The present application claims priority from Japanese application JP 2004-197750 filed on Jul. 5, 2004, the content of which is hereby incorporated by reference into this application.
The present invention relates to an image display device, and more particularly to an image display device such as a liquid crystal display and an organic EL display provided with field effect transistors suitably modified to be mounted on an active matrix substrate used as a driving circuit.
An image display device provided with an active matrix substrate that has a pixel area with numerous pixels formed in a matrix and a peripheral circuit area formed around the periphery of the pixel area as an active circuit to send signals to the pixels is widely used in the field of various image display devices.
Part of the peripheral circuits for a flat panel display of medium or small size such as a portable telephone is assembled on the same substrate as for pixels, while most of it is constructed with the use of external LSI chips.
A thin film transistor that is currently used for mass goods as a semiconductor device constituting the main part of the pixels and peripheral circuits is mainly made of polycrystalline silicon that is obtained by crystallization of amorphous silicon thin film by means of excimer laser annealing (hereinafter, referred to as ELA crystal). In addition, crystalline silicon by solid phase growth that is obtained by thermal annealing of amorphous silicon thin film in an electric furnace, polycrystalline silicon formed directly by thermal chemical vapor deposition and catalytic chemical vapor deposition, and the like are also used.
It is desirable that a display device is made thinner and its frame is made narrower in view of weight reduction of a portable good and its free designing. One of the methods to achieve the above is incorporation of the peripheral circuits onto a glass substrate. The incorporation of the peripheral circuits onto the glass substrate makes it possible to reduce the number of external LSI chips and their mounting processes, thereby allowing a display device to be manufactured at a lower cost.
In order to realize incorporation of more peripheral circuits onto a glass substrate and an image display device having a larger number of pixels, a circuit that operates at higher driving frequencies is required. To increase driving frequencies of a circuit, an increase of power voltage or an enhancement of performance of a thin film transistor is required.
However, since portable instruments use battery as the power source, the increase of power voltage that leads to an increase of power consumption is not desirable. Therefore, it is necessary to fabricate a high performance thin film transistor that operates at a high speed and at a lower voltage on the same glass substrate as for the pixel area.
The development of a high performance thin film transistor that operates at a high speed and at a lower voltage has been widely conducted to date. As a method to obtain a transistor with high performance and high reliability that is not limited to a thin film transistor, there is a method in which trap levels present at the interface between a gate oxide film and a semiconductor film are terminated with mono-valent atoms such as hydrogen and fluorine.
An atom used for the termination is preferred to be as small as possible from a standpoint that peripheral atomic arrangement is not disturbed and that diffusion is easy. Accordingly, termination of the trap levels with the use of hydrogen is popular at present.
However, an bond between hydrogen and silicon atoms is cleaved upon collision with carriers that have come to have an energy higher than a thermal temperature produced by a high electric field in a channel (hereinafter, referred to as hot carrier), thereby loosing its termination effect. As a result, characteristics of the thin film transistor deteriorate, and display characteristics of an image display device deteriorate. In view of endurance of the termination effect, it is desirable to have a stronger bond to silicon atom. Accordingly, fluorine that is not too large and forms a strong bond to silicon is suitable for the atomused for the termination. As for documents related to this technology, Patent Document 1 [Japanese Patent No. 2846329 (Japanese Patent Laid-Open No. H2(1990)-205016)] is listed.
A method to enhance the performance of a thin film transistor by introducing fluorine into it is described in Patent Documents 2 (Japanese Patent Laid-Open No. H5(1993)-152333) and 3 (Japanese Patent Laid-Open No. H11(1999)-330474), in both of which fluorine is introduced into thin film transistors using granular crystals obtained by ELA. For this reason, substantial enhancement in performance and reliability could not be attained, and therefore it was difficult to install a circuit driven at a low voltage.
The present inventors have previously proposed in Patent Document 4 (Japanese Patent Laid-Open No. 2002-222959) that a crystalline thin film approximately in a band shape is applied to a channel for a thin film transistor constituting a driving circuit in order to drive pixels of an image display device at a high speed. In the subsequent research and development, they realized that realization of a thin film transistor that satisfies both of further steep transfer characteristic and excellent resistance to hot carriers for the thin film transistor constituting the driving circuit was an urgent matter.
