Laser Terahertz Emission Research Articles

The combinational or selective usage of the laser SQUID microscope, the non-bias laser terahertz emission microscope, and fault simulations in non-electrical-contact fault localization

Recently, in the field of fault localization of LSI chips, several non-electrical contact techniques have been proposed. The techniques include the laser SQUID microscope (L-SQ) and the non-bias laser terahertz emission microscope (NB-LTEM). Both techniques have pros and cons. The L-SQ, for examples, can localize open defects in some cases, but it requires closed circuit. The NB-LTEM can localize open defects and short defects, and not requires closed circuit. The NB-LTEM, however, cannot localize open defects in some cases. The fault simulation specially designed for the L-SQ or the NB-LTEM makes it precise or efficient to localize defects. The combinational or selective usage of the L-SQ, the NB-LTEM and the related simulations makes it possible to localize defects in many cases. In this paper, we would like to review our results and organize them from the viewpoint of failure mode and defect sites.

Interaction of hot spots and terahertz waves inBi2Sr2CaCu2O8intrinsic Josephson junction stacks of various geometry

At high enough input power in stacks of ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8}$ intrinsic Josephson junctions a hot spot (a region heated to above the superconducting transition temperature) coexists with regions being still in the superconducting state. In the ``cold'' regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong and stable coherent terahertz (THz) emission. We investigate the interplay of hot spots and standing electromagnetic waves by low-temperature scanning laser microscopy and THz emission measurements, using stacks of various geometries. Standing electromagnetic wave patterns and THz emission are observed for a disk-shaped sample. The growth of a hot spot with increasing input power is monitored by small detector junctions surrounding a large rectangular mesa. For two rectangular mesas equipped with two current injectors and one arrow-shaped structure we show that the standing wave can be turned on and off in various regions of the stack structure, depending on the hot-spot position. The results support the picture of the hot spot acting as a reflective termination of the cavity, formed by the cold part of the mesa.

Laser THz emission microscope as a novel tool for LSI failure analysis

We have proposed and developed a novel technique for a non-contact inspection of defective interconnections in an LSI chip using a laser terahertz emission microscope (LTEM). The LTEM measures the THz emission images of an LSI chip by scanning it with fs laser pulses. When a fs laser pulse irradiates a p–n junction in an LSI chip, transient photocurrent flows into interconnections resulting in the emission of the THz pulse into free space. We investigated the characteristics of the THz emissions from simple test element group samples which consist of p–n junctions connected to metal lines. It was found that the metallic lines connected to photo-excited p–n junctions worked as THz emission antennae which enhance the emission efficiency of THz pulses near their resonant frequencies corresponding to the line lengths. This result indicates that THz emission signals from p–n junctions in circuits strongly depend on the structure of the interconnections. We show the successful results on the inspection of defective interconnections in MOSFET devices and C7552 ISCAS’85 benchmark circuits using LTEM. By comparing the THz emission images between a normal circuit and a defective one, it is possible to identify the p–n junctions connected to the defective interconnections without electrical contacts.

Backside observation of large-scale integrated circuits with multilayered interconnections using laser terahertz emission microscope

We have developed a laser terahertz emission microscope utilizing excitation laser pulses at 1.06 μm wavelength for the inspection and localization of electrical failures in large-scale integrated circuits with multilayered interconnection structures. The system enables to measure terahertz emission images from the backside of a large-scale integrated circuits chip with a multilayered interconnection structure that prevents the observation from the front side. By comparing the terahertz emission images, we successfully distinguish a normal circuit from damaged ones with different positions of the interconnection defects without any electrical probing.

Noncontact inspection technique for electrical failures in semiconductor devices using a laser terahertz emission microscope

We have proposed and demonstrated a novel technique for the noncontact inspection of electrical failures in semiconductor devices using a laser terahertz emission microscope. It was found that the waveforms of the terahertz pulses, emitted by exciting p-n junctions in semiconductor circuits with focused ultrafast laser pulses, depend on the interconnection structures of the circuits. We successfully distinguished damaged silicon metal-oxide-semiconductor field effect transistor circuits with disconnected wires from normal ones by comparing the images of terahertz emission amplitudes between a normal chip and a defective one.

Chemical sensing plate with a laser-terahertz monitoring system

A new type of laser-terahertz emission system for noncontact investigations of chemical solutions has been developed. The system monitors terahertz emission from a sensing plate, which consists of silicon oxide and silicon thin film layers on a sapphire substrate. Sensing of chemical solutions with pH values between 1.68 and 10.01 was demonstrated. The amplitude of the terahertz emission from the sensing plate increased with increasing pH value. This change in the amplitude was caused by a change in the depletion layers of the silicon thin film when protons were adsorbed on the surface of the sensing plate. This study demonstrates that full noncontact monitoring of chemical solutions is possible using the laser-terahertz emission system.

