Contact resistance of polysilicon silicon interconnections

Electrical contacts to poly are an important part of current silicon technology. In this letter we present a method of calculating the electrical characteristics for a planar poly to silicon contact. An interesting and significant result that is derived and discussed is a minimum contact resistance that is a strong function not only of the specific contact resistance of the contact interface but also of the contact geometry.     

Electromigration and low-frequency resistance fluctuations in aluminum thin-film interconnections

Low-frequency noise spectra originating from resistance fluctuations in Al films during electromigration were measured in the absolute temperature and current density intervals327 leq T leq 396K and1.34 times 10^{6} leq j leq 2.22 times 10^{6}A/cm². The values of SR, the resistance power spectral density, at 20 × 10^-3Hz allowed the construction of an Arrhenius plot from which a grain-boundary activation energy value of about 0.6 eV was deduced. This value lies in the range of values found by other authors using different techniques. A first attempt to model the observed dependence of SRonjandTis also described.     

Contact resistance of LPCVD W/Al and PtSi/W/Al metallization

The sensitivity of measured specific contact resistivity to surface doping concentration has been investigated for selectively deposited LPCVD W contacts to n+ and p +Si with surface concentrations from 10¹⁸to 10²⁰cm^-3. W contact resistance to n+ Si is about a factor of 20 lower than that of Al; W contact resistance to p +Si is comparable to that of Al. Ultralow resistance, stable contacts with self-aligned PtSi, and W to p +Si are demonstrated.     

High performance a-Si:H thin film transistors based on aluminum gate metallization

We present a systematic study of the sputter deposition conditions for aluminum thin films employed as gate metallization for high performance a-Si:H thin film transistors (TFTs). Here, we vary sputtering parameters such as deposition temperature, process pressure, and power, all of which have a strong bearing on the surface roughness of the film, including hillock generation induced by thermal processing. For example, at a low deposition temperature (30°C) and a low process pressure (5 mTorr), the surface roughness appeared to be significantly reduced. Transistors with gate metallization deposited under these conditions show a low leakage current (10 fA), an ON/OFF ratio better than 10⁸, and a mobility of 1.1 cm²/V s. In contrast, films deposited at 150°C and 10 mTorr, yield a degradation in mobility to 0.77 cm²/V s and an increase in leakage current to 1 pA, caused by the high interface roughness of the TFT channel due to hillock formation on the Al gate.     

Simulation of elevated temperature aluminum metallization usingSIMBAD

A ballistic deposition model, SIMBAD, has been extended to simulate physical vapor deposition onto substrates at elevated temperatures. The model has been expanded to account for the effect of film curvature on surface diffusion. The effects on via coverage and filling have been simulated for aluminum films, and complete planarization of a 1:1 aspect ratio via is predicted for a temperature of 550°C. Via aspect ratio and sidewall taper can also strongly affect coverage and filling. Biased sputtering has also been incorporated into the model and shows that a primary effect is a substantial reduction in the temperature required to achieve full planarization. However, void formation and substrate damage are problems predicted to occur under some bias sputter conditions     

The effects of dielectric overcoating on electromigration in aluminum interconnections

The phenomena of electromigration is self-diffusion effect, and has an associated activation energy. Although the activation energy for bulk self-diffusion in aluminum is 1.48 eV, experimentally measured energies for films range approximately from 0.5 to 1.2 eV. This lower observed energy may be or has been ascribed by investigators to crystalline imperfections in and on the surface of the aluminum film. Of the three contributions to the effective activation energy, i.e., the surface, grain boundary, and bulk component, the surface component is here investigated. In light of the requirements for LSI and multilevel integrated circuitry, i.e., narrower, thinner metallizafion implying a larger surface to bulk atom ratio, correlation is made between surface treatment and effective activation energy of the conductor. Surface treatment is accomplished through vapor plating of dielectric overcoatings of P2O5-SiO2and Al2O3-SiO2glass. A study is made of the effects of dielectric overcoating as a function of aluminum thickness. It is seen that surface treatment is effective for ahtminum less than 5000 Å thick. It is further pointed out that for aluminum thickness greater than 6000 Å, surface passivation will have no effect with respect to electromigration. It has been shown that the type of aluminum deposition technique and deposition and post-deposition heat treatment is critical in obtaining the maximum bulk component of the activation energy. The dielectric overcoating, in combination with optimal bulk properties, increases the mean time between failure of the aluminum stripes by one to two orders of magnitude at high current densities and elevated ambients, than that found in many conventional integrated circuits.     

