`Kobayashi et a].
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,948,482
`Aug. 14, 1990
`
`.
`
`.,
`
`.
`
`, pp.
`
`63-132433 6/1988 Japan ................................. .. 437/241
`OTHER PUBLICATIONS
`S M Hu et al J Electrochem Soc vol 114
`826_833 (1967).
`Primary Examiner-Aaron Weisstuch
`Attorney, Agent, or Firm—Nix0n & Vanderhye
`[57]
`ABSTRACT
`Silicon nitride ?lms are formed by controlling the inter
`nal stress more precisely than conventional methods
`without varying its optic 31 properties, mechanical
`strength, composition and density. The ?lm is formed
`by sputtering, using an inert gas or a mixture of an inert
`gas and nitrogen, onto a substrate while keeping the
`substrate temperature within a given temperature range
`according to the pressure of the sputtering gas or gas
`mixture, the two being interrelated, thus carefully and
`precisely controlling the internal stress of the film
`fmmed
`
`.
`
`.
`
`.,
`
`.
`
`[54] METHOD FOR FORMING SILICON
`NITRmE FILM
`[75] Inventors: Masato Kobayashi; Yoichi
`Yamaguclu, both of Tokyo, Japan
`[73] Ass1gnee: Hoya Corporation, Tokyo, Japan
`[21] Appl. N0.: 287,017
`[22] Filed:
`Dec. 21, 1988
`[30]
`Foreign Applicg?on Priority Data
`
`.
`
`Japan ................. .. 62-333733
`Dec. 29, 1987 [JP]
`[51] Int. Cl.5 ......................................... .. C23C 14/34
`[52] US. Cl. ................... .. 204/ 192.23; 437/241
`[58] Field of Search ................. .. 204/ 192.23; 437/241;
`428/428, 446
`
`[56]
`
`.
`Referenca Clted
`FOREIGN PATENT DOCUMENTS
`59-114829 7/1984 Japan ................................. .. 437/241
`60-12737 l/ 1985 Japan .................... ..
`62-81033 4/1987 Japan ................................. .. 437/241
`
`7
`
`2 Drawing Sheets
`
`
`
`INTERNAL STRESS
`
`COMPRESSIVE 5 o
`
`o
`
`400
`300
`200
`100
`SUBSTRATE TEMPERATURE (°C)
`
`MICRON ET AL. EXHIBIT 1047
`Page 1 of 6
`
`
`
`US. Patent Aug. 14, 1990
`
`Sheet 1 of2
`FIG.
`
`4,948,482
`
`:53 559 V:
`
`0- O4Pc|
`
`1&- 0.65130
`
`~I- LOPQ
`
`+30 —
`
`-0- O.5F’c1
`
`D- O 7 Po
`
`~0- 0 (SP0
`
`{I- 08 Pa
`
`I00
`200
`300
`SUBSTRATE TEMPERATURE
`
`400
`(°C)
`
`MICRON ET AL. EXHIBIT 1047
`Page 2 of 6
`
`
`
`I US. Patent Aug. 14, 1990
`
`Sheet 2 of2
`
`4,948,482
`
`FIG. 2
`
`+ 0.6Pd
`
`I00
`200
`300
`SUBSTRATE TEMPERATURE (°C)
`
`400
`
`2.1 _
`
`0. 2
`
`
`
`XmQZ_ MZPQdWEmE
`
`T
`
`MICRON ET AL. EXHIBIT 1047
`Page 3 of 6
`
`
`
`1
`
`4,948,482
`
`METHOD FOR FORMING SILICON NITRIDE
`FILM
`
`2
`obtained. For example, in the substrate dissolution step
`in the production of an X-ray lithography mask using a
`silicon nitride ?lm as an X-ray transmission ?lm, partial
`dissolution of the silicon nitride film takes place,
`thereby allowing the ?lm to have flaws. Further, the
`impurities in silicon nitride ?lms are easily eliminated by
`the application of an ionizing radiation, thereby causing
`a change in the composition, optical transparency and
`physical properties of the ?lms.
