CA1134320A - Method of producing a microstructured surface and the article produced thereby - Google Patents

Abstract

ABSTRACT
Microstructured antireflecting surface such as shown in Figure 1 made by depositing a discontinuous layer of a material exhibiting a relatively low rate of sputter-etching on a substrate exhibiting a relatively high rate of etching under the same conditions and thereafter sputter-etching the composite surface to produce a topography of pyramid-like micropedestals which are random in height and separation.

Description

~ 32~ 915,226 METHOD OF PRODUCING A
MICROSTRUCTURED SURFACE AND THE ARTICL~ i --P~ODUCED THEREBY

The present invention relates to a method ~or producing an article having a microstructured surface, and the resultant article. The surface o~ the article thus ~orms an interface between the article and the ad~acent medium, which if of dif~ering indices of re~raction, results in enhanced light transmission and decreased re~lectance without producing significant dif~use scatterin~. ;
Various types of coatings to reduce re~lectivity and improve the transparency o~ articles such as lenses and windows, and to lmprove the e~ficiency o~ solar cells and solar absorption panels are well known. Perhaps the best known are the sin~ley or pre~erably multiple, layer inter~erence coatings used on optical lenses, ~ilters and as antireflecting fllms used on windows. While such coatings are desirable in that they are durable and are known to provide an extremely low re~lectivlty at speclfic wavelengths, they exhibit a number of limltations. For example, the optical characteristics o~ such single layer ~ilms are hlghly sensitive to the wavelength, such that rnultiple layer coatings must be employed. However~ lf ~uch multiple layer coatings are used~ a signi~icant sensitlvity to the direction o~ incident light results.
Interference coatings providlng antireflecting characteristics which are simultaneously independent of 3~3Z~

the lncident wavelength and in which the antireflection is substantially uniform over a wide range of incident angles, are, therefore, not known. Furthermore, such interferrometric films are relatively expensive to produce, requiring careful control of the thickness of the coating as well as multiple coating operations.
In addltion to such articles in which the re~lectance therefrom is reduced via a coating having optical interference characteristics, it is also known to provide articles in which the reflectance is reduced by providing a microstructured surface over wh~ch the effective index of refraction varies continuously from the substrate to the amblent environment. See, for example, U.S. Patent 2,432,484 (Moulton) and the above referenced patent to Maffitt et al, wh~ch patent is assigned to the present assignee. It is believed that the highly sensitlve vislon of nocturnal insects, such as moths, is at least partly due to the low reflectivity from the surrace of the eyes due to the presence o~ such a microstructure on the surface of the eye. ~T. C. Bernhard et al, Acta Physiologica Scand., Vol. 63 243, pp. 1-75 (1965).
Another example of a method of producing an anti-re~lective surface utilizing a regular array of mlcropro-tuberances is disclosed in U.S. Patent 4~013~465 (Clapham).

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Solar collectors utilizing porous coatings to increase the absorptivity and to minimize the radiation loss due to reverse reflected radiation ~visible or IR) are also known. It is also known to utilize ~.
micropores, groovesor other "textural" effects in such devices to effect an increase in absorptance. J. Vac. Sci. Tech.~ Vol. 12, No. 1, Jan/Feb (1975). For example, United States Patent 3,490,982 (Sauveniere et al) d.iscloses a method of treating a glass surface to provide a microstructured surface exhibiting reduced reflectivity. Commercial acceptance of some of :
the coatings, surface treatments and the like disclosed in the above cited references have not proven commercially acceptable, possibly due to the instability of the surfaces, cost or inability to provide uniform surfaces over extended areas. `
Articles having a microstructured surface are also disclosed in United States Patent Specifications Serial No's. ~,190,3~1 (February 26, 1980, Dorer ~ Mikelsons) and 4,252,843 (February 24, 1981, Dorer ~ Mikelsons) which application was filed February 18, 1977 and is assigned to the same assignee as the present invention. That application discloses the treat-ment of an aluminum surface to form thereon an aluminum hydrate, or boehmite composition having a plurality of randomly positioned leaflets which give the treated surface an antireflecti.ng characteristic. In a somewhat similar manner, United States Patents 3,871,881, 3,975,197 and 4,054,467 disclose prior inventions o:E Mikelsons in which aluminum sur~aces are treated to provide .3~3~
rnicrostructured boehmite surfaces by which other coati.ngs, applied to the aluminum prior to treatment, become tenaclously bonded to the aluminum. Also, U.S. Patent No.
3,664,888 (Oga et al) depicts an electrochemical process for treating aluminum or aluminum alloy surfaces which etches the sur~ace, leaving minute irregularities and pinhole cavities which are said to pro~ide mechanical anchorage for subsequently applied resin coatin~s.
In contrast to prior art microstructured articles such as that of ~affitt et al (U.S. Patent No. 4,114~983)~
in which a homo~enous polymeric article is provided ~lth a microstructured surface by repllcation of a master surface lnto a polymerlc material, the present invention ~s directed to an artlcle ln whlch a durable, mlcrostructured sur~ace ls formed directly on the article itsel~ thus eliminaking any necd .~or repllca~lon.
Such an article, preferably formed of a VarietY o~
polymers such as are increasingly commercially important, is formed, according to the method of the present invention, by L`irst selectlng a substan~ially transparent polymeric substrate having a predetermined ra~e o~ sputter etching uncler a ~iven set o~ sputtering condltions. Discontinuous island~ of mater~l selected ~rom the group consistin~ of metal oxides, refractory metals and noble metals, having a ;~
rate of sputter etching lower than the predetermined rate under the same set of conditions are then applied onto the substrate in an average thickness in the range o~ 0.1 to 10 nm~ to form a composite sur~ace on which portions of the underlying substrate are exposed between the discontinuous - . ,, . . . . . . .

