THE PROPERTIES OF THIN FILM DIELECTRIC LAYERS PREPARED BY SPUTTERING Ivana BESHAJOVÁ PELIKÁNOVÁ a, Libor VALENTA a a KATEDRA ELEKTROTECHNOLOGIE, ČVUT FEL, Technická 2, 166 27 Praha 6, Česká republika, pelikano@fel.cvut.cz Abstract Technologies and processes concerning preparation thin film layers are the topic work. Experimental part work is focused on analysis properties dielectric corundum layers. The set samples capacitors with vapor deposited aluminum electrodes and with sputtered dielectric layer on glass substrates was prepared. Dielectric corundum layers were deposited by apparatus for high frequency sputtering equipped with magnetron for enhancement efficiency sputtering. The investigation is concerned to analysis electrical capacitance corundum layer in dependence on condition s layer deposition mainly. The conditions sputtering influencing the thickness and thus capacitance are included power plasma generator, time sputtering, distance between substrate and target etc. The thickness dielectric layer was measured an analyzed in connection with capacitance. Permittivity is important parameter with regard to character thin film layer. Properties thin film layers are influenced by their thickness significantly. The analysis thin film dielectric layers is included frequency dependence capacitance too. 1. INTRODUCTION Thin film layers are used in many area production, for example in optics, electronics, mechanical engineering etc. As some example can be mentioned the electric passive thin film components in electronic circuits. Furthermore, the nanostructures for build-up sensors can be growing in thin film layers with certain treatment such thin film layer. Nanostructures can be used to enlarge surface area sensors electrodes [1]. Thin film layer we consider as layer with thickness in range approximately from tens nm to ones µm. Generally the thin film layer is possible to characterize as layer, whose properties are markedly influenced also by thickness in contrast to bulk material. 2. TECHNOLOGIES FOR THIN FILM LAYER PREPARATION A number technologies for creation thin film layers are used. The properties deposited thin film layer and elaborateness are influenced by type method. Technologies used for sample preparation (sputtering and vacuum evaporation) belong to group methods called as PVD physical vapor deposition. Principle technology vacuum evaporation is follow. During heating deposited material in vacuum chamber the kinetic energy particle in surface layer deposited material is enhanced. The energy is increased until to release atoms or molecules. Vaporized material is condensate on surface substrate with lower temperature. Released atoms are moved in recipient by straight course at sufficiently low pressure i.e. pressure lower than 10-3 Pa.
Next thin film method is sputtering. Deposited material is there in form target. Target i.e. source material deposited layer is connected to one electrode and substrate is connected to second electrode. The glow discharge is kept between those electrodes. At bombardment cathode - target by positive ions working gas happens to pulse transfer energy to surface atoms target and to dispersion elements into recipient. Direct current sputtering is impossible to use for sputtering non-conductive materials. At direct current sputtering the very thin film layer on substrate is created and is charged. The electric field restraining to settling next atoms deposited layer is arose around substrate. Therefore high frequency sputtering is used for deposition non-conductive materials. Frequency is chosen usually 13 MHz. Time change polarity voltage are followed only quicker electrons but not slower positive ions. 3. SAMPLE PREPARATION The set thin film capacitors with corundum dielectric was prepared. Microscopic support glass was used as substrate for thin film samples. First layer on surface are aluminum electrode. Their position on surface is shown in fig. 1. These electrodes were created by evaporation aluminum wire through metal mask. Middle Fig. 1. bottom electrodes on substrate Fig. 2. upper electrode on substrate layer is sputtered corundum dielectric. Third layer is again evaporated aluminum electrode common for all capacitors on substrate. Shape and placement electrode is shown in fig. 2. Tested capacitors are created by crossing aluminum electrodes, as is shown in fig. 3. Single capacitors are marked by numbers from 1 to 10 in fig. 3. Sputtered corundum dielectric layer is in fig. 3 illustrating by grey color. Dielectric layer was not coat whole surface substrate. Interface between dielectric layer and substrate is created for measurement thickness dielectric layer. Fig. 3. Sample with electrodes and dielectric layer (illustrated by grey color) Dielectric layer was deposited from aluminum target. The sputtering corundum layer was proceed at residual pressure atmospheres 2,5 10-3 Pa and pressure working gas argon 2 10-3 Pa. Magnitude power plasma was changed in range from 25W to 150W and time sputtering was ranged from 10 minutes to 50 minutes. Some samples were created at lower distance between target and substrate in recipient
