United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,958,155
`
`Kawamata et al.
`
`[45] Date of Patent:
`
`Sep.28,1999
`
`US005958155A
`
`J54] PROCESS FOR PRODUCING THIN FILM
`
`75]
`
`Inventors: Ken Kawamata; Nobuaki Mitamura,
`both of I-Iachioji, Japan
`
`73] Assignee: Olympus Optical Co., Ltd., Japan
`
`21] Appl. No.: 08/683,195
`
`22]
`
`Filed:
`
`Jul. 18, 1996
`
`Foreign Application Priority Data
`30]
`Japan ...................... ..
`Jul. 20, 1995
`pm
`
`7-18-4454
`
`Int. Cl.5 ................................................... .. C22C 23/00
`1]
`
`52] US. Cl.
`. . . . . .
`. . . .. 148/420; 420/402; 359/580;
`204/192.12; 204/192.26; 204/192.29; 204/298.09
`Field of Search ......... .. 204/192.12, 192.26~l92.29,
`204/298.09; 148/420; 420/402; 359/580
`
`58]
`
`56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3/1972 Sadagopltan ..................... .. 2t)-4/'l‘)2.26
`3,649,501
`
`6/1994 7.hang ct al.
`437/233
`5,320,984
`..
`.... .. 428/216
`12/1994 Dickey et al.
`5,372,874
`4/1996 Abe ....................................... AI 359/580
`5,513,038
`FOREIGN PATENT DOCUMENTS
`
`Japan .
`53438060 12/1978
`Japan .
`61—032242
`2/I986
`Japan .
`6l—O47645
`3/1986
`Japan .
`6l—127862
`6/1986
`Japan .
`62—1'/7168
`/1987
`Japan .
`4223401
`/1992
`OTHER PU BLICATI ONS
`
`C.W. Pitt et al, '1'/tin Solid Films, vol. 26, pp. 25-51 (1975)
`Vacuum, Dec. 1985. UK., vol. 35, No. 12, “Thin lilms
`prepared by sputtering Mgl’./sub 3/ in an Rl’ planar magen-
`tron".
`
`Journal of Applied Physics, Oct. 1993., U.S.A., vol. 74, No.
`8, “Radio frequency sputter depostiton and properties of
`calcium lluoride thin films”.
`
`Journal of Materials Science Letters, Aug. 1994, UK., vol.
`13, No. 16, Temperature dependence of sputtering yield of
`(la/\s under 30 keV ar/sup +bombardment..
`
`I’/‘i/Itrlry IiL\'(I/rIir1er~—Nam Nguyen
`/\.S'.S‘[.S'[{1I’lt [,:'.\'rm1/r1er——Steven H. VerSteeg
`/trio/'r7e_y, Agent, or Firm»—Ostrolenk, Faber, Gerb & Soffen,
`I.,I.P
`
`{$7}
`
`ABSTRACT
`
`Asubstrate (2) is rotatably installed in a vacuum chamber (1)
`at an upper part thereof MgF3 granules (3) as a lilm source
`material are placed in a quartz boat (4) and mounted on a
`magnetron cathode (5). The magnetron cathode (5) is con-
`nected through a matching box (6) to a 13.56 MH7. radio
`frequency power source (7). (Tooling water (8) for holding
`the temperature of the magnetron cathode (5) constant flows
`against a lower face of the magnetron cathode (5). A side
`wall of the vacuum chamber (1)
`is provided with gas
`introduction ports (9),
`(10)
`for
`introducing gas in the
`vacuum chamber (1). A shutter (11) is placed between the
`magnetron cathode (5) and the substrate (2). This structure
`provides a process enabling forming a thin film at a high
`speed by sputtering, especially a high speed sputtering
`process enabling forming a thin fluoride film free of light
`absorption.
