Liquid crystal display device comprising a TFT with a barrier metal formed of a first layer and a second layer wherein the first layer is denser than the second layer

A contact resistance in a through-hole with a source or a drain electrode connected to a TFT is decreased, thereby improving the operation efficiency of a display device.In the through-hole, a source portion of the TFT is connected to a source electrode 8. The source electrode 8 is formed of three layers comprising a barrier metal, an Al alloy 82, and a cap metal 83. The barrier metal is divided into a lower layer 81a in contact with the semiconductor layer and an upper layer 81b in contact with the Al alloy. The lower layer 81a of the barrier metal is formed by sputtering, the lower layer 81a is heat-treated and, subsequently, an upper layer 81b of the base metal, the Al alloy 82, and the cap metal 83 are formed continuously by sputtering. Since the upper layer 81b of the barrier metal in contact with the Al alloy 82 is not oxidized, increase in the contact resistance in the through-hole can be prevented.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent Application JP 2010-032439 filed on Feb. 17, 2010, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device. The invention more particularly relates to an active matrix type display device which is improved in the response speed or the emission efficiency by decreasing the contact resistance in a through-hole.

2. Description of the Related Art

In an organic EL display device or a liquid crystal display device, scanning lines are extended in a first direction and arranged in a second direction. Further, video signal lines are extended in the second direction and arranged in the first direction. Areas surrounded by the scanning lines and the video lines define pixels and a thin film transistor (TFT) is formed for each of the pixels.

The organic EL display device is configured to control current supplied to an organic EL layer by a TFT while the liquid crystal display device is operated to select signals on images supplied to pixel electrodes. The TFT is formed of a-Si or poly-Si.

In the organic EL display device, since the organic EL layer is driven by the current, it is necessary to increase the mobility of a semiconductor constituting the TFT. Further, a small-sized liquid crystal display device is such that a driving circuit is mounted in a liquid crystal display panel. Since high speed operation is necessary for the TFT for the driving circuit, it is necessary to increase the mobility of the semiconductor. Accordingly, a poly-Si TFT is used in the organic EL display device or the small-sized liquid crystal display device.

Generally, to form a poly-Si semiconductor layer, an a-Si semiconductor layer is first formed and then excimer laser is radiated to the a-Si semiconductor laser to convert a-Si into poly-Si. Installation investment is necessary and process control is difficult for the radiation of the excimer laser. In view of this, JP-A-2007-142059 that a semiconductor layer of poly-Si is formed by radiating an a-Si semiconductor layer not by the excimer laser but by light of a lamp.

SUMMARY OF THE INVENTION

Generally, to form the poly-Si TFT, a semiconductor layer is formed first and a gate electrode is formed above the semiconductor layer, with a gate insulation film being sandwiched therebetween. An interlayer insulation film is then formed above the gate electrode. A source electrode or a drain electrode (hereinafter represented typically by the source electrode) is formed above the interlayer insulation film. The source electrode is in a layer in which the video signal line extends. An inorganic SiN passivation film is formed above the source electrode for protection of the entire TFT and an organic passivation film is formed above the inorganic SiN passivation film.

The source electrode is connected by way of a through-hole formed in the interlayer insulation film to a source portion of the semiconductor layer. An Al alloy is used for the source electrode which is formed in a layer in which the video signal line extends so as to decrease the resistance. However, Al atoms tend to migrate. The Al atoms are diffused to a semiconductor layer when they are passed through a thermal step or operated for a long period of time, resulting in the characteristics of the TFT changed.

In order to prevent this, a barrier metal such as MoW is formed as an underlayer for the Al alloy in the source electrode. While the barrier metal is formed by sputtering, the barrier metal in the state as sputtered has no sufficient barrier effect for Al. Accordingly, after the barrier metal is formed, it is necessary to subject the same to a heat treatment to increase the density of the barrier metal thereby improving the barrier effect. However, the heat treatment step results in a phenomenon of oxidizing the battier metal. When the barrier metal is oxidized, Al is oxidized thereby resulting in a problem of increasing the contact resistance with respect to the Al alloy.

FIGS. 8 to 12show a process of forming a poly-Si TFT according to the existent method described above. InFIG. 8, a first underlayer2of SiN is first formed on a substrate1, and a second underlayer3of SiO2is formed on the first underlayer2. They are formed for preventing impurities contained in the glass substrate1from contaminating a semiconductor layer4. Subsequently, an a-Si film is formed as the semiconductor layer4. An excimer layer is radiated to the a-Si film to convert it into a poly-Si film. Then, the semiconductor layer4is patterned by photolithography.

