WO2021031507A1 - Formation of fine pitch traces using ultra-thin paa modified fully additive process - Google Patents

Formation of fine pitch traces using ultra-thin paa modified fully additive process Download PDF

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Publication number
WO2021031507A1
WO2021031507A1 PCT/CN2020/000184 CN2020000184W WO2021031507A1 WO 2021031507 A1 WO2021031507 A1 WO 2021031507A1 CN 2020000184 W CN2020000184 W CN 2020000184W WO 2021031507 A1 WO2021031507 A1 WO 2021031507A1
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Prior art keywords
layer
copper
paa
traces
layers
Prior art date
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PCT/CN2020/000184
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English (en)
French (fr)
Inventor
Po Leung Pun KELVIN
Wah Cheung CHEE
Original Assignee
Compass Technology Company Limited
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Publication date
Priority claimed from US16/548,373 external-priority patent/US10636734B2/en
Application filed by Compass Technology Company Limited filed Critical Compass Technology Company Limited
Priority to KR1020227009098A priority Critical patent/KR20220061992A/ko
Priority to CN202080057766.7A priority patent/CN114616662A/zh
Priority to JP2022511270A priority patent/JP2022545799A/ja
Publication of WO2021031507A1 publication Critical patent/WO2021031507A1/en

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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
    • H01L2224/73—Means for bonding being of different types provided for in two or more of groups H01L2224/10, H01L2224/18, H01L2224/26, H01L2224/34, H01L2224/42, H01L2224/50, H01L2224/63, H01L2224/71
    • H01L2224/732—Location after the connecting process
    • H01L2224/73201—Location after the connecting process on the same surface
    • H01L2224/73203—Bump and layer connectors
    • H01L2224/73204—Bump and layer connectors the bump connector being embedded into the layer connector
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
    • H01L2224/73—Means for bonding being of different types provided for in two or more of groups H01L2224/10, H01L2224/18, H01L2224/26, H01L2224/34, H01L2224/42, H01L2224/50, H01L2224/63, H01L2224/71
    • H01L2224/732—Location after the connecting process
    • H01L2224/73251—Location after the connecting process on different surfaces
    • H01L2224/73265—Layer and wire connectors

Definitions

  • This application relates to producing a flexible substrate with fine copper traces, and more particularly, to producing a semiconductor package with solid state diffusion bonding using a flexible substrate with fine copper traces.
  • Chip-on-film (COF) packaging constitutes a substantial technology to cope with the future demands of higher function, lower power consumption and miniaturization; in particular, high resolution and increasing I/O count of touch integrated circuit (IC) and display-drive IC integrated modules (TDDI) requiring extremely fine pitch COF packages.
  • COF Chip-on-film
  • a flexible circuit is fabricated in a subtractive method where the copper trace pattern is formed by etching.
  • this subtractive method has an inherent problem in sidewall geometry control.
  • SAP semi-additive process
  • SAP semi-additive process
  • the deformable layer must provide the requisite electrical properties with good trace integrity. It must be able to withstand sufficient pressure during contact and hence there must be enough top width on the trace so that a full contact interface with proper creep deformation and void elimination on the bonding zone is achieved. As the bond pitch reduces, the semi-additive and subtractive methods have limitations to maintain the top to bottom width (T/B) ratio as close to 1 while achieving a reasonable yield.
  • An alternative method is a full additive process (FAP) , in which the copper pattern can be formed by electroless plating.
  • FAP full additive process
  • PI polyimide
  • the PI which is comprised of an imide ring can be easily opened by the incoming nucleophilic hydroxide ion forming polyamic acid salt (PAA) . Since the carboxylate group on this polyamic acid is an ion exchange group, it can be reduced to deposit a Pd catalyst when treated in an aqueous Pd (II) ion solution. Once the catalyst is deposited, subsequent electroless plating is then possible.
  • PDA nucleophilic hydroxide ion forming polyamic acid salt
  • a principal object of the present disclosure is to provide a method of producing a plurality of fine traces on a flexible substrate for a chip on flex (COF) package.
  • Another object of the disclosure is to provide a fully additive method of plating up fine and robust copper traces on a flexible substrate for a chip on flex (COF) package.
