Ion beam etching for manufacturing of ultra-thin lithium tantalate for IR Sensors

High-sensitivity sensors for radiation detection in the infrared (IR) range have a wide field of application e.g. for Detectors in IR spectroscopy or contact-free temperature measurement. A common material for those sensors is the pyroelectric lithium tantalate (LT).

To achieve an optimum sensor performance, a reduction of the thermal inertial and hence the LT thickness is required. Classical slicing works down to a thickness of 300 µm, which can be further decreased to about 25 µm by using chemical mechanical polishing (CMP). At this thickness, the probability of crack formation dramatically increases.

The method of choice for further minimizing the LT thickness is ion beam milling with a scia Mill 150/200. By argon etching, the LT is removed with a rate of about 1 µm/h. The substrates require a suitable preparation and masking before they are transferred into the process chamber. Due to the helium backside cooling the substrate temperature is kept low which allows the utilization of a photoresist. For the required LT thickness reduction, a process time of ten to twenty hours is necessary. Via power regulation of the ion beam source, the ion current can be stabilized allowing a precise thickness removal over tens of hours. In addition, smooth etch trenches emerge which are required for a process integration.

For studying the influence of ion beam etching on the IR sensor performance, the specific detectivity D* has to be considered. D* is an expression of the signal-to-noise ratio of a pyroelectric detector and should get maximum. In figure 1, a comparison of D* before (25 µm) and after (5 µm) ion beam etching is shown. It can clearly be seen that D* improves by a factor of two due to the thinning of LT beyond the limit of CMP by ion beam milling.

DIAS Infrared GmbH is using a scia Mill 150 for IR sensor production and are kindly thanked for the provided data.

Related product: scia Mill 150 & scia Mill 200

  • Full substrate ion beam milling with superior homogeneity
  • Etching with inert gases to avoid after-corrosion
  • Thinning of materials like LT beyond the limits of CMP
  • Stable process over tens of hours for removal of several µm of material
  • Helium cooling of substrates for use of photoresist
  • Reactive gas compatibility in RIBE and CAIBE processing
  • Ion beam source with high stability, adjustable ion energy and ion current density
  • Full software integration and automated processes via recipe

Fig. 1: D* of LT Detectors

D* of LT detectors, thickness dp 5 µm and 25 µm.

Reference

DIAS Infrared GmbH

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