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`ELSEVIER
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`Surface and Coatings Technology 93 ( i998 ) 12454250
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`AYIIIHIIE
`a£'J477NE3
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`Asymmetric bipolar pulsed DC: the enabling technology for reactive PVD
`
`J. Sellers *
`
`EN], 4150 Freidrich Ln., Suite J, Austin. TX 78744, USA
`
`
`
`Abstract
`
`The use of reactive DC sputtering for the deposition of insulators from conductive targets has been limited by the intrinsic
`problem of target poisoning and the consequent arcing and process instabilities. The need to deposit high quality dielectric films
`rapidly is becoming more important as technology pushes forward. Asymmetric bipolar pulsed DC eliminates target poisoning
`through preferential sputtering, enabling existing PVD tools to produce the high—quality, low-defect dielectric films needed for
`next generation processes. Typical films being produced with asymmetric bipolar pulsed,DC from metallic targets include A1203,
`AIN, Si02, SiN, Ta2O,, DLC. TaN, TiN and ITO. The mechanisms of target poisoning and dielectric arcing are explained in this
`paper, and solutions are given. © 1998 Published by Elsevier Science S.A.
`
`Keywords: Sputter; Reactive; Magnetron; Thin film; PVD
`
`1. Introduction
`
`Asymmetric bipolar pulsed DC technology has been
`quickly proving its effectiveness
`in a wide variety
`of standard and reactive
`sputtering applications.
`Acceptance of this technology has been extremely rapid
`due to its elimination of many long-standing process
`limitations. Asymmetric bipolar pulsed DC was specifi-
`cally developed to optimize the deposition of insulating
`films from conductive targets with reactive sputtering.
`Further, it is important to understand that this technol-
`ogy did not evolve as an afterthought to control arcing:
`rather, it was conceived with a definite understanding
`of the electrical and physical process requirements. This
`understanding led to a unique solution in which the
`power source is given the dual properties of a current
`source and a voltage source, depending on which plasma
`constituent is being driven. Thus, both ions and electrons
`are driven in their optimal manner, and the plasma’s
`asymmetry of masses (ions versus electrons) is matched
`by the supply’s source dual characteristics (current
`source (forward sputter) versus voltage source (reverse
`bias). Films successfully reactively sputtered with this
`technology include A1203, Ta2O5, BST, PZT, Ta,O5,
`TaN, TiO2, TiN and ITO. Additional applications for
`
`the technique include etching/cleaning, CVD bias, and
`substrate bias for sputtered films.
`
`2. Target poisoning
`
`The key to the successful production of insulating
`films from metallic targets is the elimination of target
`poisoning. Poisoning is the build-up of insulating layers
`on the target surface. In simple metallic sputtering, the
`target (or cathode) is driven by the sputtering supply to
`a specific DC voltage based on the power, chamber
`pressure, magnetron design. etc. This voltage accelerates
`the argon ions into the target with sufficient kinetic
`energy to cause them to knock atoms from the target.
`The freed target atoms then condense on the substrate
`to form the desired film. The free atoms also deposit on
`the walls of the chamber and back on the target. In
`pure metallic sputtering, this redeposition on the target
`does not represent a problem, since the target and the
`redeposited atoms are the same material. But in reactive
`sputtering, the deposited film is a compound, and there-
`fore a different material
`to the target. For example,
`aluminium oxide, a ceramic, has properties which are
`quite dilferent from metallic aluminum. If the reactive
`film is an insulator, such as aluminum oxide, then the
`situation becomes intolerable.
`
`“ Corresponding author. Tel: +1 512 4622191; Fax: + I 512 2462941 I.
`
`When an insulator is deposited on the surface of the
`
`0257-8972/98/$19.00 © 1998 Published by Elsevier Science S.A. All rights reserved.
`Pl! S0257-8972(97)00403-9
`
`GILLETTE 1022
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`GILLETTE 1022
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`

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`1245
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`.1. Sellers /' Sur/(we and Coutings Tet‘/inology 98 ( I 998) I245-1 250
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`target, a capacitor is formed (see Fig. l). The target acts
`as one conductor, the plasma acts as the other conduc-
`tor, and the insulating film forms the dielectric of the
`capacitor. DC current cannot flow through a capacitor.
`This results in two problems. First, current flow is ion
`current, and therefore, if no argon ions strike the area.
