WO2019034574A1

WO2019034574A1 - Method for producing ultra-thin layers on substrates

Info

Publication number: WO2019034574A1
Application number: PCT/EP2018/071859
Authority: WO
Inventors: Sebastian Runde; Heiko AHRENS; Christiane A. Helm
Original assignee: Universität Greifswald
Priority date: 2017-08-14
Filing date: 2018-08-13
Publication date: 2019-02-21
Also published as: DE102017118471A1

Abstract

A method is proposed for producing ultra-thin layers consisting of metal/metal oxides/metal hydroxides on substrates (1) by utilizing the properties of the starting materials. Different building-up possibilities are proposed with regard to the number of layers, the sequence of layers, the material composition and the thickness of the individual layers. The method makes it possible to build up layer systems of which the boundary and surface roughnesses are less than those of the substrate used.

Description

Method for producing ultra-thin layers on substrates

The invention relates to the production of ultra-thin layers of metals, semimetals and their alloys on substrates by means of the new method "forced wetting with provoked film contraction".

Metal oxide layers are important for modern industrial purposes. Examples are the passivating zinc oxide layer on aluminum as corrosion protection,

Oxide coating process as abrasion protection for tools or machine components or antireflection coatings for radiation protection. The fields of application of these inorganic compounds are currently and in the future almost limitless. Particularly in the field of semiconductor technology, transparent conductive thin-film systems are the key element for energy production (photovoltaics) as well as for information transfer. In addition, applications of radiation absorption material in passive armor technology are known to reduce the reflection cross section with respect to radar radiation.

Established methods for surface coating various substrates with thin or even ultrathin layers are physical vapor deposition, including RF magnetron sputter deposition, electron beam evaporation, laser ablation, thermal evaporation, etc., as well as chemical vapor deposition, including ion layer gas reaction (ILGAR) and plasma enhanced chemical vapor deposition ,

A disadvantage of the previous methods is the technological and energetic effort required to produce these high quality layers. In addition, the handling of vacuum technology is mandatory. For example, the effort to realize the operation of a plasma reactor and a vacuum system, considerably.

The coating process is bound to island growth in the examples given, i. H. In order to produce a homogeneous and self-contained thin film, a minimum layer thickness is necessary.

There is already known a method by means of which fine metal lines can be deposited on a substrate. In particular, conductor patterns of metallic conductors are to be applied to an insulating substrate by means of a nib, the nib being wetted with molten metal (DE 693 06 565 T2). A production of layers is not mentioned.

Also known is an electroplating process for depositing copper-indium-gallium-containing thin films on substrates. It is a prerequisite that the substrate is electrically conductive, since it forms the anode (EP 2 393 964 Bl).

The invention is based on the object to provide a method for producing ultra-thin layers on substrates, which is low in terms of apparatus and energy Can carry out effort and with the help of which layers can be produced with a thickness which is smaller than the layer thickness of the previously known methods.

To solve this problem, the invention proposes a method for producing ultra-thin layers with the method steps mentioned in claim 1. Further developments of the invention are the subject of dependent claims.

The method proposed by the invention makes it possible to produce ultrathin layers in the range of 3 nm to 25 nm thickness.

In addition, the method is able to manage without the technical and energy expense of conventional coating techniques.

The invention is suitable for the production of layers of different metals / semimetals and their alloys. Specifically, examples may be mentioned: gallium, indium, bismuth, tin, zinc, aluminum, germanium and their alloys. These inorganic substances described here have the following common features:

- when used properly, they are non-toxic and non-carcinogenic

- They are low melting with temperatures below 1000 ° C and have a high surface tension in the liquid state

they show a tendency to form a passivating oxide layer

- The formed oxides / hydroxides form polymorphic crystal structures

- They are alloyable with the melting point degrading eutectics of less than 300 ° C.

- The transparency of the metallic oxides / hydroxides can be influenced by annealing

- The metallic oxides / hydroxides may have semiconductor properties.

