Patent Publication Number: US-2005117222-A1

Title: Precise optical grating element and method of making a stamper thereof

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to a precise optical grating element and a method for making a stamper thereof. In particular, it relates to a method for making a precise optical grating element and its stamper through a similar fabrication process of read-only optical disc. In addition, the signal of the optical grating element can be read through a common optical pick-up head.  
      2. Related Art  
      An optical encoder, which resolution is determined by the scale of the optical grating element, is composed of an optical grating element and an optical reading device. A finer scale gives a higher resolution. Optical encoders are usually used in the precise control systems for detecting angles, speed or positions.  
      Conventionally, optical grating elements used in optical encoders are made by plastic mold-injection, machining or semiconductor process.  
      The optical grating elements made by the plastic mold-injection are easy and low cost, while their fine scale is limited by the precision of tooling and injection. Therefore, they cannot meet the high-resolution requirements to be used in the precise control systems.  
      The optical grating elements made by machining are also limited to the precision of machining and the fine scale of optical grating elements can not be obtained. As a result, they cannot be used in the high-precision control systems as well. Further, machining is not optimal for mass production, and the cost is high.  
      Semiconductor process for making the optical grating elements is illustrated in  FIGS. 1A  to  1 G. First, in  FIG. 1A , a glass or metal substrate is provided with a flat surface after polishing. Then, the surface is cleaned to be free from dusts.  
      In  FIG. 1B , a photoresist layer is coated on the surface of the substrate, then, heated and cured there, as shown in  FIG. 1C , through a curing process.  
      Then, in  FIG. 1D , using a photomask, which carries an optical grating pattern, on the substrate to expose and transfer the optical grating pattern onto the surface of the substrate after developing, as shown in  FIG. 1E .  
      Further, in  FIG. 1F , depositing a metallic film on the surface of the substrate as a reflective layer through a sputtering or evaporating process. The reflective layer makes the optical grating pattern readable by light beam. Then, coating a protective layer on the substrate to protect the optical grating element, as shown in  FIG. 1G .  
      Though the aforesaid semiconductor process can produce the optical grating element with fine scales to meet the high-resolution requirements, the optical signal is weak that a highly precise optical reading device is necessary for reading the weak signal. The delicate structure makes the product expensive and hard for mass production. Moreover, the optical signal is weak and sensitive to vibration that usually causes reading failure. In order to read the weak signal of the optical grating element, a complicated optical reading device including lens, mirrors and prisms, etc., such as that disclosed in U.S. Pat. No. 4,829,342 and U.S. Pat. No. 4,868,385, is required. However, it costs very high and is difficult in alignment and sensitive to vibration.  
     SUMMARY OF THE INVENTION  
      The object of the invention is to provide a new method for making an optical grating element and its stamper. The method is modified from the fabrication process of read-only optical disc. The optical grating element and the stamper have fine scales suitable for being used in the high-resolution control systems while resistant to vibration conditions.  
      The stamper can be used as a mold for the plastic injection so that optical grating elements can be easily made through the mold-injection and deposition process. Mass production is possible and cost less. Further, the optical grating element made from the stamper has fine patterns similar to that made by semiconductor process and gives high resolution. Besides, the weak signal from the optical grating element can be easily read using a common optical pick-up head. The common optical pick-up head is reliable and resistant to vibration so as to meet the requirements of high precision control systems and to overcome the problems of prior arts.  
      A comparison among fabrication processes of mold-injection, machining, semiconductor process and read-only optical disc process for optical grating elements is listed as follows.  
                                              Characteristics                                                     Mass   Reading       Different process   Process   Resolution   Price   production   device               Plastic mold-injection   Easy   Low   Low   Easy   Simple       Machining   Easy   Low   High   Difficult   Simple       Semiconductor process   Difficult   High   Expensive   Difficult   Complicated       Read-only optical disc process   Easy   High   Low   Easy   Simple                  
 
      From the table, it is clear that the optical grating element made by using the read-only optical disc process of the invention has easy process, high resolution, low cost, easy for mass production and the optical reading device is simple and reliable.  
