DE102012017894A1 - Method for applying an oxide layer to a surface - Google Patents

Abstract

The method comprises plasma coating a surface (4) of a substrate (2) with a preparation layer (12) using a first process gas containing base element in a gaseous compound so that the preparation layer includes the base element, and plasma treating the preparation layer using a second process gas containing molecular oxygen. The first process gas contains a gaseous precursor, and a carrier gas. The element includes a metal or a semiconductor. The second process gas is air. The substrate is made of a metallic, semiconducting, ceramic or organic material, and includes micro- or nanoparticles.

Description

The invention relates to a method for applying an oxide layer of an oxide of a base element to a surface of a substrate. The coating of surfaces of substrates with oxide layers of oxides of various basic elements, such as titanium, silicon or the like is used today in a variety of applications of modern technology. For example, roof tiles can be provided with a titanium dioxide particle coating, which must then be subjected to thermal treatments. Oxide layers can also be used to produce dielectric layers for solar cell applications and for field effect transistors in microelectronics, hydrophobic passivation layers for a wide variety of applications, antireflection coatings on glasses or biofunctional layers, for example on medical implant workpieces. These applications relate solely to the use of silicon oxide layers. Of course, other basic elements, such as germanium, arsenic, boron, phosphorus or tellurium can be used, resulting in further applications.

However, the production of oxide layers on an industrial scale is technically complex and expensive, above all because of the required purities of the applied layers. Particularly in the field of microelectronic components and products, the applied layers must be particularly pure, since even the slightest influence on the applied layers between or during the production steps can not be neglected.

Conventionally, oxide layers are applied to the surface of the substrate to be coated by plasma coating methods. In this case, the base element whose oxide is to form the oxide layer, in the form of gaseous compounds, so-called precursors provided. In the case of silicon, this may be silane, siloxanes or a derivative of these compounds. These precursors are also used at least as process gas for the plasma coating. A major problem in the production of such oxide layers by means of plasma coating is that premature direct contact of the precursor with oxygen must be prevented, since this would lead to precipitation of nanoparticles, for example in the case of silicon oxide layers.

To solve this problem, different approaches have been discussed in the literature.

For example, should be prevented by special flow processes within the coating device direct premature contact between the precursor and oxygen. However, this has a relatively complicated and thus costly and time-consuming construction result. Other approaches attempt to use other gases in the production of the oxide layer. For example, it has been proposed to use as oxygen sources carbon dioxide and nitrogen oxides instead of molecular oxygen. In place of the very reactive precursors, such as silane used for depositing a silicon oxide layer, for example, other, less reactive precursors, such as gaseous siloxanes or equivalents of other basic elements, are proposed.

However, the proposed solutions either have the disadvantage that they require complicated devices and / or process guides, or that due to the lower reactivity of the process gases used a significantly increased time required for the coating of the surfaces of the substrate. In addition, the use of less reactive precursors may lead to unwanted deposits of other substances on the surface to be coated. The production of layers with particularly high purity and quality is thus possible with these methods only with difficulty and with the greatest effort.

The invention is therefore based on the object to propose a method for applying an oxide layer that is fast, easy and inexpensive feasible for a variety of materials and basic elements and still layers of almost any desired quality and purity can be applied to the surface to be coated.

• a) Plasmabeschichten der Oberfläche des Substrates mit einer Vorbereitungsschicht, wobei ein erstes Prozessgas verwendet wird, das das Grundelement in einer gasförmigen Verbindung enthält, so dass die Vorbereitungsschicht das Grundelement enthält,
• b) Plasmabehandeln der Vorbereitungsschicht, wobei ein zweites Prozessgas verwendet wird, das Sauerstoff enthält.

The invention achieves the stated object by a generic method which has the following steps:

• a) plasma coating the surface of the substrate with a preparation layer, wherein a first process gas is used, which contains the base element in a gaseous compound, so that the preparation layer contains the base element,
• b) plasma treating the preparation layer using a second process gas containing oxygen.

The invention is based on the finding that the oxide layer of an oxide of the base element does not have to be applied to the surface in a single step, but that first of all a preparatory layer can be applied, which can then be easily converted into an oxide. This has the technical significant advantage that for applying the preparation layer, a process gas can be used that contains no oxygen, so that the risk of premature oxidation of the base element is excluded. This can not lead to the precipitation of nanoparticles or other disturbances in the process flow. The apparatus design for carrying out such a method is very simple since, for example, only two plasma devices have to be used one after the other. Complicated equipment structures, for example, to achieve certain flow characteristics and paths of the process gases to prevent premature contact of the base element with oxygen, are unnecessary. In this case, dielectrically impeded plasmas, whose handling from the prior art has been known for a long time, can be used in a particularly simple manner.

