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`P‘IRApplied Phys“ 5 .
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`_
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`\ Letters
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`Evolution of the electron energy distribution and plasma parameters in a
`pulsed magnetron discharge
`J. T. Gudmundsson, J. Alami, and U. Helmersson
`
`Citation: Appl. Phys. Lett. 78, 3427 (2001); doi: 10.1063/1.1376150
`
`View online: http://dx.doi.org/10.1063/1.1376150
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`

`

`APPLIED PHYSICS LETTERS
`
`VOLUME 78, NUIVLBER 22
`
`28 MAY 2001
`
`Evolution of the electron energy distribution and plasma parameters
`in a pulsed magnetron discharge
`
`J. T. Gudmundssona)
`Science Institute, University of Iceland, Dunhaga 3, IS—107 Reykjavik, Iceland
`
`J. Alami and U. Helmersson
`Department of Physics, Link5ping University, SE—581 83 Link5ping Sweden
`
`(Received 27 February 2001; accepted for publication 2 April 2001)
`
`We demonstrate the creation of high—density plasma in a pulsed magnetron discharge. A 2.4 MW
`pulse, 100 ,us wide, with a repetition frequency of 50 Hz is applied to a planar magnetron discharge
`to study the temporal behavior of the plasma parameters: the electron energy distribution function,
`the electron density, and the average electron energy. The electron density in the vicinity of the
`substrate, 20 cm below the cathode target, peaks at 8 X 1017 m’3, 127 ,us after initiating the pulse.
`Towards the end of the pulse two energy groups of electrons are present with a corresponding peak
`in average electron energy. With the disapperance of the high—energy electron group, the electron
`density peaks, and the electron energy distribution appears to be Maxwellian like. Following the
`electron density peak, the plasma becomes more Druyvesteyn like with a higher average electron
`energy. © 2001 American Institute of Physics.
`[D012 10.1063/1.1376150]
`
`The dc magnetron sputtering discharge has found wide—
`spread use in coating processes, particularly in the deposition
`of thin metallic films. In magnetron sputter deposition, atoms
`are sputtered from the cathode target by ions drawn from
`a magnetically
`confined
`plasma. A dense
`plasma
`(~ 1018 m’3) is generally trapped close to the cathode—target
`surface. However, the plasma densities close to the sample to
`be deposited (~ 5— 10 cm below the target) are several orders
`of magnitude lower (1015—1016m’3). Furthermore, the ion—
`ized fraction of the sputtered species is small (~1%—10%)
`and the majority of the species extracted on the negatively
`biased substrates are ions of the discharge gas.
`Recently, pulsing the magnetron has been shown to in—
`crease the ion density significantly.l’2 By pulsing the magne—
`tron, very high plasma densities (~ 1018m’3) have been ob—
`tained 6—10 cm away from the target with a degree of
`ionization of 30%—70%.2’3 Furthermore, the target utilization
`is improved.1 The pulsed magnetron has been demostrated
`for use in high—aspect—ratio filling applications and improved
`thickness homogenity of deposited films compared to con—
`ventional dc magnetrons.1 However, the energetics of the
`discharge, the composition of the plasma, and the reactions
`among the species remain to be investigated. The fundamen—
`tal plasma characteristic for better understanding of the
`plasma chemistry is the electron energy distribution function.
`Measurements in a conventional dc magnetron indicate that
`the electron energy distribution on axis is strongly assym—
`metric, representing a net electron drift from the cathode to
`the anode.4 A non—Maxwellian electron energy distribution is
`to be expected since the source is localized to the magnetic
`trap region, and at this low neutral pressure (1—5 mTorr) the
`electron mean—free path is relatively long. The electron en—
`ergy distribution in a dc argon discharge in the vicinity of the
`substrate has been measured by Ivanov et a].5 They report
`
`a)Electzronic mail: tumi@hi.is
`
`the presence of two energy groups of electrons in the plasma.
