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The dry cleaning process at a given laser wavelength shows good efficiency in the removal of selected organic contaminants from technological substrates. The cleaning efficiency obtained in the removal of metallic particulate contaminants from these same substrates varies according to the particle and substrate types. In general, the cleaning efficiency achieved with this process depends on the deposit/substrate combination and the wavelength and excellent results can be obtained it these ( and other process parameters) are properly selected.
The results of electron microscopy studies at MATS (UK) have shown that laser cleaning under the irradiation conditions undertaken does not inflict any crystalline damage to the underlying silicon.
We have also shown the efficiency of standard dry laser cleaning under various cleaning conditions for the removal of micron scale particles of gold, tungsten, copper and nickel on silicon, gallium arsenide and silicon oxide. The results have shown the process to be extremely efficient for the removal of gold, nickel and copper but less so for tungsten.
We have also shown that whilst wet cleaning methods are efficient in removing organic molecules from the surface of silicon they may leave behind undesirable organics as residues.
The successful development of a range of laser (and flash-lamp) power supplies of various average powers and levels of compactness.
The power supplies developed incorporate computer control (via RS232), power factor correction, thyristor or IGBT switching and various levels of safety interlocking. A standard 1.4kW PSU is now in use in the company's pulsed medical and large scale laser cleaning products. A high repetition rate 2.5kW version has also been specially developed for the company's next generation fibre-delivered large-scale laser cleaning product. These results could also be exploited in any commercial LMC device to power a power a high repetition rate flash-lamp-pumped laser micro-cleaning system.
Laser cleaning efficiency can be improved by altering the method of application of the laser pulse to the surface to be cleaned. This (together with laser wavelength, cover fluid type and laser fluence) is the main factor governing the efficiency of removal. Compared with application of the laser fluence at normal incidence, angular laser cleaning increases the area of the surface intersected by the laser and hence increases the removal rate. In laser shock cleaning, a completely different principle is used to generate the forces required for particle removal from the surface: the laser is fired parallel to the surface and the fluence is adjusted to produce laser induced breakdown of the ambient air. Shock waves from this event produce strong forces that allow particle removal in a manner that is independent of contaminant / substrate type and which minimises any substrate damage.
Details of the effect of laser wavelength and other operating parameters on laser cleaning efficiency for a range of contaminant / substrate combinations is shared by University of Liverpool, ARTT and Instituto Superior Tecnico.
When applying a liquid layer before the laser pulse, the mechanism involves thermal energy transfer along with explosive vaporisation finally resulting in the removal of particles from the surface.
With steam cleaning a cleaning efficiency of nearly 100% is achieved with a reduced number of pulses, provided the substrate is a strong absorber in the UV wavelength range, in order to ensure efficient superheating of the liquid film deposited onto it prior to each laser pulse. This process is clearly more efficient than dry cleaning in the removal of particulate contaminants from technological substrates, due to the ability to achieve complete removal with a reduced number of pulses, independently of the type of contaminant. Surface deterioration is also minimised, but it was evident that careful optimisation of the processing parameters is required to avoid contamination of the substrate by re-deposition of vaporised metals.
Laboratory demonstration systems for laser cleaning have been built at Liverpool, IST and ARTT based on excimer, Nd:YAG and ND:YAG pumped OPO sources.
These demonstrator prototypes could be used as the basis for commercial systems for laser cleaning in industrial concerns.
The successful use of a scanner in a laser beam delivery system to enable pulses of low energy and high repetition rate to mimic the (e.g.) cleaning effectiveness of high energy pulses at low repetition rates.
In effect, the scanner uses many small spots of a particular critical fluence to build up the same large spot size on the work piece that a high energy pulse can do in a single pulse at the same critical fluence. The lower pulse energies can be transmitted through fibres without damage, thereby simultaneously allowing remote operation and improved beam homogenisation. This result could be exploited in a commercial LMC device to clean the entire area of a semiconductor wafer.
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