Page 1 of 19
`
`Samsung Exhibit 10(cid:19)(cid:28)
`Samsung Electronics Co., Ltd. v. Daniel L. Flamm
`
`

`
`U.S. Patent
`
`9.5IenUJ
`
`3991..
`
`Sheet 1 of 11
`
`5,219,485
`
`H:3
`
`mssmmE330«S>imam
`
`>imam
`
`m~*
`
`Page 2 of 19
`
`

`
`U.S. Patent
`
`June 15, 1993
`
`Sheet 2 of 11
`
`5,219,485
`
`22
`
`23 21
`
`22B 233
`
`213
`
`22A 23A
`
`21A
`
`It /4
`
`figure 2521
`
`figure 299
`
`T f
`
`igure 2
`
`figure 3
`
`figure 3% figure 393
`
`figure 454
`
`figure 413
`
`T f
`
`igure 4
`
`i
`25
`;
`3. _1 1
`T e = 60°
`9=30°
`’
`
`29A
`/1'
`
`.
`
`293
`,1‘
`to-\
`'«
`3
`9 = 75°
`
`figure 5 figure 554 ” figure 558
`
`Page 3 of 19
`
`

`
`U.S. Patent
`
`June 15,1993
`
`,
`
`Sheet 3 of 11
`
`5,219,485
`
`Page 4 of 19
`
`

`
`.Iune 15, 1993
`
`Sheet 4 of 11
`
`5,219,485
`
`REQUIREDETCHFIATERATIOOFSIL/C/DETOPOLYSILICON
`
`2
`
`‘L
`
`9%l~
`O
`35;
`“$1”
`n:c3'
`5%
`l~l-
`'~“u4
`:39
`E5’
`3‘
`833
`0:0
`
`CONSTANT FILM
`THICKNESS RA T/O:
`I71
`3
`hg-2
`
`15 30 45 60 75 90
`
`TOPOGRAPHICAL ANGLE 9
`
`9-figure 8
`
`1
`
`.3
`3
`
`CONSTANT STEP
`HEIGHT:
`
`h5=h1+h2
`
`15 30 45 60 75 Q0‘
`
`TOPOGRAPHICAL ANGLE 9
`
`figure 9
`
`Page 5 of 19
`
`

`
`U.S. Patent
`
`June 15, 1993
`
`Sheet 5 of 11
`
`5,219,485
`
`—L Q.QQ
`
`ETCHHATE(A/MIN)
`
`HCI + BCI3 + C/2
`75 + 0 + 30 SCCM
`
`15 MT
`
`-300V
`
`72.5°C
`
`so/3 FLOW: so
`
`4o
`
`50 SCCM
`
`{Figure 10
`
`HCI + BCI3 + CI2
`75 + 40 + 30 SCCM
`
`25 MT
`
`'
`
`-300V
`
`725°C
`
`'3‘
`E
`‘S.
`
`ED
`
`: EKL
`
`LI
`
`so
`so/3 + 0/2 FLOW:
`(BCI3 + C12)/T07,’ FLOW: 0.2
`
`0.5
`
`A figure 11
`
`ETCHRATE(A/MIN)
`
`BCI3
`
`4o soc/w
`
`c/3 FLOW:
`
`7
`
`20 so 40 so so 70 so socm
`
`BCI3/Clg RATIO: 2 1.5
`
`1
`
`0.8 0.67 o.57 0.5
`
`{Figure 12
`
`Page 6 of 19
`
`

`
`U.S. Patent
`
`June 15, 1993
`
`Sheet 6 of 11
`
`5,219,485
`
`BCI3 + CI2
`{'51, 40 + 40 = 80 SCCM
`
`25 MT
`
`ETCHRATE(A/MIN)
`
`~tM(0OO0OQQC)QQ
`
`DC BIAS:
`
`-250
`
`-300
`
`-350
`
`RF POWER:
`
`750
`
`900
`
`1250
`
`figure 13
`
`1
`B013 + 0/2
`40 + 40 = so SCCM
`
`-350v
`
`45
`
`Page 7 of 19
`
`

