PERFORATED METAL PRODUCT OBTAINED BY ETCHING, PROCESS FOR FORMING STARTING MATERIAL THEREFOR AND ETCHING PROCESS

Abstract: Described is a metal screen material obtained by photoetching having a ratio of depth etching to lateral etching between 1,5 and 5, the etching factor. Also is described a process for producing ametal starting material for manufacturing such screen material by photoetching in which the starting material is formed by electrodeposition of metal from a bath comprising one or more substances which influence the etching factor towards higher values. In order to reach a high etching factor the concentration of the relevant substance in the deposit is made to increase oppositely to the direction of later etching. Suitable substances are inter alia class I and class II brighteners.

Description: Perforated metal product obtained by etching, process for forming starting material therefor and etching process The present invention relates firstly to a metal screen material which is obtained by photoetching star¬ ting from an unperforated material. The manufacture of metal perforated products by photoetching is generally known. A perforated material of this type is obtained by depositing a photoresist pattern on a metal base and then etching away the parts not covered by the photoresist pattern with the aid of an etchant such as FeCl 3 . In the forming of known products of this type, there is the disadvantage that the degree of fineness thereof is in general relatively limited since during the etching procedure, etching occurs not only in the depth direction of the metal substrate but also in the lateral direction, i.e. under the photoresist pattern. For a targeted hole size, this phenomenon limits the so-called transmission percentage. Bearing in mind the rigidity of the final pro¬ duct, it is necessary to place the openings in the photoresist pattern none too close together so that, after the etching operation is complete, the bridge width between the different perforations is still adequate. In particular, if the substrate to be etched through is fairly thick, this problem will take on more serious forms and it will only be possible to manufacture a material with a relatively low degree of fineness. The object of the invention is to provide a screen material of the type referred to in the preamble having a high degree of fineness accompanied by large thickness. According to the invention, a material of this type is characterized in that the cross-sectional area of each of the openings in the screen material varies over the thickness according to a predetermined pattern as a function of a ratio between the final etching depth and the length of the lateral etching under the edge of an etch pattern (the etching factor) of between 1,5 and 5. As will be also explained in detail later, it has been found that it is possible to formulate the material to be etched in a manner such that etching factors much greater than 1,5 are obtained. The etching factor is defined as the ratio between the final etching depth and the length of lateral etching under a photoresist pattern and is in general equal to 1.5, i.e. an etching of approximately 33 micrometres also occurs in the lateral direction for an etching of 50 micrometres in the depth direction. It has now been found that it is possible to influence the etching factor by means of the composition of the metal to be etched in a manner such that the depth etching proceeds more rapidly than the lateral etching, in other words, an etching factor of greater than 1.5 is achieved. In particular, the cross-sectional area varies in accordance with an etching factor of between 1.8 and 4.0. If a screen material is formed by double-sided etching, the material composition may advantageously be such that two patterns are obtained by etching, each of the patterns extending from one side of the material over a part of the thickness of the material. Advantageously the etching patterns obtained with double-sided etching are mirror images and adjoin each other and each extend over an equal part of the thickness of the material, for example half thereof. However, as will be indicated below, the starting material for the screen material may also be constructed in a manner such that there is a central material layer which does not contain any substances which influence the etching factor or contains a fixed concentration thereof, while said central layer is clad with material which does in fact contain a structured concentration variation of sub¬ stances which influence the etching factor. Said mater¬ ial layers may have different thickness but may also be of equal thickness. Moreover the concentration patterns of substance which influences the etching factor may be identical or different in the two cladding layers on either side of the central layer. It will be clear that the screen material accor¬ ding to the invention may have a variety of forms, it being possible to vary the cross-sectional area of the openings over the thickness of the material as desired. In this manner, a metal screen material can be obtained which combines a large thickness with a high degree of fineness. The invention also relates to a process for forming a metal starting material for use in manufac¬ turing a screen material by photoetching, in which process the starting material is formed by electrolyti- cally