1. Field of the Invention
The present invention relates to a method for bonding wafers, and more particularly, to a method that utilizes a photosensitive masking-and-bonding pattern to bond a first wafer and a second wafer. The photosensitive masking-and-bonding pattern functions as a mask pattern and a bonding layer, with which a wafer pattern of the first wafer is formed, the first wafer being then directly bonded to the second wafer without an additional bonding layer.
2. Description of the Prior Art
As the rapid development of micro-electromechanical system (MEMS) technologies progresses, the manufacturing processes of MEMS devices become increasingly complex. For the most part, semiconductor devices can be completely fabricated in a single wafer. On the contrary, MEMS devices, e.g. return path structures, have to be formed separately in two wafers, the two wafers then being bonded together to complete fabrication. Therefore, the conventional method for bonding wafers suffers from misalignment or limitations to the surface conditions of the wafers to be bonded or strict process conditions.
Please refer to FIG. 1 through FIG. 5. FIG. 1 through FIG. 5 are schematic diagrams illustrating a conventional method for bonding wafers. As shown in FIG. 1, a first wafer 10 is provided, and a photoresist layer 12 is coated onto the surface of the first wafer 10. As shown in FIG. 2, an exposure-and-development process is carried out to form a photoresist pattern 14 on the surface of the first wafer 10.
As shown in FIG. 3, an etching process is performed using the photoresist pattern 14 as a hard mask to etch regions of the first wafer 10 not protected by the photoresist pattern 14, thereby forming a wafer pattern 16 on the surface of the first wafer 10. As shown in FIG. 4, the photoresist pattern 14 is stripped, and a bonding pattern 18 is subsequently formed onto the surface of the first wafer 10. The material of the bonding pattern 18 is metal, solder, resin, or glass glue. The bonding pattern 18 is coated onto the surface of the first wafer 10, but does not overlapped the wafer pattern 16, using a shadow mask or by means of halftone printing. As shown in FIG. 5, a second wafer 20 is then affixed to the surface of the first wafer 10. The first wafer 10 and the second wafer 20 are therefore bonded together with the bonding pattern 18.
According to the conventional method, the bonding pattern 18 is coated onto the first wafer 10 in the areas around the wafer pattern 16 subsequent to removing the photoresist pattern 14. However, MEMS devices have sophisticated structures, and thus the critical dimensions of the wafer pattern 16 are extremely small. Consequently, the conventional method which utilizes a shadow mask or halftone printing to form the bonding pattern 18 usually leads to a misalignment problem (as indicated by the dotted line shown in FIG. 4). As long as the misalignment of the bonding pattern 18 occurs, the wafer pattern 16 will deviate (as indicated by the dotted line shown in FIG. 5) after the first wafer 10 and the second wafer 20 are bonded. This could degrade performance of the MEMS device. In view of the aforementioned problem, the conventional method is not suitable for use in bonding two wafers where the wafer pattern 16 is delicate. In addition, the conventional method is only suitable when the surface condition of the first wafer 10, e.g. the roughness of the first wafer 10, is proper.
In addition to the aforementioned conventional method, another conventional method which bonds two wafers directly is also well known. Please refer to FIG. 6 through FIG. 10. FIG. 6 through FIG. 10 are schematic diagrams illustrating another conventional method for bonding wafers. As shown in FIG. 6, a first wafer 30 is provided, and a photoresist layer 32 is coated onto the surface of the first wafer 30. As shown in FIG. 7, an exposure-and-development process is then performed to form a photoresist pattern 34 on the surface of the first wafer 30.
As shown in FIG. 8, an etching process is performed using the photoresist pattern 34 as a hard mask to etch the first wafer 30 not protected by the photoresist pattern 34 to form a wafer pattern 36 on the surface of the first wafer 30. As shown in FIG. 9, the photoresist pattern 34 is then stripped. As shown in FIG. 10, the first wafer 30 and a second wafer 38 are bonded in a direct manner. Direct bonding techniques include anodic bonding and fusion bonding. However, both the anodic bonding technique and the fusion bonding technique require numerous process steps, and have to be implemented when the surface condition is suitable. For example, the anodic bonding process has to be performed at a temperature of higher than 400° C. and an operation voltage of larger than 1000 V. The fusion bonding process has to be performed at a temperature of higher than 1000° C. Consequently, the direct bonding technologies are not suitable for fabricating devices incapable of resisting heat and high voltage.