Query: Amorphous Silicon Thin-Film Transistors on Compliant Polyimide Foil Substrates

Candidate papers:

abstract: Much of the mechanical strain in semiconductor devices can be relieved when they are made on compliant substrates. We demonstrate this strain relief with amorphous silicon thin film transistors made on 25 /spl mu/m thick polyimide foil, which can be bent to radii of curvature R down to 0.5 mm without substantial change in electrical characteristics.
author_names: Helena Gleskova

Sigurd Wagner
title: Amorphous silicon thin film transistors on compliant polyimide foil substrates
venue: IEEE Electron Device Letters
year: 1999

abstract: Flexible electronics will have inorganic devices grown at elevated temperatures on free standing foil substrates. The thermal contraction mismatch between the substrate and the deposited device films, and the built in stresses in these films, cause curving and a change in the in plane dimensions of the workpiece. This change causes misalignment between the device layers. The thinner and more compliant the substrate, the larger the curvature and the misalignment. We model this situation with the theory of a bimetallic strip, which suggests that the misalignment can be minimized by tailoring the built in stress introduced during film growth. Amorphous silicon thin film transistors (a Si:H TFTs) fabricated on stainless steel or polyimide (PI) (Kapton E(r) foils need tensile built in stress to compensate for the differential thermal contraction between the silicon films and the substrate. Experiments show that by varying the built in stress in just one device layer, the gate silicon nitride (SiNx) one can reduce the misalignment between the source/drain and the gate levels from ~400 parts per million to ~100 parts per million.
author_names: Helena Gleskova

I-Chun Cheng

Sigurd Wagner

Zhigang Suo
title: Mechanical theory of the film on substrate foil structure: curvature and overlay alignment in amorphous silicon thin film devices fabricated on free standing foil substrates
year: 2009

abstract: We have fabricated high performance amorphous silicon thin film transistors (a Si:H TFTs) on 2 mil. (51 mm) thick polyimide foil substrates. The TFT structure was deposited by r.f. excited plasma enhanced chemical vapor deposition (PECVD) All TFT layers, including the gate silicon nitride, the undoped, and the n+ amorphous silicon were deposited at a substrate temperature of 150 degC. The transistors have inverted staggered back channel etch structure. The TFT off current is approximately 10 12 A, the on off current ratio is >107, the threshold voltage is 3.5 V, the sub threshold slope is approximately 0.5 V/decade, and the linear regime mobility is approximately 0.5 cm2 V 1 s 1. We compare the mechanical behavior of a thin film on a stiff and on a compliant substrate. The thin film stress can be reduced to one half by changing from a stiff to a compliant substrate. A new equation is developed for the radius of curvature of thin films on compliant substrates.
author_names: Helena Gleskova

Sigurd Wagner

Zhigang Suo
title: a Si:H TFTs made on polyimide foil by PE CVD at 150 degC
year: 1998

abstract: Large area electronic circuits on thin foil substrates can be made with techniques adapted from conventional printing. Conventional printing can provide a resolution and an overlay registration of 10 mm and 5 mm, respectively, which will allow making thin film transistors (TFTs) at densities above 10 000 per square centimeter. An early example is the use of laser printed toner etch masks for the fabrication of amorphous silicon TFTs. Patterned devices can be made by direct printing, as demonstrated by the jet printing of the active polymer for organic light emitting diodes (OLEDs) Paper thin foils of glass, steel, and polyimide can serve as substrates for making TFTs with characteristics comparable to those made on glass plates. Materials options for thin foil substrates are described. A study of the mechanics of films on stiff and compliant foil substrates shows that particularly rugged and flexible device structures can be made when the foils are very thin. Integrating OLEDs with thin film transistors on steel foil substrates provides an early example of 3 D integrated components for macroelectronics.
author_names: Sigurd Wagner

Helena Gleskova

James C Sturm

Zhigang Suo
title: Novel processing technology for microelectronics
year: 2000

abstract: We report the successful fabrication of high quality a Si:H thin film transistors (TFTs) on stainless steel foil substrates. TFTs with an inverted staggered structure were grown on 200 /spl mu/m thick stainless steel foil. These TFTs show typical ON/OFF current ratios of 10/sup 7/ OFF currents on the order of 10/sup 12/ A, good linear and saturation current behavior, subthreshold slopes of 0.5 V/decade, and linear channel mobilities of 0.5 cm/sup 2//V. In addition, we have demonstrated that these TFTs are capable of withstanding significant mechanical shocks, as well as macroscopic deformation of the substrate, while remaining functional. This work demonstrates that transistor circuits can be made on a flexible, nonbreakable substrate. Such circuits would be highly useful in reflective or emissive displays, and in other applications that require rugged macroelectronic circuits.
author_names: Steven D Theiss

Sigurd Wagner
title: Amorphous silicon thin film transistors on steel foil substrates
venue: IEEE Electron Device Letters
year: 1996

abstract: We have fabricated TFTs of nanocrystalline silicon (nc Si) at 150degC on clear polymer substrates (coefficients of thermal expansion, a~45 to 55ppm/K) on Kapton(r) 200E (a=17ppm/K) and on Corning 1737 glass (a=3ppm/K) for comparison. Because thermally stable polymers, such as Kapton(r) 200E polyimide, have glass transition temperatures as high as 325degC, they are candidates for direct substitution of display glass. The stresses developed in the substrate and device layers, due to a, are reduced by decreasing the thickness of the active layers, by cutting the layers into islands separated by exposed substrate, and by designing stresses, via plasma conditions, into the SiNx passivating layers. By using these three techniques we have made nc Si TFTs on high Tg, and high a, clear polymer foils with electron mobilities of up to 18cm 2 /Vs. When integrated with bottom emitting organic light emitting diodes, such devices will allow for a 10x reduction in pixel TFT areas, compared to TFTs of amorphous silicon.
author_names: Alex Z Kattamis

