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`Global Foundaries US v. Godo Kaisha
`Global Ex. 1011
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`Page 1 of 10
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`US. Patent—Sep. 18, 1990 Sheetlof2 4,957,590
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`44
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`SS
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`rzeree
`Lip nd
`UPIESSESS
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`<LALMGy
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`NAKnH7r
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`i,
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`<
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`3 WrzLp
`y SASSIZ
`iLilliteiels.
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`SSS
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`Page 2 of 10
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`Page 2 of 10
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`US. Patent—Sep. 18, 1990 Sheet 2 of 2 4,957,590
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`1
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`4,957,590
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`METHOD FOR FORMING LOCAL
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`INTERCONNECTS USING SELECTIVE
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`ANISOTROPY
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`This application is a continuation-in-part of applica-
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`tion S.N. 273,287 filed Nov. 17, 1988, now U.S. Pat. No.
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`4,863,559, which is a continuation of application S.N.
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`159,282 filed Feb. 22, 1988, now U.S. Pat. No. 4,793,896
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`issued Dec. 27, 1988.
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`This invention relates in general to semiconductor
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`devices, and in particular to an improved method for
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`forming local
`interconnects using chlorine bearing
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`agents.
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`2
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`with the n-type dopant through thesilicide intercon-
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`nect.
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`Another known method uses molybdenum metalas a
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`interconnect material. Molybdenum, however,
`local
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`also acts as a diffusion conduit through which phospho-
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`rus, used to dope n-type regions of the semiconductor
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`device, can diffuse. The molybdenum interconnect
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`therefore is not an effective local interconnect between
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`n-type and p-type regions, as the p-type regions can be
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`undesirably counterdoped by the phosphorousdiffusing
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`through the molybdenum,similarly as the silicide strap
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`interconnect.
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`Anotherlocal interconnection method is disclosed in
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`U.S. Pat. No. 4,675,073, issued to me on June 23, 1987,
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`and assigned to Texas Instruments Incorporated, incor-
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`BACKGROUND OF THE INVENTION
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`porated herein by this reference. As disclosed therein,
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`the desired local interconnect is formed by patterning
`Increasing the number of levels of interconnects
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`the residual titanium compound, for example titanium
`(both intra-level and inter-level) in integrated circuits
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`nitride, from the direct reaction forming titanium sili-
`provides additional routing capability, more compact
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`cide cladding of the diffusions and polysilicon gates.
`layouts, better circuit performance and greater use of
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`Thetitanium nitride is sufficiently conductive so thatit
`circuit design within a given integrated circuit surface
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`is useful to make local interconnections between neigh-
`area. A particularly useful level of connection is com-
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`monly called local interconnection, where neighboring
`boring regions. The disclosed process uses carbontetra-
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`diffused areas are connected to one another, and to
`fluoride (CF4) as the reactant in a plasma etch to re-
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`move the undesired titanium nitride faster than titanium
`neighboring polysilicon and metal lines.
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`silicide. This plasma etch using carbon tetrafluoride
`For example, a conventional method for creating
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`local interconnects uses metal interconnection of dif-
`etches titanium nitride or titanium oxide at approxi-
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`mately twice the rate it removes titanium silicide. This
`fused regions to one another, as well as to other layers.
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`technique also etches silicon oxides at twice the rate,
`The metal interconnection is formed by etching vias
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`through a thick oxide layer to the locations to be inter-
`and photoresist at five times the rate, as it etches tita-
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`connected. A conductor is then formedto fill the vias
`nium nitride or titanium oxide. Additionally, products
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`of the etching process include solids that tend to adhere
`and make the connection. This method is limited, for
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`to the etching device. This requires extra maintenance
`purposesof reducing the area required for such connec-
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`and cleanup time that is nonproductive. Thus, a need
`tion, by the state of the technology of etching contact
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`has arisen for a method for producing a local intercon-
`holes and the planarization of interlevel dielectrics.
