1. U. Kim RF and Millimeter-wave Integrated Systems Lab. 1 Q Enhancement in Spiral Inductors q Why Inductors need? q Inductors Used in RFIC, MMIC q Spiral Inductor Modeling q Degradation of Q q Q Enhancement Techniques q Conclusion Microwave Devices Term Project Unha Kim (2004-21475) edmaun1@snu.ac.kr RF and Millimeter-wave Integrated Systems Lab.

2. U. Kim RF and Millimeter-wave Integrated Systems Lab. 2 Why Inductors Need? Typical Design Example A single-chip GPS Receiver CMOS technology Freq = 1.57542GHz Used more than 10 inductors About 25% of chip area Impedance matching DC biasing (RF choke) Phase shifting Filtering LC tank

3. U. Kim RF and Millimeter-wave Integrated Systems Lab. 3 Inductors Used in RFIC, MMIC Inductors Ribbon Inductor Loop Inductor Meandered Inductor Spiral Inductor Bondwire Inductor Active Inductor Considerations Inductance Quality Factor Self Resonant Frequency

4. U. Kim RF and Millimeter-wave Integrated Systems Lab. 4 Some Types of Inductors Ribbon inductor  Less than 1nH  High Z 0 needed  Relatively ‘pure’ inductance (low parastics)  Often used in distributed amplifiers Loop inductor  Used extensively in the early days of MMICs  Inefficient use of chip area  Recently, it is used very little Meandered track inductor  Can get more than 1nH  L meandered < L straight track with same length

5. U. Kim RF and Millimeter-wave Integrated Systems Lab. 5 Some Types of Inductors Bondwire inductor  Diameter = 1mil (0.001 inches)  More surface area per length than spirals  Less resistive loss, Higher Q  L = 1nH / mm Active inductor  Higher noise  Power consumption  Limited linearity - Distortion  L = C / (g m1 g m2 ) d Discrete inductor  L = 2 ~ 100nH with 2 ~10% tolerance  Q = 50 to 200 (1 to 2GHz)  SRF = 4 to 10GHz

6. U. Kim RF and Millimeter-wave Integrated Systems Lab. 6 Spiral Inductor The most frequently used High inductance per unit area Square, octagon, circular type Q circular > Q octagon > Q square Air bridge crossover or dielectric spaced underpass D in : Inner dimension D out : Outer dimension S : Spacing W : Width t : Thickness n : Number of turns

7. U. Kim RF and Millimeter-wave Integrated Systems Lab. 7 Spiral Inductor Modeling L s : Mutual Couplings R s : DC & AC resistance (skin effect) C s : Series Capacitance C ox : Oxide Capacitance C si : Si Substrate Capacitance R si : Si Substrate Ohmic Loss C.Patrick Yue, “Physical Modeling of Spiral Inductors on Silicon”

8. U. Kim RF and Millimeter-wave Integrated Systems Lab. 8 Degradation of Q Problems Limitation on the number of turns Occupies large area Series (DC + AC) resistance Substrate loss Some Proposed solutions Patterned ground shield Differentially driven inductor Copper metallization Three dimensional inductor

9. U. Kim RF and Millimeter-wave Integrated Systems Lab. 9 Dominant Effects on Spiral Inductor R s, C s effect dominant C si, R si effect dominant  Low frequency : series resistance effect  High frequency : substrate loss effect  Conductive Si substrate have a defect!  How can we reduce the substrate loss? SRF

10. U. Kim RF and Millimeter-wave Integrated Systems Lab. 10 Other Dimensional Effects on Spiral Inductor 12 3 1.Size dependency larger size, larger substrate loss 2.Oxide thickness dependency thicker oxide, lower substrate loss 3.Metal thickness dependency thicker metal, lower R s Or, using Cu instead of Al, lower R s

11. U. Kim RF and Millimeter-wave Integrated Systems Lab. 11 Solid Ground Shield Severe substrate loss at high freq. Si substrate is vulnerable Usually ρ < 20Ω·cm GaAs substrate is less vulnerable Semi-Insulating Substrate SGS To reduce substrate loss Conductive ground shield between oxide and substrate Metal or polysilicon deposition Eddy current ☞ L↓ Q↓ Capacitance increases ☞ SRF↓

12. U. Kim RF and Millimeter-wave Integrated Systems Lab. 12 Eddy Current  Eddy current occurs when a conductor is subjected to time-varying-magnetic field and is governed by Faraday’s law.  Eddy currents produce their own magnetic fields to oppose the original field  Eddy currents reduce the net current flow in the conductor  Increase the ac resistance

13. U. Kim RF and Millimeter-wave Integrated Systems Lab. 13 Patterned Ground Shield PGS Orthogonal to spiral (block eddy current) Avoid attenuation of the magnetic field Isolates between inductor and ground termination of the electric field Aluminum metal or polysilicon (better) Capacitance increases ☞ SRF↓ C.Patrick Yue, “On-Chip Spiral Inductors with Patterned Ground Shields for Si-Based RFICs”

