Patent Application
20090003801
The present invention relates to techniques for controlling crystallization processes and, in particular, to methods and systems for optical control of sequential lateral solidification (SLS) crystallization processes.
Thin film transistor (TFT) technology is the basis for high-resolution, high-performance liquid crystal display (LCD) screens, providing the best resolution of the various flat panel display technologies that are currently available. Advanced thin film transistor technology is based upon polycrystalline silicon.
Polycrystalline silicon may be formed using laser recrystallization techniques, such as excimer laser annealing. In excimer laser annealing, a high-power laser beam is scanned over the surface of a substrate that is coated with amorphous silicon. The amorphous silicon is heated, melts and then recrystallizes to form polycrystalline silicon.
A more recently introduced laser recrystallization technique is 2-shot sequential lateral solidification (SLS). In a typical application of this technique, a two-dimensional mask pattern is imaged upon the amorphous silicon film using an imaging lens. Only the irradiated areas of the amorphous silicon melt and recrystallize. By repetitive irradiation of different areas, the entire substrate silicon film can be recrystallized in the desired pattern. The quality of the resulting patterned polycrystalline silicon film exceeds that of excimer laser annealing processed material in various parameters.
Thin film transistor technology requires extremely high quality processes and high process speeds. These requirements place great demands on the process control of SLS system. It is particularly important to be able to control the crystallization process continuously to avoid defect structures in the recrystallized areas.
One conventional technique for controlling recrystallization processes is the use of off-situ incident light microscopes with color charge coupled device (CCD) cameras. The inspected area is illuminated in light field or in dark field. Different phases of crystallization show different colors and contrasts in the image. Amorphous areas of a silicon substrate usually exhibit a red/orange color, whereas crystallized areas exhibit a bright orange/yellow color. One drawback of this conventional method is that it is difficult to automate; the images usually must be inspected by a qualified person.
There are a number of challenges associated with an optical approach to recrystallization control. First, the inspected structures of recrystallization are rather small. Deviations from the predetermined structures are even smaller. High quality optical systems are needed to detect those deviations. In addition, the inspected areas are comparatively small with respect to the size of the entire substrate. As mentioned above, a high inspection rate is necessary to gain effective control over the recrystallization process.
The object of the present invention is, therefore, to provide methods and systems for optical crystallization control that overcome the drawbacks of the known methods and systems and that enable fast and reliable control with equipment that is cheaper and easier to use.
Methods and systems in accordance with the present invention enable accurate and reliable inspection of recrystallization processes. The invention is generally based upon optical control methods.
One aspect of the invention relates to a method for characterization of a crystallization process with optical methods, in which a processed substrate is illuminated with polychromatic light. An image of the illuminated substrate is recorded using an image acquisition system that is capable of acquiring color images. The recorded image is then transmitted to an image processing unit that analyzes the image color selectively. This enables faster scans and more reliable results than analyzing color images with conventional methods. The image processing unit analyzes the green and/or the blue channel of the recorded image separately. It has been found that different stages of recrystallization show different optical properties, especially in the green and blue channel of a color image. The red channel exhibits only very small contrasts. Limiting the analysis to the green and/or blue channel saves calculation resources and improves the contrast between the different phases. It is particularly advantageous to further optimize the contrast of the blue and/or the green channel of the image. The regions of interest, particularly the borders between crystallized areas and amorphous areas and uncrystallized spots in the areas to be crystallized, are more visible after contrast optimization.
Another aspect of the invention relates to a method for characterization of a crystallization process with optical methods in which a crystallizing substrate is illuminated with a substantially monochromatic light or a narrow band polychromatic light. An image of the substrate is recorded using an image acquisition system and the recorded image is transmitted to an image processing unit. The image processing unit analyzes the recorded image. This aspect of the invention enables the use of black and white cameras that usually have a higher resolution than color cameras, since color cameras usually use three sensor elements for one pixel of an image. Furthermore, black and white cameras are cheaper than color cameras because the image sensor technology is less complex.
