United States Patent
`Flamm et al.
`
`119
`
`US005711849A
`
`{11] Patent Number:
`
`145] Date of Patent:
`
`5,711,849
`Jan. 27, 1998
`
`[54] PROCESS OPTIMIZATION IN GAS PHASE
`DRY ETCHING
`
`Thompsonetal. “Introduction to Microlithography” © 1983
`ACS pp. 228-235.
`
`[75]
`
`Inventors: Daniel L. Flamm, 476 Green View Dr.,
`Walnut Creek, Calif. 94596; John P.
`Verboncoeur, Hayward, Calif.
`
`[73] Assignee: Daniel L. Flamm, Walnut Creek, Calif.
`
`[21] Appl. No.: 433,623
`
`[22] Filed:
`May3, 1995
`P51]
`Tint, Cho iccccssscsssnsessssecsscnsssersnesesseesee HO1L 21/3065
`P52] US. C0.
`ccescccsssssssssssssssecessssseseeese 156/643.1; 156/625.1;
`156/346 P; 156/626.1; 156/659.11; 156/646.1;
`204/298.31; 204/298.32; 216/58; 216/59
`[58] Field of Search ................. 204/298.31, 298.32;
`216/39, 58, 74, 79, 246 P, 625.1, 643.1,
`626.1, 646.1, 659.1, 662.1, 659.11
`
`Giapis et al. Appl. Phys Lett 57(10) 983-985 (Sep. 1990).
`
`Greguset al. Plasma Chem. Plasma Process. 13(3) 521-537
`(1993).
`
`Babanovet al. Plasma Chem. Plasma Process. 13(1) 37-59
`(1993).
`
`Ha et al. Plasma Chem. Plasma Process. 11(2) 311-321
`(1991).
`
`Raynet al. Plasma Chem. Plasma Process. 10(2) 207-229
`(1990).
`
`Elliott “Integrated Circuit Fabrication Technology” © 1982
`pp. 242-243 and 258-271.
`
`Manoset al. “Plasma Etching, An Introduction”, © 1989,
`Academic Press, pp. 91-183.
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`Primary Examiner—Martin Angebranndt
`Attorney, Agent, or Firm—Townsend and Townsend and
`Crew LLP
`
`3/1980 Horiike ........cssssssscsssesseaeeees 156/643.1
`4,192,706
`10/1980 Mogab.......
`we 156/643.1
`4,226,665
`1/1981 Ikeda etal. ..
`w+ 156/345 P
`4,243,506
`4,297,162 10/1981 Mundtetal. .....
`.. 156/345 P
`4,340,461
`7/1982 Hendricks etal.
`.. 156/345 P
`5,147,520
`9/1992 Bobbio.........
`- 204/298.32
`5,330,606
`7/1994 Kubota etal. ....
`.- 156/345 P
`
`....
`.. 156/626.1
`5,399,229
`3/1995 Stefani et al.
`........sssssecseee 156/622.1
`5,445,709
`8/1995 Kojima et all.
`
`
`
`
`OTHER PUBLICATIONS
`
`Birdet al., Transport Phenomena, 1960, John Wiley & Sons,
`p. 510.
`Manos, D. M. and Flamm, D. L., Plasma Etchings, An
`Introduction, 1989, Academic Press, (title page and table of
`contents only).
`
`[57]
`
`ABSTRACT
`
`A method of designing a reactor 10. The present reactor
`design method includes steps of providing a first plasma
`etching apparatus 10 having a substrate 21 therein. The
`substrate includes a top surface and a film overlying the top
`surface, and the film having a top film surface. The present
`reactor design method also includes chemical etching the top
`film surface to define a profile 27 on the film, and defining
`etch rate data from the profile region. A step of extracting a
`reaction rate constant from the etch rate data, and a step of
`using the reaction rate constant in designing a second plasma
`etching apparatus is also included.
`
`29 Claims, 8 Drawing Sheets
`
`
`
`Page | of 19
`
`Samsung Exhibit 1009
`
`Page 1 of 19
`
`Samsung Exhibit 1009
`
`

