CA2084537A1

CA2084537A1 - Durable low-emissivity solar control thin film coating

Info

Publication number: CA2084537A1
Application number: CA002084537A
Authority: CA
Inventors: Jesse D. Wolfe; Abraham I. Belkind; Ronald E. Laird
Original assignee: BOC Group Inc
Current assignee: Linde LLC
Priority date: 1992-03-04
Filing date: 1992-12-04
Publication date: 1993-09-05
Also published as: MX9300283A; FI930941A0; JPH063523A; EP0560534A1; AU2989092A; FI930941A; KR930020179A; AU657014B2; NO924849L; NO924849D0; TW221703B

Abstract

DURABLE LOW-EMISSIVITY SOLAR CONTROL THIN FILM COATING
Abstract of the Disclosure An infrared reflecting interference filter capable of transmitting a desired proportion of visible radiation while reflecting a large portion of incident solar radiation is provided. The filter consists of a transparent substrate coated first with a dielectric layer, next a partially metal reflectance layer, and finally an outer protective dielectric layer. In addition, between each metal-dielectric interface is deposited a nucleation or glue layer that facilitates adhesions and improves chemical and mechanical resistance. The interference filters are durable and can be modified to provide a full range of optical and electrical characteristics. The dielectric layer can comprise of composite films consisting of silicon nitride in combination with zirconium nitride, titanium nitride, and/or hafnium nitride.

Description

,rl IN ~HE UNITED STATES PATENT AND ~RADEMARR OFFICE
APPLICATION FO~ PATENT

InYentors: Jesse D. Wol~e ~braham ~elkind Ronald E. Laird s This application i8 a con~inuation-in-part o~
Serial No. 522,266, filed ~ay 10, 1990, ~nd has a ~ommon assignee.

~ackqxou~d Q~_~hQ Inven~ion This ~nvention relates generally to visibly transparent infrared reflecting interference filters, and more particularly, to a durable low-emissivity filter.
The use of transparent panels in buildings, ~ehicles and other structures ~or controlling solar radiation i8 quite prevalent today. The goal of ~olar control is to transmit liqht while excluding much of the solar energy, thus decreasing the ~mount of air condition or cooling required, and conserving energy.
In nddition, modified glass as ~ ~tructural ~aterial prov$de~ the color flexibility architects desire.
Various process0s have been employed to ~lter the optical propert~e~ of these panels, lncl~ding coating glas~ or plastic ~ubstrates by various techniques such as electrolysi~, ~hemical vapor deposition and physical vapor deposition, including ~putterin~ with planar ~agnetrons. For instance, thin metal films have ~een deposited on glass or plastic to increase the reflectance of ~olar radiation. Windows deposited with a multi-layer dielectric-metal-dielectric 2 ~

coating that exhibits h$gh visible transmittance, ~nd high reflectivity and low ~missivity in the infrared ran~e, are ~ven Dora energy ~fficient. The index of refraction of the dielectr~c layer i8 prefer~bly 2.0 or greater in order to minimize the visible reflectance and ~nhance the visible tr~nsmittance o~ the window. This dielectric layer which often con~ists o~ ~etal oxide coating also offers additional protection to the fragile metal ~l~s. The optical properties o~ panels can also be modified by altering the composition o~ the substr~te ~aterial. Nevertheless, ~nter~erence ~ilter panels manufactured by the above-described methods have ~een only partially successful in reflecting ~olar radiation to the degree required ~or signi~icant energy lS conservation. For example, Apfel et ~1., U.S. Patent 3,682,528, issued August 8, 1972, described an ~nfra-red interference filter with visible light transmission of only approximately 72% and with ~n~ra-red transmission of approximat~ly 8%.
..
Summary_Qf the Invention It is a primary object of the present invention to provide a durable, thin-~ilm $nterference f~lter which transmits v~sible light while reflecting infrared radiation.
It i~ another ob~ect o~ the present invention to provide an inter~erance ~lter that i8 userul in ~rchitectural panels which give~ l~ss reflected color of visible light over ~ wide band.
~hese and additional objects ~re accomplished by the present invention which provides a durable, thin-$ilm interference ~ilter which comprises a substrate onto which is deposited a dielectric layer, followed by ~etal nnd dielectric layers. In between each of the dielectric and ~etal layers is deposited a "nucleation"
or glue layer that promotes adhesion between the dielectric to the metal. In one pre~erred e~bodimen~ of the invention, the interference filter comprises a glass 6ubstrate onto which is depo~ited a thin-film desiqn consisting of ~ive layers, n~ely: titaniuD oxide, nickel-chromiu~ alloy, ~ilver, nickel-chromium alloy, and ~ilicon nitride.
Another pre~erred ~mbodiment of the ~nter~erence filter comprises of a ~ive layer ~tructure where$n on~ or both Or the dielectric layers is for~ed of a composite material containing zirconium nitride and silicon nitride. It was found that mixing zircGnium nitride with silicon nitride create~ a composite layer that has a high refractive index ~nd excellent tr~nsparency in the vlsible region. Moreover, ~e optical properties o~ thi3 co~posite l~yer ~an be ~djusted by varying the rel~tiv~ amounts of zirconium nitrid~ and ~ilicon ~itride.
The dielectric layers of the inventive inter~erences ~ilters can ~e reactively ~puttered by a rotatable cylindrical ~agnetron. Composite layers can be formed by cosputtering from dual cathode targets or ~ro~ one or ~ore alloy targets. A feature of the inventive process i8 that by reducing the intrinsic 6tress o~ the ~econd dielectric layer, an extremely hard and chemically resi~tant thin fil~ coating is produced.
In sputtering illcon nitride a~ the second dielectric layer, lt wa~ de~onstrated that the intrinsic stre~s o~
this layer can be reduced by orienting tbe ~agnetic assembly of the cathode at an ncute angle v~s~ is the substrate.
Additional ob~ects, ~dvantages and features of the present invention will become apparent from the ~ollowing det~iled exemplary description, wbich description should be taken in conjunction with the accompanying drawings.