The size effect of liftoff metallization of sputtered aluminum films

Liftoff metallization of sputtered aluminum films successfully produces small patterns with tapered sidewalls. In this metallization, a size effect occurs wherein pattern height is influenced by pattern width. This article reports experiments regarding size effect dependences on liftoff parameters of pattern width, film thickness, photoresist thickness, and sputtering argon pressure. The size effect is favorably suppressed by reducting photoresist thickness and sputtering argon pressure. In addition, the effect is discussed by a model taking into account shadowing of sputtered atoms due to photoresist patterns and aluminum film itself deposited on the photoresist patterns.     

Healing of voids in the aluminum metallization of integratedcircuit chips

A thermal treatment for healing voids in the aluminum metallization of integrated circuit (IC) chips has been discovered. The aluminum metallization is alloyed with nominally 1 wt.% of silicon. This discovery arose from efforts to cause further growth of preexisting voids in IC RAMs intended for long-term unattended spacecraft applications. The experimental effort was intended to cause further void propagation for the purpose of establishing a time/temperature propagation relationship, but it resulted instead in a healing of the voids. The thermal treatment consisted of heating IC chips with voids in the aluminum/silicon metallization to temperatures in excess of 200°C, followed by quick immersion into liquid nitrogen. The thermal treatment is described, and a theory based on silicon solubility and migration in aluminum is advanced to explain both the formation and the healing of voids in the aluminum metallization of IC chips     

Effect of hydrogen-argon mixing for sputtered aluminum metallization on MOS devices

Effects of aluminum films deposited by RF diode sputtering in hydrogen and argon mixed gases have been investigated in n-channel silicon-gate MOS transistors and capacitors. Positive fixed oxide charge Qoxand acceptor-type surface states Nsscreated by the sputter metallization process have been confirmed experimentally. The threshold voltage Vthof a transistor metallized with aluminum sputtered in pure argon was found to be shifted toward positive voltages and its transconductance was markedly lowered due to acceptor-type surface states. Hydrogen mixing in the sputtering argon gas minimizes Qoxand Nss, resulting in small Vthand gmchanges.     

Electromigration performance of electroless platedcopper/Pd-silicide metallization

The electromigration reliability of Cu interconnects has been studied under DC, pulse-DC, and bipolar current stressing conditions. Electroless plating was used to selectively deposit Cu in oxide trenches by using Pd silicide as a catalytic layer at the bottom of the trenches to initiate copper deposition. The DC and pulse-DC lifetimes of Cu are found to be about two orders of magnitude longer than that of Al-2%Si at 275°C, and about four orders of magnitude longer than that of Al-2%Si when extrapolated to room temperature. On the other hand, Cu AC lifetimes are found to be comparable to the AC lifetimes of Al-2%Si. The pulse-DC lifetime of copper interconnects follows the similar frequency and duty factor dependence as aluminium and the prediction of the vacancy relaxation model     

The contact resistance and reliability of anisotropicallyconductive film (ACF)

The effect of bonding pressure on the electrical and mechanical properties of anisotropic conductive film (ACF) joint using nickel particles and metal-coated polymer ball-filled ACFs was investigated. The contact resistance decreases as the bonding pressure increases. Contact resistance of ACF is determined by the contact area change between particles and contact substrates. Electrical conduction through the pressure engaged contact area between conductive particles and conductor substrates is the main conduction mechanism in ACF interconnection. In addition, environmental effects on contact resistance and adhesion strength such as thermal aging, high temperature/humidity aging and temperature cycling were also investigated. Interestingly, the contact resistances of the excessively bonded samples deteriorated more than those of optimally bonded ones. Increasing contact resistance and decreasing adhesion strength after harsh environmental tests were mainly due to the loss of contact by thermal stress effect and moisture absorption, and also partially due to the formation of metal oxide on the conductive particles     