`
`11 Claims, 10 Drawing Shoots
`
`
`
`Reflection(%)
`
`350400
`
`450
`
`500
`
`550
`
`600
`
`650
`
`700
`
`750
`
`800
`
`Wavelength(nm)
`
`INTEL 1109
`
`INTEL 1109
`
`

`
`U.S. Patent
`
`Sep.28, 1999
`
`Sheet 1 of 10
`
`5,958,155
`
`FIG.1
`
`1
`
`! Rotated
`5 2
`
`3
`
`,———1__
`
`|==.--_.:
`
`1 1
`
`4
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`
`box
`
`6
`
`5
`
`9
`
`10
`
`I
`Radio frequency
`power source
`
`7
`
`Reflecti0n(%)
`
`350400
`
`450
`
`500
`
`550600650 700
`
`750800
`
`Wavelength(nm)
`
`

`
`U.S. Patent
`
`Sep.28,1999
`
`Sheet 2 of 10
`
`5,958,155
`
`~65
`
`(uyux/um) 91111 uopysodaq
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`

`
`U.S. Patent
`
`Sep. 28, 1999
`
`Sheet 3 of 10
`
`5,958,155
`
`F I G . 4
`
`1.4
`
`1.39
`
`1.38
`
`1.37
`
`1.36
`
`1.35
`
`
`
`350
`
`450
`
`550
`
`650
`
`750
`
`nm
`
`F I G . 5
`
`

`
`U.S. Patent
`
`Sep. 28, 1999
`
`Sheet 4 of 10
`
`5,958,155
`
`0.)
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`Deposition
`
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`

`
`U.S. Patent
`
`Sep. 28, 1999
`
`Sheet 5 of 10
`
`5,958,155
`
`F I G . 7
`
`1
`
`Carriable
`
`1a
`
`9
`
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`
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`
`FIG.8
`
`(%)
`Reflection
`
`500
`
`550
`
`600
`
`650
`
`700
`
`750
`
`800
`
`Wavelength (nm)
`
`

`
`U.S. Patent
`
`Sep.28, 1999
`
`Sheet 6 of 10
`
`5,958,155
`
`FIGg9
`
`(%)
`Reflection
`
`400
`
`450
`
`500
`
`550
`
`600
`
`650
`
`700
`
`Wavelength (nm)_
`
`FIG;1O
`
`(%)
`Reflection
`
`O 4
`
`00
`
`450
`
`500
`
`550
`
`600
`
`650
`
`700
`
`Wavelength (nm)
`
`

`
`U.S. Patent
`
`Sep. 28, 1999
`
`Sheet 7 of 10
`
`5,958,155
`
`FIG.11
`
`Reflection
`
`(%)
`
`Wavelength (nm)
`
`FIG.12
`
`%
`
`lb
`
`-1.
`Sb 7
`
`O‘
`
`

`
`U.S. Patent
`
`Sep. 28, 1999
`
`Sheet 8 of 10
`
`5,958,155
`
`FIG.13
`
`2c
`
`1C
`
`15
`
`14
`
`11¢
`
`16
`
`9c
`
`Ar gas
`
`

`
`U.S. Patent
`
`Sep.28, 1999
`
`Sheet 9 of 10
`
`5,958,155
`
`FIG.14(a)
`
`In 02
`
` ,3‘
`
`3 EI
`
`11
`
`.5
`.5.
`
`300
`
`400
`
`500
`
`600
`
`700
`
`800
`
`Wavelength (nm)
`
`F I G. 1 4 (b)
`
`In Ar
`
`3*
`
`LU
`
`.E
`
`.5
`
`"E
`
`300
`
`400
`
`500
`
`600
`
`700
`
`800
`
`Wavelength (nm)
`
`

`
`U.S. Patent
`
`Sep.28,1999
`
`Sheet 10 of 10
`
`5,958,155
`
`F I G . 1 5 (3)
`
`Sputtering
`
`.+§‘
`
`» E.
`.5
`§
`8.1‘
`5
`
`20.0
`
`25.0
`
`30.0
`
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`
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`
`45.0
`
`50.0
`
`Angle (2 8)
`
`F I G . 1 5 ( b)
`
`Vac. evaporation
`
`intensity
`Diffraction
`
`20.0
`
`25.0
`
`30.0
`
`35.0
`
`40.0
`
`45.0
`
`50.0
`
`Angle (2 Q)
`
`

`
`1
`PROCICSS FOR PR()I)UCING THIN FILM
`
`BACKGROUNI) OI‘ 'I‘Ill.i INVENTION
`
`1. Field of the Invention
`
`The present invention relates to a process for producing a
`thin film by sputtering at
`a high speed and a thin film
`produced thereby, especially, an optical thin film such as an
`antirefiection coating film. More particularly,
`the present
`invention is concerned with a process in which a surface of
`a film source material is heated and the heated surface is
`sputtered by ions to thereby produce a thin film and a thin
`film produced thereby. Further, the present invention relates
`to an optical instrument including such a thin film.
