Characterizing acid mobility in chemically amplified resists via spectroscopic methods

Abstract

State-of-the-art microlithographic processes used to make features smaller than 0.25 microns are based upon deep-UV lithography and chemically amplified resists (CARs). In these resists, photoacid generated during exposure initiates cascading deprotection reactions during post exposure bake (PEB) to form a developable image. Reaction may not be limited to the illuminated areas since the photo-generated protons may diffuse outside this region; therefore, it is important to understand the diffusional characteristics of the photoacid. In this coniribution, the photoacid generation during exposure was characterized using acid-sensitive fluorescence of fluorescein, and the acid mobility and concentration during PEB were studied using crystal violet. These probes were added to the Shipley Apex-E 2408 DUV photoresist, and measurements were carried out in situ on quartz substrates. For fluorescein, characteristic changes in the fluorescence peaks intensities were used to determine the acid concentration during exposure. Crystal violet is a triphenylmethane "propeller" molecule whose excited state lifetime increases as the local free volume decreases, and molecular rotation is hindered. This feature can be used to characterize the free volume in the photoresist using ground state recovery experiments. In addition, crystal violet has three protolytic forms, each with a unique absorption spectrum, and a calibrathxi of the absorption spectrum as a function of acid concentration was used to measure the acid concentration during PEB. These studies illustrate the utility of spectroscopic techniques based upon carefully selected molecular probes to characterize CARs in situ.

Using advanced simulation to aid microlithography development

“…Since the mid-1980s, a number of advances have been made, particularly: 1) in the area of new physical conditions, such as for high NA situations [43]- [46], [25], which involves the inherent vector propagation of electromagnetic radiation [47], [48]; 2) the very significant advances in numerical methods, such as enabling 3-D solutions over nonplanar topographies [33], [35] for tackling "reflective notching" problems; and 3) the characterizations of new photoresist chemistries [49], [50], [1] and optical conditions [51], [52]. Regarding 3), new photoresist materials and processes were developed, such as negative, silylated, and chemically amplified photoresists, 1 all requiring new simulation models and capabilities.…”

Highly Efficient Bicolor (Green−Blue) Fluorescence Imaging in Polymeric Films

“…In addition to the development of new photoresist materials capable for exposure at deep, and lately vacuum, UV spectral regions, increased interest has also arisen in the development of suitable metrology tools and methodolologies to monitor and control physicochemical changes occurring inside the resist films. One of the areas where intense activity is reported lately is related with the introduction of suitable probes, including fluorescence probes, for the study of photogenerated acid formation and diffusion aiming at the optimization of lithographic materials and processes. − …”

“…Fluorescent organic dyes dispersed in the polymer film or moieties pending to the polymer itself have been used so far in fluorescence imaging, either in the area of lithographic processing or in one of the organic displays, including fluoresceins, acridins, cumarins, anthracene derivatives, quinizarins, and recently pyridylbenzoxazoles . The acid-induced changes of emission properties in all these cases result in production of fluorescent and nonfluorescent areas (acid-induced dye quenching).…”

Expanding the Process Window and Reducing the Optical Proximity Effect by Post-Exposure Delay

“…Therefore, the diffusion behavior of photogenerated acid has been widely investigated for both high and low activation energy (E a ) resist systems. [13][14][15][16][17][18][19][20][21][22][23][24][25][26] Base additives can reduce the linewidth slimming of low E a systems such as acetal-based resists by reducing the acid diffusion. 12,22 Additional base components cannot only quench photogenerated acid, but can also suppress the acid diffusion reaction within the resist film.…”