~ 5 - ~ ~3~3;~i3 ic~oislands o~ the rnaterial. ~lnally, the composite surface is sputter etched under the given set o~ sputtering conditions in a partial atmosphere of a reactiv~ gas to promote the formation of a top layer on the microislands having a desirably low sputtering rate and to pre~erentiallY
etch the exposed portions of the hi~her sputtering rate substrate, while the dlscontinuous mlcroislands are etched at a lower rate, resulting in a random topography of micro-pedestals which vary :in height within a range of approxl-mately 0.01 and 0.2 ~m and which are separated from adJacentmicropedestals a dlstance within a range of approximately 0.05 to 0.5 ~m. Such a topography o~ micropedestal~ has been found to provide a surface exhi~iting substantially decreased specular re~lectance, without an attendant lncrease in di~fuse scattering. The use o~ a substrate selected of a substantlally transparent organlc polymer~ ;~
such as a clear acrylic, also resul~s in the microstructu~ed sur~ace exhlbltlng enhanced transmittance as ~ell as decreased rerlectance.
I~urther, lt has been found preferable to utilize a re~ractory metal such as chromium to form the discontin~
uous lslands. When applled to most polymers, such a metal, either ln its metallic state or as converted to a metallic oxide, exhibits a rate of sputter etching which is typically at least an order of magnltude less than that of the polymer, thus resultlng in the rapid formation of the micropedestals during the sputter etchlng operation. This operation is desirably carrled out in a reactive akmosphere, e.g. oxygen.
Such an atmosphere is belleved to promote the formation of 1 . - . . ... . . . ... .

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metallic oxldes which frequently have an appreciably lower rate o~ sputter etching than that of the metal. Also, such a reac~ive atmosphere is believed to promote general degradation of polymeric substrates such that the rate of sputter etching is increased.
The optical article of the present invention is characterized by a microstructured sur~ace thereon exhibit-ing antirerlective characteristlcs, said article comprising a substantially transparent, polymeric substrate character- -ized by a predetermined rate of sputter etching under a given set o~ sputtering conditions and having there~n a random topography of discrete micropedestals varyin~ in height within a range o~ approximatel~ 0,01 and 0,2 ~m?
randomly separated ~rom adJacent micropedestals a distance within a range of approx~mately 0.05 to 0.5 ~m. The micro-pedestals have in the vicinity of the peaks thereo~ a generall.y detectable material compri~ing ~etal oxides~ noble metals, and mixtures and alloys thereo~ the material having a rate of sputter etching lower than the predetermined rate under the ~iven set of sputtering conditions. The resultant topography results in the microstructured surface, which exhiblt~ substantlal:Ly decreased specular re~lectance without an attendant increase in di~use scattering, resuLting in enhanced transmissivity.
FIGURE 1: An electron micrograph of a micro-structured surrace Or an article prepared according to one embodiment of this invention;

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~ 3 FIGURE 2: Curves A and B respectively show the percent o~ total re~lectance as a function o~ wavelength for a prior art9 untreated surface of a polycarbonate article and for surfaces of a polycarbonate article, one surface of which was treated with chromium pursuant to one embodiment of the present invention;
FIGURE 3: Curves A and B respectively show the percent of total transmi~slon as a function of wavelength ~or a prior art untreated polycarbonate article and for a polycarbonate article treated on one surface with chromiurn pursuant to one embodiment of the present invention;
FIGURE 4: Curves A, B, C and D respectively, show extent of diffuse scatterlng, i.e. scattering as a function of angle off normal incidence for the unscattered beam (A) the extent o~ scattering for a prior art untreated polycarbonate article (B), a polycarbonate article treated on both surfaces pursuant one embodiment o* the present invention (C), and a polycarbonate article treated under conditions outside the llrnits of the present lnventlon (D);
FIGURE 5: Curves A and B respectively, show the percent total:reflectance as a function of wavelength for prior art untreated surface of an oriented polyester article and for surfaces of an oriented polye~ter article, both of which were treated using chromium pursuant one embodiment Or the present invention;

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FIGURE 6: Curves A and B respectively show khe percent total transmission as a function of wavelength for a prior art untreated oriented polyester article and for an oriented polyester article treated on both surfaces with chrornium pursuant one embodiment of the present invention;
FIGURE 7: Curves A and B respectively show the percent total reflectance as a funckion of wavelength for prior art untreated surfaces of an oriented polyester article and for sur~aces of an oriented polyester article, both of which were treated using glass pursuant one embodiment of the present invention; and FIGURE 8: Curves A and B respectively, show the percent total transmission as a function of wavelength for a prior art untreated oriented polyester article and for an oriented polyester article treated on both surfaces with glass pursuant one embodiment of the present invention.
In the present invention, a variety of composite ~urfaces have been ~ound to provide the required dif~erential rates of sputter etching. Such differences in etch rate or sputter yield are controlled by local variations in composition or crystallinity. While the preferred method of producing and controlling such variations is directed to the placement of a discontinuous metal or metal oxide film on an organic polymeric surface, other techniques are similarly within the scope of the ~:

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~ 3 _9_ present invention. ~'or example, discrete rnetal particles may be applied to an organic polymer substrate. Such particles, however, are usually relatively large in size and often rearrange in clumps such that the resultant dlscontinuous mlcroislands are sufficiently large that ~fter the composite surface is sputter etched, the micropedestals are so large that the reflection characteristics of the surface are outside the limits desired of the invention.
Slmilarly~ the sputter etching rate o~
crystalline polymers has been found to be different in many instances from that o~ the non-crystalline analog thereof. Accordingly, i~ a polymer i9 provided in which both crystalline and non-crystalline regions are present~
the difrerence ln sputter etching rate may be utilized to provide the requi~ite micropedestals. However, since the differences ln sputter etching rate for most materials is rather small, the tlme required to provide the desired amplitude o~ micropedestals may be much longer than that neces~ary utilizing other methods.
Another technique involves the preparation of a polymer with metal oxide particles ranging in diameter `~
between 10 and 50 nanometers uniformly dispersed within .,.~
the polymer. Upon sputter etching, the metal oxide 25 particles will be sputter etched at a rate less than that ;
of a surroundin~ polymer. However, while such composites are available, the number of polymer choices is somewhat 3~ '3 limited, thus restricting the utility o~ such a technique.
In a preferred embodiment, the structure required to reduce specular reflections below a desired level of 1% per surface across the visible spectrum is random in height within the limits of 0.01 to 0.2 micrometers~ and wherein a predominant number of micropedestals in the structure are in the range of 0.1 to 0.2 micrometers. The peak-to-peak separation is also random and preferably ranges between the limitation of 0.05 to 0.5 micrometers, with the preerred separation being in the range of 0.1 to 0.2 micrometers.
In a preferred embodiment, such structures are preferably forrned by the following series of steps: A
substrate having a range of sputter etching under a given set of sputtering conditions is irst selected.
Preferably such a substrate is an organic polymer such as polyesterl cellulose acetate butyrate~ acrylics, and polycarbonates. Onto such a substrate is then applied, such as by vacuum e~aporation or sputtering deposition, discontinuous microislands of a material having a rate of sputter etching lower than that of the substrate under the sarne set of sputter etching conditions. Such a material is applied in an average thickness in the range of 0.1 to 10 nanometers, with a preEerred thickness in most cases being less than 2.0 nm. Such an average thickness is suf~iciently thin that the material is deposited in the aEorementioned discontinuous microislands~ While the .... , .~

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particular method by whlch the microislands are formed is not overly crit~cal~ it has been ~ound that sputter deposition is preferred due to the fact that improved control is obtainable Generally~ sputter deposition 5 proceeds at a lower rate. Furthermore, sputter deposited material is believed to arrive at the substrate sur~ace ~ith a higher kinetic energy than that of evaporated atoms, for example, and hence have a higher mobility.
This higher mobillty apparently allows the deposited 10 material to move about on the substrate surface to coalesce with other material 3 thus remainin~ as discontinuou~ microislands having larger average thicknesses than that obtainable with evaporated coatings.
The thus ~ormed composite surface is then 15 sputter etched. Since the discontinuous microislands formed from the deposited films or deposition of ~ine partlcles or the like are formed from the materials having a rate of sputter etching whlch is lower than that o~ the substrate, the exposed portions of the underlying ~ub~trate then etch at a rate which is greater than that of the microislands. m is differential etching rate results in the formation of a random topography o~
mlcropedestals which vary in height within a range of approximately 0.01 and 0.2 nm. The micropedestals are 25 separated from the ad~acent micropedestals a distance within the range of approximately 0.05 to 0.5 nm. The peak-to-peak spacing of the resultant micropedestals is .

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controlled by the spacing of the discontinuous mlcroislands, whereas the overall height of the micropedestals is controlled by a combination of the sputter etching time and power, and the difference in the sputtering yield between the material used to form the microisland~ and that o~ the underlyin~ substrate.
The desired dif~erences in the sputter etching rate o~ the substrate an~ that o~ the materials applled to provide the discontinuous microislands thereon ls typlcally ln the range o~ a ~actor of lO - 1000. For example, most suitable polymers have been found to sputter etch at a rate ranging between 150 and 300 nanometers per minute under- conditions o~ approxlmately 0.4 watts per square centimeter at a pressure of 5 to 10 microns o~
oxy~en. Such sputter etching rates are generally a factor of 2 to 4 tlmes less under similar sputter etching conditlons when a partial atmosphere o~ an lnert gas such as argon is used. Where microislands formed o~ a noble metal are utllized, the sputter etching rate is approxlmately 1/10 to 1/25 that o.f typical polymersO If a re~ractory metal such as chromium is utilized, the sputter etching rate has been found to be typically less than 1/10 that of such polymers, and where a metal oxide is provided, the sputterin~ rate may be typically as low as one-one hundredth that of the underlying substrateO
While organic polymeric substrates are o~
primary importance in the present invention, known 13- ~3~L3~

inorganic substrates are similarly encornpassed within the present invention. For example, quartz substrates may be utilized by overcoating the substrate with discontinuous rnicroislands o~ a polymer, a~ter which the composite surface is di~ferentially sputter etched using a plasma containing a material such as trifluoromethane.
Maximization of di~ferences in etch rate, thus reducing the time required to produce a rei~lection reducing microstructure on many polymer substrates is best achieved by reactively sputter etching in oxygen. The use of oxygen causes an oxide to ~orm on the di~continuous film coating, thus reducing its etch rate while simul-taneously reacting with the polymer and increasing its etch rate. Typically, the etch rate of polymers such as polyester and CR-39 i8 two to four times hi~her in oxygen t~an in argon.
The average ~ilm thickness required to form a discontinuous fllm sultable for production of reflection reducing mlcrostructures is a function of the material belng deposited, the composltion and structure of the substrate, the substrate temperature, the deposition method and rate and vacuum condltions.
Some non-limiting combinatlons which have been found to produce the desired microstructures are listed below.

