Gallium nitride is starting to make broader inroads in the lower-end of the high-voltage, wide-bandgap power FET market, where silicon carbide has been the technology of choice. This shift is driven by lower costs and processes that are more compatible with bulk silicon.

Efficiency, power density (size), and cost are the three major concerns in power electronics, and GaN can meet all three criteria. However, to satisfy all of those criteria consistently, the semiconductor ecosystem needs to develop best practices for test, inspection, and metrology, determining what works best for which applications and under varying conditions.

Power ICs play an essential role in stepping up and down voltage levels from one power source to another. GaN is used extensively today in smart phone and laptop adapters, but market opportunities are beginning to widen for this technology. GaN likely will play a significant role in both data centers and automotive applications [1]. Data centers are expanding rapidly due to the focus on AI and a build-out at the edge. And automotive is keen to use GaN power ICs for inverter modules because they will be cheaper than SiC, as well as for onboard battery chargers (OBCs) and various DC-DC conversions from the battery to different applications in the vehicle.

Fig. 1: Current and future fields of interest for GaN and SiC power devices. Source A. Meixner/Semiconductor Engineering

But to enter new markets, GaN device manufactures need to more quickly ramp up new processes and their associated products. Because GaN for power transistors is a developing process technology, measurement data is critical to qualify both the manufacturing process and the reliability of the new semiconductor technology and resulting product.

Much of GaN’s success will depend on metrology and inspection solutions that offer high throughput, as well as non-destructive testing methods such as optical and X-ray. Electron microscopy is useful for drilling down into key device parameters and defect mechanisms. And electrical tests provide complementary data that assists with product/process validation, reliability and qualification, system-level validation, as well as being used for production screening.

Silicon carbide (SiC) remains the material of choice for very high-voltage applications. It offers better performance and higher efficiency than silicon. But SiC is expensive. It requires different equipment than silicon, it’s difficult to grow SiC ingots, and today there is limited wafer capacity.

In contrast, GaN offers some of the same desirable characteristics as SiC and can operate at even higher switching speeds. GaN wafer production is cheaper because it can be created on a silicon substrate utilizing typical silicon processing equipment other than the GaN epitaxial deposition tool. That enables a fab/foundry with a silicon CMOS process to ramp a GaN process with an engineering team experienced in GaN.

The cost comparison isn’t entirely apples-to-apples, of course. The highest-voltage GaN on the market today uses silicon on sapphire (SoS) or other engineered substrates, which are more expensive. But below those voltages, GaN typically has a cost advantage, and that has sparked renewed interest in this technology.

“GaN-based products increase the performance envelopes relative to the incumbent and mature silicon-based technologies,” said Vineet Pancholi, senior director of test technology at Amkor. “Switching speeds with GaN enable the application in ways never possible with silicon. But as the GaN production volumes ramp, these products have extreme economic pressures. The production test list includes static attributes. However, the transient and dynamic attributes are the primary benefit of GaN in the end application.”

Others agree. “The world needs cheaper material, and GaN is easy to build,” said Frank Heidemann, vice president and technology leader of SET at NI/Emerson Test & Measurement. “Gallium nitride has a huge success in the lower voltages ranges — anything up to 500V. This is where the GaN process is very well under control. The problem now is building in higher voltages is a challenge. In the near future there will be products at even higher voltage levels.”

Those higher-voltage applications require new process recipes, new power IC designs, and subsequently product/process validation and qualification.

GaN HEMT properties
Improving the processes needed to create GaN high-electron-mobility transistors (HEMTs) requires a deep understanding of the material properties and the manufacturing consequences of layering these materials.

The underlying physics and structure of wide-bandgap devices significantly differs from silicon high-voltage transistors. Silicon transistors rely on doping of p and n materials. When voltage is applied at the gate, it creates a channel for current to flow from source to drain. In contrast, wide-bandgap transistors are built by layering thin films of different materials, which differ in their bandgap energy. [2] Applying a voltage to the gate enables an electron exchange between the two materials, driving those electrons along the channel between source and drain.

