Thesis Organization

This thesis consists of six chapters. Following the introduction in chapter 1, chapter 2 presents in detail the low-pressure CVD system used for this thesis work. Chapters 3, 4 and 5 contain the major results of this thesis study. Chapter 3 deals with the effect of iodine on the characteristics of Cu nucleation and film growth. In chapter 4, we characterize the Cu films deposited using iodine as a catalyst. Chapter 5 contains the study on the via-filling capability of Cu-CVD using the iodoethane (C₂H₅I) additive. Finally, chapter 6 gives the conclusion of this thesis work and the suggestions for future study.

Fig. 1-1 Intrinsic gate delay vs. interconnect RC delay at minimum design rules of each technology node.

Chapter 2

Multi-Chamber Cu CVD System and Cu Precursor

2.1 Multi-Chamber Low Pressure Cu CVD System

A multi-chamber low pressure CVD system with a warm wall reaction chamber built in our laboratory is used for this thesis study. Figure 2-1 shows the schematic diagram of this CVD system, and the photographs of the apparatus are given in Fig. 2-2 and Fig. 2-3. There are four chambers in the CVD systems: (1) loading chamber, (2) transfer chamber, (3) plasma pretreatment chamber, and (4) reaction chamber. Besides these four chambers, the apparatus also contains a precursor delivery system and a catalyst injection system. The details of each chamber and sub-system are described as follows.

(1) Loading Chamber

The loading chamber is a buffer between the controlled vacuum environment inside the CVD system and the atmospheric environment outside the system. The function of this chamber is to handle the substrate input and output. The substrate wafer is placed on an aluminum made substrate holder; either a whole piece of wafer up to an 8-inch diameter or broken pieces of wafer can be placed on the substrate holder. The substrate wafer (together with the substrate holder) can be transferred to any desired chamber via a robot arm that is set in the transfer chamber.

(2) Transfer Chamber

This is a hexagonal shaped chamber directly connected to each of the other chambers of this multi-chamber CVD system. The transfer chamber houses a robot arm, the function of which is to carry the substrate holder and transfer it to the designated chamber. The operation of the robot arm is controlled and monitored by a robot controller via a host computer system. Figure 2-4 is a photograph showing the robot arm in the transfer chamber.

(3) Pre-treatment Chamber

Prior to the deposition of copper films, the substrate wafers can be pre-treated by plasma in this chamber. Figure 2-5 shows the schematic diagram of the pre-treatment chamber. The chamber wall is grounded, and a 600W power generator operated at a radio frequency of 13.56 MHz is used for the plasma generation. The plasma can be induced in the pre-treatment chamber by a negative DC bias [17]. Various gases, including Ar, N2 and H2, can be introduced into the chamber for the generation of different plasma [18,19]. The plasma treatment on the substrate surface can be classified into two different types:

physical sputtering treatment (such as Ar-plasma) and chemical reaction treatment (e.g. H2-plasma).

(4) Reaction Chamber

This is the chamber where reaction of precursor takes place to deposit the Cu film. Figure 2-6 shows a schematic diagram of the reaction chamber used for Cu film deposition. On top of the chamber there is a shower-head injector, through which the Cu precursor carried by the carrier gas is introduced into the reaction

chamber. Under the injector there is a substrate holder, which can be heated by a resistive heating element. The substrate holder is rotatable so as to obtain a better uniformity of the deposited film. It can also be moved in vertical direction so that the distance between the injector and the substrate can be adjusted. The side wall of the cylindrical reaction chamber as well as the Cu precursor injector is kept at a temperature of 45℃ by a circulating warming water to prevent Cu precipitation.

The reaction chamber of this CVD system can be pumped down to a background pressure of 10^-7 Torr before the deposition process to ensure the contamination free deposition environment.

(5) Direct Liquid Injection (DLI) Delivery System of Precursor

The liquid Cu precursor is delivered by a direct liquid injection (DLI) system, which is composed of a liquid flow meter (LFM) and a controlled evaporation mixer (CEM). A schematic diagram of the DLI system is shown in Fig. 2-7.

Initially, the liquid precursor is propelled by the nitrogen gas through the LFM, which can precisely control the mass flow of the liquid precursor. The precursor is then vaporized in the CEM unit and mixed with the carrier gas. Finally, the carrier gas with its carried precursor is directed into the reaction chamber through the gas injector.