The object of the present invention is to obtain a thin film transistor with excellent characteristics and reliability that is suitable for driving pixels on the same active matrix substrate as for a pixel area in order to realize a high performance image display device having a large number of pixels at a low cost.
That is, the object of the present invention is to realize a thin film transistor having both steep transfer characteristic and excellent resistance to hot carriers and further both high performance and high reliability that is suitable for driving pixels and to form various circuits that operate at a low electric power and at a high speed on the same active matrix substrate as for the pixel area in order to obtain a system-in-display having high performance and multifunction at a low cost.
A feature of the present invention to achieve the above object is that, for a thin film transistor suitable to form a driving circuit for driving pixels, fluorine is introduced into the interface between a gate oxide film and a semiconductor thin film forming a channel, interface trap levels are terminated by the fluorine, and the semiconductor thin film makes use of a crystalline semiconductor thin film approximately in a band shape to take advantage of the effect of enhancement in performance of the thin film transistor due to the interface termination.
In other words, the driving circuit of the image display device makes use of a thin film transistor in which the concentration of fluorine at the interface between the gate oxide film and the semiconductor thin film forming the channel is higher than that in the inside of the semiconductor thin film and the semiconductor thin film is a crystalline thin film approximately in a band shape in which crystal grains are allowed to grow and crystallize in the direction nearly parallel to the direction of current conduction.
Construction of the semiconductor thin film forming the channel approximately in a band shape is characterized in that the main alignment with respect to the surface of the semiconductor thin film is {110} as proposed previously in Patent Document 4 by the present inventors (Japanese Patent Laid-Open No. 2002-222959).
Further, an outline of the present invention will be specifically explained below by exemplifying an n-channel thin film transistor with reference to the accompanying drawings and in comparison with a conventional technology.
The ELA crystals are granular crystals having a number of columnar grain boundaries GB in the thickness direction of the film as shown in
When a positive voltage is applied to the gate electrode GT to open the channel in the n-channel thin film transistor, the Fermi level EF rises, and the trap levels of the grain boundaries and the interface are occupied by electrons. Particularly, the negatively charged trap levels at the grain boundaries GBT form potential barrier against electrons as shown by EC (lower end of conduction band) in
Since current conduction of a thin film transistor making use of ELA crystal is mainly governed by scattering due to the potential barrier of the grain boundaries crossing the channel, terminating the trap levels IT at the interface between the gate oxide film and the ELA crystal with fluorine atom provides little effect on the characteristics of the thin film transistor.
Next, a thin film transistor in which the channel is formed with a semiconductor thin film consisted of crystals approximately in a band shape according to Patent Document 4 (Japanese Patent Laid-Open No. 2002-222959) proposed previously by the present inventors and the interface termination is performed with fluorine will be explained according to
In the thin film transistor that uses the crystals approximately in a band shape so that the growth direction of the crystal may become nearly parallel to the channel direction, the number of the grain boundaries GB crossing the channel is reduced as shown by the line A-A′ in
Even in the crystals approximately in a band shape, potential barriers are formed due to trap levels GBT present in the grain boundaries GB. However, current conduction of the thin film transistor becomes not to be governed only by scattering caused by the potential barriers of the grain boundaries for two reasons that the number of the grain boundaries crossing the channel is small as described above and there are regions where carriers can move from the source region to the drain region without crossing potential barriers of the grain boundaries.
What influences the characteristics of the thin film transistor besides scattering caused by the potential barriers of the grain boundaries is weakening of the relation between gate voltage and surface potential by the interface trap levels IT. When there exist many interface trap levels IT, an application of a positive voltage to the gate electrode GT to open the channel raises the Fermi level EF at the interface and the trap levels at the interface are occupied by electrons, thereby inhibiting channel formation. The interface levels are energetically distributed, and the higher the Fermi level of the interface is raised, the more levels become lower than the Fermi level. Therefore, the number of electrons that are captured by the interface trap levels increases. As a result, the larger the number of the interface trap levels becomes, the weaker the relation between the gate voltage and the surface potential becomes, resulting in deterioration of the transfer characteristics of the thin film transistor.
