GB2178187A

GB2178187A - Optical coating by ionised cluster beam deposition

Info

Publication number: GB2178187A
Application number: GB08616253A
Authority: GB
Inventors: Ewan Mackinnon Waddell; Brian Cochrane Monachan
Original assignee: Thales Optronics Ltd
Current assignee: Thales Optronics Ltd
Priority date: 1985-07-17
Filing date: 1986-07-03
Publication date: 1987-02-04
Also published as: GB8616253D0; GB2178187B

Abstract

An optical coating, particularly an antireflective coating, having a high threshold to damage by a laser beam, is deposited on a substrate by the ionised cluster beam deposition method where evaporant is coded and made to condense into clusters of atoms or molecules by adiabatic expansion through a small hole in a heated reservoir, and, after being ionised, is electrostatically impelled onto a substrate.

Description

SPECIFICATION Optical coating This invention relates to optical coatings and more particularly to anti-reflective coatings for optical components utilised in lasers or subjected to laser radiation.

There is a growing need for lasers operating at higher power than hitherto with energy densities of the order fo 50 J cm--2 which is about an order of magnitude greater than conventional lasers. With high power lasers the laser beam is likely to damage the laser's optical components, particularly the optical coatings which form part of these components, because the coatings used in conventional lasers have a finite capacity to absorb. If even a small fraction of a high-power laser beam is absorbed by a conventional coating the coating tends to evaporate, melt or ablate (blow apart) thereby rendering the laser useless.

An object of the present invention is to provide an optical coating having a high threshold to damage by a laser beam.

According to the present invention an optical coating having a high threshold to damage by a laser beam is provided by depositing the coating on a substrate using the ionised cluster beam deposition method.

In the ionised cluster deposition method evaporant is cooled and made to condense into clusters of atoms or molecules by adiabatic expansion through a small hole in a heated reservoir, and, after being ionised, is electrostatically impelled onto the substrate. These clusters are loosely bound, and they break up upon impact. Since they are generally singly ionised, and are typically 500-2000 atoms in size, and the electrostatic field 1-10kV, the average energy available to each atom on impact is around 0.5-20eV. This is enough to enable the atoms to migrate large distances over the surface of the growing film until they find a stable growth site. While doing so they dislodge adsorbed residual gases.The result is that a film prepared by ionised cluster beam deposition under normal high vacuum conditions (say 1OGTorr) has a high purity and crystalline perfection. In particular it is substantially free of voids which would absorb water and it is this absence of voids which is important if high laser damage thresholds are to be achieved.

Coatings produced in accordance with the present invention are particularly suitable for anit-reflection purposes since reflectivity of less than 1% can be achieved with absorption of less than 1%.

Sources of absorption within conventional optical coatings are known to be introduced during the deposition process by the following mechanisms: a) Gas absorption onto the film surface as it grows, causing both impurities and structural disorder; b) Structural disorder introduced due to insufficient adatom energy; c) Variation in the stoichiometry of the film; d) Substrate surface defects causing defective film growth.

The advantage of ionised cluster beam deposition lies in the unique combination of physical conditions which are present in the process, viz: a) the process is capable of producing a high flux of atoms at energies in the range of a few eV to a few hundred eV; b) the adatom mobility is high because of the kinetic energy resulting from impinging atoms; c) the process is carried out at a low pressure; d) the growing film surface is continuously bombarded thereby removing absorbed impurities.

Not all of these favourable conditions exist simultaneously in other known deposition processes. For example, in conventional vacuum evaporation the adatom energy is supplied mainly by thermal excitation and this is insufficient to produce near defect free films in high vacuum conditions. In ultra high vacuum conditions combinations of thin film materials and substrate can be found which produce very highly ordered films and this has been demonstrated in molecular beam epitaxy (MBE) deposition but the problem with MBE lies in the difficulty of producing multilayer coatings and the highly specialised and expensive nature of the equipment. Ion beam techniques offer the prospect of increased adatom energy and this occurs in RF sputtering and RF plasma deposition. However, both these RF processes take place in poor vacuum conditions and highly disordered films result.Ion plating also uses ions to increase adatom energy and can be performed under high vacuum conditions.

However, here high energy ions are utilised (few kV) which can cause structural damage, implantation and forward sputtering in the film and substrate. This disadvantage also applies to ion assisted deposition techniques such as dual ion beam deposition. It is important to realise that such techniques must use high energy ions (few kV) because space charge limits the possible flux of a low energy ion beam to impracticaliy low vaiues. lonised cluster beam deposition avoids this problem because the clusters have a very high mass to charge ratio.