`For sputter deposition of thin films, knowledge of the elec—
`tron energy distribution and plasma parameters in the near—
`substrate vicinity are of great importance for determining the
`process parameters. The aim of this work is to investigate the
`temporal evolution of the electron energy distribution func—
`tion (EEDF) and the plasma parameter electron density ne ,
`average electron energy (8), and plasma potential Vpl for a
`pulsed high—density plasma in a magnetron sputtering dis—
`charge in the substrate vicinity.
`The standard balanced planar magnetron source is oper—
`ated with a tantalum target of 150 mm diam. The cathode is
`located inside a stainless—steel sputtering chamber of radius
`R =60 cm and height L=75 cm. Argon of 99.9997% purity
`is used as the discharge gas. The magnetron cathode was
`driven by a pulsed power supply that can deliver peak power
`pulses of up to 2.4 MW (2000 V and 1200 A) at a repetition
`frequency of 50 Hz and a pulse width in the range of 50— 100
`,us. For the measurents presented here, the average power
`was 300 W, pulse width 100 ,us, and repitition frequency 50
`Hz. The peak voltage was roughly 800 V, and the peak cur—
`rent about 100 A. The argon pressure was 2 mTorr. A cylin—
`drical Langmuir probe, which is a cylindrical tungsten rod of
`
`length lpr: 5.5 mm and radius rpr= 50 ,um, was applied for
`the measurements. The probe holder is an alumina tube with
`
`outer radius rpm: 0.5 mm and 1.9 cm long. The probe is
`designed to fulfill the basic requirements for Langmuir—probe
`diagnostics
`as
`discussed
`by Godyak,6
`rpm<1pr
`and
`rpr,rprh,)\De<)\e~l cm. Here, AD6~ 14— 100 ,um is the De—
`bye length and )xe~ 1 cm is the electron mean—free path. The
`probe is positioned perpendicluar to the discharge axis, and
`thus to the electric— and magnetic—field lines 20 cm below the
`target. The magnetic field at this position is <0.2 mT, which
`leads to a gyroradius of ag= (irmekTe/2)1/2/eB~ 140 ,um,
`and thus rpI/ag~ 0.4. Therefore, we can neglect the error in
`the measured electron density caused by the magnetic field.7
`The time—resolved probe current was recorded for 500 ,us
`
`© 2001 American Institute of Physics
`0003-6951/2001/78(22)/3427/3/$18.00
`Downloaded 02 Oct 2013 to 216.185.156.28. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.0rg/about/rightsiandipermissions
`
`TSMC-1015 / Page 2 of 4
`3427
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`TSMC-1015 / Page 2 of 4
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`

`

`3428
`
`Appl. Phys. Lett., Vol. 78, No. 22, 28 May 2001
`
`Gudmundsson, Alami, and Helmersson
`
`0.06
`
`0.05
`
`0.04
`
`0.03
`
`0.02
`
`0.01
`
`0.06
`
`0.05
`
`0.04
`
`0.03
`
`0.02
`
`
`
`NormalizedEEDF
`
`Normalized
`
`Normalized
`
`EEDF
`EEDF
`
`after initiating the pulse at 1 ,us intervals for a fixed voltage.
`This was repeated in the voltage range from — 30 to 20 V at
`0.1 V intervals. For each time value, the I—V curve was
`reconstructed. The measured 1— V curve was smoothed by
`convoluting a Blackman window to the measured data.8 The
`second derivative of the I— V curve was calculated and the
`
`electron energy distribution function 82(5) found. The EEDF
`is given by the Druyvesteyn formula as9’10
`
`2m 2eV l’ldzle
`m
`eZAp,
`dVZ’
`
`ge(V)=
`
`(1)
`
`where 5 is the electron energy in equivalent voltage units.