`
`US. Patent
`
`June 15, 1993
`
`Sheet 7 of 11
`
`5,219,485
`
`ETCHRATE(A/MIN)
`
`BCI3 + CI2
`40 + 40 = 80 SCCM
`
`25 MT
`
`-350V
`
`NF3 FLOW: 10
`
`20
`
`30
`
`40
`
`SCCM
`
`figure 15
`
`WSix _/fl
`
`,/
`
`50
`
`/.
`POLY
`
`HCI + BCI3 + CI2
`75 + 40 + 40 = 155 SCCM
`
`25 MT
`
`-350V
`45°C
`
`_—.a_....g_._—uEgj
`
`49
`
`3 § 1
`
`?.
`UJ
`|~
`
`1000
`
`§ §
`
`500
`
`lu
`
`0;-'4 +35% 02,- 10
`
`20
`
`30 SCCM
`
`figure 16
`
`Page 8 of 19
`
`

`
`U.S. Patent
`
`June 15, 1993
`
`Sheet 8 of 11
`
`5,219,485
`
`N)OOQ
`
`ETCHRATE(A/MIN)
`
`R301Q
`
`E §
`
`'3.
`
`~L QQQ
`
`8':<3
`
`E0
`
`:
`:1:
`
`8Q
`
`Em
`
`HCI + BCI3 + CI2
`75 + 40 + 40 = 155 SCCM
`
`30 MT
`
`-300 V
`
`15
`
`20
`
`SCCM
`
`figure 1 7
`
`HCI + BCI3 + C12
`75 + 40 + 30 = 145 SCCM
`
`25 MT
`
`350 V
`
`figure 18
`
`72°C
`’
`CF4 + 8.5% 03: 0
`20
`40 SCCM
`
`Page 9 of 19
`
`

`
`U.S. Patent
`
`June 15, 1993
`
`_
`
`Sheet 9 of 11
`
`5,219,485
`
`Mo Six
`
`§
`
`-
`
`HCI + BCI3 + CI2
`75 + 40 + 40 = 155 SCCM
`
`25 MT
`
`-300 V
`
`45°C
`
`0
`
`20CC
`
`ZOCC
`
`CF4 + 8.5% 02
`
`CF4 + 17% O2
`
`figure 19
`
`HE"/HO//BCI3/CI2 (SCCM)
`0/75/40/30 ----- --
`
`30/75/3o/25 — — — — —
`
`25 MT, -300 v
`
`ETCHRATE(A/MIN)
`
`ETCHRATE(A/MIN)
`
`HEXODE TEMP: 45
`
`55
`
`65
`
`75 °C _
`
`figure 20
`
`Page 10 of 19
`
`

`
`June 15, 1993
`
`Sheet 10 of 11
`
`5,219,485
`
`/
`
`/
`
`,’L_/64
`
`5- 63
`
`HCI - 75 SCCM
`so/3 — 4o SCCM
`c/2 - 40 SCCM
`(CF4 + 3.5% 02) - 2o SCCM
`25 MT
`-300 V
`
`ETCHRATE(A/MIN) 3QQ
`
`HEXODE TEMP: 45 50
`
`60
`
`70
`
`80° C
`
`figure 21
`
`Wsi
`
`..__.
`
`ETCHRATE(A/MIN) 3QQ
`
`.
`
`°
`
`X 56
`67
`MO Six _C _ _ _
`POLY /‘)1’ 65
`BCI3 (scam)
`C12
`(SCCM)
`(CF4 + 3.5% 02) (SCCM)
`PRESS. (M
`D. 0. BIAS (- )
`
`-
`WSIX
`50
`10
`25
`350
`
`40
`
`40
`
`POLY
`50
`10
`25
`350
`
`HClFLOW.' 25
`
`50
`
`75 90 SCCM
`
`figure 22
`
`Page 11 of 19
`
`

