United States Patent
`Blalock et al.
`
`15)
`
`[54] PROCESS FOR SELECTIVELY ETCHING A
`LAYER OF SILICON DIOXIDE ON AN
`UNDERLYING STOP LAYER OF SILICON
`NITRIDE
`
`[75]
`
`Inventors: Guy Blalock; David S. Becker; Fred
`Roe, all of Boise, Id.
`
`[73] Assignee: Micron Technology, Inc., Boise, Id.
`
`[21] Appl. No.: 898,505
`
`[22] Filed:
`
`Jun. 15, 1992
`
`[52] Unt, C15 ooecccccecsccscsssssssssvesssssssseseeseeees HO1L 21/00
`[52] U.S. Ch.
`iecccccccscsssnseesessssnnnn 156/657; 156/643;
`156/646; 156/662; 156/656; 156/659. 1
`[58] Field of Search............ 156/643, 646, 662, 659.1,
`156/652, 653, 657, 655, 654
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`US005286344A
`
`{11] Patent Number:
`
`[45] Date of Patent:
`
`5,286,344
`Feb. 15, 1994
`
`for 1.5-V
`“Crown-Shaped Stacked-Capacitor Cell
`Operation 64-Mb Drams”, Kagaet al., 1991 IEEE.
`“VLSI Devoced Fabricator Using Unique, Highly-se-
`lective Sis3Nq Dry Etching” (T. Kuret al.), Proceeding
`of the International Electron Devices Meeting (IEDM),
`1983, pp. 757-759.
`“Formation of Contracts in a Planarized Si02/Si3N4-
`/SiO2 Dielectric Structure” (Paul E. Riley, Konrad K.
`Young, and Charles C. Liu) J. Electrochem. Soc., vol.
`139, No. 9, Sep. 1992.
`“Self-Aligned Betline Contact for 4 MBit Dram”, K.
`H. Kuesters, H. M. Mueklhoff, G. Enders, E. G. Mohr,
`W. Mueller, pp. 640-649, 1987.
`“A Buried-Plate Trench Cell for a 64-Mb Dram”,
`Kenneyet al., 1992 Symposium of VLSI, IEEE.
`
`Primary Examiner—Brian E. Hearn
`Assistant Examiner—George Goudreau
`Attorney, Agent, or Firm—Michael W. Starkweather
`
`148/1.5
`156/643
`156/643
`156/643
`156/643
`156/657
`156/643
`
`ABSTRACT
`[57]
`Morespecifically, a process is provided for etching a
`.u...ccececcsesccneeeseeeseee 156/11
`3,479,237 11/1969 Berg,
`multilayer structure to form a predetermined etched
`
`4,180,432 12/1979 Clark ...cceecceee «
`pattern therein. The subject process comprises provid-
`4,244,752
`1/1981 Hendersonetal.
`
`ing the multilayer structure having a plurality of struc-
`.......
`4,324,611
`4/1982 Vogel et al.
`
`tural
`layers. The structural
`layers of the multilayer
`.
`4,374,698
`2/1983 Sanders et al.
`structure comprise a silicon dioxide outer layer on an
`4,568,410 2/1986 Thornquist.....
`
`4,581,101
`4/1986 Senoueetal. ..
`underlying silicon nitride stop layer. Then, a chemical
`4,734,152
`3/1988 Geis etal.
`......
`etchantprotective layer is formed on a major surface of
`
`
`the multilayer structure having a predetermined pattern
`3/1988 Carbaughetal. .
`4,734,137
`YOU <assseeeey
`4,789,560 12/1988
`of openings, thereby exposing areasofthe silicon diox-
`4,877,641 10/1989
`Dory..
`ide outer layer corresponding to the predetermined
`
`we 437/44
`4,912,061
`3/1990 Nasr...........
`pattern of openings. The exposed areas ofthe silicon
`4,971,655 11/1990 Stefano et al.
`.
`.- 156/659.1
`
`
`dioxide outer layer are then etched downto thesilicon
`4,978,420 12/1990 Bach oo. ecseeceseseeeeresereeeees 156/643
`
`nitride stop layer, at a high SiO2 etch rate and at a high
`5,013,398 5/1991 Long et ab.oo.eeeeeteeseee 156/643
`
`level of selectivity of the SiO2 etch rate with respect to
`........