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`nect with increased selectivity to the refractory metal
`These limitations include the alignment tolerance of the
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`compound ofthe local interconnect(e.g., titanium ni-
`vias to the underlying region to be connected, the size
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`tride or titanium oxide) relative to silicides, silicon ox-
`of the via required (and accordingly the size of the
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`ides and photoresist, so that an additional layer of inter-
`contact area in the underlying region) which can be
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`connection may be more consistently manufactured
`reliably etched, and the step coverage of the conductor
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`with precisely located interconnects and improved pla-
`in filling the via and making good ohmic contact to the
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`narization compatible with sub-micron technology.
`underlying region. Also, the additional layer of a metal-
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`For purposes of forming such smail feature sizes for
`lic conductor across the dielectric contributes to a loss
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`the interconnect,it is well knownthat etches which are
`of planarization in subsequentlevels.
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`substantially anisotropic (i.e., directional) are preferred.
`An alternative methed developed by Hewlett Pack-
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`Anisotropic etches provide improved control in fabri-
`ard and published at page 118 of the 1984 [EDM Pro-
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`cation, since the patterned masking material more
`ceedings uses additional patterned silicon to provide
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`closely defines the feature to remain after the etch;
`conductive silicide regions extending over the field
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`etches which are more isotropic tend, of course,
`to
`oxide as desired. A layer of titanium is deposited over
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`undercut the mask, requiring that the size of the pat-
`the substrate and, prior to the direct reaction of the
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`terned masking material be made larger than that of the
`titanium with the underlyingsilicon to form thesilicide,
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`desired feature in order to compensate for the line width
`a thin layerof silicon is patterned on top ofthe titanium
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`loss resulting from the undercut.
`metal to define an interconnect extending overasilicon
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`However, substantially anisotropic etches can leave
`dioxide region separating the two regions to be inter-
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`filaments of the material being etched. Such filaments
`connected. Wherethis silicon layer remains,a silicide is
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`tend to occur at locations where the etched material
`formed during the reaction process extending over the
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`makesa step over surface topography.In the case of the
`oxides. This method requires the deposition and pat-
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`local interconnect discussed above, such filaments can
`terning of the additional layer ofsilicon to define the
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`create a short circuit between conductive structures at
`local interconnection. In addition, the resulting silicide
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`the top of, and at the bottom of, a step where no such
`strap provides a conduit through which typical n-type
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`connection is desired. In addition, filaments may be
`dopants such as phosphorouscan diffuse, since titanium
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`present
`laterally, connecting two otherwise uncon-
`silicide is a very poor diffusion barrier to conventional
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`nected structures.
`semiconductor dopants. If a silicide strap is used to
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`It should also be noted that the titanium nitride may
`connect n-type regions to p-type regions, for example
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`be used as a gate material, especially in the case where
`n-doped polysilicon to p-type diffusion, subsequent
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`it is deposited. Similar problems concerning the etch as
`processing must be doneatrelatively low temperatures
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`described above will also affect the etch of such tita-
`to minimize the counterdoping of the p-type region
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`Page 4 of 10
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`Page 4 of 10
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`3
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`nium gate electrodes, as well as other structures for
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`which a conductive titanium compound may be used.
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`It is therefore an object of this invention to provide
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`an etch for the residual material over insulating layers
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`remaining from the direct react silicidation which is
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`anisotropic at the locations where the masking layeris
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`in place to define the interconnect, but which is iso-
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`tropic elsewhere so that filaments of the interconnect
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`material are removed.
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`It is a further object of this invention to provide such
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`an etch which also has improvedetch selectivity (i.e.,
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`an increased etch rate ratio) of the interconnectrelative
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`to underlying conducting layers.
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`It is a further object of the present invention to pro-
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`vide such an etch with improvedselectivity to titanium
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`nitride or titanium oxide with respect to titanium sili-
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`cide, silicon dioxide and photoresist.
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`It is a further object of the present invention to pro-
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`vide such an etch which reduces the preventative main-
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`tenance and cleanup schedules and procedures by the
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`use of a chlorine bearing agent as opposed to a flourine
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`agent.