depositing metal on amandrel, followedby removal of a metal deposit from the mandrel, which is characterized in that the metal deposit is obtained by using an electro¬ lytic bath which contains one or more substances which influence the etching factor of the metal deposit formed, and the concentration of substance influencing the etching factor is varied according to a predetermined pattern during the forming of the deposit. In particular, it has been found that there are substances which, when incorporated in an electrolyti- cally formed metal deposit, influence the etching factor to a high degree and which ensure, in particular, that the degree of lateral etching associated with a par¬ ticular degree of depth etching is significantly less than the depth etching. This implies that the etching factor is much greater than 1 in cases of this type. As has been discussed previously, the etching factor may be adjusted between 1.5 and 5, in particular between 1.8 and 4, as desired by choosing a suitable concentration variation of substance influencing the etching factor. As regards the type of substance which can be used to influence the etching factor, in particular substances which exhibit properties of brightening agents of the first and second class are satisfactory, although other organic additives which are normally used in electrolytic baths also have the property that they influence the etching rate. A number of additives of this type is mentioned in the book entitled Modern Electroplating, 3rd edition, 1973, published by John Wiley & Son Incorporated, pages 296 et seq. In particular, good results have been obtained if the substance which influences the etching factor is chosen from the group comprising butynediol, ethylene- cyanohydrin and disodium metabenzenedisulphonic acid. In particular, to obtain a high degree of fine¬ ness, it is important that the concentration of substance influencing the etching factor in the metal deposit increases in a direction which is opposite to the etching direction. In this manner, the result is achieved that the top of a perforation is only slightly larger than the opening in the associated etching pattern and that it is even possible to obtain a hole with walls extending virtually parallel to the etching direction or even walls receding in the etching direction. In particular, in the procedure according to the invention a starting material is formed by electrolyti- cally depositing a base deposit which does not contain any substance which, influences the etching factor or contains a fixed concentration thereof and said base layer is thickened in a bath or a series of consecutive baths, the concentration of substance influencing the etching factor being varied according to a predetermined pattern in said bath or in said series of consecutive baths. During the build up of a metal deposit, the concentration of substance influencing the etching factor can be increased with time during growth in order to ensure that the highest concentration of substance influencing the etching factor is situated at the surface which first comes into contact with the etching liquid. Of course, a concentration variation of this type may also be present in a series of consecutive baths in which a part of the thickening operation is carried out in each case. In an advantageous embodiment of the process for forming a starting material, the base deposit is removed from the mandrel onwhich it is formed, after which said base deposit is subjected, after clamping, to a thickening operation or a series of consecutive thicken¬ ing operations in a series of baths containing an increasing concentration of substance influencing the etching factor. In the bath or the series of baths the concentra¬ tion of substance influencing the etching factor should, on the one hand, be increased to compensate for the loss of said substance by incorporation in the deposit, while on the other hand, an additional increase is necessary to provide the desired concentration profile of substance influencing the etching factor during the build up of the metal layer. In a very advantageous embodiment of the proce¬ dure according to the invention the basic deposit removed from a mandrel is providedwith a metal deposit on two sides in a thickening operation, a suitable predetermined concentration variation of substance influencing the etching factor being formed in said metal deposit. Of course, a basic deposit of this type clad on two sides may alsobe left on themandrel for cladding on one side, after which the basic layer thus thickened on one si d e is removed from the mandrel and the other side is provided with a deposit; this procedure can be used, in particular, in the case of flat sheet-type screen mate¬ rials. Finally, the invention also relates to an etching procedure for forming a metal screen material, in which etching procedure a metal starting material is provided with an etch resist pattern, after which etching is carried out, which etching procedure is characterized in that a starting material is used as starting material which is obtained with the aid of the procedure according to the invention as described above. The invention will now be explained by reference to a drawing, in which: Figure 1 is a graph of the normal concentration varia- tion of substance influencing the etching factor over the thickness