Submicron Contact Holes Research Articles

Integration and electrical characterization of carbon nanotube via interconnects

Carbon nanotubes (CNTs) are considered a promising material for interconnects in the future generations of microchips because of their low electrical resistance and excellent mechanical stability. In particular, CNT-based contacts appear advantageous when compared with current tungsten or copper technologies and could therefore find an application as metal contacts interconnecting the transistors with the back end of line of the microchip. In this work, the integration of vertical CNT bundles in sub-micron contact holes is evaluated at wafer scale and the major integration challenges encountered in the practical realization of the process are discussed. Nickel PVD films were used to selectively grow CNT into the contact holes at temperatures as low as 400 °C, which is the thermal budget available for contacts. The height of the contacts and the length of the CNT are controlled by a chemical mechanical polishing step (CMP) after embedding the CNT into SiO 2. Ti/Au metal pads are then formed onto the CNT bundles by PVD and lift-off. The integrated CNT are electrically characterized and an annealing treatment was found to improve the CNT-via resistance. As the electrical properties of the CNT can be evaluated, the structure and the process presented constitute a test vehicle for the development of high-quality CNT-contacts.

Customized illumination aperture filter design for through-pitch focus latitude enhancement of deep submicron contact hole printing

A simple graphic analysis technique named the illumination chart method is introduced to aid the customization of the illumination aperture filter for synergistic combination with a high transmission rim-type attenuated phase-shifting mask (PSM) for deep submicron contact hole printing. This graphic method gives direct visualization of the relationship between the interference condition in the pupil and the incident angle of illumination. The working ranges of oblique illuminations with different numbers of diffraction beams taking part in imaging can be easily clarified by this graphic method, which explains the dependence of depth of focus (DOF) on pattern duty. A customized illumination aperture filter (CIF) is synthesized by collecting the effective source elements for every pattern pitch to remedy the inability of the attenuated PSM for dense patterns. To preserve the merits of off-axis illumination to dense patterns and on-axis illuminations to sparse patterns in a single exposure, the illumination chart suggests a zeroth-order-reduction mask design for dense hole pattern. We applied this integrated resolution enhancement technique to 0.17 μm contact hole printing in 248 nm wavelength, 0.55 numerical aperture optics. The experimental results show our CIF illumination not only balances the DOF enhancement throughout the pattern pitches but also suppresses the best focus shift due to spherical aberration.

Atomic-Layer Chemical-Vapor-Deposition of TiN Thin Films on Si(100) and Si(111)

The microelectronic industry has shown an increasing interest in TiN as a diffusion barrier for aluminum and copper metallization in ultra-large-scale integration (ULSI) devices because of its low electrical resistivity, high melting point, thermal stability, and good adhesion properties [1,2]. It is also used as a diffusion barrier for capacitor electrodes for high-k dielectric materials, such as Ta2O5 or BST [3], and is used as both a barrier against WF6 attack and a nucleation layer in the chemical vapor deposition (CVD) W plug filling process [4]. Both physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques have been applied to grow TiN film. Conventionally, TiN films have been deposited either by the reactive ion sputtering or by thermal nitridation of sputtered titanium. As integrated feature sizes have shrunk to submicron dimensions, the limitations of films produced by these physical techniques have become apparent. The poor step coverage of sputtered films, especially in submicron contact holes with

Correlation between agglomeration of a thin film and reflow filling in a contact hole for sputtered Al films

Al reflow filling in contact holes by annealing after Al deposition at room temperature was investigated with relation to agglomeration properties of very thin films. A strong correlation between filling achievement and agglomeration property was observed when the underlayer TiN film property was changed. When rapid thermal nitridation (RTN) was carried out after reactive sputtering of TiN, excellent filling in submicron contact holes was achieved at annealing temperatures around 400 °C for the case of thick Al film, and agglomeration into small islands was observed for thin Al films. The activation energy of atomic mass transport for the filling process was 0.36 eV, which suggests a surface diffusion dominated filling process. On the other hand, when no RTN was carried out for TiN, reflow filling was achieved at temperatures around 600 °C which were close to the melting point of Al, and agglomeration into islands in thin films was suppressed at 400 °C. The activation energy of the latter case was 0.90 eV, which was between those of lattice diffusion and grain boundary diffusion. The correlation between filling achievement and agglomeration is ascribed to a large surface diffusion atomic flux in the case with RTN.

