CA1243273A

CA1243273A - Method of bonding silver to glass and mirrors produced according to this method

Info

Publication number: CA1243273A
Application number: CA000485832A
Authority: CA
Inventors: John R. Pitts; Terence M. Thomas; Alvin W. Czanderna
Original assignee: US Department of Energy
Current assignee: US Department of Energy
Priority date: 1984-07-31
Filing date: 1985-06-28
Publication date: 1988-10-18
Also published as: GB2162546B; GB8518415D0; IT1187698B; JPS61266334A; DE3527260A1; US4547432A; IT8521655A0; GB2162546A; FR2574067A1

Abstract

METHOD OF BONDING SILVER TO GLASS AND MIRRORS PRODUCED ACCORDING TO THIS METHOD A method for adhering silver to a glass substrate for producing mirrors includes attaining a silicon enriched substrate surface by reducing the oxygen therein in a vacuum and then vacuum depositing a silver layer onto the silicon enriched surface. The silicon enrichment can be attained by electron beam bombardment. It can also be attained by depositing a metal, such as aluminum, on the substrate surface, allowing the metal to oxidize by pulling oxygen from the substrate surface, thereby leaving a silicon enriched surface, and then etching or eroding the metal oxide layer away to expose the silicon enriched surface. Ultraviolet rays can be used to maintain dangling silicon bonds on the enriched surface until covalent bonding with the silver can occur. This disclosure also includes encapsulated mirrors with diffusion layers built therein. One of these mirrors is assembled on a polymer substrate.

Description

: L~432'73 METHOD OF BONDING SILVER TO GLASS AND
MIRRORS PRODUCED ACCORDING TO THIS METHOD

~AC~RG~RO~nrD OF '~EE rY~ IO~
1. FI~T.~ ~F ~ IN ~NTTON
The present inven~ion relates to mirrors, and more part~cularlY,to a met~od of. adherinq silver to qlass.

2. DES~RIPTI~N OF ~E PP.I~ AP~
Mirrors, which are liqht-refle~tive surraces, have been most commonly ~ade over the DaSt century bv Provl~in~ a metallic or amalgam backina on a a~ass surface. ~ silver backinq ia preferred and is ca~able of providin~ reElection of the highest qualitY~
~he conventiona? state-o~-the-art methods of coatinq silver lavers on ql~ass surfaces orior to this inventiQn generall.v fa.ll into three cateaories, inc'.ud~n~ wet che~ica?.

