Domande d'esame surface technology

Surface technology questions and answers (a.s 2019/2020)

1. What is free mean path? Considerations of used pressure levels and

deposition methods.

The mean free path () is the average distance travelled by the gas molecules between

collisions

k T

is the Boltzmann constant, is the absolute temperature, is the cross section for the



P -3

collision and is the gas pressure. At pressure below 10 mbar, is so large that



molecules typically collide only with the walls of the vacuum chamber.

Higher is the P and higher will be the number of collisions between the molecules in the

atmosphere, usually if we want to have a controlled deposition we have to avoid this

phenomenon because for every collision we have the loss of E with a low efficiency for

the process. The most part of the deposition methods use low values of P, all the PVD

processes (vacuum evaporation, sputtering deposition, cathodic arc, ion beam deposition

-3 -6

etc) use high vacuum (10 -10 mbar). There are some techniques which perform in

atmospheric condition of P 1mbar-1 bar (atmospheric pressure plasma jets), but is good

to figure out that these techniques usually are used as a complementary option to low P

plasmas systems.

2. Vacuum gauges (especially high vacuum one)

There are 3 distinct devices most used:

Capacitance Vacuum gauge, -5

1) which operates in the range of atmospheric P – 10

mbar. Direct measure of the pressure using a metallic diaphragm and an

-7

electrode(capacitor) with an reference pressure 10 mbar.

Pirani vacuum -4

2) gauge used for relatively low P values (10 -1 bar) heated filament,

it measures the current and so the change in the Resistance of the wire at the

current passage and associate it with the P, it is possible only for low values of the

Kn number (0,01-10) because for this range the rate of heat transfer is prop to the

P. it need to be calibrated with direct meausure since gases have different heat

transfer coeff.

Ionization vacuum gauges -2

3) are used in high and ultrahigh vacuum regimes (10 to

-10

10 mbar), gas molecules are ionized through electrons and the ion current

formed in the chamber are attracted by a grid anode and passing through it they

are collected by a collector. Here the current found out is linked to the presence of

the gas molecule and so to the effective P inside the chamber. There are 2 types of

ionization vacuum: cold cathode, the e- are emitted by a high voltage (3kV) applied

between 2 electrodes, and the hot cathode where the e- are emitted by the

thermionic effect.

3. How do I create vacuum, why using two pumps? example with two choice

pumps and explain them.

I create vacuum taking a well isolated chamber and taking out all the air/gas

molecules, as much as possible when we arrive at the ultimate P, typical for any kind

of pump and chamber condition. As long as is very difficult to reach very high values

of vacuum we can use 2 pumps, one for reaching the medium-vacuum (usually

mechanical pumps) and one to reach the high-vacuum (diffusion and turbomolecular).

One choice of 2 pumps could be using the Rotary pump together with a diffusion

pump.

The rotary pump is considered mechanical pump because it isolate a small quantity of

gas from the system, compress and discharge it to the atmosphere using a rotation

system with a piston or a vane, which transport the gas without letting it turn back.

The diffusion pump utilizes a vapor jet to increase the momentum to the gas

molecules and force them to pump outlet (connected with a mechanical

forepump).This kind of pump utilizes an oil as vaporized material for the jet, this is

carried out in the pump chamber through multistage nozzles. The vapor jet collides

the residual gas molecules, which are driven toward the bottom of the pump and

compressed at the exit. The biggest problem for this kind of pump is the back-

streaming of the oil in the chamber, cold baffles are used to avoid this phenomenon

(cooled by liquid N ).

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4. Why we sometimes need vacuum and how do we create it (all the pumps).

The vacuum is required usually for many deposition techniques in order to avoid the

presence of contaminants gases in the atmosphere, so in this way we have a very

clean working environment, and to have a very controlled deposition from the point of

view of the time and deposition rate. Because we are working on the mean path of the

particles and on the possible collisions which can take place. Higher is the P and lower

will be the mean path of the particles, this will lead to have more collisions between

particles so a higher E dispersion (=scattering phenomenon).