`
`is the voltage where the second
`The plasma potential Vpl
`derivative of the electron current Ie is zero, and the floating
`potential Vfl is where the probe draws equal ion and electron
`currents. The electron density 118 is determined as
`
`”2: J0 ge(€)d€,
`
`and the average electron energy (8) is determined as
`
`1
`
`m
`
`<s>= n— ] sgmds.
`
`(2)
`
`(3)
`
`Figure 1 shows the evolution of the electron energy dis—
`tribution function with time from initiating the pulse.
`Ini—
`tially, the distribution can be described by a single peaked
`distribution [Fig. 1(a)]. At around 95 ,us, a second group of
`high—energy electrons appears. This high—energy group re—
`mains until roughly 115 ,us after initiating the pulse. This
`presence of two energy groups can be seen at 96 ,us in Fig.
`1(a) and at 105 and 110 ,us in Fig. 1(b). At roughly 120 ,us
`after initiating the pulse the electron energy distribution
`shows a single group of electrons. At roughly 250 ,us after
`initiating the pulse the electron energy distribution reaches
`the shape that remains for the following 250 ,us, as seen in
`Fig. 1(c). Higher—energy peaks are seen at 350 and 450 ,us
`[Fig. 1(c)]. The evolution of the electron density with time
`from the initiation of the pulse is shown in Fig. 2(a). The
`electron density peaks at 8 X 1017 m73 127 ,us after initiating
`the pulse. The electron density decreases again and falls to
`8><1016m’3 at 500 ,us after initiating the pulse. The mea—
`sured electron energy distribution function can be fitted to
`the function
`
`g,<8>=af£exp(—bsa,
`
`(4)
`
`FIG. 1. Normalized EEDF measured (a) during pulses 60, 80, and 100 us
`after initiating the pulse; (b) around the electron density maximum 105, 110,
`and 130 as afler initiating the pulse; and (c) 250, 350, and 450 us after
`initiating the pulse. Pulse length, 100 as; average power, 300 W; and pres-
`sure 2 mTorr.
`
`where a, b, and x are constants. For x= 1, we have a Max-
`wellian electron energy distribution,
`and for x= 2
`a
`Druyvesteyn distribution. The value of x was determined by
`performing a least—squares analysis of ln[gf(c‘,')/ J3] vs Ex for
`various x to find the best fit. During the pulse, 50—90 ,us
`after initiating the pulse, the parameter x is ~2, indicating a
`Druyvesteyn—like energy distribution. The fitting parameter
`is x~1 in the range of 115 ,us, until 150 ,us after initiating
`roughly one order of magnitude.8 The best fit to Eq. (4) is
`the pulse, indicating a Maxwellian—like electron energy dis—
`then interpolated to zero electron energy. The interpolated
`tribution. Thus, when the electron density is the most dense,
`electron energy distribution function is used to extend the
`3 —8 X 1017 m’3, the electron energy distribution is Maxwell—
`measured electron energy distribution function to low en—
`ian like. From roughly 200 ,us until 500 ,us we find x
`ergy. The electron density and the averaged electron energy
`~2.5—3. The smoothing method introduces distortion to the
`are then calculated using the extended electron energy distri—
`electron energy distribution function around the plasma po—
`tential. Due to this distortion, the average electron energy is
`bution function in Eqs. (2) and (3), respectively. The average
`Downloaded 02 Oct 2013 to 216.185.156.28. This article is copyrighted ESMQ 171104541. Reagfllarngné subject to the terms at: http://ap|.aip.0rg/about/rightsiandipermissions
`
`somewhat overestimated. To correct for this smoothing error,
`Eq. (4) is fitted to the measured electron energy distribution
`function from the electron energy where the electron energy
`distribution function has a maximum value until it has fallen
`
`TSMC-1015 / Page 3 of 4
`
`

`

`Appl. Phys. Lett., Vol. 78, No.22, 28 May 2001
`
`Gudmundsson, Alami, and Helmersson
`
`3429
`
`again with time. The average electron energy peaks at 3.5 eV
`roughly 100 ,us after initiating the pulse. This peak in the
`average energy coincides with the presence of the high-
`energy group of electrons appearent in the electron energy
`distribution. At 127 ,us, when the electron density peaks, the
`average electron energy has decreased to ~2 eV. The aver—
`age electron energy reaches a minimum of about 1.5 eV at
`240 ,us. It increases again until it reaches a plateau of 2.4 eV
`at roughly 290 ,us, which remains for the following 210 ,us.
