HNO3

The wet chemical etching of silicon using HNO3-rich HF/HNO3 mixtures has been studied. The effect of different parameters on the etch rate of silicon,...
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J. Phys. Chem. C 2007, 111, 2133-2140

2133

Study on the Mechanism of Silicon Etching in HNO3-Rich HF/HNO3 Mixtures M. Steinert, J. Acker,* S. Oswald, and K. Wetzig Leibniz Institute for Solid State and Material Research Dresden (IFW Dresden), P.O. Box 270116, D-01171 Dresden, Germany ReceiVed: September 27, 2006; In Final Form: NoVember 8, 2006

The wet chemical etching of silicon using HNO3-rich HF/HNO3 mixtures has been studied. The effect of different parameters on the etch rate of silicon, for example, the HF/HNO3 mixing ratio, the silicon content of the etchant, temperature, and stirring speed in these solutions, has been examined and discussed in light of a previous study on etching in HF-rich HF/HNO3 mixtures. Nitrogen(III) intermediates are generated owing to the dissolution of silicon and the decomposition if the solution is exposed to air. The nitrite ion concentration, measured in diluted etchant solution by ion chromatography, acts as a sum parameter for the reactive N(III) species in the concentrated etchant. The etch rate shows two different correlations to the nitrite concentration. In the region of high nitrite concentrations, the etch rate decreases slightly with decreasing nitrite concentration, whereas at lower nitrite concentrations, the etch rate increases linearly with further decreasing nitrite concentration. Stirring experiments and the determination of activation energies show that the etching of silicon in HNO3-rich etchants is controlled by diffusion. X-ray photoelectron spectroscopy measurements of the silicon surface after etching revealed a hydrogen termination independent of the concentration of reactive species and the content of HNO3 in the etchant. Si-O containing surface species were not found. A combined electrochemical (injection of holes into the valence band of silicon) and chemical (Si-Si back-bond breaking by an attack of HF) reaction mechanism of silicon etching without generation of SiO2 is proposed.

1. Introduction The production of clean and smooth surfaces plays an increasingly important role in today’s microelectronic industry for the elimination of defects and impurities that can degrade the electronic properties. The HF/HNO3 etching system perhaps is the most widely used isotropic etchant for silicon.1 In the semiconductor industry, HF/HNO3 mixtures were applied in a dipping bath for the removal of contaminations and lattice defects generated by the lapping of silicon wafers.2 Further applications are the removal of work damage or roughness (after sawing of ingots) and the texturing of the surface of multicrystalline silicon wafers for solar cell fabrication.3,4 In the literature, the etching of silicon in HF/HNO3 mixtures is described as a two-step chemical process including (i) the formal oxidation of silicon to SiO2 by nitric acid (eq 1) and (ii) the subsequent dissolution of formed SiO2 by HF (eq 2). The overall reaction, as written in eq 3, shows that formally the only reaction products are water, nitrogen monoxide, and hexafluorosilicic acid.5-7

3Si + 4HNO3 f 3SiO2 + 4NO + 2H2O

(1)

SiO2 + 6HF f H2SiF6 + 2H2O

(2)

3Si + 4NHO3 + 18HF f 3H2SiF6 + 4NO + 8H2O (3) The formalized description of the silicon etching chemistry, as stated above, can be traced back to the original studies by Robbins and Schwartz.5-7 The step of SiO2 dissolution by HF solutions was widely investigated and out of debate.8-14 * Address correspondence to this author. Phone: +49 351 4659-694. Fax: +49 351 4659-452. E-mail: [email protected].

However, the crucial and yet unresolved step in this reaction is the oxidation of silicon by nitric acid. The injection of holes into the valence band of the semiconductor by the reduction of nitric acid at local cathodic sites and by the dissolution of silicon at local anodic sites shows the electrochemical nature of that process and was first mentioned by Turner15 and reassumed by Kooij et al.1 Shih et al. showed that the chemical etching of silicon causes, under certain conditions, the formation of a porous layer that exhibits photoluminescent properties known before only to come from electrochemical etching. The porous layer formation was explained by a mechanism based on the generation and injection of holes into the valence band of silicon.16,17 Abel et al.18-20 suggested, as a mechanistic approach, that several combined equilibria between different nitrogen oxides lead to nitrous acid as the reactive species in the etching process. The occurrence of an induction period, observed by a significantly lower etch rate because of the etching of silicon in a freshly prepared etch mixture, can be prevented by adding NaNO2 as the catalyst5,6 that yields the formation of the assumed reactive agent nitrous acid. Once the reaction has initiated, HNO2 is assumed to be generated autocatalytically by the etch process itself. Kelly and co-workers disputed the HNO2 pathway and concluded the increased etch rate on the formation of a stronger oxidizing agent, the nitrosonium ion NO+.16 Furthermore, unresolved is the question of the final state of HNO3 reduction after its reaction with silicon. A quantitative analysis of the gas atmosphere during the acidic etching of multi-crystalline silicon with HF-HNO3-H2O gave a considerable amount of N2O as a reaction product as well as NO and NO2 (in the absence of air or oxygen).21 This short selection out of the available literature emphasizes the need for a deeper understanding of

