United States Patent c19J
`Collins et al.
`
`[54] METHOD FOR PLASMA PROCESSING
`USING MAGNETICALLY ENHANCED
`PLASMA CHEMICAL VAPOR DEPOSffiON
`
`[75]
`
`Inventors: Kenneth S. Collins, San Jose;
`Chan-Lon Yang, Los Gatos; John M.
`White, Hayward, all of Calif.
`
`[73] Assignee: Applied Materials, Inc., Santa Clara,
`Calif.
`
`[21] Appl. No.: 618,142
`
`[22] Filed:
`
`Nov. 23, 1990
`
`Related U.S. Application Data
`[63] Continuation-in-part of Ser. No. 559,947, Jul. 31, 1990,
`abandoned, and a continuation-in-part of Ser. No.
`560,530, Jul. 31, 1990, abandoned, each is a continua(cid:173)
`tion-in-part of Ser. No. 416,750, Oct. 3, 1989, aban(cid:173)
`doned.
`
`[51]
`
`Int. Cl.s ..................... HOlL 21/00; HOlL 21/02;
`HOlL 21/302;. HOlL 21/463
`[52] U.S. Cl ..................................... 437/225; 437/228;
`427/571
`[58] Field of Search ....................... 437 /225, 228, 233;
`427/571
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,860,507 1/1975 Vossen, Jr. .......................... 437/225
`4,369,205 1/1983 Winterling et al. ................. 427/571
`4,521,286 6/1985 Horwitz .............................. 437/225
`4,526,643 7/1985 Okano et al. ........................ 156/345
`4,572,759 2/1986 Benzing ............................... 156/345
`4,623,417 11/1986 Spencer et al. ..................... 156/345
`4,632,719 12/1986 Chow et al. ........................ 156/345
`4,673,589 6/1987 Standley ................................ 427/41
`4,761,219 8/1988 Sasaki et al. ........................ 204/298
`4,776,918 10/1988 Otsubo et al. ....................... 156/345
`4,808,258 2/1989 Otsubo et al. ....................... 156/643
`4,842,683 6/1989 Cheng et al. ........................ 156/345
`4,874,494 10/1989 Ohmi .............................. 204/192.12
`4,891,095 1/1990 Ishida et al. ......................... 156/643
`5,011,705 4/1991 Tanaka ................................ 427/571
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US005312778A
`5,312,778
`[11] Patent Number:
`[45] Date of Patent: May 17, 1994
`
`FOREIGN PATENT DOCUMENTS
`0272142 6/1988 European Pat. Off ..
`0326405 8/1989 European Pat. Off ..
`0343017 11/1989 European Pat. Off ..
`0396919 11/1990 European Pat. Off ..
`0421430 4/1991 European Pat. Off ..
`0208125 12/1982 Japan ................................... 427/571
`0176224 9/1985 Japan ................................... 427/571
`0026597 2/1986 Japan ................................... 427/571
`0185915 7/1989 Japan ................................... 427/571
`0237117 9/1990 Japan ................................... 427/571
`0146661 6/1991 Japan ................................... 427/571
`
`OTHER PUBLICATIONS
`Ohmi, From Alchemy To Science: Technological Chal(cid:173)
`lenges Sep. l, 1989.
`Primary Examiner-Brian E. Hearn
`Assistant Examiner-B. Everhart -
`Attorney, Agent, or Firm-Noel F. Heal
`ABSTRACT
`[57]
`A method for plasma processing characterized by the
`steps of disposing a wafer proximate to a cathode within
`a process chamber, releasing a gas into the chamber,
`applying R.F. power in the VHF/UHF frequency
`range to the cathode to form a plasma within the cham(cid:173)
`ber, developing a magnetic field within the chamber
`having flux lines substantially perpendicular to the sur(cid:173)
`face of the wafer, and varying the strength of the mag(cid:173)
`netic field until a desired cathode sheath voltage is at(cid:173)
`tained. The apparatus includes a chamber, a wafer-sup(cid:173)
`porting cathode disposed within the chamber, a mecha(cid:173)
`nism for introducing gas into the chamber, an R.F.
