RU2530784C1 - Method of durability assessment for thin protective coatings on materials under high energy effects - Google Patents

Abstract

FIELD: measuring instrumentation.

SUBSTANCE: high energy effect is achieved by focused ion beam, and scanning probe microscope with measurement result processing means is used as an instrument for determination of physical destruction parameters in small areas of samples. Destruction of thin protective submicron coatings in the form of grooves made by the focused ion beam is determined by groove depth and/or microrelief along centrelines compared with similar groove parameters of a reference sample. Further, time rate of protective coating destruction is determined for sample and reference sample. Durability of the protective coating in comparison to a reference sample material is deduced by comparing these rates.

EFFECT: developed method of comparative quantitative parametrical assessment of durability of thin submicron protective coatings under corpuscular high energy flows, mainly in vacuum, simulating open space conditions.

5 cl, 4 dwg, 1 tbl

Description

Technical field

The invention relates to a method for evaluating the protective properties of thin coatings from surface degradation (destruction, erosion, spraying) of protected materials when exposed to high-energy radiation, mainly in vacuum.

State of the art

Currently, much attention is paid to the issues of resistance of materials of external structures used in spacecraft (SC) to streams of corpuscular high-energy radiation in open space. In outer space, spacecraft materials are exposed to such cosmic factors as ultrahigh vacuum, high-energy radiation, sudden temperature changes, micrometeorites, etc., which lead to a decrease in the operational life of both individual nodes and the entire spacecraft. The influence of outer space factors leads to the destruction of materials and the loss of their physical and mechanical characteristics, as well as to an increase in the gas evolution products of materials of the spacecraft’s external structures and, accordingly, to an increase in the density of the spacecraft’s own external atmosphere, which, when deposited on the optical elements of the spacecraft, causes a deterioration in the optical characteristics of spacecraft devices .

Among the means of protecting spacecraft materials from the effects of outer space factors, apart from the selection of special materials resistant to KP factors, the creation of ion-modified surface layers with thin coatings (films) on materials used in spacecraft is also distinguished.

The choice of materials is carried out according to applicability in the design of spacecraft used in open space. The selection of basic materials for use in spacecraft is carried out according to the following criteria: - according to the parameter of mass loss, it is recommended to use non-metallic materials used for external structures of the spacecraft with a total mass loss of not more than 1.0%. It is allowed to use materials with an excess of the total mass loss of 1.0% if it is impossible to replace them with similar materials with satisfactory rates.

The choice of protective thin coatings is carried out on the basis of the a priori characteristics of the coating materials in terms of resistance to space factors and their good adhesion to the material to be protected. The methods for obtaining and quantifying these characteristics are associated with great difficulties, primarily because of the small micron, submicron, or even nanoscale thickness of these coatings.

To work with such small thicknesses of materials, there are currently a number of general methods, among which we can primarily distinguish:

- for controlled fractures of small material thicknesses - focused ion beam method (FIP, English version: Focused Ion Beam (FIB) or Focused Particle Beam);

- for research and evaluation of various characteristics of such materials - scanning probe (contact) or electron (non-contact) microscopy methods.

For the use of FIP to remove small thicknesses of materials, there are a fairly large number of patents in the field of lithography, for example, US patent 5482802 (IPC G03F 1/00, G03F 9/00, published 09.01.1996) for the invention "Removing material by focused particle beams" (Material removal with focused particle beams).

However, these patents do not use scanning microscopy.

According to the simultaneous use of FIP and scanning microscopy (electron, but not probe), only 1 US patent 7069155 (IPC G01B 5/00; G03F 1/00; G06K 9/00, published on 06/27/2006) for the invention "Real-time analytical monitor for inspection of soft defects on the grid ”(Real time analytical monitor for soft defects on reticle during reticle inspection).

However, this invention relates to the field of semiconductor technology for chemical (rather than physical) analysis of defects in photolithographic grids.

The closest analogue (prototype) for the object of research - the protective coating of the material, the physical orientation of the tasks, the sequence of actions and a set of essential features can be recognized as the method according to US patent 4704297 (IPC G01B 11/06; G01N 19/04; G01N 21/94; G01N 21 / 95; B05D 1/04, published 03.11.1987) “Assessing powder coatings”, which includes focused high-energy impact on small areas of two samples with a coating and a reference (without coating) to compare and determine the parameters of destruction (erosion ) after exposure. This method is intended to quantify the adhesion of the coating to the substrate and assess the thickness of the coating itself. The essence of the prototype method is in the direction of the destructive high-energy impact - focused hydrojets under pressure on the surface of the coating from a fixed distance for a given period of time. A tool for comparative control of the destruction parameters of the material coating sample and the second reference sample is presented as an infrared radiation source and a detector for the passage of this radiation through the areas of exposure of the samples after the destructive effect. Mostly experiments are carried out with repetitions and in a sealed enclosure to exclude the influence of environmental factors.

