Thesis organization… - 不同表面處理對二氧化鉿與二氧化鋯推疊式高介電常數材料薄膜之效果

In this thesis, following chapters were primarily organized :

In chapter 2, we indicated our motivation for using plasma fluorination and plasma nitridation.

In chapter 3, we described the details of our experiment. Dual electron gun evaporation system was used to deposit ZrO2/HfO2 stack dielectric and plasma-enhanced chemical vapor deposition (PECVD) system was used to plasma fluorination and high-density plasma source of HDPCVD was used to plasma nitridation.

In chapter 4, we discuss the electrical characteristics of ZrO2/HfO2 stack insulator by Metal Insulator Semiconductor (MIS) capacitors.

In chapter 5, we make the conclusions for this thesis and provide some suggestions for future work.

Chapter 2 Motivation

2.1 Plasma fluorination

The MOS devices have properties with the result that there is often Nit or Dit in the interface, imperfect bonding of interface usually makes the characteristic of the device deteriorate. For example, charge will be trapped by the defects of the interface, it produces flat band voltage shift and also reduces the carrier mobility. Another shortcoming is that these dangling bonds will easily bond with oxygen atoms in the following high temperature environment. The extra chemical reaction will let the interfacial oxide growth, and it will reduce the C value because of the lower dielectric constant.In addition, the quality of interfacial layer formed by oxidation is worse and there still will be the more problems of charge trapping.

HfO2 was a good gate dielectric due to its high dielectric constant [30-35], wide energy bandgap (~5.68 eV), and high stability with the Si surface. Unfortunately, they suffered from a high density of charge traps which caused flat-band voltage shift, threshold voltage instability, Coulombic scattering of carriers in the channel and hysteresis problems [36].

On the other hand, the unavoidable formation of interfacial layer (IL) was another critical issue, limiting the reduction of the effective oxide thickness. The reasons for this unwanted layer were the presence of excess oxygen during the film growth that initially oxidized the Si surface and Si diffusion into the film producing a silicate layer [37].

Because the quality of interfacial layer formed by oxidation was worse, it caused the problem of charge trapping. Plasma fluorination and plasma nitridation were considered to be effective methods for melioration of properties of HfO2 gate dielectric.

HfO2 gate dielectric with fluorine incorporation exhibited better reliability and

performance [36-40]: (a) Fluorine was believed to form stronger Hf–F and Si–F bonds than Hf–H and Si–H bonds, leading to improvement of the reliability of HfO2/SiO2, (b) The interfacial layer (IL) between HfO2 film and Si substrate could effectively be suppressed by a pre-deposition CF4 plasma treatment, (c) The charge trapping effect could greatly be eliminated for the HfO2 gate dielectric by the post-deposition CF4

plasma treatment, (d) The thermal stability of HfO2 gate dielectric could be much improved by fluorine ion implantation on the silicon surface.

An inner-interface trapping model was presented to explain the hysteresis [39], as shown in Fig. 2-1. It shows that the voltage was swept from accumulation to inversion, the C-V curve shifted negatively, in contrary to the C-V curve shifted positively when the voltage was swept from inversion to accumulation. The positive and negative carrier trappings caused the C-V curve to have a hysteresis loop. After CF4 plasma treatment, Fluorine passivated the dangling bond and accumulated at the interfacial layer, as shown in Fig. 2-2. It appeared that fluorine incorporation improved the interface quality and resulted in less hysteresis.

2.2 Plasma nitridation

Nitridation was also a common method to improve the properties of HfO2 gate dielectric. Nitrogen incorporation into Hf-based high-k dielectrics has been intensively studied.According to traditional view of improving SiO2 device performance, we could find that nitridation is a common method to improve the interface [41]. Property with the result that there is often Nit or Dit in the interface, imperfect bonding of interface usually makes the characteristic of the device deteriorate.

In order to solve these problems, nitridation treatment could let the atom of nitrogen bond with these dangling bonds and fix it while entering the interface layer, and then improve the stability and reliability of interface.

