Properties of atomic layer deposited Al 2 O 3 /ZnO dielectric films grown at low temperature for RF MEMS

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1 Properties of atomic layer deposited Al O /ZnO dielectric films grown at low temperature for RF MEMS Cari F. Herrmann *a,b, Frank W. DelRio a, Steven M. George b,c, Victor M. Bright a a Department of Mechanical Engineering, University of Colorado, Campus Box 7 b Department of Chemistry and Biochemistry, University of Colorado, Campus Box 15 c Department of Chemical and Biological Engineering, University of Colorado, Campus Box Boulder, CO 89, USA ABSTRACT Al O /ZnO alloy films were grown at 1 C using atomic layer deposition (ALD) techniques. It has been previously established that the resistivity of these films can be tuned over a wide range by varying the amount of Zn in the film. Al O /ZnO ALD alloy films can therefore be designed with a dielectric constant high enough to provide a large downstate capacitance and a resistivity low enough to promote the dissipation of trapped charges. The material and electrical properties of the Al O /ZnO ALD films were investigated using Auger electron spectroscopy (AES), nanoindentation, and mercury probe measurements. Chemical analysis using AES confirmed the presence of both Al and Zn in the alloys. The nanoindentation measurements were used to calculate the Young s modulus and hardness of the films. Pure Al O ALD was determined to have a modulus between 15 and 155 GPa and a hardness of ~8 GPa, while the results for pure ZnO ALD indicated a modulus between 1 and 1 GPa and a hardness of ~5 GPa. An Al O /ZnO ALD alloy displayed a modulus of 1 15 GPa, which falls between the two pure films, and a hardness of ~8 GPa, which is similar to the pure Al O film. The dielectric constants of the ALD films were calculated from the mercury probe measurements and were determined to be around 6.8. These properties indicate that the Al O /ZnO ALD films can be engineered as a property specific dielectric layer for RF MEMS devices. Keywords: atomic layer deposition, Auger electron spectroscopy, dielectric constant, hardness, MEMS, nanoindentation, Young s modulus 1. INTRODUCTION Reliability of RF MEMS devices is a major concern for successful commercialization. One of the major hurdles limiting RF MEMS device reliability is stiction between contact surfaces. Dielectric charging is one of the major elements that causes stiction in RF MEMS capacitive switches. Charging of the dielectric layer is caused by the high electric field, on the order of 1 to MV/cm, that exists across the dielectric layer during device operation. 1 This charging phenomenon screens the applied potential and eventually caused the device to stick and fail. Choosing a proper dielectric layer is an important element in minimizing the dielectric charging problem. PECVD silicon dioxide is commonly chosen as the dielectric layer for RF MEMS switches because it is known to have a lower trap density than PECVD silicon nitride. Devices made with silicon dioxide dielectric layers, therefore, have fewer charging problems. However, silicon dioxide has a lower dielectric constant than silicon nitride, which leads to a decrease in the down-state capacitance. Ideally, a high dielectric constant material with a low trap density is desired in order to maximize the down-state capacitance and minimize the dielectric charging. This paper reports on an alternative dielectric for RF MEMS switches created by the atomic layer deposition (ALD) of Al O /ZnO alloy films. ALD is a gas-phase deposition technique based on sequential, self-limiting surface reactions. ALD has recently been utilized for MEMS applications because it is an ideal technique for coating high surface areato-volume ratio structures. ALD films are ultra conformal and can be deposited on almost any substrate at almost any * cari.herrmann@colorado.edu; phone ; fax ; Micromachining and Microfabrication Process Technology X, edited by Mary-Ann Maher, Harold D. Stewart, Proc. of SPIE Vol (SPIE, Bellingham, WA, 5) X/5/$15 doi: /