Fig. 2. Cross-sectional animation of e-mode GaN HEMT device. Source: Zeiss Microscopy

“GaN devices rely on two-dimensional electron gas (2DEG) created at the GaN and AlGaN interface to conduct current at high speed,” said Jiangtao Hu, senior director of product marketing at Onto Innovation. “To enable high electron mobility, the epitaxy process creating complex multi-layer crystalline films must be carefully monitored and controlled, ensuring critical film properties such as thickness, composition, and interface roughness are within a tight spec. The ongoing trend of expanding wafer sizes further requires the measurement to be on-product and non-destructive for uniformity control.”

Fig. 3: SEM cross-section of enhancement-mode GaN HEMT built on silicon which requires a superlattice. Source: Zeiss Microscopy

Furthermore, each layer’s electrical properties need to be understood. “It is of utmost importance to determine, as early as possible in the manufacturing process, the electrical characteristics of the structures, the sheet resistance of the 2DEG, the carrier concentration, and the mobility of carriers in the channel, preferably at the wafer level in a non-destructive assessment,” said Christophe Maleville, CTO and senior executive vice president of innovation at Soitec.

Developing process recipes for GaN HEMT devices at higher operating ranges require measurements taken during wafer manufacturing and device testing, both for qualification of a process/product and production manufacturing. Inspection, metrology, and electrical tests focus on process anomalies and defects, which impact the device performance.

“Crystal defects such as dislocations and stacking faults, which can form during deposition and subsequently be grown over and buried, can create long-term reliability concerns even if the devices pass initial testing,” said David Taraci, business development manager of electronics strategic accounts at ZEISS Research Microscopy Solutions. “Gate oxides can pinch off during deposition, creating voids which may not manifest as an issue immediately.”

The quality of the buffer layer is critical because it affects the breakdown voltage. “The maximum breakdown voltage of the devices will be ultimately limited by the breakdown of the buffer layer grown in between the Si substrate and the GaN channel,” said Soitec’s Maleville. “An electrical assessment (IV at high voltage) requires destructive measurements as well as device isolation. This is performed on a sample basis only.”

One way to raise the voltage limit of a GaN device is to add a ‘gate driver’ which keeps it reliable at higher voltages. But to further expand GaN technology’s performance envelope to higher voltage operation engineers need to comprehend a new GaN device reliability properties.

“We are supporting GaN lifetime validation, which is the prediction of a mission characteristic of lifetime for gallium nitride power devices,” said Emerson’s Heidemann. “Engineers build physics-based failure models of these devices. Next, they investigate the acceleration factors. How can we really make tests and verification properly so that we can assess lifetime health?”

The qualification procedures necessitate life-stressing testing, which duplicates predicated mission profile usage, as well as electrical testing, after each life-stress period. That allows engineers to determine shifts in transistor characteristics and outright failures. For example, life stress periods could start with 4,000 hours and increase in 1,000-hour increments to 12,000 hours, during which time the device is turned on/off with specific durations of ‘on’ times.

“Reliability predictions are based upon application mission profiles,” said Stephanie Watts Butler, independent consultant and vice president of industry and standards in the IEEE Power Electronics Society. “In some cases, GaN is going into a new application, or being used differently than silicon, and the mission profile needs to be elucidated. This is one area that the industry is focused upon together.”

As an example of this effort, Butler pointed to JEDEC JEP186 spec [3], which provides guidelines for specifying the breakdown voltage for GaN HEMT devices. “JEDEC and IEC both are issuing guideline documents for methods for test and characterization of wide-bandgap devices, as well as reliability and qualification procedures, and datasheet parameters to enable wide bandgap devices, including GaN, to ramp faster with higher quality in the marketplace,” she said.

