GEORG HASS

nary work of this nature has already been reported.**With this technique it should be possible to follow thecourse of a chemical reaction where certain radicals arefree and active only during the reaction, and are notavailable or detectable before or after the reaction.

Preliminary observations indicate that for short ex-posure times the orthicon is about fifty times more

** Agnew, Franklin, Benn, and Bazarian, J. Opt. Soc. Am. 39,409 (1949).

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

sensitive than the photographic plate, if the orthiconresponse is measured above noise and the plate responseabove background. The orthicon has the further ad-vantage that the entire region between 3500A and10,OOOA can be covered at the same time-no singlephotographic emulsion will provide this coverage.Errors due to the use of several plates of different types,especially if used at different times, or to low sensitivityin the overlap regions, are thus avoided.

VOLUME 39, NUMBER 7 JULY, 1949

On the Preparation of Hard Oxide Films with Precisely ControlledThickness on Evaporated Aluminum Mirrors*

GEORG HASSEngineer Research and Development Laboratories, Te Engineer Center and Fort Belvoir, Fort Belvoir, Virginia

(Received April 7, 1949)

By increasing the thickness of the natural oxide film onevaporated aluminum mirrors, a much better surface protec-tion can be achieved. Heat treatment in air is not practicalfor producing protective coatings on aluminum because thetemperature required is more than 400'C and oxide filmsformed at this temperature are rough. Oxide films with pre-cisely controlled thickness can be produced on evaporatedaluminum mirrors by anodic oxidation in electrolytes such asammonium tartrate. The thickness of the oxide layer formedin a given time increases linearly with the applied voltage andis 13.0 A-units/volt for 2 minutes anodizing time. The currentefficiency of the anodic process in ammonium tartrate is inthe neighborhood of 80 percent. The thickness of the oxide

INTRODUCTION

THE fact that evaporated aluminum mirrors doTnot tarnish in normal atmosphere is a result

of the good protective qualities of their naturaloxide films. For many mirror applications, however,the natural oxide film is too thin to furnish suffi-cient mechanical and chemical protection. There-fore, various methods have been developed to coatthe aluminum mirror surface with a hard and ad-herent dielectric layer. Since the extremely thinnatural oxide film has a relatively good protectiveeffect, a very satisfactory protection of evaporatedaluminum mirrors must be expected after increasingthe thickness of their oxide coating. Electrolyti-cally brightened massive aluminum reflectors showvery good scratch and corrosion resistance due totheir hard anodically produced surface layer ofaluminum oxide. 1

The thickness of the oxide film on aluminum canbe increased by chemical methods such as heattreatment in air or by anodic oxidation. The present

* Presented at the Winter Meeting of the Optical Societyof America, New York, New York, March 10-12, 1949.

l J. D. Edwards, Trans. I. E. S. 29, 351 (1934), and N. D.Pullen, J. Inst. Metals (London) 59, 151 (1936).

coatings is 1.38 times thicker than that of the aluminum layerreplaced. The anodic oxide films formed in ammonium tartrateare amorphous and free of pores. They exhibit no noticeableabsorption in the ultraviolet, visible, and infra-red. Theirrefractive index between X = 3000A and X = 6000A varies from1.67 to 1.62. Anodized aluminum mirrors show excellentabrasion resistance. To obtain highest reflectivity in thevisible the anodic oxidation must be performed with 120 voltsfor 2 minutes. The precisely controlled uniform anodic coat-ings on opaque aluminum mirrors are suitable for the prepara-tion and investigation of reflection type interference filters.The thickness of a pure aluminum film can be determined bythe voltage required for complete oxidation.

paper presents data on the formation and proper-ties of oxide films produced on evaporated alumi-num mirrors at various temperatures in air and byanodic oxidation in ammonium tartrate. The appli-cations of anodically produced films, the thicknessof which can be precisely controlled, will bediscussed.

