Surface Analysis of Metals in the Glass on Smart Phone Screens

This article discusses the analysis of elemental concentration in the near surface region of smart-phone display glass using the technique of depth profiling. The distribution of alkali and alkali earth metals is studied, especially potassium  which is an important element in the glass toughening process. Here, monatomic Ar and Ar cluster depth profiling methods are compared to understand the ion bombardment effects of these two techniques. This article highlights the significance of ion choice when depth profiling inorganic materials, especially materials consisting of light alkali metals.

Introduction

Traditionally, soda-lime glass has been employed in architectural and automotive windows but recently, it has been employed in photovoltaics and smart displays. Due to this shift in application it has become increasingly important to understand the processes taking place in the surface of the glass for quality control and failure analysis. The distribution and mobility of alkali and alkali-earth metals is especially important for electronic devices as they can have a significant impact on the electrical properties of glass.

Soda-lime glass typically contains Si, O, Na, Mg and Ca, but the element K is also included in smart phone displays to minimize the possibility of fracture under impact. The manufacturing method involves the ion-exchange process whereby Na in the glass surface is replaced by K. The occupation of more space in the glass structure by the larger K ions creates residual compressive strength (Figure 1).

Figure 1. Ion-exchange mechanism.

This article aims to understand the metal distribution in toughened glass. Depth profiling  is used to analyse the distribution of elements in the surface region. Monatomic Ar⁺ is traditionally used for this process as the impinging ion but the advantages of using large Argon cluster ions are also discussed in this article.

Experiment

A Kratos AXIS Nova photoelectron spectrometer was used to acquire all measurements. It incorporates several features which make high resolution spectroscopic analysis of these kinds of challenging samples a routine process:

• A 500 mm Rowland circle Al X-ray monochromator
• Delay line detector
• Magnetic lens
• Coaxial charge neutralization system
• 165 mm hemispherical analyzer

Thanks to the large surface area of the sample, the entire smartphone screen was loaded without the need for cleavage and possible loss in integrity. The smart phone screen was placed on the platen, secured by two copper clips (Figure 2).

Figure 2. Smartphone screen loaded on sample platen.

The new Gas Cluster Ion Source was used to perform depth profiling. The ions used were 5 kV Ar⁺ and 20 kV Ar₅₀₀⁺. The size of the crater for both profiles was 1 mm x 1mm, with an analysis area of 220 µm diameter in the center of each crater. Using a SiO₂ standard, the etch rate was found to be 13 nm/minute for 20 kV Ar₅₀₀⁺.

Results and Discussion

Figure 3a shows the XPS depth profile using 5 kV Ar⁺.

Figure 3. 5 kV monatomic depth profile.

Si and O, as expected, are the primary components. The low concentration elements are enlarged in Figure 3b. K has the highest concentration (~2.4%) at the surface of the display and its concentration decreases to <0.5% at depths greater than 5 µm. Concurrent to the decrease in K concentration is a rise in Na concentration. Interestingly, there is little difference in other elements present in the surface region that remain unchanged, highlighting the relationship between K and Na as the key participants in the ion-exchange mechanism. After the removal of the outer K-treated layer, a steady-state Na concentration is reached (<4%) – a value considerably less than the stoichiometric ratio expected for the soda-lime glass of ~9%.

In earlier studies, decreased Na+ ions have been reported in the surface region under monatomic Ar⁺ bombardment^2,3. The highly mobile Na⁺ ions are repelled into the bulk of the glass due to the build-up of positive charge in the near-surface region. When an ion approaches the surface of the glass, a positive surface layer is created on the glass due to electron transfer from the surface. Na ions are repelled into the glass matrix due to this positive region, creating a sodium-depleted zone. This process is commonly called field-induced migration, causing an underestimation in the concentration of alkali elements.

For comparison, 20 kV Ar₅₀₀⁺ cluster ions were used to perform a depth profile. Here also K showed the highest concentration at the surface (3.6%), which is 33% more than was observed in the monatomic Ar⁺ depth profile.

Figure 4. 20 kV Ar₅₀₀⁺ depth profile.

Most significant is the difference in bulk Na concentration using AR₅₀₀⁺, 8.8% compared to <4% using monatomic Ar⁺. This value is also similar to the expected stoichiometric concentration, suggesting the occurrence of negligible field-induced migration under cluster bombardment. The variation in ion current between the depth profiling methods is the reason behind this reduction in migration. The surface experiences an ion current of several µA under monatomic Ar⁺ but the current is typically 100 times less in cluster mode. This reduced ion current leads to less charge build-up and therefore, decreased migration. Figure 5 shows a comparison of the two modes for Na and K.

Figure 5. Comparison of K and Na profiles for monatomic (red) and cluster (blue) depth profiles.

Conclusions

This article has discussed the analysis of a smart phone display to investigate metal distribution in the top surface using two different depth profiling methods. The K distribution in the surface was of particular interest as this element serves as a toughening agent. The presence of K was confirmed but its concentration reduced to <0.5% at depths greater than 5 µm, revealing shallow penetration of K in the ion-exchange method.

For the two modes, there was a discrepancy in surface concentration of K with the cluster profile showing considerably more surface K. This result, along with monatomic Ar⁺ underestimating bulk Na concentration, emphasizes the limitations of monatomic depth profiling caused by unwanted effects of field-induced migration of Alkali metal. The effect is not as pronounced as observed with Na due to a reduction in ion mobility with increasing ionic radii.

From these results, it is evident that the combination of XPS and Ar cluster ion sputtering provides a simple, reliable depth analysis method for alkali glass.

References and Further Reading

1. Y. Yamamoto, K. Yamamoto, J. Non. Cry. Sol., 356 2010, 14.
2. J. Counsell et al. J. Surf. Anal., 20, 2014, 211.
3. L. S. Drake, D. E. King, J. R. Pitts, A. W. Czanderna, Ion Beam Effects – Plenum Press NY.

This information has been sourced, reviewed and adapted from materials provided by Kratos Analytical, Ltd.

For more information on this source, please visit Kratos Analytical, Ltd.