Transmission FTIR Manual
for Atomic Layer Deposition
Version 0.1 by Menno Bouman, Byungchang Kang
February 16
Table of Contents
1 Vacuum system ............................................................................................................ 2
Components ................................................................................................................ 2
Vacuum system set-up ................................................................................................ 2
2 ALD Gas delivery lines ............................................................................................... 3
Components ................................................................................................................ 3
Gas delivery system .................................................................................................... 3
3 Substrate preparation ................................................................................................... 4
Components ................................................................................................................ 4
4 Reactor assembly ......................................................................................................... 6
Components ................................................................................................................ 6
5 Precursor gasses and ALD reaction ............................................................................. 7
6 FTIR instrument ........................................................................................................... 7
7 Experimental procedure ............................................................................................... 8
Temperature calibration of the sample ....................................................................... 8
Set up, pumping and system purging .......................................................................... 8
System purging test ..................................................................................................... 9
Substrate drying .......................................................................................................... 9
System gas line, precursor bottle and substrate heating ............................................. 9
ALD on the substrate .................................................................................................. 9
1 Vacuum system
Components
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Mechanical pump
Molecular sieve trap
Rubber tube
Oil trap
Exhausting gas line
Vacuum system set-up
A mechanical pump that is capable of creating a vacuum as low as 10 to 20 mTorr
evacuates the ALD system. The pressure is measured by using a thermocouple vacuum
gauge. The system is equipped with 2 of such gauges, one is near the reactor cell and the
other is attached to a line near the pump. In order to reduce the vibrations caused by the
mechanical pump a rubber tube is used to connect the molecular sieve with the pump. A
schematic view of the vacuum system is shown in Figure 1.
Figure 1: a schematic view of the ALD vacuum system.
2 ALD Gas delivery lines
Components
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¼” Stainless steel tubes
2 thermocouple vacuum gauges
Vacuum valves (Bellows-Sealed Valves (conical), 3-way Ball Valves)
Precursor bottle
N2 gas tank with regulator, 99.9%
NH3 gas tank with regulator, 99.99%
Gas delivery system
The nitrogen gas is used as a purging gas for the system as well as a carrier gas for the
precursor gasses. The flow of the nitrogen gas and ammonia gas are controlled using 3
regulator valves each. Since there are no gas flow controllers as MFC and automatic
valves available, regular hand controlled valves are used. Though this will not affect the
ALD process. A more detailed view of the gas delivery system is given in Figure 2.
Figure 2: a detailed view of the gas delivery system.
3 Substrate preparation
Components
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Nickel grid; dimensions 1” x 0.5”, thickness 0.002”, grid spacing 80 lines per inch
Silicon dioxide powder (SiO2, also called silica)
Nickel wire; diameter 1 mm, 99.9+%
Ceramic insulator
Two copper blocks
Copper wires
Thermocouple (Al-Cr)
Power transformer
Setscrews
The ALD reaction is carried out on a silica (SiO2) pallet. Silica has a large surface area
and is transparent for infrared light which makes it very suitable for FTIR studies. The
silica is mounted on a nickel grid with dimensions of 1” x 0.5”. The thickness of the
nickel threads is 0.002” with a density of 80 lines per inch. The silica is mounted onto the
grid using a manual press (room 1.08, physical science building, ask Ricardo Morales for
key). Some silica powder on both sides of the grid is pressed with a pressure of 4000–
5000 psi as shown in Figure 3.
Figure 3: schematic picture of making a silica substrate.
The nickel grid with the silica pallet is then welded onto two nickel wires (1 mm dia.)
using a spot welder to provide electrical contact and facilitate resistive heating. This
sample is then tightened between two copper blocks and a ceramic insulator to prevent
the sample from creating a short circuit. The fixation is made using setscrews as shown in
Figure 4. The electric power is fed through two copper wires. An Aluminium –
Chromium thermocouple is welded onto the grid to measure the substrate’s temperature.
Welding conditions: Low voltage, manual, ~ 5 Watt.
Figure 4: a schematic view of the sample a used for FTIR studies. Silica is mounted on a nickel grid.
The grid is spot welded onto nickel wires. Electric power is used to heat up the nickel grid and the
silica pallet with it. The temperature is measured using a thermocouple. The nickel and copper wires
are attached using setscrews.
4 Reactor assembly
Components
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Two 2.75” CF flanges
Two NaCl windows
Four O-rings
Double side CF flange with power and thermocouple feed trough and two tubes
attached.
The reactor is the place where the ALD process actually takes places. The ALD reactor
consists of a double site flange clamped between two NaCl windows and two CF flanges.
Four O-rings are placed between the NaCl windows and the flanges. The assembly is
shown in figure 5.
Figure 5: the assembly of the NaCl windows, O-rings and CF flanges.
