Comparison between ZEP and PMMA resists for nanoscale electron beam lithography experimentally and by numerical modeling

Abstract

A modern alternative to the positive-tone PMMA resist is the ZEP 520A (Nippon Zeon) brand co-polymer resist, which offers a higher sensitivity and etch durability for electron beam lithography. However, the molecular mechanisms are not entirely understood, and the relative performance of two resists for various process conditions of nanofabrication is not readily predictable. The authors report a thorough experimental comparison of the performance of PMMA 950k and ZEP 520A resists in MIBK:IPA, ZED, and IPA:water developers. Interestingly, ZEP resist performance was found to depend significantly on the developer. ZED developer increases the sensitivity, whereas IPA:water optimizes line edge roughness and conceivably the resolution at the expense of sensitivity. The authors also describe two alternative numerical models, one assuming an enhancement of the main chain scission in ZEP as a result of electronic excitations in side groups, and another without such enhancement. In the second case, the differences in ZEP and PMMA resists performance are attributed to their different interaction with the developers. Using both approaches, the authors parameterize the respective models of ZEP development by fitting numerical results to the experimental resist morphologies, and analyze the outcomes.

Towards a novel positive tone resist mr-PosEBR for high resolution electron-beam lithography

“…dose-to-clear: 60 µC/cm 2 at 30 kV under the conditions applied in this work)) and especially more etch resistant but at the same time it provides a similar ultimate resolution as PMMA. [12] The higher sensitivity of the ZEP520A as compared to PMMA is ascribed to the presence of the acrylic MCA comonomer. Due to the electron withdrawing effect of the chlorine atom on the quarternary carbon the main chain stability of the polymer is weakened.…”

SML resist processing for high-aspect-ratio and high-sensitivity electron beam lithography

“…The quantity contrast-weighted sensitivity has been introduced as our figure of merit to factor in sensitivity while selecting the developer with the best contrast. The IPA/water developer has other merits including cost, safety, and experience of the EBL community using it as a developer for PMMA [1,19,21] and ZEP [19,22] at both ambient and cold development conditions. In addition to the aforementioned developers, the development of SML in MIBK/IPA (1:3) at -15°C cold development conditions was also attempted; however, due to the extremely low sensitivity (clearance onset >1,000 μC/cm 2 ), it was abandoned.…”

“…This represents a greater than two times improvement over benchmark PMMA resist; however, its sensitivity and resolution are lower than those of PMMA using supplier-recommended conditions. Similar to other positive-tone resists such as PMMA [18], PMGI [8], and ZEP [19], SML may be developed in methyl isobutyl ketone (MIBK)/isopropyl alcohol (IPA) (1:3) solution and rinsed in IPA [20]. …”

Fabricating a high-resolution mask with improved line-edge roughness by using a nonchemically amplified resist and a postexposure bake

“…17,18 To reduce the surface roughness of the granular structure, the resist must be baked at its glass-transition temperature or higher; for ZEP520A, this temperature is 105 C. 6 When a positive-tone chain-scission-type resist, such as ZEP520A or PMMA, is exposed to an electron beam, its main-chain C-C bonds break in response to impacts of primary and secondary electrons as well as electrons backscattered from the substrate. 15,16 This chain-scission reduces the molecular weight of the resist, eventually producing the granular structure. 18,19 The resist should develop from the granular units.…”