`
`U.S. Patent
`
`June 15, 1993
`
`Sheet 11 of 11
`
`5,219,485
`
`HCI/BCI3/CI2/CF4 + 02 (SCCM)
`-—uj---—-o——---—
`25/60/60/30 }__..,._..,__
`75/40/40/20 }....n_..n__
`
`! I I Q E E5 §u
`
`:
`to
`|~
`‘L3
`
`Eo
`
`:
`
`oETCHRATE(A/MIN)
`
`N WAFERS:
`
`1
`
`2
`
`3
`
`
`
`jfigure 23
`
`HO! + BCI3 + CC/2F2
`50+30+5=85SCCM
`
`ETCHRATE(A/MIN)72——
`
`~L OC) Q
`
`E:3
`
`8<3
`
`£30
`
`PRESSURE: 10
`
`20
`
`30
`
`40
`
`50
`
`60
`
`70 MT
`
`figure 24
`
`Page 12 of 19
`
`

`
`1
`
`5,219,485
`
`MATERIALS AND METHODS FOR ETCHING
`SILICIDES, POLYCRYSTALLINE SILICON AND
`POLYCIDES
`
`5
`
`This is a divisional application of Ser. No. 443,811,
`filed Nov. 29, 1989, now US. Pat. No. 5,112,435, which
`is a continuation of Ser. No. 185,256, filed Apr. 19, 1988,
`now abandoned, which is a continuation of Ser. No.
`786,783, filed Oct. 11, l985, now abandoned.
`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
`field 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‘, filed 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.
`
`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
`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 IC 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, TaSi,; 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
`profile (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.
`
`Page 13 of 19
`
`

`
`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 profiles 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 fluorinated gas
`chemistry. However, while fluorinated 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.
`BCI3/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 difficult 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.
`
`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. Specifically, we have determined that
`the filaments or fences frequently formed adjacent the
`steps in polycide etching are eliminated by a sufficiently
`high silicide:polysilicon etch rate ratio, R. We have
`quantified that etch rate ratio as a function of polycide
`film thickness, topographic step height and angle.
`FIGS. 2-5, 2A-SA 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 (9=30°) with the oxide underlayer
`
`l0
`
`4
`25 and substrate 26. FIGS. 2A and 2B are similar to
`FIG. 2 except that the topographical angles are 6=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 sufficiently 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 filament or
`fence along the step. See filaments 29A and 29B shown
`in phantom‘in FIGS. 5A and 5B. The filament 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 silicide:poly-
`cide etch rate ratio is required to provide residue-free
`removal of silicide over topographical steps and to
`avoid filaments along the steps. However, the prior art
`has not quantified the relationship of the silicide etch
`selectivity to ‘polysilicon and, in fact, is believed not to
`have recognized the existence of a specific 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
`filaments 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.
`
`Page 14 of 19
`
`