I. ON THE OXIDATION OF EVAPORATEDAL-MIRRORS IN AIR AT VARIOUS

TEMPERATURES

The thickness of oxide layers formed in air onevaporated aluminum films was studied by Drude'soptical polarization method. The special vacuumapparatus in which the films can be evaporatedand examined before and after initial exposure toair has been described in a previous paper. Theoxide film formed on aluminum when exposed toair at normal temperature for two hours is about10 A thick. It continues to grow very slowly andstops almost completely in about one month. Thethickness is then about 45 A. The same thicknessfor the oxide film formed on evaporated aluminum

2 G. Hass, Zeits. f. anorg. allgen. Chemie 254, 96 (1947).

532

PREPARATION OF HARD OXIDE FILMS

100

.4

2U)

I-

80

60

40

20

20 100 200 300

TEMPERATURE C

400 500

FIG. 1. Thickness of oxide film formed on evaporatedAl-mirror after two hours exposure to air as function oftemperature.

was calculated from the increase of the electricalresistance caused by oxidation in air. The factthat the oxide film on aluminum reaches a limitingthickness of about 45 A is in good agreement withthe theory of the formation of oxide films on metalsdeveloped by Mott.3

Figure 1 shows the thickness of oxide filmsformed on aluminum at different temperaturesafter two hours exposure to air. The rate of oxida-tion of a fresh aluminum surface changes very littlebelow 300° C. From 3000 C, it increases slowly andfrom 450° C very rapidly. An increase of humidityin the air increases the rate of oxidation and shiftsthe beginning of more rapid oxidation to lowertemperatures. By introducing steam into the oxida-tion furnace a rapid oxidation could be detected at3500 C. The oxidation of aluminum in oxygen atvarious temperatures and pressures has beenstudied by Gulbransen and Wysong.4 They, too,-found a rapid increase of oxidation rates at hightemperature beginning at about 4750 C. The factthat high humidity increases the rate of oxidationof aluminum slightly at low and strongly at highertemperatures has been reported previously.- Theelectron diffraction investigations of aluminumoxide films showed the following results. The oxidefilms formed on poly-crystalline aluminum at atemperature region where the oxidation rate islow are amorphous and those formed at highertemperatures are crystalline consisting of y-A1203with a = 7.90 A. At low humidity the formation ofcrystalline y-Al203 begins at about 450° to 5000 C,while in the presence of steam y-A1203 and rapidoxidation can be found at 3500 C. The change instructure of the aluminum oxide film seems to beresponsible for the rapid change in oxidation rateswith increasing temperature. Furthermore, the elec-tron diffraction investigations showed that during

3 N. F. Mott, Trans. Faraday Soc. 36, 472 (1940), and 43,429 (1947).

4 E. A. Gulbransen and W. S. Wysong, J. Phys. and ColloidChem. 51, 1087 (1947).

5 H. Rohrig and G. Lux, Korrosion metallischer WerkstoffeBand 3 (Verlag S. Hirzel, Leipsig, 1940), and N. Cabrera andJ. Hammon, C. R. Acad. Sci., Paris 225, 59 (1947).

c -ou- i_,_'Al MIRROR

TO BEL PUR . , L-*E AL, ANODIZ ED.

3% TARTARIC ACID + N H, OHTO pH- 5.5

FIG. 2. Sketch of arrangement used to anodizeevaporated Al-mirrors.

the formation of crystalline -y-AI203 in air at highertemperatures the lattice constant of the underlyingaluminum increased from 4.04 A to 4.10it0.02 A.The increase of the Al-lattice constant must becaused by the diffusion of oxygen into aluminumand may be considered as the first oxidation step.

Heat treatment in air is not a practical methodfor producing protective coatings on evaporatedaluminum mirrors. The temperature required forproducing the desired thickness of oxide is veryhigh, and the films thus produced are rough andnon-uniform resulting in diffuse reflection.

II. ON THE ANODIC OXIDATION OF EVAPORATEDALUMINUM MIRRORS IN 3 PERCENT

AMMONIUM TARTRATE

Unlike aluminum oxide films produced by oxida-tion at high temperatures, those formed anodicallyin a suitable bath are smooth and uniform and canbe prepared in precisely controlled thicknesses.The method used to anodize evaporated aluminummirrors and data on the properties of A1203-filmsthus formed, together with their applications, aregiven in the following.