  • COF chip on flex
  • a further object of the disclosure is to provide a fully additive method of plating up fine and robust copper traces on a flexible substrate for a chip on flex (COF) package using electroless Ni-P and a reliable nano-size polyamic acid (PAA) anchoring layer on the dielectric/Ni-P interface.
  • COF chip on flex
  • PAA nano-size polyamic acid
  • a method to produce a substrate suitable for various interconnection methods including thermocompression bonding, wire bonding, adhesive bonding, and soldering is achieved.
  • a flexible dielectric substrate is provided.
  • An alkaline modification is applied to the dielectric substrrate to form a polyamic acid (PAA) anchoring layer on a surface of the dielectric substrate.
  • a Ni-P seed layer is elecrolessly plated on the PAA layer.
  • Copper traces are plated within a photoresist pattern on the Ni-P seed layer.
  • a surface finishing layer is electrolytically plated on the copper traces. The photoresist pattern and Ni-P seed layer not covered by the copper traces are removed to complete the substrate suitable for diffusion bonding.
  • a method of manufacturing a 2ML (metal layer) substrate suitable for various interconnection methods including thermocomrpession bonding, wire bonding, adhesive bonding, and soldering is achieved.
  • a flexible dielectric substrate is provided.
  • At least one via opening is laser drilled all the way through the dielectric substrate.
  • An alkaline modification is applied to the dielectric substrrate to form a polyamic acid (PAA) anchoring layer on top and bottom surfaces of the dielectric substrate.
  • a Ni-P seed layer is elecrolessly plated on top and bottom PAA layers.
  • Photoresist dry resist/wet resist
  • Copper traces are plated within photoresist patterns on the top and bottom Ni-P seed layers and through the at least one via opening.
  • a surface finishing layer is electrolytically plated at least on one side of the copper traces. The photoresist patterns and Ni-P seed layers not covered by the copper traces are removed to complete the substrate suitable for diffusion bonding.
  • a method of manufacturing a multiplayer substrate suitable for various interconnection methods including thermocomrpession bonding, wire bonding, adhesive bonding, and soldering is achieved.
  • a flexible dielectric substrate is provided. At least one via opening is laser drilled all the way through the dielectric substrate.
  • An alkaline modification is applied to the dielectric substrrate to form a polyamic acid (PAA) anchoring layer on top and bottom surfaces of the dielectric substrate.
  • a Ni-P seed layer is elecrolessly plated on top and bottom PAA layers.
  • Photoresist dry resist/wet resist
  • Copper traces are plated within photoresist patterns on the top and bottom Ni-P seed layers and through the at least one via opening.
  • the photoresist patterns and Ni-P seed layers not covered by the copper traces are removed to complete the substrate suitable for diffusion bonding.
  • a bonding film is laminated on top and bottom surfaces of the first copper traces.
  • a dielectric layer (PI) is laminated on top and bottom of the bonding films.
  • At least one second via opening is laser drilled all the way through the dielectric layer and bonding film to contact the first copper traces on top and bottom of the substrate.
  • an alkaline modification is applied to the dielectric layers to form a second polyamic acid (PAA) anchoring layer on top and bottom surfaces of the dielectric layers and within the at least one second via openings.
  • a second Ni-P seed layer is electrolessly plated on top and bottom of the second PAA layers.
  • a second photoresist pattern is formed on top and bottom of the second Ni-P seed layers.
  • Second copper traces are plated within the second photoresist patterns and through the at least one second via opening.
  • a surface finishing layer is plated on the second copper traces. The second photoresist patterns are removed and the second Ni-P seed layers not covered by the second copper traces are etched away to complete the flexible substrate.
  • a 2ML (metal layer) chip on film (COF) is achieved.
  • the COF comprises a flexible dielectric substrate having a first polyamic acid (PAA) anchoring layer on its top surface, at least one first copper trace on a first Ni-P seed layer on the first PAA layer and having a surface finishing layer on a top surface of the at least one first copper trace, and at least one die mounted on the dielectric substrate through diffusion bonding with the at least one first copper trace.
  • PAA polyamic acid
  • a multilayer chip on film comprises a flexible dielectric substrate having a first polyamic acid (PAA) anchoring layer on its top surface and a second PAA layer on its bottom surface, at least one first copper trace on a first Ni-P seed layer on the first PAA layer and at least one second copper trace on a second Ni-P seed layer on the second PAA layer wherein the first and second copper traces are interconnected through a via through the dielectric substrate, having a surface finishing layer on a top surface of the at least one first copper trace, and at least one die mounted on the dielectric substrate through diffusion bonding with the at least one first copper trace.
