United States Patent
`
`
`Kobayashi et al.
`
`[19]
`
`
`
`
`
`
`[11] Patent Number:
`
`
`
`[45] Date of Patent:
`
`4,948,482
`
`
`Aug. 14, 1990
`
`
`
`
`[75]
`
`
`
`
`
`[54] METHOD FOR FORMING SILICON
`
`
`
`
`
`NITRIDE FILM
`
`
`Inventors: Masato Kobayaslii; Yoichi
`
`
`
`
`Yamaguchi, both of Tokyo, Japan
`
`
`
`[73] Assignee: Hoya Corporation, Tokyo, Japan
`
`
`
`
`
`[21] App]. No.: 287,017
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`
`
`
`[22] Filed:
`Dec. 21, 1988
`
`
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`
`
`[30]
`Foreign Application Priority Data
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`
`
`
`
`Dec. 29, 1987 [JP]
`Japan ................................ 62-333733
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`Int. CL5 .............................................. C23C14/34
`[51]
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`[52] US. Cl. ............ ..
`204/192.23;437/241
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`[58] Field of Search ................... 204/192.23; 437/241;
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`428/428, 446
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`
`[56]
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`
`
`Referenc Cited
`
`
`FOREIGN PATENT DOCUMENTS
`
`
`
`59-114829 7/1984 Japan ................................... 437/241
`
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`60-12737
`1/1985 Japan .
`. 437/241
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`
`62-81033 4/1987 Japan ................................... 437/241
`
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`63-132433
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`6/1988 Japan ...................................437/241
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`
`
`OTHER PUBLICATIONS
`
`
`
`
`
`
`
`
`
`S. M. Hu et al., J. Electrochem. Soc., vol. 114, pp.
`
`
`826-833 (1967).
`
`
`
`Primary Examiner—Aaron Weisstuch
`Attorney, Agent, or Firm—Nixon & Vanderhye
`
`
`
`
`ABSTRACT
`
`
`Silicon nitride films are formed by controlling the inter-
`
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`
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`
`
`
`
`nal stress more precisely than conventional methods
`
`
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`
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`
`
`
`
`
`
`
`
`without varying its optical properties, mechanical
`strength, composition and density. The film is formed
`
`
`
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`
`
`by sputtering, using an inert gas or a mixture of an inert
`
`
`
`
`
`
`
`gas and nitrogen, onto a substrate while keeping the
`
`
`
`
`
`
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`
`substrate temperature within a given temperature range
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`
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`
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`according to the pressure of the sputtering gas or gas
`
`
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`
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`
`
`
`
`
`
`
`
`
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`mixture, the two being interrelated, thus carefully and
`precisely controlling the internal stress of the film
`
`
`
`
`
`
`
`
`formed.
`
`
`[57]
`
`
`
`7 Claims, 2 Drawing Sheets
`
`
`
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`
`
`-0- O.4Po
`
`-0- O.5Pa
`-0- O.6Po
`
`‘A’ 0.65130
`
`‘I-
`
`-|'_'l- 0,? Pa
`
`-5- 0.8 Pa
`
`100
`
`200
`
`300
`
`
`
`
`SUBSTRATE TEMPERATURE
`
`
`
`
`
`
`(°C)
`
`
`
`+ 01O
`
`+ NO 1
`
`:7
`_ EU
`:>.
`‘UW
`
`\v
`
`9X
`
`
`STRESSCOMPRESSIVETENSILE 5
`INTERNAL
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`T
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`SAMSUNG ET AL. EXHIBIT 1047
`Page 1 of 6
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`

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`US. Patent
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`
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`Aug. 14,1990
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`
`Sheet 1 of2
`
`
`I
`
`FIG.
