RU2556697C1 - Method of making flexible micro-printed circuit boards - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of making flexible micro-printed circuit board includes pre-oxidising a monocrystalline silicon plate with thickness of 20-100 mcm, diameter of 200-300 mcm and orientation <100>; applying a coating; performing photolithography; coating the obtained electronic circuit with a polymer layer; dissolving the silicon dioxide film and peeling the silicon plate to form a flexible micro-printed circuit board on the polymer film.

EFFECT: obtaining highly dense packaging with the width of electroconductive tracks of less than 50 mcm and a shorter process cycle.

Description

The invention relates to the field of instrumentation and electronics and can be used in the manufacture of flexible printed circuit boards used in the manufacture of secondary converters of micromechanical accelerometers, microgyroscopes, integrated pressure sensors and other products.

A known manufacturing method, which consists in the manufacture of a metal pattern on a dielectric substrate by selective etching of individual sections of copper foil glued to the base of the dielectric [1]. The areas of the foil that should not be etched and which form the desired electrically conductive pattern of the electronic circuit are protected from the action of the etching solution by a photoresist resistant to it. After etching and removing the photoresist layer from the conductive tracks, an electronic circuit pattern is obtained.

The disadvantage of this method is that the printed circuit boards on fiberglass do not have flexibility and break when bent. A known method of producing boards on a metal base, which consists in sequentially applying a dielectric oxide-chromium and electrically conductive metal nickel coating to a metal plate [2].

The disadvantage of this method is that the base itself is flexible, however, when it is bent, peeling and destruction of the metallic nickel, cobalt coating forming an electronic circuit is observed.

The electrically conductive tracks on the board, obtained in this way, are covered with a protective layer of metal resist only from above, and the sides are unprotected.

The known method, consisting of sequential deposition on a metal plate by thermal decomposition of an aluminum metal coating with a thickness of 4-5 microns and conductive copper or molybdenum with a thickness of 20-30 microns. Then, by means of photolithography, a drawing of an electronic circuit is obtained and it is coated with a polymer layer 80-100 μm thick, after which a polymer film with an electronic circuit and a metal-resistive coating is separated by dissolving the aluminum layer [3].

The disadvantage of this method is lateral etching, as well as the inability to obtain an electronic circuit with a track width of less than 100 microns.

The problem to which the invention is directed, is to obtain high-density installation with a width of electrically conductive tracks less than 50 microns, a reduction in the technological cycle.

The problem is solved due to the fact that in the method of manufacturing a flexible micro-printed circuit board, which consists of sequentially applying metal-resisting nickel or cobalt coatings to a plate with a thermal decomposition and an electrically conductive copper or molybdenum coating, by means of photolithography, a drawing of an electrically conductive circuit is obtained, followed by coating with a polymer film and separation of the resulting flexible microprobe from the plate, a thin 20-100 micron single-crystal silicon diameter plate is used as the plate 200-300 mm, <100> orientation, pre-oxidized, with an oxide thickness of 1-2 microns, photolithography is carried out after coating, then coated with a polymer layer, then the silicon dioxide film is dissolved, followed by peeling of the silicon wafer and subsequent separation of the polymer film with an electrically conductive circuit and metal coating.

Instead of a metal plate, a single-crystal silicon wafer with a diameter of 200 to 300 mm and a thickness of 20-100 μm orientation <100> is used. The plate is pre-oxidized to 1-2 microns of oxide thickness. Before carrying out coating operations on one side of the wafer, the oxide film is etched. Next, operations are carried out coating and forming a micro-printed circuit board. Moreover, part of the operations can be carried out through masks, which increases the accuracy of the reproduction of electronic circuit elements, since there are no operations for applying a photoresist, its manifestation, followed by etching through a photoresist mask. In this case, ghosting is excluded. The final step is to dissolve the silicon dioxide film, followed by peeling off the silicon wafer and then separating the polymer film with an electrically conductive circuit and a metal resist coating. Currently, a number of companies, such as Wafer World and Twin Creeks Technologies, produce thin silicon wafers from 20 microns. The use of single-crystal silicon wafers is due to the fact that, based on the requirements for creating high density micro-printed circuit boards and precision topology elements, it is advisable to use microelectronic technology methods, which include: using liquid photoresists with high sensitivity, using a wide arsenal of methods for applying photoresist onto a plate (centrifuge, immersion , pulverization), a combination of positive and negative photoresists, exposure (possibly in the gap) with approx neniem kolimirovannogo beam of UV radiation, application of the glass (in conjunction with film) photomask, ensuring high accuracy image transfer plazmohimicheskim plasma and ion-selective material processing. When a photoresist is applied to a plate by free immersion and drawing at a low speed, a photoresist film with a thickness of 1-3 μm is formed with minimal stresses and defects. When drying such films, no noticeable distortion of the flatness of the plates occurs. Group processing is possible. The possibility of combining positive and negative photoresists in the manufacturing process provides technological flexibility: in the manufacture of photomasks, when combining layers, when using acid or alkaline etchants. The process of exposure of thin films of photoresist is performed on installations of the EM-576 or EM-565 type with one-sided and two-sided exposure. Such installations are equipped with an optical system that ensures a strictly parallel beam of cold UV radiation in the exposure zone. The diameter of the light spot in the exposure zone is 100-110 mm.

