Robert S. Howell

Polysilicon TFT technology for active matrix OLED displays

The integration of active matrix polysilicon TFT technology with organic light emitting diode (OLED) displays has been investigated with the goal of producing displays of uniform brightness. This work identifies and addresses several process integration issues unique to this type of display which are important in achieving bright and uniform displays. Rapid thermal processing has been incorporated to achieve uniform polysilicon microstructure, along with silicides to reduce parasitic source and drain series resistance. Using these processes, TFT drain current nonuniformity has been reduced below 5% for 90% of the devices. This work also introduces transition metals to produce low resistance contacts to ITO and to eliminate hillock formation in the aluminum metallization. These processes, along with spin on glasses for planarization, have been used to produce functional active matrix arrays for OLED displays. The final array pixel performance is also presented.

Polysilicon VGA active matrix OLED displays-technology and performance

The first VGA polysilicon active matrix organic light emitting diode (AM-OLED) display will be presented. The goal of this work is to investigate the technology and performance of polysilicon active matrix OLED displays along with a new driving method.

Poly-Si thin-film transistors on steel substrates

We report the successful fabrication of poly-Si thin-film transistors (TFTs) on stainless steel substrates. The TFTs were fabricated on a 500 /spl mu/m thick polished stainless steel substrate using furnace crystallized amorphous Si deposited by PECVD. These devices typically have threshold voltages of 8.6 V, linear effective mobilities of 6.2 cm/sup 2//V/spl middot/s and subthreshold slopes of 0.93 decade/V. This work demonstrates the feasibility of poly-Si TFTs on stainless steel substrates and identifies some critical issues involved in poly Si processing on stainless steel. This will enable the fabrication of arrays with integrated drivers on a cheap, flexible and durable substrate for various displays and other large area array microelectronic applications.

Silicidation reactions with Co–Ni bilayers for low thermal budget microelectronic applications

Interfacial reactions between ultra-thin Ni/Co bilayer films and single crystal Si (100) substrate were studied at 600°C using X-ray photoelectron spectroscopy (XPS). In very thin bilayer films (<20 nm), only CoSi 2 and NiSi 2 were detected in the reaction layer. When the total thickness of the film was increased to 30 nm along with an increase of thickness of the Co layer with respect to the Ni layer, NiSi formed along with CoSi 2 and NiSi 2 . The resulting silicide film also had a very low sheet resistance. The concept of effective heat of formation has been used to explain the experimentally observed reaction layer structure.

LP-1: Late-News Poster: Polysilicon TFT Display Driver Circuits on Stainless Steel Foil Substrates

We have demonstrated the performance of circuits on stainless steel and compared them to those fabricated on quartz substrates. We have modeled the substrate dependence of the minimum and maximum operating frequency of a Shift Register, and found close agreement with our experimental measurements. This model can be used as a design guideline for future development of circuits on conducting substrates.

Development of cap-free sputtered GeTe films for inline phase change switch based RF circuits

Germanium telluride (GeTe) films have been recently demonstrated as the active element in low-loss RF switches where a 7.3 THz cut-off frequency (Fco) was achieved. In order to simultaneously realize the low ON-state transmission loss and large OFF-state isolation required for this application, significant optimization of the GeTe films was required. In particular, minimizing contact resistance (Rc) and sheet resistivity (Rsheet) without the use of a capping layer is a necessity. Varying the GeTe deposition conditions led to a wide range of structural, chemical, and electrical properties, which ultimately enabled the demonstration of a capless GeTe inline phase change switch (IPCS) structure. Conversely, improper deposition conditions led to extensive oxidation which would push Rc and Rsheet to unacceptable levels. In addition to its relevance for IPCS devices, this work has implications for the environmental stability of GeTe as a function of its physical morphology.

Elimination of hillock formation in Al interconnects using Ni or Co

The effect of using bilayer Ni/Al-1%Cu or Co/Al-1%Cu metallization stacks with regard to hillock formation is investigated in this study and compared to hillock suppression capabilities of Al-1%Si and Al-1%Cu single layer films and Ti/Al-1%Cu bilayer metallization stacks. Various heat treatments during and immediately after depositions were used to study the resistances of different metallization stack specimens to hillock formation. The densities and sizes of hillocks were characterized using scanning electron microscopy and atomic force microscopy, while interfacial reactions between the transition metal layer and Al-1%Cu in the metallization stack were examined using glancing angle x-ray diffraction. Resistivities of the bilayer metallization stacks were characterized using conventional four-point probe measurements. Our studies indicate that metallization processes involving bilayer metallization stacks of nickel and cobalt with Al-1%Cu are more robust compared to those involving titanium with Al-1%Cu. The mechanism of hillock formation is analyzed in light of intermetallic compound formation and stress-strain considerations.

Low-loss non-volatile phase-change RF switching technology for system reconfigurability and reliability

A novel phase-change microelectronics technology is described to enable wideband reconfigurable RF systems and components for EW, RADAR and communications applications. This technology can lower the development time and cost of DoD systems for new missions by enabling factory or mission re-programmability. It can also support component redundancy in system architectures with little impact to system performance.

Reaction Mechanisms in Aluminum‐Indium Tin Oxide Ohmic Contact Metallization with Co and Ni Barrier Layers for Active‐Matrix‐Display Applications

Reaction mechanisms within aluminum‐indium tin oxide (Al‐ITO) metallization schemes were investigated in this work using X‐ray photoelectron spectroscopy and X‐ray diffraction. Examination of Al‐ITO, aluminum‐nickel‐indium tin oxide (Al‐Ni‐ITO), and aluminum‐cobalt‐indium tin oxide (Al‐Co‐ITO) interfaces reveals that considerable atomic rearrangement at the ITO interface with Al or with barrier layer transition (CO or Ni) metals occurs due to reaction. Metal‐oxygen bonds in crystalline ITO are broken and the interface is rearranged. In Al‐Co‐ITO stacks, reduced intermetallic species were detected at the Co‐ITO interface. In Al‐Ni‐‐ITO stacks, such species were not detected at the Ni‐ITO interface, although formed at the Al‐Ni interface. These reactions within metallization stacks are important because they determine the electrical quality of ITO contacts with Al alloy data lines in polysilicon thin film transistor displays.

Advanced polysilicon TFT technology for active matrix organic light-emitting diode displays

This work discusses the features of a low temperature polysilicon thin film transistor (TFT) technology suitable for application in the new Active Matrix Organic Light Emitting Diode (AMOLED) displays. The most important facet of this work is the preparation of polysilicon films by the method of solid phase crystallization of amorphous silicon films using rapid thermal processing (RTP). It is shown that amorphous silicon films can be crystallized by RTP at temperatures compatible with glass substrates yielding polysilicon TFT performance suitable for AMOLED. The use of transition metals for achieving aluminum lines with no hillock and low contact resistance to indium tin oxide, two important features for AMOLED displays is discussed.