Magnetron sputtering: basic physics and application to
`cylindrical magnetrons
`
`John A. Thornton
`
`Telic Corporation, 1631 Colorado Avenue, Santa Monica, California 90404
`(Received 22 September 1977; accepted 7 December 1977)
`
`Magnetron sputtering sources can be defined as diode devices in which magnetic fields are
`used in concert with the cathode surface to form electron traps which are so configured that
`the EXB electron-drift currents close on themselves. Coaxial cylindrical magnetron sputtering
`sources in which post or hollow cathodes are operated in axial magnetic fields have been
`reported for a number of years. However,
`their performance is limited by end losses. A
`remarkable performance is achieved when the end losses are eliminated by proper shaping of
`the magnetic field or by using suitably placed electron-reflecting surfaces. High currents and
`sputtering rates can be obtained, nearly independent of voltage, even at low pressures. This
`characterizes what has been defined as the magnetron mode of operation. This paper reviews
`the basic principles that underly the operation of do sputtering sources in the magnetron mode
`with particular emphasis on cylindrical magnetrons. The impOrtant attributes of these devices
`as sputtering sources are also reviewed.
`
`PACS numbers: 81.15.—z, 52.75.—d
`
`I.
`
`INTRODUCTION
`
`Magnetron sputtering sources can be defined as diode devices
`in which magnetic fields are used in concert with the cathode
`surface to form electron traps which are so configured that
`the E X B electron-drift currents can close on themselves.1
`
`Cylindrical sputtering sources with post2‘10 or hollow11
`cathodes, coaxial anodes, and axial magnetic fields have been
`reported for a number of years. Their configurations, shown
`in Figs. 1(a) and (b), are of a general magnetron form, but
`their performance is limited by end losses. Remarkable per—
`formance is achieved when the end losses are eliminated by
`proper shaping of the magnetic field, as for example in the
`configurations shown in Figs. 1(c) and (d), or by using suitably
`placed electron reflecting surfaces as shown in Figs. 1(c) and
`(f). High currents and sputtering rates can be obtained, at
`moderate and near constant voltages, even at low pressures.
`This characterizes what has been defined as the magnetron
`mode of operation. 1 It can also be obtained for plasma rings
`magnetically confined over planar surfaces (planar magne-
`trons)12 or over short cylinders or within cylindrical surface
`cavities (sputter guns).13
`Cylindrical magnetron configurations have been used for
`microwave oscillators?“16 pressure gauges,”20 and sputter
`ion pumpsm‘23 The first operation in the magnetron mode
`for sputtering appears to have been achieved by Penning and
`Moubis using the configurations of Figs. 1(c)24 and (f).“’5 Very
`active development of cylindrical magnetron sputtering
`technology has occurred over the period from 1969 to the
`present.l This work has involved primarily the dc operation
`of apparatus of the basic types shown in Figs. l(c),26—28 ((1)36)28
`(e),27’28“36 and (f),28’29)31‘33 although rf operation has been
`explored with both the post— and hollow—cathode configura—
`tions.1’23
`
`171
`
`J. Vac. Sci. Technol. 15(2) March/April 1978
`
`0022-5355/78/1502-0171501.00
`
`This paper reviews the basic principles that underlie the
`operation of dc sputtering sources in the magnetron mode.
`The discussion concentrates on the cylindrical magnetrons
`with post cathodes and electrostatic reflecting surfaces [Fig
`1(c)] because these devices possess a relatively simple geom-
`etry. Following previous nomenclaturel these devices, and
`their inverted counterparts [Fig l(f)], will be referred to as
`cylindrical-post and cylindrical-hollow magnetrons, respec-
`tively. However, the discussion also applies to other cylindrical
`magnetrons and generally to planar magnetrons and sputter
`guns
`
`SI units are used except for electron energies and temper—
`atures which are expressed in eV. (1 eV = 11 600 K.)
