Substrate with antireflection coating and method for producing same

Abstract

A substrate is provided with an abrasion resistance antireflection coating. The coated substrate includes a multilayer antireflection coating on at least one side. The coating has layers with different refractive indices, wherein higher refractive index layers alternate with lower refractive index layers. The layers having a lower refractive index are formed of silicon oxide with a proportion of aluminum, with a ratio of the amounts of aluminum to silicon is greater than 0.05, preferably greater than 0.08, but with the amount of silicon predominant relative to the amount of aluminum. The layers having a higher refractive index include a silicide, an oxide, or a nitride.

Claims

1. A method for producing a coated substrate, comprising: depositing a multilayer antireflection coating to at least one side of a substrate, the coating comprising successively deposited layers having different refractive indices, with layers having a higher refractive index alternating with layers having a lower refractive index, wherein the layers having the lower refractive index consist of silicon oxide with a proportion of aluminum and a ratio of aluminum to silicon determined according to a relationship: n(Al)/(n(Si)+n(Al)) that is greater than 0.05, but with an amount of silicon predominant relative to an amount of aluminum, wherein the multilayer antireflection coating comprises an upper surface formed by a layer of the lower refractive index that has the proportion of aluminum providing a particularly good antireflection effect and avoiding rapid abrasion of the upper surface; and wherein the step of depositing the multilayer antireflection coating comprises depositing the layers having the higher refractive index consisting of silicon nitride with a proportion of aluminum and a second ratio of an amount of aluminum to an amount of silicon determined according to the relationship that is greater than 0.05.

2. The method as claimed in claim 1, wherein the ratio is greater than 0.08.

3. The method as claimed in claim 1, wherein the ratio of aluminum to silicon does not exceed 0.2.

4. The method as claimed in claim 1, wherein the step of depositing comprises magnetron sputtering with a pulsed field exciting of a plasma and having a power density of at least 100 watts per square centimeter of a target surface.

5. The method as claimed in claim 4, wherein the plasma is maintained in intervals between pulses.

6. The method as claimed in claim 1, wherein the second ratio is greater than 0.08.

7. The method as claimed in claim 1, wherein the step of depositing the multilayer antireflection coating comprises depositing the coating to a total coating thickness ranging from 200 to 400 nm.

8. The method as claimed in claim 1, wherein the step of depositing the multilayer antireflection coating comprises depositing at least two layers having the higher refractive index and at least two layers having the lower refractive index.

9. The method as claimed in claim 1, wherein the step of depositing the multilayer antireflection coating comprises depositing a stack of four successive layers having a bottom layer, a first layer on the bottom layer, a second layer on the first layer, and a top layer on the second layer, the bottom layer being the higher refractive index layer, the first layer being the lower refractive index layer, the second layer being the higher refractive index layer and having a layer thickness that is the largest of the four successive layers within the stack, the top layer being the lower refractive index layer and having a layer thickness that is the second largest of the four successive layers within the stack, and wherein the bottom layer and the first layer have a combined layer thickness that is thinner than the layer thickness of the top layer.

10. The method as claimed in claim 9, wherein the step of depositing the multilayer antireflection coating further comprises depositing an aluminum-containing silicon oxide layer on the substrate and depositing the stack of four successive layers on the aluminum-containing silicon oxide layer.

11. The method as claimed in claim 1, further comprising providing the substrate of a material selected from the group consisting of glass, glass ceramic, sapphire glass panel, synthetic quartz glass substrate, crystal, optical glass, and filter glass.

12. The method as claimed in claim 11, further comprising providing the substrate with a curved surface on the at least one side having the multilayer antireflection coating deposited thereon.

13. The method as claimed in claim 12, wherein the step of providing the substrate with the curved surface comprises providing a lenticular surface.

14. The method as claimed in claim 1, wherein the substrate is a sapphire glass substrate.

15. The method as claimed in claim 1, wherein the step of depositing the multilayer antireflection coating comprises depositing the layer having the lower refractive index as the upper surface with an average roughness and a root mean squared roughness of less than 1.5 nm each, based on an area of one square micrometer.

