Deposition Techniques

8/17/2019 Deposition Techniques

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DEPOSITION TECHNIQUES

PHYSICAL VAPOR DEPOSITION

In this chapter we will see various deposition techniques that are used in microelectronic

fabrication. If a material like copper or tungsten has to be deposited onto the silicon wafer,

there are multiple ways to achieve that goal. The deposition methods used in semiconductor

industry can be divided into four groups.

1. Physical Vapor Deposition (PVD)

2. Chemical Vapor Deposition (CVD)

3. Electrochemical Deposition (ECD)

4. Spin-on coating

Among these four processes, PVD and spin-on coating are purely physical processes. CVD and

ECD are chemical processes. That means in CVD and ECD, chemical reactions occur.

In IC manufacturing, the actual quantity of material to be deposited in any single step is very

low. The deposited material will be in the form of a thin film, almost like the painting on a

surface and in reality much thinner than the paint coating.

Thin film requirements: Any deposition method must satisfy certain requirements:

The deposition must be uniform throughout the wafer

A very good control is necessary. If we want 1000 nm of the material deposited on thewafer, then the variation must be within one or two percentage. i.e. the thickness of the

film after deposition must be at the minimum 980 nm and at the most 1020 nm.

In the places where trenches or vias are made, the side wall coverage must be good.

This is explained in the later section (figure 3.11)

The material should adhere to the wafer well and should not peel off

Dust particles should not fall onto the wafer during the deposition process

The crystal structure of the film deposited must be of sufficient quality because it will

affect the properties of the film. For example, when copper is deposited, we need large

grain size since it will result in less electrical resistance.

If we are depositing alloys, then the composition must be uniform throughout theprocess

In general, PVD is used to deposit titanium, titanium nitrate, tantalum, tantalum nitrate,

aluminum and a very thin film of copper called seed layer. CVD is used to deposit tungsten,

titanium, again copper seed layer, silicon di-oxide, silicon nitride, etc. Electrochemical deposition


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is used for depositing copper. Spin-on coating is used to deposit insulators or dielectric

materials, which are usually organic in nature.

Historically, some of the metals were evaporated and then the vapor was deposited onto the

wafer. The wafer would be kept at lower temperature and the vapor will cool down and form

the solid metal. This methods used large amount of energy and had other difficulties. It couldnot be used for all materials, and particularly for alloys. The material will also deposit on other

places like the chamber walls. Hence, it is not currently used for IC manufacturing.

If the raw material is taken in gaseous form and the final material is deposited on the wafer

after reaction, it is called chemical vapor deposition (CVD). If the material is taken in solid state,

but sent as very small particles or atoms and deposited on the wafer without any reaction, it is

called physical vapor deposition (PVD). We will first see the details of the PVD.

Physical Vapor Deposition:

DC sputtering

The PVD equipment will be about 4 ft in height and 4 ft in diameter. The material to be

deposited (e.g. titanium) will be at the top, as shown in schematic Fig 3.1

The tungsten will be in the form of a disc of 1 inch thickness and 5 or 6 inches diameter. At the

bottom, silicon wafer will be kept. Apart from these, there will be facilities to allow gases into

the chamber and to evacuate the chamber with vacuum pump and electrical connections toapply very high voltage (of the order of 10000 V). The negative plate will be near the tungsten

and the positive plate will be near the wafer. Tungsten (or any other material in its place) is

called target . Why is it called target? How is it deposited onto the wafer?

Let us consider an example. In old houses, if we stand inside the house and throw a ball at the

top of the house, some of the dust material will fall on the floor. If we throw the ball


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repeatedly, after some time, the whole floor will be covered with dust. Some dust particles may

be coarse and may not stick well to the floor. The fine dust particles will stick well to the floor.

In PVD, we can visualize a similar process: Instead of the ball, argon ions are used. The ceiling

represents the tungsten target. When the ions hit the target, a few atoms will break away from

the target and fall on the wafer. This is a very simplified description of PVD process. PVD is alsocalled sputtering.

First the air in the chamber must be removed and vacuum must be created. Then argon gas

sent inside and a low pressure will be maintained. If high voltage is applied to the plates, a

plasma will be generated. The plasma will have electrons and positive argon ions. The plasma

cannot be generated by normal 230 volts. The positive argon ions will be attracted towards the

negative plate. They will move towards the negative plates and hit the tungsten with high force.

