Supervisors: Ir. J. Neggers Dr. ir. J.P.M. Hoefnagels 16 May 2011

Microstructure analysis of PVD deposited thin copper films Bergshoeff, N.D. MT 11.17

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Content

1. Introduction ......................................................................................................................................... 3

2. Goal...................................................................................................................................................... 4

3. Literature ............................................................................................................................................. 5

Substrate temperature ........................................................................................................................ 5

Background pressure ........................................................................................................................... 5

Sample rotation ................................................................................................................................... 6

4. Setup .................................................................................................................................................... 7

TEER Deposition Machine ................................................................................................................... 7

Pumping system .............................................................................................................................. 7

Sputter Etching ................................................................................................................................ 8

Magnetron Sputtering ..................................................................................................................... 9

Scanning Electron Microscope .......................................................................................................... 10

Electron Backscatter Diffraction ........................................................................................................ 11

5. Results ............................................................................................................................................... 13

Sputter Deposition ............................................................................................................................ 13

Effect of changing rotation ............................................................................................................ 13

Effect of changing duration ........................................................................................................... 13

Effect of changing pressure ........................................................................................................... 14

Heat Treatments................................................................................................................................ 16

Effect of changing temperature .................................................................................................... 16

Effect of changing duration ........................................................................................................... 17

Effect of changing heating up time ............................................................................................... 18

Effect of changing purge gas ......................................................................................................... 19

6. Conclusion ......................................................................................................................................... 20

References ............................................................................................................................................. 21

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1. Introduction Thin film materials are currently widely used in industry, because they form an essential ingredient for

advanced technological products. Common applications are electric semiconductors and optical coatings.

Examples are integrated circuit (IC) chips, micro electro mechanical systems (MEMS), hard disk drives and

flexible monitors. Some pictures of these products are shown in figure 1.

Figure 1: Examples of products made with the use of thin film technology: a flexible screen,

an IC-chip and a MEMS thermal actuator

Thin films are materials with a thickness ranging from nanometers till micrometers and made of different

materials, like silicon, oxides, nitrides, metals and polymers. The complexity of the application of thin films

requires that the mechanical behavior is fully understood. The mechanical properties of these thin films are

different from their bulk counterparts because they typically have one or more dimensions smaller than the

intrinsic length scale of the microstructure. Understanding the formation of the microstructure during

production is therefore very important.

The act of applying a thin film to a surface is called thin film deposition, a technique for depositing a thin film of

material on a substrate or previous layers. Most deposition techniques control layer thickness within a few tens

of nanometers. There are two categories of deposition techniques; chemical and physical. Magnetron

sputtering is a form of physical vapor deposition, which will be used in this project. Sputtering relies on a

plasma, using an inert gas like argon, to knock material from a target and deposit it onto a substrate. A

schematic view of this process can be found in figure 2.

Figure 2: Overview of a magnetron sputter device with a target, a substrate

and a plasma created by argon [http://www.reade.com]

In this report the microstructure of deposited layers is studied, especially the effect of deposition parameters

and different heat treatment methods on the thin film microstructure.

4

2. Goal Before this project another student worked with the bulge tester and the magnetron sputter device during his

bachelor end project. His mission was to investigate size effects of thin copper films by measuring the stress-

strain curve of thin Cu films [Ruybalid, p. 1]. He used the bulge test device to deflect a thin SiN membrane and

after that a membrane with a copper layer on top. The copper layers of various thicknesses are deposited using

the magnetron sputter device.

The first batch of deposited samples showed very brittle fracture behavior for samples with Cu layers thicker

than 500 nm. They also fracture without any visible plasticity. Looking at the samples with the SEM shows that

the grains were very small and the grain boundaries were poorly formed [Ruybalid, p.3]. New settings for

deposition are used which resulted in very similar microstructure. Bulge tests with the new samples result in

some plastic strain before fracturing, although most still fractured without yielding. The stress-strain curve

showing the problems can be found in figure 3.