(6) Catalyst Storage and Injection System

Liquid C2H5I was used in this study to provide iodine (I2) as catalyst for the CVD of Cu films. C₂H₅I has a high vapor pressure of 137.52mmHg at 25^oC. In this study, the container of the liquid C2H5I was maintained at a constant temperature of 25^oC, and a mass-flow controller (MFC) is connected to the container through a valve, as shown schematically in Fig. 2-8. When the valve is

opened, the iodine containing vapor will enter the reaction chamber by diffusion.

The parameters of the mass-flow controller are set ready at the beginning and the vapor flow is totally controlled by the valve.

2.2 Real-time Automatic Monitor and Controller System

Figure 2-9 shows the automatic controlling system of the multi-chamber Cu-CVD system (an automatic remote monitor and controller (ARMC) system).

The design concept of this automatic controlling system is focused on the requirements of gas transport, vacuum procedure sequences controlling, substrate plasma treatment, substrate heating control, pressure control, safety protection handling, users friendly interface, and convenient maintenance.

The ARMC system is composed of a host computer, a subsidiary controller, and a unit controller of the Cu CVD apparatus. A communication program is designed to link together the functions of the host computer and the subsidiary controller. Therefore, data and commands can be transferred between them through a dynamic data exchange (DDE) server in Windows98 operation system.

In the host computer, a friendly interface program with monitor and control function is operated under the Windows98 operation system, as shown in Fig.

2-10. The deposition process and the procedure sequences are under automatic control by this friendly interface program. What the user needs to do is to program the recipes and process steps correctly and precisely, and the host computer can monitor all of the operation conditions of the Cu CVD system.

2.3 Reaction of Cu precursor

The Cu precursor used in this thesis study is a liquid Cu(I) precursor of

Cu(1,1,1,5,5,5-hexafluoroacetylacetonate)trimethylvinylsilane [Cu^I(hfac)TMVS]

plus 2.4wt%TMVS developed by SCHUMACHER company [11,19]. The 2.4wt%TMVS additive promotes stability of the precursor.

The Cu^I(β-diketonates) is one of the most promising Cu precursors [19]. The group of Cu(I) precursors has a form of Cu^I(β-diketonates)L, where L is a neutral ligand weakly bonded to Cu. Hexafluoroacetylacetonate (hfac) is the most commonly used β-diketonates. The structure of Cu^I(hfac)L is similar to that of Cu^Ⅱ(hfac)₂, with the L ligand replacing one of the hfac rings. Depending on the L ligand, CVD of Cu films may have a selective deposition property. Low resistivity Cu films can be deposited from Cu^I(hfac)TMVS at temperatures below 200℃.

It has been proposed that the complete disproportionation reaction of the Cu^I(hfac)TMVS precursor consists of four steps as follows [20].

2Cu⁺¹(hfac)TMVS(g) → 2Cu⁺¹(hfac)(g) + 2TMVS(g) (dissociation) (2-1) 2Cu⁺¹(hfac)(g) → 2Cu⁺¹(hfac)(s) (adsorption) (2-2) 2Cu⁺¹(hfac)(s) → Cu (s) + Cu⁺²(hfac)2 (s) (disproportionation) (2-3) Cu⁺²(hfac)2 (s) → Cu⁺²(hfac)2 (g) (desorption) (2-4)

where (g) denotes “gas phase” and (s) denotes “adsorbed on substrate surface.”

The first step involves the decomposition of Cu(hfac)TMVS into Cu⁺¹(hfac) and TMVS species by the gas phase reaction (Eq. 2-1); this is because Cu-TMVS in the Cu(hfac)TMVS precursor is a weak bond compared to the Cu-hfac bond. This is followed by the adsorption of the Cu⁺¹(hfac) species on the substrate surface

(Eq. 2-2). The Cu⁺¹(hfac) species then undergo a disproportionation reaction to produce the Cu metal and the Cu(II) species [Cu⁺²(hfac)2] (Eq. 2-3). Finally, the volatile Cu⁺²(hfac)₂ by-product desorbs from the substrate surface (Eq. 2-4). The reaction step 3 (Eq. 2-3) is the key step of Cu nucleation on the substrate surface, which involves a process of electron exchange between the adsorbed Cu⁺¹(hfac) and the substrate surface. Thus, it is easier to deposit Cu film on a conducting substrate than on an insulating substrate. The overall disproportionation reaction of Cu film deposition is given by the following equation:

2Cu⁺¹(hfac)TMVS_(g) → Cu (s) + Cu⁺²(hfac)_{2 (g)} + 2TMVS_(g) (2-5)

Figure 2-11 illustrates the four steps of Cu film deposition by Cu(hfac)TMVS precursor.

C

Fig. 2-1 Schematic diagram of the multi-chamber low pressure CVD system used in the study of this thesis.