That is why tungsten is called target in this process.

Since the argon ions impinge on the target with large force, some of the target atoms will break

and come out, as shown in Fig 3.2. How many tungsten atoms will come for each argon ion

hitting the target? This number is called sputtering yield. It depends on the speed of the argon

ions, the angle of the impact and also on the bond strength of the target. Tungsten is one of

the hard materials. If a relatively soft material such as copper is used as target, then the yield

will be higher.

The atoms from the target will come towards the wafer with some force. Not all of them will

deposit on the wafer. Some will be deposited (Figure 3. 3), while some will bounce back (Figure

3.4). Some may even bounce back and remove some of the materials already deposited on the

wafer (Figure 3.5)


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Among the tungsten atoms that fall on the wafer, the fraction that stick to the wafer is called

sticking coefficient. If all the atoms that fall on the wafer stick to it, then the sticking coefficient

is one. If none of them stick, then the sticking coefficient is zero. Typically, the sticking

coefficient is about 0.7 to 0.8.


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Physical Vapor Deposition Continued

Certain techniques are used to ensure that the deposition is uniform. The wafer can be rotated

slowly. If the wafer is heated, then the deposited layer can soften and move and also stick well.

What is the advantage of using argon to hit the target and deposit the material on the wafer?Why not just use evaporation?

Materials such as tungsten have very high melting point (3422 oC) and boiling points (5555 oC).

It will be very difficult and energy intensive to deposit tungsten by evaporation. Besides, if an

alloy has to be deposited, the composition of the vapor will not be the same as that of alloy.

Materials with low boiling point will tend to evaporate early. However, in PVD, since the material

is removed by the bombardment of argon ions, the operation can be conducted at relatively low

temperature. Alloys can be deposited without difficulty.

Why do we have to use argon? Argon is an inert gas which will not react with the target or the

wafer surface. If we use oxygen or nitrogen instead, they may react with the target and or thewafer surface. We can use other inert gases such as helium or neon. However, it is more

difficult to ionize helium or neon. They are also more expensive than argon. Hence argon is

used for sputtering.

PVD can be used for depositing tungsten, titanium, tantalum, copper, Titanium nitride and

tantalum nitride. Currently titanium and tantalum metals and their nitrides are deposited using

PVD. Tungsten is deposited using chemical vapor deposition or CVD. A thin layer of copper,

called seed layer, is deposited using PVD while the remaining copper is deposited using

electrochemical deposition.

RF sputtering: All the materials we saw above are electrical conductors. PVD can be modified

so that insulators such as silicon dioxide or silicon nitride can also be deposited.


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What is the problem in using regular PVD for depositing an insulator like silicon dioxide?

Let us look into the figure 3.6. This explains the problem in using the normal PVD chamber to

deposit and insulator like silicon dioxide. In the beginning, argon ions will come and hit the

silicon dioxide and some of the silicon dioxide will be removed from there and deposited on the

wafer. But, the argon will get stuck on the silicon dioxide. Since it is an insulator, the charge willnot be removed. The silicon dioxide will acquire a positive charge. The backside of the glass will

have negative charge, but there will not be any electrical conduction. Very soon, the entire

surface will be positively charged and argon ions, which are also positively charged, cannot

approach the glass. The argon ions will not be able to hit the target and deposit the target

material on the wafer. If we were using a metal, then the positive charge will be neutralized

because the metal will conduct the electron from the backside to the plasma side.

In order to deposit the insulator by PVD, electrons must be supplied to the surface of the

insulator on a regular basis. This is achieved by using radio frequency (RF) AC voltage. This is

sometimes called as RF sputtering.

How does the RF PVD function? The chamber, which uses RF sputtering, looks very similar to

the one with normal sputtering, but the voltage applied will be different in this case. This is

explained in the schematics shown in figure 3.7 and 3.8. If we take an AC voltage and add it to

a DC voltage, then we will get a combined voltage as shown in this figure. If we apply only a

pure AC voltage, then 50% of the time the wafer will be positive and 50% of the time the

target will be positive. But when we apply the combined DC plus AC voltage, the wafer will be

positive 75% of the time and the target will be negative for most of the time. For a short time,

the target will be positive and the wafer will be negative.