during sputtering. Used sputtering device is equipped by magnetron. The magnetic field is arisen in target area. Electrons generated on cathode by thermal excitation during classical sputtering are escaped from target area straightly. Electrons are moved in spiral along field lines in consequence Lorentz force in case magnetron sputtering. Thereby the path electron in closeness target is enlarged and thus time keeping on in area discharge plasma and probability ionization other atoms working gas is enhanced. This provides possibility to keep discharge at lower pressure gas in recipient and at lower power plasma generator. 4. THICKNESS OF THIN FILM DIELECTRIC LAYER The thickness thin film layer directly influence the barrier effect in coating, his compactness, porosity and others physical-mechanical feature layer. Thickness dielectric thin film layer chosen sample was measured. All samples were not measured from reason high time-consuming measurement. The electrical permittivity ε corundum dielectric is possible to determine from result thickness and electrical capacity measurement. The surface thin film capacitors S is given by mask used at deposition aluminum electrodes and has magnitude 0,75mm 2. Relative permittivity then can be calculated from wellknow relationship, where C is electrical capacity, l is thickness dielectric layer and ε 0 is permittivity vacuum and is 8,854 10-12 F m -1 ε = l S C. ε 0 Device Talstep was used for thickness measurement. The device represents nondestructive contacting method. Measuring sensitive tip pinned by fine force to sample is moved on surface sample in area interface dielectric layer with substrate. Change high tip is scanned and converted to electrical signal that is evaluated subsequently. Thickness measured chosen dielectric layer is ranged from 9nm at power plasma generator 75W and time sputtering 30 minutes to 48nm at power 125W and time sputtering 40 minutes. Relative permittivity was evaluated in range values from 10 to 20. This is result comparable with tabular value that is determine in interval 9 to 12. 5. RESULTS AND CONCLUSIONS Number types dependences parameters dielectric thin film layer was analyzed. The dependence electrical capacity on power plasma generator, dependence capacity on time sputtering, frequency dependence capacity were investigated. The dependence electrical capacity on power plasma generator is shown in fig 4. Values for constant time sputtering 30 minutes are shown in graph. Significant drop capacitance with increasing power plasma generator can be observed. The highest capacity is reached at lowest power plasma generator. This result from fact that deposited layer is thinnest at lowest power plasma generator. The values 25W and 150W can be taken as boundary limit power plasma generator under given conditions. Too low power is insufficient for creation continuous layer. On the contrary too large power can be lead to target destruction by too high energy elements attacking the target.
De pen den ce ele ctri cal cap acit y on tim e spu tteri ng at con sta Fig. 4. Dependence electrical capacity on power plasma generator nt val ue po wer pla sm a gen erat or 125 W is sho wn in fig. 5. Fig. 5. Dependence electrical capacity on time sputtering De pendence is decreasing according to expectation. Thicker layer is deposited at longer deposition and therefore electrical capacity will be lower. Time sputtering is ranged from 10 minutes to 50 minutes. Non
continuous layer is created in time shorter than 10 minutes. The deposited layer is not significantly grown in time deposition above 50 minutes. Than the enhancement time sputtering is not effective. Now we compare the influence power plasma generator and time sputtering. It was found that larger changes are occurred at changes power plasma generator in comparison with influence time sputtering. Higher power and shorter time sputtering is preferable for layer deposition. The combination values 25W and 20 minutes or 50W and 10 minutes indicate the lower limit for continuous layer. Non continuous layer is created below these combinations values and such layer is not able to isolate the electrodes and is lead to their short circuit. Fig. 6. Frequency dependence electrical capacity and dielectric loss Frequency dependence electrical capacity and dielectric loss is shown in fig. 6. Displayed curves are concerned to sample with dielectric sputtered at power plasma generator 50W and during 50 minutes. Small decreasing electrical capacity with increasing frequency is possible to observe. This dependence is not too marked. LITERATURA [1] KLOSOVÁ, K., HUBÁLEK, J. Advanced electrodes with nanostructured surfaces for electrochemical microsensors. physica status solidi, 2008, č. 205, s. 1435-1438 [2] SESHAN, K. Handbook thin film deposition, William Andrew Publishing, 2002