Nanometer Scale Industrial Automation for Optical Device Manufacturing

Author: RJ Hardt, Director of 3D Metrology Systems, Aerotech Inc.

Manufacturers of optical devices know the importance of mature industrialized processes built around the cost savings of automation. These processes have made manufacturing optical devices economical in a number of speed and bandwidth-sensitive markets such as data communications, telecommunications, and commercial sensing. The technology that goes into these devices is constantly evolving to meet the future needs of these markets, and this forces product manufacturing practices to follow suit.

Disruptive technologies, including the advancement of chip-level optical functions and the miniaturization of device interconnections, have forced the adoption of new and unproven manufacturing processes in mass production environments. Component assembly and alignment tolerances that were once measured in tens of micrometers are now scrutinized in terms of nanometers. For example, chip based photonics are commonly designed with waveguides that are only a few hundred nanometers wide and have alignment tolerances to external components or fibers that can be tens of nanometers. Not achieving these tolerances during the assembly process causes attenuation and signal loss that affect device functionality. Optical research laboratories have made many process advancements at the nanometer scale, but taking these same techniques and applying them to an industrial automation process is no small task and carries significant risk. Fortunately, the risks of developing production platforms with enough robustness and throughput can be mitigated by choosing the appropriate automation equipment.

Why Nanometer-Scale Manufacturing Poses New Challenges

Processes such as optical alignment, component placement, and automated test pose new hurdles on a nanometer-scale mainly because external influences are more pronounced at a nanometer level when compared to their predecessor micrometer processes. The finite mechanical rigidity of automation hardware, tight coordination requirements between automation and process tools, and the electrical noise induced by the various control system components all exacerbate parasitic errors at the nanometer level. Furthermore, a production environment will induce more environmental challenges that are not seen in an ideal lab setting. Production-related influences including vibration from coexisting machinery, temperature variation, and harsh machine throughput requirements are exponentially more difficult to negate at a nanometer scale.

The current high volume manufacturing ecosystem is limited by the scale at which it can operate. Moving to a nanometer assembly scale requires pinpoint automation determinism and control capabilities that much of the current optical device manufacturing space has not yet mastered. The challenges are plentiful, but many strides have been made to mitigate risk involved in nanometer-level optical device manufacturing processes.

Transfering Laboratory Processes to Industry

There are a number of automation technologies that are suitable for assembling optical devices in a laboratory setting. Keep in mind that many of these technologies have flaws when moved to an industrial atmosphere, but for laboratory qualifications they will perform as expected. Removing manufacturing throughput requirements and the harsh environment of a production floor allows R&D process engineers to select from a wide variety of automation equipment. From a component positioning equipment perspective, utilizing servo, stepper, or piezoelectric technologies are all acceptable choices for high-precision component alignment and placement. For example, highly-flexible, six degree-of-freedom, servo-driven hexapods are capable of 20 nanometer incremental motions and high-accuracy, multi-axis positioning (see Figure 1). As another example, piezoelectric actuators can make incredibly small moves with extremely fast settling times (see Figure 2).

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Ultimately, the purpose of automation in a laboratory is to prove a process that can be used to manufacture optical devices in higher quantities to promote economy of scale cost savings. With this in mind, it is important to choose automation technologies at the R&D level that are also suitable for production environments. These environments require robust manufacturing equipment and algorithms that can withstand physical disturbances and operator errors. Furthermore, the automation equipment selected must satisfy capital equipment cost drivers such as throughput, lifetime, and maintenance. There are tradeoffs behind each automation technology applied to optical device manufacturing, but here are some general guidelines to select the right equipment.

