It has been found that good quality silicon nitride films can be deposited at room temperature, with an alternate electron cyclotron resonance (ECR) plasma source, called multipolar ECR. The effects of several deposition conditions on physical and electrical properties were studied in order to optimise the deposition process.\ud
The layers were characterised by a refractive index of 1.97, dielectric constant of 7.1, Si/N ratio values of around 0.78 and very low hydrogen content (under 1%). The lowest oxygen contamination (2%) was obtained for the highest nitrogen flow. A decrease in refractive index was observed at high pressure, probably because of insufficient energy to dissociate the nitrogen molecules. The density of interface charge was estimated to be in the range of 3-11•1011 cm-2 and the breakdown field was calculated to be around 12 MV/cm.\u

Holleman, J.

Isai, I.G.

Wallinga, H.

Woerlee, P.H.

It has been found that good quality silicon nitride films can be deposited at room temperature, with an alternate electron cyclotron resonance (ECR) plasma source, called multipolar ECR. The effects of several deposition conditions on physical and electrical properties were studied in order to optimise the deposition process. The layers were characterised by a refractive index of 1.97, dielectric constant of 7.1, Si/N ratio values of around 0.78 and very low hydrogen content (under 1%). The lowest oxygen contamination (2%) was obtained for the highest nitrogen flow. A decrease in refractive index was observed at high pressure, probably because of insufficient energy to dissociate the nitrogen molecules. The density of interface charge was estimated to be in the range of 3-11•1011 cm-2 and the breakdown field was calculated to be around 12 MV/cm

Wallinga, Hans

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Silicon nitride layers obtained by ECR PECVD

