Roman Gouk

Facile synthesis of bimetallic Ag/Ni core/sheath nanowires and their magnetic and electrical properties.

This paper describes a facile method for coating Ag nanowires with uniform, ferromagnetic sheaths made of polycrystalline Ni. A typical sample of these core/sheath nanowires had a saturation magnetization around 33 emu g(-1). We also demonstrated the use of this magnetic property to align the nanowires by simply placing a suspension of the nanowires on a substrate in a magnetic field and allowing the solvent to evaporate. The electrical conductivity of these core/sheath nanowires (2 × 10(3) S cm(-1)) was two orders of magnitude lower than that of bulk Ag (6.3 × 10(5) S cm(-1)) and Ni (1.4 × 10(5) S cm(-1)). This is likely caused by the transfer of electrons from the Ag core to the Ni sheath due to the difference in work function between the two metals. The electrons are expected to experience an increased resistance due to spin-dependent scattering caused by the randomized magnetic domains in the polycrystalline, ferromagnetic Ni sheath. Studies on the structural changes to the Ni coating over time under different storage conditions show that storage of the nanowires on a substrate under ambient conditions leads to very little Ni oxidation after 6 months. These Ag/Ni core/sheath nanowires show promise in areas such as electronics, spintronics, and displays.

Contactless bottom-up electrodeposition of nickel for 3D integrated circuits

Packaging applications in the semiconductor industry rely on electrodepositing metals into high aspect ratio (HAR) vias without the formation of any defects or voids. The process and economic efficiency of conventional methodologies are limited by the ability to achieve high deposition rates along with uniformity of the deposited metal layer. In this work, a contactless and scalable electrodeposition technique has been developed to deposit metallic nickel onto p-doped silicon wafers. The effect of various process variables such as deposition and etchant solution composition and concentration, solution temperature and stirring on nickel deposition rates have been investigated. The importance of backside silicon oxidation and subsequent oxide etching on the kinetics of nickel deposition on frontside silicon has been highlighted.

Using megasonics for particle and residue removal in single wafer cleaning

Introduction With smaller device geometries, the number of cleaning steps increases and is reaching >100 steps in some recent process flows. Increasing the number of cleaning cycles contributes to additional cycle time, cumulative silicon and oxide loss, and damage to fragile structures. Therefore, a shorter, more efficient cleaning process is critical to achieving high-productivity device manufacturing. In a conventional wet bench, the megasonics crystals typically are mounted on the bottom of the tank, and their energy is dispersed over 50-100 wafers. In a single-wafer horizontal spin system, the acoustic energy can be focused uniformly over a wafer. In single wafer cleaning, effectiveness of megasonics is typically the most critical part of process reliability because of the short cycle time. This requires that all causes and effects in the megasonic process are well understood. In this work we have attempted to test the mechanism of cavitation and its role in particle removal in a single wafer system. Tests described in this paper were conducted using a single wafer process chamber with full coverage, 300 mm megasonic transducer, which operates in parallel to the processing wafer. The effect of DI water gas content on particle removal was studied.

Advanced mask cleaning techniques for sub-100-nm technology nodes

Sub-pellicle defects and haze increase due to photon reaction with cleaning chemistry residues are especially problematic on photomasks for 193 nm and shorter exposure wavelengths. In addition to mask cleaning, these chemistries are also used for photoresist stripping from photomasks. In this paper sulfuric acid free processes are shown to be effective for mask cleaning and photoresist removal. Bulk removal of photoresist was accomplished with both oxygen based dry plasma stripping and with wet oxidizing chemistry. Surface preparation prior to the main cleaning step was necessary to render Cr surface hydrophilic and enable targeted cleaning performance. This was accomplished with an O3/DI pre-treatment step. Full mask megasonics improved particle removal efficiency of moderately to heavily contaminated masks.

Novel Texture Etch Chemistry for Transparent Conducting Oxides Used in Photovoltaic Cells

In thin film photovoltaic silicon stacks, the sun facing contact needs to be transparent and textured. Typically transparent metal oxides are used for this purpose. When using sputtered ZnO as the transparent conducting contact typically an acid etch is used to texture etch the surface. This texturing enables light trapping in the cell and greatly enhances the photoresponse. Traditionally dilute HCl has been used for this purpose. In this paper we present the work on a novel etchant for this purpose consisting of HNO3 and Acetic Acid. This greatly enhances the texturing and hence the light trapping in the cell.

