Machine Design

Fast frame grabbers force wafer surfaces to reveal their secrets

A digital camera records images from the phosphor screen of a RHEEM (right) which catches electrons diffracted from the crystalline structure on the surface of a silicon wafer. Captured images go to a Data Translation DT3152 PCI-based frame grabber. Its programmable resolution lets the board capture the camera's full 768 pixels/line of horizontal resolution.

Software devised by k-Space lets researchers analyze images captured via frame grabbers to calculate factors such as atomic spacing, thickness, and surface roughness.

Super-powerful systems able to analyze the surface of silicon wafers in real time have become practical thanks in part to framegrabber technology. Systems integrator k-Space Associates in Ann Arbor, Mich., devised a system based on a PC, camera, and frame grabber that hooks up to the phosphor screen of a RHEED (reflection high-energy electron diffraction) device. The camera and frame grabber capture images of diffraction patterns that represent the crystalline structure of the wafer surface. Researchers process the digitized images to study parameters such as atomic spacing, thickness, and roughness.

The heart of the system, called the kSA 400, is a Data Translation Mach Series DT3155 or DT3157 PCI frame grabber which captures images from the digital camera. Custom software monitors the process.

Developers at k-Space say they used the Data Translation frame grabbers to get enough accuracy for the detailed measurements required. For example, the boards can record every pixel of data produced by a digital camera's array to get the best possible spatial resolution accuracy. Because much of the imaging concerns rotating wafer surfaces, the asynchronous grab accuracy of the boards is also important. Developers at k-Space wrote software that monitors the pulse that triggers an image capture, the camera frame-enable pulse, and the positional variation of the grabbed image. The resulting digitized diffraction images from the frame grabber are inspected and used to calculate the atomic spacing of the atoms on the surface of the wafer.

A software development kit from Data Translation, Marlboro, Mass., let k-Space meet special requirements by using different frame grabbers as needed, without rewriting existing code. For example, some applications need more than 8-bit depth in intensity to more finely analyze diffraction patterns. Such needs call for use of a 12-bit camera and DT3157 frame grabber to get full 4,096 levels of depth, compared to the standard 8-bits of depth with an analog camera and the DT3152. This swapping of frame grabbers takes place with little or no code rewriting.

High-performance electron defraction systems have only become powerful enough to provide such capabilities in the last few years, says k-Space. Developments contributing to their feasibility include the advent of CPUs clocked at 1 GHz or more, lowercost high-end digital cameras with high spatial resolution and signal-to-noise ratios, and large hard disks speedy enough to record full digital movies of diffraction pattern evolution.

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