This work presents results of investigations of electronic properties of undoped boron nitride (BN) films produced on Si substrates in the course of radio frequency (rf) PACVD process with boron triethyl (C2H5)3B as the boron source. The influence of the deposition process parameters on thickness and electronic properties (resistivity r, dielectric strength EBR) of BN films based on ellipsometry and I-V curve measurements at room temperature is studied. The obtained results show that proper selection of deposition process parameters allows BN layers with the required thickness and advantageous values of r and EBR to be fabricated. BN becomes therefore an interesting material for microelectronics applications

Firek, Piotr

Kwietniewski, Norbert

Szmidt, Jan

Werbowy, Aleksander

Biblioteka Cyfrowa Instytutu Łączności / National Institute of Telecomunications: Digital Library

Paper Influence of the depositionprocess parameters on electronic propertiesof BN films obtained by means of RF PACVDPiotr Firek, Aleksander Werbowy, Jan Szmidt, and Norbert KwietniewskiAbstract—This work presents results of investigations of elec-tronic properties of undoped boron nitride (BN) films pro-duced on Si substrates in the course of radio frequency (rf)PACVD process with boron triethyl (C2H5)3B as the boronsource. The influence of the deposition process parameterson thickness and electronic properties (resistivity ρ , dielec-tric strength EBR) of BN films based on ellipsometry andI-V curve measurements at room temperature is studied. Theobtained results show that proper selection of deposition pro-cess parameters allows BN layers with the required thick-ness and advantageous values of ρ and EBR to be fabricated.BN becomes therefore an interesting material for microelec-tronics applications.Keywords—III-nitrides, thin BN films, electronic properties,RF PACVD.1. IntroductionDynamic development of microelectronics creates an in-creased demand for new materials. New areas of appli-cation emerge, where typical semiconductor devices runa risk of high-temperature and/or high power emission.Unfortunately, classic silicon-based devices are not capa-ble of working in such operation regimes. One of the po-tential ways to overcome this problem is to develop fab-rication technologies and applications involving the use ofwide band-gap materials like diamond or III-group nitrides(GaN, AlN and in particular BN).Cubic boron nitride (c-BN) with its unique properties, suchas wide band gap (6.4 eV), chemical inertness, good ther-mal stability and thermal conductivity (∼1300 W/mK), isconsidered a prospective material for electronic applica-tions in high-temperature and high-power electronics, par-ticularly in combination with other wide band-gap materi-als [1, 2]. It is worth to emphasize that the expected work-ing temperature of BN-based devices can reach even upto 900◦C! [2].2. Experimental detailsBoron nitride films were produced on p-type Si (〈100〉,ρ = 6− 8 Ωcm) substrates using radio frequency plasmaassisted chemical vapor deposition (RF PACVD)method [3–5]. Layers were synthesized from borontriethyl (C2H5)3B vapors carried by nitrogen N2. Thencircular, aluminum (Al) electrodes (diameter = 1 mm, gatearea = 7.85 ·10−3 cm2) were evaporated on the top of thedeposited layers. As a result metal-insulator-semiconductor(MIS) structures with BN thin films acting as the insulatorwere fabricated.Table 1Process parameters of RF PACVD depositionParametersVoltage Time N2 flow (C2H5)3B flow[V] [s] [cm3/min] [a.u.]Value 1 70 30 10 200Value 2 105 60 20 250Value 3 140 90 30 300Table 2Thickness of the deposited BN layersPro- Voltage Time N2 flow (C2H5)3BThicknesscess [V] [s] [cm3/min] flowx/(x+ i ·period);[a.u.]i = 1, 2, 3 . . .