Bottom Of Contact Holes Research Articles

On-wafer monitoring of electron and ion energy distribution at the bottom of contact hole

In situ on-wafer monitoring of the electron and ion energies at the contact-hole bottom is primarily achieved in Ar ultrahigh-frequency plasma. The on-wafer probe reveals a lower electron density and higher electron temperature at the contact-hole bottom due to the electron-shading effect, as compared with that in the bulk plasma. The on-wafer probe also shows the ion energy distribution function (IEDF) at the contact-hole bottom. The peak energy of IEDF corresponded to the sheath potential. Accordingly, the authors found that the on-wafer probe is a very effective tool for investigating the electron and ion energies in real SiO2 contact structures.

Real-time monitoring of charge accumulation during pulse-time-modulated plasma

The authors investigated real-time monitoring of charge accumulation during pulse-time-modulated plasma processes by using their developed on-wafer monitoring chip. The charge accumulation potential between the top surface and the bottom in a SiO2 contact structure was measured during pulse-time-modulated plasma exposure with an on-wafer monitoring device. In conventional plasma with rf bias, the electron shading effect could be clearly observed as the potential difference between the wafer surface and the contact-hole bottom. Conversely, the accumulated charge in the pulse-time-modulated operation was drastically decreased. Time-resolved electron and ion flows to the SiO2 contact hole were clarified by the on-wafer monitoring. Accordingly, it was confirmed that the on-wafer monitoring is a very effective tool for investigating the local charge accumulation in actual device structures.

Removal of Fluoropolimers from the Surface of Silicon Structures by Treatment in an Atomic Hydrogen Flow

The possibility of successful removal of fluoropolimers from the surface of silicon structures by treatment in an atomic hydrogen flow is investigated. It is ascertained that the treatment of samples in a direct atomic hydrogen flow with a density of 2 × 1015 cm−2 s−1 at temperatures from 20 to 100°C leads to a decrease in the content of fluorocarbon residues (in particular, CF) by 5 orders of magnitude. Fluorocarbon residues are removed from both the planar surface of silicon structures and the lateral walls and the bottom of contact holes with a diameter of 0.3–0.25 μm and larger and a depth of 0.9 μm, opened in a SiO2 layer by reactive ion etching. A treatment time of 2 min is sufficient for complete removal of the fluoropolimers. This process of dry cleaning can be recommended for use in the fabrication of integrated circuits containing an interlayer of a low-permittivity insulator.

Mitigation of accumulated electric charge by deposited fluorocarbon film during SiO2 etching

SiO 2 contact-hole etching with a high-aspect ratio is a key process in fabricating ultra-large scale integrated devices. However, charge accumulation in contact holes during plasma etching causes serious problems, such as charge-build-up damage, etching-stop, and microloading effects. Therefore, understanding the mechanism behind this electric charge accumulation and controlling the plasma etching processes would be very important to achieve the next-generation semiconductor devices. We found, through our previous research, that deposited fluorocarbon film in contact holes had high electric conductivity because of ion bombardment. In this experiment, we investigated the build up of charging potential during plasma processes by in situ on-wafer monitoring to control charge accumulation in the contact holes. We developed an on-wafer monitoring device to measure the charging potential in SiO2 contact holes (aspect ratio=5.7). The dc potential of the SiO2 contact hole top and bottom surfaces were measured during plasma exposure with/without deposited fluorocarbon film in the holes. The results revealed that the sidewall deposited fluorocarbon film has high electric conductivity that may mitigate electric charge accumulation at the bottom of contact holes during SiO2 etching processes.

Mechanism of highly selective SiO2 contact hole etching

To investigate the mechanism of highly selective SiO2 contact hole etching, we analysed surfaces exposed to C4F8/Ar/O2 plasma in a dual-frequency parallel-plate etching system. The thickness and composition of the fluorocarbon layer on large areas and at the bottom of contact holes were quantitatively analysed, in conjunction with the flux of CFx chemical species and the etch rates. In a highly selective etch processes, the thickness of the fluorocarbon layer on the SiO2 surface is less than 1 nm, while that on the Si surface is 4–6 nm. We found that the etch rate is strongly affected by the thickness of the fluorocarbon film on the SiO2 and Si surfaces at the bottom of the contact holes, as well as on the large areas. However, a small increase of C4F8 gas flow causes a larger increase in the fluorocarbon film thickness at an aspect ratio of 4 than that on the large flat area surface. Furthermore, we observed a slightly thinner and C-rich film at an aspect ratio of 10. This was probably caused by the decrease in the radical flux passing through the small hole. This aspect-ratio dependence is the cause that the process window for highly selective hole etching tends to be narrow.

