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aPhys. Status Solidi C 8, No. 3, 784–787 (2011) / DOI 10.1002/pssc.201000343

Investigation of current-voltage characteristics of p-type silicon during electrochemical anodization and application to doping profiling Ahmed Gharbi* , 1, Boudjemaa Remaki1, Aomar Halimaoui2, Daniel Bensahel2, and Abdelkader Souifi**, 1 1 Université de Lyon, Institut des Nanotechnologies de Lyon, INL-UMR5270 CNRS, INSA de Lyon, 69621 Villeurbanne, France 2 STMicroelectronics, 38920 Crolles, France

Received 18 May 2010, revised 29 June 2010, accepted 1 July 2010 Published online 25 January 2011

Keywords porous silicon, electrochemical anodization, I-V characteristics, doping profiling ** Corresponding author: e-mail ahmed.gharbi@insa-lyon.fr, Phone: +33 472 436 443, Fax: +33 472 436 082 ** e-mail abdelkader.souifi@insa-lyon.fr, Phone: +00 472 438 962, Fax: +33 472 436 082

The analysis of anodic current-potential I-V characteris-tics of p-type silicon in concentrated hydrofluoric acid solution shows that the mechanism at the origin of porous silicon PS formation is determined by the charge ex-change at the silicon surface over a Schottky barrier formed at the interface through a thermionic emission process. In addition, a negative potential shift in the I-V characteristics is observed with increasing doping con-centration which can explain the selective formation of PS towards the doping level of silicon. In fact, when a silicon wafer of different regions of doping concentration is anodized at a fixed current or potential, the dissolution current will be more important in regions of higher dop-

ing level and PS formation can only take place there. The property of selectivity is used in this work in order to de-termine the p-type doping impurity profiles. During ano-dization under galvanostatic conditions of a specific test sample presenting dopant concentration variations, it is shown that the measured anodization potential varies ac-cording to silicon doping concentration. Consequently, by converting potential values to dopant concentration, and the electrolysis time to a depth scale, a doping impu-rity profile can be extracted. The profile using this tech-nique appears to be in a good agreement with the secon-dary ion mass spectrometry (SIMS) analysis.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Porous silicon PS is known to form during electrochemical dissolution of silicon in hydroflu-oric acid (HF) solutions. This material has attracted in-creasing interest for a wide spectrum of potential applica-tions since its discovery 5 decades ago. The most promis-ing applications are related to photoluminescence and gas sensitivity [1, 2]. In fact, by controlling the porosity, it is possible to control the color of the emitted light and from the large surface to volume ratio of PS, one can perform gas sensors. Furthermore, one can mention Bragg reflec-tors and Fabry-Perot filters realized by a modulation of the PS porosity as well as the new perspectives for micro-machining of free standing silicon structures in MEMS fabrication since the PS layer can be a sacrificial layer formed by anodization and selectively removed underneath a silicon membrane [3]. The material can be also used for the electrical isolation of CMOS circuits in the so called

“IPOS” « Isolation by Porous Oxidized Silicon » process [4, 5] where local p+ wells containing in p-region can be selectively turned into PS during anodization and then oxi-dized. The latter property of PS formation selectivity, which is explained by a dependence of the anodization po-tential vs. the doping level of silicon, is also used for the determination of impurity concentration profile in p-type silicon [6].

In the first part of this work, current-voltage I-V char-acteristics during electrochemical anodization of p-type silicon in the condition corresponding to PS formation are analyzed and then discussed according to a model pro-posed by Gaspard et al. [7] in order to understand the property of PS formation selectivity with doping concen-tration. In the second part, we take advantage of this prop-erty to determine the p-type doping profile of a specific

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test sample using the recorded anodization potential during PS formation at a constant current density.

2 Experimental Anodization of silicon samples is

performed in a single-tank Teflon cell, in the dark, at room temperature. The silicon sample surface area in contact with the electrolyte is 0.5 cm2 and the counter electrode is a cylindrical platinum grid, parallel and far enough to the silicon electrode to ensure uniform anodic current distribu-tion. The electrolyte is a mixture of one volume of HF 49% weight and one volume of absolute ethanol; so an ethanoic solution of 25% HF.

