Chapter 1: Introduction
1.1.2 Theory Background
The first theory of electron field emission from metals based on quantum mechanics was published by R. H. Fowler and L. W. Nordheim in 1928 [1.7]. The Fowler-Nordheim tunneling field emission is a quantum tunneling phenomenon that when the electric field of conductive solid surface is extremely high, electrons could tunnel through the narrow barrier into vacuum directly of the conductive solid surface, such as metals or semiconductors, even under a very low temperature. On the contrary, thermionic emission is the hot electron emission under high temperature to acquire sufficient energy to overcome the barrier and its electric field is low.
Fig. 1-2(a) showed the band diagram of a metal-vacuum system without an external bias applied. The energy required to make electrons be extracted from the Fermi level of a metal to a rest position of vacuum is named as the work function φ. When a bias is applied, vacuum energy level is lowered and therefore the potential barrier at the surface becomes narrower as shown in Fig. 1-2(b). Afterward, an electron which acquires energy “W” has a finite probability to tunnel through the surface barrier. Fowler and Nordheim deriving the famous F-N equation (1.1) is as followed [1.7]:
( )
exp[ 2 ( )/ ] can be approximated as [1.16]),
Typically, the field emission current I is measured as a function of the applied voltage V.
Substituting relationships of J = I/α and E = βV into Eq.(1-1), where α is the emitting area and β is the local field enhancement factor of the emitting surface, the following equation can be obtained
From Eq. (1-5), the slope of a Fowler-Nordheim (F-N) plot is given by
)
The parameter β could be evaluated from the slope S of the measured F-N plot if the work function φ was known
The emission area α can be subsequently extracted from a rearrangement of Eq. (1-5)
10 )
For example, the electric field at the surface of a spherical emitter of radius r concentric with a spherical anode (or gate) of radius r+d can be represented analytically by
)
Though a realistic electric field in the emitter tip is more complicated than above equation, we could multiply Eq.(1-9) by a geometric factor β` to approximate the real condition.
tip ≡
where r is the tip radius of emitter tip, d is the emitter-anode(gate) distance and β` is a geometric correction factor [1.17].
For a very sharp conical tip emitter, where d >> r, Etip approaches to β`(V/r). And for r>>d, Etip approaches to β`(V/d) which is the solution for a parallel-plate capacitor and for a diode operation in a small anode-to-cathode spacing. As the gated field emission array (FEA) with very sharp tip radius, Eq. (1-10) can be approximated as:
Etip =β`(V/r). (1-11) Combining E= βV and Eq. (1-11), we can obtain the relationship:
Etip =βV =β`(V/r), and β`= βr. (1-12)
The tip radius r is usually in the range from a few nm to 50 nm, corresponding to the parameter β` ranging from 10-1 to 10-2.
Besides, transconductanceg of a field emission device is defined as the change in anode m current due to a change in gate voltage [1.6].
g
Transconductance of a FED is a figure of merit that gives as an indication of the amount
of current charge that can be accomplished by a given change in grid voltage. The transconductance could be increased by using multiple tips or decreasing the cathode-to gate spacing for a given cathode-to-anode spacing.
According to the equations mentioned previously (especially Eq.1-5), some approaches could be taken to reduce the operating voltage of the field emission devices, including increasing the aspect ratio of emission cathodes, lowering the work function of emitter materials, fabricating protrusions, increasing emission sites, narrowing the cone angles of field emitters.
1.2 The Application of Vacuum Microelectronics
Vacuum microelectronics devices may contain two parts: one is that devices require large area of cathodes and in the meanwhile use micro-fabricated sources to achieve high current density, fast current control with low voltages. These kinds of devices include electron guns for microwave beam tubes, emission sources for displays etc.. The other is that devices themselves are small and hence require small electron source. Such devices contain micro-triodes, power generators for infrared or optical frequencies etc..
1.2.1 Overview of the Application of Vacuum Microelectronics
Owing to the superior properties of vacuum microelectronic devices, such as fast carrier drift velocity, temperature insensitivity, and radiation hardness, there are plenty of potential applications for vacuum microelectronics. A great number of applications have been proposed, including high efficiency microwave amplifier and generator [1.18-1.20] (Fig.1-3), high brightness flat-panel display [1.21-1.25] (Fig.1-4), scanning electron microscopy, electron beam lithography, micro-sensor [1.26-1.27] (Fig.1-5), ultra-fast computer, intense electron/ion sources [1.28-1.29] (Fig.1-6), temperature insensitive electronics, and radiation hardness analog and digital circuits.
