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Through-glass Via(TGV) - A Critical Technology For Advanced Packaging

In the field of advanced packaging, TGV is generally considered by the semiconductor industry to be the key technology for next-generation 3D integration. TGV can be used in optical communications, RF front-ends, optical systems, MEMS advanced packaging, consumer electronics, medical equipment and other fields. In this article, we will deep into the through-glass via(TGV) technology.

Table of Contents

What is through-glass via(TGV)?

TGV, the abbreviation “Through-Glass Via”, is the vertical electrical interconnection through the glass substrate. Corresponding to TSV(through Silicon Via), as a material that may replace silicon substrates, through glass via (TGV) three-dimensional interconnection technology is becoming a current research hotspot due to its various advantages.

Since silicon is a semiconductor material, the carriers around TSV can move freely under the action of electric field or magnetic field, which will interfere with adjacent circuits or signals and affect chip performance.

However, the glass material does not have free moving charges, and there is no need to deposit an insulating layer. It has excellent dielectric properties. Meanwhile, the thermal expansion coefficient of glass can be adjusted, reducing the thermal mismatch with different materials. Due to the easy availability of large-sized glass panels, the glass cost is about 1/8 of that of silicon substrates. Strong mechanical stability, Even when the thickness is less than 100µm, the warpage is still small.

The key challenge TGV technology is facing is that there is no deep etching process similar to silicon, and it is difficult to quickly fabricate glass deep holes or trenches with high aspect ratios. The traditional sandblasting method, wet etching method, and laser drilling method all have certain limitations.

Inductively coupled plasma (ICP) dry etching technology has high control precision, smooth etching surface, and good verticality. It is often used to etch high aspect ratio structures, but isotropic etching is serious. Due to the limitation of the mask deposition process on the glass substrate, a certain etching selectivity is required when etching deep holes. Improving the glass etching rate while ensuring the verticality of the sidewall and the etching selectivity ratio has become a difficult point in current research.

What is through glass vias

What are the strengths of TGV?

Compared with silicon substrates, the strength of through-glass via(TGV) are mainly includes the following:

1)Excellent high-frequency electrical characteristics.
Glass material is an insulator material, the dielectric constant is only about 1/3 of that of silicon material, and the loss factor is 2-3 orders of magnitude lower than that of silicon material, which greatly reduces substrate loss and parasitic effects and ensures the integrity of transmitted signals.

2)Large-scale ultra-thin glass substrates are easy to obtain.
Glass manufacturers such as Corning, Asahi, and SCHOTT can provide ultra-large (>2m × 2m) and ultra-thin (<50µm) panel glass and ultra-thin flexible glass materials.

3) Low cost
Benefiting from the easy availability of large-size ultra-thin panel glass and the need not to deposit an insulating layer, the manufacturing cost of a glass interposer is only about 1/8 of that of a silicon-based interposer.

4)The process flow is simple.
There is no need to deposit an insulating layer on the surface of the substrate and the inner wall of the  through-glass via(TGV), and no thinning is required in the ultra-thin interposer.

5)Strong mechanical stability.
The warpage is still small when the interposer thickness is less than 100µm.

6)Wide range of application fields.
In addition to having good application prospects in the high-frequency field, as a transparent material, it can also be used in optoelectronic system integration. The advantages of air tightness and corrosion resistance make glass substrates have great potential in MEMS packaging.

Where can TGV be used?

Where can TGV be used
Where can TGV be used
  • 3D Integrated Passive Components on Glass Substrates

The glass substrate has excellent high-frequency electrical properties. Compared with the two-dimensional planar inductor, the three-dimensional inductor with the  through-glass via(TGV) structure has a better quality factor.

Glass has a lower dielectric constant and higher resistivity than silicon, giving it better high-frequency performance. Passive components such as filters and duplexers using  through-glass via(TGV) can be kept small in size while ensuring low in-band insertion loss and high out-of-band rejection.

Therefore, it is widely used in integrated passive devices (IPD). Secondly, the warpage of the glass plate can be controlled within 1mm, and there is no obvious structural peeling and delamination.