Brie~_Pescription of the Drawinqs - Figure la is a cross-sect~onal view of a five layer design thin-f~ nterference ilter produced in accordance wit~ t~is invention.
Figure lb is ~ graph illustrating the spectral transmittancs ~nd ~eflectanco o~ ~ thin-fil~
interferencQ ~ilter.
Figure 2 i3 ~ cross-sectional view of ~athode asse~bly.
Figure 3 ~8 a graph illustr~ting the spectral tr nsmission ~n the visible light region for a composite rilm.
Figure 4 i~ a graph illustrating the spectral reflection in tha visible light reg~on for a compos~te ~llm.
Figure 5 is a graph illustrating the spectral adsorption in the visible light region for a composite ~ilm.

~esc~ipti~n of the Preferred Embodiments ~0 A thin-film interference filter incorporat$ng the present invent~on is hown in Figure la. As shown therein, the filter consists o~ a transparent substrate 2 which i provided with two planar parallel surfaces 4 and 6, ~n which ~urface 4 ~ exposed to the medium ~nd ~urface 6 i8 coated. The sub~trate c~n be formed of any 6ultabla transparent material; however, the substrate is preferably a ~aterial which ~as ~uperior ~tructural properties and ~in~mum absorpt~on in the visible and ~ear-infrared ~pectra regi~ns where the solar energy $s concentrated. Crystalline quartz, fused silica, soda-lime silicate glass, and plastics 6uch as polycarbonates and ~crylates, are ~11 preferre~ substrate materials.
Depos~ted onto the ubstrate surface 6 is a first dielectric layer 8 that is preferably made of a material having an index of refraction of greater than ,d ~

about 2.0, and most preferably between 2.4 and 2.7.
Suitable dielectric layer material~ include metal oxides such as titanium oxide, tln oxidQ, zinc oxide, indium oxide (optionally doped with tin oxide), bismuth oxide, and zirconium oxide. See Hart, U.S. Patent 4,462,883, issued July 31, 1~84, Which i~ ineorporated herein by ref~rence. Yet another ~uita~le ~aterial ~ sil~con n$trlde. A particularly su~table dielectrio material compri6es a thin composite ~ilm containing zirconium nitride ~nd ~ilicon nitride (collectively referred to herein as ~SiZrN") that i~ fabricated by cosputtering from dual targets or from a ~ingle alloy target of ~ dc cylindrical magnetro~, as described herein.
Zirconium nitride is an electrically conductiv~ material which ~as very good optical reflectance in the infrared 6pectru~; however, this materi~ very absorbing in the visible portion of the Qpectrum and cannot be u~ed on device~ requiring high transparency. Silicon nitride, on the other hand, is very transparent ln the near W through the near IR
spectrum (350 nm, 2.0 microns). It was discovered that ~ixing z~rconium nitride with the ~ilicon nitride creates a composite film that has ~ high index of refraction (22.10) and excellent transparency in the vis~ble ~pectrum. ~h~ film also demonstr~tes good chemic~l and mechanical ~urability. FurtherDore, by employing co~puttering with dual cathode targets, the ~ndex of refraction of the film can be adjusted by vsrying the ~mount of power to c~ch cathode ~nd/or the gases used in the process. The index of refraction of the film 80 fabricated ranges ~rom approximately 2.00 to 2.45.
Besides SiZrN, composite fil~s comprising titanium nitride and silicon nitride (collectively 3S re~erred to ~erein as "SiTiN") or comprising hafnium nitride and ~ilicon nitride (collectively referred to 2 ~ 3 3 ~

