Through-silicon via submount for the CuO/Cu2O nanostructured field emission display

A three dimensional (3D) field emission display structure was prepared using CuO/Cu2O composite nanowires (NWs) and a three dimensional through silicon via (3D-TSV) technique. The experimental results indicated that the diameter and length of the Si via were about 100 μm and 200 μm, respectively. For the 3D field emission structure, high-density CuO/Cu2O composite nanowires (NWs) were grown on the concave TSV structure using thermal oxidation. The field emission turn-on field and enhancement factor of the CuO/Cu2O composite NWs were 15 V μm−1 and ∼1748, respectively. With regard to field emission displays, we successfully used the 3D field emission structure to excite the orange phosphors.


Introduction
Semiconductor nanowires (NWs) have been investigated for a broad range of potential applications, such as in electronics, 1 optoelectronics, 2 eld emission, 3 the biosciences, and energy sciences. p-Type semiconductors with cupric oxide (CuO) 4 have attracted much attention because of their interesting properties and potential applications in eld emission devices, solar cells, superconductors, and photo detectors. Cupric oxide (CuO) and cuprous oxide (Cu 2 O) are p-type semiconductors with monoclinic and cubic crystalline structures, respectively. In the last few years, various CuO nanostructures have drawn research attention for use in applications such as solar cells, 5 high-temperature superconductors, 6 gas sensors, 7 nanorods, 8 and eld emission devices. 9,10 CuO is a suitable eld emitter for such uses due to its rather small bandgap and good conductivity compared to those of other metal oxide materials. 11,12 NWs are attractive as components of eld emitters due to their high emission current density and ease of fabrication. [13][14][15] Field emission displays (FED) are at plane displays that have the advantages of low powerconsumption, high brightness, good color rendition, short response time, and wide operating temperature range. 16,17 In recent years, the development of integrated circuits (IC) has followed Moore's law. However, current semiconductor processing technologies face new challenges when the size of electronic components is downscaled to 10 nm and beyond. 3-D integration using through silicon via (TSV) technology is one solution to overcome the scaling limit, and to realize functional diversication for Si-based ICs. TSV technology has been used in microelectromechanical systems (MEMS), 18 and can enhance the performance of 3D ICs. A lot of effort has thus been devoted to simplifying the TSV process to increase stability and reduce power consumption.
In this work, we integrated the CuO/Cu 2 O composite nanowires (NWs) and the three-dimensional through silicon via (3D-TSV) technique to fabricate a 3D eld emission structure. CuO/Cu 2 O composite nanowires are grown on a concave TSV structure. The detailed fabrication of the TSV and the electro properties of the fabricated materials are also discussed. These structures are then combined with phosphors to complete a 3D at display. Fig. 1 shows a schematic of the fabricated 3D eld emission display. To fabricate the concave TSV structure, a 6-inch silicon substrate with a (100) orientation was wet cleaned in a standard RCA process. A 300 nm thick Al layer was then deposited by sputtering deposition for use as the etching barrier layer. Standard photolithography was used to make a mask for etching the Al layer. The exposed Al was then wet etched using aluminum etch, aer using acetone to remove the positive type photoresist. For the TSV process, the ow of SF 6 gas, O 2 gas, substrate temperature, etching time, electrode gap, RF power, and chamber pressure were set at 6 sccm, 50 sccm, À117 C, 180 min, 7 cm, 1200 W, and 10 mTorr, respectively. The patterning Al layer was then removed by aluminum etch. Next, a SiO 2 isolation layer with a thickness of 1 mm was formed by thermal oxidation. The Ti adhesion layer was deposited by thermal evaporation with a thickness of 100 nm. The Cu seed layer was also deposited by thermal evaporation, with a thickness of 500 nm. Aer the Cu electroplating process, a 15 mm thick negative-type PR was spin-coated on the TSV structure, and then standard photolithography was used to dene pattern. For the Cu electroplating process, the solution consisted of CuSO 4 , H 2 SO 4 and suitable organic additives. The plating current and temperature were xed at 0.1 A and 25 C, respectively. The backside of the substrate was then polished the backside using chemical-mechanical planarization (CMP). Finally, the wafer was put into H 2 SO 4 solution to etch the copper and thus achieve a concave structure.

