Manufacturing - Lithography
to note the difference in thermal expansion coefficient between quartz and soda lime glass, with quartz glass being an order of magnitude better. For soda lime, a 10C change in temperature results in a 1um expansion over a 4” distance. If soda lime glass is being used, it is useful to advise the photomask manufacturer of the temperature at which the photomask will be used, as this can be compensated for as the photomask is written. If the temperature is likely to vary while the mask is in use, or if the mask is being used with a deeper UV wavelength (i-line or lower) then quartz material should always be used. There is no such thing as a defect free photomask. The question is: at what level of detail do you want to inspect the mask? At some, possibly sub-micron level, there will always be some defects caused by imperfections in the photomask blank, the coating or the development and etching process. In the case of reticles or 1X photomasks with no repeating patterns, a typical defect specification would be “zero defects greater than x microns”. In the case of advanced photomasks “x” might be hundreds of nanometres, and for semiconductor photomasks “x” will usually be 1 micron or less. If there are repeating patterns, then the mask defectivity is usually expressed as being a maximum acceptable level of “y” defects per square “unit” at “z” microns, where “y” is the number of allowable defects, “z” is a defect size in microns, and “unit” is typically “cm” or “inch”. (e.g. 1 defect per square inch at 1 micron). In some causes it is also desirable to use both a defect density specification and a maximum allowable defect size, e.g. 1 defect per square inch at 1 micron and 0 defects > 5um. Any defects detected outside of this specification can either be repaired or the photomask rejected and rewritten. If there is a repeated pattern, and the mask is only being used for development or research purposed, the existence of small numbers of defects may be non-critical. If this is
the case the customer can request no defect inspection, and thus save the cost associated with a significant process step. Again ordering photomasks with no inspection is most practical when you are using a major photomask manufacturer where the SPC can show that all of their masks are being manufactured to very low levels of defectivity.
It can also be very useful to advise the photomask manufacturer what type of technology the mask will be used for. Examples are optical wave guide masks where high resolution when writing the mask is important in achieving smooth curved features; or surface acoustic wave devices where any breaks or bridges in the fingers can cause fatal device errors. Larger manufacturers will already be familiar with many similar issues, based on the extensive experience of their workforce. Once all of the technical information has been assembled, there are a number of methods for passing it on for manufacture. For many smaller customers, the instruction to apply the photomask manufacturer’s standards may be sufficient (although some items such as mask titles, tone required and a minimum feature size are always required). Larger manufacturers offer online ordering systems which can capture the data required in a user friendly manner. If ordering large quantities of masks, standards such as the SEMI P10 format can be used, but the software effort required to generate this data is probably hard to justify for smaller customers.
Getting the photomasks you desire is never a trivial task, but some consideration to the requirements and qualification options required can pay dividends in achieving cheaper and reliable photomasks.
© 2011 Angel Business Communications. Permission required.
Gordon Hughes is the CAD Systems Manager at the Compugraphics International Glenrothes site. He graduated with a BSc in Computer Science from Edinburgh University. Following graduation, continued to work for Edinburgh University developing and supporting their CAD tools that were used as part of an MSc course. He then joined Lattice Logic (subsequently) European Silicon Structures where he developed the Shapesmith software for the processing of mask data. He has been responsible for the CAD and Data Processing systems within Compugraphics for over 20 years, pursuing his interest in automation of the data processing and customer requirements from order forms and specifications.
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