The sample is positioned in the middle of the reactor facing both NaCl windows, as
shown in Figure 6. The double site CF flange is equipped with two tubes; an inlet for the
nitrogen and precursor gasses and an outlet attached to the vacuum pump. A third tube,
diameter 0.5”, is attached for the feed trough and thermocouple.
Figure 6: a schematic view of the reactor cell front face (left) and side face (right).
5 Precursor gasses and ALD reaction
In atomic layer deposition (ALD) gaseous precursors are dosed alternately onto a surface
(in this case silica) on which they undergo self-limiting reactions with each other. Films
grown by ALD have a very uniform thickness and excellent conformality.
In this experiment higher nitrides of zirconium are produced by atomic layer deposition
from Zr(NEtMe)4 - metal(IV) complex and ammonia (NH3) at low substrate temperatures
(150–250°C).
The precursor tetrakis(ethylmethylamido)zirconium is liquid at room temperature. To
increase the vapor pressure and the reactivity of Zr(NEtMe)4 the liquid heated to 95°C. It
is commercially available from Sigma Aldrich Co. It is highly flammable, irritant and
reacts violently with water. Therefore the precursor should be kept in a glove box filled
with inert gas.
The film that is grown by applying these precursors is Zr3N4. This material is insulating,
transparent and colored.
6 FTIR instrument
Infrared transmittance spectra of the sample are taken using a Fourier Transform Infrared
(FTIR) spectrometer. The spectrometer is controlled using the software program OPUS.
7 Experimental procedure
Temperature calibration of the sample
The temperature of the substrate is measured with a K-type thermocouple that is mounted
on the nickel mesh. However the temperature of the silica may differ from the
temperature of the nickel mesh. The relation between the temperature of the nickel mesh
and silica substrate can be calibrated using a pyrometer. The pyrometer (OMEGA,
OSP100) has the smallest area of measurement at a distance of 9 inch from the object.
The temperature is measured with the pyrometer and K-type thermocouple as function of
the voltage applied on the feed through. This way a relation between the temperature
measured with pyrometer and thermocouple can be derived.
Set up, pumping and system purging
Once the reactor cell is placed in the FTIR spectrometer and the tubes are connected, the
mechanical pump can start pumping the system vacuum. After about 30 minutes the
system reaches a vacuum around 20 to 10 mTorr.
To prevent the NaCl glasses to react with water the space between the glasses and the
spectrometer will be sealed using two foam tubes. These tubes are fed, or purged, with
nitrogen gas by small tubes that are pinned in the foam, as shown in figure 7.
Figure 7: FTIR nitrogen purging. The foam tubes with nitrogen feed are placed in the FTIR
spectrometer to protect the NaCl glasses from contact with water vapor.
System purging test
Once the system has reached a stable vacuum of approximately 20 mTorr, the system can
be purged with nitrogen. Using the regulator valves of the nitrogen gas line a stable gas
flow of 200 mTorr can be established. The stability of the system can be checked taking
FTIR spectra every several minutes. By comparing these spectra relatively to each other
the fluctuations become visible. After about 2 hours the fluctuations will be leveled off to
less than 1%. The system has become stable, this process is called system purging test.
Substrate drying
In order to remove water and other impurities and pollutions from the silica pallet, the
substrate is heated up to 400–500°C. This is done for about two hours long. The greater
part of the former present water is then disappeared. The effect of the heating can be
checked taking FTIR spectra and comparing them relatively. Water shows peaks and
bands in FTIR spectra at 3900–3600 and 1900–1300 cm-1.
System gas line, precursor bottle and substrate heating
The next step in the preparations to an ALD reaction is the heating of the precursor and
substrate. An electric radiator is wrapped around the precursor bottle and the gas lines
from the bottle to the reactor. The temperature of the bottle is measured using a K-type
thermocouple. Heating the gas lines will cause some pollution attached to the inner walls
of the lines to come of. Closing the valve towards the reactor will prevent these pollutions
from contacting the substrate. Applying a voltage on the feed through of the reactor will
heat the substrate up to the desired temperature of 150–250°C. Again a system-purging
test is necessary before the ALD reaction can start.
ALD on the substrate
The ALD process can be described in a few alternating steps:
I. The first step is making a FTIR spectrum of the stable situation before the ALD
starts. This spectrum will be used as a reference, or background, spectrum for the
coming spectra. The conditions where under this spectra will be taken:
i.
Temperature of substrate: 150–250°C
ii. Temperature of Zirconium precursor: 95°C
iii. Nitrogen gas flow pressure: 200 mTorr
II. Expose the substrate to the precursor gas (Zr(NetMe)4 or NH3, depending on the
position in the ALD process)
III. Wait several minutes to let the system level off back to the base pressure
IV. Take a FTIR spectrum of the sample
V. Go to II until experiment is completed.