Method for producing semiconductor device including step for removing contaminant

A method for removing contaminant compounds respectively having a benzene ring therein from the surface of an Si layer, the method containing enter a step for causing the Si layer to contact with the air, oxygen or ozone under a heated condition or a step for causing the Si layer to contact with a mixture of sulfuric acid and hydrogen peroxide or a mixture of pure water and ozone. The method can be applicable to methods for providing a field effect transistor.

FIELD OF THE INVENTION
 This invention relates to methods for removing contaminant compounds
 respectively having a benzene ring therein from the surface of an Si layer
 and methods for producing semiconductor devices, the methods respectively
 including one or more steps for removing contaminant compounds
 respectively having benzene ring therein from the surface of an Si layer.
 BACKGROUND OF THE INVENTION
 The production processes of a semiconductor device are conducted in a clean
 room, and the semiconductor wafers are frequently washed with pure water,
 because contaminants such as dusts, mists, and fine particles composed of
 conductive compound atoms readily jeopardize the property such as the
 insulation characteristics of the semiconductor devices produced employing
 the foregoing semiconductor wafers. Unfortunately, however, it is not easy
 to keep the internal atmosphere of a clean room absolutely clean, and the
 pure water employed for washing the semiconductor wafers is gradually
 contaminated. Exemplary contaminants jeopardizing the insulation of a gate
 insulator layer of a field effect transistor having a gate insulated from
 a channel layer and/or the insulation of a capacitor are phthalic acid
 ester which is employed for a draught test of a clean room, dibutyl
 phthalate which is employed as a plasticizer employable for producing
 various plastic vessels, boxes, other apparatus or the like, and butyl
 hydroxy toluene which is employed as a plasticizer employable for
 producing wafer cases or the like.
 For the purpose to identify the contaminants of a contaminated
 semiconductor wafer before and after a step for producing a gate insulator
 layer of a field effect transistor, a wafer-heating gas-removing gas
 chromatography mass spectrometer was employed. FIG. 1 is a gas
 chromatogram of a gas evaporated from an Si substrate just after being
 washed with pure water and FIG. 2 is a gas chromatogram of a gas
 evaporated from an Si substrate on which a gate insulator layer has been
 produced. It is well known that the Y axis of a chromatogram represents
 relative strength or detected quantity of a detected compound and the X
 axis of a chromatogram represents the time at which the foregoing compound
 was detected or the mass of the compound detected.
 For the purpose to identify the contaminants remained unevaporated on an Si
 water after heating it in N.sub.2 gas for 10 minutes, a wafer-heating
 gas-removing gas chromatography mass spectrometer was employed. FIG. 3 is
 a gas chromatogram of a gas evaporated from an Si substrate heated at a
 temperature range of 500.degree. C. through 700.degree. C. in N.sub.2 gas
 for 10 minutes. Benchtop/PBM search results show the substances
 represented respectively by A and B shown in FIG. 3 are 1-(phenyl
 thio)-1-(trimethylsilyl)-2-propene and (trimethylsilyl) adamantone
 respectively having a molecular construction respectively shown in FIG. 4A
 and in FIG. 4B.
 Results of an analysis applied to FIGS. 1, 2 and 3 are itemized below.
 1. Major contaminants of an Si wafer under progress of a production process
 for producing a semiconductor device are compounds each of which has a
 benzene ring therein.
 2. A high temperature process conducted for producing a gate insulator
 layer is effective to remove some volatile sorts of contaminants from the
 surface of an Si wafer.
 3. A heating process applied to an Si wafer on which a gate insulator layer
 has been produced, the heating process being conducted at a temperature
 range of 500.degree. C. through 700.degree. C. in N.sub.2 gas for 10
 minutes, causes silylyzation to occur for reacting with some of the
 functional groups alone of the compounds each of which has a benzene ring
 therein, resultantly converting the compounds to other non-volatile sorts
 of compounds, such as 1-(phenyl thio)-1-(trimethylsilyl)-2-propene, and
 trimethylsilyl adamantone.
 The above results imply a reaction which cleaves benzene rings of the
 contaminant compounds respectively having benzene rings therein would
 convert the contaminant compounds to volatile compounds which can readily
 be removed from the surface of an Si wafer.