Proton channeling into the Si substrate at the bottom of ultrahigh-aspect-ratio contact holes during plasma etching

Etching-induced physical damage at the bottom of ultrahigh-aspect-ratio contact holes was investigated by transmission electron microscopy, and a characteristic hemispherical lattice disorder was discovered in the Si substrate. Depth profiles of impurities in this disordered region were analyzed by secondary ion mass spectrometry, and it was found that this phenomenon is characteristic only for hydrogen-containing plasma. The mechanism of such damage formation is explained by proton (H +) channeling into the substrate at the ultrahigh-aspect-ratio contact hole bottom.

New Ar-Plasma Cleaning Process for Reduction of Al/TiSi2 Contact Resistance

RF-Ar-plasma cleaning lowers Al/TiSi2 contact resistivity by 70% in comparison to conventional buffered-HF (BHF) cleaning. This improvement is due to effective removal of contaminants at the bottom of contact holes. Application of the Ar-plasma treatment to the Si surface before silicidation leads to a decrease in contact resistance down to 25 Ω for 0.3-µm-diameter contact holes. The decrease is believed to be due to the fact that the effective barrier height between TiSi2 and n+-Si is reduced by defects induced by Ar cleaning.

X-ray photoelectron spectroscopy analyses of silicon dioxide contact holes etched in a magnetically enhanced reactive ion etching reactor

High aspect ratio SiO2 contacts are etched with a C4F8/CO/O2/Ar chemistry using a magnetically enhanced reactive ion etching (MERIE) plasma source with a good selectivity to the underlying silicon. X-ray photoelectron spectroscopy (XPS) studies allow chemical analysis of high aspect ratio SiO2 contact holes. Using the charging effect, a complete separation of the XPS peaks originating from the resist mask and from the bottom of the contact holes is possible. XPS analyses show that the fluorination of the polymer on the bottom of the contact is low (F/C around 0.2) and independent of the aspect ratio of contact holes. Furthermore, XPS spectra also show that the full width at half maximum of the Si 2p doublet measured at the bottom of contact holes strongly increases, showing that the highly energetic ion bombardment induced by the MERIE source lead to a severe amorphization of the silicon surface. Finally, comparisons between MERIE and high density plasma are discussed.

Ａｒイオンソフトエッチングを用いた微細接続孔内の自然酸化膜除去効果

An anisotropic inductive coupled plasma (ICP) soft etching using high-density Ar plasma and low incident energy, generally performed before metallization of interconnection, and which can uniformly remove the native oxide at the bottom of fine contact holes, is introduced. As a result, excellent ohmic characteristics can be achieved using ICP soft etching. A cross-sectional TEM (Transmission Electron Microscopy) -EELS (Electron Energy Loss Spectroscopy) was performed in order to compare interface layer generation at the contact hole in the ICP system with that generated in standard RF (Radio Frequency) diode etching. In the sample treated by ICP soft etching, reacted Ti silicide was observed at the bottom of the contact holes. Furthermore, the formation mechanism of stable ohmic contacts was theoretically analyzed using a Monte-Carlo topological simulation. From the results, we found that it is possible to prevent rem-iner denosition on the bottom of holes with 0.1μm diameter.

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.

Dry Post-Etch Treatment using SF6 Chemistry for 0.2 µm Contact Hole

SF6 dry post-etch treatment after contact hole etching has been compared with both NF3 and CF4+O2 post-etch treatment (PET) through measurement of electrical characteristics. It is found that SF6 PET produces the smallest contact resistance at 0.2 µ m contact hole. This phenomenon is analyzed by using secondary ion mass spectroscopy (SIMS) and atomic force microscopy (AFM). According to SIMS analysis, the removal effect of the contaminative carbon and fluorine is almost the same for the three types of PETs. AFM results show that the surface roughness of the contact hole bottom after SF6 PET is the greatest among the three different PETs. This indicates that SF6 PET is very effective in reducing the contact resistance by increasing the surface area of contact hole bottom, since resistance decreases with increasing surface area.

