1. Introduction

Metamaterials are a class of engineered materials capable of manipulating electromagnetic waves. They are often assembled from cellular units in repeating patterns and derive their properties from the materials involved and their structure. While metamaterial research aims at developing tunable properties [1], passive components still prevail in laboratory and consumer applications. Terahertz (THz) optoelectronics is essential for communication, security, chemical analysis, and medical applications [2]. Low-pass, band-pass and high-pass filters enhance the signal-to-noise ratio of detectors and cameras by suppressing background radiation. They are also used in spectrally sensitive experiments. However, the center frequency and the bandwidth vary from one application or experiment to the next, which asks for tunable devices or an agile design and production strategy. The design and fabrication of THz band-pass metamaterials have recently been reviewed [3].

Most THz filters are based on metallic meshes or similar structures manufactured by photolithography and electroplating [4,5] or laser machining [6]. Some commercial products are fabricated from metallic foils, meshes, or wire grids. These devices need to be several micrometers thick for mechanical stability. While metallic meshes are freestanding, they are very delicate to handle unless mounted in a frame. If they are unprotected, they are prone to damage upon handling. Typically, the manufacturer warranty is voided when the sealed container is opened. Some vendors deliver a pair of protective windows to sandwich the element and mount it inside a metallic housing. This turns the element into a heavy and clumsy device, only allowing standard geometries. Therefore, attempts were made to ruggedize the devices by using a rigid substrate. The use of quartz [7], silicon [8], barium fluoride [9], or silicon nitride [10] has been reported.

The disadvantages of rigid substrates include their brittleness and a pre-defined substrate size to avoid machining. Their back and front surface lead to multiple beam reflections, which can introduce Fabry-Perot artifacts, especially in broad-band operation. Moreover, stacking of metamaterials is either limited to the two substrate surfaces or requires stacking of another rigid element, generating multiple interferences. As stacked metamaterials are expected to have improved functionalities [11], polymer films being low-loss dielectrics were introduced as substrates for metamaterials [12]. Due to their widespread use in flexible electronics, polydimethylsiloxane (PDMS) and polyimide are the most common flexible substrates. Other materials include metaflex, polyethylene naphthalene (PEN), polyethylene terephthalate (PET), polymethylmethacrylate (PMMA), and polystyrene. Commercial thin-film polarizers are based on an array of metal stripes on polymer membranes (e.g., from Microtech). The array of metal strips is fabricated using an ultrafast laser micromachining technique. However, laser machining is limited in lateral resolution and structure quality, and is based on thick metal films. Dielectric materials and fabrication techniques for THz metasurfaces based on resonant structures have recently been reviewed [13]. Recently, Yao et al. reported THz filters based on polyimide films with sandwiched cellular structures made of aluminum [14]. In their design, metamaterial layers were stacked using a cyclic olefin copolymer (COC) spacer and hot pressing to compound the device. Stacking of copper structures using consecutive spin-coating of PDMS and metallization was used to fabricate second-order THz band-pass filters [15]. Cyclic olefin copolymers have been well researched in the THz regime [16,17] because they have a very low absorption from UV to THz, except for some absorption lines in the infrared, they are biocompatible and have high chemical stability. Cyclic olefin polymer substrates were used, among others, for polarizers [18], antennae [19], and THz filters [20,21]. They allowed for patterning resonators of gold thin films on spin-on COC and can be extended to multilayer structures [20], although the latter seems complicated to achieve [13].

A significant disadvantage seen in the reported work is the mounting of the fabricated planar devices. Pavanello et al. [20] use an annular metal holder, while Ebrahimi et al. [15] use a large clamp. Therefore, we suggest 3D printed frames, as 3D printing of COC is a versatile method for producing THz devices such as waveguides and lenses [22].

In this work, we combine microfabrication techniques on COC films with 3D printed frames using COC filament, and we develop free-standing THz band-pass filters with thickness as low as 1 µm and transmission T > 90%. We focus on designing, fabricating, framing, and characterizing free-standing band-pass filters to be integrated into THz experiments. We implemented cross-slit designs and established a processing technology to fabricate the device on cleanroom-compatible materials. These methods allow the THz filters to be fabricated with small to medium-sized batches of elements. Then, the 3D printed frames can be directly 3D printed on the THz band-pass filters. THz time-domain spectroscopy (THz-TDS) was applied to assess the filter performance.

