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Reflectance and durability of Ag mirrors coated with thin layers of Al2O3 plus reactively deposited silicon oxide
A technique for producing adherent and well-prtected and well-protected front surface Ag mirrors with an evaporated Al2O3+SiOx overcoat and an evaporated Al2O3 underlayer is described. A thin Al2O3 layer promotes adhesion between the Ag film and its substrate. THE optimum thickness of each outer layer necessary to give good adhesion and protection to the Ag surface with minimal loss in reflectance due to ir absorption was found to be about 300a for the Al2O3 layer and between 1000a and 2000a for the siox film. ag surfaces coated 450nm to the far ir, even when exposed to harsh sulfied and humidity environments. calculations demonstrate the advantage of using protected front surface ag in comparison to more durable metal reflectors.such as al or rh.in terms of high reflectance and low visible region palarization.however,a very pronounced restrahlen reflectance and absorption effect in the thicker siox film is responsible for da calculated drop in reflectance from 98.5%to about 65% at =8.1um and 45 incidence for the ag +al2o3+siox coating.


evaporated silver films ate know to have the highest reflectance of any metallic mirror surface farir.silver mirrors also introduce less polarization into an optical system than do other metallic reflectors,such an aluminum or rhodium.however.the use of evaporated silver as a front surface mirror coating has been severely restricted by its poor ashesion to mirror substrates and its susceptibility to sulfide tarnishing.attempts to use evaporated silicon oxide or magnesium fluoride as protective films have been generally unsuccessful due to their poor adhesion to silver
new requirements for exceedingly high reflecting low palarizing mirror elements have stimulated research to overcome thest inherent restrictions.a recent publicatiom demonstrated that evaporated silver could be overcoated with double layers of rather thick al2o3 and silicon oxide films to produce surfaces with low solar absorptivity and hihe hemispherical emissivity. evaporated al2o3 was used both as an intermediate binder layer between the silver film and the substrate and also to provide an adherent overcoat on silver for the outer silicom oxide layer. this combination produced well-adherent silver films on metal,glass,or plastic substrates. to maintain high ir reflectance wihe rhis same ag+al2o3+siox combination,it was essential to reduce the dielectric film thickness in order to minimeze ir absorptiom and interference effects.it has been known from earlier work that thin al2o3films dehere well to ag,but do not offer sufficient protection against moisture damage,whereas silicon oxide is resistant to moisture attack,but does not adhere well to silver.the adbantage of using a combined aluminum oxide plus silicon oxide overcoat to protect front surfacesilvermirrors has been recognized by other ,and a patent has been recently issude to h,adams of dentou vacuum,inc.,for such a film combination, in this paper we will show that a composita al2o3+siox coating with an evaporated al2o3 underlayer can produce well-adherent and protected front surface silver mirrors that have low polarization in the eisible region and high normal incidence reflectance from the short wavelength region of the visible to the far ir
analysis on the reflectance behavior of front surface mirrors
the effect of a single-layer overcoat on the mormal incidence refectances of silver ,aluminum,and rhodium mirror surfaces at 546nm is shown in fig.1 reflectances were calculated with optical constants for ag(n=0.005-i3.32)3,aln=0.82-i5.99,and rh() obtained from the indicated references.the metals were assumed to have beenovercoated with effective quarter-wave and half-wave thicknesses of a nonabsorbing surface film that had an index of refraction between 1.0and3.0 compared to other typical metal mirror coatings,such as evaporated al or rh,silver is least affected by the presence of a nonabsorbing surface layer. it is this property of silver that makes it possible to construcet protected ag mirrors that have near-normal reflectances greater than 95%throughout the visible and ir regions.
a typical overcoating material,such as al2o3,deposited to a thickness of or an odd multiple of 1/4,will reduce the reflectance of ag from 98.5%to96.0%;of al from 92%to80%;and of rh from79%to 55%.
in fig.2,the calculatede reflectance of silver as a function of incidence angle is presented for the perpendicular(Rs),parallel(Rp),and average(Rav)components of reflectance at a=550nm.the calculations were performed for bare ag and for ag over-coated tiht an effective1/4.2/4.and a typocal high/low index reflectance enhancing combination.it is immediately evident that an wvaporated silver mirror surface,whether coated or uncoated,will exhibit a negligible decrease in reflectance at 550nm for all angles of incidence.
