A systematic study on metal-assisted chemical etching of high aspect ratio silicon nanostructures

Accepted Manuscript A systematic study on metal-assisted chemical etching of high aspect ratio silicon nanostructures Ali Ghafarinazari, Masoud Mozafari PII: DOI: Reference:

S0925-8388(14)01623-5 http://dx.doi.org/10.1016/j.jallcom.2014.07.044 JALCOM 31672

To appear in:

Journal of Alloys and Compounds

Received Date: Revised Date: Accepted Date:

9 May 2014 21 June 2014 5 July 2014

Please cite this article as: A. Ghafarinazari, M. Mozafari, A systematic study on metal-assisted chemical etching of high aspect ratio silicon nanostructures, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/ j.jallcom.2014.07.044

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A systematic study on metal-assisted chemical etching of high aspect ratio silicon nanostructures Ali Ghafarinazari 1,2,*, Masoud Mozafari 2,** 1

Department of Mechanical and Industrial Engineering, University of Brescia, Via Branze 38, 25123 Brescia, Italy 2

Bioengineering Research Group, Nanotechnology and Advanced Materials Department, Materials and Energy Research Center (MERC), P.O. Box 14155-4777, Tehran, Iran *

Corresponding author: E-mail: [email protected] Tel.: (+39-38) 86598606 **

Co-corresponding author: E-mail: [email protected] Tel: (+98-912) 6490679 Fax: (+98-263) 6280033

Abstract Metal-assisted chemical etching as an anisotropic wet etching method is an important step in semiconductor device processing capable of producing high aspect ratio semiconductor nanostructures. Silicon (Si) nanostructures as the most important material for current semiconductor industry, have been widely used in different ways. This study describes experiments on the etching rate and morphology of Si nanostructures produced by metal-assisted chemical etching approach. In addition, the effects of the synthesis conditions such as noble metal and hydrogen peroxide concentration were investigated by Taguchi method. The obtained results indicated that the rate of etching and homogeneity of the structures assisted by silver was much better than the structures assisted by platinum. Keywords: Metal-assisted etching; Silicon nanowires; Hydrogen peroxide; Taguchi method 1

1. Introduction Semiconductor nanostructures are well-documented as promising building blocks for many devices, since they are promising candidates for improving the performance and cost of several types of electronic devices. In particular, silicon (Si) nanostructures with high aspect ratios are continuously explored for the applications in solar cell [1, 2], chemical sensing [3-6], thermoelectric [7-9], photovoltaic [10], bone growth media [11] etc. Since controllable fabrication of Si nanostructures is a requirement for their device applications, several approaches have been developed for controlled fabrication of Si nanostructures [10, 12]. In this regard, several methods have been developed to fabricate Si nanostructures using top-down or bottomup approaches. Metal-assisted chemical etching is a simple, low-cost and powerful technique, offering better controllability of structural parameters without electrical bias [1, 13]. Based on the mechanism of this technique, metal can drill through Si if the metal stays closely in contact with the substrate. This makes the aspect ratio of the structures basically related to the etching time. Under appropriate etching conditions, the metal ions are reduced, and the ions inter the holes into the valence band of the Si substrate. This localized microscopic electrochemical processes leads to self-assemble nanowire [1] or nanoporous [14] structures, via formation of an intermediate Si oxide formation [11] (see Figure 1).

(Figure 1)

To the best of our knowledge, no explicit mechanism for evaluating the difference in the etching rate and morphology of Si nanostructures has yet been well-described in the literature. Thus, Taguchi method has been used as a robust design approach for the understanding of the effects of different factors on the synthesis of Si nanostructures. The Taguchi method, 2

established by Genichi Taguchi (1924-2012) [15], has been generally adopted to optimize the design variables because this systematic approach can significantly minimize the overall testing time and cost [8]. In general two different kinds of noble metals are usually used in this approach [12]. The first group makes dendrite structures on the surface of Si substrates that contains silver (Ag) and gold (Au). The second category makes a thick layer on the surface, such as platinum (Pt) and copper (Cu). In this study, Ag and Pt were selected for the purpose of comparison of the two groups; because Ag [11, 16-18] and Pt [10, 19] are more common. Beside, three different factors such as time, temperature, and H2O2 concentration were selected as conventional parameters [12]. Due to the selection of four factors, i.e.: noble metal, time, temperature, and H2O2 concentration, with the three levels, full factorial of this process requires 81 (34) experiments to achieve the main effects. In contrast, Taguchi method chooses only 9 tests [20]. While, response surface methodology, as another robust design method, requires 27 experiments [21].