2 temperature. Low temperature deposition is particularly important to reduce any thermally-induced device deformation when complete structures are coated with ALD. In a previous study, Al O /ZnO ALD alloy films were reported to exhibit a widely tunable range of physical properties by adjusting the concentration of Zn in the film. Specifically, the resistivity of the Al O /ZnO ALD films was found to vary anywhere from 1 - Ω cm for pure ZnO ALD to 1 16 Ω cm for pure Al O ALD depending on the concentration of Zn. The alloy films, therefore, can be engineered to have a resistivity that will promote the dissipation of trapped charges. If these alloys maintain a high dielectric constant, it could be used as a charge dissipative dielectric layer for RF MEMS devices without sacrificing the down-state capacitance.. EXPERIMENTAL.1 Atomic layer deposition (ALD) ALD is a vapor-phase, thin film deposition technique that can deposit conformal and pinhole free films with atomicscale control. 5 ALD is based on a series of two self-limiting reactions between gas-phase precursor molecules and a solid surface as illustrated in Figure 1. Sequential reactions can be designed where the product of the first surface reaction becomes a reactant for the second surface reaction. If the second surface reaction returns the surface back to the initial state, then atomic layer-controlled growth can be achieved using an alternating ABAB, etc., reaction sequence. As the reactions are self-limiting, ALD does not require line-of-sight for deposition and high surface areato-volume ratio structures and complex geometries can be conformally coated. Furthermore, only one reactant is present in the chamber at a time. This prevents any unwanted gas-phase reactions such as chemical vapor deposition, which can lead to particle formation and inferior device performance. ALD techniques exist for depositing a variety of substances including oxides, nitrides, and metals. The results presented in this paper concentrate on ALD films of Al O and ZnO deposited at 1ºC. A B Figure 1: A schematic drawing of the AB reaction sequence during atomic layer deposition Al O ALD films are deposited using alternating exposures of trimethylaluminum (Al(CH ), TMA) and water (H O). The chemistry of Al O ALD occurs via the following equations where the asterisk represents surface species AlOH * + Al + CH (1A) ( CH ) AlOAl( CH) * * H O AlOH * CH AlCH + + (1B) At a deposition temperature of 1ºC, the growth rate for pure Al O ALD determined using an in-situ quartz crystal microbalance (QCM) is 1.1 Å/cycle and the typical cycle time is 8 seconds. 6 Al O films grown by ALD techniques are insulating, amorphous and smooth. 7 Furthermore, the Al O ALD surface chemistry is very favorable to growth 16 Proc. of SPIE Vol. 5715

3 on a wide variety of substrates including oxides, nitrides, metals, semiconductors, 8 and polymeric surfaces. 9 allows for devices of almost any material to be coated with ALD films. This Diethylzinc (Zn(CH CH ), DEZ) and water are the reactants used during ZnO ALD. The AB reaction sequence for ZnO ALD is as follows ZnOH * + Zn( CH + CH CH (A) CH ) * ZnOZn( CH CH ) * CH ) * H O ZnOH * CH Zn ( CH + + CH (B) At a deposition temperature of 1ºC, the growth rate for pure ZnO ALD is. Å/cycle as determined with an in-situ QCM. A typical AB cycle takes 8 seconds. In contrast to the Al O ALD films, ZnO ALD films are conducting, polycrystalline and rough. 7. Al O /ZnO ALD Alloys In order to create alloys of Al O ALD and ZnO ALD, a percentage of the TMA exposures are replaced by exposures of DEZ. 1 Figure (a) illustrates the exposure sequence for pure Al O ALD were TMA exposures are alternated with water exposures. An Al O /ZnO alloy with % DEZ exposures, for example, can be grown by substituting every third TMA exposure with a DEZ exposure as shown in Figure (b). By evenly distributing the DEZ exposures, the Al O /ZnO ALD alloy is as homogeneous as possible. For this study, alloys containing, 1, 5,, 5, 67, and 1% DEZ exposures were grown on HF-etched Si(1) wafers. (a) TMA H O DEZ (b) TMA H O DEZ Figure : Schematic drawings of the exposure sequence for (a) Al O ALD and (b) Al O /ZnO ALD with % DEZ exposures. RESULTS AND DISCUSSION.1 Auger electron spectroscopy Auger electron spectroscopy (AES) is a thin film chemical analysis technique that is based on the Auger radiationless process. An electron beam is used to ionize the core level of a surface atom, which causes the atom to decay to a lower energy state through an electronic rearrangement. The energy of the resulting ejected Auger electrons can be used to identify the composition of the solid surface. The AES system consists of an ultrahigh vacuum chamber, an electron gun for specimen excitation, and an energy analyzer for the detection of the Auger electrons. AES was used to determine the chemical composition of the Al O /ZnO ALD alloy films. ALD films containing, 5, 67, and 1% DEZ exposures were grown on Si(1) substrates with thicknesses of ~5 nm. AES is a surface sensitive technique, therefore, only the first few nm are analyzed. Figure shows the AES for pure Al O ALD and pure ZnO ALD. The predominant peak for the pure Al O ALD film occurs at 191 ev, which corresponds to the KLL Auger transition for aluminum. The zinc LMM Auger transition can be seen at an electron energy of 99 ev in the ZnO ALD spectrum. The AES for the Al O /ZnO alloy films exhibit two main peaks in the 7-15 ev energy range, as shown in Figure. These peaks correspond to the Auger transitions for aluminum and zinc as discussed above. This indicates that the alloys are mixtures of both Al O and ZnO. The peak intensities vary based on the atomic concentration of the Proc. of SPIE Vol

