EP0956375A1

EP0956375A1 - Reactive magnetron sputtering apparatus and method

Info

Publication number: EP0956375A1
Application number: EP96918398A
Authority: EP
Inventors: Michael A. Scobey
Original assignee: Corning Oca Corp
Current assignee: Corning Oca Corp
Priority date: 1996-06-10
Filing date: 1996-06-10
Publication date: 1999-11-17
Also published as: WO1997047781A1; CA2254354A1; JP2001502754A

Abstract

A method and apparatus for producing optical films on substrates having extremely high packing densities of the same quality as those films produced by ion beam sputtering including a vacuum chamber with a conventional magnetron sputtering system and unusually high speed vaccum pump means. The low pressure of inert gas created by said high speed vacuum pump means being in the range of 5 x 10-5 Torr to 2.0 x 10-4 Torr and the magnetron sputtering system being at least 20'' from said substrates. A gas manifold around the magnetron and target material confines the inert working gas in the vicinity of the magnetron and as the gas diffuses and expands into the chamber the high speed vacuum pump means removes the expanded gas from the chamber at the high speed. An ion gun directs ionized reactant gas toward the substrates which has the effect of improving film stoichiometry as well as reducing reactant gas at the magnetron. Multiple magnetron assemblies, multiple target materials and compound target materials may be used.

Description

REACTIVE MAGNETRON SPUTTERING APPARATUS AND METHOD

FIELD OF THE INVENTION

The present invention relates to extremely low pressure reactive magnetron sputtering apparatus and method for fabricating dielectric optical coatings on substrates. The invention in this application deals with the specialized field of optical interference filters suitable for

such applications as, for example, laser mirrors and output couplers. In these types of films,

film scatter, absoφtion, and defects must be kept to a minimum. To date, the only known type of film process which has succeeded in this endeavor is Ion Beam Sputtering or (IBS) and to a lesser degree Ion Assisted Deposition (IAD). IBS will be further explained

hereinafter.

Prior Art Optical coatings requiring very low levels of scatter and absorption have been

traditionally manufactured using IBS With this method, in a very high vacuum environment,

a high energy ion beam in the energy range of 500 eV to 1500 eV is directed at a target (source) composed of desired coating materials. The effect of the ion bombardment is to

sputter or remove atoms (species or particles) from the target due to momentum exchange

in the target lattice. The sputtered species then condenses onto substrates. The pressure in

the chamber is desired to be maintained at a very low level to prevent gas-phase collisions of

the sputtered particles with background gases. There is an extensive volume of references to reasons for the improved optical film

performance that IBS provides over other coating techniques such as evaporation and other methods of sputtering.

Wei et al initially recognized the benefits of IBS for laser high reflectors in the U.S. Patent No. Re 32,849 "Method of Fabricating Multi-layer Optical Films." In Wei et al,

quarterwave stacks used for laser mirrors are produced using only a single ion gun illuminating a target. Fig. 1, which is a reproduction of Fig. 1 of Wei et al, shows an ion gun

"A," target "B" and substrate "C." With this system, background levels of argon (inert gas) was kept at the extremely low pressure level of 1.5 x 10"* Torr. Reactive gas pressure

(oxygen) was set to a level to insure proper stoichiometry of the depositing levels, in the range of 5 x 10^"5 Torr for high index materials and 3 x 10^"5 for low index materials.

Another IBS patent by Scott et al, No. 4,793,908 "Multiple Ion Source Method and Apparatus for Fabricating Multilayer Optical Films" uses the method of Wei et al with the

addition of a second ion beam directed at the substrate which is partly composed of the required reactive species. The second ion beam provides improved optical properties Fig

2, which is a reproduction of Fig. 2 of Scott et al, shows ion gun "A," target "B," substrate

"C" and the second ion gun "D." In this patent, Scott et al teaches that IBS is improved over

conventional magnetron sputtering as the ". . . gas pressure in the chamber, e.g. at the substrate surface, using this approach can be in the tenths or hundredths of a millitorr range.

