Enhanced transmission line model structures for accurate resistanceevaluation of small-size contacts and for more reliable fabrication

Two new transmission line model (TLM) techniques are presented, The first structure, LT-TLM, provides very accurate determination of the transfer length LT (within 5%) and the contact resistivity ρ_C (within 12%) of lateral contacts, as compared to typical errors of 30% and 50%, respectively, for conventional TLM patterns, accurate determination of these quantities is important for the optimal design of small lateral contacts in high-performance devices, such as heterojunction bipolar transistors (HBTs). The second structure, a modified concentric TLM pattern, provides the same results as conventional TLM patterns; however, they do not require mesa isolation, and they can be fabricated extremely reliably. In contrast, conventional concentric TLM patterns are often shorted together or distorted by lift-off metallization processes. Both theoretical analysis and experimental verification with analysis of the accuracy of the extracted data is presented for each technique, demonstrating the advantages of these approaches     

Statistical modeling of transmission line model test structures.II. TLM test structure with four or more terminals: a novel method tocharacterize nonideal planar contacts in presence ofinhomogeneities

For pt.I see ibid., vol.37, pp.2350-2360, Nov. 1990. A modified transmission line model (redundant TLM or RTLM) test structure (with four or more terminals) and a novel data extraction method are suggested to improve the accuracy of the TLM method. Statistical modeling by both statistical simulation and the method of error propagation shows that with four terminals and independently known sheet resistances between the contacts, the errors of the extracted parameters can be reduced considerably. With the use of five or more terminals, the accuracy of the parameter extraction can also be determined by a single structure, allowing the separation of inhomogeneities within a test structure from the inhomogeneities over the wafer. Experimental data gathered by Kelvin cross-bridge resistors and by traditional and modified transmission line model structures on Al-Si-Cu/TiW/p-n Si contacts clearly show the advantage of the RTLM method over the traditional TLM method. On a wafer with small inhomogeneities, both methods gave consistent results     

Error analysis leading to design criteria for transmission linemodel characterization of ohmic contacts

The transmission line model (TLM) is a standard method for planar specific contact resistance measurement. Although widely used, the accuracy of a measurement is typically not stated. In addition to contributions from random errors, there can be substantial contributions from systematic errors in typical TLM measurements. In this paper, we develop an analytical model for the experimental uncertainty from the fundamental TLM expressions in order to understand and calculate the uncertainty associated with the specific contact resistance and sheet resistance derived by the TLM method. The experimental uncertainties in measured resistances, together with the pad width and pad spacing, are the dominant contributions to the total uncertainty. Analytical expressions for relative random and systematic uncertainties in contact resistance and sheet resistance are developed in terms of the error contributions and the parameters of the TLM geometry. Expressions for minimum uncertainty, with associated optimum widths and sheet resistances, serve as a basis for the design of TLM structures with minimum uncertainty. The model quantifies the increase in relative uncertainty associated with decreasing contact resistance. Simulations of uncertainty under various sheet resistance, contact resistance, and pad width are implemented and uncertainties are calculated for realistic data sets     

Impact of injection limitations on the contact resistance and the carrier mobility of organic field effect transistors

The contact resistance as well as the mobility have developed to key performance indicators for benchmarking organic field-effect transistors. Typically, conventional methods for silicon transistors are employed for their extraction thereby ignoring the peculiarities of organic transistors. This work outlines the required conditions for using conventional extraction techniques for the contact resistance and the mobility based on TCAD simulations and experimental data. Our experimental data contain both staggered and coplanar structures fabricated by exploiting different optimization techniques like SAM treated electrodes, different shearing speeds, PS blending and silicon oxide functionalization. In addition, the work clarifies how injection limited current–voltage characteristics can affect high-performance organic field-effect transistors. Finally, we introduce a semi-physical model for the contact resistance to accurately interpret extracted benchmark parameters by means of the transfer length method (TLM). Guidelines to use conventional extraction techniques with special emphasis on TLM are also provided.     

