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So we will continue our discussion on micromachining of silicon. In my last lecture I told you
there are two micromachining processes. One is bulk micromachining, other is surface micromachining.
So out of those two processes, today I will discuss in detail bulk micromachining process
for silicon. So this particular technology there are various chemical etchants we use
for bulk micromachining. So one of such chemical used or etchant used for bulk micromachining
is ethylenediamine pyrocatechol or in short EDP etching. So here we use three chemicals;
one is ethylene diamine, another is pyrocatechol and the third is water. So they are mixed
in a certain stoichiometric ratio and then we go for etching silicon at a particular
temperature.
This particularly technology has got certain advantage. One is it is highly selective over
materials like silicon dioxide, silicon nitrite, chromium and gold. So all these three materials
can be used for masking purpose that means it can be protected. This particular layer
can be used for protection of silicon where you do not want to etch that material, means
silicon material. So it is very good masking material and in this particular technique
EDP etching etch stop technique is very simple; it is not that much complicated. So these
are the two basic advantages for EDP etching. So now some other features are there for EDP
etching. Those features are mentioned here one by one.
Etch rate of the silicon material depends on temperature composition of etchant and
density of atomic bonds on exposed silicon plane. What does it mean by that? Density
of atomic bonds on exposed silicon plane means it will depend on crystallographic orientation
because in different crystallographic plane the atomic density the silicons are different.
That means the etch rate will be different for 1 0 0 oriented plane crystal plane in
silicon 1 1 1 and 1 1 0 the etch rate of these three planes will be different for getting
different structure and shape of silicon microstructure. Now orientation size and shape of the oxide
opening on the wafer surface determine the type of hole formed that I have explained.
I think in my earlier one video graph that complete structures etch structure will depend
on the shape of the opening on silicon dioxide mask. If you want to membrane or if you want
to have the v group or want to have the micro nozzle.
So accordingly you have to shape the masking oxide material and silicon will referential
etch in different crystallographic direction. So as a results of which you will get different
kind of shape in the etch group. So that means your design or mask should be such that at
the end of etch process you will get your desired structure out of the silicon. Now
third one is very thin membrane of uniform thickness can be created by forming heavily
boron P plus layer. That means here the P plus boron layer will act as an etch stop
layer. I will discuss in detail the etch stop mechanism and etch stop process, what are
the various etch stop techniques used in micromachining in my next lecture. Now let us discuss on
the the setup, EDP etching setup.
So in this diagram you can see the laboratory level EDP etching apparatus. It is not commercial.
What we use here, you can see your conical flask is here. So there we put the etching
solution and there is a wafer carrier which is held in a hook here and which is basically
the insulating rubber or some polypropylene or some other hard ceramic material, then
some cork is here. Now inside that the thermometer is there which can measure the temperature
inside the chamber. Now the wafers are like this in this figure, it is just vertically
you can dip the area of wafers into to the solution. Now there are two other things here
you can see, in the right side there is an arrangement which is the condenser arrangement
and the left is another arrangement which can give you the liquid nitrogen will be generated
here and those nitrogen will be flown into the etching chamber.
Now as I mentioned that the etching of silicon in EDP is dependent on the temperature. That
means ion concentration of the etching solution. So to ensure that we have to have a heating
arrangement, so the heating arrangement is here in the heater and thermometer can measure
the temperature, so that you can adjust the temperature by controlling the heater and
you can measure the temperature with the thermometer. Now other thing as I told you just concentration
of etching is also important. That means you have to ensure that concentration of the etching
solution will remain constant throughout your etching process. That means once you make
the solution during etching the concentration may degrade of various reason. Because of
the reactions and at the same time since you are doing this etching at a high temperat0ure
in the range of that temperature is nearly 100 degrees centigrade, 100 to 110 degree
centigrade.
So there some of the etching solution will evaporate and if the etching solution evaporates,
then automatically concentration will increase. So the evaporation has to be stopped. So for
that reason in condensational, arrangement has been made here. So whatever the etching
solution will evaporate, it will because this side is closed top and the only path is through
this and when you will, the vapour of EDP solution, when you will passes through that,
is cold water circulation around that tube. So again it will condense and it will come
back to its original conical container. So the evaporated EDP solution it goes through
there again it condensed and it will come back to its original location. So that, in
that way we can prevent these going out of the EDP etching solution and after evaporation
at higher temperature and so that the concentration of the etching solution will remain constant
and another arrangement here, this liquid nitrogen evaporator has been used here.