[A˚]1 70 30 10 200 391 / 25792 70 60 20 250 1770 / 25403 70 90 30 300 111 / 15674 105 30 20 300 2220 / 25685 105 60 30 200 513 / 25146 105 90 10 250 985 / 25387 140 30 30 250 1039 / 24558 140 60 10 300 1515 / 25069 140 90 20 200 474 / 25241bis 70 30 10 200 633 / 2532Deposition parameters subsequently used in the Taguchi-based process analysis method [6] are presented in Table 1,whereas the results of BN film thickness measurements arecollected in Table 2.3. ResultsApplying Taguchi’s method allowed the influence of in-put process parameters (self-bias potential, deposition time,flow rates of reactant vapors N2 and (C2H5)3B) on its out-put parameters – in this case thickness x, resistivity ρ andcritical electric breakdown field EBR (being the measure ofdielectric strength) of deposited BN layers to be analyzed.The dependence of the film thickness on process parametersis illustrated in Fig. 1.33Piotr Firek, Aleksander Werbowy, Jan Szmidt, and Norbert KwietniewskiFig. 1. Influence of the: (a) self-bias potential; (b) deposition time; c) N2 flow rate; (d) (C2H5)3B flow rate on thickness of BN layers.Fig. 2. Influence of the: (a) self-bias potential; (b) deposition time; c) N2 flow rate; (d) (C2H5)3B flow rate on resistivity of BN layers.34Influence of the deposition process parameters on electronic properties of BN films obtained by means of RF PACVDFig. 3. Influence of the: (a) self-bias potential; (b) deposition time; c) N2 flow rate; (d) (C2H5)3B flow rate on critical electricbreakdown field of BN layers.The obtained results show clearly that increasing depositiontime results in thicker layers. The same is also observedwhen boron triethyl flow rate increases. On the other hand,the correlation between film thickness and self-bias poten-tial, as well as nitrogen flow rate is more complicated. Itsuggests the presence of opposing phenomena during BNlayer formation and needs further investigation.Electrical characterization (current-voltage (I-V ) measure-ments) of the fabricated MIS structures was performed toextract the resistivity ρ and critical electric breakdownfield EBR of BN layers. In order to determine the influ-ence of the deposition process parameters on ρ and EBRTaguchi’s method was used again. The results are shownin Figs. 2 and 3.As seen in Fig. 2, increasing self-bias potential (i.e., sup-plied power) leads to a decrease of the resistivity of BNlayers. The opposite trend is observed in the case of borontriethyl flow rate growth.As far as the dependence of deposition time on resistivityis concerned, the initial low value of this parameter mightbe attributed to the fact that for short growth times (there-fore for thinner films) one has weaker control over the syn-thesis process, which usually results in inferior values ofresistivity.Obviously the nitrogen flow rate affects chemical composi-tion of the layer and thus its parameters. At the same timenitrogen flow rate exerts influence on the pressure underwhich the coating is formed. The obtained results suggesthowever that higher resistivity is obtained when N2 flowrate is increased to 30 cm3/min.In the case of the dielectric strength of BN films (Fig. 3) thecharacter of its dependence on the deposition process pa-rameters is more complicated as the obtained curves are notmonotonic, showing minima or maxima instead. Neverthe-less it can be seen that a certain set of process parameters(self-bias potential – 140 V, N2 flow rate – 20 cm3/min,boron triethyl flow rate – 300 arbitrary units) results in thehighest value of critical electric breakdown field. On theother hand, Fig. 2 shows that ρ will not reach the requiredhighest level with this combination of parameters. Thisindicates that it is necessary to find a trade-off betweenthese parameters. Taking into account both resistivity anddielectric strength it appears that the best results are ob-tained with the following process parameters: self-bias po-tential – 70 V (substantial gain of with relatively smallEBR loss), N2 flow 30 cm3/min (in spite of a slight dropof the dielectric strength, the resistivity increases almosttwice). For the same reason using the maximum (300 ar-bitrary units) boron triethyl flow rate is the most advanta-geous.4. ConclusionsThe obtained results show that it is possible to achievegood electrophysical parameters of BN layers grown bymeans of RF PACVD. The method is relatively inexpensiveand does not require substrate heating. Proper selection35Piotr Firek, Aleksander Werbowy, Jan Szmidt, and Norbert Kwietniewskiof process parameters allows the quality of the fabricatedfilms to be controlled, although further efforts (in particu-lar involving the use of more advanced Taguchi procedures)towards process optimization are required.References[1] S. N. Mohammad, “Electrical characteristics of thin film cubic boronnitride”, Solid-State Electron., vol. 46, pp. 203–222, 2002.