Enhancing THz Micro‑Bolometer Performance with Spiral‑Type Antenna‑Coupled Micro‑Bridge Designs

Background

Terahertz radiation (0.1–10 THz) offers unique spectral features—broad bandwidth, low‑energy penetration, and distinct absorption signatures—that make it valuable for molecular spectroscopy, medical diagnostics, and security imaging. Yet, the full potential of this band remains untapped due to limited THz sources and detectors. Advances in ultrafast electronics, laser science, and semiconductor processing have produced quantum cascade lasers (QCLs) and powerful far‑infrared CO₂ lasers capable of delivering tunable or high‑power THz beams.

THz detection typically relies on photon‑based devices, such as SIS junctions and quantum‑well photoconductors, which achieve high sensitivity and rapid response but require cryogenic cooling and exhibit narrow spectral selectivity. Photothermal detectors—including pyroelectric elements and micro‑bolometers—operate at room temperature and possess wideband spectral response, making them attractive for large‑area focal‑plane arrays (FPAs). However, conventional micro‑bolometers suffer from low THz absorption because the standard micro‑bridge architecture is optimized for mid‑infrared wavelengths.

Enhancing THz absorption in micro‑bolometers has been pursued through impedance‑matching metal films, metamaterial absorbers, and antenna coupling. Antenna‑coupled micro‑bolometers combine the high absorption of an engineered antenna with the thermal sensitivity of a micro‑bridge, yielding superior performance. Prior work has demonstrated antenna‑coupled VOx bolometers at 94 GHz and metal‑oxide semiconductor FET bolometers in the 0.5–1.5 THz range, as well as real‑time 2.5 THz imaging with QCL illumination.

While planar antennas are easy to fabricate, wire‑type antennas with reduced bulk volume accelerate heating and lower thermal time constants. Building on our earlier work with a 35 µm × 35 µm pixel, we now present a 25 µm × 25 µm micro‑bridge with three spiral‑type antenna variants that enhance THz absorption and enable frequency modulation around 2.52 THz.

Results and Discussion

The micro‑bridge pixel, illustrated in Fig. 1a, features a 20 µm × 20 µm sensitive area supported by two long legs. The sensitive region comprises a 0.4 µm Si₃N₄ support, a 70 nm VOx thermal layer, and an 0.05 µm Al spiral antenna. A 0.2 µm NiCr reflector below the active area forms a 2 µm high resonant cavity that optimizes infrared absorption while isolating THz heating. Spiral antennas are confined within an 18 µm diameter on the Si₃N₄ support. Fig. 1c–e show the three proposed antenna configurations with linear elements on the bridge legs.

Design of spiral‑type antenna‑coupled micro‑bridge structures. a Model of micro‑bridge structure. b Spiral antenna on the support layer. c Spiral antenna with a single linear antenna on one leg. d Spiral antenna with two separate linear antennas on the legs. e Spiral antenna with two connected linear antennas on the legs. f Directions of electric and magnetic fields for vertical incidence.

Spiral‑Type Antenna on the Support Layer

We first optimized the standard spiral antenna on the support layer, varying the rotation angle (360° × n, 0.5 ≤ n ≤ 2.0), line width (w), and spacing (g). Figure 2a shows that increasing n from 0.5 to 0.9 shifts the primary absorption peak downward and reduces the peak absorption from 90 % to 65 %. At n = 1.6, the peak aligns with 2.52 THz, albeit with only 30 % absorption. Further adjustment of w and g (Fig. 3) demonstrates that larger antenna footprints lower the resonance frequency but provide only modest gains in absorption efficiency. The 25 µm × 25 µm pixel’s smaller active area limits the maximum attainable absorption compared to our previous 35 µm × 35 µm design.

Spiral‑Type Antenna with a Single Separate Linear Antenna on One Leg

Inserting a single linear antenna on one bridge leg expands the effective antenna area and introduces an additional low‑frequency resonance. For n = 1.6 and a line width of 1 µm, positioning the antenna port within 2 µm of the sensitive area merges the two resonances into a broad peak centered at 2.52 THz, achieving over 40 % absorption across a 0.4 THz bandwidth (Fig. 4b). The extra resonance is driven by electric‑field coupling at the antenna‑leg interface, as confirmed by energy‑density maps (Fig. 6c‑d).

Spiral‑Type Antenna with Two Separate Linear Antennas on Both Legs

Doubling the linear antennas further enlarges the absorption area. At n = 1.1, the combined structure reaches absorption rates above 90 % at lower frequencies (Fig. 5a). With n = 1.6, the two resonances overlap to form an even wider, flat absorption band around 2.52 THz (Fig. 5b). Energy‑density analysis (Fig. 6e‑f) shows that the electric‑field absorption now occurs primarily along both legs, while magnetic‑field absorption remains concentrated in the central spiral. The resulting power‑loss distribution (Fig. 6g‑h) is confined to the sensitive area, ensuring efficient conversion of THz energy into a temperature rise in the VOx layer.

Spiral‑Type Antenna with Two Connected Linear Antennas on Both Legs

Connecting the linear antennas on the legs yields a more compact antenna with a single metal layer that serves as both antenna and electrode lead. Figure 8 shows that varying the line width from 0.8 µm to 1.1 µm produces absorption rates above 70 % at 2.52 THz, demonstrating fabrication tolerance. Energy‑density maps (Fig. 9) reveal that the central spiral dominates magnetic‑field absorption while the linear sections contribute mainly to electric‑field coupling, again localizing the loss to the active area. This configuration offers a streamlined, high‑integration design with minimal impact on thermal response.

Thermal Response Considerations

The thermal time constant τ = C_tot / G_eff is governed by the total heat capacity C_tot and effective thermal conductance G_eff. For a fixed bridge geometry, C_tot is dominated by the antenna volume. By replacing planar antennas with slender linear elements, we reduce C_tot and achieve a 16.3 % shorter τ relative to conventional dual‑metal‑layer designs. This improvement translates directly into faster detector response times, essential for real‑time THz imaging.

Conclusions

We have systematically designed, simulated, and optimized four spiral‑type antenna configurations for 25 µm × 25 µm micro‑bolometer pixels operating at 2.52 THz. Introducing linear antennas on the bridge legs expands the effective antenna area, lowers the resonance frequency, and enhances absorption. The dual‑linear‑antenna layout delivers a wide, high‑efficiency absorption band, while the connected‑antenna design offers a stable, high‑absorption peak with simplified fabrication and integration. These architectures promise room‑temperature THz FPAs with rapid response and high sensitivity, advancing the practicality of THz imaging and sensing systems.

Methods

Finite‑element simulations were performed in CST Microwave Studio 2016 using a 25 µm × 25 µm unit cell with periodic boundary conditions along x and y. The micro‑bridge structure was illuminated from the top (port 1) and transmitted through the bottom (port 2). Complex S‑parameters were extracted, and absorption was calculated as A = 1 − |S₂₁|² − |S₁₁|². Materials were modeled with realistic conductivities: σ_Al = 3.56 × 10⁷ S/m, σ_NiCr = 1 × 10⁷ S/m, and Si₃N₄ with a second‑order dispersion model (ε ≈ 2.0, μ = 1). The 2 µm vacuum cavity was represented by ε = 1.