Application Note | Coating Adhesion
Micro-Scrape Test of Polymeric Coating on Silicon Wafer
Measuring Cohesion and Adhesion of a Soft Coating on a Brittle Substrate Without Cracking the Wafer
Research & Experimental Testing
Duanjie Li
Visual Design & Editorial
Andrew Shore
Introduction
Polymeric coatings are extensively used to enhance the mechanical, tribological, optical and chemical properties of different substrates in a wide variety of industries, such as semiconductor, solar energy, optics and biomedicine. The intrinsic quality of the coating and the interfacial integrity of the coating/substrate system determine the service performance and lifetime of the polymeric coating.
Why Soft Coatings on Brittle Substrates Need a New Test
Scratch and micro-scratch testing has developed into one of the most widely applied methods to evaluate the cohesive and adhesive strength of the coatings. The critical load, at which a certain type of coating failure occurs as the applied load progressively increases, is widely regarded as a reliable tool to determine and compare the adhesive and cohesive properties of the coatings.
The most commonly used indenter for scratch testing is the conical Rockwell diamond indenter. However, when the scratch test is performed on the soft polymeric coating deposited on a brittle substrate such as silicon wafer, the conical indenter tends to plough through the coating forming grooves rather than creating cracks or delamination. Cracking of the brittle silicon wafer takes place when the load further increases. Therefore, it is vital to develop a new technique to evaluate the cohesion or adhesion properties of soft coatings on a brittle substrate.
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Measurement Objective
In this application, the cohesive and adhesive strength of a polymeric coating on the silicon wafer substrate is evaluated by micro-scrape test using the NANOVEA Mechanical Tester.
NANOVEA PB1000 Large Platform
Mechanical Tester
Test Conditions
The NANOVEA Mechanical Tester was used to perform the micro-scrape tests on a polymer coating sample on the silicon wafer substrate using the test parameters summarized in Table 1. A new sample is used for the micro-scrape tests at each load.
| Test parameter | Value |
|---|---|
| Load type | Constant |
| Loads | 0.25, 0.5, 1, 2, 3, 3.5, 3.75 and 4 N |
| Sliding speed | 3 mm/min |
| Sliding distance | 1 mm |
| Indenter geometry | Knife |
| Indenter material (tip) | Diamond |
| Indenter tip angle | 90° |
| Atmosphere | Air |
| Temperature | 24 °C (room) |
Table 1: Test parameters of the micro-scrape measurements.
Results and Discussion
The adhesion of the polymer coating on the silicon substrate is measured in this study. Due to the brittle characteristics of the silicon wafer, a diamond stylus of a horizontal knife-shape edge is used to avoid penetration of the silicon substrate that may be caused by the sharp conical indenter. The full scrape tracks on the polymer coating sample at different constant loads are shown in Fig. 2. The polymer coating has a 90° triangle shape pattern on the silicon wafer substrate.
It can be observed that the polymer coating shows signs of cohesive failure at the tip of the triangle shape at a normal load of 0.25 N. As the constant normal load during the micro-scrape test increases from 0.25 to 2.0 N, the cohesive failure region on the coating progressively expands from the tip of the triangle to its bottom as indicated by the increased size of the dark area. When the load further increases from 2.0 to 3.75 N, signs of delamination of the polymer coating start to appear at the tip of the triangle. The coating completely delaminates from the substrate at a normal load of 4.0 N. Such critical loads at which typical forms of coating failures occur can be used to quantitatively compare the cohesive and adhesive strength of the coating to select the best candidate for targeted applications.
Fig. 3 plots the evolution of COF during the micro-scrape test that may provide more insight in the progression of coating failures. At a low load of 0.25 N, the COF remains constant as the knife tip scrapes in the silicon substrate and the polymer coating areas. However, when the knife tip scrapes at the tip of the triangle, i.e. the edge of the coating, a high COF peak is measured due to the elevated height by the coating. A small cohesive failure of the polymer coating takes place at the same time. At a higher load of 2.0 N, cohesive failure takes place on the whole surface of the polymer coating, leading to higher COF value measured in the coated area. As the normal load further increases to 4.0 N, the polymer coating is completely scraped off from the silicon substrate.
Conclusion
In this study, we showcased that the NANOVEA Mechanical Tester performs reliable micro-scrape tests on a polymer coating deposited on a silicon wafer substrate. A knife-shape diamond tip of a horizontal edge is selected to perform the test so as to avoid penetration through the brittle silicon substrate. By applying different constant loads, the micro-scrape measurement allows users to identify the load at which typical cohesive and adhesive coating failure occurs. It provides a superior tool to quantitatively evaluate and compare the intrinsic quality of the polymer coating and the interfacial integrity of the coating/substrate system.
The Nano, Micro or Macro modules of the NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing available in a single system. NANOVEA’s unmatched range is an ideal solution for determining the full range of mechanical properties of thin or thick, soft or hard coatings, films and substrates, including hardness, Young’s modulus, fracture toughness, adhesion, wear resistance and many others.
Frequently Asked Questions About Micro-Scrape Testing
Why not use a standard scratch test on soft coatings over silicon?
A conical Rockwell indenter tends to plough grooves through a soft polymer coating instead of creating cracks or delamination, and the brittle silicon wafer cracks as the load increases. A different technique is needed.
What is a micro-scrape test?
A diamond stylus with a horizontal knife-shaped edge is drawn across the coating at a constant load, so the coating is scraped rather than penetrated and the brittle substrate is not cut.
At what loads did the polymer coating fail?
Cohesive failure appeared at the tip of the coating pattern at 0.25 N and spread across the coating up to 2.0 N; delamination started between 2.0 and 3.75 N, and the coating fully delaminated at 4.0 N.
What does the friction signal show?
At 0.25 N the COF stays constant except for a peak at the coating edge; at 2.0 N cohesive failure over the whole coating raises the COF in the coated area; at 4.0 N the coating is scraped off.
Which test parameters were used?
Constant loads from 0.25 to 4 N, one new sample per load, a 1 mm scrape at 3 mm/min with a 90° diamond knife tip, in air at room temperature.
What are the results used for?
Critical loads for cohesive and adhesive failure quantitatively compare coating quality and coating/substrate integrity, to select the best candidate for a targeted application.