the final curve of fig.2 presents the reflectance of a double-layer reflectance enhancing combination,with n1=1.6(al2o3)and n2=2.3(ceo2).the walue of normal incidence reflectance,RAV=99.2%,remains virtually unchanged to incidence angles up to 50.
figures 3and 4 contain similarly calculated angular reflectance behavior for al and rh,respectively.both metals show the same genetal trend as ag with and without surface films,but in each case the average reflectances are noticeably lower than they are for ag,and there is a gerater difference in reflectance between the two polarized components with incresaing angle of incidence.
it can be seen from figs.2,3and 4 that as the diwlectric layer thickness increases form 0to 2 1/4,
there are two reversals in the magnitudes of the Rs an Rp components.the reversal points are of interest because the polarization is zero at there thick-ness values.for example,a ga mirror used at 45 and protected with a single-layer overcoat (n=1.6)approximately either 500a or 900a thick would have a reflectance of about 96.6%and zero polarization.
the preceding series of figures clearly indicates that not only do evaporated ag films have high reflectance,but they also introduce less polarization into an optical system than either al or rh surfaces.this is summarized in table i,where values for Rs,Rp,and Rp/Rs are presented for 550nm and 45.
the preceding discussion has centered on the feefct of nonabsorbing surface films on the visible region reflectances fo ag,al and rh . surface films of this type with n=1.6or 1.6 at 552nm,such as wvaporated silicon oxide (siox)or aluminum oxide (al2o30,become strongly absorbing at certain ir wavelenghts. the reduction in normal incidence reflectance presents no insurmountable diffculties if the films are kept thin enough,however,the effect of even thin absorbing films on the angular reflectance can be surprisingly large,as demonstrated in table2.calculations were made for a film arrangement consisting of ag+300al2o3+1500siox,which is a typical example of the surfaces studied in this investigation.a wavelength of 8.10um was chosen because saparate calculations over a broader wavelenght region shower the effect to be most severe at this wavelength region showed the effect to be most severe at this wavelenght for the given film combination.optical constants for ag (n=8056-i49.0),al2o3(n=1.64-i0.00)and sio2(n=0.40-i0.37)6,necessary for the calculations,were obtained from the indicated references.n and k walues for sio2,rather than siox were used bacause they were available in the literature and should be quite similar.the tabulated data clearly incicate that a thin surface film,nonabsorbing in the visible region but absorbing in the ir,will have little effect on the visible reflectance at 0or 45 or the ir reflectance at 0 .but the reflectance decrease for 45 is extremely severe at 8.1um.the drop in Rp/Rs of 0.309.it should be noted that the thin al2o3 layer is only slightly absorbing in this spectral region,and it is the thicker siox film that in this wavelength region has a very pronounced restrahlen reflectance and absorption that is responsible for the severe drop in reflectance.from these calculations one must conclude that a film such as siox can not be used on mirrors at high angles of incidence in this spectral region. the salient points and practical ramifications of this phenomenon will be discussed in a future publication by g.hass.j.t cox and w.hunter.
experimental techniques and results
three different substrate materials were used in this investigation-ordinary fire polished glass(microscope slides) , highly polishide cer-vit,and fused silica.
evaporations were performed in a 1.8-m vacuum chamber and films were condensed onto substrates mounted 80-100cm above the evaporation sources the al2o3 films underneath and on top of the ag film were applied by electron-beam evaporation.the nonabsorbing outer al2o3layer was deposited at a slightly faster rate and lower pressure.silver was evaporated from a heated tungsten boat to a thickness between 800a and 1000a at a rate of 100-200a per sec. as a final protective film,silicon monixide was reactively evaporated from a tungsten coil heated source box and deposited onto the al2o3 layer at a rate of 3-5a per sec. and oxygen pressures of 7-9*10torr. the basic principles and methods of producing the thin films used in these studied have been thoroughtly discussed in earlier publication.
two methods of monitoring film thickness were used during the deposition process.the thicker films were monitored potically by reflectance measurements made with monochromatic light of 500nm. thinner films were monitored with a quartz crystal microbalance that was seperately calibrated for each materal with a fizeau interferometer.
after removal from the evaporation chamber, bare,unprotected ag test samples were stored in an evacuated desiccator.nevertheless,aslight drop in reflectance of the ag occurred before the samples were exposed to h2s an humidity testing.silver samples with protective overcoats were kept at essentially ambient laboratory conditions.