For the electrical connection, a three-electrode configu-ration with an Ag/AgCl reference electrode is used; the current is supplied between the working and the counter electrode and the potential of the working electrode is measured against the reference electrode. Taking advan-tage of its small dimension and its resistivity to HF degra-dation, the reference electrode is maintained close to the anodized silicon surface. So, there is no need for ohmic drop correction due to the electrolyte resistivity when tak-ing potentiostatic (i.e. scanning of the potential) I-V char-acteristics. All potentials values and the notation V given in the text are referred to the working electrode potential vs. the reference electrode.

All wafers are CZ grown, <100> oriented, boron doped silicon. Two different types of samples are studied, pre-senting different doping concentration profiles. The first type is bulk silicon substrates presenting uniform doping concentrations. Substrates with different doping levels ranging between 1 x 1015 cm-3 and 4 x 1019 cm-3 are ana-lyzed. The second type of sample studied in the second part of this paper is a specially designed wafer with a stair-case doping profile containing three p-type regions of 250 nm width and doping concentrations of 1 x 1017 cm-3, 1 x 1018 cm-3 and 1 x 1019 cm-3 which are grown by RP-CVD (reduced pressure chemical vapour deposition) over a sub-strate of doping concentration of 5 x 1015 cm-3. For all wa-fers, an ohmic back contact has been achieved by alumin-ium vacuum deposition followed by a rapid thermal an-nealing at 700 °C under nitrogen atmosphere.

3 Current-potential characteristics of p-type

silicon during PS formation The first part of this paper is devoted to the analysis of the current-voltage I-V charac-teristics measured on a p-type silicon electrode during anodization in the regime of PS formation. Figure 1 shows typical characteristics over a large current density range obtained for samples with different doping concentrations. It is important to mention that successive potential sweeps on the same electrode lead to reproducible I-V characteris-tic. This suggests that the potential drop in the electrolyte within the porous silicon network can be neglected that means that the anodization potential is not modified by the thickness of the formed PS layer and is only determined by the electrochemical reaction at the silicon electrolyte inter-face.

Figure 1 Anodic current-voltage characteristics of p-type silicon in hydrofluoric acid solution for different doping concentrations.

Three different regimes can be distinguished from the

semi logarithmic plot of the I-V curves as shown in Fig. 2. The regime observed for current densities of about 1 mA/cm2 to 100 mA/cm2 corresponds to a linear variation with a slope of 59 mV/decade at room temperature, in agreement with a classical metal/semiconductor contact slope of (kT/q) x Ln(10) (where q is the electron charge, k the Boltzmann constant, and T the absolute temperature). This result is indicative, as suggested by several authors [1, 7-9], of a current which flows during PS formation regime over the Schottky barrier at the depleted silicon surface through thermionic emission.

Figure 2 Logarithmic variations in anodic current as a function of anodization potential. Full line is drawn with a slope corre-sponding to a variation of 59 mV/decade.

The regime corresponding to low current densities be-low 1 mA/cm2, and in which no porous silicon is formed, is attributed to the leakage current of the junction. Whereas, the one obtained at higher current densities, and in which the current increases vs. the electrode potential slows down, is related to the ohmic drop due mainly to the substrate re-sistivity. Indeed, it must be kept in mind that a few tenths of an ohm can correspond to a potential drop of several tens of millivolts at current densities of about 100 mA/cm2.

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786 A. Gharbi et al.: Current-voltage characteristics of p-type silicon during electrochemical anodization

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However, the simple Schottky theory cannot solely ex-plain the shift observed in the I-V characteristics towards negative potential when the doping concentration increases. Gaspard et al. [7] have attributed that to an increase in the potential drop across the Helmholtz layer present at the sil-icon electrolyte interface. In fact, according to the Schottky model, the current which is limited by the majority carrier supply over the potential barrier Ψs at the silicon surface can be expressed as

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The variations in the Helmholtz drop voltage VH re-main very small compared to the anodic potential of a same sample. Therefore, the linear regime in the semi loga-rithmic plot of the I-V characteristic is always obtained whatever the doping concentration is. On the other hand, the doping level influences the value of K’’ through VH, and so the different characteristics obtained for different doping concentrations will be shifted with respect to each other by an amount equal to the VH variations resulting from the concentration variations.

4 Application of PS formation selectivity to

impurity profiling The aim of this part is to present an impurity profiling method based on simple potential meas-urements during the electrochemical anodization of a p-type silicon sample which presents staircase doping con-centration variations.

