Appunti completi surface technology

Surface Source and Deposition

The distance from the source. So, in the point source we assume that our source is concentrated in a single point and atoms are emitted in the space with a uniform distribution. If the substrate is not coincident with the tangent plane we have to consider a projection factor, since atoms collide within dA and then are distributed in a large area dA.

Cs surface source. The mass deposited per unit area from a depends on two angles, the emission angle φ and the incidence angle θ:

Boat sources and wide crucibles approximate surface sources in practice.

So we have the same probability of deposition for different object position, only if the radius of these objects are small compared to the sphere, otherwise we have to consider the different deposition between the edge and the central position.

In many cases a cos φ dependence is more realistic:

When the number n is large, the vapor flux is highly directed; n is related to the crucible geometry and increases with the ratio of.

the melt depth (below crucible top) to the melt surfacewidth (h/w).

3.1.4 Thickness uniformity

The film thickness d is given by

where ρ is the density of the deposit.

Formulas reported above can be used to calculate the thickness distribution:

d is the maximum film thickness (l=0), q=3/2 for a point0source and q = 2 for a surface source.

It is apparent that less thickness uniformity can be expected with the surface source.

Two principal methods for optimizing film uniformity over large areas involve varying the geometric location of the sources and interposing static and rotating shutters between evaporationsources and substrates (e.g. cylinders). In the last we eliminate part of the evaporating flux reducing the maximum in the layer thickness curve for a 0,6 and 0,7 ratios.

Computer calculations have proven useful in locating sources and designing shutter contours to meet the stringent demands of optical coatings (thickness deviation of ±1%).

3.1.5 Conformal coverage

Because of

3.1.6 Film purity

The chemical purity of evaporated films depends on:

• the level of impurities initially present in the source;
• contamination of the source from the heater, crucible or support materials;
• residual gases present in the vacuum system.

As regards the residual gases, the atomic

fraction of the impurity (x ) is given by the ratio between the flux of the gas molecules and the flux of the vapour atoms:

where A is a coefficient, P is the partial pressure of the gas impurity, M and M are the molar masses of the vapour and impurity, respectively, ρ is the film density and d is the deposition rate (thickness/time).

So, we can reduce the residual gas with a higher vacuum level and deposition rate.

The impurity fraction reported above is probably overestimated, because the sticking coefficient of gas species can be about 0,1 or less, however the effect of the process parameters is correctly predicted.

To produce very pure films, it is important to deposit at very high rates while maintaining very low background pressures of residual gases such as O₂, N₂, H₂O and CO₂.

Both these requirements can be fulfilled in vacuum evaporation, where deposition rates from electron-beam sources can reach 0,1 μm/s at chamber pressures of ~10 mbar.-8

Instead, very high gas

1. incorporation can occur at high residual pressure (~10 mbar). This fact is exploited in reactive evaporation processes, where intentionally added gases promote reactions with the vaporizing metal and control the film stoichiometry.
2. Evaporation sources
• Common heating techniques for evaporation include:
• resistive heating,
• electron beams,
• inductive (rf) heating.
• Ideally, heaters should:
• reach the vaporization temperature while having a negligible vapor pressure (otherwise we have vapors of the heater with the source ones),
• not contaminate the evaporant by reaction or alloying,
• not release gases such as O₂, N₂ or H₂ at the evaporation temperature.
• Refractory metals (W, Ta, Mo) and graphite are typical resistive heater materials.
3. Resistively-heated sources

In this sources we pass a current through our heater, then by joule effect we have heat dissipation and an increasing in T of heater and material itself.

by taking a filament of W e.g., then it is dipped in a bath of the molten metal to be evaporated. Therefore we extract the filament with a thin layer of the metal to be evaporated and we put this inside our chamber connected to a power supply system (low V and high I in order to have electric power converted into heat), so having the evaporation of the external material layer.

In principle it is very useful if the molten metal is able to wet the filament, so it can distribute spontaneously over the entire surface of the filament, otherwise we can use a basket.