`

`U.S. Patent
`
`Jan. 27, 1998
`
`Sheet 1 of 8
`
`5,711,849
`
`17~\_ PlateStack
`
`10
`
`oo
`
`13
`>
`Plasma
`_ Generating
`
`cenceecunensnaneeasnanamesRCSASLACAALERTATRRTRERRTARTA
`ontenedaaneeanne
`
`
`
`
`
`CONCENTRATION
`
`n
`
`00
`
`ny(tzZ)
`
`z|
`
`21
`
`J
`
`25
`
`af
`
`23
`
`FIG. 1A
`
`Page 2 of 19
`
`Page 2 of 19
`
`

`

`U.S. Patent
`
`Jan. 27, 1998
`
`Sheet 2 of 8
`
`5,711,849
`
`
`
`
` Tegan coe Manne SMAAS “KAKA WAKA SAKA RCKAT MAHAL OKADA DOO DOR
`DAM ARAN Sanne ARAM ARMA, UA AARNE ARRAY ARE TRAN nt ont
`;
`
`zi
`
`Page 3 of 19
`
`Page 3 of 19
`
`

`

`U.S. Patent
`
`Jan. 27, 1998
`
`Sheet 3 of 8
`
`5,711,849
`
`Measure etchprofile Y 100
`
` Eich film at constant pressure and preferably
`constant plasma source characteristics and:
`
`
`|. do not etch to endpoint or 2. Etch exactly to
`first sign of endpoint. Best to do isothermally,
`
`but can probably handle complex caseif
`measure temperature-time history.
`
`101
`
`
`
`
`
`
`
`107
`
`
`
`
`Get etch rate profile data as (Etch rate,x,y) or
`(Etch rate, r, 6) triads, ann x 3 array where
`n is the numberof points sampled. lf
`temperature variesit will be average etch rate.
`
`New temperatures
`in orderto get
`
`Fit to model with kyo/D and
`105
`ks{T2), kg(T3)...
`
`etch rate as parameters
`
`
`
`
`
`Run a least squaresfit using differences between the
`data array and analytical modelfor etch rate to
`compute: kv)/D and the etch rate at edge of the plate
`
`
`or wafer.
`
`Calculate D and with
`it kvo & Ks
`
`
`
`Calculate D from Chapman-Enskog kinetic theory
`formulas (see R. B. Bird, W. E. Steward, E. N.
`
`
`Lightfoot, pp. 510-513, Transport Phenomena, Wiley
`(1960). Multiply kyo/D by D(T,P) to obtain
`Kyo(T.dgap) = kg(T)(Area/Volume)=Ks/dgap
`
`5
`t
`pre-exp & Eg
`
`
` From Kg(T 4), Kg(T9)... do least squarefit to
`extract Eg and preexponential where
`
`kg(T)=A(T! i2)nebakT Or, if Eg is known,
`just extract preexponential.
`
`108
`
`111
`
`eparate
`
`109
`
`of the wafer.
`
`Use ER(edge)=kyo No to
`deduce no, the atom
`concentration at the edge
`
`FIG. 3
`
`Page 4 of 19
`
`Page 4 of 19
`
`

`

`U.S. Patent
`
`Jan. 27, 1998
`
`Sheet 4 of 8
`
`5,711,849
`
`200
`
`201 J
`
`
`
`
`Measure etch rate vs. effective area of
`substrate.Plasma source temperature, pressure
`
`
`(T,P), power held constant. However substrate stack
`or single substrate T,P may be varied to alter Agz.
`
`
`
`
`
`
`Attegap(cd) between] Change number of||g, simultaneously)suppor
`
`
`
`member dimensions
`‘above
`watersin the reactor
`
`
`
`:
`.
`208
`
`Is uniformity very high
`
`(>90-95%)?
`
`Measureetching profile
`and compute Aeff
`
`(loading eqn)
`
`
`
`Aeff =A substr
`
`been measured? (n> 3)?
`
`Try new plasma source or
`plasma source parameters.
`
`
`
`
`
` Has data forn Aggf values
`
`
`
`Fit to eqn. 5, & from slope
`
`(m Aes) obtain 1/ST, the supply of
`
`
`
`JS 203
`etchant from source to the reactor.
`
`
`
`
`Modify chamber materials etc. to
`reduceslope of the numerator,
`(kr Ar + F) or reduce flow if F term is
`
`major etchantloss.
`
`205
`
`FIG. 4
`
`Page 5 of 19
`
`Page 5 of 19
`
`