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`Other objects and advantages of the invention will
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`become apparent to those of ordinary skill in the art
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`having reference to the following specification in con-
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`junction with the drawings.
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`SUMMARYOF THE INVENTION
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`The invention may be incorporated into a method for
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`forming a local interconnect on a semiconductorsur-
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`face. A dielectric layer of a prefabricated integrated
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`circuit is covered with a conductive chemical com-
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`pound ofa refractory metal, such as titanium. The com-
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`pound may be formed by deposition, or as a by-product
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`of the silicidation of the refractory metal at locations
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`whereit is in contact with the underlying semiconduc-
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`tor. Photoresist is placed over this chemical compound
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`layer to protect a specific portion thereof which will
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`serve as an interconnect. A chlorine bearing agentis
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`used to etch the exposed conductive chemical com-
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`pound layer, which occurs in substantially an aniso-
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`tropic manner at those locations where the photoresist
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`is in place. At those locations where the photoresistis
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`absent, the etch is substantially isotropic. Accordingly,
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`the etch removes filaments which may otherwise re-
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`main upon clearing of the surfaces, without undercut-
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`ting the photoresist mask. The chlorine bearing agent
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`etches the conductive chemical compound at a greater
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`rate than the underlyingsilicide, or other material used
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`as a conductor.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`For a more complete understanding of the present
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`invention, and for further advantages thereof, reference
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`is now madeto the following description taken in con-
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`junction with the accompanying drawings in which:
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`FIG. 1 is a cross-sectional view of a partially fabri-
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`cated integrated circuit with a titanium chemical com-
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`pound formed over the entire surface;
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`FIG.2 is a cross-sectional view of the device of FIG.
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`1 with patterned masking material added over the area
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`to be protected;
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`FIG.3 is a cross-sectional view of an integrated cir-
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`cuit with a local interconnect formed in accordance
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`with the present invention.
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`FIGS.4a and 46 are cross-sectional views of an inte-
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`grated circuit with a local interconnect formed accord-
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`ing to an alternate embodiment of the invention.
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`15
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`45
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`65
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`4,957,590
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`4
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`FIG. 5 is a cross-sectional SEM microphotograph
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`illustrating the anisotropy of the etch described herein.
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`DETAILED DESCRIPTION OF THE
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`PREFERRED EMBODIMENTS
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`FIG.1 showsthefirst step utilizing the method of the
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`preferred embodiment of the present invention, as di-
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`rected to an integrated circuit wafer 44. Wafer 44 con-
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`tains a semiconducting substrate 10 formed ofsilicon.
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`is
`Field oxide 12, preferably silicon dioxide (SiOz),
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`grownor deposited in selected portions of the surface of
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`the substrate 10 for isolation of active regions from one
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`another according to the well known local oxidation
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`(LOCOS)isolation technique; of course, other isolation
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`techniques suchas trenchisolation mayalternatively be
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`used. The active transistors of the integrated circuit
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`device are formed into the locations of the surface of
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`substrate 10 not covered with field oxide 12, such loca-
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`tions commonly called moat regions. In FIG. 1, a tran-
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`sistor 44 is shown having source and drain regions 14
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`and 16, respectively, diffused into the moat region be-
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`tween two portionsof field oxide 12. Source and drain
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`regions 14 and 16 are generally implanted and subse-
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`quently diffused after the placementofpolysilicon gate
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`electrode 30 over gate dielectric 24, so that source and
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`drain regions 14 and 16 areself-aligned relative to gate
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`electrode 30. As described in U.S. Pat. No. 4,356,623,
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`issued Nov. 11, 1982 and assigned to Texas Instruments
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`Incorporated, the incorporation of sidewall oxide fila-
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`ments 26 along the side of gate electrode 30 provide for
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`a graded junction, as shown in FIG. 1. FIG. 1 further
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`illustrates a polysilicon layer 42 overlying field oxide 12
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`serving as an interconnect to another portion of the
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`integrated circuit, for example extending to another
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`moat region (not shown) and serving as the gate elec-
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`trode for a transistor.
