A semiconductor device including a control circuit for carrying out gamma correction of a supplied signal and a memory for storing data used in the gamma correction. The control circuit and the memory are constituted by TFTs and are a integrally formed on the same insulating substrate. A semiconductor display device including a pixel region in which a plurality of TFTs are arranged in matrix; a driver for switching the plurality of TFTs; a picture signal supply source for supplying a picture signal; a control circuit for carrying out gamma correction of the picture signal; and a memory for storing data used in the gamma correction of the picture signal. The plurality of TFTs, the driver, the control circuit, and the memory are integrally formed on the same insulating substrate.
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1. A semiconductor display device comprising:
a pixel region in which a plurality of TFTs are arranged in matrix; a driver for switching the plurality of TFTs; a picture signal supply source for supplying a picture signal; a control circuit for carrying out gamma correction of the picture signal; and a memory for storing data used in the gamma correction of the picture signal, wherein the plurality of TFTs, the driver, the control circuit, and the memory are integrally disposed on a same insulating substrate.
11. An electroluminescence display device comprising:
a pixel region in which a plurality of TFTs are arranged in matrix; a driver for switching the plurality of TFTs; a picture signal supply source for supplying a picture signal; a control circuit for carrying out gamma correction of the picture signal; and a memory for storing data used in the gamma correction of the picture signal, wherein the plurality of TFTs, the driver, the control circuit, and the memory are integrally disposed on a same insulating substrate.
7. A semiconductor display device comprising:
a pixel region in which a plurality of TFTs are arranged in matrix; a driver for switching the plurality of TFTs; a digital picture signal supply source for supplying a digital picture signal; a conversion circuit for converting the digital picture signal to an analog signal; a control circuit for carrying out gamma correction of the digital picture signal; and a memory for storing data used in the gamma correction of the digital picture signal, wherein the conversion circuit includes a plurality of voltage lines for supplying different voltages to source lines of the plurality of TFTs; and wherein the plurality of TFTs, the driver, the control circuit, and the memory are integrally disposed on a same insulating substrate.
17. An electroluminescence display device comprising:
a pixel region in which a plurality of TFTs are arranged in matrix; a driver for switching the plurality of FTFs; a digital picture signal supply source for supplying a digital picture signal; a conversion circuit for converting the digital picture signal to an analog signal; a control circuit for carrying out gamma correction of the digital picture signal; and a memory for storing data used in the gamma correction of the digital picture signal, wherein the conversion circuit includes a plurality of voltage lines for supplying different voltages to source lines of the plurality of TFTs; and wherein the plurality of TFTs, the driver, the control circuit, and the memory are integrally disposed on a same insulating substrate.
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1. Field of the Invention
The present invention relates to a semiconductor device and a semiconductor display device. More specifically, the present invention relates to a semiconductor device in which a nonvolatile memory for storing data and other logic circuits are integrally formed on an insulating substrate by using a SOI (Silicon On Insulator) technique. The present invention also relates to a semiconductor display device in which pixels and peripheral circuits such as a driving circuit and a memory are integrally formed on an insulating substrate by using the SOI technique.
2. Description of the Related Art
In recent years, techniques for manufacturing a semiconductor device, such as a thin film transistor (hereinafter referred to as a TFT), in which a semiconductor thin film is formed on an inexpensive glass substrate, have been rapidly developing. The reason is that the demand for an active matrix type liquid crystal display panel (liquid crystal panel) has increased.
The active matrix type liquid crystal panel is constructed such that a TFT is provided for each of the several tens to several millions of pixel regions arranged in the matrix, and an electric charge going in and out of each pixel electrode is controlled by the switching function of the TFT.
The source line side driver and the gate line side driver include a shift register, a buffer circuit, and the like, and have been integrally formed on the same substrate as an active matrix circuit in recent years.
Thin film transistors using amorphous silicon formed on a glass substrate are used in the active matrix circuit.
It is also known to use a structure in which quartz is used for a substrate and a thin film transistor is formed of a polycrystalline silicon film. In this case, both a peripheral driving circuit and an active matrix circuit are constituted by thin film transistors formed on the quartz substrate.
A technique in which a thin film transistor using a crystalline silicon film is formed on a glass substrate using a technique such as laser annealing is also known. When this technique is used, an active matrix circuit and a peripheral driving circuit can be integrated on a glass substrate.
In the structure as shown in
The picture signal supplied to the source signal line is selected by a thin film transistor of a pixel and is written into the designated pixel electrode.
The thin film transistor of the pixel is operated by a selection signal supplied from a shift register of the gate line side driver (shift register for vertical scanning) through a gate signal line.
This operation is sequentially repeated at a suitable time by signals from the shift register of the source line side driver and signals from the shift register of the gate line side driver, so that information is sequentially written into the respective pixels arranged in the matrix.
In recent years, an active matrix type liquid crystal display device has often been used for a note-sized personal computer. In the personal computer, a multi-gradation liquid crystal display device is required to realize such functions that plural pieces of software are concurrently started up or a picture from a digital camera is taken in and is processed.
Moreover, the demand for a liquid crystal projector which can project a television signal such as a high-definition television signal and can meet a large screen, has increased. In this case as well, the quality of the supplied picture depends on the degree of fineness of the gradation display.
Like this, for the purpose of providing a high quality picture, it is important to what degree the gradation display can be made fine. As a method of gradation display, there is the example (analog gradation) of supplying an analog signal such as a video signal or a television signal to a source line and (digital gradation) of supplying a digital signal such as a data signal from a personal computer or the like.
In the analog gradation, as described above, an analog picture signal to be supplied to the picture signal line is sequentially selected by a signal from the source driver, and the designated picture signal is supplied to the corresponding source line.
In the digital gradation, a digital signal to be supplied to the picture signal line is sequentially selected, and after the selected signal is D/A converted, the designated picture signal is supplied to the corresponding source line.
In the case of the liquid crystal display device, even when any gradation display is used, there is a relation between the voltage (V) applied to a liquid crystal and the strength of transmitted light as indicated by a dotted line in FIG. 15. However, it is assumed that the liquid crystal display device uses a TN (twisted nematic) mode and a normally white mode in which the device is in a light state when a voltage is not applied.
