Thin film technology
The thin-film technology , rarely thin film technology called is engaged in the manufacture and processing of thin layers of different materials, such as metallic , dielectric and semiconducting materials. The thickness of such layers is typically in the range from a few micrometers to a few nanometers.
The layers are usually deposited over the entire surface of a substrate using physical (PVD, e.g. thermal evaporation or sputtering ) and chemical vapor deposition (CVD) processes. Further processing of the layers can then take place, including post-treatments such as tempering , recrystallization or doping of the layer, as well as targeted material removal, for example with the help of chemical-mechanical polishing . Especially in the manufacture of products for semiconductor electronics (such as integrated circuits or thin-film solar cells ) and microsystem technology (sensors, actuators), the layers are also structured, that is, the layer material is deliberately removed in some places. The structure can be generated by the photolithography common in semiconductor technology or directly by laser or electron beam processing. Resistances are often calibrated using an electron beam, which enables the highest levels of accuracy (0.1%) to be achieved.
Demarcation
So-called thin layers (<1 µm) are used in many areas (optics, catalysts, ICs , cylindrical resistors, capacitor foils, packaging). However, the term thin-film technology is usually only used for flat electronic components and circuits made of “thin layers” on substrates such as a wafer or a printed circuit board .
The so-called thick - film technology also uses insulator substrates; However, resistors and conductor tracks are produced using printed and fired so-called glass frits (powder mixture of metal and glass). In contrast, thin-film technology includes not only additive processes such as sputtering, but also subtractive processes such as etching. The importance of the cleaning process should also be noted.
Additive processes
Processes are referred to as additive in which layers are applied over a large area or in a structured manner (e.g. lift-off process ) on a substrate. This usually happens through the chemical reaction or condensation of gaseous substances on the substrate surface. Furthermore, processes for the separation from the liquid phase are also widespread.
The quality of a thin film depends on three factors:
- the physical state of the surface of the substrate (surface roughness)
- of the activation energy for surface and volume diffusion of the layer atoms
- on the binding energy between adsorbed atom and substrate surface
The most important process groups are briefly described below.
Chemical vapor deposition (CVD)
Chemical vapor deposition (CVD) is a gas phase reaction (mostly on or near the substrate surface). The reaction gases are simultaneously fed into the reaction chamber with the substrate to be coated. Most of the preheated gases are thermally activated by the heated substrate and react with one another. The desired material is deposited and chemically bound ( chemisorption ).
In addition to countless CVD variants, which differ in terms of working pressure and other process parameters, there are also some coating processes that are more or less heavily modified CVD processes:
- Plasma polymerisation : In this process, gaseous monomers excited by a plasma form a highly cross-linked layer on a substrate.
- Atomic layer deposition : Atomic layer deposition is a heavily modified CVD process in which the reaction or sorption on the surface stops automatically after the surface is completely covered. This self-limiting reaction is carried out in several cycles (with rinsing steps in between), so very good aspect ratios and exact layer thicknesses can be achieved.
Physical vapor deposition (PVD)
In contrast to CVD processes, PVD processes are based on purely physical action processes; as a rule, it is a material vapor that condenses on the substrate surface. One differentiates:
- Thermal evaporation: In thermal evaporation , the evaporation material is heated until it evaporates with a suitable evaporation rate. There are three “sub-processes” depending on the evaporator used (inductive, resistance or electron beam evaporator). In order to ensure the deposition of high quality and homogeneous layers, it is necessary to keep the space between the evaporator and the substrate as free of material as possible (i.e. vacuum ). Interactions (mostly collisions) of the particles with residual gas atoms can bind them or scatter them in such a way that the reproducibility of the coating cannot be guaranteed. Quartz oscillators are often used to measure and control the vapor deposition rate and layer thickness (alternatively, optical monitoring ).
- Sputter deposition : In sputtering (also called cathode atomization), particles are removed from the surface by ion bombardment. By this method the surface can e.g. B. of oxides or water that have entered the material during manufacture, processing or storage. This physical process is also used in thin-film technology to sputter material from the target, i.e. i.e., to convert it into the gas phase. The resulting gaseous material is then fed onto the substrate to be coated and condenses there. This coating process is called sputter deposition and has the advantage over vapor deposition that alloys are alsotransferred to the wafer in "equal proportions". It must be noted, however, that different materials have different sputtering coefficients, i.e. can be sputtered to different degrees. The layer thickness is often controlled by a time switch.
- Ion plating : Ion plating is a vacuum-based and plasma-supported PVD process for metals and metal compounds. In this process, vaporized metal (e.g. by means of an arc discharge) is fed into a plasma. There part of the metal vapor cloud ionizes and is accelerated in the direction of the substrate. The metal ions form a layer on the substrate surface, which is initially sputtered back together with the substrate material by the constant bombardment by metal ions.
- ICB technology ( ionized cluster beam deposition , ICBD): ICB technology is a modified vapor deposition process. The crucible used for evaporation is kept closed. The heating of the evaporation material creates an overpressure in the closed crucible. If this steam is let off through a nozzle,a sudden coolingoccurs due to an adiabatic expansion. Neutral clusters of atoms are formed, which partially dissolve when they hit the substrate surface and are deposited over the surface.
- Molecular beam epitaxy (Engl. Molecular beam epitaxy , MBE)
Galvanic process
In addition to the deposition from the gas phase, there are also numerous deposition processes from the liquid phase. One of the most important process groups is electroplating (electroplating for short). It comprises all processes for the electrochemical deposition of electrically conductive layers (usually metals ) on a substrate. For this purpose, the substrate is immersed in an electrolytic bath and applied with an electrical voltage. An electric current flows in the resulting electric circuit, which is primarily formed in the electrolyte by the movement of positive metal ions. When voltage is applied, the dissolved metal ions move to the negative pole (cathode), the substrate to be coated, and are deposited there.
A variety of materials can be used as substrates. The most important prerequisite, however, is at least low electrical conductivity on the surface. For this reason, in the case of non-conductive substrates such as plastics, a thin conductive layer is first applied using other methods (see, for example, plastic metallization ). The surface conductivity also has an influence on the homogeneity of the deposit. In general, the layer thickness achieved depends on the current strength used and the process duration as well as the bath composition. The deposition in holes and trenches can also be influenced via bath additives. This is used, for example, in semiconductor technology when cutting off the copper conductor tracks, where special bath additives accelerate the deposition of the copper layer on the bottom and corners of contact holes or hinder the deposition on the top.
Sol-gel process
Inorganic and hybrid polymer layers can be produced from colloidal solutions by wet chemical coating processes and subsequent curing . The underlying sol-gel process is to be understood as part of chemical nanotechnology .
Applications
- Thin-film solar cells ( amorphous silicon layers)
- Resistor networks , high-precision individual resistors
- Strain gauges
- Photoresistors
- Platinum temperature measuring resistors
- Medical technology
- semiconductor
- Membranes, e.g. B. for micro pressure sensors , condenser and electret microphones
- LCDs