When the interface trap levels of the thin film semiconductor making use of the crystals approximately in a band shape are terminated by fluorine according to the present invention, the influence of scattering due to the potential barriers of the grain boundaries is weakened, and therefore a greater effect on transistor characteristics can be acquired compared to a case where the interface trap levels of a thin film semiconductor making use of granular crystals are terminated.
It is desirable that not only is the interface level density at the interface between the gate oxide film and the semiconductor thin film that constitute the above thin film transistor equal to or lower than 7×1101/cm2/eV at the mid of the band-gap but also the above semiconductor thin film is a crystalline thin film approximately in a band shape in which crystal grains are allowed to grow and crystallize in the direction nearly parallel to the direction of current conduction.
In particular, characteristics of the rising edge in the subthreshold region become steep and excellent, and it is possible to obtain a transistor having a high driving capability even at a low voltage. At the same time, resistance to hot carriers is improved, and a transistor with high reliability can be obtained.
Further, it is desirable that the atomic concentration of fluorine relative to silicon at the interface is equal to or higher than 0.05%.
In the foregoing, the effect of the present invention on n-channel thin film transistor is described. A similar effect is also achieved on p-channel thin film transistor.
According to the present invention, it is possible to realize a thin film transistor having both steep transfer characteristic and excellent resistance to hot carriers, to form various circuits on the same active matrix substrate as for the pixel area, and to obtain a system-in-display having high performance and multifunction at a low cost.
FIGS. 10(1) and 10(2) are graphs representing transfer characteristics of p-channel thin film transistors that show an effect of the present invention, where
FIGS. 11 (1) and 11(2) are graphs representing transfer characteristics of n-channel thin film transistors that show another effect of the present invention, where
Hereinafter, representative embodiments of the present invention are explained.
(1) In order to achieve the object of the present invention, an image display device according to a first aspect of the invention is characterized in that the image display device is provided with a substrate having a pixel area with numerous pixels formed in a matrix and a peripheral circuit area formed around the periphery of the pixel area and having circuits formed to drive the pixels; a semiconductor device formed at least in the peripheral circuit area includes a thin film transistor having a crystalline thin film approximately in a band shape, in which crystal grains are allowed to grow and crystallize in the direction nearly parallel to the direction of current conduction and which serves as a semiconductor thin film constituting a channel, and a gate oxide film formed on the crystalline thin film approximately in a band shape; and fluorine is introduced into at least the interface between the gate oxide film and the semiconductor thin film constituting the channel.
It is desirable that the concentration of fluorine at the interface between the gate oxide film and the semiconductor thin film is higher than that in the inside of the semiconductor thin film.
It is desirable that the atomic concentration of fluorine at the interface between the gate oxide film and the semiconductor thin film of the thin film transistor is equal to or higher than 0.05% relative to that of silicon. More preferably it is approximately from 0.5 to 3.0%.
(2)-In order to achieve the object of the present invention, the image display devices according to a second aspect of the invention is characterized in that part or the whole of the driving circuits are formed on the same substrate as for the pixel portions; the driving circuit on the substrate is provided with a thin film transistor having a crystalline semiconductor thin film approximately in a band shape in which crystal grains are allowed to grow and crystallize in the direction nearly parallel to the direction of current conduction as well as a concentration distribution of fluorine higher at the interface between the gate oxide film and the semiconductor thin film than in the inside of the semiconductor thin film; and a pixel portion other than the above driving circuit on the substrate is provided with a thin film transistor making use of a granular crystalline semiconductor thin film.
The image display device according to the above (2) is characterized in that part or the whole of the driving circuits are formed on the same substrate as for the pixel portions; the driving circuits on the substrate are provided with two kinds of thin film transistors consisting of the thin film transistor having a crystalline semiconductor thin film approximately in a band shape in which crystal grains are allowed to grow and crystallize in the direction nearly parallel to the direction of current conduction as well as the concentration distribution of fluorine higher at the interface between the gate oxide film and the semiconductor thin film than in the inside of the semiconductor thin film and the thin film transistor making use of the granular crystalline semiconductor thin film; and the pixel portions other than the above driving circuit on the substrate are provided with the thin film transistor making use of the granular crystalline semiconductor thin film.