10.1021/jp066348j CCC: $37.00 © 2007 American Chemical Society Published on Web 01/12/2007

2134 J. Phys. Chem. C, Vol. 111, No. 5, 2007 the yet only formally described step of silicon oxidation in the etching process. One approach to address this issue was given in our previous studies about the impact of N(III) intermediates on the etching of silicon in HF-rich etch solutions.22,23 It was observed that etch solutions can turn color into blue or green during etching in the cold. By means of Raman and UV-vis spectroscopy, N2O3 was identified to cause the blue color. Furthermore, a complex N(III) species (3NO+‚NO3-) denoted as [N4O62+] is observed in these solutions, and from a linear relationship between etch rate and [N4O62+] concentration, NO+ is considered a reactive species in the rate-limiting step.23 The dilution of an etchant causes the complete conversion of all N(III) intermediates into nitrite ions as quantified by eq 4. This underlines the importance of the nitrite concentration as a useful and easily measurable sum parameter to characterize the reactivity of etch mixtures.

([NO2-])diluted solution ) ([N2O3] + [NO+] + [HN2O3+] + +

[H2NO2 ] + [HNO2])concentrated solution (4) The obtained correlation between the etch rate and the nitrite concentration provided the first explanation that an etch solution of given concentrations of HF, HNO3, and dissolved silicon can behave in an entirely different manner and apparently as a function of time. Hence, the induction period is the consequence of the enrichment of the reactive N(III) species formed during the initially slow dissolution in freshly prepared HF/HNO3 mixtures. The present paper is devoted to the etching of silicon in HNO3-rich, concentrated HF/HNO3 mixtures. A clear differentiation from the previously investigated HF-rich system is done. The presented correlations between etch rate, nitrite concentration, and stirring speed lead to a new insight into the reaction mechanism of isotropic etching in these kinds of solutions. 2. Experimental Section Analytical grade nitric acid (65% (w/w), 14.45 M) and hydrofluoric acid (40% (w/w), 22.77 M) were used for all etch mixtures reported herein as a volume percentage (% (v/v)). A volume of 50 mL of etch solution, prepared by the mixing of certain amounts of HF and HNO3, was placed into widemouthed bottles of HDPE (high-density polyethylene) and thermostated during the series of experiments to the reaction temperature by using a cryostat (Polystat K12-2, Huber Ka¨ltemaschinen GmbH). The pre-aging of an etch mixture (in order to avoid an induction period by the generation of a sufficiently high amount of reactive species) with a defined silicon content was performed by slow dissolution of small silicon wafer pieces of approximately 70 mg (boron doped, thickness 675 µm, resistivity 24-36 Ω‚cm) one after another. Because the next silicon piece was not added until the first one was completely dissolved, the whole procedure of pre-aging up to 1.0-1.4 g of silicon took 2 h. The advantageous side effects are the minimization of an uncontrollable warming up of the mixture during the dissolution as well as losses of SiF4. During the dissolution and the whole series of experiments, the reaction vessel was covered by a Teflon cap. For the determination of the etch rate, a 10 × 10 mm silicon sample ((111) orientation, boron doped, thickness 325 µm, resistivity 10 Ω‚cm) was held between the ends of tweezers and immersed in the etch solution for 5-180 s (tetch). The

Steinert et al.

Figure 1. Logarithmic view of the initial etch rate as a function of the HNO3 content.

reaction was quenched by immersing the sample into a large volume of deionized water with ample rinsing thereafter. To perform this series of experiments under quasi-constant silicon content, the etch time had to be adjusted to the etch rate so that for every etch experiment only 1-7 mg of silicon were dissolved additionally. A typical value for such a series is the total dissolution of 50 mg of Si during all etch rate measurements in a pre-aged solution of an initial content of 1 g of Si. The etch rate r in this paper (eq 5) is the quotient of the etched silicon layer (in nanometers) and the immersion time (tetch). The removed silicon thickness (∆d) is calculated from the mass loss obtained by differential weighing (A ) 1 cm2; F(Si) ) 2.33 g‚cm-3).