`2. Discussion of Related Art
`
`The vacuum evaporation process has been widely
`employed in the formation of thin films, especially, optical
`thin films such as an antirellection coating film, a half
`mirror, or an edge filter because not only is the processing
`easy but also the deposition for film formation can be
`conducted at a high rate.
`In recent years, the demand for coating by sputtering in
`the formation of an optical thin film and other thin films is
`increasing because of its advantages over the vacuum evapo-
`ration process in terms of ease of automation, energy saving,
`and applicability to substrates with large surface areas.
`llowever, the sputtering process has a drawback in that
`the film formation is slower than in the vacuum evaporation
`process In the formation of a metal coating film, the sput-
`tering process is still practicable. However, in the formation
`of other coating films, the extreme slowness of lilm forina-
`tion has tended to delay the industrial spread of the sput-
`tering process. Moreover, the sputtering process has encoun-
`tered the problem that, in the sputtering of a fluoride such as
`MgF3 of low refractive index which provides a
`typical
`optical thin film, dissociation into E and species such as Mg
`occurs so that F is deficient in the coating film to thereby
`Cause the coating film to suffer from absorption of visible
`radiation.
`
`The above drawback and problem have been a serious
`obstacle in the application of the sputtering process to
`formation of optical thin films.
`For example, an invention in which the sputtering process
`has been applied to optical thin films is disclosed in Japanese
`Patent Application Laid-Open Specification No. 223401/’
`1992.
`
`In the above specification, it is disclosed that, although
`sputtering of Mg17._, per se leads to absorption of visible
`radiation, sputtering of a target composed of Mgfz doped
`with Si enables formation of a coating film of low refractive
`index substantially free of light absorption.
`In the invention of Japanese Patent Application Laid-
`Open Specification No. 223401/"1992, however, the highest
`rate of deposition for film formation is only 10 nm/min or
`less even when a radio frequency power of 500 W (2.8
`W/cmfi) is supplied to a 6-inch target. That is, the invention
`has not overcome the slow film formation drawback of the
`sputtering process.
`When the deposition rate is only 10 nm/min or less, for
`example, formation of a monolayer antirellection coating
`lilm used in the visible region would take 10 min or more.
`Thus, it is clear why the industrial spread of the disclosed
`sputtering process is difficult.
`liurther, follow—up experiments made by the instant appli-
`cant iri accordance with this prior art have demonstrated that,
`when a MgF3 plate having an Si wafer disposed thereon is
`
`5,958,155
`
`7..
`
`light absorption in the
`target and is sputtered,
`used as a
`visible region is practically negligible but a coating film with
`a refractive index of 1.4 or less cannot be formed.
`
`SUMMARY OF THE INVENTION
`
`Objects of the present invention are to provide a process
`for producing a thin film, especially a thin fluoride him free
`of light absorption, by sputtering at a high speed and a thin
`film produced thereby,, especially an optical thin film such
`as an antirellcction coating film.
