Plasma treatment of flexures for hard disk drives

Abstract

A method for manufacturing a flexible circuit may include treating a surface of the flexible circuit with at least one of an atmospheric plasma and an ion beam. The atmospheric plasma is formed by directing a gas stream between a motor and the surface of the flexible circuit and generating a voltage between the motor and the flexible circuit to generate plasma from the gas. The average ion energy of the ions varies from about 500 electron volts to about 1,500 electron volts.

Description

[0001] This application is a divisional application of a Chinese patent application filed on May 6, 2016, with application number 201680035848.5 and titled “Plasma Treatment of Flexure Portions of Hard Disk Drives”.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to Provisional Application No. 62 / 157,819, filed May 6, 2015, the entire contents of which are incorporated herein by reference. Technical Field

[0004] The present invention relates to plasma processing in the manufacture of flexible circuits. More particularly, the present invention relates to plasma processing of flexures for hard disk drives. Background Art

[0005] A flexible circuit typically includes a conductor layer and an insulating layer that may separate at least portions of the conductor layer and may provide environmental protection for the conductor. A flexure is a structure that flexibly supports a read / write transducer proximate a rotating disk while also supporting a flexible circuit for conducting electrical signals to and from the transducer. The desire for smaller and higher density disk drives has led to a need for smaller flexures with smaller features. As flexures become smaller, it may become more challenging to provide smaller low resistance electrical connections between the conductive layers and to ensure good adhesion between the various conductive layers of the flexure and the insulating layer. Summary of the invention

[0006] Various embodiments are directed to methods of manufacturing flexible circuits. Such methods may include treating a surface of the flexible circuit with an atmospheric plasma. The atmospheric plasma is formed by directing a gas stream between an electrode and the surface of the flexible circuit and generating a voltage between the electrode and the flexible circuit to generate plasma from the gas.

[0007] In some embodiments where the flexible circuit includes a flexible metal substrate and a dielectric polymer layer disposed on the metal substrate, the method may further include: forming at least one opening in the dielectric polymer layer to expose a portion of a surface of the metal substrate; treating the exposed portion of the surface of the metal substrate using an atmospheric plasma to remove contaminants from the exposed portion of the metal substrate; depositing a seed layer on the dielectric polymer layer and the treated surface of the metal substrate; and forming a plurality of conductive traces on the seed layer, wherein at least one of the conductive traces extends to the treated surface of the metal substrate to form electrical contact between the conductive trace and the metal substrate.

[0008] In some embodiments where the flexible circuit includes a flexible metal substrate and a dielectric polymer layer disposed on the metal substrate, the method may further include: depositing a seed layer on the dielectric polymer layer; covering a portion of the seed layer with a patterned photoresist layer; electroplating a conductive metal onto a plurality of portions of the seed layer not covered by the photoresist layer to form a plurality of conductive traces; stripping off the patterned photoresist layer to expose a portion of the seed layer; etching away the exposed portion of the seed layer to expose a portion of the surface of the dielectric polymer layer; and treating the exposed portion of the surface of the dielectric polymer layer with an atmospheric plasma, wherein the atmospheric plasma removes conductive contaminants from the exposed portion of the surface of the dielectric polymer layer between adjacent conductive traces. This method may further include plating another conductive metal onto the plurality of conductive traces by electroless plating.

[0009] In some embodiments where the flexible circuit includes a flexible metal substrate, a dielectric polymer layer disposed on the flexible metal substrate, and a plurality of conductive traces disposed on the dielectric polymer layer, the method may further include: treating a surface of the dielectric polymer layer using an atmospheric plasma, wherein the atmospheric plasma functionalizes the surface of the dielectric polymer layer; and forming a dielectric polymer coating that covers the plurality of conductive traces and a portion of the functionalized surface of the dielectric polymer layer adjacent to each of the plurality of conductive traces.

[0010] In some embodiments where the flexible circuit includes a flexible metal substrate, a dielectric polymer layer disposed on the metal substrate, a seed layer, and a plurality of conductive traces disposed on the seed layer, the method may further include: forming at least one opening through the flexible metal substrate on a side opposite to the dielectric polymer layer; etching the dielectric polymer layer through the at least one opening to expose a portion of a surface of the seed layer of one of the plurality of conductive traces; and treating the exposed portion of the surface of the seed layer using an atmospheric plasma, wherein the atmospheric plasma removes contaminants from the exposed portion of the surface of the seed layer to enable formation of an electrical connection to the conductive trace.

[0011] In some embodiments, the gas consists essentially of oxygen.In some embodiments, the flexible circuit is fabricated by web processing, and a surface of the flexible circuit is treated with the atmospheric plasma as the flexible circuit moves through the atmospheric plasma.

[0012] Various embodiments are directed to methods of manufacturing a flexible circuit that include treating a surface of the flexible circuit with a beam of ions, wherein the average ion energy of the ions varies from about 500 electron volts to about 1,500 electron volts.

[0013] In some embodiments where the flexible circuit includes a flexible metal substrate and a dielectric polymer layer disposed on the metal substrate, the method may further include: forming at least one opening in the dielectric polymer layer to expose a portion of the surface of the metal substrate; treating the exposed portion of the surface of the metal substrate and the surface of the dielectric polymer layer using an ion beam, wherein the ion beam removes contaminants and native oxides from the exposed portion of the surface of the metal substrate and modifies the surface of the dielectric polymer layer to form a graphite-like layer; depositing a seed layer on the graphite-like layer and the treated surface of the metal substrate; and forming a plurality of conductive traces on the graphite-like layer, wherein at least one of the conductive traces extends to the treated surface of the metal substrate so as to form electrical contact between the conductive trace and the metal substrate.