  • PAA polyamic acid
  • Fig. 1 is a flowchart of steps in a first alternative of a first preferred embodiment of the present disclosure.
  • Figs. 2A-2G schematically illustrate in oblique representation steps in a first alternative of the first preferred embodiment of the present disclosure.
  • Figs. 2H-2J schematically illustrate in oblique representation additional steps in a first alternative of the first preferred embodiment of the present disclosure.
  • Fig. 3 is a flowchart of steps in a second alternative of the first preferred embodiment of the present disclosure.
  • Figs. 2K-2M schematically illustrate in oblique representation additional steps in the second alternative of the first preferred embodiment of the present disclosure.
  • Fig. 4 is a flowchart of steps in a first alternative of a second preferred embodiment of the present disclosure.
  • Figs. 5A-5H schematically illustrate in oblique representation steps in the second preferred embodiment of the present disclosure.
  • Figs. 5I-5K schematically illustrate in oblique representation additional steps in the first alternative of the second preferred embodiment of the present disclosure.
  • Fig. 6 is a flowchart of steps in a second alternative of the second preferred embodiment of the present disclosure.
  • Figs. 5L-5N schematically illustrate in oblique representation additional steps in the second alternative of the second preferred embodiment of the present disclosure.
  • Figs. 7A-7K schematically illustrate in oblique representation additional steps in a third preferred embodiment of the present disclosure.
  • Figs. 7L-7N schematically illustrate in oblique representation additional steps in a first alternative of the third preferred embodiment of the present disclosure.
  • Figs. 7O-7Q schematically illustrate in oblique representation additional steps in a second alternative of the third preferred embodiment of the present disclosure.
  • Fig. 8 is an oblique representation of a completed flexible substrate of the second preferred embodiment of the present disclosure.
  • Fig. 9 graphically illustrates the peel strength of traces produced by the method of the present disclosure before and after reliability testing.
  • Fig. 10 graphically illustrates bending endurance before and after annealing of the flexible substrate of the present disclosure as compared to a traditional subtractive process.
  • Fig. 11 graphically illustrates strain as a function of pressure of the present disclosure as compared to a traditional subtractive process.
  • Fig. 12 graphically illustrates strain as a function of temperature of the present disclosure as compared to a traditional subtractive process.
  • Fig. 13 is an oblique representation of a completed COF using the flexible substrate of the present disclosure.
  • the present disclosure discloses a method of producing a plurality of fine traces on a flexible substrate, specifically for chip on flex (COF) packages.
  • This process will plate up reliable and robust copper traces with a trace pitch as fine as 8 ⁇ m and top to bottom width ratio close to 1.
  • the copper traces are built up by a fully additive process using electroless Ni-P as a seed layer on a modified dielectric material with a specific thickness that is capable of producing a reliable nano-size polyamic acid (PAA) anchoring layer on the dielectric/Ni-P interface.
  • PAA nano-size polyamic acid
  • the proposed process is compatible with a wide range of dielectric and surface finishing materials.
  • the traces formed are suitable for various interconnection methods including thermocompression bonding, wire bonding, adhesive bonding, and soldering of IC/chip to form a semiconductor package.
  • This formation of fine pitch COF is targeted for future demand of miniaturization in numerous sectors including organic light emitting diodes (OLED) , active matrix organic light emitting diode (AMOLED) , liquid crystal display thin film transistor (LCD/TFT) , smart wearable, medical imaging, and IoTs packaging.
  • OLED organic light emitting diodes
  • AMOLED active matrix organic light emitting diode
  • LCD/TFT liquid crystal display thin film transistor
  • smart wearable smart wearable, medical imaging, and IoTs packaging.
  • a fine pitch chip on flex is formed using a full additive process which is able to form reliable adhesion that ensures robust precision formation of fine traces on the flexible substrate and provides unique opportunities for ultra-fine pitch and high electrical performance interconnects.
  • each embodiment may include either electrolytic surface finishing or electroless surface finishing.
  • the process begins with a flexible dielectric substrate 10.
