Research
FBH research: 26.08.2010
- Cross-section of a UV LED with nanopixel contacts
- Characteristics of LEDs with conventional contact (a) and nanopixel contacts of different sizes and distances: 4 µm (b), 2 µm (c) and 1 µm (d)
Light emitting diodes (LEDs) emitting in the ultra-violet (UV) spectral region are interesting for application fields such as water purification, curing of plastics, spectroscopy, and bioanalysis. For many of these applications the currently low efficiency of UV LEDs has to be significantly increased. One of the issues to be solved is that a large proportion of the generated UV light does not escape the chip but gets lost due to internal absorption. Following a novel design approach, a breakthrough has now been achieved with FBH’s semiconductor chip. Instead of a large-area electrical contact above the light-emitting area as commonly used in conventional UV LEDs, an array of nano contacts made of palladium (nanopixels) has been utilized whose spacing is filled with aluminum. Whereas the nanopixels ensure efficient current injection, the aluminum acts as efficient reflector for UV light, so that absorption losses at the chip surface are reduced.
The proof of the nanopixel concept was verified by means of LEDs emitting at a wavelength of 390 nm [see Lobo et al., Appl. Phys. Lett. 96, 081109 (2010)]. When size and distance of the nanopixels were in the 1 µm range, the efficiency of corresponding LEDs could be increased by up to 90% compared to conventional contact designs. Theoretical calculations have shown that only for those small dimensions the currents from the individual nanopixels overlap in the active region, and the reflector takes effect on the emission between the nanopixels. Currently, LEDs with nanopixels are developed for shorter wavelengths, and the concept is combined with other procedures in order to enhance light extraction and reduce thermal resistance.
FBH research: 03.08.2010
- Transistor gate made of p-type GaN with metallization on top
- Flip-chip mounted 50 A / 250 V GaN power transistor
FBH successfully developed gallium nitride (GaN) power transistors offering up to 1000 V breakdown strength. An additional advantage is their normally-off characteristic. Due to safety reasons, the transistor can only be opened by applying a positive gate voltage of more than 1 V, which is a precondition for use in power electronics.
The newly developed gate module made of p-type GaN causes the required normally-off characteristic. The high breakdown strength was obtained by increasing the confinement of the transistor channel.
Because of its high band gap and breakdown strength, gallium nitride as semiconductor material has advantages compared to silicon for power electronics. GaN transistors can be operated at higher temperatures, higher voltages and higher current densities than silicon devices of the same size. Hence, less power losses appear and lower parasitic capacitances and better electron transport properties lead to faster switching speed. Converter modules can thus become smaller. Due to the higher possible operating temperature, cooling efforts scale down, weight and size of the converter modules further decrease. For an electrically powered car, this leads to a significant energy saving.
Power converters based on GaN transistors may thus offer a higher efficiency compared to Si-based systems. They are more robust, faster and more efficient and thus highly attractive for industry.
FBH research: 16.07.2010
- New e-beam SB251 at FBH
- 150 nm T-gate generated with direct write e-beam technology
In June 2010, the new Vistec SB251 electron beam lithography system (e-beam) passed all FBH specific acceptance tests in due time. The shaped beam system allows direct write applications with a minimum feature size of down to <50 nm on up to 8 inch wafers. It provides the technological basis for further development of state-of-the-art devices.
Specific tests demonstrated the capability of the system for gate technologies of high-power transistors and for grating technologies of opto-electronical devices. Based on FBH’s technology 50 nm wide metal lines were generated by metal lift-off and T-gates with a foot width of 150 nm were demonstrated. Gratings with 70 nm line width (lines and spaces) were processed using one of FBH’s standard resist process.
Before moving in the e-beam system, very demanding installation requirements concerning temperature stability, magnetic disturbances, and mechanical vibrations had to be ensured. For preparing these superior environmental conditions, substantial reconstruction took place in fall 2009, including a complete shut-down of the cleanroom to build a new foundation and to modernize the air-conditioning system. Early in December 2009, the e-beam was moved into the cleanroom. After passing all standard acceptance tests in April 2010, the SB251 has been used for photomask writing.
FBH research: 24.06.2010
- Atmospheric plasma source in operation
- Interior view of the plasma source
Plasmas at atmospheric pressure open up new applications in medical and industrial fields and are presently the subject of various research activities. At FBH, a new microwave plasma source for operation at atmospheric pressure was developed together with Aurion Anlagentechnik. The source contains an integrated microwave oscillator, based on a GaN power transistor fabricated at FBH. The integrated concept allows cost-efficient manufacturing, furthermore a safe operation without high voltages is possible.
This is presently the only plasma source worldwide which uses a GaN transistor for power generation. The supply voltage is only 24 V and the oscillator delivers a power of approximately 10 W to the plasma. Inside the source, the microplasma achieves a temperature of more than 1200°C which is significantly higher than for example at barrier discharges and enables interesting plasma chemical applications. Nevertheless, the temperature at the treatment location outside the source is lower than 60°C.
Key to success was the development of new microwave measurement methods to characterize the nonlinear and dynamic impedance behavior of the plasma load. This allowed an efficient circuit design of the power oscillator.
Subsequent to the optimization of several prototypes, a first pilot series has been manufactured. After assembly and electrical set up at FBH, the process performance of the sources was analyzed quantitatively by Aurion. High activation capabilities and good reproducibility within the different samples were found. Presently, samples are provided to customers for tests and development of applications.