3~

L3~73 e].ectroless ~rocesses. organometallic processes and vacuum deposition. ~here are other well-known methods o~ p.latinq glass surfaces ~ith silver, such as e~ectrolysis; however, the costs of such other processes are prohlbitive Cor commercial use.
~ ach o~ the conventional prior art methods of coating a glass surface has specific a~vantages and disadvantaqes.
~owever, thev have been used successfull~J and to great advantaqe for man~ years in ~roducinq hi~?h guality mirrors, most of w~ich in the past have been used indoors. 'dith the advent of increased interest in the use o~ m;rrors outdoors for solar collectors, such as he.l.iostats and other solar col?ector apalications over the ~ast decade, the di~advanta~es and deficiencies of mirrors prcduce~ by these conventional methods have become more signi~icant an-? more acute.
~ or exam~le, manv conventiona' silvere~-qlass mirror.s are in the form of comDosite structures. ~hese structures usuall~ consist of a substrate qlass layer or pane, a thin reflector laver of 57 lver on the ~lass surface, a layer of copper over the silver laver, and a ~rotective laYer of a paint substance oompr.sed primarily of polv~ers, carbon, ?ead, barium, titantum, and other heavv ~etals. '~hile th.is ~ind of mirror structure is satisfac~orv ~or ,~any vears o.~
service indocrs. the reflectance and other desirably ~ 432~3 o~tica1. pro~ertie~ of these mirror structures deqrade wb~n olaced ~n a terrestrial environment outdoors. In particular, silver is susceptible to agglomeration, the formation of oxides, sulfides, and chlorides, diffusion between component layers, and de-adhesion or separation of the silver layer from the glas~ sub trate. One example of such conditions involves en~ironment~l gases or po~lutan~s, such as combinations o.~
moisture and oxygen, hydrogen sulf ide f~2C), sul~ure dioxide (.SO2), and/or hy~rogen ch]oride (~l), which diffuse through the layers of paint to react with the cop~er an~ the si7ver ref7ector layer. ~.ectrochemica1. reactions also occur at the interface region o. the silver reflector 7.ayer an~ the qlass substrate, as well as at the side edqes Oe the compo-site mirror structure. The degra~ation tvpicalty occurs within a few .months to a ~ew vears, which is an unacceotab1Y
sk.ort time when a aesired lifetime for such equipment is 2 to 3~ years.
Although the s~lvered mi.rrors oroduced by the wet che-mical electrol.ess processes and hy the organometallic metho~s are susce~tible to chemical deqradat.ion resulting i.n loss o~ reflectivitY, mirrors oroduced bv vacuum deposi~ion ~rocesses, such as sputtering and vacuum evaporation, which generally have suPerior o~tical qualities and fewer impurities, are more sus~eptible to de-adhesion of the silver layers from the glass so that the silver layers flake or peel off the glass substrate ~ 5 -~ur~aces. ~ecause Oe the very large surface areas of mlrrors reauired ~or commercial scale ~eliostats and other solar collectors, it is necessary to increase t~e ~urabilitv of mirrors manyfold before s~ch installations become econo-mically feasibte.
.S~MMAR~ O~ T~E ~NYE~TIO~
Accordinqly, it is a qene~al ob~ect of the presentinvention to Provide a method of producina silvered mirrors that are resistant to chemical degrada~ion, ~laking, or ~eelinq when used outdoors in terrestrial environments.
rt is also a general ob~ect of the present invention to examine the causes of t~e degradation Problems in conven-tional silvered mirror structures and to ~rovide corrective methods to alleviate such de~radation ~roblems.
It is a more specific ob~ect o~ the present invention to minimize the impurities th~t are incor~orate~ in t~e silver/q~ass inter~ace durina fabrication of silvered q7ass mirrors and to maximize t~e stabilitv of the resultinq multilaver mirror configuration.
It is a~so a soeci~ic ob~ect of the ~resent invention to obtain a si~nificantlY increas~d number of si7ver-to--~ilicon bonds in a vacuum de~osition process to eli~inate f1aking and ~eelina of the silver layer from the g1ass surface.
~dditional obiects, advantaq~s an~ novel features Oc ~243273 the invention shall be set eorth in part in the des~rlption that ~ollow~, an~ in part will become aD~arent to those skilled in the art upon examination o~ the eollowing or may be learned by the practice of the invention. ~he objects and the ~dvantages of the invention ma~ be realized and attained bY means of the instrumental'-ties an~ in combina-tions particularly pointe~ out in the appended claims.
~ o achieve the ~oregoing and other ob~ects and in accordance with the purpose of the present invention, as embodied an~ broad1v described herein~ the method and pro-~uct Oe this invention may comorise fabricatin~ a mirror ~Y