`As stated above, in conventional methods for forming
`a silicon nitride ?lm, it has been very dif?cult to control
`the internal stress of the film.
`
`SUMMARY OF THE INVENTION
`An object of the present invention is to provide a
`method for forming a silicon nitride ?lm with a highly
`controlled internal stress.
`Other objects will be apparent from the following
`description and drawings.
`The present invention resides in a method for forming
`a silicon nitride ?lm which comprises depositing a sili
`con nitride ?lm on a substrate by a sputtering method
`using, as a sputtering gas, an inert gas or a mixed gas of
`an inert gas and nitrogen gas, said method further com
`prising, during the deposition of said silicon nitride ?lm,
`keeping the substrate temperature at a given tempera
`ture range appropriate for the pressure of the sputtering _
`gas to control the internal stress of the film formed.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a graph showing the change of the internal
`stress of a silicon nitride ?lm as a function of the change
`of the substrate temperature in the Example of the pres
`ent invention.
`FIG. 2 is a graph showing the change of the refrac
`tive index of a silicon nitride ?lm as a function of the
`change of the substrate temperature in the Example of
`the present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`The present invention utilizes the fact that in deposit
`ing a silicon nitride ?lm on a substrate by a sputtering
`method using, as a sputtering gas, an inert gas or a
`mixed gas of an inert gas and nitrogen gas, the internal
`stress of the ?lm formed can be controlled by keeping
`the substrate temperature at a given temperature range
`appropriate for the pressure of the sputtering gas used.
`In the present method, the substrate temperature can
`be controlled precisely and the change of the internal
`stress of the silicon nitride ?lm as a function of the
`change of the substrate temperature is small; therefore,
`the internal stress of the ?lm can be controlled pre
`cisely. Further, the change of the substrate temperature
`causes no change in properties of the silicon nitride ?lm
`other than internal stress, such as refractive index, com
`position, visible light transmission and the like. It is
`preferred, however, that ?lm deposition be effected at a
`low sputtering gas pressure in order to prevent the
`trapping of impurities in the ?lm and at a high substrate
`temperature in order to obtain a ?lm of high chemical
`stability.
`The present invention is illustrated in more detail by
`way of Example. However, the present invention is in
`no way restricted to the Example.
`
`5
`
`20
`
`25
`
`35
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to a method for forming
`a silicon nitride ?lm used as, for example, a protective
`?lm for semiconductor chips or memory disks and an 10
`X-ray transmission ?lm. More particularly, the present
`invention relates to a method for forming a silicon ni
`tnide ?lms with a highly controlled internal stress.
`2. Description of the Prior Art
`conventionally, the CVD method and the sputtering
`method have often been used for formation of a silicon
`nitride film. The silicon nitride ?lm formed by the CVD
`method, however, has had the following drawbacks.
`Firstly, there are used, as the raw material gases, a sili
`con compound such as a silicon hydride (e.g. silane
`SiH4), a silicon ?uoride (e.g. SiF4) or a silicon chloride
`(e.g. SiCl4), ammonia (NH3) and nitrogen (N2); that is,
`the easily decomposable raw material gases contain not
`only silicon and nitrogen which are constituent ele
`ments of silicon nitride (SixNy) but also other elements;
`consequently, the silicon nitride ?lms formed by the
`CVD method inevitably contains impurities in princi
`ple. The silicon nitride ?lms containing impurities be
`sides silicon and nitrogen largely vary in internal stress
`depending upon the content of the impurities. In order
`to precisely control the impurities content in the silicon
`nitride ?lms, it is necessary to always make the ?lm
`formation (deposition) conditions constant; that is, in
`the thermal CVD method, for example, it is required to
`always make constant the deposition temperature, gas
`composition, gas ?ow rate and gas pressure. In the
`plasma CVD method, not only the deposition tempera
`ture, gas composition, gas ?ow rate and gas pressure but
`also the plasma state must be made constant. To always
`keep these parameters constant is extremely dif?cult
`and the impurities content in the silicon nitride cannot
`be kept constant. Thus, the precise control of internal
`stress of silicon nitride ?lms has been impossible in the
`CVD method. Secondly, the impurities in the silicon
`nitride ?lms signi?cantly reduce the chemical stability
`of the ?lms. For example, in the substrate dissolution
`45
`step in the production of an X-ray lithography mask
`using a silicon nitride ?lm as an X-ray transmission ?lm,
`the partial dissolution of the silicon nitride ?lm takes
`place, thereby allowing the ?lm to have flaws. Further,
`the impurities in silicon nitride ?lms 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 ?lm.
