Investigation On The Preparation And Properties Of Boron Carbon Nitride Thin Films

Diamond is the hardest material, but one disadvantage is its reactionwith ferrous materials at high temperature. Therefore, diamond is notsuitable for the deposition on cutting tools operated at high temperature.Cubic boron nitride (c-BN), which is the second hardest material only nextto diamond, shows higher chemical stability at high temperature and lowersolubility on ferrous materials than diamond. But high internal stress inc-BN films and poor adhesion are obstacles for the application of c-BNfilms. Boron carbon nitride thin films are expected to overcome thesedifficulties and possess properties between those of diamond and c-BN.That is to say, boron carbon nitride thin film may possess higher hardness,higher chemical stability and lower internal stress, making it an idealcandidate for cutting tools operated at high temperature. Therefore,researchers began to investigate boron carbon nitride materials.In recent years, boron carbon nitride films have attracted widespreadinterests. But there are still many problems to be solved. First, someresearchers couldn't prepare the boron carbon nitride films, but themixture of hexagonal boron nitride and carbon. Second, little systematicresearch work on the compostion of boron carbon nitride films at differentexperimental parameters has been reported so far. Last, less attentionhas been focused on the optical and electrical prorperties. In order toconsiderably understand the boron carbon nitride films, we deeply andsystematically investigate the preparation and properties of boron carbonnitride films. In this thesis, both r.f. magnetron sputtering and r.f. reactivemagnetron sputtering are used to prepare boron carbon nitride thin films.The influence of experimental parameters on the compositon andthickness of boron carbon nitride thin films is investigated. The opticalproperties, electrical properties, internal stress, and hardness of boroncarbon nitride thin films are also studied.Boron carbon nitride thin films were deposited by radio frequencymagnetron sputtering on Si(100) and glass substrates using acomposite target consisting of h-BN and graphite. During the deposition,Ar and N2 were used as working gas.The appropriate depositionparameters for the preparation of boron carbon nitride films have beenobtained. They are target-substrate space of 65mm, working pressure of0.4~2.5Pa, rf power of 90~150W, N2 partial pressure of 0%~75%,deposition temperature of 300~600℃, sputtering time of 60min~120minand substrate bias of 0~-150V. The samples have been characterized andanalyzed by Fourier Transformed Infrared Spectroscopy, X-ray diffraction,Atomic Force Microscopy and Raman Spectroscopy, etc. The resultsindicate that the three elements of B, C, N are chemically bonded witheach other and atomic level hybrids have been formed in the films. Theboron carbon nitride films prepared in the present experiment possess anamorphous structure and a smooth surface.Boron carbon nitride thin films have also been deposited by radiofrequency reactive magnetron sputtering on Si(100) and glass substratesusing h-BN target. During the deposition, Ar and CH4 were used asworking gas. The deposition parameters for the preparation of boroncarbon nitride films have been obtained. They are target-substrate spaceof 65mm, working pressure of 0.8~4.0Pa, rf power of 100~140W, CH4partial pressure of 0.1%~2%, deposition temperature of 500℃, sputteringtime of 120min and substrate bias of 0V. The results indicate that the threeelements of B, C, N are chemically bonded with each other and atomiclevel hybrids have been formed in the films.With the r.f. magnetron sputtering method the deposition of boroncarbon nitride films is influenced by many experimental parameters, suchas nitrogen partial pressure, working pressure, substrate temperature,bias voltage, r.f. power and so on. Many serials experiments reach thefollowing results: (i), With the increase of nitrogen partial pressure, thethickness of the films first increases and then decreases. The atomicfractions of B, N increase and the fraction of C decreases with increasingnitrogen partial pressure. (ii), With increasing working pressure, theatomic fractions of B, N first increase and then decrease, and the fractionof C first decreases and then increases, and the thickness of the films firstincreases and then decreases. (iii), With the increase of substratetemperature, the thickness of the films first decreases and then increases.The atomic fractions of B, C and N change irregularly with increasingsubstrate temperature. (iv), With the increase of bias volatage, thethickness of film first increases and then decreases. The boron carbonnitride films exhibit lower C content and higher B, N contents at lower biasvoltages. And the boron carbon nitride films show higher C