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Fast Flexible Transistors with a Nanotrench Structure.

Seo JH, Ling T, Gong S, Zhou W, Ma AL, Guo LJ, Ma Z - Sci Rep (2016)

Bottom Line: On the other hand, recent advances in nanoimprinting lithography (NIL) may enable the fabrication of large-area nanoelectronics, especially flexible RF electronics with finely defined patterns, thereby significantly broadening RF applications.A unique 3-dimensional etched-trench-channel configuration was used to allow for TFT fabrication compatible with flexible substrates.Optimal device parameters were obtained through device simulation to understand the underlying device physics and to enhance device controllability.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA.

ABSTRACT
The simplification of fabrication processes that can define very fine patterns for large-area flexible radio-frequency (RF) applications is very desirable because it is generally very challenging to realize submicron scale patterns on flexible substrates. Conventional nanoscale patterning methods, such as e-beam lithography, cannot be easily applied to such applications. On the other hand, recent advances in nanoimprinting lithography (NIL) may enable the fabrication of large-area nanoelectronics, especially flexible RF electronics with finely defined patterns, thereby significantly broadening RF applications. Here we report a generic strategy for fabricating high-performance flexible Si nanomembrane (NM)-based RF thin-film transistors (TFTs), capable of over 100 GHz operation in theory, with NIL patterned deep-submicron-scale channel lengths. A unique 3-dimensional etched-trench-channel configuration was used to allow for TFT fabrication compatible with flexible substrates. Optimal device parameters were obtained through device simulation to understand the underlying device physics and to enhance device controllability. Experimentally, a record-breaking 38 GHz maximum oscillation frequency fmax value has been successfully demonstrated from TFTs with a 2 μm gate length built with flexible Si NM on plastic substrates.

No MeSH data available.


Comparison of the device structures (cross-sectional view) and fabrication processes between (a) 3-D nano trench Si NM flexible RF TFTs, and (b) conventional 2-D TFTs. The effective channel lengths Lch are marked in red in (a3,b3). The smallest Lch of the nano trench TFT can reach down to 50 nm via NIL and that of the conventional TFT can only reach down to about 1.5 μm. (a1) Blanket phosphorous ion implantation and thermal anneal. (a2) Nano trench formation via nanoimprint. (a3) Final structure of nano trench TFT where the channel length Lch is defined by nanoimprint. (b1) Photolithography to define S/D regions for ion implantation. (b2) Selective ion implantation and thermal anneal. (b3) Final structure of conventional TFT where the channel length Lch is limited by gate electrode and dopant out-diffusion during ion implantation and thermal anneal.
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f1: Comparison of the device structures (cross-sectional view) and fabrication processes between (a) 3-D nano trench Si NM flexible RF TFTs, and (b) conventional 2-D TFTs. The effective channel lengths Lch are marked in red in (a3,b3). The smallest Lch of the nano trench TFT can reach down to 50 nm via NIL and that of the conventional TFT can only reach down to about 1.5 μm. (a1) Blanket phosphorous ion implantation and thermal anneal. (a2) Nano trench formation via nanoimprint. (a3) Final structure of nano trench TFT where the channel length Lch is defined by nanoimprint. (b1) Photolithography to define S/D regions for ion implantation. (b2) Selective ion implantation and thermal anneal. (b3) Final structure of conventional TFT where the channel length Lch is limited by gate electrode and dopant out-diffusion during ion implantation and thermal anneal.

Mentions: In recent years, flexible electronics have gained popularity with various applications ranging from flexible displays, wearable electronics and identification tags, biomedical devices, to structural health monitoring123456. Many flexible electronics applications generally do not require the use of very high speed devices, but the flexibility of the electronics is of critical importance. Typically, the low speed flexible electronics are based on organic or low temperature deposition-compatible amorphous semiconductor (e.g., a-Si) or metal oxide materials, which can be processed with large area printing, coating, and deposition techniques567. On the other hand, radio-frequency (RF) capable flexible transistors, due to their wider signal handling capability, can extend flexible electronics applications toward wireless data transmission and wireless power transfer, or allow circuits to operate with much lower power consumption. The main challenges in the pursuit of RF flexible electronics included: (1) a lack of materials with sufficient mobility and simultaneous mechanical flexibility, and (2) difficulties in defining a fine channel region using a scalable fabrication process. Some solutions have been found to overcome the first challenge over the past decade. Flexible single crystalline semiconductor nanomembranes (NM) have adequately fulfilled the desired requirements8. However, patterning deep submicron scale features on the nanomembranes on flexible substrates using conventional fine lithography techniques91011 has been very challenging due to the difficulties encountered in the fabrication process, such as the diffraction of exposed light on the plastic substrate and particularly the thermal plasticity of the flexible substrates under even moderate temperatures that are essential for photolithography. In addition, the conventional selective doping process via ion implantation and thermal diffusion can lead to unwanted short circuit due to easy merging between source and drain n+ wells (as shown in Fig. 1(b2))9101112. Such challenges associated with the conventional field effect transistor structure and its standard processes become more critical when dimensions are scaled down, thereby limiting the performance of flexible electronics (Fig. 1b). As of today, the smallest channel length of flexible transistors made on plastic substrates using the semiconductor nanomembranes is about 1 μm91011.


