Process for preparing an epitaxial alpha-quartz layer on a solid support, material obtained and uses thereof

Abstract

The present invention relates to a process for preparing epitaxial -quartz layers on a solid substrate, to the material obtained according to this process, and to the various uses thereof, especially in the electronics field.

Claims

1. Process for preparing an epitaxial -quartz layer on a solid support, comprising the following steps: i) a step of preparing a composition containing, in a solvent, at least one silica and/or colloidal silica precursor; ii) a step of depositing a layer of the composition obtained above in step i) onto at least part of the surface of a substrate and the formation of an amorphous silica matrix layer; iii) a step of heat treatment of the amorphous silica matrix layer obtained in step ii) to obtain an epitaxial -quartz layer, wherein: the substrate is a self-supporting mono-oriented crystalline silicon substrate comprising a layer of native amorphous SiO.sub.2, and in that the step of heat treatment of the amorphous silica layer is performed at a temperature of greater than or equal to 800 C., in the presence of at least one catalyst based on one of the following elements in oxidation state (+2): strontium, barium, calcium, magnesium, beryllium, or one of the following elements in oxidation state (+1): caesium, rubidium, lithium, sodium or potassium, or capable of generating one of the following elements in oxidation state (+2): strontium, barium, calcium, magnesium, beryllium, or one of the following elements in oxidation state (+1): caesium, rubidium, sodium, potassium or lithium, said catalyst being present in said amorphous silica matrix.

2. Process according to claim 1, wherein the heat treatment step is performed in the presence of oxygen.

3. Process according to claim 1, wherein the catalyst is present in the composition prepared in step i) and represents from 0.2 mol % to 30 mol % relative to the silica precursor and/or the colloidal silica precursor.

4. Process according to claim 1, wherein the catalyst is introduced into the amorphous silica matrix at the end of step ii) and before performing the heat treatment step iii) and the amorphous silica matrix layer is impregnated with an impregnation solution containing at least said catalyst in a solvent.

5. Process according to claim 4, wherein the concentration of catalyst in said impregnation solution ranges from 0.2 mol % to 50 mol %.

6. Process according to claim 1, wherein the catalyst(s) present in the composition prepared in step i) or in the impregnation solution for the amorphous silica matrix layer are strontium, barium, calcium, magnesium or beryllium salts, chosen from the nitrates, sulfates, carbonates, hydroxides, chlorides, acetates, perchlorates, oxides and alkoxides.

7. Process according to claim 1, wherein the catalyst(s) present in the composition prepared in step i) or in the impregnation solution for the amorphous silica matrix layer are caesium, rubidium, sodium, potassium or lithium salts chosen from the nitrates, sulfates, carbonates, hydroxides, chlorides, acetates, perchlorates, oxides and alkoxides.

8. Process according to claim 1, wherein the silica precursor(s) that may be used in the composition prepared in step i) are chosen from silicon alkoxides, silicon tetrachloride, silicates and silicic acid, and mixtures thereof.

9. Process according to claim 8, wherein the silicon alkoxides are chosen from tetramethoxysilane, tetraethoxyorthosilane, (3-mercaptopropyl)-trimethoxysilane, (3-aminopropyl)triethoxysilane, N-(3-trimethoxysilylpropyl)pyrrole, 3-(2,4-dinitrophenylamino)propyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, phenyltriethoxysilane and methyltriethoxysilane.

10. Process according to claim 1, wherein the silica and/or colloidal silica precursor(s) represent from 0.01% to 95% by mass relative to the total mass of the composition prepared in step i).

11. Process according to claim 1, wherein the composition prepared in step i) also contains one or more additives chosen from pH regulators, structuring or modifying agents, and porosity promoters.

12. Process according to claim 1, wherein the heat treatment step iii) is performed at a temperature ranging from 800 to 1200 C.

13. Process according to claim 1, wherein steps i) to iii) are repeated one or more times on the same substrate so as to form successive quartz layers superposed one on the other, or else on different zones of the same substrate.

