WO2002086190A1

WO2002086190A1 - Method for preparing thin porous layers of inorganic oxides

Info

Publication number: WO2002086190A1
Application number: PCT/ES2002/000193
Authority: WO
Inventors: Angel Barranco Quero; Francisco Yubero Valencia; Juan Pedro Espinos Manzorro; Agustín RODRÍGUEZ GONZALEZ-ELIPE; José COTRINO BAUTISTA
Original assignee: Consejo Superior De Investigaciones Científicas; Universidad De Sevilla
Priority date: 2001-04-19
Filing date: 2002-04-18
Publication date: 2002-10-31
Also published as: ES2186530A1; ES2186530B1

Abstract

The invention relates to a method for preparing thin layers of inorganic oxides on substrates by means of plasma-assisted vapour phase deposition. The inventive method is different from other common methods in that organic layer deposition steps are included between the successive inorganic oxide deposition steps such that, during the inorganic layer deposition step, the previously-deposited organic part is eliminated by combustion. Said method is suitable for developing selective membranes that are used to separate and purify fluids and to produce and modify sensors for gases and humidity and electronic components.

Description

TITLE

Procedure for the preparation of fine porous layers of inorganic oxides.

TECHNICAL SECTOR A first sector of application of the procedure is framed, within the new chemical technologies, in the development of selective membranes for fluid separation and purification. Other sectors of interest are related to the manufacture and modification of gas and humidity sensors and physical properties modification technologies of optical fibers and electronic components.

OBJECT OF THE INVENTION

The object of the present invention is a process for the preparation of thin layers of inorganic oxides on substrates by means of plasma-assisted vapor phase deposition that differs from the usual procedures in that they are intercalated between the successive stages of deposition of inorganic oxide stages of deposition of organic layers, so that during the deposition step of the inorganic layer the combustion removal of the previously deposited organic part occurs.

STATE OF THE ART

The methods of deposition of thin sol / gel type sheets, use of materials in powder form and the like based on chemical methods have been previously used for the synthesis of porous layers of interest such as membranes, sensors, catalyst support, etc. (CG Gólner, M. Antonietti, Avd. Mater. 9 (1997) 432, S. Singh, DY Sasaki, J. Cesarano III, AJ Hurd, Thin Solids Films, 339 (1999) 209, A. Julbe, C. Guizard , A. Larbot, L. Cot, A. Giroir-Fendler, J. Membrane Sci. 77 (1993) 137, P. Piagio, A. Bottino, G. Capanneli, E. Carosini, A. Julbe, Langmuir 11 (1995 ) 3970, H. Lao, C. Detellier, T. Matsuura, AY Tremblay, J. Mater. Sci. Lect 13 (1994) 895. A. Sayari, Che. Mater. 8 (1996) 1840, MS Dutraive, R. Lalauze, C Pijolat, Sensors and Actuators B 26/27 (1995) 38; G. Behr, W. Fliegel, Sensors and Actuators B 26/27 (1995) 33; WF Maier, IC. Tilgner, M. Wiedorn, H- Ch. Ko, Advanced materials 5 (1993) 726). In all these processes it is necessary to proceed, at some stage of the preparation, to heat the material to either activate chemical reactions (combustion) and / or to accelerate the restructuring and sintering of the layer. An alternative method that allows working either at room temperature or under heating of the substrate is the use of cold plasmas that, by decomposing volatile metal precursors, enable the deposition of thin layers of metal oxides or nitrides or similar systems (EC Tracy , J A. Turner, DK Benson, P. Liu, J-G. Zhang, W09919534, 1999; A. Koganei, Z. Motomiya, JP9272978, 1997; MS Ameen, J. Hillman, US5989652, 1999; R. Avni, D. Morvan, J. Amouroux, S. Miralai, FR2729400, 1996; JC Alonso, SJ Ramírez, M. García, A Ortiz, J. Vac. Sci. Technol. A 13 (1995) 2924; M. Matsuoka, SI. Tohno, J. Vac. Sci. Technol. A 13 (1995) 2427; U. Beck, DT Smith, G. Reiners, SJ Dapkunas, Thin Sol. Films 332

