CH683006A5 - Ion assisted depostion of optical oxide, nitride or oxynitride - using kinetic energy and impinging density adjusted to give min. optical absorption at given wavelength and max. density - Google Patents

Abstract

Coating a substrate with optical film(s) of oxide, nitride or oxynitride involves vaporising a component (I) forming this type of cpd. in vacuo and reaction with O2 and/or N2, (partial) ionisation of the particles of reaction prod. and/or starting material and building up the film by deposition of ions. The average kinetic energy of the ions impinging on the substrate or film built up on this and the average impinging density are adjusted so that the film has min. optical absorption at a given wavelength of light and max. density. The film consists of SiO2, A12O3, ZrO2, TiO2 and/or Ta2O5. The optical absorption is adjusted to a max. of the impinging density/absorption function, a min. of the kinetic energy/absorption function, a min. of the impinging density/kinetic energy/absorption function, a max. of the impinging density/film density function, a max. of the kinetic energy/film density function and/or a max. of the impinging density/kinetic energy/film density function. ADVANTAGE - The films are of optimum quality, i.e. have min. water absorption and min. optical losses and good adhesion to the substrate.

Description

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description

The present invention relates to a method according to the preamble of claim 1 and an optical layer according to that of claim 14.

Coating processes of the type mentioned, i.e. in which a solid is evaporated in a vacuum, reacted there with a gas or gas mixture and after or under ionization, the layer is built up to a significant extent by ions that are deposited, are ion-assisted under the term ion-assisted, reactive coating processes déposition, IAD.

The so-called ion beam assisted coating (IBAD) differs from this in that a more or less focused ion beam is directed at the substrate or the layer being built up and not, as in the first case, in an extended area ionization is sought before the substrate or workpiece to be coated.

With regard to the ion-assisted coating process of interest, reference can be made to the following references:

- "Synthesis of Silicon nitride and Silicon oxide films by ion-assisted depositian", R. P. Netterfield et al., Applied Optics / Vol. 25, No. 21/1. November 1986,

- "Plasma Ion Assisted Déposition - A Promising Technique For Optical Coatings", S. Pongratz et al., Reprint 38th AVS Symposium, Seattle 1991,

- "Summary Abstract: Properties of ion plated oxide films", H.K. Pulker et al., J. Vac. Be. Technol. A 3 (6), Nov / Dec 1985, 1985 American Vacuum Society,

- "Plasma enhances ion-assisted deposition", G. Pfister et al., Laser Focus World, September 1991, pp. 115/116,

- "Advanced plasma source enhances ion-assisted deposition", European News: Optics, Detectors, and Imaging, Laser Focus World European Electro-Optics, Sommer 1991, pp. 20-22,

- "Reactive ion-assisted deposition processes for interference coatings", H. Schwiecker et al., IPAT 91, 8th International Conference on Ion & Plasma Assisted Techniques, Brussels, May 1991, p. 147 ff.

- «Reactive lon-plating for the production of nitride and oxide layers», R. Buhl et al., Special print from the magazine Metallfläche, 42nd year 1988/10, Karl Hanser Verlag, Munich,

- «Optical Thin Films III: New Developments, Johannes Edlinger et al., SPIE Vol. 1323, Proceedings Reprint, The International Society for Optical Engineering, 9-11 July 1990, San Diego, California.

From the following articles

(A): "Ion-assisted deposition of optical thin films: low energy vs high energy bombardment", John R. McNeil et al., Applied Optics, Vol. 23, No. 4, 15 February 1984,

(B): “Optical coatings deposited using ion assisted deposition”, James J. McNally et al., J. Vac. Be. Technol. A 5 (4), Jul / Aug 1987,

(C): "Summary Abstract: Ion-assisted deposition of Ta 2O5 and AI2O3 thin films", J.J. McNally et al., J. Vac. Be. Technol. A 4 (3), May / Jun 1986,

Studies on the properties of optical layers, in particular from SÌO2 and TÌO2 (A), AI2O3, Ta2Ü5 (B) or Ta20s and AI2O3 (C) and manufactured by the IAD process, have become known.

From (A) it can be seen that the transmission in a light wave range from 200 to 300 nm of SiO 2 layers, produced by the ion-assisted reactive coating process (IAD), is a function of the average ion impingement density on the substrate or the layer being built up Maximum point at a density in the range of 0.075 mA / cm2 substrate area. It can also be seen that as the average kinetic energy of the impinging ions decreases from 500 eV to 30 eV, the optical transmission of the layer increases, i.e. the optical absorption of the layer decreases.

