DE3134477C2 - Absorbent anti-reflective coating for a metallic layer - Google Patents

Abstract

A metallic anti-reflective film is described which consists of a relatively thick, i.e., opaque, metal layer on which there is at least a thin second metallic layer. The second metallic layer on one or on both sides of the thick metal layer expediently consists of chromium oxide. The described antireflective film facilitates production, especially when it is used for lens hoods or annular gaps in optical instruments such as endoscopes or microscopes.

Description

30th

The invention relates to .pinen absorbent Anti-reflective coating according to the preamble of claim 1.

In particular, this is a coating on the surface of optical components such as Diaphragms or annular gap elements, such as those for endoscopes or microscopes or similar optical instruments be used. Its purpose is to prevent light in the visible range from being reflected from the surface.

It is usually used to produce a diaphragm or an annular gap, such as those used for an endoscope or microscope are required, a lens surface or a flat glass surface coated with black paint and an opening part through which light is to pass is cut out for peeling off the paint. One can but for the production of the diaphragm or the annular gap also on the lens surface or on the flat glass surface evaporate a metal film, in which case a dielectric film on the surface of the metal film Anti-reflective coating was applied and then an opening was formed by photoetching. With this photo-etching method to form the opening, the dielectric film and the metal film had to be separated from each other Using different etching solutions for the dielectric film or for the metal can be etched.

From the book "Thin layers of door optics" by Dr. H. Anders, 1965, page 67 it is known that one does not only by using homogeneous absorbent layers, but also by applying a thin one Metal layer on an absorption-free layer the disturbing high degree of reflection from as scales. Masks and Can reduce metal layers serving for diaphragms. Because here only one for a certain wavelength Zero point of reflection is attainable. while a large reflection remains for white light, should first on a glass surface an inhomogeneous absorbent layer with a continuously rising layer from the glass Refractive index and then the metal layer are applied.

The invention is based on the object of providing an absorbent anti-reflective coating for an anti-reflective coating to create a metallic layer.

This object is achieved by the anti-reflective coating characterized in claim 1.

The anti-reflective coating described here has the advantage that the reflection factor in the range of visible light can be kept between almost zero and less than 10% depending on the wavelength and the parameters ri and A- in the range of visible light, and ... was without it Necessity to adhere to special conditions with regard to the opaque metallic layer, in particular its refractive index. The anti-reflective coating can be used in particular for a lens hood or for an annular gap, the production of which is simplified in that the anti-reflective coating can be treated with the same etchant as the thick metallic layer.

Embodiments of the invention are explained below with reference to the drawing. Show it:

1 to 3 sectional views of three different exemplary embodiments of the invention;

FIGS. 4 to 6 show the curves of the spectral reflection factor which arise in the case of the three exemplary embodiments mentioned result; and

7 shows the course of the spectral reflection factor in an arrangement without anti-reflection measures.

The invention is intended to prevent the reflection of light on a metal surface with a metallic or chromium oxide coating. According to FIG. 1, the covering I is applied to a metallic substrate in the form of a thick layer II which is essentially impermeable to light. The complex refractive index of the coating I is h ₂ = n ₂ - / A ₂ . the coating thickness is </ ,, the complex index of the thick metallic layer II is A ₁ = H ₃ -Jk ₃ and the index of the medium (air or a glass plate) over the surface A of the coating I is / i ,. The reflection factor R related to the total thickness then corresponds to the following relationship (1) for light incident perpendicularly on the surface A. where p _l2 and p _{23 are} the absolute values of the complex ratio of the amplitudes of the reflected and incident waves on the surfaces of covering I and layer II and φ ₁₂ and φ _{2ί are} the associated phases:

+ k \

₁₂ p ₂₃ cos (<i) ₁₂ + ^ ₂ 3-2 ;; ₂ i /) (1)

n \ + k \ - n \

_{2 =} (n ₃ -n ₂ ) ² + (k ₃ -k ₂ ) ²

2π

So that reflection can be prevented, the factors n ₂ , A ₂ and (I _{1 can} be chosen so that they satisfy the amplitude relationship and the phase relationship according to the two following formulas »(2) and (3):

3 4

p \ -, e ^lk y + p \ e ~ ^2t 2 ⁿ -Ip ₁ -Jp ₂₃ = O (2) curve α actually measured values and curve b

φ ~ _φ _-> _ _{+ π} (3) calculated values. The illustration shows that the actual

² actually measured values are not significantly different from the

calculated values differ.

