EDITION 2
MEMS Developments in advanced functional materials for MEMS applications EXTRACTS FROM LAYERS 2016
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MEMS
MEMS
Maurus Tschirky, Senior Manager Product Marketing
Micro-Electro-Mechanical Systems, or MEMS, is a technology that in its most general form can be described as miniaturized mechanical and electromechanical elements (i.e., devices and structures). MEMS devices can vary from relatively simple structures having no moving elements, to highly complex electromechanical systems with multiple moving elements under the control of integrated microelectronics. A more imaginative perspective might see in MEMS no less than another “REVOLUTION�. They are the bridge between the digital and the real world. Look at inertial instruments for example: Such complex mechanical and optical instruments once weighing several kilos and restricted to submarines and missile war-heads for those who could afford it, are now size of a fraction of a fingernail and cost a few tens of cents. Such devices allow for more natural and intuitive communication with machines. The next step here is already demonstrated by yet another concept involving optics for gesture recognition. All of this is summarized as Heterogeneous Integration of various physical stimuli into a single device where the integration actually happens on silicon wafers. Thin film technology is at the core of the MEMS fabrication and Evatec delivers deposition solutions for advanced functionality materials with optical, magnetic, piezoelectric, thermoelectric and chemical properties to enable the required multidisciplinary approaches now and in the future. We strive to advance MEMS fabrication methods that promise an enormous design freedom wherein any type of sensor or actuator can be merged onto a single substrate combining approaches in microelectronics, photonics and nanotechnology.
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Challenges in characterization of piezoelectric thin films Professor Paul Muralt, from Ecole Polytechnique Fédérale de Lausanne talks about measurement techniques and explains the challenges in proper characterization and measurement of thin piezoelectric films like AlN and AlScN approach is long, complex, and not practical for materials research targeting at processproperty relationships for a large number of process parameters. For this reason, materials scientists use a simplified approach, i.e. to produce controlled deformations without making much of micromachining before (thus film on full substrate), and to measure resulting charges, or to apply an electric field in a well defined capacitor structure, and measure the resulting strain or stress.
Figure 1: Schematic drawing of the wurtzite structure as e.g. for an AlN-ScN alloy having the Al position partially occupied by Sc. The polar characteristics of wurtzite is based on the fact that all tetrahedrons of the same type (either formed by 4 N, or by 4 metal ions) are pointing in the same direction. In the drawing, N tetrahedrons are pointing up, metal tetrahedrons are pointing down.
Piezoelectric coefficients are the most important property parameters for a piezoelectric material. Unfortunately, the electromechanical response is not a simple one because dielectric, elastic, and piezoelectric properties are linked together. For the characterization of a single crystal material, one would prepare several samples (platelets) with different orientations, in order to measure the elastic, dielectric, and piezoelectric properties (11 in total for AlN) along all major directions. For a thin film, this approach is not feasible for all orientations. The most easy one to realize is the geometry of a thin film bulk acoustic wave resonator, i.e. a platelet with (0001) faces and measure e33, c33D, ε33S and ε33T through the excitation of standing waves along the thickness of the plate, and measurement of the impedance or admittance as a function of frequency. With other vibration modes one can derive further properties. This
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However, this approach is not as simple as measuring a bulk sample. We can see the difference considering the assessment of the direct piezoelectric effect with a bulk sample equipped with 2 parallel electrodes (see fig. 2), and which is based on the following constituent equation:
D3 means the displacement field or charge density produced at electrodes perpendicular to the 3-axis, equal to the charge Q3 divided by the electrode area. When applying an exclusive T3 stress, and measure the charge at a virtual short (which is the case when using a charge amplifier), then we get , or when applying an exclusive stress T1, then we can derive also. When we try to do the same with a thin film sample we have several problems. In theory, it is possible to apply T3, because the stress will be the same in the substrate and in the thin film. However, the substrate will expand in the transverse direction (S1, S2 different from zero as T1=T2=0) differently as the piezoelectric thin film would do alone, because the respective s31 compliance values are different in general. The strain of the substrate is imposed to the piezoelectric thin film, causing more charges on the electrodes due to the transverse piezoelectric effect. The latter is best expressed in terms of the e-coefficients in this case:
The second problem is that we cannot a defined apply a transverse stress (T1, T2) to the piezoelectric film, because the substrate will dictate the elastic reaction, and not the film. What we can do, however, is to impose a defined transverse strain (S1, S2) to the substrate, meaning that we rather get e31, than d31. The ideal variables for the thin film problem are thus T3, S1 and S2. One can show that the constituent equation can be transformed to the following form:
where
We note that the effective thin film coefficient d33,f is always smaller than d33, and that the effective thin film e31,f has a larger value that e31. For measuring e31,f one has provide an experiment with a well known substrate deformation, to put T3=0 (this is the case at 1 atmosphere pressure (=0.1 MPa)), and E3=0 when using a charge amplifier. The controlled deformation is conveniently achieved by a bending experiment. The most simple experiment with a well defined strain is the 4-point bending test [1]. It works also be single point bending [2] and plate flexural bending [3]. The cantilever for bending experiments have a defined long direction along which the main strain is develops (say S1). Then, in a wide range of thickness to width ratios, the strain S2 is then given by the Poisson ratio νsub of the substrate: S2=- νsub S1. It follows that
So besides the curvature, one needs to know also the Poisson ratio of the substrate. In case of cantilevers obtained from (001) silicon wafers, the Poisson ratio amounts to 0.28 when the cantilevers point along [100], and 0.064 when they point along [110]. For measuring d33,f we consider the converse effect:
Applying E3 (voltage V3), and measuring the thickness change Δtp by double side interferometry, we can derive d33,f as Δtp/V3 provided that (1) S1=S2=0 and (2) the substrate does not change its thickness. The reason is that one measures the complete thickness change, the sum of the substrate and of the thin film. Unfortunately, these 2 conditions are normally not met. They would be true in the limiting case of an infinitely stiff substrate. For AlN/Si this is definitely not the case. So the only way to get around this problem is to perform finite element calculations. The elastic properties of the substrate are usually well known, and e31,f can be measured before. Let us call this new method FEM supported interferometry.
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Table 1: Thin film coefficients for AlN, derived from data as obtained at single crystals, epitaxial AlN thin films on sapphire (SAW device characterization), from direct thin film measurements (fat), and from ab-initio studies. Values in italics stand for derived values. The bold numbers are either measured directly (kt2 from TFBARs).
For AlScN thin film measurements one can first test the methods on pure AlN, for which properties are quite well known by now. The first complete determination of AlN properties was carried out at epitaxial AlN thin films on sapphire substrates by Tsubouchi and Mikoshiba [5] already in the early 1980’s. They characterized surface acoustic wave resonators. It does not require much effort in micromachining, but a large effort in characterization and fitting procedures to reproduce impedance curves of SAW resonators. A multitude of different SAW geometries, eventually also different substrates, combined with the simulation of wave propagation is needed to derive all property coefficients. The pioneering work of Tsubouchi and Mikoshiba [5, 12] shows that such work can be realized with very good results. In their work, AlN thin films were deposited by MOCVD at temperatures between 1000 and 1180 °C. Their values remained reference data, are reported in compendia of piezoelectric materials [13], and included in material libraries of finite element modeling programs. For a long time they were without competition by single crystal data. A new trend is to use ab-initio calculations based on density functional theories (DFT). They become increasingly powerful, and tend to meet experimental values more and more precisely. Recently, it became also possible to grow high quality AlN crystals. Their piezoelectric properties are not yet as high as in thin films, but close. Table 1 shows that crystal data, epitaxial film data, and ab-initio calculations are rather underestimating the coupling coefficient kt2. There is tremendous data collection of thickness mode resonators, because they are the heart of a large portion of RF filters used in mobile phones. The coupling coefficient is frequently reported to be around 6.5% for resonators near 2 GHz. Lakin et al [7] reported even a value of 7.0 % obtained with solidly mounted resonators. Electrode material (acoustic impedance) and thickness need to be chosen
optimally for getting the best possible coupling at a given frequency (AlN thickness). With higher frequency, electrodes tend to reduce the coupling as their relative volume in the resonator tends to increase. But even at 8 GHz, a coupling of kt2 = 5.9 % was realized [14]. The difference with respect to the T&M value is most likely due to a too high dielectric constant in the latter data set. A relative ε33S of 9.6 (corresponding to ε33,f = 10.3) would better fit to single crystal and sputtered thin film values, and leads to a kt2 of 6.7 % using otherwise T&M data. For resonator data, d33,f is derived between 3.86 pm/V (kt2 = 6.5 %) and 4.00 pm/V (kt2 = 7.0 %). The true values of AlN are with high precision: d33,f = 4.0 pm/V and e31,f = 1.05 C/m2.