`
`II. SOME BASIC CONCEPTS FROM PLASMA
`PHYSICS
`
`A glow discharge plasma can be defined as a region of
`relatively low—temperature gas in which a degree of ionization
`is sustained by the presence of energetic electrons. Such
`plasmas are neutral and their state is characterized by (l) the
`degree of ionization or = Ne/(Ni + NA), where Ne is the
`electron density, N,- = Ne is the ion density, and NA is the gas
`density; and (2) the electron energy distribution, which can
`often be approximated by an electron temperature Te. The
`long-range Coulomb forces between charged particles give
`plasmas a propensity for collective behavior.3738 A condition
`for dominance of Coulomb collisions is that a >> 1.7 X 10160”
`Te 2, where 08A is the electron—atom collision cross section.39
`Electrostatic waves can be expected to develop if the plasma
`angular frequency, defined as cop = 56.4 Nel/Z, is much larger
`than the electron—atom collision frequency. Another impor-
`tant parameter, the Debye length X0 = 7430 (Te/Ne)1/2,
`provides a measure of the distance over which si nificant
`ILLETTE 1116
`171
`
`© 1978 American Vacuum Society
`
`GILLETTE 1116
`
`

`

`172
`
`John A. Thornton: Magnetron sputtering
`
`CATHDDE
`
`CATHODE
`
`MAGNETIC
`FIELD
`
`
`
`
`
`NIACNFT I C
`FIELD
`
`Ex 5'
`ELECTRON
`MOTION
`
`E' x 3'
`ELECTRON
`MOTl 0N
`
`CATHODE
`
`.
`
`-
`
`E‘égRON
`MOTION
`
`ANODE
`
`'
`
`
`
`
`Fx 8'
`HECTRON \,_
`MOTION
`
`FLASTTA
`
`RINGS
`
`I‘MONETVC
`FlFLD
`
`/
`— PLASMA
`RINGS
`
`
`
`
`
`
`
`/
`
`PLASMA
`
`.
`.
`\EXEELECTRON
`MOTION
`
`ANODE
`
`CATHODE
`
`NIACNETI C
`HELD
`
`
`ELECTRON
`REFLECTING
`SURFACE
`
`\ ‘
`MAGNETIC
`FIELD
`
`
`
`
`
`ELECTRON
`REFLECTING
`
`.
`,«
`Ex B
`ELECTRON
`MOTION
`
`ANODE
`
`6
`
`SURFACE
`
`f
`
`FIG. 1. The configuration of various cylindrical magnetron sputtering
`sources. (a) and (b) have general magnetron configurations but do not operate
`in the magnetron mode because of end losses. (b), (d), and (f) are often called
`inverted magnetrons and sometimes hollow cathodes. e is referred to here
`as a cylindrical-post magnetron and f as a cylindrical-hollow magnetron.
`
`departures from charge neutrality can occur. Magnetron
`plasmas generally have higher degrees of ionization and are
`much richer in collective behavior than are the plasmas in
`conventional planar diode sputtering sources.
`Despite the importance of collective phenomena an ex-
`amination of single-particle motion37‘42 provides useful in—
`
`172
`
`sight into the behavior of magnetron plasmas. Only the
`electrons are significantly influenced by magnetic fields of
`the strengths used in magnetron mode sputtering sources. An
`electron in a uniform magnetic field B will drift along the
`field lines with a speed 1)“ that is unaffected by the field, and
`orbit them at a gyro or Larmor radius (1g = 3.37 X 10—6
`W L”2/B), and with a gyro or cyclotron angular frequency
`(me = 1.76 X 1011 B), where W J_ is the energy associated with
`the electron motion perpendicular to the field [see Fig. 2(a)].
`When there is a component of electric field E J_ perpendicular
`to B, a drift of speed 013 = E i/ B develops in a direction
`perpendicular to both E l and B and combines with the or-
`biting motion as shown in Fig. 2(b). This is the E X B drift.
`The motion of an electron created at rest in uniform and
`
`perpendicular E and B fields is the cycloid generated by a
`circle of radius rg(vE) moving with velocity 1);; as shown in
`Fig. 2(0), where rg(vE) is the value obtained by setting W J_
`= 1/2 121305 2. Electrons are emitted from a magnetron cathode
`with energies that are small compared to that acquired as they
`are accelerated through the cathode dark space (CDS).
