Evaluation of I-line and DUV photoresists for high-density optical disc mastering

Abstract

The next generation in the optical media family which follows Compact Disc and DVD formats, will be a high density format that contains up to 25 GB of information (high definition digital video) on a surface area similar to that of a Compact Disc. The information is recorded as a spiral track of pits on a master disc by applying a lithographic process where a focussed and modulated laser spot exposes a positive photoresist ('mastering' process). Typical pit dimensions are shown below. Compact Disc DVD Next generation (25GB) Pit depth [nm] 140 130 ∼ 85 Pit width [nm] 500 250 ∼ 130 Pitch [nm] 1600 740 ∼ 320Compact Disc and DVD formats can typically be mastered with 413nm Kr-laser and conventional broadband novolak/DNQphotoresists. When striving for higher densities, a reduction in exposure wavelength is needed to obtain a smaller laser spot and thus to create smaller pits. A frequency-doubled Ar laser is therefore chosen as recording laser, λ = 257 nm. The design of a DUV exposure tool will be outlined in chapter 2. The wavelength reduction as such, is insufficient to obtain the required reduction in feature size. Consequently, a higher contrast photoresist process is necessary. Different types of broadband, I-line and DUV photoresists have been evaluated for this specific application (chapter 3). The main target is to obtain maximum resist contrast (γ) and minimal feature size. A maximum resist contrast of 3 has been realized with I-line resist and 8 using a chemically amplified type. Features of 120nm wide and 85 nm deep have been mastered. The photoresist layer thickness varies between 40 nm and 85 nm. The I-line photoresist with the highest contrast is Shipley Ultra-i-123. With this type of photoresist an optical disc with a capacity of 20.7GB of information has been produced. The quality of such a disc is indicated by the time resolution in the readout signal, expressed as the so-called jitter (= time error/clock time). For the optical disc with 20.7GB of information the measured jitter equals 7.5% which is regarded as acceptable in optical read-out signals. For these results the process has been optimized by varying developer concentration, developer type and the exposure dose.Decreasing the concentration of hydroxide in metal ion containing developer (sodium and potassium based) increases the contrast and decreases the surface roughness. Due to the limits of I-line photoresist at 257nm wavelength further experiments with chemically amplified (3.2) and 257nm photoresists (3.3) have been carried out. Airborne molecular contamination causes some problems with the use of chemically amplified photoresist. * roland.tacken@toolex.com; phone +31 40 2581742; fax +31 40 2509805; http://www.toolex.com; Toolex International NV,