Electrical tests remain essential to screening for both time-zero and reliability-associated defects (e.g. infant mortality and reduced lifetime). This holds true for screening wafers, singulated die, and packaged devices. And test content includes tests specific to GaN HEMT power devices performance specifications and tests more directed at defect detection.

Due to inherent device differences, the GaN test list varies in some significant ways from Si and SiC power ICs. Assessing GaN health for qualification and manufacturing purposes requires both static and dynamic tests (SiC DC and AC). A partial list includes zero gate voltage drain leakage current, rise time, fall time, dynamic RDS_on, and dielectric integrity tests.

“These are very time-intensive measurement techniques for GaN devices,” said Tom Tran, product manager for power discrete test products at Teradyne. “On top of the static measurement techniques is the concern about trapped charge — both for functionality and efficiency — revealed through dynamic RDS_on testing.”

Transient tests are necessary for qualification and production purposes due to the high electron mobility, which is what gives GaN HEMT its high switching speed. “From a test standpoint, static test failures indicate basic processing failures, while transient switching failures indicate marginal or process excursions,” said Amkor’s Vineet Pancholi. “Both tests continue to be important to our customers until process maturity is achieved. With the extended range of voltage, current, and switching operations, mainstream test equipment suppliers have been adding complementary instrumentation capabilities.”

And ATE suppliers look to reduce test time, which reduces cost. “Both static and dynamic test requirements drive very high test times,” said Teradyne’s Tran. “But the GaN of today is very different than GaN from a decade ago. We’re able to accelerate this testing just due to the core nature of our ATE architecture. We think there is the possibility further reducing the cost of test for our customers.”

Tools for process control and quality management
GaN HEMT devices’ reliance on thin-film processes highlights the need to understand the material properties and the nature of the interfaces between each layer. That requires tools for process control, yield management, and failure analysis.

“GaN device performance is highly reflective of the film characteristics used in its manufacture,” said Mike McIntyre, director of software product management at Onto Innovation. “The smallest process variations when it comes to film thickness, film stress, line width or even crystalline make-up, can have a dramatic impact on how the device performs, or even if it is usable in its target market. This lack of tolerance to any variation places a greater burden on engineers to understand the factors that correlate to device performance and its profitability.”

Inspection methods that are non-destructive vary in throughput time and in the level of detail provided for engineers to make decisions. While optical methods are fast and provide full wafer coverage, they cannot accurately classify chemical or structural defects for engineers/technicians to review. In contrast, destructive methods provide the information that’s needed to truly understand the nature of the defects. For example, conductive atomic force microscopy (AFM) probing remains slow, but it can identify electrical nature of a defect. And to truly comprehend crystallographic defects and the chemical nature of impurities, engineers can turn to electron microscopy based methods.

One way to assess thin films is with X-rays. “High resolution X-ray measurements are useful to provide production control of the wafer crystalline quality and defects in the buffer, said Soitec’s Maleville. “Minor changes in composition of the buffer, barrier, or capping layer, as well as their layer thickness, can result in significant deviations in device performance. Thickness of the layers, in particular the top cap, barrier, and spacer layers, are typically measured by XRD. However, the throughput of XRD systems is low. Alternatively, ellipsometry offers a reasonably good throughput measurement with more data points for both development and production mode scenarios.”

Optical techniques have been the standard for thin film assessment in the semiconductor industry. Inspection equipment providers have long been on the continuation improvement always evolving journey to improve accuracy, precision and throughput. Providing better metrology tools helps device makers with process control and yield management.

“Recently, we successfully developed a non-destructive on product measurement capability for GaN epi process monitoring,” said Onto’s Hu. “It takes advantage of our advanced optical film experience and our modeling software to simultaneously measure multi-layer epi film thickness, composition, and interface roughness on product wafers.”

