Fabrication Techniques for Mems-based Sensors: Clinical Perspective

Prof. Hardik J Pandya

Department of Electronic Systems Engineering
Indian Institute of Science, Bangalore

Lecture – 05
Silicon, silicon di-oxide and photolithography contd

Welcome to this module and in this module, we will be looking at how we can grow
silicon dioxide and what is the characterization method to understand or to measure the
thickness of the grown oxide.

In the last module what we have seen, we have seen the importance of silicon dioxide or
you can say the application of silicon dioxide such as a masking oxide, a gate oxide, pad
oxides and also it can work as a mask against diffusion, against dopant. Either it is a
diffusion method or it is an ion implantation method.

So, we also saw how we can grow oxide depending on whether we are using wet
technology or we are using dry technology. So that means that if we use vapor, then we
have wet oxidation; if we just use oxygen, we have dry oxidation. An advantage of dry
oxidation over wet oxidation when it comes to the quality of the oxide.

Now, again in wet oxidation and dry oxidation, the growth is done in a furnace. It has the
net high temperature around 900 degree centigrade to 1100 degree centigrade. So, when
you talk about furnace if the furnace is vertical it is called vertical tube furnace. If the
furnace is horizontal it is called horizontal tube furnace. So, this is done in a horizontal
tube furnace.
(Refer to Slide Time: 02:10)

So, if you see the screen, you can see thermal oxidation methods. The first one is dry
oxidation where you can see that when we have to load the first stage, we have to load
the wafers inside the furnace. Now when you are loading the wafers, you have to purge
out the unwanted oxygen, because we do not want to start the growth of an oxide layer at
lower temperatures until all the wafers are at a particular temperature to have uniform
growth.

So, we will first introduce nitrogen and we will purge out the oxygen and other gases
from the chamber only nitrogen would be there. Now we will insert wafers or a wafer
holder; wafers will be kept on a wafer holder. The wafer holder would have slits to hold
the wafers. So, if you see a cross-section, you will just see like this which is shown here.

So, that is why whenever we see oxide, oxide grows on both, the front of the surface and
back whether it is polished or unpolished, it does not matter.

Second, so once the furnace reaches a temperature of 1,100 o centigrade, we will stop
nitrogen and we will start the flowing of oxygen; So, that silicon dioxide forms on the
silicon wafer; so, first thing. Second is wet oxidation. In wet oxidation, there is a bubbler
and a heating mantle on the outside, this is a heating mantle here and there is water
inside the bubbler. Now we heat this water and the vapors are created. Now initially, this
valve is closed, this valve is opened and this valve is closed. So, oxygen cannot pass only
nitrogen is allowed to pass. So, nitrogen flows into the chamber similar to this process.
And once we reach an appropriate temperature say 1100 degrees centigrade, then what
do we do? We keep this open, we stop nitrogen, we start oxygen; we stop this or close
this valve open this valve and open this one. So, what will happen?

Now, the oxygen will flow through this to the bubbler, it will take the vapor out and
vapor will go up and it will reach the chamber to form the oxide. So, here we are using
the vapor which is H 2 O gas to create ann oxide and this is your wet oxidation.

Another one is pyrogenic oxidation where you allow the H 2 to react with O 2 and form
H 2 O and then you know when the oxygen is flowing you allow H2 to pass through it
react it and then, you can grow oxide. However, pyrogenic oxidation is not so popular
like dry oxidation and wet oxidation. So, we will just concentrate on dry and wet
oxidation we will not consider pyrogenic, we will not talk about it later.

Now, this is the actual photograph of the horizontal multi-zone furnace used to grow
thermal oxide on silicon wafer. This is horizontal tube furnace, this is a vertical tube
furnace, this one is horizontal furnace one and number two is vertical tube furnace.
Vertical tube furnaces are not too popular in growing silicon dioxide. So, we will
concentrate on horizontal tube furnace.

Now, in horizontal tube furnace there are three zones, a three-tube horizontal furnace
with multi-zone temperature control see tube 1, tube 2, tube 3; and multi-zone. We will
see in the next slide; you can control the temperature, you can program the temperature
using your system and you can grow multiple oxide thickness and multiple thicknesses
of oxide. One thickness can be on tube 1, second would be on tube 2, third would be tube
3. So, the throughput increases and the throughput would be high.
(Refer Slide Time: 08:25)

Now, you can see here we have wafers loaded onto the wafer holder. And this wafer
holder is inserted inside this horizontal tube furnace. It is already heated and you can
here see that the operator is pushing the wafers inside the horizontal tube furnace. Now
the tube is generally made up of quartz; the gases are inlet from one side and gases go to
the outlet on another side and the silicon rod is used to push the wafers inside the
horizontal tube furnace.

The typical velocity of oxygen or water vapor that flows through the reactor is around
one centimeter per second. This is a typical velocity for the oxygen or water vapor to
flow through the reactor.

So, once we have an oxide, let us say I have silicon wafer and the oxide will grow on
both sides because gases react with both the surfaces of the silicon wafer. You can see
here on both the surfaces of silicon wafer the gases react forming oxide on both sides.
So, once we have oxide grown on the silicon wafer, it is silicon dioxide.