InFIG. 9, after a gate insulation film5is formed on the semiconductor4layer, ions are injected into the semiconductor4layer by implantation. The conduction type of the channel portion is defined to either a p-type or n-type by the ion injection. After the ion injection, a heat treatment is applied for the entire substrate1. This step is performed for driving atoms injected by ion implantation into the semiconductor layer4and activating them.

Subsequently, a gate metal is formed over the gate insulation film5by sputtering, and the gate metal is fabricated by photolithography to form a gate electrode6. Although not illustrated inFIG. 9, an impurity such as P or Bo is injected by ion implantation using the gate electrode6as a mask to form a drain portion or a source portion in the semiconductor layer4. The drain portion or the source portion is in contact with a drain electrode or a source electrode8.

InFIG. 10, an interlayer insulation film7is formed on the gate electrode6by plasma CVD, etc. After the interlayer insulation film7is formed, a heat treatment is performed for the entire substrate1and the ions injected into the drain portion or the source portion of the semiconductor layer4are driven into the inside and activated. Subsequently, a through-hole is formed in the interlayer insulation film7and the gate insulation film5such that the drain portion or the source portion of the semiconductor layer4can be in contact with the drain electrode or the source electrode8.

InFIG. 11, a barrier metal is formed on the interlayer insulation film7by sputtering. The barrier metal is formed, for example, of MoW. The barrier metal can be also formed by high melting point metals typically represented by Mo, Ti, Ta, Mn, Ru, V, and Co, and compounds or alloys thereof. Since the barrier metal is sputtered also in the through-hole in the interlayer insulation film7, the drain electrode or the source electrode8can be in contact with the drain portion or the source portion of the semiconductor layer4.

While the barrier metal serves as a barrier for an Al alloy82to be sputtered subsequently, the barrier metal does not form a sufficiently dense film only by sputtering. To attain the barrier effect, it is necessary to subject the sputtered barrier metal to a heat treatment.

The heat treatment is performed, for example, in an N2atmosphere after the substrate1is taken out from a sputtering device into atmospheric air. The surface of the barrier metal is oxidized when the substrate1is placed under the atmospheric air.

As shown inFIG. 12, an Al alloy82used to form the drain electrode or the source electrode8is sputtered over the barrier metal. If the surface of the barrier metal is oxidized, then this oxidizes Al in contact with the barrier metal to increase the contact resistance in the through-hole. If the contact resistance increases, current that allows an organic El layer to emit light cannot be sufficiently obtained in an organic EL display device and the switching characteristics are deteriorated in a liquid crystal display device.

The present invention intends to prevent the increase of the contact resistance in the through-hole.

In accordance with the invention, to overcome the problem described above, a source electrode to be connected with the source portion of a TFT is formed with three layers comprising a barrier metal, an Al alloy, and a cap metal. The barrier metal is formed in such a manner as to be divided into two layers comprising a lower layer in contact with the source portion of the TFT and an upper layer in contact with the Al alloy.

First, after the lower layer of the barrier metal is formed by sputtering, the substrate is taken out of a chamber and subjected to a heat treatment. The heat treatment is performed for increasing the density of the film structure in the lower layer of the barrier metal to provide the barrier effect. Subsequently, the substrate is again loaded in the vacuum chamber, the upper layer, the Al alloy, and the cap metal of the barrier metal are formed continuously by sputtering.

While the lower layer of the barrier metal is oxidized when taken out into the atmospheric air, the layer is covered by the upper layer of the barrier metal sputtered subsequently in vacuum, and the barrier metal and the Al alloy are not in contact with each other. Thus the Al in the Al alloy is not oxidized and the contact resistance can be kept lower.

According to the invention, since the contact resistance between the TFT and the drain electrode or the source electrode is decreased, rise of the driving voltage can be suppressed in the organic EL display device and, as a result, the emission efficiency of the organic EL display device can be improved.

Further, in the liquid crystal display device, since the contact resistance between the TFT and the source electrode in the through-hole can be decreased, the operation speed of the driving circuit can be increased, particularly, when the driving circuit is formed by the TFT.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is to be described for an example of applying the invention to an organic EL display device. Before explanation of a specific example, outline of the organic EL display device is to be described.FIG. 1is a cross sectional view for a pixel portion in the organic EL display device to which the invention is applied. One type of the organic EL display device is a top emission type in which light from an organic EL layer14is emitted on the side opposite to the side of a substrate1. Another type of the organic EL display device is a bottom emission type in which light from the organic EL layer14is emitted on the side of the substrate1.

