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Welcome to the 46th lecture of surface engineering in the previous one, we discussed the process
of physical vapor deposition, where we realized that we can always try toheat a material with
reasonably low boiling temperature and high vapor pressures. So, that it goes into the
vapor state and then ah; obviously, the vapor would like to come back to the ground state
which is the liquid or the solid state. So, in the in the process it actually will
be looking for a colder substrate and get deposited onto the colder substrate and that
is how we can create few micrometer thick layers which we call till films on all kinds
of materials or not not necessarily conducting materials also non conducting materials. The
whole process wasah based on heating the species that we want to coat hm. There are applications
whereexposing material to such high temperature is not always desirable.
In fact, we may require particularly for semiconductor industry we may require certain thin coatings
to be developed which will not be exposed to anythermal activation at all. Because that
may damage theperformance of the device. So, in a situation like that we adopt a specialcategory
of PVD which we call sputtering. Now, sputtering is in in simple words it is
like you arehere is a process, where you scoop the material from the surfaceliterally atom
by atomby way of shooting certain time little projectiles. And in the process those scooped
out species or atoms neutrals will find it is their way on to hm to deposit ontocolder
substrate and that is the process of sputtering. So, let us try and explain what exactly it
is. So,first is we have a chamber we have an enclosed chamber which we whichwe can actually
we need to evacuate ah. So, this is how we can pump down to very low pressure let us
say typically 10 raise to minus 6 millibar pressure . So, after evacuationso there is
no oxygen there is no other reactive gases inside very low pressure . So, now, we actuallyplace
the target. So, so thisisthetarget which from which we want totake out maybe let us say
aluminium or let us saymaybe gold or maybe carbon or some otherwhatever is a desired
material. So, we place it over here and thenalso we
have a stage on which we have our material which is to be coated. So, let us say this
is thethis is the component that we want to coat . Now, wecreate an electricalcircuit
such a way that the target is connected to the negative electrode the cathode and component
to be coated is connected, or is actuallygrounded or can be considered to be in the positive
side the anode. So, we usually pulse at very high rate and when we pulseelectrical current
at very high rate then during the pulse on time the there will be ah. So, once we create
a very high in the electrical bias inside the chamber and we would have back filled
with certain amount of argon or usually argon. So, the argon gets ionized and these ions
actually get during the pulse on time the positive pulse they gets they get very highly
accelerated charge they move at a very high velocity towards thethe negativelybiased electrode
the cathode. So, these are thethese are theargon ions which arenow energizedat a very high
velocity accelerated towards the cathode. And when a heat the cathode when they heat
the cathode here or here or wherever it is they will be able todislodge some of theatoms
sitting onto the surface. Now we are we already studied that while discussing
structure of solids that the surface atoms are indeed at relatively higher energy state
than the atoms at the core and they have a higher reactivity because they enjoy a 1degree
of freedomcompared to higher 1 degree of higher freedom then the atoms in the surface because
they have atoms missingabove or below the surface. So, these atoms which when they are
heat by these projectiles individual projectiles of argon ions, they are some of them will
actuallyah experience an impact which will have an energy that can over the overcome
the binding energy of the atom to the surface. So, that would allow the those few atoms to
get detached from the surface. And so, they get detached and then they come out. So, when
this process happens then you actually see a number of such atoms from the target surfaceto
come out and theseatoms actuallynow come and deposit onto the surface. So, the so you first
heat the surface with very high velocity these large atoms and these atoms now come anddeposit
on to the surface. So, thistransport can actually be pretty fast.
In fact, the whole if the PVD process takes let us say for a particularto deposit a particular
thickness if it takes an hour here it can be done in few tens of a minute. So, it can
be much faster . So, the important part is that stepwise we first have to evacuate, then
we create thisah highpotential difference between the 2 electrodes, we feed in certain
gas which is usually argon then we create the plasma and now we are ready. So, once
we start pulsing at very high rate thenthe this process ofremoving atom by atom from
the surface and then bringing them onto the vapor or the substrate to be deposited, then
they create a very thin coating . So, the process depends upon this ejection
of atoms from the surface and this surface of the targets. So, which is basically the
source material and the ejected atoms should come and sit on the substrate that we want
to coat. So, the ejected atoms depending upon whether the projectile is heatinghead on 90
degree directly normal or through a grazing angle. The kinetic energy of the of the ejected
atom will greatly depend upon the energy with which the projectile had heat the surface
and made it come out. So, also it will depend upon the distance
which it has to cover from the center to the periphery. So, there could be a wideah distribution
of energy andthis because of thiswide distribution energy there could be variation in thickness
and evencomposition, if it is a if it is deposition if the sputtering is from an alloy. Anyway
we need to create a vacuum and then backfill with an inert gas usually argon. The interface
just like in the previous PVD process this is also nothing, but a PVD process will also
be a sharp. So, you too create a sharp substrate deposit interface .