`
`4,948,482
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`
`
`
`
`
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`INTERNALSTRESS(XlO8dyn/CFTIZ)
`
`
`
`-0- O.4Pa
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`
`
`fir O.65Pc1
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`
`
`
`-I |.OPa
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`
`
`
`
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`-0- O.5Pa
`
`
`
`
`U 0.7 PCI
`
`
`-0- O.6Pa
`
`
`
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`-T} 0.8 PCI
`
`
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`
`
`COMPRESSIVETENSILE
`
`
`
`
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`SUBSTRATE TEMPERATURE
`
`
`
`(°C)
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`
`
`
`
`SAMSUNG ET AL. EXHIBIT 1047
`Page 2 of 6
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`

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`4 US. Patent
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`_Aug.14, 1990
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`Sheet 2 of2
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`4,948,482
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`
`FIG. 2
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`L
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`100
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`
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`L.
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`200
`
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`REFRACHVEINDEX
`
`
`I
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`
`SUBSTRATE TEMPERATURE
`
`300
`
`
`
`400
`
`
`
`PC)
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`
`
`SAMSUNG ET AL. EXHIBIT 1047
`Page 3 of 6
`
`

`
`1
`
`
`METHOD FOR FORMING SILICON NITRIDE
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`
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`
`
`FILM
`
`
`4,948,482
`
`
`
`
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`2
`
`obtained. For example, in the substrate dissolution step
`
`
`
`
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`
`
`in the production of an X-ray lithography mask using a
`
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`silicon nitride film as an X-ray transmission film, partial
`
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`dissolution of the silicon nitride film takes place,
`
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`
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`thereby allowing the film to have flaws. Further, the
`
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`
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`impurities in silicon nitride films are easily eliminated by
`
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`
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`the application of an ionizing radiation, thereby causing
`
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`a change in the composition, optical transparency and
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`physical properties of the films.
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`As stated above, in conventional methods for forming
`
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`a silicon nitride film, it has been very difficult to control
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`the internal stress of the film.
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`SUMMARY OF THE INVENTION
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`An object of the present invention is to provide a
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`method for forming a silicon nitride film with a highly
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`controlled internal stress.
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`Other objects will be apparent from the following
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`description and drawings.
`The present invention resides in a method for forming
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`a silicon nitride film which comprises depositing a sili-
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`con nitride film on a substrate by a sputtering method
`
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`using, as a sputtering gas, an inert gas or a mixed gas of
`
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`an inert gas and nitrogen gas, said method further com-
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`prising, during the deposition of said silicon nitride fihn,
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`keeping the substrate temperature at a given tempera-
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`ture range appropriate for the pressure of the sputtering
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`gas to control the internal stress of the film formed.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG. 1 is a graph showing the change of the internal
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`stress of a silicon nitride film as a function of the change
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`of the substrate temperature in the Example of the pres-
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`ent invention.
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`
`FIG. 2 is a graph showing the change of the refrac-
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`tive index of a silicon nitride film as a function of the
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`change of the substrate temperature in the Example of
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`
`the present invention.
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`
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`DETAILED DESCRIPTION OF TIDE
`
`
`INVENTION
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`The present invention utilizes the fact that in deposit-
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`ing a silicon nitride film on a substrate by a sputtering
`
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`
`method using, as a sputtering gas, an inert gas or a
`
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`mixed gas of an inert gas and nitrogen gas, the internal
`
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`stress of the film formed can be controlled by keeping
`
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`
`the substrate temperature at a given temperature range
`
`
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`appropriate for the pressure of the sputtering gas used.
`
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`In the present method, the substrate temperature can
`
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`be controlled precisely and the change of the internal
`
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`stress of the silicon nitride film as a function of the
`
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`change of the substrate temperature is small; therefore,
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`the internal stress of the film can be controlled pre-
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`cisely. Further, the change of the substrate temperature
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`
`causes no change in properties of the silicon nitride film
`
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`other than internal stress, such as refractive index, com-
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`
`position, visible light transmission and the like. It is
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`preferred, however, that film deposition be effected at a
`
`
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`low sputtering gas pressure in order to prevent the
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`
`trapping of impurities in the film and at a high substrate
`
`
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`temperature in order to obtain a film of high chemical
`
`
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`
`
`
`stability.