herein ~ "SiHfN~) can ~l~o be used. SiTiN and SiHfN
composite films ~re ~160 prepared by cosputtering from dual or ~ingle targets. Finally, ~ composite film comprising ~ mixtur~ of silicon nitride, zirconium nitride, titanium nitride, and/or hafniu~ nitride can be used ~s th~ first dielectric layer. As ~ill be descri~ed further below, th~ re~racti~ index o~ She composite film~ will VAry depending on the relative amounts of th~ di~ferent nitrides that comprise each film.
It has been found ~hat when 8ili~0n nitride i8 used a~ the first dielectric layer, the visible light transmission of the inventive filtQr is slightly less than the trans~ission when titan~u~ oxide or a composite fil~ is used.
~ he th~ckness of the rirst dielectric layer ranges from approximately 200 to 500 A, and more preferably ~rom approximately 300 to 350 ~
A3 shown in Fig. la, the inventi~e ~ilter next comprises of a ~irst metal precoat 10 that is deposited over t~e first dielectric layer. Precoat layer 10 ~8 preferably maintained a~ thin AS possible ~o that it will have very little, lf nny, ~dver~e effect upon the optical characteri tics of the ~lter or the ~ubseguent metal layer. Pr~coat layer3 with thicknesses ranging rrom approxi~ately 5 to 20 A h~ve been 6atisfactory;
~ore prefer~bly, the thickness 1~ between approximately 8 to 16 A. Thi~ thin precoat layer can be formed ~rom any number of ~etals. It has been gound that nickel-chromium alloy comprising ~pproximately 1 to 80 percent~ickel and approxi~ately 1 to 20 percent chromium can be used ~s a precoat; more preferably, the alloy content is - approxi~ately 80 percent nicXel and 20 percent chromiuffl.
Other metals and alloys thereof that can be used as a preooat include nickel, chromiu~, rhodium, platinum, tung~ten, molybdenum, and tantalum. See Hart, U.S.

r~

Patent 4,462,883, issued July 31, 1984. The precoat layer apparently acts a~ a glue or nucleat~on layer and as a stress reducing l~yer. It is believed that while the precoat layer is thin enough not to adversely affect th~ optical propertieC Or the ~ilter, it causes the metal.~ 12 :t~ behAve. a~ -if ~t werc a homogeneous ~e~aL.sl~b~
Next, a partially x~flective metal layer 12 is deposited onto the first precoat layer. ~he metal layer reflects infrared-radiation, yet ~llows for ~uficient visible light tra~s~ission. ~hc metal layer can be formed fro~ ~ number of ~aterials, with ~ilver being particularly ~atlsfactory. Other ~etal~ which also ca~
~.util~zed: include gold, copper and platinum. The thickness of the metal layer ranges from approximately 40 to l5o A, and more preferably, from approximately 90 to llo ~.
In ~hi~ preferred embodi~ent, ~ 6econd ~etal precoat layer 14 i8 then deposited onto the metal layer which i~ followed by the ~inal dielectric layer 16.
Thi~ second metal precoat layer can be ~ormed from the same material and ~n the ~ame thicknesR range as precoat layer 10. ~he second dielectric layer can be made of ~ilicon nitrid2 that i8 ~ormed by reactive sputtering a cylindrical magnetron. This layer has a thickness from ~pproximately 350 to 500 A, and moro prefera~ly from approximately 4so to 475 A. The above re~erenced composite films can also be used although the relative proportion o~ ~ilicon nitr~de ~n each film is ad~usted 30 fiO that the re~r~ctive index ranges preferably ~rom ~pproxi~ately 2.04 to 2.10. When a composite film is used, its thickness should be from approximately 300 to 500 A, preferably 350 to 375 ~. However, whether ~ilicon nitride or a composite substance is used as the second dielectric layer, the layer ~ost preferably exhibits low intrinsic stress as described further 2i~`?3~

