THIN FILM ARTWORK COMPOUNDS

Abstract: A graphic artwork includes a thin film coating layer thereon which includes a copper oxide which is substantially opaque to ultraviolet light and partially transmissive to visible light and suitable for printing circuiting on a printed circuit board or for other graphic artwork. In the case where the thin film ­ pound includes unoxidized copper, etching thereof to achieve a predetermined pattern on the graphic artwork is facilitated by using a mixture of ammonium chloride and hydrogen peroxide.

Description: The present invention relates to copper oxide masks and methods of making such masks. Heretofore, thin film coatings for producing graphic artworks such as masks for printed circuits or photo tools for graphic printing have suffered from a variety of known disadvantages. For example, germanium is a well known compound which is opaque to ultraviolet light and hence useful for producing patterned graphic artworks, but has an inherent disadvantage due to its high cost. Metals such as zinc or aluminum could be used for the production of graphic artworks since they are also opaque to ultraviolet light, but an accompanying disadvantage is that rather strong caustic solutions are required to appropriately etch these metals. Zinc has an additional disadvantage in that it is soft. Also, metals are not transparent at visible light wavelengths and hence it is difficult to align a mask or graphic artwork using metals if the product being masked requires several superimposed pattern layers. EP-A-0057528 describes films for micrographics and graphic arts having a layer comprising a dispersion of metal and metal oxide or sulfide and a photoresist over the said layer. Copper is not mentioned among the several metals proposed. N. Feldstein et al, in "Fabrication of semitransparent masks", J. Electrochem Soc. Vol. 120, No. 12, December 1973, pp 1654-1657, and in the US-A-3882155, describe masks of copper oxide alone. The invention provides a method of making a mask by (a) sputtering a coating layer containing copper oxide onto a substrate; (b) forming a patterned resist material onto selected regions of the coating layer so as to form a predetermined pattern on the coating, leaving the remainder of the coating layer exposed; and (c) etching the exposed coating layer to form a mask, characterized in that the substrate is transparent to ultraviolet light and in that the coating layer comprises unoxidized copper metal together with the copper oxide and is at least partially transparent to visible light, and has less than 10% transmittance to light at wavelengths between 300 and 400 nanometers. The invention also provides a mask comprising (a) a substrate and (b) a light blocking layer containing copper oxide disposed on a surface of the substrate, characterized in that the substrate is transparent to ultraviolet light and in that the coating layer comprises unoxidized copper metal together with the copper oxide and is at least partially transparent to visible light, and has less than 10% transmittance to light at wavelengths between 300 and 400 nanometers. According to a preferred embodiment of the invention, the coating layer is etched using ammonium chloride mixed with hydrogen peroxide, the hydrogen peroxide oxidizing the nonoxidixed copper atoms. Preferably, the coating layer is deposited by planar magnetron reactive sputtering whereby a target being sputtered comprises essentially pure copper metal, with oxygen being introduced into the sputtering chamber as a gas. Optimum optical properties of the coating layer are determined by varying a sputtering power and an oxygen partial pressure with the sputtering chamber while measuring the resultant optical properties of the coating layers so produced using feedback control. Specifically, the coating layer can be tailored so as to have a transmission to light having a wavelength between 300 and 400 nanometers which is less than 10%, preferably no greater than 1% (e.g. between 200 and 400 nanometers), more preferably no greater than 0.1% (e.g. between 240 and 400 nanometers), and optimally less than 0.01%. Also, the coating layer can be tailored so as to have a transmission to visible light, e.g. light having a wavelength between 600 and 700 nanometers, which is greater than 20%, preferably no less than 30%, or greater than 40%, optimally greater than 60%. Figure 1 illustrates a substrate having a coating layer of the invention disposed thereon; Figure 2 illustrates the embodiment of Figure 1 on which has been disposed a patterned resist material; Figure 3 illustrates the embodiment of Figure 2 after etching; Figure 4 illustrates a mask as illustrated in Figure 3 being used for producing a predetermined pattern on a printed circuit