Realization of sensors by the transfer technique and by sputtering

The whole equipment includes, an evaporation frame (picture 1), a sputtering frame (picture 2), a polishing workshop, a HF characterization bench as well as various metrology control systems: thickness measurer, profilometer and optical means (picture 3).
The realization of sensors by the technique known as transfer constitutes an essential know-how of our laboratory.

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Principle

Cold bonding of a thick piezoelectric single crystal, on a substrate constituting  the propagation medium, by metallic diffusion
. Thinning, by abrasion, to the desired thickness often equal to half a wavelength in the piezoelectric medium. The limit of piezoelectric thicknesses (a few micrometers), is set by the technological difficulties related to thinning by abrasion.

Alternative

Sputter deposition, which conversely is limited, on the one hand by the difficulty of growing piezoelectric thin films of more than a few micrometers (often subject to internal stresses) and on the other hand to control the crystalline orientation.

Advantage of the carry-over technique

The cross-section of the single-crystal transducer is independent of the substrate cross-section, making the choice of the elastic wave mode free. (LiNbO3) is the ideal candidate due to its excellent electromechanical coupling coefficients, quasi-Longitudinal (KL=0.49) and quasi-transverse (KT=0.62).

Acoustic and electrical characterization of the translators

The acoustic characterization bench is shown in photo 4.

Electrical characterization of sensors consists of measuring their electrical impedance via a network analyzer and comparing it to previously established simulation results.

The strong evolution of acoustics activities towards system integration leads our laboratory to develop innovative technology processes based on the integration of small size and high frequency acoustic components.

This leads us to realize HF translators resonating around the GHz bonded on thin silicon substrates. A wide band (300KHz to 8GHz) pulse network analyzer, shown in picture 5, allows electrical measurements on small size (100 mm) sub-tip translators. In Figure 1 we can see the electrical response obtained on a resonant sensor at 1 GHz thinned to 2.7 mm.

Figure 1: Electrical impedance of a resonant transducer at 1 GHz