US008125155B2
`
`(12) Unlted States Patent
`(10) Patent No.:
`US 8,125,155 B2
`
`Chistyakov
`(45) Date of Patent:
`*Feb. 28, 2012
`
`(54) METHODS AND APPARATUS FOR
`GENERATING STRONGLY-IONIZED
`
`PLASMAS WITH IONIZATIONAL
`INSTABILITIES
`
`5,192,894 A *
`5,212,425 A *
`5,286,360 A
`5,330,800 A
`5,427,669 A
`
`3/1993 Teschner .................. 315/111.21
`5/1993 Goebel et a1.
`............ 315/111.21
`2/1994 Szczyrbowski et a1.
`7/1994 Schumacher et a1.
`6/1995 Drummond
`
`(75)
`
`Inventor: Roman Chistyakov, Andover, MA (US)
`
`(Continued)
`
`(73) Assignee: Zond, Inc., Mansfield, MA (US)
`
`DE
`
`FOREIGN PATENT DOCUMENTS
`3210351 A1
`9/1983
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(Continued)
`
`OTHER PUBLICATIONS
`
`This patent is Sllbjeet to a terminal (115'
`claimer.
`
`.
`(21) Appl. NO" 12/870’388
`
`(22)
`
`Filed:
`
`Aug. 27, 2010
`
`(65)
`
`Prior Publication Data
`US 2011/0019332 A1
`Jan. 273 2011
`
`Related US. Application Data
`(60) Division of application No. 11/465,574, filed on Aug.
`18, 2006, now Pat. No. 7,808,184, which is a
`continuation of application No. 10/708,281, filed on
`Feb. 223 2004, now Pat. No. 7,095,179-
`
`51
`
`Int. Cl.
`
`(2006.01)
`H05B 31/26
`(52) US. Cl.
`.......... 315/111.21; 315/111.41; 315/111.71
`(58) Field
`of
`Classification
`Search ................ 315/11121711191; 204/192.12,
`204/298.08
`See application file for complete search history.
`
`(56)
`
`References Cited
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`
`(Continued)
`
`Primary Examiner i Tung X Le
`(74) Attorney, Agent, or Firm iKurt Rauschenbach;
`Rauschenbach Patent Law Group, LLP
`
`ABSTRACT
`(57)
`Methods and apparatus for generating strongly-ionized plas-
`mas are disclosed. A strongly-ionized plasma generator
`according to one embodiment includes a chamber for confin-
`ing a feed gas. An anode and a cathode assembly are posi-
`tioned inside the chamber. A ulsed ower su
`l
`is electri-
`P
`P
`PP y
`cally connected between the anode and the cathode assembly.
`The pulsed power supply generates a multi-stage Voltage
`pulse that includes a low-power stage with a first peak voltage
`having a magnitude and a rise time that is sufficient to gen-
`erate a weakly-ionized plasma from the feed gas. The multi-
`stage voltage pulse also includes a transient stage with a
`second peak voltage having a magnitude and a rise time that
`is sufficient to shift an electron energy distribution in the
`weakly-ionized plasma to higher energies that increase an
`ionization rate which results in a rapid increase in electron
`density and a formation of a strongly-ionized plasma.
`
`19 Claims, 16 Drawing Sheets
`
`COOLING
`SYSTEM
`
`[500
`501
`PULSED
`
`POWER
`SUPPLY
`
`
`
`
`
`.\
`
`
`
`
` TEMPERATURE
`
`.......
`
`
`
`51B
`
`, CONTROLLER
`
`
`/
`
`'4."