2. Experimental procedure 2.1. Material preparation The Si nanostructures were synthesized on a single-crystalline [100] p-type wafer (15-25 Ω cm, 79.2 mg, 520 mm, 675 µm thickness). Initially, the wafer ultrasonically washed in acetone and isopropyl alcohol for 30 min, respectively. Then, it immersed in a mixture of 5 M hydrofluoric acid (HF) aqueous and 0.02 M noble metal from silver nitrate (AgNO3) and/or hexachloroplatinic (IV) acid (H2PtCl6), along with H2O2 aqueous solution, in different time durations and temperatures. All of the parameters with levels are listed in Table 1. Following the wet electroless etching process, the wafer wrapped with a thick film of noble metals. The asprepared samples were dipped into a 30 wt% nitro-hydrochloric acid aqueous solution to remove 3

the capped metal. Finally, the samples were cleaned with deionized water and blown dry with nitrogen. The surface and cross-sectional observations of the samples were performed with the field emission scanning electron microscopy (SEM) by a LEO 1521 SEM-FEG high resolution instrument equipped with and an energy dispersive X-ray spectroscopy (EDX, Rontec, Germany) device for chemical analysis.

(Table I)

2.2. Experimental design by Taguchi method Taguchi method utilizes orthogonal arrays in order to simplify designing process and statistical calculations. In this study, four factors with three levels of each factor have been evaluated (Table 1), and appropriate arrays designed by Taguchi method. Table 2 shows each experiment with parameter conditions. At the end, the model was validated by some other experiments.

(Table 2)

From the quality point of view, there are three possible categories of the characteristics: 1) smaller better; 2) nominal better; and 3) bigger better. The Taguchi method uses the signal-tonoise ratio (S/N) to express the scatter around a target value. A high value of S/N implies that the signal is much higher than the random effects of the noise factors. The noise is usually due to the uncontrollable factors, which exist in the environment, often cannot eliminate and causes variations in the output. In this study, the main goal of the design was to maximize the etching

4

performance, i.e. less residual sample mass; the S/N ratio with “smaller better” characteristics was required, which is given by [22]:

1 n S / N = −10log (∑ Yi 2 ) n i =1

(1)

where n is the number of repetitions under the same experimental conditions, Y represents the results of measurement, which is the mass of the sample. While, we carried out just 9 experiments, instead of 81, the simulation of other results is possible with the prediction formula of Taguchi method [23]:

L

S / N P = S / N m + ∑ [ S / Ni − S / N m ]

(2)

i =1

In Equation (2), S/Nm is the total average of S/N ratio, S/Ni is the mean S/N ratio at the predicted level, and L represents the number of main design parameters that affect the quality characteristics. Utilizing of this equation for validation of statistical calculation is required. The analysis of mean statistical approach adopted here in order to construct the main effects. Therefore, the mean of the S/N ratio of each factor at a certain level should be calculated. For =i example, Mlevel the mean of the S/N ratio of factor I at level i, given by: factor = I

=i M level factor = I =

1 k Ii

k Ii

∑ [S / N

level = i factor = I

(3)

]j

j =1

5

=i where kIi represents the number of appearances of factor I at level i, and [ S / N level factor = I ] j is the S/N

ratio of factor I at level i; its appearance sequence in Table 1 is the jth. By the same measurement, the mean of the S/N ratios of other factors were determined at a certain level.