Figure 3: Stress-strain data for bulge tests performed on the first batch of SiN/Cu bilayer membranes

for various thicknesses [Ruybalid, Figure D.2]

To get more useful results from the bulge tests, decent SiN-Cu samples are needed to study the yielding

behavior of Cu films at different thicknesses in detail. Cu layers with a grain size of the whole thickness of the

copper layer will probably give better results, so the goal of the project is to produce Cu layers with the new

microstructure and grains over the whole thickness of the layer by vary deposition settings with the TEER

magnetron sputter device.

5

3. Literature Many deposition parameters may affect the structure of thin films. Examples of these parameters are

background pressure, sample rotation, deposition rate, substrate material, deposition temperature, surface

cleanliness and film thickness [Machlin, p. 1]. Only the deposition parameters which are varied in this project to

reach the goal of large grains will be discussed in this chapter.

Substrate temperature The substrate temperature is the deposition parameter that has the greatest effect on the deposited thin film.

Substrate temperature will rise if the magnetron power will be increased and when the deposition without

rotation will last longer. Adatom (an atom which lies on a crystal surface) diffusion on the substrate surface,

the rate of adatom desorption and the rate of nucleation control the grain size at impingement and are all

exponentially dependent on temperature [Thompson, p. 182]. The grain size at impingement increases with

increasing substrate temperature. Movchan & Demchishin made a structure zone model of the relationship

between substrate temperature and structure evolution during deposition, it is shown in figure 4a.

Figure 4: Movchan & Demchishin zone models which shows a relation between temperature and structure evolution. The

right map is the zone model for FCC metals under ultra-high vacuum conditions [Thompson, p. 184]

The zone diagram of Movchan & Demchishin is a useful tool to illustrate the expectations for structural

evolution as a function of the process parameters. However, such diagrams are developed for a specific

material or class of materials and specific processing conditions. For materials like copper, which is mainly used

in this project, the structure zone model of figure 4b would be a better representation. Reason for this is the

higher atomic mobilities characteristic of FCC materials compared with BCC materials [Thompson, p. 183].

Background pressure The background pressure of the inert gas during deposition affects the film microstructure in several ways.

When the pressure is increased the material will result in a more open structure and it has been suggested that

it also decreases adatom mobilities [Ohring, p. 499]. A reduction in gas pressure results in increased energy of

particles which densifies the film.

6

In figure 5 the Thornton structure

zone model is shown. It does not only

show the effect of substrate

temperature on the film structure, but

also background pressure as variable.

Thornton noted that by sputter

deposition the adatom mobility is

affected by the energy with which the

atoms arrive at the film surface. This

energy depends on the number of

collisions of the atom which scales

with the gas pressure [Thompson,

p.183]. This is illustrated by adding

another dimension to the Movchan &

Demchishin structure zone model.

Sample rotation Sample rotation is a deposition

parameter which also affects the

microstructure of the deposited

material. When the rotation of the

samples is stopped during deposition

the substrate temperature will be

much higher due to the constant flux

of high energy atoms to the surface.

The effects of increasing temperature

can be found in the first part of this

chapter.

The line-of-sight travel of the

incoming particles is another aspect

which is affected by the rotation of the samples. In line-of-sight travel of particles that are deposit on a thin

film, projections on the surface can shadow other parts of the film surface from the incoming particles and a

network of voids separating fibers or columns of film material is formed in the shadowed regions. When all the

particles have the same direction, it is possible to change the crystallographic texture and the orientation of

columnar boundaries by changing the incoming angle of the depositing atoms [Machlin, p.4]. When the

substrate is right in front of the target, straight columnar boundaries will form.

Line-of-sight travel of atoms applies only when the chamber pressure is less than about 0.01 mbar. The mean-

free path between gas collisions exceeds about one centimeter at lower chamber pressure [Machlin, p.4].

When sputtering with the standard argon pressure of the used setup of 0.008 mbar this will be the case.