[0014] In such an embodiment, the average ion energy of the ions may vary from about 1,000 electron volts to about 1,500 electron volts. In such an embodiment, forming a plurality of conductive traces on the graphite-like layer may include: covering a portion of the seed layer with a patterned photoresist layer; electroplating a first conductive metal onto a plurality of portions of the seed layer not covered by the photoresist layer to form a plurality of conductive traces; stripping away the patterned photoresist layer to expose a portion of the seed layer; etching away the exposed portion of the seed layer to expose a portion of the graphite-like layer; and treating the exposed portion of the graphite-like layer with an oxygen plasma, wherein the oxygen plasma achieves at least one of: removing at least a portion of the graphite-like layer; and oxidizing the graphite-like layer between adjacent conductive traces. This method may also include plating another conductive metal onto the plurality of conductive traces by electroless plating. In such an embodiment, the oxygen plasma may be an atmospheric oxygen plasma formed by directing a stream of oxygen between the electrode and a surface of the flexible circuit and generating a voltage between the electrode and the metal substrate to generate plasma from the oxygen.

[0015] In such an embodiment, the method may further include: forming at least one opening through the metal substrate on a side opposite to the dielectric polymer layer; etching the dielectric polymer layer through the at least one opening to expose a portion of a surface of the seed layer of one of the plurality of conductive traces; and treating the exposed portion of the surface of the seed layer with an atmospheric plasma, wherein the atmospheric plasma removes contaminants from the exposed portion of the surface of the seed layer to enable formation of an electrical connection to the conductive trace.

[0016] In some embodiments, the ions consist essentially of at least one of argon ions and nitrogen ions. In some embodiments, the dielectric polymer layer is a polyimide layer. In some embodiments, the flexible circuit is a flexure of a hard disk drive. In some embodiments, the flexible circuit is made by web processing and the surface of the flexible circuit is treated with the ion beam as the flexible circuit moves through the ion beam.

[0017] Various embodiments are directed to a method of manufacturing a flexible circuit, the method comprising treating a surface of the flexible circuit with an atmospheric plasma and with a beam of ions. The atmospheric plasma is formed by directing a gas stream between an electrode and a surface of the flexible circuit and generating a voltage between the electrode and the flexible circuit to produce a plasma from the gas. The average ion energy of the ions in the ion beam varies from about 500 electron volts to about 1,500 electron volts. The flexible circuit is made by web processing, and the surface of the flexible circuit is treated with the atmospheric plasma as the flexible circuit moves through the atmospheric plasma, and the surface of the flexible circuit is treated with the ion beam as the flexible circuit moves through the ion beam.

[0018] While multiple embodiments are disclosed, other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description which describes illustrative embodiments of the present invention. Therefore, the detailed description is to be regarded as illustrative rather than restrictive in nature. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic cross-sectional view of a web containing a flexible circuit treated by atmospheric plasma is shown in accordance with some embodiments.

[0020] Figure 2 is a cross-sectional view of a portion of a flexible circuit showing at least one opening in a dielectric polymer layer according to some embodiments.

[0021] Figure 3 is a graph of defective ground feature ratio as a function of dielectric opening size for ground features processed with and without atmospheric plasma, according to some embodiments.

[0022] Figure 4 According to some embodiments, after the seed layer is deposited Figure 3 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0023] Figure 5 According to some embodiments, after forming a patterned photoresist layer Figure 4 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0024] Figure 6 After electroplating a conductive metal onto portions of the seed layer to form conductive traces according to some embodiments Figure 5 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0025] Figure 7According to some embodiments, after removing the patterned photoresist layer from a portion of the seed layer Figure 6 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0026] Figure 8 According to some embodiments, after electroless deposition of another conductive metal onto the conductive traces Figure 7 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0027] Fig. 9 According to some embodiments, after forming a surface coating on the conductive trace Figure 8 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0028] Fig.10 is a graph illustrating the maximum shear force between a dielectric polymer layer and a dielectric polymer coating for samples treated with atmospheric plasma, according to some embodiments.

[0029] Fig.11 After forming an opening in the metal substrate on a side opposite to the dielectric polymer layer and etching the dielectric polymer layer through the opening in the metal substrate according to some embodiments. Fig. 9 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0030] Fig. 12A and Fig. 12B A schematic cross-sectional view of a web containing a flexible circuit that has been processed by an ion beam is shown in accordance with some embodiments.

[0031] Fig.13 is a cross-sectional view of a portion of a flexible circuit showing at least one opening in a dielectric polymer layer and the formation of a surface modification layer according to some embodiments.

[0032] Fig.14 is a graph illustrating ground feature resistance of flexures subjected to various plasma and ion beam treatments according to an embodiment.

[0033] Fig.15 is a graph showing peel strength for various ion beam treatments according to an embodiment.

[0034] Fig.16 According to some embodiments, after depositing the seed layer Fig.13 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0035] Fig.17 After forming the conductive traces and removing the exposed portions of the seed layer according to some embodiments Fig.16 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0036] Fig.18 After forming an opening in the metal substrate on a side opposite to the dielectric polymer layer and etching the dielectric polymer layer through the opening in the metal substrate according to some embodiments. Fig.17 A cross-sectional view of the portion of the flexible circuit shown in FIG.

[0037] Although the present invention may be modified into a variety of modifications and alternative forms, specific embodiments have been shown as examples in the accompanying drawings and are described in detail below. However, the present invention is not intended to limit the present invention to the specific embodiments described. On the contrary, the present invention is intended to cover all modifications, equivalents and substitutes falling within the scope of the present invention defined by the appended claims. DETAILED DESCRIPTION

[0038] The embodiments described below disclose plasma cleaning and treatment processes and their use in the manufacture of flexible circuits. In some embodiments, the flexible circuit is a flexure of a hard disk drive suspension, such as the suspension described in any of U.S. Patent No. 9,296,188 and U.S. Patent No. 8,891,206, both of which are incorporated herein by reference in their entirety.

