United States Patent [191
`Wang
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,992,391
`Feb. 12, 1991
`
`[54] PROCESS FOR FABRICATING A CONTROL
`GATE FOR A FLOATING GATE FET
`_
`_
`[75] Inventor: Hsmgya A- Wang, San Jose, Cahf-
`[73] Assignee: Advanced Micro Devices, Inc.,
`Sunnyvale, Calif.
`
`[21] Appl. No.: 442,903
`
`FOREIGN PATENT DOCUMENTS
`0160965 11/1985 European Pat. Off. .....
`437/200
`2077993 12/1981 United Kingdom .............. .. 437/200
`Primary Examiner-Olik Chaudhuri
`Assistant Examiner-—T. N. Quach
`Attorney, Agent, or Firm-Fliesler, Dubb, Meyer &
`Lovejoy
`.
`
`[22] Filed:
`
`Nov. 29, 1989
`
`[57]
`
`_ ABSTRACT
`
`[51] Int. Cl.5 ......................................... .. H01L 21/336 A Process of forming a floating gate ?eld'effect transis"
`[52] US. Cl. .................................... .. 437/43; 437/193;
`tor having a multi-layer control gate line is disclosed
`37 200
`The multi-layer control gate line includes a ?rst
`4 /
`.
`.
`.
`[58] Field of Search ............... .. 437/200, 43, 193, 192,
`polysilicon layer, a slliclde layer provlded on the ?rst
`437/40, 41, 42, 50; 357/71, 67, 14; 148/DIG.
`polysilicon layer, and a second polysilicon layer pre
`17, DIG, 147
`vided on the silicide layer. The ?rst and second polysih
`con layers are formed as undoped polysilicon to im
`prove the adhesion of the polysilicon layers to the sili
`cide layers sandwiched therebetween. After all three
`layers are formed, the polyslhcon layers are doped in an
`environment including POC13_ Because the ?rst and
`second polysilicon layers are formed as undoped layers,
`all three layers of the control gate line may be formed
`using a single pump-down.
`
`[56]
`
`References Cited
`U_S_ PATENT DOCUMENTS
`
`i?gLéég 5/1935 Nuttalllet al. .......... .. l48/DIG. 147
`4’37g’zsl
`Gauss 6“ "" "
`4’389’257 6/1983
`414031394 9/1983
`4,640,844 2/1987
`4,740,479 4/ 1988
`
`15 Claims, 3 Drawing Sheets
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`U.S. Patent
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`Fefi. 12, 1991
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`‘Sheet 1 of 3
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`Feb. 12, 1991
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`Sheet 2 of 3
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`Patent
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`Feb. 12, 1991
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`Sheet 3 of 3
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`4,992,391
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`4,992,391
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`strate in the buried contact region as the wafer is in
`serted into a hot furnace tube will ruin a die.
`
`PROCESS FOR FABRICATING A CONTROL GATE
`FOR A FLOATING GATE FET
`
`SUMMARY OF THE INVENTION
`It is therefore, an object of the present invention to
`provide an improved method of fabricating a floating
`gate ?eld effect transistor.
`A further object of the present invention is to provide
`a method of fabricating a ?oating gate FET having a
`multi-layer- control gate including a highly doped
`polysilicon layer adjacent to the inter-gate oxide.
`Another object of the present invention is to provide
`a method of fabricating a ?oating gate FET having a
`multi-layer control gate which improves the adhesion
`of a silicide layer to an underlying polysilicon layer.
`Another object of the present invention is to provide
`a method of fabricating a multi-layer conductive line
`with a single vacuum chamber pump down.
