Overview
Optical lithography requires the fabrication of a mask. Generally, photomasks consist of a piece of glass of some type, coated with a film in which the pattern is formed. A layer of sputtered Cr about 100 nm thick coats the glass plate. Resist is then spun on the plate, and the exposure is made. After development, the Cr is removed from the unprotected areas with an acid etch, and an image of the pattern is left in the Cr. The exposure can be made with either an optical or an electron beam lithography tool. The lithography requirements at CNF usually allow you to make a mask faster, cheaper, and more easily with optical tools than with an electron beam lithography tool. There are some specific exceptions to this that are covered in the electron beam lithography mask making section.
Optical Mask Making Tools
The CNF currently operates two optical mask making tools: a GCA/Mann 3600F Pattern Generator (PG) and a Heidelberg Instruments DWL 66. General information on these two tools is provided below. Information on selecting which of these tools is better suited for your mask making needs is covered in a pdf file entitled Chosing an Optical Pattern Generator. In addition to these two tools, two of the CNF steppers can be used in conjunction with the PG and DWL 66 to further reduce the amount of time to takes to produce masks for certain applications.
GCA/Mann 3600F Pattern Generator (PG): The PG exposes variably sized rotated rectangles onto the mask blank defined by a set of precision controlled moveable aperture blades. Pattern data is fractured into a format such that the entire pattern can be described by these rotated rectangles. Consequently, pattern write time is entirely dependent on the both the complexity and amount of exposed areas. The rate of exposure is usually in the range of 4000 - 5000 exposures/hour where each exposure is one rotated rectangle.
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Heidelberg Instruments DWL 66: The DWL 66 is a high-resolution direct-write optical imaging system where over half a million dpi is achieved using a 40-nanometer address grid for exposing chrome plates or wafers. The DWL 66 will accommodate substrates up to 8 x 8 inches. Design data is produced in CAD and is converted into a proprietary data format. There are two write lenses, with a 2 µm or 0.5 µm spot size. For the 2 µm spot size a mask can be exposed at a rate of 1 square inch every 20 minutes regardless of the amount of exposed area. Using the .5 um spot size the write time is on the order of 1 square inch every 1.5 hrs. The write times are fairly insensitive to the complexity of the pattern data. However, large arrays of repetitive shapes can take up to 5 times longer to expose than normally expected.
Click here for more information on the DWL 66.
GCA/Mann 6300F in Photorepeater Mode: Both GCA 6300 steppers have a mode in which resist-coated chrome masks can be exposed using a 5:1 or 10:1 "master." This is useful for masks on which an individual pattern is repeated many times across a mask. Good examples are arrays of dots, and large field gratings, which are time-consuming to make full size on the PG or DWL 66. Making a subset of the array on one mask using the PG or DWL 66, and then repeating it in this mode on another mask using the stepper can be much less expensive. See the appropriate staff member for details if you think your pattern fits in this category.
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Electron Beam Lithography Mask Making Tools at the CNF
Electron beam lithography (EBL) provides a flexible high resolution tool for producing photomasks with feature sizes well below 50 nm. The EBL tools at the CNF are optimized for high resolution, not throughput. Using these EBL systems to produce most photomasks is not cost effective or an efficient use of time. However, these tools are the option of choice for making masks for next generation lithography exposure systems. Some specific examples include:
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Nanoimprint lithography templates
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Stencil masks for charged particle lithography
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Reflective masks for extreme ultraviolet lithography (EUVL)
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Advanced masks for phase shift and optical proximity corrected (OPC) deep UV (DUV) photolithography
Information on our EBL lithography capabilities can be found on the EBL Lithography Process Technology pages and on the Advanced Mask Making Page.
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Mask Materials Considerations
It is sometimes important to consider the type of glass out of which the mask is made. There are two considerations here: the thermal expansion of the glass and its transmission at the exposure wavelength.
Thermal coefficients for different types of glass are:
Soda-lime: 9.3 ppm/˚C
Borosilicate: 3.7 ppm/˚C
Quartz: 0.5 ppm/˚C
The worst case here is soda-lime glass, which gives a 1.2 µm change across a 5 inch mask for every 1˚C variation in temperature. This is about twice the PG image placement error, but we still use soda-lime in most cases because it's much cheaper. Thermal effects are limited because the PG is in an environmental chamber (as are the steppers), and it uses a temperature compensation scheme.
Quartz glass is usually used for masks made on electron beam tools. Since the masks are exposed under vacuum, heat is not transferred as readily, so a lower expansion glass is desirable. These masks are about twice as expensive as soda-lime.
Deep UV exposures require fused Silica instead of other types of glass, because glass begins to absorb strongly at wavelengths below about 350 nm. However, fused Silica (quartz) masks are several times more expensive than glass.
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