`
`The present invention is illustrated in more detail by
`
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`
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`way of Example. However, the present invention is in
`
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`no way restricted to the Example.
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`
`
`BACKGROUND OF THE INVENTION
`
`
`1. Field of the Invention
`
`
`
`
`The present invention relates to a method for forming
`
`
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`
`
`
`a silicon nitride film used as, for example, a protective
`
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`
`film for semiconductor chips or memory disks and an
`
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`X-ray transmission film. More particularly, the present
`
`
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`
`
`invention relates to a method for forming a silicon ni-
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`
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`
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`tride films with a highly controlled internal stress.
`
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`
`
`2. Description of the Prior Art
`
`
`
`
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`Conventionally, the CVD method and the sputtering
`
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`
`
`
`method have often been used for formation of a silicon
`
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`
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`nitride film. The silicon nitride film formed by the CVD
`
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`method, however, has had the following drawbacks.
`Firstly, there are used, as the raw material gases, a sili-
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`con compound such as a silicon hydride (e.g. silane
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`SiH4), a silicon fluoride (e.g. SiF4) or a silicon chloride
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`(e.g. SiCl4), ammonia (NH3) and nitrogen (N2); that is,
`the easily decomposable raw material gases contain not
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`only silicon and nitrogen which are constituent ele-
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`ments of silicon nitride (SixN_y) but also other elements;
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`consequently, the silicon nitride films formed by the
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`CVD method inevitably contains impurities in princi-
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`ple. The silicon nitride films containing impurities be-
`sides silicon and nitrogen largely vary in internal stress
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`depending upon the content of the impurities. In order
`
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`to precisely control the impurities content in the silicon
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`nitride films, it is necessary to always make the film
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`formation (deposition) conditions constant; that is, in
`
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`the thermal CVD method, for example, it is required to
`
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`always make constant the deposition temperature, gas
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`composition, gas flow rate and gas pressure. In the
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`plasma CVD method, not only the deposition tempera-
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`ture, gas composition, gas flow rate and gas pressure but
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`also the plasma state must be made constant. To always
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`keep these parameters constant is extremely difficult
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`and the impurities content in the silicon nitride cannot
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`be kept constant. Thus, the precise control of internal
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`
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`stress of silicon nitride films has been impossible in the
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`
`CVD method. Secondly, the impurities in the silicon
`
`
`
`
`
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`
`
`nitride films significantly reduce the chemical stability
`
`
`
`
`
`
`
`of the films. For example, in the substrate dissolution
`
`
`
`
`
`
`
`
`step in the production of an X-ray lithography mask
`
`
`
`
`
`
`
`using a silicon nitride film as an X-ray transmission film,
`
`
`
`
`
`
`
`the partial dissolution of the silicon nitride film takes
`
`
`
`
`
`
`
`
`
`place, thereby allowing the film to have flaws. Further,
`
`
`
`
`
`
`
`
`the impurities in silicon nitride films are easily elimi-
`
`
`
`
`
`
`
`
`nated by the application of an ionizing radiation,
`
`
`
`
`
`
`
`
`thereby causing a change in the composition, optical
`
`
`
`
`
`
`transparency and physical properties of the film.
