We investigated electrophysical properties of the SiO₂-GaAs and Au-Ti-SiO₂-GaAs structures that are used in technological process when manufacturing transmission lines for microwave integrated circuits. The SiO₂-GaAs structures were formed using different techniques, namely, (i) monosilane oxidation in oxygen, (ii) high-temperature tetraethoxysilane decomposition, (iii) high-frequency cathode sputtering of quartz in argon plasma, and (iv) electron-beam evaporation of quartz in a vacuum. The SiO₂ films obtained using monosilane oxidation or electron-beam evaporation of quartz demonstrated better properties. For the Au-Ti-SiO₂-GaAs structures a layer structure transformation was shown to occur, with formation of TiOx-SiO₂ junction and gold atoms penetration over the whole adhesion layer thickness. The microwave oscillator modules made using the Au-Ti-SiO₂-GaAs (Si) structures demonstrated output power of 10-60 mW in the 8 mm wavelength range

Boltovets, N.S.

Kashin, G.N.

Konakova, R.V.

Lyapin, V.G.

Milenin, V.V.

Soloviev, E.A.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

183' 2002, Institute of Semiconductor Physics, National Academy of Sciences of UkraineSemiconductor Physics, Quantum Electronics & Optoelectronics. 2002. V. 5, N 2. P. 183-187.PACS: 68.65.AProperties of SiO2-GaAs and Au-Ti-SiO2-GaAs structuresused in production of transmission linesN.S. Boltovets1), G.N. Kashin2), R.V. Konakova2), V.G. Lyapin2), V.V. Milenin2), E.A. Soloviev2)1)State Scientific & Research Institute Orion, 8a Eugene Pottier St., 03057 Kiev, UkraineE-mail: bms@i.kiev.ua2)Institute of Semiconductor Physics, NAS of Ukraine, 45 prospect Nauky, 03028 Kiev, UkrainePhone.: +(380) 44 265-61-82; fax: +(380) 44 265-83-42; e-mail: konakova@isp.kiev.uaAbstract. We investigated electrophysical properties of the SiO2-GaAs and Au-Ti-SiO2-GaAsstructures that are used in technological process when manufacturing transmission lines formicrowave integrated circuits. The SiO2-GaAs structures were formed using different tech-niques, namely, (i) monosilane oxidation in oxygen, (ii) high-temperature tetraethoxysilanedecomposition, (iii) high-frequency cathode sputtering of quartz in argon plasma, and (iv)electron-beam evaporation of quartz in a vacuum. The SiO2 films obtained using monosilaneoxidation or electron-beam evaporation of quartz demonstrated better properties. For theAu-Ti-SiO2-GaAs structures a layer structure transformation was shown to occur, with for-mation of TiOx-SiO2 junction and gold atoms penetration over the whole adhesion layerthickness. The microwave oscillator modules made using the Au-Ti-SiO2-GaAs (Si) structuresdemonstrated output power of 10-60 mW in the 8 mm wavelength range.Keywords: microwave diode, oscillator module, interface between phases, SiO2−GaAs layer structure,Au−Ti−SiO2−GaAs multilayer structure.Paper received 13.03.02; revised manuscript received 24.05.02; accepted for publication 25.06.02.1. IntroductionWhen developing GaAs-based microwave integrated cir-cuits, much consideration is being given to the develop-ment of transmission lines based on the metallized SiO2-GaAs structures. In this case, along with formation of ahigh-quality metallization to this structure, heavy de-mands are imposed upon the SiO2 films and SiO2-GaAsinterface. Among these demands are: high insulating prop-erties; low density of surface states at the SiO2-GaAs in-terface; chemical inactivity to GaAs and metallization;high density; low porosity; high adhesion to GaAs; highthermal and radiation tolerance; compositional and sur-face uniformity of SiO2 films; manufacturability of thefabrication technique. A considerable