As is understood from
In order to compensate for the above, a means such as gamma correction is adopted. In gamma correction, a picture signal is gained, and correction is made so that the strength of transmission light is linearly changed according to an applied voltage. By this gamma correction, excellent gradation display can be obtained. The relation between the applied voltage and the strength of the transmitted light in the case where the gamma correction is carried out is indicated by a solid line in FIG. 15.
However, in order to apply the gamma correction to a picture signal, an IC circuit is additionally required so that a circuit must be provided on the outside of the liquid crystal panel. Thus, it has been impossible to miniaturize a product.
In view of the above, an object of the present invention is to provide a semiconductor display device, particularly a liquid crystal display device, which is able to carry out excellent gradation display and which is able to be miniaturized.
According to an aspect of the present invention, a semiconductor device comprises a memory for storing data, and a logic circuit for controlling the data, wherein the memory and the logic circuit are constituted by TFTs, and are integrally formed on the same insulating substrate. The above object is achieved by this structure.
The memory may be a nonvolatile memory.
The nonvolatile memory may include a plurality of FAMOS type TFTs.
The thickness of an active layer of the TFT may be 10 to 100 nm.
According to another aspect of the present invention, a semiconductor device comprises a memory for storing data, and a logic circuit for controlling the data, wherein the memory and the logic circuit are constituted by TFTs, and are integrally formed on the same insulating substrate, and wherein the thickness of an active layer of the TFT is 10 to 100 nm so that it becomes easy to carry out impact ionization. The above object is achieved by this structure.
The memory may be a nonvolatile memory.
The nonvolatile memory may include a plurality of FAMOS type TFTs.
According to still another aspect of the present invention, a semiconductor device comprises a control circuit for carrying out gamma correction of a supplied signal, and a memory for storing data used in the gamma correction, wherein the control circuit and the memory are constituted by TFTs, and are integrally formed on the same insulating substrate. The above object is achieved by this structure.
The memory may be a nonvolatile memory.
The nonvolatile memory may include a plurality of FAMOS type TFTs.
The signal may be a digital signal.
The signal may be an analog signal and the semiconductor device may further comprise a conversion circuit for converting the analog signal to a digital signal.
According to still another aspect of the present invention, a semiconductor display device comprises a pixel region where a plurality of TFTs are arranged in matrix; a driver for switching the plurality of TFTs; a picture signal supply source for supplying a picture signal; a control circuit for carrying out gamma correction of the picture signal; and a memory for storing data used in the gamma correction of the picture signal, wherein the plurality of TFTs, the driver, the control circuit, and the memory are integrally formed on the same insulating substrate. The above object is achieved by this structure.
The memory may be a nonvolatile memory.
The nonvolatile memory may include a plurality of FAMOS type TFTs.
The picture signal may be a digital signal.
The picture signal may be an analog signal, and the semiconductor display device may further comprise a conversion circuit for converting the analog signal to a digital signal.
The thickness of an active layer of the TFT may be 10 to 100 nm.
According to still another aspect of the present invention, a semiconductor display device comprises a pixel region where a plurality of TFTs are arranged in matrix; a driver for switching the plurality of TFTS; a digital picture signal supply source for supplying a digital picture signal; a conversion circuit for converting the digital picture signal to an analog signal; a control circuit for carrying out gamma correction of the digital picture signal; and a memory for storing data used in the gamma correction of the digital picture signal, wherein the conversion circuit includes a plurality of voltage lines for supplying different voltages to source lines of the plurality of TFTs, and wherein the plurality of TFTs, the driver, the control circuit, and the memory are integrally formed on the same insulating substrate. The above object is achieved by this structure.
The memory may be a nonvolatile memory.
The nonvolatile memory may include a plurality of FAMOS type TFTs.
The thickness of an active layer of the TFT may be 10 to 100 nm.
In the accompanying drawings:
Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.
Embodiment 1
In this embodiment, a semiconductor device, in which a memory for storing data is integrally formed on an insulating substrate by using a SOI (Silicon On Insulator) technique, will be described. Various semiconductor devices, especially a liquid crystal display device, will be described. More specifically, a liquid crystal display device, in which the above technique is used for a memory for storing data used in gamma correction of picture data will be described.
First, the inventors of the present invention confirmed a memory effect of a P-channel TFT described in this embodiment (FIGS. 20A and 20B). As shown in
Next, when the characteristics were measured in this state, the result was as shown in FIG. 20B. The measurement result of an initial state is shown in FIG. 20A. From these graphs, the variation of a threshold value of a TFT was found.
The inventors of the present invention provided a floating gate to this P-channel TFT to develop a memory.
Reference will be made to FIG. 1.
Reference numeral 104 denotes a gamma correction circuit, and 105 denotes a 4k-bit memory. Based on gamma correction data stored in the memory 105, the gamma correction control circuit carries out gamma correction of a digital picture signal supplied from the A/D conversion circuit 102 or the digital picture signal supply source, and transmits the corrected signal to a source signal side driver 106. In this embodiment, although the 4K-bit memory is used as the memory 105, the storage capacity of the memory 105 is not limited to this value. A memory having storage capacitance of not larger than 4 k bits or not less than 4K bits may be used as the memory 105.
The source signal line side driver 106 is constituted by a shift register, a buffer, a digital decoder, a D/A converter, and the like. A gate signal line side driver 107 is constituted by a shift register, a buffer, and the like. The source signal line side driver 106 and the gate signal line side driver 107 are provided with other circuits as necessary.
Reference numeral 108 denotes a pixel region which is constituted by a plurality of thin film transistors (TFTs) arranged in a matrix. The pixel region 108 is also called a pixel matrix circuit. In this embodiment, the number of pixels was made 1024×768 in length and breadth. Although explanation will be made with respect to a liquid crystal display device having the foregoing number of pixels in this embodiment, the present invention is not limited to a liquid crystal display device having the foregoing number of pixels.
In the liquid crystal display device of this embodiment, any of the pixel region 108, the source signal line side driver 106, the gate signal line side driver 107, the gamma correction control circuit 104, and the memory 105 are constituted by TFTs, and are integrally formed on a substrate. The A/D conversion circuit 102 may be mounted as an IC chip on the substrate, or may be integrally formed by TFTs on the substrate. Other peripheral circuits may also be integrally formed by TFTs on the substrate. Moreover, the other peripheral circuits may be mounted as an IC chip on the substrate.
Next, the operation of the liquid crystal display device of this embodiment will be described. Reference will be made to FIG. 2.