16. A coated substrate comprising: a substrate; and a multilayer antireflection coating on at least one side of the substrate, the multilayer antireflection coating comprising layers having a higher refractive index alternated with layers having a lower refractive index, wherein the layers having a lower refractive index are formed of silicon oxide with a proportion of aluminum and have a first ratio of an amount of aluminum to an amount of silicon determined according to a relationship: n(Al)/(n(Si)+n(Al)) that is greater than 0.05, but with the amount of silicon being predominant relative to the amount of aluminum, and wherein the layers having a higher refractive index comprise silicon nitride with a proportion of aluminum, wherein the layers having the higher refractive index have a second ratio of aluminum to silicon determined according to the relationship that is substantially the same as the first ratio, and wherein the multilayer antireflection coating comprises an interface between the layers having the higher and lower refractive indexes, the interface comprising silicon nitride and silicon oxide.

17. The coated substrate as claimed in claim 16, wherein the first ratio is greater than 0.08.

18. The coated substrate as claimed in claim 16, wherein the second ratio is greater than 0.08.

19. The coated substrate as claimed in claim 16, wherein the antireflection coating has an upper surface formed by a lower refractive index layer of silicon oxide with a proportion of aluminum.

20. A method for producing a coated substrate, comprising: selecting an aluminum-doped silicon target; reactive pulsed sputtering while maintaining a plasma during pulse pauses to sputter a multilayer antireflection coating from the aluminum-doped silicon target to at least one side of a substrate, the coating comprising successively deposited layers having different refractive indices, with layers having a higher refractive index alternating with layers having a lower refractive index, the reactive sputtering comprising changeovers between the layers having different refractive indices by changing a composition of a process gas, wherein the layers having the lower refractive index are formed of silicon oxide with a proportion of aluminum and a ratio of aluminum to silicon determined according to a relationship: n(Al)/(n(Si)+n(Al)) that is greater than 0.05, but with an amount of silicon predominant relative to an amount of aluminum.

21. The method as claimed in claim 20, wherein the ratio is greater than 0.08.

22. The method as claimed in claim 20, wherein the ratio is 0.075 to 0.125.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in more detail by exemplary embodiments and with reference to the accompanying drawings. In the drawings, the same reference numerals designate the same or equivalent elements.

(2) FIG. 1 illustrates an embodiment of a substrate having an antireflection coating;

(3) FIG. 2 shows reflectances of an uncoated and a coated glass substrate in function of the wavelength;

(4) FIG. 3 shows bar charts of the percentage increase in haze value after different abrasive stresses of uncoated and coated glass substrates;

(5) FIGS. 4A to 4F show AFM images of coatings deposited according to the invention;

(6) FIGS. 5A to 5C show exemplary embodiments of principal voltage distributions at the sputtering target for deposition of antireflection coatings; and

(7) FIGS. 6 and 7 show profiles of the proportion of layer constituents in a direction perpendicular to the surface.

DETAILED DESCRIPTION OF THE DISCLOSURE

(8) The example of a product 1 with a coated substrate 3 as shown in FIG. 1 is based on an antireflection coating 5 comprising a total of four layers. Typically, substrate 3 is pane- or plate-shaped, and antireflection coating 5 is deposited on one side 31 of substrate 3. In antireflection coating 5 layers having a higher refractive index alternate with layers having a lower refractive index. Here, layers 52, 54 are composed of silicon oxide with a proportion of aluminum so that the ratio of the amounts of aluminum to silicon is greater than 0.05, preferably greater than 0.08, but with the amount of silicon predominating the amount of aluminum. Preferably, the ratio of the amounts of aluminum to silicon is about 0.075 to 0.125, more preferably about 0.1. Thus, these layers 52, 54 function as low refractive index layers, because they primarily include silicon oxide.

(9) Layers 51, 53, on the other hand, are layers having a higher refractive index and comprise silicon nitride, also with a proportion of aluminum. Preferably, the ratio of the amounts of aluminum to silicon is substantially the same in all layers.

(10) In order to obtain the highest possible mechanical resistance, the initial layer is formed by the mechanically more stable component, i.e. silicon nitride or aluminum-doped silicon nitride as a thin layer, since it is this layer that determines the growth of the rest of the alternating layer system. Subsequently, a thin aluminum-doped SiO.sub.2 layer is applied, followed by a thick aluminum-doped Si.sub.3N.sub.4 coating which provides the resistance to the outside. A subsequent thinner aluminum-doped SiO.sub.2 layer is deposited so that the desired antireflective effect is provided and at the same time the rest of the system does not appear visually more conspicuous upon a possible removal of this layer.