However, in this invention there is no use of the FIP method and the method of scanning microscopy, which were not yet practically used in the 1980s. Therefore, this method is not applicable to assess the resistance of coatings with thicknesses less than 1 μm, currently used in the space industry. Since the effect used in the indicated method is quite large-scale, it does not allow to achieve an estimate of the submicro-sized areas exposed. The use of FIP as the acting radiation on the coating and scanning microscopy as a means of analysis will eliminate these limitations and use the proposed method for evaluating micro- and nanoscale coatings.

Disclosure of invention

The objective of the invention is the development of a method of comparative quantitative parametric assessment of the resistance of thin submicron protective coatings of materials to the effects of corpuscular high-energy flows, mainly in vacuum, simulating the conditions of outer space.

The main difference between the proposed method and the prototype method is the simultaneous use of a focused ion beam in a single technological cycle as a controlled high-energy micro-action on protective coatings and materials and a quantitative assessment of the resistance of various coatings to the effects of FIP corpuscular radiation through the use of scanning probe (contact) microscopy or profilometry. The advantage of using a probe microscope in comparison with a prototype, in which there is no method for determining the profile of the sprayed area, is the high resolution of determining the depth of the groove obtained during the destruction (spraying) from the influence of FIP, thus it is possible to identify the depth of the groove with a height of up to 1 nm and speeds destruction of several tens of picometers per second (pm / s).

The essence of the proposed method is to compare the rate of destruction of the material and the material with a protective coating when irradiating surface areas with a high-energy flow of directed ions, mainly gallium, and the subsequent comparative analysis of these areas according to the depth parameters of grooves and / or grooves formed from the influence of FIP according to the values of their middle lines .

The method for assessing the durability of thin protective coatings of materials under high-energy exposure to them includes a single technological cycle consisting of a controlled focused high-energy exposure over a specified period of time on small areas of two compared samples: test coated and reference, instrumental determination and comparative analysis of the physical parameters of fractures in small areas of the samples as a result of exposure. In this case, a focused ion beam is used as an impact, and a scanning probe microscope with means for processing measurement results is used as a tool for determining the physical parameters of fractures in small areas of samples. The destruction of thin protective submicron coatings in the form of grooves in them from the action of a focused ion beam is determined by the parameters of the depth and / or microrelief of the grooves by the values of their middle lines in comparison with the similar parameters of the grooves of the reference sample. Next, determine the destruction rate of the protective coating and the reference sample in time. Compared to these destruction rates, the degree of resistance of the protective coating is judged in comparison with the material of the reference sample.

Advantageously, the same backing material protected by the coating is selected as a reference sample, but already without coating.

As thin protective coatings, coatings obtained by deposition from carbon, or metal, or semiconductor, or polymeric materials can be used.

They can carry out repeated tests of the same samples with varying the controlled parameters of the focused ion beam exposure and the time of its exposure to the samples in the same or in different small areas of the samples.

Mostly test samples in sealed vacuum conditions with the possibility of modeling the factors of outer space.

List of figures

Figure 1 - external view of the installation for implementing the method.

Figure 2 - arrangement of the coating and substrate when exposed to FIP.

Figure 3 - scheme of scanning probe coating and substrate with grooves after exposure to FIP.

Figure 4 - 3D model of the microscopic relief of the surface area of the coating and the substrate with grooves after exposure to FIP.

The implementation of the invention

To implement the proposed method, it is necessary to use the methods of FIP and scanning probe microscopy, implemented in a single technological cycle. To do this, we used the Nanofab-100 lithographic nanotechnological complex manufactured by NT-MDT (Russia, Zelenograd MO), which has a chamber for loading samples and preliminary evacuation, a system for transporting and moving samples, locking systems and focused working chambers ion beam and scanning probe microscopy. Figure 1 shows the appearance of this complex, which consists of a vacuum system 1, a lock chamber for loading samples of materials 2, a lock chamber for loading probe sensors 3, a transport system for moving samples 4, a working chamber for a focused ion beam 5, and a working chamber for scanning probe microscopy 6. The vacuum system includes fore-vacuum and ion pumps providing a vacuum of 2 · 10 ^-7 Pa. The sample transfer system performs precise positioning of the sample under the influence of a high-energy radiation beam and moves the sample from the loading chamber to the working chamber.