The method is not limited to the said elements / element compounds but also applicable to other elements, such as antimony, arsenic, boron, tellurium and especially thallium and alloys of these elements.

The method proposed by the invention can be due to the low

Performing requirements for equipment and energy also faster, so that in this way also a cost saving occurs.

In a development of the invention, it can be provided that the layer material is heated and then dropped in liquid form onto the surface of the substrate to be coated. The heating can be done for example in a bottle or other container.

The application of the drops can take place at a larger surface to be coated in several places. In order to distribute the liquid applied to the surface layer material over the surface, it may be provided in a development to do this using a scraper. The scraper can be moved over the substrate to distribute the liquid. On the other hand, the substrate can also be moved.

It can be provided that a crystal wafer broken in a crystal plane is used as the scraper.

After distributing the liquid layer material over substantially the entire surface of the substrate to be coated, the surface of the liquid layer is touched in an embodiment of the invention, for example with an edge of the scraper. This creates the tearing of the liquid film of material with the adjoining one

Retreatment of the material.

To improve the outflow of the material film, the substrate can be made oblique in a further embodiment of the invention.

In a further development of the invention it can be provided that, after the application of an ultrathin layer in the described manner, a second layer is applied in exactly the same way, wherein the process can also be repeated for applying a plurality of layers.

The multiple layers may be layers of the same material or different materials. It is also possible to use the same but differently pre-processed or preprocessed material for the further layers.

In particular, it can be provided that a layer of alternating materials is produced.

In a further embodiment of the invention it can be provided that the substrate is heated before and / or during the application of the layer.

As already mentioned, the method can be carried out in a normal atmosphere and / or under normal conditions.

In a further embodiment of the invention can be provided that the configuration of the individual layers using the process parameters temperature, humidity,

Oxygen partial pressure and coating time is changed.

In a further development of the invention can be provided that the properties of

Layer materials are changed by their stoichiometric composition.

It can also be provided that the conductivity duration of a layer system is preset by means of the selection of the stoichiometric composition of the materials used and / or the number of layers deposited. It has been found that a multilayer produced by the process progressively has a lower surface roughness from layer to layer than the substrate on which the layers were deposited. Therefore, the method of reducing the surface roughness of the final product from the substrate can be used.

Further features, details and advantages of the invention will become apparent from the

Claims and the abstract, the wording of which is incorporated by reference into the content of the description, the following description of preferred

Embodiments of the invention and with reference to the drawing. Hereby show:

Figure 1A is a schematic representation of the method proposed by the invention for producing ultrathin layers;

Figure 1B is a schematic representation of the method from another direction;

Figure 2 is an illustration of a glass substrate having a stepped triple layer;

FIG. 3 shows a graphic representation for demonstrating the achieved layer thicknesses in a single layer as well as in multilayer systems by means of X-ray reflectometry;

Figure 4 is a table of the results of X-ray reflectometry of Figure 3 and a

Model presentation of the thicknesses and electron densities of the alternating layers in the multilayer system;

FIG. 5 shows a graphical representation for indicating the possible conductivities of FIG

Layers / multilayers by means of current-voltage measurement;

FIG. 6 shows a graphical representation for indicating the possible conductivities of FIG

Layers multilayers compared to model over a period of 60 days after preparation;

FIG. 7 shows a graphic representation for specifying the transparency of multilayers;

FIG. 8 shows a graphic representation of the achieved layer thicknesses in a single layer and in multilayer systems by means of X-ray reflectometry;

9 shows a graphic representation of the achieved layer thicknesses in the case of a single layer and in multilayer systems by means of X-ray reflectometry.

The exemplary embodiments illustrated in FIGS. 1-7 were carried out with the aid of gallium or gallium hydroxide. FIG. 8 relates to a gallium-indium alloy and FIG. 9 to a gallium-indium-tin alloy.

