Sliced, diced and wire bonded 4-20-98Sliced, diced and wire bonded 4-20-98

April 20, 1998 Design News

SEMICONDUCTOR MANUFACTURING/COVER STORY

Sliced diced & wire bonded

Precise positioning is critical to the efficient manufacture of ICs

David J. Bak, International Editor

Laptop computers, CD and mini-disc players, cellular phones--few associate such high-profile electronic gear, integral to life in the '90s, with something as mundane as motion control. Yet without precise positioning, the tiny chips that drive these products might not be so reliable or reasonably priced, nor the products themselves so abundant.

Think of the process. During chip manufacture, etched wafers, generally measuring about 200 mm in diameter, mount onto aluminum carriers. Dicing saws cut the wafers into hundreds or even thousands of individual dies, or chips. How are the carrier and saw aligned? A positioning table.

Cut wafers, still mounted on their carriers, next move to die bonders. These machines pick good dies off the carriers and bond them to a package--for example, a stamped metal or ceramic leadframe, depending on the application. Wire bonding, followed by encapsulation and final testing, completes the process. Conductors of very fine gold or aluminum wire connect bond pads on the die to corresponding leads on the package. Successful bonding, again, depends on the positioning table.

"Different stages of the chip manufacturing process demand different positioning requirements," says Robert Chylak, director of engineering at Kulicke & Soffa Industries, a leading supplier of semiconductor assembly solutions. Wafer inspection and the cutting of dies demand adherence to extremely tight positioning tolerances. Wire bonding, on the other hand, calls for extremely fast acceleration and deceleration. The higher the bonding speeds, Chylak notes, the greater the throughput.

One example of a precision positioning system specifically designed for the high speeds demanded by wire bonding is a rigid, yet lightweight table called the NXT. Developed by Schneeberger Inc., Bedford, MA, and currently under evaluation at several beta sites, including Kulicke & Soffa, the NXT can move a bonding head 2 mm from stop-to-stop in 18 msec--with high precision and repeatability. Three design factors make this possible: a dual guideway system, cage creep control, and 2-D encoder.

Light weight for high speed. Both top and bottom (X and Y) stages of the NXT feature two complete linear bearing systems, one at either side. Each bearing system sandwiches a V-groove center rail between a pair of crossed roller bearings.

"Instead of the usual four rails per stage, the NXT incorporates six," says George Jaffe, Schneeberger executive vice president and managing director. Doubling the number of bearings, he notes, adds stiffness. More importantly, it reduces table weight by approximately 50% compared to conventional designs.

"When you tighten the preload screws on a four-rail stage," Jaffe explains, "the stress propagates across the entire table. Table mass, consequently, must be sized to prevent flexing which can lead to distortion and poor repeatability." Six rails per stage--i.e., individually preloaded bearing sets at either end--localizes preload forces. This results in the use of lighter weight materials with thinner cross sections, permitting acceleration and deceleration to 5G.

To combat cage creep, the NXT incorporates a cage assist design. High-speed oscillatory motion, combined with crossed roller bearings, can promote cage migration, or movement with respect to the bearing system's sliding elements. As the roller cage creeps away from center, it will begin to restrict the slide's travel. Eventually, the cage contacts the end piece, dramatically increasing the force required to move the slide.

The cage of each NXT bearing set, therefore, incorporates a pinion gear that runs in a rack. During operation, these plastic components force the cage to move a prescribed distance, maintaining the relative position between cage and sliding pieces.

NXT's third design element, a grid encoder, reads in two directions simultaneously to eliminate orthogonal positioning errors. Made by Heidenhain Corp. (Shaumburg, IL), the two-coordinate encoder encompasses one grid plate and one scanning head. The grid plate offers an accuracy of ±2 microns; the scanning head provides two 90-degree phase-shifted sinusoidal output signals.

Air bearings for accuracy. Because they have no friction or stick slip, air bearings offer motion resolutions in the nano meter range when combined with electronic feedback and control systems. Additionally, the air film averages small manufacturing irregularities--an important consideration for long-travel positioning stages.

These requirements, coupled to the semiconductor industry's use of increasingly larger wafers for economy of scale, explains the reasoning behind Dover Instrument's new 300-mm, single-plane X-Y stage. Originally introduced for flat-panel display applications two years ago, the single-plane X-Y stage offers the large travel needed to inspect 300-mm wafers, without any fall-off in accuracy.

"Stacked stages," says Applications Engineer Doug Gurley, "are fine for smaller wafers, but with longer travel, they can introduce error." Why? With stacked stages, Gurley explains, the upper axis overhangs the lower axis which may lead to some error at the end of travel. Additionally, flatness of travel in the upper axis depends on lower-axis accuracy; error in the lower axis will be compounded in the upper axis.

In a single-plane X-Y stage, both axes vertically reference the same granite base, which is lapped to within 0.0001 inch for a high level of flatness. As a result, the system accommodates relatively long travel with no compromise in performance. Building blocks include three permanent magnet, brushless dc linear motors, each equipped with linear encoders, and precision air-bearing ways.

Two of the linear motors, located at either end of the X-axis air-bearing guiderail, move the payload carriage in the Y direction. The third linear motor moves the payload along the X axis.

"Driving the X-axis guiderail from both sides," says Gurley, "counteracts potential moment forces due to large loads and high acceleration." Conversely, magnetic preloading at one end only--allowing the other end to "float"--accommodates thermal expansion and prevents binding. To compensate for guideway mechanical errors and correct for yaw between the two X-axis motors, Dover employs a proprietary servo control algorithm.

The algorithm is part of the company's DMM-2100 fully programmable, DSP-based motion controller designed to govern the single plane X-Y stage. A PMAC control card from Delta Tau (Northridge, CA) or Motion Engineering Inc. (Santa Barbara, CA) commands the dual-loop system. Each servo loop, says Gurley, can be tuned individually due to the decoupled dynamics of the stage's linear and rotational degrees of freedom. Yaw motion between the dual X-Y axis motors can be dynamically constrained to zero or forced to small angle correction values.

The brains behind Dover's controller? A tiny chip, of course.

Nowhere without software

Table design is just one element to accurate positioning; software is the other. So says systems integrator Jim Saudargas. His company, Concepts in Computing, South Beloit, IL, writes the source code that ties together all elements necessary for wafer fabrication: the vision system camera and lens; frame-grabber cards that convert camera output into digital data the computer can manipulate; X-Y table drives that position the camera and wire bonder; and the software platform, be it Windows 95 or NT.

Saudargas notes there are many ways to accomplish precise positioning. In one scenario, the semiconductor manufacturer may opt for a medium-range positioning system "just to get in the general vicinity of the fiducials, or reference marks on the wafer." From there, the manufacturer may employ the vision system to move in closer, making relative measurements from the fiducials.

"Suppose a vision system provides a quarter-pixel resolution," Saudargas speculates. "If the field of view is 300 microns and it is 480 pixels high, that equates to about 0.6 microns per pixel. One fourth of that allows submicron readings, even though the stage and scale itself are not that accurate."

Regardless of positioning scenario, Saudargas claims software is the key. "You can have the best hardware positioning solutions available," he says, "but it will not be helpful unless you have the right software driving it."