Fabrication and characterization of the Pd-silicided emitters for field-emission devices

Fabrication and Characterization of the Pd-Siljcjded Emitters for Field-Emission Devices Chih-Chong Wang, Tzu-Kun Ku, Ming-Shiann Feng*, ling-Jar Hsieh**, and Huang-Chung Cheng

Department of Electronics Engineering and Institute of Electronics, *Iflsfitute of Material Science and Engineering,

National Chiao Tung University, Hsinchu ,Taiwan,Republicof China. **Depjen of Electrical Engineering, Chung-Hua Polytechnic Institute,

Hsinchu ,Taiwan

_,

_Republicof China.

ABSTRACT

The structure of Pd-silicided field emitters based on the silicon micromachining technology has been demonstrated. The uniform and extremely sharp silicided emitters are formed using wet chemical etching, low-temperature oxidation sharpening (LTOS), coating metal and furnace annealing in N2 ambient. The sheet resistance and Auger electron spectroscope (AES) results depict the transformation of siicidation. Transmission electron microscope (TEM) of bright-field, dark-field, and diffraction pattern show the formation of silicided emitters. These emitters have potential applications in vacuum microelectronics to obtain superior lifetime, reliability, and stability.

Keywords: Pd, silicided, field emitter, micromachining, sharpening, annealing, ThM, AES

1. INTRODUCTION

Vacuum microelectronic devices have been extensively studied in recent years because of its potential applications in high frequency devices, flat panel displays, microscopy, lithography, and microsensors. Field-emission flat panel displays are the most interesting one for its ultra-high resolution, thin thickness, low power consumption, and good color image. Such properties are particularly desirable for portable computer and electronic avionics displays 1)•

For most applications, vacuum microelectronic devices had to be maintained in

ultrahigh-vacuum conditions which was less than iO ton. To diminish this problem, it was thus important to choose the appropriate emitter materials that were suitable for a given state and possibly to stabilize the emission. There were some methods to fabricate the field emitters, which consisted of metal, silicon, metal-coated and silicided ones. The first field emitter satisfied the requirements of field-emission devices was the metal emitters, commonly called as "Spindt type" 2)• The advantage of this type was the high current density that could be achieved for their lower surface work function; the disadvantages

included the relatively complicated and non-standard fabrication process to the

very-large-scale-integration (VLSI). The emitters were formed by using special electron beam deposition system to

deposit metal through near-micron-size gate apertures that were defined lithographically. Hence,

precise control of the tip radii and heights of the cones were hard to achieve. The strong temperature dependency of molybdenum emitters was another reliability problem on emission 3)• On the other hand, the fabrication process of the pyramid-shaped silicon was relatively easily controlled for the tip radii and pyramid heights. The native oxide layer, however, was feasibly produced in the surface of silicon

emitters and made the devices deviated from the classical Fowler-Nordheim behavior slightly. Furthermore, the metal-coated microtips with the advantages of silicon one and without the

disadvantages of metal one were reported. But if the native oxide layer was not removed completely in the dual-electron-gun evaporation system, the emitter surface would be broken and removed like a result of emitter explosions in the operation 46) Therefore, the consideration of the reliability and the fabrication technology of the field emitters, made the silicided emitters more suitable for the field emission devices.

Some interesting aspect was to explore the silicides as field emitting materials. High-temperature stability of silicides throughout IC fabrication and actual usage of the devices have been well

established. High-temperature solid-state reactions between deposited metals and Si emitter can be activated by sintering. The silicide formation causes the suicide-silicon interface to be free of surface imperfections and contamination. In addition, the work function of the Mo silicides formed by heating the Si/Mo tips are about 10 % smaller than that of a clean Mo surface. The strong metal-Si bonds are moreover indicative of the excellence of the field emitting materials 7)• The topics of this study are

therefore .focused_{on the Pd silicided-emitter. The sheet resistance (Rs), Auger electron spectroscope} (AES) and transmission electron microscope (ThM) were used to examine the emitter microstructures and constituents before and after silicidation.

2. FABRICATION

The silicon field emission cathode reported here were fabricated on 3-inch, (100)-oriented, 5 7

ohm-cm_{phosphorus-doped silicon wafers. After a standard RCA initial cleaning process, 3000 A-thick} layers were thennally grown on the wafers in an 02 H2 ambient at 1 100 C to provide a masking material for the following wet chemical etching process, as shown in Figure 1(a). The samples were then photolithographically patterned. The oxide layer was etched using buffer HF acid (or BOE) to

form a 40x25 array with 2'5mm_{square pads as shown in Figure 1(b). After the photo-resist was} removed, the residual oxide pad became the self-aligned etching mask. The silicon pyramids were formed by the isotropic etchant. The etchant consisted of 95%_HNO3,3% CH3COOH, and 2% HF (vol.

%).The_{etching process was monitored and terminated when the oxide caps were undercuued almost} completely or a few oxide caps in the arrays had been already released. Then, the initial pyramids, as shown in Figure 1(c), would be formed after these processes. In order to obtain the Fowler-Nordheim emission at moderate voltages, a sharp emitter tip is ieeded. Low-temperature oxidation sharpening technique (LTOS) is used to sharpen the emitter radius ,as_{shown in Figure 1(d). The LTOS included}

reoxidizing the emitter in dry 02 at 950Cfor 2.5 hrand subsequent oxide stripping. This procedure can be repeated as often as necessary to achieve the desired sharpness. Immediately after the sharpened tips were dipped in a concentrated HF, as shown in Figure 1(e), 300 A-thick Pd film was deposited onto by dual-electron-gun evaporation system, as shown in Figure 1(f). In addition to the substrate with tips, flat Si substrates were also installed into the chamber for comparative studies. The deposition rate of Pd layer was 1.0 A/s. The vacuum during deposition was maintained to be better than 4*106 torr. Then, the annealing processes at the temperatures ranging from 200 C to 800 C in N2 ambient for 60 mm were used to form the silicided emitters.