(1998) 164). These methods are often recognized under the generic English name "plasma enhanced Chemical Vapor Deposition" (PECVD). In addition to the preparation of layers of metal compounds, these procedures can also be used for the preparation of layers of organic polymers (P. Giordano, RC Smierciak, US4784769, 1988; Y. Hayashi, US4347139, 1982; CG Guizard, EHJ Durand, IHLecacheux, JD Martin, EP1013335, 2000; H. Nomura, US5439736, 1995; R. Nakase, EP1035569, 2000; FUZ Chowdhury, AH Bhuiyaa Thin Sol. Films, 360 (2000) 69; T. Schwarz-Selinger, A. Von Keudell, W. Jacob, J. Appl. Physics 86 (1999) 3988; N. Tanigaki, H. Kyotani, M. Wada, A. Kaito, Y. Yoshida, EM. Han, K. Abe, K. Yase, Thin Sol. Films 331 (1998) 229). The type of volatile precursor, the plasma gas precursor and the working conditions and the type of the plasma source are the variables that allow one type or another of preparation to proceed. PECVD methods have previously been used to obtain porous layers of inorganic oxides and siloxane-like composites, of interest as gas separation membranes (Ch. Nelson, R. Domanik, US5002652, 1991; R. Vos, DE19603829, 1997; S. Matsuo, M. Kiuchi, M. Sekimoto, US4543266, 1985; Z. Chu, P. Gao, R. Ye, CN1185041, 1998; S. Roualdes, A. Van der Lee, R. Berjoan, j. Sánchez, J. Durand, AIChE Journal 45

(1999) 1566, O. Górbig, S. Nehlsen, J. Müller, J. Memb. Sci. 138 (1998) 115, A.S. da Silva Sobrino, G. Czremuszkin, M. Latreche, M.R. Wertheimer, J. Vac. Sci. Technol. TO

18 (2000) 149), having in this case achieved some control over the porous structure of the deposited layer by managing adequate plasma conditions (working pressure, plasma gas, etc.) during the decomposition process of the inorganic precursor. The method claimed here involves performing successive deposition cycles of organic and inorganic parts, achieving control of the texture of the deposited layer by means of adequate sequencing of these cycles.

EXPLANATION OF THE INVENTION

An object of the present invention is a process for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor phase deposition consisting of the following steps: a) placement of the substrate in the plasma reaction chamber b ) introduction of a volatile precursor containing the inorganic element whose oxide is to be deposited c) supply with oxygen or O ₂ / inert gas mixtures from the plasma source in the plasma reaction chamber d) activation of the plasma source in said chamber to react the precursor gas with the oxygen plasma causing the inorganic oxide layer to deposit on the substrate

Between successive stages of deposition of the inorganic oxide, stages of deposition of organic layers are interspersed, so that during the deposition stage of the inorganic layer, the combustion removal of the previously deposited organic part occurs. The deposition of the organic layer consists of the following steps: a) introduction of a volatile precursor containing the organic compound to be deposited b) feeding with oxygen or O ₂ / inert gas mixtures from the plasma source into the reaction chamber plasma c) activation of the plasma source in said chamber to react the precursor gas with the oxygen plasma causing deposition of the organic compound layer on the substrate.

The substrate can be any metallic, ceramic, glass or optical fiber material whose shape can be flat, tube-shaped or with other geometries.

