- Email: [email protected]
Novel field emission structure based on tetrahedrally bonded amorphous carbon
Diamond and Related Materials 12 (2003) 195–200
Novel field emission structure based on tetrahedrally bonded amorphous carbon W.I. Milne*, J.T.H. Tsai, K.B.K. Teo Engineering Department, University of Cambridge, CB21PZ Cambridge, UK
Abstract This paper demonstrates how tetrahedrally bonded amorphous carbon (ta-C) can be utilised in the manufacture of novel field emission structures which can be used in flat panel displays. By utilising 2-D conventional processes a 3-D device can be produced. We describe how to control the field emission from such structures. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Field emission; Tetrahedrally bonded amorphous carbon
1. Introduction Over the past 10 or so years various carbon based films, including, hydrogenated diamond like carbon w1x, tetrahedrally bonded amorphous carbon (ta-C) w2x, nitrogen containing ta-C w3x, nanostructured carbon w4x, diamond w5x and graphite w6x have been tested for field emission applications because of their potentially low electron affinity or work function. However, although most of these emit at reasonably low fields, the emission is always ‘spotty’ and it is impossible to accurately control the emission site density. Several attempts have been made to overcome this problem, for example the use of high energy ion beams to generate conduction regions in highly sp3 bonded ta-C w7x, and we have also recently published some preliminary results on a novel self-assembled thin film edge emitter (SATFEE) w8x. Although these structures lead to controlled field emission with a high site density, during the wet etch process surface tension effects can pull down several of the cantilevers leading to a reduced yield. In this paper we extend the work reported there to improve the yield and here we include a multilayer SATFEE produced from a combination of silicon nitride and ta-C thin layers and also a new metal–carbon–metal (MCM) emitting structure which allows control of the emission current using only a small control voltage. An 8=8 pixel prototype *Corresponding author. Tel.: q44-1223-332757; fax: q44-1223766207. E-mail address: [email protected] (W.I. Milne).
field emission display based on this structure will also be described. As this novel structure uses hot electrons for emission, the mechanism is different from that utilised in standard FEDs. 2. Experimental work The ta-C is deposited using a double bend filtered cathodic arc deposition system, the details of which are described elsewhere w9x. Fig. 1 shows the SEM image of an array of edge emitters based on ta-C cantilevers with a tensile stressed silicon nitride layer w8x deposited on top of the ta-C to produce bilayers which have an increased stress which has the effect of reducing the surface tension effects. The as deposited ta-C has a ˚ with excellent uniformity over thickness of order 300 A a 60=60 mm2 area on a nqq-Si wafer. The plasma beam is scanned over the area to produce films with ˚ thick film of silicon nitride uniformity "4.5%. A 650 A then was deposited using a conventional PECVD system. The curvature of the cantilevers can be tuned by altering the thickness of the two layers. A high yield can therefore be obtained. A typical array is shown in Fig. 1. In Fig. 1, the array of edge emitters has been tested for field emission performance utilising a diode type test structure and a 7 mm diameter phosphor coated ITO screen as the anode. Fig. 2 shows the FE test structure and the uniform field emission provided from such an array. The bright dots which appear on the screen are due to some of the phosphor powder falling
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 3 . 0 0 0 2 2 - 0
196
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
Fig. 1. Self-structured thin film cantilever array, the stress mismatch provides a force which causes the cantilevers curl up.
down onto the cathode during assembly of the FE system in the vacuum chamber. Individual emitters can easily be distinguished showing that the emission does indeed emanate from the edges of the cantilevers. Fig. 3 compares emission from two similar arrays but with different ta-C thickness—structure (a) uses 10-nmthick ta-C and (b) uses a 30-nm ta-C layer. The thinner ta-C layer provides a higher effective field enhancement as discussed in Ref. w8x and this is verified from the current vs. voltage characteristics. The screen brightness depends on the phosphors used but has been measured using a digital light meter and compared with the emission brightness from a typical CRT display (also shown in Fig. 3b). As can be seen the brightness of these SATFEE structures can easily reach the required levels for such an application using this phosphor. Although the structures shown above exhibit excellent FE performance to complete the fabrication of a practical FE display we need a controllable technique to switch each individual emitter in order to provide the required dynamic images.
