AVS Thin Films History

The components in the display included:

Vacuum coated jewelry (photo )

Color correctors for TV cameras (1948)

Decorative Metallized parts

Infra-red system for tanks

Fabry-Perot Interference filters

Drapery lining coated with Al

Polystyrene rolls coated with Al for "Wheaties"

Electrosensitive paper coated with Al Bausch & Lomb gradient density inconel coated sunglasses

License Plates

Non-absorbing beam splitter (1945)

Aircraft sight (1952)

OCLI Variable Bandpass Filters

OCLI AR coated glass

The subject of this paper is the early history of the industrial use of thin films. The emphasis, is on optical applications (mirrors, antireflection coatings, beam splitters, etc.) which became, during the second world war, the first industrial application on a large scale. Marketing data for some important applications are given at the end in order to illustrate the economic importance which thin films have today.

1) INTRODUCTION -

The production of thin films is a modern technology that was born from research, but today, we will not discuss the scientific side, we will discuss the production and application of thin films.

Applications for the optics will stand in the foreground because they were the starting point for the industrial inset of thin films and they determined the technical progress for a long .time (development of proper sources, measuring instruments, etc.).

The statements center on the development in Europe and especially in Germany. The historical specifications about the development of thin films in general, is referred to the known books (3, 4, 5).

2) DEFINITIONS

Thin films are usually denoted as films with thicknesses of less than some

micrometers, which are produced in the vacuum through evaporation or cathodic
sputtering. During the evaporation, the material is heated to a high temperature in a crucible so that it evaporates. These vapors spread in the high vacuum straight out and condense on all cooler spots of the apparatus. High vacuum is essential so that compact films develop (hard and abrasion resistant films). If the base on which the vapors condense is shiny and smooth, thin films develop that are also shiny, i.e., target reflective).

During the cathodic sputtering, an argon gas is ignited through glow discharge.
Through bombardment with Argon ions, the material (of which the film is supposed to be made) gets sputtered. The great advantage of this method is, that it is a cold
procedure, where the film material doesn't have to be heated.

Newer procedures could be interpreted as a combination of both processes: Through an activating process, the vapor gets partially ionized to increase the chemical capacity for reaction. In so called ion plating, the condensation takes place under a

bombardment through ions (argon and evaporation material) which add energetic
particles to the process.

The technology of film production is going through a phase of intense development right now. Many new ideas will in the long term surely help to control the main
problem of industrial application of thin films: the need for better reproduction of
film properties. These thoughts are already a view into the future, but today we
especially want to turn to the past.

PRODUCTION OF METALLIC CONDUCTIVE COATINGS

Who can be surprised that we come across the name Thomas Edison at the beginning of a technology that starts with our century?

He has not only invented the phonograph, but also a procedure for metallizing wax molds using cathodic sputtering. He applied in 1903 for US patent No.767216. This patent proposes a bell jar installation with two vacuum containers are evacuated with a pumpstand (without valve) to sputter alternately. The development of the cathodic sputtering process is illustrated in picture 1,2, & 3.

In the middle of the thirties, there existed a firm in Hanau which did metallizing of the wax molds, and also of paper and textiles (still worthy of respect today).

The pictures are from the magazine DIE UMSCHAU from the year 1936. I couldn't determine the name of the company.

METALLIC MIRRORS

The first use of thin films for optical purposes can be dated to exactly 1912. Pohl and Pringsheim published the well known work about the production of mirrors in which they vaporize metals like Ag and Al out of a MgO crucible in high vacuum.

After that almost all metals have been examined for their usability for mirror production especially Cr, Ni, Pt, Rh, Pd, Sn, Au, etc. Greater industrial importance was achieved only as the Rh, Ag, and Al. Rhodium-mirrors were introduced in 1936 by M. Auwaerter and produced by the firm W.C. Heraeus/Hanau to a great extent for partial translucent mirrors for view and range finders. In addition, these mirrors were used as full mirrors to change the direction of light in Rollei and Voigtlaender cameras.

