CA1106668A

CA1106668A - Microstructured transmission and reflectance modifying coating

Info

Publication number: CA1106668A
Application number: CA297,255A
Authority: CA
Inventors: Gary L. Dorer; Valdis Mikelsons
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1977-02-18
Filing date: 1978-02-17
Publication date: 1981-08-11
Also published as: IT1104102B; IT7848102A0; AU3339878A; AU517534B2; JPS53103754A; JPS6148124B2; FR2381322B1; US4190321A; FR2381322A1; DE2807413A1; GB1597646A

Abstract

913,318 ABSTRACT

An article and method of making the same is disclosed, which article has a substantially antireflecting surface and comprises a substrate having a surface coating of a metal oxide such as an oxide of aluminum magnesium or zinc. The surface coating is a substantially complete conversion of a thin film of the selected metal wherein the thickness of the thin film prior to conversion is at least 5 nanometers. After conversion, the surface is characterized by a plurality of randomly positioned discrete leaflets of varying heights and sizes which extend from the surface a distance of not less than 20 nanometers, the bases of the leaflets contacting the bases of substantially all adjacent leaflets. The antireflecting properties thus provided by such a microstructured surface are such that the total specular reflectance of the coating of visible light incident and normal to the surface is less than 1%.

Description

MICROSTRUCTURED TRANSMISSION AND
REFLECTANCE MODI~YIN~ COATING

This invention relates to articles havlng coatings thereon to reduce ref'lection and, if desired, ~o increase transparency. The invention particularly relates to such single layer coatings as are believed to provide a variable gradient index of refraction at the surface o~ the articles.
Various types of coatings to reduce reflecti-vity and improve the transparency of articles such as lenses and windows, and to improve the ef'ficiency of' solar cells and -solar absorption panels are well known. Perhaps the best ; 10 known are the single, or preferably multiple, layer inter-ference coatings used on optical lenses, f'ilters and as anti-reflecting films used on windows. While such coatings are desirable in that they are durable and are known to provide an extremely low reflectivity at specific wa~elengths, they exhibit a number of limitations. For example, the optical characteristics o~ such single layer ~ilms are highly sensi-tive to the wavelength, such that multiple la~7er coatings must be ernployedO However, if such multiple layer coatings are used, a signif'lcant sensitivity to the direction of' incident light results. It has not heretofore been possible to achieve wavelength lndependent~ wide angle response simultaneously.
; ~ Furthermore, such films are relatively expensive to produce, requiring careful control of' the thickness of the coating as well as multiple coating operatlons.
In addition to such articles in which the ref'lect-ance therefrom is reduced via a coating having optical inter-ference characteristics, it is also known to provide articles in which the reLlectance is reduced by providing a micro-structured surf'ace over which the e~ective index of' 6~3 refraction varies continuously from the substrate to the ambient en~ironment. See, for example, U.S. Patent No.

2,432,484 (Moulton). It is believed that the highly sensl-tive vision o~ nocturnal insects such as moths is at least partly due to the low re~lectivity from the surface of the eyes due to the presence o~ such a microstructure on the surface of the eye. (G. ~. Bernhard et al, Acta Physiologica Scand., Vol. 63 243, pp. 1-75 (1965)). More recen~ly, a technique has been disclosed for reducing sur~ace reflection by applying a layer of photosensitive mater~al to the sur~ace, after which the layer is exposed to a regular pattern to light and developed to convert the regular light pattern into a regular array o~ protuberances (Clapham and Hutley, U.S. Patent No. 45013,465).
While investigators have thus appreciated the desir-ability of forming such surfaces for antireflective applica-tions, it has not heretofore been known how to provide such surfaces havlng an acceptable degree o~ uniformity across an extended surface nor how to produce sur~aces in a commerci-ally acceptable manner.
Solar collectors utilizing porous coatings to ~ - increase the absorptlvity and to m~n~mize the radiation loss ; due to reverse reflected radlation tv~sible or ~R) are also known. It is also known to utilize micropores~ grooves or other "textural" effects in such devices to effect an increa~e in absorptance. (J. ~ac. Sci. Tech., ~ol. 12, No. 1, Jan/
Feb (1975~). For example, U.S. Patent No. 3,490,982 (Sauveniere et al) discloses a method of treating a glass surface to provide a microstructured surface exhibiting reduced re~lectlvity. Such coatings, sur~ace treatment3 and 6~