(11) Such a layer structure with four layers has proved to be very durable and enables to decrease the reflectance of a glass substrate such as a borosilicate glass to below 1%, in the visible spectral range. In addition, layer structures that include four layers can still be produced cost-efficiently.

(12) Other designs based on 4 layers which for example start with a low refractive index aluminum-doped silicon oxide layer as an adhesion promoting layer exhibit a lower resistance on borosilicate glasses.

(13) According to the principle structure shown in FIG. 1, an embodiment of the invention, without limitation to the specific layer thicknesses of the example shown in FIG. 1, comprises a layer stack of four successive layers wherein the bottom layer is a silicon nitride-containing higher refractive index layer, the other silicon nitride-containing higher refractive index layer which constitutes the uppermost high refractive index layer of the layer stack has the largest layer thickness in the layer stack, and the top layer of the layer stack is a lower refractive index layer of silicon oxide with a proportion of aluminum and has the second largest layer thickness among the layers of the layer stack, and wherein the first layer and the second layer, the latter like the top layer being a lower refractive index layer of silicon oxide with a proportion of aluminum, have a combined layer thickness which is thinner than the layer thickness of the top layer.

(14) The bottom layer and the subsequent layer which, like the top layer, is a lower refractive index layer of silicon oxide with a proportion of aluminum are preferably very thin, in particular the combined layer thickness of both layers is thinner than the layer thickness of the uppermost, fourth layer.

(15) In the specific example shown in FIG. 1, the layer thickness of the bottom layer is 13 nm, the layer thickness of the subsequent layer is 34 nm, the layer thickness of the other silicon nitride-containing higher refractive index layer which is the second uppermost layer of the layer stack and thus the uppermost high refractive index layer of the layer stack is 127 nm, and the layer thickness of the top layer is 84 nm. Accordingly, the combined thickness of the lowermost and subsequent layers, with 47 nm as described above, is thinner than the layer thickness of the uppermost layer.

(16) Without being limited to the illustrated example, the following ranges of layer thicknesses are preferred: for the bottom layer from 5 to 40 nm, for the subsequent layer from 10 to 40 nm, for the subsequent layer which is the second uppermost layer of the layer stack and thus the uppermost high refractive index layer of the layer stack from 100 to 200 nm, preferably more than 120 nm, and for the uppermost layer from 60 to 120 nm.

(17) The layer design described above and exemplified in FIG. 1 is particularly well suited for glass and glass ceramic substrates. If however the substrate is very hard, such as in case of sapphire glasses, for example for use as watch glasses, it is advantageous in terms of strength and abrasion resistance to use an additional thin layer.

(18) In this case, deposition starts with a thin silicon oxide layer in contact with the substrate on which a layer stack of four layers as described above is deposited.

(19) Therefore, according to a refinement of the invention a substrate, in particular a hard substrate, more preferably a sapphire glass substrate is provided, on which an aluminum-containing silicon oxide layer is deposited, and on the aluminum-containing silicon oxide layer a layer stack with the above thickness ratios. So the layer stack deposited on the aluminum-containing silicon oxide layer again comprises four successive layers of which the bottom layer is a silicon nitride-containing higher refractive index layer, wherein the other silicon nitride-containing higher refractive index layer which constitutes the uppermost high refractive index layer of the layer stack has the largest layer thickness within the layer stack, and wherein the top layer of the layer stack is a lower refractive index layer of silicon oxide with a proportion of aluminum and has the second largest layer thickness among the layers of the layer stack, and wherein the first layer and the second layer which like the top layer is a lower refractive index layer of silicon oxide with a proportion of aluminum, have a combined layer thickness which is thinner than the layer thickness of the top layer. Not only is the combined layer thickness of the two lowermost layers of the layer stack preferably smaller than the layer thickness of the uppermost layer, but also the combined thickness of the two lowermost layers of the layer stack and of the aluminum-containing silicon oxide layer deposited in contact with the substrate.

(20) The curves of spectral reflectance obtained on the basis of a design with a four layer antireflection coating as shown in FIG. 1, are illustrated in FIG. 2. Curve 10 represents the spectral reflectance of an uncoated borosilicate glass substrate. Curve 11 represents the reflectance of such a glass substrate which is coated with a conventional three-layer antireflection coating. Curve 12 shows the reflectance of an inventive, four-layer antireflection coating deposited on a borosilicate glass substrate using magnetron sputtering. Curve 13 also shows the reflectance of such a coating, but in this case deposited by high-energy pulses, or the HiPIMS technique.