3. Results 3.1. Before acid washing On morphological point of view, the samples have been divided into three groups based upon noble metals. In the samples #1-3 that silver implemented, dendrite structure created on the Si samples (Fig. 2a). Due to the low stability of this layer, few seconds of acid washing is needed for the removal of the layer, but Ag particles also exhibited on the Si nanowires (Fig. 2b). Opposite of Huang et al. outputs [1], the EDX results confirmed that removing these particles completely was not achieved after just one minute acid washing. In the samples #4-6, Pt makes a thick layer on the surfaces (Fig. 2c). Unlike the previous samples, the Si structures had no uniform shape under the coating layer. Removing of Pt was harder than Ag, because of the higher chemical stability of Pt, which makes this a close-packed layer. Therefore, the time of acid washing increased to 12 hours for all of the samples [9]. The progressive coating structure was for the samples #7-9 (when Ag and Pt utilized together). In this noble structure, there was a thick layer of platinum on the surface of the Si substrate (Fig. 2d). The wire shape precipitated on this layer based on the existence of Ag (Fig. 2e). In comparison with the Ag dendrites (Fig. 2a), this structure is not branched and it is thicker, suggesting that Pt is covered the entire surface of the silver and act as a barrier. In general, this structure is very promising for photocatalysis applications.

(Figure 2) 6

3.2. After acid washing The samples were also categorized into 3 groups based on noble metals. As it can be seen in Fig. 3a, the sample #1 has a Si nanowire forest with 29.47 nm diameter and 1.64 µm length in average (R = 55) with high homogeneity (standard deviations are 5.27 and 0.06, respectively, at more than 10 nanowires from 3 different pictures). By increasing the etching conditions at the sample #2 (Fig. 3b) the micro wires of Si were created, with an about 382 nm diameter and 58.2 µ m height (R = 152). In comparison with the sample #1, not only aspect ratio (R) increased but also the distance between the wires increased, indeed the sample #1 has a very close-packed structure based on less amount of corrosion. As can see in Fig. 3c, the morphology of the sample #3 altered to a totally rough and non-homogenous structure with about 68% thickness reduction. This wire-shaped structure has 12 µm diameters, is full of nanoporosity with about 90 nm diameter (Fig. 3d). In the literature, no observable porous layer was found around the pores or wires etched from Ag-coated substrates [24]. It seems that, boiling of the solution is due to high temperature and interaction with H2O2 molecules.

(Figure 3)

The implementation of Taguchi design (Table 2) leads to complicated structural comparisons in the samples. In the second three samples, which Pt just used, the degree of homogeneity was scarce. Finding wires could be possible in only some parts of the samples #5 and 6, even in the sample #4 there was no wire (Fig. 2c), that could be related to matter of low time duration. Moreover, the sample #5 (Fig. 4a) has nanowires about 62.5 nm diameter and 13.5 µ m length (R = 216). In addition, the sample #6 (Fig. 4b), which has more time and H2O2 concentration than the sample #5, has higher aspect ratio (R = 755) with overall 93.25 nm 7

diameter and 70.46 µm length; and also, large porosity areas (Fig. 4c) with about 50 nm holes diameter [25, 26]. The sample #6 is a prime example of inhomogeneity from Pt, nanowire and porous area.

(Figure 4)

The comprehension about the last three samples is much more complex on account of the fact that both noble metals utilized together, and therefore the design of other parameters is more confusing. The microstructure in these cases is very incongruous; some areas have wires, and the others have porosity. Therefore, discussion based on one image from a limited area would not be valid.

3.3. Statistical calculations The weight of the samples after 12 hours acid washing has been utilized, that is related to the corrosion rate, in order to understand the effect of each factor in this complex reaction [11]. The amounts of Yi in Table 3 represent the mass of the samples after acid washing. With the aim of decreasing the errors of the tests, each experiment repeated three times [27]. By substituting the results of the measurement and the number of experimental repetitions (n = 3) into Equation (1), the S/N ratio of each test condition was determined (Table 3). Subsequently, the values of the S/N ratio were substituted into Equation (3) and the mean of the S/N ratios of a certain factor in level=i the ith level, M factor = I , was obtained in order to evaluate the main effect of each factor (Fig. 5).

