Off-axis illumination
Encyclopedia
In photolithography
, off-axis illumination is an optical system setup in which the incoming light strikes the photomask
at an oblique angle rather than perpendicularly to it, that is to say, the incident light is not parallel to the axis of the optical system.
The advantages of off-axis illumination can be explained in the context where the pattern on the photomask is a diffraction grating
with a small pitch. The light that strikes the grating is diffracted in various directions. If the incident light is at a normal angle (along the axis of the optical system), then the zero-th diffracted order continues to be along the optical system axis, while the other orders are diffracted sideways, with the amount of deviation increasing as the pitch of the grating is decreasing. For sufficiently small pitch, only the zero-th diffraction order manages to make it through the projection lens, with the other orders being lost. The result is that no pattern is created on the wafer, since the zero-th diffraction order only contains the average of the photomask pattern.
By making the illumination off-axis, all the diffraction orders are tilted, which makes it more likely that the higher diffraction orders can make it through the projection lens and help form the image of the mask onto the wafer.
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
, off-axis illumination is an optical system setup in which the incoming light strikes the photomask
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
at an oblique angle rather than perpendicularly to it, that is to say, the incident light is not parallel to the axis of the optical system.
The advantages of off-axis illumination can be explained in the context where the pattern on the photomask is a diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
with a small pitch. The light that strikes the grating is diffracted in various directions. If the incident light is at a normal angle (along the axis of the optical system), then the zero-th diffracted order continues to be along the optical system axis, while the other orders are diffracted sideways, with the amount of deviation increasing as the pitch of the grating is decreasing. For sufficiently small pitch, only the zero-th diffraction order manages to make it through the projection lens, with the other orders being lost. The result is that no pattern is created on the wafer, since the zero-th diffraction order only contains the average of the photomask pattern.
By making the illumination off-axis, all the diffraction orders are tilted, which makes it more likely that the higher diffraction orders can make it through the projection lens and help form the image of the mask onto the wafer.