1. METHOD FOR ANODIZING EVAPORATEDALUMINUM FILMS

When aluminum is made the anode in an elec-trolyte containing oxygen-bearing anions, a coatingconsisting predominantly of aluminum oxide isformed on its surface as long as a current flows.The electrolytes used for the anodic oxidation mustbe divided into two classes: Those which have anappreciable solvent effect on the oxide films formed,

+ CONTACTAL- WEDGE

>~~~~* \\\ ~.4 GLASS

t-PLACED IN ELECTROLYTE

FIG. 3. Al-wedge to measure thickness of anodic coatingsas function of voltage and time.

533

GEORG HASS

2400

2000

E 15 Mi Anodizing:t 13.5 A/ Volt"'

5 1600

2 Min. Anodizing:tz 13.0 A/Volt

el0c_ .- 30 Sec. Anodizing:t- 12.2 A/Volt

800

40C1

zu 140 60 80 100VOLTS

120 140 160 180

FIG. 4. Thickness of anodic coating formed in 3 percent ammonium tartrate as30 sec., 2 min., and 15 min. anodizing time.

such as dilute sulfuric and oxalic acid, and thosewhich have no solvent action on aluminum oxide,such as boric acid and solutions of acidified organicammonium salts. The oxide films formed in elec-trolytes such as dilute sulfuric acid are characterizedby substantial thickness and porous structure.Within quite a wide range, these oxide coatingscontinue to grow as long as potential is applied andfilm thicknesses of more than 100 microns can beattained under suitable conditions. In electrolytesin which the aluminum oxide is insoluble the di-electric coating forms rapidly and the high initialcurrent which flows when contact is first made de-creases to less than 1 ma/dM 2 in a few minutes,indicating that the growth of the coating has al-most ceased. The thickness of the oxide films pro-duced in a given time depends only on the appliedanodizing voltage and therefore can be controlled.This type of oxide film is non-porous and is calledan anodic barrier-layer. It has been found to beespecially suitable as protective coating on thinevaporated aluminum mirrors.

The electrolyte used for producing anodic barrier-layers on evaporated aluminum is a solution of 3percent tartaric acid with ammonium hydroxide

function of voltage for

added to make the pH about 5.5. It permits theformation of uniform films up to about 5000 A thickon evaporated aluminum mirrors of any size.

Figure 2 is a sketch of the anodizing arrangement.The evaporated aluminum surface is made theanode in the electrolyte and a plate of aluminum isused as cathode. The strong initial current islimited to 2 amperes by a resistor. As the oxidationincreases, the current decreases to a few ma/dm inone minute. Continued application of the appliedvoltage after the first minute produces little changein film thickness.

The glass form on which the aluminum film isevaporated must be extremely clean; otherwise, thealuminum film will loosen during the anodic oxida--tion process. Also impurities will cause pinholes bythe anodic treatment. Thus, the anodic oxidationprovides an excellent means for checking the ad-herence and purity of evaporated aluminum mirrors.

2. THICKNESS OF THE ANODIC COATINGS PRO-DUCED IN 3 PERCENT AMMONIUM TARTRATE

The thickness of A12 03-layers formed in 3 percentammonium tartrate as a function of voltage andtime was determined by means of multiple reflec-

7 J A__ A . _ I. . I ... L..._ . __ _ 200

534

I

PREPARATION OF HARD OXIDE FILMS

tion fringes. This optical method was developed byTolansky6 and is based on the consideration ofthese fringes as Newton's rings greatly sharpenedthrough a multiple-beam action between two highlyreflecting surfaces placed close together. The ex-treme sharpness of these fringes permits a veryaccurate determination of film thicknesses. Thethicknesses of the A1203-coatings thus determinedare accurate to about 10 A. The technique is illus-trated in Fig. 3.-

