A METHOD FOR FORMING AN ABRASION RESISTANT COATING ON A TRANSPARENT SUBSTRATE

Abstract: A method for depositing an abrasion resistant coating (54) on a transparent substrate (41) includes directing a first stream of argon ions (50) at an aluminum oxide target (48), thereby raising the temperature of the aluminum oxide until aluminum oxide molecules are released for deposit on a rotating transparent substrate (41) positioned adjacent the target (48). A second stream of argon ions (58) is applied to the deposited aluminum oxide for providing a uniform surface to the layer of deposited aluminum oxide (54). The first stream of ions (50) applied to the aluminum oxide target (48) operates at an intensity level which is greater than the intensity level of the second stream of ions (58) applied to the deposited aluminum oxide layer (54). A glass substrate (24) provided with an abrasion resistant coating according to the above method may be used in a checkout counter (28) for serving as a support for merchandise items (40) being scanned by an optical scanner (42, 44).

Description: A METHOD FOR FORMING AN ABRASION RESISTANT COATING ON A TRANSPARENT SUBSTRATE Technical Field This invention relates to a coating process and more particularly to a process for coating a glass substrate in order to improve the abrasion resistant characteristic of the surface of the glass substrate. Background Art In merchandise check-out systems presently employed in supermarkets or the like, coded labels are attached to merchandise items such as cans of food or the like for use in processing the purchase of such items. The coded label contains data which is used in retrieving from a price look-up table the price of the item to which the coded label is attached. To increase the speed of the check-out operation, optical scanner devices have been incorporated into the check-out counters found in such check-out systems, in which a scanning light beam is projected through a transparent window located in the surface of the counter for scanning the coded label on the merchandise item being purchased. Movement of the merchandise item past the transparent window results in the reading of the coded label. In the case where the merchandise item is a metal can or other type of metal container, it has been found that the transparent window, which normally takes the form of a glass or plastic substrate, becomes scratched as a result of the movement of the can across the surface of the substrate, which interferes with the scanning light beam resulting in the generation of invalid scanning readings. This condition has limited the life of the glass window, thereby producing high maintenance cost as a result of replacing the window. In order to overcome this problem, windows composed of a sapphire sheet-glass laminate have been used to eliminate this scratching condition since the hardness of the sapphire is much greater than any material commonly used in the packaging of merchandise items. Such windows are very expensive and therefore are limited to relatively small window dimensions. Less expensive coatings such as aluminum phosphate or other types of metallic coating applied to a transparent substrate have been used. However each of these coatings tends to exhibit low optical quality and high light absorption together with scattering of the light beams as they are transmitted through the coating. The scattering diminishes the effectiveness of the light beam in scanning the coded label. Disclosure of Invention It is therefore an object of this invention to provide a method for constructing a transparent substrate having a high abrasion resistant surface in which the above disadvantages are alleviated and which is inexpensive to manufacture. Thus, according to the invention, there is provided a method for forming an abrasion resistant coating on a transparent substrate including the steps of: mounting a transparent substrate within a vacuum on a support member; positioning a body of aluminum' oxide material adjacent the substrate, the body having a planar surface facing the substrate; and directing a first stream of energy beams having a first intensity level at the planar surface of the body of aluminum oxide to raise the temperature of the body of aluminum oxide, enabling the molecules of aluminum oxide to be deposited on the surface of the substrate; characterized by directing a second stream of energy beams having a second intensity level at the surface of the substrate for providing a uniform surface of the aluminum oxide deposited on the substrate. The preferred method of fabricating an abrasion resistant transparent substrate in which a thin film of aluminum oxide is applied to the surface of the substrate includes the steps of bombarding an aluminum oxide target with argon ion beams, thereby causing the ejection of oxide molecules from the target surface for deposit on a rotating transparent substrate positioned adjacent the oxide target. A first argon-ion sputtering gun, which is directed at the target of the substrate, is used in sputtering the molecules of the target of aluminum oxide located within a vacuum chamber for deposit on the surface of the substrate. A second argon-ion sputtering gun is used to form a relatively uniform surface on the substrate after the aluminum oxide has been deposited thereon by the first gun. The first ion gun is operated at an intensity level of 800-1000eV while the second gun is operated at an intensity level of 20- lOOeV. Brief Description of the Drawing An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a portion of a check-out counter showing the general arrangement of an optical scanner assembly; Fig. 2 is a schematic diagram of the ion gun arrangement for the deposition of the aluminum oxide on the glass substrate; Fig. 3 is an enlarged sectional view of the substrate manufactured according to the present invention, showing the crystalline structure of the initial deposition of aluminum oxide on the glass substrate. Fig. 4 is another enlarged sectional view of the substrate showing the crystalline structure of the finished layer of aluminum oxide. Best Mode for Carrying out the Invention Referring now to Fig. 1, there is shown a perspective view of a portion of a typical check-out counter found in a merchandise check-out system in which is located an optical scanner assembly for scanning a coded label on a merchandise item purchased by a customer. The system includes a light source 20 emitting an optical scanning light beam 22 in the visible or near infrared spectrum, the light beam being directed through a transparent substrate 24 which may take the form of a glass or plastic window which is mounted flush with the " supporting surface 26 of the check-out counter generally indicated by the numeral 28. The light source 20 may. be a helium neon laser that is pumped to produce a continuous laser scanning beam 22 of monochromatic light of approximately 6328 angstroms wavelength. In a manner that is well known in the art, the light beam 22 produced by the source 20 may be focused by a lens system 30 onto a multi-faced mirror 32. The mirror 32 is mounted on the shaft 34 of a motor 36 which rotates the mirror 32 at a substantially constant speed. The mirror 32 is positioned to intercept the light beam 22 and project same through the substrate 24 to scan encoded indicia located on a label 38 affixed to a merchandise item 40, the encoded indicia comprising a plurality of black and white coded areas (not shown) representing data concerning the identity of the merchandise item. The rotation of the mirror 32 causes a succession of light beams 22 to scan any encoded label 38 positioned over the substrate 24. The light beams 22 are reflected off the label 38 and back through the substrate 24 and an optical filter 42 to a photo- responsive pick-up device such as a photo-multiplier 44 which converts the reflective light beams into electrical signals in a manner that is well-known in the art. Movement of the merchandise item 40 across the substrate 24 enables the light beams 22 to scan the complete label 38. If the merchandise item 40 is a metal container, it has been found that the glass window 24 becomes scratched and pitted, thereby interfering with the scanning of the label 28 by scattering the light beams 22 as they are projected through the window 24, thus preventing the scanning system from properly reading the label. It has been found that by depositing a protective thin film layer of aluminum oxide (AI2O3) on a suitable transparent substrate such as glass, the above disadvantages can be overcome. One method of depositing such a film is shown in Fig. 2 in which a glass substrate 41 is mounted on a rotatably mounted support member 43 which in turn is secured to the shaft 45 of a motor 46, all of which are positioned within a vacuum chamber 47. The atmosphere within the chamber 47 is a mixture of oxygen and argon gas with the oxygen comprising 30-40% of the mixture. The motor 46 will rotate the support member 43 when operated. A target 48 composed of solid aluminum oxide is mounted adjacent the support member 43 and has a planar surface portion 49 which is orientated at a predetermined angle to the substrate 41. The target 48 is bombarded with ion beams 50 generated by an argon-ion sputtering gun 52. The gun 52 is operated at an intensity level of between 800-1000 eV, plus or minus 10 eV. The ion beams 50 from the gun 52 will, upon striking the target 48, impart energy to the surface molecules of the target, thereby heating the target to a temperature of between 10 and 150 degrees C, causing the molecules to be sputtered or ejected from the material, which molecules are then deposited on the rotating glass substrate 41. As shown in Fig. 3, the molecules of aluminum oxide are deposited on the glass substrate 41 in the form of a filamentary layer structure 54 whose filament structure 56 results in reduced hardness and film strength. In order to improve the abrasive resistance of the deposited layer structure 54, the substrate 41 is bombarded with ion beams 58 from a second argon-ion sputtering gun 60 (Fig. 2) which is operated at an energy level of between 20-100 eV plus or minus 10 eV, which operation disrupts the growth of the filaments 56 in the layer structure 54. This bombardment allows aluminum oxide to form between the filaments 56 resulting in a relatively uniform surface 62 (Fig. 4) whose index of refraction approximates that of aluminum oxide bulk material, thereby increasing the hardness of the structure 54. It is desirable that the glass substrate 41 and the layer structure 54 have similar indices of refraction to eliminate any reflection losses of the light beams as they travel through the layered substrate 41. The bombardment of the target 48 and the substrate 41 takes place in a vacuum of 1 X 10 -4 to 92 x 10 ~7 Torr. It has been found that with this method, the index of refraction of the deposited layer is between 1.71 and 1.73, which is close to that of bulk sapphire, which is 1.77. The glass substrate 42 is constructed to have a similar index. If the crystalline structure 54 does not have an index of refraction close to 1.77, while the substrate 41 has such an index, the hardness and durability of the deposited film are greatly reduced, together with increasing the attenuation of the scanning light beams as they pass through the substrate, thereby producing errors in the reading operation of the scanning device. It has been found that with an ion energy output of 600 eV from the first gun 52, the resulting thickness of the crystalline structure 54 is between .5 and 1.5 microns, with the aluminum oxide atoms being deposited on the substrate 41 at a rate of approximately 4 angstroms per minute.