  • the dielectric may be any kind of polyimide (PI) , such as Kapton PI or Upisel PI, modified PI (MPI) , cyclo olefin polymer (COP) , or liquid crystal polymer (LCP) .
  • PI polyimide
  • MPI modified PI
  • COP cyclo olefin polymer
  • LCP liquid crystal polymer
  • the polyimide surface is modified by applying a KOH/alkaline base chemical to the PI surface.
  • Concentration of the modifier chemical is optimized to produce a PAA layer thickness of less than 10 nmin order to achieve the desirable trace integrity performance.
  • the modification layer of the present disclosure is extremely thin ( ⁇ 10 nm) , which can prevent the degradation of chemical bonding by reducing water intake on the layer during heat treatment and consequently minimizing the effect induced by the coefficient of hygroscopic expansion. Therefore, high adhesion can still be maintained after heat treatment.
  • the PAA layer could be more than 10nm, but should be less than 100 nm.
  • a catalyst layer is deposited on the PAA layer by immersion into an ionic metal solution.
  • Palladium (Pd) or Nickel (Ni) is deposited to activate the surface for subsequent electroless Ni-P plating.
  • an autocatalytic nickel-phosphorus (Ni-P) seed layer 14 is applied over the modified polyimide film using an electroless plating process.
  • the thickness of the Ni-P layer is ideally 0.1 ⁇ m +/-10%.
  • the composition of Ni-P in the seed layer is Ni: 96.5 ⁇ 97.5 wt%, P: 2.5 ⁇ 3.5 wt%.
  • step 104 the substrate is annealed at about 200 °C for a duration of at least ten minutes to at most two hours.
  • step 105 as shown in Fig. 2D, a layer of photoresist 16, preferably a positive-acting photoresist, is applied to the seed layer surface of the substrate.
  • the photoresist may be a dry film or a liquid photoresist.
  • the photoresist is exposed (step 106 and Fig. 2E) and developed (step 107 and Fig. 2F) to form a fine pitch trace for circuitization.
  • the plating is employed only on the areas of the spacing which are not covered by the photoresist. In some applications, the plating is controlled to be at an aspect ratio of close to 1. The ratio of the top to bottom widths of the traces using this method can be close to 1.
  • the copper is a fine-grained deposit with highly ductile properties.
  • the thickness of copper is about 8 ⁇ m. In some applications, the thickness of electrolytic copper can be in a range of 2-18 ⁇ m.
  • the elongation strength of the copper deposit is over 15%with a tensile strength of between 290-340 N/mm 2 .
  • the hardness of electrolytic copper is 100 in vicker hardness with a purity of more than 99.9%.
  • the electrolytic copper plating process enables high speed plating that enables mass production of the fine pitch COF.
  • step 109 the surfaces of the traces are finished by plating electrolytic Ni/Au, electrolytic Palladium, electrolytic Titanium, electrolytic Tin, or electrolytic Rhodium as shown by 22 in Fig. 2H.
  • the photoresist layer 16 is stripped, as shown in step 110 and Fig. 2I, followed by etching away the Ni-P seed layer 14 using a hydrogen peroxide acidic base solution that is strictly controlled to etch the Ni-P seed layer in a unidirectional manner with no or minimal etch on the copper trace to maintain the copper trace aspect ratio of close to 1, as shown in step 111 and Fig. 2J. This completes formation of the traces on the flexible substrate.
  • the inner lead bonding (ILB) pitch between the traces is a pitch defining a center to center distance between two adjacent traces, each respective trace having a respective surface layer.
  • the ILB of the substrate of the present disclosure is less than about 8 ⁇ m. In some applications, the ILB pitch can be 4-30 ⁇ m.
  • the COF is assembled.
  • the traces are compatible with various interconnection methods including thermocompression bonding, adhesive bonding, wire bonding and soldering of die or dies to form the semiconductor package.
  • Fig. 13 shows a completed COF using the flexible substrate of the present disclosure.
  • Copper traces 20 with surface finishing 22 on substrate 10 with PAA surface treatment are used to connect with several components.
  • Die 204 is shown with thermocompressive bonding to copper traces 20a through gold bumps 202.
  • Solder mask 200 and underfill 205 is shown.
  • Die 206 is bonded to a copper trace 20b preferably using epoxy.
  • Gold wires 208 are bonded to copper traces 20c.
