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Plasma-Enhanced Chemical Vapor Deposition of Boron Nitride Thin Films from B2H6–H2–NH3 and B2H6–N2 Gas Mixtures

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Plasma-Enhanced Chemical Vapor Deposition of Boron Nitride Thin Films from B2H6–H2–NH3 and B2H6–N2 Gas Mixtures Plasma-enhanced chemical vapor deposition of boron nitride thin films from B2H6–H2–NH3 and B2H6–N2 gas mixtures J. L. Andu´jar,a) E. Bertran, and M. C. Polo Departament de Fı´sica Aplicada i Electro`nica, Universitat de Barcelona, Avinguda Diagonal 647, E-08028 Barcelona, Spain ~Received 17 July 1997; accepted 21 November 1997! Highly transparent and stoichiometric boron nitride ~BN! films were deposited on both electrodes ~anode and cathode! of a radio-frequency parallel-plate plasma reactor by the glow discharge decomposition of two gas mixtures: B2H6–H2–NH3 and B2H6–N2. The chemical, optical, and structural properties of the films, as well as their stability under long exposition to humid atmosphere, were analyzed by x-ray photoelectron, infrared, and Raman spectroscopies; scanning and transmission electron microscopies; and optical transmittance spectrophotometry. It was found that the BN films grown on the anode using the B2H6–H2–NH3 mixture were smooth, dense, adhered well to substrates, and had a textured hexagonal structure with the basal planes perpendicular to the film surface. These films were chemically stable to moisture, even after an exposition period of two years. In contrast, the films grown on the anode from the B2H6–N2 mixture showed tensile stress failure and were very unstable in the presence of moisture. However, the films grown on the cathode from B2H6–H2–NH3 gases suffered from compressive stress failure on exposure to air; whereas with B2H6–N2 gases, adherent and stable cathodic BN films were obtained with the same crystallographic texture as anodic films prepared from the B2H6–H2–NH3 mixture. These results are discussed in terms of the origin of film stress,the effects of ion bombardment on the growing films, and the surface chemical effects of hydrogen atoms present in the gas discharge. © 1998 American Vacuum Society. @S0734-2101~98!02302-1# I. INTRODUCTION and bias sputtering,9 because the ion bombardment during film growth is considered an essential condition for obtaining Thin films of boron nitride ~BN! have recently attracted a c-BN. Nevertheless, the high compressive stress caused by lot of interest due to the combination of properties offered by ion bombardment generally leads to poor adhesion and often this nonoxide ceramic material, such as low density, optical to delamination of the films.10 However, uniform high-purity transparency, hardness, chemical inertness, high electrical re- noncubic BN coatings on irregularly shaped surfaces can be sistivity, and high thermal conductivity.1 BN has several obtained with CVD methods. Several gas sources have been crystalline structures similar to the structures in carbon. Cu- used, the most common being BCl –NH ,BF–NH , and bic BN ~c-BN!, analogous to diamond, is extremely hard and 3 3 3 3 B H –NH .3 Borazine (B N H ) and organometallic precur- more stable than diamond at high temperatures.2 Hexagonal 2 6 3 3 3 6 sors have also been employed. Thermal CVD of BN is usu- BN ~h-BN! has a layered structure similar to graphite, but is a transparent, very refractory corrosion-resistant material ally performed at processing temperatures of about 1000 °C, with good dielectric properties.3 In addition, disordered tur- which is a limiting factor for many uses. Nevertheless, the bostratic modifications of h-BN, which range from perfectly lower temperatures involved in plasma-enhanced CVD oriented h-BN to completely disordered amorphous BN ~PECVD! processes, typically, in the 200–300 °C range, al- ~a-BN!, are commonly found in BN films. low a great variety of substrates used in the semiconductor BN coatings have been successfully used in high- and metallurgical industries to be BN coated. With capaci- temperature crucibles, masks for x-ray lithography, heat tively coupled radio-frequency ~rf! discharges, by far the sinks, and sodium barriers in microelectronics.4 Newer ap- most widely used plasma source for low-pressure materials 11–15 plications of BN films include their use in microelectronic processing, noncubic BN films are usually obtained. devices, not only as passivation layers but also as active PECVD films with some fraction of the c-BN phase have dielectric layers,5 as well as wear and corrosion-protective been obtained using higher-density plasma sources such as 16 coatings for cutting tools and optical windows.6 magnetic-field-enhanced rf glow discharges, inductively Excellent reports on the preparation of BN films by low- coupled rf discharges,17 and electron cyclotron resonance pressure physical vapor deposition ~PVD! and chemical va- plasmas.18 por deposition ~CVD! processes can be found in the recent Owing to the complexity of the PECVD processes, the literature.2–4,6,7 Most successful methods for the preparation properties of the films are strongly dependent on the