BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more fully understood from the detailed description given herein below illustration only, and thus is not limitative of the present invention, and wherein:
FIGS. 1A to 1D are schematic illustrations showing a conventional excimer laser annealing method;
FIG. 2 is a graph showing the relationship between the laser light with different wavelengths and the transmittance ratio of the amorphous silicon according to a preferred embodiment of the invention;
FIG. 3 is a schematic illustration showing the structure of a LTPS-TFT display panel according to the preferred embodiment of the invention;
FIG. 4 is a flow chart showing an annealing method for LTPS according to the preferred embodiment of the invention;
FIGS. 5A to 5E are schematic illustrations showing the annealing method for LTPS and crystal grains of the polysilicon film layer of the LTPS panel according to the preferred embodiment of the invention;
FIG. 6 is a schematic illustration showing the structure of a LTPS panel according to another preferred embodiment of the invention;
FIG. 7 is a schematic illustration showing the structure of a LTPS panel according to still another embodiment of the invention;
FIG. 8 is a schematic illustration showing a liquid crystal display device according to the preferred embodiment of the invention; and
FIG. 9 is a schematic illustration showing a system for displaying image according to the preferred embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements.
Referring to FIG. 3, an annealing method for LTPS according to an embodiment of the invention is applied to an LTPS-TFT substrate 20. The LTPS-TFT substrate 20 includes a glass substrate 21, a buffer layer 24 formed on the glass substrate 21, a first metal layer 22 disposed on the buffer layer 24, a first insulating layer 25 formed on the first metal layer 22, and a silicon film layer 23 disposed on the first insulating layer 25. The laser light L disposed above the glass substrate illuminates the silicon film layer 23.
Referring to FIGS. 3 and 4, the annealing method for LTPS includes the following steps. In step S01, the silicon film layer 23 is illuminated by the laser light L having a wavelength larger than 400 nm. The silicon film layer 23 is heated to melt by absorbing a part of the laser light L, and is heated to re-crystallize (the range defined by the dashed lines of FIG. 3) by absorbing another part of the laser light L, which passes through the silicon film layer 23 and is reflected from the first metal layer 22 to the silicon film layer 23. In this embodiment, the laser light L is the solid-state laser light. The portion of the silicon film layer 23, which does not correspond to the first metal layer 22, absorbs one part of the laser light L. The other part of the laser light L passes through silicon film layer 23 and is not reflected.
In step S02, the silicon film layer 23 is placed to cool the silicon film layer 23 down to the room temperature after being illuminated by the laser light L.
Finally, the silicon film layer 23 crystallizes and transforms into a polysilicon film layer after the laser annealing process.
In addition, in order to make the invention be more easily understood, the annealing method for LTPS of this embodiment will be described with reference to the steps of FIGS. 5A to 5E. Referring to FIGS. 4 and 5A, the silicon film layer 23 of this embodiment has a first region 231, a second region 232 and a third region 233. The first region 231 is located between the second region 232 and the third region 233, and it is disposed opposite to the first metal layer 22. The thickness of the first metal layer 22 is larger than 100 angstroms such that the first metal layer 22 can reflect the laser light L. The laser light L includes one part L1 of the laser light and the other part L2 of the laser light. When the laser light L illuminates the silicon film layer 23, the first region 231, the second region 232 and the third region 233 are heated by the illumination of the laser light L. Then, the first region 231, the second region 232 and the third region 233 are melted and start to crystallize. However, because the solid-state laser light is applied, the amorphous silicon has a poor laser absorptivity with respect to the laser light having the wavelength above 400 nm as shown in FIG. 2. Thus, the silicon film layer 23 only absorbs a part L1 of the laser light.
As shown in FIG. 5B, the first region 231 absorbs the part L1 of the laser light to crystallize, and the other part L2 of the laser light passes through the first region 231 and illuminates the first metal layer 22. Because the thickness of the first metal layer 22 is large enough, the laser light L cannot penetrate through the first metal layer 22. Instead, the other part L2 of the laser light can be reflected to the first region 231 by the first metal layer 22. As shown in FIGS. 5C and 5D, the first region 231 absorbs the other part L2 of the reflected laser light to heat the first region 231. Such that the first region 231 is kept melted for a period of time, which is longer than the time when each of the second region and the third region is kept melted, and the first region 231 is re-crystallized after the annealing process. As shown in FIG. 5B, when the laser light L illuminates the second region 232 and the third region 233, the part L1 of the laser light is also absorbed by the second region 232 and the third region 233. The other part L2 of the laser light passes through the second region 232 and the third region 233, and it is not reflected because the second region 232 and the third region 233 are not disposed above the first metal layer 22. The second region 232 and the third region 233 are heated to melt and to crystallize. Finally, as shown in FIG. 5E, the silicon film layer 23 transforms from the amorphous silicon into a polysilicon film layer 23′ after the annealing process. Thus, the first region 231 of amorphous silicon transforms into the first region 231′ of polysilicon after it is melted and re-crystallized. The second region 232 and the third region 233 of amorphous silicon also transform into a second region 232′ and a third region 233′ of polysilicon.
Referring again to FIG. 5E, because the first region 231′ almost completely absorbs the laser light L, the crystal grains of the first region 231′ are larger than those of the second region 232′ and the third region 233′. Furthermore, because the first metal layer 22 reflects the laser light L to the first region 231′, the part of the first region 231′ adjacent to the first metal layer 22 is illuminated and heated to crystallize, and it thus gets a longer period of melting time than the other part of the first region 231′. Thus, the crystal grains of the part of the first region 231′ adjacent to the first metal layer 22 absorb much more energy of the laser light L than those of the other part of the first region 231′. Accordingly, the crystal grains of the one part of the first region 231′ adjacent to the first metal layer 22 are also larger than the crystal grains of the other part of the first region 231′. Finally, a TFT is formed by way of doping after the annealing method for LTPS. In this embodiment, the first metal layer 22 is the gate of the transistor, the second region 232′ and the third region 233′ are respectively the source and the drain of the transistor, and the first region 231′ is the channel region of the transistor.