Now, let us say we have optimized a parameter and we have grown 1 micrometer of
silicon dioxide. How do you know it is 1 micrometer?

So, to understand and measure the thickness of silicon dioxide we have to use some
characterization tool, thickness measurement tool correct. So, what are those tools? So
let us see.
(Refer Slide Time: 11:06)

The first technique is called a surface profilometer which is also called the alpha step
profilometer in which we use a stylus to scan the wafer and get the thickness. However,
we need to perform a process before we can use stylus to measure the thickness, what is
this process.

So, let us see here let us say this is the wafer and we have an oxide. Then before we use
stylus we have to create a step in the oxide using photolithography. I will teach you
photolithography in the following lectures now just know that there is a step. We have
removed oxide from this surface and we have retained oxide in this area. In this area we
have removed and in this area we have retained the oxide.

Now, what do we do? Now we will take this stylus and we will scan the wafer with the
stylus. So, what will happen when it goes here? There is a step here. So, because of this
step, the stylus will bend and corresponding to the bending of the stylus we know the z
direction, and corresponding to that we can know the thickness of the silicon dioxide.

So, you can see here that the oxide is etched away by HF hydrofluoric acid over part of
the wafer and mechanical stylus is dragged over the resulting step. There is a traveling
stage there is a stylus here and there is a scanning line through which stylus is scanning
there is a 2 micron per millimetre slope. And you can see the photograph of a styler
mirror image of a styler here; And you can see it is scanning through the stage and when
we can change the speed of scanning, we can change the speed of the z-direction,
illumination, scan stage, zooming

Whether you want to have a display at 100 percent, 50 percent 25 percent and you can
calculate the thickness of the oxide. If I move this stylus from this direction to this
direction, you can see my pen moving here. So, from this to this direction there is a step,
because of this step we can know the thickness.

However, this way of measuring the thickness of the silicon wafer comes with some kind
of drawbacks. And the drawback is that it cannot measure the thickness of silicon with
ultra-precision accuracy, precise value. And for example, with the precision of 1
nanometer or 2 nanometers or 10-nanometer scan, it cannot achieve. So, it can be used
for crude measurements.

Now, this is still better, but sometimes in a lab, you will observe that just looking at the
wafer you can tell it is 1 micron or it is 0.5 micron. Because the colour of the wafer
would change since the silicon is light gray in colour or you can say little bit darker light
lighter gray or gray in colour and then you grow 1-micron oxide, you will see a greenish
colour of silicon.

So, this is another wafer and this is purple in colour; So, this thickness of oxide is
different in this case.

(Refer Slide Time: 15:56)

Silicon colour would be greyish. So, if I put this one now, you can clearly see the
difference when an oxide is grown. The difference when the oxide is grown can be seen
clearly as difference in colour. Silicon oxide on silicon, can be very easily identified on
the backside.

So, we have etched the oxide from the front side and there is an oxide on the backside;
Here there is no oxide on the front side, but there is oxide on the backside. So, now what
is the thickness of the oxide, I cannot tell what is the thickness of oxide just by looking at
the colour of oxide. Or if I am working from a long time in a laboratory with the
experience, I can tell it can be 0.5 micron it can be 0.8 micron or close to 1 micron, but I
cannot tell it with accuracy.

(Refer Slide Time: 17:17)

So, here the thickness determination is by looking at the colour, relative illumination
intensity of silicon dioxide. When you grow silicon dioxide you can clearly see here
because of the transparent thin-film the oxide thickness for constructive interference

viewed from an angle of about 0 degrees is , n =1.46 for k =1 to 3. So, I

can tell the difference between two films having ten nanometers of thickness difference.
However, it is very difficult to exactly differentiate ten nanometers, but like I said about
200 nanometers difference 0.2 microns we can tell if we have enough experience
working in the lab.

(Refer to Slide Time: 18:05)

So, what to do? Then to have an extremely precise measurement of the thickness of
silicon, we have to use something called ellipsometer the technique is called ellipsometry
and the equipment is called ellipsometer where there is a light source, filter, polarizer,
quarter wave plate; there is a reflection, there is analyzer and detector the film is
measured. So, after the quarter-wave plate is linearised, so later when you insert the light
the linear polarized light becomes circularly polarized which is incident on the oxide.
And the polarization of light which depends on the thickness and refractive index of the
oxide layer is determined and used to calculate the oxide thickness. Now, at multiple
wavelengths, incident angles can be used to measure thickness or RI of each film in a
multi film stack.
(Refer Slide Time: 19:10)

The advantage of this particular technique is its accuracy of 1 nanometer. So, this is what
we have to understand that when it comes to measuring the thickness of oxide, we have a
few techniques starting from alpha step profilometer or surface profilometer, we can
understand using the colour of the wafer or we can use ellipsometer to understand the
thickness of the wafer.

Now in the next class, let us see what is physical vapour deposition. It is very important
because we want to understand what is photolithography; but for performing
photolithography, first we let us see what is silicon, silicon dioxide, physical vapour
deposition, then we move to photolithography. Till then you take care I will see you in
next class bye.