United States Patent
`Barnes et al.
`
`[19]
`
`[11] Patent Number:
`
`5,505,816
`
`[45] Date of Patent:
`
`Apr. 9, 1996
`
`llllllllllllllllllllllIllll|||||Illllllllllllllllllllllll||||l||||l||l|||||
`U8005505816A
`
`ETCHING 0F SILICON DIOXIDE
`SELECTIVELY T0 SILICON NITRIDE AND
`POLYSILICON
`
`7/1993 Kadomura ............................... 156/643
`5,250,772
`2/1994 Jeng etal.
`.............................. 156/646
`5,282,925
`FOREIGN PATENT DOCUMENTS
`
`[54]
`
`175]
`
`Inventors: Michael S. Barnes, San Francisco,
`Calif.; John H. Keller, Poughkeepsie,
`N.Y.; William M. Holber, Boston,
`Mass; Tina J. Cotler, Newburgh, N.Y.;
`Jonathan D. Chapple-Sokol,
`Poughkeepsie, N.Y.; Dragan Podlesnik,
`New York City, NY.
`
`[73]
`
`Assignee:
`
`International Business Machines
`Corporation, Armonk, NY.
`
`[21]
`
`[22]
`
`[51]
`[52]
`
`[581
`
`[56]
`
`Appl. No.2 168,887
`
`Filed:
`
`Dec. 16, 1993
`
`Int. Cl.6 ..................................................... H01L 21/00
`US. Cl.
`..................................... 156/662.1; 156/6431;
`156/646.1; 437/225
`Field of Search .............................. 156/6431, 646.1,
`156/662.1; 437/225
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,352,724
`4,615,764
`4,711,698
`4,807,016
`5,030,319
`5,122,225
`5,128,744
`5,173,151
`
`10/1982
`10/1986
`12/1987
`2/1989
`7/1991
`6/1992
`7/1992
`12/1992
`
`..................... 156/643
`Sugishima et a1.
`156/643
`Bobbio et a1.
`.. 156/643
`Douglas .......
`Douglas ............. 357/67
`Nishino et a1.
`156/635
`Douglas
`.. 156/643
`.
`Asano et a1.
`..... 357/54
`Namose .......
`.. 156/643
`
`58-056317
`59—033833
`
`Japan.
`4/1983
`Japan.
`2/1984
`OTHER PUBLICATIONS
`
`“Plasma Chemical Aspects of Magnetron Ion Etching with
`CF4/O2 and CF4/H2h”; Plasma Processing and Synthesis of
`Materials; Material Research Soc; XVL; 1987; abstract
`only; Bright et al.
`
`Primary Examiner—R. Bruce Breneman
`Assistant Examiner—George Goudrean
`Attorney, Agent, or Firm—Whitham, Curtis, Whitham &
`McGinn; Harold Huberfeld
`
`[57]
`
`ABSTRACT
`
`Silicon dioxide on a substrate is directionally etched using a
`hydrogen halide plasma which is created within an etch
`chamber. The method selectively etches silicon dioxide
`relative to polysilicon and silicon nitride. A substrate and the
`combination of NH3 and NE, gases or the combination of
`CF4 and O2 gases mixed with H2 and N2 gases are located
`within an etch chamber. An electrical field is created within
`the etch chamber causing the gas mixture to form a plasma.
`The negative charge at the bottom of the chamber attracts the
`positively charged plasma, thereby etching the substrate in
`the downward direction. The result is an anisotropic product.
`The method is also shown to be efiective in non-selectively
`etching thermal and deposited oxides, resulting in a similar
`etch rate for the different types of oxides.
`
`15 Claims, 7 Drawing Sheets
`
`54
`
`56
`
` ///:
`
`
`
`
`IP Bridge Exhibit 2227
`IP Bridge Exhibit 2227
`TSMC v. Godo Kaisha IP Bridge 1
`TSMC v. Godo Kaisha IP Bridge 1
`IPR2017-01843
`IPR2017-01843
`
`

`

`US. Patent
`
`Apr. 9, 1996
`
`Sheet 1 of 7
`
`5,505,816
`
`14
`_/
`
` FIG.1A
`
`2°
`
`PRIOR ART
`
`
`
`

`

`V
`
`

`

`US. Patent
`
`Apr. 9, 1996
`
`Sheet 3 of 7
`
`5,505,816
`
`800
`
`0 Silicon Dioxide
`
`A Polysmcon-
`
`9 Silicon Nitride ’E 600
`
`400
`
`200
`
`E 3 g
`
`0
`
`0
`
`2°
`
`40
`
`50
`Time (sec)
`
`80
`
`100
`
`FIG.3
`
`