Parameters of the working chamber of a focused ion beam:

 ion source Ga  chamber pressure ≤1.3 × 10 ^-9 Pa  FIP operating pressure ≤10 ^-6 Pa  ion beam energy 3 ... 30 kV  ion current 1 pA to 40 pA  minimum spot size <10 nm at a current of 1 pA

Installation work

Ultra-high vacuum is constantly maintained in the working chambers. Loading is carried out through the loading chamber, samples are vacuumized to a pressure of 2 · 10 ^-7 Pa. Samples using manipulators are transported into the working chamber of a focused ion beam. In the regime of high-energy ion irradiation of the selected portion of the sample in the optimal mode, the ion emission current is from 1 pA to 40 nA. Ions are accelerated to a nominal energy of 30 keV. Part of the beam is cut out by an aperture diaphragm and is focused by electrostatic lenses. To deflect the beam and correct astigmatism, a stigma-deflector is used.

Parameters of high-energy ion radiation:

ion energy Ga ⁺ 30 keV  time variable  extractor voltage 9 ± 0.3 kV  suppressor voltage 1 ± 0.1 kV  capacitor voltage 10 kV  lens voltage 15888 ± 10 V  aperture 10 microns  beam current 2 μA  operating pressure 10 ^-9 mbar  beam diameter 10 nm

In preparation for comparative tests, two samples of uncoated material and a material with a thin protective coating are selected, which are placed in a vacuum chamber.

To implement the proposed method, in contrast to the technology of pure lithography, for which Nanofab-100 was originally intended, it was necessary to make a number of significant changes and adjustments to the initial operation of the complex, namely, to create a high-energy effect in order to determine the durability of thin protective coatings, an additional adjustment was made the ion column of the source to control the beam parameters, since the assessment of the resistance of micro-dimensional coatings as a result of exposure requires repeatability and troliruemosti parameters of influence, and lithography is practically required. And also, a special table for samples was prepared and built into the unit for the purpose of precise control of their position during accurate comparative analysis of the samples by scanning probe microscopy after exposure to FIP. The area of influence of the sample has microdimensional parameters, which requires high accuracy of positioning in contrast to lithography, where the impact is of a group nature in area, which is two orders of magnitude more than the microdimensional areas of exposure according to the proposed method.

An example of the method

Figure 2, 3 shows the scheme of the FIP effect on the samples and scanning with a probe samples with grooves after the FIP action (substrate 7, coating 8, FIP 9, probe 10), figure 4 is a 3D model of the microscopic relief of the coating surface and substrate after exposure to FIP.

A silicon diamond-like material with a thickness of 80 nm was chosen as a coating for the substrate — a silicon wafer. The selected material as a coating on a silicon wafer is irradiated with a high-energy focused flux of gallium ions so that both the coating material and the silicon substrate are irradiated. Thus, the impact of high-energy corpuscular ionized flows simulates the factor of open space. This allows you to irradiate the material with ionized streams and control the rate of destruction of the material. To create a high-energy focused flux of gallium ions, a special separate source is used, which is a column design that includes a block of liquid metal ion sources (LMMI) from gallium, which provides high uniformity, intensity and controllability of the flow, and an electronic magnetic lens system. Upon receipt of a high negative voltage on the extracting electrode, the LMIC is exposed to an electric field. An electric field of sufficient strength forms a Taylor cone from the gallium substrate covering the substrate. The deformation of a liquid metal on a substrate into a cone occurs only after exposure to an electric field of sufficient power. After reaching such a critical voltage, a cone is formed and the emission of the FIP begins. At the beginning of the emission, the current rises to about 200 μA / kV of the applied voltage. At this stage, the current density on the surface of the liquid reaches approximately 10 ⁸ A per square centimeter. A very small tip (incipient stream) with a radius at the end of about 20 Å protrudes from the end of the cone. At this point in the nucleation of the jet, ion emission occurs due to evaporation in an electric field. The source simulates corpuscular radiation in the energy range from 3 to 30 keV.