[0039] Figure 1 A schematic cross-sectional view of a web containing a flexible circuit treated by atmospheric plasma (also referred to as atmospheric pressure plasma) is shown in accordance with some embodiments. Figure 1 A web 10 is shown moving through an atmospheric plasma system 12. The web 10 includes a plurality of flexible circuits ( Figure 1 ), such as a flexible circuit 40 ( Figure 2-11 ) or flexible circuit 122 ( Figure 13-18 ). The web 10 can be moved through the atmospheric plasma system 12 at any speed required to produce a residence time within the atmospheric plasma suitable for a particular plasma cleaning or treatment, wherein a slower speed results in a longer residence time and a longer plasma cleaning or treatment for any particular flexible circuit. The atmospheric plasma system 12 includes a first electrode 14, a second electrode 16, and gas ports 18 and 20. The first electrode 14 can be a plate over which the web 10 travels closely or a roller in physical contact with the web 10 to support the web 10 and / or drive the web through the atmospheric plasma system 12. The second electrode 16 is disposed away from the web 10. In some embodiments, the second electrode 16 is disposed about 0.04 inches from the web 10.

[0040] Gas port 18 penetrates second electrode 16 to provide gas flow to the space between the second electrode and web 10. Gas port 20 is separated from second electrode 16 and is oriented to provide gas flow to the space between the second electrode and web 10. Figure 1 The atmospheric plasma system 12 of FIG. 1 includes both the gas port 18 and the gas port 20 , but in some embodiments, the atmospheric plasma system 12 may include only the gas port 18 or the gas port 20 .

[0041] In operation, the gas port 18 and / or the gas port 20 directs a gas flow 22 between the second electrode 16 and the web 10 as the web 10 moves through the atmospheric plasma system 12. Figure 1 In the embodiment shown in , the second electrode 16 is electrically connected to an AC voltage source 24 to provide an AC voltage to the second electrode 16, while the first electrode 14 is electrically connected to a ground line 26. In some embodiments, the web 10 is also electrically connected to the ground line 26 by contacting the first electrode 14. The potential voltage difference thus generated between the first electrode 14 and the second electrode 16 is sufficient to ionize the gas and generate an atmospheric plasma 28 between the web 10 and the second electrode 16. In the atmospheric plasma 28, active species may be formed from the gas. Some of the active species may be generated by stripping electrons, resulting in active ions of positive charge. In some embodiments, the atmospheric plasma 28 may be at a positive voltage compared to the first electrode 14, thereby generating an electric field that accelerates the charged active ions toward the web 10 to clean and / or process the flexible circuit included in the web 10.

[0042] The ion bombardment energy of the ions in the atmospheric plasma 28 may be, for example, a function of the power applied to the second electrode 16 and the flow rate of the gas stream 22. In some embodiments, the ion bombardment energy of the ions may vary from about 1 electron volt (eV) to about 100 eV, the power may vary from about 1,000 watts to about 4,000 watts, and the gas flow rate may vary from about 10 liters per minute (min) to about 20 liters per minute.

[0043] Despite Figure 1 In the embodiment shown in , the AC voltage source 24 is configured to provide an AC voltage to the second electrode 16 while electrically connecting the first electrode 14 to the ground line 26, but in other embodiments, the AC voltage source 24 can be configured to provide an AC voltage between the first electrode 14 and the second electrode 16.

[0044] exist Figure 1In the embodiment shown in , the gas consists essentially of oxygen (O2) 30 in diatomic form. By consisting essentially of oxygen 30, it is meant that the gas supplied to the chamber is pure oxygen that may include negligible amounts of impurities (e.g., water vapor, nitrogen, carbon dioxide, etc.) that do not have a substantial effect on the effectiveness of plasma cleaning or treatment. Once in the space between the web 10 and the second electrode 16, electrons 32 are separated from the oxygen 30, thereby generating an atmospheric plasma 28. The atmospheric plasma 28 includes a variety of active species, including diatomic ions 34, monatomic ions 36, and monatomic oxygen 38. The atmospheric plasma 28 may be at a positive voltage compared to the first electrode 14, thereby generating an electric field that will accelerate the positive diatomic ions 34 and monatomic ions 36 toward the web 10 to clean and / or treat the flexible circuits moving across the web 10.

[0045] The web 10 can freely and easily move through the atmospheric plasma system 12 without delays for evacuating and venting a vacuum chamber or gas lock. However, because the atmospheric plasma system 12 operates at atmospheric pressure and without a pressure vessel, generally insignificant amounts of the surrounding atmosphere may enter the plasma, for example as a boundary layer carried along the web. Thus, although the gas stream directed at the chamber may consist essentially of oxygen, the atmospheric plasma 28 itself may include detectable amounts of atmospheric gases.

[0046] Although the use of a gas consisting essentially of oxygen shows Figure 1 However, it is understood that other embodiments may employ a gas comprising a mixture of oxygen, nitrogen, hydrogen, any of the inert gases, or any of the halogens mixed in any suitable combination and ratio.

[0047] Figure 2 is a cross-sectional view of a portion of a flexible circuit showing at least one opening in a dielectric polymer layer according to some embodiments. Figure 2 A flexible circuit 40 is shown that includes a metal substrate 42, a dielectric polymer layer 44, and an opening 46. The metal substrate 42 can be a flexible metal substrate. In some embodiments, for example, where the flexible circuit 40 is a flexure, the metal substrate 42 can be stainless steel. In other flexure embodiments, the metal substrate 42 can be other types of metals, such as copper, phosphor bronze, or nitinol. The dielectric polymer layer 44 can be made of any suitable curable polymer, such as polyimide. The dielectric polymer layer 44 is disposed on a surface 48 of the metal substrate 42. The opening 46 is an opening in the dielectric polymer layer 44 that extends through the dielectric polymer layer 44 to expose a portion of the surface 48. The opening 46 can be used to form conductive traces (conductive traces 56a, 56b) on the dielectric polymer layer 44. Figure 5 ) to establish electrical connection with metal substrate 42. In some embodiments, dielectric polymer layer 44 can be formed by depositing a photoimageable polyimide precursor onto surface 48 and then by a photolithography process known in the art, which includes exposing the polyimide precursor through a photomask and developing to form openings 46. Once openings 46 are formed, the polyimide precursor is cured to form polyimide.

