Subsurface optical microscopy of semiconductor integrated circuits

Thesis (Ph.D.)--Boston UniversityThe semiconductor industry continues to scale integrated circuits (ICs) in accordance with Moore's Law, and is currently developing the processing infrastructure at the 14nm technology node and smaller. In the wake of such rapid progress, a number of challenges have arisen for the optical failure analysis methods to meet the requirements of the advancing process technology. Most notably, complex circuits with shrinking critical dimensions will demand higher resolution signal localization currently beyond the capability of the existing optical techniques. This dissertation aims to develop novel optical systems to address the challenges of non-destructive circuit diagnostics at the 14nm technology node and beyond. Backside imaging through the silicon substrate has become an industry standard due to the dense multi-level metal wiring and the packaging requirements. The solid immersion lens is a plano-convex lens placed on the planar silicon substrate to enhance the subsurface focusing and collection of light in back-side imaging of ICs. The silicon and gallium-arsenide aplanatic solid immersion lenses (aSILs) were investigated in detail for the subsurface laser-scanning, voltage modulation, photon emission and dark-field IC imaging applications. Wave-front sensing and shaping techniques were developed to evaluate and mitigate optical aberrations originating from practical issues. Furthermore, the method of pupil function tailoring was explored for sub-diffraction spatial resolution. Super-resolving annular phase and amplitude pupil masks were developed and experimentally implemented. A record-breaking light confinement of 0.02 λ2 0(λ 0 refers to the free-space wavelength) was demonstrated using the vortex beams. The beam invasiveness is a critical issue in the optical circuit probing as the localized heat due to the absorption of the focused beams may unwittingly interfere with the circuit operation in the course of a measurement. A dual-phase interferometry assisted circuit probing was developed to enhance the signal extraction sensitivity by as much as an order of magnitude. Thus, the power requirement of the probe beam is significantly reduced to avert the consequences of the beam invasiveness. The optical systems and methods developed in this dissertation were successfully demonstrated using a number of modern ICs including devices of 14nm, 22nm, 28nm and 32nm technology nodes

Laser Reflectance Modulation in Silicon Integrated Circuits

Ph.DDOCTOR OF PHILOSOPH

Scanning thermal microscopy using nanofabricated probes

Novel atomic force microscope (AFM) probes with integrated thin film thermal sensors are presented. Silicon micromachining and high resolution electron beam lithography (EBL) have been used to make batch fabricated, functionalised AFM probes. The AFM tips, situated at the ends of Si3N4 cantilevers, are shaped either as truncated pyramids or sharp triangular asperites. The former gives good thermalisation of the sensor to the specimen for flat specimens whereas the latter gives improved access to highly topographic specimens. Tip radii for the different probes are 1 m and 50 nm respectively. A variety of metal structures have been deposited on the tips using EBL and lift-off to form Au/Pd thermocouples and Pd resistance thermometer/heaters. Sensor dimensions down to 35 nm have been demonstrated. In the case of the sharp triangular tips, holes were etched into parts of the cantilever in order to provide self alignment of the sensor to the tip. On the pyramidal tips it has been shown that multiple sensors can be made on a single tip with good definition and matching between sensors. A conventional AFM was constructed in order to test the micromachined thermal probes. During scans of a photothermal test specimen using improved access thermocouple probes, 80 nm period metal gratings were thermally resolved. This is equivalent to a thermal lateral resolution of 40 nm. Pyramidal tips with a resistance thermometer/heater, which were made for the microscopy and analysis of polymers, have been showed by others to produce high resolution thermal conductivity images. The probes have also been shown to be capable of locally heating a polymer specimen and thermomechanically measuring phase changes in small volumes of material. Also presented here is a study of scanning thermal microscopy of semiconductor structures using a commercial AFM. Included are scans of several specimens using both commercial andthe new micromachined probes. Subsurface images of voids buried under a SiO2 passivation layer were taken. It is shown that contrast caused by thermal conductivity differences in the specimen may be detected at a depth of over 200 nm

High Performance InP Photonic Integrated Circuits and Devices for Free Space Communications and Sensing

