Conventional contact interconnect technology as an alternative tocontact plug (W) technology for 0.85 μm CMOS EPROM IC devices

The conventional (plug-less) and tungsten (W) plug contact interconnect technologies were studied for the fabrication of 0.85 μm CMOS EPROM integrated circuit devices. 4 Mbit EPROM devices and appropriate test structures were fabricated using these two interconnect architectures and were evaluated for process simplicity, associated problems/solutions, contact electrical characteristics, and circuit yield and speed. The most important process issue for the conventional contact technology was the overlay accuracy of the stepper used for printing the contacts. It was found that a misalignment of <0.3 μm was essential if contacts were to be reflowed after the contact etch process in a way that: a) did not violate the geometrical design rules, and b) did not result in bulging of the contacts, or an increase in the contact profile angle which would degrade metal step coverage. Electrical characteristics of the contacts were studied through contact resistance, specific contact resistivity, contact failure rate, and junction leakage measurements for both contact interconnect architectures. The data presented indicated that both processes produced contacts with similar characteristics. Finally, the results of this work indicated that the conventional contact interconnect technology could be reliably used for fabrication of 0.85 μm CMOS EPROM devices. This process was simpler, less expensive, and as structurally reliable as the W contact plug technology     

On the failure mechanisms of titanium nitride/titanium silicidebarrier contacts under high current stress

Structures with a single pair of titanium nitride/titanium silicide barrier contacts with connecting narrow diffusion widths (⩽3.0 μm) have been investigated for their electromigration resistance and reliability performance by applying current ramping and high current stress at different temperatures. Under a current ramping, the contact structures show a decrease in contact resistance after being virtually constant in the low-current regime and a steady increase of contact resistance toward the higher-current regime due to Joule heating in the diffusion region. The transition occurs when the temperature in the diffusion region reaches a value (the critical point) above which the intrinsic carriers become a dominant for conduction, especially for the region near the p-n junction. This point is followed by an unstable region in which the contact resistance continues to reduce. As the current rises further, a catastrophic structural failure, similar to a thermally initiated second breakdown in a transistor, is observed. The magnitude of the critical current is linearly proportional to the diffusion width and is independent of the type of impurity used to dope the diffusion region     

High current pressure contacts to Ag pads on thin filmsuperconductors

High current, low resistance, nonmagnetic, and nondestructive pressure contacts to Ag pads on YBa₂Cu₃O_7-δ (YBCO) thin film superconductors were developed in this study. The contact resistance reported here includes the resistance of the current lead/Ag pad interface, the Ag pad/YBCO interface, and the bulk resistance of the contact material. This total contact resistance is the relevant parameter which determines power dissipation during critical-current measurements. It was found that regardless of the optimization of the Ag pad/YBCO interface through annealing, a pressure contact can yield a lower total resistance than a soldered contact. The lowest resistance obtained with pressure contacts was 3 μΩ (for a 2×4 mm ² contact). These contacts may be useful for many different high temperature superconductor (HTS) studies where high-current contacts with low heating are needed     

Experimental verification of a novel electrical test structure formeasuring contact size

An electrical test structure that measures contact size without reliance on contact resistivity measurements is presented. A comparison with SEM photographs shows that the structure measures contact size in the range of 1.0 μm with a resolution close to 0.2 μm. Results are shown for measuring the size of contacts to poly, but the concept should also apply to measuring contacts to active areas, or contacts between two metal layers. In addition to the size of contacts to poly, the structure presented also measures the linewidth and sheet resistance of the poly, and misalignment of contacts to poly. It consists of a digital vernier of 72 samples and a linewidth structure and is implemented without active circuitry in a 2 by 12 pad array using three masking levels     

Ohmic contacts to p-CuInSe2 crystals

We report here on the optimization of ohmic contacts to p-CuInSe₂ (CISe) single crystals. A low resistance ohmic contact is required to minimize current losses due to series resistance; e.g. in Schottky diodes. Both In-Ga (eutectic)/CISe and gold (evaporatedVCISe contacts have been fabricated on crystals with different orientations and bulk properties. Gold contacts were found to have a lower resistance and to be more stable than In-Ga ones, from the slope of the linear current-voltage plot of the junctions. The resistance of the Au/CISe ohmic contact was decreased by etching the CISe crystal surface chemically in a 0.5% solution of Br₂ in methanol for 30 sec at room temperature, prior to gold deposition, while that of the In-Ga contact increased by this etch. Wetting experiments and contact angle measurements showed evidence for changes in the polarity of the surface due to chemical etches.     