r)

∆d ∆m 1 ) tetch tetch AF(Si)

(5)

In some cases, a magnetic stirrer (Micro 20, H+P Labortechnik AG) was used and positioned directly below the reaction vessel (completely immersed into the cryostat cooling liquid). The concentrations of nitrite, nitrate, and fluoride ions in the etch solution were determined by ion chromatography (Deutsche METROHM GmbH & Co. KG) by dilution of an aliquot of 0.5 mL of the etch mixture with deionized water to a factor of 1:5000. All species written in squared brackets in the text or in figures denote concentrations in the unit grams per liter. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI 5600 CI (Physical Electronics) system using a hemispheric energy analyzer. Typical measurement parameters included an excitation with monochromatic Al KR X-rays at 350 W, a measuring area of about 800 µm in diameter, a pass energy of 12 eV, and a base pressure of 2 × 10-8 Pa. The samples were measured immediately after drying, however, after a short (some minutes) transport through air. 3. Results and Discussion 3.1. Etch Experiments. The reliable and precise measurement of the etch rate is only possible in the absence of an induction period. The addition of a catalyst (NaNO2) may help to avoid the induction period;6,7 however, this process does not lead to reproducible concentrations of the N(III) species. A feasible and reproducible way to generate the reactive species for etching is to dissolve a small amount of silicon in a freshly prepared etch solution, an approach that was chosen again in the present study.22 The so-called initial etch rates displayed in Figure 1

Silicon Etching in HNO3-Rich HF/HNO3 Mixtures

Figure 2. Dependency of the etch rate r from the nitrite concentration at different temperatures (m(Si)dissolved ) 1.0 g, 50 mL 50% (v/v) HF, 50% (v/v) HNO3).

were determined after 30-50 mg of silicon had been dissolved in 50 mL of a freshly prepared etchant over a period of 1-5 min. Figure 1 shows the plot of the logarithm of the initial etch rate versus the nitric acid content of the etch solutions. The etch rate maximum divides the system into two regions. The region left of the maximum is synonymous to the already mentioned HF-rich etch solutions (dotted curve) that were previously investigated. Here, the etch rate is controlled by the chemical reaction as shown by the strong dependence on the concentration of the intermediate N(III) species.22 This is contrary to the finding by Robbins and Schwartz who proposed a regime controlled by the diffusion of nitric acid to the silicon surface.5 The linear behavior between etch rate and nitric acid content right of the maximum (dashed curve) points toward a diffusion controlled etch regime in good agreement with results by Robbins and Schwartz.5 All studied HNO3-rich etchants ranging from 60:40 to 10:90 (% (v/v)) HF/HNO3 exhibit a uniform etch behavior with respect to temperature, stirring speed, and concentration of the intermediate N(III) species so that all of these solutions are grouped together for further discussion. All results throughout this paper are exemplarily shown for an etch mixture of 50:50 (% (v/v)) HF/HNO3. In the following studies, the nitrite concentration measured in a 1:5000 dilution of the original etchant serves as the easily accessible sum parameter representing the total content of N(III) intermediates (N2O3 and NO+) stable only in the concentrated etch solution (eq 4).22,23 The etch solutions used for the following kinetic studies were prepared by the slow dissolution of a certain amount of silicon in 50 mL of the etch mixture. After the last piece had been dissolved, the nitrite concentration was measured and considered as the maximum nitrite concentration, serving as a starting point for the kinetic study. The time-dependent decay of the N(III) intermediates by exposing the etch solution to air, and therefore a decreasing nitrite concentration as a function of time, is used to investigate the effect of intermediates on the etch rate. The nitrite concentration and the etch rate were determined at the same time in intervals of 1-2 h after the starting point. Hence, to follow the proceeding series of experiments on a time scale, the x-axis of Figure 2 has to be read from right to left. The influence of temperature in the region between -10 and 35 °C on the etch rate as a function of nitrite concentration is shown in Figure 2. In general, the etch rate increases and the maximum nitrite concentration decreases with rising tempera-

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Figure 3. Activation energy vs nitrite concentration [m(Si)dissolved ) 1.0 g, 50 mL 50:50 (% (v/v)) HF/HNO3].