The volatile precursor can be any compound of metallic elements from the periodic table that decomposed by an oxidizing plasma causes an oxide. The plasma source is a microwave generator, a radio frequency generator or a direct current source that is supported by a magnetic field or an electron source and which can work in cyclotron electronic resonance mode ("electron cyclotron resonance", ECR ) at a pressure between 10 ^"2 and 10 ^{" 4} Torr and at a temperature between room temperature and the decomposition or graphitization temperature of the organic compound or in the conventional regime at a pressure between 100 and 10 ^"2 Torr and at a temperature also comprised between the ambient temperature and the decomposition or graphitization temperature of the organic compound.Oxygen is supplied in a ratio of at least 4: 1 with respect to the volatile precursor and at least 10: 1 with respect to the organic compound.

The organic compound can be any volatile organic compound formed by carbon, hydrogen and / or oxygen, in particular aliphatic, alicyclic or aromatic hydrocarbons and in particular toluene. The procedure can be applied to a volatile single metal precursor or to metallic precursors of two or more metals.

Another object of the present invention is the porous thin layers prepared by the described procedure, as well as the use of said porous thin layers as gas sensors (in particular deposited on a fiber optic substrate), as gas separation membranes or as materials for microelectronic use.

BRIEF DESCRIPTION OF THE FIGURE

Figure 1: Schematic of the experimental system, in which: 1. Plasma source

2. Reaction chamber in stainless steel

3. Precursor dosing system

4. Pumping system based on turbomolecular and rotary pumps

5. Gas supply for the plasma source 6. Sample holder for substrate placement ' Figure 2: Scanning electron microscopy (SEM) images of the layers obtained in the embodiment of the invention.

Figure 3: Experimental results of the analysis by means of spectroscopic ellipsometry of a porous SiO ₂ layer exposed under controlled conditions to atmospheres with different degrees of relative humidity from 5 to 76%.

DETAILED DESCRIPTION OF THE INVENTION

Precise control of the porosity in thin sheets of metal oxides and other inorganic materials is very important for many applications of such systems. For example, the size and shape of the pores is a critical parameter when defining the separation capacity of inorganic membranes. The porous structure itself, together with the thickness of the layer, determines the sensitivity and responsiveness of the sensors in the case of using thin film technology for their preparation or to modify their response. On the other hand, the fact that the proposed method involves synthesis procedures at room temperature represents an undoubted advantage over chemical synthesis methods under aggressive conditions or others that involve heating the parts to high temperatures. Indeed, due to the characteristics of their use, the porous thin layers have to be prepared on substrates, these acting as active elements, as in the case of sensors, or merely passive as carriers of the layers. In the first case, the need to avoid causing changes in the functionality, composition and other characteristics of the sensors is evident. In the second case, it is important to maintain low preparation temperatures, when the substrates cannot be altered by chemical reaction. This is the case, for example, in the use of metal fiber agglomerates as a substrate for filter membranes, or also the case of ceramic micro-porous substrates when any reaction that alters its composition should be avoided.

The characteristics of the method make it suitable for preparing layers of variable chemical composition in depth. This possibility may be of great interest to chemically make compatible a porous layer and a specific substrate, avoiding delamination problems. Also, this possibility allows to alleviate problems of mechanical stress between materials with different coefficients of expansion when the layer / substrate system has to be used at high temperatures

Since it is a sequential procedure, it is also possible to prepare layers with controlled porosity and variable in thickness from the inside to the outside of the same. This possibility may be of great interest to limit clogging processes with ash or powders or, simply, to improve the abrasion resistance of the porous layer.

The method of preparation developed consists of the deposition from the vapor phase induced by an oxygen plasma. This method is based on the decomposition of volatile precursors dosed in a controlled manner on the substrate to be covered by the porous layer. A diagram of the experimental system is presented in Figure 1. A 16 cm diameter microwave-excited external plasma source (1) is used, which can operate in the ECR regime at low pressures of the order of 10 ^"3 Torrs and in the conventional mode at pressures of some Torrs. The deposition cycles of the organic material are carried out under high pressure conditions, using toluene as the organic precursor and oxygen as the gas that feeds the plasma source. This process constitutes one of the novel points of procedure since the formation of organic polymeric layers is generally not achieved using oxygen plasmas; under these conditions, the removal of the formed organic layer usually occurs.The microwave power used is of the order of 700 w, with the oxygen flows being the source and. of toluene on the substrate in a ratio of at least ten to one. Typical values of oxygen fluxes used are in the ord 50 sccm In order to achieve the necessary working pressures, it may be necessary to decrease the pumping capacity of the system by throttling it.

