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 of roughness of surface.

Although the field emission characteristics of Cr thin film edge emitters were better than those of W/Ti thin film edge emitters, the electrode material was selected as W/Ti because of the failures of Cr thin film emitters due to joule heating or local arcing produced by large field emission current density passing through small emission area during measurement [2.7-2.9]

(Fig.2-16). The failure phenomena of W is not obvious attributed to the higher melting point of W compared with Cr and therefore W showed higher enduring ability of joule heating.

2.4.2 The Discussion of Field Emitter Material (1): Co

Cobalt thin film (Co, 50 Å) was deposited on the W/Ti (500/100 Å) to expected to improve field emission characteristics by increase of roughness of the surface. However, when Co thin film treated with C2H4 and H2 (the forming process for Cr and W/Ti) in the same time, carbon nanotubes would tend to be produced and resulted in short circuit between anode and cathode easily (Fig.2-17). Consequently, the forming processes were designed as treated with H2 and treated with C2H4 individually to improve field emission characteristics of Co thin film field emitters.

There were two kinds of forming processes mentioned in 2.3 for Co thin film edge field emitters to be treated. The SEM images of Co thin film before and after forming processes are shown in Fig. 2-18 and from them could be observed that morphologies of Co thin film became rougher after the two forming processes. Besides, the morphologies were also examined by AFM images of Co thin film before and after forming processes shown in (Fig.

2-19). The mean roughness of Co thin film before forming was 1.021 nm (Rms = 1.291 nm) and became 4.291 nm (Rms = 6.439 nm) and 7.111 nm (Rms = 9.134 nm) after treated with H2 and C2H4, respectively. After treated with H2 at 550℃, Co thin film was transformed into nano-particles due to thermal energy therefore increasing the roughness of metal surface and H2 was used to reduce the metal. After treated with C2H4, Co thin film was assumed to increase roughness by carbide formation [2.10-2.12] and graphite layers production [2.12-2.13]. At the beginning of C2H4 treatment, C2H4 was decomposed on the surface of Co and diffused into Co particles, and carbide phase and graphite layers was produced during cooling [2.12-2.13]; therefore, these phenomena increased the roughness of Co surface at the same time. The field emission characteristics of Co thin film edge emitters before and after treated with H2 and C2H4 are shown in Fig. 2-20 and Fig. 2-21, respectively. The turn-on voltages of Co thin film field emitters treated with both forming processes were shown in

Table 2-3. The turn-on voltage of Co thin film edge emitter with emitter-to-collector gap fixed at 150 nm before forming process was 112 V and reduced to 12 V after treated with H2, which has the field emission current as high as 1.63×10^-6 A when the collector was biased at 40 V.

Furthermore, the increase of field emission current and reduce of the turn-on voltage accompanied the narrowing of emitter-to-collector gap due to ease of extraction of electrons.

The turn-on voltage of Co thin film emitters were 28 and 50 V with gap fixed at 300 and 450 nm, and it reduced to 12 V when the gap was fixed at 150 nm.

The turn-on voltage of Co emitter with gap fixed at 150 nm reduced to 8 V after treated with C2H4 and the field emission current could achieve 1.9×10^-5 A when the collector was biased at 40 V. Besides, the turn-on voltage of Co thin film emitters were 20 and 43 V with gap fixed at 300 and 450 nm, and reduced to 8 V when the gap was fixed at 150 nm.

The corresponding F-N plots of the Co thin film edge emitters are shown in Fig.2-20(b) and Fig. 2-21(b). It revealed that straight lines with negative slope at large voltage region, and also indicated the field emission phenomena of thin film edge emitters.

Field emission characteristics were improved by forming processes to increase roughness of the surface of Co thin film. In addition, Co thin film edge emitters treated with C2H4

showed better field emission characteristics than those treated with H2 owing to rougher surface. The roughness was attributed to the carbide and graphite layer formation to enlarge the field enhancement factor.

In addition, the main reason for improvement of field emission characteristics of Co was conjectured as increase of roughness instead of reduction of work function due to carbide formation. The reason was the same as the reason mentioned in 2.4.1. There was no obvious difference of work function between Co and C, and therefore the reduction of work function due to carbide formation wasn’t apparent.