  • Component 212 is soldered (210) to copper traces 20d.
  • the first alternative process included an electrolytic surface finishing.
  • the second alternative process includes an electroless surface finishing.
  • Fig. 3 shows that the steps in the process of the second alternative are identical to the first alternative through step 108, copper plating, as shown in Fig. 2G.
  • step 112 photoresist 16 is stripped from the substrate, leaving copper traces 20 on the Ni-P layer 14, as shown in Fig. 2K.
  • step 113 the Ni-P layer is etched away from the substrate, as shown in Fig. 2L.
  • step 114 the surfaces of the traces are finished by full body or selective surface finishing by immersion Tin (Sn) , electroless plating of Ni/Au, electroless Nickel/Immersion gold (ENIG) , Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) , Electroless Palladium/Autocatalytic Gold (EPAG) , or Immersion Gold/Electroless Palladium/Immersion Gold (IGEPIG) , as shown by 22 in Fig. 2M.
  • immersion Tin Sn
  • electroless plating of Ni/Au electroless Nickel/Immersion gold
  • EPIG Electroless Nickel/Electroless Palladium/Immersion Gold
  • EPAG Electroless Palladium/Autocatalytic Gold
  • IGEPIG Immersion Gold/Electroless Palladium/Immersion Gold
  • the electroless process of the second alternative requires a thinner surface finishing thickness but has a slower plating rate as compared to electrolytic plating.
  • the first embodiment shows a method of manufacturing a flexible substrate having at least one metal layer.
  • the metal layer can be one conductive metal layer or more than one conductive metal layer.
  • the flexible substrate can have double sided conductive metal layers or more than two stack-up conductive metal layers.
  • the second embodiment of the present disclosure shows a double sided (2 ML) metal layer process.
  • the process begins with a flexible dielectric substrate 10.
  • the dielectric may be any kind of polyimide (PI) , such as Kapton PI or Upisel PI, modified PI (MPI) , cyclo olefin polymer (COP) , or liquid crystal polymer (LCP) ,
  • PI polyimide
  • MPI modified PI
  • COP cyclo olefin polymer
  • LCP liquid crystal polymer
  • Dielectric 10 has a preferred thickness of between about 12.5 and 100 ⁇ m, as shown in Fig. 5A.
  • step 401 of Fig. 4 via openings 11 are laser drilled through the substrate 10, as shown in Fig. 5B. Vias will electrically connect the metal layer on either side of the substrate.
  • the polyimide surface is modified by applying a KOH/alkaline base chemical to the PI surface. This alters the molecular bond forming a polyamic acid (PAA) layer 12 on the top side of the substrate and 13 on the bottom side of the substrate, as shown in Fig. 5C, as well as within the via openings.
  • Concentration of the modifier chemical is optimized to produce a PAA layer thickness of less than10nm in order to achieve the desirable trace integrity performance.
  • the PAA layer could be more than 10nm, but should be less than 100 nm.
  • step 403 catalyst layers, not shown, are deposited on the PAA layers 12 and 13 by immersion into an ionic metal solution. Typically, Palladium (Pd) or Nickel (Ni) is deposited to activate the surface for subsequent electroless Ni-P plating.
  • an autocatalytic nickel-phosphorus (Ni-P) seed layer 14, 15 is applied over both sides of the modified polyimide film and within the via holes 11 using an electroless plating process.
  • the thickness of the Ni-P layer is ideally 0.1 ⁇ m +/-10%.
  • the composition of Ni-P in the seed layer is Ni: 96.5 ⁇ 97.5 wt%, P: 2.5 ⁇ 3.5 wt%.
  • step 405 the substrate is annealed at about 200 °C for at least ten minutes and at most two hours.
  • a layer of photoresist 16, 17, preferably a positive-acting photoresist is applied to the top and bottom seed layer surfaces of the substrate, respectively.
  • the photoresist may be a dry film or a liquid photoresist.
  • the photoresist is exposed (step 407 and Fig. 5F) and developed (step 408 and Fig. 5G) to form a fine pitch trace for circuitization.
  • step 409 and Fig. 5H layers of conductive metal 20, 21, including a plurality of traces for active bonding and a soldering pad, are plated up to the desired thickness on top and bottom of the substrate, respectively, using electrolytic copper plating.