From (A) it can also be seen that the density of the SiO 2 layer, as is customarily measured in at% of the hydrogen which is bonded in total in the layer, increases with increasing ion current density, then decreases. Correspondingly, it follows that the density of the layer first increases and then decreases, and that up to a certain ion current density of oxygen ions impinging on a mean kinetic energy of 500 eV, the layer density is smaller than for energies of 30 eV, and this ratio reverses from a certain ion current density of approx. 0.1 mA / cm2.

Without going into the density of the Al2O3 or Ta205 layers produced, it can be seen from (B) and (C) that here too, in function of the oxygen ion current density, the optical reflection has maximum points - and thus the absorption has minimal points - at 350 or 400 nm.

While in the investigations on AI2O3, on the respective maxima of the refractive index - corresponding to minima of the optical absorption - at the highest mean kinetic ion energies of 1000eV (B) and (C), the highest and lowest values are reached, for Ta20s- Layers have the highest indices of refraction at the maximum points mentioned, corresponding to the lowest absorption values, given average kinetic energies between 200 eV and 500 eV.

The aim of the present invention is to optimize the quality of the optical oxide, nitride2

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or to form oxynitride layer. Optimal quality is defined by simultaneously optimizing the following sizes:

- highest possible density, i.e. Lowest possible water sorption: This achieves a stable spectral layer characteristic under varying environmental conditions, e.g. with changes in temperature and humidity.

- Lowest possible optical losses, i.e. lowest possible optical absorption, for example measured by the transmission losses.

At the same time, it should be ensured that the quality-optimized layer has good adhesion to the substrate surface under normal stresses and environmental conditions.

This object is achieved by the method of the type mentioned at the outset, which is characterized in accordance with the characterizing part of claim 1.

According to the invention, it was recognized that oxide, nitride or oxynitride layers produced by ion-assisted, reactive coating, on the one hand, as a function of the average kinetic energy with which the ions strike the substrate or the layer being formed, and, on the other hand, as a function of the average impact density of the ions , the optical absorption of the layer for a given light wave length and the layer density can be optimized simultaneously, namely the optical absorption to a minimum, the density to a maximum.

According to the invention, it was therefore recognized that by selectively adjusting or selecting the average kinetic energy on the one hand and the ion impingement density on the other hand, minimal optical absorption and at the same time maximum density of the layer can be achieved for a given light wavelength.

Preferred embodiments of this quality optimization are specified in claims 2 to 7.

According to the wording of claim 8, the range of the mean kinetic energy in particular should be chosen to be surprisingly low.

Preferably, according to the wording of claim 9, the setting of the average kinetic energy and the average impact density is chosen such that the hydrogen content of the layer is at most 5 at%, preferably at most 1 at%.

Following the wording of claim 10, transmission attenuation through the layer of at most 7 dB / cm, preferably at most 3 dB / cm, is realized, preferably at the same time.

The optimization according to the invention can be carried out in particular for layers according to the wording of claim 11.

In particular for the production of optical tantalum pentoxide layers, the wording of claim 12 is followed.

All known ion-assisted coating processes of the type mentioned at the outset are suitable for carrying out the process according to the invention, but processes are particularly suitable in which the oxide, nitride or oxynitride former is a solid, whether as a metal or semiconductor, or in a suboxidic or Subnitridischen or suboxinitridischen form is evaporated and the ionization is generated with the aid of an ion source, such as a Kaufmann ion source, but preferably by a gas discharge, in a preferred manner today by a low-voltage arc discharge plasma. In the aforementioned preferred embodiment, the evaporation is carried out by means of an electron beam, which is operated in combination with a hot cathode low-voltage discharge.

The substrate can be operated electrically, floating, or can be electrically bound by then being placed on DC bias and / or on an AC potential, a medium or high frequency potential or on a chopped DC voltage potential.

With regard to a system which is preferred today for carrying out the method according to the invention according to the wording of claim 13, reference is made to DE-PS 3 543 316, which is declared an integral part of the present description with regard to system configuration.

A layer according to the invention is further distinguished by the wording of claim 14, preferred training variants according to claims 15 to 18.

The invention is subsequently explained, for example, using figures.

Show it:

1a qualitatively shows the course of the inverse quality, as defined below, as a function of the average kinetic energy E and of the ion impingement density J, as is used according to the invention in a first variant;

1 b shows a further variant of the course used according to the invention according to FIG. 1 a;

1c shows a further, preferably utilized course in analogy to FIGS. 1a and 1b;

2 in the coordinate system, indicating on the one hand the average kinetic energy E of the layer-forming ions, on the other hand their impact density J, the area used according to the invention for obtaining optimally slightly absorbent and optimally dense layers;

Fig. 3 is a graphical representation of the layer density, given in at% H, the refractive index N, the layer-internal compressive stress ratios and the transmission losses, at 633 nm, as a function of the impact ion density and at the kinetic energy of 17 eV preferably used for the production of TaaOs layer , the course of the inverse quality Q_1.

