AVS1997 Session TF1-WeA: Mechanical Properties of Thin Films

Wednesday, October 22, 1997 2:00 PM in Room B1/2

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		TF1-WeA-1 New Observations on the Effect of HF Exposure on the Mechanical Properties of Polysilicon Thin Films B. Yuan, W.N. Sharpe, Jr. (The Johns Hopkins University) It has been reported that the mechanical properties of polysilicon films including Young's modulus and strength are influenced due to exposure to hydrofluoric acid [1, 2]. Walker et al. [1] used a membrane technique to test undoped LPCVD polysilicon films exposed to various concentration of hydrofluoric acid including pure HF (49%). They found that the average value of Young's modulus changed from 190 GPa for films with no treatment to 240 GPa for films exposed to pure HF, and the average value of fracture strain decreased from 0.101% for unexposed films to 0.043% for those exposed to pure HF. Jones et al. [2] tested static cantilever beams of heavily phosphorus doped polysilicon exposed to HF for various durations and found that the average fracture strain dropped from 1.1% for a 10 minute exposure to HF to 0.84% for a 2 hour exposure to HF. Nevertheless, our measurements of Young's modulus, tensile strength and Poisson's ratio show that no significant differences exist between the mechanical properties of those exposed and unexposed specimens. The specimens tested in this paper are heavily phosphorous doped LPCVD polysilicon thin films exposed to pure HF (49%) for various durations. Twelve specimens with no HF exposure, two specimens exposed to HF for 30 minutes, three specimens for 60 minutes, and four specimens for 120 minutes were tested. The testing techniques used uniaxial dogbone tensile specimens with direct strain measurement by the Interferometric Strain/Displacement Gage (ISDG), a linear air bearing in the loading mechanism to eliminate friction, and a 1 lb load cell for load measurement. Details about the testing techniques can be found in our recent report [3]. The test results, SEM images and optical photographs for the specimens, and experimental techniques will be presented. [1] J.A. Walker, K.J. Gabriel, and M. Mehregany, "Mechanical Integrity of Polysilicon Films Exposed to Hydrofluoric Acid Solutions", Proceedings IEEE Micro Electro Mechanical Systems, Napa Valley, California, 1990, pp. 56-60. [2] P.T. Jones, G.C. Johnson, and R.T. Howe, "Micromechanical structures for fracture testing of brittle thin films", Micro Electro Mechanical Systems, ASME International Mechanical Congress and Exposition, Atlanta GA, 1996, DAC-Vol. pp. 325-330. [3] W.N. Sharpe, Jr., Bin Yuan, R. Vaidyanathan, and R.L. Edwards, "New test structures of techniques for measurement of mechanical properties of MEMS materials", Proceedings of SPIE Symposium on Micromachining and Microfabrication, Volume 2880, pp. 78-91.
2:20 PM		TF1-WeA-2 Synthesis and Characterization of BN and BCN Thin Films A. Tempez, N. Badi, A. Bensaoula, J. Kulik, S. Lee, S.S. Perry (University of Houston); K. Waters, A. Shultz (Ionwerks) Boron nitride and boron carbonitride thin films have been investigated for electronic and tribological characteristics. Nitrogen species were delivered by a gridless end hall ion source and an electron cyclotron resonance (ECR -ASTEX) plasma source. The former has been used for ions of energy between 40 and 60 eV and ion beam current ranging from 110 mA to 190 mA. The ECR source was used for species with lower energy and beam current. The boron and carbon were obtained by electron beam evaporation of pure boron (99.999 %) and pure carbon (99.999 %), respectively. BN and BCN films were achieved at a controlled boron rate of 0.2 Å/s and a varied range of carbon rate of 0.02 to 0.1 Å/s, respectively. The composition of the deposited films has been investigated in-situ by Auger technique and ex-situ using electron probe microanalysis (EPMA) and x-ray photoelectron spectroscopy (XPS) techniques. Most of the films were off- stoichiometry favoring boron rich material . The effect of in-situ vacuum annealing to up to 900 °C on surface stability of BN and BCN samples has been studied by means of Auger technique. Most of the films regardless of their thicknesses or nitrogen content showed a good thermal stability. Thicker BN films ( ~ 1 micron ) on sapphire have been subjected for the first time to a laser transient photoconductivity experiment and promising results for high temperature optoelectronic devices have been obtained. For tribological applications, results based on atomic force microscopy (AFM) friction studies for BN and BCN films, show that films deposited on sapphire present the lowest friction coefficient. The lower the B/N and C/N ratios are, the better the surface morphology and friction coefficient.