`
`
`
`
`
`
`Meanwhile, the conventional sputtering method for
`
`
`
`
`
`
`forming silicon nitride films has had the following prob-
`
`
`
`
`
`
`
`
`
`lems. Firstly, in the conventional sputtering method, the
`
`
`
`
`
`
`
`internal stress of the silicon nitride films is controlled by
`
`
`
`
`
`
`
`the pressure of the sputtering gas used. In this case, the
`
`
`
`
`
`
`
`
`
`precise control of the internal stress is impossible be-
`
`
`
`
`
`
`cause the internal stress is greatly changed even by the
`
`
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`
`
`
`
`
`slight change of the gas pressure, and the control of the
`
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`
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`
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`
`
`internal stress has been possible only in the order of, for
`
`
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`
`
`
`
`
`
`example, about l0xl03 dyn/cmz. Secondly, the gas pres-
`
`
`
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`
`
`
`sure must be fairly large (at least 10 Pa) in order for the
`
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`internal stress to be a tensile stress; use of a large gas
`
`
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`
`pressure incurs trapping of impurities (e.g. hydrogen,
`
`
`
`
`
`
`
`oxygen) in silicon nitride; these impurities significantly
`
`
`
`
`
`
`reduce the chemical stability of the silicon nitride film
`
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`
`SAMSUNG ET AL. EXHIBIT 1047
`Page 4 of 6
`
`

`
`3
`
`
`EXAMPLE
`
`4,948,482
`
`
`A silicon nitride film of 2 pm in thickness was depos-
`
`
`
`
`
`
`
`ited on a silicon substrate by a rf magnetron sputtering
`
`
`
`
`
`method. The sputtering target was a single crystal sili-
`
`
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`
`
`
`
`
`con target. The sputtering gas was a mixed gas of Ar gas
`
`
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`
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`
`
`(an inert gas) and N2 gas. The flow rate of this mixed gas
`
`
`
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`
`
`was constant (Ar gas flow rate= 12.0 sccm (cc at stan-
`
`
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`
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`
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`dard condition/min), N2 gas flow rate=4.0 sccm, ratio
`
`
`
`
`
`
`of N2 gas to total gas flow rate (N2/(Ar+N2))=0.25).
`
`
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`
`
`
`The internal stress of silicon nitride film was controlled
`
`
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`
`
`by the control of the substrate temperature. In this Ex-
`
`
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`ample, the control of the substrate temperature was
`
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`made by fixing an electric heating wire to a plate for
`
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`holding the substrate so as to ensure uniform heating of
`
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`the substrate and supplying an electric current to the
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`heating wire. The rf power was constant at 16.72
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`W/cmz but there were employed various sputtering gas
`
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`pressures, i.e. 0.4 Pa, 0.5 Pa, 0.6 Pa, 0.65 Pa, 0.7 Pa, 0.8
`
`
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`Pa and 1.0 Pa.
`
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`The change of internal stress of silicon nitride film by
`
`
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`the change of substrate temperature is shown in FIG. 1.
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`
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`
`
`Incidentally, the internal stress was measured by the
`
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`Newton rings method.
`
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`
`In the case of sputtering gas pressure=0.4 Pa, the
`
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`internal stress was a compressive stress at a substrate
`
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`temperature of 100‘ C.; and the compressive stress in-
`
`
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`creased gradually as the substrate temperature was in-
`
`
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`creased. In the case of sputtering gas temperature =0.5
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`Pa, the internal stress was a tensile stress at a substrate
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`temperature of 100° C.;
`the tensile stress decreased
`
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`gradually as the substrate temperature was increased:
`
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`
`and the internal stress transform from a tensile stress to
`
`
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`
`a compressive stress at a substrate temperature of 290‘
`
`
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`C. The compressive stress showed a gradual increase
`
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`
`with the further increase of the substrate temperature.
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`In this case of 0.5 Pa, since the internal stress changes
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`linearly with the change of the substrate temperature,
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`the internal stress can be controlled at a precision of
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`40
`0.5x103 dyn/cm2 for a substrate temperature change of
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`10' C. In the case of sputtering gas pressure=0.6 Pa, the
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`internal stress was a tensile stress at a substrate tempera-
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`ture of 100" C.; the tensile stress increased gradually as
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`the substrate temperature was increased and became a
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`maximum at a substrate temperature of 200° C; and with
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`the further increase of the substrate temperature, the
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`tensile stress decreased gradually. In the case of sputter-
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`ing gas pressure=O.8 Pa, the internal stress was a tensile
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`stress at a substrate temperature of 100° C.; and the
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`tensile stress increased gradually as the substrate tem-
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`perature was increased. As seen from the above, the
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`change of internal stress as a function of the change of
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`substrate temperature difl'ers according to the pressure
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`level of the sputtering gas employed. As is clear from
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`FIG. 1, however, the change of internal stress as a func-
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`tion of the change of substrate temperature is very small
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`when the sputtering gas pressure, is less than 1.0 Pa.