`However, their motion is not exactly cycloidal because the
`CDS electric field is not uniform. In general, the distance to
`the turning point dt will exceed the CDS thickness ds, as
`shown in Fig. 2(d). For planar cathodes, (1.; equals the Tg value
`that is obtained with W i set equal to the CDS voltage drop.43
`For thin sheaths (d5 << 1g) over cylindrical cathodes of radius
`Bo, the turning distance d, varies from rg, when R0 >> (1,, to
`21g, when Ro << d, as shown in Fig. 2(e).43
`Curved magnetic fields of the type shown in Figs. 1(c) and
`(d) have a perpendicular gradient VB i and a parallel gradient
`VB”, as shown in Fig. 2(f). Electrons in such a field experience
`a VB i X B drift that is proportional to 0 i2. This combines
`with an identically directed drift, due to the centrifugal force
`associated with the speed 0 ll, and yields a drift identified as
`on in Fig. 2(f).44 Electrons tend to conserve their magnetic
`moment uM (MM = meo i 2/B). Thus conservation of energy
`may cause electrons passing in the direction of VB” to be re-
`flected by the magnetic field45 before they reach the cathode
`surface.
`
`Important collective behavior includes charged particle
`
`MAGNETIC FIELD
`@ UPWARDS
`
`
`
`ELECTRON
`STARTING
`AT REST
`
`C
`
`[I ECTRUH
`T“.\I\(ZNFT| (i
`li‘EFlf CTION
`
`Ii
`
`
`
`MAGNETIC
`FIELD
`
`
`CATHODF
`
`
`I
`"; ‘— '9
`
`V“
`
`V.
`
`a
`
` Ex 3'
`
`FIG. 2. Electron motion in static electric and
`magnetic fields.
`
`MAGNETIC FIEI D
`UW‘AR D S
`
`PIAQMA
`
`
`
`CATHODE
`
`J. Vac. Sci. Technol., Vol. 15, No. 2, March/April 1978
`
`

`

`173
`
`John A. Thornton: Magnetron sputtering
`
`VIRTUAL
`ANDDE
`
`CATHODE DARK SPACE
`
`PRIMARY ELECTRON
`LOST AT CATHODE
`
`PRIMARY ELECTRON
`FIRST ORBIT
`MDMENTLIM
`TRANSFER
`
`,
`
`‘
`
`ION NEUTRALIZATION
`AND REFLECTION
`WITH LARGE ANGLE
`SCATTERING
`
`S PUTTERED
`
`g¢§=
`é FLUX
`NEGATIVE GLOW
`TV PE REG ION
`
`
`
`PRIMARY ELECTRON
`TRAPPED IN CIRCULAR
`ORBIT
`
`\
`
`\
`
`‘
`
`
`
`MAGNETIC FIELD
`
`
`LOW ENERGY
`‘7L
`UPWARDS
`ELECTRONS
`ELECTRON
`CURRENT
`DENSITIES
`
`FIG. 3. Schematic representation of electron transport processes in a cold-
`cathode discharge between a cylindrical cathode and a coaxial anode in a
`uniform axial magnetic field [configuration shown in Fig. l(e)].
`
`current flow and diffusion, sheath formation, and plasma
`oscillations. When an electric field E J_ is applied perpen-
`dicular to a magnetic field of sufficient strength to affect the
`electrons but not the ions, a current density
`
`JJ. = eNel-‘eLEL + eNi/MLEJ.
`
`(1)
`
`will flow in the direction of E i and an electron Hall cur-
`rent46
`
`JH = (dc/”e JeJ. = “Jo/”e 3N2 HeJ. EJ.
`
`(2)
`
`will flow in the E X B direction. In Eq. (1) the mobility per-
`pendicular to the magnetic field is given by“7
`
`IN. = M/[1 + (we/”>21,
`
`(3)
`
`where u = e/ (mu) is the mobility in the absence of a magnetic
`field or along a field line (an) and u is the collision frequency
`for the species in question (m and e are the appropriate mass
`and charge). The diffusion coefficients can be written as D
`= ukT/e (k is Boltzman constant) and therefore obey rela-
`tionships similar to Eq. (3)4748
`Because of their difference in mass, electrons and ions tend
`to pass from a plasma to an adjacent surface at different rates.