Hybrid optical-electrical overlay test structure

This paper describes the exploratory use of electrical test structures to enable the calibration of optical overlay instruments of the type used to monitor semiconductor-device fabrication processes. Such optical instruments are known to be vulnerable to hard-to-determine systematic process- and instrument-specific errors known as shifts. However, these shift errors generally do not affect electrical test-structure measurements extracted from the same features. Thus the opportunity exists to configure physical standards having overlay targets that can be certified by electrical means, thereby enabling estimates of the shifts prevailing in a particular application. In this work, a new hybrid test structure, meaning one from which overlay measurements can be extracted electrically, as well as by optical instruments, has been designed and fabricated with built-in overlay values ranging from -60 to +60 nm. A selection of structures constituting a test chip has been patterned in a single conducting film with CD (critical dimension) design rules ranging from 1.0 μm to 2.0 μm and fabricated and tested. Electrical overlay parameters, derived from multiple step-and-repeat die-site measurements, generally match the corresponding optical measurements to within several nanometers, subject to limitations of the pattern-replication process. This paper focuses on the extraction of overlay from the electrical measurements, the dependence of the measurements on CD design rules, and their comparison with the corresponding measurements made both by a commercial optical-overlay instrument and by a coordinate-measurement system having measurements traceable to absolute dimensional standards. It is presented as a first step toward the use of electrical measurements for advancing shift management in optical overlay of features patterned in separate lithographic processes     

Excess noise in AlGaAs/GaAs heterojunction bipolar transistors andassociated TLM test structures

Noise measurements both on transmission line model (TLM) test structures and on associated HBT's are presented. Contact noise is proved to be negligible in the TLM's related to the base structure of transistors. A Hooge parameter for p⁺⁺ doped GaAs is extracted. Activation energies are calculated from results versus temperature. Considering the TLM related to the structure of the emitter, it is shown that the g-r levels observed originate from the AlGaAs layer. Noise measurements on HBT's also exhibit excess noise. A value of the cutoff frequency between the equivalent input current white noise and the 1/f component is given. The base current dependencies associated with different measurement configurations suggest the 1/f noise to come from the base or the emitter-base junction. The g-r components are studied as a function of temperature. Activation energies are deduced. Finally a comparison of the TLM and HBT noise results is presented. The presence of the complex DX center and of g-r levels in the base region are proposed as possible origins for the g-r noise in HBT's     

The effect of lateral current spreading on the specific contact resistivity in D-resistor Kelvin devices

The effects of lateral current spreading on the determination of ohmic contact resistivity when Using Kelvin devices with a D-resistor layout are studied by numerical simulation and experimental measurements. Simulations show that the extracted contact resistivity can be higher or lower than the actual resistivity. The extracted contact resistivity is found to be a strong function of the geometrical layout, the semiconductor sheet resistance, and the actual contact resistivity. Futhermore, it is shown that the actual contact resistivity cannot be determined below some minimum value and the effect of misalignment during processing is shown to produce significant errors. Good agreement with the experiment is found for As-implanted ohmic contacts to Si. Guidelines for the design, processing, and evaluation of D-resistor Kelvin devices are presented.     

An analytical model for alloyed ohmic contacts using a trilayer transmission line model

This paper describes a Transmission Line Model approach to the modeling and analysis of alloyed planar ohmic contacts. It briefly reviews the standard Transmission Line Model (TLM) commonly used to characterize a planar ohmic contact. It is shown that in the case of a typical Au-Ge-Ni alloyed ohmic contact, a more realistic model based on the TLM should take into account the presence of the alloyed layer at the metal-semiconductor interface. In this paper, such a model is described. It is based on three layers and the two interfaces between them, thus forming a Tri-Layer Transmission Line Model (TLTLM). Analytical expressions are derived for the contact resistance Re and the contact end resistance Rc of this structure, together with a current division factor, S. Values for the contact parameters of this TLTLM model are inferred from experimentally reported values of Re and Re for two types of contact. Using the analytical outcomes of the TLTLM, it is shown that the experimental results obtained using a standard TLM can have considerable discrepancies.<>     

Evaluation of Transmission Line Model Structures for Silicide-to-Silicon Specific Contact Resistance Extraction

In order to measure silicide-to-silicon specific contact resistance rho_c, transmission line model (TLM) structures were proposed as attractive candidates for embedding in CMOS processes. We optimized TLM structures for nickel silicide and platinum silicide and evaluated them for various doping levels of n- and p-type Si. The measurement limitations and accuracy of the specific contact resistance extraction from the optimized TLM structures are discussed in this paper.     