`A process for fabricating a ?oating gate ?eld-effect
`transistor in accordance with the present invention
`comprises the steps of (a) providing a gate oxide on the
`substrate, (b) providing a ?oating gate line on the gate
`oxide, (0) providing an intergate oxide layer overlying
`the gate oxide and the ?oating gate line, ((1) providing
`control gate layers, including a ?rst undoped polysili
`con layer overlying the intergate oxide layer, a silicide
`layer overlying the ?rst polysilicon layer, and a second
`undoped polysilicon layer overlying the silicide layer,
`(e) annealing the control gate layers in an environment
`including POC13, (t) etching the control gate layers to
`form a control gate line, (g) etching the ?oating gate
`line using the control gate line as a mask to form a
`?oating gate, and (h) implanting source and drain re
`gions using the control gate line as a mask.
`
`5
`
`20
`
`m 0
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to processes for fabri
`eating semiconductor devices; more particularly, pro
`cesses for fabricating control gate lines for ?oating gate
`?eld effect transistors.
`2. Description of the Related Art
`The gate structure of a conventional ?oating gate
`?eld effect transistor (FET) includes a gate oxide layer
`provided on a substrate, a ?oating gate provided on the
`gate oxide layer, and a control gate separated from the
`?oating gate by an inter-gate oxide layer. The control
`gate has conventionally been formed of a polysilicon
`layer or a polysilicon layer with a silicide layer overly
`ing the polysilicon layer‘. The control gate is usually
`fabricated with a polysilicon layer adjacent to the inter
`gate oxide in order to maintain the device characteris
`tics provided by a polysilicon gate.
`The desire to increase the speed and to reduce the
`power consumption of semiconductor devices has
`prompted the use of multi-layer structures, including a
`silicide layer overlying the polysilicon layer, to take
`advantage of the lower resistivity of the silicide. Several
`problems are associated with forming a silicide layer on
`a polysilicon layer. One such problem is that the doping
`level of the polysilicon must be low to insure that the
`silicide will adhere to the polysilicon. Poor adhesion
`results in silicide lift-off and device failure. Doping
`levels up to approximately 5><l0l9 cm-3 have been
`utilized; however, greater doping levels increase the
`probability of device failures beyond acceptable limits.
`Doping levels below 5X10l9 cm-3 for the polysilicon
`layer create a large resistivity and power consumption
`and reduce speed. Further, since polysilicon is usually
`doped with an N-type dopant, in the fabrication of
`CMOS devices, the low doping level of the polysilicon
`layer allows P-type dopants (used to form the source
`and drain regions in P-channel devices) to neutralize, or
`invert, the doping (or conductivity type) of the polysili
`con layer. An inversion of the conductivity of the
`polysilicon layer from N-type to P-type doping radi
`cally changes the threshold voltage (V 1) of the device.
`A further problem associated with the formation of a
`multi-layer control gate is that the device must be re
`moved from the furnace tube, or vacuum chamber, after
`the deposition of the polysilicon layer to allow the
`polysilicon layer to be doped before the silicide layer is
`deposited. Each time the device is removed from the
`furnace tube one of two problems arise. The cooling of
`the furnace tube to insert the wafers causes the polysili
`con accumulated on the tube walls to warp or break the
`furnace tube due to the divergent coefficients of ther—
`mal expansion of polysilicon and quartz. Alternatively,
`if the tube is maintained at a high temperature and the
`60
`wafers are inserted into a hot tube there is a high risk of
`wafer oxidation, even if a ?ow of an inert gas is pro
`vided, which causes yield problems.
`The problem of oxidation is more severe if buried
`contacts are formed. Buried contacts require the re
`moval of the inter-gate oxide and the gate oxide in the
`region where the buried contact is to be formed. This
`leaves the substrate exposed and oxidation of the sub
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIGS. 1-4 are cross-sectional views useful for de
`scribing the process of the present invention;
`FIG. 5A is a cross-sectional view along line SA-SA
`in FIG. 6 useful in describing the process of the present
`invention;
`FIG. 5B is a cross-sectional view along line SB-SB
`in FIG. 6 useful in describing an alternative embodi
`ment of the process of the present invention;
`FIG. 6 is a simpli?ed plan view of a semiconductor
`device fabricated in accordance with the process of the
`present invention; and
`FIG. 7 is a simpli?ed plan view useful in describing
`the process of the present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT(S)
`The present invention will be described with refer
`ence to FIGS. 1-7. The process of the present invention
`is particularly useful and is described below in the fabri
`cation of ?oating gate ?eld effect transistors. However,
`the process of the present invention is also applicable to
`the formation of any conductive line for a semiconduc
`tor device in either a bipolar or an MOS process. For
`example, the process of the present invention may be
`used to fabricate gate structures for non-floating gate
`?eld effect transistors or conductive lines for bipolar
`devices.