In FIG. 1, the process for the production of ultrathin layers of metal and metallic oxides / hydroxides is provoked by the forced wetting

Film contraction shown. An example is elemental gallium. In the case of alloys Before stoichiometric weighing of the products are necessary, which are then tempered together and melted down to an alloy.

In advance of the layer production of Ga / Ga (OH) x shown in FIGS. 1A and 1B, gallium is liquefied in a polyethylene bottle. The polyethylene bottle with gallium is in a

Set water bath on a laboratory hot plate and warmed to about 60 ° C. This

ensures a continuous liquefaction of the entire gallium volume in the polyethylene bottle. The liquefied gallium is poured onto the wet-chemically cleaned substrates 1 by means of a Pasteur pipette made of clear soda lime glass. The wet-chemical

Pretreatment of substrates 1 is performed with RCA cleaning. The substrates 1 are located on any smooth base 10 (see FIG. 1B) on a hotplate which provides a constant process temperature of 35 ° C. for working with pure gallium. For certain alloys, the temperature may be higher. This will help prevent potential undesirable solidification by crystallization of the metal on substrate 1. The amount of liquid gallium used will vary depending on the intended sample size.

In FIG. 1A, it can be seen how the deposition of the ultrathin layer of gallium 2 on various substrates 1 with a silicon wafer as a preparation instrument, hereinafter referred to as scraper 3, is realized. One necessary for the preparation

To ensure positive engagement between the substrate 1 and scraper 3, a straight as possible breaking edge on scraper 3 is to be used. This scraper 3, to whose

Realization of the silicon wafer is broken in the crystal plane and RCA is cleaned, is placed at an angle of about 90 ° on the substrate 1 and brought to the already on the substrate 1 located liquid gallium 2, see the position 1 on the left in Figure 1A. The scraper 3 is arranged to act on the substrate 1 only by contact with the liquid gallium 2, but no direct contact between it

Scraper 3 and the substrate 1 takes place. This would lead to unwanted scratch-like defects on or in the layers. To avoid this direct contact spacers can be used, which are, for example, glass slide 5 with a thickness of 1 mm and thereon unpolished

Silicon wafer 6 can act with a thickness of about 0.4 mm. Due to their rough surface structure, these silicon wafers 6 are not wetted by gallium.

The scraper 3 is then tilted against the gallium 2, so that it is in contact with the gallium 2 over a larger area. The scraper 3 is now held at an angle of about 60 ° to 80 ° to the substrate 1. The substrate 1 is then slowly and constantly moved under the scraper 3, as attempted by the various positions 1 to 4 in Figure 1A. The angle between the scraper 3 and the substrate 1 is reduced during this movement to about 45 °. As a pure example, a speed of 3.5 mm / s can be assumed. The thus accumulated liquid gallium 2 is applied evenly on the substrate 1 by the scraper 3. This creates an approximately 1.5 mm thick and closed liquid gallium cover on the entire substrate. 1

With any edge of the scraper 3, an edge area of the liquid gallium ceiling is now touched. The thus generated anomaly in the energy landscape of the surface of the gallium 2 provokes rupture of the 1.5 mm thick gallium film and generation of a notch 7 to be seen schematically in the position 3 in Figure 1A.

This results in a retraction tendency of the still liquid gallium 2 to the ends of the substrate, which is caused by the high surface tension of the liquid gallium 2. In addition, by tilting the substrate 1, a slope can be generated, whereby the coated area on the substrate 1 is increased. The process to be observed is similar to dewatering the liquid gallium 2 from the substrate 1, but leaving an ultrathin gallium gallium (hydr) oxide film 9 between the liquid phases 8 at the substrate ends which continues to oxidize instantly.

In this way, an ultrathin gallium coating is produced on top of the substrate 1. The process can be repeated several times, whereby the same material or another material can be used for the following layers.

The conclusion at the interface between layer and air forms a cover layer with defined electron density. The final overcoat of a multilayer can provide protection to the underlying layers and provide for sustained electrical conductivity of the multilayers.

