Abstract: As III-V compound semiconductor technology extends its reach into the millimeter wavelength (MMW) frequency application space, solid-state power amplifier designs rely heavily on the ability to reduce transistor lateral and vertical dimensions while maintaining high yield. In highelectron-mobility transistors (HEMTs), T-shaped gates are often used to simultaneously attain short gate length and large gate cross-sectional area to reduce electron transit delay time and ensure low gate resistance, respectively. To fabricate sub-0.25 micron T-gates that are necessary for MMW transistor operation, e-beam lithography is often used to define either a bi-layer or tri-layer resist stack for metal liftoff. A common problem that occurs as gate stem length is reduced is known as “metal cathedraling,” where metal builds up laterally on the top corners of the bottom resist layer feature and eventually results in a narrow junction between the T-gate stem and head. This undesirable junction can lead to poor electrical contact between the stem and head of the T-gate, and in extreme cases can result in low yield due to detachment/liftoff of the gate head from the stem. In this presentation, we will discuss the development of a simple technique that helps improve the yield of sub-0.25 micron T-gates by reflow rounding of the bottom ZEP resist feature in a bi-layer ZEP/UV5 stack. This fabrication procedure is outlined in figure 1 below. By using this reflow technique, we have been able to successfully demonstrate 40 nm T-gates with reduced metal cathedraling and improved yield. Process parameters such as ZEP reflow time/temperature, e-beam dose, and the effect of O2 plasma descum were evaluated during the course of this study. Reflow temperature was found to have the largest effect on reducing ZEP feature size and profile rounding. An optimized process parameter window was determined where ZEP feature size could be controllably reduced by 20 – 40% (depending on initial feature size) as a result of reflow. The final portion of this talk will discuss experimental results of vertically-scaled AlN/GaN HEMT devices with T-gates fabricated using the reflowed ZEP technique. In devices with T-gate length of 120 nm, we have been able to measure pad de-embedded fT and fmax in excess of 160 GHz with maximum current density of 1.7 A/mm.

Modulating retro-reflectors (MRR) allow free space optical links with no need for pointing, tracking or a laser on one end of the link. They work by coupling a passive optical retro-reflector with an optical modulator. The most common kind of MRR uses a corner cube retro-reflector. These devices must have a modulator whose active area is as large as the area of the corner cube. This limits the ability to close longer range high speed links because the large aperture need to return sufficient light implies a large modulator capacitance.To overcome this limitation we developed the concept of a cat's eye MRR. Cat's eye MRRs place the modulator in the focal plane of a lens system designed to passively retro-reflect light. Because the light focuses onto the modulator, a small, low capacitance, modulator can be used with a large optical aperture. However, the position of the focal spot varies with the angle of incidence so an array of modulators must be placed in the focal plane, In addition, to avoid having to drive all the modulator pixels, an angle of arrival sensor must be used.We discuss several cat's eye MRR systems with near diffraction limited performance and bandwidths of 45 Mbps. We also discuss a link to a cat's eye MRR over a 7 Km range.

Modulating retro-reflectors (MRR) couple passive optical retro-reflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing/acquisition/tracking system required on only one end of the link. In operation a conventional free space optical communications terminal, the interrogator, is used on one end of the link to illuminate the MRR on the other end of the link with a cw beam. The MRR imposes a modulation on the interrogating beam and passively retro-reflects it back to the interrogator. These types of systems are attractive for a asymmetric communication links for which one end of the link cannot afford the weight, power or expense of a conventional free-space optical communication terminal. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large area MQW placed in front of the aperture of a corner-cube. For the MQW MRR, the maximum modulation can range into the gigahertz, limited only by the RC time constant of the device. This limitation, however, is a serious one. The optical aperture of an MRR cannot be too small or the amount of light retro-reflected will be insufficient to close the link. For typical corner-cube MQW MRR devices the modulator has a diameter between 0.5-1 cm and maximum modulation rates less than 10 Mbps. In this paper we describe a new kind of MQW MRR that uses a cat’s eye retro-reflector with the MQW in the focal plane of the cat’s eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 10 Mbps free space link over a range of 1 km is demonstrated. In addition a laboratory of a 70 Mbps MQW focal plane is described.

We demonstrate that a surface-normal coupled-quantum-well InGaAs/InAlAs electroabsorptive modulator can provide optical modulation that is comparable to a square-well modulator at 1.55 /spl mu/m; but at much lower driving voltages.

Modulating retro-reflectors provide means for free space optical communication without the need for a laser, telescope or pointer tracker on one end of the link. These systems work by coupling a retro-reflector with an electro- optic shutter. The modulating retro-reflector is then interrogated by a cw laser beam from a conventional optical communications system and returns a modulated signal beam to the interrogator. Over the last few years the Naval Research Laboratory has developed modulating retro-reflector based on corner cubes and large area Transmissive InGaAs multiple quantum well modulators. These devices can allow optical links at speeds up to about 10 Mbps. We will discuss the critical performance characteristics of such systems including modulating rate, power consumption, optical contrast ratio and operating wavelength. In addition a new modulating retro-reflector architecture based upon cat s eye retroreflectors will be discussed. This architecture has the possibility for data rates of hundreds of megabits per second at power consumptions below 100 mW.

Data transfer on-board a spacecraft is typically done using wiring harnesses (where the wires may be copper for electrical signals or fiber optic for optical signals) using a serial protocol such as 1553 or 1773. This approach has several limitations. The wiring harnesses themselves are undesirable because of their mass and large moment of inertia. Also having an astronaut repair or add new connections once the spacecraft has been launched is quite difficult. The nature of the serial bus is also limiting because only one instrument can talk at a time limiting the aggregate data rate of the network. The object of the Cat's Eye Modulating Retro-reflector program is to develop the components for a new type of spacecraft data network that utilizes free space optical data transfer. This free space optical network will allow the elimination of long wiring harnesses, enable point-to-point data connections, allow new data nodes to be added to a spacecraft after launch with relative ease, be immune to RF interference and allow data networks to easily extend outside of the spacecraft or to parts of distributed spacecrafts. In addition the network will not have the very accurate alignment tolerances between nodes that is generally required by free space optics.