Figure 5: Thornton structure zone model with substrate temperature and

inert gas pressure as variable deposition parameters [Ohring, p.500]

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4. Setup

TEER Deposition Machine

Figure 6: Picture of the TEER coatings Ltd. magnetron sputter device in W-Hoog 0.053 [Made by Jan Neggers]

Figure 6 shows a picture of the setup of the TEER magnetron sputter device which can be found in W-Hoog

0.053. The setup consists of a lot of different devices, at the left side the inflow of argon and nitrogen and the

cooling system, in the middle the chamber, the pumps and pressure sensors and on the right a cabinet with the

control unit and the AC and DC magnetrons. In the next paragraphs there is a short explanation about the most

important parts of the setup: using the turbo pump to create a vacuum, clean the specimens in the chamber by

sputter etching and the deposition process using the AC and DC magnetrons.

Pumping system

The pumping system of the deposition system consists of two pumps: a mechanical rotary piston pump and a

turbo molecular pump. Figure 7 shows a schematic view of the placement of the pumps and how they are

connected. It is very important that the turbo pump should never be in direct contact with atmospheric

pressure when on and also the backing pump should always be running when the turbo pump is on [Neggers p.

1]. When the turbo pump is exhausted directly to the atmosphere it stalls and breaks, so the rotary pump is

used to produce the minimum vacuum level required (about 10e-1 mbar) to let the turbo molecular pump

operate.

Figure 7: Schematic view of the placement of the rotary pump and the turbo pump and how they are connected with the

chamber and each other (a turbo molecular pump is used instead of a multistage diffusion pump) [Ohring, p. 81]

In the rotary piston pump gas is drawn into a space when

isolated from the inlet after one revolution, then compre

molecular pump work on the principle that gas molecules can be given molecular motion in a preferred

direction. This impulse is caused by impact with a rapidly rotating turbine rotor with rotating speeds

30.000 revolutions per minute [Ohring, p. 76

this type of pump.

Sputter Etching

When the samples are placed and there is a high vacuum in the chamber, the samples have to be cleaned first

before deposition. This is called etching, where atoms will be removed from the film or substrate surface in a

plasma to get a flat surface without pollution

cathodes during sputtering in low-pressu

sensitive to the magnitude of atomic bonding forces and structure. For this reason there is no selectivity in the

removal of surface atoms. Figure 8 shows the mechanism of sputter etc

Figure 8: Mechanism of sputter etching, where an ion strikes the surface

and removes atoms

8

view of the placement of the rotary pump and the turbo pump and how they are connected with the

(a turbo molecular pump is used instead of a multistage diffusion pump) [Ohring, p. 81]

In the rotary piston pump gas is drawn into a space when the piston rotates [Ohring, p. 71

isolated from the inlet after one revolution, then compressed and exhausted during the next cycle.

work on the principle that gas molecules can be given molecular motion in a preferred

direction. This impulse is caused by impact with a rapidly rotating turbine rotor with rotating speeds

lutions per minute [Ohring, p. 76]. Extreme low pressures below 1e-10 mbar

When the samples are placed and there is a high vacuum in the chamber, the samples have to be cleaned first

This is called etching, where atoms will be removed from the film or substrate surface in a

to get a flat surface without pollution. According to [Ohring, p. 238], sputter etching happens at

pressure plasmas. Because their energies are high, ions striking a surface are

sensitive to the magnitude of atomic bonding forces and structure. For this reason there is no selectivity in the

shows the mechanism of sputter etching.

: Mechanism of sputter etching, where an ion strikes the surface

and removes atoms from the surface [Ohring, p.239]

view of the placement of the rotary pump and the turbo pump and how they are connected with the

(a turbo molecular pump is used instead of a multistage diffusion pump) [Ohring, p. 81]

p. 71]. There the gas is

ssed and exhausted during the next cycle. The turbo

work on the principle that gas molecules can be given molecular motion in a preferred

direction. This impulse is caused by impact with a rapidly rotating turbine rotor with rotating speeds up to

10 mbar are achievable with

When the samples are placed and there is a high vacuum in the chamber, the samples have to be cleaned first

This is called etching, where atoms will be removed from the film or substrate surface in a

sputter etching happens at

re plasmas. Because their energies are high, ions striking a surface are

sensitive to the magnitude of atomic bonding forces and structure. For this reason there is no selectivity in the

: Mechanism of sputter etching, where an ion strikes the surface

9

Magnetron Sputtering

Sputtering is a mechanism by which atoms or molecules are ejected from a target material by high-energy

particle bombardment and condense on a substrate as a thin film. Magnetron sputtering is the most widely

used variant of DC sputtering, because deposition rates are much higher for the same applied voltage and the

operating pressures can be reduced [Ohring, p. 223].