[0048] like Figure 2 As shown in FIG. 4 , opening 46 may include contaminants 50 on the exposed portion of surface 48. Contaminants 50 may include, for example, residues of a polyimide precursor remaining after a photolithography process or byproducts from a cured polyimide deposited on surface 48. It has been found that if these contaminants are not removed, contaminants 50 may increase the conductivity of conductive traces (conductive traces 56a, 56b) formed on dielectric polymer layer 44. Figure 5 ) and the resistance of the electrical connection between the metal substrate 42. Therefore, after forming the opening 46, using the above reference Figure 1 Atmospheric plasma 28 is described to treat exposed portions of surface 48 of metal substrate 42 to remove at least some of contaminants 50 .

[0049] In some embodiments, treatment with atmospheric plasma 28 to remove contaminants 50 may be accomplished at a power of about 2,000 watts and a flow rate of about 15 liters per minute of a gas consisting essentially of oxygen, with a residence time of about 5 seconds for the flexible circuit 40 in the atmospheric plasma 28. Limiting the energy and exposure time may remove contaminants 50 without damaging the dielectric polymer layer 44.

[0050] Figure 3 Based on the above reference Figure 2 A graph of the defective ground feature ratio as a function of dielectric opening size of the ground features processed with and without atmospheric plasma 28 for the described embodiment. The ground features are electrical connections between the conductive traces formed on the dielectric polymer layer 44 and the metal substrate 42. Figure 3 As shown in , the use of atmospheric plasma 28 significantly reduces the rate of ground feature defects, also at openings 46 with sizes ranging from 20 microns to 70 microns.

[0051] Figure 4 In some embodiments, the above reference Figure 2 A cross-sectional view of the portion of the flexible circuit 40 after additional processing as described. Figure 4A seed layer 52 is shown deposited onto the dielectric layer 44 and the exposed portions of the surface 48 of the metal substrate 42 after the exposed portions of the surface 48 are treated with the atmospheric plasma 28 to remove contaminants 50. With the contaminants 50 removed, the seed layer 52 is capable of forming a low resistance electrical connection with the metal substrate 42. The seed layer 52 may be formed, for example, by sputtering a chromium layer onto the exposed portions of the surface 48 of the dielectric layer 44 and the metal substrate 42, followed by sputtering a copper layer onto the chromium layer.

[0052] Figure 5 is shown in some embodiments. Figure 4 4 is a cross-sectional view of the portion of the flexible circuit 40 after additional processing as described above. Figure 5 A patterned photoresist layer 54 is shown formed on the seed layer 52. The patterned photoresist layer 54 may be formed by photolithography techniques known in the art.

[0053] Figure 6 It is shown that in some embodiments Figure 5 4 is a cross-sectional view of the portion of the flexible circuit 40 after additional processing as described above. Figure 6 The formation of a plurality of conductive traces 56a, 56b on the seed layer 52 is shown. The plurality of conductive traces 56a, 56b may be formed by electroplating a conductive metal, such as copper, onto portions of the seed layer 52 that are not covered by the patterned photoresist layer 54. The patterned photoresist layer 54 prevents the conductive metal from being deposited onto the seed layer 52. Although only two conductive traces 56a and 56b are shown for ease of illustration, it is understood that embodiments may include more than two conductive traces.

[0054] Figure 7 is shown in some embodiments. Figure 6 4 is a cross-sectional view of the portion of the flexible circuit 40 after additional processing as described above. Figure 7 The flexible circuit 40 is shown after removing the patterned photoresist layer 54 and removing a portion of the seed layer 52. The patterned photoresist layer 54 can be removed by any of a variety of chemical photoresist strippers known in the art. After removing the patterned photoresist layer 54, thereby exposing a portion of the seed layer 52, the exposed portion of the seed layer 52 is etched away by a chemical etchant known in the art to expose a portion of the dielectric polymer layer 44.

[0055] In some embodiments, after etching away the seed layer 52, the conductive contaminants 60 may remain on the exposed portions 62 of the surface of the dielectric polymer layer 44. The conductive contaminants 60 may include, for example, residues of the seed layer 52 that were not completely removed by the chemical etching. It has been found that even if the residues are insufficient to directly reduce the resistance between adjacent conductive traces such as conductive traces 56a and 56b, the conductive contaminants 60 may remain on the exposed portions 62 of the surface of the dielectric polymer layer 44 after etching away the seed layer 52. The conductive contaminants 60 may include, for example, residues of the seed layer 52 that were not completely removed by the chemical etching. Figure 6 During subsequent processing such as electroless plating by ), the residue can serve as a nucleation site for the growth of conductive structures or stringers between conductive traces 56a and 56b. Such conductive stringers can significantly reduce the resistance between conductive traces 56a and 56b, thereby negatively affecting the performance of flexible circuit 40. Therefore, in some embodiments, after etching away seed layer 52, the conductive structure or stringer can be used to form a conductive structure between conductive traces 56a and 56b. Figure 1 The atmospheric plasma 28 in which the gas includes oxygen treats the exposed portions 62 of the surface of the dielectric polymer layer 44. Without wishing to be bound by any theory, it is believed that the residues are oxidized by the atmospheric plasma 28 and once the residues are oxidized, they are no longer suitable for use as nucleation sites for growing the conductive stand.