`With reference to FIG. 1, the process of the present
`invention begins with a substrate 10 which is thermally
`oxidized to form a gate oxide 12. Alternatively, the gate
`oxide 12 may be a deposited oxide; however, thermal
`oxides are considered to be higher-quality oxides more
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`suitable for use as a gate oxide. Then, ?eld oxide regions
`(not shown) are provided to de?ne the active regions
`where individual ?eld effect transistors would be
`formed.
`A ?oating gate material layer 14 is provided over
`gate oxide 12. In the preferred embodiment of the pres
`ent invention ?oating gate material layer 14 is formed of
`polysilicon. The ?oating gate material layer 14 may
`have aa thickness ranging from approximately 1,000 to
`3,000 A, and is approximately 1,500 A in the preferred
`embodiment. The ?oating gate material layer 14 is then
`doped by annealing the device in an atmosphere com
`prising phosphorous oxychloride (P0013). This anneal
`is conducted for a time suf?cient to provide ?oating
`gate material layer 14 with a doping concentration of
`approximately 1X1020 cm-3. In the preferred embodi
`ment, the POC13 concentration is approximately 0.1%,
`and the device is annealed at a temperature of approxi
`mately 875° C. for approximately ?fteen (15) minutes.
`Thereafter, ?oating gate material layer 14 is etched to
`form ?oating gate lines 14 as shown in the simpli?ed
`plan view of FIG. 7.
`An inter-gate oxide layer 16 is provided over gate
`oxide 12 and ?oating gate lines 14. In the preferred
`embodiment, intergate oxide layer 16 is a 200 A thermal
`oxide grown by annealing the device at approximately
`ll00° C. for approximately ten minutes in an environ~
`ment including dry oxygen and HCl. Alternatively,
`intergate oxide 16 may be a deposited oxide.
`The formation of the layers which ultimately form
`the control gate lines will be described with reference to
`FIG. 2. First, an undoped polysilicon layer 18 havingoa
`thickness ranging approximately from 50 to 5,000A,
`2,000 A in the preferred embodiment, is provided over
`intergate oxide 18. Polysilicon layer 18 is formed as an
`undoped layer using conventional deposition tech
`niques. Then, a silicide layer 20 is provided on polysili
`con layer 18. The silicide may be selected from the
`group including TaSi2, WSi2, TiSiQ, and MoSi2; WSig,
`the preferred silicide, is deposited, for example, in an
`environment including SiH2Cl2 and WF6 at a tempera
`ture of approximately 600° C. The thickness of silicide
`layer 20 may range from approximately 500 to 5,000 A;
`in the preferred embodimentosilicide layer 20 is a thick
`ness of approximately 2,000 A. An undoped polysilicon
`45
`layer 22 is then provided over silicide layer 20. The
`thickness of lysilicon layer 22 may be approximately
`50 to 2,500 A, with the preferred embodiment having a
`thickness of approximately 1,000 A.
`Polysilicon layers 18 and 22 are formed as undoped
`polysilicon in order to improve the adhesion of these
`layers to silicide layer 20. After the three control gate
`layers, including polysilicon layer 18, silicide layer 20,
`and polysilicon layer 22, are formed, polysilicon layers
`18 and 22 are doped by annealing the device in a POCl3
`environment. The desired impurity concentration (dop
`ing level) in polysilicon layers 18 and 22 is greater than
`approximately 5X 1019 cm‘3 and in the preferred em
`bodiment the doping level is approximately 3-5 X lo20
`cm-3. In the preferred embodiment, polysilicon layers
`18 and 22 are doped in an environment including a
`nitrogen carrier, approximately 0.1% POC13, and ap
`proximately 5% oxygen. The anneal is performed at
`approximately 875‘ C. for approximately thirty (30)
`minutes. The doping of polysilicon layer 18 is not ad
`versely affected by the presence of silicide layer 20.