This is a great advantage since the finished film tends to contain fewer gas atoms and have

an improved range structure and atomic packing density." Col. 2, lines 14 - 19. So-called "low loss" films can be produced by the foregoing techniques, such as films or coatings having less than 500 ppm or even 100 ppm loss. When used in an application

such as high reflector laser mirrors, for example, the films produced by the foregoing techniques can have total losses well less than 100 ppm. As used here (unless otherwise indicated by context), references to "low loss" films or coatings means films or coatings having less than 500 ppm loss.

"Loss" refers to everything other than reflection or

Total loss = 1-R where R = 1-T-A-S and where R is reflection, T is transmission, A is absoφtion and S is scatter

Vossen and Kern in the book "Thin Film Processes," Academic Press, New York

1978 at page 189 describe IBS as differing from other sputtering processes due to the fact that "low background pressure gives less gas incoφoration and less scattering of sputtered particles on the way to the substrates " ("low background pressure" [sic] are in italics in the

original). As discussed previously, this is of great advantage for depositing optical films Non-sputtering techniques such as evaporation tests do not produce film suitable for

high quality applications. Evaporation techniques, where the coating material is heated under

vacuum to the point at which it evaporates, do not impart nearly the kinetic energy of

sputtering and the films tend to grow in a porous columnar manner In addition, the evaporation processes tend to eject small particles from the hot source due to small source

explosions of source materials which can be caused by expanding trapped gases or differential

heating. For these reasons, evaporation is used only for the production of relatively low

tolerance coatings. DC or magnetron sputtering has also been used to produce dielectric coatings for many low tolerance or low film quality applications. In general, these methods involve filling a chamber with inert gas which is then ionized to form a low energy plasma. A target is then

charged to a negative potential in the range of 400 to 900 volts which has the effect of bombarding the target with energetic charged ions and sputter atomic or molecular particles from the target. The sputtered particles then condense onto substrates. DC sputtering is used to sputter metals. RF sputtering utilizes oscillating target voltage with a net zero DC current

to allow dielectric targets to be sputtered.

In the case of reactive DC sputtering, where reactive gas(es) are added to the chamber to form a compound film at the substrate, it is desired to have a reaction take place on the substrate and not the target, as a severe reduction in deposition rate as well as an increase in target arcing will take place when the target becomes covered with a reactive dielectric species. Many techniques in the prior art exist to cope with this problem, which are all some

form of target and substrate isolation, where the reactive gas pressure at the target is maintained at a low level to prevent target "poisoning" and the reactive gas pressure is kept

high at the substrate to effect reaction

In the U.S. Patent to Scobey et al, No 4,851,095, "Magnetron Sputtering Apparatus

and Process," parts are shuttled on a high speed drum between a deposition zone maintained

at a very high pressure of argon and a reaction zone containing an energetic reactive gas

plasma.

In the U.S. Patent to Maniv et al, No. 4,392,931, "Reactive Deposition Method and

Apparatus," target material is sputtered through an orifice or aperture onto a rotating drum. Inert working gas is bled into the target chamber, and a reactive gas is bled into the rest of the chamber. The aperture limits the amount of reactive gas to the target. A field is

established on the drum to ionize reactive gas and increase film transparency.

Scherer et al U.S. Patent No. 4,931,169, "Apparatus for Coating a Substrate With

Dielectrics," also discloses a sputtering through an orifice and produces an alternating current field (AC) component to the DC drive voltage to prevent arcing. The AC field is said in

Scherer et al patent to have the added effect of increasing deposition rate due to an increase in collisions between oscillating electrons and the working gas. The field is said to have the further effect of allowing a reduction in the coating pressure to as low as 0.5 Torr

The U.S. Patent to Dietrich et al, No. 4,946,576, "Apparatus for the Application of

Thin Layers to a Substrate," also discloses the use of an aperture between the cathode and the substrate and adds a positive voltage near the substrate over which the reactive gas flows. The reactive gas becomes ionized by the anode which has the effect of improving film

stoichiometry. Another U.S. Patent to Dietrich et al, No. 4,572,840, "Method and Apparatus

for Reactive Vapor Deposition of Compounds of Metal and Semi-Conductors" uses a flow

restriction between the magnetron and substrate equal to at least 40% of the cross-section of

the space.