THEORY & PROPERTIES OF AlScN ALLOY THIN FILMS Recently, it was discovered that Al substitution by Sc allows an increase of the piezoelectric response [15, 16]. Apparently, it needs a 3+ ion whose nitride structure exhibits a higher coordination than 4. Sc in AlN spreads the nitrogen tetrahedron to find more space. It is mainly the lattice constant a that increases. The length of c is almost untouched [8]. The Sc ion thus tends to approach the nitrogen base plane, increasing the u-parameter from 0.38 towards 0.5, as shown by ab-initio calculations [10]. Centring the Sc in the base plane would nearly cause a 5-fold coordination. The enlargement of the unit cell volume increases the distance between ions, and leads to a softening of the material. DFT works advance the interpretation that there is a competition of Al3+ and Sc3+ ions about the coordination of nitrogen resulting in a kind of frustrated system [10]. With increasing Sc concentration in the wurtzite phase, the potential wells of the ions become less deep, and thus the ionic displacements in an electric field become larger, leading to larger piezoelectric strains and dielectric responses. For piezoelectric characterization, Akiyama et al. applied a measurement technique that is applied for bulk materials (typically PZT ceramics). The sample was squeezed between two spherically shaped stamps, one serving at the same time a contact for charge collection. The curved shape served to avoid that curved substrates reacted by bending the sample, rather than reducing the thickness (which of course gives large contributions from strains S1 and S2 in eq. 1). When the curvature of the substrate is not changed, the Poisson effect also
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causes a lateral extension leading to an effective strain imposed to the thin film as defined by the difference in transverse compliance coefficients s13 ((s) stands for the substrate):
The corresponding transverse piezoelectric contribution must be added to eq. 1. These strains are positive when T3 is negative. As e31,f is negative, and d33,f is positive, the transverse effect gives the same charge sign as the longitudinal effect. Such test set up measures a larger d33,f than really present, the value is indeed even larger than d33. The interesting point is the s13 values of AlScN and Si are very close at 40 % Sc, when considering the predicted s13E of Caro et. al. for Al0.6Sc0.4N. This would mean that the value of 21.1 pm/V published in [16] for this composition would be free of d31 contribution! DFT calculations are predicting an even higher value of 29.3 pm/V [11]. Both are extraordinary high values for nonferroelectric materials, and even more spectacular considering the low dielectric constant! In fig. 3, the so far published piezoelectric coefficients d33,f and e31,f for AlScN thin films are shown. The curves were corrected when there was too large a deviation from the true values for pure AlN. The values were then compared with published DFT results. It is interesting to note that ab-initio calculations rather underestimate piezoelectric properties at larger Sc concentrations. Figure 3: Longitudinal, clamped coefficient d33,f=e33/c33E. The DFT Caro curve is from ref. [11], and the DFT Tasnadi curve from ref. [10]. EPFL data are from[8, 18]. Experimental data were adjusted proportionally when not having close to 4.0 pm/V for d33,f, and close to 1.05 C/m2 for e31,f.
