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Silica films with various types of alkyl groups were grown from the liquid phase, at room temperature, using alkyl tri-alkoxy silane compounds. From Fourier transform infrared (FTIR) absorption measurements, a majority of Si-alkyl bonds in the source molecules were found to remain in the grown film, while Si-alkoxy bonds were completely removed. Electrical and thermal properties have been measured comparatively for various films having dense alkyl groups. The films with the methyl group are promising for silicon production in ultra large-scale integrated circuits (Si-ULSIs) and the film having the vinyl group for non-heat-tolerant compound-semiconductor large-scale integrated circuits (LSIs).

Chemical vapor deposition parameters have been investigated for low (∼200°C) temperature deposited and hydrogen-free SiO 2 films from a mixture of tetra-iso-cyanate-silane and various kinds of tertiary-alkyl-amine. Di-methyl mono-ethyl amine was found the most suitable partner gas for H free low temprature CVD source gas mixture. Addition of small amount of H 2 O (0.5% of tertiary-alkyl-amine) was effective for decreasing the deposition temperature up to 50°C without serious drawbacks on the film properties such as hydration and electric properties. The deposition rate increased to higher temperatures by using those best partner gases, di-methyl mono-ethyl amine, with tetra-iso-cyanate-silane. The resistivity defined at 1 MV cm −2 was >10 16 Ω cm, and breakdown field strength at 1 μA cm −2 was >6 MV cm −2 . The fixed-charge density and interface sate density near midgap were ∼4 × 10 12 cm −2 and ∼10 12 eV −1 cm −2 respectively.

Reaction process has been studied for an atomic hydrogen-induced abstraction of methyl groups on the Ge surface. The reaction rate estimated from experiments had a very large activation energy and depended on a surface coverage of methyl groups; they are not expected for a `pure' Eley–Rideal mechanism. The process can be explained by the `modified' Eley–Rideal mechanism or the `precursor' mechanism based on two-dimensionally bound hydrogen atoms on the surface.

We achieved the atomic-layer-deposition of SiO2 with an extremely low H content for the first time by using Si(NCO)4 and N(C2H5)3. The deposition rate was independent of the exposure times of both sources. The saturated deposition rate was about 1.2 A/cycle at 150°C. AFM observation revealed that the increase of roughness after 50 deposition cycles was only about 0.4 A in root mean square. The FT-IR spectra showed that the deposited film was SiO2 without Si–H, Si–OH or NCO bond.

Based on the first result of the layer-by-layer deposition of silicon dioxide films by a cyclic exposure of water and tetra isocyanate silane, Si(NCO) 4 , new isocyanate compounds preferable to Si(NCO) 4 have been proposed for ideal layer-by-layer deposition characteristics. Preliminary results are also presented for one of the substances.

Two methods are proposed and demonstrated successfully for low temperature atomic layer epitaxy (ALE) of Si, where H atoms play essential roles. The first method is the use of H as a self-limiting factor. Trisilane (Si3H8) was used as source gas and the substrate temperature was modulated in order to alternate steps in an ALE cycle. When the temperature was less than 380°C in the adsorption step and more than 520°C in the desorption step, respectively, the grown layer thickness per cycle was 0.8 ML/cycle. The second method is the use of atomic H as an active reducer of a self-limiting factor. A clean surface was exposed to dichlorosilane (SiH2Cl2) as source gas to grow an Si monolayer covered with CI. Next, atomic H was injected to reduce CI from the surface. An ideal monolayer growth was obtained with the substrate temperature over 540°C.

SiO2 films were deposited layer by layer from a new silicon source gas, i.e. tetra-iso-cyanate-silane (Si(NCO)4). An average growth rate of about 0.17 nm per cycle was achieved by a cyclic process of alternating reaction of the substrate surface with Si(NCO)4 and H2O respectively. The detailed deposition characteristics together with chemical and physical properties of the deposited film were evaluated with ellipsometry, Fourier transform IR spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy.

We have found that an undesirable SiNH component is reduced drastically from the low-temperature-deposited SiN surface by intense ArF excimer-laser irradiation on the low-temperature-deposited SiN film. This pre-annealing of the SiN film is very effective in forming an abrupt Si/SiN interface, and thus plays a very important role in high-performance excimer-laser-crystallized poly-Si/SiN thin-film transistors aiming at high-quality liquid-crystal displays. Annealing characteristics are also presented for the various SiN film thickness and for both the ArF and KrF excimer-laser lights.