Figure 9: Schematic view of the principle of DC magnetron sputtering [Institute of materials & machine mechanics, Slovak

academy of sciences, http://www.umms.sav.sk]

Figure 9 shows a schematic view of the DC magnetron sputter process. When the chamber is filled with

pressurized argon the magnetic field in combination with the voltage of the direct current creates a glow

discharge (purple color in figure 9). A glow discharge is a self sustaining type of plasma, creating free electrons

(red dots) within the discharge region. The glow discharge also generates high-energy particles (Ar+ ions) which

are directed at the target material. The ions sputter atoms from the target and they are transported (blue

arrow) through a region of reduced pressure to the substrate. When the atoms arrive at the substrate they

condense on it, forming a thin film.

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Scanning Electron Microscope A scanning electron microscope (SEM) is a type of microscope which doesn’t work with light, as conventional

microscopes do, but it uses a beam of electrons to get an image of the specimen. It is primarily used to study

the surface or near surface structure of materials.

According to [Goodhew, p. 106], an electron gun produces

electrons and accelerates them to energy between about 2

keV and 40 keV. Two or three condenser lenses reduce the

electron beam to a diameter of only 2e-10 m. The beam is

scanned across the specimen while a detector counts the

number of radiations given off from each point of the

surface. This radiation could be secondary electrons,

backscattered electrons or X-rays. At the same time the

spot of a cathode ray tube (crt) is scanned across the

screen. The electron beam and the crt spot are both

scanned in a similar way to a television receiver, which

creates an image of the specimen on the screen to study

the surface. In figure 10 a schematic view can be found of

the main components of a scanning electron microscope.

Image processing

The copper samples from which the surface will be studied

at the SEM have a maximum thickness of about 1 µm and

some of them have a thickness of about 100nm (0,1 µm).

The region into which the electrons penetrate the

specimen is called the interaction volume and it’s shown in

figure 11. According to [Goodhew, p.110], electrons will

not be backscattered out of the specimen if they have

penetrated more than a fraction of a micron and

experiments with materials of medium atomic weight, like

copper, indicate this will be about 0.1 µm. For secondary

electrons this is even worse, the detected signal originates mainly from a region which is little larger than the

diameter of the incident beam, like the top of the teardrop-shaped volume in figure 11.

Conclusion for use with thin film copper in the range of

hundreds of nanometers will be that only secondary electrons

will give a reliable view of the surface of the sample. However,

this will not be a problem because secondary electrons give the

best spatial resolution caused by their small sampling volume.

Therefore the secondary electron signal is most used at

scanning electron microscopes.

Figure 10: Schematic view of the main components of

a scanning electron microscope

[http://www.purdue.edu]

Figure 11: Drawing of the interaction volume

and the regions from which secondary electrons,

backscattered electrons and X-rays may be

detected [Goodhew, p. 110]

11

Electron Backscatter Diffraction When studying the surface of a sample is not sufficient, it is also possible to get some information about crystal

orientation of the material with a scanning electron microscope. When the electron beam penetrates the

material the electrons undergo various interactions with the atoms in the crystal lattice. Some of these

electrons emerge from the sample and when they are collected on a phosphor screen a pattern is formed. This

diffraction pattern (also called Kikuchi pattern) can be formed because the intensity of the emerging electrons

varies with direction. To get enough contrast in this pattern the specimen must be tilted through a large angle

(>70 degrees). The diffraction pattern can be used to obtain information about crystal orientations and

measuring grain size, which is very useful for this project. An example of a very clear diffraction pattern of

copper at 20keV can be found in figure 12.