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What is the use of this? What will happen when this RF AC voltage is applied along with the

DC? When the target is mostly negative, argon ions will come and hit it. Some of the electrons

will go and hit the wafer. The glass will be removed by the argon ion and they will go and

deposit on the wafer. For a short time, the electrode will be positive near the target; that time

the electrons in the plasma will be attracted to the target. They will hit the target. The targethas argon ions and the electrons will go and neutralize the argon ions, as shown in Fig. 3.9.

Hence, the target will become neutral. Essentially, the electrons are supplied to the target

surface from the plasma side and not from the electrical contact at the back.

Now will the argon ions go and hit the wafer in the same time? Because argon is much more

heavier than electron, and because the duration is very short, it will not hit the wafer thatmuch. Since the electrons are very light (low mass), they will not cause any damage and no

atom will be taken out because of the electrons.


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We saw that argon ions will not hit the wafer because it’s for a shorter duration. Even if they

hit, they will hit with less force and few hits will occur. In the beginning, in fact, the wafer will

be made negative and the target will be made positive. Because of this, the argon will hit the

wafer a little, and remove some material and make it rough. This actually facilitates good

deposition of the material in the later stages. The initial roughening of the wafer will also help

in cleaning any of the contaminant deposited on the material before this process.

Now the AC has to be applied at a very high frequency. That is why it is called RF or radio

frequency. Now almost all of the sputtering equipment are RF sputtering equipment.


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PHYSICAL VAPOR DEPOSITION AND CHEMICAL VAPOR DEPOSITION

Collimated and Ionized Metal Plasma (IMP):

There are some more advanced techniques in the PVD. One is called collimated PVD. Another is

called ionized metal plasma or IMP PVD.

During deposition, we want uniform deposition on the wafer. i.e. the deposited film thickness

must be the same at the centre of the wafer, at the edge of the wafer and anywhere in-

between.

If the target is very small compared to the wafer, it will look like a point source for the

deposited material. In that case, the wafer centre will have higher deposition and the wafer

edge will have lower deposition.

One way to improve this is to increase the target size. If the target is large, then thedeposition will be uniform. In that case, the target can be near the wafer and still we

can get uniform deposition.

On the other hand, we can pull the target away. If it is too far away, then most of the

wafer will be more or less at the same distance from the target and hence, the

deposition will be uniform. This is called “long throw”.

Additionally, the wafers can be rotated during the process, and this also helps in

obtaining uniform film thickness.

However if we increase the target distance, then the chamber will become very large and to

evacuate the chamber and operate, one will spend correspondingly high energy. The goal ofuniform deposition can be achieved by another method called collimated beam. A schematic is

shown in the figure 3.11.


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In this, a plate with holes is placed between the target and the wafers. The target atoms, which

are coming towards the wafer, will have to come in straight line. Only then they will pass

through the collimated plate. Otherwise, they will be stopped by the plate. This ensures that all

of them are coming at similar angle and the film deposited will be uniform.

The most advanced method to obtain uniform thickness is called ionized metal plasma or IMP. A

schematic of the IMP sputtering equipment is shown in figure 3.12. In this, the metal clusters

that are coming towards the wafers are ionized using ionizing coils. Thus the titanium or

aluminum metal atoms that are coming are ionized. They become charged and they will be

attracted towards the wafer. Hence, even if they start at a different angle when they ejected

out of the target, they will all move vertically towards the wafer because of the attractiveforces. Since they are all coming at similar angles, the deposition will be uniform. The best

quality of the film is obtained when IMP sputtering is used.

In the next section we will look into the second deposition technique, viz. CVD.

Chemical Vapor Deposition

In the PVD method described above, it is not easy to deposit a material on the side walls as

shown in fig 3.13. This is especially true if the depth is very high and the opening si small. If

the ratio of diameter (or width) to the depth, known as aspect ratio, is very high, PVD cannot

give a good side wall coverage (Figure 3.14). However, CVD can be used to get a good sidewall coverage (Figure 3.15).


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Chemical Vapor Deposition Cntd.)

