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Ic Transformer

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Diplomarbeit

Design of Monolithic IntegratedLumped Transformers

in Silicon-based Technologiesup to 20 GHz

Ausgefuhrt zum Zwecke der Erlangung des akademischen Grades eines

Diplom-Ingenieurs unter Leitung von

Werner Simburger und Arpad L. Scholtz

E389

Institut fur Nachrichtentechnik und Hochfrequenztechnik

eingereicht an der Technischen Universitat Wien

Fakultat fur Elektrotechnik

von

Daniel Kehrer

9526730Hacklweg 9, 4081 Hartkirchen

Wien, im Dezember 2000

Contents

1 Introduction 1

2 Design of Integrated Transformers 52.1 Silicon-based Technology and Metallization . . . . . . . . . . . . . 52.2 Transformer Construction . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1 Basic Electrical Characteristics . . . . . . . . . . . . . . . 72.2.2 Planar Winding Scheme . . . . . . . . . . . . . . . . . . . 92.2.3 Metallization Structure . . . . . . . . . . . . . . . . . . . . 16

2.3 Transformer Model . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3 Parameter Extraction 253.1 Inductance Calculation . . . . . . . . . . . . . . . . . . . . . . . . 253.2 Skineect and Current Crowding . . . . . . . . . . . . . . . . . . 31

3.2.1 Equivalent Series Resistance . . . . . . . . . . . . . . . . . 323.3 Substrate Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.3.1 Conductor on Substrat . . . . . . . . . . . . . . . . . . . . 343.3.2 Conductor Suspended in Dielectric . . . . . . . . . . . . . 37

3.4 Capacity Extraction . . . . . . . . . . . . . . . . . . . . . . . . . 413.5 Test-Structures for Measurement and Characterization . . . . . . 43

4 Design Examples 464.1 Monolithic Transformer BL62S005 . . . . . . . . . . . . . . . . . 46

4.1.1 Measurement and Simulation Results up to 5GHZ . . . . . 484.2 Monolithic Transformer N3M2 . . . . . . . . . . . . . . . . . . . . 55

4.2.1 Measurement and Simulation Results up to 20GHZ . . . . 56

Conclusion 62

Appendix 63

FastTrafo Manual 63

Bibliography 80

i

Abstract

Monolithic integrated lumped planar transformers have several advantageswhen using them in power ampliers, mixers, oscillators and other radio frequencycircuits. But, up to now there was no way to get an accurate prediction and modelof the electrical characteristic of on-chip transformers.

For the rst time this work presents the modeling and model verication of in-tegrated lumped planar transformers in silicon which have excellent performancecharacteristics in the 1-20 GHz frequency range. A new method for characteriza-tion of monolithic lumped planar transformers is proposed in this work.

The metallization of a semiconductor process denes the possibilities for trans-former design. Planar and layer construction of common monolithic lumped trans-formers and its optimization techniques are considered in detail. A lumped low-order equivalent circuit is presented which results from the transformer geome-tries.

The aim of precise and fast transient analysis of RF circuits using monolithictransformers was reached with the compact lumped low order model which con-sists of 24 elements. The model gives accurate prediction of the electrical behaviorand ensures fast transient analysis. An excellent prediction accuracy is achieved.

Background details about extraction of all elements used in the equivalent cir-cuit are given. The inductance of transformers built up from straight conductorsis derived. Skin eect and current crowding cause losses in the conductors. Theconductive substrate causes additional losses. Some thesis about the parasiticcapacitive coupling between the windings and the substrate are presented.

The parameter extraction for the equivalent circuit is based on a tool devel-oped by the author which uses a new expression for the substrate loss and twonite element method cores.

The modeling and parameter extraction of monolithic transformers has beenveried by multiple transformers. In this work the measurement of two types oftransformers is presented as example. The rst type oers a high coupling perfor-mance up to 4 GHz. The second type of transformer oers a high self resonancefrequency of 20 GHz. Measurement and simulation of the two transformers showexcellent agreement.

