Thin films

Thin film deposition is a process whereby materials - generally precious metals or oxides - are deposited onto a substrate or lithography sample. This can be achieved in a variety of ways such as sputtering, electroplating, evaporation, or CVD. MCN offers a range of different thin film deposition techniques and instruments, which are briefly explained here.

Atomic layer deposition

The process

Atomic Layer Deposition (ALD) involves the deposition of materials one atomic monolayer at a time. It forms extremely uniform, conformal, pin-hole-free coatings on high aspect ratio structures. This is achieved by pulsing a chemical precursor onto a hydroxylated substrate surface. The precursor reacts with this surface but not with itself resulting in a monolayer of material. The precursor is purged from the chamber and then the surface is again hydroxylated with water vapour or oxygen, followed by another purge. These two steps are then repeated until the desired thickness of material is achieved.

The uses

ALD has a vast array of applications from semiconductors, MEMS, nanostructures and optics through to wear-resistant coatings.

The instruments

MCN has two ALD instruments – the Fiji F200 and the Savannah S100, both from Cambridge Nanotech. The Savannah system is thermal only and is integrated into a glovebox system so that samples can be kept in an inert environment. The Fiji F200 is capable of both thermal and Plasma Assisted ALD (PA-ALD).

PA-ALD expands the window for materials by decreasing activation energy and allows for deposition at lower temperatures to reduce precursor decomposition, deposition times and film contaminations. Both systems are equipped to enable Cambridge Nanotech’s unique Exposure Mode™ for thin film deposition on ultra high aspect ratio substrates.

Electron beam evaporation

The process

Electron-beam evaporation is a physical vapour deposition method for depositing thin films of metals, oxides and semiconductors in a high vacuum environment. Ultra high purity coating material is placed inside a vacuum chamber, typically as pellets in a crucible. Electron energy is used to heat these pellets, causing the coating material to enter the gas phase. Due to the vacuum environment, the evaporated particles can travel to the substrate without colliding with foreign particles, where they then condense on the substrate surface in a thin film.

The uses

Electron beam evaporation is used to deposit electronic and optical films for the semiconductor industry and has applications in displays and photovoltaics. High melting point materials can be deposited at high deposition rates, making this a preferred process for refractory metal and ceramic films.

The instrument

The Intlvac Nanochrome II electron beam evaporation system consists of a 10kV power supply and supports up to sixteen 4 inch wafers or fifteen 6 inch wafers or wafer mounted samples. In addition, the system also supports ion beam assisted deposition and substrate pre-cleaning. Single films or multi-material stacks are easily created using simple layer definitions, while users can also make use of ion-beam pre-cleaning and sample heating. Coating thickness are from 2nm up to 200nm (thickness >200nm by approval).

Currently available in the materials library are metals (Al, Cr, Ti, Au, Ag), oxides (SiO2, TiO2, Al2O3, ITO), fluorides (MgF2) and semiconductors (Si, Ge).

Thermal evaporation

The process

Resistive thermal evaporation is another form of physical vapour deposition and can be used to deposit metals, organic, and inorganic polymers. In this method, electrical energy is used to heat a filament which in turn heats a deposition material to the point of evaporation. The process can be performed at very high levels of vacuum to eliminate collisions with foreign particles and therefore fewer tendencies to introduce film impurities. High deposition rates can be achieved and lower energy particles can reduce substrate damage.

The uses

Polymer solar cells and flexible electronics.

The instrument

MCN’s thermal evaporation system is the Angstrom Engineering EvoVac. This system is integrated with glove box and has six resistive sources; three of which are standard resistive source locations and three of which are Luxel Radak furnaces. It supports sequential and co-deposition of thin films. The deposition process is controlled using quartz rate monitors. Currently available in the materials library are metal (Ag, Al) and organic materials (Ca, BCP, TIPS-DBC, C60).

Polymer electronics glovebox

The instruments

The Angstrom Engineering polymer dual glovebox system for polymer electronics fabrication provides an inert atmosphere for spin coating, electrode or counter-electrode deposition and assembly of organic photovoltaic and other flexible electronic devices. The glovebox system maintains a pure inert atmosphere by circulating dry nitrogen gas through a purifier and solvent trap. Integrated into this system is a high vacuum chamber with mask transfer system for the thermal evaporative deposition of patterned electrodes and the ALD Savannah S100 system for counter electrode deposition.

The uses

Useful for oxygen-sensitive chemistries and the assembly of organic photovoltaics and flexible transistor fabrication.

Electroplating

The process

Electroplating uses electrical current to reduce dissolved metal ions from an electrolyte to form a coherent metal coating on a material. The cathode, or part to be plated, and plating metal are immersed in an electrolyte containing dissolved metal salts as well as other ions that permit the flow of electricity. When a current is applied to the electrolyte, the free metal ions move through the solution to the cathode and are reduced to metal, creating a thin metal covering the surface.

The uses

Electroplating is a common process in the jewellery, automotive, and food industries but it is also highly useful in the production of semi-conducting electronics and optics. Different metals can also be used to grow shims for use in hot embossing and nano-imprint lithography, while they can also help to protect surfaces from aggressive etching processes. Nickel electroplating can also be used to cast PDMS in the creation of fluidic cells.