~elow. A suitable composite f ilm is SiZrN comprising ~pproximately 80-83% by weight silicon nitride ~nd the balance zlrconiu~ nitride. This particulAr ~ilm has a refr~ctlv~ index o~ approxiBately 1. 85 to 2.2. A
preferred SiZrN composite ~il~ has 2 refractivQ index o~
about 2.08. A3 will be described below, the inventive ~ilters offer excellent ~echanical and corrosion resistance.
- T~e precoat ~nd ~etal layers were deposited with ~ D.C. planar ~agnetron. Other technigues including E-beam evaporation could have also been employed. The dielectric layers of the inventive filter ~ere prep~red by DC-re~ctive Eputtering with a rotating ~ylindrical DagnetrOn- The ~agnetron reactive ~puttering technique iB particularly useful for depo~iting dielectric films. While there are other technigues for depositing the dielectric layers ~uch as thermal oxidation and LPCVD (low pr~ssure chemical vapor deposition), these method3 ~uffer fro~, a~ong other things, 810w depos$tion rates. Moreover, RF planar ~agnetron 6puttering ~or depositing dielectric material ~6 impr~Gtical for l~rge-scale industrial applications bec~use o~ the enormous power reguire~ents and RF
radi~tion hazards. A description of n cylindrical magnetron suitable for deposit~ng eubstrates with th~
di~lectric ~ateri~ 8 ~ound ln Wolfe et ~1., U.S.
Patent S,047,131, i~sued Septomber 10, 1991, incorporated herein by reference. To provide additional protection to the lnventive filter, a plastic laminate can be applied to the filter of Fig. la. See Young et al., U.S. P~tent 4,965,121, issued October 23, 1990 incorporated herein by reference.
In ~abricating the lnventive filter, it was ~ound that by reducing the intrinsic ~tress of the ~econd dielectric layer 16~ an extremely hard and che~ically resistant thin ~ilm coating is produced.

Stress i8 an important Yariable that i8 inherent in each layer of ~- thin film stac~. There are generally two stres~ states; ~1). compressive, where the film i~
tryi~g to ~xpand on th~ substrate ~nd, ~2) tensile, S where the.film ~.trying to contract. In ~a~netron ~yste~s, the pressure o~:the vacuum depositing chamber i8 ~n impQrt~nt ~ctor which influences stress. It is beli~ed that ~t ~ufficiently low pres~ures, sputtered atoms and reflected neutral gas ~toms lmpinge on the film at nearly normal ~ncidence with high energy because at lower pressure thers are ~ewer.collisions within the plasma (larger ~ean fre~ path). This mechanism, as reported by Hoffman and ~horton in Thin solid F~lms, ~0, 3~5 ~1977), ~ ~nown aS ~atomic peening", ~nd is lS believed to cause compression i~ ~ilms.
At higher worX~ng pressures, the sputtered atoms collide with atoms in the plas~a more frequently.
Sputtered mater~al reache~ the ~ubstrate at oblique ~ncidence and with lower energies. The decrease in ~inetic energy of t~e ~ncident atoms ~akes the peening ~echanism inoperative. The decrease in the ~lux o~
atoms ~rriving at normal incidence results in "shadowing~ -- voids remaining from the nucleation cta~e of ~ilm growth are not filled ~ecause nucleation ~ites 2~ shadow the obliguely ~rriving atoms. Shadowing and 'competinq ~one growth~ can lead to isolated columnar grain ~tructures and ~n extensive void network. Messier and Yehoda, ~. Appl. Phys., 58, 3739 ~1985).
- Whatever the cause o~ internal ~tress in ~puttered fil~s, there ~5~ ~or a given set of system parameters (e.g., ~agnetro~ geometry, deposit~on rate, film thickness, gas pressure), ~n abrupt transition from co~pression to tension at a critical pressure which depends on the ~tomic ~a~s of the material. (Hoffman and Thorton, Thin Solid Films, 45, 387 (1977); Hoffman and Thorton, J. Vac~ Sci. Technol., 20, 35~ ~1982);

~ ~ S ~

i Hoffman and T~orton, J. Yac. Sc~. Technol., 17, 380 (1980).) Above thi8 critical pressure, tensile stresses gradually decrease to zero. The relaxat~on o~ 8tre5s beyond some ~axi~um tensil~ ~tres5 point was reported for chromium sputtered $n argon and molybdenu~ sputtered ~n- xenon- Shl~ ~t al. f ~Propertie8- of Cr-N Films Produced by Reactivo Sputtering~, ~. V~c. Sc~. Technol.
A4 (3), May/JunQ 1986, 564-567.
In depositing ~ilicon nitride as the second lQ dielectric layer wi~h a rotatable cylindrical magnetron, it was found that the intrinC$~ stress of the ailicon nitride layer can be reduced by orienting the magnetic asse~bly of the ca~hode ~t an acute angle. As shown in Fig. 2, w~ich ~ a cross-~ectional view of cathode 20 and gubstrate 21, the maynetic assembly 18 has a "Wl~
conf~guration with three elongated magnetics 24, 26, and 28. The permanent magnetics used ~ormed an unbalanced system which i6 typical for rotatable cylindrical magnetrons. As is ~pparent, the assembly is oriented at an acute angle ~-o~ ~pproximately 45 ~o as to direct sputtere~ ~aterial tow~rds the substrate 29 as it enters the deposit~on chamber. ~ngle ~ can range from approxi~ately 30 to 80. Silicon nitride layers ~o deposited have ~pproximately one-fourth the intrinsic ~tress of ~ilicon nitride layer~ produced when tha aBaembly 15 ~t ~ nor~al ~ngle r~latlve to th~ ~ub~trate.