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`Accordingly,
`the control of internal stress by the
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`change of substrate temperature provides an excellent
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`method for the control of internal stress. Moreover, the
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`internal stress obtained by this method has an excellent
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`reproducibility.
`As is clear from FIG. 1, when the sputtering gas
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`pressure is 1.0 Pa, the internal stress changes more
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`diamatically as a function of the change of substrate
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`temperature than when the sputtering gas pressure is
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`less than 1.0 Pa, making the control of internal stress by
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`the control of substrate temperature more difficult.
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`4
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`Nevertheless, the above control of internal stress by the
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`control of substrate temperature is far superior to the
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`conventional methods for the control of internal stress
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`by, for example, sputtering gas pressure alone.
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`The silicon nitride film formed by the above sputter-
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`ing method can be used as an X-ray transmission film
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`for an X-ray lithography mask. This X-ray transmission
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`film preferably has an internal stress of 10.0x103
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`dyn/cm2 or less in terms of tensile stress. According to
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`the preferable substrate temperature range
`FIG. 1,
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`which enables the production of a silicon nitride film
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`(an X-ray transmission film) having an internal tensile
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`stress of l0.0xl03 dyn/cm2 or less is about 100° C. to
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`about 290° C. in the case of sputtering gas pressure=0.5
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`Pa, about 340' C. to about 380‘ C. in the case of sputter-
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`ing gas pressure=0.6 Pa, and about 100° C. to about
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`170° C. in the case of sputtering gas pressure=O.8 Pa.
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`Especially in the case of sputtering gas pressure=O.5
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`the internal stress can be controlled at 5x103
`Pa,
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`dyn/cm1 or less in terms of tensile stress over a wide
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`substrate temperature range of 200-290‘ C.; the change
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`of internal stress as a function of the change of substrate
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`temperature is linear; and the internal stress can be con-
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`trolled at a precision of 0.5x1O3 dyn/cm? for a substrate
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`temperature change of 10‘ C. Hence, the sputtering gas
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`pressure is most preferably 0.5 Pa or its vicinity (e.g.
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`0.45-0.55 Pa) when the silicon nitride film formed is
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`used as an X-ray transmission film.
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`Shown in FIG. 2 is the change of refractive index of
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`silicon nitride film as a function of the change of sub-
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`strate temperature. As seen in FIG. 2, the refractive
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`index of silicon nitride film has substantially no depen-
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`dency on the substrate temperature and was 2.0 for both
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`cases of sputtering gas pressure=0.5 Pa 0.6 Pa. In addi-
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`tion, these silicon nitride films were transparent in the
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`visible region.
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`The Fourier transform infrared absorption spectra of
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`the above silicon nitride films confirmed that the films
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`contained no impurities. Therefore, when the films are
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`applied to an X-ray lithography mask, there occurs no
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`mask strain, no compositional change, no reduction in
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`optical transparency and no change in physical proper-
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`ties by the application of an ionizing radiation.
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`When film formation was conducted at substrate
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`temperatures of 200' C. or more, the resulting films
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`showed significant improvement of chemical stability as
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`well as improvement of optical property and mechani-
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`cal strength. The silicon nitride sample formed at a
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`substrate temperature of 100' C. and the silicon nitride
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`sample formed at a substrate temperature higher than
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`200° C. were immersed in a 50% NaOH solution at 100"
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`C. for 3 hours; the former sample dissolved partially
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`followed by film breakage while the latter sample saw
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`no change. Hence, in order to obtain a silicon nitride
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`film with satisfactory internal stress and chemical stabil-
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`ity, the preferable substrate temperature is 200° C. to
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`290' C. in the case of 0.5 Pa and 340° C. to 380° C. in the
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`case of 0.6 Pa.
