Crystallization of amorphous silicon thin films by various methods has fostered enhancements in the electrical characteristics over their amorphous counterparts. For example, carrier mobilities ranging from 10 to >100 cm2/V·sec have been reported for laser crystallized films. The rather large variability of the transport characteristics with crystallization processing conditions is not well understood and, as a result, greatly complicates device process debugging. In addition, while it is generally believed that defects inherent in the grain boundaries provide the primary barriers degrading transport properties relative to single crystal silicon, the specific nature of these defects is not known. In this paper, we present data on the temperature dependence of the Hall mobility of thin silicon films crystallized by thermal and excimer laser processing. Hall data for the laser-crystallized phosphorus-doped material show a temperature dependence which differs dramatically from that for thermally crystallized materials, while the effects of hydrogenation are similar, reducing the barriers at the grain boundaries.REFERENCES

Nuclear magnetic resonance (NMR) has provided essential information on the local atomic bonding and microstructure of hydrogen in hydrogenated amorphous silicon (a-Si:H). Here we describe results from NMR and Raman spectroscopy on the hydrogen distribution and bonding in a-Si:H prepared by remote hydrogen plasma (RHP) deposition and contrast the results with those from a-Si:H prepared by conventional glow discharge (GD) deposition. The films prepared by the two techniques have similar H bonding except for the presence in the RHP sample of about 1 atomic % molecular hydrogen, a factor of ten higher than in GD material. For RHP samples prepared from a deuterium plasma rather than a hydrogen plasma, substantial differences in the hydrogen NMR spectra, hydrogen spin lattice relaxation time and Raman spectra are observed. The hydrogen which necessarily originates from the silane has a dramatically altered spectra.

This paper describes new results on the relationship between the grain size, mobility, and Si (111) x-ray peak intensity of laser crystallized amorphous silicon as a function of the laser fluence, shot density, substrate temperature, and film thickness. These observations include an unexpected narrow peak found in the silicon (111) x-ray peak intensity, which occurs at a specific laser fluence for a given film thickness and substrate temperature. Amorphous silicon materials processed at laser energy densities defined by this peak exhibit exceptionally large grain sizes and electron mobilities that cannot be obtained at any other energy and shot density combination above or below the energy at which the Si (111) x-ray peak intensity maximum occurs.ACKNOWLEDGMENT

Additive manufacturing and 3D printing are poised to reshape entire manufacturing value chains. To be truly disruptive, additive manufacturing has to move beyond shapes and colors. Novel printing technologies are beginning to emerge that enable conformal electronics and even printing with inks containing microchips. This in turn also creates new openings for the progress of electronics itself. Over the last 50 years silicon microelectronics advanced through shrinking device dimensions and packing more and more functionality into tiny spaces. Printing technologies open up exciting new ways of scaling electronics “Beyond Moore”, through the integration of micro and macro, creating new form factors, complex shapes, conformal devices and distributed systems. Printed, hybrid electronics systems will enable new classes of sensor systems, structural electronics and wearable devices, where the “system is the package”.

The effects of fast-pulse excimer laser annealing of a-Si:H were investigated by measurements of electronic transport properties and impurity concentration depth profiles as a function of incident laser energy density. The dc dark conductivity of laser-annealed, highly-doped a-Si:H increases by a factor of ∼350 above a sharp laser energy density threshold whose magnitude increases with decreasing impurity concentration and which correlates with the onset of hydrogen evolution from and crystallization of the near-surface layer. The similarities between the preparation and properties of laser-crystallized a-Si:H and pc-Si:H are discussed.

The need for low cost, flexible, thin film transistor (TFT) display backplanes has focused attention on new processing techniques and materials. We report the development of TFT backplane technology based entirely on jet-printing, using a combination of additive and subtractive processing, to print active materials or etch masks. The technique eliminates the use of photolithography and has the potential to reduce the array manufacturing cost. The printing technique is demonstrated with both amorphous silicon and polymer semiconductor TFT arrays, and we show results of small prototype displays.

Jet printing is an interesting patterning technique for electronic devices because it requires no physical mask, has digital control of ejection, and provides good layer-to-layer registration. It also has the potential to reduce display manufacturing costs and enable roll-to-roll processing. The technique is illustrated with examples of prototype printed displays using amorphous silicon and polymer semiconductors.

