Low-Temperature Polycrystalline Silicon Thin-Film Transistors and Circuits on Flexible Substrates

Abstract

Low-defect-density polycrystalline Si on flexible substrates can be instrumental in realizing the full potential of macroelectronics. Direct deposition or solid-phase crystallization techniques are often incompatible with polymers and produce materials with high defect densities. Excimer-laser annealing is capable of producing films of reasonable quality directly on polymer and metallic substrates. Sequential lateral solidification (SLS) is an advanced pulsed-laser-crystallization technique capable of producing Si films on polymers with lower defect density than can be obtained via excimer-laser annealing. Circuits built directly on polymers using these SLS films show the highest performance reported to date. For more information, see http://www.mrs.org/bulletin_ads For more information, see http://www.mrs.org/bulletin_ads

Inorganic Semiconductors for Flexible Electronics

“…[17] In this approach, known as [18] Transport in films of a-Si and nc-Si grown using these procedures can be improved significantly by transforming them into pc-Si films with techniques such as solid-phase crystallization (SPC), excimer-laser annealing (ELA), or sequential lateral solidification (SLS). [19] SPC involves the solid-state transformation of the energetically metastable a-Si phase into crystalline Si. Although this process can yield large-grain materials and good performance (e.g., TFTs made of n-type pc-Si films crystallized at 650°C show mobilities of 64 cm 2 V -1 s -1 in both linear and saturated regimes [20a] ) in TFTs on flexible steel foils, [20] the high intragrain defect density can limit the characteristics.…”

“…Figure 2A presents a scanning electron microscopy (SEM) image of a pc-Si film formed by ELA, clearly showing grains with characteristic sizes of a few hundred nanometers. [19] The grain size formed by ELA is usually similar to the laser wavelength, resulting in large grain boundaries (i.e., a high density of defects) in the channel regions of the TFTs. The correlation between grain size and laser wavelength is thought to originate from interference effects in waves scattered at surface protrusions formed as a result of the volume expansion upon lateral solidification.…”

“…As a result, SLS always produces grains with much larger sizes, better-controlled shapes, and higher degrees of uniformity compared to ELA. [23] Figure 2B shows an SEM image of a Si film converted through line-scan SLS, indicating the formation of stripes of crystalline Si with high aspect ratios (i.e., the ratio of length to width), [19] which, when oriented along the channel in a transistor, can offer high mobility transport. In addition, other microstructures of crystalline Si can be obtained via various SLS schemes, as controlled by the patterns of the laser beams and the sequences of the pulses.…”

Introduction to Solution‐Deposited Inorganic Electronics

“…40 Signifi cant literature is already available that describes efforts at improving the device/circuit characteristics of conventional polysilicon -based TFTs. The interested reader is referred to several review articles 27,37,39 and to several excellent reference volumes 41 that describe the efforts to circumvent process temperature issues by fabrication of LTPS devices via laser -induced crystallization. We note that display manufacturers are now introducing products with on -display LTPS driver and demux circuits, with built -in memory expected in the near future.…”

“…43 -45 For applications that do not need transparent substrates, thin metal foils can replace the plastic. 39 LTPS circuits on stainless steel foil that switch at rates of several hundred megahertz up to a gigahertz have been fabricated. Devices fabricated with this approach also provide performance that is satisfactory for operation of analog circuits.…”