Electrical Characterization of Platinum Thin Films Deposited by Focused Ion Beam. (Unicamp), , BRAZIL

Transcription

1 / The Electrochemical Society Electrical Characterization of Platinum Thin Films Deposited by Focused Ion Beam *M. M. da Silva a b, A. R. Vaz b, S. A. Moshkalev b, J. W. Swart a b, a School of Electrical and Computer Engineering, State University of Campinas, b Center for Semiconductor Components (CCS), State University of Campinas (Unicamp), , BRAZIL *macchi@ccs.unicamp.br Dual beam FIB (focused ion beam)/sem (scanning elelctron microscope) systems are commonly used for imaging, selective etch and deposition of materials like platinum. The paper presents the results of electrical characterization of platinum thin films deposited by focused ion beam. For measurements, two types of test structures were fabricated: (i) 150x150 µm and 20x20 µm squares with thickness of 5, 10, 30 and 100 nm, and (ii) 30 µm long resistors with variable cross section (50 nm x 50nm to 1µm x 1µm). The Pt film resistivity has been measured by a four points probe method, to give the value of ~10 x 10-4 Ω.cm. Introduction In 1974 Selinger and Fleming demonstrated a unique microfabrication technique by incidence of a focused ion beam (FIB) on a silicon surface (1). Doped regions written in a silicon substrate by focused boron beam were obtained. This result led to a maskless implantation of dopants into a silicon surface layer. A continuous development of the focused ion beam (FIB) technology has been observed since then. A FIB system is similar to a scanning electron microscope (SEM) operating with a focused electron beam. Usually, gallium is used to generate the ion beam (Ga+) produced by a liquid metal ion source (LMIS). The FIB system can also be used for a local metal deposition from a precursor gas, which is injected into the vacuum chamber through a needle close to the sample surface. This gas is adsorbed onto the surface and decomposes by the scanning ion beam to leave the deposited metal, as illustrated in Fig. 1 (2). Platinum is the most common choice for use in integrated circuit repair and modification because it is inert in air and does not cause contamination of the circuit (3). Platinum deposition with a focused ion beam results in disordered platinum, contaminated with gallium from the ion beam, carbon from the precursor gas and oxygen when the base pressure is not optimal (2,3). The resistivity of an ion-beam deposited platinum film was measured previously to vary from 0,7 to 7x10-4 Ω.cm (3), while the bulk resistivity of pure platinum is approximately 10 µω.cm (3). 235

2 Fig1: The process of deposition. A precursor gas is injected into the system and is absorbed to the substrate surface. Wherever the ion beam hits the surface the platinum is deposited (2). In the present work, the method for measure the resistivity is the Van Der Pauw method. This method can be used in a flat sample with an arbitrary shape (4). The sample is connected at four corners, and the resistance is measured. From the measured resistance values R V/I, the sheet resistance R S can be derived according to the relation (4): π Rs = R V [1] ln 2 ( ) I The resistivity can be calculated using the relation ρ Rs = [2] t where ρ is the resistivity and t is the thickness. The motivation of this study is to create and study test structures for electrical characterization of thin platinum films and patterns deposited by FIB for future research in nanodevices. Experiment The FIB system model used in our experiment is a Dual Beam of FEI (model Nova NanoLab 200). This system combines an ion beam column with a scanning electron microscope (SEM) focused at the same point on the sample surface. This allows us to monitor ion beam processing with simultaneous SEM imaging. All structures were fabricated on a 400 nm thick SiO 2 substrate. The 150x150 µm square structures were fabricated varying the thickness from 5 nm to 100 nm, at 30 kv ion energy and current of 0.5 na (thickness of 5, 10, 30 nm) or 3nA (100 nm). To measure the resistance in the cases of 20x20 µm squares and linear resistors, it was necessary to fabricate contact Au pads with area of 100x100 µm and thickness of 50 nm. These pads were fabricated by a conventional lithography and lift-off technique. The example of a 20x20 µm square test structure (in the center) with thickness of 5 nm is shown in Fig. 2 (a). 236

3 (a) (b) (c) Fig 2: (a) 20x20um square structures (in the center) of 5nm thickness, (b) 1x1 µm 2 cross-section resistor, (c) parallel linear structures with 150x150nm 2 cross-sections with distance between the structures 300nm. The 30 µm long resistors were fabricated using a current of 0.5 na for 1x1µm 2 cross-section. For resistors with smaller cross-section, lower current values were used: 0.3 na for 500x500 nm, 0.1 na for 300x300 nm, 50 pa for 100x100 nm and 30 pa for 50x50nm cross-section. The 1x1 µm 2 resistor is shown in Fig. 2 (b). To analyze the leakage between the Pt lines, test structures shown in Fig. 2 (c) were fabricated using the same conditions that for fabrication of resistors, varying both the cross-section and the distance between the lines. To study the effect of surface roughness on the leakage, these structures were also fabricated after SiO 2 substrate milling (5 nm deep) by the ion beam. A Keythley 4200 SCS station is used to make the electrical measures. Results To measure the platinum film resistivity, squares and linear resistors of different thickness and cross-section were deposited. Figures 3 (a) and 3 (b) show results for 20x20 µm and 150x150 µm squares, respectively. The results show that the resistivity starts to increase sharply after the critical thickness (below 30 nm) is reached. This happens when the thickness of the film becomes comparable to or smaller than the mean free path for electrons ~10 nm (5, 6, 9). Note that for very thin films (5 nm) the measurements using a conventional probe are not safe, and the very high resistivity for 5 nm thick and 150x150 µm wide sample was measured probably because the probe of Keythley station might damage the film surface. For films with thickness above 10 nm a constant resistivity of 10x10-4 Ω.cm was found. We also compared our results with values obtained in other studies using similar FIB systems for platinum deposition and the results are found to be similar, varying in the range ~0.1 to 1 mω.cm (1, 2, 7). 237

