David Eric Schwartz

From Printed Transistors to Printed Smart Systems

Printing as a manufacturing technique is a promising approach to fabricate low-cost, flexible, and large area electronics. Over the last two decades, a wide range of applications has been explored, among them displays, sensors, and printed radio-frequency identification devices. Some of these turned out to be challenging to commercialize due to the required infrastructure investment, accuracy or performance expectations compared to incumbent technologies. However, the progress in terms of material science, device, and process technology now makes it possible to target some realistic applications such as printed sensor labels. The journey leading to this exciting opportunity has been complex. This review describes the experience and current efforts in developing the technology at PARC, a Xerox Company. Printed smart labels open up low-cost solutions for tracking and sensing applications that require high volumes and/or would benefit from disposability. Examples include radiation tags, one-time use medical sensors, tracking the temperature of pharmaceuticals at the item level, and monitoring food sources for spoilage and contamination. Higher performance can be achieved with printed hybrid electronics, integrating microchip-based signal processing, wireless communication, sensing, multiplexing, as well as ancillary passive elements for low-profile microelectronic devices, opening up further applications. This technology offers custom circuitry for demanding applications and is complementary to mass printed transistor circuits. As an example, we describe a prototype sense-and-transmit system, focusing particularly on issues of integration, such as impedance matching between the sensor and circuits, robust printed interconnection of the chips, and compatible interface electronics between printed and discrete parts. Next-generation technologies will enable printing of entire smart systems using microchip inks. A new printing concept for the directed assembly of silicon microchips into functional circuits is described. The process is scalable and has the potential to enable additive, digital manufacturing of high-performance electronic systems.

A heat-switch-based electrocaloric cooler

A heat-switch-based electrocaloric cooler is reported in this letter. The device consists of two silicon heat switches and an electrocaloric module based on BaTO3 multilayer capacitors (MLCs). To operate the cooler, the heat switches are actuated synchronously with the application of electric fields across the MLCs. Heat flux versus temperature lift is fully characterized. With an electric field strength of 277 kV/cm, the system achieves a maximum heat flux of 36 mW and maximum temperature lift of greater than 0.3 °C, close to the expected MLC adiabatic temperature change of 0.5 °C. The cooler is shown to work reliably over thousands of actuation cycles.

Utilizing high resolution and reconfigurable patterns in combination with inkjet printing to produce high performance circuits

Inkjet printing on pre-fabricated high-resolution substrate is developed to improve the operational speed of printed organic transistors. The high-resolution features are designed to define transistor critical dimensions, while maintaining the flexibility to incorporate different circuit constructions. Logic gate and ring oscillator circuits fabricated by inkjet printing on the high-resolution substrate are demonstrated, to show that the same high resolution pattern can be adapted for constructing different electronic circuits.

Pulsed voltage multiplier based on printed organic devices

To enable the integration of components with different supply voltage requirements, and to optimize power consumption in printed flexible electronics, we demonstrate an inkjet-printed pulsed voltage multiplier that boosts the voltage at specific circuit nodes above the supply voltage. A five-stage pulsed voltage multiplier is shown to provide an output voltage up to 18 V from a supply voltage of 10 V, with minimum 10 ms pulse rise time for a 70 pF load. This circuit allows a single power source to deliver multiple voltage levels and enables integration of low-voltage logic with components that require higher operating voltage. A key requirement for the pulsed voltage multiplier circuit is low device leakage to boost the output voltage level. To this end, the composition of the transistor semiconducting layer is modified by blending an insulating polymer with the small molecule semiconductor. This modification allows control over the transistor turn-on voltage, which enables low leakage current required for operation of the circuit.

Flexible Hybrid Electronic Circuits and Systems

Printed organic electronics are being explored for a wide range of possible applications, with much of the current focus on smart labels, wearables, health monitoring, sensors and displays. These applications typically integrate various types of sensors and often include silicon integrated circuits (IC) for computation and wireless communications. Organic thin film transistors (TFT), particularly when printed, have performance and yield limitations that must be accommodated by the circuit design. The circuit design also needs to select sensor technology, ICs and other circuit elements to integrate with the TFTs and match the functional and performance requirements of the application. This paper describes organic TFT properties and strategies for circuit and sensor design, with examples from various sensor systems.

