Sara Lotfi

Electrical and thermal characterization of 150 mm Silicon-on-polycrystalline-Silicon Carbide hybrid substrates

150 mm Silicon-on-polycrystalline-Silicon Carbide (poly-SiC) hybrid substrates, without intermediate oxide layers have been realized by hydrophilic wafer bonding of SOI- and poly-SiC wafers. A novel rapid thermal treatment step has been introduced before furnace annealing to avoid bubble formation, cracks and breakage. The final substrates are shown to be stress-free. Electrical and thermal characterization of devices manufactured on the substrate using a MOS process show excellent performance.

A micromachined dual-axis beam steering actuator for use in a miniaturized optical space communication system

The design, fabrication and evaluation of an electrothermally actuated micromachined beam steering device for use in a free-space optical communication system intended for use on micro- and nanospacecraft in kilometer-sized formations are presented. SU-8 confined in v-grooves is heated to create bending movement in two orthogonal directions for two-axial steering with large static bending angles and low actuation voltages. Standard MEMS processing is used to fabricate the devices with square mirror side lengths of 1, 3.5 and 5 mm. In addition, a method to prevent thermal damage to SU-8 during deep reactive ion etching has been successfully developed. Characterization shows optical scan ranges larger than 40° in both directions with the maximum driving voltage of 16 V corresponding to a total power consumption of 1.14 W. Infrared imaging is used to investigate thermal cross-talk between actuators for the two scanning directions. It is found that a silicon backbone on the joint backside is crucial for device performance. Differences from expected performance are believed to arise from the SU-8 curing process and excessive heating during fabrication. A finite element method simulation is used to find the eigenfrequencies of the structures, and these are in good agreement with the measured frequency response.

Hybrid microtransmitter for free-space optical spacecraft communication ― design, manufacturing and characterization

Optical intra-communication links are investigated by several currently operational qualification missions. Compared with RF communication systems, the optical domain obtains a wider bandwidth, enables miniaturized spacecraft and reduced power consumption. In this project, a microtransmitter is designed and manufactured for formation flying spacecraft with transmission rates of 1 Gbit/s. Simulations in Matlab and Simulink show that a BER of 10-9 can be achieved with aperture sizes of 1 cm and a transmitter output peak power of 12 mW for a distance of 10 km. The results show that the performance of the communication link decreases due to mechanical vibrations in the spacecraft together with a narrow laser beam. A dual-axis microactuator designed as a deflectable mirror has been developed for the laser beam steering where the fabrication is based on a double-sided, bulk micromachining process. The mirror actuates by joints consisting of v-grooves filled with SU-8 polymer. The deflection is controlled by integrated resistive heaters in the joints causing the polymer to expand thermally. Results show that the mirror actuates 20-30° in the temperature interval 25-250°C. Flat Fresnel lenses made of Pyrex 7740 are used to collimate the laser beam. These lenses are simulated in the Comsol software and optimized for a 670 nm red VCSEL. The lenses are manufactured using lithography and reactive ion etching. All tests are made in a normal laboratory environment, but the effect of the space environment is discussed.

Power performance of 65 nm CMOS integrated LDMOS transistors at WLAN and X-band frequencies

