Edwin Jan Klein - Universiteit Twente

123
Fabrication
In the first exposure step the wafer stepper is used to define ASML-specific alignment
markers that are used to align all future images. In addition a special marker, located
in reticle image 5 (see fig. 5.16) and shown enlarged in Figure 5.17a, is placed on the
wafer in two locations: -25000, -500 and +25000, -500 (x,y-in microns, offset relative
to the center of the wafer). To these markers the contact mask that defines the vertical
tapers is then aligned.
During the second exposure step the contact mask, which defines the vertical tapers
for the OADM devices, is aligned to these markers. The following two critical stepper
exposure steps 3 and 4 that define the waveguide and resonator layers use the ASML
markers created in step 1. Next, in the fifth exposure step, the mask used by the
contact lithography to define the chromium heaters is aligned to the markers present
in the OADM resonator reticle image which is shown enlarged in Figure 5.17b. The
mask used in this step is also shown in Figure 5.18 to show how the various device
groups are positioned across a wafer. Each of the squares visible on the mask
represents a device group: 58 OADM groups and 2 router groups. The layout follows
the typical pattern of a wafer stepper where many identical device groups are created
adjacent to one another in consecutive exposure steps. The sixth and final exposure
uses a similar mask to define the gold electrodes. In this instance the mask is aligned
to alignment markers created in the chromium heater layer.
Figure 5.18. Layout of a mask used for contact lithography. The typical pattern produced by
the stepper, creating many of the same images side by side which also dictates the layout of
this mask, is clearly visible.