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`In the above method for the control of internal stress
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`of silicon nitride films by the control of substrate tem-
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`perature, the properties of the formed films other than
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`chemical stability, for example, the film composition
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`and density, were confirmed to exhibit no change.
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`The above Example can be modified as follows.
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`In the above Example, the Si target was used as a
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`sputtering target and the mixed gas of Ar gas and N2 gas
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`was used as a sputtering gas. It is possible to use, as a
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`sputtering target, a SixN_y target of desired composition
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`15
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`50
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`SAMSUNG ET AL. EXHIBIT 1047
`Page 5 of 6
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`4,948,482
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`5
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`and, as a sputtering gas, only an inert gas such as Ar gas
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`or the like. It is also possible to use a substrate other
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`than Si substrate, i.e. a SiO2 substrate (glass wafer).
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`The present method for forming films silicon nitride
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`film can control the internal stress of the formed silicon
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`nitride more precisely than the conventional methods.
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`Further, the present method can effect the precise
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`control of the internal stress of the films without vary-
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`ing the optical properties, mechanical strength, compo-
`sition aud density. Accordingly, the present method has
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`excellent practical uses.
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`The silicon nitride films obtained by the present
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`method are transparent in a visible region and have
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`excellent chemical stability and mechanical strength,
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`and therefore are suitable for use as an X-ray transmis-
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`sion film for X-ray lithography mask.
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`The above-described embodiments are just an exam-
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`ple of the present invention, and therefore, it will be
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`apparent for those skilled in the art that many modifica-
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`tions and variations may be made without departing
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`from the scope of the present invention.
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`What is claimed is:
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`1. A method for forming a silicon nitride film which
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`comprises depositing a silicon nitride film having a
`preselected amount of internal stress on a substrate by a
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`sputtering method using, as a sputtering gas, an inert gas
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`or a mixed gas of an inert gas and nitrogen gas, said
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`method further comprising, during the deposition of 30
`said silicon nitride flm, keeping the substrate tempera-
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`ture at a given temperature range according to the pres-
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`sure of the sputtering gas to control the internal stress of
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`6
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`the film formed substantially to said preselected
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`amount.
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`2. A method according to claim 1, wherein the sub-
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`strate temperature is kept in the range of about 100° C.
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`to 400° C. when the pressure of the sputtering gas is less
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`than 1.0 Pa.
`.
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`3. A method according to claim 2, wherein the pres-
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`sure of the sputtering gas is 0.5 Pa or its vicinity.
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`4. A method according to claim 2, wherein the range
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`of the substrate temperature is about 100° C. to about
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`290° C. in the case of the pressure of sputtering gas of
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`0.5 Pa, about 340° C. to about 380' C. in the case of the
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`pressure of the sputtering gas of 0.6 Pa and about 100°
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`C. to about 170° C. in the case of the pressure of the
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`sputtering gas of 0.8 Pa.
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`5. A method according to claim 4, wherein the sub-
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`strate temperature is in the range of 200° C. to 290° C.
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`in the case of the pressure of the sputtering gas of 0.5 Pa.
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`6. A method according to claim 1, wherein said sub-
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`strate is a Si substrate or a Si02 substrate.
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`7. A method of forming a silicon nitride film on an Si
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`or SiO2 substrate and carefully controlling the internal
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`stress of the silicon nitride film so formed to a prese-
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`lected value, comprising sputtering, using an inert gas
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`or a mixture of an inert gas and nitrogen gas, as the
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`sputtering gas, wherein during sputtering, while the
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`silicon nitride film is being formed, maintaining the
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`substrate at a temperature in the range of about 100° C.
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`to 400° C. and the pressure of the sputtering gas less
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`than 1.0 Pa and thereby controlling the internal stress of
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`the silicon nitride film thus formed substantially to said
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`preselected value.
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`I
`V
`3
`1
`i
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`SAMSUNG ET AL. EXHIBIT 1047
`Page 6 of 6