the like have not proven commercially accepta~le, possibl-l due to the lnstability of the surfaces, C05t or lnabillt~ tG
provide uniform surfaces over e~tended areas.
While not heretofore directed to the formatiorJ of ar~icles having desirable optical properties, lt ls known to treat metal surfaces such as aluminum with water and to there-by form a porous oxide or hydroxide (boehmite) surface layerO
See, for example, U.S. Patent Nos. 3,871,881 and 3~957,197.
The present invention is directed to an art~cle exhibiting improved antireflective and, if provided ~1th a transparent substrate, transmissive characteristics over that available heretofore, which articles are exceptionally stable, low cost, can be provided over extended areas, and can have complex surface shapes.
The articles of the present invention comprise a substrate which may be of virtually any construction, i.eO, transparent or opaque, insulator, semiconductor or metal~
having a flat, curved or complex shape, and have formed - thereon an oxide coating of a metal selected from the group consisting of aluminum, magnesium or sinc or alloys thereof.
The oxide coating is formed by the substantially complete conversion of a thin-film of a metal or meta~ alloy, the thickness of which th~-film prio~ to conversion ~as at least five nanometers. In the present invent~on, it s~r-prisingly has been found that ar~icles having essentiallythe same antiref~ecting and transmitting character~stics may e formed from thin-films having thicknesses Prior to con-vers~on of as much as 200 nm, and from thin-f~lms ln ~hich the thickness varies appreciably over the surface of the article. Thus, careful control of the thickness of the starting thin-film is not necessary. The articîes of the present inventlon are characterized by a surface containing a plurality of randomly positioned discrete leaflets of^
varying heights and shapes, which leaflets extend from said substrate a distance of not less than twenty nm, an~ the bases of which are in substantial contact wlth the bases of each of ad~acent leafletsO When so structured, the article preferably exhibits a total reflectance from the coa~ed sur~ace o~ less than one percent when normally incident radiation is varied over a range of wavelengths extending at least between 380-700 nm.
~ n one embodiment o~ the present invention, the article comprises a transparent substrate of g1ass or quartz ; or other inorganic transparent material or of polyimide, polyester, polystyrene, polymethylmethacrylate, polycarbonate, polypropylene or other polymeric material. When so con-structed, such that internal absorption of inc~dent radiatlon may be appreciably ignored, the article preferably exhibits a total transmissivity of normal incident light of not less than 98% over the same range of wavelengths.
hile such articles have broad general utility~
the present invention is further directed to a variety of specific devlces ut~lizing one or more such articles and the antireflecting/transmissive surfaces thereof. One general type of such devices may be categorized as passive devices, i.e., those in which the substrate or other com-ponents of the device do not actively interact with incident ~adiation~ to convert it to other forms of energyO ~n one example thereo~, a transparent substrate is shaped to pro-v-ide an optical element such as a lens or prism, or . ~

particularly, a Fresnel lens wherein the antireflectlng coating is applied to at least one i~ not all of the radia-tion transmissive surfaces, such as on a grooved Fresnel surface.
When utiliæed in "active" devices, i.e., those in which incident radiation is converted to enable the energy to be transported to another site, such as solar collectors, heat pipes, photovoltaic cells, etc., the substrate may be either transparent or opaque as appropriate ~or the given application. For example, a flat plate solar collector having exceptional characteristics, made according to the present invention, comprlses the antireflecting coating and one or more radiation absorption surfaces. In an alterna-tive device, the active construction may include heat pipes, radlators or other heat transfer components to which is ; directly applied the antireflecting coating, or which are used in con~unction with transparent substrates having the antiref~lecting coating. In a further active construction, the devlce may comprise a semiconductor substrate which is treated to have the antireflecting coating.
The articles o~ the present invention are made via a very simple and lnexpenslve process. In this process, à
thin-film of a metal (Al~ Mg or Zn, or alloys thereo~) is deposited onto a selected substrate. As mentioned herein-above, the substrate may be virtually any material. If`desired, primers or appropriate pretreatments may be applied to the selected substrate to improve the adherence - or uniformity o~ the thin-~ilm to the substrate Typically, such metal films may be provided via conventional evapora-tion, sputtering or chemica~ vapor deposition me~ho~s;
.

-, , .

~ J~ 6~ ~

however, other methods may similarly be emplo~ed.
The deposited metal thin-~llm is then conver~ed to an oxide or hydroxide coating by a chemical or chemical~
electrochemical method whereby the coating has a rough and structured topography possessing the requisite antireflectlng and transmissive characteristics. For example, in the pre-sent invention, a uniform conversion is accomplished by exposing the thin-film to water for a duration and at a temperature sufficient to convert the thin-film to a sub-stantially transparent oxide coating which, in a preferredembodiment, has a reflectivity less than 1%. If the coating is on a transparent substrate such that internal absorption can be ignored, the article preferably exhibits a transmissivity of not less than 98%.
In one preferred embodiment, the complete conver-sion of the metal thin-film is effected by applying a wetting agent to the thin-film and thereafter exposing the film to saturated steam for a duration ranging between 0.5 and 60 mlnutes, at a temperature ranging between 85 and 98C. In an alternative preferred embodiment, the thin-film is immersed in an aqueous oxidizing solution at a temperature of not less than 25C for a duration of not less than 0.5 minutes to achieve the conversion.
Figure 1 is a reproduction of a transmission electron micrograph of a cross section of an article having an antireflecting coating thereon pursuant the present invention;
Figures 2 and 3 are spectra of the percent reflectance and transmission of a polyester sheet in an uncoated state and when coated on both sides with the antireflectlng coating of the present lnvention;
Figure 4 is a plot of the ~ncrease in percer,t transmission as a function of the initial thickness OL
an Al thin-fllm used to form an antireflecting coating pursuant the present invention;
Figure 5 is a plot of the reflectance as a function o~ the angle of incidence for an uncoated glass plate and a similar plate coated pursuant the present invention;
10Figures 6 and 7 are spectra of the percent reflectance and transmission of a quartz glass sheet in an uncoated and coated state pursuant the present inven tion; and Figures 8 and 9 are spectra of the percent refleotance and transmission of four parallel spaced sheets of polymethylmethacrylate in an uncoated and coated state pursuant the present invention.
Flgure 1 is a reproduction of a transmission : electron micrograph of a typical cross-section of an , : 20 article of the present invention. As shown in Figure 1, the artlcles exhibit a surface morphology which can gen-erally be descrlbed as being a plurality of randomly positioned discrete leaflets of varying heights and shapes, the base Or each leaflet be~ng ln substantial contact ~: 25 with the base of ad~acent leaflets. The leaflets extend ~;~ from a substrate a di~tance o~ not less than 20 nanometersg and pre~erably extend from the substrate a distance varying ~ from that corresponding to the wavelength of llght down to ,~ 1/10 that wavelength, such as approximately 1.00 to 150 ~: 30 nanometers. ln such articles, the re~lectivity is - :~

