Characterization of different porous silicon structures by spectroscopic ellipsometry

i[ ELSEVIER

Thin Solid Films 276 (1996) 223-227

Characterization of different porous silicon structures by spectroscopic ellipsometry M. Fried ", T. Lohner ", O. Polg~r ", P. Petrik a'~, I~. V~zsonyi ", I. B ~ s o n y ", J.P. Piel b, J.L. Stehle b Research Institutefor Materials Science, P.O.B 49, H-1525 Budapest, Hungary b SOPRA S.A., 26 Rue Pierre Joigneaux, F-92270 Bois-Colombes, France c Fraunhofer-lnstitutfiir lntegrierte Schaltungen, Schottkystr. 10, D-91058 Erlangen, Germany

Abstract The results of multiparameter fitting of spectroscopic ellipsometric (SE) spectra on porous silicon layers (PSL) were connected with the processing parameters (oxidation, etching time, porosity, argon implantation dose). Two optically different types of silicon forms, a bulk-type silicon (c-Si) and polycrystalline-like silicon with enhanced absorption in the grain boundaries (p-Si) needed to be mixed with voids in the appropriate ratio, and the PSL had to be divided in depth in several different sections in order to obtain the best fit. The sectioning reflects the effect of upper and lower interfaces or inhomogeneity in depth. The effective porosity and the sublayer thicknesses are determined with high precision. In the ease of argon implantation, we used the dielectric function of c-Si and implanted amorphous silicon (a-Si) mixed with voids. The regression analysis of SE spectra dearly shows the effect of different doses of implantation. The sectioning reflects that to the full range of argon ions the porous silicon became amorphous and denser. The overall thickness of the originally porous layer also s~gnificantly redtw.ed (from 670 nm to 320 nm) due to the argon implantation. Keywords: Ellipsometry; Silicon; Ion implantation

1. Introduction Non-contact and non-destructive diagnostic methods of thin films are essential for the determination of layer thicknesses and the optical and electrical properties of the films. Recently, spectroscopic ellipsometry (SE) was applied successfully for characterizing porous silicon layers (PSL) [ 1-8]. In this paper we demonstrate the possibility to investigate PSL by SE. The results of multiparameter fitting of SE spectra were connected with the processing parameters (oxidation, etching time, implantation). Two optically different types of silicon forms, a bulk-type silicon (c-Si) and fine-grain polycrystalline silicon [9] with enhanced absorption in the grain boundaries (p-Si) needed to be mixed with voids in the appropriate ratio, and the PSL had to be divided in depth in several different sections in order to obtain the best fit. The sectioning reflects the effect of upper and lower interfaces or inhomogeneity in depth. The effective porosity (the optical model does not distinguish the voids from the non-absorbing oxide) and the sublayer thicknesses are determined with high precision. Comparing the 0040-6090/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD!0040-6090 ( 9 5 ) 0805 8-9

results of the reference sample and samples subjected to oxidation, an increase of the void fraction (effective porosity) is observed. In the case of argon implantation, we used the dielectric function of c-Si and implanted amorphous silicon (a-Si) [ 10] mixed with voids.

2. Experimental PSL was formed on p-type silicon wafers by anodization in ethanol containing aqueous electrolyte with HF content chosen between 12.5 and 35%. The duration of etching was in the range of 10 s to 6 min. The nominal porosity was determined by gravimetry. The ellipsometric measurements were made using a rotating analyzer ellipsometer at Twente University, or at SOPRA with an ES4G device (rotating polarizer type). Rapid thermal oxidation (RTO) was carried out in a highvacuum load-locked, cold-wall stainless-steel process chamber equipped with a Peak Systems LXU-35 arc lamp and pyrometer control. Before the low-pressure oxidation at

224

M. Fried et ai. / Thin Solid Fihns 276 (1996) 223-227

T= 950 °(2 started using ramp-up and cool-down rates of 150 *(2 s - t an 02 pressure of 8 Torr was set. The ion implantation (in the 6 × 10 ~4 to 6 × l& 5 atom era-" range) was made in a Varian-type implanter at a 7 ° tilt angle by electrostatically scanned Ar + beam of 100 keV.