The sputtered ions which actually are only a fraction of the total number of ejected
ions, will fly ballistically in other words they basically aredependent entirely upon
the energy with which they are heat. So, there is no thermal activation here, andthey will
move in straight lines and then impact the substrate or the chamber wall. So, they can
go and deposit on the chamber wall or they can come and sit onto the substrate whichevercomes
on their path of light. But if theif themomentum is fairly high then
there could be a possibility of re sputtering or re emission of atoms from thesubstrate
on which you are depositing; that means, you are now re sputtering atoms from the substrate
surface which you do not want. So, you have to control the energy very carefully . And
this, so this high energy bombardment needs to be avoided . So, at higher gas pressure
the ions can collide with the gas atoms, which is used as a moderator and diffuse to the
substrate. And when actually it heats and goes and sits
insideit actually cancreatecertaindiffusion gradient and in the process it can actually
move inside the substrate and proceed up to a certain depth . The applications could be
typically in semiconductor industryfor various kinds of functional and decorational coating.
It can beused for surface analysis and also has wide application in fusion plasma and
space physics. So, sputtering is used also to create a conducting coating or a non conducting
coating. So, the applications can be multifaceted ah, but the most of the applications are in
the semiconductor industry anyway. So, so the typical process would require that
as I was saying that this is the target and these are thecomponents on which you are depositing.
Say for example, silicon wafers , you may have an auxiliary heater herewhich actually
allow certain diffusionof the depositand the substrate below and in the process the sharp
interface can be replaced with a fairly diffuse interface with a better bonding.
So, this argon plasma that you create is it covers the entire distance in between except
thin layers, some kind ofah sheet which actually is divide of very high density of these ions.
Now the activation can be through direct current. So, in that case you call it simply dc sputtering
you can also use high frequency inductively coupled radiofrequency plasma. So, it can
be an RF sputteredRFinduced sputtering ah, but the process remains the same that you
actuallywill have a sputtering target whichwill take the heats of these argon ions will allowejection
of these species neutral species which go and sit onon the surface of the cold substrate.
So, you can deposit metal you can deposit plastics or ceramics. So, that is the beauty
you this actually independent of the chemical nature of the coating that you want to deposit.
So, you use low pressure under a magnetic field to concentrate high energy ions this
is typically done in case of magnetron sputtering, where the magnetron high flux magnets actually
confine the plasma in a smaller region. And in the process make the deposit more uniform
andcompositionally morecorrect . So, the the application wise you actually
use it either forcreating a conducting layer or an insulating layer or change the electricalresistance
or the band gap you can also use it for improving the corrosion resistance or wear resistance
and so on. You can even create it just as a possible substitute for let us say, galvanized
you can create actually an cathodic or anodic layer to prevent corrosion and oxidation,
but very thin and the advantage the other advantage is that you you not exposing to
any aqueous path you are not subjecting to any high temperature you are doing more or
less at room temperature and most importantly you actually are able to maintainvery thin
precise layer. So,this isthe space I was talking about which
is called the dark space or the sheet. So, you create a high density plasma all over
so if this is the anode or the positive side on which you have placed your substrates to
be coated. Let us say silicon wafers and this is the target from which you are actuallyahextracting
ions or making theargonions heat the surface and dislodge atoms from this may be aluminium,
may be carbon or any other material. So, there will be a little region in between which will
have very low density and that is typically called the cathode dark space or sheet. the
exact thickness and the location and the width of these region, largely affects the overall
coating thickness and the uniformity of the coating.
One other thing is very important is that the if you can look at this substrate here
or this substrate here they are tiny little much smaller compared to the overall dimension
of the plasma that you create. And this is how you actually can make sure that there
is uniform coverage andno shadowing effectno non uniformity. So, this plasma that we create
is by ionizing a gas which is usually argon which I have already mentioned the reason
why we use argon is because it does not have any solubility in the materials that we are
using either as target or as the substrate .
So, like I was saying that here is a situation where let us say this is an aluminum target
and this is a silicon wafer and we would like to create thin layers of aluminum in certain
portions and not all over the surface. So, where we do not want aluminum to get deposited
we willcover it up with a mask andrest of the regions will be exposed. So, we shoot
argon ion and it dislodges and neutral aluminium and the aluminum arrives onto the surface
like this. It can be a straight one like that or it actually can come through certaincircuitry
path some spiral path, but in the process it actually can heat the the atmosphere insidethe
atoms inside andas since we are dealing with plasma so; obviously, we have these argon
ions, cations and also we have electrons. So, they actually the density of them actually
willvastly control thethe rate and also the uniformity and the composition with which
they arrive. So, this can be controlled like; obviously, one possibility is that you confine
the plasma by way of applyinghigh fluxhighpower magnet and you in typically a magnetron and
then you call it a magnetron sputtering. Similarly the frequencies of the sputtering can be changed
from some normal level of using a normal electrical power supply to an RF couple power supply.