(21) As is evident from FIG. 2, the antireflection layer stacks according to the invention allow to achieve antireflective effects with a reduction in reflectance of more than 3%, and with neutral color impression. Reflectance in the visible spectral range is to a large extend less than 1%.

(22) The effect of the mechanical resistance of the antireflection coating according to the invention was analyzed using the Taber test represented by the ANSI/SAE Z26.1-1996 standard, as well as a sand trickling test and an abrasion test (PEI test, Bayer test). The reason for this is that there are different types of mechanical stress and the Taber test only reflects the Taber wheels running on the surface. The sand trickling test simulates the effect of sand grains on coated panes such as windows of vehicles, and the abrasion test simulates the abrasion stress due to e.g. sand grains on windshield wipers.

(23) The different tests were performed to obtain a meaningful statement about the resilience of the surface by joint assessment of the tests.

(24) By using the sputter technique which generally enables to produce very dense layers and layer systems, it is possible to improve the mechanical resistance of the surface by a thin AR layer system with total coating thicknesses in the range of 250-300 nm. In contrast to tribological coatings which are generally much thicker than 1 micrometer (m), this even works with comparatively thin coating thicknesses.

(25) The test results are illustrated in FIG. 3 in a bar chart. A measurement of the percentage increase in haze value was carried out on the same substrates on which the measurement illustrated in FIG. 2 was based. In FIG. 3, the bars for each sample show, from left to right, the increase of the Haze value after a Bayer test, after a sand trickling test, after a Taber test using hard wheels, and after a Taber test using soft wheels. The haze measurement was carried out according to the ASTM D1003-95 standard. The proportion of scattered light in the light transmitted through the sample is compared to the intensity of the total transmitted light. Thus, the measurement values of FIG. 3 represent the percentage increase of the proportion of scattered light due to scratches and other damage of the sample surface after each of the wearing tests.

(26) In the Taber test, metal wheels roll on the coating along a circular path. Additionally, the running direction of the wheels is tilted relative to the tangent of the path, so that during rolling friction occurs between the running surface of the wheels and the surface to be tested. In the sand trickling test, sand falls from a specified height, which was two meters for the illustrated measurements, onto the surface to be tested. In the abrasion test according to the Bayer test, the substrate to be tested is placed in a trough and is covered with sand. The trough is oscillated so that the sand which, in compliance with ASTM F735, is piled to more than 1 cm in the trough, grinds on the surface to be tested under its own weight.

(27) As can be seen from FIG. 3, a conventional antireflection coating (sample 2) in terms of abrasion resistance performs even worse in all tests than the uncoated borosilicate glass (sample 1). The sample coated using magnetron sputtering according to the invention (sample 3) shows a very clear enhancement of resistance in the abrasion test. The other tests show at least a noticeable improvement. Only in the Taber test with soft wheels, the improvement is not clearly visible, however already at a very low level.

(28) For sample 4 which is a sample provided with an antireflection coating according to the invention like sample 3 but deposited using HiPIMS, a further significant improvement in abrasion resistance has been found in all tests. In particular in the Taber tests, an improvement can be seen relative to the layers of sample 3 deposited using conventional magnetron sputtering. The sample even passes the Taber tests with a barely measurable increase in haze value. Generally, in all tests on sample 4 the increase of the haze value is so low that signs of abrasion are not or hardly visible to the naked eye.

(29) Especially the high resistance of sample 4 with respect to the Taber tests may probably be attributed to the extremely smooth surface of the antireflection coating.

(30) Table 1 below shows roughness values for three different flat glass samples (samples: 01-A, 02-H, and 03-F) which were measured using an atomic force microscope, wherein each of the coatings on the samples were deposited using the HiPIMS process, i.e. with a excitation of the sputtering plasma by a pulsed field with pulses of a power density of at least 100 watts per square centimeter of the target surface. The table shows the measurement values of two measurements for each sample (measurement 1, measurement 2).

(31) TABLE-US-00001 TABLE 1 Measurement 1 Measurement 2 Sample RMS [nm] Ra [nm] RMS [nm] Ra [nm] 01 - A 0.9 0.8 1 0.8 02 - H 0.5 0.4 0.6 0.4 03 - F 0.3 0.2 0.3 0.3

(32) The absolute error of the measurement is roughly estimated at 30%.