  • the plating is employed only on the areas of the spacing which are not covered by the photoresist. In some applications, the plating is controlled to be at an aspect ratio of close to 1. The ratio of the top to bottom widths of the traces using this method can be close to 1.
  • Plating continues through the via openings resulting in an electrical connection between the top and bottom copper layers.
  • the copper is a fine-grained deposit with highly ductile properties.
  • the thickness of copper is about 8 ⁇ m.
  • the thickness of electrolytic copper can be in a range of 2-18 ⁇ m.
  • the elongation strength of the copper deposit is over 15%with a tensile strength of between 290-340 N/mm 2 .
  • the hardness of electrolytic copper is 100 in vicker hardness with a purity of more than 99.9%.
  • step 410 the surfaces of the traces 20 are finished by plating electrolytic Ni/Au, electrolytic Palladium, electrolytic Titanium, electrolytic Tin, or Electrolytic Rhodium, as shown by 22 in Fig. 5I. At least one of the traces on the top and the traces on the bottom of the substrate are finished with the surface finishing 22.
  • the photoresist layers 16, 17 are stripped, as shown in step 411 and Fig. 5J, followed by etching away the Ni-P seed layer 14, 15 using a hydrogen peroxide acidic base solution that is strictly controlled to etch the Ni-P seed layer in unidirectional manner with no or minimal etch on the copper trace to maintain the copper trace aspect ratio of close to 1, as shown in step 412 and Fig. 5K. This completes formation of the traces on the flexible substrate.
  • the first alternative process included an electrolytic surface finishing.
  • the second alternative process includes an electroless surface finishing.
  • Fig. 6 shows that the steps in the process of the second alternative are identical to the first alternative through step 409, copper plating, as shown in Fig. 5H.
  • step 413 photoresist 16, 17 are stripped from the substrate, leaving copper traces 20, 21 on the Ni-P layer 14, 15, as shown in Fig. 5L.
  • step 414 the Ni-P layer is etched away from the substrate, as shown in Fig. 5M.
  • step 415 the surfaces of the traces are finished by full body or selective surface finishing by immersion Tin, electroless plating of Ni/Au, electroless Nickel/Immersion gold (ENIG) , Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) , Electroless Palladium/Autocatalytic Gold (EPAG) , or Immersion Gold/Electroless Palladium/Immersion Gold (IGEPIG) as shown by 22 in Fig. 5N.
  • ENIG electroless Nickel/Immersion gold
  • ENEPIG Electroless Nickel/Electroless Palladium/Immersion Gold
  • EPAG Electroless Palladium/Autocatalytic Gold
  • IGEPIG Immersion Gold/Electroless Palladium/Immersion Gold
  • the third embodiment of the present disclosure shows more than two stack-up conductive metal layers.
  • Figs. 5A-5H and 7A-7N the third preferred embodiment in the process of the present disclosure will be described in detail.
  • the steps in the process of the third embodiment are identical to the second embodiment through step 409, copper plating, as shown in Fig. 5H.
  • photoresist layers 16, 17 are stripped, followed by etching away the Ni-P seed layer 14, 15 using a hydrogen peroxide acidic base solution that is strictly controlled to etch the Ni-P seed layer in a unidirectional manner with no or minimal etch on the copper trace to maintain the copper trace aspect ratio of close to 1, as shown in Fig. 7B.
  • a bonding film 70, 71 is laminated onto the top and bottom surfaces, respectively.
  • the bonding film can be any kind of dielectric material including polyimide, fluoropolymer, polyester, and so on.
  • the bonding material can be any kind of modified epoxy or thermoset adhesive film reinforced with fibers, such as epoxy, cyanide ester, or acrylic adhesive.
  • the bonding film will have a low coefficient of thermal expansion (CTE) and high glass transition temperature (Tg) .
  • the bonding film can be an Ajinomoto Bonding film (ABF) , an epoxy resin-based film consisting of. : Bisphenol A epoxy resin: 9 wt. %, Petroleum naphtha: under 5.0 wt.
  • Another polyimide base film 73, 74 is laminated onto the top and bottom bonding films, respectively, as shown in Fig. 7D.
  • polyimide (PI) liquid crystal polymer (LCP) may be used as layer 73, 74.
