Chapter 1 Introduction
1.6 Motivation and Strategies
1.6.2 Strategies
In this study, a few impurity doped transition metal oxides produced by Sol-Gel method
are investigated and some properties of samples are connected to the mechanisms. First, we
use the Sol-Gel method to spin coat the impurity doped transition metal oxides on a substrate;
the I-V characteristic are employed to find out bistable electrical properties. X-ray diffraction
is, then, used to identify the film type and the crystallized states. The thickness and surface
morphology of film are also observed and measured by SEM. In the end, some properties of
the film are discussed and concluded.
Chapter 2
Experimental Detail
In this chapter, we introduce the fabrication of bottom electrode using sputtering and
present how to deposit a transition metal oxide film by Sol-Gel method. Both the physical and
the electrical characteristic of the film were analyzed by X-Ray diffraction to identify the
structure and the I-V measurement was used to observe its bistable properties.
The precursor solution is prepared by using both the acetic acid and the acetylacetone as
solvent and some kinds of materials which include acetate, nitrate, propoxide, and ethoxide
were also used as solutes.
2.1 Bottom Electrode
2.1.1 Sputtering System
RF magnetron sputter is used in deposition of a variety of metallic films in VLSI
fabrication, and it is also used in some applications to deposit dielectric film because
sputtering method is not only matured but also simple and easy to apply. In our work, it is
used for deposition of a bottom electrode on a substrate. We introduce it as the following.
The mechanism of the RF magnetron sputter is based on sputtering. First, the RF
magnetron sputter system produces secondary electrons between two electrode plates where
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the substrate and the target are seated respectively, and then the collision between the
electrons and the inert gas which is pre-filled in the sputter chamber makes the inert gas be
ionized. These ionized molecules move to the cathode and collide with it due to the Coulomb
force. After the crash between the cathode and ions, the atoms on the target get the kinetic
energy and thus escape from the cathode. Finally, these atoms would be deposited on the
anode (substrate) and the film is therefore formed.
The RF magnetron sputter composed of 4 systems as discussed below.
(1) Vacuum system is mainly made up of a vacuum pump system. This system consisting
of two major parts, a diffusion pump and a mechanical pump, and can control the pressure in
the chamber.
(2) Plasma generator system, composed of a RF power generator providing a maximum
power of 600W is primarily employed to produce plasma. The system not only uses the
secondary electrons to collide with the inert gas to produce plasma but utilizes RF coil to
increase the residence time of the sputtered metallic particle.
(3) Gas flow system, which is mainly composed of the mass flow meter is used to
control the ratio and flow rate of the gases during the deposition process.
(4) Cooling system, this part is used to cool the diffusion pump in the vacuum system,
the magnet in the chamber and the metal chamber. When the diffusion pump operates, the
pump increases the temperature to evaporate the oil and afterwards, the cooling system is used
to cool and control the pump temperature to prevent it from being too high. Furthermore, as
the sputtering is in progress, the magnet temperature rises. It is well-known that if the
temperature of a magnet material is too high, the magnetic force will be degraded. Therefore,
the cooling system can protect the temperature of the magnet from being too high.
2.1.2 Preparation of Target
The sputtering target of LNO is prepared by conventional solid state ceramic
powder-mixing way. La2O3 is blended with NiO in a stoichiometric ratio at La2O3 : NiO = 0.5 :
1. Then, the blended powder is ball milled for 12 hours to ascertain the particle size reduction
and the homogeneity of the mixing. The blended powder is calcined at 1300 0C for 4 hours in
air. Finally, the calcined powder is mixed into a disc of a 3-inch diameter.
2.1.3 Deposition of Bottom Electrode
After the preparation of target, the La2O3 was deposited by sputtering on SiO2 which is
grown on a 4-inch bare silicon wafer, and then its thickness grows to about 2000 Ao .
2.2 Transition Metal Oxide Film Samples
2.2.1 The Preparation of Solution
Acetic acid [C2H4O2] and acetylacetone [C5H8O2] are used as solvent to dissolve
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different kinds of dopants and starting chemicals including Zirkon (IV) propylate, propoxide,
Nickel (II) acetate tetrahydrate, chromium (III) nitrate nonahydrate, niobium (V) ethoxide,
molybdenum (II) acetate diamer, vanadium (V) oxide, magnesium acetate tetrahydrate and
titanium (IV) ethoxide [Ti(OCH5)4] whose ionic radius is shown in table 2-1.
Acetic acid, first, is heated at the temperature of 80 oC for 10 minutes in order to remove
partial water content. Materials, shown in table 2.2.1-2 are used as dopants to the solvent.
Then, the solution is stirred and heated at temperature 80 oC for 30 minutes for synthesis.
Finally, acetylacetone [C5H8O2] and Zirkon (IV) propylate, propoxide [Zr(OCH2CH2CH3)4]
is mixed with the solution at temperature 80 oC for 1 hour.