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Film Deposition Substrate Composition _mposition Method Polyester ~oriented) Cr, glass, AlSputtering Polyester (amorphous) Au Sputtering Cellulose Acetate Glass, Cr Sputtering Butyrate Acrylic (Rohm and Haas) Cr Sputtering or Type 147F Evaporation methyl methacrylate Polycarbonate Cr Sputtering CR-39, a proprietary polycarbonate produced by Pittsburg Plate Glass Inc. (PPG Corp.) especially for optical lens, etc., and which i8 composed of diallyl glycol carbonate.
As will become more apparent when the results of the specific examples to be discussed later are shown, the method oE this invention has the following advantages over heretofore taught methods for producing antireElecting surfaces:
1. The method can be applied to any material that has a sputtering yield higher than that of metal oxides.

2. Microstructure surfaces can be produced on polymers, such as oriented polyester, which are difficult to embos s .

3. The method is adaptable to on-line continuous processing of a web.
~. The resu~ting microstructured surfaces appear to be more rugged than prior art microstructured surfaces.
5. The need for an expensive mold subject to wear and -filling is eliminated.

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6. ~he microstructure dimensions can be varied over a broad range.
7. The substrate may be of any shape so long a~ the surface can be coated.
A better understanding of the lmportance of the topographlc control o~ a microstructured surface provided in this invention can be attained by reference to the ~ollowing specific examples and accompanying figures. ;~
Figure 1 is a scanning electron micrograph ~howing a typical microstructured surface of an article of the present inventlon. As ~hown in Figure 1, a typical polymeric optlcal article according to the present lnvention has a microstructured sur~ace topographic which can ~enerally be described as a plurality of randomly positioned peaks, a predominant number or whlch range in amplltude between 0.020 to 0.20 ~m. In such articles, the reflectivity is ~lgni~icantly reduced from similar but untreated articles, and if the artlcles comprise a transparent substrate, the transmissivity ls appreclably increa~ed. It is believed that these characterlstics are lue to a gradation in the index of re~raction between that of the medlum outside the surface of the artlcle and that of` the bulk of the article. In the present invention, the change~ in the effective index of refraction varies over a distance ranging between the wavelength of light down to one-tenth that wavelength. Accordin~ly, it is believed that it is the property of a graded change in the ...~

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refractlve index over this distance which renders the article~of the present invention antire~lecting, and, under certain conditions, more transmitting over an extended range of optical wavelengths.
Example l A protective paper covering having a pressure ~ensitive backlng was stripped ~rom a 15 cm x 20 cm x 0.16 cm piece of Homalite~ type 911 (an ophthalmic grade polycarbonate, generally known as CR 39) obtained from the SGL Industrles Inc~ Wilmington~ Delaware. The small amounts of adhesive remainlng on the polymer surface were removed by scrubbing the surface using 95% ethanol. The sur~ace was then further cleaned with a mild detergent and water, followed by a water rinse and a ~inal 95% ethanol (0.8 micron ~iltered) rinse. The sample was blown dry with nitrogen gas and, i~ not ~urther processed, stored in a clean lamlnar flow hood until ~urther processing.
Further process~ng was done in a Vecco~ model 776 radio frequency diode sputtering apparatuæ operating at a frequency of 13.56 MHz, modified to include a variable impedence matching network. The apparatus included two substantially parallel shielded circular aluminum electrodes 40.6 cm in diameter with a 5 cm gap between them. The electrodes were housed in a glass ~ar provided with R.F. shlelding. The bell ~ar was e~acuatable by means of a mechanical fore/roughing pump with a water cooled trap and oil diffusion pump. The 3~
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cathode pedestal was cooled by circulating water, and covered by a plate of double strength window glass to prevent sputtering of the underlying aluminum electrodes.
The sample CR-39 panel was centrally attached to the aluminum anode pla~e by means of small pieces of pres-sure sensitiv~ adhesive tape at the corners of the sample, with the surface of the CR-39 panel to which a sputtered film was to be applied acing the cathode electrode. The source of the material to be sputter deposited was an evaporated chromium coating in excess of 0.05~um thick on a plate of double strength window glass, which plate was placed over the glass covered cathode electrode, with the Cr coating facing the CR-39 panel on the anode.
The system was then evacuated to 2 x lO 5 torr, and argon gas introduced through a needle valve. An equilibrium pressure of 6 to 9 x lO 3 torr was maintained as argon was continuously introduced and pumped through the system R.F~ energy was capacitively coupled to the cathode, initiating a plasma and was increased until a cathode power density of 0.38 watts/cm2 is reached, thus causing chromium to be sputtered from the cathode and deposited on the opposing anode. The sputter deposition Oe chromium metal onto the sample was continued for seven minutes ~ ten seconds. Reflected power was less than 2%.
The coupling capacitance was continuously manually adjusted to maintain the above stated power density.