`power source coupled to the cathode operating in the
`frequency from about 50-800 megahertz, an electro(cid:173)
`magnetic coil disposed around the chamber adapted to
`develop a magnetic field within the chamber which is
`substantially perpendicular to the wafer and a variable
`output power supply coupled to the coil to vary the
`magnetic field strength and therefore the cathode
`sheath voltage within the chamber.
`
`15 Claims, 5 Drawing Sheets
`
`12
`
`"~ 241
`
`32
`
`34
`
`?
`10/
`
`~
`~
`
`Ex.1024 p.1
`
`

`
`U.S. Patent
`
`May 17, 1994
`
`Sheet 1 of 5
`
`5,312,778
`
`12
`
`p
`1
`
`32
`
`34
`
`42
`
`'\: ... :.'.: ~ :· .': ~ ... ':. '.: .' :·. '.. .'_.. ; . '.-.. ':.. : . "(."
`-........:. .
`.
`.
`. .
`. . _,,,,.--
`. .
`.
`!
`·"":'·.·. ·.:.·.· .. ·.·.·.··:·······.·.· 14·.· .. ·.·.·r.: 1 _.
`. ·. ·.:·: .: ·.' ..... >.:·: .: ·.': · ... ·.:·: .: ·.' ... ·. ~8·~
`· ~sa·.·.·.·.·.·
`·.·.·.·.·.· .. ·.·~
`r:-·····
`. :-. · : . ._ : . : . :-. . : P LAS M A · · : . : . :· .. : : _-. · .
`... ··~
`A
`. : . ·._ '. ." : . : : . . . E 46 I S E
`. : .. : . ·._ · . ." :
`. .
`. . . .
`
`/.{
`
`0
`
`42
`
`36
`~
`
`26
`CURRENT
`SUPPLY
`
`64
`
`50
`
`65
`
`10 /
`
`I
`MATCHING
`NETWORK
`
`51
`
`48
`
`R. F.
`SUPPLY
`
`Figure 1
`
`Ex.1024 p.2
`
`

`
`U.S. Patent
`
`May 17, 1994
`
`Sheet 2 of 5
`
`5,312,778
`
`Figure 2a
`
`2
`
`L
`
`s
`
`N
`
`s
`
`N
`
`2
`_j
`
`A
`..... l•--R--...
`
`60'
`
`62
`
`D
`
`I
`A
`
`Figure 2b
`
`Ex.1024 p.3
`
`

`
`U.S. Patent
`
`May 17, 1994
`
`Sheet 3 of 5 ·
`
`5,312,778
`
`s
`
`N
`
`N
`
`s
`
`60
`
`8
`
`)!l 46
`7~ If( Figure 2c
`
`62
`
`Figure 2d ~~ ~s
`_
`)
`- -, ------- - -
`72)) ! \\
`I•
`•I
`.,_--------D--------_.
`
`p
`
`1 60
`
`/,,_,,_ _____ I-"
`
`d
`
`SHOWERHEAD -----+---___.,~---1
`34 SURFACE
`(ANODE)
`
`PLASMA
`POTENTIAL
`
`Figure 3
`
`_!_
`SHEATH
`
`CATHODE
`..,_,....__SHEATH--~
`VOLTAGE
`
`D. C.
`BIAS-~
`
`CATHODE16
`SURFACE
`
`40 ov
`
`500
`
`Ex.1024 p.4
`
`

`
`U.S. Patent
`
`May 17, 1994
`
`Sheet 4 of 5
`
`5,312,778
`
`34
`
`22
`
`;:
`
`~ 14 --
`
`22
`
`i
`
`+-0
`f
`
`D. C. BIAS VOLTAGE (V)
`700 --~~~~~~~~~~~~~~~~----.,
`600 ·············
`............................................................................................................................................................. .
`
`500
`400
`300
`200
`100 ··················································································································································································
`0 - - - . i
`I
`0 '--~~~-'-~~~~~ ...... ~~-"-~~------i
`4
`8
`12
`16
`20
`0
`COIL CURRENT {amp)
`
`Figure 5
`
`Ex.1024 p.5
`
`

`
`U.S. Patent
`
`May 17, 1994
`
`Sheet 5 of 5
`
`5,312,778
`
`PROVIDE A GAS
`
`80
`
`COUPLE R.F. ENERGY INTO THE GAS WITH
`A CATHODE TO FORM A PLASMA AND A
`SHEATH NEAR THE CATHODE
`
`,. " ' -
`
`82
`
`DEVELOP A MAGNETIC FIELD WITHIN THE
`PLASMA
`
`- " '
`84
`
`VARY THE STRENGTH OF THE MAGNETIC
`FIELD TO VARY THE POTENTIAL OF
`THE SHEATH.