`

`US. Patent
`
`Jan. 27, 1998
`
`Sheet 5 of 8
`
`5,711,849
`
`uniformity L 301 J 300
`
`
`
`
`
`
`
`
`Compute locus of highest T vs. P and
`d; and highest P vs. T and d where
`uniformity meets specification.
`
`
`
`
`
`Specify: uniformity limit manifold. Select
`d values and adjust locus of P and T
`values below the calculated manifold
`by a predetermined amountto allow for
`
`
`statistical and experimetnal error and
`
`processdrift. (Po,To)
`
`
`
`Determine max edgeetch rate & supply
`
`
`of etchant from plasma source (sl) for
`RF power, flow values and [Pj,Tj](d)
`
`
`within uniformity limits manifold.
`
`
`
`Locate intersection space of P<P; ,T<Tj
`(uniformity manifold) and maximum
`etch rate or etchant supply at selected
`RF power (flow) values.
`
`
`
`
`
`
`If the resulting etch rates are too
`low, change power and/or reduce
`effective etchable area (example:
`
`
`increase d, decrease number of
`substrates).
`
`
`303
`
`307
`
`309
`
`313
`
`315
`
`Specify required
`
`Input predetermined Kg or determine
`kg from uniformity measurementsat
`one or more temperatures
`
`FIG. 5
`
`Page 6 of 19
`
`Page 6 of 19
`
`

`

`U.S. Patent
`
`Jan. 27, 1998
`
`Sheet 6 of 8
`
`5,711,849
`
`Loading Effect Data
`
`1/AshRate
`
`LCD Plates
`
`FIG. 6
`
`
`
`FIG. 7
`
`FIG. 8
`
`Page 7 of 19
`
`Page 7 of 19
`
`

`

`US. Patent
`
`Jan. 27, 1998
`
`Sheet 7 of 8
`
`5,711,849
`
`
`
`NormalizedStrippingRate,Dimensonless
`
`
`
`
`
`0.9
`
`0.8
`
`0.7
`
`0.6
`
`0.5
`
`0.4
`
`0.3
`
`0.2
`
`0.1
`
`100
`50
`Radial Distance from the Wafer Center, millimeters
`
`150
`
`FIG. 9
`
`V.d=dyap
`~* d=0
`
`Nel
`
`b/e
`
`x
`
`-b/2
`
`-a/2
`
`a/2
`
`FIG. 11
`
`4
`
`FIG. 10
`
`Page 8 of 19
`
`Page 8 of 19
`
`

`

`US. Patent
`
`Jan. 27, 1998
`
`Sheet 8 of 8
`
`5,711,849
`
`ol
`| ean
`
`
`a Hl uF
`
`
`ee NAH
`
`
`
`
`
`FIG. 12
`
`Page 9 of 19
`
`Page 9 of 19
`
`