1.2.2 Vacuum Microelectronics Devices for High Frequency Application
Both solid-state devices and vacuum devices could generate power at frequency in the GHz range, but only vacuum devices remain the technology available for high power and high frequency applications. This is because that when the applied voltages are high, there is enough energy for electrons to accelerate to the velocity faster than the saturation velocity in semiconductor and therefore the fast electrons could reduce transit time for high frequencyoperation. These devices contain traditional multi-terminal vacuum tubes, such as triodes, pentodes, and beam power tubes, and distributed-interaction devices, including traveling wave tubes (TWTs), klystrons, backward-wave oscillators (BWOs).
The performance of conventionally modulated power tubes using FEAs, such as TWT, is determined basically by emission current and current density capability. On the other hand, the application of FEAs in the microwave tubes in which modulation of the beam is accomplished by capacitance and transconductance. A gated FEA in a 10 GHz TWT amplifier with conventional modulation of electron beam has been proposed by NEC Corporation of Japan [1.30]. The amplifier utilized a modified Spindt-type Mo cathode which incorporated a resistive poly-Si layer as a current limiting element. The emission current from the cathode was 58.6 mA. Besides, the structure owns a low resistance under normal operation, so it could operate at 10.5 GHz with the output power of 27.5 W and the gain of 19.5 dB.
1.3 Lateral Field Emission Array
1.3.1 Comparison between Lateral and Vertical Field Emission Array
There are two basic types of field emission arrays: one is vertical type, and the other is lateral one. In the vertical field emission array, the distance between anode and cathode is difficult to control exactly, and the incorporation of a collector needs extra complicated steps to form a cantilevered electrode [1.31-1.32] (Fig.1-7). Therefore, this kind of vertical structure increases fabrication complexity and cost. Compared with vertical field emitters, lateral-type [1.33] (Fig. 1-8) emitters own a great number of advantages, including ease of fabrication, design versatility of electrode geometry and better gap-controlling ability than vertical type emitters.
1.3.2 Fabrication of Lateral Field Emitters
In order to reduce the operation voltage of field emission array to lower power consumption, there are a great many of methods have been proposed to fabricate a small distance between anode and cathode for lateral field emission array. A gap could be produced by high resolution electron beam lithography (EBL) and etching process [1.34] (Fig.1-9).
Besides, focused ion beam (FIB) is another method to fabricate wedge type field emitters [1.35] (Fig.1-10). In addition, some small gaps were produced by stress to cleave during annealing (Fig.1-11), oxidation (Fig. 1-12), and phase transformation [1.36-1.38] (Fig.1-13).
1.4 Motivation
In order to improve field emission characteristics of field emission devices to apply on high performance devices, lower operation voltage and higher field emission current are necessary in field emission devices. Consequently, the aim of this essay is concentrated on reducing operation voltage and increasing field emission current density of field emitters.
Moreover, lateral types of field emitters are adopted due to the advantages of lateral field emitters in comparison with vertical field emitters mentioned previously. In addition, the fabrication temperatures are also controlled to be as low as possible to reduce cost and utilize glass substrate to replace silicon substrate in the future.
1.4.1 Lower Operation Voltage of Field Emitter by Structure Improvement
For the sake of reducing operation voltage, it is essential to reduce the distance between anode and cathode of field emitters. There are lots of methods have been proposed to produce the small anode-to-cathode gaps, including high resolution electron beam lithography (EBL), focused ion beam (FIB) and producing small cracks by thermal or phase transformation stress.
Nevertheless, the throughput of EBL and FIB is low, and the gaps fabricated by thermal and phase transformation are difficult to be controlled exactly. Besides, extra cost will be increased during long-time and high-temperature process, such as annealing and oxidation.
In this essay, thin-film edge emitters are proposed to reduce the operation voltage. In these kinds of structures, the distance between anode and cathode is controlled by the thickness of inter layer and hence could be defined precisely. Furthermore, different metal thin films are deposited to improve field emission characteristics by being treated with hydrogen and ethylene to change the morphologies of surface or physical properties. In addition, the throughput of these structures is higher than that of structures fabricated by EBL or FIB.
1.4.2 Enhance Field Emission Current Density by Carbon Nanotubes
Since carbon nanotubes were discovered by Sumio Iijima in 1991 [1.39] (Fig.1-14), they have attracted a great deal of interest due to their unique properties and potential applications.
Carbon nanotube is considered as one of the most promising field emission materials owing to its special properties, such as high aspect ratio, strong mechanical strength, chemical inertness and good thermal conductivity. Consequently, two types of CNT-based lateral field emission devices are proposed in this essay to enhance field emission current density.
1.5 Thesis Organization
In chapter 1, the overview of vacuum microelectronics including history, basic theory background, applications and motivation is described. In chapter 2, thin-film edge field emitters are proposed to reduce the operation voltage. In these kinds of structures, the distance between anode and cathode could be defined precisely by controlling the thickness of inter layer. Besides, different metal thin films are deposited to enhance field emission characteristics. Afterward, the forming processes which followed metal thin-film deposition to improve field emission characteristics are inspected.