`then no target atoms can be freed and no sputtering can
`occur. Consequently, this area of the target is poisoned.
`Second,
`the parasitic capacitor may not have enough
`dielectric capability to charge all
`the way up to the
`applied voltage. If not. the breakdown of the insulator
`will cause a sudden release of charge carriers. forcing
`the local current density to increase into the are dis-
`charge region, which results in arcing with all its atten-
`dant particulate problems.
`Asymmetric bipolar pulsed DC is the optimum solu-
`tion to the target poisoning problem because it sets up
`conditions which cause the insulators on the target to
`be sputtered first and with a higher sputter yield than
`the base material (a mechanism called preferential sput~
`tering). This eliminates target poisoning by removing
`the cause.
`
`3. Preferential sputtering
`
`Preferential sputtering is accomplished by adding a
`reverse voltage bias pulse to the normal DC waveform
`(see Fig. 2). First,
`if typical sputtering runs at about
`
`-400 V. then the conditions are as shown. Argon ions
`accelerate towards the target at ——400 V, striking the
`target and sputtering the aluminum. However, the rede-
`posited insulating film on the target behaves in a very
`different manner. As the film forms, it tends to collect
`
`low-energy ions on its surface, charging the parasitic
`capacitor towards the applied voltage. As capacitor
`voltage climbs, the available energy with which the ions
`can strike the insulator is decreased by the voltage on
`the capacitor, thus reducing the likelihood of the film
`being sputtered off. Even worse, as the charge builds
`up,
`the ions are actually repelled by the electrostatic
`repulsion of the Ar* ions and the positive capacitor
`voltage. Next, the polarity is rapidly reversed to about
`+100 V (see Fig. 3), causing the plasma—facing surface
`of the dielectric film (parasitic capacitor) to be charged
`up to the opposite polarity (— 100 V). The magic occurs
`as the reverse pulse ends and the voltage returns to
`sputter mode (-400 V), as shown in Fig. 4. Since the
`plasma side of the parasitic capacitor is now charged to
`-100 V. when the target reaches —40OV the effective
`voltage on the plasma side of the parasitic cap is
`— 500 V. Thus, the argon ions are drawn by electrostatic
`attraction to the insulators, and strike with extra energy
`(500 V versus 400 V), thereby sputtering the insulators
`off the target first, eliminating target poisoning.
`The effectiveness of asymmetric bipolar pulsed DC is
`also dependent on pulse frequency. The charging pulses
`must occur frequently enough to prevent the build-up
`
`Dielectric breakdown
`(Micro-Arcing)
`
`Plasma
`
`
`W (dielectri /%C)
`
`tare
`
`
`
`A1403 (dielectric)j
`
`
`
`Fig. 1. Fundamental capacitor model.
`
`
`
`Time
`
`Fig. 2. Normal sputter mode.
`
`Vsurface =Vsputter
`
`
`
`

`
`J. Sellers / Surface and Coatings Technology 98 ( 1998) 12454250
`
`l247
`
`surface = '
`
`Vreversal
`
`
`73
`T0
`T1 T2
`Time
`
`Fig. 4. Return to sputter mode.
`
`be driven by a very still‘ current source, giving a constant
`current regardless of the voltage required, and allowing
`the full forward current to be re-established immediately.
`These opposite drive requirements reflect the asymmetriv
`cal nature of the plasma itself (i.e. electron mass versus
`ion mass).
`In actual use, asymmetric bipolar pulsed DC technol-
`ogy has demonstrated reactive deposition rates of
`60~l00% of the metallic deposition rate [3]. A typical
`example would be reactive TaN film (see Table I). This
`is a much higher rate than any competitive techno-
`logy. Bipolar dual cathode systems have difiiculty
`approaching 50% due to the lost duty cycle in the
`transitions from positive to negative sputter voltage.
`Also, they require two magnetron target assemblies (see
`
`Table l
`Rate and unifomtity comparison (TaN)
`
`
`
`Technique
`
`Rate
`
`Uniformity (%)
`
`of voltage on the parasitic capacitors from exceeding
`their breakdown [1]. Typical process frequencies are
`80-150 kHz. As this technology has been applied to
`materials other than true insulators, it has become clear
`that preferential sputtering is also effective for resistive
`films. Slightly higher frequencies are needed due to the
`tendency of the resistive film to self-discharge. TaN,
`TiN and ITO seem to work best at around 150-180 kHz.