Both ends of each of the memory elements arranged in the respective columns are connected to signal lines A0, B0 to A63, B63. Gate electrodes of the respective memory elements arranged in the respective rows are connected to signal lines C0, D0 to C63, D63. As shown in
The respective signal lines A0, B0 to A63, B63, and C0, D0 to 063, D63 are connected to th e X-address decoder 201 and the Y-address decoder 202, respectively. The address of the memory element is specified by the X-address decoder 201 and the Y-address decoder 202, and writing or reading of data is carried out.
Next, the operation of the memory 105 will be described with reference to a memory element (1, 1) as an example.
First, in the case where data are written into the memory element (1, 1), a high voltage of 50 V is applied to the signal line C1. Further, a voltage of 5 V is also applied to the signal line D1. When the signal line 31 is connected to the GND, and a voltage of -5 V is applied to the signal line A1, an electric charge is stored in the floating gate of the TFT Tr1.
Next, in the case where data are read from the memory element (1, 1), a voltage of 0 V is applied to the signal line C1, and a voltage of 5 V is applied to the signal line D1. When the signal line B1 is connected to the GND, a stored signal is read from the signal line A1.
The above operation is summarized in the following table.
| TABLE 1 | |||||
| A1 (V) | B1 (V) | C1 (V) | D1 (V) | ||
| At writing | 0/-5 | GND | 50 | 5 | |
| At reading | -- | GND | 0 | 5 | |
Incidentally, the storage content stored in the memory element can be erased by irradiation of X-rays, ultraviolet rays, electron beams, or the like to the memory 105.
The memory 105 stores data for gamma correction of a digital picture signal. This is data intrinsic to the liquid crystal display device, and is written into the memory 105 at the production.
Next, reference will be made to FIG. 3.
A corresponding pixel TFT 308 is selected by a signal voltage corresponding to a designated gradation supplied to a source signal line and a signal from the gate signal line side shift register 306. In this way, picture information corresponding to the designated gradation is written into the respective pixels.
Next, manufacturing steps for the liquid crystal display device of this embodiment will be described.
In this embodiment, an example, in which a plurality of TFTs are formed on a substrate having an insulating surface, and a matrix circuit of the pixel region and a peripheral circuit including a driver circuit are monolithically formed, will be described with reference to
Reference will be made to
Reference numeral 402 denotes an amorphous silicon film. Adjustment is made so that the final film thickness (film thickness takes the film decrease after thermal oxidation into consideration) becomes 10 to 100 nm (preferably 15 to 45 nm). During film formation, it is important to thoroughly manage the concentration of impurities in the film. When the film thickness is adjusted to 10 to 100 nm, it is possible to facilitate the impact ionization to a FAMOS type TFT constituting a memory element, to lower the applied voltage, and to facilitate injection of electric charges.
In this embodiment, management is done so that the concentration of each of C (carbon), N (nitrogen), O (oxygen), and S (sulfur), which are typical impurities in the amorphous silicon film 402, becomes less than 5×1018 atoms/cm3 (preferably not larger than 1×1018 atoms/cm3). This is preferred because if the concentration of any one of the impurities exceeds the above value, the impurity may have a bad influence on the film at crystallization and may cause the film quality to be degraded after the crystallization.
The concentration of hydrogen in the amorphous silicon film 402 is also a very important parameter, and it appears that as the hydrogen content is made low, a film with superior crystallinity is obtained. Thus, it is preferable to form the amorphous silicon film 402 by a low pressure CVD method. A plasma CVD method may be used if the film formation condition is optimized.
Next, the amorphous silicon film 402 is crystallized. A technique disclosed in Japanese Patent Unexamined Publication No. Hei. 7-130652 is used as a means for crystallization. Although any means of embodiment 1 or embodiment 2 disclosed in the publication may be used, in this embodiment, it is preferable to use the technical content (described in detail in Japanese Patent Unexamined Publication No. Hei. 8-78329) set forth in embodiment 2 of the publication.
According to the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-78329, a mask insulating film 403 for selecting an added region of a catalytic element is first formed. The mask insulating film 403 has a plurality of openings for adding the catalytic element. The positions of the openings can determine the positions of the crystal regions.
A solution containing nickel (Ni) as the catalytic element for promoting the crystallization of the amorphous silicon film is applied by a spin coating method to form a Ni containing layer 404. As the catalytic element, cobalt (Co), iron (Fe), palladium (Pd), platinum (Pt), copper (Cu), gold (Au), germanium (Ge), or the like may be used instead of nickel (FIG. 4A).
As the adding step for the above-mentioned catalytic element, an ion implantation method or a plasma doping method using a resist mask may also be used. In this case, since it becomes easy to decrease the occupied area of an added region and to control the growth distance of a lateral growth region, the method is an effective technique when a minute circuit is formed.
Next, after the adding step of the catalytic element is concluded, dehydrogenating is carried out at about 450°C C. for 1 hour. Then, a heat treatment is carried out in an inert gas atmosphere, a hydrogen atmosphere, or an oxygen atmosphere at a temperature of 500 to 700°C C. (typically 550 to 650°C C.) for 4 to 24 hours to crystallize the amorphous silicon film 402. In this embodiment, the heat treatment is carried out in a nitrogen atmosphere at 570°C C. for 14 hours.
At this time, crystallization of the amorphous silicon film 402 proceeds first from nuclei produced in regions 405 and 406 added with nickel, and crystal regions 407 and 408 grown almost parallel to the surface of the substrate 401 are formed. The crystal regions 407 and 408 are respectively referred to as a lateral growth region. The lateral growth region has such an advantage that the total crystallinity is superior since respective crystals are gathered in a comparatively uniform state (FIG. 4B).
Incidentally, even in the case where the technique set forth in embodiment 1 of the above-mentioned Japanese Patent Unexamined Publication No. Hei. 7-130652 is used, a region which can be called a lateral growth region is microscopically formed. However, since the production of nuclei occurs irregularly on the surface, it is difficult to control crystal grain boundaries.
After the heat treatment for crystallization is ended, the mask insulating film 403 is removed and patterning is carried out, so that island-like semiconductor layers (active layers) 409, 410, and 411 made of the lateral growth regions 407 and 408, are formed (FIG. 4C).
Reference numeral 409 denotes the active layer of a P-type TFT constituting a CMOS circuit, while 410 denotes the active layer for an N-type TFT constituting the CMOS circuit, and 411 denotes the active layer for an N-type TFT (pixel TFT) for constituting a pixel matrix circuit.