Claims: CLAIMS : 1. A method for forming an abrasion resistant coating on a transparent substrate including the steps of: mounting a transparent substrate (41) within a vacuum on a support member (43); positioning a body of aluminum oxide material (48) adjacent the substrate (41), the body having a planar surface (49) facing the substrate (41), and directing a first stream of energy beams (50) having a first intensity level at the planar surface (49) of the body of aluminum oxide (48) to raise the temperature of the body of aluminum oxide (48), enabling the molecules of aluminum oxide to be deposited on the surface of the substrate (41); characterized by directing a second stream of energy beams (58) having a second intensity level at the surface of the substrate (41) for providing a uniform surface of the aluminum oxide deposited on the substrate (41). 2. Method according to claim 1, characterized in that the second intensity level is less than the first intensity level. 3. Method according to claim 2 , characterized in that the first intensity level is between 800 and 1000 eV. 4. Method according to claim 2, characterized in that the second intensity level is between 20 and 100 Ξv. 5. Method according to claim 1, characterized in that said support member (43) is rotated during deposition. 6. Method according to claim 1, characterized in that the layer of aluminum oxide deposited on the substrate (41) is between .5 and 1.5 microns thick. 7. Method according to claim 6, characterized in that the aluminum oxide is deposited on the substrate (41) at a rate of 4 Angstroms per minute. 8. Method according to claim 1, characterized in that said first and second streams of energy beams are beams of argon ions generated by respective argon-ion sputtering guns (52, 60). 9. A glass substrate (41) provided with an abrasion resistant coating (54) claimed in any one of the preceding claims. 10. The use of a glass substrate (41) according to claim 9 in a checkout counter (28) in which said substrate (24) serves as a support for merchandise items (40) being scanned by an optical scanning apparatus (42, 44).

Inventors: FERRANTE RONALD ALFRED, FERRANTE, RONALD, ALFRED
Applicants: NCR CO, NCR CORPORATION
Application year: 1986
Main classification: C23C14
IPC codes: C03C17, C23C14