Lateral field emitters combined with carbon nanotubes are proposed in chapter 3. Carbon nanotubes are applied on these kinds of field emitters to take advantages of field enhancement factor. For the sake of improving field emission current density, the trench-type lateral field emission device is developed based on the co-planar-type lateral field emission device mentioned previously. In the end, the summary and conclusions are explicated in chapter 4.
Chapter 2
Fabrication and Field Emission Characteristics of Thin Film Edge Emitters
2.1 Introduction
For the sake of improving the field emission characteristics of devices to be applied on high performance devices, it is essential to reduce the operation voltage of field emitters and hence the distance between anode and cathode must be small enough. In this chapter, a kind of thin film edge emitters with a small gap between emitter and collector is proposed. The size of the gaps is controlled by the thickness of inter layer (amorphous silicon) and could be defined precisely. Furthermore, different emitter materials are deposited to take advantages of their own field emission characteristics. In addition, the samples are loaded into thermal CVD chamber to change the morphologies and chemical properties of the emitter materials to improve field emission characteristics. The schematic diagram of the whole experimental procedures is illustrated in Fig. 2-1
2.2 Experimental Procedures
Fig. 2-2 shows the schematic diagram of the fabrication procedures of a thin film edge emitter. In the beginning, wet oxide (200 nm), amorphous silicon and TEOS oxide (500 nm) were sequentially deposited on the (100) p-type wafer by furnace. In this deposition step, the small gaps between collector and emitter were decided by the thickness of amorphous silicon as 150, 300, and 450 nm and could be defined precisely owing to stable deposition parameters.
After the first lithography, TEOS oxide was wet-etched to become hard mask and then the photo-resist was removed by acetone. Then, amorphous silicon was dry-etched by dielectric reactive ion etcher (dielectric RIE; SAMCO RIE200L) with TEOS oxide as hard mask to produce undercut of amorphous silicon. After the second photolithography, the metal thin films as field emitter materials were deposited by dual E-gun evaporator (JAPAN ULVAC EBX-10C) and magnetron sputtering (Ion Tech Microvac 450CB). In the end, samples with different field emitter materials were loaded into thermal CVD system to be treated and this step is called forming process. The atmospheric pressure thermal CVD system is composed of a 2-in.-diameter horizontal quartz tube, an electric heating system, reaction gas supply and related mass flow controllers (Fig. 2-3).
The morphologies of the samples with different field emitter materials which were untreated or treated by thermal CVD system were characterized by scanning electron microscopy (SEM; Hitachi S-4700I) and atomic force microscope (AFM; Veeco Dimension 5000 Scanning Probe Microscope (D5000) ). Field emission characteristics of thin film edge field emitters were measured with the pressure of 5×10-6 Torr. Anode voltages were applied with a source measure unit (Keithley 237) while the cathode was biased at 0 V. The high vacuum measurement system is shown in Fig. 2-4
2.3 Experimental Design
After the structures of thin film field emitters were fabricated, different field emitter materials were deposited on the samples and the patterns of these field emitter materials were defined through photolithograph and lift off process.
In the first part of experiment of thin film edge field emitters, the Cr thin film (1000 Å) and the bi-layered thin film (W/Ti, 500/100 Å) which were composed of Ti and W sequentially deposited were chosen to be electrodes. Both the samples with Cr thin film and bi-layered thin film were loaded into thermal CVD system to be treated at 550℃ for 30 minutes with 50 sccm C2H4 and 100 sccm H2 (shown in Fig. 2-5) to improve field emission characteristics. One of the electrode materials was selected to be electrodes according to its better electrical characteristics.
In the second part of experiment of thin film edge field emitters, extra thin films, including Co (50 Å) and Pd (50 Å) were deposited individually on the electrodes to be expected to improve field emission characteristics. After deposition, the samples with different field emitter materials were also treated by forming process to improve field emission characteristics. There were two kinds of forming processes; one of the forming processes was that samples were treated at 550℃ for 30 minutes with 50 sccm H2 (shown in Fig. 2-6) and the other forming process was that treated at 550℃ for 30 minutes with 20 sccm C2H4 and 500 sccm N2 (shown in Fig. 2-7). These forming processes were expected to change the morphologies of the surface and chemical properties of the thin films.
2.4 Results and Discussion
The tilted and cross-sectional SEM images of thin film edge emitters are shown in Fig.
2-8(a) and Fig.2-8(b), respectively. The length of emission edge was fixed at 200 μm. The small gap between emitter and collector was fabricated by dry-etching of amorphous-silicon layer to produce undercut. In order to clarify the field emission characteristics of thin film edge emitters, three kinds of gaps were designed by change of the thickness of amorphous-silicon layer as 150, 300, 450 nm (Fig. 2-9) and could be controlled precisely due to the stable amorphous-silicon deposition parameters.
