THICK FILM CERAMIC

Thick Film Ceramic (Alumina/Ltcc) Pressure Sensors



Thick film ceramic (alumina/LTCC) pressure sensors

Abstract

Thick film technology has been the traditional method to manufacture microwave circuitry for many years. An outstanding line resolution, excellent conductor edge definition combined with superb ceramic substrate properties in regard of high frequency and thermal behaviour are ideal prerequisites for these applications. On the other hand, this approach can be critical in terms of cost. Complex modules are often assembled in a special hermetic housing using a patch work arrangement of certain substrates. Multilayer substrates based on Low Temperature Cofired Ceramics (LTCC) offer a variety of design options for microwave designs. However, fine line printing resolution and associated tolerances might be the bottle neck. Though lines and spaces down to 50 micron are achievable, this is not sufficient for certain elements like edge coupled filters, couplers etc.

Introduction

LTCC Technology

Low Temperature Cofired Ceramics is a multilayer technology that offers numerous options for the design of circuits. For microwave applications DC-connections and digital control functions can be implemented in separate layers, chip tailored cavities can improve the return loss of the signal interconnections, various transmission line types as well as wave guides are available and the hermetic substrate itself can be used as a part of the package with integrated feed through (Jaakola, 2003, 84). Embedded resistors and capacitors are additional features to further shrink the designs. Though, the bulk materials have a medium thermal conductivity (2-4 W/mK) LTCC provides options to remove heat from active components effectively. By using thermal vias underneath the die, the thermal conductivity can be increased by a factor of 10 or more [1]. Additionally, a very low inductive ground interconnect is established by the fully metallised vias. The matched thermal coefficient of expansion (TCE) to both GaAs and Si leads to highly reliable die to substrate interconnections. The conductive pattern is usually structured by screen printing thus providing the cost and environment advantages of an additive process. However, line resolution is limited to a standard line width and space of about 100 µm. Special screens can be used to extend this range to about 50 µm (Fig. 1). New screen developments in conjunction with fine particle pastes are being made to further reduce the feature sizes and tolerances. Screen-printing processes offer many advantages in producing directly patterned and integrated thick films, and hence are important methods of preparing piezoelectric ceramic thick films. It is particularly interesting to deposit PZT thick films directly on silicon substrates to develop silicon-based piezoelectric MEMS. However, the processing warmth for completely sintering piezoelectric PZT ceramics is overhead 1200 °C. In positions where a silicon substrate is utilised, temperatures of this magnitude are improper because of important lead diffusion into the silicon substrate and oxidation of the steel electrode layers.

Other approaches are focussing on subtractive methods by photodefined conductors to increase the line resolution. By using chrome glass masks and processes used in the semiconductor industry extremely fine conductor tracks, typically 10 µm across, can be deposited ...