2.2.2 TMO Film
First, on a 4-inch silicon substrate, 2000 Ao thick SiO2 is grown and subsequently, LNO is sputtered as bottom electrode. The electrode is cut into 2.2 x 2.2 cm2 square areas and then
the squares are put in an oven in order to degas. After 2 hours, the film is spun on the
substrate at two steps spin rates which are 1000 rpm and 3500 rpm respectively and whose
duration are 5 seconds and 1 min separately. It is, then, heated on a hot plate at temperature
125 oC for 10 minutes so as to remove the wet and partial acetate content.
Second, the film undergoes two step heat treatments in a furnace. The first one is in order
to remove residual water content and its temperature is 200 oC for 10 minutes. After the first
step, the pyrolysis, proceeds successively at temperature varying from 400 oC to 700 oC in
order to remove organic compounds.
2.2.3 Deposition of Top Electrode
After the TMO film has formed, the top electrode, Al, is deposited on it with metal
masks by a thermal coater. The metal masks are used to define different areas, and therefore
the probe of the probe station could probe on these areas to measure electrical properties. The
thermal coater is, first, pumped down to a pressure smaller than 10-6 and then the current
passes through tungsten boat to heat the aluminum bullet and evaporate aluminum of about
3000 Ao onto the TMO film at a deposition rate of about 8 /s.
o
A
2.3 Physical characteristics
2.3.1 Scanning Electron Microscopy (SEM)
The film thickness is examined by scanning electron microscopy (SEM) using Hitachi
model S4700I. The microstructures of thin films in the cross section are investigated by SEM.
Besides, In order to know the relationship between the thickness of the films and solutions of
different mole concentrations, we deposited different layers with mole concentrations varying
from 0.05 mole/l to 0.5 mole/l.
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2.3.2 X-Ray Diffraction Analysis
The film structure and the crystallization related to temperature is investigated by X-Ray
diffraction. First, we use the solutions of 0.05 mole/l, 0.1 mole/l, 0.3 mole/l and 0.5 mole/l to
deposit 1, 2 and 3 layers of the films. The samples are subsequently thermaly treated in a high
temperature furnace and in a rapid thermal processing (RTA) furnace, respectively, with
temperatures varying from 400 to 800 oC. Finally, these samples, were cut into about 0.5 x 0.5
cm2, analyzed by a Rigaku Dmmax-B diffractometer with 0.02 degree beam divergence and
operated at 30 KV x 20 mA with Cu K radiation. α
2.4 Measurement System
The system of measurement is composed of three parts, measurement machines, a
personal computer, and Agilent VEE software. Measurement machines are made up of a probe
station, one Agilent 4155C semiconductor parameter analyzer, one Agilent Agilent E5250A
low leakage switch and one Agilent 81110A pulse generator. However, Agilent VEE software
is used to control the measurement machines.
2.5 Electrical Measurement
In the RRAM, electrical properties can be divided into two parts, static and transient
characteristics. The static one is measured by applying a static voltage with a rate of 0.5 V/s
sweeping from positive maximum to negative minimum and then scanning adversely. The
waveform is shown in fig 2.5.0-1.
On the other hand, the transient property is measured by a sequence of steps interpreted
in the following. First, a positive voltage whose magnitude is just a threshold voltage (will be
explained in the next chapter) is applied on the sample with a short duration which can be
adjusted by the Agilent VEE software and can vary from 1us to 0.99 s. The readout voltage is
subsequently applied on the sample to extract the conductance state from the film.
Second, a negative voltage whose magnitude is also just a threshold voltage is applied on
the sample for a short duration. After this step, the readout voltage is again applied on the
sample to trace out another conductance state.
In conclusion, we, first, introduced how to sputter a LNO film involving sputtering
system, preparation of target, and deposition of bottom electrode, and we also presented the
fabrication of a transition metal oxide film sample, including how to make up solutions and
the process of deposition of the sample. Furthermore, X-Ray and SEM are used to analyze the
structure, crystallized properties, thickness and morphologies of the film. In the end, electrical
analysis presented two kinds of states, that is, the static state and the transient state. The
difference between these two states depends on what kind of wave form is applied.
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Chapter 3
Results and Discussion
In this chapter, we show and discuss the results of X-Ray, SEM, electrical properties.
First, the results of X-Ray related to different solutions are used to identify whether the
structure of the films is correct or not. Furthermore, we investigate crystallization properties
related to different temperatures. Second, the plane view of scanning electron microscopy
(SEM) is employed to observe surface morphology and to measure the thickness of films with
different solutions of mole concentration. Finally, we show the electrical characteristic of the
film with different parameters of solution, thickness of film, RTA temperature, and furnace
temperature and further discuss their relation.
