HSQ double patterning process for 12 nm resolution x-ray zone plates Page: 4 of 23
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but also accommodates the negative tonality nature of HSQ. The differences result in a
single electroplating step to form all absorptive metal zones, in comparison to two
separate steps in the PMMA process. This simplification eliminates the possibility of two
different plating thickness for the two zone sets. Fig. 2 illustrates the process flow of the
HSQ double patterning process. First, alignment marks are fabricated on a 100 nm thick
low stress membrane window wafer [Fig. 2(I.1)]. The wafer, consisted of an array of 0.5
mm membrane windows, is pre-coated with a 5 nm thick chromium adhesion layer and a
7 nm thick gold plating base using electron beam evaporation. Alignment marks are
fabricated at the four corners of a major field (114 m field size, 1.75 nm pixels) on each
membrane window, using a standard positive resist process. The marks, as seen in Fig. 3,
are variants of four two-dimensional Barker alignment mark series21, each of which is a
rotated copy of the other, and a similar set of marks closer to the center. The smallest
elements in the Barker marks are 60 nm squares. The marks are electroplated with gold to
about 90 nm thick and form a high contrast transmitted electron signal.
Next, the resist plating mold is removed [Fig 2(I.2)]. This is a crucial step to
forming a high-quality, continuous 3-MPT adhesion layer on the wafer surface in the
following procedure. We strip the resist by flood exposing the wafer with ultraviolet light
and developing the resist. This recipe removes the resist rather effectively without
damaging the plating base. In the case where resist residue is still present, a fresh gold
layer of 5 nm is evaporated onto the wafer surface.
Once a clean gold substrate is prepared, the wafer surface is primed by soaking in
0.02M 3-MPT in ethanol for 2 hours [Fig. 2(11.1)]. Excessive 3-MPT is then washed off
with ethanol, and the wafer is blow dried with N2. To complete the priming procedure,
the wafer is soaked in 0.0 IN hydrochloric acid at 300C for 5 hours, and the residue acid
after soaking is removed by rinsing the wafer with ethanol and drying in N2. The wafer is
now ready for the first zone set patterning with HSQ.
To start the fabrication, a 50 nm thick HSQ film is spin-coated on the wafer,
which is then placed in a vacuum chamber of 10-6 torr at 250C [Fig. 2(111.1)] to evaporate
the solvents. Before zone set I is exposed, the orthogonality, scaling, rotation and offset
of the electron beam deflection are calibrated with the inner set of the alignment marks
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Chao, Weilun; Kim, Jihoon; Rekawa, Senajith; Fischer, Peter & Anderson, Erik H. HSQ double patterning process for 12 nm resolution x-ray zone plates, article, June 16, 2009; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc933871/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.