Page 115 - Sami Franssila Introduction to Microfabrication

94 Introduction to Microfabrication

rectangle. Vector scanning enables skipping of empty optics aberrations and diffraction for highly collimated
(non-exposed) spaces, making the system much faster, beams. Interactions in solid further limit minimum size:
at the expense of system complexity. Variable shaped effective beam diameter is given by
beam is another improvement over raster scan: when 1.5
larger than minimum pixel size structures are drawn, d eff (nm) = 0.9 (t/V ) (8.1)
writing speed is enhanced dramatically.
Electron beam (and laser beam) writing area is very resist thickness t is in nm and voltage in kV.
small: ca. 250 × 250 µm area, that is, the area that can Some electrons experience backscattering (large angle
be scanned electromagnetically (e-beam) or acousto- scattering) with ca. micrometre ranges. Exposure dose
thus depends on the neighbouring structures. This is
optically (laser beam). If an area larger than 250 ×
250 µm needs to be drawn, additional movements must known as the proximity effect. The proximity effect can
be introduced (Figure 8.1). The stage scan is a mechan- be combated by biasing structures smaller or larger so
ical movement, controlled by an interferometer. Pattern that the final pattern is of desired size and shape.
placement in different subfields is thus a sum of two
rather different mechanisms. 8.3 PHOTOMASK FABRICATION

Instead of direct writing of millions of pixels on a wafer,
8.1.1 Alignment beam writers can be used to write photomasks for optical
lithography. The simplest photomasks are just laser-
Alignment is a major criterion in all lithography tech- printed overhead transparencies: they are suitable for
niques. In Electron beam lithography (EBL), alignment structures in the size range of hundreds of micrometres
relies on electron scattering from alignment marks. It and for simple demos, for example, in a student lab. The
can be done in two basic ways. Global alignment uses printed circuit board industry uses more advanced laser
marks placed on wafer edges. This is fast if ultimate plotters and polyester transparency films, with minimum
accuracy is not necessary. Chip-alignment uses align- lines of ca. 30 to 50 µm. Polymer-based masks suffer
ment marks at each chip location. The accuracy can be from wear and tear and from dimensional instability.
further increased if alignment marks are visited regu- Photomasks proper are glass plates with chromium
larly during writing, rather than just at the beginning (ca. 100 nm thick) on them. Soda lime glass is used
of writing. Processing usually begins with a zero layer for larger linewidths (>3 µm) and quartz is the material
lithography: only alignment marks are exposed on the of choice for micron and submicron work. Optical
zero layer and etched into the wafer, for example, 1 µm lithography with photomasks is the dominant patterning
deep, 10 µm wide and 100 µm long. These may deteri- technology because optical exposure is fast: illumination
orate as more layers are deposited and etched, but their through a photomask exposes up to 10 10 pixels in a
global nature makes them better than a sequential layer- one second exposure. But the original mask pattern
to-layer alignment scheme. that optical lithography so efficiently reproduces must
be written slowly feature by feature. The enormous
throughput difference warrants making the mask plates,
8.2 ELECTRON BEAM PHYSICS
which can be costly: a set of 15 plates (corresponding
to 1 µm CMOS process) costs 15 000 USD; and a set of
Electrons are light mass objects, and when they hit
resist with high energy (10–50 kV typical), they scatter 25 plates for 0.25 µm CMOS costs ten times more.
forward (recall Figure 2.12). Even though the beam spot Writing time for a mask plate can be limited by
on resist top surface is very small, scattering broadens several factors, which depend on the pixel size, total
the beam inside the resist and the resist is exposed on a area, resist sensitivity and electronic and mechanical
larger area than the beam spot. Forward scattering is not, scan speeds
however, the major component of resist exposure: most τ 1 = AS/I (8.2)
of the resist exposure comes from secondary electrons where A is area, S is the exposure dose, I is beam
that have been created when the beam slows down. current.
These 2 to 50 eV electrons have a range of a few Exposed pixel size, d, affects writing time via
nanometres in resist.
Beam spots in the 5 nm range are available. This is τ 2 = A/fd 2 (8.3)
not limited by the wavelength of electrons (λ = 8 pm
for 25 kV) but rather by electron source size and electron where f is the beam incrementing rate (up to 500 MHz).