I-Chun Cheng

Ke Long

James C Sturm

Sigurd Wagner
title: Nanocrystalline Silicon Thin Film Transistors on Optically Clear Polymer Foil Substrates
year: 2005

abstract: We have developed a 150degC technology for amorphous silicon thin film transistors (a Si:H TFTs) on polyimide substrates deposited by plasma enhanced chemical vapor deposition. The silicon nitride gate dielectric and the a Si:H channel material were tailored to provide the least leakage current and midgap defect density, respectively. In addition, we conducted experiments on the TFT structure and fabrication with the aim of obtaining high electron mobility. TFTs with back channel etch and channel passivated structures were fabricated on glass or 51 mm thick polyimide foil. The a Si:H TFTs have an on/off current ratio of ~10 7 and an electron mobility of ~0.7 cm 2/V s.
author_names: Helena Gleskova

Sigurd Wagner

Vladimir Gasparik

P Kovac
title: 150degC Amorphous Silicon Thin Film Transistor Technology for Polyimide Substrates
year: 2001

abstract: Thin film transistors (TFTs) were fabricated on polyimide and glass substrates at low temperatures using microwave ECR CVD deposited amorphous and nanocrystalline silicon as active layers. The amorphous Si TFT fabricated at 200 /spl deg/C on the polyimide foil had a saturation region field effect mobility of 4.5 cm/sup 2//V s, a linear region mobility of 5.1 cm/sup 2//V s, a threshold voltage of 3.7 V, a subthreshold swing of 0.69 V/decade, and an ON/OFF current ratio of 7.9 /spl times/ 10/sup 6/ This large mobility and high ON/OFF current ratio were attributed to the high quality channel materials with less dangling bond defect states. Nanocrystalline Si TFTs fabricated on glass substrates at 400 /spl deg/C showed a saturation region mobility of 14.1 cm/sup 2//V s, a linear region mobility of 15.3 cm/sup 2//V s, a threshold voltage of 3.6 V, and an ON/OFF current ratio of 6.7 /spl times/ 10/sup 6/ TFT performance was mostly independent of substrate type when fabrication conditions were the same.
author_names: Wayne A Anderson
title: Thin film transistors on plastic and glass substrates using silicon deposited by microwave plasma ECR CVD
venue: IEEE Electron Device Letters
year: 2003

abstract: We applied strain ranging from 1% compressive to ~0.3% tensile to a Si:H TFTs on polyimide foils by bending them inward or outward, or by stretching them in a microstrain tester. We also applied strain to a Si:H TFTs by deforming a flat substrate into a spherical dome. In each case, compression lowered and tension raised the on current and hence the electron field effect mobility. We conclude that compressive strain broadens both the valence and conduction band tails of the a Si:H channel material, and thus reduces the effective electron mobility. We show that the mobility can be used as an indicator of local mechanical strain.
author_names: Helena Gleskova

P I Hsu

Z Xi

James C Sturm

Zhigang Suo

Sigurd Wagner
title: Field effect mobility of amorphous silicon thin film transistors under strain
year: 2004

abstract: Flexible electronics and displays have attracted attention due to merits such as light weight, thin profiles, portability, and the ability to form conformable shapes, all of which allow novel applications. Lightweight, bendable thin film transistors (TFTs) are a must for the circuitry that drives such displays. Emerging TFT technology based on oxide semiconductors is promising for this application due to advantages such as high mobility in the amorphous phase, low processing temperatures, and relative compatibility with existing fabrication methods.1 4 Most reports on flexible oxide TFTs involve the use of steel foil substrates or of fabrication techniques such as lift off and shadow masking. These processes are relatively primitive and cannot be easily applied for mass production. Steel foils, meanwhile, suffer limited flexibility and issues of parasitic capacitance. Recently, there have been some reports of oxide TFTs on polyimide substrates, which are compatible with higher processing temperatures compared with other common plastic substrates. Although polyimides can, in general, withstand higher processing temperatures, to date most of these materials are tinted or not transparent, limiting applications in displays and other flexible electronics. We have used fully colorless and transparent polyimide based nanocomposite substrates to make high performance, highly flexible amorphous indium gallium zinc oxide (a IGZO) TFTs: see Figure 1.5 The lithographic and etching processes we use are compatible with existing TFT fabrication technologies. The first step is to coat smooth polyimide films tens of microns thick on glass substrates to which they adhere strongly. To Figure 1. (a) Flexible amorphous indium gallium zinc oxide thinfilm transistors (a IGZO TFTs) using a titanium/indium zinc oxide (Ti/ZnO) stacked electrode on a freestanding nanocomposite substrate. (b) Structure of the a IGZO TFT. SiNx: Silicon nitride.
author_names: Chih Wei Chien

Cheng-han Wu
title: High performance, highly rollable oxide thin film transistors
year: 2012