`Thus a space charge region or sheath, in which one species is
`largely excluded, forms adjacent to such surfaces.49 The po—
`tential variation between the surface and the plasma is largely
`confined to this layer. The nature of the sheath depends on
`the current passing across it. Therefore the current density
`at the anode establishes the anode sheath voltage drop and
`thus the plasma potential. For floating surfaces the sheath
`potential drop is the well-known floating potential and is
`negative (relative to plasma potential) except for cases where
`the magnetic field causes the total electron flux to be less than
`the ion flux. The CD8 is a positive space charge sheath (neg-
`ative potential drop relative to plasma). The ion current
`density j,- is related to the sheath thickness d, and potential
`drop V, by the Child—Langmuir law37 [first term at right in
`Eq. (4)] and approximately to the plasma electron density and
`temperature by the Bohm relationship”,50 (second term at
`right):
`
`J. Vac. Sci. Technol., Vol. 15, No. 2, March/April 1978
`
` 40 1/2 V 3/2
`,- = 8.6 x10-9 (—)
`s
`M
`d,2
`J
`
`401;
`=lA8X1046Ne<
`)UzA/ma
`M
`
`173
`
`(n
`
`where M is the ion molecular weight.
`The plasma state is rich in wave phenomena, particularly
`when a magnetic field is present.37=38-51 Electron electrostatic
`waves can propagate along field lines with frequencies of the
`order of cop, or across field lines with a frequency of the order
`of (mp2 + wc2)1/2. Ion electrostatic waves can pass in all di-
`rections at a velocity of the order of (kTe/m¢)1/2. Electron
`drifts perpendicular to the magnetic field develop in the
`presence of a perpendicular electric field, as discussed above,
`or a density gradient.37 Such drifts are inherently unstable,
`since any departure from charge neutrality in the form of
`charge bunching and separation (over distances of the order
`of the Debye length) create electric fields which cause sec-
`ond-order E X B drifts that can exacerbate the perturbation.
`These instabilities are often referred to as gradient—drift and
`neutral-drag instabilities.52 Thus a charge perturbation as-
`sociated with the azimuthal 19 Hall current in a cylindrical
`magnetron can cause an E 9 perturbation field which interacts
`with the axial magnetic field Bz, to produce radial electron
`drift waves.53—55 Drifts driven by the two density gradients
`associated with a maximum in the radial electron density
`distribution can interact to cause the diocotron instability.“-58
`If feedback mechanisms are present, the oscillations associated
`with these instabilities can grow to produce macroscopic
`motions capable of enhancing electron transport across the
`magnetic field.
`Plasma instabilities can also be important in permitting a
`collisionless energy exchange within a plasma. Examples are
`the two-stream instability and Landau damping.”38 Evi-
`dence of such energy transfer has been seen between primary
`electrons and the negative glow over thermionic cath-
`odes.59’60
`
`Plasma oscillations and instabilities are believed to play an
`important role in the operation of magnetrons, as will be
`discussed in the next section.61'71
`
`III. THE PLASMA DISCHARGE
`
`Consider a cold-cathode magnetron mode discharge in a
`cylindrical-post magnetron of the type shown in Fig. l(e). The
`discharge cross section in the r—0 plane (cylindrical coordi-
`nates) is shown in Fig. 3. The uniform axial magnetic field is
`assumed to be of such strength that d, < d, < W and we >> 1!,
`where W is the width of the end reflector.
`
`A low-pressure cold-cathode discharge is maintained pri—
`marily by secondary electrons emitted from the cathode by
`ion bombardment. These electrons are accelerated in the CD8
`
`and enter the plasma where, known as primary electrons, they
`must produce sufficient ions to release one further electron
`from the cathode.72 This requirement can be expressed by the
`following relationship for the minimum potential to sustain
`such a discharge:73
`
`Vmin = 60/111952
`
`(5)
`
`

`

`174
`
`John A. Thornton: Magnetron sputtering
`
`where F,- is the number of secondary electrons per incident
`ion which leave the cathode, (3’0 is the average energy required
`for producing ions (about 30 eV for Ar+ 74), e, is the ion col—
`lection efficiency, and 52 is the fraction of the full complement
`of ions V/ 6 0 that is made by the average primary electron
`before it is lost from the system. The primary electrons are
`emitted from the cathode with energies of a few eV75 and
`move in cycloidal-like trajectories, as shown at A in Fig. 3.
`There is a strong probability of recapture at the cathode, since
`the path length is much smaller than the mean free path.