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`In this embodimentof the invention, source and drain
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`regions 14 and 16, and gate electrodes 30 and 42, are
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`clad with a refractory metal silicide such as titanium
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`silicide. This cladding is performed by depositing a
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`layer of the refractory metal, and subsequently heating
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`the wafer 44 so that the metal directly reacts with the
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`underlying silicon to form thesilicide, as described in
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`U.S. Pat. No. 4,384,301, issued on May 17, 1983 and
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`assigned to Texas Instruments Incorporated. An exam-
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`ple of the conditions for such direct reaction is heating
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`the wafer 44 in a nitrogen and argon ambient at a tem-
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`perature on the order of 675° C. Other methods of
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`achieving the direct reaction may alternatively be used,
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`for example, by use of a single-wafer Rapid Thermal
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`Processor (RTP) where the wafer 44 is rapidly heated
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`to the appropriate temperature for a sufficient amount
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`of time to perform the direct reaction described above.
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`It has been determined by physical analysis that the
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`titanium silicide formed in this manner may notbestoi-
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`chiometrically, or chemically, pure titanium silicide, but
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`that measurable amounts of other materials such as
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`silicon oxide and other oxides are also present in the
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`film.
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`As described in said U.S. Pat. No. 4,675,073, where
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`titanium is used as the refractory metal, as a result of the
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`direct reaction process a layer of a conductive titanium
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`compound covers the surface of the wafer 44,including
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`the silicide regions. Referring to FIG. 1, source region
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`14, drain region 16, and gate electrodes 28 and 42 are
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`shownclad with titanium silicide film 20, 22, 28 and 40,
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`respectively. A layer 43 of residual material containing,
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`for example, titanium nitride if the direct reaction is
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`Page 5 of 10
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`Page 5 of 10
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`

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`4,957,590
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`5
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`performed in a nitrogen atmosphere, remains over the
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`surface; if a layer of oxide (not shown) is provided over
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`the top of the titanium metal layer prior to the direct
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`reaction, as described in U.S. Pat. No. 4,690,730 issued
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`Sept. 1, 1987 and assigned to Texas Instruments Incor-
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`porated, layer 43 may contain titanium oxide. When
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`formed in the nitrogen atmosphere, in the event that
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`oxygen is present (at contamination-level concentra-
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`tion) in the titanium metal or in the atmosphere, or from
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`the underlying silicon dioxide, layer 43 mayalso include
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`titanium oxide, for example in the form of titanium
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`oxynitride (TiO,Ny).
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`Layer 43 will have a thickness on the order of 100 nm
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`over field oxide 12, and will be thinner (e.g., on the
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`order of 40 nm thick) in the locations oversilicide film
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`20, 22, 28 and 40 cladding the underlying silicon or
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`polysilicon. If desired, the thickness of layer 43 may be
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`increased by a second deposition of titanium metal after
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`the initial direct reaction, followed by a second direct
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`reaction in a nitrogen atmosphere, as described in U.S.
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`Pat. No. 4,676,866 issued June 30, 1987 and assigned to
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`Texas Instruments Incorporated.
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`It should further be noted that the instant invention is
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`also applicable to other structures, as well as to similar
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`interconnection structures formed in a different manner.
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`For example, layer 43 may alternatively be formed by
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`way of deposition, for example by chemical vapor de-
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`position or by sputtering of titanium nitride. If layer 43
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`is deposited, it should be noted that the structure may
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`not necessarily include a silicide cladding on the sur-
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`faces of the source/drain regions 14 and 16, or on the
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`gate electrode 30. However, for such a structure the
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`etch process described herein is also beneficial for the
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`selective etching of the interconnect layer 43.
