Sputtergeschwindigkeiten
To use these charts, locate the material for which known conditions are available. Then multiply the rate by the relative factors to arrive at the estimated rate for the new material. For example, with previous data showing 3.5A/s aluminum at l00W, then titanium at similar conditions will generate approximately (0,53/1,00) • 3,5 Å/s ≅ 2 Å/s.
The rates in this table are calculated based on a 500V cathode potential. As the power is increased greater than two times the original rate, the relative rate will drop slightly (up to 10%). For example, aluminum at 250W.
Al250 W = 0,9 • AI100 W • (P1/P0)
0,9 • 3,5 Å/s • (250/100) ≅ 7,4 Å/s
The rates in the ceramics table assume the use of an RF power supply and account for the partial duty cycle of the RF generator as compared to a DC supply. A pulsed DC supply will yield slightly higher effective rates.
The magnetic materials table shows the rate for DC operation with a new target. As the magnetic target erodes, the influence of the remaining material on the magnetic confinement field will change, leading to variations in sputter rate, operation voltage, and ignition pressure.
This information is for general planning purposes only. The Kurt J. Lesker Company makes no guarantees of the correctness of these numbers in your process. Contact the Kurt J. Lesker Company for specific assistance in setting up your process.
| NON-MAGNETIC MATERIALS* | ||
| Werkstoff | Name | Rate |
| Ag | Silber | 2,88 |
| Al | Aluminium | 1,00 |
| Au | Gold | 1.74 |
| Be | Beryllium | 0,21 |
| C | Kohlenstoff | 0,23 |
| Cu | Kupfer | 1,42 |
| GaAs | Gallium Arsenide {100} | 1,03 |
| GaAs | Gallium Arsenide {110} | 1,03 |
| Ge | Germanium | 1,50 |
| Mo | Molybdän | 0,66 |
| Nb | Niob | 0,76 |
| Pd | Palladium | 1,77 |
| Pt | Platin | 1,00 |
| Re | Rhenium | 0,84 |
| Rh | Rhodium | 1,16 |
| Ru | Ruthenium | 0,98 |
| Si | Silizium | 0,60 |
| Sm | Samarium | 1.74 |
| Ta | Tantal | 0,67 |
| Th | Thorium | 1,31 |
| Ti | Titan | 0,53 |
| V | Vanadium | 0,50 |
| W | Wolfram | 0,57 |
| Y | Yttrium | 1,53 |
| Zr | Zirkonium | 0,88 |
* All rates in this table are relative to aluminum.
| OXIDES AND CERAMICS | ||
| Werkstoff | Name | Rate |
| Al2O3 | Aluminiumoxid | 0,05 |
| SiC | Siliciumcarbid | 0,22 |
| SiO2 | Siliziumdioxid | 0,21 |
| Tac | Tantalcarbid | 0,09 |
| Ta2O5 | Tantalpentoxid | 0,39 |
| MAGNETIC MATERIALS | |||
| Werkstoff | Name | Mag Moment | Rate |
| Co | Kobalt | Niedrig | 0,73 |
| Cr | Chrom | Med | 0,87 |
| Fe | Eisen | Hoch | 0,57 |
| Mn | Mangan | Med | 0,14 |
| Ni | Nickel | Niedrig | 0,86 |
| Ni80Fe20 | Permalloy | Hoch | 0,80 |
Es gibt einige Möglichkeiten, die Sputterrate von Werkstoffen zu erhöhen bzw. zu maximieren.
1. Erhöhen der Energie: Jeder Werkstoff hat zwar relativ zu seinen Werkstoffeigenschaften eine maximal zulässige Leistung, dank der Kühlungseffizienz können Sie das Target jedoch mit der höchstmöglichen Energiedichte betreiben. Als erstes müssen Sie den Targetwerkstoff direkt kühlen. Dazu können Sie ein Bolt-On-System oder eine gebondete Targetkonfiguration verwenden. Dies in Kombination mit der Verwendung von Leitpaste oder Epoxidverklebung kann die Wärmeleitfähigkeit maximiert und auf diese Weise Energiedichte auf das maximale Niveau erhöht werden, das vom Targetwerkstoff erreicht werden kann.