An under-gate structure has been previously proposed w8x in order to fabricate the SATFEE into a matrix where we are able to address each pixel. However using this structure, a strong e-field can easily break down the dielectric layer between the ground plate and the SATFEEs. Furthermore, due to the large parasitic capacitance associated with the ground electrode–dielectric–SATFEE structure, the switching speed is slow. To develop a more controllable field emitter structure using the SATFEE, a MCM structure was built directly on the SATFEE. In this device, a bottom Cr metal electrode defined as the data line is pre-deposited onto a c-Si substrate and this is followed by the thin film deposition of the ta-C and silicon nitride. A photomask ‘via’ is used to generate hole patterns which allows a CF4 plasma to etch through the silicon nitride to clear out the small area. A top Cr metal thin film is then deposited and patterned into the scan line. A standard photolithography process is then defined to produce micro-cantilevers using these thin films. The next process is to etch the unwanted area through the top Cr layer, silicon nitride, ta-C and bottom Cr layer. A KOH etching process is applied to undercut the silicon substrate to generate a 3-D SATFEE structure. The field emission from this novel controllable field emitter may be related to two major field emission mechanisms. While applying a voltage across the metal electrodes, the control current heats up the emitter tip by passing current through the ta-C thin film which increases the temperature in a confined area. The conduction electrons are therefore promoted into higher energy states. Under this situation, the MCM acts as a micro-heater which eventually provides a better platform for field emission. Furthermore, there is a Schottky barrier between the Cr and ta-C. Therefore when a control current flows through the interface, the electrons have to overcome this barrier. The applied voltage pushes those conduction electrons to overcome the barrier by giving the electrons higher energy. Therefore, the electron moves either in the conduction band or
Fig. 2. The field emission from a SATFEE in the test chamber and a close-up image from the phosphor anode.
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
197
Fig. 3. (a) The I–V tests from the multilayer SATFEE. The thin ta-C SATFEE provides higher field emission current. (b) High screen brightness can be achieved by using a thin ta-C SATFEE. Comparison with the brightness of a commercial CRT monitor is shown in the dashed box. The brightness of the SATFEE can reach the required level for such an application.
hops through the defect states. Once the electrons are in the higher states, the probability of tunnelling through the thin vacuum barrier is increased. Thus a higher emission current can be obtained. The band diagram for the MCM–SATFEE is shown in Fig. 4. Four layers of photolithography are needed to complete the MCM–SATFEE fabrication. Hence a high degree of accuracy for the overlay across the four layers is required. A photomask has been designed with critical dimension of 10 mm and an overlay error tolerance of 2 mm. The fabrication process of the MCM–SATFEE is relatively complex. There is an additional sacrificial layer that has to be deposited directly on the glass substrate. The function of this sacrificial layer is to
provide a gap between the SATFEE and the glass substrate that allows the micro-cantilever to separate itself from the substrate. To generate a 3-D structure, a wet etching process was carried out to undercut the sacrificial layer which is similar to the KOH etching process when using the silicon substrate. Two kinds of thin film have been used to obtain an optimised sacrificial layer for our SATFEE structure: silicon dioxide and amorphous silicon (a-Si). Silicon dioxide has good adhesion on the glass substrate and the first Cr electrode layer. Hence it can provide a steady anchor in order to support the freestanding cantilever. Although the buffered HF can etch SiO2 properly, the etching rate is relatively low when having
198
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
Fig. 4. The band diagram of the field emission from MCM–SATFEE.