`The average electron energy we report
`in the pulsed
`magntron is comparable to what is observed by Sheridan,
`Goeckner, and Goree.11 The time evolution of the plasma
`potential and the floating potential from initiating the pulse is
`shown in Fig. 2(c). As the energy of ions bombarding a
`substrate at the floating potential is determined by the differ—
`ence between the floating potential and the plasma potential
`(Vpl— Vfl), this value is plotted in Fig. 2(c) as well.
`In conclusion, we have measured the temporal behavior
`of the electron energy distribution function in a pulsed mag—
`netron. Towards the end of the pulse, two energy groups of
`electrons are present with a corresponding peak in average
`electron energy. With the disapperance of the high—energy
`electron group, the electron density peaks, and the electron
`energy distribution appears to be Maxwellian like. Eventu—
`ally, the plasma becomes more Druyvesteyn like with lower
`electron density and higher average electron energy.
`
`This work was partially supported by the Swedish Foun—
`dation for Strategic Research and the University of Iceland
`Research Fund. The company Chemfilt R & D is acknowl—
`edged for the use of the power supply.
`
`lV. Kouznetzov, K. Macak, J. M. Schnider, U. Helmersson, and I. Petrov,
`Surf. Coat. Technol. 122, 290 (1999).
`2K. Macak, V. Kouznetzov, J. M. Schnider, U. Helmersson, and I. Petrov,
`J. Vac. Sci. Technol. A 18, 1533 (2000).
`in The Third International
`3U. Helmersson, Z. S. Kahn, and J. Alami,
`Euroconference on Advanced Semiconductor Devices and Microsystems,
`Somolenice Castle Slovakia (2000), p. 191, IEEE Catalog No. 00EX386.
`4T. E. Sheridan, M. J. Goeckner, and J. Goree, Jpn. J. Appl. Phys., Part 1
`34, 4977 (1995).
`5 I. Ivanov, S. Statev, V. Orlinov, and R. Shkevov, Vacuum 43, 837 (1992).
`(’V. A. Godyak, in Plasma—Surface Interactions and Processing of Mate—
`rials, edited by O. Auciello (Kluwer Academic, Dordrecht, 1990), pp.
`95—134.
`7E. Passoth, P. Kudma, C. Csambal, J. F. Behnke, M. Tichu, and V.
`Helbig, J. Phys. D 30, 1763 (1997).
`SJ. T. Gudmundsson, Memorandum No. UCB/ERL M97/38, Electron
`Research Laboratory, University of California, Berkeley (1997).
`9M. J. Druyvesteyn, Z. Phys. 64, 781 (1930).
`10M. A. Lieberman and A. J. Lichtenberg, Principles ofPlasma Discharges
`and Materials Processing (Wiley, New York, 1994).
`H T. E. Sheridan, M. J. Goeckner, and J. Goree, J. Vac. Sci. Technol. A 34,
`2173 (1998).
`
`0
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`100
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`200
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`300
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`400
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`500
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`t [#51
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`(b) average electron energy, and (c)
`(a) Electron density,
`FIG. 2.
`+ floating potential V“ , X plasma potential Vpl , and * potential difference
`(Vpl— V“) as a fimction of time from initiation of the pulse. Target current
`pulse length, 100 as; average power, 300 W; and pressure, 2 mTorr.
`
`electron energy (8) is shown versus time from initiating the
`pulse in Fig. 2(b). The average electron energy decreases
`during the pulse, down to 2.5 eV at 92 ,us, where it increases
`
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