Automation of optoelectronic assembly
07/01/2001
Cover Story
Advances in automating first-level interconnection of optoelectronic components are essential to reducing costs in the optical network
BY BRUCE W. HUENERS
As the demand for photonics components grows, increasing emphasis is being placed on the automation of this assembly process to drive down component costs and thereby increase volume. Currently, optoelectronic component manufacturers are using automated equipment to process and assemble multi-chip eutectic die attach packages in a single pass on die attach equipment with mechanized substrate and die handling. In the future, other processes that currently require sub-micron resolution and involve complex sets of iterative alignment steps, such as fiber alignment and active sub-component alignment, will be performed by automated systems. This equipment will incorporate several technological advances that reduce assembly time and increase yield. The package accounts for 60 to 80 percent of current manufacturing expenses in optoelectronic component assembly, and advances in automation will be essential to reducing component costs in the optical network.
Background
Recently, demand for optical components has skyrocketed, and its growth rate is forecast to continue at a compound annual rate of 40 to 50 percent for the next several years. Established component manufacturers have quickly discovered that simply increasing their manual workforce will not meet the demand. Start-ups have realized that a manual assembly process cannot be used to meet the time-to-volume requirements necessary for them to establish market share rapidly.
 Table 1. Materials for optoelectronics. |
The optoelectronic component industry is currently capacity-constrained and labor-intensive, with predominantly piece-part assembly that is not very scalable. Consequently, component manufacturers, both large and small, are investing in automation to reduce cost, increase yield and gain productivity.
 Table 2. Processes for optoelectronic assembly. |
Parallels to the present optoelectronics industry can be drawn from the early electronics industry where automation was scarce, with a myriad of discrete components, relatively low volume and low production yields. Where optoelectronic assembly seriously diverges from traditional semiconductor assembly is in the area of packaging. The value of a semiconductor product is predominantly in the chip itself. Wafer fabrication equipment constitutes about 90 percent of all semiconductor manufacturing equipment sold each year. Conversely, the value of an optoelectronic device is in the package, with typically 60 to 80 percent of manufacturing expenses contained in the packaging. Hence, packaging is the key to reducing optoelectronic component costs, and automation of packaging is one of the key enablers.
Automation Challenges
Fiber optic manufacturing today is predominantly a process of manual handling and assembly. Proprietary methods and high intellectual property barriers are prevalent in the industry. These methods include custom wafer processing, thin film processing, device and subassembly packaging (including eutectic and epoxy component attach and wire bonding), fiber handling and alignment, and the finishing steps of tuning, adjusting and testing. Multiple fabrication techniques and processes are common, coupled with a lack of package and material handling standards.
 Figure 1. Generic assembly process. |
Industrial engineering approaches common in other technology industries are largely absent from today's photonics manufacturing environment. These practices include process engineering, design for manufacturing, design for test, standardization, outsourcing and automation. These are highly interrelated, and any discussion of automation must include these practices as well as the specific types of materials employed in optoelectronic components (Table 1).
 Figure 2. Deep-access wedge bonding. |
Processes for optoelectronic assembly today include many of the traditional microelectronic assembly processes, with the added complications of fiber and active alignment of components (Table 2).
The applications of optoelectronic devices - ranging from high-performance, long-haul to metro and access devices - include a large mix of package types (Table 3). These vary from custom, high-cost microwave-style packages to standard transistor-outline packages (TO cans).
Automation Strategies
A generic assembly process in fiberoptic device assembly is shown in Figure 1. With tangible benefits capable of being realized today, optoelectronic manufacturers are pulling automation into their factories, but they are facing considerable challenges.
Some of the factors that affect photonic package assembly strategy include:
- Product volume
- Product complexity
- Product handling
- Product mix
- Design changes requiring customization
- Cost of labor
- Degree of assembly/operator skill required
- Device functional performance.
Accelerating the use of automation in fiber optic component manufacturing will include the following elements:
- Accommodating complex proprietary methods in automated equipment
- Finding commonalities in multiple fabrication techniques and processes
- Designing components for automation
- Establishing package and material handling standards.
