Elsevier

Microelectronic Engineering

Volume 98, October 2012, Pages 329-333
Microelectronic Engineering

A new process for the fabrication of planar antenna coupled Ni–NiOx–Ni tunnel junction devices

https://doi.org/10.1016/j.mee.2012.07.078Get rights and content

Abstract

Antenna coupled metal–insulator–metal (ACMIM) tunnel junctions are fast electromagnetic wave detectors shown to respond to radiation of wavelength as short as 1.6 μm. In the design and fabrication of these devices, it is crucial to keep the RC time constant of the tunnel junction small to achieve the requisite cut-off frequency and adequate rectification efficiency. Junction geometry and the properties of the insulation layer between the metal antenna parts play an important role in determining both the time constant and the rectification effectiveness. In this paper we have designed and fabricated Ni–NiOx–Ni devices and performed three oxidation processes to optimize the insulation layer. Ease of manufacture over large areas is also a requirement for successful implementation. We detail two simple and low cost processes for the fabrication of Ni–NiOx–Ni tunnel junctions. These methods allow for the large area array implementations of the antenna coupled MIMs for the application in low-cost energy harvesting.

Highlights

► Methods to fabricate planar, geometrically asymmetric Ni–NiOx–Ni tunnel diodes are developed. ► Planar structure decreased the parasitic capacitance enabling high cut-off frequency. ► Ni electrodes provided the surface oxidation control and low band edge offset with its native oxide. ► Asymmetric geometry of the electrodes resulted in electrical asymmetry across zero bias. ► Three Ni oxidation methods are studied to build a thin, but durable oxide for the tunnel barrier.

Introduction

Antenna coupled metal–insulator–metal (ACMIM) tunnel junction devices are in active development as infrared detectors and energy harvesters [1], [2], [3], [4]. Detection wavelengths as short as 1.6 μm may be possible. In addition to high-frequency sensitivity, these devices offer the possibility of cheap, large area manufacture. Given these benefits, these structures may receive and rectify the infrared portion of the out-of-visible solar spectrum as well as radiation from the cooling earth (10μm in wavelength). In this paper our primary application goal is IR–RF energy scavenging. As shown here, design simplicity, cheap processes that can be realized over large areas, low cost material, and efficient zero bias operation move ACMIM junctions closer to reality as energy harvesting devices.

ACMIM tunnel junctions utilize a fast tunneling mechanism that enables the rectification of terahertz signal coupled to the micro-scale antenna. Although the electron transport through the insulator is extremely fast (10–15 fs) [5], the junction time constant (RjCj) defines a cut-off frequency. It is crucial to decrease both the resistance and the capacitance for the high frequency operation. However, when the junction is formed by overlapping electrodes, junction resistance and capacitance are linked together by geometry: increasing the junction area decreases junction resistance, but it also increases capacitance. It is not possible to decrease the time constant by simple junction area modulation.

In our design, we utilize a geometrically asymmetric field enhancing technique in a completely planar device. The planarity significantly reduces the parasitic capacitance effects, thereby reducing the time constant. The asymmetry in the geometry enhances the junction electric field, reducing the effective junction resistance. This enables zero bias rectification operation.

Section snippets

Background

In the early development of thin film antenna coupled MIM diodes, the main challenge was to pattern micro-scale antennas that respond to terahertz frequencies [6]. Developments in electron beam lithography (EBL) have made micro-scale patterning straightforward. Currently, the main fabrication challenges are the fabrication of an asymmetric tunnel junction and small area junction formation.

Two new methods aimed at overcoming the resolution limits of the EBL have been offered recently. Bean et

Device fabrication

Contrary to the common approach, where an overlap between antenna metal films forms the tunnel junction and the conduction takes place in the vertical direction over the overlap area, in our processes the conduction is in the lateral direction.

Both processes start with isolating the silicon substrate in order to minimize the leakage current. We use 100 nm SiO2 and 50 nm Si3N4 as a dual dielectric insulation layer. The resulting 150 nm insulator film does not sustain a resolution-degrading charging

Results

Among the two fabrication methods described above, the double step lithography process gives more sustainable and higher yield results. A typical IV curve measured on a device with zero bias resistance of 200 kΩ, made with the second process is shown in Fig. 5. In these plots it can be seen that the nonlinear and asymmetric current behavior across the zero-bias results in non-zero sensitivity. Here, we define an asymmetry factor by point-wise dividing the forward current by the reverse current.

Conclusion

In this work, we have developed two different lithography techniques to fabricate perfectly planar antenna coupled Ni–NiOx–Ni tunnel diodes in order to decouple the capacitance from the resistance of the junction. In the first technique, a single metal deposition is used. It was difficult to sustain high yield with this approach. In the second method, fabrication is performed in two lithography steps and the parasitic capacitance due to the overlap of the two metals is eliminated by utilizing

Acknowledgements

This work has been supported by US Navy Naval Air Systems (NAVAIR), National Science Foundation (NSF) and Office of Naval Research (ONR).

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