Enabling reliable normally-off recessed gate MISHEMT fabrication for power electronics applications

What does GaN offer to the power electronics market?

Our communication-centric and data-driven world has created a massive demand for efficient, reliable, and cost-effective power supplies and amplifiers. Wide bandgap materials like gallium nitride (GaN) have significant benefits over traditional silicon-based power electronics. In terms of transistor properties, GaN offers higher breakdown voltage, faster switching speed, and lower on-resistance, delivering high voltage, high frequency device performance with improved efficiency and smaller device geometries. GaN devices also generate less heat, and can operate at higher temperatures, making them ideal to achieve high-performance in harsh environments.

Although GaN technology has been in use for many years in light emitting diodes¹ its implementation in the power electronics field was slower due to the higher substrate cost (relative to Si) and limited manufacturing capacity. However, the increasing demand from the power electronics industry have driven down costs and accelerated adoption. An example of this is the adoption of GaN in high volume consumer electronics solutions such as mobile device chargers due to the advantages it delivers of faster, smaller chargers compared to Si-based device iterations. It is predicted the GaN power device market will reach $2B by 2027² driven primarily by the consumer applications. In addition to the consumer electronics markets the expansion of the implementation of GaN power devices is predicted by a broader range of markets including renewable energy, data centres, electric vehicles, and infrastructure for 5G and 6G networks (Figure 1). To support the explosion in GaN applications there are a variety of device architectures available in the market each with their own processing and performance benefits and challenges. We present the recent developments by Oxford Instruments that overcome a key technical hurdle – low damage controlled etching with sub-nm accuracy and endpoint control to enable the partially-recessed GaN MISHEMT device structure.

Fig. 1 — Applications for GaN HEMTs in power electronics and RF devices, including 5G base stations, fast chargers for EV, efficient power suppliers for data centres, and fast mobile device charging.

GaN HEMT device solutions – challenges and benefits

The advantageous material properties of GaN translate into transistor device benefits including high breakdown field owing to its large band gap and high electron mobility and saturation velocity³ (Table 1) which is a result of the 2-dimensional electron gas (2DEG) formed in the GaN/AlGaN heterostructure (GaN HEMT). These fundamental properties result in GaN-based devices with lower switching losses and enable energy efficient devices compared to other semiconductor materials.

However, despite the superior material qualities of GaN, the presence of the 2DEG results in GaN HEMTs being normally-on, depletion-mode devices (D-mode), which is highly undesirable. For fail-safe operations it is crucial that these devices are normally-off, and a positive threshold voltage is achieved. Currently multiple device architectures have been proposed to overcome the normally-on operation, of which cascode and p-GaN HEMT (Figure 2(a)) devices have been the most widely used. However, there are limitations to both device types; cascode devices have been reported to add undesirable interconnect parasitic inductances and p-GaN devices have been reported to suffer from high gate leakage current.^4,5

Another important device type is a recessed MISHEMT (Figure 2(b)), which is beneficial, as it can achieve positive threshold voltages, reach higher breakdown voltages, possess high gate voltage swing, and enables simple gate driving circuitry to be used.⁴ A recessed MISHEMT is formed by thinning the AlGaN barrier in the gate region to achieve a normally-off enhancement-mode device (E-mode). A gate dielectric (such as Al₂O₃ by atomic layer deposition) is then deposited to reduce leakage currents. Realisation of stable and reproducible recessed MISHEMTs requires a precise controllable and repeatable low damaging etch process with minimal interfacial traps and a high-quality dielectric with minimal defects.

Table 1 — Comparison of material properties of Si, GaAs, SiC, and GaN³.Introducing Atomic Layer Etch & Etchpoint®

Atomic layer etch (ALE) is cyclical etch process that allows for very fine control of the etched thickness, typically with an etch rate on the order of Å/cycle (Figure 3). This allows for excellent depth control which is beneficial to applications such as a recessed gate MISHEMT where control of both the remaining AlGaN thickness and limiting the GaN/AlGaN surface damage during processing are equally important. Anisotropic plasma-based ALE is typically used for the etching of the AlGaN gate recess utilising process gases such as Cl₂ plasma and Ar+ ions. Cl radicals populate the surface of the AlGaN and then Ar+ ions remove this chlorinated layer. The self-limiting nature of ALE is achieved by chlorination of the top surface; the subsurface is slow to chlorinate. The ion energy chosen for Ar is sufficient for removal of the top chlorinated layer, but below the sputter threshold for the bulk material.⁶