`Furthermore the retarding field reduces the returning elec—
`tron energy to a value which is generally too low to produce
`significant secondary electron emission. It is believed that the
`recapture probability is reduced by interactions with plasma
`oscillations. On a random phase basis about one-half will
`undergo an interaction which promotes their return to the
`cathode, while about half will exchange sufficient momentum
`so that they cannot return. F,- is called the effective secondary
`emission coefficient.7€*78 It is equal to the secondary emission
`coefficient due to ion bombardment 7,- times a factor which
`accounts for the probability of recapture. 7; is typically 1/10
`for low—energy argon ions incident on metal surfaces. I} is
`perhaps close to 1/20 for the cylindrical cathode surface. Sec-
`ondary emission from the end reflectors (unaffected by the
`magnetic field) is of little significance in most geometries
`because of the relatively small end-reflector area in contact
`with the intense plasma.
`Primary electrons which enter the plasma are believed to
`become trapped on cycloidal-like paths which orbit but do
`not reach the cathode, as shown at B in Fig. 3. Collisions in the
`radial electric field beyond the CD5 and plasma oscillations
`cause the electrons to migrate to the anode. Such discharges
`exhibit a bright glow which extends a distance of about 2d,
`from the cathode, and a dimmer glow from there to the
`anode. The bright glow is believed to be the main region of
`energy exchange for the primary electrons. Thus, following
`conventional terminology,”79 the regions can be identified
`as having negative glow and positive column—type charac~
`teristics, respectively. Since the primary electrons can escape
`the magnetic trap only by exchanging energy, Q ~ 1; and
`since the primary energy exchange is close to the cathode, 61
`~ 1. Thus magnetron discharges are very efficient and operate
`in Ar at voltages near the Vmin of 600 V predicted by Eq. (5)
`for 60 = 30 eV /ion and 1‘,- = 1/20 (see Sec. IV). The high oper—
`ating voltages of conventional planar diode sources result from
`low values of ee and 6,.1
`.
`A typical cylindrical magnetron discharge operates at a
`pressure of 013 Pa (1 mTorr) with a magnetic field strength
`of 0.02 T (200 C). Thus we = 8.5 X 109 rad/s. The total elec—
`tron—atom cross section of electrons of a typical energy of 10
`eV in argon is about 2 X 1019 m2,”4 and the collision fre-
`quency V is about 107 5’1. Thus «20/ V ~ 300. Accordingly, the
`Hall current density, Eq. (2), is significant and electrons circle
`the cathode many times in passing to the anode. The total Hall
`current is typically an order of magnitude larger than the
`discharge current and constitutes a solenoidal current that can
`cause a reduction of the magnetic field strength in the vicinity
`of the cathode by ~10%.
`Equation (3) predicts He J_ ~ 10—5ue at wc/ 11 ~ 300. If the
`
`J. Vac. Sci. Technol., Vol. 15, No. 2, March/April 1978
`
`174
`
`electron mobility were this low, it would be less than the Ar
`ion mobility, and operation of the discharge in the positive
`space charge mode would be endangered.1 Similarly, the
`radial current predicted by Eq. (1) (using measured values
`for E i and Ne in the vicinity of the anode) or by electron
`diffusion (using electron density gradients estimated from
`probe measurements) fails to account for the discharge current
`by a factor of more than two orders of magnitude.80 The an-
`swer to these dilemmas is believed to be plasma oscillations
`which promote electron migration across magnetic fields
`strong enough to restrain the motion of the primary electrons.l
`Oscillations at frequencies in the 50—500 kHz range, with a
`maximum frequency that varies inversely with the square root
`of the working gas atomic mass, have been detected using
`electrostatic probes in cylindrical—post magnetrons operating
`with Ar, 02, and He.80 This frequency range is consistent with
`electrostatic ion waves oscillating in the discharge cavity and
`about equal to the ion—ion collision frequency.
`Electrostatic probe measurements of the electron density
`in the discharge region are consistent with Eq. (4) and indicate
`that the degree of ionization is a few percent or less. At low
`pressures the primary electrons in a well—designed trap are
`estimated to travel as much as 100 m before their energy is
`totally exchanged although, as with conventional negative
`glows, the primary electron density is typically only a few
`percent of the density of ions and low—energy electrons. Atom
`flux calculations indicate that an average atom passing
`through the discharge region of a cylindrical-post magnetron
`must have a 1% to 80% chance of being ionized, depending
`on the discharge current.1 Probabilities of being either excited
`or ionized are about twice as large. This is particularly relevant
`to reactive sputtering.
