Daniel N. Pascual, SÜSS MicroTec
George A. Riley
This tutorial describes the capabilities and limitations of a Flip Chip bonder for MEMS bonding, and gives an overview of bonding techniques successfully used to assemble a variety of complex, sensitive MEMS devices.
Flip-chip bonding has been successfully used as an enabler for MEMS assembly.  A commercially available flip-chip bonder, the FC150, meets the stringent requirements of MEMS assembly.
For more than twenty years, the FC150 has been used to attach planar infrared detector arrays to read-out circuitry. Like MEMS, the delicate infrared detectors and fine pitch attachment require sophisticated systems for handling, maneuvering, aligning, and bonding. The equipment and techniques developed and perfected for infrared detector arrays have now been applied to 3D MEMS assembly.
The following sections describe the capabilities and limitations of the FC150 for MEMS bonding, and gives an overview of MEMS bonding techniques successfully in this application.
FC150 Capabilities and Limitations
The first step in MEMS assembly is to hold the parts securely. The FC150 holds the upper part of the assembly with vacuum tool on a moveable arm, and the lower part with a moveable vacuum chuck. Vacuum pickup requires that the upper part have an adequate touchable surface area that can have vacuum applied to it.
Each vacuum tool is custom made to fit a specific application. The tool material depends on the application. Silicon carbide tools are suitable when high heat or high force is required. Silicon carbide combines high hardness, good thermal conductivity, low coefficient of thermal expansion, and an ability to remain flat after repeated temperature cycling.
Quartz tools are suitable for applications requiring ultraviolet illumination, since quartz has high transparency at those wavelengths. Other tool materials may be required for a particular application.
Once a suitable material is chosen, the tool is sized and shaped to fit the parts and to provide a path for vacuum channels
The FC150 can position the parts in all six degrees of motion, with translation and rotation about the x, y, and z-axis.
A variety of machine arms are available, with the choice governed by the bonding force and the accuracy required.
The FC150 Solder Reflow Arm (SRA) is most common for MEMS assembly because it combines precise control of low forces with 1 m m post-bond accuracy. The SRA controls the vertical (z-axis) motion of the upper part, and the rotations about the x and y-axes (pitch and roll). The minimum step in the z direction is 0.5m m while the minimum step for pitch and roll is 0.05m rad with a range of ± 0.6° .
The chuck positions the lower part along the x- and y-axis as well as rotating it about the z-axis (theta). A minimum step of 0.1m m is possible with the high-accuracy stage. The minimum step in theta is 9 m rad, with a standard range of ± 7° .
Alignment is the most critical aspect in MEMS assembly. The FC150 uses a set of optical systems, shown in Figure 2, to align the parts in all six degrees of freedom.
This high accuracy passive alignment method requires precise and consistent machine performance. Passive alignment is faster than active alignment, which requires the components to be active during alignment.
Figure 2 FC150 alignment system
The primary alignment system on the FC150 is a bi-directional microscope used for x and y alignment. The bi-directional microscope is positioned between the parts before aligning, and withdrawn before bonding. It allows simultaneously viewing both the upper and the lower parts, while positioning them in x and y.
Theta alignment requires aligning two pairs of alignment marks. To maximize theta alignment accuracy, the distance between the two pairs of alignment marks should be as large as possible
The FC150 offers two alignment systems for leveling (pitch and roll) alignment. The first is an autocollimator, which projects an image onto both parts.
The image of a cross is projected onto the surface of the upper and lowers parts. The two crosses can be observed on either the video screen or through eye-pieces.
One cross, reflected from the lower part, is used as the reference. The second cross, reflected from the upper part, can be positioned relative to the reference by adjusting the leveling.
The parts are aligned parallel when the two crosses overlay. Since the collimated beam of light is about 13mm in diameter, a fairly large and reflective surface area is required for autocollimation.
Laser leveling may be used if there is insufficient reflective surface area for autocollimation. A laser beam measures the distance between the upper and lower part at three locations. The upper part is leveled until the three distance measurements are within tolerance.
Although the laser-leveling system does not require a large reflective surface, it does require at least three highly reflective targets, such as 50 m m square gold pads.
Once the parts are aligned, the last step is to bring them together along the z-axis for bonding. The FC150 can bring the parts together, under the control of either a contact force sensor or a z-motion control.
Bonding requires the application of heat, force, or ultraviolet energy. The vacuum tooling is heated by infrared radiation supplied by halogen lights located behind the tooling. The arm and the chuck heating can be independently, controlled from room temperature up to 450° C. The tooling can be cooled passively or with air jets.
Force is applied by driving the arm down with a lead screw. A maximum force of 450 gf can be applied with the SRA. Higher force is available using other arms.
Ultraviolet energy is provided by a separate source via a fiber optic cable. It produces a 20 mm diameter spot size with 80mW/cm2 intensity. Since the ultraviolet light is projected from behind the arm, quartz tooling must be used to allow the light to pass with minimal obstruction.
Bonding Methods for MEMS Assembly
The FC150 offers a variety of bonding methods to cope with the range of mechanical, electrical, and thermal requirements for MEMS assembly.
Intermediate-layer bonding uses an additional material layer to join the components being bonded. The most common intermediate materials are metal, glass, and polymers.
Intermediate-layer bonding on the FC150 includes metals (eutectic and non-eutectic solders), glass frit, and polymer bonding. The FC150 also offers limited direct bonding, with no intermediate layer.
Solders can be deposited onto components by sputtering, evaporation, electroplating, or as solder preforms.
Eutectic solder bonding combines two or more metals to create an alloy with a lower melting temperature than its constituents. Eutectic bonds provide good electrical and thermal conductance. Another metal bond is thermo-compression of thin soft metals like gold.
Glass frit is a low melting temperature glass is used for hermetically packaging MEMS devices. The frit is commonly screen printed onto the components, but this makes fine lines and accurate patterning difficult.
Polymerbonds may be either thermo-curing or thermo-plastic. Thermo-curing polymers are organic compounds that harden or cure with the application of heat or ultraviolet radiation. Thermo-plastics soften when heated, and re-harden when cooled.
Direct bonds such as silicon fusion bonding occur in two steps: pre-bonding and annealing. Direct bonds require extremely clean and smooth (<5 Å rms roughness) mating surfaces.
Pre-bonding can be done on the FC150. Annealing requires an oven to reach the 1100° C annealing temperatures.
Using the techniques described above, the FC150 has successfully assembled a variety of MEMS, including rotary engine components, an optical switch, and adaptive optics. Consult the original paper (see below) for extensive details and photographs of these assemblies.
The FC150 device bonder can assemble complex MEMS devices. Processing these devices requires specialized tooling to ensure proper handling. The FC150 alignment system allows positioning of components along all degrees of motion, including leveling. Several bonding methods are available, including heat, force, or ultraviolet light.
 K. Boustedt, K. Persson, and D. Stranneby, “Flip Chip as an Enabler for MEMS Packaging”, IEEE, Proc. 2002 Electronic Components and Technology Conference, 2002
This tutorial is based upon the paper, ” Flip-Chip Bonder for MEMS Assembly,” presented at IMAPS International 2004, Long Beach California, in November 2004.