`
`5,219,485
`
`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.
`
`10
`
`5
`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 HCI/Cl; gas chemistry as the basic gas chem-
`istry for etching refractory metal silicides and polysili-
`con. The use of HCI/C12 gas chemistry in the polysili-
`con etch step and BCI3-containing HCI/Cl; 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. I-ICl/C12 gas chem-
`istry provides high rate anisotropic etching of the un-
`derlying polysilicon with selectivity to underlying ox-
`ides such as gate oxide layers. BCI3-containing I-lCl/Cl;
`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, filament free etch. In
`addition, the BCI3/HCl/Cl; 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 profiles, minimum linewidth loss, cleanliness,
`uniformity and reproducibility.
`BRIEF DESCRIPTION OF THE DRAWING
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`1. Determination of R (Silicide:Polysilicon 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 films 22 and 23
`(FIG. 2) have conformal coverage; h1 and h; are the
`thickness of the silicide and polysilicon, respectively; hs
`is the topographic height of oxide step 24; and 0 is the
`topographic angle (0 —§ 0 § 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
`cosd
`
`l - cos0
`cost)
`
`j.
`
`The required etch rate ratio of silicide to polysilicon is
`then equal to the remaining silicide to polysilicon thick-
`ness ratio:
`
`If the boundary point A exceeds the foot point C, as
`shown in FIG. 7, that is, when the topographic angle 0
`is high, the thickest remaining silicide is in the curved
`portion and the thickness will be:
`
`2
`I
`h
`lI'=[(/Ii +h2)2~(fa-“£9-+h2un-g—) J +h;-h2—h1.
`
`(3)
`
`The required etch rate ratio then will be:
`
`45
`
`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 6 =30°, 60° and 75°, respectively;
`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 flow
`for molydenum silicide;
`FIG. 11 depicts etch rate as a function of (BCl3—+— Clz)
`flow for molybdenum silicide;
`FIG. 12 depicts etch rate as a function of chlorine
`flow 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 NF; flow
`for tungsten silicide and polysilicon;
`FIGS. 16-19 depict tungsten/molybdenum silicide
`etch rates and polysilicon etch rates as a function of
`additive gas flow at various hexode temperatures;
`
`65
`
`Page 15 of 19
`
`