We can use different pumps to create vacuum in a hermetic chamber:

1) Rotary pumps

2) Turbomolecular pumps

As the rotary pumps can be considered mechanical pumps, they use many rotors

mounted in series, each one is made by many blades with different inclination. The rotor

speed is very high, on the range of 24 000 to 90 000 rpm. The gas captures in the upper

stages is transferred to the lower one, in this way all the gas is compressed and taken

out. The compression ratio of a single stage increases exponentially with the product of

(Vb)

the tangential rotor speed and square root of the molecular weight of the gas and

s/b.

also depends on the angle and the ratio The compression ratios are important



because the ultimate pressure is determined by the compression ratio for light gases and

9 3

by the amount of outgassing. (N2 and H2 are 10 and 10 ) Molecular flow must prevail in

the pump(avoid losing momentum transferred by the blades), the pumping speed is

weakly dependent on the molecular mass of the pumped gas, because of the

compensating effect given by higher arrival rate (conductance) of light gases.

3) Diffusion pumps

4) Cryogenic pumps

Cryogenic pumps are capable of creating very clean vacuum in the high-vacuum or

ultrahigh-vacuum range. These pumps rely on the condensation or adsorption of vapor

molecules on surfaces cooled below 120 K. Pump surfaces are cooled by liquid N2 (77 K),

He gas refrigerators or liquid He (4.2 K). In two-stage pumps, the first stage (warmer)

condenses H2O (and CO2) and thermally shields the second stage (colder). An initial low

pressure is required, in order to reduce the thermal load on the refrigerant and to avoid

the accumulation of a thick ice layer on the cryopanels. All gases, except He, condense to

form a solid phase at sufficiently low temperatures (cryocondensation). A temperature of

20 K is low enough to achieve UHV conditions with all gases, except He, Ne and H . The

2

effectiveness of a cryopump is increased very significantly by cryosorption: a gas is

adsorbed on a cooled solid at temperatures even higher than the boiling point of the gas

itself. Adsorbants with high surface area, like activated charcoal or molecular sieves, are

so commonly used.

5. How and why do we regenerate cryopumps.

Regeneration of a cryopump is the process of evaporating the trapped gases. During a

regeneration cycle, the cryopump is warmed to room temperature or higher, allowing

trapped gases to change from a solid state to a gaseous state and thereby be released

from the cryopump through a pressure relief valve into the atmosphere. Most production

equipment utilizing a cryopump have a means to isolate the cryopump from the vacuum

chamber, so regeneration takes place without exposing the vacuum system to released

gasses such as water vapor.

6. Why high vacuum pumps also need a forepump?

The forepump is an auxiliary used by a vacuum pump to be more effective, it supplies a

first stage of exhaustion. In the case of the diffusion pump it helps the compressed gas at

the end to be ejected and avoid the hot oil to be stopped in front (or close) to the nozzle.

In order to push gas molecules towards the lower section of the pump, we need a

sufficiently long mean free path, so we must reduce the pressure, that mean that we

need a preliminary pump. This is the region for which we can’t use diffusion pumps at

atmospheric pressure. Also in the case of cyropumps a forepump is required in order to

have already a low P, otherwise we lose efficiency creating a too thick layer on the wall of

the pump or resulting in doing too many cycles(high time and costs)

6. Why turbomolecular pumps are preferred over diffusion ones, even

though the latter have no moving parts?

Because we can achieve the same values of vacuum but in the case of the

turbomolecular we don’t have the problem of the backstreaming of the oil in the

chamber, this could be a very difficult passage for us because is difficult to obtain a clean

environment of working.

7. Vacuum pumps: all + focus on cryopumps work between liquid N2 and

liquid He.

N2 arrives at 77K(removing H2O and CO2) while liquid He can reach 4,2 K(removing H2

and other gases). The T which has to be reached in order to achieve UHV with almost all

the gasses is 20K, but this value could be increased in the case of the cryosoprtion

( when we have adsorbants on the surface).

8. Why we use high rpm in turbomolecular pumps?

Because this type of pumps give to the gas molecules a high velocity by a momentum

transfer from a fast-moving solid surface, which are the blades of the rotors. The normal

speed goes from 24000 to 90000 rpm. The compression ratio of every single stage

increases exponentially with the tangential rotor speed. The stator is needed to reduce

velocity molecule and let the second rotor to be more efficient.