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A quantity Q is defined as the layer quality Q at a light wavelength Xo, which is proportional to the layer density and inversely proportional to the absorption a at Xo. Taking into account that the density of the layer is usually measured by the hydrogen content S (H) of the layer, the result is

Q-1 = prop. {S (H) • a (Xo)}.

Mean:

Q: for example defined quality size;

S (H): hydrogen content in at%;

a (Xo): optical absorption at the light wavelength Xo, for example measured in dB / cm.

In FIG. 1 a, the quality-inverse quantity Q_1 (xo) as a function of the ion impingement density J and the average kinetic impingement energy of the ions is shown purely qualitatively to explain the method according to the invention. It also denotes aJ, for example, an ion current density range registered and used according to the invention and aE a range of kinetic energy E used according to the invention.

As can be seen from this, the inventive search of a two-dimensional J / E adjustment area optimizes the quality Q of the layer as defined above. A setting range aJ / aE according to FIG. 1 a for at least approximated the highest possible quality Q can lie, for example, on the minimum position of the ion impingement density / absorption function. Likewise, the range mentioned can lie at a maximum point of the ion impingement density / layer density function.

In Fig. 1 b, the analogous relationships to Fig. 1 a are shown, which can occur when the utilized area is at a minimum point in the function between kinetic energy E and absorption or at a maximum point of the kinetic energy / layer density function.

1c can occur if in the two-variable function ion impingement density / kinetic energy / layer absorption there is a minimum point in the range used according to the invention or if in the two-dimensional range used according to the invention there is a maximum point of the two-variable function ion impact density / average kinetic energy / layer density occurs. Of course, both of the two-dimensional functions mentioned can possibly become extremal in the mentioned area used according to the invention.

In FIG. 2, the area is shown hatched as a function of the ion impingement density J, specified in mA / cm 2, and as a function of the average kinetic energy E, specified in eV, with which ions strike the substrate or the layer being formed , in which it was found according to the invention that an optimal layer quality Q is produced in the sense of FIG. 1. As will be shown from the example, the compressive stresses within the layer are also optimally low, which ensures good layer / substrate adhesion.

Within the hatched area of FIG. 2, it was found, in particular for TaaOs, but also for silicon oxide, aluminum oxide, zirconium oxide and titanium oxide, that layers with minimal optical transmission loss or minimal optical absorption at a given wavelength, for example of 633 nm, are formed are optimally tight at the same time.

With a system, as shown in the above-mentioned DEPS 3 543 316, was used to produce a TaaOs layer, after previously evaluating the optimal value for the mean kinetic energy on the substrate or the layer of ions forming in the The following table sets the parameters set and the results measured on the layer.

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Parameters and results

Current density [mA / cm2]

Hydrogen [at% H]

Stress [GPa]

N [633]

TEO loss [dB / cm]

Bow [A]

Q-1

[at% • dB / cm]

0

6.3

0.027

0

0.05

4.9

0.064

1,856

15

0.09

3.4

0

2.01

3.4

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11.6

0.21

1

-0.03

2,184

1.8

40

1.8

0.33

0.9

-0.46

2,214

6.2

60

5.6

0.36

0.9

7.7

65

6.9

All tests were carried out with the following parameters:

PAr = 3 * 10-4 mbar

P02 = 1.2 * 10-3 mbar

Rate = 0.3 nm / sec

T-substrate = 20-60 ° C

The only parameter that was varied was the arc current

The floating substrate was not heated, resulting in a substrate temperature of 20 to 60 ° C. The arc current of the hot cathode low-voltage arc discharge was varied with, as mentioned, the mean kinetic ion energy E held constant, the impact density of the ions J given in mA / cm 2. The results are shown graphically in FIG. 3. The left vertical axis applies to both the at% H and the quantity indicating the density of the layer, as well as for dB / cm for the transmission losses at Xo = 633 nm, and further for the quality-inverse value Q — 1 in at% H • dB /cm.

The table and the graph show that the mean kinetic energy of the ions of 17 eV found to be optimal on the tantalum pentoxide layer produced in this way with currents in the hatched area in FIG. 3, that is to say approximately in the area of 0 , 15 mA / cm2 to 0.3 mA / cm2, optimal conditions in terms of the quality of the layer occur. It can also be seen that the internal stresses of the layer drop markedly in the shaded area.

Optimally, the Ta20s layer is produced with an impact ion density of approx. 0.2 mA / cm2.

