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2019

03/22

A New Solution to the Difficulty of Wafer-level Packaging

Nowadays, people are fully aware of the challenges of advanced packaging. However, overcoming these challenges at the wafer level before thinning device chips are packaged can further increase value and performance while reducing cost of ownership.

This paper consists of three parts. It will introduce specific challenges, new packaging methods, and examples of new high temperature resistant materials and solutions for device manufacturing. These new materials have shown better performance than other existing solutions. With examples, we will explore the applications that benefit from the use of chips manufactured with new technologies.

Summary

Better system performance and functionality, lower power consumption and smaller size are the main factors driving today's packaging technology. Wafer-level packaging (WLP) technology, widely used in mass production, is currently mainly used to manufacture consumer products, such as smartphones, tablets and other handheld devices. Many packaging platforms are being deployed to enable higher-performance packaging, lower cost, smaller shape and size, and higher-level integration.

Wafer-level chip size package (WLCSP) is attractive because of its cost-performance ratio and substrate-free packaging, but it is limited by chip size. Another alternative, Fan Out Wafer Level Packaging (FOWLP), is being developed and applied because it allows for increased I/O density by interconnecting Fan Out with external pads. Ultimately, it has smaller shape size and lower power consumption. Heterogeneous integrated semiconductor packaging technologies, such as system-level packaging (SIP) and stacked packaging (PoP) infrastructure, face major challenges due to increasingly complex integration.

Wafer-level packaging challenges

For many of these technologies, substrate treatment of thin devices is a major challenge in the manufacturing process. Thinning silicon wafers to less than 50 microns (m) or creating a redistribution layer (RDL) using an RDL-first process requires great care and high manufacturing costs. The process requires the use of temporary bonding and de-bonding (TBDB) technology to process the supporting substrates to facilitate the construction of complex packaging infrastructure [1].

Temporary bonding materials made from thermoplastic polymers are usually used in TB/DB processes. When used together with carrier substrates, they can provide thermal mechanical stability and make thin device substrates easier to handle. However, at higher temperatures, these materials behave more like liquids. As the melt viscosity decreases, the mechanical stability gradually disappears and the material softens, thus reducing the stability of the bonding layer. Deformation and delamination may occur in the wafer of the device, resulting in problems in downstream processes [2].

Now we have done a high-level research on some of the challenges of advanced packaging. Next, we will explore some of these technologies more deeply. Part II will examine the differences between chip-first and chip-last process flows and why the latter is more popular.

Chip-First or Chip-Last processes

Two main types of FOWLP technology are chip-first and chip-last technology, also known as RDL-first. Chip-first and chip-last processes require high temperature and high vacuum processes to create redistribution layers (RDL). Today's FOWLP process requires materials that can withstand high temperatures and harsh chemical conditions while maintaining mechanical support for device substrates.

For chip-first process, a single chip is placed on a substrate treated with temporary bonding material or thermal release tape (TRT) before it is coated with epoxy resin forming compound (EMC) and solidified. High temperature dielectric treatment can cause stress and warpage between carrier wafer and EMC. In EMC process, chip movement and deviation due to warping of substrate and softening of bonding material will cause dislocation between RDL and embedded chip.

After the wafer is processed in the wafer factory, the chip is cut into small pieces. Then, the chip is placed on a new 200 mm or 300 mm wafer based on epoxy resin moulding plastics through the fetching and placing system. The packaging process is carried out on this new wafer and the chip is cut to obtain the chip in the fan-out packaging.

Although chip-first packaging has been used in production for the past 10 years, there are also some challenges in this process. In the process, wafers may warp and embedded chips may displace, resulting in a decline in yield.

On the other hand, Chip-last/RDL-first has not been widely used yet, but interest in this method is increasing because it uses a very different process from chip-first. RDL-first is an ideal technology for chip manufacturers who want to transition from chip-first FOWLP to 2.5D/3D packaging.

In the RDL-first process, glass carrier wafers are coated with removable laser demoulding materials, and RDL will be built on this basis. Laser demoulding materials need good thermal stability, mechanical stability and chemical stability to withstand the process of thinning, backside medium and precipitation. First build RDL, then install the chip. In this process, RDL structure can be used for both electronic testing and visual inspection to determine yield loss, so as to avoid putting good chips in bad positions. This process is especially suitable for large I/O chips whose yield is critical.