FIG. 2 shows a graphic representation of a soda-lime glass slide 1. A multi-layer system composed of triple Ga / Ga (OH) x is deposited on the slide 1 and designated accordingly. The short sides of the glass slide still show the liquid residue of spent gallium. In between, you can clearly see the homogeneous

Ga / Ga (OH) x triple layer. The left side of the image shows an uncoated part of the glass slide 1, followed to the right by a single layer, then a double layer and finally the homogeneous triple layer.

FIG. 3 shows the results of X-ray reflectometry. Shown on the left side are the measured Ga / Ga (OH) x multilayers of single to eightfold layers (1 L to 8 L). The closed symbols here are data points, solid lines the

corresponding fit curves, from which the electron density profile and layer thicknesses (both on the right side) were determined. For a clear presentation, the

Reflection curves each by a factor of 100 and the electron density profiles each shifted by 0.5 e ^~ / ° A ³ vertically. The electron density profiles clearly show the periodically changing A / B formation of the deposited multilayers. From separated measurements of the surface roughness of the substrates used and in comparison to the here The results presented show an extraordinary behavior of the Ga / Ga (OH) x multilayers. Layer by layer, the originally high roughness of the substrate is compensated. As a result, the multilayers have lower interfacial and surface roughness than the pure substrates.

The graph at the bottom right in FIG. 3 shows the monotonous increase in the total layer thickness with the number of coating steps. In this case, the final thickness of a single layer is about 3.5 nm and the thickness of an eightfold layer is about 23 nm. The findings from reflectometry are systematically incorporated into the generation of the model concept from FIG.

The results for the layer thicknesses from X-ray reflectometry are listed in tabular form in FIG. 4.

FIG. 4 below shows a model of a multilayer. This model was developed to describe the Ga / Ga (OH) x system, but it also applies to the low-melting alloys in a first approximation.

Where:

Ig = total thickness of the layer system

11 = high electron density component (for example, gallium)

1 ₂ = low electron density component (for example, gallium hydroxide)

Irep = repeating unit of Ii + l ₂ (generated per coating cycle)

Itop = passivating cap at the film / air interface with lowest

Electron density throughout the layer system.

FIG. 5 now shows the dependencies of the electrical resistivity with the number of layers produced. It can be seen that the single layer (1 L) oxidizes within 60 days. Although this process is associated with the increase in transparency, but with loss of electrical conductivity. The latter comes to a halt after about 60 days. The double layer (2 L) shows not only a lower resistivity than the single layer but also a reinforced one

Insensitivity to environmental influences, causing only small changes in resistivity over the course of 60 days. From the triple layer, a certain robustness of the electrical conductivity can be assumed. The layer (s) at the film / air interface seem to conserve the electrical conductivity of the layers below in the mold so that from a certain number of layers (for Ga / Ga (OH) x = triple layer) a permeation barrier to reactive H ₂ 0 , OH ^" and O2 molecules can be observed.

FIG. 6 summarizes the results from FIG. 5 with the addition of the results of the individual layer thicknesses from X-ray reflectometry (FIG. 3) and shows the trend of FIG Development of the electrical resistivity between the day 0 and the day 60 since the layer production. In this case, the specific resistance of the electrically conductive portion (gallium) is plotted against the determined total layer thickness. The graph also corroborates the statement that the specific resistance decreases per layer number or, conversely, that the electrical conductivity increases. Again, it can be seen again that, starting from a triple layer (3 L), a protective mechanism with respect to the electrical conductivity is established which, starting at a quadruple layer (4 L), ensures stability of the conductivity over the time period shown. Finally, the

Six-layer after the measurement period, approximately the same

Conductivity as at the beginning of the measurements. The gray parallel to the abscissa (bulk gallium) represents the literature value of resistivity for pure gallium. This value is 141 nQm and serves as a reference for the means presented here

Preparation method of prepared layers. It is noteworthy that 141 nQm holds for gallium in volume, but this value is even almost reached by a deposited sixfold layer (6 L) Ga / Ga (OH) x. It should be noted that the six-layer (6 L) has just about 17 nm total layer thickness and even more than 50% electromagnetic radiation in the wavelength range from 400 nm to 2200 nm

can transmit (see Figure 7). This wavelength interval is also the working range of photovoltaic and transparent conductive components, with which the layers produced here appear formally predestined for such applications as electrode material.

