György Bognár

Thermal investigation of high power Optical Devices by transient testing

In case of opto-electronic devices, the power applied on the device leaves in a parallel heat and light transport, the interpretation of R/sub th/ is not obvious. The paper shows results of a combined optical and thermal measurement for the characterization of power light emitting diodes (LEDs). A model explaining R/sub th/ changes at different current levels is proposed.

Improved thermal characterization method of integrated microscale heat sinks

The thermal management of semiconductor devices is still a hot topic. Most designers, who are aware of the thermal aspects of IC design, know that new, cheaper and more efficient methods are required to keep the temperature of electronic systems low. Research by different teams regarding the cooling of stacked die structures is in progress.In this paper an improved thermal characterization method will be presented to determine the flow dependent partial thermal resistance of integrated microchannel based heat sinks. This reliable characterization method does not demand thermal isolation during the measurements, only constant environment conditions. The measurements are based on the industrial standard thermal transient testing method.On the other hand we present an approach to realize an integrated microfluidic channel based heat sink, which can be realized in the backside of the silicon chip itself. The approach is based on a cheap wet etching process instead of reactive ion etching or LIGA technologies, which enables batch processing.

Comprehensive design of a high frequency PLL synthesizer for ZigBee application

Time-to-market demands shorter design period of applied semiconductor devices. For this reason the iteration steps, the simulation time and repetitions have to be minimized along the design flow. In this paper a new design method of a phase locked loop (PLL) frequency synthesizer and its complete design flow is introduced. This synthesizer is designed mainly for ZigBee application. In this case the top-down design methodology was used. At higher hierarchy levels the Verilog-AMS hardware description language and various Matlab tools were applied. This analogue system will have to be realized on 0.35mum CMOS technology, so the complete layout has to be designed as well. The work has been focused on the main critical element of the PLL circuit, therefore two different types of a voltage controlled oscillator (VCO) blocks were realized. The carrier frequency is around 2.4 GHz during transmission according to the ZigBee standard, thus the VCOs were designed for operating on this frequency. In this paper, the mixed-signal simulation results of the entire system will be introduced, as well

Thermal management in System-on-Package structures by applying microscale heat sink. Part I

Since the first microchannel heat sink was introduced, intense research on microchannel based heat sinks has been conducted to study the heat transfer mechanism and fluid flow characteristics in microscale channel structures. In this paper the determination of proper channel length of the microscale heat sink(s) is presented. The main aim is to introduce a methodology to define the maximum applicable channel length in order to achieve the maximum heat transfer from the substrate to the fluid without realizing unnecessarily long channels. It is important because superfluous parts of the channel(s) from the heat transfer point of view occupy important areas where further channel(s) can be realized. The proposed characteristic channel length depends on the fluid properties, the flow rate and the channel geometries. This approach could be more important in the case of the more-than-Moore 3D integration (e.g.: System-in-Packages, stacked dies structure, etc.) where removing the heat from the inner layers or forming homogenous temperature distribution within a selected die is the most up-to-date question nowadays. The determination of the proper length of the channel(s) gives a basis to design the optimal architecture of microscale heat sink structures. The commonly applied microscale cooling structures with radial arrangement and one inlet usually reach the highest achievable heat transfer at low pressure drop but occupy more surface area than needed at applied flow rates.

Enhanced thermal characterization method of microscale heatsink structures

In the frame of thermal management of electronic devices, finding efficient cooling solutions of the next generation equipment is a hot topic. If a new or improved solution is presented it always requires efficient characterization methods to prove the benefits compared to its predecessor. In case of microscale heatsink structures which are integral parts of modern chip or package level cooling concepts, an efficient measurement method is needed to analyse the performance of structures with different layouts and/or manufacturing technologies. This paper presents an enhanced thermal characterization method of microchannel based cooling structures, determining relevant partial thermal resistances from structure functions obtained by thermal transient testing. Our prior microscale heatsink characterization method was recently improved, accounting e.g. for possible nonlinearities of the heat transfer processes. This paper presents how we have improved our measurements setup in detail to deal with these phenomena compared to the previous setup.

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 %.

Generalization of the thermal model of infrared radiation sensors

In many theories and applications, generalized models can give a good head start for further research where the implementation of new elements and/or boundary conditions could become quite complex. In this paper the development of a compact thermal model of an infrared sensor will be presented. This thermal model includes not only the thermal resistances and capacitances of the sensor structure itself but the radiative and convective thermal resistances to the ambience and between the sensor plate and the heat source (thermal transfer impedance) which is important when the heat source and the sensor are in close proximity. Limitations and the applicability of the proposed model are also discussed. We also aim to present how the proposed model can be used for other IR sensor structures as well.

Contactless characterization of MEMS devices using optical microscopy

In this paper a new approach for measuring depth values of cavities of Micro-Electro Mechanical System (MEMS) is presented. This measurement was done by using a simple optical microscope and image processing techniques. The sample need not to be treated with any foreign material such as reflective or conductive coating.

Fabrication and characterization of microscale heat sinks

In the field of thermal management, engineers are well aware of the challenges posed by the increasing level of dissipation. Among the many possible solutions to counter the threat of overheating, one is dealing with the usage of microscale heat exchangers, where the forced air or liquid cooling solution is integrated into the electronic package itself. As the System-on-Package integration is not a straightforward task, many fabrication steps have to be fully developed before a successful chip-level cooling system is ready to be used. In this paper, as one of these many steps, we present a refined manufacturing technology which gives the possibility to create the microscale heatsink integrated together with the electronic devices. With the refined manufacturing technology several channel patterns can be created easily. Nevertheless, only a simple channel pattern is presented now which is tested with the enhanced thermal characterization method developed for microchannel based cooling structures last year.

Die attach quality testing by fully contact-less measurement method

The paper presents a novel, fully contact-less method for detecting die attach problems of semiconductor devices by measuring the dilatation resulting from thermal expansion. Laser interferometer measuring system was used to measure the thermal dilatation caused by infrared radiation directed onto the measured structure. By using the contact based stylus measurement method the previously got results have been cross-verified