A large CVD chamber may be of 6 ft long and 3 ft diameter. In this, a material may be

deposited on a lot (i.e. a batch) of 25 wafers. The CVD chamber will have pipes and automatic

valves to create vacuum and to supply desired gases at controlled rates. The material to be

deposited (e.g. tungsten) will be in the form of a compound, in the gaseous form. The CVD

chamber will also have facilities to heat the wafers in a controlled fashion. A simple schematicof a CVD chamber is given in Figure 3.16

Operation: First the wafers are kept in the chamber and the chamber is evacuated. Then the

wafers will be heated to the desired temperature. Next the gases are supplied and they react

only on the surface of the wafer and deposit the material. For example, let us assume that we

want to deposit tungsten on the wafer. For this, a mixture of tungsten hexa fluoride, hydrogen

and nitrogen (all in gaseous form) will be supplied to the chamber at low pressure. The

temperature of the wafer would be maintained between 150 to 300 oC. In the CVD technology,

the phrase “high temperature” refers to 800+ oC and low temperature may refer to “less than

400 oC”. Very low temperature may refer to room temperature.

In most of the CVD tools, all the chamber walls will be maintained at relatively lower

temperature compared to the wafer. Only the wafers would be heated. WF6 and H2 will react

only at high temperature. Hence the reaction will occur only on the wafer and not on the

chamber walls. This type of CVD process is called low pressure CVD or LPCVD. Using this

method many materials such as W, Ti, TiN, Cu, SiO2, Si3N4, Si etc can be deposited on wafer.

The side wall coverage will also be good.

Some example reactions to deposit various materials are given below.

1. To deposit silicon

SiCl4 + 2 H2 → Si + 4 HCl

SiH4 → Si + 2H2

2. To deposit poly silicon

SiH4 → Si + 2H2

3. To deposit silicon di oxide


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SiH4 + O2 → SiO2 + 2H2

SiH4 + 2N2O → SiO2 + 2N2 + 2H2

SiH2Cl2 + 2 N2O → SiO2 + 2N2 + 2HCl

4. To deposit Silicon nitride

3 SiCl2H2 + 4 NH2 → Si2N2 + 6H2+ 6 HCl

SiH2 + NH2 → SixNyHz+ H2 (some hydrogen is incorporated in the film)5. To deposit Tungsten

WF2 + 3 Si → 2W + 3 SiF4

WF6 + 3 H2 → W + 6HF

There are certain disadvantages associated with the CVD process. Many of the gases used in

CVD and highly corrosive and toxic. Hence good safety measures need to be taken. Also, the

cost of the high purity chemicals is also high.

If the temperature and pressure are not controlled well, the reaction will occur in air itself and

form tungsten (or whatever we are trying to deposit). These tungsten will appear like a dustand will fall on wafer. This dust particle will not stick well to the wafer and will degrade the

deposited film quality.

The temperature cannot be raised arbitrarily during chip manufacturing. This is because a

transistor is made with impurities doped in particular location. When the silicon wafer is heated,

the dopants will move and the transistor will not function properly as shown in Figure 3.17 and

Figure 3.18 . To avoid this, the wafer must not be heated. Then how do we deposit the

material? Plasma comes to help here.


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If the WF6 and other gases are passed while a high electric voltage is applied in the chamber,

plasma will form. This is similar to creating plasma in PVD. Now the reaction will occur even at

low temperature. This type of CVD is called Plasma Enhanced CVD or PE-CVD. In this method,

high magnetic field can be applied to increase the density of plasma and improve the deposited

film quality. This process is called High Denisty Plasma CVD or HDP-CVD.

We saw the materials such as silicon dioxide can be deposited using CVD. For example, Tetra

Ethyl Oxy Silane (TEOS) can be heated to high temperature (or disintegrate in presence of

plasma) and form silicon dioxide. Similarly silane gas (SiH4) and ammonia will react to form

silicon nitride. These films would be called as LP nitride, PE-nitride or HDP-nitride depending on

the method used to deposit them.

CVD, Electrochemical Deposition and Spin-On coating

APCVD and LPCVD: There is also another CVD method run at normal pressure. It is called atmospheric pressured

CVD or AP-CVD. Here the temperature is relatively higher, in the range of 600 to 800 oC. In

this method, a thick film can be formed quickly. However, there is risk of generating many dust

particles and the film quality will be poorer. This method is used only in a few select cases.