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`Referring to FIG. 2, the next step in the preferred
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`embodiment of the present invention is illustrated. A
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`layer of masking material is deposited over layer 43, and
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`is patterned according to conventional techniques to
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`remain in the locations of the eventual local intercon-
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`nect, as shown by patterned masking material 46 in
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`FIG. 2. The masking material 46 serves to protect the
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`covered portion of layer 43 from subsequent etching.
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`The patterned masking material 46 may be photoresist,
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`or a hardmask material such assilicon dioxide.
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`When photoresist serves as the masking material 46,it
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`is preferable to clean the surface of wafer 44 prior to
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`beginning the etching process, by use of an Oz based
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`resist descum to establish etch rate uniformity, to pre-
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`vent extended etch initiation periods and to eliminate
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`post-etch scumming problemsor incomplete etching of
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`layer 48. An O2 descum time equal to the removal of
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`approximately 400A ofresist is sufficient. Wafer 44 is
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`then hardbakedat 120° C. for approximately 60 minutes
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`in an oven and then O2 descummedagain.It is necessary
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`to redo the descum if more than a three hour delay is
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`incurred after the descum before the local interconnect
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`dry etch described below to avoid incomplete etching
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`of the local interconnect. It should also be noted that
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`the oxygen descum, if performed after hard bake, will
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`removethe resist anisotropically.
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`After patterning masking material 46, wafer 44 is
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`placed in any appropriate plasma etching device such as
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`plasma modeetcher, a reactive ion etcher or a micro-
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`waveafter glow, which are well known in the art. In
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`the preferred embodiment, a plasma mode etcher, not
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`shown, utilized. The plasma mode etcher comprises a
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`symmetrical parallel plate reactor, for example with the
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`substrate grounded. The powered top plate comprises
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`6
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`graphite or titanium, and the bottom plate may com-
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`prise bare aluminum. The plates are spaced approxi-
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`mately one centimeter apart, and approximately 200
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`watts of poweris applied. Radio frequency (RF) energy
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`is transmitted between the parallel plates by an RF
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`generator. The powered electrode also serves as a gas
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`dispersal source similar to a shower head. According to
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`the invention, a chlorine bearing agent such as carbon
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`tetrachloride (CCl4) is used as the dry etch reactant in
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`the plasma mode etcher. Wafer 44 is placed on the
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`groundedplate which is spaced apart from the powered
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`plate by approximately one centimeter. The preferred
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`substrate temperature is on the order of 50° C.
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`Carbontetrachloride is a particularly useful chlorine-
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`bearing agent for this etch because it can exhibit low
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`etch rates of photoresist and silicon dioxide. Carbon
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`tetrachloride also has a carbon center which will serve
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`to “poison” the titanium silicide first layers 20, 22, 28
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`and 40 once layer 43 of the titanium compoundlying
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`over them has been etched. This poisoning, or surface
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`passivation, is due to carbon, probably in the form of a
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`partially dissociated carbon tetrachloride molecule
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`(such as CCl, where x=1, 2 or 3) reacting with either
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`the titanium orthesilicon in the titaniumsilicide film 20,
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`22, 28 and 40 to form silicon carbide or titanium carbide.
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`Since silicon carbide and titanium carbide are both non-
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`volatile, the reactive sites on the titanium silicide first
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`layers 20, 22, 28 and 40 are effectively “tied up”, con-
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`sumed or covered, thus preventing reaction of layers 20,
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`22, 28 and 40 with the other reactive speciesin the etch,
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`namely chlorine. If not passivated, the chiorine in the
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`etch is capable of volatilization and etching ofthe tita-
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`nium or the silicon in silicide film 20, 22, 28 and 40.
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`Moreover, the surface passivation from the chlorocar-
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`bon reagents can be of such a magnitude that a polymer
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`film is formed, so that the surface passivation is on a
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`macroscopic scale rather than a molecular scale.
