Vibration spectroscopy for surface layers on Si

Applied SurfaceScience 39 (1989) 127-134 North-Holland, Amsterdam

127

VIBRATION S P E C T R O S C O P Y F O R SURFACE LAYERS O N Si F. MULLER, N. SCHWAP, Z, V. PETROVA-KOCH and F. K O C H Phys~k.Department El6, Technische Univemitat Mflnchen, 8046 GarcMn~ Fed. Rep. of Germany

Received l0 April 1989; accepted for publication 24 April 1989 We show that thin infrared-active layer~ on Si can be observed sensitively and with high resolution using FTtR spectroscopy (Fourier-transform infrared), Experiments make use of radiation polarized perpendicular to the interface in a multiple internal reflection geometry.We analyze the ease of thin SiPz on Si to show how signals must be interpreted. We compare the spectra of several native oxides.

LHnlmdnefinn Vibration spectroscopy is a natural tool for the identification c,f polar chemical bonds such as St-O, St-H, etc. For the analysis of monolayers of surface contaminants on Si high sensitivity is needed. The EELS (electron energy loss spectroscopy) measurement has a high inherent sensitivity but suffers from limited resolution. Higher resolution is achieved with IR and especially F T I R (Fourier-transform 11t). The problem with liR-spectroscopy is the low sensitivity of a conventional transmission geometry. Only SiP 2 with thickness greater than 100 A has been successfully measured in transmission {1,2]. With the recog.Mtion that the chemical composition and physical constitution of the interracial region in St-SiP 2 MOS heterostructures decisively controls device properties, there is a challenge to apply F T I R spectroscopy for very thin surface layers. In the literature the question of an ordered interface oxide structure (tridymite) has been raised [3]. It is claimed that native oxides on MBE(moleeular-beam epitaxy)-grown Si have a strained, crystalline interfacial region for the first 1-2 monolayers. Vibration spectroscopy should be able to identify such mt interlayer. It is expected that also OH- or H-related complexes in thermal or CVD (chemical vapour deposition) oxides can be monitored. The experimental challenge is to improve the sensitivity of the F T I R measurement. Polarized light and proper choice of boundary conditions can help. Francis and Ellison [4] discuss the advantages of IR measurements using metallic boundary conditions. H a r f c k and du Pr~ [5,6] use total internal 0169-4332/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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reflection in combination with polarized light. In a very recent publication Olsen and Shimura [7] emp¿o)' Harrick's approach for measuring SiO 2 on Si in the thickness range 20-400 A. The authors note a doublet peak feature with lines centered near 1080 and 1220 c m - k With decreasing thickness down ,o 20 A the high energy hand becomes relatively stronger and they conclude "that the spectral difference resulted from a structural change in the oxide and not from some unknown optical artifact". We propose to explain the effect as straightforward optics in section 2. We follow the basic ideas involved in subband-rcsonance spectroscopy [10,11] and demonstrate its application for IR vibrations of surface layers. We concentrate on natural oxides and surface contaminants on both MBE-grown and pol/shed Si(100) wafer surfaces. Although initially motivated to search for differences in the structure of native oxides on polished and MBE-grown Si surfaces, the work has evolved into a demonstration of "monolayer" sensitivity and a correct analysis of the signals. Section 2 deals with the experiments and interpretation. Section 3 discusses results on several native oxides.

2. E×pefimental and ~natys[s For highest sensitivity the experiments make use of multiple internal reflection geometry and metallic boundary conditions as in the insert in fig. 1. Polarized IR light is focussed on the edge of the Si wafer. It traverses the - 6 . 5 mm long sample after multiple internal reflections from the surface.

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Fig, 1. Experimental arrangement and electric field configurations in the metal- and vacuum.type interfaces (~ i = 900 cm 1). (Note the change of length scale at the Si-SiO 2 interface.)