The reason is that, this particular etching is highly sensitive to the environment. That
means, environment means, if the etching the solution is done in open atmosphere, so there,
it will have lot of oxygen also along the hydrogen. So if oxygen is there, so this oxygen
gas will oxidize the etching solution, as a result of which the etch rate will vary
in order to prevent the oxidation. What we have to do? The complete etching process should
be done; etching should be done in nitrogen environment. So above the etching solution
the space has to be filled by nitrogen for that the easiest two techniques, either you
connect a gas cylinder, nitrogen gas cylinder and you can pass nitrogen through that. So
that may be little bit expensive. So on easier technique we adopted in our laboratory. That
is a liquid nitrogen bath. So liquid nitrogen is very cheap and easily available in our
institute is no problem.
So here what we have done, you allow the insulated thermocol chamber. There we put liquid nitrogen
and then what has been done? A metal cover, this is basically cylindrical funnel so cylindrical
funnel is inserted and this portion is metallic. So what will happen at room temperature? So
this, it is a metallic, means highly conducting. So that means a room temperature a liquid
nitrogen temperature is very low, you know. So in outside, is a room temperature. So you
because of the temperature difference, the heat will be conducted from through the hollow
metallic cylinder into the liquid nitrogen. As a result of which the liquid nitrogen will
evaporate. So that liquid nitrogen through this cylindrical space it will flow into that,
to this path into these etching chamber. We do not need the high pressure nitrogen inside
the chamber just the environment inside the etching chamber should be nitrogen.
So that is why this is the very simple technique and using that if you do go for etching for
long time the consumption of the liquid nitrogen is not that much. So the nitrogen will flow
slowly into the chamber. So as a result of which inside the chamber it will be in nitrogen
ambient and temperature is controlled here, condensation you are preventing here, so this
is the complete the setup it is a laboratory scale setup and you can for EDP etching.
Now what are the compositions of EDP etching? So the composition of the EDP solution is
given here, 50 mole percent water, 40 mole percent ethylene diamine and 4 mole percent
pyrocatechol. So if you calculate from their formulae, then you will get the 387 cc of
the the ethylene diamine which is liquid, 55 gram of pyrocatechol which is solid and
112 cc of water is mixed and this pyrocatechol is dissolved into the solution and that is
the EDP etching solution and etch temperature we used normally used here in 100 degree centigrade,
etch environment is nitrogen etch rate of 1 0 0 silicon plane is found to be 25 micrometer
per hour. That is the etch rate there.
Now another technique is KOH potassium hydroxide silicon etchant. But EDP although it is a
very simple process, although is a masking of silicon is very simple in EDP etching,
but there are certain problems. What are those problems? Problem is this etching solution
is highly toxic. So you have to have very good exhaust system into the etching room
as well as the complete etching apparatus. What are the vapour come it has to be exhausted
properly otherwise it will be health hazardous. That means this particular solution you have
to have certain specific arrangement for etching for prevention of your health, protection
of your health. That is why many laboratories they do not EDP etching. They go for very
simple user friendly etching solution which is KOH potassium hydroxide is a very well
known solution and is not expensive also and potassium hydroxide is a highly popular as
silicon anisotropic etchant in micromachining of silicon. So here the advantage of KOH is
easy to handle. With KOH you can get a smooth edge profile, but it attack aluminum metal.
So aluminum metal or in some cases gold metal also cannot be used for passivation.
In EDP you can use chromium, gold but not aluminum. Aluminum passivation is not allowed
either EDP or KOH. Next is much higher 1 0 0 to 1 1 1 etch rate ratio. That means anisotropic
is very high 1 0 0 to 1 1 1, the etch rate ratio is large compared to EDP, Silicon dioxide
etch rate in KOH is higher than EDP. That is one advantage. KOH is much useful to etch
deep trenches in 1 1 0 silicon. If you want to have deep trench, so to you need etching
solution whose etch rate is relatively high. So that advantage is than KOH. Now normally
the KOH concentration is used 10 to 50 percent of KOH solution is used for the micromachining
of silicon. In KOH we sometimes add some organic chemical which is isopropyl alcohol and that
isopropyl alcohol will help you getting more selectivity. This particular solution will
improve selectivity, means selectivity with respect to what, with respect to passivation
layer will masking layer and silicon and at the same time selectivity is, that is you
can say anisotropic between different crystallographic planes. So particularly for that reason a
small amount of isopropyl alcohol is sometimes added into the KOH solution.