In all of the above cited prior part, the source to substrate distance is short. In Scobey et al, the distance is approximately 10 cm; in Maniv et al, the distance is 10 cm; in Scherer et

al, the distance is 4 cm, and Dietrich et al '842 uses an example of 6 cm while Dietrich et al

'576 does not list a distance. Attempts to utilizer magnetron sputtering at low pressure and long throw distances have been described. Pond et al, Advanced Coating Development Program, Final Report

(June, 1994) Phillips Laboratory, Kirtland Air Force Base, New Mexico, USA (PL-TR-93-

1033), developed a magnetron sputtering process using 8 inch magnetrons to coat moderate size substrates of 8 inches or less in diameter To achieve adequate deposition rates, Pond tilted the magnetrons to a 20° angle Pond reduced reactive gas pressure at the magnetron by a "gas separation process" where reactive gas is introduced through a manifold in or near the substrate plain and opposite of the magnetron assemblies and inert working gas is introduced through a shield at the magnetron This was done in an attempt to reduce poisoning and arcing at the target as well as to help provide for a fully reacted and stoichiometπc film As higher background gas pressures reduced film density, Pond attempted to reduce the inert gas pressure in the chamber with the use of magnetrons outfitted with special high strength magnets The Pond system was characterized by low coating rates of typically 0 5 to 1 5 A/sec These low coating rates can in general be attributed to poisoning of the target, arcing, poor film reaction and low applied power levels

In addition, in all of the above cited prior art, the total pressure is maintained at conventional sputtering pressures of approximately 3 x 10^'3 Torr between the substrate and target, except for the Scherer et al patent and the Pond et al report

SUMMARY OF THE INVENTION

It is a primary object of this invention to produce optical films having extremely high

packing densities, smooth surfaces, and low scatter on the order of the quality produced by IBS, but in a DC magnetron sputtering system. The method and apparatus which accomplishes this object includes a conventional magnetron sputtering system in a vacuum tank outfitted with an unusually high pumping

speed vacuum pump. A gas manifold around the magnetron and target material confines the inert working gas (argon) in the vicinity of the magnetron As the gas diffuses and expands from the area of the magnetron, the unusually high pumping speed vacuum removes the expanding gas from the chamber at a high speed. The pressure in the chamber is then a

function of the pumping speed of the vacuum pump and the confinement efficiency of the magnetron baffle. Reactive gas enters the chamber through an ion gun which ionizes the gas and directs it toward the substrate This has the effect of reducing the amount of gas required

to provide the film with proper stoichiometry as well as reducing the reactive gas at the

magnetron

This invention distinguishes shaφly from the known prior magnetron sputtering

techniques and from conventional ion beam techniques It is characterized by extremely low chamber pressures, including extremely low reactive gas pressure and extremely low inert gas

pressure The reactive gas pressure, such as O₂, N₂, NO, etc. (measured at the substrate surface being coated) is preferably in the range of 2.0 x 10 ^s to 1.5 x 10"⁴ Torr, more preferably 3 x 10^'5 to 9 x 10^"5 Torr. This advantageously reduces or eliminates arcing at the magnetron and "poisoning" of the source by the reactive gas. The inert gas, such as argon, krypton, xenon, etc., is in preferred embodiments introduced primarily at the

magnetron. A shaφ pressure drop is established for the inert gas, preferably having a pressure (measured at the substrate surface being coated) in the range of 5.0 x 10^'5 Torr to 2.0 x 10^"4 Torr, more preferably 5 x 10^"5 Torr to 1.5 x lO^Torr. Such low chamber