4 elements in the film. While both films appear to contain a larger concentration of aluminum than zinc, the film with 67% DEZ exposures clearly shows more zinc than the film with 5% DEZ exposures. The atomic concentration of zinc, C Zn, can be estimated from the Auger spectra using the following equation C I Zn S Zn Zn = () IZn I + Al SZn S Al where I x is the Auger peak intensity for element X and S x is the sensitivity factor for element X. 11 From the spectra in Figure, the concentration of zinc was determined to be about 11% for the alloy containing 5% DEZ exposures and % for the alloy containing 67% DEZ exposures. Pure Al O ALD dn(e)/de Pure ZnO ALD Zn LMM (Zn-O) Al KLL (Al-O) Kinetic Energy (ev) Figure : Auger electron spectra from pure Al O ALD and pure ZnO ALD 67% DEZ exposures dn(e)/de 5% DEZ exposures Pure Al O ALD Kinetic Energy (ev) Figure : Auger electron spectra for pure Al O ALD and Al O /ZnO ALD alloys containing 5 and 67% DEZ exposures These percentages are remarkably lower than expected using the rule of mixtures. One explanation stems from the oxidation affinities of aluminum and zinc. As AES is a surface sensitive technique, only the first few nm are being 16 Proc. of SPIE Vol. 5715

5 sampled. In this region, aluminum may have a tendency to accumulate because it has a reduction potential of V, which is more negative than the V reduction potential of zinc. 1 This means that aluminum is more likely to give up electrons to oxygen than zinc. When the sample is exposed to the atmosphere, the surface becomes oxygen-rich and the aluminum will migrate towards the surface. A better correlation between % DEZ exposures and % zinc concentration could be established using depth profiling techniques such as Rutherford backscattering spectroscopy or X-ray photoelectron spectroscopy.. Young s modulus and hardness The Young s modulus and hardness of the ALD films were obtained from nanoindentation measurements carried out using a NanoXP from MTS. 1 ALD films containing, 5, and 1% DEZ exposures were grown and tested. The film thickness for all of the samples was ~ nm. The Berkovich tip was displacement controlled to a final contact depth of nm. According to a conservative rule of thumb proposed by Pharr and Oliver, 1 the contact depth should be less than 1% of the film thickness to avoid substrate effects. The results of the load versus displacement curves, therefore, can only be interpreted up to a contact depth of nm. Additionally, the initial depth measurements for the Young s modulus of the films should be discarded due to oxidation and contamination on the surface. The results of the Young s modulus versus contact depth measurements for pure Al O ALD, pure ZnO ALD and the Al O /ZnO ALD alloy with 5% DEZ exposures are shown by the squares in Figures 5, 6, and 7, respectively. The curve for the pure Al O ALD film reached a plateau between contact depths of and nm, corresponding to a Young s modulus between 15 and 155 GPa. The results for the pure ZnO ALD film never reached a plateau. This behavior may be due to the polycrystalline nature of the ZnO ALD films. 15 The results just before the 1% threshold indicate the Young s modulus of the ZnO ALD film was between 1 and 1 GPa. These values are consistent with 16, 17 previous results for the modulus of Al O and ZnO films Young s Modulus (GPa) Hardness (GPa) Contact Depth (nm) Figure 5: Modulus and hardness vs. contact depth measurements for pure Al O ALD The measurements on the Al O /ZnO ALD film with 5% DEZ exposures displayed characteristics of both pure Al O ALD and pure ZnO ALD. The modulus versus contact depth curve, seen in Figure 7, reached a plateau between depths of and nm, corresponding to a Young s modulus between 1 and 15 GPa. The measurements for the alloy, however, exhibited a high uncertainty similar to the pure ZnO ALD film. This variation is also attributed to the polycrystalline nature of the ZnO ALD in the film. Proc. of SPIE Vol