TABLE 1: Activation Energies of Etch Rate Determined for Different Nitrite Concentrations [NO2-] (g‚L-1) 25 20 14 10 7.5 2.5 EA (kJ‚mol-1) 17 ( 2 19 ( 4 21 ( 4 23 ( 5 24 ( 4 34 ( 4

ture. The decrease in temperature yields a stabilization of N(III) intermediates and therefore higher initial nitrite concentrations. The sole exception shown is the series at -10 °C where the maximum nitrite concentration is lower than that at 1 °C. This is explained by the relatively low etch rate resulting in a comparably long time for the pre-aging step and the simultaneously proceeding degradation of the reactive intermediates (see below). The general shape of all curves in Figure 2 remains unchanged for all temperatures and can be divided into two regions. Beginning from the maximum nitrite concentration, the etch rate decreases slightly with decreasing nitrite concentration denoted as region 2. At a certain nitrite concentration, different for each reaction temperature, the slope of the function etch rate versus nitrite concentration inverts. In this region of low nitrite concentrations, denoted as region 1, the etch rate increases significantly with a further decreasing nitrite concentration. After the complete decomposition of N(III) intermediates, that is, a nitrite concentration close to 0 g‚L-1, the etch rate is similar to or even higher than the initial etch rate at the maximum nitrite concentration. Activation energies in regions 1 and 2 were determined at constant nitrite concentrations. The activation energy undergoes a significant change from a quite low value of 17 kJ‚mol-1 at 25 g‚L-1 NO2- (region 2) to 34 kJ‚mol-1 at 2.5 g‚L-1 NO2(region 1, Table 1). This behavior is explained as a transition from a regime controlled by diffusion at high nitrite concentrations to a regime mainly controlled by the reaction at low nitrite concentrations. As a consequence, the reactive N(III) species are located in excess near the silicon surface and act as a diffusion barrier for the attack of the minority component HF. The results from Figures 2 and 3 lead to the assumption that the intermediary N(III) species are not the rate-determining species in the studied HNO3-rich etch mixtures. To clarify this issue, the etch solutions were stirred at different stirring speeds during the etch rate measurement to examine the role of the mass transport on the etch rate. Figure 4 shows the effect of stirring on the etch rate as a function of the nitrite content at a constant silicon content ([Si] ) 20 g‚L-1) at 1 °C. The lowest curve in Figure 4 displays as a reference the etch rate behavior without agitation. For stirring, the general shape of the plotted functions remains unchanged regardless of the applied stirring speed. The etch rate increases with increasing stirring speed

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Figure 4. Effect of stirring speed on the etch rate r (ϑ ) 1 °C, 50 mL, 50% (v/v) HF, 50% (v/v) HNO3, m(Si)dissolved ) 1.0 g).

Figure 5. Variation of the etch rate r at different nitrite concentrations of the solution in dependence on the Si content (ϑ ) 1 °C, 50 mL of etch solution).

underlining the importance of a forced transport of reactive species to the silicon surface and supporting the model of a process controlled by diffusion. Remarkably, stirring in HFrich etch solutions was found to have an opposite effect since the etch rate decreased with increasing stirring speed by stirring the reactive N(III) intermediates away from the silicon surface.22 Figure 5 compares the etch rate as a function of the dissolved silicon content at a maximum nitrite concentration and at a nitrite concentration close to zero for HF-rich and HNO3-rich etch mixtures. The etch rates decay exponentially with increasing silicon content in the etchant regardless of the nitrite concentration. Figure 5 exemplarily shows, for a 70:30 (% (v/v)) HF/ HNO3 mixture (dotted lines), that the etch rate at a maximum nitrite concentration is considerably higher than that in the absence of N(III) intermediates. In the case of HNO3-rich mixtures exemplarily shown for a 50:50 (% (v/v)) HF/HNO3 mixture (dashed lines) in Figure 5, the displayed curves for the minimum and maximum nitrite concentration are close together regardless of the silicon content in the etchant. This underlines the enormous differences in the impact of the N(III) intermediates on the etching of silicon depending on the composition of the etchant. The surface morphology of each etched sample was determined for a 3 × 3 mm area, and the arithmetic average roughness value (Ra) was obtained. The surface of initially polished silicon samples (Ra ) 0.7 nm) at 1 °C became, owing to etching, significantly rougher regardless of the composition

Steinert et al.