From 1940 on the production of these films was done in fully air-conditioned rooms with installations shown in figures 4 and 5. This equipment was designed in 1942 by M. Auwaerter and operated from 1942 by 0. Wenkle, and from 1943 by A. Ross. In total there were four installations with chamber dimensions of 1.8 m and two installations with diameters of 2.2 m. The chambers were made out of hammered copper. Substrate rotation was accomplished with a magnetic drive. The Rh films were produced through sublimation from three Rh strips (each 5 x 0.2 x 40 cm).

From 1942 on the fabrication of the full mirrors have been converted to protected
aluminum mirrors whereby the protective layer was produced following the proposal of G. Haas through evaporation of SiO.

The partial Rh-mirrors also were replaced, especially by Fe₂0 (and in smaller circumference) by Ti0₂ films. The production consisted with a total capacity of approximately 10,000 square meter/year: about 3/4 protected Al-mirrors and 1/4 beam splitter. The production plant was totally destroyed by a bomb attack at the end of 1944.

Back to the subject of mirror production: of all metals, only two succeeded in the long run: Al and Ag. Silver mirrors only are used in a very small volume for the highest reflection in visible and infrared. For all other applications, Al is used. This metal is very difficult to vaporize because it is aggressive in the liquid state and it oxidizes easily during condensation because of its high reactivity. Therefore, the best vacuum conditions and high deposition rates during production are necessary , especially for mirrors that exhibit high reflection in the UV.

The technical development can be summarized as follows:

1912 Vaporization from MgO crucible: Pohe and Pringsheim;

1930 High UV reflection of Al discovered: Coblentz and Stair;

1933 Al vaporized from tungsten filament: Strong;

1941 AI protected by evaporation of SiO:Hass;

1955-1965 UV reflection of Al layers is optimized; MgF₂ or LiF used as protective layers; and MgF₂ and CeO₂ used as reflection enhancing layers in the visible.

With that the development of Al-films for mirrors is practically finished. More works follow to increase productivity relative to hardness and lower costs (through use of automatic in-line processes).

REFLECTIVITY REDUCTION

The reflection of a glass surface is approximately 4%. Already in the 19th century it was observed that weathered glass surfaces has a lower reflection than fresh polished glass. They could produce this effect through an etching process. But, it needed more than some decades to understand why it worked. A layer with a lower refractive index than the glass reduces the reflection through interference (for complete
extinction n_layer = n_glass^0.5 at a layer thickness of one quarterwave).

The technical interest to reduce the disturbing reflections is without any doubt
enormously great, because lower reflection means higher light transmission. The contrast in an optical system with many lens to air surfaces is greatly increased by reducing reflection and increasing transmission.

Surprisingly, these initial steps of knowledge did not result in technical application in thin films, until more or less at the same time, independently in Germany and USA, technical useable solutions were found. After this, rapid development started, summed up in a few key words as follows:

SINGLE LAYERS

1935 Zeiss (Smakula), German Patent No.605761 claimed on 1/11/1935 vapor
deposition of CaF₂;

1936 Strong CaF₂ and formula n_film=n_substrate^0.5 specified. Article submitted 25/9/1935: The reflection will be reduced using this layer from 4.2% to 0.6%;

1938 Cartwright and Turner published a whole class of usable materials including MgF₂ and Cryolite;

The vapor deposition of MgF₂ on hot glass surfaces is still state-of-the-art today. This procedure was unknown in Germany during the last war. Therefore they primarily used Cryolite which lowers the reflection, but is not as hard.

DOUBLE LAYERS

For a single layer, one only has a few choices of materials. With a double layer, there are more choices using high and low reflective indexes. The three reflections at the three interfaces can always add to zero by varying film thicknesses. This is true for at least one wavelength. The milestone developments were:

1938 Research Corporation, New York announced without mentioning any inventor, the principal of solving the three vector calculations and mentioned materials
that can be used (Swiss Patent No.221992, issued June 30, 1942);

1939 Cartwright and Turner - a short publication, describing the principle of using Al₂O₃/SiO₂, which was not practical;

1940- 1941

Zeiss (Smakula) and Schott (Geffcken) and Steinkeil (Schneider) experimented
with double and triple layers; double layers never got into production in
Germany or the U.S.;

1949 Balzers (Auwaerter) introduced the double layer "Transmar" (MgF₂ plus rare earth oxide) and for many years had a leading position with this method.