`
`~~
`86
`
`Figure 6
`
`Ex.1024 p.6
`
`

`
`1
`
`5,312,778
`
`METHOD FOR PLASMA PROCESSING USING
`MAGNETICALLY ENHANCED PLASMA
`CHEMICAL VAPOR DEPOSffiON
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`This application is a continuation-in-part of two U.S.
`patent application Ser. Nos. 07/559,947 now abandoned
`and 07 /560,530 now abandoned, both of which were
`filed on Jul. 31, 1990 and assigned to the assignee of the
`present invention and both of which are continuation(cid:173)
`s-in-part of U.S. patent application Ser. No. 07/416,750
`filed Oct. 3, 1989 now abandoned and assigned to the
`assignee of the present invention.
`
`2
`circuitry. In contrast, systems operated in the micro(cid:173)
`wave range of about 900 megahertz to 2.5 gigahertz,
`such as electron cyclotron resonance (ECR) systems,
`have sheath voltages so low that an auxiliary bias on the
`5 cathode is often required to provide commercially use(cid:173)
`ful etch rates.
`Of the frequency range choices, the high frequency
`range of 13-40 megahertz is most often employed in
`plasma etch systems. By far the most popular choice for
`10 an RIE system operating frequency is the ISM (indus(cid:173)
`try, scientific, medical) standard frequency of 13.56
`megahertz. However, the potentially damaging sheath
`voltages of such systems limits their usefulness in per-
`forming certain sensitive etch processes, such as a
`polysilicon over silicon dioxide ("oxide") etch.
`The cathode sheath voltage can be reduced by the
`BACKGROUND OF THE INVENTION
`use of magnetic confinement techniques such as those
`disclosed in U.S. Pat. No. 4,842,683 entitled "Magnetic
`This invention relates generally to the plasma pro-
`Field-Enhanced Plasma Etch Reactor" of Cheng et al.
`cessing of semiconductor wafers and more particularly
`to methods and apparatus for plasma processing semi- 20 . which teaches the use of a rotating magnetic field above
`the surface of a wafer having magnetic flux lines sub-
`conductor wafers in a reactive ion etch mode.
`Integrated circuits (ICs) are fabricated on semicon-
`stantially parallel to the wafer surface. The magnetic
`field of Cheng et al. decreases the cathode sheath volt-
`ductor wafers by subjecting the wafers to a precise
`sequence of processes. These processes can include, but
`age 25-30 percent, i.e. to about 700 volts, while it in-
`are not limited to, epitaxial deposition, lithographic 25 creases the etch rate by as much as 50 percent.
`patterning, chemical vapor deposition, sputter deposi-
`A problem with a magnetic enhancement system as
`tion, ion implantation and etch processes.
`disclosed by Cheng et al. is that the electric (E) field
`There is a seemingly inexorable trend in the IC indus-
`created within the cathode sheath is substantially per-
`pendicular to the wafer surface and is therefore at sub-
`try to produce more powerful integrated circuits by
`packing ever greater numbers of active and passive 30 stantially perpendicular to the applied magnetic (B)
`devices into each integrated circuit. This is typically
`field. The EXB force created by the interaction of these
`accomplished by both reducing the sizes of the devices
`two fields causes the well-known electron/ion drift
`within an IC and by arranging the devices more closely
`effect, which is a major source of etch non-uniformity in
`together.
`As IC devices become smaller and more densely 35 magnetically enhanced RIE systems. The aforemen-
`tioned rotation of the magnetic field reduces, but does
`packed they also become more susceptible to damage
`during the aforementioned processing steps. For exam-
`not eliminate, etch non-uniformity by averaging the
`ple, when the minimum feature size (such as a line
`effects of the electron/ion drift over the surface of the
`width) of an IC reaches about l micron, the devices of
`wafer.
