Nano-oxide thin films deposited via atomic layer deposition on microchannel plates
© Yan et al.; licensee Springer. 2015
Received: 14 December 2014
Accepted: 18 March 2015
Published: 2 April 2015
Microchannel plate (MCP) as a key part is a kind of electron multiplied device applied in many scientific fields. Oxide thin films such as zinc oxide doped with aluminum oxide (ZnO:Al2O3) as conductive layer and pure aluminum oxide (Al2O3) as secondary electron emission (SEE) layer were prepared in the pores of MCP via atomic layer deposition (ALD) which is a method that can precisely control thin film thickness on a substrate with a high aspect ratio structure. In this paper, nano-oxide thin films ZnO:Al2O3 and Al2O3 were prepared onto varied kinds of substrates by ALD technique, and the morphology, element distribution, structure, and surface chemical states of samples were systematically investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoemission spectroscopy (XPS), respectively. Finally, electrical properties of an MCP device as a function of nano-oxide thin film thickness were firstly studied, and the electrical measurement results showed that the average gain of MCP was greater than 2,000 at DC 800 V with nano-oxide thin film thickness approximately 122 nm. During electrical measurement, current jitter was observed, and possible reasons were preliminarily proposed to explain the observed experimental phenomenon.
KeywordsMicrochannel plate (MCP) Atomic layer deposition (ALD) Thin film High aspect ratio Electrical performance
Microchannel plate (MCP) is a thin glass plate with thickness of about 0.5 mm consisting of several millions of pores of a cylinder geometry with a 4 ~ 25 μm diameter and with a bias angle usually 5° ~ 13° to the normal of the plate surface; the open area ratio of the plate is up to 60%, and the high aspect ratio in each pore is about 20:1 to 100:1. MCP usually as a kind of electron multiplied device can be used in many scientific applications, such as microchannel plate photomultipliers (MCP-PMT), night vision devices, electron microscopy, and fluorescent electron imager [1-5]. Traditional MCP is made of lead silicate glass, and the production process is complex . However, a new type of MCP is made of borosilicate glass in the form of hollow tubes and then fabricated in a typical drawing/stacking/fusing/slicing process, without extensive hydrogen reduction chemical processing .
One can see from Figure 2 that the substrate surface is initially covered with hydroxyl (OH) groups which react with trimethylaluminum (TMA) to deposit a mono-layer of aluminum-methyl groups and give off methane (CH4) as a byproduct. This new surface is exposed to water regenerating the initial hydroxyl-terminated surface and again releasing CH4. Then, one mono-layer of Al2O3 is deposited on the surface. And Figure 2 also shows the steric hindrance of the ligands which is a factor to cause the saturation of the surface, and another factor is the number of reactive surface site (not shown in Figure 2) [18-20]. Due to the absorbing mechanism in ALD method, the saturation of the absorbed precursor is very important for the growth rate of the thin films. For fabricating composite films, such as ZnO:Al2O3 alloys, by adjusting the ALD pulse sequence, ‘etching’ phenomenon has been observed that many of the ZnO:Al2O3 samples prepared in a viscous flow ALD reactor showed lower than expected Zn film content and were thinner than predicted by the ALD growth rates of the pure ZnO and Al2O3 films [21-23]. The Zn deficiency may result from the etching of Zn by the TMA during the Al2O3 cycles by quartz crystal microbalance measurements, and the lower than expected film thickness is caused by a reduced initial growth rate for ZnO ALD on Al2O3 surface and vice versa. So, precisely controlling the thickness and composition of ZnO:Al2O3 films is difficult, especially on complex substrate if a viscous flow reactor is utilized.
The optimization of ALD-MCP with conductive layers and SEE layers is a complicated process because nano-oxide thin film performance depending upon a lot of ALD process parameters was influenced by the existence of an ‘etching’ phenomenon. The conductive and SEE layers as nano-engineered thin films were shown previously to be successfully and uniformly deposited by Sullivan et al. onto non-lead glass MCPs [24,25] and plastic MCPs . And the Argonne National Laboratory has systematically studied conductive layers on a broad range of oxides, such as ZnO:Al2O3, W:Al2O3, and MoO3 − x :Al2O3 [27,28], and characterized the SEE properties for MgO, Al2O3, and multilayered MgO/TiO2 structures to serve as electron emissive layers in the channels of the MCPs . However, the relationship of electrical performance of an MCP device as a function of nano-oxide thin film thickness has not been reported.
In this study, ZnO:Al2O3 as conductive layer and Al2O3 as SEE layer were prepared by ALD technique in the pores of the MCP with 33 mm in diameter. The morphology, element distribution, structure, and surface chemical states of ALD-deposited oxide thin films and the electrical performance of ALD-MCP were systematically investigated.
A commercial hot-wall atomic layer deposition system was used to prepare nano-oxide thin films onto MCP, glass, and polished n-Si (100) substrates. The glass substrates were ultrasonically cleaned in an ethanol/acetone solution and then rinsed in deionized water. The polished Si substrates were dipped in hydrofluoric acid for 30 s and then placed in an ALD chamber waiting for deposition. The bare MCPs (thickness = 0.5 mm, pore size = 20 μm, aspect ratio = 25, bias angle = 8°) were heated to 350°C for 2 h prior to growing nano-oxide thin films. According to the complex structure of MCP, controlling the thickness and composition of ZnO:Al2O3 films on MCP is harder than controlling those on planar substrate [21,22]. During ALD-ZnO:Al2O3 alloy film fabrication process with certain Zn content, the ‘etching’ phenomenon is unavoidable, and the actual thickness and composition of films are lower than the expected values. By our ex situ measurements, the actual results can match the ‘expected’ one with good repeatability. And two approaches were adopted to make thickness and composition as far as possible be uniformly distributed in the pores of MCP. One is extending TMA (or diethyl zinc (DEZ)) and water exposure time for each ALD cycle. Second is using a force flow chamber, a similar chamber structure with ref. , for ALD-MCP instead of a viscous flow reactor.