experience gainedduring investigations of SiO2-GaAs structures intendedfor various functional purposes evidences that one of thefeatures of these structures is high imperfection of theinterface. This imperfection appears during the physico-chemical processes used when growing SiO2 films [1-4].Choice of techniques to form SiO2-GaAs structuresand exert control over the interface properties seems tobe topical because the transmission line characteristicsessentially depend on the SiO2 film quality. That is whyour objective was to choose a technique for formation ofthose SiO2-GaAs structures that would meet the require-ments of microwave engineering, and to study physico-chemical properties of the Au-Ti metallization sputteredonto the SiO2-GaAs structure.2. Experimental procedureThe epitaxial n-n+-GaAs structures have been chosen toserve as substrates because it is rather convenient to studysuch subjects for determination of optimal manufactur-ing technology for SiO2-GaAs contacts. Silicon dioxidefilms were sputtered onto the surface of the above struc-tures using the following four techniques: (i) monosilaneoxidation in oxygen at T = 350 °C for t = 15 min.; (ii)high-temperature tetraethoxysilane decomposition at T= 680 °C for t = 150 min.; (iii) high-frequency cathodesputtering of quartz in argon plasma; (iv) electron-beamevaporation of quartz in a vacuum at T = 400 °C. The184 SQO, 5(2), 2002N.S. Boltovets et al.: Properties of SiO2-GaAs and Au-Ti-SiO2-GaAs structures...SiO2 films were about 300 nm thick. Au-Ti contactmetallization was formed on the SiO2-GaAs structuresthat were obtained using the electron-beam evaporationof quartz.Evidence on chemical composition and structure ofthe grown SiO2 layers was obtained using Auger electronspectroscopy (AES) and secondary ion mass spectrometry(SIMS) (argon ions were used for sputtering). The dataon the properties of the GaAs near-contact layers wereobtained from the photoreflection and photoluminescencespectra.3. Results and discussionThe AES and SIMS measurements have shown that thefilm composition was close to that of SiO2. Etching ofSiO2 film in peroxyammonia showed for every of the fourtypes of samples that GaAs surface was depleted of gal-lium; the degree of depletion depended on the sampletype. To illustrate, the ratio between gallium and arsenicconcentrations at the surface after etch removal of filmsobtained by techniques (i) and (iv) was about 0.4-0.45,while that for the case of technique (iii) was about 0.1.The results of photoluminescence studies also indicate atchanges in the structural-impurity composition of theGaAs near-surface layer. The photoluminescence spec-iS selpmasfoepyT retemaraP yG,esoD0 01 3 01 4 01 5 01 61negyxoninoitadixoenalisonoM).nim51=t,C°053=T(Γµ3200633200630200140200042200832 noitisopmocedenalisyxohtearteT ).nim051=t,C°086=T(Γµ5300820300135200433200638200233 niztrauqfognirettupsedohtaCamsalpnograΓµ3300920300038200139200033300824 fonoitaropavemaeb-nortcelE )C°004=T(muucavniztrauqΓµ520053120093020093020093 420063Table 1. Impact parameter Γ (meV) and Hall mobility of charge carriers µ (cm2/V⋅s) for the GaAs near-surface layerFig. 