Can You Measure Thickness of Biologically Active Films?

Biologically active films are crucial in the field of medical devices and sensors, where precision, reliability, and compatibility are essential. Films are usually comprised of biomaterials or functional coatings and are incorporated into a variety of applications, including implantable medical devices and diagnostic sensors.

The accurate measurement of film thickness is not just a technical necessity; it is a fundamental requirement that directly affects the performance and safety of these essential biomedical tools.

For example, when developing implantable medical devices, such as bioresorbable scaffolds or drug-eluting stents, the controlled deposition of biologically active coatings dictates their drug release kinetics and biocompatibility.

In diagnostic sensors, the precise thickness of functional layers is critical for optimal sensing performance, sensitivity, and selectivity. As a result, characterizing these films goes well beyond simple metrology; it is a vital aspect of the design and quality assurance processes in the biomedical field.

This article investigates the significance of controlling film thickness when producing glucose sensors, tissue sealers, and blood sensors. The advantages of innovative reflectometry and ellipsometry technologies for assuring consistency, quality, and performance of medical devices are also highlighted.

Whole-Wafer Mapping of Film Thickness for Glucose Sensors

Continuous glucose monitoring (CGM) technology has transformed the management of diabetes, providing continuous insights into blood glucose levels in real-time.

Biologically active films are crucial to the effectiveness of continuous glucose monitoring (CGM) devices, where they are integrated into the glucose sensors. Engineered to selectively react to glucose molecules while minimizing interference from other substances in the physiological environment, the thickness of these films directly impacts the sensor’s sensitivity, response time, and overall performance.

Meticulous control and accurate, repeatable measurements of film thickness are essential for ensuring consistent sensor output.

Optical techniques are ideal for process control in medical device manufacturing, where measuring film thickness requires non-contact and non-destructive methods.

However, even two of the most common optical methods still encounter issues in these environments:

  • Optical profilometry techniques are problematic because the films are thick (several microns or more), so a clear step is not easily available.
  • Standard reflectometry and ellipsometry techniques often struggle to provide accurate readings, as biologically active films are typically deposited on rough metal electrode surfaces, causing variations in the film’s thickness across the measurement area, which in turn causes the reflected light to become incoherent.

To obtain consistent mirrored reflection from a thick film and rough metal surface, the optical technique must have a small spot size and an aligned beam.

The Bruker FilmTek 2000M spectroscopic reflectometer is an ideal choice when it comes to overcoming these issues. The patented optical design integrates a measurement spot size as small as 1 x 2 µm and an aligned beam. Accurate, non-contact measurements of thick films, even on rough substrates, is possible.

Automated wafer handling, along with 1D/2D barcode scanners and pattern recognition, enables straightforward measurements across an entire device wafer. This eliminates the need to infer overall performance from a limited sample area.

Integrated SECS/GEM software facilitates automated factory control of recipe selection (including recipe change control) and output data management.

Film thickness data for many implantable glucose sensors over an entire wafer

Figure 1. Film thickness data for many implantable glucose sensors over an entire wafer. Image Credit: Bruker Nano Surfaces and Metrology

In Figure 1, FilmTek 2000M was utilized to produce implantable glucose sensors for diabetes patients. The film thickness was measured automatically at the active sensor area of each device across the entire product wafer.

Coating Thickness Mapping on the Surface of Metal Jaws

Tissue sealers are critical in laparoscopic surgeries for sealing and dividing blood vessels and tissue structures. They are designed to increase hemostasis, decrease postoperative complications, and improve the accuracy of the entire surgical process.

Utilizing tissue sealers during laparoscopic procedures gives surgeons the reliability needed for effective hemostasis with minimized tissue damage. Films or coatings add lubrication and shielding when applied to the surface of these devices. Accurate film thickness measurement is thus vital for device performance and consistency, biocompatibility, tissue trauma reduction, and compliance with regulations.

Film thickness data for a nonstick coating on metal jaws, mapped over the surgical device

Figure 2. Film thickness data for a nonstick coating on metal jaws, mapped over the surgical device. Image Credit: Bruker Nano Surfaces and Metrology

The use of small-spot, collimated-beam technology in FilmTek metrology tools allows for application on a wide variety of substrates, including silicon, metal, and glass.