[2] P. Gielisse, “Wide bandgap materials in future electronic appli-cations”, in Proc. XXIV Int. Conf. IMAPS, Rytro, Poland, 2000,pp. 115–123.[3] S. Mitura, E. Mitura, and A. Mitura, “Manufacture of amorphouscarbon layers by r.f. dense plasma CVD”, Diam. Rel. Mater., no. 4,pp. 202–203, 1995.[4] J. Szmidt, “Electronic properties of nanocrystalline layers ofwide band gap materials”, Chaos, Sol. Fract., vol. 10, no. 12,pp. 2099–2152, 1999.[5] S. Mitura, A. Mitura, P. Niedzielski, and P. Couvart, “Nanocrystallinediamond coatings”, Chaos, Sol. Fract., vol. 10, no. 12, pp. 2165–2176,1999.[6] G. S. Peace, Taguchi Methods – A Hands – On Approach. Reading:Addison-Wesley, 1993.Piotr Firek was born in RawaMazowiecka, Poland, in 1977.He received the M.Sc. de-gree in microelectronics fromthe Faculty of Electronics andInformation Technology, War-saw University of Technology(WUT), Poland, in 2004, wherehe is currently working to-wards a Ph.D. degree. His re-search concentrates on fabrica-tion, characterization, processing and application of thinand thick film materials (e.g., barium titanate, boron ni-tride, diamond) in microelectronic devices.e-mail: pfirek@elka.pw.edu.plInstitute of Microelectronics and OptoelectronicsWarsaw University of TechnologyKoszykowa st 7500-662 Warsaw, PolandAleksander Werbowy receivedthe M.Sc. and Ph.D. degrees inmicroelectronics from the Fac-ulty of Electronics and Informa-tion Technology, Warsaw Uni-versity of Technology (WUT),Poland, in 1994 and 1999,respectively. His main researchinterests are nanotechnologiessynthesis, characterization, pro-cessing as well as application of thin films of wideband-gap materials in microelectronics and nanoelectronicsstructures.e-mail: werbowy@imio.pw.edu.plInstitute of Microelectronics and OptoelectronicsWarsaw University of TechnologyKoszykowa st 7500-662 Warsaw, PolandJan Szmidt received the M.Sc.degree in the electronics fromthe Faculty of Electronics, War-saw University of Technology(WUT), Poland, in 1976. Fromthe same university, he receivedthe Ph.D. and D.Sc. degrees in1984 and 1995, respectively. In1999, he became an AssociateProfessor. Since 2002 he hasbeen an Associate Dean for De-velopment of the Faculty of Electronics and InformationTechnology, Warsaw University of Technology. Since 2006he has been a Head of Electronic Materials and Microsys-tem Technology Division, Institute of Microelectronics andOptoelectronics, WUT. His research interests concentrateon characterization of thin films for electronics, especiallyfor microelectronics and nanoelectronics, as well as ontheir application in microelectronic and nanoelectronicstructures.e-mail: j.szmidt@elka.pw.edu.plInstitute of Microelectronics and OptoelectronicsWarsaw University of TechnologyKoszykowa st 7500-662 Warsaw, PolandNorbert Kwietniewski wasborn in Sandomierz, Poland, in1980. He received the M.Sc.degree in microelectronics fromthe Faculty of Electronics andInformation Technology, War-saw University of Technology(WUT), Poland, in 2007. Hisresearch concentrates on fab-rication, characterization, pro-cessing and application of wideband-gap materials (e.g., silicon carbide) materials inmicroelectronic devices.e-mail: nkwietni@ite.waw.plInstitute of Electron TechnologyLotników av. 32/4602-668 Warsaw, Poland36

Influence of the deposition process parameters on electronic properties of BN films obtained by means of RF PACVD, Journal of Telecommunications and Information Technology, 2007, nr 3

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Influence of the deposition process parameters on electronic properties of BN films obtained by means of RF PACVD, Journal of Telecommunications and Information Technology, 2007, nr 3

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