Page 106 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH09.qxd 04/04/2005 19:42 Page 9.5

MICROLITHOGRAPHY

MICROLITHOGRAPHY 9.5

of parameters can have significant impact on photoresist thickness uniformity and control. There is
a choice among static dispense (wafer stationary while resist is dispensed) or dynamic dispense
(wafer spinning while resist is dispensed), spin speeds and times, and accelerations to each of the
spin speeds. Also, the volume of the resist dispensed and properties of the resist (such as viscosity,
percent solids, and solvent composition) and the substrate (substrate material and topography) play
an important role in the resist thickness uniformity. At the end of this cycle a thick, solvent-rich film
of photoresist covers the wafer, ready for postapply bake. Although theory exists to describe the spin
coat process rheologically, in practical terms the variation of photoresist thickness and uniformity
with the process parameters must be determined experimentally. The photoresist spin speed curve is
an essential tool for setting the spin speed to obtain the desired resist thickness. The final resist thick-
ness varies as one over the square root of the spin speed and is roughly proportional to the liquid
photoresist viscosity.

9.1.3 Postapply Bake

After coating, the resulting resist film will contain between 20 to 40 percent of weight solvent. The
postapply bake process, also called a softbake or a prebake, involves drying the photoresist after the
spin coat process by removing this excess solvent. There are four major effects of removing solvent
from a photoresist film—(1) film thickness is reduced, (2) postexposure bake and development prop-
erties are changed, (3) adhesion is improved, and (4) the film becomes less tacky and thus less sus-
ceptible to particulate contamination. Typical prebake processes leave between 3 and 8 percent
residual solvent in the resist film, sufficiently small to keep the film stable during subsequent litho-
graphic processing.
Unfortunately, there are other consequences of baking some photoresists. At temperatures greater
than about 70°C, the photosensitive component of a typical conventional resist mixture, called the
photoactive compound (PAC), may begin to decompose. Thus, one must search for the optimum pre-
bake conditions that will maximize the benefits of solvent evaporation and minimize the detriments
of resist decomposition. For chemically amplified resists, residual solvent can significantly influence
diffusion and reaction properties during the postexposure bake, necessitating careful control over the
postapply bake process. Fortunately, these modern resists do not suffer from significant decomposi-
tion of the light-sensitive components during prebake.
Although the use of convection ovens for the prebaking of the photoresist was once quite com-
mon, currently, the most popular bake method is the hot plate. The wafer is brought either into inti-
mate vacuum contact with or close proximity to a hot, high-mass metal plate. Due to the high thermal
conductivity of silicon, the photoresist is heated to near the hot plate temperature quickly (about 5 s
for hard contact, or about 20 s for proximity baking). The greatest advantage of this method is an
order of magnitude decrease in the required bake time over convection ovens, to about 1 min, and
the improved uniformity of the bake. In general, proximity baking is preferred to reduce the possi-
bility of particle generation caused by contact with the backside of the wafer.

9.1.4 Alignment and Exposure
The basic principle behind the operation of a photoresist is the change in the solubility of the resist in
a developer on exposure to light (or other types of exposing radiation). In the case of the standard dia-
zonaphthoquinone positive photoresist, PAC, which is not soluble in the aqueous base developer, is
converted to a carboxylic acid on exposure to UV light in the range of 350 to 450 nm. The carboxylic
acid product is very soluble in the basic developer. Thus, a spatial variation in the light energy inci-
dent on the photoresist will cause a spatial variation in the solubility of the resist in the developer.
Contact and proximity lithography are the simplest methods of exposing a photoresist through a
master pattern called a photomask (Fig. 9.2). Contact lithography offers high resolution (down to
about the wavelength of the radiation), but practical problems such as mask damage and resulting
low yield make this process unusable in most production environments. Proximity printing reduces
mask damage by keeping the mask a set distance above the wafer (e.g., 20 µm). Unfortunately,

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.