`

`US. Patent
`
`Apr. 9, 1996
`
`Sheet 4 of 7
`
`5,505,816
`
`'
`
`695.0
`
`69 I .0
`
`687.0
`
`68 .0
`
`679.0
`
`2000
`
`FLUORINE - INITIAL
`
`1 00000
`
`
`
`FLUORINE — PROCESSED
`
`5000
`
`
`
`695.0
`
`691.0
`
`687.0
`
`68 .0
`
`679.0
`
`FLUORINE — BAKED
`
`FIG.4A
`
`

`

`US. Patent
`
`Apr. 9, 1996
`
`Sheet 5 of 7
`
`5,505,816
`
`100000
`
`
`
`542.0
`
`538.0
`
`534.0
`
`53 O .
`
`OXYGEN — INITIAL
`
`5000
`
`20x
`
`542.0
`
`538.0
`
`534.0
`
`530.0
`
`526.0
`
`522.0
`
`OXYGEN - PROCESSED
`
`5000
`
`20x
`
`542.0
`
`538.0
`
`534.0
`
`530.0
`
`526.0
`
`522.0
`
`OXYGEN — BAKED
`
`FIG.4B
`
`

`

`US. Patent
`
`Apr. 9, 1996
`
`Sheet 6 of 7
`
`5,505,816
`
`50000
`
`
`
`10 .0
`
`98.0
`
`SILICON - INII'IAL
`
`20000
`
`2.5 x
`
` 9.0 110.0
`
`10 .0
`
`98.0
`
`.
`
`SIUCON - PROCESSED
`
`
`
`10 .
`
`102.0
`
`98.0
`
`94.0
`
`SILICON — BAKED
`
`FIG/IO
`
`