FIG. 1is an example of the top emission type in which light is emitted in the direction of a blank arrow L. The top emission type is advantageous in terms of screen brightness since a large emission area is available. The organic EL display device has a plurality of organic layers sandwiched between a lower electrode13and an upper electrode15. The upper electrode15is referred to as a top anode in the case where the upper electrode15is an anode, and referred to as a top cathode in the case where the upper electrode15is a cathode.

Generally, the top cathode type is often used since the anode side is stable to the circumstance. That is, it is convenient to use the lower electrode13as a stable anode because, in some cases, a certain manufacturer prepares TFT substrates including portions formed up to the lower electrode13and sells them another manufacturer, and another manufacturer forms the organic EL layer14and the upper electrode15thereon.

InFIG. 1, a first underlayer2formed of SiN and a second underlayer3formed of SiO2are deposited over a device substrate1formed of glass. Both the first underlayer2and the second underlayer3play a role of preventing impurities from the glass from contaminating the semiconductor layer4.

A poly-Si semiconductor layer4is formed on the second underlayer3. An a-Si semiconductor layer4is first formed, and converted into a poly-Si film by annealing the a-Si film using an excimer laser. Ion implantation and subsequent heat treatment are applied to the poly-Si thereby defining the conduction type of the semiconductor layer4to the p-type or the n-type.

A gate insulation film5is formed over the semiconductor layer4. A gate electrode6is formed on the gate insulation film5. P or B are injected by ion implantation using the gate electrode6as a mask and then activated by a heat treatment to form a drain portion or a source portion of high electroconductivity to the semiconductor layer4.

An interlayer insulation film7is formed on the gate electrode6. The interlayer insulation film7plays a role of insulating the gate electrode6or a gate interconnection from the video signal line or the drain electrode and the source electrode8which are formed in a layer in which the video signal line extends. A through-hole is formed in the interlayer insulation film7by photolithography.

Video signal lines and the drain electrode and the source electrode8and which are formed in a layer in which the video signal line extends are formed on the interlayer insulation film7. Since only the source electrode8is shown on the interlayer insulation film7inFIG. 1, they are typically represented by the source electrode8. The source electrode8is formed of an Al alloy82with a large thickness in order to decrease the electric resistance.

Al is instable and tends to diffuse into other layers. Since Al deteriorates the characteristics of the TFT when it diffuses into the semiconductor layer4, a barrier metal is formed so that Al does not diffuse into the semiconductor layer4. As described later, the invention has a feature of forming the barrier metal with two layers, suppressing the increase in the contact resistance between the Al alloy and the barrier metal, and decreasing the contact resistance in the through-hole.

Further, Al generates hillock when subjected to a heat treatment. When the hillock is generated, it breaks through the insulation film to short circuit the source electrode8and the electroconduction layer of a counter electrode. In order to prevent the hillock, a cap metal83is formed on the Al alloy82to suppress the development of hillock of Al. In this example, while both the barrier metal and the cap metal83are formed of MoW, this is not particularly restricted and they may be formed of a high melting point metal typically represented, for example, by Mo, Ti, Ta, Mn, Ru, V, or Co, and compounds or alloys thereof.

An inorganic passivation film9is formed by CVD to cover the source electrode8. The inorganic passivation film9protects the entire TFT. An organic passivation film10is formed to cover the inorganic passivation film9, with the organic passivation film10also serving as a planarizing film. Since the organic passivation film10plays a role of serving as a planarizing film, the organic passivation film10formed has a thickness as large as about 1 to 4 μm.

To establish contact between the source electrode8and the lower electrode13of the organic EL layer14, a through-hole is formed in the organic passivation film10and the inorganic passivation film9. Subsequently, an Al alloy is formed by a sputtering method or the like to establish the contact between the lower electrode13of the organic EL layer14and the source electrode8. Since the Al alloy has a high reflectance, the Al alloy plays a role of serving as a reflection electrode12for the top emission type.

ITO is deposited as the lower electrode13of the organic EL layer14on the Al alloy as the reflection electrode12. Since ITO has a large work function, ITO can be used as an anode for the organic EL layer14. After deposition of ITO, ITO is subjected to a heat treatment such that it has a decreased resistivity.

