Yutaka Miyata

Low-temperature polycrystalline silicon thin-film transistors for large-area liquid crystal display

A low-temperature poly-Si thin-film transistor (TFT), having inverted-staggered structure, has been developed successfully using excimer laser annealing and ion doping. This TFT is suitable for pixel transistors of large-area and high-resolution LCDs. The maximum process temperature of the TFT fabrication steps is less than 450°C, so the same glass substrate on which amorphous Si TFT arrays are formed can be used in this poly-Si TFT process. Furthermore, most of the procedures, equipment and thin-film materials used to fabricate amorphous Si TFTs are compatible with fabrication of the poly-Si TFTs. On the other hand, some investigation of the CMOS driver circuit has been done, and it has been found that the threshold voltage of these poly-Si TFTs can be controlled easily by lightly doping of B ion into the channel region using the ion doping system.

Bottom-gate poly-Si thin film transistors using XeCl excimer laser annealing and ion doping techniques

High mobility bottom-gate poly-Si thin film transistors (TFTs) have been successfully fabricated on a hard glass substrate using XeCl excimer laser annealing and ion doping techniques. The authors used an a-Si:H film which is deposited by a plasma-enhanced chemical vapor deposition (PECVD) as a precursor film, and then they crystallized the a-Si film by XeCl excimer laser annealing. The maximum field effect mobility and grain size obtained were 200 cm/sup 2//V-s (n-channel), and 250 nm, respectively. The poly-Si TFTs showed excellent transfer characteristics, and an ON/OFF current ratio of over 10/sup 6/ was obtained. Successful control of the threshold voltage within 4 V using an ion doping technique is also demonstrated. >

Polycrystalline silicon recrystallized with excimer laser irradiation and impurity doping using ion doping method

Polycrystalline silicon (poly‐Si) thin films having large grain size have been obtained by XeCl excimer laser annealing of a‐Si deposited by plasma enhanced chemical vapor deposition on SiNx. The grain size of poly‐Si reaches to about 300 nm with excimer laser irradiation around 500 mJ/cm2. For excimer laser irradiation from 500 to 600 mJ/cm2, instead of further enlargement of grains, amorphous regions appear and increase in the grains. Most of the Si layer becomes an amorphous state at pulse energy density over 600 mJ/cm2. On the other hand, impurity implantation to a‐Si layers using non‐mass‐separated ion doping equipment and a subsequent activation step with excimer laser irradiation enable us to obtain recrystallized poly‐Si layers having low resistivity. By using ion doping and excimer laser annealing, we have obtained excellent n‐channel thin film transistors, the field effect mobility in excess of 40 cm2/V s and the ON/OFF current ratio of more than 106, respectively.

A high-sensitivity solid-state image sensor using a thin-film ZnSe-Zn 1-x Cd x Te heterojunction photosensor

A high-sensitivity solid-state image sensor in which a charge-transfer device is overlaid by a thin-film photoconductor for the photosensor has been developed. The scanning registers are composed of interline transfer-type bucket-brigade registers (BBD) in the imaging area and charge-coupled registers (CCD) in the horizontal scanning circuit. The special feature of this device is that the thin-film photoconductor of ZnSe-Zn1-xCdxTe heterojunction is directly formed not only on the Si-diode area but also on the scanning circuit area of BBD to obtain a high-aperture ratio. This solid-state image sensor has 506V× 413Hpicture elements and the imaging area is about 10.4V× 13.5Hmm which corresponds to that of a 1-in vidicon. High sensitivity of 0.46 µA/lx (2856 K) and large blooming control capability have been obtained by this structure. The characteristics such as sensitivity, dark current, resolution, and blooming peculiar to the structure of a solid-state imager overlaid by a photoconductor are also discussed.