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Subsequent measurements using an Airco Temescal FDC 8000 Film Deposition Controller to monitor film thicknesses as a ~unction of time under identical conditions revealed that the sputtered discontinuous film was being deposited at a rate of approximately 0.13 nm/minutes~ In seven minutes, the average film thickness was, thereEore, approximately 0.9 nm.
The R.F. power was ~hen shut off, the argon needle valve closed and the system let up to atmospheric pressure using 0.2 micron filtered air. The chromium coated double strength window glass was removed, revealing the clean uncoated glass covering the aluminum cathode plate. The sample was detached from the anode and placed on the clean glass covered cathode such that the surface was sputter deposited chromium on it faced the anode.
The system was next evacuated to 2 x 10 5 torr and oxygen introduced by means of a needle valve. An oxygen equilibrium pressures of 6 x 10 3 torr was maintained in the system and R.F. energy capacitively coupled to the cathode, initiating a plasma. The energy was increased until a cathode power density of 0.31 wattsJcm2 was reached. The reactive sputter etching was continued Eor 60 seconds ~ 3 seconds.
A microstructured surface consisting of chromium or chromium oxide capped pyramid-like micropedestals having a peak-to-peak spacing small compared with the wavelengths of the visible light, such as shown in Figure ~ ;

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1, was thus formed.
The articles produced by the method of the present invention as demonstrated in Example 1 are characterized by a marked decrease in interface reflectance, an increase in total transmission, and no significant increase of optical scattering. The reflectivity of the air/substrate inter~ace over the range of wavelengths extending between 400 and 700 nm for a prior art nonstructured CR-39 surface and for the micro-structured surface described above is shown in Figure 2, curves A and B respectively. As can be seen, a dramatic ;~
reduction in interface reflectivity resulted, wherein the reflectanca is essentially reduced to zero for the 400 -520 nm region and does not increase to more than 0.7~ for the rest of the wavelength region. In optical elements, it is most often desired to increase the interface trans-mittance and decrease the specular reflection. In such instances, diffuse reflection is to be avoided. The fact that this is indeed the case for the product o this invention is demonstrated by Figure 3 in which the transmission for an untreated sheet of CR-39 and the sheet treated as in Example 1 are shown.
Further ~confirmation o~ the relative lack of diffuse scattering is shown in Figure 4, where the int~nsity of light (HeNe laser at a wavelength of 633 nm) ~;
scattered from a given object is plotted semilogarith mically as a function of the angle off the normal. In , `` -20~ 3~

Curve A of Figure 4, the intensity of the light without an object in the path of the beam is plotted~ Curve B shows the scattering of the light for a prior art control panel Or CR-39 in which neither sur~ace had been treated. In contrast, Curve C shows the intensity of light scattered from a CR-39 panel where both surfaces were kreated as set forth in Example 1. Qs may there may be seen, the inten~ity of light scattered at 5 off the normal is approximately five orders of magnitude below the peak intensity at normal. Curve D shows the result when an undesirable sur~ace microstructure is produced. In this case, the dif~erential sputtering was continued for nine minutes, rather than the 60 seconds as in Example 1, to lntentionally produce pyramids lar~er than the preferred range of this inventlon. As can be seen, the off normal scattering is approximately two orders of ma~nitude greater than that for the preferred article. The microstructured surface thus produced according to the method of this invention provides an interface whose reflectivity ls relatively lndependent of the an~le o~
incLdence, slmilar to microstructured surfaces produced by other means, such as, for example, that disclosed in U.S.
Patent No. 4,114,983 (Maffitt et al).
Example 2 A CR-39 polycarbonate plano-convex lens blank was substituted for the planar sample of Example 1. Each surface of the lens blank was microstructured according to : -. j , ~ .... . , .~ , - ; . , :.. :-.. :; ,, :, : ' ' ~ :: . .' . i , ,' ~: .: : : , :

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the procedure outlined in Example 1, except that the sputter deposition of chromium was continued for three minutes and the sputter etching was continued for 90 seconds, rather than the 60 seconds of Example 1. The topography of each surface was observed to be substan-tially the same as that of Figure 1. Since each surface is microstructured, the transmission of an optical beam over a wavelength region of 400 ~ 700 nm was very near 100%, with essentially no off axis scattering.
Example 3 In this Example, a 10 cm x 10 cm x 0.2 cm piece of Type 147F pure extruded polymethylmethacrylate sheet from E.I. DuPont Corporation was substltuted for the polycarbonate substrates of the previous examples. The sheet waæ scrubbed clean in mild detergent and water as in Example 1. The sample was rinsed in distilled, deionized and flltered water, and subsequently blown dry with nitro~en gas. Chromium was then sputter deposited on the sample as in Example l; however, the deposition time was continued ~or five minutes to provide an average film thickness o~ about o.6 nm. Further processing was the ~ame as in Example 1, with the exception that the sputter etch tlme was about 135 seconds. The air/sample interface produced by thiæ method was characterized by a decrease in lnterface reflectance, lncrease in interface transmission and no significant lncrease of optical scattering, similar to the results as produced in Example 1~

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Example 4 To show the utility Or the method of the invention, using other techniques for depositin~ the discontinuous microisland3, in this Example all the materials, processing steps, etc. were the same as in Example 3, except that the discontinuous chromium metal film was produced by resistive evaporation from a tungsten boat in a vacuum of about 2 x 10 5 torr. Using aforementioned Alrco Temescal ~DC 8000 film of about 0.1 nm of chromium was deposited. A~ter sputter etching as before, reflectivity of the air/acrylic interface over the wavelength region 400 - 700 nm was observed to vary from about 1~ to 2.5%. Thus a significantly decreased reflec-tance, and hence improved transmission, was demonstrated, although it was not quite as dramatic as in the preferred sputter deposlted case.
Example 5 ~ ~ .
In thls example, both-maJor surfaces of a sheet of 100 ~rn thick orlented polyester were treated according to the followin~ preferred embodiment of this invention.
the surfaces Or the polyester were clean as received and thus needed no further cleanin~ prior to treatment. This sample was treated as in Example 1, except that the dlscontinuous chromium filrn was produced by sputter deposition for eight minutes from an evaporated chromium cathode at 0.38 watts/cm2 in 5 - 6 x 10 3 torr of Ar to produce an average thickness of about 1.0 nm. The . ! , ' ' .: ' :.' '; ~ , ` . . . ', i, ` .

