Page 125 - Sami Franssila Introduction to Microfabrication

104 Introduction to Microfabrication

(a) (b) (c)
Figure 9.7 Alignment operation: (a) wafer with alignment marks; (b) photomask with alignment marks and (c) after
linear translation and rotation of the wafer the alignment marks on wafer and mask coincide

photomask. The lenses in the optical path of the smaller and larger structures so that process robustness
exposure tool are subject to constant UV flood, and they and linearity can be checked. Optical microscopy
too need to be thermally stabilized. and scanning electron microscopy (SEM) are standard
Alignment needs to be discussed from two rather methods. Even when linewidths are below optical
different points of view: microscopy resolution, it is useful as an initial check:
for instance, resist adhesion loss, delamination and
1. Equipment view: This is an optomechanical problem other gross errors can be seen. Linewidth control is
of finding alignment marks on the mask and on the usually accepted as ±10% of design value. Linewidth
wafer, and manipulating them to coincide. measurements by stylus/AFM or SEM form the basis
2. Device design view: This is a design issue and it of lithography process control. Resist thickness has a
depends on overlaps and spacings that structures need profound effect on linewidth, as will be discussed in the
for the device to operate, for instance metallization next chapter.
has to overlap contacts.

Alignment could be done using the devices themselves, 9.5 EXERCISES
but this is impractical because of micrometre dimensions
and multiple identical structures. Therefore separate 1. What is the best possible resolution in optical contact
alignment marks are used. Alignment marks are much lithography?
larger than device features because they exist only for 2. What is the diffraction limited resolution of 10 nm
alignment, and have nothing to do with resolution. X-ray photons?
Alignment is usually done on a wafer level, with two 3. 100 mm diameter silicon wafer has 1 µm lines
alignment marks as far from each other as possible, to fabricated on it. The photomask is made of soda lime
increase theta (rotational) resolution (Figure 9.7). glass with a coefficient of thermal expansion (CTE)
−6 ◦
Alignment sequence determines which layers are of 10 ppm (10 × 10 / C). How accurately must the
aligned to each other. Layers are not necessarily aligned temperature in the patterning process be controlled
sequentially to a preceding layer, but to some important in order to keep distortions from thermal expansion
previous layer. A contact hole is aligned to a resistor, but over 100 mm wafer below 0.3 µm? Silicon CTE is
−6 ◦
the metal layer can be aligned either to the contact hole, 2.5 × 10 / C.
to make sure that the whole contact hole is covered, but 4. Make a graphical presentation of projection lithogra-
it can also be aligned to the resistor; after all, the metal phy resolution versus depth of focus!
has to make contact with the resistor. These issues will 5. A 50 µm thick resist must be used in an electroplating
be dealt with in Chapter 24. process. What is the minimum feature size that can
be used?

9.4.1 Lithography metrology
REFERENCES AND RELATED READINGS
Lithography produces test structures of itself. Test
structures must include resolution structures with the Helbert, J.N.: Handbook of VLSI Micro lithography, Noyes
same dimensions as the devices themselves, but also Publications, 2001.