Maskless DNA Array Synthesizer Using An Object-Plane Microlens Array
Publication Date: 2001-Mar-24
The IP.com Prior Art Database
An object-plane microlens array can significantly improve the spot size resolution of a maskless DNA array synthesizer by effectively reducing the SLM's fill factor and concentrating illumination energy at the center of each microspot. A similar optical design configuration could also function as a massively-parallel, confocal fluorescence microscope for chip readout.
Maskless DNA Array Synthesizer Using an Object-Plane Microlens Array
Author: Kenneth C. Johnson
An object-plane microlen array can significantly improve the spot size resolution of a maskless DNA array synthesizer by effectively reducing the SLM's fill factor and concentrating illumination energy at the center of each microspot. A similar optical design configuration could also function as a massively-parallel, confocal fluorescence microscope for chip readout.
Maskless lithography systems are useful for high-resolution digital printing and microfabrication applications such as the fabrication of oligonucleotide microarrays ("DNA chips"). A system of this type, the "maskless array synthesizer" (MAS), is currently under development by NimbleGen Systems. (The system is described in Nat Biotechnol 199 Oct; 17(10):953.) Fig. 1 shows an optical schematic of the system. Off-axis UV illumination 101 illuminates a spatial light modulator (SLM) 102, which is imaged by a projection system 103 onto an image plane 104. The DNA chip 105 that is being synthesized is positioned at the image plane. Individual SLM pixels are imaged onto corresponding microspots on the DNA chip, and the SLM exposure pattern determines the DNA base pair sequence that is synthesized at each microspot.
The projection system is preferably a unit-magnification Offner Catropic System. This system comprises two spherical mirrors, a large primary mirror 106 and a small secondary mirror 107. The axial displacement between the mirrors is D, and the SLM and image plane are both located at an axial displacement of 2·D from the primary mirror. Both mirrors are spherical (or approximately spherical). The curvature radius of the primary is 2·D, and the curvature radius of the secondary is D.
The SLM currently used in NimbleGen's MAS is a Digital Micromirror Device (DMD), manufactured by Texas Instruments. The DMD is an array of tilt-actuated micromirrors, each 16 microns square. Each micromirror has two tilt positions, an ON position in which the mirror is oriented to direct illumination into the projection system, and an OFF position in which the illumination is diverted out of the projection optics.
The DMD is designed to have a high pixel fill factor, with a gap of just 1 micron between micromirrors. However, the small gap is problematic for the MAS application because the printed microspots must be sufficiently well separated to be easily distinguishable by the chip reader. Although the microspot dimension is approximately 16 microns, the lithographic projection system and the chip reader must have micron-scale optical resolution to resolve the spaces between microspots.
The effective pixel fill factor can be reduced without overfilling the micromirror apertures by using the DMD in conjunction with an object-plane microlens array, as illustrated in Fig's. 2 and 3. The improvement comprises a microlens array 201 positioned at the projection system's object plane; see Fig. 2. Fig. 3...