Experimental
To produce the 3D eld emission display, the concave TSV structure was then put into a furnace and heat-treated at 550 C in ambient air for 6 h to grow CuO/Cu 2 O NWs as an emission structure. For the light layer structure, off-the-shelf orange phosphor was deposited on an indium tin oxide (ITO) electrode. Finally, we integrated the emission structure and light layer structure to form the 3D eld emission structure.
The morphology, crystallinity, and optical properties were measured using eld-emission scanning electron microscopy (FESEM, JEOL JSM-7000F) and X-ray diffraction (XRD, Rigaku D/ MAX2500). The current-voltage (I-V) and eld emission measurements were conducted using a high-voltage source meter (Keithley 4210) under a vacuum. The side length and width of the TSVs were about 240 mm and 120 mm, respectively. To carry out the TSV principle etching mechanism of the cryogenic DRIE, both SF 6 and O 2 were provided as a continuous gas ow inside the reactor. During the process these gases react with silicon and form a solid passivation layer of SiO x F y at surface temperatures below À117 C. Due to directed kinetic energy transfer by the ions, the bottom is by far more easily cracked than the sidewalls. Therefore, the DRIE etch reactions for the TSV structure are as shown in the following equations: 19 Fig. 2(b) shows the cross-sectional SEM image of the Cu TSV plating aer etching with a concave structure. It can be seen that the diameter and depth of the concave structure were about 120 mm and 40 mm, respectively. According the past thermal oxidation reports, copper oxide based NWs grow within the temperature range 350-700 C, and the results of the current study are compatible with previous reports. The atmospheric oxygen reacted with the Cu TSV, the surface of which quickly oxidized to copper oxide based NWs. Fig. 2(c) shows the highdensity copper oxide based NWs were grown on the top of the Cu TSV aer heat treatment at 550 C for 6 h. The average diameter and length of the NWs were $100 nm and $20 mm. The chemical reactions for the CuO composite NWs grown are as follows: Fig. 3 shows the XRD scan pattern of the copper oxide based NWs. The XRD peaks demonstrate that the NWs have the CuO monoclinic and Cu 2 O cubic crystalline structures (JCPD card no. 89-2530 and 65-3288, respectively). The Cu 2 O (111) peaks are much more intense than the other peaks. Fig. 2 The cross-sectional SEM images of (a) a Cu/TSV, (b) a concave structure, (c) the high-density copper oxide based NWs were grown on the top of the concave structure. Fig. 3 The XRD scan pattern of the copper oxide based NWs. Fig. 4(a) shows the eld emission spectra of the CuO/Cu 2 O composite NWs measured at room temperature in the dark. The eld emission turn-on eld of the CuO/Cu 2 O composite NWs was 14.5 V mm À1 . The eld emissions are described by the Fowler-Nordheim (F-N) equation: 20

Results and discussion
where J is the current density (A m À2 ), a ¼ 1.54 Â 10 À6 (A eV V À2 ), b is the eld enhancement factor, E is the applied electric eld, f is the work function (eV), and b ¼ 6.83 Â 10 À3 (V mm À1 eV À3/2 ). Fig. 4(b) shows the F-N plot. The work function of CuO is 5.31 eV. The enhancement factor b is 1748, as calculated from the curve slope.
To obtain a 3D nanostructure eld emission display, the light layer structure was placed on top of the CuO/Cu 2 O nanostructure eld emission structure, and then the 3D nanostructure eld emission display was put into a vacuum chamber. Fig. 5 Fig. 5(e). As such, it can be concluded that this study successfully fabricated a 3D eld emission display using TSV and the CuO/Cu 2 O nanostructure technology. Furthermore, this study showed that we can integrate this with a controller, circuit design, and layout design to easily control the words or images shown.

Conclusions
In summary, a 3D eld emission display structure was prepared in this work using CuO/Cu 2 O composite nanowires and the 3D-TSV technique. With regard to the structure of the 3D-TSV, the diameter and length of the Si via were about 100 mm and 200 mm, respectively. For the 3D eld emission structure, highdensity CuO/Cu 2 O composite nanowires (NWs) were grown on the concave TSV structure using thermal oxidation. The eld emission turn-on eld and enhancement factor of the CuO/ Cu 2 O composite NWs were 15 V mm À1 and $1748, respectively. We successfully obtained a light layer structure covering the CuO/Cu 2 O nanostructure eld emission structure, thus producing a 3D eld emission display. Moreover, we found that it is relatively simple to control the positions of the rear electrodes, and thus show the words or images that are wanted.

Conflicts of interest
There are no conicts to declare.