`the integrated circuit may be susceptible to damage if 40 Another problem encountered with the system of
`exposed to voltage levels over 200 volts. Since it is not
`Cheng et al. is that, even with magnetic confinement, a
`cathode sheath voltage at about 700 volts is still too
`unusual for conventional semiconductor processing
`equipment, such as a reactive ion etch (RIE) system, to
`large to avoid damaging IC devices during certain types
`of processes. Unfortunately, since the electron/ion drift
`develop considerable voltage levels during operation,
`steps must be taken to prevent damage to the devices of 45 is caused by the EX B force, raising B to lower the
`the integrated circuits.
`cathode sheath voltage will increase the electron/ion
`In a reactive ion etch system a process gas is released
`drift effect a corresponding amount. In consequence,
`into a process chamber and a radio-frequency (RF)
`the system of Cheng et al. cannot reduce the cathode
`power source is coupled to a cathode located within the
`sheath voltage much below 700 volts by further increas-
`chamber to create a plasma from the process gas. A 50 ing the B field strength without causing an unaccept-
`semiconductor wafer can be supported by the cathode
`ably high etch non-uniformity over the surface of the
`and positive ions formed within the plasma can be accel-
`wafer.
`erated to the surface of the wafer to provide a very
`One approach to reducing the cathode sheath voltage
`anisotropic etch of the wafer's surface. Conventional
`to acceptably low levels is disclosed in parent applica-
`RIE systems have been operated at a number of fre- 55 tion U.S. Ser. No. 07/559,947, filed Jul. 31, 1990, of
`quencies including a low frequency range from about
`Collins et al. and entitled "VHF /UHF Reactor Sys-
`10-400 kilohertz and a high frequency range from about
`tern", the disclosure of which is incorporated herein by
`13-40 megahertz.
`reference. It is known that the cathode sheath voltage is
`Both ions and electrons within a plasma are acceler-
`a function of the RF impedance (ZRF) of the plasma
`ated in systems operated in the low frequency range of 60 which is given by the following relationship:
`10-400 kilohertz creating the risk of potential damage to
`IC devices caused by the impact of heavy, high-energy
`ions against the surface of the wafer. In high frequency
`operation in the 13-40 megahertz range a steady state
`cathode sheath is formed near the cathode which typi- 65
`cally develops a magnitude of over 1000 volts at 1 kilo(cid:173)
`watt of power. As mentioned previously, voltages of
`this magnitude can be very damaging to high-density IC
`
`15
`
`where R is the resistive component of the plasma impe(cid:173)
`dance and x is the reactive component of the plasma
`impedance. Therefore, an increase in RF frequency
`causes a decrease in ZRpand a consequent reduction in
`the cathode sheath voltage. Collins et al. teach that
`
`Ex.1024 p.7
`
`

`
`5,312,778
`
`4
`nets can be used to vary the B field strength within the
`process chamber.
`By applying a B field which is substantially parallel to
`the E field of the cathode sheath, the effects of electron(cid:173)
`s /ion drift are minimized. A vertical B field furthermore
`reduces radial diffusion losses of the free electrons,
`thereby increasing plasma density and etch rate.
`These and other advantages of the present invention
`will become clear to those skilled in the art upon a study
`to of the detailed description of the invention and of the
`several figures of the drawings.
`
`3
`operating a RIE system at VHF /UHF frequencies from
`about 50 megahertz to about 800 megahertz will result
`in lower cathode sheath voltages resulting in a softer,
`less damaging etch processes.
`While the system of Collins et al. performs very well,
`it suffers from the drawback that it requires a variable
`frequency R.F. power supply or multiple frequency
`R.F. sources which, for the required power and fre(cid:173)
`quencies, are very large and very expensive. Also, their
`impedance matching network was, to some extent, a
`compromise over the range of operating frequencies,
`resulting in less than optimal impedance matching at
`any one frequency within the range.
`In consequence, there was heretobefore an unsatisfied
`need for a plasma processing system in which the cath- IS
`ode sheath voltage could be controlled both inexpen(cid:173)
`sively and effectively.