`The value of the reverse bias voltage is critical. It must
`be chosen to be low enough not to generate backsputter—
`ing, and yet high enough to maintain preferential sput-
`tering. Experience on many different chambers has
`shown 75«lO0V to be effective and safe. Full bipolar
`pulses have enough voltage to sputter in both directions.
`Clearly, if there is enough voltage to sputter one way,
`then there is enough to sputter in the other direction
`also. Most magnetrons are asymmetrical in their voltage
`withstand, and will tolerate up to about + l50 V.
`
`4. Deposition rate
`
`It is equally critical to return to forward current (ion
`current) flow as quickly as possible after the reverse
`bias pulse in order to maximize the deposition rate. This
`drives the requirement for the forward sputter power to
`
`2
`1.06
`RF diode
`ll.2
`2.00
`RF magnetron
`Not long—term stable
`3.53
`DC magnetron
`
`
`3.48Pulse DC magnetron l.97
`
`

`
`1248
`
`J. Sellers / Surface and Coutings Teclmology 98 ( 1998) 1245-1250
`
`Fig. 5). Asymmetric bipolar pulsed DC technology was
`optimized to sputter insulators from a conductive target.
`
`5. Uniformity enhancement
`
`Asymmetric bipolar pulsed DC technology has proven
`to be particularly beneficial for the enhancement of film
`qualities. Film uniformity (deposition uniformity and
`film characteristics) has demonstrated benefits from the
`use of pulsed DC techniques. There are two mechanisms
`at work in these results. First, in the case of a complex
`film (the deposited film is a compound), the additional
`ionization of the pulsed plasma results in a hotter
`(greater electron temperature) and more chemically
`active plasma, which tends to improve the consistency
`of the film chemistry. This ionization enhancement
`comes from the high-frequency (edge rates) and the mid-
`frequency (pulse rate) components of the waveforms.
`The second mechanism is the eiTects of peak to average
`power ratio on the deposition pattern. The greater the
`power applied to a magnetron, the wider the erosion
`zone (race track). This occurs due to the opposing
`electrostatic and electromagnetic forces of the magnet-
`
`+400
`
`Amps
`
`ron and the plasma ions. The magnetron has a preferred
`location for the ions to strike due to the geometry of
`the intersection of the electrical and magnetic fields.
`This “sweet spot" is located at the juncture of the peak
`magnetic and electrical fields, at the point where E x B
`is a maximum (see Fig. 6). At low power density the
`ions will simply form a narrow ring at this “sweet spot”
`radius. resulting in a very narrow erosion zone. As the
`power is increased, the ions begin to crowd together in
`the “sweet spot”, but the electrostatic repulsion for the
`like-charged ions forces them to stay apart. Thus, the
`ions are forced into a widening pattern around the
`“sweet spot” radius (see Fig. 7). As the ions are forced
`apart, the additional energy required to generate ioniza-
`tion at the lower magnetic field intensity is compensated
`by an increase in plasma voltage. As the erosion zone
`(race track) becomes wider,
`the deposition pattern at
`the substrate also becomes more uniform. Another effect
`
`
`
`Full Bipolar
`
`"Sweet Spot": Max Point of EXB
`
`Fig. 6. "Sweet spot".
`
`
`
` — Racetrack
`
`Low Power
`
`High Power
`
`Fig. 5. Waveform comparison curves.
`
`Fig. 7. Uniformity.
`
`

`
`J. Sellers / Surface and Caating: Technology 98 (1998) 1245-1250
`
`1249
`
`Table 2
`Pulse DC etch results
`
`Substrate Contaminant Press (mT) Frequency (kHz) PW (us) Power (W)Run Approx. etch rate (nm 5‘ ‘)
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`1
`2
`3
`4
`5
`6
`
`Silicon
`Silicon
`Silicon
`Steel
`Steel
`Steel
`
`AL,(),
`ALZO-3
`AL,O3
`Light oils
`Heavy rust
`Heavy rust
`
`30
`70
`46
`33
`39
`75
`
`162
`162
`185
`215
`2l5
`63
`
`1.32
`l .3
`1.2
`1.01
`1.01
`l.6
`
`192
`193
`308
`385
`413
`l20
`
`0.02
`0.01
`0.056
`0.2
`0,2
`0.05
`
`processes. Work is presently in progress to harness the
`clear benefits of the technique, which include no RF, no
`RF match, direct DC—type control of the substrate bias
`energy and the rate improvements due to the increased
`bias duty factor. Several types of films, including DLC
`and B4C, have shown benefit from asymmetric bipolar
`pulsed DC bias.