`

`5,711,849
`
`1
`PROCESS OPTIMIZATION IN GAS PHASE
`DRY ETCHING
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to integrated circuits and
`their manufacture. The present invention is illustrated in an
`example with regard to plasma etching, and more particu-
`larly to plasma etching in resist strippers in semiconductor
`processing. But it will be recognized that the invention has
`a wider range of applicability in other technologies such as
`flat panel displays, large area substrate processing, and the
`like. Merely by way of example, the invention may be
`applied in plasma etching of materials such as silicon,
`silicon dioxide, silicon nitride, polysilicon, photoresist,
`polyimide, tungsten, among others.
`Industry utilizes or has proposed several techniques for
`plasma etching. One such method is conventional chemical
`gas phase dry etching. Conventional chemical gas phase dry
`etching relies upon a reaction between a neutral gas phase
`species and a surface material layer, typically for removal.
`The reaction generally forms volatile products with the
`surface material layer for its removal. In such method, the
`neutral gas phase species may be formed by wayof a plasma
`discharge.
`A limitation with the conventional plasma etching tech-
`nique is obtaining and maintaining etching uniformity
`within selected predetermined limits. In fact, the conven-
`tional
`technique for obtaining and maintaining uniform
`etching relies upon a “trial and error” process. Thetrial and
`error process often cannot anticipate the effects of parameter
`changes for actual wafer production. Accordingly, the con-
`ventional technique for obtaining and maintaining etching
`uniformity is often costly, laborious, and difficult-to achieve.
`Another limitation with the conventional plasma etching
`technique is reaction rates between the etching species and
`the etched material are often not available. Accordingly,it is
`often impossible to anticipate actual etch rates from reaction
`rate constants since no accurate reaction rate constants are
`available. In fact, conventional techniques require the actual
`construction and operation of an etching apparatus to obtain
`accurate etch rates. Full scale prototype equipment and the
`use of actual semiconductor wafers are often required,
`thereby being an expensive and time consuming process.
`From the aboveit is seen that a method and apparatus of
`etching semiconductor wafers that is easy, reliable, faster,
`predictable, and costeffective is often desired.
`
`SUMMARY OF THE INVENTION
`
`According to the present invention, a plasma etching
`method that includes determining a reaction rate coefficient
`based uponetchprofile data is provided. The present plasma
`etching method provides for an easy and cost effective way
`to select appropriate etching parameters such as reactor
`dimensions,
`temperature, pressure, radio frequency (rf)
`power,flow rate and the like by way of the etch profile data.
`In a specific embodiment, the present invention provides
`an integrated circuit fabrication method. The present method
`includes steps. of providing a plasma etching apparatus
`having a substrate therein. The substrate includes a top
`surface and a film overlying the top surface. The film
`includesa top film surface. The present method also includes
`chemically etching the top film surface to define an etching
`profile on the film, and defining etch rate data which
`includes an etch rate and a spatial coordinate from the
`etching profile. A step of extracting a reaction rate constant
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`2
`from the etch rate data, and using the reaction rate constant
`in adjusting a plasma etching apparatusis also included.
`In analternative specific embodiment, the present inven-
`tion also provides a method of designing a reactor. The
`present method includes providing a first plasma etching
`apparatus having a substrate therein. The substrate has a top
`surface and a film overlying the top surface. The film has a
`top film surface. The present method also includes chemi-
`cally etching the top film surface to define an etching profile
`on the film, anddefining etch rate data which hasan etch rate
`anda spatial coordinate from the etching profile. A step of
`extracting a reaction rate constant from the etch rate data,
`and using the reaction rate constant in designing a second
`plasma etching apparatus is also included.
`A further alternative embodiment provides another
`method of fabricating an integrated circuit device. The
`present method includes steps of providing a uniformity
`value for an etching reaction. The etching reaction includes
`a substrate and etchant species. The present method also
`includes defining etching parameter ranges providing the
`uniformity value. A step of adjusting at least one of the
`etching parameters to produce a selected etching rate is also
`included. The etching rate provides an etching condition for
`fabrication of an integrated circuit device.
`The present
`invention achieves these benefits in the
`context of known process technology. However, a further
`understanding of the nature and advantages of the present
`invention may be realized by referenceto the latter portions
`of the specification and attached drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a simplified diagram of a plasma etching
`apparatus according to the present invention;
`FIG. 1A is a simplified cross-sectional view of a wafer
`profile according to the plasma etching apparatus of FIG.1;
`FIG. 2 is a simplified diagram of an alternative embodi-
`ment of a plasma etching apparatus according to the prevent
`invention;
`FIGS.3-5are simplified flow diagrams ofplasma etching
`methods according to the present invention;
`FIG.5Ais a plot of uniformity, temperature, pressure, and
`gap for an etching process according to the present inven-
`tion;
`FIG. 6 is a simplified plot of i/ash rate vs. LCD plate
`number according to the present invention;
`FIGS. 7-9 illustrate an example with regard to circular
`substrates according to the present invention; and
`FIGS. 10-12 illustrate an example with regard to rectan-
`gular substrates according to the present invention.
`
`DESCRIPTION OF THE SPECIFIC
`EMBODIMENT
`
`Plasma Etching Apparatus
`FIG. 1 is a simplified diagram of a plasma etching
`apparatus 10 according to the present invention. The plasma
`etching apparatus also knownas a co-axial reactor includes
`at least three processing zones. The three processing zones
`are defined as a plasma generating zone (PG) 13,a transport
`zone (TZ) 15, a plate stack zone (PS) 17, and others. Also
`shown are a chemical feed F and exhaust E. The plasma
`generating zone provides for reactant species in plasma form
`and others. Excitation is often derived from a 13.56 MHzrf
`discharge 8 and may use either capacitor plates or a wrapped
`coil, but can also be derived from other sources. The co-axial
`
`Page 10 of 19
`
`Page 10 of 19
`
`