`« 437/241
`5,013,692
`5/1991
`Ide et al.
`
`the Si3N4 etch rate, with a fluorinated chemical etchant
`wes 357/54
`5,040,046
`8/1991 Chhabraet al.
`system. The fluorinated chemical etchant system in-
`8/1991) Butler oo... eeeeseeeeeeeneee 357/68
`$,043,790
`cludes an etchant material and an additive material. The
`additive material comprises a fluorocarbon material in
`which the number of hydrogen atomsis equal to or
`greater than the numberoffluorine atoms. The etching
`step formsa substantially predetermined etch pattern in
`the silicon dioxide outer layer in which the contact
`sidewalls of said SiO2 outer layer are substantially up-
`right.
`
`OTHER PUBLICATIONS
`
`“A Method of Obtaining High Oxide to Nitride Selec-
`tivity in an Menle Reactor,” by Becker, Blalock to be
`presented at the spring Electrochemical Society Meet-
`ing, May 1993.
`“Selective Oxide: Nitride Dry Etching in a High Den-
`sity Plasma Reactor” by M. Armocost, J. Marks, May
`1993.
`
`31 Claims, 1 Drawing Sheet
`
`12
`
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`
`a14
`
`
`NeeeSoe
`
`
`
`
`
`19 17 19
`
`19
`
`17
`
`TSMC 1329
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`TSMC 1329
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`

`

`U.S. Patent
`
`Feb. 15, 1994
`
`5,286,344
`
` Cae:
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`SSS
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`1
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`5,286,344
`
`BACKGROUNDOF THE INVENTION
`
`PROCESS FOR SELECTIVELY ETCHING A
`LAYER OF SILICON DIOXIDE ON AN
`UNDERLYING STOP LAYER OF SILICON
`NITRIDE
`
`2
`etch stop material is silicon nitride becauseit’s proper-
`ties are well knownandit is currently used for semicon-
`ductor fabrication. The preferred outerlayeris silicon
`dioxide.
`With respect to etching of a multilayer structure
`including a silicon dioxide layer on an underlying sili-
`con nitride layer, a problem which occurs and which
`must be overcomeis profile control. Prior art methods
`This invention relates to a process for selectively
`of obtaining high oxide to nitride selectivity rely on
`etching a silicon dioxide layer deposited onasilicon
`pure chemical etching (such as hydrofluoric acid). Pro-
`nitride layer, and more particularly to a process for
`file control using this method producedstructures that
`effectively and efficiently etching such silicon dioxide
`do not have vertical sidewalls. Dry etch processing
`layer at a high etch rate and high selectivity of silicon
`usually produces a more vertical profile because of the
`dioxide with respect to silicon nitride, particularly in a
`ion bombardment aspect of the process. However, the
`multilayer structure.
`dry etch process can produce a contact wall that slopes
`It is known in the prior art that the manufacture of
`multilayer multilayer structures typically involves pat-
`out from the bottom instead of being 90 if the wrong
`terned etching of areas of the semiconductor surface
`mix of process parameters are used. These parameters
`can include, but are not limited to, CF4, CHF3, RF
`which are not covered by a pattern of photoresist pro-
`tective material. These etching techniquesuse liquid or
`Power, and pressure.
`wet etching materials, or dry etching with halogens or
`The same ion bombardment aspect of the dry etch
`halogen-containing compounds, of certain layers of
`process used to producestraight sidewalls has a very
`these devices. For example, one well known etching
`negative effect on oxide to nitride selectivity. High
`material is chlorine which can exist in the etching pro-
`energy ions needed to etch both oxide and nitride do so
`cess as either chlorine gas or HCl, etc. Chlorine etches
`by disassociating a chemical bond at the oxide and/or
`the semiconductor isotropically,
`i.c., in both a lateral
`nitride surface. However the disassociation energy
`and vertical direction. This results in an etched feature
`needed for nitride is less than that required for oxide.
`which has a line width which is smaller than the ex-
`Hencethe addition of CH2F;to offset the disassociation
`posed resist image.