`The foregoing and other objects, features and advantages
`of the present
`invention will become apparent
`from the
`following detailed description and appended claims taken in
`connection with the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the drawings:
`FIG. 1 is a schematic diagram of the structure of the
`apparatus employed in Embodiment I of the present inven-
`tion;
`FIG. 2 is a graph showing the relationship between input
`power and surface temperature measured in Embodiment I
`of the present invention;
`MG. 3 is a graph showing the relationship between
`wavelength and reflection measured in I‘.mbodnncnt
`I of the
`present invention;
`HO. 4 is a graph showing the rel:-itionship bctwccn
`wavelength and refractive index measured in liinbodii'iicnt 1
`of the present invention;
`FIG. 5 is
`a graph showing the relatioiiship between
`wavelength and absorption coeilicient measured in l‘{mbodi-
`ment
`1 of the present invention;
`FIG. 6 is a graph showing the relationship between input
`power and surface temperature measured in Comparative
`Example '1;
`FIG. 7 is a schematic diagram of the structure of the
`apparatus employed in Embodiments 3 to 5 of the present
`invention;
`FIG. 8 is a graph showing the relationship between
`wavelength and reflection measured in Embodiment 3 of the
`present invention;
`FIG. 9 is a graph showing the relationship between
`wavelength and reflection measured in Embodiment 4 of the
`present invention;
`FIG. 10 is a graph showing the relationship between
`wavelength and reflection measured in Embodiment 5 of the
`present invention;
`FIG. 11 is a graph showing the relationship between
`wavelength and reflection measured in Embodiment 7 of the
`present invention;
`FIG. 12 is a schematic diagram of the structure of the
`apparatus employed in Embodiment 9 of the present inven-
`tion;
`FIG. 13 is a schematic diagram of the structure of the
`apparatus employed in Iimbodiment 10 ofthe present inven-
`tion;
`MG. 14 ((1) and MG. 14 ([7) show plasma emission spectra
`obtained by sputtering with introduction of oxygen on the
`one hand and without introduction of oxygen on the other
`hand; and
`FIG. 15 ((1) and FIG. 15 (b) show XRD measurement data
`with respect to thin films obtained by the present invention
`on the one hand and by the conventional vacuum evapora-
`tion.
`
`tJ '1:
`
`5U
`
`U.)Ur
`
`40
`
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`
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`
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`
`

`
`5,958,155
`
`3
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`there is
`invention,
`In the first aspect of the present
`provided a process for producing a thin film which com-
`prises heating a surface of a film source material, causing
`ions to sputter the surface of the film source material so that
`at
`least a part of the film source material
`is ejected in
`molecular form and depositing the film source material in
`molecular form on a substrate to thereby form a thin film on
`the substrate.
`
`an
`
`there is
`invention,
`In the second aspect of the present
`provided a process as recited above, wherein the film source
`material is sputtered by positive ions while not only apply-
`ing an alternating voltage to an electrode having the film
`source material disposed thereon to thereby cause the elec-
`trode to have a negative potential but also applying alter—
`nating current power so as to generate plasma over the film
`source material to thereby cause the surface of the film
`source material to have its temperature raised by the plasma.
`In the third aspect of the present
`invention,
`there is
`provided a process as recited above, wherein the film source
`material is in the form of granules having an average grain
`size of 0.1 to I0 mm.
`
`invention, there is
`In the fourth aspect of the present
`provided a process as recited above, wherein the sputtering
`is conducted in an atmosphere while introducing thereinto a
`gas containing at least one of oxygen, nitrogen, and hydro-
`gen.
`there is
`invention,
`In the fifth aspect of the present
`provided a process as recited above, wherein the plasma is
`generated over the film source material by radio frequency
`power.
`there is
`invention,
`In the sixth aspect of the present
`provided a process for producing a thin film which com-
`prises providing Mglig, preferably Mgliz in the form of
`granules having an average grain size of 0.1 to 10 mm as a
`film source material, generating plasma over the Mgl“: by
`alternating current power, preferably radio frequency power,
`in an atmosphere while introducing thereinto at least one gas
`selected from the group consisting of oxygen and nitrogen
`so that
`the Mglg has its surface heated at
`a constant
`temperature ranging from 650 to 110()‘’ C. by the plasma,
`causing positive ions to sputter the MgF2 so that at least a
`part of the Mgfiz is ejected in molecular form and depositing
`the Mgl7,_ in molecular form on a substrate to thereby form
`a thin film on the substrate.
`
`In the seventh aspect of the present invention, there are
`provided thin films produced by the above processes.
`Each of these thin films is used as a monolayer antirc-
`flection coating film or combined with a layer of high
`refractive index or the like for use as an optical thin film of
`variable performance such as an antireflection coating film,
`a beam splitter, or a filter.
`In the eighth aspect of the present invention, each thin
`film has a composition of Mgfix wherein x is a number
`ranging from 1.8 to L95, and the film is not crystallized.