The inventive method also has the advantage that in the first plasma coating, a reducing plasma is used, while in the second step, an oxidizing plasma is used. This results in at least nearly complete cleaning of the surface of the substrate of adsorbates and other contaminants that traditionally accumulate on the substrate over time. An expensive prior cleaning of the surface to be coated is therefore not necessary, whereby the coating process further streamlined and the coating costs are further reduced.

Advantageously, the first process gas consists of the gaseous compound of the base element. This means that within the scope of the achievable chemical purities, the process gas merely consists of the gaseous compound of the basic element. In this way, it is possible, for example, to apply particularly pure preparatory layers which consist almost exclusively of the base element.

However, it may also be advantageous that the first process gas includes a gaseous precursor containing the base member in the gaseous compound and a carrier gas. The carrier gas is preferably nitrogen or a noble gas. In this way, the preparation layer, which is applied to the surface of the substrate during the first plasma coating process, can be influenced. If, for example, a mixture of silane (SiH ₄ ) as precursor and nitrogen (N ₂ ) as the carrier gas is used as the first process gas, 97% nitrogen and 3% of the precursor being used, this leads, for example, to an almost stoichiometric Si ₃ N in the first plasma coating process _4- layer. Of course, other, preferably inert, carrier gases and other mixing ratios of the precursor with the carrier gas are possible. The advantage of such a silicon nitride layer over a pure silicon layer is that when the silicon is oxidized in the second plasma coating process, the nitrogen contained in the preparation layer is replaced by the oxygen. Although the desired silica has a different crystal structure and stoichiometric distribution than the silicon nitride, both compounds have approximately the same atomic density. This means that the applied preparation layer already has the almost desired density of atoms of the base element, in this case silicon. Of course, appropriate choices for the carrier gas used and the precursor used are also possible for other primitives and desired oxide layers.

The primitive is advantageously a metal or semiconductor. It has proven to be particularly advantageous if the gaseous compound is a compound of the base element with hydrogen. Depending on the basic element used, silanes for silicon, but also germanium for germanium, arsenic for arsenic, borane for boron, bismuthine for bismuth, phosphane for phosphorus, lead for lead, selenium for selenium, stannane for tin, stibane for antimony and tellane for tellurium as the respective basic element. Of course, derivatives of said gases or other, less reactive process gases can be used. Which chemical composition the first process gas should actually have, can be selected individually for the desired oxide layer to be applied and is irrelevant to the operation of the present invention, as long as the first process gas has the base element in a gaseous compound and preferably no oxygen. Further criteria may be the subsequent use of the thus coated substrate and, for example, thermal and / or mechanical stresses to which the substrate will later be exposed.

As the second process gas, air is advantageously used. For a method according to an embodiment of the present invention, the air to be used as the second process gas need not be pretreated, cleaned or otherwise processed. The second process gas is therefore available for free in unlimited quantities. Experiments have shown that, for example, contamination by nitrogen present in the air or by other elements in the oxide layer to be applied does not occur or only to a negligible extent. Interestingly, the plasma produced from the second process gas air has in a vast majority only reactive oxygen, while reactive constituents of others Elements, such as Sticksoff, virtually do not occur. However, even if such reactive moieties are contained in the plasma generated from air, they have almost no possibility of precipitating on the surface of the substrate due to the high reactivity of the oxygen in the air plasma and contaminating the oxide layer in any way.

For other applications, it may be advantageous to use molecular oxygen as the second process gas. This can be used exclusively or as a mixture within the chemically achievable purity. For example, it is conceivable to mix pure molecular oxygen with inert gases and to use this mixture as the second process gas. Naturally, pure oxygen refers to the chemically achievable purities available on the market. By using pure oxygen, for example, it may be possible to reduce interstitial hydrogen contained in the applied preparation layer and to achieve even higher purity of the applied oxide layer. Experiments in which a silicon oxide layer has been applied to a surface of a substrate have shown that both atmospheric air and pure oxygen as the second process gas result in an almost stoichiometric silicon dioxide layer. Typical exposure times of the plasmas on non-porous surfaces are below one second for layer thicknesses of a few, for example 3 to 7 nm. In particular, when using atmospheric air, it has been found that aerosols contained in the air due to self-cleaning processes and the high reactivity of the oxygen do not cause contamination of the deposited oxide layer.