KOH etch selectivity of 1 1 0 over 1 1 1 crystal plane is much higher of the order of 500 than
that of EDP. As etch selectivity over silicon dioxide is less than 500 at various concentrations
of KOH silicon dioxide etch mask is not adequate for long etching. Here the silicon dioxide
if we will not protect the layer ideally, if you go for long etching. For example if
you, for 4 inch wafer, complete etching whose thickness is nearly 400, 500 micrometer then
silicon dioxide will be attacked by the KOH also. But for say 1 micron, 2 micron or even
say 10 micron, 15 micron, 20 micron etching, it will not create any problem. In that case,
another passivation layer is prescribed for KOH micromachining that is silicon nitride.
Silicon nitride is an effective masking film for KOH etching.
Now other than those KOH and EDP, some other alkaline solutions may be used for silicon.
They are namely sodium hydroxide NH4OH sodium hydroxide NaOH, ammonium hydroxide is NH4OH,
hydrazine N2H4H2O. But, these alkaline solutions are not very much popular because these alkaline
etchants affects metal interconnection lines. Not only than out of these, the hydrazine
is extremely health hazardous chemical. So that is people try to avoid these chemical,
that is why these ammonia is also not user friendly chemical. So because of the hazardous
nature of these chemicals people do not use these chemicals for silicon etching. EDP does
not attack gold, but it does attack aluminum as I mentioned. So gold or chromium gold can
be used as masking layer in case of EDP. So another promising silicon micromachining etching
solution is TMAH tetramethyl ammonium hydroxide. It does not attack aluminum and is a promising
silicon etchant with aluminum masking layer.
So we will discuss little bit on TMAH solution now. So TMAH has come into the micromachining
process late than KOH and EDP. The TMAH basically is a organic etching solution and this particular
etchant has one biggest advantage is that, it does not attack aluminum. That means after
complete metallization of the silicon wafer, that means interconnect lines has been pattern
then you go for micromachining. So the aluminum fine lines which are used for interconnection
will never be attacked or never be disturbed. Because of that reason, we say the TMAH is
CMOS compatible micromachining etchant solution. Many cases now days as I mentioned earlier
also that the sensor and the signal conditioning circuits are fabricated side by side and they
are integrated together. In that case you can fabricate the CMOS signal conditioning
circuit.
Then sensor definition and then at the end you can go for the micromachining or etching
of silicon. In that case you have to protect the CMOS chip interconnect metallization lines.
So people who are looking for a long time, for a particular etching solution which can
be used in presence of aluminum, which will not attack aluminum. Ultimately we found that
TMAH is a very good etchant alternative which is highly CMOS compatible and this is gaining
considerable interest because of its excellent silicon etch rate. Etch selectivity to masking
layers even with aluminum film degree of anisotropy and relatively low toxicity. Because it is
an organic chemical etchant, so it is not toxic also, that is another advantage. So
we always try to avoid the toxic etchant because of the health point of view.
Now the characteristics of TMAH etching are as follows. Influence of TMAH concentration
is there on etching process. Quality of silicon etched surface has to be studied. Because
if you go for the VLSI realization along with the sensor realization. So silicon etch surface
quality should be extremely good. Smooth surface you have to get for various reasons selectivity
to aluminum lower 1 0 0 to 1 1 1 etch rate ratio. Anisotropic etchant for silicon, anisotropic
etchant means it is like selectivity over the different crystallographic plane, low
toxicity, highly selective to oxide and nitride compared to KOH. The selectivity to oxide
and nitride is more in TMAH compared to KOH. So that means you can go for either oxide
masking or nitride masking or aluminum masking or gold masking. You have lot of freedom if
you use TMAH. But the total standardization technique of the TMAH etching in silicon is
not very simple is not that much easy.
