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CGH Photomasks
General Description
CGH Photomasks are the basis of our diffractive optic products and are also used directly as diffractive optical elements. CGH Photomasks are encoded using our HoloMask software and written using a MEBES e-beam lithography system. The MEBES is the standard photomask tool in the microelectronics industry. In resolution, accuracy and format size, it is unsurpassed for CGH writing. Unfortunately, the MEBES pattern vocabulary is limited to trapezoids. Our HoloMask software accurately encodes a diffractive optic element using a minimum number of these trapezoids.
Resolution and Feature Size
Patterns are written in a raster manner. The range of address resolution, or pixel size, is 0.10 to 1.1 micron. Common address sizes are 0.50, 0.25 and 0.10 micron.
Because of exposure proximity effects, the minimum feature size should be not less than 5 pixels. This is seldom a limitation for CGH patterns since a minimum of 10 pixels per grating period (5 pixels per line or space) is required to achieve a typical recording accuracy of 0.1 fringe. Feature sizes below 0.7 microns are extremely difficult to control in processing.
Aperture
Within a rectangular data field, apertures of any shape may be encoded. Patterns may not exceed the MEBES physical reticle limit of 155x155 mm.
Optical Prescription
The optical phase function is specified by an equation. Virtually any functional form F(x,y) can be accommodated. Simple zone plate lenses may be specified by giving the focal length and wavelength. More general phase functions are described by radial or cartesian monomials or by Zernike polynomials. We will accept Sweatt model wavefront prescriptions. We can also write a custom wavefront evaluation subroutine or link to your code.
Most often, phase function coefficients are generated by an optical design program. We are familiar with Code V, OSLO and ZEMAX formats. If your phase function requires a large number of coefficients, we ask that you supply it by e-mail.
Units and Dimensions
We usually design in millimeters but can accomodate any unit of measure. Wavelength is in microns or nanometers. The MEBES works in microns.
Artwork Generation
In addition to the phase function, other image elements such as fiducials, title blocks, etc. may be included. A conversion program for customer generated AutoCAD DXF files is available, or we can produce CAD layouts from your sketches. Points and zero width lines are ignored. Text is converted to a monospace font.
Multiple images may be combined to form an overall pattern. Images may overlap and can be either positive (digitized area exposed/clear) or negative (digitized area unexposed/dark). It is difficult to add dark features within a positive pattern or clear features within a negative pattern. The MEBES writes most efficiently when patterns repeat in a rectangular array. This is often true of lenslet arrays or linear gratings.
Pattern Accuracy
Overall pattern accuracy is determined by encoding, digitization, and registraton errors. The registration accuracy of the MEBES is better than 0.15 microns (3 sigma) for an address size of 0.25 micron. Allow for somewhat less accuracy when using larger address sizes. Encoding accuracy is user specified in phase and/or linear units. Tighter accuracy specs will result in larger data sets and longer write times.
Complexity
Because of the inefficiency of the MEBES data format in describing CGHs, greater time may be required to input and rasterize the data than in actual writing. It is relatively easy to specify a CGH pattern which, while possible, would exceed most budgets. Pattern complexity depends on a number of factors including pixel size, encoding accuracy, and encoding algorithm. For off-axis CGHs, encoding efficiencies result when fringes are oriented parallel to the y axis. We strive to select the most cost effective encoding parameters to meet your requirements.
Verification
Our encoding and digitization algorithms are rigorously verified through both internal and external checks. Violations of user specified encoding tolerances are reported to a log file.
Phase function coefficients are echoed to a log file. A table of phase function values at customer specified aperture locations can also be generated.
Test plots of any pattern region can be provided. Please specify plot scale. We offer AutoCAD DXF plots on floppy disk or letter size 300 dpi laser plots.
Nikon 2i marks are usually included in CGH patterns. Their locations may be measured to verify positional metrology.
Substrates
We use synthetic fused silica photomask blanks because of their ultra-low thermal expansion, high index homogeneity, and superior etching properties. These are coated to an optical density of 3.0 with either anti-reflective (11%) or bright (55%) chrome.
Photomask sizes are listed below. We use commercial blanks of the highest available flatness grades. The flatness spec refers to the face side, although the back side is typically of similar quality. Flatness is better over smaller apertures.