Tameichi Ochiai

Design of high-performance chemically amplified resist

The lithographic performance of chemically amplified positive resists has nearly reached at the level of application to quarter-micron level with regard to their resolution and sensitivity. However, it is hard to say that the problem of post exposure delay (PED), which contains formation of T-top shape or foot profiles, has been completely solved. We studied structure effect of both a dissolution inhibitor and a protecting group on the problem. It was shown that a resist film having a dissolution inhibitor derived from trisphenol looses less amount of acid by evaporation compared with one having bisphenol type dissolution inhibitor. With regard to easiness of cleavage of the protecting group, IR measurement has confirmed that the dissociation of t-BOC occurs during PEB whereas that of THP occurs during exposure. Molecular orbital calculation showed that acetal group can be protonated easier than carbonate group and that both of them will have no barrier to cleave after protonation. Bases on the above findings, we have obtained the high performance resist by applying these findings.

Environmentally stable chemically amplified resist

The dissolution characteristics and the free volume of the chemically amplified resists were studied for the improvement of their environmental stability. In the case of the environmentally unstable t-Boc type resist, we have found that the resist film after 1 hour PED under the 10 ppb NH3 atmosphere showed the decrease of dissolution rates with increasing exposure energies. This fact suggests that it is important to select an appropriate combination of a photoacid generator (PAG) which generates a weak acid and a protecting group which can be easily deprotected under the weak acidity condition. As an example for the resist which deprotection activation energy was small, we examined the development behavior of acetal type resists. Among acetal type resists there are differences for the airborne contamination stability. These differences are caused by the kind of PAGs or solvents. Using the measured results, CD changes and pattern profiles were calculated by photolithography simulator PROLITH/2. It was able to explain the real behavior of the resists. As for the kinds of resist solvents, some structural changes in the resist film may occur by the resist solvent. Especially, the free volume in the resist film correlates to the amount of airborne basic contamination. To estimate the free volume in the resist film, we applied a new technique which was called a positron annihilation. We observed that the free volume in the resist film depended on kinds of solvents. Using these experimental results, a resist which had good PED stability was developed. This resist resolved 0.20 micrometer lines and spaces pattern with good profile using KrF excimer laser stepper and it had also excellent stability for airborne basic contamination and substrate influences.

Design of acid labile protecting groups in chemically amplified resists

The relation between the characters of the protecting groups and lithographic performance in chemically amplified resist was investigated by computational techniques. The stability of de-blocking reaction was estimated by means of molecular orbital calculation (MO). Their calculated results showed that electron withdrawing group increases the activation energy of de-blocking reaction. And the resists which contain these protecting groups were evaluated with acidic top-coating. Though the stability of de-blocking reaction correlated with the sensitivity of the resist, it was not necessarily effective to acidity of top-coating. Furthermore, we investigated the effect of the bulkiness and hydrophobicity of protecting groups to resist performance. As the bulkiness of the protecting group increased, the top shape of lines became better. However, without top coating, too bulky protecting group formed T-top shapes. These results show that the bulkiness of the protecting groups has relation to formation of the insoluble layer. The stabilization energy in the interaction of protecting groups with hydroxyl group of phenol ((Delta) H) was calculated by means of MO calculation. The highest value of (Delta) H between carbonate group and OH group is larger than that between acetal group and OH group by 1 kcal/mol. This result suggests that the interaction between carbonate group and OH of phenol contribute to the ability of dissolution inhibition.