Automated Semiconductor Wafer Sorting Using NI LabVIEW with Synchronized Motion, Vision, and DAQ

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"This project wouldn’t have been economically viable without LabVIEW and NI synchronized motion, vision, and DAQ products."

- Jeff Long, Automation Works, Inc.

The Challenge:
Sorting semiconductor wafers automatically into categories based on physical and electrical characteristics such as thickness, bow, warp, total thickness variation (TTV), and type (N-type or P-type) in addition to matching the precision and repeatability of industry-standard equipment with greater throughput, flexibility, and user friendliness at much lower cost.

The Solution:
Taking advantage of NI LabVIEW software, toolkits, and advanced analysis capabilities with tightly synchronized motion, vision, and data acquisition hardware to create a PC-based system that sets a new standard for semiconductor wafer sorting

Author(s):
Jeff Long - Automation Works, Inc.

In semiconductor manufacturing, the push for greater efficiency and higher yield of silicon semiconductor material is never ending. As circuit features shrink in size and global price competition intensifies, wafer processes push the physical and operational limits of equipment manufacturers. One result is increasingly narrow tolerances for incoming wafer physical and electrical parameters in delicate process steps, such as mask and etch.

To accommodate tight process step tolerances, wafers must be pre-sorted into narrow categories based on electrical and mechanical parametric values such as thickness, bow, warp, TTV, and type after semiconductor wafers are sawn from an ingot and before processing. Gigamat Technologies, Inc., a Milpitas, CA, manufacturer of sorting, polishing, and edge grinding equipment undertook the task of developing the model 200TRT, a new generation of automated, high-accuracy, high-throughput, full-scan wafer sorting machines with the help of Jeff Long of AutomationWorks, Inc.

System Requirements

Measuring the bow, warp, and TTV of a wafer requires performing a full dimensional measurement scan of the wafer top and bottom surfaces. This is not only technically challenging, but represents a significant increase in process time compared to simple, single-point measurements that were previously sufficient. For these measurements to be useful, they must match industry-standard benchtop instruments, which have the luxury of taking a great deal of time to ensure measurements are precise. Gigamat’s challenge was to automatically sort wafers from cassettes using full-scan measurements at high throughput rates, with industry-standard accuracy and repeatability.

LabVIEW with Synchronized DAQ, Motion, and Vision on a single PC

LabVIEW running on a PC was the key to integrating all of the high-performance technologies required to make this project a success. Combining the hardware synchronization of PCI boards controlling eight NI motion axes with two NI data acquisition boards and one vision board, the inherent multi-tasking and re-entrant execution capabilities and DAQmx task, timing, and triggering programming simplicity in LabVIEW gave engineers an ideal platform to rapidly implement, test, and validate multiple iterations of process code.

The measurement process is comprised of two steps, wafer alignment and wafer measurement. Wafer alignment identifies the location and orientation of the wafer relative to a vacuum chuck on which it is held and repositions the wafer exactly on the chuck center and with its primary fiducial precisely oriented.

The second functional step in the measurement process is the performance of the full wafer scan. This step involves acquiring top and bottom distance measurements from many points across the surface of the wafer and performing analysis on them to derive results.

Wafer Alignment

Wafer alignment is performed using three axes of motion and a linescan camera. A wafer is aligned by rotating it in the field of view of the camera. By synchronizing camera scans with chuck rotation, a 6 Megapixel image of the wafer edge is composed in a single revolution , which takes about one second. Because camera scans are synchronized with chuck position, they are independent of chuck velocity and may be acquired during chuck acceleration and deceleration ramps to save time. The wafer center, flat and other features are identified from image data using LabVIEW vision, math, and advanced analysis tools. The wafer is then rotated and indexed in two short moves to bring it into perfect alignment for the measurement station.

Wafer Measurement

A full measurement scan is performed by gripping a wafer from beneath with a rotational chuck and spinning it between top and bottom probes which measure the distance to the wafer surface with a resolution of <0.0001mm. Accurate and repeatable wafer measurements require high measurement density, and throughput requires high data acquisition rates. Measurements must be associated with the position on the wafer from which they were acquired. The solution was to synchronize the NI PCI-6115 four-channel simultaneous acquisition board with the measurement chuck position to acquire many sets of position-related measurements per chuck revolution. Controlling the position of a servo translational carriage on which the chuck was mounted as data was acquired allowed for full surface scanning. We used NI Motion-based contoured moves to create seamless combinations of circular and spiral trajectories employing sometimes one and at other times two axes of motion for optimal throughput and measurement integrity. We then used LabVIEW advanced analysis and math tools extensively to calculate measurement results from the more than many thousands of measurement sets acquired from each wafer.

Process Control

In addition to wafer alignment and measurement, we developed a complete application executive using LabVIEW that integrates:

  • Graphical touchscreen user interface (via front panel events)
  • Control of 12 wafer elevators via RS-485
  • Independent control of 62 servo and stepper motors
  • Digital I/O control of vacuum, light tower, machine power, etc.
  • Powerful recipe management tools that allow the sorter to operate in many different modes on wafers of varying diameters and shapes

Data Management

The Gigamat wafer sorter makes extensive use of the LabVIEW Database Connectivity Toolkit to interface with Microsoft Access for storage and recall of all machine configuration parameters, process recipes, and measurement results. Customers can easily search and sort resulting database files using Access, Excel, or text-based tools as required in this highly flexible and customizable data management architecture.

“This project wouldn’t have been economically viable without LabVIEW and NI synchronized motion, vision, and DAQ products.” said Edmond Abrahamians, president and CEO of Gigamat, “The amount of time and money saved by choosing to integrate off-the-shelf National Instruments products versus developing custom-engineered solutions of satisfactory performance made the difference between success and failure. The timeline of this project was minimized with the extensive use of built-in LabVIEW tools that provided our engineers with the ability to rapidly implement and visualize solutions to problems, and to use measurement data as a closed-loop feedback tool for machine and process development. The result is a wafer sorter that matches accuracy and repeatability of standalone systems, but with full automation and high throughput rates."

Author Information:
Jeff Long
Automation Works, Inc.
266 Bayview Avenue
San Jose, CA 95127
Tel: (408) 937-7765
jeff.long@automationworks.com

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