In combination with the fine process control of ALE, a clear need was identified for an end-pointing solution that is capable of delivering the accuracy required in the remaining post-etch thickness of AlGaN for a partially-recessed MISHEMT. Etchpoint® is a patent-pending UV reflectance-based endpoint technique with the optimised wavelength selected to allow for unrivalled accuracy of etch layer depth for GaN and AlGaN. Other endpoint solutions can achieve ±1 nm accuracy in the remaining thickness after layer etching which limits the capability to reliably fabricate some GaN HEMT device structures which require a recess in the gate region with thickness accuracy of the AlGaN of ±0.5 nm. This new etch-depth monitoring solution has been exclusively developed and optimised by Oxford Instruments in collaboration with LayTec and is fully integrated with both the hardware and software of the PlasmaPro® 100 ALE system (Figure 7). The capability of Etchpoint has been successfully validated through repeat measurements of the remaining AlGaN thickness with a target of 5 nm with all samples confirmed by transmission electron microscopy (TEM) as being within the specification of 5 nm ±0.5 nm (Figure 4).

To further demonstrate the capability of ALE and Etchpoint partially-recessed MISHEMTs were fabricated with our collaboration partners at Industrial Technology Research Institute in Taiwan (ITRI).

Fig. 2 — (a) pGaN HEMT, (b) partially recessed MISHEMT.

Fig. 3 — Typical ALE cycle for anisotropic etching of AlGaN using Cl₂/Ar process chemistry.

Fig. 4 — Remaining AlGaN thickness of 5 nm ±0.5 nm verified by TEM for 3 samples.

How Atomic Layer Etch and Etchpoint enable successful fabrication of partially-recessed normally-off MISHEMT!

When etching <30 nm of an AlGaN layer to a critical depth specification to form a partially-recessed gate, ALE is the ideal solution because it is a low etch rate, controlled and low damage process compared to conventional ICP etching. The final critical capability required is to stop the AlGaN etch process on precisely at the desired AlGaN etch depth even with variation in thickness of the incoming AlGaN layer. Using Etchpoint we have developed the capability to monitor the etch depth and stop the process to within ±0.5 nm of the depth specification to ensure the target threshold voltage (V_th) can be achieved to enable normally-off behaviour. Figure 5 demonstrates our endpoint capability to etch precisely 4 nm remaining AlGaN barrier to control positive V_th and achieve normally-off operation. A 17 nm layer of Al₂O₃ was deposited by plasma atomic layer deposition (ALD) as the gate dielectric. We have achieved remarkable MISHEMT performance indicated by +0.7 V of V_th (Figure 5), 1.2 mΩ.cm² of specific on resistance, 600 mA/mm of drive current and 1190 V of breakdown voltage (Figure 6).

Fig. 5 — Normally-off MISHEMT performance indicated by +0.7 V of V_th with 4 nm ±0.5 nm remaining AlGaN.

Fig. 6 — High breakdown voltage of 1190 V achieved for partially-recessed MISHEMT.

The combined solution of ALE and Etchpoint provide key processing and performance benefits that enable fabrication of device geometries previously deemed too challenging! As GaN HEMTs move further into emerging applications such consumer electronics, audio amplifiers, EV, defence, satellite communications, and 5G mobile networks the necessity of atomic scale process control and monitoring in combination with the trusted performance of our production-proven solutions uniquely positions Oxford Instruments to serve the GaN ecosystem.

References

1. The birth of the blue LED | Nature Photonics, Yasushi Nanishi, Nature Photonics volume 8, pages 884–886 (2014).
2. Yole Intelligence "2021 Status of Power Electronics Market" and "Power GaN 2022".
3. Meneghini et al, "GaN-based power devices: Physics, reliability, and perspective", Journal of Applied Physics 130, 181101 (2021).
4. Zhang et al., "Hybrid Gate p-GaN Power HEMTs Technology for Enhanced V_th Stability", 2022 International Electron Devices Meeting IEDM, pp. 35.4.1–35.4.4 (2022).
5. Xia et al, "Investigation of high threshold voltage E-mode AlGaN/GaN MIS-HEMT with triple barrier layer", Results in Physics, Volume 25, 104189 (2021).
6. Atomic Layer Deposition and Atomic Layer Etching for GaN Power Electronics. White Paper. Oxford Instruments Plasma Technology June 2019.