Large-area femtosecond laser milling of silicon employing trench analysis

A femtosecond laser is a powerful tool for micromachining of silicon. In this work, large-area laser ablation of crystalline silicon is comprehensively studied using a laser source of pulse width 300 fs at two wavelengths of 343 nm and 1030 nm. We develop a unique approach to gain insight into the laser milling process by means of detailed analysis of trenches. Laser scribed trenches and milled areas are characterized using optical profilometry to extract dimensional and roughness parameters with accuracy and repeatability. In a first step, multiple measures of the trench including the average depth, the volume of recast material, the average longitudinal profile roughness, the inner trench width and the volume removal rate are studied. This allows for delineation of ablation regimes and associated characteristics allowing to determine the impact of fluence and repetition rate on laser milling. In a second step, additional factors of debris formation and material redeposition that come into play during laser milling are further elucidated. These results are utilized for processing large-area (up to few mm2) with milling depths up to 200 {\mu}m to enable the fabrication of cavities with low surface roughness at high removal rates of up to 6.9 {\mu}m3 {\mu}s-1. Finally, laser processing in combination with XeF2 etching is applied on SOI-CMOS technology in the fabrication of radio-frequency (RF) functions standing on suspended membranes. Performance is considerably improved on different functions like RF switch (23 dB improvement in 2nd harmonic), inductors (near doubling of Q-factor) and LNA (noise figure improvement of 0.1 dB) demonstrating the applicability of milling to radio-frequency applications.Comment: 23 pages, 16 figure

Substrate engineering of inductors on SOI for improvement of Q-factor and application in LNA

Renatech Network, Laboratoire commun ST-Microelectronics - IEMNInternational audienceHigh Q-factor inductors are critical in designing high performance RF/microwave circuits on SOI technology. Substrate losses is a key limiting factor when designing inductors with high Q-factors. In this context, we report a substrate engineering method that enables improvement of quality factors of already fabricated inductors on SOI. A novel femtosecond laser milling process is utilized for the fabrication of locally suspended membranes of inductors with handler silicon completely etched. Such flexible membranes suspended freely on the BOX show up to 92 % improvement in Q-factor for single turn inductor. The improvement in Q-factor is reported on large sized inductors due to reduced parallel capacitance which allows enhanced operation of inductors at high frequencies. A compact model extraction methodology has been developed to model inductor membranes. These membranes have been utilized for the improvement of noise performance of LNA working in the 4.9-5.9 GHz range. A 0.1 dB improvement in noise figure has been reported by taking an existing design and suspending the input side inductors of the LNA circuit. The substrate engineering method reported in this work is not only applicable to inductors but also to active circuits, making it a powerful tool for enhancement of RF devices

Functional packaging of RF, mmW and photonic functions based on femtosecond laser micromachining