It is particularly advantageous to use light emitting diodes for illumination. Diodes which irradiate blue and green light have shown best results. The use of color light diodes enables faster switching between different illumination colors than the use of, for example, different color filter elements in front of white light or broad band illumination systems.
It is particularly advantageous for both aspects of the invention to automatically recognize the image in the image processing unit.
The aforementioned methods are especially suitable for inspecting SLS crystallization processes.
Another aspect of the invention relates to an SLS system that includes an optical control system. The control system further includes an illumination system to illuminate a substrate, image acquisition means to record images of the illuminated substrate, and an image processing unit to analyze the acquired images from the image acquisition system. Preferably, the image acquisition system is a color image acquisition system. The illumination system is polychromatic and the image processing unit is capable of color selective analysis of the recorded image. In an alternative embodiment, the image acquisition system is a black and white image acquisition system. The illumination system irradiates monochromatic or narrow band polychromatic light.
Since the characterization technique of the present invention can be automated and works without human interaction, it can be used as a base for automatized system performance monitoring or control of the SLS process.
The features and advantages of the various aspects of the present invention will be more fully understood and appreciated upon consideration of the following detailed description of the invention and the accompanying drawings, which set forth illustrative embodiments in which the concepts of the invention are utilized.
For quality purposes, it is important that the profile of the recrystallized substrate remains homogeneous over the full substrate area. That is, position and width of the protrusion lines 4 and the borders 5 should not change. Inhomogenities will lead to visible deviations of cell performance, e.g., in TFT displays, and, therefore, are not desirable. (The black bent line in the top left part of
The color channels shown in
Substrate 2 is treated by an SLS imaging unit 10. The SLS imaging unit 10 is controlled by SLS control unit 11. An image recognition system 12 comprises a camera 13, an illumination system 14 and a calculation unit 15. The calculation unit 15 comprises an image processing unit 16 and an image recognition unit 17. The calculation unit 15 is connected to an SLS control unit 11.
According to a first aspect of the present invention, the illumination system 14 illuminates the inspected area of the substrate 2 with polychromatic light and camera 13 is a color camera, such as, for example, a standard Sony XC-555P CCIR video camera.
According to a second aspect of the invention, the illumination system 14 illuminates the inspected area with monochromatic or narrow band polychromatic light and the camera 13 is a black and white camera, such as, for example, a standard Sony XC-ST50CE CCIR video camera. The illumination system 14 may be a switchable color illumination system or may comprise separate illuminators means for each color. For example, color light emitting diodes have shown good results. The use of a white light source combined with color filters would be less advantageous, since it is easier to switch on and off a light diode than to mechanically move a color filter. In the most preferred embodiment of the invention, images are taken of the same area with different illumination colors.
The image taken by the camera 13 is transmitted to the calculation unit 15. Calculation unit 15 can be a general purpose desk top computer or a similar data processing system. Image processing unit 16 of calculation unit 15 processes the image in different steps. If the image is a color image, the color information of the image is separated into its red, blue and green color channels and the red channel is discarded. Then, the blue and the green channel images are contrast optimized. If the image is a black and white image, the calculation unit 15 recognizes the illumination color in which the image has been taken and the image is contrast optimized depending upon the illumination color. Those skilled in the art will be familiar with commonly available image processing units and contrast optimization techniques that can be used in this application.
The processed images are transmitted to the image recognition system 16 which checks the areas of protrusion. Critical points of the protrusions 4 are the boundaries between the amorphous areas 3 of the silicon and the recrystallized areas 4 of the substrate 2. Further, it is important that the ridge 5 of the protrusion is intact. A constant line thickness and a continuous line indicate usable structures. The image recognition system 16 further checks to determine if spots of amorphous silicon are in the areas of protrusion 4. The results of the image recognition are fed to the SLS control system 11 in order to optimize the SLS process, e.g., by readjusting the focus of the SLS system 10.
It should be understood that the particular embodiments of the invention described above have been provided by way of example and that other modifications may occur to a person skilled in the art without departing from the spirit and scope of the invention as expressed in the appended claims and their equivalents.