`
`25
`
`30
`
`SUMMARY OF THE INVENTION
`The method of the present invention couples radio- 20
`frequency (RF) energy into a process gas by means of a
`cathode to form a plasma, develops a magnetic field
`within the plasma and varies the strength of the mag(cid:173)
`netic field to vary the cathode sheath voltage. The
`magnetic field strength and the cathode sheath voltage
`are inversely related over a selected operating range of
`the system. Preferably, the flux lines of the magnetic
`field (B) are parallel to the electric field lines (E) of the
`cathode sheath to eliminate EX B electron/ion drift.
`The preferred frequency of operation is in the
`VHF /UHF radio frequency band of approximately
`50-800 megahertz. Within this range, the relationship
`between the ~athode sheath voltage and the magnetic
`field allow the cathode sheath voltage to be reduced 35
`75% or more from its maximum value, which does not
`appear to be possible at lower operating frequencies for
`RIE systems. Preferably, an RF frequency is chosen
`which, in the absence of a magnetic field, produces a
`cathode sheath voltage greater than or equal to the 40
`maximum value of the different sheath voltages which
`may be desired to perform different processes. Pro(cid:173)
`cesses requiring cathode sheath voltages lower than this
`maximum value can be obtained by increasing the mag-
`netic field strength.
`The apparatus of the present invention includes a
`process chamber, a wafer-supporting cathode disposed
`within the process chamber, a mechanism for releasing
`a process gas within the chamber, a RF power supply
`operating in the 50-800 megahertz range coupled to the so
`cathode, and a magnet adapted to develop a magnetic
`field in the chamber which has flux lines substantially
`perpendicular to the cathode surface. Preferably, a pair
`of coils coupled to a variable power supply are used to
`provide a variable magnetic field within the process 55
`chamber.
`The choice of operating frequencies is an important
`aspect of the present invention. By operating the system
`at frequencies within the range of 50-800 megahertz a
`strong resonance phenomenon is observed which per- 60
`mits the cathode sheath voltage to be varied over a
`much greater range than if the system was operated at
`lower frequencies.
`An advantage of this invention is that the cathode
`sheath voltage can be controlled with a relatively inex- 65
`pensive D.C. constant' current source instead of the
`relatively expensive R.F. power source as disclosed by
`Collins et al. Alternatively, replaceable permanent mag-
`
`45
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. l is a cross-sectional view of a plasma process(cid:173)
`ing apparatus in accordance with the present invention;
`FIG. la is a top plan view illustrating an orientation
`of an electromagnetic coil and a wafer of the plasma
`processing apparatus of FIG. l;
`FIG. lb is a front elevational view of a first vertical
`magnetic field coil pair configuration as seen along line
`l-l of FIG. la;
`FIG. le is a front elevational view of a second verti(cid:173)
`cal magnetic field coil pair configuration as seen along
`line 2-2 of FIG. la;
`FIG. 2d is a front elevational view of a single vertical
`magnetic field coil configuration as seen along line 2-2
`of FIG. la;
`FIG. 3 is a graph illustrating the D.C. bias of the
`apparatus of FIG. l;
`FIG. 4 is a diagram illustrating the magnetic flux lines
`of the apparatus of FIG. l;
`FIG. 5 is a graph illustrating the relationship between
`coil current and D.C. bias voltage on the cathode of the
`apparatus of FIG. l; and
`FIG. 6 is a flow chart of a method for forming a
`plasma in accordance with the present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`In FIG. l, a plasma processing apparatus 10 in accor(cid:173)
`dance with the present invention includes an enclosure
`12 defining a processing chamber 14, a cathode 16, a
`radio-frequency (R.F.) supply system 18 coupled to the
`cathode 16, and a magnetic enhancement system 20 for
`providing a magnetic field within chamber 14. As will
`be explained in more detail subsequently, the plasma
`processing apparatus 10 typically operates in the well(cid:173)
`known reactive ion etch (R.l.E.) mode wherein positive
`ions of a process gas are accelerated to the surface of a
`wafer to provide a highly anisotropic etch of the sur-
`face.
`·
`Enclosure 12 includes sidewalls 22, lid l4 and base 26.
`An apertured plate l8 divides the process chamber 14
`from an exhaust manifold 30 within the enclosure 12
`and an insulating ring 29 insulates the plate l8 from the
`cathode 16. A process gas P is released into an inlet
`manifold 32 and is dispersed through an apertured plate
`or showerhead 34 into the process chamber 14. Gasses,
`ions, particulates and other matter can be exhausted
`from the chamber 14 through the exhaust manifold 30
`an out an exhaust conduit 36 by means of a pump 38. A
`throttle valve 40 can be used to regulate the flow rate of
`the exhaust through the exhaust conduit 36.