The organic EL layer14is formed on the lower electrode13. The organic EL layer14generally has a 5-layered structure comprising a hole injection layer, a hole transport layer, an emission layer, an electron transport layer, and an electron injection layer from the side of the lower electrode13. IZO is deposited as a cathode on the electron injection layer. The cathode is an upper electrode15.

A bank11is formed between pixels in order to prevent destruction of the organic EL layer14caused by disconnection of step at the end. The bank11may be formed of an organic material or formed of an inorganic material such as SiN. When the organic material is used, the bank11is generally formed of an acrylic resin.

The emission characteristics of the organic El layer14are degraded when moisture is present. Accordingly, for the substrate1shown inFIG. 1, the side thereof formed with the TFT and the organic El layer is sealed by a sealing substrate formed of glass (not shown). The substrate1having the organic EL layer14, etc. formed thereover and the sealing substrate1are sealed at the periphery of them.

The present invention is configured to prevent increase in the contact resistance in the through-hole portion that establishes the contact between the source electrode8and the source portion of the semiconductor layer4at the portion indicated by A. In the invention, this is true not only for the portion A but also for the contact, for example, between the drain electrode and the drain portion of the semiconductor layer4inFIG. 1.

The present invention is to be described specifically by way of examples.

FIGS. 2 to 6show a process up to the formation of the source electrode8in the organic EL display device shown inFIG. 1. SinceFIGS. 2 to 4are identical withFIGS. 8 to 10that have been explained for the existent example, explanation for them is to be omitted. That is, the process up to the state inFIG. 4where the through-hole is formed in the interlayer insulation film7and the gate insulation film5is identical with that in the existent example.

Similarly to the existent example, the source electrode8has the three layered structure comprising the barrier metal, the Al alloy82, and the cap metal83. The feature of the invention is to deposit the barrier metal film divided into two layers. That is, a lower layer81aof the battier metal serves to prevent the Al atoms from diffusing into the poly-Si semiconductor layer4. This is a primary role as the barrier metal. An upper layer81bof the barrier metal serves to decrease the contact resistance in the through-hole by enhancing the contact between the upper layer81bof the barrier metal and the Al alloy82. The term Al alloy82is used herein in the meaning of Al-containing metals.

InFIG. 5, the lower layer81aof the barrier metal is first formed by sputtering MoW. The thickness of the lower layer81aof the barrier metal is, for example, about 30 nm. Since MoW after being deposited by sputtering is in an amorphous state, MoW has an insufficient barrier effect. Then, the substrate1sputtered with MoW is taken out of a sputtering apparatus, loaded in a heating furnace and subjected to a heat treatment to convert the MoW film into a dense film, whereby a sufficient barrier effect is provided.

However, since the substrate1is taken out into atmospheric air in a state where MoW is deposited, the surface of MoW is oxidized. Further, while heating for the substrate1is performed in a nitrogen atmosphere, since oxygen remains slightly in nitrogen, oxidation of MoW also proceeds during heating for the substrate1. When the Al alloy82is deposited over the thus oxidized MoW, Al in the Al alloy82is oxidized to form Al2O3. Since Al2O3is an insulator, the contact resistance between the Al alloy82and MoW increases.

In the organic EL display device, if the contact resistance in the through-hole increases, voltage rises when a current is supplied to the organic EL layer14. Further, the power consumption also increases. Accordingly, the contact resistance in the through-hole should be as low as possible.

In the present invention, after the lower layer81aof the barrier metal is formed of MoW and then subjected to a heat treatment such that it has a barrier effect, the upper layer81bof the barrier metal is formed of MoW before formation of the Al alloy82. The upper layer81bof the barrier metal is shown by a dotted line inFIG. 5. The thickness for MoW of the upper layer81bof the barrier metal is, for example, 13 nm.

The upper layer81bof the barrier metal serves to enhance the contact with the Al alloy82and, and the lower layer81aof the barrier metal serves to provide the barrier effect against Al. Thus the upper layer81bdoes not need have a large thickness. By contrast, since it is necessary that the lower layer81aof the barrier metal provide the essential barrier effect, the lower layer81aneeds to have a predetermined thickness. That is, the lower layer81aof the barrier metal is formed to a thickness larger than that of the upper layer81bof the barrier metal.

