United States Patent r191
`Chen et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,741,799
`May 3, 1988
`
`[54] ANISOTROPIC SILICON ETCHING IN
`FLUORINATED PLASMA
`Inventors: Lee Chen; Gangadhara S. Mathad,
`both of Poughkeepsie, N.Y.
`
`[75]
`
`[73] Assignee:
`
`International Business Machines
`Corporation, Armonk, N.Y.
`[21] Appl. No.: 931,909
`[22] Filed:
`Nov. 17, 1986
`
`Related U.S. Application Data
`[63] Continuation of Ser. No. 730,988, May 6, 1985, aban(cid:173)
`doned.
`[51]
`Int. Q,4 ...•.................. HOlL 21/306; B44C 1/22;
`C03C 15/00; C03C 25/06
`[52] U.S. a ..................................... 156/643; 156/646;
`156/657; 156/659.1; 156/662; 204/192.37;
`252/79.1; 427/38
`[58] Field of Search ............ 156/643, 646, 657, 659.1,
`156/662,345;252/79.1;204/192.37;427/38,39
`References Cited
`U.S. PATENT DOCUMENTS
`4,431,477 2/1984 Zajac .............................. 156/662 X
`4,473,435 9/1984 Zafiropoulo et al. .......... 156/657 X
`4,522,681 6/1985 Gorowitz et al ................... 156/643
`4,528,066 7/1985 Merkling, Jr. et al ......... 156/657 X
`4,529,475 7/1985 Okano et al ................. 204/192 EX
`
`[56]
`
`4,534,816 8/1985 Chen et al. .......................... 156/345
`
`OTHER PUBLICATIONS
`Mogab et al., "Anisotropic plasma etching of polysili(cid:173)
`con", J. Vac. Sci. Technol., vol. 17, No. 3, May/Jun.
`1980, pp. 721-730.
`Mathad, G. S., "Review of Single Wafer Reactor Tech(cid:173)
`nology ... ", Solid State Technology, Apr. 1985, pp.
`221-225.
`Bruce et al., "High Rate Anisotropic Aluminum Etch(cid:173)
`ing'', J. Electrochem. Soc., vol. 130, No. 6, pp.
`1369-1372 (1983).
`Primary Examiner-William A. Powell
`Attorney, Agent, or Firm-Graham S. Jones, II
`
`ABSTRACT
`[57]
`A method of high rate anisotropic etching of silicon in
`a high pressure plasma is described. In one embodiment
`the etching ambient is a mixture of either NF 3 or SF 6, an
`inert gas such as nitrogen, and a polymerizing gas such
`as CHF3 that creates conditions necessary for anisot(cid:173)
`ropy not normally possible with nonpolymerizing fluo(cid:173)
`rinated gases in a high pressure regime. The etch pro(cid:173)
`cess is characterized by high etch rates and good unifor(cid:173)
`mity utilizing photoresist or similar materials as a mask.
`The present process may advantageously be used to
`etch deep trenches in silicon using a photoresist mask.
`
`7 Oaims, 1 Drawing Sheet
`
`Page 1 of 4
`
`Samsung Exhibit 1025
`
`

`

`U.S. Patent
`
`May 3, 1988
`
`4,741,799
`
`FIG.1
`
`14
`
`REACTIVE GAS
`
`i COOLING FLUID
`i
`
`/iO
`
`12
`
`17
`
`FIG.2
`
`Page 2 of 4
`
`

