Agnitron’s New Showerhead: Advancing MOCVD Technology
A novel showerhead featuring innovative gas injection accelerates MOCVD
growth while expanding the palette of epitaxial materials to include
BY AARON FINE, FIKADU ALEMA, WILL BRAND, VITALI SOUKHOVEEV, PAUL FABIANO AND ANDREI OSINSKY FROM AGNITRON
POWER ELECTRONICS is undergoing a transformation. Silicon is being replaced with the likes of GaN and SiC, members of a family of wide bandgap materials that are enabling devices to be smaller, more efficient and high-performing. But that’s just the beginning, with the focus shifting to the new frontier of ultra-wide bandgap materials, primarily AlN and Ga2O3.
Collectively, silicon’s superiors are grabbing an increasing share of the power device market, with total sales projected to to exceed $10 billion within just a few years. These high-performance alternatives are finding deployment in many sectors, including the military, electric vehicles, power supplies and photovoltaic inverters.
Playing a pivotal role within this dynamic landscape is our company, Agnitron Technology. We are developing and manufacturing an expanding portfolio of OEM semiconductor growth tools, including but not limited to MOCVD, CVD and HVPE technologies.
One of these three forms of deposition, MOCVD, is widely used for the manufacture of power electronics, as well as LEDs and lasers. This epitaxial growth technique, involving the deposition of substances onto a surface in a high-temperature chamber, ensures meticulous control of the film’s thickness, its composition, and its quality. These assets make MOCVD ideal for crafting high-performance layered structures in various devices.
It’s far from trivial to extend the use of MOCVD to the likes of AlN and Ga2O3, due to persistent hardware challenges. At Agnitron, we have recognised and started to address these specific challenges.
The Agnitron Agilis 100 features a Gen III Close Space Injection Showerhead, aiding the growth of an AlN film.
Gen III CIS
The biggest challenges in hardware design are associated with the showerheads, arguably the most crucial component for epitaxial semiconductor material growth. Traditional showerhead design focuses on facilitating multiple separate, consistent, uniform flows of temperature-regulated gases, by utilising various complex machining methods. Unfortunately, these designs result in expensive, fragile showerheads with a limited lifetime. Success has been held back by traditional manufacturing practices, which hamper efforts to realise faster growth rates, uniform film thickness and smooth surfaces – and the ability to grow films previously not possible with standard commercial MOCVD reactors.
Our team at Agnitron has broken through this impasse with an innovative, patent-pending showerhead technology that we incorporate into our vertical CIS Agilis reactors. Our triumph stems from advances in the initial design of the showerhead based on our previous Gen II CIS. When developing this particular reactor, which features a close injection showerhead (CIS), we focused on exploring a novel method for controlling the well-established hydride and alkyl delivery system. The immediate outcomes, soon discussed, showcase that this design provides precise control over thickness uniformity and surface roughness, as well as enhancing film growth rates and reducing the particle count on the wafer.
The new showerhead can be applied across different Agilis reactors, for the growth of materials such as GaN, AlN and AlGaN alloys, as well as for oxide-based materials, like Ga2O3 and AlGaO alloys. This innovative showerhead offers tremendous versatility, demonstrated by its ability to be heated to temperatures exceeding 150 °C that allow for the pre-heating of ammonia. This attribute also facilitates the growth of films utilising low vapor pressure precursors. An illustrative case is the growth of ternary and quaternary nitrides, including ScAlN, and YAlN, which have garnered a lot of interest recently for expanding the application possibilities of AlN.
The Agnitron Agilis 100 MOCVD
One of our most prominent tools, the Agnitron’s Agilis 100 MOCVD, is recognised for its compact footprint and outstanding adaptability, and is capable of the growth of an incredibly wide variety of materials. Designed for R&D with smaller substrates but capable of accommodating larger wafers up to 100 mm in diameter, this tool can be used to grow: Ga2O3, AlN, AlGaN, GaN, InGaN, III-V As/P alloys, MgZnO, ZnO, II-IV compounds; transition metal dichalcogenides, such as MoS2, MoSe2, WS2, WSe2; phosphorene, two-dimensional hexagonal BN, SiC, ScAlN, and YAlN.
Offering enhanced usability, our Agnitron Agilis 100 integrates optical reflectometry in the UV and blue with Imperium analytical control software to provide real-time measurements of growth rate and thickness. Additionally, the Agilis 100 offers UV light exposure during growth, a feature that ensures effective control of epitaxial film purity by reducing residual carbon incorporation and managing point defects. Moreover, our Agilis 100, along with all our MOCVD/CVD systems, facilitates seamless plug-and-play switching between sources and gas distribution (showerhead) configurations – this is realised within just a few hours of maintenance. Another notable feature of the Agilis 100 is the option to add precursor distribution nozzles. This enables process refinement and wafer carrier cleaning, enhancing the appeal of these tools across a diverse range of applications.
Figure 1. Ga2O3 film growth in an Agilis 100 reactor with the cutting-edge CIS showerhead. The growth rate (a) and uniformity (b) dependencies on chamber pressure for TEGa and TMGa. Growth rate and uniformity versus TMGa molar flow rate (c) for layers grown at substrate temperatures of 800 °C and 900 °C and constant growth pressure of 30 Torr, achieving around 20 µm/hr at 540 µmol/min with less than 1 percent, 1σ uniformity.
There are two interchangeable configurations associated with the Agilis 100: the remote injection showerhead (RIS), and the CIS. The primary distinction is the placement of the showerhead relative to the wafer carrier. This ensures adaptable film growth options for meeting various research and development requirements.
Our RIS Agilis 100 reactor, capable of temperatures of over 1600 °C, features a distinctive remote injection showerhead that incorporates an optimised hydride or oxygen injector and uniformly distributes around it metalorganic precursors, such as trimethylgallium and trimethylaluminium. Commonly used for Ga2O3 growth, this configuration encounters two primary challenges. Firstly, film growth is impeded, due to higher gas-phase reactions, resulting from the increased gap between the showerhead and the wafer carrier. Secondly, it is harder to realise uniform film thickness and doping, due to difficulties in maintaining flow laminarity across the wider showerhead-to-wafer carrier gap.
Despite these limitations, our RIS Agilis 100 MOCVD reactor offers exceptional thickness uniformity within a tight process window. This is accomplished by lowering the reactor pressure and introducing high total gas flows to suppress buoyancy, which threatens to lead to flow instabilities. However, while the RIS Agilis 100 enables uniformity below 2 percent across a 50 mm substrate, process efficiency falls, due to high gas and precursor consumption, coupled with the narrow process window.
As mentioned earlier, the challenges just discussed can be addressed by equipping the Agilis 100 with the new Gen III CIS showerhead. Note that this is available as an upgrade. We have conducted extensive testing of our Agnitron Agilis 100 reactor for Ga2O3 growth, with evaluation showcasing the enhanced performance that comes from introducing the new showerhead. This design overcomes the limitations associated with the RIS configuration.