Guide to Gray Scale Photolithography and
Mass Fabrication of 3D Microstructures
(CMI Product Information 04-100)
HEBS-glass gray scale photomasks offer the highest resolution in gray
and have a spatial resolution of 0.1 micron
HEBS-glass gray scale photomask is a true gray scale photomask having a continuous tone
of gray levels. The difference in gray scale resolution between a HEBS-glass gray scale
photomask and a halftone gray scale photomask is apparent by observing both types of gray
scale masks under a microscope at a magnification of more than about 200X.
Photos 1 and 2 of CMI Product Information 04-101 (photo a
at 1000x and photo b at 400x) exhibit a sinusoidal transmittance grating of 80 cycles per
mm in a HEBS-glass gray scale mask and in a high resolution photographic emulsion plate,
respectively. Whereas the transmittance profile is smooth and continuous in a HEBS-glass
mask (See Photos 1), the transmittance profile is rough and
not continuous (See Photo 2) due to the graininess of an
emulsion plate. We compare the size of gray scale resolution elements in the table
Table: Gray Scale Resolution Element
* Only in a certain defocused state, the half tone gray scale mask may produce a
quasi-uniform gray level within a gray scale resolution element provided the scattering
as well as the diffraction of pinholes and opaque spots can be ignored.
** Any one of more than 1000 gray levels can be assigned to and produced in each of 0.1
micron spots within a HEBS-glass plate.
*** Fixed Optical Density from spot to spot.
**** Variable Optical Density from spot to spot, there are more than 1000 gray levels to
choose from for each gray scale resolution element of 0.1 micron spot.
A state of the art halftone chrome mask may consist of mixtures of 0.5 micron x 0.5 micron
chrome spots which are totally opaque and 0.5micron x 0.5 micron clear spots which are
totally transparent due to the absence of chrome film coating on the glass photomask
substrate. The transmittance of a gray level in a halftone chrome mask is determined by
the ratio of the number of chrome spots to clear spots within a gray scale resolution
element. For a gray scale chrome mask having 16 gray levels, a gray scale resolution element
must consist of 16 binary spots (or a multiple of 16 binary spots). A binary spot of the
chrome mask is either a chrome spot or a clear spot. When all 16 binary spots in a gray
scale resolution element are chrome spots, the gray scale resolution element is totally opaque.
When one chrome spot is replaced by a clear spot, the transmittance is 0.0625 or (1/16).
As more chrome spots in a gray scale resolution element are replaced by clear spots, the
transmittance increases in steps of 0.0625, assuming the scattering and diffraction of
chrome spots and chrome pinholes can be ignored. Therefore, a gray scale resolution element
of a chrome mask having 16 gray scale levels is at least 2 micron x 2 micron in lateral
(i.e. x and y) dimensions. This is because the gray scale resolution element consists of
at least 16 binary spots, each of 0.5 micron x 0.5 micron. As listed in the table above,
a gray scale resolution element of a chrome mask having 16, 64, 100 and 1000 gray levels is
>/= 2 micron, >/= 4 micron, >/= 5 micron and >/= 15.8 micron, respectively, in its lateral
dimensions.scattering and diffraction of chrome spots and chrome pinholes can be ignored.
Therefore, a gray scale resolution element of a chrome mask having 16 gray scale levels is
at least 2 micron x 2 micron in lateral (i.e. x and y) dimensions. This is because the
gray scale resolution element consists of at least 16 binary spots, each of 0.5 micron x
0.5 micron. As listed in the table above, a gray scale resolution element of a chrome mask
having 16, 64, 100 and 1000 gray levels is >/= 2 micron, >/= 4 micron, >/= 5 micron
and >/= 15.8 micron, respectively, in its lateral dimensions.
Gray scale patterns in a photo-emulsion film or a photographic emulsion glass plate are halftone gray scale patterns, since each silver grain in a developed photographic emulsion is totally opaque. The gray scale is produced by varying the number density of the silver grains. The spacing between the grains are transparent. The size of silver grains is not uniform and may range from, for example, about 1 micron to about 5 micron in a high resolution photoemulsion plate. Therefore, the gray scale resolution element of a photographic emulsion plate having 16, 64, 100 and 1000 gray levels is >/= 4 micron, >/= 8 micron, >/= 10 micron and >/= 31.6 micron , respectively in its lateral (i.e. x and y) dimensions.
As shown in the table, the size of the gray scale resolution element of a HEBS-glass gray scale photomask is independent of the number of gray scale levels, and is 0.1 micron in each lateral (x and y) dimensions. Any one of more than 1000 gray levels can be assigned to and produced in each of 0.1 micron spots within a HEBS-glass plate. This is because the silver specks in the HEBS-glass is of atomic dimensions. Monatomic, diatomic, and polyatomic species of nano-silver-particles are stable only in certain silicate glass compositions of HEBS-glass.
The fabrication of tapered structures in optoelectronic components, and of microoptical elements such as refractive microlens arrays, diffractive optical elements, prism couples, and three-dimensional microstructures in general can be realized with the existing micro-fabrication methods normally used for the production of microelectronics, provided a gray scale photomask of desired resolution is constructed. The well-established microfabrication technologies that have been adapted for gray scale method of 3D microstructure fabrication include a photolithographic step and a reactive ion etching step. Reactive ion etching is employed to transfer and/or replicate patterns in photoresist into the underlying substrate material. Commercially available systems include plasma etchers, inductive coupled plasma (ICP) and chemically assisted ion beam etchers (CAIBE). In the past, the difficulty of the fabrication of micro-optical elements resulted from the lack of a true gray scale photomask. Custom designed HEBS-glass gray scale photomasks have been available from Canyon Materials, Inc. since 1997. Using a HEBS-glass gray scale photomask, one can fabricate any arbitrary shaped 3D microstructure with the choice of using any one of the existing photolithographic tools including contact mask aligners, projection printers, as well as 1x or reduction steppers.
A Short List of Publications on HEBS-Glass and LDW Glass Gray Scale Photomasks
More About the Process & Results of Gray Scale Lithography