Figure 12: Very clear backscatter diffraction pattern from copper

at 20 keV [http://www.crystaltexture.com]

Image processing

When using electron backscatter diffraction at thin film copper layers the problem with the interaction volume

occurs again, because this time backscattered electrons instead of secondary electrons will be used. As stated

in the last chapter about the SEM, the source of backscattered electrons is about 1-2 µm under the surface.

When using copper layers with a thickness of under 1 µm, this could be a problem, in particular when the

specimen is tilted seventy degrees. When using high energies and beam size the beam will go through the

copper layer and backscattered electrons from the underlying silicon will be collected at the detector. To get

the best results a beam energy of 15 kV and a spot (indicator for beam size) of 4 is used.

To get an overview of the crystal orientation in an indicated area of the specimen, the electron beam scans

over the sample and measures the orientation of the diffraction pattern at each point. The scan area must be

big enough to scan multiple different grains to measure a reliable grain size, for example 10x10 µm. When the

scan is completed the dataset can be processed to get the right information from the material. To obtain a

reliable grain size from the dataset some parameters has to be set in the software: confidence index (CI), grain

tolerance angle and minimum grain size.

The confidence index indicates how reliable the patterns of the different grains at one point are. Therefore low

confidence indices has to be filtered out of the dataset to calculate grain size. In practice a confidence index

>0,1 is acceptable, so all points in the dataset with a CI <0,1 will be filtered out of the grain size calculation. The

grain tolerance angle is the angle between grains at which two adjacent grains will be different grains. In the

manual of the OIM software an angle of fifteen degrees is assumed to be the grain tolerance angle for most

12

materials. The minimum grain size is the minimum number of scan points a grain must include to be called a

grain. By looking at the inversed pole figures a number of ten points at one grain will be a good assumption.

With these settings the dataset will be filtered to calculate a reliable grain size.

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5. Results

Sputter Deposition

Effect of changing rotation

To work on the goal of larger grains, one of the options is to stop sputtering with rotation of the mill which hold

the samples. As explained in chapter 3, this will have great influence on the microstructure of the material

because temperature during deposition will rise and the direction of impact of the copper particles will change.

The mill is positioned at an angle where the samples are right in front of the copper target to get the best

transport of copper through the chamber. The flow of copper atoms to the surface in this situation will be a lot

bigger compared to a rotating mill. To get samples with the same thickness, the duration of the sputter session

has to be shorter. The samples are at a length of 2/3π in the field of the floating copper particles, so they will

receive copper particles about one third of the time. To compare the samples with and without rotation, the

other deposition parameters will be the same, 300W DC, 50W AC and 8*10-6

mbar argon pressure.

Figure 13: ESEM pictures of sputtered copper samples, 300W DC, 50W AC

Left: 4 minutes deposition time, with rotation, layer thickness ~50 nm, grain size 20±5 nm

Right: 1,5 minutes deposition time, without rotation, layer thickness ~80 nm, grain size 22±5 nm

One sputter session was done with rotation and 4 minutes deposition time, while the next session was without

rotation and 1,5 minutes deposition time, about three times shorter. Results are shown in figure 13. Grain size

of both samples is almost the same and far from big enough. Conclusion is that only stopping the rotation will

not give large grains because the temperature increase is not high enough.

Effect of changing duration

Only stop the rotation will not give large grains, so the temperature has to rise in another way to let the grains

grow. One of these ways is to use a longer deposition time. At the first sessions 4 minutes with rotation and 1,5

minutes without rotation is used, but the next time the deposition time will be three times longer to get a quick

look of which time range will be good. Rotation will still be switched off because without rotation the

temperature will rise a lot faster.