Now there are some features of TMAH you have to do in the lab. Some experiment to improve
the selectivity as well as more etch rate as well as surface finish of the etch surface
should be very good. For that we did lot of experiment in our laboratory and some of the
results will be shown now. We have seen the surface roughness increases with the decrease
of TMAH concentration. Here is a plot of etch rate versus TMAH concentration. You can see
one typical thing over different temperature we did it 70 degrees, 80 degree and 90 degree
Celsius. We found the TMAH concentration increases the etch rate falls. That means at low concentration
TMAH the etch rate is higher which is normally not true in many of the etchant solution.
But at the same time if you decrease the the TMAH concentration surface roughness increases.
So if we need high etch rate ratio as well as good surface then we have to go for certain
extra technique. We have to adapt extra technique, extra mechanism. What is that? We use here
silicic acid and ammonium persulphate as a dopant into the TMAH solution. Basically silicic
acid, ammonium persulphate has got different purpose or different action on total TMAH
etching process. What are those?
First let us talk about silicic acid. Silicic acid material is a highly helpful for aluminum
passivation. It helps in formation of aluminosilicate on the exposed aluminum layer. Although TMAH
has got the property of not attacking aluminum, but even then if you want ideal masking property,
the aluminum should not attack at all. Then you add little bit silicic acid, so that silicic
acid with that TMAH it will form aluminosilicate and a thin layer of aluminosilicate over the
exposed aluminum layer will help further passivation or masking properties of this particular layer.
But it has got certain limitation. Silicon etch rate falls due to lowering of pH of the
doped solution. If you add silicic acid, then etch rate will be little bit at a downward
trend because, the adding silicic acid will lower the pH value of the TMAH solution. That
is one of the thing we have to adjust by proper mixing of silicic acid.
You cannot use silicic acid as much as possible for get 100 percent selectivity over aluminum.
Some compromise you have to do. Now, the role of ammonium persulphate. That a particular
chemical increases silicon etches rate and surface smoothness. If you add silicic acid
it decreases the etch rate. But in addition if you add the ammonium persulphate, it will
improve again etch rate. At the same time surface smoothness. Etch surface smoothness
is also an important criteria I told you. So that we will get it by adding small amount
of ammonium persulphate and this ammonium persulphate is basically an oxidizing agent
that eliminates hillock formation on silicon surface. Because surface smoothness will be
disturbed if there are certain hillocks on the surface of the silicon and that hillock
formation will be prevented by ammonium persulphate because it is you know oxidising agent.
That means we found in TMAH if we add judiciously, silicic acid and ammonium persulphate with
proper stoichiometric ratio, then at the same time we can achieve different objectives.
Number one etch rate will be increased number two the surface smoothness will be improved
and number three what we can get that etch rate surface smoothness and aluminum passivation.
All the three things can be achieved by judicious solution of the silicic acid and the ammonium
persulphate.
Some of the experimental studies we shown here in this plot we can see here. The dissolved
ammonium persulphate silicon etch rate. Here the micron per minute. So ammonium persulphate
gram per liter with 5 percent TMAH because as I shown you the TMAH concentration increase
the etch rate also falls. So that is why we confined 5 percent TMAH etching solution silicic
acid we added 38 gram per liter at 80 degree Celsius. Then if we go on adding ammonium
persulphate in this ratio 2 gram per liter, 4, 6, 8 like that silicon etch rate is like
that. So that means here with addition of more AP the silicon etch rate also increases.
On the other hand the aluminum etch rate you can see dissolved silicic acid. As I told
you the silicic acid will formed an aluminosilicate film on the bare aluminum and it will prevent
etching of aluminum into the TMAH solution.
So how much silicic acid is to be added that will depend from this characterization curve.
You see aluminum etch rate goes down drastically if you go on dissolving silicic acid. So here
also 5 percent TMAH etching is solution is used and AP is used 7 nearly here in this
region middle of that some 7 gram per liter ammonium persulphate. Then the silicic acid,
addition of silicic acid you go on changing accordingly the aluminum etch rate will fall.
Aluminum etch rate will fall is desire thing, that means it is not attacking aluminum at
all we will get perfect passivation layer.