significantly reduced from similar but un~reated articles, and if the articles comprise a transparent substrate, the transmissivity is appreciably increased. It is believed that these characteristics are due to a gradation in the ~ndex of refraction between that of the medium outside the sur~ace of the article and that of the substrate of the article. A better understanding of the importance of such a leaflet structure and of its effect on the reflection and transmission of light can be obtained by analyzing the propagation of light incident on a boundary separatlng the media having differlng indices of refraction. Equations describing such propagation are, for example, set ~orth in "Principles of Optics", M. Born and E. ~olf, Second Revised Edition, Pergamon Press, New York, New York (1964), - -PP. 23 ff. As is there described, the light re~lected and transmitted at a boundary can be approximated by applying boundary conditions to the solutions o~ Maxwell's Equations.
Even where the discontinuity is not mathematically sharp, so long as the change ln the refractive ind~x takes place over a distance which is very small ~less than 1/10 the wavelength of light) compared with the wavelength of light used, such a theoretical treatment closely corresponds with experimental results. In the present invention, the change in the e~ective index o~ refraction varies over a distance ; 25 ranging between the wavelength of light down to one-tenth that wavelength. Accordlngly, it is believed that lt is the property o~ a graded change in the refractive index over this distance which renders the article of the present invention antireflectlng and~ under certain conditions, more transmitting over an e~tended range of optical wavelengths.

-~3~

According to a preferred process of the present invention, the article depicted in the micrograph repre-sented in Figure 1 may be formed according to the followlng steps: -a) A thin-film o~ a metal, such as aluminum, is first deposited onto an appropriately selected substrate to a thlckness which is deslrably greater than 5 nanometers and which may range upwards to at least 200 nanometers (nm) Preferably, the film thickness is between 30 and 50 nm.
While a uniform thickness ~ilm is desired, a nonuniform thickness film has been found to result in high quality optical properties. In one example, a thin-film, approxim-ately 30 nm thick was vacuum evaporated onto a polyester (biaxially oriented polyethylene terephthalate) sheet. In alternative embodiments, thin-films of magnesium and zinc may also be deposited: Thin-~ilms of other materials - --may also be deposited and chemlcally treated to produce simllar microstructured coatings. The thin-films may be deposlted by electroplating, sputtering, chemical vapor deposition, and other techniques. The substrate may be ,~
any material includlng other polymers such as polyimide, polystyrene, polymethylmethacrylate, polycarbonate, poly-propylene, as well as inorganic materials such as glass3 quartz, and semiconductors. The only limitation to such substrates appears to be that the material be stable under the~condi~ions under~which the metal film is deposited and subsequently converted. The substrate may also be of any ::
shape, so long as the sur~ace can be coated.
b) The thus deposited metal thin-film is then converted to a coating or metal oxide or hydroxide by _9_ .

exposing the film to water for an appropriate durat~on and at a temperature until the metal layer becomes transparent and exhlbits the requisite decrease in reflect1vity. The conversion may be accomplished by exposing the metal thln-film to saturated steam or by immersing the film in aqueousoxidizing solutions. The required duration of treatment ranges frorn half a minute upward depending upon the thick-ness and type of metal thin-film used.
As will be more apparent from the specific examples set forth hereinafter, the articles of the present invention include optical elements such as lenses, prisms, beam splitters, couplers and transmissive devices such as instru-ment windows, solar transmitting and absorption panels, etc.
All such articles have the following advantages over prior art articles exhlbiting antireflecting properties:
1. Through the use of a single layer, surfaces may ~e rendered antireflecting and more transmitting o-ver a very wide range of wavelengths extending from the UV
through the visible spectrum and up to at least 2.25 micrometers. T.e., the article of the present invention exhibits little or no dependence on the wavelength of incident light.
2. Furthermore, the reflectance from surfaces treated with such a single layer remain substantially the same as that for normal incident light over a very wide range of angles of incidence without an attendant increase in diffuse scatte~in~, thereby making the articles highly deslrable for use with curved optical surfaces, and for renderin~ flat surfaces non-reflecting even when viewed from wide angles such as with cover glasses for picture frames, etc.

3. The article of the present invention ls significantly hydrophilic, and hence exhibits desirable nonfogging characteristics rendering it further useful in diverse optical applications.

4. The articles may be ~rmed o~ a flexible substrate such that antireflecting screens and the like may be provided for "roll-up" applications, ease of storage, etc.

5. The article may further be used as a master to provide replicated articles having similar antireflecting characteristics.

6. The method of the present invention is extremely simple and inexpensive, requiring but two element-ary operations and yet achieves a highly desirable optical article.

7. The method may be used with almost any type of substrate, having surfaces even containing complex geometries.

8. The method will produce antireflecting coatings which are stable, i.e., insoluble in water and useful even at appreciahly elevated temperatures, and which adhere well to virtually any substrate.

