We present, for the first time, the fabrication

process for a submicron planar Gunn diode in In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As on an InP substrate operating at 265 GHz. A novel two stage lift off method has been developed to achieve a submicron gaps between

contacts down to 135 nm with widths up to 120 μm

Cumming, D.R.S.

Khalid, A.

Macintyre, D.

Thayne, I.G.

Thoms, S.

English

Ata Khalid

S. Thoms

D. Macintyre

I. G. Thayne

D. R. S. Cumming

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Fabrication of submicron planar Gunn diode

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    Khalid, A., Thoms, S., Macintyre, D., Thayne, I.G., and Cumming, D.R.S. (2014) Fabrication of submicron planar Gunn diode. In: 26th International Conference on Indium Phosphide and Related Materials, 11-15 May 2014, Montpelier, France.   Copyright © 2014 The Authors   A copy can be downloaded for personal non-commercial research or study, without prior permission or charge  Content must not be changed in any way or reproduced in any format or medium without the formal permission of the copyright holder(s)   When referring to this work, full bibliographic details must be given     http://eprints.gla.ac.uk/94667    Deposited on:  10 September 2014                 Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Fabrication of Submicron Planar Gunn Diode Ata. Khalid1*, S. Thoms1, D. Macintyre1, I. G. Thayne1 and D.R.S. Cumming1 1: School of Engineering, University of Glasgow, Glasgow, United Kingdom , G12 8LT e-mail: ata.khalid@glasgow.ac.uk  Short-Abstract—We present, for the first time, the fabrication process for a submicron planar Gunn diode in In0.53Ga0.47As on an InP substrate operating at 265 GHz. A novel two stage lift off method has been developed to achieve a submicron gaps between contacts down to 135 nm with widths up to 120 µm. I. INTRODUCTION  Semiconductor device fabrication technology is capable of very small feature sizes but some III-V devices such as Gunn diodes require dimensions longer than a micron with submicron sized gaps.  Writing narrow gaps between long and wide contacts is difficult by electron beam lithography despite the small beam diameter due to electron beam proximity effects. The effective dose within the gap is 50% of the clearing dose, and with a conventional polymethylmethacrylate (PMMA) lift off process this leads to rapid resist erosion and a process window of around 1% in dose. Unfortunately, it is relatively difficult to etch high resolution gold patterns, and because of this, small gaps between metal contacts are difficult to fabricate using the conventional subtractive processing flow of metal deposition followed by resist patterning and metal etch. Thoms et al [1] previously showed that 100 nm gaps between 100 µm square metal contacts can be realized using a tri-layer of polydimethylglutarimide (PMGI) silicon nitride and UVIII. The UVIII resist layer is patterned by electron beam lithography. Reactive ion etching (RIE) is used to etch through the silicon nitride layer and a low bias oxygen RIE process is used to etch the sacrificial PMGI layer. The silicon nitride layer is not etched by the plasma and so enables the PMGI layer to be etched back by a controlled amount to give profiles optimal for metal lift off. In this paper, we demonstrate the application of the sacrificial PMGI layer technique used to produce undercut profiles for metal lift off to the fabrication of submicron planar Gunn diodes using an epitaxial layer of In0.53Ga0.47As on an InP substrate. We show that 700 nm long devices oscillate at 264GHz with a measured rf power of -10dBm. These planar Gunn devices are excellent candidates for many applications requiring a solid-state THz source for chip integration.. II. EXPERIMENTAL A. Device Fabrication InGaAs layers were grown using molecular beam epitaxy (MBE) on a 600 µm thick semi-insulating InP substrate, as shown in Figure 1. The active channel layer was 300 nm thick with a doping level of 8×1016 cm-3, followed by a 200 nm thick cap layer of In0.53Ga0.47As with a doping level of 2×1018 cm-3. The cap layer helps to achieve lower Ohmic contact resistance to the devices. The channel layers are designed to keep the nLac product of the devices above 1012 cm-2, where n is the free carrier density and Lac is the separation distance between the anode and cathode [2].  Substrates were initially spin coated with a 300 nm thick layer of PMGI SF6 (supplied by Microchem) and baked at 180˚C for 2 hrs. A 50 nm layer of silicon nitride was then applied using a room temperature PECVD process.  A 300 nm thick layer of UVIII resist (supplied by Chestech LTD) was applied to the substrates from a mixture of 60:40 UVIII to ethyl lactate by weight.  Substrates were spin coated at 3000 rpm for 60s and baked on a vacuum hot plate at 145˚C for 60s . A Vistec VB6 UHR EWF large area electron beam lithography tool was then used to pattern anode and cathode pads with dimension 120 µm x 80 µm and a separation of 700 nm. Pattern proximity correction and fracturing was carried out using Layout Beamer software from GenIsys GMBH. After ebeam exposure substrates were given a post exposure bake at 145˚C for 60s on a vacuum hot plate. The UVIII layer was developed for 60s in CD26 at room temperature and washed in DI water. The parting layer of silicon nitride was then etched for 90s in CHF3/O2 50/5 sccm, @150 W, 55mT, using an Oxford Instruments RIE80+ tool. To create an undercut profile suitable for metal lift off the same RIE tool was used with a low bias oxygen etch process  (20W at 10mT and an oxygen flow rate of 10sccm for 15 mins) to etch back the sacrificial PMGI layer by a controlled amount. To ensure the PMGI layer was completely etched down to the cap layer an interferometer was used to monitor the etch process throughout.  