gas pressures provide long mean free path, (MFP) and correspondingly allow advantageously long throw distances without undue collisions between the chamber gasses and the sputtered material. Advantageously good coating uniformity is achieved via long throw distance, preferably greater than 12", more preferably 20" or longer. The extremely low chamber

pressures enable the use of long throw distances. That is, notwithstanding the use of such long throw distances, advantageously high coating deposition rates can be achieved with correspondingly high magnetron power levels The loss of films or coating quality normally expected to result from higher magnetron power levels and longer throw distances is avoided

by the novel use of extremely low chamber pressures. Thus, preferred embodiments of this

invention duplicate several key process conditions of IBS (which, for example, operates in

the same pressure range as described above), but uses a DC magnetron sputtering system

This novel system, based on magnetron sputtering substantially improves the coating speed

and correspondingly cost and throughput of depositing high film quality coatings BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a reproduction of Fig. 1 of Wei et al, as mentioned in the Background of the

Invention;

Fig. 2 is a reproduction of Fig. 2 of Scott et al, as mentioned in the Background of the Invention;

Fig. 3 is a cross-sectional schematic illustration of the apparatus of this invention;

Fig. 4 is a schematic representation in cross-section of a magnetron sputtering apparatus of this invention;

Fig. 5 is a cross-sectional schematic illustration of the apparatus of this invention with

multiple magnetron sputtering assemblies;

Fig. 6 is a graph showing the relationship between chamber pressure and chamber pumping speed assuming the magnetron pressure of 0.7 microns and a magnetron assembly conductance (C_M) of 3000 1/sec; and

Fig. 7 is a graph showing the relationship between chamber pressure and chamber pumping speed assuming a magnetron pressure of 0.4 microns and a magnetron assembly

conductance (C_M) of 3000 1/sec.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As mentioned previously, Figs 1 and 2 show IBS systems capable of producing high

quality dielectric coatings on substrates to form mirrors which are usable in ring laser

gyroscopes. The present invention now being described is capable of producing the same

high quality coatings, but using a DC reactive magnetron sputtering system instead of IBS. Background pressures of inert gas (e.g., argon) can be maintained at or about the same levels or lower than those disclosed in the Wei et al and Scott et al patents. Films made with this invention have comparable properties to IBS coatings in that they have extremely high

packing density, as well as smooth surfaces and low scatter. Total losses for a high reflector laser mirror made in accordance with preferred embodiments of the method disclosed here, for example, are well less than 0.01% or 100 ppm.

Figs. 3 and 4 show the method and apparatus of preferred embodiments of the method

disclosed here. The housing 10 forms a vacuum chamber 1 1 containing a low pressure magnetron assembly 12 and a planetary substrate holder 13 with a plurality of rotatable planets 14. Each planet 14 holds a substrate facing the magnetron assembly 12. In this embodiment, the distance between the top of the magnetron assembly 12 and the planets is 16". The magnetron assembly 12 is connected to a source of working gas 16 by conduit 17. In this embodiment, the housing 10 is shown spherical with a radius of 48", but other

configurations are equally appropriate. The housing 10 has a lower sleeve 18 which opens into the vacuum chamber 11 and

contains a high speed vacuum pump 20 with a gate valve 21 located between it and the

vacuum chamber 11. The vacuum pump is of course used to lower and maintain the pressure

in the vacuum chamber at a very low level in the inert gas pressure range of 5 x 10^"5 Torr to

1.5 x lO^"4 Torr.