to undercut through a 10 mm wide cantilever. Undiluted HF has been used instead to undercut the sacrificial layer. A very fast etching rate can damage the glass substrate which will easily destroy the micro-cantilever structure as well. An alternative material for use as the sacrificial layer is a-Si, as the KOH solution not only reacts with the silicon but also etchs a-Si. The final structure can be achieved with perfect yield, with a high aspect ratio. Fig. 5 shows the completed MCM–SATFEE device on a glass substrate using a-Si as the sacrificial layer. 3. Results and discussion The MCM–SATFEE sample was made on a 2-in. glass substrate. The overall device size is 2=2 cm2 including the contact pads. This prototype FED contains an 8=8 pixel array of size 160 mm2. Each pixel contains nine MCM–SATFEE micro-cantilevers. The pitch of each MCM–SATFEE is 80 mm so as to observe field emission from each individual site. The two metal electrodes contribute a certain mechanical strength to the micro-cantilever. Hence a higher
multilayer thin film stress is required to curl up the cantilever into a 3-D structure. Therefore, the cantilever has to be formed in one photolithography step and several etching steps must be carried out continuously to etch through the multilayer thin film. Field emission tests were then carried out under a vacuum of less than 10y7 Torr. The field emission was obtained while the anode voltage was fixed at 1450 V and the emission current is controlled by the voltage which is applied between the data lines and the scan lines. Fig. 6 shows the field emission from the MCM– SATFEE built on a glass substrate using a-Si as the sacrificial layer. Data lines were linked in parallel and biased with a voltage from 0 to y4.5 V and all the scan lines are grounded. Therefore all pixels are turned on at the same time. The I–V data from this sample is shown in Fig. 7. From measurement of the total control current and the emission current, the efficiency of the MCM–SATFEE can be calculated. At a maximum data voltage of y4.5 V, the device is able to provide a total efficiency of up to 20%. To prevent potential breakdown of the SiNx layer, the data voltage is limited by the
Fig. 5. (a) The complete 8=8 pixels MCM–SATFEE built on the glass substrate using the a-Si as the sacrificial layer. (b) An optical microscope image of one pixel (before the SATFEE becomes freestanding).
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
199
Table 1 Materials use in the TFT and the MCM–SATFEE
Fig. 6. Eight by eight pixels are turned on while y4.5 V is applied on the data lines.
thickness of the SiNx insulator layer. A compromise is required to optimise the thickness of the SiNx layer: the SATFEE requires a certain SiNx thickness combined with the thin ta-C to provide enough stress to pull up the micro-cantilever but a thinner ta-C will give a higher field emission current. The data voltage has to be limited in order to control the current without breaking down the SiNx. The active matrix structure in a liquid crystal display allows pixels to switch their onyoff state in the millisecond range. In the case of the MCM–SATFEE display, applying a control current at one data line will easily leak to the other data lines when these neighbourhood lines are grounded. Some pixels will be turned on by this leakage current and cause crosstalk error. To prevent this it is also proposed to use thin film transistors to control the data current in the MCM–SATFEE display. The embedded TFT blocks the leakage current and allows the pixel to turn on while enabling one scan line. Hence, the problem of crosstalk will be solved. The
Material_device
TFT
MCM–SATFEE
Glass substrate a-Si nq-a-Si SiNx Cr ta-C
Yes Yes (active layer) Yes Yes (gate insulator) Yes (electrode) No
Yes Yes No Yes Yes Yes
(sacrificial layer) (source-drain insulator) (electrode) (emitter)
materials to fabricate the TFT and the MCM–SATFEE are shown in Table 1. The similarity of the materials used in the fabrication of these two devices will simplify the integration process while combining these devices into a display. 4. Conclusions A novel approach to fabricate a device called ‘the self-assembled thin film edge emitter’ has been demonstrated. This approach allows a simple but effective method to fabricate a 3-D device, using the thin film stress mismatch to deform a micro-cantilever in order to make it stand up vertically. This produces a sharp thin film edge pointing directly towards the anode. Hence an emitter with a very high aspect ratio structure can easily emit electrons due to the high field enhancement factor. Also, the emitting sites directly relate to the location of the micro-cantilevers, providing a uniform field emission pattern on the anode. To control the SATFEE properly and effectively, a MCM structure has been built on the micro-cantilever. Because this structure is positioned at the high field enhancement point, a thinner vacuum barrier is formed. By applying a current through the ta-C thin film, hot electrons can tunnel through the thin energy barrier and
Fig. 7. The field emission and I–V data of the MCM–SATFEE.