`

`5,711,849
`
`3
`reactor 10 also includes a chemical controller 14 and a
`temperature and pressure control 12, among other features.
`Chemical effects are often enhanced over ion induced
`effects and other effects by way of perforated metal shields
`18 to confine the discharge to a region between an outer wall
`16 and shields 18. The co-axial reactor relies substantially
`upon diffusion to obtain the desired etching uniformity. The
`co-axial reactor also relies upon a chemical etch rate which
`is diffusion limited. In particular, the chemical etch rate is
`generally defined as a chemical reaction rate of etchant
`species plusat least a diffusion rate of etchant species. When
`the diffusion rate of etchant species is much greater than the
`chemical reaction rate,
`the chemical etch rate is often
`determinedby the diffusion rate. A more detailed analysis of
`such chemical etch rate will be described by way of the
`subsequent embodiments.
`Etchant species from the plasma generating zone diffuse
`through the transport zone 15 ofthe reaction chamber, and
`enter the plate stack zone space over surfaces of substrates
`21. A concentration of etchant in the transport zone, which
`is often annular, between the plasma generating zone and the
`plate stack zone is defined as no. AS etchant diffuses
`radially from the transport zoneinto the plate stack zone and
`over surfaces of the substrates, it is consumed by an etching
`reaction. A reactant concentration above the substrate can be
`defined as n,(1,z), where 1 is the distance from the center of
`the substrate and z is the distance above the substrate. A
`diffusive velocity v, of etchant species in the plate stack
`zone is characterized by Fick’s law.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`¥o = —D
`
`
`Vito
`No
`
`In a specific embodiment, a gap d,,,, above the substrate
`is much less than the lateral extent d,,,<<r and gas phase
`mass transfer resistance across the small axial distance is
`negligible so that the axial (z-direction) term of the concen-
`tration gradient can be ignored. The embodiment can be
`applied without this restriction; however, numerical mesh
`computer solutions are then required to evaluate the reaction
`rate constant and uniformity. In the embodiment,the surface
`etching reaction bears a first order form:
`0+850
`
`where
`S is a substrate atom (e.g., resist unit “mer”); and
`O is the gas-phase etchant (for example oxygen atoms)
`with certain etching kinetics. Thefirst order etching reaction
`can be defined as follows:
`
`Ros = oA Nr e-FACYRT
`
`where
`R,, defines a reaction rate;
`n, defines a concentration;
`A defines a reaction rate constant;
`T defines a temperature;
`E,cr defines an activation energy; and
`R defines a gas constant. An example of thefirst reaction
`is described in D. L. Flamm and D. M. Manos “Plasma
`Etching,” (1989), which is hereby incorporated by reference
`for all purposes. Of course, other order reactions, reaction
`relations, and models may be applied depending upon the
`particular application.
`An example of an etched substrate 21 from the plate stack
`zoneis illustrated by FIG. 1A. The substrate 21 is defined in
`
`Page 11 of 19
`
`4
`spatial coordinates such as z and r. The substrate includes a
`bottom surface 23, sides 25, and a top surfacefilm 27. As can
`be seen, the top surface film includes a convex region, or
`etching profile. The etching profile occurs by way of differ-
`ent etch rates along the r-direction of the substrate corre-
`sponding to different etchant species concentrations. A con-
`centration profile n,(1,z) is also shown where the greatest
