Alignment technique for masked ion beam lithography

Abstract

The ion beam which performs the printing on the resist through the mask is also used to perform the alignment function. Alignment marks are initially provided on the wafer of a material which emits light when an ion beam impinges thereon, such as silicon dioxide. An ion mask, preferably of silicon, is then positioned over the wafer and alignment marks and ions are directed to the alignment marks through the mask. The degree of alignment is determined by the amount of light emitted by the alignment marks since more ions will strike the alignment marks with increased alignment. The emitted light is detected and +X, -X, +Y, -Y and +theta and -theta error signals are provided on a continuous basis for closed loop control of the mask relative to the wafer under proper alignment is achieved. The control is provided by scanning the emitted light from six appropriately positioned alignment marks on the wafer, comparing the intensity of the emitted light in terms of +X amd -X signals to provide an X error signal, doing the same with the Y and theta signals to provide Y and theta error signals and then digitizing each of the error signals in a digital to analog converter. The digitized signals are then applied to a central processing unit wherein they are analyzed in accordance with a predetermined program and provide signals to a motor controller. The motor controller proceeds to provide output signals to properly position the wafer in the X and Y directions as well as to rotate the mask to provide the desired alignment between the mask and wafer.