preparinq the surface of a gJass substrate in a vacuum to be silicon enr;ched and then dePositing a laver o~ silver on the silicon-enriched subst.~ate sur~ace while it is in the vacuum. It ls sometimes desirable, althouqh not considered to always be necessary, to keep the substrate-enriched surface e~Posed to ultravio1et ravs ~uring this orocess.
There are several methods o~ sllicon enrichina the substrate surace by removi.ng tke oxyqen there~rom, in-2~ çluding electron bea~ bombardment, ion beam bombard~ent, andneutral beam ~ombardment. Silicon enrichment of the sub-strate sureace can a~so be accomp1ished bv depositinq a meta?, such as aluminum, that has a areater aFfinitv ~or oxYgen than does silicon on the substrate sur~ace. The aluminum wi1.l ?U]l the ox~gen awav From the si.7icon to ~orm ~2~3273 an a1uminum oxide layer on the substrate surface ~hat is underlaid with a 3il.icon enriched layer ~rom which the oxYgen was removed bv the aluminum oxidation process. ~eat can be used to enhance this oxidation process. ~he aluminum oxide layer can then be etched or eroded awav, such as by ion beam bombardment in the vacuum, thereby leaving the silicon-enriched substrate surface exposed: Another alternative for attainina a silicon-enriched substrate surface i.s to vacuu~ depo~.t a silicon-rich layer onto the substrate surface.
This invention also includes encapsulatin~ a mirror structure with a Polymer. One more desirable mirror struc-ture fa~ricated accor~inq to this invention has a transparent Do1vmer substrate with a vacuum deoosited transparent metal oxide dif~usion barrier laver on the su~-strate surface, sucb as si1icon oxide or all~m~num o~ide.
~he si.lver layer t S deposited on this dirfus.ion ~arrier layer, and another similar ~irfusion barrier is de~osite~ on the exposed silver surface. rhe si.lver and ~i~fusion .l.avers are ~hen enca~sulated ~v a polvmer shel 1 to hermeticallv sea~ all the layer surfaces, edges, an~ interface edges.
~RIEF ~SCRIP~ION O~ T9E DR~ GS
The accomPanying ~raw;nqs, which are incorDorate~ in and form a ~art o~ the s~eci~cations i.l1ustrate r~referred 2~ em~odi~ents o~ the Present invention, and together wlth the 3;i~73 description, ~erve to e~plain the princiPleS of the lnven-tion. In the drawings:
Fiqure 1 i5 a cro~s-sectlonal illu~tratio~ o~ a ty~ical prior art si.l~ere~ qla-~s mirror structure;
~i~ure 2 is an illustrati~n o~ the atomic l~ttice structure of glasss ~i~ure~ 3 through 9 illustrate the chan~e~ in the atomic lattice structure as silver is applied to a ~las~
substrate in a wet chemic~7. e~ectro~.ess process;
Figures la throu~h l3 il7ustrate in cross sec~ion the phases of mirror construction accor2in~ to one Oe tha ~ro-cesses o~ t~is invention;
Figure 14 illu~trates in cross section a m~rror constructed accordin~ to a varatlon of a process oF ths invention5 Fi~ure l.S illustrates the resul~i~g a~omic lattice structure of the mirror in Figure ~.4;
Figure 1~ illustrates in cross qection a di.fusion layer ~rotected silver laver in a mirror structure:
~ gure 17 illustrates another form o~ .eusion laYer protected mirror structure similar to ~iqure l~, appearing with Figures 6 - 9;
Figure 18 illustrates in cross section a polymer encapsulated silvered glass mirror structure according to this invention, appearing with Figures 6 - 9; and Figure 19 illustrates in cross section a polymer encapsulated mirror structure on a transparent polymer substrate ~2~32~3 constructed according to this invention and appearing with Figures l - 5.
DETAIL~D DES~RIP~I~N OR ~E P~EFERR~D ~ODI'.~
In order to ap~r~ciate tbe advancement in the art of silv~red mirror production accordinq ~o this invention, it i-~ also benefic~al ~o consi~er the inventors' discovery o~
the deficiencie~ inherent in the pr30r art mirror productlon methodq that cau~e the degradation, f1aklnq, and peeling ~roblems when ex~osed to terrestrial environments. ~he nature of these problem-causing deflciencies has not been recoqnized or understood prior to t~i~ invention. In ~act, the nature an~ characteristics of ehe silver-to-a~ass bond have not been rec~gnl2ed or understood DriQr to t~is inven-tion. The understandinq now gaineA, as will be described ~elow, contribute~ to tbe ln~ovations in mirror construction accor~ng to this invention.
~ ~irror structure lO ~abricated accordjna to a conven-tional wet chemical electro1e~s ~rocess is illustrated in ~igure 1. The g1ass pane or substrate 12 has a silver t~q~
laYer 14 on its surface. A co~er (ru) layer 16 is provi~e~
over the silver layer 14, and a protective paint coat 18 is spread over the copPer laver l~. ~he pai nt coat 18 is usuallv comDrisea o~ a ~olvmer ~ixe~ with carbon an~ several heavy metals, such as ~e~d, barium, and t~tanium to provide a thick, dense barrie~ to oxvqen and ~ater.

