GREYSCALE TARGETS

A greyscale image is one where each pixel in the image represents a different rate of intensity. These can be represented in many ways, but a general consensus is to use a range where 0% - is totally clear and 100% is totally black, with various ranges in-between. This does not cover all aspects, because with printing we also use the term 256 level greyscale, meaning 256 individual steps between white (clear) and black (opaque).

Our process is a binary one, whereby we can only image light on, light off. It is true that we can change the intensity of the light, but this can only be done on a whole mask basis – not pixel by pixel, so we cannot create a normal greyscale image as defined by "printers" or "photography". However, we DO have a solution that works in the vast majority of cases, and that is pixelation or dithering.

There are 2 different types of customers who want greyscale masks. One of these is optical - where the customer wants a section of the photomask to appear grey to an eye or camera/lens/video. This is the part that we can certainly help with because we use a differing - or halftone -  process and the trick is to make the pixel that creates the "grey" smaller than the lens can resolve (normally 50% smaller) - so the lens is tricked into thinking it is looking at a grey tone. This generally works very well.

The second type of customer needs a transmission grey - where different amounts of light are transmitted through the photomask to expose a photoresist underneath, usually building up 3D exposures. This method is more difficult to overcome with the dithering process because it depends upon the type and thickness of the resist used. It can partially work, but generally needs lots of experimentation with resist speeds, resist thickness, and pixel size by the customer.