Justin C. Kuo

Chip-scale sonic communication using AlN transducers

On-chip wired interconnects presents a bottleneck for VLSI integrated circuits. An additional channel with which to communicate information would be beneficial to supplement traditional wired designs. Utilizing virtual, reconfigurable ultrasonic interconnects operating at high bit rate could open new vistas for computer architecture and low power computing. The first step to this goal has been demonstrated in this paper by using ultrasonic pulses to communicate between two aluminum nitride thin film transducers on a silicon wafer representative of a VLSI substrate. Direct output voltages on receive pixels were on the order of 40-60 μVpp for a drive voltage on transmit pixels of 0.5 Vpp at 900 MHz. An FEA model was used to verify the time-of-flight and signal amplitudes to demonstrate that the primary mechanism is bulk acoustic waves travelling through the silicon substrate.

Towards ultrasonic through-silicon vias (UTSV)

3D interconnects between stacked silicon chips in 3D integrated circuits (3DICs) have been realized with through-silicon vias (TSVs). These call for special processing microfabriation techniques requiring through wafer etching and metal filling, which add to the cost and can lead to lower chip reliability due to thermal expansion mismatch related strain fields. In this paper, we present a novel interconnect for 3DICs - an ultrasonic TSV implemented with piezoelectric aluminum nitride (AlN) thin film transducers on silicon. Whereas TSVs require a physical via through the entire thickness of a wafer, the ultrasonic TSV works by propagating signals from one aluminum nitride (AlN) transducer to another with bulk acoustic waves through the silicon wafer. This technology has the potential to simplify processing in 3DIC manufacturing and integration and increase TSV density.

Chip-scale reconfigurable phased-array sonic communication

As CMOS electronics scale to smaller dimensions, wired interconnect density, power consumption, and delays have introduced bottlenecks in performance. Ultrasonic communication links integrated within the silicon substrate offer an opportunity to create low power high data rate channels. We have utilized ultrasonic phased arrays, micro-sonars, for reconfigurable communication links. Brand new vistas in computer architecture and low power computing are enabled by these links, including brain inspired computing. This paper demonstrates the use of phased array elements to create a reconfigurable 120 Mbit per second channel in silicon. The array is capable of selectively communicating to four different locations on chip.

Chipscale GHz ultrasonic channels for fingerprint scanning

In this paper we present 1-3 GHz frequency ultrasonic interrogation of surface ultrasonic impedances. The chipscale and CMOS integration of GHz transducers can enable surface identification imaging for many applications. We use aluminum nitride piezoelectric thin films driven at maximum amplitudes of 4-Vpp to launch and measure pulse packets. In this paper we first use the contrast in ultrasonic impedance between air and skin to create an image of a fingerprint. As a second application we directly measure the reflection coefficient for different liquids to demonstrate the ability to measure the ultrasonic impedance and distinguish between three different liquids. Using a rubber phantom the image of a portion of a fingerprint is captured by measuring changes in signal levels at the resonance frequency of the piezoelectric transducers 2.7 GHz. Reflected amplitude waves from air and skin differ by factors of 1.8-2. The measurements for three different liquids; water, isopropyl alcohol, and acetone show that the three liquids have sufficiently different acoustic impedances to be able to identify them.

Towards a CMOS compatible ultrasonic delay line memory

In this paper, we present the concept and demonstrate the use of recirculating ultrasonic pulses as memory elements, stored in the thickness of a silicon wafer. This memory can be called SIUM, Substrate Integrated Ultrasonic Memory, and can be integrated within CMOS. We outline a theory of maximum SIUM spatial density. In this paper, we demonstrate a 2 bit delay line memory with the bulk of the silicon wafer as the delay line medium, using high frequency, thin film aluminum nitride (AlN) transducers. Pulse lengths of 16 ns was used at a carrier frequency of 1.3 GHz, at 2.1 volts peak to peak applied to the AlN films. These voltages and frequency ranges can be implemented in silicon CMOS for complete integration within an integrated circuit.

Optimized response of AlN stack for chipscale GHz ultrasonics

In this paper we present designs of an aluminum nitride (AlN) based transducer stack for ultrasonic transmit/receive applications integrated in silicon. By optimal design of the mechanical layer thickness and material properties, channel gain, center frequency and bandwidth can be controlled to allow for the use of lower gain and on-chip power electronics for integrated ultrasonic information processing. Certain materials in the stack were fixed due to the fabrication processing capability, however some of the passive layers, and the thicknesses of the layers could be controlled. Simulations were done to select the desired thicknesses of each layer and the resulting chip was fabricated and verified. Previous tape-outs from the fab had resulted in receive signal levels of 200 μV at 1.3 GHz, whereas the current stack had signal levels of 18 mV at 1.3 GHz.

Wideband material detection for spoof resistance in GHz ultrasonic fingerprint sensing

One of the primary motivations for using ultrasound reflectometry for fingerprint imaging is the promise of increased spoof resistance over conventional optical or capacitive sensing approaches due to the ability for ultrasound to determine the elastic impedance of the imaged material. A fake 3D printed plastic finger can therefore be easily distinguished from a real finger. However, ultrasonic sensors are still vulnerable to materials that are similar in impedance to tissue, such as water or rubber. Previously we demonstrated an ultrasonic fingerprint reader operating with 1.3GHz ultrasound based on pulse echo impedance imaging on the backside silicon interface. In this work, we utilize the large bandwidth of these sensors to differentiate between a finger and materials with similar impedances using the frequency response of elastic impedance obtained by transducer excitation with a wideband RF chirp signal. The reflected signal is a strong function of impedance mismatch and absorption [Hoople 2015].

Thermal wavefront imaging using GHz ultrasonics

In this paper an ultrasonic reflectometry approach to measure the thermal conductivity and temperature of a material by contact is presented. This device can be utilized for monitoring material properties during manufacturing, and robotic handling of objects. The sensor comprises an AlN piezoelectric ultrasonic transducer operating at GHz frequencies, situated on one side of a silicon chip. The transducer transmits a N-period ultrasonic pulse that propagates through the silicon bulk, reflects from the opposing side, and is received by a secondary receiving transducer. The time of flight (TOF) of the ultrasonic pulse and its return amplitude are measured. As the temperature distribution in the silicon bulk changes, by contacting the sensed object, the TOF changes due to the thermal coefficient of expansion and speed of sound. This paper demonstrates that the TOF temperature dependence can be used to determine the thermal properties of the contacted object. Analysis show that the sensor can detect different materials like silicon and glass. Experiments show that, by measuring the thermal time constant, the sensor can differentiate between a bare finger and a finger with rubber gloves. The temperature coefficient of the TOF in the device is 6 ps/°C.