Detailed ALD experimental parameters for conductive and SEE layers
Percentage of ZnO cycles = 75%
Pure Al2O3, approximately 8 nm
n-Si (100), glass, MCPs
SEM, XPS, EDS, and electrical characterization for MCPs
The surfaces of MCP samples were examined by scanning electron microscopy (SEM; Hitachi S4800, Hitachi, Ltd., Chiyoda, Tokyo, Japan). The film thickness and elemental composition were measured by cross-sectional SEM method and energy-dispersive X-ray spectroscopy (EDS; Oxford Aztec, Oxford Instruments, Oxfordshire, UK). The structure of thin films deposited on varied kinds of substrates was examined by X-ray diffraction (XRD; D8 ADVANCE from Bruker, Madison, USA). The surface chemical composition of samples prepared on silicon was measured by X-ray photoemission spectroscopy (XPS) at 4B9B beamline of Beijing Synchrotron Radiation Facility.
After ALD functionalization, the nickel chromium (NiCr) layer with 250 nm as electrodes was prepared on both MCP sides by electron beam evaporation system for MCP electrical characterization. The MCP resistances were measured using a Keithley Model 6517B electrometer under 10−6 Torr vacuum (Keithley, Cleveland, OH, USA).
Results and discussion
The studies of morphology, composition, chemical state, and structure of nano-oxide thin films prepared via atomic layer deposition
Thickness variation of nano-oxide thin films in the MCP pores
Thicknesses of coatings located in different locations of a pore (nm)
Average value (nm)
According to the measured results, we can conclude that the nano-oxide thin films deposited on MCP pores by ALD technique with conditions 3 and 4 are more even than others. And the uniform thicknesses of conductive and SEE layers are crucial for MCP application.
The average value of Zn/Al atomic ratio is about 2.2 lower than the expected value of 3. There are two reasons for this. One is the existence of the ‘etching’ phenomenon which results in Zn element deficiency. Another more important is the influence of SEE layer using pure Al2O3 on the Al content that results in large Al atomic percentage and lower Zn/Al atomic ratio.
All of the ALD-deposited nano-oxide thin films on silicon substrates in this study have an amorphous structure according to the XRD analysis shown in Table 1. The amorphous structure of all samples probably resulted from low deposition temperature , high aluminum content , and thin thickness . In Table 2, with nano-oxide thin film thicknesses increasing from 91.5 to 249.3 nm, the uniformities decrease slowly from 2.26% to 1.07%. This probably can be explained with the relationship between the structure and surface roughness of ZnO:Al2O3 film. The percentage of ZnO cycle is 75% in this study, and the XRD results show the amorphous structure for all samples suggesting that the ZnO nanocrystal growth is interrupted by the Al2O3 layers and the amorphous structure results in smoother surface when thickness increases .
The shift to lower binding energy of Al 2p and O 1 s peaks after etching treatment is 5.3 eV as shown in Figure 9B and 5.4 eV as shown in Figure 9C, respectively. The results imply that the charge effect is obviously existing when Ar+ is etching the surface without a neutralizing gun.
Electrical performance of ALD-MCP with NiCr electrode
And we also observed current jitter phenomenon shown in Figure 12A during electrical measurement which is equivalent to resistance jitter. To our knowledge, this phenomenon has not been reported and the mechanism of this phenomenon is not fully understood. But we have considered possible reasons to explain this phenomenon.
Figure 12B shows an ideal band gap diagram of conductive and SEE layers. Without carbon atom contamination, the junction of conductive and SEE layer presents a typical band gap diagram. With carbon atom contamination, the recombination center and surface defect states appear, and the different band gap diagram is shown in Figure 12C. According to generalized Ohm’s law , the flow of charge is caused by Fermi level difference which is related to working temperature, and the valence band gap offset is influenced by temperature change and defect states produced by carbon atom contamination. Current jitter is probably associated with temperature change when MCP channels are bombarded by multiplied electrons. Moreover, resistance coefficient of nano-oxide thin films increases with temperature. So, the material resistance is changing with temperature. However, the relationship of Fermi level difference and resistance coefficient of nano-oxide thin films in MCP pores as a function of temperature variation is not present in this work and is studied on the way.
The morphology, composition, chemical state, and structure of nano-oxide thin films ZnO:Al2O3 and Al2O3 prepared via atomic layer deposition were investigated. The nano-oxide thin film thickness uniformities for all MCP samples were less than 3%. The results of Al and Zn contents at different locations along the pore surface signified that the elements were nearly uniformly distributed. The results implied that the ALD technique was capable of depositing homogeneous nano-oxide thin films on substrates with a complex structure, such as glass MCP. The electrical properties of the MCP device as a function of nano-oxide thin film thickness were firstly studied. The electrical measurement results showed that the average gain of MCP was greater than 2,000 at DC 800 V with nano-oxide thin film thickness approximately 122 nm. And current jitter phenomenon was observed in this study. The steady resistance coefficient of nano-oxide thin films and free of contamination production process are probably important for eliminating current jitter. The mechanism of this phenomenon should be studied further.
The authors would like to thank the Strategic Priority Research Program of the Chinese Academy of Sciences for financially supporting this research under Contract No. XDA10010400 and the State Key Laboratory of Particle Detection and Electronics of Institute of High Energy Physics of Chinese Academy of Sciences and University of Science and Technology of China under Contract No. H929420ETD. We also thank Jiaou Wang who provided XPS measurement services at 4B9B beamline of Beijing Synchrotron Radiation Facility.
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