With increasing amount of user data and applications in wireless communication technology, demands are growing on performance and fabrication costs. One way to decrease cost is to integrate the building blocks in an RF system where digital blocks and high power amplifiers then are combined on one chip. This thesis presents LDMOS transistors integrated in a 65 nm CMOS process without adding extra process steps or masks. High power performance of the LDMOS is demonstrated for an integrated WLAN-PA design at 2.45 GHz with 32.8 dBm output power and measurements also showed that high output power is achievable at 5.8 GHz. For the first time, this kind of device is moreover demonstrated at X-band with over 300 mW/mm output power, targeting communication and radar systems at 8 GHz. As SOI is increasing in popularity due to better device performance and RF benefits, the buried oxide can cause thermal problems, especially for high power devices. To deal with self-heating effects and decrease the RF substrate losses further, this thesis presents a hybrid substrate consisting of silicon on top of polycrystalline silicon carbide (Si-on-poly-SiC). This hybrid substrate utilizes the high thermal conductivity of poly-SiC to reduce device self-heating and the semi-insulating properties to reduce RF losses. Hybrid substrates were successfully fabricated for the first time in 150 mm wafer size by wafer bonding and evaluation was performed in terms of both electrical and thermal measurements and compared to a SOI reference. Successful LDMOS transistors were fabricated for the first time on this type of hybrid substrate where no degradation in electrical performance was seen comparing the LDMOS to identical transistors on the SOI reference. Measurements on calibrated resistors showed that the thermal conductivity was 2.5 times better for the hybrid substrate compared to the SOI substrate. Moreover, RF performance of the hybrid substrate was investigated and the semi-insulating property of poly-SiC showed to be beneficial in achieving a high equivalent substrate parallel resistance and thereby low substrate losses. In a transistor this would be equal to better efficiency and output power. In terms of integration, the hybrid substrate also opens up the possibility of heterogeneous integration where silicon devices and GaN devices can be fabricated on the same chip.

Investigating Reliability and Stress Mechanisms of DC and Large-Signal Stressed CMOS 65-nm RF-LDMOS by Gate Current Characterization

This paper presents reliability measurements under the dc and large-signal conditions of an LDMOS transistor integrated in the 65-nm CMOS process. The gate current was measured with a high resolution across the whole operation area with an atto-sense unit, and distinct behavior was seen in the gate current characteristics due to hot-carrier injection and Fowler-Nordheim tunneling. Several bias points were chosen for the dc stress of the transistor, and the degradation of important parameters in terms of an RF operation was studied. Furthermore, the behavior from the dc stress was compared with the large-signal stress of the device in class AB, where the output power was monitored. Results show that the operation at a supply voltage of 3.3 V shows no significant drift of transistor parameters, whereas the operation at 5 V shows an increase in the on-resistance but no changes in the quiescent current or the threshold voltage. These results are in coherence with what the dc stress at quiescent bias points for class AB showed and may imply that dc-stress measurements are sufficient in order to understand the transistor reliability during an RF operation.

Dynamic characterization and modelling of a dual-axis beam steering device for performance understanding, optimization and control design

This paper presents a lumped thermal model of a dual-axis laser micromirror device for beam steering in a free-space optical (FSO) communication system, designed for fractionated spacecraft. An FSO communication system provides several advantages, such as larger bandwidth, smaller size and weight of the communication payload and less power consumption. A dual-axis mirror device is designed and realized using microelectromechanical systems technology. The fabrication is based on a double-sided, bulk micromachining process, where the mirror actuates thermally by joints consisting of v-grooves filled with the SU-8 polymer. The size of the device, consisting of a mirror, which is deflectable versus its frame in one direction, and through deflection of the frame in the other, is 15.4 ? 10.4 ? 0.3?mm3. In order to further characterize and understand the micromirror device, a Simulink state-space model of the actuator is set up using thermal and mechanical properties from a realized actuator. A deviation of less than 2% between the modelled and measured devices was obtained in an actuating temperature range of 20?200 ?C. The model of the physical device was examined by evaluating its performance in vacuum, and by changing physical parameters, such as thickness and material composition. By this, design parameters were evaluated for performance gain and usability. For example, the crosstalk between the two actuators deflecting the mirror along its two axes in atmospheric pressure is projected to go down from 97% to 6% when changing the frame material from silicon to silicon dioxide. A feedback control system was also designed around the model in order to examine the possibility to make a robust control system for the physical device. In conclusion, the model of the actuator presented in this paper can be used for further understanding and development of the actuator system.

Mobility profiles and thermal characterization of SOI and Si-on-SiC hybrid substrates

This paper presents new results revealing the electrical properties of the silicon-on-polycrystalline silicon carbide hybrid substrate. The thermal resistance in the substrate was measured and compared to simulations and is linked to the measured reduced self-heating in LDMOS transistors. The mobility in the device layer was extracted and shows slightly lower values in the hybrid compared to the SOI. Furthermore, the gate oxide integrity was evaluated suggesting that the poly-Si layer in the Si/SiC hybrid may act as a gettering layer for impurities due to the lower QBD spread.