0.12-μm gate III-V nitride HFET's with high contact resistances

HFET's with 0.12-μm gate length were fabricated on a III-V nitride wafer. The contact resistance from unannealed Ti/Au ohmic contact was 10 Ω·mm. Even with this relatively high contact resistance, f_T of 46.9 GHz and f_max of 103 GHz were measured with the Ti/Au contacts, the highest yet achieved on III-V nitride FETs. The improvement in the frequency response was mainly due to the decrease in the gate length (0.12 μm). In addition, the effects of high contact resistances at high frequency are discussed     

Reliability problems of polysilicon/Al contacts due tograin-boundary enhanced thermomigration effects

This paper presents results of a reliability study of n+polysilicon/Al contacts. The contact resistance of this structure ranged dramatically from sample to sample, and in some cases the contact resistance was extremely large (e.g. 80 kΩ.μm²). In addition, important changes in contact resistance were caused by temperature stress. This variation in contact resistance poses a serious problem in the manufacturability of accurate polysilicon resistors. This paper briefly describes the measurement procedures and measurement data. The measurements used to deduce and analyze the reliability problem include differential resistance and thermal stress. The samples were obtained from three industrial 2 μm CMOS sources. Finally, the paper discusses the data in detail and gives a reason for this reliability problem     

Improving charge injection in organic thin-film transistors with thiol-based self-assembled monolayers

The aim of this work is to improve charge injection by interposing an appropriately oriented dipole layer between contact and semiconductor in organic thin-film transistors (OTFTs). OTFTs are fabricated with pentacene semiconductor and gold source and drain contacts. The contacts are modified with self-assembled monolayers (SAMs) made of alkane or fluorinated alkane thiols. Ultraviolet photoelectron spectroscopy (UPS) shows a respective decrease and increase of the work function of the electrodes. Consistent with these results, we observe an increase and a decrease, respectively, of the contact resistance of the OTFTs, and a further decrease when shortening the length of the fluorinated molecule.     

Low Resistance Contacts to p-CulnSe2 and p-CdTe Crystals

Room temperature formation of ohmic contacts by electroplating gold on chemically treated surfaces of p-CuInSe₂ and p-CdTe single crystals is reported. The effect of Br₂/methanol and KOH+KCN+H₂O treatments prior to plating was analyzed in the case of CuInSe₂. It is shown that the former treatment yields better ohmic contacts, with lower contact resistance, than the latter. While annealing these contacts made them highly non-ohmic, the method gives reasonably ohmic contacts on surfaces, that were purposely oxidized prior to contact preparation. In the case of p-CdTe stable, low resistance ohmic contacts were obtained at room temperature by electrochemical diffusion of Hg from solution, prior to gold plating. The treatment forms a highly degenerated p^+- HgCdTe layer. The contacts, which have a very low contact to bulk resistivity ratio, were further improved by vacuum annealing.     

Study of contacts to CdZnTe radiation detectors

This study characterizes, for the first time, contacts to CdZnTe radiation detectors by measuring the dark noise spectra as a function of the applied bias. The noise currents are correlated with the dc dark current-voltage characteristics of CdZnTe x-ray and gamma-ray detectors. In order to identify and separate the role of the contacts in the overall performance, the measured noise phenomena is correlated with detector configuration and contact design as well as the growth method of the CdZnTe crystals, contact technology, and passivation. Several contact technologies (electroless gold, and a number of evaporated metallic contacts including gold, indium, zinc, titanium, aluminum, and platinum contacts) are compared. Contacts to CdZnTe crystals grown by high pressure Bridgman are compared with contacts to CdZnTe crystals grown by modified Bridgman. Contacts of resistive detectors as well as of Schottky detectors are reported. Large area symmetric contacts are compared with small area pixelized contacts. The role of the metallization used for contacts, the role of surface effects and passivation, and the role of contact design are discussed.     

Measurement and analysis of contact resistance in wafer probe testing

When testing IC chips using a wafer probe card, maintaining a low and stable contact resistance is essential. However, the electrical contact between the probe and the bonding pad of the IC chip becomes unstable following repeated probing operations since particles from the chip surface gradually accumulate on the probe tip. The contamination caused by these particles causes the contact resistance to increase. Accordingly, this study develops an experimental procedure for investigating the effect of the particle contamination on the magnitude and stability of the contact resistance. Initially, an experiment is performed to establish the contact resistance between a clean tungsten probe and various specimen surfaces, i.e. aluminum, gold and copper, at various levels of overdrive. Subsequently, an experiment is conducted to investigate the accumulation of surface particles on the probe tip following multiple contacts of the probe with the wafer surface. The extent of particle contamination following 10,000, 30,000 and 50,000 contacts, respectively, is examined using a scanning electron microscope (SEM). The contact resistance of the contaminated probes is then measured at various levels of overdrive. The experimental results are then integrated to establish a suitable tradeoff between the contact resistance, the overdrive displacement, and the number of contacts.The results from the contact resistance experiment conducted using a clean tungsten probe indicate that the surface specimens with a lower resistively generate a lower contact resistance. For example, the contact resistance between the tungsten probe and the copper foil is approximately 100 mΩ, and becomes stable at an overdrive of 45 μm. However, the contact resistance increases with an increasing number of contacts. In general, the probe should be removed for cleaning following 30,000 contacts to ensure that a contact resistance of less than 1 Ω is maintained.     

