.
`
`United States Patent [19]
`Wang et al.
`
`lIllllllllllllIlllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`[in Patent Number:
`5,219,485
`[45] Date of Patent:
`Jun. 15, 1993
`
`USOO5219485A
`
`[54] MATERIALS AND METHODS FOR
`ETCHING SILICIDES POLYCRYSTALLINE
`SILICON AND POLYCIDES
`
`[56]
`
`[75] Invent0r5= David N- Wang, Cupe?ino; Mei
`Chang, San Jose; T. K. Leong,
`deceased, late of Santa Clara, all of
`Calif.; Peter P. Leong, executor,
`Vancouver, Canada
`_
`_
`_
`‘[73] Ass1gnee: .egpilfled Materials, Inc., Santa Clara,
`
`References Cited
`U'S' PATENT DOCUMENTS
`3,951,709 4/1976 J
`b ................................. .. 156 643
`4,203,800 5/1980 lzictgher et a1. .
`....... .. 156§643
`4,478,678 10/1984 Watanabe ...................... .. 156/643
`4,543,597 9/1985 Shibata . . . . . . . . . . .
`. . . .. 156/643 X
`4,615,764 10/1986 Bobbio et al. ..................... .. 156/643
`
`Primary Examiner-Thi Dang
`Attorney, Agent, or Firm__philip A_ Dalton
`
`[21] Appl. No.: 778,326
`[22] Filed:
`Oct 17, 1991
`
`.
`Related Us. Application Data
`
`[60]
`
`,
`,
`_
`DlVlSlOl'l of Ser. No. 443,811, Nov. 29, 1989, Pat. No.
`5,112,435, which is a continuation of Sei'. No. 185,256,
`Apt 19, 1988, abandoned, which is a continuation of
`Ser- No. 786,783, Oct. 11, 1985, abandoned,
`
`ABSTRACT
`[57]
`Gas chemistry and a related RIE mode process is de
`scnbed for etching silicides of the refractory metals
`titanium, tantalum, tungsten and aluminum and for etch
`ing composites of these silicides on polycrystalline sili
`-
`-
`co“ layers‘ Bch ‘5 .afided t° the HCV (3.12 gas °.h.em‘s"y
`used for the polyslllcon etch along with additives se
`lected from fluorinated gases and oxygen to satisfy the
`‘ multiple requirement of the two-step silicide-polysilicon
`etch process, including the silicide-to-polysilicon etch
`5267540;
`(gis """""""""""""""""
`156/646; 156/656; 156/657; 156/662 M
`[58] Field of Search ............. .. 156/643, 646, 656, 657,
`156/662; 252/793
`
`.
`
`.
`
`. ................................. .,
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`.
`
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`tic requirement
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`
`‘HEXODE TEMP: 45 50
`
`LAM Exh 1006-pg 1
`
`

`
`US. Patent
`
`June 15, 1993
`
`Sheet 1 of 11
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`5,219,485
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`
`June 15, 1993
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`
`LAM Exh 1006-pg 3
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`

`
`US. Patent
`
`June 15,1993
`
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`june 15, 1993
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`LAM Exh 1006-pg 5
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`

`
`US. Patent
`
`June 15, 1993
`
`Sheet 5 of 11.
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`5,219,485
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`
`LAM Exh 1006-pg 6
`
`

`
`US. Patent
`
`June 15, ‘1993
`
`Sheet 6 of 11
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`5,219,485
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`LAM Exh 1006-pg 7
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`

`
`U.S. Patent
`
`June 15, 1993
`
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`
`LAM Exh 1006-pg 8
`
`

`
`US. Patent
`
`June 15, 1993
`
`Sheet 8 of 11
`
`5,219,485
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`figure 18
`
`LAM Exh 1006-pg 9
`
`

`
`US. Patent
`
`June 15, 1993
`
`Sheet 9 of 11
`
`5,219,485
`
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`
`LAM Exh 1006-pg 10
`
`

`
`US. Patent
`
`J0me 15, 1993
`
`Sheet 10 of 11
`
`5,219,485
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`
`LAM Exh 1006-pg 11
`
`

`
`LAM Exh 1006-pg 12
`
`

`
`1
`
`MATERIALS AND METHODS FOR ETCHING
`SILICIDES, POLYCRYSTALLINE SILICON AND
`POLYCIDES
`
`This is a divisional application of Ser. No. 443,811,
`?led Nov. 29, 1989, now U.S. Pat. No. 5,112,435, which
`is a continuation of Ser. No. 185,256, ?led Apr. 19, 1988,
`now abandoned, which is a continuation of Ser. No.