Application of Selective Liquid‐Phase Deposition to Fabricate Contact Holes Without Plasma Damage

This work develops an alternative method, selective liquid‐phase deposition (S-LPD), to fabricate contact holes instead of reactive ion etching. In preliminary experiments, deep junction diodes with contact holes prepared by S-LPD exhibit much less reverse current, unity ideality factor, larger forward current, lower contact resistance, and higher thermal stability than those prepared by reactive ion etching. Further superiority of plasma damage‐free near‐surface regions is also investigated using Schottky and ultra‐shallow junction diodes. Experimental results indicate that S-LPD can be applied to the submicron contact‐hole process. The data after reverse bias temperature stress reveals the satisfactory reliability of S-LPD contact holes. This work demonstrates that the S-LPD technology is a highly promising means of replacing reactive ion etching processes to form submicron contact holes as reliably as those by wet‐etching. © 1999 The Electrochemical Society. All rights reserved.

Aspect ratio effects in submicron contact hole plasma etching investigated by quantitative x-ray photoelectron spectroscopy

The submicron contact hole etching process using CHF3–CF4–Ar plasma was performed in a commercial parallel plate reactive ion etching reactor (LAM 4520). Response surface methodology was employed to correlate input parameters with etching results. For the aspect ratio investigations, a special test pattern consisting of line/space and contact/space arrays of various dimensions (1 and 0.6 μm) was used. Aspect ratio dependent etching effects were observed. The bottom surface chemistry of SiO2/Si patterned structures after plasma etching have been investigated by using quantitative x-ray photoelectron spectroscopy (XPS). After the SiO2 etching treatments, the silicon surface of the contact (or trench) bottom presents modifications similar to that observed on unpatterned SiO2/Si samples. These modifications are described by a two-layer model involving a fluorocarbon overlayer and an interfacial oxide layer. The fluorocarbon layer thickness strongly increases when the surface area of silicon substrate exposed to the plasma decreases whereas, as unexpected, the fluorine density of the fluorocarbon film decreases. A refluorination mechanism depending on the diffusion of the fluorine atoms from the plasma through the fluorocarbon overlayer is proposed. This work points out the interest of XPS surface analysis in improving the contact hole etching process.

Cu Deposition Characteristics into Submicron Contact Holes Employing Self-Sputtering With a High Ionization Rate

AbstractCu deposition profiles in submicron contact holes are investigated employing the Cu self-sputtering which do not need any inert gas e.g. Ar during sputtering. Excellent bottom coverage in the high aspect ratio contact holes was obtained at a large target-substrate distance in the selfsputtering due to a long mean free path of Cu ions and atoms, although a coverage is poor in Ar sputtering at 6 mTorr. It is also shown that the self-sputtering has low resputtering effect and high self-diffusivity of Cu, while the resputtering predominates in the case of Ar sputtering when DC bias is applied on the substrate. As a consequence, the bottom coverages of the self-sputtered films are much improved than the Ar sputtered ones. The present work strongly suggests that the self-sputtering is promising to fill Cu in sub micron via and contact holes.

Anomalous Selective Tungsten- Growth by -Chemical- Vapor -Deposition

AbstractSelective tungsten chemical vapor deposition (W-CVD) is one of the most attractive technique for filling deep submicron contact hole and via hole for the ULSI application. In this work, we firstly find out the anomalous selective W growth for the contact hole. The tungsten only nucleates from the side ring of the contact hole bottom. Several predeposition treatments prior to W growth can improve this anomalous feature and excellently selective W growth can be achieved. Auger electron spectroscopy (AES) is utilized to investigate the interface of Si/W. A model has been proposed to explain this anomalous selective tungsten growth.

Al-Reflow Process with a “Cap-Clamp” for Sub-Micron Contact Holes

The use of “cap-clamp” has been demonstrated for extending the process margin of Al-reflow process. To evaluate the process window of Al-reflow with and without the “cap-clamp”, two major factors of in-situ annealing temperature and time were varied between 500-560° C and 40-90 seconds, respectively. Sub-micron contact holes were completely filled by heating wafers at 500° C or higher temperatures by adopting the “cap-clamp” to the conventional conduction heating system. The effect of the “cap-clamp” comes from the improved heating efficiency of the wafer by introducing hot Ar gas into the inner space of “cap-clamp”, resulting in faster ramp-up as well as more effective heating of the front-side of the wafer. Al-reflow with the “cap-clamp” gives wider process window than that of the conventional heating method and realizes the contact planarization of 256 Mbit dynamic random access memory (DRAM) and beyond.

Direct analysis of contamination in submicron contact holes by thermal desorption spectroscopy

Thermal desorption spectroscopy (TDS) has been applied to detect contamination, produced in deep-submicron contact holes during plasma etching. AlF and other Al-containing fragments were observed as constituents of the contamination. The contamination is found to be accumulated in the holes, depending on an overetch time, acting as an inhibitor against achieving sufficiently low contact resistance. The effects of cleaning by Ar+ ion sputtering after contact-hole fabrication are also investigated. The sputtering improves the contact resistance significantly. The sharp TDS peaks of AlF and AlC clearly decreased in good relation to the improvement in the contact resistance. The TDS method has proven to be an effective tool in evaluating contamination in the submicron contact holes.