Experimental Re~ul~
- A low-e~issiv~ty interference filter having the ~tructure ~5 shown in Fig. la compri~ing a glass ~ub~trate, a titaniu~ oxide first dielectric layer, nickel-chromium alloy precoat layers, a silver ~etal layer, and a 6ilicon nitride 6econd dielectric layer was ~abricated ~n an in-line magnetron system manufactured by Airco Coating Technology, a division of Assignee. It is known that Tio2 ~s the predominant form o~ titanium oxide created i~ the 6puttering process. However, it iB
believed that other forms are produced a~ well. Thus, unless ot~erwise stated, TiO2 will represent all forms o~ titanium oxide produced. The system comprises of S five magnetron~ arran~ed in series, with each ~agnetron depositing one o~ the ~iv~ layers of the filter. The ~econd, third, and ~ourtb are planar magnetrons for depo~iting th~ first precoat, ~etal, and second precoat layer~ respectively. The planar magnetrons, each comprising of a ~odel HRC-3000 unit, were manufactured by Airco Coating Technology. The first and fifth magnetrons are cylindrical ~agnetrons to dep~sit the dielectric layers. The cylindrical magnetrons, each comprised of a C-Mag~ model 3000 cathode, also manufactured by Airco Coating Technology.
The target ~8) for each of the cylindrical magnetrons was conditioned using an inert ga, thereafter the process ga~ was added until the desired partial pressure was reached. The process was operated at that point until the process was stabilized. The substrate was then ~ntroduced to the coat zone of the first cylindrical ~agnetron and the fil~ was applied.
The substrate used wa~ soda lime glass.
Fcr depositing ~ ~irst dielectric layer comprising o~ titanium oxide, a C-MAG~- rotatable ~agnetron employing a titanium target was used.
Alternatively, ~ planar magnetron can be employed.
Argon was the inert gas and oxygen was the reactant gas.
When depositing ~ilicon nitride in the cylindrical magnetron, ~rgon was used as an inert gas ~nd nitrogen was used as the reactant gas. The partial pressure of the gas was determined ~y the transition fro~ the nitride mode to the metallic ~ode. Experiments were run as close to that transition as practicable. The pressure and flow rate of the sputtering gases were controlled by conventional devices.

Becaus~ the electrical conductivity of pure ~ilicon i8 80 low that it $~ unsultable for ~puttering with direct current, the ~ con target was impregnated or doped with a ~mall amount o~ aluminum in the range of from 2-4%. Th~ tarqet was prepared by plas~a 6pray.
The sputtering source wa~ connected to ~n appropr~ate direct curr~nt po~er ~ource ~aving provision for au~o-~atically malnt~inlng t~e voltage, current or power, as desired. The ~agnet assembly o~ the single cathode was oriented at ~n angle of approximately 45~ from nor~al.
With nitrogen a~ the 6puttering ~as, the coating ~ontained a ~ixture Or ~luminum and ~ilicon nitrides. All of these ~o~ponents are relatively hard and form an ~morphou fil~ that acts as a strong barrier. However, the amount of aluminum in the film did not interfere with for~ation of the desired silicon based compound film~. In the course of the experiments, films were sent out for independent RBS (Rutherford Back-Scattering) 6ampling to determine the composition of the compound. The ~ilicon nitride ~easured 42%
Si/57% N, which ~s very close to the theoretical 3:4 ratio for nitride (Si3N~).
Table 1 ~ets forth the process datA for deposition of ~n inventive filter.