Low emitter resistance GaAs based HBT's without InGaAs caps

Low emitter resistance is demonstrated for AlGaAs/GaAs heterojunction bipolar transistors using Pd/Ge contacts on a GaAs contact layer. The contact resistivity to 2-10×10¹⁸ cm ^-3 n-type GaAs is 4-1×10^-7 Ω-cm² . These are comparable to contact resistivities obtained with non-alloyed contacts on InGaAs layers. The non-spiking Pd/Ge contact demonstrates thermal stability and area independent resistivity suitable for scaled devices. The substitution of Pd/Ge for AuGe/Ni GaAs emitter and collector contacts reduced by an order of magnitude the emitter-base offset voltage at high current densities and increased f_t by more than 15% with significantly improved uniformity for devices with 2 and 2.6 μm wide emitters having lengths two, four and six times the width     

Pressure Contact Micro-Springs in Small Pitch Flip-Chip Packages

This work investigates electrical pressure contacts based on a micro-spring with orders of magnitude smaller pitch and force than conventional pressure contacts. The springs are beams which curl out of the surface and can be used for wafer-scale testing and packaging. They are fabricated with standard wafer-scale thin film techniques and have been previously demonstrated on active silicon integrated circuits. Single springs and their electrical contacts are characterized with force versus compression and compression versus resistance measurements. Flip-chip packages with hundreds of micro-springs were assembled with 20-mum pad pitch and 40-mum spring pitch. Each spring operates with a force of approximately 0.01 g and contacts a gold pad. These packages are shown to have stable resistance values during both in-situ thermocycle (0degC to 125degC) and humidity testing (60degC at 95%RH). Spring electrical contacts inside the package are shown not to degrade during environmental testing through measurements of four-wire resistance and electrical isolation structures. High-speed glitch measurements are performed to confirm that the pressure contact does not have intermittent opens during thermocycling. These results suggest that a low-force solder-free pressure spring contact is a viable technology for next generation flip-chip packaging     

High-performance submicrometer gatelength GaInAs/InP compositechannel HEMT's with regrown ohmic contacts

This letter reports DC and RF performance of 0.25 μm gatelength GaInAs/InP composite channel HEMT's with nonalloyed, regrown ohmic contacts by MOCVD. Regrown channel contacts are used to achieve low contact resistance (0.35 Ω-mm) to (50 Å) GaInAs/(150 Å) InP composite channel HEMT's. High transconductance (600 mS/mm), high full channel current (650 mA/mm), and high peak cut-off frequencies (F_t=70 GHz, F_max=170 GHz) are observed. Contact transfer resistance of regrown channel contacts is compared to conventional alloyed contacts for varying GaInAs/InP channel composition     

Effect of the periphery of metal-semiconductor contacts with Schottky barriers on their static current-voltage characteristic

Kelvin probe atomic-force microscopy of the electrostatic surface potential of gold Schottky contacts on n-GaAs showed that there is an extended transition area (halo) (tens of micrometers) around contacts in which the surface potential varies from the n-GaAs free surface potential to the gold contact surface potential. The contact potential and its distribution in the surrounding halo are controlled by the contact structure. The study of spreading currents showed that there is a high-conductance area (periphery) around the contact perimeter due to strong electric fields of the halo, which causes leakage currents. The conductivity of the main contact area is caused by 100- to 200-nm local areas with higher and lower conducting abilities. Mesa formation around contacts causes a decrease in the work function, a decrease in the halo extent and electric field strength, which is accompanied by spreading and decreasing of the peripheral area conductance. This results in disappearance of leakage currents and a decrease in the ideality index. In contrast, protection of the peripheral area by a SiO₂ insulating film 0.5 μm thick increases the work function, which is accompanied by the formation of potential lobes around the contact in two mutually perpendicular crystallographic directions. A stronger penetration of halo electric fields into the contact area results in an increase in the ideality index and disappearance of high-conductance peripheral area and leakage currents. The difference between the electrical properties of the periphery, gold grains, and their boundaries controls the contact switching mechanism when applying forward or reverse biases.     