  • Dielectric 73, 74 has a preferred thickness of between about 12.5 and 100 ⁇ m, as shown in Fig. 7D.
  • via openings 75 are laser drilled through the PI layer and bonding layer on both top and bottom of the substrate 10, as shown in Fig. 7E. Vias will electrically connect the additional metal layers on either side of the substrate to the metal layers 20.
  • the polyimide surfaces 73, 74 are modified by applying a KOH/alkaline base chemical to the PI surface. This alters the molecular bond forming polyamic acid (PAA) anchoring layers 76, 77, as shown in Fig. 7F. Concentration of the modifier chemical is optimized to produce PAA layer thickness of less than 10 nm in order to achieve the desirable trace integrity performance.
  • PAA polyamic acid
  • a catalyst layer is deposited on the PAA layers 76, 77 by immersion into an ionic metal solution.
  • Palladium (Pd) or Nickel (Ni) is deposited to activate the surface for subsequent electroless Ni-P plating.
  • an autocatalytic nickel-phosphorus (Ni-P) seed layer 78, 79 is applied over the modified polyimide films 76, 77 on top and bottom of the substrate, respectively, using an electroless plating process.
  • the thickness of the Ni-P layer is ideally 0.1 ⁇ m +/-10%.
  • the composition of Ni-P in the seed layer is Ni: 96.5 ⁇ 97.5 wt%, P: 2.5 ⁇ 3.5 wt%.
  • the substrate is annealed at about 200 °C for a duration of at least ten minutes and at most two hours.
  • a layer of photoresist 82, 83 preferably a positive-acting photoresist, is applied to the seed layer surface 78, 79 on top and bottom, respectively, of the substrate.
  • the photoresist may be a dry film or a liquid photoresist.
  • the photoresist is exposed (Fig. 7I) and developed (Fig. 7J) to form fine pitch traces for circuitization on top and bottom surfaces of the substrate.
  • a layer of conductive metal 90, 91 including a plurality of traces for active bonding and a soldering pad, is plated up to the desired thickness on top and bottom of the substrate, respectively, using electrolytic copper plating.
  • the plating is employed only on the areas of the spacing which are not covered by the photoresist. In some applications, the plating is controlled to be at an aspect ratio of close to 1. The ratio of the top to bottom widths of the traces using this method can be close to 1.
  • the copper is a fine-grained deposit with highly ductile properties.
  • the thickness of copper is about 8 ⁇ m. Copper composition details are as described above.
  • the surfaces of the traces are finished by plating immersion Tin, electrolytic Ni/Au, electrolytic Palladium, electrolytic Titanium, electrolytic Tin, or electrolytic Rhodium, as shown by 92 in Fig. 7L. At least one of the traces on the top and the traces on the bottom of the substrate are finished with the surface finishing 92.
  • the photoresist layers 82, 83 are stripped, as shown in Fig. 7M, followed by etching away the Ni-P seed layer 78, 79 using a hydrogen peroxide acidic base solution that is strictly controlled to etch the Ni-P seed layer in a unidirectional manner with no or minimal etch on the copper trace to maintain the copper trace aspect ratio of close to 1, as shown in Fig. 7N. This completes formation of four level metal traces on the flexible substrate.
  • the first alternative process included an electrolytic surface finishing.
  • the second alternative process includes an electroless surface finishing.
  • the steps in the process of the second alternative are identical to the first alternative through copper plating, as shown in Fig. 7K.
  • photoresist 78, 79 are stripped from the substrate, leaving copper traces 90, 91 on the Ni-P layer 78, 79, as shown in Fig. 7O.
  • the Ni-P layer is etched away from the substrate, as shown in Fig. 7P.
  • the surfaces of the traces are finished by selective surface finishing by immersion Tin, electroless plating of Ni/Au, electroless Nickel/Immersion gold (ENIG) , Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) , Electroless Palladium/Autocatalytic Gold (EPAG) , or Immersion Gold/Electroless Palladium/Immersion Gold (IGEPIG) as shown by 92 in Fig. 7Q. At least one of the traces on the top and the traces on the bottom of the substrate are finished with the surface finishing 92.
  • a cover coat such as solder resist or coverlay, is formed to act as a barrier between adjacent copper traces to protect the traces and prevent electrical shorts.
  • the flexible substrate of the present disclosure is suitable for any cover coat material.
