Xun Gu

Effect of Additives in Organic Acid Solutions for Post-CMP Cleaning on Polymer Low-k Fluorocarbon

The appropriate post- chemical mechanical polishing (CMP) cleaning solution for an advanced nonporous polymer, ultralow-k fluorocarbon film is proposed. While decrease in fluorine content measured by high-resolution X-ray photoelectron spectroscopy and increase in dielectric constant were observed after additives cleaning with oxalic acid, no significant change of fluorine content and dielectric constant were found after citric acid with different additives and acids without additives cleanings. This reveals that additives play an important role in degradation of the fluorocarbon film. An appropriate selection of additives is important to clean the fluorocarbon film in post-CMP cleaning in advanced large-scale integrated generations. To complete successful integration of the post-CMP cleaning on new dielectric materials such as nonporous low-k fluorocarbon film, an appropriate solution and additives should be carefully selected to avoid electrical degradation in subsequent process.

Reduction of Scratch on Brush Scrubbing in Post CMP Cleaning by Analyzing Contact Kinetics on Ultra Low-k Dielectric

As technology node progressing, the Chemical Mechanical Planarization (CMP) and post CMP clean with brush scrubbing become a crucial in an integrated circuit fabrication. Especially after implementation of ultra low-k dielectrics in Cu damascene structure, damage reduction during planarization is strongly required to avoid scratch generation leading to yield loss. In this work, kinetics of brush contact on substrate and behavior of shear force during brush scrubbing are studied to avoid damage generation in post CMP process. The 200mm wafer available brush scrubber in this experiment is employed with the polyvinyl alcohol brushes with wafer rotation mechanism. A loadcell is placed between wafer holder and fixed plate to be able to measure shear force in real time generated by friction force between brush and wafer. Figure 1 shows shear force as a function of brush rotation rate in dry, which means no ultra pure water (UPW) flows and in wet, which means UPW flows. In lower brush rotation rate ( 200rpm), shear force increased significantly in wet, while shear force in dry remains same value. To investigate these results, a 237mm square pressure sensing sheet with 1600 points tiny sensors was placed between brush and wafer. The pressured area is measured as a sum of area of each sensor at which pressure increase. Down force is calculated by average down pressure and pressured area. Figure 2 shows average down pressure, pressured area, and down force as a function of brush rotation rate in dry and wet. Similar to Fig.1, down force decreased significantly in lower brush rotation rate, while they increased in higher brush rotation rate. Decrease in down force is interpreted by decrease in average down pressure in lower brush rotation rate, while increase in it is interpreted by increase pressured area in high brush rotation rate. Therefore, increase in shear force in Fig.1 is considered to be a result in increase in down force led by increase in pressure area at higher rotation rate. Figure 3 shows pressured area images in (a) dry, (b) 50rpm in wet and (c) 500rpm in wet. In Fig.3 (a), each brush nodule was clearly observed. On the other hand, in Fig.3 (b), pressured area was generated between nodules and these areas are considered to contribute to increase total pressured area. Moreover, this phenomenon becomes obvious at 500rpm, shown in Fig.3(c). Increase in pressured area between nodules can be considered to result that higher velocity of fluid increase pressure on non-contact area. An ultra high speed camcorder (<8000fps) was used to investigate contact characteristic of brush and wafer. At higher brush rotation rate, UPW on wafer is observed to move to follow nodule movement and this observation supports hydrodynamic model. Moreover, clear stick-slip phenomena were observed at lower brush rotation rate region. Stick-slip is considered to decrease shear force in Fig.1 and down force in Fig.2, because stick-slip is reported to decrease at higher velocity. Particle removal and scratches were qualified with various down pressures and rotation rates on slurry contaminated ultra low-k in Table I. At 200rpm and 6.9kPa, serious scratches were observed although showing enough particle removal efficiency. Decreasing down force helps reduction of scratches, but particle removal efficiency also decreased. Increase brush rotation rate at lower down pressure can satisfy both particle removal efficiency and scratch reduction. These results reveal that increase contribution of fluid flow by increasing brush rotation rate with lowering down force can be direction for future low-k compatible devices.

End-point detection of Ta/TaN chemical mechanical planarization via forces analysis

This study explores the transition of shear force spectral fingerprints during tantalum (Ta) and/or tantalum nitride (TaN) chemical mechanical planarization on patterned wafers using a polisher and tribometer that has the unique ability to measure shear force and down force in real-time. Fast Fourier Transformation is performed to convert the raw force data from time domain to frequency domain and to illustrate the amplitude distribution of shear force and down force. Results show that coefficient of friction, variance of shear force and variance of down force increase during polishing when the Ta/TaN layer is removed thus exposing the inter-layer dielectric layer. Unique and consistent spectral fingerprints are generated from shear force data showing significant changes in several fundamental peaks before, during and after Ta/TaN clearing. Results show that a combination of unique spectral fingerprinting, coefficient of friction and analysis of force variance can be used to monitor in real-time the polishing progress during Ta/TaN chemical mechanical planarization for optimal polishing time.