3.1 X-Ray Results
3.1.1 Different Solution
As we mentioned in section 1.3, in the binary System, some transition metal oxides
including TiO2, ZrO2, Nb2O5, NiO2 and SiO etc. had been found to exhibit bistable
phenomenon so that we use the different kinds of solutes, containing Zirkon (IV) propylate,
propoxide, Nickel (II) acetate tetrahydrate and titanium (IV) ethoxide [Ti(OCH5)4], to make
different types of solutions but the process of the solutions is the same as mentioned in section
2.2.2.
First of all, Fig. 3.1.1-1 ~ Fig. 3.1.1-6 show X-Ray diffraction patterns versus 2-theta of
the samples made up with titanium (IV) ethoxide [Ti(OCH5)4] solvent. Here, all the samples
are divided into two parts, Fig. 3.1.1-1, 3.1.1-3 and 3.1.1-5 and Fig. 3.1.1-2, 3.1.1-4 and
3.1.1-6, respectively by two rapid thermal annealing at temperatures of 500 and 700 oC. Fig.
3.1.1-1 and 3.1.1-2 present the X-Ray patterns of the samples which are fabricated with the
solution of 0.05 mole concentration and Fig. 3.1.1-3 ~ Fig. 3.1.1-4 present the X-Ray
diffraction patterns of the samples that are fabricated with the solution of 0.1 mole
concentration. Finally, The Fig. 3.1.1-5 and Fig. 3.1.1-6, show the X-Ray pattern of the
samples which are fabricated with the solution of 0.5 mole concentration. In these figures, the
peaks at 2-theta are at about 23, 33 and 48 degrees are the diffraction pattern of LNO (100), Si
(100) and LNO (200) planes, respectively, but there are no peaks of Ti oxide.
Next, Fig. 3.1.1-7 shows the X-Ray diffraction pattern of the samples using a solution
with different mole concentrations and with the solvent of Zirkon (IV) propylate, propoxide,
after rapid thermal annealing at temperature 700 oC. In this figure, the sample with 0.5 mole
concentration solution has a weak peak at 2-theta of about 31 degree but there are no peaks
for the samples with 0.05 and 0.1 mole concentration solutions. This result reveals that the
film of the samples is ZrO2 (111) plane but the samples with 0.05 and 0.1 mole concentration
solutions are too thin to have the peak. We further present the relation between thickness and
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mole concentration of the solution. Fig. 3.1.1-8 shows the thickness (measured by SEM and
will show and discuss in the next section) versus the different mole concentrations of the
solutions. Moreover, the weak peaks in Fig. 3.1.1-7 exhibits that the film is not crystallized
well.
3.1.2 The Properties of Crystallization
In order to find out the properties of crystallization of the zircon oxide film with different
temperatures of thermal treatment, we utilize different temperatures of a RTA furnace and a
high temperature furnace to crystallize the film. Fig. 3.1.1-9 ~ Fig. 3.1.1-12 show X-Ray
diffraction patterns with different RTA temperatures, 500, 600, 700 and 800 oC, of the 0.5
mole concentration solution. The peak of the X-Ray diffraction is too weak to observe but in
these diagrams it is apparent that the higher the temperature of the thermal treatment is, the
more crystallized the film is.
3.2 Morphology and Thickness of the ZrO
2Film
In this section, the plane view of SEM is employed not only to study the properties of
crystallization, directly and to compare with the X-Ray diffraction pattern mentioned in the
above section, but also to observe the morphology of the ZrO2 films. Besides, the cross
section picture of SEM is used to measure the thickness of the films deposited by different
Fig. 3.2-1 ~ Fig. 3.2-4 show the plane view of SEM of samples with temperatures of
thermal treatment varied from 400 oC to 700 oC, respectively. First, there is no apparent grain
in Fig. 3.2-1 and Fig. 3.2-2, implying that the crystallization of the film at temperature 400 oC
and 500 oC is not complete but in Fig. 3.2-3 and Fig. 3.2-4 where the samples after thermal
treatment at temperature 600 oC and 700 oC, the grain formation is more evident. To compare
with X-Ray diffraction pattern, Fig. 3.2-5 and Fig. 3.2-6, we can further ascertain the
properties of crystallization of ZrO2 film made up by sol-gel method.
In the cross section view of SEM, as shown in Fig. 3.2-7 ~ Fig. 3.2-9, We deposited five,
five and four layers of samples with respective solutions of 0.05, 0.1 and 0.5 mole
concentration to get the average value of the thickness of the ZrO2 film. Using this result, the
figure 3.1.1-7 shows the relation between the thickness of the film and the solution of the
mole concentration.
3.3 Electrical properties
3.3.1 Static Properties of ZrO2 Film
The electrical property of static state is measured by a voltage of double sweeps from the
magnitude of the maximum to the minimum at a rate of 0.5 V/s. The sample, first, was
fabricated by the thermal coater with the metal mask to define the active region.