~l24~273 g - The surface layer of gl.ass in a natural. atmosphere has a gel layer thereon ~ormed of hydroxi~e ions (~~) honded to the .sili~on (Si) atoms Oe the glass silicon ox~de (SiOx~
lattice. ~herefore, before the silver laYer is deDosite~, S the surface of the ~lass is scrubbed in the conventiona' wet chemical electro'ess process ~or several minutes with an abrasiue to remove the gel layer. Most of the qel layer can be remove~ with this abras've scrub; however, the presence of oxygen and water a.lwavs leaves at least one layer o*
hY~roxi~e ions (O~~) on the surface of tb.e glass ~ as shown in Figure 2.
The next step ~n the conventiona.l wet chemical electro-tess Drocess is to wash the glass surface with a stannous chlor~de (Cn~12 2~20) solution and then to rinse with water. S'lver nitrite is then added wi.th a reducina aqent,such as su~ar, and an additional source of ~-, such as sodlu~ h~aroxiAe (~aO~, to form the si'ver laver 14. The coDper (~u) .layer 1~ is put on top of the silver tAq) layer ?4 bY usina a coor,Jer su.~.fate so,tution with a re~ucinq aqent, suc~ as ca?cium hydriZe and iron filings. ~he structure is then dried under infrared la~ps and painteR.
As ~iscusse~ above, mirrors ~roduced accor~inq to this conventiona.t wet chemical e].ectroless ~rocess ~roviZe excel-lent o~tical ~ualities, but the~ ~eqr~de ra~i~lv in outdoor environments. T_ has not previouslY been !~nown how ~:se ~43273 above process works or what constitutes its deficiencies.
Rowever, the present inventors have now determined how this process works, what deficiencies contribute to rapi~ deqra-dation, and how such prob~ems can be avoide~. ~his 2escription wil~ now ~rovide the inventors ana~.~sis of the wet chemical electrole4s proce4s and an explanation o~ its deficiencies.
As described above, t~e lattice shown in Fi~ure ~
-lustrates the top layers o~ the gtass (~iOx~ suostrate aft*r the qel is scrubbed off the surface leavinq a surface tayer of hydroxide (~~) ions bonded to the silicon (si! atoms at the next adiacent atomic laver. ~hen the sur~ace Is washed with the ~ annous ch~ori~e (~Sn~t.3) solutibn, the (.Sn~13~
co~P~ex inserts itse'f into the covalent bond between the si atom and the O~ ion, as illustrated in ~igure ~. ~he result is the SnCl~t~ co~lex bonde~ to the c; atom, as shown in ~iaure 4.
mhe surface is then rinsed with water which eliminates the ~1- ions, as ~ollows:
~2 + C~ 1 +~~
The result ;s that the Cl ions attached to-the .Sn are washed away and re~laced by hy~roxide ions O~~, as shown in ~igure 5.
An additiona~ O~ converts the f-.ve coordinate ~n com-plex to a six coord nate ~n com~lex, shown i.n ~lgure 6,wit~