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composite surface was then sputter etched for 105 seconds at 0.31 watts/cm2 in 5 to 6JU oxygen. The results of the interface reflection reduction and the attendant trans-mission increase are shown in Figures 5 and 6. In Flgure 5, curve A shows that the total reflectance from both surfaces o~ an untreated sheet was about 13%, whereas after the surfaces were thus treated (Curve B~, the total reflectance was reduced to about 3%. In Figure 6, the transmlssion of an untreated sheet is shown in Curve A.
In Curve B the transmlssion of the treated sheet is shown to be significantly increased.
Example 6 To show the applicability of the pre~ent method using non-metallic materials to provide the discontinuous mlcroislands, in this example microislands of glass were provided. As in Example 51 two major surfaces of a 100 ~m thick oriented polyester sheet were treated. The surfaces were clean as received and needed no further cleaning. A
dlscontinuous glass film on the polyester surfaces was produced by sputter deposition ~or eight minutes from a window glass cathode at 0.38 watts/cm2 in 6 to 7 ~m argon to provide a discontinuous ~lass film having an average thickness o~ 0.7 nm. The sputter etching was carried out for 150 seconds at 0.31 watts/cm2 in 5 to 6 ~ oxygen. The optical results ~or this example are shown in Figures 7 and 8, wherein ~igure 7, Curve B, shows a total reflectance o~ about 4% over the visible spectrum ~or the , . , ~ , ; ,, : ' ,, :

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treated sarnple, and Figure 8~ Curve B, shows an attendant increase in transmittance.
_ ample 7 The use o~ other metals, particularly those which readily convert to a metal oxide having a very low sputter etching rate, ~s shown in this Example. An oriented polyester sheet as in Examples 5 and 6 was coated on one maJor surface with a discontinuous layer of aluminum by sputter deposition for ten rninutes ~rom an lO aluminum plate at 0.23 watts/cm2 in 6 x 10 3 torr of` Ar.
Under such conditions, Al is deposited at a rate of about 0.1 nm per minute; hence, a discontinuous film having an avera~e thickness of about 1.0 nm was produced. The composite sur~ace was then sputter etched ~or four minutes 15 at 0O23 watts/cm2 in 6 x 10 3 torr o~ oxygen. The decrease in reflectance from the treated surface of about 5% and an attendant increase in transmittance of about 4%
~or the optical wavelength range of 400 to 700 nm was observed.
Exarnple 8 The applicability of the present invention to another type polymer and discontinuous film formlng mat~rial i.s ~hown in this Example. Here, a thin, extruded ~heet consisting o~ a layer o~ an arnorphous mixture of 80 terephthalate and 20% isophthalate on an oriented polyester substrate was coated by a 30 second sputter deposition of gold from a gold cathode at 0.38 watts/cm~

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~3~3~3 in 6 x 10 3 torr of Ar to provlde an average film thlckness of about 2.8 nm. This surface was then sputter etched for one to three minutes at 0.31 watts/cm2 in 5 to 6 x 10 3 torr of oxygen. The microstructured surface whlch was produced resulted in a surface reflectance reduction as in previous examples.
Example 9 In this Example, a base resin of Cellulose Acetate Butyrate (CAB) with no additives for extruding was extruded into a rough, approximately 250 ~m thick, sheet.
The sheet was then thermally flattened in a press at 150 C and 9 kg/cm2 between chrome plated steel backed plates.
The sheet was glass coated by sputter deposition for five minutes from a æoft glass cathode in 5 to 6 x 10 3 torr of argon at 0.38 watts/cm2 to provide a discontinuous glass film havlng an average thickness of about 1.2 nm. The coated sur~ace was then sputter etched ~or three minutes in 6 x 10 3 torr o~ oxygen at 0.31 watts/cm2 to ~orm the microstructured surface. An interface reduction in reflectance and transmission increase in transmittance resulted a3 in the previous examples.
Example 10 The applicability of the invention to a layered substrate is shown in thls Example in which a 30% solids solution of CAB and 50/50 MEK and toluene was cast onto a CR-39 substrate and allowed to dry at 30 C in a nitrogen ;
atmosphere. me sample was then treated as in Example 9 except that it was coated with a discontinuous film of chromium about .15 nm thick by sputter deposition for 75 seconds from a chromium cathode at 0.38 watts/cm2 in 6 to 8 x 10 Torr of Ar. The composite surface was sputter etched for 2.25 minutes at 0.31 watts/cm2 in 8 x 10 3 Torr of oxygen. Again, ~he surface exhibited a decreased -reflectance and increased ~ransmittance as a result of the resultant microstructure. - -The utility of the present invention to provide a primed surface exhibiting enhanced adhesion is demonstrated i;
in the following additional examples:
Example 11 Two 10 x 30 cm pieces o 0.762 mm thick "Tuffak"
Brand polycarbonate film were manufactured by Rohm & Haas Company and sputter-etched under the following conditions.
Discontinuous microislands of Cr metal were first deposited on the film in an RF diode sputtering apparatus operating at 13~56 MH in an Ar gas plasma. A deposition time of 6 ;`
minutes at a power density of 0.4 W/cm2 was used. This was immediately followed by a reactive etch in an 2 gas plasma ;`
for 2.5 minutes at a power density of 0.32 W/cm2 to produce a desired microstructure.
one piece of the microstructured polycarbonate Eilm was subjected to an adhesion tape peel test as ~ollows. A 10 cm piece oE Scotch~ brand Magic mending tape `
manufactured by Minnesota Mining & Manufacturing Company was folded over itself for 1/3 of its length. The ;~
remaining length of exposed adhesive was firmly adhered to ~ ~ 3~