`The portions of enclosure 12 which are exposed to
`the process chamber 14 should be made from process(cid:173)
`compatible materials. For example, the sidewalls 22,
`plate 28 and showerhead 34 are preferably made from
`anodized aluminum. The vacuum integrity of the enclo-
`
`Ex.1024 p.8
`
`

`
`5,312,778
`
`s
`sure 12 is ensured by a number of seals 42 between
`abutting surfaces of the sidewalls, lid, base, etc.
`The cathode 16 is an elongated, conductive member
`having an upper surface 44 which can be exposed to the
`process chamber 14. It is therefore important that at 5
`least the upper surface 44 of the cathode 16 be made
`from a process compatible material, such as the afore(cid:173)
`mentioned anodized aluminum. A semiconductor wafer
`46 can be supported by the upper surface 44 of the
`cathode 16 for subsequent processing.
`The R.F. supply system 18 includes an R.F. power
`supply 48 coupled to a matching network SO which
`matches the impedance of the power supply 48 to the
`impedance of the rest of the plasma processing appara-
`tus 10. Typically, a coaxial transmission line 51 having a 15
`characteristic impedance substantially the same as the
`output impedance of the power supply 48 is used to
`couple the R.F. power supply 48 to the matching net(cid:173)
`work 50. The cathode 16 cooperates with an insulating
`sleeve 52 and a conductive sleeve 54 to serve as a coax- 20
`ial transmission line 56 which couples the matching
`network to the process chamber 14. The sleeve 54 is
`electrically coupled to the base 26 and the plate 28. The
`sidewalls 22 are electrically coupled to the lid 24, show(cid:173)
`erhead 34, plate 28 and the base 26. When R.F. power is 25
`applied to cathode 16 the process gas P within process
`chamber 14 is ionized to form a plasma due to the accel(cid:173)
`eration of free electrons which undergo collisions with
`the gas molecules to create positive ions of the gas mol(cid:173)
`ecules and more free electrons. The cathode sheath S 30
`above the cathode 16 creates an electric field E which is
`substantially perpendicular to the cathode surface. A
`current flows along a current path 58 comprising the
`cathode 16, the plasma within process chamber 14,
`showerhead 34, sidewalls 22, plate 28 and conductive 35
`sleeve 54.
`Magnetic enhancement system 20 preferably includes
`a pair of electromagnetic coils 60 and 62 and a D.C.
`power supply 64. As will be discussed in greater detail
`subsequently, the coils 60 and 62 develop a magnetic 40
`field within process chamber 14 to confine more free
`electrons above the cathode 16 thereby reducing the
`D.C. bias voltage on the cathode. The coils 60 and 62 of
`this preferred embodiment are approximately 17 inches
`in diameter and comprise 180 turns of # 12 square mag- 45
`net wire forming a toroid having cross-sectional dimen(cid:173)
`sions of about 1.5 inches in height and 1 inch in width
`and an unshielded inductance of approximately 19-20
`millihenrys each. Preferably, the coils are covered with
`a magnetic shield 66 made from a high magnetic perme- 50
`ability material such as soft iron or carbon steel to mini(cid:173)
`mize magnetic coupling and interference with nearby
`equipment and to shield the process chamber from ex(cid:173)
`ternal magnetic 'influences. When the coils 60 and 62 are
`covered with the shield 66, the inductance of each coil 55
`drops to about 14 millihenrys.
`The D.C. power supply 64 is preferably a variable,
`current regulated supply capable of supplying currents
`in the range of 0 to about 20 amperes. A suitable D.C.
`power supply is commercially available from the Soren- 60
`sen Company of Chicago, Ill. as product DCS 40-25. In
`can be controlled by an input 65 which can be a manual
`input from a knob or a signal input from a control de(cid:173)
`vice such as a computer.