2. Design and simulation

Starting from the well-known cross-shape design of band-pass filters [21,23], the geometric parameters were adapted to different substrate materials, such as quartz, silicon, and TOPAS, with thicknesses from 1 to 200 µm to match the design wavelength. The metasurface material was implemented as a perfect conductor or as gold with a thickness of 200 nm. For all numerical simulations, we used the Radiofrequency module of COMSOL Multiphysics [24]. The dielectric permittivity ε= 2.33 was calculated using the values of the complex refractive index of TOPAS [22]. For the simulations, the thickness of TOPAS was set at 1 µm, and similar results can be obtained with thicknesses >20 µm. An empiric formula for the band-pass center frequency f₀ is given by [3]

The geometric parameters are defined in Fig. 1(a), with L being the overall length of the limbs, W their width, and P the pitch of the repeating structure. Varying the geometry of the elements allowed for adapting the design to the desired center frequency of the band-pass filter. In our work, we designed filters from 0.3 to 4.8 THz. Figure 1(b) shows the result for a band-pass filter at 0.37 and 1.05 THz. While the center frequency is given by the geometry of the individual cross, Eq. (1), the transmission peaks at higher frequencies are guided-mode resonances (GMR). They occur at frequencies where the higher-order diffracted waves are phase-matched to modes guided in the dielectric substrate [21].

 

Fig. 1. (a) Geometry of the unit cell; (b) Comsol simulations using the geometry shown in (a) with 200 nm Au outside the crosses on a TOPAS substrate, illustrating two center frequencies and the effect of a protective coating.

Download Full Size | PDF

We also simulated a design with an additional layer of TOPAS on top of the crosses as a protective coating for the filter with center frequency f₀ = 1.05 THz. Figure 1(b) shows that the additional layer shifts the center frequency down to f₀ = 0.86 THz. The dimensions for the filters are given in Table 1.

Table 1. Values for L, W, and P (Fig. 1 a) for filters with center frequency f₀ = 0.370 THz and 1.05 THz.

View Table

3. Experimental

To confirm the suitability of TOPAS for a filter substrate, we characterized its optical properties in the THz regime using a THz-TDS system (Toptica, Gräfelfing, Germany). The instrument was also used to measure the spectral transmission of TOPAS substrates with various thicknesses from d = 1 µm to 2 mm and for filter performance measurements. We used a spectroscopic ellipsometer (M-2000-VI, J.A. Woollam, Lincoln NE, USA) for precise thickness measurements of thin substrates. Figure 2 summarizes the fabrication process described in the text.

 

Fig. 2. Schematic representation of the fabrication process. a) Deposition of a thin layer of PMMA on a Si substrate. b) Deposition of TOPAS on the top of the PMMA layer. c) The device after the laser writing process and the Au deposition. d) Deposition of an additional layer of TOPAS (optional). e) Direct 3D printing of a holder using TOPAS filament on top of the device. f) Final filter after removing the substrate in an acetone bath.

Download Full Size | PDF

While polymer substrates of a given thickness are available, here, we spin-coated a thin layer of THz transparent cyclic olefin copolymer TOPAS (micro resist technology GmbH, Berlin, Germany) onto a separation layer of PMMA on a 4-inch Si carrier wafer, Fig. 2(a) and (b). Then, the film thickness was adapted by selecting the appropriate TOPAS dilution and the processing parameters, such as rotational speed and time. This allowed us to obtain thinner substrates than reported, e.g., by Ferraro et al. for Zeonor [21]. Finally, the spin-coated film was baked at 100  °C for 10 mins.

Further processing was performed in a cleanroom facility of ETH's FIRST lab. The filter design was transferred to a spin-coated photoresist layer (AZ 5214 E) using a laser writer DWL66+ (Heidelberg Instruments Mikrotechnik GmbH, Heidelberg, Germany) with optimized power. The device was then baked and exposed to light with 10 mW/cm² for 25 s before development with MF319 MICROPOSIT developer for roughly 1 min. First, oxygen plasma was applied to remove resist residues from the developed areas. Then, a 5 nm Ti layer was deposited, followed by a 200 nm Au layer by e-beam evaporation. Afterward, the wafer was left in hot acetone to lift off the excess gold, Fig. 2(c). After development and metallization, the wafer was post-processed with a protective layer of the same TOPAS polymer, Fig. 5(d). By repeating the process [20], we achieved stacking two metamaterial layers to sharpen the cut-off frequency edge of low-pass filters.