`The anode in a magnetron should be placed within the
`magnetic field so that it terminates the trap at a point where
`the primary electrons have dissipated their energy.1 Anode
`placement is aided by the fact that Mell >> ”vei- Thus all
`electrons reaching the radius R A in Fig. 3 are swept into the
`anode; i.e., a virtual anode, that does not disrupt the sputtered
`flux, separates the substrates from the plasma and reduces
`plasma substrate heating (ions are electrostatically forced to
`stay in the vicinity of the electrons). An anode of insufficient
`size or with poor placement can cause a significant anode
`voltage drop.
`
`IV. CURRENT—VOLTAGE CHARACTERISTICS
`
`The current—voltage characteristic reveals a great deal
`about the ionization processes in a plasma discharge. Dis-
`charges operating in the magnetron mode obey an I—V rela-
`tionship of the form I 0: V", where n is an index to the per-
`formance of the electron trap and is typically in the range 5
`to 9. The voltage is related to the ionization mechanisms by
`Eq. (5).
`Typical I—V curves for cylindrical—post magnetrons oper—
`ating with various magnetic field strengths and pressures are
`shown in Fig. 4 and compared with a I—V curve for a planar
`diode. If the magnetic field is too weak, the discharge will
`leave the magnetron mode and the voltage will increase
`abruptly, as shown at A.1~24~35 Figure 5 shows the variation in
`
`

`

`175
`
`8 1°
`
`PRESSURE ImTorrI
`2
`2
`A
`6 81°
`CYLINDRICALvPOSTMAGNETRON
`COPPER CATHODE (32 mm diam. X 300 mm |g.|
`CURRENT DENSITY ’ 100 A/rn2 I10 mA/cmzl
`ARGON
`
`4
`
`6
`
`STABLE DISCHARGE
`
`10
`
`2
`
`4
`
`e 81
`
`2
`8
`
`6 4
`
`I
`K;
`rg/w ~{7 \;
`DISCHARGE
`EXTINGUISHES
`
`ImTI
`
`
`MAGNETICFIELDSTRENGTH
`
`175
`
`John A. Thornton: Magnetron sputtering
`
`1
`
`10
`
`2
`
`4681
`
`
`
`DISCHARGEVOLTAGE
`
`PLANAR DIODE
`ALUMINUM CATHODE
`ARGON - 6. 67 Pa (50 mTorrI
`
`CURRENT DENSITV (mA/cmZI
`46810
`2
`2
`46810
`CYLINDRICAL-POST MAGNETRONS
`COPPER CATHODES
`ARGON
`110 mTorrI
`I 1.33 Pa
`o 0.067 Pa (0.5 mTorrJ
`
`2
`
`rq/w ~ 0.9
`SmT (som/
`
`rg/w~o.7
`[/6 mT (606)
`
`rglw~0.2
`”Of/20 mT IZOOGI
`., 10 mT 11000)
`~20 mT (2006!
`I. “all
`" 30 mT 430001
`
`——°‘°
`
`IVOIIS‘
`
`2
`
`4
`
`6
`
`a
`
`2
`
`z
`
`A
`
`a
`
`a
`
`3
`
`2
`
`'1
`
`2
`
`8
`(7
`4
`PRESSURE lPaI
`
`2
`
`4
`
`6
`
`8
`
`2
`
`CURRENT DENSITY Wm?)
`
`FIG. 4. Typical current voltage characteristics for a planar diode sputtering
`source and for cylindrical-post magnetron sputtering sources operating under
`various conditions of pressure and magnetic field strength.
`
`FIG. 6. Conditions of pressure and magnetic field strength for stable oper-
`ation of a representative cylindrical-post magnetron at fixed current (from
`Ref. 1).
`
`voltage with pressure at constant magnetic field strength. A
`similar abrupt voltage increase is seen at B. These voltage
`behaviors are a result of electrons escaping from the trap
`without making sufficient ionization (low 63), because their
`gyro radii are too large compared to the end reflector size W.
