Jongjoo Lee

Suppression of coupled-slotline mode on CPW using air-bridges measured by picosecond photoconductive sampling

For the first time, the effect of air-bridges on the suppression of the unwanted coupled-slotline (CSL) mode on a coplanar waveguide (CPW) is experimentally observed up to 250-GHz bandwidth, based on the picosecond photoconductive sampling technique. The CSL mode is initially originated from the asymmetric pulse generation on the CPW using ultrashort laser pulses and photoconductive switches. We find that more than two crossover air-bridges are needed to remove the unwanted CSL modes. The parasitic capacitance for the crossover air-bridge over the CPW and the parasitic inductance for the ground connecting air-bridge along the CPW are crucial in determining the signal distortion.

A study on wafer level molding for realizing 3-D integration

3D-IC packaging using through silicon via technology has been extensively developed to meet small form factor and low power consumption demands for next generation devices. A wafer molding technology is required for 3D chip integration. Wafer molding is carried out in the chip-to-wafer process to ensure suitable levels of mechanical strength are reached. The key to wafer level mold processing is the reduction of warpage. This paper discusses the material developments and steps taken for process optimization in the wafer molding process to reduce warpage. A 2 chip stack arrangement was used for this investigation, with a 12 inch bottom wafer containing vias, and 9×9 mm top chip wafers. The evaluation was run systematically in three major phases. In the first phase, the levels of warpage experienced during the packaging processes were simulated. The evaluation of three different types of material (Epoxy, Silicone and Hybrid) was carried out in the second phase. The third and final phase involved the testing for warpage at room and high temperature conditions of the epoxy and hybrid based resins. The silicone based resin was also evaluated with varying amounts of filler and adhesion promoter. The modulus and coefficient of thermal expansion (CTE) were found to be extremely important, since lowering this property would result in low warpage levels, both at room and high temperatures, which control the water absorption and temperature cycle reliability in the silicone based resin were established.

Closed-Form Expressions for the Noise Voltage Caused by a Burst Train of IC Switching Currents on a Power Distribution Network

A burst stimulus of data is a common activity in circuits and systems. The supply noise voltage waveform induced by a burst train of integrated circuit (IC) switching currents is rigorously derived for a power distribution network (PDN) with power traces, commonly used in handheld devices. Closed-form expressions are proposed to quickly estimate the amount of voltage drop and overshoot and to develop more complete PDN design methodology as improved target impedances. The proposed PDN noise expressions are also validated with SPICE simulation and measurements using a fabricated IC and PCB.

Picosecond time-domain characterization of CPW bends using a photoconductive near-field mapping probe

Propagation and reflection characteristics of right-angle coplanar waveguide (CPW) bends were measured using a novel photoconductive near-field probe with picosecond temporal resolution and pin spatial resolution. The probe can measure the transverse electric-field components existing over devices under test. Time-varying transverse electric field maps for different CPW bending structures were acquired by varying the probe position. The CSL mode generation and a difference in flight time of propagating pulses on two slots of the CPW bends were observed. Further, it was found that there exists a considerable unexpected pulse caused by the bent line structure, which has opposite polarity to the input pulse and exists only at the inner ground plane. The undesirable phenomena originated from the bend discontinuity were adequately reduced by bend smoothing techniques.Propagation and reflection characteristics of right-angle coplanar waveguide (CPW) bends were measured using a novel photoconductive near-field probe with picosecond temporal resolution and pin spatial resolution. The probe can measure the transverse electric-field components existing over devices under test. Time-varying transverse electric field maps for different CPW bending structures were acquired by varying the probe position. The CSL mode generation and a difference in flight time of propagating pulses on two slots of the CPW bends were observed. Further, it was found that there exists a considerable unexpected pulse caused by the bent line structure, which has opposite polarity to the input pulse and exists only at the inner ground plane. The undesirable phenomena originated from the bend discontinuity were adequately reduced by bend smoothing techniques.

A micromachined photoconductive near-field probe for picosecond pulse propagation measurement on coplanar transmission lines

For the first time, a micromachined near-field probe has been developed, based on picosecond photoconductive sampling using low-temperature-grown GaAs (LT-GaAs). By changing the direction of the probe, it is capable of scanning/mapping independent orthogonal components of free-space electric fields. The micromachined 1-/spl mu/m-thick LT-GaAs epilayer substrate of the probe provides a minimal loading effect for measuring picosecond electric fields. The use of optical fibers, for guiding laser pulses to the photoconductive switch on the probe and for electrical connections, enables the probe to be positioned freely with uniform sensitivity. We have demonstrated by finite-difference time-domain simulation that the measurement process of the developed probe is dominated by the switching action of the carriers generated in the photoconductive switch of the probe. Using the probe, propagating picosecond pulse waveforms and field distribution images on coplanar transmission lines were successfully obtained. The probe measurement provides a useful understanding, which cannot be obtained from conventional external-port access test instruments, of electromagnetic phenomena related to wave propagation.