`786,783, ?led Oct. 11, 1985, now abandoned.
`
`25
`
`35
`
`5,219,485
`2
`FIG. 1 schematically illustrates an etching system 10
`that is one presently preferred system for reactive ion
`etching. This system 10 is available commercially from
`Applied Materials, Inc. of Santa Clara, Calif. as the 8100
`Series System. This system 10 utilizes a cylindrical reac
`tion chamber 11 and a hexagonal cathode 12 which is
`connected to an RF power supply 13. An exhaust port
`14 communicates between the interior of the reaction
`chamber and a vacuum pump. The walls of the reaction
`chamber 11 and the base plate 16 form the grounded
`anode of this system. A supply of reactive gas from gas
`supply 17 is communicated to the interior of the cham
`ber 10 through an entrance port 18 and by a conduit
`arrangement 19 to a gas distribution ring 20 at the top of
`the chamber. The reactor 10 is asymmetric. That is, the
`anode-to-cathode ratio is slightly greater than two
`to-one, resulting in high energy bombardment of the
`hexagonal cathode surface 12 relative to the anode
`surface 11. Such a design provides lower power density
`20
`and better etch uniformity, decreases contamination of
`and from the chamber walls and promotes a higher
`anisotropy. Additionally, the cathode structure config
`uration allows all wafers to be vertically oriented dur
`ing the process to minimize wafer exposure to particu
`lates.
`Despite the availability of plasma etching systems
`such as the AME 8100 System 10, the microelectronics
`polycrystalline silicon processing technology, like the
`rest of the classic microelectronics technology, has been
`strained by the increasing levels of silicon integrated
`circuit integration. Polysilicon has been and is widely
`used in both bipolar and CISFET 1G technology, for
`example in conductors, such as gate electrodes; in single
`level and multi-layer interconnects; in resistors; in bur
`ied contacts; and in the formation of emitter structures
`such as shallow self-aligned emitters and self-aligned
`emitter-contact structures. However, meeting the sheet
`resistance requirements in very small devices and con
`ductors requires very high polysilicon doping levels
`which are obtained at the cost of isotropic etch behav
`ior and precise pattern transfer.
`Over the last several years, the microelectronics in
`dustry has been developing polycide technology as a
`substitute for polysilicon technology in a number of
`applications, in part because polycides have much
`lower sheet sensitivities than doped polysilicon. Poly
`cide is a layer of metal silicide over a layer of polysili
`con. Of primary interest here are the refractory metal
`silicides (typically disilicides): titanium silicide, TiSix;
`tantalum silicide, TaSix; molybdenum silicide, MoSix;
`and tungsten silicide, WSix.
`POLYCIDE ETCH REQUIREMENTS
`In general, there are certain requirements which must
`be satisfied when etching any material, including con
`ductive layers, during intergrated circuit fabrication.
`The conductive layer should be etched to an anisotropic
`pro?le (vertical or slopping) with the minimum line
`width loss in the masking layer and etched material.
`There should be good etch selectivity to overlying
`layers (principally the mask) and to underlying layers.
`Preferably there is a moderate to high etching rate asso
`ciated with the etching step (multiple steps in the case of
`polycide). The etch step(s) should be residue free. In
`addition, the etch process must provide uniform charac
`teristics which are reproducible from run-to-run. Also,
`device damage must be avoided.
`
`60
`
`BACKGROUND OF THE INVENTION
`The present invention relates in general to a process
`for etching conductive layers used in semiconductor
`integrated circuits (IC). In particular, the invention
`relates to methods for etching metal silicides, polycrys
`talline silicon (polysilicon) and composite silicide
`polysilicon (polycide)_structures and to reactive plasma
`gas chemistry for use in such methods.