200
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
cause emission. This device gave a reasonable efficiency (of order 20%) indicating that this approach could indeed be applied to FED manufacture with low production cost. References w1x K.C. Park, J.H. Moon, S.J. Chung, M.H. Oh, W.I. Milne, J. Jang, J. Vac. Sci. Technol. B 15 (1997) 428. w2x B.S. Satyanarayana, A. Hart, W.I. Milne, J. Robertson, Appl. Phys. Lett. 71 (1997) 1430.
w3x K.C. Park, J.H. Moon, S.J. Chung, et al., J. Vac. Sci. Technol. B 15 (1997) 431. w4x B.S. Satyanarayana, J. Robertson, W.I. Milne, J. Appl. Phys. 87 (2000) 3126. w5x C. Wang, A. Garcia, D.C. Ingram, M. Lake, M.E. Kordesch, Electron. Lett. 27 (1991) 1459. w6x J.B. Cui, J. Robertson, J. Vac. Sci. Technol. B 20 (2002) 238. w7x W.I. Milne, K.B.K. Teo, M. Chhowalla, et al., New Diamond Front. Carbon Technol. 11 (4) (2001) 235–247. w8x J.T.H. Tsai, K.B.K. Teo, W.I. Milne, J. Vac. Sci. Technol. B 20 (2002) 1. w9x M.M.M. Bilek, M. Chhowalla, W.I. Milne, Appl. Phys. Lett. 71 (1997) 1777.
Novel field emission structure based on tetrahedrally bonded amorphous carbon W.I. Milne*, J.T.H. Tsai, K.B.K. Teo Engineering Department, University of Cambridge, CB21PZ Cambridge, UK
Abstract This paper demonstrates how tetrahedrally bonded amorphous carbon (ta-C) can be utilised in the manufacture of novel field emission structures which can be used in flat panel displays. By utilising 2-D conventional processes a 3-D device can be produced. We describe how to control the field emission from such structures. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Field emission; Tetrahedrally bonded amorphous carbon
1. Introduction Over the past 10 or so years various carbon based films, including, hydrogenated diamond like carbon w1x, tetrahedrally bonded amorphous carbon (ta-C) w2x, nitrogen containing ta-C w3x, nanostructured carbon w4x, diamond w5x and graphite w6x have been tested for field emission applications because of their potentially low electron affinity or work function. However, although most of these emit at reasonably low fields, the emission is always ‘spotty’ and it is impossible to accurately control the emission site density. Several attempts have been made to overcome this problem, for example the use of high energy ion beams to generate conduction regions in highly sp3 bonded ta-C w7x, and we have also recently published some preliminary results on a novel self-assembled thin film edge emitter (SATFEE) w8x. Although these structures lead to controlled field emission with a high site density, during the wet etch process surface tension effects can pull down several of the cantilevers leading to a reduced yield. In this paper we extend the work reported there to improve the yield and here we include a multilayer SATFEE produced from a combination of silicon nitride and ta-C thin layers and also a new metal–carbon–metal (MCM) emitting structure which allows control of the emission current using only a small control voltage. An 8=8 pixel prototype *Corresponding author. Tel.: q44-1223-332757; fax: q44-1223766207. E-mail address: [email protected] (W.I. Milne).