`A preferred method for operating the magnetic en- 65
`hancement system 20 will be discussed with reference
`to FIGS. 1, 2a and 2b. In the top plan view of FIG. 2a,
`the top of wafer 46 (which is resting on upper surface 44
`
`6
`of cathode 16) can be seen through the inner circumfer(cid:173)
`ence 68 of coil 60. Current is supplied to coils 60 and 62
`by the D.C. power supply 64 by power lines 70. In this
`preferred embodiment, the magnetic fields developed
`by coils 60 and 62 aid each other, i.e. dissimilar mag(cid:173)
`netic poles of the two coils face each other. This can be
`seen in FIG. 2b where the north pole of coil 60 faces the
`south pole of coil 62 thereby developing a magnetic
`field B which is substantially perpendicular to the upper
`10 surface 72 of the wafer 46. In this preferred embodi(cid:173)
`ment, the coils 60 and 62 share a common magnetic axis
`A which coincides with an axis of symmetry A of both
`the wafer 46 and the cathode 16. It should be noted that
`the magnetic polarities of the coils 60 and 62 can be
`reversed, i.e. the B field can be pointing upwardly in
`FIG. 2b, without noticeable effect on the process.
`Preferably, the coils 60 and 62 are arranged in a
`Helmholtz configuration where the distance D between
`the coils is approximately equal to the radius R of each
`of the coils. The Helmholtz configuration results in a
`region X between the two coils where the magnetic flux
`lines f of the B field are substantially parallel to each
`other and substantially normal to the planes of the two
`coils. It is important that the B field be uniform in the
`proximity of the wafer 46 to provide etch uniformity
`over the surface of the wafer.
`An alternative method for operating the magnetic
`enhancement system 20 of FIG. 1 is illustrated in FIG.
`2c. In this embodiment, the magnetic fields produced by
`coils 60 and 62 oppose each other, i.e. the north poles of
`the two coils face each other. This creates a magnetic
`mirror within the process chamber proximate the wafer
`46. While the magnetic mirror is advantageous in that it
`enhances the concentration of energetic electrons near
`the surface 72 of the wafer, it increasingly suffers from
`the electron/ion drift effect away from the center of the
`wafer. A description of a method and apparatus for the
`magnetic enhancement of a plasma etching system is
`found in co-pending U.S. patent application Ser. No.
`07/349,010, filed May 8, 1989, of Hanley et al. entitled
`"Magnetically Enhanced Plasma Reactor System for
`Semiconductor Processing (as amended)" and assigned
`to the assignee of this application, the disclosure of
`which is hereby incorporated herein by reference.
`Another alternative method for operating a magnetic
`enhancement system for a plasma processing system is
`illustrated in FIG. 2d wherein only a single coil 60 lying
`in a plane P of the wafer 46 is used to create the mag(cid:173)
`netic field B. If the coil 60 diameter Dis large compared
`to the wafer diameter d a magnetic field B is created
`proximate the center of coil 60 having flux lines fwhich
`are substantially parallel and perpendicular to the wafer
`suiface 72 of the wafer 46. A drawback of this single
`coil arrangement is that unless D is much greater than d,
`e.g. at least four times as large, and unless the anode
`surface is very close to the· cathode that the B field will
`be insufficiently uniform to provide good etch unifor(cid:173)
`mity. Therefore, the single coil configuration of FIG.
`2d is best adapted for standalone single wafer etch sys(cid:173)
`tems where the size of the coil 60 is less important than
`in multi-chamber etch systems.
`As can be seen above, there are a great number of
`ways that the variable magnetic enhancement system of
`the present invention can be implemented. For example,
`on plasma etch system chambers which are not cylindri(cid:173)
`cal the coils can be made to follow the external contours
`of the chamber. Since an Applied Materials Precision
`Etch 5000 has octagonal chamber shapes, electromag-
`
`Ex.1024 p.9
`
`

`
`5,312,778
`
`8
`versely related to the strength of the B field within the
`operating range.
`Briefly, ZRFis directly to the density of the electrons
`and the ions within the plasma. Since the electrons are
`5 constrained to move along the magnetic flux line f,
`fewer electrons will be lost to the sidewalls 22, creating
`a greater density of electrons in the plasma. While the
`ions are not similarly constrained, the higher density of
`electrons will also produce a higher density of ions due
`to more frequent electron/gas molecule collisions. In
`consequence, the B field increases electron and ion
`density in the plasma, which decreases ZRF which, in
`turn, decreases the cathode sheath voltage.