`,
`Over the past several years, the silicon integrated
`circuit technologies used in manufacturing conductor
`insulator-semiconductor ?eld effect transistor (CIS
`FET) devices and bipolar transistor devices have devel
`oped to the point that they provide very small geome
`try, highly dense integrated circuits. The continued
`improvement in silicon integrated circuit integration
`has been made possible by advances in the manufactur
`ing equipment, as well as in the materials and methods
`used in processing semiconductor wafers and IC chips.
`At the same time, however, the increasingly stringent
`requirements imposed by the improvements in the sili
`con integrated circuit integration and density have
`strained much of the classic microelectronics process
`ing technology. For example, with the trend toward
`greater device densities and smaller minimum feature
`sizes and smaller separations in integrated circuits, the
`sheet resistance of multi-level interconnects and gate
`electrodes and‘other conductors becomes a primary
`factor affecting frequency characteristics and power
`consumption, and in limiting device speed. Thus, to
`successfully implement greater density without ad
`versely affecting such characteristics, it is necessary to
`reduce the sheet resistance of the gate and conductor
`materials.
`Another requirement which must be met to achieve
`the increasingly small minimum feature sizes and mini
`mum separations is that the lithographic pattern-trans
`fer process must be very precise. In addition to factors
`such as the lithographic process itself and the wafer
`topography, satisfaction of this requirement necessitates
`in general the use of an anisotropic plasma or dry etch
`ing technology that is capable of precisely replicating
`the mask dimensions and size in the etched layer with
`out degradation of the mask and loss of line width.
`The two basic types of plasma etching systems
`plasma etching itself in which the chemical etching
`component is dominant and reactive ion etching in
`which physical ion bombardment is dominant-are
`described in commonly assigned U.S. Pat. No.
`4,376,672, entitled, ‘Material and Methods for Plasma
`Etching of Oxides and Nitrides of Silicon‘, ?led Oct. 26,
`1981 and issued Mar. 15, 1983. That description is
`hereby incorporated by reference. Of the different types
`of plasma etching systems, it is believed that reactive
`ion etching systems are the preferred systems for
`achieving high resolution replication of photoresist
`patterns, for example, in electrically conductive materi
`als.
`
`45
`
`50
`
`55
`
`65
`
`LAM Exh 1006-pg 13
`
`

`
`5,219,485
`3
`The art has used fluorinated CF4/O2 gas mixtures in
`a high pressure plasma etching mode to provide aniso
`tropic etch pro?les of titanium silicide. In addition, at
`lower pressures, typically 3-200 millitorr, using the
`reactive ion etching (RIE) mode, tungsten, molybde
`num and tantalum silicides have been etched anisotropi
`cally (as has titanium silicide) using ?uorinated gas
`chemistry. However, while ?uorinated chemistry ap
`plied in a low pressure, RIE mode can provide anis
`tropic clean etching of silicides, there is a tendency to
`undercut polysilicon and to low selectivity to oxide.
`BCl3/CL2 gas chemistry has been used to etch tanta
`lum polycide anisotropically. In general, chlorinated
`gas chemistry has a lesser tendency to undercut polysili
`con and provides higher selectivity to oxide, but has a
`tendency to leave residues.
`In short, it is very dif?cult to obtain both anisotropy
`and high selectivity to oxide during the silicide etch. At
`least in part because of this difficulty, the art has utilized
`multi-step processes for polycide etching. Using this
`approach, high selectivity to oxide is a requirement
`during etching of the underlying polysilicon, but not
`during the silicide etch step. In fact, it is desirable that
`the silicide etch be able to etch oxide, to remove any
`residual native oxide on the silicide surface and at the
`silicide-polysilicon interface. That is, it is actually pref
`erable to have a low etch selectivity to oxide, rather
`than a high selectivity to oxide, during the silicide etch
`step.
`While the use of a multi-step process for etching
`silicide and polysilicon somewhat eases the difficulties
`associated with single step etching, it substitutes a dif
`ferent set of requirements for each step. First, high
`selectivity to the polysilicon is critical during the sili
`cide etch step. Also, high selectivity to the underlying
`layer such as oxide is critical only during the polysili
`cide etch step. In addition, compatibility between the
`silicide and polysilicon etch steps is required in that the
`silicide etch must not undercut the poly or affect the
`poly etch step performance, and the poly etch in turn
`must not undercut the silicide.