After the active layers 409, 410 and 411 are formed, a gate insulating film 412, made of an insulating film containing silicon, is formed thereon.
Next, as shown in
In order to sufficiently obtain the gettering effect by the halogen element, it is preferable to carry out the above heat treatment at a temperature exceeding 700°C C. If the temperature is not higher than 700°C C., it becomes difficult to decompose a halogen compound in the processing atmosphere, so that there is a fear that the gettering effect can not be obtained.
Thus, in this embodiment, the heat treatment is carried out at a temperature exceeding 700°C C., preferably 800 to 1000°C C. (typically 950°C C.), and a processing time is 0.1 to 6 hours, typically 0.5 to 1 hour.
In this embodiment, an example is shown in which a heat treatment is carried out in an oxygen atmosphere containing hydrogen chlorine (HCl) of 0.5 to 10 vol % (in this embodiment, 3 vol %) at 950°C C. for 30 minutes. If the concentration of HCl is higher than the above-mentioned concentration, roughness comparable to a film thickness is produced on the surfaces of the active layers 409, 410 and 411. Thus, such a high concentration is not preferred.
Although an example is shown in which the HCl gas is used as a compound containing a halogen element, one or more plural kinds of the gases selected from compounds containing halogen, such as HF, NF3, HBr, Cl2, ClF3, BCl3, F2, and Br2, may be used other than the HCl gas.
In this step, it is conceivable that nickel is removed in such a manner that nickel in the active layers 409, 410 and 411 is gettered by the action of chlorine and is transformed into volatile nickel chloride which is released into the air. By this step, the concentration of nickel in the active layers 409, 410 and 411 is lowered to 5×1017 atoms/cm3 or less.
Incidentally, the value of 5×1017 atoms/cm3 is the lower limit of detection for SIMS (Secondary Ion Mass Spectroscopy). As a result of analysis of TFTs experimentally produced by the inventors of the present application, it was discovered that when the concentration is not higher than 1×1018 atoms/cm3 (preferably 5×1017 atoms/cm3 or less), the influence of nickel upon the TFT characteristics can not be seen. However, it should be noted that the concentration of an impurity in the present specification is defined as a minimum value in measurement results of the SIMS analysis.
Moreover, by the above heat treatment, a thermal oxidation reaction proceeds at the interface between the gate insulating film 412 and the active layers 409, 410 and 411, so that the thickness of the gate insulating film 412 is increased by the thickness of a thermal oxidation film. When the thermal oxidation film is formed in this way, it is possible to obtain an interface of a semiconductor/insulating film which has very few interfacial levels. Moreover, there is also the effect of preventing inferior formation (edge thinning) of a thermal oxidation film at the end of the active layer.
Further, it is also effective that after the heat treatment in the above-mentioned halogen atmosphere is carried out, a heat treatment approximately at 950°C C. for one hour is carried out in a nitrogen atmosphere to improve the film quality of the gate insulating film 412.
Incidentally, it is also confirmed by the SIMS analysis that the halogen element, which was used for the gettering process, having a concentration of 1×1015 to 1×1020 atoms/cm3 remains in the active layers 409, 410 and 411. Moreover, it is confirmed by the SIMS analysis that at that time, the foregoing halogen element with a high concentration distributes between the active layers 409, 410 and 411 and the thermal oxidation film formed by the heat treatment.
As a result of a SIMS analysis for other elements, it was confirmed that the concentration of any of C (carbon), N (nitrogen), O (oxygen), and S (sulfur) as typical impurities was less than 5×1018 atoms/cm3 (typically 1×1018 atoms/cm3 or less).
Next, reference will be made to FIGS. 5A and SD. A not-shown metal film mainly composed of aluminum is formed, and originals 413, 414 and 415 for subsequent gate electrodes are formed by patterning. In this embodiment, an aluminum film containing 2 wt % scandium is used (FIG. 5A). The original 413 subsequently becomes a floating gate of a P-channel FAMOS type TFT.
Next, by the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 7-135318, porous anodic oxidation films 416, 417 and 418, nonporous anodic oxidation films 419, 420 and 421, and gate electrodes 422, 423 and 424 are formed (FIG. 5B).
After the state shown in
Next, the gate electrode is divided into parts to form a floating gate.
Next, reference will be made to
In this embodiment, the addition of impurity is divided and is carried out two times. The first impurity addition (P (phosphorus) is used in this embodiment) is carried out at a high acceleration voltage of about 80 KeV to form a n- region. Adjustment is made so that the concentration of p ion impurity in the n- region becomes 1×1018 to 1×1019 atoms/cm3.
Further, the second impurity addition is carried out at a low acceleration voltage of about 10 KeV to form a n+ region. Since the acceleration voltage is low at this time, the gate insulating film functions as a mask. Adjustment is made so that the sheet resistance of the n+ region becomes 500 Ω or less (preferably 300 Ω or less).
Through the above described steps, a source region 428, a drain region 429, a low concentration impurity region 430, and a channel formation region 431 of the N-type TFT constituting the CMOS circuit are formed. Moreover, a source region 432, a drain region 433, a low concentration impurity region 434, and a channel formation region 435 of the N-type TFT constituting the pixel TFT are defined (FIG. 6A).
In the state shown in
Next, as shown in
Although this step is also divided and is carried out twice like the foregoing adding step of the impurity, since the N type must be inverted to the P type, a B (boron) ion, with a concentration several times that of the foregoing added concentration of the P ion, is added.
In this way, a source region 437, a drain region 438, a low concentration impurity region 439, and a channel formation region 440 of the P-type TFT constituting the CMOS circuit are formed (FIG. 6B).
After the active layer is completed in the manner described above, activation of the impurity ions is made by a combination of furnace annealing, laser annealing, lamp annealing, and the like. At the same time, damage to the active layer caused in the adding steps also is repaired.
Next, as an interlayer insulating film 441, a lamination film of a silicon oxide film and a silicon nitride film are formed (FIG. 6C). Next, contact holes are formed in the interlayer insulating film 441, and then, source electrodes 442, 443 and 444, drain electrodes 445 and 446, and a gate electrode 447 are formed to obtain the state shown in FIG. 6D.
Next, reference will be made to
Next, a silicon nitride film 450 with a thickness of 10 to 50 nm and a black mask 449 are formed (FIG. 7A).