Fig. 4: Metrology measurements on GaN for roughness and for Al concentration. Source: Onto Innovation

Assessing the electrical characteristics — 2DEG sheet resistance, channel carrier mobility, and concentration are required for controlling the manufacturing process. A non-destructive assessment would be an improvement over currently used destructive techniques (e.g. SEM). The solutions used for other power ICs do not work for GaN HEMT. As of today, no one has come up with a commercial solution.

Inspection looks for yield impacting defects, as well as defects that affect wafer acceptance in the case of companies that provide engineered substrates.

“Defect inspection for incoming silicon wafers looks for particles, scratches, and other anomalies that might seed imperfections in the subsequent buffer and crystal growth,” said Antonio Mani, business development manager at Thermo Fisher Scientific. “After the growth of the buffer and termination layers, followed by the growth of the doped GaN layers, another set of inspections is carried out. In this case, it is more focused on the detection of cracks, other macroscopic defects (micropipes, carrots), and looking for micro-pits, which are associated to threading dislocations that have survived the buffer layer and are surfacing at the top GaN surface.”

Mani noted that follow-up inspection methods for Si and GaN devices are similar. The difference is the importance in connecting observations back to post-epi results.

More accurate defect libraries would shorten inspection time. “The lack of standardization of surface defect analysis impedes progress,” said Soitec’s Maleville. “Different tools are available on the market, while defect libraries are still being developed essentially by the different user. This lack of globally accepted method and standard defect library for surface defect analysis is slowing down the GaN surface qualification process.”

Whether it involves a manufacturing test failure or a field return, the necessary steps for determining root cause on a problematic packaged part begins with fault isolation. “Given the direct nature of the bandgap of GaN and its operating window in terms of voltage/frequency/power density, classical methods of fault isolation (e.g. optical emission spectroscopy) are forced to focus on different wavelengths and different ranges of excitation of the typical electrical defects,” said Thermo Fisher’s Mani. “Hot carrier pairs are just one example, which highlights the radical difference between GaN and silicon devices.”

In addition to fault isolation there are challenges in creating a device cross-section with focused-ion beam milling methods.

“Several challenges exist in FA for GaN power ICs,” said Zeiss’ Taraci. “In any completed device, in particular, there are numerous materials and layers present for stress mitigation/relaxation and thermal management, depending on whether we are talking enhancement- or depletion-mode devices. Length-scale can be difficult to manage as you are working with these samples, because they have structures of varying dimension present in close proximity. Many of the structures are quite unique to power GaN and can pose challenges themselves in cross-section and analyses. Beam-milling approaches have to be tailored to prevent heavy re-deposition and masking, and are dependent on material, lattice orientation, current, geometry, etc.”

Conclusion
To be successful in bringing new GaN power ICs to new application space engineers and their equipment suppliers need faster process development and a reduction in overall costs. For HEMT devices, it’s understanding the resulting layers and their material properties. This requires a host of metrology, inspection, test, and failure analysis steps to comprehend the issues, and to provide feedback data from experiments and qualifications for process and design improvements.

References

[1] M. Buffolo et al., “Review and Outlook on GaN and SiC Power Devices: Industrial State-of-the-Art, Applications, and Perspectives,” in IEEE Transactions on Electron Devices, March 2024, open access, https://ieeexplore.ieee.org/document/10388225

[2] High electron mobility transistor (HEMT) https://en.wikipedia.org/wiki/High-electron-mobility_transistor

[3] Guideline to specify a transient off-state withstand voltage robustness indicated in datasheets for lateral GaN power conversion devices, JEP186, version 1.0, December 2021. https://www.jedec.org/standards-documents/docs/jep186

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Successive versions of vertical transistors are emerging as the likely successor to finFETs, combining lower leakage with significant area reduction.

A stacked nanosheet transistor, introduced at N3, uses multiple channel layers to maintain the overall channel length and necessary drive current while continuing to reduce the standard cell footprint. The follow-on technology, the CFET, pushes further up the z axis, stacking n-channel and p-channel transistors on top of each other, rather than side by side.

