Page 117 - Sami Franssila Introduction to Microfabrication

96 Introduction to Microfabrication

Figure 8.3 Test structures for inductor coils: the process engineer is interested in different linewidths and spacings; the
device engineer wants to test different coil shapes and see the effect of the number of coil turns

Writing shapes other than rectangles can be difficult perfect mask plate (known as die-to-die) or a compar-
for mask makers. Photomasks are written by machines ison between design data and the finished mask plate
designed to do XY-orthogonal structures. The CAD (die-to-data). There are usually hundreds of identical
programs for IC design support drawing on XY-grid, chips on a photomask plate and if they have been inde-
and even data conversion from design program to pendently drawn, it would be statistically improbable
mask writer program can be difficult for non-rectilinear that they would have defects at the same locations. This
shapes. Photomasks are, however, not necessarily XY- could be the case, however, if there is a systematic error
symmetric. For instance, stitching of subfields can in the data, for example, structures that are beyond the
be made as small as 6 nm in X-direction, but not capability of the mask writer system (e.g., too narrow
in Y-direction, because the former depends on beam lines have been designed, or too narrow spaces between
scanning, but the latter on the mechanical stage the lines).
movement. Smoothly curving lines needed in integrated When defects are detected on a mask plate, it is
optics are difficult, and circles and arbitrary angles pose often financially attractive to repair them rather than to
difficulties, too. Edge definition of structures other than write a new plate. Defects come in many guises, but
XY-lines can, of course, be increased by using smaller from a repair point of view there are two grand classes
writing grid, or double exposure, both of which increase of defects:
writing time considerably.
• missing chrome
• extra chrome.
8.5 PHOTOMASK INSPECTION, DEFECTS
AND REPAIR
The former requires the deposition of a layer that
Photomask fabrication requires, in addition to a scanning will prevent light transmission. Usually, a metallic layer
beam equipment, a repertoire of inspection and repair is deposited, for example, tungsten. The latter defect
equipment. Three basic control measurements for masks type requires the removal of extra chrome. Both can be
are linewidth, position and defects. Linewidth is a accomplished with focused ion beam (FIB) techniques
local measurement, over a test structure pattern. With but the real difficulty lies in guiding the FIB to a detected
linewidths in the micrometre range, measurement should defect site.
be able to discern ca. 10 nm. Pattern position is a global Geometric/topological classification of defects (see
measurement and it is usually fixed to a mask writing Figure 8.4):
tool, controlled by a stage interferometer, and measured
to ca. 10 nm accuracy over 10 cm mask plate size. • protrusion (extra chrome attached to a feature)
Defects on the mask are fatal because they will be • intrusion (partial loss of chrome in a feature)
reproduced on the wafers. Defects can be classified into • bridge (chrome connecting two features)
two broad categories of hard defects and soft defects. • necking (discontinuity in a line)
Soft defects are mainly particles or resist residues that • pinhole (hole in a chrome)
can be cleaned away. Hard defects are permanent spots • pin spot (extra chrome on a light field area).
or scratches in chrome or in quartz.
Two basic inspection strategies are used: optical From the yield and reliability point of view not
inspection combined with a comparison to a known all defects are equal. Defect must be understood as a