Next, a third interlayer insulating film 450 made of one of a silicon oxide film, a silicon nitride film, an organic film, or a lamination film of these and having a thickness of 0.1 to 0.3 μm is formed. After a contact hole is formed in the interlayer insulating film 450, a formed conductive film is patterned to form a pixel electrode 451. Since this embodiment relates to a transmission type, a transparent conductive film such as an ITO is used as a conductive film constituting the pixel electrode 451.
In the structure shown in
In the structure as shown in
Next, the entire substrate is heated in a hydrogen atmosphere at a temperature of 350°C C. for 1 to 2 hours to hydrogenate the entire device, so that the dangling bonds (unpaired bonds) in the film (especially in the active layer) are compensated. Through the above steps, it is possible to manufacture the CMOS circuit and the pixel matrix circuit on the same substrate.
Next, as shown in
An oriented film 452 is formed on the active matrix substrate in the state of FIG. 7A. In this embodiment, polyimide is used for the oriented film 452. Next, an opposite substrate is prepared. The opposite substrate is constituted by a glass substrate 453, a transparent conductive film 454, and an oriented film 455.
In this embodiment, the polyimide film in which liquid crystal molecules are vertically oriented to the substrate, is used as the oriented film. Incidentally, after the oriented film is formed, a rubbing process is carried out so that the liquid crystal molecules are vertically oriented with a fixed pretilt angle.
Although a black mask, a color filter and the like are formed on the opposite substrate according to necessity, they are omitted here.
Next, the active matrix substrate and the opposite substrate obtained through the above steps are bonded to each other by a known cell fabricating process through a sealing material or a spacer (not shown). Thereafter, a liquid crystal material 456 is injected between both the substrate and is completely sealed with a sealing agent (not shown). Thus, the transmission type liquid crystal panel as shown in
In this embodiment, the liquid crystal panel is designed to make the display with a TN mode. Thus, a pair of polarizing plates (not shown) are disposed so that the liquid crystal panel is held between the polarizing plates in cross Nicol (state in which polarizing axes of a pair of polarizing plates are orthogonal to each other).
Thus, it is understood that in this embodiment, the display is made in a normally white mode in which the liquid crystal panel is in a light state when a voltage is not applied thereto.
Although the logic circuit 805 includes all logical circuits constituted by TFTs in a wide sense, in order to distinguish the logic circuit from such a circuit as is conventionally called a pixel matrix circuit or a driver circuit, the logic circuit in the present specification indicates signal processing circuits other than such a circuit.
Even in the case where Si is used for a floating gate of a FAMOS type TFT used in a memory, the memory has the same structure as a peripheral circuit and a logic circuit, and the present invention can be applied to such a case.
In this embodiment, although explanation has been made to the case where the memory including the FAMOS type TFT is used, other types of TFT may be used for the memory.
A FPC (Flexible Print Circuit) terminal is attached to the liquid crystal panel formed in this way. In general, what is called a liquid crystal module is a liquid crystal panel in the state where the FPC is attached.
In
As so, in this embodiment, the gamma correction control circuit and the memory for storing gamma correction data are integrally formed on the substrate. Thus, it is possible to miniaturize the liquid crystal display device.
It is effective to add an impurity element (an element in group 13, typically boron or an element in group 15, typically phosphorus) for controlling the threshold voltage (Vth) of a TFT. The amount of the addition must be determined in view of Vth in the case where the above impurity for Vth control is not added.
In this embodiment, although the liquid crystal display device among semiconductor devices equipped with the memory of the present invention has been described, the FAMOS type memory of the present invention can be used for any semiconductor device which requires a memory.
Here, a semiconductor thin film manufactured by a manufacturing method of this embodiment will be described. According to the manufacturing method of this embodiment, it is possible to obtain a crystalline silicon film called by the present applicant "continuous grain boundary crystalline silicon so-called Continuous Grain Silicon: CGS)" by crystallizing an amorphous silicon film.
The lateral growth region of a semiconductor thin film obtained by the manufacturing method of this embodiment shows a specific crystal structure composed of a collective of rod-like or flattened rod-like crystals. The features will be described below.
[Findings as to Crystal Structure of an Active Layer]
The lateral growth region formed in accordance with the foregoing manufacturing method has microscopically a crystal structure in which a plurality of rod-like (or flattened rod-like) crystals are arranged almost parallel to each other and with regularity in a specific direction. This can be easily confirmed by observation with a TEM (Transmission Electron Microscope).
The inventors of the present invention magnified the crystal grain boundaries of the semiconductor thin film obtained by the foregoing manufacturing method eight million times by using an HR-TEM (High Resolution Transmission Electron Microscope) and observed them in detail (FIG. 21A). In this specification, the crystal grain boundary is defined as a grain boundary formed at an interface where different rod-like crystals are in contact with each other, unless specified otherwise. Thus, the crystal grin boundary is regarded as different from, for example, a macroscopic grain boundary formed by collision of separate lateral growth regions.
The foregoing HR-TEM (High Resolution Transmission Electron Microscope) is a method in which a sample is vertically irradiated with an electron beam and the arrangement of atoms and molecules is estimated by using interference of transmission electrons or elastically scattered electrons. By using this method, it is possible to observe the state of arrangement of crystal lattices as lattice stripes. Thus, by observing the crystal grain boundary, it is possible to infer the bonding state of atoms at the crystal grain boundary.
In the TEM photograph (
In the observation of lattice stripes by the TEM photograph as described above, lattice stripes corresponding to a {111} plane are observed in the {110} plane. The lattice stripe corresponding to the {111} plane indicates such a lattice stripe that when crystal grain is cut along the lattice stripe, the {111} plane appears in the section. It is possible to simply confirm by the distance between the lattice stripes the plane to which the lattice stripe corresponds.
At this time, the inventors of the present invention observed in detail the TEM photograph of the semiconductor thin film obtained through the foregoing manufacturing method, and as a result, very interesting findings were obtained. In any of the two different crystal grains seen in the photograph, lattice stripes corresponding to the {111} plane were seen. And it was observed that the lattice stripes were clearly parallel to each other.
Further, irrespective of the existence of the crystal grain boundary, lattice stripes of the two different crystal grains were connected to each other so as to cross the crystal grain boundary. That is, it was confirmed that almost all lattice stripes observed to cross the crystal grain boundary were linearly continuous in spite of the fact that they were lattice stripes of different crystal grains. This is the case with any crystal grain boundary, and 90% or more (typically 95% or more) of lattice stripes in total keep continuity at the crystal grain boundary.