`
`4
`25 and substrate 26. FIGS. 2A and 2B are similar to
`FIG. 2 except that the topographical angles are 0=60°
`and 75°, respectively. The step etch sequence begins in
`FIGS. 3, 3A and 3B using reactive ion etching to re
`move the silicide layer 22._
`As shown in FIG. 3, when the polysilicon is just
`exposed by the silicide etch, a thin silicide residue 27
`remains on the sidewall or riser of the 30° step. Then the
`residue is removed by a subsequent silicide overetch,
`and an RIE mode polysilicon etch is used to remove the
`polysilicon 23. FIG. 4 illustrates when the gate oxide is
`just exposed by the polysilicon etch. At this point, be
`cause of the silicide residual layer 27 and the step angle,
`a residual layer of polysilicon 28 remains on the side
`wall. This residual 28 is removed by a polysilicon ove
`retch of about 20 percent, FIG. 5. Because the polysili
`con etch exposes the gate oxide 25, a high selectivity to
`oxide is required to prevent degradation/destruction of
`the oxide during the poly overetch.
`Similar etch sequences are employed for the 60° and
`75° step structures, but result in thicker silicide sidewall
`residual layers 27A and 27B and thicker polycide resid
`ual layers 28A and 28B. Assuming a suf?ciently high
`selectivity to oxide during the polysilicon etch, these
`residuals can be removed by the polysilicon overetch
`without destroying the gate oxide.
`'
`The result is quite different if, during the silicide etch,
`the silicide-to-polysilicon etch rate ratio is too low to
`remove the silicide residual. That is, if the silicide etch
`ing step cannot meet the etch rate requirement for a
`given topography and therefore does not remove the
`silicide residual on the sidewall/riser before etching
`through the poly and exposing the gate oxide, the
`polysilicon etch step must replace the silicide etch step
`to save the gate oxide. Typically, the polysilicon etch
`step does not effectively etch silicide. The silicide resi
`due along the topographic step thus acts as a micromask
`to form a free-standing silicide-polysilicon ?lament or
`fence along the step. See filaments 29A and 29B shown
`in phantom‘in FIGS. 5A and 5B. The ?lament has a
`height similar to the polysilicon layer thickness and
`departs from the topographic step about the same dis
`tance
`It has been known in the art that a high silicidezpoly
`cide etch rate ratio is required to provide residue-free
`removal of silicide over topographical steps and to
`avoid ?laments along the steps. However, the prior art
`has not quanti?ed the relationship of the silicide etch
`selectivity to vpolysilicon and, in fact, is believed not to
`have recognized the existence of a speci?c relationship
`between this selectivity and the step geometry and
`thickness. Furthermore, the prior art has not provided
`an etch chemistry capable of eliminating the silicide
`?laments and satisfying the other silicide etch require
`ments.
`
`SUMMARY OF THE INVENTION
`In view of the above discussion, it is a primary object
`of the present invention to etch the refractory metal
`silicide-polysilicon sandwich structure while keeping
`linewidth loss to a minimum and with the required
`selectivity to oxide to maintain gate oxide integrity.
`It is also an object of the present invention to provide
`an etching gas composition and an associated plasma
`process for etching silicides of tungsten, molybdenum,
`titanium and tantalum with the high selectivity for
`polysilicon which is required to etch topographical
`structures without leaving silicide residue.
`
`20
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`25
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`35
`
`TOPOGRAPHY ETCH REQUIREMENTS
`In addition to the above requirements, which are
`known in the art, we have discovered a less obvious
`silicide-to-polysilicon etch selectivity requirement. This
`requirement pertains to the step of etching the silicide
`component of polycide structures that are formed on
`non-planar, stepped topography. Stepped topography is
`associated, for example, with recessed or semi-recessed
`dielectric isolation and with multi-layer metal intercon
`nect structures. Speci?cally, we have determined that
`the ?laments or fences frequently formed adjacent the
`steps in polycide etching are eliminated by a sufficiently
`high silicide=polysilicon etch rate ratio, R. We have
`quanti?ed that etch rate ratio as a function of polycide
`?lm thickness, topographic step height and angle.