[0056] In some embodiments, treatment with the atmospheric plasma 28 to remove the conductive contaminants 60 may be accomplished at a power of about 3,000 watts and a flow rate of about 15 liters per minute of a gas consisting essentially of oxygen, with a residence time of about 27 seconds for the flexible circuit 40 in the atmospheric plasma 28. Increased energy and extended exposure times may be required to adequately oxidize the conductive contaminants 60 as compared to some other atmospheric plasma processes described herein.

[0057] In some embodiments, the gas may include more than 50%, more than 60%, more than 70%, more than 90%, or more than 99% oxygen, or any percentage of oxygen greater than any of the foregoing percentages. All gas percentages are given by stream 22 ( Figure 1 ) as a percentage of the total gas flow provided. In some embodiments, the gas consists essentially of oxygen. It has been found that when the gas consists essentially of oxygen, the atmospheric plasma 28 is more effective in preventing subsequent growth of conductive stands than a gas comprising a mixture of 50% oxygen and 50% argon (Ar).

[0058] Figure 8 is shown in some embodiments. Figure 7 4 is a cross-sectional view of the portion of the flexible circuit 40 after additional processing as described above. Figure 8The flexible circuit 40 is shown after another conductive metal 64 is electrolessly deposited onto the conductive traces 56a, 56b. After the conductive metal 64 is deposited, the flexible circuit 40 may be further deposited using the method described above with reference to FIG. Figure 1 The exposed portion 62 of the surface of the dielectric polymer layer 44 is treated by the atmospheric plasma 28 described, in which the gas includes oxygen. In some embodiments, the gas consists essentially of oxygen. Since the ionized oxygen atoms break the carbon chains of the dielectric polymer layer 44, thereby generating carbon radicals at the ends of the broken chains, the treatment functionalizes the exposed portion 62. The ionized oxygen atoms combine with the carbon radicals, thereby incorporating additional oxygen atoms into the exposed portion 62. These additional oxygen atoms can serve as bonding sites for polymer chains that are subsequently deposited on the exposed portion 62 of the surface of the dielectric polymer layer 44.

[0059] In some embodiments, treatment with the atmospheric plasma 28 to functionalize the exposed portion 62 of the surface of the dielectric polymer layer 44 can be achieved at a power of about 2,000 watts and an oxygen gas flow rate of about 15 liters per minute, with a residence time of about 5 seconds for the flexible circuit 40 in the atmospheric plasma 28. Limiting the energy and exposure time allows the surface to be functionalized without damaging the dielectric polymer layer 44.

[0060] Fig. 9 is shown in some embodiments. Figure 8 4. A cross-sectional view of the portion of the flexible circuit 40 after additional processing described above. After functionalizing the exposed portion 62 of the surface of the dielectric polymer layer 44, a dielectric polymer coating 66 may be formed on the conductive traces 56a, 56b and at least a portion of the exposed portion 62 adjacent to the conductive traces 56a, 56b to form a surface coating. The dielectric polymer coating 66 may be made of any suitable curable polymer, such as a polyimide. In some embodiments, the dielectric polymer coating 66 may be formed by depositing a photoimageable polyimide precursor onto the conductive traces 56a, 56b and the exposed portion 62, and then by a photolithography process known in the art, which includes exposing the polyimide precursor through a photomask and developing to define the dielectric polymer coating 66. Once the dielectric polymer coating 66 is formed, the polyimide precursor is cured to form a polyimide, and the dielectric polymer coating 66 is thermally bonded to the dielectric polymer layer 44.

[0061] The stronger the bond between the dielectric polymer coating 66 and the dielectric polymer layer 44, the more durable and effective the surface coating will be in protecting the conductive traces 56a, 56b. Forming the dielectric polymer coating 66 on the functionalized exposed portions 62 of the surface of the dielectric polymer layer 44 forms a stronger bond because the additional oxygen atoms of the functionalized surface bond with the polymer chains of the dielectric polymer coating 66.

[0062] Although Figure 8 and Fig. 9 The embodiment shown in FIG. 4 shows functionalization of the exposed portions 62 and application of the dielectric polymer coating 66 after electroless deposition of the conductive metal 64, but in other embodiments, the conductive metal 64 may be omitted. In such an embodiment, the dielectric polymer coating 66 is formed directly on the conductive traces 56 a, 56 b after treatment with the atmospheric plasma 28 to functionalize the exposed portions 62 of the surface of the dielectric polymer layer 44.

[0063] Fig.10 is a graph showing the maximum shear force between dielectric polymer layer 44 and dielectric polymer coating 66 samples that were treated with atmospheric plasma 28 using various powers and dwell times to functionalize the exposed portion 62 of the surface of dielectric polymer layer 44 prior to forming dielectric polymer coating 66. Fig.10 As shown in , the sample treated with atmospheric plasma 28 shows greater shear force compared to the sample treated with a nitrogen ion beam (control standard).

[0064] Fig.11 is shown in some embodiments. Fig. 9 4 is a cross-sectional view of the portion of the flexible circuit 40 after additional processing as described above. Fig.11 The formation of an access hole through the metal substrate 42 and the dielectric layer 44 is shown. An opening 68 through the metal substrate 42 can be formed on the side opposite to the dielectric polymer layer 44 (the back side). The opening 68 can be formed by, for example, patterning a photoresist on the back side of the flexible circuit 40 and then etching the metal substrate 42 using an etchant suitable for the material of the metal substrate 42. For example, if the metal substrate 42 is made of stainless steel, a suitable etchant will dissolve the stainless steel and will not aggressively attack the photoresist. Such etchants are known in the art. After the opening 68 is formed through the metal substrate 42, the metal substrate 42 can be used as a mask to etch the dielectric polymer layer 44 to extend the opening 68, thereby exposing a portion 70 of the seed layer 52 located below the conductive trace 56b.