`Meanwhile, the conventional sputtering method for
`forming silicon nitride ?lms has had the following prob
`lems. Firstly, in the conventional sputtering method, the
`internal stress of the silicon nitride ?lms 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
`slight change of the gas pressure, and the control of the
`internal stress has been possible only in the order of, for
`example, about 10x108 dyn/cmz. Secondly, the gas pres
`sure must be fairly large (at least 10 Pa) in order for the
`internal stress to be a tensile stress; use of a large gas
`pressure incurs trapping of impurities (e.g. hydrogen,
`oxygen) in silicon nitride; these impurities signi?cantly
`reduce the chemical stability of the silicon nitride ?lm
`
`50
`
`60
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`MICRON ET AL. EXHIBIT 1047
`Page 4 of 6
`
`
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`0
`
`35
`
`45
`
`3
`EXAMPLE
`A silicon nitride ?lm 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
`con target. The sputtering gas was a mixed gas of Ar gas
`(an inert gas) and N2 gas. The flow rate of this mixed gas
`was constant (Ar gas flow rate= 12.0 sccm (cc at stan
`dard condition/min), N2 gas ?ow rate=4.0 sccm, ratio
`of N2 gas to total gas ?ow rate (N7/(Ar+N2))=0.25).
`The internal stress of silicon nitride ?lm was controlled
`by the control of the substrate temperature. In this Ex
`ample, the control of the substrate temperature was
`made by ?xing an electric heating wire to a plate for
`holding the substrate so as to ensure uniform heating of
`the substrate and supplying an electric current to the
`heating wire. The rf power was constant at 16.72
`W/cm2 but there were employed various sputtering gas
`pressures, i.e. 0.4 Pa, 0.5 Pa, 0.6 Pa, 0.65 Pa, 0.7 Pa, 0.8
`Pa and 1.0 Pa.
`20
`The change of internal stress of silicon nitride ?lm by
`the change of substrate temperature is shown in FIG. 1.
`Incidentally, the internal stress was measured by the
`Newton rings method.
`In the case of sputtering gas pressure=0.4 Pa, the
`internal stress was a compressive stress at a substrate
`temperature of 100° C.; and the compressive stress in
`creased gradually as the substrate temperature was in
`creased. In the case of sputtering gas temperature =0.5
`Pa, the internal stress was a tensile stress at a substrate
`temperature of 100° C.; the tensile stress decreased
`gradually as the substrate temperature was increased:
`and the internal stress transform from a tensile stress to
`a compressive stress at a substrate temperature of 290°
`C. The compressive stress showed a gradual increase
`with the further increase of the substrate temperature.
`In this case of 0.5 Pa, since the internal stress changes
`linearly with the change of the substrate temperature,
`the internal stress can be controlled at a precision of
`0.511108 dyn/cm2 for a substrate temperature change of
`40
`10° C. In the case of sputtering gas pressure=0.6 Pa, the
`internal stress was a tensile stress at a substrate tempera
`ture of 100° C.; the tensile stress increased gradually as
`the substrate temperature was increased and became a
`maximum at a substrate temperature of 200° C; and with
`the further increase of the substrate temperature, the
`tensile stress decreased gradually. In the case of sputter
`ing gas pressure=0.8 Pa, the internal stress was a tensile
`stress at a substrate temperature of 100° C.; and the
`tensile stress increased gradually as the substrate tem
`perature was increased. As seen from the above, the
`change of internal stress as a function of the change of
`substrate temperature differs according to the pressure
`level of the sputtering gas employed. As is clear from
`FIG. 1, however, the change of internal stress as a func
`tion of the change of substrate temperature is very small
`when the sputtering gas pressure, is less than 1.0 Pa.
