ITO‑Film‑Based Heaters
Due to the tiny dimensions of micro‑vapor cells, they contain an extremely small number of atoms at room temperature (typically around 35 000 000 atoms). A low atom count results in low light absorption and correspondingly weak CPT signals. Since atomic density rises with temperature, the cells are normally operated within 80 °C to 130 °C to boost the atom population until the absorption rate approaches 50 %. Micro‑cell heaters are adopted to heat the cells.
Such heaters are fabricated by depositing an indium‑tin‑oxide (ITO) layer onto a glass substrate via electron‑beam evaporation. ITO is a transparent conductive material, which enables the formation of resistive heaters distributed across the substrate surface while permitting light transmission. An approximately 30 nm‑thick ITO film yields a sheet resistance of 100 Ω/sq. Gold bus‑bars deposited near the edges of the finished heater element allow gold wires to be bonded to the assembly. The opposite ends of the gold wires are connected to the CSAC substrate to establish electrical pathways.
Fabrication of Magnetic‑Field‑Canceling Heater
Two glass substrates with patterned ITO are bonded together using non‑conductive epoxy to fabricate a magnetic‑field‑canceling heater. The faint‑colored lines within the heater correspond to regions where ITO has been removed by laser patterning. After ITO patterning, gold pads are deposited for wire bonding. A 2 μm benzocyclobutene (BCB) insulating layer is deposited over the remaining exposed ITO, so that electrical contact between the upper and lower ITO layers is established exclusively through the gold pads. Large arrows mark the positions where current enters and exits the heater. Conductive epoxy provides electrical connection between the top‑side and bottom‑side ITO layers.
Heater design is critical for magnetometer operation. Magnetic fields generated by any electric current near the vapor cell can be sensed by atoms; therefore special care must be taken to minimize disturbances induced by heater current. To largely cancel the magnetic field produced by the ITO‑heater current, the current path is arranged for magnetic self‑compensation. This is accomplished by laser‑patterning the ITO to form 120 μm‑wide ITO strips (Figure 2). The meandering current path yields a high heater resistance (2 kΩ), which reduces current magnitude and dissipates power across most of the heater for uniform heating of the cell window. To create a return‑current path offset by only approximately 10 μm relative to the incoming‑current path, two patterned ITO heater chips are bonded together, where the second heater carries a mirror‑image pattern of the first. Consequently, the magnetic field generated by current flowing in the lower ITO layer is counter‑balanced by the field from the return current flowing directly above it in the upper ITO layer.
To further suppress interference from ITO‑heater current, alternating‑current (AC) power at 2 kHz‑10 kHz is supplied to the heater. Magnetic‑field measurements under AC and DC heater excitation were compared. Under DC operation, the heater produces a magnetic field of 23 nT and introduces 110 Hz broadening of magnetic resonance (against total resonance broadening of 1800 Hz).
As an alternative to ITO‑based heaters, we have developed monolithic “in‑situ” heaters integrated inside the cell unit.
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