1. Photoluminescence spectra (intensity vs photon energy)for GaAs films: 1  initial n-n+-GaAs structure; 2  after removalof a 300 nm thick SiO2 layer (obtained by monosilane oxidationin oxygen at 350 °C) in the peroxyammonia etchant. The meas-urements were performed at 77 K.0120.8 1.0 1.2 1.4 1.6 h , eVν2 1PL intensity, arb. un.N.S. Boltovets et al.: Properties of SiO2-GaAs and Au-Ti-SiO2-GaAs structures...185SQO, 5(2), 2002tra taken at T = 77 K on the SiO2-GaAs structures afterremoval of the SiO2 layer obtained by technique (i) showthat in this case the properties of the GaAs near-surfacelayer are close to those of the starting GaAs material(Fig. 1). Along with the principal band, an impurity bandis observed in the photoluminescence spectra. This bandis related to the VGaTeAs complex [5] (V stands for va-cancy, and TeAs for a Te atom occupying the As site). Inthe GaAs near-surface layer of the structures obtainedby technique (ii) the intensity of the ~1.33 eV band in-creased (according to literature, this band is related tothe VAsCuGaVAs complex). At the same time, the princi-pal band in these structures was substantially quenched;this fact indicates at intense defect production in the GaAsnear-surface layer (Fig. 2).An analysis of the photoreflection spectra for the SiO2-GaAs structures prepared using the above techniquesshowed that the samples obtained using monosilane oxi-dation (sample 1 in Table 1) and electron-beam evapora-tion of quartz (sample 4 in Table 1) demonstrated thebest structural perfection. The Hall mobility µ of chargecarriers in the GaAs near-surface layers of these sampleswas 1.2÷1.3 times higher than that for the samples 2 and3. The impact parameter Γ of the photoluminescence spec-tra broadening may serve as a degree of structural per-fection of the near-surface layers. The lower is Γ, themore perfect is the GaAs near-surface layer. One can seefrom Table 1 that minimal Γ values are demonstrated bythe samples 1 and 4. Thus, according to the above resultsobtained by analytical methods and from the photolumi-nescence and photoreflection spectra, the best parametersare demonstrated by the SiO2-GaAs structures obtainedusing monosilane oxidation in oxygen and electron-beamevaporation of quartz in a vacuum. The SiO2-GaAs struc-ture obtained with technique (iv) was exposed to Timetallization followed by gold layer sputtering.Shown in Fig. 3 are the Auger concentration depthprofiles of the components of the Au-Ti-SiO2 structureFig. 2. Photoluminescence spectra (intensity vs photon energy)for GaAs films: 1  initial n-n+-GaAs structure; 2  after removalof a 300 nm thick SiO2 layer (obtained by tetraethoxysilane de-composition at 680 °C) in the peroxyammonia etchant. The meas-urements were performed at 77 K.S p u tte rin g  tim e ,  sAt, %A u -T i-S iO   -G a A s2A uT iCOS iG aA s0.8 1.0 d /d020406000 400 800 1200 1600 2000020406080100Concentration, at. %Fig. 3. Auger concentration depth profiles of the Au-Ti-SiO2 structure components.0120.8 1.0 1.2 1.4 1.6 h , eVν12PL intensity, arb. un.186 SQO, 5(2), 2002N.S. Boltovets et al.: Properties of SiO2-GaAs and Au-Ti-SiO2-GaAs structures...formed in the technological mode. Strongly pronouncedintermixing effects should be noted. They completelydestroy the layered structures of the samples studied. Oneshould expect a substantial spreading of the Ti-SiO2 in-terface due to pronounced gettering properties of tita-nium. But for the chemically passive Au-Ti pair such adeep (over the whole thickness of the Ti film) penetrationof gold atoms seems to be quite unexpected.One may suggest that the reasons for such strong ef-fects of diffusion redistribution of atoms in the contact-ing layers result from phase changes in the Ti layer. In-deed, it is well known that the Ti-O bonds are intenselyformed even at room temperature. So one may assumethat during the Ti film deposition and further treatmentsthe Si-O bonds break and new oxides are formed accord-ing to the following reaction: SiO2 Si ( ρ ≥ 5 ⋅10 3 Ω⋅cm) or GaAs ( ρ ≥ 5 ⋅10 7 Ω⋅cm) TiAu TiAu Au2 Au2 1 2 3 4 5 6 Fig. 4. Technological route for oscillator module manufacturing: 1 - SiO2 formation on high-resistance