Even if the above formula (2) does not strictly meet 5 that formed according to the second exemplary embodiment

should be, a far arrangement is shown in FIG. 2 when the formula (3) is fulfilled. Here both should

Going reflection prevention possible if reflection for surfaces of the thick metallic layer

1 ^ n ₁ ^ 2 prevent the factors n ₂ , A- ,, «j and λ-j in the following.

Limits are kept: opposite side of layer II an anti-reflective coating

<-, <? "<- * n ^ i- <m 10 III is applied. The indices and thicknesses of the relevant

'- ~ "~ _,",. The following values are approximated in the following films:

Oo ^ n ₃ ^ 4. 2 ^ Ar ₃ SlO

_ß . . " ^l " ¹ I _94-04 .- _d _ ₄₇ -> "" * £ gj gA J ^ OjJ £ "l

In practice it may even be sufficient if the formula

_ß I _{94-04 d} _ ₄₇ > "

(3) likewise not strictly, but only essentially * £ gj "gA J ^ _OjJ £" genüeel _dkk

or is approximately fulfilled. 15 _{Belae In Cr o} _-> ₄ /% Ί

To prevent reflection on both upper surfaces, YES ", ^{C 2} ° ³ " ⁴ I, ';

2, the metallic layer II

used as a base and on each of its surfaces depending on the characteristics of the spectral reflection factor

an anti-reflective coating I or III can be applied. (calculated values) of the second embodiment

With G in Fig. 2, a glass plate is designated. 20 are shown in FIG. There represents the curve

The above explanations related to einschich- a _x general reflection factors "uf in Betige coverings. The covering I can, however, also contain several layers with a side standing in contact with air and the curve a ₂ including a dielectric layer. those on the glass side.

If, in the case of a multi-layer covering, the then complementary and for comparison, Fig. 7 shows the reflective

Equivalence false curves resulting from the corresponding combination of those not protected against reflection

lente complex index in the limits given above samples. Here, too, the curve α represents the fact

is held for / j, and k ₂ , the result is the same reflectors on the air side and the curve a ₂ those

xion prevention effect as with a single layer. on the glass side. The achieved according to the invention

In the case of such a multi-layer covering, a noticeable anti-reflective effect can also be derived from a comparison

a metal 30 of FIGS. 7 and 5 is particularly expedient for the first layer.

are used in which the above limits for the The third exemplary embodiment according to the invention

Metal are adhered to. formed film is shown in FIG. Here is

The following is a detailed description of the Hp anti-reflective coating I made of two layers. The indices

Exemplary embodiments explained. In the first embodiment and film thicknesses in this embodiment

In the example according to Fig. 1, the coating I has the following values:
xen index from 2.4 to 0.41, a thickness of 44.7 ηιμ and
consists of Cr ₂ O ₃ . Layer II is so thick that im
essentially unable to penetrate light, has one
complex index from 1.97 to 3.31 and consists of

an alloy (Inconel) made of 75.3% Ni. 15.6% Cr. ₄₀

7.7% Fe and 1.4% other elements. A glass The spectral reflectance curves (calculated

Plate with the index n, = 1.52 is on the values) of the third exemplary embodiment are shown in FIG. 6

Impact side of the covering I. The curve of the spectral shown. According to the invention can also in this case

Reflection factor of an anti-reflection reflection formed in this way can be prevented, although a metal-containing one

Lung lining is shown in FIG. 4. There the 45 topping shows that two or more layers are used.

air "l = 1 el -, = 45 ΐτμ thick Topping I '': = 2.4-0.4 /. d'i = 5.1 µm «'■>' = 1.97-3.3 /, dy :enough layer II «j = 0.82-5.99 /.

In addition 5 sheets of drawings

Claims (3)

Patent claims: 1. Absorbent anti-reflective coating for a thick metallic layer (II) which is essentially impermeable to light, characterized in that the complex refractive index (, n'-ik) of the anti-reflective coating (I) satisfies the following relationship: 10 0 <A "^ 0.7. 2. Anti-reflective coating according to claim 1, characterized in that its thickness d _x is chosen so that essentially the phase relationship 2π with η = - where φ _α and ^ _{12 are} the phase of the complex ratio of the amplitudes of the reflected and incident waves on the surface of the thick layer (II) or the anti-reflective coating (I) and A _{0 denotes} a given wavelength. 3. Anti-reflective coating according to claim 1 or 2, characterized in that it is made of chromium oxide consists.