`properties of nitride as compared to oxide. The CH2F2
`Etching of the multilayer structures can also be con-
`produces a polymer deposition on the nitride surface
`ducted in a gas phase using known techniques such as
`that acts to passivate the nitride surface and thereby
`plasmaetching, ion beam etching, and reactive ion etch-
`reduce the dry etch removal rate. However,the silicon
`ing. The use of gas plasma technology provides substan-
`dioxide etch rate is sustained at a much higherrate than
`tially anisotropic etching using gaseous ions, typically
`that of silicon nitride.
`generated by an RF discharge. In gas plasma etching
`Hereis a discussion of various prior art processes for
`the requisite portion of the surface to be etched is re-
`etching silicon dioxide and/orsilicon nitride. In U.S.
`moved by a chemical reaction between the gaseousions
`Pat. No. 4,789,560 to Yen, for example, a fusion stop
`and the subject surface. In the anisotropic process, etch-
`methodis provided for forming silicon oxide during the
`ing takes place only or primarily in the vertical direc- -
`fabrication of integrated circuit devices. A diffusion
`tion so that feature widths substantially match the pho-
`stop layer of thermalsilicon oxide is formed during the
`toresist pattern widths. Anisotropic etching is utilized
`fabrication of integrated circuit device prior to the de-
`when feature sizing after etching must be maintained
`position of the poly layer to be oxidized. The nitride
`within specific limits in order not to violate alignment
`isolates the substrate from diffused oxygen within the
`tolerances or design rules. For example, in U.S. Pat. No.
`poly layer during oxidation, permitting a non-critical
`4,734,157 an elemental silicon-containing layer, such as
`oxidation time.
`a layer of polysilicon or silicide, is etched anisotropi-
`U.S. Pat. No. 4,877,641 to Dory discloses a plasma
`cally employing a gas plasma comprising a gaseous
`CVDfor formingsilicon nitride or silicon dioxide films.
`chlorofluorocarbon, capable of supplying CF, and
`onto a substrate using a reactant gas including di-tert
`chlorine ions, and ammonia. Profile controlofa silicon
`butylsilane and at least one other reactant gas.
`layer is controlled by the use of this etching mode.
`US. Pat. No. 4,324,611 to Vogel et al. discloses a
`Higher density multilayer structures such as 64 and
`process and gas mixture for etching silicon dioxide and-
`256 Megabit DRAM will require an additional amount
`/or silicon nitride in a plasma environmentin a planar
`of alignment tolerance which can not be addressed by
`reactor using a carbon fluorine gas comprising C2Fe,
`photolithography means. In such applications, an etch
`CFs4, C3Fs, C4Fio, CaF, and combinations thereof.
`stop technology could be used to supply the desired
`U.S. Pat. No. 4,912,061 to Nasr discloses a method of
`tolerance. In an etch stop system an etch stop layeris
`forming a salicided self-aligned metal oxide multilayer
`deposited on underlying structures. The outer layer is
`structure using a disposable silicon nitride spacer.
`deposited over the underlying etch stop layer through
`U.S. Pat. No. 4,568,410 to Thornquist relates to the
`which the desired patterns will be defined. The etch
`selective gaseous plasma etching with nitrogen fluoride
`stop layer will then be used to terminate the etch pro-
`and an oxygen sourcegas ofsilicon nitride.in the pres-
`cess once the outer layer has been completely removed
`ence ofsilicon oxide.
`in the desired pattern locations. Thus the etch stop layer
`U.S. Pat. No. 3,479,237 to Bergh et al. discloses etch-
`acts to protected structures underlying the etch stop
`ing silicon oxide on silicon nitride using a hydrofluoric
`layer from damagedueto the outer layer dry chemical
`acid solution.
`etch. The process used to perform this etch must have
`U.S. Pat. No. 4,971,655 to Stefano et al. discloses a
`three basic properties, namely, (1) a high outer layer
`method for protecting a refractory metal silicide during
`etch rate which (2) produces substantially upright side-
`high-temperature processing using a dual-layer cap of
`walls and (3) has a high selectivity of the outer layer
`silicon nitride on silicon dioxide.
`being etched downto the etch stop layer. The preferred
`
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`5,286,344
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`25
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`3
`4
`U.S. Pat. No. 5,013,398 to Long et al. discloses a
`system, substantially high oxide to nitride selectivities
`plasmaetch process to anisotropically etch a sandwich
`can be achieved, with high etch rate, and substantially
`structure of silicon dioxide, polycrystalline silicon and
`upright sidewall profiles.