So now some picture is same photomicrographs are shown is here. The surface roughness increases
with decrease of TMAH concentration in a, you see here again as I told you the etching
of etch rate will be higher at lower TMAH concentration, that we have seen in the curve.
So when the etch rate will be higher, then the problem will be the surface roughness
also appear. Because etch rate is very fast and this picture shows 5 percent TMAH and
you can see lot of the hillocks and these black portion are basically the groups that
is why it is a black. So these are the island so the silicone at silicon atoms are disperse
into in different direction and these are the blank portion and as a result which surface
roughness will deteriorate. But other in all other case you can see here 10 percent TMAH
solution those the black regions are missing here. That means here surface smoothness is
good in this particular picture.
Now here is again the silicon micro roughness study has been made in this diagram as well
as the masking property of silicon dioxide is also shown in this diagram. You can see
if you go higher and higher temperature etching then silicon dioxide etch rate will be more
and more. That means silicon dioxide etch rate more and more means, the silicon will
not be a good masking layer. It should not etch at all. So high temperature etching may
create problem if you go for silicon dioxide masking layer. But here although that is micron
per hour unit it is angstrom per hour. So although the silicon dioxide etch rate increases
with temperature, but at a small rate. Now in this diagram you see roughness of the etch
silicon surface in kilo angstrom and here is TMAH concentration.
So here you see lower concentration of TMAH the higher etch rate and surface roughness
also is higher. Because roughness of the etch silicone kilo angstrom, means the tops option
bottoms are there inside roughness. So that is in this range, so in kilo angstrom unit.
So that is very large at lower TMAH concentration and low at higher TMAH concentration. That
means using those characteristics curves with temperature, with TMAH concentration, with
silicic acid and with ammonium persulphate a good study has to be made in any lab to
get a perfect etching solution which will satisfy all the criteria.
Now here is again some photomicrograph study. You can see SEM of etched surface of silicon
with the addition of ammonium persulphate. One is 1 gram per liter is a 5 gram per liter,
1 gram per liter surface roughness is more because you can see here the lot black spaces,
means lot of holes are created. But here it is not that much because you see the holes
are less in this particular picture black regions are less.
Now the aluminum masking property one photograph as we will take using the scanning electron
microscopy and here you can see the aluminum line in this particular picture has not affected
at all and the black region is a silicon and the colored, these line say aluminum line.
Say this shows that this particular etching has been done with 5 weight percent TMAH 50
gram per liter of silicic acid to passivate exposed aluminum and 1 0 0 etch rate was 2.5
micron per hour at 85 degrees centigrade. That is one photograph taken in our laboratory
after etching silicon using aluminum as a masking layer.
Now this particular table shows the anisotropic etching characteristics. Here we discussed
on KOH, H2O, KOH, EDP hydrazine and ammonium hydroxide. The temperature used is here 80
degree, 75, 110 degree, 118 degree and 75 degree Celsius and 1 0 0, 1 1 0 and 1 1 1,
these three surface etch rates are very important and the etch rate in micron per hour is mentioned
here. You can see 1 0 0 silicon in KOH water 84 micron per hour. Only KOH if you use 25
to 42 EDP is 51 and hydrazine is highest even then it is not used because as I told you
is a not user friendly chemical is extremely health hazardous chemical. Now you can see
here the EDP is 1 0 0 to 1 1 0 the etch are almost similar so that 1 1 0 and 1 0 0, that
selectivity is not there in case of EDP. But 1 1 1 and 1 0 0 you can see the high selectivity.
51 micron per hour here is 1.25 micron per hour. On the other hand here in KOH also it
is in the range of 25 to 42 and 1 1 1 is 0.5 micron per hour. But if you go for the high
selectivity of 1 0 0 and 1 1 0 then you to have dilute KOH solution. KOH plus H2O which
will give you 84 and 126, 1 0 0 and 1 1 0. Depending on your application, depending on
the etching structure you have to choose which solution you are going to use for your desired
microstructure.
Now another technology I will discuss today that is a LIGA micromachining technique. LIGA
is again bulk micromachining it is not conventional etching technique which is used in EDP. KOH
or TMAH is a altogether complete different technique for making microstructure. In this
particular LIGA process you can get very high aspect ratio 3 dimensional structures. Basically
many of thus mechanical structure which is used in watch are now being made with the
help of LIGA micromachining process. The complete name of the LIGA is lithographie galvanoformung
and abformung. These are German words and it English equivalent is "lithographie" is
lithography. Galvanoformung is "electroplating" and "abformung" is a molding. So LIGA basically
lithography, electroplating and molding. Let us discuss now how this particular in this
particular technique we get the exact 3D structure with high aspect ratio.