Fig. 3.3.1-1 shows an I-V curve of a ZrO2 film with thickness of 82 nm at the double
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sweeping of the first time. When the sweeping voltage had decreased to the negative threshold
voltage, about -25 Volts in this sample, the magnitude of current density switched to a higher
one.
In the opposite, when the sweeping voltage has increased to the positive threshold, the
magnitude of current density switch to original state, low current density state. This bistable
switching current density phenomenon is similar to that of Cr doped SrTiO3 film. Fig. 3.1.1-2
shows the I-V curve of ZrO2 film after 5 times’ sweeping. The same, sample can change the
current density with ratio of 1~2 orders by applying voltage up to threshold voltage, but it
sometimes occurs that the curve at the maximal voltage is not closed. Moreover, the curve in
the region of the positive voltage is more unstable than that in the opposed region. As shown
in figure 3.3.1-1 or 3.3.1-2, the curve is not smooth in the region of the positive voltage so
that the measurement of electrical properties is often in the region of the negative voltage.
3.3.2 Dynamic Properties of ZrO2 Film
The device is first formed by forming process that applies a pulse voltage with
magnitude of threshold voltage causing the device in the low current density state. A sequence
of voltage pulse, write-read-erase-read, with a proper pulse width and a designated cycle
delay between the write and erase pulse voltage are employed to extract the dynamic
properties from the ZrO2 film.
Fig. 3.3.2-1 shows the dynamic properties of the ZrO2 film with thickness of 30 nm
measured by pulse voltage with pulse width of 0.5 s, pulse magnitude of 15 volts, readout
voltage of -1 volt and cycle delay 1 sec. Different pulse widths have also been tested on the
film with thickness of 30 nm, but the bistable switching phenomenon did not occur (the ratio
of two states was less than 5 times). Besides, in the samples with thickness of about 30 nm,
the fatigue property is often the pulse number of 10, and the maximum value of that is always
less than pulse number of 20 with the 1 order ratio of two states.
Fig. 3.3.2-2 presents the dynamic properties of the ZrO2 film with thickness of 45 nm. In
the sample of 45 nm, the magnitude of pulse voltage is up to 15 volts that equaled to its
threshold voltage of static property and the pulse width is also 0.5s. Fatigue for thickness of
about 45 nm is often pulse number of 30.
3.3.3 Effect of Doping
First, we doped the different kinds of transition metals to be an impurity into ZrO2. As
we had mentioned, in section 1.4.4 states modulation, the fact that if a voltage is applied on
the Cr doped SrTiO3, one mechanism assumes that Cr may act as an impurity and could
release electrons leading to current conduction in the oxide. When an applied voltage up to
threshold voltage, the valence of Cr could be transformed from initial type to another type
causing free carriers in the film increasing or decreasing and thus resistivity is varied.
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Transition metals, molybdenum, chromium, and vanadium were doped into ZrO2 film.
Fig. 3.3.3-1 ~ Fig. 3.3.3-4 show the I-V curve of the first time of a double sweeping (i.e.
voltage sweep from positive maximal voltage to negative min voltage and sweep again with
reverse process) on the non-dpoed, V, Cr and Mo doped ZrO2 films of one layer deposited by
solutions of 0.1 mole concentration. There are not only forming phenomena in the impurities
doped films but also in the non-doped one. Fig. 3.3.3-5 ~ Fig. 3.3.3-8 show the I-V curves of
the 5th time of the double sweeping on the films and the results are not stable; that is, the
bistable switching phenomenon is observed in some samples but not in others.
Indeed, there is the bistable switching phenomenon in the ZrO2 film and it does not need
doping impurity into the film.
3.3.4 Effect of Thickness
The effect of thickness related to the I-V characteristic is studied on the samples
fabricated by Sol-Gel method with solutions of 0.05, 0.1, 0.3 and 0.5 mole concentration.
Fig. 3.3.4-1~3.3.4-4 and Fig. 3.3.4-5~3.3.4-8 show the I-V curve of variant thickness, 20
nm, 30 nm, 45 nm and 82 nm, of ZrO2 film correspond to first and the 5th sweeping time,
respectively. It is apparent that the forming voltage increases as the thickness of ZrO2 film
increases. Besides, before and after the forming process, the current order of the high states of
the samples respecting to different thickness is quite near. Fig. 3.3.4-9 and Fig. 3.3.4-10
exhibit the diagram of the order of current density at sweeping voltage of -1 volt versus
variant thickness of the film at the 1st and the 5th sweeping time, respectively.
The magnitude of the high state current density is quite near and the thickness of the film
do not affect the high state current suggesting that the bistable switching phenomenon in the
do not affect the high state current suggesting that the bistable switching phenomenon in the