Mass transfer control vs Reaction control: The following steps occur during CVD. First the

reactant molecules diffuse through the boundary layer near gas-solid interface. Next the adsorb

on the surface. In the third step, they diffuse on the surface. In the fourth step, they react with

each other and the solid product is formed. Any gaseous byproduct formed may be adsorbed on

the surface. Next, they desorb and diffuse outwards into the gas stream and get carried away.When high temperatures are used, the reaction rate is very high and the rate of diffusion of the

reactants through the boundary later decides the film growth rate. On the other hand, when

the temperatures are lower, the mass transfer rate will decrease a bit, but the reaction rate will

decrease a lot, and the surface reaction rate will decide the film growth rate. A plot of film

growth rate vs inverse of temperature will appear as shown below.


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The two different zones where diffusion is rate limiting and kinetics are rate limiting are

marked.

APCVD is usually operated in high temperature regime, i.e. the film growth rate is diffusion

controlled or mass transfer controlled. LPCVD is usually operated in relatively low temperatures

and is reaction rate controlled.

We saw that sometimes the temperature has to be raised to anywhere between 150 to 1000

oC. For lower temperatures, normal heaters may be used. For high temperatures, heating

lamps are used. The lamps enable us to start and stop the heating very quickly. This in turn

results in good control of the deposition process.

Of late, organic gases containing metal atoms or ions are used in CVD. These are called metal-

organic or MO-CVD. Based on a process that is similar to CVD, it is possible to create films of

certain materials with exactly one atom thickness! This is called atomic layer deposition (ALD).

Atomic Layer Deposition: ALD can be thought of as a CVD technique with precise flowcontrol. Consider a reaction between water vapor and trimethyl aluminum (TMA) to produce

alumina (Al2O3) layer. The overall reaction is

If both reactants are supplied to the chamber containing wafer, alumina layer will form on the

wafer, but the thickness will not be easy to control. However, in ALD, first only water vapor is

sent to the chamber. A monolayer of water will chemisorb onto the wafer. Then the chamber is

evacuated and all the water vapor present, except the adsorbed molecules, will be removed.

Next, a pulse of TMA is introduced in the chamber. Now, the TMA will react with limited watermolecules present on the wafer and produce exactly one layer of alumina. The chamber is

evacuated again and the water vapor and TMA are sent in sequence to grow the exact number

of alumina layers required. At present, ALD is used for growing gate oxides in the advanced

chips. Instead of SiO2, other materials such as HfO2 (hafnium oxide) are used as gate oxide in

these chips and ALD offers the control necessary to deposit thin layers. The reaction between

tetrakis dimethyl amido hafnium (structure given below) and water or ozone results in hafnium

oxide.


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Similarly, tetrakis dimethyl amido zirconium and water can be used as two reactants to obtain

zirconia (zirconium oxide) in ALD.

Molecular beam epitaxy: Another technique in research stage is called molecular beam

epitaxy (MBE). This is done in very high vacuum (10-8 Pa). The material to be deposited isheated and the molecules (or atoms) will evaporate. The wafer is kept at a lower temperature.

Due to the high vacuum, the molecules will have a long mean free path and will not interact

with one another. They will deposit on the wafer and due to the slow growth rate, it is possible

to get single crystal growth using MBE. The main disadvantage of MBE is that it is a very slow

process and hence is not yet suitable for implementation in semiconductor industry.

Copper deposited by PVD or CVD method has slightly higher resistance than the film deposited

using electrochemical methods. Hence electrochemical deposition (ECD) is used for coating the

wafer with copper.

Electrochemical deposition (ECD):

Copper is used as interconnect material in the ICs. Copper can be deposited by PVD or CVD.

However, the copper deposited by ECD has a lower resistivity and a better fill characteristic.

The basic principle of electrochemical deposition is very simple. One can even try it at

home. You can take two normal batteries and stack one on top of another and get a 3 volt

battery combination. Two metal plates (e.g. key or shaving blade) should be connected to the

positive and negative end by wires. In a glass beaker, some water and copper sulfate should be

taken. If the metal plates are dipped inside the solution, without touching each other and then

the connected to the batteries, then copper will deposit onto the metal connected to the

negative terminal and oxygen will evolve from the metal connected to the positive terminal. Inthis system, the metal connected to the negative terminal is called cathode and the metal

connected to the positive terminal is called anode. Using the same principle, gold or silver is

also coated on inexpensive ornaments.