In accordingly there could be certain differences. So, this arrival will create the overall geometry
or overall thickness and uniformity will govern that. So, what is important is that this angle
theta actuallyat which the species is arriving is important. So, they can be uniform with
isotropic arrival distribution or they can be directed with an isotropic arrival distribution.
So that means, in certain cases I want a normaldepositionwith certain area coverage or I can make it an
incline orarrival at an angle and there at the repulsion will be different and that may
be required in a situation like this. So, in this kind of a situation if you want uniform
deposition coming from the top, thenthese trenches here the opening being so small,
may not be able toallow entry of sufficient numberof atomsinto the trenches. And in the
process you get very poor coverage in thebottom of the trench.
On the other hand if you can create a directeddepositionthenactuallyyou actually do see that the deposit here could
bewider or or more uniform. So,with the narrower angle of arrival you can have a better bottom
filling or flooding with a wider angle of arrival you can have poorer bottom filling.
So, this kind of geometry or consideration becomes assumes very importantvery very high
importance when these dimensions are extremely small.
Now, generally when you are talking about a cutting tool or a large manufacturing device
the dimensions are typically in millimeters if not centimeters, but when you are talking
about a semiconductor device maybe as some reflector or maybe some photovoltaic device
or some sensor. There these dimensions could be typicallymicrometer or even less than a
micrometer . So, the arrival direction or the directionality of the flow of these atoms
during sputtering becomes assumes very high importance.
So, in that case we may use either wider or shallowerangle of deposition and that is how
we can make sure that we actually can create wider coverage here . Andand not end up having
incomplete coverage at the bottom of the trench like here. So, depending on the features that
we want to develop and the dimension of the features we have to select the exactarrangements
for deposition . So, there could be various types of sputtering
like I was saying that Radio Frequency couplesputtering;RF sputtering. Magnetron sputtering confined
iron beam sputtering where we actually use an iron beam not just individual argon ions,
butdirected focused ion beam coming onto the target for sputtering. We can usewe can allow
entry of reactive gases and then create some kind of a situation wherewe create an oxide
or a nitride or some other reaction layer onto the surface .
Similarly, deposition could be assisted by an ion beam. So, we actually where the ion
beam is not a part of the same chamber it it is created elsewhere and that ion beam
can actually come andassist the deposition. We can create a separate tart a separateactivationsource
for creating very high target utilization for sputtering also we can have a very high
impulse or using a magnetic magnetron sputtering unit to create a very high impulse for deposition
the deposition rate would be much higher in that case.
So, these are the various possibilities of sputtering the by and large the process remains
the same that we actually createionscreate a plasma we direct the plasma by way ofnegatively
pulse negative pulses electrical pulses forced them to heat the surface of the target dislodge
atoms eject atoms from those targets and those atoms will flyballistically and then come
on to the colder substrate and then deposit. And we actually can depositsmulti layers or
we can have multiple composition orwe need not actually use the same target we can use
multiple targets for example, in in in situations like this instead of having a single target
covering the entire area we can have multiple targets here. And we actually canuse the pulses
let us say this is 1 2 3 we can sequentially create pulsing between this or this or this
and in the process we can hm seeah deposition of 1 2 3 or A B C and in certain sequences
. So, time to recapitulate what we have discussed
is a special type ofPVD technique called sputtering. Wherein we actually use we take the material
to the vapor state not by thermal evaporation, but by hmion ion ion induced ejection, which
is typicallythe process involving high velocity ions heating the target andejecting some of
the atoms in the form of neutral atoms. So, these atoms now fly and then come and sit
and that is how we create a deposit layer. The typical process parameters would be the
distance the the density of the plasma we create whether we are heating any we using
any auxiliary heating processto heat the substrate during deposition hmthen; obviously, the material
that we use it is vapor pressure melting temperature vaporizing temperatures and so on. Then we
alsohave to understand that the one of the biggest advantage is the fact that, we do
not necessarily arethis not an electrolytic process is not an electronic process. But
it is a process where we are using certain discharge and by way of creating a very large
potential difference between two electrodes. So, it can be used in special cases also for
non conducting materials ah, even plastics can be quoted by sputtering provided we have
what we have created certain provisions ofhaving electrodes implanted onto thesubstrate on
which we want to code. So, this is useful that we actually can be hm this is not just
confined only to metals, but these useful to metal ceramicssemiconductors polymers all
kinds of engineering solids so. Thank you very much.