Figure 6. Influence of temperature on the decomposition of nitrite (m(Si)dissolved ) 1.0 g, 50 mL, 50% (v/v) HF, 50% (v/v) HNO3).

of the etchant. In HF-rich etch mixtures, an evolution of the surface morphology, in dependence on the nitrite concentration, was observed.23 At a nitrite concentration above 9 g‚L-1, the etched slices exhibit a smooth morphology with an Ra value of about 0.08 µm. Below 9 g‚L-1 of [NO2-], the etch rate of a HF-rich mixture diminishes with further decreasing nitrite content, and the evolution of etch pits results in a roughening of the surface up to 0.4 µm. In contrast, no changes in surface morphology of etched silicon were obtained in HNO3-rich mixtures yielding Ra values lower than 0.1 µm independent of the nitrite concentration. This finding underlines the polishing property of HNO3-rich etch mixtures. Stirring enhances the etch rate by a forced transport of the reactive species to the silicon surface as shown in Figure 4, but the Ra value remains comparable to etching in a non-stirred solution. This fact is consistent with the supposed diffusional control of the dissolution mechanism in HNO3-rich etch solutions. 3.2. Decomposition of Nitrite. The N(III) generated during the dissolution of silicon is not stable and decomposes by exposing the solution to air. This becomes easily visible since the blue or green-blue colored solutions fade out beginning at the liquid-air interface.22,23 As in the previous experiments, the nitrite concentration measured after the last piece of silicon had been dissolved is set as the starting point at zero time for the kinetic measurements. In Figure 6, the influence of temperature on the nitrite decay for a HNO3-rich etch mixture is shown at a constant silicon content. The higher the temperature, the lower is the initial nitrite concentration at zero time, and the decomposition of the N(III) intermediates proceeds faster. The degradation follows a first-order kinetics as shown by the linear slopes for each temperature in the ln[NO2-] versus time plot in Figure 6. The calculation of an activation energy from an Arrhenius diagram provided a value of 16.6 ( 2.2 kJ‚mol-1 which is comparable to the value for the HF-rich system of 20.1 ( 0.9 kJ‚mol-1.22 Remarkably, N(III) intermediates exhibit a considerably higher stability in HNO3-rich etch solutions than in HF-rich solutions. Table 2 compares the rate constants k and half-life time t1/2 for the decomposition at different temperatures for a mixture consisting of 70% (v/v) HF and 30% (v/v) HNO3 (A) and a mixture of 50% (v/v) HF and 50% (v/v) HNO3 (B) at constant silicon content. To emphasize the observations, the decay of the nitrite concentration was investigated as a function of the etch bath composition at 1 °C. As it can be seen in Figure 7, the mixing ratio of HF and HNO3 has a significant influence on the lifetime of the intermediate N(III) species yielding a decrease of the

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TABLE 2: Rate Constants (k (min-1)) of Nitrite Decomposition (First-Order Reaction) and Half-Life Timea mixture A

mixture B

temperature (°C)

k (min-1)

t1/2 (min)

k (min-1)

t1/2 (min)

-10 1 8 15 25 35

0.0011

630

0.0008 0.0012

866 578

0.0020 0.0024 0.0034

347 289 204

0.0016 0.0017 0.0027

433 408 257

a A: 50 mL, 70% (v/v) HF, 30% (v/v) HNO3, m(Si)dissolved ) 1.4 g. B: 50 mL, 50% (v/v) HF, 50% (v/v) HNO3, m(Si)dissolved ) 1.0 g.

Figure 8. Maximum nitrite concentration in dependence on the etch bath composition and the silicon content (50 mL, 1 °C).

Figure 7. Decomposition of nitrite in various etch mixtures (50 mL etchant, m(Si)dissolved ) 1.0 g, 1 °C).

TABLE 3: Rate Constants k of Nitrite Decomposition (First-Order Reaction) and Half-Life Time for Different Reaction Conditions [50 mL, 50:50 (% (v/v)) HF/HNO3, [Si] ) 20 g‚L-1]

k (min-1) t1/2 (min)

reaction vessel covered by Teflon cap

stirred solution (500 rpm) covered by Teflon cap

tightly sealed vessel

0.0011 630

0.0019 365

2.596 × 10-5 26 704

nitrite decomposition rate k (right plot) with increasing nitric acid content. As a consequence, HNO3-rich solutions remain their color for several days in contrast to the fast decoloration, within 1 day, for a HF-rich solution. The intermediate N(III) species were found to degrade by oxidation at the liquid-air interface under the formation of nitrate ions. The formation of nitrate ions follows a first-order kinetics with exactly the same rate constant as the nitrite concentration decays. The molar ratios between the formed nitrate and the decomposed nitrite for given time intervals amount to values ranging from 0.9 to close to unity pointing to an apparent direct oxidation of the N(III) intermediates. Molar ratios below 1 indicate the existence of a marginal, parallel pathway of nitrite decay, either by disproportionation or by outgassing of nitric oxides into the gas phase.22 The results in Table 3 show that the same mechanism is valid for the decay of the N(III) intermediates in the HNO3-rich solution. Stirring the etch solution at 500 rpm increases the area of the liquid-gas interface, and a higher amount of oxygen accelerates the nitrite decay compared to the nonstirred system by a factor of 2. If the oxygen supply to the etch solution is prevented by a tight sealing of the reaction vessel, an interruption of the nitrite decay occurs. Such an etch solution holds its color at 1 °C up to several weeks.