~A~E_1 Flo~- ~lo~- Sub Th~ck- r-t~ rrte rot- ~ot-n- Pres- trrte ~. ~sca) ssc~) tsca~ tlal Po~r ur- ~o. S~
r ~ tV~ (kl~) tu~ Passes t1n/~nin~
T~02 32r71 0 131 -371 ~0 1.5 ~ r lI~Cr ~2 1700 0 -U~ 1 3.0 1 15~
A~100 i9 0 0 -552 10 1.5 1 156 3 0II~Cr lZ 1700 O -~4 1 3.0 1 154 5~3N4 Ul 12~0 0 -38r 15~J~2) 5.0 2 31 ~ ~ ~?~

The abov~ filter had th~ following optical and electr$cal characteristics:
82.4 % Transmittance (integrated ~65 ~ource) 6.1 % Reflectanc~ of the film co~ered side 511.5 % Absorbanc~
- - 10.5 n/0 Electric~l ~heet resistance O.09: E~i siv~ty The dur~bility o~ thQ inventivQ filter of Table 1 was tested. The procedures of the chemical and mechanical tests that were performed are described in Table 2. The ~nventive fllter passed ~11 the tests.
Curve 1 ~n Fig. lb illustrates the reflectance of the interferance fllter produc~d under the parameters set forth in Table 1 as ~ro~ thQ ~ ide. Curve 3 is the reflectance of the uncoated 6ubstrate slde and curve S is the transmittance. The measurement~ were performed with a scanning spectrophotometer.

Test Conditions and Scorina Procedures ~. HuDidity Test Exposures in a humidity cabinet for: (1) 24 hrs. at 90C and 98% RH
and (23 96 hrs. ~t 60C and 98% RH.
2. Salt ~og Test 20% Salt Fog, 9S-98F for 72 hrs.
3. W Exposure Expo~ure ~or 24 ~r~. with cycles Test o~ 4 hrs. ~ondensation until failur~ or 120 hrs.

4. Ammonium Test Samples Are placed upright in closed container of 50% a D onium hydroxide solution at room temperature for 5 hrs.
S. Salt Dot Test A 1% ~alt ~olution is applied to a filter paper dot placed on the film with the sample placed in a con-~tant humidity environment for 24 hrs.

; 5 3 r~

Evaluations o~ the above tests are based on both ~icro-scopic ~valuation and amissivity measurements. The detail~ o~ thQ evaluations are:
A. Sample~ ~r~ ~cored for evidence o~ micro-fiCOpiC eorrosion as seen under 200x magni-fication on a scale o~ 1 to 10, where 10 is unaffected and 1 ~8 completely corroded.
B. Measure the change ~n ~missivity due to corrosion. ~he s~oring i8 based on:
Emissivity Score - 10 (Emi~s. ~e~ore/~miss. ~fter) C. Recorded scores are an average of 1 and 2 6. Taber Abrasion Samples are subjected to a total o~
SO revolutions on the Taber abrader, using the standard 500 gra~ weight and CS-lOP wheels.

.
Evaluation is based on the average number of ~cratches seen under 50x magnification in 4 inch2 areas. Using the equation below gives a score of o for more than 55 ~cratches ~n a ~" ~guare area and 10 for none:
Taber Score ~ 10 - tt~ ~cratches) x ~0.18~]

