1. Field of the Invention
The present invention is related to the processing of light patterning devices.
2. Background Art
A patterning device is used to pattern incoming light. A static patterning device can include reticles or masks. A dynamic patterning device can include an array of individually controllable elements (or, Spatial Light Modulators) that generate a pattern through receipt of analog or digital signals. Example environments for use of the patterning device can be, but are not limited to, a lithographic apparatus, a maskless lithographic apparatus, a projector, a projection display apparatus, or the like.
Currently, arrays of Spatial Light Modulators (SLMs) can comprise various types of mirrors, including pistoning mirrors, tilting mirrors and tilting phase step mirrors.
Pistoning mirrors have a pure phase modulation effect, but amplitude modulation can also be obtained by combining piston mirrors into one large pixel. This results in a loss of resolution as well as limits the ability in replicating the effect of assist features (e.g., features intended to improve lithography on a customer wafer, for example, optical proximity correction features, serifs, hammerheads, scattering bars, anti-scattering bars, etc.) smaller than the largest pixel. There is also significant throughput loss with this approach.
Tilting mirrors are used to produce different amplitudes and/or phases of reflected light at an image plane and/or collected (captured) at projection optics. At different phases, an amplitude of reflected light, as seen at an image plane and/or collected at projection optics, is considered to have positive or negative light intensity. For example, when a mirror is untilted (e.g., resting) light at the image plane and/or collected at projection optics is considered to have a positive intensity with zero phase. During tilting of the mirror, there is a tilt angle at which no light is directed toward the image plane and/or is collected at projection optics, so the amplitude of the light at the image plane goes to zero. Then, as the mirror continues to tilt, out of phase light reaches the image plane and/or is collected at projection optics, which is considered to be negative light intensity or negative amplitude light.
One type of tilting mirror, as discussed above, is a phase step tilting mirror (λ/4 phase step), for example, proposed by Micronic Laser Systems of Sweden. When at rest a phase step mirror reflects no light to a pupil of a projection system because, due to the step, half the light has a zero degree phase and the other half of the light has a 180 degree phase. As the mirror is tilted, light is captured or collected by the projection system, where a direction of tilt determines the amplitude and/or phase of light that is captured or collected.
Processing phase step tilting mirrors requires at least two steps. First, flat SLM mirrors are created by MEMS techniques. Then, these SLM mirrors have the phase step either etched into the existing surface, or deposited as an increase in thickness by adding material. Applying photoresist and patterning the photo resist layer is not an effective procedure to produce phase steps on an array of SLM mirrors. Since there are gaps between adjacent mirrors in the array, photoresist applied to the entire array will have zones of varying thickness. These thickness defects in the photoresist layer can cause etching or deposition where it is not desired. In addition, there is also a severe yield risk to otherwise functional mirrors due to the wet processing steps involved with the photoresist process.
Therefore, what is needed is a is a method and apparatus for processing an array of SLM mirrors to produce a phase step in respective mirrors in the array.