the rnicrostructured polycarbonate sur~ace. The tape was then re~noved from the surface uslng a forcible upward motion, resulting in the delamination of the adhe~ive ~rom the tape backing over the entire impressed microstructured area. No adhesive was delaminated when the same test was performed on the unstructured polycarbonate sur~ace.
The ~econd piece of mlcrostructured polycarbonate was overcoated with an epoxy terminated silane UV polymerizable composition and al]owed to cure to provide a hard, abrasion resistant layer. The cured overlayer was scored horizontally and vertically with a minimum of 10 lines/in. over an area at least 2.5 cm2 (cross-hatchLng). Scotch~ brand Magic transparent tape was then firmly adhered to the scored area. Upon removal of the tape, delamination of the adhesive was observed, with no evidence of removal o~ the overlayer. The same test applied to a simll.ar overlayer coated onto unstructured polycarbonate film resul~ed ln a 100% removal of the overlayer and no adhesive delaminat~on.

Example 12 Two 10 cm x 10 cm x 0.63 cm pieces of Homalite~
type 911 CR-39 (diallyl glycol carbonate), obtained from SGL Industries, were sputter-etched as in Example 11~ In th:Ls Example~ the Cr deposition time was increased to 7 minutes, and the 2 etch time reduced to 1.25 minutes. A
microstructure resulted.
One piece of CR-39 was submitted to the adhesive , . , - - - - . . -.
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tape peel test as in Exarnple 11. The adhesive applied to the microstructured surface was delaminated from the tape backin~, whereas it remained on the tape backing when applied to the unstructured surface.
The second piece of microstructured CR-39 was coated with an epoxy terminated silane composition and allowed to cure as in Example 11. The coated surface was then scored and adhesive tape firmly adhered to it as in Example 11. Upon removal of the tape, the adhesive delaminated, and no evidence o~ removal or peeling of the coating was noticed. Submitting coated, unstructured CR-39 to the same test resulted in 100% coating removal (~ailure).

Example 13 Two 10 x 30 cm pieces of 0.2 cm type 147F
acrylic sheet such as described in Example 3 were sputter-etched as in Example ll, except that in this example, microislands of so~t glass were deposited over a period o~ 505 minutes. A~ter sputter etching in 2 as in Example 11, a microstructure resulted.
One plece o~ the microstructured acrylic sheet was sub~ected to khe adhesive tape peel test as descrlbed ln Example 11. The adhesive was delaminated by the micro-structure surface but not by the unstructured surface, in a manner identical to the results in Example 11.
The second piece of microstructured acrylic sheet was also coated with an epoxy-termirated silane ~L~ 3~3~2 composition and allowed to cure. The coated surrace wa~
then scored and adhesive tape firmly adhered to it as in Example 11. Upon removal of the tape, no evidence of coating removal or peel was noticed. Submitting a similarly coated, unstructured acrylic sheet to the same test resulted in 100% coated removal (failure).

Example 14 Two 10 x 30 cm pieces of 0.10 mm polyester film were sputter-etched as ln Example 11, except that in this Example the 2 etch time was decreased to 1.75 minutes. A
micro~structure resulted.
/ One piece of the microstructured polyester film was sub~ected to the adheslve tape peel test as described in Example 11. The adhesive applied to the microstructured surface was delaminated, while that applied to the unstructured surface was not.
The second piece of microstructured polyester film was coated with the epoxy-terminated silane composition and allowed to cure. The coated surface was then scored and adhesive tape firmly adhered to it as in Examp].e 11. Upon removal of the tape, no evidence of coating removal or peel was noticed. Submitting coated, unstructured polyester film to the same test resulted in 100% coating removal (failure).

25Example 15 A 0.076 rnm film of polyvinylidene fluoride was .. ' ,', , '' "' ' '' ~ , . '.' '` ' . ' `: ' : ' " `
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RF sputter coated with SiO2 for 6 minutes at 0.38 w/cm2 in 5 ~ Ar. Subsequently, the masked film was etched in an RF
generated oxygen plasma for 3 minutes at 0.31 w/cm2 in 5 2 to provide a microstructured surface. When tested in the same manner as in Example 11, the resulting film surface was found to delaminate the adhesive from Scotch~
Brand Magic Mending Tape, whereas the same test applied to an untreated sample of the same film resulted in no adhesive delamination.

Example 16 ..
0.076 mm films of polyethyleneterephthalate and polybutyleneterephthalate were RF sputter coated with SiO2 for 6 minutes at 0.38 w/cm in 5JU Ar. The Eilms were then RF sputter etched in an oxygen plasma for 3 minutes at 0.31 w/cm and 5~u 2 to provide a microstructured surface. When tested as in Example 11, resulting microstructures were found to delaminate the adhesive from adhesive tape, - whereas untreated samples would not delaminate the adhesive when identically tested.
Example 17 :
A 2.5 mm thick piece of nylon resin, specifically Vydyne~Brand RP 260 manufactured by Monsanto Corporation, was RF sputter coated with 6 min. SiO~ at 0.38 w/cm2 in 5~ ~ !
Ar. ~he film was then RF sputter etched in an oxygen plasma for 3 minutes at 0.31 w/cm2 and 5~ 2 to provide a -~
microstructured surface. The resulting surface was tested as before and was found to delaminate -. 3 ~ .a3~ 3 adhesive ~rorn the adhesive tape, whereas the untreated surface would not.