The image display device according to the above (1) is characterized by a liquid crystal display or an organic EL display.
Further, the image display device according to the above (1) is characterized in that the power voltage for the driving circuit provided with the thin film transistor in which the semiconductor thin film having the crystal structure approximately in a band shape is utilized as the channel is in the range of 1.0 to 6.0 V.
Still further, the image display device according to the above (1) is characterized in that the concentration of fluorine per unit volume of the gate oxide film constituting the above thin film transistor for the driving circuit in the region of 10 nm thickness from the interface with the semiconductor thin film is higher than that in the inside of the above semiconductor thin film, and this semiconductor thin film is composed of a crystalline thin film approximately in a band shape in which crystal grains are allowed to grow and crystallize in the direction nearly parallel to the direction of current conduction.
Still further, the image display device according to the above (1) is characterized in that not only is the interface level density at the interface between the gate oxide film and the semiconductor thin film that constitute the thin film transistor for the above driving circuit equal to or lower than 7×1011/cm2/eV at the mid of the band-gap but also the above semiconductor thin film is composed of the crystalline thin film approximately in a band shape in which crystal grains are allowed to grow and crystallize in the direction nearly parallel to the direction of current conduction.
(3) In order to achieve the object of the present invention, according to a third aspect of the invention, a method for manufacturing the image display device having the substrate with the thin film transistor formed thereon as the driving circuit to drive pixels around the periphery of a pixel area is characterized in that the process to manufacture the thin film transistor includes the steps of forming a first semiconductor thin film made of an amorphous or polycrystalline material on an insulating substrate; allowing a second semiconductor thin film to grow as crystals approximately in a band shape by irradiating a continuous wave laser to an arbitrary region of the first semiconductor thin film, scanning the continuous wave laser relatively to the above substrate, and crystallizing in the direction nearly parallel to the scanning direction; forming the gate oxide film on the second semiconductor thin film made of crystals approximately in a band shape; and introducing fluorine into at least the vicinity of the interface between the gate oxide film and the second semiconductor thin film via thermal diffusion by implanting an ion of a molecule containing fluorine ion or fluorine atom into the depth proximal to the surface of the gate oxide film using an ion implantation apparatus.
Hereinafter, embodiments of the present invention applied to a liquid crystal display and an organic EL display are described. Here, a glass substrate is used for an active matrix substrate. However, it is needless to say that the present invention is also applicable to an image display device that uses an insulating substrate such as plastic substrate.
The present embodiment shows a manufacturing example in which a thin film transistor to drive pixels is formed on an active matrix substrate. The manufacturing method explained here exemplifies the production of a CMOS thin film transistor where an n-type thin film transistor is formed by self-aligned GOLDD (gate overlapped lightly doped drain) and a p-type thin film transistor is formed by counter doping. Hereinafter, a sequential manufacturing process is explained step by step according to
Preferably, a SiN film having a thickness of about 140 nm and a SiO film having a thickness of about 100 nm that function as thermal and chemical barrier films are successively and uniformly deposited by a chemical vapor deposition (CVD) method over this glass substrate SUB1. An amorphous silicon film ASI is formed on this glass substrate SUB1 by means of CVD and the like.
In place of the excimer laser beam ELA, alternatives that could be employed include, for example, crystallization by solid pulse laser annealing, Cat-CVD film turned into polysilicon film at the time of silicon film formation, and SiGe film.
Next, Implantation PE to control a threshold voltage is carried out in the region where the p-type thin film transistor Q2 is formed. At this time, the photoresist RNE that covers the region where the p-type thin film transistor Q2 is formed is removed, and instead the region where the n-type thin film transistor Q1 is formed is covered with a photoresist RPE not shown in a manner similar to that when the n-type thin film transistor Q1 is formed, and then implantation PE is carried out.
