Picosecond pulse propagation on coplanar striplines fabricated onlossy semiconductor substrates: modeling and experiments

A simple model for the propagation of high-frequency signals on coplanar striplines with lossy semiconductor substrates is proposed and demonstrated. This model incorporates the effect of a conductive substrate through the loss tangent in a distributed-circuit analysis extended to high frequencies. Very strong attenuation and dispersion due to the substrate are observed even when the GaAs conductance is only 1 mho/cm, corresponding to a doping density of around 10¹⁵ cm ^-3. The accuracy of this model is tested with a direct comparison to experimental data of picosecond pulse propagation on a doped GaAs coplanar stripline (CPS) measured in the time domain using the electro-optic (EO) sampling technique. Good agreement is found in terms of the attenuation and phase velocity of the distorted pulses at four propagation distances up to 300 μm. The pulse propagation on a multiple modulation-doped layer is also studied experimentally as a prototype of high-frequency signal propagation on the gate of a modulation-doped field-effect transistor (MODFET). The attenuation shows linear frequency dependence up to 1.0 THz, contrary to the cubic or quadratic dependence of coplanar transmission lines on low-loss substrates     

Quasi-TEM Analysis of "Slow-Wave" Mode Propagation on Coplanar Microstructure MIS Transmission Lines

We present a simple quasi-TEM analysis of "slow-wave" mode propagation on micron-size coplanar MIS transmission lines on heavily doped semiconductors and compare theoretical results with measurements on four such structures at frequencies from 1.0 to 12.4 GHz. Excellent agreement is found, which shows that the "slow-wave" mode propagating on these transmission lines is, in fact, a quasi-TEM mode. Relatively low-loss propagation along with significant wavelength reduction is observed. Conduction losses of the metal, which have been tacitly ignored in previously published "full-wave" treatments of "slow-wave" mode propagation, are included in the theory and are shown to dominate the attenuation at frequencies below 25 GHz and to still be significant at frequencies up to at least 100 GHz.     

Low-loss slow-wave propagation along a microstructure transmission line on a silicon surface

We report observations of relatively low-loss propagation in the frequency range of 1.0 to 12.4 GHz using a micrometer-size coplanar MIS transmission line fabricated on a heavily doped N + silicon surface. This low-loss mode of propagation is found to be accompanied by significant wavelength reduction which suggests that such lines may be useful as transmission media for distributed components in silicon monolithic microwave integrated circuits (MMICs).     

W-band finite ground coplanar monolithic multipliers

This paper describes the design, fabrication, and experimental evaluation of W-band planar monolithic varactor frequency multipliers based on finite ground coplanar (FGC) lines. These lines are a low-loss low-dispersion alternative of a planar transmission line to more conventional microstrip of coplanar waveguide lines at millimeter-wave frequencies. The near transverse-electromagnetic nature of propagation of the FGC lines simplifies circuit design and layout. Two-diode W-band varactor multipliers with input Q's of two and three and FGC input and output have been realized. The multiplier with input Q=2 has an output power of 72 mW, an efficiency of 16.3% near 80 GHz, and a -3-dB bandwidth greater than 10 GHz, while the multiplier with Q=3 has an efficiency of 21.5% near 70 GHz and a 6-GHz bandwidth. This paper briefly describes the characteristics of the FGC lines, the design of the multipliers and their radiofrequency performance     

Modeling of picosecond pulse propagation in microstripinterconnections of integrated circuits

Theoretical time-domain analyses of the dispersion and loss of square-wave and exponential pulses on microstrip transmission line interconnections on silicon integrated-circuit substrates, performed using the quasi-TEM approximation, are discussed. Geometric dispersion and conductor line width, as well as losses from conductor resistance, conductor skin effect, and substrate conductance, are considered over the frequency range from 100 MHz to 100 GHz. Results show the enormous significance of the substrate losses and demonstrate the need for substrate resistivities >10 Ω-cm for high-performance circuits. The results also show the effects of geometric dispersion for frequencies above 10 GHz, the unimportance of conductor skin-effect losses for frequencies up to 100 GHz, and the transition from a high-frequency regime where losses do not affect phase velocity to a low-frequency regime where the ratio of he conductor and substrate loss coefficients determines phase velocity     

Exploiting CMOS reverse interconnect scaling in multigigahertzamplifier and oscillator design