Fig. 2-2 The panorama photograph of the multi-chamber Cu CVD apparatus.

Fig. 2-3 Photograph showing the cluster-chamber, DLI system, and gas piping system.

Fig. 2-4 Photograph showing the robot arm in the transfer chamber.

RF generator 13.56 MHz 600W Power

plasma

substrate holder

Temp. range

20 ~ 400°C 4 cm

gas

Fig. 2-5 Schematic diagram of the pretreatment chamber.

a: carrier gas (with precursor)

h: distance between substrate and precursor injector

Fig. 2-6 Schematic diagram of the reaction chamber (copper deposition chamber).

Precursor (liquid)

Fig. 2-7 Schematic diagram of the direct liquid injection (DLI) precursor delivery system.

To Reaction Chamber MFC

Catalyst

(C

2

H

5

I) Container maintained at 25

^o

C

Fig. 2-8 Schematic diagram of the catalyst storage and injection system.

Unit

controller

^Robot controller

Pre-treatment Chamber Loading

Chamber

Transfer Chamber

Reaction Chamber

Subsidiary controller

Host Computer

Fig. 2-9 Schematic diagram of the automatic remote monitor and controller (ARMC) system, which is composed of a host computer, a subsidiary controller, and a unit controller of the Cu CVD apparatus.

Fig. 2-10 The friendly monitor and control program displays in the host computer screen.

Cu(hfac)TMVS

Gas Phase

Intermediate Formation

Cu

⁺¹

(hfac) + TMVS

Adsorption

Desorption

Substrate

+

(hfac)

Cu

⁺¹

(hfac)

Cu

⁺¹

Surface Reaction

Cu

Cu

⁺²

(hfac)

2

Fig. 2-11 The deposition mechanism of Cu film by Cu(hfac)TMVS precursor.

Chapter 3

Nucleation of Cu Using Iodoethane as Catalyst

3.1 Introduction

When a metal film is deposited by CVD method, the film property is directly affected by the nucleation process, which is very sensitive to the substrate surface condition [21, 22]. In this thesis, iodoethane (C₂H₅I) is used as catalyst for substrate surface treatment prior to the deposition process of Cu films. Effects of iodine adatoms on the nucleation of Cu on TaN substrate are investigated in this chapter by measuring the wetting angle of Cu grains in the nucleation stage of Cu-CVD.

3.2 Experimental Details

The TaN-layer-covered Si wafer was used as the substrate for the nucleation study of Cu-CVD. The TaN layer was deposited to a thickness of 25 nm on a 500-nm-thick thermal SiO₂-covered Si wafer (TaN(25nm)/SiO₂(500nm)/Si). The TaN film was sputter-deposited using a Ta target in an Ar/N2 gas mixture; the flow rates of Ar and N₂ were 55 and 2.5 sccm, respectively, for making the gas mixture. The as-deposited TaN film has a resistivity of 288µΩ-cm, and the film’s composition is TaN_0.85 as determined by RBS.

The TaN substrate wafer (TaN/SiO2/Si) placed on the substrate holder was loaded into the load-locked chamber of the Cu CVD system. When the pressure of all relevant chambers was pumped down to 2×10^-6 torr, the substrate wafer was transferred to the reaction chamber via the transfer chamber. Then, helium (He)

was introduced into the reaction chamber and the chamber pressure was maintained at 150 mtorr, while the substrate holder was heated to the deposition temperature of 160^oC. It took approximately 70 min for the substrate holder to reach the temperature of 160^oC. If the substrate surface is to be treated with catalyst (C₂H₅I was used as catalyst in this study), the reaction chamber was pumped down to 10 mtorr after the substrate holder had reached the desired deposition temperature, and the valve on the pipeline connecting to the catalyst container was opened so that the vaporized C2H5I was introduced into the reaction chamber. Then, after the pressure in the reaction chamber had reached 1 torr, the chamber was pumped down again and kept at 150 mtorr until the valve was closed. In this work, the valve opening time was 3 min. Following the close of the valve to stop the inflow of catalyst, the pressure of the reaction chamber was pumped down to 10 mtorr again, and then helium (He) gas was introduced into the chamber. When the chamber pressure was stabilized at 150 mtorr, the valve to the precursor pipeline was opened so that the liquid precursor was propelled by nitrogen gas through the LFC and was then vaporized in the CEM and mixed with the He carrier gas. The precursor-saturated carrier gas was introduced into the reaction chamber through the gas injector to proceed with the Cu CVD process. Table 3-1 summarizes the major parameters and processing conditions for Cu CVD in this study.