`In the ninth aspect of the present
`invention,
`there is
`provided an optical instrument comprising an optical part
`comprising the above thin films.
`With respect to the first aspect of the present invention,
`further description will be made below.
`it has been
`In the conventional sputtering process,
`required that collision of ions with a target break intcratomic
`bonds of the target to thereby cause atoms to be ejected from
`the target. Thus, part of the energy of the accelerated ions is
`
`30
`
`35
`
`40
`
`'1:O
`
`Jt ‘Jr
`
`(80
`
`4
`consumed in breaking interatomic bonds, so that the sput-
`tering yield is lowered with the unfavorable result that the
`film formation is slow.
`
`By contrast, in the present invention, the temperature of
`the film source material
`a target
`is raised to thereby
`weaken interatomic bonds of the target prior to the collision
`of ions with the target. Therefore, most of the energy of the
`accelerated ions is used in the sputtering, so that sputtering
`yield is enhanced with the favorable result
`that
`the film
`formation cart be very rapid as compared with that of the
`conventional sputtering process.
`Moreover,
`in the conventional sputtering process, inter-
`atomic bortds are broken to thereby cause atoms to be
`ejected from the target.
`By contrast, in the present invention, an increase of the
`temperature of the film source material excites thermal
`vibrations to thereby form zones of strong bonds and zones
`of weak bonds, so particles may be ejected from the target
`in molecular form. The terminology “molecular form” used
`herein means not only a monorriolecular form but also a
`polymolecular form constituting a cluster aggregate. The
`form of each molecule ejected from the target would be
`substantially identical with that ofeach molecule evaporated
`by heating.
`With respect to the second aspect of the present invention,
`further description will be made below.
`Sputtering the film source material by positive ions while
`applying an alternating voltage to an electrode having the
`film source material disposed thereon to thereby cause the
`electrode to have a negative potential is based on the same
`principle as in the generally known radio frequency sput-
`tering. The terminology “radio frequency" used herein
`means not only the customary 13.56 MH7. radio frequency
`but also medium frequency of tens of kilohertz (kllz).
`Although with respect to the first aspect of the present
`invention the means for heating the film source material is
`not particularly limited, that is, the heating can be conducted
`by arty of resistance heating,
`induction heating and an
`infrared heater,
`in the sputtering according to the second
`aspect of the present invention, alternating current power is
`applied to the electrode having the film source material
`disposed thereon so
`to generate plasma over the film
`source material to thereby cause the surface of the film
`source material to have its temperature raised by the plasma.
`With respect to the third aspect of the present invention,
`further description will be made below.
`When the film source material is in the form of granules,
`the temperature thereof can easily be raised because of poor
`heat conduction and centralization of electric and magnetic
`fields on a large number of edge portions present
`in the
`target.
`When the granules are too small in si'/.e, these are easily
`dislodged and are converted to particles in the vacuum
`chamber. Therefore, it is preferred that the average grain size
`of the granules be at least 0.1 mm, especially at least 0.5 mm.
`On the other hand, when the granules are too large in size,
`the adiabatic effect thereof is lowered and the number of
`edge portions is reduced to thereby decrease the effect of the
`centralization of electric and magnetic fields. Therefore, it is
`preferred that the average grain size of the granules be not
`greater than 10 mm, especially not greater than 5 mm. The
`granules do not necessarily have to be uriifortn in grain size
`and configuration.
`With respect to the fourth aspect of the present invention,
`further description will be made below.
`
`

`
`5,958,155
`
`S
`
`In optical applications, it is generally preferred that the
`light absorption of the thin film be low, Therefore, when the
`composition of the film source material is identical with that
`of the desired thin film, particles ejected from the film source
`material are preferred to be in molecular form rather than in
`atomic form resulting from complete dissociation, because
`completely dissociated matter is not always restored to the
`original.
`As a result of extensive studies, it has been found that the
`form of particles ejected from the film source material
`depends on the type of gas introduced in the sputtering. An
`inert gas such as Ar customarily employed in sputtering is
`likely to cause particles ejected from the film source material
`to break up into atomic form. On the other hand, oxygen,
`nitrogen, hydrogen and gases containing these cause such
`particles to be ejected from the film source material
`in
`molecular form without being broken up. Therefore, intro~
`duction of a gas containing at
`least one member selected
`from the group consisting of oxygen, nitrogen and hydrogen
`is preferred especially in the production of optical thin films.
