Vacuum and Thin Film Technology

Transcription

1 1 Instructions for the Laboratory Course Vacuum and Thin Film Technology (Vakuum- und Dünnschichttechnik) Part of the Course Advanced Laboratory Exercises (Praktikum für Fortgeschrittene) Graz University of Technology Institute of Solid State Physics Supervisor: Assist. Prof. Dr. Bettina Friedel (Office PH ,

2 2 1 Objectives of this Course The goal of this one-day course is the acquirement of basic skills in the use of vacuum equipment and the practice of different thin film deposition methods, leading towards applications in thin film electronics as one example. Computer-supported data acquisition (Labview) and analysis is trained. It is expected that the students familiarize themselves with the following topics before start of the experimental work: vacuum - definition, terms and theoretical basics - components and their function - applications thin films - definition and properties - vacuum-based deposition techniques - non-vacuum-based deposition techniques - methods for thickness determination of thin films - applications of thin films The fundamentals given below (Chapter 3) are for guideline only; the students are encouraged to gain further information from literature/ electronic resources. 2 Experimental Tasks a) Acquisition of pressure development data for the recipient depending on different starting conditions, filled either with (i) air or (ii) Ar (or CO 2 ). b) Determination of pumping speed and leakage rate of the system from pressure-timecharacteristics. c) Deposition of a transparent (!) metal film on glass via thermal evaporation method, thickness monitored by quartz microbalance. (Alternatively dip-coating deposition of transparent conducting oxide film; as soon as available.) d) Quantification of its transparency with a UV-VIS spectrophotometer. e) Spin-coating deposition of a photoactive polymer film. f) Measurement of its optical absorption via UV-VIS spectrophotometer and thickness estimation from a (provided) calibration series. g) Deposition of a reflective (!) metal film on glass via thermal evaporation method, thickness monitored by quartz microbalance. h) Electrical characterization of the derived thin film photodiode (dark IV, light IV). Further instructions are given on-site.

3 3 3 Fundamentals 3.1 Vacuum Fields, Units, Terms and Theory Vacuum is simply defined as reduced pressure. The vacuum achieved by common vacuum systems is generally far from being empty from matter. However, vacuum technology is an extremly important tool in many areas of physics. In condensed matter physics and materials science, vacuum systems are used in many surface processing steps. Without the vacuum, such processes as sputtering, evaporative metal deposition, ion beam implantation, and electron beam lithography would be impossible. A high vacuum is required in particle accelerators, from the cyclotrons used to create radionuclides in hospitals up to the gigantic high-energy physics colliders such as the LHC. Vacuum systems are also used in many precision physics applications where the scattering and forces induced by the atmosphere would introduce a major background to precise measurements. [1] The measures for vacuum are possibly the most versatile ones in physics. The most recent addition is the unit Pascal (1971) which is expressible in SI-base units as N kg 1Pa= 1 = m m s Nevertheless, plenty of other units are (still) used. The most popular ones and their equivalencies are found in table 1. Pascal Bar Technical atmosphere Standard atmosphere Torr Pound per square inch Pa bar at atm torr psi 1 Pa 1 N/m bar dyn/cm at kp/cm atm p Torr = 1 mm Hg psi lb F /in 2 Table 1: Commonly used pressure units and their equivalencies [3]. Vacuum is usually classified into different pressure ranges, as seen in the table 2. Table 2: Vacuum regimes and their pressure ranges [4]. The definition of these pressure ranges may vary, occasionally only low vacuum LV (e.g. used in industrial chip production lines, as transfer medium), high vacuum HV (e.g. in thin deposition) and ultrahigh vacuum UHV (e.g. essential for surface science) are distinguished, but the values can be taken as a rule of thumb.

