Optical and optoelectronic (OE) devices are being rapidly integrated into many facets of everyday life. From telecommunications to sensor applications, these devices are expected to perform accurately and reliably for long periods of time.
Common optoelectronic components such as drop/add filters, fiber amplifiers, variable optical amplifiers (VOAs), and Vertical-External-Cavity Surface-Emitting-Lasers (VECSELs) are used in dense wavelength division muliplex (DWDM) telecom infrastructure. As the functionality of these devices increases, the number of new materials used also increases. In all of these cases, the materials must be put together in a specific design and packaged to ensure reliability and function. The assembly normally includes bonding at the interfaces of the various materials. Typically, the bonding is done with adhesives or with solders; however, both of these approaches have their shortcomings.
Organic-based adhesives can join a variety of materials, but often lack the long-term stability required for optoelectronic components. On the other hand, solders are mechanically more robust, but not capable of bonding to some materials without surface preparation such as metallization or flux. Optical/optoelectronic components can comprise a number of different materials. In addition to SiO2, ceramics, optically transparent materials (e.g. LiNbO3, MgF2, CaF2, ZnSe, and ZnS) and heat sink substrates (e.g. diamond and SiC) are used. Traditional solder cannot bond to these materials and there is a need for a more universal solder.
The use of reactive solders, which are capable of bonding to a host of materials, is described below, as well as the function and advantages of the solder in two integrated photonic devices.
Value in the Package
Long-term stability in packaging and assembly is critical to the performance of optical/optoelectronic devices. Gradual misalignment of optical components over time can lead to reduced signal strength or a complete loss of transmission intensity (i.e. device failure). As such, a robust solution is required to prevent such catastrophic losses.
While Nobel prizes typically go to groundbreaking basic scientific research and businesses get excited about new optical disc formats, scientists and engineers understand that their financial future often rests on the mundane topics of assembly, packaging, and interconnection of components and systems. So, while new bonding technologies may not receive the fanfare of discoveries in spintronics or lasers, being aware of the latest bonding methods and materials is often the difference between success and failure.
From Canada balsam and sealing wax to direct diffusion, the range of bonding materials (and bonding without added material) is vast. Adhera Technologies and the Fiber Optic Center have focused on materials (AdherAlloy) that look like solder and stick to ceramics like frit glass.
Metallic bonding can offer several advantages over conventional polymer-based adhesives. These include:
- Strength — in tension, compression, shear
- Stability — dimensional constancy, low creep, nil internal stress
- Inertness/hermeticity — nil outgassing or permeability
- Thermal/electrical conductivity — superior to adhesives
As stated earlier, most solders require pre-treatment such as etching/metallization in order to form a useful joint; with fluxing and post-cleaning, a single bond may require many costly and yield-reducing process steps and inspections. A potentially more efficient approach is to use reactive solders that bond directly to most metals, ceramics, diamond, and other materials without pre-treatment beyond simple cleaning.
Reactive solders are a new addition to the device designer’s portfolio. Emerging recently from the laboratory, these are now finding broad application in important areas from armor to MEMS (micro-electromechanical systems).
These solders typically are based on familiar solder compositions, like lead-free SAC, low-creep gold, and stress-relieving indium compositions. These are specially formulated with proprietary mixtures of rare-earth (RE) elements that create nanodisperse intermetallic phases. When the solders are melted, these migrate to the bonding surface, where the REs attack oxides and other interfacial layers. Due to the special chemistry of REs, the result is a bond that is intrinsically strong and thermodynamically stable. Equally important, these solders bond directly to glasses, ceramics, and specialty alloys, making a vacuum-tight bond at lower temperature than required by frit glass or brazing.
Figure 1 shows a schematic of a laser-fiber optic coupling device. Manufacturing such a device requires many bonding operations, including element mounting and alignment, heater/heat sink attachment, and final package sealing. In this example, the reactive solder was used for all three (packaging not shown). By virtue of its low-creep properties, alignment is maintained over time and 64 W/m-°K conductivity heat flow is managed. Adhera’s solder is capable of forming strong bonds with all of the materials tested and listed in Figure 1. Additionally the solder provides a hermetic seal between the fiber and the Kovar sleeve. It should be noted that typical lead-free solder is not capable of bonding to any of these material interfaces.
For another good example of what RE-based, lead-free solders can do, consider the fiber Bragg grating (FBG) and temperature compensator in Figure 2. In it, the FBG-written fiber is attached to a bimetallic temperature compensator (Kovar-SS) with a negative CTE using creep-resistant solder. The plot on the right-hand side of Figure 3 indicates that there is no change in the signal intensity and shape after the fiber is strained and mounted. Only the predicted shift in wavelength is observed, which is consistent with the equation shown in the left side of Figure 3.
Applications like this also require extreme delicacy: surface damage, excess stress on, or penetration into will crack the fiber. Since the device is by nature a sensor, integrity of signal traces before and after secure bonding indicates good construction that is free of residual stresses. The assembly is expected to be dimensionally and chemically stable for the full life of the finished product.
As illustrated here, rare-earth-based, reactive solders have been proven effective in assembling several optoelectronic devices, including a laser-fiber coupler and a strained FBG system, with no compromise to any performance parameters. The same advantages are available for diverse materials, structures, and systems.