Low-temperature Bonding Methods Based on Cu Crystal Thin Film Modification Layers and Porous Ag Layers

XIAO Jin; LUO Jia; FANG Yan; CHEN Siyuan; HOU Xiaoli; ZHONG He

doi:10.16490/j.cnki.issn.1001-3660.2026.06.020

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Surface Technology ›› 2026, Vol. 55 ›› Issue (6) : 260-269. DOI: 10.16490/j.cnki.issn.1001-3660.2026.06.020

Functional Surfaces and Technology

Low-temperature Bonding Methods Based on Cu Crystal Thin Film Modification Layers and Porous Ag Layers

XIAO Jin¹, LUO Jia^1,*, FANG Yan², CHEN Siyuan¹, HOU Xiaoli³, ZHONG He¹

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Abstract

A thin-film-modified Cu bump serves as one end of the bonding pair, while a Cu substrate coated with a specific thickness of Ag layer functions as the other end. Cu-Cu bonding interconnects are achieved under low-temperature and low-pressure conditions. This approach resolves thermal shock and defect issues induced by high-temperature and high-pressure bonding conditions, ensuring the reliability of sensitive thin chips during packaging. The work aims to introduce nanoscale structures and materials into the bonding process to explore a low-temperature, no-solder, no-flux insertion bonding method. Compared to other Cu-Ag bonding techniques, this bonding process can be completed at low temperatures and on flat surfaces due to the nanoscale characteristics of the Cu bump modification layer and the deformation effect of the porous Ag sheet.
The preparation of thin-film-modified Cu bumps was achieved by electroplating Cu columns and applying a chemical copper plating process to form a micrometer-thick film on patterned chips. This method was based on a needle-cone-shaped Cu crystal film deposited on the Cu bump with an average height of approximately 2 μm and an average root diameter of approximately 500-800 nm, and a 2 μm thick porous Ag layer deposited on the Cu substrate. Cu-Cu interconnect bonding was achieved by bonding for 25 minutes at 200 ℃ and 5 MPa pressure in an air atmosphere. The connection between the Cu micro-cone array structure on the Cu bump and the porous Ag layer was achieved. Upon completion of bonding, the average shear strength at the bonding interface was measured with a destructive shear test method. The cutting tool was pushed down on the Cu bump at a speed of 1.2 mm/min, with the tool sensor recording the maximum shear force. The microstructure, chemical composition, and related properties of the bonded contact layer was investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thickness of the coating was measured with an X-ray fluorescence thickness gauge.
The modified layer deposited on the Cu bump exhibited a Cu crystalline layer with a needle-cone array structure. The height and distribution of the needle-cone structure were uniform and consistent. The apex angles were relatively small, exhibiting a pronounced tip effect. The Ag layer deposited on the Cu substrate formed a uniform porous nano-Ag flake structure with a spindle-shaped texture. During bonding, the harder Cu needle cones penetrated the softer Ag layer while retaining their distinct conical profiles. Sufficient atomic-scale interlocking and bonding occurred between the Cu micro-cone structure and Ag, creating a mechanical interlocking effect. Under optimal parameter conditions, the average shear strength at the bonding interface reached 38.9 MPa, with an extremely dense bonding interface exhibiting virtually no gaps. Due to the unique morphology of the Cu crystal surface modification layer, the nanoscale effects of the flake-like Ag layer, and the deformation effects, bonding is achieved under low-temperature and low-pressure conditions in an air atmosphere. This method eliminates the need for ultra-high vacuum, ultra-flat surfaces, and complex subsequent heat treatment processes to enhance bonding interface shear strength, thereby improving package reliability. This technology holds promise for extensive practical applications.

Key words

Cu crystal / Ag layer / bonding / electronic packaging / diffusion

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XIAO Jin, LUO Jia, FANG Yan, CHEN Siyuan, HOU Xiaoli, ZHONG He. Low-temperature Bonding Methods Based on Cu Crystal Thin Film Modification Layers and Porous Ag Layers[J]. Surface Technology. 2026, 55(6): 260-269

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Funding

Special Funds for Intelligent Manufacturing Modern Industrial College (0220119); Guangdong Provincial Education Science Planning Project: Higher Education Special Program (2023GXJK643)