Photomasks: Made to be Used with Care

When I sat down at the beginning of January and reviewed the requests for technical support I’d received from customers during 2014, one topic stood out from the rest – mask handling and cleaning.

The lithography processes used by our customers, regardless of sector, demand that the mask has a level of pattern integrity that enables devices to be manufactured without any imperfections which will affect performance. Maintaining mask quality is a challenge and while photomasks are robust, they are not indestructible.

So what advice can we offer to customers that will prolong the life of your masks, maximise your productivity and minimise your costs?

There are some material characteristics that you should be aware of when handling and cleaning photomasks. The choice of substrate, typically between soda lime glass and quartz, is often made on the difference in thermal expansion coefficient (94 vs. 5 x10-7/°C) or % transmittance at short wavelengths, but the difference in hardness of the substrate material (540 vs. 615 kg/mm2 measured using the Knoop hardness test) may be significant in specific applications such as hard contact.

The opaque film, composed of a bright chrome ‘base’ layer with a low-reflective chrome oxide ‘top’ layer, is approximately 100nm thick. The low-reflective ‘top’ layer will be the first to be removed by the mask cleaning process, which will result in a progressive increase in reflectivity. As the chrome oxide layer does not contribute to the opacity of the overall thin film, this change in appearance is unlikely to affect the print performance characteristics of the photomask. Nevertheless, as the chrome oxide coating is approximately it is 1/10 or less of the total film thickness, the change in appearance when compared to the number of cleaning cycles the mask has been exposed to can be used to give some indication of the rate of chrome removal and therefore the potential life of the photomask.

Routine inspection, using either the naked eye combined with a bright-light source, a simple stereomicroscope with dark-field illumination or more sophisticated automatic inspections systems, is essential to gain an understanding of any degradation in mask quality with use and the effectiveness of any changes to current practice you may consider.

“Prevention is better than cure” is an old saying but still true. Minimising particulate through good handling is always preferable to removing contamination by repeated cleaning. With so much choice out there, selecting the right cleanroom gloves for your application can be a challenge. Having been through this process, we are happy to share our selection criteria and choices with you. If you are interested in more information please get in touch. Handling masks only by the edges or alternatively using a mask pick will also help reduce contamination levels. Masks used in contact will become more contaminated than those used in proximity or projection systems, as they will suffer from direct transfer from the substrates being imaged in addition to any handling or environmental contaminants.

The cleaning processes used during the mask manufacturing process at Compugraphics are based on combining both chemical and physical methods. A mixture of Sulphuric Acid and Hydrogen Peroxide is used to remove photoresist and organic contaminants. Followed by a DI water rinse, high pressure spray, a rotating PVA sponge clean then a final DI water rinse and spin dry to complete the process. While we use purpose designed automatic single substrate mask cleaning tools, dispensing chemistry in a ‘one-shot’ mode, with application it is possible to replicate this mask cleaning sequence in a ‘laboratory style’ process for lower volumes. The ratio of Sulphuric Acid to Hydrogen Peroxide, the temperature of the acid/peroxide chemistry and the duration of this step will affect both the effectiveness of the clean and rate of chrome removal. Like all processes, these parameters need to be controlled to ensure consistency of results.

We are aware that subsequent handling and cleaning can affect the long-term performance of the mask and ultimately customer satisfaction. Sharing our experience with you is one thing we can do to help.

Written by Neil Holmes, Continuous Improvement & Technical Support Manager

Neil holmes