9. Compression ratio and molecular weight relation.

Compression ratio = ratio of molecules leaving /entering the rotor

When the molecular weight is low the molecules have higher velocities, so they can pass

through the rotor without getting compressed to other rotor, so the compression ratio is

smaller for them. If compression ratio is high the pump works efficiently, if it's small the

energy is not utilized properly so, we say pumping process is not efficient. So if molecules

have low molecular weight pump should run at low rmp. If molecules have higher

molecular weight then pump should run at higher rpm (to push away the molecules to

next rotors pump needs more speed) and to maintain the efficiency of the pump.

10. How to reach high vacuum pressures?

First rotary pump (or another forepump), then another pump can be used in order to

achieve better performance as the turbomolecular pumps/cryopumps. Another

alternative to be used in replacement of rotary pumps are diaphragm pumps

, employed as oil free. This allows

them to be used without added lubrication in contact with the air, so the compressed air

produced can be guaranteed clean.

11. Focus on densification mechanisms in sintering.

Sintering is a kinetic process that leads to the reduction of porosity in a material which

contains voids. In materials technology it is exploited to produce near-net-shape

components starting from materials in powder form.

Densification is a process which provides to increase density and strength of the

compacted powders, by reducing the pore space inside the material. The material

transport is essential in this process, essentially we deal with diffusional type (Surface,

Volume and GB diffusion). Pay attention that the redistribution of material over the

surface (by surface/volume diffusion) will not result in the shrinkage of the compact or in

the reduction of the pore space, they only increase the strength and reduce the sharp

pore contours. Densification occurs when the material is removed from the V between the

particle contact and it diffuse to close the pore.

There are 2 different stages for the sintering process, during the initial stage of sintering,

the neck grow and the radius increases during time with a power law, smaller particles

have a faster neck growth and larger neck radius, grain boundaries develop at the

contact regions, and the grain structure in the particles changes by grain growth or

recrystallization. The microstructure is then characterized by a continuous pore space

with grain boundaries located at the small cross sections of the solid. The decisive part of

shrinkage takes place during an intermediate stage, the second, where the individual

necks and powder particles grow together and cannot be clearly identified any longer.

The pore space then is still a continuous network embedded in a continuous solid. Grain

growth and pore shrinkage (resulting from diffusional mechanisms) are assumed to occur

simultaneously. In the very late stages of sintering (below 10% porosity), isolated and

geometrically well defined pores are present.

Breakaway of grain boundaries from the pores is usually the main obstacle to complete

densification. Local breakaway may result in exaggerated grain growth or secondary

recrystallization. The pressure allows lower temperatures than in pressure less sintering

to be employed, thus normal grain growth can be reduced and exaggerated grain growth,

caused by breakaway of grain boundaries from pores, may be avoided.

12. The effect of the energetic particles bombardment in deposition

process.

In many deposition processes the growing film is bombarded by energetic particles in this

way we have:

-Increased film density

-Modification or disruption of columnar microstructure

-Enhanced adhesion to the substrate

- Improvement in step coverage

But also negative aspects have to be considered:

- Creation of lattice defects (e.g. interstitials and vacancies)

- Increase in residual stress

13. Thermal barrier coating (TBC).

The TBC is usually used in the aerospace field above other coatings, as oxidation resistant

ones (MCrAlY). The most used now days is the Yttria stabilized zirconia one (ZrO2 at 8%)

8YSZ, because it shows a low density, low thermal conductivity, high melting point, good

thermal shock resistance and excellent erosion resistance.

The coating can be applied by plasma spray or EB-PVD. In the first case the coating

shows a lower mechanical strength and so a reduced erosion resistance, but a lower

initial thermal conductivity because of the presence of many voids and high porosity

inside the coating and we know that air gaps are good thermal insulator compared with

zirconia. While the TBC produced with EB-PVD is divided in two zone with different

microstructures: the inner zone is the early part of multiple nucleation and exhibits lower

thermal conductivity (around 1.0 W m-1 K-1). The outer part is characterized by a

columnar grain structure and its thermal conductivity approaches that of bulk zirconia. A

microstructure characterized by periodic interfaces and