A wedge of aluminum is evaporated on a flatglass plate while a narrow central area of the glassis shielded. The top of the wedge is increased inthickness to furnish good contact for the anodizingprocess. The wedge coated glass is immersed in theammonium tartrate bath and the aluminum oxi-dized anodically with constant voltage. The thinnerpart of the aluminum wedge becomes transparentand completely oxidized in a few seconds. Theboundary up to which the evaporated aluminum iscompletely changed to its oxide moves rapidlyduring the first 10 seconds and then slower andslower up to the thicker part of the aluminum film.The thickness of the coating on glass at thisboundary represents the thickness of the aluminumoxide film formed under the applied voltage and

time. It is measured by the relative fringe dis-placement, which is observed at the boundary ofthe narrow uncoated central area after a highreflecting Al-film is evaporated over the wholeplate and a similar plate with a 90 percent re-flecting silver coating is placed close to it. Thus,the thickness of the anodic coatings produced withvarious voltages in various times was determined.The results are presented in Fig. 4. The thicknessof the oxide layer formed in a given time onaluminum in 3 percent ammonium tartrate in-creases linearly with the applied voltage. Layersproduced in two minutes have a thickness of13.0 A-units/volt. The thickness increase in thenext 15 minutes is only 5 percent. After about 40minutes the anodizing current reaches a constantvery small value resulting in a constant increasein thickness of only about 1 percent per hour.Walkenhorst7 determined the thickness of anodicbarrier-layers formed on Al in ammonium citrate tobe 13.7 A-units/volt, and Taylor and Edwards8found for films produced in aqueous solution ofboric acid a thickness of about 11.6-A/volt. Thethickness of oxide coatings formed in different elec-trolytes in which aluminum oxide is insolubleseems to be in the same order of magnitude.

60 70TIME - Seconds

FIG. 5. Calculated and measured thickness of Al203-coating formed in 3 percent ammonium tartrate asfunction of time for 50 volts and 100 volts anodizing voltage.

6 S. Tolansky, J. Sci. Instr. 22, 161 (1945).7 W. Walkenhorst, Naturwiss. 34, 373 (1947).8 C. S. Taylor and J. D. Edwards, Heating, Piping and Air Conditioning 11, 59 (1939).

535

GEORG HASS

3. CURRENT EFFICIENCY OF THE ANODIC PROCESS

The amount, i, of A1203 formed on aluminumduring the anodic oxidation should be, accordingto Faraday's law, proportional to the quantity ofelectricity passing through the electrolyte: n= te.The electrochemical equivalent e for A1203 is17.6 X 10-5 g/amp.-sec.

This means that the thickness of the oxide coat-ing increases with the amount of electricity ac-cording to the following relation:

17.61t(ma-sec.)tAI2o3 (in A-units) =

pA12 03 F

where PA1203 = density of A120 3 in g/cm 3 andF=area of the aluminum surface to be anodizedin cm2 . The density of -y-A12 03 calculated from x-raydata is 3.62 g/cm3 .9 The values measured for anodiccoatings are smaller, about 3.0 g/cm3 for the poroustype of films'" and between 3.1 and 3.3 g/cm3 fornonporous anodic barrier layers." Assuming thedensity is 3.2 g/cm3 for the films formed in 3 per-cent ammonium tartrate, their thickness may becalculated from:

5.501t(ma-sec.)tA12o3 (in A-units) =

F(cm2 )

100

80

c

0X 60

I

I-

o 40-JIL

20

In electrolytes which have a solvent effect on A12 03the increase in thickness must be smaller, while inthose in which the coating formed is insoluble anagreement with the values calculated by Faraday'slaw may be expected. The amount of amp.-sec.was measured with a recently described coulom-eter'2 and checked with a fast recording ammeter.Figure 5 shows the measured and the theoreticalthickness increase of A120 3-films as function oftime for formation voltages of 50 volts and 100volts. The apparent current efficiency is in theneighborhood of 80 percent only, though no solventeffect on A1203 by ammonium tartrate could bemeasured after 65 hours of exposure.

4. RELATION BETWEEN THICKNESS OF ANODICCOATING FORMED AND THICKNESS OF

ALUMINUM LAYER REMOVED

If all of the aluminum reacting appears as oxide,the thickness ratio of the oxide film formed to thealuminum layer replaced should be:

tA12 O3 MIVlA12o 3 pAl= =1.60,

tAl 2 MAIPA12 0 3

where MA12 03 , the molecular weight of A1203

= 101.94, MAI, the atomic weight of Al = 26.97,

200

WAVELENGTH - Millimicrons

FIG. 6. Reflectivity of evaporated Al-mirrors coated with anodic A1203-films of different thicknesses asfunction of wave-length from 220 mAu to 1000 mg.