6 Young s Modulus (GPa) Contact Depth (nm) Hardness (GPa) Figure 6: Modulus and hardness vs. contact depth measurements for pure ZnO ALD 18 1 Young s Modulus (GPa) Hardness (GPa) Contact Depth (nm) Figure 7: Modulus and hardness vs. contact depth measurements for an Al O /ZnO ALD alloy with 5% DEZ exposures The nanoindentation measurements were also used to determine the hardness of the ALD films. The triangles in Figures 5, 6, and 7 show the hardness versus contact depth for pure Al O ALD, pure ZnO ALD and the Al O /ZnO ALD alloy with 5% DEZ exposures, respectively. At a contact depth of to nm, the pure Al O ALD film exhibited a hardness of approximately 8 GPa. The hardness for the pure ZnO, like the Young s modulus, displayed a high uncertainty and never reached a plateau between contact depths of and nm. The hardness value for the ZnO just before the 1% threshold, however, was approximately 5 GPa. The hardness results for the Al O /ZnO ALD alloy shows a small plateau before nm, corresponding to a hardness of ~8 GPa.. Dielectric constant In order to perform as an effective dielectric layer in RF MEMS switches, the Al O /ZnO ALD films must have a high dielectric constant similar to pure Al O ALD. The dielectric constants of the films were determined from electrical measurements 8 performed using a MDC 811 Mercury Probe from Materials Development Corporation. 18 Capacitance versus voltage (CV) measurements were carried out using a Stanford Research SR7 LCR Meter 19 and were controlled using LabVIEW from National Instruments. ALD films containing, 1, 5,, and 5% DEZ 16 Proc. of SPIE Vol. 5715

7 exposures were grown on heavily doped n-type Si(1) substrates. The film thicknesses ranged from 59 to 65 nm, corresponding to 6 AB cycles for all of the samples. The dielectric constants of the ALD films were calculated using the CV measurements and the following equation Cd k = () A ε where C is the capacitance, d is the thickness of the film measured using a stylus profilometer, ε is the permittivity of free space and A is the area of the Hg column,.7 mm. Table 1 shows the dielectric constants for the Al O /ZnO ALD alloy films. The approximate resistivities of the films based on the results reported in Reference are also listed in Table 1. The pure Al O ALD film had a dielectric constant of 6.8. The dielectric constants for the films containing zinc are very similar to the value for the pure Al O ALD film. This indicates that the Al O /ZnO ALD alloy films should perform as effective dielectric layers for RF MEMS switches. Table 1: Dielectric constants and resistivities for Al O /ZnO ALD films % DEZ exposures Dielectric Constant 6.8 ± ±. 6.9 ±. 7. ±. 6.6 ±. Resistivity (Ω cm) ~1 16 ~5 x 1 15 ~5 x 1 1 ~1 1 ~1 1. CONCLUSIONS Atomic layer deposition (ALD) was used to create Al O /ZnO alloy thin films for use as charge dissipative dielectric layers on RF MEMS switches. These alloys exhibit a widely tunable range of physical properties, allowing the deposition of a material capable of dissipating trapped charges and maximizing the down-state capacitance of the switch. The chemical composition of the alloy films was investigated using Auger electron spectroscopy (AES). The AES results confirmed that the alloys were a mixture of Al O and ZnO, however the exact atomic percentage of Zn could not be reliably determined. Nanoindentation was used to determine the hardness and Young s modulus of the ALD films. Pure Al O ALD was determined to have a hardness of approximately 8 GPa and a modulus between 15 and 155 GPa, while the results for the pure ZnO ALD indicated a hardness around 5 GPa and a modulus between 1 and 1 GPa. The Al O /ZnO ALD alloy containing 5% DEZ exposures exhibited a hardness of ~8 GPa and a modulus between 1 and 15 GPa. Finally, the dielectric constants of the alloy ALD films were determined using CV measurements from a mercury probe. The dielectric constants for films containing various amounts of Zn were very similar to the dielectric constant of 6.8 for pure Al O. The properties reported here indicate that Al O /ZnO ALD films can be engineered to be effective charge dissipative dielectric layers for RF MEMS devices. 5. ACKNOWLEDGEMENTS The authors would like to thank DARPA for funding under grants # F C-1518 and # NBCH1. The authors would also like to thank G. Bruce Rayner at the University of Colorado for the Auger electron spectroscopy and Megan Cordill at the University of Minnesota for the nanoindentation measurements. REFERENCES 1. C. Goldsmith, J. Ehmke, A. Malczewski, B. Pillans, S. Eshelman, Z. Yao, J. Brank and M. Eberly, Lifetime characterization of capacitive RF MEMS switches, IEEE MTT-S International Microwave Symposium Digest, 1, pp. 77-, 1.. G. M. Rebeiz, RF MEMS Theory, Design, and Technology, John Wiley and Sons, New Jersey,.. N. D. Hoivik, J. W. Elam, R. J. Linderman, V. M. Bright, S. M. George, Y. C. Lee, Atomic layer deposited protective coatings for micro-electromechanical systems, Sensors and Actuators A 1, pp.1-18,.. J. W. Elam, D. Routkevitch, and S. M. George, Properties of ZnO/Al O alloy films grown using atomic layer deposition techniques, J. Electrochem. Soc. 15, pp. G9-G7,. Proc. of SPIE Vol