For several years there have been many efforts to employ ink jet technologies in the fabrication of consumer electronics. The potential of displacing large and expensive pieces of electronic fabrication equipment and processes with seemingly appropriately scaled inexpensive alternatives is attractive. However, of course, the devil is in the details. Feature size, accuracy, registration and materials all have severe impacts on design rules, processing, performance and the types of devices appropriate to the technology. In this article, we describe aspects of the jet-printing technology for large-are electronic device processing that have been developed at PARC. These aspects include fine feature patterning, multi-layer registration for thin-film transistor device fabrication, printing of solution processable semiconductor and conductive materials, and printer color filters. The focus of this work is to demonstrate the wide range of applications for jet printing in the area of device processing.. Examples of working proto types of displays, imagers and microfluidic devices produced through ink jet printing are given and we discuss the tools used to design these devices.http://www.ingentaconnect.com/search/article?option1=tka&value1=Toolset+for+printed+electronics&pageSize=10&index=1

The development of electronic paper has been a long-term goal and electrophoretic displays are a promising candidate for this application. Apart from developing the electrophoretic display medium there are important issues to solve regarding the overall system. The importance of the electronic addressing method increases with the demand for arbitrary images or text. We have developed and tested active-matrix backplanes based on amorphous silicon technology, as well as the driver electronics. The electrophoretic ink is combined with the backplane employing polymer MEMS cell structures. This system allows us to display high-resolution images and it is a good test bed for investigating various parameters of the electrophoretic display medium and of the electronics.

A novel jet-printing process to fabricate amorphous silicon thin-film transistor arrays is described, in which device features are defined by printed etch masks in place of conventional photo-lithography. The wax-based etch masks with a minimum feature size of ∼40 μm were printed using a multi-ejector piezoelectric print-head. Bottom-gate TFTs in a 64 × 64 pixel array of 338 μm pixel dimension, were created using a four-mask process. TFTs have current-voltage characteristics comparable to conventional devices, with mobility 0.7 cm2/Vs and on/off ratio exceeding 108. The arrays were configured for X-ray imaging by adding an a-Si:H p-i-n photodiode layer. Similar arrays can be configured for active matrix displays. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase-change wax-based printed masks, in place of conventional photoresist masks, were used to fabricate a-Si:H thin-film transistors (TFTs). Printed wax-mask features with a minimum feature size of ∼20 μm were achieved using an acoustic-ink-printing process. Both discrete and matrix addressing structured bottom-gate TFTs with sourcedrain contacts overlapping the channel were created using a four-mask process. The TFTs had I-V characteristics comparable to photolithographically patterned devices, with mobility of 0.6-1 cm2/V·s, threshold voltage of 2-3 V, and on/off ratios exceeding 107 for devices with channel lengths below 50μm. The wax-mask process was also used to fabricate self-aligned TFT devices, eliminating the sourcedrain contact overlap constraint.

Conventional MEMS devices are based on silicon micro-machining and their maximum size is limited by the wafer. In contrast, we are exploring micro-machining for large area applications on substrates such as glass using polymeric materials. Our research is focused on the photopolymer SU-8, and we apply the MEMS fabrication technology to large area image sensors and displays. There are many challenges concerning the materials and processes, since large area compatibility is essential and integration with large area electronics may be required. The adhesion of SU-8 to the underlying layers as well as stress in the SU-8 are important issues and surface treatments have been investigated. Two applications of SU-8 MEMS are discussed to illustrate large area applications:. First, in the fabrication of an X-ray imager, high aspect ratio SU-8 walls form a micro-patterned phosphor screen to increase the image resolution. Second, a similar approach of patterning SU-8 into arrays of micro-cells is applied to an electrophoretic display.

We report studies of the image-blur effects caused by lateral cross-talk between neighboring pixels of large-area amorphous silicon (a-Si:H) image sensors. The lateral conduction is attributed to three effects: conduction along the interface between the a-Si:H film and the underlying passivation; field-dependent electron injection at the edge of the sensor; and a field enhancement of the interface conduction due to the bias applied to the address lines. We show that the cross-talk can be controlled by choice of the operating conditions and optimization of the materials.

The pixel cross-talk is investigated in two-dimensional amorphous silicon (a-Si:H) imager arrays based on the new high fill factor design. In this configuration a continuous a-Si:H sensor extends over the whole surface of the imager, and a buried insulator material with low dielectric constant is used to separate the sensor from the underlying active matrix readout circuit. We find that the lateral conduction between neighboring pixels is mainly determined by the quality of the buried insulator-sensor interface, rather than the specific buried material itself. Minimum cross-talk values below 1% are obtained for different insulator materials including silicon oxynitride and thicker polymer based resins. The quality of this interface also affects trapping and recombination of the photogenerated carriers, influencing important imager properties such as sensitivity and image lag.