4 4 As vacuum is described as reduced pressure it might be worth a thought, what actually defines pressure. For detailed theory the dedicated reader is referred to according literature [5], as for our practical applications a few basic concepts will do the job. According to the ideal gas law pressure can be expressed by p = nrt / V = Nk B T/V = ñk B T p is the absolute pressure of the gas n is the amount of substance T is the absolute temperature V is the volume R is the ideal gas constant N is the number of particles k B is the Boltzmann constant ñ is the particle density per volume. The law from Boyle and Mariotte shows that for a constant temperature and amount of substance the product of pressure p and volume V remains constant p 1 V 1 = p 2 V 2. According to Daltons law the total pressure p in a given volume is either the pressure of a pure gas or the sum of the partial pressures of different gaseous species (vapours and gases) in a mixture p total = p 1 + p 2 + : : : = Σp i (e.g. air, see composition in table 3). Table 3: Composition of air at normal pressure and partial pressures of the components [6]. A more figurative view of pressure is provided from the kinetic gas theory which basically characterizes pressure p by the average force F in Newton (or the rate of normal momentum transfer) exerted by gas molecules impacting on a surface of unit area A (in m 2 ). Therefore, pressure can be expressed as: p = F /A (Pa). Thereby some assumptions are made: 1. Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion. 2. These particles move in a straight line until they collide with another particle or the walls of the container. 3. These particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space. 4. There is no force of attraction between gas particles or between the particles and the walls of the container. 5. Collisions between gas particles or collisions with the walls of the container are perfectly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container. 6. The average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else. Figure 1: Random move of gas atoms in an enclosed volume (left), demonstration of one gas atom moving in x-direction in a cube of side length L (right). [7]

5 5 According to Newton, force F is the time rate of change of the momentum ᵽ (not to be mixed up with pressure p) F = dᵽ/dt = ma. The momentum change is equal to the momentum after collision mv minus the momentum before collision mv. Accordingly we get ᵽ = mv x (-mv x ) = 2mv x. As the collisions with the wall happen with a frequency f = v x /2L, we get for the force F in x- direction F = ᵽ/ t = ᵽ*f = 2mv x *(v x /2L) = mv x 2 /L and so consequently for the pressure p of a single atom/molecule in the box p = F/A = mv x 2 /V. Taking N atoms/molecules into account (via mean square speed) and all three dimensions we end up with p = Nmῡ 2 /3V. As the speed of the atoms in this model is purely dependent on the temperature with the kinetic energy given E kin = mv 2 /2 = 3/2* k B T, we finally get for the temperature dependent pressure derived from kinetic gas theory p = Nmῡ 2 /3V = N/3V * 2E kin = N k B T/V So, we are back to the ideal gas law (originally phenomenologically derived from thermodynamics). Another term which can be derived from kinetic gas theory and the Maxwell-Boltzmann velocity distribution is the mean free path λ of our gas particles, the distance they can travel before the next collision with eachother after a time τ. Naturally this would be a function of the density of particles ñ, the mean velocity ν and the size d of the gaseous particles λ = ντ = (1/ñπd 2 τ) *τ = 1/ñπd 2. But this is only the formula for one moving particle while the others are all fixed. With introduction of a factor 1/ 2 (which can be non-trivially derived from Maxwell-Boltzmann equation) also the movement of the other particles is accounted for and we use the ideal gas equation in the form p = ñk B T to get λ = kbt / 2πd 2 p. For air for example (for a rough estimation data are taken from nitrogen), the mean free path at room temperature is only 66 nm at normal atmosphere while being 6.7 m at 10-3 Pa [1]. Many interesting applications of vacuum technology depend on the mean free path being much larger than other dimensions in the system. For example, thermal evaporation deposition of metal films onto substrates is only efficient if the mean free path is significantly larger than the distance between the evaporator element and the substrate, often a distance on the order of 10 cm. The relation between the mean free path and the characteristic dimension d of the vacuum system (e.g. a tube diameter) results in a specific flow behaviour. The Knudsen number is a dimensionless measure therefore: K n = λ/d Figure 2: The three flow regimes of gases depending on the atoms/molecules mean free path. Accordingly, if K n <<1 (λ<<d), a gas molecule will encounter many collisions with other molecules during transport between two walls of a container, while when K n >1 (λ>d), the collisions between gas molecules can be neglected and the collisions with walls dominate (Figure 2). The regimes are called viscous/continous flow (K n <<1), Knudsen flow (K n <1) and molecular flow (K n >1).

