Photomasks - Seeking Projection Perfection
In chipmaking, the same process is used, except that that hundreds of exposures are made, printing the same pattern stepwise on every chip across the wafer. To make the photomask, the litho equivalent of the photographic negative, the desired pattern is “written” into a layer of chemically-active photoresist, using a laser or electron beam. Using a high-vacuum plasma process, the pattern in the fragile photoresist is then etched into more durable metallic layers underneath to form the final photomask.
Photomask etch technology is therefore critical to the quality of the photomask and indeed to the success of the whole chipmaking process. The mask must be as close to perfect as possible: any defects will be faithfully reproduced on every single chip.
Recently Applied launched its new photomask etch system, the Applied Centura Tetra X, designed to keep the industry supplied with cutting-edge masks to the 22nm technology node and beyond. Read the press release for more information.
Applied’s Tetra systems have been used by mask makers worldwide to etch the vast majority of high-end masks over the last five years including virtually every 32nm node. Perhaps every smartphone should carry a “Tetra Inside” sticker?
Photomask inspection is vital to decide if a mask meets specification and is fit to be used to make pattern chips. If not, a lot of time and expensive silicon could be wasted.
However, mask inspection is not an easy task. Chipmakers have shrunk circuit features in pursuit or Moore’s Law to the point where chip features are now much smaller that the 193nm wavelength of the light used to project the mask image onto the wafer. Generally speaking, you can’t get a sharp image as the features size approaches the imaging wavelength – diffraction effects cause the image to blur until the pattern is lost.
Fortunately, lithographers have become quite adept at defying the laws of physics and deploy a battery of optical tricks to create sharply-defined features. Unfortunately, the pattern on advanced masks that use these “resolution-enhancement techniques" bear almost no resemblance to the final pattern on the wafer, so it’s hard to draw conclusions about a mask just be looking at it, no matter how closely you look.
The Aera3 system uses a technique called “aerial imaging” to sidestep all that complexity. The system essentially replicates the “projector” used to pattern the wafers but places an image sensor where the wafer would be. Thus, the system doesn’t inspect the mask itself, but the image the mask produces, providing “what you see is what you print” results.