`Accordingly, the control of internal stress by the
`change of substrate temperature provides an excellent
`method for the control of internal stress. Moreover, the
`internal stress obtained by this method has an excellent
`reproducibility.
`As is clear from FIG. 1, when the sputtering gas
`pressure is 1.0 Pa, the internal stress changes more
`diamatically as a function of the change of substrate
`temperature than when the sputtering gas pressure is
`less than 1.0 Pa, making the control of internal stress by
`the control of substrate temperature more difficult.
`
`4,948,482
`4
`Nevertheless, the above control of internal stress by the
`control of substrate temperature is far superior to the
`conventional methods for the control of internal stress
`by, for example, sputtering gas pressure alone.
`The silicon nitride ?lm formed by the above sputter~=
`ing method can be used as an X-ray transmission ?lm
`for an X-ray lithography mask. This X-ray transmission
`?lm preferably has an internal stress of 100x108
`dyn/cm2 or less in terms of tensile stress. According to
`FIG. 1, the preferable substrate temperature range
`which enables the production of a silicon nitride ?lm
`(an X-ray transmission ?lm) having an internal tensile
`stress of 10.0x108 dyn/cm2 or less is about 100° C. to
`about 290° C. in the case of sputtering gas pressure=0.5
`Pa, about 340° C. to about 380° C. in the case of sputter
`ing gas pressure=0.6 Pa, and about 100° C. to about
`170° C. in the case of sputtering gas pressure=0.8 Pa.
`Especially in the case of sputtering gas pressure=0.5
`Pa, the internal stress can be controlled at 51:108
`dyn/cm2 or less in terms of tensile stress over a wide
`substrate temperature range of ZOO-290° C.; the change
`of internal stress as a function of the change of substrate
`temperature is linear; and the internal stress can be con
`trolled at a precision of 0.5xl08 dyn/cm2 for a substrate
`temperature change of 10° C. Hence, the sputtering gas
`pressure is most preferably 0.5 Pa or its vicinity (e.g.
`0.45-0.55 Pa) when the silicon nitride ?lm formed is
`used as an X-ray transmission ?lm.
`Shown in FIG. 2 is the change of refractive index of
`silicon nitride ?lm as a function of the change of sub
`strate temperature. As seen in FIG. 2, the refractive
`index of silicon nitride ?lm has substantially no depen
`dency on the substrate temperature and was 2.0 for both
`cases of sputtering gas pressure=0.5 Pa 0.6 Pa. In addi
`tion, these silicon nitride ?lms were transparent in the
`visible region.
`The Fourier transform infrared absorption spectra of
`the above silicon nitride ?lms con?rmed that the ?lms
`contained no impurities. Therefore, when the ?lms are
`applied to an X-ray lithography mask, there occurs no
`mask strain, no compositional change, no reduction in
`optical transparency and no change in physical proper
`ties by the application of an ionizing radiation.
`When ?lm formation was conducted at substrate
`temperatures of 200' C. or more, the resulting ?lms
`showed signi?cant improvement of chemical stability as
`well as improvement of optical property and mechani
`cal strength. The silicon nitride sample formed at a
`substrate temperature of 100° C. and the silicon nitride
`sample formed at a substrate temperature higher than
`200° C. were immersed in a 50% NaOH solution at 100°
`C. for 3 hours; the former sample dissolved partially
`followed by ?lm breakage while the latter sample saw
`no change. Hence, in order to obtain a silicon nitride
`?lm with satisfactory internal stress and chemical stabil
`ity, the preferable substrate temperature is 200° C. to
`290' C. in the case of 0.5 Pa and 340° C. to 380° C. in the
`case of 0.6 Pa.