Although the mechanism of the apparent oxidation from the N(III) intermediates to nitrate ions is yet unresolved, the decomposition of dissolved N2O3(aq) with the generation of NO(aq) and NO2(aq) (eq 6) might be assumed ((aq) denotes that all compounds are present as dissolved species in aqueous media). Nitrogen dioxide or its dimer N2O4(aq) (eq 7) decomposes, by disproportionation in the reaction with water, into nitric and nitrous acids (eq 8). Nitrous acid, only stable in a highly diluted solution and at low temperatures and furthermore not detected by Raman spectroscopy,23 decomposes by disproportionation into nitric acid and NO(aq) (eq 9). Finally, the fraction of dissolved NO(aq) is oxidized by dissolved oxygen near the interface to NO2(aq) (eq 10). Hence, the decomposition of the N(III) species can be understood as a formal oxidation of N2O3 to nitric acid (eq 11).

Figure 8 summarizes the relationship between the etch rate, that is, the rate of the N(III) species generation, and the decomposition rate for these species. Each point in Figure 8 represents a single experiment in which a certain amount of silicon was dissolved at 1 °C and the nitrite concentration was measured thereafter. It should be noted that, as a consequence of the different etch rates, the individual points do not correlate to a uniform time scale. Starting with a freshly prepared acid mixture, silicon is quickly dissolved, and a high concentration is established since the decomposition of the intermediates proceeds much slower than the dissolution of silicon. The more silicon an etch solution contains, the lower is the etch rate (Figure 5). Then, the maximum in the curves shown in Figure 8 is the consequence of a now faster proceeding decomposition than silicon dissolution. With a further increasing silicon content, the maximum nitrite concentration drops because of a further decreasing etch rate so that, in the same time interval, more nitrite is consumed than generated. 3.3. XPS Study of Etched Silicon. The composition of the etchant has a distinct influence on the etch rate (Figure 1) and

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Steinert et al. 4. Conclusions

Figure 9. XPS Si 2p spectra of etched Si(111) samples as a function of etchant composition.

Figure 10. (a) Correlation among etch rate r and nitrite concentration, and (b) related XPS Si 2p spectra of etched Si(111) samples [ϑ ) 1 °C, 50 mL, 50:50 (% (v/v)) HF/HNO3, m(Si)dissolved ) 1.0 g]. Selected samples are numerated.

on the surface morphology of the silicon wafer after the etching process (structuring or polishing). Moreover, if one assumes that the proposed model by Robbins and Schwartz of a twostep dissolution mechanism for Si (oxidation followed by dissolution of generated SiO2) is valid, then the Si surface should be hydrogen terminated after etching in a HF-rich solution and oxygen terminated after etching in a HNO3-rich solution or for a high N(III) intermediate concentration. Etched silicon slices were studied by XPS, and a complete hydrogen termination independent of the etchant composition was revealed (Figure 9). The investigation of etched Si in a HNO3-rich mixture [50: 50 (% (v/v)) HF/HNO3] during the decomposition of N(III) showed also that the surface coverage with Si-H bonds is always present independently of the nitrite concentration (Figure 10). The same result was received in a former study on HFrich etch mixtures.23 The fact that neither Si-O bonds nor Si-F or Si-O-F bonds (103.3-105 eV) could be detected even if nitric acid was present in considerable excess is surprising. If Si-F bonds were formed during the etching of silicon, replacement with Si-OH bonds would occur during water rinsing, and a change in the Si 2p peak should occur.24,25 Only a complete hydrogen termination of the silicon surface and its resistivity against oxidation explains why no native oxide was observed in the Si 2p spectra.

