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.
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.
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 today's display processing, leading to a new generation of low cost, high performance versatile display systems.
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.
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.
Lateral polysilicon p+-n-n+ and p+-p- n+ diodes in a series combination of 2 to 5 were fabricated and their electronic properties such as ON- resistance, reverse current and ideality factor were studied. Since the multiple diodes are in series with each other, they have a reduced reverse current. Such a series combination can be used as a single device in applications such as x-ray sensing arrays, which need switching devices with reverse current lower than 1 by 10-12 A and forward current more than 1 by 10-6 A. The ON- resistance and the ideality facto increase linearly with the number of diodes in series. This behavior is attributed to the effect of series combination of diodes.
Organic light emitting diodes are a new flat panel display technology that offers high luminous efficiencies. In this paper, a VGA format polysilicon active matrix organic light emitting diode display will be presented. The display design and pixel structure will be discussed as well as the integration of the polysilicon TFT with the OLED display process. The method used to drive the display will be presented along with the active matrix display performance.
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.
Organic light emitting diodes are a new flat panel display technology that offers high luminescent efficiencies. In this paper, with aspects of this new technology are reviewed and the limitations of the currently used passive matrix addressing are identified. New active matrix addressed organic light emitting diode displays are proposed that are based on the polysilicon TFT technology. Different polysilicon TFT active matrix pixel structures for OLED applications are described and their advantages and disadvantages are discussed. The characteristics of fabricated polysilicon TFT arrays for driving OLED are presented.
It has been reported that single crystal silicon transistors whose gate oxides were grown in N2O or treated to a post oxidation anneal in N2O demonstrated better resiliency under electrical stress. However, normal oxidation in single crystal silicon processing to form gate oxides is incompatible with the low temperature processes required for polysilicon TFTs designed for AMLCDs. In this work, we have used our previously reported process for a double layer gate oxidation, the bottom layer being a grown oxide formed in a low temperature, high density oxygen plasma and the top layer being a deposited oxide, to build the first low temperature polysilicon TFTs with a nitrogen passivated oxide/polysilicon interface. Our devices were constructed with the bottom 10 nm layer of gate oxide being grown in either an N2O or an O2 plasma, followed by a deposition of 100 nm of SiO2 by PECVD. The devices were then subjected to prolonged periods of stress at Vds equals Vgs equals 20 V. While having similar pre-stress device characteristics, the devices made with the oxide grown in the N2O plasma demonstrated a greater degree of stability than those with an oxide grown in an oxide grown in an O2 plasma.
Silicides are proposed for display applications in order to reduce the series and contact resistances of TFTs fabricated on thin polysilicon islands which would otherwise limit the on current of these transistors. The use of two likely candidates, nickel and cobalt silicide was investigates in order to determine their suitability for TFT-LCD applications. Cobalt silicide was formed at the boundary of the thermal budget by requiring annealing at 600 degrees C, but the silicide is stable and does not degrade with further anneals as would occur during an implant anneal after silicidation. Nickel silicide is formed at 400 degrees C but its sheet resistance was observed to degrade when subjected to a 600 degree C anneal after the silicidation. This problem was partially over come by depositing enough nickel to completely consume the polysilicon contact area during silicidation, though the nickel silicide was still observed to grow in volume during prolonged post silicidation anneals. Comparisons between the relative abilities of cobalt and nickel silicide to act as silicon dioxide etchant stops have been made, and while cobalt silicide films are destroyed during both dry etching in CF4 and wet etching in HF, nickel silicide films demonstrated only a slow degradation. Devices with silicides have been made and their results compared to non-silicided counterparts.