F M~ller et aL / Vibration spectroscopy for surface layers on Si

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Fig. 2. Dielectric function of SiO2 [3] ~nd calculatt.~d spectra for transnfission and internal reflection, When the latter is metal-clad only p-polarization is avail#_ le to excite vibrational modes in the surface layer. As in the figure, both options of boundary conditions are available. They can be chosen to suit the problem. High sensitivity for excitation with E± results because the boundary condition requites the electrical displacement D± = eE± to be continuous. Inside the thin dielectric the local E± driving the resonance is enhanced. The increase is largest in the speciral range where the real part of the dielectric function has a zero. The resultant absorption signal is frequency-shifted to the vicinity of this zero. Fig. 1 shows how effectively the metallic boundary assures a large Ej. for the p-polarized radiation. In this configuration the signal from the metal-covered surface will dominate. We illustrate the signal enhancement effect and the frequency-shifting by making use of the dielectric fanclion for bulk, vitreous SiO2 at room temperature [8]. For the known Re{e} and Im{¢} we calculate the spectrum for the SiO 2 bending and stretching modes. The two modes are at 810 and 1070 cm -~ w h e ~ Ira{e} has a peak. Fig. 2 gives the transmission spectrum at normal incidence for a single passage through a model "monolayer" of the thi~!:ness 3 ,~. Minima appear in the positions where Ira{e} lias a maximum. Ampli-

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Fig. 3. Internal reflection signal in the p-mode for thicknesses 10, tO0 and l{~} ,~ of the oxide layers, tudes are in the range 10-4-10 -3 of the transmitted signal. In the lower curve we give the result for a single reflection at 45 ° incidence in the p-polarization for metallic boundary conditions. The stretching mode resonance is more than two orders of magnitude larger than in the transmission case. It appears shifted to 1250 cm -1 where Re{c} = 0 . Because of its smaller oscillator strength the bending mode shifts only a small amount. It also lacks the dramatic enhancement of amplitude that occurs when there is a pole of the dielectric function. In fig. 3 calculations for different thicknesses of the SiO 2 film are presented. Increasing from 10 A to 100 and 1000 A shows the structure at 1250 cm -~ to broaden and shift. Note the asymmetric lineshape. For thicknesses above 100 .A an additional peak arises at 1080 cm - l . In films with finite thickness the electric field configuration is changed relative to that in fig. 1. The doublet results from bu:h perpendicular and parallel excitation of the stretching vibration. For very thin layers only the plasma-shifted, perpendicularly excited contribution is observable. This provides a natural explanation for the results in ref. [7]. For experiments with IR light passing in multiple reflections along the wafer, it is necessary to separate small surface signals from resonance absorption occurring in the bulk. For this purpose the metal boundaries are particularly useful. Bulk absorption signals, such as from interstitially dissolved oxygen, phonon absorption, or electronic absorption, wkich occur equally in both the s- and p-modes of polarization can be eliminated by taking the p / s ratio. Since only the p-mode has an E± in the thin surface layer, the ratio will be selectively sensitive to the surface. Fig. 4 illustrates how the ratio emphasizes surface signals. The sample is a FZ(floafing zone)-grown wafer, which is polished, RCA etch-cleaned and handled at ambient conditions before

I~ Mailer er aL / Vibration spectroscopy for surface layers on Si

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Si (100) polashed FZ wafer, T=aBK t.O ~

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Fig. 4. Spectra for p- and s-polarizationand their ratio for a native oxide on Sift00). The wafer has a metalcoating. coating both sides with aluminum. The broad structure centered at 1170 c m - t is expected to contain the contribution of the St-O-St stretching mode. In the discussion related to the following fig. 5 we return to exanfine the energy position of this signal and to relate it to the 1230, em -t in fig. 2.

3. Com~'~son o~"some ~6nalive oxides" In order to demonstrate the use of vibration spectroscopy as an analytical tool for thin ( ~ 10 ,~) surface layers on St, we compare in fig. 5 the p / s spectra of three differently prepared samples. Curve A is tile previously discussed spectrum. The signal obtained from a part of the same wafer for which one side has been overgrown with - 1 ~m of epitaxial Si is shown in curve B. The sample has been rinsed ill water and treated in a RCA etch-cleaning. Natural oxide has been allowed to form before Muminum

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F. MUller et al, / Vibration spectroscopy for surface layers o. Si

500

~000 WavenumbeP [cm-I]