In situ Observation of Grain Growth on Electroplated Cu Film by Electron Backscatter Diffraction

In situ observations using an automated electron backscatter diffraction (EBSD) system with a heating stage were carried out to clarify the mechanism of grain growth on electroplated Cu and Cu deposited by physical vapor deposition (PVD). Hardness measurements and EBSD observations revealed that the strain in electroplated Cu and Cu deposited by PVD was released by annealing. Little preferential orientation of the texture was observed in electroplated Cu, while Cu deposited by PVD showed strong (111) texture. A greater number of twin boundaries were observed in electroplated Cu than in Cu deposited by PVD. Grains surrounded by random grain boundaries were more enlarged by annealing in electroplated Cu than those in Cu deposited by PVD. Since segregation of impurities with a low melting point, such as sulfur, to grain boundaries could promote grain growth, which is often accompanied by twin boundary formation, the reason that many twin boundaries are found in electroplated Cu is considered to be the existence of impurities such as sulfur at grain boundaries. The effect of a twin grain boundary on resistivity was calculated and the contribution of a twin grain boundary to resistivity was found to be much less than that of a random grain boundary.

Tribological Effects of Brush Scrubbing in Post Chemical Mechanical Planarization Cleaning on Electrical Characteristics in Novel Non-porous Low-k Dielectric Fluorocarbon on Cu Interconnects

Damage reduction during planarization is strongly required to avoid scratch generation and the variation in the electrical properties of low-k dielectrics leading to yield loss in an integrated circuit after the implementation of an ultralow-k dielectric in Cu damascene interconnects. An optimum process condition to reduce damage on brush scrubbing in post-chemical–mechanical-planarization (post-CMP) cleaning was proposed for advanced nonporous organic ultralow-k dielectric fluorocarbon/Cu interconnects. Increasing brush rotation rate by decreasing down pressures results in the improvement in both electric properties and particle removal efficiency. The tribological effects of brush scrubbing in post-CMP cleaning on the electrical characteristics were explored. The brush scrubbing condition of a high brush rotation rate at low down pressures contributes to the suppression of damage generation.

Damage-Free Post-CMP Cleaning Solution for Low-k Fluorocarbon on Advanced Interconnects

As technology node progressing, ultra low-k film has been implemented to reduce RC delay in LSI circuit. A fluorocarbon (CFx) film is proposed as foreground ultra low-k film because of non-porous structure [1]. Although CFx film is expected to be stable for its structure advantage, damage-less process is anticipated to avoid dielectric constants change in subsequence process steps [2]. CMP and post CMP process are concerned to bring damage on devices, so the effect of post CMP cleaning solutions on CFx structure and electric property is evaluated.

Electrical Characteristics of Novel Non-porous Low-

A novel non-porous low-k dielectric, fluorocarbon, deposited by new microwave excited plasma enhanced chemical vapor deposition was successfully integrated into Cu damascene interconnects for the first time. Electrical characteristics of fluorocarbon/Cu damascene lines are investigated. A compatible line to line leakage current to the one with porous low-k carbon doped silicon oxide and a low effective dielectric constant as a value of 2.5 are achieved. The novel non-porous ultralow-k dielectric, fluorocarbon, is considered as a promising candidate to extendible for 22 nm generation and beyond.

k

An integration of an organic nonporous ultralow-k dielectric fluorocarbon (k = 2.2) deposited by microwave-excited plasma-enhanced CVD into Cu single damascene interconnects is developed in this paper. The changing of the chemical structure of the fluorocarbon was found after dry etching, which resulted in the degradation of electrical properties during the postetching cleaning process. A nitrogen plasma treatment was applied as a postetching process to minimize damage introduction to the fluorocarbon in the following damascene fabrication processes, and a line-to-line leakage current was significantly improved without the variance of effective dielectric constant (keff = 2.5) in Cu lines. In a thermal stress test at 350°C after Cu-interconnect fabrication, no degradation of the Cu line resistance and line-to-line capacitance was found, which indicated a sufficient thermal stability of the fluorocarbon film in Cu single damascene interconnects. Therefore, this robust organic nonporous fluorocarbon film is considered as one of the promising candidates of ultralow-k dielectrics for high-performance Cu interconnects in the future.

Dielectric Fluorocarbon on Cu Interconnects for 22 nm Generation and Beyond

A novel end-point detection method based on a combination of shear force and its spectral amplitude was proposed for barrier metal polishing on copper damascene structures. Under some polishing conditions, the shear force changed significantly with polished substrate. On the other hand, the change in shear force was insignificant under certain polishing conditions. Therefore, a complementary end-point detection method by monitoring oscillation frequency of shear force was proposed. It was found that the shear force fluctuated in unique frequencies depending on polished substrates. Using Fast Fourier Transformation, the shear force data was converted from time domain to frequency domain. The amplitude of spectral frequencies corresponding to the rotational rate of wafer carrier and platen was monitored. Significant frequency amplitude changes were observed before, during and after the polished layer transition from barrier film to silicon dioxide film. The results indicated that a combination of shear force and its spectral amplitude analyses provided effective end-point detection for barrier CMP process.

Cu Single Damascene Integration of an Organic Nonporous Ultralow-

Brush scrubbing has been widely used in post chemical mechanical planarization (CMP) applications to remove contaminations, such as slurry residues and particles, from the wafer surface. During brush scrubbing, particle removal results from direct contact between a soft poly vinyl alcohol (PVA) brush and the wafer surface in which the brush asperities engulf the particles while the rotating motion of the brush, as well as the cleaning fluid at the surface, dislodge and carry the particles away from the wafer. The cleaning performance of brush scrubbing depends heavily on the choice of the cleaning solution and brush scrubber kinematics. In this work, the effect of various cleaning solutions and brush scrubber kinematics on the frictional attributes of post copper CMP cleaning process was investigated.