Sputter Coater

Sputter coaters are devices which deposit a substance (target) onto a substrate by means of ion etching.

Types

There are many variantions on the design of Sputtering devices. Many with advantages and disadvantages.

Simple Diode

The Diode sputtering setup is the most simple of all the sputtering setups. It consists, in essence a plate capacitor within a vacuum chamber. The Cathode plate is made of the material that is to be deposited and the Anode is the object to be coated. These are then placed into a vacuum chamber filled, usually, with an Inert Gas Atmosphere. Typically Argon is used, though other gases, including air, can be used with various effects. See the section on working gasses for more details.

Diode type sputtering setups have various downsides, among these are ones inherrit to all DC type sputtering setups, in that only conductive targets may be used. Another major downside is that the Substrate (object to be coated) is strongly heated by Electron bombardment from the Cathode. This can cause structural changes in the object which can be unacceptable in electron microscopy. Another downside to the Diode sputtering setup is that the sputtering rate is rather low compared to other setups.

This effect of Diode sputtering is observed in any plasma device opperated at DC. Thus if sputtering is to be minimized, a metal with very low sputtering yield is to be used. Examples include, among others, Tantalum and Tungsten.

Cold Diode / Magnetron

This advancement of the Diode setup is what is now commonly used in electron microscope preperation, as it elimminated, or at least minimizses the effect of electron bombardment heating of the substrate.

The Cold Diode consists, again much like the pure Diode of a plate capacitor. However, unlike the pure Diode the Cathode is fitted with a magnet structure creating a field which traps electrons and ion in the vecinity of the cathode plate. This has several effects, these include the reduction of electron bombardment heating of the substrate, though this can still occoure depending on the magnetic field and geometry of the setup. The second effect is that, due to concentrating the electron and ion flux on the cathode, the rate at which it is etched away is increased. Thus shorter sputtering times can be achieved. The cathode and magnet structure is typically surrounded by a shielding can with an opening exposing the target, but shielding the rest of the structure. Thus preventing the supttering of superstructure materials together with the target.

Due to trapping the electrons and ions around the cathode, this is much more strongly heated then in the pure diode. This usually nesessitates the use of water cooling to keep the cathode cold enough not to demagnetize the magnetic structure due to heating it past the Curie temperature as well as preventing damage to the target.

Due to the need of a magnetic field in the vecinity of the Target, the sputtering of high permiability materials is only possible if these are of thing sheets which can be saturated with only a small portion of the magnetic structures field. If the entire field is contained within the target material, the Cold Diode no longer opperates in the cold Diode mode, but instead reverts back to the simple diode, thus negating the benefits the cold diode setup has over the simple diode.

Triode

The Triode sputtering setup was one of the ways that electron bombardment heating of the substrate was reduced. Unlike the Simple Diode and Cold Diode, this setup introduces a third electrode. This third electrode is usually held at a potential zwice as high as the substrate, thus prefferentially atracting the electrons to it, rather then the substrate.

This third electrode is typically in the form of a ring elevated above the substrate carrier, which sits at its center. Sputtering rates in this proscess are not as high as that of the Cold Diode, and the heating of the substrate is still present, all be it reduced in comparison to the Pure Diode. Thus this technique has not found common useage for Electron Microscopy.

Electron Assisted Triode

This form of sputtering setup is a further refinement of the above sputtering setups. In order to increase the sputtering rate of the target, an electron gun, such as in the form of a ring shaped filament, is placed close to the target, but shielded in line of sight to the substrate. This electron gun increases the amount of electrons avalible to the target, and thus also the amount of Ions hitting it. It also has the benefit of allowing lower opperating pressures to be used, since the sputtering setup no longer relies on plasma discharge Ionoization to create the working ion stream.