`
`5,219,485
`
`7
`-continued
`
`I!
`[(111 + h)2 -— (“:6 + I12 tan -2-
`
`8
`crease in molybdenum silicide etch rate which is
`achieved for BCI3/Cl;/HCI chemistry by adding in-
`creasing amounts of BCl3+C12 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+Cl2 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 figure illustrates the above-
`mentioned dichotomy in the etch behavior of titanium
`and tantalum silicides on the one hand, and tungsten and
`molybdenum silicides on the other. That is, this figure
`depicts the increases in the silicide etch rate and in the
`silicide:polysilicon etch rate ratio which are achieved
`for BC];/C12 gas chemistry by L adding increasing
`amounts of chlorine to the BCI3 flow of 40 sccm. Curve
`35 shows the variation in polysilicon etch rate as a func-
`tion of chlorine flow rate using 25 millitorr pressure and
`— 350 volts DC bias. Curves 36 and 37 illustrate the etch
`rates of molybdenum silicide and tungsten silicide under
`the same conditions..The molybdenum etch rate curve
`36 and the tungsten curve 37 nearly parallel the polysili-
`con curve 35 and have etch rate ratios, R, relative to the
`polysilicon of slightly greater than l:l. In contrast,
`under the same conditions, the tantalum curve 38, has
`R222] for chlorine flows greater than about 20 seem
`30
`(that is, over the entire range of flow rates of FIG. 12).
`‘ Similarly, for the titanium curve 39, R22 over the
`entire range of flow rates of FIG. 12 and R==(3—4):l
`over much of the flow rate range.
`Furthermore, FIGS. 13 and 14 illustrate that these
`5 etch rate ratios are relatively insensitive to RF power
`(DC bias voltage) and to chamber pressure, at least over
`a substantial range of conditions. The data of FIG. 13
`were taken using BCI3 and Cl; flow rates of 4-0 sccm
`each and a chamber pressure of 25 millitorr. The change
`in the etch rates of the titanium curve 43 and the molyb-
`denum curve 42 as a function of DC bias voltage do not
`differ significantly from the slope of the polysilicon etch
`rate curve 41. Also, the titanium, molybdenum and
`polysilicon curves 46, 45 and 44 of FIG. 14, associated
`with BCI3 and Cl; flow rates of 40 sccm and -350 volts
`DC bias, have similar relative slopes. Consequently, DC
`bias and pressure per se cannot be relied upon under
`these conditions to increase the etch rate ratio of molyb-
`denum and, presumably, of tungsten silicides. (It should
`be noted that despite the relative insensitivity of the
`titanium etch rate ratio and the tantalum etch rate ratio
`to power (DC bias voltage) and pressure, those etch
`rate ratios were at an excellent, high level throughout
`FIGS. 13 an 14.)
`In short, the BC];/C12 chemistry provides excellent
`silicide:polysilicon etch rate ratios for titanium and
`tantalum silicides which are believed sufficient for es-
`sentially all present topographical geometries. How-
`ever, the tungsten and molybdenum silicide:polysilicon
`etch rate ratio of about l:l is still too low for many
`topographical IC devices. It should also be noted that,
`even where very high etch rate ratios (of about 2:1) are
`not required because of the particular topography (for
`example, when low 0 is acceptable), a high etch rate
`ratio of silicide to polysilicon is still preferable to clean
`up the silicide residue with a minimum overetch.
`
`When the topographical step is vertical, i.e. 0=90°:
`
`R = 1:17“: — on + hz) + [(111 + ml — 1:221’).
`
`‘5’
`
`b. Discussion of Process Requirements
`Using equation (2) and (4), the required etch rate
`ratios are plotted as a function of 0 in FIG. 8 and FIG.
`9, using hi/hz (curve 34, FIG. 8) and hs/h1+h2 (curve
`35, FIG. 9) as discrete parameters.
`The required etch rate ratio monatonically increases
`when topographical angle 0 increases. The steepest rate
`of increase is around 45°—60°. For small 0, the required
`etch rate ratio is independent of topographical step
`height and is related to the layer thickness ratio of sili-
`cide to polysilicon. At high 0, both the step height and
`thickness ratio play roles in the required etch rate ratio.
`Using equation (2) and (4), one can calculate the re-
`quired etch rate ratio for a given device topographical
`structure. As a typical example, for 3,000 Angstroms
`silicide and 2,000 Angstroms doped polysilicon over a
`5,000 Angstroms, ~ 90‘ steep step the required etch rate
`ratio is about 2.3 to etch clean. In general, R of (l—2):l
`and, preferably, ~22] is required for present topogra-
`phies. Using BCI3-containing HCl/C12 gas chemistry R
`values of (1—2):l are readily obtained for tantalum sili-
`cide and titanium silicide, but not for molybdenum and
`tungsten silicides.
`
`25
`
`4-0
`
`55
`
`2. Gas Chemistry for Required Silicide:Polysilicon R 3
`a. BCI3-Containing Cl; Gas Chemistry
`We have found that including BCI3 in the chlorinated
`etching gas solution promotes cleanliness of the sub-
`strate during the etch process and maintains the desired
`profile of the etched material (that is, prevents under-
`cutting). Specifically, we have found that this dual pur-
`pose is satisfied by using BCl3 and Cl; flow rate ratios in
`which BCl3Cl2§ 1:1.
`The BC]; does not substantially enhance the etch rate
`of the silicides. This is illustrated by curve 32 of FIG.
`10, which depicts the molybdenum silicide etch rates
`which are achieved for Cl;/I-ICl gas chemistry by add-
`ing increasing amounts of BCl3 to the Cl; and HCl flow
`of 30 sccm and 75 sccm at -15 millitorr pressure and
`-300 volts DC bias. The curve indicates the BCI3 flow
`rate has little effect on the molybdenum silicide etch
`rate. If anything, the molybdenum silicide etch rate is
`decreased slightly by increasing the flow of BCI3.
`Data listed here for silicide and polysilicon polycide
`films were for films which were formed on oxide layers
`formed on single crystal silicon substrates. The polysili-
`con films were highly doped n-type with phosphorus to
`a concentration which would provide sheet resistivity
`of about 15-20 ohms per square for 5000 Angstrom
`thick films. The silicide and polysilicon films were
`etched by using photoresist masks about one micron
`thick and etching using the above-described Applied
`Materials, Inc. 8100 Series reactor 10. The process was
`also demonstrated on silicon-on-sapphire structures.
`In contrast to the effect of BCI3 alone, the silicide
`etch rate is influenced by the total BCl3+Cl2 flow rate.
`This is illustrated in FIG. 11. Curve 33 depicts the in-
`
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`