Improvement in Step Coverage at Submicron Contact Holes by Switching Bias Sputtering

W film deposition by switching bias sputtering, which features alternating operation of standard and bias sputtering, was investigated to develop a process which better fits the requirements of ultra very large scale integration circuits applications. Step coverage is improved above 50% for the holes with 0.3 μm diameter and 1 μm depth by adopting most suitable conditions in this sputtering. The bias time ratio in this method is found to be a dominant factor to raise the step coverage. The quality of W films deposited by this method is the same level as that of deposited by conventional DC sputtering. It is found that the method does not produce damages during deposition but gives low contact resistances to n + and p + type Si. The W metallization using this method is considered to be quite promising for submicron LSIs.

A Process Model for Thin Film Deposition by Sputtering: Study for Bottom Coverage of Sub-Micron Contact Holes

ABSTRACTTitanium (Ti) is widely used to improve the contact resistance with the underlying substrate in contact or via filling applications. As the critical dimensions of the electronic devices fall below 0.5 micron and the aspect ratio of the contact holes increases, it becomes more difficult to obtain a good coverage at the bottom of the contact. In our continuing effort to develop a better technology for the metallization of sub-micron liners [1, 2] using the sputtering process, we have constructed a feature scale model and have initiated an extensive study on the coverage of the contact liners. Our objective is to search for an alternative process technique to replace the current, widely-used collimated sputtering with the help of a realistic model that accounts for all important deposition phenomena such as the effect of the working pressure and the ion bombardment. Since the mean free path of the deposited atoms, particularly in a low pressure system, is comparable to the surface feature of the substrate, particle methods are more appropriate for the modeling of thin film formation phenomena and predictions of microstructures than the continuum approach. This is specially true in the case of sputter-deposition. In this paper, a simple two dimensional molecular dynamics model is used to predict the growth of the thin film. It allows us to study the effects of the physical parameters such as working pressure and ion energy as well as the influence of the geometric configuration, e.g., distance between the target and the substrate and the size of the contact hole. Simulations for long-throw/low-pressure sputtering and ionized magnetron sputtering are presented together with the experimental results for collimated sputtering.

Auger depth profiles of TiN/Ti films in submicron contact holes: A comparison of collimated and uncollimated deposition processes

Auger depth profiles are used to compare sputtered Ti, reactively sputtered TiN, and TiN/Ti bilayer films in submicron (0.8 and 0.4 μm) contact/via holes, deposited with and without a collimator. Samples were cleaved and mounted to minimize difficulties associated with the sputter profiling of and data acquisition from recessed features. The Ti and N concentrations were calculated with a subtract-and-weight routine which uses dN(E)/d(E) peak-to-peak intensities. Transmission electron microscopy film thicknesses measurements were used to confirm that the sputter rates were comparable for the various sample mounting geometries. The thickness of films deposited on the contact base, relative to those on the surrounding oxide was greater when the collimator was used. For 0.8 μm contacts, the film thickness ratio obtained using the collimator was nearly twice that obtained with the uncollimated deposition (0.5 versus 0.25 for TiN; 0.8 versus 0.6 for Ti). The benefits of the collimator, for obtaining good coverage of the base of the hole, are more pronounced in smaller geometries. For films deposited with the collimated on 0.4 μm contacts the film thickness ratio was ten times that for films deposited without the collimator (0.4 versus 0.05 for TiN, 0.35 versus 0.03 for Ti). There was no evidence of variations in the TiN stoichiometry for films deposited in confined geometries versus the surrounding oxide for either deposition configuration. There was an increase in the depth of surface oxidation for the Ti-only film deposited on the sidewalls using the collimator. Shadowing effects cause an increase in film porosity when the direction of collimated metal flux is parallel to the sample surface. On exposure to atmosphere, this porosity allows oxidation below the surface of the film.

Rapid Thermal Chemical Vapor Deposition of Titanium Nitride for Barrier Application

ABSTRACTToday there are many investigations of titanium nitride (TiN) deposition as diffusion barriers in microelectronics, especially with Chemical Vapor Deposition (CVD) techniques. In our newly developed Rapid Thermal CVD (RTCVD) process, we combine the conventional LPCVD process of TiN using titanium (IV) chloride and ammonia with the advantages of a RTCVD reactor. With regard to the ability of fast temperature change especially to reach the anneal temperature and to cool down to room temperature in the annealing ambient, it is possible to perform the entire processing sequence within one single processing chamber. The influences of deposition temperature, as well as the effects of the temperature during a subsequent in situ anneal step on the properties of the layers is analyzed. TiN layers with a specific resistivity as low as 250 μΩ-cm even at deposition temperatures of 450 °C are obtained. The resistivity of the layers and the chlorine content is nearly half of the films without an anneal step. The capability of these layers for ULSI application is shown by depositing TiN in submicron contact holes with a step coverage of nearly 100 %.