The increasing number of interconnect layers that are needed in a CMOS process to meet the routing and power requirements of large digital circuits also yield significant advantages for analog applications. The reverse thickness scaling of the top metal layer can be exploited in the design of low-loss transmission lines. Coplanar transmission lines in the top metal layers take advantage of a low metal resistance and a large separation from the heavily doped silicon substrate. They are therefore fully compatible with current and future CMOS process technologies. To investigate the feasibility of extending CMOS designs beyond 10 GHz, a wide range of coplanar transmission lines are characterized. The effect of the substrate resistivity on coplanar wave propagation is explained. After achieving a record loss of 0.3 dB/mm at 50 GHz, coplanar lines are used in the design of distributed amplifiers and oscillators. They are the first to achieve higher than 10 GHz operating frequencies in a conventional CMOS technology     

High-temperature superconducting delay lines and filters onsapphire and thinned LaAlO3 substrates

The very low microwave surface resistance of high-temperature-superconductor (HTS) thin films allows the realization of microwave devices with performance superior to those made by conventional technology. Superconducting delay lines, for example, have very low propagation loss and dispersion. Long, low-loss, superconducting delay lines on both thinned LaAlO₃ and sapphire substrates are presented. Delay lines with 27- and 44-ns delay have been made, for the first time, on 5-cm-diameter 254- and 127-μm-thick LaAlO₃ substrates, respectively. The insertion losses at 77 K and 6 GHz are 6 and 16 dB, respectively. Delay lines with 9-ns delay have, for the first time, been produced on M-plane sapphire substrates and demonstrate, at 77 K, an insertion loss of 1.0 dB at 6 GHz. A 2.5%-bandwidth 10 GHz four pole edge-coupled bandpass filter on M-plane sapphire substrates is also reported. The filter has minimum insertion loss of less than 0.5 dB at 9.75 GHz and 71 K     

The Generation and Propagation of Acoustic Surface Waves at Microwave Frequencies

The generation and propagation of acoustic surface waves is reviewed with particular emphasis on the microwave-frequency range. Theoretical work on optimizing the generation efficiency and the bandwidth of interdigital transducers is compared with recent experimental results. The minimum Iinewidth of 0.9 /spl mu/m which can be produced by optical photolithographic techniques places an upper limit of about 1 GHz on the maximum frequency that can be generated at the fundamental mode. Overtone operation has been used to generate 3 GHz surface waves on LiNbO/sub 3/ but this method has the disadvantage of reduced efficiency plus the complication of volume-wave generation. A better solution for generation above 1 GHz is the fabrication of interdigital transducers by means of electron beam exposure of the photoresist. The surface-wave propagation loss gives a significant contribution to the total insertion loss of delay lines operating at microwave frequencies. Losses of 1.1 dB//spl mu/s and 3.8 dB//spl mu/s at 0.9 GHz and 2 GHz, respectively, have been measured for propagation along the Z-direction of Y-cut LiNbO/sub 3/ by means of a laser deflection method. Larger losses have been observed for quartz. The additional complexities for surface-wave propagation due to the anisotropic single-crystal substrates which are necessary at microwave frequencies are also described.     

A Comprehensive Technique to Determine the Broadband Physically Consistent Material Characteristics of Microstrip Lines

This paper describes a method to extract the relative complex dielectric permittivity from propagation coefficient measurements on microstrip lines. The material characteristics of microstrip lines fabricated on two different types of substrates commonly used in microwave circuit and printed circuit boards are investigated. The mechanisms that cause the effective permittivity of microstrip lines to be dispersive are explored. The technique includes creating closed-form effective permittivity equations to relate the effective permittivity of the microstrip lines to the real part of the dielectric permittivity of the substrate. Curve-fitting methods are used to create causal dielectric material models that relate the imaginary part of the dielectric permittivity to its real part. The methods developed in this paper can be used to characterize low-loss dielectric materials whose polarization is dominantly dipolar within the microwave frequency range in high-speed packaging applications.      

CPW MMIC coupler based on offset broadside air-gap couplingfabricated by standard airbridge processes

A new low-loss CPW-based MMIC coupler is developed. Offset broadside coupling using the air gap between the two lines in employed to obtain tight coupling as well as low conductor loss. Moreover, the air-gap coupling is achieved using a standard MMIC airbridge process, eliminating the need for an additional dielectric process. The fabricated Ka-band coupler showed transmission and coupling losses of 3.6±0.4 dB over a wide frequency range from 20 to 39 GHz. This is better than the previous CPW coupler loss in this frequency range     

A 5-μm-wide 18-cm-long low-loss YBa2Cu3O7-δ coplanar line for future multichip moduletechnology