The driving force of this reaction is total energy stabilization with charge compensation. In this charge compensation, charge transfer arises from two electrons of the induced Vo gap state falling into the two vacant N states at the top of the valence band as shown in Fig. 2-3. As a result, a large amount of N induces a large amount of Vo. The key point is the increase in Vo–Vo interactions with the increase in the amount of Vo. A large amount of N induces the deterioration of the CB offset owing to Vo–Vo

interaction as shown in Fig. 2-4 [42]. If we use N doped HfO2, the deterioration of the VB offset is inevitable. If Vo can move easily, total energy is stabilized by local crystallization around Vo owing to disturbance by the movement of Vo, because the crystal state is lower in energy than the amorphous state. A pair of Fs and Ns can passivate the Vo with no excess charges as shown in Fig. 2-5. The main passivation energy comes from one excess electron transfer of CB bottom of Fs to VB top of Ns as shown in Fig. 2-5. The amount of N is not needed large, because Fs fills up Vo. Moreover, CB lowering owing to Vo–Vo interaction can be avoided. Defects formation in HfO2 should be affected \by its crystal structure. But the fact that the CB of HfO2 is composed of Hf 5d states, and the VB is composed of O 2p states is not affected by its crystal structure. The stabilization mechanism by the electron transfer is not affected by its crystal structure. That is to say, fluorines can fill up oxygen vacancies in HfO2 by exothermic reaction, but induce distribution of the positive charges in HfO2 [43]. The most important point is that we should fill up Vo by fluorine (Fs), and Fs donates an excess electron to other acceptor type dopant. This passivation process completely eliminates Vo with no excess charges in gate oxide. The amount of N is not needed large, because Fs fills up all oxygen vacancies [43]. According to [44], we could understand that the effect of plasma nitridation is better than thermal nitridation. The reason is that high-k materials could not sustain high thermal stress. As long as the temperature reaches certain degree, we would see the phenomenon of crystallization. The

crystallization of dielectric would increase leakage current substantially, because it offers the path of leakage current. On the other hand, the meaning of plasma nitridation is to activate the source gas first. The high activation energy of radical will provide better mend which is better than nitridation at high temperature. For all these reasons, we adopt plasma nitridation in present experience.

2.3 Dual plasma treatment

In this thesis, we tended to employ CF4 plasma for pre-deposition of ZrO2

treatment in order to eliminate the low dielectric constant interfacial layer and improved the quality of silicon substrate surface. In the next step, we used the NH3, N2O and N2

plasma for post-deposition of HfO2 treatment because of the capability of nitrogen incorporated HfO2 gate dielectric to reduce oxygen vacancies and increase the crystallization temperature of HfO2.

The method that combined pre-deposition plasma fluorination and post-deposition plasma nitridation was called dual plasma treatment.

Chapter 3 Experimental of Ti/HfO

₂

/ZrO

₂

/Si MIS capacitors

3.1 PECVD system for plasma treatment

Plasma-enhanced chemical vapor deposition (PECVD) is a process used to deposit thin films on a substrate. After creation of the plasma of the reacting gases, chemical reactions are induced into the process. The plasma is generally created by AC (RF) frequency or DC glow discharge between two electrodes. The chamber space between two electrodes is filled with the reacting gases.

Plasma is any neutral gas which is partially ionized and contains the positive charges (ions), negative charges (electrons) and neutral particles. Density of the charged particles is high enough to produce Coulomb interaction. This phenomenon induces the charged particles to reveal behaviors of fluid, resulting in the properties of the plasma.

In generally, the plasma is at neutral state, that is, the density of negative charged particles and the density of positively charged particles are the same. When some particles undergo external force, other particles will be affected. Thus the plasma has a good electrical conductivity and thermal conductivity.

Plasma is generated by electrons which are accelerated by electric field to very high kinetic energy and then impact with gas atoms or molecules to produce ionic reaction. The gas atoms or molecules which are impacted by electrons dissociate as a positive charged ion and an electron. The results of this dissociation will produce two free electrons, they will be accelerated again and the impact of other gas molecules or atoms so that it can produce four free electrons. By the continuous reaction, gases within the zone of reactive chamber are ionized.

Plasma treatment had two advantages which were different from the traditional thermal ones :

(1) When using plasma process for the chemical reactions, electrons impacted the gas molecules and produced the high reactive species, leading to the reactions which occur difficultly in the thermodynamics could be generated.

(2) Traditional thermal chemical reactions usually needed high thermal energy to control the conditions of reaction. Therefore the high temperature was required but this was not what we want due to the high-k materials could not sustain high thermal stress. Plasma process could effectively induce chemical reactions without high temperature, so we tended to use plasma fluorination.

We used the SAMCOs PD-220N PECVD system for our plasma treatment. Fig.

3-1 shows the Schematic illustration of the PECVD system. This is a diode-type plasma system. The process gas and carrier gas (inert gas) flowed into chamber. Here we used CF4 as the process gases. RF power source was applied at the top electrode and the bottom electrode connects to ground. The process gases got energy from the RF power source and then generated the plasma. Wafer was placed on the bottom electrode and the charged particles within plasma accelerated toward to the bottom electrode due to potential difference between top electrode and bottom electrode. Thus plasma fluorination was completed during the plasma process.