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REFERENCES
1. K. Prume, P. Muralt, C. F., T. Schmitz-Kempen and S. Tiedke, Piezoelectric thin films: evaluation of electrical and electromechanical characteristics for MEMS devices. IEEE Trans. UFFC, 2007. 54: p. 8-14. 2. M.-A. Dubois and P. Muralt, Measurement of the effective transverse piezoelectric coefficient e31,f of AlN and PZT thin films. Sensors and Actuators A, 1999. 77: p. 106-112. 3. J.F. Shepard, P.J. Moses and S. Trolier-Mckinstry, The wafer flexure technique for the determination of the transverse piezoelectric coefficient (d31) of PZT thin films. Sensors and Actuators A, 1998. 71: p. 133-138. 4. A.V. Sotnikov, H. Schmidt, M. Weihnacht, E.P. Smirnova, T.Y. Chemekova and Y.N. Makarov, Elastic and piezoelectric properties of AlN and LiAlO2 single crystals. IEEE Trans. UFFC, 2010. 57(4): p. 808-811. 5. K. Tsubouchi and N. Mikoshiba, Zero-temperature coefficient SAW devices on AlN epitaxial films. IEEE Trans. Sonics and Ultrasonics, 1985. SU-32: p. 634-644. 6. A. Mazzalai, D. Balma, N. Chidambaram, R. Matloub and P. Muralt, Characterization and fatigue of the converse piezoelectric effect in PZT films for MEMS applications. J.MEMS, 2015. 24: p. 831838. 7. K.M. Lakin, J. Belsick, J.F. Mcdonald and K.T. Mccarron. Improved bulk wave resonator coupling coefficient for wide bandwidth filters. in IEEE Ultrasonics symposium. 2001. Atlanta (GA), USA: IEEE. 8. R. Matloub, M. Hadad, A. Mazzalai, N. Chidambaram, G. Moulard, C. Sandu, T. Metzger and P. Muralt, Piezoelectric AlScN thin films: A semiconductor compatible solution for mechanical energy harvesting and sensors. Appl.Phys.Lett., 2013. 102: p. 152903. 9. F. Bernardini, Spontaneous and piezoelectric polarization: Basic Theory vs. Practical Recipes, in Nitride Semiconductor devices, J. Piprek, Editor 2007, Wiley-VCH: Newark, USA. p. 49-68. 10. F. Tasnadi, B. Alling, C. Hรถglund, G. Wingqvist, J. Birch, L. Hultman and A. Abrikosov, Origin of the anomalous piezoelectric response in wurtzite ScAlN alloys. Phys.Rev.Lett., 2010. 104. 11. M.A. Caro, S. Zhang, M. Ylilammi, T. Riekkinen, M. Moram, O. Lopez-Acevedo, J. Molarius and T. Laurila, Piezoelectric coefficients and spontaneous polarization of AlScN. J.Phys.: Condens. Matter, 2015. 27: p. 245901. 12. K. Tsubouchi, K. Sugai and N. Mikoshiba. AlN material constants evaluation and SAW properties of AlN/Al2O3 and AlN/Si. in IEEE Ultrasonics Symposium. 1981. 13. J.G. Gualtieri, J.A. Kosinski and A. Ballato, Piezoelectric materials for acoustic wave applications. IEEE UFFC, 1994. 41: p. 53-59. 14. R. Lanz and P. Muralt, Bandpass filters for 8 GHz using solidly mounted bulk acoustic wave resonators. IEEE Trans. UFFC, 2005. 52: p. 936-946.