List of Abbreviations

Al AluminumCAD Computer Aided Design Depth of Current Penetration in [m]DECT Digital Enhanced Cordless TelecommunicationDC Direct CurrentDUT Device Under Test"r Relative Permittivity in [1]FEM Finite Element MethodfOp Operation Frequency in [Hz]GaAs Gallium-ArsenideGMD Geometric Mean Distance in [m]IC Integrated Circuitk Coupling Coecient in [1]L Inductance in [H]g Guided Wavelength in [m]n Turn Ratio in [1]N Number of Turns in [1]M Mutual Inductance in [H] Permeability in [Vs/Am]Q Quality Factor in [1]RF Radio Frequency Specic Resistivity in [m] Conductivity in [S/m]Si SiliconSi3N4 SiliconnitrideSiO2 SilicondioxideVLSI Very Large Scale Integration

Chapter 1

Introduction

Transformers have been used in radio frequency circuits since the early days oftelegraphy. Normally transformers are relatively large and expensive componentsin a circuit or system. But there are several outstanding advantages using trans-formers in circuit design:

DC isolation between primary and secondary winding

BALUN function

Impedance transformation and matching

No power consumption

The requirements of nowadays telecommunication systems needs a high degreeof monolithic integration. Today it is possible to integrate lumped planar trans-formers in Si- and GaAs-based IC technologies which have excellent performancecharacteristics in the 1-20 GHz frequency range. The outer dimensions are in therange of about 500m down to 60m diameter depending on the frequency ofoperation and the IC technology.

Monolithic integrated lumped planar transformers are introduced by [Rabjohn 89].A review of the electrical performance of passive planar transformers in IC tech-nology was presented by [Long 00]. Ampliers and mixers using monolithic trans-formers are presented in [McRory 99], [Long 99]. A monolithic 2 GHz Meissner-type voltage controlled oscilallator is realised in [Wohlmuth 99]. The transformercoupled push-pull type amplier was invented in the early days of tubes. Re-cent designs in monolithic integration of this concept shows a high performance[Simburger 99],[Simburger 00],[Heinz 00].

Figure 1.1(a) shows the schematic diagram of a monolithic transformer coupledRF power Amplier for 2 GHZ in SI-bipolar [Simburger 00].The circuit consistsof a transformer X1 as input-balun, a driver stage T1 and T2, a transformer

1

CHAPTER 1. INTRODUCTION 2

X2 as interstage matching network and a power output stage T3 and T4. Thetransformers X1 and X2 are of the same kind.

Figure 1.1b shows the chip micrograph of the power amplier. The key elementsof this circuit are two high performance on-chip transformers of the same kind,which work as input balun and for interstage matching.

A detailed micrograph of the on-chip transformer is shown in Figure 1.2. A moredetailed description of this transformer is given in Sect. 4.1.

SubstrateVEE

RFIN+

VCCD

VEED E

KPBKDP

PBDB

X1N=6:2

X2N=6:2

R1R2

CIN CISD1

D2

T1

T2

T3

T4

RFIN-

RFOUT-

RFOUT+

Figure 1.1: (a) Schematic diagram of a power amplier using integratedlumped planar transformers (b) Micrograph of the 2 GHz RF power amplier[Simburger 00]

CHAPTER 1. INTRODUCTION 3

Figure 1.2: Micrograph of an on-chip transformer. Size: 205m diameter

The operation of a lumped transformer is based upon the mutual inductancebetween two or more conductors or windings.

In contrast to an ideal transformer, monolithic integrated transformers have par-asitic eects and imperfect coupling between the windings which results in acoupling coecient less than one. However, monolithic integrated lumped trans-formers have not the characteristics of an ideal component. To create a successfuldesign including a integrated transformer it is not enough to know the trans-formers turn ratio. A sucient specication includes at least the main electricalparameters, which are inductance and coupling coecients of multiple coupledinductors of the windings, ohmic loss in the conductor material of the windingsdue to skin eect and current crowding, parasitic capacitive coupling between thewindings and parasitic capacitive coupling into the substrate and nally substrateloss.

The limitations of monolithic integrated transformers on silicon must be clearlyunderstood by the circuit designer in order to get an overall successful circuitdesign. Up to now there was no way to get an accurate prediction and models of

CHAPTER 1. INTRODUCTION 4

the electrical characteristic of the on-chip transformers.

For the rst time this work presents the modeling and model verication of inte-grated lumped planar transformers in silicon. A lumped low order model whichconsists of 24 elements is presented. The model gives accurate prediction of theelectrical behavior and ensures fast transient analysis, because of the low com-plexity. This work presents a method of parameter extraction for the equivalentcircuit. The method is based on a tool developed by the author. The tool, calledFastTrafo, uses a new expression for the substrate loss and two nite elementmethod (FEM) cores available from Massachusetts Institute of Technology calledFastHenry [MIT 96] and FastCap [MIT 92].

FastTrafo consists of three program modules. The rst module computes theself inductances and mutual inductances of the primary and secondary windingand the ohmic loss due to the r

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