FIG. 7 now shows results from the spectroscopic measurement, especially in the range VIS / NIR (wavelength spectrum of the visible and the near infrared light). The figure shows that, although the absorption of light with increasing

Number of layers increases, but up to the eight-fold layer (8 L), a significant proportion of the radiation is transmitted. Furthermore, a decreasing absorption curve can be seen beginning at about 700 nm towards smaller wavelengths. This affects all layers from the triple layer (3 L) upwards and indicates a metallic conductor, since this characteristic is characteristic of such. In contrast to this, the absorption behavior in the case of a single layer (1 L) is constantly low over the entire wavelength range and in the case of a double layer (2 L) an ascending curve can be recognized even from the wavelength of 700 nm downwards.

FIG. 8 shows the results of X-ray reflectometry. Shown on the left side are the measured Galn / Galn (OH) x multilayers from single to quadruple layer (1 L to 4 L). For clarity, the Fresnel-normalized reflection cures were shifted vertically. The layer thicknesses were determined by the amplitude distances of the

Oscillations calculated and graphed on the right side of the figure. The stoichiometric ratio of the gallium / indium alloy used is 1: 1.

FIG. 9 shows the results of X-ray reflectometry. Shown in the upper illustration are the measured GalnSn / GalnSn (OH) x multilayers from single to Quadruple layer (1 L to 4 L). For clarity, the Fresnel normalized reflection cures were shifted vertically. The layer thicknesses were over the

Amplitude distances of the oscillation calculated and shown graphically in the lower part of the figure. The stoichiometric ratio of the alloy used

Gallium / indium / tin is 7: 2: 1. This corresponds approximately to the eutectic of the system.

Possible practical applications of the method proposed by the invention:

Advantages for industrial production:

Special features of the presented coating method can save energy, equipment, personnel and time. Thus, the coating method can replace or supplement the known conventional coating methods. In addition, new applications result from special internal structures of multilayers prepared by forced wetting with provoked film contraction.

Application as electrode material for photovoltaic elements: most of the production methods of materials for this application area are based on an epitaxial growth of the layers. This has the consequence that one

crystallographic repetition between the individual layers is generated. The consequences are causal, d. H. layers with identical properties are produced,

for example, identical electron densities. This is different in the forced wetting with provoked film contraction, that is, the method proposed by the invention. Here, by means of described influence in the layer deposition, each multilayer can be changed, the multilayer from layer to layer

can have different properties.

The starting materials used or their layered products have so-called light-trapping structures and are thus able, for example, to utilize solar photons energetically. Thus, it could be possible, a kind of innovative

Tandem solar cell with this method and from the described

Metals / semi-metals / alloys. This can be realized if the layer sequence is deliberately chosen so that strong light-absorbing alloys are deposited near the substrate and overlying layers of alloys are deposited, which successively absorb less light. A tandem solar cell of this shape would have great potential to use a wider range of the solar electromagnetic spectrum than was previously possible.

The use of the layer systems produced according to the invention as so-called radiation absorption material (RAM) seems to be possible. These materials are used in passive armor engineering to increase the backscatter cross section of

military vehicles and in particular of aircraft to a fraction to decrease. As a result, and coupled with certain geometrical effects, it is possible, for example, to reduce an aircraft to the radar signature of a bird of prey. RAMs have structural properties that produce phase jumps of radiation through the material. Hereby it is possible by refraction effects on layers, whose distance corresponds to the wavelength or an integral multiple thereof, to have offset the extruding beam by 180 ° with respect to the intruding beam. This results in a destructive interference of the amplitudes of the coherent radiation and the backscatter signal is greatly reduced. The invention shows that distances of the

metallic components of a layer can be adjusted by controlling the growth of the oxide or hydroxide. This results in homogeneous and lateral extended layers with vertical alternating composition in defined

Distance to each other are produced, which is beneficial to the RAM concept.