9. The method has a wide process latitude and will produce desirable results o~er a wide range of thick-nesses of the me~al thin~~ilm precursors as well as over a wide range Or conVersion techniques.
Example 1 - :
An article having the cross section shown in the reproduced micrograph o~ Figure 1 was prepared as ~ollows:

A .05 millimeter thick sheet of' b'laxially or1ented polyethylene terephthalate (PET) was vapor coated ~lth 33 nanometers (nm) of aluminum on one side and w~th 32 nm of aluminum on the opposite side. These aluminum fllms were then sub~ected to saturated steam at a temperature of approximately 95C ~or 3 to 4 minutes, during which time the films were completely converted to structured layers of boehmite (AlO(OH)), approximately 120 nm thick. The resultant sheet was visibly more transparent than an un-coated control sheet of PET having the same thickness.
In order to determine the nature of the boehmitestructure thus prepared, a section of the sheet was placed in a transparent epoxy potting compound, and, after curing, was cut into a thin section extending normal to the surface of the sheet. This thin section was then analyzed via trans-mission electron microscopy to produce an electron micro-graph such as that reproduced in Figure 1.
The spectacular reduction in re~lectivity and similarly spectacular increase in light transmission for such an article are shown in Figures 2 and 3, respectivelyO
It is there shown that the total reflectivity of a beam traversing the article so as to include the contribution f'rom both sides o~ a thus coated sheet is approximately less than 2% over a range o~ wavelengths extending between 350 and 700 nm. The reflectivity f~rom a single surface would be less than 1% over the same range o~ wavelengths~
In compar1son therewith, Figure 2 also indicates that an uncoated'she~e't of PET would exhibit a ref'lectivity o~
approxlmately 12% ~or both surfaces or a reflectivity of~
approximately 6~ ~or one such surf'ace. As shown in Figure 3, the PET sheet coated pursuant the oresent lnven-tion exhibits a light transmissivity in excess of approxlm-ately 95% over the same range of wavelengths, whereas the uncoated sheet of PET exhib~ted a transmissivlty of not less than 85%. An increase in transmissivity o~ approxlm-ately 10% throughout the entire vislble spectrum was thus achieved. A similar increase in transmission is present in the infrared region of electromagnetic radiation, i.e., extending out to approximately 2600 nm.
As mentioned hereinabove, the transmissivity and reflectivity of the articles of the present invention are markedly independent of the thickness of the initial metal thin-film layer. In one set of tests, thin-films of aluminum on PRT substrates having initial thicknesses ranging between

10 and 100 nm have been prepared and converted via steam treatment as discussed hereinabove. As shown in Figure 4, the resultant increase in percent transmission was found to be approximately 9% over this entire range of thicknesses.
~urthermore, where thicker alumi,num thin-films such as 200 nm were prepared and were converted by immersion of the film-substrate combination in an aqueous solutlon held at 60C ~or approximately one and one half hours, after whlch the films were found ~o be transparentized and hence to be fully converted, a similar lncrease ln the percent trans-mission was observed.
The articles of the present invention are charac-; terized not only by ~he marked decrease in re~lectivity and increase ln transmissivity, but also by remarkable adhesion of the converted layer to the substrate. For example, where a typical Al film on polyester is completely converted to the boehmite structure, and a section of pressure sensitlve adhes~ve transparent tape is applied to the ~oehmlte coating and is then peeled back at an angle of 180, the boehmite layer is not removed from the substrate. Rather, the adhesive is found to be stripped from the backing.
It has also been found that the converted oxlde layer, such as that of boehmite, exhibits an appreciable hydrophilic nature, such that it is desirably used on optical elements and the like where an advantageous anti-fogging characteristic is provided.
Figure 5 provides a further indication of theindependence of the reflectivity of the articles of the present invention as a function of the angle of incidenceO
In preparing the curves labeled A and B in that figure, a beam of light having a wavelength of 632.8 nm provided by a He-Ne laser polarized perpendicular (curves A and B) and parallel tcurves A' and B') to the plane of incidence was lmplnged upon representative sur~aces and the intensity of radiatlon specularly reflected was then determined. As shown ln Figure 5,~ when an uncoated glass surface was thus measured, the re~lectlvlty Or llght polarized perpendicular ~;~ to the plane of lncldence was found to be approximately ~ 4% from one surface when incident normal to the surface, `~ and increased steadily to approxlmately 100% at an angle of ~ 25 incidence of 90 wi~h respect to khe normal (curve A)o ~ .
Correspondingly, the reflectivity of light polarized parallel to the plane Or lncidence was approximately 4%
from one surface when lncident normal to the surface, decreased gradually to approximatelY 0.0l% at 57~ with respect to the normal due to the Brewster effect and -l4-'':