For this reason, helical coils are used for metals that wet tungsten readily (the molten metal is retained by surface tension forces), whereas conical baskets are better adapted to contain poorly wetting materials.

Refractory metal sheets (W, Ta, Mo) are used to produce a variety of shapes, including dimpled strips, boats and deep-folded configurations (for a large amount of evaporated material). Strips are a very good example of surface.

Sources with low values of h/w (n->1). Crucibles commonly consist in cylindrical cups composed of oxides, graphite or refractory metals need when we have no compatibility with the material to be evaporated and the heater material. These crucibles (that contain the material to be evaporated) are normally heated by an external W wire wound closely around them. High-frequency induction heating is also used.

In sublimation sources, the solid material is frequently a sintered body, which is heated by resistance heating, direct contact with a hot surface, or radiant heating from a hot surface. Generally, sintered materials are porous and have appreciable outgassing. Shutters (a plate usually) can be used to isolate the substrate from the source and allow outgassing of source material without contaminating the substrate.

03-04-203.1.7.2 Electron-beam sources Disadvantages of resistively-heated sources include possible contamination by crucibles, heaters and support materials and the

limitation of relatively low power input (T < ~1500°C). As a result, deposition of pure films or evaporation of materials with high melting point may be difficult.

Electron-beam heating eliminates these drawbacks and is necessary for the evaporation of most ceramics, glasses, carbon and refractory metals.

When vaporizing solid sources of electrically insulating materials, local charge build-up can occur on the source surface leading to arcing that can produce particulate contamination in the deposition system (that is not desired). In this case we can use a pulse electron beam in order to leave time for the surface charge to accommodate on our source material.

The vaporizing charge is typically placed in a water-cooled copper hearth, where a small amount of the source material melts or sublimes.

The effective crucible is the shell of unmelted material next to the cooled hearth, hence the evaporant is not contaminated by copper. Multiple-source units are used for sequential or parallel

deposition of more than one material. Electrons are thermoionically emitted by a heated filament and are accelerated by high voltages (10-20 kV). Electric and magnetic fields can be applied to focus and deflect the electron beam onto the source material. The electron beam source is located below the crucible in order to avoid that the material emitted from the source could be captured by the electron beam source.

3.1.8 Vacuum evaporation configurations

The principal components of a batch-type vacuum deposition chamber are shown in this illustration. The substrate holder is moved to randomize the substrate position and angle with respect to the direction of the vapour flux. This results in improved surface coverage and more uniform thickness distribution.

Substrates placed as close as possible to the vaporization source will intercept the maximum amount of evaporated material (better material utilization), but can result in excessive heating during deposition.

Generally, vacuum evaporation chambers are

largebecause a long distance between the heated source and the substrate is necessary.

In some special cases (e.g. web coating), the source-substrate distance may be short becausethe substrate is moving rapidly.

A shutter is usually interposed between the source and the substrate in order to catch thecontaminants emitted from the source in the first transient stage. Then it will rotate and thedeposition can start.

Process control

Important process variables are:

• substrate temperature;
• deposition rate.

Thermocouples (that can gives a continuous measure) embedded in the substrate fixture give aninaccurate indication of the substrate temperature if this one rotate.

Optical IR pyrometers allow the estimate of the temperature if the surface emissivity andadsorption in the optics are constant and known (radiation is measured, so the T), but it needs acalibration. Alternatively, the IR pyrometer can be used to establish a reproducible temperatureeven if the value is not

known accurately (we measure the T with thermocouple and we record the signal with the IR; then we relate these value in order to have a calibration).

The quartz crystal monitor (QCM) is the most commonly used device to control the deposition rate.

Single-crystal quartz is a piezoelectric material; if the voltage is applied at a high frequency (5MHz), the movement will resonate with a frequency that depends on the crystalline orientation of the quartz slab and its thickness.

QCM measures the change in resonant frequency as the film is added to the crystal face.

Ideally the QCM probe should be placed in a substrate position in order to collect atoms as the substrate will do, but in this way we will have a shadowing effect on the substrate itself.