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High-Power Varactors for Fast Adaptive Impedance Matching at 13.56 MHz

Kienemund, Daniel (2019):
High-Power Varactors for Fast Adaptive Impedance Matching at 13.56 MHz.
Darmstadt, Technische Universität, [Online-Edition: https://tuprints.ulb.tu-darmstadt.de/8860],
[Ph.D. Thesis]

Abstract

Solid state varactors, applicable in rapidly adjustable high-power impedance matching circuits at 13.56MHz are presented in this work. Tunable impedance matching networks are necessary for RF based plasma processes in the semiconductor industry. With increasing integration level, the speed at which the matching circuit can be adjusted gains impact as constant power transfer is pivotal during ignition and extinction of the plasma. State-of-the-art mechanical vacuum varactors have a limited tuning time of minimum 1ms but are hardly replaceable, as they introduce lower losses and higher linearity compared to solid state varactors. However, as tuning speed gains impact, the substitution with fast but lossy solid state varactors becomes feasible. Within the group of tunable high-power compatible solids, barium strontium titanate (BST) is a possible candidate, due to its high tunability and low dielectric losses in the desired frequency range. The material gains its tunability from an electric field dependent permittivity, which can be adjusted with a DC biasing voltage. When biased, BST exhibits piezoelectricity, resulting in acoustical excitation with an applied RF voltage. As a result, the electrical Q-factor locally decreases and additional losses are introduced. At high-power operation, this property represents a significant problem. In this work, novel fully-printed BST thick film and bulk ceramic disk varactors are presented and evaluated in regard to their compatibility in high-power impedance matching networks. The general applicability of thick film based varactors under high-power conditions up to 1kW is shown. At this power level, an acoustically optimized varactor design achieves a 63% dissipated power reduction from 37.1W to 13.5W, while remaining electrically tunable with 18%. By transient characterization, a response time for thick film varactors of less than 1.5μs is demonstrated in the range of C(0V) to C(120V). Bulk ceramic disk varactors are presented, employed on a printed circuit board for high-power operation. For that, a composite material comprised of BST and a nontunable, low loss, magnesium based metal oxide dielectric shows favorable properties, with a significantly reduced acoustic activity above the single digit MHz region. The feasibility, however, is shown with a thermally stabilized varactor module, employing bulk ceramic pellets based on a commercially available dielectric. At 700W input power a tunability of 12% is obtained with an input power and biasing voltage independent relative loss of 1.8%. A transient response time of less than 1.2μs is measured for a tuning range of C(0V) to C(1.1 kV).

Item Type: Ph.D. Thesis
Erschienen: 2019
Creators: Kienemund, Daniel
Title: High-Power Varactors for Fast Adaptive Impedance Matching at 13.56 MHz
Language: English
Abstract:

Solid state varactors, applicable in rapidly adjustable high-power impedance matching circuits at 13.56MHz are presented in this work. Tunable impedance matching networks are necessary for RF based plasma processes in the semiconductor industry. With increasing integration level, the speed at which the matching circuit can be adjusted gains impact as constant power transfer is pivotal during ignition and extinction of the plasma. State-of-the-art mechanical vacuum varactors have a limited tuning time of minimum 1ms but are hardly replaceable, as they introduce lower losses and higher linearity compared to solid state varactors. However, as tuning speed gains impact, the substitution with fast but lossy solid state varactors becomes feasible. Within the group of tunable high-power compatible solids, barium strontium titanate (BST) is a possible candidate, due to its high tunability and low dielectric losses in the desired frequency range. The material gains its tunability from an electric field dependent permittivity, which can be adjusted with a DC biasing voltage. When biased, BST exhibits piezoelectricity, resulting in acoustical excitation with an applied RF voltage. As a result, the electrical Q-factor locally decreases and additional losses are introduced. At high-power operation, this property represents a significant problem. In this work, novel fully-printed BST thick film and bulk ceramic disk varactors are presented and evaluated in regard to their compatibility in high-power impedance matching networks. The general applicability of thick film based varactors under high-power conditions up to 1kW is shown. At this power level, an acoustically optimized varactor design achieves a 63% dissipated power reduction from 37.1W to 13.5W, while remaining electrically tunable with 18%. By transient characterization, a response time for thick film varactors of less than 1.5μs is demonstrated in the range of C(0V) to C(120V). Bulk ceramic disk varactors are presented, employed on a printed circuit board for high-power operation. For that, a composite material comprised of BST and a nontunable, low loss, magnesium based metal oxide dielectric shows favorable properties, with a significantly reduced acoustic activity above the single digit MHz region. The feasibility, however, is shown with a thermally stabilized varactor module, employing bulk ceramic pellets based on a commercially available dielectric. At 700W input power a tunability of 12% is obtained with an input power and biasing voltage independent relative loss of 1.8%. A transient response time of less than 1.2μs is measured for a tuning range of C(0V) to C(1.1 kV).