then gradually increased to 100% at 90 (curve A'). T~lhere one sur~ace of the glass plate was prepared with a slngle layer coatlng according to the present inventlon, the reflectivity at 0 wlth respect to the normal was approx~m-ately 0.1% for both perpendicular and parallel polarizedllght. The reflectivity measured with perpendicularly polarized light was found to gradually lncrease to approxlm-ately 100% at 90~ but remained at less than 1% up to approximately 55 (curve B). The reflectivlty measured with parallel polarized light was found to exh1blt a sllght decrease up to approxlmately 30, and agaln remained at less than 1% up to approximately 60~ (curve B').
Example 2 Desirable characteristlcs obtalned wlth the pre-sent invention incorporatlng an inorganic substrate are indicated in an example ln whlch a quartz plate was vapor coated in a conventlonal manner on both sldes with 32.5 nm of aluminum. These aluminum ~llms ~ere pre-wet wlth an aqueous solution conkaining a wetting agent and were then sub~ected to saturated steam for three to four mlnutes to form structured layers of boehmite approximately 120 nm thick. Results o~ the light reflectivity and transmissivity measurementsmade on this sample are shown in Figures 6 and 73 respectively. A~ain, as can be seen in these figures, a spectacular decroase in re~lectivity and lncrease in transmlssivlty has ~een accomplished over the wa-velength range from 400 to 700 nm. The re~lectivity is less than 0.5 percent per surface. Due to the low absorptivity of quartz over extended wavelengths 3 the low reflectlvity and high transmissivity was observed to be substantially ,: . ' : ' ~ - ' "

J ~

unchanged from 350 nm through 2250 nm; hence, it is expected that similar properties extend further lnto the W and IR
ranges.
Example 3 The utility of the present invention ln produclng antireflecting surfaces on optical elements having a complex geometry was demonstrated by coating a section of a fresnel lens formed of polymethylmethacrylate with approximately 39.5 nm of aluminum on the grooved or structured surface 10 of the fresnel lens. Since such a material would warp unacceptably under the temperature (approximately 96C) typically used during the saturated steam treatment dis-cussed in Examples 1 and 2 hereinabove, in this example the aluminum was converted to the boehmite coating by 15 immersing the coated lens section in a 5% aqueous solution o~ NaC102 held at approximately 78C for approximately 5 minutes.
The ant~ire~lection properties o~ the grooved surface accomplished by the above process were very appar-20 ent to visual observation. The results of measurements of .
the total reflectlvity and transmissivity were very similar to those shown in Flgures 2 and 3 and 6 and 7 ~n that the ; properties were substantially independent o~ wavelength.
In that the grooved 8urface of the fresnel lens produces 25 appreciable scQttering off normal incidence~ the total re~lectivity of an uncoated sample (both surfaces) is , :
typically approxim~tely 8~. In contrast, the reflect-ivity from a sample having the coating on the fresnel surface is typically approximatel~ 6%. Accordingly, while 30 the coating treatme~t of the present invention resulted in ~, ~
- -16- ~
.~ "

a reduction in the re~lectivity ln excess o~ 2%, thls was not as marked a change as observed with planar samples, ~xample 4 In this example, a .05 mm thick sheet o~ poly-ethylene terephthalate was vapor coated in a conventionalvacuum evaporation process with 27 nm of magnesium on one side of the polyester sheet. This thin film was then completely converted to an oxide form by sub.~ectlng the film to saturated steam for three to four minutes at approxlmately 90C. The reflectivity and transmisslvlty of the resultant converted coating again were found to be substantially like that shown in Figures 2 and 3 and 6 and 7 inasmuch as they exhlblted virtually no dependence upon wavelength. In thls example, the total reflectlvity (both surfaces) of the sheet thus coated on one side only was found to have decreased from approximately 12% to approxim- -ately 8% while the transmissivit~ was found to have ln- --creased from approximately 85% ko ln excess of approxlmately 90%.
Example 5 In a ~urther modlfication o~ the process o~ the present ~nvention~ a .Q5 mm thick sheet of polyethylene terephthalate was vapor coated as in the above examples ; with 120 nm Qf aluminum. This evaporated thln-fllm was then electrolyt~¢ally converted by electrically connecting the film to the p~sltive terminal of a 7.0 vol~ potential ~ source a~ter which it was slowly immersed ln an electrolyte ;~ conslsting of 50% (by volume) H2S04 in water. The negative ; electrode was provlded by a lead plate immersed in the electrolyte solution, ~y so doing, the aluminum thin-film :

:

was substantially converted to aluminum oxlde.
A small amount of residual aluminum remalning after the electrolytic process was subsequently converted by immersing the sample in a diluted solution of NaClO2 ln water at 80C for 10 minutes. After this treatment, the anodized area of the sample was found to be clear and to exhibit a significant reduction in the surface reflect-ivity and an appreciable increase in the transmissivity.
Electron micrographs of this sample indicated that the resultant structured layer was coarser than those described in Example 1 hereinabove and as shown in Figure 1.
Example 6 -; In a solar energy transmitting panel in which articles of the present invention were assembled to pro-vide a multilayer assembly, four sheets of 1.6 mm thick polymethylmethacrylate were cleaned and were then vapor coated with 32 nm of aluminum on both sides of each sheet.
These aluminum thin-fllms were then converted to the structured boehmite layers~by the saturated steam process described in Example 1 hereinabove. The results in the reduction in the reflectivity and the increase in trans-missivity for a stack of these four sheets (8 sur~aces) when assembled with approximately one-half inch ~ap between each of the sheets are shown in Figures 8 and 9 respectively. In Figure 8, it may thus be seen that whereas the assemblage o~ four such methylmethacrylate sheets in the uncoated form exhibited a re~lectivity of approximately 25%, in the coated ~orm the reflecti~ity was less than one percent. Similarly~ whereas the trans-missivlty of an uncoated assemblage of such sheets was approximately 75%, the transmissivity o~ the asse~led coated sheets was approximately 98%.
Example 7 To demonstrate the utility o~ the present invention wherein a thin-~ilm o~ zinc was employed as a precursor, a layer of silver less than 1 atomic layer thick was initially deposited on a polyethylene terephthalate sheet to facilitate the subsequent deposition of the thin-film of zinc. Such a zinc film was then vapor coated onto the silver nucleated surface via conventional vapor depo-sition processes to a thickness o~ approximately 45 nm.
The zinc thin-film was then converted by exposure to steam at a temperature of approximately 96C for 30 minutes, a~ter which the film was completely converted and substantially transparentized. It was subsequently found that the re~lectivity of the coated sample was approximately 3 percent per surface across the visible range, in contrast to a reflectivity for the uncoated sample o~ approximately 6.7% per sur~ace.
Example 8 The sultability of the~present invention in providing antirePlecting coatings on a substrate of a polylmide sheet was demonstrated in a test in which a 45 nm film of aluminum was vapor coated onto a polyimide sheet~ after which ~he aluminum film was converted to a ; boehmite structure by immersion of the coated sheet in deionized water at 60C for 20 minutes. The aluminum film was then substantially completely converted and was found to exhibit a total transmissivity at 700 nm of approximately 90%~ whereas the untreated sheet had a :