The use of a lower opperating pressure has the advantage of better layer control, reduced granularity, higher purity of the deposition layer, but comes at the cost of a more directional coating effect from the target owing to the reduction in scatting events within the proscess gas. Thus depending on the pressure used, and the size distance ratio of the Target to the Substrate, similar directional effects to that of Evaporation can be seen. I.e. only coating in line of sight to the target.

Ion Gun Sputtering

The Ion Gun Sputtering setup is among the most advanced of all the setups. It consists of a Target, usually held at the same potential as the Substrate. The target is usually tilted slightly off axis in relation to the substrate owing to the need for an ion beam to impact the target, but not the substrate.

Off axis from the Target substrate axis, usually sits between one and several ion guns. These ion guns are fed with the proscess gas, such as argon, and project a semi focused beam of ions onto the target. The target and substrate may be held at high or even ultra high vacuum, since the ion stream is no longer created in the coating chamber but instead within the ion gun(s). This results in the best purity of the coating layer. However it also has the disadvantage of the previously mentioned electron assisted Diode, in that depending on the Target to Substrate ratio, as well as size and geometry of the Ion beam, shading effects much like those from Evaporation can occour.

In contrast to the above mentioned sputtering setups, the Ion Gun Sputtering setup can also be modified to sputter non conducting Targets by means of a neutralizing electron gun. This electron gun keep the charge buildup on the target at managable levels during the coating, thus still allowing the Ions to sputter away the target. If the electron gun is switched off or the current set incorrectly, the non conductive target would build up a charge from the ion beam, ultimately becoming an Ion mirror, completely reflecting the ion beam away from the target.

Open Field Cold Diode / Magnetron

This variation on the Cold Diode has the difference to the above, in that additional magnetic elements are incorperated into the target (Magnetron head) to change the field geometry to one that includes, usually to a variable extent, the substrate within the magnetrons field. This setup may further be expended by means of Electromagnets so that the field may be extended away from and contracted back to the Target in a time variable fassion. Among the advantages of this methode are better adhesion of the target material on the substrate.

Opperating Modes

DC

This opperating mode is the simplest and most common mode in which Sputtering setups are opperated. The high voltage supply from a transformer is rectified and smoothed prior to being fed to the sputtering setup. This mode of opperation has the downside that only conductive Targets can be used, owing to charge buildup within the target slowly leading to develpment of an ion mirror, prventing further sputtering.

AC

AC sputtering addresses the above charge buildup problem by means of time varying the Electric field Polarity within the setup. During the Negative part of cycle the Target is bombared with Positively charged Ions, which remove the target material, but also accumulated a positive charge. The Positive part of the cycle then bombards this positively charged target with electrons, neutralizing the charge, and or changing it to be negative. The polarity then reverses once more, and the target is once again bombarded with positive ions.

Typically this AC sputtering is done in the radio frequency region, as apposed to the low frequency region such as that of mains power, or typical switching power supplies. Due to this, the RMS voltage needed to ionize the working gas is much lower then what would typically be needed for DC opperation. The power is typically derived from a RF power amplifier connected to the sputtering setup by means of an impedence matching network. This then opperates the sputtering setup as an Antenna coupling the RF power into the working gas, thus ionizing it.

Pulsed DC

This mode of opperation is similar to steady state DC, but instead of continuasly supplying the setup with power, the power is switched on and off in various ratios and intervals. This allows to use of much higher peak ion currents then steady state DC. The target and substrate having time to cool down inbetween the high current pulses. By these means a higher effective sputtering rate can be achieved in comparison to steady state DC.

With Pulsed DC, it is common to not fully extinguage the plasma discharge between high current pulses. Thus making higher repetition rates easier to accomplish since the gas is already ionized before the next high current pulse begins.

Pulsed AC

Pulsed AC is basically the same as Pulsed DC, but instead of turning on and off a DC voltage, the RF power is raised and lowered instead.

Process Gas

The choice of proscess gas depends on numerous factors, including the desired rate of sputtering, avalibility, time and or the need for the gas to react with the target material in the plasma state in order to form a new compound.