`540 JZ—105
`
` l-v‘) VACUUM PUMP
`
`GILLETTE 1001
`
`GILLETTE 1001
`
`

`

`US 8,125,155 B2
`
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`

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`US. Patent
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`Feb. 28, 2012
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`Sheet 2 of 16
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`US 8,125,155 B2
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`U.S. Patent
`
`Feb. 28, 2012
`
`Sheet 3 of 16
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`US 8,125,155 B2
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`U.S. Patent
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`Feb. 28, 2012
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`US 8,125,155 B2
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`U.S. Patent
`
`Feb. 28, 2012
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`U.S. Patent
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`Feb. 28, 2012
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`US 8,125,155 B2
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`U.S. Patent
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`Feb. 28, 2012
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`US 8,125,155 B2
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`U.S. Patent
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`Feb. 28, 2012
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`US 8,125,155 B2
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`U.S. Patent
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`US 8,125,155 B2
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`US. Patent
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`Feb. 28, 2012
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`Sheet 14 of 16
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`US 8,125,155 B2
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`562
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`PRIMARY SECONDARY
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`PRIOR ART
`
`

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`U.S. Patent
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`Feb. 28, 2012
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`Sheet 16 of 16
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`US 8,125,155 B2
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`US 8,125,155 B2
`
`1
`METHODS AND APPARATUS FOR
`GENERATING STRONGLY-IONIZED
`PLASMAS WITH IONIZATIONAL
`INSTABILITIES
`
`RELATED APPLICATION SECTION
`
`This application is a divisional application of US. patent
`application Ser. No. 11/465,574, filed on Aug. 18, 2006,
`entitled “Methods And Apparatus For Generating Strongly-
`Ionized Plasmas With Ionizational Instabilities” which is a
`
`continuation application of US. patent application Ser. No.
`10/708,281, filed on Feb. 22, 2004, entitled “Methods and
`Apparatus for Generating Strongly Ionized Plasmas with Ion-
`izational Instabilities,” the entire applications of which are
`incorporated herein by reference.
`
`BACKGROUND OF INVENTION
`
`A plasma can be created in a chamber by igniting a direct
`current (DC) electrical discharge between two electrodes in
`the presence of a feed gas. The electrical discharge generates
`electrons in the feed gas that ionize atoms thereby creating the
`plasma. The electrons in the plasma provide a path for an
`electric current to pass through the plasma. The energy sup-
`plied to the plasma must be relatively high for applications,
`such as magnetron plasma sputtering. Applying high electri-
`cal currents through a plasma can result in overheating the
`electrodes as well as overheating the work piece in the cham-
`ber. Complex cooling mechanisms can be used to cool the
`electrodes and the work piece. However, the cooling can
`cause temperature gradients in the chamber. These tempera-
`ture gradients can cause non-uniformities in the plasma den-
`sity which can cause non-uniform plasma process.
`Temperature gradients can be reduced by pulsing DC
`power to the electrodes. Pulsing the DC power can allow the
`use of lower average power. This results in a lower tempera-
`ture plasma process. However, pulsed DC power systems are
`prone to arcing at plasma ignition and plasma termination,
`especially when working with high-power pulses. Arcing can
`result in the release of undesirable particles in the chamber
`that can contaminate the work piece.
`Plasma density in known plasma systems is typically
`increased by increasing the electrode voltage. The increased
`electrode voltage increases the discharge current and thus the
`plasma density. However, the electrode voltage is limited in
`many applications because high electrode voltages can effect
`the properties of films being deposited or etched. In addition,
`high electrode voltages can also cause arcing which can dam-
`age the electrode and contaminate the work piece.
`
`BRIEF DESCRIPTION OF DRAWINGS
`
`This invention is described with particularity in the
`detailed description and claims. The above and further advan-
`tages of this invention may be better understood by referring
`to the following description in conjunction with the accom-
`panying drawings, in which like numerals indicate like struc-
`tural elements and features in various figures. The drawings
`are not necessarily to scale, emphasis instead being placed
`upon illustrating the principles of the invention.
`FIG. 1 illustrates a cross-sectional view of a plasma sput-
`tering apparatus having a pulsed direct current (DC) power
`supply according to one embodiment of the invention.
`FIG. 2 is measured data of discharge voltage as a function
`of discharge current for a prior art low-current plasma and a
`high-current plasma according to the present invention.
`
`10
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`FIG. 3 is measured data of a particular voltage pulse gen-
`erated by the pulsed power supply of FIG. 1 operating in a
`low-power voltage mode.