14. Process according to claim 1, wherein the process is a sol-gel process in which: the composition used in step i) is a sol-gel composition containing, in a solvent, at least one silica precursor, the formation of the amorphous silica matrix layer on the surface of the substrate is performed by evaporating the solvent contained in the sol-gel composition, the substrate is a mono-oriented crystalline silicon substrate, the step of heat treatment of the silica layer is performed in the presence of oxygen and at atmospheric pressure, and the catalyst is a catalyst based on strontium, barium or calcium in oxidation state (+2) or which is capable of generating strontium, barium or calcium in oxidation state (+2).

15. Process according to claim 14, wherein the catalyst is a catalyst based on strontium or barium in oxidation state (+2) or capable of generating strontium or barium in oxidation state (+2).

16. Process according to claim 14, wherein the catalyst is present in the composition prepared in step i), and said composition also contains one or more surfactants.

17. Process according to claim 16, wherein the surfactant(s) are chosen from cationic surfactants chosen from tetradecyltrimethylammonium bromide, dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide; anionic surfactants chosen from sodium dodecyl sulfate, sodium dodecylsulfonate and sodium dioctylsulfosuccinate; and nonionic surfactants chosen from block copolymers of ethylene glycol and of propylene glycol.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1a and 1b are the AFM images of the surface of the -quartz layer and FIG. 1c shows the curve of the AFM analysis of the surface profile of the layer, from example 1, in accordance with one embodiment;

(2) FIGS. 2a-2d and 3a-3d show the results of the structural and microstructural study of the quartz layer obtained in example 1, in accordance with one embodiment;

(3) FIGS. 4a and 4b gives the results of the study of the thickness of the epitaxial -quartz dense layer from example 2, in accordance with one embodiment; and

(4) FIGS. 5a-5c and 6a-6d show the images of the porous quartz layer of example 3, in accordance with one embodiment.

DETAILED DESCRIPTION

(5) The present invention is illustrated by the following implementation examples, to which it is, however, not limited.

EXAMPLES

(6) Starting Materials Used in the Examples:

(7) 98% tetraethoxyorthosilane (TEOS): Sigma-Aldrich company,

(8) ethanol (EtOH),

(9) hydrochloric acid (HCl),

(10) strontium nitrate (Sr(NO.sub.3).sub.2): Sigma-Aldrich company,

(11) barium hydroxide (Ba(OH).sub.2): Sigma-Aldrich company,

(12) cetyltrimethylammonium bromide (CTAB): Sigma-Aldrich company,

(13) polyethylene glycol monohexadecyl ether sold under the trade name Brij-56 by the Sigma-Aldrich company.

Example 1

Preparation of an -Quartz Layer According to the Process of the Invention

(14) A precursor solution having the following initial (molar) composition was prepared: 1TEOS, 25EtOH, 5H.sub.2O, 0.18HCl, 0.062Sr(NO.sub.3).sub.2.

(15) Deposition of the precursor solution was performed on a silicon substrate (Si(100)) (dimensions: 0.7 mm thick and area of 3 cm5 cm) comprising a layer of native SiO.sub.2 2.2 mm thick by dipping and withdrawal in dry air at room temperature, at a rate of 2 mm.Math.s.sup.1.

(16) After depositing the precursor solution, the silicon substrate was subjected to the following heat treatment in a tubular oven, under air and at atmospheric pressure: temperature rise from room temperature to 1000 C. at a rate of 3 C./min, followed by maintenance at 1000 C. for 5 hours.

(17) The oven was then switched off and the substrate was allowed to cool to 25 C. at a rate of 3 C./min.

(18) A silicon (1 0 0) support covered, with a layer of -quartz was obtained, and was then characterized.

(19) The thickness and refractive index measurements were taken by ellipsometry using a spectroscopic ellipsometer sold under the trade name 2000U Woollam by the company VASE or by scanning electron microscopy-field emission (SEM-FE) with an SU6600 scanning electron microscope from the company Hitachi.