`

`
`
`

`

`5,505,816
`
`1
`ETCHING OF SILICON DIOXIDE
`SELECTIVELY T0 SILICON NITRIDE AND
`POLYSILICON
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention generally relates to a surface pro-
`cessing technique of a silicon wafer substrate. Particularly,
`this invention relates to a technique for directionally remov—
`ing or dry etching of an oxide film, using a plasma of NH3
`and NF3 or CF4 and 02 mixed with H2 and N2.
`2. Description of the Prior Art
`The exposure of a surface of a silicon (Si) substrate to
`ambient air causes a layer of native silicon dioxide (SiOz) to
`form On its surface. The formation of the native SiO2 layer
`presents certain manufacturing difliculties since many semi-
`conductor processes, such as low-temperature epitaxy, poly-
`silicon deposition, and silicidation, require the silicon wafer
`surface to be free of all native SiOz. The prior art establishes
`that many difierent processes for etching or cleaning of the
`native SiO2 from Si surfaces have been developed. One of
`the most commonly used techniques for etching SiO2 is to
`expose the substrate to hydrofluoric acid (HF), either wet or
`dry. However, the use of liquid or vapor HF has major
`drawbacks for the semiconductor manufacturer because it is
`both highly toxic and highly corrosive. The high toxicity of
`HF requires the semiconductor manufacturer to implement
`safety procedures and devices which ensure that people and
`the environment are not injured by the HF gas. In addition,
`since HF is highly corrosive, it must be stored in containers
`and delivered to a cleaning or etch chamber in tubing which
`is resistant to corrosion. This type of equipment is generally
`more expensive than containers and tubing that do not
`possess anti—corrosive properties.
`It is also known in the prior an to selectively etch silicon
`dioxide using mixtures of fluorocarbon gases. However, this
`technique is also problematic due to the reaction inhibiting
`polymer which forms when the silicon or silicon nitride
`substrate is exposed to the fluorocarbon gases. In this
`technique, when oxide film is present, the oxygen in the
`oxide film prevents the formation of a reaction inhibiting
`polymer. However, once the oxide is etched through, the
`reaction inhibiting polymer can form and is deposited on the
`silicon or silicon nitride,
`inhibiting the etching of these
`films. The reaction inhibiting polymer film is diflicult to
`remove from the wafer. The polymer film can also passivate
`the chamber wall and electrode surfaces and is a source of
`particles. Finally,
`the etch selectivity,~in this process, is
`under 20:1 for SiO2 relative to polysilicon or silicon nitride.
`The use of wet etching techniques, as described above, to
`remove a thermal or native oxide film also present problems
`due to the variation in the wet etch rates of different oxide
`films. There are many types of oxide films currently in use,
`with two common ones being thermal oxide and deposited
`tetraethylorthosilicate TEOS) oxide. The etch rate of these
`films can vary by a factor of three, with the thermal oxides
`being slower. Since each wet etch performed reduces the
`film thickness of any exposed deposited oxide in proportion
`to the etch rate milo, it is usually necessary to adjust the
`initial thickness of the deposited oxide. Therefore, it would
`be desirable for the deposited oxides to etch at the same or
`a lower rate than the thermal oxides.
`
`US. Pat. No. 5,030,319 to Nishino et a1. and in the article
`to Nishino et 31., 1989 Dry Process Symposium IV-2 90-92
`(1989), discloses a method of selectively etching native
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`SiO2 from a silicon substrate using fluorine atoms and
`nitrogen hydrides produced by a NH3 and NF3 microwave
`discharge. This technique, as shown in FIG. 1A, discloses
`the use of a source 12 remotely positioned from etch
`chamber 16 to excite a gaseous NH3 and NF3 mixture into
`a plasma prior to its transport to the etch chamber 16. The
`substrate 18 is located on the floor 20 of etch chamber 16.
`The remote excitement of the gases causes the excited gases
`to randomly diifuse over the substrate which is to be etched.
`There is no directional control over the etching process. FIG.
`2B provides an example of etching using this method. Since,
`the excited particles etch in all directions an isotropic
`product is created. As shown, the process is effective in
`etching to the surface of the substrate 52, but results in the
`undercutting of the wall 58 in the area beneath resist pattern
`54 which should not to be etched. It is desirable for the
`
`etching product to have an anisotropic profile.
`
`SUMMARY OF THE INVENTION
`
`It is therefore an object of the present invention to use a
`gas mixture of NH3 and NF3 or CF4 and 02 mixed with H2
`and N2 to provide an improved means for selectively etching
`silicon dioxide.
`
`Another object of this invention is to provide a means for
`etching vias and trenches in SiO2 with a high degree of
`anisotropy.
`According to the invention, SiO2 is selectively etched
`from a substrate using a plasma created from NH3 and NF3
`gases, the substrate being located in an etch chamber where
`the plasma is formed. The creation of the NH3 and NF3
`plasma within the etch chamber provides for enhanced
`directional etching capabilities and results in improved
`anisotropic etching of $0,. A field source is used to
`generate an electric field within the chamber and to excite
`the plasma. The creation of a negative charge at the bottom
`of the chamber forces the positive radicals in the downward
`direction towards the substrate. A passivation layer of
`ammonium hexafluorosilicate (AHFS) will be formed at the
`point where the plasma contacts exposed Si02. The lapse of
`a sufficient amount of time for the reaction allows the
`passivation layer to extend to the Si or silicon nitride etch
`stop layer. If desired, the passivation layer can be removed
`by either heating the passivation layer at a temperature
`greater than 100° C. or by rinsing the passivation layer with
`deionized water. The removal of the ammonium hexafluo-
`rosilicate passivation layer results in anisotropic trenches
`and vias in the SiOz. It is also possible to leave the layer of
`AHFS in place and use this method for epitaxial growth
`pre—treatment or other process steps requiring an oxide free
`surface.
`
`Furthermore, the use of the combination of NH3 and NF3
`gases, as is disclosed in this application, provides a means
`for achieving the non-selective etching of dilferent types of
`oxides, such as oxides deposited by TEOS, or by chemical
`vapor and doped oxides, such as phosphosilicate glass
`(PSG), thereby allowing a thermal oxide to be etched at
`approximately the same rate as a deposited oxide.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The foregoing and other objects, aspects and advantages
`will be better understood from the following detailed
`description of a preferred embodiment of the invention with
`reference to the drawings, in which:
`FIG. 1A illustrates the prior art etch chamber as disclosed
`in US. Pat. No. 5,030,319 to Nishino et al.;
`
`