~, ' ~ ' ' ' '', 3~'73 ~ an addtional ~~ ion attac'ned to the Sn. ~his ste~ is accelerated by raising the p~, such as bY addinq sodiura hYdroxide (t~aO~ 'he Ag then attacks and bonds with the ~, thus d~ splacnq the 0~~ ion ~reviously bonded to the Si, 5 as illustrated in ~iaure 7, and the two disl~7aced f~~ ions70in the Sn atom. ~he ~n then leaves wi.th six ~ ions in the form of qn~-~M)2~ leaving ~7ehind an CIR- grou~ to bonc~ to the Si in Place ol~ the ~n. ~be result is two Ag atoms ~onded to Si atom~ for every one Sn complexr bonded to a ~i 10 atom, as shown in ~glare 8. ~inally, the reducer plates more Ag atoms to a thickness of a~proximate7Y 10n.~, as shown in 7~igure ~. In other words, Ag ions pick up electrons erom the reducer to form silver ~netal. as Follows:
A + + ~ ~ A
~onsequent~y, as ~iscovered bv these inven~ors, what-ever adhesion there is between silver laver 14 to a~ass substrate 12 obtained in this l~rocess resu7ts directly Crom the ~g to Ci covalent bonds linkinq these two elements.
~ests ?~y the inventor~ have shown ~hat in the convent~ional 20 wet cbemical e!ectro7ess ?rocess as describe~ above, ~ow-ever, less than ~0 ~ercent o the available ~i atoms are bonded to Sn complexes. 'rhis ~eans that less than ~ er-cent of the silver can be bon~e~ to the surface si1icon. In 'act, as shown in ~igures ~ and a, the maximum ~heoretical 25 ~q- to-Si bondinq availa!~e un~er r~erfec~ conditions in this wet chemical electroless process is only two out o~ thr~e or 67 percent o~ the .~ atoms on the top layer, since one out of these three Si atoms retains an OH ion ln the ~rocess.
~onsequently, the a~herence of the si~ver taver 14 to the 5 glass substrate 1~ has a relativelJ low theoretica:l maximum strenqth and an even lower l~ractical. 4trenat'..
. ~urther, the characterlstic susceptib.U.~ty to chemical degradation in mirrors ln fabricated with this ~in~l o~' tet chemical electroless process is due to impurities, 5nch as 10 the Sn+2 and ~1- ions left at the ~g-to-~Si i.nter~ace in the fabrication process. ~~ooper and imsurities . ~om the adjacent co!?per laver, as wel.7 as impurities from the paint layer, al50 contribute to acce3.erated dearac~ation o~ ~he silver layer. n'here~ore, these chemical impurities are 15 placed in the mirror ln an~ are inherent i.n the nature of this fabrication process. 'onseauentlv, t!~ese conventional ~irrors 10 are constructecl with bot:1 inherentlv ~eak silqer-to-g~ass bonding, as well as with the chemically active impur-ties built in to cause se3.f-~lestruction i~ a ter-20 restrial environment.
~ he organometall~c method o~ producing m~rrors basical-ly involves the apolication o:~ complexe~3 si7ver in organ.ic !nc lecules to the ~ass suhstrate. ~hese organometallic solutisns are t~ermally ~ecomposed on the surface o~ the 25 glass sul~strate under carefull.v controll.ed conditions o' ~43Z73 solution, composition, firing temperature, and firins atmosphere. For exampl.e, the glass substrate is eirst cleaned with chromic acid and rinsed with distilled water and alcohol. A solution of organic material with comp'exed 5 silver metal therein is then aT~?lied to the subst- ate bv spin-on or spray techni~ues, either as a single coatinq or as nsuJtiP~e layers. '~inally, the coated substrates are heated to temperatures just be]ow the softening point of ql ass . or approximatelY 40û to 700 ~.
~estlng and analysis by t~e inventors of the mirrors fabricated bv this organometallic process ~roduce-3 results tkat confirmed their discoverY that e_fective silver-to-glass adhesis~n or b-~ndinq must he the resu't Oe Aq- to- ~L
covalent bonding at the sil~er-to-g?ass inter ar:e. P.dhesion 15 testinq with a ~ebastian adhesion tester in~icate¢i the ~ailure surface under tensile stress at the silver-to- qJass interface is about ~5 ~ glass and; ~ silqer. ~herefore, the adhesion test fai].ure is ~rimarilv cohes've in the glass and inaicates strong Aq-to-Ci bonding at the silver'glass inter-20 ace.
~ he mechanism for the adhesion or bonding at thesilver/~lass inter~ace in ~nirrors Labricated with the orqanometa~lic orocess ~as ~iscovered by the inventors to result Crom enr~ckment o~ c; at the ?'ass substrate surface 25 b~ destructiJe oxidat-on of tl.e orqan;c com~Lex. ~heir ;i~ ~, , - - testing and analvsis revealed that glass inherentlY has a high oxygen content. ~owever, the surface layers o~ glass at the silver-glaYs inter'ace of mirrors constructed bv the organometa~lic process as described above are nearl.y devoi~
5 of oxygen. T~uring thermal decompositi.on oÇ the organic complex at 400 to 700 C, reactlons occur which attaclc the glass surface and deo~ete the oXvqen, thereby leaving a - silicon- rich layer. .Silver in the solution then forms - - covalent bonds at the oxygen- depleted Si gites. ~nce a 10 sufficient portion of the glass surface has been covered bv covalen~l.y bonded ci~.ver, the sil.ver meta.~ plates electro-les~ly onto this surface to ~orm a thick, re1ativet.y pure ~lm. , b~irrors fabricated by this organometaUic method also 15 exhibit si,q,nificant susceptibility to chemical degradation I.n te restrlal environm ents due to several factors. ~, ases produced durin~ the therma.l decom~osition of the orqano-meta:llic so7.ution cause or at least contribute to voicls an-l rou,qhness in the silver layer surface causinq a hazy or 20 m~ cv appearance and reduced reflectivitv. which decrease the ef f ectiveness of such mirrors FOr solar collector app~.i-cations where hiqh reflectivitv is re~uired for economic a~plications. Also, impurities in the ,qlass substrate mi,qrate to the surface ~ur1ng the oraanometa~-ic ~tatj.ng ~rocess, 25 which renders ~he s~ver/ql.ass inter:tace susce~tlb.le to .