field emission display based on this structure will also be described. As this novel structure uses hot electrons for emission, the mechanism is different from that utilised in standard FEDs. 2. Experimental work The ta-C is deposited using a double bend filtered cathodic arc deposition system, the details of which are described elsewhere w9x. Fig. 1 shows the SEM image of an array of edge emitters based on ta-C cantilevers with a tensile stressed silicon nitride layer w8x deposited on top of the ta-C to produce bilayers which have an increased stress which has the effect of reducing the surface tension effects. The as deposited ta-C has a ˚ with excellent uniformity over thickness of order 300 A a 60=60 mm2 area on a nqq-Si wafer. The plasma beam is scanned over the area to produce films with ˚ thick film of silicon nitride uniformity "4.5%. A 650 A then was deposited using a conventional PECVD system. The curvature of the cantilevers can be tuned by altering the thickness of the two layers. A high yield can therefore be obtained. A typical array is shown in Fig. 1. In Fig. 1, the array of edge emitters has been tested for field emission performance utilising a diode type test structure and a 7 mm diameter phosphor coated ITO screen as the anode. Fig. 2 shows the FE test structure and the uniform field emission provided from such an array. The bright dots which appear on the screen are due to some of the phosphor powder falling
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 3 . 0 0 0 2 2 - 0
196
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
Fig. 1. Self-structured thin film cantilever array, the stress mismatch provides a force which causes the cantilevers curl up.
down onto the cathode during assembly of the FE system in the vacuum chamber. Individual emitters can easily be distinguished showing that the emission does indeed emanate from the edges of the cantilevers. Fig. 3 compares emission from two similar arrays but with different ta-C thickness—structure (a) uses 10-nmthick ta-C and (b) uses a 30-nm ta-C layer. The thinner ta-C layer provides a higher effective field enhancement as discussed in Ref. w8x and this is verified from the current vs. voltage characteristics. The screen brightness depends on the phosphors used but has been measured using a digital light meter and compared with the emission brightness from a typical CRT display (also shown in Fig. 3b). As can be seen the brightness of these SATFEE structures can easily reach the required levels for such an application using this phosphor. Although the structures shown above exhibit excellent FE performance to complete the fabrication of a practical FE display we need a controllable technique to switch each individual emitter in order to provide the required dynamic images.
An under-gate structure has been previously proposed w8x in order to fabricate the SATFEE into a matrix where we are able to address each pixel. However using this structure, a strong e-field can easily break down the dielectric layer between the ground plate and the SATFEEs. Furthermore, due to the large parasitic capacitance associated with the ground electrode–dielectric–SATFEE structure, the switching speed is slow. To develop a more controllable field emitter structure using the SATFEE, a MCM structure was built directly on the SATFEE. In this device, a bottom Cr metal electrode defined as the data line is pre-deposited onto a c-Si substrate and this is followed by the thin film deposition of the ta-C and silicon nitride. A photomask ‘via’ is used to generate hole patterns which allows a CF4 plasma to etch through the silicon nitride to clear out the small area. A top Cr metal thin film is then deposited and patterned into the scan line. A standard photolithography process is then defined to produce micro-cantilevers using these thin films. The next process is to etch the unwanted area through the top Cr layer, silicon nitride, ta-C and bottom Cr layer. A KOH etching process is applied to undercut the silicon substrate to generate a 3-D SATFEE structure. The field emission from this novel controllable field emitter may be related to two major field emission mechanisms. While applying a voltage across the metal electrodes, the control current heats up the emitter tip by passing current through the ta-C thin film which increases the temperature in a confined area. The conduction electrons are therefore promoted into higher energy states. Under this situation, the MCM acts as a micro-heater which eventually provides a better platform for field emission. Furthermore, there is a Schottky barrier between the Cr and ta-C. Therefore when a control current flows through the interface, the electrons have to overcome this barrier. The applied voltage pushes those conduction electrons to overcome the barrier by giving the electrons higher energy. Therefore, the electron moves either in the conduction band or
Fig. 2. The field emission from a SATFEE in the test chamber and a close-up image from the phosphor anode.