When setting the kinetic energy E using the low-voltage arc current on the system according to DE-PS 3 543 316, for example, the density of the ions striking the substrate or the layer being built up is varied by varying the evaporator power, for example by varying the Electron beam power, set. Since, in the example shown, tantalum or sub-stoichiometric tantalum pentoxide is vaporized, preferably by means of an electron beam, and in which the ionization is carried out with the aid of an arc discharge, preferably a low-voltage arc discharge, with or without magnetic field support, the amount becomes in the gaseous state transferred solid essentially given by the evaporation capacity. By providing a defined or controllable magnetic field, the plasma density is specifically influenced, in particular increased, in the low-voltage discharge. Since the vaporized particles are released essentially in an electrically neutral manner, but essentially with a predetermined kinetic energy, the ionization density and thus the density of ions driven against the substrate to be coated is essentially dependent on the power or the current density of the arc discharge.

The arc discharge is operated so that a certain, e.g. homogeneous plasma distribution is present, which can be supported by providing the magnetic fields.

Claims (18)

Claims 1. A method for coating a substrate with at least one optical layer of an oxide, nitride or oxynitride, in which an oxide, nitride or oxynitride generator is evaporated in vacuo, there is reacted with oxygen, nitrogen or a mixture of these gases, and, after at least partial ionization of the reaction product particles and / or of the reaction starting materials that are formed, the layer is built up to a significant extent by ions that are deposited, characterized in that the average kinetic energy of the ions impinging on the substrate or on the layer that builds up and their mean impact density are adjusted so that, for a given light wavelength, the optical absorption of the layer becomes minimal and at the same time the density of the layer becomes maximal. 2. The method according to claim 1, characterized in that the optical absorption is set to a minimum point of the impingement density / absorption function. 5 5 10th 15 20th 25th 30th 35 40 45 50 55 CH 683 006 A5 3. The method according to any one of claims 1 or 2, characterized in that the optical absorption is set to a minimum point of the kinetic energy / absorption function. 4. The method according to any one of claims 1 to 3, characterized in that the optical absorption is set to a minimum point of the impingement density / kinetic energy / absorption function. 5. The method according to any one of claims 1 to 4, characterized in that the density is set to a maximum point of the impingement density / layer density function. 6. The method according to any one of claims 1 to 5, characterized in that the density of the layer is set to a maximum point of the kinetic energy / layer density function. 7. The method according to any one of claims 1 to 6, characterized in that the density of the layer is set to a maximum point of the function impingement density / kinetic energy / density of the layer. 8. The method according to any one of claims 1 to 7, characterized in that the average kinetic energy (E) of the ions 5 eV <E <60 eV is selected and the average impingement current density (5) is 0.01 mA / cm2 <J <0.3 mA / cm2. 9. The method according to any one of claims 1 to 8, characterized in that the average impingement current density is set so that the hydrogen content of the layer is at most 5 at%, preferably at most 1 at%. 10. The method according to any one of claims 1 to 9, characterized in that the optical absorption at the light wavelength is selected so that the transmission loss through the layer is a maximum of 7 dB / cm, preferably a maximum of 3 dB / cm. 11. The method according to any one of claims 1 to 10, characterized in that at least one SÌO2, Al2O3, ZrC> 2, TÌO2 or TaaOs layer is formed. 12. The method according to any one of claims 1 to 11, characterized in that a TazOs layer is formed and the average kinetic energy (E) to 15 eV <E <20 eV is selected, preferably to approximately 17 eV, and the ion current density J to 0.15 mA / cm2 <J <0.3 mA / cm2, preferably about 0.2 mA / cm2. 13. The method according to any one of claims 1 to 12, characterized in that the oxide, nitride or oxynitride generator is evaporated, preferably electron beam evaporated, and the ionization with the aid of a gas discharge, such as an arc discharge, preferably a low-voltage arc discharge, he follows. 14. Optical layer made of an oxide, nitride or oxynitride, produced by a method according to one of claims 1 to 13, for at least one predetermined light wavelength, characterized in that its hydrogen content is at most 5 at% and the optical transmission loss at the wavelength of 633 nm is at most 7 dB / cm. 15. Optical layer according to claim 14, characterized in that the transmission attenuation at the wavelength of 633 nm is at most 3 dB / cm. 16. Layer according to one of claims 14 or 15, characterized in that the hydrogen content is at most 1 at%. 17. Layer according to one of claims 14 to 16, characterized in that the layer consists of SÌO2, AI2O3, Zr02, TÌO2 or TaaOs. 18. Layer according to one of claims 14 to 17 made of Ta20s, characterized in that the hydrogen content is at most 1 at%, the transmission loss at the considered wavelength of 633 nm is at most 5 dB / cm, preferably less than 2 dB / cm. 6