`In accordance with the present invention it is possible
`to form polysilicon layer 18, silicide layer 20, and
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`4,992,391
`4
`polysilicon layer 22 using a single pump-down cycle
`because the device is not required to be removed from
`the vacuum chamber used to deposit these layers in
`order to dope the individual layers as they are formed.
`After polysilicon layers 18 and 22 are doped, a cap
`ping oxide layer is formed over polysilicon layer 22. In
`the preferred embodiment, capping oxide layer 24 is a
`thermal oxide having a thickness of approximately 1,000
`A, formed by oxidation in an atmosphere including dry
`oxygen and HCl at a temperature of approximately 900°
`C. for a period of approximately ?fty (50) minutes.
`The etching of the control gate layers, 18, 20, 22 and
`oxide layer 24 to form control gate lines will be de
`scribed with reference to FIG. 3. A photo-resist layer
`(not shown) is formed on oxide layer 24, and is then
`patterned using a conventional lift-off process so that
`the photo-resist remains on the portions of the control
`gate layers and oxide layer 24 which will form control
`gate lines. Then, oxide layer 24 is etched using an etch
`ant which is effective for silicon oxide, and subse
`quently polysilicon layer 18, silicide layer 20, and
`polysilicon layer 22 are etched using an etchant which is
`effective for etching polysilicon. Because etchants
`which are effective for etching polysilicon are generally
`not selective for silicon oxide, intergate oxide layer 16
`serves as an etch-stop layer. This etching process forms
`control gate lines 26 (FIG. 6). The relationship of a
`?oating gate line 14 and a control gate line 26 is shown
`in the plan view of FIG. 7. The sectional view of FIG.
`3 is in the direction of arrow A in FIG. 7.
`With reference to FIG. 4, intergate oxide layer 16 is
`etched using the control gate line 26 as a mask, and then
`the portions of ?oating gate lines 14 which do not un
`derlie control gate line 26 are removed by etching; the
`portions of the ?oating gate line which are removed by
`etching are designated 151 and 152 in FIG. 7. The etch
`ing of ?oating gate line 14 to form a ?oating gate 114
`completes the fabrication of gate structure 28. FIG. 5A,
`which corresponds to a view in the direction of arrow
`B in FIG. 7, shows ?oating gate 114 under inter-gate
`oxide 16 and control gate line 26 (including layers 18,
`20, and 22). Gate structure 28 is then used as a mask to
`implant self-aligned source and drain regions 36, 38.
`Then, the source/drain implant is driven and a thick
`thermal oxide (not shown) is grown on the sides of gate
`structure 28 to electrically insulate the ?oating gates
`114. The fabrication of the ?eld-effect transistor is com
`pleted by providing metallization and passivation layers
`in accordance with conventional techniques.
`The utilization of the present invention to form ?oat
`ing gate ?eld effect transistors having buried contacts
`will be described with reference to FIGS. 5B and 6. As
`shown in FIG. 6, one example of a buried contact is an
`extension 27 of control gate line 262 which contacts the
`drain region 381 of a ?eld effect transistor formed on
`adjacent control gate line 261. A simpli?ed cross-sec
`tional view of a ?oating gate ?eld effect transistor hav
`ing a buried contact is shown in the cross-sectional view
`of FIG. 5B, taken along line 5B-5B in FIG. 6. In order
`to form buried contact 40, a portion of intergate oxide
`16 and gate oxide 12 are removed by etching prior to
`the formation of the control gate material layers. This
`etching is performed by providing a photo-resist layer,
`which is patterned using conventional lift-off tech
`niques so that only the buried contact regions are ex
`posed. Polysilicon layer 18, silicide layer 20, and
`polysilicon layer 22 are then formed in accordance with
`the process described above. The use of a load lock
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`chamber is particularly useful for providing the control
`layers with a doping concentration of at least 5><1O2O
`cm-3.
