RU2555013C1 - Method of producing hydrophobic or hydrophilic porous silicon - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method includes treating the surface of crystalline silicon by electrochemical etching in hydrofluoric acid solution with concentration of 20-30% while supplying current with surface density of 750-1000 mA/cm² for 5-30 s to obtain hydrophobic silicon or supplying current with surface density of not more than 650 mA/cm² for 5-30 s to obtain hydrophilic silicon.

EFFECT: method enables to obtain a surface with multimodal nano- or microporosity in a single step.

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Description

The invention relates to the field of chemistry, in particular to methods of nanostructuring and modification of surface properties.

The invention allows you to change the wettability of the silicon surface by changing the surface porosity, including the production of hydrophilic and hydrophobic surfaces based on crystalline silicon, which can be used in microfluidics devices and for protective coatings in silicon electronics.

The development of technologies for producing new materials with various wetting properties has been significantly developed after the discovery of the lotus effect. This effect occurs on the surface of a lotus leaf and other plants: water droplets practically do not wet the surface, because the latter has a complex relief and is covered with wax, as a result, the drop takes the form of a ball and easily rolls. Due to this effect, self-cleaning of the surface with water drops is observed, which aroused interest in this subject area for the practical use of surfaces with low wetting. In the literature [1], it is customary to call surfaces with a contact angle greater than 90 ° hydrophobic, and with a contact angle less than 90 ° - hydrophilic. It is known [2] that no smooth surfaces make it possible to reach edge angles of more than 120 °, for this it is necessary to have a rough, embossed surface.

The interaction of the microrelief of a hydrophobic surface with water is described by two states - the Wenzel state and the Cassie state. In the Cassi state, the surface of the water does not interact with the recesses of the surface and contacts only with the upper part of the relief, the most prominent elements. In Wenzel’s state, water penetrates into micro-depressions of the surface, i.e. the surface of the water repeats the microrelief of the surface of a solid.

The development of nanostructuring methods has led to the development of technologies to create surfaces with a lotus effect and superhydrophobic properties. This effect is achieved by creating a stable Cassie state on the surface, which means heterogeneous wetting of the surface, in which there are air gaps between the liquid and the surface.

A known method (US 20130292839 A1, H01L 21/768, published July 1, 2003) for producing hydrophobic porous silicon for a wide class of functional applications, including as an electrode for liquid solar cells. The method is based on a two-stage process: obtaining porous silicon in the first stage and subsequent hydrophobization in the second. To obtain superhydrophobic coatings, the method is also used (WO 2010022107 A2, B05D 7/00, published February 25, 2010), which consists of the following steps: applying a silicon coating to the surface by centrifugation, surface treatment with ultraviolet radiation, annealing at 300 ° C and subsequent etching of silicon layer with the formation of nanowires. The technology for creating nanowires can be changed using alumina instead of silicon, and the formed nanowires can be coated with metal or polymer to enhance the effect. In addition, one-step chemical methods have been proposed for creating superhydrophobic coatings without preliminary treatment, in this case, the surface of a siloxane rubber insulator, by applying a special composition (RU 2400510, C09D 183/08, published September 27, 2010), in which they are present as a water-repellent water-repellent component, and nanosized particles that create micro-sized surface roughness.

The closest to the proposed method is a method for producing hydrophobic and hydrophilic surfaces (US 20110033663 A1, B32B 3/10, publ. 02/10/2011), in which the method of forming polymer surfaces with different porosity is used to create hydrophilic and hydrophobic surfaces. To obtain hydrophobic properties, a multimodally structured (structuring with several characteristic spatial dimensions) porous relief is created on the surface. To obtain hydrophilic surfaces, a porous relief with one characteristic spatial scale is created on the surface. The disadvantage of this technique is the presence of a polymer layer on the surface, which can interfere if the working surface of crystalline silicon is important, for example in silicon electronics, on the surfaces of silicon solar cells. Also, a polymer coating often requires special conditions; it cannot be subjected to temperature heating or laser radiation. And finally, the presence of the polymer as a whole complicates the method of producing a hydrophobic or hydrophilic surface by the need to use an additional substance to form a surface with a given wettability.

In the course of the experiments, the possibility of modifying the surface of crystalline silicon by electrochemical etching to obtain various types of porosity was revealed. This method allows to obtain surfaces with multimodal porosity, which has the characteristic dimensions of both nano- and microscale, which is important for the possibility of creating superhydrophobic surfaces. It has been found that a surface having multimodal porosity can be obtained using electrochemical etching of crystalline silicon through a silicon dioxide film on a silicon surface. In this etching mode, the oxide film is destroyed at the first stage and the surface microrelief simultaneously appears, and at the second step, with the continued electrochemical etching of microstructured silicon, a nanorelief is created on its surface. Thus, it is possible to create a multimodally structured silicon surface. The advantage of this method is its one-stage; the possibility of applying the method to obtain both hydrophilic and hydrophobic coatings; no need to use additional water-repellent substances.