8 5. S. M. George, A. W. Ott, J. W. Klaus, Surface chemistry for atomic layer growth, J. Phys. Chem. 1, pp , M. D. Groner, F. H. Fabreguette, J. W. Elam and S. M. George, Low temperature Al O atomic layer deposition, Chem. Mater. 16, pp ,. 7. J. W. Elam, Z. A. Sechrist, and S. M. George, ZnO/Al O nanolaminates fabricated by atomic layer deposition: growth and surface roughness measurements, Thin Solid Films 1, pp. -55,. 8. M. D. Groner, J. W. Elam, F. H. Fabreguette and S. M. George, Electrical characterization of thin Al O films grown by atomic layer deposition on silicon and various metal substrates, Thin Solid Films 1, pp ,. 9. J. W. Elam, C. A. Wilson, M. Schuisky, Z. A. Sechrist and S. M. George, Improved nucleation of TiN atomic layer deposition films on SiLK low-k polymer dielectric using an Al O atomic layer deposition adhesion layer, J. Vac. Sci. Technol. B 18, pp. -,. 1. J. W. Elam and S. M. George, Growth of ZnO/Al O alloy films using atomic layer deposition techniques, Chem. Mater. 15, pp. 1-18,. 11. L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, Handbook of Auger Electron Spectroscopy, Perkin-Elmer Corporation, Minnesota, R. C. Weast, Ed., CRC Handbook of Chemistry and Physics, 6 th Edition, CRC Press, Florida, pg. D16, MTS Systems Corporation, 1 Technology Drive, Eden Prairie, MN G. M. Pharr and W. C. Oliver, Measurements of thin film mechanical properties using nanoindentation, MRS Bulletin 17, pp. 8-, M. J. Mayo, R. W. Siegel, Y. X. Liao and W. D. Nix, Nanoindentation of Nanocrystalline ZnO, J. Mater. Res. 7, pp. 97-9, J. C. Barbour, J. A. Knapp, D. M. Follstaedt, T. M. Mayer, K. G. Minor, and D. L. Linam, The mechanical properties of alumina films formed by plasma deposition and by ion irradiation of sapphire, Nuc. Instr. and Meth. Phsy. Res. B , pp. 1-17,. 17. L. P. Martin, D. Dadon, M. Rosen, D. Gershon, A. Birman, B. Levush, and Y. Carmel, Ultrasonic and dielectric characterization of microwave-sintered and conventionally sintered zinc oxide, J. Am. Ceram. Soc. 79, pp , Material Development Coroporation, 151 Nordhoff Street, B, Chatsworth, CA Stanford Research Systems, Inc., 19-D Reamwood Ave., Sunnyvale, CA 989. National Instruments Corporation, 115 N Mopac Expwy, Austin, TX Proc. of SPIE Vol. 5715

Alternative dielectric films for rf MEMS capacitive switches deposited using atomic layer deposited Al 2 O 3 /ZnO alloys

Sensors and Actuators A 135 (2007) 262 272 Alternative dielectric films for rf MEMS capacitive switches deposited using atomic layer deposited Al 2 O 3 /ZnO alloys Cari F. Herrmann a,b, Frank W. DelRio