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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 Heidelberg Instruments DWL 66fs and a Heidelberg Instruments DWL 2000.  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 Choosing an Optical Pattern Generator.  In addition to these two tools, one of the CNF steppers can be used in conjunction with the Heidelbergs to further reduce the amount of time to takes to produce masks for certain applications.

 

Heidelberg Instruments DWL 66fs:The DWL 66FS is a high-resolution imaging system where over a million dpi is achieved using a 25-nanometer writeable address grid for exposing metallized plates or wafers. The DWL 66FS will accommodate media up to 6 x 6 inches. Design data can be created with any program using DXF, HPGL, Gerber, GDSII, or CIF format and is converted into a LIC format on a “CONVERT“ workstation. Design data transfer to the system is then realized via an FTP connection.

The writing is performed using a combination of raster and scanning techniques. Substrates are mechanically moved beneath the writing lens in one axis while the beam is scanned in the transverse direction. The resolution of the tool and the width of the scan are determined by the final lens. CNF has 10 mm working distance lens that produces a 2.4 um diameter spot.

Due to the writing strategy, exposures are independent of the complexity of the patterned features. Shapes that have been traditionally difficult to pattern at the CNF-circles, curves, spirals and arcs-can be written efficiently and accurately using the DWL 66FS. This capability should be of great benefit to research in microfluidics and integrated optical devices. An example result is shown in the following figures.

The DWL 66FS also provides a comprehensive set of metrology functions including line width, overlay, stitching and pattern placement measurements. Front to backside alignment is also possible for direct write applications using a video backside alignment system.

 

Click here for more information on the DWL 66fs.

 

Heidelberg Instruments DWL 2000: The Heidelberg Instruments DWL 2000 is an economical, high resolution direct write pattern generator for direct writing on photosensitive Cr mask plates. The tool can handle mask sizes from 3 to 9 inches. The unit has an interchangeable write head that can accommodate different feature sizes for writing. The smallest feature size is 0.7 microns and 0.6 microns with the 4mm and 2mm write heads, respectively.

DWL 2000 system consists of the main system unit, an XP-based PC for running job files, a Linux based PC for data conversion and a pattern generator (located in the service chase). The tool accepts files of various format, for best results use the semiconductor standard GDSII format.

 

Click here for more information on the DWL 2000.

 

 

Photorepeater Mode: The GCA AS200 stepper has a mode in which resist-coated chrome masks can be exposed using a 5:1 "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 laser writers.  Making a subset of the array on one mask using onof the Heidelbergs, 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.

 

Click here to go back to the CNF Process Technologies page.

 

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:

  • Nanoimprint lithography templates
  • Stencil masks for charged particle lithography
  • Reflective masks for extreme ultraviolet lithography (EUVL)
  • 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.

 

CNF currently sells only fused Silica mask blanks.

 

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