board; Figure 5 illustrates apparatus for depositing the coating of the invention on a substrate; Figure 6 is a graph illustrating transmission characteristics as a function of light wavelength for a coating of the invention produced according to Example 1; and Figure 7 illustrates transmission characteristics of a coating as a function of light wavelength for a coating produced according to Example 2. Figures 1-4 generally illustrate a process for making a mask of the invention. Figure 1 illustrates a substrate 1 on which has been formed a coating layer 2, and Figure 2 illustrates the article of Figure 1 subsequent to a resist material 3 being formed ontop of the coating layer 2 in a predetermined pattern. Thereafter, the substrate 1, the coating layer 2, and the pattern resist material 3 are exposed to an etching solution which selectively etches exposed portions 4 of the coating 2 rather than the resist material 3 so as to form the predetermined pattern on the substrate 1 and also on the coating layer 2, as illustrated in Figure 3. The mask 15 so produced is then used to transfer the predetermined pattern into an appropriate medium, such as a printed circuit board 9 illustrated in Figure 4 so as to print circuit paths and circuit features corresponding to the predetermined pattern onto conducting layer 12 of the board 9. The printing is accomplished by disposing the mask 15 over the printed circuit board 9 and exposing both the mask 15 and the printed circuit board 9 to ultraviolet light radiation 8. The radiation is transmitted through the mask 15 at all areas thereof where the coating layer 2 has previously been etched away thereby exposing an upper ultraviolet sensitive layer 13 of the printed circuit board along these areas, the ultraviolet light radiation being blocked by the mask at all locations where the coating layer 2 had not been previously removed. After ultraviolet light radiation exposure, exposed portions of the ultraviolet sensitive layer 13 of the printed circuit board 9 are removed by conventional techniques, and portions of the metal layer 12 so exposed are then etched away so as to form the predetermined pattern within the metal layer 12 of the printed circuit board. In practice, it is not atypical for a printed circuit board to have a plurality of stacked metal layers thereon each individually patterned in a preselected way, the plurality of layers being interconnected by vias or vertical circuit lines. Accordingly, it can be appreciated that for producing such multilayer printed circuit boards it is advantageous that the masks used for printing the circuits on the various board layers be at least partially transparent at visible light wavelengths so as to assist in aligning the masks with alignment markings on the board or other patterns of a set of masks. Of course the masks of the invention can be used in additional applications as well. For example, masks are used for producing color printing plates. Since a plurality of such masks and associated plates need to be used to superimpose a plurality of colors for a single image to achieve good color resolution, it is particularly advantageous that the masks used to create these plates be semitransparent at visual wavelengths so as to allow the masks associated with any one image to be stacked to insure a partial image of each mask appropriately registers with partial images of each of the other masks. According to the invention, the coating layer 2 of the mask 15 comprises a copper oxide thin film, optimally having a thickness of between 300 and 3000 angstroms, preferably between 500 and 2000 angstroms, e.g. between 600 and 1200 angstroms, the copper oxide essentially corresponding to Cu₂O. Though the oxide Cu₂O is known to be formed according to the first embodiment since a color of the coating is yellowish, it is conceivable that the other oxide of copper, CuO, any also exist in small amounts in the coating layer. The coating layer 2 also comprises unoxidized copper metal particles, the coating layer having a volume average composition Cu₂O x where x is less than 2.0, preferably less than 1.0. It should be noted there are a variety of ways of characterizing the crystalline structure of the coating layer, e.g., as a suboxide or as a cermet, due to its extremely thin layer structure and an inevitable random nature in which it must be deposited. However, the characterization of the coating layer as only one of a suboxide and a cermet is actually of only academic concern, the important characteristic of the coating being that unoxidized copper does not exist, and hence the volume average composition of the coating layer comprises CuO x where x is less than 1.0, and preferably less than 0.5. Preferred materials for the substrate include