of a deposit as a consequence of depletion of the electrolysis bath. Figure 2 is a concentration variation of substance influencing the etching factor as a consequence of extra additions during the growth operation. Figure 3 shows diagrammatically a hole in a metal deposit formed by etching. Figure 4 shows diagrammatically the variation of the etching process while forming a screen material according to the invention. Figure 5 shows a cross section of an opening which has been etched in a base deposit clad with metal on two sides. Figure 1 shows the concentration variation of substance influencing the etching factor over the thick¬ ness of a metal deposit formed by electrodeposition. The metal deposit may be composed, for example, of nickel, butynediol being used as substance influencing the etching factor. It can be seen that as a result of depletion phenomena in the bath, the concentration of butynediol decreases over the thickness of the deposit formed; the concentration varies from C B to C z so that the last metal layer deposited has the lowest concentration of substance influencing the etching factor. The con¬ centration of substance influencing the etching factor is indicated here in arbitrary units. Figure 2 shows the concentration variation in a metal deposit when the procedure according to the inven- tion is used, the bath concentration being increased in a controlled manner during the growth of the deposit in order to compensate for depletion and to provide an increase in concentration in the metal deposit during the deposition thereof. It can be seen that the concentration has risen to C E for the final thickness D E . In both Figures it has been assumed that the starting material is formed by forming a thickening metal layer on a base deposit having a thickness D B so that the resultant total thickness of the material is D E after the deposition process. Figure 3 indicates that a cavity has been etched in a metal deposit 1 composed, for example, of nickel or copper on which there is a resist pattern having an opening therein through which etching liquid, for example FeCl 3 , can reach the metal. It can be seen that an underetching has taken place over a length O for the final etching depth D. The etching factor is now defined as D/0, a factor of 1 to 1,5 being usual under normal conditions without special measures being taken, whereas values between 1,5 and 5 can be achieved at every point over the etching depth according to the invention. Incidentally, it is pointed out that it is known in the art to include so-called "Flankenschutzmittel" ("Edge protection agents") in the etching liquid in an attempt to slow down the lateral etching during the etching. No data can be derived from the literature which describes the use of "Flankenschutzmittel" of this type during " etching on the incorporation of specific agents which slow down the etching in the metal which has to be etched through. Figure 4 diagrammatically shows the etching variation if a metal substrate is etched through using a photoresist pattern. In forming the starting material, a base deposit 40 was first formed on a mandrel, after which a thickening deposit 41 was formed on the base deposit using a bath or a series of baths in order to ensure that a pre-required concentration variation of substance influencing the etching factor is present in the deposit, in which case the highest concentration of substance influencing the etching factor will be situated at the side of the deposit which is opposite to the base layer 40 and which forms the top of the thickening deposit. To carry out the etching operation, a photoresist pattern 42 with openings 46 therein is applied to the top of the metal deposit. An etching liquid, for example FeCl 3 , is sprayed with the aid of an etching liquid source 43 against the surface carrying the photoresist pattern, the spraying etching liquid being diagrammatically indicated by 44. The etching operation forms an opening 47, the course of the etching process being indicated by the broken lines. It can be seen that in this case a large etching depth is combined with a relatively low underetching of the photoresist pattern 42. Figure 5 shows the cross section of a hole which has been formed in a metal deposit using a double-sided etching operation. The metal deposit is composed of a base layer 50 on which metal deposits 51 and 52 have been formed on two sides, the base deposit 50 not containing any substance influencing the etching factor while a desired concentration variation of substance influencing the etching factor is provided in the thickening deposits 51 and 52. A photoresist pattern 53 or 54 respectively is formed on both sides and this contains openings 55 and 56 situated opposite each other. Etching now forms the opening " 57, the concentration variation of substance influencing the etching factor in the layers 51 and 52 being adjusted so that a very low degree of underetching is combined with a large depth etching. The screen material formed in Figure 5 has a total thickness of, for example, 1300 micrometres, the base deposit being 100 micrometres thick, with a deposit 600 micrometres thick being grown thereon. The pitch spacing of the openings 55 and 56 was, in a typical case, 1,000 micrometres, so that the final degree of fineness of the screen material obtained is approximately 25 mesh (the mesh