(33) FIGS. 4A to 4F show the corresponding AFM images, the designations of the sample and the number of the measurements corresponding to the above table. The illustrated portion of the layer surface is 1 micron times 1 micron in each case.

(34) From FIGS. 4C to 4F it can be seen that the roughness of the layers is barely measurable, even with an atomic force microscope. In FIGS. 4E and 4F, the microscope image therefore appears in a nearly uniform shade of gray. At least the two samples 02-H and 03-F show that using the HiPIMS process aluminum-doped silicon oxide and silicon nitride layers according to the invention can be produced that exhibit roughness values Ra and RMS of significantly less than 1 nm, substantially even less than 0.75 nm.

(35) Referring to FIGS. 5A to 5C, the voltage characteristics for different exemplary embodiments of HiPIMS sputtering that can be used for depositing the inventive antireflection coatings will now be explained. In each of the figures, the cathode voltage or voltage applied at the sputtering target is plotted in function of time.

(36) FIG. 5A shows a typical voltage waveform, with pulses 11 of high negative voltages that are periodically applied at the sputtering target for a time t.sub.on. Between these pulses, i.e. during a pulse pause, the voltage is shut off for a time t.sub.off. Optionally, as shown in FIG. 5A, a slightly positive voltage can be applied during the pulse pause. The duty cycle, i.e. the ratio of durations t.sub.on/t.sub.off, is preferably less than 1/5. The repetition frequency of the pulses is preferably between 500 Hz and 10 kHz.

(37) In the example shown in FIG. 5B, the plasma is maintained during the pulse pauses, i.e. during time t.sub.off. This is advantageous, since in this case due to the ions that are present the sputtering process instantaneously starts with the beginning of an HiPIMS pulse. This has proved to be favorable in terms of deposition rates.

(38) In order to maintain the plasma, a DC voltage, or as shown in FIG. 5B an AC voltage, is applied to the target, however with an input power substantially less than the power of the pulses. Preferably, the power density during the pulse pause is smaller than that of the pulses by at least a factor of 10, more preferably by at least a factor of 50.

(39) FIG. 5C shows another variation. Here, pulses 11 are split into a plurality of even shorter pulses 110 with very short time intervals compared to the pulse pause. Otherwise stated, in this case pulses 11 are produced as a sequence of pulses.

(40) This variation has proven advantageous to obtain steep switch-on pulse edges and to avoid a drop of currents during the pulse length. This avoids production of lower energy particles since such lower energy particles would cause less smooth layers. Of course, the variations of FIGS. 5B and 5C may be combined.

(41) FIGS. 6 and 7 show profiles of the amounts or concentrations of layer constituents of the antireflection coatings according to the invention. The profiles were obtained using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The time values plotted on the abscissa represent the duration of the sputter process. Accordingly, measurement begins at time t=0 at the surface of the antireflection coating.

(42) In both measurements a layer system as shown in FIG. 1 was used.

(43) Both figures show the measurement of the concentration profiles of the constituents silicon oxide, silicon nitride, and aluminum. Since for the illustrated profiles signals of aluminum ions were evaluated, the figures do not reflect whether the aluminum exists as an oxide or a nitride in the respective layers. The corresponding profiles for aluminum nitride and aluminum oxide have been omitted for the sake of clarity, however, like the profiles of silicon oxide and silicon nitride, they show a corresponding correlation with the individual layers, with aluminum substantially being present as aluminum nitride in the silicon nitride layers, and substantially being present as aluminum oxide in the silicon oxide layers.

(44) The sample used for the measurement values shown in FIG. 6 had an antireflection coating deposited using conventional magnetron sputtering, and the sample used for the measurement values shown in FIG. 7 had an antireflection coating deposited using HiPIMS. The antireflection coating deposited by HiPIMS which is more complex to be produced, shows sharper transitions at the interfaces between the individual layers. In the example shown in FIG. 7, the interfaces discernible as steps of the concentration profiles are reached at sputtering times of about 220, 520, 620, and 660 seconds. According to the TOF-SIMS measurement shown in FIG. 6, the somewhat less sharply defined interfaces of the layers result in a slight mixing of the phases. In other words, the silicon nitride layers include small amounts of silicon oxide, and vice versa, the silicon oxide layers include small amounts of silicon nitride. Due to the enhanced densification of the layers that were deposited using HiPIMS, a higher abrasion resistance results. However, this is at the expense of higher manufacturing costs, since HiPIMS deposition is more energy intensive and provides lower deposition rates.