Fast Flexible Transistors with a Nanotrench Structure.

Seo JH, Ling T, Gong S, Zhou W, Ma AL, Guo LJ, Ma Z - Sci Rep (2016)

Comparison of the device structures (cross-sectional view) and fabrication processes between (a) 3-D nano trench Si NM flexible RF TFTs, and (b) conventional 2-D TFTs. The effective channel lengths Lch are marked in red in (a3,b3). The smallest Lch of the nano trench TFT can reach down to 50 nm via NIL and that of the conventional TFT can only reach down to about 1.5 μm. (a1) Blanket phosphorous ion implantation and thermal anneal. (a2) Nano trench formation via nanoimprint. (a3) Final structure of nano trench TFT where the channel length Lch is defined by nanoimprint. (b1) Photolithography to define S/D regions for ion implantation. (b2) Selective ion implantation and thermal anneal. (b3) Final structure of conventional TFT where the channel length Lch is limited by gate electrode and dopant out-diffusion during ion implantation and thermal anneal.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4837400&req=5

f1: Comparison of the device structures (cross-sectional view) and fabrication processes between (a) 3-D nano trench Si NM flexible RF TFTs, and (b) conventional 2-D TFTs. The effective channel lengths Lch are marked in red in (a3,b3). The smallest Lch of the nano trench TFT can reach down to 50 nm via NIL and that of the conventional TFT can only reach down to about 1.5 μm. (a1) Blanket phosphorous ion implantation and thermal anneal. (a2) Nano trench formation via nanoimprint. (a3) Final structure of nano trench TFT where the channel length Lch is defined by nanoimprint. (b1) Photolithography to define S/D regions for ion implantation. (b2) Selective ion implantation and thermal anneal. (b3) Final structure of conventional TFT where the channel length Lch is limited by gate electrode and dopant out-diffusion during ion implantation and thermal anneal.
Mentions: In recent years, flexible electronics have gained popularity with various applications ranging from flexible displays, wearable electronics and identification tags, biomedical devices, to structural health monitoring123456. Many flexible electronics applications generally do not require the use of very high speed devices, but the flexibility of the electronics is of critical importance. Typically, the low speed flexible electronics are based on organic or low temperature deposition-compatible amorphous semiconductor (e.g., a-Si) or metal oxide materials, which can be processed with large area printing, coating, and deposition techniques567. On the other hand, radio-frequency (RF) capable flexible transistors, due to their wider signal handling capability, can extend flexible electronics applications toward wireless data transmission and wireless power transfer, or allow circuits to operate with much lower power consumption. The main challenges in the pursuit of RF flexible electronics included: (1) a lack of materials with sufficient mobility and simultaneous mechanical flexibility, and (2) difficulties in defining a fine channel region using a scalable fabrication process. Some solutions have been found to overcome the first challenge over the past decade. Flexible single crystalline semiconductor nanomembranes (NM) have adequately fulfilled the desired requirements8. However, patterning deep submicron scale features on the nanomembranes on flexible substrates using conventional fine lithography techniques91011 has been very challenging due to the difficulties encountered in the fabrication process, such as the diffraction of exposed light on the plastic substrate and particularly the thermal plasticity of the flexible substrates under even moderate temperatures that are essential for photolithography. In addition, the conventional selective doping process via ion implantation and thermal diffusion can lead to unwanted short circuit due to easy merging between source and drain n+ wells (as shown in Fig. 1(b2))9101112. Such challenges associated with the conventional field effect transistor structure and its standard processes become more critical when dimensions are scaled down, thereby limiting the performance of flexible electronics (Fig. 1b). As of today, the smallest channel length of flexible transistors made on plastic substrates using the semiconductor nanomembranes is about 1 μm91011.

Bottom Line: On the other hand, recent advances in nanoimprinting lithography (NIL) may enable the fabrication of large-area nanoelectronics, especially flexible RF electronics with finely defined patterns, thereby significantly broadening RF applications.A unique 3-dimensional etched-trench-channel configuration was used to allow for TFT fabrication compatible with flexible substrates.Optimal device parameters were obtained through device simulation to understand the underlying device physics and to enhance device controllability.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA.

ABSTRACT
The simplification of fabrication processes that can define very fine patterns for large-area flexible radio-frequency (RF) applications is very desirable because it is generally very challenging to realize submicron scale patterns on flexible substrates. Conventional nanoscale patterning methods, such as e-beam lithography, cannot be easily applied to such applications. On the other hand, recent advances in nanoimprinting lithography (NIL) may enable the fabrication of large-area nanoelectronics, especially flexible RF electronics with finely defined patterns, thereby significantly broadening RF applications. Here we report a generic strategy for fabricating high-performance flexible Si nanomembrane (NM)-based RF thin-film transistors (TFTs), capable of over 100 GHz operation in theory, with NIL patterned deep-submicron-scale channel lengths. A unique 3-dimensional etched-trench-channel configuration was used to allow for TFT fabrication compatible with flexible substrates. Optimal device parameters were obtained through device simulation to understand the underlying device physics and to enhance device controllability. Experimentally, a record-breaking 38 GHz maximum oscillation frequency fmax value has been successfully demonstrated from TFTs with a 2 μm gate length built with flexible Si NM on plastic substrates.

No MeSH data available.