`For example, FIG. 14 (a) shows a plasma emission
`spectrum in which the emission intensity is plotted against
`wavelength with respect
`to sputtering carried out while
`introducing oxygen. In the figure, not only pcalcs ascribed to
`atomic Mg and 0 but also a peak ascribed to molecular MgF
`is observed. On the other hand, FIG. 14 (b) shows a plasma
`emission spectrum in which the emission intensity is plotted
`against wavelength with respect to sputtering carried out
`while introducing argon only. In this figure, no peak ascribed
`to molecular MgF is observed.
`Oxygen, nitrogen, hydrogen, and gases containing these
`would be used only in the sputtering of the film source
`material, and the probability is nearly zero that they combine
`with the film source material. Therefore, the optical thin film
`produced by the sputtering while introducing such gases
`would have a composition substantially identical with that of
`the thin film formed on a substrate by heating the film source
`material to thereby evaporate the same.
`There is no particular limitation on the film source mate-
`rial. Basically, any kind of film source material can be used.
`A suitable film source material can be selected according to
`use. For example, a fluoride, an oxide, a nitride, or a sulfide
`can be employed in the formation of an optical thin film.
`With respect to the fifth and sixth aspects of the present
`invention, further description will be made below.
`When the lilm source material
`is a conductor such as a
`metal, both direct current power and alternating current
`power can be used for generating plasma over the film
`source material
`to thereby conduct sputtering.
`llowever,
`when the film source material is an insulator such as Mglg,
`alternating current power must be used.
`It
`is preferred to
`employ radio frequency power from amongst alternating
`current power varieties. The reason is that the use of radio
`frequency power produces stronger negative bias on the film
`source material because of the difference in mobility
`between electrons and positive ions.
`Therefore, in the sputtering conducted while generating
`plasma over the film source material by radio frequency
`power, not only a conductor such as a metal but also a
`dielectric substance can be used as the film source material.
`In the sputtering of MgF: especially suitable for forma-
`tion of an optical thin film, the rate of deposition for film
`formation can be very high as long as the temperature of the
`surface of the target is 650° C. or higher.
`On the other hand, when the temperature is ll(l0° (.7. or
`higher,
`the vapor pressure of the film source material
`is
`
`LII
`
`10
`
`It)
`
`toU:
`
`beUr
`
`40
`
`St)
`
`'J\‘Jr
`
`60
`
`6
`increased so as to be as high as the pressure ofthe introduced
`gas, with the result that evaporated molecules directly reach
`the substrate In that case, there would be no difference. from
`the customary vacuum evaporation. The vacuum evapora-
`tion would provide a thin film of poor scuff resistance,
`different from that obtained by the sputtering process.
`In the customary vacuum evaporation of MgF2 the sub-
`strate must be heated up to about 300° C. because otherwise
`the thin film would have extremely poor scuff resistance and
`thus would not be able to serve in practical use. However, a
`thin film of high scuff resistance can be obtained irrespective
`of the temperature of the substrate when the temperature of
`the surface of the target is held at ll()(l° (T. or below in the
`present process. Among the above introduced gases, oxygen
`and nitrogen are preferred from the viewpoint of economy,
`availability, and safety.
`Occasionally,
`the selllsputtering phenomenon would
`occur in which molecules ejected from the target material
`collide with electrons present in the plasma to thereby form
`positive ions and thereafter collide with the target to thereby
`effect the sputtering. When the film source material is Mgl-'3‘,
`the form of particles ejected from the target by the self-
`sputtering would be molecular, so that
`the phenomenon
`would not cause any particular problem.
`With respect to the seventh to ninth aspects of the present
`invention, further description will be made below.
`The thus produced optical thin film is nearly stoichiomet—
`ric and is substantially free of light absorption in the visible
`region, and its refractive index is approximately I38.