Automated Optoelectronic Assembly Roadmap
For the near term, the required die bond accuracy will hover around ±2 microns until the need for single mode fiber passive alignment arises. Emphasis will be on increasing throughput without impacting accuracy. The preferred method of chip interconnect will be gold ribbon bonding and high-speed, deep-access ball bonding. (This parallels the technology found in high-frequency RF components.)
 Table 3. Optoelectronic components and package types. |
Fiber attachment to the package will include automated fiber handling and the emergence of fiber attachment techniques that eliminate rework. In certain applications, passive alignment for single-mode fibers will be automated in production environments.
 Figure 3. Effect of beam displacement from fiber core. |
The processes that precede and follow assembly and alignment will be incorporated into integrated, in-line solutions. These include device inspection, test and binning before die attach, and device test after fiber attach.
The Packaging Process
The packaging process itself further challenges the move to automation. Because the purpose of an optical component is to manipulate light, the design rules for packaging are significantly more complex than those found in the semiconductor industry. With semiconductors, advances in wafer processing drive technology, and packaging is very automated and planar. With optical components, the front-end process is significant, but equally important for current photonic devices are the package design and the critical assembly tolerances.
Package Design
As previously mentioned, the history of optical component manufacture is filled with manual processes. Equipment suppliers and component manufacturers are currently facing the challenge of automating package designs that are optimized for human assembly, not robotic assembly. Some of the more common problems experienced in the industry are vertical obstructions within the package, component placement and wire bonding inside deep packages, visual obstructions within the package, and the lack of visual references that can support machine vision.
 Figure 4. Semiconductor laser diodes typically emit light in a vertical ellipse. |
Package depth and height variations within the package also have proven difficult to handle for automated equipment. For instance, because most automated wire bonders and die bonders available on the market today were designed for the relatively planar assembly of semiconductors, adjustments for varying heights and package depths were generally not designed into the machines. Specifically, ball bonders that use a pivoting motion may be unable to make acceptable bonds at different heights within the package. Also, wedge bonding at any feed angle other than 90 degrees is generally prohibited by the confined space and high package walls. As a result, automated optoelectronic wire bonding, both ball and wedge, has generally been limited to machines that are designed to adjust for variations in height and move in the vertical plane to facilitate a deep access bonding capability (Figure 2).
As data rates increase and designing for RF becomes more important, ribbon bonding, which can only be done on a wedge bonder, may play a significant role in first-level interconnects for optoelectronic packages. Consequently, even some of the wire bonders available today that are used in optoelectronic packaging will not be able to support the requirements of the optical component industry in the near future. Advances in high-speed, deep-access wedge bonding will be necessary to enable low-cost packaging for the next generation of high-performance components.
A challenge for machine vision in optoelectronic packaging is "seeing" the package and materials. Because machine vision generally "sees" by converting grayscale into code and then matching similar patterns, the visual references that can be used by manual assemblers often do not support the requirements for machine vision.
Piece parts and packages without metallization or other patterns are difficult to identify and make poor references for the machine. Without the ability to use machine vision to determine locations within the package, automated assembly cannot be successful. Consequently, component manufacturers are beginning to incorporate accurate and repeatable fiducial markings on packages and piece parts to facilitate vision and improve placement accuracy.
Meeting the Critical Assembly Tolerances
The assembly tolerances for optical component assembly can be exceptionally stringent (Figure 3). For component placement, required accuracies are usually measured in microns, with requirements on rotational accuracy specified in tenths of degrees. Adding these additional strict tolerances to dimensions that have historically been unimportant is another challenge that automated equipment must overcome to support optoelectronic assembly.