 As a matter of fact, since most compounds respectively having a benzene
 ring therein are conductive, such compound contaminants readily jeopardize
 the insulation of an insulator layer located between a conductor layer of
 a semiconductor device e.g. a gate insulator layer of a field effect
 transistor and an insulator layer composing a capacitor produced in a
 semiconductor device.
 OBJECTS AND SUMMARY OF THE INVENTION
 Accordingly, a first object of this invention is to provide a method for
 removing contaminant compounds each of which has one or more benzene rings
 therein from the surface of an Si layer.
 A second object of this invention is to provide a method for producing a
 field effect transistor, the method including a step for removing
 contaminant compounds each of which has one or more benzene rings therein
 from the surface of an Si layer.
 A third object of this invention is to provide a method for producing a
 semiconductor device containing a capacitor produced therein, the method
 including a step for removing contaminant compounds each of which has one
 or more benzene rings therein from the surface of an Si layer.
 This invention is based on a concept described below.
 1. Major contaminants remaining on an Si wafer after a high temperature
 process for producing a gate insulator layer are non-volatile compounds
 respectively having a benzene ring therein, such as 1-(phenyl
 thio)-1-(trimethylsilyl)-2-propene, trimethylsilyl adamantone and the
 like.
 2. An action to cause an Si wafer having the foregoing contaminants thereon
 to contact with a gas such as the air, oxygen and ozone under a
 temperature range of 500.degree. C. through 700.degree. C. or with a
 liquid such as a mixture of sulfuric acid and hydrogen peroxide and a
 mixture of pure water and ozone causes silylyzation to occur to cleave
 some of the benzene rings of the foregoing non-volatile compounds,
 resultantly converting the non-volatile compounds to chain silyl esters or
 the like which are volatile.
 Accordingly, a first method for removing contaminants from a surface of an
 Si layer in accordance with this invention comprises: a step for causing
 the Si layer to contact with a gas selected from a group of the air,
 oxygen and ozone under a heated condition, for causing silylyzation to
 occur for the contaminants to cleave a benzene ring of at least one of the
 contaminants, during a process for producing a semiconductor device.
 It is realistic to conduct the foregoing process during a process for
 producing a gate insulator layer of a field effect transistor or a process
 for decompose a silane for depositing an SiO.sub.2 layer on an Si layer.
 A second method for removing contaminants from a surface of an Si layer
 comprises: a step for causing an Si layer to contact with a liquid
 selected from a group of a mixture of sulfuric acid and hydrogen peroxide
 and a mixture of pure water and ozone for causing silylyzation to occur
 for the contaminants to cleave a benzene ring of at least one of the
 contaminants, during a process for producing a semiconductor device.
 It is realistic to conduct the foregoing process during a process for
 producing a gate insulator layer of a field effect transistor.
 A first method for producing a field effect transistor in accordance with
 this invention includes the foregoing first method for removing
 contaminants from a surface of an Si layer in accordance with this
 invention.
 A second method for producing a field effect transistor in accordance with
 this invention includes the foregoing second method for removing
 contaminants from a surface of an Si layer in accordance with this
 invention
 A first method for producing a semiconductor device containing a capacitor
 therein in accordance with this invention includes the foregoing first
 method for removing contaminants from a surface of an Si layer in
 accordance with this invention.
 A second method for producing a semiconductor device containing a capacitor
 therein in accordance with this invention includes the foregoing second
 method for removing contaminants from a surface of an Si layer in
 accordance with this invention.

DETAILED DESCRIPTION OF THE PREFERRED
 EMBODIMENTS Referring to drawings, methods for respectively producing field
 effect transistors in accordance with a first and second embodiments of
 this invention and methods for respectively producing field effect
 transistors attached by a capacitor connected therewith in accordance with
 a third and fourth embodiments of this invention will be described below.
 FIRST EMBODIMENT
 A method for producing a field effect transistor, the method containing a
 step for removing contaminants from a surface of an Si substrate, by
 causing the Si wafer to contact with the air, oxygen or ozone at a
 temperature range of 500.degree. C. through 700.degree. C. preferably
 under a condition in which a silane is available.