`FIG. 4 is measured data of a multi-stage voltage pulse that
`is generated by the pulsed power supply of FIG. 1 that creates
`a strongly-ionized plasma according to the present invention.
`FIG. 5A-FIG. 5C are measured data of other illustrative
`
`multi-stage voltage pulses generated by the pulsed power
`supply of FIG. 1.
`FIG. 6A and FIG. 6B are measured data of multi-stage
`voltage pulses generated by the pulsed power supply of FIG.
`1 that illustrate the effect of pulse duration in the transient
`stage of the pulse on the plasma discharge current.
`FIG. 7A and FIG. 7B are measured data of multi-stage
`voltage pulses generated by the pulsed power supply of FIG.
`1 that show the effect of the pulsed power supply operating
`mode on the plasma discharge current.
`FIG. 8 is measured data for an exemplary single-stage
`voltage pulse generated by the pulsed power supply of FIG. 1
`that produces a high-density plasma according to the inven-
`tion that is useful for high-deposition rate sputtering.
`FIG. 9 illustrates a cross-sectional view of a plasma sput-
`tering apparatus having a pulsed direct current (DC) power
`supply according to another embodiment of the invention.
`FIG. 10 illustrates a schematic diagram of a pulsed power
`supply that can generate multi-step voltage pulses according
`to the present invention.
`FIG. 11 illustrates a schematic diagram of a pulsed power
`supply having a magnetic compression network for supplying
`high-power pulses.
`FIG. 12 illustrates a schematic diagram of a pulsed power
`supply having a Blumlein generator for supplying high-
`power pulses.
`FIG. 13 illustrates a schematic diagram of a pulsed power
`supply having a pulse cascade generator for supplying high-
`power pulses.
`
`DETAILED DESCRIPTION
`
`FIG. 1 illustrates a cross-sectional view of a plasma sput-
`tering apparatus 100 having a pulsed direct current (DC)
`power supply 102 according to one embodiment ofthe inven-
`tion. The plasma sputtering apparatus 100 includes a vacuum
`chamber 104 for containing a plasma. The vacuum chamber
`104 can be coupled to ground 105. The vacuum chamber 104
`is positioned in fluid communication with a vacuum pump
`106 that is used to evacuate the vacuum chamber 104 to high
`vacuum. The pressure inside the vacuum chamber 104 is
`generally less than 10'1 Torr for most plasma operating con-
`ditions. A process or feed gas 108 is introduced into the
`vacuum chamber 104 through a gas inlet 112 from a feed gas
`source 110, such as an argon gas source. The flow of the feed
`gas is controlled by a valve 114. In some embodiments, the
`gas source is an excited atom or metastable atom source.
`The plasma sputtering apparatus 100 also includes a cath-
`ode assembly 116. The cathode assembly 116 shown in FIG.
`1 is formed in the shape of a circular disk, but can be formed
`in other shapes. In some embodiments, the cathode assembly
`116 includes a target 118 for sputtering. The cathode assem-
`bly 116 is electrically connected to a first terminal 120 of the
`pulsed power supply 102 with an electrical transmission line
`122.
`
`A ring-shaped anode 124 is positioned in the vacuum
`chamber 104 proximate to the cathode assembly 116. The
`anode 124 is electrically connected to ground 105. A second
`terminal 125 of the pulsed power supply 102 is also electri-
`cally connected to ground 105. In other embodiments, the
`
`

`

`US 8,125,155 B2
`
`3
`anode 124 is electrically connected to the second terminal
`125 of the pulsed power supply 102 which is not at ground
`potential.
`A housing 126 surrounds the cathode assembly 116. The
`anode 124 can be integrated with or electrically connected to
`the housing 126. The outer edge 127 of the cathode assembly
`116 is electrically isolated from the housing 126 with insula-
`tors 128. The gap 129 between the outer edge 127 of the
`cathode assembly 116 and the housing 126 can be an air gap
`or can include a dielectric material.