The objective of the invention is to provide a method for producing hydrophilic and hydrophobic coatings of the surface of crystalline silicon in one technological step.

The solution to this problem, that is, the creation of a silicon surface with different wettability, is possible through the use of the method of electrochemical etching of crystalline silicon, which results in the formation of porous silicon of various types of porosity using various etching modes.

To implement this method, the traditional method of electrochemical anodic etching of crystalline silicon in the electrochemical cell shown in FIG. 1. In the cell, etching of the silicon surface with an area of about 600 mm ^{2 is} possible. A solution of hydrofluoric acid in ethyl alcohol with a volume of about 100 ml is used as an electrolyte. Before the electrochemical etching procedure, the silicon surface is subjected to standard chemical cleaning procedures that remove or not remove a layer of silicon dioxide from the surface. By changing the etching current density and etching time, one can obtain both a porous silicon surface with pore sizes of one characteristic nanometer or submicron scale, and a porous surface with multimodal structuring, in which pores with a characteristic size of the nanometer scale and pores with a characteristic size of the micrometer scale are formed. In this case, in the first case, when the surface interacts with water, a stable Wenzel state is achieved and the surface acquires hydrophilic properties, which is manifested in a decrease in the wetting angle compared to the smooth surface of crystalline silicon. In the second case, it is possible to achieve a stable Cassie state on a multimodal structured surface, which is not achievable on a porous surface with one characteristic pore micro-size. For an example of using this technology, crystalline silicon of orientation (100) was used with a specific conductivity of 0.005 Ohm / cm. The surface was cleaned in an aqueous solution of ethyl alcohol using an ultrasonic bath. After that, the silicon wafer was placed in an electrochemical cell with a diameter of 13 mm, into which a 25% solution of hydrofluoric acid in ethyl alcohol was poured. A copper plate was used as an anode, and a platinum spiral immersed in an electrolyte as a cathode. Various etching modes were used to create hydrophobic and hydrophilic surfaces. To create a multimodal hydrophobic surface, an etching mode was used with an etching current density close to the separation current density of a silicon film. In this mode, the diameter of the formed pores increased, as a result, they overlapped and the particles of porous silicon detached from the surface. In this experimental configuration, this corresponded to the value of the surface etching current in the range I = 750-1000 mA / cm ² . The etching time can vary from 5 to 30 s. A microscopic image of the surface thus obtained by etching for 16 s with a surface current of 900 mA / cm ^{2 is} shown in FIG. 2. The value of the contact angle on this surface was measured by analyzing the image of a 10 μl drop of water lying on this surface. This image is shown in FIG. 3, and the average value of the contact angle is 136 °.

When using a different etching mode with an etching current density less than critical, corresponding to tearing off the film, surfaces with increased hydrophilicity were obtained. In this experimental configuration, this corresponded to a surface etching current strength of not more than 650 mA / cm ² . The etching time can vary from 5 to 30 s. In FIG. 4 shows the image of the edge of a 10 μl drop of water on the surface of the structure thus obtained. The surface was made by etching with a surface current strength I = 650 mA / cm ² for 16 s. The wetting angle measured photographically by the method was 14 °.

Thus, opportunities are created for creating surfaces with controlled wettability in a one-step process by electrochemical etching of porous silicon. These features are significant and are interconnected with the formation of a stable set of essential features sufficient to obtain the desired technical result. The present invention is illustrated by a specific example of execution, which, however, is not the only possible, but clearly demonstrates the possibility of achieving the desired technical result.

This method allows in the applied plan to get the opportunity to create elements of silicon microfluidics devices that are applicable for controlling fluid microflows in micromechanical and microfluidic devices, in particular in biochips.

List of references

1. L.B. Boynovich, A.I. Emelianenko. Hydrophobic materials and coatings: principles of creation, properties and applications // Advances in Chemistry, 77 (7) 2008, pp. 619-638.

2. O.I. Vinogradova, A.L. Dubov. Superhydrophobic textures for microfluidics // Mendeleev Communications, 2012, Vol. 22, No. 5, p. 229-236.

Claims (1)

A method of producing hydrophobic or hydrophilic porous silicon, including surface treatment of the material to create different wettability, characterized in that the surface treatment uses a one-step method of electrochemical etching of crystalline silicon in a solution of hydrofluoric acid with a concentration of from 20% to 30% by applying a current with a surface density of 750- 1000 mA / cm ² for 5-30 seconds to obtain hydrophobic silicon or applying a current with a surface density of not more than 650 mA / cm ² for 5-30 seconds to obtain hydrophilic silicon.

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