In work presented at December’s IEEE Electron Device Meeting, researchers at TSMC estimated that CFETs give a 1.5X to 2X overall size reduction at constant gate dimensions. [1] Those are significant area benefits for any digital logic, but manufacturing these new transistor structures will be a challenge.

Monolithic 3D integration is the simplest integration scheme, and the one likely to see production first. In monolithic 3D integration, the entire structure is assembled on a single piece of silicon. This approach can also be used to fabricate compute-in-memory designs where memory devices are fabricated as part of the metallization layers for a conventional CMOS circuit. While individual layers in monolithic 3D designs can incorporate new technologies — the integration of ReRAM devices, for example — the overall CMOS flow is preserved. All of the materials and processes used must be compatible with that rubric.

Adding more nanosheets for complementary devices
The overall process in this kind of scheme is similar to a stacked nanosheet transistor flow. It starts with a stack of eight or more alternating silicon and silicon germanium layers (four pairs), compared to a stacked nanosheet NFET or PFET, which might have only four such layers (two pairs). In a CFET flow, however, middle dielectric layer is inserted halfway through the stack.

This layer, separating the n-type and p-type transistors, is probably the most important difference from a standard nanosheet transistor flow. To minimize parasitic capacitance, the middle dielectric layer should be as thin as possible, said imec’s Naoto Horiguchi. If it’s too thin, though, edge placement errors can cause isolation failures, landing contacts for the top devices onto bottom devices. [2]

In TSMC’s process, the Si/SiGe superlattice includes a high-germanium SiGe layer as a placeholder for the middle dielectric. After the source/drain etch, a highly selective etch removes this layer and oxidizes the silicon on either side of it to form the middle dielectric.

The inner spacer recess etch, which follows middle dielectric formation in the TSMC process, indents the SiGe layers relative to the silicon nanosheets, defining the gate length and junction overlap.

While TSMC emphasized it has not yet made fully metallized integrated CFET circuits, it did report that more than 90% of the transistors survived.

Fig. 1: TSMC used monolithic integration to stack NFET and PFET devices. [1]

Depositing the nanosheet stack is straightforward. Etching it with the precision required is not. A less-than-vertical etch profile will change the relative channel lengths of the top and bottom devices, leading to asymmetric switching characteristics.

Stacking wafers for more flexibility
The alternative, sequential 3D integration is a bit more flexible. While monolithic 3D integration uses a single device layer, sequential 3D integration bonds an additional tier on top of the first. Sequential 3D integration is different from three-dimensional wafer-level packaging and chip stacking, though. In WLP, the component devices are finished, passivated, and tested. The component chips are fully functional as independent circuits. In sequential 3D integration, the two tiers are part of a single integrated circuit.

Often, though not always, the second tier is an unprocessed bare wafer with no devices at all. Ionut Radu, director of research and external collaborations at Soitec, said his company used its SmartCut process to transfer sub-micron silicon layers. [3] One of the advantages of sequential integration, though, is that it opens the door to other possibilities. For example, the second layer could use a different silicon lattice orientation to facilitate stress engineering for improved carrier mobility. It also could use an alternative channel material, such as GaAs or a two-dimensional semiconductor. And up until the transfer occurs, processing of the second wafer has no effect on the thermal budget of the first.

After bonding, the second tier’s process temperature generally must remain below 500° C. Tadeu Mota-Frutuoso, process integration engineer at CEA-Leti, said researchers were able to achieve this benchmark in a conventional CMOS process by using laser annealing for the source/drain activation steps. [4]

While sequential 3D integration can be used to realize CFET devices, the top layers also can contain independent circuitry. Still, as in monolithic integration, the dielectric layer between the two circuit tiers is a critical process step. Analysts at KAIST found that reducing the thickness of the interlayer dielectric improves heat dissipation. It also facilitates the use of a bottom gate to control the top tier devices. On the other hand, the dielectric layer lies at the interface between the original wafer and the transferred layer. Thickness control depends on the polishing step used to prepare the transfer surface. Such precise control is extremely challenging for CMP. [5]

Re-driving wafers without contamination
While the second circuit tier can be added at any point in the process flow, the insertion point constrains not only the first and second tier devices, but also the fab as a whole. When the second layer does not yet contain devices, alignment to the first layer is relatively easy. In contrast, Horiguchi said, aligning one device wafer on top of another imposes an area penalty to accommodate potential overlay error. The total device thickness of sequential 3D structures tends to be greater, as well.

