How Diamond Heat Sinks Revolutionize Advanced Packaging Cooling

2024-09-05

Cutting-edge technologies like AI, deep learning, and cloud computing rely on high-performance chips. Global tech companies such as Google and Amazon, along with Chinese giants like Huawei and Alibaba, are heavily investing in this field. As Moore's Law slows down, chip technology faces challenges. Advanced packaging techniques, like 2.5D packaging, help integrate multiple chips densely. Diamond, known for its exceptional thermal conductivity, is seen as an ideal future solution for chip cooling issues.

The deceleration of Moore's Law

Since the invention of semiconductors and chips, the main development direction has been the extension of Moore's Law. The continually shrinking transistor process allows for smaller chip sizes, increases the number of transistors a chip can hold, thus enhancing chip computing power, speed, and performance, while reducing power consumption and cost.

As semiconductor process technology (the equivalent width of gate or channel) advances into the nanometer scale, improvements in the process become increasingly challenging, primarily due to two factors. First, quantum tunneling effects (a type of short-channel effect) lead to transistor leakage and increased chip heating, which results in reduced chip performance and higher power consumption. Although some small-scale breakthroughs have been achieved in certain laboratories using new materials like silicon carbide, these developments have not yet reached a commercializable stage.

Another reason is that the development and manufacturing costs of advanced process chips remain high, while yields continue to decline. According to predictions by IBS and Gartner, the total design cost for 5nm technology can reach up to $500 million. Additionally, the prices of EUV lithography machines, masks, and other equipment continue to rise with technological advancements, increasing the cost of chip foundry services. Moreover, Korean media Chosunbiz reports that the yields for 3nm semiconductors from Samsung and TSMC struggle to exceed 60% (with TSMC's 3nm yield reportedly around 55%). Low yields significantly increase manufacturing costs and sales pressure for chips, leading Apple to negotiate lower prices for its A17 processor chip.

As Moore's Law slows down and chip feature sizes approach physical limits, advanced packaging technologies have become a crucial way to continue the progress of Moore's Law. Leading manufacturers aim to use packaging technologies to reduce costs while maintaining high performance, small size, and low power consumption of small process chips, thereby addressing the challenges of advancing process technology.

Advanced packaging refers to cutting-edge packaging methods and technologies that enhance the connection density and integration level of integrated circuits by optimizing connections and integrating different materials and line widths within the same package. Currently, packaging methods such as flip-chip (FC) structures, wafer-level packaging (WLP), system-in-package (SiP), and 2.5D/3D packaging are considered advanced packaging. Among these, 2.5D/3D packaging is experiencing the fastest growth within the advanced packaging subfields.

Diamond Heat Sinks Enhancing Thermal Management in Advanced Packaging (1)

What is 2.5D packaging?

From a historical development perspective, 2.5D packaging technology began in the 2010s and is an advanced heterogeneous chip packaging method that integrates multiple chips into a single package with high-density signal connections. Its main features include a three-layer structure:

1)Main Chips: Multiple chips are mounted using flip-chip technology with microbumps.

2)Interposer Layer: A dielectric layer with through-silicon vias (TSVs) that supports bumps or solder balls, connecting the upper and lower layers.

3)Mounting: The interposer layer is mounted onto the substrate.

The adoption of such a complex packaging structure has several reasons and highlights some of the advantages of 2.5D packaging. To meet the increasing demands for computational speed, the physical distance between memory and the main chip has been reduced. This reduction in distance results in decreased latency and improved signal quality, enabling higher speeds and reduced energy consumption. The development of silicon interposer technology to meet these requirements has ushered in the 2.5D packaging era.

Diamond Heat Sinks Enhancing Thermal Management in Advanced Packaging (2)

Diamond heat sink cooling solution

1. Currently, common semiconductor materials like Si, SiC, and GaN have relatively low thermal conductivities, generally not exceeding 500 W·m⁻¹K⁻¹, while the power density of high-power electronic devices can reach up to 1000 W·cm⁻². Additionally, the variation in power density across different functional regions can lead to uneven temperature distribution within the chip, with local hotspots potentially reaching 5 to 10 times the average power density of the chip.

2. Diamond heat sink materials, with a thermal conductivity of 2000 W·m⁻¹K⁻¹ at room temperature, excellent dielectric properties, and a low thermal expansion coefficient, represent the highest thermal conductivity materials found in nature. These materials are expected to effectively dissipate accumulated heat, achieving ideal cooling effects, and are widely regarded as one of the future solutions for enhancing the thermal management capabilities of semiconductor devices. Both single-crystal and polycrystalline diamonds have significantly higher thermal conductivities than other substrate materials, making them superior alternatives for heat sink substrates.

3. The effectiveness of heat dissipation is largely determined by the method used to connect the diamond to the semiconductor device. If the diamond can be directly connected to the semiconductor material, it can fully leverage its high thermal conductivity, which is why direct bonding processes have been a major focus of research. The main methods of direct bonding between diamond and semiconductor include:

1) Direct connection through deposition processes between the diamond and semiconductor.

2) Direct connection via low-temperature bonding.

Mature process in the semiconductor industry.

Depositing a layer of diamond film directly on the prepared semiconductor device or depositing a diamond passivation layer on the device front can enhance the upward heat dissipation capability of the device. However, the mismatch in thermal expansion can still cause cracking in the epitaxial layer. Additionally, when depositing a diamond heat dissipation layer using the CVD process, high temperatures (over 700°C) and a high-concentration hydrogen plasma environment are typically required, which can severely etch semiconductors like Si, SiC, and GaN, leading to significant degradation in their electrical properties.

To avoid the high temperatures and hydrogen plasma environment required for direct epitaxial growth, a widely researched approach involves first depositing semiconductor materials on a substrate using epitaxial growth processes, then removing the substrate and bonding the semiconductor material to a diamond substrate using low-temperature bonding techniques. Both polycrystalline and single-crystal diamonds can serve as heat sink substrates for low-temperature bonding, significantly reducing the difficulty of preparing diamond substrates. Moreover, the semiconductor epitaxial layer and the diamond heat sink substrate can be prepared independently before bonding, streamlining the process for diamond-based semiconductor devices.

While low-temperature bonding techniques circumvent the challenges of epitaxial growth, they require that the surfaces of the diamond heat sink substrate and semiconductor epitaxial layer be flat, with minimal warping and low surface roughness (less than 1 nm). This presents a significant challenge for current processing techniques. Additionally, controlling the pressure and hold time during direct bonding is difficult, which can lead to sample breakage during bonding and low yield rates. This is particularly problematic for large-sized samples, which remain in the experimental research phase and have only been successfully applied to small-scale chips at the millimeter scale, preventing large-scale application.

Although the ideal application of diamond heat sinks is direct connection with the chip, indirect connection packaging using metal between the chip and the substrate is a more mature process in the semiconductor industry. Common methods include soft soldering using solder (such as tin-lead or lead-free), transient liquid phase diffusion bonding using low-melting intermediate layers (such as gold-tin eutectic alloys), and low-temperature sintering with nano-silver.

Conventional nano-silver requires pressure-assisted sintering, and the sintering temperature exceeds 250°C. It can be successfully applied to the connection of SiC and GaN chips, which have high packaging and operating temperatures (typically greater than 250°C), but it is not suitable for large-area low-temperature connections of silicon chips. Low-temperature, pressure-free, and low-temperature, low-pressure sintering techniques for large-area nano-silver are both hot topics and challenges in nano-silver sintering processes and are considered key research directions for the future.