1. Field of the Invention
The present invention relates to a method for fabricating a grating pattern.
2. Discussion of Related Art
An optical pick-up unit is an element, which is constructed of a semiconductor laser, an object lens, an optical diode converting an optical signal into an electrical signal, and an actuator. This is also called an optical head as it becomes small. The optical head is divided into a separation type which generally names the optical pick-up unit including optical systems, a one-body type in which the object lens, actuator and optical elements are mounted on a single base, and a hologram laser in which a prism is replaced by a flat hologram (grating).
The optical pick-up unit is used in the appliances using optical disks such as CD-ROM player, optical magnetic disk, video CD, and MD, as well as the audio field like CD player. Furthermore, there is growing demand for the optical pick-up unit for DVD. Currently, optical pick-up elements become small, thin, light and high-speed.
FIGS. 1A, 1B, 1C and 1D are cross-sectional views showing a conventional method of fabricating a grating pattern. Referring to FIG. 1A, a silicon oxide layer 11 is deposited on a glass substrate 10 by sputtering process, photoresist is coated thereon by spin coating method and heat treatment is performed, to form a photoresist layer 12. Referring to FIG. 1B, exposure is carried out through a mask 13 in order to fabricate a grating pattern. Referring to FIG. 1C, exposed photoresist layer 12 is developed, and silicon oxide layer 11 is selectively etched by 15:1 buffered oxide etchant(BOE) using the photoresist layer as a mask. Referring to FIG. 1D, the photoresist used as the mask is developed to form silicon oxide layer pattern 11, thereby fabricating the grating pattern.
FIGS. 2A, 2B, 2C and 2D are cross-sectional views showing another conventional method of fabricating a grating pattern. Referring to FIG. 2A, photoresist is coated on the glass substrate 20 by spin coating method and heat treatment is performed, to form a photoresist layer 21. Referring to FIG. 2B, photoresist layer 21 is exposed through a mask 22 in order to form a grating pattern. Referring to FIG. 2C, exposed photoresist layer 21 is developed, and a silicon oxide layer 23 and 23xe2x80x2 is deposited by sputtering process. Referring to FIG. 2D, photoresist is coated on the silicon oxide layer 23 and 23xe2x80x2, silicon oxide layer 23xe2x80x2 formed on the photoresist layer 21 is etched using 15:1 BOE by masking process, and the photoresist layer (not shown) which was coated on the silicon oxide layer 23 and used as the mask is developed to form silicon oxide layer pattern 23, thereby fabricating the grating pattern.
In the above-mentioned conventional methods of fabricating the grating pattern, however, the silicon oxide layer is not uniformly deposited over the entire glass substrate because it is formed by sputtering process. That is, its thickness at the center of the substrate is different from that at the peripheral region. Furthermore, the fabrication of the silicon oxide layer requires an expensive vacuum apparatus, deteriorating productivity.
An object of the present invention is to provide a method for fabricating a grating pattern in which SOG solution is coated on a glass substrate by spin-coating method and heat-treated to easily form a silicon oxide layer.
In order to achieve the above object of the present invention, the present invention provides a method for fabricating a grating pattern, including the steps of: coating an SOG solution on a glass substrate by spin coating method; heat-treating the SOG thin film coated on the glass substrate, to form a silicon oxide layer; coating a photoresist on the silicon oxide layer and exposing the photoresist layer by masking process; developing the exposed photoresist layer and wet-etching the silicon oxide layer; and developing the photoresist layer used as a mask.
The SOG thin film is heat-treated at a temperature of 80-170xc2x0 C. The silicon oxide layer has a wet etch selectivity to the glass substrate of above 100:1, and its refractive index varies according to a gas atmosphere for the heat treatment. The silicon oxide layer is a multilevel silicon oxide layer, which is fabricated in such manner that the heat treatment is repeated at the temperature range of 80-170xc2x0 C., carrying out from a high temperature to a low temperature (each heat treatment being conducted a lower temperature than the prior treatment). Also, the silicon oxide layer pattern is formed a step shape by one-time wet etching.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.