Narrow and low-loss YBa₂Cu₃O_7-δ (YBCO) coplanar lines, which can be used in multichip module technology for future high-density and high-speed digital circuits, have been developed. Etch-back planarization and a patterning process combining Ar-ion milling and wet-etching enabled us to form an 18-cm-long 5-μm-wide YBCO coplanar line without electrical shorts, even for the narrow spacing of 2.5 μm. The surface resistance of this line was kept at a level comparable to that of 10- or 25-μm-wide YBCO coplanar lines and also comparable to that of unpatterned films. This indicates successful fabrication of the 5-μm-wide YBCO coplanar line without notable loss increase resulting from process damage. The 5-μm-wide line showed a low-transmission loss of 0.49 dB at 10 GHz and 55 K. This level of loss is similar to that in Cu coaxial cables. No significant increase in transmission loss was observed up to an input power level of 16 mW at 10 GHz and 55 K. This input power is comparable to the power-handling capability required for transmitting high-speed digital signals through the lines with characteristic impedance of 50 Ω. These results show that the narrow 5-μm-wide YBCO coplanar line has great potential for high-density and high-speed digital circuits     

Ultra-wideband (from DC to 110 GHz) CPW to CPS transition

A new ultra-wideband, low-loss and small-size coplanar waveguide (CPW) to coplanar strip (CPS) transition which can be used from DC to 110 GHz is presented. The proposed transition connects CPW with CPS by the reformed air-bridge. Two ground planes of CPW are tied at their ends by a line and the centre of the line is connected to the ground strip of CPS by another line. Owing to the symmetry of the proposed structure, the currents of two ground planes of CPW are combined with the same phase and transferred to the ground strip of CPS. With height of 3 μm, the signal line of CPW passes over two connecting lines and is connected to the signal strip of CPS. For the back-to-back transition structure, insertion loss <1 dB and return loss >15 dB are obtained from 0.5 to 110 GHz     

Cutoff frequency and propagation delay time of 1.5-nm gate oxideCMOS

The high-frequency AC characteristics of 1.5-nm direct-tunneling gate SiO₂ CMOS are described. Very high cutoff frequencies of 170 GHz and 235 GHz were obtained for 0.08-μm and 0.06-μm gate length nMOSFETs at room temperature. Cutoff frequency of 65 GHz was obtained for 0.15-μm gate length pMOSFETs using 1.5-nm gate SiO₂ for the first time. The normal oscillations of the 1.5-nm gate SiO₂ CMOS ring oscillators were also confirmed. In addition, this paper investigates the cutoff frequency and propagation delay time in recent small-geometry CMOS and discusses the effect of gate oxide thinning. The importance of reducing the gate oxide thickness in the direct-tunneling regime is discussed for sub-0.1-μm gate length CMOS in terms of high-frequency, high-speed operation     

An equivalent circuit model for terminated hybrid-modemulticonductor transmission lines

An equivalent circuit for terminated hybrid-mode multiconductor transmission lines is presented. Existing CAD packages, such as SPICE, can be used for its implementation. Model parameters can be found from either a TEM or a full-wave analysis of the transmission lines. The equivalent circuit is used to simulate multiconductor microstrip for applications in high-speed integrated circuits. The accuracy of the TEM approximation is investigated for time-harmonic as well as pulse signal propagation. For some cases examined, the time-harmonic signal propagation data obtained using the TEM approximation become quite inaccurate at frequencies in excess of 20 GHz. The TEM approximation is adequate, however, for pulse propagation with 15-ps risetime and falltime pulses on the geometries investigated     

Low-loss Transmission Lines for High-power Terahertz Radiation

Applications of high-power Terahertz (THz) sources require low-loss transmission lines to minimize loss, prevent overheating and preserve the purity of the transmission mode. Concepts for THz transmission lines are reviewed with special emphasis on overmoded, metallic, corrugated transmission lines. Using the fundamental HE₁₁ mode, these transmission lines have been successfully implemented with very low-loss at high average power levels on plasma heating experiments and THz dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments. Loss in these lines occurs directly, due to ohmic loss in the fundamental mode, and indirectly, due to mode conversion into high order modes whose ohmic loss increases as the square of the mode index. An analytic expression is derived for ohmic loss in the modes of a corrugated, metallic waveguide, including loss on both the waveguide inner surfaces and grooves. Simulations of loss with the numerical code HFSS are in good agreement with the analytic expression. Experimental tests were conducted to determine the loss of the HE₁₁ mode in a 19 mm diameter, helically-tapped, three meter long brass waveguide with a design frequency of 330 GHz. The measured loss at 250 GHz was 0.029 ± 0.009 dB/m using a vector network analyzer approach and 0.047 ± 0.01 dB/m using a radiometer. The experimental results are in reasonable agreement with theory. These values of loss, amounting to about 1% or less per meter, are acceptable for the DNP NMR application. Loss in a practical transmission line may be much higher than the loss calculated for the HE₁₁ mode due to mode conversion to higher order modes caused by waveguide imperfections or miter bends.     