14

Figure 14: ESEM pictures of sputtered copper samples, 300W DC, 50W AC

Left: 4,5 minutes deposition time, without rotation, layer thickness ~250 nm, grain size 28±5 nm

Right: 15 minutes deposition time, without rotation, layer thickness ~800 nm, grain size 2±0.5 µm

Result of the 4,5 minutes sputter session is shown in figure 14a. Grain size is still almost the same as 1,5

minutes deposition time. The deposition time of the next session will be again three times longer, so the

temperature is high enough to reach a point where the grains will grow substantially. Figure 14b shows the

copper surface measured by ESEM, showing that for extreme sputter times without sample rotation the

surface will reach temperatures where the copper layer will show grain growth or even recrystallisation.

Nevertheless, this method is not desired because the high temperatures causes too large residual stresses to

remain in the sample at room temperature. More importantly, this method of grain growth will only occur for

very thick films in the range of micrometers and is thus not possible over the entire film thickness range.

Effect of changing pressure

Another parameter during the sputter process which can be modified is the pressure of the argon in the

chamber. In chapter 3 the influence of this parameter on the microstructure is explained. To get large grains

without large residual stresses and over the entire film thickness range, the argon pressure has to go up instead

of deposition time.

In the previous sputter sessions an argon pressure of 8e-6 mbar is used, this time pressures of 1e-5 ,3e-5 and

5e-5 mbar will be used. Higher pressures are not possible due to the maximum argon flow of the Mass Flow

Control (MFC).

15

Figure 15: HRSEM pictures of the copper surface, 500W DC

Left: 5 minutes deposition, argon pressure 50e-6 mbar, with rotation, grain size100±20 nm

Right: 5 minutes deposition, argon pressure 30e-6 mbar, with rotation, grain size 50±10 nm

During sputtering the plasma shows a total different color. The plasma at 30e-6 mbar shows a light green color,

while the plasma at 50e-6 mbar shows a intense dark green color. Sputtering with standard argon pressure

gives a light purple color. Figure 15 shows the results of the sputter sessions with 50e-6 mbar and 30e-6 mbar.

With grain sizes of about 100 and 50 nm, the grains are significantly larger compared with the standard

pressure. They are still not big enough to reach the goal, but the grains are about four times larger than with

the standard argon pressure so larger grains will occur at lower temperatures with higher argon pressure.

16

Heat Treatments After several sputter sessions the choice is made to stop with varying parameters of the process and find

another way to produce copper layers with big grains. Problems as too much residual stress, too thick layers

and small grains at low temperatures don’t give the desired result. Heat treatment of the copper layers is

another way to let the grains grow.

Information about thin film heat treatment is not available, so typical heat treatment temperatures and

durations for bulk materials are used to start with. According to [Callister, p. 198], the recrystallisation

temperature Tr of copper is 120°C and the melting temperature Tm is 1200°C. Typical temperatures for heat

treatments for grain growth are about 0,5 Tm, a typical duration is one hour.

To compare the results of the different heat treatments every time the same samples will be used, sputtered at

300W DC and 50W AC and a deposition time of 45 minutes. Alternating current is used to get samples with

nicely formed grains and to get some small stress in the material to trigger grain growth. To avoid the forming

of oxidation at the surface of the sample the vacuum furnace will be used.

Effect of changing temperature

For the first heat treatment a temperature a bit below 0,5Tm will be used. According to [Callister, p.197], there

will be some recrystallisation and grain growth in bulk materials already above Tr. To look if anything happens

with grain size 400°C is chosen to start with. The duration will be one hour, a standard heat treatment time for

bulk materials.

Figure 16: ESEM images of copper layers after heat treatments

Left: One hour heat treatment at 400°C, grain size at surface 35±10 nm

Right: One hour heat treatment at 500°C, oxidation at the surface

An ESEM picture of the surface of the copper layer treated one hour at 400°C is shown in figure 16a. Grain size

is about 35 nm, which means almost no grain growth: from 30 nm to 35 nm. This setting of heat treatment will

not give large grains, so the next attempt will be with higher temperature: 500°C. Figure 16b shows the surface

of the sample heat treated at 500°C. It is a total different result compared with the surface of the previous

sample. The particles at the surface are significantly bigger, but there form are different than before.

Unfortunately the particles at the surface seems to be oxidation of copper caused by a little bit present oxygen

in the vacuum chamber.