`silicon dioxide “‘in situ”, that is, in a single etch cham-
`Morespecifically, a process is provided for etching a
`ber.
`multilayer structure to form a predetermined etched
`U.S. Pat. No. 5,040,046 to Chhabra et al. discloses a
`pattern therein. The subject process comprises provid-
`process for forming silicon dioxide, or silicon nitride
`ing the multilayer structure having a plurality of struc-
`layers on selected substrates employing C4H)2Si and an
`tural
`layers. The structural
`layers of the multilayer
`QO2 source.
`structure comprise a silicon dioxide outer layer on an
`U.S. Pat. No. 5,013,692 to Ide et al. discloses a pro-
`underlying silicon nitride stop layer. Then, a chemical
`cess for preparing film for a semiconductor memory
`etchant protective layer is formed on a majorsurface of
`device which comprises formingasilicon nitride film
`the multilayer structure having a predetermined pattern
`over a substrate by a chemical vapor deposition tech-
`of openings, thereby exposing areas ofthe silicon diox-
`nique, oxidizing the surfaceofthesilicon nitride film to
`ide outer layer corresponding to the predetermined
`form a silicon oxide layer over the film, and removing
`pattern of openings. The exposed areas of the silicon
`the silicon oxide layer by etching to form an improved
`dioxide outer layer are then etched downto thesilicon
`silicon nitride film.
`nitride stoplayer, at a high SiO, etch rate, and at a high
`U.S. Pat. No. 4,244,752 to Henderson, Sr. et al. dis-
`level of selectivity of the SiO2 etch rate with respect to
`closes a method of fabricating an integrated circuit
`the SijNg etch rate, with a fluorinated chemical etchant
`wherein a silicon oxide-silicon nitride layer is formed on
`system. The etching step forms a substantially predeter-
`the surface of a silicon wafer.
`mined etch pattern in the silicon dioxide layer in which
`U.S. Pat. No. 4,374,698 to Sanders,et al. relates to the
`the contact sidewalls of said SiO2 are substantially up-
`etching of SiO2 or Si3Ng with CF4, CF2C}2 or CF3Br,
`right.
`and O2, while U.S. Pat. No. 4,581,101 to Senoueetal.
`The fluorinated chemical etchant system includes an
`etches the same materials with a fluorinated ether.
`etchant material and an additive material. The additive
`U.S. Pat. No. 5,043,790 to Butler uses upper and
`material comprises a fluorocarbon material in which the
`lower nitride layers in the formation of sealed self-
`number of hydrogen atomsis equal to or greater than
`the numberof fluorine atoms. Fluorocarbon materials
`aligned contacts. The upper non-conductive nitride
`layer is composed ofsilicon nitride which acts as an
`comprise carbon, hydrogen and fluorine atomsin differ-
`etch stop layer for an isotropic silicon dioxide wet etch.
`ing relative ratios. For example, the preferred fluoro-
`The lowernitride layer is a titanium nitride layer on a
`carbon material employed as the additive materia! is
`titanium silicide layer, both of which are conductive
`CH2F2. In case of CH2F2,
`the number of hydrogen
`materials. The titanium nitride layer acts as an etch stop
`atoms (2) is equal to the numberoffluorine atoms(2).
`Anotherfluorocarbon material which can be used as the
`during an anisotropic dry etch of the silicon dioxide
`layer.
`additive material in the present invention is CH2F2. As
`Current etch process technology for etching an SiO2
`to CH3F, the number of hydrogen atoms(3) is greater
`outer layer on an underlying Si3Ng layer using a dry
`than the numberoffluorine atoms(1).