(20) The dimensions, the roughness and the appearance of the crystals were determined using an atomic force microscope (AFM) sold by the company Veeco and an optical microscope.

(21) The epitaxy was determined by wide-angle x-ray scattering via a diffractometer sold under the trade name GADDS D8 in Brker mounting, copper irradiation 1.54056 .

(22) The -quartz layer thus obtained had a thickness of 180 nm and a refractive index of 1.545, which is very close to the refractive index of dense quartz (1.55 at =700 nm).

(23) The attached FIG. 1 gives the results of the topographic study of the epitaxial dense layer of -quartz thus obtained: FIGS. 1a and 1b are the AFM images of the surface of the -quartz layer and FIG. 1c shows the curve of the AFM analysis of the surface profile of the layer (height Z in nm as a function of the length X in m).

(24) It emerges from FIGS. 1a and 1b that the quartz polycrystals have a lateral dimension ranging from 450 nm to 500 nm.

(25) The image of the AFM profile (FIG. 1c) shows that the quartz layer obtained has very good homogeneity: perfect optical quality and low roughness (RMS=8 nm).

(26) The results of the structural and microstructural study of the quartz layer obtained are given, respectively, by the attached FIGS. 2 and 3.

(27) FIG. 2a shows the XRD spectrum (intensity in arbitrary units as a function of the angle 2 in degrees); FIG. 2b is the SEM microscopy image of the vertical section of the quartz layer; FIG. 2c is a pole figure: -quartz (1 0 0) 2=20.8, : 50, : 0; silicon substrate (1 1 1) : 60, : 45 2=28.7; -quartz (1 0 1) : 58, : 45 2=26.8; -quartz (1 0 1) : 36, : 0, 2=26.8; and FIG. 2d is a model of two-dimensional representation of the orientation of the epitaxial dense layer of -quartz.

(28) FIG. 3a shows a TEM image at low magnification (in bright-field mode) of the cross section of a quartz layer on a silicon substrate oriented along the crystallographic direction (100). FIGS. 3b, c and d show high-resolution TEM images of the quartz-silicon interface across the axes of crystallographic zones [001] and [011] of -quartz, respectively. The framed figure in FIG. 3c shows the Fourier transform (FFT) of the image of METHR 3c. FIG. 3d shows the FFT of FIG. 3b which confirms the epitaxial relationship between quartz and silicon.

Example 2

Preparation of an -Quartz Layer According to the Process of the Invention (Impregnation of a Mesoporous Layer of Amorphous Silica with Barium (Ba2+) Salts)

(29) In this example, it is shown that the impregnation of a mesoporous layer of amorphous silica with a solution containing barium Ba.sup.2+ makes it possible to form an -quartz layer.

(30) A silica precursor solution having the following initial (molar) composition was prepared: 1TEOS, 25EtOH, 5H.sub.2O, 0.18HCl, 0.05 Brij-56.

(31) Deposition of the solution was performed on a silicon substrate (Si(100)) (dimensions: 0.7 mm thick and area of 3 cm5 cm) comprising a layer of native SiO.sub.2 2.2 mm thick by dipping and withdrawal in dry air at room temperature, at a rate of 2 mm.Math.s.sup.1.

(32) After deposition of the silica precursor solution, the silicon substrate was subjected to the following heat treatment in an open oven under air and at atmospheric pressure: 5 minutes at 500 C.

(33) A silicon (100) support covered with a layer of mesoporous amorphous silica 60 mm thick and comprising pores of about 3 nm interconnected in a 3D network was obtained.

(34) Following the formation of the silica sol-gel mesoporous layer, the porosity was impregnated with a 1M solution of barium hydroxide Ba(OH).sub.2 in acetic acid.

(35) The impregnation was performed by dipping and withdrawal in dry air at room temperature, at a rate of 2 mm.Math.s.sup.1.

(36) The silicon substrate was then subjected to the following heat treatment in a tubular oven, under air and at atmospheric pressure: temperature rise from room temperature to 1000 C. at a rate of 3 C./min, for 5 hours.