International audienceThe increasing difficulty to pursue the aggressive objectives of Moore's law (More-Moore) has in parallel favored the emergence of integration solutions grouped under the term More-than-Moore, allowing to enrich silicon technologies by heterogeneous co-integration with new functionalities such as mixed digital/analog/RF/mmW circuits, antennas, sensors, actuators, embedded memory and other various microsystems). Still, it is clear that the performance of electronic systems (SoB - Systemon-Board) is far from having followed the same progression, thus marking a gap with monolithic nanometric CMOS technologies. To bridge this gap, it is now a question of producing gains in functionality, performance and compactness at the system level by heterogeneous integration of components in elemental system building blocks. Sometimes referred to as ‘System Moore’, this approach thus conceives the package not only as a simple encapsulation function but more precisely integrates the package as a functional system block. The dimensional level of functional packaging typically covers the 1-1000 µm range for which the use of microelectronics fabrication techniques are oversized, expensive and unable to efficiently handle thicknesses of a few tens of microns. In this context, laser micromachining is an increasingly used tool for micro/nanostructuring of materials for the packaging of integrated functions in photonics, microelectronics, RF and mmW. In this paper, we will first provide an overview of laser machining techniques in microelectronics and we will detailthe main characteristics of this technique with respect to the laser source and beam conditioning. It will be shown that the use of ultrashort laser processing with pulse width in femtosecond range is advantageous because it can be applied to a virtually unlimited range of materials like metals,semiconductors, dielectrics, alloys, and ceramics. The range of surface processing is diverse varying from a small scale (a few nm2) to large scale (a few mm2). The unwanted heat affected zone is greatly reduced for ultrashort lasers which allows for enhanced machining quality with low thermal impact of laser radiation on material. In a second step, we detail a selection of laser micromachining applications allowing either to increase the performance of RF and mmW components, or to introduce an innovative and distinctive manufacturing method promoting compactness and cost reduction:i) The first illustrated application is the fabrication of ultra-thin free standing membranes of SOICMOS RF circuits/functions on Silicon-on-Insulator (SOI) wafers. It will be shown that tremendous performance improvements can be obtained on a range of RF components like integrated inductances and power switches as well as in terms of cross-talk reduction.ii) The second example describes the packaging of an electro-optical transceiver using a glass electrooptical interposer to connect a silicon photonic chip to a single mode optical fiber.iii) Finally, the third illustration covers the fabrication of structures integrating mmW waveguides in G and J band [140-220 GHz, 220-320 GHz] with silicon chips and transitions allowing the conversion from coplanar to a guided mode in rectangular waveguides (WR5.1 and WR3.1)

Functional packaging of RF, mmW and photonic functions based on femtosecond laser micromachining

International audienceThe increasing difficulty to pursue the aggressive objectives of Moore's law (More-Moore) has in parallel favored the emergence of integration solutions grouped under the term More-than-Moore, allowing to enrich silicon technologies by heterogeneous co-integration with new functionalities such as mixed digital/analog/RF/mmW circuits, antennas, sensors, actuators, embedded memory and other various microsystems). Still, it is clear that the performance of electronic systems (SoB - Systemon-Board) is far from having followed the same progression, thus marking a gap with monolithic nanometric CMOS technologies. To bridge this gap, it is now a question of producing gains in functionality, performance and compactness at the system level by heterogeneous integration of components in elemental system building blocks. Sometimes referred to as ‘System Moore’, this approach thus conceives the package not only as a simple encapsulation function but more precisely integrates the package as a functional system block. The dimensional level of functional packaging typically covers the 1-1000 µm range for which the use of microelectronics fabrication techniques are oversized, expensive and unable to efficiently handle thicknesses of a few tens of microns. In this context, laser micromachining is an increasingly used tool for micro/nanostructuring of materials for the packaging of integrated functions in photonics, microelectronics, RF and mmW. In this paper, we will first provide an overview of laser machining techniques in microelectronics and we will detailthe main characteristics of this technique with respect to the laser source and beam conditioning. It will be shown that the use of ultrashort laser processing with pulse width in femtosecond range is advantageous because it can be applied to a virtually unlimited range of materials like metals,semiconductors, dielectrics, alloys, and ceramics. The range of surface processing is diverse varying from a small scale (a few nm2) to large scale (a few mm2). The unwanted heat affected zone is greatly reduced for ultrashort lasers which allows for enhanced machining quality with low thermal impact of laser radiation on material. In a second step, we detail a selection of laser micromachining applications allowing either to increase the performance of RF and mmW components, or to introduce an innovative and distinctive manufacturing method promoting compactness and cost reduction:i) The first illustrated application is the fabrication of ultra-thin free standing membranes of SOICMOS RF circuits/functions on Silicon-on-Insulator (SOI) wafers. It will be shown that tremendous performance improvements can be obtained on a range of RF components like integrated inductances and power switches as well as in terms of cross-talk reduction.ii) The second example describes the packaging of an electro-optical transceiver using a glass electrooptical interposer to connect a silicon photonic chip to a single mode optical fiber.iii) Finally, the third illustration covers the fabrication of structures integrating mmW waveguides in G and J band [140-220 GHz, 220-320 GHz] with silicon chips and transitions allowing the conversion from coplanar to a guided mode in rectangular waveguides (WR5.1 and WR3.1)