Resist Image Profiles

The above-mentioned linewidth information is generally obtained by using a non-destructive top-down SEM. However, since the three-dimensional profile is what counts in lithography, cross-sectional SEM can provide the most important information. Chemical amplification resists tend to exhibit a foot in the case of positive systems and notching in negative systems when imaged on nitride substrates such as silicon nitride and titanium nitride, which can be examined only by cross-sectional SEM. 

AFM has emerged as a non-destructive (sometimes destructive) alternative of SEM in observation of the three-dimensional relief images and surface roughness. Furthermore, AFM has been utilized in observation of pattern collapse during rinse as mentioned earlier. 

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A.irbome Basic Chemical Contamination. A critical, and at-first pu22ling problem, was encountered during early manufacturing trials of CA resists. Sporadically, severely distorted resist profiles would be formed in positive-tone CA resists, displaying what seemed to be a cap on the upper surface of the resist image (Fig. 26). In severe cases this cap or T-top would appear as a kin or cmst over the entire wafer surface that prevented development of the pattern. The magnitude of the effect varied dramatically between laboratories and appeared to grow more severe as the time interval between exposure and post-exposure bake was increased. 

Lin (83) studied the MRS resist (RD2000N) to determine the effect of optical density on performance. Resist images were delineated by 222-nm KrCl and 308-nm XeCl excimer laser radiation as shown in Figure 3.72. At 308 nm, nearly vertical resist profiles were produced. This result provides... 

Figure 3.3 Photoresist profiles. (A) Positive resist (a) desired resist profile for lift-off i.e. exposure-controlled profile, also called overcut (b) perfect image transfer by applying a normal exposure dose and relying moderately on the developer (c) receding photoresist structure with thinning of the resist layer i.e. developer control, also called undercut. (B) Negative resist the profile is mainly determined by the exposure. Development swells the resist a little but has otherwise no influence on the wall profile.

Distribution of PAG in spin-cast resist film can affect image profiles. Thus, efforts have been made to tailor the PAG structure for adequate polarity and to... 

As mentioned earlier, the COMA positive resists tend to have higher optical absorption at 193 nm than polymethacrylate and COBRA systems, which would produce a tapered image profile. To overcome this potential problem, the T-top formation by absorption of base into the top layer (see above) has been intentionally incorporated in the lithographic process (amine gradient process) [281]. Poly(acrylic acid-co-methyl acrylate) and L-proline were dissolved in water and spin-cast on a COMA resist. During PEB the amine in the overcoat diffuses into the COMA resist layer and compensates for the acid gradient caused by illumination, providing a vertical profile. 

The development process produces final three-dimensional resist images. This is a critical process as a sinusoidal latent image generated by exposure must be converted to a step function in order to produce resist images with vertical wall profiles. Thus, it is important to understand the dissolution behavior of resist films in a developer (especially aqueous base). 

In point of fact, the reaction of ketene with the anhydrous base resin has been exploited to produce negative resist images with strongly undercut profiles, which have found applications in lift-off processes (Figure 7.3). Furthermore, Itoh et al. have reported that exposure of a novolac-DNQ positive resist to DUV (250 nm) radiation, to which it is ve absorptive, leads to the creation of a cross-linked crust on the top of the resist. ... 

Image Blur in CA Resists. The mobility of the acid catalyst in the film influences CA resist performance. At the molecular level, the proton must move from site to site to effect acid-catalyzed reactions in the resist film. If the mobility is too great, however, the initial spatial profile of photogenerated acid will be blurred and the final developed resist image will appear distorted. The problem becomes more acute at finer resolution. In those situations where diffusion of the... 

The impact on negative-CA resists of airborne base contamination differs qualitatively from their positive tone counterparts. Suppression of acid-catalyzed chemistry at the surface of a negative resist results in some film erosion at the top of the exposed fields and in some cases an apparent loss of photosensitivity, but in general the reUef images formed exhibit the expected cross-sectional profile. This is in sharp contrast with the typical behavior seen with positive-tone CA resists, where suppression of acid-catalyzed chemistry at the surface causes an insoluble surface skin. 

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