Semiconductor probe needle inspection and gauging
Combining motion 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 being 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 micro machining services and products for small diameter wire and tubing to the semiconductor, medical and biotech communities—has recently applied motion and vision to a new area in the semiconductor industry: probe needle inspection.
Formerly, the company relied on a combination of manual video inspection systems and several optical comparators to inspect probe needle geometry. This method was slow, labor intensive, required extensive operator training, and was error prone due to operator bias.
By integrating Cognex machine vision with motion control in an automated needle inspection station, called PointScanTM, the design team created a system that inspects needles faster, provides more accurate measurements, requires less operator training, automatically documents each measurement, all while improving repeatability and providing feedback for process improvements.
Eyeing the needle
Semiconductor manufacturers rely on probe needles to test Integrated Circuits (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 that tapers down into a sharp point. They play the vital role of establishing an electrical connection between tester and IC by contacting the metal bond pads on the wafer.
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 upon contact with the wafer causing the tips to slide across the metal pads. In this way, probe tip diameter is a critical dimension that determines the area of the pad that is scrubbed.
Next, the needles’ diameter and taper shape determine how much it will flex, and the amount of force with which 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 how well the probe tip will break through a thin layer of Aluminum oxide on the metal bond pads.
Finally, the probe tips must be precisely bent before assembling the probes onto the probe card; this requires precise and careful work. Most probe card manufacturers use 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 poor test data consistency.
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 meet stringent requirements. However, as IC production volumes rise and demand for probe needles increases, it became obvious that 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 sights on automation
In late 2003, Point Technologies initiated an investigation of available options for improving the needle inspection process. Commercially available equipment that could do the job was much too cumbersome, slow, and expensive to offer a viable solution, so the team set about designing a system to better meet their needs. With a clear vision and well-defined goals, the design team managed to garner the financial support necessary to succeed.
The primary design goals were set to increase measurement throughput, accuracy and repeatability, while reducing inspection time. The system would also automatically document all measurements onto a shared server, minimize operator training, and maximize operator comfort. Finally, it would automatically plot the measurement data on a graph for comparison to customer specifications, and provide statistical analysis for process improvements.
With the time and resources in hand, the engineering team set out to master the art of integrating motion control and machine vision. Up-front work was done to evaluate the feasibility of using a machine vision system to improve the measurement efficiency and still get repeatable measurements within 0.00002 inches. With little to no machine vision experience, it made sense to start with an In-Sight® vision system from Cognex because it was easy to learn and comes complete with user software.
In-Sight is a high-performance machine vision sensor that consists of a DSP-based vision processing unit, high-speed digital camera, onboard light control, and built-in discrete I/O. It also provides a standard VGA output for real-time display, built-in Ethernet communications, and an onboard serial port.
Without attending any formal training classes, it took approximately 30 hours to get comfortable enough with In-Sight to set up and set up the inspection routine. Reviewing the tutorials that came with it and using the on-phone technical support provided by Cognex was very helpful in gaining familiarity with the unique vision spreadsheet development environment, library of vision software tools, and the built-in operator interface.
The In-Sight vision system was a good validation tool, however wasn’t the right product for the final application. Though it measured the needles and output the data, the design team chose to go with a Cognex VisionPro® PC-based vision system instead, because it offered higher performance and provided more seamless integration with the motion control system. Moving up to the VisionPro platform took a lot more time to learn, and required a couple of weeks of help from a Cognex Certified Vision Integrator (CVI). The expert CVI was instrumental in developing the vision code required, and was well worth the investment, especially considering that the vision code was used again for subsequent systems.
The PC-based vision system uses a Cognex MVS-8100D Series frame grabber inside of a basic Dell PC, and a high-resolution (1280x1024) Cognex CDC-200 digital camera. One of the most challenging parts of any machine vision application is selecting the image formation system to produce a good image. After some lighting and optics experimentation, the team implemented a high-magnification telecentric lens that eliminates optical distortions and a 2x2 inch diffuse LED backlight to provide suitable images with the necessary resolution.
Vision meets motion
The motion control system of the PointScan probe needle inspection station uses limit switches, but all other input comes from the vision system. Engineers blended the vision, motion, operator interface, network communications, and database using MicroSoft Visual Basic 6.0 on the PC.
All logic and control is managed with Visual Basic, which allows stepper motors to move the camera and position the needles. Vision-guided motion applications such as this require a very fast vision system that will synchronize image capture, analysis, and measurement with the motion required to find the needle, focus the needle, and move from needle to needle.
