Compound Semiconductor Devices

Over the years, compound semiconductors have played a key role in driving innovative electronics in segments such as high frequency, high power, photonics, and sensor devices.

A major motivation for using compound semiconductors in the industry is their superior material properties compared to Si with respect to thermal, optical, and electrical properties. In addition, these properties can be tailored to specific device performance parameters by utilizing a large variety of different compound semiconductor materials or even varying compositions within specific material systems.

Advanced crystal growth methodologies such as MBE and MOCVD are applied to grow high-purity crystal layers and combine complex layer stacks of different compound semiconductor materials. The resulting heterostructures comprise quantum wells and superlattices, providing an unmatched opportunity for advanced bandgap engineering of innovative devices.

T-shape NiAu-structure

Prominent examples of high frequency devices are high electron mobility transistors (HEMTs) based on GaAs/AlGaAs, InP, or GaN material systems. HEMT devices are fabricated as discrete single devices as well as in microwave monolithic integrated circuits (MMICs), incorporating structures like T-gates or Γ-gates with minimum feature sizes below 100 nm. These gate structures are highly critical for achieving good high frequency device performance. Other compound semiconductor devices with similar minimum feature sizes and similar requirements for high-precision patterning are photonic integrated circuits (PICs) and distributed feedback (DFB) lasers. Both PICs and DFB lasers are key elements in state-of-the-art optical data communication.

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E-beam lithography in compound semiconductor fabrication

Typical techniques for sub 100 nm structuring in optical lithography require special highly complex processes, such as optical proximity correction (OPC) structures or multi patterning. These processes are established in the Si industry and mainly used for high-volume manufacturing on 12” Si substrates. On smaller substrates like 2” to 6” wafers, and in particular for compound semiconductor devices with sub-100 nm features, e-beam lithography (EBL) plays a key role in development and manufacturing.

Excellent high resolution and precise pattern placement capabilities of state-of-the-art EBL systems enable high wafer level yields to be achieved. EBL systems in combination with high-end laser stages meet system stability requirements for process uniformity and placement precision in large-area mix-and-match patterning processes. In addition, based on the nature of maskless lithography, EBL systems are ideal tools for rapid prototyping and extensive design of experiment exposures in device development.

SEM image of a 2 finger transistor
2 finger transistor

High frequency compound semiconductor devices

HEMTs are a prominent structure widely used in compound semiconductor MMICs and discrete high frequency devices. In typical HEMT structures such as GaAs/AlGaAs heterostructures, a two dimensional electron gas (2DEG) is formed at the interface between GaAs and the AlGaAs layer. Electrons in this 2DEG channel exhibit very high electron mobility, which can even be increased by applying special processing such as modulation doping for reduced electron scattering in the conductance band.

Based on the high electron mobility in the 2DEG channel of the heterostructure substrate, high-frequency field effect transistors can be fabricated using standard chip processing technologies. The most critical design feature for achieving good high-frequency performance well above 100 GHz is a low capacitance gate structure, often in the form of a T shape (T gate).

SEM image of a HEMT device
High Electron Mobility Transistor (HEMT) device with 75nm T-gate; IUHFSE, Moscow, Russia

With EBL structuring, low-capacitance T gates can be precisely built with small gate length lg, a critical parameter for high-bandwidth HEMTs. While larger structures such as source and drain metal contacts in MMICs or discrete HEMTs can be patterned with standard optical lithography, gate length pattern fidelity and uniformity across the wafer is crucial to guarantee a high wafer yield. This is where modern EBL systems play a key role, in particular when it comes to sub-100 nm minimum feature size chip manufacturing.

Our solutions for compound semiconductor fabrication

Raith offers a variety of electron beam solutions, from leading-edge high-throughput, high-precision EBL systems to versatile EBL / imaging tools for multipurpose applications, all incorporating proven high-stability technology. Our dedicated EBL systems are designed to provide precise large-area patterning at high throughput for volume production, advanced prototyping, and research.

Do you have any questions about how the Raith systems will fulfill your specific research or fabrication need? Get in touch with us now and we will happily evaluate your requirements.

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