polyester and other types of thin plastic films. Examples of resist materials include Riston® manufactured by DuPont and AZ1350 liquid photoresist manufactured by Shipley. A preferred method for forming the coating layer 2 on the substrate 1 is by planar magnetron reactive sputtering using an apparatus such as that illustrated in Figure 5. Referring more particularly to Figure 5, a target 2, which comprises a cathode, consists essentially of copper metal, and is located directly above a magnet 21 utilized for confining ionized gases in a region of the target 22, the gases to be ionized being injected into sputtering chamber 31 via any one of inlet ports 27-29. An outlet port 30 is provided for evacuating the sputtering chamber 31. The copper is deposited onto the substrate 1 as the layer 2 by maintaining a voltage differential between the cathode copper target 22 and an anode 33 while introducing at least one inert gas such as argon into the sputtering chamber which is ionized by electric fields created within the sputtering chamber 1. The ionized gas causes metal particles to be knocked off the target 22 and to be sputter deposited onto the substrate 1. Figure 5 further shows a substrate holder 23, 24, a seal 25 for sealing between the holder portion 24 and a wall 26 of the sputtering chamber 31, a D.C. high voltage power source 35, insulation 36, a shielding box 37 disposed around the copper cathode target 22, and electrical connection cables 38 connected to the power source 35. An important feature of the invention is that oxygen is introduced into the sputtering chamber 31 via any one of the inlet ports 27-29 so that the copper metal particles are deposited onto the substrate as an oxide or suboxide, depending upon the partial pressure of oxygen within the sputtering chamber 31 and the sputtering power. Accordingly, if the substrate 1 is continuously transported across a sputtering region within the sputtering chamber 31 while a partial pressure of oxygen within the sputtering chamber 31 and/or a sputtering power is varied, it is possible to deposit various different coating layer compositions onto the substrate 1 so as to obtain optimum optical properties. In addition, if the opaqueness or transparency of the coated substrate 1 is measured soon after sputtering at various light wavelengths, feedback control can also be incorporated so as to continuously vary various deposition parameters until an optimum coating layer composition is achieved. The copper oxide masks including nonoxidized copper according to the invention have several significant advantages over coating layers previously used for masks. First the present coating layers are extremely environmentally stable, second they are easily produced and optimized using reactive magnetron sputtering techniques, and third it is possible to etch such coating layers with materials that are non-caustic and relatively environmentally benign. In addition, the coating layers of the invention can be produced as extremely thin layers, as mentioned typically between 300 and 3000 angstroms thick, preferably between 500 and 1500 angstroms thick, optimally between 600 and 1200 angstroms thick, and are very fine grained, so that they can be etched to form extremely fine line patterns. And, as mentioned, optical properties of these coating layers can be readily adapted to meet optimum requirements for any particular photo reproductive system. These coating layers may be etched by using a solution of ammonium chloride and hydrogen peroxide, ammonium chloride alone not being capable of etching the unoxidized copper metal particles, and accordingly the use of hydrogen peroxide in conjunction with the ammonium chloride results in the copper being fully oxidizible and hence fully removable in the solution. The invention will be further described with reference to particular examples. Example 1 An eighteen inch diameter stainless steel bell jar vacuum chamber was fitted with a planar magnetron sputtering source with an 8" x 3" target surface and a 12" diameter rotating drum substrate holder which a substrate of flexible film would be wrapped. The target to substrate separation was 2½". The planar magnetron source was filled with a 99.9% pure copper target, and a substrate of 4 mil thick PET polyester film substrate was wrapped around the drum. The chamber was evacuated to a pressure of 1.8 x 10⁻⁵ Torr, then the following coating conditions were met: partial pressure of argon gas, 2.0 mTorr; partial pressure of oxygen gas, 0.30 mTorr; sputtering power, 500 watts; drum speed, 0.19 rpm; coating time, 25 sec. The resulting coating had a composition consisting essentially of Cu₂O, a visible light transmission of 60%, a yellow color, and a transmission spectrum as shown in