By changing the way the system works, the organic layer is burned at room temperature through the oxygen plasma while the inorganic oxide in question is simultaneously deposited. At that time toluene is replaced by a volatile precursor of the metal to be deposited and the plasma working mode is established within the ECR conditions at low pressure. The working pressures in this case are of the order of 1 * 10 ^"2 Torr, being the ratio of flows between the oxygen that feeds the source and the volatile metallic precursor of at least 4 The process of Pore generation is the result of the evolution of the combustion products of the previously deposited organic part (C0 ₂ and H ₂ 0) and the simultaneous deposition of the inorganic component.

Controlling the porous structure of the layer is achieved by regulating the number of cycles and the deposition time of the organic and inorganic layers within each cycle. A larger pore volume is achieved by increasing the relative thickness of the organic layer and decreasing the deposition time of the oxide layer with each cycle.

By conveniently varying the deposition cycle times of the organic and inorganic layers, depth-variable porosity layers could be prepared using the proposed methodology. Also, by combining inorganic precursors of different chemical elements, it is possible to control the chemical composition of the layer in depth and / or prepare layers of mixed oxides. In the latter case, two or more inorganic precursors would be dosed simultaneously into the plasma chamber.

Example of embodiment of the invention

An example of application of the procedure is the preparation of porous SiO ₂ layers. As a volatile precursor of Si, Si (CH ₃ ) ₃ Cl has been used, the flow of this compound being 5 cubic centimeters under standard conditions per minute (sccm) when the inorganic part is deposited. ECR working conditions were achieved with a microwave power of 400 w, and a source feed oxygen flow of 20 sccm. With the pumping conditions used, these flow values resulted in a pressure in the plasma chamber of 1 * 10 ^"2 Torr. To obtain the organic deposit, a toluene flow of 5 sccm and an oxygen flow of 50 sccm After throttling the pumping system, a working pressure was obtained within the plasma chamber of 1.5 Torrs An example of cycles used to control the porosity of the layer consisted of the following protocols:

Protocol a) 1.- SiO ₂ 75 nm 2.- Polymer 10 nm 3.- SiO ₂ 150 nm 4.- Polymer 10 nm 5.- SiO ₂ 375 nm Protocol b) 1 - SiO ₂ 75 nm 2.- Polymer 20 nm 3.- SiO? 150 nm 4.- Polymer

20 nm 5.- SiO ₂ 375 nm Protocol c) 1.- SiO ₂ 75 nm 2.- Polymer 200 nm 3.- SiO ₂ 150 nm 4.- Polymer

200 nm 5.- SiO ₂ 375 nm Figure 2 shows a series of SEM microscopy images of the layers obtained according to the three previous protocols. It can be clearly seen that the porosity increases from protocol a) to c), in the sense that the deposition time of the organic layer or layers increases. The characterization of these layers by adsorption methods (BET method) shows that they have specific surfaces greater than 30 m ² g ^"1 and a well defined pore structure.