[0065] like Fig.11 As shown in , opening 68 may include contaminants 72 on exposed portion 70 of seed layer 52. Contaminants 72 may include, for example, residues from removal of dielectric polymer layer 44. It has been found that if contaminants 72 are not removed, they increase the resistance of the backside electrical connection to conductive trace 56b. Therefore, after forming opening 68, the backside electrical connection 70 may be formed using the method described above with reference to FIG. Figure 1The atmospheric plasma 28 is used to treat the exposed portion 70 of the seed layer 52 to remove contaminants 72 .

[0066] In some embodiments, treatment with atmospheric plasma 28 to remove contaminants 72 may be accomplished at a power of about 3,000 watts and a flow rate of about 15 liters per minute of a gas consisting essentially of oxygen, with a residence time of flexible circuit 40 in atmospheric plasma 28 of about 38 seconds.

[0067] Fig. 12A and Fig. 12B A schematic cross-sectional view of a web containing a flexible circuit that has been processed by an ion beam is shown in accordance with some embodiments. Fig. 12A and Fig. 12B 1 and 2. Web 110 is shown moving through vacuum system 112. Web 110 includes a plurality of flexible circuits, such as flexible circuit 122 ( Fig.13 ). The web 110 may be moved through the vacuum system 112 at any speed necessary to produce a residence time within the vacuum system 112 suitable for a particular ion beam treatment. The vacuum system 112 includes an ion source 114. The ion source 114 ionizes a gas 116 to produce a plasma including positively charged ions 118, and then accelerates the ions 118 away from the ion source 114 to form an ion beam 120. The ion source 114 may be any type of ion source known in the art, including those that employ a powered grid to accelerate the positively charged ions 118 by attraction and those that employ a powered anode to accelerate the positively charged ions 118 by repulsion.

[0068] In operation, as the web 110 moves through the vacuum system 112, the ion source 114 accelerates an ion beam 120 toward the web 110 to clean and / or treat the flexible circuits moving over the web 110. The average ion energy can vary from about 500 eV to about 1,500 eV, which is significantly higher than the ion energy associated with the atmospheric plasma 28 discussed above. As described below, such high energy ions can produce beneficial processes in the manufacture of flexible circuits. The energy of the ions 118 in the ion beam 120 can be controlled to provide a desired treatment to the web 110. For example, Fig. 12A As shown in , the diffuse ion beam 120 can be formed by, for example, increasing the gas flow through the ion source 114 to produce a denser ion 118. The greater density of ions 118 can result in increased collisions between ions 118, resulting in lower ion energy and a wider diffuse beam. The diffuse ion beam 120 can have an ion bombardment energy ranging from about 500 eV to less than about 1,000 eV. Fig. 12BAs shown in , for processes requiring a higher energy ion beam, a collimated ion beam 120 can be formed. The collimated ion beam 120 can be formed by, for example, reducing the gas flow through the ion source 114 to produce a lower density of ions 118. The lower density of ions 118 can result in reduced collisions between ions 118, resulting in higher ion energy and a narrow collimated beam. The collimated ion beam 120 can have an ion bombardment energy ranging from about 1,000 eV to as high as about 1,500 eV.

[0069] During cleaning and / or processing by the ion beam 120, the pressure in the vacuum system 112 can be maintained. In some embodiments, the pressure can be less than 200 mTorr. In some embodiments, the pressure can be less than about 50 mTorr.

[0070] although Fig. 12A and Fig. 12B The embodiment shown is ion 118 that is always substantially composed of positively charged argon ions, but it is understood that other embodiments may employ ions of other gases, including any mixture of any of the inert gases in any suitable combination and ratio with any of oxygen, nitrogen (N2), hydrogen or halogens.

[0071] Fig.13 is a cross-sectional view of a portion of a flexible circuit showing at least one opening in a dielectric polymer layer according to some embodiments. Fig.13 1 and 2. A flexible circuit 122 is shown that includes a metal substrate 42, a dielectric polymer layer 44, and an opening 46. The metal substrate 42, the dielectric polymer layer 44, and the opening 46 may be as described above with respect to FIG. Figure 2 The dielectric polymer layer 44 is disposed on the surface 48 of the metal substrate 42. Fig.13 As shown in FIG. 4 , the opening 46 may include contaminants 50 located on the exposed portion of the surface 48, as described above with reference to FIG. Figure 2 As described above, if the contaminants 50 are not removed, the contaminants 50 will increase the resistance of the electrical connection between the conductive traces formed on the dielectric polymer layer 44 and the metal substrate 42. Therefore, after the openings 46 are formed, the above-mentioned reference Fig. 12A and Fig. 12B The ion beam 120 processes the exposed portion of the surface 48 of the metal substrate 42 to remove the contaminants 50. In some embodiments, after the opening 46 is formed, the ion beam 120 may be used as described above with reference to Figure 2 The atmospheric plasma 28 is used to treat the exposed portion of the surface 48 of the metal substrate 42, and then the above-mentioned reference Fig. 12A and Fig. 12BThe ion beam 120 processes the exposed portion of the surface 48 of the metal substrate 42 to remove the contaminants 50 .

[0072] In some embodiments, metal substrate 42 is made of a material such as stainless steel that forms a native oxide. It has been found that treating the exposed portion of surface 48 with ion beam 120 removes and / or destroys the native oxide. After the ion beam treatment and before exposing surface 42 to oxygen, a substrate such as described above with reference to Figure 4 Deposition of a conductive layer such as the seed layer 52 onto the exposed surface 42 further significantly improves the contact resistance, as described below with reference to Fig.14 This can be accomplished, for example, by combining an ion source 114 with metal sputter deposition capabilities in the vacuum system 112 .

[0073] In some embodiments, treatment with ion beam 120 to remove contaminants 50 and native oxides can be accomplished with ion energies varying from about 500 eV to about 1,500 eV. These relatively high average ion energies are sufficient to break metal oxide bonds in an effective manner. In some embodiments, ion beam 120 may consist essentially of argon ions. In other embodiments, ion beam 120 may consist essentially of nitrogen ions. In still other embodiments, ion beam 120 may consist essentially of a mixture of argon ions and nitrogen ions.

