Page 119 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH09.qxd 04/04/2005 19:42 Page 9.18

MICROLITHOGRAPHY

9.18 WAFER PROCESSING

The diazonaphthoquinone absorbs a photon and a water molecule, releases nitrogen gas and pro-
duces a carboxylic acid (that will exhibit high solubility in a developer made of water and base).
The kinetics of the chemical reaction represented by Eq. (9.14) are first order:
dm
=− CIm (9.15)
dt

where the relative PAC concentration m (the actual concentration divided by the initial preexposed
concentration) has been used and C is the exposure rate constant. A solution to the exposure rate
equation (Eq. (9.15)) is simple if the intensity within the resist is constant throughout the exposure.

m = e −CIt (9.16)
This result illustrates an important property of first-order exposure kinetics called reciprocity.
The amount of chemical change is controlled by the product of light intensity and exposure time.
Doubling the intensity and cutting the exposure time in half will result in the exact same amount of
chemical change. This product of intensity and exposure time is called the exposure dose.

9.3.2 Chemically Amplified Resists
Unlike conventional resists, such as the diazonaphthoquinone system discussed earlier, chemically
amplified resists require two separate chemical reactions to change the solubility of the resist. First,
exposure turns an aerial image into a latent image of exposure reaction products. Although very sim-
ilar to conventional resists, the reaction products of exposure for a chemically amplified resist do not
change the solubility of the resist. Instead, a second reaction during a postexposure bake is catalyzed
by the exposure reaction products. The result of the postexposure bake reaction is a change in the
solubility of the resist. This two-step sensitization process has some interesting characteristics and
challenges.
For chemically amplified photoresists, the sensitizer is called a photoacid generator (PAG). As
the name implies, the PAG forms a strong acid when exposed to deep-UV light. The reaction of a
common (though simplified) PAG is shown in Eq. (9.17):

Ph
hν
+ − (9.17)
Ph S CF COO CF COOH + others
3
3
Ph
The acid generated in this case (trifluoroacetic acid) is a derivative of acetic acid where the
electron-drawing properties of the fluorines are used to greatly increase the acidity of the molecule.
The PAG is mixed with the polymer resin at a concentration of typically 5 to 15 percent by weight
for 248 nm resists, with 10 percent as a typical formulation. For 193 nm resists, PAG loading is kept
lower at 1 to 5 percent by weight to keep the optical absorbance of the resist within desired levels.
The kinetics of the exposure reaction are presumed to be standard first order.
Exposure of the resist with an aerial image I(x) results in an acid latent image H(x). A postexpo-
sure bake (PEB) is then used to thermally induce a chemical reaction. This may be the activation of
a cross-linking agent for a negative resist or the deblocking of the polymer resin for a positive resist.
The defining characteristic of a chemically amplified resist is that this reaction is catalyzed by the
acid so that the acid is not consumed by the reaction and, to first order, H remains constant. A base
polymer such as polyhydroxystyrene (PHS) is used, which is very soluble in an aqueous base
developer. It is the hydroxyl groups that give the PHS its high solubility, so by “blocking” these
sites (by reacting the hydroxyl group with some longer chain molecule) the solubility can be reduced.
Early chemically amplified resists employed a t-butoxycarbonyl group (t-BOC), resulting in a very

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.