6 6 If there is a flow of gas, it has to be distinguished between dynamic and static pressure (i.e. static if gauge reading of the pressure is stationary, but dynamic, if it moves with the same velocity as the flow). Similarly, one speaks of a steady-state pressure, if the pressure at different locations in a vacuum system remains constant with time. So, if pressure is indeed the sum of forces from a bombardment of the recipients walls by the atoms/molecules of a gas and vacuum is reduced pressure, then the task of vacuum technology must be the reduction of the number of atoms/molecules from the system. This job is done by all sorts of pumps (see next chapter). The gas removal is quantified by three characteristics: the pumping speed, the gas throughput and the pumping capacity. The pumping speed S = dv/dt (or V/ t) refers to the volume flow rate of a pump at its inlet and is measured in volume per unit of time (e.g. L/s, m 3 /h). It might vary with the composition of the gas pumped. The throughput q PV refers to the pumping speed multiplied by the gas pressure at the inlet, Q = S p = (d/dt) pv (=(d/dt) mrt), measured in units of pressure volume/unit time (e.g. mbar L/s). Similar to electrical circuits also vacuum systems are characterized by a sort of conductance their ability to transport gas. This conductance C of a vacuum system depends on the geometry of the vacuum chamber, its surface, length and diameter of plumbing, valves bends and traps. Omnipresent loss mechanisms in vacuum systems are leaks, occurring at every kind of connection to the outsideworld. In those cases, throughput refers to the volume leak rate multiplied by the pressure at the vacuum side of the leak, so the leak throughput can be compared to the pump throughput. One last important feature, especially of interest in ultrahigh vacuum technology is the recovering time τ. It describes the time required for the adsorption of a monolayer of atoms or molecules on a formerly gas-free surface. Assuming that every particle sticks to the surface this is p τ = const 10-6 mbar s. A last remark: It is a common misunderstanding that vacuum pump sucks gas from a chamber. Until an atom/molecule, driven by random collisions, enters the pumping mechanism of a pump, it cannot be removed from the chamber. The pump does not reach out, grab a molecule from outside and suck it in. When gas molecules in one section of an enclosed volume are removed (e.g. at the entrance of the pump), molecules from the remaining volume will occasionally migrate to the lower pressure space, via their normal random flight, colliding and bouncing off walls and eachother. 3.2 Vacuum Generation Figure 3: General build of a high vacuum system and associated function. [2] Common high vacuum systems are built from a vacuum chamber (also called recipient) connected to a systems of pumps. Usually a roughing pump is used two ways: to create a pre-vacuum before the high vacuum pump is started and to carry away the exhaust gas from the high vacuum (or

7 7 secondary) pump. As it is handy to be able to open the vacuum chamber (e.g. for changing samples or modifications) without turning the pumps off, a range of valves are installed, serving to separate parts of the system, to evacuate or vent them. If for instance the recipient had been separated from the rest of the system and vented, it needs to be pre-evacuated again with the roughing pump, before opening the valve to the high vacuum side (which otherwise would cause heavy damage to certain high vacuum pumps), for that purpose these systems usually have a bypass between roughing pump and recipient. A schematic view of the common built of such a vacuum system is shown in Figure 3. Pumps can be divided into three functional types Positive displacement pump (capturing, compressing and expelling the gas molecules): Mechanical pumps Momentum transfer pump (giving the gas molecule a preferential direction): e.g. Diffusion pump, Turbo-molecular pump, Aspiration pump Adsorption or reaction pump (Capturing and keeping the gas molecules): e.g. Cryopump, Sorption pump, Ion pump, Evaporative getter pump, Absorption pump, Getter pump Positive displacement (mechanical) pumps are used for low to medium vacuum applications, the most common ones are the rotary vane pump and the diaphragm pump. Function of rotary vane pump (figuren 4 top): The rotary vane modules are immersed in an oil bath and driven by a motor with about rpm. The oil serves as coolant, lubricant and sealing. The pumping process works in 4 stages: intake, transfer, compression and exhaust (see Figure 4 bottom). Rotary vane pumps are available either singular (single stage) or two (or more) compression cycles (double stage) in series. Different to the diaphragm pump (described in the following), rotary vane pumps should not be used to pump continiously at high inlet-pressure (as gas transport pump), as gases and moisture might condensate in the compression process and lead to corrosion and damage of the pump. Figure 4: Built of a rotary vane pump (top) and the four stages of its function (bottom). [8][9]