`

`1
`
`4,741,799
`
`ANISOTROPIC SILICON ETCHING IN
`FLUORINATED PLASMA
`
`This application is a continuation of copending appli- 5
`cation Ser. No. 730,988, filed on May 6, 1985, now
`abandoned.
`
`BACKGROUND OF THE INVENTION
`The present invention relates to the fabrication of 10
`integrated circuit devices such as LSI or VLSI semi(cid:173)
`conductor chips, and more particularly to plasma etch
`processes designed to define microscopic patterns in
`such devices.
`Many dry etching processes for etching silicon are 15
`known, typically involving plasmas in the reactive ion
`etch (RIE) regime of relatively low pressure, approxi(cid:173)
`mately 30-100 microns, and low power density, about
`0.01-0.5 watt/cm2. Recently, much attention has been
`directed in the semiconductor industry to plasma etch- 20
`ing using high pressure, 1 torr and above, and high
`power density, 2-10 watt/cm2, resulting in substantially
`higher etch rates than previously possible.
`In plasma etch processes, two removal components
`contribute to form the resulting etch profile in the target 25
`film: a chemical component, due to the chemical reac(cid:173)
`tion of the plasma generated species with the surface
`material to be removed, and a physical component, due
`to the momentum transfer of the charged particles
`formed in the plasma and accelerated through the 30
`sheath to the target material. Plasma etch processes
`carried out in the high pressure regime are distinguished
`by the much greater importance of the chemical com(cid:173)
`ponent in etching than in the low pressure RIE pro-
`cesses.
`In the conventional fluorinated gas chemistry, as
`exemplified by U.S. Pat. No. 4,310,380 to Flamm et al.,
`etching is isotropic in nature, with comparable lateral
`and vertical etch rates in silicon. In the disclosed pro(cid:173)
`cess the chemical component of the readily dissociated 40
`NF3 ambient is very strong, even in the low pressure
`RIE type process, where one would normally expect a
`greater vertical etch rate than lateral rate due to the
`strength of the physical bombardment. In a high pres(cid:173)
`sure regime, such a gas chemistry will become even 45
`more isotropic. While isotropic etching is useful in some
`silicon etch steps, it is not desirable where deep etching
`of silicon (3 to 5 microns) of small dimensions is re(cid:173)
`quired, such as in isolation trench etching. In such a
`process, a trench is etched around a transistor or other 50
`device which is then filled with a dielectric material to
`electrically isolate the device. The trench cuts verti(cid:173)
`cally through several layers of differently doped
`polysilicon or silicon. An etch plasma which uses chlo(cid:173)
`rinated gases to control undercut will undercut each 55
`layer a different amount depending on that layer's reac(cid:173)
`tivity with fluorine and chlorine. These and other prob(cid:173)
`lems are overcome by the present invention.
`
`35
`
`60
`
`SUMMARY OF THE INVENTION
`One object of the present invention is to provide an
`improved plasma etching process for silicon, particu(cid:173)
`larly when multiple silicon layers having different dop(cid:173)
`ing characteristics are present.
`According to one embodiment of the invention, the 65
`etchant gas composition includes three major constitu(cid:173)
`ents: the etchant species, for example, NF3 or SF6; an
`inert gas, such as Nz; and a polymerizing gas such as
`
`2
`CHF3. Nitrogen trifluoride (NF3) readily dissociates in
`a plasma releasing free fluorine and fluorine-containing
`radicals in greater quantities than alternate fluorine
`sources. It is also much safer than CIF3, BrF3, or IF3
`which are potentially explosive gases not suitable in a
`manufacturing environment. In fact, the extremely
`rapid dissociation of NF3 in a high pressure plasma
`proves to cause rather nonuniform etching without the
`dilution by an inert gas. Nitrogen was found to yield
`somewhat better uniformity than argon or helium.
`The addition of a small amount of a polymerizing gas
`to the high pressure plasma gives the present process its
`anisotropic character. The choice of polymerizing gases
`is determined by the type of mask used. A fluorine-con(cid:173)
`taining gas is preferred for photoresist, aluminum or
`chromium masks, while a silicon dioxide mask will
`necessitate the addition of a chlorine-containing gas to
`the fluorine etchant mixture. In the plasma, the gas will
`form a polymer which is subsequently conformally
`deposited on the target suface. In the vertical direction,
`the polymer is etched away leaving a polymer passiv(cid:173)
`ated sidewall. The sidewall is protected from lateral
`etching and undercutting of the silicon is thus elimi(cid:173)
`nated. In the particular case of trench isolation process(cid:173)
`ing, the etchant species does not attack the sidewall of
`the differently doped polysilicon and so there is no
`variable undercut such as that encountered in processes
`using chlorinated gases to control anisotropy.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a cross-sectional view in elevation of a high
`pressure single wafer reactor used to practice the pro(cid:173)
`cess of present invention; and
`FIG. 2 is a cross-sectional view of a portion of an
`integrated circuit device etched in accordance with the
`present invention, specifically, for trench isolation.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`In accordance with the principles of the present in(cid:173)
`vention, etching is performed in a high pressure, high
`plasma density single wafer reactor of the type shown in
`FIG. 1. The reactor is similar in design to that described
`in greater detail in the copending U.S. patent applica(cid:173)
`tion of Chen et al., Ser. No. 623,670, filed June 22, 1984,
`now U.S. Pat. No. 4,534,816, which is assigned to the
`present assignee, and is incorporated herein by refer(cid:173)
`ence.
`Referring now to the drawings, there is shown in
`FIG. 1 a single wafer reactor 10, wherein a circular,
`electrically grounded upper electrode 11 is attached to
`a cylindrical housing 12. Housing 12 has a gas distribut(cid:173)
`ing baffle 13, a reactive gas inlet 14, and a cooling fluid
`inlet (not shown) and outlet 15. This assembly is con(cid:173)
`tained within an insulating housing 16. The lower elec(cid:173)
`trode 17 includes a conductive upper section 18 and an
`insulating lower section 19. Upper section 18 includes
`cooling channels 20 and a raised portion surrounded by
`an insulating ring 21 having gas exhaust channels 2la.
`The spacing 22 between the upper 11 and lower 17
`electrodes is set at approximately 4 mm. An insulating
`ring 23, which electrically isolates the two electrodes, is
`formed of conduits 24 for exhausting the gas from the
`inter-electrode spacing. These conduits 24 open into a
`gap 25 between inner housing 16 and outer housing 26.
`The reacted gases are exhausted from the system
`through a port 27.
`
`Page 3 of 4
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`