An Enhanced Statistical Analysis Method for I/O Links Considering Supply Voltage Fluctuations and Intersymbol Interference

Statistical link analysis methods were previously developed for effective computation of bit error rate due to intersymbol interference (ISI). In addition to ISI, supply voltage fluctuations at output drivers can cause jitter and amplitude uncertainty in I/O links. In this paper, the enhanced statistical link analysis method considering both ISI and supply voltage fluctuations is clearly reformulated and experimentally validated step by step by various measurements. A silicon integrated circuit (IC) is designed, fabricated, and assembled on a manufactured printed circuit board (PCB). The supply voltage fluctuations on the IC with regard to the receiver reference voltage are extracted from measurements at the IC and PCB. Also, the impulse response of the total output channel is extracted from the measurements of the driver and channel characteristics. The statistical eye diagrams of the channel output including both ISI effects and the supply voltage fluctuations are then calculated and validated by comparison with the direct eye measurements.

Characterization of picosecond electric-pulse propagation on CPW components by transient near-field mapping

For the first time, the picosecond electric-pulse propagation characteristics of a few coplanar waveguide (CPW) components, including transmission lines, capacitive open and short terminations and a right-angle bend, have been successfully measured by mapping the time-dependent tangential near-electric-field distributions. The measurement was performed using a two-dimensional photoconductive (PC) near-field mapping system incorporating a recently developed novel PC electric-field probe, which could measure the separate picosecond orthogonal electric-field components with minimal loading effects. The resulting field-images mapped over the structural capacitive open termination and the short termination, showed previously unknown, and quite remarkable, transient picosecond electric-pulse reflection phenomena, which cannot be measured using conventional test instruments. In addition, the images showed that reflection-phenomenon itself required about 1 ps of propagation delay, and that the pulse-reflection from the capacitive open-termination required additional time delay matched to the length of the open-gap between the two slots of the CPW.

Pulse-coupling measurement of coupled microstrip lines using a micromachined picosecond optical near-field probe

By measuring the transverse electric-field distributions using a newly developed micromachined optical near-field mapping probe, pulse-coupling phenomena on coupled microstrip lines are reported for the first time. The measured field distribution of the propagating coupled pulse provides useful information to aid understanding of the coupling phenomena; this cannot be obtained by conventional external-port access test instruments. The measurement is performed based on the picosecond photoconductivity of low-temperature-grown GaAs (LT-GaAs). A system for the measurement of the internal electric field distribution using the optical near-field probe is described and characterized. It is capable of measuring independent orthogonal components of free-space electric fields with less than 2-ps temporal resolution and with minimal loading effects. The loading effects of the probe are minimized by adopting a micromachining technique for the use of a 1-/spl mu/m-thick LT-GaAs epilayer as a substrate, and by using silver-paint-coated optical fibers for electrical connections.

Picosecond-domain radiation pattern measurement using fiber-coupled photoconductive antenna

The photoconductive terahertz measurement technique was used for measurement of the radiation pattern from thick metal slits. Picosecond pulses were generated and detected by an ultrafast optoelectronic technique using a femtosecond pulse laser and a subpicosecond photoconductor. Photoconductive sampling measurements of the polarization-dependent diffraction of the picosecond electromagnetic pulses showed the highly polarized radiation characteristics of the photoconductive probing antenna (PCPA) compared with the finite-difference time-domain simulation. The radiation patterns from the slits were measured using a fiber-coupled PCPA. Investigation of the spatiotemporal radiation pattern from a thick metal slit shows the usefulness of the measurement set-up for time-domain and broadband antenna characterization. Together with the time-domain radiation pattern, the frequency-domain radiation pattern from the experimental transient response is presented in both amplitude and phase. It is demonstrated that the time-domain radiation pattern measurement using the photoconductive terahertz radiation technique is a promising characterization method for millimeter-wave and submillimeter-wave antenna time-domain testing.

High-performance Substrate Design for DRAM Flip-chip Interconnection using Etch-back Process

To apply an Au-stud bumping, which has the merit of being a supportable fine pad/bump pitch comparable to that of conventional wire-bonding, in the high-reliable, low-cost flip-chip packaging of high-speed DRAMs with a central dual-inline chip pad configuration, a new design method of the flip-chip package substrate was developed. In the method, a narrow, through-center plating line was formed between dual-in-line bump pads, all of which were connected to the central plating line. After thick electroplating of the bump pads for the reliable joint formation between an Au-stud bump and a package substrate, the central plating line was etched out. The Au-stud flip-chip substrate design method was applied to a 512 Mb GDDR4 DRAM, together with the PCB interconnect design to obtain balanced parasitics and improved power delivery, and the resulting 2-layer flip-chip package, showed improved performance, especially, at low supply voltage over the conventional 2-layer BOC package for the device.