The Accuracy of TLM Analysis of Finned Rectangular Waveguides

This paper investigates three sources of error affecting the Transmission Line Matrix (TLM) analysis of finned rectangular waveguides. It is shown how truncation and velocity errors can be minimized, and a diagram for maximum coarseness error affecting the TLM analysis is presented. After error correction, cutoff frequencies obtained with the TLM method are in excellent agreement with results obtained with the Transverse Resonance Method.     

Electrical properties of titanium-HgCdTe contacts

Transfer length method (TLM) structures were fabricated to characterize the Ti-HgCdTe contacts. Low-temperature measurement of contact resistance was found to be affected by the background-generated carriers in long wavelength infrared HgCdTe material. Measurements carried out by keeping the TLM structures behind a cold shield showed low contact resistance indicative of the formation of a good “Ohmic” contact. Low specific contact resistance of the order of 10⁻⁴ Ω-cm² makes this contact scheme suitable for the fabrication of photoconductive as well as photovoltaic HgCdTe detectors. Annealing the contacts in air at 60°C for 15 days yielded the specific contact resistance values of the same order of magnitude at room temperature; however, low-temperature measurements show a minor change in the specific contact resistance. The current-voltage measurements show that current transport is dominated by the thermionic field emission mechanism.     

异质结n+多晶硅/n型4H-SiC欧姆接触的制备

从理论和实验的角度研究了n型4H-SiC上的多晶硅欧姆接触.在P型4H-SiC外延层上使用P+离子注入来形成TLM结构的n阱.使用LPCVD淀积多晶硅并通过P+离子注入及扩散进行掺杂,得到的多晶硅方块电阻为22Ω/□.得到的n+多晶硅/n-SiC欧姆接触的比接触电阻为3.82×10-5Ω·cm2,接触下的注入层的方块电阻为4.9kΩ/□.对n+多晶硅/n-SiC欧姆接触形成的机理进行了讨论.     

The generation of optimal microwave topologies using time-domain field synthesis

We present a novel approach to the design of microwave structures using time-domain field synthesis. A standard transmission-line matrix (TLM) electromagnetic analysis of the starting geometry yields the structure response and the field distribution on the optimizable boundary parts. A number of characteristic frequencies equal to the number of designable parameters of the structure are determined first. For narrow-band structures, these frequencies may be natural resonance frequencies. For wide-band structures, we create appropriate resonance conditions. The target response of the structure allows us to identify the desirable values of these frequencies. For each parameter, a synthesis phase is then performed. In this phase, the optimizable boundary parts are replaced by matched TLM sources that inject sampled sinusoidal streams at the desired characteristic frequency. The TLM field model generates an electromagnetic field pattern. The synthesized geometry is obtained by examining the envelope of that field pattern. Our approach is illustrated by means of several examples.     

Allowing for current spreading in semiconductors during measurements of the contact resistivity of ohmic contacts

A modification of the contact-area pattern with radial geometry, which has certain advantages in determining the contact resistivity of ohmic contacts (ρ _c) fabricated on substrates and low-resistance semiconductor layers, is proposed. Different variants of its application for both the transmission line method (TLM) and methods based on a numerical calculation of the resistance of the semiconductor with allowance for current spreading are considered. It is shown that the transmission line method makes it possible to obtain an upper estimate of the contact resistivity on substrates. The errors of such estimates are also calculated as a function of the parameters of the semiconductor and the contact. The TLM estimate is a good first approximation for determining the exact value of ρ _c by numerically calculating the resistance of the semiconductor. The results obtained are used to study the contact resistivity of Ni-based ohmic contacts on n-6H-SiC substrates. Fiz. Tekh. Poluprovodn. 32, 832–837 (July 1998)     