2:40 PM		TF1-WeA-3 A Hardness Enhancement by Compositionally Modulated Structure of TiN/Ti Multilayer Films E. Kusano, M. Kitagawa, H. Nanto, A. Kinbara (Kanazawa Institute of Technology, Japan); I. Kondo (Denso Co., Ltd., Japan) A compositionally modulated multilayer film is one type of films with a nano-scale layer structures. For this type of films it is expected that hardness is affected by a compositional modulation frequency due to an effect of the modulation on mechanical properties. In this paper results of an investigation of effects of multilayer structures of compositionally modulated TiN/Ti films on their hardness will be presented. Compositionally modulated TiN/Ti multilayer films were deposited by a reactive gas flow rate modulation magnetron sputtering method using a combination of a Ti metal target and an Ar-N₂ mixture discharge gas. In this method, a N₂ gas flow rate was automatically controlled by a computer-controlled mass flow controller to produce films with an optimized composition distribution. Borosilicate glass and alumina plate were used as substrates. Hardness of prepared films were measured by a nano-indentation method. Composition distribution of prepared films was estimated by Auger electron spectroscopy. TiN singlelayer films were also deposited to compare its hardness with those of the multilayer TiN/Ti films. By changing N₂ reactive gas flow rate modulation frequency, films with a modulation period of 5nm, 7.5nm, 10nm, and 20nm were deposited onto unheated substrates. By nano-indentation measurements micro hardness values of the multilayer films with a modulation period of 7.5nm, 10nm, and 20nm were found to be higher than that of the singlelayer film(45GPa). The highest value of about 78GPa was obtained for the multilayer film deposited on an alumina substrate with a modulation period of 10nm. The similar results were obtained for the films both deposited on the two types of substrates. The results obtained in this study indicate that the hardness of compositionally modulated multilayer TiN/Ti films strongly depends on modulation frequency.
3:00 PM		TF1-WeA-4 Morphological Instability in InAs/GaSb Heterostructures M.E. Twigg, B.R. Bennett, R. Magno (Naval Research Laboratory) Most single-component heteroepitaxial systems exhibiting elastic instability, as evidenced by strain-relieving surface undulations, involve misfits of several percent, although multi-component films may exhibit similar features with smaller misfits. Here we report a study of morphological instability in a single-component small-misfit (0.6%) heteroepitaxial system: InAs grown on GaSb. Using transmission electron microscopy, we examined several different InAs films grown on (100) GaSb using molecular beam epitaxy, with thicknesses ranging from 3 to 40 monolayers (ML). The growth conditions included deposition rates of 0.2 ML/s and 0.03 ML/s, and temperatures of 400C, 425C, and 450C. Only at a sufficiently low growth rate (0.03 ML/s) and higher temperatures (425-450C), did the InAs layers experience elastic strain-relief. The film surface was imaged with reflection electron microscopy and atomic force microscopy. The wavelength of the surface undulations was found to range from 100-800 nm with an amplitude of less than 4 nm. Our observations of the onset of morphological instability as a function of temperature and growth rate are found to be in quantitative agreement with the linear stability theory of Spencer et al.¹, when the InAs cation diffusion length data of Shen and Nishinaga² are used. The linear nature of the theory is well-suited to the study of the long-wavelength small-amplitude surface undulations that are present in small-misfit heteroepitaxial systems such as InAs/GaSb. The absence of faceted high-aspect islands in the InAs/GaSb growths may contribute to the success in predicting the onset of elastic instability. ¹B. J. Spencer et al., J. Appl. Phys. 73, 4955 (1993). 2. X. Q. Shen and T. Nishinaga, J. Cryst. Growth 146, 374 (1995). ² X. Q. Shen and T. Nishinaga, J. Cryst. Growth 146, 374 (1995).
3:20 PM		TF1-WeA-5 Surface Characterization of Single Crystal ZnO and Nanocrystlline ZnO Thin Films Using Atomic Force Microscopy and Lateral Force Microscopy. J.J. Nainaparampil, J.S. Zabinski (Systran, Wright Laboratory, Materials Directorate, USAF); S.V. Prasad (UDRI, Wright Laboratory, Materials Directorate, USAF) Atomic Force Microscopy is used to record inter-atomic forces between atoms of a sharp cantilevered tip and atoms on the surface of a specimen. These forces are recorded as the deflection of the cantilever as the sample is scanned by the tip. Lateral force microscopy evolved from Atomic Force Microscopy as a means to characterize surface forces in relation to friction and lubrication. The significance of this approach is the understanding of the tribology of surfaces at a fundamental level. In this work, the friction characteristics of ZnO single crystal surfaces are evaluated as a function of crystal face and crystallographic direction. Lateral force measurements are made on different planes of ZnO single crystals while the crystal is rotated. The variation of friction force with crystal structure and direction will be discussed. The results of friction force measurements indicate the dependence of crystallographic directions on nano scale friction. Results of the lateral force microscopy in relation to recently obtained low friction ZnO nano polycrystalline thin films will also be presented.