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`Conversely, the reactive carbon containing species
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`will react with the nitrogen componentofthe titanium
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`nitride in layer 43 to form CN or CN2,each of whichis
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`volatile. If oxygen is present in layer 43, the reactive
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`carbon containing species will react with the oxygen to
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`form carbon monoxide, also volatile. The residual
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`atomic chlorine radicals will react with the titanium
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`componentin the titanium nitride of layer 43 to form
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`titanium chloride (TiCl, where x= 1-4), which is also
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`volatile. Thus, the titanium nitride of layer 43 whichis
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`not protected by patterned masking material 46 will be
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`etched, while leaving the titanium silicide first layers 20,
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`22 28 and 40, field oxide 12, and patterned masking
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`material 46.
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`This mechanism, where the carbon from the carbon
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`tetrachloride builds a residue on the material which is
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`not to be substantially etched, but which reacts with
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`nitrogen in the titanium nitride film to form volatile
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`cyanogen (and with oxygen to form volatile carbon
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`monoxide), can apply to other underlying layers having
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`the same property as the titanium silicide. These other
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`underlying layers can be either insulating or conduc-
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`tive. Examples of conductive material which are be-
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`lieved to have the property of being unable to volatilize
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`the carbon, and which therefore will have a residue
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`buildup thereupon when the titanium nitride is etched
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`with the carbon tetrachloride etch described above,
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`include silicon, cobalt silicide, tungsten, molybdenum,
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`among others.
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`In order to initiate the plasma, it is necessary to de-
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`tach electrons from the chlorocarbon anions present in
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`20
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`25
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`30
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`35
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`45
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`50
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`55
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`65
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`Page6 of 10
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`Page 6 of 10
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`

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`7
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`the plasma gas. Because neutral chlorocarbon species
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`have high cross-sections for electron attachment, the
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`reaction chamber tends to become depleted of free
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`electrons, which are crucial to the ignition of the plasma
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`by the mechanism of electron-impact ionization of other
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`atomic and molecular species. Such electron-impact
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`ionization is generally required to ignite the plasma
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`once the avalanche condition is reached. As is well
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`knownin the art, the electron is an important charged
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`species in plasma ignition since its low massallowsit to
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`be sufficiently energized by an RFelectric field to ion-
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`ize a neutral species.
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`A useful method according to the invention for gen-
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`erating sufficient free electrons to ignite the CCl4
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`plasmais to illuminate the reaction chamberwith a light
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`source, with the light having a wavelength in the range
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`of 180 to 1200 nanometers. In the preferred embodi-
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`ment, an intense emission in the ultraviolet end of the
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`spectrum, such as from a mercury/argon light source,
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`will photodetach electrons from anions in the plasma,
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`such anions having a high cross-section for photoelec-
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`tron detachment. Hence, the light source illumination
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`effectively provides a sufficient free electron concentra-
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`tion by photodetachment from the anions, to permit
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`facile plasma ignition. Alternative techniques for re-
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`plenishing the free electron concentration for otherwise
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`detaching electrons, by introducing new electrons from
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`an auxiliary source or by temporarily increasing the
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`powerof operation to increase the electron energy of a
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`deficient concentration of electrons, can also be used for
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`facilitating ignition.
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`Once the ignition begins, a plasma is formedresulting
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`in electron-impact dissociation of CCl, where x= 1-3,
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`and atomic chlorine radicals. It is also advantageous to
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`add an inert gas, such as helium (He), argon (Ar) or
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`nitrogen (N32) to the carbontetrachloride to add stabil-
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`ity to the plasma and also to improve the etch proper-
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`ties.
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`Molecular and atomic chlorine each can react with
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`titanium orsilicon to form volatile gases. The presence
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`of excess chlorine can thus undesirably etch titanium
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`silicide film 20, 22, 28, and 40. Hence, in addition to
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`passivating the surfaceofsilicide film 20, 22, 28 and 40
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`as described above with the carbon from the CCl, etch-
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`ant,it is also desirable to reduce the chlorine concentra-
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`tion in the reaction chamber, further improving the
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`titanium nitride to titanium silicide etch rate ratio. This
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`can be accomplished by adding chlorine scavenging
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`reagents to the carbontetrachloride. Chlorine scaveng-
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`ing reagents are rea