`FIGS. 2-5, 2A-5A and 2B-5B schematically depict
`the process of sequentially etching polycide structures
`formed over topographical steps of different angles,
`0:30’, 60' and 75°. A multiple step reactive ion etch
`process is assumed, involving separate silicide and
`polysilicon etch steps and a silicide etch step which
`provides a silicide:polysilicon etch rate ratio of about
`2:1. The structure 21 of FIG. 2 is a starting silicide
`22-on‘polysilicon 23 sandwich formed over a topo
`graphical step 24 of silicon oxide that forms a relatively
`shallow step angle (0:30“) with the ‘oxide underlayer
`
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`It is a related object of the present invention to pro
`vide a gas etching composition and an associated plasma
`etching process for anisotropically etching silicides
`with a high selectivity for overlying photoresist mask
`layers and for underlying polysilicon layers, with a
`relatively low-to-moderate selectivity for oxide.
`It is another related object of the present invention to
`provide a silicide etching gas composition and an associ
`ated plasma etch process for silicide, of the type de
`scribed above, in combination with a process for etch
`ing polysilicon anisotropically, without undercutting of
`the silicide and with a high selectivity to oxide; and in
`an overall process that provides high throughput and
`utilizes the same base gas chemistry for the silicide and
`polysilicon etch steps.
`In one aspect, the present invention relates in part to
`the use of I'ICl/Clg gas chemistry as the basic gas chem
`istry for etching refractory metal silicides and polysili
`con. The use of HCl/Clz gas chemistry in the polysili
`con etch step and BCl3-containing HCl/Clz gas chemis
`try in the silicide etch step permits an essentially contin
`uous silicide and polysilicon etch process in the same
`chamber without breaking vacuum. HCl/Cl; gas chem
`istry provides high rate anisotropic etching of the un
`derlying polysilicon with selectivity to underlying ox
`ides such as gate oxide layers. BClg-containing I'ICl/Cl2
`gas chemistry selectively doped with relatively small
`volumetric amounts of dopant gas not only etches all
`four refractory metal silicides anisotropically, but also
`provides the required high etch rate ratio of silicide to
`polysilicon to provide a clean, ?lament free etch. In
`addition, the BCl3/HCl/Cl2 gas chemistry provides a
`high selectivity to organic photoresist masks and the
`desired etchability of oxide. In more general terms, the
`process is consistent with requirements of anisotropic
`polycide pro?les, minimum linewidth loss, cleanliness,
`uniformity and reproducibility.
`
`15
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`25
`
`BRIEF DESCRIPTION OF THE DRAWING
`FIG. 1 is a schematic illustration of a plasma etcher
`which can be used to etch silicide and polysilicon in
`accordance with the present invention;
`FIGS. 2-5; 2A-SA; and 2B—5B schematically depict
`the process of etching polycide structures formed on
`topographical steps of 0=30°, 60° and 75°, respectively;
`45
`FIGS. 6 and 7 illustrate the dimensions and geometri
`cal relationships associated with step topography of
`small angles and large angles, respectively;
`FIGS. 8 and 9 graphically illustrate the required etch
`rate ratio of silicide to polysilicon for different topo
`graphical angles 0;
`FIG. 10 depicts etch rate as a function of BCl3 ?ow
`for molydenum silicide;
`FIG. 11 depicts etch rate as a function of (BCl3+Cl2)
`flow for molybdenum silicide;
`FIG. 12 depicts etch rate as a function of chlorine
`?ow for silicides and polysilicon;
`FIG. 13 depicts etch rate as a function of DC bias
`(RF power) for titanium and molybdenum silicides and
`polysilicon;
`‘
`FIG. 14 depicts etch rate as a function of chamber
`pressure for titanium and molybdenum silicides and -
`polysilicon;
`FIG. 15 depicts etch rate as a function of NF3 ?ow
`for tungsten silicide and polysilicon;
`65
`FIGS. 16-19 depict tungsten/molybdenum silicide
`etch rates and polysilicon etch rates as a function of
`additive gas flow at various hexode temperatures;
`
`55
`
`6
`FIGS. 20 and 21 illustrate the effect of hexode tem
`perature on polysilicon etch rate and on molybdenum
`silicide etch rate for etching gases which are devoid of
`and contain a small volume percentage of additive gas,
`respectively;
`FIG. 22 illustrates the effect of HCl flow on the etch
`rates of polysilicon, molybdenum silicide and tungsten
`silicide;
`‘FIG. 23 depicts the loading effect on polysilicon and
`tungsten silicide etch rates of different BCl3+Cl2 flow
`rates using the same total reactant gas flow rate; and
`FIG. 24 depicts the effect of pressure on the etch rate
`of molybdenum silicide and on the overall selectivity
`over photoresist.