We have to remember that silicon is a semiconductor and the wafer must be made conductive

for electrochemical deposition. Therefore, a thin layer of copper is deposited on the wafer using

PVD or CVD first. This is called seed layer. Then the wafer is kept in a tank containing copper

sulfate solution. The negative terminal of a voltage controlling system is connected to the wafer

while the positive terminal will be connected to a copper block. When copper is deposited on

the wafer, the copper content of the solution will decrease. If we use copper block as anode,then copper will dissolve from it and the solution will have a uniform and constant copper

content. By controlling the temperature of the bath and the voltage applied, the thickness of

the copper deposited can be controlled.

Apart from copper sulfate, a few other chemicals are usually added to the bath. This enables

the deposited film to have good quality, without voids. Fig 3.20 shows a poor quality deposit


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and the good quality deposit. Certain large organic chemicals, called macromolecules, are

added to the bath to obtain a leveled surface. These molecules tend to adsorb on the surface

and suppress the deposition rates. They are more likely to adsorb on the flat surface and less

likely to adsorb inside trenches and holes. Thus, they reduce the deposition rate on the top

surface, but do not affect the deposition rate inside the trench and holes significantly. These are

called suppressors or levelers. Certain chemicals called accelerators are also added to theelectrochemical bath. They improve the deposition rate. They compete with suppressors in

adsorbing to the surface and tend to adsorb more inside the trenches and holes. They also lead

to less surface roughness and more uniform grain size of the deposit. They are sometimes

called as brighteners. The addition of suppressor and brighteners in appropriate concentration

makes the ‘bottom up’ fill possible in electrochemical deposition of copper. Note that the

mechanism of action of suppressors and brighteners are not always supported by experimental

evidence and in many cases remain as hypothesis. However since they result in good quality

film, they are used commercially.

FIGURE 3.20. Examples of poor quality deposits and good quality deposits

While PVD or CVD can be used to coat copper on the wafer, it is used only for coating the seed

layer. This is because superior quality copper is obtained by electrochemical deposition. For

example, the electrical resistance of the copper obtained by electrochemical deposition is lower

than that obtained by CVD or PVD. Another important property called electromigration-

resistance, is more for electrochemically deposited copper. Electromigration is the tendency of

the material to move with flow of current. If electromigration is high, then the wire will degrade

soon and hence is not suitable. If electromigration resistance is high, then the material is

considered as suitable. Thus, the ideal interconnect material should have low electrical

resistance and high electromigration resistance. If the grain size of the deposited metal is large,it will tend to have better electromigration resistance. Since electrochemically deposited copper

has large grain size and achieves low electrical resistance and high electromigration resistance,

this method is used in semiconductor industry for depositing copper.

Spin on coating:

This method is used for depositing organic materials. The equipment is similar to the tool used


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in lithography for coating photo resist on to the wafer. First, the material to be deposited is

dissolved in a solvent such as acetone or ethanol. Next, the wafer is kept on a platform and is

rotated at about 500 rpm. While the wafer is spinning, the solution is poured on the wafer. Due

to the rotation, the solution will spread on the wafer. Then the wafer is rotated at higher speed

(about 5000 rpm). This controls the thickness of the solution. If the wafer is also heated, the

solvent will evaporate, leaving the material on top of the wafer in the form of a film.

Traditionally silicon dioxide is used as the insulator between copper wires. Recently some

materials called low-k materials, which have low dielectric constant, are used as insulators.

Some examples of low-k materials are poly-tetrafluroethylene (PTFE, also called Teflon), benzo

cyclobutene and polyimide (structure given below) This enables the chip to run faster and

reduce the electrical current loss. Spin-on method is used to deposit the low-k materials.

(a) (b)

Fig 3.21 Structure of (a) benzocyclobutene and (b) imide

In the next section, we will see the techniques used to remove excess materials, under the

topic “removal methods”.

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Deposition Techniques