Communication needs have grown tremendously over the past few decades and will continue to increase in the future. In order to address these needs, 5G mobile communication systems are moving towards higher carrier frequencies in the millimeter wave (mmW) regime (30 – 300 GHz). Unlike traditional microwave frequencies, which have a relatively isotropic radiation pattern, the highly directional free space propagation characteristics of mmWs requires beemsteering and tracking between transmitters and receivers. One technology that is promising for future mobile communication systems is optical beam forming networks (OBFN). This technology uses photonic components to provide wide bandwidth and eliminate beam squint associated with RF methods to drive phased array antennas. The optical signals from the OBFN are down-converted using high speed photodiodes, which require high bandwidth, efficiency and RF output power. Here we present results on waveguide uni-traveling-carrier photodiodes integrated with mode converters for efficient coupling to a silicon nitride OBFN photonic integrated circuit (PIC). We demonstrate greater than 67 GHz bandwidth and extract efficiency limitations due to the space charge effect of the high carrier density under large optical input power.In addition to communication, highly directional beams can be used for free space sensors including LIDAR. While various frequency ranges provide benefits for specific applications, by increasing the frequency from the mmW regime to the near infrared (~193 THz), beam size can be further reduced to provide high resolution imaging and sensing. We present an indium phosphide transceiver PIC that incorporates a tunable laser, frequency discriminator, and receiver that can be used for frequency modulated continuous wave (FMCW) LIDAR when integrated with an optical phased array for 2D beamsteering. The transceiver provides wavelength tuning over 40 nm, a method for stabilizing the lasing frequency and imparting frequency modulation, and a balanced receiver for coherent detection. The components of the PIC will be discussed along with experimental verification of the functionality of this transceiver

Secure and Unclonable Integrated Circuits

Semiconductor manufacturing is increasingly reliant in offshore foundries, which has raised concerns with counterfeiting, piracy, and unauthorized overproduction by the contract foundry. The recent shortage of semiconductors has aggravated such problems, with the electronic components market being flooded by recycled, remarked, or even out-of-spec, and defective parts. Moreover, modern internet connected applications require mechanisms that enable secure communication, which must be protected by security countermeasures to mitigate various types of attacks. In this thesis, we describe techniques to aid counterfeit prevention, and mitigate secret extraction attacks that exploit power consumption information. Counterfeit prevention requires simple and trustworthy identification. Physical unclonable functions (PUFs) harvest process variation to create a unique and unclonable digital fingerprint of an IC. However, learning attacks can model the PUF behavior, invalidating its unclonability claims. In this thesis, we research circuits and architectures to make PUFs more resilient to learning attacks. First, we propose the concept of non-monotonic response quantization, where responses not always encode the best performing circuit structure. Then, we explore the design space of PUF compositions, assessing the trade-off between stability and resilience to learning attacks. Finally, we introduce a lightweight key based challenge obfuscation technique that uses a chip unique secret to construct PUFs which are more resilient to learning attacks. Modern internet protocols demand message integrity, confidentiality, and (often) non-repudiation. Adding support for such mechanisms requires on-chip storage of a secret key. Even if the key is produced by a PUF, it will be subject to key extraction attacks that use power consumption information. Secure integrated circuits must address power analysis attacks with appropriate countermeasures. Traditional mitigation techniques have limited scope of protection, and impose several restrictions on how sensitive data must be manipulated. We demonstrate a bit-serial RISC-V microprocessor implementation with no plain-text data in the clear, where all values are protected using Boolean masking and differential domino logic. Software can run with little to no countermeasures, reducing code size and performance overheads. Our methodology is fully automated and can be applied to designs of arbitrary size or complexity. We also provide details on other key components such as clock randomizer, memory protection, and random number generator

Implantable Neural Probes for Electrical Recording and Optical Stimulation of Cellular Level Neural Circuitry in Behaving Animals.

In order to advance the understanding of brain function, it is critical to monitor how neural circuits work together and perform computational processing. For the past few decades, a wide variety of neural probes have been developed to study the electrophysiology of the brain. This work is focused on two important objectives to improve the brain-computer interface: 1) to enhance the reliability of recording electrodes by optimizing the shank structure; 2) to incorporate optical stimulation capability in addition to electrical recording for applications involving optogenetics. For the first objective, a flexible 64-channel parylene probe was designed with unique geometries for reduced tissue reactions. In order to provide the mechanical stiffness necessary to penetrate the brain, the miniaturized, flexible probes were coated with a lithographically patterned silk fibroin, which served as a biodegradable insertion shuttle. Because the penetration strength is independent from the properties of the probe itself, the material and geometry of the probe structure can be optimally designed without constraints. These probes were successfully implanted into the layer-V of motor cortex in 6 rats and recorded neural activities in vivo for 6 weeks. For the second objective, either optical waveguides or μLEDs were monolithically integrated on the probe shanks for optogenetic applications. Compared to existing methods, this work can offer high spatial-temporal resolution to record and stimulate from even subcellular neural structures. In the experiments using wild type animals, despite optimized recording of spontaneous neural activities, the cells never responded to illumination. In contrast, for the ChR2 expressed animals, light activation of neural activities was extremely robust and local, which phase-locked to the light waveform whenever the cell was close to the light source. In particular, the probes integrated with μLEDs were capable of driving different neural circuit behaviors using two adjacent μLEDs separated only by a 60-μm-pitch. With 3 μLEDs integrated at the tip of each of the 4 probe shanks, this novel optogenetic probe can provide more than 480 million (12!) different spiking sequences at the sub-cellular resolution, which is ideal to manipulate high density neural network with versatility and precision.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111604/1/wufan_1.pd

