Accuracy performance analysis of a chrome on glass dot grid calibration target

To demonstrate the accuracy of our Chrome on Glass Calibration Target, the circle center locations of dot grid plate are measured by high accuracy Optical Measurement Machine (OMM). Then the feature (X, Y) locations are analyzed for a better understanding of the performance.

In this report, dot center positions (x, y) in the calibration target are measured as shown below:
Ø  Dot Diameter: 1.0mm
Ø  Dot Spacing: 2.0mm
Ø  Array size: 11x6
Ø  Pattern Area: 20x10mm
Ø  Type: chrome-on-glass calibration target
Ø  OMM machine used: Hexagon OMM machine with uncertainty E_xy= ±(0.6+L/900) um, L is the measured object length in milimeter. (Laboratory temerature 20℃, humidity 49%)
Ø  OMM measuring 3rd party is accredited in accordance with ISO/IEC 17025: 2005 General Requirement for the competence of Testing and Calibration Laboratories
Ø  Test Report(Original data) of this chrome dot grid calibration target ➡ Go Link

 
Figure 1. Ideal location of chrome on glass dot grid target

Figure 2. OMM Measured results of all circle centers

For most measurement results, the measured (x, y) positions may generally have an inclination or eccentricity effect regarding its ideal positions. The original (x, y) results are transformed by a best-fit 2D rigid transformation, which will not change relative position of all points towards their ideal (x, y) value. After this transformation, the inclination and eccentricity effect of measured value can be eliminated. 

After this pre-processing, the accuracy of chrome on glass dot grid calibration target is analyzed from 3 aspects as follows:

1. Feature Position Accuracy among Entire Pattern Area
The error of every point is analyzed by calculating the distance between ideal (x, y) location and actual (x, y) location. The result is shown in Figure 3. 

 
Figure 3. Positioning error of the measured dot grid plate

To have a better understanding, the error vectors of all points are also presented in Figure 4. The error vector is pointing from a point’s ideal location to its actual location. In order to have a better visualization, all the vector are enlarged with a scale of 1844. As shown in Figure 4, the maximum error is marked in red circle. The maximum error of this plate is 0.81um, so it is enlarged to 1.5mm for a better visualization. 

Standard deviation of these 63 distance errors is also calculated, the 1x standard deviation is 0.15um.

Figure 4. Vector error of all points

2. Feature Spacing Accuracy of Any 2 Points among entire pattern area
In order to quantify the accuracy of this plate among the entire area, the distance errors are analyzed for any 2 point-pairs and illustrated in Figure 5. In this plate, there are 63X62/2=1953 pairs of any 2 point-pair distances. 

The minimum and maximum errors are -1.0um and +1.0um. That means for whole plate area, for any two-point pairs the production error is less than [-1.0um, +1.0um]. A negative value means the actual two-neighbor-point distance is less than ideal distance. A positive one means this distance is greater than the ideal one. 

Standard deviation of these 1953 errors is also calculated, the 1x standard deviation is 0.36um.

Figure 5. Feature Spacing Error of Any 2 Points of the dot grid calibration target

3. Feature Spacing Accuracy of Any 2 Neighbor Points
The nearest-neighbor distance errors are also analyzed and illustrated in Figure 6. In this plate, there are 108 pairs of nearest-neighbor distances, whose ideal values are all 2mm.

The minimum and maximum errors are -0.80um and +0.80um. That means for arbitrary two-nearest-neighbored-point pair whose ideal distance is 2mm, the production error is less than [-0.80um, +0.80um]. A negative value means the actual two-neighbor-point distance is less than ideal distance (2mm in this plate). A positive one means this distance is greater than the ideal one. 

Standard deviation of these 108 errors is also calculated, the 1x standard deviation is 0.26um.

Figure 6. Nearest-neighbor distance errors of the dot grid calibration target

Conclusion:
1. From a whole plate view, Feature Position Accuracy among Entire Pattern Area is 0.81um. That means all circle centers have a high accuracy of <±1um. You can achieve similar results by using your optical measurement system with our precise calibration targets. 
Corresponding standard deviation is 0.15um;
2. From another aspect of overview, Feature Spacing Accuracy of Any 2 Points is [-1.0um, +1.0um]. That means the maximum deviation of the actual distances and ideal values among all circle center distances of any two circles shall not exceed [-1.0um, +1.0um]. 
Corresponding standard deviation is 0.36um.
3.  From a small range aspect, the performance of plate is also analyzed. The conclusion is that Feature Spacing Accuracy of Any 2 Neighbor Points is [-0.80um, +0.80um].
Corresponding standard deviation is 0.26um.