content andlower B, N contents at higher bias voltages. (v), With the increase of r.f.power, the atomic fractions of B, N first increase and then decrease, andfraction of C first decreases and then increases. The thickness of the filmsfirst increases and then decreases with increasing r.f. power.With the r.f. reactive magnetron sputtering method the deposition ofboron caron nitride films is also influenced by many experimentalparameters, such as CH4 partial pressure, r.f. power, working pressureand so on. Many serials experiments reach the following conclusions: (i),With the increase of CH4 partial pressure, the thickness of the filmsincreases. The atomic fractions of B, N decrease and the fraction of Cincreases. (ii), With the increase of r.f. power, the atomic fractions of B, Nfirst increase and then decrease, and fraction of C first decreases andthen increases. The thickness of the films first increases and thendecreases with increasing r.f. power. (iii), With increasing workingpressure, the atomic fractions of B, N first increase and then decrease,and the fraction of C first decreases and then increases, and the thicknessof the films first increases and then decreases.The electrical resistivity of boron carbon nitride films is measured byver der Pauw method. The electrical resistivity of boron carbon nitridefilms prepared at different nitrogen partial pressure, bias voltage, r.f.power and sputtering time is investigated. Generally speaking, thethickness and carbon content of the boron carbon nitride films are majorfactors that influence the electrical resistivity. The electrical resistivityincreases with the increase of thickness. And the electrical resistivitydecreases with increasing carbon content in the film. Therefore, theelectrical resistivity of the films can be adjusted by the thickness andcarbon content.The field emission characteristics of boron carbon nitride films arestudied in this paper. The experimental results indicate that not all theboron carbon nitride films have field emission characteristics. Only thosefilms with appropriate carbon content possess field emissioncharacteristics. The films preprared at 10% and 25% nitrogen partialpressure by r.f. magnetron sputtering method have field emission. Thethreshold electric field is 18V/μm and the largest emission current densityis 196μA/cm2 for the film deposited at 10% nitrogen partial pressure.The threshold electric field is 12V/μm and the largest emission currentdensity is 206μA/cm2 for the film deposited at 25% nitrogen partialpressure. Both the films follow the F-N theory in the region of low electricfield. However, the F-N data deviate from the F-N theory in the region ofhigh electric field. This may be due to that the resistace of the sampledecreases with increasing applied electric field. The machanism on fieldemission of boron carbon nitride films is also investigated in this paper.The optical properties of the boron carbon nitride films areinvestigated. The transmission spectra of the samples are measured byUV-VIS-NIR spectrometer. The transmission ratio of the boron carbonnitride films deposited by r.f. magnetron sputtering is about 81% in thevisible region. For the films deposited by r.f. reactive magnetron sputtering,the transmission ratio increases monotonously during the 300nm-1000nmwavelength. Compared with the films deposited by r.f. magnetronsputtering, the films deposited by r.f. reactive magnetron sputtering showlower transmission ratio during 300nm-1000nm wavelength. The reasonwill be investigated in the future.The hardness of boron carbon nitride film is measured by NanoIndenter. From the results we know that BCxNY film with y>1 possesseshigher hardness than that with y<0.4. The hardness increases with theincrease of carbon content for BCxNY film with y>1. The hardness alsoincreases with the increase of carbon content for BCxNY film with y<0.4.The hardness of BC1.59N1.24 film at a certain spot have attained 57.1GPa,which is higher than the reported hardness of boron carbon nitride films. Itcan be concluded that boron carrbon nitride film with appropriate contentand structure may be a kind of superhard material, and at the same time ithas good adhesion with the substrate, so it is a potential candidate forcutting tools.In this work, the stresses in boron carbon nitride films are investigatedvia FTIR spectra. According to the literature, the boron carbon nitride filmsusually peeled off in several hours even several minutes after being takenfrom the chamber. It is found that most of the films we have preparedhave lower internal stress and have not peeled off even after one year,which offers fundermental conditions for the practical application. Only afew samples have larger internal stress. It is concluded that boron carbonnitride film with lower stress and better adhesive property can be preparedat appropriate conditions.