We describe new amorphous silicon (a-Si:H) image sensor arrays which are the highest resolution imagers so far reported. The pixel sizes of 64 micrometer (resolution 8 lp/mm) and 75 micrometer (6.7 lp/mm) are made possible using a photodiode technology that enables high sensor fill factor even in very small pixels. This approach allows the a-Si:H imagers to satisfy high resolution requirements of digital mammography. Each array contains 512 X 512 pixels with matrix addressing provided by a-Si:H thin film transistors (TFT). The high fill factor structure contains a continuous a-Si:H photodiode layer grown on top of the TFT array, with contacts to each pixel through a patterned metal/n+ layer. X-ray detection is accomplished by use of a phosphor layer superimposed on the array. The continuous photodiode layer maximizes light absorption from the phosphor and provides high x-ray conversion efficiency. Since the photodiode forms a continuous layer, crosstalk between adjacent pixels due to the lack of isolation is a particular concern, and has been extensively studied. We find that the high fill factor structure can be made such that the lateral charge leakage is minimal in the dark or under moderate illumination, although small amount of charge spreading is observed under conditions of sensor saturation. The measured MTF for optical illumination exceeds 60% at the Nyquist frequency, even for long integration times.

Amorphous silicon large area sensor arrays are in production for x-ray medical imaging. The most common pixel design works very well for many applications but is limited in spatial resolution because the available sensor area (the fill factor) vanishes in small pixels. One solution is a 3-dimensional structure in which the sensor is placed above the active matrix addressing. However, such high fill factor designs have previously introduce cross talk between pixels. We present data for a design in which the a-Si:H p-i-n photodiode sensor layer has a continuous i-layer and top p + -layer, and a patterned n + -layer contact to the pixel. Arrays of 64 µm and 75µm pitch have been fabricated and are the highest resolution a-Si:H arrays reported to date. The resolution matches the pixel size, and sensitivity has been improved by the high fill factor. Comparison is made between arrays with standard TFTs and TFTs with self-aligned source and drain contacts. Data line capacitance is improved by use of the self-aligned contacts. Measurements are included on the contact to bias capacitance. The high fill factor design greatly suppresses lateral leakage currents, while retaining ease of processing. Provided illumination levels remain below saturation, the resolution matches expectation for the pixel size.

Deuteron magnetic resonance (DMR) has been used to further examine hydrogen (deuteron) populations in amorphous-silicon (a-Si) and in n -type crystalline silicon (x-Si). In both a-Si and x-Si DMR shows central components arising at least in part from isolated molecular deuterium and sharp doublet features from Si-bonded hydrogen (D). Our new results include the observation in x-Si of molecule-specific DMR multiple echoes from trapped ortho-D 2 . A second new result is the observation in x-Si of a substantialn Si-bonded D population with splittings between 46 and 80 kHz and perhaps arising from (deuterated) hydrogen bond-centered and antibonding configurations.

This paper will review electronic device characteristics of recently developed large-area, amorphous Silicon (a-Si) TFT/photodiode X-ray image sensors, and discuss some of the imaging characteristics which such devices can achieve.

Recent advances in the understanding of hydrogen in single-crystal silicon have introduced new perspectives for interpreting the role of hydrogen in a-Si:H. Properties of H that have been demonstrated in c-Si and which should be relevant in a-Si:H include the existence of dopant-H complexes, charge states of isolated migrating H, metastable diatomic hydrogen (H-2*), and H-induced platelets.

Fast-pulse laser crystallization of amorphous silicon thin films on non-crystalline substrates provides a low-temperature process for generating polycrystalline silicon. This process can be augmented by including laser doping to reduce the number of process steps in the fabrication of thin-film polysilicon devices. We have studied the simultaneous laser crystallization and laser doping process, starting with amorphous silicon on fused silica substrates and using the gas immersion technique for the doping. n-type and p-type doping employed PF5 and BF3 gases, respectively. Films were characterized both structurally and electrically. The grain size increases with increasing laser energy density as the film becomes fully melted and reaches a peak value, similar to laser crystallization without doping. The dopant concentration increases with the number of laser shots and, with 100 shots, achieves a high dose with a low sheet resistance below 1000 ohms/square, appropriate for devices. The dopant profile extends to a depth comparable to the melt depth, beyond which it falls off to the background level. Therefore, the doping depth and concentration can be controlled with the laser parameters.