`concentration of reactant species exists at the outer periph-
`ery of the top surface film. In the present invention, an etch
`rate constant may be obtained by correlation to the etching
`profile. Byway of the etch rate constant, other etching
`parameters such as certain reactor dimensions including a
`distance between substrates, pressure, temperature, and the
`like are easily calculated.
`FIG. 2 illustrates an alternative example of an etching
`apparatus 50 accordingto the present invention. The etching
`apparatus 50 is a single wafer etching apparatus with ele-
`ments such as a chamber 53, a top electrode 55, a bottom
`electrode 57, a power source 59,a platen 64, and others. The
`bottom electrode 57 is at a ground potential, and the top
`electrode is operably coupled to the power source 59 ata
`high voltage potential. A plasma exists in a region 54
`between the top electrode 55 and the bottom electrode 57,
`which is often a grid configuration or the like. Reactant
`species are directed via power source from a plasma source
`to a wafer substrate 61 by diffusion. A temperature and
`pressure controller 67 and a flow controller 69 are also
`shown. The etching apparatus also includes a chemical
`source feed F and a exhaust E. Of course, other elements
`may also be available based upon the particular application.
`By way of a plate 63 interposed between the wafer
`substrate 61 and the bottom electrode 57,
`the reactant
`species do not directly bombard the wafer substrate. The
`plate is preferably made of an inert material appropriate for
`the particular etching such as pyrex or glass for resist ashing,
`alurnina for fluorine atom etching ofsilicon, silicon nitride,
`or silicon dioxide, and the like. In an ashing reaction, the
`plate is placed at a distance ranging from about 5 mm to 50
`mm and less from the wafer substrate 61. Of course, other
`dimensions may be used depending upon the particular
`application. The reactant species are transported via diffu-
`sion from the plasma source to the wafer substrate around
`the periphery of the plate 63. Accordingly, the reaction rate
`at the wafer substrate is controlled by a balance between
`chemical reaction and diffusion effects, rather than direc-
`tional bombardment.
`By way of the diffusion effects, an etching rate constant
`may be obtained for the etching apparatus 50 of FIG. 2. In
`particular, the etching rate constant derives from a etching
`profile 65, which can be measured by conventional tech-
`niques. The present invention uses the etching rate constant
`to select other etching rate parameters such as reactor
`dimensions, spacing between the substrate and its adjacent
`surface,
`temperatures, pressures, and the like. But the
`present invention can be used with other reactor types where
`etching may not be controlled by diffusion. For example, the
`present invention provides a reaction rate which can be used
`in the design of reactors where diffusion does not control
`such as a directional etcher and the like. The reaction rate
`constant may also be used in the directional etcher to predict
`an extent of, for example, undercutting of unprotected
`sidewalls while ion bombardment drives reaction in a ver-
`tical direction. Of course, the invention may be applied to
`other reactors such as large batch, high pressure, chemical,
`single wafer, and others. The invention can also be applied
`to different substrate materials, and the like.
`Plasma Etching Method
`FIGS. 3-5 illustrate simplified flow diagrams of plasma
`etching methods according to the present invention. The
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`Page 11 of 19
`
`