The outcome of main effects after validation in section 3.4, are explained in section 4. (Table 3) (Figure 5) 8

3.4. Model validation The Taguchi method represents a model for the estimation of other experiments in which Equation (2) can be implemented. Then, it should be validated by using protocols already reported [8,20, 22]. The method could apply on samples prepared by Ag as etchant, due to final uniform structure. As a first validation (V1), we carried out a new synthesis like conditions of the sample #4, but Ag utilized as assisted instead of Pt (Table 4). In order to predict the result of this condition by Taguchi formula, an average of S/N, S/Nm, in Table 3 is 25. In addition, the main effect of etchant in the case of Ag is 29.51 (Fig. 5); and the main effects of time, temperature, and H2O2 are 22.79, 22.96, and 29.32 respectively. Therefore, S/NV1 is equal to 29.58 (25 + (29.51 - 25) + (22.79 - 25) + (22.96 - 25) + (29.32 - 25)). By implementation of Equation (1) in reverse, i.e. Y = 10-(S/N)/20, the mass amount predicted about 33.18 mg which is very near to real data from experiment that was 34.7 mg (Table 4). As it is clear, the prediction of Taguchi in this complicated condition is valid. From morphological point of view, Fig. 6a, in comparison with the structure of the sample #4, which has no wire or porosity (Fig. 2c), the V1 has nanowires about 130.84 nm diameter and 9.35 µm length (R = 71.46). As it can be seen, the noble metal completely changed the structure and weight as a main parameter.

(Table 4) (Figure 6)

For further validation, two other experiments carried out (V2 and V3). As can be seen in Table 4, the amounts of the sample weight were near the prediction results. Based on Fig. 6b, the V2 has nanowires about 89.28 nm in diameter and 8.19 µm in length (R = 91.76). The V2 parameter conditions were near the sample #2 except for temperature. As a comparison in 9

FESEM micrographs, Fig. 3b and 6b, it can be noticed that temperature could have significant effects on the size of nanowires and also a little on the distance of wires. In fact, with increasing the temperature, the size of wires increased in all dimensions. The other validation test, V3, carried out similar to the sample #3, but without H2O2. Given in Table 4, mass amount of V3 is very close to the estimated result by the Taguchi method. In nutshell, this statistical calculation provides supports for the modeling and simulation of complicated reactions at the selected conditions, particularly for the main effect behavior that is the fundamental of the discussion section. According to our experiments and the reported works in the literature, at high temperatures, the decomposition of H2O2 is the main reason for the creation of porous surfaces [12, 28]. It is clear that, V3 has the biggest wires between the samples with the length of 107.0 µm and the diameter of 458 nm (R = 233.6), with high degree of homogeneity as compared the wires formed by Pt or Ag-Pt assisted.

4. Discussion Based on the experimental results and validated statistical calculations concerning getting the main effect of each factor, it is possible to discuss about the influence of each factor individually, and compare them with the reported data in the literature.

4.1. Effect of noble metal It is known that noble metals participate as catalysts during the Si etching process in HF solution. The reduction of metal nanoparticles and the anodic dissolution of Si at the surface of the samples occur simultaneously [13]. From the obtained results here and the reported data in the literature, we can describe the effect of noble metals in this electrochemical process in two main visions as: (1) making porous surfaces of Si, (2) synthesis of micro or nanowires. 10