`The reason why a stronger magnetic field reduces
`ZRF more than a weaker magnetic field has to do with
`the average radius of the spiraling electrons and with
`the mean free path between particles within the plasma.
`The radius of gyration of an electron around a magnetic
`flux line is given, in meters, by the Lamor radius RD
`
`25
`
`7
`netic coils can be made in a matching octagonal shape
`to closely engage the outer surface of the chamber.
`When placing such coils in a Helmholtz configuration,
`the distance between opposite faces of the octagonally
`shaped coils are used as the coil radius R.
`It should further be noted that it is possible to replace
`one or more of the electromagnetic coils of the mag(cid:173)
`netic enhancement system with permanent magnets.
`For example, one or both of the magnetic coils 60 and
`62 of FIG. 2b can be replaced or aided with one or more 10
`permanent magnets as taught in the Hanley et al. patent,
`supra, the disclosure of which has been incorporated by
`reference. The strength of the magnetic field in perma(cid:173)
`nent magnet embodiments of the present invention can
`be changed by physically changing the permanent mag- 15
`nets in the apparatus 10 as part of a process kit. For
`example, in Hanley et al., the permanent magnet 246
`within cathode 216 could be replaced with another
`permanent magnet having greater or lesser magnetic
`field strength. Another potential location for a perma- 20
`nent magnet is within a suitable enlarged inlet manifold
`32, where the magnet would be positioned so as not to
`prevent the process gas P from flowing into the cham(cid:173)
`ber 14. A process kit for forming windows in silicon
`dioxide layers can use weak permanent magnets to cre(cid:173)
`ate a high sheath voltage while a process kit for etching
`polysilicon can have strong permanent magnets to cre(cid:173)
`ate a low sheath voltage. Specific processes in accor(cid:173)
`dance with the present invention will be discussed in 30
`greater detail subsequently.
`In FIG. 3 is a graph illustrating the various D.C.
`potentials within chamber 14 of apparatus 10 with no
`magnetic field, i.e. with 0 amperes applied to the coils
`60 and 62. At the inner surface ofshowerhead 34, which 35
`serves here as an anode, the voltage is zero. Within the
`body of the plasma within chamber 14 the plasma po(cid:173)
`tential is typically in the range of +20 to +40 volts.
`The sheath layer near the surface of cathode 16 is
`strongly negative, often obtaining a cathode sheath 40
`voltage of 500 volts. The difference between the plasma
`potential and the cathode sheath voltage (i.e. plasma
`potential minus the cathode sheath voltage) is the D.C.
`bias within the process chamber 13. As can be seen, in
`RIE systems the cathode sheath voltage is very nearly 45
`the same as the D.C. bias. As will be discussed subse(cid:173)
`quently with reference to FIG. 4, the magnetic en(cid:173)
`hancement of the present invention reduces the cathode
`sheath voltage level and, in consequence, the D.C. bias
`on the cathode.
`FIG. 4 illustrates the magnetic field B developed by
`the magnetic enhancement system 20 of FIG. 1. The
`effect of the magnetic field B is to cause free electrons
`e to spiral along the flux lines f of the B field. The elec(cid:173)
`trons e can equally well move up or down the flux lines 55
`but are constrained from crossing the flux lines f. The
`much heavier positive ions i, however, are essentially
`free to move around the process chamber 14 and to
`cross the flux lines f. The ions i can therefore impact
`upon the sidewalls 22 of the apparatus while the elec- 60
`trans e are constrained to impacting upon the shower(cid:173)
`head 34 and wafer 46 if they are sufficiently energetic.
`As noted previously, the cathode sheath voltage de(cid:173)
`creases with decreasing R.F. impedance of the plasma.
`It was also noted that the cathode sheath voltage is 65
`inversely related to the strength of the magnetic field B
`within an operating range to be discussed subsequently.
`This is because R.F. impedance ZRF of a plasma is in-
`
`where m is the mass of the electron in kilograms, v is its
`initial velocity in meters/second, q is the charge of the
`electron in coulombs, and B is the magnitude of the
`magnetic flux density in tesla. In consequence, the aver-
`age radius of the electrons spiraling along the magnetic
`flux lines f is inversely proportional to the strength of
`the B field. If the mean free path (MFP) between