`DETAILED DESCRIPTION OF THE
`INVENTION
`1. Determination of R (SilicidezPolysilicon Etch Ratio)
`
`a. Formulation
`Referring to FIGS. 6 and 7, to quantitatively describe
`the required etch rate ratio, R, of silicide to polysilicon,
`it is assumed the silicide and polysilicon ?lms 22 and 23
`(FIG. 2) have conformal coverage; b1 and hg are the
`thickness of the silicide and polysilicon, respectively; hs
`is the topographic height of oxide step 24; and 0 is the
`topographic angle (O-EO 2 90°).
`When the polysilicon 23 is just exposed during the
`silicide etching step, the thickest remaining silicide is
`always located at the foot point C as shown in FIGS. 6
`and 7. There are two portions on the topographic step:
`one 23C is curved, the other 23L is linear; they are
`separated at boundary point A.
`If the boundary point A is behind the foot point C as
`shown in FIG. 6, that is, when the topographic angle 0
`is low, then the thickest remaining silicide will be lo
`cated in the linear portion and is given by:
`
`hi
`h, :
`0050
`
`l - c050
`c050
`
`(1)
`
`The required etch rate ratio of silicide to polysilicon is
`then equal to the remaining silicide to polysilicon thick
`ness ratio:
`
`1 — cos6
`e050
`
`If the boundary point A exceeds the foot point C, as
`shown in FIG. 7, that is, whenthe topographic angle 0
`is high, the thickest remaining silicide is in the curved
`portion and the thickness will be:
`
`2
`
`l
`
`The required etch rate ratio then will be:
`
`(4)
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`-continued
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`5,219,485
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`2
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`1
`
`When the topographical step is vertical, i.e. 0:90":
`
`5
`
`,2 {b2 — (hl + k2) + [(111 + m2 - 112214}.
`
`(5)
`
`10
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`20
`
`25
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`35
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`40
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`45
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`55
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`h;
`0
`[(111 + k3)2 —— (lane + h; tan?) 1
`
`8
`crease in molybdenum silicide etch rate which is
`achieved for BCl3/Cl;/HC1 chemistry by adding in
`creasing amounts of BCl3+Cl; to the HCl flow of 75
`sccm at -25 millitorr pressure and — 300 volts DC bias.
`Using the stated conditions, increasing the volume per
`centage of BCl3+Cl; from 20 percent to 50 percent of
`the total gas flow increased the silicide etch rate from
`about 900 to 1100 Angstroms per minute.
`FIG. 12 illustrates the effect of chlorine on silicide
`etch rates. In addition, the ?gure illustrates the above
`b. Discussion of Process Requirements
`mentioned dichotomy in the etch behavior of titanium
`and tantalum silicides on the one hand, and tungsten and
`Using equation (2) and (4), the required etch rate
`molybdenum silicides on the other. That is, this ?gure
`ratios are plotted as a function of 0 in FIG. 8 and FIG.
`depicts the increases in the silicide etch rate and in the
`9, using hl/h; (curve 34, FIG. 8) and hS/h1+h; (curve
`silicidezpolysilicon etch rate ratio which are achieved
`35, FIG. 9) as discrete parameters.
`The required etch rate ratio monatonically increases
`for BCl3/Cl; gas chemistry by L adding increasing
`when topographical angle 0 increases. The steepest rate
`amounts of chlorine to the BC]; ?ow of 40 sccm. Curve
`of increase is around 45°—60°. For small 0, the required
`35 shows the variation in polysilicon etch rate as a func
`etch rate ratio is independent of topographical step
`tion of chlorine flow rate using 25 millitorr pressure and
`height and is related to the layer thickness ratio of sili
`-— 350 volts DC bias. Curves 36 and 37 illustrate the etch
`cide to polysilicon. At high 0, both the step height and
`rates of molybdenum silicide and tungsten silicide under
`thickness ratio play roles in the required etch rate ratio.