The etching of silicon using HF/HNO3 etch mixtures is characterized by an etch rate maximum at 45% (v/v) HNO3 (Figure 1). The etching process at the HF-rich side is controlled by the chemical reaction by means of the concentration of the reactive N(III) intermediates.22,23 The linear dependence between the logarithm of the etch rate and the composition of the HNO3rich etch mixture with HNO3 contents higher than 45% (v/v) indicates a diffusional control of the etch process (Figure 1). Contrary to HF-rich etch mixtures, the etch rate is widely independent of the concentration of reactive nitrogen intermediates, expressed by the nitrite concentration (Figure 2). However, at nitrite concentrations lower than 5-7 g‚L-1, a strong increase of the etch rate is observed. The activation energy rises gradually with decreasing nitrite concentration and increases suddenly when the nitrite concentration is below a critical limit of 5-7 g‚L-1 (Table 1, Figure 3). This observation is not considered as an indication for a change in the reaction mechanism but rather for a transition from the diffusional controlled etch regime at high concentrations of N(III) [EA ) 17 ( 2 kJ‚mol-1] to a more and more reactionally controlled dissolution of silicon at lower N(III) concentrations [EA ) 34 ( 4 kJ‚mol-1]. Comparable values of the activation energy were determined earlier for the reaction controlled regime of HF-rich etch mixtures as 41 ( 1 kJ‚mol-1 for high nitrite concentrations and 44 ( 6 kJ‚mol-1 for low nitrite concentrations.22 Stirring experiments support this hypothesis because the etch rate increases with enhanced agitation of the etch solution (Figure 4). As a consequence, the reactive N(III) intermediates are not involved in the rate-determining step as it was observed for HF-rich etch solutions.22,23 Moreover, they act as inhibitors in HNO3-rich etch mixtures. Since N(III) intermediates are generated in an electrochemical reaction between Si and HNO3 on the silicon surface, it is very likely to assume that their concentrations are enriched close to the silicon surface. Therefore, the N(III) species prevent the attack of hydrofluoric acid. Indeed the attack of nitric acid molecules should also be hindered, but this has presumably no significant effect on the reaction rate since the N(III) intermediates and HNO3 (N(V)) are both very reactive species. This confirms the results by Schwartz and Robbins for the HNO3-rich etch regime.7 A remarkable outcome of the present study is the discovery of the increase of the lifetime of N(III) intermediates with increasing nitric acid content (Figures 6 and 7), that is, their dramatically enhanced stability against oxidation by oxygen from air, against disproportionation, and against outgassing of NOx. Although the reason for this behavior is still ambiguous, it can be assumed that the intermediates are stabilized by the formation of complex solution species. The existence of [3NO+‚NO3-] in HF-rich mixtures and its role as a stable26 and etch-rate determining intermediate23 support this hypothesis. From the presented results, conclusions for an improved industrial etch process control can be drawn. The behavior of an HNO3-rich etch bath is quite constant insofar as the silicon content does not change significantly during the etching and the concentration of the N(III) intermediates is higher than approximately 7 g‚L-1 NO2-. If the etch solution enriches with silicon during wafer processing, the etch rate decreases as the acids are consumed. However, the etch behavior remains basically the same within one etch batch as long as the nitrite concentration is above the limit. If it decreases, either by outgassing due to a high bath temperature during etching or by a long stagnation of the production process, the subsequent etch process might turn into a less controllable regime. Because of

Silicon Etching in HNO3-Rich HF/HNO3 Mixtures

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Figure 11. Proposed reaction scheme of silicon dissolution as a combined electrochemical and chemical reaction under hydrogen evolution (adapted after Lehmann32).

the low nitrite concentration, the generation of new intermediates during etching, and the warming of the etch bath, the etch behavior is practically uncontrollable. Finally, as an issue of safety, utilized etch mixtures of high HNO3 contents contain large amounts of N(III) intermediates that slowly release over time as nitric oxides. Such solutions can contain dissolved nitric oxides even if they visibly appear colorless. Precaution should be taken to avoid an overpressure in the storing tank or a decomposition of the intermediates by the addition of hydrogen peroxide. Proposal of Reaction Mechanism. The XPS results of etched silicon surfaces are of particular interest for the formulation of a precise model of the wet chemical etching of silicon by mixtures of HF/HNO3 since they demand a revision of the yet generally accepted formalized model of an intermediate silicon oxide formation according to eq 2. Figure 10 shows that neither an etch solution with a high nitric acid concentration nor one with a high amount of reactive N(III) intermediates leads to a detectable formation of Si-O species at the silicon surface. The effect of the etchant composition of freshly prepared etch mixtures with HNO3/HF ratios ranging from 10:90 to 90:10 (% (v/v)) on the surface composition of the etched silicon slice is that no silicon oxide species were found (Figure 9). In any case, the silicon surfaces were found to be completely hydrogen terminated after etching. The consequence is that the etching of silicon considered from the standpoint of silicon proceeds by a uniform reaction mechanism. Then, any other observed effects on the etch behavior arising from the HF/HNO3 mixing ratios, the nature of the reactive species (N(III) or N(V)), the temperature, or the content of already dissolved silicon in the etchant have to be attributed to the chemistry or the transport properties of the etch mixture but do not affect the chemistry at the silicon surface. The summary of the effects, (i) the persistent presence of Si-H termination, (ii) the absence of any Si-O species even in the presence of a highly oxidizing solution species, and (iii) that the etching of silicon in HNO3-rich etch mixtures is limited by the diffusion of the HF species to the Si surface, points toward the mechanism of the divalent electrochemical dissolution of Si in a HF solution at low current densities (Figure 11)27-32 and to the (photo)electrochemical mechanism at low light intensity by Kolasinski and Nahidi.33-35 In the mechanism of divalent electrochemical dissolution, the silicon surface is persistently terminated by hydrogen (the etching at high current densities leading to the formation of an oxide layer is denoted as a tetravalent electrochemical dissolution32). The dissolution is initiated by a hole (h+) introduced by the external voltage and formally located at a surface silicon atom. This allows the nucleophilic attack of the reactive species HF or HF2- under the formation of a Si-F surface bond as the rate-limiting step. Subsequently, the second Si-H bond is replaced by a Si-F bond and the polarized Si-Si bonds are attacked again by two HF2- species. The SiF4 finally formed is complexed by HF as H2SiF6. Two consequences arise if one accepts this argumentation that the etching of silicon by mixtures