2. Decrease source-substrate distance: Je näher sich das Target am Substrat befindet, desto höher die Sputterrate. Im Allgemeinen befindet sich das Plasma in einem Abstand von bis zu 2" über der Targetoberfläche. Many sputtering applications utilize a 3"-4" source-substrate distance. Assuming a 4" source-substrate distance, the sputtering rate will fall off by approximately 25% for every inch beyond 4". Die Rate erhöht sich jedoch normalerweise mit jedem Zoll, mit dem Sie den Abstand von 4" verringern, um rund 35 %.
3. Lower operating pressures: Beim Sputtern gilt: Je mehr Gas sich in der Kammer befindet, desto mehr Kollisionen zwischen Atomen und Ionen entstehen. Durch diese Stöße reduziert sich die Aufdampfrate, mit der Werkstoffatome von der Targetoberfläche abgetragen werden und sich auf dem Substrat niederschlagen. Durch Reduzieren des Gasdurchflusses werden diese Kollisionen reduziert und erhöhen die endgültigen Sputterraten, die erzielt werden können.
4. Increase the number of magnetrons in the chamber: Die Aufdampfraten steigen linear zur Anzahl der Magnetrone, die Ihrer Anwendung hinzugefügt werden. In production applications with specific yield requirements, once the power and source-substrate parameters have been fully maximized, increasing the number of magnetrons is a parameter that can be utilized to enhance sputtering rates.
Homogenität
Um die Homogenität in Sputteranwendungen zu verbessern, sind viele Variablen erforderlich. Einige dieser Variablen werden vom Magnetron selbst beeinflusst. Manche Variablen sind jedoch auch vom Design des Systems bzw. der Kammer und der Strömungsdynamik abhängig, die in einigen Fällen nicht gesteuert werden kann. Es gibt jedoch einige Techniken, mit denen die Homogenität in Ihrer Anwendung erhöht werden kann. Die folgende Beispielliste umfasst einige Vorschläge, welche Parameter angepasst werden können, um die Homogenität positiv zu beeinflussen. Es ist wichtig, zunächst einmal zu erkennen, dass es zwei völlig unterschiedliche Depositionskonfigurationen gibt, die ganz verschiedene Ansätze zur Verbesserung der Homogenität bieten. Daher werden wir diese Konfigurationen unabhängig voneinander betrachten.
Homogenität auf statischem Substrat
Abbildung 2
Abbildung 1
Beim Sputtern eines statischen Substrats wird die allgemeine Beschichtungshomogenität von den folgenden Parametern beeinflusst:
1. Ausrichtung des Magnetrons zum Substrat: Das Magnetron und das Substrat müssen sich mittig auf ihrer Achse befinden, um eine optimale Homogenität zu erzielen.
2. Magnetron Size: Das Target muss größer als das Substrat sein, um eine optimale Homogenität zu erzielen. Ein typisches Beschichtungsprofil fällt an den Kanten ab und ist in der Mitte am homogensten (siehe Abbildung 1).
Je mehr Überhang am Substrat vorhanden ist, desto homogener die Beschichtung.
3. source-substrate distance: If magnetron to substrate overhang is non-existent or limited due to existing chamber design or equipment, increasing the source-substrate distance will help improve uniformity. Je größer der Abstand, desto mehr Kollisionen zwischen den Argonionen, Elektronen und Werkstoffatomen finden statt, wodurch die Randomisierung auf der gesputterten Schicht zustande kommt, die sich auf dem Substrat niederschlägt. Im Endeffekt wird so auch eine bessere Homogenität erzielt. Der Nachteil dabei ist jedoch, dass, je weiter weg Sie sich vom Substrat befinden, die Sputterraten desto niedriger ausfallen.
4. Masking: Das Maskieren ist eine Technik, die zur Verbesserung der Homogenität eingesetzt werden kann. Dabei wird verhindert, dass Werkstoff aus bestimmten Bereichen des Targets sich auf dem Substrat absetzen. Beispielsweise fällt die Materialdeposition aus den Randbereichen des Targets her aufgrund des Magnetfeldprofils und der daraus folgenden aktiven Erosionszone sowie dem resultierenden Flussprofil typischerweise ab. Durch den Einsatz von Maskierungen im mittleren Bereich des Targets lässt sich das Erosionsprofil verflachen (siehe Abbildung 2).
