Ion Beam Etching for Manufacturing of Ultra-Thin LT 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 pyroelectric lithium tantalate (LT).

To achieve an optimum sensor performance, a reduction of the thermal inertial, and hence the thickness of the lithium tantalate is required. Classical slicing, based on a grown single crystal of a certain diameter, works down to a thickness of 300 µm, which can be further decreased to about 25 µm by using chemical mechanical polishing (CMP). Achieving a smaller thickness of the LT substrate is not possible due to the probability of crack formation, which dramatically increases at this point. The method of choice for further minimizing LT thickness in order to increase D* is Ar+ ion beam etching, also known as ion beam milling.

Before the etching process the LT substrates had a thickness of about 25 μm. It was coated with a metal layer as back electrode and a structured photoresist (PR) mask on top. Figure 1 shows a sketch of the integration of an ion beam etched LT sample in a pyroelectric sensor. The top electrode is deposited after ion beam etching. While the sensitive area in the center should be as thin as possible, the thicker frame is necessary for mechanical stability of the sensor chip. 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 helium backside cooling contact the substrate temperature is kept low, which allows the utilization of a photoresist. For the required LT thickness reduction a long term stable process over several hours is required. 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.

The long term stability test was evaluated by etching a reference wafer with thermally oxidized silicon for three minutes. Afterwards the etch profile and uniformity were analyzed. In the following, a silicon wafer with PR mask was etched for twelve hours with a substrate rotation of 3 rpm. The homogeneity measurement was performed by two line scans using a profiler. In Figure 2 the distribution of the removed material on the silicon wafer is shown. A uniformity of ± 0.7 % (sigma/mean) across the 150 mm wafer was obtained, with a mean removal rate of 1.3 μm/h. A comparison with the reference wafer shows that both wafers exhibit a similar homogeneity and etching profile. This confirms the long-term stability of the ion beam source.

For studying the influence ion beam etching on IR sensor performance, the specific detectivity D* has to be considered.The specific detectivity D* of a piezoelectric element is an expression of the signal-to-noise ratio of a pyroelectric detector and should be maximum. In Figure 3 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 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.