Further developments are well known. Today large volumes of anti-reflection coatings up to six layers are produced. Not only optical instruments but consumer goods like binoculars and cameras use anti-reflection coatings (modern camera lenses have the label “Imulti-coated”).

The biggest use (in dollar value) of thin films from 1950 to 1970 was certainly anti-reflection coatings.

BEAM SPLITTER

With thin film coatings, reflection cannot only be decreased, but also increased. You then get so-called "beam splitters", which are used for example in range-finders, camera view finders, or for binoculars or microscopes.

For this, you put on a layer with a higher refractive index than the glass. Originally,

there were semi-transparent metal coatings using Ag, Cr, Rh, Pt, etc. These all caused

losses through absorption.

The development of dielectric layers, and with no losses from absorption, were
demonstrated by:

1934 P fund evaporates ZnS and proposes TiO₂ and Sb₂S₃ (22)

(About 1940) Steinheil (Hammer) introduces to production Fe₂O₃ and TiO₂ coatings with the following characteristics (2,4,23)

Reflection Transmission Absorption
49% 43% 8%
41% 57% 2%

For production, pure metals were evaporated up and the Fe or Ti-coatings were
oxidized in air at about 400° C.

Fe₂O₃ coatings were also set up using the installation shown in picture 4. For this Fe layers were produced through sublimation of pure iron (ARMCO - iron) and later
thermally oxidized. Those coatings were almost only used for the production of sights and range finders.

USE OF REACTIVE GAS

Oxides often produce very hard, abrasion resistant coatings, but the problem is that oxides can disassociate during evaporation so the coating grows with an oxygen deficit. This leads to layers, which absorb transmitted light and thereby leads to unwelcome losses. It was known on the one hand that bad vacuum conditions (water vapor or oxygen) during evaporation led to partial oxidation of the layers. On the other hand it was also known that bad vacuum leads to porous coatings and thence, a good high vacuum is needed to get compact and hard, abrasion resistant coatings. Therefore, it couldn't be foreseen that it could be possible to set the oxygen pressure just right that the coatings on the one hand are fully oxidized, on the other hand are compact.

In practice it was shown that a small pressure region existed in which it was possible to produce oxide layers in very good quality. Figure 7 shows the corresponding US patent, which protects this process of reactive evaporation. With this process was possible to interchange the relative soft, high refractive index ZnS the much harder TiO₂ and MgF₂ with SiO₂. It took a long time (until after the reactive gas patent expired) before hard oxide coatings were introduced into general coating technique, because reactive evaporation of reproductive absorption free coatings with constant refractive index was difficult. Over a long period of time, the way was cleared for production of high grade coatings, with which many complex optical systems can be produced. Figure 8 shows that a good agreement can be made today between calculated and actual values of multi-layer systems.

In the end, I would like to go on to two consumer products, which are of interest in today's discussion...

ABSORBING COATINGS FOR SUNGLASSES

From the user of a highly sophisticated optical device such as a binocular or camera, it can be expected that the tool is carefully handled. With a product like sunglasses, however, this is not the case. Therefore the use of thin film coatings for this application is proof that modem coatings are sufficient for the hardest demands for rub-and-scratch resistance as well as resistance against solvents. The question is can complex coatings dim the light enough. It is necessary to use the high absorption of metal coatings. The disturbing metallic reflection can be lessened through the addition of dielectric layers. Using Cermet (ceramic-metal) coatings allowed reproducibility of colors and the desired light reduction.

The patent history for these layers is not free of a certain irony - after the last world war four companies* applied almost at the same time for a patent for this type of layer. During examination it was determined that a so called "highway man patent" from the year 1917 existed, that means a patent, which anticipated an idea, without finding a useful way to realize it, so that it never came to an application. However, no new patent was able to be granted. It can be remarked today that the economic value of these absorbing layers for sunglasses is much less than antireflection coatings for glasses.

· Balzers, Metallix, Holler, Zeiss)

Figs
1. cathode of silver wires used to coat wax records

production equipment for Cathode Sputtering (1935)
Equipment for coating textile rolls by cathode sputtering
Thin film production evaporation equipment at W.C. Heraeus Co. in Hanau, Germany 1940-1944