Similar thickness of a silicon chromium, or SiCr, film does not guarantee matching electrical performance inside BCD devices. Films that appear similar in thickness may differ in density, composition, microstructure, and surface roughness—factors that can affect the temperature coefficient of resistance (TCR), resistor matching, and long-term stability. For this reason, a technical article published by Semiconductor Engineering and written by engineers from Onto Innovation presents an approach that combines thickness measurement with simultaneous reflectance measurement.
Picosecond ultrasonics uses laser pulses to generate acoustic waves inside the wafer. By analyzing the acoustic echo within the SiCr layer, the film thickness can be estimated in a noncontact, nondestructive manner on production wafers. However, the same signal also provides reflectance data, making it possible to add a qualitative indicator of surface quality without carrying out a separate measurement step.
What Does Thickness Measurement Not Reveal?
The properties of SiCr film are affected by deposition conditions, including the flow of specialized gases during the sputtering process. A change in flow can produce variations in microstructure, composition, and density, even when the thickness remains close to the target value. The article indicates that properties such as grain growth and the spacing between grain boundaries can affect TCR behavior, meaning that electrical differences may appear between wafers that do not look significantly different when thickness alone is examined.
The practical problem is that these deviations may not be apparent during deposition, but may emerge later in electrical testing or product reliability. Therefore, adding simultaneous reflectance measurement may provide process engineers with an early signal of changes that thickness metrology alone does not capture.
What Did the Comparison Between the Two N₂ Flow Conditions Show?
The experiment described in the article collected thickness, acoustic, and reflectance data from SiCr wafers processed under two different N₂ gas flow conditions. The target thicknesses were similar, but the reflectance signatures differed clearly. The condition with higher reflectance was associated with a smoother, denser surface, while the condition with lower reflectance was associated with greater surface roughness.
The article states that the film was thinner under the higher N₂ flow condition. This flow may affect silicide or nitride formation and grain spacing, and therefore can change thickness and reflectance together. According to the presentation, reflectance is not used here as a complete direct measurement of composition or electrical performance, but rather as a qualitative indicator that helps detect process differences and monitor the effect of gas flow.
What Changes in Practice for BCD Manufacturers?
The primary change is not to replace thickness measurement, but to expand what can be extracted from the same measurement platform. Instead of treating thickness as the sole indicator of deposition uniformity, the manufacturing team can compare thickness maps with reflectance maps across the wafer. A difference in reflectance with stable thickness may help direct the investigation toward changes in surface roughness, density, or microstructure before their effects reach electrical testing.
This matters for applications that use BCD to integrate analog circuits, digital control, and power management, including automotive electronics, medical products, and consumer electronics. In these devices, inconsistency in SiCr resistors may affect analog accuracy, thermal behavior, or long-term stability—not merely the nominal thickness value.
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The technical value of this proposal lies in linking physical measurement with an additional indicator of film quality within a single step, rather than waiting for the problem to appear in final electrical measurements. However, the limits of the conclusion are important: the article is published in the form of a Sponsor Blog by Onto Innovation and presents a qualitative comparison between two flow conditions, without providing detailed figures for thickness or reflectance, or electrical results demonstrating the extent of improvement in TCR or manufacturing yield.
Therefore, the technique can be considered a promising means of increasing the visibility of deposition deviations, not a proven replacement for all film-characterization tools. Questions such as the quantitative relationship between reflectance and electrical performance, and the extent to which the indicator can be generalized to different compositions and manufacturing conditions, still require additional data. What the article clearly demonstrates is that similar thickness does not necessarily mean similar film quality, and that combining reflectance with acoustic measurement may provide an early warning of these differences.