`Additionally, the use of substrate bias can allow the
`elimination of some adhesion layers due to the enhance-
`ment of adhesion levels achieved by the use of high-
`energy bias during the growth of the sputtered films.
`The mechanisms of this adhesion enhancement are the
`dual effects of surface activation of the substrate through
`pre-etching and the additional energy available to the
`growing film from the substrate ion current. As the film
`begins to form, the early stages of formation are the
`most critical to the characteristics of the film, since the
`
`first layer will tend to set the form (crystallography) and
`adhesion for the entire film [2,4]. The addition of energy
`to this early film growth allows the atoms to achieve
`greater mobility and find the low-energy wells on the
`substrate surface where the adhesion will be maximized.
`
`Also, the high flux of energetic ions will tend to wash
`away any poorly adhered film atoms. These effects are
`also enhanced because of the increased ionization of the
`
`asymmetric bipolar pulsed DC. Strongly ionized film
`atoms can be implanted into the substrate due to the
`bias energy adding to the energy of the film atoms.
`Clearly, there is never a “free lunch”, so the negative
`efl"ect of using high—energy bias is a reduction in the
`overall deposition rate due to the resputtering of the
`growing film. The use of bias to generate crystalline
`films at low substrate temperatures is also progressing
`well.
`
`7. Conclusion
`
`Asymmetric bipolar pulsed DC technology is the
`future of DC plasma processing because of its ability to
`extend the range of DC processes and to broaden the
`use of sputtered film applications into entirely new
`markets.
`
`of the high power density ion crowding is an increase
`in the ionization of the sputtered material. This is caused
`by an increase in collisions between the freed target
`atoms and the incoming argon ions. As the density of
`freed target material and ions increases, the frequency
`of collisions between ions and molecules also increases.
`These effects are applicable to asymmetric bipolar pulsed
`DC simply due to the high peak to average power ratios
`attainable with the technology (see Fig. 8).
`
`6. Other applications
`
`6.1. Etching/cleaning
`
`Just as it can remove insulating coatings from conduc-
`tive sputter targets, asymmetric bipolar pulsed DC tech-
`nology can remove non-conductive contamination from
`steel tools, silicon wafers and other conductive materials
`(see Table 2).
`
`6.2. Bias/source
`
`The AC characteristics of the technology also allow
`it to be applied to substrate bias of insulating CVD
`
`Current
`
`llIII
`
`Peak Power = 4.8kW
`Av. Power = 1kW
`Peak — Av. Ratio = 4.8
`
`
` Voltage
`
`Fig. 8. Peak power.
`
`

`
`1250
`
`References
`
`J. Sellers / Surface and Coatings Technology 98 ( I998) 1245-1250
`
`_
`.
`‘
`_
`{1} S. Schiller. K. Goedicke. J. Reschke. V. Krrchofi, S. Schneider, F.
`Milde. Pulsed Magnetron DC Sputtering Technology, FhG FEP.
`ICMCTF. San Diego. CA. I993. PPr9-10-
`[2] 5- Fessman. 0. Kamschulte. J. Ebbemik. Improved control of TiN
`coating properties using cathodic arc evaporation with a pulsed
`bias by W. Olbrich_ Surf, Coat, Technol, 49 (1991) 258,
`
`[3] J. Schneider, W. Sproul, A. Lefkow, Birl, A. Matthews. U. Hui].
`‘
`Scaleable Process for Pulsed DC Magnetron Sputtering of Non-
`
`Conductive Oxides, 39th l996 SUC Tech. Cont". Proceedings,
`phma_ PA p_,6g_
`[4] Ammina.chmmica coatings deposited by reactive magnetron sput-
`tering. J.M. Schneider. WODA Sproul. Birl, Northwestern Univer-
`sity, Evanslon IL; J. Sellers. ENI-SW. Austin. TX and A. Mathews,
`RCSE.Hul1University. United Kingdom. 1997 Society of Vacuum
`Coaters, 40th Technical Conference Proceedings 1997.