`

`5,711,849
`
`6
`
`where
`a and b define substrate lengths in, respectively, an
`x-direction and a y-direction.
`
`u(ay) =
`
`Le
`
`+
`
`In cylindrical coordinates, the relationship between the etch
`constant over diffusivity k,/D andthe relative etch rate u(r)
`is defined as follows:
`
`u(r) =
`
`AE»)
`We)
`
`ais an outer radius (or edge) of the substrate and I, is
`modified Bessel function of the first kind.
`Tn step 106, a diffusivity is calculated for the particular
`etchants. The binary diffusivity D,, may be calculated based
`upon the well known Chapman-Enskog kinetic theory equa-
`tion:
`
`1
`
`i
`
`bao=( 2 ) Das
`
`5
`present methods provide for improved etching conditions by
`wayof areaction rate constant derived from, for example, an
`etching profile. It should be noted that the present methods
`as illustrated should not be construed as limiting the inven-
`tion as defined in the claims. One of ordinary skill in the art
`would easily recognize other applications of the inventions
`described here.
`cosh(Nk/D+(nmby 2x]
`In a specific embodiment, a method of extracting a rate
`constant 10¢ for a plasma etching step according to the
`cosh [Nko/D+(ontop S |
`present invention is illustrated by the flow diagram of FIG.
`3. A substrate with an overlying film is placed into a plasma
`is 3|>
`etching apparatus or the like. The overlying film is defined
`InTlx
`coshk,/D+(nmayy]
`
`as an etchingfilm. In the present embodiment, the overlying
`cos
`a
`film is a photoresist film, but can also be other films such as
`cosh [NUWolD+ (nmi |
`a silicon film, a polysilicon film, silicon nitride, silicon
`oxide, polyimide, and the like.
`A step of plasma etching the film is performed by step
`101. The plasma etching step occurs at constant pressure and
`preferably constant plasma source characteristics. More
`preferably, the plasma etching stép occurs isothermally at
`temperature T,, but can also be performed with changing
`temperatures where temperature and time histories can be
`monitored. Plasma etching of the film stops before the
`endpoint (or etch stop). Alternatively, plasma etching stops
`at a first sign of the endpoint (or etch stop). The plasma
`etching step preferably stops before etching into an etch stop
`layer underlying thefilm to define a “clean” etching profile.
`The substrate including etched film is removed from the
`chamber of the plasma etching apparatus. The etched film
`includes an etching profile (step 103) made by way of
`plasma etching (step 101). The etching profile converts into
`a relative etch rate, relative concentration ratio, a relative
`etch depth, andthe like at selected spatial coordinates. The
`relative etch rate is defined as an etch rate at a selected
`spatial coordinate over an etch rate at the substrate edge. The
`relative concentration ratio is defined as a concentration of
`(sir +a)
`etchant species at a selected spatial coordinate over a con-
`centration of etchant at the substrate edge.
`Dap = 2.2646 x 10-5—alba
`In x-y-z coordinates,
`the relative etch rate in the
`z-direction, and the spatial coordinates are defined in the x-y
`coordinates. The etching profile is thereby characterized as
`a relative etch rate u, a x-location, and a y-location u, (x, y).
`In cylindrical coordinates, the relative etch rate is also in the
`z-direction, and the spatial coordinates are defined in the r
`and 6 coordinates. The etching profile is characterized as a
`relative etch rate u, ar-location, and a @-location (u,r, 9). An
`array of data points in either the x-y coordinates or 1-0
`coordinates define the etching profile. The array of data
`points can be defined as an nx3 array, where n represents the
`number of points sampled and 3 represents the etch rate and
`two spatial dimensions. Of course, the choice of coordinates
`depends upon the particular application.
`Optionally,in a non-isothermal condition, an average etch
`rate is measured. By approximate integration of a time
`dependent etch rate, suitable starting point approximations
`for an etching rate constant pre-exponential and activation
`energy can be selected. The etch rate is integrated over time
`(and temperature) using measured temperature-time data (or
`history). An etched depth profile and the etching rate from
`the integration can then be compared with actual data. A rate
`constant is appropriately readjusted and the aforementioned
`method is repeated as necessary.
`An etch constant (or a reaction rate constant) over diffu-
`sivity (k,,/D) and an etch rate at an edge is calculated at step
`105. The etch constant over diffusivity correlates with data
`points representing the etch rate profile. In x-y coordinates,
`the relationship between k,,/D and the relative etch rate
`u(x,y) is often defined as follows:
`
`where
`T is a temperature;
`c is a total molar concentration;
`M, and Mgare molecular weights;
`Dgz is a binary diffusivity;
`Ox, is a collision diameter; and
`Qpap is a collision integral.
`The Chapman-Enskog kinetic theory equation is described
`in detail in part I of R. B. Bird, W. E. Stewart, and E. N.
`Lightfoot, “Transport Phenomena,” Wiley (1960) which is
`hereby incorporated by reference forall purposes. Of course,
`other techniques for calculating a diffusivity may also be
`used. The equivalent volumetric reaction rate constant k,,, is
`derived from the diffusivity as follows.
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`30
`
`55
`
`Once the reaction rate constant k,, is extracted, the surface
`reaction rate constant k, may be isolated from the previous
`equation as follows.
`
`KGoigap
`
`65
`
`Repeat steps 101-106 at different temperatures T.,
`T3...T,, to calculate additional reaction rate constants k(T.),
`k(T;) .. . K(T,,). The steps are repeated at least two times and
`more, and preferably at least three times and more. Each
`temperature is at least 5° C. greater than the previous
`
`Page 12 of 19
`
`Page 12 of 19
`
`