~3~73 --,,5--chemlca1 degradation when exposed to ~he u7traviolet rays of the sun and other terrestrial environmental conditlons.
Tt waq found that mirrors produced by conventional vacuum deposition processes, such as sputtering OT vacuum evaporation, exhlbited the best reflectivitv (in the range of q7~) and o~her o~tical propert~es desirable 4Or solar co~ector use. ~owever, adhesion of the s~ver to the glass substrate is very poor. In ~act, it was ~ound that adhesion of tbe silver f~ms from the oraanometa1l;c p1a~ing ~rocess is about ten times stronger than films ~roduced bv vacuu~
evaDorating si-ver onto alass. ~parently there is very little, if any, ~q- to-~i covalent bon~ing in mirrors ~ro-duced bY convent~ona1 vacuum de~osition processes.
~he inventors' discovery o~ the Aa-to-~i covalent bond as the mechanism for superior si'ver-to- glass adhesicn in mirrors produce~ bv t~e wet chem7ca~ electroless process an~
bv the orqanometa~ic process, alonq ~ith their ~iscoverv o~
the causes of susce~tibilitv of those mirrors to degra~a~ion in outdoor use as solar coUector~, contrihute~ to the;r innovation of the nove~ processes for adhering silver to glass according to this invention and the vastlY superior and durable mirrors pro~uced according to the processes of this invention. ~hese mirrors have extr2~elv strong and a ~igh ~ercentaae of covalent Ag-to- ci honds for strona adhesion of the silver 1aver to the alass suhs.rate, vir-2~3273 tuall~r no impurities at the si~ver/glass interface to contribute to chemical degradation, and the superior optical qualitites of vacuum - deposite<~ silver laYer mirrors.
The processes of this invention are directed Jco maximi-zing Si enrichment on the qlass substrate sur~ace .~hile keeping contaminants and impurities away and vacuum de~ositinq an Ag laver on tke ~i-enriched surface to obtain maximum Ag-'co-~Si covalent bondinq. ~our related methods of adhering silver to glass accordinq to this. invention are described be~ow.
~he '~irst methoA is accom~lishe~ bY initiaLlY scrubb~ng the ~ass with an abrasive, such as cerium oxide, to remove any sur~ace impuritles and ~el. ~f course, whenever the glass is in air there will be a surface layer of O ~~ ions, as shown in Figure 2. that cannot be eliminated due to mo;sture in t~e air. ~he scrubbed ~lass substrate is then place~ in a vacuum chamber and a vacuu~ is pu11ed A vacuum in the ran~e of 10 4 Pa to 10~ Pa prov~des satisfactorv results in norma} circunlstances.
T~71t~ the glass substrate ~n the evacuated cham~er, ~he surface of the substrate is bombarded with an electron beam to reduce the sur~ace, i.e., to eliminate the oxvgen from the surface. .~ ~eam in the ranae of l~ootl to 5,00/~ volts should be sufficient ~or this Durpose. ~he electron beam 25 ruptures in-?ivldual~ sillcon-oxvgen bonds. ~ince the sur ace ~Z~3;~73 oxvgen has onlv one bon-3 att~ching it to the substrate, the o~y~en leaves, while the silicon remain~s ln the matrix because of the silicon's other bonds. This ~roce~s contlnues in the same manner to release oxvgen from the silicon oxicle 5 (siox) of the glas~ substrate, thus leavin~ the sur~2ce Yer of the substrate enriched with Ci, which provides llan~lin9 bonds or partiallv fi~led electron or~its on the sub~trate surface ~ith which Ag wi~l b~nd ~uite rea~ily.
~rhe next ste~ there~ore is to vacuum deposit ~ on the ~Si 10 enriche~ sub3trate surface, such as by sputterinq or vacuum eva~oration. ~he Aq and Si readilY form a stron~ covalent bond to adhe~e the silver to the su~strate surface as ton~
as the substrate remains cLean, i.e., as long as the sub-strate is maintained in a vacuum. Of course, once the 15 sur'ace is covered with Ag, deposition continues to bu;ld UD additional 7avers of Ag to the desired thic~ness.
It is qu;te feasible in th~s Process to achieve removal o~ substantial amounts the oxygen 'rom the su~strate surface b~v electron beam bombardment so that the ~q ~onds to nearly 20 100 ~ of the ava~lah7e silicon s~tes. ~urther, since no other chemicals are introduced and a vacuum is maintained throughout this entire process, there are virtuallY no i m-purities to contaminate the Aq-to-5i inter~ace. ~he result is 2 very stronglv adherent and extreme7Y pure Ag- to- ~1 25 interface, and the resulting sil.ver laver has all of the ~L2~3;Z73 - very hiqh aualitv reflectivitv and other ootical Properties o~ vacuum denositPd mirroes so desirable and even necessarv eor effective sol.ar co~lector apDtications.
~'c is possible with a ~?assaqe of time durin~ the pro-5 cess that the .Si ions at the substrate surface could - restruc:ture themselveq to eliminate the danaling .bon-ls or parti.aLl.y fille~ electron orbits. ~owever, since Aa- to-~i bonding is more favorable enerqetical'Y than S1- to- ~i bonding, suc~ .~i restructurin~ ~rior to ~g d~ostion usuaLly 10 will not hinder the ultimate for~ation of Aq- to- ~i bon~lnq as l.onq as t~e Ci enrichment at the substrate surface is almost ~ure with virtual~y al~ of the oxyaen removed. . Tn the event there is a concern in this req. ard, ultraviole~
light wiU hreak .S~.-to- ~i bonds. ~herefore, exposure o~ the 15 Si- enriche~ substrate surface to ultraviolet light clurinq the process ~ill maintain the tlangling ~i bonds and keeo them available F~or coval.ent bond~nq to the ~g atoms.
The secon~ metho~ Oc t!~is invention is very similar to the first n-ethod. ~owever, rather than using electron beam 20 bombardment for removi!la o~cvgen and silicon enrichinq the substate surFace, an ion beam is use~ to bomhard the sub-strate surface. The e~ection o~ oxyqen From the su~?strate surface using this ion beam l~ombardment technioue results from a mechanism or ~ri.ving force similar to the el.ectron 25 beam tec~ni~ue, but it is enhance-3 hv the mass o~ the kont-~ .