Preferably, the substrate is a metallic, semiconductive, ceramic or organic material. Particularly advantageously, nanoparticles which have an average particle diameter of, for example, 10 to 50 nm can also be used as the substrate; or micro particles are used. In principle, all surfaces to which a layer can be applied by means of a plasma coating process can also be coated by a method according to the present invention. This applies to three-dimensionally shaped components made of almost any material, such as metal, ceramic, wood or other organic materials, as well as porous surfaces and micro- and nanoparticles.

Compared to prior art plasma coating processes in which an oxide layer of a primer is applied to the surface to be coated in a single coating operation, layers applied by a process according to the present invention have a further advantage. In a method according to the invention, the preparation layer is first applied to the surface of the substrate in which the base element is located. Subsequently, this layer is converted by the second plasma treatment into the desired oxide layer. The reactive oxygen from the plasma generated from the second process gas thus impinges on the surface of the preparation layer and, in order to be able to completely convert it into the desired oxide layer, has to penetrate into this layer. The penetration depth up to which the preparation layer is converted into the desired oxide layer naturally depends on the selected elements and materials and on the exposure time of the oxygen-containing plasma. This means that with non-oxidic surfaces between the substrate and the applied oxide layer there is no sharp and clear oxide boundary, but that the "degree of oxidation" from the surface of the applied layer into the material decreases. The result is a so-called gradient material, which has the advantage that the physical properties, such as compressive and tensile strength, elasticity and thermal expansion coefficient, steadily from the values of the applied oxide layer on the values of the underlying substrate, so that this coating in particular thermal and mechanical loads much better withstand than a deposited oxide layer in one step.

Thus, with a method according to the present invention, it is possible to apply almost any desired oxide layer to almost any surface to be coated, without the need for complex experimental setups. In addition, almost any gaseous compound of the base element can be used at least as part of the first process gas, while in particular air can be used as the second process gas. The method can thus be used almost universally for all desired oxide layers and simple, safe and cost-effective in particular feasible.

With the aid of a drawing, an embodiment of the present invention will be explained in more detail below. Show it

1 - The schematic representation of an apparatus for carrying out a method according to an embodiment of the present invention and

2a to 2c - X-ray photoelectron spectra of a surface to different Times during the process according to an embodiment of the present invention.

1 shows a substrate 2 that has a surface 4 has, on the one oxide layer 6 should be applied. For this it becomes along the direction of an arrow 8th moved by a dedicated device. First, the surface 4 with a first plasma device 10 , in the 1 is shown schematically, a preparation layer 12 applied. In the first plasma device 10 In this case, a first plasma is generated from a first process gas, wherein the first process gas is the basic element, from whose oxide the oxide layer 6 is to be built up, containing in a gaseous compound. Also the preparatory shift 12 contains the primitive. The preparatory shift 12 may be, for example, a pure layer of this element, such as titanium or silicon, or contain deposits of another material, for example, from a carrier gas, which is part of the first process gas, on the surface 4 is deposited. Subsequently, the substrate passes through 2 with the preparatory shift 12 on the surface 4 a second plasma device 14 in which a second plasma is generated from a second process gas, which is on the preparation layer 12 on the surface 4 of the substrate 2 is steered. This is the preparatory shift 12 edited so that the oxide layer 6 arises from the desired oxide.

Should the oxide layer 6 applied to the surface of nano- or microparticles, these can be used as powders by the in 1 blown schematically shown device. Of course, the actual apparatus design must be changed and adapted to the changed substrate properties. Thus, it is possible to provide both the surface of nanoparticles with a preparation layer, as well as to convert this preparation layer into the desired oxide layer. Advantageously, all of this can be done in a production plant into which only the corresponding gases which are to be used as the first process gas and as the second process gas are introduced.

The 2a to 2c show, by way of example, three X-ray photoelectron spectra at different points in time in a method according to an exemplary embodiment of the present invention, with which a silicon dioxide layer is applied to anatase titanium dioxide nanoparticles having a mean particle diameter of 21 nm. In all spectra, a count rate of released electrons is plotted over their binding energy.

2a shows a spectrum of the nanoparticles in the initial state. It can be seen at a binding energy of about 290 electron volts (eV) a carbon peak 16 indicative of atmospheric contaminants located on the surface of titanium dioxide nanoparticles. The remaining three peaks in 2a are shown correspond to two titanium peaks 18 and an oxygen peak 20 ,

In this context, for example, a titanium peak means that electrons of this binding energy were dissolved by the incident X-ray of titanium atoms. From the area of these peaks, statements about the proportion of the respective element on the surface of the nanoparticles irradiated with the X-ray radiation can be made.