`gate layers when buried contacts are formed so that
`oxidation of the substrate in the region of buried contact
`9. A process according to claim 7, wherein said step
`(e) comprises providing the ?rst and second polysilicon
`40 is prevented.
`‘
`The disclosed embodiments of the present invention
`layers with a doping concentration of greater than ap
`proximately 1 X 1020 cm~3.
`are intended to be illustrative and not restrictive, and
`the scope of the invention is de?ned by the following
`10. A process according to claim 7, wherein said step
`claims rather than by the foregoing description.
`(e) comprises providing the ?rst and second polysilicon
`I claim:
`layers with a doping concentration of approximately
`1. A process for fabricating a control gate line for a
`3-5 X 10230 cm—3.
`?oating gate ?eld effect transistor formed in a semicon
`11. A process for fabricating an integrated circuit
`ductor substrate, comprising the steps of:
`having a plurality of transistors including at least one
`(a) providing a ?oating gate overlying the substrate;
`?oating gate ?eld-effect transistor formed in a substrate,
`(b) providing an inter-gate oxide overlying the ?oat
`comprising of the steps of:
`ing gate;
`(a) providing a gate oxide on the substrate;
`(c) providing a ?rst undoped polysilicon layer over
`(b) providing ?oating gate lines on the gate oxide;
`lying the inter-gate oxide;
`(0) providing an intergate oxide layer overlying the
`(d) providing a silicide layer overlying the ?rst
`gate oxide and the ?oating gate lines;
`polysilicon layer;
`((1) removing selected portions of the gate oxide and
`(e) providing a second undoped polysilicon layer
`the intergate oxide corresponding to the positions
`overlying the silicide layer;
`of buried contacts;
`‘
`(t) doping the ?rst and second polysilicon layers after
`(e) providing control gate layers, including a ?rst
`said steps (d) and (e); and
`undoped polysilicon layer overlying the intergate
`(g) etching the ?rst polysilicon layer, the silicide
`oxide layer and and contacting the substrate in the
`layer and the second polysilicon layer to form a
`regions where the gate oxide and intergate oxide
`conductive line after said step (i).
`are remove,, a silicide layer overlying the ?rst
`2. A process according to claim 1, wherein said step
`polysilicon layer, and a second undoped polysili
`(f) comprises annealing the ?rst and second polysilicon
`con layer overlying the silicide layer;
`layers in an environment including POC13.
`(0 annealing the control gate layers in an environ
`3. A process according to claim 1, wherein said step
`ment including POC13;
`(t) comprises concurrently doping the ?rst and second
`(g) thermally oxidizing the second polysilicon layer
`polysilicon layers.
`to form a capping oxide;
`4. A process according to claim 1, wherein said step
`(h) etching the capping oxide and the control gate
`(i) comprises providing the ?rst and second polysilicon
`layers to form control gate lines and buried
`layers with a doping concentration of at least approxi
`contacts;
`mately 5 X 1019 cm—3.
`(i) etching the ?oating gate lines using the capping
`5. A process according to claim 1, wherein said step
`oxide and the control gate line as a mask to form
`(f) comprises providing the ?rst and second polysilicon
`?oating gates underlying the control gate lines; and
`layers with a doping concentration of greater than ap
`(j) implanting source and drain regions using the
`proximately 1X 1020 cm—3.
`capping oxide and the control gate lines as masks.
`6. A process according to claim 1, wherein said step
`12. A process for fabricating a ?oating gate ?eld
`(f) comprises providing the ?rst and second polysilicon
`effect transistor (FET) on a substrate, comprising the
`layers with a doping concentration of approximately
`sequential steps of:
`3-5 X 1020 cm-3.