Argon

Argon, or technical Argon is the standard Proscess gas for sputtering. It offeres high sputtering rates and is chemically innert, thus not reacting with the target material.

Air

Air is used in low end sputtering setups as a cheap alternative to Argon. It has markedly reduced sputtering rates in comparison to argon, as well as having the downside of only being usable with noble metals as targets, such as Gold or Platnium.

Oxygen

Oxygen is used, either pure or in combination with other gases like Argon to perform reactive Sputtering. In this proscess the target material is chemically reacted with the oxygen to form a new compound which then depoisits on the substrate. An example of this is the use of a Silicon Target in oxygen atmosphere. Running this setup results in the deposition of SiliconDioxide (Quart) on the substrate as apposed to pure silicon. This is one way of coating an insulating material by means of a DC operated Sputtering setup.

Opperating pressure

Sputtering is usually performed in a fine vacuum in the region between 1mBar and 1e-2mBar. Ataining this pressure is done in a few ways, depending on the requiroments on proscess gas purity.

Fine Vacuum

In low to mid range coaters, the vacuum is produced by means of a mechanical vacuum pump, such as a 2 stage rotory vane pump. This vacuum is then balanced to the opperating pressure by means of a needle valve. The downsides of this methode are numerous, such as there always being a slightly oil mist within the coating chamber, leading to a contaminted layer on the substrate. Additionally, owing to the incomplete removal of air from the chamber, a residual amount of air is still present, this can cause problems with reactive targets as well as addition further contamination to the substrate layer.

The oil mist backstreaming problem form oil sealed rotory vane pumps my be reduced by means of a Sorption pump or other means of oil backstreaming foreline trap prior to the sputter coater. Alternaively a oil free pump such as membrane or scroll pump may be used.

The residual air contamination may be reduced by means of flushing the vacuum chamber several times with the prosccess gas prior to sputtering. To do this, the chamber is first evacuated, then backfilled with proscess gas, fallowed by another evacuation. This may be repeated several times to remove the concentration of air within the camber. It is reccomended to not bring the chamber pressure up to ambiant, as this would break the seals and introduce air into the system once more, thus making the effort pointless. Thus only backfilling to a rough vacuum is reccomended. When combined with the above oil free vacuum pumping system, high quality layers my be produced with low to mid range sputter coaters at the cost of more gas and time usage.

High Vacuum

Some sputtering schemes require High Vacuum to opperate, such as Ion Beam Sputtering. If other types of sputtering are attempted at such low pressures the amount of power and or voltage needed to initiate a plasma discharge becomes too large to be feasable.

High end sputter coaters of the Cold Diode type are usually fitted with a high vacuum pumping system, which is used to evacuate the proscess chamber to a high vacuum, prior to being backfilled with the proscess gas for subsequent coating. This leads to a marked reduction in layer contamination in comparison to the more simple approch of using a single roughing pump.

Some low to mid range sputter coaters may be modified to opperate in this manner by means of an external High Vacuum pumping system connected instead of the simple roughing pump. These setups usually require the use of a Turbomolecular pump which is switched off prior to backfilling with the proscess gas, or a Diffusion pump which is isolated from the proscess chamber prior to backfilling.

The use of a turbomolecular pump is strongly reccomended due to strongly reduced oil contamination within the proscess chamber when compared to Diffusion pumps. IF a diffusion pump is to be used, the coating quality will be infirrior to that of Turbomolecular pump opperated systems due to minute quanteties of oil which backstream from the Diffision pump. This backstreaming can be further reduced in diffusion pump systems by means of a cold trap placed inline of the diffusion pump. This cold trap is typically cooled by liquid nitrogen and traps the oil molecules reaching it in line of sight by adhearing them to the cold surface, Thus preventing backstreaming.

For further details regarding pumping systems, their advanted, disadvantage and opperation see Vacuum Pumps.