According to the foregoing process, a CMOS thin film transistor in which fluorine is introduced into the interface between the crystalline silicon film SPSI approximately in a band shape and the gate oxide film GI is formed.
Incidentally, deterioration of n-type thin film transistors is generally serious. This deterioration is reduced by forming a LDD region between the channel and each of the source and drain regions. GOLDD has a structure in which the gate electrode covers the LDD region. In this case, performance degradation observed with LDD is reduced.
Deterioration of p-type thin film transistors is not so serious as that of n-type thin film transistors, and therefore the LDD region and GOLDD are not generally employed. Although the GOLDD structure is used in this embodiment, the effect of the present invention can also be obtained even when a single drain structure or LDD structure is used. The characteristics of the transistor fabricated as above are shown below.
FIGS. 10(1), 10(2), 11(1), and 11(2) show the results of experiments on the thin film transistors according to the present invention. In FIGS. 10(1), 10(2), the present invention is applied to a p-channel thin film transistor, and dependence of its transconductance on the gate voltage is measured. The dimension of the thin film transistor is 4 μm in channel length and 4 μm in channel width, and the drain voltage at the time of measurement of transconductance is 0.1 V.
In
Despite the fact that fluorine is introduced, its effect is not seen in
In FIGS. 11(1) and 11(2), the present invention is applied to an n-channel thin film transistor, and dependence of its transconductance on the gate voltage is measured. In
These can not be observed, however, in
This embodiment represents an application of the present invention to a liquid crystal display. Hereinafter, the embodiment is explained according to
In
On the active matrix substrate SUB1 having any one of the structures in the embodiment described above, a liquid crystal alignment layer is formed by a known process, and an alignment controlling force is provided thereto by means of rubbing and the like. After forming a sealing member around the periphery of a pixel area AR, the opposing substrate SUB2 on which an alignment layer is similarly formed is placed opposite with a predetermined gap, a liquid crystal is sealed in this gap, and an opening of the sealing member is closed with a sealant.
The polarizing plates POL1 and POL2 are laminated on the surface and the back surface of the liquid crystal cell PNL constructed in this way as shown in
Data and timing signals are supplied to the driving circuits arranged around the liquid crystal cell via flexible print-circuit boards FPC1 and FPC2 as shown in
The liquid crystal display of the present embodiment that makes use of the active matrix substrate SUB1 is excellent in current driving capability by arranging the excellent thin film transistor circuit described above for its pixel circuits, and therefore it is suitable for a high speed operation. A further feature is that the liquid crystal display can be supplied at a low cost by reducing the number of LSI.
This embodiment represents an application of the present invention to an organic EL display. Hereinafter, the embodiment is explained according to
The organic EL device is formed on pixel electrodes provided on any one of the active matrix substrates SUB1 in each of the embodiments described above. The organic EL device is composed of a laminate in which a hole transport layer, emissive layer, electron transport layer, metal cathode, and the like are deposited in turn on the surface of the pixel electrodes.
Around the periphery of a pixel area PAR formed with such a laminate on the active matrix substrate SUB1, a sealing member is placed, followed by sealing with a sealing substrate SUBX or a sealing canister. A protective film may also be used in place of these.
In this organic EL display, signals for display from an external signal source are supplied to its peripheral circuit areas DDR and GDR by a print-circuit board PLB. An interface circuit chip CTL is mounted on this print-circuit board. Unifying these with a shield frame SHD, i.e. an upper case and a lower case CAS makes up the organic EL display.
Since the organic EL device makes use of a current-driven light emitting system in the active matrix driving for the organic EL display, employment of a high-performance pixel circuit is essential for provision of images with high quality, and therefore, it is desired to use a pixel circuit for CMOS-type thin film transistor. Further, the thin film transistor circuit formed in the peripheral circuit area is also essential for high-speed and high definition. The active matrix substrate SUB1 of the present embodiment has a high performance to meet such requirements. The organic EL display with the use of the active matrix substrate SUB1 fabricated according to the manufacturing method in the preceding first embodiment is one of the display devices that make the best possible use of the feature of the present embodiment.
Number | Date | Country | Kind |
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2004-197750 | Jul 2004 | JP | national |