[0074] Fig.14 is a graph showing ground feature resistance of flexures subjected to various plasma and ion beam treatments according to an embodiment. Fig.14 The resistance of the sample at various points during processing is shown, including after forming the conductive traces (initial), after curing the dielectric polymer coating (CCC), and after exposing the finished flexure to an environment with a temperature of 85°C and a humidity of 85% for a long time (85 / 85). The flexure was treated with a plasma (designated "MF") or with a diffuse ion beam (designated "dif") or a collimated ion beam (designated "col"), where argon, nitrogen, or oxygen were used to generate the corresponding ions. Fig.14As shown in , flexures treated with an ion beam consisting essentially of argon ions or an ion beam consisting essentially of nitrogen ions have significantly lower ground feature resistance than flexures treated with a plasma or with an ion beam having oxygen ions. Without wishing to be bound by any theory, it is believed that the low average ion energy in the plasma (less than 100 eV) is insufficient to remove and / or damage the native oxide, while the average ion energy in the diffuse ion beam (500 eV to 1,000 eV) and the average ion energy in the collimated ion beam (1,000 eV to 1,500 eV) are more than sufficient. An ion beam containing oxygen ions may have sufficient average ion energy, but the oxygen ions appear to reorganize the oxide, resulting in a higher ground feature resistance.

[0075] Return to reference Fig.13 In some embodiments, the treatment with ion beam 120 may modify the surface of dielectric polymer layer 44 to form surface modified layer 124. As described below with reference to Fig.15 As described, the surface modification layer 124 can improve the peel strength of the conductive traces. The surface modification layer 124 can be a low-oxygen graphite-like layer formed by the action of the ion beam 120 on a material such as polyimide of the dielectric polymer layer 44. The surface modification layer 124 can be formed with a relatively high average ion energy. In some embodiments, the treatment with the ion beam 120 to form the surface modification layer 124 can be achieved with an ion energy ranging from about 1,000 eV to about 1,500 eV. In some embodiments, the ion beam 120 can consist essentially of argon ions. In other embodiments, the ion beam 120 can consist essentially of nitrogen ions. In still other embodiments, the ion beam 120 can consist essentially of a mixture of argon ions and nitrogen ions.

[0076] Fig.15 is a graph showing peel strength for various ion beam treatments according to an embodiment. Fig.15 The peel strength of conductive traces such as conductive traces 56a, 56b for the flexures of the samples at various points during processing are shown, including after forming the conductive traces (initial), after curing the dielectric polymer coating (CCC), and after exposing the finished flexure to an environment with a temperature of 85°C and a humidity of 85% for a long time (85 / 85). The flexures were treated with diffuse ion beams (designated "dif") or collimated ion beams (designated "col") and with argon ions, nitrogen ions, or a mixture of argon and nitrogen ions. Fig.15As shown in , after the finished flexure was exposed to an environment with a temperature of 85° C. and a humidity of 85% for a long time (85 / 85), the treatment with a collimated ion beam caused the flexure to have a peel strength of greater than 80 g / mm, which exceeded 100 g / mm. In contrast, after the finished flexure was exposed to an environment with a temperature of 85° C. and a humidity of 85% for a long time (85 / 85), the treatment with a diffuse ion beam caused the flexure to have a peel strength of less than 80 g / mm, which did not exceed 60 g / mm. Without wishing to be bound by any theory, it is believed that in order to produce the graphite-like surface modification layer 124, a higher average ion energy of 1,000 eV to 1,500 eV is required in the collimated beam.

[0077] Fig.16 In some embodiments, the Fig.13 A cross-sectional view of the portion of the flexible circuit 122 after additional processing as described. Fig.16 A seed layer 52 is shown deposited onto the surface modification layer 124 and the exposed portion of the surface 48 of the metal substrate 42 after being treated with the ion beam 120 to clean and / or modify the surface. With the contaminants 50 and native oxides removed, the seed layer 52 is capable of forming a low resistance electrical connection with the metal substrate 42. The seed layer 52 may be formed, for example, by sputter depositing a chromium layer onto the surface modification layer 124 and the exposed portion of the surface 48 of the metal substrate 42, followed by sputter depositing a copper layer onto the chromium layer.

[0078] Fig.17 In some embodiments, the Fig.16 A cross-sectional view of the portion of the flexible circuit 122 after additional processing as described. Fig.17 As shown in the above reference Figures 5 to 7 The flexible circuit 122 after forming the conductive traces 56a, 56b and removing the exposed portion of the seed layer 52. As described above, it is believed that the surface modification layer 124 acts as a tie layer between the dielectric polymer layer 44 and the seed layer 52 to improve the adhesion between the seed layer 52 and the dielectric polymer layer 44, thereby resulting in a greater peel strength ( Fig.15 ).

[0079] like Fig.17As shown in , etching away the seed layer 52 exposes a portion of the surface modification layer 124. As described above, the surface modification layer 124 is a graphite-like layer and is slightly conductive. The conductive surface modification layer 124 can significantly reduce the resistance between the conductive traces 56a and 56b, thereby negatively affecting the performance of the flexible circuit 122. Therefore, in some embodiments, after etching away the seed layer 52, the surface modification layer 124 can be used as described above with reference to Figure 1 The atmospheric plasma 28, in which the gas includes oxygen, treats the exposed portion of the surface modification layer 124. Without wishing to be bound by any theory, it is believed that the atmospheric plasma 28 may remove at least some of the surface modification layer 124 and / or may oxidize the remaining portion of the surface modification layer 124, thereby rendering the surface modification layer 124 no longer conductive.