1. An optional coating having a high threshold to damage by a laser beam, the coating having been deposited on a substrate by the ionised cluster beam deposition method.

2. A multilayer optical coating having a high threshold to damage by a laser beam,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical coating This invention relates to optical coatings and more particularly to anti-reflective coatings for optical components utilised in lasers or subjected to laser radiation. There is a growing need for lasers operating at higher power than hitherto with energy densities of the order fo 50 J cm--2 which is about an order of magnitude greater than conventional lasers. With high power lasers the laser beam is likely to damage the laser's optical components, particularly the optical coatings which form part of these components, because the coatings used in conventional lasers have a finite capacity to absorb. If even a small fraction of a high-power laser beam is absorbed by a conventional coating the coating tends to evaporate, melt or ablate (blow apart) thereby rendering the laser useless. An object of the present invention is to provide an optical coating having a high threshold to damage by a laser beam. According to the present invention an optical coating having a high threshold to damage by a laser beam is provided by depositing the coating on a substrate using the ionised cluster beam deposition method. In the ionised cluster deposition method evaporant is cooled and made to condense into clusters of atoms or molecules by adiabatic expansion through a small hole in a heated reservoir, and, after being ionised, is electrostatically impelled onto the substrate. These clusters are loosely bound, and they break up upon impact. Since they are generally singly ionised, and are typically 500-2000 atoms in size, and the electrostatic field 1-10kV, the average energy available to each atom on impact is around 0.5-20eV. This is enough to enable the atoms to migrate large distances over the surface of the growing film until they find a stable growth site. While doing so they dislodge adsorbed residual gases.The result is that a film prepared by ionised cluster beam deposition under normal high vacuum conditions (say 1OGTorr) has a high purity and crystalline perfection. In particular it is substantially free of voids which would absorb water and it is this absence of voids which is important if high laser damage thresholds are to be achieved. Coatings produced in accordance with the present invention are particularly suitable for anit-reflection purposes since reflectivity of less than 1% can be achieved with absorption of less than 1%. Sources of absorption within conventional optical coatings are known to be introduced during the deposition process by the following mechanisms: a) Gas absorption onto the film surface as it grows, causing both impurities and structural disorder; b) Structural disorder introduced due to insufficient adatom energy; c) Variation in the stoichiometry of the film; d) Substrate surface defects causing defective film growth. The advantage of ionised cluster beam deposition lies in the unique combination of physical conditions which are present in the process, viz: a) the process is capable of producing a high flux of atoms at energies in the range of a few eV to a few hundred eV; b) the adatom mobility is high because of the kinetic energy resulting from impinging atoms; c) the process is carried out at a low pressure; d) the growing film surface is continuously bombarded thereby removing absorbed impurities. Not all of these favourable conditions exist simultaneously in other known deposition processes. For example, in conventional vacuum evaporation the adatom energy is supplied mainly by thermal excitation and this is insufficient to produce near defect free films in high vacuum conditions. In ultra high vacuum conditions combinations of thin film materials and substrate can be found which produce very highly ordered films and this has been demonstrated in molecular beam epitaxy (MBE) deposition but the problem with MBE lies in the difficulty of producing multilayer coatings and the highly specialised and expensive nature of the equipment. Ion beam techniques offer the prospect of increased adatom energy and this occurs in RF sputtering and RF plasma deposition. However, both these RF processes take place in poor vacuum conditions and highly disordered films result.Ion plating also uses ions to increase adatom energy and can be performed under high vacuum conditions. However, here high energy ions are utilised (few kV) which can cause structural damage, implantation and forward sputtering in the film and substrate. This disadvantage also applies to ion assisted deposition techniques such as dual ion beam deposition. It is important to realise that such techniques must use high energy ions (few kV) because space charge limits the possible flux of a low energy ion beam to impracticaliy low vaiues. lonised cluster beam deposition avoids this problem because the clusters have a very high mass to charge ratio. CLAIMS

2. A multilayer optical coating having a high threshold to damage by a laser beam, each coating layer having been deposited by the ionised cluster beam deposition method.

3. A coating as claimed in either preceding claim, wherein the deposition method is effected under high vacuum conditions.

4. An optical component incorporating an optical coating as claimed in any preceding claim.