FIG. 6 is a schematic illustration showing the structure of an LTPS-TPT substrate 30 according to another embodiment of the invention. After the annealing method (FIG. 5E), a second insulating layer 26 is disposed on the polysilicon film layer 23′ and a second metal layer 27 is disposed on the second insulating layer 26 such that another aspect of manufacturing process is built. Then, a TFT is formed by way of doping. Herein, the first metal layer 22 and the second metal layer 27 serve as the gate of the transistor, the second region 232′ and the third region 233′ are respectively the source and the drain of the transistor, and the first region 231′ is the channel region of the transistor. Besides, in other embodiments, the first metal layer 22 is light shading metal and the second metal layer serves as the gate.
FIG. 7 is a schematic illustration showing the structure of an LTPS-TFT substrate 40 according to still another embodiment of the invention. In this structure, the buffer layer 24 is formed on the glass substrate 21, the polysilicon film layer 23′ is disposed on the buffer layer 24, an insulating layer 25′ is disposed on the polysilicon film layer 23′, and then the first metal layer 22 is disposed on the insulating layer 25′. When the laser annealing method is being performed, the laser light L below the glass substrate 21 illuminates the silicon film layer 23. The silicon film layer 23 transforms into the polysilicon film layer 23′ after the annealing method is performed. Finally, a TFT is formed by way of doping. In this case, the first metal layer 22 is the gate of the transistor, the second region 232′ and the third region 233′ are respectively the source and the drain of the transistor, and the first region 231′ is the channel region of the transistor.
Because the laser light L is used for illumination, the silicon film layer 23 of amorphous silicon crystallizes and transforms into the polysilicon film layer 23′. Then, the crystallized first region 231 of the silicon film layer 23 is re-crystallized because the first metal layer 22 reflects the laser light L. Thus, the crystal grains of the first region 231′ after the annealing process not only get larger but may also be distributed over the first region 231′ more evenly, such that the carrier mobility of the transistor is enhanced.
FIG. 8 is a schematic illustration showing a system for displaying image according to the various embodiments of the invention. The system includes a liquid crystal display device 5. Referring to FIG. 8, the liquid crystal display device has an LTPS-TFT display panel 2 and a backlight module 6, which is disposed at one side of the LTPS-TFT display panel 2.
The LTPS-TFT display panel 2 has the LTPS-TFT substrate 20, a liquid crystal layer 28 and a color filter substrate 29. In the embodiment, the LTPS-TFT substrate 20 has the glass substrate 21, the first metal layer 22, the polysilicon film layer 23′. The first metal layer 22 and a polysilicon film layer 23′ are formed on the glass substrate 21, and the liquid crystal layer 28 and the color filter substrate 29 are formed on the polysilicon film layer.
The liquid crystal display device 5 of this embodiment uses the backlight module 6 as the light source, as indicated by the arrow of FIG. 8. The light coming from the light source passes through the LTPS-TFT substrate 20, the liquid crystal layer 28 and the color filter substrate 29. Thus the liquid crystal display device displays images. Because the liquid crystal display device 5 comprises the LTPS-TFT display panel 2, the carrier mobility is enhanced, the electroconductivity is good, the power may be saved, and the displayed image looks better.
The LTPS-TFT substrate 20 is characterized in that the polysilicon film layer has a first region, a second region and a third region. The first region is located between the second region and the third region and disposed opposite to the first metal layer. The crystal grains of the first region are larger than those of the second region and the third region. The LTPS-TFT display panel of this embodiment is manufactured according to the annealing method for LTPS, as shown in FIGS. 4 and 5A to 5E, and detailed descriptions thereof are omitted.
FIG. 9 is a schematic illustration showing a system for displaying image according to the preferred embodiment of the invention. The system further includes an electronic device 7. The electronic device 7 includes the LTPS-TFT display panel 2 and an input unit 8. The input unit 8 is coupled to the LTPS-TFT display panel 2 and provides input signals (e.g., an image signal) to the LTPS-TFT display panel 2 to generate images. The electronic device 7 may be a mobile phone, digital camera, PDA (personal data assistant), notebook computer, desktop computer, television, car display, or portable DVD player, for example.
In summary, according to the system for displaying image and the laser annealing method for LTPS, the laser light is used for illumination and the first metal layer is used to reflect the laser light such that the silicon film layer absorbs a part of the laser light to crystallize, and the first metal layer reflects another part of the laser light to the first region of the silicon film layer such that the first region is kept at the melted state for a period of time longer than the time when each of the second region and the third region is kept at the melted state. Thus, the crystal particles of the first region are larger than those of the second region and the third region. Then, the silicon film layer is placed to cool the silicon film layer down to the room temperature. In this case, the silicon film layer transforms into the polysilicon film layer after being illuminated by the laser light. Compared with the prior art, because the first metal layer can reflects a part of the laser light, which is not absorbed by the silicon film layer, back to the silicon film layer, so that the silicon film layer can further absorb the reflected light. After several times of absorption and reflection, the energy of the laser light is almost absorbed by the silicon film layer. Thus, the usage of the laser light is enhanced, and the cost can be decreased because the solid-state laser light is used. The first region of the silicon film layer also absorbs the laser light several times and is thus heated, so that the melting time of the first region is lengthened. Accordingly, the crystallized first region obtains larger and smoother crystal particles and the polysilicon film layer with a lower defect density. Furthermore, the electron mobility of the TFT can be enhanced.
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.
Patent Application
20070285592