      The prior arts of making a read-only optical disc include the following steps. First, polishing a glass substrate to get a flat surface. Washing and removing dusts from the surface of the substrate. Then, coating a photoresist layer on the surface of the substrate. Heating and curing the photoresist layer. Further, using a high power laser to write data patterns into a spiral track on the substrate. Forming the data patterns on the substrate through a developing process. Then, depositing a metallic film as an electrically conductive layer on the substrate through a sputtering or evaporating process. Further, electroplating the substrate to increase the thickness of the metallic film and forming a metallic plate. Then, separating the metallic plate from the substrate. The metallic plate carries the reversed patterns of the data patterns on the substrate. Cleaning the metallic plate, polishing its back, punching the central hole and removing the circumference excessive portion to finish a stamper for making the read-only optical disc.  
      When making a read-only optical disc, the stamper is used as a mold for the plastic injection and getting a plastic substrate having a correspondent pattern to that of the stamper. Then, the plastic substrate is coated with a metallic film as a reflective layer so that a laser beam can read the data patterns from the optical disc. Finally, a protective layer is formed on the optical disc to finish a common read-only optical disc. The fabrication process is inexpensive because the plastic mold-injection is easy and suitable for mass production.  
      The fabrication process of the invention is similar to that of making a read-only optical disc. But before the developing process, a photomask with the optical grating patterns is used for exposure to transfer the optical data patterns onto the substrate instead of using a laser beam to write the spiral data patterns. Finally, a stamper of optical grating element is made through the rest procedures as described above for making a stamper of read-only optical disc.  
      When making an optical grating element, the stamper is used as a mold for the plastic injection and getting a plastic substrate having a correspondent pattern to that of the stamper. Then, the plastic substrate is coated with a metallic film as a reflective layer so that a laser beam can read the data pattern from the optical disc. Finally, a protective layer is formed on the optical disc to finish an optical grating element.  
      The optical grating element made by the invention has fine scales for being used in the precise control systems. Though the optical signals are weak, they can be read using a common optical pick-up head so that no special or complicated reading device is needed. The current optical disc technology is well developed, and the optical pick-up head can read very small signals. For example, the track size of 1.6 μm in CD has been reduced to 0.74 μm in DVD, and has been further reduced to 0.32 μm in blu-ray disc. The common optical pick-up heads are adequate for reading the weak signals of the optical grating element accordingly and they are reliable and low-cost.  
      Moreover, the focal lens in the optical pick-up head can be finely adjusted by using an actuator when the focus of reading is not well aligned. The fine adjustment helps the optical grating element free from the poor signal problems caused by uneven surface or vibration. Therefore, using the optical pick-up head is the only way to prevent from the vibration problem of prior arts.  
      The optical grating element made by using the read-only optical disc fabrication process of the invention has fine scales suitable for optical encoders and meets the high-resolution requirements of precise measuring systems. Common inexpensive and practical optical pick-up head can be used for reading the weak signals of the optical grating element.  
      The optical grating element is also applicable to a sliding mechanism and used as a control element of precise displacement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will become more fully understood from the detailed description given hereinbelow. However, this description is for purposes of illustration only, and thus is not limitative of the invention, wherein:  
       FIGS. 1A  to  1 G are sequential views of a semiconductor fabrication process for making an optical grating element;  
       FIGS. 2A  to  2 L are sequential views of a modified read-only optical disc fabrication process for making a stamper of optical grating element;  
       FIGS. 3A  to  3 E are sequential views of a fabrication process of mold-injection and film-deposition for making an optical grating element from a stamper made by using the process of the invention;  
       FIGS. 4A  to  4 D are sequential views of a fabrication process of mold-injection, film-deposition and adhesion for making an optical grating element from a stamper made by using the process of the invention; and  
       FIGS. 5A and 5B  are examples of photomask patterns for making different kinds of optical grating elements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIGS. 2A  to  2 L are sequential views of a modified read-only optical disc fabrication process of the invention for making a stamper of optical grating element. First, in  FIG. 2A , polishing a glass substrate  10  to get a flat surface. Washing and removing dusts from the surface of the substrate  10 . Then, in  FIG. 2B , spin-coating a photoresist layer  20  on the surface of the substrate  10 . Heating and curing the photoresist layer  20  on the substrate  10 , as shown in  FIG. 2C . Further, in  FIG. 2D , a photomask  30  with an optical grating pattern is used for light exposure so that the optical grating pattern is transferred to the substrate  10 . In  FIG. 2E , forming an optical grating pattern on the substrate  10  through a developing process and removing excessive photoresist layer  20 . The optical grating pattern is correspondent to the shape and resolution of the photomask  30 . As shown in  FIG. 2F , an optical grating pattern is formed. Then, in  FIG. 2G , depositing a metallic (usually nickel) film  40  as an electrically conductive layer on the substrate  10  through a sputtering or evaporating process. Further, in  FIG. 2H , electroplating the substrate  10  to increase the thickness of the metallic film  40  and forming a metallic plate  50 . Then, in  FIG. 21 , separating the metallic plate  50  from the substrate  10 . The metallic plate  50  carries a reversed pattern of the optical grating pattern on the substrate  10 . Further, in  FIG. 2J , cleaning the metallic plate  50 , polishing its back, punching the central hole and removing the circumference excessive portion, as shown in  FIG. 2K , to finish a stamper, as shown in  FIG. 2L , for making the optical grating elements.  