University of Twente Research Information

Abstract— It has been found that good quality siliconnitride films can be deposited at room temperature, withan alternate electron cyclotron resonance (ECR) plasmasource, called multipolar ECR. The effects of severaldeposition conditions on physical and electrical propertieswere studied in order to optimise the deposition process.The layers were characterised by a refractive index of1.97, dielectric constant of 7.1, Si/N ratio values of around0.78 and very low hydrogen content (under 1%). Thelowest oxygen contamination (2%) was obtained for thehighest nitrogen flow. A decrease in refractive index wasobserved at high pressure, probably because of insufficientenergy to dissociate the nitrogen molecules. The density ofinterface charge was estimated to be in the range of 3-11·1011 cm-2 and the breakdown field was calculated to bearound 12 MV/cm.Keywords — silicon nitride, PECVD, Poole-Frenkel,room temperature.I. INTRODUCTIONThe demand for high mobility TFTs realised ontemperature unstable substrates is increasing [1]. Thesedevices require thin, low-temperature, high-quality gatedielectrics.Plasma-enhanced chemical vapour deposition(PECVD) with radio-frequency excitation is the mostknown method used for obtaining TFTs materials attemperatures of 300oC. However, the rf-PECVDdielectrics contain a large amount of hydrogen (20-30%)and have high defect densities due to the high-energyions impinging on the film surface during the filmgrowth.Multipolar ECR plasma enhanced deposition [2] waschosen for our research because it is a low processpressure, with low electron and ion energy [3], and highelectronic density [4]. Because of the ECR plasmascharacteristics ("soft" and "dense") [5] we believe thatthe deposition temperature can be lowered, withoutdegrading the film quality or increasing theconcentration of unwanted hydrogen bonds.The influence of different deposition parameters onfilm quality was investigated in order to find the optimaldeposition conditions.II. EXPERIMENTAL DETAILSThe deposition system, equipped with a MPDR-300plasma source has been described elsewhere [6]. N2 wasadmitted directly into the quartz dome. SiH4 wasintroduced through a dispersive ring downstream fromthe plasma.3-inch (100)-oriented n-type Si wafers having aresistivity of 1-10 Ω*cm were used for the experiments.The wafer preparation included a 10 minute dip in 100%fuming HNO3, a 10 minute dip in boiling 69% HNO3,followed by a 1% HF etch in order to remove the nativeoxide.The silicon nitride layers were obtained at floatingtemperature, without external heating. Due to plasmaheating, the substrate temperature, measured by athermocouple reached a maximum of 60oC. During theexperiments, the flow rate of 2% SiH4-in-helium wasmaintained at a constant value of 5 sccm and themicrowave power was adjusted at 400 Watts. The totalpressure was in the range of 5-23 mTorr and the flowrate of nitrogen varied between 30 and 110 sccm.Silicon nitride films with thickness of 30-50 nm weredeposited. The refractive index was measured with anautomatic PLASMOS elipsometer and the atomiccomposition was detected by X-ray photoelectronspectroscopy (XPS).MNS capacitors with aluminum gate electrodes wereSilicon nitride layers obtained by ECRPECVDGratiela Isai1, Jisk Holleman1, Pierre Woerlee2, Hans Wallinga11 MESA+ Research Institute, Twente University, P.O. Box 217, 7500 AE, Enschede, TheNetherlands2Royal Philips Electronics, Groenewoudseweg 1, 5621 BA, Eindhoven, The NetherlandsTel.: 31-53-4894394, Fax: 31-53-4891034E-mail: G.I.Isai@el.utwente.nl39manufactured in order to measure the electricalproperties of the SiO2 layers. All samples weresubjected to an aluminium-sintering step for 5 minutes,at 400oC in wet N2 ambient. The CV and IV curves wereobtained with an MDC system and an HP 4156.III. RESULTS AND DISCUSSIONA. Film compositionIn figure 1 the refractive index of the layers and thenitrogen atomic concentration are plotted versuspressure The refractive index decreases with increasingthe pressure as a result of less nitrogen incorporation inthe layers. Apparently, at high pressures, due to morecollisions, the energy provided by the plasma isinsufficient for ionizing the nitrogen molecules [7]; andoxygen atoms, originating from leaks, saturate insteadthe silicon bonds.1.651.701.751.801.851.901.954 9 14 19 24Pressure, mTorrRefractiveindex404550556065Nitrogencontent,at%Refractive indexNitrogenFigure 1 Refractive index and nitrogen atomicconcentration versus pressure for nitrides deposited with30 sccm N2.A typical depth profile of an ECR layer is shown infigure 2 and as it can be seen the concentration ofnitrogen, silicon and oxygen does not vary with the filmthickness.The refractive index increased until 1.97 and theoxygen contamination decreased from 5% until 2%,when the nitrogen flow rose from 30 sccm to 110 sccm.This decline in oxygen content is due a reduction ofoxygen partial pressure, proving that the oxygenoriginates from the system leaks. Improving the basepressure can furthermore diminish the oxygencontamination [8].0204060801000 20 40 60 80 100Thickness, minConcentration,at.%NitrogenSiliconOxygenFigure 2 XPS depth profile for a nitride deposited with 55sccm N2, at 14 mTorr.The hydrogen content, measured by elastic recoildetection (ERD) was less than 1%, independent ofdeposition conditions. These nitrides were deposited atroom temperature and did not suffer any annealingprocess, however they exhibited extremely lowhydrogen content, proving that ECR PECVD is a verygood method for obtaining films with a low thermalbudget and good properties.B. Electrical propertiesHigh frequency and quasistatic C-V measurementshave been investigated in order to determine the SiO2/Siinterface properties.00.20.40.60.81-5 -3 -1 1 3 5Voltage, VC/Cmax,pF18 mTorr23 mTorrFigure 3 CV curves for two ECR films deposited with 30sccm N2 at different pressure.Figure 3 shows capacitance-voltage characteristics oftwo silicon nitride layers deposited at different pressure.40As one can see, at higher pressure the flatband voltageshift, the positive charge and the interface trap densityare smaller. This is a result of more oxygenincorporation, which relaxes the stressed interface andlimits the number of silicon dangling bonds. Thepositive charge present at the interface varied from2·1011cm-2 for high pressure and low nitrogen flows to1.1·1012cm-2 for low pressures and high N2 flows.Figure 4 presents the current passing through nitridelayers deposited at different pressure. It can be observedthat while the deposition pressure does not significantlyinfluence the breakdown field, the critical field, definedas the electric field for which the current that passesthrough the nitride film is higher than 10-8A/cm2,increases with amplifying the pressure. This effect canbe caused by the amount of oxygen present in the film. Itis known that silicon oxide has a higher critical field (6-7 MV/cm) than silicon nitride (4 MV/cm) [9], thereforeit is normal for nitrides deposited at higher pressure withmore oxygen contamination, to exhibit also a largercritical field.Nitrogen flow rate does not influence the layers bulkproperties.1.E-111.E-091.E-071.E-051.E-031.E-011.E+010 2 4 6 8 10 12Electric field, MV/cmCurrentdensity,A/cm210 mTorr23 mTorrFigure 4 IV curves for two ECR films deposited with 30sccm N2 at different pressure.IV. CONCLUSIONSHigh electrical quality silicon nitride films withrefractive index of 1.97, dielectric constant of 7.1, 2%oxygen content and 1% hydrogen content, critical fieldof 12 MV/cm, resistivity of 1015 Ωcm, and net chargedensity in the order of 1.1·1012 cm-2 were deposited by amultipolar ECR plasma source at room temperature. Inorder to optimise the deposition process, the effects ofdifferent deposition parameters on film properties werestudied. At lower pressure and higher N2/SiH4 ratio, weobtained the best silicon nitride films from the physicaland electrical point of view. We explained the effects ofdeposition parameters on the film electrical propertiesbased on film composition.V. ACKNOWLEDGEMENTSThis work was financially supported by theFoundation for Fundamental Research on Matter (FOM)in a joined project with DIMES, TU Delft. We wish tothank B. Stannowsky, A. A. I. Aarnink, M. H. H.Weusthof, A. van den Berg and the MESA+ Cleanroomstaff for their help with measurements.VI. REFERENCES[1] C.S. Yang, L.L. Smith, C.B. Arther and G.N.Parsons, Journal of Vacuum Science and Technology BVol.18, p.683, 2000.[2] J. Asmussen, Journal of Vacuum Science andTechnology A Vol.7, p.883, 1989.[3] Y. Kawai and Y. Ueda, Surface and CoatingsTechnology Vol.131, pp. 12, 2000.[4] N. Jiang, B. Agius, M. C. Hugon, J. Olivier and M.Puech , Journal of Applied Physics, Vol.76, p.1847,1994.[5] S. Garcia, I. Martil, G. Gonzalez Diaz, E. Castan, S.Duenas and M. Fernandez , Journal of Applied PhysicsVol.83. p.332, 1998.[6] G. Isai, A. Kovalgin, J. Holleman, P.W. Woerlee andH. Wallinga, Journal of Physics IV Vol. 11 p. 747, 2001.[12] J.R. Flemish, R.L. Pfeffer, Journal of AppliedPhysics Vol.74, p. 3277, 1993.[8] J.C. Barbour, H.J. Stein. O.A. Popov, M. Yoder andC.A. Outten, Journal of Vacuum Science andTechnology A Vol.9, p.480, 1991.[9] D.G. Farber, S. Bae, M. Okandan, D.M. Reber, T.Kuzma and S.J. Fonash, Journal of ElectrochemicalSociety Vol.146 p.2254, 1999.41

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Silicon nitride layers obtained by ECR PECVD

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