reported fluorine SF6/Ar and chlorine Cl2/Ar electron cyclotron resonance etching of Ca-manganites that are very similar to pure Ar IBE with no chemical enhancement. This is attributed to the high melting points beyond several hundreds °C of the reacted products of fluorides and chlorides. Naoe et al. [83] recently proposed a CO/NH3 gas RIE process with formation of

Figure 7. Scanning electron microscope (SEM) pictures of 2-jU.m-diameter dots patterned in a La0 7Sr0 3MnO3 thin film grown on SrTiO3 recorded after (a) the XRL nanolithography replication in the resist (step 2) and (b) the IBE etching (step 6).

Figure 8. SEM top view of a La0 7Sr03MnO3 spin filter patterned using conventional SEBL and high resolution EBL at 200 keV. Reprinted with permission from [81], J. Wolfman et al., J. Appl. Phys. 89, 6955 (2001). © 2001, American Institute of Physics.

Figure 7. Scanning electron microscope (SEM) pictures of 2-jU.m-diameter dots patterned in a La0 7Sr0 3MnO3 thin film grown on SrTiO3 recorded after (a) the XRL nanolithography replication in the resist (step 2) and (b) the IBE etching (step 6).

Figure 8. SEM top view of a La0 7Sr03MnO3 spin filter patterned using conventional SEBL and high resolution EBL at 200 keV. Reprinted with permission from [81], J. Wolfman et al., J. Appl. Phys. 89, 6955 (2001). © 2001, American Institute of Physics.

metal-carbonyls that exhibit low melting points (several tens °C) and a high volatility. These authors demonstrate an enhancement of selectivity up to 4.7 that is comparable to those of ferromagnetic metals, with a significant reduction of the peak-to-peak surface roughness. Such recent RIE processes have not been applied yet to magnetoresistive devices. Thus a comparison of pure Ar and reactive gases etchings in terms of lateral damages in manganite nanodevices cannot be done for the time being.

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