SPUTTER COATING IN EM: A TECHNICAL BRIEF
When a target is bombarded with fast heavy particles, erosion of the target material occurs, this is termed sputtering. The arrangements of the systems are such that some of the sputtered atoms will condense on the surface of the specimen to be coated.
The above process occurs under conditions of a gaseous glow discharge between an anode and cathode. It can be enhanced by the choice of a suitable gas and target material, which together with other developments of the technique, allows the deposition of a suitable coating to increase the electrical conductivity and increase "Z" number for greater resolution. These are common and important requirement for Scanning Electron Microscopy.
The development of sputter coating systems embodies significant empirical design features, however, an understanding of terms such as "glow discharge characteristics" are important to these systems and may assist in the comparison of differing systems.
If an inert gas such as argon is included in a cathode gas tube, the free ions and electrons are attracted to opposite electrodes and a small current is produced. As the voltage is increased some ionisation is produced by collision of electrons with gas atoms, the 'Townsend' discharge. When the voltage across the tube exceeds the breakdown potential, a self sustaining glow discharge occurs, characterised by a luminous glow.
The current density and voltage drop remains relatively constant, the increase in total current being satisfied by the area of the glow increasing. Increasing the supply voltage increases current density and voltage drop, this is the abnormal glow region. Further increase in supply voltage concentrates the glow into a cathode spot and arc discharge is apparent. The operating parameters of sputter coaters are within the glow discharge regions of the characteristic described.
Once the condition for a sustained discharge is met, the tube exhibits the characteristic glow discharge, so called because of the associated luminous glow. It has been established that free ions and electrons are attracted to opposite electrodes producing a discharge; however, for a discharge to be self-sustaining requires regeneration of the electrons by the positive ion bombardment of the cathode. This produces secondary electrons and enhances ionisation. The resulting positive ion excess creates a positive space charge near the cathode. The voltage drop experienced is termed the cathode fall. If the discharge is established in a long narrow tube it exhibits the characteristics indicated.
The positive ion density in the Crookes dark space is very high, as a result a significant voltage drop is experienced between it and the cathode. The resulting electric field accelerates the positive ions which produce secondary electron emission from the cathode. These electrons are accelerated in the direction of the anode and cause ionisation, generating positive ions to sustain a discharge. Subsequently, excitation of the gas results in intense illumination in the negative glow region. From this stage the electrons have insufficient exciting or ionising energy, resulting in the Faraday dark space. Towards the anode, a small accelerating field can produce ionisation and excitation, the gas again becoming luminous.
It has been indicated that under conditions of glow discharge, ion bombardment of the cathode will occur, this results in the erosion of the cathode material and is termed plasma sputtering, the subsequent omni-directional deposition of the sputtered atoms forming coatings of the cathode material.
This process is enhanced in sputter coaters for use in SEM. For these applications the objective is the provision of an electrically conductive thin film representative of the surface topography of the specimen. Such films inhibit 'charging', reduce thermal damage, and enhance secondary electron emission.
The most common arrangement for a D.C. (Direct Current) sputter coater is to make the negative cathode the target material to be sputtered (typically gold) and the location of the specimens the anode (which is usually 'earthed' to the system, so the specimens are effectively at 'ground' potential). The desired operating pressure (relative vacuum) is obtained usually a two stage rotary pump. An inert gas, such as argon, is admitted to the chamber by a fine control valve.
The glow discharge in sputtering is significantly dependent on the work function of the target material and pressure of the environmental gas. A range of target materials are used including gold, gold-palladium, platinum and silver, although gold is the most common having the most effective electrical conductivity characteristics. The sputter head and sputter power supply should be effective over a range of anticipated target materials. The deposition rate is current dependant and when operated in the correct glow region of the characteristic previously described, several fold changes in current should be available for a relatively small change in sputtering voltage. The deposition rate should not be sensitive to small changes in pressure which may be experienced in the system.
If an efficient sputter head design, operating on low voltage and as a result low energy input, is achieve, then radiant heating from the target and high energy electrons, (potentially the most significant sources of damage to delicate specimens) should be considerably reduced.
There is evidence that such a sputter head system may also produce finer grain size for a given target material. The presence of an inert gas which will not decompose in the glow discharge is obviously desirable. Argon, having a relatively high atomic weight, provides a suitable source of ions for effective bombardment of the target material. The effectiveness is also dependent on the mean.
The Micrograph (shown on right) is 3-day old concrete, freshly fractured. This is a typically difficult sample as the surface is highly granular and uneven and therefore susceptible to charging during SEM. However, after coating in the K550 such problems were not encountered. (Coating conditions: Gold, 20mA, 2 minutes, 0.1 torr, coating thickness 11nm)
It is, of course, possible to satisfy very precise parameters by the selection of target material, voltage deposition current and vacuum. Under these conditions, it is possible to achieve thin films to 10nm with grain sizes better than 2nm and temperature rises of less than 1°C.
The application of sputter coating has been well established. However, the improved performance of conventional Scanning Electron Microscopes requires the enhanced capabilities of EMITECH series of sputter coaters. Cathode target material is commonly gold, however, to achieve finer grain size, and thinner continuous coatings for Field-emission SEM (FSEM), it is advantageous to use cathode target materials such as chromium. To achieve sputtering with this target material requires vacuums better than those achievable with a Rotary Vacuum Pump and best provided by a turbo pump system.
For chromium coating these systems require further refinements and they are offered as "Chromium Sputter Coaters".
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