Themis Afentakis

Poly-silicon TFT AM-OLED on thin flexible metal substrates

Thin metal foils present an excellent alternative to polymers for the fabrication of large area, flexible displays. Their main advantage spurs from their ability to withstand higher temperatures during processing; microelectronic fabrication at elevated temperatures offers the ability to utilize a variety of crystallization processes for the active layer of devices and thermally grown gate dielectrics. This can lead to high performance (high mobility, low threshold voltage) low cost and highly reliable thin film transistors. In some cases, the conductive substrate can also be used to provide power to the active devices, thus reducing layout complexity. This paper discusses the first successful attempt to design and fabricate a variety of active matrix organic light emitting diode displays on thin, flexible stainless steel foils. Different pixel architectures, such as two- and four-transistor implementations, and addressing modes, such as voltage- or current-driven schemese are examined. This work clearly demonstrates the advantages associated with the fabrication of OLED displays on thin metal foils, which - through roll-to-roll processing - can potentially result in revolutionizing todays display processing, leading to a new generation of low cost, high performance versatile display systems.

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.

High-performance poly-silicon circuits on thin metal foils

In recent years, there has been a growing interest in microelectronic fabrication of thin, flexible substrates. The utilization of flexible materials in processing is motivated by the need to have low weight, high strength microelectronic circuitry compatible with roll-to-roll processing. This can lead to a new era in the fabrication of reliable, low cost and highly versatile circuits for a wide variety of applications. This metal foils offer a number of significant advantages over polymers, their main contender in this field. The most important asset of metals for substrate application is their compatibility with high temperature processing (up to 1000°C), which can lead to high mobility, low drift devices. This paper examines the performance of a variety of circuits fabricated on flexible metal foils, such as stainless steel, using laser crystallized polycrystalline silicon films. The basic performance characteristiscs and architecture of fabricated static and dynamic shift registers and ring oscillators are discussed. N-channel thin film transistors with an average mobility of 200cm2/Vs were measured. Ring oscillator measurements indicated an average propagation delay of 1.38ns per inverter stage at a supply voltage of 15V. Both static and dynamic shift registers exhibit a maximum clock frequency beyond 1MHz. These circuits play a pivotal role for the fabrication of integrated display systems and most other large area electronics. This is the first time circuits of this complexity and performance have been successfully fabricated on flexible metal substrates.

Low Temperature Flat Panel Display Driver Circuits in RTP Crystallized Polysilicon

The crystallization of amorphous silicon by rapid thermal processing has some important advantages over other techniques such as laser annealing. This work investigates the characteristics of low temperature polysilicon flat panel display driver circuits fabricated in RTP crystallized polysilicon. Both digital (shift register) and analog (operational amplifier) circuits are presented.

Active-matrix organic light-emitting displays on flexible metal foils

This paper describes the development of a 3.5 inch diagonal Active Matrix Organic Light Emitting Diode Display on flexible metal foils. The active matrix array had the VGA format and was fabricated using the polysilicon TFT technology. The advantages that the metal foil substrates offer for flexible display applications will first be discussed, followed by a discussion on the multilayer coatings that were investigated in order to achieve a high quality insulating layer on the metal foil substrate prior to TFT fabrication. Then the polysilicon TFT device performance will be presented as a function of the polysilicon crystallization method. Both laser crystallized polysilicon and solid phased crystallized polysilicon films were investigated for the TFT device fabrication. Due to the opaque nature of the metal foil substrates the display had a top emission structure. Both small molecule and polymer based organic material were investigated for the display emissive part. The former were evaporated while the latter were applied by spin-cast. Various transparent multi-layer metal films were investigated as the top cathode. The approach used to package the finished AMOLED display in order to protect the organic layers from environmental degradation will be described. The display had integrated polysilicon TFT scan drivers consisting of shift registers and buffers but external data drivers. The driving approach of the display will be discussed in detail. The performance of the finished display will be discussed as a function of the various materials and fabrication processes that were investigated.

A simple analytical model for the dependence of the propagation delay of the polycrystalline silicon CMOS inverter on temperature

Abstract We present a simple analytical model that accounts for the effects of temperature on the propagation delay of the CMOS inverter, implemented with polycrystalline silicon (polysilicon) thin film transistors. We have also fabricated polysilicon CMOS inverters with a variety of crystallization methods. We measure their switching speed as a function of temperature, and compare the results with the analytical model’s predictions. The experimental data are shown to be in excellent agreement with the developed model, which constitutes an effective tool for modeling the impact of temperature on polysilicon digital circuits.

Reliability of polysilicon thin film transistors on stainless steel foil substrates

We have successfully fabricated polysilicon thin film transistors on a flexible stainless steel foil substrate. Both - and p-channel devices have been subjected to DC and AC voltage stressing, in order to provide for a basic measurement of their performance. We have compared these characteristics with results obtained from devices we have previously fabricated on quartz substrates, and have found no evidence that the stainless steel substrate has affected the reliability of the transistors. We therefore believe that polysilicon devices and circuits on steel present an attractive alternative to transistors fabricated on more expensive substrates, without any reliability compromise.

Modeling and performance of polysilicon thin film transistor circuits on stainless steel foil substrates

We have successfully fabricated a variety of analog and digital thin film transistor circuits on a flexible stainless steel foil substrate, the majority of which serve display driver purposes. We have modeled the operation of a number of different circuits, in order to determine how the substrate affects their performance. We have verified that a major performance-limiting factor in this type of circuits is the parasitic capacitance to the conductive substrate. The results of our analysis can also provide for some basic guidelines to aid future design and development.

Trade off between polysilicon film quality and thin film transistor operational amplifier DC gain

Abstract We have fabricated thin film transistor operational amplifiers on a variety of polycrystalline silicon films. We have examined the open loop DC gain of these modules, and have observed that higher quality polycrystalline silicon films usually cause a negative impact on the DC gain of the amplifier. In this paper we have attempted to quantify this relationship, presenting the gain as a function of the transistor mobility, threshold voltage and channel length modulation parameter, which collectively can describe the quality of the active film. We have found that primarily the saturation characteristics of the transistor, as represented by the channel length modulation parameter, and the device threshold voltage have the biggest impact on amplifier gain.