8 8 Function of a diaphragm pump: A diapragm is lifted and lowered by a motor, thereby opening/closing inlet and outlet valve accordingly, to get gas in and exhaust gas out (see figure 5). These pumps are mostly used to support gas transport. Figure 5: Function of a diaphragm pump. [1] Momentum transfer pumps are used in medium to high and ultra high vacuum applications. The most commonly used ones are the oil-diffusion pump and the turbo molecular pump. Function of a oil-diffusion pump: The pump oil reservoir on the bottom of the pump is heated and creates an oil vapour which is ejected with high speed through nozzels on different levels of the pump. Thereby the oil vapour droplets/molecules (which are heavier and larger compared to general gas molecules) force the gas molecules by collisions downwards within the pump (see figure 6), producing a vertical pressure gradient with an enrichment of gas molecules at the bottom. At its exhaust it needs to be carried away by a backing pump. As the walls of the pump and the exhaust are cooled the oil vapour condensates and runs back into the reservoir. These pumps are advantageous because they hardly need any maintenance. Oppositely the danger of getting oil vapour accidentially into the apparatus is a drawback. Figure 6: Built and function of an oil-diffusion pump. [2][9]

9 9 Function of a turbomolecular pump: An arrangement of turbine blades is rotating at high speed (up to 60000rpm), another set of blades is installed stationary at the walls of the housing (stator). As the blades have a speed near molecular motion and particular angles they hit the gas molecules i.e. transfer momentum onto them to force them progressively down towards the exhaust. Also this pump needs a backing pump at the exhaust. The turbomolecular pump is a delicate system, the thin blades move with a high speed, any imbalance can cause heavy damage to the pump. Especially a large pressure gradient between the top and bottom of the pump causes a large amount of gas suddenly streaming into one end of the blade system, leading to an imbalance in the rotor system, which at that high speed transfers a lot of force onto the housing. For this reaon it is built extremely robust. Though the blades keep contained on those occassions they will be destroyed. Turbomolecular pumps are also not meant to pump at high pressure for a longer period, as the large amount of blade-gas collisions heats up the pump and might damage it. Figure 7: Built and function of a turbomolecular pump. [10][9] Adsorption or reaction pumps are used for high to ultrahigh vacuum applications. The biggest disadvantage of these pumps is that they enrich with captured gas molecules over time and accordingly require regeneration cycles. The most commonly used one is the cryopump. Function of a cryopump: A cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface. They are only effective on selective gases, depending on the freezing and boiling points of the gas relative to the cryopump's temperature. In a very simple form they are used sometimes to block particular contaminents, for example in front of a diffusion pump to trap backstreaming oil. In this function, they are called a cryotrap or cold trap, even though the physical mechanism is the same as for a cryopump. Cryopumps are commonly cooled by liquid helium or liquid nitrogen, or stand-alone versions may include a built-in cryocooler. Over time, the surface eventually saturates with condensate and the pumping speed gradually drops to zero. It will hold the trapped gases as long as it remains cold, but it will not condense fresh gases from leaks or backstreaming until it is regenerated. Saturation happens very quickly in low vacuums, so cryopumps are usually only used in high or ultrahigh vacuum systems. Regeneration of a cryopump is the process of evaporating the trapped gases. This can be done at room temperature and pressure, or the process can be made more complete by exposure to vacuum and faster by elevated temperatures.

10 10 Figure 8: Built and function of a cryosorption pump. [11][12][2] The pressure ranges of most available pumps can be seen in figure 9. Figure 9: Pressure ranges of vacuum pumps. [13]