3.2 Electron beam evaporation system

Fig. 3-2(a) shows a schematic illustration of a simple vacuum evaporator. It is mainly constituted by an evaporator chamber and a vacuum system which provides the required degree of vacuum. In the evaporator chamber, the solid materials used to deposit are called evaporation sources. They will be placed in the crucible manufactured from refractory. The crucible constituted by the conductive materials connects directly to DC power outside the chamber. When appropriate current flowing into the crucible, heat generated due to the resistance effect of the crucible. Source placed inside the

crucible will be heated until it is close to the melting point. At this time, solid sources will evaporate and become vapor. Therefore, we can deposit thin film which we want onto the wafer.

In general, electron beam evaporation (EBE) system was popular for the applications of semiconductor industry. The basic structure is the same as Fig. 3-2 (a).

The major distinction is that EBE system heats the evaporation source by electron beam, as shown in Fig. 3-2 (b).

3.2.1 Dual electron gun evaporation system

A Dual electron gun evaporation system is used for electron beam evaporations.

This system provides the capability for the evaporation of high melting point materials.

The source has two crucibles, which enable multiple evaporations to be performed. The electron beam gun (e-gun) is within a high vacuum chamber.

3.3 Rapid thermal annealing (RTA) system

Metal RTA-AG610 is a single-wafer lamp-heated and computer-controlled rapid thermal processing (RTP) system. Generic RTP reactor is shown in Fig. 3-3.

Water and compressed dry air (CDA) cooling system are used to cool down the quartz chamber. High intensity visible radiation heating and cold-heating chamber walls allow fast wafer heating and cooling rate. The tungsten halogen lamps are distinguished into five groups, and the relative percentage of lamp intensity can be adjusted individually for each group to achieve uniform temperature distribution. Temperature is obtained from pyrometer and precise controlled by computer. Two gas lines are used in the system which can be switched between Ar and N2. Before RTA process starts, one minute N2 gas purge is performed to minimize the water vapor introduced during wafer loading and also sweep unwanted particles induced during process. A fast heating rate of 150^oC/s was chosen in this work. When anneal was complete, chamber temperature

was quickly cooled down from 600^oC to 400^oC by 30 seconds N2 purge. Then, the chamber was slowly cooled down to 300^oC without N2 purge to avoid creaking of films.

After five minutes later, wafers were taken out from the chamber. Films’ creak could be avoided by two-steps-cooling method.

3.4 Inductively coupled plasma (ICP) system

The high-density plasma source of HDPCVD is come from inductively coupled plasma(ICP) system. The inductive coils shown in Fig. 3-4[45] serve just like the initial coils of a transformer. When an RF current flows in the coils, it generates a changing magnetic field. The inductively coupled electric field accelerates electrons and causes ionization collisions. Since the electric field is in the angular direction, electrons are accelerated in the angular direction, which allows electrons to travel a long distance without collisions with the chamber wall or electrode.[46] 13.56 MHz RF power was coupled to the top electrode through a matching network. After the sample load to reactor, the system was pumped down to keep the chamber clean enough. Subsequently, the source gas was become radical by the plasma system, as the chamber pressure was 100 mTorr and the substrate temperature was 300°C so that to achieve the goal of low temperature process. The power of working plasma was kept constant at 200W and the flow rate of source gas was 100 sccm. While the process of plasma treatment was finished, these samples were brought to thermal treatment to reduce plasma damage.

3.5 Experiment Details

3.5.1 Post deposition annealing (PDA) effect on ZrO

MIS capacitors with Ti/HfO2/ZrO2/Si structure was fabricated on (100) oriented p-type silicon wafers which were one side polished and their resistivity was 0.1 to 1 ohm-cm. Fig. 3-5, Fig. 3-6, and Fig.3-7 shows that flowchart of fabrication of MIS capacitors in our experiment.

Prior to the growth of HfO2/ZrO2 stack dielectrics, the native oxide was cleaned by the conventional RCA cleaning and diluted HF etching in sequence for the removal of particles and native oxides. After standard initial RCA clean, a 1nm ZrO2 thin film was grown on Si substrate by Dual E-gun evaporate deposition system. After that, some samples were subjected to post deposition annealing (PDA) treatment in O2 ambient at 500°C for 5 minute, and the others without PDA. Then, a 4nm HfO2 thin film was grown on Si substrate by Dual E-gun evaporate deposition system. After the thin films were deposited, all samples were annealed in O2 ambient for 5 minute at 500°C after deposition (PDA, Post deposition anneal) again.