2.4.1 The Selection of Field Emitter Electrode: Cr and W/Ti
The SEM images of Cr thin film and bi-layered thin film (W/Ti, 500/100 Å) are shown in Fig. 2-10 and Fig. 2-11, respectively. It could be observed that after forming process, the morphologies of Cr and W/Ti became rougher. To know more details, the AFM images of Cr thin film and bi-layered thin film (W/Ti, 500/100 Å) before and after forming process shown in Fig. 2-12 and Fig. 2-13 are also checked. The Rms of Cr and W/Ti are also showed in Table 2-1. The scan area of AFM for the samples was 3×3 μm2. The mean roughness of Cr thin film before forming was 0.987 nm (Rms = 1.286 nm) and was 2.086 nm (Rms = 2.629 nm) after forming. The Rms of W/Ti before forming was 0.94 nm and was 1.258 nm after forming.
These results were consistent with SEM images. The increase of surface roughness of Cr thin film and the bi-layered thin film (W/Ti) was conjectured as formation of carbide phase by thermal reaction after forming process [2.1] and therefore could improve the field emission characteristics [2.2-2.3]. In addition, the thickness of W was not so thin and the moving ability of W atoms on the surface was not strong due to the higher melting point and hence the stronger binding energy, and therefore the roughness of W was not obvious as that of Cr after forming process.
The field emission characteristics of Cr thin film and bi-layered thin film (W/Ti, 500/100 Å) with different emitter-to-collector gaps were shown in Fig.2-14 and Fig. 2-15, respectively.
The field emission currents (I) were measured as a function of collector voltage (V) sweep from zero to appropriate positive voltages. The length of emission region is designed as 200 μm, and the turn-on voltage is defined as the emission current achieved 10-7 A. The turn-on voltages of Cr and W/Ti thin film field emitters were shown in Table 2-2.
From Fig. 2-14(a), it revealed that field emission characteristics of Cr thin film edge field emitters with emitter-to-collector gap fixed at 150 nm were improved greatly after forming process, including increase of field emission current and reduce of turn-on voltage. The turn-on voltage of Cr thin film edge emitter before forming process was 115 V and reduced to 10 V after forming process, which has the field emission current could as high as 1.44×10-5 A when the collector was biased at 30 V. Besides, the field emission current was increased and the turn-on voltage was reduced accompanied the narrowing of emitter-to-collector gap. The turn-on voltage of Cr thin film emitters were 15 and 17.5 V with gap fixed at 300 and 450 nm, and it reduced to 10 V when the gap was fixed at 150 nm. Because of the lowering of gaps, electrons were more easily to be extracted from metals.
From Fig. 2-15(a), it could be observed that field emission characteristics of W/Ti thin film edge field emitters with emitter-to-collector gap fixed at 150 nm were also improved after forming process. The turn-on voltage of W/Ti thin film edge emitter before forming process was 62 V and reduced to 15 V after forming process, which has the field emission current as high as 9.73×10-6 A when the collector was biased at 30 V. Furthermore, due to the narrowing of emitter-to-collector gaps, the field emission characteristics of W/Ti thin film emitters were improved. The turn-on voltage of W/Ti emitters were 40 and 62 V with gap fixed at 300 and 450 nm, and it reduced to 15 V when the gap was fixed at 150 nm.
The corresponding F-N plots of the Cr and W/Ti thin film edge emitters are shown in Fig.2-14(b) and Fig.2-15(b), respectively. As could be seen, nearly straight lines with negative
slope at large voltage region were observed, indicating the field emission phenomena of thin film edge emitters.
The improvement of field emission characteristics of thin film edge emitters may because the increase of roughness on the surface of metal thin films enhanced field emission due to formation of carbide phase after forming process. The improvement of field emission characteristics of W/Ti emitters was not obvious as that of Cr emitters due to less increase of roughness of W/Ti surface.
In addition, the improvement of field emission characteristics for both Cr and W was conjectured as increase of roughness rather than reduction of work function. This was because that the work function of carbide phase was almost the same or slightly lower than that of pure metal thin film [2.4-2.5]. Furthermore, the change of work function of binary compound was affected by both work functions of the elements [2.6]. Since the work function of carbon is thought as 4.7~5 eV and almost the same as that of the metal thin film used in the experiments, the improvement of field emission characteristics was regarded as the increase
In addition, the improvement of field emission characteristics for both Cr and W was conjectured as increase of roughness rather than reduction of work function. This was because that the work function of carbide phase was almost the same or slightly lower than that of pure metal thin film [2.4-2.5]. Furthermore, the change of work function of binary compound was affected by both work functions of the elements [2.6]. Since the work function of carbon is thought as 4.7~5 eV and almost the same as that of the metal thin film used in the experiments, the improvement of field emission characteristics was regarded as the increase