Such a crystal structure (more precisely the structure of crystal grain boundary) indicates that two different crystal grains are in contact with each other with excellent conformity in the crystal grain boundary. That is, the crystal lattices are continuously connected to each other in the crystal grain boundary, so that such a structure is formed that trap levels caused by crystal defects or the like are not easily formed. In other words, it can be said that the crystal lattices are continuous in the crystal grain boundary.
In
The inventors of the present invention refer to the bonding state of atoms in the case where the lattice stripes correspond to each other with good conformity, like the semiconductor thin film obtained by the foregoing manufacturing method, as conformity bonding, and refer to a bond at that time as a conformity bond. In contrast, the inventors of the present invention refer to the bonding state of atoms in the case where the lattice stripes do not correspond to each other with good conformity often seen in a conventional polycrystalline silicon film as unconformity bonding, and refer to a bond at that time as an unconformity bond (or unpaired bond).
Since the semiconductor thin film used in the present invention is particularly excellent in conformity at the crystal grain, the foregoing unconformity bonds are very few. As a result of study for arbitrary plural crystal grain boundaries conducted by the inventors of the present invention, the existing ratio of the unconformity bonds to the total bonds was 10% or less (preferably 5% or less, more preferably 3% or less). That is, 90% or more of the total bonds (preferably 95% or more, more preferably 97% or more) are constituted by the conformity bonds.
In
In the case of
Although the respective spots have small concentric expanses, it is presumed that this is caused from a distribution of rotation angles of some degree. The degree of the expansion is within 5°C from the evaluation of the pattern.
Among many observations, there was a case where the diffraction spots were partially not seen (a part of the diffraction spots is also not seen in FIG. 22A). It appears that although the crystal has substantially the {110} orientation, since crystal axes are slightly shifted, the diffraction pattern becomes unobservable.
Based on the fact that the {111} plane is almost always included in a crystal plane, the inventors of the present invention presume that the shift of rotation angles around the <111> axis causes such a phenomenon.
On the other hand, in the case of the electron beam diffraction pattern shown in
As shown from these results, the feature of the crystalline silicon film of the present invention is that almost all crystal grains are oriented roughly in the {110} plane, and the lattice has continuity in the crystal grain boundary. These features are not seen in a conventional polysilicon film.
As described above, the semiconductor thin film manufactured by the foregoing manufacturing method is a semiconductor thin film having a crystal structure (more precisely a structure of a crystal grain boundary ) quite different from a conventional semiconductor thin film. The inventors after present invention have explained the result of analysis as to the semiconductor thin film used in the present invention in Japanese Patent Application Nos. Hei. 9-55633, Hei. 9-165216 and Hei. 9-212428 as well.
The inventors of the present invention performed X-ray diffraction in accordance with a method disclosed in Japanese Patent Unexamined Publication No. Hei. 7-321339, and calculated the ratio of orientation with respect to the crystalline silicon film manufactured by the foregoing manufacturing method. In the publication, the ratio of orientation is defined with the calculation method as indicated by the following expression 1:
{111} orientation existence ratio=(relative strength of {111} to {220} of a sample)/(relative strength of {111} to {220} of powder),
{311} orientation existence ratio=(relative strength of {311} to {220} of a sample)/(relative strength of {311} to {220} of powder), and
{220} orientation ratio=({220} orientation existence ratio)/({220} orientation existence ratio+{111} orientation existence ratio+{311} orientation existence ratio).
As has been described above, it is understood that the crystalline silicon film formed by the manufacturing method of this embodiment has a crystal structure (crystal constitution) quite different from a conventional polysilicon film. Also from this point, it may be said that the crystalline silicon film of the present invention is a quite novel semiconductor film.
In the formation of the foregoing semiconductor thin film, an annealing step at a temperature above a crystallizing temperature plays an important role with respect to lowering defects in a crystal grain. This is described below.
Although such defects are mainly lamination defects in which the order of lamination of atoms on a silicon crystal lattice plane is discrepant, there is also a case of dislocation. It appears that
On the other hand, as shown in
That is, in the crystalline silicon film shown in
Like this, in the crystalline silicon films shown in the photographs of
The thus obtained crystalline silicon film (
The difference in the number of defects appears as the difference in spin density by the analysis of ESR (Electron Spin Resonance). In the present circumstances, it is ascertained that the spin density of the crystalline silicon film by the manufacturing method of this embodiment is at most 5×1017 spins/cm3 (preferably 3×1017 spins/cm3 or less). However, since this measurement value is near the detection limit of an existing measuring device, it is expected that the actual spin density is lower than this value.
Applicant now refers to the foregoing crystalline silicon film having the above described crystal structure and the features as a continuous grain boundary crystalline silicon (Continuous Grain Silicon: CGS).
In a conventional semiconductor thin film, although a crystal grain boundary serves as a barrier for blocking the movement of carriers, since such a crystal grain boundary does not substantially exist in the semiconductor thin film according to the manufacturing method of this embodiment, high carrier mobility can be realized. Thus, the electrical characteristics of a TFT manufactured by using the semiconductor thin film according to the manufacturing method of this embodiment have excellent values. This will be described below.
[Findings as to Electrical Characteristics of a TFT]
Since the semiconductor thin film according to the manufacturing method of this embodiment can be regarded substantially as a single crystal (crystal grain boundaries do not exist substantially), a TFT using the semiconductor thin film as an active layer shows electrical characteristics comparable with a MOSFET using a single crystal silicon. Data as shown below are obtained from TFTs experimentally formed by the present inventors.
(1) The subthreshold coefficient as an index showing switching performance (promptness in switching of on/off operation) of a TFT is as small as 60 to 100 mV/decade (typically 60 to 85 mV/decade) for both a N-channel TFT and a P-channel TFT.
(2) The field effect mobility (μFE) as an index showing an operation speed of a TFT is as large as 200 to 650 cm2/Vs (typically 250 to 300 cm2/Vs) for a N-channel TFT and 100 to 300 cm2/Vs (typically 150 to 200 cm2/Vs) for a P-channel TFT.
(3) The threshold voltage (Vth) as an index indicating a driving voltage of a TFT is as small as -0.5 to 1.5 V for a N-channel TFT and -1.5 to 0.5 V for a P-channel TFT.
