With the rapid development of semiconductor technology, the demand for miniaturization and high performance of electronic devices is growing increasingly [1-3]. The operating environments of high-performance devices are more demanding, requiring more reliable and durable packaging technologies. At present, with the rapid development of the integrated circuit industry, packaging technology has transitioned from traditional two-dimensional planar packaging to three-dimensional packaging. Glass substrates, owing to their excellent electrical, mechanical, and thermal properties, have gradually become an emerging advanced packaging material. Among them, TGV can achieve high-density, low-loss vertical interconnections and is one of the important technical means for realizing high-density packaging and three-dimensional integration [4].
The core of TGV technology lies in via processing and filling, as these two steps directly determine the stability and electrical performance of chip packaging. Via processing technology affects the size, shape, and processing efficiency of the vias, while via filling technology determines the electrical performance and mechanical strength of the interconnections.
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Glass Substrate Via Processing Technologies
Via processing technology is a key process for TGV to achieve multilayer interconnection and high-density packaging. It requires high processing efficiency and high precision, and the resulting vias should have good surface quality as well as excellent verticality and aspect ratio. The main via processing technologies include sandblasting, electrochemical discharge machining, groove sinking and grinding, laser ablation, and laser-induced etching.
2.1 Sandblasting
Sandblasting is a processing technique that etches glass substrates by high-pressure spraying of fine particles to form vias [5]. A solid-liquid mixture is sprayed at high speed through a nozzle to impact the glass substrate surface, gradually etching and removing material until the predetermined hole structure is formed.
Sandblasting has excellent processing adaptability and can handle glass substrates of different sizes and thicknesses. It offers high processing flexibility, can process vias of various shapes and sizes, and features low equipment cost and simple operation, making it suitable for mass production. However, the TGV surfaces produced by this method have uneven roughness, and there are also problems such as dust pollution, material waste, and low processing efficiency.
2.2 Electrochemical Discharge Machining
Electrochemical discharge machining is a non-traditional processing method that combines the advantages of electrochemical machining and electrical discharge machining, and is particularly suitable for processing hard and brittle materials such as glass. Its basic principle is as follows: under the action of an applied voltage, the tool electrode undergoes spark discharge, releasing enormous energy; the high temperature and high pressure generated in the tiny gap formed between the tool electrode and the material cause the material to melt or vaporize, which is then removed by high-pressure impact; at the same time, the high temperature accelerates chemical reactions, making the chemical corrosion of the material by the alkaline solution more significant, thereby completing the processing process under the combined physical and chemical effects.
Electrochemical discharge machining is widely used for processing surface microstructures of insulating hard and brittle materials, such as micro-pits, micro-holes, and micro-grooves. Through this method, complex microstructures can be precisely manufactured to meet high-precision processing requirements. However, this method also has several shortcomings, such as rapid electrode wear, difficult process control, high equipment cost, and relatively slow processing speed.
2.3 Groove Sinking and Grinding
Groove sinking and grinding is a method in which a material (usually in liquid or softened state) is deposited into grooves of a mold or substrate with a specific morphology, formed into the desired shape and structure through cooling or solidification, and then subjected to planarization and grinding treatment [6]. The advantages of groove sinking and grinding include good electrical insulation performance and excellent surface quality, and this process can simplify the manufacturing of complex three-dimensional structures. However, in high-aspect-ratio structures, there may be problems such as incomplete filling, long process time, and thermal stress generation.
2.4 Laser Ablation
Laser ablation is a technology that uses a high-energy laser beam to remove material to manufacture glass vias. Its principle is to focus a high-energy laser beam on the glass substrate, generating extremely high temperatures in a localized area, thereby causing the material to instantly melt or vaporize and be removed through ejection or evaporation [7].
Laser ablation offers high processing precision, strong flexibility, no mechanical stress, and fast processing speed, but it also has defects such as protrusions, debris, micro-cracks, and burns. These defects can negatively affect the quality of subsequent bonding processes with other materials (such as metals, ceramics, or composites), leading to reduced bonding strength and decreased durability.
2.5 Laser-Induced Etching
LPKF of Germany proposed laser-induced etching [8]. The process steps are as follows: first, clean the glass substrate to ensure the surface is free of dust and oil, and cover it with a mask to protect areas that do not need processing; then select an appropriate laser and set the parameters, and use computer-controlled laser beam positioning to irradiate the glass and change its structure; finally, immerse the glass substrate in a chemical etching solution to preferentially dissolve the laser-treated areas, controlling the etching time and solution concentration to achieve the required etching depth and structure.
Compared with sandblasting, electrochemical discharge machining, groove sinking and grinding, and laser ablation, laser-induced etching has significant advantages in high precision, processing flexibility, and surface quality. Laser-induced etching combines the precision of laser pretreatment with the selectivity of chemical etching. This method can achieve high-quality TGV processing and overcome the shortcomings of traditional methods in precision, efficiency, and material applicability. However, this method has the problem of low verticality of the processed vias, and the equipment used for laser modification is expensive.
