Woo-Suk Sul

Guard-Ring Structures for Silicon Photomultipliers

Si photomultipliers with three different guard-ring structures are fabricated, and a detailed comparative study on their device performances is performed. The virtual guard-ring structure shows a high-resolution full width at half maximum in the gamma spectrum and a high breakdown voltage of ~ 66 V but the lowest fill factor of 46.6%-59.8% among the examined structures. The best charge conversion performance, gain, and fill factor (67.1%) are achieved with the trench guard-ring structure. However, this structure shows a low energy resolution, which is supposed to be due to the trench-associated defects. The performance of the N-implantation guard-ring structure is intermediate in most aspects of the device performance compared to the other structures.

A Technique for Converting Perhydropolysilazane to SiO x at Low Temperature

Spin-coated perhydropolysilazane films on Si(100) substrates were prepared by a dibutyl ether solution and converted into SiO x using a variety of low temperature curing methods. From the Fourier transform IR spectroscopy and the refractive index (RI) measurements, the successful conversion to a high density silica network was observed from the curing methods by dipping the coatings into either diluted H 2 O 2 (for > 10 min) or deionized water under a 405 nm UV irradiation (for >60 min) at near room temperature. The measured RI values of the cured SiO x films were 1.45-1.47, and the X-ray photoelectron spectroscopy showed that the O/Si stoichiometries of the cured SiO x films were in the range of 1.5-1.7.

A high performance V-band monolithic quadruple sub-harmonic mixer

In this paper, we present a high performance V-band quadruple sub-harmonic mixer monolithic circuit which is designed and fabricated for the millimeter wave down converter applications. While the typical sub-harmonic mixers use a half of fundamental frequency, we adopt a quarter of the fundamental frequency. The proposed circuit is based on sub-harmonic mixer with APDP (anti parallel diode pair). Upon the typical mixer design, additional stubs are placed with the modification of original stub length. And the 0.1 /spl mu/m pseudomorphic high electron mobility transistors (PHEMTs) providing better gain are positioned to each port. Used lumped elements at IF port, it provides selectivity of IF frequency, and increases isolation. Maximum conversion gain of 0.8 dB at a LO frequency of 14.5 GHz and at a RF frequency of 60.4 GHz is measured. Both LO-to-RF and LO-to-IF isolations are higher than 40 dB. These conversion gain results and isolation characteristic are the best performances reported among the quadruple sub-harmonic mixers operating in the V-band millimeter wave frequency thus far.

Influence of Guard-Ring Structure on the Dark Count Rates of Silicon Photomultipliers

We present the diverse characteristics of silicon photomultipliers using three different structures. Three different trench gap-filled materials are fabricated, and a detailed comparative analysis on their device performances is carried out. The high energy resolution in the gamma spectrum (16%-17%), and the highest fill factor (73.6%) are achieved with the trench-type guard-ring structure. However, due to its trench-associated defects, the trench-type guard-ring structure showed the lowest dark count rate characteristic in the single microcell, which dramatically slowed due to the decreased probability of crosstalk in the 4 × 4 matrix array Si photomultipliers. In particular, the performance of the oxide + polysilicon gap-filled trench-type guard-ring structure is intermediate in most aspects of the performance compared to the other types of guard-ring structures.

Influence of Silicon Nitride Passivation on DC and RF Characteristics of 0.1 μm Pseudomorphic HEMTs

DC and radio frequency (rf) characteristics of pseudomorphic high electron mobility transistors (HEMTs) are investigated before and after silicon nitride (Si 3 N 4 ) passivation. After the passivation, we observed significant degradation of cut off frequency and noise performance of the HEMTs. We also observed clear increases in the drain-source saturation current at a gate voltage of 0 V and in the extrinsic transconductance at a drain voltage of 1 V from 325 and 264 to 365 mA/mm and 304 mS/mm, respectively, with no significant variation in pinchoff voltage. We propose that the observed variations in the dc characteristics are due to the positively charged surface states after deposition of the Si 3 N 4 passivation film. Also, the degradation of rf and noise performance is associated with the increase of gate-source capacitance. Hydrodynamic device model simulations were performed based on the proposed mechanisms for the change in electrical behavior, and the calculated results show good agreement with the experimental results.

