Machine Vision

Manufacturing Proble Needles with Vision

August 1, 2005 By: Dave Senders, Steve Neely, John Lewis Sensors

Combining motion control and machine vision expands inspection equipment capabilities.

Motion control and machine vision are used throughout the semiconductor manufacturing process, from monitoring the diameter of ingots as they are 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 that the various pieces of equipment can do their tasks.

Point Technologies, Inc.—a supplier of precision electrochemical pointing and micromachining services and products for small-diameter wire and tubing to the semiconductor, medical, and biotech industries—has recently applied motion and vision to the new area of probe needle inspection for the semiconductor industry.

Eyeing the Needle

Semiconductor manufacturers rely on probe needles to test ICs during the final phase of production to ensure that only the functional ones get packaged for final use. A probe needle is a straight, small-diameter metal wire with one end tapering down into a sharp point (see Figure 1). Probe needles establish an electrical connection between tester and IC by contacting the metal bond pads on the wafer.

 Figure 1. If probe geometry is inconsistent, the reference diameter, bend angle, tip diameter, and tip length will be inconsistent.
Figure 1. If probe geometry is inconsistent, the reference diameter, bend angle, tip diameter, and tip length will be inconsistent.

Generally hundreds or thousands of probe needles are assembled into an array on a device called a probe card, which is tailored to interface between the specific type of IC being tested and the wafer prober. During testing, precise needle geometry is essential to ensure test data reliability and consistency for several reasons. First, when the prober aligns with the wafer being tested and then lowers the probe card onto the IC, the needles flex on contact with the wafer, causing the tips to slide across the metal pads. Probe tip diameter is a critical dimension that determines the area of the pad that is scrubbed.

Next, the needle's diameter and taper shape determine how much it will flex, and how much force it will apply as it touches down on the wafer. This is called the Balance Contact Force, a critical contact pressure specification set by the manufacturers that affects probe card life and the probe tip's ability to break through a thin layer of aluminum oxide on the metal bond pads to the metal beneath.

Finally, the probe tips must be precisely bent before assembly onto the probe card; this requires precise and careful work. Most probe card manufacturers use a reference diameter to determine where to bend the probe. If probe geometry is inconsistent, the reference diameter, bend angle, tip diameter, and tip length will be inconsistent. Any of these problems can cause probe misalignment and result in inconsistent test data.

Because probe needle geometry is vital for a successful test operation, Point Technologies, in the past, relied on a combination of manual video inspection systems and optical comparators to provide probe card manufacturers with needles that met stringent requirements. However, with rising IC production volumes and increasing demand for probe needles, this process was far too labor-intensive. Because needle inspection accounted for a significant proportion of the total manufacturing time, it was a prime candidate for automation.

Setting Its Sights on Automation

In late 2003, Point Technologies started investigating ways to improve its needle inspection process. Commercially available equipment was too cumbersome, slow, and expensive for its needs, so an engineering team from the company set about designing their own system.

The major design goals were to increase measurement throughput, accuracy, and repeatability while reducing inspection time. The system should also automatically document all measurements onto a shared server, minimize operator training, and maximize operator comfort. Finally, it should automatically plot the measurement data on a graph for comparison to customer specifications and to provide statistical analysis for process improvements.

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