  • Fig. 8 illustrates a completed oblique view of the four level conductive layer flexible substrate of the third embodiment. It can be seen that the metal layers 90, 20, 21, 91 (top down) are electrically connected through vias 75 and 11. Surface finishing 92 is shown on the exposed top copper traces 90. Cover coat, such as solder resist, 93 covers portions of the top copper traces 90 and covers the bottom copper traces 91. In this example, these areas are not used for bonding, so they do not need the relatively more expensive surface finishing 92.
  • a flexible substrate having multiple conductive layers more than four can be achieved by sequentially repeating the steps of the third embodiment on the completed copper formation of the third embodiment.
  • the process of the present disclosure can achieve an extremely smooth surface (Ra ⁇ 100 nm) without compromising the trace adhesion. This smooth surface is able to minimize the conductor loss during signal transmission.
  • the traces are compatible with various interconnection methods including thermocompression bonding, adhesive bonding, wire bonding and soldering of die or dies to form the semiconductor package.
  • TEM images of the substrate in the process of the present disclosure showed the thickness of the Ni-P seed layer of about 100 nm and the thickness of the PAA anchoring layer of about 3-4 nm before and after 300°C annealing. No degradation of the PAA anchoring layer was observed after annealing.
  • Fig. 10 graphically illustrates the bending endurance before and after annealing of the flexible substrate with the proposed method (Full Additive) using direct metallization as compared to a traditional Subtractive process using a sputtering type base film material.
  • the traditional method is shown on the left of the graph.
  • Bending endurance is shown before annealing (301) , after annealing at 200°C for 24 hours (302) , and after annealing at 300°Cfor 24 hours (303) .
  • On the right is shown the bonding endurance of the full additive method of the present disclosure before annealing (305) , after annealing at 200°C for 24 hours (306) , and after annealing at 300°C for 24 hours (307) . It can be seen that the process of the present disclosure provides improved bending endurance in all cases.
  • Fig. 11 graphically shows the plastic deformation characteristic of the thermocompression bonding of the fully additive process of the present disclosure 311 as compared to a conventional subtractive (sputtering) process 313.
  • temperature is constant at 345 °C and the pressure is varied.
  • Fig. 12 shows the deformation strain of the process of the present disclosure 321 as compared to a conventional subtractive process 323 at a constant pressure of 140 MPa at various temperatures.
  • Trace adhesion strength and bend durability of the process of the disclosure is similar to if not better than the substrate fabricated by a conventional subtractive process with a sputtering type base film material. Likewise, similar plastic deformation behavior after thermcompression bonding is observed as compared to a substrate fabricated by a conventional subtractive process with a sputtering type base film material. Reliable adhesion strength (on both sides for a two or more metal layer substrate) is maintained particularly due to the stability of the PAA anchoring layer after a 300°C heat treatment for 24 hours.
  • the flexible substrate of the present disclosure is suitable for various interconnection methods including thermocompression bonding, wire bonding, adhesive bonding, and soldering of the IC/Chips to form a semiconductor package.
  • the manufacturing process of the present disclosure results in an extremely smooth surface of the copper trace (Ra ⁇ 100 nm) without compromising the trace adhesion. This smooth surface is able to minimize the conductor loss during signal transmission.
  • the present disclosure has described a method of manufacturing a flexible substrate with fine traces for COF that can be integrated into AMOLED, OLED, TFT/LCD and at least one of: a smart phone device, portable devices, IoT packaging, smart wearables, tablets, UHD TV, micro display, optoelectronics, medical devices, industrials (building &machinery monitoring) , and IC packaging/3D IC integration modules.

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PCT/CN2020/000184 2019-08-22 2020-08-19 Formation of fine pitch traces using ultra-thin paa modified fully additive process WO2021031507A1 (en)

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KR1020227009098A KR20220061992A (ko) 2019-08-22 2020-08-19 초박형 paa 변형된 완전 애디티브 공정을 사용하는 미세 피치 트레이스들의 형성
CN202080057766.7A CN114616662A (zh) 2019-08-22 2020-08-19 使用超细paa改性全加成法制造精细间距走线的方法
JP2022511270A JP2022545799A (ja) 2019-08-22 2020-08-19 完全に付加的なプロセスで修正された極薄ｐａａを使用した微細ピッチトレースの形成

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