number indicates the number of openings in a screen material per linear inch) . In this case the screen material was composed of nickel, while the substance influencing the etching factor in the layers 51 and 52 was butynediol. While the layers 51 and 52 were being formed, the concentration of butynediol in the bath used was allowed to increase from 0 mg/1 to 180 mg/1. The etching factor achieved at the end of the etching was 2.7 on both sides. It is further pointed out that the substrates described here may have either a flat shape or a cylin- drical shape. In the case of cylindrical substrates, the base material used may have a thickness of up to approximately 200 micrometres. A thickening layer with good adhesion is then deposited. The photoresist pattern is formed on the basis of generally known photolithographic methods for forming an etch resist pattern, for example using a photoresist. To form an etch resist pattern, however, use may also be made of a laser engraving procedure in which the portions of a continuous resist layer through which the etching liquid has later to come into contact with the metal base has been burned away with the aid of a laser. In addition, a laser exposure method can also be used in which parts of a continuous photosensitive layer are selectively cured or rendered chemically soluble using a negative-working or positive-working photoresist suitable for this purpose. It will also be possible to make use of a laser engraving procedure or a laser exposure procedure for the accurate positioning of the openings in an etch resist pattern on both sides of a substrate since procedures of this type make very accurate positioning possible. For an accurate positioning of this type, reference is made to the Dutch Patent Application No. 8802928 of the Applicant filed simultaneously with the present application. Claims

Claims: Claims 1. Metal screen material which is obtained by photoetching starting from an unperforated material, characterized in that the cross-sectional area of each of the openings (47, 56) in the screen material varies over the thickness according to a predetermined pattern as a function of a ratio between final etching depth (D) and the length of lateral etching (O) under the edge of an etch pattern (2) (the etching factor) of between 1.5 and 5. 2. Screen material according to Claim 1, charac¬ terized in that the cross-sectional area varies in accordance with an etching factor of between 1.8 and 4.0. 3. Screen material according to Claim 1 - 2, charac- terized in that two patterns are obtained by double-sided etching in which the cross-sectional area varies, each of the patterns extending over a part of the thickness. 4. Screen material according to Claim 3, charac¬ terized in that the two patterns are mirror images and adjoin each other. 5. Process for forming a metal starting material for use in the manufacturing of a screen material by photo¬ etching, in which process the starting material is formed by electrolytically depositing metal on a mandrel, followed by removal of the metal deposit from the mandrel, characterized in that the metal deposit is obtained by using an electrolytic bath which contains one or more substances which influence the etching factor of the metal deposit formed, and the concentration of substance influencing the etching factor is varied according to a predetermined pattern during the forming of the deposit. 6. Process according to Claim 5, characterized in that the substance which influences the etching f ctor is a substance or a mixture of substances having properties of brightening agents of the first and second class. 7. Process according to Claim 6, characterized in that the substance or a mixture of substances is chosen from the group comprising butynediol, ethylenecyanohydrin and disodium metabenzenedisulphonic acid. 8. Process according to Claim 5 - 7, characterized in that the concentration of substance influencing the etching factor is increased according to a predetermined pattern during the forming of the deposit. 9. Process according to one or more of the prece¬ ding claims, characterized in that the metal deposit is formed by electrolytically depositing a base deposit which does not contain any substance influencing the etching factor or contains a fixed concentration thereof and the base deposit is thickened in a bath or a series of consecutive baths and the concentration of substance influencing the etching factor is varied according to a predetermined factor. iθ. Process according to Claim 9, characterized in that thebase deposit is removed fromthemandrel onwhich it is formed, after which the base deposit is suitably clamped for the subsequent thickening operation. 11. Process according to Claim 10, characterized in that the base deposit is provided with a metal deposit on two sides in a thickening operation. 12. Etching process for forming a metal screen material, in which process a metal starting material is provided with an etch resist pattern and is etched, characterized in that a starting material is used as starting material which is obtained with the aid of the process according to one or more of Claims 5 - 11.

Inventors: THUIS HENRICUS HERMANUS WILHEL, THUIS, HENRICUS, HERMANUS, WILHELMUS
Applicants: STORK SCREENS BV, STORK SCREENS B.V.
Application year: 1989
Main classification: C23F1
IPC codes: B07B1, C23F1, C25D5