`Therefore, this optical thin film has a satisfactory antirellec-
`tive effect even if it is in the form of a monolayer and can
`be used as an antireflection coating film in optical parts and
`in struments such as a
`lens,
`a prism, optical
`fibers,
`spectacles, sunglasses and goggles, displays such as a cath-
`ode ray tube and a liquid crystal device, window materials,
`screens, etc. Moreover, this thin film can be combined with
`a film of high refractive index so as to form a multilayer
`structure, thereby providing an antireflection coating film of
`superior performance or an optical thin film such as a half
`mirror or an edge filter.
`the
`is not necessary to heat
`it
`As described above,
`substrate in the present invention, so that there is no par-
`ticular limitation on the material of the substrate. That is, the
`substrate can be composed of any material, e.g., a glass such
`as an optical glass or a window glass, any of various resins
`such as PM MA, polycarbonate and polyolelins, a metal or a
`ceramic. The shape of the substrate is also not particularly
`limited and it may be film—lil<c, spherical, or plate—lil\'e.
`l£l5I*'l.i(."l" OI“ 'l'l*Ili lNVl.iN'l‘l()N
`
`The process according to the first to sixth aspects of the
`present
`invention has the elliect
`that a solid lilm source
`material is heated in advance and sputtered, so that most of
`the energy of accelerated ions is used in the sputtering,
`thereby rendering the sputtering yield high. As a result, the
`film formation can be much faster than in the conventional
`process. Moreover, in the present invention, an increase of
`the temperature of the film source material excites thermal
`vibration to thereby form zones of strong bonds and zones
`of weak bonds, so that particles are ejected from the target
`in molecular form. Therefore, a thin film of substantially the
`same composition as that of the starting film source material
`can be formed without suffering from dissociation even if
`the film source material is one which is completely disso-
`ciated in the customary sputtering. The process of the
`present invention is especially efiective in the formation of
`
`

`
`5,958,155
`
`an optical thin film of a lluoride such as MgI’,_ according to
`the sputtering process and enables easily obtaining a coating
`film of low refractive index which is free of light absorption.
`
`PREFERRED EMBODIMENT OF THE
`INVENTION
`
`'.I\
`
`invention will now be described in greater
`The present
`detail with reference to the following Embodiments and
`Comparative Examples, which should not be construed as
`limiting the scope of the invention.
`Embodiment 1
`1 denotes a vacuum chamber. A
`In FIG. 1, numeral
`substrate 2 was rotatably set in the vacuum chamber 1 at an
`upper part thereof. MgF2 granules 3 oi’ I
`to 5 mm in grain
`size as a film source material were placed in a quartz boat 4
`and mounted on a magnetron cathode 5 of 4 inches (about
`100 mm) in diameter.
`The magnetron cathode 5 was connected through a match-
`ing box 6 to a 13.56 Mlelz radio frequency power source 7.
`Cooling water 8 maintained at 20:(l.5° C. was caused to
`flow on a lower face of the magnetron cathode 5 so that the
`temperature of the magnetron cathode 5 was held constant.
`A side wall oi‘ the vacuum chamber 1 was provided with gas
`introduction ports 9, 10 for introducing gas in the vacuum
`chamber 1. liurther, a shutter 11 was provided between the
`magnetron cathode 5 and the substrate 2. The substrate 2 was
`not provided with a heater and was not heated at all.
`For the production of a thin tilm with the use of the
`apparatus of the above structure, an La optical glass of
`refractive index of 1.75 was employed as the substrate 2, and
`the vacuum chamber 1 was evacuated to 7x10“5 Pa. Then,
`03 gas was introduced through the gas introduction port 9
`into the vacuum chamber 1 so that the internal pressure of
`the vacuum chamber 1 was 4><10‘1 Pa. Subsequently,
`the
`magnetron cathode 5 was supplied with power by the radio
`frequency power source 7, thereby generating plasma. The
`Mgliz granules 3 were heated by the plasma with their
`temperature maintained by a balance between plasma heat-
`ing and cooling by cooling water 8 llowing on the lower lace
`of the magnetron cathode 5 and thus sputtered.
`In this situation, the substrate 2 was rotated and the shutter
`11 was opened while continuing the rotation, so that a Mgliz
`film was formed on the substrate 2. When the optical film
`thickness increased to 130 nm, the shutter 11 was closed.