If Robustness Is Paramount

Robustness in an automation system is the ability of the mechanical hardware and associated control system to withstand external influences such as mechanical wear and electrical noise over time. There are ways to specify a robust platform, but there are also many potential pitfalls. For example, choosing a contact screw or gear-based positioning actuator increases the chance of wear and reduces the chances of nanometer-level positioning over time. Another pitfall is choosing a control system that induces electrical noise that may contaminate the signals from process equipment such as optical power meters. One ideal solution for robustness is to use a direct-drive, noncontact motor coupled with a low-noise amplifier. These motors have no contact points that induce wear, and the low-noise linear amplifier removes harmful electrical switching noises associated with common servomotor pulse width modulation methods. These types of systems are capable of achieving long-term, nanometer-level linear and rotational alignment tolerances (see Figure 3 and 4).

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Figure 4.

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Upfront equipment cost is always a concern, but an automation platform’s robustness can significantly reduce the overall cost of ownership of the machine and should be taken into consideration for upfront budgeting. Incorporating servo driven, noncontact motors reduces the need for extensive maintenance or replacement after the automation platform has aged. This allows the same equipment to be reused to automate the assembly o multiple optical device generations.

As an example of a platform with limited robustness we can turn to piezoelectric stepping actuators with strain-gauge feedback. They are often selected due to lower cost and a smaller footprint, but the long-term costs can be significantly higher than other technologies. They are prone to shorter life in industrial settings due to the contact wear of the stepping mechanism and fatigue wear of the strain gage sensor, and may need to be replaced frequently. Ultimately, this makes long-term use difficult, and utilizing other available technologies, such as servomotors or flexure based piezo actuators, with a higher up-front cost becomes more attractive.

If Throughput Is Paramount

Often the cost of manufacturing an optical device can be reduced by increasing the throughput of a machine using automation. However, increasing process dynamics while operating on a nanometer scale is no easy task. First, selecting the right mechanical hardware will have a substantial impact on achievable system dynamics. Second, the process equipment must be tightly tied to the automated positioning equipment in order for the manufacturing platform to make quick and informed decisions. Optical power detectors and other tools are used to make process decisions, and any associated latency between these devices and the automation tasks will increase the time it takes to manufacture an optical device.

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For high-dynamic, nanometer-level applications, piezoelectric and direct-drive servomotor platforms are ideal mechanical components. They utilize more bandwidth from the control system than other technologies such as screw- or gear-driven systems. Ultimately, this leads to reduced motion profile execution times and higher machine throughput. Furthermore, selecting a control system that allows for the direct integration of high-speed process tools into the automation algorithms is paramount. Coordinating the position of the parts being assembled with the output from the various process devices needs to be done with as little latency as possible. Here, built-in firmware-level alignment and data collection algorithms reduce the time for a process to complete. Figure 5 shows an ideal control network for the coordination of process tools and automation equipment with an emphasis on throughput.

The Future of Nanometer Processes

As optical devices continue to pave the way for product innovations in a number of different industries, it will be important to address the manufacturing concerns in a timely manner. Bringing these new devices to market faster and in higher volumes will be important to the growth of the optical device market as a whole. Companies that can incorporate a production mentality into their R&D efforts will have a head start on their competition. Additionally, prioritizing an automation system’s robustness and throughput will help guide the selection of the appropriate equipment. Each has its place in determining performance on a nanoscale level for the next generation of optical device manufacturing processes.

Figure 1. A plot of the XY plane position error using an Aerotech HEX300-230 six axis hexapod. The error is kept below 2.3 microns for the 50 x 50 mm area.

Figure 2. Plot showcasing an Aerotech QNP-L linear piezo actuated stage performing a 1 micrometer move and settling to ±10 nanometers in less than 2 milliseconds.

Figure 3. [Left] A plot showing a linear servomotor mechanical stage making single nanometer motions. [Center] An Aerotech FiberMaxHP six-degree-of-freedom direct-drive positioning system and linear amplifier. [Right] A plot showing a rotational direct-drive stage making 0.05 arc second incremental motions.

Figure 4. An Aerotech BLMSC linear servomotor including a noncontact array of magnets and a dense copper coil pattern. Both are separated by a thin layer of air to reduce long-term wear.

Figure 5. Schematic of Aerotech’s industrial machine controller and associated hardware. This system coordinates high-speed signals from optical process equipment with high-precision positioning devices.