`etcher, such as an RIE or MRIEetcher, cannot produce
`In the process ofthis invention the fluorinated chemi-
`Si02-to-Si3Ng selectivities above 3:1 with adequate pro-
`cal etchant system preferably comprises from about
`file and SiO2 etch rate characteristics. Therefore, a need
`70-90%, and morepreferably from about 75-85%, of
`exists for a process for etching a SiOlayer on an under-
`the etchant material, and from about 10-30%, and more
`lying Si3Nqlayer, at a high SiOzetchrate, and at a high
`preferably from about 15-25% of the additive material,
`based onthetotal flow of the fluorinated chemical etch-
`selectivity of SiO2 with respect to the underlying Si3N4,
`to form an etched multilayer structure at a controlled
`ant system. The amount of the additive material,
`predetermined profile in which the sidewalls are sub-
`CH2F?, based on thetotal flow of fluorinated chemical
`stantially upright.
`etchant system, is preferably at least about 3%, more
`preferably at least about 12%, and most preferably at
`SUMMARYOF THE INVENTION
`least about 20%. Preferably, the etchant material of the
`fluorinated chemical etchant system of this invention
`comprises at least one of CHF:, CF, and Ar. In the
`preferred CHF3-Ar-CF4 system, the amount of CHF;in
`the gas flow mixture is preferably about 3%, morepref-
`erably about 6%, and most preferably at least about
`10% of the total gas flow. With respect to argon, the
`flow rate should be at least about 33%, more preferably
`at least about 50%, and most preferably at least about
`60% of the total gas flow. Finally, as to the flowrate of
`CFy4, it should preferably be at least about 10%, more
`preferably at least about 16%, and most preferably at
`least about 22% ofthe total gas flow.
`The total pressure of this etching process preferably
`Tanges from 0.001-0.5 torr, more preferably 0.01-0.3
`torr., with the most preferred range being 0.05-0.25
`torr. As for the magnetic gauss level, it can be prefera-
`bly be at a set point range of 35-150 gauss.
`invention
`The multilayer structure of the present
`generally includes a silicon wafer. Preferably, the tem-
`perature of the silicon wafer during the etching process
`
`Theprocessof the present invention meets the above-
`described existing needs by forming the above-
`described etched multilayer structure in which theside-
`walls of the SiO? layer are substantially upright at a high
`SiO. etch rate and at a high selectivity of SiO2 with
`respect to the underlying Si3N4. This is accomplished
`by employing a process for etching the SiOz layer down
`to the Si3Nq stop layer as hereinafter described.
`In two published articles “Crown-Shaped Capacitor
`Cell for 1.5 V Operation 64 Mb DRAMS”byT. Kaga,
`et al in JEEE Transactions On Electron Devices, Vol. 38,
`No. 2, February 1991, and “VSLI Device Fabricator
`Using Unique, Highly Selective Si3N4 Dry Etching”by
`T. Kure, et al Proceeding of the International Electron
`Devices Meeting (IEDM), 1983, pp. 757-759., a highly
`selective anisotropic dry etching techniqueis described
`for etching a Si3Ng layer down to an underlying SiO?
`stop layer using a CH2F2 plasma. However, applicants
`have unexpectedly discovered that when CH>Fis em-
`ployed as an additive in a fluorinated chemical etchant
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`5
`is important in producing high selectivity of silicon
`dioxide to silicon nitride. It is also important in the
`formation of a good profile. It has been determined in
`the subject process that when higher etch temperatures
`are employed, the high selectivity previously described
`herein can be readily maintained. For example, in the
`case of certain preferred systems such as the MERIE
`system, a preferable temperature range of the silicon
`wafer in the multilayer structure during the etching step
`is about 20-80 degrees C., more preferably about 30-60
`degrees C., and most preferably about 35-50 degrees C.
`This is the temperature of the bottom electrode adjacent
`to the silicon wafer location during the etching process.
`In the process ofthis invention the high level of selec-
`tivity of the SiO2 etch rate with respect to said Si3N4
`etch rate is preferably at least about 10:1, more prefera-
`bly at least about 20:1, and most preferably at least
`about 50:1. The process also produces a preferred high
`SiQ2 etch rate which is at least about 2500 angstroms of
`SiOz per minute, more preferably at least about 3000
`angstroms of SiQ2 per minute, and most preferably at
`least about 4000 angstromsof SiO2 per minute. Further-
`more,the selectivity of the SiO2 etch rate with respect
`to the Si3N4 etch rate for etching the silicon dioxide
`outerlayer to the silicon nitride stop layer, employing a
`fluorinated chemical etchant system including an etch-
`ant material and an additive material,
`is preferably at
`least about 500%, more preferably at
`least about
`1000%, and most preferably at
`least about 1500%,
`higher than the selectivity of said SiOetch rate with
`respect to said Si3N4 etch rate for etching the silicon
`dioxide outer layer to the silicon nitride stop layer,
`employing a fluorinated chemical etchant system in-
`cluding the above-described etchant material, but with-
`out the subject additive material.
