Diffusion, photoacid

Figure 17.23 Exposure process of a CAR, illustrating photoacid diffusion beyond the exposed areas.

As a thermally driven process, the photoacid diffusion length and rate in CARs is temperature dependent, as illustrated in Fig. 17.24 for the diffusion of perfluoro-octane sulfonate photoacid in partially protected poly(4-t-butyloxycarbonyloxy-strene)-based resist. Within normal processing conditions, the higher the PEB temperature, the faster is the rate of the photoacid diffusion and the longer is the diffusion length. Although the PEB temperature can be used to modulate photoacid diffusion and consequently resist sensitivity, it nevertheless involves a trade-off... 

Elucidating how photoacid diffusion leads to resist contrast and resolution loss... 

Figure 17.25 Average photoacid concentration, or line spread function, due to photoacid diffusion as a function of scaled coordinate x/ Dtf. (Adapted from Ref. 47.)...

An important result from the application of the continuum theory to this diffusion problem is the fact that shrinking the feature pitch can only be accommodated as long as the photoacid diffusion length is also comparably shrunk. For a pitch of 45 nm, one requires a diffusion length of about 7 run or less. The diffusion... 

Photoacid diffusion behavior in t-BOC-blocked chemically amplified positive DUV resists under various conditions was studied. Based on the experimental results, it was confirmed that only one mechanism dominated the acid diffusion in the resist film, and two diffusion paths, i.e., the remaining solvent in the resist film and hydrophilic OH sites of base phenolic resin, existed. Moreover, the effects of molecular weight dispersion, acid structure, and additional base component on both acid-diffusion behavior and lithographic performance were revealed. Finally, the acid diffusion behavior in the resist film was clarified and the acid diffusion length that affected the resist performance could be controlled. 

Figure 39 Example of a two-component blended CAR (Resist 1 and SEM image (1)) and the analogous CAR containing the same type of PAG which has been covalently incorporated into the polymer backbone (Resist 2 and SEM image (2)). The so-called polymer-bound PAG resist (i.e., Resist 2), in which the photoacid cation is covalently attached to the polymer, exhibits substantially improved minimum feature resolution as demonstrated by the SEM images of the minimum size features produced in both resists using e-beam lithography. It Is believed that the reduction in photoacid diffusivity afforded by this covalent attachment of the photoacid cation is responsible for the difference.

Fig. 3. Decay of the photoacid band at 1486 cm 1 (left panel), the rise of the conjugated photobase band at 1503 cm 1 (centre panel) and acetic acid band at 1720 cm 1 (right panel) as function of acetate concentration. The photoacid and photobase signals are in addition affected by rotational diffusion.

Fig. 4. Schematic representation of the observed dynamics. Initially uncomplexed photoacid first forms an encounter complex through diffusion. This loose complex either rearranges to a tight complex, or reacts via the hydrogen bonded network between photoacid and base. A pre-formed photoacid-base complex can directly react with extremely fast rates.

The sharply increased acidity of phenols in the excited state can be used to lower the pH of aqueous solutions by a pulsed light source within nanoseconds (photoacid). However, the equilibrium is rapidly re-established in the ground state by diffusion-controlled recombination of the released protons with the basic phenolates. 

Proton transfer dynamics of photoacids to the solvent have thus, being reversible in nature, been modelled using the Debye-von Smoluchowski equation for diffusion-assisted reaction dynamics in a large body of experimental work on HPTS [84—87] and naphthols [88-92], with additional studies on the temperature dependence [93-98], and the pressure dependence [99-101], as well as the effects of special media such as reverse micelles [102] or chiral environments [103]. Moreover, results modelled with the Debye-von Smoluchowski approach have also been reported for proton acceptors triggered by optical excitation (photobases) [104, 105], and for molecular compounds with both photoacid and photobase functionalities, such as lO-hydroxycamptothecin [106] and coumarin 4 [107]. It can be expected that proton diffusion also plays a role in hydroxyquinoline compounds [108-112]. Finally, proton diffusion has been suggested in the long time dynamics of green fluorescent protein [113], where the chromophore functions as a photoacid [23,114], with an initial proton release on a 3-20 ps time scale [115,116]. 

Some PAGs play quite diverse roles in addition to their primary function of generating photoacids. Some PAGs have been reported to act as dissolution inhibi-tors of phenolic and other acidic resins in aqueous developers. Furthermore, the use of alkylsulfonium iodides in combination with PAGs has been reported to improve resist contrast and acid diffusion in electron-beam systems. ... 

One of the most important ester-protected polyhydroxystyrene-based resist copolymers, ESCAP (environmentally stable chemically amplified photoresist), developed at IBM, is based on the random copolymerization of 4-hydroxystyrene with tert-butyl acrylate (XXX).On exposure, this resist copolymer is converted to a copolymer of 4-hydroxystyrene with acrylic acid through photoinduced acid-catalyzed deprotection of the tert-butyl group (see Scheme 7.34). Because this resist system can be annealed at temperatures near its Tg in a process that hlls up the free volumes (voids in the resist matrix), thus preventing the out-diffusion of photoacids from the matrix and in-diffusion of airborne bases into the resist, neutralization reactions between the photoacids and bases in the resist matrix (otherwise known as poisoning) are reduced, thus allowing... 

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