`
`In some embodiments, the plasma sputtering apparatus
`100 includes a magnet assembly 130 that generates a mag-
`netic field 132 proximate to the target 118. The magnetic field
`132 is less parallel to the surface ofthe cathode assembly 116
`at the poles of the magnets in the magnet assembly 130 and
`more parallel to the surface ofthe cathode assembly 116 in the
`region 134 between the poles of the magnets in the magnetic
`assembly 130. The magnetic field 132 is shaped to trap and
`concentrate secondary electrons emitted from the target 118
`that are proximate to the target surface 133. The magnet
`assembly can consist of rotating magnets.
`The magnetic field 132 increases the density of electrons
`and therefore, increases the plasma density in the region 134
`that is proximate to the target surface 133. The magnetic field
`132 can also induce an electron Hall current 135 that is
`
`formed by the crossed electric and magnetic fields. The
`strength of the electron Hall current 135 depends, at least in
`part, on the density of the plasma and the strength of the
`crossed electric and magnetic fields.
`The plasma sputtering apparatus 100 also includes a sub-
`strate support 136 that holds a substrate 138 or other work
`piece for plasma processing. In some embodiments, the sub-
`strate support 136 is biased with a RF field. In these embodi-
`ments, the substrate support 136 is electrically connected to
`an output 140 of a RF power supply 142 with an electrical
`transmission line 144. A matching network (not shown) may
`be used to coupled the RF power supply 142 to the substrate
`support 136. In some embodiments, a temperature controller
`148 is thermally coupled to the substrate support 136. The
`temperature controller 148 regulates the temperature of the
`substrate 138.
`
`In some embodiments, the plasma sputtering apparatus
`100 includes an energy storage device 147 that provides a
`source of energy that can be controllably released into the
`plasma. The energy storage device 147 is electrically coupled
`to the cathode assembly 116. In one embodiment, the energy
`storage device 147 includes a capacitor bank.
`In operation, the vacuum pump 106 evacuates the chamber
`104 to the desired operating pressure. The feed gas source 110
`injects feed gas 108 into the chamber 104 through the gas inlet
`112. The pulsed power supply 102 applies voltage pulses to
`the cathode assembly 116 that cause an electric field 149 to
`develop between the target 118 and the anode 124. The mag-
`nitude, duration and rise time of the initial voltage pulse are
`chosen such that the resulting electric field 149 ionizes the
`feed gas 108, thus igniting the plasma in the chamber 104.
`In one embodiment, ignition of the plasma is enhanced by
`one or more methods described in co-pending U.S. patent
`application Ser. No. 10/065,277, entitled High-Power Pulsed
`Magnetron Sputtering, and co-pending U.S. patent applica-
`tion Ser. No. 10/065,629, entitled Methods and Apparatus for
`Generating High-Density Plasma which are assigned to the
`present assignee. The entire disclosures of U.S. patent appli-
`cation Ser. No. 10/065,277 and U.S. patent application Ser.
`No. 10/065,629 are incorporated herein by reference. U.S.
`patent application Ser. No. 10/065,629 describes a method of
`accelerating the ignition of the plasma by increasing the feed
`
`10
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`65
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`gas pressure for a short period oftime and/or flowing feed gas
`directly through a gap between an anode and a cathode assem-
`bly. In addition, U.S. patent application Ser. No. 10/065,277
`describes a method of using pre-ionization electrodes to
`accelerate the ignition of the plasma.
`The characteristics of the voltage pulses generated by the
`pulsed power supply 102 and the resulting plasmas are dis-
`cussed in connection with the following figures. The pulsed
`power supply 102 can include circuitry that minimizes or
`eliminates the probability of arcing in the chamber 104. Arc-
`ing is generally undesirable because it can damage the anode
`124 and cathode assembly 1 16 and can contaminate the wafer
`or work piece being processed. In one embodiment, the cir-
`cuitry of the pulse supply 102 limits the plasma discharge
`current up to a certain level, and if this limit is exceeded, the
`voltage generated by the power supply 102 drops for a certain
`period of time.
`The plasma is maintained by electrons generated by the
`electric field 149 and also by secondary electron emission
`from the target 1