2 B shows an X-ray photoelectron spectrum of titanium dioxide nanoparticles after being plasma-coated for the first time. The process gas used was a mixture of 3% silane (SiH ₄ ) and 97% nitrogen (N ₂ ). It can be seen that the in 2a still clearly visible carbon peak 16 hardly any more here. So there is a cleaning effect of the surface 4 without having been previously pretreated.

In 2 B are clearly more peaks visible than this in 2a the case is. In addition to the still existing titanium peaks 18 and the oxygen peak 20 are now two silicon peaks 22 and a nitrogen peak 24 recognizable. After the first plasma coating of the surface 4 of the substrate 2 Consequently, part of the nitrogen from the carrier gas (N ₂ ) and silicon from the precursor silane on the surface have with the process gas mentioned 4 dejected.

2c shows the X-ray photoelectron spectrum after the nanoparticles have been plasma-treated a second time, using atmospheric air as the second process gas. In addition to the now clearly emerging oxygen peak 20 and the two titanium peaks 18 are also the two silicon peaks 22 and the nitrogen peak 24 still there. The nitrogen peak 24 now, however, has become significantly lower, especially in comparison to the other peaks.

After the first plasma coating, a slightly substoichiometric silicon nitride preparation layer is present on the surface of the nanoparticles, since in the substrate 2 which consists of titanium dioxide, containing oxygen.

More detailed not shown here further evaluations of the properties of the layer after completion of the plasma treatment have shown that in the silicon dioxide layer, the oxide layer 6 on the substrate 2 forms a proportion of less than 3% of silicon nitride. The oxide layer 6 contains almost exclusively silicon in the Si ⁴⁺ -form, which proves stoichiometric silicon dioxide (SiO ₂ ) formation. Based on the Titanpeaks 18 it could be shown that there are still pure particles and no further chemical influences occur. Microscopic images confirmed complete coverage of the surface. The proportion of carbon is only 2.3%. Thus, it has been shown that the use of a process gas with 3% silane as precursor and 97% nitrogen as carrier gas for the first process step, a cleaning of the surface 4 the substrates 2 caused by almost all oxidic adsorbates. During layer growth, the remaining residues of the adsorbates were in the form of atomic carbon on the surface of the prepared preparation layer 12 carried to the outside. It comes to a floating of these carbon residues on the preparation layer 12 ,

The second process step, in which the preparatory shift 12 is processed by plasma treatment with an oxygen-containing plasma, in addition to an almost complete conversion of the preparation layer 12 in the desired oxide layer 6 in addition, the remaining atomic carbon radicals, for example in the form of carbon dioxide removed from the surface. In addition, stoichiometric defects on the particle surface were completely cured by the oxidizing plasma in the second process step. As already stated, with a method according to an embodiment of the present invention, a very wide variety of surfaces of very different materials can be coated with a multiplicity of different oxide layers of different basic elements. Possible layer thicknesses are between individual monolayers and several, for example fifteen or twenty nanometers. For most applications, such as corrosion protection, layer thicknesses of 2 to 5 nm are sufficient.

LIST OF REFERENCE NUMBERS

2

substratum

4

surface

6

oxide

8th

arrow

10

First plasma device

12

preparation layer

14

Second plasma device

16

Carbon peak

18

Titan peak

20

peak oxygen

22

silicon peak

24

nitrogen peak

Claims (10)

Method for applying an oxide layer ( 6 ) of an oxide of a primer on a surface ( 4 ) of a substrate ( 2 ), the method comprising the following steps: a) plasma coating the surface ( 4 ) of the substrate ( 2 ) with a preparatory shift ( 12 ), wherein a first process gas is used, which contains the base element in a gaseous compound, so that the preparation layer ( 12 ) contains the base element, b) plasma treating the preparation layer ( 12 ) using a second process gas containing oxygen. A method according to claim 1, characterized in that the first process gas consists of the gaseous compound of the base element. A method according to claim 1, characterized in that the first process gas includes a gaseous precursor containing the base member in the gaseous compound and a carrier gas. A method according to claim 3, characterized in that the carrier gas is nitrogen or a noble gas. Method according to one of the preceding claims, characterized in that the basic element is a metal or a semiconductor. Method according to one of the preceding claims, characterized in that the gaseous compound is a compound of the base element with hydrogen. Method according to one of the preceding claims, characterized in that the second process gas is air. Method according to one of the preceding claims, characterized in that the second process gas is molecular oxygen. Process according to one of the preceding claims, characterized in that the substrate ( 2 ) is a metallic, semiconducting, ceramic or organic material. Method according to one of the preceding claims, characterized in that as substrate ( 2 ) Micro or nanoparticles are used.

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