ELECTRONICS LETTERS 4th February 1999 Vol. 35 No. 3 Terahertz time-domain spectroscopy of films fabricated from SU-8 S. Arscott, F. Garet, P. Mounaix, L. Duvillaret, J.-L. Coutaz and D. Lippens Terahertz time-domain spectroscopy has been used to characterise the refractive index and absorption coefficient of samples fabricated using the negative photoresist SU-8, in the frequency range 0.1– 1.6THz. At a frequency of 1THz, the corresponding dielectric constant and dielectric loss tangent are 2.9 and 6.3 × 10 –6 , respectively. Introduction: The negative photoresist NANO™ XP SU-8 is attract- ing much attention for applications which range from microstruc- tures for micromachining [1] to being a potential component in ultra-high frequency microcircuits [2]. The high aspect ratios (>10:1) and ultra-thick layers (2000 μm) which are attainable by patterning the photoresist facilitates the manufacture of 3D features. Such fea- tures could be utilised in conjunction with ultra-high frequency devices [3] for the next generation of integrated circuits with target operating frequencies in the terahertz (10 12 Hz) region. To our knowledge no studies have been carried out into the dielectric prop- erties of SU-8 in this frequency domain. In this Letter, we have char- acterised the refractive index and absorption coefficient of SU-8 from 0.1 to 1.6THz using terahertz time-domain spectroscopy. Fabrication of samples: SU-8 is a three-component epoxy based neg- ative photoresist formed by dissolving the resin (SU-8) in an organic solvent γ-butyrolactone (GBL) and finally adding a photo-initiator (triaryl sulphonium) which makes up 10% of the resin mass. The resist can be easily spun using a conventional spin-coater, photolith- ographically patterned and annealed to form the resultant cross- linked polymer, which is an extremely durable material [4]. In prac- tice, the samples used for this study were fabricated using SU-8 100 resist which has silicon wafers (φ = 2 in) which had been successively cleaned in acetone, isopropanol and water and dried at 120°C prior to resist deposition. Approximately 5ml of the resist was delivered onto each wafer to obtain full coverage. A 60s spin was then per- formed at 400rpm and 500rpm in order to obtain two samples hav- ing a nominal thickness of 620 and 520 μm, respectively. The SU-8 layers were then photolithographically patterned into the correct size required for the terahertz time-domain spectroscopic measurements (20mm |20mm). The resultant SU-8 samples were released from the silicon wafers using a lift-off technique which involved etching the silicon completely away in a KOH:H 2 O (50g:100ml) solution at a temperature of 65°C. There is an extremely high etch-selectivity between the materials since cross-linked SU-8 is inert to such hot alkaline solutions. F ig. 1a shows an SEM image of typical features which we are able to produce. A high aspect ratio (15:1) and high uniformity in the resist thickness across the wafer have been achieved by careful opti- misation of the processing conditions. Fig. 1b shows an image of a novel feature that we are able to fabricate by using a multi-exposure technique on a single layer of resist. By varying the exposure time we are able to control the thickness of the membrane-like feature. M easurement: Terahertz time-domain spectroscopy has already been used to characterise dielectric films such as polyimide. Here we measured the refractive index n and the absorption coefficient α of the SU-8 samples with a typical experimental setup [5] schematically shown in Fig. 2. Sub-picosecond electromagnetic pulses are gener- ated by pulsed optical excitation of an LT-GaAs photoconductive switch using a self-modelocked Ti:sapphire laser. The collimation of the terahertz signal is obtained by a hyperhemispherical lens fash- ioned from high-resistivity silicon. The inset to Fig. 2 shows the elec- tromagnetic spectrum which is generated for measurement, demonstrating that a signal covers the bandwidth 0.1–2.5THz. Results: Fig. 3 shows the variations of refractive index and absorp- tion coefficient of the SU-8 samples with frequency between 100GHz and 1.6THz. Above this frequency the magnitude of the detected signal becomes too low. The precision is ~2% for the refrac- tive index and 5% for the absorption. Over the measured frequency range, it can be seen that n is a relatively flat function of frequency, decreasing from 1.8 to 1.7 (variation of 5%) over a bandwidth of 0.1–1.6THz. The value of α is 25cm –1 at 1THz; α is seen to increase with frequency in an approximately linear fashion. The value of α, at a measurement frequency of 1THz, can be compared with values of absorption coefficient for other materials. Notably it is of the same order of magnitude as epoxy-glass or PEEK (polymer polyethyl- ethylketone) but much higher than quartz [6]. The weak oscillations observed for both index and absorption curves can be attributed to a slight lack of parallelism in the sample. Functions of dielectric constant ε R and dielectric loss tangent tan(δ) against frequency can be calculated from these measured parameters to give meaningful data for future terahertz frequency design. The dielectric constant is seen to be very constant over a large frequency range, e.g. varying from 2.8 to 3 in the range 0.1– 1.6THz for the 520 μm thick sample. The associated dielectric losses (tanδ) is 6.3 × 10 –6 at 1THz. Fig. 1 SEM micrographs of high aspect ratio features and novel mem- brane-like features which can be formed using S U-8 a High aspect ratio features b Membrane-like features Fig. 2 Schematic diagram of experimental setup for terahertz time-domain spectroscopy Inset: typical generated electromagnetic spectrum Fig. 3 M easured variation of refractive index n and absorption coefficient α of S U-8 samples against frequency 620 μm sample thickness ——— 520 μm sample thickness