Si or semi-insulating GaAswafers; 2 - sputtering of Ti-Au1 contact layers; 3 - two-sided photolithography, topology formation; 4 - two-sided photolithography,hole etching; 5 - die separation; 6 - assembling of oscillator module.N.S. Boltovets et al.: Properties of SiO2-GaAs and Au-Ti-SiO2-GaAs structures...187SQO, 5(2), 2002Si-O + Ti → Si + Ti-O.That this reaction occurs is also supported by the val-ues of thermochemical parameters of titanium oxide for-mation. It was stated in [6] that for TiO2 the heat of for-mation is 218 kcal/mole, i.e., it is much higher than thatfor titanium silicides (TiSi2: - 32 kcal/mole; TiSi: - 31kcal/mole; Ti3Si3: -138 kcal/mole). Quick oxygen diffu-sion into Ti layer leads to the titanium oxide formation.At the same time no titanium silicides are formed (takinginto account the sample preparation conditions). Thus atthermal equilibrium the layered heterostructure is trans-forming with formation of the TiOx-SiO2 junction. Thisprocess is accompanied by disruption of the titanium-based diffusion barrier that prevented Au diffusion deepinside the heterophase structure. As a result, gold atomspenetrate into the whole adhesion layer.Similar structure of the Au-Ti-SiO2 contacts was ob-served in the Au-Ti-SiO2-Si samples. Both structures (Au-Ti-SiO2-GaAs and Au-Ti-SiO2-Si) were used when pro-ducing microwave oscillator modules. The technologi-cal route for oscillator module manufacturing is presentedin Fig. 4. The high-resistance Si and semi-insulating GaAswafers 200 + 10 µm thick (resistivity no less than 5⋅103and 5⋅107 Ω⋅cm, respectively) served as starting materi-als. Two-sided photolithography and local chemical depo-sition of gold were used to form topology. The holes weremade using two-sided alignment and chemical-dynamicetching. The IMPATT diode chip was mounted on theheat sink using diffusion welding. The gold lead waswelded to mesas and topology elements usingthermocompression. The output power of such oscillatormodules was from 10 to 60 mW in the 8 mm wavelengthrange (frequency of 38 GHz).8. ConclusionThus we have developed and realized the technologicalroute for production of oscillator modules (intended tooperate in the millimeter wavelength range) using high-resistance Si (GaAs) substrates coated with SiO2-Ti-Aulayers.AcknowledgementsThis work was supported by the INCO-COPERNICUS Program (Project No 977131MEMSWAVE).References1. N.A. Kolobov, M.M. Samokhvalov, Diffusion and Oxida-tion of Semiconductors, Metallurgiya, Moscow (1975) (inRussian).2. GaAs Microelectronics (VLSI Electronics MicrostructureScience, v. 11), Eds. N.G. Einspruch, W.R. Wisseman, Aca-demic Press, New York (1985).3. S.A. Vasilkovskii, I. Ivancho, Raman spectroscopy investi-gation of the effect of SiO2, Si3N4 insulation films depositionand low doses of g-irradiation on the GaAs near-surfaceregion // Zhurn. Tekhn. Fiz., 63(6), pp. 80-86 (1993) (in Rus-sian).4. J. Breza, P.I. Didenko, R.V. Konakova, V.V. Milenin, G.F.Romanova, Effect of g-irradiation on the properties of SiO2-GaAs structures // Zhurn. Tekhn. Fiz., 65(5), pp. 122-131(1995) (in Russian).5. V.I. Gavrilenko, A.M. Grekhov, D.V. Korbutyak, V.G.Litovchenko, Optical Properties of Semiconductors (A Hand-book), Naukova Dumka, Kiev (1987) (in Russian).6. V.I. Perelman, A Concise Handbook for Chemists, StateChemical Publ., Moscow (1963) (in Russian).

Properties of SiO₂-GaAs and Au-Ti-SiO₂-GaAs structures used in production  of transmission lines

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Properties of SiO₂-GaAs and Au-Ti-SiO₂-GaAs structures used in production of transmission lines

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