Correlative Framework of Techniques for the Inspection, Evaluation, and Design of Micro-electronic Devices

Trillions of micro- and nano-electronic devices are manufactured every year. They service countless electronic systems across a diverse range of applications ranging from civilian, military, and medical sectors. Examples of these devices include: packaged and board-mounted semiconductor devices such as ceramic capacitors, CPUs, GPUs, DSPs, etc., biomedical implantable electrochemical devices such as pacemakers, defibrillators, and neural stimulators, electromechanical sensors such as MEMS/NEMS accelerometers and positioning systems and many others. Though a diverse collection of devices, they are unified by their length scale. Particularly, with respect to the ever-present objectives of device miniaturization and performance improvement. Pressures to meet these objectives have left significant room for the development of widely applicable inspection and evaluation techniques to accurately and reliably probe new and failed devices on an ever-shrinking length scale. Presented in this study is a framework of correlative, cross-modality microscopy workflows coupled with novel in-situ experimentation and testing, and computational reverse engineering and modeling methods, aimed at addressing the current and future challenges of evaluating micro- and nano-electronic devices. The current challenges are presented through a unique series of micro- and nano-electronic devices from a wide range of applications with ties to industrial relevance. Solutions were reached for the challenges and through the development of these workflows, they were successfully expanded to areas outside the immediate area of the original project. Limitations on techniques and capabilities were noted to contextualize the applicability of these workflows to other current and future challenges

Single Event Transients in Linear Integrated Circuits

On November 5, 2001, a processor reset occurred on board the Microwave Anisotropy Probe (MAP), a NASA mission to measure the anisotropy of the microwave radiation left over from the Big Bang. The reset caused the spacecraft to enter a safehold mode from which it took several days to recover. Were that to happen regularly, the entire mission would be compromised, so it was important to find the cause of the reset and, if possible, to mitigate it. NASA assembled a team of engineers that included experts in radiation effects to tackle the problem. The first clue was the observation that the processor reset occurred during a solar event characterized by large increases in the proton and heavy ion fluxes emitted by the sun. To the radiation effects engineers on the team, this strongly suggested that particle radiation might be the culprit, particularly when it was discovered that the reset circuit contained three voltage comparators (LM139). Previous testing revealed that large voltage transients, or glitches appeared at the output of the LM139 when it was exposed to a beam of heavy ions [NI96]. The function of the reset circuit was to monitor the supply voltage and to issue a reset command to the processor should the voltage fall below a reference of 2.5 V [PO02]. Eventually, the team of engineers concluded that ionizing particle radiation from the solar event produced a negative voltage transient on the output of one of the LM139s sufficiently large to reset the processor on MAP. Fortunately, as of the end of 2004, only two such resets have occurred. The reset on MAP was not the first malfunction on a spacecraft attributed to a transient. That occurred shortly after the launch of NASA s TOPEX/Poseidon satellite in 1992. It was suspected, and later confirmed, that an anomaly in the Earth Sensor was caused by a transient in an operational amplifier (OP-15) [KO93]. Over the next few years, problems on TDRS, CASSINI, [PR02] SOHO [HA99,HA01] and TERRA were also attributed to transients. In some cases, such events produced resets by falsely triggering circuits designed to protect against over- voltage or over-current. On at least three occasions, transients caused satellites to switch into "safe mode" in which most of the systems on board the satellites were powered down for an extended period. By the time the satellites were reconfigured and returned to full operational state, much scientific data had been lost. Fortunately, no permanent damage occurred in any of the systems and they were all successfully re-activated