Lithographic Evaluation Of Copolymers With Enhanced Dry Etch Resistance

Abstract

Alternating copolymers of alphamethylstyrene (AMS) with maleic anhydride (MA) and methyl maleate (MeM) are evaluated as positive electron resists. The chain scission efficiency (Gs) of P(AMS-MA), determined by exposure to 50 keV electrons, is 0.90 scissions/100 eV. When the maleic anhydride in the copolymer is reacted with sodium methoxide to form its methyl ester, P(AMS-MeM), the Gs increases to 2.9 for electrons and to 3.5 for gamma radiation. Based on these G-scission values, this copolymer is expected to exhibit enhanced sensitivity, while having good dry etch resistance due to the aromatic nature of alphamethylstyrene. Lithographically, P(AMS-MeM) is more sensitive than P(AMS-MA), as expected from G-scission data. Film properties such as adhesion are also superior for P(AMS-MeM). Using a one hour prebake at 140°C, 10% thinning of unexposed P(AMS-MeM) occurs upon development of pads exposed to an incident electron dose of 8 jC/cm2 (accelerating voltage = 20 kV). The contrast (1) is 2.0 for development of 12 iiC/cm exposur2es. In comparison, P(AMS-MA) exhibited 10% thinning for an incident dose of 40 pC/cm, which is similar to observations with PMMA. The copolymers are developed with mixtures of ethyl 3-ethoxypropionate and 1-methoxy-2-propanol acetate. The dry etch rate of P(AMS-MA) in CFI.' plasma with 8% 02 varies from 45 to 53% of the etch rate of a PMMA standard. The etch rate of P(AMS-MeM) after a 140°C prebake is about 65% that of PMMA. Thus, much of the etch resistance of alphamethylstyrene is maintained in copolymers with maleic anhydride or methyl maleate, while the copolymer with methyl maleate also exhibits significantly enhanced sensitivity.