A microstructure has been introduced into the channel of amorphous-silicon thin-film transistors with poly-silicon sources and drains. A slant-photolithography method was introduced for fabricating this device without additional photomasks. The off-characteristics have been improved without sacrificing originally good on-characteristics. Temperature dependence is also discussed for the transistor characteristics. >

The characteristics of amorphous silicon thin-film transistors fabricated by chemical vapor deposition method is discussed in detail in conjunction with posthydrogenation. As the hydrogenation around an active interface starts, the transistor performance is abruptly improved and then does not change with further hydrogenation. A critical condition for hydrogenation for such an abrupt improvement may be accounted for based on a simple formula using a transit time of hydrogen atoms. A desirable thickness of amorphous silicon films for high performance TFT is thicker than 100 nm. In addition, it is recommended that hydrogenation is to be made at a temperature as low as possible as long as it can be possible.

Hot ion-implantation has been applied to threshold voltage control of amorphous-silicon thin-film transistors. A threshold voltage shift as large as 13 V has been achieved without deterioration of the field-effect mobility. The technique was also used to form distributed-threshold voltage transistors which have a microstructure inside the channel. It was verified that the off-characteristics were greatly improved. >

Low-temperature CVD method has been investigated for silicon-nitride films by using higher silanes and hydrazine aiming at thin-film transistor application. The deposition temperature has been reduced to as low as 350°C, i.e., by about 400°C lower than the typical temperature. Atomic N/Si ratio was more than 4/3, i.e., the film was stoichiometric SiN, and hydrogen content was as high as 20 atomic%. Breakdown field strength, specific resistivity and MIS interface state density were about 6MV/cm, 6x1015 Ωcm and 1x1011cm-2eV-1, respectively.

Amorphous silicon (a-Si) Thin Film Transistors (TFT) will continue to play an important role in large-area liquid crystal displays (LCD), however, there is also a strong demand for ultra-high performance TFTs aimed at intelligent small (or medium) size displays. For this reason, “Crystal Si on glass” has become an increasingly attractive solution. This paper reviews recent work performed at ALTEDEC (Advanced LCD Technologies Development Center Co., Ltd, with regard to growing an array of large Si grains at low temperature. The results obtained so far indicate that satisfactory progress has been made towards the achievement of this goal.

Stoichiometric boron‐nitride films have been successfully deposited at temperatures as low as 400 °C by chemical vapor deposition using diborane (B2H6) and hydrogen azide (HN3). The film deposited on the silicon substrate at 475 °C was amorphous and contained hydrogen atoms with a density of 1.3×1022 cm−3. The breakdown field strength and the low‐field resistivity were 2.8 MV/cm and 1015 Ω cm, respectively. The optical and low‐frequency dielectric constants were 3.6 and 4.0, respectively. Metal‐insulator‐metal device equipped with this film showed steep current‐voltage characteristics.

The application of chemical-vapor-deposited (CVD) amorphous-silicon and silicon-nitride films to active layers of thin-film transistors on a glass substrate is discussed. The maximum process temperature was 485 degrees C. The maximum field-effect mobility and the typical on-off current ratio were more than 0.9 cm/sup 2//V-s and 10/sup 6/, respectively. Advantages of applying the fully plasma-free CVD method in the amorphous-silicon thin-film transistor process are discussed. >

Time-delay problems will become extremely severe in active-matrix-addressed ultra-large liquid-crystal display-panels. The problem caused by large crossover capacitance can be solved by moving the signal bus-line to the facing glass plate. The problem caused by large gate capacitance can be solved only by reducing the TFT channel much shorter than the conventional value. This requirement can be satisfied by a novel vertical TFT fabricated by using a selective excimer-laser crystallization method. Switching characteristics have been evaluated for the prototype devices, and good on-characteristics have been confirmed for the O.5μm-long gate device. Improved device structures are also presented.

A new source gas mixture of Si(NCO)4 and N(CH3)3 has been investigated for low-temperature chemical vapor deposition of silicon dioxide. Hydrogen-free films have been successfully deposited at 200° C for the first time. Evaluated results are also presented for deposition characteristics and deposited film properties. The breakdown-field strength and low-field resistivity were as high as 5.9 MV/cm and 6×1016 Ωcm, respectively, for the as-deposited film.

We report, for the first time, that ArF excimer laser annealing can improve silicon-nitride film properties. It is shown that the 15-nm-thick top region of the laser preannealed film had a lower hydrogen content and a much lower etching rate than the as-deposited film. The laser preannealed film is very useful as the gate insulator of high-performance bottom-gate thin-film transistors fabricated with the laser-recrystallized poly-Si film. The field-effect mobility of electrons was as high as 200 cm2/V·s, while the mobility was as low as 40 cm2/V·s without preannealing the silicon-nitride film.