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15. M. Akiyama, T. Kamohara, K. Kano, A. Teshigahara, Y. Takeuchi and N. Kawahara, Enhancement of piezoelectric response in scandium aluminum nitride alloy thin films perpared by dual reactive cosputtering. Adv.Mat., 2009. 21: p. 593-596. 16. M. Akiyama, K. Kano and A. Teshigahara, influence of growth temperature and scandium concentration on piezoelectric response of ScAlN alloy thin films. Appl.Phys.Lett., 2009. 95: p. 162107. 17. M. Moreira, J. Bjurstrom, I. Katardjiev and V. Yantchev, Aluminum scandium nitride thin film bulk acoustic resonators for wide band applications. Vacuum, 2011. 86: p. 23-26. 18. R. Matloub, A. Artieda, E. Milyutin and P. Muralt, Electromechanical properties of Al0.9Sc0.1N thin films evaluated at 2.5 GHz film bulk acoustic wave resonators. Appl.Phys.Lett. , 2011. 99.
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Fig. 1 MSQ 200 Multisource
A promising future for AlScN in MEMS Aluminum Nitride (AlN) is a promising material for MEMS applications due to its good piezoelectric performance and superior dielectric properties, e.g. low permittivity and low dielectric losses. Forming AlScN by substituting a fraction of Aluminium with Scandium is a viable way to enhance the material’s properties even further. Dr. Steffen Chemnitz from University of Kiel explains how this enables benefits for sensing, actuating and energy harvesting in numerous MEMS devices and reports results for AlScN film properties achieved to date.
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At the same time, the permittivity rises more moderately by a factor of 1.5 and the loss angle remains constant up to the maximal piezoelectric activity. In our experiments, the favorable behavior of the material’s dielectric properties translated into an optimal performance of AlScN based energy harvesting and sensing MEMS at 27% Sc. Here, the sensing FOM has increased by a factor of 1.7 and the harvesting FOM by a factor of 3.2. By varying pressure and Ar/N2 ratio during deposition, we were also able to control film stress over a wide range from strongly tensile to strongly compressive.
PIEZOELECTRIC BEATS ELECTROSTATIC Sensing the position of moving parts within MEMS devices typically requires relatively large “electrostatic sensors” but smaller “piezoelectric sensors” would save space and therefore reduce costs. The same is valid for actuation, where piezoelectric materials can introduce significant forces into MEMS enabling new devices with better actuation performance. Work on AlScN at University of Kiel is currently focused on energy harvesting applications where the main goal is to enable an integrated power supply to support wireless MEMS sensors. For Scandium doped AlN, a significant increase in the respective figures of merit (FOM) could be expected. This is especially true for energy harvesting, where the piezoelectric performance increases according to its FOMEH = (e31,Ɛ2)/ (Ɛ Ɛ0). In sensor applications too, where the FOM describes the limit of detection and can be defined as FOMSA = (e31,Ɛ)/(√ Ɛ Ɛ0 tan Δ), a significant improvement could also be expected, as long as the dielectric loss angle delta is controlled.
Fig. 2: (a) top view of 400 nm thick AlN (left) and Al0.73Sc0.27N (right) films (b) AFM recorded topography of the same Al0.73Sc0.27N sample
Fig. 4: Mean and maximal transversal clamped piezoelectric coefficient e31,f, dielectric constant ε and (b) dielectric loss tangent of Al1−xScxN as a function of Sc content
GETTING THE COMPOSITION RIGHT To test this theory, AlScN films were fabricated on standard 200mm wafers with Sc content of up to 37% on top of a platinum bottom electrode using an Evatec MSQ 200 Multisource installed on CLUSTERLINE® sputtering tool. Precise control of Sc concentration over such a large range was made feasible by the possibility of co-sputtering from two pulsed DC-Cathodes with varying power. At concentrations up to 27% Sc, our XRD measurements showed that films of a constant good quality could be deposited revealing exclusively polar texture (Fig. 2) as well as narrow Rocking-Curve (Fig. 3).
