17.1.4 Applications

Application areas of thick film devices include piezoelectric strain gauges for force, weight or pressure; resistors for fluid level gauges, flow meters and position sensors; gas and humidity sensors (for example to detect toxic gases, tin oxide ceramic is used as a thick film on an alumina substrate, operated at between 300 and 500 ºC), and for the heating element in high speed kettles where the dielectric film is deposited onto a stainless steel carrier for the water side and a spiral heating element fired on to the other side of the dielectric.

Multi-level co-fired ceramic circuits were first used for military applications. LTCCs were used in the early 80’s mainly in the military and medical fields, but in the 90’s broke into cell phones and wireless devices (Wide Area Networks) due to their good high-frequency performance, high packing density and stability. They are now used for top end commercial applications such as high-speed digital computer circuitry, digital cameras, medical (hearing aids), avionics and automotive (car engine control units, anti-lock braking systems and fuel injection units). Military uses are airborne communications, cockpit displays and satellite controls.

LTCC satellite synthesiser
- source IMST Keramis project

HTCC and LTCC ceramic devices in power
converter system of an electric car
- source Siemens

17.2 Thin Film Circuits and Components

17.2.1 Introduction

Initially, thin film metallisation on ceramic substrates was developed to meet higher circuit densities, reduced size and weight and reduced delay times by shortening signal paths. A thick film co-fired ceramic package of 33 layers to interconnect 100 semiconductor devices could be replaced by a thin film circuit having only 5-10 layers. The technology, which took off in the 1980’s, demanded tight tolerances of deposited and etched metals and other materials. With polished, high-purity alumina substrates, lines and spacings of 10 to 20 microns can be achieved with bulk grain size of 2 to 2.5 microns. The substrate surface had to be polished until it was very flat, then it was metalised, covered with a light sensitive resist, photo imaged with the required pattern and the unwanted metal etched away. Subsequently other materials such as tantalum nitride and nickel-chromium were used as thin films (0.1 or less to 2 microns) for resistors, and silica or nitrides for passivation and capacitors. Apart from reduced size, other advantages of thin film technology are lower weight, compatibility with silicon technology, ease of mass fabrication and lower overall cost. However, as thick film circuits can withstand higher voltages they can achieve greater power levels than thin film ones..

The first ferroelectric thin films were produced in 1977. More exotic ceramic thin films were investigated in the mid-80s and commercial devices based on them were available from the mid-90s. Two particular application areas triggered this interest, superconducting ceramic thin films and the potential of ferroelectric memories.