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![7. Cleaning thresholds and process efficiency A systematic determination of cleaning thresholds in both DLC and SLC should provide key information for the application of laser cleaning, as it allows to predict the minimum particle size that ca n be removed and to judge which of the two processes DLC or SLC is more efficient. On the basis of our measurements this comparison ca n be done for the first time and for a large size interval of particles. Perhaps the most striking differenc e in the two laser cleaning methods is the dependence of the cleaning threshold fluence on particl e size. Whereas in SLC this threshold appeared to be universal, i.e. size- and material-independent for the investigated particles, in DLC we found in agreement with other authors a size dependent threshold, with smaller particles being harder to remove than larger ones. Fro m this it is obvious that SLC is a more efficient method for small particles, i.e. for particles smaller than about 400 nm in diameter (for particles larger than 400 nm see below ) which is the most interesting size regarding the cleaning of bare silicon wafers in the semiconductor industry. In addition, SLC is superior to DLC in the minimum particle size that could be cleaned from silicon wafers. Recalling that the current minimum line width in ICs is 13 0 nm, which means that particles of about 60-70 nm in size have to be removed, this is a key information on the quality of a cleaning method. The lower size limit of particles that could be remove d by DLC was found to be 110 nm, compared to 60 nm and an efficiency above 90% in SLC. Summarizing the above, SL C is superior to DLC due to three crucial characteristics: its universal cleaning threshold, its lower threshold fluences for the relevant particle sizes, and its capability of removing sub 100 nm-particles. 5.2. Consequences of cleaning mechanisms involved Although in DLC no particles smaller than 100 nm could be removed, at a first glance it seems to be the more appropriate method for larger particles as its cleaning thresholds are distinctly lower than th e universal SLC threshold. However, for a judgement of the perspectives of SLC and DLC it is not sufficient to solely determine and compare cleaning efficiency and laser cleaning threshold fluence. On the contrary, as our studies above show very clearly, this comparison must be put into perspective by taking a closer look at the cleaning mechanisms involved. The most important physical process not taken into account in traditional investigations and only recently [34, 35, 38-40] studied is the local substrate ablation due to the enhancement of the laser intensity in the near field of the particles. The first, and most obvious, consequence of field enhancement is a locally increased laser fluence underneath the particle, and hence a decrease in the incident laser fluence necessary for particle removal. At a first sight this looks like a positive effect, but obviously a locally enhanced laser intensity drastically lowers the threshold for surface damage, and indeed we did observe surface damage caused either by melting (small particles) or local substrate ablation (large particles)](https://images.tandf.co.uk/common/jackets/crclarge/978906764/9789067643764.jpg "7. Cleaning thresholds and process efficiency A systematic determination of cleaning thresholds in both DLC and SLC should provide key information for the application of laser cleaning, as it allows to predict the minimum particle size that ca n be removed and to judge which of the two processes DLC or SLC is more efficient. On the basis of our measurements this comparison ca n be done for the first time and for a large size interval of particles. Perhaps the most striking differenc e in the two laser cleaning methods is the dependence of the cleaning threshold fluence on particl e size. Whereas in SLC this threshold appeared to be universal, i.e. size- and material-independent for the investigated particles, in DLC we found in agreement with other authors a size dependent threshold, with smaller particles being harder to remove than larger ones. Fro m this it is obvious that SLC is a more efficient method for small particles, i.e. for particles smaller than about 400 nm in diameter (for particles larger than 400 nm see below ) which is the most interesting size regarding the cleaning of bare silicon wafers in the semiconductor industry. In addition, SLC is superior to DLC in the minimum particle size that could be cleaned from silicon wafers. Recalling that the current minimum line width in ICs is 13 0 nm, which means that particles of about 60-70 nm in size have to be removed, this is a key information on the quality of a cleaning method. The lower size limit of particles that could be remove d by DLC was found to be 110 nm, compared to 60 nm and an efficiency above 90% in SLC. Summarizing the above, SL C is superior to DLC due to three crucial characteristics: its universal cleaning threshold, its lower threshold fluences for the relevant particle sizes, and its capability of removing sub 100 nm-particles. 