Enhancement of lithographic performance

In the electron beam writing technology, the negative tone resist is usually used to fabricate the line or low density patterns, especially for the gate line. In this study, the commercial NEB-22 resist is a negative type, and the fullerene molecules such as C60 and C70 are incorporated into the commercial resist. Figure3.1(a) depicts the resist sensitivity curve. The dose (D_C) for the “NEB” sample that the polymer constituent begins cross-linkage is 5.2 µC/cm², while the dose (DO) that the polymer can achieve 100% cross-linkage is 6.2 µC/cm². As to the “NEB+Toluene” sample, both the DC and DO are increased. This observation is attributed to the dilution of acid generator in the sample, and therefore, reduces the sensitivity. Interestingly, the sensitivity for 0.01% w/v “NEB+C60” or “NEB+C70” sample is significantly enhanced after nano-material modification. Both the D_C and D_O are decreased to 4.6µC/cm² and 5.4µC/cm², respectively. This finding suggests that the incorporation of fullerene molecules into resist can effectively enhance the process throughput.

Another the electron beam writing technology, positive tone resist is usually used to fabricate the contact hole [3]. In this study, the commercial DSE-1010 electron

beam resist is a positive type, and the fullerene molecules such as C60 and C70 are incorporated into the commercial resist. Figure3.1(b) depicts the resist sensitivity curve. The irradiation dose (D_i) for the “DSE” resist that the acid generator and functional group in the polymer begins reaction is 2.3 µC/cm², while the dose (Dc) that the polymer film can fully dissolve is 3.4 µC/cm². As to the “DSE+Toluene”

sample, both Di and Dc are increased. This observation is attributed to the dilution of acid generator by toluene solvent, and therefore, reduces the sensitivity. Interestingly, the sensitivity for 0.01% w/v “DSE+C60” or “DSE+C70” sample is significantly enhanced after spiking the fullerene molecules. Both D_i and D_c are decreased to 1.4 µC/cm² and 2.2 µC/cm², respectively. This finding suggests that the incorporation of fullerene molecules into resist can effectively enhance the process throughput.

What happens to the decrease of addressing dose for the resist after spiking with fullerene molecules? In the resist, the electron beam activates the bond of acid generator to produce acid, and the acid induces the functional group reaction of the polymer. The irradiated electron beam easily penetrates through the void region embedded in the resist film, and degrades the throughput. We infer the fullerene molecules with sub-nanometer sizes (0.7-0.8 nm) are embedded into the void of resist sample (in Fig. 3.2) The C60 or C70 fullerene embedded in the void region has a higher electron affinity ~2.6 eV, and therefore, facilitate the bond activation for the acid generator. As we know the electron accelerating voltage can influence the sensitivity, the higher accelerating voltage can improve the resist resolution but deteriorate the resist sensitivity [4]. However, the incorporation of fullerene molecules can shorten the resist exposure time

Figure3.3 indicates the line width increases with the exposure dose. We define the dose range for the “nominal line ± (10%)(nominal line)” as the process window.

Table 3.1 suggests the fullerene-incorporated resist has wider dose window than

unadulterated resist. The resist with C60 modification in Fig. 3.3 can fabricate sub-50nm line with respect to C70. This phenomenon implies the C60 with smaller size is better for the resist void filling. The SEM images for the resist with 0.02% w/v fullerene modification are illustrated in Fig. 3.4. The print of sub-50nm lines can not achieve from Fig. 3.4a. The line without fullerene tends to pattern collapse at aspect ratio of 5.75 due to the insufficient adhesion at interface. Figure 3.4b demonstrates the resist without fullerene can resolve 53nm lines at aspect ratio of 4.3, but the line has serious line edge roughness problem. The line edge roughness can lead to higher leakage current for the future nano-devices. However, the resist with 0.02% C60 modification can print 46nm lines (Fig. 3.4c), and 0.02% C70 can print 51nm lines (Fig. 3.4d). The problem of line edge roughness is not seen for the resist with fullerene modification. This observation is dependent on the void of resist polymer of which is filled with fullerene. The fullerene on the sidewall also can minimize the extent of protrusion of polymer. In addition, the interfacial adhesion between fullerene-incorporated resist and the substrate is stronger than the unadulterated resist and the substrate.

Figure 3.5a indicates the hole dimension is significantly influenced with the exposure dose, while the fullerence-incorporated resists in Fig. 3.5b and 3.5c are not.

The resist without embedded fullerene molecules can not resolve 60 nm contact hole.

The electron beam doses at 7 µC/cm² and 8.5 µC/cm² can resolve 50 nm contact hole for the resist with 0.01% C60 and C70 modification, respectively. We define the dose range for the “nominal hole ± 10% x (nominal hole)” as the process window. The fullerene-incorporated resists have wider dose windows (i.e. 8-9.5 µC/cm² for 0.01%

C60, and 9.5-11.5 µC/cm² for 0.01% C70) for 60 nm contact hole formation than unadulterated resist. These phenomena are all attributed to the high electron affinity of fullerene molecules.

The C70 fullerene molecule is chosen for further studies due to its better process window. The SEM images for the resist with 0.01% w/v fullerene modification are illustrated in Fig. 3.6. The fabrication yield of 60 nm nominal hole in Fig. 3.6a is not satisfactory due to the lack of electron affinity fullerene. Figure 3.6b demonstrates the resist with C70 fullerene molecules can resolve 53 nm hole.

The stress for various fullerene-incorporated films is demonstrated in Fig.3.7.

The resist film without spiking fullerene has large tensile stress. The spiking of fullerene molecules such as C60 or C70 can prevent the stress, and is beneficial for the surface flatness. The fullerene molecules can fill the void of resist and minimize the deformation as spin-coating the resist.