`

`5,505,816
`
`3
`FIG. 1B is a schematic drawing of the etch chamber of the
`present invention;
`FIG. 2A depicts a substrate with a silicon dioxide layer;
`FIG. ZB illustrates isotropic etching of the substrate
`depicted in FIG. 2A when using the prior art technique and
`apparatus as depicted in FIG. 1A;
`FIG. 2C is a schematic drawing of anisotropic etching of
`the substrate depicted in FIG. 2A when using the technique
`and apparatus of this invention depicted in FIG. 1B;
`FIG. 3 is a graph which shows the respective etch rates of
`silicon dioxide, polysilicon and silicon nitride when exposed
`to a plasma of NH3 and NF3;
`FIGS. 4A, 4B and 4C are examples is an example of an
`XPS analysis which demonstrates the effective removal of
`native oxide achieved by the method of the present inven-
`tion; and
`FIGS. SA and 5B illustrate a selective silicon dioxide
`etch.
`
`DETAILED DESCRIPTION OF A PREFERRED
`EMBODIMENT OF THE INVENTION
`
`Refening now to the drawings, and more particularly to
`FIG. 1B, the invention is directed to the selective etching of
`silicon dioxide, wherein a gaseous mixture of NH3 and NF3
`14 is formed in the chamber 16 where the substrate 18 is
`located. It is also possible to use the combination of CF4 and
`02 mixed with H2 and N2 as a substitute for the NH3 and
`NF3 gas mixture. This process is also shown to be non—
`selective with respect to the etching of different oxides. A
`power source 26 is used to create an electrical field within
`the chamber 16, thereby exciting the gaseous mixture of
`NH3 and NF3 to create a plasma of NH4F and similar
`radicals. The plasma created has fields within the plasma and
`a sheath above the substrate 18, thereby accelerating the
`ions, such as NH4+or any ion containing N, H or F, down-
`ward onto the substrate surface. The reaction of the radicals
`and ions with the SiO2 layer converts the layer to ammonium
`hexafluorosilicate (AI-IFS), (NH4)ZSiF6. If desired, as in a
`selective oxide etch application, the ammonium hexafluo-
`rosilicate can easily be removed by heating to a temperature
`of 100° C. or by rinsing with deionized water. This results
`in the release of NH3, HF and SiF4. However, it is also
`possible to leave the passivating product layer in place and
`use it as a pre—treatrnent for epitaxial growth or other process
`steps requiring an oxide free surface.
`FIG. 2A illustrates a substrate 52 with a SiO2 layer 50
`prior to the etching to the substrate. A resist pattern 54 has
`been created on the SiO2 layer 50 in preparation for selective
`etching.
`FIG. 2C illustrates the resulting product after it has been
`etched using the method and apparatus of this invention. As
`shown, an anisotropic product is created, with the silicon
`dioxide 50 being etched straight down to the substrate
`surface 56. The selective etching of the silicon dioxide
`results in the formation of straight walls 58 which do not
`extend beneath the desired resist pattern 54. The selective
`etching shown results from the directional control provided
`by exciting the NF3 and NH3 gas mixture within the etch
`chamber.
`
`The graph shown in FIG. 3, provides a comparison of the
`etch rates of silicon dioxide, silicon nitride and polysilicon
`and illustrates the variance in the etch rate over time. As can
`be interpreted from this graph, a selectivity of greater than
`fifty (50) for silicon dioxide over polysilicon and of greater
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`than five (5) for silicon dioxide over silicon nitride have
`been achieved.
`The effective removal of a native oxide has been demon—
`strated using XPS analysis, with an example of the results
`shown in FIG. 4. The initial surface of the substrate was
`cleaned using the RCA or Huang process and the substrate
`was characterized as containing silicon, oxygen, fluorine and
`nitrogen in the respective ratio of 58.72:37.56:0.49:0. The
`oxygen peak 70 is characteristic of silicon dioxide. Follow-
`ing the reaction of the NH4F+with the SiO2 layer, the ratio
`of Si, O, F, and N in the surface was 27.66:2.96:48.46:9.44,
`respectively. After the removal of the AHFS by baking or
`rinsing, the surface contained Si, O, F and N in the ratio of
`68.51:6.08:3.94;1.03. This graph shows an oxygen peak
`which is characteristic of subsurface oxygen.
`FIGS. 5A and 5B further illustrate the selective etching
`effect of this invention. FIG. 5A illustrates a substrate 100
`with deposits of silicon nitride (SiN4) 102 and polysilicon
`(Si) 104 and a film of silicon dioxide (SiOZ). The ions supply
`energy to the surfaces which are parallel to the sheaths thus
`causing the reaction to be anisotropic.
`FIG. 5B shows the substrate after it has been etched. The
`charged particles etch the substrate in only one direction.
`The exposed surface of SiO2 has been etched down to
`substrate surface 112. In addition, the SiO2 has been etched
`to the surface of the polysilicon 104. Furthermore, walls 108
`and 110, respectively beneath Si3N4 102 and polysilicon 104
`have not been undercut during the etch process. The process
`disclosed in this invention etches only the SiO2 which is
`exposed on the upper surface of the substrate.
`The selective etching of the silicon dioxide with respect to
`silicon and silicon nitride, using the direct activation tech-
`nique discussed supra, also provides a means whereby a
`thermal oxide is non—selectively etched with respect to a
`deposited oxide. This method has produced the etch rates of
`50 for thermal oxide, 60 for PECVD TEOS oxide and 56
`PECVD PSG relative to the etch rate of polysilicon. The
`consistency of the etch rates among these different types of
`oxides is desirable.
`While the invention has been described in terms of its
`preferred embodiments, those skilled in the art will recog-
`nize that the invention can be practiced with modification
`within the spirit and scope of the appended claims.
`Having thus described our invention, what we claim as
`new and desire to secure by Letters Patent is as follows:
`1. A method of producing a silicon dioxide free surface,
`comprising the steps of:
`placing a substrate with a silicon dioxide layer in a
`chamber, said substrate including an etch stop layer
`which is selected from the grOup consisting of silicon
`nitride and polysilicon;
`generating a plasma of reactive radicals from a gas
`mixture, said generating occurring in said chamber; and
`exposing said silicon dioxide layer on said substrate to
`said plasma for a suflicient length of time wherein a
`portion of said silicon dioxide layer forms product
`layers
`containing
`ammonium hexafluorosilicate
`whereby said step of generating said plasma in said
`chamber produces enhanced etching anisotropy during
`said step of exposing in comparison to exposing said
`substrate to a plasma generated remotely from said
`chamber, and wherein said silicon nitride and polysili-
`con layers are substantially afiected than said oxide
`layer.
`2. A method of etching an oxide layer, comprising the
`steps of:
`
`