5.2. Consequences of cleaning mechanisms involved Although in DLC no particles smaller than 100 nm could be removed, at a first glance it seems to be the more appropriate method for larger particles as its cleaning thresholds are distinctly lower than th e universal SLC threshold. However, for a judgement of the perspectives of SLC and DLC it is not sufficient to solely determine and compare cleaning efficiency and laser cleaning threshold fluence. On the contrary, as our studies above show very clearly, this comparison must be put into perspective by taking a closer look at the cleaning mechanisms involved. The most important physical process not taken into account in traditional investigations and only recently [34, 35, 38-40] studied is the local substrate ablation due to the enhancement of the laser intensity in the near field of the particles. The first, and most obvious, consequence of field enhancement is a locally increased laser fluence underneath the particle, and hence a decrease in the incident laser fluence necessary for particle removal. At a first sight this looks like a positive effect, but obviously a locally enhanced laser intensity drastically lowers the threshold for surface damage, and indeed we did observe surface damage caused either by melting (small particles) or local substrate ablation (large particles)")
Chapter
7. Cleaning thresholds and process efficiency A systematic determination of cleaning thresholds in both DLC and SLC should provide key information for the application of laser cleaning, as it allows to predict the minimum particle size that ca n be removed and to judge which of the two processes DLC or SLC is more efficient. On the basis of our measurements this comparison ca n be done for the first time and for a large size interval of particles. Perhaps the most striking differenc e in the two laser cleaning methods is the dependence of the cleaning threshold fluence on particl e size. Whereas in SLC this threshold appeared to be universal, i.e. size- and material-independent for the investigated particles, in DLC we found in agreement with other authors a size dependent threshold, with smaller particles being harder to remove than larger ones. Fro m this it is obvious that SLC is a more efficient method for small particles, i.e. for particles smaller than about 400 nm in diameter (for particles larger than 400 nm see below ) which is the most interesting size regarding the cleaning of bare silicon wafers in the semiconductor industry. In addition, SLC is superior to DLC in the minimum particle size that could be cleaned from silicon wafers. Recalling that the current minimum line width in ICs is 13 0 nm, which means that particles of about 60-70 nm in size have to be removed, this is a key information on the quality of a cleaning method. The lower size limit of particles that could be remove d by DLC was found to be 110 nm, compared to 60 nm and an efficiency above 90% in SLC. Summarizing the above, SL C is superior to DLC due to three crucial characteristics: its universal cleaning threshold, its lower threshold fluences for the relevant particle sizes, and its capability of removing sub 100 nm-particles. 5.2. Consequences of cleaning mechanisms involved Although in DLC no particles smaller than 100 nm could be removed, at a first glance it seems to be the more appropriate method for larger particles as its cleaning thresholds are distinctly lower than th e universal SLC threshold. However, for a judgement of the perspectives of SLC and DLC it is not sufficient to solely determine and compare cleaning efficiency and laser cleaning threshold fluence. On the contrary, as our studies above show very clearly, this comparison must be put into perspective by taking a closer look at the cleaning mechanisms involved. The most important physical process not taken into account in traditional investigations and only recently [34, 35, 38-40] studied is the local substrate ablation due to the enhancement of the laser intensity in the near field of the particles. The first, and most obvious, consequence of field enhancement is a locally increased laser fluence underneath the particle, and hence a decrease in the incident laser fluence necessary for particle removal. At a first sight this looks like a positive effect, but obviously a locally enhanced laser intensity drastically lowers the threshold for surface damage, and indeed we did observe surface damage caused either by melting (small particles) or local substrate ablation (large particles)
DOI link for 7. Cleaning thresholds and process efficiency A systematic determination of cleaning thresholds in both DLC and SLC should provide key information for the application of laser cleaning, as it allows to predict the minimum particle size that ca n be removed and to judge which of the two processes DLC or SLC is more efficient. On the basis of our measurements this comparison ca n be done for the first time and for a large size interval of particles. Perhaps the most striking differenc e in the two laser cleaning methods is the dependence of the cleaning threshold fluence on particl e size. Whereas in SLC this threshold appeared to be universal, i.e. size- and material-independent for the investigated particles, in DLC we found in agreement with other authors a size dependent threshold, with smaller particles being harder to remove than larger ones. Fro m this it is obvious that SLC is a more efficient method for small particles, i.e. for particles smaller than about 400 nm in diameter (for particles larger than 400 nm see below ) which is the most interesting size regarding the cleaning of bare silicon wafers in the semiconductor industry. In addition, SLC is superior to DLC in the minimum particle size that could be cleaned from silicon wafers. Recalling that the current minimum line width in ICs is 13 0 nm, which means that particles of about 60-70 nm in size have to be removed, this is a key information on the quality of a cleaning method. The lower size limit of particles that could be remove d by DLC was found to be 110 nm, compared to 60 nm and an efficiency above 90% in SLC. Summarizing the above, SL C is superior to DLC due to three crucial characteristics: its universal cleaning threshold, its lower threshold fluences for the relevant particle sizes, and its capability of removing sub 100 nm-particles. 