11 Vacuum Quantification Figure 10: Pressure ranges of common vacuum gauges. There is a wide range of instruments for pressure measurement in vacuum applications. None of these is capable of covering all vacuum ranges; accordingly on most vacuum systems combinations of two (or more) gauges are used. As all gauges can only deliver total pressures, partial pressures must be determined with mass spectrometer-based systems. Gauges are divided into two groups. The first group are the direct/absolute gauges, which measure pressure classically via the force-per-area (liquid and mechanical vacuometers). These methods are independent of the type of gaseous species, as they only depent on the number of particles (see theory part chapter 3.1). The second group are the indirect gauges, which determine the pressure from measuring a gas concentration - more precisely, a density dependent property (thermal conductivity, probability of ionization, electrical conductivity); these are dependent on the gas. They are usually calibrated on one specific sort of gas and need correction factors if other gases are involved. The function of vacuum gauges is based on one of the following mechanisms: Pressure exerted on a surface with respect to a reference (e.g. capacitance manometers) Thermal conductivity of system gases (e.g. thermocouple and Pirani gauge) Ionization and collecting of ions (e.g. hot and cold cathode ionization gauges) The most commonly used ones are the - mechanical-type capacitance diaphragm manometer Baratron (10 +5 to 10-6 mbar, accur. ±0.2-2%), - thermal conductivity-based Pirani gauge (operating 1000 to 10-4 mbar, accur. ±5%) and thermocouple gauge - hot cathode ionization Bayard-Alpert gauge (10-1 to 10-9 mbar, accur. ±1%) - cold cathode ionization Penning gauge (10-3 to mbar, accur. ±1%).

12 12 (a) (b) (c) Figure 11: Schematics of the five most common vacuum gauges. (a) Baratron, (b) Pirani, (c) Thermocouple, (d) Bayard-Alpert and (e) Penning gauge. [14][6] (d) (e) A capacitance manometer contains a thin metal diaphragm that is deflected when the pressure changes. The deflection is sensed electronically via a change in capacitance between the diaphragm and one or more fixed electrodes. One side of the diaphragm is maintained at a reference pressure, and the measured capacitance therefore allows an absolute pressure to be determined. Pirani gauges contain a metal wire that is heated by an electrical current. At the same time, collisions with the surrounding gas carry heat away from the wire and cool it, with the net effect being that the wire temperature settles at some equilibrium value. If the pressure is lowered, heat is carried away less effectively and the temperature of the wire increases, while an increase in pressure leads to more effective cooling and a decrease in the wire temperature. The temperature of the wire may therefore be used to measure the pressure. In practice this is achieved by monitoring the electrical resistance of the wire, which is temperature-dependent. Thermocouple gauges work in a very similar way to Pirani gauges, except that a thermocouple is used to measure the temperature of the wire directly, rather than inferring the temperature from a measurement of the resistance. The Bayard-Alpert gauge consists of a heated filament that emits electrons, an acceleration grid, and a thin wire detector. Electrons emitted from the filament are accelerated towards the grid, and ionise gas molecules along the way. The ions are collected at the detection wire, and the measured ion current is proportional to the gas pressure. This type of ionization gauge has the advantage that there is a linear dependence of the ion current on the gas pressure. Like any ionization gauge, correction factors need to be applied for different gases to account for differences in electron-impact ionization probability. The Penning gauge works on a similar principle to a hot cathode gauge, but the mechanism of ionization is somewhat different. There is no filament to produce electrons, simply a detection rod (the anode) and a cylindrical cathode, to which a high voltage (~4kV) is applied. Ionization is

13 13 initiated randomly by a cosmic ray or some other ionizing particle entering the gauge head (this occurs more frequently than you might think!). The electrons formed are accelerated towards the anode. A magnetic field causes them to follow spiral trajectories, increasing the path length through the gas, and therefore the chance of ionizing collisions. The ions are accelerated towards the cathode, where they are detected. More free electrons are emitted as the ions bombard the cathode, further increasing the signal. Eventually a steady state is reached, with the ion current being related to the background gas pressure. The relationship is not a linear one as in the case of a hot cathode gauge, and the pressure reading is only accurate to within around a factor of two. However, in its favour, the Penning gauge is more damage resistant than a hot cathode gauge. 3.4 Thin Films General Terms and Properties A thin film is a microscopically thin layer of material deposited on a carrier (substrate) as it is (normally) not capable of self-standing. The thickness of a thin film usually ranges from a couple of nanometers up to 1 micron. The material can be a metal, ceramic, semiconductor or organic and accordingly it can be conductive or dielectric (non-conductive), transparent, absorbing or reflective. A main characteristic of thin films is that the surfaces (and interfaces) have a considerable influence on its physical properties, oppositely to thick films (> 1micron) and bulk materials which show usually the bulk properties of a material. Traditional single crystal based technology is quite costly and complex, as it requires the growth of defect-free large single crystals of high purity material, to subsequently cut them into discs, followed by time consuming polishing processes to get electronic grade wafers. In recent years it gets more and more replaced by the introduction of thin film technology which brings a range of benefits for today s electronics: lower production costs and energy consumption - less material required - (partially) cheaper raw materials - lower technical effort - less logistics - few and shorter processes (no single crystal growth, no polishing) - lower process temperatures required flexibility in application and design - material can be deposited on almost every substrate - low film thickness enables thin and flexible bendable electronics Certainly also thin films have to meet specific requirements, to be adequate for applications in electronics and/or optics. Thereby the focus is on properties like - film uniformity - film quality: stress, adhesion, density, defects, stoichiometry, grain size, boundary property, orientation - electrical film properties: breakdown voltage, impurity/dopant level, spatial preferences 3.5 Thin Films Deposition Methods Vacuum-based The vacuum-based techniques can be sub-devided into two fields:

14 14 In physical vapour deposition (PVD) the film is formed by atoms directly transported from source to the substrate through a gas phase. Common methods with this type of deposition are Evaporation (thermal evaporation or e-beam evaporation) Sputtering (DC sputtering, DC Magnetron sputtering, RF sputtering) Reactive PVD During chemical vapour deposition (CVD) a film is formed by chemical reaction on the surface of substrate. The following methods use this type of deposition process. Low-Pressure CVD (LPCVD) Plasma-Enhanced CVD (PECVD) Atmosphere-Pressure CVD (APCVD) Metal-Organic CVD (MOCVD) Atomic Layer Deposition (ALD) As it would be beyond the scope of this script to introduce all above mentioned techniques, we focus on the most prominent PVD-based deposition techniques, which are thermal evaporation and DC diode sputtering. Below (figure 12) you see schematics of a typical thermal evaporation setup (a) and a sputter coater (b). A thermal evaporator consists of a vacuum chamber with a resistance-heated evaporation source on its bottom, a substrate holder in the top (heatable), which is mostly equipped with a shutter unit (for better deposition control) and a quartz crystal monitor for in-situ thickness determination (sometimes water-cooled). The evaporation sources come usually in forms of boats, crucibles (plus metal holder), boxes, baskets or wires, made from a high temperature stable metal (tungsten, molybdenum or tantalum) with an infinite number of different designs (see fig. 12c). The chamber is evacuated down to pressures of about 10-6 mbar (or below) and a voltage is applied to the source. The onset of evaporation is detectable by a frequency shift of the crystal monitor and a short raise of chamber pressure. After adjustment of an adequate evaporation rate, the shutter to the sample is opened and a layer deposited, controlled by the quartz crystal monitor. Materials which can be deposited with this method are metals, organics and low evaporation temperature inorganic compounds, as the achievable source temperature with the resistance heating is limited to about 1800 C. Higher temperatures can be reached with an e-beam evaporator (up to 3000 C) which enables a larger range of materials.[16] (a) (b)

15 15 Figure 12: Typical design of a thermal evaporator (a) and a DC sputter coater (b). Below the sources of a thermal evaporator (c) and of a sputter coater (d) are shown. [15] Also a DC sputter coater is built within a vacuum chamber, though it needs argon for operation, its pressure has to be low enough to enable the sputtered material to reach the substrate (mean free path, see Chapter 3.1). Here, the substrate holder (anode) is located on the bottom of the chamber, while the sputter target/source (cathode), is mounted at the top of the chamber. The target is basically a mountable metal plate with a coating (2 mm-14 mm thick) of the desired high purity material. A high voltage (~2-5 kv) is applied, generating an electric field between anode and cathode. Then a low pressure argon atmosphere is established within the chamber. Free electrons in the chamber are accelerated by the E-field and collide inelastically Ar atoms, causing ionization of Ar to Ar + with generation of a secondary electron. The generated Ar + ions are then accelerated towards the cathode target and impact, thereby blasting-off target atoms. These released target atoms then condensate on the substrate. As the ionized argon recombines occasionally with free electrons a glowing is visible (glow discharge plasma). Unfortunately this method qualifies only for metal deposition as otherwise an opposing electrical field is generated, hindering the process. [15] The table below concludes the characteristics of different vacuum-based methods (table 4). (c) (d) Table 4: List of specific characteristics of different vacuum-based thin film deposition methods. [16] Non-vacuum-based Compared to traditional vacuum based deposition methods, some of the non-vacuum deposition techniques are rather new - at least to electronic applications. However, some fields, like organic electronics, are unimaginable without them. The most common non-vacuum-based methods are: - Oxidation, - Electroplating - Spin coating - Dip coating - Doctor blading - Spray (pyrolysis) deposition