14. U. Kim RF and Millimeter-wave Integrated Systems Lab. 14 Patterned Ground Shield (cont’) Q factor up to 33% SRF decrease SRF

15. U. Kim RF and Millimeter-wave Integrated Systems Lab. 15 Patterned Ground Shield (cont’) 1.Parallel LC resonator at 2GHz There are many advantages in designing oscillator. 2.Reduce the substrate coupling b/w two adjacent inductors by 25dB Using PGS has both advantages and disadvantages. 1 2

16. U. Kim RF and Millimeter-wave Integrated Systems Lab. 16 Differentially Driven Inductors Differential circuits have robustness and superior noise rejection properties Can get greater Q without altering the fabrication process Differential signal path requires extra chip area compared to a single-ended Symmetrical inductor has better performance than asymmetric inductor. Adjacent conducting strips : voltage (anti-phase), current (same direction) Reinforces the magnetic field by the parallel groups of conductors Increases the overall inductance per unit area asymmetric symmetric High V difference Same I direction Low V difference Same I direction

17. U. Kim RF and Millimeter-wave Integrated Systems Lab. 17 Differentially Driven Inductors (cont’) Mina Danesh, “Differentially Driven Symmetric Microstrip Inductors” L sub.s Spiral inductor modeling Single-ended L sub.d Differential excitations L series.d L series. s L series.d and L series.s are similar low-freq. dominant factor L sub.d is less than L sub.s up to 2 times high-freq. dominant factor Low freq. performance is similar (c) is superior at high freq.

18. U. Kim RF and Millimeter-wave Integrated Systems Lab. 18 Differentially Driven Inductors (cont’) Less affected by substrate parastics 50% grater Q factor than single-ended Broader range of operating frequencies common node

19. U. Kim RF and Millimeter-wave Integrated Systems Lab. 19 Circular Shaped Inductors 1GHz R circular and R octaogonal is smaller by 10% than R square Decreasing conductor spacing is better than increasing conductor width C IW ↑ C ox, C si ↑ S. Chaki, “Experimental Study on Spiral Inductors”

20. U. Kim RF and Millimeter-wave Integrated Systems Lab. 20 Reducing Line Resistance AC resistance ☞ W, t ＞ 2 δ s DC resistance The four best conducting metal resistivities are Silver : 1.62 μΩ·cm Copper : 1.72 μΩ·cm Gold : 2.44 μΩ·cm Aluminum : 2.62 μΩ·cm If we use Cu instead of Al, R s would be reduced significantly Some paper proposed that ( 3um-thick Al ) = ( 1um-thick Cu ) But thicker metal, larger C IW Damascene Cu metallization Cu metallization is not mature in RFIC & MMIC

21. U. Kim RF and Millimeter-wave Integrated Systems Lab. 21 Cu Damascene Interconnects (a) Etch trenches and via holes (b) Ta barrier layer and Cu seed layer (c) Electrochemical plating Cu (d) CMP Cu and Ta, CVD nitride Example : Cu metallization in VLSI technology  Better conductor than aluminum  Higher speed and less power consumption  Higher electomigration resistance  Diffusing freely in silicon and silicon dioxide, causing heavy metal contamination, need diffusion barrier layer  Hard to dry etch, no simple gaseous chemical compounds

22. U. Kim RF and Millimeter-wave Integrated Systems Lab. 22 High Q Inductor in Single Damascene Snezana Jenei, “High Q Inductor Add-on Module in Thick Cu/SiLK TM single damascnene” Al sheet resistance : 20~100 Q factors up to 24 at 2.8nH by using think metal layer Non-effective unless the substrate losses are lowered sufficiently

23. U. Kim RF and Millimeter-wave Integrated Systems Lab. 23 Conclusion q Inductors are needed in RFICs & MMICs q High Q inductors are required for high performance q Spiral inductors are mostly used q The Q of spiral inductor is very low q Substrate loss and series resistance are major effects on Q q Some Q-enhancement techniques are suggested q PGS + Cu-metal + Octagonal shaped inductor is best performance q It will be trade-off relation between high-Q process and cost

24. U. Kim RF and Millimeter-wave Integrated Systems Lab. 24 References  C. Patrick Yue, “Physical Modeling of Spiral Inductors on Silicon”  Mina Danesh, “Differentially Driven Symmetric Microstrip Inductors”  C. Patrick Yue, “On-Chip Spiral Inductors for Silicon-Based RFICs”  Snezana Jenei, “High Q Inductor Add-on Module in Thick Cu/SiLK single damascene”  Daniel C. Edelstein, “Spiral and Solenoidal Inductor Structures on Silicon Using Cu-Damascene Interconnects”  Joachim N. Burghartz, “On the Design of RF Spiral Inductors on Silicon”  S. Chaki, “Experimental Study on Spiral Inductors”  C. Patrick Yue, “On-Chip Spiral Inductors with Patterned Ground Shields for Si-Based RFICs”  John Rogers, “Radio Frequency Integrated Circuit Degisn”, Artech House  Thomas H. Lee, “The Design of CMOS RFICs”, Cambridge Univ. press  I. D. Robertson, “MMIC Design”, IEE press