`

`_ 5,505,816
`
`5
`forming an oxide layer on a substrate;
`positioning said substrate within a chamber;
`generating, in said chamber, a plasma of reactive radicals
`from a gas mixture selected from the group consisting
`of the combination of NH3 and NF3 and the combina—
`tion of CF4, 02, H2 and N2;
`exposing said oxide layer to said plasma for an amount of
`time to form a removable product layer, said step of
`exposing being performed independent of a process
`used in said forming step and proceeding at the same
`rate for oxide layers formed by difierent processes
`during said forming step.
`3. A method, as recited in claim 2, wherein said step of
`forming is performed by a process selected from the group
`consisting of thermally growing oxides and depositing tet-
`raorthosilicate oxide and depositing phosphosilicate glass.
`4. A method, as recited in claim 2, further comprising the
`step of removing said removable product layer.
`5. A method, as recited in claim 4, wherein said step of
`removing said product layers includes baking at a tempera-
`ture greater than 100° C.
`6. A method, as recited in claim 4, wherein said step of
`removing said product layers includes rinsing in water.
`7. A method of non-selectively etching silicon dioxides,
`comprising the step of:
`exposing a substrate with at least two different oxide types
`to reactive radicals, said oxide types being selected
`from the group consisting of thermal oxides and depos—
`ited oxides, whereby said exposing consumes said
`oxide types at a substantially equal rate, the consumed
`oxides forming product layers.
`8. A method of non-selectively etching oxides, compris-
`ing the step of:
`exposing a substrate with at least two different oxide types
`to reactive radicals, said oxide types being selected
`from the group consisting of thermal oxides and depos-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`6
`ited oxides, whereby said exposing consumes said
`oxide types at a substantially equal rate, the consumed
`oxides forming product layers, and wherein said reac-
`tive radicals are formed by exciting a gas mixture
`selected from the group consisting of the combination
`of NH3 and NF3 and the combination of CF4, 02, H2
`and N2, whereby said exposing consumes said oxide
`types at a substantially equal rate, the consumed oxides
`forming product layers.
`9. A method of non-selectively etching oxides, compris-
`ing the steps of:
`exposing a substrate with at least two difierent oxide types
`to reactive radicals, said oxide types being selected
`from the group consisting of thermal oxides and depos—
`ited oxides, whereby said exposing consumes said
`oxide types at a substantially equal rate, the consumed
`oxides forming product layers; and
`removing said product layers.
`10. A method, as recited in claim 9, wherein said step of
`removing said product layers includes rinsing in water.
`11. A method, as recited in claim 9, wherein said step of
`removing said product layers includes baking at a tempera—
`ture greater than 100° C.
`12. A method, as recited in claim 8, further comprising the
`step of removing said product layers.
`13. A method, as recited in claim 12, wherein said step of
`removing said product layers includes baking at a tempera—
`ture greater than 1000 C.
`14. A method, as recited in claim 12, wherein said step of
`removing said product layers includes rinsing in water.
`15. A method, as recited in claim 7, further comprising the
`step of removing said product layers.
`*
`*
`*
`*
`*
`
`