5.2. Consequences of cleaning mechanisms involved Although in DLC no particles smaller than 100 nm could be removed, at a first glance it seems to be the more appropriate method for larger particles as its cleaning thresholds are distinctly lower than th e universal SLC threshold. However, for a judgement of the perspectives of SLC and DLC it is not sufficient to solely determine and compare cleaning efficiency and laser cleaning threshold fluence. On the contrary, as our studies above show very clearly, this comparison must be put into perspective by taking a closer look at the cleaning mechanisms involved. The most important physical process not taken into account in traditional investigations and only recently [34, 35, 38-40] studied is the local substrate ablation due to the enhancement of the laser intensity in the near field of the particles. The first, and most obvious, consequence of field enhancement is a locally increased laser fluence underneath the particle, and hence a decrease in the incident laser fluence necessary for particle removal. At a first sight this looks like a positive effect, but obviously a locally enhanced laser intensity drastically lowers the threshold for surface damage, and indeed we did observe surface damage caused either by melting (small particles) or local substrate ablation (large particles)
7. Cleaning thresholds and process efficiency A systematic determination of cleaning thresholds in both DLC and SLC should provide key information for the application of laser cleaning, as it allows to predict the minimum particle size that ca n be removed and to judge which of the two processes DLC or SLC is more efficient. On the basis of our measurements this comparison ca n be done for the first time and for a large size interval of particles. Perhaps the most striking differenc e in the two laser cleaning methods is the dependence of the cleaning threshold fluence on particl e size. Whereas in SLC this threshold appeared to be universal, i.e. size- and material-independent for the investigated particles, in DLC we found in agreement with other authors a size dependent threshold, with smaller particles being harder to remove than larger ones. Fro m this it is obvious that SLC is a more efficient method for small particles, i.e. for particles smaller than about 400 nm in diameter (for particles larger than 400 nm see below ) which is the most interesting size regarding the cleaning of bare silicon wafers in the semiconductor industry. In addition, SLC is superior to DLC in the minimum particle size that could be cleaned from silicon wafers. Recalling that the current minimum line width in ICs is 13 0 nm, which means that particles of about 60-70 nm in size have to be removed, this is a key information on the quality of a cleaning method. The lower size limit of particles that could be remove d by DLC was found to be 110 nm, compared to 60 nm and an efficiency above 90% in SLC. Summarizing the above, SL C is superior to DLC due to three crucial characteristics: its universal cleaning threshold, its lower threshold fluences for the relevant particle sizes, and its capability of removing sub 100 nm-particles. 5.2. Consequences of cleaning mechanisms involved Although in DLC no particles smaller than 100 nm could be removed, at a first glance it seems to be the more appropriate method for larger particles as its cleaning thresholds are distinctly lower than th e universal SLC threshold. However, for a judgement of the perspectives of SLC and DLC it is not sufficient to solely determine and compare cleaning efficiency and laser cleaning threshold fluence. On the contrary, as our studies above show very clearly, this comparison must be put into perspective by taking a closer look at the cleaning mechanisms involved. The most important physical process not taken into account in traditional investigations and only recently [34, 35, 38-40] studied is the local substrate ablation due to the enhancement of the laser intensity in the near field of the particles. The first, and most obvious, consequence of field enhancement is a locally increased laser fluence underneath the particle, and hence a decrease in the incident laser fluence necessary for particle removal. At a first sight this looks like a positive effect, but obviously a locally enhanced laser intensity drastically lowers the threshold for surface damage, and indeed we did observe surface damage caused either by melting (small particles) or local substrate ablation (large particles)
ABSTRACT
A second consequence of field enhancement also argues against a technological application of DLC. The DLC results obtained both in ambient conditions and in HV confirm a general trend already obtained by other authors [6, 7, 19, 31, 33, 64] and predicted by their models: cleaning thresholds for smaller particles tend to be higher than for larger ones. Yet this is not a strict rule. In contrast to the above cited experiments we used a large variety of particle diameters ranging from 110 nm to 4100 nm and could show that the dependence of the cleaning threshold as a function of the particle diameter was non-monotonous as a consequence of the optical resonances in the near field of the particles. This non-monotonous behaviour in DLC makes it difficult to apply the correct cleaning fluence for the removal of a specific particle size, unlike the universal threshold in SLC.