Homogenität auf rotierendem Substrat (einachsige Drehung)
Bei der Beschichtung eines rotierenden Substrats können die folgenden Techniken eingesetzt werden, um die Homogenität zu verbessern:
1. Außeraxiale oder konfokale Ausrichtung des Magnetrons zum Substrat: Der Hauptvorteil beim Rotieren des Substrats ist, dass Sie ein wesentlich kleineres Magnetron benötigen, um eine optimale Homogenität zu erzielen und zwar indem Sie das Target achsenversetzt zum Substrat anbringen. Der Einsatz einer Einzelachsenrotation des Substrats ist für die folgenden Techniken erforderlich:
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A. Offset (Versatz): Durch Ändern des Abstands der Mittelachsen von Traget zu Substrat hin zu einem Versatz in Bezug auf den Radius lässt sich die Homogenität über die gesamte Substratoberfläche hinweg verbessern, wobei der Fokus nur auf der Hälfte oder einem Teil des Substrats liegt.
- i. source-substrate distance: Adjusting the source-substrate distance will also help dial in the uniformity.
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B. Konfokal: Bei einer konfokalen Ausrichtung wird das Magnetron in einem Winkel zum Substratradius angeordnet. Die idealen Parameter für das konfokale Sputtern lauten folgendermaßen:
- i. 30 Grad außeraxiale Neigung zur Mitte des Substratradius
- ii. 4" source-substrate distance
Hinweis: Adjustments to the angle and source-substrate distance may be required based on chamber design and flow dynamics.
Bei einer konfokalen Ausrichtung können mit Magnetronen von 3" die folgenden Homogenitäten erzielt werden:
| Substratgröße | Homogenität |
| 4" Außendurchmesser | +/- 1–2 % |
| 6" Außendurchmesser | +/- 3–5 % |
| 8" Außendurchmesser | +/- 7 % |
Beim konfokalen oder außeraxialen Sputtern können für das gleichzeitige Sputtern mehrerer Schichten aus variablen Werkstoffen mehrere Magnetrone eingesetzt werden und/oder es können die Sputterraten desselben Targetwerkstoffs erhöht werden.
Typically, when sputtering a 6" substrate wafer, it is possible to mount up to (4) four sputtering cathodes at a 4" source-substrate distance. It is critical to have the ability to adjust source-substrate distance and the angle of the sputtering cathode to have maximum ability to dial in uniformity.
In off-axis sputtering, having the ability adjust the off-set to the substrate, angle of the source, and source-substrate distance, are all critical parameters in dialing in the uniformity. Nachstehend finden Sie einige Beispiele dafür, wie das Anpassen dieser Variablen sowohl die Homogenität als auch die Aufdampfrate erheblich beeinflussen können:
| Konfiguration 1 |
| Substrat: 6" |
| Offset: 3" |
| (Mitte des Targets – Mitte des Substrats) |
| source-substrate distance: 4" |
| Winkel des Kopfes: 0 Grad |
| Targetmaterial: Aluminium |
| Homogenität: +/- 4,7 % |
| Aufdampfrate: 4,5 Angstrom/s |
| Konfiguration 2 |
| Substrat: 6" |
| Offset: 3" |
| (Mitte des Targets – Mitte des Substrats) |
| source-substrate distance: 4" |
| Winkel des Kopfes: -5 Grad |
| Targetmaterial: Aluminium |
| Homogenität: +/- 2,2 % |
| Aufdampfrate: 3,5 Angstrom/s |
| Konfiguration 3 |
| Substrat: 6" |
| Offset: 3,5" |
| (Mitte des Targets – Mitte des Substrats) |
| source-substrate distance: 3" |
| Winkel des Kopfes: -5 Grad |
| Targetmaterial: Aluminium |
| Homogenität: +/- 3,5 % |
| Aufdampfrate: 5,5 Angstrom/s |
| Konfiguration 4 |
| Substrat: 6" |
| Offset: 3,5" |
| (Mitte des Targets – Mitte des Substrats) |
| source-substrate distance: 4" |
| Winkel des Kopfes: 0 Grad |
| Targetmaterial: Aluminium |
| Homogenität: +/- 1,3 % |
| Aufdampfrate: 3,6 Angstrom/s |
| Konfiguration 5 |
| Substrat: 6" |
| Offset: 4,25" |
| (Mitte des Targets – Mitte des Substrats) |
| source-substrate distance: 3" |
| Winkel des Kopfes: 15 Grad |
| Targetmaterial: Aluminium |
| Homogenität: +/- 2,5 % |
| Aufdampfrate: 2,8 Angstrom/s |
| Konfiguration 6 |
| Substrat: 6" |
| Offset: 4" |
| (Mitte des Targets – Mitte des Substrats) |
| source-substrate distance: 6" |
| Winkel des Kopfes: -5 Grad |
| Targetmaterial: Aluminium |
| Homogenität: +/- 1,9 % |
| Aufdampfrate: 0,9 Angstrom/s |
Hinweis: Alle obigen Konfigurationen verwenden die Einzelachsen-Substratrotation.