Silicon nitride films have been successfully deposited at a temperature as low as 300°C by chemical-vapor-deposition using tctra-silane (Si4 H10) and hydrogen azidc (HN3). Atomic ratio (N/Si) of the film deposited at 400°C was 1.47, i.e., the film was N-rich. Total hydrogen content was about 25atomic%. The breakdown-field strength was 6.5MV/cm at leakage-current density of 1μA/cm2, and the low-field resistivity was more than 1015 Ωcm. Similar electrical characteristics were obtained from films deposited at a temperature range between 300°C and 500°C. Amorphous silicon thin-film transistors equipped with this film as the gate dielectric showed good transfer characteristics.

Chemical-vapor-deposited (CVD) amorphous-silicon films have been applied as the active layer in thin-film transistors with a top-gate structure for the first time. The maximum field-effect mobility and the typical on/off current ratio were more than 0.5 cm2·V-1·s-1 and 107, respectively. The characteristics were better than the ones based on plasma-enhanced CVD.

Amorphous-silicon thin-film transistors (a-Si TFTs) have been fabricated by using an a-Si layer deposited by low-temperature thermal-CVD method using higher silanes. The TFT with thermally grown SiO2 gate was operated in both the n-channel mode and the p-channel mode. The maximum field-effect mobility and typical on/off current ratio were 1.5 cm2/Vs and 107 for the n-channel operation, and 0.2 cm2/Vs and 106 for the p-channel operation, respectively. Free electron mobility in the conduction band and free hole mobility in the valence band have been estimated, using the temperature dependence of the field-effect mobilities. It was found that the hole mobility is as high as the electron mobility.

XeF2 has been proposed as a catalyst for low-temperature dehydration of SiO2 films. The critical dehydration temperature was about 200°C less XeF2 dehydration-annealing than for vacuum dehydration-annealing. The SiO2 film deposited from tetraethylorthosilicate (TEOS) and ozone at 270°C showed OH-free and good insulating characteristics after XeF2-annealing at 400°C.

We have successfully demonstrated chemical vapor deposition of a silica film in which some of oxygen atoms of the pure silica network are replaced to methylene (-CH 2 ,) groups. The as-deposited film was thermally unstable owing to the unwanted decomposition reaction of the methylene group with the dense H 2 O in the film. It was found that the film could be dehydrated without decomposition of the methylene group by low temperature anncaling in a XeF 2 ambient. The dehydrated film showed not only good insulating and low-k characteristics (resistivity of 10 15 Ω cm, breakdown field of 3.3 MV/cm, and k of 2.8), but also good thermal properties, such as good thermal stability and high thermal conductivity.

A new sourcegas has been presented for quasi hydrogen-free chemical-vapor deposition of organic-silica films. The content of the methyl groups incorporated in the film was as much as 25% of that of Si, and the as-deposited film contained only a few silanol and cyanate groups. Thus it had good insulating characteristics with a dielectric constant, k, as low as 3.5 even under as-deposited conditions. These characterisitcs were further improved by vacuum annealing. And, the value of k became became 3.0 with a low-field resistivity of more than 1015 Ωcm, after 500°C annealing.

Silicon-dioxide films have been liquid-phase deposited from a halogen-free silicon source. Tetra-ethyl orthosilicate was introduced, for deposition, into water with NH3 and HCOOH. The deposition rate was as high as 150 nm/h. The deposited film was stoichiometric SiO2 with no C and N. The film properties are also discussed.

Step-coverage characteristics have been investigated for hydrogen-free SiO2 films deposited by a newly-developed low-temperature chemical-vapor-deposition (CVD) method using a gas mixture of tetra-iso-cyanate-silane and tertiary-methyl-amine. The effective diffusion length of CVD source molecules estimated from open-cavity experiments was around 300 µm. Scanning electron microscopy (SEM) observation showed that the film filled a 1-µm-wide and 2.7-µm-deep trench with excellent conformity.

A new method has been proposed for fabricating short-channel amorphous-silicon vertical thin-film transistors (TFTs). The channel is formed at the side wall of the gate electrode, but the source and drain are formed on the flat glass and the gate electrode regions, respectively. Due to the desirable shadowing effects of the side wall, the channel remains in an amorphous state, but the source and drain are crystallized selectively by excimer-laser irradiation. The 1.5 µm-long TFTs had field-effect mobility as high as 0.7 cm2/(V·s) and no current crowding effects.