 Referring to FIG. 5, a selective oxidation process is conducted to produce
 a field insulator layer 2 surrounding active areas in each of which a
 field effect transistor is scheduled to be produced, on the surface of an
 Si substrate 1 containing impurities of one conductivity. After the Si
 substrate 1 (Hereinafter referred to as an Si wafer under some situation.)
 is washed with pure water, a wet oxidation process or a pyrogenic
 oxidation process is conducted to produce a thin SiO.sub.2 layer 3.
 The Si wafer 1 is charged in a CVD furnace in which a low pressure CVD
 process is conducted to produce a conductive Si layer 4 (See FIG. 8) on
 the thin SiO.sub.2 layer 3. In the CVD furnace, the Si wafer 1 is allowed
 to contact with a mixture of the air and a silane e.g. Si H.sub.4 at a
 temperature range of 500.degree. C. through 700.degree. C. for a
 relatively long time e.g. 10 min.
 As a result, silylyzation occurs to cleave some of the benzene rings of
 compounds such as 1-(phenyl thio)-1-(trimethylsilyl)-2-propene,
 trimethylsilyl adamantone, C.sub.6 H.sub.7 SC (Si (CH.sub.3).sub.3)
 HC.sub.2 H.sub.3 or the like, resultantly converting the compounds to
 2-piperidinecarborylic acid, 1-(trimethylsilyl)-, trimethylsilyl ester,
 ethanedjoic acid, bis (trimethylsilyl) ester, (CH.sub.3).sub.3 SiOCOC (Si
 (CH.sub.3).sub.3) HCOO Si (CH.sub.3).sub.3, (CH.sub.3).sub.3 Si OCO Si
 (CH.sub.3).sub.3 or the like. Since the latters are volatile, the
 contaminants are readily removed. FIG. 6 is a gas chromatogram of a gas
 evaporated from the foregoing Si wafer.
 A gas chromatogram of a gas evaporated from the foregoing Si wafer is shown
 in FIG. 6. In this specification, a process for removing contaminant
 compounds from the surface of an Si layer by causing the Si layer to
 contact with the air, oxygen or ozone, is defined as a first phase of this
 invention.
 For the purpose of comparison, a gas chromatogram of a gas evaporated from
 an Si wafer kept in a CVD furnace in which N.sub.2 gas is filled, is shown
 in FIG. 7.
 Referring to FIG. 8, a CVD process is conducted to produce a conductive Si
 layer 4 on the thin SiO.sub.2 layer 3.
 Referring to FIG. 9, a piled layer consisting of the conductive Si layer 4
 and the thin SiO.sub.2 layer is patterned into a shape of a gate
 electrode. An ion implantation process is conducted to produce a source 5
 and a drain 6 along the top surface of the Si layer 1. A metal layer is
 produced to cover the Si wafer and the metal layer is patterned into
 shapes of a source electrode 8 and a drain electrode 9.
 A dielectric strength test is conducted for plural specimens of the field
 effect transistor produced in a manner described above. Results of the
 tests are shown in a histogram shown in FIG. 10. Nearly 100% of the
 specimens show a satisfactory result.
 For the purpose of comparison, plural field effect transistors are produced
 employing Si wafers which were kept for a while in a CVD furnace in which
 N.sub.2 gas is filled. A gas chromatogram of a gas evaporated from the Si
 wafer is shown in FIG. 7. A dielectric strength test is conducted for the
 foregoing plural field effect transistors produced employing Si wafers
 which were kept for a while in a CVD furnace in which N.sub.2 gas is
 filled. Results of the tests are shown in FIG. 11. It is clear the results
 are entirely unsatisfactory.
 Plural experiments have been conducted to prove that the process in which
 an Si wafer is contacted with the air at a temperature range of
 500.degree. C. through 700.degree. C. preferably in a CVD furnace in which
 some quantity of a silane may remain, can be replaced by a similar process
 in which an Si wafer is contacted with oxygen or ozone under similar
 conditions. It is noted that the length of period required for removing
 the contaminants is shortened, in each of the foregoing cases. The
 magnitude of the time shortening effect is remarkable in the case of
 ozone. In reality, a period of 5 through 10 minutes is enough for the case
 of oxygen and a period of 3 through 5 minutes is enough for the case of
 ozone.