3:40 PM		TF1-WeA-6 Effects of Ta Crystallites at the TaAl/Al-Cu Interface of Hyper-Texture Aluminum J.P. Allain (University of Illinois); D.S. Gardner, T.N. Marieb (Intel Corporation); H. Wang, X. Sun, A. Fischer-Colbrie, G.W. Ray (Hewlett Packard Company) The electromigration lifetime of aluminum interconnect lines shows a strong dependence on the degree of texture in the metal. One method to enhance the (111) crystallographic texture of Al-alloy films is by deposition on an amorphous TaAl underlayer. Several effects on aluminum texture have been found to be related to thin (< 100 Å) Ta crystallites at the TaAl/Al-Cu interface. Lower concentration Ta films show complete crystallization at temperatures below the melting point of aluminum determined using temperature vs. stress measurements. The films were sputter deposited in an ultra-high vacuum system at base pressures below 10^-10 Torr. The metal and underlayer depositions used an Al-0.5at%Cu target and a Ta-Al target respectively. Crystallites form at the TaAl/ Al-Cu interface and were determined to be bcc-Ta using electron microdiffraction. Alpha scan and rocking curve measurements of the (111) peak indexed by the full width at half-maximum (FWHM) were used to determine Al texture. The measurements are a reliable quantitative measure of texture and values down to 0.42 degrees were obtained. The results suggest that crystallites formed at the TaAl/ Al-Cu interface have an impact on aluminum texture. In this paper, hyper-texture aluminum will be correlated to the microstructure and character of the Ta crystallites.
4:00 PM		TF1-WeA-7 Micromechanical Properties of Silicate-Glass Films on Sapphire Substrates A.V. Zagrebelny, C.B. Carter (University of Minnesota) It has been shown that internal residual stresses may compromise the performance of integrated circuits, magnetic media, etc. In addition, the presence of residual and thermal stresses between the matrix and intergranular films in structural multiphase ceramics is a common mechanism of failure that often causes deformation and fracture. As a result, understanding of mechanical properties of thin films on substrates requires an understanding of the stresses in the film structures as well as a knowledge of mechanisms by which thin films deform. Silicate-glass films of anorthite (CaAl₂Si₂O₈) composition with thicknesses ranging 100-200 nm have been grown on elastically strained single-crystal Al₂O₃ (R-plane) by pulsed-laser deposition (PLD). After the growth, the substrates were unloaded causing a uniaxial stress imposed on the film so it can match the dimensions of the substrates. The experimental set-up developed for this study allows to control the magnitude of the residual stress introduced into the films precisely. The micromechanical properties of these films have been studied with nanoindentation and in-situ AFM imaging. This testing technique allows stress-related changes in surface morphology observed with AFM to be correlated to the irregularities observed in corresponding load-displacement responses produced with nanoindentations. This new approach provides additional means of studying the film's properties, both structural and mechanical and relating the changes in the surface morphology to overall mechanical properties associated with the film strain and relaxation.
4:20 PM		TF1-WeA-8 Residual Stress Effects on Adhesion and Fracture of Thin Electronic Ceramic Films. N.R. Moody, R.Q. Hwang (Sandia National Laboratories); S.K. Venkataraman (Applied Materials); J.E. Angelo (Seagate Technologies); W.W. Gerberich (University of Minnesota) Residual stresses can markedly alter thin film properties. However, their effects are not well-defined due to limitations in test techniques. Recent work shows that as-deposited tantalum nitride films are in a high compressive residual stress state and provide a good opportunity to study residual stress effects. We therefore used these films in a study on thin film durability. The films were sputter deposited at room temperature onto single crystal sapphire monitors to a thickness of 600 nm during production runs. Some films were left in the as-deposited condition while the remaining films were vacuum annealed at 300°C to simulate processing effects. Indentation fracture and continuous nanoscratch test techniques were then used to evaluate the susceptibility of these films to fracture. The results revealed a more than ten-fold increase in applied energies for fracture going from the as-deposited to the vacuum annealed films. Although the fracture stresses differed, high resolution TEM revealed no change in the structure of the films or of the interfaces. The only discernible change in structure was a surface rearrangement of atoms into parallel arrays of striations on the vacuum annealed samples revealing a high compressive residual stress state after deposition. Comparison of results and application of mechanics-based models showed that the high compressive residual stresses promoted the susceptibility to fracture and decreased the measured fracture energies of the as-deposited films. The models also provided a good measure of the residual stress levels and interfacial strain energy release rates. This work supported by U.S. DOE Contract DE-AC04-94AL85000.
4:40 PM		TF1-WeA-9 Microstructural Effects on the Strength of Diamond Thin Films M.D. Drory (Crystallume) Synthetic diamond films grown by low pressure CVD methods enables relatively large windows to be developed with exceptional optical (broad spectrum) transmission, strength, thermal conductivity and erosion resistance. A critical issue is the strength of free standing diamond for these applications. It has been proposed that at least 1mm thickness is required for safe design of 1atm. pressure of relatively small diameter windows. A thinner window would greatly reduce raw material and fabrication costs. The strength of thin free-standing diamond is explored for a range of thicknesses with sufficient sampling to enable Weibull statistics to be performed. The relationship between microstructure (e.g. critical flaws) and strength will be discussed.