This article appeared previously in Electronics Design.
The road to a billion qubits
Quantum computing is arguably one of the hottest topics in research right now. Quantum computers harness the power of quantum mechanical phenomena to solve certain problems faster than the best classical computers. If developed to a sufficient level of maturity, they could one day help compose the perfect medicine on a molecular level and could help solve other –currently intractable– problems in materials research, chemistry, numerical mathematics and cryptographic spaces. The keyword here is ‘could’. Before quantum computers become practical (that is, capable of solving currently intractable problems), major hurdles must be cleared. Notably, scaling up the number of qubits -the information units in these devices- remains a key barrier.
Qubits are notoriously unstable and prone to mistakes, and, therefore, require many millions of qubits working in unison to correct said mistakes and thus faithfully perform the transformational calculations quantum computers promise. To obtain a billion qubits, fabrication must be taken from lab to fab thereby addressing yield and variance issues. Advanced industrial CMOS process flows allow wafer-scale uniformity and high device yield, yet in a very different and highly optimized space: off-the-shelf transistor processes cannot be directly transferred to qubit structures due to the different designs and operation conditions in those devices, compared to quantum devices. Yet modifications of those processes, when properly matched to the requirements of qubits, could significantly address current qubit limitations.
Recent work of imec researchers now demonstrates the successful integration of semiconducting and superconducting qubits with a customized 300mm wafer fabrication line. Moreover, imec’s silicon (Si)-quantum-dot-spin-based qubits display record-low charge noise levels (average charge noise 0.61 μeV/√Hz), a critical parameter to maintain quantum coherence.
Si spin qubits manufactured with state-of-the-art 300mm integration flows.
Silicon spin qubits
Tackling charge noise and gate defects
Imec’s Si spin qubits are defined by semiconductor quantum dot structures that trap a single spin of an electron or hole. A major advantage of these qubits for fabrication in semiconductor fabs or industrial lines lies in their close compatibility with existing Si-based manufacturing processes. Though there are several demonstrations of fab-made Si spin qubits, the final qubit performance typically shows higher noise and therefore lower coherence and fidelity compared to qubits made in a lab environment or academic cleanroom. Coherence is the metric for the ability of qubits to be able to store quantum information for a long time, which is essential for performing quantum information processing tasks. It directly affects the fidelity of qubits – another quantum metric that defines the faithfulness with which a qubit can perform a certain operation.
Charge noise has been identified as one of the main culprits for reduced coherence. To keep the charge noise as low as possible -and thus achieve long coherence times- the quality of the Si/SiO2-based metal-oxide-semiconductor (MOS) gate stack turns out to be paramount. While this can be realized through lab-based techniques such as lift-off processes, which allow, in principle, for ‘gentle’ processing, industrial manufacturing techniques like subtractive etch in reactive ion plasmas and lithography-based patterning have shown to easily result in degradation of the Si/SiO2 interface quality, presumably due to the injection of charged defects during the plasma processing.
Through full gate stack optimization, including detailed analysis of the defectivity induced by different process steps, imec researchers demonstrate that the Si/SiO2 interface -even with industrial processing- could provide very low-noise environments for qubit operations. This approach also offers the added advantage of higher yield and lower variance, which are the main arguments used in industrial CMOS processing to move away from lift-off and towards subtractive etching-based patterning. The researchers found out that a medium-thick layer of high quality, thermally grown oxide combined with polysilicon gates (instead of commonly used metal gates) and specific cleaning and etch processes yielded the best results. The optimized gate stack also showed a reduction in density of so-called spurious dots, unintended quantum dots defined by localized defects that form in the vicinity of desired quantum dots. Spurious dots affect the scalability of the qubits because they interfere with coupling of qubits, for example, in two-qubit gates between neighboring qubits.
The resulting qubits display charge noise that is up to an order of magnitude lower compared to previous state-of-the-art fab-based Si quantum dot structures and achieve remarkably uniform quantum dot operation, with high fidelity. In collaboration with researchers in Australia, the imec-team demonstrated a record-high qubit control fidelity of 99.91% on the optimized devices – a result that was recently demonstrated at the 2024 Silicon Nanoelectronics Workshop attached to the annual VLSI conference This confirms the maturity of industrial fabrication techniques for qubit development.
Scanning electron microscope (SEM) image of the overlapping qubit structure.
Further optimization and characterization
In the next steps, the researchers will further characterize the qubits and optimize the process, as well as assess larger quantum bit arrays. One of the outstanding questions remains how reduced charge noise could affect other metrics. For example, while reducing charge noise by using a thicker SiO2 layer (from 8 to 20nm) helps, it also increases crosstalk between different gates at the quantum dot, which can limit the fidelity of two-qubit gates.
Further, the current process uses 300 mm industrial e-beam lithography, since that offers the flexibility to change the design at every iteration. However, the limited throughput of e-beam, and inherent limitations to the alignment accuracy of the gates are drawbacks. In contrast, optical lithography (the main workhorse of industrial CMOS technology) has been heavily optimized to reduce alignment errors. Therefore, moving from e-beam to optical lithography (in this case EUV lithography) should help in pushing the maturity of the processes.
Finally, upscaling and maturing single qubits is just the first step. Another challenge lies in integrating these qubits into functional arrays and letting them talk to each other. The current overlapping gate architecture is perfect to demonstrate single and two-qubit operations but will hit a limit going to tens or hundreds of qubits. Therefore, the team is developing architectures that ensure scaling up to much larger arrays of qubits. Since such architectures require modified processes and development of a ‘quantum back end of line’, compatibility with high-fidelity qubits will remain the primary focus in this effort.
