Semiconductor metrology

Integrated circuits form the bedrock of the Information Age. Silicon wafer sizes are increasing while the device feature sizes decrease, reflecting Moore’s Law and a driving need for more precise metrological and inspection sources. As the complexity on the wafer surfaces grows, it is essential that fabricated devices are monitored for optimization and quality control, both during and after processing.

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Cost, and energy, conservation is a priority - where the development of smaller, more efficient laser technology allows the optimization of production and makes the manufacturing process more efficient, lowering the cost per unit.

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Lasers drive the inspection and measurement at nearly every step of semiconductor wafer fabrication.

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Critical parameters - such as thin film thickness or non-uniformity in deposition, defects, holes and scratches, overall flatness, deviations in the crystal structure or consistency of doping - can be detected and optimized using several interferometric techniques.

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Laser interferometry, and the monitoring of an interference pattern from the wafer surface, is essential for controlling a number of these parameters. Ellipsometry allows sub-wavelength resolution of these features, below the diffraction limit of light. Similarly, confocal microscopy detects subsurface errors and provides thin film quality diagnostics. Controlling the accuracy and uniformity of layer thickness optimizes the material use, improves performance, and reduces the number of in-process failures.

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This brings a need for high resolution imaging and requires consistent measurement precision and accuracy. As such, the selection of an appropriate laser source requires the examination of several key characteristics:

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1.    Wavelength

Wavelength requirements for the semiconductor industry are, largely, in the UV range – however, inefficiencies in harmonic conversion are problematic at these lower wavelengths. This is where modern, continuous-wave single frequency DPSS lasers can fill the gaps and reduce errors in defect detection. Single frequency sources ensure a precise interference pattern during inspection and measurement.

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2.    Low Noise

Wafer inspection lasers must emit low noise to help minimize detection errors and prevent inaccuracy in the analysis between signal noise and laser source. Low noise level, in combination with narrow linewidths, increases the signal to noise ratio and enhances measurement and inspection sensitivity.

3.    Stability

These lasers also require exceptional spectral and power stability, as well as long coherence lengths, to eliminate errors in prolonged measurement and ensure stable operation. Ultra-stable laser sources that provide excellent power stability and long-term wavelength stability are well suited for applications that demand precise high-resolution measurements.

4.    Small footprint

Compact laser sources allow for integration into existing systems without augmentation of the existing set-up, as well as reducing bench-space requirements. The ability for modern DPSS lasers to produce excellent beam quality at high power from a small footprint, allows for maximum flexibility in deployment and operation.

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5.    Low maintenance

The removal of an ongoing maintenance schedule helps avoid unscheduled downtime for semiconductor manufacturers - conserving production time and costs.

Additionally, the lack of consumables needed for operation with solid state lasers mean that there is little downtime during processing and operation, with no need to stop for the replenishment of the gas used, or refurbishment of other optical components. This allows faster production while maintaining a high yield, while also increasing the quality of measurements made and decreasing the need for complex analysis of the data gathered.

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Skylark design and manufacture high power, single frequency laser sources with unrivalled wavelength stability, narrow linewidths and long coherence lengths over a range of wavelengths within a small footprint – making them well suited for integration into existing systems. We currently offer single frequency lasers in the infrared, as well as red area of the visible spectrum with our 640 Series Laser; to complement our range of UV sources, such as the 349 NX Laser, to facilitate metrology at decreasing feature sizes.