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Fig. 5. Comparisonof the p/s signalsof three differentlyhandled" nativeoxides".SamplesA and Bhave been RCA-etchcleaned. Band C are epitaxialsurfaces. deposition on both sides. C is from a sample that also has an epitaxial surface layer of Si. It has been rinsed only in organic solvents prior to metaUization. In ease C the data have been obtained at 300 K. The comparison of signals A and B with curve C makes clear that the first two spectra contain contributions from species other than SiOz. The large signals in the 800-900 cm -1 range are not linked with the oxide. There ig no signal of such magnitude expected from the calculations in figs. 2 and 3 for me thin, native oxide layers. In particular, their absence in curve C leaves no doubt. The extra signals must come from some contaminant. It is likely to be an OH-related vibration [9]. Originally the motivation for the comparison of samples A and C was the search for vibrational features related to oxide structuring on the MBE-grown surface [3]. Curves A and B show that care must be taken to control and avoid chemical contamination of the surface layer in order to clearly identify the surface oxide vibrations. It appears that only curve C relates closely to the calculations. It has the proper asymmetry of lineshape and very small bending mode contribution expected for the SiO2 surface layer. It is clear that in curve A the stretching vibration is distorted by superposition of an additional signal. Its position should not be compared directly with the calculated 1250 cm -].

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For curve C we note that the peak appears about 20 cm - ~ below the expected value. This does imply some small difference between the dielectric properties of this native oxide film and the vitreous, blflk oxide for which th,; calculations apply. We can say with some certainty that there is nothing special in the vibration spectrum of the native oxide on MBE St. ~'t appears unlikely that it grows pscudomorphically for even two atomic layers. On the other hand, fig. 5 does show that vibration spectroscopy can provide a fingerprint identification of the state of the surface layer. In the course of the work we have learned that the infrared spectra must be carefully modelled in order to obtain the vibration frequencies. The plasmashifting effect must bc identified and linked with the oscillator strength in order to conclude anything about the frequency. In this sense the perpendicularly-excited spectra arc more difficuR to interpret than simple transmission at normal incidence. The present experiments only point out the possibilities and a great deal more can be done. In future work we intend to provide well defined condilions of surface oxidation and contamination. Because metallized surfaces provide a means of applying electric fields in a MOS configuration they permit vibration spectroscopy on the oxides under MOS working conditions. For the very thin oxides intended for use in future devices, surface preparation prior to oxidation is a nontrivial step. It may be that FTIR spectroscopy (along with established methods like EELS) will prove a useful monitoring tool for wafer surface conditions before they are oxidized.

Acknowledgements We thank Professor I. Eisele and Dr. H.P. Zeindl for providing the MBE St. Useful discussions with Dr. Griif on surface contamination arc acknowledged,

References [I] G. Lucovsky,M.J. Mantini, J.K. Srivastavaand E.A. Irene, J. Vacuum Sci. Technol. B 5 (1987) 530. [2) I.W. Boydand J.L~. Wilson,J. Appl. Phys. 62 (1987) 3195. [3] A. Ourmazd, D.W. Taylor, J.A. Reotschlerand J. Bey/c,Phys, Rev. Letters 59 (1987) 213. [4] S.A. Francisand A.H. Ellison,J. Opt. See. Am. 49 (1959) 131. [5] N.J. Harrick and F.K. du Pr6, Appl.Opt. 5 (1966) 1739. [6] N.J, Harrick, J. Opt. Sac. Am. 55 (1965) 851. [7] J.E. Olsenand F. Shimura,Appl. Phys. Letters 53 (1988) 1934. [8] H.R. Philipp, in: Handbook of Optical Constants or Solids (Academic Press. New York, 1985), [9] D. Gr~lf,M. Grundnerand R. Schulz,J. Vacuum Sci.Technol. A 7 (1989)808.

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[10l P. MUller, V. Petrova-Koch, M, Zachau, F. Koch, D. Griitzmacher, R. Mayer, H. Jtirgensen and P. Balk, 8emicond. 8ci. Technol. 3 (1988) 1132. [11] G. Tempel, N. 8chwarz, F. MUller, F. Koch, P. Zeindl and I. Eisele, in: Pro¢. E-MRS 89, Strasbour 8, to be published.