Now it has got certain advantage over other techniques. What are those? Ability to create
3D structures as thick as bulk micromachine devices while remaining the same degree of
design freedom as surface micromachining. Design freedom of surface micromachining and
etch depth is similar to bulk micromachining you get it. Microstructures with feature sizes
of several microns have been made with a thickness in excess of 300 micron with the LIGA process.
More than 3 micron thickness is easily obtained by the LIGA technique.
Now let us see the process steps of LIGA micromachining technique. First we take a substrate is a
bottom. One is a substrate and on the substrate you put a conductive, electrically conductive
layer. That means some metal plating has to be done at bottom. After that you coat photoresist
and that photoresist thickness is 300 to 500 micrometer. That means one important point
I would again mention they are not on bare silicon, not on bare ceramic insulator material.
You cannot coat these PMMA photoresist and it first your substrate has to be coated with
conductive layer because in the next stage you are going to form an electroplating. So
electroplating means some cathode anode will be there. So until unless you coat on conducting
layer you cannot use as either cathode or anode. So whatever the substrate is used either
silicon or ceramic or other material first you coat with some conducting electrically
conducting layer then you deposit the thick photoresist.
Normal photoresist which we use for VLSI process, those photoresist cannot be used in LIGA process.
Here you will need photoresist thickness of the order of 300 to 500 micrometer. So that
means you need a very high viscous photoresist and PMMA polymethyl methacrylate is one such
resist, SU8 is another resist which is used for LIGA process. So that a photoresist will
give you a thick layer of the film after spinning and drying. So now the PMMA of thickness greater
than sometimes 500 hundred micrometer on this may conductive layer coated substrate is made.
Then the step two. What is the step two? This is the mask you can see the green color is
the mask and then you have to expose. Through the mask you have to radiate the PMMA with
x-ray radiation. Normally UV lithography cannot be done here. You need here x-ray lithography.
So x-rays are collimated and it will penetrate to the thick resist in well-defined sidewall.
This is the mask, so through that mask x-ray passes. Because here the thickness of the
photoresist is 500 micrometer. So here this 500 micrometer thick photoresist should be
reacted with the radiation. So for that you need x-radiation. So that x-radiation will
penetrate through that thick layer and complete reaction will take place that is polymerization
will take place. So that is the X-radiation you expose it, then here there
is a much contrast red color means through that X-ray penetrates into the layer and then
it reacted here and as a result of which it will be polymerized.
Now step three. Here you have to develop, after exposing next step is developed. So
you develop the desire develop the photoresist after exposing. Then during development as
you have seen in case of negative photoresist, so our positive photoresist is there, whatever
you it is some kind of positive photoresist in nature. So where you expose, those such
portion will dissolve. So here also this portion where exposed by x-ray radiation has been
dissolved by developer solution of the PMMA. So then here hole has been formed. Step four.
Step four is what? Metal electroplated on the exposed conductive substrate surface.
So metal is electroplating. So after that the bottom is a conductive layer, you for
electroplating.
What electroplating, which metal you want to fill this group, this thick developed structure.
So go for electroplating. So electroplating will help you depositing that particular metallic
film into the groups as higher thickness as you wish. So long time electroplating you
do it so there you can increase the etch rate, you can adjust the electroplating process
by the what are their variants. One is the concentration of these electroplating solution,
another is the temperature and another is a current. The current through that electroplating
is basically electrolysis process. So if you change the current, so the rate of deposition
will also change. So by adjusting those parameters you can get the thick layer.
So after that, then step five. So in step five what you are doing? So the photoresist
is removed. You see photoresist has been removed and then only the metallic structures is there
which is which is basically fixed on this bottom metal plate. So now here the sacrificial
techniques are combined with the basic LIGA process to create partially freed flexure
suspended structures or completely freed devices. Now this can be used as a mold. This is one
mechanical structure that easily can be made. You see here thickness is a large and this
hinges can be very small then the aspect ratio is very large so that can be made using the
LIGA process. Here you do not need that mask aligning you do not need the conventional
machine which is used is normally lithography process. But here some important feature which
are different from normal lithography techniques are first is a x-radiation you need it, second
is electroplating you need it, third is a different kind photoresist you need it. Those
are the different form the normal the lithography and etching process.