Submicron contact hole filling by electron bias sputtering

A new electron bias sputtering method capable of filling Al into submicron contact holes without degradation of the quality has been developed for the formation of reliable interconnections of very large scale integrated circuits. The method features intermittent electron irradiation to enhance Al flow during sputtering. The Al film deposited contains little Ar and has low resistivity. Contact holes with 0.4-μm diameter and 0.9-μm depth can be filled with Al.

Vertical and lateral hole aluminum filling characteristics employing electron cyclotron resonance plasma sputtering with high magnetic field

Filling of submicrometer vertical and lateral holes was investigated by employing ECR (electron cyclotron resonance) aluminum sputtering with two times higher magnetic field than the usual condition of 875 G. Deep submicron contact holes with no adhesive metal layer were satisfactorily filled at 350 °C, which was a much lower temperature than conventional methods. Buried lateral trenches with 1-μm depth were also completely filled at 300 °C. The filling mechanism is considered to be a cooperative effect of an enhanced diffusion by an ion bombardment effect, and the formation of dense aluminum islands at the inner wall which act as an adhesive layer.

Process integration for barrier layers and Al-alloys using a sputtering cluster tool

This report describes an integrated Ti/TiN barrier and aluminum contact hole filling process that promises significant advantages in barrier layer quality and aluminum planarization. A process sequence in a cluster tool is developped that consists of a stress-free Ti/TiN barrier layer, a Ti interlayer of optimized thickness and an AlSiCu metallization that fills submicron contact holes and shows improved electromigration resistance.

Analysis for effects of mask defects to resist pattern using a three-dimensional photolithography simulator

A three‐dimensional (3D) photolithography simulator was developed which is composed of the two‐dimensional (2D) mask aerial image simulator, simulator for 2D intensity pattern in aligner (STIPAL), and the 3D resist image simulator, resist process 3D simulator (RESPROT). STIPAL can calculate the mask aerial image in projection printing considering lens aberrations and reticle defects which are very important factors influencing submicron pattern transfer, in addition to the parameters of lens NA (numerical aperture), source wavelength λ, partial coherency σ and defocusing. RESPROT can simulate the 3D resist image in a conventional resist process and contrast enhancement lithography (CEL) using the mask aerial image data calculated by STIPAL. Resist pattern printability or fidelity in the submicron process can be analyzed by these simulators. Resist linewidth shifts caused by reticle defects and lens aberrations were analyzed. Effects of a dark defect (a surplus pattern) or a clear defect (a lack of pattern) on the resist linewidth shifts are almost the same when defocusing and/or lens aberration do not occur. When defocusing and/or lens aberration occur, however the resist linewidth shift caused by a dark defect is larger than that caused by a clear defect. A high resolution photoresist and CEL can decrease the resist linewidth shift caused by mask defects. Further, CEL can decrease oval distortion of the submicron contact hole pattern caused by lens aberration.

Blanket Tungsten Film Evaluation for Application to Submicron Contact Filling

ABSTRACTThe ptanarization of metal interconnect Layers has recently been a subject of intensive development effort in semiconductor processing. Although layer thicknesses have not decreased In proportion, lateral dimensions have been reduced to the point that aspect ratios are too large for effective step coverage by conventional film deposition techniques. The most difficult structure to peanarize is submicron contact holes in thick dielectrics.A dual-Layer CVD blanket tungsten film was shown to be viable for use as an interconnect material in submicron processes. A single-wafer reactor operating at a pressure of 80 Torr was used to deposit the film. The film exhibits superior properties in terms of its basic characteristics. The film completely fits submicron contacts to silicon, yet has a low surface roughness that would minimize concomitant photolithography and etching problems. Film adhesion is excellent, uniformity and reproducibility are >95%, depgsition rate is greater than 400 nm/mmn., stress is below 8×109 dynes/cm2, and resistivity is 11 μ-ohm-cm or less.

Excimer laser planarization of AlSi, AlSiTi and AlSiCu alloys

In this work we use a XeCl excimer laser (λ = 308 nm) to planarize three Al alloys, namely AlSi(1%), and AlSi(1%)Ti(0.1%), and AlSi(1%)Cu(0.5%). AlSi(1%) is currently the dominant first level metalization used in production because of its low resistance and compatibility with conventional Si processes. The Ti and Cu are added to suppress hillocking and improve electromigration resistance. Our results show that excellent planarization is achieved for all topologies including submicron contact hole structures, however, for the AlSi alloy spot ablation occurs which is characterized by the removal of metal. This is not seen for the AlSiCu and the AlSiTi alloys. The best morphology is obtained for the AlSiTi alloy with large areas completely free of ablation and pitting, which indicates that excellent planarization over large device areas can be achieved using this alloy.