`the same conditions..The molybdenum etch rate curve
`Using equation (2) and (4), one can calculate the re
`36 and the tungsten curve 37 nearly parallel the polysili
`quired etch rate ratio for a given device topographical
`con curve 35 and have etch rate ratios, R, relative to the
`structure. As a typical example, for 3,000 Angstroms
`polysilicon of slightly greater than 1:1. In contrast,
`silicide and 2,000 Angstroms doped polysilicon over a
`under the same conditions, the tantalum curve 38, has
`5,000 Angstroms, ~ 90“ steep step the required etch rate
`R2221 for chlorine flows greater than about 20 sccm
`ratio is about 2.3 to etch clean. In general, R of (1-2):1
`and, preferably, ~2zl is required for present topogra
`(that is, over the entire range of ?ow rates of FIG. 12).
`30
`phies. Using BCl3-containing HCl/Cl; gas chemistry R
`‘ Similarly, for the titanium curve 39, R22 over the
`values of (l—2):l are readily obtained for tantalum sili
`entire range of flow rates of FIG. 12 and R=(3-4):l
`cide and titanium silicide, but not for molybdenum and
`over much of the flow rate range.
`tungsten silicides.
`Furthermore, FIGS. 13 and 14 illustrate that these
`etch rate ratios are relatively insensitive to RF power
`2. Gas Chemistry for Required SilicidezPolysilicon R
`(DC bias voltage) and to chamber pressure, at least over
`a. BCl3-Containing Cl; Gas Chemistry
`a substantial range of conditions. The data of FIG. 13
`We have found that including BC13 in the chlorinated
`were taken using BCl3 and Cl; ?ow rates of 40 sccm
`etching gas solution promotes cleanliness of the sub
`each and a chamber pressure of 25 millitorr. The change
`strate during the etch process and maintains the desired
`in the etch rates of the titanium curve 43 and the molyb
`pro?le of the etched material (that is, prevents under
`denum curve 42 as a function of DC bias voltage do not
`cutting). Speci?cally, we have found that this dual pur
`differ signi?cantly from the slope of the polysilicon etch
`pose is satis?ed by using BC]; and Cl; ?ow rate ratios in
`rate curve 41. Also, the titanium, molybdenum and
`which BCI3C12§ 1:1.
`polysilicon curves 46, 45 and 44 of FIG. 14, associated
`The BCl3 does not substantially enhance the etch rate
`with BCl3 and Cl; flow rates of 40 sccm and —350 volts
`of the silicides. This is illustrated by curve 32 of FIG.
`DC bias, have similar relative slopes. Consequently, DC
`10, which depicts the molybdenum silicide etch rates
`bias and pressure per se cannot be relied upon under
`which are achieved for Cl;/HCl gas chemistry by add
`these conditions to increase the etch rate ratio of molyb
`ing increasing amounts of BC13 to the Cl; and HCl flow
`denum and, presumably, of tungsten silicides. (It should
`of 30 sccm and 75 sccm at — 15 millitorr pressure and
`be noted that despite the relative insensitivity of the
`—300 volts DC bias. The curve indicates the BC]; ?ow
`rate has little effect on the molybdenum silicide etch
`titanium etch rate ratio and the tantalum etch rate ratio
`rate. If anything, the molybdenum silicide etch rate is
`to power (DC bias voltage) and pressure, those etch
`decreased slightly by increasing the ?ow of BC13.
`rate ratios were at an excellent, high level throughout
`Data listed here for silicide and polysilicon polycide
`FIGS. 13 an 14.)
`?lms were for ?lms which were formed on oxide layers
`In short, the BCl3/Cl2 chemistry provides excellent
`formed on single crystal silicon substrates. The polysili
`silicidezpolysilicon etch rate ratios for titanium and
`con ?lms were highly doped n-type with phosphorus to
`tantalum silicides which are believed sufficient for es
`a concentration which would provide sheet resistivity
`sentially all present topographical geometries. How
`of about 15-20 ohms per square for 5000 Angstrom
`ever, the tungsten and molybdenum silicidezpolysilicon
`thick ?lms. The silicide and polysilicon ?lms were
`etch rate ratio of about 1:1 is still too low for many
`etched by using photoresist masks about one micron
`topographical IC devices. It should also be noted that,
`thick and etching using the above-described Applied