A~ ~tated above, in other embodiments o~ the inventive f~lter, one or both o~ the dielectric layers can comprise of composite ~ilms o~ elther SiZrN, SiTiN, SiHfN, or mixtures thereof. For each composite, the relative ~mount o~ ~ilicon nitride ranges from ~pproximat~ly 60-95% by weight depending on whether the compositQ io used as t~ ~$rst or second dielectric layer. The index of re~ract~on of the composite ~ilm correspondingly ranges from approximately 2.4 (60%
silicon nitride) to zpproximately 2.05 (95% silicon nitride).
One method of depositing composite films is cosputtering of a cylindrical ~agnetron employing dual targets with one target being made of silicon ~nd the other target being made o~ oither zirconium, titanium, hafnium, or mixtures thereo~. When cosputtering ~ith dual cathodes with nitrogen ~5 th2 reactant gas, the angle of th8 ~agnetic assembly of ~ach target can be ~djusted to get ho~ogeneou~ ~omposition distribution.
See Belkind et ~1., U.S. Patent Application Serial No.
~71,360, filad March 19, 1991, of common assignee, and Belkind et Al., ~Reactiv~ Co-Sputtering of Oxides and Nitrides using a C-MAG~ Rotabable Cylindrical Cathode,~
Surface ~nd Coating Technology, ~9 (1991), 155-160.
Another ~ethod of depositing composite films to have one or more ~lloy targetY, ~ach coated with ~ilicon and either zirconium, titanium, hafnium, or a mixture thereof. A process for fabricating cylindrical alloy targets involves doping silicon and another metal (or other metals) to form a conductive silicide. For instance, doping ~ilicon ~nd zirconium results in forming ZrSi2, a conductive cilicide that possesse~ a bulk resistivity oP ~pproximately 160 micro ohm am.
~his material i~ conductive enough to be sputtered ~y a magnetron. The silicide can ~e synthesized by heating zirconiu~ and 3ilicon together (hot press technique) to a sufficient temperature to form ZnSi2. Thereafter, the sili~ide i~ grounded to a powder and sprayed onto 8 ~tainles~ ~teel backing tube to form a ~omogeneous coating.
ZnSiN compositQ fllms were formed by ~o-sputtering a C-MAG rotatable magnetron ~ystem ~anufactured by Airco Coating ~echnology. The system employed dual cathode targets wherein the angle the magnetic assembly o~ each target was 6et at approxi-mately 45~ relative to normal ~o ~s to focus the ZrN and Si3N~ ~olecul~s onto the glass 6ubstrates. It is believed that ZrN is the predomin~nt form of zirconium nitride created in the sputtering process, although other forms may be produced as well. Thus, unless otherwise ~tated, ZrN will represent all forms o~
zirconium nitride ~uttered.
- With dual target~, tho relative amounts o~
reactively ~puttered material deposited frc~ each target can be regulated, in.partj ~y-ad~usting th~ power to e~ch target. Employin~ thi~ technique, three di~ferent ZrSiN co~posit~ films were depo~ited.~. The first ~ilm comprised o~ approximat~ly 60% Si~N~ and 40~ ZrN (60/40), the second comprised of approxi~ately 72% si~N~ and 28%
Zr~, and the third comprised of approximately 83% Si3N~
and 17% ZrN (83/17).
- Curves 30 and 32 ~n F~g. 3 illustrate the percentage transmission in the visible llght region for fil~s one-(60/40) ~nd three ~83~17), respectively;
15curves 40 and 42 in Fig. 4 illustrate the percentage re~lection in the visible light region for f~lms one (60/40) and three ~3/17), respect~vely; and curves 50 and 52 in Fig. 5 illustrate the percentage absorption for films one (60/50) ~nd three (83/1~), respectively.
20Table-3 sets forth the refractlve index (n) and extinction ccefficient (~) values versus wavelength or the first composite film (6~ Si3N~, 40% ZrN), ~nd Table 4 ~ets forth the optical values versus wavelength for the second composite film (72% Si3N~, 28~
2S ZrN). (The optical v~lues were measured ~y ~n ellip~o~eter.) 2~ r~ 3~

.: . ; .
a . _ n k : i80 - 2.600 0.0500 ^ . 40Q. 2.566 - 0.0500 s . 420 2.55~ . 0.0400 -- -44~ -- . 2.54z 0.0350 . . 460.. . .2.521 : . 0.0300 480 2.500 0~0250 ~ - 500 - 2.472 0.0200 lo - - 520 . . 2.463 0.0150 s40 2.44g 0.0150 -` 560 - 2.436 0.0150 580 2.424 0.0100 600 2.412 0.~110 620 2.404 o.ooso 640 . . 2.396 0.0080 660 2.389 0.0070 680 2.382 0.0060 700 2~376 0.0060 720 2.371 0.0060 - 740 2.366 0.0060 760 2.361 0.0050 780 2.356 0.0040 800 2.353 0.0030 . 820 2.349 0.0030 840 2.347 0.0001 860 2.344 0.0000 ~80 2.341 0.0000 900 2.338 o~oooo , 920 2.337 0.0000 940 ~.335 o.oooo 960 2.332 0.0000 980 2.332 0.0000 looo 2.329 0.0000 2000 2.300 0.0000 2~

~ . n k 300 2.4972 0.1768 350 2.3298 0.0718 4002 . 2752 0 . 0400 4502 . 2298 0 . 0156 5002 . 2122 0. 0071 5502 . 195~ 0. 000 6002 . 1886 0 . 0028 6502 . 1813 0. 0051 700 2.1779 0.0060 800 - 2.1724 0.0070 1000 2.1673 0.0070 2000 2.1500 0.0070 As is apparent, refractive index in the visible region was higher for the f~rst composite film which ~as less Si3N~.

Although the invention has been described wit~
respect to ~ts preferred ~mbodi~ents, it will be understood tbat the invention is to be protected within the full ~cope of the appended claims.

Claims

1. A thin film interference filter having a substantially neutral visible reflected color, comprising:
a transparent substrate;
a first substantially transparent dielectric layer having a refractive index within a range of approximately 2.0 and 2.7;
a first metal precoat layer;
a partially reflective metal layer;
a second precoat layer; and a second substantially transparent dielectric layer.