Superconducting qubits
Bringing superconducting qubits from lab to fab
While Si spin qubits are very promising in the long run because of their advantage for upscaling with their smaller size, long coherence times and high integration density, as well as their close compatibility with CMOS manufacturing technologies, superconducting quantum circuits are arguably the most developed platform at the moment. The energy states of superconducting qubits are easy to control and made to interact with each other, and that combination has led to demonstrations of medium scale quantum processors coupling tens to hundreds of superconducting qubits together. These qubits show -in a lab environment- long coherence times (up to several 100µs) and high gate fidelities (99.995% for single qubits), two important benchmarks for quantum computing.
Most of the superconducting qubits are, however, still made in laboratories where techniques such as shadow evaporation and lift-off - ‘gentle’ processes that help produce extremely clean interfaces. This is particularly critical for the key element of a superconducting qubit: the Josephson junction (JJ). To maximize coherence time, the various interfaces contained in the structures that make up the junction and the rest of the qubit must be as clean as possible. Even a single unlucky atomic defect present at one of the interfaces can cause the qubit to lose coherence. Unfortunately, these fabrication techniques are not fab-compatible and are generally difficult to scale up, thereby challenging the production of larger numbers of qubits needed for quantum computing.
Earlier work from imec researchers already demonstrated the feasibility of creating the JJ using only CMOS-compatible materials and techniques in the lab. This so-called overlap JJ is created by overlapping a top electrode and a bottom electrode with a thin insulator layer in between. Now, the researchers successfully transferred this process to a full fab environment, manufacturing superconducting qubits on 300mm silicon wafers using exclusively optical lithography and reactive ion etching with an impressive 98.25% qubit yield.
The qubits display excellent coherent times in excess of 100µs, thereby establishing that the quality of the interfaces is sufficiently clean using fab-compatible processing steps. Additionally, the imec team observed very good stability or aging of the JJ, significantly better than in the lab. Shadow-evaporated JJs often show notable aging: increases in resistances of the junction over time that indicate how it will behave when placed in a qubit and cooled down. These room temperature resistances can drift from one day to another day by tens of percent, while the new overlap JJ is very reliable with only a few percent aging.
(a) Photograph of the 300 mm wafer and (b) photograph of one die with highlights of Josephson Junction and qubit subdies. (c) Tilted scanning electron microscopy (SEM) image of an overlap Josephson junction and (d) a cross-sectional transmission electron microscopy (TEM) image of the junction.
Improving process control and exploring 3D
The researchers established for the first time a fabrication process on 300 mm silicon wafers achieving high coherence and across-wafer yield. The next step is to improve process control. This will tackle, for example, the observed qubit-to-qubit and wafer-to-wafer variability. In addition, various surface treatment, cleaning techniques and new materials will be further explored, to keep improving the interface and with that qubit coherence time.
In the longer term, when moving beyond a few tens of qubits, 3D integration techniques will be paramount. First, to connect the qubits. Current solutions such as air bridges are not ideal; they introduce crosstalk and surface losses. Second, to ramp up the number of qubits, keeping in mind that superconducting qubits are very large (mm-size) compared to, for example, transistors. These cannot be significantly scaled down in 2D. Even though moving to fab processing opens up access to state-of-the-art 3D technology, 3D integration remains very complex. One of the critical hurdles is protecting the JJ during stacking. Any 3D processing that happens after fabrication of the JJ, cannot affect or destroy it. For example, one hurdle to consider is that the thermal budget is restricted to 200°C while established processes require significantly higher temperatures.
Conclusion
Imec shows the successful integration of semiconducting and superconducting qubits with full industrial 300 mm wafer fabrication, achieving reproducible and high-yield qubits. The results confirm the maturity of industrial fabrication techniques for qubit development.
This is a necessary leap to make systems based on hundreds and thousands of qubits work. Imec is further optimizing the process to scale up state-of-the-art performing qubits and the necessary technology to connect them.
Want to know more?
- Read about earlier updates of superconducting qubits.
- This article talks about the previous work to make semiconducting and superconducting qubits fab-compatible.
- Results on the high qubit control fidelity in collaboration with Diraq and UNSW were presented at the 2024 Silicon Nanoelectronics Workshop in June. N.D. Stuyck, F. Mengke, W. H. Lim, S.S. Ramirez, C. Escott, T. Botzem, T. Tanttu, H. Yang, A. Saraiva, A. Laucht, S. Kubicek, J. Jussot, S. Beyne, B. Raes, C. Godfrin, D. Wan, K.D. Greve, A. Dzurak. “Demonstration of 99.9% Single Qubit Control Fidelity of a Silicon Quantum Dot Spin Qubit Made in a 300 mm Foundry Process”, 2024 Silicon Nanoelectronics Workshop.
- Read the details of the work on Si spin qubits in the recent publication in Nature Publishing Group’s NPJ Quantum Information: A. Elsayed, M.M.K. Shehata, C. Godfrin et al. “Low charge noise quantum dots with industrial CMOS manufacturing”. npj Quantum Inf 10, 70 (2024). https://doi.org/10.1038/s41534-024-00864-3
- Read the details of the work on superconducting qubits in the recent publication in Nature: J. Van Damme, S. Massar, R. Acharya, et al. “Advanced CMOS manufacturing of superconducting qubits on 300 mm wafers.” Nature (2024). https://doi.org/10.1038/s41586-024-07941-9
