Composite diamond films for short-mm wave and THz traveling wave tube windows

•SEM was used to study the growing process of UNCD on PCD at an early stage•Effect of UNCD thickness on fracture strength and thermal conductivity were studied.•A 100 μm composite film was designed and fabricated, and two 180 GHz TWT windows fabricated.•Two diamond windows were tested for RF performance and mechanical properties.

With an increase in frequency, the diamond thickness of the microwave windows for short-mm wave and THz traveling wave tubes (TWTs) approaches 100 μm or even tens of μm. This poses problems of mechanical strength and air tightness to the polycrystalline diamond (PCD) window. To overcome these problems, we have studied a composite diamond film that consists of PCD and ultra-nanocrystalline diamond (UNCD). First, SEM was used to examine the early growing process of UNCD on PCD. The 5 μm thick UNCD grown on 40 μm PCD exhibited a hillock structure with densely packed ≤ 20 nm granules, in contrast to the PCD layer showing randomly packed, micrometer sized grains. Then, the effect of UNCD thickness on fracture strength and thermal conductivity was studied using the test samples with thin layers of UNCD having thicknesses of 1, 2.5, 5, and 10 μm on 100 μm thick PCD films, respectively. The fracture strengths of all the films are 2–3 times higher than that of the PCD films, which is 350 ± 150 MPa. As expected, the thermal conductivity of the samples measured at ~ 20 °C decreases with an increase in UNCD thickness, particularly in the range of 0 to 2.5 μm. At a thickness of 10 μm, the thermal conductivity was found to be ~ 10 W/cm ∗ K. Finally, a 100 μm sandwich-like structure with a total UNCD thickness of 10 μm was fabricated and two 180 GHz TWT windows were assembled. RF tests show that for the operating frequency range of 175 to 185 GHz, the transmission loss (S21) was found to be ≤ 1.22 and ≤ 1.71 dB, respectively, indicating an excellent RF performance. Mechanical strength and air tightness of the windows were also found improved and able to meet the requirement of the device. This work provides a novel approach for fabricating relatively thin diamond films for RF applications, such as TWT windows.