[0080] In some embodiments, the gas used for the atmospheric plasma 28 may include more than 50%, more than 60%, more than 70%, more than 90%, or more than 99% oxygen, or any percentage of oxygen greater than any of the foregoing percentages. All gas percentages are obtained from the stream 22 ( Figure 1 ) is a percentage of the total gas flow provided. In some embodiments, the gas consists essentially of oxygen.

[0081] In some embodiments, treatment with the atmospheric plasma 28 to remove and / or oxidize the surface modification layer 124 may be accomplished at a power of about 3,000 watts and a flow rate of about 15 liters per minute of a gas consisting essentially of oxygen, with a residence time of about 27 seconds for the flexible circuit 122 in the atmospheric plasma 28. Increased energy and extended exposure times may be required to adequately remove and / or oxidize the surface modification layer 124 as compared to some other atmospheric plasma processes described herein.

[0082] Fig.18 In some embodiments, the Fig.17 A cross-sectional view of the portion of the flexible circuit 122 after additional processing as described. Fig.18 Flexible circuit 122 is shown after surface modification layer 124 has been removed and / or oxidized from between conductive traces 56a and 56b. Fig.18 It is also shown in the above reference Figure 8 and Fig. 9 Flexible circuit 122 after electrolessly depositing another conductive metal 64 onto conductive traces 56a, 56b, functionalizing the surface of dielectric polymer layer 44, and forming dielectric polymer coating 66 as described. Fig.18 Also shown as above reference Fig.11The formation of the inspection hole (i.e., opening 68) through the metal substrate 42 and the dielectric layer 44 is as described. Fig.18 As shown in , opening 68 may include contaminants 126 located on exposed portion 70 of seed layer 52. Contaminants 126 may include, for example, residues from removal of dielectric polymer layer 44 and surface modification layer 124. It has been found that if contaminants 126 are not removed, contaminants 126 may increase the resistance of the backside electrical connection connected to conductive trace 56b. Therefore, after forming opening 68, the above-referenced method may be used. Figure 1 The atmospheric plasma 28 treats the exposed portion 70 of the seed layer 52 to remove the contaminants 126. Alternatively, after the opening 68 is formed, the above-mentioned reference Fig. 12A and Fig. 12B The ion beam 120 processes the exposed portion 70 of the seed layer 52 to remove contaminants 126 .

[0083] In some embodiments, treatment with atmospheric plasma 28 to remove contaminants 126 may be accomplished at a power of about 3,000 watts and a flow rate of about 15 liters per minute of a gas consisting essentially of oxygen, wherein the residence time of flexible circuit 40 in atmospheric plasma 28 is about 38 seconds.

[0084] Various modifications and additions may be made to the exemplary embodiments discussed without departing from the scope of the invention. For example, although the above-described embodiments relate to specific features, the scope of the invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Therefore, the scope of the invention is intended to cover all such substitutions, modifications and variations that fall within the scope of the claims and all equivalents thereof.

Claims

1. A method for manufacturing a flexible circuit, the method comprising: Treating the surface of the flexible circuit using an ion beam, wherein the average ion energy of ions in the ion beam is between 500 electron volts and 1,500 electron volts, the flexible circuit comprising a flexible metal substrate and a dielectric polymer layer disposed on the metal substrate, forming at least one opening in the dielectric polymer layer to expose a portion of the surface of the metal substrate; treating the exposed portion of the surface of the metal substrate and the surface of the dielectric polymer layer using the ion beam, wherein the ion beam removes contaminants and native oxides from the exposed portion of the surface of the metal substrate and modifies the surface of the dielectric polymer layer to form a graphite-like layer; depositing a seed layer on the graphite-like layer and the treated surface of the metal substrate; and forming a plurality of conductive traces on the graphite-like layer, wherein at least one of the conductive traces extends to the treated surface of the metal substrate to form an electrical contact between the conductive trace and the metal substrate.

2. The method according to claim 1, wherein: The average ion energy of the ions is between 1,000 electron volts and 1,500 electron volts.

3. The method according to claim 1, wherein: Forming the multiple conductive traces on the graphite-like layer includes: covering a portion of the seed layer with a patterned photoresist layer; electroplating a first conductive metal onto multiple portions of the seed layer not covered by the photoresist layer to form a plurality of conductive traces; stripping off the patterned photoresist layer to expose a portion of the seed layer; etching away the exposed portion of the seed layer to expose a portion of the graphite-like layer; and treating the exposed portion of the graphite-like layer with oxygen plasma, wherein the oxygen plasma achieves removal of at least a portion of the graphite-like layer and oxidation of at least one of the graphite-like layers between adjacent conductive traces.

4. The method according to claim 3, wherein: The method also includes plating another conductive metal onto the plurality of conductive traces by electroless plating.

5. The method according to claim 3, wherein: The oxygen plasma is an atmospheric oxygen plasma formed by directing a flow of oxygen between an electrode and the surface of the flexible circuit; and generating a voltage between the electrode and the flexible circuit to generate plasma from the oxygen.

6. The method according to claim 5, wherein: The method also includes: forming at least one opening through the metal substrate on a side opposite to the dielectric polymer layer; etching the dielectric polymer layer through the at least one opening to expose a portion of the surface of the seed layer of one of the plurality of conductive traces; and treating the exposed portion of the surface of the seed layer with the atmospheric oxygen plasma, wherein the atmospheric oxygen plasma removes contaminants from the exposed portion of the surface of the seed layer so as to enable an electrical connection to be formed to the conductive trace.

7. The method according to claim 1, wherein: The ions are composed of at least one of argon ions and nitrogen ions.

8. The method according to claim 1, wherein: The dielectric polymer layer is a polyimide layer.

9. The method according to claim 1, wherein: The flexible circuit is the flexure of the hard disk drive.

10. The method according to claim 1, wherein: The flexible circuit is fabricated by web processing and the surface of the flexible circuit is treated with the ion beam as the flexible circuit moves through the ion beam.

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