By Dave Senders, Point Technologies
MOTION CONTROL and machine vision are used throughout the semiconductor manufacturing process, from monitoring the diameter of ingots as they’re formed from a crystal seed to aligning a die lead frame prior to wire bonding. In nearly every step of the process, motion and vision can be found working together to align, inspect, measure, and identify wafers and die, so the various pieces of production equipment can perform their tasks.
These jobs weren’t always easy or labor-efficient. Our company, Point Technologies, recently applied a much-needed integrated motion and vision solution to a critical area of the semiconductor industry: probe needle inspection. Based in Boulder, Colo., Point supplies precision electrochemical pointing and micro machining services and products for semiconductor, medical and biotech applications.
Eyeing the Needle
Semiconductor manufacturers rely on probe needles for wafer-level testing during initial production phases to ensure that only fully functional product is packaged for final use.
A probe needle is a straight, small-diameter (0.002-0.012 in.) metal wire with one end tapered to a sharp point. Each probe plays the vital role of establishing an electrical connection between the tester and the wafer. Generally hundreds or thousands of probe needles are precisely bent and set into an array on a device known as a probe card. Each probe card is tailored to interface between the specific wafer being tested and the wafer probing machine, or “prober.” During testing, precise needle geometry is essential to ensure test data reliability and consistency.
Most probe card manufacturers use reference diameters to determine where to bend each probe (See Figure 1 below). If the geometry of a group of probe needles is inconsistent, then the reference diameter, as well as the bend angle, tip diameter and tip length, might be inconsistent. A change in any of these variables can cause probe misalignment and result in poor test data consistency.
FIGURE 1: SPECS MUST LOOK SHARP
A probe needle is a straight, small-diameter (0.002-0.012 in.) metal wire with one end tapered to a sharp point used to establish an electrical connection between tester and wafer. Inconsistent probe needle geometry can cause probe misalignment and poor test data consistency.
TO TEST a wafer, both the probe card and wafer are loaded into the prober. Then, the prober drives the probe needles slightly past the point at which the needle tips contact the wafer, causing the needle body to flex and the needle tips to slide across the metal bond pads on the wafer. Because of the nature of this motion, probe tip diameter becomes a critical dimension. The probe tip diameter helps determine the width of the area of each pad that is scrubbed or scratched up on contact with the needles.
The needle body diameter and taper shape help determine how much each needle will flex and the amount of force (known as balance contact force) with which the needles will touch down on the wafer. This is a critical specification set by the wafer manufacturer. It affects probe needle life, and, therefore, probe card life, and determines how well the probe tip will break through the thin layer of aluminum oxide on the metal bond pads.
As a manufacturer of probe needles, Point has quality control inspection standards to ensure proper probe needle performance. The company must measure locations on the needle that define the taper shape, as well as measuring the diameter at specified distances back from the tip, prior to bending. The probe diameter and the taper length (the distance from the tip to the end of the taper) also are measured before bending.
In recent years, demand for wafer production grew, causing an increase in demand for probe needles. Since inspection is a significant proportion of total needle manufacturing time, it was clear to Point’s management that inspection would be a prime candidate for automation.
The Old Days
Before starting its recent improvement project, the company relied on a combination of manual video-inspection systems and several optical comparators to inspect probe needle geometry. “We had two inspection methods,” says Edgar Miller, Point’s quality manager. “An optical comparator enlarged a shadow of a part by 50X, 100X or 250X. An operator would place a fixture on the comparator, and then align and focus the part to be measured. Numerical measurements could be taken using a calibrated ruler, or we could use charts with the required needle shape and tolerance, so the operator could ensure part acceptance.”
Video inspection was an alternate method. “The video inspection system would magnify the part up to 720X,” adds Miller. “The operator would place a fixture on the video inspection system, focus, and align the part. He then would pick points on the taper to compare with the required specifications.”
These inspection processes clearly were slow, labor intensive, required extensive operator training, and were error-prone due to operator bias. They had to go.
Setting Sights on Automation
Point subsequently investigated available options for improving the needle inspection process. “Commercially available equipment that could do the job was cumbersome, slow and too expensive, so our team designed a system to better meet our needs,” says Steve Neely, Point’s semiconductor division manager. “With a clear vision and well-defined goals, our design team won the financial support necessary to succeed and move ahead.”