In order to ensure the success of FOWLP, whether chip-first or RDL-first methods are used, it is very important to use appropriate bonding materials to ensure the stability and uniformity of the reconstituted wafers. Brewer Science has developed a series of materials for this purpose.

BrewerBOND < T1100 and BrewerBOND < C1300 series materials represent a new generation of bonding systems, which can provide higher yield and thermal stability. These materials provide better mechanical stability and chemical tolerance at higher process temperatures. Whether wafer-level or panel-level processes, they can bond and decouple at room temperature. The lower total thickness variation (TTV) and the increase of mechanical strength of the system can make the ultra-thin back wafer thinner and realize the wafer thickness less than 50 m after grinding.

BrewerBUILD () material is a single layer high absorption material, which can be used for the construction and assembly of RDL, and is specially designed for laser ablation process. These materials increase the absorbance of 308 to 355 nm and provide protection for device wafers in laser ablation process. In addition to improving the performance in laser ablation process, the new generation of materials also have strong solvent resistance, high adhesion to various materials, and good solvent cleaning effect after ablation [2].

Materials for FOWLP Process

In this section, we will further explore the BrewerBOND < T1100 > and BrewerBOND < C1300 series of materials introduced above for Fan-Out Wafer-Level Packaging (FOWLP).

The system consists of a low Tg thermosetting material applied to the glass carrier wafer, which is then bonded to the device wafer, which has been coated with a corresponding high Tg bonding material. After bonding at room temperature, the bonding pair can be either exposed to ultraviolet (UV) light or baked on a hot plate to solidify the thermosetting material (Fig. 1).

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Figure 1: Typical process flow

When treated at 350 C, Brewer BOND < T1100 series materials are still soluble in solvents, and there is almost no melt flow below 300 C. After coating, the material can be highly conformal, or even thin coated to cover a severely uneven surface. Figure 2 shows the cross section of 2.15-m film of BrewerBOND < T1100 series material processed on 80-m solder bump pad by scanning electron microscopy (SEM).

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Figure 2: Conformal coating of Brewer BOND < T1100 material

BrewerBOND < C1300 series materials have high melt fluidity (low Tg), which is liquid before curing. In this way, BrewerBOND < T1100 series materials can be bonded at room temperature without pressure. After bonding, the material needs a curing process to form a bonding layer. This characteristic enables the system to have high mechanical strength even at high temperatures (Table 1).

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Table 1: Material properties

Dielectric treatment, metal precipitation and metal annealing are processes requiring high temperatures. These new generation Brewer BOND #3 materials can maintain the integrity of the bonding layer without decomposition, gas release or reflux. Isothermal thermogravimetric analysis (TGA) in nitrogen showed that the weight loss of these materials was less than 6% after three hours of heating at 350 degree C. In FOWLP technology, the adhesion of materials to organic and inorganic substrates and metal layers is also necessary. As shown in Figure 3, the new bonding material exhibits good adhesion to copper sheet and epoxy resin forming compound (EMC).

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Figure 3: Copper precipitation on bonding materials: (A) poor adhesion, showing defects; (B) good adhesion, no defects.

In the bonding process, BrewerBOND < T1100 series materials together with BrewerBOND < C1300 series materials show tolerance to common downstream wet chemical processes. After the process is completed, the carrier substrate can be removed from the thinned device by mechanical release or laser ablation technology. Both processes can be accomplished at room temperature and are light-weight technologies that can be used with thinned substrates.

When laser ablation process is used, BrewerBOND < T1100 series materials absorb the energy used in laser ablation for bonding at 308 and 355 nm, thus preventing laser direct damage to device wafers.

After debonding, BrewerBOND < T1100 series materials can be removed from device substrates by solvent or oxygen plasma etching. The Brewer BOND < C1300 series material can be removed from the carrier substrate using Dynaloy's Dynasolve < 220 detergent material.

summary

Brewer Science is developing new temporary bonding materials and processes to pave the way for the development of FOWLP technology. When used as a system, these materials can improve the mechanical stability of thin, bonded wafers processed under high vacuum and high temperature. The combination of chemical resistance and room temperature bonding and de-bonding technology provides additional value and improves performance while reducing cost of ownership. For RDL-first process, Brewer Science recently introduced Brewer BUILD () material for construction and assembly, which can be a preferred alternative to heat release tape. These new materials promote the low energy laser de-bonding process and provide better protection for wafers with low carbon residues. Brewer Science will continue to promote the development of wafer-level packaging (WLP) technology by providing a new generation of materials to support FOWLP technology.