Increasing Sputter Rates
When sputtering dielectric targets using RF power, it is quite possible for the maximum deposition rate on the substrate to be less than 0,1 Å/sec. That is, depositing a film 100nm thick may take over 2,5 hours. It is no surprise, therefore, that we are frequently asked, "How can I increase the sputter rate?"
Actually, what the questioner wants is to increase the deposition rate, but we're not about to argue semantics with a frustrated researcher.
(But to segue into semantics for a moment: we will use sputters the adjectival form, as in sputter yield, sputter rate, sputter gun, rather than sputtering yield etc.)
In this issue we review ways to increase deposition rates and look at conditions where maximizing one parameter inadvertently affects something else.
While the substrates can be static or rotating, these suggestions apply only to circular sputter guns with flat disc targets and stationary magnet assemblies. Sputter guns with targets of other shapes and configurations, moving magnet assemblies, and linear sputter guns, have their own performance attributes that are not directly addressed here.
The Ar+ ions have sufficient energy to eject target atoms into gas phase
Sputter sources in co-deposition 'action'
Sputter Yield
First, we must understand that each material has its own characteristic sputter yield - the number of atoms (or molecules) leaving the target for each ion that hits it. The sputter yield value depends on: the material; the mass of the incoming ion; the voltage through which the ion is accelerated; and its angle of incidence on the target.
For Ar+ ions striking a target at 45° through a potential of 500eV, the sputter yields of most elements are between 1 - 10, roughly.
Materials that are chemical compounds such as oxides can have much lower sputter yields! For example, Maissel and Glang's book Handbook of Thin Film Technology quotes the sputter yield for SiO2 as 0,13 and Al2O3 as 0,04.
Extending the concept of sputter yield, we will later refer to a material's sputter rate, which is its sputter yield multiplied by the ion current to the target.
Throw Distance Changes
A representation of the effect of deposition rate when the Throw Distance is halved
Reducing the target-to-substrate distance (often called throw distance) is a simple, direct way to increase deposition rate. To fully understand this effect, the angular distribution of sputtered particles must be known. Regrettably, this is a complex subject since material is ejected from a circular 'trench' around the target and terms like over-cosine and under-cosine are used in the literature to describe a sputtered material's flux distribution.
For these notes, however, it is sufficient to understand that the sputtered particles' arrival rate (per unit area of substrate) varies as the inverse square of the throw distance. That is, halving the throw distance quadruples the material's arrival rate at the substrate and the film's thickness grows at 4x the previous rate!
However, it is important to consider the shorter throw distance's affect on the film's (thickness) uniformity. If, for example, material leaves the target in roughly a cosine distribution pattern, then the larger the throw distance, the higher the number of thermalizing collisions between sputtered atoms and sputter gas atoms. These collisions tend to 'flatten out' the cosine distribution making the deposition more uniform across the substrate. Since a shorter throw distance means fewer collisions, film uniformity at shorter distances may be worse.
In addition, at shorter throw distances substrates may see: higher energy sputter particles; more stray electrons; more plasma ions and 'hot' neutrals; and higher thermal radiation heat transfer from the plasma and target surface. So the adverse effects of a shorter throw distance include:
- Excessive substrate outgassing
- Increase in compressive stress in the growing film
- Films beneath the present one damaged by electron bombardment
- Substrate melting!
However, shorter throw distances (and, therefore, higher substrate temperatures) can have beneficial effects too:
- Films may grow as successive monolayers (called Frank-van der Merwe growth, a frequently desirable nucleation mode)
- The film's tensile stress may be reduced
- Film adhesion may improve due to the higher energy of arriving atoms
- Films may be 'densified' by bombardment with higher energy plasma ions and 'hot' neutral
Increasing Power
Doubling the power applied to the target roughly doubles the sputter rate and this always appears to be the 'easy option' when faced with low deposition rates.