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3. Results and discussion

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A series of samples was prepared with the same porosity (nominally 60%), the etching time, i.e. the layer thickness was varied between nominally 100 nm and 2 mm choosing an anodization duration between 10 s and 3 min 24 s. The spectrum corresponding to the thinnest layer is displayed in Fig. 1. The inset shows the surface of the unbiased estimator using a grid search on the thickness-porosity plane for a simple one-layer model of this thinnest layer. One can observe a local minimum as well as an absolute minimum on the contour projection of the surface on the thickness-porosit)' plane. This shows the importance of proper grid search

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prior to the fitting procedure even in this relatively simple case. Fig. 2 displays the spectra of a sample with a nominal layer thickness of 1 mm. The PSL was divided into three sublayers to obtain proper agreement between measured and simulated spectra.

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Fig. 1, $E spectra of the thinnest PS layer together with the simulated spectrum based on the best fitted parameters (shown in the lower inset) using a two-layer optical model. The upper inset shows the surface of the o'(lhickness, porosity) using a simple one-layer model.

Fig. 3 represents the reference sample (without oxidation), Fig. 4 shows the effect of 120 s RTO. The PSL had to be divided into three different sections in order to obtain the best fit. The insets show the best fitted parameters of the fourlayer model. SE spectra of samples oxidized for 240 s and 480 s are similar. Comparing the results of the reference sample and of the sample subjected to 120 s RTO, an increase of the void fraction and of the surface oxide thickness is observed. (The optical model does not distinguish the voids from the non-absorbing oxide.) The light absorption of the PSL is also reduced on the effect of RTO. The same tendencies can be observed on the longer RTO processing. The effect of anodic oxidation was investigated on a PSL of 1 mm nominal thickness and 68% porosity. Fig. 5 displays the spectra of the as-prepared PSL. Sufficiently good agree-

M. Fried et al. / Thin Solid Films 276 (1996) 223-227

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M. Fried et al. / Thin Solid Fihns 276 (1996) 223-227

ment was obtained between measured and calculated spectra using a four-layer optical model. The spectra of the anodically oxidized PSL are shown in Fig. 6 together with the inset representing the parameters obtained using an optical model of native oxide film and four sublayers. The agreement is better for the tan qe spectra than for cos A spec,a. The ,'onsiderable increase in void fraction ~ ows the effect at oxiclarion. The enhanced oxygen content was also observed by FTIR-SE.

3.3. Effect o f ion implantation

In the case of argon implantation, we used the dielectric function of c-Si and implanted amorphous silicon (a-Si) mixed with voids. The unimplanted reference has nominally less than 50% porosity, but the SE measurement showed similar porosity to the ps273 on Fig. 6. It seems to be oxidized in the air during storage. Its thickness was 670 nm, the porosity was 77%. This relatively high amount of oxygen cannot be separated from the " r e a l " porosity, so the voids mean a mixture of voids and silicon oxide.

The implanted PS layers were divided four parts. The deepest, next to the bulk, was considered as c-Si mixed with voids, the upper three proved to be a-Si (even in the case of the lowest dose) mixed with voids. The regression analysis of SE spectra (Fig. 7) clearly shows the effect of different doses (in the 6 × 1014 to 6 × 10 ~s atom c m - 2 range) of implantation. (We think that the quality of the fitting can be enhanced by using more sophisticated optical models.) The sectioning in the inset reflects that from the surface to the full range of argon ions the porous silicon became denser. The effect of densification is seen mainly at around the equivalent (considering the porosity) projected range. (The full range of 100 keV Ar is about 150 nm in compact silicon. This thickness is in good agreement with the values in the inset of Fig. 7, in the case of the highest dose.) Integrating the full amount of silicon in the inset of Fig. 7 (thickness × volume fraction) we obtain nearly the same value at all doses with only a 10% statistical error. The overall thickness of the originally porous layer also significantly reduced (from 670 nm to 320 nm) due to the argon implantation. This thickness reduction was detected independently by step height analyser in a similar experiment [ 11 ].

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It is demonstrated that SE is very sensitive to changes caused by different processes (oxidization, etching time, ion implantation) in porous silicon layers. Using appropriate multilayer models, one can evaluate the thicknesses, effective porosity and different phases, even in the case of inhomogeneity in depth like ion implantation caused densification.

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Acknowledgements

Partial support from OTKA grants (N#F4378 and N#T016821 ) is greatly appreciated. Joint work with SOPRA was supported by BALATON-APAPE exchange program.

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References

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Wovelength [nrn] Fig. 7. SE spectra of selected argon implanted PS layers. Results of SE analysisof argon-implantedPS layers are shown in the inset. The first (next to the bulk) sublayeris consideredas c-Si mixedwith voids,the upperthree as a-Si mixed with voids,

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