A thermomechanical study of the electrical resistance of Cu lead interconnections

The choice of liquid crystal display (LCD) driver packaging technology significantly influences the display performance of flat panel displays. Tape automated bonding (TAB) is generally the method of choice for connecting the LCD and the LCD driver circuit in flat panel displays. To achieve a finer pitch, an easier assembly, and a greater connection reliability, the design of the inner Cu lead must not only consider thermomechanical failure aspects, but must also maintain an acceptable joint resistance. This paper proposes an analytical model to predict the unit change in resistance of the copper foils used for TAB inner lead interconnections under various thermal environments and stressstrain states. The analytical model is based on a constitutive equation of the copper foil and the working principle of strain gages. Copper foil specimens are tensile tested at temperatures of 25°C, 50°C, 75°C, and 100°C at strain rates of 0.2/min. and 0.5/min., respectively, to confirm the validity of the developed analytical model. The numerical results and the experimental data are found to be in good agreement. Hence, the analytical method provides the means of predicting the thermal effect on the electrical and mechanical properties of the copper foils. Finally, by implementing finite-element method (FEM) solutions in the developed analytical model, this study constructs electrical resistance design charts to predict the variation in the electrical resistance of the copper foils under different thermal-mechanical conditions.     

Electromigration in aluminum/silicon/copper metallization due to the presence of a thin oxide layer

The effect of a thin layer of SiO₂ (50 nm) on the electromigration behavior of Al/ 0.8wt.%Si/0.5wt.%Cu metallization, passivated by spin-on-glass, phosphorus silicate glass and silicon nitride as part of the complementary metal oxide semiconductor technology fabrication process was studied. It is found that voids were formed along the edge of the metallization line as opposed to formation at triple point of grain boundaries. At the same stress current of 1 × 10⁶ A/cm², thicker metallization layer (600 nm) showed an improvement in median time to failure (MTF) (1.4 times) with smaller void size (0.2 to 0.4 μm) over one without an underlying oxide, whereas if the metallization thickness is thin (300 nm), the MTF is degraded (0.6 times) with larger void size formed (0.3 to 1.0 μm).     

Enhancement of electrical stability of anisotropic conductive film (ACF) interconnections with viscosity-controlled and high Tg ACFs in fine-pitch chip-on-glass applications

This paper describes how anisotropic conductive film (ACF) properties including viscosity affect the electrical stability of ACF interconnections for fine pitch chip-on-glass (COG) applications. In this study, new ACFs for COG applications were designed by combining a high viscosity ACF layer and a low viscosity NCF layer to prevent the electrical shortage between bumps. As expected, the viscosity-controlled ACF showed better electrical insulation stability than a conventional ACF in fine pitch COG assemblies. According to the results of thermo-mechanical analysis (TMA) and dynamic-mechanical analysis (DMA), the viscosity-controlled ACF showed the improved thermo-mechanical properties such as lower coefficient of thermal expansion (CTE), higher storage modulus (E′) at higher temperature region, and higher glass transition temperature (T_g) than the conventional ACF. Furthermore, hot air reliability test and pressure cooker test (PCT) results showed that the viscosity-controlled ACF with higher T_g had better hot air test and PCT reliabilities than the conventional ACF.     

Testing the electromigration resistance of Al metallization patterns by electrical-resistance measurement

The results are presented of an experimental evaluation of electromigration resistance for two types of Al metallization pattern used in the ICs of the 1554TBM and 1594T series. The experimental procedure is based on electrical-resistance measurements. Gamma-percent lifetime is calculated.     

Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection

The effects of different bonding parameters, such as temperature, pressure, curing time, bonding temperature ramp and post-processing, on the electrical performance and the adhesive strengths of anisotropic conductive film (ACF) interconnection are investigated. The test results show that the contact resistances change slightly, but the adhesive strengths increase with the bonding temperature increased. The curing time has great influence on the adhesive strength of ACF joints. The contact resistance and adhesive strength both are improved with the bonding pressure increased, but the adhesive strengths decrease if the bonding pressure is over 0.25 MPa. The optimum temperature, pressure, and curing time ranges for ACF bonding are concluded to be at 180–200 °C, 0.15–0.2 MPa, and 18–25 s, respectively. The effects of different Teflon thickness and post-processing on the contact resistance and adhesive strength of anisotropic conductive film (ACF) joints are studied. It is shown that the contact resistance and the adhesive strength both become deteriorated with the Teflon thickness increased. The tests of different post-processing conditions show that the specimens kept in 120 °C chamber for 30 min present the best performance of the ACF joints. The thermal cycling (−40 to 125 °C) and the high temperature/humidity (85 °C, 85% RH) aging test are conducted to evaluate the reliability of the specimens with different bonding parameters. It is shown that the high temperature/humidity is the worst condition to the ACF interconnection.     

Diffusion barrier performance of W/Ta-W-N double layers for Cu metallization

In this work, the properties of Cu/W/Ta-W-N/Si film stacks were studied. Adding a thin W layer to a stable Ta-W-N diffusion barrier significantly affected the whole metallization system. The introduction of a thin W interlayer caused a significant change of the system while increasing the stability of the film. The tandem barrier was demonstrated to be stable up to 800 °C by the performed analytical barrier tests.     

Neutron irradiation-induced effects on the reliability performance of electrochemical metallization memory devices

In this work,electrochemical metallization memory(ECM)devices with an Ag/AgInSbTe(AIST)/amorphous carbon(a-C)/Pt structure were irradiated with 14 MeV neutrons.The switching reliability performance before and after neutron irradiation was compared and analyzed in detail.The results show that the irradiated memory cells functioned properly,and the initial resistance,the resistance at the low-resistance state(LRS),the RESET voltage and the data retention performance showed little degradation even when the total neutron fluence was as high as 2.5×1011 n/cm2.Other switching characteristics such as the forming voltage,the resistance at the high-resistance state(HRS),and the SET voltage were also studied,all of which merely showed a slight parameter drift.Irradiation-induced Ag ions doping of the a-C layer is proposed to explain the damaging effects of neutron irradiation.The excellent hard characteristics of these Ag/AIST/a-C/Pt-based ECM devices suggest potential beneficial applications in the aerospace and nuclear industries.     

Contact resistance of thin metal film contacts

To be able to reduce the size of products having electronic devices, it becomes more and more important to miniaturize the electromechanical parts of the system. The use of micromechanical connectors and contact structures implies the need of methods for estimating the properties of such devices. This work will, by use of finite element modeling, treat the influence of a thin film constituting at least one of the contacting members of an electrical contact. The error introduced by using the traditional Maxwell/Holm contact constriction resistance theory will be investigated. Numerical methods are used to present a way to approximate the total resistance for the thin metal film contact.     

The performance of GaAs power MESFET’s using backside copper metallization

The performance of GaAs power MESFET’s using backside copper metallization has been evaluated. 10 nm Ta metal was used as the diffusion barrier between GaAs and Cu for copper film metallization in this study. Microstructural characterization shows that the Cu/Ta films with GaAs remained stable up to 400 °C, indicating that Ta is a good diffusion barrier for Cu in GaAs MESFET’s. A copper metallized 6 mm power MESFET was thermal stressed to test the device stability. After annealing at 200 °C for 3 h, the devices showed very little degradation in power performance, and the thermal resistance of the device was 65 °C mm/W with 1.4 W/mm DC input power. Results in this study demonstrate that the feasibility of using Cu/Ta films for the backside metallization of GaAs power devices with stable electrical and thermal characteristics.