`(a) providing a gate oxide overlying the substrate;
`7. A process for fabricating a ?eld-effect transistor,
`(b) providing a ?rst polysilicon layer overlying the
`comprising the sequential steps of:
`gate oxide layer;
`(a) providing a gate oxide on the substrate;
`(0) annealing the ?rst polysilicon layer in an environ
`(b) providing a ?oating gate line on the gate oxide;
`ment including POC13;
`(c) providing an intergate oxide layer overlying the
`(d) etching the ?rst polysilicon layer to form ?oating
`gate oxide and the ?oating gate line;
`gate lines;
`(d) providing control gate layers, including a ?rst
`(e) providing an inter-gate oxide layer overlying the
`undoped polysilicon layer overlying the intergate
`?oating gate lines and the gate oxide;
`oxide layer, a silicide layer overlying the ?rst
`(l) placing the substrate in a vacuum chamber and
`polysilicon layer, and a second undoped polysili
`reducing the pressure in the vacuum chamber;
`con layer overlying the silicide layer;
`(g) providing control gate material layers overlying
`(e) annealing the control gate layers in an environ
`the intergate oxide layer, the control gate material
`ment including ROC13;
`layers including a second undoped polysilicon
`(t) providing a capping oxide layer overlying the
`layer, a silicide layer overlying the second polysili
`control gate layers;
`con layer and a third undoped polysilicon layer
`(g) etching the control gate layers to form a control
`overlying the silicide layer, without breaking the
`gate line;
`(11) etching the ?oating gate line using the capping
`vacuum formed in said step (f);
`(h) annealing the substrate in an environment includ
`oxide layer and the control gate line as a mask to
`ing POCl3 to provide the second and third polysili
`form a ?oating gate; and
`(i) implanting source and drain regions using the
`con layers with a doping concentration greater
`than approximately 1 X 1020 cm-3;
`control gate line as a mask.
`'
`(i) thermally oxidizing the third polysilicon layer to
`8. A process according to claim 7, wherein said step
`(e) comprises providing the ?rst and second polysilicon
`form a capping oxide layer;
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`said step (d) comprises:
`(j) etching the capping oxide layer, the control gate
`( 1) providing a ?rst undoped polysilicon layer
`material layers, and the inter-gate oxide layer to
`overlying the inter-gate oxide layer,
`form control gate lines;
`,
`(2) providing a silicide layer overlying the ?rst
`(k) etching the ?oating gate lines using the cappin
`polysilicon layer, and
`oxide and the control gate lines as masks to form
`(3) providing a second undoped polysilicon layer
`?oating gates; and
`overlying the silicide layer; and
`(l) implanting source and drain regions using the
`the ?rst and second polysilicon layers are doped
`control gate lines as masks.
`after said step (d) and before said step (e).
`13. An improved process for fabricating a floating
`14. A process according to claim 13, further charac
`gate transistor in a substrate including the steps of (a)
`terized in that the ?rst undoped polysilicon layer, the
`providing a gate oxide layer overlying the substrate, (b)
`silicide layer and the second polysilicon layer provided
`providing a ?oating gate material layer overlying the
`in said step (d) are provided in a sealed vacuum cham
`gate oxide, (c) providing an inter-gate oxide layer over
`ber without breaking the seal.
`lying the ?oating gate material layer, (d) providing
`15. A process according to claim 13, further charac
`control gate material layers overlying the inter-gate
`terized that said step (d) further comprises (4) providing
`oxide layer (e) etching the control gate material layer to
`the ?rst and second polysilicon layer with a doping
`form a control gate and (f) etching the ?oating gate
`concentration greater than approximately 1X 1020
`cm—3.
`material layer to form a ?oating gate, characterized in
`that:
`
`10
`
`20
`
`# $ 1 i i
`
`25
`
`35
`
`45
`
`50
`
`55
`
`65
`
`LAM Exh 1008-pg 8