transmissivity of approximately 86% at that waveleng~h~
Corresponding increases in transmissivity were also found over a range of wavelengths extending from approxlm-ately 350 nm to 2200 nm.
Example 9 The applicability o~ the present invention to provide antireflecting surfaces on a substrate of cellu-lose acetate butyrate (CAB) is demonstrated in a sample in which a thin sheet o~ such a substrate was provided with a 45 nm film of aluminum vià conventional vapor coating processes. This coating was then converted to a boehmite layer by immersion in deionized water at 60C
for 20 minutes. Upon thus treating one side of the CAB
substrate~ the transmissivity was found to have increased throughout the visible region from approximately 91% for the untreated substrate to approximately 95% ~or the treated substrate.
Example 10 The utility of the present inventlon utilizing a polycarbonate substrate was further found wherein such a substrate was provided with a 45 nm thick film of aluminum ; via conventional vapor coating processesO This thin-film was then converted to a boehmite layer ~y first wetting the film with an a~ueous solution of a wetting agent followed by exposure to steam at 96C ~or approximately 2 mlnutes. The transmissivity o~ such a sheet having the coating on one sur~ace was found to have increased from approximately 80% ~r the untreated sheet to approximately 85% at a wavelength of approximately 500 nm. A similar increase ln the transmissivity was observed from -20_ approximately 420 nm, at which this polycarbonate sample was significantly absorptive, through a wa~telength of approximately 1,000 nm.
Example ll The utility of the present invention in pro-viding an antireflecting surface on a polystyrene sub-strate was demonstrated in an example in which one surface of such a substrate was provided with a 45 nm film of alumlnum via conventional vapor deposition. This film was then converted to a boehmite structure via immersion in deionized H20 ~or 20 minutes at 60C. The untreated sheet of polystyrene was observed to have a transmissivity of approximately 85% at a wavelength of 500 nm. The transmissivlty of the converted coating/substrate was - 15 approxlmately 90% at a wavelength of 500 nm, in contrast to that of 85% for an untreated substrate. A similar increase in the transmissivity was observed over a range of wavelengths extending from approximately 400 to at least 1,000 nm.
Exam~le 12 In a further embodiment of the present invention, a fresnel lens package including a glass cover plate, three linear fresnel lenses formed of an acrylate copolymer3 i.e.~ Type DR 61 obtained from K~S-H Inc., St. Lou~s, Missouri, and a fourth linear fresnel lens formed of poly-; methylmethacrylate, Type 147F, abtained from Rohm ~ Haas Company, were provlded with antireflecting coatings on ; ~ all inter~or sur~aces as follows: The five elements were first coated with a 45 nm aluminum film via conventional vapor deposltion. ~he glass cover plate and the acrylate ;