`Plasma emission spectra obtained during the film forma-
`tion were analyzed with respect to the wavelength. When the
`input power was 400 W or higher, emission was recognized
`not only from Mg atoms but also from MgF molecules.
`Thus, it was confirmed that at least part of the film source
`material was ejected (sputtered) in molecular form.
`FIG. 2 shows what changes of the surface temperature of
`granules 3 and the rate of film formation on the substrate 2
`are brought about by changes of the input power. When the
`input power is 400 W or higher, it
`is seen that
`the surface
`temperature of granules 3 rises to about 650° C. or higher
`with the result
`that
`the rate of film formation is rapidly
`increased. When the input power is 800 W,
`the surface
`temperature of granules 3 rises to about 1100” (,7.
`The coating films formed with 400 to 800 W input power
`were analyzed by EPMA. As a result, it was found that, with
`respect to the component proportion, Mg: 17 was 111.8 to 1.95
`and the F content was increased with the increase of the
`input power. Further,
`it was confirmed that oxygen intro-
`duced as a process gas was substantially not present in the
`coating films. Moreover, as a result of l4T—IR analysis, the
`Mg—l" bond was observed but
`the Mg~—O bond was not
`observed in the coating films.
`
`31)
`
`1.4Ut
`
`4t)
`
`60
`
`(35
`
`8
`Still further, XRD analysis was conducted. ("rystallization
`was hardly observed and no clear peak ascribed to crystals
`was recognized. FIG. 15 shows XRI) measurement data.
`FIG. 15 (a) shows that the crystallization of the thin films of
`this Embodiment is slight, while FIG. 15 (b) shows that a
`thin film formed by the customary vacuum evaporation
`process has peaks ascribed to crystalli/.ation.
`Each coating film was subjected to a tape peeling test in
`which a cellophane tape was adhered to the coating lilm and
`was peeled off at an angle of 90°. With respect to all
`the
`coating films produced in varied conditions, peeling of the
`coating filnt from the substrate did not occur. Further, each
`coating lilm was subjected to a scull‘ resistance test in which
`the coating tilm was rubbed with a lens cleaning paper
`wetted with alcohol and the surface of the rubbed coating
`film was visually inspected. In this test, the coating films
`produced with less than 800 W input power were completely
`free of flaws but the coating film produced with 800 W input
`power was observed to have slight flaws. Further, with
`respect
`to the coating film produced with 900 W input
`power, it easily peeled from the substrate.
`With respect to an antirellection coating film produced by
`the above procedure oi‘ this Embodiment, the spectral rcllec—
`tion was measured and also the refractive index n and
`absorption coellicient k were measured by spectroellipsom~
`etry. Measurement results are shown in FIGS. 3 to 5.
`The reflection dropped to 0.2% or less at
`the central
`wavelengths, so that it can be stated that the coating him has
`excellent antirefiection properties.
`The refractive index n was about 1.38 and the absorption
`coctlicicnt k was not greater than 10"", so that it can be
`stated that these are at a practical level from the viewpoint
`of the use of the coating film as an optical film of low
`refractive index.
`Similar results were obtained as long as use was made of
`granules having a grain size ranging from 0.1 to 10 mm, and
`no problems were observed in this Embodiment.
`Antirellection coating films with excellent optical char-
`acteristics and durability were obtained as long as the
`pressure of introduced oxygen (02) gas ranged from 5><l(t':i
`to 5x100 Pa, although the required input power had to be
`slightly varied.
`
`Comparative Example I
`
`A similar experiment was conducted with the use of
`sintered MgF2 in place of MgF2 granules 3. FIG. 6 shows the
`relationships between input power and surface temperature
`and between input power and measured rate of deposition.
`With the use of sintered Mglg, dillerent from the use of
`granules,
`the heating thereof was not effective and the
`temperature rise was slight irrespective of the employment
`of high input power. The sputtering etliectively was the same
`as conventionally carried out and the rate of deposition for
`film formation was too low to be practicable. l“or example,
`although the film formation took only 18 sec with 600 W
`input power in limbodiment
`I,
`ll min was required for
`obtaining the same lilm thickness in this (‘omparative
`Example.
`The thin film produced in this Comparative Example had
`absorption in