Example 18 A 2.5 mrn piece of acrylonitrile-butadienestyrene copolymer was RF sputter coated with 6 mlnutes SiO2 at 0.38 w/crn2 in 5~ Ar~ m e film was then RF sputter etched in an oxygen plasma ~or 3 minutes at 0.31 w/cm2 in 5~ 2 to provide a microstructured surface. This surface was again found to delamlnate the adhesive ~rorn the adhesive tape while an untreated sample did not.

Example 19 A 2.5 mm piece of a phenylene oxide-based resin (Noryl~) (type PN-235 manufactured by G. E. Corp.) was RF
sputter coated with 6 minutes SiO~ at 0.38 w/cm2 in 5~ Ar.
The sarnple was then RF sputter etched in an oxygen plasma for 3 minutes at 0.31 w/cm2 in 5~ 0~ to provide a microstructured surface. As in the preceding examples, the treated surface was ~ound to delaminate adhesive from the adhesive tape while an untreated saMple did not.

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Claims (11)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A method for forming a microstructured surface having antireflective characteristics comprising a) selecting a substantially transparent polymeric substrate having a predetermined rate of sputter etching under a given set of sputtering conditions, b) applying onto said substrate discontinuous microislands of a material selected from the group consist-ing of metal oxides, refractory metals, and noble metals, having a rate of sputter etching lower than said pre-determined rate under said given set of sputtering conditions to form a composite surface on which portions of the underlying substrate are exposed between the discontinuitics of said microislands, said material being applied in an average thickness in the range of 0.1 to 10 nm; and c) sputter etching said composite surface under said given set of sputtering conditions in a partial atmosphere of a reactive gas to promote the formation of a top layer on said microislands having a desirably low sputtering rate and to preferentially etch the exposed portions of the substrate, while said discontinuous micro-islands are etched at a lower rate, resulting in a random topography of micropedestals which vary in height within a range of approximately 0,01 and 0.2 µm, and are separated from adjacent micropedestals a distance within a range of approximately 0.05 to 0.5 µm and which exhibit substantially decreased specular reflectance without an attendant increase in diffuse scattering.

2. A method according to claim 1, comprising applying said discontinuous islands by vapor deposition unto said substrate of a said material having a said lower rate of sputter etching.

3. A method according to claim 1, comprising applying said discontinuous islands by sputtering onto said substrate a said material having a said lower rate of sputter etching.

4. A method according to claim 1 comprising applying microislands of chromium, aluminum or glass.

5. A method according to claim 1, further comprising the step of cleansing said substrate prior to applying said microislands to remove contaminating oils, particulate matter or the like which may provide non-uniform conditions affecting the steps of applying said microislands or said sputter etching.

6. A method according to claim 1, comprising sputter etching said composite surface in a partial atmosphere of oxygen.

7. A method according to claim 1 wherein said step of sputter etching comprises positioning said substrate on the cathode electrode of an RF diode sputtering apparatus, enclosing said electrode within an evacuatable container, evacuating said container to a pressure less than 10-4 torr, backfilling with oxygen to a pressure in the range of 10-2 torr, coupling RF between the anode and cathode electrodes of said apparatus to initiate a plasma there-between, and maintaining said plasma at a predetermined power density level for a given duration of time.

8. A method according to claim 1 wherein said applying step comprises;
a) providing an RF sputtering apparatus having within an evacuatable container substantially parallel and separate cathode and anode electrodes, b) positioning said substrate on said anode.
electrode, c) positioning a source of said lower sputter etching rate material on said cathode electrode, said source having overall dimensions at least as large as said substrate and having a surface profile substantially like that of the exposed surface of said substrate such that all portions thereof are substantially equispaced from said source, d) enclosing said electrodes within said container and establishing therein a partial atmosphere of an inert gas at a pressure in the range of 10-2 - 10-3 torr, e) coupling RF between the electrodes to initiate a plasma therebetween and maintaining said plasma at a predetermined power density level for a given duration of time during which said source material is sputter deposited onto said exposed surface to form said micro-islands.

9. A method according to claim 1, wherein said selecting step comprises forming said substrate on a separate base member.

10. An optical article having a microstructured surface thereon exhibiting antireflective characteristics, said article comprising a substantially transparent, poly-meric substrate characterized by a predetermined rate of sputter etching under a given set of sputtering conditions and having thereon a random topography of discrete micro-pedestals varying in height within a range of approximately 0.01 and 0.2 µm, randomly separated from adjacent micro-pedestals a distance within a range of approximately 0.05 to 0.5 µm, wherein said micropedestals have in the vicinity of the peaks thereof a generally detectable material comprising metal oxides, noble metals, and mixtures and alloys thereof, said material having a rate of sputter etching lower than said predetermined rate under said given set of sputtering conditions, and wherein said topography results in a said microstructured surface which exhibits substantially decreased specular reflectance without an attendant increase in diffuse scattering resulting in enhanced transulissivity.

11. An article comprising a substantially transparent, polymeric substrate characterized by a predetermined rate of sputter etching under a given set of sputtering conditions and having thereon a topography of discrete micropedestals varying in height within a range of approximately 0.01 and 0.2 µm, randomly separated from adjacent micropedestals a distance within a range of approximately 0.05 to 0.5 µm, said micropedestals having associated therewith a generally detectable material comprising metal oxides, noble metals and mixtures and alloys thereof, said material having a rate of sputter .
etching lower than said predetermined rate under said given set of sputtering conditions, and a layer bonded to said substrate, wherein the presence of said micropedestals results in enhanced bonding of said layer while also provid-ing an interface between the substrate and layer, having a gradation in the optical index of refraction such that the interface is substantially invisible.