p~

147F ~resnel lens were immersed ~or five minutes at 8~C
in a solution o~ 1% NaN02 in deionized water~ one drop G~
a wetting agent such as a quaternary ammonium hydroxlde surfactant, Type X-100 manufactured by Rohm & Haas CoO per liter and a buffer such as 1% sodium acetate/acetic acid.
The pH of the solution was adlusted to 6.65 with acetic acid. Following the conversion to the boehmite micro-structure, these elements were then rinsed by dipping in deionized water at 80C and were air dried. The DR61 acrylic lenses were converted via immersion for six minutes at 70C in deionized water in which the pH was ad~usted to 8.65 with sodlum hydroxide. Following the ; conversion o~ the coating to boehmite, these elements were ~-allowed to cool slowly in a humid environment for two to three minutes and were ~en dried in a stream o~ dry nitro-gen. The thus coated lens elements were then assembled and were tested in an overhead pro,~ector in which the intensity of light transmitted through the lens assembl~ was measured both at the center and corner portions. A 20% increase in the llght intensity over the entire microstructured area was observed compared to the intensl~y for a similar but uncoated lens assem~ly. ~urthermore, a signi~icant reduc-~, tion in glare was achieved, and inter~erence patterns preyiously observed due to slightly contacting elements eliminated.
~, In all of the examples set ~orth hereinabove, the su~strate surfaces are desirably cleaned prior to the deposition o~ the metal thin-~ilms to improve the adherence and uniformity o~ the metal deposit~on. It has further ~ ~ , ~ ~ 30 been found that surraoe oontaminatlon deleterlously arfects the uniformlty of the resultant converted coatings and hence decreases the optical performances. The partlcular cleaning procedures are desirably modified depending upon the substrate selected.
Example 13 In a further embodiment illustrative of the applicability of the present invention to large area sur-- faces such as architectural windows, a 1828 m roll of 130 cm wide polyethylene terephthalate sheet .05 mm thick was vapor coated on each side with about 35 nm of aluminum using conventional vapor deposition processes. 60 cm wide sections of this roll were then converted via exposing the sect~ons to saturates steam as described hereinabove. These converted sections were measured for the percent trans-mission of the solar energy spectrum from 0.3 microns out to 2.2 microns. The film was found to be transmissive of 96.8% of the incident solar energy, compared to approxim-ately 85% for good quality window glass.
Example_14 The articles of the present invent~on are ~'urther use~ul ~n forming a master having a corresponding micro-structured surface whlch may then be used to replicate articles having corresponding microstructured~ anti~
reflectlve surfaces. In such an embodiment, a 0.05 mm sheet of PET was coated with a 30 nm thin-~ilm of Mg, whioh film was then converted to a structured surface ~y pre~letting the sur~ace with an aqueous solution o~
a wetting agen~ ? Pollowed ~y exposing the wet surface to saturates steam at approximately 90C for 20 seconds.
The film was then completely transparentized, and ~as then -;:

washed in isopropyl alcohol and drled~ The structured ~ur-face was then placed in contact with a 0.025 mm sheet o~
cellulose acetate butyrate (CAB), was placed ln a press and was pressed at a temperature o~ 120C for 60 seconds. The sheets were then removed and separated. Optical measure-ments o~ the CAB surface indicated a correspondingly low reflectivity and high transmissivity.
Alternatively, a stamping master ~or use in making replicas may be prepared from a boehmite coatlng on glass prepared as discussed hereinabove, which coating ~s successively overcoated with a 80 nm film of Cr, 40 nm film of Ni and a 80 nm of Cu. The conductive outer Cu film was then used as an electrode and a 500 micrometers thick layer of Ni electroplated thereon. The glass substrate was then strlpped away and residual ~oehmite bound to the now exposed Cr surface etched away. ~his resultant stamper ~ras then used to thermally emboss large quantities of polymeric articles with a structured surf~ace having similar antire~lecting characteristicsO
Such replicated articles are desirable improve-ments o~er articles having the microstructured sur~aces coated directl~ thereon9 in that the coated surfaces are relatively sens~tive to abrasion, scratching and the like~ -Such coatings are thus primarily suitable ~or use in applications where the coatings are protected, such as on interior sur~aces of lenses, windows, and the~ike. ln contrast, the replicated surfaces are more durable and less brittle and may be used in more exposed locations.
Example 15 In another embodiment, the utilit~ o~ the present invention was demonstrated on an opaque svbstrate A
steel plate was coated with glossy black paint and the painted layer was subsequently provided with a ~ilm of aluminum 50 nm thick. This film was then converted to boehmite via exposing the steam at a temperature of 96C
for three minutes. Subsequent to conversion, the re~lect-ivity of the glossy paint layer and that of the resultant boehmite layer were compared with that obtained from a black velvet cloth. The glossy black paint was found to exhibit a reflectivity at normal incidence of 4.7% at 500 nm. In contrast, the black velvet cloth exhibited a reflectlvity at the same wavelength of approximately 0.2%. The converted boehmite layer on top of the glossy black paint exhibited a reflectivity ~ust slightly in excess of that for the black velvet, i.e., approximately 0.25%. The reflectivity over a range of wavelengths extending from 350 to 700 nm was substantially the same, hence it is expected that a similar reflectlvity will extend into the W and IR ranges, dependent somewhat on the reflectivlty characteristics of the paint layer.

In a ~ur~her test to determlne the stability of ; the converted coating, thin-~ilms o~ 35 nm of aluminum were coated on one slde of a number of glass plates and were subse~uentIy converted to boehmite by exposure to ;; ~ steam at 96C for two mlnutes. The reflectivity o~ the untreated glass plates at 5Q0 nm was determined to be approximately 8% for both surfaces. In contrast, the re~lectivitles of t~e coabed slides were found to be approximately 4 8% for both surfaces. These plates were , .

' :

then sub~ected to prolonged exposure at varlous elevated temperatures as follows: One plate was exposed at 210C
for 93 hours a~ter which the reflectivity had increased to approximately 4.85% for both surfaces. Another plate was heated at 460C for 66 hours, after which the reflect-ivity was determined to be approximately Ll . 5% for both surfaces. Another plate was treated at 560C for 22 1/2 hours, after which the reflectivity was determined to be approximately 4.4% for both surfaces. These above examples thus demonstrate the exceptional stability of the coatings of the present invention even at elevated temperatures, which propertles are of significant importance in solar : applications.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An article having a substantially antireflecting surface, wherein said article comprises a substrate having a surface coating of an oxide of a metal selected from the group consisting of aluminum, magnesium and zinc or alloys thereof, which coating is a substantially complete conversion of a thin-film of said metal, the thickness of said thin-film prior to conversion being at least 5 nanometers and exhibiting a surface after said conversion characterized by a plurality of randomly positioned discrete leaflets of varying heights and shapes, which leaflets extend from said surface a distance of not less than 20 nanometers, and the bases of which contact the bases of substantially all adjacent leaflets.