常用触点材料表面腐蚀物微动电特性研究

针对常用连接器触点材料(镀金、镀镍、镀锡),研究工业环境对其表面的腐蚀性及对触点微动电特性的影响。经过长期室内自然暴露后,镀金、镀镍、镀锡表面生成了离散的呈岛状分布的腐蚀产物。在腐蚀产物上进行了微动实验,发现在自然腐蚀产物的表面初始电阻高于通常的失效标准(10 mW),有的甚至达到1 W。当岛状腐蚀产物在微动过程中被逐渐磨掉后,接触电阻也由跳动渐渐降低至有效值。而当磨损碎屑堆积在微动痕迹附近,或接触表面之间嵌入尘土颗粒,或者在接触区内的腐蚀产物经微动后反而被挤压得更致密时,接触电阻会升高,甚至开路,造成电接触失效。腐蚀产物在微动中的去除与腐蚀物形貌及其机械特性直接相关。     

Recovery of severely degraded tin-lead plated connector contactsdue to fretting corrosion

The susceptibility of connectors with tin-lead plated contacts to fretting corrosion is well known. It would be advantageous to be able to recover such degraded tin-lead connectors to service. This possibility has been explored with bench apparatus using plated tin-lead and plated gold to tin-lead contacts mated in a crossed rod configuration. The crossed rods were fretted until the contact resistance severely degraded. Then the crossed rods were separated and various treatments were applied to the fretted surfaces. Included were a commercial contact cleaner and several different lubricants. After carefully returning the rods to their original position, several long movements were made to simulate the mating and unmating of a connector, and fretting was resumed. Preliminary results show that a contact cleaner may temporarily recover a degraded contact, but a lubricant is more effective. Although the avoidance of fretting degradation by optimal choice of materials and design is best, this work demonstrates that it may be possible to restore tin-lead connector systems to service     

The contact resistance force relationship of an intrinsically conducting polymer interface

Investigations on contact connector materials for different applications such as in the automotive industry have focused toward the increasing interest of using conducting polymers, as compared to conventional metallic contacts. The aim is to achieve overall improvements in performance as well as cost effectiveness. Currently, extrinsic conducting polymers (ECPs) are employed as conductive coats or adhesives at contact interfaces. However, frictional abrasion within the metal doped polymer (ECP) causes fretting corrosion, which leads to instability in the contact resistance. To overcome this, intrinsically conducting polymers (ICPs) are explored. Hemispherical contact coatings were fabricated using poly(3,4-ethylenedioxythiopene) (PEDOT) or polyaniline/polyvinylchloride (PANI/PVC) commodity blends. Contact resistances were taken using four-wire resistance measurement techniques. The conductivities of in-house fabricated ICP contacts were found to be in the range of 10/sup -2/ S/spl middot/cm/sup -1/. The response relating the change of contact resistance under varying compression force appeared to be repeatable with minimum deviation of 2%. The surface profiles of the ICP contacts were also recorded by an optical confocal system. The initial investigation results presented in this paper were used to evaluate and validate the hypothesis of employing ICP contacts to eliminate or minimize wearing and fretting effects.     

Integration of screen-printing and rapid thermal processingtechnologies for silicon solar cell fabrication

For the first time, the potentially cost-effective technologies of rapid thermal processing (RTP) and screen-printing (SP) have been combined into a single process sequence to achieve solar cell efficiencies as high as 14.7% on 0.2 Ω-cm FZ and 14.8% on 3 Ω-cm Cz silicon. These results were achieved without application of a nonhomogeneous (selective) emitter, texturing, or oxide passivation. By tailoring the RTP thermal cycles for emitter diffusion and firing of the screen-printed silver contacts, fill factor values >0.79 were realized on emitters with a sheet resistance (ρ_s ) of ~20 Ω/□ and grid shading <6%. Such high fill factors clearly demonstrate that screen-printed contacts can be fired on extremely shallow RTP emitters (x_j=0.25-0.3 μm) without shunting cells. IQE analysis depicts a strong preference for shallow emitter junction depths to achieve optimal short wavelength response of these unpassivated emitters. In some cases, front contacts were printed through plasma enhanced chemical vapor deposited (PECVD) SiN/SiO₂ dielectrics which prevented the shunting of shallow emitters by serving as partial barriers minimizing the diffusion of metallic species from the contacts. The firing of screen-printed contacts through these PECVD films, achieved the multiple purposes of contact formation, efficient front surface passivation due to annealing of the SiN, and high quality antireflection (AR). Research is presently underway to further optimize the RTP emitter design for screen-printing and develop techniques for implementing selective emitter and oxide passivation technologies for higher efficiency cells     

Contact noise in n-type Hg1-xCdxTe photoconductors at 77 K

When gold wires are bonded to indium contacts on n-type Hg1-xCdxTe photoconductors with silver paste, the contacts are very noisy when carrying current. In all cases, the noisy contacts inject electrons into the sample when the contact acts as a cathode; in the poorest contacts, the noisy contact also injects holes when the polarity is reversed. The problem can be avoided by soldering the gold wires to the Hg1-xCdxTe samples with In solder or by bonding the gold wire to evaporated In contacts with the help of silver epoxy.