'J 12. . W. Verwey, elts . rist. 01, 6 (39).10 J. D. Edwards, Proc. A. S. T. M. 40, 14 (1940)."Burgers, Claasen, and Zernike, Zeits f. Physik 74, 593 (1932).

12 E. L. Abers and R. F. Newton, J. Phys. and Colloid Chem. 52, 955 (1948).

300 400 500 600 700 800 900 1000

536

PREPARATION OF HARD OXIDE FILMS

4 5WAVELENGTH- Microns

10

FIG. 7. Reflectivity of evaporated Al-mirror coated with an anodic A1203-filmfunction of wave-length from 0.2 ,u to 10A.

1560 A thick as

PAl, the density of Al= 2.7 g/cm', (aluminum filmsproduced by fast evaporation under good vacuumconditions have, according to Walkenhorst," nor-mal density), PA 2,0, the density of A1203 = 3.2 g/cm'.

By measuring the thickness of 10 different filmbefore and after complete oxidation in ammoniumtartrate the thickness ratio was determined to:

(tAl 2 o,/tAl) = 1.3840.03.

The measured value is considerably smaller thanthe theoretical one. An agreement between the twovalues could be obtained by assuming a density of3.70 g/cm3 for the anodic films. But this value ishigher than any one known for anodic aluminumoxide films and since the refractive index of thesefilms is very low (see later) this assumption is notjustified. The fact that the thickness of the okideis only 1.38 instead of 1.60 times greater than thatof the aluminum from which it is formed may beexplained by a theory recently developed by Ander-son'4 According to this theory one third of theanodizing current is carried through the anodicalbarrier-layer by Al-ions and the rest is transportedby oxygen ions, and only two out of every threealuminum ions that leaves the parent metal com-bine with oxygen ions to form A1203 while one dif-fuses through the oxide film into the electrolyte,where it may become lost for oxide formation. Theloss of one out of three aluminum ions during theoxide formation would lead to a thickness ratiowhich is agreeable with the one measured here.

13 W. Walkenhorst, Zeits. Tech. Physik. 22, 14 (1941), andDiss. Hannover (1940).

14 Scott Anderson, J. App. Phys. 15, 477 (1944).

The thickness of an evaporatedtAl can be readily determined fromquired for its complete oxidationtartrate, since

tAl = 0. 72 5tA12 o,and.

aluminum filmthe voltage re-in ammonium

tAl 2o3 (in A) = 12.2 Xvolts for 30 sec. anodizing time,

tAl (in A) = 8.85 X volts.

For complete anodic oxidation of an aluminumfilm that is just opaque to a bright tungsten fila-ment, a 30-second application of about 70 volts isrequired. Its thickness is therefore about 620 A.

5. STRUCTURE OF THE ANODIC COATINGSPRODUCED IN AMMONIUM TARTRATE

Anodic coatings produced with various voltages(10 to 150 volts) were released from their substrateby dissolving the aluminum in a solution of mer-curic chloride and were examined by the electrondiffraction method and with the electron micro-scope.

The electron diffraction patterns of all theseanodic coatings consists only of diffuse rings in-dicating an amorphous, or glass-like structure. Thefilms become crystalline when subjected to tem-peratures of 7000 C or higher. The electron diffrac-tion patterns of films heat treated at temperaturesbetween 7000 and 950° C correspond with those for-y-A1203 while at higher temperatures a-A1203 isformed.

The electron micrographs show no evidence ofpores or structure in the oxide layer even at mag-

00)

0.

I-

I-0-JU.al

537

GEORG HASS

160_ _ _ _ _ _ _ _ _ _ _ _

140" _ _o

1200 600_ _ _ _ __ _ __ _

"l00o _ _ _ __ _ _ 00

- : / \ in Angstrom, e r < f X ~~~~~~~~~~~~~~6I Un itDs

k6air-A203 A 2%3 A1 I nt

8 _____ -- ~~~~~~~~~~~~~~~~~~~~~~~400'

. paetThickness Increase

200

( 6 ir-Al2036 A12 O.