For example, in packaging a pump laser, the required placement accuracy for the laser diode on the sub-mount can be between 1 and 2 microns in the X and Y assembly plane. This is about an order of magnitude more precise than high accuracy semiconductor applications, which rarely require accuracies better than 12 microns. The laser must be within 0.5 or 0.25 degrees of parallel to the sub-mount in all other dimensions (pitch, roll and yaw). These strict tolerances are caused by the beam profile and heat dissipation requirements of the laser diode. Thermal dissipation is critical for laser diodes because the output frequency of the laser is affected by changes in temperature.
Accuracy requirements are also driven by the elliptical beam profile that is typical of a semiconductor laser diode. Because light is generated in a horizontal "stripe" region within the diode, it is emitted as a vertical ellipse, usually diverging about 40 degrees in the vertical and about 10 degrees in the horizontal (Figure 4).
Managing the Light Path
For light to be coupled successfully into a fiber, the light must pass into the core of the fiber with an angle of incidence less than or equal to the numerical aperture (NA) of the fiber (Figure 5). The typical NA for single mode fiber with a core diameter of nine microns is about 12. That means that the only light that will be propagated through the fiber is light that has entered the nine-micron core at an incidence angle less than 12 degrees. Any light that impacts the cladding or enters the core at an angle greater than 12 degrees is lost. Consequently, coupling an elliptical beam diverging at 40 degrees into the fiber core is a difficult task.
What Can Be Automated Today
The first true automation in the industry for first-level interconnect has come from adapting existing automated die attach and wire bonding equipment to meet the needs of optoelectronic component manufacturers. The industry has found that the wire bonding and die bonding equipment that was designed during the past decade for the high-performance hybrid, multi-chip module and microwave industry can be used as off-the-shelf solutions for automating certain processes in the optical industry.
For example, automated eutectic die attach is currently used throughout the industry for attaching high-performance laser diodes to sub-mounts (Figure 6). It is commonly done either using a batch process, where piece parts are loaded onto a work area and then mechanized material handling is used to assemble the subcomponents, or as an island of automation, in which piece parts are automatically indexed into the work area, assembled and then automatically indexed out of the work area. Key factors for success in this process include machine accuracy, programmable pulse heat control and low-force bonding.
 Figure 7. Gold wire bonding to an InP laser chip. |
The following example also illustrates some of the cost savings that automation can bring to the optoelectronic assembly industry. One component manufacturer demonstrated that one automated eutectic die attach machine could produce the same total output as four manual stations that require 20 operators for round-the-clock production. Furthermore, when using the automated process, yielded output went up by 50 percent, and there was a 67-percent reduction in occupied floor space in the clean room. These savings resulted in the equipment paying for itself in less than three months.
Manufacturers have also been able to incorporate automatic wire bonding stations into their manufacturing processes to deliver repeatable first-level interconnects significantly faster than manual stations. This deep access wire bonding was another process that was easily adopted into the optoelectronic component manufacturing industry based on years of previous development in the high-frequency wireless and hybrid semiconductor markets (Figure 7). With bit-rates projected to reach 40 Gbps and higher this year, predictable and repeatable wire and ribbon bond lengths, loop profiles, and tail lengths will be an essential part of component design and packaging. These critical parameters are nearly impossible to achieve manually, and this will necessitate the proliferation of automated solutions throughout the industry.
Summary
While automation will eventually be able to provide complete process solutions for the optical component assembly industry, currently there are significant challenges to automation. The combination of extreme accuracy requirements, significant variations in height, unusual form factors, unusual materials and the inherent difficulties arising from the requirement to guide light have prevented most equipment available today from being used in optical component packaging.
Fortunately, we are now seeing the emergence of an equipment supplier base that is able to support the stringent tolerances of the optoelectronic component assembly industry. The equipment that these suppliers are designing today will not only facilitate the efficient manufacture of optical components, but it will enable the component designers to continue to push the envelope of communications technology into the foreseeable future. AP
For more information, contact Bruce W. Hueners, director of marketing, at Palomar Technologies Inc., 2230 Oak Ridge Way, Vista, CA 92083; 800-854-3467, Fax: 760-931-3444; E-mail: [email protected].