Analysis of a CPW on electric and magnetic biaxial substrate

A formulation of the spectral domain technique (SDT) for the analysis of the general case of uniaxial/biaxial, electric/magnetic anisotropic multilayer planar structures is presented. As an illustration of the capabilities of this formulation it is applied to the analysis of shielded coplanar waveguide (CPW) structures. The dispersive properties of the fundamental and higher-order modes in various cases of electric/magnetic anisotropy as well as the induction electric and magnetic lines are calculated. Such results, and particularly the induction lines, may be used to predict the behavior of the structure studied as well as to point out its sensitive geometrical and electrical parameters. Some general rules which lead to a better understanding of the effect of anisotropy are discussed     

Characterization of extremely low contact resistances on modulation-doped FET's

The measurement and affects of extremely low contact resistances on GaAs/AlGaAs modulation-doped FET's (MODFET's) were studied. Ohmic contacts were characterized by a variety of test structures in close proximity to each other. Measurements of front, end, and interfacial contact resistance were used to calculate the specific contact resistance by the transmission line model (TLM) and the two-dimensional resistor network model. The validity of these models in describing contacts to GaAs is discussed. Normalized contact resistance as low as 0.06 Ω . mm in MODFET's exhibiting semiconductor sheet resistance of 536 Ω/ are reported, contributing only 10 percent to the correspondingly reduced parasitic resistances. This leads to high extrinsic transconductances of 240mS/mm at 300 K in devices with a gate length of 1.4 µm and to extraordinary small drain-source saturation voltages of about 0.6 V. Both effects lead to much improved MODFET performance which would be not only advantageous in small-signal, but especially in high-speed/low-power logic circuits.     

A method for designing high-power offset Cassegrain antennas

Starting from the geometrical parameters of the feed and the desired gain of the Cassegrain antenna, a method to determine the design starting parameters for a high-power offset Cassegrain antennas is discussed. Then, by numerically solving the equations governing the antenna geometry, a set of design curves are obtained, with which the structure of the antenna can be determined by trading off the electrical and geometrical system requirements. The method presented can be applied to both high or low gain feeds for constructing a high power microwave electromagnetic environment, where complex electronic systems can be tested. For linearly polarized feeds with high gain, the structure parameters of antenna with zero cross polarization can be obtained, which is equal to the design results of Brown and Prata (see IEEE Trans. Antennas Propagat., vol.42, no.8, p.1145-53, 1994)     

Modeling and characterization of the bonding-wire interconnection

In this paper, the bonding-wire interconnection has been studied from the points of view of its modeling and electrical characterization. Both singleand double-wire structures have been considered, the latter under the assumption of parallel wires. Two electrical models of the bonding wire are discussed. First, the finite-difference time-domain (FDTD) method is proposed for the rigorous analysis of such structures. This method uses a suitable discretization technique, which accounts for the wire curvature by means of a polygonal approximation. A quasi-static model of the bonding wire, suitable for commercial microwave computer-aided-design tools is then proposed. This model is based on the representation of the structure with four sections of a uniform transmission line and the model parameters are evaluated analytically from the dimensions of the interconnection. Accuracy and applicability of the quasi-static model have been assessed by analyzing several test structures, the reference results being obtained with the FDTD method. Finally, the quasi-static model has been used to provide an extensive electrical characterization of the bonding wire versus its main geometrical parameters. This characterization is given in terms of an equivalent series inductance and two equivalent shunt capacitances forming a π low-pass network. This representation is particularly useful in the matching of the bonding-wire discontinuity     

Scattering from an anisotropic cylindrical dielectric shell

The electromagnetic scattering from an anisotropic cylindrical dielectric shell is formulated by using the wave functions for anisotropic media and the boundary-value method. The cylindrical shell is assumed to be infinite in length, and it is illuminated by a plane wave or a cylindrical wave from a line source. The problem is two-dimensional and the solutions to both types of polarization (TE and TM) are presented. Numerical results for the effects of various geometrical and electrical parameters on the bistatic radar cross section are presented.