This high-energy ion radiation was exposed to samples of the starting material (silicon wafers) and a material with a protective thin coating (diamond-like carbon) in an amount of three regions for each sample with an irradiation area larger than the size of the ion beam. A section of up to 400 μm wide and 400 μm long was irradiated by a focused flux of gallium ions. After that, the samples were transferred to the working chamber of a scanning probe microscopy, where the regions were scanned after exposure to FIP (see Fig. 4). During the scan, the depths of the formed grooves and the roughness parameters of the microrelief of the ion exposure region — the average deviation of the profile — were determined. Based on the results of determining the depth and surface microrelief of the surface of the grooves, we estimated the comparative rate of destruction of thin protective coatings under the simulated effect of high-energy radiation on them.

The silicon substrate acts as a reference sample with a known and constant rate of destruction (sputtering, erosion) and serves to comparatively evaluate the rate of destruction of the coating relative to the rate of destruction of the substrate. Since silicon is a fairly well-studied material at present in the field of sputtering by a focused ion beam with known calculated data, the use of this method for evaluating various thin protective coatings, including composite ones, in comparison with known data on silicon can characterize the coating material for resistance to the effects of corpuscular high-energy flows.

In the given example, the character of sputtering under the influence of an ion beam of both the substrate material (silicon) and the coating material (diamond-like carbon) turned out to be such that the irradiated surface has a minimum roughness of the relief surface (see Fig. 4, individual high columns in the 3D model these are single failures in the operation of the probe microscope, which should not be taken into account). Therefore, in this case, to assess the spraying speed of the material, the depths of the grooves were determined (see table 1). There are cases when the nature of the spraying of the material is such that the surface relief of the bottom of the grooves after exposure will have a very large roughness. For example, this is characteristic of polymeric materials. In this case, to determine the spraying speed of the material, statistical processing of data on the topology of the surface exposed to ionic exposure will be required in order to determine the average spraying depth.

For the given example, the results obtained suggesting a higher resistance of the coating to ionic effects in comparison with the resistance of single-crystal silicon. Thus, the sputtering rate of the silicon substrate (from 40 to 53 pm / s) was 2 to 2.5 times higher than the spraying rate of the coating material (from 15 to 26 pm / s), which indicates a sufficiently high resistance and potential applicability of the coating for materials used on spacecraft.

Table 1 Area number FIP current, pA Zone of influence, microns The total time of exposure to FIP, s Depth of sprayed material in the zone of influence of FIP, nm Spraying speed, pm / s calculated for the substrate real for the substrate real to cover the substrate coverings one 46.5 10 × 30 325 15.38 13 5 40 fifteen 2 47.3 661 31.82 31 eleven 47 17 3 47.0 1321 63.19 70 35 53 26

As a result, the proposed method can be used to assess the resistance of protective coatings with a thickness of less than 1 μm and has a high degree of reproducibility of the results due to the precise setting of the parameters of the focused action and scanning probe microscopy.

Claims (5)

1. A method for assessing the resistance of thin protective coatings of materials under high-energy exposure to them, consisting in a single technological cycle consisting of a controlled focused high-energy exposure over a specified period of time on small areas of two compared samples: test coated and reference, instrumental determination and comparative analysis physical parameters of damage in small areas of the samples as a result of exposure, characterized in that as the impact and use a focused ion beam, as a tool for determining the physical parameters of destruction in small areas of the samples, use a scanning probe microscope with means for processing measurement results, the destruction of thin protective submicron coatings in the form of grooves in them from the action of the focused ion beam is determined by the parameters of depth and / or microrelief grooves according to the values of their midlines in comparison with similar parameters of the grooves of the reference sample; Further, the destruction rates of the protective coating and the reference sample are determined in time, compared with these destruction rates, the degree of resistance of the protective coating is judged in comparison with the material of the reference sample. 2. The method according to claim 1, characterized in that as the reference sample, the same substrate material protected by the coating is selected, but already without coating. 3. The method according to claim 1 or claim 2, characterized in that the coatings obtained by deposition from carbon, or metal, or semiconductor, or polymeric materials are used as thin protective coatings. 4. The method according to claim 1, characterized in that repeated tests of the same samples are performed with varying the controlled parameters of the focused ion beam exposure and the time of its exposure to the samples in the same or in different small areas of the samples. 5. The method according to claim 1, characterized in that the test samples in sealed vacuum conditions with the possibility of modeling space factors.

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