`

`4,741,799
`
`15
`
`3
`FIG. 2 is a greatly enlarged cross-sectional represen(cid:173)
`tation of a portion of a silicon wafer showing a trench
`etched according to the principles of the present inven(cid:173)
`tion. A pattern mask layer 28, for example, photoresist,
`aluminum, chromium or silicon dioxide, is formed on
`the surface of a heavily n-doped layer 30 of polycrystal(cid:173)
`line silicon. The mask layer 28, of course, must be resis(cid:173)
`tant to the etching gas mixture. Layer 30 overlies a
`lightly p-doped monocrystalline silicon layer 32 which
`is formed on a silicon wafer 34. According to one em- 10
`bodiment of the present invention, the unmasked areas
`of layers 30 and 32 are etched anisotropically to form a
`trench 36 having substantially vertical sidewalls.
`Trench 36 may typically have a width of about 5 mi-
`crons.
`Fluorine-containing polymerizing gases such as
`CHF3, C2F4. C2F6 and C3Fs have been found to be
`advantageous in etching silicon or doped polysilicon
`through photoresist, aluminum or chromium mask lay- 20
`ers. It has been found that a silicon dioxide mask layer
`required, in addition, an amount of a chlorine contain(cid:173)
`ing gas such as CCl4, CFCl3,' CF2Cl2 or C2HCl3, the
`latter being a halogenated hydrocarbon containing one
`or more fluorine atoms.
`It is to be understood that the specific sequence of
`doped or undoped layers are illustrative only, and that
`any sequence or number or layers of undoped polysili(cid:173)
`con, doped polysilicon, and monocrystalline silicon
`may be etched utilizing the principles of this invention. 30
`In accordance with a preferred embodiment of the
`present invention, a pressure of about 1 torr is estab(cid:173)
`lished in plasma reactor 10. The etchant gas is intro(cid:173)
`duced at a total flow rate of about 24 SCCM; 10 SCCM
`of NF3, 10 SCCM of Nz, and 4 SCCM of CHF3. In the 35
`single wafer reactor, a power density of about 2
`watts/cm2 is generated at the wafer surface. The upper
`electrode 11 temperature is maintained at about 0 de(cid:173)
`grees C., and the temperature of the lower electrode at
`about -10 degrees C. The silicon wafer is partially 40
`masked and placed on the lower electrode 17. For the
`process conditions specified above, anisotropic etch
`rates of about 1.6 microns/min. have been observed.
`The above example is illustrative only. More gener(cid:173)
`ally, etching can be carried out by selecting pressures, 45
`total gas flow rates, and power densities in the ranges of
`
`25
`
`4
`0.5 to 5 torr, 10 to 100 SCCM, and l to 10 watts/cm2,
`respectively.
`While the present invention has been particµlarly
`shown and described with reference to the preferred
`5 embodiments thereof, it will be understood by those
`skilled in the art that various changes in form and details
`may be made therein without departing from the spirit
`and scope of the invention.
`What is claimed is:
`1. A mehtod of anisotropically etching a silicon body
`in a single etching step, comprising:
`placing said silicon body in a gaseous plasma environ-
`ment at a constant gas pressure and composition
`consisting essentially of a chlorine-free fluorine(cid:173)
`containing etchant gas, and a polymer-forming gas
`for substantially limiting lateral etching of said
`silicon body, said polymer-forming gas consisting
`of one or more halogenated hydrocarbons wherein
`the halogen in each is fluorine.
`2. The method of claim 1, wherein said etchant gas is
`selected from the group consisting of NF 3, SF 6 and the
`combination thereof.
`3. The method of claim l, wherein said polymer(cid:173)
`forming gas comprises CHF3.
`4. The method of claim l, wherein said gaseous
`plasma environment comprises a pressure of about 1-5
`torr and a power density of about 2-5 watts/cm2 at the
`surface of said silicon body.
`5. The method of claim 1, wherein said silicon body
`to be etched comprises multiple layers of differently
`doped monocrystalline silicon or polycrystalline silicon
`or combinations thereof.
`6. A method of anisotropically etching a silicon body
`in a single etching step, comprising:
`placing said silicon body in a gaseous plasma environ(cid:173)
`ment at a constant gas pressure and composition
`consisting essentially of an inert gas, and a chlo(cid:173)
`rine-free fluorine-containing etchant gas, and a
`polymer-forming gas for substantially limiting lat(cid:173)
`eral etching of said silicon body, said polymer(cid:173)
`forming gas consisting of one or more halogenated
`hydrocarbons wherein the halogen in each is fluo-
`rine.
`7. The method of claim 6, wherein said
`comprises nitrogen.
`
`inert gas
`
`* * * * *
`
`50
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`55
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`60
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`65
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`Page 4 of 4
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