1 µ m-long amorphous-silicon thin-film transistors with a distributed-threshold-voltage (DTV) structure have been evaluated in detail, for the first time, and found to have sufficiently low cut-off characteristics, simultaneously with good turn-on characteristics. The dynamic range, that is, the on/off current ratio, was as high as 8 orders in magnitude. This favorable property is due to the partially doped channel in the DTV structure and the selectively polycrystallized Si source and drain.

We have successfully deposited SiNx:H films at temperatures as low as 350°C by the chemical-vapor-deposition (CVD) method using hexachloro-disilane (Si2Cl6) and hydrazine (N2H4). The atomic ratio (N/Si) of the film deposited at 400°C was 1.26 with a total hydrogen content of about 30 at.%. The breakdown-field strength was 5.3 MV/cm at a leakage-current density of 1 µA/cm2, and the low-field resistivity was more than 1015 Ω·cm. Amorphous-silicon thin-film transistors equipped with this film as the gate dielectric showed clear transfer characteristics.

We propose alkoxyl-silane-isocyanate [(RO) n Si(NCO)4-n ] as a new silicon source material for low-temperature chemical-vapor deposition of silicon dioxide. Detailed deposition characteristics and properties of the deposited film evaluated by auger electron spectroscopy (AES), X-ray photoemission spectroscopy (XPS) and Fourier transform IR (FTIR) are presented for ethoxyl-silane-isocyanate (R=C2H5). The deposition rate took a maximum value of 20 nm/h around 120°C and then decreased gradually with increasing deposition temperature T. The atomic ratio O/Si evaluated by AES was constant and about 2 for T≧200°C. The breakdown-field strength and low-field resistivity were 3.2 MV/cm and 1.3×1015 Ωcm, respectively.

XeF excimer-laser annealing was found to be very effective for activating ion-implanted dopants in hydrogenated amorphous-silicon (a-Si:H) films at room temperature, because of the low photon energy of the XeF excimer-laser light. The XeF excimer-laser-annealed a-Si:H films showed conductivity as high as 5×10-3 S/cm with the phosphorus atom dosage of 1×10-6 cm-2 without post-hydrogenation. With XeF excimer-laser annealing and ion implantation, the threshold voltage of a-Si:H thin-film transistors could be changed by more than 20 V without deterioration of on-characteristics.

A novel post-hydrogenation method for chemical-vapor-deposited amorphous-silicon (CVD a-Si) thin-film transistors (TFTs) has been proposed. Samples were annealed in the presence of the atomic hydrogen generated by a hot tungsten filament. The dependence of the amount of adsorbed hydrogen on annealing period, filament temperature and total gas pressure was described. A-Si TFTs which were post-hydrogenated by means of the hot-filament method showed an electron mobility of 1.1 cm2/V·s. The lifetime and diffusion length of atomic hydrogen were estimated.

Hydrogenated amorphous-silicon films have been deposited by the plasma-free chemical-vapor-deposition method using tetrasilane (Si4H10) at temperatures as low as 350°C. The film deposited at 350°C and 9 Torr had hydrogen content as high as 15 at.%, optical bandgap of 1.78 eV, logarithmic ratio of photoconductivity (at a light intensity of 100 mW/cm2 under AM1 conditions) to dark conductivity of 3.2, activation energy of 0.78 eV and the Urbach tail slope as small as 56 meV. Thin-film transistors have been fabricated using the film deposited at 350°C. The electron mobility was 0.6 cm2/V s under as-deposited conditions.

New fabrication processes for selfaligned amorphous silicon TFTs are proposed. The TFTs have a polysilicon source and drain which are formed by ArF excimer laser annealing. They exhibit a field-effect mobility of 0.8 cm/sup 2//Vs, threshold voltage of 11 V, and on/off current ratio of higher than 10/sup 6/. >

We have studied post-hydrogenation characteristics and photo-absorption characteristics of amorphous-silicon films thermally deposited from disilane at 500°C. There is a critical post-hydrogenation temperature of about 400°C, below which both the density of hydrogen (deuterium) atoms on the surface and the activation energy of their diffusion constants are constant. Post-hydrogenation drastically enhances the decrease of the photo-absorption coefficient for photon energies less than the optical bandgap. The optimum density of hydrogen atoms introduced by post-hydrogenation was between 3 at% and 3.5 at%. The Urbach tail slope E 0 was 42 meV, i.e., about 8 meV lower than the typical value of the film deposited by plasma enhanced chemical vapor deposition (PECVD) method.

