Zsolt Kohári

CMOS sensors for on-line thermal monitoring of VLSI circuits

The paper presents appropriate sensors for the realization of the design principle of design for thermal testability (DfTT). After a short overview of the available CMOS temperature sensors, a new family of temperature sensors will be presented, developed by the authors especially for the purpose of thermal monitoring of VLSI chips. These sensors are characterized by the very low silicon area of about 0.003-0.02 mm/sup 2/ and the low power consumption (200 /spl mu/W). The accuracy is in the order of 1/spl deg/C. Using the frequency-output versions an easy interfacing of digital test circuitry is assured. They can be very easily incorporated into the usual test circuitry, via the boundary-scan architecture. The paper presents measured results obtained by the experimental circuits. The facilities provided by the sensor connected to the boundary-scan test circuitry are also demonstrated experimentally.

High-aspect-ratio metal microchannel plates for microelectronic cooling applications

A new manufacturing process and the characterization of high-aspect-ratio metal microchannel plates for microelectronic cooling applications are reported in this article. A nickel-based microchannel cooling plate, with channels of width 20 µm and aspect ratio up to 3.6:1, has been successfully fabricated using a modified UV-LIGA process. Similar metal microstructures, based on electroplated copper, have also been obtained with a width of 15 µm and an aspect ratio of up to 5:1. In both cases, an over-plate technology was used to electroform the metallic microchannel plates in a single manufacturing step. Hydrodynamic and cooling characteristics of the microchannel plates such as flow rate and heat resistance have been measured. A heat transfer coefficient of 511 W m−2 K−1 for a flow rate of 120 l h−1 has been obtained for the 20 µm wide nickel-based microchannel.

Nonlinear electro-thermal modeling and field-simulation of OLEDs for lighting applications I: Algorithmic fundamentals

Large area OLEDs aimed at lighting applications should provide homogeneous luminance-homogeneity is one of the quality metrics of such devices. Local light generation depends on both the local temperature and the local voltage drop across the light emitting polymer(s) in the device. Therefore the thermal and electrical engineering of OLEDs aimed at lighting applications is critical. Due to the large area of these devices the coupled electrical and the thermal simulation problem is of distributed nature. Electrical characteristics of organic semiconductor materials used in OLED devices are highly nonlinear, and their nonlinear temperature-dependence is significant. In our present approach to distributed electro-thermal field simulation we address special needs of OLEDs, which is not yet the case with widely used, commercially available simulation tools. In this paper we present the latest version of our SUNRED electro-thermal field solver algorithm capable of handling coupled, non-linear electro-thermal problems. The new features of the algorithm are demonstrated by modeling some research OLED samples available to us in the Fast2Light project-this way simulation results are compared against measured data.

Fast field solver for the simulation of large-area OLEDs

Lighting purpose organic light-emitting devices need special engineering because of the high electrical and thermal requirements of the operation. Our electro-thermal field simulation software is better to satisfy these special demands than the widely used commercial tools. This article surveys the special simulation needs of lighting purpose OLEDs, presents the electro-thermal extension of the FDM-based SUNRED thermal field simulator and the significant algorithmic changes for speed up the program and make it more flexible. The simulation of an existing OLED closes the paper.

Thermal characterization of a radial micro-channel cooling plate

The thermal behavior of a square nickel plate micro-cooler holding 128 micro-channels in radial arrangement has been investigated. The device is to be used in microelectronic packaging cooling applications. In our study it was attached to a power transistor which was used as a dissipator and a temperature sensor. The thermal transient response to a dissipation step of the transistor was recorded in the measurement. The measured transients (cooling curves) were transformed into structure functions from which the partial thermal resistance corresponding to the cooling assembly was identified. The measurement and the partial thermal resistance identification was carried out at different flow-rates of nitrogen gas forced through the micro-channels. This way thermal resistance versus flow-rate and heat-transfer coefficient versus flowrate characteristics of the investigated micro-channel cooler were derived.

How thermal environment affects OLEDs' operational characteristics

In recent years great effort has been put into development of organic light emitting diodes (OLEDs) worldwide. Among many concerns of developers is heat-removal from the thin film structure of the active layers of OLEDs which are typically realized on low thermal conductivity substrates such as glass or polymer foils. The other issue is to provide the OLEDs with transparent, yet high electrical conductivity electric power supply structure, therefore metallic shunting grids are added to the layer stack of OLEDs. These two major issues necessitate self-consistent electro-thermal simulation of large area OLEDs in which the temperature dependent I-V characteristics of the light emitting polymer layers are also considered. In this paper we discuss details of our 2.5D field-solver algorithm extended with such capabilities which was developed for the European funded Fast2Light project. The paper also presents a measurement and simulation example for a glass-based research OLED sample.

Integrated microscale cooling for concentrator solar cells

The work presents a new solution proposal to the cooling of concentrator photovoltaic cells. In our concept the microscale channels are integrated into the back surface metallization, the microscale channels are formed by electroplating copper around a photoresist channel pattern. This approach has the advantage that it has no restrictions regarding the solar cell material and technology. In this work we give a description on the process technology, perform mechanical simulations for the feasibility of our approach, optimize the channel geometry for a 20 × 20 mm concentrator solar cell and estimate the cooling performance of the microscale channel structure at different operating conditions. We found, that the proposed cooling solution would have a calculated thermal resistance of 0.26 K/W at pressure drop of 100 kPa. This would result in a temperature raise of less than 8 K in case of a concentration level of 100 suns and a solar cell efficiency of 25 %.

Vertical natural convection models and their effect on failure analysis in electro-thermal simulation of large-surface OLEDs

Abstract Besides classical inorganic LEDs, intelligent light sources can be also based on organic LEDs. OLEDs function as surface light sources and manufacturing of large area light sources is feasible with OLED technologies. Despite their lower luminous efficacy their other properties make OLEDs still an attractive option especially in high end indoor applications. In natural convection environment the temperature difference in the same OLED panel can reach 20–30 °C which can result in up to 30–40% difference in current density and thus, in the luminance. This difference in temperature and current density leads to differential ageing of the organic materials. CFD simulation is the obvious way to investigate natural convection environments but integration of a CFD solver in an OLED simulator may be difficult and the solution times are high. As a possible workaround to this problem, in this paper the application of five natural convection models for vertical plates in an electro-thermal field solver based OLED simulator as thermal boundary condition are presented. Steady state and transient simulation results of a free-standing 50 × 50 mm2 active surface OLED, surrounded by still air, are compared with measurement results. A typical failure type of OLEDs is thermal runaway caused by e.g. manufacturing problems, operational damages or overcurrent. The paper presents the effect of the natural convection model on the overcurrent caused thermal runaway simulation results.