A Plassys 450MB ebeam evaporator was used to deposit the following metal films to the patterned substrates Au (10 nm), Ge (14 nm), Au (10 nm), Ni (14 nm)  and Au (80 nm).  A two step lift off process was found to give reliable metal lift off.  In the first step substrates were soaked in acetone which removed the UVIII layer and the excess metal film. A second soak in SVC-14 was used to remove the PMGI resist and wash away the nitride layer. This step was carried out using ultrasound agitation for 5 seconds to ensure substrates were perfectly clean after the lift-off process.  The quality of lifted off metal patterns was good. The fabrication process described was found to be very successful at defining wide anode-cathode structures up to 120 µm with Lac as short as 130 nm. Figure 2. a shows a typical submicron structure obtained and this has an anode cathode separation of 130 nm. These results are an improvement on those previously reported for the fabrication of 1.3 µm long planar Gunn diodes [3]. The contacts were not annealed and contact resistances of the order of 0.12 Ω mm were achieved. A mesa was etched using 1:1:10::H2SO4:H2O2:H2O at an etch rate of 60 nm/s. A 50 Ω coplanar waveguide (CPW) feed structure was then  deposited using 800 nm of gold to form the probe pads for subsequent on-wafer RF and dc measurements. The CPW had a ground-to-signal separation of 15 µm and a central signal track width of 50 µm. Finally, the unwanted In0.53Ga0.47As contact layer between the anode and cathode was carefully etched away using 1:1:8 H3PO4:H2O2:H2O solution. B. DC & RF Measurements DC characteristics of the devices were measured with a semiconductor device analyzer (Agilent Technologies B1500A) on a semi-automated probe station (Cascade Microtech Summit 12K). Figure 3 shows measured current-voltage characteristics of fabricated 700nm long and 120µm wide planar Gunn diode.  Conventional vertical Gunn oscillators are frequency band limited because they are constructed in rectangular waveguide cavities; planar Gunn diodes, on the other hand, are integrated with CPWs that have no cut-off frequency limitation. This configuration allows device oscillation frequency to be measured for the entire band using on wafer probing. Figure 4. shows measured spectrum of a submicron planar Gunn diode oscillating above 265GHz. III. DISCUSSION The experimental results presented in this work explore the function of Gunn operation at submicron lengths. Simulations using Monte Carlo methods have already shown that Gunn diodes may function with submicron transit regions [4].  Since the transit region length is intrinsically related to the frequency at which the devices operate, the new submicron devices discussed in this paper allow increases in the frequency of operation. As the experimental planar Gunn device have a quite highly doped transit region, thermal generation within such a device may be significant, and one area worth further study would be the transport of heat within such a device. IV. CONCLUSIONS In conclusion, the first submicron (700 nm × 120 µm) In0.53Ga0.47As/InP planar Gunn diode has been fabricated and found to oscillate above 265 GHz. The highest observed power was above -10dBm. The fabrication technique lends itself to device arrays to make very high power sources. The proposed In0.53Ga0.47As/InP planar Gunn diode with a higher level of output power shows great potential as a solid-state source of terahertz radiation. Figure 1.  A schematic diagram of the layer structure of the In0.53Ga0.47As device.  Figure 2.  A SEM image taken using FEI scanning electron microscope for a typical submicron planar Gunn diode. The measured gap is ~140nm.  Figure 3.  Measured current-voltage characteristics of a completed 700nm x 120µm planar Gunn diode  using 500µs pulse voltage.  Figure 4.  Measured on-wafer spectrum & power measurement of 700nm planar Gunn diodes using a mixer. The insertion is de-embedded as specified by the manufacturer. ACKNOWLEDGMENT This work was supported by UK EPSRC and e2v Technologies (UK) Ltd.  REFERENCES [1] S.Thoms and D.S.Macintyre, “Long nanoscale gaps on III-V substrates by electron beam lithography”J.Vac. Sci. Technol, B 30 06F305 (2012)  [2] W. Kowalsky and A. Schlachetzki, “InGaAs Gunn oscillators,” Electron. Lett., vol. 20, no. 12, pp. 502-503, Jun. 1984 [3] A. Khalid, C. Li, V. Papageogiou, G. M. Dunn, M. J. Steer, I. G. Thayne, M.  Kuball, C. H. Oxley, M. M. Bajo, A. Stephen, J. Glover, and D. R. S. Cumming, “In0.53Ga0.47As planar Gunn diodes operating at a fundamental frequency of 164 GHz” IEEE Electron Device Lett., vol. 34, no. 1,  pp. 39-41, Jan. 2013. [4] S. Perez, T. González, D. Pardo, and J. Mateos, “Terahertz Gunn-like oscillations in InGaAs/InAlAs planar diode,” J. Appl. Phys., vol.103, no.9, pp.4516-4518, May 2008.   

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Fabrication of submicron planar Gunn diode

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