17

At the surface the grain size can’t be measured anymore because the surface hardly changes during heat

treatment and the presence of oxidation, but with other electron microscopy techniques it is possible. One of

them is orientation imaging microscopy (OIM) which uses the principle of EBSD, more information about this

technique and how it is used can be found in chapter 4.

Figure 17 shows the inversed pole figure of the copper

sample heat treated one hour at 500°C. The different

colors in the figure shows the different directions of the

grains in the material. With another function in the OIM

software the grain size can be determined, which is about

250nm. This means notable grain growth with grains of

about half the thickness of the copper layer. It’s a quite

good result, but the goal of grains over the total thickness

(about 800 nm) of the layer is not reached.

When the temperature gets increased further, the

residual stresses in the material will be bigger. At 500°C

this was almost critical for the copper layer. To avoid the

same problem as during sputtering the temperature

cannot be increased further at the next heat treatment.

Effect of changing duration

Another way to get more grain growth without increasing temperature is to treat the material longer. One hour

is too short to let the grains grow to the desired size, so the next heat treatment will be with duration of two

hours at 400°C. At this temperature there was almost no grain growth at the surface, but to be sure there will

not be too much residual stress in the material afterwards the temperature can’t go up even more.

Figure 18: Inversed Pole Figures of copper samples

Left: Deposited half an hour with 300W AC/50W DC, heat treated two hours at 400°C

Right: Deposited half an hour with 300W AC/50W DC, heat treated ten hours at 400°C

Figure 17: Inverse Pole Figure of a copper sample,

deposited 300W AC/50W DC, heat treated one hour

at 500°C

18

An inverse pole figure of the copper sample heat treated two hours at 400°C can be found in figure 18a. The

grain size is about 200 nm, which is six times bigger than the grains without heat treatment. At the top layer

the grains are also measured by ESEM and the size of them are about 35 nm. Conclusion is that the grains at

the surface of the copper layer almost don’t grow. Still the grains are not large enough, so a longer heat

treatment will be the next step.

To get a quick view of which duration of the heat treatment at 400°C will give the desired grain size, the next

treatment will be ten hours long. Figure 18b shows the inverse pole figure of the copper sample heat treated

10 hours. Grain size from the software is about 300 nm. This grain size still not satisfy the goal of grains over

the thickness of the layer, so this ten hour treatment concludes that a lot more time will not give more grain

growth and the goal will not be reached at 400°C, no matter how long the heat treatment will be.

Effect of changing heating up time

One of the reasons for getting moderate grain growth with heat treatments could be the long heat up time of

the used furnace. It takes the furnace about half an hour to get from room temperature to the desired

temperature. Due to recrystallisation the stress in the material after deposition will disappear, which prevents

the grains from growth. Solution for this problem will be to get a really high temperature gradient in the

material so recrystallisation is minimized. Internal stresses from deposition will remain in the material and will

effectuate grain growth. The vacuum furnace is not suitable for this way of heat treatment, for this reason a

atmospheric furnace with nitrogen purge is used.

The goal of the heat treatments with shorter heating up time is to get large grain growth by putting the

samples in the furnace at the desired temperature. This will be 500°C and the duration will be one hour, to

compare the results with that from the previous heating up time.

Figure 19: SEM picture of oxidation at the surface of a copper layer, deposited half an hour with 500W DC and heat

treated one hour at 500°C

Result of this heat treatment is a lot oxidation or copper nitride (CuN) on the copper surface which is even

visible with the naked eye. Figure 19 shows a SEM picture of oxidation or CuN on the surface. As a result of the

oxidation it is impossible to get a good signal during the oim scan, so unfortunately the effect of this heat

treatment on the grain size can't be measured. Three treatments with higher nitrogen pressure (0.5 bar, 1.0

bar and 1.5 bar) do not help to expel the oxygen from the chamber and prevent oxidation.