"~L2~3273 bardina ions colliding with the oxvqen to a~so physicaLly erode or preferentially sputter the oxygen ~rom the sur'ac~.
~ variation o~ this second method that a1so is ef~ec-tive is to use a neutral beam hombardmeslt, such as hel~um, 5 neon, or argon, to sputter the oxygen from the substrate surface, thus leaving silicon- enriched outer layer-~ at the substrate surface. In fact, anv technique capab1e o~
preferential1y removinc oxyqen from the substrate sur~ace in a vacuum without introducinq any imDUritieS or contaminants lO would be effective to provi~e tbe silicon- enriched surface neces~ary for this process.
A thir~ method of si~lcon enrichinq the g.~ass substrate sur~ace is to first scrub the glass surface w ith an abra~ive to remove as much gel as possible. ~en p]ace the substrate 15 4~ in a vacuum cham ber and evaporate or sputter a layer of a1uminum (Al) 44 onto the surface, as sbown in ~iqu~e lO. rnhis A.l 1 ~yer 4~ can ranqe anvwhere Crom a monolayer of A'.atoms to lOOOA th~.ck or more as deslred, as iltustrat~d in .~iq~.~r~? 1 n 20~he next step i~ to heat t~e su~strate 4 ~ an~ ~.1 laver 44 to a temperature iust ~low the softenin~ tem~erature o~
he q.lass, such as ap~rox7mately 500 r, The Al wi11 react with the o:~yqen in the outer l.ayers of the sub~trate to ~ozm aluminum oxide (A1~0~3, therebv extractinc the o~Yqen from 25the si1.i.con. mhe resulting structu~e, as shown in $'igure - . : - , .
i7 32'^~3 11, has three lavers. ~he ~lass substrate 4~ is si.licon oxi~e (SiOx). An intermediate zone 46 of pure silicon (Si) is formed where the oxyqen was extracted erom the ~ubstrate.
A layer of aluminum ox~ e r~l2(~3) 48 is ~ormed where the 5 oriqinal Al laYer ~4 was dePosited.
~ hile ~aintain~ng the vacuum, the laYer 48 of A12~13 is et~hed away bY ion beam or neutral beam bombardment to ex~ose the 7ure layer 46 of $i, a~ shown ~.n ~iaure 12.
Finally, the Ag laYer 50 is vacuum deposi.ted `onto the .Si 10 laYer 4~ where it ~or~ns a strong covalent Ag- to- ~i bond, as shown in ~ic?ure l3 and as lescribed in the methods discussed above. ~gain, s1nce this method ~ro~ides a pure sil~.con enriched su~strate surface in a vacuu.~ wlthout introducinq imourit~es or contaminants, the resultlnq Ag- to- ~i covalent 15 bond is verv strong and ~urable and has the hiqh ~ualit~J
o~tical prol~erties characteristic o:~ vacuum - de~osite~ sil-vered mirrors. It should be noted that met27s other than Al that also have a stron~er affinity to oxvc~en than silicon, such as zirconium, chromium,.or titanium, could also ZO be used instead of aluminum.
A variation o~ th.is third method is to ~eave t~.e A~.233 layer d8 shown in ~igure ll, ratl1e: than etchinc7 it off.
~hen, since ?1 acts like .~i in this kind o~ lattice struc-ture, an ~g layer ~0 can !le vacuum de~osited on the ~1?03 25 ?.aver 4R, as shown in ~igure la. ~he resu.7tina l.att~ce 32~73 structure would be something like that illustrated in Fiqure 15 .
fourth method o~ ~roviding a .s~icon- enriched layer - on the substrate sur~ace is to vacuum depos.~t pure Si on the 5 scrubhe~J ~urface of the ~ix substrate to ohtain a structure such as that shown in ~i~ure 12. ~hen a~ ~9 layer 5~ can be vacuum del~ositetl on the ~i layer 'co obtain the m7rror struc-ture shown .in ~igure 13.
Once the mirror structure s obtainecl accord~n~ to this 10 invention as described above, the silvered refl~ctive la~Jer 50 can be protected and ma~e more durable by encapsulatlnq it in a material iml~ervious to air and moisture, hecause silver is reactive with oxygen and wi?t. corrode or tarnish.
In this wav, copl~er and paint layers with their associated 15 de?eterious eefects on the silver laYer, as ~escribea a50ve, are avoided. For examole, as shown in ~igure 1~, a thin ~?ass laver ;l can be lasninated over the silver ?.aver 5t~ to sandwich the silver laYer 5tt between the atass substrate 42 and the qlass laYer 51. A.nother sealing structure is shown 20 in ~igure 17, wherein a ci~2 la~er ~2 is vacuum ~eoosite~l over the silver laYee 50. ~he encls or edges are sealed bY
ena caPs 54 ~ormed of a thick eooxy or acrylic hon~ing materia7 or bv heat or laser fusion o the glass edges.
~nother sea?ing structure is shown in ~i.gure l8 wherein 25 a clast;c material 55, sucn as ~olvmethvlmethacrvlate ~l24;~73 (21~fA), is coated on top of the sjlver layer ~0. ~MA, - which is one o~ ~he most diffusion- resistan~ polysners available com merically, can be applie~ as a l.iauid in solu-tion, and then the solvent can be allowed to evaporate 5 leaving a tou~h, substantiallY imoer;neable seal layer ;~.
~ novel composite sealed silver mirror structure 60 fabricated accordin~ to thls invention is shown in Fi~ure 19, which is durable, lightweighi~, and protected fro~n de-qradatton ~n the terrestrial environment. A ootY~er sub- -10 strate 52, such as P~A. has vacuum de~osited thereon a laver ~4 of ~iO2 that serves as a di~fusion barrier. The surface ~S o~ the ~iO2 l.ayer 64 is silicon enriched as described above, and the silver layer ~6 is vacuum dePosited on this silicon-enriched surface. Another difÇusion barrier 15 layer ~8 Oe ~ciO2 is vacuum deposite2 on the silver layer ~6.
~his steo can be initiated or ~recede~ ~v vacuum de~ositin~
a silicon- enriched coatins 57 on the sil~rer laver ~6 to oromote enbanced adhesion of the ~sin2 layer 68 to the si.lver ayer ~. This assembly can then be remove~ rrom the vacuum and 20 - coated with a oolymer superstrate laver 70 on the toP and over the edges 7~.
It is imoor~ant that the orocess of this invention eor silicon enrichina a glass substrate and deoositin~ silver in the silicon enriched surface in a vacuu~ is aoo?icable to 25 a~-'t process reauirina stronalv adherina silver to a silica ~jL%4~3zr73 '' - substrate, such as thermall~ qrown silicon dioxicle in the production of el ectronic circuits.
The foregoing is considered as illustrative onlY of the principles o~ the invent.ion. ~urther, since numerous 5 moaifications and changes wil.l readi.lv occur to those skilled in the ar~, it is not desired to ~.imit the invention to the exact construction an~ operation shown and described, and accordingly all suitab~.e modifications anA equivalence - may he resorted to fal1ina with.in the scope o~ the invention 10 as defined by the claims which follo~.