of HF/HNO3 follows the divalent electrochemical pathway. First, nitric acid plays the role as external source to supply holes (remove electrons) to oxidize silicon. Then, the reduction of the nitric acid should proceed quite unspecifically since all intermediates are strong oxidizing agents of comparable oxidation potential. Then, the observed reaction products, NO2, NO, and N2O, would be the consequence of these somehow unspecific reductions, and their formation is quite arbitrary depending upon the parameters affecting the lifetimes of the reacting intermediates. Second, the divalent electrochemical dissolution requires the formation of 1 mol of hydrogen per 1 mol of silicon from the recombination of hydridic and protic hydrogen. The only evidence for the formation of hydrogen in silicon etching by HF/HNO3 mixtures is given by Kooij et al.1 The presented experimental results are in full agreement with the (photo)electrochemical mechanism at low light intensity.33 Here, the oxidant injects holes into the valence band of silicon without oxide formation and thereby induces the subsequent nucleophilic attack of HF and HF2-. In contrast to the first mentioned mechanism, no hydrogen is formed. That such a pathway also exists if a chemical oxidant is used instead of light or electrical potential is shown in recent studies of silicon etching by metal ion/HF solutions.34,35 It was found that silicon can be dissolved without the formation of hydrogen if Fe3+ or MnO4- are used as oxidants. This finding underlines again the complex role of the oxidant that goes far beyond that of simply injecting holes into the valence band of silicon. Further efforts, particularly on the evolution of hydrogen among the other gaseous products of HNO3 reduction, are necessary to clarify the open questions and to decide which mechanistic picture is more suitable to describe the chemical etching in HF/HNO3 mixtures. Acknowledgment. The authors thank, in particular, A. Rietig and I. Faltin for their support of the realization of etch experiments and for ion chromatographic measurements. The authors gratefully acknowledge the European Regional Development Fund 2000-2006 and the Free State of Saxony for funding within the project “SILCYCLE” under Contract No. 8323/1293 at the Sa¨chsische Aufbaubank (SAB). References and Notes (1) Kooij, E. S.; Butter, K.; Kelly, J. J. Silicon Etching in HNO3/HF Solution: Charge Balance for the Oxidations Reaction. Electrochem. SolidState Lett. 1999, 2 (4), 178-180. (2) Hilleringmann, U. Silizium-Halbleitertechnologie; 4. Auflage, ed.; B. G. Teubner Verlag: Wiesbaden, Germany, 2004. (3) Einhaus, R.; Vazsonyi, E.; Duerinckx, F.; Horzel, J.; Kerschaver, E. V.; Szlufcik, J.; Nijs, J.; Mertens, R. Recent progress with acidic texturing solutions on different mc-Si materials including ribbons. Proceedings of the 2nd World Conference and Exhibition on PhotoVoltaic Solar Energy ConVersion, Vienna, Austria, July 6-10, 1998; pp 1630-1633. (4) Tool, C. J. J.; Coletti, G.; Granek, F. J.; Hoornstra, J.; Koppes, M.; Riffe, H. C.; Romijin, I. G.; Weeber, A. W. 17% mc-Si solar cell efficiency using full in-line processing with improved texturing and screen-printing. Proceedings of the 20th European PhotoVoltaic Solar Energy Conference, Barcelona, Spain, June 6-10, 2005; pp 578-583.

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