Antireflection coatings for optical applications in research and technology are also conceivable with the coating method described. For example, such coated glass substrates as a beam splitter (coupling out partial beams of the

Beam geometry), where their performance can be regulated by the number of layers deposited and by the use of defined materials / alloys.

The inventive method can also serve as a coating method for glasses of all kinds (windows, building facades, vehicle windows), for glazed clay tiles or polished metal components, provided that they have sufficient smooth and wettable surfaces.

The method can be used in applications where a high microhardness is required for a small layer thickness. The layers tend to be thermally animated

Oxidation not only increases its transparency, but also becomes

Thickness growth excited, where they continue to be stable as a solid body.

Another application may find the method in cases where it is necessary to use a material that changes over time from an electrical conductor to an insulator, but without losing any other properties. This is made possible by the processual implementation of the coating steps by means of the invention and by the use of the already known starting materials. Single or double layers are suitable for this because they lose their conductivity in a chronologically manageable framework.

Other use cases:

transparent electrodes in organic light emitting diodes, touch screens and

liquid crystal displays

- Cover electrodes for photodiodes and thin-film solar cells - Thermal protection on window glass panes

- invisible antenna material

- Inhibiting layer against electrostatic charge.

Claims

claims

1. A method for producing ultra-thin layers of materials of the group of metals and semimetals and their alloys on substrates (1), with the following

Steps:

on the substrate (1) the layer material is applied in liquid form,

- The liquid coating material is over the surface to be coated of the

Distributed substrate (1) in a thin layer, whereby the surface of the substrate is wetted and the thin layer forms a liquid film of material,

- In the surface of the liquid film of material, a notch (7) is introduced,

- The material of the liquid film of material is due to his

Return surface tension to the edges,

- At the edges remain liquid phases (8) (metal drops) back and

in between an ultra-thin film of material (9).

2. The method of claim 1, wherein the layer material is dropped in liquid form onto the substrate (1).

3. The method of claim 1 or 2, wherein the on the substrate (1) applied layer material using a scraper (3) over the surface to be coated of the substrate (1) preferably with simultaneous relative movement between the scraper (3) and the substrate ( 1) is distributed.

A method according to claim 3, wherein an edge (4) of the scraper (3) is used in the surface of the liquid film of material to form a notch (7).

5. The method according to any one of the preceding claims, wherein the substrate (1) is inclined before or after the introduction of the notch (7) in the surface of the liquid material film (9) for improving the flow behavior of the material film (9).

6. The method according to any one of the preceding claims, in which a plurality of ultrathin layers are produced in succession by the same method.

The method of claim 6, wherein the multiple layers are made of different materials and / or differently treated materials.

8. The method according to claim 6 or 7, wherein a layer structure of a

alternating layer composition is produced.

9. The method according to any one of the preceding claims, wherein the substrate (1) is heated during the production of the ultrathin layer.

10. The method according to any one of the preceding claims, which in a

Normal atmosphere and / or carried out under normal conditions.

11. The method according to any one of the preceding claims, wherein the configuration of the individual layers using the process parameters temperature, humidity, oxygen partial pressure and coating time is changed.

12. The method according to any one of the preceding claims, wherein the properties of the produced material film (9), namely the electrical conductivity, the thickness and the surface roughness, are changed by the stoichiometric composition of the layer material used.

13. The method according to any one of the preceding claims, wherein the conductivity of the produced material film (9) or a layer system by means of the stoichiometric composition of the materials used and / or the number of deposited layers is preset.

14. The method according to any one of the preceding claims, wherein by the

Application of a plurality of material films (9), the resulting surface roughness of the uppermost material film (9) relative to the substrate (1) is reduced.