Fig. 3: X-ray diffraction patterns for samples with varying Sc content from x=0 to x=0.37 and (b) corresponding rocking curve
SEM and AFM imaging also confirmed a uniform surface structure [Fig2.]. Above this concentration however, the crystalline quality started to decrease, with barely any polar contribution observed at 37% Sc. Coinciding with these observations, the measured transversal piezoelectric coefficient e31,f peaks at 27% Sc with 3 C/m² [Fig 4], corresponding to a significant increase by a factor of 2.4.
BUT THAT’S NOT THE WHOLE STORY Of course, the deposition and characterization of feasible AlScN layers is only one link of a chain for successful process integration. Issues related to reliability, etching, stress control and compatibility to other processes are also of great importance and our work goes on in collaboration with the Fraunhofer ISIT for better understanding of these topics to provide better AlScN based MEMS devices in future.
“So Is AlScN the solution? - just watch this space to find out!”
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Delivering perfectly tailored magnetic films Senior Scientists R&D, Hartmut Rohrmann and Dr. Claudiu Valentin Falub, talk about the features at their disposal on the LLS EVO II for the process engineering of soft magnetic thin film properties. They present results using Evatec’s new Aligning Magnetic Field Technology with case studies for CoZrTa/Al2O3 and CoFeB/Al2O3 laminated films. The miniaturization and rapid performance improvement of electronic devices for RF communications at high and ultrahigh frequencies from a few MHz up to several GHz has paved the way for micro-magnetic devices such as inductors and transformers based on thin films. Both miniaturization and performance could be further improved by using cores based on soft-magnetic materials. The term “soft-magnetic” refers to the fact that the lowest possible effort is required to switch between magnetized and de-magnetized states. In the absence of an external magnetic field, the magnetization – the vector field that expresses the density of magnetic dipoles – should orient along an easy axis. While the coercivity (Hc, the field to reverse the magnetization along the easy axis) should be as low as possible, the anisotropy field (Hk, the field to reverse the magnetization perpendicular on the easy axis) should be tuned by the process conditions depending on the desired application (i.e. operation frequency) and the choice of soft magnetic material. Thus, by matching the maximum possible magnetization, so called saturation magnetization (4πMs), defined by the magnetic material itself, and the Hk, one is able to adapt to different requirements. The third item is a proper orientation of the easy axis (preferred direction of magnetization).
Such configuration results in both deposition under oblique incidence as well as textured seed and intermediate layers. In addition we have now introduced an aligning magnetic field that is applied during the deposition itself.
THE QUEST FOR THE IDEAL SOFT-MAGNETIC PROPERTIES There’s a number of perquisites to achieve ideal soft-magnetic properties. The choice of a magnetic alloy with low crystalline magnetic anisotropy and magnetostriction, a low film stress and small crystallites of a size below the width of magnetic Bloch walls proves to be essential for reducing the hysteretic loss. While small crystallites result from low deposition temperatures and high rates, the controlled use of reactive gas is also decisive. The alloy composition itself requires a small amount of amorphisation atoms such as B, Zr, Al or Si. We can also use seed magnetic layers to induce a favorable microstructure (e.g. CoFe on NiFe), or intermediate laminating layers (e.g. AlN, Al2O3, SiO2, etc.) to stop the crystallite growth. To influence the anisotropy energy Ku (and anisotropy field Hk), we can control the degree of pair ordering by controlling the angular distribution of the deposited material using temperature, sputter pressure, sputter power and bias.
MAGNETIC ALIGNMENT ON THE LLS
Hard magnetic material
Soft magnetic material
Anisotropies in different crystallographic directions
The LLS EVO II is a well established industrial deposition platform to achieve and tailor magnetic anisotropies using the geometries related to its unique concept in conjunction with a collimator.
While the use of oriented seeds, interface layers and collimators was introduced a while ago, Evatec’s latest feature achieves the required pair ordering through the application of an aligning magnetic field. Since the shadowing effects normally exhibited for deposition with linear collimator are no longer a concern, we can boost the sputter rate of the soft magnetic material by a factor of ~2.5 with respect to the case of a collimator with an aspect ratio of 1:1. The newly developed aligning magnetic field has already proven stable and easy to handle in industrial applications and is a powerful tool for tailoring the magnetic properties during deposition.