The instruments

MCN has the capability to provide high quality Nickel coatings and embossing shims using the Digital Matrix PMT-16 and SA1000.

Sputtering

The process

Sputtering is a physical vapour deposition method that involves depositing thin films in a vacuum environment. During this process, a solid material and substrate are positioned separately within a vacuum system. A high-energy argon ion plasma stream is targeted at the material, resulting in the subject material being ejected and deposited onto the substrate, creating a thin film.

As this is not an evaporative process, the temperatures required for sputtering are lower than evaporation methods. This makes it one of the most flexible deposition processes and it is particularly useful for depositing materials with a high melting point or a mixture of materials, as compounds that may evaporate at different rates, can be sputtered at the same rate. Certain processes will benefit from improved film adhesion due to higher impact energy.

The uses

The sputtering process is used extensively in the semiconductor industry, screen displays, photovoltaics and magnetic data storage. Sputtering can be used to deposit a wide variety of thin films including metals, oxides, nitrides and alloys.

The instruments

MCN has two sputter deposition systems, the Intlvac Nanochrome AC/DC system and the Anatech Hummer BC-20 RF/DC System. These systems consist of a RF and dual AC and DC power supply and support samples sizes up to 8 inch diameter. Different process gases are available including argon, oxygen and nitrogen, allowing the deposition of pure metals, oxides, nitrides, alloys and customised combinatorial compositions.

In addition, the system also supports ion beam assisted deposition. The thickness of the coating can vary from ~10nm up to 5µm (thickness greater than 1µm should be discussed with the process engineer). The materials library currently available includes metals (Al, Cr, Ti, Cu, W, Mo, Co, Ni, Fe), oxides (ZnO, TiO2, WoO3, MoO3, CoO, Al2O3) and nitrides (AlN, TiN).

Furnace systems

The process

Furnace systems are used to treat materials at high temperatures to change their properties through diffusion, annealing and chemical vapour deposition.

The instruments

MCN has two furnace systems - the HiTech furnace and the SVCS four-stack horizontal furnace.

The HiTech furnace system is used to create layers of silicon dioxide on a silicon substrate. This can be achieved through wet or dry oxidation. The furnace can operate up to 1200^oC and can perform dry oxidation (O₂ is oxidant) and wet oxidation (steam is oxidant). For wet oxidation, water vapour is generated using a torch which burns O₂ and H₂. The system can accept 4 inch wafers (3 and 6 inch wafers on approval by the process engineer).

The SVCS four-stack horizontal furnace system provides access to high temperature processing of silicon and other substrates, with small batch processing options. The four processing tubes cater for atmospheric pressure diffusion, atmospheric pressure annealing and the low-pressure chemical vapour deposition of low strain and stoichiometric silicon nitride. The tool has a HEPA controlled loading station.

Plasma enhanced chemical vapour deposition

The process

Plasma Enhanced Chemical Vapour Deposition (PECVD) combines silicon with oxygen or nitrogen to create a plasma that deposits a thin film of silicon dioxide or silicon nitrate onto a substrate. PECVD uses lower temperatures than the furnace systems to achieve an insulating layer on a variety of materials.

The uses

PECVD is used in optics, microelectronics, energy applications, packaging and chemistry for the deposition of anti-reflective coatings, scratch resistant transparent coatings, electronically active layers, passivation layers, dielectric layers, isolating layers, etch stop layers, encapsulation and chemical protective coatings.

The instruments

The PECVD system at MCN is an Oxford Instruments PLASMALAB100 PECVD and it is capable of depositing silicon dioxide and silicon nitride using a combination of gases. The system caters for a range of sample sizes up to and including eight inch wafers. Deposition temperatures range from 25°C to 350°C.

Microwave plasma chemical vapour diamond deposition

The process

Diamond deposition is a chemical vapour deposition process which uses a continuous microwave source to heat hydrogen and carbon (in the form of methane) to between 700°C and 1000°C. This breaks down the atoms to become a highly reactive plasma. When the plasma interacts with an existing diamond sample surface, the hydrogen stabilises the diamond structure, while the methane adds new carbon to the diamond lattice. The systems are capable of fabricating either single or polycrystalline diamond as well as boron doped diamond.

The uses

Diamond is highly useful in a wide range of applications, such the study of quantum phenomena, electronics and medical bionics. Diamond is an ideal material for bionic devices which need to be both biocompatible and suitable for long-term implant. It is a highly robust and hermetic material which makes it perfect for encasing and protecting electronics from body fluids. Furthermore, when doped with boron it can conduct electrons, predisposing it to use in electronics.

Nitrogen vacancy diamond displays interesting photo-luminescence properties which can be exploited for quantum information systems.

The instruments

MCN has two Seki Diamond Microwave Systems - AX6500X and AX6300. The AX6500X is used for growing ultra-high purity and nitrogen vacancy diamond, while the AX6300 is used for growing boron-doped diamond.