`At high magnetic field strengths the discharge will extinguish
`if the pressure is too low, as seen at C in Fig. 5. This is believed
`to be the result of electrons not exchanging sufficient m0-
`mentum with the plasma during their first cycloidal orbit (low
`Pi). Note that the low-pressure extinction is surprisingly in-
`dependent of gas species (i.e., collision cross section). This is
`attributed to the role of plasma oscillations in establishing I‘i.
`Figure 6 summarizes the pressure-magnetic field require-
`ments of a small cylindrical-post magnetron operating at a
`current density of 100 A /m2. Increasing the magnetic field
`strength lowers the allowable operating pressure until the
`length of the first cyloidal orbit becomes too small to permit
`adequate energy exchange. At normal operating pressures
`(>0.07 Pa) low field strengths can be used for larger devices
`(design requirement is that W be a mutliple of rg1’29).
`At a fixed voltage and magnetic field strength the current
`
`-
`
`10
`
`PRESSURE (mTorr)
`1
`1
`8
`o
`4
`2
`2
`4
`6 810
`CYLINDRICAL-POST MAGNETRON
`COPPER CATHODE (32 mm diam. X 300 mm lg.|
`I CURRENT - 3A, ARGON
`A CURRENT - 1A. ARGON
`0 CURRENT -1A, OXYGEN
`
`2
`
`4
`
`a 31°
`
`2
`
`
`
`DISCHARGEVOLTAGE
`
`10 mT (100(3)
`20 mT (2006) K,
`5,.
`20 mT IZOOGI
`A
`M‘
`O\o‘~
`DISCHARGE
`EXTINGUISHES
`
`WET (SCSI
`
`' ‘°"°‘°'°~°-—2o m (2006)
`
`(Volts)
`
`-1
`
`2
`
`8
`6
`4
`PRESSURE (Pal
`
`FIG. 5. Operating characteristic of a cylindrical-post magnetron sputtering
`source, showing the variation of discharge voltage with pressure at constant
`discharge current and magnetic field strength (from Ref. 1).
`
`J. Vac. Sci. Technol., Vol. 15, No. 2, March/April 1978
`
`increases with pressure (more collision targets). At a fixed
`voltage and pressure the current increases with magnetic field
`strength (higher 6;), at least for moderate fields (<20 mT). An
`electrostatic probe placed over the anode of a cylindrical-post
`magnetron operating at 700 V indicated that the primary
`electrons had exchanged essentially all of their energy in the
`discharge, therefore implying that ee ~ 1.81 Taking 1‘, = 1/20
`and 60 = 30 eV/ ion, in Eq. (5) yields q ~ 0.9. This ion-col—
`lection efficiency is generally consistent with electrostatic
`probe measurements at positions beyond the virtual anode.
`With proper Choice of magnetic field, magnetron mode
`operation with a near common I—V characteristic has been
`achieved for a wide range of apparatus sizes in both cylin-
`drical-post and cylindrical-hollow configurations.l
`
`V. PERFORMANCE AS SPUTTERING SOURCE
`
`With sufficiently uniform magnetic fields, the cylindrical
`magnetron configurations shown in Figs. 1(e) and 1(f) provide
`uniform sputtering over the cathode surface. This has been
`verified by measuring the cathode erosion profiles after ex—
`tended sputtering]!82 (Reports of nonuniform erosion are
`believed to be due to nonuniform magnetic fields”) A large
`inventory of coating material can be stored in a cylindrical
`target and used efficiently. Current densities are typically 50
`to 500 A/In2 with erosion rates of 33 nm/s and deposition rates
`of 3.3 nm/s (2000 A /min). However, a copper cylindrical-post
`cathode has been operated with a uniform current density of
`over 2000 A/m2 and an erosion rate of over 333 nm/s (200 000
`A /min).