2.4.3 The Discussion of Field Emitter Material (2): Pd

Palladium thin film (Pd, 50 Å) was another field emitter material deposited on the W/Ti (500/100 Å) to improve field emission characteristics and the two kinds of forming processes mentioned previously were also adopted.

The SEM images of Pd thin film before and after the two forming processes are shown in Fig. 2-22. It revealed that the increase of roughness after the two forming processes was not very obvious. After treatments, the phase transformation of Pd would take place, the carbon atoms would diffuse into the interstitial positions and hydrogen atoms would diffuse into both the interstitial and substantial positions of Pd and resulted in lattice expansion of Pd [2.14-2.15]. However, the roughness of Pd thin film after treatment was not apparent. This was conjectured that the under layer of Pd, W, has the higher surface energy than Pd and hence the roughness of Pd was not so obvious. The morphologies were also checked by AFM images shown in Fig. 2-23. The mean roughness of Pd was 0.802 nm (Rms = 1.028 nm) before forming process, and was 1.106 (Rms = 1.394 nm) and 0.909 nm (Rms = 1.152 nm) after forming treated with H2 and C2H4, respectively. These results were also consistent with SEM images.

The field emission characteristics of Pd thin film edge emitters before and after treated with H2 and treated with C2H4 are shown in Fig. 2-24, and Fig.2-25, respectively. The turn-on voltages of Pd thin film field emitters treated with both forming processes were shown in Table 2-4. The turn-on voltage of Pd thin film edge emitter with emitter-to-collector gap fixed at 150 nm before forming process was 114 V and reduced to 25.5 V after treated with C2H4, which has the field emission current as high as 3.59×10^-7 A when the collector was biased at 40 V. Besides, the turn-on voltage of Co thin film emitters were 71 and 84 V with gap fixed at 300 and 450 nm, and it reduced to25.5 V when the gap was fixed at 150 nm.

On the other hand, the great improvement of field emission characteristics of Pd treated

with H2 could be observed in Fig. 2-24(a). The turn-on voltage of Pd emitter with emitter-to-collector gap fixed at 150 nm before forming process was 114 V and reduced to 6.5 V after treated with H2, which has the field emission current as high as 5.67×10^-5 A when the collector was biased at 40 V. Besides, the turn-on voltage of Pd thin film emitters were 10.5 and 16 V with gap fixed at 300 and 450 nm, and it reduced to 6.5 V when the gap was fixed at 150 nm.

The corresponding F-N plots of the Pd thin film edge emitters are shown in Fig.2-24(b) and Fig. 2-25(b). The straight lines with negative slope at large voltage region could be seen, and it indicated the field emission phenomena of thin film edge emitters.

Although there was no apparent difference between the morphologies of Pd surface after the two forming processes, the improvement of field emission characteristics of the samples treated with H2 were greater than those treated with C2H4, including increase of field emission current and reduce of turn-on voltage. This was conjectured that PdHX would be produced after Pd thin film treated with H2, and the work function of PdHX was lower than pure Pd [2.16-2.18]. Besides, although the treatment of C2H4 would provide hydrogen atoms for Pd thin film, the PdHX production would be suppressed by the competition between PdCX and PdHX [2.14-2.15]. Additionally, the flow rate of H2 (50 sccm) was larger than the flow rate of C2H4 (20 sccm) and hence the formation of PdHX for Pd treated with C2H4 was not obvious.

Consequently, the field emission characteristics of Pd thin film edge emitters treated with H2

were improved greatly than those treated with C2H4.

2.4.4 The Comparison for Different Field Emitter Materials

Due to the thickness of electrode materials was much thicker than Co and Pd thin films, the comparison was divided into two parts: one was about electrode material, and the other was about emitter thin film.

The original work function and the morphologies of Cr and W/Ti electrode materials are shown in Table Table 2-5. The increase of roughness was due to formation of carbide phase after treatment and therefore improved the field emission characteristics. The roughness of Cr after forming process was larger than that of W. This was inferred that melting point of W was much larger than Cr and thus the weaker moving ability of atoms due to stronger bonding energy resulted in less increase of roughness. Therefore, the improvement of field emission characteristics of Cr was greater than that of W due to larger roughness.

The original work function of Co and Pd thin films and the morphologies of Co and Pd

The original work function of Co and Pd thin films and the morphologies of Co and Pd