High-Precision Digital Printing Processes for Electronics Fabrication

This thesis investigates the capabilities of high-precision digital printing technologies in the fabrication of miniaturized components for electronics packaging, transistor intercon- nects and monolithically integrated lab-on-skin systems for biosignal monitoring. In gen- eral, the printing technologies suffer from poor resolution compared to conventional lith- ographic fabrication methods, which limits the level of miniaturization for printed elec- tronics components, devices, and circuits. This leads to their significantly lower perfor- mance compared to conventional electronics. However, certain application areas exist where pushing the envelope of printing technologies towards higher resolution and pre- cision would result in the addition of new functionalities.

Replacing lithographic fabrication of high-density circuitries of electronics packages with high-resolution electrohydrodynamic inkjet (E-jet) printing could result in higher levels of customizability and reduced environmental impacts. In this thesis, the parameters affect- ing E-jet printing resolution were studied using statistical tools; the resulting regression model applied for droplet diameters of 3.5 µm to 20 µm and had a coefficient of determi- nation (R2) of 94 % with a residual of 1.1 µm. Finally, the combination of E-jet and inkjet printing is demonstrated in the fabrication of a high-density (5/5 µm width/spacing) mul- tilayer redistribution layer (RDL) for a silicon interposer.

E-jet printing could be also used to enhance the interconnect density, and concomitant performance of application specific printed electronic circuits (ASPEC), which in them- selves are already an enhancement of the existing application specific integrated circuits (ASIC) in that they allow field configurability of the prefabricated logic circuits. In this thesis, E-jet printing was compared to aerosol jet (AJ), piezoelectric inkjet and litho- graphic fabrication methods for the fabrication of ASPECs. Two different interconnect structures were used and in both cases the E-jet printing compared favourably to AJ and piezoelectric inkjet printing technologies.

Piezoelectric inkjet printing cannot be considered a true high-resolution technology sim- ilar to E-jet printing due to its large droplet volume (pL vs. fL), However, it may still be used to print small (i.e., high-precision) structures required for example in transistor fab- rication. The high-precision printing capability coupled with a large droplet volume ena- bles higher throughput when fabricating amplifiers with monolithically integrated active and passive components. In this thesis, a piezoelectric inkjet was used for the fabrication of source/drain (S/D) electrodes for transistors with ~10 µm channel length together with monolithically integrated large area parallel plate capacitors and resistors. The resulting charge amplifier optimized for pulse wave (PW) measurements had a gain of 1.6 V/nC with a pass band of 50 MHz to 32 Hz. Furthermore, the performance of the amplifier was evaluated for PW measurements by amplifying a PW signal recorded using piezoelectric poly(vinylidene-trifluoroethylene) (P(VDF-TrFE) pressure from the radial artery at the wrist and analyzing the amplified signal for clinically relevant PW features. As a support- ing study, the PW signal generated by a fully printed P(VDF-TrFE) pressure sensor was evaluated in a pre-clinical study with a statistically significant number of study subjects (22). Clinically relevant indices were calculated from the PW signal generated by the P(VDF-TrFE) sensor and these were compared to concurrent measurement with a ref- erence PW sensor. Good agreement between the PW sensors could be found in the case of the stiffness index (SI) and radial augmentation index (rAIx).