Electrical Characteristics of the 0.1 µm Gate Length Pseudomorphic High-Electron-Mobility Transistors with Low-Dielectric-Constant Benzo-Cyclo-Butene Passivations

We examine the effects of thin-film passivation on the DC, RF and noise characteristics of 0.1-µm-gate-length pseudomorphic high-electron-mobility transistors (PHEMTs) when either conventional Si3N4 or low-dielectric-constant benzo-cyclo-butene (BCB) is used for the passivation layer. A significant shift in pinch-off voltage of ~-0.4 V is observed from the PHEMTs after the Si3N4 passivation, whereas no such variation in DC parameters is observed in the case of BCB passivation. Noise figures of the devices are very stable after the BCB passivation and are only ~60% of those in the case of Si3N4 passivation when measured at 55–62 GHz. Other RF parameters are not significantly affected by the passivation materials at a frequency of 50 GHz. We propose that the degradation in DC and noise characteristics can be due to the high density of surface states in the Si3N4 passivation layers; on the other hand, excellent stability in electrical characteristics in the BCB passivations can attribute to the minimized surface states in the passivation layer.

Low-characteristic impedance transmission line fabricated using benzo-cyclo-butene thin film

Transmission lines of very low characteristic impedance (Z/sub 0/) are fabricated and analyzed. The transmission lines are fabricated on the benzo-cyclo-butene (BCB) films of a low dielectric constant. Two types of coplanar waveguide (CPW) structures are fabricated, which include bottom-ground and double-ground type. Measurement shows that Z/sub 0/ values for each CPW type are 7.3 and 9.4 /spl Omega/, respectively, at a signal line width of 100 /spl mu/m. When the ratio between the spacing of bottom-ground and the signal line width becomes greater than 2.5, the Z/sub 0/ is nearly saturated. In addition, thin film microstrip lines fabricated using the BCB insertion layers show very low Z/sub 0/ of 25.5 /spl Omega/, and this impedance is /spl sim/64 % of the values obtained from the BCB-based CPW structures of the same line width.

Development of P-on-N silicon photomultiplier prototype for blue light detection

In this paper, we report a preliminary study on the electrical and optical properties of the first P-on-N SiPM prototype developed at KAIST with a collaboration of NNFC. The sensors were fabricated on a 200 mm n-type silicon epitaxial-layer wafer via customized CMOS process at NNFC. Measurements on the reverse current were carried out on a wafer-level with an auto-probing station and breakdown voltage was found as 32.3 V. As for optical characterization, gain, dark count rate, and photon detection efficiency have been measured as a function of bias voltage at room temperature. In particular, we show that the device had a comparable gain of ~ 106 with respect to conventional PMTs and had a peak sensitivity in blue light regime. Furthermore, we attempt to explain possible causes of some of phenomena seen from the device characterization.

Silicon photomultiplier modules for MRI-compatible PET

Silicon photomultiplier (SiPM) modules were developed for use in positron emission tomography-magnetic resonance imaging (PET-MRI), which is a hybrid medical imaging technology. A PET-MRI is very efficient in the early diagnosis of representative senile diseases, including cancer, Alzheimers disease, and Parkinsons disease. SiPMs comprise the core image sensor for MR-compatible PET applications since they have a low operational voltage, high gain, good timing resolution, ruggedness, insensitivity to magnetic fields, compactness, and low cost. In PET systems, SiPM microcells can be optimized by making a trade-off between photon detection efficiency (PDE) and dynamic range. The SiPM modules used in this study were fabricated at the National NanoFab Center (NNFC) of South Korea by using a customized CMOS processes. The SiPM modules were evaluated by first packaging them with a cost-effective PCB package instead of with a conventional ceramic package. Measurements on 1,400 SiPMs indicated a uniform breakdown voltage of 20.54 V with a standard deviation of 0.07 V. Moreover, the SiPM modules present a high and uniform energy resolution of 13.6% with a standard deviation of 0.5% at 511 keV with 3 × 3 × 20 mm3 cerium-doped lutetium-yttrium oxyorthosilicate (Lu2(1−x)Y2xSiO5:Ce, LYSO) crystal coupling. These results indicated that the proposed devices offer adequate performance to form the foundation of an image sensor technology for MRI-compatible PET.

High performance Q-band subharmonic receiver chip set

A high conversion gain receiver chip set for Q-band millimeter-wave wireless communication systems is designed and fabricated by using 0.1 /spl mu/m GaAs-based pseudomorphic high electron mobility transistors (PHEMTs) and the coplanar waveguide (CPW) library. The fabricated receiver chip set consists of a subharmonic mixer circuit and a low noise amplifier. From the device characterization, the subharmonic mixer shows a maximum conversion gain of /spl sim/4.8 dB at an RF frequency of 40 GHz for a local oscillation (LO) power of 10 dBm at 17.5 GHz. The subharmonic mixer also exhibits a high degree of isolation characteristic of -35.8 dB for LO-to-IF and -40.5 dB for LO-to-RF, respectively, at a LO frequency of 17.5 GHz. The low noise amplifier shows a S/sub 21/ gain of /spl sim/25.6 dB at a RF frequency of 40 GHz. Due to the high performances of the circuits, the fabricated receiver chip set produces a high conversion gain of 30.4 dB for the Q-band application purpose.