As discussed in the first part, the anodization potential corresponding to the porous silicon formation regime is found to be determined by the interface electrochemical reaction and dependent on the silicon doping level. This is also confirmed by the measurements of the anode potential versus the electrolysis time during PS formation at con-stant current density (10 mA/cm2) for substrate of different doping levels. Figure 3 shows that the electrolysis potential remains constant and independent of the formed porous layer thickness, up to several microns and is found to de-crease when the doping level increases.

Consequently, if porous silicon is formed at a constant current density in silicon presenting dopant concentration variations, the anodization potential is expected to vary ac-cording to the concentration profile. Figure 4 shows the re-

Figure 3 Electrode potential vs. electrolysis time during porous silicon formation on homogeneously p-doped substrates of differ-ent doping concentration at constant current density (10mA/cm2).

cording of potential variations during the anodization of the staircase doping sample at a constant current density of 10 mA/cm2. These variations correspond well to the pene-tration of PS/Si interface into a depth of varying doping level.

Figure 4 Potential variations during anodization of a staircase doping profile sample at constant current density of 10 mA/cm2.

In order to transform this potential recording into an impurity profile representation, converting of potentials to doping concentrations and the electrolysis time scale to a depth scale has to be done. Conversion of potentials is achieved using the proportionality of the anodization po-tential V versus the square root of the doping concentration Nsub which has been theoretically justified in [7] and ex-perimentally obtained according to Fig. 5.

In fact, as in practical profile analysis where the bulk concentration is generally known, one can have the doping concentration of the staircase sample substrate as a known value, and so by taking the constant potential correspond-ing to the anodization of the bulk of the sample as a refer-ence, one can determine the doping concentration related to each measured potential at a given time of anodization just by measuring the potential shift from this reference value.

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Figure 5 Variations of anodization potential V for a current den-sity of 10mA/cm2 as a function of the square root of the silicon doping concentration Nsub. Full line is the calculated curve ac-cording to Gaspard et al. [7]. The profile determination necessitates also the knowledge of the anodized depth as a function of time during elec-trolysis. This depth scale has been obtained by determining the thickness of the porous layer formed on bare p-type silicon samples of different doping concentration anodized at constant current density of 10 mA/cm2. The results ob-tained show that the rate of PS formation is first constant with depth, at least for thickness of several microns and it also varies little over a range studied of doping concentra-tion from 1 x 1017 cm-3 to 1 x 1019 cm-3. Consequently, as-suming a same PS rate at 10 mA/cm2 equal to 0.5 µm/min for all doping levels in the staircase sample, one can obtain a resulting doping profile of the staircase sample as shown in Fig. 6.

Figure 6 Impurity profile of the doping staircase sample as de-termined by the porous silicon method (curve a) and compared to SIMS analysis (curve b).

Through these results, profiling by using PS method formation appears to be a very sensitive method which gives accurate profiles comparable to SIMS analysis. Al-though the low resolution obtained, the method is of inter-est in that it is much simpler than other techniques, mainly because it only requires a very simple electrochemical

anodization in a hydrofluoric acid solution and no complex apparatus.

5 Conclusion This paper presents experimental re-

sults arguing the idea that the mechanism at the origin of PS formation on p-type silicon in concentrated hydroflu-oric acid solution is determined by the charge exchange at the silicon surface over the Schottky barrier which is formed at the interface through a thermionic process. An-other important result obtained is the I-V characteristics shift with the silicon doping concentration which is ex-plained by the potential drop variations across the Helm-holtz layer present at the electrolyte/silicon interface. This result was at the origin of an impurity profiling method us-ing electrochemical anodization: in fact, it was shown that monitoring the electrode potential during PS formation at a constant current density appears to be well suited for pro-filing impurity concentration in a p-type doping staircase sample.

Such a shift can also explain the selectivity of porous silicon formation towards the doping level of p-type sili-con: with a correct choice of anodization potential, the dis-solution reaction can be restricted to the higher doping level region in a sample of different doping concentrations regions. So that, as a new approach for SOI substrate fabri-cation, a buried highly doped p+ layer in p-/p+/p- structure can be preferentially anodized and subsequently oxidized in order to produce silicon islands on a silicon dioxide layer. Our next work will be focused on this application with trying to extend it for n-type doped silicon.

Acknowledgements The authors gratefully acknowledge Dr. C. Dubois and Pr. B. Gautier for SIMS analysis. This study has been performed in the frame of CNRS/STMicroelectronics partnership.

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