Claims

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

1. The method of adhering silver to the surface of a glass substrate, comprising the steps of:
placing the glass substrate in an evacuated chamber;
enriching the surface of the glass substrate with silicon atoms while the surface of the glass substrate is in the evacuated chamber so as to substantially eliminate the oxygen from the surface of the glass substrate and to sub-stantially protect the surface from impurities and contaminants;
and depositing silver atoms on the silicon-enriched surface of the glass substrate while the glass substrate is in the evacuated chamber, and wherein eliminating the oxygen from the surface of the glass substrate and protecting the surface from impurities enables a bond to be formed between the silicon atoms and silver atoms which has sufficient strength and durability to withstand degradation caused by terrestrial environmental conditions.

2. The method of Claim 1, wherein the step of silicon enriching the substrate surface is accomplished by the step of bombarding the surface of the glass substrate with a beam of electrons.

3. The method of Claim 1, wherein the step of silicon enriching the substrate surface is accomplished by the step of bombarding the surface of the glass substrate with a beam of ions.

4. The method of Claim 1, wherein the step of silicon enriching the substrate surface is accomplished by the step of bombarding the surface of the glass substrate with a neutral beam.

5. The method of Claim 1, wherein the step of silicon enriching the substrate surface is accomplished by the step of depositing a layer of silicon on the surface of the glass substrate.

6. The method of Claim 1, wherein the step of silicon enriching the substrate surface is accomplished by the step of depositing a layer of metal that has a greater affinity for oxygen than silicon on the surface of the glass substrate, allowing the metal to react with the oxygen in the glass sub-strate surface layer to form metal oxide over a layer of oxygen-reduced, silicon-enriched substrate.

7. The method of Claim 6, including the additional step of etching the layer of metal oxide off the substrate to expose the silicon-enriched surface layer of the substrate.

8. The method of Claim 7, wherein the step of etching the metal oxide off the substrate is accomplished by ion beam bombardment of the metal oxide layer until the metal oxide layer is completely removed from the substrate.

9. The method of Claim 7, wherein the step of depositing a layer of metal on the substrate surface is accomplished by depositing aluminum on the substrate surface.

10. The method of Claim 7, including the additional step of heating the substrate after the metal layer is deposited thereon to a temperature approaching, but not reaching, the softening temperature of the glass substrate to enhance oxidation of the metal on the surface of the substrate.

11. The method of Claim 1, including the step of shining ultraviolet rays on the substrate surface during the steps of silicon enriching the substrate surface and depositing silver thereon.

12. A process for forming a silvered-glass mirror having a silver layer substantially impervious to separation due to environmental weathering, said process comprising the steps of:
placing a glass substrate in an evacuated chamber;
reducing a surface of the glass substrate in the chamber such that the surface of the glass substrate is sub-stantially devoid of oxygen and comprised substantially of only a metal-like silicon layer;

depositing and bonding a layer of silicon on said reduced surface to form a composite structure so as to effect formation of direct silver-to-silicon covalent bonding, and wherein reducing the surface of the glass substrate substantially enhances adhesion of the silver layer to the glass substrate as defined by the properties of such covalent silver-to-silicon covalent bonds.

13. The process of Claim 12, including the additional steps of dissolving a polymer that is free of heavy metals, fluorides, chlorides, and sulfides in a suitable solvent to obtain a liquid form, spreading the dissolved polymer over the silver layer and over the edges of the silver layer and the silver-to-glass interface, and allowing the solvent to evaporate leaving the silver layer and the edges of the silver and the silver-to-glass interface encapsulated and sealed by hardened polymer.

14. A silvered-glass mirror capable of providing protection against moisture and other forms of environmental weathering having a structure fabricated by a process comprising the steps of:
placing a polymer substrate in an evacuated chamber;
depositing a layer of transparent metal oxide on a surface of the polymer substrate;
enriching the exposed surface of the metal oxide layer with silicon atoms;
depositing a silver layer on the silicon-enriched surface of the metal oxide layer;

depositing a metal oxide layer over the exposed surface of the silver layer;
removing this composite structure of metal oxide and silver layers bonded to the polymer substrate from the evacuated chamber and encapsulating the composite structure, including the interface edges of all said layers with each other and with the substrate with a dissolved polymer in liquid form, and allowing the solvent to evaporate, leaving a hardened polymer shell that hermetically seals the metal oxide and silver layers and the layer-to-substrate interface edges from the atmosphere.

15. The mirror structure of Claim 14, wherein the metal oxide deposited on said substrate surface is silicon oxide.

16. The mirror structure of Claim 14, wherein the metal oxide deposited on said substrate surface is aluminum oxide.

17. The mirror structure of Claim 15, wherein the fabricating process also includes the step of depositing a silicon-rich layer on the exposed surface of the silver layer and then depositing a layer of silicon oxide on the silicon-rich layer.