16 16 - Drop casting - Printing Though all these mentioned methods are used for thin film electronics, a detailed description of each one of them would be beyond the scope of this script. Therefore only short overviews of the most important ones: Oxidation This one is certainly not new and probably the best self-running thin film growth existing. Many metals (like aluminium) and semiconductors (like silicon) are subject to unavoidable oxidation. In Si-based transistors this is used as one part of the process, as the silicon dioxide layer generated on top of the silicon serves as the gate dielectric. Oxidation can be accelerated/triggered by use of oxidizing chemicals or an oxygen-plasma-etcher. Spin coating In this method the substrate to be coated is placed on the spinner chuck and fixed by a low vacuum. After a drop of material is deposited on the substrate, the spinner chuck is put on rotation with high speed (usual rpm). By centrifugal forces the liquid spreads and covers the substrate homogeneously (in the optimum case), the excess liquid flies off. After an liquid layer is established, the film begins to evaporate the solvent and the film solidifies. Though this appears to be quite simple, the film forming processes are complex and equilibrium based and so very sensitive to the substrate surface (morphology and energy), the solvent/s (viscosity, surface tension, boiling point, vapour pressure), the solute (surface energy, concentration), the environment (gas, temperature) and time (acceleration, speed). The thickness of a spin coated film for instance, can be altered by the concentration of the solution, the boiling point of the solvent, the spin speed and the temperature. Figure 13: Spin coater and schematic of the spin casting process. [17] Dip coating In the dip coating process a substrate is fixed at a vertical lifting stage. The substrate is dipped into the according solution of the material to be deposited and is subsequently withdrawn from the solution very slowly. The solid film is formed at the very end of the liquid meniscus a kind of drying zone (see figure 14). Oppositely to spin casting, which is the major process used currently in solution processible electronics research, dip coating is also applicable for larger substrates, thus of higher interest for industrial use. The disadvantage of dip coating is that the coating process takes quite long, depending on the evaporation rate (vapour pressure) of the solvent, and is consequently affected easily by air turbulence, vibration and shocks. Furthermore, the decreasing concentration during the coating process can be an obstacle, especially if the amount of dipping solution is not several magnitudes larger than the amount evaporated/deposited during the process.

17 17 The film forming properties themselves are affected by the same factors as in spin coating, apart from the spin-speed which is replaced by the retraction speed. Figure 14: Dip coating apparatus and film forming process. [18] spray coating Also this method is applicable on large areas. Basically a spray gun is used to deposit fine droplets of solution on a substrate. The challenge here is to achieve a homogeneous thin film. The method is mostly used for the deposition of transparent conductive oxides. Therefore the droplet size has to be small enough (to obtain optical transparency, avoid scattering grains ) and the solvent has to dry off as soon as the droplet hits the substrate. Accordingly the substrate is heated in most cases. Figure 15: Automated spray gun and schematic spray coating process. [19] printing Printing is so far the most promising approach to large area solution-processed optoelectronic devices on flexible substrates and seen as a serious alternative to conventional multistep lithographic methods. Therefore the printing techniques developed in the last 10 years are quite versatile. One differentiates between a large range of sub-techniques, e.g. screen printing, flexography, gravure, offset lithography and inkjet printing.