In the provided example, the thickness of a nonstick coating on a tissue sealer device was mapped across the metal jaw (Figure 2). FilmTek optical technology delivers nanometer-level thickness accuracy for film coatings, even when these vary by several thousand microns across the device.

Film Thickness Determination for Multi-Layer Stacks in Single-Use Blood Sensors

Handheld blood sensors are crucial instruments in hospital settings. They provide healthcare professionals with a fast and accurate way to analyze vital blood parameters, including a critical biomarker for cardiac health and troponin levels.

In the context of cardiac care, troponin response becomes a key indicator of myocardial injury, helping medical professionals diagnose and manage cardiovascular conditions in a timely manner.

This point-of-care capability is crucial in emergency departments, where immediate assessment and diagnosis are essential for patients who are presenting with chest pain or symptoms indicative of potential cardiac issues.

Handheld blood sensors rely on the precision of production, where film thickness metrology is instrumental in reliable data. Sensors with thin films applied are vital interfaces between the device and the blood sample that influence accuracy and sensitivity.

The reliability of handheld blood sensors is heavily dependent on the precision of their manufacturing processes, where film thickness metrology is crucial. The thin films on these sensors act as essential interfaces with the blood sample, directly influencing the device’s accuracy and sensitivity.

Accurate measurement of each film’s thickness is vital because it affects the sensor’s interaction with blood constituents and its ability to deliver accurate results. This underscores the need for advanced metrological techniques to precisely measure and control film thickness during manufacturing.

In this context, film thickness metrology is not just a quality control measure but a fundamental element in ensuring the effectiveness of handheld blood sensors.

Reflectometry and ellipsometry techniques are suitable for non-contact thickness measurement. Spectroscopic ellipsometry can measure thin films (<250 nm), and reflectometry can rapidly measure thicker films (>5 nm).

However, the accuracy of thickness measurements and the ability to resolve multi-layer stacks largely depend on the accuracy of the refractive index, which may be unknown for advanced biologically active films.

When characterizing the refractive index using these methods, encountering multiple solutions is a common issue, introducing uncertainty. This uncertainty limits both the precision of thickness measurements and the capacity to clearly resolve multi-layer stacks.

Film thickness mapping for a very thin film in a blood sensor device

Figure 3. Film thickness mapping for a very thin film in a blood sensor device. Image Credit: Bruker Nano Surfaces and Metrology

In Figure 3, FilmTek 2000 PAR-SE was utilized in the manufacturing process of single-use blood sensors on silicon wafers. FilmTek’s multi-angle and multi-modal measurement technology is suitable for the accurate measurement of thin films and multi-layer stacks that contain the active sensor area of the devices.

Concurrent multi-angle calculations produce a wavelength shift between spectra that is only a function of refractive index. Combined with proprietary modeling algorithms, the added information permits the absolute determination of unknown materials and multi-layer stacks. In Figure 3, a thin 30 Å film is mapped over the wafer with a standard deviation of ~1 Å.

Layer-by-layer analysis (three layers) for a multilayer film in a blood sensor device

Figure 4. Layer-by-layer analysis (three layers) for a multilayer film in a blood sensor device. Image Credit: Bruker Nano Surfaces and Metrology

In Figure 4, the thicknesses of a multi-layer stack at the active sensor region are unambiguously determined across the wafer. Accurate and reliable film thickness measurement is crucial for ensuring consistent manufacturing, regulatory compliance, and maintaining the overall quality of these medical devices.

Conclusion

Film thickness metrology tools are crucial in the accurate production of medical devices and sensors. This article examined the accuracy and robustness of thickness measurements to the sub-angstrom level using revolutionary reflectometry with small, aligned beam technology and multi-angle reflectometry/ellipsometry technology.

Companies can improve the quality, consistency, and performance of manufactured devices by using these revolutionary metrology tools and solutions. This will contribute to healthcare technology innovations and protect the safety and reliability of medical devices in real-world applications.

 

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces and Metrology.

For more information on this source, please visit Bruker Nano Surfaces and Metrology.

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