Place of Publication: Darmstadt
Divisions: 18 Department of Electrical Engineering and Information Technology
18 Department of Electrical Engineering and Information Technology > Institute for Microwave Engineering and Photonics > Microwave Engineering
18 Department of Electrical Engineering and Information Technology > Institute for Microwave Engineering and Photonics
Date Deposited: 11 Aug 2019 19:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/8860
URN: urn:nbn:de:tuda-tuprints-88601
Referees: Jakoby, Prof. Dr. Rolf and Kölpin, Prof. Dr. Alexander
Refereed / Verteidigung / mdl. Prüfung: 9 July 2019
Alternative Abstract:
Alternative abstract Language
In dieser Arbeit werden Varaktoren entwickelt, die als steuerbare Komponenten in schnellen Hochleistungs-Anpassnetzwerken bei 13.56MHz eingesetzt werden sollen. Steuerbare Anpassnetzwerke finden ihre Anwendung vor allem in der Prozessierung mikro- und nanoelektrischer Schaltungen und gewinnen an Bedeutung durch deren steigende Integration. In diesem Zusammenhang werden die limitierten Steuerzeiten der eingesetzten Vakuum-Varaktoren von mehr als 1ms zunehmend zu einem Problem. Bisher war die Substitution der Vakuum-Varaktoren durch Feststoffvaraktoren vor allem durch die höheren dielektrischen Verluste Letztgenannter erschwert. Aufgrund der steigenden Anforderungen an die Varaktorsteuerzeiten, wird jedoch ein Austausch der vakuumbasierten Varaktoren unumgänglich. Feststoffe, die sich als Dielektrikum in Hochleistungsvaraktoren einsetzen lassen, finden sich vor allem in der Gruppe der Ferroelektrika. Hier nimmt Barium Strontium Titanat (BST) als hochsteuerbares, niedrigverlustbehaftetes Mischkristallsystem eine spezielle Rolle ein. Die Kapazität eines BST basierten Varaktors wird durch Anpassung der Permittivität unter Belastung eines DC Steuerfeldes eingestellt. Die dadurch induzierte Piezoelektrizität stellt ein maßgebliches Problem für den Einsatz unter Hochleistung dar. Als Folge entstehen akustische Schwingungen im Material, die lokal den elektrischen Gütefaktor vermindern. In dieser Arbeit werden sowohl neuartige, vollgedruckte BST Dickschicht-, als auch Vollkeramikvaraktoren entwickelt und in Bezug auf ihre Hochleistungseignung in steuerbaren Anpassnetzwerken bewertet. Es zeigt sich, dass der Einsatz dickschichtbasierter Varaktoren unter Hochleistung bis mindestens 1kW grundsätzlich möglich ist. Durch akustische Optimierung der Varaktoren kann bei Eingangsleistungen von bis zu 1kW eine Verminderung der Verlustleistung um 63% von 37.1W auf 13.5W erzielt werden, bei gleichzeitiger elektrischer Steuerbarkeit von 18%. Die hergestellten Varaktoren werden hinsichtlich ihrer Steuerzeit charakterisiert und mit weniger als 1.5μs für den Kapazitätsbereich C(0V) bis C(120V) gemessen. Vollkeramiken werden in Form zylindrischer Pellets, integriert mit Platinenmaterial zu Varaktormodulen, für den Hochleistungseinsatz vorgestellt. Ein Komposit, bestehend aus BST und einem verlustarmen, nicht steuerbaren, magnesiumbasierten Metalloxiddielektrikum stellt sich als besonders geeignet heraus, durch seine Eigenschaft akustisch nur bis in den einstelligen MHz Bereich aktiv zu sein. Die Machbarkeit wird jedoch an einem temperaturstabilisierten Varaktormodulprototyp auf Basis eines kommerziellen Dielektrikums gezeigt. Bei einer Eingangsleistung von 700W wird eine Steuerbarkeit von 12% gemessen und ein relativer Verlust von 1.8%, unabhängig von Eingangsleistung und Steuerspannung. Darüber hinaus wird eine minimale Steuerzeit von weniger als 1.2μs für den Bereich C(0 V) bis C(1.1 kV) gemessen.German
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