(37) The oven was then switched off and the substrate was allowed to cool to 25 C. at a rate of 3 C./min.

(38) A silicon (100) support covered with an -quartz layer was obtained, and was then characterized (thickness, refractive index and epitaxy) as described above in Example 1.

(39) The -quartz layer thus obtained had a thickness of 235 nm and a refractive index of 1.52, which is very close to the refractive index of dense quartz (1.55 at =700 nm).

(40) The attached FIG. 4 gives the results of the study of the thickness of the epitaxial -quartz dense layer thus obtained: FIG. 4a is the SEM microscopy image of the cross section of the quartz layer, FIG. 4b is the image of the XRD spectrum (intensity given in arbitrary units as a function of the angle 2 in degrees).

Example 3

Preparation of a Structured -Quartz Layer According to the Process of the Invention

(41) In this example, it is shown that the presence of a cationic surfactant in the precursor solution enables micro- or nano-structuring of the epitaxial -quartz layer. Specifically, during the evaporation of the precursor solution at the surface of the silicon substrate, phase separation takes place between a silica-rich phase and a strontium-rich second phase, stabilized by the presence of a surfactant such as CTAB. During the heat treatment, the silica-rich phase becomes converted into epitaxial quartz. Alternatively, a longer heat treatment makes it possible to transfer to the Si substrate the structure formed by phase separation (formation of pores). Without wishing to be bound by any theory, the inventors suppose that the mechanism which is at the origin of this replication probably proceeds via a surface oxidation of the Si to SiO.sub.2, followed by its conversion into quartz under the quartz crystals already formed by means of the presence of the catalyst in the cavities and of the diffusion of oxygen through these same cavities. However, this mechanism remains to be confirmed. Thickening of the layer is thus envisaged by the gradual consumption of the silicon substrate.

(42) A precursor solution having the following initial (molar) composition was prepared: 1TEOS, 25EtOH, 5H.sub.2O, 0.18HCl, 0.114CTAB 0.062Sr(NO.sub.3).sub.2.

(43) Deposition of the quartz layer was then performed under exactly the same conditions as those of Example 1 above, it being understood that the heat treatment was performed over a period of 5 hours, and also over a period of 20 hours.

(44) The structured (porous) quartz layers have the same characteristics (crystal size, orientation, thickness) as the dense layers apart from the presence of the pores. The pure quartz layer has cavities of homogeneous diameters relatively well dispersed over the surface, derived from the phase separation.

(45) FIGS. 5 and 6 show the images of the porous quartz layer thus obtained.

(46) FIG. 5a is an image taken by SEM and shows the appearance of the layer before heat treatment. The phase segregation which gave rise to a nanoporous layer of amorphous silica with pores 780 nm in diameter and hexagonal stacking is observed. FIGS. 5b and Sc are AFM microscopy images.

(47) FIG. 6a is an SEM image of the cross section of the epitaxial -quartz porous layer; FIG. 6b gives the XRD spectrum (intensity in arbitrary units as a function of the angle 2 in degrees); FIG. 6c shows the AFM microscopy image of the epitaxial quartz porous layer and FIG. 6d gives the analysis of the thickness profile of the layer (height Z in nm as a function of the length X in m).

(48) The characteristics of the epitaxial -quartz layer thus obtained were as follows:

(49) thickness of the layer after 5 hours of heat treatment (ellipsometry)=250 nm,

(50) thickness of the layer after 20 hours of heat treatment (ellipsometry)=500 nm (cf. electron microscopy image, FIG. 6a and profile of the AFM microscope FIG. 6d),

(51) lateral dimension of the crystals: 40 nm (cf. electron microscopy image, FIG. 5a),

(52) lateral dimension of the nanopores: 200-500 nm (cf. electron microscopy image and AFM, FIG. 5c),

(53) epitaxy: (100) (cf. XRD spectrum, FIG. 6b).