As described above, it is confirmed that the TFT obtained in the present invention can realize highly superior switching characteristics and high speed operation characteristics.
Incidentally, in the formation of the CGS, the foregoing annealing step at a temperature above the crystallizing temperature (700 to 1100°C) plays an important role with respect to lowering defects in the crystal grain. This will be described below.
From the above, it is understood that the gettering process of a catalytic element is an indispensable step in the formation of the CGS. The inventors of the present invention consider the following model for a phenomenon occurring in this step.
First, in the state shown in
However, when Ni, existing in the defects, is removed by carrying out the gettering process of the catalytic element, the bond of Si--Ni is cut. Thus, the remaining bond of silicon immediately forms a Si--Si bond and becomes stable. In this way, the defects disappear.
It is known that the defects in a crystalline silicon film disappear by thermal annealing at a high temperature. Further, since bonds with nickel are cut and many unpaired bonds are generated, the recombination of silicon is smoothly carried out.
The inventors of the present invention also contemplate a model in which the crystalline silicon film is bonded to its under layer by a heat treatment at a temperature (700 to 1100°C C.) above the crystallizing temperature, and adhesiveness is increased, so that the defects disappear.
[Findings as to the Relation Between TFT Characteristics and CGS]
The above described excellent TFT characteristics mainly depend on the use of the semiconductor thin film having continuity of crystal lattices in the crystal grain boundary as an active layer of the TFT. The reason will be discussed below.
The continuity of crystal lattices in the crystal grain boundary is caused from the fact that the crystal grain boundary is a grain boundary called "plane grain boundary". The definition of the plane grain boundary in the present specification is given as "Planar boundary" set forth in "Characterization of High-Efficiency Cast-Si Solar Cell Wafers by MBIC Measurement; Ryuichi Shimokawa and Yutaka Hayashi, Japanese Journal of Applied Physics, vol. 27, No. 5, pp. 751-758, 1988".
According to the above paper, the planar boundary includes a {111} twin boundary, {111} lamination defect, {221} twin boundary, {221} twist boundary and the like. This planar boundary has the feature that it is electrically inactive. That is, since the boundary does not function as a trap for blocking the movement of carriers though it is a crystal grain boundary, the boundary can be regarded as substantially not existing.
More specifically, the {111} twin boundary is also called a corresponding boundary of Σ3, and the {221} twin boundary is called a corresponding boundary of Σ9. The Σ value is a parameter which becomes an index showing the degree of conformity of a corresponding boundary. It is known that when the Σ value is small, the conformity of the boundary is superior.
As a result of the observation of a semiconductor thin film according to the manufacturing method of this embodiment conducted in detail by the present inventors, it was found that almost all crystal grain boundaries (90% or more, typically 95% or more) are the corresponding boundaries of Σ3, i.e., the {111} twin boundaries.
In the crystal grain boundary formed between two crystal grains, when the plane orientations of both crystals are {110}, and if an angle formed by lattice stripes corresponding to the {111} plane is θ, it is known that when θ is 70.5°C, the boundary becomes the corresponding boundary of Σ3.
Thus, in the crystal grain boundary shown in the TEM photograph of
Incidentally, when θ is 38.9°C, the boundary becomes the corresponding boundary of Σ9. Such other crystal grain boundary also existed.
Such a corresponding boundary is formed only between crystal grains of the same plane orientation. That is, since the plane orientation of the semiconductor thin film of the present invention is uniform roughly as {110}, such a corresponding boundary can be formed over a wide range. This feature can not be obtained by other polysilicon films in which the plane orientation is irregular.
The remarkable feature in
On the other hand,
By repeating observation and measurement over a number of regions other than the measured points shown in
Embodiment 2
In this embodiment, a liquid crystal display device, in which an analog picture signal supplied from an analog picture signal supply source is directly subjected to gamma correction and analog gradation can be realized, will be described.
Reference will be made to FIG. 10. Reference numeral 1001 denotes an analog signal supply source for supplying an analog picture signal, such as a video signal or a television signal. Reference numeral 1002 denotes a gamma correction control circuit for gamma correcting the analog signal supplied from the analog picture signal supply source 1001. Reference numeral 1003 denotes a D/A conversion circuit, and 1004 denotes a memory. The memory 1004 is similar to that in Embodiment 1. Reference numeral 1005 denotes a source signal line side driver, and 1006 denotes a gate signal line side driver. Reference numeral 1007 denotes a pixel region which is constituted by a plurality of thin film transistors (TFTs) arranged in a matrix. The pixel region 1007 is also called a pixel matrix circuit. In this embodiment, the number of pixels is made 1024×768 in length and breadth. In this embodiment, although the liquid crystal display device having the foregoing number of pixels will be described, the present invention is not limited to a liquid crystal display device having the foregoing number of pixels.
In the liquid crystal display device of this embodiment, any of the pixel region 1007, the source signal line side driver 1005, the gate signal line side driver 1006, the gamma correction control circuit 1002, the D/A conversion circuit 1003, and the memory 1004 are constituted by TFTs and are integrally formed on a substrate. The D/A conversion circuit 1003 may be mounted as an IC chip on the substrate, or may be integrally formed by TFTs on the substrate. Other peripheral circuits can be integrally formed by TFTs on the substrate. Moreover, the other peripheral circuits may be mounted as an IC chip on the substrate.
An analog picture signal supplied from the analog picture signal supply source 1001 is supplied to the gamma correction control circuit 1002. The memory 1004 is a 4-bit memory, and 1003 is an A/D conversion circuit. The gamma correction control circuit 1002 corrects the analog picture signal supplied from the analog picture signal supply source 1001 based on the gamma correction data stored in the memory 1004 and transmits the corrected signal to the source signal line side driver 1005. The data for gamma correction stored in the memory 1004 are converted into analog signals by the D/A conversion circuit 1003 and are transmitted to the gamma correction control circuit 1002.
In the gamma correction control circuit 1002, an analog signal is processed, a picture signal is subjected to gamma correction, and the analog picture signal subjected to the gamma correction is supplied to the source signal line side driver.
The analog signal supplied to the source signal line side driver is selected by a signal from a shift register of the source signal line side driver and is supplied to a source signal line. Then, according to a signal from a shift register of the gate signal line side driver, a desired pixel is turned on.