Electrical properties of coplanar transmission lines on lossless and lossy substrates

The characteristic impedance and the propagation constant of coplanar transmission lines on lossless and lossy substrates are given and compared with computed values within the frequency region 0?8 GHz. Al2O3 and n-Si substrates are used.     

Study of the DC biasing effect on insertion losses in high-frequency interconnections

The paper deals with an experimental investigation of the behavior of high-frequency Si/SiO₂/Al based interconnects when an extra DC bias voltage is applied, by means of which the conductor line changes the surface properties of the semiconductor substrate. By superposing a DC bias to the high-speed signal applied to the line, the insertion losses caused by the semiconductor substrate show a significant decrease over the observed frequency range. In order to study this effect a number of test samples containing several microstrip asymmetric transmission lines were prepared and measured. The obtained results suggest a way of controlling the performance and energy propagation of interconnects on semiconductor substrates. The observed effect can be successfully applied in high-speed blocks with tunable parameters.     

High Performance Inductors on CMOS-Grade Trenched Silicon Substrate

This paper reports on a new implementation of high-quality factor copper inductors on CMOS-grade silicon substrates (p = 10-20 Omega ldr cm) using a CMOS-compatible process. A low-temperature fabrication sequence (<300degC) is used to reduce the loss in silicon at RF frequencies by trenching the silicon substrate. The high aspect-ratio (30:1) trenches are subsequently bridged over or refilled with a low-loss dielectric to close the open areas and create a rigid low-loss island, referred to as Trenched Si Island. This method does not require air suspension of the inductors, resulting in mechanically-robust structures that are compatible with any packaging technology. A one-turn 0.8 nH inductor fabricated on a Trenched Silicon Island exhibits a very high peak quality factor of 71 at 8.75 GHz with a self-resonant frequency larger than 15 GHz.     

Altered phase velocity lines for low crosstalk microstrip interconnection of high-speed digital circuits: design and experimental validation

Altered phase velocity lines are a novel kind of parallel microstrip lines for high-speed interconnection of digital circuits, on which the crosstalk is reduced by the different phase velocities of propagation on the adjacent lines. In this paper, a design method is proposed to optimize the geometry sizes of the altered phase velocity lines. The measured results of a prototype altered phase velocity pair designed by the proposed method are presented to validate the design method. And the effects of the process variation are simulated to analyze the robustness of the prototype in fabrication. The altered phase velocity lines outperform the symmetric parallel microstrip lines in terms of the lower far-end crosstalk (FrdCtk) and the lower dielectric loss. This technique reduces the FrdCtk in the pair of the microstrip transmission lines and does not significantly improve the near-end crosstalk. The prototype works at the speed of 2 Gbps for low crosstalk digital signal transmission, while it can transmit the high-speed clock signal at 10.5 GHz, so the altered phase velocity lines are a useful supplementary to the existing low crosstalk interconnection concepts in the scenario that the parallel microstrips have to be placed closely on printed circuit board.     

Permalloy loaded transmission lines for high-speed interconnect applications

The mutual inductance and self-inductance of global interconnects are important but difficult to extract and model in deep submicrometer very large scale integration (VLSI) designs. The absence of effective mutual magnetic field shielding limits the maximum unbuffered interconnect line length. In this paper, we propose and demonstrate that permalloy-loaded transmission lines can be used for high-speed interconnect applications to overcome these limitations. Permalloy films were incorporated into planar transmission lines using a CMOS-compatible process. The line characteristics show that eddy-current effects are the limiting factors for the high-frequency permalloy applications when ferromagnetic resonance are restrained through geometry design. Patterning permalloy films effectively extends their application to above 20 GHz. The line characteristic impedances are about /spl sim/90 /spl Omega/. Under 50 mA dc current biases, the line parameters did not change much. Moreover, the patterned permalloy reduces the magnetic field coupling between two adjacent transmission lines by about 10 dB in our design. The demonstrated operation frequency range, current carrying capability and magnetic field shielding properties indicate that the permalloy loaded lines are suitable for high-speed interconnect applications in CMOS technologies.