19

Effect of changing purge gas

In practice inserting of the sample in a hot surface will work, but despite the nitrogen purge there is still too

much oxygen left in the chamber which causes oxidation on the copper surface. To research the effect of a high

temperature gradient during heating up on grain growth, a solution could be to purge the chamber with argon

instead of nitrogen. Argon is an inert gas, which means that it almost do not react with other substances and

will reduce the forming of oxidation.

Temperature and duration will again be 500°C and one hour to compare the results with previous treatments.

Figure 20: SEM picture of the surface of a broken copper layer, deposited half an hour with 500W DC and heat treated

one hour at 500°C

Unfortunately the heat treatment with argon purge did not work. Too much oxygen is present in the chamber

which can't be expelled by the argon flow. For this reason there is still too much oxidation on the surface of the

copper samples to get a usable signal for OIM scanning.

Another way to reduce the forming of oxidation is to speed up the cooling down process by laying the copper

sample on a block of aluminum directly after the heat treatment. When the temperature gradient in the

material is very high there is less time to oxidize during cooling down. Figure 20 shows the result of this fast

cooling down process; the stresses in the material during cooling down are getting too big and the copper layer

fractured. For thin film materials the conclusion is that the fast cooling down method will not work.

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6. Conclusion Varying the background pressure and stopping the rotation of the substrate during deposition did not lead to

the desired grain size at a layer thickness of about 100 nm. However, the combination of longer duration times

and no rotation creates a higher substrate temperature which is necessary for producing copper layers with the

desired grain size. After fifteen minutes of deposition at medium background pressure a grain size of about two

μm was obtained, which achieves the goal of columnar grains at a layer thickness of about 800 nm. However,

after cooling the samples there was too much residual stress in the material and, additionally, this grain size

will only occur for very thick films.

Heat treatments of the copper layers after deposition are another way to increase the grain size of the small

grains created by sputtering. Heat treatments in vacuum gave some moderate grain growths to grains of

approximately 200-300 nm. Nevertheless, regarding the 750 nm thick copper layers, this is not large enough to

achieve the goal. An attempt was made to increase the grow speed by reducing the ramp-time. But, all of these

samples fractured due to too high internal stresses in combination with surface oxidation, making it impossible

to measure the grain growth.

The temperature during deposition was identified as the biggest influence on the microstructure of the

sputtered material and in the current setup the substrate temperature is completely unknown. Therefore it is

recommended to add a temperature control module to the current deposition setup to allow for temperature

controlled deposition. A recommendation for heat treatment is to use a furnace where a short ramp time can

be used in a vacuum. That would solve the problem of oxidation. Another way of heat treatment is possible in

the chamber of the sputter device, where the radio-frequency (RF) generator could be used to give the

material a heat treatment directly after deposition. Additionally, if possible, using a different metal material

could also be a solution to create the desired microstructure.

Finally, it can be concluded that while the desired microstructure could not be reached, using the equipment of

the MultiScaleLab, for these thin copper films, instead a lot of experience is gained about using the TEER

magnetron sputter device. Also, working with the electron microscopes has lead to useful insights into the

highly specialized settings needed when using orientation imaging microscopy for these thin film materials.

21

References

Books:

[Machlin] Machlin, E.S., Materials Science in Microelectronics I, Elsevier, 2005

[Ohring] Ohring, M., Materials Science of Thin Films; Deposition and Structure, Academic Press, 2002

[Goodhew] Goodhew, P.J. & Humpfreys, F.J., Electron Microscopy and Analysis; Second Edition, Taylor &

Francis, 1988

[Callister] Callister Jr., William D., Materials Science and Engineering; An Introduction Seventh Edition,

John Wiley & Sons, Inc., 2007

Articles:

[Thompson] Thompson, C.V., Structure Evolution During Processing of Polycrystalline Films, Annu. Rev.

Mater. Sci. 30:159-190, 2000

[Neggers] Neggers, J., Manual for the TEER PVD deposition machine located at wh0.053, May 2009

[Ruybalid] Ruybalid, A., Appendix D: Measuring size effects in thin Cu films, 2010

Websites:

[1] http://www.siliconfareast.com/sputtering.htm (Consulted march 2011)