2. The thin film interference filter as defined in claim 1 wherein said first dielectric layer comprises of titanium oxide.

3. The thin film interference filter as defined in claim 1 wherein said first dielectric layer comprises of silicon nitride.

4. The thin film interference filter as defined in claim 1 wherein said first dielectric layer is a composite comprising of silicon nitride and zirconium nitride.

5. The thin film interference filter as defined in claim 1 wherein said first dielectric layer is a composite comprising of silicon nitride and one or more other nitrides selected from the group consisting of zirconium nitride, titanium nitride, and hafnium nitride, wherein said first dielectric layer comprises of approximately 60 to 95% by weight of silicon nitride.

6. The thin film interference filter as defined in claim 2 wherein said second dielectric layer comprises of silicon nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG. and the second dielectric layer has a thickness ranging from approximately 350 to 500 .ANG..

7. The thin film interference filter as defined in claim 2 wherein said second dielectric layer is a composite comprising of silicon nitride and zirconium nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG. and the second dielectric layer has a thickness ranging from approximately 300 to 500 .ANG..

8. The thin film interference filter as defined in claim 2 wherein said second dielectric layer is a composite comprising of silicon nitride and one or more other nitrides selected from the group consisting of zirconium nitride, titanium nitride, and hafnium nitride, and wherein second dielectric layer comprises of approximately 60 to 95% by weight of silicon nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG. and the second dielectric layer has a thickness ranging from approximately 300 to 500 .ANG..

9. The thin film interference filter as defined in claim 3 wherein said second dielectric layer comprises of silicon nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG. and the second dielectric layer has a thickness ranging from approximately 350 to 500 .ANG..

10. The thin film interference filter as defined in claim 3 wherein said second dielectric layer is a composite comprising of silicon nitride and zirconium nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG. and the second dielectric layer has a thickness ranging from approximately 300 to 500 .ANG..

11. The thin film interference filter as defined in claim 3 wherein said second dielectric layer is a composite comprising of silicon nitride and one or more other nitrides selected from the group consisting of zirconium nitride, titanium nitride, and hafnium nitride, and wherein said second dielectric layer comprises of approximately 60 to 95% by weight of silicon nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG.
and the second dielectric layer has a thickness ranging from approximately 300 to 500 .ANG..

12. The thin film interference filter as defined in claim 5 wherein said second dielectric layer comprises of silicon nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG. and the second dielectric layer has a thickness ranging from approximately 350 to 500 .ANG..

13. The thin film interference filter as defined in claim 5 wherein said second dielectric layer is a composite comprising of silicon nitride and zirconium nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG. and the second dielectric layer has a thickness ranging from approximately 300 to 500 .ANG..

14. The thin film interference filter as defined in claim 5 wherein said second dielectric layer is a composite comprising of silicon nitride and one or more other nitrides selected from the group consisting of zirconium nitride, titanium nitride, and hafnium nitride, and wherein said second dielectric layer comprises of approximately 60 to 95% by weight of silicon nitride, and wherein the first dielectric layer has a thickness ranging from approximately 200 to 500 .ANG.
and the second dielectric layer has a thickness ranging from approximately 300 to 500 .ANG..

15. The thin film interference filter as defined in either claims 8, 11, or 14, wherein one or both metal precoat layer is formed from a metal selected from the group consisting of nickel, chromium, tungsten, and platinum and wherein said partially reflective metal layer is formed from a metal selected from the group consisting of silver, gold, copper, and platinum.

16. The thin film interference filter as defined in claim 15 wherein one or both precoat layer is a metal film wherein the metal elements comprise approximately 80 to 95 weight % nickel and 5 to 20 %
chromium.

17. A method for the production of a durable thin film interference filter on a transparent substrate, with said filter having a substantially neutral visible reflected color, comprising the steps, in sequence, of:
reactively sputtering a first substantially transparent dielectric layer having a refractive index within a range of approximately 2.0 to 2.7 onto said substrate;
depositing a first metal precoat layer;

depositing a partially reflective metal layer;
depositing a second metal precoat layer; and reactively sputtering a second substantially transparent protective dielectric layer onto said metal precoat layer.

18. The method of producing a durable interference filter as defined in claim 17 wherein the step of reactively sputtering said second dielectric layer comprises the steps of:
providing a cylindrical magnetron having a silicon coated rotatable target and having magnetic means disposed at an angle of approximately 30° to 80°
from normal relative to said substrate; and moving said substrate towards the rotatable target so that dielectric material reactively sputtered from the target is focused unto the substrate as it approaches the target.