Specimens for cross-sectional ThM observations were prepared and a JEOL-2000FX scanning transmission electron microscope operating at 200 KY was used to examine the microstructures of the formed emitters. In order to protect the silicide emitters, a photoresist was used to cover the whole sample before specimen cleaving. Since the silicided emitters would be milled by the powder which was lapped from the samples. This specimen was cleaved and then lapped from the sides to assure the emitters remained. Then, the specimen was mounted on its side onto a copper ring using epoxy. The acetone was used to remove the photoresist on the siicide emitters and clean the specimen surface. The silicide emitters were studied with the electron beam normal to the emitter axis. The thinning procedure was not necessary because the region near the apex was already electron transparent. The specimen with flat surface for comparison was prepared by the usual process of bonding the identical pieces of

the sample face to face, mechanical thinning, dimpling, and fmally argon ion milling to electron

transparency 9)

3. RESULTS AND DISCUSSION

Figure 2(a) shows a silicon emitter after low-temperature oxidation sharpening and removing the oxide layer with buffer HF solution. The silicon emitter is not sharpened enough in the oxidation procedure. Therefore, it has a small neck on the apex and the tip radius is in the order of 500 A. The

apex of this silicon tip will be further sharpened when the oxidation time is added. The sharpening mechanism agrees with the result of Marcus 10) Thefinal radius of the tip is ranging from 200 A to 300 A. The fringes in Figure 2(a) is the thickness effect. Figure 2(b) shows the ThM diffraction pattern corresponding to Figure 2(a). It is the [1 1OJ-orientation of silicon, which is normal to the orientation of the emitter. Figure 3(a) shows a ThM bright-field micrograph of a tip deposited with a 300 A-thick palladium layer. The roughness of the palladium surface is attributed to the Pd grain morphology. Selected area diffraction and clark field images show that the Pd has a grain size less than 100 A. It can also be seen that the Pd film completely covers all the tip surface. In addition, the emitter has a tip redius less than 300 A. Figure 3(b) shows a ThM diffraction pattern corresponding to Figure 3(a). The {1 1 1}, (200), {220}, {3 1 1}-oriented diffraction rings of polycrystalline palladium are observed. Figure 4(a) shows the ThM micrograph of silicon emitter coated with Pd layer and annealed at 500 C for 60 mm. From the dark-field image and the concerning diffraction pattern in Fig. 4(b), it can be seen that the suicide phase PdSi has completely be formed on the whole emitter surface. In addition, theapex of the emitter is also converted into palladium silicide and the radius of the emitter is keep even after the siliciciation. In order to further identify the phase of siicided emitter, a flat silicon substrate deposited with a 300 A-thick Pd layer and subsequent annealed in N2 ambient at 200 C for 60 mm were also analyzed with ThM. Consequently, the bright-field and diffraction pattern, as shown in Figs. 5(a) and (I,), indicate the same PdSi phase, as those in Figs. 4(a) and (b). R. A. King and his co-authors also reported that the palladium-suicide-coated silicon emitters were identified by the pulse laser atom probe

")._{However,the shape of the emitter was round and the tip radius of the emitter was larger than 1500 A}

due to the different fabrication technique of emitter for the King's report.

In order to understand the constituent of the emitters, the performance of Auger electron

spectroscope (A.ES) was undertaken. Figure 6 shows the ABS depth profile for the siicided emitters formed by annealing at 200 C for 60 mm. It was observed that the emitters had the palladium atoms twice time more than silicon ones. The phase of these field emitters was thus PdSi, which has been confirmed by the images of transmission electron microscope. Four-point-probe (4PP) was used to measure the sheet resistance of the silicide films. The sheet resistance results could depict the silicide transformation. The sheet resistance (Rs) slightly increased with annealing temperature initially due to the initial phase transition from Pd to Pd-suicide. Then, the sheet resistance decreased slightly with increasing annealing temperature up to 600 C. It can be attributed to the grain growth of Pd silicide. The sheet resistance increased rapidly when the temperature rises above 600 C. It is due to the island formation of the Pd silicide at 700 C and 800 C annealings. The results was also consistent with the ThM micrographs.

4. CONCLUSIONS

In summary, the palladium-silicided emitters was completely fabricated by etching, low-•

temperature oxidation sharpening, coating metal and furnace annealing in N2 ambient. The silicide emitters were demonstrated by bright-field, dark-field images and diffraction pattern of transmission electron microscope, Auger electron spectroscope and sheet resistance. The silicided phase of the field emitter was first identified by the transmission electron microscopy. In addition, the ThM results are consistent with Auger electron spectroscopy. The siicided emitter can be concluded to be PdSi. In the consideration of reliability and stability, the silicided emitter is more suitable for the field emission devices.

5. ACKNOWLEDGMENTS

This research was supported in part by the Republic of China National Science Council (ROC NSC) under the Contract No. NSC84-0404-E-O09-O11. The technical supports from the National Nano Device Laboratory, ROC NSC and the Semiconductor Research Center in National Chiao_Tung University were also acknowledged.

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