 Further, it is proved that the process for removing contaminant compounds
 from a surface of an Si layer can be effectively conducted before a
 process for producing a thin SiO2 layer. In other words, the process for
 removing contaminant compounds from a surface of an Si layer can be
 conducted either before or after a process for producing an insulator
 layer of which the insulation is required to be improved.
 SECOND EMBODIMENT
 A method for producing a field effect transistor, the method containing a
 step for removing contaminants from a surface of an Si substrate, by
 causing an Si layer to contact with a mixture of sulfuric acid and
 hydrogen peroxide or a mixture of pure water and ozone.
 Referring to FIG. 5 again, a selective oxidation process is conducted to
 produce a field insulator layer 2 surrounding active areas in each of
 which a field effect transistor is scheduled to be produced, on the
 surface of an Si substrate 1 containing impurities of one conductivity.
 After the Si substrate or the Si wafer 1 is washed with pure water, a wet
 oxidation process or a pyrogenic oxidation process is conducted to produce
 a thin SiO.sub.2 layer 3.
 The Si wafer is contacted with a liquid which is a mixture of sulfuric acid
 and hydrogen peroxide or a mixture of pure water and ozone. As a result,
 silylyzation occurs to cleave some of the benzene rings of compounds such
 as 1-(phenyl thio)-1-(trimethylsilyl)-2-propene, trimethylsilyl
 adamantone, C.sub.6 H.sub.7 SC (Si(CH.sub.3).sub.3) HC.sub.2 H.sub.3 or
 the like, resultantly converting the compounds to 2-piperidinecarborylic
 acid, 1-(trimethylsilyl)-, trimethylsilyl ester, ethanedioic acid, bis
 (trimethylsilyl) ester, (CH.sub.3).sub.3 SiOCOC (Si(CH.sub.3).sub.3) HCOO
 Si (CH.sub.3).sub.3, (CH.sub.3).sub.3 Si OCO Si (CH.sub.3).sub.3 or the
 like. Since the latters are volatile, the contaminants are readily
 removed.
 To prove the effect for removing the contaminant compounds from a surface
 of an Si layer, gas chromatography tests were conducted respectively for
 an Si wafer just after being washed with pure water, for an Si wafer after
 a gate insulator layer is produced and for an Si wafer after a process for
 causing an Si wafer to contact with a mixture of sulfuric acid and
 hydrogen or a mixture of pure water and ozone.
 Referring to FIGS. 12A, B and C, a gas chromatogram of a gas evaporated
 from the Si wafer just after being washed with pure water is shown in FIG.
 12A, a gas chromatogram of a gas evaporated from the Si wafer after a gate
 insulator layer is produced is shown in FIG. 12B and a gas chromatogram of
 a gas evaporated from the Si wafer after a process for causing an Si wafer
 to contact with a mixture of sulfuric acid and hydrogen peroxide or a
 mixture of pure water and ozone is shown in FIG. 12C. It is clear from
 FIG. 12C that nearly 100% of the contaminants are removed by a process
 specific to this invention.
 Referring again to FIG. 8, a CVD process is conducted to produce a
 conductive Si layer 4 on the thin SiO.sub.2 layer 3.
 Referring to FIG. 9, a piled layer consisting of the conductive Si layer 4
 and the thin SiO2 layer is patterned into a shape of a gate electrode. An
 ion implantation process is conducted to produce a source 5 and a drain 6
 along the top surface of the Si layer 1. A metal layer is produced to
 cover the Si wafer and the metal layer is patterned into shapes of a
 source electrode 8 and a drain electrode 9.
 In the foregoing manner, a field effect transistor having a gate insulator
 layer of which the magnitude of insulation is improved, is successfully
 produced.
 Dielectric strength tests are conducted for a group of field effect
 transistors produced in the foregoing manner. Results of the tests are
 shown in a histogram shown in FIG. 13. Nearly 100% of the specimens show a
 satisfactory result.