Figure 7. It was less than 1% transparent to ultraviolet radiation of wavelengths less than 400 nm. This coating could be completely removed by immersing it for 20 sec. in a solution of 27% (by weight) of ammonium chloride in water at 20 degrees for 20 seconds. The coating was however quite stable, and showed no change in properties after extended periods in air, at 80 degrees dry heat, and at 60 degrees C, 95% RH environments. Example 2 The apparatus used in Example 1 was used to coat a piece of 4 mil thick PET polyester film in the same manner as was used in Example 1, using the following conditions. The chamber was evacuated to a pressure of 1.8 x 10⁻⁵ Torr, then backfilled with argon gas to a partial pressure of 2.0 mTorr and oxygen gas to a partial pressure os 0.8 mTorr. The target was then sputtered in this atmosphere at a power of 1000 watts for twelve minutes, while the substrate drum was rotated at a speed of 0.5 rpm. The resulting coating had a composition CuO x where x was less than 0.5, had an optical density of 2.67 for visible light, and a transmission spectrum as shown in Figure 2. It was less than 0.1% transparent to ultraviolet radiation of wavelengths less than 400 nm. This coating could be completely removed by immersing it for 20 seconds in a solution of 27% ammonium chloride in water plus 7 ml of 3% hydrogen peroxide solution per 100 ml of ammonium chloride solution, at 20 degrees C. The coating was however quite stable, and showed no change in properties after extended periods in air, 80 degrees C dry heat and 60 degrees C 95% RH, environments.

Claims: A method of making a mask by (a) sputtering a coating layer containing copper oxide onto a substrate; (b) forming a patterned resist material onto selected regions of the coating layer so as to form a predetermined pattern on the coating, leaving the remainder of the coating layer exposed; and (c) etching the exposed coating layer to form a mask, characterized in that the substrate is transparent to ultraviolet light and in that the coating layer comprises unoxidized copper metal together with the copper oxide and is at least partially transparent to visible light, and has less than 10% transmittance to light at wavelengths between 300 and 400 nanometers. The method of claim 1, wherein the unoxidized copper metal is in the form of unoxidized copper metal particles. The method of claim 1 or 2, the coating layer consisting essentially of unoxidized copper metal particles and the copper oxide. The method of any preceding claim, the exposed coating layer being etched using ammonium chloride. The method of any preceding claim, the exposed coating layer being etched using a mixture of ammonium chloride and hydrogen peroxide. The method of any preceding claim, the coating layer being formed by sputtering a copper target in an ionized atmosphere containing a partial pressure of oxygen, a level of the partial pressure of oxygen being controlled so as to precisely control optical characteristics of the coating layer being formed, the coating layer having a thickness less than 3000 angstroms. The method of any preceding claim, the coating layer having a radiation transmittance to light having a wavelength between 200 and 400 nanometers which is no greater than 1%. The method of claim 7, the coating layer having a radiation transmittance to light having a wavelength between 240 and 400 nanometers which is no greater than 0.1%. The method of any preceding claim, the coating layer having a radiation transmittance to light having a wavelength between 600 and 700 nanometers which is no less than 30%. A mask comprising (a) a substrate and (b) a light blocking layer containing copper oxide disposed on a surface of the substrate, characterized in that the substrate in transparent to ultraviolet light and in that the coating layer comprises unoxidized copper metal together with the copper oxide and is at least partially transparent to visible light, and has less than 10% transmittance to light at wavelengths between 300 and 400 nanometers. The mask of claim 10, the light blocking layer having a thickness less than 3000 angstroms and a light transmission no greater than 1% to light radiation having a wavelength between 300 and 400 nanometers, the light blocking layer having a transmittance no less than 30% to light radiation having a wavelength between 600 and 700 nanometers. The mask of claim 10 or 11, the light blocking layer having a thickness less than 3000 angstroms and a transmittance no greater than 0.1% to light radiation having a wavelength between 300 and 400 nanometers.

Inventors: CZUKOR, JANOS, KITTLER, WILFRED C., JR., STODDARD, DARRELL
Applicants: ANDUS CORPORATION
Application year: 1988
Main classification: G03F1
IPC codes: G03F1, G03F7, H05K3