Piezoelectric control technology aligns many optical junctions in parallel

Silicon photonics has evolved over the past 30 years from the comparatively simple interconnects of the late 1980s to the more advanced devices available today, with photonic logic and quantum computing now both on the (surprisingly close) horizon. The advantages are clear—the ability to carry more data in less time while using less energy than electrical conductors is invaluable to providers trying to meet demand for instant and unlimited data from ever-smaller devices. In addition, moving from conventional electrical conductors to photonic interconnects will pave the way for photonic-based computing, quantum communications, and quantum computing.

However, the manufacture of silicon photonic devices is expensive, with the need for accurate nanoscale alignment during probing (prior to packaging), testing (during packaging), and the packaging process itself eclipsing the cost of wafers. Similar challenges are emerging in manufacturing fields as diverse as smartphone cameras and industrial lasers, where many elements must be aligned in multiple degrees of freedom while keeping costs under control. As a result, fast and accurate alignment is essential to ensure product quality and profitability.

The industry has begun to move away from conveying data via electrical conductors to using photonic interconnects, to the point where photonic devices are increasingly incorporated onto chips alongside conventional microelectronics. Broadly speaking, the initial design and materials challenges associated with manufacturing silicon photonics have now been met, but the industry is still struggling with the practicalities of testing and packaging these complex components.

With all microelectronics, every device must be evaluated at the very start of the manufacturing process while it’s still on the wafer to make sure it is operating correctly, in a process referred to as “probing the wafer.” For conventional microelectronics, electrical probes are brought into contact with the devices, which are then stimulated to determine if they are working and to assess their quality, thus avoiding the significant downstream costs that would be incurred from packaging faulty components. Manufacturers therefore need to determine with great accuracy and reliability whether devices are working correctly before the wafer is further processed.

The small matter of precision

Wafer probing to check electrical interconnects is a relatively straightforward process; the probe must be positioned with an accuracy of around 10 µm to make the necessary electrical contact. This can be achieved quickly and easily with modern precision positioning systems, allowing cost-effective chip manufacture. However, if photonic elements are added into the mix, then it’s no longer possible to just take a probe and make electrical contact; light must also be coupled into and out of the device while it is still on the wafer. In almost all cases, this needs to be a noncontact process, with positioning requirements 10- to 1000-fold finer than for electrical probing. None of this was an issue 20 or even 5 years ago, but with silicon photonics, there could be thousands of devices on a wafer, all needing noncontact optical probing with unprecedented accuracy. This requires test rigs (wafer probers) that can accurately find and align photonic elements within tens of nanometers for the effective assessment of the components while ensuring absolutely no contact with the wafer, which is easily damaged and represents millions of dollars in value.

This challenge is further complicated by the numerous parallel optical paths with interacting inputs and outputs allowed by silicon photonics technologies, and the need for rapid alignment at multiple points in the production chain to ensure chips can be manufactured cost-effectively. This includes during packaging, where alignments are necessary for assembly to ensure that everything is still working, and that all devices still meet the manufacturing specifications.

The power of piezo components

As with traditional electrical probing and packaging, piezo nanopositioners are an enabling technology. The fast, stable, precise, and reproducible nanoscale motion control and long life offered by piezo actuators provide the positioning capabilities and throughput required for economical optical or electrical manufacturing applications and, as such, are helping to ensure good production economics for silicon photonics.

Physik Instrumente (PI; Auburn, MA), which provides nanopositioning systems to the semiconductor industry, and many of its customers, are tackling the challenges of precision automation for the purposes of probing and packaging photonics (see Fig. 1). For example, the company’s recently introduced Fast Multichannel Photonics Alignment (FMPA) control technology uses a fully parallel gradient search to simultaneously align multiple couplings, multiple degrees of freedom, and multiple channels, even if they interact. Unlike traditional positioning systems that are programmed to move from point A to point B and so on in an inevitable pattern, the new approach uses its own advanced algorithms to autonomously align numerous optical junctions in parallel.

Inside any package is the chip, but also potentially fibers, lasers, detectors, waveguides, mirrors, diffractive elements, and lenses. Each of these needs to be aligned; crucially, FMPA technology itself achieves the multiple adjustments required for correct alignment in a few steps (or often even a single step) with simple commands, compared to previous setups where the couplings had to be optimized in a sequential fashion, resulting in iteratively going back and forth and readjusting at every stage to gradually reach a consensus. This took a considerable amount of time, which had a significant impact on price, overall efficiency, and scalability. In contrast, by embedding these algorithms within the firmware of the multiaxis positioning systems used for alignment, the FMPA engine can do in a few seconds what once took minutes or hours (see Fig. 2).

A key advantage of this technology is that a single command is completely internalized to the controller. For example, the first step might instruct the handling robot to take a wafer, place it accordingly in the tool, and tell the alignment system to focus on a nominal position within roughly 100 μm from where the center should be. The system then builds a picture of the coupling and performs analyses to determine the true center. A coarse scan is enough to localize that and can be typically performed within 300 to 400 ms.

Accounting for lens tip

Next comes the optimization process, because the light coming through is not yet as efficiently coupled as it needs to be. A second command performs a gradient search, where one device (such as a fiber or array probe looking at a wafer) is moved in a small circle, typically on the order of a micron or two in diameter. This circular motion, which is performed at many tens of hertz, causes a modulation of the light observed by the controller. The system uses the phase information in this modulation to determine in real time how to move the objects so that they end up perfectly aligned, again, in fractions of a second. A significant advantage of FMPA is that it continues to track the alignment throughout the device testing so that, for example, if conditions change and cause the components to move, such as heat from a laser, then the gradient search remains locked on. Similarly, while the involvement of lenses also complicates alignment because lenses are sensitive to angles, this technology can rapidly account for that, achieving automatic, simultaneous optimization of multiple degrees of freedom, inputs, outputs, and elements.

In summary, FMPA is an enabling technology, allowing multiple couplings, multiple degrees of freedom, and multiple elements to all be aligned in as little as one step through parallel automation of alignments. The system has been recognized by several industry bodies for its cost reductions, whether for making the wafers, the packaging apparatus, or the tooling that performs the testing and the packaging. As manufacturers chase production economics and scale up production, FMPA is advancing production economics by reducing overall alignment times down to the subsecond range.

What’s next

This is by no means the end of the silicon photonics story. The next step after resolving connectivity infrastructure challenges is optical computing, where the logic itself is performed optically. This is then likely to be transcended by quantum computing, which is approaching faster than anyone could have imagined. Both these research areas will have a significant photonics aspect. In terms of probing and packaging, what’s next will be more of the same—greater speed, smaller sizes, and more embeddable configurations that can be easily slotted into equipment.