1. Field of the Invention
The present invention relates to a method of separating (or peeling) a lamination body and a method of manufacturing a semiconductor device formed using a plastic substrate.
2. Description of the Related Art
In recent years, a technique of forming a thin film transistor (TFT) using a semiconductor thin film (with a thickness of from approximately several nm to several hundreds nm), which is formed over a substrate with an insulated surface, has been attracting attention. The thin film transistor has been widely applied in various electronic devices such as an IC and a display device. In particular, development related to the thin film transistor as a switching element for an image display device has been carried out hurriedly.
Various applications of such an image display device have been expected, and particularly, application to a portable device has been attracting much attention. A glass substrate and a quartz substrate has been typically used for forming the image display device now, however, these substrates have some drawbacks of being fragile and heavy. Further, these substrates are unsuitable for mass-production since the surface area thereof is difficulty enlarged. Therefore, it has been tried to form a semiconductor element, e.g., a TFT on a substrate having flexibility as typified by a flexible plastic film.
In the case of using the flexible plastic substrate, however, the maximum temperature of the process should be lowered since the plastic film has low heat resistant properties. Accordingly, it has been impossible to form a semiconductor element, e.g., a TFT having as good electric characteristics as that formed over a glass substrate. Thus, a high-performance semiconductor device, e.g., a liquid crystal display device or light emitting element using a plastic film has been developed.
Various kinds of methods for separating a lamination body, which is formed over a substrate through a separation body, from the substrate have been already proposed. For example, there is a technique as disclosed in patent document 1 and patent document 2, wherein a separation layer is formed on a transparent substrate by using amorphous silicon (or polysilicon), a lamination body is formed thereon, and laser light is irradiated from a side of the substrate to discharge hydrogen contained in the amorphous silicon so that a gap is caused between the separation layer and the substrate and the substrate is separated from the lamination body.
[Patent Document 1]: Japanese Patent Application Laid-Open No. Hei 10-125929
[Patent Document 2]: Japanese Patent Application Laid-Open No. Hei 10-125931
In the above-mentioned separation method, however, a substrate having a high light-transmitting property is absolutely required. Further, a step for irradiating relatively high-energy laser beam on an entire surface of the substrate is required to apply sufficient energy such that laser beam transmits through the substrate and hydrogen contained in the amorphous silicon is discharged. This might damage the lamination body. In the case where an element is formed on the separation layer according to the above-mentioned separation method, when the element is formed by a heat treatment at high processing temperatures, hydrogen contained in the separation layer is dispersed and reduced. Accordingly, poor separation might be caused even when laser beam is irradiated to the separation layer, which results in reduced yield.