2. An article according to claim 1, wherein said article is character-ized by a total reflectance from the coating of visible light incident normal to the surface of less than one percent.

3. An article according to claim 1 wherein said substrate is a trans-parent material comprising polyimide, polystyrene, polyester, polymethyl-methacrylate, polycarbonate, polypropylene, quartz or glass through which the total transmissivity of said normal incident light is greater than that of the material without the coating.

4. An article according to any of claims 1 to 3, wherein said substrate is an insulating or semiconductor inorganic material.

5. An article according to any of claims 1 to 3, wherein said substrate is a semiconductor inorganic material having photovoltaic properties such that when provided in a photovoltaic cell, the antireflecting coating effects an increase in the transmission of radiant energy into the cell and thereby increases the effective efficiency of the cell.

6. An article according to any of claims 1 to 3 wherein said substrate is silicon.

7. An article according to claim 1, wherein said substrate is opaque and wherein the coating thereon decreases the reflectance of radiation there-from such that the absorption of radiation by the substrate is maximized.

8. An article according to any of claims 1 to 3, wherein said sub-strate is selected of a transparent material and is shaped to form an optical element, and wherein the coating thereby decreases the reflectance from a surface of the element and improves the transmissivity thereof over a similar but uncoated element.

9. An article according to any of claims 1 to 3, wherein said substrate is of a transparent material and is shaped to form a grooved fresnel-type lens having the coating on at least the grooved surface, which coating de-creases the reflectance from a surface of the element and improves the trans-missivity thereof over a similar but uncoated element.

10. An article according to any of claims 1 to 3, wherein said substrate is of a transparent material and is shaped to form a plurality of fresnel-type lenses and at least one planar cover plate, wherein a* least some of the lenses and the cover plate are spaced apart to provide a compound lens assembly, and wherein all interior surfaces of the lenses and the cover plate are provided with said coating.

11. An article according to claim 1, wherein said substrate is an in-sulating, solar energy transmitting panel and wherein the surface coating provides antireflecting characteristics to prevent reflection of appreciable amounts of incoming radiant energy, thereby adapting the panel for use in passive solar heating systems.

12. An article according to claim 11, wherein said article is further characterized by a total transmissivity of said normal incident light of not less than 98%.

13. An article according to claim 11, wherein said panel further com-prises outer panels of at least an optically translucent, if not transparent, nature and at least one said substrate having a said coating thereon sand-wiched between the panels.

14. A method of forming an article having a substantially antireflect-ing surface comprising the steps of (a) coating onto a substrate a metallic thin-film having a thick-ness of at least 5 nanometers of a metal selected from the group consisting of aluminum, magnesium and zinc or alloys thereof, and (b) exposing the metallic thin-film to water for a duration and at a temperature sufficient to convert the metallic thin-film to an oxide or hydroxide coating, such that the converted coating is characterized by a total reflectivity of visible light incident normal to the surface coating of less than one percent.

15. A method according to claim 14, wherein the exposing step results in an oxide coating having a reflectivity less than 1.0% such that if said coating is on a transparent substrate, allowing internal absorption to be appreciably ignored, the article exhibits a transmissivity of not less than 98%.

16. A method according to either claim 14 or 15, wherein the step of exposing the thin-film comprises applying to the thin-film a layer of a wet-ting agent and exposing the film to saturated steam for a duration ranging between 0.5 and 60 minutes and at a temperature ranging between 85°C and 98°C.

17. A method according to either claim 14 or 15, wherein the step of exposing the thin-film comprises immersing the thin-film-substrate combina-tion in an aqueous oxidizing solution at a temperature of not less than 25°C

for a duration of at least 0.5 minutes.

18. An article according to claim 1 comprising a transparent flexible substrate having thereon a coating of boehmite, wherein the coating is char-acterized by a plurality of closely spaced and randomly positioned discrete leaflets of varying heights and shapes, which leaflets extend from the sub-strate a distance of not less than 20 nm, and the bases of which contact the bases of substantially all adjacent leaflets to provide the article with a total reflectance of visible light incident normal to the surface coating of less than one percent, and a transmissivity of said normal incident light of not less than 98%, and wherein the surface coating is substantially hydro-philic, such that the reflectance is decreased and transmissivity is enhanced over similar but uncoated articles to enable greater amounts of light and/or radiant energy to be passed therethrough, whereby the articles may be useful as optical lenses, solar absorbers, photovoltaic cells and the like.

19. A grooved fresnel-type lens comprising a polymeric body having on at least one surface thereof an array of grooves forming a fresnel-type lens, wherein there is present on at least the grooved surface a coating of an oxide or hydroxide of a metal selected from the group consisting of aluminum, mag-nesium and zinc or alloys thereof, which coating is a substantially complete converstion of a thin-film of said metal, the thickness of said thin-film prior to conversion being at least 5 nanometers and exhibiting a surface after said conversion characterized by a plurality of randomly positioned discrete leaflets of varying heights and shapes, which leaflets extend from the surface a distance of not less than 20 nanometers and the bases of which contact the bases of substantially all adjacent leaflets.