In this embodiment, the source signal line side driver 1005, the gate signal line side driver 1006, the pixel matrix circuit (pixel region) 1007, the gamma correction control circuit 1002, the D/A conversion circuit 1003, and the memory 1004 are integrally formed by TFTs on the substrate. The D/A conversion circuit 1003 may be mounted on the substrate by means of an IC chip including the D/A conversion circuit. Moreover, as necessary, other peripheral circuits are integrally formed. Further, as necessary, an IC chip including other peripheral circuits, is mounted on the substrate.
The liquid crystal display device of this embodiment can also be manufactured by the manufacturing method of embodiment 1.
In this embodiment, among the semiconductor devices equipped with the memory of the present invention, although the liquid crystal display device has been specifically described, the FAMOS type TFT memory of the present invention may be used in any semiconductor device requiring a memory.
Embodiment 3
In this embodiment, an other embodiment of a liquid crystal display device of digital gradation will be described.
Reference will be made to FIG. 11.
A corresponding pixel TFT 1110 is selected by a signal voltage corresponding to a designated gradation supplied to the source signal line and a signal from a gate signal line side shift register 1108. In this way, the picture information corresponding to the designated gradation is written into the respective pixels.
In this embodiment, the digital signal supplied to the signal line 1102 is not subjected to gamma correction. In this embodiment, equal voltages are not applied to the respective voltage lines DC1 to DC16 to be selected by the switching circuits 1104, but it is designed so that the voltage lines are nonlinearly applied with voltages. By doing so, the picture signal can be subjected to gamma correction.
Reference will be made to FIG. 12.
The gradation voltage control circuit 1105 is constituted by a plurality of TFTs Tr1.1 to Tr15.4 and a plurality of resistors connected to the voltage lines DC1 to DC16 and is designed so that according to a TFT selected by the gamma correction control circuit, voltage applied to the voltage lines DC1 to DC16 is subjected to gamma correction.
The data for gamma correction are stored in the memory 1107, and according to the supplied digital picture signal, necessary data are read. A memory similar to one used in embodiment 1 is used for the memory 1107.
Reference will be made to FIG. 13.
In this embodiment, gamma correction is performed to the gradation voltage applied to the voltage lines DC1 to DC16, so that the voltage line selected by the switching circuit 1104 according to the digital picture signal, can supply a desired gradation voltage to the source signal line.
Although the memory 1107 of this embodiment stores data for gamma correction, the data are stored by the method described in embodiment 1. In this embodiment as well, peripheral circuits such as the memory, gamma correction control circuit, and gradation voltage control circuit are integrally formed on the substrate at the same time as TFTs arranged in matrix in the pixel region and TFTs constituting the driver circuit. The manufacturing method may be the method described in embodiment 1.
In this embodiment, among semiconductor devices equipped with the memory of the present invention, although the liquid crystal display device has been specifically described, the FAMOS type TFT memory of the present invention may be used in any semiconductor device requiring a memory.
Embodiment 4
The liquid crystal display device used in the above embodiments may be included and used in a projection type liquid crystal display device. Also in this case, it is possible to miniaturize the device and make an excellent gradation display.
In this embodiment, among semiconductor devices equipped with the memory of the present invention, although the projection type liquid crystal display device has been specifically described, the FAMOS type TFT memory of the present invention may be used in any semiconductor device requiring a memory.
Embodiment 5
In the foregoing embodiments 1 to 4, although explanation has been made to cases where a liquid crystal is used as a display medium, a mixed layer of a liquid crystal and a high polymer may also be used for the semiconductor display device of the present invention to make a so-called polymer dispersion type liquid crystal display device. Moreover, the present invention may be used for a display device equipped with any display medium in which the optical characteristics can be modulated in response to an applied voltage. For example, the present invention may be applied to a display device equipped with an electroluminescence element as a display medium. Also in this case, the steps described in embodiment 1 are used for manufacturing of an active matrix substrate including a memory, a peripheral circuit and the like.
In this embodiment, among semiconductor devices equipped with the memory of the present invention, although the display device has been specifically described, the FAMOS type TFT memory of the present invention may be used in any semiconductor device requiring a memory.
Embodiment 6
The semiconductor device or semiconductor display device constituted by the nonvolatile memory and the gamma correction circuit of embodiments 1, 2, 3 and 5 have various uses. In this embodiment, these semiconductor devices will be described.
As such semiconductor devices, a video camera, a still camera, a projector, a head mount display, a car navigation system, a personal computer, a portable information terminal (mobile computer, portable telephone, etc.) and the like are enumerated.
Embodiment 7
In this embodiment, in the manufacturing steps described in embodiment 1, a case where Ta (tantalum) or Ta alloy is used for the gate electrode will be described.
If Ta or Ta alloy is used for the gate electrode, it is possible to carry out thermal oxidation at about 450°C C. to about 600°C C., so that an oxide film having excellent film quality, such as a Ta2O3 film, is formed on the gate electrode. It is known that this oxide film has a film quality better than that of the oxide film formed when Al (aluminum) is used for the gate electrode, as described in the above embodiment 1.
This has been found from the fact that in the J-E characteristics (current density--electric field strength characteristics) which is one of withstand voltage estimations of an insulating film, the oxide film of Ta or Ta alloy has characteristics superior to the oxide film of Al.
Since Ta2O3 has a specific dielectric constant of about 11.6, and the capacitance C3 (see the expression 1 in embodiment 1) between the floating gate and control gate is large, there is also an advantage that an electric charge is more easily injected to the floating gate as compared with the case where Al is used for the gate electrode.
Moreover, in the case where Ta is used for the gate electrode, it is also possible to carry out anodic oxidation as in the above embodiment.
In the semiconductor devices of the foregoing embodiments 1 to 6, Ta or Ta alloy may be used for the gate electrode.
According to the present invention, in a semiconductor device requiring a memory, it is possible to easily manufacture a FAMOS type TFT memory without increasing steps, so that it is possible to increase the performance of the semiconductor device and to miniaturize the semiconductor device.
Moreover, according to the present invention, since an arithmetic circuit for carrying out gamma correction and a memory for storing data used in the gamma correction are integrally formed concurrently with a pixel TFT, a driver circuit, and other peripheral circuits, it is possible to realize a liquid crystal display device having excellent gradation display while the liquid crystal display device can be miniaturized.
Yamazaki, Shunpei, Koyama, Jun
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