  • Through Glass Via MEMS Packaging

The wafer-level packaging of RF MEMS devices is realized by using glass through-hole technology, and the electroplating scheme is used to completely fill the through holes.

  • Embedded glass fan-out

Through-glass via(TGV) can also create cavities on the glass, thereby providing a new solution for embedded glass fan-out (eGFO) for chip packaging. Realize high I/O density and high-performance glass panel fan-out packaging, and effectively control chip offset and warpage.

The process flow is shown in the figure below: In a glass wafer with a thickness of 180um, a glass cavity is first formed by laser-induced glass denaturation and chemical etching, and then a 175um-high chip is placed in the glass cavity. The gap between the chip and the glass is filled with a composite material without creating a void while protecting the back of the chip. The top surface of the wafer is peeled off to form copper RDL, and finally, subsequent circuit fabrication, ball grid array (BGA) fabrication and wafer slicing are performed.

  • Integrated Antenna for TGV

Firstly, the waveguide slot array antenna glass substrate is prepared by laser-induced etching, and a continuous denatured area is induced on the glass by laser, and then the denatured glass is etched in dilute hydrofluoric acid, and the glass will fall off in pieces and thus form the target via structure.

Secondly, physical vapor deposition is used to sputter a copper layer on each layer of waveguide slot array antenna glass substrate, and after oxygen plasma cleaning to completely remove organic matter and other particles on the surface of the pad, and to produce a certain roughness on the surface of the wafer, as the seed Create conditions for good adhesion of layers.

The cleaned wafer was baked in an oven at 150°C for 60 minutes to completely remove moisture. Then in the magnetron sputtering equipment, a layer of copper with a thickness of about 5 μm is sputtered on the surface of the wafer.

Finally, 5 wafers were bonded using technical solder bonding technology (specifically: use a scraper and a silk screen to print 10 μm thick tin solder on the surface of the wafer, and then use a vacuum chamber of the bonding machine at 240 ° C to Heating at high temperature, pressing with a pressure of 40N for 5 minutes to melt or interdiffuse the solder to achieve the purpose of bonding).

  • Multilayer Glass Substrate Multichip Package

Fill the through-glass holes with conductive glue to realize stacking of multi-layer glass substrates. During the reflow process, the warping of the multi-layer glass substrates produced by this scheme is smaller than that of traditional organic substrates. High-density wiring and high reliability can be achieved through this technology.

What methods are commonly used in TGV?

Methods for preparing TGV through holes include sandblasting, mechanical drilling, dry etching, wet etching, focused discharge, etc. However, the above methods have obvious shortcomings. At present, there are 2 widely used method for  through-glass via(TGV) technology that are laser ablation and laser-induced denaturation.

Laser ablation
Principle: The atoms of the glass substrate generate high-frequency oscillations and rapidly heat up under the excitation of the laser, and the atoms are detached from the substrate and are ablated and volatilized.

Advantages: No film is required, reducing process cost; Deep holes with a certain inclination angle can be naturally formed, which is conducive to subsequent filling of metal in deep holes with high aspect ratio.

  • Disadvantages:
    Serial processing is required, which affects the etching efficiency and yield;
  • The molten products and residues caused by laser ablation adhere to the opening of the deep hole, and the surface residues need to be removed after the etching is completed.

 Laser-Induced-Deep Etching
The ultrashort pulse laser (picosecond) induces the glass to produce a continuous denatured area. Compared with the glass in the undenatured area, the denatured glass has a faster etching rate in hydrofluoric acid.

Advantages
There is no special requirement for the glass material itself, and the speed of achieving through holes is relatively fast.

Conclusion

Due to the successful development of innovative low-cost through-hole processing technology,  through-glass via(TGV) integration technology will be widely used in the field of radio frequency.

FAQ

TGV, the abbreviation "Through-Glass Via", is the vertical electrical interconnection through the glass substrate.

1)Excellent high-frequency electrical characteristics.

2)Large-scale ultra-thin glass substrates are easy to obtain.

3) Low cost

4)The process flow is simple.

5)Strong mechanical stability.

6)Wide range of application fields.

TGV can be used in optical communications, RF front-ends, optical systems, MEMS advanced packaging, consumer electronics, medical equipment and other fields.

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