particu- of films containing some amount of c-BN are based on ion- lar deposition system and the gas chemistry used. Therefore, assisted PVD techniques, such as ion-assisted evaporation8 the optimized conditions found in a given system cannot generally be extrapolated to another. Further, in capacitive rf a!Electronic mail: [email protected] discharges, the location of substrates on the grounded elec- 578 J. Vac. Sci. Technol. A 16„2…, Mar/Apr 1998 0734-2101/98/16„2…/578/9/$10.00 ©1998 American Vacuum Society 578 579 Andu´ jar, Bertran, and Polo: Plasma-enhanced chemical vapor deposition 579 trode ~anode! or on the powered electrode ~cathode! greatly TABLE I. Deposition conditions for the B2H6–H2–NH3 gas system. affects the structure and properties of deposited films, since, Substrate position: Anode Cathode as a result of the negative self-bias voltage developed in the cathode, the impact energy of ions striking the cathode could rf power density (W/cm2): 0.25–0.5 0.3–0.5 be up to several hundreds of eV higher than when striking Gas pressure ~Pa!: 30, 60 20–60 B H ~1% in H ! flow ~sccm!: 50, 60, 100 50, 60 the anode. In fact, optoelectronic quality hydrogenated amor- 2 6 2 NH3 flow ~sccm!: 2.5, 5 2.5, 5 phous silicon films are obtained in anodic conditions under B2H6 /NH3 flow ratio: 0.2, 0.4 0.1–0.4 soft ion bombardment, whereas the growth of diamondlike Deposition rate ~nm/min!: 0.6–1.4 2–6 amorphous carbon films requires cathodic conditions under high ion bombardment. Although BN films have been depos- ited on substrates placed on both grounded and powered electrodes, to our knowledge no systematic study of the ef- was adjusted by a throttle valve and measured by an absolute fect of the substrate electrode has been reported. In addition, capacitance manometer. Several kinds of substrates ~fused despite the work done in characterizing the properties of BN silica, Corning 7059 glass, crystalline silicon polished on one films, relatively few reports deal with their stability when and both sides, and NiCr-coated silicon! were used in order aging and exposed to atmosphere. Several stability studies to perform the different film analyses. Before deposition, the have been reported for BN films obtained by thermal substrates were cleaned with acetone and methanol, blown CVD,19–21 but there is a lack of similar studies for PECVD dry with nitrogen, and heated at 300 °C in vacuum 24 BN films. Stability is crucial in determining the suitability of (10 Pa) for several hours. a thin-film material for application as a protective coating. For this study, more than 30 rf discharges were run. The The present study investigates the preparation, properties, deposition conditions corresponding to each gas chemistry and stability of PECVD BN films obtained on both elec- and substrate position ~cathode or anode! are listed in Tables trodes ~anode and cathode! of a parallel-plate rf reactor by I and II. The range of deposition parameters explored was glow discharge decomposition of two different gas mixtures: chosen to obtain transparent films of nearly stoichiometric B2H6–H2–NH3 and B2H6–N2. This deposition technique is composition. All the films were grown at a 300 °C substrate used worldwide in the semiconductor industry and the above temperature. The film thicknesses, measured by a stylus pro- gases are commonly employed, in combination with silane filometer and optical transmittance, ranged from 100 to 2000 gas, for the preparation of silicon nitride and amorphous nm. silicon-based devices. Thus, the findings of this study could also be of interest for the use of BN films in the fabrication of semiconductor devices. The properties of the BN films B. Film characterization were analyzed by Fourier transform infrared ~FTIR! spec- Film surface morphology was studied using an Hitachi troscopy, Raman spectroscopy, scanning electron micros- 2300 scanning electron microscope working at 15 kV accel- copy ~SEM!, high-resolution transmission electron micros- eration voltage. Cross-sectional HRTEM and dark-field ~DF! copy ~HRTEM!, x-ray photoelectron spectroscopy ~XPS!, images as well as selected area diffraction ~SAD! patterns and optical transmittance spectrophotometry in the visible– were recorded using a Philips CM 30 transmission electron near-UV region. microscope operating at 300 kV. The optical transmittance of films deposited on fused II. EXPERIMENT silica and Corning 7059 substrates was measured by a Shi- A. Film preparation mazdu 2101 PC spectrometer in the 200–800 nm range. The infrared properties were studied with a DA3 Bomem FTIR The deposition system consists of a capacitively coupled spectrometer working within the 500– 4000 cm21 wave rf plasma reactor provided with a computer-controlled gas numbers with a resolution of 4 cm21. FTIR transmittance 22 supply and vacuum system. The reactor chamber is a cy- spectra were measured under vacuum conditions using a pol- lindrical stainless-steel vacuum vessel 30 cm in diameter ished on both sides ~100! silicon wafer ~50% transmittance! with two planar electrodes placed vertically and 4 cm apart.
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