scotch tape ashesion tests were performed on each coating and substrate combination before and after every stage of testing.
accelerated sulfide tarnishing was accomplished with an apparatus and recipe similar to one described by h.e. bennett et al hydrogen sulfide gas was humidified in passage through a water vessel and mixed with air in a 1:9 ratio.test specimens were exposed to a flowing mixture of this moist h2s gas in an otherwise sealed glass cylinder that remained in the temperature range of 20-25 for periods up to 20h.humidity exposures were performed in accordance with mil-c-675a; that is ,at 50 and 95% relative humidity for 24h.an aminco type pc climate lab was used for the humidity exposures
the reflectance of the evaporated coatings was measured in the 0.35um to 20um wavelength region with perkin-elmer model 350 and model 621 double-beam spectrophotometers equipped with specular reflectance attachments. reflectance data obtained in this manner were relative to a freshly evaporated al standard of well-know reflectance . the normal incidence reflectance of freshly deposited silver and silver coated with a protective film of 300a al2o3+1500a siox is presented in fig.5 for the wavelength region 0.36-20um .the protected silver film was deposited onto a glass slid with an intermediate adhesive layer of al2o3 about 900a thick.figure 5 shows that the reflectance of the protected silver surface remains above 95% from 0.45um to 20um . below 0.45um,the reflectance decreases due to a combination ofeffects--a roughness related surface plasmon excitation in the ag film coupled with a thickness related interference minimun.
the thickness dependency of the ir absorption bands at 3um and 9.6um forces an optimization process to be followed in choosing the proper siox thickness necessary to provide sufficient protection for the ag layer while maintaining high reflectance. the band at 3um is caused by water absorbed throughout the siox film,whereas the ~9.6um band is due to the intrinsic absorption of the 1500a thick siox layer.
an example of the effects of intense h2s exposure on a al film protected with 300a of al2o3 alone,compared to an unprotected ag surface,is presented in fig.6. the relatively rapid deterioration of the unprotected film is an indication of the severity of the environment . the protected silver film experienced a less than 1% drop in reflectance confined to the region of the surface plasmon effect.this indicates that a thin,well adherent single layer of al2o3 can inhibit tarnish growth on ag.
the type of sturctural damage inflicted on inadequately protected ag mirror as the result of h2s and humidity exposure can be seen in fig.7. the photomicrographs refer to a film combination of ag +300a al2o3 plus an additional 300a of siox for moisture protection.the deterioration is highly localized corresponding to gaps or pinholes in the various thin films the number of these sites is strongly related to the substrate cleaning and handling procedure prior to evaporation. figure7(a)
shows that the ag has been tarnished by the h2s ,whereas in (b)the high humidity and temperature have combined to cause the ag to locally delaminate with no apparent tarnishing.the photographs also show the film to be intact and in uniformly good condition away from the pinhole sites. when the siox layer thickness is increased above 1000a, good protection is obtained. as indicated by zero loss in reflectance and microscopic analysis
results obtained from exposure tests similar to those described above demonstrated that nichrome (80%ni,20%cr) or an equivalent alloy could be substituted for al2o3 as a binder film between the substrate and ag layers with no impairment of adhesive optical quality .
the authors wish to acknowledge the valuable assistance rendered by w. gilchrist in preparing the evaporated coatings,and by s.shenker in conducting the h2s experiments.
this paper was presented at the october 1974 annual meeting of the optical society of america.houston ,texas




Fig.1. Calculated reflectances of Ag ,and Rh coated with effective quarter wave or odd multiple of 1/4 and half-wave or even multiple of 1/4 thicknesses of a nonabsorbing sruface film
Fig.2 calculated reflectance of ag and ag plus three typical nonabsorbing surface films as a function of incidence angle at 550.
Fig.3calculated reflectance of al and al plus three typical nonabsorbing surface films as a function of incidence angle at
Fig.4 calculated reflectance of rh an rh plus three typical nonabsorbing surface films as a function of incidence angle at 546
Fig.5 measured reflectance of freshly deposited ag and ag +300a+1500a siox from 0.36um to 20um
Fig.6 measured reflectance of unprotected ag and ag protected with 300a of al2o3 before and after h2s exposure.
Fig.7 photomicrographs of defect sites on ag nirror surfaces overcoated with 300a al2o3+300a siox.(a)after exposure to moist h2s gas for 18h at 20 (b)after exposure to 95%relatie humidity at 50 for 24 h.