In the first approach, when the concentration of the metal salt is very low as 5 × 10−4 M [11, 13, 29-33], and there are some well delineated articles about the mechanism and the effects of noble metal, such as the report by Yae et al. [13, 29-33]. With this amount, the metal precipitated in nano scales. Therefore, electronegativity, chemical stability, catalyst activity, shape, and size of the metal particles are dependent on the etching rate and the morphology of the pores. Then, the rate of etching assisted by Pt was faster than assisted by Ag [13, 34]. From other viewpoint, when the amount of metal salt is about 2 × 10−2 M [1, 9]. For comparison, these articles focused just on the application of nanostructures, such as thermoelectric properties [9]. At this quantity, the metal precipitated as a dendrite or thick layer structure instead of nanoparticles, and leads to isolated nanowires [35]. The effect of noble metal, in particular catalyst activity, has totally changed. In this amount of noble metal, which we worked for higher corrosion, the etching rate assisted by Ag was faster than Pt, as shown in Fig. 5. Exactly opposite of the first view, the key role is mass transfer. When Pt utilized, the thick layer acted as a barrier for the mass transfer and efficiency of Ag, which was much better due to the branched structure (refer to the section 3.1). Even if the amount of Pt decried to half and added Ag, in A/3, the efficiency would not have significant difference from just Pt (A/2) due to the barrier layer of Pt. A little reduction of A/3 was probably related to the nucleation effects of Ag for thicker Pt layer. In comparison with other factors, noble metals could make a significant difference in the output; i.e. S/Nmax – S/Nmin for metal is the highest amount [15]. It means that, the main parameter is noble metal. Following, the second parameter is H2O2 (especially at high temperatures). This convection leads to remove byproducts and increase the activity of etchant.

11

4.2. Effect of hydrogen peroxide H2O2 is commonly utilized as an oxidizing agent allowing hole injection into Si at the cathode sites, i.e. the metals [12], but other oxidizing agents are also reported in the literature, such as potassium dichromate [36], permanganate [37], or dissolved oxygen gas [13]. In thermodynamics, the electrochemical potential of H2O2 (1.763 V) is much more positive than the valence band of Si and oxidants, usually used in stain etching of Si, while in kinetics, the etching rate is lower than 10 nm/h [12]. Indeed, the reduction of [OH]- on the surface of the noble metals is faster than the bare Si, based on the catalytic activity, rather than the electrochemical potential [11]. In making porosity on the surface of Si; i.e. low amount of metal, the H2O2 for dissolution of Si is unavoidable [38]. At this configuration, H2O2 is the second main parameter based on the Taguchi method [18], also the efficiency of corrosion enhanced exponentially (Fig. 5). The disproportionation of H2O2 into O2 and H2O is a source of gas evolution, which known to be catalyzed by noble metals. As a consequence, the gas evolution is probably related to the dissolution mechanism of Si at anodic sites [11]. This decomposition has two benefits for etching, first of all, O2 is a strong oxidizing agent [13], secondly leads to a convection that enhances the homogenization of solution and removing byproducts. It is known that another source of gas evolution could be the release of H2 [39]. As a summery, when the concentration of H2O2 increases, the driving force for etching reaction increases. Therefore, the etching reaction becomes faster, and more Si dissolves.

4.3. Effect of temperature It has been reported that increasing the temperature of etching bath improves the diffusion coefficient and chemical activity, and then dramatically enhances the corrosion. For instance Lehman et al. reported that when the etch rate raised linear by 2 from 20 °C to 40 °C, the 12

morphology was shorter, rougher, and sparser [28]. In contrast, in the samples #1-3, increasing H2O2 and time combined with increasing temperature leads to longer structures due to the interaction effects. It has been reported that, at low temperatures (0 - 50 °C), there is also a linear relationship for etching time and length of nanowires at all temperatures [40]. On the other hand, at high temperatures, the SiF62- ions will decompose to the volatile SiF4 gas, which also induces faster etching reactions [18]. Based on the statistical calculations (Fig. 5), the efficiency of temperature is slightly less than noble metals and H2O2 concentration. Also, the observed etching rate increased by increasing the etching temperature. It is clear that, the temperature and the main effects lead to a linear relationship between them.

4.4. Effect of time Based on the reaction mechanisms, the metal can drill through Si as far as etching time allows, which makes the aspect ratio of the created structure essentially determined by etching time. In another word, the characteristics of the etched morphology varied with time. Cheng et al. [40] reported that the length of Si nanowires increased approximately linearly with the etching time. From the formation mechanism, the etching time mainly affects the homogeneity of the Si microstructure [18]. Nevertheless, in our case that all parameters changed together, it experimentally proved that, not only the length and uniformity but also other morphological properties such as porosity are important. In addition, from the statistical viewpoint, the behavior of time is almost the same with the effect of temperature.