`The process of the present invention preferably in-
`cludes the step of etching the exposed areasofthesili-
`con dioxide outer layer downto thesilicon nitride stop
`layer employing a dry etching process conducted in a
`magnetically-enhanced etching chamber, more prefera-
`bly an RJE or an MERIEetching chamber.
`The foregoing and other objects, features and advan-
`tages of the invention will become morereadily appar-
`ent from the following detailed description of a pre-
`ferred embodiment which proceeds with reference to
`the drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG.1 is a pictorial representation of a multilayer
`structure of the present invention, such as a semicon-
`ductor profile, having a silicon dioxide outer layer on a
`silicon nitride etch stop layer, prior to etching with the
`fluorinated chemical etchant system of the present in-
`vention.
`FIG.2 is a pictorial representation of the multilayer
`structure of FIG. 1 after etching the silicon dioxide
`outer layer down to the silicon nitride etch stop layer
`using the fluorinated chemical etchant system of the
`present invention.
`DETAILED DESCRIPTION OF A PREFERRED
`EMBODIMENT
`
`The inventive process herein is directed towards
`anisotropically etching a multilayer structure compris-
`ing a silicon dioxide outer layer on an underlying silicon
`nitride stop layer. Referring now to FIG. 1, a schematic
`representation of a multilayer structure, which is
`formed by conventional deposition techniques,
`is de-
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`picted. The multilayer structure of FIG. 1, generally
`designated as “10”, is shown prior to conducting the
`subject etching operations. The multilayer structure 10
`comprises a plurality of structural layers which are
`sequentially deposited onto an underlyingsilicon struc-
`ture 18. Multilayer structure 10 comprisesa plurality of
`structural layers including an outer layer 14 having a
`major outer surface 14a. Structural layer 14 is fabri-
`cated of SiO2. Basically, SiO2 (oxide) can be described
`as being either undoped or dopedglass. In the semicon-
`ductorindustry, the term oxide is generally used instead
`of glass. Generally an undoped oxide is either a field
`oxide or gate oxide which is usually grownin a furnace.
`Doped oxide include BPSG, PSG,etc. which are gen-
`erally deposited on the silicon wafer with a dopant
`gas(es) during a deposition process.
`The outer structural layer 14 is deposited onto an
`adjacent intermediate structural layer 16. Layer 16 in-
`cludes sidewalls and is fabricated of an etch stop layer
`of silicon nitride. Also shown in FIG. 1 is a chemical
`etchant protective patterned layer 12 which comprises a
`photoresist layer having a predetermined arrangement
`of openings 12a for forming a predetermined pattern in
`multilayer structure 10. Typically, this is accomplished
`using a semiconductor photomask and known conven-
`tional etch mask patterning techniques. The etch stop
`layer is deposited onto field oxide 15, silicon substrate
`18, and onto a plurality of polysilicon lines 17 having
`located adjacent their respective sidewalls spacer ele-
`ments 19.
`Asseen in FIG.2, preferred mannerofetching of the
`SiO»structural SiO2 layer 14 downto etch stop layer 16
`is by plasma etch. The gas plasma etch technique em-
`ployed herein typically has an etching area in a plasma
`and is generated under vacuum within the confines of
`an RF discharge unit. The preferred plasma etch tech-
`nique employed herein may include the use of ECR,
`Electron Cyclotron Resonance, RIE, MIE, MERIE,
`PE reactive ion, point plasma etching, magnetically
`confined helicon and helical resonator, PE, or magne-
`tron PE. In plasma dry etchers, typically the upper
`electrode is powered while the lower electrode is
`grounded. In RIE (Reactive Jon Etchers), the lower
`electrode is powered while the upper electrode is
`grounded. In triode dry etchers, the upper and lower
`electrodes can be powered as well as the sidewall. In
`MERIE (magnetically enhanced reactive ion etch)
`magnets are used to increase the ion density of the
`plasma. In ECR (Electron Cyclotron Resonance), the
`plasma is generated upstream from the main reaction
`chamber. This produces a low ion energy to reduce
`damageto the wafer.