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Glass Substrate Via Filling Technologies
After TGV via formation, they need to be filled to achieve electrical interconnection. The filling process is a key step for realizing high-performance multilayer interconnection and high-density packaging, and mainly includes metal electroplating filling and conductive paste filling.
3.1 Metal Electroplating Filling
After forming the vias, a metal seed layer needs to be deposited on the inner walls of the vias, usually by physical vapor deposition (PVD) or chemical vapor deposition (CVD). The seed layer material is generally copper or nickel, and copper and nickel are filled into the vias through an electroplating process. During electroplating, metal ions are reduced on the surface of the seed layer under the action of an electric field, gradually filling the entire via. Because this process adopts a bottom-up filling method, it is necessary to prevent the formation of voids during filling, so it is time-consuming and costly. TGV vias do not necessarily need to be completely filled; they can be filled along the sidewalls or with a semi-closed structure. This filling method is called partial filling. Partial filling technology is already close to full filling technology in electrical performance and can optimize electroplating time and cost.
Metal electroplating filling technology, with its excellent electrical conductivity, mature process flow, and efficient filling capability, has become the main method for TGV via filling at present. This is because the bottom-up filling mode can effectively avoid the generation of voids and ensure the integrity of the via structure. However, the traditional full filling method has the limitations of long process time and high cost, and is prone to pores or surface defects under high current density. Partial filling technology achieves electrical performance comparable to full filling by optimizing process time and cost, and combined with the application of new materials such as silver nanowires, further reduces manufacturing cost and process complexity.
3.2 Conductive Paste Filling
Conductive paste filling of TGV refers to forming conductive paths by filling conductive paste into glass vias. Conductive paste usually contains conductive particles (such as silver, copper, etc.) and binders (such as epoxy resin or organic polymers). After high-precision vias are formed in the glass substrate, the inner walls of the vias should first be surface-treated to enhance the adhesion of the conductive paste, and then the conductive paste is filled into the glass vias by screen printing, injection filling, or printing coating. To ensure uniform distribution of the conductive paste in the vias, vacuum-assisted filling technology is usually adopted to eliminate air entrapment and avoid the formation of voids. After the conductive paste filling is completed, a curing treatment is required. Common curing methods include thermal curing, ultraviolet curing, or chemical curing. Through appropriate curing treatment, the conductive paste forms a stable conductive path in the vias. The vias are then polished and cleaned to ensure a flat via surface and meet the requirements of subsequent packaging and integration.
Conductive paste filling has adhesiveness, and after curing, the coefficient of thermal expansion of the conductive vias can be adjusted to be close to that of the substrate. However, the electrical conductivity of conductive paste is usually lower than that of metal conductors, which will have a certain impact on the performance of electronic devices; in addition, vias filled with conductive paste may develop reliability problems such as cracks or delamination during long-term use, and these defects limit the application scope of this method.
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Summary
Each glass substrate via processing technology has its significant advantages as well as corresponding limitations.
Sandblasting has a simple process, low equipment cost, and flexible operation, and is suitable for processing large-diameter vias, especially having advantages in mass production. However, this method still has problems such as susceptibility to cracks, uneven surface roughness, serious dust pollution, large material waste, and low processing efficiency.
Electrochemical discharge machining can produce glass vias with high aspect ratios and high processing precision. However, this method has disadvantages such as relatively complex process, fast tool electrode wear, high cost, troublesome waste liquid treatment, and low processing efficiency.
Groove sinking and grinding has advantages such as high processing precision, excellent surface quality, and good electrical performance, and is suitable for precision manufacturing of complex three-dimensional structures. However, in high-aspect-ratio structures, problems such as incomplete filling, long process cycles, and thermal stress generation may occur.
Laser ablation has advantages such as high processing precision, strong flexibility, and high efficiency, but due to its large heat-affected zone, it is prone to thermal stress and micro-cracks, and the equipment cost is high. At the same time, some glass materials have low absorption rates for lasers, resulting in decreased processing quality.
Laser-induced etching has advantages such as high precision, rapid via formation, and damage-free processing. The finished products processed by this method have excellent surface quality and are suitable for manufacturing high-aspect-ratio glass vias. However, this method has high processing cost and expensive processing equipment.
Different glass substrate via filling technologies also have their respective advantages and disadvantages.
Metal electroplating filling technology has the characteristics of excellent electrical conductivity, mature process flow, and efficient filling capability, and has become the main method for TGV via filling. The traditional full filling method has limitations such as long time consumption and high cost. In comparison, partial filling technology greatly reduces process complexity and cost through optimized processes and material applications while maintaining excellent electrical performance and structural integrity.
Conductive paste filling technology has advantages such as simple process and relatively low cost, and has application potential in three-dimensional packaging and passive device integration. However, its application is limited by low electrical conductivity, poor stability, and insufficient mechanical strength.