Typical high speed vacuum pumps useful in the embodiments disclosed here include turbopumps, cryopumps and diffusion pumps. One larger pump, such as a 16" cryopump or

16" turbopump or 16" diffusion pump or, more preferably, in this invention are 16" cryopumps or 16" diffusion pumps. Pumping speeds with these pumps are on the order of

5000 liters/second (nitrogen) for a 16" cryopump and 10000 liters/second for a 16" diffusion pump (ref. Leybold Product and Vacuum Technology reference book, 1993). Larger pumps can be used such as a 20" pump having pumping speeds of 10000 liters/second for cryopumps (N2) and 17500 liters/second for diffusion pumps (N2) (ref. Varian Vacuum Products

Catalogue 1991-92). Pumping speeds referenced above are at the throat of the pump.

The magnetron assembly 12 is in vertical alignment with the axis of rotation (main center line 22) of the planetary substrate holder 13 and with a holder for monitoring witness chip 23. In this embodiment, the throw or the distance between the top of the magnetron

assembly and the planets is 16". Each planet and its substrate rote about their own center line

24. Such planetary holders are conventional and need not be described further except to point

out that, in this embodiment, the planets are 15" and the substrates are 15" or any size less than 15" in diameter, and the center line of each planet is 14" from the center line 22 to

accommodate large substrates. Larger planets can be used, for example, 24 inch planets, with sizes correspondingly increased substrates and throw distances, whereby even greater

throughput improvements can be achieved.

An ion gun 26 whose output, represented by dashed lines 27, is directed obliquely

toward the substrate holder 13 and whose input in connected to a source of reactive gas

mixture 28 by conduit 30 The ion gun is positioned such that its output of ions and gas mixture cover the entire substrate holder 13 and in this embodiment the top of the ion gun is

210" from the planets. The principal function of the ion gun is twofold. The first is to modify

and improve film properties in a manner similar in concept to the Scott et al patent The second function may be more important, which is to serve to maintain low reactive gas background pressure. With the ion gun, reactive gas is ionized and directed toward the substratφ). The momentum of the reactive gas then, carries it only toward the substrate(s)

and not toward the magnetron, where it has the effect of causing arcing and rate reduction. The small amount of gas which diffuses toward the magnetron does not noticeably affect its operation. Typical reactive gas pressures are in the range of 2 x 10^"$ Torr to 1.5 x 10"⁴ Torr, preferably, 3 x 10⁵ Torr to 8 x 10^"5 Torr

A suitable hot cathode pressure gauge 31 is also connected to the vacuum chamber 11 to measure the pressure within the vacuum chamber. Also, vacuum chamber is provided with a shutter 32 oscillatible about a stem 33 blocking the output of the magnetron assembly

12, represented by dashed lines 34 The stem 33 is connected in any suitable manner to a platform 35 and to a means for oscillating the stem (not shown) The shutter is used to pre- sputter the source(s) to remove contaminates from the target which may have condensed, etc., onto the surface of the target while the apparatus was idle between layers being

deposited on the substrate

As shown in Fig 4, the magnetron assembly 12 comprises a target holder 36 having

a cavity 37 formed by walls 38 and target material 40 Centrally within the cavity 36 are

conventional magnets 41 which are water cooled by the circulating flow of water in and out

of the cavity 36 through passages 42 and 43. The metallic target material 40, clamped by the

holder, also is water cooled A manifold 44, spaced slightly from the holder 36, and sealed

by insulators 45, is connected to the source of working gas 16 by conduit 17 (Fig. 3) which

enables the gas to flow entirely around the top of the holder and over the metallic target material 40. The manifold 44 has an opening 45 substantially the size of the metallic target material so that sputtered target material and working gas is emitted as represented by the

lines 34. The magnetron is available from Material Sciences of Boulder, Colorado and is typically 6" to 8" in diameter with high strength magnets.