3.3 Results and Discussion

The nucleation process of Cu film on the TaN substrate with and without catalyst treatment was investigated by SEM observation on the surface morphology as well as measuring the wetting angle of the Cu grains nucleated on

the substrate surface. Figure 3-1 and Fig. 3-2 illustrate the top view SEM micrographs showing the Cu grain nucleation process on the control (as-deposited) and C₂H₅I-treated TaN substrates, respectively. Hereafter in this thesis, the as-deposited TaN substrate without the C2H5I pretreatment is to be designated as control TaN substrate. It can be seen from Fig. 3-1 and Fig. 3-2 that the Cu species from the precursor are more easily adsorbed on the C2H5I-treated substrate, leading to a denser distribution of Cu grains. This indicates that iodine adatoms enhanced the chemisorption of Cu-containing adspecies [Cu(hfac)] on the substrate surface. Thus, Cu can be more easily and uniformly nucleated on the C2H5I-treated TaN substrate surface, resulting in shorter incubation time and densely and uniformly distributed Cu grains. This behavior directly affects the formation and growth of Cu film. After the Cu-CVD for 3 min, the nucleation of Cu on the C₂H₅I-treated substrate surface had almost developed into a Cu film, whereas on the control substrate surface the Cu grains were still sparely distributed and also characterized by nonuniform grain size. This is presumably because the subsequent Cu species are much more easily absorbed on the existing Cu grains than the control TaN substrate surface. The nonuniform size of the nucleated Cu grain will presumably affect the Cu film property, especially the film resistivity, because the highly nonuniform size of the nucleated Cu grain may result in voids in the Cu film and rough film surface, leading to higher resistivity. More discussion in this regard is to be given in chapter 4. On the other hand, the denser and more uniform-sized Cu grains on the C2H5I-treated TaN substrate resulted in easier merging of the Cu grains to form a continuous Cu film.

The Cu CVD process and the property of CVD Cu films are closely related to the shape of nucleated Cu grains in the nucleation process. Figure 3-3 shows a

schema of nucleated Cu grain as well as the Young’s equation and the definition of wetting angle. According to the Young’s equation [23], we have

(4-1)

where

θ

is the wetting angle of the copper grain,

σ

s and

σ

c are respectively the substrate surface energy and the Cu grain surface energy, and

σ

i is the interfacial energy between the Cu grain and the substrate. The oblique view SEM micrographs in Fig. 3-4 show the Cu grains nucleation at 160^oC and 150 mtorr on TaN substrate with and without the C₂H₅I treatment. The wetting angle of the Cu grains on the control TaN substrate was measured to be about 63^o while that on the C₂H₅I-treated TaN substrate was about 94^o. The increased wetting angle for the nucleated Cu grains on the C2H5I-treated substrate indicates that the C2H5I treatment resulted in reduced substrate surface energy and/or increased interfacial energy, thus enhancing the Cu film growth in three dimensions and forming less stable (200) orientation [24]. The Cu grains nucleated on the C₂H₅I-treated substrate look like pillars; this is because the Cu grains nucleated on the C₂H₅I-treated TaN substrate have a large wetting angle. Besides, a larger obtuse wetting angle may imply a smaller contact area between the Cu grain and the substrate surface, resulting in degraded film adhesion [25]. In other words, Cu films grown on the C2H5I-treated TaN substrate surface may have a degraded adhesion to the substrate, which was confirmed by Scotch tape adhesion test in chapter 4.

σ

c

σ

s

- σ

cos θ =

ⁱ

3.4 Summary

This chapter investigates the Cu nucleation on the TaN substrate with and without the C2H5I catalyst surface treatment. It was found that the C2H5I treatment enhanced the chemisorption of Cu-containing adspecies, and thus reduced the incubation time and accelerated the Cu film formation. For the Cu grains nucleated on the C₂H₅I-treated substrate, the wetting angle increased from 63^o to 94^o, implying that the C2H5I-treated substrate has a reduced surface energy and/or increased interfacial energy; this might promote the vertical growth of nucleated Cu grains, degraded the adhesion to substrate, and enhanced (200) orientation packed configuration.

Table 3-1 Major parameters and processing conditions of Cu CVD used in this study.