Silicon nitride films were deposited at temperatures as low as 350°C by chemical vapor deposition using a new source gas, hydrogen azide (HN3). Silicon nitride film deposited at 425°C was nitrogen-rich and showed hydrogen content of about 28 atomic%. The breakdown field strength was as high as 8.7 MV/cm and the resistivity was as high as 1015 Ω·cm. Amorphous silicon thin-film transistors equipped with this film as the gate dielectric showed good transistor characteristics.

Hot-wall chemical vapor deposition of amorphous-silicon has been investigated, aiming at a batch process for thin-film transistors (TFTs). High-performance TFTs (mobility=1.7 cm2/Vs, threshold voltage =9 V and subthreshold voltage swing =0.8 V/decade) have been successfully fabricated. Similar good TFT characteristics were obtained over a wide range of a-Si deposition conditions.

A new atomic layer epitaxy (ALE) method of silicon has been investigated. Substrate temperature T s is increased to more than a critical temperature, and decreased to less than another critical temperature alternatively, and trisilane gas is injected as source molecules only in a short interval within the low-T s phase. This method separates an adsorption phase of silicon hydrides with one-monolayer thickness from a desorption phase of hydrogen, resulting in the monolayer growth per cycle. ALE growth conditions were estimated from gas-source MBE characteristics, and ALE growth (with 0.8 monolayer/cycle) was demonstrated. The grown layer had good surface morphology and crystallinity.

Hot ion-implantation into amorphous silicon (a-Si) has been studied for chemical-vapor-deposited films. Conductivity of as high as 0.4 S/cm (100 times higher than the previous result) was achieved at the phosphorus atom concentration of 8×1020 cm-3, when the substrate temperature was elevated up to 450°C during the ion-implantation. Activation energy was 0.12 eV.

Transient temperature profiles have been studied theoretically in silicon film heated by a single shot of a laser light pulse for several substrate structures. The molten silicon region is very shallow and disappears completely within a few tens of ns in (poly)crystalline silicon. It exists for more than a few hundred ns in amorphous silicon and also in silicon films on an oxide substrate. In silicon/oxide/conductive-substrate structures, the sustaining time of the molten silicon region is shortened by thinning the oxide film. Numerical results are compared with approximate relations derived from a simple physical picture.

Amorphous silicon films have been prepared by low pressure chemical vapour deposition of monosilane, disilane and trisilane. The deposition temperatures ranged from 385°C to 555°C. The hydrogen content in the films was approximately 3 atomic percent and the optical band-gap was 1.6∼1.7 eV. Simple inverted staggered thin-film transistors were made with thermal silicon dioxide as the gate insulator. After annealing in a hydrogen radical rich ambient the field-effect mobilities for electrons were as high as 2.0 cm2/V·s and almost independent of temperature from 385°C to 500°C. At higher temperatures the mobility decreased rapidly and was less than 0.1 cm2/V·s at 555°C. These results appeared to be independent of the source gas.

This letter describes that photogenerated hydrogen-radical annealing (HRA) is very effective for improving the electric properties of chemical-vapor-deposited amorphous silicon films. The ratio of photoconductivity to dark conductivity (at a light intensity of 100 mW/cm2) changed from 100 to 6×104 upon annealing in a hydrogen-radical-rich ambient at 280°C. Thin-film transistor (TFT) characteristics were also improved by HRA.

The optimum condition has been investigated for chemical vapor deposition (CVD) of amorphous-silicon (a-Si) films to form high-performance a-Si thin-film transistors (TFTs) by a fully plasma-free process. It was found that hydrogen annealings improve the TFT characteristics a great deal and that there is an optimum deposition temperature and gas flow rate. The maximum field-effect mobility of electrons and on/off current-ratio were more than 0.8 cm2V-1s-1 and 106, respectively.

High-performance bottom-gate thin-film transistors (TFTs) with both polysilicon (poly-Si) and amorphous silicon (a-Si) have been fabricated, for the first time, using chemical-vapor-deposited a-Si film as a starting semiconductor material. The feature of this technology is that both TFTs can be produced in the same process and at the same time on a single substrate except for the local laser crystallization step of the a-Si. The field-effect mobilities were 60 cm2/V ·s for the poly-Si TFT and 1.3 cm2/V ·s for the a-Si TFT, respectively. Their on-off current ratios were more than 106.

Amorphous silicon layers have been deposited by low pressure chemical vapour deposition at 450°C using disilane as the only source gas. Simple inverted staggered thin-film transistors were made with thermal silicon dioxide as the gate insulator. Field-effect mobilities for electrons and holes were 1.4 cm2/V ·s and 0.1 cm2/V ·s, respectively. In order to obtain these high mobilities the transistor structures were carefully annealed in a hydrogen-radical rich ambient.