When it is realized that this invention has the capability of producing extremely high quality film coatings by magnetron sputtering without the constraints of IBS or other known techniques, it will also be realized that this invention is a major advance over the prior art

The foregoing dimensions and pressures of this embodiment— a throw distance of 16", 15" diameter planets, 15" or less diameter substrates and the distance of 20" from the top of the ion gun to the planets along with extremely low reactant gas pressures in the range of 2

x 10^'5 Torr to 1 5 x 10^ Torr and extremely low inert gas pressures of 5 x \(f to 2 x 10* Torr— also show the great difference between this invention and the prior art

Compare also the throughput of preferred embodiments of this invention with the throughput of a typical IBS system in making laser quality mirrors

From the foregoing it can be seen that the throughput of this invention is 20 to 120 times

faster than the throughput of the typical IBS system Coating throughput is a function of

coating rate and substrate area. Furthermore, the method of this invention scales easily to larger apparatus dimensions All of the dimensions above can be easily increased at least by a factor of two to allow coating of optical substrates of 30" diameter or even large with laser low loss coatings having good uniformity Scaling is a simple linear issue A larger system uses larger magnetrons and more

process gas (e.g , argon) The vacuum pumps need to be correspondingly increased to accommodate the larger chamber and the increase in process gas flow

Thus, as is apparent, this invention is capable of producing, for example, laser quality mirrors which are many times greater in diameter than those known to be made by current IBS systems The long throw of 16" and more and low chamber pressures of preferred embodiments of this invention allow two or more materials to be concurrently deposited to form high optical films composed of mixtures of materials Fig 5 shows two sources, magnetron

assembly 12 and magnetron assembly 12a in vacuum chamber 1 1 as an example of multiple sources (The subscripts to the added source and the use of all other reference numerals as in Fig 3 are to simplify the descπption herein)

By controlling the level of power of each source which effectively controls the

deposition rate, a layer of selected refraαive index can be formed as a mixture of two or more materials The mixture can be homogenous throughout the layer to form a film of selected

index, or inhomogeneous where the layer composition and hence the refractive index varies

throughout the film One common form of inhomogeneous film is called a "rugate" filter,

where the refractive index varies in a sinusoidal manner which has the effect of forming a

narrow notch reflector To maintain a low pressure for such a multi-source system, the pumping speed must be roughly increased by a factor of two for two concurrent deposition sources, or a factor of N forN sources. Given the benefit of the disclosure, adding pumping speed will be a simple

exercise for those skilled in the art, involving generally either increasing the size of the pump or adding more pumps to the chamber. In practice, however, two concurrent sources need not be powered at a level equal to that used for a simple source to maintain coating rate, as

the rate from the sources is additive, and hence the sources can be sized to smaller levels which use less gas.

Another device which may be used in this invention is an arc reducing electronic device sold by Advanced Energy of Boulder, Colorado under the trademark SPARC-LE. In

Fig. 3, the SPARC-LE 46 is shown connected to the magnetron assemblies 12 by an electrical conductor 47 with its own DC power supply 48. The SPARC-LE is connected similarly to the two magnetron assemblies 12 and 12a as shown in Fig 4. Such a device helps in reducing arcing but it is not necessary in the method and apparatus of this invention.

From the foregoing, it can be appreciated that the magnetron system operates at

extremely low pressures. The chamber pressure of the inert gas will be a function of the magnetron pressure. Most importantly in this invention, the low total pressure region 50 (A

+ O₂) is always much less than the higher argon pressure region 52 as depicted in Fig. 4.

Pressure in the chamber can be modeled using the well known pressure-flow equations (see Leybold Product & Technology Reference Book, page 18 - 5, 1993): ^pChamber ⁼ """Ar / ^CP

Magnetron ^""""Ar / ^CM ^{+ p}Chamber

Where:

^pchamber is the pressure in the chamber;

"""Ar is the flow of argon into the chamber (through the magnetron);

^CP is the conductance of the high vacuum pump (chamber pumping speed),

^pMagnetron is the pressure in the magnetron,

^CM is the conductance due to gas confinement at the magnetron (confinement

efficiency of the magnetron)

substituting terms, the chamber pressure can be written as

^pChamber = Magnetron / (^cp / ^CM + 1 )