`In the above method for the control of internal stress
`of silicon nitride ?lms by the control of substrate tem
`perature, the properties of the formed ?lms other than
`chemical stability, for example, the ?lm composition
`and density, were con?rmed to exhibit no change.
`The above Example can be modi?ed as follows.
`In the above Example, the Si target was used as a
`sputtering target and the mixed gas of Ar gas and N2 gas
`was used as a sputtering gas. It is possible to use, as a
`sputtering target, a SixNy target of desired composition
`
`60
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`MICRON ET AL. EXHIBIT 1047
`Page 5 of 6
`
`
`
`5
`and, as a sputtering gas, only an inert gas such as Ar gas
`or the like. It is also possible to use a substrate other
`than Si substrate, i.e. a Si02 substrate (glass wafer).
`The present method for forming ?lms silicon nitride
`?lm can control the internal stress of the formed silicon
`nitride more precisely than the conventional methods.
`Further, the present method can effect the precise
`control of the internal stress of the ?lms without vary
`ing the optical properties, mechanical strength, compo
`sition and density. Accordingly, the present method has
`excellent practical uses.
`The silicon nitride ?lms obtained by the present
`method are transparent in a visible region and have
`excellent chemical stability and mechanical strength,
`and therefore are suitable for use as an X-ray transmis
`sion ?lm for X~ray lithography mask.
`The above-described embodiments are just an exam
`ple of the present invention, and therefore, it will be
`apparent for those skilled in the art that many modi?ca
`tions and variations may be made without departing
`from the scope of the present invention.
`What is claimed is:
`1. A method for forming a silicon nitride ?lm which
`comprises depositing a silicon nitride ?lm having a
`preselected amount of internal stress on a substrate by a
`sputtering method using, as a sputtering gas, an inert gas
`or a mixed gas of an inert gas and nitrogen gas, said
`method further comprising, during the deposition of
`said silicon nitride flm, keeping the substrate tempera
`ture at a given temperature range according to the pres
`sure of the sputtering gas to control the internal stress of
`
`4,948,482
`6
`the ?lm formed substantially to said preselected
`amount.
`2. A method according to claim 1, wherein the sub
`strate temperature is kept in the range of about 100° C.
`to 400° C. when the pressure of the sputtering gas is less
`than 1.0 Pa.
`.
`3. A method according to claim 2, wherein the pres
`sure of the sputtering gas is 0.5 Pa or its vicinity.
`4. A method according to claim 2, wherein the range
`of the substrate temperature is about 100° C. to about
`290° C. in the case of the pressure of sputtering gas of
`0.5 Pa, about 340° C. to about 380° C. in the case of the
`pressure of the sputtering gas of 0.6 Pa and about 100°
`C. to about 170° C. in the case of the pressure of the
`sputtering gas of 0.8 Pa.
`5. A method according to claim 4, wherein the sub
`strate temperature is in the range of 200° C. to 290' C.
`in the case of the pressure of the sputtering gas of 0.5 Pa.
`6. A method according to claim 1, wherein said sub
`strate is a Si substrate or a SiOz substrate.
`7. A method of forming a silicon nitride ?lm on an Si
`or SiOz substrate and carefully controlling the internal
`stress of the silicon nitride ?lm so formed to a prese
`lected value, comprising sputtering, using an inert gas
`or a mixture of an inert gas and nitrogen gas, as the
`sputtering gas, wherein during sputtering, while the
`silicon nitride ?lm is being formed, maintaining the
`substrate at a temperature in the range of about 100° C.
`to 400° C. and the pressure of the sputtering gas less
`than 1.0 Pa and thereby controlling the internal stress of
`the silicon nitride ?lm thus formed substantially to said
`preselected value.
`
`* i i i i
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`MICRON ET AL. EXHIBIT 1047
`Page 6 of 6