`A semiconductor device can then located in the de-
`sired etcher, within an etching area, and is etched with
`a fluorinated chemical etchant system to form a prede-
`termined pattern therein. The fluorinated chemical
`etchant system comprises a chemical etchant composi-
`tion of the type described above such as CHF3—CF-
`4—Ar, and a CH2F) additive material. The fluorinated
`chemical etchant system is in a substantially gas phase
`during the etching of the multilayer structure.
`The exposed SiOz layer is selectively etched at a
`relatively high etch rate down to the Si3Nq4 etch stop
`layer by removing predetermined portions of the SiO2
`layer by chemically enhanced ionic bombardment.
`Someareas of the wafer continue to have SiOavailable
`to be etched while otherareasof the wafer have already
`reached the nitride Jayer where the etching process
`
`

`

`5,286,344
`
`7
`effectively stops because of polymer formation on the
`nitride surface. In this way, the etching process can
`provide for the formation of the upright sidewalls in
`etched layers which have a profile which is substan-
`tially vertical.
`
`EXAMPLE 1}
`
`A preferred etching system which is employed in the
`process of this invention is the Applied Materials Preci-
`sion 5000, a single wafer plasma etching apparatus man-
`ufactured by Applied Materials of Santa Clara, Calif.
`This apparatus comprises a mobile, double cassette plat-
`form, a transport chamber with an 8 wafer storage ele-
`vator, and from 1-4 plasma etching chambers.
`The mobile cassette platform is maintained at atmo-
`spheric pressure during the entire operation of the appa-
`ratus. It holds two cassettes of wafers, each capable of
`holding up to 25 wafers. The platform can be raised or
`lowered and moved laterally so that any particular
`wafer maybe lined up with a narrow door between the
`platform and the transport chamber.
`Nitrogen gas is fed through a flow control valve into
`the transport chamber to vent the chamber to atmo-
`sphere. A robot transfer arm in the transport chamber
`transfers wafers from the cassette on the mobile cassette
`platform to the storage elevator in the transport cham-
`ber. The transport chamberis connected to a twostage
`evacuation pump which is used to evacuate the trans-
`port chamberand a maintain it at a suitable pressure for
`transporting wafers from the elevator to the plasma
`etching chamber. This pressure was maintained at
`75-125 mTorr.
`The plasma etching chamberis connected to a turbo
`pump and the two stage pump which evacuates the
`chamberto a lower pressure than that of the transport
`chamber. This pressure was typically less than 10
`mTorr. When the transport chamber and the plasma
`etching chamber have reached suitable pressures for
`wafer transfer, the robot arm transfers a wafer from the
`wafer storage elevator to the plasma etch chamber.
`The plasma etching chamber contains an upper, elec-
`trically grounded electrode which also serves as the
`chambersidewalls, and a lower, RF powered electrode
`upon which the wafer is clamped during the plasma
`etch process, and a set of electromagnetic coils placed
`around the chamber sidewalls. The chamber also con-
`tains a gas distribution plate connected to the lid ofthe
`chamber, through which suitable feed gas mixtures are
`fed into the chamber from a connected gas supply mani-
`fold.
`When RFenergy is applied to the lower electrode,
`the gas fed into the chamber via the gas distribution
`plate is converted to plasma. The plasma contains reac-
`tive chemical species which etch selected unmasked
`portions of the wafer clamped to the lower electrode.
`Electric power is applied to the electromagnetic coils
`which surround the chamber sidewalls. The magnetic
`field generated by the coils increases the density of the
`plasma near the wafer surface. A throttle valve located
`between the plasma etching chamberregulates the pres-
`sure of the chamber to processing values, generally in
`the range of 10-350 mTorr.
`The lower electrode is connected to a wafer cooling
`system designed to maintain the wafer at a constant
`temperature during the plasma etch process. This sys-
`tem consists of three parts. The first is an apparatus
`providing a temperature controlled fluid which circu-
`lates through a tunnel in the lower electrode. The sec-
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`20
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`8
`ond part is an apparatus providing a pressure and flow
`controlled inert gas (typically helium) of high therma!