Then, an Al alloy82playing a primary role of the source electrode8is formed by sputtering. In the invention, the Al alloy82is AlSi. The Al alloy82is formed to a thickness as large as about 250 nm to decrease the resistance. Subsequently, MoW is formed as a cap metal83to a thickness of about 75 nm on the Al alloy82by sputtering. The cap metal83plays a role of suppressing hillock of Al. Then, a metal layer comprising the barrier metal, the Al alloy82, and the cap metal83is patterned by photolithography to form a source electrode8, a drain electrode, a video signal line, etc.

Since the upper layer81bof the barrier metal serves to enhance the contact with the Al alloy82and the lower layer81aof the barrier metal serves to provide the barrier effect, the upper layer81bmay be in an amorphous state as sputtered. That is, after sputtering it is not necessary to subject the barrier metal (upper layer81b) to a heat treatment to form a dense film. Accordingly, the upper layer81bof the barrier metal, the Al alloy82and, further, the cap metal83can be formed continuously in the chamber without breaking a vacuum state.

In the invention, the barrier metal is formed of two layers. However, as described above, the upper layer81bof the barrier metal can be sputtered continuously with the Al alloy82and the cap metal83in one identical vacuum chamber. Since the thickness of the upper layer81bof the barrier metal is 13 nm, the thickness of the Al alloy82is 250 nm, and the thickness of the cap metal83is 75 nm, increase in the tact time caused by the sputtering of the upper layer81bof the barrier metal is extremely small. That is, the process time scarcely increases even when the invention is practiced.

FIG. 7is a cross sectional view showing only the source electrode8inFIG. 6. InFIG. 7, the source electrode8comprises a barrier metal, an Al alloy82, and a cap metal83in which the barrier metal is divided into a lower layer81aand an upper layer81b. InFIG. 7, the thickness tb1of the lower layer81aof the barrier metal is 26 nm, the thickness tb2of the upper layer81bof the barrier metal is 13 nm, the thickness to of the Al alloy82is 250 nm, and the thickness tc of the cap metal83is 75 nm.

InFIG. 7, since the lower layer81aof the barrier metal undergoes the heat treatment, a dense crystal structure is formed. By contrast, since the upper layer81bof the barrier metal is in a state as sputtered, this is in a substantially amorphous state. Further, since the crystal grain boundary formed in the lower layer81aof the barrier metal is disconnected at a portion of the dotted line B corresponding to the boundary between the lower layer81aand the upper layer81bshown inFIG. 7, the boundary can be observed, for example, by TEM and SEM.

Further, a great amount of oxygen is contained in the lower layer81aof the barrier metal and in the boundary between the barrier metal lower layer81a/barrier metal upper layer81bshown inFIG. 7, whereas oxygen is scarcely contained in the upper layer81bof the barrier metal and in the boundary between the barrier metal upper layer81b/Al alloy82. Accordingly, by executing elemental analysis for the upper layer81band the lower layer81aof the barrier metal with the use of SIMS or the like it can be confirmed that the two layers of the lower layer81aand the upper layer81bare formed.

The serial resistance of a plurality of through-hole portions for use in the source electrode8according to the configuration of the invention as inFIG. 7was compared with the serial resistance of a plurality of through-hole portions in which the source electrode8according to the existent configuration is used. As a result, while the serial resistance of the through-hole portions according to the configuration of the invention was 27.8Ω, the serial resistance of the through-hole portions according to the existent configuration was 112.9Ω. The serial resistance in the invention is ¼ or less of the serial resistance in the existent example and the effect of the invention is remarkable.

Description has been made of the example of the through-hole near the TFT in the display area of the organic EL display device. In the case where a driving circuit is formed by a TFT at the periphery of the display area, the present invention is applicable in the same manner also to the through-hole at the periphery of the TFT of the driving circuit.

In the foregoing description, while the organic EL display device has been explained as an example, the TFT is used also in a liquid crystal display device in the same manner as in the organic EL display device. In particular, in a liquid crystal display device incorporating a driving circuit at the periphery of the display area by using a poly-Si TFT, the contact resistance in the through-hole portion gives a significant effect on the operation speed of the driving TFT. When the invention is applied to such a liquid crystal display device, a liquid crystal display device incorporating a high performance driving circuit can be attained. When the invention is applied to the driving TFT of the driving circuit in such a liquid crystal display device, the invention is applicable also to the switching TFT in the display area.