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
197
Fig. 3. (a) The I–V tests from the multilayer SATFEE. The thin ta-C SATFEE provides higher field emission current. (b) High screen brightness can be achieved by using a thin ta-C SATFEE. Comparison with the brightness of a commercial CRT monitor is shown in the dashed box. The brightness of the SATFEE can reach the required level for such an application.
hops through the defect states. Once the electrons are in the higher states, the probability of tunnelling through the thin vacuum barrier is increased. Thus a higher emission current can be obtained. The band diagram for the MCM–SATFEE is shown in Fig. 4. Four layers of photolithography are needed to complete the MCM–SATFEE fabrication. Hence a high degree of accuracy for the overlay across the four layers is required. A photomask has been designed with critical dimension of 10 mm and an overlay error tolerance of 2 mm. The fabrication process of the MCM–SATFEE is relatively complex. There is an additional sacrificial layer that has to be deposited directly on the glass substrate. The function of this sacrificial layer is to
provide a gap between the SATFEE and the glass substrate that allows the micro-cantilever to separate itself from the substrate. To generate a 3-D structure, a wet etching process was carried out to undercut the sacrificial layer which is similar to the KOH etching process when using the silicon substrate. Two kinds of thin film have been used to obtain an optimised sacrificial layer for our SATFEE structure: silicon dioxide and amorphous silicon (a-Si). Silicon dioxide has good adhesion on the glass substrate and the first Cr electrode layer. Hence it can provide a steady anchor in order to support the freestanding cantilever. Although the buffered HF can etch SiO2 properly, the etching rate is relatively low when having
198
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
Fig. 4. The band diagram of the field emission from MCM–SATFEE.
to undercut through a 10 mm wide cantilever. Undiluted HF has been used instead to undercut the sacrificial layer. A very fast etching rate can damage the glass substrate which will easily destroy the micro-cantilever structure as well. An alternative material for use as the sacrificial layer is a-Si, as the KOH solution not only reacts with the silicon but also etchs a-Si. The final structure can be achieved with perfect yield, with a high aspect ratio. Fig. 5 shows the completed MCM–SATFEE device on a glass substrate using a-Si as the sacrificial layer. 3. Results and discussion The MCM–SATFEE sample was made on a 2-in. glass substrate. The overall device size is 2=2 cm2 including the contact pads. This prototype FED contains an 8=8 pixel array of size 160 mm2. Each pixel contains nine MCM–SATFEE micro-cantilevers. The pitch of each MCM–SATFEE is 80 mm so as to observe field emission from each individual site. The two metal electrodes contribute a certain mechanical strength to the micro-cantilever. Hence a higher
multilayer thin film stress is required to curl up the cantilever into a 3-D structure. Therefore, the cantilever has to be formed in one photolithography step and several etching steps must be carried out continuously to etch through the multilayer thin film. Field emission tests were then carried out under a vacuum of less than 10y7 Torr. The field emission was obtained while the anode voltage was fixed at 1450 V and the emission current is controlled by the voltage which is applied between the data lines and the scan lines. Fig. 6 shows the field emission from the MCM– SATFEE built on a glass substrate using a-Si as the sacrificial layer. Data lines were linked in parallel and biased with a voltage from 0 to y4.5 V and all the scan lines are grounded. Therefore all pixels are turned on at the same time. The I–V data from this sample is shown in Fig. 7. From measurement of the total control current and the emission current, the efficiency of the MCM–SATFEE can be calculated. At a maximum data voltage of y4.5 V, the device is able to provide a total efficiency of up to 20%. To prevent potential breakdown of the SiNx layer, the data voltage is limited by the
Fig. 5. (a) The complete 8=8 pixels MCM–SATFEE built on the glass substrate using the a-Si as the sacrificial layer. (b) An optical microscope image of one pixel (before the SATFEE becomes freestanding).
