The invention relates generally to digital microprinters and more specifically to digital microprinters such as maskless projection lithography systems that use spatial light modulators (SLMs) for optical pattern generation (Refs. 1-6).
Such microprinters can be adversely affected by pixel cross talk resulting from coherent interactions between adjacent pixels. Such interactions could be mitigated by spacing the SLM pixels sufficiently far apart that there is no significant overlap between adjacent pixels"" point spread functions at the image plane. Or alternatively, pixel exposures could be sequenced so that adjacent pixels are never on at the same time. However, these approaches can result in very low optical efficiency because illumination energy that intercepts the SLM between pixels, or on pixels that are in their off state, is wasted. Printing speed is limited by optical efficiency, so it would be desirable to use closely spaced pixels and to allow adjacent pixels to be simultaneously on in order to optimize efficiency.
This invention provides an optical technique for mitigating pixel cross talk resulting from coherent interactions between adjacent pixels in a digital microprinter. No loss in optical efficiency or printing speed is incurred.
The invention is applicable to a printer system that uses a spatial light modulator (SLM) for image generation. The SLM comprises an array of source pixel elements, each of which is digitally controlled to modulate illuminating radiation intercepting the pixel. Either a transmission-type or reflection-type of SLM can be used. A projection system conveys the modulated radiation to a printing surface wherein each source pixel generates a corresponding modulated exposure field that exposes the surface. (In some embodiments the SLM may be imaged directly onto the printing surface. Alternatively, the SLM may be imaged onto an intermediate image plane proximate to a microlens array, wherein each microlens condenses the illumination from a corresponding source pixel onto a focal point on the printing surface.) Due to the projection system""s limited optical resolution, adjacent pixels"" exposure fields may overlap, resulting in pixel cross talk effects due to coherent interaction between the overlapping fields.
The coherent effects are significantly mitigated by configuring the SLM to induce an optical phase shift on alternate pixels, wherein the phase shift induces a phase offset of approximately xcfx80/2 between adjacent pixels"" exposure fields. (Typically, the phase shift would be exactly xcfx80/2 at a specific wavelength within the illumination""s spectral band.) If the exposure fields (without the phase shift) have a uniform phase distribution across the printing surface, this technique can substantially eliminate the coherent interaction between adjacent pixels. If the phase distribution is not uniform, an alternative technique can be employed in which two exposures are made, with the positional relationship between the SLM and printing surface altered between exposures so that the induced phase offset is sign-inverted.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.