Before removing the filters from the carrier wafer, frames were 3D printed around each filter element with a 3D printer (RepRap Industrial, Kühling&Kühling, Kiel, Germany) using a TOPAS filament with a diameter of 2.85 mm (Creamelt, Rapperswil-Jona, Switzerland), Fig. 2(e). Using the same type of material as the protection layer guaranteed excellent adhesion. Finally, we left the wafer in an acetone bath for one day to remove the device from the carrier, Fig. 2(f).

4. Results

The optical properties of TOPAS in the THz regime were obtained from a d = 2 mm sheet, Fig. 3(a). The measurement confirmed the low absorption of $\alpha < $ 1 cm⁻¹. Also, we measured the THz transmission of TOPAS films of various thicknesses, Fig. 3(b). The etalon effect is appreciable for thicknesses > 25 µm, and the field transmission is $T < \; $1. However, for a TOPAS film with $d = \; ($1.00 ± 0.02) µm, the transmission is close to $1.$ Note that the dips appearing at 1.7 and 1.9 THz are artifacts caused by water absorption lines. The various total thicknesses of the TOPAS films in Fig. 4(b) are given by the films that were available from TOPAS Advanced Polymers GmbH, namely the films with the thicknesses 50 µm, 100 µm, 400 µm, and 2 mm and thicknesses of films that we developed (1 µm and 25 µm), using cyclic olefin copolymer TOPAS resist.

 

Fig. 3. (a) Refractive index and absorption of a TOPAS substrate with thickness d = 2 mm. (b) Transmission of TOPAS films of thickness d = 1 µm, 25 µm, 50 µm, 100 µm, 400 µm, 2  mm.

Download Full Size | PDF

 

Fig. 4. Band-pass filters. (a) Microscope image of a 1 THz BPF; (b) Measured transmission spectra of bandpass filters with a center frequency of 0.35 THz (red line) and 1.0 THz before (blue) and after (dotted) applying a protective polymer layer, and a 1 THz filter on a 1 µm thick substrate (black).

Download Full Size | PDF

After fabrication, the quality of the THz filters was assessed using optical microscopy, Fig. 4(a), while their performance was characterized using THz-TDS, Fig. 4(b). The measured filter parameters were compared to the simulation results to identify any deviations, e.g., due to geometrical inaccuracies or inappropriate material parameters used in the simulation. After iterating the simulations, the design specifications were met. Figure 4(b) shows the transmission of a THz filter on a 25 µm TOPAS film with center frequency ${f_0} = $ 0.35 THz and a filter before coating with a center frequency ${f_0} = $ 1 THz and after coating ${f_0} = $ 0.85 THz. The coating film thickness was measured at 20 µm. These results are similar to the simulated results shown in Fig. 1(b), with the first filter with center frequency ${f_0} = $ 0.35 THz, showing ${f_0} = $ 0.37 THz in the simulations and the second filter before and after coating showing frequencies ${f_0} = $ 1.05 THz (in simulations ${f_0} = $ 1 THz) and ${f_0} = $ 0.86 THz (in simulations ${f_0} = $ 0.85 THz) respectively. We also show a ${f_0} = $ 1 THz filter on a TOPAS film with thickness $d = $1 µm. As can be seen, the transmission is higher than 0.90.

We designed different frames using CAD software. These frames allow for easy handling and mounting to standard optomechanical holders. For example, Fig. 5(b) shows quadratic frames for mounting in 50 mm filter holders, whereas Fig. 5(c) shows a customized 1-inch holder with an integrated thread mounted in the THz-TDS setup.

 

Fig. 5. (a) Si wafer with a set of nine band-pass filters after clean-room fabrication; (b) 3D printed quadratic frames around diffraction gratings for mounting to filter holders; (c) a freestanding filter with a 3D printed 1-inch frame including a mounting thread in the THz-TDS setup.

Download Full Size | PDF

5. Conclusions

Combining TOPAS and gold led to robust metamaterials such as THz band-pass filters. In addition, TOPAS is an efficient moisture barrier with a glass temperature above 120°C, which promises excellent humidity and temperature stability. The project resulted in band-pass and low-pass filters for integration in THz experiments or devices such as cameras. The developed process flow will provide dedicated high-end filters for camera integration and free-standing filters for the standard THz market. However, the process described here is not limited to the THz regime or wavelength filters. Based on standard microfabrication techniques, it has a high potential for various wavefront engineering applications such as reflectors, polarizers, 2D lenses, or waveplates.