The use of glass masks, which determine the precision level of the elements of microprinted flexible boards, allows you to reproduce topology elements with a minimum size of 25 ÷ 30 microns. A positive photoresist is used.

The creation of interlayer transitions is the most time-consuming in terms of the number and nature of technological operations. The stage includes double-sided photo printing with localization of junctions, etching of copper in junctions with a minimum diameter of 0.05 mm, etching of through holes in a polyimide (thickness of 0.025 mm) and selective galvanic build-up of copper in interlayer junctions. Thus, the method excludes machining operations, and this reduces the technological cycle and even reduces the complexity of the product. Since all of the above operations are performed using aggressive acidic and alkaline media, the most stable (especially in alkaline) negative photoresist FN-11C was used. The through hole in the polyimide as a result of etching increases by the size of the polyimide film thickness (0.025 mm), and the minimum diameter of the transition grows to 0.01-0.015 mm. As a result, a reliable interlayer transition is formed. When forming a precision microrelief, it is more advisable to use a positive photoresist FP-051K, as having an increased photosensitivity, to achieve high precision elements. At the same time, due to the greater viscosity and thickness, the positive photoresist reliably protects the formed interlayer transitions during etching of copper conductors. When forming a microrelief with a small width of conductors, it should be provided that the etching channels have a uniform width.

The method is as follows. A nickel or cobalt coating is deposited onto a pre-oxidized silicon single crystal wafer. An electrically conductive coating is applied to the metal resist coating. Then, using a photolithography method, an electronic circuit pattern is formed. Then the resulting electronic circuit is coated with a thin layer of polymer.

A silicon wafer with an electronic circuit and a polymer film is placed in a hydrofluoric acid solution - a silicon dioxide film is removed. The silicon wafer is peeling off and a flexible printed circuit board with an electronic circuit on a polymer film is formed.

Thus, the manufacture and use of flexible micro-printed circuit boards in finished products confirms the productivity of using microelectronic technologies for the production of flexible micro-printed polyimide boards.

The experimental results confirmed the possibility of using integrated processing methods at all technological stages, creating high-precision micro-printed circuit boards, obtaining high-density mounting with a width of electrically conductive tracks of less than 50 microns, reducing the technological cycle and reducing the overall laboriousness of circuit boards manufacturing. Thus, it is possible to form electronic circuits on a polymer film with an area of up to 700 square meters. cm.

Information sources

1. Fedulova A. A., Kotova E. A., Yavich E. R. Multilayer printed circuit boards. M .: Sov. Radio, 1977.S. 248.

2. RF patent No. 2231939.

3. RF patent No. 2277764 (prototype).

Claims (1)

A method of manufacturing a flexible micro-printed circuit board, consisting of sequentially applying a metal-resisting nickel or cobalt coating and an electrically conductive copper or molybdenum coating to the plate by thermal photolithography, producing, by photolithography, a drawing of an electrically conductive circuit followed by coating with a polymer film and separating the resulting flexible micro-printed circuit board from the plate, characterized in that as a plate, a thin 20-100 μm single-crystal silicon plate with a diameter of 200-300 mm, <100> orientation is used and, pre-oxidized, with an oxide thickness of 1-2 μm, photolithography is carried out after coating, then it is coated with a polymer layer, then the silicon dioxide film is dissolved, followed by peeling of the silicon wafer and subsequent separation of the polymer film with an electrically conductive circuit and a metal resistive coating.