4 (a) Fig 3. (a)the 20x20 µm resistivity versus thickness, (b) 150x150 µm resistivity versus thickness. For this, the resistance measurements for 30µm long resistor were compared with the resistance calculated using the resistivity value of ρ = Ω.cm and using the relation R = ρ.(l/a), for varying cross-section, see Fig. 4 (a). The results show that for 50x50 nm resistors the measured resistance is much higher than calculated, probably due to increased contribution of scattering in the low dimensions structure. Figure 4 (b) shows the resistivity calculated from the measured resistance for the linear test resistors. In this case, good agreement can be seen. To analyze the leakage between the linear structures with 5 µm overlapping and various cross-sections, a potential varying for -4 V to 4 V was applied between the pads. The result can be seen in Table 1. The platinum patterns deposited by ion beam FIB are surrounded by `halo` structures which contains Ga and C, resulting in notable leakage between them. To reduce the spreading of the impurities (C and Ga) over the oxide surface during the deposition, the samples were previously milled by ion beam. This procedure increases the surface roughness, and thus reduces the diffusion of impurities (b) 238

5 along the surface. It can be seen that in some cases (wider separation between the lines), the reduction of leakage is considerable. An effective method to remove the impurities is to use O 2 plasma, which can burn out the carbon deposit and oxidize or sputter Ga deposit (8). (a) Fig. 4. (a) Resistance measured for 30µm long resistor compared with resistance calculated using ρ = Ω.cm as functions of length/ cross section, (b) resistivity calculated using the measured resistance as a function of thickness. The results of leakage measurements after processing in O 2 plasma (capacitively coupled asymmetric plasma reactor, 150 mtorr, 30 W, 200 V, 2 min.) are shown in Table 2. The results show that O 2 plasma efficiently removes the carbon/gallium deposits around the Pt structures, resulting in notably lower leakage currents. (b) 239

6 Table 1. Leakage measurements. Thickness Space between the lines (nm) (nm) 100 Resistance (Ω) K 33K K 53K K 78K K 72K Resistance (Ω) with 5nm Milling M Not Connected Table 2. Leakage measurements after processing in O 2 plasma. Thickness (nm) Space between the structures (nm) Resistance (Ω) 100 Resistance (Ω) with 5nm Milling K 58K K 157K K 450K K 1,5M 1000 Not Connected Not Connected Conclusions Resistivity of various platinum patterns deposited by a focused ion beam has been measured. From these measurements, the resistivity of Pt film was calculated to be ρ 10 x 10-4 Ω.cm. We also compared the measured results with calculated results and we compare our result with values obtained in other studies using FIB system for platinum deposition (1, 2, 7). The motivation of this study is to determine the conditions when ion-beam deposited Pt cam be used for nanocontacts. The low resistance in FIB prodeced resistors can be compared with the intrinsic resistance of multi-wall carbon nanotubes that ranges from ~10 to 10 2 kω.µm. The use of oxygen plasma or shallow milling of the oxide surface prior to the Pt deposition decreases the leakage between Pt structures that occurs due to the halo effect (spreading of carbon and gallium impurities over the oxide surface surrounding the deposited patterns). References 1. R. L. Selinger and W. P Fleming, Journal of Applied Physics vol.45, , (1974). 2. M. S. H. Go, Focused ion beam fabrication of junctions in the charge density wave conductor NbSe, M.S. thesis, Delft Univ. of Technol., Delft, The Netherlands, (2001). 3. T. Tao, J. S. Ro, J. Melngailis, Z. Xue, and H. D. Kaesz, J. Vac. Sci. Technol., vol. B 8, n. 6, , (1990). 4. L. J. van der Pauw, Phillips Res. Rep., vol. 13, 1 9, (1958). 5. M. Tay, K. Li and Y. Wu, J. Vac. Sci. Technol., vol. B 8, n. 23(4), , (2005). 6. T. Kanagawa, R. Hobara, I. Matsuda, T. Tanikawa, A. Natori and S. Hasegawa, Physical Review Letters, vol 91, n 3, (2003). 240

7 7. S. Smith, A. J. Walton, S. Bond, A. W. S. Ross, J. T. M. Stevenson and A. M. Gundlach, IEEE Transactions on Semiconductor Manufacturing, vol. 16, n. 2, , (2003). 8. D. Ko, Y. M. Park, S. Kim, Y Kim, Ultramicroscopy, vol.107, , (2007). 9. M. C. Salvadori, A. R. Vaz, R. J. C. Farias and M.Cattani, Surface Review and Letters, vol. 11, n 2, , (2004). 241