`At typical operating pressures the sputtered flux passes to
`substrates with little gas scattering. This has been verified by
`observing the post—cathode images that are formed by the
`coatings on substrates placed behind shields with small ap-
`ertures in pin-hole camera configurations.1 Thus mechanical
`masks can be effectively used to accurately define deposition
`areas. The deposition flux at various points surrounding a post
`cathode can be predicted with considerable accuracy by as-
`suming a uniform cathode current density and a cosine
`emission of sputtered flux with collisionless transport.83
`
`

`

`176
`
`John A. Thornton: Magnetron sputtering
`RADIAL
`POSITION
`X=U2
`XIR=6
`
`X‘L
`XIR=12
`
`CATHODE
`
`
`
`
`
`
`
`RELATIVE /
`umromm
`
`
`
`FIG. 7. Theoretical coating uniformity profiles at various radii surrounding
`a cylindrical—post magnetron sputtering source with a uniform cathode
`current density. Cosine emission and collisionless transport are assumed.
`Profiles show the influence of end effects. Absolute deposition rate incor-
`porates an X/R dependence not shown. Such profiles have been verified
`experirnentallyfi3
`
`Typical calculated profiles are shown in Fig. 7. A hollow
`cathode with uniform cosine emission has the interesting
`characteristic that the coating flux at all points within the
`cathode that are unaffected by end losses is equal to the
`cathode erosion flux, independent of the working gas pres-
`sure.84 This behavior has been experimentally verifieds4 It
`makes hollow cathodes particularly effective for coating
`objects of complex shape. All surfaces having an unobstructed
`view of the cathode receive a uniform coating.
`An important attribute of magnetrons is the reduced sub-
`strate plasma bombardment. No primary electrons reach
`substrates located at radii beyond the virtual anode of 9. cy-
`lindrical post magnetron. Floating potentials are a few volts
`positive with respect to the anode. Ion densities are typically
`50—100 times less than in the discharge. When sputtering
`metals, ion bombardment fluxes are typically only about 1/10
`to 1/5 of the deposition flux, even with a negative substrate bias.
`Therefore, auxiliary discharges are generally required for
`sputter cleaning and bias sputtering.34 In the case of cylin-
`drical-hollow magnetrons the charged particle density on the
`axis is typically 1/3 that adjacent to the target surface. Floating
`potentials are —20 to —50 V relative to the anode. For metals
`the ion diffusion flux to the substrate is typically l/5—1/3 the
`sputtered atom flux.
`The substrate heating is much reduced in cylindrical-post
`magnetrons by the absence of plasma bombardment. Plastics
`and other heat-sensitive substrates can be effectively coatedf“5
`The heating flux has been found to be proportional to the
`sputtered flux and to vary from 15 to 25 eV/atom for metals
`such as Ti, Cr, and Cu, to about 50 eV/atom for heavy metals
`such as Mo, Ta, and W, where ion neutralization and reflec-
`tion86‘89 become more significant and where the sputtered
`atom KE is larger.90 Atom reflection is believed to be rela-
`tively important for cylindrical-post magnetrons because of
`the low operating pressure and the fact that ions undergoing
`
`J. Vac. Sol. Technol., Vol. 15, No. 2, March/April 1978
`
`176
`
`reflections of considerably less than 180° pass in the direction
`of the substrates1 as indicated in Fig. 3.
`
`VI. SUMMARY
`
`An operation defined as the magnetron mode can be
`achieved in diode sputtering sources that are configured so
`that magnetic fields in concert with the cathode surface form
`electron traps of such geometry that the E X B electron—drift
`currents close on themselves. The ionization process is very
`efficient, and high currents and sputtering rates can be
`achieved at relatively low voltages, even at low pressures.
`Plasma collective behavior is believed to play an important
`role in the discharge operation.
`Uniform current densities can be achieved over post or
`hollow cathodes, thereby permitting a large inventory of
`coating material to be stored and efficiently used. Post cath-
`odes can be scaled to large sizes, permitting large areas to be
`uniformly coated. Hollow cathodes are effective for coating
`substrates of complex shape. Substrate plasma bombardment
`is much reduced, particularly with cylindrical-post magne-
`trons.
`
`ACKNOWLEDGMENT
`
`The author gratefully acknowledges many stimulating and
`fruitful discussions with Dr. Alan S. Penfold of Telic Corpo—
`ration concerning plasma physics in general and magnetrons
`in particular.
`
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`
`

`

`177
`
`John A. Thornton: Magnetron sputtering
`
`26U. Heisig, K. Goedicke, and S. Schiller, Proc. 7th Intl. Symp. Electron and
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`27N. Hosok