A CCD imager overlaid with a thin-film photodetector of a heterojunction ZnSe-Zn 1-x Cd x Te

A CCD imager which is composed of an interline transfer type scanner in the imaging area and a thin-film photodetector of a heterojunctjon ZnSe-Zn1-xCdxTe has been developed. The array consists of506^{V} \times 404^{H}picture elements. The imaging area is about6.7^{V} \times 9.0^{H}mm2in size which corresponds to that of a 2/3-in vidicon. For the device to achieve high performance, necessary conditions between the parameters of the overlaid thin-film photoconductor and the scanner have been analyzed. Blooming phenomenon is deeply related to the sensitivity. The blooming has been suppressed without sacrificing the sensitivity by applying the pulse operation of the heterojunction. As a result, excellent performances such as high sensitivity and large blooming suppression have been realized. The scene illumination if F 1.4 100 1x (S/N ratio is 46 dB for luminance signal) in a single-chip color camera. The blooming control capability is 250 times of the saturation exposure.

Blooming characteristics of a solid-state imager overlaid with a photoconductor

The control mechanisms of blooming phenomena in a solid-state imager overlaid with a photoconductor have been analyzed. The device studied is composed of an interline transfer type of scanner and a thin-film heterojunction ZnSe-Zn1-xCdxTe photoconductor. Two types of operation exist depending on the dc voltage or the pulse voltage applied to the ITO electrode on the photoconductor and it has been proved that pulse mode operation is superior to dc mode to sustain normal sensitivity under blooming suppressed condition. From the analysis of the operation it is made clear that one condition for blooming control in this device is based upon the principle of biasing the photoconductor so that in strong light the readout transistor is held in the cut-off condition except during the readout period. In the case that this condition is met, additional blooming can still occur due to the carriers excited in the Si substrate by the light that passed through the photoconductor and through the gap area between the charge collecting electrodes. By special shielding the blooming control capability can be effective up to 1100 times the saturation exposure compared to 120 times without the special shielding. The remaining blooming signal is clearly shown to be due to photo-excited carriers in the photoconductor during the blanking period.

Effect of Residual Phosphorus on Amorphous Silicon Thin Film Transistors

Effects of residual phosphorus in the channel region of amorphous silicon thin film transistors(a-Si TFTs) on the TFT characteristics were quantitatively investigated. Concentration and the depth profile of the residual phosphorus were measured by high resolution secondary ion mass spectroscopy(SIMS). The OFF characteristics of a-Si TFTs were also measured. The SIMS data showed that the phosphorus exists about 100nm deep into intrinsic a-Si(i-a-Si), but the OFF characteristics showed that the activity of the residual phosphorus is 4 order of magnitude lower than that of heavily phosphorus doped a-Si( + -a-Si). The residual phosphorus is found to be inactive and stable, and has little effect on a-Si TFT characteristics. These results enabled us to fabricate inverted staggered a-Si TFTs by the simplest process using only 2 photo-mask steps and 1 self-aligned exposure.

A CCD Imager Using ZnSe-Zn1-xCdxTe Heterojunction Photoconductor

A new CCD imager, having ZnSe-ZnCdTe heterojunction photoconductor as a photosensor which is vacuum-deposited on the CCD scanner, has been fabricated. The device has 404 (H) × 506 (V) scanning elements, each element measures 24 µm(H) by 14 µm(V), and the imaging area is 9.0 mm(H) × 6.7 mm(V) in size which is applicable to 2/3 inch camera format. The CCD scanner is fabricated with denuded zone which defines the defect-free device. A glass-reflow process is introduced to obtain the high breakdown of the heterojunction diode. The experimental result are as follows,; The sensitivity is 0. 16 µA/lx (3200 K). The horizontal and the vertical resolution are 280 and 450 TV lines, respectively. Dark current is 2 nA/cm2 for the photosensor (27°C). The signal to noise ratio is above 60 dB. The smearing suppression is effective for reducing the blooming phenomenon under the intense light illumination. The successful blooming suppression is achieved by this PLOSS structure device with metal screen grid. The high light exposure is allowable up to 1000 times as intense as the saturation exposure.