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CHOOSING THE RIGHT MAGNETIC MATERIALS
HW modification and its respective integration into an LLS EVO (Figure 1):
The soft magnetic material should have high saturization magnetizartion (4πMs) for the high permeability (µ) required to guide the magnetic field efficiently, and a high resisitivity (ρ) to reduce the Eddy current losses. In practice, this is done by laminating the magnetic material with intermediate dielectric interlayers, e.g. Al2O3, AlN, SiO2, Ta2O5, etc. In addition, the material should be compatible with the MEMS (e.g. Cu TSVs) and CMOS technologies (e.g. low deposition temperatures), and exhibit no deterioration upon subsequent annealing steps that might be necessary during device manufacturing. Therefore, magnetic materials of interest are CoZrTa, CoFeB, NiFe and similar amorphous alloys. Amongst these materials CoZrTa and CoFeB are the most promising in view of their high saturation magnetizations of ~1.5 T and ~2 T (the exact values will depend on the exact alloy composition), respectively, and their high electrical resistivity of more than 100 µΩcm.
CASE STUDY NO. 1- CoZrTa/AL2O3 MULTILAYERED FILMS USING THE ALIGNING MAGNETIC FIELD In this study, PM1 module was equipped with a CoZrTa-target, while an Al2O3 target was used in PM5 to deposit the intermediate Al2O3 layers. Around 50 periods of CoZrT(80nm)/Al2O3(2nm) resulted in a multilayer stack of a total thickness of about 4 µm. The magnetic layers were sputtered by a DC process (1 kW sputter power), whereas the Al2O3 interlayers were sputtered by RF plasma (2.5 kW). The applied field during deposition was approximately 100 Gauss, with an angular deviation on 8” wafer of less than ± 0.3º. Typical results achieved including hysteresis curves along the easy and hard axes and distributions of the easy axis and anisotropy field are shown in Figs. 2 and 3. Thus, we obtained a small coercivity field Hc ~ 0.1 - 0.2 Oe, anisotropy field Hk ~16.5 Oe (standard deviation of 3.6%), and a small deviation from the easy axis (< ± 2º).
Figure 2: Typical magnetization curves for the 4 µm thick CoZrTa/Al2O3 soft magnetic multilayer sputtered with the new aligning field (measured by magnetooptic Kerr effect measurements).
Figure 3: Distribution of the easy axis (left) and anisotropy field Hk (right) for a 4 µm thick CoZrTa/Al2O3 multilayer deposited on a 8” SiO2 wafer using the new aligning field. The 8” sputtered wafer was mapped by magneto-optic Kerr effect measurements on a mesh containing 80 points (edge exclusion 30 mm).
CASE STUDY NO. 2- CoFeB/Al2O3 MULTILAYER FILMS USING THE ALIGNING MAGNETIC FIELD In this example, PM1 module was equipped with a CoFeB target to sputter the soft magnetic layers, whereas an Al2O3 target was used in PM5 to deposit the intermediate Al2O3 layers. Around 10 periods of CoFeB(100nm)/Al2O3(4nm) resulted in a multilayer stack of a total thickness of about 1 µm. The process conditions were similar to the CoZrTa/Al2O3 case. Typical hysteresis curves along the easy and hard axes for this soft magnetic material are shown in Fig. 4. Just like for CoZrTa/Al2O3 we obtained a small coercivity field Hc ~ 0.15 - 0.25 Oe, which should lead to very low losses, and for the anisotropy field we got Hk ~26 Oe. Figure 4: Typical magnetization curves for the 1 µm thick CoFeB/Al2O3 soft magnetic multilayer sputtered with the new aligning field (measured by magneto-optic Kerr effect measurements).
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