`
`9
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`5,219,485
`
`b. Dopant:Additive Effects
`The molybdenum and tungsten silicides-to-polysili-
`con etch rate ratios are increased by adding small volu-
`metric amounts of oxygen and fluorinated dopant/addi-
`tive gases to the BCl3-containing chlorine gas. FIG. 15
`depicts the increases in etch rates and in the relative
`etch rate ratio of tungsten silicide to polysilicon by
`adding increasing amounts of NF3 to BCI3/Cl; gas
`chemistry. Curve 47 shows the variation in polysilicon
`etch rate as a function of NF3 flow rate at 25 millitorr
`chamber pressure, -350 volts DC bias and BCI3 and
`C12 flow rates of 40 sccm each. Similarly, curve 48
`shows the variation in the tungsten etch rate as a func-
`tion of the NF; flow rate under the same conditions.
`The silicidezpolysilicon etch rate ratio is greater than
`about 2:1 for NF3 flow rates greater than about 20 sccm
`(greater than about 20 volume percent of the total
`BCl3+Cl2+NF3 additive gas flow rate of 100 sccm).
`Similar results were obtained for other flow rate 20
`combinations and other fluorinated additives. FIG. 16
`depicts the increase in etch rates and etch rate ratio of
`tungsten silicide and polysilicon for BCI3/Cl;/HCI gas
`chemistry by adding increasing amounts of the gas mix-
`ture CF4+ 8.5% Oz. The etch conditions were 25 milli-
`torr, -350 volts DC bias, 45° C. hexode temperature
`and HCl/BCI3/C12 flow rates of 75/40/40 sccm. As the
`(CF4+ 8.5% Oz) flow rate was increased, an etch rate
`ratio of about 2:1 was reached at dopant gas flow rates
`of about 25-30 sccm (about 14-16 percent by volume of 30
`the total BCl3+Cl2+I-lCl+additive gas flow rate of
`180-185 sccm).
`Fluorinated additives have also been used to increase
`the etch rate of molybdenum silicide, but oxygen addi-
`tives have proven more effective than iluorinated
`for this purpose. As shown in FIGS. 17-20, the oxy-
`gen additive effect on molybdenum silicides has proven
`to be sensitive to the hexode temperature, and is suffi-
`cient to effectively double the molybdenum silicide etch
`rate without substantially affecting the polysilicon etch
`rate.
`
`10
`FIGS. 19 through 21 indicate that the etch rate ratio-
`enhancing effect shown in FIGS. 17 and 18 is the result
`of the additive effect of temperature as well as the dop-
`ant gas. FIG. 19 illustrates that adding increasing
`amounts of (CF4+ 8.5% 02) may in fact slightly de-
`crease the etch rate ratio of molybdenum silicide, curve
`59, to polysilicon, curve 58.
`The molybdenum and polysilicon etch data shown in
`curves 62 and 61 of FIG. 20 illustrate that increasing the
`hexode temperature over the range 45° C. to 75° C_. in
`the absence of additive dopant gas increases the silici-
`de:polysi1icon etch rate ratio.
`Comparing FIG. 21 to FIG. 20 (and to FIG. 17)
`illustrates the double additive effect provided by the
`presence of a fixed volumetric percentage of the dopant
`gas (CF4+ 8.5% Oz) in the BCl3/C12 etching gas mix-
`ture as the hexode temperature is increased. The FIG.
`21
`etch
`data
`were
`taken
`for
`I-ICl/BC13/C12/(CF4+B.5% O2)
`flow rates
`of
`75/40/40/20 sccm, 2