Silicon Heat Switches for Electrocaloric Cooling

This paper presents two versions of a silicon mechanical heat switch designed for electrocaloric cooling. The first design, which consists of two 10-mm-by-12.8-mm micromachined silicon parts, allows investigation of the performance of a reciprocating solid thermal shunt device. This heat switch has a measured thermal contrast ratio in the range of 34-59. The second design adds self-alignment features that constrain the motion of the switch to facilitate fabrication and integration. The self-aligned heat switch has a thermal contrast ratio >28. It has been successfully operated for >18 000 cycles and employed in an electrocaloric cooler. Design, fabrication, and characterization of both heat switches are reported. [2016-0218]

Printed Organic Circuits for Reading Ferroelectric Rewritable Memory Capacitors

We demonstrate an inkjet-printed organic thin-film transistor (OTFT) circuit for reading ferroelectric (FE) nonvolatile rewritable memories. With the large difference in polarization charge between FE memory states, we implement a single-OTFT gain stage with latch and show that a gain of −2.8 is sufficient to distinguish memory states. This paper evaluates the effect of device variations on the yield of this readout circuit.

Digital Fabrication and Integration of a Flexible Wireless Sensing Device

In this paper, we combine high functionality c-Si CMOS and digitally printed components and interconnects to create an mostly printed integrated electronic system on a flexible substrate that can read and process multiple discrete sensors. Our approach is to create an integrated platform for the fabrication of mechanically flexible sensor tags that can be powered and interrogated wirelessly, precluding the need of a separate on-board power source. The high level system design is aimed at minimizing the number of non-printed components and reducing power consumption to enable energy harvesting from the RF field. Digital fabrication of these systems requires a range of materials, feature sizes, and electrical characteristics. In order to integrate the various printed components on a single substrate, we developed an integrated printer to accommodate a range of inks for printing the antenna, different types of sensors, chip interconnects, and wiring. For chip attachment to the flexible substrate, a method of integrating the die within the thickness of the substrate was developed. With proper system design and fabrication, a complete integrated tag for wireless sensing of temperature, strain, and touch was demonstrated. Our approach facilitates customization to a wide variety of sensors and user interfaces suitable for a broad range of applications including remote monitoring of health, structures, and the environment.

Methods for fabrication of flexible hybrid electronics

Printed and flexible hybrid electronics is an emerging technology with potential applications in smart labels, wearable electronics, soft robotics, and prosthetics. Printed solution-based materials are compatible with plastic film substrates that are flexible, soft, and stretchable, thus enabling conformal integration with non-planar objects. In addition, manufacturing by printing is scalable to large areas and is amenable to low-cost sheet-fed and roll-to-roll processes. FHE includes display and sensory components to interface with users and environments. On the system level, devices also require electronic circuits for power, memory, signal conditioning, and communications. Those electronic components can be integrated onto a flexible substrate by either assembly or printing. PARC has developed systems and processes for realizing both approaches. This talk presents fabrication methods with an emphasis on techniques recently developed for the assembly of off-the-shelf chips. A few examples of systems fabricated with this approach are also described.

Additive printing of organic complementary circuits for temperature sensor tag

With the recent improvements in printed devices, it is now possible to build integrated circuit systems out of printed devices. The combination of sensor, logic, and rewritable memory will greatly enhance the functionalities of printed electronics. We have demonstrated integrated sensor tags based on organic complementary circuits patterned by inkjet printing. One example is a temperature threshold sensor tag, wherein if the thermistor temperature exceeds a pre-set threshold, the control circuit generates a pulse to write into a nonvolatile ferroelectric memory cell. The trigger temperature is set by adjusting the bias voltage across the thermistor bridge to match the trigger voltage of the printed threshold circuit, and the threshold temperatures has been tuned between 8 °C and 45 °C with a bias voltage below 30V.