Cu CVD processing conditions

Substrate temperature (^oC) 160

Operation pressure (mTorr) 150

Cu precursor flow rate (ml/min) 0.4

Carrier gas (He) flow rate (sccm) 25 Substrate holder rotation speed (rpm) 10 Gas-injector/susceptor distance (cm) 2 CEM temperature (^oC) 70 Precursor’s delivery line temperature (^oC) 72 Reactor (reaction chamber) wall temperature (^oC) 45 C2H5I storage temperature (^oC) 25 Chamber pressure during catalyst treatment (mTorr) 150

(c) (b) (a)

Fig. 3-1 Top view SEM micrographs showing Cu grain nucleation on control TaN substrate

(a)

(b)

(c)

Fig. 3-2 Top view SEM micrographs showing Cu grain nucleation on C2H5I-treated TaN substrate for (a) 1 min, (b) 2 min, and (c) 3 min of Cu-CVD.

σ

_c

Substrate Cu grain σ

_i

θ σ

_s

θ : wetting angle (contact angle) σ

s

: surface energy of substrate σ

c

: surface energy of Cu grain

σ

i

: interfacial energy between Cu grain and substrate

cos

^{s i}

c

σ σ

θ σ

= −

Fig. 3-3 Schematic illustration of Cu nucleation and Young’s equation.

200nm (a)

θ～63

^o

(b)

θ～94

^o

Fig. 3-4 Oblique view SEM micrographs showing Cu grain nucleation for (a) 3 min on as-deposited and (b) 1 min on C2H5I-treated TaN substrates.

Chapter 4

CVD Cu Films on C

₂

H

₅

I –Treated TaN Substrate

4.1 Introduction

The properties of CVD Cu films are closely related to the initial nucleation of Cu on the substrates [26-28]. Variation of nucleation and growth mechanism of CVD Cu on different substrate conditions leads to different properties of the deposited Cu films. In chapter 3, we have found that C2H5I treatment on TaN substrate resulted in different Cu nucleation and film-forming process. In this chapter, properties of Cu films deposited on the C2H5I-treated TaN substrate, such as growth rate, surface morphology, film resistivity and film texture, are investigated.

4.2 Experimental Details

CVD Cu films were deposited on the same TaN surface of the substrate wafer TaN(25nm)/SiO₂(500nm)/Si, as was used in the study of chapter 3. The TaN substrate wafer (TaN/SiO2/Si) placed on the substrate holder was loaded into the load-locked chamber of the Cu CVD system. When the pressure of all relevant chambers was pumped down to 2×10^-6 torr, the substrate wafer was transferred to the reaction chamber via the transfer chamber. Then, helium (He) gas was introduced into the reaction chamber so as to maintain the chamber pressure at 150 mtorr, while the heating element was turned on to heat the substrate holder to the desired reaction temperature. As the temperature of the substrate holder reached the desired reaction temperature, which usually took

about one hour, the reaction chamber was pumped down to 10 mtorr. Then, the control valve to the catalyst container was opened to guide the vapor phase catalyst of C₂H₅I into the reaction chamber. As the chamber pressure rose to 1 torr in a short time, it was pumped down again and maintained at 150 mtorr until the valve was closed to turn off the catalyst supply. In this study for the effect of catalyst, the control valve to the catalyst container was opened for 3 min. Then, the chamber pressure was pumped down to 10 mtorr again, and He gas was introduced into the chamber. When the chamber pressure was stabilized at 150 mtorr, the control valve to the Cu precursor was opened, and the liquid precursor was propelled by the pushing nitrogen gas through the LFM. It was then vaporized in the CEM and mixed with the He carrier gas. The precursor-saturated carrier gas was introduced into the reaction chamber through the gas injector to proceed with the Cu CVD. In this study, Cu CVD was usually conducted for 10 min. The general processing conditions of Cu CVD for the study in this chapter are the same as those used in chapter 3, as shown in Table 3-1.

about one hour, the reaction chamber was pumped down to 10 mtorr. Then, the control valve to the catalyst container was opened to guide the vapor phase catalyst of C₂H₅I into the reaction chamber. As the chamber pressure rose to 1 torr in a short time, it was pumped down again and maintained at 150 mtorr until the valve was closed to turn off the catalyst supply. In this study for the effect of catalyst, the control valve to the catalyst container was opened for 3 min. Then, the chamber pressure was pumped down to 10 mtorr again, and He gas was introduced into the chamber. When the chamber pressure was stabilized at 150 mtorr, the control valve to the Cu precursor was opened, and the liquid precursor was propelled by the pushing nitrogen gas through the LFM. It was then vaporized in the CEM and mixed with the He carrier gas. The precursor-saturated carrier gas was introduced into the reaction chamber through the gas injector to proceed with the Cu CVD. In this study, Cu CVD was usually conducted for 10 min. The general processing conditions of Cu CVD for the study in this chapter are the same as those used in chapter 3, as shown in Table 3-1.

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