Now as I mentioned process requirements are x-ray lithography and thick photoresist PMMA
or SU8, that is one of the requirements. Second is electroplating with precious controls on
current density. Because the deposition on the metal, on the base plate depends on the
current density, temperature, concentration of the electrolyte solution, composition of
the plating solution to avoid hydrogen bubbles which may result fetal defects. So here all
the temperature concentration composition will decide whether hydrogen bubbles are coming
from the deposition process. If hydrogen bubbles are coming more, then automatically they will
create some holes and that will form a defect into the mold. So the rigidity of the mold
will be less and it will be porous, the total mold which you got it. Isn't it? So that is
why you have to control or standardize the complete electroplating process by adjusting
temperature, by adjusting current density, concentration and composition of the plating
solution.
Now just in a cyclic way, how do you proceed that is shown again. The lithography galvanoformung
and abformung. So is cast PMMA on metal base then coming here x-radiation then after that
develop the PMMA solution. After developing you get the holes then you come here. Then
is electroplate through PMMA and after electroplating you this photoresist is completely removed,
dissolved then you will get this structure separate metal from PMMA. This is the LIGA
micromachining process which is used now days for making mechanical structure, mostly metallic
structure. You cannot get ceramic structure out of that because you see electroplating
only for metal deposition. If you want to have, then you have to go for this LIGA process.
Now another technique I will discuss today, so that is the laser micromachining. So we
initially discussed on the conventional chemical etching process, silicon etching process that
is KOH or TMAH or EDP or some other solution hydrazine or other materials. Then you went
for different in total different technique, that is the LIGA method and now I will discuss
on laser micromachining. Now days people are in some cases they are using the laser micromachining.
So laser micromachining means here you have to have high concentrated high power laser
and laser basically the laser ray if is incident on some structure, some micro explosion will
take place and a result of which that will evaporate or that will be ejected from the
basic material. So laser drill is available and that laser can be used for getting a mechanical
structure. Here one of advantage of the laser micromachining obviously, this particular
micromachining will not. I should say etch simple class a single class a metal like it
earlier we see silicon in KOH or TMAH. Only silicone, ceramic cannot be done for example
electroplating will micromachining structure can only metal ceramic or semiconductor cannot
be done. But if you use laser there semiconductor can be done, metal can be done, ceramic can
be done, irrespective of the material because it is a mechanical process, mechanical machining
process.
It will not obviously just the etching rate or removal rate of the material will be different
for ceramic or metal or the semiconductor. But any kind of those materials you can easily
make using the laser micromachining process. But here another point you cannot get the
etch selectivity kind of thing. You do not need any masking layer also. Because the laser
material when it a hitting a material that will be removed. So that means there is no
questing of masking. There is no question of the mask aligner. There is no question
of developing. So all these things are not there. So basically this is useful only for
materials which cannot be micromachining. Using the conventional techniques, popular
technique the laser micromachining. Obviously here one disadvantage is there. Very small
microstructure of say small dimension cannot be made using laser micromachining. Why because,
there you need the laser beams to be focus to a singular point of diameter. May be sometimes
less than of the order of micrometer. If you cannot focus there, so automatically you cannot
there you cannot get very fine line. So those problems are there even now days in some cases
people are people use these laser micromachining technique.
Here basically the process is laser ablation using high power laser pulses of short wavelength
or nanosecond pulsed gas laser at 157 to 353 nanometer or femtosecond solid state laser
at 266 to 1060 nanometer. That is the wavelength of the laser normally used in case of laser
micromachining technique. No lithography CNC programmed micromachining that is basically
the structure is CNC programmed machine is available in mechanical laboratory. Mechanical
machining laboratory the CNC machines are available all. Though is a computer programmed
control the CNC machine which is used for cut a metal cutting and structure making the
same machine is used for micromachining for guiding the laser beam. Not governed by crystallographic
orientation, I discuss it is not governed by crystallographic orientation. Suitable
for silicon and non-silicon materials. Another point to note it is suitable for silicon and
non-silicon. Both materials, ultra short laser pulses produce micro explosions which causes
ejection of solid and gaseous particles without significant thermal degradation. So that is
the basic principle or basic thing based on which the materials are ejected.