Example of use as optical sensors

Thanks to the porous microstructure, the SiO ₂ layers can be used as optical gas sensors. A typical application is for use as a humidity sensor. Figure 3 shows the experimental results of the analysis by means of spectroscopic ellipsometry of a porous SiO ₂ layer exposed under controlled conditions to atmospheres with different degrees of relative humidity from 5 to 76%. The optical parameters tanψ and eos Δ vary significantly and reversibly with the degree of humidity, being able to establish a direct correlation between the optical characteristics of the layer and the degree of ambient humidity. The observed evolution has its origin in the reversible adsorption of water vapor in the porous structure of the layer. In the specific case of Figure 3, the preparation conditions of the layer whose behavior is reflected are:

- polymeric layer: thickness 5nm

- SiO ₂ layer: thickness 150 nm

- polymeric layer: thickness 5 nm - SiO ₂ layer: thickness 300 nm

Claims

1.- Procedure for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma-assisted vapor deposition consisting of the following stages: a) placement of the substrate in the plasma reaction chamber b) introduction of a precursor volatile ue contains the inorganic element whose oxide is to be deposited c) feeding with oxygen or mixtures O ₂ / inert gas from the plasma source in the plasma reaction chamber d) activation of the plasma source in said chamber to react the precursor gas with the oxygen plasma causing the deposition of the inorganic oxide layer on the substrate and characterized in that the stages of deposition of organic layers are intercalated between the successive stages of deposition of the organic layers, so that during the deposition stage of the Inorganic layer occurs combustion removal of previously deposited organic part.

2 - Procedure for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claim 1, characterized in that the deposition of the organic layer consists of the following steps: a) introduction of a volatile precursor containing the organic compound to be deposited b) feeding with oxygen or O ₂ / inert gas mixtures of the plasma source in the plasma reaction chamber c) activation of the plasma source in said chamber to react the precursor gas with the oxygen plasma causing the deposition of the organic compound layer on the substrate.

3 - Procedure for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma-assisted vapor deposition according to claims 1 and 2, characterized in that the substrate can be any metallic, ceramic, glass or optical fiber material whose shape can be be flat, tube-shaped or with other geometries.

4 - Process for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claims 1-3, characterized in that the volatile precursor is any composite of metallic elements of the periodic table that decomposed by a Oxidizing plasma causes an oxide.

5. Process for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claims 1-4, characterized in that the plasma source is a microwave generator, a radio frequency generator or a DC power source

6 - Method for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claim 5, characterized in that the plasma source is supported by a magnetic field or an electron source.

7. Procedure for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claims 5 and 6, characterized in that the plasma source works in cyclotronic electron resonance regime ("electron cyclotron resonance"", ECR) at a pressure between 10 ^{" 2} and 10 ^"4 Torr and at a temperature between room temperature and the decomposition or graphite temperature of the organic compound.

8 - Procedure for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claims 5 and 6, characterized in that the plasma source works in a conventional regime at a pressure between 100 and 10 " ² Torr and at a temperature between room temperature and the decomposition or graphite temperature of the organic compound.

9. Process for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma-assisted vapor deposition according to claims 1-8, characterized in that the oxygen is fed in a ratio of at least 4: 1 with respect to the volatile precursor

10. Process for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claims 1-9, characterized in that the oxygen is fed in a ratio of at least 10: 1 with respect to the organic compound

11 - Process for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claims 1-10, characterized in that the organic compound is any volatile organic compound formed by carbon, hydrogen and / or oxygen , in particular aliphatic, alicyclic or aromatic hydrocarbons.

12. Process for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claim 11, characterized in that the organic compound is toluene.

13.- Method for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claims 1-12, characterized in that the procedure is applied to a single metal volatile precursor.

14. Process for the preparation of porous thin layers of inorganic oxides on substrates by means of plasma assisted vapor deposition according to claims 1-12, characterized in that the process is applied to metal precursors of two or more metals.

15 - Porous thin layers of inorganic oxides prepared by a process according to claims 1-14.

16. Use of the porous thin layers of inorganic oxides according to claim 15 as gas sensors, in particular deposited on a fiber optic substrate.

17. Use of the porous thin layers of inorganic oxides according to claim 15 as gas separating membranes.

18 - Use of the porous thin layers of inorganic oxides according to claim 15 as microelectronic materials.