`conductivity which is fed to the underside of wafer
`during etch via a channel through the lowerelectrode,
`opening to grooves on the top face of the lowerelec-
`trode. The third part of the wafer cooling system is an
`o-ring seal which lies partially in a circular groove in
`the lower electrode. The lowerelectrode is constructed
`in such a way that it may be raised so that the wafer
`placed on its top surface is held against a clamp ring
`supported above the wafer. When the lowerelectrodeis
`raised to clamp the wafer against the clamp ring, the
`wafer underside is held tightly against the o-ring seal.
`This seal prohibits leakage of the inert gas from under-
`neath the wafer to the plasma etch cavity.
`The machine is governed by a programmable com-
`puter that is programmed to prompt the machine to
`evacuate and vent the transport chamber and plasma
`etching chamber, transfer wafers to and from the cas-
`settes, elevator, and etch chamber, control the delivery
`of process gas, RF power, and magnetic field to the
`plasma etching chamber, and maintain the temperature
`of the waferin the plasma etching chamber,all at appro-
`priate times and in appropriate sequence.
`A multilayer structure is then located within the
`plasma etching chamber and is etched with a fluori-
`nated chemical etchant system to form a predetermined
`pattern therein, The fluorinated chemical etchant sys-
`tem comprises a chemical etchant composition, such as
`CHF;, CF, and Ar, and an additive material as de-
`scribed above. The fluorinated chemical etchant system
`is in a substantially gas phase during the etching of the
`multilayer structure.
`In the case of the chemical etchant composition in-
`cluding CHF3, CF, and Ar, and an additive material
`comprising CH2F2, the exposed SiQ2layeris selectively
`etchedat a relatively high etch rate and highselectivity
`downto the Si3N4 etch stop layer by removing prede-
`termined portions of the SiO4 layer using chemically
`enhanced ionic bombardmentof the gas phase etchant
`material. Some areas of the wafer continue to have
`SiOavailable to be etched while other areas of the
`wafer have already reachedthe nitride layer where the
`etch process effectively stops because of polymer for-
`mation on the nitride surface. In this way, the etching
`process can provide for the formation of the upright
`sidewalls in etched layers which have a profile which is
`substantially vertical.
`Representative etch parameters were employed in
`the process for etching a multilayer structure of the
`present invention,as set forth above in this EXAMPLE
`1. The flow rates of the component gases, based on the
`total gas flow of the fluorinated chemical etchant sys-
`tem, used herein was as follows: an etchant material
`comprised of 16% CF4, 57% Ar, and 9% CHF, at a
`total pressure in the system of 200 mTorr, magnetic
`gases maintained at 150 gauss, and RF powerapplied at
`500 watts.
`When 20% ofthe total gas flow of CH2F2 was em-
`ployed as the additive material, a silicon dioxide to-sili-
`con nitride selectivity of more than 30:1, and a silicon
`dioxide etch rate of over 4,000 angstroms per minute
`resulted.
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`65
`
`EXAMPLE2
`
`When the process of EXAMPLE1 wasrepeated as
`described above, except that no additive material was
`introduced along with the etchant material in the feed
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`roy 5
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`gas, the selectivity was determined to be about 1.2:1,
`5. The process of claim 1, wherein said high SiO2 etch
`and the silicon dioxide etch rate was also about 4000
`rate is at least about 2,500 angstroms of SiO» per minute.
`angstroms per minute.
`6. The process of claim 1, wherein said additive mate-
`Therefore, when the process of the present invention
`rial comprises CH3F.
`7. The process of claim 1, wherein said etching of the
`was employed in EXAMPLE1, a selectivity of greater
`exposed areas of the silicon dioxide outer layer to the
`than 30:1 was achieved, as compared an selectivity of
`silicon nitride stop layer comprises a dry etching pro-
`1.2:1 for the process of EXAMPLE2. This is an in-
`crease in selectivity of greater than 2400%. In spite of
`cess conducted in a magnetically-enhanced etching
`chamber.
`this overwhelming disparity in selectivity, the processes
`—_ 0
`of EXAMPLES1|and 2 each hadasilicon dioxide etch
`8. The process of claim 1,