5. Conclusion This study investigated the effects of noble metals, H2O2 concentration, temperature, and time for the metal-assisted HF etching of Si by the Taguchi method. The catalytic activity of the 13

selected controllable parameters for etching in a HF solution increased in the order of noble metal, H2O2 concentration, temperature, and time. The degree of homogeneity and catalytic activity of Ag was much higher than Pt, based on the mass transformation. The correlation between the parameters with the amount of etching for H2O2 was exponential and for others (time and temperature) was linear.

6. Acknowledgment The authors wish to express their appreciation to Prof. Ivano Alessandri and Prof. Laura Depero from University of Brescia who provided comments on drafts of this paper and helpful supports to this work. Furthermore, we are thankful to University of Brescia for the financial support of this project.

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Figure captions: Figure 1. Schematic of Si etching by increasing the corrosion conditions Figure 2. FESEM micrographs of the noble metals on the Si structures for the samples: (a) #2; (b) #2, after few seconds acid washing; (c) #4; (d) #8; (e) #8, high magnification of a thick layer. Figure 3. FESEM micrographs of the Si structures by Ag assisted for samples: (a) #1, 53.8˚ tilted; (b) #2, cross-section; (c) #3, cross-sectional view (low magnification); (d) #3, high magnification Figure 4. FESEM cross-sectional micrographs of the Si for samples: (a) #5; (b) #6, nanowire and (c) porosity area Figure 5. Main effect plots for each factor. Abbreviation in horizontal axes related to Table 1, for example: B/1 represents the time duration of 10 min Figure 6. FESEM micrographs from the validated samples: (a) V1, (b) V2, and (c) V3

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Table:

Table 1. Experimental parameters (factors) and levels Levels

Parameters

1

2

3

A

Noble metal

0.02 M Ag

0.02 M Pt

0.01 M Ag + 0.01 M Pt

B

Time (min)

10

60

300

C Temperature (˚C)

27

45

90

D

0

0.15

0.3

H2O2 (M)

Table 2. Test conditions Tests

Parameters Noble metal

1

Time (min) Temp. (˚C) H2O2 (M) 10

27

0

60

45

0.15

3

300

90

0.3

4

10

45

0.3

60

90

0

300

27

0.15

10

90

0.15

60

27

0.3

300

45

0

2

5

0.02 M Ag

0.02 M Pt

6 7 8 9

0.01 M Ag + 0.01 M Pt

21

Table 3. The S/N ratio of each test Parameters

Yi (mg)

Tests

S/N

Noble metal 1

Time (min) Temp. (˚C) H2O2 (M) Y1

Y2

Y3

10

27

0

75.8 76.3 76

60

45

0.15

64.8 66.9 65.5 23.829

3

300

90

0.3

8.3

4

10

45

0.3

72.5 71.9 72.6 22.852

60

90

0

75.3 74.9 75.2 22.537

300

27

0.15

72.2 71.9 72.7 22.933

10

90

0.15

70.7 70.3 70.5 23.098

60

27

0.3

73.0 72.4 72.6 22.813

300

45

0

78.4 78.7 78.6 22.195

2

5

0.02 M Ag

0.02 M Pt

6 7

8.1

7.8

22.421

42.291

0.01 M Ag + 8 0.01 M Pt 9

Table 4. Tests for validation of Taguchi prediction Parameters Tests

S/Np Noble metal

V1 V2 V3

0.02 M Ag

Yp (mg) Yr (mg)

Time

Temp.

H2O2

10 min

45 ˚C

0.3 M

29.58

33.18

35.6

60

27 ˚C

0.15 M 23.58

66.20

61.9

300

90 ˚C

17.08

19.1

22

0

35.35

23

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

29

Highlights


The effects of synthetic conditions were implemented by Taguchi method




The etching rate and homogeneity of the structure assisted by silver is shown to be better than the structure assisted by platinum




The correlation between the parameters with amount of etching for H2O2 is exponential




The correlation between parameters with amount of etching for time and temperature is linear

30