Returning a first-tier wafer with contacts and other metallization to FEOL tools for fabrication of a second transistor layer poses a substantial cross contamination risk. Even if the top surface is well encapsulated, Mota-Frutuoso explained in an interview that the sidewalls and bevels of the bottom tier can still expose metal layers to FEOL processes. CEA-Leti’s proposed bevel contamination wrap (BCW) scheme first cleans the wafer edge, then encapsulates it and the sidewall in a protective oxide layer.

Fig. 2: CEA-Leti’s sequential 3D integration stacked silicon CMOS on an industrial 28nm FDSOI wafer. [4]

Controlling heat dissipation
Heat dissipation is a major challenge for both monolithic and sequential 3D devices. Generalizations are difficult because thermal characteristics depend on the specific integration scheme and even the circuit design. Wei-Yen Woon, senior manager at TSMC, and his colleagues evaluated AlN and diamond as possible thermal dissipation layers. While both have been used in power devices, they are new to CMOS process flows. They achieved good quality columnar AlN films with a low temperature sputtering process, though the columnar structure did impede in-plane heat transport. While diamond offers extremely high thermal conductivity, it also can require extremely high process temperatures. The TSMC group deposited thin films with acceptable quality at BEOL compatible temperatures by using pre-deposited diamond nuclei, but they have not yet attempted to integrate these films with working devices.^[6]

What’s next?
In the short term, monolithic 3D integration offers a relatively straightforward path to CFET fabrication, building on existing nanosheet transistor process flows. Even proponents of sequential 3D integration expect the monolithic approach to reach production first. For the longer term, though, the ability to use a completely different material for the second device layer gives device designers many more process optimization knobs.

However it is achieved, the idea that active devices no longer need to confine themselves to a single planar layer has implications far beyond logic transistors. From compute-in-memory modules to image sensors, 3D integration is an important tool for “More than Moore” devices.

References

^[1] S. Liao et al., “Complementary Field-Effect Transistor (CFET) Demonstration at 48nm Gate Pitch for Future Logic Technology Scaling,” 2023 International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2023, pp. 1-4, doi: 10.1109/IEDM45741.2023.10413672.

^[2] N. Horiguchi et al., “3D Stacked Devices and MOL Innovations for Post-Nanosheet CMOS Scaling,” 2023 International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2023, pp. 1-4, doi: 10.1109/IEDM45741.2023.10413701.

^[3] I. Radu et al., “Ultimate Layer Stacking Technology for High Density Sequential 3D Integration,” 2023 International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2023, pp. 1-4, doi: 10.1109/IEDM45741.2023.10413807.

^[4] T. Mota-Frutuoso et al., “3D sequential integration with Si CMOS stacked on 28nm industrial FDSOI with Cu-ULK iBEOL featuring RO and HDR pixel,” 2023 International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2023, pp. 1-4, doi: 10.1109/IEDM45741.2023.10413864.

^[5] S. K. Kim et al., “Role of Inter-Layer Dielectric on the Electrical and Heat Dissipation Characteristics in the Heterogeneous 3D Sequential CFETs with Ge p-FETs on Si n-FETs,” 2023 International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2023, pp. 1-4, doi: 10.1109/IEDM45741.2023.10413845.

^[6] W. Y. Woon et al., “Thermal dissipation in stacked devices,” 2023 International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2023, pp. 1-4, doi: 10.1109/IEDM45741.2023.10413721.

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