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
199
Table 1 Materials use in the TFT and the MCM–SATFEE
Fig. 6. Eight by eight pixels are turned on while y4.5 V is applied on the data lines.
thickness of the SiNx insulator layer. A compromise is required to optimise the thickness of the SiNx layer: the SATFEE requires a certain SiNx thickness combined with the thin ta-C to provide enough stress to pull up the micro-cantilever but a thinner ta-C will give a higher field emission current. The data voltage has to be limited in order to control the current without breaking down the SiNx. The active matrix structure in a liquid crystal display allows pixels to switch their onyoff state in the millisecond range. In the case of the MCM–SATFEE display, applying a control current at one data line will easily leak to the other data lines when these neighbourhood lines are grounded. Some pixels will be turned on by this leakage current and cause crosstalk error. To prevent this it is also proposed to use thin film transistors to control the data current in the MCM–SATFEE display. The embedded TFT blocks the leakage current and allows the pixel to turn on while enabling one scan line. Hence, the problem of crosstalk will be solved. The
Material_device
TFT
MCM–SATFEE
Glass substrate a-Si nq-a-Si SiNx Cr ta-C
Yes Yes (active layer) Yes Yes (gate insulator) Yes (electrode) No
Yes Yes No Yes Yes Yes
(sacrificial layer) (source-drain insulator) (electrode) (emitter)
materials to fabricate the TFT and the MCM–SATFEE are shown in Table 1. The similarity of the materials used in the fabrication of these two devices will simplify the integration process while combining these devices into a display. 4. Conclusions A novel approach to fabricate a device called ‘the self-assembled thin film edge emitter’ has been demonstrated. This approach allows a simple but effective method to fabricate a 3-D device, using the thin film stress mismatch to deform a micro-cantilever in order to make it stand up vertically. This produces a sharp thin film edge pointing directly towards the anode. Hence an emitter with a very high aspect ratio structure can easily emit electrons due to the high field enhancement factor. Also, the emitting sites directly relate to the location of the micro-cantilevers, providing a uniform field emission pattern on the anode. To control the SATFEE properly and effectively, a MCM structure has been built on the micro-cantilever. Because this structure is positioned at the high field enhancement point, a thinner vacuum barrier is formed. By applying a current through the ta-C thin film, hot electrons can tunnel through the thin energy barrier and
Fig. 7. The field emission and I–V data of the MCM–SATFEE.
200
W.I. Milne et al. / Diamond and Related Materials 12 (2003) 195–200
cause emission. This device gave a reasonable efficiency (of order 20%) indicating that this approach could indeed be applied to FED manufacture with low production cost. References w1x K.C. Park, J.H. Moon, S.J. Chung, M.H. Oh, W.I. Milne, J. Jang, J. Vac. Sci. Technol. B 15 (1997) 428. w2x B.S. Satyanarayana, A. Hart, W.I. Milne, J. Robertson, Appl. Phys. Lett. 71 (1997) 1430.
w3x K.C. Park, J.H. Moon, S.J. Chung, et al., J. Vac. Sci. Technol. B 15 (1997) 431. w4x B.S. Satyanarayana, J. Robertson, W.I. Milne, J. Appl. Phys. 87 (2000) 3126. w5x C. Wang, A. Garcia, D.C. Ingram, M. Lake, M.E. Kordesch, Electron. Lett. 27 (1991) 1459. w6x J.B. Cui, J. Robertson, J. Vac. Sci. Technol. B 20 (2002) 238. w7x W.I. Milne, K.B.K. Teo, M. Chhowalla, et al., New Diamond Front. Carbon Technol. 11 (4) (2001) 235–247. w8x J.T.H. Tsai, K.B.K. Teo, W.I. Milne, J. Vac. Sci. Technol. B 20 (2002) 1. w9x M.M.M. Bilek, M. Chhowalla, W.I. Milne, Appl. Phys. Lett. 71 (1997) 1777.