What is deposition uniformity and how is it measured? Optimize Film Consistency & Quality Control

Deposition uniformity is a metric used to evaluate the consistency of a thin film across a substrate. While it most frequently refers to the evenness of film thickness, it can also quantify the consistency of other physical properties, such as the index of refraction. This measurement is essential for determining the quality and control of the deposition process.

Uniformity provides a statistical snapshot of process stability. It is measured by analyzing deviations from an average value across the wafer, calculated either through standard deviation or a range-based formula, while strictly excluding edge effects to ensure accuracy.

Quantifying Uniformity

To determine how consistent a film is, engineers typically rely on one of two primary calculation methods. The choice depends on the specific requirements of the application and the level of statistical rigor needed.

The Statistical Approach (Sigma)

This method is widely used for robust process control. It involves collecting data points across the wafer and calculating the average.

Uniformity is then expressed using standard deviation (sigma). Depending on the stringency of the process limits, engineers may look at one, two, or three sigma values to represent the deviation from the mean.

The Range-Based Calculation

For a more direct assessment of the spread between the highest and lowest points, an alternative formula is used.

This calculation is defined as: ((Maximum Value – Minimum Value) / (2 x Average Value)). This method highlights the absolute range of variation relative to the film's average thickness.

Critical Measurement Protocols

Obtaining a raw number is not enough; the method of data collection dictates the validity of the measurement.

The Importance of Edge Exclusion

When performing metrology, it is vital to exclude a specific zone near the substrate's edge.

Mechanical clamping and other edge-related phenomena can cause localized irregularities. Including this data can lead to misleading results, skewing the uniformity metric and masking the true performance of the deposition across the usable area of the wafer.

Making the Right Choice for Your Goal

Select your measurement strategy based on the specific data fidelity your process requires.

• If your primary focus is statistical process control: Rely on the standard deviation (sigma) method to track consistency and trends over time.
• If your primary focus is absolute variance: Use the ((Max – Min) / (2 x Avg)) formula to identify the worst-case spread across the substrate.
• If your primary focus is data accuracy: Ensure you implement a strict edge exclusion zone to prevent clamping artifacts from corrupting your quality metrics.

Accurate uniformity measurement is the baseline for ensuring reliable, high-yield thin film deposition.

Summary Table:

Metric Type	Calculation Method	Best Used For
Statistical (Sigma)	Standard Deviation from Mean	Robust process control and tracking long-term trends.
Range-Based	((Max - Min) / (2 * Avg))	Identifying absolute variance and worst-case spread.
Edge Exclusion	Data Point Filtering	Ensuring accuracy by removing mechanical clamping artifacts.

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