Industrial applications of ICB deposition for the fabrication of electronic devices

Technologies for forming functional layers are required to increase functions and to achieve miniaturization of devices. Ionized cluster beam (ICB) deposition is suitable for these needs and has been used to form thin films with unique characteristics compared to films formed by other techniques. Here, the ICB technique for forming a TiN Ti layer at the bottom of contact holes in DRAMs is discussed. A simple deposition model predicted that the narrow angular distribution of evaporated species and deposition at low vacuum pressure are essential conditions for sufficient coverage of the contacts. The bottom coverage of TiN Ti films has been improved by over 10 times by using the ICB technique in comparison with films produced by conventional sputtering methods. Other applications of ICB to high reflective optical mirrors with an atomically flat surface and high-temperature superconductors with high critical current density within a wide range are also discussed.

Self-Aligned Rapid Thermal Nitridation of TiSi2 in NH3 Ambient as a Diffusion Barrier Layer for Selective CVD-Al Contact Plug Formation

Diffusion barrier properties and selectivity required for selective chemical vapor deposition (CVD)-Al contact plug formation are remarkably improved by the self-aligned rapid thermal nitridation (RTN) of TiSi2 layer in NH3 ambient at 865° C for 30 s. This treatment enables the formation of a nonstoichiometric TiSix layer with a low concentration of nitrogen within 10-20 nm below its surface. However, nitridation is thought to occur particularly along the grain boundaries, resulting in high diffusion-barrier integrity against aluminum diffusion through the grain boundaries. In addition, this treatment provides favorable selective growth from the bottom of contact holes by reducing the number of nucleation sites on the inner side of contact holes. Therefore, our newly developed process is very promising for contact plug formation of complementary metal-oxide-semiconductor (CMOS) using the salicide process.

Observation of the bottom surface of contact holes by hopping scanning atomic force microscopy with a ZnO whisker tip

In this article, it is demonstrated that atomic force microscopy (AFM) can be useful for observing surface microstructures on the bottom of contact holes used for large scale integrated devices. The AFM employs a zinc–oxide (ZnO) whisker tip with high‐aspect ratio and a hopping scanning mode operation. In the hopping scanning mode operation, the probing tip is moved up and down with a period synchronized with the movement in the x direction to avoid collisions against the acute protrusions. Some kinds of bottom surfaces of the contact holes with a diameter of 1 μm and a depth of 1.5 μm have been successfully imaged with nanometer‐scale resolution by this AFM technique.

Improvement of the Interface between Selectively Deposited Aluminum and Silicon by Annealing

The interaction between Al deposited selectively by chemical vapor deposition (CVD) and Si has been investigated. The structure of Al depended on the size of contact holes. Single-crystalline Al was formed in 0.7 µm contact holes, but polycrystalline Al in 1.2 µm and 2.0 µm holes. In the case of single-crystalline Al, Si at the bottom of contact holes was eroded uniformly by Al after deposition and after annealing at 400° C. In the case of polycrystalline Al, Si was eroded non uniformly and Al spikes were formed. However, after annealing at 400° C, Al spikes disappeared and the surface of eroded Si became extremely flat. Junction leakage current of the 2.0 µm n+ contact was found to increase after Al deposition. However, after annealing at 400° C, increase of junction leakage current was not found to 10 V. This change may be due to the improvement of the interface. These results indicate that selective Al-CVD can be used as a contact-hole-filling method.

Thickness determination of thin oxide layers at the bottom of contact holes of semiconductor devices by Auger electron spectroscopy

Fabrication of electrical contacts in semiconductor device technology can be hampered by the presence of thin oxide layers that may form at the bottom of the contact or via holes as a consequence of the reactive ion etching process. In order to achieve low contact resistances after metallization this oxide thickness must be below some critical value. This article describes a method based upon Auger electron spectroscopy for the thickness determination of such thin oxide films at the bottom of micron-sized contact holes prior to metallization. In this technique, the different Si KLL line shapes of elemental and oxidized silicon are utilized for fitting the measured spectrum with a superposition of Si and SiO2 spectra; the oxide thickness follows from the fitting coefficients. The method is applicable to oxide thicknesses up to ∼10 nm and to contact holes with aspect ratios below ∼1. Systematic errors due to scattering and shadowing effects, and to electron-beam induced damaging are discussed. The method has proven to be extremely important in optimizing reactive ion etching processes.