 For the purpose of comparison, a group of field effect transistors produced
 employing Si wafers for which the process for removing contaminants from
 the surface of an Si wafer in accordance with this invention is not
 conduced. Dielectric strength tests are conducted for the field effect
 transistors produced employing Si wafers for which the process for
 removing contaminants from the surface of an Si wafer in accordance with
 this invention is not conduced. Results of the tests are shown in FIG. 14.
 It is clear the results are unsatisfactory.
 Plural experiments have been conducted to prove that the process in which
 an Si wafer is contacted with a mixture of sulfuric acid and hydrogen
 peroxide or a mixture of pure water and ozone can be conducted either
 before or after a process for producing an insulator layer of which the
 insulation is required to be improved.
 THIRD EMBODIMENT
 A method for producing a field effect transistor attached by a capacitor
 connected therewith, the method containing a step for removing
 contaminants from a surface of an Si substrate, by causing an Si layer to
 contact with the air, oxygen or ozone at a temperature range of
 500.degree. C. through 700.degree. C. preferably under a condition in
 which a silane is available.
 Referring again to FIGS. 5 and 8, a process similar to that which was
 employed for the first and second embodiments is employed to produce a
 field effect transistor.
 Referring to FIG. 15, after the gate electrode 4 is covered by an insulator
 layer 7, a contact hole is produced only for the drain 6. A doped poly
 crystalline Si layer is produced on top of the Si substrate 1, which is
 patterned into a shape of a first electrode 10 of a capacitor. An
 SiO.sub.2 layer is produced to cover the first electrode 10, before a
 metal layer is produced on the entire surface of the Si wafer 1. The metal
 layer is patterned to shapes of a source electrode 8 and a second
 electrode 12 of the capacitor.
 When the field effect transistor composes a memory cell having a one
 transistor one capacitor structure, the second electrode 12 of the
 capacitor is grounded, and the gate electrode 4 works as a word line and
 the source electrode 8 is connected with a bit line.
 During the foregoing process, a process for causing an Si wafer to contact
 with the air, oxygen or ozone at a temperature range of 500.degree. C.
 through 700.degree. C. is conducted either before or after the process for
 producing an insulator layer 3 or 11, for the purpose to remove
 contaminants from the surface of the Si wafer. When the process is
 conducted for producing a gate electrode 4, it is preferable to conduct
 the process in a CVD furnace which is employed for producing the gate
 electrode 4.
 Further, it is proved that the length of period required for removing
 contaminants is remarkably shortened, when oxygen particularly ozone is
 employed rather than the air as the active gas.
 In the foregoing manner, a field effect transistor attached by a capacitor
 connected therewith having a gate insulator layer and an insulator layer
 of the capacitor of which the insulation is improved, is successfully
 produced.
 Results similar to those of the first embodiments of this invention have
 been experimentally proved.
 FOURTH EMBODIMENT
 A method for producing a field effect transistor attached by a capacitor
 connected therewith, the method containing a step for removing
 contaminants from a surface of an Si substrate by causing an Si layer to
 contact with a mixture of sulfuric acid and hydrogen peroxide or a mixture
 of pure water and ozone.
 Referring to FIG. 5 for the fourth time, a process similar to that which
 was employed for the third embodiment is employed to produce a field
 effect transistor attached by a capacitor connected therewith.
 During the foregoing process, a process for causing an Si wafer to contact
 with a mixture of sulfuric acid and hydrogen peroxide or a mixture of pure
 water and ozone, for the purpose to remove contaminants from the surface
 of the Si wafer. When the process is conducted for producing a gate
 electrode 4, it is preferable to conduct the process in a CVD furnace
 which is employed for producing the gate electrode 4.
 In the foregoing manner, a field effect transistor attached by a capacitor
 connected therewith having a gate insulator layer and an insulator layer
 of the capacitor of which the insulation is improved, is successfully
 produced.
 Results similar to those of the second embodiments of this invention have
 been experimentally proved.
 The foregoing description has clarified that methods for removing
 contaminant compounds respectively having a benzene ring therein from the
 surface of an Si layer and methods for producing semiconductor devices,
 the methods respectively including one or more steps for removing
 contaminant compounds respectively having benzene ring therein from the
 surface of an Si layer, have been successfully provided by this invention.