Unfortunately, arbitrarily increasing power has many adverse effects.All power applied to the gun must dissipate somewhere in the system. It is claimed that roughly 75% ends up heating the gun's cooling water. That is, 75% of the total power dumped into the target's front face must transfer through the target to reach the water! Clearly, the target's thermal conductivity, thermal coefficient of expansion, mechanical strength characteristics, and melting point, are critical issues.
- Thermal conductivity helps determine the temperature difference between the target's front and rear faces. The larger that difference the higher the thermal stress in the material
- Thermal coefficient of expansion partly determines the mechanical stresses resulting from the thermal stress
- Mechanical strength determines how the mechanical stresses are dissipated (usually by bowing, warping, chipping, or cracking
- Melting point (obviously) determines if the target will melt at the temperature generated by the applied power level - and a molten target can ruin a sputter gun
Another major concern is the 'thermal conductance' of the interface between the target's rear face and the sputter gun's cooling well. Results tabulated in A Heat Transfer Textbook by Lienhard & Lienhard indicate the thermal conductance between two lightly clamped, flat metal surfaces is (a) not very high, and (b) depends significantly on air between the surfaces.
Evacuate that interface - that is, put the sputter gun under operating conditions - and the thermal conductance of the interface between the target and the cooling well may drop to 1/20 to 1/50th of its 'with air' value.
Some target materials are so fragile they crack no matter what sputter power is used. Bonding such materials to copper backing plates may allow their continued use even though cracked. However, if pieces chip off or the cracks become wide enough to expose bonding agent or copper backing plate, the target must be replaced.
Too high sputter power is the most common cause of target and sputter gun damage. Given the target/interface thermal limitations, such damage can be reduced/eliminated by using an appropriate maximum power (see Maximum Power Levels). However, 'appropriate' often equates to 'low' and low power means low deposition rates.
One final point about applying power to a target. Once the appropriate power has been established for a given target/gun, never switch on and immediate increase power to that value! Always increase power slowly to its maximum value through a series of ramps and soaks.
Kurt J. Lesker Company RF power supply for non-conducting targets
Sputter Gas Pressure
Lowering the sputter gas pressure causes a modest increase in deposition rate by a two-fold mechanism:
- Sputtered atoms leaving the target will undergo fewer thermalizing collisions.They are less likely to scatter 'sideways' and a larger percentage will continue to the substrate, slightly increasing the deposition rates
- In power control mode, using either RF or DC power, the plasma-to-target voltage will increase slightly. Ions bombarding the target will, therefore, have a higher energy which slightly increases the sputter yield and consequently the sputter rate
One potential side-effect of lowering the gas pressure is a change in film uniformity. Whether it improves or worsens is typically not predictable because there are many factors involved. But one obvious aspect is a reduction in the number of thermalizing collisions.
An adverse effect of lower gas pressure/higher plasma-to-target voltage combination is the greater likelihood of arcs occurring near the target.
Increasing Target Size
As a method of increasing deposition rate, this option is not easily implemented and is expensive since it requires a new sputter gun, sufficient room to install it in the chamber, and possibly a larger power supply.
For a given power density (see Power & Power Density), the larger the target diameter the higher the sputter rate. The explanation is simple. A larger target diameter means a larger sputter trench area and, for a given power density, increased trench area means increased sputter rate.
Number of Guns
The majority of R&D deposition systems have more than one sputter gun installed. Typically, the user installs different target materials in each gun. However, putting the same target material on two or more guns and operating them simultaneously can double, triple, etc. the sputter rate and resulting deposition rate.
The drawback is, many multi-gun systems were not built for co-deposition work and have just one power supply. Buying additional supplies for simultaneous operation may make this option expensive.
Schlussfolgerung
Yes, there are ways to increase deposition rates. Unfortunately the easy winding-up-the-power option, if misused, at best leaves your targets looking a little sad. At worst, your sputter gun splutters to a stop, water leaks into the chamber, or the power supply fries. No, I jest! At worst, all three happen simultaneously.
Cerium oxide target bonded to copper backing plate but sputtered at a power that melted the indium bond and cracked the target
3" diameter Indium tin oxide target sputtered at 1000 W (roughly 7x the maximum recommended power)
Aluminum doped zinc oxide target given the 'tough love' treatment of inadequate cooling (at the 10:00 o'clock position) and excessive power
As always, if you have questions or comments email techinfo@lesker.com and they will be forwarded to the author.