100

X Experimentalo Galculated

300 400 500WAVELENGTH - Millimicrons

FIG. 8. Phase changes occurring on reflection at A03 coated Al as function of wave-tength from 300 m to 650 m.

nifications of 50,000 diameters. A characteristicgrain structure appears only when the films areheated to more than 7000 C. They are therefore suit-able as temperature-resistant support films forelectron diffraction and electron microscope in-vestigations." 6 When used for this purpose thefilms are made 100 A to 200 A thick.

6. PROPERTIES OF ALUMINUM OXIDE PROTECTEDALUMINUM FRONT SURFACE MIRRORS

Figures 6 and 7 show the reflectivity in the wave-length region from 0.22,u to 10A of evaporatedaluminum mirrors protected by anodic films ofvarious thicknesses. The interference maxima andminima which shift with increasing thickness ofthe aluminum oxide coating to longer wave-lengthcan be adjusted to a desired position by the appliedanodizing voltage. To obtain highest reflectivity inthe visible, the anodic oxidation must be performedwith 120 volts (2 minutes). The oxide film formedwith this voltage is 1560 A thick and yields a firstorder maximum at X=550 m Front surface mir-rors for use in the ultraviolet and infra-red can be

"I G. Hass and H. Kehler, Kolloid Zeits. 95, 26 (1941), and97.27 (1941).

advantageously protected with an anodic coatingbecause the thin layer of aluminum oxide used forprotection does not show any appreciable absorp-tion. The infra-red reflectivity from 2 to l0,4 (seeFig. 7) is therefore practically the same as that ofan unprotected aluminum mirror, since the A1203coating is too thin to produce any interferenceminimum in this region.

The abrasion resistance of evaporated aluminummirrors protected with aluminum oxide is verygood. They cannot be damaged by strong rubbingwith rough linen and are extremely resistant toscratching with most metal points. They are, how-ever, more sensitive to boiling water and saltspray than silicon monoxide protected aluminummirrors.

7. REFRACTIVE INDEX OF THE ANODICALLYPRODUCED AL20, FILMS

The refractive index of the anodic coatings wasdetermined by dividing the difference in opticalthickness, n(t 2-tl) of two films produced withdifferent voltages by their actual difference inthickness, t2 -tl. To eliminate the phase change onreflection from the calculations, the thickness of

IL

II

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IdCDz

6.)

U'

0.

538

600 700

PREPARATION OF HARD OXIDE FILMS

the oxide films were adj usted so that interferencemaxima or minima of different orders were pro-duced at the same wave-length. Thus the differencein optical thickness of two films can be calculatedfrom:

n(t2 -ti) = X/4(m2 -ml)where m2 -ml represents the difference in the inter-ference order. The actual difference in thicknesswas determined by the difference in voltage usedfor the anodic oxidation. The refractive index,which was determined to be n = 1.62 for X = 600 mu,increases uniformly with decreasing wave-length ton = 1.67 for X = 300 mi. These values are smallerthan those known for -y-A1203 (n=1.73 for X=.589mAu'6) but are in agreement with the refractiveindices for anodic coatings as determined byEdwards.' 7

8. ABSOLUTE PHASE CHANGE OF LIGHT WAVES ATBOUNDARY AL2 03 -AL AND ITS EFFECT ON

THE POSITION OF REFLECTIONMINIMA AND MAXIMA

If light is reflected at normal incidence from ametal surface covered with a thin dielectric film ofthe optical thickness nt the spectral position ofreflection minima and maxima can be calculatedby the following equation:

2nlt+86-6 2 =m(X/2), (1)

where m is the interference order, 61 the absolutephase change at the boundary air-dielectric film,and 2 the absolute phase change at the boundary

CI?0

0

I-

ILJ

100I II

90AII

80

70

60

50

40

30 1I 20

10 - - - -''A

.2 .3 .4 .5 .

dielectric film-metal surface. While 61 has a con-stant value of /2 or 180°, 2 depends upon theoptical constants of the metal n, K and the re-fractive index nl of the dielectric and is defined by:

2nnKtan 62 =…

nli2 -n 2- (nK)2

(2)

62 can be determined directly from Eq. (1) whereit is the only unknown quantity and can be calcu-lated from Eq. (2) using the optical constants ofaluminum, which have been reported in a recentpaper.' 8 Figure 8 shows the 6-values in degrees andA-units (units of the wave-length) for the systemAl+A1 203 as function of wave-length from 300 to650 m. There is a good agreement betweenthe calculated and directly determined 8-values.(bair-A1203-5A1 203-A1) in A-units is almost fixedthroughout the wave-length region studied andbehaves like a constant increase in optical thick-ness of the A12 03-film of 250 A. Therefore Eq. (1)becomes simple for this special case: 2(nlt+250)=m(X/2). (t and X in A-units.)

9. REFLECTION TYPE INTERFERENCE FILTERSWITH AL20 3 DIELECTRIC LAYER

A reflection type interference filter consists in itsnormal form of a highly reflecting opaque mirrorcoating on which is first applied a thin layer ofdielectric material and then a very thin semi-transparent metal film.9 The spectral positions ofminimum and maximum reflection and the shape

.7.8.9 1

WAVELENGTH- Microns

2 3 4 5 6 7 8 9 10

FIG. 9. Reflectivity of reflection-type interference filter as function of wave-length from 0.2A to lOu. FilterAl (opaque) +A1 2 0 3 (580 A thick) +AI (transparent appr. 50 A thick).

16 E. Kordes, Zeits f. Krist. 91, 193 (1935).17 J. D. Edwards, Monthly Rev. Am. Electroplater' Soc. 26, 513 (1939).18 G. Hass, Optik 1, 2 (1946).19 L. N. Hadley and D. M. Dennison, J. Opt. Soc. Am. 37, 451 (1947) and 38, 483 (1948).

consists of

539

GEORG HASS

C

E0

0n

C

IWU)4

0U)U)z0I-

20 25

TIME- Hours

FIG. 0. Dissolving rates of anodic A12 0 3-films in H2SO4 -solutions of various concentrations and temperatures.

of the spectral reflectivity curve depend on theoptical thickness of the dielectric layer and thethickness of the semi-transparent metal film. Theanodizing process permits the production of uni-form A1203-layers, of desired thickness on opaquealuminum mirrors, thus providing a simple methodfor shifting the spectral positions of maximum andminimum reflectivity to desired wave-lengths andto study the influence of the thickness of thetransparent metal film on the reflection propertiesof reflection type interference filters. Figure 9 showsthe reflection properties of a very simple reflectionfilter.

It consists of an evaporated aluminum mirrorwhich has been oxidized anodically with 45 volts(585 A A 203) and coated with aluminum about50 A thick. This type of reflection filter reduces thevisible reflectivity to less than 2 percent but ex-hibits high reflectivity in the ultraviolet and infra-red where it may have useful applications.

10. SOLUBILITY OF ANODIC FILMS IN DILUTESOLUTIONS OF SULFURIC ACID

As mentioned above very thick coatings ofaluminum oxide can only be produced in electro-

lytes in which the oxide has an appreciable solu-bility, such as solutions of sulfuric acid. All com-mercial anodic protective coatings are made inthese electrolytes and have a thickness from 2 to20A. An exact study of the solubility of aluminumoxide in sulfuric acid of various concentration andtemperature is therefore of technical interest.Alumirfum mirrors, coated anodically in ammoniumtartrate with oxide films 2000 to 4000 A thick, wereexposed for various times to sulfuric acid solutionsof various concentrations and temperatures. Thewave-length positions of reflection minima andmaxima were determined before and after exposureand their shift used to calculate the decrease inthickness caused by the sulfuric acid treatment.The decrease in thickness thus determined is ac-curate to 1 20 A which corresponds to accuracy inweight decrease of 4t6X10- 7 g/cm3 . The results ofthe measurements are represented in Fig. 10. Therate of dissolving increases slightly with the con-centration but very rapidly with the temperatureof the sulfuric acid solution. The aluminum oxidefilms decrease very uniformly in thickness duringtheir exposure to sulfuric acid.

E010,

IU)4

a

0I3l

540