Optical element having antireflection film

Details Optical element having antireflection film Optical element having antireflection film

2003-02-21

2004-02-17

Juba, John (Department: 2872)

Optical: systems and elements

Light interference

Produced by coating or lamina

C359S580000, C359S586000

Reexamination Certificate

active

06693747

ABSTRACT:

The present invention relates to an optical element having an antireflection film. It relates to an optical element having an antireflection film that has excellent adhesiveness between a plastic substrate and the antireflection film, abrasion resistance, heat resistance, alkali resistance and impact resistance.
Heretofore known are optical elements having an antireflection film provided on a plastic substrate. Also known are optical elements having a thin metal film layer provided on the surface of a plastic substrate for enhancing the adhesiveness between the plastic substrate and the antireflection film. For example, Japanese Patent Laid-Open No.186202/1987 discloses an antireflection film for an optical element having a thin metal film layer provided on the surface of a plastic substrate, in which the metal layer is made of a metal selected from the group consisting of copper (Cu), aluminum (Al), nickel (Ni), gold (Au), chromium (Cr), palladium (Pd) and tin (Sn).
However, these optical elements having an antireflection film are unsatisfactory with respect to their heat resistance and impact resistance. Therefore, it is desirable to provide optical elements having an antireflection film that have improved physical properties such as heat resistance, abrasion resistance, alkali resistance and impact resistance.
Heretofore, in general, a basic layer made of SiO
2
has been provided in a plastic lens for enhancing the strength of coating films. However, the basic layer made of SiO
2
has a drawback of lowering the heat resistance of the plastic lens.
The present invention provides an optical element having an antireflection film having excellent adhesiveness between a plastic substrate and the antireflection film, heat resistance, abrasion resistance, alkali resistance and impact resistance.
The present invention addresses the problems noted above. The inventors have determined that when a layer made of niobium (Nb) is provided between a plastic substrate and an antireflection film to form an optical element, the adhesiveness between the plastic substrate and the antireflection film, the heat resistance, abrasive resistance, alkali resistance and the impact resistance of the optical element are improved.
The optical element of the invention has a basic layer made of Nb, and therefore, has not only excellent adhesiveness between the plastic substrate and the antireflection film, heat resistance and impact resistance, but also excellent alkali and abrasion resistance and properties such that an absorbance index inherent to metals is low.
The basic layer may consist of Nb (that is 100% by weight of Nb), or may comprise a mixture of niobium and up to 50% by weight, preferably 25% by weight of other elements such as aluminum(Al), chromium(Cr), tantalum(Ta) and mixtures of two or more thereof. The antireflection film may also be comprised of multi-layers, and at least one of the layers is obtainable by an ion-assisted process. The basic layer comprising Nb may also be formed by an ion-assisted process.
The “ion-assisted process” referred to herein is a well known process also called “ion beam assisted vapor deposition process”. According to this process, a material is deposited on a substrate, such as a lens substrate, by vapor deposition using an ion plasma in a gas atmosphere, such as argon (Ar) and/or oxygen. In a common apparatus suitable to perform this process, preferred vapor deposition conditions are an accelerating voltage of 100-250V, and an accelerating current of 50-150 mA. A detailed description is given in e.g. U.S. Pat. No. 5,268,781. Further details can be derived from M. Fliedner et al., Society of Vacuum Coaters, Albuquerque, N.M., USA. p237-241, 1995 as well as from the references cited therein.
In the ion-assisted process, argon (Ar) maybe used as the ionizing gas for preventing oxidation of films being formed. Although argon is preferred, other ionizing gases such as oxygen and nitrogen, or mixtures of these gases could also be used. This stabilizes the quality of the films formed and enables easy control of the thickness of the films by the use of an optical film thickness meter.
For ensuring good adhesiveness between the plastic substrate and the basic layer and for ensuring good uniformity of the initial film morphology in vapor deposition in the ion-assisted process, the plastic substrate may be subjected to ion gun pretreatment before the basic layer is formed thereon. The ionizing gas in the ion gun pretreatment may be any of oxygen, nitrogen, Ar, or mixtures thereof. For the preferred power range, the accelerating voltage is from 50 V to 200 V, and the accelerating current is from 50 mA to 150 mA. If the accelerating voltage is lower than 50 V, or the accelerating current is lower than 50 mA, an effect for improving the adhesiveness between the plastic substrate and the basic layer formed thereon may not be sufficient. However, if the accelerating voltage exceeds 200 V, or the accelerating current exceeds 150 mA, the plastic substrate and also the cured film and the hard coat layer thereon may possibly be yellowed, or the abrasion resistance of the optical element may possibly be lowered.
In the invention, after the basic layer comprising Nb has been formed on the substrate, an antireflection layer is formed by any suitable process. For example, it may be formed by vapor deposition, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD), or by other methods such as ion plating vapor deposition.
In one embodiment, the antireflection film has at least one SiO
2
layer as a low-refraction layer and at least one TiO
2
layer as a high-refraction layer. If desired, the antireflection film may have a metal layer comprising Nb.
For relieving the stress within the low-refraction layer such as an SiO
2
layer, when the SiO
2
layer is formed in an ion-assisted process in which Ar is used for the ionizing gas for SiO
2
deposition, the abrasion resistance can be improved. Regarding the ion-assisting condition for obtaining the result, the ion current density on the dome in the vapor deposition device is from 15 to 35 &mgr;A, and the accelerating voltage is from 400 to 700 V. If the ion current density is lower than 15 &mgr;A or the accelerating voltage is lower than 400 V, both an effect for relieving the stress and an effect for improving the abrasion resistance may be hardly obtained. If, however, the ion current density exceeds 35 &mgr;A or the accelerating voltage exceeds 700 V, the plastic substrate may possibly be yellowed, or the optical performance may possibly be adversely affected.
The high-refraction layer such as a TiO
2
layer may also be formed in an ion-assisted process. For the ionizing gas in the ion-assisted process for forming the high-refraction layer, a mixed gas of O
2
and Ar is used. The mixing ratio of O
2
to Ar based on the volume of flowing gases preferably ranges from 1:0.5-2. It is possible to improve the refractive index of the high-refraction layer formed and to promote the improvement of the abrasion resistance by using an ion-assisted process. Materials for forming the high-refraction layer are TiO
2
, Nb
2
O
5
, Ta
2
O
5
, ZrO
2
, Y
2
O
3
, and mixtures thereof. Preferred examples include TiO
2
, Nb
2
O
5
, Ta
2
O
5
and mixtures thereof.
As a suitable ion-assisting condition for using TiO
2
, Nb
2
O
5
or their mixtures as the metal oxide, the ion current density on the dome in the vapor deposition device is from 8 to 15 &mgr;A, and the accelerating voltage is from 300 to 700 V. The volume ratio of O
2
to Ar in the ionizing gas mixture is from 1/0.7 to 1/1.0. If the ion current density, the accelerating voltage and the ionizing gas ratio overstep the defined ranges, the intended refractive index may not be obtained, and, in addition, its absorbance index may likely increase, and its abrasion resistance may possibly be lowered.
As a suitable ion-assisting condition for using Ta
2
O
5
or its mixtures as the metal oxide, the ion current density on the dome in the vapor deposition device is from 12 to 20 &mgr;A,

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