`

`5,711,849
`
`8
`takes another etch rate measurementat a different effective
`area. The flow diagram then turns to step 203.
`In step 203, a supply of etchant S? in the reactor is
`calculated. Based upon the different etchable areas a slope
`mA, deduces from the loading effect relationship as fol-
`lows.
`
`1
`1
`_
`Ros) sto
`
`kA +E
`kST
`
`mAg
`st
`
`*
`
`7
`temperature. Of course, the selection of temperatures and.
`trial numbers depend upon the particular application.
`Extract an activation energy E,,,, for a first order reaction
`from the data k(T,), K(T) ... K(T,,) at T2, T .. .T,, collected
`via step 109 by way of the following equation:
`Lace
`k(D=ANT © F
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`The activation energy is preferably calculated by a least
`squarefit of data collected at step 109 or any other suitable
`statistical technique. By way of the same equation,
`the
`present methodcalculates surface reaction rate constant k, at
`any temperature.
`In step 111, a concentration n, at the substrate edge is
`calculated. The concentration n, deduces from the following
`relationship:
`
`n=Ry5/K,
`
`where
`R,,, is an etch rate.
`From the concentration and the surface reaction rate, the
`particular etching step can be improved by wayof adjusting
`selected etching parameters.
`In an alternative specific embodiment, a method to “tune”
`a plasma source using a loading effect relationship (or
`equation)is illustrated by the simplified flow diagram 200 of
`FIG.4. The method includes a step 201 of measuring an etch
`rate against an effective etchable area A,,. The effective
`etchable area changes by varying the number m of wafers in
`the reactor, varying the size of the wafer, or the like. The
`effective area can be changed 209 by altering a gap between
`a wafer andits above surface 211, changing wafer quantity
`in the reactor 213, and varying substrate support member
`dimensions 215. The method preferably occurs at constant
`temperature and pressure. However, the effective etchable
`area may also be varied by way of changing a temperature
`and/or a pressure.
`The method calculates a uniformity value (step 217) from
`the measured values of etch rate vs. effective area in steps
`211, 213, and 215. The uniformity is calculated by, for
`example:
`
`where R,,(m)is the etchingrate at the boundary between the
`plate stack zone and transport zone when m substrates are
`present in the reactor. Thefirst term includes a recombina-
`tion term proportional to the total effective area A, which
`acts to catalyze loss of etchant on reactor surfaces in the
`reactor plus a convection term F. The second term is the
`loading effect relation, where the reciprocal etch rate is
`proportional to the amount of effective etchable substrate
`areaA,,times the number of substrates m. Whenthe etching
`across a substrate is uniform, A,,is the geometrical substrate
`areaA,,. When etching is nonuniform, on the other hand, A,»
`is a function of k,,/D and geometrical reactor dimensions.
`The supply of etchant S? may be calculated for a different
`plasma source or plasma source parameters such as
`temperature, pressure, or the like by repetition 207 of steps
`201 and 203. By way of the supply of etchant to the reactor,
`other plasma source parameters may be varied to obtain
`desired etching rates and uniformity for the particular reac-
`tor.
`Step 205 provides for the modification of chamber mate-
`rials and the like to reduce slope numerator (k,, A-+F) in
`selecting the desired etching conditions. The chamber mate-
`rials can be modified to reduce, for example, the recombi-
`nation rate in the reactor. The recombinationrate is directly
`related to the effective reactor recombination area A,. In step
`205, the recombination rate can be adjusted by changing A,
`via changing chamber material, coating chamber surfaces
`with, for example, a product sold under the trademark.
`TEFLON™ or KALREZ™ and the like, among others.
`Alternatively, the slope numerator flow term F is reduced
`whenF contributes as a substantial loss term. Of course, the
`particular materials used depend upon the application.
`In step 207, the method changes plasma source param-
`eters such as rfpower,flow rate, and the like to select desired
`etching conditions. Once one of the aforementioned param-
`eters is adjusted, the method returns to step 201 via branch
`208. At step 201, an etch rate vs. effective etchable area is
`measured and the method continues through the steps until
`desired etching condition are achieved. Of course, other
`sequences of the aforementioned step for tuning the plasma
`source may also exist depending upon the particular appli-
`cation.
`FIG. 5 is a simplified flow diagram for a method of
`selecting a desired uniformity and desired etching param-
`eters within selected ranges to provide a desired etch rate for
`a particular etching process. The etching parameters include
`process variables such as reactor dimensions, a pressure, a
`temperature, and the like for a particular substrate and
`reactants. Other etching parameters may also be used
`depending upon the particular application.
`In step 301, select a uniformity for the selected substrate
`andthe reactants. The selected uniformity becomes an upper
`operating limit for the reaction according to the present
`method. The upper operating limit ensures a “worst case”
`uniformity value for an etched substrate according this
`method. Uniformity can be defined by, for example:
`
`uniformity = 100[1 ~ *
`
`Ruax — Ruin
`m R;
`25 —1 ™
`
`where
`Ryax is a maximum etch rate;
`Rag is a minimum etch rate;
`m is a sample number;
`R, is a general etch rate for an ith sample; uniformity is a
`planarity me