*2.2. Manufacturing*

Figure 2 depicts a schematic cross-section of the photoacoustic transducer. Its centerpiece is a thin, homogeneous glass membrane that has been wet-etched from a borosilicate glass wafer with a thickness of approximately 500 μm. A silicon nitride (SiN) hard mask was applied from the top and the bottom side to the glass wafer and was structured by lithography. At the openings of the hard mask, an anisotropic etch process symmetrically created a cavity on both sides of the wafer, leaving residual thin membranes with a diameter of approximately 3.6 mm. Defined by the duration of the etching, thicknesses between about 10 μm and 70 μm could be achieved. Within each membrane, the thickness tolerance was found to be ±1 μm. For investigation of the concept, we focused on samples with the thinnest membrane thickness, as they are the most flexible and have the highest mechanical compliance.

After etching, the glass wafer was anodically bonded to a blank silicon wafer, which was dry-polished in order to achieve the highest bond quality. Furthermore, for the purpose of maximizing the infrared light transmission, the used Si wafer was low doped and backside-grounded to a thickness of 250 μm. The anodic bond was formed at 300 °C while applying a CO2 bias atmosphere of 2 bar. Thus, after cooling down to ambient temperature, a CO2 filling of about 1 bar was captured in the gas-tight cavity between the glass membrane and silicon lid. After bonding, the wafer was mechanically sawed to form transducers of the size 5.5 mm by 5.5 mm.

Using Fourier transformation infrared (FTIR) spectroscopy, an absorbing behavior at the CO2 absorption bands was confirmed as shown in the transmission spectrum (Figure A1).The characteristic shape of the CO2 absorption at 4.26 μm was clearly visible with a relative absorption of about 15 % to 20 % compared to the background. The spectrum was limited at the lower end by the natural transparency of silicon and the capabilities of the used FTIR spectrometer and on the upper end by the borosilicate glass, which is nontransparent for light with wavelengths above 6.6 μm.

**Figure 2.** Proposed detector element comprising the photoacoustic transducer, a microphone, sealant compounds and a carrier PCB (not to scale).

The singularized transducers were then treated by means of physical evaporation and deposition to form a nickel-gold coating as a nontransparent, reflecting layer for the infrared light. In this way, it was ensured that no IR light could hit and potentially disturb the microphone membrane and, additionally, that reflection caused a second pass of the light through the absorption cavity, increasing the absorption inside.