Finally, all samples were deposited 200nm thick Ti layer defined as shadow mask by E-gun. The active region pad of etch capacitors’area was 1.33×10^-4/cm ².

3.5.2 Plasma treatment on HK stack w/ or w/o ZrO

₂

PDA

MIS capacitors with Ti/HfO2/ZrO2/Si structure was fabricated on (100) oriented p-type silicon wafers which were one side polished and their resistivity was 0.1 to 1 ohm-cm. Fig. 3-8, Fig. 3-9, Fig. 3-10 and Fig. 3-11 shows that flowchart of fabrication of MIS capacitors in our experiment.

The next step was using HDPCVD to add plasma N2 treatment in order to improve the electrical properties of dielectric. The deposition source N2 flow rate was 100 sccm, ICP power was 200 W, bias was 0 W, process pressure was 100 mTorr, and process time was 30 sec and 90 sec, temperature was 300°C. After nitridation, we also annealed these samples to reduce the plasma damage (PNA, Post nitridation anneal).

Finally, all samples were deposited 200nm thick Ti layer defined as shadow mask by E-gun. The active region pad of etch capacitors’area was 1.33×10^-4/cm ².

3.5.3 Post deposition annealing (PDA) effect on ZrO

₂

after plasma fluorination pretreatment

MIS capacitors with Ti/HfO2/ZrO2/Si structure was fabricated on (100) oriented p-type silicon wafers which were one side polished and their resistivity was 0.1 to 1 ohm-cm. Fig. 3-12, Fig. 3-13, Fig. 3-14, and Fig. 3-15 shows that flowchart of fabrication of MIS capacitors in our experiment.

Prior to the growth of HfO2/ZrO2 stack dielectrics, the native oxide was cleaned by the conventional RCA cleaning and diluted HF etching in sequence for the removal of particles and native oxides. After standard initial RCA clean, wafers are placed into the chamber of PECVD. All samples were prepared in CF4 plasma. The reactive pressure and flow rate of CF4 was 67 Pa and 100 sccm, respectively. The substrate temperature was increased to 300^oC and the RF power was 20 W for CF4 plasma exposure times of 10 seconds.

After CF4 plasma pre-deposition treatment, a 1nm ZrO2 thin film was grown on Si substrate by Dual E-gun evaporate deposition system. After that, some samples were subjected to post deposition annealing (PDA) treatment in O2 ambient at 500°C for 5 minute, and the others without PDA. Then, a 4nm HfO2 thin film was grown on Si substrate by Dual E-gun evaporate deposition system. After the thin films were

deposited, all samples were annealed in O2 ambient for 5 minute at 500°C after deposition (PDA, Post deposition anneal) again.

Finally, all samples were deposited 200nm thick Ti layer defined as shadow mask by E-gun. The active region pad of etch capacitors’area was 1.33×10^-4/cm ².

3.5.4 Plasma treatment on HK stack w/ or w/o ZrO

PDA after plasma fluorination pretreatment

MIS capacitors with Ti/HfO2/ZrO2/Si structure was fabricated on (100) oriented p-type silicon wafers which were one side polished and their resistivity was 0.1 to 1 ohm-cm. Fig. 3-16, Fig. 3-17, Fig. 3-18, Fig. 3-19 and Fig. 3-20 shows that flowchart of fabrication of MIS capacitors in our experiment.

Prior to the growth of HfO2/ZrO2 stack dielectrics, the native oxide was cleaned by the conventional RCA cleaning and diluted HF etching in sequence for the removal of particles and native oxides. After standard initial RCA clean, wafers are placed into the chamber of PECVD. All samples were prepared in CF4 plasma. The reactive pressure and flow rate of CF4 was 67 Pa and 100 sccm, respectively. The substrate temperature was increased to 300^oC and the RF power was 20 W for CF4 plasma exposure times of 10 seconds.

After CF4 plasma pre-deposition treatment, a 10Å ZrO2 thin film was grown on Si substrate by Dual E-gun evaporate deposition system. After that, some samples were subjected to post deposition annealing (PDA) treatment in O2 ambient at 500°C for 5 minute, and the others without PDA. Then, a 40Å HfO2 thin film was grown on Si substrate by Dual E-gun evaporate deposition system. After the thin films were deposited, all samples were annealed in O2 ambient for 5 minute at 500°C after

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