This is an important relationship, because it shows that the pressure in the chamber

is dependent upon the pumping speed of the chamber (^cp) It also shows that if the pumping

speed of the chamber were low, then the pressure in the chamber would be approximately

equal to the pressure in the magnetron Such low pumping speed type systems are known in

the prior art, where throttle valve mechanisms are placed in front of the pump to reduce

pumping speed. See Vossen and Kern, above, at page 156 However, if the pumping speed

in the chamber is large, as taught by this invention, then the chamber pressure becomes low

relative to the magnetron pressure

Using the equations above, chamber pressures can be determined approximately for

any new chamber with known pumping speed, as shown in Figs 6 and 7 As is clearly evident from the figures shown, any suitable desired pressure can be achieved by increasing the pumping speed of the chamber. If the operating inert gas pressure in the magnetron is

lowered, it is possible with certain magnetron types, then the entire pressure curve is correspondingly lowered. This is shown by the comparison of the pressure curve of Fig. 6 for magnetron pressure of 0.7 microns and a magnetron assembly conductance (C_M) of 3000 1/sec. with the pressure curve of Fig. 7 for a magnetron pressure of 0.4 microns and a

magnetron assembly conductance (C_M) of 3000 1/sec. The pumping speeds shown on the abscissa are quite achievable— for example, a commonly used 20" diffusion pump is rated at 17500 1/sec, and 32" diffusion pump is rated at 32000 1/sec.

It will be apparent from the above the above discussion that various additions and

modifications can be made to the optical multiplexing devices described here in detail, without departing from the true scope and spirit of this invention. All such modifications and additions are intended to be covered by the following claims.

Claims

7/4778118 CLAIMS

1. A method of depositing sputtered particles on a substrate to form a low loss optical coating, comprising the steps of: positioning the substrate in a vacuum chamber having a magnetron and source for

sputtered particles and an inert gas shroud means for partially enveloping the magnetron, the substrate having a surface facing the source at a long throw distance therefrom, operating the magnetron to sputter particles from the source for coating the substrate surface, including introducing inert gas to the shroud means;

rapidly withdrawing and depleting the inert gas from the vacuum chamber by high speed, high vacuum pump; and directing ionized reactant gas to the substrate surface to facilitate reactive coating whereby a low-loss optical coating is obtained on the substrate surface.

2 The method as claimed in claim 1 wherein the throw distance between the source and the substrates is at least 20"

3. The method as claimed in claim 1 where said source means is a compound source

means for depositing compound sputtered particles on said substrates

4 The method as claimed in claim 1 further including the step of providing a plurality of magnetrons for a plurality of sources of sputtered particles for depositing on said

substrates.

5. The method as claimed in claim 1 including the further step of rotating the substrate with respect to said chamber.

6. The method as claimed in claim 1 wherein the inert gas pressure in the chamber is maintained less than 2.0 x 10^"4 Torr and greater than 5 x 10^'5 Torr.

7. The method as claimed in claim 1 wherein the substrate surface is positioned stationary in the vacuum chamber at a position laterally offset from the magnetron.

8. An apparatus for magnetron sputtering to obtain a coating on a substrate surface, comprising: a vacuum chamber and having a magnetron system therein,

target material in the magnetron system for the formation of sputtering particles, means for introducing inert gas adjacent the target material,

means for maintaining the pressure in the chamber in the range of 5 x 10^"5 Torr to 2.0

x lO^ Torr,

means for directing ionized reactant gas to the substrate surface, and

substrate positioning means in the chamber spaced a substantial distance from the

target material, whereby the sputtered particles travel a long mean free path of at least 12"

from said target material to the substrate.

9. The apparatus as claimed in claim 8 wherein the means for directing the ionized reactant gas comprises an ion gun.

10. The apparatus as claimed in claim 8 wherein a manifold partially envelopes the magnetron system, having an opening into the vacuum chamber facing the substrate

11. The apparatus as claimed in claim 8 further including an arc reduction means