Tunable Droplet Momentum and Cavitation Process for Damage-free Cleaning of Challenging Particles

Particle removal without damage has been demonstrated for <60nm photomask sub-resolution assist features with droplet momentum cleaning technology that employs NanoDropletTM mixed-fluid jet nozzle. Although 99%+ particle removal efficiency can be achieved for standard Si3N4 particles with broad size distribution, there are some cleaning challenges with small (<100nm) and large contact area (>500nm) particles. It was found that tunable uniform cavitation can provide the additional physical assist force needed to improve cleaning efficiency of these challenging particles while meeting the damage-fee cleaning requirement. An integrated cleaning process was developed that combines both droplet momentum and damage-free cavitation technology. Cleaning tests were performed with different types of challenging particles. The results showed 5-8% particle removal efficiency improvement as compared to momentum based only cleaning. All masks were processed using the TetraTM mask cleaning tool configured with NanoDropletTM mixed fluid jet technology and full face megasonics.

Advanced damage-free photomask cleaning for 45/32nm technology nodes

High particle removal efficiency (PRE) up to 99%+ without damage to sub-50 nm linewidth features has been demonstrated using a mixed fluid jet technology and sulfur-free chemistry. This high PRE was achieved with several types of deposited particles, including polystyrene latex spheres. Damage-free cleaning was demonstrated on binary and phase shift masks with Cr and MoSi structures. All masks were processed using the TetraTM mask cleaning tool configured with the NanoDropletTM mixed fluid jet technology.

Advanced processes for photomask damage-free cleaning and photoresist removal

Photon induced haze resulting from sulfur residues that remain after cleaning and photoresist stripping is a key challenge for 193 nm photomasks. In previously reported work, sulfur-free processes for cleaning and photoresist removal on mask blanks were shown. Additional characterization and development of the cleaning and strip/clean processes are presented here. For cleaning the particle adder stability, ammonia chemistry residue levels, and chrome oxide anti-reflection coating (ARC) layer integrity were characterized. It was found that process modification was needed to provide acceptable post-clean ammonia levels and reflectivity change per clean. A strip/clean process with acceptable window for complete resist removal without ARC layer damage was found to be challenging and dependent on the mask photoresist/ARC stack. Dry strip, wet strip, and combined dry/wet stripping approaches (all followed by wet clean) were investigated. Oxidizing dry strip chemistry, while easily removing the bulk photoresist layer, gave unacceptable ARC attack. For FEP photoresist an all-wet process was demonstrated, and for iP and NEB resists, promising results were achieved with less oxidizing dry strip chemistry.

Mechanism and Principles of Post Etch Al Cleaning with Inorganic Acids

In the metallurgical industry, Al cleaning has been carried out traditionally with inorganic acids, such as sulfuric acid and phosphoric acid, quite often with an oxidizer added such as sodium or potassium dichromate [1], e.g. before bonding aluminum surfaces. In the semiconductor industry, the use of inorganic acids for cleaning aluminum with mixtures such as phosphoric/chromic and fuming nitric acid was used since the early 1980’s. However, it was rather the use of hydroxylamine (NH2OH) for cleaning aluminum that found widespread use, since it could be used for both postaluminum etch and post via-etch cleaning processes. In 1998, David Rath and Ravikumar Ramachandran patented a formulation using the traditional sulfuric acid and an oxidizer for aluminum cleaning as a basis, but they added a very small amount of HF (0-100ppm) [2]. This together with other work reinvigorated the interest in using inorganic chemicals for aluminum cleaning.

High Efficiency Wafer Backside Cleaning with Full Coverage Megasonics Spin Process

As the technology node scales down to 45nm and beyond, removal of particles and metal contamination from wafer backsides have become critical for advanced lithography with smaller depth of focus, as well as to enable efficient metrology tool sharing between various applications. Smaller particles are usually held by van der Waals’ force, and removal of these particles usually requires mechanical force, such as acoustic energy which creates cavitation. Just as the wafer frontside, backside cleaning requires full liquid coverage for high efficiency cleaning. Backside clean with full coverage backside megasonics can achieve higher particle removal efficiency (PRE), especially for smaller particles. In this paper, we report a novel full coverage backside megasonic spin process that demonstrates high PRE and metal contamination removal and achieve high PRE for advanced lithography and wafer metrology.