Laser terahertz emission microscopy

Laser terahertz (THz) emission microscopy (LTEM) is reviewed. Femtosecond lasers can excite THz waves in various electronic materials due to ultrafast current modulation. The current modulation is realized by the acceleration or deceleration of photo-excited carriers, and thus LTEM visualizes the dynamic photo-response of substances. We construct a free-space type and scanning probe type with transmission or reflection modes. The developed systems have a minimum spatial resolution better than 2 μm, which is defined by the laser beam diameter. We present some examples of LTEM applications, such as ferroelectric domain imaging, quantitative supercurrent distribution in high- T c superconductors, defect detection of MOS devices as well as the visualization of the photo-responses in materials and devices. To cite this article: H. Murakami, M. Tonouchi, C. R. Physique 9 (2008).

Transmission-Type Laser THz Emission Microscope Using a Solid Immersion Lens

A recently developed laser terahertz (THz) emission microscope (LTEM) is a new type of inspection tool for semiconductor integrated circuits by 2-D mapping of THz emission excited by femtosecond (fs) laser pulse. For high-resolution imaging, we demonstrate the combination of a hemispherical solid immersion lens (SIL) and an LTEM with transmission-type detection mode. Unlike reflective LTEM geometries, in which both the probing fs laser pulses and the generated THz emissions utilize the same optical components, the transmission system enables flexibility in the construction of both the fs laser and the THz detection optics: The optics on the incident side of the sample are used only to manipulate the fs pulses, while those on the transmission side are meant only for manipulating the THz emission. We could improve the spatial resolution by less than 1.5 mum by combining the objective lens with the hemispherical SIL with a refractive index n = 1.98 at 780 nm.

Laser Terahertz Emission Microscope

Developments of laser terahertz (THz) emission microscope (LTEM) systems are reviewed. Femtosecond lasers can excite the THz wave emission from various electronic materials, such as semiconductors, high-temperature superconductors, manganites, multiferroic oxides, etc., due to ultrafast current modulation. Limiting the topic to semiconductors, the current modulation is realized by acceleration or deceleration of photoexcited carriers due to the local electric field extrinsically or intrinsically induced at the laser illumination spot. Thus, LTEM has a potential to visualize the local electric field distribution and photoresponse without any contacts or damages. We have ever constructed prototype free-space type and scanning fiber-probe (SFP) type LTEM systems with transmission or reflection mode. The system performance of the SFP-LTEM has been greatly improved compared with that for the prototype one. The spatial resolution of the SFP-LTEM system has a minimum spatial resolution less than 3 mum , which is defined by the laser beam diameter. The compact SFP-LTEM system, in particular the reflection system, has the potential to be utilized for wide applications as well as various materials. In this review paper, we introduce the details of the LTEM systems and example applications for the evaluation of electric field distribution in integrated circuits and supercurrent distribution in high-temperature superconductors.

Laser terahertz emission system to investigate hydrogen gas sensors

A laser terahertz emission system is proposed to investigate the catalytic metal/semiconductor interfaces of hydrogen sensors. Samples were fabricated by depositing a catalytic metal thin film on a semi-insulating silicon substrate. A femtosecond laser was used to radiate terahertz waves from the sample in a gas cell filled with a hydrogen and nitrogen gas mixture. The peak amplitude of the terahertz waves decreased with increasing hydrogen concentration. We also fabricated a metal-oxide-semiconductor field effect transistor hydrogen sensor, and compared its properties with the terahertz radiation properties. These results suggest that the laser terahertz emission system is a potential tool to investigate catalytic metal/semiconductor interfaces.

Imaging of large-scale integrated circuits using laser-terahertz emission microscopy

We present the redesign and improved performance of the laser terahertz emission microscope (LTEM), which is a potential tool for locating electrical failures in integrated circuits. The LTEM produces an image of the THz waves emitted when the circuit is irradiated by a femtosecond laser; the amplitude of the THz emission is proportional to the local electric field. By redesigning the optical setup and improving the spatial resolution of the system to below 3 microm, we could extend its application to examining of large-scale integration circuits. As example we show the THz emission pattern of the electric field in an 8-bit microprocessor chip under bias voltage.

Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits.

A laser terahertz-emission microscope (LTEM) system is proposed and developed for inspecting electrical faults in integrated circuits (IC). We test a commercial operational amplifier while the system is operating. Two-dimensional terahertz-emission images of the IC chip are clearly observed while the chip is scanned with a femtosecond laser. When one of the interconnection lines is cut, the damaged chip has a LTEM image different from that of normal chips. The results indicate that the LTEM system is a potential tool for IC inspection.