18 18 Figure 16: Printing techniques in solution processed electronics, examples: printed structures on PET foil (top left), inkjet printing droplet formation and printed circuit (top right), flat and rotary screen printing (middle right), gravure printing schematic and a resulting flexible OLED display. [20] 3.6 Thickness Determination of Thin Films non-optical e.g. - weighting - quartz monitor - imaging (AFM, SEM) - stylus profilometer optical e.g. - interference - transparency or reflectivity measurement Methods applied during this course are the quartz monitor and optical transparency and will be discussed in more detail in the following. The quartz monitor actually operates by measuring the shift of the resonance frequency of an oscillating quartz crystal plate, due to mass gain by deposition of a thin film. A relation between the resonance frequency shift f and this mass gain m was initially suggested by G. Sauerbrey (1959) [22]: The Sauerbrey equation is defined as (approximation for thin rigid films, f/f < 0.02, m F <<m q ): f = 2 mf A ρ µ q q = f ρ µ q 2 0 q m = C A SB m A f 0 Resonant frequency (Hz) (typically between 5 and 10 MHz) f Frequency change (Hz) m Mass change (g) A Piezoelectrically active crystal area (Area between electrodes, cm 2 ) ρ q Density of quartz (ρ q = g/cm 3 ) µ q Shear modulus of quartz for AT-cut crystal (µ q = 2.947x10 11 g/cm.s 2 ) C SB Sauerbrey constant or Sensitivity factor

19 19 The Sauerbrey constant depends on the quartz cut, for an AT-cut (see figure 17) 5MHz quartz 2 Hz cm crystal it is C SB =56.6 µ g this is then related to the film thickness via C m0 - Sauerbrey constant ρ F - film material density h F = film thickness m= ρ F Ah F Figure 17: Quartz monitor and crystal (left), schematic of the concept (right). [21] Optical measurements not only via interference methods - can be a useful tool for thickness determination. One simple way is by optical absorption. A calibration curve is needed, which is created by optical measurement This is easy if there are a couple (2-4) films of that material with known thicknesses, which can serve as a calibration curve. Because the law of Lambert-Beer says A= log I I α l = = αl = εlc (A ist he absorbance, I 0 is the initial incidence, I 1 the transmitted incidence, α the absorption constant and l the film thickness) As the density of absorbers of a thin film of the identical material (and treatment) stays (about) constant, this implies that the absorbance goes linear with the thickness of the film. So this can be used for extracting the thickness of an unknown film from an absorbance measurement by comparison to a calibrated thickness-absorbance series. 3.7 Photodiodes Figure 18: Schematic built of a thin film photodiode, a manufactured example and typical IV curves in the dark and under illumination. [23]

20 20 A photodiode is probably one of the simplest electrical devices which can be made using thin film technology. The schematic structure of such a device is shown in figure 18 (top left). A glass substrate is coated with a transparent electrode (anode), then a photoactive layer (in our case a blend of organic semiconductors) and finally a reflective back electrode (cathode). The result can look like the one shown in figure 18 (bottom left). To characterize such a photodiode, the easiest way is to measure its current-voltage behaviour, by contacting one electrode to the transparent anode and the other to the reflective cathode. IF the device works as it should, meaning: 1) it is not shortened, 2) there is a contact at all, 3) and in the optimum case a current is generated during illumination, THEN the achieved IV-curves should look similar to those in figure Literature, Electronic Resources [1] [2] Surface%20Engineering%20and%20Coatings/Surface%20Engineering%20and%20Coatings%20pdf/ Surface%20Engineering%20and%20Coatings%20pdf/L02a.pdf [3] [4] F. Weber, Dictionary of High Vacuum Science and Technology, American Elsevier Publishing Co., New York, 1968 [5] John F. O'Hanlon, A user's guide to vacuum technology, Wiley-Interscience; 3 edition (2003); Dorothy M. Hoffman,Bawa Singh,John H. Thomas, Handbook of vacuum science and technology, Academic Press; 1st edition (1997) [6] VacuumSystems.pdf [7] [8] Technical_Articles/Mechanical%20vacuum%20pumps.pdf [9] [10] [11] [12] [13] [14] [15] [16] %20Deposition-1.pdf [17] [18] _04_blaschke.pdf [19] DOI: / /19/6/019 DOI: /

21 21 [20] [21] [22] G. Sauerbrey, Z. Phys. 155 (1959) 206 [23] Scientific Talk, Dr B. Friedel, MRS Fall Meeting, Boston, 2008