       FIGS. 3A  to  3 E are sequential views of a fabrication process of mold-injection and film-deposition for making an optical grating element from a stamper made by using the process of the invention.  
      As shown in  FIG. 3A , when making an optical grating element, the stamper  60  is used as a mold for the plastic injection and getting a plastic substrate  70  as shown in FIG.  3 B. The plastic substrate  70  is made of a polycarbonate with the thickness of 0.6 to 5 mm. Then, in  FIG. 3C , the plastic substrate  70  having a correspondent pattern to that of the stamper  60  is separated from the stamper  60 . In  FIG. 3D , the plastic substrate  70  is coated with a metallic film  80  as a reflective layer so that a laser beam can read the data signals from the plastic substrate  70 . The metallic film  80  is usually made of an aluminum having a reflectivity over 35%. Finally, in  FIG. 3E , a protective layer  90  is formed through spinning coating on the metallic film  80  to finish an optical grating element. The material of the protective layer  90  is an acrylic photocurable resin.  
      As shown in  FIGS. 4A  to  4 D, the optical grating element can also be made by adhesion. First, in  FIG. 4A , making a blank stamper  100  by using a similar process of the aforesaid read-only optical disc fabrication process. Then, in  FIG. 4B , using the blank stamper  100  as a mold for the plastic injection and getting a plastic blank substrate  110 . The blank substrate  110  is made of a polycarbonate with the thickness of 0.6 to 5 mm. Then, in  FIG. 4C , the blank substrate  110  is separated from the blank stamper  100 . In  FIG. 4D , adhering the blank substrate  110  to a plastic substrate  70  with a metallic film  80  made from the aforesaid process of  FIG. 3D  and finishing the optical grating element. The optical grating element made by adhesion is stronger than that made with a protective layer through spinning coating.  
      The optical grating element made by using the read-only optical disc fabrication process of the invention has fine scales suitable for optical encoders and meets the high-resolution requirements of precise measuring systems. The optical grating element is also applicable to a sliding mechanism and used as a control element of precise displacement.  
      As the optical grating element of the invention being made through the read-only optical disc fabrication process, the weak signals of optical grating element can be read using a common optical pick-up head. In comparison with the complicated optical reading system composed of lens, mirrors and prisms, etc., for reading the fine scale in prior arts. The common optical pick-up head is adequate for optical grating elements of the invention in reading data and it is practical, inexpensive and convenient.  
      Moreover, the focal lens in the optical pick-up head can be finely adjusted by using an actuator when the focus of reading is not well aligned. The fine adjustment helps the grating element free from the poor signal problems caused by uneven surface or vibration. Therefore, using the common optical pick-up head is the only way to prevent from the vibration problem of prior arts.  
       FIGS. 5A and 5B  are examples of photomask patterns  30  for making different kinds of optical grating elements. The photomask pattern  30  in  FIG. 5A  has a single scale so that an optical grating element made form it has a fixed fine scale. While, in  FIG. 5B , the photomask pattern  30  has multiple scales so that an optical grating element made form it has multiple scales. Users can choose the different scale regions for different needs and expand the applications of an optical encoder.  
      In the photomask patterns  30  of  FIGS. 5A and 5B , there is a circular reflective region  31  at the center of the patterns. The reflective region  31  has no optical grating patterns but a plane for checking the performance of the laser diode in the optical pick-up head. When the laser diode decays, the reflection from the reflective region  31  decreases and indicates that the laser diode has to be replaced. Therefore, there is no need to use additional components for calibration of the laser source. The reflective region  31  is not necessarily at the center portion. It can be located at the circumference, for example.  
      The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.