PointScan’s motion control system is essentially a tabletop, Cartesian robot that uses precision stepper motors to drive three axes of motion. One axis moves the loading table into and out of the inspection area. The other two axes control camera movement. One moves horizontally. The other moves vertically. The motion system combines assorted components such as a three-axis motion controller, precision linear slides, leadscrews, and stepper motors from various vendors.
Modeling every last component using Autodesk Inventor® 7.0 helped to streamline the process, and reduce design time. In all, the team was able to go from concept to working system in about six months. Since late 2004, five systems have been built and are being used in production. The systems are primarily used for measuring probe needles, but they are also used for measuring electro-surgery needles for the medical device industry.
The design team’s previous experience building multi-axis automation systems made the mechanical design pretty straightforward, though there were some challenges. For example, due to the very small size of the probe needles and the high magnification of the vision system, special care was taken to eliminate sources of vibration in the machine design, and by using vibration dampers.
When considering camera resolution, optics and working distance to achieve the desired field of view (FOV) another challenge arose. The entire length of part being measured didn’t fit into the FOV, so the part must be moved a fixed distance under the camera to complete the taper length measurement. This complicated the application because it required combining the information from more than one image.
Integrating vision and motion also requires that engineers carefully calibrate the vision system with the motion system. If there’s optical distortion from the lens, or perspective changes due to camera mounting angle, this involves more than coming up with a scaling factor that relates pixels to a measured dimension. The vision system tool suite includes special algorithms to correct for these errors.
Lights, camera, inspection
Prior to inspection, 200 blank straightened wires are loaded into a transferable fixture, or block, and then tapered in a proprietary Electro-Chemical Pointing (ECPTM) process. The needles are inspected in their holding fixtures in process and after completion. For inspection, the operator carries the block of probes to the PointScan, and presses their name on a 17-inch flat panel touch screen interface. The specification for every job are stored in a shared database, so after setting the fixture down on the loading table, the operator simply presses ‘Go’ to start the inspection.
The loading table automatically indexes the probe needles into the inspection area. The camera then moves over the top of the needles from the side. When its FOV intersects the first needle, the table backs up until the point is located. Tight coupling of the vision with the motion allows the two systems to communicate in order to locate the point, and then center the point in the FOV.
To optimize measurement accuracy, repeatability, and reproducibility, PointScan also includes an automatic focus routine. During this step, the vision system communicates with the motion system as it moves the camera up and down to focus on the needle. Once focused, the vision system acquires the image, analyses the image, and makes the appropriate measurements with the required accuracy. The data is automatically recorded, and the camera jogs to the next needle to be measured.
As measurements are recorded, PointScan automatically plots the data on the screen so that the operator can see if they are within the specification. At the end of the inspection, or if the operator presses ‘Stop’, PointScan automatically unloads the needles, and the pins are accepted or rejected.
If the parts are rejected, the operator will need to rework, or make new parts, and can use the inspection data to adjust their process. If the parts are accepted, a label is printed that allows for future reference of the inspection data. At the end of the job, the system generates an inspection report to accompany each shipment that includes all of the data for the measured parts and compares it with customer specification.
The report helps streamline quality issues by clearly communicating specification and measurements to the customer. Providing actual measurement data distributions helps customers to better model their process. In addition, the reports give customers confidence in the process, and is attractive to potential new customers because it saves the customer time since they don’t have to do their own internal inspection.
Machine vision has enabled the design team to create an inspection system where a computer does all of the inspection and most of the data analysis. Tests indicate more than a ten-fold reduction in inspection time.
It used to take a person about 20 minutes to measure 12 needles and record the data on paper. PointScan measures 12 needles in less than 2 minutes, records the data, and automatically graphs out the data for the operator.
Machine vision has proven invaluable for accurately gauging very small parts and providing quantitative results that can be used to track parts, productivity, and to help troubleshoot inefficiencies in the process.
The data gathered by the vision system provides great insight into the production process. In fact, PointScan provides the feedback needed for operators and the quality team to successfully “re-model” systems and processes with more built-in improvements and efficiencies.
As a result, Point Technologies sees machine vision and motion control as key for the future of manufacturing. Integrating vision with motion is not as difficult as some would have you believe. However, it does take time, money, and expertise, but done right, even the most challenging applications can pay off.