Thick Epi and Aluminum Films Drive Progress in Power Devices
Mike Rosa Ph.D.
Although advanced semiconductor technology developments tend to focus largely on 300mm logic and memory devices, significant efforts are also concentrated on developing new generations of power devices using 200mm production tools.
These developments are driven by urgent calls for the smaller form factors, faster switching frequencies and higher voltage capabilities needed to meet the evolving power requirements of a host of fast-growing applications. One example is the unceasing demand for increased functionality in mobile devices, where users have come to expect rich features such as streaming video and long battery life.
Improved power devices are also needed for enhanced power control capabilities in industrial, transportation and power generation applications, and to effectively manage and conserve power for the vast number of sensors and ICs that will underpin the Internet of Things (IoT).
Due to the growth in these application markets, two types of power semiconductors are gaining popularity: insulated gate bipolar transistors (IGBTs) and more recently, super-junction (SJ) MOSFETs. IGBT devices are more commonly used today, and although they have a slower switching speed, they offer higher voltage- and current-handling abilities than SJ MOSFETs. They are often used for electric power control and conditioning in higher-voltage applications like switching power supplies and motor control.
Alternatively, SJ MOSFETs offer faster switching speeds than standard planar MOSFET designs. They are most often used in applications where ever-smaller device sizes and increased switching speeds at higher voltages are paramount, such as in power adapters and battery chargers for consumer electronics equipment.
Applied Materials is developing and integrating a number of complementary technologies to increase the performance and reduce the physical size of these power devices. Two enabling technologies now in advanced stages of development and testing with multiple customers are thick epitaxial (epi) films and thick aluminum (Al) metallization.
Thick epi enables higher voltages and faster switching in smaller form factors. Today, epi films more than 20µm thick are regarded as thick epi, although developmental targets are for epi films ranging up to about ~150µm in thickness.
Meanwhile, thick Al metallization on both the front and backside of a wafer provides enhanced current-carrying and heat dissipation characteristics. Thick Al films are generally considered to be 5µm or thicker (frontside) and 10µm or thicker (backside), growing to ever-thicker layers in the future that are limited only by available deposition technologies.
This work is being performed on 200mm tool technology, which helps customers extend the lifecycle and maximize the value of that relatively inexpensive capital equipment.
In contrast to the well-like structures used to build conventional planar MOSFET cells, the body of a SJ MOSFET features cells with a columnar structure, where p-type columns extend vertically through a relatively thicker n-doped epitaxial region to the substrate. The thicker epi region enables higher breakdown voltages, while placing taller, thinner pillars closer together allows voltage- and current-handling capabilities to be maintained as die sizes shrink. Reduced on-resistance is achieved by reducing the overall thickness of the die.
At present, the predominant approach to building such a thick epi region for an SJ MOSFET is to do it a layer at a time with a sequence of growth-implant-anneal steps repeated for each layer.
Applied Materials and others are working both to refine that process, and also to advance an emerging alternative: an integrated etch-based approach that is faster, more flexible and yields more uniform devices with better performance (see Figure 1).
Figure 1. The two main methods used to produce the thick epitaxial films required by superjunction MOSFETs. (Courtesy of Yole Développement)
Conceptually, a thick epitaxial layer is grown and deep high-aspect-ratio trenches are etched into it via deep reactive ion etching (DRIE). Then the trenches are backfilled with highly doped void-free material. Although simple in concept there are many practical issues – key requirements are an increased pressure atmosphere to build the n layer, and precise sidewall profile control and a reduced pressure atmosphere for fast, void-free filling. Applied Materials has demonstrated aspect ratios for the etching of these trenches as high as 40:1, with critical dimensions of about one micron, in trenches 44µm deep.
Figure 2 gives a general overview of the thick epi fabrication results achieved to date using the Applied Centura platform.
Figure 2. High film-growth rates and low film non-uniformity numbers are critical to the growing thick epi needs of the power device market. The images above show process results that deliver growth rates in excess of 5.9µm/min for films over 100µm, with thickness non-uniformity and film resistivity non-uniformity of <1%.
Thick blanket layers of Al are desirable on the backside of power semiconductors for enhanced heat dissipation, and thick Al is also needed on the front side to build high current-carrying plugs to fill vias in high-density interconnects.
However, the deposition of thick Al films on silicon (Si) comes with three major materials challenges: thermal deformation of the Al film at grain boundaries; “whisker” defects from high film stress and oxygen contamination; and hillocks in the Al caused by hills of Si that form under the deposited Al layer (see Figure 3).
Figure 3.Three of the major materials challenges that come with depositing thick aluminum films on silicon. (a) shows thermal grooving, or deformation of the Al film at grain boundaries because of high temperature and stress levels; (b) shows a “whisker” defect due to high film stress and oxygen contamination; and (c) shows hillocks, which can form when Al is deposited on silicon and a eutectic composition forms that pulls silicon from the underlayer, such that a “hill” of Si forms under the Al.
Applied Materials has demonstrated techniques to deposit high-quality thick (>5um) Al films on power semiconductors with high uniformity, tight control of all relevant material parameters, and at a rate of >2µm/minute.
Figure 4. Shown above are (a) Sheet Resistance Rs and (b) Reflectivity measurements for 5µm thick Aluminum films deposited at 250°C using Applied Materials new thick Al process chamber. The Rs measurement in (a) demonstrates a nu. of ~1%, which can be directly correlated to thickness non-uniformity for thick Al films. The Reflectivity measurement shown in (b), while influenced by the amount of grooving and surface roughness is generally of concern based on subsequent lithography requirements. Each data point in shown (a) and (b) is based on average of 9pt measurement across a 200mm wafer with 3mm EE.
Developments such as these thick epi and aluminum films will help to bring about enhancements in the power semiconductors which are the prerequisites for today’s most exciting and fast-growing electronics applications.
As important, the ability to apply these innovations using cost-effective workhorse 200mm tools will maximize resources. It also will continue to extend the reach of familiar silicon technology into speed and power regimes where more exotic GaN- or SiC-based compound semiconductors once were thought to be needed.
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The author thanks Gary Dagastine for his editorial support.