So this video graph I am showing you some of the structure which has been made using
the laser micromachining, they call it as laser LIGA. So KrF laser has been used here
and this structure is nickel motor turbine. So miniature micro motor and which is structured
from nickel, that has been made using these laser micromachining techniques and this picture
has been taken from this particular paper reference and you can see here the structure
is not extremely small, here the size of this 375 micrometer may be the group. But total
structure in the range of a few 100 micrometer to millimeter range. So that is made using
the laser LIGA process. So now the basic bulk micromachining processes I discussed now today.
Those are normal chemical anisotropic etchant solutions and then LIGA and then laser micromachining.
So we compare the advantage, disadvantage of the various anisotropic silicon etchants
and as well as now you can just differentiate the LIGA process as well as the laser micromachining
process.
What to be used to what? That you have to select depending on your requirement. Out
of all these techniques I should now conclude with that TMAH is the best choice of laser
best choice of micromachining technique. So for as the integrated microsensor fabrication
is concerned and it is the basically the organic etching solution which will not at all toxic,
which is not health hazardous and biggest advantage is that. It is done in laboratory
in normal environment and biggest advantage is aluminum passivation. So with this I will
just stop today. In the next class we will continue on micromachining. Particularly the
surface micromachining technique, we will discuss and then we will go for the etch stop
techniques which is also another important aspect for micromachining to get different
structure. So thank you very much.
Etch Stop Techniques and Microstructure Fabrication.
Today we will discuss on etch stop techniques and microstructure fabrication. Etch stop
is an very important aspect in making microstructure. I have told you earlier that in MEMS in many
cases we need membranes and flexures or cantilevers of certain thickness and that thickness varies
in case of surface micromachining may be 2 micron, 3 micron in case of bulk micromachining.
Sometimes we need membrane of 10 micron, 20 micron or 30 micron and thus those 10, 20
or 30 micron is coming from the bulk thickness of the wafer which is nearly 300 to 500 micrometer
depending on the wafer size. If it is a 2 inch diameter wafer the thickness is nearly
280 to 300 micrometer. If it is a 4 inch diameter wafer, the thickness of the wafer is nearly
500 micrometer.
If it is 6 inch or 8 inch, then it is further you you will get more thickness of the silicon
wafer. So from that thickness it has to come down to 10, 20, or 30 micrometer. So somewhere
you have to stop the etching process. Then there are two ways; one is a mechanical process,
that means you observe the time if you know the etch rate of that film basically silicon.
Here if you now the etch rate of silicon in that particular etching solution then you
can note down the time, how much time you etch then after that you take out the wafer
and then you measure the thickness you can get it. The other way is automatic stopping.
So you see automatic means it will continue etching. But after certain point, that point
has to be decided by electronically or electrically.
So now you go for printing just like your stamp you use it. You printing here and this
is the gold coated this is substrate is a gold coated. Now if you print here that thiol
or thioether ink will make some passivation layer, monolayer and after that go for wet
etching. This particular portion will be stopped and here etching will be done. That is basically
micro contact printing no lithography no exposure no machine nothing all required very simple
technique. Isn't it? Is a not at all the high expenditure equipment required nothing required.
So next is some issue.
I will just these are known as soft lithography. The novel fast developing soft lithography
technique invented by this people. This is mpl or stamp and lithography process. Stamp
poly dimethyl siloxane PDMS elastomer that is the stamp material. Fast stamp material
has been found then you are going to use a ink mostly use long chain alkanethiols 18
to 20 methyl group soaking in ethanolic solution drying and contact inking, that is the ink.
Printing self assembly monolayer SAM on gold, silver, copper, palladium protecting against
etchants what I just showed you. So that technique is is getting importance now a days that does
not have lot of money. Lot of capital equipment is not required, so easy technique you can
get some micro fabrication using these two methods. So let we stop here today. We will
continue in the next class on the micromachining surface micromachining basically. Thank you
very much.